Department of Pharmacology and Therapeutics, McGill University, Montreal, Quebec H3G 1Y6, Canada
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ABSTRACT |
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Jackson, Michael F.,
Barbara Esplin, and
Radan apek.
Inhibitory nature of tiagabine-augmented
GABAA receptor-mediated depolarizing responses in
hippocampal pyramidal cells. Tiagabine is a potent GABA uptake
inhibitor with demonstrated anticonvulsant activity. GABA uptake
inhibitors are believed to produce their anticonvulsant effects by
prolonging the postsynaptic actions of GABA, released during episodes
of neuronal hyperexcitability. However, tiagabine has recently been
reported to facilitate the depolarizing actions of GABA in the CNS of
adult rats following the stimulation of inhibitory pathways at a
frequency (100 Hz) intended to mimic interneuronal activation during
epileptiform activity. In the present study, we performed extracellular
and whole cell recordings from CA1 pyramidal neurons in rat hippocampal slices to examine the functional consequences of tiagabine-augmented GABA-mediated depolarizing responses. Orthodromic population spikes (PSs), elicited from the stratum radiatum, were inhibited following the
activation of recurrent inhibitory pathways by antidromic conditioning
stimulation of the alveus, which consisted of either a single stimulus
or a train of stimuli delivered at high-frequency (100 Hz, 200 ms). The
inhibition of orthodromic PSs produced by high-frequency conditioning
stimulation (HFS), which was always of much greater strength and
duration than that produced by a single conditioning stimulus, was
greatly enhanced following the bath application of tiagabine (2-100
µM). Thus, in the presence of tiagabine (20 µM), orthodromic PSs,
evoked 200 and 800 ms following HFS, were inhibited to 7.8 ± 2.6% (mean ± SE) and 34.4 ± 18.5% of their unconditioned
amplitudes compared with only 35.4 ± 12.7% and 98.8 ± 12.4% in control. Whole cell recordings revealed that the bath
application of tiagabine (20 µM) either caused the appearance or
greatly enhanced the amplitude of GABA-mediated depolarizing responses
(DR). Excitatory postsynaptic potentials (EPSPs) evoked from stratum
radiatum at time points that coincided with the DR were inhibited to
below the threshold for action-potential firing. Independently of the
stimulus intensity with which they were evoked, the charge transferred
to the soma by excitatory postsynaptic currents (EPSCs), elicited in
the presence of tiagabine (20 µM) during the large (1,428 ± 331 pA) inward currents that underlie the DRs, was decreased on the average
by 90.8 ± 1.7%. Such inhibition occurred despite the presence of
the GABAB receptor antagonist, CGP 52 432 (10 µM),
indicating that GABAB heteroreceptors, located on
glutamatergic terminals, do not mediate the observed reduction in the
amplitude of excitatory postsynaptic responses. The present results
suggest that despite facilitating the induction of GABA-mediated depolarizations, tiagabine application may nevertheless increase the
effectiveness of synaptic inhibition during the synchronous high-frequency activation of inhibitory interneurons by enhanced shunting.
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INTRODUCTION |
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-Aminobutyric acid (GABA)-mediated inhibition plays a critical
role in the control of CNS excitability as demonstrated by the
convulsions resulting from the administration, in both animals and
humans, of compounds that decrease the efficacy of this
neurotransmitter system (for review see Gale 1992
). The
development of pharmacological agents capable of potentiating GABAergic
neurotransmission has therefore been an important strategy in the
search for novel anticonvulsants. Such a mechanism is thought to
underlie or contribute to the effectiveness of the barbiturates and
benzodiazepines (Rogawski and Porter 1990
), both
allosteric modulators of GABAA receptor function, as well as to the more recently developed antiepileptic vigabatrin
(Grant and Heel 1991
), an inhibitor of GABA metabolism.
