The role of transcriptional and translational mechanisms in flumazenil-induced up-regulation of recombinant GABA(A) receptors

The aim of this study was to further elucidate the mechanisms involved in adaptive changes of GABA(A) receptors following prolonged exposure to flumazenil, the antagonist of benzodiazepine binding sites on GABA(A) receptors. The effects of prolonged flumazenil treatment were studied on recombinant alpha(1)beta(2)gamma(2S) GABA(A) receptors stably expressed in human embryonic kidney (HEK 293) cells. Using radioligand binding experiments we found enhancement in the maximum number of [(3)H]muscimol labeled binding sites in different preparations of HEK 293 cells. The parallel increase of [(3)H]flunitrazepam binding sites in the membranes was reduced in the presence of actinomycin D and cycloheximide, inhibitors of RNA and protein synthesis, respectively. Chronic flumazenil also raised the steady-state level of mRNA encoding alpha(1) receptor subunit. The results suggest that the up-regulation of GABA(A) receptors, observed after prolonged flumazenil treatment is at least partly due to increased de novo synthesis of receptor proteins at both transcriptional and translational level.

Low concentrations of ethanol do not affect radioligand binding to the delta-subunit-containing GABAA receptors in the rat brain

In the present study, we investigated the co-localization pattern of the delta subunit with other subunits of GABA(A) receptors in the rat brain using immunoprecipitation and Western blotting techniques. Furthermore, we investigated whether low concentrations of ethanol affect the delta-subunit-containing GABA(A) receptor assemblies in the rat brain using radioligand binding to the rat brain membrane homogenates as well as to the immunoprecipitated receptor assemblies. Our results revealed that delta subunit is not co-localized with gamma(2) subunit but it is associated with the alpha(1), alpha(4) or alpha(6), beta(2) and/or beta(3) subunit(s) of GABA(A) receptors in the rat brain. Ethanol (1-50 mM) neither affected [(3)H]muscimol (3 nM) binding nor diazepam-insensitive [(3)H]Ro 15-4513 (2 nM) binding in the rat cerebellum and cerebral cortex membranes. However, a higher concentration of ethanol (500 mM) inhibited the binding of these radioligands to the GABA(A) receptors partially in the rat cerebellum and cerebral cortex. Similarly, ethanol (up to 50 mM) did not affect [(3)H]muscimol (15 nM) binding to the immunoprecipitated delta-subunit-containing GABA(A) receptor assemblies in the rat cerebellum and hippocampus but it inhibited the binding partially at a higher concentration (500 mM). These results suggest that the native delta-subunit-containing GABA(A) receptors do not play a major role in the pharmacology of clinically relevant low concentrations of ethanol.

The effects of antipsychotic drugs on GABAA receptor binding depend on period of drug treatment and binding site examined

Changes in GABA(A) receptors are observed in schizophrenia, with benzodiazepine-sensitive GABA(A) receptor subtypes being affected differently to other subtypes. However, long-term antipsychotic drug use in schizophrenia may underlie these changes. To test this, we examined the effects of administering a typical (haloperidol) and an atypical (olanzapine) antipsychotic drug on the GABA(A) receptor agonist (orthosteric) and benzodiazepine (allosteric) binding sites in rat prefrontal cortex. As antipsychotic drugs have delayed maximal therapeutic effects we also examined different drug treatment periods. Male SD rats received a sucrose solution containing either haloperidol (1.5 mg/kg), olanzapine (6.5 mg/kg) or no drug daily for either 7, 14 or 28 days. Sections of rat brain were then labelled with [(3)H]muscimol, which labels the total population of GABA(A) receptors, or the benzodiazepine site ligand [(3)H]flunitrazepam in separate saturation binding experiments using quantitative receptor autoradiography. [(3)H]Muscimol binding was enhanced in the prefrontal cortex after 7 days but no differences were observed after longer periods of drug administration. In contrast there was a delayed increase in density of benzodiazepine-sensitive GABA(A) receptors in the PFC, suggesting that antipsychotic drugs have different effects on different GABA(A) receptor subtypes. These changes in the properties of GABA(A) receptor binding following antipsychotic drug administration are not consistent with those observed in schizophrenia and suggest a 'reshuffling' in GABA(A) receptor subtypes over time.

Local distribution and toxicity of prolonged hippocampal infusion of muscimol

OBJECT

The activity of gamma-aminobutyric acid (GABA), the principal inhibitory neurotransmitter, is reduced in the hippocampus in patients with complex partial seizures from mesial temporal sclerosis. To provide preliminary safety and distribution data on using convection-enhanced delivery of agents to treat complex partial seizures and to test the efficacy and safety of regional selective neuronal suppression, the authors infused muscimol, a GABA-A receptor agonist, directly into the hippocampus of nonhuman primates using an integrated catheter electrode.

METHODS

Ten rhesus monkeys were divided into three groups: 1) use of catheter electrode alone (four monkeys); 2) infusion of escalating concentrations of muscimol followed by vehicle (three monkeys); and 3) infusion of vehicle and subsequent muscimol mixed with muscimol tracer (three monkeys). Infusions were begun 5 days after catheter electrode placement and continued for 5.6 days before switching to the other agent. Head magnetic resonance (MR) images and electroencephalography recordings were obtained before and during the infusions. Brain histological studies and quantitative autoradiography were performed. Neurological function was normal in controls and when muscimol concentrations were 0.125 mM or less, whereas higher concentrations (0.5 and 1 mM) produced reversible apathy and somnolence. Fluid distribution was demonstrated on MR images and muscimol distribution was demonstrated on autoradiographs throughout the hippocampus and adjacent white matter.

CONCLUSIONS

Targeted modulation of neuronal activity is a reasonable research strategy for the investigation and treatment of medically intractable epilepsy.

Regulation of native GABAA receptors by PKC and protein phosphatase activity

RATIONALE AND OBJECTIVE

Protein kinase C (PKC) modulation of ionotropic receptors is a common mechanism for regulation of channel function. The effects of PKC and phosphatase activation on native gamma-aminobutyric acid (GABA(A)) receptors in adult brain are unknown. Previous studies of recombinant GABA(A) receptors have provided evidence that PKC activation inhibits receptor function, whereas other studies suggest that PKC either increases or does not alter GABA(A) receptor function. The present study explored (a) the effects of PKC and phosphatase activity on GABA-mediated (36)Cl(-) uptake in cerebral cortical synaptoneurosomes and (b) the effect of PKC activity on muscimol-induced loss of righting reflex (LORR) in adult rats.

METHODS

GABA(A) receptor function in vitro was measured by muscimol-induced (36)Cl(-) uptake into cerebral cortical synaptoneurosomes. The in vivo effect of PKC on GABA(A)-mediated function was measured by intracerebroventricular (i.c.v.) injection of 4-beta-phorbol-12,13-dibutyrate (PDBu) or calphostin C followed by determination of muscimol-induced LORR.

RESULTS

Adenosine triphosphate (ATP) and PDBu produced a concentration-dependent and specific reduction in muscimol-stimulated (36)Cl(-) uptake that was blocked by the PKC inhibitor calphostin C. Both adenosine diphosphate and 4alphaPDBu were ineffective. Phosphatase inhibition produced similar inhibition of muscimol responses. Furthermore, i.c.v. administration of PDBu and calphostin C produced opposing effects on both the onset and the duration of muscimol-induced LORR in rats.

CONCLUSIONS

The present study provides evidence that PKC activation reduces GABA(A) receptor function in native receptors both in vitro and in vivo. Phosphatase inhibitors decrease muscimol-mediated Cl(-) uptake in GABA(A) receptors demonstrating coordinated regulation of native receptors by PKC and phosphatases.

Hippocampal inactivation disrupts the acquisition and contextual encoding of fear extinction

In recent studies, inactivation of the dorsal hippocampus before the retrieval of extinguished fear memories disrupted the context-dependent expression of these memories. In the present experiments, we examined the role of the dorsal hippocampus in the acquisition of extinction. After pairing an auditory conditional stimulus (CS) with an aversive footshock [unconditional stimulus (US)], rats received an extinction session in which the CS was presented without the US. In experiment 1, infusion of muscimol, a GABAA receptor agonist, into the dorsal hippocampus before the extinction training session decreased the rate of extinction. Moreover, when later tested for fear to the extinguished CS, all rats that had received hippocampal inactivation before extinction training demonstrated renewed fear regardless of the context in which testing took place. This suggests a role for the dorsal hippocampus in both acquiring the extinction memory and encoding the CS-context relationship that yields the context dependence of extinction. In experiment 2, inactivation of the dorsal hippocampus before testing also disrupted the context dependence of fear to the extinguished CS. In experiment 3, quantitative autoradiography revealed the boundaries of muscimol diffusion after infusion into the dorsal hippocampus. Together, these results reveal that the dorsal hippocampus is involved in the acquisition, contextual encoding, and context-dependent retrieval of fear extinction. Learning and remembering when and where aversive events occur is essential for adaptive emotional regulation.

