Analysis of the hippocampal GABAA receptor system in kindled rats by autoradiographic and in situ hybridization techniques: contingent tolerance to carbamazepine

Characterization of the GABAergic system in Asian clam Corbicula fluminea: Phylogenetic analysis, tissue distribution, and response to the aquatic contaminant carbamazepine

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter involved in the neuro-endocrine-immune (NEI) system. In this study, we sequenced the partial length of cDNA fragments of three genes involved in GABA neurotransmitter system of the Asian clam (Corbicula fluminea) (GABA_A receptor-associated protein (GABARAP), GABARAPL2 and GABA transporter (GAT-1)). These genes exhibited high amino acid sequence identity compared with other invertebrate orthologs. Expression patterns of the three genes were determined in mantle, gill, gonad, digestive gland and muscle, and the steady state levels of mRNA for each were determined to be highest in gonad and lowest in muscle. To determine their regulation by pharmaceuticals that are present as contaminants in waterways, clams were exposed to carbamazepine (CBZ) for 30 days. CBZ is an agonist for GABA receptors and is an anticonvulsant pharmaceutical that is often detected in aquatic ecosystems. GABARAP and GABARAPL2 mRNA levels were significantly downregulated by 5 and 50 μg/L CBZ in mantle and gill (p < 0.05), while in the gonad and digestive gland, steady state levels (p < 0.05) were decreased with exposure to all three doses. GAT-1 mRNA was upregulated by CBZ (p < 0.05) in the mantle and gill at all three doses tested and in the gonad and digestive system with 5 and 50 μg/L. These data suggest that CBZ disrupt the expression of the GABAergic neurotransmitter system in C. fluminea. Moreover, GABARAP, GABARAPL2 and GAT-1 may be useful biomarkers for the screening of substances that are hazardous to the NEI system of mollusks.

Deep brain stimulation restores the glutamatergic and GABAergic synaptic transmission and plasticity to normal levels in kindled rats

Background

The precise effect of low frequency stimulation (LFS) as a newly postulated, anticonvulsant therapeutic approach on seizure-induced changes in synaptic transmission has not been completely determined.

Hypothesis

In this study, the LFS effect on impaired, synaptic plasticity in kindled rats was investigated.

Methods

Hippocampal kindled rats received LFS (4 trials consisting of one train of 200 monophasic square waves, 0.1 ms pulse duration, 1 Hz) on four occasions. LTP induction was evaluated using whole-cell recordings of evoked excitatory and inhibitory post-synaptic potentials (EPSPs and IPSPs respectively) in CA1 neurons in hippocampal slices. In addition, the hippocampal excitatory and inhibitory post-synaptic currents (EPSCs and IPSCs), and the gene expression of NR₂A, GluR₂ and γ₂ were evaluated.

Results

LTP induction was attenuated in excitatory and inhibitory synapses in hippocampal slices of kindled rats. When LFS was applied in kindled animals, LTP was induced in EPSPs and IPSPs. Moreover, LFS increased and decreased the threshold intensities of EPSCs and IPSCs respectively. In kindled animals, NR₂A gene expression increased, while γ₂ gene expression decreased. GluR2 gene expression did not significantly change. Applying LFS in kindled animals mitigated these changes: No significant differences were observed in NR₂A, γ₂ and GluR₂ gene expression in the kindled+LFS and control groups.

Conclusion

The application of LFS in kindled animals restored LTP induction in both EPSPs and IPSPs, and returned the threshold intensity for induction of EPSCs, IPSCs and gene expression to similar levels as controls.

Tolerance to the prophylactic effects of carbamazepine and related mood stabilizers in the treatment of bipolar disorders

Tolerance development after successful long‐term treatment of bipolar disorder is under recognized, as are ways to prevent or show its occurrence or reverse it once it has occurred. We review the clinical literature which suggests that tolerance can develop to most treatment approaches in bipolar illness and present an animal model of tolerance development to anticonvulsant effects of carbamazepine or lamotrigine on amgydala‐kindled seizures. In this model tolerance does not have a pharmacokinetic basis, but is contingent upon the drug being present in the brain at the time of amygdala stimulation. The occurrence of seizures in the absence of drug is sufficient to reverse tolerance and re‐establish anticonvulsant efficacy. Based on the model, we hypothesize that some episode‐induced compensatory adaptive changes in gene expression fail to occur in tolerant subjects and that episodes off medication re‐induce these changes and renew drug effectiveness. Approaches that slow or reverse tolerance development in the animal model are reviewed so that they can be tested for their applicability in the clinic. Criteria for assessing tolerance development are offered in the hope that this will facilitate a more systemic literature about its prevalence, prevention, and reversal. Careful longitudinal monitoring of episode occurrence is essential to understanding tolerance development in the affective disorder and its treatment.

Developmental psychobiology of cyclic affective illness: Implications for early therapeutic intervention

The recurrent affective disorders are discussed from the perspective of accumulating inherited and experiential effects on gene expression. Stress and episodes of affective illness are viewed as leaving biochemical and microstructural residues in the central nervous system (CNS) in relation to their patterning, severity, and recurrence. Comorbid factors such as substance abuse and developmental disturbances may also interact with these illness-related variables. In addition to the primary pathological processes, secondary adaptive changes can also be induced, which, in concert with pharmacological interventions, may be sufficient to counter episode occurrences and illness progression. We postulate that the balance of primary pathological and secondary adaptive changes at multiple levels of CNS regulation accounts for recurrence and cyclicity in the affective disorders. The importance of early, effective, long-term interventions in the recurrent affective disorders and the therapeutic potential of several new treatment modalities including repeated transcranial magnetic stimulation (rTMS) are discussed.

GABAA receptor promoter hypermethylation in suicide brain: implications for the involvement of epigenetic processes

BACKGROUND

Epigenetic mechanisms may be involved in the reprogramming of gene expression in response to stressful stimuli. This investigation determined whether epigenetic phenomena might similarly be associated with suicide/depression.

METHODS

The expression of DNA methyltransferase (DNMT) mRNA was assessed in several brain regions of individuals who had committed suicide and had been diagnosed with major depression relative to that of individuals who had died suddenly as a result of factors other than suicide.

RESULTS

The DNMT gene transcripts' expression was altered in several brains regions of suicides, including frontopolar cortex, amygdala, and the paraventricular nucleus of the hypothalamus. Importantly, an increase of both mRNA and protein expression was found in the frontopolar cortex. In addition, although transcript abundance of various forms of DNMT was highly correlated in normal control subjects, this coordination of DNMT isoform expression was diminished in suicide brain. Further, within the frontopolar cortex, gene-specific aberrations in DNA methylation were apparent in the gamma-aminobutyric acid (GABA)(A) receptor alpha1 subunit promoter region, the transcript of which is underexpressed in suicide/major depressive disorder (MDD) brains. Indeed, three cytosine/guanosine sites were hypermethylated relative to control subjects. Finally, we found that DNMT-3B mRNA abundance was inversely correlated to alpha1 mRNA abundance.