The therapeutic potential of GABA uptake inhibitors in the treatment of
epilepsy has been recognized since the early demonstrations of the
ability of small amino acids, such as nipecotic acid and guvacine, to
enhance GABA-mediated responses evoked either synaptically (Dingledine and Korn 1985; Matthews et al.
1981
) or by iontophoretic application of the neurotransmitter
(Curtis et al. 1976
; Dingledine and Korn
1985
). However, being highly polar, these compounds do not
readily cross the blood-brain barrier and therefore can only produce an
anticonvulsant effect when administered intracerebroventricularly (Croucher et al. 1983
; Frey et al. 1979
).
The addition of lipophilic anchors to these simple structures increases
not only their ability to permeate the blood-brain barrier but also
their affinity for the GABA transporter and thus allows them to exert
an anticonvulsant effect following their systemic administration
(Braestrup et al. 1990
; Suzdak et al.
1992
; Yunger et al. 1984
).
We have previously reported that in hippocampal slices, bath
application of two of these novel uptake blockers, SKF 89976A and SKF
100330A, results in a relatively selective and prolonged enhancement in
the inhibition produced following the high-frequency stimulation (100 Hz, 200 ms) of recurrent inhibitory pathways (apek and Esplin
1993
). Such a frequency-dependent mechanism of action should result in
a selective strengthening of synaptic inhibition during episodes of
high-frequency activity and allow these compounds to control seizures
while minimizing the occurrence of side effects normally associated
with drugs that produce an indiscriminate enhancement of inhibition.
Paradoxically, recent evidence has demonstrated that high-frequency
activation of inhibitory pathways, similar to that used in our previous
study, evokes large (5-20 mV) GABAA receptor-mediated
depolarizing responses (DRs) capable of triggering a burst of action
potentials (APs) in hippocampal pyramidal cells, suggesting that
GABAergic mechanisms may contribute to the generation and/or
propagation of seizure activity (Grover et al. 1993
;
Jackson et al. 1995
; Staley et al. 1995
).
Furthermore, we have recently demonstrated that tiagabine, a novel GABA
transport inhibitor with demonstrated anticonvulsant properties in
animal models of epilepsy (Faingold et al. 1994
) as well
as in human partial epilepsy (Ben-Menachem 1995
;
Schachter 1995
), greatly facilitates the depolarizing
actions of GABA following the stimulation of hippocampal interneurons
at high (100 Hz) frequency (Jackson et al. 1996
).
We therefore decided to further investigate the mechanisms by which GABA-uptake blockers produce frequency-dependent enhancement of inhibition by studying the effects of tiagabine-augmented GABA-mediated depolarizations on excitatory responses evoked at various (100-1,600 ms) time intervals following the high-frequency stimulation of hippocampal recurrent inhibitory pathways.
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METHODS |
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Extracellular field and whole cell recordings were made from the CA1 region of hippocampal slices obtained as follows. Male Sprague-Dawley rats (100-200 g) were decapitated, their brains quickly removed and placed in cooled, oxygenated (95% O2-5% CO2) artificial cerebrospinal fluid (ACSF) of the following composition (in mM): 124 NaCl, 3.3 KCl, 2.5 CaCl2, 2.4 MgSO4, 25.6 NaHCO3, 1.25 KH2PO4, and 10 glucose. Transverse slices (400-500 µm) were then prepared using either a McIlwain tissue chopper or vibratome and allowed to recover at room temperature for at least 1 h in oxygenated ACSF. For recordings, slices were transferred to a tissue chamber and maintained at 31°C at the interface between humidified 95% O2-5% CO2, and oxygenated ACSF perfused through the chamber at a flow rate of 1.5-3.5 ml/min.
