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Vol. 295, Issue 3, 1258-1266, December 2000
-Aminobutyric AcidA
Receptor Responsiveness in Genetic Models of Seizure Susceptibility
with Different Expression Patterns1
Department of Pharmacology and Toxicology, Robert C. Byrd Health Sciences Center, West Virginia University School of Medicine, Morgantown, West Virginia
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Abstract |
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 Top
 Abstract
 Introduction
Materials and Methods
Results
Discussion
References
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The genetically epilepsy-prone rat (GEPR) is a unique animal model of seizure predisposition with substrains (i.e., GEPR-NE, GEPR-3, and GEPR-9) that exhibit different seizure patterns in response to the same stimulus. Among many deficits identified in these animals, reduced responses to GABAA receptor agonists have been described in several brain regions of the GEPR-9. However, few studies have quantitatively analyzed this difference in responsiveness or have examined and compared the responsiveness of GEPR-3 neurons with the other strains. Using intracellular recording, we determined and compared the responsiveness of Purkinje neurons from GEPR-3 animals with those of control (both Sprague-Dawley and GEPR-NE) and GEPR-9 rats at different developmental ages. In GEPR-9 animals, the EC50 value for GABA and muscimol was shifted 3-fold to the right, with no reduction in maximum. In contrast, GEPR-3 animals showed a significant reduction in the maximum hyperpolarizing response to only GABA and muscimol with no change in the EC50 values. Responsiveness to glutamate, aspartate, norepinephrine, and diazepam was unchanged in both strains, indicating that the change in responsiveness was highly selective for GABAA receptor agonists. Changes in responsiveness in animals <15 days of age suggests that deficits in GABAergic function exist before the development of seizure susceptibility. In addition, the data are the first to reveal that the GEPR-3 and GEPR-9 exhibit different changes in GABAA receptor function and may provide significant insight into the cellular mechanism underlying differences between these two strains.
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Introduction |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Seizure
disorders are a heterogeneous group of syndromes characterized by
recurrent synchronous neuronal hyperexcitability, inappropriately
altering responsiveness to a variety of stimuli. Proposed mechanisms
underlying this hyperexcitability are based on the premise that a shift
occurs in the excitatory/inhibitory balance of neurotransmission in the
brain toward elevated excitation (Meldrum, 1990
; Tasker and Dudek,
1991; Olsen et al., 1999). The cellular modification(s) responsible for
this shift could be either altered sensitivity to excitatory and/or
inhibitory neurotransmitter(s), alteration in the release and/or
metabolism of neurotransmitter(s), or some combination of these
effects. A variety of animal models with enhanced propensity toward
seizure generation exist, including several genetic models (for review,
see Jobe et al., 1991).
The genetically epilepsy-prone rat (GEPR) inherits the propensity for
seizures as a polygenetic and autosomal trait (Ribak et al., 1988).
Other than a convulsive predisposition, the GEPRs exhibit no gross
evidence of any other neurological dysfunctions (Jobe et al., 1993a,b).
Three strains of the GEPR rats, each expressing different seizure
patterns after audiogenic stimulation, have been characterized (Reigel
et al., 1986; Dailey et al., 1989; Jobe et al., 1991). These are the
GEPR-9, GEPR-3, and GEPR-NE (nonepileptic) strains, the latter serving
as the genetic control strain. The GEPR-9 exhibits full tonic clonic
seizures when presented with audiogenic stimuli, whereas the GEPR-3
exhibits only clonic seizures after an identical stimulus (Dailey et
al., 1989). In addition to sensitivity to audiogenic stimuli, the
animals develop spontaneous seizures (Jobe et al., 1986; Dailey et al.,
1989) and exhibit heightened responsiveness to other convulsive stimuli such as kindling (Savage et al., 1986), electroshock, or
pentylenetetrazol (Browning et al., 1990).
