Maryland Psychiatric Research Center and the Department of Psychiatry, University of Maryland School of Medicine, Baltimore, Maryland 21228
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ABSTRACT |
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Ping, Han Xian and
Paul D. Shepard.
Blockade of SK-type Ca2+-activated K+ channels
uncovers a Ca2+-dependent slow afterdepolarization in
nigral dopamine neurons. Sharp electrode current-clamp
recording techniques were used to characterize the response of nigral
dopamine (DA)-containing neurons in rat brain slices to injected
current pulses applied in the presence of TTX (2 µM) and under
conditions in which apamin-sensitive Ca2+-activated
K+ channels were blocked. Addition of apamin (100-300 nM)
to perfusion solutions containing TTX blocked the pacemaker oscillation
in membrane voltage evoked by depolarizing current pulses and revealed an afterdepolarization (ADP) that appeared as a shoulder on the falling
phase of the voltage response. ADP were preceded by a ramp-shaped slow
depolarization and followed by an apamin-insensitive hyperpolarizing
afterpotential (HAP). Although ADPs were observed in all apamin-treated
cells, the duration of the response varied considerably between
individual neurons and was strongly potentiated by the addition of TEA
(2-3 mM). In the presence of TTX, TEA, and apamin, optimal stimulus
parameters (0.1 nA, 200-ms duration at 55 to
68 mV) evoked ADP
ranging from 80 to 1,020 ms in duration (355.3 ± 56.5 ms,
n = 16). Both the ramp-shaped slow depolarization and
the ensuing ADP were markedly voltage dependent but appeared to be
mediated by separate conductance mechanisms. Thus, although bath
application of nifedipine (10-30 µM) or low Ca2+, high
Mg2+ Ringer blocked the ADP without affecting the ramp
potential, equimolar substitution of Co2+ for
Ca2+ blocked both components of the voltage response.
Nominal Ca2+ Ringer containing Co2+ also
blocked the HAP evoked between
55 and
68 mV. We conclude that the
ADP elicited in DA neurons after blockade of apamin-sensitive Ca2+-activated K+ channels is mediated by a
voltage-dependent, L-type Ca2+ channel and represents a
transient form of the regenerative plateau oscillation in membrane
potential previously shown to underlie apamin-induced bursting
activity. These data provide further support for the notion that
modulation of apamin-sensitive Ca2+-activated
K+ channels in DA neurons exerts a permissive effect on the
conductances that are involved in the expression of phasic activity.
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INTRODUCTION |
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Mesencephalic dopamine (DA)-containing neurons in
vivo exhibit a variety of activity patterns ranging from a tonic single spike to a multiple spike bursting discharge (Bunney et al.
1973; Grace and Bunney 1984a
,b
; Shepard
and German 1988
; Wilson et al. 1977
). During the
past decade, converging lines of evidence suggested that alterations in
DA neuronal firing pattern are of considerable physiological importance
(reviewed by Overton and Clark 1997
). Increases in the
incidence and intensity of bursting activity are observed in a variety
of species in response to auditory and visual stimuli (Freeman
et al. 1985
; Horvitz et al. 1997
;
Mirenowicz and Schultz 1996
). In the primate, transient
increases in bursting activity are linked to novel appetitive stimuli
whose rewarding properties are unexpected, suggesting that changes in
the temporal organization of DA neuronal spike trains may encode errors
in reward prediction (Schultz et al. 1997
).
Functionally, bursting activity is associated with enhanced DA release
in terminal field areas (Chergui et al. 1994
;
Garris et al. 1994
; Gonon 1988
;
Gonon and Buda 1985
; Manley et al. 1992
;
Nissbrandt et al. 1994
) and a regionally selective
increase in c-fos expression (Chergui et al. 1996
). It
was also recently shown that DA released during bursts of action
potentials exerts a delayed excitatory effect on striatal neurons via
activation of extrasynaptic D1 receptors (Gonon 1997
).
