Download Excitatory Amino Acid Receptors at a Feedback Pathway in the

Excitatory Amino Acid Receptors at a Feedback Pathway in the
Electrosensory System: Implications for the Searchlight Hypothesis
NEIL J. BERMAN, JAMES PLANT, RAY W. TURNER, AND LEONARD MALER
Department of Cellular and Molecular Medicine, University of Ottawa, Ottawa, Ontario K1H 8M5, Canada
Berman, Neil J., James Plant, Ray W. Turner, and Leonard
Maler. Excitatory amino acid receptors at a feedback pathway in
the electrosensory system: implications for the searchlight hypothesis. J. Neurophysiol. 78: 1869–1881, 1997. The electrosensory
lateral line lobe (ELL) of the South American gymnotiform fish
Apteronotus leptorhynchus has a laminar structure: electroreceptor
afferents terminate ventrally whereas feedback input distributes to
a superficial molecular layer containing the dendrites of the ELL
principle (pyramidal) cells. There are two feedback pathways: a
direct feedback projection that enters the ELL via a myelinated
tract (stratum fibrosum, StF) and terminates in the ventral molecular layer (VML) and an indirect projection that enters as parallel
fibers and terminates in the dorsal molecular layer. It has been
proposed that the direct feedback pathway serves as a ‘‘searchlight’’ mechanism. This study characterizes StF synaptic transmission to determine whether the physiology of the direct feedback
projection is consistent with this hypothesis. We used field and
intracellular recordings from the ELL to investigate synaptic transmission of the StF in an in vitro slice preparation. Stimulation of the
StF produced field potentials with a maximal negativity confined to
a narrow band of tissue dorsal to the StF. Current source density
analysis revealed two current sinks: an early sink within the StF
and a later sink that corresponded to the anatomically defined
VML. Field potential recordings from VML demonstrated that
stimulation of the StF evoked an excitatory postsynaptic potential
(EPSP) that peaked at a latency of 4–7 ms with a slow decay
( Ç50 ms) to baseline. Intracellular recordings from pyramidal cells
revealed that StF-evoked EPSPs consisted of at least two components: a fast gap junction mediated EPSP (peak 1.2–1.8 ms) and
a chemical synaptic potential (peak 4–7 ms) with a slow decay
phase ( Ç50 ms). The amplitudes of the peak and decay phases of
the chemical EPSP were increased by depolarizing current injection. Pharmacological studies demonstrated that the chemical EPSP
was mainly due to ionotropic glutamate receptors with both
N-methyl-D-aspartate (NMDA) and non-NMDA components.
NMDA receptors contributed substantially to both the early and
late phase of the EPSP, whereas non-NMDA receptors contributed
mainly to the early phase. Stimulation of the StF at physiological
rates (100–200 Hz, 100 ms) produced an augmenting depolarization of the membrane potential of pyramidal cells. Temporal summation and a voltage-dependent enhancement of later EPSPs in
the stimulus train permitted the compound EPSP to reach spike
threshold. The nonlinear behavior of StF synaptic potentials is
appropriate for the putative role of the direct feedback pathway as
part of a searchlight mechanism allowing these fish to increase the
electrodetectability of scanned objects.
INTRODUCTION
Vertebrate sensory transmission consists of ascending
pathways leading from receptors to higher ‘‘integrative’’
regions and eventually to motor areas of the brain. At each
level, local neuronal interactions generate receptive fields
that characteristically increase in selectivity and complexity
at succeeding levels of the neuraxis. Sensory systems also
have extensive feedback projections and long-range horizontal interactions. These connections may permit regions
outside the cell’s classic receptive field to modulate responses to receptive field input and may underlie ‘‘higher
level’’ effects such as attention and adaptation to sustained
input.
This paper focuses on the synaptic physiology of a feedback projection in the electrosensory system. Electric fish
generate electric organ discharges (EODs) used for electrocommunication (Heiligenberg 1991) and electrolocation
(Bastian 1986a). Distortions in the fish’s EOD are detected
by electroreceptors, which project to the medullary electrosensory lobe (ELL) (Carr and Maler 1986). The ELL is a
laminar structure and consists of four segments (medial,
centromedial, centrolateral, and lateral segments) optimized
for processing of different features of the electrosensory input (Maler 1989; Shumway 1989). Pyramidal cells are the
principle output neurons of the ELL and project topographically to the midbrain (torus semicircularis) and the isthmic
nucleus praeminentialis (Pd) (Carr and Maler 1986). The
ascending electrosensory projections have been mapped in
detail up to sensorimotor interfaces in the optic tectum and
diencephalon (tectum: Bastian 1982; diencephalon: Heiligenberg 1991; Rose and Heligenberg 1988), and recordings
from periphery to diencephalon reveal the elaboration of
receptive fields leading to outputs suitable for driving motor
responses (Heiligenberg 1991).
There are two excitatory feedback pathways to the ELL,
both emanating from at least three cell types in Pd (Carr
and Maler 1986; Sas and Maler 1983, 1987): stellate,
multipolar, and tufted cells. Stellate cells project directly and
topographically back to the ventral molecular layer (VML)
of the ELL (direct feedback) via a myelinated fiber bundle
(tractus stratum fibrosum, tSF; within the ELL this tract is
designated the stratum fibrosum or StF) and terminates
densely in the ventral-most portion of the ELL molecular
layer (VML; Fig. 1) (Maler 1979; Maler et al. 1981b).
Multipolar and tufted cells project to cerebellar granule cells
overlying the ELL, and these cells in turn project parallel
fibers to the dorsal molecular layer (DML) of the ELL (indirect feedback). There is also a direct inhibitory feedback
projection from GABAergic bipolar cells in the hilar region
of Pd, via fibers lying at the ventral aspect of tSF, to pyramidal cell somata (Maler and Mugnaini 1994).
Bastian and coworkers have used a variety of physiological electrosensory inputs to characterize the responses of the
cells giving rise to the direct (Bratton and Bastian 1990)
0022-3077/97 $5.00 Copyright q 1997 The American Physiological Society
/ 9k1d$$oc22 J277-7
09-05-97 11:58:59
neupa
LP-Neurophys
1869
1870
N. J. BERMAN, J. PLANT, R. W. TURNER, AND L. MALER
FIG . 1. Sites for recording and stimulating the stratum fibrosum (StF) projection in an electrosensory lateral line lobe
(ELL) transverse section (level T-7) (Maler et al. 1991). Injection of wheat germ agglutinin (WGA)-horseradish peroxidase
in isthmic nucleus praeminentialis (Pd) anterogradely labeled the stratum fibrosum, which terminated in the ventral molecular
layer (VML) and retrogradely labeled pyramidal cells (Maler 1979; Maler et al. 1992). One basilar pyramidal cell ( right
of – – – ) shows the orthogonal orientation ( – – – ) of their dendritic arbors to the ELL laminae. Inset: simultaneous
recordings obtained from a pyramidal cell proximal apical dendrite ( bottom traces) and the mid-VML field potential ( top
traces) in the centromedial segment during low- (thick trace) and high-intensity (thin traces) StF stimulation in the medial
segment (*). AS, antidromic spike; BS, brain stem; EGp, eminentia granularis posterior (cerebellar granule cells); CLS,
centrolateral segment; CMS, centromedial segment; DFL, deep fiber layer; DML, dorsal molecular layer; DNL, deep neuropil
layer; GCL, granule cell layer; LS, lateral segment; MS, medial segment; PA, primary (electroreceptor) afferents; pf, parallel
fiber; PCL, pyramidal cell layer; and PlL, plexiform layer.
and indirect (Bastian and Bratton 1990) feedback pathways.
Stellate cells have small receptive fields with phasic responses and appear to code for the movement of objects
across their receptive fields; their response properties and the
topographic nature of their feedback projections prompted
Bratton and Bastian (1990) to hypothesize that the direct
feedback pathway was involved in focusing the electrosensory system, i.e., a ‘‘searchlight’’ mechanism (Crick 1984;
see also Maler and Mugnaini 1993, 1994). The indirect
feedback pathway appears to regulate more global changes
in electrosensory processing (Bastian 1986b,c; Bastian and
Bratton 1990).
Both direct and indirect feedback projections to the ELL
molecular layer probably use the excitatory amino acid
(EAA) glutamate as a neurotransmitter (Wang and Maler
1994). Studies using receptor binding (Maler and Monaghan
1991), in vivo pharmacological analysis (Bastian 1993),
and in situ hybridization (Bottai et al. 1995–1997) have
suggested that both N-methyl-D-aspartate (NMDA) and aamino-3-hydroxy-5-methyl-4-isoxazolepropionic
acid
(AMPA) receptors are associated with the excitatory feedback input to the ELL.
The development of selective ionotropic EAA receptor
agonists and antagonists has facilitated analysis of their role
in sensory systems; however, the interpretation of these studies has been hampered by the complexity of sensory processing in mammals. The simpler electrosensory system is
more tractable for the study of low-level sensory processing.
This study focuses on the synaptic physiology of the StF
/ 9k1d$$oc22 J277-7
(direct) feedback projection to the ELL and on the properties
of the ionotropic EAA receptors involved in the putative
electrosensory searchlight.
METHODS
Preparation of ELL slices
A total of 87 weakly electric fish of the species Apteronotus
leptoryhchus (Brown Ghost Knife Fish) were used. Transverse
slices of the ELL were prepared as previously described (Mathieson and Maler 1988; Turner et al. 1994). Fish were anesthetized
(MS-222), transferred to a foam-rubber–lined holder, and respirated with 11–14 ml/min of oxygenated water containing anesthetic during surgery. The brain stem was transected, glued to an
aluminum block and 350–500 mm transverse ELL slices cut on a
Vibratome into oxygenated ice-cold artificial cerebrospinal fluid
[ACSF, which contained (in mM) 124 NaCl, 24 NaHCO4 , 10
D-glucose 1.25 KH2PO4 , 2 KCl, 2 MgSO4 , and 2 CaCl2 ; all chemicals from Sigma unless otherwise noted].
