Download Properties of Primary Sensory (Lemniscal) Synapses in the

J Neurophysiol
87: 946 –953, 2002; 10.1152/jn.00426.2001.
Properties of Primary Sensory (Lemniscal) Synapses in the
Ventrobasal Thalamus and the Relay of High-Frequency
Sensory Inputs
MANUEL A. CASTRO-ALAMANCOS
Department of Neurology and Neurosurgery, Montreal Neurological Institute, McGill University,
Montreal, Quebec H3A 2B4, Canada
Received 23 May 2001; accepted in final form 14 September 2001
Castro-Alamancos, Manuel A. Properties of primary sensory (lemniscal) synapses in the ventrobasal thalamus and the relay of highfrequency sensory inputs. J Neurophysiol 87: 946 –953, 2002;
10.1152/jn.00426.2001. The main role of the thalamus is to relay
sensory inputs to the neocortex. In the primary somatosensory thalamus (ventrobasal thalamus), sensory inputs deliver tactile information
through the medial lemniscus tract. The transmission of sensory
information through this pathway is affected by behavioral state. For
instance, the relay of high-frequency somatosensory inputs through
the thalamus is suppressed during anesthesia or quiescent states but
allowed during behaviorally activated states. This change may be due
to the effects of modulators on the efficacy of lemniscal synapses.
Here I show that lemniscal synapses of adult rodents studied in vitro
produce large amplitude-highly secure unitary excitatory postsynaptic
potentials (EPSPs), which depress in response to repetitive stimulation
at frequencies ⬎2 Hz. Acetylcholine and norepinephrine, which are
important thalamic modulators, have no effect on the efficacy of
lemniscal EPSPs but reduce evoked inhibitory postsynaptic potentials
and corticothalamic EPSPs. Although acetylcholine and norepinephrine do not affect lemniscal synapses, the postsynaptic depolarization
they produce on thalamocortical neurons serves to warrant the relay of
lemniscal inputs at high-frequency rates by bringing the depressed
lemniscal EPSPs close to firing threshold. In conclusion, acetylcholine
and norepinephrine released during activated states selectively enhance sensory transmission through the lemniscal pathway by depolarizing thalamocortical neurons and simultaneously depressing the
other afferent pathways.
The rodent ventroposterior medial thalamus (VPM) receives
sensory information about the whiskers from the principal
trigeminal nucleus through the medial lemniscus tract. Lemniscal terminals form glutamatergic synaptic contacts with the
soma and proximal dendrites of VPM neurons (Chiaia et al.
1991; Diamond 1995; Feldman and Kruger 1980; Liu et al.
1995; Spacek and Lieberman 1974; Veinante and Deschenes
1999; Williams et al. 1994b). Although the whisker system of
rodents is among the most widely investigated sensory systems, the electrophysiological properties of lemniscal synapses
have not been investigated in vitro. In addition to lemniscal
synapses, thalamocortical neurons in the ventrobasal thalamus
receive a massive excitatory input from the neocortex via
corticothalamic synapses and an inhibitory input from the
nucleus reticularis of the thalamus (nRt). Moreover, several
neuromodulatory systems from the brain stem and basal forebrain project to the ventrobasal thalamus; cholinergic and
noradrenergic fibers innervate the rodent ventrobasal thalamus
to different degrees depending on the species and nucleus
(Bennett-Clarke et al. 1999; Hallanger et al. 1990; Simpson et
al. 1997). Cholinergic and noradrenergic cell populations discharge vigorously during activated states (Aston-Jones et al.
1991; Buzsaki et al. 1988) and thus the levels of these modulators increase in the thalamus during activated states (Williams et al. 1994a). We have recently shown that cholinergic
and noradrenergic inputs from the brain stem regulate corticothalamic synapses (Castro-Alamancos and Calcagnotto 2001).
The extent to which these neuromodulatory inputs regulate
lemniscal and inhibitory synapses is unknown.
Studies conducted in vivo suggest that the lemniscal pathway may be regulated by neuromodulators because the relay of
high-frequency sensory inputs through this pathway is affected
by behavioral state. For instance, Poggio and Mountcastle
(1963) demonstrated that the capacity for frequency following
of tactile stimuli is dramatically different for thalamic cells in
the waking as compared with the anesthetized monkey. More
recent work in the freely behaving rat using electrical stimulation of the infraorbital nerve has shown that VPM sensory
responses at frequencies ⬎10 Hz were suppressed during quiescent states but not during active exploration (Fanselow and
Nicolelis 1999). These results indicate that during quiescent
states, the relay of high-frequency sensory inputs is impeded
but allowed during activated states. This may be due to the
effects of neuromodulators released in the thalamus during
behavioral activation, such as acetylcholine and norepinephrine. Thus the lemniscal pathway may be regulated by these
neuromodulators so that the relay of high-frequency inputs is
possible during behaviorally activated states.
The present study explored the properties of lemniscal synapses in slices of rodent tissue and how acetylcholine and
norepinephrine affect these synapses and the relay of highfrequency lemniscal inputs.
Address for reprint requests: Montreal Neurological Institute, 3801 University St., Rm. WB210, Montreal, Quebec H3A 2B4, Canada (E-mail:
[email protected]).
The costs of publication of this article were defrayed in part by the payment
of page charges. The article must therefore be hereby marked ‘‘advertisement’’
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
INTRODUCTION
946
0022-3077/02 $5.00 Copyright © 2002 The American Physiological Society
www.jn.org
LEMNISCAL SYNAPSES
METHODS
Horizontal slices were prepared from adult (ⱖ7 wk) BALB/C mice.