Thus, by transiently increasing release, bursting activity could
trigger a change in the nature of DA neurotransmission from a
conventional synaptic to a paracrine modality (Fuxe and Agnati
1991
). Taken together, these data indicate that changes in the
temporal organization of DA neuronal spike trains represent a mechanism
through which these neurons alter their influence on target cells in
the forebrain.
The mechanisms responsible for generation of bursting activity in DA
neurons are incompletely understood but are likely to involve both
extrinsic and intrinsic components. Activation of afferent projections
are undoubtedly involved in mediating the transient increases in
bursting activity evoked by sensory stimuli. Unpatterned synaptic input
may also contribute to the spontaneous firing patterns exhibited by
these cells because DA neurons recorded in coronal brain slices, a
preparation devoid of medium- and long-length afferents, exhibit a
homogenous pacemaker-like firing pattern that is not observed in the
intact animal (Grace and Onn 1989; Kita et al.
1986
; Sanghera et al. 1984
; Shepard and
Bunney 1988
). On the other hand, the ability of DA neurons to
exhibit spontaneous spiking in the absence of synaptic input indicates
that intrinsic mechanisms also contribute to the firing properties
exhibited by these cells. Although DA neurons possess a variety of
voltage- and ligand-gated conductances (Silva et al.
1990
), their respective contributions to the
electrophysiological properties exhibited by these neurons are for the
most part incompletely understood. One notable exception is the small
conductance Ca2+-activated K+ channel
(gKCa2+), which is potently antagonized by the
neurotoxin apamin (Hugues et al. 1982
;
Köhler et al. 1996
; Sah 1996
).
Blockade of gKCa2+ inhibits the postspike afterhyperpolarization (AHP) and changes the firing pattern of DA cells
in brain slices from a pacemaker-like discharge to an irregular single
spike or a multiple spike bursting pattern (Gu et al.
1992
; Nedergaard et al. 1993
; Shepard and
Bunney 1988
, 1991
). Apamin-induced bursting activity appears to
result from a modification in the cell's autogenous pacemaker
mechanisms. Thus, in contrast to the sinusoidal oscillation in membrane
voltage characteristic of pacemaking DA neurons (Fujimura and
Matsuda 1989
; Harris et al. 1989
; Kang
and Kitai 1993a
; Yung et al. 1991
), bursting
activity, elicited after blockade of gKCa2+, is
driven by a plateau-like oscillation in membrane potential (Ping
and Shepard 1996
). Although spontaneous plateau oscillations are not observed among nonbursting DA neurons, they can be evoked by
intracellular injection of hyperpolarizing bias currents. These data
suggest that all DA neurons possess the capacity to generate plateau
potentials and imply that conditions favoring their development (i.e.,
inhibition of gKCa2+) may be associated with a
change in the electroresponsive properties of the neuron, as was
demonstrated for other cells exhibiting bistable membrane characteristics (Hsiao et al. 1998
; Russo and
Hounsgaard 1996
). To test this hypothesis, current-clamp
recording techniques were used to characterize the response of DA
neurons to injected current pulses applied after blockade of fast
Na+ channels and gKCa2+. The
results show that apamin-induced blockade of
gKCa2+ results in the development of an
afterdepolarization (ADP) that appears to be mediated by the same
conductance mechanisms underlying bursting plateau potentials.
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METHODS |
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Brain slices were prepared from male Sprague-Dawley rats
(Charles River, Raleigh, NC) as previously described (Shepard
and Bunney 1991; Wu et al. 1994
). Tissue
harvesting procedures were conducted in accordance with the Guide
for the Care and Use of Laboratory Animals (NIH Publication No.