Deficits in a number of neurotransmitter systems have been identified
in the GEPRs (Jobe et al., 1991). One abnormality that has been
identified in the GEPR, an alteration in the function of the inhibitory
neurotransmitter 
-aminobutyric acid (GABA), is of particular
interest because it appears to be shared with a number of other animal
models of seizure disorders (DeDyn et al., 1990; Houser, 1991; Jobe et
al., 1991; Gale, 1992; Snodgrass, 1992). A decrease in neuronal
responsiveness to GABAergic agents has been described in the inferior
colliculus (Faingold et al., 1986a,b), cerebral cortex (Waterhouse,
1986), hippocampus (Evans et al., 1994; Vermu-Ahuja et al., 1998), and
cerebellum (Gould et al., 1991, 1995) of the GEPR-9. Other alterations
in GABAergic function have been identified, including both reductions
in GABA levels in several brain areas (Lasley, 1991) and changes in the GABAA receptor population ranging from an
increase in the cerebral cortex (Booker et al., 1986) to no change in
the cerebellum (Gould et al., 1995). Changes in GABAergic function have
been investigated far less in the GEPR-3 than in the GEPR-9 and not at
all from an electrophysiological standpoint. Therefore, examination of GABA responsiveness in these two strains would provide valuable new information.
GEPR animals do not exhibit seizure predisposition before postnatal day
15 (Jobe et al., 1991). Examining neuronal responsiveness at different
ages during development offers two advantages: 1) animals <15 days of
age would have no prior seizure history, thereby ensuring that
alterations in neuronal responsiveness were innate differences rather
than consequences of prior seizure activity; and 2) sensitivity of
seizure-prone animals can be compared with both controls and animals of
the same strain that had not yet developed the propensity for seizures.
Most of the electrophysiological studies to date have been conducted in
adult animals. Verma-Ahuja et al. (1998) have reported increased
excitability in hippocampal CA3 neurons of young (i.e., <15-day-old)
GEPR-9 animals that was due, at least in part, to reduced GABAergic inhibition.
The present study quantitatively examines the basic
electrophysiological properties and sensitivity characteristics of
Purkinje neurons in cerebellar slices of GEPR-NE, GEPR-3, GEPR-9, and
control Sprague-Dawley rats. The cerebellum was chosen to study based upon existing data from this laboratory describing changes in GABA
responsiveness (Gould et al., 1991, 1995) and the well documented changes that occur during development of the cerebellum. The results of
these studies will provide the first set of data quantitatively comparing the responsiveness of any GEPR-3 neuronal population to that
of both strains of control animals and the GEPR-9. In addition, the
ability to compare the electrical and pharmacological properties of
these neurons at different developmental ages provides an opportunity
to correlate any changes in cellular responsiveness with seizure predisposition.
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Materials and Methods |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Animals.
GEPR-NE, GEPR-3, and GEPR-9 animals were purchased
from the colonies maintained at the University of Illinois at Peoria.
The GEPR strain was derived from Sprague-Dawley stock by selective breeding for audiogenic seizure susceptibility (Laird and Jobe, 1987).
Therefore, Sprague-Dawley animals purchased from Harlan Sprague-Dawley
(Indianapolis, IN) were used as an additional control group. Most
animals used in these studies were received as part of a mother and
litter shipment because a significant component of the study was
developmental and there was a need to make certain that the animals
were age-matched. The animals were housed in plastic cages to reduce
the possibility of seizure production resulting from normal care as the
animals age. Care was taken in the selection of animals for specific
experiments to ensure that neurons included for analysis were obtained
from different preparations taken from animals from different litters
to reduce the bias provided by multiple neuron recording from either a
single animal or within a single litter. All procedures involving
animals were conducted according to protocols approved by the
Institutional Animal Care and Use Committee.
Brain Slices.