Slices were cut in ice-cold buffer using a vibratome and kept in a
holding chamber for ⱖ1 h. Experiments were performed in an interface chamber at 32°C. The slices were perfused constantly (1–1.5
ml/min) with artificial cerebrospinal fluid (ACSF) containing (in mM)
126 NaCl, 3 KCl, 1.25 NaH2PO4, 26 NaHCO3, 1.3 MgSO4 7H2O, 10
dextrose, 2.5 CaCl2 2H2O. The ACSF was bubbled with 95% O2-5%
CO2. Synaptic responses were induced using a concentric stimulating
electrode placed in the medial lemniscus (Fig. 1). Another stimulating
electrode was placed in some cases in the thalamic radiation to evoke
corticothalamic responses onto the same neuron. The stimulus consisted of a 200-␮s pulse of ⬍70 ␮A unless otherwise indicated. The
947
ventrobasal thalamus and medial lemniscus were easily and clearly
identifiable with a dissecting microscope. Intracellular recordings
were performed using sharp electrodes (60 – 80 M⍀) filled with potassium-acetate (3 M) or with cesium-acetate (1 M) and QX-314
(50 –100 mM). In some experiments, the GABAA receptor antagonist,
bicuculline methobromide (BMI; 10 –20 ␮M), was included in the
ACSF. Inhibitory postsynaptic potentials (IPSPs) were isolated with
bath application of 6-cyano-7-nitroquinoxalene-2,3-dione (CNQX, 20
␮M) and 2-amino-5-phosphonovaleric acid (AP5, 50 ␮M) and evoked
by stimulating nRt fibers within the ventrobasal thalamus. In some
experiments, a cut was produced to excise the nRt from the slice.
RESULTS
Properties of lemniscal EPSPs
FIG. 1. Intracellular responses evoked in ventrobasal thalamic neurons by
stimulating the medial lemniscus in vitro. A: diagram of the slice preparation
used in the present study. The typical locations for simulating electrodes in the
medial lemniscus and in the thalamic radiation are shown. Also shown is the
placement of the recording electrode in the ventrobasal thalamus. B: excitatory
postsynaptic potential (EPSP) triggered by stimulating the medial lemniscus
(below) and the action potential it triggers when firing threshold is reached
(above). Note the different y axis scales. C: comparison of lemniscal and
corticothalamic EPSPs evoked on the same neuron by a pair of stimuli at
50-ms interstimulus interval. D: effect of bath-applied 6-cyano-7-nitroquinoxalene-2,3-dione (CNQX, 10 ␮M) on a lemniscal EPSP. Neurons were recorded
using K⫹-acetate-filled pipettes.
J Neurophysiol • VOL
Figure 1A shows a schematic representation of the horizontal
slice preparation used in the present study. Stimulation of the
medial lemniscus produced a very short-latency (⬃1 ms),
fast-rising EPSP that peaked at ⬃2 ms. When the EPSP
reaches firing threshold it produces an action potential at a
latency of ⬃2 ms (Fig. 1B). Thus lemniscal synapses are
extremely fast (Sabatini and Regehr 1999). Corticothalamic
synapses formed onto neurons of the ventrobasal thalamus
display paired-pulse facilitation (Castro-Alamancos and Calcagnotto 1999). The next experiments (n ⫽ 10 neurons) explored the frequency-dependent properties of the lemniscal
response and compared it with the corticothalamic response.
Figure 1C illustrates the effect of a pair of stimuli delivered to
the medial lemniscus and to the thalamic radiation to activate
corticothalamic fibers onto the same cell. The lemniscal response shows paired-pulse depression while the corticothalamic response shows facilitation. When both responses are
overlaid, several characteristic differences are apparent. The
lemniscal response to the first stimulus has a larger amplitude
and shorter latency and rises faster than the corticothalamic
response. As a consequence of facilitation and depression, the
excitatory postsynaptic potential (EPSP) amplitudes for both
pathways become similar after the first stimulus (see Fig. 1C
overlay), but the difference in latency remains. In every case
tested (n ⫽ 4), the lemniscal response was completely abolished by bath application of 10 ␮M CNQX (Fig. 1D).
To further investigate the properties of the lemniscal EPSP,
recordings were performed from ventrobasal neurons (n ⫽ 8)
filled with CS⫹ acetate and QX-314, which suppress K⫹ and
Na⫹ currents. The intensity, frequency, and voltage dependency of the lemniscal response was investigated. Manipulation of the intensity of the stimulus revealed that the lemniscal
response is all or none (Fig. 2A). Threshold stimulation produces a unitary event that always has the same amplitude.
When stimulation was delivered above threshold intensity, the
probability of occurrence of this event was 100% and its
amplitude was unchanged (i.e., between trials it varied ⬍5% at
0.1 Hz). The average amplitude of the unitary event across
cells was 11.87 ⫾ 2 mV (mean ⫾ SD; n ⫽ 8). As mentioned
in the preceding text, the lemniscal unitary EPSP depressed
slightly but significantly at frequencies ⬎2 Hz, and the amount
of depression increased with frequency and was particularly
strong at ⱖ10 Hz (Fig. 2B). Manipulation of the voltage with
current injection revealed that the unitary event occurred at
both hyperpolarized and depolarized potentials and was able to
trigger low-threshold potentials (presumably low-threshold
87 • FEBRUARY 2002 •
www.jn.org
948
M. A. CASTRO-ALAMANCOS
FIG. 2. Intensity, frequency, and voltage dependency of lemniscal EPSPs in vitro. A: effects
of varying the stimulus intensity on the evoked
lemniscal response. Note that the response does
not increase steadily with current intensity. Instead, it is an all-or-none event. In this experiment, the maximum current used was 250 ␮A,
which produced the same effect as 65 ␮A. Superimposed are 10 stimulus trials at each intensity. B: effects of varying the frequency of the
applied stimuli on the lemniscal EPSPs. The
graph below displays the percentage of change in
amplitude between the 1st stimulus and the following 3 stimuli (average of 3 neurons; 5– 40 Hz
are significantly depressed as compared with 1
Hz, P ⬍ 0.0001). C: voltage dependency of
lemniscal EPSPs. Three trials are superimposed
at each voltage. Neurons were recorded using
Cs⫹- and QX-314-filled pipettes.
calcium currents) when hyperpolarized and high-threshold potentials (presumably high-threshold calcium currents) when
depolarized (Fig. 2C). The low- and high-threshold potentials
were also triggered with the application of current injection and
thus they did not require synaptic input to be evoked (not
shown).