Slices were positioned rostral side up in an interface slice chamber and maintained at room temperature in oxygenated ACSF for
a minimum of 1 h before recordings were begun. The StF was
clearly evident (Mathieson and Maler 1988) and served as a landmark for the placement of stimulating and recording electrodes.
Recordings were obtained exclusively from the centromedial segment of the ELL (Figs. 1 and 2).
Stimulation of StF
Stimulating electrodes were either bipolar (65 mm nichrome
wire) or a monopolar tungsten electrode ( õ5-mm tip diameter).
09-05-97 11:58:59
neupa
LP-Neurophys
EXCITATORY AMINO ACID RECEPTORS AT A FEEDBACK PATHWAY
1871
ACSF) were 2 kynurenic acid, 1 3-((RS)-2-carboxypiperazin4-yl)-propyl-1-phosphonic acid (CPP, Research Biochemicals International, MA), 2 DL-2-amino-5-phosphonopentanoic acid
(APV, Tocris, UK), and 1 6-cyano-7-nitroquinoxaline-2,3-dione
[CNQX, Tocris; predissolved in 100 ml dimethyl sulfoxide]. Control microdroplet applications of ACSF alone had no effect on StFevoked responses.
Recording of synaptic potentials
Potentials recorded by glass micropipettes (1 M NaCl; 2–10 MV ) were filtered (DC-10 kHz),
amplified (Axoclamp-2A, Axon Instruments), digitized (10–25
kHz), and stored on disk for off-line analysis (CLAMPAN, Axon
Instruments; A/Dvance; IgorPro, Wavemetrics, OR).
ANALYSIS OF FIELD POTENTIALS. Under each stimulus condition, 10–15 consecutive StF-evoked field potentials were averaged.
Field potentials were mapped along the dendro-somatic axis of
ELL pyramidal cells (dashed line in Fig. 1) by two methods. 1)
StF field potentials were recorded from positions corresponding
to visually identifiable anatomic structures within the ELL slice
preparation, allowing for the construction of low-resolution spatial
maps of StF-evoked field potentials (n Å 8). 2) High-resolution
maps constructed by recording field potentials (50 mm depth) every
25 mm along most of the ELL pyramidal cell axis (n Å 3) were
used for one-dimensional current source density (CSD) analysis.
Standard methods for CSD analysis in slices were followed (BodeGreul et al. 1987; Richardson et al. 1987; Taube and Schwartkroin
1988) using the equation (Bode-Greul et al. 1987) d 2 (P)/dz 2 Å
P(z / n ∗ Dz) 0 2 ∗ P(z) / P(z 0 n ∗ Dz)/(n ∗ Dz) 2 , where P is
the evoked field potential, z is the spatial location, Dz is the sampling interval (25 mm), and n is the integration grid (n Å 3).
INTRACELLULAR RECORDING. Intracellular recording pipettes (3
M potassium acetate, 70–200 MV ) were advanced into the CMS
pyramidal cell layer using a microdrive (Burleigh Instruments,
NY). Recordings from pyramidal cell somata or proximal apical
dendrites (Turner et al. 1994) were amplified by an Axoclamp 2A
preamplifier, filtered (DC-10 or 25 kHz) and digitized for off-line
analysis. Results are given as means { SD, and statistical analysis
is by Student’s t-test.
FIELD POTENTIAL RECORDING.
FIG . 2. Laminar profile and current source density (CSD) analysis of
StF-evoked currents. A: schematic diagram of a tissue slice at the level of
the ELL. Recordings were made in the centromedial segment along a track
parallel to the pyramidal cell soma-dendritic axis (rrr). Refer to Fig. 1
for abbreviations. B: field potentials recorded at several key sites from a
laminar profile of StF-evoked responses with distance noted with respect
to the StF recording site; r, stimulus time (in this and subsequent figures,
artifacts were digitally suppressed). The VML response ( – – – ) is displayed at half the gain of other potentials (4 mV on calibration bar). *,
peak negativity in StF (presumed fiber volley). C: superimposed CSD
measurements from 3 locations shown in B. D: a spatial profile of CSD
over the pyramidal cell axis at a latency corresponding to the peak of the
negative-going field potential in VML shown in B. Profile is aligned with
a schematic of a basilar pyramidal cell, shown with StF afferents contacting
its apical dendrites in the VML.
The stimulating electrode was positioned at the dorsal aspect of
the StF within the medial segment to prevent direct electrical activation of centromedial segment interneurons, pyramidal cell efferent axons within the plexiform layer (Turner et al. 1994) or
GABAergic axons from Pd bipolar cells (Maler and Mugnaini
1994). Stimulus timing was computer-controlled (pClamp, Axon
Instruments, CA; A/Dvance, McKellar Designs, BC; Master-8,
AMPI, Israel) and delivered via constant voltage or current stimulus isolation units (Digitimer, UK; 10–80 V or 30–800 mA;
0.1–0.5 Hz; tetanic stimulation: 100–200 Hz for 100 ms).
Drug application
Drugs were applied using two methods. 1) Bath applications for
field potential experiments. After 1 h of normal ACSF, perfusate
was switched for ú1 h to either 4 mM Mn 2/ -ACSF solution [containing (in mM) 129 NaCl, 10 D-glucose, 3.25 KCl, 0.2 CaCl2 ,
11.4 tris(hydroxymethyl)aminomethane, and 20 N-2-hydroxyethylpiperazine-N *-2-ethanesulfonic acid] or Mg 2/ -free ACSF. 2)
Micro-droplet applications: Drugs were applied by brief pressure
ejection ( õ10 psi; 80–190 ms) from pipettes (3- to 7-mm tips)
placed at the surface of the slice centered in the VML dorsal to
the recording site (Turner et al. 1994) (NeuroPhore BH-2, Medical
Systems, NY).
For microdroplet application, drug concentrations (in mM in
/ 9k1d$$oc22 J277-7
RESULTS
The ELL is a laminar structure with separate layers for
electrosensory afferent (deep fiber layer) and feedback input
(VML, DML). The majority of efferent neurons are within
the pyramidal cell layer, whereas most interneurons are located in the granular cell layer. The StF is a myelinated
fiber tract the unmyelinated terminal fibers of which branch
densely in the VML, which occupies Ç100–150 mm (15–
20%) of the total molecular layer width (measured along
the pyramidal cell dendritic axis: dashed line, Fig. 1). Pyramidal cell axons run in the plexiform layer to exit at the
medial aspect of the ELL. As seen in Fig. 1, the dendritic
axis of pyramidal cells is orthogonal to the ELL cellular
and fiber laminae (Maler 1979); this geometric arrangement
allows field potential recordings of activity evoked by stimulation of input or output fiber tracts.
Because the StF is in close proximity to both the plexiform
layer (pyramidal cell efferents) and the DML, it was important to establish a means of selectively stimulating StF
afferent inputs. As shown by Fig. 1, inset, StF stimulation
at relatively low intensities ( õ70 V) selectively activated a
prominent EPSP in a recording from a pyramidal cell proxi-
09-05-97 11:58:59
neupa
LP-Neurophys
1872
N. J. BERMAN, J. PLANT, R. W. TURNER, AND L. MALER
mal apical dendrite. Stimulation at intensities ú 70 V evoked
both a short-latency (antidromic) spike and a reduced excitatory postsynaptic potential (EPSP). These two potentials
are reflected in field recordings in the mid-VML as a shortlatency negativity (0.96 { 0.42 ms; n Å 8) and a later,
longer lasting field EPSP (3.2 { 0.56 ms; n Å 44). The first
response results from current spread to the plexiform layer,
which elicits an antidromic spike that backpropagates into
dendritic regions (Turner et al. 1994). The second response
represents synaptic depolarization of pyramidal cell apical
dendrites in the VML (see below). These identifying characteristics allowed stimulus intensity in all subsequent experiments to be set at a level that selectively activated StF afferent inputs.
StF-evoked EPSPs
FIELD POTENTIAL RECORDINGS. Laminar profiles of StFevoked field potentials were constructed, and CSD analysis
carried out to examine the site for termination and the nature
of synaptic inputs activated by StF stimulation (Fig. 2). The
largest field potential response evoked by StF stimulation
always was recorded in the VML as a biphasic potential
consisting of a short-duration positivity followed by a rapid
negative-going potential (peak latency 3.2 { 0.56 ms,
03.02 { 1.50 mV; n Å 44) with a long duration ( ú30 ms;
Fig. 2B). The initial positivity in the VML was correlated
temporally with a sharp, short-duration negativity in the StF
(1.7 { 0.30 ms; n Å 35; Fig. 2B, StF trace, *). The next
negative peak in the StF response was lower in amplitude
and longer lasting, decaying to baseline over Ç20 ms. A
small broad field positivity was recorded at the level of the
pyramidal cell body layer and the proximal DML with a
peak latency slightly delayed to that recorded in the VML
(4–7 ms; Fig. 2B, PCL trace, *). In the pyramidal cell body
layer, single-unit spike discharge was sometimes superimposed on the peak of the positivity (not shown). Much
smaller potentials were recorded in the mid-DML and granular cell body layers, most often as a triphasic potential or
monophasic positivity, respectively; no field potentials could
be detected in the deep fiber layer (DFL) or distal DML.
