1Montreal Neurological Institute, Department of Neurology and Neurosurgery, and Department of Physiology, McGill University, Montreal, Quebec H3A 2B4, Canada; 2Centre Paul Broca, Institut National de la Santé et de la Recherche Médicale U109, 75014 Paris, France; 3Dipartimento di Neuroscienze, Università degli Studi di Roma `Tor Vergata', 00173 Rome, Italy; and 4Institut National de la Santé et de la Recherche Médicale U398, 67000 Strasbourg, France
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ABSTRACT |
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Motalli, Rita, Jacques Louvel, Virginia Tancredi, Irène Kurcewicz, Doreen Wan-Chow-Wah, René Pumain, and Massimo Avoli. GABAB Receptor Activation Promotes Seizure Activity in the Juvenile Rat Hippocampus. J. Neurophysiol. 82: 638-647, 1999. We analyzed how the GABAB receptor agonist baclofen (10-50 µM) influences the activity induced by 4-aminopyridine (4-AP, 50 µM) in the CA3 area of hippocampal slices obtained from 12- to 25-day-old rats. Interictal and ictal discharges along with synchronous GABA-mediated potentials occurred spontaneously in the presence of 4-AP. Baclofen abolished interictal activity (n = 29 slices) and either disclosed (n = 21/29) or prolonged ictal discharges (n = 8/29), whereas GABA-mediated potentials occurred at a decreased rate. The N-methyl-D-aspartate (NMDA) receptor antagonist 3,3-(2-carboxypiperazine-4-yl)-propyl-1-phosphate (CPP, 10 µM, n = 8) did not modify the GABA-mediated potentials or the ictal events recorded in 4-AP + baclofen. In contrast ictal, activity, but not GABA-mediated potentials, was blocked by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 µM, n = 5). Most baclofen effects were reversed by the GABAB receptor antagonist CGP 35348 (1 mM; n = 4). Baseline and transient increases in [K+]o associated with the 4-AP-induced synchronous activity were unaffected by baclofen. Baclofen hyperpolarized CA3 pyramids (n = 8) recorded with K-acetate-filled electrodes by 4.8 ± 1.3 mV and made spontaneous, asynchronous hyperpolarizing and depolarizing potentials disappear along with interictal depolarizations. GABA-mediated synchronous long-lasting depolarizations (LLDs) and asynchronous depolarizations were also studied with KCl-filled electrodes in 4-AP + CPP + CNQX (n = 6); under these conditions baclofen did not reduce LLD amplitude but abolished the asynchronous events. Dentate hilus stimulation at 0.2-0.8 Hz suppressed the ictal activity recorded in 4-AP + baclofen (n = 8). Our data indicate that GABAB receptor activation by baclofen decreases transmitter release leading to disappearance of interictal activity along with asynchronous excitatory and inhibitory potentials. By contrast, GABA-mediated LLDs and ictal events, which reflect intense action potential firing invading presynaptic inhibitory and excitatory terminals respectively, are not abolished. We propose that the proconvulsant action of baclofen results from 1) block of asynchronous GABA-mediated potentials causing disinhibition and 2) activity-dependent changes in hippocampal network excitability.
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INTRODUCTION |
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GABA is a ubiquitous inhibitory transmitter in the
CNS where it acts mainly on two receptor subtypes (termed A and B) that are located pre- and postsynaptically on both local and long-axoned cells (Bowery 1993; Kaila 1994
;
Macdonald and Olsen 1994
; Misgeld et al.
1995
). Epileptiform activity ensues when GABAergic mechanisms weaken (Krnjevic 1991
), and several anticonvulsant drugs
including some of the newest compounds, may enhance
GABAA-mediated inhibition (Olsen and Avoli
1997
).
GABAB receptor-mediated mechanisms are involved in the
generation of focal seizures and in epileptogenesis (Haas et al.
1996; McLean et al. 1996
; Scanziani et
al. 1994
; Velísková et al. 1996
).
