Download Octopus Cells of the Mammalian Ventral Cochlear Nucleus Sense

J Neurophysiol
87: 2262–2270, 2002; 10.1152/jn.00587.2001.
Octopus Cells of the Mammalian Ventral Cochlear Nucleus Sense
the Rate of Depolarization
MICHAEL J. FERRAGAMO AND DONATA OERTEL
Department of Physiology, University of Wisconsin, Madison, Wisconsin 53706
Received 17 July 2001; accepted in final form 17 December 2001
INTRODUCTION
The tonotopic array of tuned auditory nerve fibers brings to
the brain a spectral and temporal representation of sound from
which biologically useful information must be extracted. Extraction of the location and meaning of sounds from the spatiotemporal pattern of activation of auditory nerve fibers begins
in the cochlear nuclei, where all auditory nerve fibers terminate
and distribute information to several parallel ascending pathways. Octopus cells form a pathway to the superior paraolivary
nucleus and to the ventral nucleus of the lateral lemniscus
(Adams 1997; Oertel 1999; Schofield 1995; Schofield and Cant
1997; Vater et al. 1997; Warr 1969). This pathway is present in
all mammals and is especially prominent in humans (Adams
1997). Octopus cells convey features of sound that are critical
for the recognition of natural sounds including speech. They
convey the presence of acoustic transients, periodicity, and
Address for reprint requests: D. Oertel, Dept. of Physiology, University of
Wisconsin Medical School, 1300 University Ave., Madison, WI 53706 (Email: [email protected]).
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direction of frequency sweeps in their temporal firing patterns
(Godfrey et al. 1975; Oertel et al. 2000; Rhode 1994, 1998;
Rhode and Smith 1986; Rhode et al. 1983; Smith et al. 1993).
Octopus cells detect the coincident input of auditory nerve
fibers. The dendrites of octopus cells cross the bundles of
auditory nerve fibers and are thus accessible to input from
many fibers (Brawer et al. 1974; Golding et al. 1995; Oertel et
al. 2000; Osen 1969; Willott and Bross 1990). Summation of
multiple auditory nerve inputs is required to bring an octopus
cell to threshold (Golding et al. 1995). Auditory nerve fibers
excite octopus cells through glutamate receptors of the AMPA
subtype that have rapid kinetics (decay time constants average
350 ␮s) (Gardner et al. 1999, 2001). The low input resistances
and short time constants of octopus cells allow the rapid
synaptic currents to produce synaptic potentials whose duration
is short, generally about 1 ms. With patch-clamp recordings,
input resistances were measured to be between about 2 and 6
M⍀ and time constants to be about 200 ␮s (Bal and Oertel
2000; Golding et al. 1999). The low input resistance arises
largely from two voltage-sensitive conductances that are partly
activated at rest, a Cs⫹- and ZD7288-sensitive, inward rectifier
(gh) and a 4-aminopyridine (4AP) and ␣-dendrotoxin-sensitive,
low-threshold potassium conductance, gKL (Bal and Oertel
2000, 2001; Golding et al. 1995, 1999). These conductances
have similar properties but are larger than those recorded in
other neurons, including other brain stem auditory neurons
with especially sharp timing. The partial activation of these
two, strong voltage-sensitive conductances at rest indicated
that the firing of octopus cells might be sensitive to the rate at
which they are depolarized. The present study shows that this
is true.
METHODS
Slices were made from young mice of the strains CBA and ICR.
Animals between 17 and 26 days old were decapitated, and their
brains were dissected in physiological saline at 31°C. The block of
tissue that contained the cochlear nuclei was glued to a Teflon block
at the transverse cut through the caudal inferior colliculus. Coronal
slices, 150 –200 ␮m thick, that contained the caudal posteroventral
cochlear nucleus were cut with an oscillating tissue slicer (Frederick
Haer, New Brunswick, ME). Slices were allowed to recover for about
1 h in a recording chamber containing about 0.3 ml, which was
continuously superfused with oxygenated physiological saline, 33°C,
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0022-3077/02 $5.00 Copyright © 2002 The American Physiological Society
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Ferragamo, Michael J. and Donata Oertel. Octopus cells of the
mammalian ventral cochlear nucleus sense the rate of depolarization.