Cerebellar slices were prepared from animals
of the various strains according to the methods described by Llinas and
Sugimori (1980a,b), Crepel et al. (1981), Dupont et al. (1987), and
Kano and Konnerth (1992) with some modifications as described by Molnar et al. (1999). Briefly, animals were anesthetized with
CO2 and subsequently decapitated with a
guillotine. After decapitation, the skull and dura mater overlying the
cerebellum were removed, the cerebellar peduncles were severed using a
sterile scalpel, and the cerebellum removed. Immediately upon removal,
the cerebellum was placed into aerated (95% O2,
5% CO2) artificial cerebrospinal fluid (aCSF)
kept on ice at approximately 4°C. Artificial cerebrospinal fluid
contained 126 mM NaCl, 5 mM KCl, 1.3 mM MgSO4,
1.2 mM NaH2PO4, 26 mM
NaHCO3, 2.4 mM CaCl2, and
10 mM glucose. The cerebellum, still in iced aCSF, was then blocked by
sagittal cuts at each vermian vein and one of the cut ends glued to a
Teflon chuck using cyanoacrylate adhesive. The cerebellum was supported
on the chuck by a notched agar block previously prepared and glued to
the chuck. Agar cutting blocks were prepared by pouring molten agar
(5% by weight in 150 mM NaCl) into 10-ml plastic syringes cut
off at one end, cooled, and then cut to an appropriate size as needed. Using a Vibroslice oscillating tissue slicer (World Precision Instruments, Sarasota, FL), 300-µm-thick sagittal sections of the
cerebellar vermis were cut while maintained in aerated, 4°C aCSF.
Upon obtaining each slice, it was immediately transferred, using a wide
mouthed, fire-polished glass pipette, to a plastic-mesh basket
submerged in a plastic beaker containing continuously aerated aCSF and
kept on ice. All slices used were cut and transferred to the incubation
beaker within 10 min after decapitation. The incubation beaker was kept
on ice until 15 min postdecapitation, and then removed from ice and
allowed to slowly (over a 2-h period) warm to room temperature
(20-22°C) before initial recordings.
Electrophysiological Recording. After the recovery period, slices were individually transferred to a holding apparatus. The holding apparatus with the brain slice was then placed in a 1.5-ml laminar flow recording chamber continuously superfused with aerated aCSF maintained at 22-25°C. Temperature was continuously monitored using a Yellow Springs Instruments thermistor probe placed in the recording chamber. Perfusion of aCSF was maintained at 3 ml/min via a pressure-assisted gravity feed system. The slice was then viewed under a dissecting microscope (Nikon SMZ-2T) using a Dolan-Jenner Fiber-Lite fiber optic light source and a bipolar stimulating electrode (Teflon-coated platinum wire) for antidromic identification of neurons was placed on the white matter.
Standard intracellular recording was performed using single-barreled borosilicate glass microelectrodes filled with 3 M potassium acetate with resistances of 80 to 110 M
. The electrical activity and
cellular currents generated were then amplified and monitored under the
bridge recording configuration using an AXOCLAMP-2A amplifier (Axon
Instruments, Foster City, CA). Electrophysiological signals were
monitored on an oscilloscope (Tektronix 5110, 5111A) and led to a
computer after A/D conversion through a TL-1 DMA interface (Axon
Instruments) for data acquisition, real time analysis, and/or storage
for subsequent off-line analysis using Axotape 2.0 (Axon Instruments).
Cells at least 50 µm, but no greater than 250 µm, from the surface
of the slice were impaled with the assistance of a Narishige hydraulic
micromanipulator and were identified as Purkinje neurons based upon the
antidromic invasion of the soma after electrical stimulation of the
axon (white matter). Criteria for inclusion of a given neuron in the
population for statistical analysis included the establishment of a
stable resting membrane potential (RMP) between 
35 and 75 mV and
the production of an antidromic action potential after white matter
stimulation. Cellular input resistance (IR) was determined by regularly
(2-5-s intervals) passing several steps of depolarizing and
hyperpolarizing currents through the recording electrode. The voltage
change induced by each electrotonic potential was later quantified to
assess input resistance based upon Ohm's law. The duration of the
electrotonic potential used was individualized for each neuron so that
the membrane capacitance was fully charged for that neuron (typically
150-250 ms).
Drug Application.