However, the GABAB receptor agonist baclofen may possess a
surprising proconvulsant effect as documented in clinical practice (Kofler et al. 1994
; Rush and Gibberd
1990
) and in some models of epileptiform discharge
(Lewis et al. 1989
; Mott et al. 1989
; Swartzwelder et al. 1987
; Watts and Jefferys
1993
). It has also been proposed that the proconvulsant effect
of baclofen is caused by a presynaptic, GABAB-mediated
inhibition of GABA release from inhibitory interneurons leading to
disinhibition (Mott et al. 1989
; Watts and
Jefferys 1993
).
Although age-related differences in the ability of baclofen to modulate
seizures have been described (Velísková et al.
1996), the evidence for a proconvulsant action of this
GABAB receptor agonist mainly derives from studies in the
adult brain. Here we used extracellular and intracellular recording
techniques in conjunction with [K+]o
measurements to characterize the effects of baclofen on the activity
induced by the convulsant 4-aminopyridine (4-AP) in the CA3 area of
hippocampal slices obtained from young rats. At this age both
interictal and ictal discharges occur in vitro along with synchronous
GABA-mediated events (Avoli et al. 1993
,
1996
). Some of these findings have been published in
abstract form (Motalli et al. 1997
; Tancredi et
al. 1998
).
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METHODS |
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Preparation and maintenance of the slices
Sprague-Dawley or Wistar rats (12-25 day old) were decapitated under halothane anesthesia, and the brains were quickly removed and placed in cold oxygenated artificial cerebral spinal fluid (ACSF). Isolated, 500-µm-thick hippocampal slices were cut with a vibratome and transferred to a tissue chamber where they lay in an interface between oxygenated ACSF and humidified gas (95% O2-5% CO2) at 32-35°C (pH 7.4). ACSF composition was (in mM) 124NaCl, 2 KCl, 1.25 KH2PO4, 2 MgSO4, 2 CaCl2, 26 NaHCO3 and 10 glucose. 4-AP (50 µM), 4-amino-3-[4-chlorophenyl]-butanoic acid (baclofen, 10-50 µM), 3,3-(2-carboxypiperazine-4-yl)-propyl-1-phosphate (CPP, 10 µM), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 µM), and CGP-35348 (1 mM) were bath applied. Chemicals were acquired from Sigma with the exception of CNQX and CPP (obtained from Tocris Cookson) and CGP 35348 (kindly donated by Novartis, Basel).
Recording procedures
Extracellular field potential recordings were made in CA3
stratum radiatum with electrodes filled with 2 M NaCl or ACSF
(resistance, 2-8 M). Intracellular recordings were performed with
electrodes filled with 2 M K-acetate (resistance, 70-120 M
) or 3 M
KCl (resistance, 60-90 M
). Signals were fed to high-impedance
amplifiers with internal bridge circuit for passing intracellular
current. The bridge was monitored carefully throughout the experiment
and adjusted as required. Whenever necessary the resting membrane
potential (RMP) during any given pharmacological test was maintained at the same value as in control by injecting intracellular current.
K+-selective electrodes were prepared according
to the techniques described by Heinemann et al. (1977)
and previously used in our laboratories (Avoli et al.
1996
). For each ion-selective electrode, a double-barreled
pipette was pulled to a tip diameter of 1-2 µm and silanized. One
barrel was filled with NaCl and was used as a reference electrode. The
other barrel was filled with a K+ solution, and
the K+-selective resin, valinomycin-based
K+ ionophore I (cocktail A), was sucked into the
tip. The resin was acquired from Fluka Chemical.
K+-selective electrodes were tested and
calibrated before and after the experiment in different solutions,
which were as follows (in mM): 1) 146 NaCl and 3 KCl;
2) 146 NaCl and 30 KCl; 3) 119 NaCl and 30 KCl;
and 4) 59 NaCl and 90 KCl.
K+-selective electrodes were accepted if their
response to a 10-fold change in [K+] was
49
mV. Signals from the K+-selective electrode (that
was positioned in CA3 stratum radiatum) were fed to a Meyer & Rentz
amplifier (Frankfurt, Germany). In these experiments the field
potential was recorded through the reference channel of the
K+-selective electrode.
