 |
Introduction |
Generalized
absence seizures occur primarily in children and are manifested as
rhythmic, bilaterally synchronous 3 Hz spike-wave discharges in the EEG
associated with sudden unresponsiveness, eye blinking and myoclonic
jerks (Mirsky et al., 1986
). Absence seizures occur as
synchronized thalamocortical oscillations, and evolve most readily from
thalamic relay nuclei (such as ventrobasal nucleus) and the neocortex
(Gloor et al., 1990). Several lines of evidence indicate
that both GABAA and GABAB receptor-mediated inhibition play a role in the pathogenesis of absence seizures. For
example, thalamic relay neurons have the ability to generate GABAB receptor-mediated low threshold calcium spikes
(Crunelli and Leresche, 1991). Such calcium spikes have been found to
be responsible for the generation of absence epilepsy as well as for
physiologically normal spindles waves (McCormick, 1992). A decrease in
GABAA receptormediated inhibition in thalamic relay neurons during spindling has been shown to transform the spindle oscillations to rhythms resembling absence seizures (von Krosigk et al., 1993). A similar phenomenon occurs in the feline
penicillin model of absence epilepsy where penicillin, a weak
GABAA receptor antagonist, transforms spindles to absence
seizures (McLachlan et al., 1984). Also, weak antagonism of
GABAA receptors by low systemic doses of bicuculline or
pentylenetetrazole produces absence-like seizures in rats (Marescaux
et al., 1984; Zouhar et al., 1989).
The GABAA receptor is a macromolecular protein that forms a
chloride channel. GABAA receptors are regulated by many
positive and negative allosteric modulators, including GABA,
barbiturates, benzodiazepines and zinc (Macdonald and Olsen, 1994).
Also, certain naturally occurring steroid metabolites (neurosteroids)
and their synthetic analogs allosterically modulate GABAA
receptor binding (Majewska et al., 1986; Sapp et
al., 1992) and function (Lambert et al., 1995). In this
way, the neurosteroids may function as endogenous modulators of
GABAA receptors. Also, there is evidence that certain
neurosteroids possess potent anticonvulsant activity (Kokate et
al., 1994).
Epileptic seizures may cause plastic changes in GABAA
receptors. For example, the sensitivity of GABAA receptors
to GABA and to allosteric modulators have been shown to alter gradually
during the development of chronic epilepsy (Paul and Skolnick, 1978). Such changes have been postulated to contribute to epileptogenesis (Rabow et al., 1995). In addition to this gradually
developing plasticity, epileptic seizures also have been shown to cause
more rapid functional changes in GABAA receptors (Kapur and
Macdonald, 1997). In our study, to study potential absence
seizure-induced changes in GABAA receptors in thalamus, we
examined the modulatory effects of GABA, pentobarbital and two
neuroactive steroids, alphaxalone and THDOC (a deoxycorticosterone
metabolite), on [35S]TBPS binding in the thalamic relay
nuclei during the course of GHB-induced absence seizures in rats. GHB
is a naturally occurring metabolite of GABA that induces generalized
absence-like seizures in rats and in a number of other animal species
(Doherty et al., 1978; Snead, 1988). TBPS is a cage
convulsant, a class of compounds shown to inhibit competitively
picrotoxin binding (Ticku and Olsen, 1979). The measurement of specific
[35S]TBPS binding is used as a marker for the
GABA-associated chloride ionophore and as a pharmacological tool to
study allosteric interactions between various modulatory sites in the
GABAA receptor complex and the chloride channel (Squires
et al., 1983).
| |
Methods |
Materials.
Radioactive [35S]TBPS (70-100
Ci/mmol) was purchased from Du Pont-New England Nuclear (Boston, MA).
GBL, alphaxalone and THDOC were purchased from Sigma Chemical Co. (St.
Louis, MO). The specific GABAB receptor agonist,
(
)-baclofen and the GABAB receptor antagonist, CGP 35348 (Olpe and Karlsson, 1990; Karlsson et al., 1992) were a gift
from Dr. R. Bernasconi. All other chemicals were obtained from
standard commercial sources and were of highest available purity.
Surgery and EEG recordings of absence seizures induced by
GHB.