CSD analysis indicated that the shortest latency current
sink (net inward current) was located in the StF at a latency
similar to a current source in the VML (Fig. 2C). This was
followed by current sinks of longer duration in both the StF
and VML, which was balanced by a current source of similar
duration in the pyramidal cell body layer. Figure 2D illustrates the sink-source relationships over the pyramidal cell
axis at the latency of the peak current sink in VML. This
reveals that a current sink extended for Ç100 mm dorsal to
the pyramidal cell body layer, a location consistent with the
anatomic definition of the VML (125 mm thick in the example of Fig. 1). This current sink was flanked by prominent
current sources in both the pyramidal cell layer and the
proximal DML.
These results verify anatomic reports that the termination
field of StF afferents is restricted to the StF and VML region
(Fig. 1) (Maler 1979; Maler et al. 1982) and that our stimulus did not substantially activate the excitatory parallel fiber
input in the adjacent DML. The primary StF response is one
of excitatory synaptic drive, with the shortest latency current
/ 9k1d$$oc22 J277-7
sink located in the StF, followed by that in the VML. This
is consistent with anatomic reports that StF fibers branch
sharply to course dorsally and form synaptic contacts with
pyramidal cell apical dendrites in the StF and then VML
(Maler 1979; Maler et al. 1981a). This ventro-dorsal axonal
projection and synaptic termination pattern may account for
the coincidence of an initial sharp current sink in the StF
with a sharp current source in the VML. The latter then may
arise in part as a passive current source for active spike
discharge and/or synaptic depolarizations of dendritic membrane in the more ventral StF (see below).
PHARMACOLOGY OF STF-EVOKED FIELD POTENTIAL. The
above identification of StF-evoked responses was further
tested using blocking agents of synaptic transmission (Fig.
3). The StF-evoked VML potential was decreased significantly by bath application of 4 mM Mn 2/ in low Ca 2/ (0.2
mM)-containing medium, decreasing from 3.0 { 1.5 to
1.73 { 1 mV (P õ 0.01; n Å 6; Fig. 3A). The negative
peak of the remaining triphasic potential had a latency of
2.55 { 0.73 ms and was the expected response for a fiber
volley in StF afferent fibers (Fig. 3A; note that subsequent
FIG . 3. Pharmacology of StF-evoked extracellular field potentials in the
VML. Ai: superimposed recordings of the VML field potential under control
(1, con) conditions and in the presence of 4 mM Mn 2/ and 0.2 mM Ca 2/
medium (2, Mn 2/ ) to block synaptic transmission. Aii: synaptic component
as the difference between the control and test potentials (1 and 2). B–D:
superimposed recordings of the VML field potential under control (con)
conditions and after exposure to kynurenic acid (B, Kyn), 3-((RS)-2carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP; C), and 1 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; D). Note the partial reduction
by kynurenic acid and CPP of both an early ( B and C; õ10 ms) and late
phase (*) of the field response, whereas CNQX (D) completely blocks the
synaptic response. E: superimposed recordings of the VML field potential
under control (con) conditions and after exposure to nominally Mg 2/ -free
medium for 1 h. Note the pronounced augmentation of both the peak and
late phase of the synaptic response by low Mg 2/ medium. F: superimposed
recordings of a VML field potential augmented under 0 Mg 2/ medium and
test responses to subsequent application of CNQX and CNQX with CPP.
Under these conditions, CNQX only partially blocks the synaptic response,
and the remaining component was blocked almost entirely by CPP. Field
recordings recovered to near control levels after washout (n Å 2) of CNQX,
kynurenic acid, or Mg 2/ -free medium after 1–2 h.
09-05-97 11:58:59
neupa
LP-Neurophys
EXCITATORY AMINO ACID RECEPTORS AT A FEEDBACK PATHWAY
experiments described below indicated an additional contribution by an electrotonic synaptic component). Subtraction
of the remaining response in Fig. 3A revealed a long-lasting,
synaptically mediated component of the StF-evoked response with a peak latency of 4.3 { 1.3 ms (n Å 6), a
value similar to the peak latency of EPSPs recorded with
intradendritic impalements (see below) and significantly
greater than the latency of the fiber volley (P õ 0.05). These
data indicated that the StF fiber volley slightly overlapped
the early component of the field synaptic response, contributing to this aspect of field potential recordings in normal
medium. Nevertheless, there was sufficient separation between presynaptic fiber and synaptic potentials to pharmacologically distinguish an early and late phase of the EPSP
(see below).
Fiber volley potentials recorded during Mn 2/ perfusion
at 100-mm intervals along the StF were used to calculate a
StF conduction velocity of 0.31 m/s ( Ç237C). An additional
conduction delay of Ç0.28 ms was introduced by the unmyelinated collaterals of the StF fibers, which ascend and synapse in the VML.
Previous studies of receptor ligand binding (Maler and
Monaghan 1991) and application of EAA agonists and antagonists in vivo (Bastian 1993) indicated that EAA receptors are present in the ELL molecular layer. Bastian (1993)
in particular demonstrated that pressure ejection of the glutamate receptor agonists AMPA or NMDA in the ELL molecular layer increased the excitability of pyramidal cells (although his injections were not specifically restricted to either
the DML or VML). He also was able to antagonize the
response to these agonists with EAA antagonists [6,7-dinitroquinoxaline-2,3-dione (DNQX), APV]. We therefore
tested several antagonists of ionotropic EAA receptors to
identify the contribution of glutamate receptor subtypes to
the StF EPSP.
Pressure ejection of the broad-spectrum antagonist kynurenic acid into the VML resulted in a significant (P õ
0.001; n Å 7) and reversible reduction in the StF-evoked
field EPSP (Fig. 3B). The small residual late component of
the VML response (Fig. 3B, *) proved sensitive to subsequent perfusion of Mn 2/ -containing medium (not shown).
Note that kynurenic acid partially reduced the amplitude of
the early field positivity in the VML, again suggesting a
relationship between this potential and inward synaptic current in the underlying StF (Fig. 3B).
Pressure ejection of the NMDA-receptor antagonist CPP
also significantly (P õ 0.001; n Å 12) and reversibly reduced the peak amplitude and late phase of the StF-evoked
EPSP (early positive and negative components) to about half
of control values (Fig. 3C). In contrast, pressure ejection of
the non-NMDA–receptor antagonist CNQX into the VML
completely blocked the synaptic component of the StFevoked EPSP, leaving only the biphasic presynaptic fiber
potential (Fig. 3D; n Å 5). Subsequent application of CPP
had no effect on the response left after exposure to CNQX
(data not shown).
These studies indicated that the StF-evoked EPSP is mediated by ionotropic glutamate receptor subtypes but produced
two unexpected results. First, the ability of CPP to reduce
the amplitude of both an early ( õ10 ms) and late phase of
the EPSP suggested the unusual contribution of a fast
/ 9k1d$$oc22 J277-7
1873
NMDA-receptor component to the StF-evoked EPSP. Second, a complete blockade of the EPSP by the non-NMDA–
receptor antagonist CNQX suggests a possible cross-reactivity with NMDA receptors in this preparation. We therefore
used a perfusate nominally free of Mg 2/ to test these drugs
under conditions expected to augment NMDA-receptor
transmission. Perfusion with 0 Mg 2/ medium (Fig. 3E) significantly increased the peak of the StF-evoked VML EPSP
(133 { 23.9%; P õ 0.02; n Å 5), as well as the late phase
of the EPSP. This is consistent with the idea that NMDA
receptors contribute substantially to both the peak and late
phase of the EPSP. In 0 Mg 2/ medium, CNQX substantially
reduced the amplitude of both the peak and late phases of
the 0 Mg 2/ -enhanced EPSP (n Å 7; Fig. 3F). However,
unlike the result found in normal medium, subsequent application of CPP further reduced both the peak and late phases
of the StF-evoked EPSP (Fig. 3 F). This result suggested
that the ability of CNQX to block the NMDA-receptor component of the StF-evoked EPSP may be partially due to the
Mg 2/ -dependent voltage sensitivity of the NMDA receptor.
The 0 Mg 2/ experiments support the conclusion that
NMDA receptors mediate both an early and a late phase of
the StF EPSP. They also suggest that CNQX exhibits some
cross-reactivity with NMDA receptors, although a CNQXresistant, CPP-sensitive component can be identified in the
presence of 0 Mg 2/ . Interestingly, Bastian (1993) also reported that, although APV was a selective antagonist of
pyramidal cell responses to NMDA in vivo, DNQX (a nonNMDA receptor antagonist similar to CNQX) also antagonized the response of ELL pyramidal cells to both AMPA
and NMDA.
Intracellular recording
Recordings were obtained from ú100 somatic and 13
proximal apical dendritic penetrations of pyramidal cells (somatic vs. dendritic recordings were distinguished using criteria established by Turner et al. 1994). Resting membrane
potential (RMP) and input resistance (Rinput ) were similar
in somatic (RMP Å 072.3 { 7.9 mV, n Å 40; Rinput Å
18.8 { 7.6 MV, n Å 42) and dendritic recordings (RMP Å
072.6 { 8.7 mV; Rinput Å 20.1 { 8.7 MV ).
Stimulation of StF always evoked EPSPs and often also
evoked inhibitory postsynaptic potentials (IPSPs) of variable amplitude; data on IPSPs generated by StF stimulation
will be presented elsewhere.
Consistent with the VML field recordings (Fig. 3), StFevoked EPSPs consisted of a rapid depolarization followed
by a prolonged decay phase (Fig. 4, A and C; additional
examples in Fig. 6A). Because the late phase did not reverse
on depolarization, it was an EPSP rather than a reversed
IPSP. There were three components of the StF-evoked EPSP:
a rapid electrotonic EPSP followed by the transmitter mediated peak and late phase.