CONCLUSIONS

These data show that DNMT mRNA expression was altered in suicide brain, and this change in expression in the frontopolar cortex was associated with increased methylation of a gene whose mRNA expression has previously been shown to be reduced. These observations suggest that epigenetic mechanisms may be associated with altered gene expression in suicide/MDD.

Determination of the architecture of ionotropic receptors using AFM imaging

Fast neurotransmission in the nervous system is mediated by ionotropic receptors, all of which contain several subunits surrounding an integral ion channel. There are three major families of ionotropic receptors: the 'Cys-loop' receptors (including the nicotinic receptor for acetylcholine, the 5-HT(3) receptor, the GABA(A) receptor and the glycine receptor), the glutamate receptors (including the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, kainate and N-methyl-D: -aspartic acid receptors) and the P2X receptors for adenosine triphosphate. These receptors are often built from multiple types of subunit, raising the question of the stoichiometry and subunit arrangement within the receptors. This question is of therapeutic significance because in some cases drug-binding sites are located at subunit-subunit interfaces. In this paper, we describe a general method, based on atomic force microscopy imaging, to solve the architecture of multi-subunit proteins, such as the ionotropic receptors. Specific epitope tags are engineered onto each receptor subunit. The subunits are then expressed exogenously in cultured cells, and the receptors are isolated from detergent extracts of membrane fractions by affinity chromatography. The receptors are imaged both alone and in complex with anti-epitope antibodies. The size of the imaged particles provides an estimate of the subunit stoichiometry, whereas the geometry of the receptor-antibody complexes produces more detailed information about the receptor architecture. We use an automated, unbiased system to identify receptors and receptor-antibody complexes and to determine the geometry of the complexes. We are also able to determine the orientation of the receptors on the mica substrate, which will allow us to solve the subunit arrangement within receptors, such as the GABA(A) receptor, which contain three types of subunits.

Persistently decreased basal synaptic inhibition of hippocampal CA1 pyramidal neurons after neonatal hypoxia-induced seizures

Hypoxia is the most common cause of neonatal seizures and can lead to epilepsy, but the epileptogenic mechanisms are not yet understood. We have previously shown that hypoxia-induced seizures in the neonatal rat result in acutely decreased amplitudes and frequency of spontaneous and miniature inhibitory postsynaptic currents (sIPSCs and mIPSCs) in hippocampal CA1 pyramidal neurons. In the current study, we asked whether such changes persist for several days following hypoxia-induced seizures. Similar to the acute findings, we observed decreased frequency and amplitudes of sIPSCs and decreased mIPSC amplitudes in CA1 pyramidal neurons at 3-5 days after hypoxia. However, in contrast to the acute findings, we observed no differences between hypoxia-treated and control groups in mIPSC frequency. Additionally, by 7 days after hypoxia, sIPSC amplitudes in the hypoxia group had recovered to control levels, but sIPSC frequency remained decreased. These data indicate that the persistently decreased sIPSC frequency result from decreased firing of presynaptic inhibitory interneurons, with only transient possible changes in postsynaptic responses to GABA release.

Kindling-induced alterations in GABAAreceptor-mediated inhibition and neurosteroid activity in the rat piriform cortex

The piriform cortex makes strong interconnections with limbic structures (amygdala, entorhinal cortex and hippocampus) that are involved in memory processing. These connections have also been implicated in the development of temporal lobe epilepsy. However, little is known about how neurones in this region may change during seizure genesis. Here we tested the hypothesis that in the kindling model of temporal lobe epilepsy GABAA receptor-mediated inhibition is altered in the piriform cortex. To do this we performed whole-cell patch-clamp recordings in piriform cortex brain slices obtained from non-kindled and amygdala-kindled adult rats. We found that kindling coincided with an increase in the amplitude and duration of miniature inhibitory post-synaptic currents (mIPSCs) recorded from non-pyramidal neurones, whereas the mIPSCs occurring on pyramidal (excitatory) cells did not change. Non-stationary noise analysis of mIPSCs occurring on the non-pyramidal neurones showed that inferred unitary conductance of synaptic channels were the same before and after kindling, implying that the channel number increased significantly. Immunocytochemical analysis of the inhibitory innervation showed that it was also unaltered by seizure induction. We also found that the effect of the positive modulator tetrahydrodeoxycorticosterone was reduced on the pyramidal neurones after kindling. In contrast, the potentiating effects of tetrahydrodeoxycorticosterone on non-pyramidal cells were about the same after kindling as in control (sham) rats. These data indicate that amygdala kindling causes a shift in the inhibition 'balance' between the pyramidal and non-pyramidal cells, perhaps leading to the disinhibition of pyramidal cells.

Delta subunit susceptibility variants E177A and R220H associated with complex epilepsy alter channel gating and surface expression of alpha4beta2delta GABAA receptors

Most human idiopathic generalized epilepsies (IGEs) are polygenic, but virtually nothing is known of the molecular basis for any of the complex epilepsies. Recently, two GABAA receptor delta subunit variants (E177A, R220H) were proposed as susceptibility alleles for generalized epilepsy with febrile seizures plus and juvenile myoclonic epilepsy. In human embryonic kidney 293T cells, recombinant halpha1beta2delta(E177A) and halpha1beta2delta(R220H) receptor currents were reduced, but the basis for the current reduction was not determined. We examined the mechanistic basis for the current reduction produced by these variants using the halpha4beta2delta receptor, an isoform more physiologically relevant and linked to epileptogenesis, by characterizing the effects of these variants on receptor cell surface expression and single-channel gating properties. Expression of variant alpha4beta2delta(R220H) receptors resulted in a decrease in surface receptor proteins, and a smaller, but significant, reduction was observed for variant alpha4beta2delta(E177A) receptors. For both variants, no significant alterations of surface expression were observed for mixed population of wild-type and variant receptors. The mean open durations of alpha4beta2delta(E177A) and alpha4beta2delta(R220H) receptor single-channel currents were both significantly decreased compared to wild-type receptors. These data suggest that both delta(E177A) and delta(R220H) variants may result in disinhibition in IGEs by similar cellular and molecular mechanisms, and in heterozygously affected individuals, a reduction in channel open duration of delta subunit-containing GABAA receptors may be the major contributor to the epilepsy phenotypes.

Central-type Benzodiazepine Receptors and Epileptogenesis: Basic Mechanisms and Clinical Validity

PURPOSE

Gamma-aminobutyric acid (GABA)-A/benzodiazepine receptors (BZRs) play an important inhibitory role in epileptogenesis. [123I]Iomazenil (123I-IMZ) is a specific ligand for central-type (or neuronal-type) BNRs and is available for single-photon emission computed tomography (SPECT) in brain disorders. We demonstrated alterations of central-type BZRs in human focal epilepsies and their experimental models.