Exposure of neurons to drugs was
accomplished through the addition of known concentrations of drug to
the superfusing solution. Dead time for drug application was less than
5 s. Muscimol, GABA, glutamate, aspartate, diazepam,
norepinephrine, bicuculline, and baclofen were prepared as stock
solutions in advance in aCSF and stored at 20°C. Aliquots were
thawed and diluted to final concentrations in fresh, aerated aCSF just
before use. When multiple drugs were applied to a single neuron, the
effects of prior drug applications were allowed to completely wash out
before subsequent exposure to another agent. This was noted by a
complete return of the membrane potential back to RMP. For determining
changes in IR during drug applications, IR was compared before and
during drug application by injecting current at a regular interval
during the recording of RMP. The IR was calculated from a series of
randomly selected electrotonic potential injections (10 before drug
application while at RMP, 10 at steady-state maximal effect of drug)
and then averaged before comparison. When bicuculline (a
GABAA-selective antagonist) was applied to a
slice, no subsequent recordings of responses to any agonist other than
GABA or muscimol were made from that slice to avoid residual effects
upon other neurons in the same slice. Thus, statistical analysis of the
impact of bicuculline on GABA and muscimol responses could be made with
a paired analysis. Furthermore, concentration-response curves were made
by applying single increasing concentrations of drug (about 1 min/concentration) while allowing time for complete washout of effect
between concentrations (noted by a return to RMP), which varied with
the drug used.
. When investigating the effect of modulators of the
GABAA receptor, norepinephrine (500 µM) or
diazepam (100 µM) was superfused for 1 min and the effects on
membrane potential were measured. After a 5-min recovery period, the
appropriate estimated EC50 concentration of GABA
was superfused for 1 min and the effects on membrane potential were determined. After another 5-min recovery period, the combination of
GABA and modulator (norepinephrine or diazepam) was superfused for 1 min and the subsequent membrane polarization effects quantified.
Statistical Analysis of Data.
Two similar methods were used
for statistical analysis of collected data. For parameters that were
assessed across time (i.e., versus age), an ANOVA using a nested design
was performed to test for the effect of age. Multiple comparisons were
then made using orthogonal contrast and the 
-values were adjusted
via the method of Bonferroni. Because 10 multiple comparisons were made
in each instance where this method was used, a significant P
value was 
.005 for these comparisons. For comparing the effects of
drugs between strains, an analysis of variance with nested design was also used. However, in these instances no multiple comparisons were
performed, thus a significant P value was .05 for these comparisons. Care was taken in the planning of experiments to ensure
that neurons included in any given population for statistical analysis
included neurons recorded from at least three different slice
preparations taken from animals that came from different litters to
avoid litter and multiple neuron bias in the analysis.
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Results |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Basic Electrical Membrane Properties.
The RMP and IR of
Purkinje neurons were measured using intracellular recording in
cerebellar slices from control (Sprague-Dawley and GEPR-NE) and seizure
prone (GEPR-3 and GEPR-9) animals. These electrical membrane properties
were compared not only between animal strains but also with respect to
animal age. To provide reasonable sample sizes for valid comparisons
between animal strains, the data were grouped in 5-day periods
beginning with postnatal day 10 (P10) and including an adult group that
consisted of animals 30 days postnatal or older (P30+). This grouping
method also provided an age group consisting purely of animals with no
prior seizure history (P10-14). Table 1
presents the data from these experiments.
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of a
significant hyperpolarization of Sprague-Dawley Purkinje neurons
through an increase in sodium pump sites that was due to an increase in
the 3 subunit isoform (Biser et al., 2000)
over the same time period. The membrane became more hyperpolarized and
the membrane IR decreased with increasing animal age with the majority
of the change occurring between the P10-14- and P15-19-aged animals
(Table 1). As such, the RMP and IR measured from P10-14 animals of
each strain were significantly depolarized from all other age groups
(P < .005 by multiple comparisons).
Intracellular Comparison of Purkinje Neuron Response to Drug
Superfusion.