Learning deficits induced by sleep deprivation and recovery are not associated with altered [3H]muscimol and [3H]flunitrazepam binding

Several studies have shown that sleep deprivation produces deficits in learning tasks, but mechanisms underlying these effects remain unclear. Other lines of evidence indicate an involvement of brain GABA systems in cognitive processes. Here, we investigated the possibility that alterations in GABA(A) or benzodiazepine (BDZ) receptor binding might underlie avoidance deficits induced by sleep deprivation. Rats were deprived of sleep for 96 h using the platform method and then trained in a step-through inhibitory avoidance task, or allowed to recover sleep for 24 h before training (sleep rebound group). Thirty minutes after training, animals were given a retention test. Both sleep-deprived and sleep-recovered animals showed a significant impairment in avoidance responding compared to cage controls, and the sleep-deprived group performed significant worse than the sleep-recovered group. A separate group of animals was sacrificed either immediately after 96 h of sleep deprivation or after 96 h of sleep deprivation followed by 24 h of sleep recovery. [(3)H]muscimol and [(3)H]flunitrazepam binding were examined by quantitative autoradiography in 42 brain regions, including areas involved in cognitive processes. No significant differences among groups were found in any brain region, except for a reduction in [(3)H]flunitrazepam binding in the frontal cortex of sleep-recovered animals. These results confirm the deleterious effects of sleep loss on inhibitory avoidance learning, but suggest that such deficits cannot be attributed to altered GABA(A) or BDZ binding in brain.

Temporal properties of cerebellar-dependent memory consolidation

Classical conditioning of the nictitating membrane response in rabbits is a well defined model of cerebellar-dependent motor memory. This memory undergoes a period of consolidation after the training session, when it is sensitive to reversible inactivations of the cerebellar cortex, but not of the cerebellar nuclei, with the GABA(A) receptor agonist muscimol. Here, the temporal properties of this cerebellar cortex-dependent consolidation were examined using delayed infusions of muscimol in cortical lobule HVI. Cortical infusions delayed by 5 or 45 min after a conditioning session produced significant and very similar impairments of consolidation, but infusions delayed by 90 min produced little or no impairment. Behavioral measures indicate that the muscimol infusions produced significant effects after approximately 30 min and they lasted for a few hours. So, over a time window beginning approximately 1 hr after the end of the training session and closing 1 hr after that, intracortical activity is critical for consolidation of this motor memory.

Subunit composition and quantitative importance of GABA(A) receptor subtypes in the cerebellum of mouse and rat

In cerebellum, 13 different GABA(A) receptor subunits are expressed. The number of different receptor subtypes formed in this tissue, their subunit composition and their quantitative importance so far has not been determined. In the present study, immunodepletion by immunoaffinity chromatography, as well as immunoprecipitation and western blot analysis was performed using 13 different subunit-specific antibodies to provide an overview on the subunit composition and abundance of GABA(A) receptor subtypes in mouse and rat cerebellum. Results obtained indicate that alpha1betaxgamma2, alpha1alpha6betaxgamma2, alpha6betaxgamma2, alpha6betaxdelta and alpha1alpha6betaxdelta are the major GABA(A) receptor subtypes present in the cerebellum. In addition, small amounts of alpha1betaxdelta receptors and a series of minor receptor subtypes containing alpha2, alpha3, alpha4, alpha5, gamma1 or gamma3 subunits are also present in the cerebellum. Whereas the abundance of alpha1alpha6betaxgamma2, alpha6betaxdelta and alpha1alpha6betaxdelta receptors is different in mouse and rat cerebellum, that of other receptors is quite similar in these tissues. Data obtained for the first time provide an overview on the GABA(A) receptor subtypes present in the cerebellum and represent the basis for further studies investigating changes in receptor expression and composition under pathological conditions.

Changes in rectal temperature and ECoG spectral power of sensorimotor cortex elicited in conscious rabbits by i.c.v. injection of GABA, GABA(A) and GABA(B) agonists and antagonists

1. In order to ascertain whether both GABA(A) and GABA(B), or only GABA(B) receptors, directly modulate thermoregulation in conscious rabbits, GABA(A)/GABA(B) agonist and antagonist agents were injected intracerebroventricularly in conscious rabbits while monitoring changes in rectal temperature (RT), gross motor behaviour (GMB) and electrocorticogram (ECoG) power spectra (ps) from sensorimotor cortices. 2. GABA (48 micromol), nipecotic acid (50 nmol), THIP (60 nmol), muscimol (18 nmol) and baclofen (8 nmol) induced hypothermia (-deltaRTmax values of 1.70+/-0.1, 1.4+/-0.2, 1.0+/-0.4, 1.1+/-0.2 and 1.6+/-0.3 degrees C, respectively), accompanied by inhibition of GMB and ECoG synchronization. THIP increased ps at delta frequency band (1.1-3.3 Hz), while GABA, nipecotic acid, muscimol and baclofen did the same at both delta and (4.6-6.5 Hz) frequency bands. ECoG ps changes were concomitant or even preceded hypothermia. 3. Bicuculline (1.8 nmol) induced hyperthermia (deltaRTmax 1.2+/-0.5 degrees C) and slight excitation of GMB, while CGP35348 (1.2 micromol) did not affect RT nor GMB. Both compounds did not affect ECoG ps. 4. Bicuculline potentiated muscimol-induced hypothermia, inhibition of GMB and synchronization of ECoG, while CGP35348 fully antagonized these effects. 5. In conclusion, the present results, while confirming the prevailing role of GABA(B), also outline a direct involvement of GABA(A) receptors in the central mechanisms of thermoregulation. Ascending inhibition towards discrete cortical areas controlling muscular activity and thermogenesis may result from GABA receptor activation in neurones proximal to the ventricles, thus contributing to hypothermia, although hypothermia-induced reduction of neuronal activity of these cortical areas cannot be ruled out.

On high- and low-affinity agonist sites in GABAA receptors

GABAA receptors are activated via low-affinity binding sites for the agonists GABA or muscimol. Evidence has been provided that the amino acid residue alpha 1F64 located at the beta2(+)/alpha1(-) subunit interface forms part of this binding site. In radioactive ligand binding studies the agonist [3H]muscimol has been found to interact with the receptor via a high-affinity binding site. This site has been interpreted as a conformational variant of the low-affinity site. Alternatively, the high-affinity binding site has been located to the alpha1(+)/beta2(-) interface and the homologous residue to alpha 1F64, beta 2Y62 has been proposed to constitute an important part of this site. Here we investigated the effect of the point mutation alpha 1F64L and the homologous mutation beta 2Y62L on agonist and antagonist binding and functional properties in alpha 1 beta 2 gamma 2 GABAA receptors. While the mutation in the alpha1 subunit had drastic consequences on all studied properties, including desensitization, the mutation in the beta2 subunit had little consequence. Our observations are relevant for the relative location of high- and low-affinity agonist sites in GABAA receptors.

Normal coupling of brain benzodiazepine and neurosteroid modulatory sites on the GABA-A receptor complex in human hepatic encephalopathy

To assess the possible implication of the allosteric coupling of different modulatory sites at the GABA-A receptor complex in hepatic encephalopathy (HE), we investigated in autopsied frontal cortex of six cirrhotic patients and six appropriately-matched controls, the modulatory effects of the benzodiazepine site agonist flunitrazepam on the binding of [3H]muscimol and the effect of the neurosteroid site agonist allopregnanolone (5alpha-pregnan-3alpha-ol-20-one) on the binding of [3H]muscimol and [3H]flunitrazepam. There were no significant differences in either the magnitude E(max): 11.5+/-1.1% (controls) versus 10.2+/-2.2% (HE patients) or the efficacy EC(50): 20.2+/-5.5 nM (controls) versus 17.7+/-6.2 nM (HE patients) of flunitrazepam modulation of [3H]muscimol binding. Allopregnanolone also showed modulation of both sites to a comparable extent in brain tissue from cirrhotic patients and controls E(max): [3H]muscimol, 15.1+/-2.8% (controls) versus 13.8+/-1.9% (HE patients); [3H]flunitrazepam, 17.9+/-2.3% (controls) versus 19.1+/-2.3% (HE patients), EC(50): [3H]muscimol, 386.5+/-25.8 nM (controls) versus 373.8+/-13.1 nM (HE patients); [3H]flunitrazepam, 49.8+/-22.9 nM (controls) versus 55.5+/-14.0 nM (HE patients). These findings demonstrate unequivocally that the GABA-A sites and their benzodiazepine and neurosteroid modulatory sites manifest normal allosteric coupling in brain in human HE. Therefore, if increased "GABAergic tone" is implicated in the pathophysiology of HE, this must be the consequence of increased brain concentrations of endogenous benzodiazepine and/or neurosteroid ligands for components of the GABA-A receptor complex rather than alterations of the receptor proteins themselves.

Rat behavior in two models of anxiety and brain [3H]muscimol binding: pharmacological, correlation, and multifactor analysis

The contribution of GABAergic mechanisms to rat emotional behavior in two animal models of anxiety (open field test of neophobia and aversively conditioned freezing reaction), was confirmed by pharmacological analysis, using anxiolytic (midazolam) and anxiogenic (picrotoxin) compounds. Both substances are known to modulate GABA(A) receptors' activity in a positive or negative manner, respectively. It seemed, therefore, worthwhile to check whether the behavioral parameters measured in these animal models of anxiety correlate with [3H]muscimol binding (a highly selective GABA(A) receptor ligand) in different brain structures of nai;ve rats, with a view to establish the role of genetically determined expression of local GABA(A) receptors in the organization of rat emotional and motor behavior. Correlation analysis revealed no links between individually determined expression of GABA(A) receptors (quantitative receptor autoradiography) in the brain structures, and the emotional behavior of nai;ve, drug-free animals, in both tests. Factor analysis confirmed that animal behavior in both tests was under control of different central processes. Moreover, none of the behavioral and ligand binding parameters loaded on the same factor, confirming the negative results of the correlation study. The present results indicate that the origin of emotions is a complex phenomenon, probably involving the interaction between GABA-ergic innervation of many brain structures.