METHODS

We examined interictal 123I-IMZ SPECT in patients with mesial temporal lobe epilepsy (MTLE; n = 19) with hippocampal sclerosis and neocortical epilepsy with focal cortical dysplasia (NE-CD; n = 18), and compared those with magnetic resonance imaging (MRI) and 123I-IMP SPECT (for regional cerebral blood flow). We also investigated in vitro autoradiography with (123)I-IMZ at various time courses in the intraamygdala kainate, amygdala kindling, and in-utero irradiation models.

RESULTS

In MTLE patients, the epileptogenic hippocampus often showed decreases in both 123I-IMZ and 123I-IMP SPECT. Consistent with those, marked reduction of 125I-IMZ binding was observed in hippocampal CA1-3 regions of the kainate model, which clearly paralleled pyramidal neuronal loss. In contrast, 125I-IMZ binding was increased in the dentate gyrus at 1 month but returned to the normal level at 3-6 months, when frequent spontaneous seizures appeared. The amygdala-kindling model demonstrated similar increases in 125I-IMZ binding in the dentate gyrus without any changes in other brain regions. In NE-CD patients, the epileptogenic foci showed decreased 123I-IMZ binding with relatively normal 123I-IMP SPECT. 125I-IMZ binding also was decreased in the cerebral cortex, hippocampus (areas CA1, 2, and 4), and caudate/putamen of the in-utero irradiation model.

CONCLUSIONS

These results indicate that central-type BZRs neuroimaging is useful for detection of epileptogenic foci, but their alterations differ between epilepsy subtypes and time-courses.

Kindling and status epilepticus models of epilepsy: rewiring the brain

This review focuses on the remodeling of brain circuitry associated with epilepsy, particularly in excitatory glutamate and inhibitory GABA systems, including alterations in synaptic efficacy, growth of new connections, and loss of existing connections. From recent studies on the kindling and status epilepticus models, which have been used most extensively to investigate temporal lobe epilepsy, it is now clear that the brain reorganizes itself in response to excess neural activation, such as seizure activity. The contributing factors to this reorganization include activation of glutamate receptors, second messengers, immediate early genes, transcription factors, neurotrophic factors, axon guidance molecules, protein synthesis, neurogenesis, and synaptogenesis. Some of the resulting changes may, in turn, contribute to the permanent alterations in seizure susceptibility. There is increasing evidence that neurogenesis and synaptogenesis can appear not only in the mossy fiber pathway in the hippocampus but also in other limbic structures. Neuronal loss, induced by prolonged seizure activity, may also contribute to circuit restructuring, particularly in the status epilepticus model. However, it is unlikely that any one structure, plastic system, neurotrophin, or downstream effector pathway is uniquely critical for epileptogenesis. The sensitivity of neural systems to the modulation of inhibition makes a disinhibition hypothesis compelling for both the triggering stage of the epileptic response and the long-term changes that promote the epileptic state. Loss of selective types of interneurons, alteration of GABA receptor configuration, and/or decrease in dendritic inhibition could contribute to the development of spontaneous seizures.

Neurobiology of Seizures and Behavioral Abnormalities

Seizures are both caused by and induce a complex set of neurobiological alterations and adaptations. The animal model of amygdala kindling provides insight into the spatiotemporal evolution of these changes as a function of seizure development and progression. Intracellular, synaptic, and microstructural changes are revealed as related to both the primary pathophysiology of kindled seizure evolution and compensatory secondary, or endogenous anticonvulsant adaptations. At the level of gene expression, the balance of these pathological and adaptive processes (as augmented by exogenous medications) probably determines whether seizures will be manifest or suppressed and could account for aspects of their intermittency. As anxiety and emotion modulation are subserved by many of the same neuroanatomic substrates involved in the evolution of complex partial seizures, particularly those of the medial temporal lobe, it is readily conceptualized how vulnerability to a range of psychiatric disorders could be related to the primary or secondary neurochemical alterations associated with seizure disorders. The discrete and methodologically controlled elucidation of the cascades and spatiotemporal distributions of neurobiological alterations that accompany seizure evolution in the kindling model may help resolve some of the difficulty and complexity of elucidating these biobehavioral relationships in the clinic.

Convergences in course of illness and treatments of the epilepsies and recurrent affective disorders

The failure to achieve and maintain remission is a critical problem for a high percentage of patients with epilepsy and the primary affective disorders. Early illness onset and delayed initiation of treatment may contribute to primary treatment resistance or that associated with loss of efficacy (tolerance phenomenon). Neurobiological data and principles drawn from the amygdala kinding model of seizure progression are reviewed for their heuristic value in conceptualizing molecular mechanisms of illness progression and its prevention with pharmacological agents in the epilepsies and, indirectly, the recurrent affective disorders. Caveats in the use of this model and convergences and divergences in its predictive validity for seizures and affective disorders are noted.

Bicuculline seizure susceptibility and nigral GABAA alpha1 receptor mRNA is altered in adult prenatally morphine-exposed females

Prenatal morphine exposure (5-10 mg/kg twice daily on gestation days 11-18) can adversely affect neurological development, including seizure susceptibility. The present study examines the effects of prenatal morphine exposure on seizure susceptibility to the GABA antagonist and convulsant bicuculline and GABA(A) alpha(1) receptor mRNA in the substantia nigra (SN) of female rats. The results demonstrate that prenatally morphine-exposed ovariectomized (OVX) females and OVX females with estradiol benzoate (EB) replacement have an increased latency to seizure onset compared to controls. In addition, prenatal morphine exposure decreases the area covered by grains of GABA(A) alpha(1) receptor mRNA in the anterior SN in both OVX and EB+progesterone (P)-treated groups, and decreases the number of GABA(A) alpha(1) receptor mRNA-labeled cells/field in EB females. Furthermore, prenatally morphine- and saline-exposed EB and EB+P females had decreased GABA(A) alpha(1) receptor mRNA-labeled cells/field in the anterior SN compared to OVX animals of the same prenatal exposure. These results demonstrate that the long term effects of prenatal morphine exposure in female rats is dependent on their hormonal status, and suggest that seizure susceptibility may be altered via neuropharmacological changes in the GABA system in the SN.

GABA(B1a), GABA(B1b) AND GABA(B2) mRNA variants expression in hippocampus resected from patients with temporal lobe epilepsy

The aim of this study was to investigate the mRNA expression of the two GABA(B1) receptor isoforms and the GABA(B2) subunit, in human postmortem control hippocampal sections and in sections resected from epilepsy patients using quantitative in situ hybridisation autoradiography. Utilising human control hippocampal sections it was shown that the oligonucleotides employed were specific to the receptor. Hippocampal slices from surgical specimens obtained from patients with hippocampal sclerosis and temporal lobe epilepsy were compared with neurologically normal postmortem control subjects for neuropathology and GABA(B) mRNA expression. Neuronal loss was observed in most of the hippocampal subregions, but in the subiculum no significant difference was detected. The localisation of GABA(B1a) and GABA(B1b) isoform mRNAs in human control hippocampal sections supported and extended earlier studies using the GABA(B1) pan probe, which does not distinguish between the two GABA(B1) isoforms. Moreover, the GABA(B2) mRNA location confirmed the heterodimerisation of the receptor. Thus, although there was an apparent correlation between GABA(B1b) and GABA(B2), GABA(B1a) exhibited no such relationship. GABA(B1b) and GABA(B2) showed a similar intensity of expression whilst GABA(B1a) displayed a lower hybridisation signal. Comparison of the expression of the three mRNAs between control and epileptic subjects showed significant decreases or increases in different hippocampal subregions.GABA(B) isoforms and subunit mRNA expression per remaining neuron was significantly increased in the hilus and dentate gyrus. These results demonstrate that altered GABA(B) receptor mRNA expression occurs in human TLE; possibly the observed changes may also serve to counteract ongoing hyperexcitability.