The membrane responses to various drugs and
neurotransmitters were measured using intracellular recording
techniques and exposure to single concentrations of drug. The effects
of the GABA receptor agonists (muscimol, GABA, and baclofen), the
GABAA receptor antagonist bicuculline, and the
excitatory agonists glutamate and aspartate on membrane polarization
are presented in Table 2 for P10-14 and
P15+ animals. Animals 15 days or older were placed into a single group
because there were no differences among the 5-day age subgroups during
that period. Both GABA (10 µM) and the
GABAA-selective agonist muscimol (1 µM) induced
prominent hyperpolarizations of Sprague-Dawley and GEPR-NE Purkinje
neuron membranes, which were not statistically different between the
control animals for either age group (P > .05).
However, the response of Purkinje neurons to both 1 µM muscimol and
10 µM GABA from the GEPR-3 and GEPR-9 rats displayed a significantly
reduced hyperpolarization, irrespective of age (P < .05; Table 2). In addition, the magnitude of the hyperpolarizaton to
both agonists was significantly less in the GEPR-9 compared with the
GEPR-3 (Table 2). There were no differences within animal groups with
respect to age (P > .05).
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Discussion |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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To study the mechanisms that underlie the development of the
neuronal hyperexcitability that characterizes seizure predisposition, many investigators have chosen to examine genetic animal models of
epilepsy because many of these models share important traits with human
forms of epilepsy (Jobe et al., 1991). In the present studies, Purkinje
neurons of the cerebellum of different strains of GEPRs were
investigated electrophysiologically in an attempt to identify a general
mechanism that may account for the seizure susceptibility and
differences in seizure pattern that exist in this animal model. The
cerebellum was chosen because of the previous semiquantitative studies
from this laboratory that identified a change in responsiveness of
Purkinje neurons to GABA and muscimol (Gould et al., 1991, 1995) and
the well understood processes of development of this brain region. The
present experiments were also undertaken to determine whether GABA
responsiveness is changed in GEPR-3 Purkinje neurons and, if so, how
the change compared with that observed in the GEPR-9. The studies were
designed to provide a quantitative assessment of the magnitude and
specificity of changes in sensitivity of GEPR-9 and GEPR-3 neurons
compared with both the Sprague-Dawley and GEPR-NE control strains at
ages before and after the animals developed seizure susceptibility.
The first goal was to compare the basic membrane properties of Purkinje
neurons between GEPR and control animals both before and after seizure
susceptibility develops. These studies indicated that the membrane
properties among animal strains were equivalent for every age group
tested (Table 1). The RMP values reported here are consistent with
other reports for both young (Molnar et al., 1999) and adult animals
(Puia et al., 1994; Molnar et al., 1999). The cellular IR observed was
also similar to previously published values for Purkinje neurons
(Crepel and Delhaye-Bouchaud, 1979; Molnar et al., 1999). Thus, these
experiments indicate that any differences in agonist sensitivity are
unlikely to be the result of a general change in the excitability of
Purkinje neurons.
The next objective was to compare the membrane-polarizing effects of
single concentrations of various drugs and neurotransmitters on
Purkinje neurons from Sprague-Dawley, GEPR-NE, GEPR-3, and GEPR-9 rats.
The results indicated a specific reduction in responsiveness to
GABAA receptor activation in both groups of
seizure-prone animals (Table 2). Purkinje neurons from GEPR-9 animals
of both age groups were much less responsiveness to the hyperpolarizing
effects of both muscimol (1 µM) and GABA (10 µM) compared with
control animals. The GEPR-3 animals (Table 2) also showed a
significantly reduced response to GABAA receptor
activation that was intermediate in magnitude between the control and
the GEPR-9 responses. Neurons of both young (<P15) and adult (
P15)
GEPR-3 animals exhibited the same magnitude of reduction in maximum
response to the hyperpolarizing effects of both muscimol (1 µM) and
GABA (10 µM). The existence of this reduced response in GEPR-3 and
GEPR-9 animals <15 days of age suggests that the deficit exists before
seizure susceptibility and extends the studies of Verma-Ahuja et al.