In the present study, stimulation of the medial lemniscus did
not usually evoke inhibitory postsynaptic potentials. Moreover,
a GABAA receptor antagonist (BMI) was bath applied in
several experiments (n ⫽ 4) with no significant effects on the
evoked lemniscal response or its characteristic frequency-dependent depression (not shown). This was expected because
the lemniscal pathway does not produce feed-forward inhibition due to the lack of inhibitory interneurons in the rodent
ventrobasal thalamus (Ohara and Lieberman 1993; Spacek and
Lieberman 1974). The only source of inhibition is the nRt,
which provides feedback inhibition when activated by collaterals from thalamocortical fibers on their way to the neocortex.
To avoid stimulating directly nRt fibers, special care was taken
to place the lemniscal-stimulating electrode outside of the
ventrobasal thalamus. Thus in this preparation, feedback inhibition would only occur subsequent to the firing of sufficient
thalamocortical neurons. The reason why recurrent IPSPs are
rare could also be attributed to the orientation of the slice
preparation so that recurrent fibers between ventrobasal neurons and nRt neurons are not in the same slice plane as
lemniscal fibers. To avoid any potential confounding effects of
IPSPs, the following experiments that studied lemniscal EPSPs
were performed in the presence of BMI in the bath.
Effects of acetylcholine
The next question was whether the lemniscal response was
affected by acetylcholine or norepinephrine, which are important
thalamic modulators (Bennett-Clarke et al. 1999; Hallanger et al.
1990; McCormick 1992; Simpson et al. 1997; Steriade et al.
1997). Neurons in the ventrobasal thalamus were recorded with
J Neurophysiol • VOL
Cs⫹ and QX-314, which suppress the postsynaptic actions of
acetylcholine and norepinephrine by reducing Na⫹ and K⫹ currents (Castro-Alamancos and Calcagnotto 2001; Gil et al. 1997).
BMI (10 –20 ␮M) was bath applied. After ⬎1 h of recording to
allow the intracellular diffusion of the drugs, acetylcholine was
applied in the bath (10 mM; n ⫽ 5 neurons) for 10 min (Fig. 3A).
In every neuron tested, application of acetylcholine had no significant effects on the size (mean ⫾ SE; 97 ⫾ 3% of baseline; n ⫽
5) or shape of the lemniscal EPSP and its frequency-dependent
depression. However, corticothalamic EPSPs on the same neurons
were significantly depressed (Fig. 3A), as previously described
(Castro-Alamancos and Calcagnotto 2001). Also, input resistance
and membrane potential were not significantly affected under
these recording conditions (Castro-Alamancos and Calcagnotto
2001). The dose of acetylcholine used is effective because it
depolarizes neurons recorded using K⫹ acetate and significantly
depresses corticothalamic synapses on the same neurons.
The effect of acetylcholine was tested on IPSPs recorded
from ventrobasal neurons impaled with Cs⫹- and QX-314filled electrodes (n ⫽ 4). Isolated IPSPs were evoked by
stimulating nRt fibers in the presence of bath-applied CNQX
(20 ␮M) and AP5 (50 ␮M). Acetylcholine (10 mM) significantly reduced the amplitude of isolated IPSPs (81 ⫾ 4%
reduction; P ⬍ 0.0001; Fig. 3B). As indicated in the preceding
text, under these recording conditions, acetylcholine did not
significantly affect the input resistance of thalamocortical neurons, which was monitored by the application of a negative
current pulse (Fig. 3B). In several experiments, a cut was
produced to excise the nRt from the slice so that acetylcholine
application would only affect the fiber terminals and not the
cell bodies of nRt neurons. Under these conditions, IPSPs were
also depressed by acetylcholine (80 ⫾ 5% reduction; P ⬍
0.0001; n ⫽ 3). This indicates that acetylcholine acts at the
terminals of nRt neurons to depress IPSPs in the ventrobasal
thalamus.
87 • FEBRUARY 2002 •
www.jn.org
LEMNISCAL SYNAPSES
949
Like acetylcholine, norepinephrine depressed isolated IPSPs
(76 ⫾ 6% reduction; P ⬍ 0.0001; n ⫽ 4) evoked by stimulating nRt fibers. In several experiments, a cut was produced to
excise the nRt from the slice so that norepinephrine application
would only affect the fiber terminals and not the cell bodies of
nRt neurons. Under these conditions, IPSPs were also depressed by norepinephrine (70 ⫾ 7% reduction; P ⬍ 0.0001;
n ⫽ 4). There was a major difference however between the
experiments performed with the nRt intact and those where the
nRt had been excised. When the nRt was intact, application of
norepinephrine produced a large increase in the frequency of
spontaneous IPSPs, which bombarded the recorded thalamocortical neuron. This effect was expected because norepineph-
FIG. 3. Effect of acetylcholine on lemniscal EPSPs and on isolated inhibitory postsynaptic potentials (IPSPs) in vitro. A: lack of effect of bath-applied
acetylcholine (Ach, 10 mM) on lemniscal EPSPs, while depressing corticothalamic EPSPs onto the same neuron. The overlaid traces correspond to
before and during the application of Ach for a single lemniscal stimulus (top
left traces), for 4 lemniscal stimuli delivered at 10 Hz (bottom traces) and for
2 corticothalamic stimuli delivered at 20 Hz (top right traces). B: depression
of isolated IPSPs in the ventrobasal thalamus by bath application of Ach (10
mM). The overlaid traces show an IPSP induced by stimulating nRt fibers,
before (control) and during (Ach) the application of acetylcholine (10 mM).
