Contribution of Outward Currents to Spike-Frequency Adaptation in
Hypoglossal Motoneurons of the Rat
ANDREA SAWCZUK, RANDALL K. POWERS, AND MARC D. BINDER
Department of Physiology and Biophysics, University of Washington, School of Medicine, Seattle, Washington 98195
INTRODUCTION
In response to a constant-current stimulus, motoneurons
display a pronounced reduction in firing rate during the first
second of discharge followed by a more gradual decline in
frequency that continues during tens of seconds or even
several minutes (Granit et al. 1963; Kernell 1965a,b; Kernell
and Monster 1982; reviewed in Binder et al. 1996). The time
course of this spike-frequency adaptation in rat hypoglossal
motoneurons generally has three distinct phases: initial,
early, and late (Sawczuk et al. 1995). In these cells, initial
adaptation is a linear function of time and is complete after
the first few action potentials of sustained repetitive dis2246
charge. The subsequent early and late phases of adaptation
can be described by two exponential curves with mean time
constants of 250 ms and 20 s, respectively (Table 1) (Sawczuk et al. 1995).
The cellular mechanisms underlying spike-frequency adaptation have not been resolved completely, although it is
acknowledged generally that several different conductances
contribute to the process (Table 1) (reviewed in Binder
et al. 1996; Jack et al. 1975; Sawczuk et al. 1995). The
conductances underlying the hyperpolarization that follows
an action potential, the afterhyperpolarization or AHP, have
received the most attention. The AHP generally has more
than one phase, each of which is mediated by a different
type of K / channel (reviewed in Sah 1996) (see, in particular, Fig. 1E). The fast phase ( fAHP; cf. Fig. 3A) occurs at
the end of the repolarization of the action potential, lasts
from 1 to 10 ms, and is mediated primarily by voltage-gated
K / channels. The fAHP is followed by a more prolonged
hyperpolarization, the mAHP, which lasts from 50 to several
hundred milliseconds (cf. Fig. 3A). The mAHP is mediated
by Ca 2/ -activated K / channels. In some cells, a third phase,
the slow AHP (sAHP), occurs with a rise time of several
hundred milliseconds and lasts for several seconds. The
mAHP has been implicated strongly in the initial phase of
adaptation (Baldissera and Gustaffson 1974; Barrett et al.
1980; Kernell and Sjoholm 1973). The increase in mAHP
amplitude and its underlying conductance over the first few
interspike intervals is correlated with the decrease in firing
rate (Baldissera and Gustafsson 1971, 1974; Baldissera et
al. 1978; Barrett et al. 1980; Jack et al. 1975; Kernell and
Sjoholm 1973). This increase in mAHP amplitude would
be expected to reach its limit within the period of initial
adaptation, although long stimulus current pulses ( ú500 ms)
could prolong the duration of the mAHP and presumably
the temporal summation of the underlying conductance beyond this period (Barrett et al. 1980). However, despite the
correlation between the changes in the mAHP and initial
adaptation, there have been conflicting reports on the effects
of blocking the conductances underlying the mAHP on the
early time course (1–2 s) of adaptation (Lorenzon and
Foehring 1992; Nishimura et al. 1989; Pineda et al. 1992;
Staley et al. 1992; Viana et al. 1993).
Several processes with slower time courses have been
proposed as contributors to the later phases of adaptation in
motoneurons and several other types of neurons (Table 1).
These include slow sodium channel inactivation (Basarsky
and French 1991; Fleidervish and Gutnick 1996; Fleidervish
et al. 1996; reviewed in Jack et al. 1975), a slowly activated
potassium conductance (reviewed in Jack et al. 1975), activ-
0022-3077/97 $5.00 Copyright q 1997 The American Physiological Society
/ 9k20$$no19 J-0120-7
10-29-97 14:16:59
neupa
LP-Neurophys
Downloaded from http://jn.physiology.org/ by 10.220.33.5 on June 14, 2017
Sawczuk, Andrea, Randall K. Powers, and Marc D. Binder.
Contribution of outward currents to spike-frequency adaptation in
hypoglossal motoneurons of the rat. J. Neurophysiol. 78: 2246–
2253, 1997. Spike-frequency adaptation has been attributed to the
actions of several different membrane currents. In this study, we
assess the contributions of two of these currents: the net outward
current generated by the electrogenic Na / -K / pump and the outward current that flows through Ca 2/ -activated K / channels. In
recordings made from hypoglossal motoneurons in slices of rat
brain stem, we found that bath application of a 4–20 mM ouabain
solution produced a partial block of Na / -K / pump activity as
evidenced by a marked reduction in the postdischarge hyperpolarization that follows a period of sustained discharge. However, we
observed no significant change in either the initial, early, or late
phases of spike-frequency adaptation in the presence of ouabain.
Adaptation also has been related to increases in the duration and
magnitude of the medium-duration afterhyperpolarization (mAHP)
mediated by Ca 2/ -activated K / channels. When we replaced the 2
mM Ca 2/ in the bathing solution with Mn 2/ , there was a significant
decrease in the amplitude of the mAHP after a spike. The decrease
in mAHP amplitude resulted in a decrease in the magnitude of the
initial phase of spike-frequency adaptation as has been reported
previously by others. However, quite unexpectedly we also found
that reducing the mAHP resulted in a dramatic increase in the
magnitude of both the early and late phases of adaptation. These
changes could be reversed by restoring the normal Ca 2/ concentration in the bath. Our results with ouabain indicate that the Na / K / pump plays little, if any, role in the three phases of adaptation
in rat hypoglossal motoneurons. Our results with Ca 2/ channel
blockade support the hypothesis that initial adaptation is, in part,
controlled by conductances underlying the mAHP. However, our
failure to eliminate initial adaptation completely by blocking Ca 2/
channels suggests that other membrane mechanisms also contribute. Finally, the increase in both the early and late phases of adaptation in the presence of Mn 2/ block of Ca 2/ channels lends further
support to the hypothesis that the initial and later (i.e., early and
late) phases of spike-frequency adaptation are mediated by different cellular mechanisms.
OUTWARD CURRENTS AND SPIKE-FREQUENCY ADAPTATION
TABLE
1.