Field potential, intracellular, and [K+]o recordings were displayed on a digital oscilloscope and on a Gould WindoGraf recorder. They were also recorded on a videocassette recorder for later analysis. In some experiments a bipolar stainless steel electrode was used to deliver extracellular stimuli (90 µs; <1,800 µA) to the hilus of the dentate gyrus.
Database and analysis
Our study is based on the use of >60 slices that were analyzed
with field potential, intracellular, and/or
[K+]o recordings. The
electrophysiological characteristics of CA3 pyramidal cells recorded
with K-acetate-filled electrodes were 1) RMP measured after
electrode withdrawal was 68.1 ± 5.0 mV (mean ± SD,
n = 11); 2) action potential amplitude
calculated from the baseline of 91.7 ± 9.5 mV (n = 18); apparent input resistance obtained from the maximum voltage
response induced by small (<0.5 nA) hyperpolarizing current pulses of
27.3 ± 7.5 M
(n = 13). Injection of
depolarizing current pulses in these cells caused regular spiking
activity with adaptation that was followed by a long-lasting
afterhyperpolarization (160-250 ms) on pulse termination.
Throughout the paper measurements are expressed as means ± SD, and n indicates the number of slices used for any given pharmacological protocol, unless otherwise stated. Statistical analysis of the data obtained under control conditions, and during any experimental manipulation was performed with paired or unpaired Student's t-tests as well as with ANOVA. Data were considered significantly different if P < 0.01.
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RESULTS |
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4-aminopyridine-induced synchronous activity
Three different types of spontaneous synchronous activity were
recorded in the CA3 stratum radiatum of juvenile rat hippocampal slices
during 4-AP application (Avoli et al. 1993,
1996
). Brief, positive interictal-like discharges
(duration, 300-800 ms; rate of occurrence, 0.2-0.9 Hz; Fig.
1, A and B, arrows
in Control) and negative potentials (duration, 300-900 ms; rate of
occurrence, 0.007-0.23 Hz; Fig. 1, A and B,
asterisks in Control) were present in all slices (n = 42). In addition, ictal-like discharges (duration, 5.4-11.0 s; rate of
occurrence, 0.004-0.011 Hz) were seen in 19/42 slices (Fig.
1B, continuous line in Control). Each ictal discharge was
preceded, and thus appeared to be initiated by a negative-going field
potential (Fig. 1B, Control).
|
We have reported that both interictal-like and ictal-like (thereafter
referred to as interictal and ictal) discharges induced by 4-AP are
insensitive to NMDA receptor antagonists, but are abolished by CNQX
(Avoli et al. 1993, 1996
). We have also
shown that the negative-going potentials are insensitive to excitatory amino acid receptor antagonists, but are blocked by µ-opioid receptor activation, or GABAA receptor antagonists. Hence
we shall refer to these events as synchronous, GABA-mediated potentials.
Effects induced by baclofen on the 4-aminopyridine-induced synchronous activity
Application of the GABAB receptor agonist baclofen (10, 25, and 50 µM) decreased and eventually blocked the 4-AP-induced interictal activity in all experiments (n = 29, Fig. 1, A and B, Baclofen). We analyzed the effects induced by increasing doses of baclofen in 12 slices. With 10 µM baclofen, a decrease in the rate of occurrence of interictal discharges occurred in 5/12 slices, whereas abolishment was seen in the remaining experiments (n = 7). When interictal activity was not fully abolished by 10 µM baclofen, increasing the concentration to 25 µM caused further reduction (n = 3) or disappearance (n = 2). Finally, no interictal discharge was observed with 50 µM baclofen (n = 2). Complete blockade of interictal discharges was also induced by application of a single concentration of baclofen (25 and 50 µM in 9 and 8 slices respectively).