The benzodiazepine partial inverse agonist Ro15-4513 alters anticonvulsant and lethal effects of carbamazepine in amygdala-kindled rats

Ro15-4513 (ethyl-8-azido-5,6-dihydro-5methyl-6-oxo-4H-imidazo-[1,5-a]-1,4-benzodiazepine-3-carboxylate), a benzodiazepine partial inverse agonist of the GABA(A) receptor, is known to protect against alcohol toxicities. The present study was designed to determine the role of Ro15-4513 in preventing anticonvulsant, toxic, and lethal effects of carbamazepine (CBZ) in amygdala-kindled rats. Acute treatment with CBZ (25 mg/kg, i.p.) produced anticonvulsant effects in fully kindled rats characterized by a significant decrease in afterdischarge and seizure duration and stage. Repeated administration of this high dose of CBZ induced sedation and high (56%) lethality. The anticonvulsant and sedative effects of CBZ were strikingly suppressed by pretreatment with Ro15-4513 (2.5 and 5 mg/kg, i.p.), and there was no mortality in animals co-administrated with Ro15-4513 during the entire experimental period. These results indicate that Ro15-4513 protects against CBZ-induced sedation and lethality, while suppressing the anticonvulsant effects of CBZ, suggesting a role for the GABA(A) receptor in CBZ efficacy and side effects. The potential clinical implications for CBZ-induced toxicity and overdose remain to be explored.

Do the epilepsies, pain syndromes, and affective disorders share common kindling-like mechanisms?

Kindling, in the classical sense, involves progressively increasing responsivity to the intermittent repetition of the same 1-s subthreshold electrical stimulation over time, with the amygdala being the area most frequently studied. Such repeated subthreshold stimulation is associated with: lowering of the after-discharge (AD) threshold; lengthening and spread of the AD; marked seizure stage progression culminating in full-blown tonic-clonic forelimb convulsions with rearing and falling; and evolution from triggered to spontaneous seizures. This evolving process concomitantly involves changes in the spatio-temporal expression of immediate early genes (IEGs), neurotrophic factors, and late effector genes (LEGs), and an associated changing pattern of effectiveness of different pharmacological interventions. Since seizures are the paradigmatic behavioral manifestation of kindling, some types of pharmacological seizures, such as those induced by the local anesthetics cocaine and lidocaine, and some epileptic syndromes, are most likely homologously modeled by kindling. However, since non-epileptiform syndromes, such as recurrent episodes of affective illness and some pain syndromes possess non-homogenous elements of kindling-like evolution, some of the principles involved in kindling progression may, nonetheless, be pertinent to the understanding and treatment of these syndromes. For example, one could attempt to distinguish between the genes involved in the primary pathological processes of syndrome evolution versus those that are secondary and adaptive; such a differentiation could have important implications for the development of therapeutic approaches targeted to suppressing or enhancing these alterations, respectively. In these instances, inferences drawn from the kindling model are necessarily indirect and circumscribed because different neuroanatomical and biochemical processes are likely involved in the evolution of each neuropsychiatric syndrome. Given these recognized limitations of non-homologous models, kindling may still provide insights into the longitudinal course, progression, and treatment of some neuropsychiatric syndromes that can then be directly tested in the clinic.

GABA(A) receptor changes in delta subunit-deficient mice: altered expression of alpha4 and gamma2 subunits in the forebrain

The delta subunit is a novel subunit of the pentameric gamma-aminobutyric acid (GABA)(A) receptor that conveys special pharmacological and functional properties to recombinant receptors and may be particularly important in mediating tonic inhibition. Mice that lack the delta subunit have been produced by gene-targeting technology, and these mice were studied with immunohistochemical and immunoblot methods to determine whether changes in GABA(A) receptors were limited to deletion of the delta subunit or whether alterations in other GABA(A) receptor subunits were also present in the delta subunit knockout (delta-/-) mice. Immunohistochemical studies of wild-type mice confirmed the restricted distribution of the delta subunit in the forebrain. Regions with moderate to high levels of delta subunit expression included thalamic relay nuclei, caudate-putamen, molecular layer of the dentate gyrus, and outer layers of the cerebral cortex. Virtually no delta subunit labeling was evident in adjacent regions, such as the thalamic reticular nucleus, hypothalamus, and globus pallidus. Comparisons of the expression of other subunits in delta-/- and wild-type mice demonstrated substantial changes in the alpha4 and gamma2 subunits of the GABA(A) receptor in the delta-/- mice. gamma2 Subunit expression was increased, whereas alpha4 subunit expression was decreased in delta-/- mice. Importantly, alterations of both the alpha4 and the gamma2 subunits were confined primarily to brain regions that normally expressed the delta subunit. This suggests that the additional subunit changes are directly linked to loss of the delta subunit and could reflect local changes in subunit composition and function of GABA(A) receptors in delta-/- mice.

alpha4beta3delta GABA(A) receptors characterized by fluorescence resonance energy transfer-derived measurements of membrane potential

Selective modulators of gamma-aminobutyric acid, type A (GABA(A)) receptors containing alpha(4) subunits may provide new treatments for epilepsy and premenstrual syndrome. Using mouse L(-tk) cells, we stably expressed the native GABA(A) receptor subunit combinations alpha(3)beta(3)gamma(2,) alpha(4)beta(3)gamma(2), and, for the first time, alpha(4)beta(3)delta and characterized their properties using a novel fluorescence resonance energy transfer assay of GABA-evoked depolarizations. GABA evoked concentration-dependent decreases in fluorescence resonance energy transfer that were blocked by GABA(A) receptor antagonists and, for alpha(3)beta(3)gamma(2) and alpha(4)beta(3)gamma(2) receptors, modulated by benzodiazepines with the expected subtype specificity. When combined with alpha(4) and beta(3), delta subunits, compared with gamma(2), conferred greater sensitivity to the agonists GABA, 4,5,6,7-tetrahydroisoxazolo-[5,4-c]pyridin-3-ol (THIP), and muscimol and greater maximal efficacy to THIP. alpha(4)beta(3)delta responses were markedly modulated by steroids and anesthetics. Alphaxalone, pentobarbital, and pregnanolone were all 3-7-fold more efficacious at alpha(4)beta(3)delta compared with alpha(4)beta(3)gamma(2.) The fluorescence technique used in this study has proven valuable for extensive characterization of a novel GABA(A) receptor. For GABA(A) receptors containing alpha(4) subunits, our experiments reveal that inclusion of delta instead of gamma(2) subunits can increase the affinity and in some cases the efficacy of agonists and can increase the efficacy of allosteric modulators. Pregnanolone was a particularly efficacious modulator of alpha(4)beta(3)delta receptors, consistent with a central role for this subunit combination in premenstrual syndrome.