2247
Spike-frequency adaptation in rat hypoglossal motoneurons
Phase
Relative Magnitude, %
Time Course, s
Possible Mechanisms
Initial adaptation
74 { 27
duration Å 0.038 { 0.011
Early adaptation
13 { 16
t Å 0.24 { 0.37
Late adaptation
14 { 10
t Å 23 { 19
mAHP summation,a fast Na/ channel inactivation,b inward
rectifier (Ih) deactivationc
Persistent Na/ current inactivation,d sAHP,e m-type K/
current,f slow Na/ channel inactivation,g delayed
rectifier inactivation,h N/-K/ ATPase activity,i
saturation of Ca2/ sequestering systemsj
same as Early Adaptation
Data and nomenclature from Sawczuk et al. (1995). Relative magnitude values are means { SD. mAHP, medium-duration after hyperpolarization;
SAHP, slow AHP. Mechanism references: a Baldissera and Gustaffson 1971, Baldissera et al. 1978, Barrett et al. 1980 (cat spinal motoneurons); Kernell
and Sjoholm 1973 (cat spinal motoneurons and neuron model); Baldissera and Gustaffson 1974 (neuron model); b Schwindt and Crill 1982 (cat spinal
motoneurons); c Schwindt et al. 1988a, Spain et al. 1987 (cat cortical neurons); d Nishimura et al. 1989 (guinea pig facial motoneurons); Fleidervish and
Gutnick 1996, Fleidervish et al. 1996 (mouse and guinea pig cortex); e Safronov and Vogel 1996 (neonatal rat spinal motoneurons); f e.g., Marrion 1993
(cultured frog sympathetic neurons); g Basarski and French 1989 (cockroach sensory neuron); Fleidervish and Gutnick 1996, Fleidervish et al. 1996
(mouse and guinea pig cortex); h e.g., Marom and Levitan 1994 (rat brain K channels expressed in Xenopus oocytes); i Kernell and Monster 1982 (cat
spinal motoneurons); French 1989 (cockroach sensory neuron); j Sawczuk et al. 1995 (rat hypoglossal motoneurons).
METHODS
Our preparation of rat brain stem slices and techniques for making intracellular recordings from hypoglossal motoneurons were
identical to those detailed in two recent papers from this laboratory
(Poliakov et al. 1996; Sawczuk et al. 1995). Briefly, the brain
stems were removed from young, adult Sprague-Dawley rats (3–
8 wk old) after anesthesia by an intramuscular injection of a mixture of ketamine and xylazine ( Ç68 and 4 mg/kg, respectively).
Slices (400 mm) were cut and maintained in cooled, oxygenated
artificial sucrose-cerebral spinal fluid (S-ACSF) composed of (in
mM) 220 sucrose, 2.0 KCl, 1.25 NaH2PO3 , 26 NaHCO3 , 2.0
MgCl2 , 2.0 CaCl2 , and 10 glucose. Final incubation and electrical
/ 9k20$$no19 J-0120-7
recordings were done at room temperature in normal, artificial
cerebral spinal fluid (N-ACSF), which differs from S-ACSF by
excluding the sucrose and including 126 mM NaCl. The N-ACSF
was maintained at pH 7.4 and oxygenated by gentle bubbling with
5% CO2-95% O2 (carbogen) throughout the experiment.
Experimental protocols
Intracellular recordings were made from hypoglossal motoneurons with glass micropipettes filled with 3 M KCl (resistances of
30–80 MV ). Motoneuron identity was based on the similarity of its
intrinsic properties to the published results of previous investigators
(Haddad et al. 1990; Mosfeldt-Laursen and Rekling 1989; NunezAbadez et al. 1993; Poliakov et al. 1996; Sawczuk et al. 1995;
Viana et al. 1990, 1993). After impalement of cells with membrane
potentials ú60 mV, we measured rheobase by injecting 50-ms
current pulses, input resistance by injecting 500-ms pulses, and
the relationship between firing frequency and injected current ( fI relation) with a series of 1-s current steps of different magnitude.
We measured the time required for the membrane potential to
return to 1/e of its maximum value after the depolarizing pulses
to estimate the membrane time constant ( tm ) (cf. Mosfeldt-Laursen
and Rekling 1989). Spike-frequency adaptation was measured by
injecting a series of 60-s constant-current steps into the motoneurons. We generally used several different current levels and completed as many as 15 trials in a single cell, including trials before,
during, and after modifications were made in the bathing solution.
After each adaptation trial, rheobase and spike height were remeasured.
Ouabain block of Na / -K / ATPase
To assess the possible contribution of Na / -K / pump activity to
spike-frequency adaptation, N-ACSF containing from 4 to 20 mM
ouabain was introduced into the chamber after recording an initial
series of adaptation trials at several different current strengths.
(Ouabain concentrations ú20 mM generally resulted in broad, short
spikes and eventual cessation of repetitive discharge. In view of
this toxic effect of ouabain, trials only were accepted for analysis
if regular discharge was observed throughout the 60-s trials). At
least 5 min elapsed before retesting the responses of the cell to
another series of 60-s trials. Subsequently, the ouabain solution
was washed-out for ¢10 min, and if the cell’s discharge remained
robust, another series of adaptation trials was performed. The efficacy of the ouabain in blocking the pump was determined off-line
by comparing the postdischarge hyperpolarization (PDH) after 60s trials before, during, and after the ouabain application. The PDH
10-29-97 14:16:59
neupa
LP-Neurophys
Downloaded from http://jn.physiology.org/ by 10.220.33.5 on June 14, 2017
ity of the Na / -K / pump (French 1989; Kernell and Monster
1982; Sokolove and Cook 1971; reviewed in Jack et al.
1975), continued increases in the amplitude of the mAHP
(Hounsgaard et al. 1988), as well as the activation of a
sAHP with an even slower time course (cf. Schwindt et al.
1988b). It is likely that more than one of these hypothesized
mechanisms contribute to the later phases of adaptation, even
within a given cell type. For example, recent evidence suggests that both slow sodium channel inactivation (Fleidervish et al. 1996) and the activation of a slow afterhyperpolarization (Schwindt et al. 1988b) can contribute to the later
phases of adaptation in mammalian neocortical neurons.
The goal of this study was to assess the contributions of
two outward currents that have been proposed as contributors
to spike-frequency adaptation: the net outward current generated by the electrogenic Na / -K / pump and the outward
current that flows through Ca 2/ -activated K / channels. We
found that bath application of ouabain to slices of the rat
brain stem produced a partial block of Na / -K / pump activity but no significant change in any of the three phases of
spike-frequency adaptation. When we blocked Ca 2/ -activated K / channels with Mn 2/ , there was a significant decrease in the amplitude of the mAHP after a spike and a
decrease in the magnitude of the initial phase of spike-frequency adaptation. There was also a dramatic increase in
the magnitude of both the early and late phases of adaptation.
