Partial hippocampal kindling decreases efficacy of presynaptic GABAB autoreceptors in CA1

GABAB Receptors: are they Missing in Action in Focal Epilepsy Research?

GABA, the key inhibitory neurotransmitter in the adult forebrain, activates pre- and postsynaptic receptors that have been categorized as GABAA, which directly open ligand-gated (or receptor-operated) ion-channels, and GABA_B, which are metabotropic since they operate through second messengers. Over the last three decades, several studies have addressed the role of GABA_B receptors in the pathophysiology of generalized and focal epileptic disorders. Here, we will address their involvement in focal epileptic disorders by mainly reviewing in vitro studies that have shown: (i) how either enhancing or decreasing GABA_B receptor function can favour epileptiform synchronization and thus ictogenesis, although with different features; (ii) the surprising ability of GABA_B receptor antagonism to disclose ictal-like activity when the excitatory ionotropic transmission is abolished; and (iii) their contribution to controlling seizure-like discharges during repetitive electrical stimuli delivered in limbic structures. In spite of this evidence, the role of GABA_B receptor function in focal epileptic disorders has been attracting less interest when compared to the numerous studies that have addressed GABA_A receptor signaling. Therefore, the main aim of our mini-review is to revive interest in the function of GABA_B receptors in focal epilepsy research.

GABAB Receptors in Neurodegeneration

GABA is the main inhibitory neurotransmitter in the mammalian central nervous system (CNS) and acts via metabotropic GABA_B receptors. Neurodegenerative diseases are a major burden and affect an ever increasing number of humans. The actual therapeutic drugs available are partially effective to slow down the progression of the diseases, but there is a clear need to improve pharmacological treatment thus find alternative drug targets and develop newer pharmaco-treatments. This chapter is dedicated to reviewing the latest evidence about GABA_B receptors and their inhibitory mechanisms and pathways involved in the neurodegenerative pathologies.

Expression of the Neuronal tRNA n-Tr20 Regulates Synaptic Transmission and Seizure Susceptibility

The mammalian genome has hundreds of nuclear-encoded tRNAs, but the contribution of individual tRNA genes to cellular and organismal function remains unknown. Here, we demonstrate that mutations in a neuronally enriched arginine tRNA, n-Tr20, increased seizure threshold and altered synaptic transmission. n-Tr20 expression also modulated seizures caused by an epilepsy-linked mutation in Gabrg2, a gene encoding a GABA_A receptor subunit. Loss of n-Tr20 altered translation initiation by activating the integrated stress response and suppressing mTOR signaling, the latter of which may contribute to altered neurotransmission in mutant mice. Deletion of a highly expressed isoleucine tRNA similarly altered these signaling pathways in the brain, suggesting that regulation of translation initiation is a conserved response to tRNA loss. Our data indicate that loss of a single member of a tRNA family results in multiple cellular phenotypes, highlighting the disease-causing potential of tRNA mutations.

Presynaptic GABAB receptors underlie the antiepileptic effect of low-frequency electrical stimulation in the 4-aminopyridine model of epilepsy in brain slices of young rats

Low-frequency electrical stimulation (LFES) of the brain is one of the promising methods for helping patients with pharmacoresistant epilepsy. However, the mechanism of the antiepileptic effect of LFES is still unclear. We applied electrophysiological and pharmacological tools and mathematical modeling to investigate it. Using the 4-aminopyridine (4-AP) model of epileptiform activity in juvenile rat brain slices, we found that LFES increased the interval between ictal discharges (IDs) in the entorhinal cortex. The blockade of GABA_A, GABA_B, AMPA, or NMDA synaptic receptors strongly affected the characteristics of epileptiform discharges in slices. However, only under the blockade of GABA_B receptors, LFES becomes entirely ineffective, indicating that the activation of GABA_B receptors underlies the main LFES antiepileptic effect. Further experiments allowed us to suggest that LFES activates mostly presynaptic GABA_B receptors, which decrease the probability of glutamate release. In line with this hypothesis is the following data: 1) LFES reduces the short-term synaptic depression of excitatory postsynaptic currents similar to the agonist of GABA_B receptors SKF-97541; 2) the blockade of excitatory amino acid transporters diminishes the antiepileptic effect of LFES; 3) modeling of the effects of LFES on the probability of glutamate release with a previously proposed mathematical model of epileptiform activity Epileptor-2 also shows the increase of the interval between IDs. Our findings point out a crucial role of presynaptic GABA_B receptors in the antiepileptic effect of LFES in the 4-AP model in juvenile rat brain slices.

Long-lasting disruption of spatial memory by GABAB receptor antagonist induced seizures

The objective of this project was to test whether a drug-induced model of temporal lobe seizures, namely seizures induced by a gamma aminobutyric acid (GABA_B) receptor antagonist, CGP35348, result in long-term disruption of hippocampal memory function. Seizures were induced in experimental rats by intracerebroventricular (i.c.v.) injection of CGP35348 (0.64 μmol in 3 μL) for three consecutive days; control rats received no injection. Rats were first trained to criterion on an open radial arm maze (RAM) with 4 of the 8 arms baited, then received seizure and control treatment, and tested again on the RAM during the first week (days 1-5) and fourth week (days 22-29) after the last injection. An initial i.c.v. CGP35348 injection induced a mean of 4.4 seizures in the hippocampus, often accompanied with stages 3-5 convulsions, and sometimes with jumping; three daily CGP35348 injections induced 10.4 ± 1.8 (n = 7 rats) seizures in total. In two separate experiments, seizure-treated rats performed worse than control rats in working memory (WM) during both the 1st and 4th weeks after seizures. Reference memory (RM) deficit during the 1st week after seizures was observed in only one experiment in which RM was acquired >2 weeks ago. The memory deficits were not accompanied by gross neuronal loss in the hippocampus. In conclusion, i.c.v. injection of a GABA_B receptor antagonist in adult rats induced brief, multiple, focal hippocampal seizures that induced deficits in spatial memory for up to 4 weeks.

Long-lasting changes in hippocampal GABAB-receptor mediated inhibition following early-life seizures in kindling-prone but not kindling-resistant rats

The hypotheses that hippocampal GABA_B receptor dysfunction is a long-lasting consequence of early-life seizures, and its dependence on genetic background, were tested. Two strains of rats bred to be prone (FAST) or resistant (SLOW) to amygdala kindling were used. On postnatal day (PND) 10, control rats were injected with saline, and seizure rats with kainic acid to induce status epilepticus (SE) for 2 h. A significantly lower dose of kainic acid was found to induce SE in FAST as compared to SLOW rats. Population excitatory postsynaptic potentials (pEPSPs) and population spikes (PSs) were recorded in CA1 of hippocampal slices of adult rats in vitro, following stimulation of stratum radiatum. Input-output relation of the single-pulse pEPSP and PS did not show a significant difference between seizure and control rats, sex, or strain (FAST and SLOW). Paired-pulse PSs were significantly enhanced at 10-50 ms interpulse intervals, in FAST seizure male rats compared to FAST male controls, but not in other groups. In adult FAST but not SLOW rats, significantly lower suppression of pEPSPs at 250-300 ms following heterosynaptic burst stimulation was found in seizure rats compared to control rats; the heterosynaptic suppression of the pEPSP was blocked by selective GABA_B receptor antagonist CGP55845A. The results provide evidence that an early-life SE has a long-lasting effect in decreasing GABA_B receptor-mediated presynaptic inhibition in the hippocampus, in FAST but not in SLOW rats.