Following the IPSP is the effect of a negative current pulse (0.1 nA, 50 ms),
which was used to monitor input resistance. Two traces are overlaid corresponding to before and during Ach. The intracellular recordings were performed using Cs⫹- and QX-314-filled pipettes. IPSPs were isolated using
bath-applied CNQX and 2-amino-5-phosphonovaleric acid (AP5). EPSPs were
isolated using bath-applied bicuculline methobromide (BMI). Data are presented as the percentage of the baseline EPSP (A) and as the absolute IPSP
amplitudes (B).
Effects of norepinephrine
The next experiments explored the effects of norepinephrine
on lemniscal synapses. Like acetylcholine, norepinephrine
(100 ␮M) had no significant effect on the size (95 ⫾ 4% of
baseline; n ⫽ 7) or shape of the lemniscal EPSP and its
frequency-dependent depression (Fig. 4A). Also, input resistance and membrane potential were not significantly affected
by norepinephrine under these recording conditions (Fig. 4B),
but corticothalamic EPSPs were significantly depressed (Fig.
4A), as previously described (Castro-Alamancos and Calcagnotto 2001).
J Neurophysiol • VOL
FIG. 4. Effect of norepinephrine on lemniscal EPSPs and on isolated IPSPs
in vitro. A: lack of effect of bath-applied norepinephrine (NE, 100 ␮M) on
lemniscal EPSPs, while depressing corticothalamic EPSPs onto the same
neuron. The overlaid traces correspond to before and during the application of
NE for a single lemniscal stimulus (top left traces), for 4 lemniscal stimuli
delivered at 10 Hz (bottom traces), and for 2 corticothalamic stimuli delivered
at 20 Hz (top right traces). B: depression of isolated IPSPs in the ventrobasal
thalamus by bath application of NE (100 ␮M). The overlaid traces show an
IPSP induced by stimulating nRt fibers, before (control) and during (NE) the
application of norepinephrine. Following the IPSP is the effect of a negative
current pulse (0.3 nA, 50 ms), which was used to monitor input resistance. Two
traces are overlaid corresponding to before and during NE. The intracellular
recordings were performed using Cs⫹- and QX-314-filled pipettes. IPSPs were
isolated using bath-applied CNQX and AP5. Data are presented as the percentage of the baseline EPSP (A) and as the absolute IPSP amplitudes (B).
87 • FEBRUARY 2002 •
www.jn.org
950
M. A. CASTRO-ALAMANCOS
rine depolarizes nRt neurons increasing their firing rates
(Kayama et al. 1982; McCormick and Wang 1991). This increase in spontaneous IPSPs was never observed when the nRt
was excised. However, depression of evoked IPSPs was
present in both conditions (with or without nRT cell bodies),
indicating that norepinephrine acts at the terminals of nRt
neurons to depress IPSPs in the ventrobasal thalamus, despite
depolarizing and increasing the firing rate of nRt neurons.
In conclusion, lemniscal responses consist of a short-latency
fast-rising unitary EPSP, which is all or none, depresses at
frequencies ⬎2 Hz, and is insensitive to the modulators acetylcholine and norepinephrine. In contrast, IPSPs evoked by
stimulation of nRt fibers and corticothalamic EPSPs are
strongly reduced by acetylcholine and by norepinephrine in the
ventrobasal thalamus.
Relay of lemniscal inputs
The main function of the thalamus is to relay sensory inputs
to the neocortex. The transfer of high-frequency sensory inputs
may be jeopardized by the depression of lemniscal synapses.
However, sensory relay is functionally relevant during information processing states in which the thalamus is activated due
to the effects of modulators. The main consequence of these
modulators, such as acetylcholine and norepinephrine, is to
depolarize thalamocortical neurons (McCormick 1992; Steriade et al. 1997). Thus the next experiments explored in slices
how postsynaptic depolarization and synaptic depression interact to relay lemniscal inputs at different frequencies. Intracellular recordings (n ⫽ 5) were performed using K⫹-acetatefilled electrodes. Figure 5 shows the effect of stimuli applied to
the medial lemniscus at different frequencies for a neuron in
the bursting mode or in the tonic mode. In the bursting mode,
neurons responded robustly to the first stimulus with a burst of
action potentials but were unable to follow at frequencies ⬎2
Hz. In contrast, in the tonic mode, the firing of thalamic
neurons was highly reliable for every stimulus delivered at
frequencies ⱕ40 Hz (Fig. 5A). Thus in the tonic mode, the
lemniscal input seems to be able to overcome the synaptic
depression because the membrane potential is placed close to
firing threshold. Thalamocortical neurons are unable to follow
high-frequency inputs in the burst mode because of the properties of the currents involved in burst generation (McCormick
and Feeser 1990; Sherman 1996). However, the suppression of
high-frequency lemniscal inputs does not require thalamocortical neurons to be in the bursting mode. Indeed, synaptic
depression is effective at suppressing the transfer of lemniscal
inputs when neurons are sufficiently depolarized so they are no
longer in the bursting mode (Fig. 5B). Within the tonic mode,
the suppression of lemniscal inputs is expressed when the
neuron is sufficiently hyperpolarized. If the neuron is depolarized closer to the firing threshold, it can overcome the depression and relay sensory inputs at high frequency (Fig. 5B). Thus
because of synaptic depression, the relay of sensory information through the lemniscal pathway requires sufficient postsynaptic depolarization to overcome synaptic depression. When
these two events coincide (lemniscal release and postsynaptic
depolarization), the relay of sensory information is warranted
for high-frequency inputs. This was further quantified by calculating a correlation between the set membrane potential of
thalamic neurons and the probability of firing to the fourth
lemniscal stimulus delivered at 10 Hz. This relation was found
to be significant (r ⫽ 0.78, P ⬍ 0.0001; n ⫽ 3 neurons).