In cases where the stimulus artifact was relatively small
(n Å 17), StF stimulation resulted in a small short-latency
EPSP (1.55 { 0.59 ms; Fig. 4A) with an amplitude of
3.1 { 1.8 mV. This fast component of the EPSP could not
be blocked by excitatory amino acid antagonists (CNQX /
CPP; Fig. 4, A and B) or low Ca 2/ , high Mn 2/ application
(not shown). Its amplitude was voltage insensitive but in-
09-05-97 11:58:59
neupa
LP-Neurophys
1874
N. J. BERMAN, J. PLANT, R. W. TURNER, AND L. MALER
FIG . 4. Intracellular electrophysiology
of pyramidal cell responses to StF stimulation. A: responses to a series of graded
stimulus intensities in a different cell before
(top trace in each set, Control) and after
application of CNQX and CPP (bottom
trace in each set). Amplitudes of 2 clearly
separable peaks (1, 1.1 ms; 2, 5.6 ms) are
plotted in B. Traces are connected by
dashed line where artifact has been blanked
(this and subsequent figures). B: early
peak (top) scaled with stimulus intensity
but was not significantly affected by CNQX
and CPP. Later peak (bottom) similarly
scaled with stimulus intensity but was
strongly sensitive to CNQX and CPP. C:
somatic excitatory postsynaptic potentials
(EPSPs) with multiple peaks (r ) recorded
while cell was current clamped to various
indicated prestimulus (vertical – – – )
membrane potentials. Late phase of the
EPSP evoked at depolarized potentials
lasted 40–60 ms. Note that, in this case, at
depolarized potentials ( 066 and 062 mV)
there is a clear separation of the EPSP into
early (3 and 6 ms; arrows 1 and 2) and late
(arrow 3) peaks. D: traces ( 078, 066, and
062 mV) from C overlaid to show clearly
the enhancement of EPSPs at depolarized
potentials. E: amplitude plots of EPSPs in
C vs. prestimulus membrane potential. Earliest (3 ms) EPSP is insensitive to membrane potential, whereas the later components of the EPSP show strong inward rectification near and above resting membrane
potential ( 068 mV). F: membrane potential during the EPSPs in C–E can be reset
by the occurrence of a spike; note that the
potential decays to resting levels ( – – – ,
062 mV) after a spike (2 of the 3 current
injections evoked spikes).
creased directly with stimulating current (Fig. 4B), as would
be expected by the recruitment of additional fibers with increasing current. In the presence of CNQX / CPP, this
potential was observed to rapidly decay and was negligible
after 5–8 ms (Fig. 4A; see also Fig. 8A, *). The CNQX/
CPP-sensitive peak of the EPSP had a much longer latency
of 4.6 { 1.5 ms and an amplitude of 4.3 { 2.2 mV (data from
the same recordings). The fast depolarization is therefore
probably an electrotonic EPSP transmitted through the morphologically identified gap junctions between StF fibers and
the proximal apical dendrites of ELL pyramidal cells (Maler
et al. 1981b). Because the main EPSP peaked considerably
later than the early electrotonic component, our measurements below reflect predominantly chemical transmission in
the VML.
When evoked at RMP, the somatic EPSP had a peak
latency of 6.1 { 2.4 ms (n Å 86) and a peak amplitude of
5.89 { 1.27 mV for intensities just subthreshold for spike
discharge. The latency to peak was highly variable (e.g.,
Fig. 4, A and C; range, 2.8–15 ms; typically 4–7 ms), a
result attributable to three factors: the precise location of the
stimulating electrode, the membrane potential (see below),
and the amplitude of StF-evoked IPSPs (strong IPSPs trun-
/ 9k1d$$oc22 J277-7
cated the EPSP and shifted its apparent peak to shorter latencies). The EPSP recorded from the apical dendrite peaked
at a slightly shorter latency of 4.32 { 1.71 ms and amplitude
of 5.70 { 2.06 mV (n Å 13).
Both the early ( Ç6 ms) and late ( Ç20 ms) phases of
StF-evoked EPSPs were clearly voltage dependent (n Å 5
dendritic; n Å 18 somatic). Depolarization to more than
068 mV typically produced a dramatic increase in the amplitude of the peak and late phase ( ú10-ms latency) of the
EPSP, with the peak shifting to the late phase as it became
enhanced at depolarized levels (Fig. 4, C–E; see also Fig.
5B). The combination of these factors could result in a
plateau potential that persisted for °60 ms. Because we did
not block IPSPs that are evoked by StF stimulation (Berman
and Maler, unpublished observations), this may underestimate the duration of the plateau potential. At depolarized
levels (more than -65 mV), action potentials often were
evoked at a latency of 15–20 ms (Fig. 4F). The action
potential also reset the membrane potential, again suggesting
that the StF-evoked EPSP is highly voltage sensitive.
In some recordings (Fig. 4, C and D), depolarizing the
cell with current injection revealed an early (3 ms) and late
(5–7 ms) peak. The early peak had a considerably longer
09-05-97 11:58:59
neupa
LP-Neurophys
EXCITATORY AMINO ACID RECEPTORS AT A FEEDBACK PATHWAY
latency than the putative electrotonic response, was not voltage dependent (Fig. 4E), and could be blocked by CNQX
(see below). These results suggest that there may be at least
three components of the chemically mediated StF-evoked
EPSP: an early (3–4 ms) voltage-independent phase, a
slightly slower (5–7 ms) voltage-dependent phase, and a
voltage-dependent late phase. In most cases, however, there
was only a single clear peak of the chemical EPSP (Fig. 4 A;
other examples also can be seen Figs. 6A and 8A below).
StF fibers originate from stellate cells of Pd (Bratton and
Bastian 1990; Maler et al. 1982; Sas and Maler 1983); Bratton and Bastian (1990) have demonstrated that, with physiological sensory input, Pd stellate cells discharge in short
bursts ( Ç100 ms) at rates of 50–200 Hz. We therefore
attempted to mimic natural stimulus patterns in vitro. Field
potential recordings revealed an apparent strong paired pulse
facilitation (PPF; Fig. 5A) at a 10-ms interpulse interval
(180%); this facilitation declined to baseline by 120 ms.
Intracellular recording also revealed PPF (Fig. 5B). Because
this could be seen at hyperpolarized membrane potentials
after the EPSP had almost returned to baseline (40 ms after
stimulus onset), it suggests that there may be classic presynaptic PPF (Zucker 1989) at StF synapses. At rest or when
the cell is depolarized (Fig. 5B), the facilitation of the peak
of the second EPSP mostly can be accounted for by simple
summation of the second EPSP with the decaying late phase
of the first EPSP.
FIG . 5. Paired pulse facilitation (PPF) of StF synaptic potentials. A:
field recordings in VML (12 overlaid traces). After the initial stimulus
(first EPSP), a second stimulus was delivered at 10-ms intervals °120 ms.
Inset: summary (n Å 10) of the decay of PPF from 180% at 10 ms to
baseline at intervals ú120 ms. – – – , prestimulus baseline and control
(first) EPSP peak. B: intracellular recording with paired pulses at an interval
of 40 ms. At hyperpolarized potentials ( 079 mV, lower dashed line), the
EPSP decayed to 0.6 mV above baseline by the time of the second stimulus
onset and the second EPSP is 1.2 mV greater than the first (PPF Å 0.6
mV). At resting membrane potential (RMP; 068 mV) or depolarized potentials (not shown), the second EPSP was evoked on the late phase of the
first EPSP (1.9 mV ú prestimulus baseline); the increase in the second
EPSP (2.1 mV) can be attributed mainly to summation with the late phase
of the first EPSP. Note that in this recording both the early and late phase
of the EPSP are voltage sensitive, and there is only one clear peak. – – – ,
prestimulus baseline and control EPSP peak for hyperpolarized case.
/ 9k1d$$oc22 J277-7
1875
As apposed to single pulse stimuli (Fig. 6 A), stimulation
of StF at frequencies of 50–200 Hz produced an augmenting
depolarization of the membrane potential of pyramidal cells
that plateaued after 50–100 ms (Fig. 6, B–D). In cases
where inhibition was more prominent, the plateau depolarization was reached earlier (Fig. 6C) or replaced by a compound IPSP (e.g., Fig. 8 B) (Berman and Maler, unpublished
observations of cells depolarized with current injection).
When inhibition was not prominent (IPSPs not seen when
cell was depolarized), stimulus trains produced long depolarizing tails that could exceed 500 ms (Fig. 6, B and D).
When the cell was depolarized slightly, the slow depolarization generated repetitive spiking (Fig. 6B, inset), indicating
that it was not a reversed IPSP.
Within the stimulus train window, although the second
EPSP was typically larger than the first, the relative amplitude (from the immediately preceding membrane potential)
of subsequent individual EPSPs in the train generally remained stable (Fig. 6, C and D). Unlike the situation after
single shocks (cf. Fig. 4F), spikes evoked during a train
were not able to reset the membrane potential (Fig. 6D).
Presumably this is at least partly due to the inward currents
produced by the summating late phases of the EPSPs greatly
exceeding the spike repolarizing K / currents; a time-dependent attenuation of repolarizing K / currents during the train
(Turner et al. 1994) also may contribute to this observed
effect.
The augmenting depolarization produced by tetanic stimulation was due to both temporal summation of the evoked
EPSPs and the voltage-dependent increase in the late phase
of the EPSP. For single EPSPs during the train (Fig. 7, A
and B), if the cell became sufficiently depolarized during
the tetanus (despite the hyperpolarizing holding current: Fig.
7B), the voltage dependence of the EPSP peak became evident. When stimulation occurred with the cell at resting
potential ( –65 mV), the enhanced (due to their voltage
dependence) and summated EPSPs crossed threshold and
triggered spikes (Fig. 7C). The posttetanic depolarization
(the tail described above) was also voltage dependent (data
not shown) and was presumably due to nonlinear summation
of the late, voltage-dependent phase of single EPSPs. Thus
the voltage dependence of StF-evoked EPSPs is an important
determinant of the response of ELL pyramidal cells to physiologically patterned inputs.