The effects of tiagabine on GABA-mediated inhibition were assessed
using the antidromic-orthodromic test of inhibition as previously
described (apek and Esplin 1993
; Jackson
et al. 1994
). Briefly, field potentials were recorded with a 3 M NaCl-filled micropipette (5-15 M
) located in the pyramidal cell
layer of the CA1 region. Constant-current stimuli (100 µs) delivered
through bipolar stimulating electrodes to the Schaffer collaterals of stratum radiatum elicited orthodromic population spikes (PSs). These
were inhibited by preceding antidromic conditioning stimulation delivered to the alveus, which causes the activation of inhibitory interneurons and the subsequent release of GABA onto pyramidal cells.
Conditioning alvear stimulation was by either a single stimulus or by a
train of 20 stimuli delivered at a high (100 Hz) frequency. The delay
between the conditioning stimulus and subsequent orthodromic test
response is referred to as the interstimulus interval (ISI) and was in
the range of 10-1,600 ms. Orthodromic stimulus intensity was adjusted
to produce a half-maximal PS. At an ISI of 10 ms, antidromic stimulus
intensity was increased until it produced a 70-90% reduction in the
amplitude of the orthodromic PS. In this paradigm, inhibition is
measured indirectly as a reduction in the amplitude of the test PS
expressed as a percentage of the unconditioned PS. For each ISI,
responses were obtained before and after drug treatment. Results were
expressed as means ± SE, and the significance of the drug-induced
change was evaluated using Student's t-test.
Tight seal (>3 G) whole cell recordings were obtained from CA1
pyramidal cells using patch pipettes pulled from borosilicate glass
(WPI) with a Brown-Flaming micropipette puller (Sutter Instruments, P-80). Patch electrodes had a resistance of 2-4 M
when filled with
an internal solution of the following composition (in mM): 140-150
KMeSO4 (or CsMeSO4 where indicated), 10 N-2-hydroxyethylpiperazine-N''2-ethanesulfonic acid (HEPES), and 2 MgCl2. The solution was buffered with
KOH (or CsOH) to a pH of 7.2-7.3 and filtered through a 0.2-µm pore size filter (Nalgene). Total osmolality for all intracellular solutions
ranged from 270 to 290 mOsm. Current-clamp recordings were made with an
Axoclamp 2A amplifier (Axon Instruments) operated in bridge mode,
whereas an Axopatch 200A (Axon Instruments) was used for voltage-clamp
recordings. All signals were digitized at 18 kHz (Instrutech, VR-10A)
and stored unfiltered on videotape for later retrieval and analysis
using a PC-based acquisition and analysis system functionally similar
to that described previously (Théorêt et al.
1984
).
In all recordings, drugs used were dissolved in ACSF and superfused over slices. Tiagabine ((R)-N-[4,4-di-(3-methylthien-2-yl)but-3-enyl] nipecotic acid hydrochloride) and CGP 52 432 (3-[[[(3,4-dichlorophenyl)methyl]amino]propyl](diethoxy-methyl)) were generously supplied by Abbott Laboratories (Chicago, IL) and Novartis Pharma AG (Basel), respectively.
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RESULTS |
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Bath application of tiagabine (20 µM) had no effect on the amplitude of the unconditioned PS (Fig. 1A) and produced no change in the inhibition resulting from a single antidromic conditioning stimulation at short ISIs (10-20 ms) (Fig. 1, A and B). At longer ISIs (40-160 ms), inhibition was, however, increased resulting in a further 34.6% reduction in PS amplitude at an ISI of 160 ms (Fig. 1B). The frequency dependence of drug action was tested by stimulating the recurrent inhibitory pathways with a train of stimuli delivered at 100 Hz. The inhibition produced by such high-frequency stimulation was greater at any ISI and lasted longer than that resulting from a single stimulus. Thus, although the PS had fully recovered by 160 ms following conditioning stimulation by a single pulse, with HFS it was inhibited to <35% of its original amplitude at a more delayed ISI of 200 ms and recovered from inhibition over the next several hundred milliseconds (Fig. 1B). When tiagabine was added to the perfusate, inhibition was increased to the extent that high-frequency antidromic conditioning stimulation could now produce a near complete inhibition of the population spike at an ISI of 800 ms (Fig. 1A). The recovery from inhibition was also greatly prolonged such that the PS recovered to its original amplitude over the course of several thousand milliseconds rather than the hundreds of milliseconds time needed in control (Fig. 1B). The effects of tiagabine on recurrent inhibition were concentration-dependent over a range of 2-100 µM.