The last experiments were performed to test if indeed the
depolarization produced by acetylcholine and norepinephrine
FIG. 5. Voltage dependency of lemniscal suppression. A: stimuli were applied to the medial lemniscus and the neuron was held
hyperpolarized in the bursting mode (left) or it was depolarized to the tonic mode (right). Note the ability of thalamic neurons to
follow high-frequency lemniscal inputs in the tonic mode. The intracellular recordings were performed using K⫹-acetate-filled
pipettes. B: 4 lemniscal stimuli are applied at 10 Hz. The neuron is placed in the bursting mode (bottom) or in the tonic mode (top
3 traces) by the application of current injection. Within the tonic mode, the neuron is unable to follow the lemniscal inputs with
action potentials because of synaptic depression. However, as the neuron is further depolarized it can then follow the synaptic input.
The intracellular recordings were performed using K⫹-acetate-filled pipettes.
J Neurophysiol • VOL
87 • FEBRUARY 2002 •
www.jn.org
LEMNISCAL SYNAPSES
is able to facilitate the relay of high-frequency lemniscal inputs. The results shown in the preceding text reveal that norepinephrine and acetylcholine do not affect the efficacy of
lemniscal synapses, but the depolarization these modulators
produce may suffice to overcome synaptic depression and the
suppression of high-frequency lemniscal inputs. Intracellular
recordings (n ⫽ 6) were performed using K-acetate-filled electrodes to monitor the relay of lemniscal inputs and BMI was
present in the bath. Figure 6A shows a typical example from a
thalamocortical neuron at resting membrane potential. Notice
951
the suppression of high-frequency lemniscal inputs when the
neuron is at resting membrane potential. Application of acetylcholine (10 mM) or norepinephrine (100 ␮M) was accompanied by an increase in the neurons input resistance and
depolarization to around spiking threshold, where spontaneous
tonic firing could occur (McCormick 1992). Moreover, under
these conditions, lemniscal suppression was not apparent and
thalamocortical neurons were able to relay high-frequency
lemniscal inputs. When the neurons are hyperpolarized with
current injection, in the presence of acetylcholine or norepinephrine, lemniscal suppression is again revealed. This indicates that the postsynaptic depolarization produced by these
neuromodulators suffices to eliminate the suppression of highfrequency lemniscal inputs. Figure 6B shows population data
corresponding to the effects of acetylcholine and norepinephrine on the relay of lemniscal inputs at different frequencies
(n ⫽ 6 neurons). The results for acetylcholine and norepinephrine were pooled together because they did not differ significantly. Note that when neurons are at resting membrane potential, lemniscal suppression occurs at frequencies ⬎2 Hz, and
is quite significant at frequencies ⬎10 Hz. In contrast, in the
presence of acetylcholine or norepinephrine, lemniscal inputs
can be relayed at ⱕ40 Hz.
DISCUSSION
FIG. 6. Lemniscal suppression is eliminated due to the depolarization
caused by acetylcholine and norepinephrine. A: stimuli were applied to the
medial lemniscus when the neurons were at resting membrane potential (control). Note that application of stimuli at 10 or 20 Hz produces lemniscal
suppression. Application of acetylcholine (10 mM) depolarizes the ventrobasal
neuron so that lemniscal suppression is no longer present (Ach). Hyperpolarizing the neuron back to resting membrane potential using intracellular current
application reinstates lemniscal suppression (Ach ⫹ Hyperpol.). B: the effect
of acetylcholine on the cell’s input resistance is shown by comparing the
voltage deflection to a negative current pulse (0.3 nA; 50 ms) applied before
and after the drug. C: population data corresponding to the probability of firing
to four lemniscal stimuli delivered at 10 Hz when the neurons are at resting
membrane potential under control conditions (control) and in the presence of
acetylcholine (n ⫽ 4) or norepinephrine (n ⫽ 2; Ach-NE). D: population data
corresponding to the probability of firing to the last of 4 lemniscal stimuli
delivered at 0.1, 1, 5, 10, 20, and 40 Hz. Data correspond to 5–10 trials per
frequency before and after the application of acetylcholine (n ⫽ 4 cells) or
norepinephrine (n ⫽ 2 cells). The data for both drugs were pooled together
because they were not significantly different. The intracellular recordings were
performed using K⫹-acetate-filled pipettes.
J Neurophysiol • VOL
The present experiments reveal that lemniscal responses
consist of a short-latency fast-rising unitary EPSP that is all or
none, depresses at frequencies ⬎2 Hz, and is insensitive to the
modulators acetylcholine and norepinephrine, which selectively depress corticothalamic responses and thalamic IPSPs.
The relay of lemniscal activity through the thalamus at frequencies ⬎2 Hz requires sufficient postsynaptic depolarization
to overcome synaptic depression. In conclusion, lemniscal
synaptic depression and postsynaptic depolarization combine
to gate the flow of sensory inputs to the cortex.
Differences between the rise time and onset latency of
sensory and corticothalamic synaptic responses have been previously described in the visual system (Turner and Salt 1998)
and may be attributed to the dendritic locations of both inputs,
the conduction velocity of the fibers and the properties of the
underlying currents. Corticothalamic terminals are located at
distal dendrites, while lemniscal synapses occur more proximal
to the cell body (Liu et al. 1995; Spacek and Lieberman 1974;
Williams et al. 1994b), and consequently corticothalamic
EPSPs can be low-pass filtered by the dendritic cable (Spruston
et al. 1994). Also, corticothalamic fibers have smaller diameters than lemniscal fibers and consequently conduct slower (Sherman and Guillery 1996; Steriade et al. 1997). In addition, to
achieve their high-speed, lemniscal synapses must have optimized
the steps for synaptic transmission (Sabatini and Regehr 1999).