Adult male Sprague Dawley rats (200-300 g) were used in all
experiments. Animals were maintained on a 12-hr light/dark cycle and
given free access to food and water. Monopolar EEG recording electrodes
were surgically implanted on the surface of frontoparietal cortex under
halothane anesthesia. The tips of these electrodes were aimed at
frontal and parietal cortices bilaterally. Seven days after surgery,
EEG recordings were made continuously with the animals freely moving in
a heated shielded Plexiglas container. GHB-seizure was induced by i.p.
administration of 
-butyrolactone (GBL, 100 mg/kg), the biologically
inactive prodrug of GHB (Snead, 1991). Control animals were implanted
with epidural electrodes and had EEG recorded but received a comparable
volume of saline rather than GBL.
[35S]TBPS autoradiography.
Animals were
sacrificed at various time intervals starting at 15 min, 30 min, 1 hr,
2 hr, 3 hr, 4 hr and 6 hr after the onset of GHB-seizures. Brains were
excised and chilled in isopentane (40°C) for 1 min and brought to
20°C for cryosectioning. Coronal sections (25-µm thick) were
thaw-mounted on gelatin-coated slides and processed for binding assay.
[35S]TBPS binding was carried out according to the method
described earlier (Edgar and Schwartz, 1990). Tissue sections were
preincubated in 50 mM
K2HPO4/NaH2PO4 (pH 7.4)
buffer containing 200 mM NaCl and 1 mM EDTA for 10 min at room
temperature. The preincubation was followed by a 3-hr incubation in the
same buffer (without EDTA) containing 2 nM [35S]TBPS at
room temperature either in the presence or absence of alphaxalone/THDOC
(10 nM-100 µM), GABA or pentobarbital (1-100 µM) or other drugs.
Incubation was terminated with two 15-min washes in the same buffer and
a dip in deionized water (at room temperature). Nonspecific binding was
estimated in adjacent tissue sections in the presence of 100 µM
picrotoxin. After the binding experiments, the sections were dried and
apposed to x-ray films along with 14C standard radioactive
scales (Amersham, IL) for 5 to 6 days at room temperature.
CGP 35348 and ()-baclofen treatments.
To study the effect
of GHB-seizures on allosteric modulation of [35S]TBPS
binding by steroids, the duration of GHB-seizures was altered using the
GABAB receptor antagonist, CGP 35348 or the
GABAB receptor agonist, ()-baclofen. CGP 35348 (400 mg/kg) or ()-baclofen (5 mg/kg) was given i.p. 30 or 60 min,
respectively, before GBL administration. After GBL administration, the
duration of absence seizure was quantitated as described below. In the
control group of experiments, saline (instead of GBL) was given 30 or
60 min after CGP 35348 or ()-baclofen, respectively. CGP
35348-pretreated rats were killed 30 and 60 min after GBL or saline
treatment, although ()-baclofen-pretreated rats were sacrificed 2, 4 and 6 hr after GBL or saline treatment for [35S]TBPS assay.
Data analysis.
GHB-induced absence seizures were quantitated
as described earlier (Banerjee and Snead, 1994). Cumulative duration of
spike wave seizures (in sec) during each 20 min epoch after GBL
administration was scored. The autoradiograms were quantitated using a
microcomputer-based densitometer system (Imaging Research Inc.,
Ontario, Canada). Briefly, a standard curve between the ROD of
14C standards and their tissue radioactivity equivalents
(pmol/mg of tissue) was constructed using nonlinear regression
analysis. The average ROD values of the selected brain regions were in
the linear portion of this standard curve. The pmol/mg value in each brain region was calculated by interpolation using the image analyzer. Five to seven readings per region from both the hemispheres were averaged in triplicate sections from four to six different animals.
Statistics.
All data comparing the effect of steroids, GABA
and pentobarbital on [35S]TBPS binding at different
stages of GHB-seizures were analyzed by one-way analysis of variance
followed by Bonferroni's or Neuman-Keul's test for post
hoc comparisons between multiple group means. Two individual group
means were compared using a two tailed, independent Student's
t test.
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Results |
Modulation of [35S]TBPS binding in thalamus by
neuroactive steroids, GABA and pentobarbital.
Neuroactive steroids
enhance the function of GABAA receptors (Lambert et
al., 1995), allosterically enhance the binding of GABA and
benzodiazepine receptor site agonists and inhibit
[35S]TBPS binding in rat brain (Majewska et
al., 1986). However, more detailed concentration-response studies
have suggested that at submicromolar concentrations certain steroids
enhance [35S]TBPS binding in some brain areas, including
thalamus and deep layers of frontal cortex in rats (Sapp et
al., 1992). In our study, we also observed a biphasic effect of
steroids on [35S]TBPS binding in thalamus.