J Neurophysiol 87: 2262–2270, 2002; 10.1152/jn.00587.2001. Whole
cell patch recordings in slices show that the probability of firing of
action potentials in octopus cells of the ventral cochlear nucleus
depends on the dynamic properties of depolarization. Octopus cells
fired only when the rate of rise of a depolarization exceeded a
threshold value that varied between 5 and 15 mV/ms among cells. The
threshold rate of rise was independent of whether depolarizations
were evoked synaptically or by the intracellular injection of current.
Previous work showed that octopus cells are contacted by many
auditory nerve fibers, each providing less than 1-mV depolarization.
Summation of synaptic input from multiple fibers is required for an
octopus cell to reach threshold. In firing only when synaptic depolarization exceeds a threshold rate, octopus cells fire selectively when
synaptic input is sufficiently large and synchronized for the small,
brief unitary excitatory postsynaptic potentials (EPSPs) to sum to
produce a rapidly rising depolarization. The sensitivity to rate of
depolarization is governed by a low-threshold, ␣-dendrotoxin-sensitive potassium conductance (gKL). This conductance also shapes the
peaks of action potentials, contributing to the precision in their timing.
Firing in neighboring T stellate cells depends much less strongly on
the rate of rise. They lack strong ␣-dendrotoxin-sensitive conductances. Octopus cells appear to be specialized to detect synchronization in the activation of groups of auditory nerve fibers, a common
pattern in responses to natural sounds, and convey its occurrence with
temporal precision.
OCTOPUS CELLS SENSE RATE OF DEPOLARIZATION
V cap ⫽ Icap Relec
(1)
I cap ⫽ Celec dV/dt ⫽ Celec Relec dI/dt
(2)
The capacitance of the electrodes could be estimated from the magnitude of the balancing voltage pulse.
2
C elec ⫽ Vcap / 共Relec
dI/dt)
FIG. 1. Artifacts appear in traces in which the resistance has been balanced
but in which the capacitance has not been balanced. A: a family of responses
to ramps of equal durations whose rate of rise is varied by varying the
steady-state current. B: a family of responses to ramps whose rate of rise is
varied by varying the duration of a ramp to a steady level. In A and B, the
resistance of the electrode was balanced by subtracting the current waveform
multiplied by the resistance of the electrode. The resistance of the electrode
was determined in the conventional way by “balancing the bridge” at the offset
of the current. The membrane potential appears to be hyperpolarized in
proportion to the rate of rise of the current ramp. These “hyperpolarizations”
must be experimental artifacts because depolarizing current cannot hyperpolarize cells. In Figs. 3– 6, the capacitative artifacts were balanced by adding a
square voltage pulse whose amplitude was proportional to the rate of rise of the
ramp.
made with a camera lucida. With practice the octopus cell area could
be visualized routinely. Because octopus cells are so distinctive electrophysiologically, not all tissues were processed histologically in
later experiments.
(3)
RESULTS
The balanced capacitance was calculated in 12 recordings where it
was on average 5.6 pF.
In some experiments, inputs to neurons were activated through a
glass pipette with shocks to a fiber tract within about 100 ␮m of the
cell body. Pipettes for stimulation were similar to those used for
recording, but they were filled with extracellular saline solution;
voltage pulses between 10 and 100 V were 100 ␮s in duration.
Many octopus cells were identified morphologically. Biocytin
(Sigma; 0.1%) was often included in the pipette solution. After a
recording, slices were fixed in 4% paraformaldehyde and stored at
4°C, embedded in a gelatin-albumin mixture (Oertel et al. 1990), and
sectioned at 60 ␮m in the plane of the slice. Biocytin-filled cells were
visualized using the avidin-biotinylated horseradish peroxidase (HRP)
complex reaction (Vectastain ABC Elite Kit; Vector Laboratories,
Burlingame, CA), using nickel/cobalt-intensified 3,3⬘-diaminobenzidine tetra HCL (DAB) (Golding et al. 1995). Reconstructions were
J Neurophysiol • VOL
Patch-clamp recordings from 58 octopus cells, of which 42
were anatomically identified, form the basis of this study.