GABA(B(1)) mRNA expression in hippocampal sclerosis associated with human temporal lobe epilepsy

GABA(B) receptors act to inhibit neurotransmitter release from presynaptic terminals, and mediate the late inhibitory postsynaptic potential. Studies of GABA(B) receptor function in rodent models of temporal lobe epilepsy (TLE) suggest that GABA(B) receptor expression and/or function may be perturbed. GABA(B(1)) mRNA levels were investigated in 10 hippocampal resection samples obtained at surgery from intractable hippocampal sclerosis (HS) associated TLE patients and five neurologically normal post-mortem (PM) control samples. In situ hybridisation with a 35S-dATP-labelled oligonucleotide was carried out to measure mRNA levels, along with three-dimensional cell counting, for assessment of neuronal density in hippocampal subregions. GABA(B(1)) mRNA was significantly up-regulated in the subiculum of HS samples as compared with PM controls. When adjusted for the characteristic neuronal density changes observed in HS, GABA(B(1)) mRNA was significantly up-regulated in CA1, hilus and dentate gyrus granule cell layer of HS samples as compared with PM controls. The possibility of increased GABA(B(1)) expression suggests that changes in GABA(B) receptor mechanisms may be involved in the pathogenesis of human HS-associated TLE.

GABA(B) receptor autoradiography in hippocampal sclerosis associated with human temporal lobe epilepsy

1. Metabotropic gamma-aminobutyric acid receptors (GABA(B)) exist both pre- and postsynaptically throughout the brain, mediating the suppression of neurotransmitter release and late inhibitory postsynaptic potentials. Investigation of GABA(B) receptors in rodent models of temporal lobe epilepsy (TLE) suggests that expression or function of these receptors may be altered in the disorder. 2. The aim of the present study was to investigate the expression of GABA(B) receptors in samples of hippocampus surgically resected from patients with hippocampal sclerosis (HS) related intractable TLE, and compare this expression with samples of neurologically normal post-mortem (PM) control hippocampal tissue. Appropriate measures of neuronal loss associated with HS were investigated for comparison with receptor binding data. 3. Receptor autoradiography with [(3)H]-GABA in the presence of isoguvacine, and quantitative densitometric analysis were used to investigate GABA(B) receptor expression (B(max)) and affinity (K(D)) in 11 HS samples and eight controls. A three-dimensional cell counting technique was used to assess neuronal density in both groups. 4. GABA(B) receptor density was significantly reduced in CA1, CA2, CA3, hilus and dentate gyrus, and increased in the subiculum, of HS cases as compared with PM controls. Neuronal loss was significant in all regions measured. When adjusted for neuronal loss, CA1 GABA(B) receptor expression appeared significantly upregulated (P:<0.05). 5. In HS/TLE, GABA(B) receptor expression per remaining neurone appears increased in CA1. This finding, and increased [(3)H]-GABA affinity at CA3 and hilar GABA(B) receptors, suggests altered GABA(B) receptor function may occur in human HS/TLE, possibly as a result of synaptic reorganization.

Tolerance to the anticonvulsant effects of lamotrigine on amygdala kindled seizures: cross-tolerance to carbamazepine but not valproate or diazepam

Using an amygdala-kindled seizure paradigm, we evaluated the acute and chronic anticonvulsant effects of lamotrigine (LTG). Lamotrigine produced dose-dependent inhibitory effects on seizure stage, afterdischarge (AD), and seizure duration. Lamotrigine (15 mg/kg) also increased the afterdischarge and seizure thresholds. Following repeated LTG administration and stimulation at 48-h intervals, tolerance developed to LTG's (15 mg/kg) anticonvulsant effects, and cross-tolerance was observed to the anticonvulsant effects of carbamazepine (CBZ, 15 mg/kg). In a separate group of kindled rats, CBZ (15 mg/kg) was repeatedly administered to induce tolerance. This led to a partial cross-tolerance to LTG, manifesting as an increased rate of tolerance development to LTG, and seizures following the first injection in some animals, which were not observed in CBZ-nontolerant controls. When these rats were made fully tolerant to LTG and then exposed to higher doses of LTG (30 and 50 mg/kg), no anticonvulsant effects were observed. In contrast, higher doses of CBZ (30 mg/kg) did restore efficacy in CBZ-tolerant animals. Cross-tolerance from LTG to valproate and diazepam was not observed, although cross-tolerance from CBZ to valproate has been reported previously. These data suggest that LTG has both shared and distinct anticonvulsant mechanisms from those of CBZ on amygdala-kindled seizures. The implications of these results for clinical therapeutics remain to be evaluated.

Preferential coassembly of alpha4 and delta subunits of the gamma-aminobutyric acidA receptor in rat thalamus

Pharmacological study of rat thalamic gamma-aminobutyric acidA (GABAA) receptors revealed the presence of two distinct populations, namely, diazepam-sensitive and diazepam-insensitive [3H]Ro15-4513 binding sites accounting for 94 +/- 2% (1339 +/- 253 fmol/mg protein) and 6 +/- 2% (90 +/- 44 fmol/mg protein) of total sites, respectively. Thalamic diazepam-insensitive sites exhibited a pharmacology that was distinct from diazepam-sensitive sites but comparable to that of the alpha4beta3gamma2 subtype of the GABAA receptor stably expressed in L(tk-) cells. Immunoprecipitation experiments with a specific anti-alpha4-antiserum immunoprecipitated 20 and 7% of total thalamic [3H]muscimol and [3H]Ro15-4513 sites, respectively. Combinatorial immunoprecipitation using antisera against the alpha4, gamma2, and delta subunit revealed that alpha4delta- and alpha4gamma2-containing receptors account for 13 +/- 2 and 8 +/- 3% of [3H]muscimol sites from thalamus, respectively. It also indicated that all delta subunits coexist with an alpha4 subunit in this brain region. In conclusion, our results show that in rat thalamus both alpha4betagamma2 and alpha4betadelta subtypes are expressed but alpha4betadelta is the major alpha4-containing GABAA receptor population.