Baclofen made spontaneous ictal activity appear when absent in control (n = 21/29; Fig. 1A) or increased the duration of preexisting ictal discharges. In the latter case the rate of occurrence of the ictal events decreased (n = 8/29; Fig. 1B). The dose-dependent changes induced by baclofen on the rate of occurrence and the duration of the ictal discharges are shown in Fig. 1C. Baclofen also decreased the rate of occurrence of the GABA-mediated potential in a dose-dependent fashion (Fig. 1C). Baclofen effects could be washed out in most experiments.
Pharmacology of the activity recorded in the presence of 4-AP and baclofen
As previously reported with 4-AP only (Avoli et al.
1993, 1996
), CPP did not influence the pattern
of GABA-mediated potential/ictal discharge recorded in baclofen + 4-AP
(n = 8, Fig.
2A). Moreover, baclofen
disclosed ictal activity when slices were pretreated with CPP
(n = 5, Fig. 2B). Ictal discharges were,
however, abolished by further addition of CNQX, a procedure that did
not influence the GABA-mediated potentials (n = 5; Fig.
2C). When the GABAB receptor
antagonist CGP 35348 (1 mM; n = 4) was applied to
slices treated with 4-AP + baclofen, interictal activity reappeared
(Fig. 2B), whereas GABA-mediated potentials occurred at a
higher rate (from 0.0023 ± 0.0004 Hz during 50 µM baclofen to
0.014 ± 0.0026 Hz after adding CGP 35348, n = 4).
CGP 35348 also reduced the duration of the ictal discharges that
occurred more frequently than with baclofen only.
|
Intracellular features of the effects induced by baclofen
The intracellular counterpart of the synchronous activity induced
by 4-AP in the CA3 area of juvenile hippocampal slices has been
described (see Avoli et al. 1993). In agreement with
these earlier experiments, whenever present, ictal discharges
corresponded to prolonged depolarizations with sustained action
potential firing (n = 3 cells, not shown). By contrast,
interictal events and GABA-mediated potentials (which occurred in all
experiments) were mirrored by brief depolarizations with action
potential burst and by long-lasting depolarizations (LLDs) with no or
minimal action potential firing, respectively (n = 8 cells, Fig. 3A, Control).
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Spontaneous, asynchronous synaptic potentials were also recorded from
CA3 pyramids between synchronous events (Avoli et al. 1993; Perreault and Avoli 1991
,
1992
). Recordings with K-acetate-filled electrodes
revealed that these potentials consisted at RMP of hyperpolarizing and
depolarizing events occurring at 5-13 Hz (n = 5; Fig.
3B, Control,
58 mV trace). Membrane hyperpolarization to
values more negative than
70 mV did not appear to influence their
occurrence rate; however, most of them became depolarizing, while the
remaining hyperpolarizing events were of smaller amplitudes as compared
with RMP (Fig. 3B, Control,
75 mV trace).
Baclofen (25 µM) application to 4-AP-containing medium induced a 4.8 ± 1.3 mV, steady hyperpolarization of the RMP and disappearance of interictal activity (n = 8), whereas ictal discharges increased in duration (n = 3; not shown) or appeared whenever absent under control conditions (n = 5; Fig. 3A, Baclofen). During application of 4-AP + baclofen, ictal events were initiated by LLDs with duration that appeared to be longer than in control (not shown, but see Figs. 3A and 5A). In all experiments baclofen reduced the occurrence of, and eventually abolished, the spontaneous asynchronous synaptic potentials; these changes paralleled the disappearance of interictal activity and either the appearance or the potentiation of ictal discharges.
To better establish the effects of baclofen on 4-AP-induced asynchronous synaptic potentials and LLDs, we used excitatory amino acid receptor antagonists while recording CA3 pyramids with electrodes filled with K-acetate (n = 3) or KCl (n = 6). Both hyperpolarizing synaptic potentials and LLDs (that were initiated by a hyperpolarization) were recorded with K-acetate-filled electrodes during 4-AP + CNQX + CPP (Fig. 4A). In these experiments baclofen (25 µM) abolished the asynchronous potentials, but only caused a small reduction of the LLDs' amplitude (Fig. 4A) while decreasing their rate of occurrence (not illustrated).