Rodent lemniscal synapses have been described at the electron
microscope level as large size terminals with numerous closely
spaced synaptic contacts (Spacek and Lieberman 1974; Williams
et al. 1994b). The unitary events may result from synchronous
release at these multiple contacts, and depression may be a consequence of vesicle depletion (Thomson 2000).
It was interesting to find that application of two important
thalamic neuromodulators in vitro, acetylcholine and norepinephrine, did not directly affect the efficacy of lemniscal synapses,
although under the same conditions, they depress IPSPs and the
87 • FEBRUARY 2002 •
www.jn.org
952
M. A. CASTRO-ALAMANCOS
efficacy of corticothalamic synapses. Acetylcholine and norepinephrine have opposite effects on the firing properties of nRt
neurons (McCormick 1992). Acetylcholine hyperpolarizes nRt
neurons (Ben Ari et al. 1976; McCormick and Prince 1986), while
norepinephrine depolarizes them (Kayama et al. 1982; McCormick and Wang 1991). This was clearly apparent in our recordings from ventrobasal neurons when the nRt was intact because
application of norepinephrine, but not of acetylcholine, caused an
increase in spontaneous IPSPs, indicating the depolarization and
firing of nRt neurons. Increased firing in nRt neurons has been
shown to suppress the background activity of ventrobasal neurons,
with no significant effect on the relay of lemniscal inputs (Warren
and Jones 1994). Despite the differential effects of these modulators on the firing of nRt neurons, the present study demonstrates
that both acetylcholine and norepinephrine can depress IPSPs
within the ventrobasal thalamus at the terminals of nRt neurons.
This is an important consideration because intracellular recordings
from the rodent ventrobasal thalamus show that tactile stimulation
produces an EPSP-IPSP sequence (Salt and Eaton 1990); because
of the lack of interneurons in the ventrobasal thalamus, the longlatency IPSP is a result of feedback inhibition from the nRt. The
present results indicate that neuromodulators released within the
ventrobasal thalamus can further regulate the amplitude of feedback IPSPs coming from the nRt and thus impact the regulation
that nRt exerts on ventrobasal neurons. Stimulation of the medial
lemniscus in the horizontal slices used here does not reliably
produce feedback IPSPs, especially when care is taken to make
sure that the stimulating electrode is placed outside of the ventrobasal thalamus; if placed inside the ventrobasal thalamus, it could
directly activate nRt fibers or collaterals of thalamocortical fibers
projecting to the nRt. The lack of robust feedback inhibition in the
slice is likely due to recurrent fibers between the nRt and the
ventrobasal thalamus not being in the same plane as the lemniscal
fibers. Nonetheless, the present results demonstrate that feedback
inhibition is not required to produce lemniscal suppression of
sensory inputs in the ventrobasal thalamus because lemniscal
suppression was robust when feedback inhibition was blocked.
Although not required, feedback IPSPs in vivo contribute to
sensory suppression (Lee et al. 1994). Thus the reduction of IPSPs
produced by acetylcholine and norepinephrine in the ventrobasal
thalamus will further facilitate the relay of high-frequency sensory
inputs during activated states.
Although acetylcholine and norepinephrine do not affect
lemniscal EPSPs, the postsynaptic depolarization they cause on
thalamocortical neurons is sufficient to overcome the suppression of lemniscal inputs. This is due to the fact that the
depressed lemniscal EPSPs generated by high-frequency stimulation retain considerable amplitude, which can reach firing
threshold if aided by postsynaptic depolarization. Thus during
behaviorally activated states, when acetylcholine and norepinephrine are released in the thalamus, feedback IPSPs will be
depressed and thalamocortical neurons depolarize to facilitate
the relay of high-frequency sensory information. The present
results serve to explain previous results obtained in vivo such
as those reported by Poggio and Mountcastle (1963). These
authors found that the capacity for frequency following of
tactile stimuli is dramatically different for thalamic cells in the
waking as compared with the anesthetized monkey. Also, in
anesthetized rats some studies have shown that thalamocortical
neurons can follow whisker stimulation ⱕ12 Hz (Hartings and
Simons 1998; Simons 1985; Simons and Carvell 1989), while
J Neurophysiol • VOL
other studies report strong frequency-dependent depression at
frequencies ⬎5 Hz (Diamond et al. 1992) or 2 Hz (Ahissar et
al. 2000). The present study indicates that these discrepancies
could be explained by the anesthetic state of the preparation
and in particular the membrane potential of thalamocortical
neurons. The more efficient frequency following of lemniscal
sensory inputs during activated states as compared with quiescent states would result from the depolarization of thalamocortical neurons during that state caused by the release of
neuromodulators in the thalamus, which serves to bring depressed lemniscal EPSPs close to firing threshold.
Interestingly, the same modulators that enhance the relay of
lemniscal sensory inputs also filter corticothalamic inputs so
that only high-frequency activity (⬎5 Hz) from the cortex can
reach the thalamus (Castro-Alamancos and Calcagnotto 2001).
Thus acetylcholine and norepinephrine produce a generalized
depolarization of thalamocortical neurons that should enhance
all inputs to these neurons. However, the same modulators that
depolarize thalamocortical neurons selectively depress corticothalamic and nRt inputs, so that postsynaptic depolarization
results in a selective enhancement of the sensory input. During
behavioral activation, the thalamus will function as an effective
relay of sensory information from the periphery to the neocortex, but it will be disconnected from the neocortex unless the
neocortex sends high-frequency activity. The neocortex would
only be able to influence thalamic activity and the flow of
sensory inputs when high-frequency cortical activity is present,
perhaps through corticothalamic oscillations that occur during
certain behavioral states in waking animals (Nicolelis et al.