In control (GHB-naive) brain sections, alphaxalone and THDOC (10 nM-1
µM) increased [35S]TBPS binding by 30 to 35% (P < .05 at 0.1-1 µM concentrations) in thalamic ventrobasal nucleus.
However, at higher concentrations (10-100 µM),
[35S]TBPS binding was significantly decreased (50-60%;
P < .01) by both steroids (figs. 1,
2 A and B). A similar biphasic effect of
these steroids was also observed in other thalamic nuclei. Both GABA
and pentobarbital (1-100 µM) dose-dependently decreased [35S]TBPS binding in the ventrobasal nucleus and other
thalamic nuclei (fig. 3).
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Fig. 1.
Concentration-dependent biphasic effect of
neuroactive steroids, alphaxalone and THDOC, on [35S]TBPS
binding in GHB-naive thalamic ventrobasal nucleus. Values are mean ± S.E.M. from five rats. 0, Specific [35S]TBPS binding
in the absence of steroid.
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Fig. 2.
Effect of alphaxalone (1 µM) on
[35S]TBPS binding in control and GHB-treated brain
sections. A and B are the autoradiograms of two adjacent brain sections
from a control (GHB-naive) rat labeled with [35S]TBPS in
the absence (A) or presence of 1 µM alphaxalone (B). C and D are two
consecutive sections from a rat sacrificed 1 h after the onset of
GHB-induced absence seizures, and labeled with [35S]TBPS
in the absence (C) or presence of 1 µM alphaxalone (D). Note in D
that alphaxalone-mediated increase in [35S]TBPS binding
(during GHB-seizures) is decreased or lost only in thalamic relay
nuclei (i.e., ventrobasal nucleus, VB and central lateral
nucleus, CL) but not elsewhere in thalamus. CTX, cortex and HP,
hippocampus. R.O.D., relative optical density.
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Fig. 3.
Concentration-dependent inhibition of
[35S]TBPS binding in GHB-naive thalamic ventrobasal
nucleus by GABA and pentobarbital. 0, [35S]TBPS binding
in the absence of GABA or pentobarbital. Values are mean ± S.E.M.
from four rats.
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Absence-like seizures induced by GHB.
After systemic GBL (the
prodrug of GHB) administration, bursts of bilaterally synchronous 4 to
6 Hz spike-wave discharges appeared in the EEG as recorded from the
surface of the frontoparietal cortex. These EEG changes occurred within
5 to 10 min of GBL administration and lasted for more than 2 hr. About
3 hr after GBL administration, the animals appeared normal both
electrographically and behaviorally (fig.
4). The half life of GHB in the rat brain
after GBL administration has been shown to be approximately 15 min
(Shumate and Snead, 1977).
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Fig. 4.
EEG recordings of GHB-induced bilaterally synchronous
spike-wave discharges from a rat frontoparietal cortex. A shows the
background EEG activity. Within 10 min after systemic administration of
GBL (100 mg/kg; i.p.) high voltage, 5 to 6 Hz spike-wave complexes
evolved from cortex (B). C and D denote continuous spike-wave
discharges 30 min and 1 hr, respectively, after the onset of
GHB-seizures. Occasional low voltage spike-wave bursts were seen 2 hr
after the onset of seizures (E). F shows normalization of EEG 3 hr
after the onset of seizures. LFr-Pr and RFr-Pr,
left and right frontoparietal leads, respectively. Calibration: 150 µV, 2 sec.
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Neuroactive steroids failed to modulate [35S]TBPS
binding in thalamic relay nuclei during the course of GHB-induced
absence seizures.
We did not observe a noticeable change in the
basal [35S]TBPS binding in the thalamus during the entire
course of GHB-seizures (see fig. 5).
However, alphaxalone and THDOC (in low doses, 10-1000 nM) failed to
increase [35S]TBPS binding in the thalamic relay nuclei
during the entire course of GHB-seizures, i.e., between 0.5 to 2 hr after the onset of seizures (figs. 2, 5,
6, A and B; P < .05 at 100 nM and 1 µM concentrations). This loss of steroid effect on binding did not occur at the onset of GHB-seizures or about 15 min after the onset of
GHB-seizures, rather it becomes apparent only 30 min after the onset of
GHB-seizures (fig. 5). As the GHB-seizures resolved and EEG and
behavior normalized, i.e., about 3 hr after the onset of
seizures, the effect of the neuroactive steroids on
[35S]TBPS binding was also restored (fig. 5). At higher
concentrations of steroids (10-100 µM) the neurosteroid effect on
[35S]TBPS binding was not lost, steroids at higher
concentrations continued to modulate [35S]TBPS binding in
thalamic relay nuclei during the entire course of GHB-seizures (fig. 6,
A and B).