86-23) and policies of the Institutional Animal Care and Use Committee
of the University of Maryland School of Medicine. Rats weighing between
120 and 170 g were anesthetized with chloral hydrate (400 mg/kg
ip) and decapitated. The brain was rapidly removed and placed in an
ice-cold Ringer solution (pH 7.4) consisting of (in mM) 125 NaCl,
4.0 KCl, 1.25 NaH2PO4, 1.2 MgSO4,
26 NaHCO3, 2.5 CaCl2, and 11 glucose. The
ventral midbrain was dissected from the surrounding tissue and placed
on the stage of a manual tissue chopper for slicing. Two coronal slices
(300-400 µm thick) containing the substantia nigra (SNc) were
immediately transferred to an interface recording chamber and perfused
with normal Ringer at a rate of 2-3 ml/min. Bath temperature was
maintained between 35 and 36°C. Tissue slices were incubated for
1
h before the start of the recording studies.
Intracellular recordings were made with sharp electrodes prepared from
borosilicate tubing (1.0 mm OD, WPI, Sarasota, FL) with a horizontal
electrode puller (Sutter Instruments, Novato, CA) and filled with 3 M
KCl (DC resistance: 40-80 M). Electrodes were positioned within the
zona compacta of the substantia nigra (SNc) with the aid of a
dissecting microscope and advanced vertically with a piezoelectric
microdrive (Burleigh Instruments, Fishers, NY). Immediately after
impalement, cells were temporarily hyperpolarized to facilitate sealing
of the recording electrode. Electrode potentials were amplified with an
Axoclamp 1A amplifier (Axon Instruments, Foster City, CA). Timed
current pulses were generated with a digital stimulator (NeuroData, New
York, NY) and applied to the cell through the balanced bridge circuit
of the amplifier. Neurons were identified as dopaminergic on the basis
of their well-characterized active and passive electrical properties,
including voltage-dependent, slow depolarization preceding spike
initiation (Grace and Onn 1989
), apamin-sensitive,
postspike AHP (Shepard and Bunney 1991
), and pronounced
time-dependent anomalous rectification (Mercuri et al.
1995
).
Membrane voltage and current monitor outputs were observed continuously
with a digital oscilloscope. Analog data were digitized in real-time
with a pulse code modulator (NeuroData, New York, NY) and stored on
videotape. Recorded data were analyzed with the pCLAMP data acquisition
package (Axon Instruments, Foster City, CA). Input resistance was
estimated by measuring the change in membrane voltage produced by
rectangular current pulses of sufficient duration (>250 ms) to fully
charge membrane capacitance. Cells were hyperpolarized slightly (65
to
68 mV) to supresses spontaneous spiking during the measurements.
The following drugs were added to normal Ringer solution in the
concentrations indicated: TTX (2 µM), tetraethylammonium chloride (TEA; 2-3 mM), apamin (100-300 nM), nifedipine (10-30 µM; RBI, Natick, MA), and -conotoxin GVIA (1.7-5 µM; Alomone, Jerusalem, Israel). Low Ca2+, high Mg2+ Ringer was
prepared by reducing the Ca2+ concentration from 2.5 to 0.3 mM and increasing the Mg2+ concentration from 1.2 to 3.4 mM
(Kang and Kitai 1993a
). Co2+-containing
Ringer was prepared by equimolar substitution of CoCl2 for
CaCl2. Cesium chloride (5-10 mM) and nickel chloride (100 µM) were added directly to normal Ringer. All drugs were obtained from Sigma (St. Louis, MO) unless otherwise indicated. Compounds were
dissolved in Ringer solution and applied directly to the chamber with a
four-channel perfusion system (Adams and List, Westbury, NY). All data
are expressed as means ± SE.
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RESULTS |
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Stable intracellular recordings were obtained from a total of 101 neurons within the SNc. All cells included in this study exhibited
electrophysiological characteristics previously ascribed to
neurochemically identified DA neurons (Grace and Onn
1989; Yung et al. 1991
) and are therefore
referred to as such. Mean values for membrane resistance (154 ± 7 M
), spike threshold (
38.4 ± 0.5 mV), duration (1.75 ± 0.04 ms), and amplitude, measured from the peak of the spike to the
peak of the AHP (82.5 ± 1.3 mV), were obtained from a
representative sample of 30 DA neurons. Eighty-four percent (85/101) of
the neurons recorded exhibited spontaneous action potentials after
recovery from impalement, whereas the remaining 16 cells were silent at
rest (
42.5 ± 1.2 mV).
