THIP: a single-blind controlled trial in patients with epilepsy

Cerebellum-Specific Deletion of the GABAA Receptor δ Subunit Leads to Sex-Specific Disruption of Behavior

Granule cells (GCs) of the cerebellar input layer express high-affinity δ GABA_A subunit-containing GABA_A receptors (δGABA_ARs) that respond to ambient GABA levels and context-dependent neuromodulators like steroids. We find that GC-specific deletion of δGABA_A (cerebellar [cb] δ knockout [KO]) decreases tonic inhibition, makes GCs hyperexcitable, and in turn, leads to differential activation of cb output regions as well as many cortical and subcortical brain areas involved in cognition, anxiety-like behaviors, and the stress response. Cb δ KO mice display deficits in many behaviors, but motor function is normal. Strikingly, δGABA_A deletion alters maternal behavior as well as spontaneous, stress-related, and social behaviors specifically in females. Our findings establish that δGABA_ARs enable the cerebellum to control diverse behaviors not previously associated with the cerebellum in a sex-dependent manner. These insights may contribute to a better understanding of the mechanisms that underlie behavioral abnormalities in psychiatric and neurodevelopmental disorders that display a gender bias.

Rudolph et al. show that deletion of the neuromodulator and hormone-sensitive δGABA_A receptor subunit from cerebellar granule cells results in anxiety-like behaviors and female-specific deficits in social behavior and maternal care. δGABA_A deletion is associated with hyperexcitability of the cerebellar input layer and altered activation of many stress-related brain regions.

Basmisanil, a highly selective GABAA-α5 negative allosteric modulator: preclinical pharmacology and demonstration of functional target engagement in man

GABA_A-α5 subunit-containing receptors have been shown to play a key modulatory role in cognition and represent a promising drug target for cognitive dysfunction, as well as other disorders. Here we report on the preclinical and early clinical profile of a novel GABA_A-α5 selective negative allosteric modulator (NAM), basmisanil, which progressed into Phase II trials for intellectual disability in Down syndrome and cognitive impairment associated with schizophrenia. Preclinical pharmacology studies showed that basmisanil is the most selective GABA_A-α5 receptor NAM described so far. Basmisanil bound to recombinant human GABA_A-α5 receptors with 5 nM affinity and more than 90-fold selectivity versus α1, α2, and α3 subunit-containing receptors. Moreover, basmisanil inhibited GABA-induced currents at GABA_A-α5 yet had little or no effect at the other receptor subtypes. An in vivo occupancy study in rats showed dose-dependent target engagement and was utilized to establish the plasma exposure to receptor occupancy relationship. At estimated receptor occupancies between 30 and 65% basmisanil attenuated diazepam-induced spatial learning impairment in rats (Morris water maze), improved executive function in non-human primates (object retrieval), without showing anxiogenic or proconvulsant effects in rats. During the Phase I open-label studies, basmisanil showed good safety and tolerability in healthy volunteers at maximum GABA_A-α5 receptor occupancy as confirmed by PET analysis with the tracer [¹¹C]-Ro 15-4513. An exploratory EEG study provided evidence for functional activity of basmisanil in human brain. Therefore, these preclinical and early clinical studies show that basmisanil has an ideal profile to investigate potential clinical benefits of GABA_A-α5 receptor negative modulation.

Tonic GABAA Receptors as Potential Target for the Treatment of Temporal Lobe Epilepsy

Tonic GABA_A receptors are a subpopulation of receptors that generate long-lasting inhibition and thereby control network excitability. In recent years, these receptors have been implicated in various neurological and psychiatric disorders, including Parkinson’s disease, schizophrenia, and epilepsy. Their distinct subunit composition and function, compared to phasic GABA_A receptors, opens the possibility to specifically modulate network properties. In this review, the role of tonic GABA_A receptors in epilepsy and as potential antiepileptic target will be discussed.