INTRACELLULAR RECORDING: PHARMACOLOGY. Consistent
with the results from the extracellular studies, pressure
application of APV or CPP into the region of the VML
( n Å 44 ) consistently reduced the earliest chemical component of the StF-evoked EPSPs by 40 – 50% ( Fig. 8 A ) .
The effect on the slow component of the evoked responses
was variable and ranged from no effect to complete blockade; Fig. 8 A illustrates a typical case with a reduction of
the late phase of the EPSP. These results therefore suggest
that Ç50% of the peak EPSP ( õ10 ms ) can be accounted
for by NMDA-receptor activation. The slow voltage-dependent component of StF-evoked EPSPs also is mediated
partially by NMDA receptors, although its lability and
overlap with IPSPs makes it difficult to quantify the contribution of NMDA receptors.
Similar to the results of the field potential studies, applica-
09-05-97 11:58:59
neupa
LP-Neurophys
1876
N. J. BERMAN, J. PLANT, R. W. TURNER, AND L. MALER
FIG . 6. Intracellular electrophysiology of StF-evoked EPSPs. A: single shock evoked EPSP. Note initial fast EPSP,
followed by a slowly decaying tail depolarization (65 ms duration). B: same cell as A, but with train (10 pulses, 100 Hz)
stimulation. Summating EPSPs during the train evoke several spikes (truncated), followed by a slow tail that lasts for
hundreds of milliseconds after the last stimulus. When cell is depolarized (inset, 0.2 nA, 800 ms), the slow tail evokes
repetitive spikes. C: different cell: summation of train stimulation-evoked EPSPs caused the slow depolarizing wave (shaded
region, top). Examination of the individual EPSPs (bottom trace) shows that the profile of each EPSP is similar, but the
initiation of each EPSP rides on the depolarized tail of the previous EPSP, causing significant summation. Markers indicate
stimulus timing. D: effect of spiking on the depolarizing wave underlying the train response. With (gray trace) or without
(dark trace) a spike the membrane potential followed a similar trajectory during repetitive StF stimulation. Same cell as A
and B.
tion of CNQX (n Å 30) completely blocked the StF-evoked
EPSP remaining after CPP or APV treatment; a small, putatively gap junction, component remained in most cases (Fig.
8A). Combined application of CNQX and APV (or CPP)
often unmasked a StF-evoked IPSP (Fig. 8, A and B); this
IPSP probably emanates from Pd bipolar cells (Maler and
Mugnaini 1994) and will be described elsewhere (Berman
and Maler, unpublished observations). As already shown in
field recordings, when applied first, CNQX blocked the StFevoked EPSP, and subsequent application of CPP or APV
had no further effect (n Å 6). The small depolarization that
remained in some cases after application of CPP / CNQX
(e.g., Fig. 8A) was resistant to further block by Mn 2/ (not
shown) and therefore presumably reflects an electrotonic
component of the EPSP. We attempted to assess whether
depolarizing the cell after application of CNQX would reveal
a voltage-dependent NMDA component. However, depolarization induced numerous spontaneous transient depolarizations with a time course similar to that of StF-evoked EPSPs
(as reported by Mathieson and Maler 1988); this precluded
visualization of a possible residual NMDA component of
the EPSP.
Application of APV or CPP also attenuated the augmenting potential seen during stimulus trains (Fig. 8B) and
greatly reduced the number of spikes produced by tetanic
stimulation (Fig. 8B, inset). Subsequent application of
CNQX completely blocked train-evoked spiking (n Å 28;
Fig. 8B, inset) and left only the electrotonic EPSP. This
electrotonic EPSP is likely to cause the summation during
the tetanus that survived APV / CNQX, probably via an
interaction with intrinsic currents (e.g., INaP ) (Mathieson and
Maler 1988; Turner et al. 1994) because the EPSP decayed
rapidly ( õ5 ms, Fig. 8A, *). The long posttetanic depolarization was diminished greatly by APV or CPP (Fig. 8B).
/ 9k1d$$oc22 J277-7
Subsequent CNQX applications eliminated the remaining
slow depolarization (Fig. 8B).
DISCUSSION
In this study, we have shown that the direct feedback
pathway from the rhombencephalic n. praeminentialis remains intact in ELL brain slices. Although the StF is close to
pyramidal cell efferent fibers in the plexiform layer, correct
placement of the stimulating electrode and limiting stimulation voltages prevented antidromic activation. Physiological
(CSD) and anatomic estimates of the thickness of the VML
coincide, suggesting that StF stimulation did not recruit
DML fibers. The interpretation of our results presented below therefore is based on specific activation of the StF, the
direct feedback projection to the ELL.
We have demonstrated that StF stimulation evokes a
mixed electrotonic and chemical EPSP; this is consistent
with a previous morphological study (Maler et al. 1981b).
The role of the electrotonic component of the StF-evoked
EPSP is unclear, and the discussion below focuses on the
larger chemical component of the StF-evoked EPSP.
The pharmacology of the StF-evoked EPSP suggests that
it is mediated mainly by NMDA and non-NMDA EAA receptors. These results are consistent with the identification
of glutamate as the StF transmitter (Wang and Maler 1994),
the presence of NMDA and AMPA binding sites in the VML
[kainic acid binding sites are not present in the VML (Maler
and Monaghan 1991)], and Bastian’s (1993) demonstration
that pyramidal cells discharge vigorously in response to application of AMPA and NMDA in vivo. The CNQX-sensitive component of the EPSP had a rapid onset (latency to
peak of Ç3 ms) and decay and was not voltage sensitive,
consistent with the physiology of the mammalian AMPA
receptor.
09-05-97 11:58:59
neupa
LP-Neurophys
EXCITATORY AMINO ACID RECEPTORS AT A FEEDBACK PATHWAY
1877
to more than 060 mV before it contributes appreciable synaptic current (Hestrin et al. 1992).
Our CNQX results are perhaps not surprising as DNQX
greatly attenuates the response to application of NMDA in
the ELL molecular layer in vivo (Bastian 1993; see also
Drejer and Honore 1988). Indeed NMDA receptor (NR1
subunits) of ELL pyramidal cells contain a 5 * end insertion
(Bottai et al. 1996) that has been shown to confer marked
CNQX sensitivity to mammalian NMDA receptors (Hollmann et al. 1993). Additionally we have shown that the
CNQX block of NMDA receptors in the ELL is due partly
to the voltage sensitivity of the NMDA receptor, so that
depolarization by the non-NMDA–receptor component of
the EPSP is required to relieve their Mg 2/ block; similar
results have been reported in the hippocampal slice (Blake
et al. 1988).
The ability of CNQX to antagonize the NMDA-receptor
component of synapses in the ELL molecular layer is there-
FIG . 7. Voltage dependence of train-evoked EPSPs. A: current injection
( 00.3–0.1 nA) during tetanic stimulation of StF (100 Hz, 10 pulses, shaded
region). Note characteristic ramp of summating EPSPs at membrane potentials ranging from 074 to 065 (rest) mV; at rest 3 spikes are initiated by
the StF-evoked EPSPs. Shaded region expanded in B and C. B: when
normalized for prestimulus membrane potential, the 074 and 071 mV
traces, including EPSPs, superimpose; at these membrane potentials, the
ramp is due entirely to temporal summation of the slow tail of the EPSPs.
At 068 mV (thick trace) individual EPSPs still superimpose, but the underlying depolarizing wave shows inward rectification. C: at 065 mV (thick
trace), the peaks of individual fast EPSPs show inward rectification. Three
EPSPs evoke spikes (truncated) with prominent afterhyperpolarizations.
Stimulus timing is indicated ( m ).
The presence of a NMDA-receptor component of the StFevoked EPSP additionally is supported by the sensitivity of
the EPSP to Mg 2/ and voltage. Further, Bottai et al. (1995–
1997) recently have cloned the A. leptorhynchus NMDA
receptor (NR1 subunit) and demonstrated by in situ hybridization that NR1 mRNA is enriched highly in ELL pyramidal
cells, including their proximal apical dendrites.
There are, however, three apparently anomalous results
with respect to the physiology of NMDA-receptor–mediated
transmission in the VML. 1) CNQX appears to antagonize
completely the StF-evoked EPSP, leaving no evident residual NMDA component unless augmented by perfusion with
0 Mg 2/ ACSF. 2) The NMDA-receptor component of the
EPSP is prominent at the peak of the EPSP ( õ10-ms latency) as well as during its slow decay phase. It commonly
is asserted that NMDA receptors contribute mainly a slow
component to EAA synaptic transmission (Collingridge et
al. 1988; Edmonds et al. 1995; Forsythe and Westbrook
1988; Hestrin et al. 1992). 3) The NMDA channels contributing to the StF-evoked EPSP appear to conduct at more
negative membrane potentials (less than -60 mV) than the
hippocampal NMDA receptor, which requires depolarization
/ 9k1d$$oc22 J277-7
FIG . 8. Pharmacology of the StF-evoked EPSPs recorded intracellularly.