All of these changes could be reversed by restoring the normal Ca 2/ concentration in the bath.
Preliminary accounts of some of these data have been
previously presented (Sawczuk and Binder 1992–1994).
2248
A. SAWCZUK, R. K. POWERS, AND M. D. BINDER
was referred to the cell’s resting membrane potential just before
the onset of each period of repetitive firing.
Manganese block of calcium channels
Data analysis
Our procedures for off-line analysis of motoneuron properties,
including repetitive firing behavior and spike-frequency adaptation,
have been detailed in several recent publications from this laboratory (Poliakov et al. 1996; Powers and Binder 1995; Powers et al.
1993; Sawczuk et al. 1995). Cells were selected for detailed analysis based on the guidelines established by Mosfeldt-Laursen and
Rekling (1989): maintained resting membrane potentials ú 55
mV; input resistances ú 5 MV; and time constants ú 2 ms.
Analysis of spike-frequency adaptation as defined in Sawczuk
et al. (1995) was performed on every long trial of repetitive discharge. The initial linear decline in discharge rate with respect to
time was calculated as the difference between the initial firing rate,
fi , and the transition firing rate, ft (cf. Fig. 1). The subsequent
exponential decline in firing rate was measured as the difference
between ft and the average firing rate during the last second of
discharge ( f f ). First, a single exponential fit was attempted that
had the form
Firing Rate Å ff / KL∗exp( 0t/ tL )
RESULTS
Recordings from 15 rat hypoglossal motoneurons were
accepted for analysis based on their ability to produce
steady, repetitive discharge both before and during alteration of outward currents. All but three of these cells were
part of the database presented in Sawczuk et al. ( 1995 ) .
Unless otherwise stated, all trials followed the protocol
outlined in METHODS .
Effects of ouabain on spike-frequency adaptation
where KL is the magnitude and tL is the time constant of the later
phase of adaptation. When this single exponential fit failed to
account for the entire drop in firing rate, the time course of the
later phases of adaptation ( ft to f f ) was fit with the sum of two
exponential functions by ‘‘peeling away’’ the fit to the first exponential curve and fitting the remainder with a second exponential
function (Sawczuk et al. 1995; Spielmann et al. 1993). The inset
in Fig. 1 illustrates this method and shows that this firing rate-time
plot was best described by a double exponential function. Thus the
entire time course of adaptation for the trial in Fig. 1 consisted of
three phases: an initial linear decline in rate with a slope of 01.3 1
10 3 imp/s 2 ; an early exponential decline ( tE Å 0.2 s); and a late,
slow exponential decline ( tL Å 15.0 s).
Paired t-tests were used to compare the motoneuron responses
to identical 60-s current steps before and during the application of
ouabain (postouabain trials were generally not available). Since
the replacement of Ca 2/ with Mn 2/ changes the f-I relation (e.g.,
Nishimura et al. 1989; Viana et al. 1993) ( RESULTS ), the responses
to a range of current step amplitudes were compared before, during,
and after Mn 2/ using a one-way analysis of variance (ANOVA).
Statistical comparisons between two different conditions (i.e., PreMn 2/ vs. Mn 2/ , Mn 2/ vs. Post-Mn 2/ , etc.) were based on unpaired
t-tests, using the Bonferroni/Dunn correction for multiple comparisons.
/ 9k20$$no19 J-0120-7
FIG . 1. Characteristics of the time course of spike-frequency adaptation
in rat hypoglossal motoneurons. Gray trace shows the instantaneous firing
rate of a cell in response to a 60-s suprathreshold step of injected current.
fi is the instantaneous frequency calculated from the reciprocal of the 1st
interspike interval, ft is the frequency attained at the end of the initial
(linear) phase of spike-frequency adaptation, and f f is the average firing
frequency during the last second of discharge. Black line is a double exponential fit to the time course of firing frequency from ft to f f , obtained by
‘‘peeling’’ off the longest exponential fit and fitting a 2nd exponential curve
to the remainder (cf. METHODS ). Inset: expanded view of the 1st 2 s of
discharge illustrates that a single exponential curve does not fit the early
time course of firing rate immediately after initial adaptation.
Spike-frequency adaptation was studied before, during,
and after bath application of ouabain at concentrations of
4–20 mM. Reduction of the amplitude of the membrane
hyperpolarization after 60 s of discharge (PDH) was the
criterion used to indicate successful block of pump activity
(cf. METHODS ). Seven cells were accepted for analysis based
on ouabain-induced reduction of the PDH ú2 mV. There
was a strong correlation between the percentage of PDH
reduction (measured at 500 ms after current offset) and the
concentration of ouabain ( 04.1% reduction per mM ouabain;
r Å 0.81, P õ 0.05). However, the ouabain never completely
eliminated the PDH as previously reported for other types
of neurons (Parker et al. 1996; Schwindt et al. 1988b, 1989).
The addition of ouabain to the bathing solution had no
significant effect on spike-frequency adaptation during 60
s of repetitive discharge for any of the seven cells. The
mean magnitude of initial adaptation ( fi 0 ft , cf. Fig. 1 )
was nearly identical before and during ouabain application
( 104.9 { 36.8 imp / s and 104.0 { 30.2 imp / s, respectively;
means { SD ) . The later phases of spike-frequency adaptation were similarly unaffected by the ouabain. Figure 2A
demonstrates the similarity of the firing rate-time plots for
10-29-97 14:16:59
neupa
LP-Neurophys
Downloaded from http://jn.physiology.org/ by 10.220.33.5 on June 14, 2017
We substituted 2 mM Mn 2/ for the 2 mM Ca 2/ in the N-ACSF
bath solution to block Ca 2/ -activated K / channels. The NaH2PO3
also was eliminated from the bathing solution to prevent chelation
of the Mn 2/ . The efficacy of the Mn 2/ -block was assessed by
monitoring the reduction in amplitude of the mAHP after action
potentials evoked by short current pulses (cf. Fig. 3A). Multiple
trials were performed in the presence of Mn 2/ , both at current
levels used in the preceding control trials and whenever possible
at current levels that reproduced the initial firing frequencies generated in the control trials. Wash-out of the Mn 2/ was monitored by
observing the restoration of the mAHP amplitude to its control
value. Post-Mn 2/ adaptation trials were performed whenever the
cell remained viable.