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Given its ability to increase the depolarizing actions of GABA released
following the HFS of inhibitory interneurons (Jackson et al.
1996), tiagabine could easily have been expected to produce an
increase in pyramidal cell excitability. However, consistent with our
previous findings with the SKF series of GABA uptake blockers
(
apek and Esplin 1993
), bath application of
tiagabine resulted in a large increase in the functional inhibition
resulting from the HFS of recurrent inhibitory pathways. Using the same stimulation paradigm, whole cell recordings were made from CA1 pyramidal cells to investigate the basis for enhanced inhibition following the application of tiagabine. In the absence of preceding alvear HFS, excitatory postsynaptic potential-inhibitory postsynaptic potential (EPSP-IPSP) sequences were evoked following the single stimulation of Schaffer collaterals in stratum radiatum (Fig. 2, A1 and B1). The
evoked IPSPs were comprised of both fast and slow hyperpolarizing
components (fIPSPs and sIPSPs, respectively). As illustrated in the
insets of Fig. 2, A1 and B1, a current
intensity for Schaffer-collateral stimulation was chosen, which could
reliably elicit unconditioned EPSPs capable of triggering the firing of a single AP. When evoked 400 ms following the high-frequency activation of recurrent inhibitory pathways, the resulting EPSPs could no longer
trigger an AP (Fig. 2A2). This occurred in spite of the fact
that the amplitude and duration of the EPSPs appeared enhanced due to a
reduction in the amplitude of the overlapping fIPSP. Such a reduction
most likely occurred as a result of a decrease in the driving force for
Cl
following its intracellular accumulation during the
sustained activation of postsynaptic GABAA receptors
resulting from high-frequency alvear stimulation (McCarren and
Alger 1985
; Thompson and Gähwiler 1989
).
In contrast, when the interstimulus interval was 800 ms, preceding
high-frequency alvear stimulation no longer prevented the firing of APs
following orthodromic stimulation (Fig. 2B2). Consistent
with previous reports of the effects of GABA uptake inhibitors on
evoked IPSPs (Roepstorff and Lambert 1992
;
Thompson and Gähwiler 1992
), bath application of
tiagabine greatly prolonged the duration of IPSPs while producing
little or no effect on their amplitude (Fig. 2, A1 and
B1). Rather than producing a qualitatively similar change in
the duration of hyperpolarization following HFS, tiagabine either
caused the appearance or greatly increased the amplitude and duration
of a GABA-mediated depolarization. The presence of the DR coincided
with a prolonged period of inhibition as demonstrated by the
suppression of AP firing and large reduction in the amplitude of EPSPs
superimposed at various time points along the different phases of the
DR (Fig. 2, A2 and B2). Thus, in control,
inhibition of AP firing was observed in only 4 of 9 and 4 of 10 cells
at ISIs of 200 and 400 ms, respectively. Following the addition of
tiagabine to the superfusate, inhibition could now be seen in all cells
at these same ISIs, with AP firing now observed in only 1 of 10 cells
at 800 ms and in only 1 of 7 at 1,600 ms following HFS of the alveus
(Table 1). It is interesting to note
that, under control conditions, inhibition of cell firing was observed
in only those cells from which a DR could be elicited. Furthermore,
following the application of tiagabine, the change in the time
course of the inhibition of EPSP-induced AP firing was similar to that
of the inhibition of stratum radiatum evoked PSs (compare with Fig. 1).