Differential expression of alpha1, alpha2, alpha3, and alpha5 GABAA receptor subunits in seizure-prone and seizure-resistant rat models of temporal lobe epilepsy

Temporal lobe epilepsy remains one of the most widespread seizure disorders in man, the etiology of which is controversial. Using new rat models of temporal lobe epilepsy that are either prone or resistant to develop complex partial seizures, we provide evidence that this seizure susceptibility may arise from arrested development of the GABAA receptor system. In seizure-prone (Fast kindling) and seizure-resistant (Slow kindling) rat models, both the mRNA and protein levels of the major alpha subunit expressed in adult brain (alpha1), as well as those highly expressed during development (alpha2, alpha3, and alpha5), were differentially expressed in both models compared with normal controls. We found that alpha1 subunit mRNA expression in the Fast kindling strain was approximately half the abundance of control rats, whereas in the Slow kindling strain, it was approximately 70% greater than that of controls. However, Fast rats overexpressed the alpha2, alpha3, and alpha5 ("embryonic") subunits, having a density 50-70% greater than controls depending on brain area, whereas the converse was true of Slow rats. Using subunit-specific antibodies to alpha1 and alpha5 subunits, quantitative immunoblots and immunocytochemistry revealed a concordance with the mRNA levels. alpha1 protein expression was approximately 50% less than controls in the Fast strain, whereas it was 200% greater in the Slow strain. In contrast, alpha5 subunit protein expression was greater in the Fast strain than either the control or Slow strain. These data suggest that a major predispositional factor in the development of temporal lobe epilepsy could be a failure to complete the normal switch from the GABAA receptor alpha subunits highly expressed during development (alpha2, alpha3, and alpha5) to those highly expressed in adulthood (alpha1).

An update on GABAA receptors

Recent advances in molecular biology and complementary information derived from neuropharmacology, biochemistry and behavior have dramatically increased our understanding of various aspects of GABAA receptors. These studies have revealed that the GABAA receptor is derived from various subunits such as alpha1-alpha6, beta1-beta3, gamma1-gamma3, delta, epsilon, pi, and rho1-3. Furthermore, two additional subunits (beta4, gamma4) of GABAA receptors in chick brain, and five isoforms of the rho-subunit in the retina of white perch (Roccus americana) have been identified. Various techniques such as mutation, gene knockout and inhibition of GABAA receptor subunits by antisense oligodeoxynucleotides have been used to establish the physiological/pharmacological significance of the GABAA receptor subunits and their native receptor assemblies in vivo. Radioligand binding to the immunoprecipitated receptors, co-localization studies using immunoaffinity chromatography and immunocytochemistry techniques have been utilized to establish the composition and pharmacology of native GABAA receptor assemblies. Partial agonists of GABAA receptors are being developed as anxiolytics which have fewer and less severe side effects as compared to conventional benzodiazepines because of their lower efficacy and better selectivity for the GABAA receptor subtypes. The subunit requirement of various drugs such as anxiolytics, anticonvulsants, general anesthetics, barbiturates, ethanol and neurosteroids, which are known to elicit at least some of their pharmacological effects via the GABAA receptors, have been investigated during the last few years so as to understand their exact mechanism of action. Furthermore, the molecular determinants of clinically important drug-targets have been investigated. These aspects of GABAA receptors have been discussed in detail in this review article.

Alterations in GABAA receptor alpha 1 and alpha 4 subunit mRNA levels in thalamic relay nuclei following absence-like seizures in rats

Modification of GABAA receptor mRNA levels by seizure activity can regulate general neuronal excitability. The possibility of absence seizure-induced alteration in GABAA receptor alpha 1, alpha 4, beta 2, and gamma 2 subunit gene expression in thalamic relay nuclei was studied in a rat model of absence seizures induced by gamma-hydroxybutyric acid (GHB). We observed a marked increase in alpha 1 mRNA and a corresponding decrease in alpha 4 mRNA in thalamic relay nuclei 2-4 h after the onset of GHB-induced absence seizures (when the seizures were terminating). These changes were selective to these alpha isoforms as neither beta 2 nor gamma 2 mRNA changed following seizures and occurred only in thalamic relay nuclei but not in hippocampus, a structure from which absence seizures do not evolve. The alterations in alpha 1 and alpha 4 mRNA persisted until about 12 h, and by 24 h after the seizure-onset the mRNA levels normalized. Blocking GHB-seizures produced no change in the levels of alpha 1 and alpha 4 mRNA in thalamic relay nuclei, suggesting that seizures themselves were responsible for mRNA alterations. In order to determine if absence seizure-induced changes in alpha 1 and alpha 4 mRNA had any physiological significance, GHB was readministered in rats 6 and 24 h after the onset of seizures. The total duration of GHB-seizures was found to be significantly decreased when GHB was readministered at 6 h but not 24 h after the seizure-onset. These results suggest that absence seizures regulate GABAA receptor alpha 1 and alpha 4 gene expression in thalamic relay nuclei as a compensatory mechanism by which absence seizures are terminated.

A history of the use of anticonvulsants as mood stabilizers in the last two decades of the 20th century

Anticonvulsants have moved into an important position as alternatives and adjuncts to lithium carbonate in the treatment of bipolar illness. Work with the nonhomologous model of kindled seizures helped in the choice of carbamazepine as a potential mood stabilizer and in the study of the mechanisms of action of the second generation anticonvulsants carbamazepine and valproate, as well as the putative third generation psychotropic anticonvulsants lamotrigine and gabapentin. Anticonvulsant neuropeptides such as TRH and nonconvulsant approaches with repeated transcranial magnetic stimulation (rTMS) also appear promising.

Kindling: separate vs. shared mechanisms in affective disorders and epilepsy

Kindling is discussed in relation to affective illness as a nonhomologous model, which shares the feature of increasing illness severity and evolution over time following repeated exposures to certain forms of stimulation. This progressive aspect of kindling has proven useful in the study of approaches to pharmacotherapeutics, mechanisms and characteristics of drug tolerance, and, most recently, illness suppression through physiological rather than pharmacological strategies. Each of these themes is described and the mechanisms that have been uncovered using the kindling model are discussed in relation to how similar principles might apply in affective illness or epilepsy. It is hoped that some of the lessons from the kindling model will provide useful and novel insights into aspects of treatment and mechanisms of psychiatric and neurologic illnesses.

Sensitivity of the rat hippocampal GABA(A) receptor alpha 4 subunit to electroshock seizures

The effects of one and five electroshock seizures on [3H]flunitrazepam binding, diazepam-insensitive (DIS) [3H]Ro 15-4513 binding, and levels of mRNA for GABA(A) receptor alpha1, alpha4, beta3 and gamma2 subunits were examined in rats. No changes in any parameter were observed in the CA1 region of hippocampus or in parietal cortex. However, a single seizure produced a rapid and transient increase of alpha4 mRNA in the dentate gyrus, without altering the expression of the other subunits. The putative alpha4 protein, as measured by DIS [3H]Ro 15-4513 binding, was also elevated in the dentate gyrus by a single shock. Repeated electroshock (48-h intervals) resulted in an enhanced response of the alpha4 subunit to the seizure. Neither one nor five seizures altered [3H]flunitrazepam binding.