OUTWARD CURRENTS AND SPIKE-FREQUENCY ADAPTATION
eight cells were accepted for analysis. The remaining two
cells produced acceptable discharge before and after the
Mn 2/ substitution but would not discharge for more than a
few seconds in the presence of Mn 2/ . Changes in the mAHP
after single action potentials were monitored during Mn2/
wash-in and wash-out and between 60-s trials with Mn 2/
(cf. METHODS ). Analysis of spike-frequency adaptation in
Mn 2/ was limited to trials collected after the amplitude of
the mAHP had been reduced to õ50% of its control value.
Figure 3A shows the loss of the mAHP after a single spike
during Mn 2/ wash-in and its return during Mn 2/ wash-out.
The marked decrease in the mAHP in the presence of Mn 2/
led to a decrease in the initial phase of spike-frequency
adaptation and an increase in the later phases of adaptation.
This latter effect is illustrated in Fig. 3, B–D, which shows
the time course of spike-frequency adaptation at three different current levels before (Fig. 3 B), during (Fig. 3C), and
after (Fig. 3D) perfusion with the Mn 2/ solution. The total
amount of adaptation after the initial phase ( ft 0 f f ) is clearly
larger during Mn 2/ perfusion than it was either before or
after in the normal ACSF. This effect of Mn 2/ was significant over the entire sample of trials (one-way ANOVA, P õ
0.001): the mean magnitudes of the later phases of spikeManganese block of calcium channels
frequency adaptation ( ft 0 f f ) were 9.5 { 4.6 imp/s (n Å
Ten motoneurons were studied before and after the substi- 16) for the pre-Mn 2/ trials, 30.8 { 7.9 imp/s (n Å 13) with
tution of Mn 2/ for Ca 2/ in the bathing solution. Of these, Mn 2/ , and 12.3 { 3.7 imp/s (n Å 8) after Mn 2/ wash-out.
The values obtained during Mn 2/ perfusion were significantly different from those obtained either before or after
Mn 2/ perfusion (P õ0.001), whereas those obtained preand post-Mn 2/ did not differ significantly from one another
(P Å 0.28). In a few cases, we also recorded 60-s trials of
repetitive discharge as the Mn 2/ solution was being washed
in or out, when the mAHP was blocked incompletely. We
found that the increase in the later phases of spike-frequency
adaptation does not appear until the mAHP is reduced
by ¢50%.
At each level of injected current, the substitution of Mn 2/
for Ca 2/ led to an increase in firing rate shortly after the
onset of the current step, as has been reported previously by
others (e.g., Nishimura et al. 1989; Viana et al. 1993). Thus
to study the changes in spike-frequency adaptation produced
by the Mn 2/ at the same firing rate, we generally used a
lower range of injected current levels during Mn 2/ perfusion
than during perfusion in control ACSF. The mean values of
the initial firing rates (i.e., the 1st interspike interval) were
similar in Mn 2/ and control solutions (pre-Mn 2/ : 84.9 {
40.4 imp/s, Mn 2/ : 87.5 { 19.5 imp/s, post-Mn 2/ : 86.7 {
29.6 imp/s), although a wider range of initial firing rates
were observed in control ACSF. Figure 4A shows the relation between the initial firing rate ( fi ) and the extent of
initial adaptation ( fi 0 ft ) before ( s ), during ( ● ), and after
FIG . 2. Effects of ouabain on spike-frequency adaptation and the post( h ) the substitution of Mn 2/ for Ca 2/ . For matched initial
discharge hyperpolarization (PDH). A: ouabain block of the pump does firing rates, there was less initial adaptation in the presence
not significantly affect adaptation during repetitive discharge of hypoglossal of Mn 2/ than in the control solution. In contrast, for matched
motoneurons as these frequency curves demonstrate. Time course of spiketotal amount of adaptation
frequency adaptation before (gray) and after (black) application of ouabain initial interval firing rates, the
2/
and control conditions (Fig.
is nearly identical. B: expanded portions of the membrane potential records ( fi 0 f f ) was similar in Mn
at the onset and offset of the injected current steps that produced the dis- 4B). As a result, the initial phase of adaptation was a smaller
charge records shown in A (spikes have been truncated). During ouabain percentage of the total amount of adaptation in Mn 2/ than
application (black trace) the resting membrane potential was depolarized
in the control conditions (pre-Mn 2/ : 76.2 { 23.3%; Mn 2/ :
slightly relative to the preouabain trial (gray trace) and the hyperpolariza2/
2/
tion after discharge (measured relative to rest, see RESULTS ) was reduced 57.3 { 7.1%; post-Mn : 76.8 { 16.7%; Mn significantly
less than pre-Mn 2/ , P õ 0.01).
by 8 mV (31%) compared with the control trace.
/ 9k20$$no19 J-0120-7
10-29-97 14:16:59
neupa
LP-Neurophys
Downloaded from http://jn.physiology.org/ by 10.220.33.5 on June 14, 2017
a motoneuron in response to the same 60-s current pulse
before and during ouabain application. Figure 2B shows
the reduction of the PDH in this cell with bath application
of ouabain. The 8-mV reduction in the PDH of this cell
was Ç31% of the total PDH magnitude. The magnitude of
spike-frequency adaptation after the initial phase ( ft 0 f f ,
cf. Fig. 1 ) was not significantly affected by the application
of ouabain ( 17.7 { 9.6 imp / s before vs. 17.6 { 8.3 imp /
s during ouabain, paired t Å 0.06, P Å 0.96 ) nor was the
magnitude of the early phase of this adaptation ( 6.6 { 5.4
imp / s before ouabain vs. 5.2 { 3.8 imp / s with ouabain,
paired t Å 0.68, P Å 0.52 ) .
Due to the progressive toxicity of ouabain (see METHODS ),
we could obtain only acceptable postouabain recordings
from three of the seven cells studies. In these cells, the
characteristics of spike-frequency adaptation after ouabain
wash-out were quite similar to those obtained before and
during the ouabain treatment. All of these results indicate
that membrane hyperpolarization due to activation of the
Na / -K / pump does not make a significant contribution to
spike-frequency adaptation in rat hypoglossal motoneurons.
2249
2250
A. SAWCZUK, R. K. POWERS, AND M. D. BINDER
The decrease in initial adaptation during Mn 2/ perfusion
led to a higher firing rate at the beginning of the later phases
of adaptation ( ft ). The increase in the later phases of adaptation might have been simply a consequence of the increase
in ft , because the magnitude of a given phase of adaptation
is correlated positively with the firing rate at the beginning
of that phase (Sawczuk et al. 1995). However, Fig. 4C
illustrates that even for matched values of ft , the subsequent
phases of adaptation were increased in the presence of Mn 2/ .