1995; Rougeul-Buser and Buser 1997).
Previous work has emphasized the thalamus as the first stage
for gating sensory information (Steriade and McCarley 1990),
and the capacity of thalamic activation to enhance sensory
transmission (Eysel et al. 1986; Francesconi et al. 1988; Humphrey and Saul 1992; Pare et al. 1990; Singer 1977; Steriade
and Demetrescu 1960; Uhlrich et al. 1995). The present study
highlights the interplay between synaptic depression and
postsynaptic depolarization as a gating mechanism of sensory
information flow. Particularly interesting is that sensory transfer by lemniscal inputs is not only controlled by a simple
change between the burst-to-tonic modes of thalamic relay
neurons. The degree of depolarization within the tonic mode
seems to be an important variable. This suggests important
functional consequences in relation to information processing;
cells will relay high-frequency sensory inputs only if sufficiently depolarized within the tonic mode. On a speculative
note, this may reflect the difference between being awake and
being attentive.
This work was supported by the Medical Research Council of Canada,
Natural Sciences and Engineering Council of Canada, Fonds de la Reserche en
Sante du Quebec, Canadian Foundation for Innovation, and Savoy Foundation.
REFERENCES
AHISSAR E, SOSNIK R, AND HAIDARLIU S. Transformation from temporal to rate
coding in a somatosensory thalamocortical pathway. Nature 406: 302–306,
2000.
ASTON-JONES G, CHIANG C, AND ALEXINSKY T. Discharge of noradrenergic
locus coeruleus neurons in behaving rats and monkeys suggests a role in
vigilance. Prog Brain Res 88: 501–520, 1991.
BEN ARI Y, DINGLEDINE R, KANAZAWA I, AND KELLY JS. Inhibitory effects of
acetylcholine on neurones in the feline nucleus reticularis thalami. J Physiol
(Lond) 261: 647– 671, 1976.
87 • FEBRUARY 2002 •
www.jn.org
LEMNISCAL SYNAPSES
BENNETT-CLARKE CA, CHIAIA NL, AND RHOADES RW. Differential expression
of acetylcholinesterase in the brainstem, ventrobasal thalamus and primary
somatosensory cortex of perinatal rats, mice, and hamsters. Somatosens Mot
Res 16: 269 –279, 1999.
BUZSAKI G, BICKFORD RG, PONOMAREFF G, THAL LJ, MANDEL R, AND GAGE
FH. Nucleus basalis and thalamic control of neocortical activity in the freely
moving rat. J Neurosci 8: 4007– 4026, 1988.
CASTRO-ALAMANCOS MA AND CALCAGNOTTO ME. Presynaptic long-term potentiation in corticothalamic synapses. J Neurosci 19: 9090 –9097, 1999.
CASTRO-ALAMANCOS MA AND CALCAGNOTTO ME. High-pass filtering of corticothalamic activity by neuromodulators released in the thalamus during
arousal: in vitro and in vivo. J Neurophysiol 85: 1489 –1497, 2001.
CHIAIA NL, RHOADES RW, BENNETT-CLARKE CA, FISH SE, AND KILLACKEY
HP. Thalamic processing of vibrissal information in the rat. I. Afferent input
to the medial ventral posterior and posterior nuclei. J Comp Neurol 314:
201–216, 1991.
DIAMOND ME. Somatosensory thalamus of the rat. Cereb Cortex 11: 189 –220,
1995.
DIAMOND ME, ARMSTRONG-JAMES M, AND EBNER FF. Somatic sensory responses in the rostral sector of the posterior group (POm) and in the ventral
posterior medial nucleus (VPM) of the rat thalamus. J Comp Neurol 318:
462– 476, 1992.
EYSEL UT, PAPE HC, AND VAN SCHAYCK R. Excitatory and differential disinhibitory actions of acetylcholine in the lateral geniculate nucleus of the cat.
J Physiol (Lond) 370: 233–254, 1986.
FANSELOW EE AND NICOLELIS MA. Behavioral modulation of tactile responses
in the rat somatosensory system. J Neurosci 19: 7603–7616, 1999.
FELDMAN SG AND KRUGER L. An axonal transport study of the ascending
projection of medial lemniscal neurons in the rat. J Comp Neurol 192:
427– 454, 1980.
FRANCESCONI W, MULLER CM, AND SINGER W. Cholinergic mechanisms in the
reticular control of transmission in the cat lateral geniculate nucleus. J Neurophysiol 59: 1690 –1718, 1988.
GIL Z, CONNORS BW, AND AMITAI Y. Differential regulation of neocortical
synapses by neuromodulators and activity. Neuron 19: 679 – 686, 1997.
HALLANGER AE, PRICE SD, LEE HJ, STEININGER TL, AND WAINER BH. Ultrastructure of cholinergic synaptic terminals in the thalamic anteroventral,
ventroposterior, and dorsal lateral geniculate nuclei of the rat. J Comp
Neurol 299: 482– 492, 1990.
HARTINGS JA AND SIMONS DJ. Thalamic relay of afferent responses to 1- to
12-Hz whisker stimulation in the rat. J Neurophysiol 80: 1016 –1019, 1998.
HUMPHREY AL AND SAUL AB. Action of brain stem reticular afferents on
lagged and nonlagged cells in the cat lateral geniculate nucleus. J Neurophysiol 68: 673– 691, 1992.
KAYAMA Y, NEGI T, SUGITANI M, AND IWAMA K. Effects of locus coeruleus
stimulation on neuronal activities of dorsal lateral geniculate nucleus and
perigeniculate reticular nucleus of the rat. Neuroscience 7: 655– 666, 1982.
LEE SM, FRIEDBERG MH, AND EBNER FF. The role of GABA-mediated inhibition in the rat ventral posterior medial thalamus. II. Differential effects of
GABAA and GABAB receptor antagonists on responses of VPM neurons.