These cells had resting potentials ⫺60.6 ⫾ 5.8 mV and input
resistances of 3.2 ⫾ 1.5M⍀.
Reconstructions of four of those cells are shown in Fig. 2.
The morphology was consistent with what was reported on the
basis of earlier recordings in parasagittal and coronal slices
(Golding et al. 1995, 1999; Oertel et al. 1990). Recordings
were made from the cell bodies that lay near the posterior cut
surface of slices. The dendrites spread toward the anterior
through the slices. The dendrites were generally thick, 2– 4 ␮m
in diameter, and often terminated with an abrupt taper. The
axons of all cells headed medially and dorsally between the
deep layer of the dorsal cochlear nucleus and the restiform
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at about 8 ml/min. Physiological saline contained (in mM) 130 NaCl,
3 KCl, 1.3 MgSO4, 2.4 CaCl2, 20 NaHCO3, 3 HEPES, 10 glucose,
and 1.2 KH2PO4, pH 7.4 (Sigma, St. Louis, MO). In some experiments, ␣-dendrotoxin (Alomone Labs, Jerusalem, Israel) was added to
the normal physiological saline.
Recordings were made with an Axopatch 200A amplifier in the
“fast” current-clamp mode. A Digidata 1200 interface (Axon Instruments, Foster City, CA) controlled by pClamp software (version 6.0;
Axon Instruments) was used to control current and shock stimuli and
the sampling of membrane potential. Voltages were low-pass filtered
at 10 kHz and sampled digitally at 25 or 50 kHz. The fire-polished
borosilicate glass patch-pipettes were filled with a solution with the
following composition (in mM) 140 KGluconate, 5 NaCl, 1 MgSO4,
1 CaCl2, 11 EGTA, and 10 HEPES, at a pH of 7.25 (Sigma).
Electrodes had resistances between 5 and 10 M⍀ before they touched
a cell. All traces are corrected for a junction potential of ⫺12 mV.
In many of the present experiments, ramps as well as square pulses
of current were injected through the recording electrode. Balancing
the resistance and capacitance of the electrode in the voltage traces
was performed off-line. Resistive components were balanced by recording the injected current pulse together with the voltage response
and subtracting a voltage that was proportional to the current in the
conventional way. The proportionality factor was determined by eliminating the most rapidly falling component at the end of the current
pulse. The proportionality factor was constant in any one recording
and reflected the resistance of the recording electrode after the seal
was made (which was sometimes greater than before the seal was
made, 5–22 M⍀). In responses to depolarizing current ramps in which
the electrode resistance, but not capacitance, was balanced, the voltage appeared to be displaced by a few millivolts in the hyperpolarizing direction during the current ramps with respect to the voltage
responses before or after the end of the ramp (Fig. 1). Because all
current was in a depolarizing direction, this apparent hyperpolarization had to be an experimental artifact of unbalanced capacitance. To
balance the loss of current to the bath across the capacitance of the
microelectrode as the voltage changed during the injection of current
ramps, a constant voltage increment was added to the segments of the
traces where current ramps were injected. The voltage increment,
Vcap, corresponds in magnitude to the product of the capacitative
current, Icap, lost to the bath across the capacitance of the electrode,
Celec, and the resistance of the electrode, Relec, during the current
ramp, dI/dt
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M. J. FERRAGAMO AND D. OERTEL
body in the intermediate acoutic stria. The axons were narrow
near the cell bodies and widened to about 1 ␮m diameter.
Some axons terminated in the octopus cell area and in the
adjacent granule cell domains with local collaterals (Fig. 2, top
right).