Despite the dramatic increase in the later phases of adaptation during Mn 2/ perfusion, the time course of adaptation
was similar to that observed in the control ACSF. As described above (see METHODS ), the time course of adaptation
after the initial phase (i.e., from ft to f f ) generally could be
fit by the sum of two exponential functions, which we have
taken to represent early and late phases of adaptation (Sawczuk et al. 1995). Perfusion with Mn 2/ did not significantly
affect either the time constants or the relative magnitudes
of these two phases. The mean time constant of the early
phase of adaptation ( tE ) was 0.28 { 0.09 s (n Å 10) and
that of the late phase ( tL ) was 13.44 { 8.00 s (n Å 11)
during Mn 2/ perfusion [compared with pre-Mn 2/ : tE Å
0.27 { 0.13 (n Å 12) and tL Å 16.44 { 7.83 s (n Å 16),
and post-Mn 2/ : tE Å 0.26 { 0.16 (n Å 5) and tL Å 10.96 {
5.36 s (n Å 8)]. The early phase of adaptation accounted
for 34.0 { 19% of the decrease in firing rate from ft to f f
during Mn 2/ perfusion [compared with pre-Mn 2/ : 40.2 {
14.0% (n Å 16) and post-Mn 2/ : 21.6 { 27.4% (n Å 8)].
DISCUSSION
The intent of this study was to examine the contribution
of two different types of outward currents to the spike-frequency adaptation of rat hypoglossal motoneurons: the net
outward current mediated by activity of the Na / -K / pump
/ 9k20$$no19 J-0120-7
and the outward currents mediated by Ca 2/ -activated K /
channels. We found that bath application of a 4–20 mM
ouabain solution produced a partial block of Na / -K / pump
activity as evidenced by a marked reduction in the PDH
that follows a period of sustained discharge. However, we
observed no significant change in any of the three phases of
spike-frequency adaptation in the presence of ouabain.
The Na / -K / pump, which under physiological conditions
is thought to be activated primarily by a rise in the internal
concentration of Na / , produces a net outward current (cf.
Jack et al. 1975). The increase in internal sodium produced
by repetitive discharge therefore should lead to activation
of the pump, which in turn results in a slow development
of a hyperpolarizing current and a decrease in discharge rate.
Although there is good evidence that activity of the Na / K / pump contributes to a slow phase of spike-frequency
adaptation in invertebrate mechanoreceptors (French 1989;
Sokolove and Cooke 1971), we found that reduction of the
pump activity with ouabain did not produce a significant
change in any of the three phases of adaptation. We do not
believe that this negative result was a consequence of using
ineffective concentrations of ouabain, because we often applied concentrations ú10 mM which have been reported to
inhibit Ç70% of the pump ATPase activity in rat brain
(Sweadner 1979). In addition, we found a dose-dependent
reduction in the amplitude of the PDH; this suggests that
ouabain did inhibit pump activity. The eventual toxicity of
ouabain (see METHODS ) together with the possibility that
pump inhibition alters the concentration gradient for K / ions
(Schwindt et al. 1988b) make the effects of ouabain difficult
to interpret. Nonetheless, our results certainly do not support
a primary contribution of Na / -K / pump activity to spikefrequency adaptation in hypoglossal motoneurons.
Previous studies have indicated that Ca 2/ -activated K /
conductances (GKCa ) may contribute to spike-frequency ad-
10-29-97 14:16:59
neupa
LP-Neurophys
Downloaded from http://jn.physiology.org/ by 10.220.33.5 on June 14, 2017
FIG . 3. A: reduction of the medium-duration afterhyperpolarization (mAHP) with
Mn 2/ replacement of Ca 2/ in the bath solution. mAHP that follows the spike before
Mn 2/ is eliminated almost completely in
the presence of Mn 2/ but returns after
Mn 2/ wash-out with normal Ca 2/ in the
bath. A 5-mV, 5-ms calibration pulse precedes each spike. B–D: plots of the instantaneous firing frequency vs. time obtained
at different levels of injected current before
(B), during (C), and after (D) Mn 2/ block
of Ca 2/ channels demonstrate the dramatic
increase in the later phases of adaptation
that occurs with Mn 2/ .
OUTWARD CURRENTS AND SPIKE-FREQUENCY ADAPTATION
aptation in a variety of ways. The medium-duration hyperpolarization after an action potential (mAHP) is known to
increase in magnitude when successive spikes are evoked at
intervals shorter than the AHP duration (e.g., Baldissera
and Gustaffson 1971; Nishimura et al. 1989). Computer
simulations based on motoneuron models indicate that this
process of AHP temporal summation appears to contribute
to the initial phase of spike-frequency adaptation (e.g., Baldissera and Gustaffson 1974; Kernell and Sjoholm 1973;
Powers 1993). Temporal summation of the AHP across successive interspike intervals presumably reflects the fact that
the calcium concentration in the vicinity of the GKCa channels
does not return to its resting level by the end of the interspike
interval. Slow increases in the calcium concentration near
/ 9k20$$no19 J-0120-7
the GKCa channels would prolong the increase in GKCa beyond
the period of initial adaptation. Such increases in calcium
concentration might reflect the saturation of intracellular calcium sequestering systems (Barrett et al. 1980) or release of
calcium from intracellular stores triggered either by second
messengers (Zhang 1989) or by calcium itself (Sah and
McLachlan 1991). Finally, another GKCa conductance that is
pharmacologically distinct from that underlying the mAHP
recently has been described in mammalian neocortical and
hippocampal neurons (Madison and Nicoll 1984; Schwindt
et al. 1988b; Storm 1990). This GKCa conductance contributes both to a long-lasting AHP (sAHP) after repetitive
discharge and to slow spike-frequency adaptation in these
cells.
We found that replacement of Ca 2/ with Mn 2/ eliminated
the mAHP after single spikes and also reduced the magnitude
of the initial phase of spike-frequency adaptation. These
results support the hypothesis that initial adaptation is, in
part, controlled by conductances underlying the mAHP.
However, our failure to eliminate completely initial adaptation by blocking the mAHP suggests that other membrane
mechanisms also contribute. One possibility is that the depolarization associated with the onset of repetitive discharge
leads to the deactivation of an inward rectifier current (Ih ),
that contributes to the early decline in firing rate (Schwindt
et al. 1988a; Spain et al. 1987). Although such currents have
been shown to exist in rat hypoglossal motoneurons (Bayliss
et al. 1994), there has been no direct demonstration of their
contribution to initial adaptation. Rapid changes in the inactivation of the voltage-gated sodium channels responsible for
spike initiation also might contribute to initial adaptation.