Epilepsy-associated alterations in hippocampal excitability

The hippocampus exhibits a wide range of epilepsy-related abnormalities and is situated in the mesial temporal lobe, where limbic seizures begin. These abnormalities could affect membrane excitability and lead to overstimulation of neurons. Multiple overlapping processes refer to neural homeostatic responses develop in neurons that work together to restore neuronal firing rates to control levels. Nevertheless, homeostatic mechanisms are unable to restore normal neuronal excitability, and the epileptic hippocampus becomes hyperexcitable or hypoexcitable. Studies show that there is hyperexcitability even before starting recurrent spontaneous seizures, suggesting although hippocampal hyperexcitability may contribute to epileptogenesis, it alone is insufficient to produce epileptic seizures. This supports the concept that the hippocampus is not the only substrate for limbic seizure onset, and a broader hyperexcitable limbic structure may contribute to temporal lobe epilepsy (TLE) seizures. Nevertheless, seizures also occur in conditions where the hippocampus shows a hypoexcitable phenotype. Since TLE seizures most often originate in the hippocampus, it could therefore be assumed that both hippocampal hypoexcitability and hyperexcitability are undesirable states that make the epileptic hippocampal network less stable and may, under certain conditions, trigger seizures.

Impaired GABAB-mediated presynaptic inhibition increases excitatory strength and alters short-term plasticity in synapsin knockout mice

Synapsins are a family of synaptic vesicle phosphoproteins regulating synaptic transmission and plasticity. SYN1/2 genes are major epilepsy susceptibility genes in humans. Consistently, synapsin I/II/III triple knockout (TKO) mice are epileptic and exhibit severe impairments in phasic and tonic GABAergic inhibition that precede the appearance of the epileptic phenotype. These changes are associated with an increased strength of excitatory transmission that has never been mechanistically investigated. Here, we observed that an identical effect in excitatory transmission could be induced in wild-type (WT) Schaffer collateral-CA1 pyramidal cell synapses by blockade of GABA_B receptors (GABA_BRs). The same treatment was virtually ineffective in TKO slices, suggesting that the increased strength of the excitatory transmission results from an impairment of GABA_B presynaptic inhibition. Exogenous stimulation of GABA_BRs in excitatory autaptic neurons, where GABA spillover is negligible, demonstrated that GABA_BRs were effective in inhibiting excitatory transmission in both WT and TKO neurons. These results demonstrate that the decreased GABA release and spillover, previously observed in TKO hippocampal slices, removes the tonic brake of presynaptic GABA_BRs on glutamate transmission, making the excitation/inhibition imbalance stronger.

Effects of hippocampal partial kindling on sensory and sensorimotor gating and methamphetamine-induced locomotion in kindling-prone and kindling-resistant rats

The effects of hippocampal partial kindling on gating of hippocampal auditory-evoked potentials (AEPs), prepulse inhibition (PPI) to an acoustic startle response, and methamphetamine-induced locomotion were examined in selectively bred kindling-prone (Fast) and kindling-resistant (Slow) rats. Ten electrographic seizures (afterdischarges, ADs) induced by high-frequency stimulation of the hippocampal CA1 region resulted in deficits in gating of hippocampal AEP and PPI in Fast, but not Slow, rats. The increase in AD duration with kindling was similar in Fast and Slow rats. Kindling-induced changes in hippocampal AEP and PPI in Fast rats were abolished by pretest injection of CGP7930 (1mg/kg i.p.), a positive allosteric modulator of GABAB receptors. Injection of haloperidol (0.1mg/kg i.p.) daily before kindling also prevented kindling-induced changes in PPI and hippocampal AEP in Fast rats. Interestingly, methamphetamine-induced hyperlocomotion was enhanced by kindling in Slow, but not Fast, rats. However, the methamphetamine-induced hyperlocomotion in Slow rats was not suppressed by daily injection of 0.1mg/kg i.p. haloperidol before kindling, as compared with kindling without haloperidol. It is concluded that genetic disposition affected the behavioral consequences of repeated seizures. Fast rats required fewer hippocampal ADs to induce sensory (AEP) and sensorimotor (PPI) deficits, while Slow kindled rats were more sensitive to methamphetamine-induced locomotion. Dopaminergic blockade by haloperidol during kindling, or acute injection of CGP7930 before testing, attenuated some of the behavioral deficits induced by repeated hippocampal seizures, suggesting possible therapeutic strategies to treat the schizophrenic-like symptoms associated with temporal lobe epilepsy.

GABAB receptor subtypes differentially modulate synaptic inhibition in the dentate gyrus to enhance granule cell output

BACKGROUND AND PURPOSE

Activation of GABAB receptors in the dentate gyrus (DG) enhances granule cell (GC) activity by reducing synaptic inhibition imposed by hilar interneurons. This disinhibitory action facilitates signal transfer from the perforant path to the hippocampus. However, as the two main molecular subtypes, GABA(B(1a,2)) and GABA(B(1b,2)) receptors, prefer axonal terminal and dendritic compartments, respectively, they may modulate the hilar pathways at different synaptic localizations. We examined their relative expression and functions in the DG.

EXPERIMENTAL APPROACH

The localization of GABAB subtypes was revealed immunohistochemically using subunit-selective antibodies in GABA(B1a)(-/-) and GABA(B1b)(-/-) mice. Effects of subtype activation by the GABAB receptor agonist, baclofen, were examined on the perforant path-stimulated GC population activities in brain slices.

KEY RESULTS

GABA(B(1a,2)) receptors were concentrated in the inner molecular layer, the neuropil of the hilus and hilar neurons at the border zone; while GABA(B(1b,2)) receptors dominated the outer molecular layer and hilar neurons in the deep layer, showing their differential localization on GC dendrite and in the hilus. Baclofen enhanced the GC population spike to a larger extent in the GABA(B1b)(-/-) mice, demonstrating exclusively disinhibitory roles of the GABA(B(1a,2)) receptors. Conversely, in the GABA(B1a)(-/-) mice baclofen not only enhanced but also inhibited the population spike during GABAA blockade, revealing both disinhibitory and inhibitory effects of GABA(B(1b,2)) receptors.

CONCLUSIONS AND IMPLICATIONS

The GABA(B(1a,2)) and GABA(B(1b,2)) receptor subtypes differentially modulate GC outputs via selective axonal terminal and dendritic locations in the hilar pathways. The GABA(B(1a,2)) receptors exclusively mediate disinhibition, thereby playing a greater role in gating signal transfer for hippocampal spatial and pattern learning.

Regulating hippocampal hyperexcitability through GABAB Receptors

Disturbances of GABAergic inhibition are a major cause of epileptic seizures. GABA exerts its actions via ionotropic GABAA receptors and metabotropic G protein‐coupled GABAB receptors. Malfunction of GABAA inhibition has long been recognized in seizure genesis but the role of GABAB receptors in controlling seizure activity is still not well understood. Here, we examined the anticonvulsive, or inhibitory effects, of GABAB receptors in a mouse model of hippocampal kindling as well as mouse hippocampal slices through the use of GS 39783, a positive allosteric GABAB receptor modulator, and CGP 55845, a selective GABAB receptor antagonist. When administered via intraperitoneal injections in kindled mice, GS 39783 (5 mg/kg) did not attenuate hippocampal EEG discharges, but did reduce aberrant hippocampal spikes, whereas CGP 55845 (10 mg/kg) prolonged hippocampal discharges and increased spike incidences. When examined in hippocampal slices, neither GS 39783 at 5 μmol/L nor the GABAB receptor agonist baclofen at 0.1 μmol/L alone significantly altered repetitive excitatory field potentials, but GS 39783 and baclofen together reversibly abolished these field potentials. In contrast, CGP 55845 at 1 μmol/L facilitated induction and incidence of these field potentials. In addition, CGP 55845 attenuated the paired pulse depression of CA3 population spikes and increased the frequency of EPSCs in individual CA3 pyramidal neurons. Collectively, these data suggest that GABABB receptors regulate hippocampal hyperexcitability by inhibiting CA3 glutamatergic synapses. We postulate that positive allosteric modulation of GABAB receptors may be effective in reducing seizure‐related hyperexcitability.