Ion channels in epilepsy

There are specific alterations in the structure or function of ion channels in the epileptic brain. Some of these alterations may promote hyperexcitability, whereas others may protect neurons from the deleterious effects of epileptic discharges. With the use of human tissue resected from epilepsy patients and the comparison of cellular properties to those found in well-defined experimental models, we will continue to gain insight into the specific ion channel changes associated with epilepsies. Further genetic studies will help to elucidate the altered molecular mechanisms underlying ion channel changes in this devastating neurological disorder (Noebels, 1996). Whether it is a change in structure, function, or both, the study of ion channels in epilepsies will soon reveal specific characteristics of ion channels found only in epileptic tissue. If the altered properties of such ion channels cannot be found in control (nonepileptic) neurons, these channels might be called "epileptic" ion channels. An understanding of the specific structure, function, and pharmacology of these "epileptic" channels will yield important clues for future therapeutical approaches aimed at preventing epileptogenesis, and insight into the processes whereby ion channels become "epileptic" may finally open the way to prophylactic treatments of the epilepsies.

Carbamazepine in the treatment of neuroleptic malignant syndrome

BACKGROUND

Neuroleptic malignant syndrome (NMS) is a potentially lethal adverse effect to neuroleptic drugs.

METHODS

We report on 2 cases where NMS dramatically improved with carbamazepine. Incidental removal and reapplication of carbamazepine attests to its effectiveness for this condition.

RESULTS

A 34-year-old woman treated for a major depressive disorder experienced NMS with a phenothiazine. Her condition dramatically improved in 8 hours after she was administered carbamazepine. Since carbamazepine was discontinued, NMS recurred in 10 hours and remitted anew within less than 24 hours after reintroduction. A 31-year-old woman experiencing a schizoaffective disorder displayed NMS with aphenothiazine and a butyrophenone. NMS completely resolved within 8 hours after she was administered carbamazepine. NMS recurred within 12 hours after carbamazepine discontinuation.

CONCLUSIONS

These data thus account for a cause-effect relationship between carbamazepine administration and NMS relief, and argue against the neuroleptic withdrawal to be responsible by itself for NMS relief.

Partial hippocampal kindling decreases efficacy of presynaptic GABAB autoreceptors in CA1

The effect of partial hippocampal kindling, a model of temporal lobe seizure, on monosynaptic inhibition mediated by GABA was studied. Kindled rats were given 15 nonconvulsive hippocampal afterdischarges, and control rats were given low frequency or no stimulations. At 1-2 d after kindling, paired-pulse depression (PPD) of the IPSCs recorded in CA1 neurons in vitro was significantly smaller in kindled as compared with control rats. The difference in PPD persisted for at least 21 d after kindling. The decrease in PPD of the IPSCs after partial hippocampal kindling was likely caused by a reduced GABA autoinhibition after downregulation of presynaptic GABAB receptors. The GABAB antagonist CGP35348 (1 mM) suppressed PPD of the IPSCs more strongly in control than in kindled rats. Direct activation of the presynaptic GABAB receptors by baclofen suppressed the monosynaptic IPSCs significantly more in control than in kindled rats. The decay rate of a single-pulse IPSC was faster in kindled than in control rats on day 1 or day 21 after partial kindling. The difference in IPSC decay between kindled and control rats was found with or without a GABAB receptor antagonist. The low efficacy of the presynaptic GABAB receptors in kindled rats may provide compensatory stabilization of the postsynaptic membrane against further seizures or plasticity.

A speculative model of affective illness cyclicity based on patterns of drug tolerance observed in amygdala-kindled seizures

In this article, we discuss molecular mechanisms involved in the evolution of amygdala kindling and the episodic loss of response to pharmacological treatments during tolerance development. These phenomena allow us to consider how similar principles (in different neurochemical systems) could account for illness progression, cyclicity, and drug tolerance in affective disorders. We describe the phenomenon of amygdala-kindled seizures episodically breaking through effective daily pharmacotherapy with carbamazepine and valproate, suggesting that these observations could reflect the balance of pathological vs compensatory illness-induced changes in gene expression. Under certain circumstances, amygdala-kindled animals that were initially drug responsive can develop highly individualized patterns of seizure breakthroughs progressing toward a complete loss of drug efficacy. This initial drug efficacy may reflect the combination of drug-related exogenous neurochemical mechanisms and illness-induced endogenous compensatory mechanisms. However, we postulate that when seizures are inhibited, the endogenous illness-induced adaptations dissipate (the "time-off seizure" effect), leading to the re-emergence of seizures, a re-induction of a new, but diminished, set of endogenous compensatory mechanisms, and a temporary period of renewed drug efficacy. As this pattern repeats, an intermittent or cyclic response to the anticonvulsant treatment emerges, leading toward complete drug tolerance. We also postulate that the cyclic pattern accelerates over time because of both the failure of robust illness-induced endogenous adaptations to emerge and the progression in pathophysiological mechanisms (mediated by long-lasting changes in gene expression and their downstream consequences) as a result of repeated occurrences of seizures. In this seizure model, this pattern can be inhibited and drug responsivity can be temporarily reinstated by several manipulations, including lowering illness drive (decreasing the stimulation current), increasing drug dosage, switching to a new drug that does not show crosstolerance to the original medication, or temporarily discontinuing treatment, allowing the illness to re-emerge in an unmedicated animal. Each of these variables is discussed in relation to the potential relevance to the emergence, progression, and suppression of individual patterns of episodic cyclicity in the recurrent affective disorders. A variety of clinical studies are outlined that specifically test the hypotheses derived from this formulation. Data from animal studies suggest that illness cyclicity can develop from the relative ratio between primary pathological processes and secondary endogenous adaptations (assisted by exogenous medications). If this proposition is verified, it further suggests that illness cyclicity is inherent to the neurobiological processes of episode emergence and amelioration, and one does not need to postulate a separate defect in the biological clock. The formulation predicts that early and aggressive long-term interventions may be optimal in order to prevent illness emergence and progression and its associated accumulating neurobiological vulnerability factors.

Zinc-induced collapse of augmented inhibition by GABA in a temporal lobe epilepsy model

In the kindling model of temporal lobe epilepsy, several physiological indicators of inhibition by gamma-aminobutyric acid (GABA) in the hippocampal dentate gyrus are consistent with an augmented, rather than a diminished, inhibition. In brain slices obtained from epileptic (kindled) rats, the excitatory drive onto inhibitory interneurons was increased and was paralleled by a reduction in the presynaptic autoinhibition of GABA release. This augmented inhibition was sensitive to zinc most likely after a molecular reorganization of GABAA receptor subunits. Consequently, during seizures, inhibition by GABA may be diminished by the zinc released from aberrantly sprouted mossy fiber terminals of granule cells, which are found in many experimental models of epilepsy and in human temporal lobe epilepsy.