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Fig. 5.
Effect of GHB-seizures on GABA and alphaxalone (FAX)
modulation of [35S]TBPS binding in thalamic ventrobasal
nucleus. Values are mean ± S.E.M. from six rats. Although GABA (1 µM) continues to inhibit binding during the entire course of
GHB-seizures (P < .01 at all stages of GHB-seizures, one-way
analysis of variance followed by Bonferroni's test), the effect of
alphaxalone (1 µM) is markedly decreased during the course of
GHB-seizures (depicted by the arrows). C, Specific
[35S]TBPS binding in GHB-naive ventrobasal nucleus, ON,
onset of GHB-seizures.
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Fig. 6.
Loss of alphaxalone (A) and THDOC (B) modulation of
[35S]TBPS binding in thalamic ventrobasal nucleus 1 hr
after the onset of GHB-induced absence seizures. Values are mean ± S.E.M. from six rats. Note that the modulation of
[35S]TBPS binding by either steroid is decreased at lower
concentrations only (10-1000 nM). At higher concentrations,
steroid-modulation of binding is not significantly altered. *P < .05 when compared against respective control values (one-way analysis
of variance followed by Bonferroni's test). 0, Specific
[35S]TBPS binding in the absence of steroid.
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The observed loss of neurosteroid modulation of [35S]TBPS
binding in the thalamus was region-specific, being observed only in those thalamic areas that become involved in the generation of GHB-seizures. For example, the midline thalamus, an area from which
GHB-induced seizures evolve rarely (Banerjee et al., 1993), showed no apparent loss of steroid modulation of
[35S]TBPS binding during GHB-seizures (see fig. 2). We
also determined the effect of GABA and pentobarbital (1-100 µM) on
binding during GHB-seizures, and found that neither GABA nor
pentobarbital modulation of [35S]TBPS binding in the
ventrobasal nucleus was altered during the entire course of
GHB-seizures (see fig. 5).
To rule out the possibility that in the present assay conditions, GHB
interacted with [35S]TBPS binding and thereby reduced
steroid-modulation of binding, we performed [35S]TBPS
binding in the presence of either steroid and 200 µM GHB (a
concentration of GHB that is known to occur in the thalamus after an
i.p. injection of 100 mg/kg of GBL) (Snead, 1991). GHB did not
significantly alter alphaxalone or THDOC dose-response curves in the
[35S]TBPS assay (table 1).
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TABLE 1
Effect of GHB on THDOC and alphaxalone-mediated increase in
[35S]TBPS binding in thalamic ventrobasal nucleus
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Allosteric uncoupling of steroid and other modulatory sites within the
GABAA receptor complex has been reptorted to occur after
chronic administration of steroids in primary cultured neurons (Yu and
Ticku, 1995). Although there is no evidence if the endogenous neurosteroid levels in thalamus are altered after absence seizures, we
tested if the phenomenon of uncoupling was responsible for the observed
loss of steroid effect in our study. Preincubation of brain sections
with either alphaxalone or THDOC in vitro (10-1000 nM) for
30 to 60 min did not significantly alter the dose response of either
steroid in [35S]TBPS binding assay (data not shown).
Based on the above results it appeared that the selective loss in the
ability of neuroactive steroids to modulate [35S]TBPS
binding in specific thalamic nuclei during GHB-seizures may have
resulted due to absence seizure-induced alterations in GABAA receptors. One way to test this hypothesis was to
determine if absence seizures were critical for the development of this loss of steroid effect on binding. For this, we determined the effect
of steroids on [35S]TBPS binding in thalami of rats where
GHB-seizures had been completely blocked.
Steroid-modulation of [35S]TBPS binding in thalamic
relay nuclei was preserved when GHB-induced absence seizure was
blocked.
CGP 35348, a GABAB receptor antagonist, is
known to block absence seizures (Snead, 1992). In our study,
pretreating the rats with CGP 35348 (400 mg/kg; i.p.; 30 min before the
administration of GBL) completely antagonized the development of
GHB-seizures. We sacrificed CGP 35348-pretreated rats 30 and 60 min
after the administration of GBL or saline (control) and found that the
steroid modulation of [35S]TBPS binding was preserved in
either of the above time points in both GBL and control groups (fig.