GABAB positive modulator GS 39783 attenuated, whereas GABAB antagonist CGP55845 facilitated hippocampal EEG spikes in kindled mice and excitatory field potentials in hippocampal slices. We postulate that GABAB receptors may inhibit CA3 glutamate synapses and hence regulate hippocampal hyperexcitability.

GABA(B) autoreceptor-mediated cell type-specific reduction of inhibition in epileptic mice

GABA(B) receptors (GABA(B)Rs) mediate slow inhibitory effects on neuronal excitability and synaptic transmission in the brain. However, the GABA(B)R agonist baclofen can also promote excitability and seizure generation in human patients and animals models. Here we show that baclofen has concentration-dependent effects on the hippocampal network in a mouse model of mesial temporal lobe epilepsy. Application of baclofen at a high dose (10 mg/kg i.p.) reduced the power of γ oscillations and the frequency of pathological discharges in the Cornu Ammonis area 3 (CA3) area of freely moving epileptic mice. Unexpectedly, at a lower dose (1 mg/kg), baclofen markedly increased γ activity accompanied by a higher incidence of pathological discharges. Intracellular recordings from CA3 pyramidal cells in vitro further revealed that, although at a high concentration (10 µM), baclofen invariably resulted in hyperpolarization, at low concentrations (0.5 µM), the drug had divergent effects, producing depolarization and an increase in firing frequency in epileptic but not control mice. These excitatory effects were mediated by the selective muting of inhibitory cholecystokinin-positive basket cells (CCK(+) BCs), through enhanced inhibition of GABA release via presynaptic GABA(B)Rs. We conclude that cell type-specific up-regulation of GABA(B)R-mediated autoinhibition in CCK(+) BCs promotes aberrant high frequency oscillations and hyperexcitability in hippocampal networks of chronic epileptic mice.

Synaptic and extrasynaptic origin of the excitation/inhibition imbalance in the hippocampus of synapsin I/II/III knockout mice

Synapsins (Syn I, Syn II, and Syn III) are a family of synaptic vesicle phosphoproteins regulating synaptic transmission and plasticity. SYN1/2 genes have been identified as major epilepsy susceptibility genes in humans and synapsin I/II/III triple knockout (TKO) mice are epileptic. However, excitatory and inhibitory synaptic transmission and short-term plasticity have never been analyzed in intact neuronal circuits of TKO mice. To clarify the generation and expression of the epileptic phenotype, we performed patch-clamp recordings in the CA1 region of acute hippocampal slices from 1-month-old presymptomatic and 6-month-old epileptic TKO mice and age-matched controls. We found a strong imbalance between basal glutamatergic and γ-aminobutyric acid (GABA)ergic transmission with increased evoked excitatory postsynaptic current and impaired evoked inhibitory postsynaptic current amplitude. This imbalance was accompanied by a parallel derangement of short-term plasticity paradigms, with enhanced facilitation of glutamatergic transmission in the presymptomatic phase and milder depression of inhibitory synapses in the symptomatic phase. Interestingly, a lower tonic GABA(A) current due to the impaired GABA release is responsible for the more depolarized resting potential found in TKO CA1 neurons, which makes them more susceptible to fire. All these changes preceded the appearance of epilepsy, indicating that the distinct changes in excitatory and inhibitory transmission due to the absence of Syns initiate the epileptogenic process.

GABA(B) receptor blockade in the hippocampus affects sensory and sensorimotor gating in Long-Evans rats

RATIONALE

Sensory and sensorimotor gating deficits are observed in schizophrenia. GABA(B) receptor deficiency is also detected in the hippocampus of schizophrenic patients.

OBJECTIVES

The present study tested the hypothesis that GABA(B) receptors in the hippocampus contribute to paired-pulse gating of hippocampal auditory-evoked potentials (AEP) and auditory prepulse inhibition (PPI) in Long-Evans rats.

METHODS

Gating of hippocampal AEP, or PPI, was examined before and after administration of GABA(B) receptor antagonist, CGP56999A or CGP35348, or saline was administered either systemically (intra-peritoneally (i.p.)) or infused bilaterally into the hippocampus 15 min before gating measurements.

RESULTS

Systemic injection of CGP56999A, at a dose of 0.2 and 0.4 mg/kg i.p. resulted in reduced gating of hippocampal AEP in a dose-dependent manner. Reduced gating was found at conditioning-test interpulse intervals of 300-500 ms, but not 100-200 ms. Reduced gating of hippocampal AEP also followed bilateral infusion of CGP56999A into the hippocampus (0.1 μg/μL/side). Gating loss was attributed to a decreased conditioning response and an increased test response after systemic or local injection of CGP56999A. Robust PPI was found at prepulse-pulse intervals of 30-100 ms, and this PPI was reduced by hippocampal infusion of CGP56999A in a dose-dependent manner, as compared with saline infusion.

CONCLUSIONS

Blockade of hippocampal GABA(B) receptors led to deficits in sensory and sensorimotor gating, which are symptoms of schizophrenia.

Impaired function of GABA(B) receptors in tissues from pharmacoresistant epilepsy patients

PURPOSE

Effects of pre- and postsynaptic γ-aminobutyric acid B (GABA(B)) receptor activation were characterized in human tissue from epilepsy surgery.

METHODS

Slices of human cortical tissue were investigated in a submerged-type chamber with intracellular recordings in layers II/III. Parallel experiments were performed in rat neocortical slices with identical methods. Synaptic responses were elicited with single or paired stimulations of incrementing intervals.

RESULTS

Neurons in human epileptogenic tissue exhibited usually small inhibitory postsynaptic potentials (IPSP) mediated by GABA(B) receptor, verified by the sensitivity to the selective antagonist CGP 55845A. The IPSP(B) conductance averaged 5.8 nS in neurons from epileptogenic tissues and 15.9 nS in neurons from nonepileptogenic tissues (p < 0.0001). Application of baclofen caused small conductance increases in human neurons, which were linearly related to IPSP(B) conductances. Paired-pulse stimulation revealed constant synaptic responses in human temporal lobe epilepsy (TLE) slices at all interstimulus intervals (ISIs). Pharmacologically isolated IPSP(A) in the human tissue exhibited a small paired-pulse depression (average 10% at 500 ms ISI). Bicuculline-induced paroxysmal depolarization shifts (PDSs) were transiently depressed by 24% in human TLE tissue; and by 74% in rat neocortical slices (200 ms ISI; p = 0.015). The depressions of bicuculline-induced PDSs were antagonized by CGP 55845A in both species. Staining for GABA(B) receptors revealed significantly smaller numbers of immunopositive dots in human epileptogenic neurons versus human control neurons.

DISCUSSION

The small IPSP(B), baclofen-conductances, and paired-pulse depression of PDSs and IPSPs in human TLE tissue indicate a reduced density of post- and presynaptic GABA(B) receptors. The reduced efficacy of presynaptic GABA(B) receptors facilitates the occurrence of repetitive synaptic activity.

Activation of GABA(B) receptors reverses spontaneous gating deficits in juvenile DBA/2J mice

RATIONALE

Gamma-amino-butyric acid (GABA)(B) receptors play a key role in the pathophysiology of psychotic disorders. We previously reported that baclofen, the prototypical GABA(B) agonist, elicits antipsychotic-like effects in the rat paradigm of prepulse inhibition (PPI) of the startle, a highly validated animal model of schizophrenia.