An estimate of the internodal length of octopus cell axons
could be made. The branch and protrusions, spreading from 1
to 3 ␮m from the axon, probably indicate the location of nodes
of Ranvier (Fig. 2, arrows). The distances between the branch
and the more distally located protrusions was 110, 125, and
J Neurophysiol • VOL
145 ␮m, averaging 127 ␮m. In peripheral nerve, it has been
shown that internodal conduction time is roughly constant at
19.7 ␮s at 37°C over a wide range of internodal lengths
(Rasminsky and Sears 1972). If the relationship between internodal length and conduction velocity is similar in central
axons, the axons of octopus cells would be expected to conduct
at about 6.5 m/s.
In every octopus cell tested, firing depended strongly on the
rate at which they were depolarized. Rapid depolarizations
caused octopus cells to fire, whereas slow depolarizations did
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FIG. 2. Octopus cells labeled by internal dialysis with
biocytin were reconstructed with a camera lucida. Reconstructions of the cells are shown in reconstructions of the
cochlear nuclear complex in coronally cut slices. The
lateral border of the brain stem is represented with a thick
line. The thinner lines enclose granule cell regions. The
granule cell lamina (GrL) separates the ventral from the
dorsal cochlear nucleus and extends into the fusiform cell
layer in the dorsal cochlear nucleus. All octopus cells lie
in the octopus cell area, an area that occupies the most
caudal and dorsal part of the ventral cochlear nucleus.
Their axons exit the cochlear nucleus through the intermediate acoustic stria (IAS) in a fiber bundle that lies
interposed between the deep layer of the dorsal cochlear
nucleus and the restiform body. Protrusions in the axon at
the top right (arrows) were likely to be nodes of Ranvier.
OCTOPUS CELLS SENSE RATE OF DEPOLARIZATION
2265
trates two examples for which the duration of ramps was varied
in small increments to several current levels. Slower depolarizations were not only less likely to evoke action potentials
than more rapid ones, but the action potentials recorded in the
cell body were smaller than those evoked by rapid depolarizations. The threshold of action potentials, indicated by step
changes in the amplitude of peak responses, occurred at a
consistent rate of depolarization for any one cell. The lowest
rate of depolarization with which an action potential could be
evoked varied among cells between 5 and 15 mV/ms (mean,
10.0 ⫾ 2.5 mV/ms; n ⫽ 12).
The finding that the firing of octopus cells depends on the
rate of depolarization by current injection raised the question
whether responses to synaptic activation are similarly sensitive
to the rate of depolarization. Figure 5 illustrates that the responses in three octopus cells to synaptic depolarization and
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FIG. 3. Generation of action potentials in an octopus cell depends on the
rate of rise of depolarization. A: cell was depolarized by 3 families of current
pulses consisting of ramps of durations (t) that led to a steady current pulse as
shown schematically. Steady-state current pulses varied identically from 2 to
3.8 nA in 0.2-nA increments in each family of traces. B: ramps produced
linearly rising depolarizations that often, but not always, led to the firing of the
cell. Rapidly rising ramps were more likely to cause firing than slowly rising
ramps. These traces were adjusted off-line to balance the 8.6 M⍀ series
resistance and 5.5 pF that were contributed by the electrode. C: the amplitude
of the peak depolarization from the ⫺62-mV resting potential as a function of
the steady-state current pulse. Threshold currents for the generation of action
potentials, evident as step changes in the amplitude were smallest for square
current pulses and greatest for the longest ramps. D: the same responses were
plotted as a function of the rate of rise of the membrane potential from rest. The
rate of rise was determined from a linear fit of the depolarization in the octopus
cell that preceded the action potential. The firing threshold is a consistent
function of the rate of rise. In responses to each of the families of currents,
action potentials were generated only when the octopus cell was depolarized at
a rate 12 mV/ms or greater.
not. This observation is illustrated in Fig. 3. Figure 3B shows
responses to ramps of current that led up to an identical family
of steady current pulses, 2.0 –3.8 nA. The duration of the ramps
differed in the three sets of traces (1, 1.2, and 1.4 ms). A
comparison of the groups of traces shows that the cell fired in
response to smaller currents when the current pulse rose in 1
ms than when the current pulse rose in 1.4 ms. Plots of peak
voltage responses as a function of the steady-state current have
a step where the current reaches threshold (Fig. 3C). With
square current pulses, only 1.5 nA was required to bring this
cell to threshold whereas over 3 nA were required when the
current rose over a 1.2 ms ramp. The plot of the amplitude of
the response as a function of the rate of depolarization indicates
that the firing threshold is a consistent function of the rate of
depolarization over all groups of recordings (Fig. 3D).