7). Also, CGP 35348 (1 µM-1 mM)
in vitro did not significantly alter alphaxalone or THDOC dose-response in the [35S]TBPS assay.
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Fig. 7.
Alphaxalone modulation of [35S]TBPS
binding in thalamic ventrobasal nucleus was preserved when GHB-seizure
was blocked by CGP 35348. Values are mean ± S.E.M. from four
rats. CGP 35348- (400 mg/kg;i.p) treated rats were killed 1 hr after
either saline or GBL administration. 0, Specific
[35S]TBPS binding in the absence of alphaxalone.
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Because the loss of steroid modulation of [35S]TBPS
binding developed after GHB-seizures and persisted till the seizures
abated, we also determined if this loss of steroid effect on binding
was related to the duration of absence seizures induced by GHB. For this, we measured the modulation of [35S]TBPS binding (by
either steroid) in thalami of rats where the duration of GHB-induced
absence seizures had been prolonged.
Loss of steroid modulation of [35S]TBPS binding in
thalamic relay nuclei persisted when the duration of GHB-seizures was
prolonged.
()-Baclofen, a GABAB receptor agonist,
has been reported to significantly prolong the duration of GHB-induced
absence seizures (Snead, 1992). Pretreating the animals with 5 mg/kg
()-baclofen significantly increased the duration of GHB-seizures
(fig. 8A; P < .001 at 2 and 3 hr).
In ()-baclofen-pretreated rats, GHB-seizures continued for more than
5 hr as compared to only about 2.5 hr in nonbaclofen treated rats.
Steroid modulation of [35S]TBPS binding was determined in
thalami of baclofen-treated rats that were killed 2, 4 and 6 hr after
the onset of first GHB-seizures. Either steroid (at concentrations
ranging 10-1000 nM) failed to increase [35S]TBPS binding
in thalamic ventrobasal nucleus at 2- and 4-hr time points (fig. 8B;
P < .05 at 100 nM and 1 µM). The effect of steroids on
[35S]TBPS binding was restored only after the seizures
had dissipated (i.e., at 6 hr time point). In the control
group of rats that received saline 60 min after ()-baclofen, the
steroid's effect on [35S]TBPS binding in thalamic relay
nuclei was not lost in any of the above time points (see fig. 8B).
Also, to rule out the possibility that ()-baclofen interacted with
steroids in the [35S]TBPS binding assay under our
experimental conditions, we performed [35S]TBPS binding
in the presence of both steroids plus () baclofen (1-1000 µM) and
found that ()-baclofen had no significant effect on the dose response
of either steroid (data not shown).
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Fig. 8.
()-Baclofen induced prolongation in the duration of
GHB-induced spike-wave discharges (SWD; A). B shows persistent loss of
alphaxalone effect on [35S]TBPS binding in thalamic
ventrobasal nucleus 2 and 4 hr after the onset of GHB-seizures in
()-baclofen-pretreated rats. Values are mean ± S.E.M. from four
rats. In the control group, ()-baclofen-treated rats were killed 2 hr
after saline administration. No spike-wave discharges occurred in the
control group of rats. In B, 0 depicts specific [35S]TBPS
binding in the absence of alphaxalone. *P < .05 when compared
against respective control (BAC + SAL) values (one-way analysis of
variance followed by Bonferroni's test).
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Discussion |
Our binding results show a significant decrease in the ability of
neuroactive steroids to influence [35S]TBPS binding in
the thalamic relay nuclei during the course of GHB-induced absence
seizures. This loss of steroid effect occurred only in those thalamic
nuclei that are involved in the genesis of GHB-seizures. For example,
the midline thalamic area which is not involved in the generation of
GHB-seizures (Banerjee et al., 1993) showed no significant
decrease in steroid modulation of [35S]TBPS binding, nor
were changes observed in hippocampus, a structure not involved in
absence seizures (Banerjee et al., 1993). Also, the observed
loss of steroid-modulation of [35S]TBPS binding in
thalamus was selective to steroids only, as GABA and pentobarbital
modulation of binding in the relay nuclei was preserved during the
entire course of GHB-seizures. Further, in our study we did not observe
any significant change in the basal [35S]TBPS binding in
thalamic relay nuclei during the entire course of GHB-seizures.