Intracellular injection of depolarizing current pulses (200- to 800-ms
duration) applied from membrane potentials between 60 and
68 mV
elicited repetitive spiking in all cells tested (n = 17). Individual action potentials were preceded by a ramp-shaped slow
depolarization and followed by a postspike AHP (Fig.
1A). Addition of TTX (2 µM)
blocked current evoked spiking (Fig. 1B) and occasionally
led to the appearance of an oscillation in membrane potential that was
potentiated by addition of TEA (2-3 mM; n = 25, Fig.
1C). Addition of apamin (100-300 nM) to Ringer containing TTX and TEA blocked the stimulus-evoked oscillation in membrane potential and led to the development of a complex voltage response comprised of a ramp-shaped depolarization followed by an
afterdepolarization (ADP) that appeared as a shoulder on the falling
phase of the voltage response (Fig. 1D). ADPs were followed
by an apamin-insensitive hyperpolarization, henceforth referred to as
the hyperpolarizing afterpotential (HAP) to distinguish it from the AHP
after individual spikes. Both the ramp potential and the ADP were
observed in all cells tested (n = 65) but only in
the presence of TTX and apamin. TEA, although not required to evoke
either component of the voltage response, strongly potentiated the ADP
without affecting the ramp potential (Fig. 1D).
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Changes in membrane potential had a dramatic effect on the response of
DA neurons to constant current depolarizing pulses (0.2 nA; 400 ms)
applied in the presence of TTX, TEA, and apamin (n = 14). At depolarized membrane potentials (positive to 50 mV),
rectangular current pulses evoked an electrotonic voltage response
accompanied by intermittent high-threshold Ca2+ spiking
(Fig. 2A). Identical stimuli
applied closer to the threshold of the slow depolarization leading to
spike initiation (
52 to
55 mV) and produced a passive voltage
response followed by a slow relaxation to a prolonged HAP. Both the
ramp potential and the ADP were most prominently observed in the
voltage region between
55 and
68 mV. However, the slope of the ramp
(Fig. 2A) and the duration of the ADP (Fig. 2B)
decreased as the membrane potential approached
80 mV. Depolarizing
current pulses applied from membrane potentials below
80 mV failed to
evoke either the ramp component or the ADP, although a HAP continued to
be observed (Fig. 2A).
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The voltage-dependent nature of the ADP is further illustrated in Fig.
3. In these experiments, the amplitude of
the stimulus current was adjusted to provide a comparable net
depolarization regardless of the initial membrane potential.
Intracellular injection of increasingly negative bias currents reduced
and eventually eliminated the ADP without significantly affecting the
ramp potential (Fig. 3A, top panel). Decreases in
the duration of the ADP were accompanied by the progressive development
of a HAP, which persisted at membrane potentials approximating
EK (90 mV). Both the amplitude and the slope
of the falling phase of the HAP increased as the membrane was
progressively hyperpolarized. Bath application of Cs+, in
concentrations capable of blocking the fast hyperpolarization-activated cation current in DA neurons (5-10 mM; Fig. 3B), attenuated
the HAP (n = 4; Fig. 3A). These effects were
fully reversed after washout with control Ringer.
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Changes in stimulus intensity also had a pronounced and unexpected
effect on the temporal characteristics of the ADP. Figure 4A illustrates the response of
a representative SNc DA neuron to 400-ms depolarizing current pulses of
increasing amplitude applied from 66 mV. The duration of the ADP was
taken as the interval between termination of the stimulus pulse and the
point at which membrane voltage returned to its initial value.
Small-amplitude depolarizing current pulses (0.1 nA) evoked ADPs
ranging in duration from 81.2 to 1,020.8 ms (355.3 ± 56.5 ms,
n = 16), whereas higher-intensity stimuli (0.2-0.3 nA)
produced a significant decrease in the duration of the response
(RMANOVA, F2,30 =19.6, P < 0.001; Fig. 4C). Similar effects were observed after
constant current pulses (0.2 nA) of increasing duration (Fig.