THIP, a hypnotic and antinociceptive drug, enhances an extrasynaptic GABAA receptor-mediated conductance in mouse neocortex

THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) is a selective GABA(A) receptor agonist with a preference for delta-subunit containing GABA(A) receptors. THIP is currently being tested in human trials for its hypnotic effects, displaying advantageous tolerance and addiction properties. Since its cellular actions in the neocortex are uncertain, we studied the effects of THIP on neurons in slices of frontoparietal neocortex of 13- to 19-day-old (P13-19) mice. Using whole-cell patch-clamp recordings, we found that the clinically relevant THIP concentration of 1 muM induced a robust tonic GABA(A)-mediated current in layer 2/3 neurons. In comparison, only a minute tonic current was induced by mimicking in vivo endogenous GABA levels. Miniature IPSCs were not affected by 1 muM THIP suggesting an extrasynaptic site of action. The EC(50) for THIP was 44 muM. In accordance with the stronger expression of delta-containing receptors in superficial neocortical layers, THIP induced a 44% larger tonic current in layer 2/3 than in layer 5 neurons. Finally, monitoring spontaneously active neocortical neurons, THIP caused an overall depression of inhibitory activity, while enhancing excitatory activity prominently. Our studies suggest that THIP activates an extrasynaptic GABA(A) receptor-mediated conductance in the neocortex, which may alter the cortical network activity.

GABA(A) receptor subtypes as targets for neuropsychiatric drug development

The main inhibitory neurotransmitter system in the brain, the gamma-aminobutyric acid (GABA) system, is the target for many clinically used drugs to treat, for example, anxiety disorders and epilepsy and to induce sedation and anesthesia. These drugs facilitate the function of pentameric A-type GABA (GABA(A)) receptors that are extremely widespread in the brain and composed from the repertoire of 19 subunit variants. Modern genetic studies have found associations of various subunit gene polymorphisms with neuropsychiatric disorders, including alcoholism, schizophrenia, anxiety, and bipolar affective disorder, but these studies are still at their early phase because they still have failed to lead to validated drug development targets. Recent neurobiological studies on new animal models and receptor subunit mutations have revealed novel aspects of the GABA(A) receptors, which might allow selective targeting of the drug action in receptor subtype-selective fashion, either on the synaptic or extrasynaptic receptor populations. More precisely, the greatest advances have occurred in the clarification of the molecular and behavioral mechanisms of action of the GABA(A) receptor agonists already in the clinical use, such as benzodiazepines and anesthetics, rather than in the introduction of novel compounds to clinical practice. It is likely that these new developments will help to overcome the present problems of the chronic treatment with nonselective GABA(A) agonists, that is, the development of tolerance and dependence, and to focus the drug action on the neurobiologically and neuropathologically relevant substrates.

The EEG findings in extratemporal seizures

Extratemporal seizures originate from the frontal, central, parietal, occipital, and midline regions of the brain. The scalp EEG can show various types of interictal and ictal discharges consisting of spikes, spike and wave sharp waves, paroxysmal fast activity, or rhythmic activity in the beta, alpha, theta, or delta frequency ranges. The discharges can occur as focal, regional, lateralized, or secondarily generalized discharges. Discharges arising from the frontal region are varied and at times complex. Centro-temporal spikes associated with benign epilepsy of childhood have a characteristic blunt spike and wave appearance. Centro-parietal spikes can occur in children with benign childhood epilepsy or in association with symptomatic epilepsies at any age. Occipital spike discharges have been seen in young children with visual problems, benign occipital epilepsy of childhood, the Sturge-Weber syndrome, and other symptomatic or structural lesions involving the occipital lobe. There may be problems with detection of the source of origin of seizures secondary to the anatomy of the various regions, deep foci, small restricted foci, rapid spread of epileptiform discharges, and contaminating effects of muscle and movement artifact. Depth or intracranial recordings may help in further localization of foci.

Epilepsy care: image of the future

A number of factors have contributed to improvements in the care of epilepsy during the past decade, including the International League Against Epilepsy classifications, therapeutic antiepileptic drug (AED) monitoring and the concept of monotherapy, new AEDs with novel mechanisms of action, and new insights into etiology that suggest novel therapies. Pharmacologically "clean" AEDs acting on a single known mechanism will be an important element in the future care of patients with epilepsy. Augmentation of GABAergic inhibition is being successfully exploited by AEDs, and there remains much room for further pharmacologic innovation. The potential role of AEDs acting specifically on the GAT-1 or GAT-4 subgroup of gamma-aminobutyric acid transporters is a topic of current research. Specifically acting AEDs designed to have a single and known mode of action will permit true monotherapy, one AED with one target as opposed to one AED with several targets, and may open the way to rational polytherapy, i.e., designed use of one AED per mechanism in epilepsies with multifactorial causation. New research demonstrating a possible autoimmune basis for some forms of epilepsy illustrates the potential for novel nonpharmacologic approaches, and the role of prevention must also be emphasized. The image of the future is an optimistic one.