Distribution of muscimol, QNB, and 5HT binding in the vertebrate diencephalon: A comparative study of eight mammals and three non-mammals

The distribution of muscimol, quinuclidinyl benzilate (QNB), and serotonin (5HT)-bound receptors in the diencephalon was examined by conventional receptor-binding methods in 11 species of amniotes including 2 reptiles, 1 bird, and 8 mammals, selected mostly on the basis of their differing last common ancestor with Anthropoids. We found that receptor binding can help define major subdivisions of the forebrain. The results show that in each of these species, the distribution of muscimol and QNB binding across the four major subdivisions of the diencephalon was consistent; densest in the dorsal thalamus, with hypothalamus and then either ventral thalamus or epithalamus with successively lesser amounts. However, the binding of serotonin (5HT) was most prevalent in the hypothalamus with equivalent amounts in the other diencephalic subdivisions. Myelin- and cell-stained materials showed that the pattern of high-density binding probably is not the secondary result of non-neurochemical factors such as differences in cell or neuropil density or in total available membrane. Perhaps more importantly, the receptor distributions suggest functional roles for major subdivisions across taxa. Results show that GABA-A and muscaranic Ach receptors are common in the dorsal diencephalon across vertebrate species and, therefore, are probably responsible for the gating of information to the cortex. Results show that serotonin is predominant in the hypothalamus. The lack of it in the dorsal thalamus indicates that it is probably not responsible for gating of information to the cortex. Results also show that in nonmammals the amount of GABA-A and muscaranic Ach differs from that found in mammals. For muscaranic Ach, the labeling in marsupials differs from that in placentals. Primates differ from other species (nonmammals and mammals combined) in the amount of 5HT found in the ventral diencephalon and the hypothalamus.

Distribution and binding parameters of GABAA receptors in the thalamic nuclei of Macaca mulatta and changes caused by lesioning in the globus pallidus and reticular thalamic nucleus

Ascending output from the basal ganglia to the primate motor thalamus is carried by GABAergic nigro- and pallido-thalamic pathways, which interact with intrinsic thalamic GABAergic systems represented in primates by local circuit neurons and axons of the reticular thalamic nucleus. Disease-triggered pathological processes in the basal ganglia can compromise any of these pathways either directly or indirectly, yet the effects of basal ganglia lesioning on its thalamic afferent-receiving territories has not been studied in primates. Two GABA(A) receptor ligands, [(3)H]muscimol and [(3)H]flunitrazepam, were used to study the distribution and binding properties of the receptor in intact monkeys, those with kainic acid lesions in the globus pallidus, and those with ibotenic acid lesions in the reticular nucleus using quantitative autoradiographic technique on cryostat sections of fresh frozen brain tissue. In control monkeys the binding affinities for [(3)H]muscimol averaged 50 nM in the thalamic nuclei and 86 nM in the basal ganglia while the binding densities varied (maximum density of binding sites [Bmax] range of 99.4-1000.1 fmol/mg of tissue). Binding affinities and Bmax values for [(3)H]flunitrazepam averaged 2.02 nM and 81-113 fmol/mg of tissue, respectively. Addition of 100-microM GABA increased average affinity to 1.35 nM whereas Bmax values increased anywhere from 1-50% in different nuclei. Zolpidem (100 nM) decreased binding by 68-80%. Bmax values for both ligands were decreased at the two survival times in both medial and lateral globus pallidus implying involvement of both nuclei in the lesion. Statistically significant, 40% decrease (P=0.055) of Bmax for [(3)H]muscimol was observed in the ventral anterior nucleus pars densicellularis (VAdc, the main pallidal projection territory in the thalamus) 1 week after globus pallidus lesioning and a 36% decrease (P=0.017) 4 months post-lesioning. In contrast, [(3)H]flunitrazepam Bmax values in the VAdc of the same animals were increased by 23% (P=0.021) at 1 week and 28% (P=0.005) 4 months postlesion, respectively. One week after the reticular nucleus lesioning, the binding densities of [(3)H]muscimol and [(3)H]flunitrazepam were decreased in the thalamic nuclei receiving projections from the lesioned reticular nucleus sector by approximately 50% (P<0.05) and 10-33% (P<0.05), respectively. The results suggest that different GABA(A) receptor subtypes are associated with different GABAergic systems in the thalamus which react differently to deafferentation.

Photoaffinity labeling with a neuroactive steroid analogue. 6-azi-pregnanolone labels voltage-dependent anion channel-1 in rat brain

Neuroactive steroids modulate the function of gamma-aminobutyric acid, type A (GABA(A)) receptors in the central nervous system by an unknown mechanism. In this study we have used a novel neuroactive steroid analogue, 3 alpha,5 beta-6-azi-3-hydroxypregnan-20-one (6-AziP), as a photoaffinity labeling reagent to identify neuroactive steroid binding sites in rat brain. 6-AziP is an effective modulator of GABA(A) receptors as evidenced by its ability to inhibit binding of [(35)S]t-butylbicyclophosphorothionate to rat brain membranes and to potentiate GABA-elicited currents in Xenopus oocytes and human endothelial kidney 293 cells expressing GABA(A) receptor subunits (alpha(1)beta(2)gamma(2)). [(3)H]6-AziP produced time- and concentration-dependent photolabeling of protein bands of approximately 35 and 60 kDa in rat brain membranes. The 35-kDa band was half-maximally labeled at a [(3)H]6-AziP concentration of 1.9 microM, whereas the 60-kDa band was labeled at higher concentrations. The photolabeled 35-kDa protein was isolated from rat brain by two-dimensional PAGE and identified as voltage-dependent anion channel-1 (VDAC-1) by both matrix-assisted laser desorption ionization time-of-flight and ESI-tandem mass spectrometry. Monoclonal antibody directed against the N terminus of VDAC-1 immunoprecipitated labeled 35-kDa protein from a lysate of rat brain membranes, confirming that VDAC-1 is the species labeled by [(3)H]6-AziP. The beta(2) and beta(3) subunits of the GABA(A) receptor were co-immunoprecipitated by the VDAC-1 antibody suggesting a physical association between VDAC-1 and GABA(A) receptors in rat brain membranes. These data suggest that neuroactive steroid effects on the GABA(A) receptor may be mediated by binding to an accessory protein, VDAC-1.

Interactions of taurine and structurally related analogues with the GABAergic system and taurine binding sites of rabbit brain

1. The aim of this study was to find taurinergic compounds that do not interact with brain GABA ergic systems. 2. Washed synaptic membranes (SM) from whole rabbit brain were able to bind [(3)H]muscimol. Saturation experiments of the binding of [(3)H]GABA to GABA(B) receptors showed that SM possess two binding components; twice Triton X-100-treated SM contained 0.048 mmol endogenous taurine/kg protein and bound [(3)H]taurine in a saturable manner (K(d)=249.0+/-6.3 nM and B(max)=3.4+/-1.0 pmol mg(-1) prot). 3. Among the 19 structural analogues of taurine, 6-aminomethyl-3-methyl-4H-1,2,4-benzothiadiazine 1,1-dioxide (TAG), 2-aminoethylarsonic (AEA), 2-hydroxyethanesulfonic (ISE) and (+/-)cis-2-aminocyclohexane sulfonic acids (CAHS) displaced [(3)H]taurine binding (K(i)=0.13, 0.13, 13.5 and 4.0 micro M, respectively). These analogues did not interact with GABA(A) and GABA(B) receptors and did not affect taurine- and GABA-uptake systems and GABA-transaminase activity. 4. 3-Aminopropanesulfonic acid (OMO), beta-alanine, pyridine-3-sulfonic acid, N,N,N-trimethyltaurine (TMT), 2-(guanidino)ethanesulfonic acid (GES), ethanolamine-O-sulphate, N,N-dimethyltaurine (DMT), taurine and (+/-)piperidine-3-sulfonic acid (PSA) inhibited [(3)H]muscimol binding to GABA(A) receptors with different affinities (K(i)=0.013, 7.9, 24.6, 47.5, 52.0, 91.0, 47.5, 118.1 and 166.3 micro M, respectively). Taurine, 2-aminoethylphosphonic acid, DMT, TMT and OMO inhibited the binding of [(3)H]GABA to GABA(B) receptors with K(i)'s in the micro M range (0.8, 3.5, 4.4, 11.3 and 5.0, respectively). GES inhibited taurine uptake (IC(50)=3.72 micro M) and PSA GABA transaminase activity (IC(50)=103.0 micro M). 5. In conclusion, AEA, TAG, ISE and CAHS fulfill the criteria for taurinergic agents.