Effects of a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)
and N-methyl-D-aspartate (NMDA) antagonists on the EPSPs evoked by
single and train pulses. A: staggered drug applications showing NMDA and
non-NMDA components of the EPSP. DL-2-amino-5-phosphonopentanoic
acid (APV) blocked about half of the EPSP peak amplitude and the tail
depolarization. Subsequent addition of CNQX (APV / CNQX) blocked
the remaining potential, leaving a small depolarized ‘‘hump’’ (*) followed
by a hyperpolarization. Artifact blanked for clarity. B: train-evoked depolarizations with spike rates plotted in inset. Train stimulation (20 pulses, 200
Hz) evoked a strong excitatory response (Con), spiking (Con, inset), and
depolarization, which outlasted the stimulus. APV reduced the evoked spike
rate (inset) and blocked a major portion of the depolarization after the
train. CNQX further reduced the slow depolarizing wave, but its major
effect was to further reduce the excitatory response (and eliminate spiking,
inset) during the train. These recordings also illustrate that the large inhibitory postsynaptic potentials (IPSPs) evoked by StF stimulation are blocked
only partially by CNQX; a detailed analysis of the various components of
these IPSPs will be presented elsewhere. Traces are averages of 3 trials.
09-05-97 11:58:59
neupa
LP-Neurophys
1878
N. J. BERMAN, J. PLANT, R. W. TURNER, AND L. MALER
fore probably due to both the molecular characteristics of the
NR1 subunit expressed by pyramidal cells and the voltage
dependence of the NMDA receptor.
The slow time course of the NMDA-receptor component
of EAA transmission has been attributed to NMDA channels
opening after a long delay (reviewed in Edmonds et al.
1995). Recent kinetic experiments, however, have demonstrated that NMDA channels open, on average, Ç10 ms after
agonist binding (Dzubay and Jahr 1996); this is only slightly
longer than the apparent peak of the NMDA-receptor component of the StF-evoked EPSP. In addition, fast ( õ10 ms)
NMDA-receptor–mediated transmission has been demonstrated in mammalian sensory systems: retino-geniculate
EPSPs (Esguerra et al. 1992), somatosensory cortex (Armstrong-James et al. 1993), and visual cortex (Shirokawa et
al. 1989). Electrophysiological analysis of cloned NMDA
receptors further demonstrates that combinations of NR1
with different NR2 subunits can result in functional receptors
with markedly different kinetics (Monyer et al. 1992).
StF-evoked EPSPs decay far more rapidly ( õ60 ms) than
the NMDA component of cortical EPSPs (time constant of
60–150 ms) (Hestrin et al. 1992). This is likely due to the
activation of ELL inhibitory interneurons because application of g-aminobutyric acid-A (GABAA ) antagonists prolongs the EPSPs to ú100 ms (Berman and Maler, unpublished observations). Stimulus trains do cause prolonged
EPSPs ( ú200 ms in cases where inhibition is weak, e.g.,
Fig. 6B), which are APV and CPP sensitive, suggesting that
the physiology of the NMDA receptors associated with the
StF input is similar to that reported for mammalian neurons.
The late component of the StF-evoked EPSP ( ú20 ms)
is voltage dependent and therefore might be attributed entirely to the activation of NMDA receptors. However this
late phase was often not completely blocked by APV or
CPP. In addition, preliminary experiments have shown that
intracellular injection of lidocaine N-ethyl bromide (QX314) greatly reduces the late component of single-shock StFevoked EPSPs, the slow depolarization that follows tetanic
stimulation of StF and the depolarizing ramp during tetanic
stimulation (Plant 1994). This suggests that voltage-dependent ion channels also might contribute to the late phase of
the StF evoked EPSPs (see Hirsch and Gilbert 1991). Because QX-314 can block both Na / and Ca 2/ inward currents
(Talbot and Sayer 1996), the channels contributing to the
voltage-sensitive late phase still must be elucidated.
Initial studies emphasized the requirement of depolarization
for NMDA-receptor activation and gave rise to the idea that
NMDA receptors are activated primarily during tetanic stimulation (Collingridge and Lester 1989). However, recent in vivo
studies have suggested that NMDA receptors contribute to the
response of cortical neurons to even weak sensory input (Armstrong-James et al. 1993; Fox et al. 1990; Kwon et al. 1992),
suggesting that these receptors may operate at near RMPs.
Biophysical analysis of cloned NMDA receptors also has demonstrated a wide variation in susceptibility to Mg 2/ blockade,
and that some subunit combinations can pass appreciable current at less than 070 mV (Kutswada et al. 1992).
Thus it appears that the properties of NMDA receptors in
the StF pathway are not unusual when compared with those
in mammalian sensory systems and that, as in the mammal,
they contribute to normal sensory processing (reviewed in
/ 9k1d$$oc22 J277-7
Nelson and Sur 1992). The NR1 subunit of the NMDA
receptor of A. leptorhynchus is highly homologous to the
mammalian NR1 subunit (Bottai et al. 1995–1997); it therefore will be important to determine which specific combinations of NR1 splice variants and NR2 subunits determine
the voltage threshold, time to peak, and time constant of
decay of NMDA receptors within the VML.
Our present picture of the StF evoked EPSP is summarized
in Fig. 9. There are at least four components to this EPSP:
a very early (1.5 ms) electrotonic synapse; an early ( Ç3
ms), voltage-insensitive AMPA-receptor component; an
early ( Ç4–7 ms) voltage-sensitive NMDA-receptor component, which also contributes to the late ( ú10 ms) phase of
the EPSP; and a late, voltage-sensitive component ( Ç50 ms
after single pulse and °500 ms in duration after tetanic
stimulation) dependent on NMDA receptors and perhaps on
voltage-sensitive ion channels as well. The frequency and
duration of our tetanic stimuli mimic natural firing patterns
of StF (Bratton and Bastian 1990).
FIG . 9. A: summary diagram of the relevant contributions of gap junction and different ionotropic excitatory amino acid receptors to the StFevoked EPSP. A small electrotonic EPSP (Gap) precedes a CNQX-sensitive, voltage-insensitive EPSP mediated by an AMPA receptor component.
NMDA receptors contribute to both the early peak and the late phase of
the EPSP; the voltage sensitivity of these components is indicated by the
double arrows. B: summary of how the receptive field sizes of pyramidal
and stellate cells, time delays of reciprocal ELL-Pd connections, and voltage
sensitivity of StF-evoked EPSP might contribute to the hypothesized searchlight function of the StF feedback pathway. As the fish scans past an object,
changes in the firing rate of electroreceptors first will drive ELL pyramidal
cells with receptive fields on the left (a). The axons of these cells travel
in the lateral lemniscus to terminate on stellate cells of the contralateral
Pd. Stellate cells have larger receptive fields than ELL pyramidal cells; in
this diagram, all the indicated pyramidal cells are supposed to project to
the Pd stellate cell. The stellate cell is activated phasically and emits a burst
of spikes; this activity reaches the ELL pyramidal cell with receptive field
(RF) ‘‘b’’ (gray) with a delay due to slow conduction in the tractus stratum
fibrosum and synaptic delay; these delays are matched to the scan rate of
the fish. Feedback input therefore arrives at pyramidal cell with RF b at
the same time as does electroreceptor input generated by the object entering
its receptive field.
09-05-97 11:58:59
neupa
LP-Neurophys
EXCITATORY AMINO ACID RECEPTORS AT A FEEDBACK PATHWAY
The various components of the StF-evoked EPSPs interact
in a dynamic manner to stimulation that mimics natural activation (i.e., 100–200 Hz, 100 ms) (Bratton and Bastian
1990) of this feedback pathway. At least four mechanisms
may be involved in the typical ramp-like response to tetanic
stimulation: presynaptic facilitation may increase the amplitude of succeeding EPSPs; temporal summation of the late
phase of the EPSP will generate a rising depolarization; the
voltage sensitivity of peak and late phase of the EPSP increase the response to later stimuli in the train and thus
contribute to the ramp-like responses; and IPSPs can attenuate the response to later EPSPs in the train (Berman and
Maler, unpublished observations). It is clear that the response to StF stimulation can be regulated dynamically at
many potential sites (see below) and that it will take more
detailed physiological studies as well as modeling to understand transmission at this feedback synapse.
Relating in vitro properties of Stf-evoked EPSPs to
electroreception
The ELL is connected reciprocally and topographically to
the n. praeminentialis (Maler et al. 1982, 1991; Sas and
Maler 1983); the connections are bilateral in both directions
with contralateral projections dominating. Because connections are excitatory in both directions, this represents an
example of positive feedback. Bratton and Bastian (1990)
have recorded from stellate cells in Pd, the neurons that give
rise to the StF (Sas and Maler 1983). Stellate cells respond,
with high sensitivity and gain, to AM (AMs õ16 Hz) of
the EOD (contralateral electroreceptors). A. leptorhynchus
scans its environment with stereotyped movements, the velocity of which (10–15 cm/s) (Lannoo and Lannoo 1992)
would be expected to generate AMs of õ10 Hz (Bastian
1981). As expected from these considerations, stellate cells
respond vigorously and phasically to movement of objects
over their receptive fields on the contralateral side of the
fish’s body (Bratton and Bastian 1990). On the basis of
these data, Bratton and Bastian hypothesized that stellate
cells are optimized to detect small moving objects. Taking
into account the excitatory reciprocal connectivity between
ELL and Pd, they further hypothesized that the StF feedback
pathway might act as a searchlight to enhance the response
to salient features of the environment. A recent behavioral
study in fact has shown that lesions of Pd do affect the
electrodetection of objects (Green 1996).
The original searchlight hypothesis was enunciated by
Crick (1984) with regard to the reciprocal connections between mammalian thalamus and cortex. This theory contained three essential ingredients: reciprocal excitatory connectivity, a relatively diffuse parallel inhibitory feedback
system that can keep the feedback excitation confined to a
limited spatial domain, and a nonlinearity in the responsiveness of the lower order cells (thalamic relay neurons in
Crick’s thesis) that amplifies stronger inputs. The identification of a diffuse inhibitory (GABAergic) feedback pathway
from cells in medial Pd to ELL pyramidal cells suggests that
the second of Crick’s criteria also is met by the direct feedback system to the ELL (Maler and Mugnaini 1993, 1994).