4B). Thus ADPs evoked in response to a 200-ms stimulus pulse
(range: 124.8-868.0 ms; 383.2 ± 57 ms) were significantly longer
than those obtained with 400- to 800-ms pulses (RMANOVA,
F3,45 =23.9, P < 0.001; Fig.
4D).
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A Ca2+ conductance appeared to be principally responsible
for generation of the ADP because perfusion with a modified Ringer solution containing low Ca2+ (0.3 mM) and high
Mg2+ (3.4 mM) blocked it without affecting the initial
ramp-shaped depolarization (n = 2; Fig.
5A). By contrast, equimolar
substitution of Co2+ for Ca2+ blocked both ramp
potential and the ADP as well as the HAP (n = 5; Fig.
5B). Bath application of the L-type Ca2+ channel
blocker nifedipine (10-30 µM; n = 8) had no effect
on the ramp potential but produced a near-complete blockade of the ADP
(Fig. 6A). Notably, however,
the drug failed to prevent high-threshold Ca2+ spiking
evoked by depolarizing current pulses applied from rest (Fig.
6B). Bath application of the T- and N-type Ca2+
channel blockers Ni2+ (100-300 µM, n = 2) and -conotoxin GVIA (1.7-5 µM, n = 2),
respectively, were without effect on either the ADP or the ramp
potential (data not shown).
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DISCUSSION |
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In the current series of experiments, we examined the effects of
apamin, a potent and selective antagonist of
gKCa2+, on the subthreshold response properties
of nigral DA neurons under conditions in which fast Na+
currents were blocked. In addition to confirming the ability of apamin
to suppress the spontaneous pacemaker oscillation in membrane
potential, we show that blockade of gKCa2+ is
associated with the development of a slow ADP. ADPs were observed in
all cells that were treated with apamin but could not be evoked in TTX
and TEA alone, suggesting that blockade of voltage-activated K+ channels is not sufficient to induce the response.
However, apamin-induced ADPs were strongly potentiated by the addition
of TEA, which has also been shown to increase the amplitude of the
slow, spontaneous oscillation in membrane potential (Nedergaard
and Greenfield 1992). These effects are consistent with TEA's
ability to eliminate outward K+ currents that might
otherwise oppose the ADP. It is also possible that by increasing the
length constant of the cell TEA facilitated propagation of ADP from a
remote location, possibly a dendritic compartment.
The stimulus-evoked ADPs appear to be generated by the same mechanism
responsible for the prolonged plateau depolarization produced after
blockade of gKCa2+ (Nedergaard et al.
1993; Ping and Shepard 1996
). Both phenomena are
markedly voltage sensitive and persist in the presence of TTX and are
observed within a relatively narrow voltage band. Previous
intracellular studies have shown that plateau potentials in DA neurons
are mediated by a dihydropyridine-sensitive Ca2+ current
(Mercuri et al. 1994
; Nedergaard et al.
1993
). A Ca2+ conductance also appeared to underlie
the ADP because it could be blocked in Ringer containing low
Ca2+, high Mg2+, by replacing Ca2+
with Co2+, or after bath application of nifedipine, a
selective antagonist of L-type Ca2+ channels (Tsien
et al. 1988
). By contrast, bath application of high
concentrations of
-conotoxin GVIA had no effect on either the ADP
(this study) or the regenerative plateau potential (Nedergaard et al. 1993
). Ni2+ (300 µM) was also without
effect on the ADP. Although these data suggest that both the ADP and
the plateau potential are mediated by an L-type Ca2+
channel, the voltage range over which these potentials are observed is
considerably below that typically associated with activation of a
high-threshold Ca2+ conductance (Bertolino and
Llinás 1992
). Moreover, high-threshold Ca2+
spikes evoked by depolarizing current pulses continued to be observed
after nifedipine-induced blockade of the ADP. The identification of
multiple subtypes of dihydropyridine-sensitive, L-type Ca2+
channels may provide an explanation for these seemingly paradoxical observations (Fisher and Bourque 1996
; Williams
et al. 1992
). As heteroligomeric complexes, L-type
Ca2+ channels are comprised of multiple subunits that
appear to confer unique kinetic and voltage-dependent properties to the
resultant conductance (Miller 1997
). Thus recombinant
expression of Ca2+ channels consisting of
1D
subunits results in a dihydropyridine-sensitive Ca2+
channel that activates at voltages considerably negative to those associated with channels comprised of
1C subunits
(Tareilus and Breer 1995
; Williams et al.