A method to measure the effective spread of focally injected muscimol into the central nervous system with electrophysiology and light microscopy

A method was developed to quantitate the volume of brain inactivated by muscimol focally injected. Tritiated muscimol was injected into the cerebellum and closely spaced sequential microelectrode recordings made at different depths by penetrations in an X-Y pattern centered at the injection site to evaluate changes in spontaneous activity in the tissue volume. Animals were euthanized after survivals from 40 min to 6 h, the cerebellum sectioned in the sagittal plane, and the sections dried onto glass slides. The slides were dipped in photographic emulsion, exposed in the dark and developed. Silver grain densities were quantitated by light microscopy from measured standards. The extent and concentration of bound, labeled muscimol co-varied with the observed reduction in recorded spontaneous activity. For future studies, the distribution and density of silver grains alone can serve as an accurate spatial indicator of the area of muscimol inactivation at high spatial resolution.

Muscimol diffusion after intracerebral microinjections: a reevaluation based on electrophysiological and autoradiographic quantifications

Intracerebral muscimol injection is widely used to inactivate discrete brain structures during behavioral tasks. However, little effort has been made to quantify the extent of muscimol diffusion. The authors report here electrophysiological and autoradiographic results obtained after muscimol injection (1 microg/microl) either into the nucleus basalis magnocellularis (0.1-0.4 microl) or into the thalamic reticular nucleus (RE, 0.05-0.1 microl). In 52 rats, multiunit recordings were collected either in the RE or in the auditory thalamus during the 2 h following muscimol injection. Decreases in neuronal activity were observed up to 3 mm from the injection site; their time of occurrence was a function of the distance between the injection and recording sites. Because these decreases cannot be explained by physiological effects, they likely reflected muscimol diffusion up to the recording sites. Autoradiographic studies involved 25 rats and different experimental conditions. Optical density (OD) measures indicated that after a survival time of 15 min, a 0.05 microl injection produced a labeled area of 5.25 mm(2) at the injection site and a rostrocaudal labeling of 1.7 mm. Increasing the survival time to 60 min, or increasing the injected volume to 0.1 microl, systematically led to a larger labeled area at the injection site (8-12 mm(2)) and to a larger rostrocaudal diffusion (2.0-2.5 mm). Direct quantifications of radioactivity by a high-resolution radioimager validated the OD measures and even indicated a larger muscimol diffusion (up to 3.25 mm). Thus, these data point out that muscimol diffusion after intracerebral microinjection is larger than usually supposed. The relationships between these results and those obtained in behavioral studies are discussed.

[3H]muscimol binding to gamma-aminobutyric acid(A) receptors is upregulated in CA1 neurons of the gerbil hippocampus in the ischemia-tolerant state

BACKGROUND AND PURPOSE

Excitotoxic activation of glutamate receptors is currently thought to play a pivotal role in delayed neuronal death (DND) of highly vulnerable CA1 neurons in the gerbil hippocampus after transient global ischemia. Postischemic degeneration of these neurons can be prevented by "preconditioning" with a short sublethal ischemic stimulus. The present study was designed to test whether ischemic preconditioning is associated with specific alterations of ligand binding to excitatory glutamate and/or inhibitory gamma-aminobutyric acid (GABA)A receptors compared with ischemia severe enough to induce DND.

METHODS

With the use of quantitative receptor autoradiography, postischemic ligand binding of [3H]MK-801 and [3H]alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) to excitatory N-methyl-D-aspartate (NMDA) and AMPA receptors as well as [3H]muscimol to inhibitory GABA(A) receptors in hippocampal subfields CA1, CA3, and the dentate gyrus were analyzed in 2 experimental paradigms. Gerbils were subjected to (1) a 5-minute ischemic period resulting in DND of CA1 neurons and (2) a 2.5-minute period of ischemia mediating tolerance induction.

RESULTS

[3H]MK-801 and [3H]AMPA binding values to excitatory NMDA and AMPA receptors showed a delayed decrease in relatively ischemia-resistant CA3 and dentate gyrus despite maintained neuronal cell density. [3H]Muscimol binding to GABA(A) receptors in CA1 neurons was transiently but significantly increased after preconditioning but not after global ischemia with consecutive neuronal death.

CONCLUSIONS

Downregulation of ligand binding to glutamate receptors in relatively ischemia-resistant CA3 and dentate gyrus neurons destined to survive suggests marked synaptic reorganization processes despite maintained structural integrity. More importantly, upregulation of binding to inhibitory GABA(A) receptors in the hippocampus indicates a relative shift between inhibitory and excitatory neurotransmission that we suggest may participate in endogenous postischemic neuroprotection.

Density and distribution of hippocampal neurotransmitter receptors in autism: an autoradiographic study

Neuropathological studies in autistic brains have shown small neuronal size and increased cell packing density in a variety of limbic system structures including the hippocampus, a change consistent with curtailment of normal development. Based on these observations in the hippocampus, a series of quantitative receptor autoradiographic studies were undertaken to determine the density and distribution of eight types of neurotransmitter receptors from four neurotransmitter systems (GABAergic, serotoninergic [5-HT], cholinergic, and glutamatergic). Data from these single concentration ligand binding studies indicate that the GABAergic receptor system (3[H]-flunitrazepam labeled benzodiazepine binding sites and 3[H]-muscimol labeled GABA(A) receptors) is significantly reduced in high binding regions, marking for the first time an abnormality in the GABA system in autism. In contrast, the density and distribution of the other six receptors studied (3[H]-80H-DPAT labeled 5-HT1A receptors, 3[H]-ketanserin labeled 5-HT2 receptors, 3[H]-pirenzepine labled M1 receptors, 3[H]-hemicholinium labeled high affinity choline uptake sites, 3[H]-MK801 labeled NMDA receptors, and 3[H]-kainate labeled kainate receptors) in the hippocampus did not demonstrate any statistically significant differences in binding.

A change in the density of [(3)H]flumazenil, but not [(3)H]muscimol binding, in Brodmann's Area 9 from subjects with bipolar disorder

BACKGROUND

This study examines the hypothesis that there are changes in cortical serotonergic, GABAergic and glutamatergic systems in bipolar disorder and schizophrenia.

METHODS

In situ radioligand binding and autoradiography were used to measure neurochemical markers in Brodmann's Area (BA) 9 from control subjects and subjects with bipolar disorder or schizophrenia (n=8 per group).

RESULTS

Compared to tissue from schizophrenic (mean+/-S.E.M, 385+/-44 fmol/mg ETE) and control (383+/-44 fmol/mg ETE) subjects, there was an increase in the density of [(3)H]flumazenil binding to the benzodiazepine binding site on the GABA(A) receptor in subjects with bipolar disorder (451+/-17 fmol/mg ETE; P<0.05). There was no difference in the density of [(3)H]muscimol binding to the GABA(A) receptor or in the density of the serotonin(1A) receptor, serotonin(2A) receptor, ionotropic glutamate receptors or the serotonin transporter between the three cohorts. There was an age-related decrease in NMDA receptor density in control subjects that was absent in schizophrenia and bipolar disorder. An age-related increase in [(3)H]flumazenil binding in schizophrenia was absent in control and bipolar disorder subjects.

LIMITATIONS

This study involved a relatively small number of individuals.

CONCLUSIONS

An increase in the gamma2-receptor sub-unit in the GABA(A) receptor has been shown to increase benzodiazepine but not [(3)H]muscimol binding, this is the mismatch in binding we have shown in BA 9 from subjects with bipolar disorder. Thus, a change in the assembly of receptor subunits into GABA(A) receptors may be involved in the neuropathology of bipolar disorder. There may also be differences in age-related changes in cortical receptor density between bipolar disorder and schizophrenia.

Inhibition of the substantia nigra suppresses absences and clonic seizures in audiogenic rats, but not tonic seizures: evidence for seizure specificity of the nigral control

GABAergic inhibition of the substantia nigra pars reticulata has been shown to suppress seizures in most models of epilepsy involving forebrain networks, such as absences or clonic seizures. No such antiepileptic effects were observed, however, in genetically audiogenic rats exhibiting tonic seizures generated in the brainstem. This suggests a constitutive dysfunction of the nigral GABAergic neurotransmission in this strain of rat or a selective action of the nigral control on specific networks. In the present study, we first confirmed that bilateral injection of muscimol (700 pmol/side) in the substantia nigra had no effect in Wistar rats with audiogenic seizures (Wistar AS). [3H]Muscimol autoradiography suggested a 40% reduced density of GABA(A) receptors in the substantia nigra of Wistar AS, whereas no change was observed in the cortex and the superior colliculus (superficial and intermediate layers), as compared to control animals. In Wistar AS where 40 repetitions of audiogenic stimulations progressively induced generalised convulsive seizures with both tonic and clonic components, bilateral injection of muscimol (350 pmol/side) in the substantia nigra suppressed the clonic component but had no effect on tonic seizures. In hybrid rats issued from cross-breeding between Wistar AS and rats with spontaneous absence seizures, bilateral injection of muscimol (18 pmol/side) in the substantia nigra abolished cortical spike-and-wave discharges, but had no effect on tonic audiogenic seizures at doses up to 700 pmol/side. These results show that despite a decreased number of GABA(A) receptors in the substantia nigra, inhibition of this structure in Wistar AS still leads to inhibition of seizures involving forebrain structures. These results confirm that GABAergic inhibition of the substantia nigra has antiepileptic effects through the control of forebrain circuits. They suggest that this control mechanism has no inhibitory effect on circuits underlying audiogenic tonic seizures.