OBJECTIVES

We studied the role of GABA(B) receptors in the spontaneous PPI deficits displayed by DBA/2J mice.

MATERIALS AND METHODS

We tested the effects of baclofen (1.25-5 mg/kg, intraperitoneal [i.p.]) in DBA/2J and C57BL/6J mice, in comparison to the antipsychotic drugs haloperidol (1 mg/kg, i.p.) and clozapine (5 mg/kg, i.p.). Furthermore, we investigated the expression of GABA(B) receptors in the brain of DBA/2J and C57BL/6J mice by quantitative autoradiography.

RESULTS

Baclofen dose-dependently restored PPI deficit in DBA/2J mice, in a fashion similar to the antipsychotic clozapine (5 mg/kg, i.p.). This effect was reversed by pretreatment with the GABA(B) antagonist SCH50211 (50 mg/kg, i.p.). In contrast, baclofen did not affect PPI in C57BL/6J mice. Finally, quantitative autoradiographic analyses assessed a lower GABA(B) receptor expression in DBA/2J mice in comparison to C57BL/6J controls in the prefrontal cortex and hippocampus but not in other brain regions.

CONCLUSIONS

Our data highlight GABA(B) receptors as an important substrate for sensorimotor gating control in DBA/2J mice and encourage further investigations on the role of GABA(B) receptors in sensorimotor gating, as well as in the pathophysiology of psychotic disturbances.

Offsetting of aberrations associated with seizure proneness in rat hippocampus area CA1 by theta pulse stimulation-induced activity pattern

Epileptiform activity induces long term aberrations in hippocampal network functions. This study was conducted in pentylenetetrazol (PTZ) -kindled rats to examine offsetting of aberrations associated with seizure proneness in hippocampus area CA1 by theta pulse stimulation (TPS: 5 Hz trains for 3 min) -induced activity pattern. In hippocampal slices from both control and kindled rats, the field excitatory postsynaptic potentials (fEPSP) and population spikes (PS) were simultaneously recorded through electrodes in the apical dendrites and stratum pyramidale, respectively. The following changes in kindled vs. control slices were observed. The fEPSP needed to be greater to produce the PS recorded in the cell body layer. The fEPSP was reduced by paired stimuli whereas the PS amplitude was increased. TPS selectively depressed the PS in a lasting fashion, and shifted the fEPSP slope and the PS amplitude relation toward what was observed in controls. Both the fEPSP and PS were increased by paired stimuli at 60 min after TPS application. The lasting depressive effect of TPS on the PS amplitude was converted into facilitation by adenosine A1 receptor antagonist 8-cyclopentyl-1, 3-dipropylxanthine (CPX). Potentiation of the PS amplitude by TPS in the presence of CPX was blocked by an N-methyl-d-aspartate receptor antagonist AP5. We hypothesize that the extracellular adenosine spillover, acting through adenosine A1 receptors, during TPS-induced activity pattern could trigger a homeostatic process for correcting network imbalances caused by epileptiform activity.

Electrically evoked GABA release in rat hippocampus CA1 region and its changes during kindling epileptogenesis

Previous findings on changes in K+-induced GABA release from hippocampal slices during kindling epileptogenesis were reinvestigated using physiological electrical stimulation. For that purpose, a procedure was developed enabling neurochemical monitoring of GABA release locally in the CA1 region of rat hippocampal slices upon tetanic stimulation of Schaffer-collateral fibers. In the presence of a GABA reuptake blocker, subsequent application of short (3 s) pulses of 50-Hz stimuli induced a local transient increase in GABA release. In slices from fully kindled animals, 24 h after the last generalized seizure, tetanically stimulated GABA release was increased in comparison to control slices. In slices from long-term kindled animals, 4-5 weeks after the last seizure, tetanically stimulated GABA release had returned to control levels. Application of the broad low-affinity GABAB receptor antagonist saclofen increased the tetanically stimulated GABA release in control slices, but had no effect in fully kindled slices. In slices from long-term kindled animals, however, saclofen enhanced GABA release similarly as in control slices. We conclude that the transient increase in tetanus-induced GABA release during kindling epileptogenesis is seizure-related, and probably caused by temporarily impaired presynaptic GABAB receptors. The possible relevance of this finding for GABA transmission in epilepsy is discussed.

Environment matters: synaptic properties of neurons born in the epileptic adult brain develop to reduce excitability

Neural progenitors in the adult dentate gyrus continuously produce new functional granule cells. Here we used whole-cell patch-clamp recordings to explore whether a pathological environment influences synaptic properties of new granule cells labeled with a GFP-retroviral vector. Rats were exposed to a physiological stimulus, i.e., running, or a brain insult, i.e., status epilepticus, which gave rise to neuronal death, inflammation, and chronic seizures. Granule cells formed after these stimuli exhibited similar intrinsic membrane properties. However, the new neurons born into the pathological environment differed with respect to synaptic drive and short-term plasticity of both excitatory and inhibitory afferents. The new granule cells formed in the epileptic brain exhibited functional connectivity consistent with reduced excitability. We demonstrate a high degree of plasticity in synaptic inputs to adult-born new neurons, which could act to mitigate pathological brain function.

Depression of glutamate and GABA release by presynaptic GABAB receptors in the entorhinal cortex in normal and chronically epileptic rats

Presynaptic GABA(B) receptors (GABA(B)R) control glutamate and GABA release at many synapses in the nervous system. In the present study we used whole-cell patch-clamp recordings of spontaneous excitatory and inhibitory synaptic currents in the presence of TTX to monitor glutamate and GABA release from synapses in layer II and V of the rat entorhinal cortex (EC)in vitro. In both layers the release of both transmitters was reduced by application of GABA(B)R agonists. Quantitatively, the depression of GABA release in layer II and layer V, and of glutamate release in layer V was similar, but glutamate release in layer II was depressed to a greater extent. The data suggest that the same GABA(B)R may be present on both GABA and glutamate terminals in the EC, but that the heteroreceptor may show a greater level of expression in layer II. Studies with GABA(B)R antagonists suggested that neither the auto- nor the heteroreceptor was consistently tonically activated by ambient GABA in the presence of TTX. Studies in EC slices from rats made chronically epileptic using a pilocarpine model of temporal lobe epilepsy revealed a reduced effectiveness of both auto- and heteroreceptor function in both layers. This could suggest that enhanced glutamate and GABA release in the EC may be associated with the development of the epileptic condition.

Successive alterations of hippocampal gamma-aminobutyric acid B receptor subunits in a rat model of febrile seizure

Febrile seizure (FS) is a frequently encountered seizure type in childhood. Changes of brain function following FS have clinical importance. The recently identified gamma-aminobutyric acid B receptor (GABA(B)R) is a metabotropic receptor of GABA. In this study, we used a rat model of recurrent FS to investigate the changes of GABA(B)R1a and GABA(B)R2 subunits in hippocampus after recurrent FS by using Western blot, quantitative RT-PCR, double immunofluorescence, in situ hybridization and immunoprecipitation/Western blot. After treatment of hyperthermia and the presence of induced seizures once every 2 days for 10 times, GABA(B)R1a and GABA(B)R2 subunits in hippocampus were decreased after 24 h of the last treatment. The decrease of GABA(B)R1a lasted for 15 days but that of GABA(B)R2 persisted for more than 30 days. The binding of GABA(B)R1a to GABA(B)R2 in hippocampus was also decreased significantly after 24 h of the last treatment and lasted for more than 30 days. In situ hybridization showed that GABA(B)R1a mRNA was significantly decreased in dentate gyrus, and GABA(B)R2 mRNA was considerably reduced in CA3 region. In H10 and FS1 groups in which hyperthermia treatment was the same but no (H10 group) or only one seizure (FS(1) group) was induced, the decrease of GABA(B)R1a and GABA(B)R2 subunits and the reduced binding capability between GABA(B)R1a and GABA(B)R2 subunits were also detected but with less severity, and the time recovering from these abnormalities was shorter. We conclude that GABA(B)R1a and GABA(B)R2 subunits and the binding of the 2 subunits decrease in hippocampus for a relatively long period of time after recurrent FS in immature rats. These changes may result in long-lasting imbalance of excitation/inhibition function in hippocampus, and are derived from the consequences of recurrent febrile seizures.