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The observed reduction in the amplitude of EPSPs following the HFS of
recurrent inhibitory pathways could be explained by two possible
mechanisms: 1) reduced activation of postsynaptic excitatory
amino acid receptors due to a decrease in transmitter release caused by
the activation of presynaptic GABAB heteroreceptors present
at glutamatergic terminals, and 2) shunting of the
dendritically evoked EPSP by the concurrently active conductance
responsible for the DR. These possibilities were investigated in cells
voltage clamped near the reversal potential for GABAA
receptor-mediated inhibitory postsynaptic currents (IPSCs) and
superfused with ACSF containing 20 µM tiagabine and 10 µM of the
specific GABAB receptor antagonist CGP 52 432, a
concentration that prevents the inhibition of glutamate release by GABA
(Waldmeier et al. 1994). Under these conditions,
relatively pure excitatory postsynaptic currents (EPSCs) could be
elicited following the single stimulation of Schaffer collaterals. HFS
of the alveus evoked large (1,428 ± 331 pA, n = 5) inward currents with a time course similar to that of the DRs
recorded in current clamp and which reversed at a membrane potential of
51 mV (Fig. 3A), a value
identical to the previously reported reversal potential of
GABA-mediated DRs (Perkins and Wong 1996
; Staley
et al. 1995
). As with the EPSPs, EPSCs evoked 800 ms following
HFS of the alveus (Fig. 3B, conditioned) were of much
reduced amplitude, this in spite of the presence of CGP 52 432, indicating a lack of involvement of presynaptic GABAB receptors in the reduction of excitatory responses. This effect was
quantified by measuring the charge transferred to the soma during EPSCs
(Staley and Mody 1992
) that were superimposed on the
large HFS-evoked inward currents (Fig. 3C). The inhibition produced following HFS of the alveus was independent of the stimulus intensity used to evoke the EPSCs and reduced the charge delivered to
the soma by evoked EPSCs to only 9.2 ± 1.7% (n = 5) of unconditioned responses across all stimulus intensities used. In
an attempt to improve the quality of the space clamp, and thus record a
larger fraction of the distally generated EPSC, recordings were made using CsMeSO4-based electrode solutions. Under these
recording conditions, the inhibitory influence of the HFS-evoked inward current on evoked EPSCs was still, nevertheless, observed. Thus, across
an identical range of stimulus intensities, the mean charge transferred
was reduced to only 12.7 ± 2.9% (n = 4) of
unconditioned evoked EPSCs, a similar value to that which we observed
in the absence of intracellular Cs+. For this reason, the
data obtained under both recording conditions were pooled to generate
the graph in Fig. 3C. In separate recordings, we determined
that the membrane resistivity during the large GABAA receptor-mediated inward current, as estimated from a 10-mV, 200-ms hyperpolarizing step, was reduced to only 16.6 ± 1.2%
(n = 2) of its resting value (data not shown).
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DISCUSSION |
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In the adult rat, biphasic GABAA receptor-mediated
postsynaptic responses consisting of an initial hyperpolarization
followed by a depolarization have been observed under a number of
circumstances that have in common the prolonged activation of
postsynaptic GABAA receptors. These include the following:
4-aminopyridine-induced epileptiform activity (Perkins and Wong
1996; Perreault and Avoli 1992
; Traub et
al. 1995
), iontophoretic application of GABA (Alger and
Nicoll 1982a
; Andersen et al. 1980
;
Thalmann et al. 1981
), evoked synaptic responses in the
presence of barbiturates (Alger and Nicoll 1982b
;
Thalmann et al. 1981
) or zinc (Xie and Smart 1991
), and responses evoked by high-frequency stimulation of
inhibitory interneurons (Grover et al. 1993
;
Jackson et al. 1995
; Staley et al. 1995
).