Cellular depolarization of neurons in the locus ceruleus region of the guinea pig associated with the development of tolerance to opioids

These experiments were designed to test two hypotheses: 1) the tolerance induced by morphine pellet implantation in guinea pigs will result in subsensitivity of cells in the locus ceruleus (LC), not only to morphine, but to another agonist acting on a different receptor and transduction system, namely the gamma-aminobutyric acid(A) receptor agonist, muscimol; and 2) The nonspecific (heterologous) tolerance would be associated with a partial depolarization of the tolerant cells and a decrease in the contribution of electrogenic Na(+)/K(+) pumping. Extracellular recording from LC neurons in brain slices from animals implanted with either morphine or placebo pellets established that the tolerant preparations were subsensitive to both morphine and muscimol. Immunocytochemical analysis identified the alpha(3)-subunit as the primary isoform of the Na(+)/K(+) pump in the cells under investigation. Whole-cell patch clamp recording of neurons in brain slices demonstrated that, with electrodes containing 20 mM Na(+) (approximating [Na](i)), tolerant cells were significantly depolarized by a mean of 6.7 mV. Dialysis with antibody specific for the alpha(3)-isoform from patch pipettes produced depolarization of both control and tolerant cells. However, the depolarizing effect of the antibody was less in tolerant cells, suggesting a lesser degree of electrogenic Na(+) pumping. Furthermore, the presence of antibody reduced the membrane potentials of tolerant and placebo cells to equal values, suggesting that the diffusion potentials were not different. In contrast, antibody specific for the alpha(1)-subunit isoform in the pipettes had no effect on membrane potential in either control or tolerant cells. In conclusion, both hypotheses were supported.

Effects of linalool on [(3)H]MK801 and [(3)H] muscimol binding in mouse cortical membranes

Linalool is a monoterpene compound reported to be a major component of essential oils of several aromatic species. Several linalool-producing species are used in traditional medical systems for sedative purposes, including the interruption and prevention of seizures. Previous studies in mice revealed that linalool modulates glutamatergic (competitive antagonism of L-[(3)H]glutamate binding, delayed intraperitoneal NMDA-induced convulsions and blockade of intracerebroventricular Quin-induced convulsions) and GABAergic transmission (protection against pentylenetetrazol and picrotoxin-induced convulsions). To further clarify the anticonvulsive mechanisms of linalool, we studied the effects of linalool on binding of [(3)H]MK801 (NMDA antagonist) and [(3)H]muscimol (GABA(A) agonist) to mouse cortical membranes. Linalool showed a dose dependent non-competitive inhibition of [(3)H]MK801 binding (IC(50) = 2.97 mM) but no effect on [(3)H]muscimol binding. The data suggest that the anticonvulsant mode of action of linalool includes a direct interaction with the NMDA receptor complex. The data do not, however, support a direct interaction of linalool with GABA(A) receptors, although changes in GABA-mediated neuronal inhibition or effects on GABA release and uptake cannot be ruled out.

Chronic prenatal ethanol exposure increases GABA(A) receptor subunit protein expression in the adult guinea pig cerebral cortex

Excessive consumption of ethanol during pregnancy can produce teratogenic effects in offspring and is the leading cause of mental deficiency in the Western world. The objective of this study was to examine the effects of chronic prenatal ethanol exposure on the number of GABA(A) receptors and relative protein levels for GABA(A) receptor alpha1 and beta2/3 subunits in the adult guinea pig cerebral cortex. Timed pregnant Dunkin-Hartley strain guinea pigs were given one of the following oral treatments daily throughout gestation: 4 gm of ethanol per kilogram of maternal body weight, isocaloric-sucrose with pair feeding, or isovolumetric water with ad libitum access to food. The ethanol treatment resulted in a peak maternal blood ethanol concentration of 328 +/- 55 mg/dl (71.3 +/- 12.0 mm) on gestational day 57 (term, approximately 68 d). Chronic prenatal exposure to ethanol resulted in increased spontaneous locomotor activity throughout development and decreased cerebral cortical weight in adult offspring. The number of cerebral cortical [(3)H]muscimol binding sites was increased in adult offspring from the ethanol treatment group, and there was a corresponding increase in the amount of GABA(A) receptor alpha1 and beta2/3 subunit proteins in these same animals. For individual offspring, there were correlations between locomotor activity and cerebral cortical weight, as well as between cerebral cortical weight and GABA(A) receptor neurochemistry. There was no effect of chronic prenatal ethanol exposure on [(3)H]MK-801 binding in this tissue. These data demonstrate that chronic prenatal ethanol exposure has long-term consequences on the regulation of GABA(A) receptor expression in the cerebral cortex.

Loss of the major GABA(A) receptor subtype in the brain is not lethal in mice

The alpha1beta2gamma2 is the most abundant subtype of the GABA(A) receptor and is localized in many regions of the brain. To gain more insight into the role of this receptor subtype in the modulation of inhibitory neurotransmission, we generated mice lacking either the alpha1 or beta2 subunit. In agreement with the reported abundance of this subtype, >50% of total GABA(A) receptors are lost in both alpha1-/- and beta2-/- mice. Surprisingly, homozygotes of both mouse lines are viable, fertile, and show no spontaneous seizures. Initially half of the alpha1-/- mice died prenatally or perinatally, but they exhibited a lower mortality rate in subsequent generations, suggesting some phenotypic drift and adaptive changes. Both adult alpha1-/- and beta2-/- mice demonstrate normal performances on the rotarod, but beta2-/- mice displayed increased locomotor activity. Purkinje cells of the cerebellum primarily express alpha1beta2gamma2 receptors, and in electrophysiological recordings from alpha1-/- mice GABA currents in these neurons are dramatically reduced, and residual currents have a benzodiazepine pharmacology characteristic of alpha2- or alpha3-containing receptors. In contrast, the cerebellar Purkinje neurons from beta2-/- mice have only a relatively small reduction of GABA currents. In beta2-/- mice expression levels of all six alpha subunits are reduced by approximately 50%, suggesting that the beta2 subunit can coassemble with alpha subunits other than just alpha1. Our data confirm that alpha1beta2gamma2 is the major GABA(A) receptor subtype in the murine brain and demonstrate that, surprisingly, the loss of this receptor subtype is not lethal.

Detection and binding properties of GABA(A) receptor assembly intermediates

Density gradient centrifugation of native and recombinant gamma-aminobutyric acid, type A (GABA(A)) receptors was used to detect assembly intermediates. No such intermediates could be identified in extracts from adult rat brain or from human embryonic kidney (HEK) 293 cells transfected with alpha(1), beta(3), and gamma(2) subunits and cultured at 37 degrees C. However, subunit dimers, trimers, tetramers, and pentamers were found in extracts from the brain of 8-10-day-old rats and from alpha(1)beta(3)gamma(2) transfected HEK cells cultured at 25 degrees C. In both systems, alpha(1), beta(3), and gamma(2) subunits could be identified in subunit dimers, indicating that different subunit dimers are formed during GABA(A) receptor assembly. Co-transfection of HEK cells with various combinations of full-length and C-terminally truncated alpha(1) and beta(3) or alpha(1) and gamma(2) subunits and co-immunoprecipitation with subunit-specific antibodies indicated that even subunits containing no transmembrane domain can assemble with each other. Whereas alpha(1)gamma(2), alpha(1)Ngamma(2), alpha(1)gamma(2)N, and alpha(1)Ngamma(2)N, combinations exhibited specific [(3)H]Ro 15-1788 binding, specific [(3)H]muscimol binding could only be found in alpha(1)beta(3) and alpha(1)beta(3)N, but not in alpha(1)Nbeta(3) or alpha(1)Nbeta(3)N combinations. This seems to indicate that a full-length alpha(1) subunit is necessary for the formation of the muscimol-binding site and for the transduction of agonist binding into channel gating.

Changes of [3H]MK-801, [3H]muscimol and [3H]flunitrazepam binding in rat brain by the prolonged ventricular infusion of ginsenoside Rc and Rg1

In the present study, we have investigated the effects of centrally administered ginsenoside Rc and Rg1 on the modulation of the NMDA receptor and GABA(A)receptor binding in rat brain. The NMDA receptor binding was analysed by quantitative autoradiography using [(3)H]MK-801 binding, and the GABA(A)receptor bindings were analysed by using [(3)H]muscimol and [(3)H]flunitrazepam binding in rat brain slices. Rats were infused with ginsenoside Rc or Rg1 (10 microg/10 microl h(-1), i.c.v.) for 7 days, through pre-implanted cannula using osmotic minipumps (Alzet, model 2ML). The levels of [(3)H]MK-801 binding were highly decreased in part of the parietal layers of the cortex and cingulated by ginsenoside Rc and Rg1. The levels of [(3)H]muscimol binding were strongly elevated in almost all regions of the frontal cortex by the treatment of ginsenoside Rc but decreased by ginsenoside Rg1. However, the [(3)H]flunitrazepam binding was not modulated by ginsenoside Rc or Rg1 infusion. These results suggest that prolonged infusion of ginsenosides could differentially modulate [(3)H]MK-801 and [(3)H]muscimol binding in a region-specific manner.