Decrease of hippocampal GABA B receptor-mediated inhibition after hyperthermia-induced seizures in immature rats

PURPOSE

Whether febrile seizures have detrimental consequences on the brain is still controversial. We hypothesized that neuronal inhibition in the hippocampus is altered after hyperthermia-induced seizures in immature rats.

METHODS

Rats were given a single seizure by a heat lamp on postnatal day (PND) 15, or repeated seizures by heated air on PND 13 to 15. Fourteen or 30 days after the seizure(s), laminar field potentials were recorded by 16-channel silicon probes in CA1 and the dentate gyrus (DG), in response to the paired-pulse stimulation of the CA3 and medial perforant path, and analyzed as current source density. Gamma-aminobutyric acid (GABA)(B)-receptor antagonist CGP35348 was injected intracerebroventricularly (icv).

RESULTS

At 14 but not at 30 days after a single or after repeated hyperthermia-induced seizures, paired-pulse facilitation (PPF) of the CA1 population spikes at 100 to 200 ms interpulse intervals (IPIs) was significantly increased in seizure as compared with control rats, irrespective of the types of induced seizures. CGP35348 icv also resulted in PPF at 100 to 200 ms IPIs in CA1 of control rats, but CGP35348 had no effect on PPF in seizure rats. At 30 days after repeated seizures, paired-pulse inhibition in the DG was significantly increased at 30-ms IPI, and PPF was increased at 200-ms IPI. CGP35348 increased paired-pulse inhibition in the DG in repeated-seizure rats but not in control rats.

CONCLUSIONS

We conclude that hyperthermia-induced seizures in immature rats induced a decrease of GABA(B) receptor-mediated inhibition in CA1 and DG that lasted > or =14 to 30 days after hyperthermic seizure(s).

Presynaptic GABA(B) receptors on glutamatergic terminals of CA1 pyramidal cells decrease in efficacy after partial hippocampal kindling

We tested the hypothesis that presynaptic GABA(B) receptors on glutamatergic terminals (GABA(B) heterosynaptic receptors) decreased in efficacy after partial hippocampal kindling. Rats were implanted with chronically indwelling electrodes and 15 hippocampal afterdischarges were evoked by high-frequency electrical stimulation of hippocampal CA1. Control rats were implanted with electrodes but not given high-frequency stimulations. One to 21 days after the last afterdischarge, excitatory postsynaptic potentials (EPSPs) were recorded in CA1 of hippocampal slices in vitro, following stimulation of the stratum radiatum. Field EPSPs (fEPSPs) were recorded in CA1 stratum radiatum and intracellular EPSPs (iEPSPs) were recorded from CA1 pyramidal cells. GABA(B) receptor agonist +/- baclofen (10 microM) in the bath suppressed the fEPSPs significantly more in control than kindled rats, at 1 or 21 days after kindling. Similarly, baclofen (10 microM) suppressed iEPSPs more in the control than the kindled group of neurons recorded at 1 day after kindling. Suppression of the fEPSPs by 1 microM N(6)-cyclopentyladenosine, which acted on presynaptic A1 receptors, was not different between kindled and control rats. Activation of the GABA(B) heteroreceptors by a conditioning burst stimulation of CA3 afferents suppressed the iEPSPs evoked by a test pulse. The suppression of the iEPSPs at 250-500 ms condition-test interval was larger in control than kindled groups of neurons. It was concluded that the efficacy of presynaptic GABA(B) receptors on the glutamatergic terminals was reduced after partial hippocampal kindling. The reduction in heterosynaptic presynaptic GABA(B) receptor efficacy will increase glutamate release and seizure susceptibility, particularly during repeated neural activity.

The effect of topiramate on GABAB receptor, vesicular GABA transporter and paired-pulse inhibition in the gerbil hippocampus

To extend our understanding of the properties of topiramate (TPM), we investigated the effect of TPM on GABAergic transmission in the dentate gyrus of gerbil. TPM treatment (> or = 40 mg/kg) dramatically decreased GABA(B)R2, not GABA(B)R1, immunoreactivity in hilar interneurons. In contrast, TPM treatment increased vesicular GABA transporter immunoreactivity and the paired-pulse inhibition in the dentate gyrus of seizure prone gerbils. Furthermore, TPM effectively prevented the reduction of paired-pulse inhibition induced by baclofen treatment. These findings suggest that TPM may enhance GABA release in the dentate gyrus of gerbils by down-regulation of GABA(B) autoreceptor expression. Therefore, these properties of TPM may be another possible antiepileptic effect, which plays an important role in preventing the spread of seizure activity without proconvulsive effects.

Mechanisms contributing to the exacerbated epileptiform activity in hippocampal slices expressing a C-terminal truncated GABA(B2) receptor subunit

GABAergic synaptic transmission plays an important role in the patterning of epileptiform activity. We have previously shown that global loss of GABA(B) receptor function due to transgenic deletion of the GABA(B1) receptor subunit exacerbates epileptiform activity induced by pharmacological manipulations in hippocampal slices. Here we show that a similar hyperexcitable phenotype is observed in hippocampal slices prepared from a transgenic mouse expressing a GABA(B2) receptor subunit lacking its C terminal tail (the DeltaGB2-Ct mouse); a molecular manipulation that also produces complete loss of GABA(B) receptor function. Thus, epileptiform bursts that are sensitive to NMDA receptor antagonists (induced by either the GABA(A) receptor antagonist bicuculline (10muM) or removal of extracellular Mg(2+)) were significantly longer in duration in DeltaGB2-Ct slices relative to WT slices. We now extend these observations to demonstrate that a stimulus train induced bursting (STIB) protocol also evokes significantly longer bicuculline sensitive bursts of activity in DeltaGB2-Ct slices compared to WT. Furthermore, synchronous GABA(A) receptor-mediated potentials recorded in the presence of the potassium channel blocker 4-aminopyridine (4-AP, 100muM) and the ionotropic glutamate receptor antagonists NBQX (20muM) and D-AP5 (50muM) were significantly prolonged in duration in DeltaGB2-Ct versus WT slices. These data suggest that the loss of GABA(B) receptor function in DeltaGB2-Ct hippocampal slices promotes depolarising GABA(A) receptor-mediated events, which in turn, leads to the generation of ictal-like events, which may contribute to the epilepsy phenotype observed in vivo.