The onset of repetitive discharge in cat spinal motoneurons
is associated with an increase in the voltage threshold for
spike initiation, presumably reflecting partial inactivation of
initial segment sodium channels (Schwindt and Crill 1982).
Our results also indicate that outward currents carried by
GKCa channels are not essential for the later phases of spikefrequency adaptation, lending further support to the hypothesis that the initial and later phases of spike-frequency adaptation are mediated by different cellular mechanisms (Sawczuk et al. 1995). The continued presence of the later phases
of adaptation after Ca 2/ replacement with Mn 2/ could be
the result of an incomplete Ca 2/ block that allowed some
residual tissue-bound Ca 2/ to enter during prolonged repetitive discharge. However, the persistently reduced mAHP
after single directly evoked spikes suggests that Ca 2/ channels are blocked adequately. The relationship between the
reduction of the mAHP and the dramatic increase in the later
phases of adaptation also suggests that our results are not a
consequence of tissue-bound Ca 2/ .
Alternative mechanisms for the later phases of adaptation
The later phases of adaptation might be produced by a
progressive increase in outward currents, a progressive decrease in inward currents, or some combination of these two
mechanisms (Table 1). Possible outward currents include a
M current (Adams et al. 1986; Marrion 1993; Storm 1990)
and a Na / -activated potassium current (Schwindt et al.
1989). Although it is not yet known if significant M currents
are present in rat hypoglossal motoneurons, Na / -activated
10-29-97 14:16:59
neupa
LP-Neurophys
Downloaded from http://jn.physiology.org/ by 10.220.33.5 on June 14, 2017
FIG . 4. Relationship between different parameters of spike-frequency
adaptation before ( s ), during ( ● ), and after ( h ) replacement of Ca 2/ with
Mn 2/ . A: relationship between the initial firing rate ( fi ) and the magnitude
of initial adaptation ( fi 0 ft ). B: relationship between the initial firing rate
( fi ) and the total magnitude of spike-frequency adaptation ( fi 0 f f ). C:
relation between the frequency at the end of initial adaptation ( ft ) and the
magnitude of adaptation after the initial phase ( ft 0 f f ).
2251
2252
A. SAWCZUK, R. K. POWERS, AND M. D. BINDER
/ 9k20$$no19 J-0120-7
We thank L. Whitely for technical assistance.
This work was supported by National Institutes of Health Grants DE00161, NS-01650, and NS-26840.
Present address of A. Sawczuk: Dept. of Oral Pathology, Biology, and
Diagnostic Sciences, University of Medicine and Dentistry of New Jersey,
110 Bergen St., Newark, NJ 07103-2400.
Address reprint requests to M. D. Binder.
Received 10 February 1997; accepted in final form 2 July 1997.
REFERENCES
ADAMS, P. R., JONES, S. W., PENNEFATHER, P., BROWN, D. A., KOCH, C.,
AND LANCASTER, B. Slow synaptic transmission in frog sympathetic ganglia. J. Exp. Biol. 124: 259–285, 1986.
BALDISSERA, F. AND GUSTAFSSON, B. Regulation of repetitive firing in
motoneurones by the afterhyperpolarization conductance. Brain Res. 30:
431–434, 1971.
BALDISSERA, F., AND GUSTAFSSON, B. Firing behaviour of a neuron model
based on the afterhyperpolarization conductance time-course and algebraical summation. Adaptation and steady state firing. Acta Physiol. Scand.
92: 27–47, 1974.
BALDISSERA, F., GUSTAFSSON, B., AND PARMIGGIANI, F. Saturating summation of the afterhyperpolarization conductance in spinal motoneurones: a
mechanism for ‘‘secondary range’’ repetitive firing. Brain Res. 146: 69–
82, 1978.
BARRETT, E. F., BARRETT , J. N., AND CRILL, W. E. Voltage-sensitive outward currents in cat motoneurones. J. Physiol. (Lond.) 104: 251–276,
1980.
BASARSKY, T. A. AND FRENCH, A. S. Intracellular measurements from a
rapidly adapting sensory neuron. J. Neurophysiol. 65: 49–56, 1991.
BAYLISS, D. A., VIANA, F., BELLINGHAM, C., AND BERGER, A. J. Characteristics and postnatal development of a hyperpolarization-activated inward
current in rat hypoglossal motoneurons in vitro. J. Neurophysiol. 71:
119–128, 1994.
BINDER, M. D., HECKMAN, C. J., AND POWERS, R. K. The physiological
control of motoneuron activity. In: Handbook of Physiology. Exercise.
Regulation and Integration of Multiple Systems. New York: Am. Physiol.
Soc., 1996, sect. 12, vol. I, p. 3–53.
BRISMAR, T. Slow mechanism for sodium permeability inactivation in myelinated nerve fibre of Xenopus laevis. J. Physiol. (Lond.) 270: 283–297,
1977.
FEATHERSTONE, D. E., RICHMOND, J. E., AND RUBEN, P. C. Interaction between fast and slow inactivation in Skm1 sodium channels. Biophys. J.
71: 3098–3109, 1996.
FLEIDERVISH, I. A., FRIEDMAN, A., AND GUTNICK, M. J. Slow inactivation
of Na/ current and slow cumulative spike adaptation in mouse and
guinea-pig neocortical neurones in slices. J. Physiol. (Lond.) 493: 83–
97, 1996.
FLEIDERVISH, I. A. AND GUTNICK, M. J. Kinetics of slow inactivation of
persistent sodium current in layer V neurons of mouse neocortical slices.
J. Neurophysiol. 76: 2125–2130, 1996.
FOX, J. M. Ultra-slow inactivation of the ionic currents through the membrane of myelinated nerve. Biochim. Biophys. Acta 426: 232–244, 1976.
FRENCH, A. S. Ouabain selectively affects the slow component of sensory
adaptation in an insect mechanoreceptor. Brain Res. 504: 112–114, 1989.
GRANIT, R., KERNELL, D., AND SHORTESS, G. K. Quantitative aspects of
repetitive firing of mammalian motoneurones, caused by injected currents. J. Physiol. (Lond.) 168: 911–931, 1963.
HADDAD, G. G., DONNELLY, D. F., AND GETTING, P. A. Biophysical properties of hypoglossal neurons in vitro: intracellular studies in adult and
neonatal rats. J. Appl. Physiol. 69: 1509–1517, 1990.
HOUNSGAARD, J., KIEHN, O., AND MINTZ, I. Response properties of motoneurones in a slice preparation of the turtle spinal cord. J. Physiol. (Lond.)