Tiagabine: a novel drug with a GABAergic mechanism of action

The various possibilities for manipulating the gamma-aminobutyric acid (GABA) system to augment GABAergic inhibition have been surveyed with reference to the relevant antiepileptic compounds that have been successfully or unsuccessfully investigated in relation to these different mechanisms of action. The first clinical studies of tiagabine (TGB), a novel GABA-uptake inhibitor are now available. These studies utilized a novel design, the enrichment (Amery) design, which is put into perspective compared to classical clinical trial designs. Possible advantages and disadvantages of TGB, as seen at this stage in development, have been identified.

Effects of GABAA receptors activation on brain glucose metabolism in normal subjects and temporal lobe epilepsy (TLE) patients. A positron emission tomography (PET) study. Part I: Brain glucose metabolism is increased after GABAA receptors activation

Though gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the human central nervous system, the metabolic response to GABA system activation remains imperfectly known. We studied in vivo with positron emission tomography (PET) the variations of glucose metabolism in the human brain after stimulation of the GABAA receptors by systemic administration of the specific GABAA agonist, 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP). These investigations were performed in three normal volunteers and as part of presurgical evaluation for temporal lobe epilepsy in six patients. While clinical and electroencephalographic (EEG) monitoring showed a sedative effect and sleepiness after THIP administration, glucose metabolism was paradoxically increased in grey matter structures, which are known to have a high density of GABAA receptors. These findings suggest that the pharmacological activation of GABA pathways, although inhibitory and producing a decrease of vigilance, increases the energetic demand at least during a phase of GABA agonist action, probably at the synaptic or at the glial cell level.

Electrophysiological studies of the GABAA receptor ligand, 4-PIOL, on cultured hippocampal neurones

1. Whole-cell, patch-clamp recordings from cultured hippocampal neurones have been used to characterize the action of the GABAA ligand, 5-(4-piperidyl)isoxazol-3-ol (4-PIOL). The action of 4-PIOL was compared with that of the established GABAA agonist, isoguvacine. 2. With a symmetrical Cl- gradient across the membrane and a holding potential of -60mV, both isoguvacine and 4-PIOL evoked an inward current. The reversal potentials of the responses to both agents were identical (+8.8 mV, n = 4) and the current/voltage relationships showed outward-going rectification. 3. The response to 300 microM 4-PIOL was completely blocked by the GABAA antagonist, bicuculline methobromide (BMB, 10 microM). The pA2 of BMB was greater than 6.46. With 2 mM 4-PIOL about 15% of the response remained in the presence of 100 microM BMB. This may represent a non-specific component of the response to large concentrations of 4-PIOL. 4. 4-PIOL was about 200 times less potent as an agonist than isoguvacine. because of the rapid fade (desensitization) of isoguvacine-induced currents, the maximum response to this agonist was not determined. However, the response to 2 mM 4-PIOL was only a small fraction of that evoked by submaximal concentrations of isoguvacine. 5. Setting the response to 1 mM 4-PIOL as maximum, the EC50 for 4-PIOL was 91 microM (95% confidence limits:73-114 microM). 6. 4-PIOL antagonized the response to isoguvacine with a parallel shift to the right of the dose-response curve. The antagonist action of 4-PIOL was about 30 times weaker than that of BMB. When allowance was made for the intrinsic agonist action of 4-PIOL, the Ki was 116p microM (95% confidence limits: 102-130 microM). This was not significantly different from EC5, (P = 0.86; non-parametric Mann-Whitney test).7. It is concluded that 4-PIOL is a partial agonist at the GABAA receptor on cultured hippocampal neurones.

Benign partial epilepsy of childhood with monomorphic sharp waves in centrotemporal and other locations

We reviewed EEGs from children whose history and clinical course was compatible with benign partial epilepsy of childhood with centrotemporal spikes. In 21% of patients with a single EEG focus, the discharge was outside the centrotemporal area. In 37.5% of patients with more than one focus, one was in the centrotemporal area while the other was not. We suggest that the typical EEG features of this syndrome are the normal background, the stereotypic morphology of the sharp waves, and their activation by drowsiness and sleep, not their exclusive location in the centrotemporal regions. Insistence on a centrotemporal location for the EEG discharges in this syndrome may lead to a misclassification of the type of epilepsy in some children with implications for therapeutic decisions and prognostic statements.