The rate of depolarization can be varied systematically either
by varying the level to which the current pulse rises or by
varying the duration of the ramp toward a steady-state current.
Figure 3 illustrates an example in which the current was varied
in small increments for three ramp durations. Figure 4 illusJ Neurophysiol • VOL
FIG. 4. Systematic variation in the rate of rise with changes in duration of
the ramp confirms that the firing threshold is a consistent function of the rate
of rise of depolarization in any 1 cell but varies between cells. A: families of
current ramps were generated by varying the duration of the ramp systematically in small increments. For each cell, the upper family of traces resulted
from ramps to 2 nA and the lower family of traces from ramps to 4 nA. B and
C: the amplitude of the peak of the response as a function of rate of depolarization is shown for 2 different cells in B and C. Plots of the amplitude of the
response as a function of rate of depolarization are shown on the right. The
threshold of action potentials, reflected in the step change in the amplitude of
the response was a consistent function of the rate of rise of membrane potential
in each cell. The cell whose responses are shown in B required only a 5-mV/ms
depolarization, the lowest of the cells measured, whereas the cell in C required
12 mV/ms and had a more graded response. Off-line adjustment was made to
balance 12 M⍀ and 5.5 pF in B and 17 M⍀ and 3.4 pF in C.
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M. J. FERRAGAMO AND D. OERTEL
FIG. 5. Comparison of firing threshold when octopus cells are depolarized
synaptically with when they are depolarized with current in 3 cells, A–C. For
each cell, the upper group of traces shows 3 superimposed responses to shocks
to the auditory nerve of different strengths (arrow). Synaptic stimulation
caused EPSPs whose rate of rise from rest (dotted line) varied as a function of
shock strength. Lower group of traces show responses to families of current
ramps like those shown in Fig. 4 in the same cell. Plots of the amplitude of the
response as a function of the rate of depolarization (the slope of the dotted line)
are shown on the right. The threshold for firing in responses to shocks and
injected current matched. Stimulus artifacts were deleted in traces depicting
responses to shocks.
depolarization by current injection follow similar patterns.
Three sample responses to shocks in the vicinity of the recorded cell and three sample responses to ramps are shown for
each of the cells in the traces on the left. Shocks evoked
excitatory synaptic responses in octopus cells that rose from
the resting potential about 0.5 ms after the beginning of the
shock. As is typical of octopus cells, synaptic responses to
shocks are brief, lasting only about 1 ms, and vary in their rise
and amplitude as a function of shock strength (Golding et al.
1995). The results of these measurements are summarized in
the plots on the right. In all three cells, the firing threshold in
response to synaptic stimulation roughly matched the firing
threshold in responses to injection of current ramps (Fig. 5).
To test whether gKL plays a role in determining the threshold
rate of depolarization, the sensitivity of firing to the rate of
J Neurophysiol • VOL
⫹
FIG. 6. ␣-Dendrotoxin, a blocker of the low-threshold K current, affects
the sensitivity to rate of depolarization of an octopus cell. A: in normal saline,
responses to depolarizing current ramps had to rise within ⬍2 ms to evoke an
action potential. B: in the presence of 100 nM ␣-dendrotoxin, the same octopus
cell fired large, repetitive action potentials. Those large action potentials could
be generated even with slowly rising current ramps.
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depolarization was tested when gKL was blocked. In octopus
cells, gKL could be specifically blocked by ␣-dendrotoxin (Bal
and Oertel 2001). ␣-Dendrotoxin blocked gKL with high affinity; with 5 nM, gKL was half-maximally blocked. At concentrations over 50 nM, more than 90% of gKL was blocked.
Figure 6 shows that ␣-dendrotoxin caused the firing of octopus
cells to become less sensitive to the rate of depolarization.