Long-term and regional specific changes in [3H]flunitrazepam binding in kindled rat hippocampus

The binding of the GABAA/benzodiazepine receptor agonist [3H]flunitrazepam was studied in the hippocampus of rats kindled by daily stimulation of the Schaffer collaterals, using semi-quantitative autoradiography. Two kindled stages were investigated: (i) 24 h after the last generalized tonic-clonic seizure (fully kindled) and (ii) 28 days after the last generalized seizure (long-term). The binding of [3H]flunitrazepam was determined at two concentrations, 3 and 16 nM. In the CA1 area, we found a small but significant decrease (ca. 10%), both in the 3 and 16 nM [3H]flunitrazepam binding at the fully kindled stage. In contrast, there was a significant increase in the 3 nM binding (c. 15%) at the long-term stage. The 16 nM binding was not significantly different from control binding at this stage. In the granular and molecular layers of the fascia dentata, we found at both kindled stages a significantly increased 3 nM (ca. 9 and 19%, respectively) and 16 nM (ca. 19 and 14%, respectively) binding. Furthermore, we found that muscimol was still able to enhance the [3H]flunitrazepam binding in kindled animals, indicating that the GABAA receptor agonist binding site and benzodiazepine agonist binding site are still functionally coupled. The changes in [3H]flunitrazepam binding at the fully kindled stage are in agreement with the recently observed kindling-induced changes in [3H]muscimol binding in the hippocampal formation of the same animals [Titulaer M. N. G. et al. (1994) Neuroscience 59, 817-826] and extend these observations to the benzodiazepine modulatory site of the GABAA receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Specific subunit mRNAs of the GABAA receptor are regulated by progesterone in subfields of the hippocampus

The ability of ovarian steroids to regulate the excitability of hippocampal neurons may be mediated by alterations in the inhibitory activity of GABA. We assessed the ability of estradiol, progesterone, and 3 alpha-OH-5 alpha-pregnan-20-one (3 alpha-OH-DHP; a metabolite of progesterone) to regulate gene expression of selected GABAA receptor subunits (alpha 1, alpha 2, beta 1, beta 2, and gamma 2). Using in situ hybridization, we found that progesterone, or 3 alpha-OH-DHP, suppressed mRNA levels for the alpha 1 subunit in the CA2, CA3, and the dentate gyrus subfields of the hippocampus in animals that were pretreated with estradiol. Progesterone had a more limited effect on the alpha 2 subunit, suppressing mRNA levels in estradiol-primed animals only in the CA3 region. In contrast, progesterone increased mRNA levels for the gamma 2 subunit in the CA1, CA2, and CA3 regions of the hippocampus, but only in animals that were not estradiol-primed. Estradiol alone had no significant effect on the expression of any subunit examined. Beta 1 and beta 2 subunit mRNA levels were not altered by any of the hormones tested. These data support the conclusion that progesterone and its metabolites may regulate excitability of the hippocampus by modulating the GABAA receptor gene expression; these effects of progesterone are dependent upon the circulating levels of estradiol. Alterations in the gene expression of selective subunits may lead to changes in the density of GABAA receptor protein or to changes in receptor subunit composition which might alter receptor sensitivity to activation by GABA or modulators such as the benzodiazepines and convulsants.

Expression of GABAA receptor subunit mRNAs in hippocampal pyramidal and granular neurons in the kindling model of epileptogenesis: an in situ hybridization study

To investigate the molecular changes underlying kindling epileptogenesis in the rat hippocampus, the expression levels of the genes encoding for 13 different gamma-aminobutyric acid type-A receptor (GABAAR) subunits were measured in hippocampal principal neurons using in situ hybridization techniques and semi-quantitative analysis of the autoradiograms. Schaffer collateral-commissural pathway kindled rats were investigated at three different stages of kindling acquisition, at 24 h after the last seizure and at long-term (28 days) after termination of kindling stimulations. Changes were distinct for the different subunits in the three analyzed regions (CA1, CA3, fascia dentata) and also different for the various kindling stages. In all hippocampal areas at the early phases of kindling epileptogenesis, before the appearance of generalized seizures, an increase was found of those transcripts that constituted the majority of the expressed variants in control animals (alpha 1, alpha 2, alpha 4, beta 1, beta 2, beta 3, gamma 2/gamma 2L mRNA). In these stages, the increased levels of different variants in the granular neurons of the fascia dentata were more pronounced when compared to the pattern of changes in pyramidal cells of CA1 and CA3. In fully kindled animals, the expression levels of several subunits returned to control levels, whereas beta 3 and gamma 2/gamma 2L mRNA expression was still significantly enhanced in all areas. At long-term, few changes were encountered. The long-splice variant of gamma 2 was decreased within pyramidal and granular neurons while the total level of gamma 2 mRNA was not different from controls. The increased GABAAR subunit expression in the fascia dentata may underly the reported increased GABAAR ligand binding and the increased GABA mediated inhibition. However, the decreased GABAAR binding and the attenuation of GABAergic inhibition in CA1, could not be explained by a decrement of receptor subunit expression.

Contingent tolerance to the anticonvulsant effects of carbamazepine: relationship to loss of endogenous adaptive mechanisms

Contingent tolerance to the anticonvulsant effects of carbamazepine on amygdala kindled seizures develops when the drug is repeatedly given prior to but not after the electrical stimulation. Such tolerance can be reversed by kindling the rats for several days without drug or even by continuing to give the drug but after each seizure has occurred. Contingent tolerance can be slowed by reducing the electrical stimulus intensity and by chronic continuous (as opposed to repeated paired) drug administration. Contingent cross-tolerance has been demonstrated from carbamazepine to PK11195 (a drug active at peripheral-type benzodiazepine receptors) and valproate, but not to clonazepam and diazepam (two drugs active at central-type benzodiazepine receptors) or phenytoin. Endogenous physiological changes occur in conjunction with contingent tolerance, exemplified by the decrease in seizure threshold that returns to normal upon reversal of tolerance. We suggest that contingent tolerance is associated with a loss of seizure-induced adaptations, since many biochemical changes that occur following seizures (or in non-tolerant animals given drug after seizures) are not observed in tolerant animals. These include a loss of seizure-induced up-regulation of GABAA receptors and a loss of increases in mRNA expression for corticotropin-releasing-factor (CRF), thyrotropin-releasing-hormone (TRH), neuropeptide Y (NPY), glucocorticoid receptors and brain-derived neurotrophic factor (BDNF). Thus, several putative seizure-induced anticonvulsant adaptations, such as increases in GABAA receptors and TRH and NPY mRNA fail to occur in tolerant animals. These findings are consistent with the novel observations that, paradoxically, seizures themselves appear to facilitate the anticonvulsant effects of carbamazepine or diazepam on amygdala kindled seizures. That is, animals given a 'vacation' from seizures show a decreased response to these agents, a phenomenon we have called the 'time-off seizure' effect. Thus, seizures are postulated to induce adaptive changes that influence seizure thresholds and potentiate the anticonvulsant effects of exogenously administered drugs such as carbamazepine and diazepam. Taken together, these data suggest that seizures are associated with endogenous adaptations lasting days to weeks and that a selective failure of some of these to occur during contingent drug administration may underlie the development of contingent tolerance. These observations suggest tht endogenous illness-related mechanisms may participate both in the therapeutic responses of some agents and that their failure to occur could relate to loss of drug efficacy via tolerance; these processes may reveal new potential targets for therapeutic intervention.