|
These baclofen effects were also analyzed in six cells with KCl-filled electrodes during application of 4-AP and excitatory amino acid receptor antagonists. As shown in Fig. 4B, asynchronous, presumptive GABAA-mediated depolarizing potentials and LLDs occurred in medium containing 4-AP + CNQX + CPP. In all cases baclofen induced a steady membrane hyperpolarization (6.0 ± 2.1 mV, n = 5) and markedly reduced the amplitude and the rate of occurrence of the asynchronous events. By contrast, it only exerted a nonsignificant change in LLD amplitude while the rate of occurrence of these potentials decreased (Fig. 4, B and C).
[K+]o and baclofen effects
[K+]o
influences neuron excitability and modulates seizure activity
(McBain et al. 1993; Traynelis and Dingledine
1988
). In addition GABAA-mediated
[K+]o increases contribute to ictal discharge
initiation in the 4-AP model (Avoli et al. 1996
).
GABAB receptor activation induces an increase in
K+ conductance (Dutar and Nicoll 1988
;
Gähwiler and Brown 1985
; Newberry and
Nicoll 1984
). We therefore used [K+]o
recordings to establish whether baclofen (25 µM) effects were accompanied by changes in [K+]o baseline
and/or in the transient elevations associated with the GABA-mediated
potential/ictal discharge (Avoli et al. 1996
). [K+]o baseline was unchanged with baclofen
(n = 5 slices). In addition, the initial component
of the transient increases in [K+]o
corresponding to the GABA-mediated potentials had similar peak values
in control and during baclofen (Fig. 5,
A and B). However, the increased duration
of the ictal discharge induced by baclofen was mirrored by
[K+]o elevations that were more prolonged
than in control (arrows in Fig. 5A).
|
We also established whether baclofen modifies the transient
[K+]o increases associated with the
GABA-mediated potentials recorded during application of 4-AP + CPP + CNQX (n = 5 slices). Transient [K+]o elevations occurred in association with
the GABA-mediated potentials recorded during blockade of ionotropic
excitatory amino acid receptors (Fig.
6A) (Avoli et al.
1996). These elevations in [K+]o
decreased in amplitude by 15-25% during application of 25 µM baclofen, an effect that was reversed by baclofen wash out (Fig. 6A). The results obtained in the course of these
experiments are summarized in Fig. 6B.
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Interictal-like stimulation during application of baclofen
Ictal discharges disappear during low-frequency stimulation that
elicits interictal epileptiform responses (Barbarosie and Avoli
1997; Bragdon et al. 1992
; Swartzwelder
et al. 1987
). Hence we tested whether stimulation of the
dentate hilus at 0.2-0.8 Hz for period >15 min could abolish the
ictal activity generated by CA3 pyramidal cells during application of
4-AP+ baclofen (25 µM).
As illustrated in Fig. 7A, ictal events recorded during application of 4-AP + baclofen were reduced, or abolished during this type of electrical stimulation in all experiments (n = 8). This effect was also characterized by reduction (2/8) or disappearance (6/8) of the GABA-mediated synchronous potentials. Ictal discharges reappeared at intervals similar to those seen in control conditions (i.e., 4-AP + baclofen) on termination of electrical stimuli (Fig. 7B). Moreover, the inhibitory effect exerted by repetitive stimuli delivered in the dentate hilus on ictal discharge occurrence was reproduced by successive periods of stimulation in the same experiment.
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DISCUSSION |
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As reported in adult brain (Lewis et al. 1989;
Mott et al. 1989
; Swartzwelder et al.
1987
; Watts and Jefferys 1993
), activation of
GABAB receptors by baclofen exerts a
proconvulsant action in the CA3 area of juvenile rat hippocampus that
is characterized by the appearance or potentiation of 4-AP-induced
ictal discharges. In this study we have sought evidence for the
mechanisms underlying this phenomenon and obtained data indicating that
such an action 1) results from the activation of
GABAB receptors leading to a reduction of
spontaneous excitatory and inhibitory synaptic potentials along with
the disappearance of interictal discharges; 2) is associated with the persistence of synchronous GABA-mediated potentials; 3) is characterized by excitatory amino acid receptor
pharmacology for the ictal activity that is similar to what seen with
4-AP only; and 4) is not accompanied by measurable changes
in baseline or transient
[K+]o increases. We have
also shown that the ictal activity recorded in 4-AP + baclofen is
interrupted by low-frequency repetitive electrical stimuli, a procedure
that induces interictal-like discharges.