Water maze and radial maze learning and the density of binding sites of glutamate, GABA, and serotonin receptors in the hippocampus of inbred mouse strains

Correlations between the densities of ionotropic glutamate, GABA(A), and serotonin binding sites in the hippocampus of seven inbred mouse strains and strain-specific learning capacities in two types of maze were studied. Binding site densities were measured with quantitative receptor autoradiography. Learning capacities were determined in a water maze task as well as in spatial and nonspatial versions of an eight-arm radial maze. The densities of most binding sites differed significantly between the strains in the subfields of Ammon's horn (CA1 and CA3) and the dentate gyrus, except for serotonin binding sites in CA1. By comparing the different strains, significant receptor-behavioral correlations between the densities of the GABA(A) receptors and the activity-dependent behavior in the water maze as well as the spatial learning in the radial maze were found. The densities of D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxalone propionate (AMPA) and kainate receptors correlated positively with learning capacity in the spatial eight-arm radial maze. We conclude that hereditary variations mainly in AMPA, kainate, and GABA(A) receptor densities are involved in behavioral variations in spatial and nonspatial learning tasks.

Transient up-regulation of gamma-aminobutyric acid(A) receptor binding by lubeluzole after neocortical specify lesion in rats

The effect of the neuroprotective drug lubeluzole on cortical receptor binding was investigated in animals with photothrombotic ischemic lesions. Control animals were treated with the inactive stereoisomer of the drug. Lubeluzole was applied intravenously as a single bolus (0.31 mg/kg) followed by a 1-h infusion of 0.31 mg/kg. Lubeluzole selectively increased gamma-amino-butyric acid(A) (GABA(A)) receptor binding but had no significant and/or consistent effects on N-methyl-D-aspartate (NMDA), (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), and kainate receptors. Lubeluzole caused a significant up-regulation of GABA(A) receptor binding in the lesioned area as well as in unimpaired cortical areas of both hemispheres. This effect appeared in the hours following the lesion and peaked at 24 h. Our findings suggest that reduced cortical excitability brought about by increased binding capacities of GABA(A) receptors may contribute to the neuroprotective effect of lubeluzole.

Changes of GABA(A) receptor binding and subunit mRNA level in rat brain by infusion of subtoxic dose of MK-801

In the present study, we have investigated the effects of prolonged inhibition of NMDA receptor by infusion of subtoxic dose of MK-801 to examine the modulation of GABA(A) receptor binding and GABA(A) receptor subunit mRNA level in rat brain. It has been reported that NMDA-selective glutamate receptor stimulation alters GABA(A) receptor pharmacology in cerebellar granule neurons in vitro by altering the levels of selective subunit. However, we have investigated the effect of NMDA antagonist, MK-801, on GABA(A) receptor binding characteristics in discrete brain regions by using autoradiographic and in situ hybridization techniques. The GABA(A) receptor bindings were analyzed by quantitative autoradiography using [3H]muscimol, [3H]flunitrazepam, and [35S]TBPS in rat brain slices. Rats were infused with MK-801 (1 pmol/10 microl per h, i.c.v.) for 7 days, through pre-implanted cannula by osmotic minipumps (Alzet, model 2 ML). The levels of [3H]muscimol binding were highly elevated in almost all of brain regions including cortex, caudate putamen, thalamus, hippocampus, and cerebellum. However, the [3H]flunitrazepam binding and [35S]TBPS binding were increased only in specific regions; the former level was increased in parts of the cortex, thalamus, and hippocampus, while the latter binding sites were only slightly elevated in parts of thalamus. The levels of beta2-subunit were elevated in the frontal cortex, thalamus, hippocampus, brainstem, and cerebellar granule layers while the levels of beta3-subunit were significantly decreased in the cortex, hippocampus, and cerebellar granule layers in MK-801-infused rats. The levels of alpha6- and delta-subunits, which are highly localized in the cerebellum, were increased in the cerebellar granule layer after MK-801 treatment. These results show that the prolonged suppression of NMDA receptor function by MK-801-infusion strongly elevates [3H]muscimol binding throughout the brain, increases regional [3H]flunitrazepam and [35S]TBPS binding, and alters GABA(A) receptor subunit mRNA levels in different directions. The chronic MK-801 treatment has differential effect on various GABA(A) receptor subunits, which suggests involvement of differential regulatory mechanisms in interaction of NMDA receptor with the GABA receptors.

Expression and distribution of GABAA receptor subtypes in human alcoholic cerebral cortex

Long-term alcohol abuse is known to target specific areas of the brain such as the superior frontal cortex (SFC), resulting in neuronal cell loss. Abnormal transmission of the inhibitory neurotransmitter GABA may contribute to this damage. Previous work in our laboratory has found differential expression and distribution of certain a subunit genes of the GABAA receptor in the SFC of human alcoholic brain, suggesting that differences in GABAA receptor subunit expression could give rise to the locally altered GABAA pharmacology which is associated with alcohol abuse. A competitive RT-PCR assay has been developed to study the expression of the GABAA receptor beta-subunit genes beta1, beta2, and beta3. A single set of primers homologous to all three beta isoform sequences has been shown to amplify each of the beta isoforms from mRNA isolated from human brain tissue obtained at autopsy. An internal standard has been designed which is identical to the target except for a 61-bp deletion and a unique restriction enzyme (RE) site. This is co-amplified with the target sequences to allow amplification efficiency to be assessed and thus enable the quantitation of gene expression. A range of GABAA receptor ligands were used to look at differential distribution of receptor subtypes in the cortical laminae by autoradiography. Differences in distribution of the ligands were demonstrated, consistent with a hypothesis of alcohol-induced variations in the expression of receptor subunits.

Honokiol and magnolol increase the number of [3H] muscimol binding sites three-fold in rat forebrain membranes in vitro using a filtration assay, by allosterically increasing the affinities of low-affinity sites

1. The bark of the root and stem of various Magnolia species has been used in Traditional Chinese Medicine to treat a variety of disorders including anxiety and nervous disturbances. The biphenolic compounds honokiol (H) and magnolol (M), the main components of the Chinese medicinal plant Magnolia officinalis, interact with GABA(A) receptors in rat brain in vitro. We compared the effects of H and M on [3H]muscimol (MUS) and [3H]flunitrazepam (FNM) binding using EDTA/water dialyzed rat brain membranes in a buffer containing 150 mM NaCl plus 5 mM Tris-HCl, pH 7.5 as well as [35S]t-butylbicyclophosphorothionate (TBPS) in 200 mM KBr plus 5 mM Tris-HCl, pH 7.5. H and M had similar enhancing effects on [3H]MUS as well as on [3H]FNM binding to rat brain membrane preparations, but H was 2.5 to 5.2 times more potent than M. 2. [3H]FNM binding. GABA alone almost doubled [3H]FNM binding with EC50 = 450 nM and 200 nM using forebrain and cerebellar membranes, respectively. In the presence of 5 microM H or M the EC50 values for GABA were decreased to 79 and 89 nM, respectively, using forebrain, and 39 and 78 nM, using cerebellar membranes. H and M potently enhanced the potentiating effect of 200 nM GABA on [3H]FNM binding with EC50 values of 0.61 microM and 1.6 microM using forebrain membranes, with maximal enhancements of 33 and 47%, respectively. Using cerebellar membranes, the corresponding values were 0.25 and 1.1 microM, and 22 and 34%. 3. [3H]MUS binding. H and M increased [3H]MUS binding to whole forebrain membranes about 3-fold with EC50 values of 6.0 and 15 microM. Using cerebellar membranes, H and M increased [3H]MUS binding approximately 68% with EC50 values of 2.3 and 12 microM, respectively. Scatchard analysis revealed that the enhancements of [3H]MUS binding were due primarily to increases in the number of binding sites (Bmax values) with no effect on the high affinity binding constants (Kd values). The enhancing effect of H and M were not additive. 4. [35S]TBPS binding. H and M displaced [35S]TBPS binding from sites on whole rat forebrain membranes with IC50 values of 7.8 and 6.0 microM, respectively. Using cerebellar membranes, the corresponding IC50 values were 5.3 and 4.8 microM. These inhibitory effects were reversed by the potent GABA(A) receptor blocker R5135 (10 nM), suggesting that H and M allosterically increase the affinity of GABA(A) receptors for GABA and MUS by binding to sites in GABA(A) receptor complexes. 5. Two monophenols, the anesthetic propofol (2,6-diisopropylphenol, P) and the anti-inflammatory diflunisal (2',4'-difluoro-4-hydroxy-3-biphenyl carboxylic acid, D) also enhanced [3H]MUS binding, decreased the EC50 values for GABA in enhancing [3H]FNM binding and potentiated the enhancing effect of 200 nM GABA on [3H]FNM binding, although enhancements of [3H]MUS binding for these monophenols were smaller than those for H and M, using forebrain and cerebellar membranes. The enhancing effect of P and D on [3H]MUS binding were almost completely additive. 2,2'-biphenol was inactive on [3H]MUS and [3H]FNM binding. These, and other preliminary experiments, suggest that appropriate ortho (C2) and para (C4) substitution increases the GABA-potentiating activity of phenols. 6. The potentiation of GABAergic neurotransmission by H and M is probably involved in their previously reported anxiolytic and central depressant effects.