Experimental studies and controlled clinical testing of valproate and vigabatrin

Gamma-aminobutyric acid (GABA) is the most important inhibitory transmitter, quantitatively, in the CNS. Evidence exists that decreased GABAergic neurotransmission may play a role in some forms of epilepsy. Consequently, manipulating the GABA system may be a therapeutic possibility in the treatment of this disease. Inhibition of the major GABA degrading enzyme, GABA-transaminase (GABA-T), seems to be the most promising approach. Currently, 2 antiepileptic drugs, valproate (VPA) and vigabatrin, gamma-vinyl GABA (GVG), are available, which are supposed to inhibit the degradation of GABA. Both drugs cause an increase in the total concentration of GABA in the brain, but to a different extent. VPA produces a moderate elevation, which seems to be the result of a marked increase in the transmitter-related GABA pool, while the pronounced elevation in GABA concentration observed during treatment with GVG seems to be caused mainly by an increase in the non-transmitter-related (glial) GABA pool. In order to investigate this apparently differential influence of VPA and GVG on the GABA system, a number of studies were undertaken in selectively cultured astrocytes and neurons from mice. For both drugs neuronal GABA-T proved far more sensitive with regard to inhibition than glial GABA-T. In order to obtain a more direct measure of a potential GABAergic mechanism of action of VPA and GVG, synaptic release of endogenous GABA was determined after culturing neurons in the presence of clinically relevant concentrations of the drugs. GVG caused a significant increase in GABA release, even at concentrations as low as 25 microM. For VPA only the highest of the investigated concentrations (300 microM) augmented GABA release. It is concluded that the antiepileptic effect of GVG seems to be caused by a direct GABAergic mechanism of action. For VPA an influence on the GABA system may play a role in the antiepileptic effect of the drug. However, the lack of definite data on human brain levels of VPA after chronic treatment, combined with evidence that VPA exhibits a number of other effects that may be relevant for its antiepileptic properties, makes the interpretation of a GABAergic mechanism of action difficult. Controlled clinical trials have been increasingly applied within all areas of medicine. In 1982 a survey of the literature identified 29 studies of antiepileptic drugs, where the design involved randomization, the double-blind principle and a statistical analysis of the results.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential effect of gamma-vinyl GABA and valproate on GABA-transaminase from cultured neurones and astrocytes

A preferential effect of valproate on gamma-aminobutyric acid (GABA) in the nerve terminal compartment has been proposed. Gamma-vinyl GABA, an irreversible inhibitor of GABA-transaminase (GABA-T) causes a preferential increase in the GABA compartment of the non-nerve terminal. The aim of the present study was to investigate further this apparent differential effect on GABA-T of these compounds in neurones and glia. The investigations were undertaken in neurones and astrocytes, cultured separately. After incubation with valproate, the IC50 value for astrocytes was found to be 1202 microM of valproate and for neurones 634 microM. Assuming regional differences of concentrations of valproate in the brain, the observed IC50 values might be clinically relevant. Culturing the cells in the presence of gamma-vinyl GABA demonstrated IC50 values for astrocytes and neurones of 89 and 24 microM, respectively. The (S)isomer of gamma-vinyl GABA was the most active inhibitor of GABA-T in both glia and neurones. After withdrawal of gamma-vinyl GABA from the culture media of the cells, in neurones 50% of the activity of GABA-T was regained within 2-4 days. In astrocytes a similar time course was observed. These findings are in agreement with clinical data: the IC50 values correspond to clinically-relevant serum levels of gamma-vinyl GABA in humans only the (S)isomer showed an antiepileptic effect in animal models of epilepsy a delayed antiepileptic effect, after withdrawal of gamma-vinyl GABA, has been established in clinical studies.