Epileptiform activity triggers long-term plasticity of GABA(B) receptor signalling in the developing rat hippocampus

GABA(B) receptor (GABA(B)R)-mediated presynaptic inhibition regulates neurotransmitter release from synaptic terminals. In the neonatal hippocampus, GABA(B)R activation reduces GABA release and terminates spontaneous network discharges called giant depolarizing potentials (GDPs). Blocking GABA(B)Rs transforms GDPs into longer epileptiform discharges. Thus, GABA(B)R-mediated presynaptic inhibition of GABA release (GABA auto-inhibition) controls both spontaneous network activity and excitability in the developing hippocampus. Here we show that extensive release of endogenous GABA during epileptiform activity impairs GABA auto-inhibition, but not GABA(B)R-mediated inhibition of glutamate release, leading to hyperexcitability of the neonatal hippocampal network. Paired-pulse depression of GABA release (PPD) and heterosynaptic depression of glutamate release were used to monitor the efficacy of presynaptic GABA(B)R-mediated inhibition in slices. PPD, but not heterosynaptic depression, was dramatically reduced after potassium (K+)-induced ictal-like discharges (ILDs), suggesting a selective impairment of GABA(B)R-dependent presynaptic inhibition of GABAergic terminals. Impairing GABA auto-inhibition induced a 44% increase in GDP width and the appearance of pathological network discharges. Preventing GABA-induced activation of GABA(B)Rs during ILDs avoided PPD loss and most modifications of the network activity. In contrast, a partial block of GABA(B)Rs induced network discharges strikingly similar to those observed after K+-driven ILDs. Finally, neither loss of GABA auto-inhibition nor network hyperexcitability could be observed following synchronous release of endogenous GABA in physiological conditions (during GDPs at 1 Hz). Thus, epileptiform activity was instrumental to impair GABA(B)R-dependent presynaptic inhibition of GABAergic terminals. In conclusion, our results indicate that endogenous GABA released during epileptiform activity can reduce GABA auto-inhibition and trigger pathological network discharges in the newborn rat hippocampus. Such functional impairment may play a role in acute post-seizure plasticity.

Kindling Limits the Interictal Neuronal Temporal Response Properties in Cat Primary Auditory Cortex

PURPOSE

The present study examined the effect of electrical kindling on the interictal temporal response properties of single units recorded from primary auditory cortex (AI) of the adult cat.

METHODS

Cats were permanently implanted with electrodes in AI, kindled twice daily for 40 sessions, and the contralateral AI was subsequently mapped. Kindling stimulation consisted of 1-s trains of biphasic square-wave pulses applied at a frequency of 60 Hz, 100 microA above the afterdischarge (AD) threshold. The EEG activity was recorded during each kindling session, and the behavioral manifestation was scored. Subsequent to kindling, multiple single-unit responses were recorded under ketamine anesthesia in response to 1-s-long periodic click trains, with click rates between 2 and 64 Hz. Neuronal responses were characterized according to their ability to respond in time-locked fashion to the clicks.

RESULTS

Kindling stimulation resulted in progression of the AD characteristics and seizure behavior, with six of 10 kindled cats reaching a fully generalized state. In the fully kindled cats, the best modulation frequencies and limiting following rates for the single-unit responses were significantly lower compared with those of naive and sham controls.

CONCLUSIONS

Repeated epileptiform activity interferes with temporal processing in cat auditory cortex in the interictal state. This may have implications for people with epileptic foci in auditory-related areas.

Epileptogenic insult causes a shift in the form of long-term potentiation expression

The relationship between epilepsy, modeled here by pentylenetetrazol kindling, and learning deficits, modeled here by long-term potentiation (LTP), was studied. The field excitatory postsynaptic potentials and population spikes (PS) were recorded from strata radiatum and pyramidale, respectively, in urethane-anesthetized rat dorsal hippocampus CA1 area upon stimulation of Schaffer collaterals. To induce LTP, a 100 Hz primed-burst stimulation protocol was used. Experiments were carried out at approximately 30 days after the last pentylenetetrazol dose. The effects of voltage dependent calcium channel blocker verapamil and N-methyl-D-aspartate receptor antagonist MK-801 on LTP expression were examined. Tetanic stimulation elicited both field excitatory postsynaptic potential LTP and PS LTP in control animals, and LTP-induction of the PS in control animals was attenuated by MK-801, but not by verapamil. By contrast, kindled rats showed LTP of the PS only. MK-801 reduced the extent of potentiation of PS amplitude and verapamil inhibited the PS amplitude potentiation, completely. The results suggest that seizure induction modifies mechanisms underlying LTP induction and causes a shift in the form of LTP expression. The pentylenetetrazol-kindling-induced increase in PS LTP is sensitive to verapamil and not to MK-801 and therefore primarily dependent on activation of voltage dependent calcium channels rather N-methyl-D-aspartate receptors. Kindling may lead to a shift in synaptic plasticity thresholds much like the shift that occurs during aging, and such alterations may contribute to deficits in learning and memory.

Molecular structure and physiological functions of GABA(B) receptors

GABA(B) receptors are broadly expressed in the nervous system and have been implicated in a wide variety of neurological and psychiatric disorders. The cloning of the first GABA(B) receptor cDNAs in 1997 revived interest in these receptors and their potential as therapeutic targets. With the availability of molecular tools, rapid progress was made in our understanding of the GABA(B) system. This led to the surprising discovery that GABA(B) receptors need to assemble from distinct subunits to function and provided exciting new insights into the structure of G protein-coupled receptors (GPCRs) in general. As a consequence of this discovery, it is now widely accepted that GPCRs can exist as heterodimers. The cloning of GABA(B) receptors allowed some important questions in the field to be answered. It is now clear that molecular studies do not support the existence of pharmacologically distinct GABA(B) receptors, as predicted by work on native receptors. Advances were also made in clarifying the relationship between GABA(B) receptors and the receptors for gamma-hydroxybutyrate, an emerging drug of abuse. There are now the first indications linking GABA(B) receptor polymorphisms to epilepsy. Significantly, the cloning of GABA(B) receptors enabled identification of the first allosteric GABA(B) receptor compounds, which is expected to broaden the spectrum of therapeutic applications. Here we review current concepts on the molecular composition and function of GABA(B) receptors and discuss ongoing drug-discovery efforts.

Altered GABAB receptor immunoreactivity in the gerbil hippocampus induced by baclofen and phaclofen, not seizure activity

The present study was performed to determine whether the effects induced by GABA(B) receptor-acting drugs would be related with the alteration in GABA(B) receptor expression in the hippocampus using Mongolian gerbil, a genetic epilepsy model. The distribution patterns of both GABA(B) receptor 1A/B and GABA(B)receptor 2 immunoreactivities were similarly detected in the hippocampi of normal and seizure-prone gerbils. Following baclofen (GABA(B) receptor agonist) or phaclofen (GABA(B) receptor antagonist) treatment, GABA(B) receptor immunoreactivities were decreased or increased by dose-dependent manners, respectively. Vigabatrin (GABA transaminase inhibitor) or 3-mercaptopropionic acid (GAD inhibitor) treatment did not affect GABA(B) receptor expressions. These findings suggest that GABA(B) receptor expression in the gerbil hippocampus may be altered by baclofen or phaclofen treatment.

GABAA, not GABAB, receptor shows subunit- and spatial-specific alterations in the hippocampus of seizure prone gerbils

In the present study, we investigated site-specific expressions of GABA(A) and GABA(B) receptor subunits in the seizure-sensitive (SS) and seizure-resistant (SR) gerbil hippocampus to elucidate the function of the gamma-aminobutyric acid (GABA) receptor in seizure activity in this animal. There were no differences of the immunoreactivities of GABA(B) receptor and some GABA(A) receptor subunits (alpha3, alpha4, pan beta and delta) in the hippocampus between SR and SS gerbils. The alpha1 subunit expression was mainly detected in interneurons of stratum radiatum and hilar region of dentate gyrus in the SR gerbil. However, in SS gerbil, interneurons were nearly devoid of alpha1 subunit immunoreactivity and mainly detected in the molecular layer of dentate gyrus. In SR gerbil, alpha2 subunit immunoreactivity was detected in Ammon's horn, particularly in the CA2 region. In SS gerbil, granule cell layer of the dentate gyrus in SS gerbil showed strong alpha2 subunit immunoreactivity. The distribution of alpha5 and gamma2 subunit immunoreactivity in the hippocampus was similarly detected in SR and SS gerbil. However, alpha5 immunodensity of SR gerbil was slightly lower than that of SS gerbil in CA1 region and was slightly strong than that of SS gerbil in subiculum. These differences in distribution of GABA(A) receptor, not GABA(B) receptor, in the SR and SS gerbil hippocampus may indicate that abnormal hyperactive neuronal discharges are occurred in SS gerbil, which presumably result in spontaneous and repetitive seizure activity in this animal.