1992
). Low voltage-activated, L-type Ca2+ currents
were identified in a variety of neurons, including neurosecretory cells
of the supraoptic nucleus (Fisher and Bourque 1996
), and in certain motoneurons (Hsiao et al. 1998
; Russo
and Hounsgaard 1996
). Cells expressing this conductance exhibit
bistable shifts in membrane potential similar to those observed in DA
neurons after blockade of gKCa2+, suggesting
that these currents may provide the principal depolarizing bias
responsible for the generation of the plateau potentials associated
with some forms of bursting activity (Fisher and Bourque
1996
).
Given the marked similarity between the stimulus-evoked ADP and the
longer-duration, plateau potential it seems reasonable to conclude that
the former simply represents a transient version of the latter.
However, unlike the regenerative plateau oscillation, which is
exhibited only by those cells in which apamin induces a sustained
bursting discharge (Ping and Shepard 1996), ADPs were observed in all cells tested, implying that nonbursting DA neurons retain the capacity to generate bursting oscillations. Moreover, the
ability to reliably evoke ADP in DA neurons by intracellular current
pulses implies that the plateau properties exhibited by DA neurons
could be evoked by afferent synaptic input. The question of whether
these properties represent a modification of the mechanisms underlying
the slow oscillatory pacemaker potential or a separate process
"unmasked" after blockade of gKCa2+ remains to be determined. Evidence favoring a common mechanism of action stems
from previous intracellular studies in which nifedipine was reported to
block both the slow oscillatory potential and spontaneous pacemaker
firing in nigral DA neurons (Mercuri et al. 1994
;
Nedergaard et al. 1993
). By contrast, neither
Ni2+ nor
-conotoxin had any effect on these parameters
(Nedergaard et al. 1993
). However, it is important to
note that other investigators reported that nifedipine in
concentrations as high as 100 µM is without effect on pacemaker
activity (Fujimura and Matsuda 1989
). Similarly, Kang
and Kitai (1993b)
found that the Ca2+ current underlying
the pacemaker oscillation in membrane potential is more sensitive to
-conotoxin than to nifedipine. Clearly, additional studies will be
required to specify the nature of the contribution, if any, made by the
Ca2+ current(s) underlying the pacemaker oscillation to the
plateau properties exhibited by DA neurons.
The mechanisms underlying termination of the ADP have also yet to be
established. However, some insights can be obtained from consideration
of the voltage- and time-dependent characteristics of the response.
Although of nearly constant amplitude, ADPs evoked from 55 to
68 mV
were reduced in duration in response to increases in stimulus
intensity. Identical results were obtained in recent studies of the
plateau generating neurons in the dorsal horn of the spinal cord
(Russo and Hounsgaard 1996
). It is possible that high-intensity stimuli terminate the ADP by activating an opposing outward current such as an apamin-insensitive
gKCa2+. This hypothesis is supported by the
observation that the ADP exhibited by DA neurons is followed by a HAP
that is blocked in Ca2+-free Ringer containing
Co2+. However, the persistence of the HAP at membrane
potentials close to EK implies that multiple
conductance mechanisms may contribute to the response. Indeed, the
ability of Cs+ to attenuate the HAP evoked at membrane
potentials below
70 mV suggests that a "sag conductance,"
mediated by a hyperpolarization-activated cation current
(Ih), may indirectly contribute to the
appearance of HAP-like waveforms at hyperpolarized membrane potentials.