Development of tolerance to the anticonvulsant effect of GABAmimetic drugs in genetically epilepsy-prone gerbils

Three drugs which increase GABA-mediated inhibitory neurotransmission in the brain, namely the GABA degradation inhibitors aminooxyacetic acid (AOAA) and gamma-acetylenic GABA (GAG), and the GABA receptor agonist THIP (gaboxadol), were administered to epilepsy-prone gerbils via subcutaneously implanted osmotic minipumps for 2 weeks. The antiepileptic drugs valproic acid (VPA) and diazepam were also included in the experiments. After one day of constant rate application, all GABAmimetics markedly suppressed seizure activity induced in the gerbils by air blast stimulation, but anticonvulsant efficacy of the drugs was lost after 8 and 14 days of treatment. With VPA, only moderate anticonvulsant effects were found because only sub-therapeutic drug levels (about 40 micrograms/ml plasma) were reached via minipump administration. The experiments with diazepam could only be evaluated in part because of instability of the drug in aqueous solution. Determination of brain GABA metabolism in the gerbils indicated that reduction of GABA synthesis may be responsible, at least in part, for development of tolerance to the anticonvulsant effects of AOAA and GAG.

Comparison of progabide with other antiepileptic and GABAergic drugs

The effect of the experimental antiepileptic gamma-aminobutyric acid (GABA) agonist drug progabide, [alpha-(chloro-4-phenyl)fluor-5-hydroxy-2-benzilideneamino]-4-buty ramide, on the trigeminal complex of cats was compared with the effect of established antiepileptic drugs and with the effect of various GABA agonists and antagonists. Intravenous administration of 10-40 mg/kg progabide depressed excitatory transmission and descending periventricular inhibition, similar to carbamazepine and phenytoin. However, progabide depressed, rather than facilitated, segmental inhibition. The serum levels of progabide were comparable with those in patients receiving long-term treatment with progabide. The GABA antagonist bicuculline had the opposite effect of progabide on our experimental model, but the other GABA agonists THIP (4,5,6,7-tetrahydroisoxazolo-5,4-C-pyridine-3-ol) and muscimol did not have the same effects as progabide. THIP had no effect on excitatory transmission, periventricular inhibition, or segmental inhibition, whereas muscimol facilitated periventricular inhibition and sometimes segmental inhibition and had no effect on excitatory transmission. Our experiments thus indicate that progabide, but not THIP or muscimol, should have antiepileptic properties, in agreement with the clinical experiences that have been reported. The reason for the differential effect of these three GABA agonists remains to be elucidated.

gamma-Vinyl GABA: a double-blind placebo-controlled trial in partial epilepsy

The antiepileptic effect of gamma-vinyl gamma-aminobutyric acid (GABA), an irreversible GABA-transaminase inhibitor, was investigated in an add-on, placebo-controlled, double-blind, cross-over, fixed-dose trial. Twenty-one patients suffering from difficult to control complex partial seizures participated; 18 patients completed the trial. Serum levels of concomitant antiepileptic drugs were kept constant throughout the trial. Three patients (17%) experienced a 75% reduction in seizure frequency and in 8 (44%) the seizures were reduced by at least 50%. Two patients developed a moderate and 1 patient a marked increase in seizure frequency during treatment with gamma-vinyl GABA. Except for 2 patients who had to discontinue the trial because of adverse effects of gamma-vinyl GABA, the participants were unable to discriminate between treatment regimens with regard to side effects. gamma-vinyl GABA seems to be a promising new antiepileptic drug, and the first one to present convincing evidence of a GABAergic mechanism of action.