Changes of [3H]muscimol binding and GABA(A) receptor beta2-subunit mRNA level by tolerance to and withdrawal from pentobarbital in rats

Effects of continuous pentobarbital administration on binding characteristics of [3H]muscimol were examined by autoradiography, and levels of GABA(A) receptor beta2-subunit mRNA were investigated by in situ hybridization histochemistry in the rat brain. In order to eliminate the induction of hepatic metabolism by systemic administration of pentobarbital, an i.c.v. infusion model of tolerance to and withdrawal from pentobarbital was used. An experimental model of barbiturate tolerance and withdrawal was developed using i.c.v. infusion of pentobarbital (300 microg/10 microl/hr for 7 days) by osmotic minipumps and abrupt withdrawal from pentobarbital. The levels of [3H]muscimol binding were elevated in cingulate of frontal cortex (46%) and granule layer of cerebellum (32%) of rats 24-hr after withdrawal from pentobarbital, while it was only elevated in cingulate (58%) of tolerant rats. The GABA(A) receptor beta2-subunit mRNA was increased in the withdrawal rats only: in the cortex (9-14%), hippocampus (15-21%), inferior colliculus (21%), and granule layer of cerebellum (24%). These results show the involvement of GABA(A) receptor and its beta2-subunit up-regulations in pentobarbital withdrawal rats, and suggest that the levels of [3H]muscimol binding and GABA(A) receptor beta2-subunit mRNA are altered in a region-specific manner during pentobarbital withdrawal.

Role of the conserved lysine residue in the middle of the predicted extracellular loop between M2 and M3 in the GABA(A) receptor

In alpha1, beta2, and gamma2 subunits of the gamma-aminobutyric acid A (GABA(A)) receptor, a conserved lysine residue occupies the position in the middle of the predicted extracellular loop between the transmembrane M2 and M3 regions. In all three subunits, this residue was mutated to alanine. Whereas the mutation in alpha1 and beta2 subunits resulted each in about a sixfold shift of the concentration-response curve for GABA to higher concentrations, no significant effect by mutation in the gamma subunit was detected. The affinity for the competitive inhibitor bicuculline methiodide was not affected by the mutations in either the alpha1 subunit or the beta2 subunit. Concentration-response curves for channel activation by pentobarbital were also shifted to higher concentrations by the mutation in the alpha and beta subunits. Binding of [3H]Ro 15-1788 was unaffected by the mutation in the alpha subunit, whereas the binding of [3H]muscimol was shifted to lower affinity. Mutation of the residue in the alpha1 subunit to E, Q, or R resulted in an about eight-, 10-, or fivefold shift, respectively, to higher concentrations of the concentration-response curve for GABA. From these observations, it is concluded that the corresponding residues on the alpha1 and beta2 subunits are involved more likely in the gating of the channel by GABA than in the binding of GABA or benzodiazepines.

Acute pentylenetetrazol injection reduces rat GABAA receptor mRNA levels and GABA stimulation of benzodiazepine binding with No effect on benzodiazepine binding site density

The effects of a single convulsive dose of pentylenetetrazol (PTZ, 45 mg/kg i.p.) on rat brain gamma-aminobutyric acid type A (GABAA) receptors were studied. Selected GABAA receptor subunit mRNAs were measured by Northern blot analysis (with beta-actin mRNA as a standard). Four hours after PTZ, the GABAA receptor gamma2-mRNA was decreased in hippocampus, cerebral cortex, and cerebellum; alpha1-mRNA was decreased in cerebellum; and beta2 subunit mRNA was decreased in cortex and cerebellum. The alpha5 subunit mRNA level was not altered. Those mRNAs that had been reduced were increased in some brain regions at the 24-h time point, and these changes reverted to control levels by 48 h. PTZ effect on GABAA receptors was also studied by autoradiographic binding assay with the benzodiazepine agonist [3H]flunitrazepam (FNP), the GABAA agonist [3H]muscimol, and the benzodiazepine antagonist [3H]flumazenil. There was an overall decrease in [3H]FNP binding 12 but not 24 h after PTZ treatment. In contrast, [3H]muscimol binding was minimally affected, and [3H]flumazenil binding was unchanged after PTZ treatment. Additional binding studies were performed with well-washed cerebral cortical homogenates to minimize the amount of endogenous GABA. There was no PTZ effect on specific [3H]FNP binding. However, there was a significant reduction in the stimulation of [3H]FNP binding by GABA. The results showed that an acute injection of PTZ caused transient changes in GABAA receptor mRNA levels without altering receptor number but affected the coupling mechanism between the GABA and benzodiazepine sites of the GABAA receptor.

Partial restoration of striatal GABAA receptor balance by functional mesencephalic dopaminergic grafts in mice with hereditary parkinsonism

Levels of inhibitory amino acid receptors were studied in the weaver (wv/wv) mouse model of dopamine (DA) deficiency after unilateral intrastriatal transplantation of fetal mesencephalic cell suspensions. Graft integration was verified by turning behavior tests and from the topographical levels of the DA transporter, tagged autoradiographically with 3 nM [3H]GBR 12935. The average increase in [3H]GBR 12935 binding in grafted dorsal striatum compared to nongrafted wv/wv striatum was 60% 3 months after grafting. Autoradiography of 8 nM [3H]flunitrazepam and 12 nM [3H]muscimol binding was carried out to visualize the distribution of GABAA receptors in +/+ mice and in recipient weaver mutants. A 17% increase in [3H]flunitrazepam binding and a 20% increase in [3H]muscimol binding was found in the nongrafted dorsal striatum of weaver mutants compared to +/+. The functional mesencephalic grafts had a partial normalizing effect on both [3H]flunitrazepam and [3H]muscimol binding in the dorsal striatum of the weaver recipients. The normalization brought about by the grafts was around 20% for [3H]flunitrazepam binding and more than 40% for [3H]muscimol binding. The results are discussed in the context of the important interaction between the converging glutamatergic corticostriatal and DAergic nigrostriatal pathways in controlling the functional GABAergic output of the basal ganglia in Parkinson's disease and in experimental models of DA deficiency.

Defining affinity with the GABAA receptor

At nicotinic and glutamatergic synapses, the duration of the postsynaptic response depends on the affinity of the receptor for transmitter (Colquhoun et al., 1977;Pan et al., 1993). Affinity is often thought to be determined by the ligand unbinding rate, whereas the binding rate is assumed to be diffusion-limited. In this view, the receptor selects for those ligands that form a stable complex on binding, but binding is uniformly fast and does not itself affect selectivity. We tested these assumptions for the GABAA receptor by dissecting the contributions of microscopic binding and unbinding kinetics for agonists of equal efficacy but of widely differing affinities. Agonist pulses applied to outside-out patches of cultured rat hippocampal neurons revealed that agonist unbinding rates could not account for affinity if diffusion-limited binding was assumed. However, direct measurement of the instantaneous competition between agonists and a competitive antagonist revealed that binding rates were orders of magnitude slower than expected for free diffusion, being more steeply correlated with affinity than were the unbinding rates. The deviation from diffusion-limited binding indicates that a ligand-specific energy barrier between the unbound and bound states determines GABAA receptor selectivity. This barrier and our kinetic observations can be quantitatively modeled by requiring the participation of movable elements within a flexible GABA binding site.

Interaction of allosteric ligands with GABAA receptors containing one, two, or three different subunits

The presence of allosteric binding sites on recombinant GABAA receptors formed after transfection of human embryonic kidney (HEK) 293 cells with alpha 1-, beta 3-, or gamma 2-subunits, or with various combinations of these subunits, was systematically investigated. From all possible subunit combinations, high affinity [3H]muscimol binding sites were induced in cells transfected with alpha 1 beta 3- or alpha 1 beta 3 gamma 2-subunits only. GABAA receptor associated [3H]flunitrazepam binding sites were induced in cells after transfection with alpha 1 gamma 2- or alpha 1 beta 3, gamma 2-subunits, and [35S]r-butylbicyclophosphorothionate (TBPS) binding sites were found in cells transfected with beta 3-, beta 3 gamma 2-, alpha 1 beta 3-, or alpha 1 beta 3 gamma 2-subunits. Binding of [35S]TBPS could be inhibited by pentobarbital, etazolate, (+)-etomidate, alphaxalone, propofol, chlormethiazole, and 4'-chlorodiazepam (Ro 5-4864) with a potency which differed in cells transfected with beta 3-, beta 3 gamma 2-, alpha 1 beta 3-, or alpha 1 beta 3 gamma 2-subunits. Results obtained indicate that receptors with different subunit composition actually can be formed in HEK cells and exhibit distinct pharmacological properties.