Under control conditions, current ramps that rose to 3.5 nA in
more than 1 ms failed to evoke action potentials (Fig. 6A).
When 100 nM ␣-dendrotoxin was added to the bath, even
slow depolarizations that rose to 3.5 nA in 40 ms evoked
large, long action potentials (Fig. 6B). The threshold rate of
rise changed by about one order of magnitude, from 12.4 to
1.1 mV/ms (not shown).
The conversion of small, brief action potentials to large,
broad ones by ␣-dendrotoxin shows that gKL plays a major role
in the repolarization of action potentials at the cell body and
draws attention to two related features of action potentials in
octopus cells, the graded nature and the timing of the peaks
recorded in the cell body. The traces in Fig. 6A illustrate
characteristic action potentials recorded in cell bodies of octopus cells. Action potentials were small and varied in the height
of their peaks depending on how they were evoked. Rapid
depolarizations evoked larger action potentials than slow depolarizations. In the presence of ␣-dendrotoxin, the action
potentials were larger and the integration time of octopus cells
expanded; Fig. 6B shows that depolarization could elicit action
potentials whose peaks occurred over a period that was an
order of magnitude longer. These findings indicate that the
gradedness and small size of action potentials in octopus cells
OCTOPUS CELLS SENSE RATE OF DEPOLARIZATION
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previous action potential and the more rapidly they fired. The
experiment in Fig. 7 shows that this cell’s threshold lay in a
narrow range of voltages near ⫺52 mV. This T stellate cell,
like the two others tested, was insensitive to ␣-dendrotoxin.
Neither the shape of the action potential nor the current-voltage
relationship was significantly affected by the application of 100
nM ␣-dendrotoxin. (Fig. 8).
DISCUSSION
arise from shunting of the inward current by gKL. In shunting
all but the earliest inward current, gKL restricted the timing of
the peak of action potentials to a narrow time window near the
beginning of a depolarization (Fig. 6A). These results show that
the ␣-dendrotoxin-sensitive conductance contributes not only
to the sensitivity to the rate of rise of depolarizations but also
to the sharp timing of the occurrence of action potentials in
octopus cells under physiological conditions.
The breadth of the action potentials in the presence of
␣-dendrotoxin raised the question whether voltage-sensitive
Na⫹ channels or voltage-sensitive Ca2⫹ channels predominated in their generation. Both types of channels have been
demonstrated to be present in octopus cells (Golding et al.
1999). The observation that 1 ␮M tetrodotoxin (TTX) blocked
those action potentials indicates that the action potentials are
generated at least in part by voltage-sensitive Na⫹ channels
(data not shown).
Low-threshold potassium conductances have been shown to
prevent repetitive firing and to cause rectification in some cells
but not others (Forsythe and Barnes-Davies 1993; Manis and
Marx 1991; Rathouz and Trussell 1998). T stellate cells lie
immediately adjacent to the octopus cell area in the caudal end
of the ventral cochlear nucleus and fire repetitively when they
are depolarized with current with action potentials that have a
single undershoot (Fujino and Oertel 2001; Oertel et al. 1990).
Figure 7 shows that T stellate cells fired action potentials even
when they were depolarized slowly. The greater the injected
current, the more rapidly they reached threshold after the
J Neurophysiol • VOL
FIG. 8. T stellate cells had small or undetectable ␣-dendrotoxin-sensitive
conductances. A: the shape of action potentials in a T stellate cell is affected
little by 100 nM ␣-dendrotoxin. Note that the undershoot of the action potential
has a single component, a characteristic of T stellate cells that distinguishes
them from D stellate cells (Fujino and Oertel 2001). B: the current-voltage
relationship over the hyperpolarizing range of the same T stellate cell was
unaffected by 100 nM ␣-dendrotoxin.
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FIG. 7. In T stellate cells, action potentials are generated independently of
the rate at which the cell reaches a threshold voltage. Responses to rapidly
rising and slowly rising current pulses are superimposed. In the top traces,
current pulses were ramped quickly and slowly to a maximum of 0.2 nA; in the
bottom traces, current pulses were ramped over similar times to 0.4 nA. Ramps
affected the rate, but not the shape or threshold, of action potentials.