Plasticity of GABA(B) receptor-mediated heterosynaptic interactions at mossy fibers after status epilepticus

Several neurotransmitters, including GABA acting at presynaptic GABA(B) receptors, modulate glutamate release at synapses between hippocampal mossy fibers and CA3 pyramidal neurons. This phenomenon gates excitation of the hippocampus and may therefore prevent limbic seizure propagation. Here we report that status epilepticus, triggered by either perforant path stimulation or pilocarpine administration, was followed 24 hr later by a loss of GABA(B) receptor-mediated heterosynaptic depression among populations of mossy fibers. This was accompanied by a decrease in the sensitivity of mossy fiber transmission to the exogenous GABA(B) receptor agonist baclofen. Autoradiography revealed a reduction in GABA(B) receptor binding in the stratum lucidum after status epilepticus. Failure of GABA(B) receptor-mediated modulation of mossy fiber transmission at mossy fibers may contribute to the development of spontaneous seizures after status epilepticus.

Partial hippocampal kindling increases GABAB receptor-mediated postsynaptic currents in CA1 pyramidal cells

In previous studies, we showed that partial hippocampal kindling decreased the efficacy of the presynaptic GABAB receptors on both GABAergic and glutamatergic terminals of CA1 neurons in hippocampal slices in vitro. In this study, GABAB receptor-mediated inhibitory postsynaptic currents (GABAB-IPSCs) were assessed by whole-cell recordings in CA1 pyramidal neurons in hippocampal slices of male Long-Evans rats. The peak GABAB-IPSC evoked by a brief train of supramaximal stratum radiatum stimuli (20 pulses of 300 Hz) in the presence of picrotoxin (0.1 mM) and kynurenic acid (1 mM) was larger in neurons of kindled (65.9 +/- 5.2 pA, N=42 cells) than control (45.8 +/- 4.8 pA, N=32 cells) rats (P<0.01). Adding GABA uptake blocker nipecotic acid (1 mM) or GABAB receptor agonist baclofen (0.01 mM) in the perfusate induced outward currents that were blocked by GABAB receptor antagonist CGP 55845A (1 microM). The peak outward current induced by nipecotic acid was larger in neurons of the kindled (55.4 +/- 5.7 pA, N=30) than the control group (39.8 +/- 4.5 pA, N=28) (P<0.05). However, the magnitude of the baclofen-induced current was not different between kindled (90.8 +/- 6.9 pA, N=29) and control (87.2 +/- 5.9 pA, N=21) groups (P>0.05). We concluded that partial hippocampal kindling increased GABAB-IPSCs in hippocampal CA1 pyramidal cells via multiple presynaptic mechanisms.

Kindling alters entorhinal cortex-hippocampal interaction by increased efficacy of presynaptic GABA(B) autoreceptors in layer III of the entorhinal cortex

We studied the effect of kindling, a model of temporal lobe epilepsy, on the frequency-dependent information transfer from the entorhinal cortex to the hippocampus in vitro. In control rats repetitive synaptic activation of layer III projection cells resulted in a frequency dependent depression of the synaptic transfer of action potentials to the hippocampus. One-to-two-days after kindling this effect was strongly reduced. Although no substantial change in synaptic inhibition upon single electrical stimulation was detected in kindled rats, there was a significant depression in the prolonged inhibition following high frequency stimulation. In kindled animals, paired-pulse depression (PPD) of stimulus-evoked IPSCs in layer III neurons was significantly stronger than in control rats. The increase of PPD is most likely caused by an increased presynaptic GABA(B) receptor-mediated autoinhibition. In kindled animals activation of presynaptic GABA(B) receptors by baclofen (10 microM) suppressed monosynaptic IPSCs significantly more than in control rats. In contrast, activation of postsynaptic GABA(B) receptors by baclofen was accompanied by comparable changes of the membrane conductance in both animal groups. Thus, in kindled animals activation of the layer III-CA1 pathway is facilitated by an increased GABA(B) receptor-mediated autoinhibition leading to an enhanced activation of the monosynaptic EC-CA1 pathway.

Presynaptic gamma-aminobutyric acid type B receptor-mediated regulation of vesicular gamma-aminobutyric acid transporter expression in the gerbil hippocampus

Changes in vesicular gamma-aminobutyric acid (GABA) transporter (VGAT) expression in the gerbil hippocampus after treatment with baclofen (GABA(B) receptor agonist) or phaclofen (GABA(B) receptor antagonist) were investigated to identify the GABA(B) receptor-mediated regulation of VGAT expression. In the baclofen-treated seizure-resistant gerbils, VGAT expression was significantly reduced, as compared with the control animals, thus the VGAT immunoreactive pattern in these gerbils was similar to that in control seizure-sensitive (SS) gerbils. In the phaclofen-treated SS gerbils, VGAT expression was dramatically elevated, compared to SS gerbil controls. Our findings demonstrated that GABA(B) receptor-mediated regulation of VGAT expression may be another mechanisms for presynaptic GABA release, which is accompanied by a reduction in Ca(2+) conductance by the inhibition of voltage-gated Ca(2+) channels.

Up-regulation of GABA(B) receptor mRNA and protein in the hippocampus of cocaine- and lidocaine-kindled rats

To evaluate the effect of GABA(B) receptor in drug-kindled seizures, the gene expression of GABA(B) receptor in cocaine- and lidocaine-kindled rats was examined in this study. Rats were injected (i.p.) daily with cocaine (55 mg/kg) or lidocaine (65 mg/kg) until they experienced a motor seizure (kindling). After kindling, rats received a 1-day, 10-day, or 30-day drug washout period. The rats in the 1-day washout group were killed after the washout. Those in the 10-day and 30-day groups were challenged either with drug or saline, and killed 24 h later. Control rats were injected and challenged with saline. GABA(B)R1a, 1b and R2 mRNAs in discrete regions of brain were detected by in situ hybridization; GABA(B)R1a protein level was measured by Western blotting. Ninety percent of the cocaine-treated rats and 100% of the lidocaine-treated rats were kindled by day 12. Those rats responded to the challenge cocaine or lidocaine with a motor seizure after the 10-day and 30-day washout. GABA(B) receptor mRNA and protein levels in the hippocampus were significantly increased after the 1-day and 10-day washout, but not the 30-day washout. In addition, the levels in drug-treated and drug-challenged rats were significantly greater than those in drug-treated and saline-challenged rats after the 10-day washout. Those data suggest that changes of GABA(B) receptor gene expression could be a factor underlying the development of drug-kindled seizure, but not a necessary component for the maintenance of this phenomenon.

GABA(B) receptor 1 polymorphism (G1465A) is associated with temporal lobe epilepsy

BACKGROUND

Dysfunction of gamma-aminobutyric acid (GABA) (B) receptors has been implicated in the pathogenesis of temporal lobe epilepsy (TLE).