GABA receptor agonists: pharmacological spectrum and therapeutic actions

From the data discussed in this review it appears that GABA receptor agonists exhibit a variety of actions in the central nervous system, some of which are therapeutically useful (Table V). GABA receptor agonists, by changing the firing rate of the corresponding neurons accelerate noradrenaline turnover without changes in postsynaptic receptor density and diminish serotonin liberation with an up-regulation of 5HT2 receptors. These effects differ from those of tricyclic antidepressants which primarily block monoamine re-uptake and cause down-regulation of beta-adrenergic and 5HT2 receptors. The GABA receptor agonist progabide has been shown to exert an antidepressant action which is indistinguishable from that of imipramine in patients with major affective disorders. The fact that: (a) GABA receptor agonists and tricyclic antidepressants affect noradrenergic and serotonergic transmission differently; and (b) tricyclic antidepressants alter GABA-related parameters challenges the classical monoamine hypothesis of depression and suggests that GABA-mediated mechanisms play a role in mood disorders. Decreases in cellular excitability produced by GABAergic stimulation leads to control of seizures in practically all animal models of epilepsy. GABA receptor agonists have a wide spectrum as they antagonize not only seizures which are dependent on decreased GABA synaptic activity but also convulsant states which are apparently independent of alterations in GABA-mediated events. These results in animals are confirmed in a wide range of human epileptic syndromes. GABA receptor agonists decrease dopamine turnover in the basal ganglia and antagonize neuroleptic-induced increase in dopamine release. On repeated treatment, progabide prevents or reverses the neuroleptic-induced up-regulation of dopamine receptors in the rat striatum and antagonizes the concomitant supersensitivity to dopaminomimetics. Behaviorally, GABA receptor agonists diminish the stereotypies induced by apomorphine or L-DOPA suggesting that GABAergic stimulation results also in an antidopaminergic action which is exerted beyond the dopamine synapse. These effects of GABA receptor agonists may represent the basis of the antidyskinetic action of these compounds which, however, remains to be fully confirmed. GABA receptor agonists reduce striatal acetylcholine turnover, an effect which occurs at doses much lower than those which affect dopamine neurons. Since hyperactivity of cholinergic neurons plays a determinant role in the pathogenesis of some parkinsonian symptoms, it is conceivable that GABAergic stimulation is effective in ameliorating Parkinson's disease.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino acid neurotransmitters and new approaches to anticonvulsant drug action

Amino acids provide the most universal and important inhibitory (gamma-aminobutyric acid (GABA), glycine) and excitatory (glutamate, aspartate, cysteic acid, cysteine sulphinic acid) neurotransmitters in the brain. An anticonvulsant action may be produced (1) by enhancing inhibitory (GABAergic) processes, and (2) by diminishing excitatory transmission. Possible pharmacological mechanisms for enhancing GABA-mediated inhibition include (1) GABA agonist action, (2) GABA prodrugs, (3) drugs facilitating GABA release from terminals, (4) inhibition of GABA-transaminase, (5) allosteric enhancement of the efficacy of GABA at the receptor complex, (6) direction action on the chloride ionophore, and (7) inhibition of GABA reuptake. Examples of these approaches include the use of irreversible GABA-transaminase inhibitors, such as gamma-vinyl GABA, and the development of anticonvulsant beta-carbolines that interact with the "benzodiazepine receptor." Pharmacological mechanisms for diminishing excitatory transmission include (1) enzyme inhibitors that decrease the maximal rate of synthesis of glutamate or aspartate, (2) drugs that decrease the synaptic release of glutamate or aspartate, and (3) drugs that block the post-synaptic action of excitatory amino acids. Compounds that selectively antagonise excitation due to dicarboxylic amino acids have recently been developed. Those that selectively block excitation produced by N-methyl-D-aspartate (and aspartate) have proved to be potent anticonvulsants in many animal models of epilepsy. This provides a novel approach to the design of anticonvulsant drugs.

GABAergic agents as anticonvulsants in baboons with photosensitive epilepsy

GABAergic agents have been evaluated for acute anticonvulsant activity in baboons, Papio papio with photosensitive epilepsy. The potent GABAA agonists muscimol and THIP are proconvulsant. (-)Baclofen, 2 mg/kg suppresses myoclonic responses; higher doses facilitate EEG paroxysmal activity. (S) gamma-vinyl GABA, 100-200 mg/kg, suppresses myoclonic responses for more than 24 h. Some derivatives of esters of beta-carboline-3-carboxylate that bind to the benzodiazepine receptor, e.g. ZK 91296 and ZK 93423, suppress myoclonus with a potency at least as great as diazepam.

Diazepam enhances the action but not the binding of the GABA analog, THIP

GABA (4-aminobutyric acid) and its bicyclic analog THIP (4,5,6,7-tetrahydroisoxazolo-[4,5-c]-pyridin-3-ol) produced membrane hyperpolarization and increased chloride ion conductance of mouse spinal cord neurons in cell culture. Above 1 nM diazepam enhanced the actions of both GABA and THIP with similar potency and efficacy. Diazepam has been shown to enhance the binding of [3H]GABA to rat brain membranes over similar concentration ranges, with the EC50 values for enhancement of [3H]GABA binding and increase in membrane conductance being similar. In contrast, binding of [3H]THIP has been shown to be unaltered by diazepam under a variety of conditions. The possible reasons for such a discrepancy between these electrophysiological and neurochemical results with THIP are discussed.