J Neurophysiol 71: 1716 –1726, 1994.
LIU XB, HONDA CN, AND JONES EG. Distribution of four types of synapse on
physiologically identified relay neurons in the ventral posterior thalamic
nucleus of the cat. J Comp Neurol 352: 69 –91, 1995.
MCCORMICK DA. Neurotransmitter actions in the thalamus and cerebral cortex
and their role in neuromodulation of thalamocortical activity. Prog Neurobiol 39: 337–388, 1992.
MCCORMICK DA AND FEESER HR. Functional implications of burst firing and
single spike activity in lateral geniculate relay neurons. Neuroscience 39:
103–113, 1990.
MCCORMICK DA AND PRINCE DA. Acetylcholine induces burst firing in thalamic reticular neurones by activating a potassium conductance. Nature 319:
402– 405, 1986.
MCCORMICK DA AND WANG Z. Serotonin and noradrenaline excite GABAergic neurones of the guinea-pig and cat nucleus reticularis thalami. J Physiol
(Lond) 442: 235–255, 1991.
J Neurophysiol • VOL
953
NICOLELIS MA, BACCALA LA, LIN RC, AND CHAPIN JK. Sensorimotor encoding
by synchronous neural ensemble activity at multiple levels of the somatosensory system. Science 268: 1353–1358, 1995.
OHARA PT AND LIEBERMAN AR. Some aspects of the synaptic circuitry underlying inhibition in the ventrobasal thalamus. J Neurocytol 22: 815– 825,
1993.
PARE D, STERIADE M, DESCHENES M, AND BOUHASSIRA D. Prolonged enhancement of anterior thalamic synaptic responsiveness by stimulation of a
brain-stem cholinergic group. J Neurosci 10: 20 –33, 1990.
POGGIO GF AND MOUNTCASTLE VB. The functional properties of ventrobasal
thalamic neurons studied in unanesthetized monkeys. J Neurophysiol 26:
775– 806, 1963.
ROUGEUL-BUSER A AND BUSER P. Rhythms in the alpha band in cats and their
behavioural correlates. Int J Psychophysiol 26: 191–203, 1997.
SABATINI BL AND REGEHR WG. Timing of synaptic transmission. Annu Rev
Physiol 61: 521–542, 1999.
SALT TE AND EATON SA. Postsynaptic potentials evoked in ventrobasal thalamus neurones by natural sensory stimuli. Neurosci Lett 114: 295–299,
1990.
SHERMAN SM. Dual response modes in lateral geniculate neurons: mechanisms
and functions. Vis Neurosci 13: 205–213, 1996.
SHERMAN SM AND GUILLERY RW. Functional organization of thalamocortical
relays. J Neurophysiol 76: 1367–1395, 1996.
SIMONS DJ. Temporal and spatial integration in the rat SI vibrissa cortex.
J Neurophysiol 54: 615– 635, 1985.
SIMONS DJ AND CARVELL GE. Thalamocortical response transformation in the
rat vibrissa/barrel system. J Neurophysiol 61: 311–330, 1989.
SIMPSON KL, ALTMAN DW, WANG L, KIRIFIDES ML, LIN RC, AND WATERHOUSE BD. Lateralization and functional organization of the locus coeruleus
projection to the trigeminal somatosensory pathway in rat. J Comp Neurol
385: 135–147, 1997.
SINGER W. Control of thalamic transmission by corticofugal and ascending
reticular pathways in the visual system. Physiol Rev 57: 386 – 420, 1977.
SPACEK J AND LIEBERMAN AR. Ultrastructure and three-dimensional organization of synaptic glomeruli in rat somatosensory thalamus. J Anat 117:
487–516, 1974.
SPRUSTON N, JAFFE DB, AND JOHNSTON D. Dendritic attenuation of synaptic
potentials and currents: the role of passive membrane properties. Trends
Neurosci 17: 161–166, 1994.
STERIADE M AND DEMETRESCU M. Unspecific systems of inhibition and facilitation of potentials evoked by intermittent light. J Neurophysiol 23: 602–
617, 1960.
STERIADE M, JONES EG, AND MCCORMICK DA. Thalamus. New York: Elsevier,
1997.
STERIADE M AND MCCARLEY RW. Brainstem Control of Wakefulness and
Sleep. New York: Plenum, 1990.
THOMSON AM. Molecular frequency filters at central synapses. Prog Neurobiol
62: 159 –196, 2000.
TURNER JP AND SALT TE. Characterization of sensory and corticothalamic
excitatory inputs to rat thalamocortical neurones in vitro. J Physiol (Lond)
510: 829 – 843, 1998.
UHLRICH DJ, TAMAMAKI N, MURPHY PC, AND SHERMAN SM. Effects of brain
stem parabrachial activation on receptive field properties of cells in the cat’s
lateral geniculate nucleus. J Neurophysiol 73: 2428 –2447, 1995.
VEINANTE P AND DESCHENES M. Single- and multi-whisker channels in the
ascending projections from the principal trigeminal nucleus in the rat.
J Neurosci 19: 5085–5095, 1999.
WARREN RA AND JONES EG. Glutamate activation of cat thalamic reticular
nucleus: effects on response properties of ventroposterior neurons. Exp
Brain Res 100: 215–226, 1994.
WILLIAMS JA, COMISAROW J, DAY J, FIBIGER HC, AND REINER PB. Statedependent release of acetylcholine in rat thalamus measured by in vivo
microdialysis. J Neurosci 14: 5236 –5242, 1994a.
WILLIAMS MN, ZAHM DS, AND JACQUIN MF. Differential foci and synaptic
organization of the principal and spinal trigeminal projections to the thalamus in the rat. Eur J Neurosci 6: 429 – 453, 1994b.
87 • FEBRUARY 2002 •
www.jn.org