(1998) in hippocampal CA3 neurons to include the cerebellar Purkinje
neuron. The lack of seizure susceptibility in animals with reduced
responsiveness does not rule out the possibility that the GABAergic
deficit is the underlying cause of seizure susceptibility because the
neuronal connectivity required for the spread of depolarizing activity may simply not be present at this age.
The effects of both muscimol and GABA were due to
GABAA receptor activation because bicuculline, a
GABAA receptor-selective antagonist, fully
blocked the effects of both GABA and muscimol when applied
concomitantly (Table 2). In addition, baclofen, a
GABAB receptor-selective agonist, was without
significant effect on neurons in these control preparations similar to
previous studies in Purkinje neurons (Parfitt et al., 1990; Cheun and
Yeh, 1992; Gould et al., 1995), indicating that there was very little,
if any, GABAB receptor activity. Thus, these
studies confirm the observation that the primary effects of GABA on
Purkinje neurons of the rat cerebellum are mediated via the
GABAA receptor. It is interesting that both
GEPR-3 and GEPR-9 neurons showed statistically significant but small
modifications in response to baclofen. The switch to depolarizing
responses to baclofen is a curious finding that remains unexplained. It
is also not well defined how significant this magnitude of change in
membrane polarization will be to the operation of the cell.
The next step was to more thoroughly investigate the responsiveness of
Purkinje neurons by comparing complete concentration-response curves.
The curves constructed to GABA, muscimol, glutamate, and aspartate for
control GEPR-NE and Sprague-Dawley animals (Figs. 1-4, respectively)
demonstrate that the responsiveness of Purkinje neurons from these two
animal strains is similar. No significant differences were found in
either the maximum responses or EC50 values
(Table 3) for any of the agonists in these control strains. However,
comparing GEPR-3 and GEPR-9 Purkinje neuron responsiveness to
Sprague-Dawley and GEPR-NE under these conditions revealed some
intriguing differences. Concentration-response curves for GEPR-9
neurons displayed a 3-fold rightward shift of the
concentration-response curves for both GABA and muscimol (Figs. 1A and
2A) with no alteration in maximal response. In contrast, the
concentration-response curves for GABA and muscimol in GEPR-3 neurons
were characterized by a depressed maximum response (Figs. 1B and 2B)
without a difference in the EC50 value (Table 3).
The EC50 values for GABA and muscimol calculated
for the control strains in these studies are comparable to values
previously reported in the literature for Purkinje neurons (Cheun and
Yeh, 1992; Itier et al., 1996). The change in sensitivity in the GEPR-9
versus the change in GABA efficacy in the GEPR-3 raises the possibility
that the molecular basis underlying the change in GABA responsiveness
may differ between the two strains. Just as in GEPR-9 neurons, the
GEPR-3 neurons exhibit normal responsiveness to the excitatory agonists
glutamate and aspartate, indicating a specific change in responsiveness
to GABA. These data further reinforce the conclusion that a selective
abnormality of GABAergic neurotransmission exists in the GEPR animals
and now extends that idea to question whether different changes in this
molecular entity may account for the difference in seizure pattern
observed in the two strains.
We also examined the effects of two modulators of the action of GABA on
Purkinje neurons, diazepam (an allosteric modulator) and
norepinephrine, a biogenic amine thought to modulate the action of GABA
via presynaptic mechanisms (Mitoma and Konishi, 1999). The response to
concentrations of GABA near the EC50 value for each animal strain was compared with responses in the presence of
either norepinephrine or diazepam (Table 4). Although it was hypothesized that differences might exist for the modulatory actions of
these compounds, no such differences were found. The relatively weak
hyperpolarizing effect of diazepam is also consistent with the idea
that there is relatively low endogenous GABAergic activity in the
slices as suggested by the lack of effect of bicuculline. These studies
are the first to quantitatively investigate the interaction between
GABA and the allosteric modulator diazepam in the GEPR-9, and suggest
that the alteration in the GABAA receptor characteristics that occurs in the GEPR-9 strain does not involve those
subunits that serve as the ligand binding site for the benzodiazepines.