OBJECTIVE

To evaluate the genetic contribution of cloned human GABA(B) receptors to TLE.

METHODS

The authors genotyped 141 patients (78 women and 63 men; mean age = 49.1 +/- 18.0 years) with nonlesional TLE and 372 age- and sex-matched normal individuals for the known polymorphism G1465A in the human GABA(B) receptor 1 [GABA(B[1])] gene.

RESULTS

There was a highly significant overrepresentation of the G1465A heterozygote in patients with TLE compared with controls. The A/G genotype was found in 17% of the 141 patients with TLE and in only 0.5% of the 372 controls (p < 0.0001). The authors also found that patients carrying the A allele had a significantly higher risk (p = 0.003, OR = 6.47, 95% CI = 2.02 to 20.76) of developing drug-resistant TLE. Furthermore, the age at onset of seizures tended to be lower in patients with A/G genotype, but the difference was not significant.

CONCLUSIONS

The results of this study indicate that the GABA(B[1]) polymorphism (G1465A) confers a highly increased susceptibility to TLE. Moreover, it seems to influence the severity of this common epileptic disorder.

Kindling suppresses primed-burst-induced long-term potentiation in hippocampal CA1

In a previous study, we showed that partial hippocampal kindling in rats, a model of temporal lobe epilepsy, reduced the efficacy of presynaptic GABA(B) receptors in the CA1 area of hippocampal slices. In this study, we show that long-term potentiation (LTP) induced by theta-frequency primed bursts was suppressed in kindled as compared to control rats, but not in the presence of the GABA(B) receptor antagonists CGP35348 or CGP55845A. This is original evidence that LTP is suppressed by pathophysiological downregulation of GABA(B) autoreceptors. Control of postsynaptic inhibition by presynaptic GABA(B) receptors may provide a compensatory mechanism for controlling paroxysmal activity, with a side effect of blocking synaptic plasticity.

Molecular biology and ontogeny of gamma-aminobutyric acid (GABA) receptors in the mammalian central nervous system

gamma-Aminobutyric acid (GABA) is the predominant inhibitory neurotransmitter in the mammalian central nervous system. After release from nerve terminals, GABA binds to at least two classes of postsynaptic receptors (ie, GABAA and GABAB), which are nearly ubiquitous in the brain. GABAA receptors are postsynaptic heteropentameric complexes that display unique physiologic and pharmacologic properties based on subunit composition. Activation of GABAA receptors in mature neurons results in membrane hyperpolarization, which is mediated principally by inward chloride flux, whereas in early stages of brain development, GABAA receptor activation causes depolarization of the postsynaptic membrane. GABA, receptors reside both presynaptically and postsynaptically, exist as heterodimers and are coupled to voltage-dependent ion channels through interactions with heterotrimeric G proteins. This review summarizes the molecular biology and ontogeny of GABAA and GABAB receptors, highlighting some of their putative roles during normal brain development as well as in disease states such as epilepsy.

Studies of GABA(B) receptors labelled with [(3)H]-CGP62349 in hippocampus resected from patients with temporal lobe epilepsy

1 The aim of this study was to investigate the binding of a novel GABA(B) receptor radioligand, [(3)H]-CGP62349, to human post-mortem control and epileptic hippocampal sections using quantitative receptor autoradiography. Utilizing human control hippocampal sections it was shown that [(3)H]-CGP62349 bound with high affinity (K(D) 0.5 nM) to this tissue. 2 Hippocampal slices from surgical specimens obtained from patients with hippocampal sclerosis (HS) and temporal lobe epilepsy (TLE) were compared with neurologically normal post-mortem control subjects for neuropathology and GABA(B) receptor density and affinity. Neuronal loss was observed in most of the hippocampal subregions, but in the subiculum no significant difference was detected. 3 The localization of GABA(B) receptors with the antagonist [(3)H]-CGP62349 in human control hippocampal sections supported and extended earlier studies using the agonist ligand [(3)H]-GABA. 4 The kinetics of binding to the GABA(B) receptor in human hippocampus using this novel compound was comparable to previous data obtained in rat hippocampal membranes. 5 GABA(B) receptor density (B(max)) was significantly reduced in CA3, hilus, and dentate gyrus (DG); the affinity was increased exclusively in DG. The trend is identical in all the hippocampal subregions with the agonist and the antagonist, although significant differences with the antagonist where recorded in CA3 and hilus, whereas with the agonist a significant reduction was reported in all of the hippocampal subfields. 6 GABA(B) receptor expression per remaining neuron appeared significantly increased in CA3 and hilus. These results suggest altered GABA(B) receptor function may occur in human TLE, possibly as a result of synaptic reorganization, and may contribute to epileptogenesis.

GABABR1 receptor protein expression in human mesial temporal cortex: changes in temporal lobe epilepsy

Immunocytochemistry was used to examine gamma-aminobutyric acid beta (GABA)(B)R1a-b protein expression in the human hippocampal formation (including dentate gyrus, hippocampus proper, subicular complex, and entorhinal cortex) and perirhinal cortex. Overall, GABA(B)R1a-b immunostaining was intense and widespread but showed differential areal and laminar distributions of labeled cells. GABA(B)R1a-b-immunoreactive (-ir) neurons were found in the three main layers of the dentate gyrus, the most intense labeling being present in the polymorphic layer, whereas the granule cells were moderately immunoreactive. Except for slight variations, similar distribution patterns of GABA(B)R1a-b immunostaining were found along the different subfields of the Ammon's horn (CA1-CA4). The highest density of GABA(B)R1a-b-ir neurons was localized in the stratum pyramidale, where virtually every pyramidal cell was intensely immunoreactive, including the proximal part of the apical dendrites. Within the subicular complex, a more intense GABA(B)R1a-b immunostaining was found in the subiculum than in the presubiculum or parasubiculum, especially in the pyramidal and polymorphic cell layers. In the entorhinal cortex, distribution of GABA(B)R1a-b immunoreactivity was localized mainly in both pyramidal and nonpyramidal cells of layers II, III, and VI and in the superficial part of layer V, with layers I, IV, and deep layer V being less intensely stained. In the perirhinal cortex, the most intense GABA(B)R1a-b immunoreactivity was located in the deep part of layer III and in layer V and was mainly confined to medium-sized and large pyramidal cells. Thus, the differential expression, but widespread distribution, of GABA(B)R1a-b protein found in the present study suggests the involvement of GABA(B) receptors in many circuits of the human hippocampal formation and adjacent cortical structures. Interestingly, the hippocampal formation of epileptic patients (n = 8) with hippocampal sclerosis showed similar intensity of GABA(B)R1a-b immunostaining in the surviving neurons located within or adjacent to those regions presenting neuronal loss than in the controls. However, surviving neurons in the granule cell layer of the dentate gyrus displayed a significant reduction in immunostaining in 7 of 8 patients. Therefore, alterations in inhibitory synaptic transmission through GABA(B) receptors appears to affect differentially certain hippocampal circuits in a population of epileptic patients. This reduction in GABA(B)R1a-b expression could contribute to the pathophysiology of temporal lobe epilepsy.

Changes in GABA(B) receptor immunoreactivity after recurrent seizures in rats

GABA(B) receptors play an important role in the excitability of neuronal networks and can influence seizure activity. Here we demonstrate for the first time that kindling, an animal model for human temporal lobe epilepsy, leads to both early and delayed changes of GABA(B) receptor immunoreactivity in hippocampal and cortical areas. We propose that the altered GABA(B) receptor levels might be a compensatory mechanism to reduce excitability induced by recurrent kindled seizures, or alternatively, may promote the development of kindled epilepsy.

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.