The present experiments show that the rate at which cochlear
nuclear octopus cells are depolarized determines whether they
fire action potentials. They fire over a narrow time range when
they are depolarized rapidly and fail to fire when they are
depolarized slowly. The sensitivity to rate of rise results from
the presence of an ␣-dendrotoxin-sensitive, low-threshold potassium conductance, gKL. The gKL also determines the timing
of the peaks of action potentials in that it shunts all but the
earliest regenerative inward current and restricts the timing of
action potentials to near the beginning of a depolarization.
Neighboring T stellate cells lack an ␣-dendrotoxin-sensitive
conductance and fire action potentials in response to depolarizations independently of whether they are rapid or slow.
Cochlear nuclear octopus cells spread their dendrites perpendicularly across the array of auditory nerve fibers from
which they receive their major excitatory input. On average at
least 50 auditory nerve fibers terminate on the dendrites of one
octopus cell with small boutons that are more uniform in size
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M. J. FERRAGAMO AND D. OERTEL
J Neurophysiol • VOL
Shaker) (Grissmer et al. 1994; Hopkins 1998; Hopkins et al.
1994; Owen et al. 1997; Robertson et al. 1996; Stühmer et al.
1989; Tytgat et al. 1995). The ␣ subunits Kv1.1, Kv1.2 (Wang
et al. 1994), and Kv1.4 (Fonseca et al. 1998) have been shown
to be strongly expressed in brain stem auditory nuclei and
especially strongly expressed in the octopus cell area. The
␣-dendrotoxin-sensitive conductance is extraordinarily large in
octopus cells, the maximum conductance being on average 514
nS (Bal and Oertel 2001).
Potassium conductances with low thresholds are widespread in neurons that encode timing in the brain stem
auditory nuclei of vertebrates. Strong, voltage-sensitive currents activated by small depolarizations from rest are observed as a rectification in many of the neurons that encode
timing in the mammalian auditory pathway including bushy
cells of the mammalian ventral cochlear nucleus and its
avian homologue (Isaacson and Walmsley 1995; Manis and
Marx 1991; Oertel 1983; Rathouz and Trussell 1998; Reyes
et al. 1994; Wu and Oertel 1984; Zhang and Trussell 1994),
principal cells of the MNTB (Banks and Smith 1992; Brew
and Forsythe 1995; Forsythe and Barnes-Davies 1993;
Wang et al. 1998; Wu and Kelly 1991), principal cells of the
MSO (Smith 1995), and neurons in the ventral nucleus of
the lateral lemniscus (Wu 1999; reviewed by Oertel 1997,
1999; Trussell 1997, 1999). The low-threshold potassium
current was first identified by its sensitivity to 4AP in
isolated bushy cells (Manis and Marx 1991). In all the
auditory neurons in which it has been studied, the rectification results from the presence of a low-threshold, 4AP- and
dendrotoxin-sensitive potassium current (Bal and Oertel
2001; Banks and Smith 1992; Brew and Forsythe 1995;
Forsythe and Barnes-Davies 1993; Golding et al. 1999;
Isaacson and Walmsley 1995; Manis and Marx 1991;
Rathouz and Trussell 1998). Unlike octopus cells, which are
contacted by large numbers of excitatory inputs and whose
responses to the activation of auditory nerve fibers are
graded, other cells in this group receive fewer inputs (1 to
about 7), many of which are suprathreshold. Only in neurons
in which multiple inputs are subthreshold does the sensitivity to rate of rise increase the sensitivity of a neuron to
synchronicity. Synchronicity has been shown to enhance the
precision in the timing of firing in bushy cells (Joris et al.
1994a,b; Rothman and Young 1996). In the cells with few
suprathreshold inputs, the sharpening of the timing of the
peak of the action potential may be the major functional
consequence of the presence of gKL (Forsythe and BarnesDavies 1993; Rathouz and Trussell 1998; Reyes et al. 1996).
Like in octopus cells, gKL seems to reduce the peak of the
action potential as it sharpens its timing.