398: 575–589, 1988.
HOWE, J. R. AND RITCHIE, J. M. Multiple kinetic components of sodium
channel inactivation in rabbit Schwann cells. J. Physiol. (Lond.) 455:
529–566, 1992.
JACK, J.J.B., NOBLE, D., AND TSIEN, R. W. Electric Current Flow in Excitable Cells. Oxford, UK: Clarendon, 1975.
KERNELL, D. The adaptation and the relation between discharge frequency
and current strength of cat lumbosacral motoneurones stimulated by longlasting injected currents. Acta Physiol. Scand. 65: 65–73, 1965a.
KERNELL, D. High-frequency repetitive firing of cat lumbosacral motoneu-
10-29-97 14:16:59
neupa
LP-Neurophys
Downloaded from http://jn.physiology.org/ by 10.220.33.5 on June 14, 2017
potassium channels have been discovered recently in neonatal rat spinal motoneurons, and their activation during repetitive discharge produces a sAHP in these cells (Safronov and
Vogel 1996). However, it is not clear that an sAHP is a
prominent feature of mature motoneurons (Nishimura et al.
1989; Vianna et al. 1993; J. R. Musick, R. K. Powers, and
M. D. Binder, unpublished).
The later phases of spike-frequency adaptation also might
be mediated by a slow inactivation of inward currents (Fleidervish and Gutnick 1996; Fleidervish et al. 1996). Slow
inactivation processes in initial segment and somatic sodium
channels would be expected to lead to a progressive increase
in firing threshold along with changes in spike shape (cf.
Schwindt and Crill 1982). Although the firing threshold
cannot be accurately measured in current-clamp recordings,
late adaptation is associated with profound changes in spike
shape (Lindsay et al. 1986; Sawczuk 1993; Sawczuk and
Binder 1993). In addition, a persistent sodium current has
been described in a number of different types of neurons
(e.g., Stafstrom et al. 1985), including rodent motoneurons
(Mosfeldt-Laursen and Rekling 1989; Nishimura et al.
1989), and it undergoes slow inactivation (Fleidervish and
Gutnick 1996; Fleidervish et al. 1996). A slow decrease in
the magnitude of this persistent inward current also would
contribute to spike-frequency adaptation.
The process of slow inactivation is a common feature of
sodium channels in a variety of excitable tissues (e.g.,
Brismar 1977; Fox 1976; Featherstone et al. 1996; Howe
and Ritchie 1992; Simoncini and Stuhmer 1987), and the
increase in early and late adaptation that we observed after
the replacement of Ca 2/ with Mn 2/ are certainly consistent
with a contribution of slow sodium inactivation. The reduction of the mAHP should lead to an increase in the mean
level of membrane depolarization during the interspike interval, which in turn would be expected to accelerate the development of sodium inactivation. Between-cell variations in
the kinetics and steady state voltage dependence of slow
inactivation may explain why some rat hypoglossal motoneurons were incapable of prolonged discharge after Ca 2/
replacement, as also has been reported for turtle spinal motoneurons (Hounsgaard et al. 1988).
There may be additional mechanisms contributing to the
early and late phases of adaptation in the presence of normal
levels of extracellular Ca 2/ . Spike-frequency adaptation in
rat hypoglossal motoneurons is associated with an increase
in spike duration as well as a decrease in the maximum rate
of spike repolarization (Sawczuk 1993; unpublished results), that might reflect slow inactivation of the potassium
channels contributing to spike repolarization (e.g., Marom
and Levitan 1994). The increase in spike duration could
lead to an increase in Ca 2/ influx and an associated increase
in the mAHP.
It is clear that there are a number of potential mechanisms
that might contribute to each of the different temporal phases
of spike-frequency adaptation (Table 1). Further, it is likely
that different mechanisms may be operating in different
types of neurons. Unravelling the relative contributions of
these processes to adaptation will probably require a combination of voltage-clamp and computer simulation approaches
(e.g., Fleidervish et al. 1996).
OUTWARD CURRENTS AND SPIKE-FREQUENCY ADAPTATION
/ 9k20$$no19 J-0120-7
SAWCZUK, A. AND BINDER, M. D. Reduction of the AHP with Mn//
decreases the initial adaptation, but increases the late adaptation in rat
hypoglossal motoneuron discharge (Abstract). Physiologist 36: A-23,
1993.
SAWCZUK, A. AND BINDER, M. D. Different mechanisms produce initial and
late adaptation during motoneuron discharge (Abstract). J. Dent. Res.
73: 166, 1994.
SAWCZUK, A., POWERS, R. K., AND BINDER, M. D. Spike-frequency adaptation studied in hypoglossal motoneurons of the rat. J. Neurophysiol. 73:
1799–1810, 1995.
SCHWINDT, P. C. AND CRILL, W. E. Factors influencing motoneuron rhythmic firing: results from a voltage-clamp study. J. Neurophysiol. 48: 875–
890, 1982.
SCHWINDT, P. C., SPAIN, W. J., AND CRILL, W. E. Influence of anomalous
rectifier activation on afterhyperpolarizations of neurons from the cat
sensorimotor cortex in vitro. J. Neurophysiol. 59: 468–481, 1988a.
SCHWINDT, P. C., SPAIN, W. J., AND CRILL, W. E. Long-lasting reduction
of excitability by a sodium-dependent potassium current in cat neocortical
neurons. J. Neurophysiol. 61: 233–244, 1989.
SCHWINDT, P. C., SPAIN, W. J., FOEHRING, R. C., CHUBB, M. C., AND CRILL,
W. E. Slow conductances in neurons from cat sensorimotor cortex in
vitro and their role in slow excitability changes. J. Neurophysiol. 59:
450–67, 1988b.
SIMONCINI, L., AND STUHMER, W. Slow sodium channel inactivation in rat
fast-twitch muscle. J. Physiol. Lond. 383: 327–37, 1987.
SOKOLOVE, P. G. AND COOKE, I. M. Inhibition of impulse activity in a
sensory neuron by an electrogenic pump. J. Gen. Physiol. 57: 125–163,
1971.
SPAIN, W. J., SCHWINDT, P. C., AND CRILL, W. E. Anomalous rectification
in neurons from cat sensorimotor cortex in vitro. J Neurophysiol. 57:
1555–76, 1987.
SPAIN, W. J., SCHWINDT, P. C., AND CRILL, W. E. Post-inhibitory excitation
and inhibition in layer V pyramidal neurones from the cat sensorimotor
cortex. J. Physiol. (Lond.) 434: 609–626, 1991.