For example, depolarizing current pulses of sufficent amplitude to deactivate Ih would be expected to increase
input resistance. As a result, membrane potential would transiently
overshoot its original value immediatly after termination of the
stimulus pulse but depolarize gradually as Ih
reactivated, resulting in the appearance of a HAP-like waveform
(Maccaferri et al. 1993
; McCormick and Pape
1990
; Mosfeldt-Laursen and Rekling 1989
;
Schwindt et al. 1988
; Womble and Moises
1993
). Although as an inward current Ih would not be expected to contribute to the process underlying termination of the ADP, it could alter the time course and amplitude of
outward currents that would otherwise act to curtail the potential.
In addition to Ca2+-sensitive ADP, apamin-treated neurons
frequently exhibited a ramp-shaped slow depolarization in response to
depolarizing current pulses. The slope of the ramp varied as a function
of the membrane potential attained during the depolarization and was
observed over approximately the same voltage region as the ADP. On the
basis of these observations, it seems reasonable to conclude that the
ramp potential simply reflects slow activation of the
dihydropyridine-sensitive Ca2+ channel responsible for
generating the ADP. Although the ADP was completely blocked in low
Ca2+, high Mg2+ salines or after bath
application of nifedipine, these treatments had no effect on the ramp
potential. By contrast, Ca2+-free Ringer containing
Co2+ blocked both components of the voltage response. Thus
the ramp potential and ADP appear to be mediated by independent
conductance mechanisms. Rather than time-dependent activation of an
inward current, it is possible that the ramp potential reflects the
gradual inactivation of an outward current that opposes the
depolarizing effects of the nifedipine-sensitive Ca2+
channel. The voltage range over which the ramp potential was observed
corresponds to that associated with activation of the transient outward
K+ current or IA (Silva et
al. 1990). Although insensitive to 4-AP (Harris
1992
), the transient outward current in DA neurons has been
shown to be modulated by Co2+, which acts to shift both
activation and inactivation curves in a depolarizing direction by
30-50 mV (Silva et al. 1990
). Modulation of
IA gating could explain why
Co2+-containing Ringer is effective in blocking both the
ramp potential and the transient plateau potential, whereas nifedipine
and low Ca2+, high Mg2+ Ringer affect only the
latter component.
In summary, it is becoming increasingly clear that mesencephalic
DA-containing neurons are capable of exhibiting nonlinear response
properties including regenerative plateau potentials and
stimulus-evoked ADP. Although endogenously generated, these characteristics are uncovered only after blockade of outward
K+ currents. The ability of apamin, a potent and selective
antagonist of gKCa2+, to reliably evoke
spontaneous bursting and plateau activity in DA neurons suggests that
modulation of these channels could exert a permissive effect on the
expression of bursting activity in DA neurons. Modulation of
Ca2+-activated K+ channels by neurotransmitters
acting through G-protein-coupled receptors was demonstrated in a wide
variety of neurons and represents an important mechanism for the
regulation of neuronal excitability (reviewed by Sah
1996). Although direct modulation of
gKCa2+ in nigral DA neurons has yet to be
demonstrated, it is tempting to speculate that a similar mechanism may
be involved in the generation of some types of bursting activity
exhibited by these neurons, particularly the sustained phasic discharge
typically observed in the anesthetized preparation.
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ACKNOWLEDGMENTS |
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The authors acknowledge S. Stilling for expert secretarial assistance.
This work was supported by Institute of Mental Health Grant MH-48543.
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FOOTNOTES |
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Address for reprint requests: P. Shepard, Maryland Psychiatric Research Center, P. O. Box 21247, Baltimore, MD 21228.
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 soley to indicate this fact.
Received 8 December 1997; accepted in final form 3 November 1998.
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REFERENCES |
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