In neurons from immature animals, GABA also produces depolarizing
responses that appear to be due to the maintenance of a
Cl
gradient that is positive to the cells resting
membrane potential (RMP) (Cherubini et al. 1990
;
Mueller et al. 1984
). In the adult, however, the DR
appears to be mediated by bicarbonate (HCO3
) anions,
which in addition to Cl
are known to permeate the
GABAA ionophore (Kaila 1994
). However, given
the fact the Cl
reversal potential in CNS neurons is
typically more negative than the RMP (with dorsal root ganglion and
hippocampal granule cells being exceptions) and that GABAA
receptors are approximately five times more permeable to
Cl
anions than to HCO3
(Bormann
et al. 1987
), the inward flow of Cl
normally
dominates postsynaptic currents leading to a hyperpolarization of the
neuronal membrane. As mentioned above, DRs typically occur following
the decay of an initial hyperpolarization of variable amplitude and
duration. A recently proposed model (Staley et al. 1995
)
suggests that during the sustained activation of GABAA
receptors the inward flow of Cl
, which underlies the
initial hyperpolarization, causes a partial collapse of the gradient
for this anion resulting in a diminished flow of Cl
. This
allows a greater net flow of HCO3
, whose reversal
potential is more positive than the RMP, leading to the generation of a
depolarizing potential.
The amplitude and duration of GABA-mediated DRs are critically
dependent on both the duration of HFS as well as on the stimulus intensity used (Jackson et al. 1995). Furthermore, they
are typically much larger when evoked monosynaptically following the
blockade of excitatory amino acid receptors with a combination of
6-cyano-7-nitroquinoxaline-2,3-dione and
3-((R)-carboxypiperazin-4-yl-propyl-1-phosphonic acid, which antagonizes
-amino-3-hydroxy-5-methyl-4-isoxazolepropionic
acid/kainate and N-methyl-d-aspartate receptors (unpublished
observation). The activation of a greater number of inhibitory
interneurons due to the necessity, under these conditions, of
positioning the stimulating electrodes much closer to the cell from
which monosynaptic GABAergic responses are to be recorded, is most
likely responsible for this. Given the fact that the DRs are generated
only after such strong, sustained activation of postsynaptic
GABAA receptors, which is unlikely to occur during normal
synaptic transmission, the exact physiological role of GABA-mediated
depolarizations is unclear. Evidence has been presented suggesting that
the transient transformation of the postsynaptic actions of GABA from
mainly hyperpolarizing to depolarizing may contribute to synaptic
plasticity by relieving the voltage-dependent Mg2+ block of
NMDA receptors (Staley et al. 1995
). Although such an interaction may occur in dendritic compartments in which both GABAA and NMDA receptors are activated simultaneously, our
results suggest that a large proximal increase in input conductance
resulting from the activation of postsynaptic GABAA
receptors, such as would be expected to occur following the intense
activation of inhibitory interneurons during epileptiform activity,
results in a transient suppression in the ability of excitatory
transmission to depolarize the soma following the activation of distal
excitatory amino acid receptors.
Although the inhibition of EPSPs could have been due to the activation
of GABAB heteroreceptors located at glutamatergic
terminals, the suppression of EPSCs, which occurred despite the
presence of the GABAB receptor blocker CGP 52 432, argues
against the involvement of such a mechanism. The evidence presented
here suggests that shunting inhibition of evoked EPSPs most likely
underlies the much stronger inhibition of stratum radiatum-evoked
population spikes resulting from the activation of recurrent inhibitory
pathways with HFS compared with that produced following the activation with a single stimulus (Fig. 1). Shunting inhibition of excitatory responses can be defined as a reduction in the depolarizing effects of
the EPSC due to a drop in membrane resistance. Because the magnitude of
the EPSC is independent of the extrasynaptic membrane resistance, it
would have been impossible for us to observe shunting inhibition had
the entire dendritic membrane voltage been perfectly controlled by the
somatically applied voltage clamp. However, due to the extended
geometry of hippocampal pyramidal neurons, especially those of the CA1
region (Carnevale et al. 1997), space clamp of the
distal dendritic compartments is incomplete. Following HFS of the
recurrent inhibitory pathway, these distal compartments become even
more electrotonically remote, due to the associated decrease in
membrane resistivity and dendritic space constant. Thus less of the
charge that crosses the subsynaptic membrane at the excitatory synapses
is transferred to the soma. Changes in the charge transfer allow then
to assess shunting inhibition even under voltage-clamp conditions.