SPIELMANN, J. M., LAOURIS, Y., NORDSTROM, M. A., ROBINSON, G. A.,
REINKING, R. M., AND STUART, D. G. Adaptation of cat motoneurons to
sustained and intermittent extracellular activation. J. Physiol. (Lond.)
464: 75–120, 1993.
STAFSTROM, C. E., SCHWINDT, P. C., CHUBB, N. C., AND CRILL, W. E. Properties of persistent sodium conductance and calcium conductance of layer
V neurons from cat sensorimotor cortex in vitro. J. Neurophysiol. 53:
153–170, 1985.
STALEY, K. J., OTIS, T. S., AND MODY, I. Membrane properties of dentate
gyrus granule cells: Comparison of sharp microelectrode and whole-cell
recordings. J. Neurophysiol. 67: 1346–1358, 1992.
STORM, J. F. Potassium currents in hippocampal pyramidal cells. In: Progress in Brain Research. Understanding the Brain Through the Hippocampus, edited by J. Storm-Mathisen, J. Zimmer, and O. P. Ottersen. Amsterdam: Elsevier, 1990, vol. 83, p. 161–187.
SWEADNER, K. J. Two molecular forms of (Na / / K / )-stimulated ATPase
in brain. J. Biol. Chem. 254: 6060–6067, 1979.
VIANA, F., BAYLISS, D. A., AND BERGER, A. J. Multiple potassium conductances and their role in action potential repolarization and repetitive firing
behavior of neonatal rat hypoglossal motoneurons. J. Neurophysiol. 69:
2150–2163, 1993.
VIANA, F., GIBBS, L., AND BERGER, A. J. Double-and triple-labeling of
functionally characterized central neurons projecting to peripheral targets
studied in vitro. Neuroscience 38: 829–841, 1990.
ZHANG, L. Effects of inositol 1,4,5-triphosphate injections into cat spinal
motoneurons. Can. J. Physiol. Pharmacol. 68: 1062–1068, 1989.
10-29-97 14:16:59
neupa
LP-Neurophys
Downloaded from http://jn.physiology.org/ by 10.220.33.5 on June 14, 2017
rones stimulated by long-lasting injected currents. Acta Physiol. Scand.
65: 74–86, 1965b.
KERNELL, E. AND MONSTER, A. W. Time course and properties of late
adaptation in spinal motoneurones of the cat. Exp. Brain Res. 46: 191–
196, 1982.
KERNELL, D. AND SJOHOLM, H. Repetitive impulse firing: comparisons
between neurone models based on ‘‘voltage clamp equations’’ and spinal
motoneurones. Acta Physiol. Scand. 87: 40–56, 1973.
LINDSAY, A. D., HECKMAN, C. J., AND BINDER, M. D. Analysis of late adaptation in cat motoneurons. Soc. Neurosci. Abstr. 12: 247, 1986.
LORENZON, N. M. AND FOEHRING, R. C. Relationship between repetitive
firing and afterhyperpolarizations in human neocortical neurons. J. Neurophysiol. 67: 350–363, 1992.
MADISON, D. V. AND NICOLL, R. A. Control of the repetitive discharge of
rat CA 1 pyramidal neurones in vitro. J. Physiol. (Lond.) 354: 319–331,
1984.
MAROM, S. AND LEVITAN, I. B. State-dependent inactivation of the Kv3
potassium channel. Biophys. J. 67: 579–589, 1994.
MARRION, N. V. Selective reduction of one mode of M-channel gating by
muscarine in sympathetic neurons. Neuron 11: 77–84, 1993.
MOSFELDT-LAURSEN, A. AND REKLING, J. C. Electrophysiological properties
of hypoglossal motoneurons of guinea-pigs studied in vitro. Neuroscience
30: 619–637, 1989.
NISHIMURA, Y., SCHWINDT, P. C., AND CRILL, W. E. Electrical properties
of facial motoneurons in brainstem slices from guinea pig. Brain Res.
502: 127–142, 1989.
NUNEZ-ABADES, P. A., SPIELMAN, J., BARRIONUEVO, G., AND CAMERON,
W. E. In vitro electrophysiology of developing genioglossal motoneurons
in the rat. J. Neurophysiol. 70: 1401–1411, 1993.
PARKER, D., HILL, R., AND GRILLNER, S. Electrogenic pump and a Ca//dependent K/ conductance contribute to a posttetanic hyperpolarization
in lamprey sensory neurons. J. Neurophysiol. 76: 540–553, 1996.
PINEDA, J. C., GALARRAGA, E., BARGAS, J., CRISTANCHO, M., AND ACEVES,
J. Charybdotoxin and apamin sensitivity of the calcium-dependent repolarization and the afterhyperpolarization in neostriatal neurons. J. Neurophysiol. 68: 287–294, 1992.
POLIAKOV, A. V., POWERS, R. K., SAWCZUK, A., AND BINDER, M. D. Effects
of background noise on the response of motoneurons to excitatory current
transients. J. Physiol. (Lond.) 495: 143–157, 1996.
POWERS, R. K. A variable-threshold motoneurone model that incorporates
time- and voltage-dependent potassium and calcium conductances. J.
Neurophysiol. 70: 246–262, 1993.
POWERS, R. K. AND BINDER, M. D. Effective synaptic current and motoneuron firing rate modulation. J. Neurophysiol. 74: 793–810: 1995.
POWERS, R. K., ROBINSON, F. R., KONODI, M. A., AND BINDER, M. D. Distribution of rubrospinal synaptic input to cat triceps surae motoneurons. J.
Neurophysiol. 70: 1460–1468: 1993.
SAFRONOV, B. V. AND VOGEL, W. Properties and functions of Na/-activated
K/ channels in the soma of rat motoneurones. J. Physiol. (Lond.) 497:
727–734, 1996.
SAH, P. Ca 2/ -activated K/ currents in neurones: types, physiological roles
and modulation. Trends Neurosci. 19: 150–154, 1996.
SAH, P. AND MC LACHLAN, E. M. Ca//-activated K/ currents underlying
the afterhyperpolarization in guinea pig vagal neurons: a role for Ca//activated Ca// release. Neuron 7: 257–264, 1991.
SAWCZUK, A. Adaptation in Sustained Motoneuron Discharge (PhD dissertation). Seattle, WA: University of Washington, 1993.
SAWCZUK, A. AND BINDER, M. D. Reduction of Na/-K/ ATPase activity
does not reduce the late adaptation of motoneuron discharge. Soc. Neurosci. Abstr. 18: 512, 1992.
2253