In conclusion, the present studies demonstrate a specific 3-fold
decrease in the sensitivity of Purkinje neurons in GEPR-9 animals to
GABAA receptor activation. The results also
suggest that the underlying cause of this reduced sensitivity could be either a reduction in receptor number/density or an alteration in the
chloride channel properties. However, radioligand binding studies have
revealed either an increase (Booker et al., 1986) or no change (Gould
et al., 1995) in receptor properties in the GEPR-9 cerebral and
cerebellar cortex. In addition, the absence of a difference in the
action of diazepam either alone or as an allosteric modulator of GABA
action is consistent with the binding data of Gould et al. (1995) who
found no significant difference in the number of
[3H]muscimol or
[3H]flunitrazepam binding sites in the GEPR-9.
Thus, it is likely that a decrease in chloride channel gating is
responsible for the GABA subsensitivity in the GEPR-9.
In contrast to the GEPR-9 animals, the specific decrease in
responsiveness of the GEPR-3 animals to GABAA
receptor activation is characterized by a depression of the maximum
without an alteration in EC50 value. A decrease
in receptor affinity would cause an increase in
EC50 value without a change in maximum response.
A decrease in the density of receptors could also depress the maximum, depending upon the receptor reserve, but would also increase the EC50 value (Ruffolo, 1982). Thus, the reduced
efficacy of GABAergic neurotransmission in the GEPR-3 is likely to
reside in either the single chloride channel characteristics or the
macroscopic chloride currents rather than in the GABA receptor-binding
site per se. Although it is likely that a decrease in chloride gating is also responsible for the reduced responsiveness to GABA in the
GEPR-3 animals, the molecular abnormality leading to this reduced
responsiveness must be different from that which accounts for the
subsensitivity observed in the GEPR-9 strain. This difference in
alteration of GABA responsiveness could provide a valuable target for
examining the factors that contribute to the different patterns of
seizure expression between the strains. Furthermore, the differences in
sensitivity versus efficacy described in these studies may provide a
valuable tool to explore the impact of receptor subunit composition on
receptor function.
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Footnotes |
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Accepted for publication August 23, 2000.
Received for publication March 31, 2000.
1 This study was supported in part by National Institutes of Health Training Grant T32 GM 07039 and by funds from the Mylan Endowment.
2 Current address: Dept. of Pharmacology and Therapeutics, 100267 JHMHSC, 1600 SW Archer Rd., University of Florida, College of Medicine, Gainesville, FL 32606.
Send reprint requests to: Dr. David A. Taylor, Department of Pharmacology and Toxicology, Robert C. Byrd Health Sciences Center, P.O. Box 9223, West Virginia University School of Medicine, Morgantown, WV 26506-9223. E-mail: dtaylor{at}hsc.wvu.edu
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Abbreviations |
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GEPR, genetically epilepsy-prone rat;
GEPR-9, severe genetically epilepsy-prone rat;
GEPR-3, moderately severe
genetically epilepsy-prone rat;
GEPR-NE, genetic control;
GABA, -aminobutyric acid;
aCSF, artificial cerebrospinal fluid;
RMP, resting membrane potential;
IR, input resistance.
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References |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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3 subunit of the Na+/K+-ATPase in developing rat cerebellum.
Dev Brain Res
123:
165-172[Medline].
currents in Purkinje versus granule neurons in rat cerebellar slices.
Neuron
12:
117-126[Medline].This article has been cited by other articles:
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  M. E. Calcagnotto, M. F. Paredes, and S. C. Baraban Heterotopic Neurons with Altered Inhibitory Synaptic Function in an Animal Model of Malformation-Associated Epilepsy J. Neurosci., September 1, 2002; 22(17): 7596 - 7605. [Abstract] [Full Text] [PDF]
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