Baclofen effects on 4-AP-induced, spontaneous activity
In our study most baclofen effects were antagonized by CGP
35348, thus indicating that they were mainly caused by the activation of GABAB receptors. In particular, we propose
that baclofen abolishes 4-AP-induced asynchronous, action
potential-dependent synaptic events and interictal activity by
decreasing the release of transmitter from excitatory and inhibitory
terminals. This may be caused by activation of presynaptic
GABAB receptors inhibiting both GABA and excitatory
transmitter release (Lambert and Wilson 1993;
Lanthorn and Cotman 1981
; Thompson and
Gähwiler 1992
), and by a postsynaptic GABAB-mediated hyperpolarization that decreases
excitability of principal cells (Newberry and Nicoll 1984
,
1985
) and interneurons (Misgeld et al. 1989
;
Williams and Lacaille 1992
). Indeed, we could document
this hyperpolarizing action of baclofen in all CA3 pyramidal cells that
were recorded during continuous application of 4-AP.
It is unclear why the synchronous GABA-mediated potentials
continued to occur (although at a reduced rate) during application of
baclofen concentrations as high as 50 µM. Baclofen may
depress excitatory transmitter release to a greater extent
than GABA release (Pierau and Zimmermann 1973;
Potashner 1978
). We are, however, inclined to
exclude that GABAB receptors on
inhibitory interneuron terminals may have lower affinity for GABA (and
baclofen) as compared with those located on glutamatergic neurons,
because both inhibitory and excitatory asynchronous potentials recorded
from CA3 pyramidal cells were decreased by baclofen to a similar
extent. Such a difference in sensitivity is also at odds with the
proconvulsant action of baclofen because ictal discharges are
excitatory amino acid-mediated synchronous events. Indeed, the
resistance of 4-AP-induced GABA-mediated synchronous potentials to
baclofen may reflect the intense discharge of action potential that
occurs in interneurons during this type of synchronous activity
(Benardo 1997
). This firing, once reaching the
terminals, may overcome the inhibition of transmitter release exerted
by baclofen. Moreover, this phenomenon may be facilitated by the
concomitant increase in
[K+]o seen during the
GABA-mediated synchronous potential.
Proconvulsant action of baclofen in the juvenile rat hippocampus
A proconvulsant action of baclofen has been described in several
in vitro models of epileptiform discharge in the adult hippocampus (Lewis et al. 1989; Mott et al. 1989
;
Swartzwelder et al. 1987
; Watts and Jefferys
1993
). In particular, Watts and Jefferys (1993)
have reported that baclofen abolishes interictal activity and discloses
ictal discharges in adult rat hippocampal slices treated with 4-AP. The
similarities between our results and their findings indicate the
existence of mature GABAB-mediated mechanisms of synaptic transmission in the juvenile rat hippocampus. A proconvulsant effect of baclofen has been shown in a patient treated for spasticity who did not have any previous history of seizures (Rush and
Gibberd 1990
) and in three patients who had previously suffered
traumatic brain injury (Kofler et al. 1994
).
We have also shown that the ictal activity generated by CA3 neurons in
the presence of 4-AP + baclofen is insensitive to the NMDA receptor
antagonist CPP, but is abolished by CNQX. These features are similar to
those reported for ictal discharges in medium containing 4-AP only
(Avoli et al. 1993, 1996
). Hence
baclofen's proconvulsant action does not result from a novel
excitatory amino acid-mediated mechanism disclosed by this
GABAB-receptor agonist. In addition, the effects
of baclofen did occur during continuous CPP application, which rules
out the involvement of a GABAB
receptor-mediated/NMDA-dependent mechanism in ictal discharge
induction. Such a mechanism facilitates the occurrence of
tetanus-induced long-term potentiation in the adult hippocampus
(Davies et al. 1991
).