As discussed below, the data presented in this paper suggest
that the third criteria is met as well.
/ 9k1d$$oc22 J277-7
1879
We propose that the voltage dependence of StF synapses
in VML causes them to behave as a nonlinear thresholding
device. StF activation will only produce large, suprathreshold EPSPs when the pyramidal cell is depolarized by other
inputs. Pyramidal cells can be depolarized by input from
electroreceptors (Bastian 1981) or by indirect feedback input
to the DML. The latter alternative is certainly plausible,
however, other than its involvement in gain control (Bastian
1986b,c), little is known about the physiology of the indirect
feedback pathway. We therefore hypothesize that when electroreceptor (primary afferents) and StF feedback inputs arrive concurrently at a pyramidal cell, the StF input is enhanced greatly and therefore is very effective at bringing
that cell above spike threshold. From this viewpoint, it is
the voltage dependence of the NMDA-receptor component
of StF synapses that is critical for their function; a similar
proposal has been made for the role of NMDA receptors
associated with corticothalamic feedback fibers (to lateral
geniculate nucleus) (Esguerra and Sur 1992). The spatial
aspects of this theory are discussed below (Fig. 9).
The hypothesis discussed above depends on nonlinear
summation of EPSPS evoked by primary afferents and StF
fibers; the nonlinearity is due to the voltage dependence
of the StF-evoked EPSPs. Recent work suggests a second
nonlinearity might be present in this system. Turner et al.
(1994) found that the proximal apical dendrites of pyramidal
cells conduct Na / spikes and that inward current associated
with these spikes sourced back to the soma where they appeared as depolarizing afterpotentials that could generate the
spike bursts seen in these cells in vitro (Turner et al. 1996).
Gabbiani et al. (1996) recently have shown that ELL pyramidal cells in vivo can signal the occurrence of temporal
electrosensory ‘‘features’’ by spike bursts. They also suggest
that the feature extraction occurs in the proximal dendrites
rather than the soma (Gabbiani et al. 1996). It is thus possible that StF-evoked EPSPs, in addition to directly triggering
spikes, can enhance antidromic spike invasion of the proximal apical dendrite of ELL pyramidal cells and thus increase
the probability of burst initiation. A recent study (Magee and
Johnston 1997) has demonstrated in hippocampal pyramidal
cells that dendritic depolarization due to EPSPs can in fact
increase the amplitude of antidromic dendritic action potentials. In this case, the slow summating component of the
StF-evoked EPSP might increase the electrodetectability of
moving objects for hundreds of milliseconds by enabling
primary afferent input to trigger spike bursts.
Pyramidal cells and stellate cells have receptive field (RF)
centers with diameters of 11 and 18 mm, respectively (Bastian 1981; Bratton and Bastian 1990); the greater size of
the stellate cell’s receptive field is presumably due to convergence of pyramidal cell input (Maler et al. 1982). Because
pyramidal and stellate cells are connected reciprocally
(Maler et al. 1982), a stellate cell’s receptive field will
extend Ç3.5 mm beyond that of its concentrically matched
pyramidal cell. This suggests that an object moving past
electroreceptors can cause Pd stellate cells to discharge and
that these stellate cells can in turn excite pyramidal cells
that have not yet been activated by that object (Fig. 9). The
relative spread of the ELL pyramidal cell and Pd stellate
cell reciprocal connections will determine how far ahead
the descending input will prime ELL pyramidal cells the
09-05-97 11:58:59
neupa
LP-Neurophys
1880
N. J. BERMAN, J. PLANT, R. W. TURNER, AND L. MALER
receptive fields of which lie in the object’s future trajectory.
Further details on the precision of this connectivity is required to accurately quantify the scope of the searchlight.
Bratton and Bastian (1990) have noted that Pd stellate
cells respond to electrosensory inputs with a long latency
(11 vs. 5.2 ms for ELL pyramidal cells). The tSF projection
from Pd to ELL is Ç3,500 mm long (estimated from the
atlas of Maler et al. 1991). Our data suggests a delay of
Ç17 ms for the Pd-ELL projection (conduction velocity of
0.31 m/s produces a conduction delay of 11 ms and a delay
to EPSP peak of 6 ms). Data from Bastian suggest a shorter
delay of 10 ms. Thus activation of ELL pyramidal cell with
RF ‘‘a’’ (Fig. 9) will produce a feedback EPSP in the pyramidal cell with RF ‘‘b’’ after Ç20–28 ms. If the fish is
scanning at 10 cm/s (Lannoo and Lannoo 1992), it will
traverse Ç2–3 mm in this period, placing the object over
RF b. The duration of the late EPSP, under this hypothesis,
would determine the minimal scanning rate. Therefore, primary afferent input to a pyramidal cell may coincide with
feedback input from stellate cells representing adjacent regions of skin; given the voltage dependence of the StFevoked EPSP discussed above, this will result in large feedback EPSPs and an enhancement of the response to the
moving object. These considerations suggest that this feedback system will create a traveling beam (searchlight) of
enhanced responsiveness in pyramidal cells, priming them to
detect scanned objects. Correlative biophysical and systems
studies of the StF feedback pathway may elucidate the cellular basis of an elementary form of spatially and temporally
localized attention.
We thank Dr. Rob Dunn for insightful discussion on the molecular biology of NMDA receptors and W. Ellis for technical support.
This work was supported by Medical Research Council grants to L.
Maler and R. Turner, who is a Medical Research Council and Alberta
Heritage Foundation for Medical Research Scholar. J. Plant was supported
by a National Science and Engineering Research Council Fellowship.
Present addresses: J. Plant, Dept. of Psychology, University of Victoria,
PO Box 3050, Victoria, British Columbia V8W 3P5; R. W. Turner, Dept.
of Anatomy, University of Calgary, Calgary, Alberta T2N 1N4, Canada.
Address for reprint requests: L. Maler, Dept. of Anatomy and Neurobiology, University of Ottawa, Ottawa, Ontario K1H 8M5, Canada.
Received 7 April 1997; accepted in final form 18 June 1997.
REFERENCES
ARMSTRONG-JAMES, M., WELKER, E., AND CALLAHAN, C. A. The contribution of NMDA and non-NMDA receptors to fast and slow transmission
of sensory information in the rat S1 barrel cortex. J. Neurosci. 13: 2149–
2160, 1993.
BASTIAN, J. Electrolocation. II. The effects of moving objects and other
electrical stimuli on the activities of two categories of posterior lateral
line cells in Apteronotus albifrons. J. Comp. Physiol. [A] 144: 481–
494, 1981.
BASTIAN, J. Vision and electroreception. Integration of sensory information
in the optic tectum of the weakly electric fish Apteronotus leptorhynchus.
J. Comp. Physiol. [A] 147: 287–297, 1982.
BASTIAN, J. Electrolocation: Behaviour, anatomy and physiology. In: Electroreception, edited by T. H. Bullock and W. Heiligenberg, New York:
Wiley, 1986a, p. 577–612.
BASTIAN, J. Gain control in the electrosensory system mediated by descending inputs to the electrosensory lateral line lobe. J. Neurosci. 6: 553–
562, 1986b.
BASTIAN, J. Gain control in the electrosensory system. A role for descending
projections to the lateral electrosensory lateral line lobe. J. Comp. Physiol. [A] 158: 505–515, 1986c.
/ 9k1d$$oc22 J277-7
BASTIAN, J. The role of amino acid neurotransmitters in the descending
control of electroreception. J. Comp. Physiol. [A] 172: 409–423, 1993.
BASTIAN, J. AND BRATTON, B. Descending control of electroreception. I.
Properties of nucleus praeeminentialis neurons projecting indirectly to
the electrosensory lateral line lobe. J. Neurosci. 10: 1226–1240, 1990.
BLAKE, J. F., BROWN, M. W., AND COLLINGRIDGE, G. L. CNQX blocks
acidic amino acid induced depolarizations and synaptic components mediated by non-NMDA receptors in rat hippocampal slices. Neurosci. Lett.
89: 182–186, 1988.
BODE-GREUL, K. M., SINGER, W., AND ALDENHOFF, J. B. A current source
density analysis of field potentials in slices of visual cortex. Exp. Brain.
Res. 69: 213–219, 1987.
BOTTAI, B., ELLIS, B., MALER, L., AND DUNN, R. NMDA receptor 1 splice
variants: differential cellular expression in the central nervous system of
the electric fish Apteronotus leptorhynchus. Soc. Neurosci. Abstr. 22:
1996.
BOTTAI, B., MALER, L., AND DUNN, R. Molecular characterization of NMDA
receptors in the electric fish Apteronotus leptorhynchus. Soc. Neurosci.
Abstr. 21: 187, 1995.
BOTTAI, D., DUNN, R., ELLIS, W., AND MALER, L. NMDA R1 mRNA
distribution in the central nervous system of the weakly electric fish
Apteronotus leptorhynchus. J. Comp. Neurol. In press.
BRATTON, B. AND BASTIAN, J. Descending control of electroreception. II.
Properties of nucleus praeeminentialis neurons projecting directly to the
electrosensory lateral line lobe. J. Neurosci. 10: 1241–1253, 1990.
CARR, C. E. AND MALER, L. Electroreception in gymnotiform fish. Central
anatomy and physiology. In: Electroreception, edited by T. H. Bullock
and W. Heiligenberg, New York: Wiley, 1986, p. 319–373.
COLLINGRIDGE, G. L., HERRON, C. E., AND LESTER, R. A. J. Synaptic activation of N-methyl-D-aspartate receptors in the Schaffer collateral-commissural pathway of the hippocampus. J. Physiol. (Lond.) 399: 283–300,
1988.
COLLINGRIDGE, G. L. AND LESTER, R. A. J. Excitatory amino acid receptors
in the vertebrate central nervous system. Pharmacol. Rev. 40: 145–210,
1989.