Our colleagues have contributed immeasurably to this work with their
willingness to listen, challenge, and share. We thank N. Golding, S. Gardner,
and K. Fujino for helpful suggestions. We are grateful to I. Siggelkow, J.
Meister, and J. A. Ekleberry for making many liters of saline and helping with
numerous bits of tissue.
This work was supported by National Institute on Deafness and Other
Communication Disorders Grant DC-17590.
Present address of M. J. Ferragamo: Biology Dept., Gustavus Adolphus
College, St. Peter, MN 56082.
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than in the adjacent multipolar cell area (Golding et al. 1995;
R. E. Wickesberg and D. Oertel, unpublished results). Synaptic
excitation is through AMPA receptors with large conductance
and fast kinetics (Gardner et al. 1999, 2001). The voltage
changes produced by synaptic currents are shaped by the
biophysical properties of octopus cells. The input resistances
and time constants of octopus cells are so low that the duration
of synaptic potentials (Golding et al. 1995) is similar to the
duration of the synaptic currents (Gardner et al. 1999). EPSPs
are between 1 and 2 ms in duration (Golding et al. 1995). The
low input resistance also makes the amplitude of voltage
changes associated with the activation of individual auditory
nerve fibers small (Golding et al. 1995). Octopus cells respond
to shocks of the auditory nerve with synaptic responses that are
graded as a function of shock strength reflecting the recruitment of variable proportions of large numbers of inputs (Golding et al. 1995). To evoke an action potential requires that
between 1/10th and one-half of a cell’s total inputs be activated
synchronously with a shock (Golding et al. 1995). The more
temporal spread in the arrival of excitation, the slower the rate
of rise of the resulting EPSP and the larger the number of
activated inputs required to bring an octopus cell to threshold
(Cai et al. 1997, 2000; Levy and Kipke 1997).
The present findings from experiments in slices are consistent with what is known about the responses of octopus cells to
sound. In vivo the convergence of many auditory nerve fibers
on octopus cells is reflected in their broad tuning and in their
sensitivity to broadband stimuli (Godfrey et al. 1975; Oertel et
al. 2000; Rhode and Smith 1986; Rhode et al. 1983; Smith et
al. 1993). Octopus cells fire vigorously in response to tones at
moderate and high levels at frequencies less than 1,500 Hz,
frequencies at which phase-locking synchronizes the firing of
auditory nerve fibers (Rhode and Smith 1986). They encode
the fundamental frequency in complex sounds with exceptional
precision (Rhode 1994, 1998). Octopus cells respond to tones
at high frequencies with only a single action potential at the
onset transients of tones (Godfrey et al. 1975; Rhode and Smith
1986; Rhode et al. 1983; Smith et al. 1993). Clicks evoke
action potentials whose temporal jitter is less than 100 ␮s
(Godfrey et al. 1975; Oertel et al. 2000). Octopus cells can
encode periodic stimuli cycle-for-cycle to rates up to 800 Hz,
firing rates that are more than double the maximum firing rates
of their auditory nerve inputs (Oertel et al. 2000; Rhode and
Smith 1986).
The properties of octopus cells contrast with those of T
stellate cells, which are their immediate neighbors and which
are also driven by input from auditory nerve fibers. T stellate
cells are driven by fewer than 1/10th the number of auditory
nerve fiber inputs, between 4 and 6 (Ferragamo et al. 1998).
Correspondingly, T stellate cells are narrowly tuned and respond tonically with “chopping” (Rhode et al. 1983; Smith and
Rhode 1989). The firing properties of T stellate cells can be
modulated by small currents, whereas those in octopus cells
cannot (Fujino and Oertel 2001). The present results show that
octopus cells fire only when they are depolarized quickly,
whereas T stellate cells fire independently of how slowly or
how quickly they are depolarized.
The sensitivity to rate of rise depends strongly on a conductance that is sensitive to ␣-dendrotoxin. Dendrotoxins isolated
from the venom of the Mamba snakes specifically block potassium channels of the Kv1 family (also called KCNA or
OCTOPUS CELLS SENSE RATE OF DEPOLARIZATION
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