For a conductance to effectively shunt the potential generated by the
activation of another, it should be sufficiently large and generated
more proximally (Koch et al. 1983; Qian and
Sejnowski 1990
; Vu and Krasne 1992
). The
conductance increase resulting from the activation of GABAA
receptors following HFS stimulation was large, as demonstrated by the
substantial drop in input resistance (to only 16.6% of rest) that was
observed in our experiments. Furthermore, either directly or through
the recurrent axon collaterals of pyramidal cells, the electrical
stimulation of the alveus that we used will have caused the activation
of interneurons predominantly located within the alveus and stratum
oriens, and to a lesser extent, stratum pyramidale. Among the different
types of interneurons responding to such stimulation, GABAergic basket
and axo-axonic cells have been shown to preferentially make synaptic
contacts with the soma, proximal dendrites, and initial segment of CA1 pyramidal cells. The electrical stimulation of these interneuronal populations from an electrode positioned in the alveus is therefore likely to have caused a proximal increase in conductance following the
activation of GABAA receptors. Furthermore, our
demonstration that the percent reduction in charge transferred to the
soma was independent of the strength of the excitatory input is
consistent with predictions of the consequences of proximal shunting
inhibition on distally evoked EPSPs (Vu and Krasne
1992
). The present findings are entirely in line with our
previous results suggesting that GABA-mediated DRs generated near the
soma and proximal dendrites are inhibitory in nature whereas those
elicited more distally, following HFS in stratum radiatum, are more
likely to initiate AP discharges (Jackson et al. 1997
).
Due to their ability to directly trigger the firing of bursts of APs,
DRs have also been implicated in the generation of epileptiform activity. This is further supported by experiments demonstrating a
reduction in the duration of afterdischarges following the blockade of
GABAA receptors by the specific antagonist bicuculline
(Higashima et al. 1996; Traub et al.
1995
). In light of this, the large increase in GABA-mediated
depolarizations, observed following the application of tiagabine,
would seem contrary to the expected actions of an anticonvulsant.
However, strong inhibition was always observed in response to the HFS
of recurrent inhibitory pathways in the presence of tiagabine (Fig. 1).
Furthermore, our results suggest that during the synchronous,
high-frequency activation of proximally located interneurons, tiagabine
increases the GABAA receptor-mediated conductance to such
a large extent that the ability of cells to fire APs in response to
dendritically evoked EPSPs is suppressed, indicating that
tiagabine-facilitated DRs can retain the inhibitory character typical
of GABA action. Therefore, despite producing a facilitation of
GABA-mediated depolarizations, the potentiation of GABA-mediated
shunting inhibition following tiagabine administration may allow this
drug to produce an anticonvulsant effect by suppressing the generation
of burst discharges during epileptiform activity.
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ACKNOWLEDGMENTS |
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We thank A. Constantin for competent technical assistance and Dr. Yves De Koninck for helpful advice as well as for the use of recording equipment.
This work was supported by the Medical Research Council of Canada. M. Jackson was supported by a Fonds pour la Formation de Chercheurs et l'Aide à la Recherche studentship, and by a McGill Faculty of Medicine Internal Studentship.
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FOOTNOTES |
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Address for reprint requests: R. apek, Dept. of Pharmacology and
Therapeutics, McGill University, 3655 Drummond St., Montreal, Quebec
H3G lY6, Canada.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 9 June 1998; accepted in final form November 1998.
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REFERENCES |
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