CRICK, F. Function of the thalamic reticular complex. The searchlight hypothesis. Proc. Natl. Acad. Sci. USA, 81: 4586–5490, 1984.
DREJER, J. AND HONORE, T. New quinoxalindiones show potent antagonism
of quisqualate responses in cultured mouse cortical neurons. Neurosci.
Lett. 87: 104–108, 1988.
DZUBAY, J. A. AND JAHR, C. E. Kinetics of NMDA channel opening. J.
Neurosci. 16: 4129–4134, 1996.
EDMONDS, B., GIBB, A. J., AND COLQUHOUN, D. Mechanisms of activation
of glutamate receptors and the time course of excitatory synaptic currents.
Annu. Rev. Physiol. 57: 495–519, 1995.
ESGUERRA, M., KWON, Y. H., AND SUR, M. Retino-geniculate EPSPs recorded intracellularly in the ferret lateral geniculate nucleus in vitro. Role
of NMDA receptors. Vis. Neurosci. 8: 545–555, 1992.
ESGUERRA, M. AND SUR, M. Corticogeniculate feedback gates retinogeniculate transmission by activating NMDA receptors. Soc. Neurosci. Abstr.
16: 159, 1992.
FORSYTHE, I. D. AND WESTBROOK, G. L. Slow excitatory postsynaptic currents mediated by N-methyl-D-aspartate receptors on mouse cultured central neurons. J. Physiol. (Lond.) 396: 515–533, 1988.
FOX, K., SATO, H., AND DAW, N. The effect of varying stimulus intensity
on NMDA-receptor activity in cat visual cortex. J. Neurophysiol. 64:
1413–1428, 1990.
GABBIANI, F., METZNER, W., WESSEL, R., AND KOCH, C. From stimulus
encoding to feature extraction in weakly electric fish. Nature 384: 564–
567, 1996.
GREEN, R. L. How lesioning the nucleus praeeminetialis affects electrolocation behaviour in the weakly electric fish, Apteronotus leptorhynchus. J.
Comp. Physiol. [A] 179: 353–361, 1996.
HEILIGENBERG, W. Neural Nets in Electric Fish, Cambridge, MA: MIT
Press, 1991.
HESTRIN, S., NICOLL, R. A., PERKEL, D. J., AND SAH, P. Analysis of excitatory synaptic action in pyramidal cells using whole-cell recording from
rat hippocampal slices. J. Physiol. (Lond.) 422: 203–225, 1992.
HIRSCH, J. AND GILBERT, C. Synaptic physiology of horizontal connections
in the cat’s visual cortex. J. Neurosci. 11: 1800–1809, 1991.
HOLLMANN, M., BOULTER, J., MARON, C., BEASLEY, J., PECHT, G., AND
HEINEMANN, S. Zinc potentiates agonist-induced currents at certain splice
variants of the NMDA receptor. Neuron 10: 943–954, 1993.
KUTSWADA, T., KASHIWABUCHI, T., MORI, N. H., SAKIMURA, K., KUSHIYA,
09-05-97 11:58:59
neupa
LP-Neurophys
EXCITATORY AMINO ACID RECEPTORS AT A FEEDBACK PATHWAY
E., ARAKI, K., MEGURO, H., MASAKI, H., KUMANISHI, T., ARAK AWA, M.,
AND MISHINI, M. Molecular diversity of the NMDA receptor channel.
Nature 358: 36–41, 1992.
KWON, Y., NELSON, K., TOTH, K., AND SUR, M. Effect of stimulus contrast
and size on NMDA-receptor activity in cat lateral geniculate nucleus. J.
Neurophysiol. 68: 182–196, 1992.
LANNOO, M. J. AND LANNOO, S. J. Why do electric fish swim backwards?
An hypothesis based on gymnotiform behavior, interpreted through sensory constraints. Environ. Biol. Fishes 36: 157–165, 1992.
MAGEE, J. C. AND JOHNSTON, D. A synaptically controlled associative signal
for Hebbian plasticity in hippocampal neurons. Science 275: 209–213,
1997.
MALER, L. The posterior lateral line lobe of certain gymnotiform fish.
Quantitative light microscopy. J. Comp. Neurol. 183: 323–363, 1979.
MALER, L. The role of feedback pathways in the modulation of receptive
fields: an example from the electrosensory system. In: Neural Mechanisms of Behaviour, edited by J. Erber, R. Menzel, H.-J. Pfluger, and D.
Todt. Stuttgart, Germany: Thieme, 1989, p. 111–115.
MALER, L., COLLINS, M., AND MATHIESON, W. B. The distribution of acetylcholinesterase and choline acetyl transferase in the cerebellum and posterior lateral line lobe of weakly electric fish (Gymnotidae). Brain Res.
226: 320–325, 1981a.
MALER, L. AND MONAGHAN, D. The distribution of excitatory amino acid
binding sites in the brain of an electric fish. Apteronotus leptorhynchus.
J. Chem. Neuranat. 4: 39–61, 1991.
MALER, L. AND MUGNAINI, E. Organization and function of feedback to
the electrosensory lateral line lobe of gymnotiform fish, with emphasis
on a searchlight mechanism. J. Comp. Physiol. [A] 173: 667–670, 1993.
MALER, L. AND MUGNAINI, E. Correlating gamma-aminobutyric acidergic
circuits and sensory function in the electrosensory lateral line lobe of a
gymnotiform fish. J. Comp. Neurol. 345: 224–252, 1994.
MALER, L., SAS, E., CARR, C., AND MATSUBARA, J. Efferent projections of
the posterior lateral line lobe in a gymnotiform fish. J. Comp. Neurol.
21: 154–164, 1982.
MALER, L., SAS, E., JOHNSTON, S., AND ELLIS, W. An atlas of the brain of
the weakly electric fish. Apteronotus Leptorhynchus. J. Chem. Neuranat.
4: 1–38, 1991.
MALER, L., SAS, E. K., AND ROGERS, J. The cytology of the posterior
lateral line lobe of high frequency weakly electric fish ( Gymnotoidei):
differentiation and synaptic specificity in a simple cortex. J. Comp. Neurol. 195: 87–139, 1981b.
MATHIESON, W. B. AND MALER, L. Morphological and electrophysiological
properties of a novel in vitro preparation: the electrosensory lateral line
lobe brain slice. J. Comp. Physiol. [A] 163: 489–506, 1988.
/ 9k1d$$oc22 J277-7
1881
MONYER, H., SPRENGEL, R., SCHOEPFER, R., HERB, A., HIGUCHI, M., LOMELI, H., BURNASHEV, N., SAKMAN, B., AND SEEBURG, P. Heteromeric
NMDa receptors: molecular and functional distinction of subtypes. Science 256: 1217–1221, 1992.
NELSON, S. AND SUR, M. NMDA receptors in sensory processing. Curr.
Opin. Neurobiol. 2: 484–488, 1992.
PLANT, J. R. Fast NMDA Transmission and Non-Linear Synaptic Interactions in a Sensory Feedback Pathway (PhD thesis). Ottowa, Ontario:
Univ. of Ottawa, 1994.
RICHARDSON, T. L., TURNER, R. W., AND MILLER, J. J. Action potential
discharge in hippocampal CA1 pyramidal neurons: a current source density analysis. J. Neurophysiol. 58: 981–986, 1987.
ROSE, G. J. AND HELIGENBERG, W. Neural coding of difference frequencies
in the midbrain of the electric fish Eigenmannia. Reading the sense of
rotation in an amplitude-phase map. J. Comp. Physiol. [A] 158: 613–
624, 1988.
SAS, E. AND MALER, L. The nucleus praeeminentialis: a golgi study of a
feedback center in the electrosensory system of gymnotid fish. J. Comp.
Neurol. 221: 127–144, 1983.
SAS, E. AND MALER, L. The organization of afferent input to the caudal
lobe of the cerebellum of the gymnotid fish Apteronotus leptorhynchus.
Anat. Embryol. 177: 55–79, 1987.
SHIROK AWA, T., NISHIGORI, A., KIMURA, F., AND TSUMOTO, T. Actions of
excitatory amino acid antagonists on synaptic potentials of layer II/III
neurons of the cat’s visual cortex. Exp. Brain. Res. 78: 489–500, 1989.
SHUMWAY, C. Multiple electrosensory maps in the medulla of weakly electric Gymnotiform fish. I. Physiological differences. J. Neurosci. 9: 4388–
4399, 1989.
TALBOT, M. J. AND SAYER, R. J. Intracellular QX-314 inhibits calcium currents in hippocampal CA1 pyramidal neurons. J. Neurophysiol. 76: 2120–
2124, 1996.
TAUBE, J. S. AND SCHWARTKROIN, P. A. Mechanisms of long-term potentiation: a current source density analysis. J. Neurosci. 8: 1645–1655, 1988.
TURNER, R. W., MALER, L., DEERINCK, T., LEVINSON, S. R., AND ELLISMAN,
M. H. TTX-sensitive dendritic sodium channels underlie oscillatory discharge in a vertebrate sensory neuron. J. Neurosci. 14: 6453–6471, 1994.
TURNER, R. W., PLANT, J. R., AND MALER, L. Oscillatory and burst discharge across electrosensory topographic maps. J. Neurophysiol. 76:
2364–2382, 1996.
WANG, D. AND MALER, L. The immunocytochemical localization of glutamate in the electrosensory system of the gymnotiform fish, Apteronotus
Leptorhynchus. Brain Res. 653: 215–222, 1994.
ZUCKER, R. S. Short-term synaptic plasticity. Annu. Rev. Neurosci 12: 13–
31, 1989.
09-05-97 11:58:59
neupa
LP-Neurophys