GABAB receptor activation induces an increase in
K+ conductance (Gähwiler and Brown
1985; Newberry and Nicoll 1984
,
1985
). It is also well-established that
[K+]o modulates seizure
activity (McBain et al. 1993
; Traynelis and Dingledine 1988
). Hence the proconvulsant action of baclofen
may have resulted from changes in
[K+]o homeostasis, and
even more so because GABA-mediated
[K+]o elevations initiate
ictal discharges in the 4-AP model (Avoli et al. 1996
).
However, our [K+]o
recordings indicate that the action of baclofen is not accompanied by
any measurable change in either
[K+]o baseline or
[K+]o transient
elevations. Accordingly, the increases in
[K+]o occurring during
the GABA-mediated synchronous potential recorded in the presence of
baclofen were not larger than those seen in medium containing 4-AP
only. Moreover, only a small, although significant decrease was
appreciated when comparing the peak
[K+]o values under
control and during baclofen in slices where excitatory transmission was abolished.
It has been proposed that the proconvulsant action of baclofen is
caused by disinhibition due to a decrease in GABA release (Mott
et al. 1989; Watts and Jefferys 1993
). In
agreement with these studies, we have observed that baclofen causes a
marked decrease of asynchronous GABAA-mediated
events recorded from CA3 pyramids. However, such a disinhibitory action
must be accompanied by the ability of excitatory terminals to release
transmitter to be expressed as a proconvulsant effect. Hence, as
proposed for the synchronous GABA-mediated potentials, we are inclined to conclude that ictal activity during activation of
GABAB receptors reflects the inability of this
presynaptic mechanism to inhibit transmitter release consequent to
intense action potential firing, which in this case does occur at the
excitatory terminals of CA3 pyramidal cells.
An important mechanism underlying the proconvulsant action of baclofen
relates to activity-dependent changes in CA3 area network excitability
that occur during GABAB receptor activation. This conclusion is supported by the findings obtained with electrical stimulation of the dentate hilus, a procedure that leads to the occurrence of interictal-like responses and the concomitant suppression of the ictal activity recorded during application of 4-AP + baclofen. It is conceivable that an accumulation of glutamate-containing vesicles
docking at presynaptic terminals results from the block of interictal
activity and asynchronous excitatory synaptic potentials caused by
baclofen. This should lead to an increased availability of excitatory
transmitter. As a consequence the synchronous GABA-mediated potentials
that are generated in the presence of baclofen initiate ictal
discharges that are more robust than those recorded under control
conditions or induce ictal events ex novo. Presynaptic factors
controlling glutamate release have been proposed to regulate the
probability and duration of synchronous discharges generated by the CA3
network (Staley et al. 1998).
In clinical practice, interictal events are used to localize the brain
area from where ictal discharges originate. However, the temporal
relation between interictal and ictal discharges remains unclear. It
should be emphasized that the control exerted by interictal activity on
ictal events has been shown in previous studies including those
performed in combined hippocampus-entorhinal cortex slices treated with
either 4-AP or low Mg2+ (Barbarosie and
Avoli 1997; Bragdon et al. 1992
;
Swartzwelder et al. 1987
). In these cases as well,
activity-dependent changes in network excitability, leading to
increased availability of excitatory transmitter, may play a role in
the unexpected control exerted by interictal activity over ictal discharges.
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ACKNOWLEDGMENTS |
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We thank Dr. K. Krnjevic for constructive criticism on an early draft of this paper.
This study was supported by Medical Research Council of Canada Grant MT-8109, the Quebec Heart and Stroke Foundation, and the Savoy Foundation.
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FOOTNOTES |
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Address for reprint requests: M. Avoli, 3801 University St., Montreal, Quebec H3A 2B4, 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 8 January 1999; accepted in final form 12 April 1999.
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REFERENCES |
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