It appears from our data that the loss of steroid effect on
[35S]TBPS binding was not causally related to the
generation of GHB-induced absence seizures. The decrease in steroid
modulation of [35S]TBPS binding was not observed at the
onset of GHB-seizures, rather it developed 30 min after the onset of
GHB-seizures. Because the inclusion of GHB (200 µM, a concentration
that is attained in the thalamus at the onset of GHB-induced absence
seizures) (Snead, 1991) in vitro did not modify the
dose-response curve of steroids in the binding assay, it was unlikely
that the observed decrease in neurosteroid modulation of
[35S]TBPS binding in the thalamus (during GHB-seizures)
was due to a direct effect of GHB. GHB is metabolically derived from
GABA and is structurally similar to that compound (Doherty et
al., 1978); however, it has been shown that GHB fails to compete
for GABAA sites, nor do GABAA receptor agonists
or antagonists compete for binding at the GHB site (Snead et
al., 1992; Snead and Liu, 1993).
The mechanism for the observed decrease in neurosteroid effect on
[35S]TBPS binding after absence seizures is not clear;
however, a rapid, absence seizure-induced uncoupling of steroid and
TBPS binding sites in the GABAA receptor complex seems
likely. Allosteric uncoupling between steroid, benzodiazepine, GABA and
barbiturate sites may occur after chronic exposure to neurosteroids or
GABA in cultured neurons (Roca et al., 1990; Yu and Ticku,
1995), suggesting that ligands which modulate GABAergic transmission
may differentially trigger intracellular plasticity events that result
in altered GABAA receptor binding, coupling and/or
function. In our study, the time course of the observed loss of steroid
effect on binding was short. The decrease in steroid's ability to
modulate [35S]TBPS binding in thalamus developed within
30 min after the onset of GHB-seizures. Because there is no evidence
whether the levels of endogenous neurosteroids in thalamus alter after
absence seizures, we tested the possibility if GHB-seizures increased
the endogenous levels of steroids in thalamus and thereby caused a
rapid uncoupling of steroid and TBPS binding sites. We found that
preincubation of the section in the presence of either steroid
(10-1000 nM) in vitro for 30 to 60 min did not change the
dose-response of steroids.
The sensitivity of GABAA receptors to GABA and to
allosteric modulators have been reported to alter gradually during the
development of epileptic seizures (Rabow et al., 1995). For
example, benzodiazepines that allosterically enhance GABAA
receptor function (Macdonald and Olsen, 1994), fail to increase
chloride current in rat hippocampus during status epileticus (Kapur and
Macdonald, 1997). This functional plasticity of GABAA
receptors develops within 40 min of continuous seizures, and has been
suggested to play a role in the benzodiazepine tolerance during status
epilepticus (Kapur and Macdonald, 1997). In our study, it is possible
that the observed loss of steroid effect on [35S]TBPS
binding in thalamus may have resulted due to absence seizure-induced alterations in GABAA receptors. To test this hypothesis we
determined whether absence seizure was critical for the development of
this loss of steroid effect. Because the ability of either alphaxalone or THDOC to increase [35S]TBPS binding in thalamic relay
nuclei was preserved when GHB-seizure was completely blocked, it would
appear that the observed loss of steroid effect on binding was caused
by absence seizures. It is not known at this time if absence
seizure-induced loss in the ability of steroids to enhance
[35S]TBPS binding has any functional relevance. Studies
involving GABAA receptor-mediated chloride uptake and its
modulation by steroids during GHB-seizures are under way in our laboratory.
We wanted to determine if the observed absence seizure-induced
selective loss of steroid effect on [35S]TBPS binding was
related to the duration of absence seizures. Upon prolonging the
duration of GHB-seizure by pretreating the animals with ()-baclofen,
the loss of steroid effect on [35S]TBPS binding in
thalamus persisted until the animals become seizure-free (about 6 hr
after the seizure-onset). Only after the seizures resolved and the
animals become electrographically and behaviorally normal, was the
steroid modulation of [35S]TBPS binding in thalamus
restored. These data suggested that the duration of GHB-seizures and
the loss of the ability of steroids to influence
[35S]TBPS binding during GHB-seizures are related.
Thus, we conclude that GHB-induced generalized absence seizures result
in a rapid loss of steroid effect on [35S]TBPS binding to
GABAA receptors in thalamic relay nuclei. Our data also
suggest that absence seizure-induced rapid loss of steroid effect on
GABAA receptors may play a role in the maintenance of absence seizures.
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Abstract
Introduction