Regional distribution of the anticonvulsant and behavioural effects of muscimol injected into the substantia nigra of rats

Previous anatomical investigations have reported a direct projection from substantia nigra pars lateralis to the dorsal midbrain anticonvulsant zone. The present study tested the hypothesis that the anticonvulsant properties of nigral inhibition previously attributed to substantia nigra pars reticulata were, in fact, due to the suppression of neural activity in the adjacent pars lateralis. Using the electroshock model of epilepsy, a systematic map of the anticonvulsant effects of bilateral injections of muscimol (60 ng/0.5 mu l per side) into different parts of substantia nigra was constructed. Electroshock (1 s of 40 mA 50 Hz AC) was administered via ear-clip electrodes 5 or 60 min following injections of muscimol, or 60 min after control injections of saline. To provide insight into the functional mechanisms whereby nigral inhibition might suppress tonic seizures the behavioural effects elicited by muscimol were also noted. No evidence supporting the experimental prediction was found. The most sensitive region of substantia nigra for suppressing tonic hindlimb extension was caudal pars reticulata. These data indicate a serious mismatch between the results of microinjection mapping studies and underlying patterns of anatomical connectivity. The behavioural reaction most closely associated with tonic seizure suppression was stereotyped locomotion; both were obtained maximally from caudal pars reticulata. Rostral substantia nigra was associated more with oral stereotypy, while a raised head position was observed at lateral injection sites and a lowered positioning of the head at medial locations. These data suggest that the rat substantia nigra may contain a functional organization based on a form of somatomotor topography. This organization may influence which part of the substantia nigra is most effective in suppressing seizures expressed by different muscle groups of the body.

Changes in GABA-benzodiazepine receptor complex and in peripheral benzodiazepine receptors in male mice after copulation

We studied the effect of copulation on GABA and benzodiazepine (BZD) receptors in the male mouse. After copulation, there was an 18% increase in the in vitro number of [3H]muscimol binding sites in frontal cortex. No changes were observed in central BZD binding sites labelled either in vivo by [3H]flunitrazepam or in vitro (in olfactory bulbs and in frontal cortex) by [3H]flumazenil, but further in vitro studies demonstrated that the GABA-stimulated [3H]flunitrazepam binding was reduced in both frontal cortex and olfactory bulbs. Copulation increased the number of peripheral BZD binding sites labelled by 3H-Ro 5-4864 in olfactory bulbs by 22% and in heart by 36%, but not in frontal cortex or in testes. The changes of GABA/BZD and peripheral BZD receptors in mouse suggest that the GABAergic system may be affected by copulation.

Kavapyrone enriched extract from Piper methysticum as modulator of the GABA binding site in different regions of rat brain

Regional differences in the modulation of [3H] muscimol binding to GABAA receptor complexes by kavapyrones, compounds of the rhizome of the plant Piper methysticum which possess sedative activity, were demonstrated using membrane fractions obtained from target brain centers of kavapyrone action: hippocampus (HIP), amygdala (AMY) and medulla oblongata (MED), and from brain centers outside the main kavapyrone effects as frontal cortex (FC) and cerebellum (CER). The kava extract enhanced the binding of [3H] muscimol in a concentration-dependent manner with maximal potentiation of 358% over control in HIP followed by AMY and MED (main target brain centers). Minimal stimulation was observed in CER followed by FC. In contrast, apart from CER, the potency of kavapyrones was similar in the brain areas investigated with EC50 values ranging between 200 and 300 microM kavapyrones. Scatchard analysis revealed that the observed effects of kavapyrones were due to an increase in the number of binding sites (Bmax), rather than to a change in affinity. At a kavapyrone concentration of 500 microM the order of enhancement in Bmax was HIP = AMY > MED > FC > CER. When kavapyrones are included together with pentobarbital or HPO the two classes of compounds produced a more than additive, i.e., synergetic effect on [3H] muscimol binding. Our findings suggest that one way kavapyrones might mediate sedative effects in vivo is through effects on GABAA receptor binding.

GABA binding sites: their density, their affinity to muscimol and their behaviour against neuroactive steroids in human gliomas of different degrees of malignancy

The aim of the present investigation was to answer the question if there exists a relation between the equipment of human gliomas with GABA binding sites and the degree of malignancy of these tumours diagnostically characterized according to WHO classification. The following parameters were assessed: the density, the affinity and the sensitivity to the modulating steroids 3 alpha-hydroxy-5 alpha-pregnane-20-one (3 alpha OH-DHP) and 3 alpha,21 dihydroxy-5 alpha-pregnane-20-one (THDOC). Scatchard analysis and non linear computerization revealed that the occurrence of GABA sites was directly related to the degree of tumour malignancy: GABA sites were only detectable in lower malignant gliomas of WHO grade II but not in the very malignant glioblastomas. However, irrespective of the individual density to be detected all glioma GABA sites were sensitive to 3 alpha-hydroxy-5 alpha-pregnane-20-one (3 alpha OH-DHP) and 3 alpha,21 dihydroxy-5 alpha-pregnane-20-one (THDOC) without exception. The effects of THDOC were due to increases in the number of binding sites whereas in the presence of 3 alpha OH-DHP a decrease in affinity was noted, additionally. These findings support the view of a normal functional integrity of GABA receptors in gliomas.

Reduced bicuculline response and GABAA agonist binding in aged rat hippocampus

Extracellular field recordings from CA1 pyramidal cells in the rat hippocampal slice preparation were used to examine the effects of age on gamma-aminobutyric acid (GABA)-mediated recurrent inhibition. The actions of bicuculline (1-100 microM), a GABAA antagonist, were assessed in slices from young (1-3 months) and aged (26 months) Fischer 344 rats. Pre-drug population spike amplitudes were smaller in slices from aged rats. Bicuculline increased population spike amplitudes in slices from both age groups, but slices from young rats were more sensitive to the antagonist. Bicuculline also produced multiple population spikes in slices from both age groups, however the increase in population spike burst durations was much greater in slices from young rats than in slices from aged rats. Agonist radiolabeled GABAA binding site density was significantly decreased in hippocampal tissue from aged rats. Our results suggest there is a reduction in GABAergic inhibition in hippocampal slices from aged rats, possibly mediated by a decrease in GABAA receptors.

Effects of Kamikihito, a traditional Chinese medicine, on neurotransmitter receptor binding in the aged rat brain determined by in vitro autoradiography (2): changes in GABAA and benzodiazepine receptor binding

We investigated the effects of the long-term administration of Kamikihito (KKT) on the specific binding of [3H]muscimol and [3H]flunitrazepam in the brains of young and aged rats using in vitro quantitative autoradiography. Specific [3H]muscimol binding in aged rats was decreased in all brain regions examined compared with that in young rats, whereas [3H]flunitrazepam binding did not change in any of the brain regions. Scatchard analysis revealed that the maximal number of [3H]muscimol binding sites in the cortex and thalamus was significantly decreased in aged rats compared with young rats, while its affinity remained unchanged. Long-term administration of KKT in young rats had no effect on either [3H]muscimol or [3H]flunitrazepam binding. In contrast, the same treatment in aged rats produced a significant increase in [3H]flunitrazepam binding to the cortex, caudate/putamen and accumbens, and it tended to decrease the [3H]muscimol binding. These results suggest that the selective reduction of specific [3H]muscimol binding in the brain may be responsible, at least in part, for anxiety-related behavior in aged rats. Furthermore, it appears that the significant increase in specific [3H]flunitrazepam binding produced in the brains of aged rats by the long-term administration of KKT may be responsible for the anxiolytic effects of this agent.

Heterogeneity of GABAA receptor in goldfish retina

The possibility of GABAA receptor heterogeneity in goldfish retina was studied with immunocytochemical and biochemical approaches: 1) immunoblotted membrane particulates of goldfish retina with mAb 62-3G1; 2) immunoprecipitation of the detergent-solubilized membrane proteins with mAb 62-3G1 followed by the receptor binding assay; 3) photoaffinity labeling of the membrane particulates with 3H-flunitrazepam (FNZ) and visualization of the labeled receptors by SDS-PAGE and fluorography; 4) dry autoradiography of 3H-muscimol and 3H-FNZ binding sites on frozen sections. Immunoblots showed that 62-3G1 reacted with 55-57.5 kDa M(r) polypeptides, similar to the muscimol-binding subunit of the receptor complex in bovine brain; while 3H-FNZ photoaffinity labeled the 52.5 kDa and 41-43 kDa M(r) polypeptides. Immunoprecipitated receptors bound only 3H-muscimol, not 3H-FNZ. An attempt to precipitate the 3H-FNZ photolabeled polypeptides failed. Dry autoradiography showed 3H-FNZ binding only in the inner plexiform layer (IPL); the binding was enhanced with gamma-aminobutyric acid (GABA) and blocked by clonazepam. In contrast, 3H-muscimol was bound in both the outer plexiform layer (OPL) and IPL, similar to that observed with 62-3G1 immunocytochemistry. We suggest that there are two subtypes of GABAA receptor in the goldfish retina: 1) GABAA receptors that are not linked to a benzodiazepine (BZD) receptor are located in the OPL and at amacrine-to-amacrine and amacrine-to-ganglion cell synapses in the IPL and are recognized by 62-3G1; 2) GABAA receptors that are linked to a BZD receptor are located only in the IPL, largely at amacrine-to-bipolar cell synapses and are not recognized by mAb 62-3G1.

Reduction of GABAA receptor binding of [3H]muscimol in the barrel field of mice after peripheral denervation: transient and long-lasting effects

The effect of peripheral sensory deprivation upon GABAA receptor binding of [3H]muscimol was investigated in the barrel cortex--cortical representation of mystacial vibrissae of mice--by means of in vitro quantitative autoradiography. Unilateral lesions of all vibrissae or selected rows of whiskers were performed neonatally or in adulthood. [3H]muscimol binding was examined after various survival times up to 60 days. Both types of lesions performed in adult mice resulted in a transient decrease (10-25%) of binding values in the deafferented areas of the barrel field as compared with the unoperated control side. Sixty days after denervation [3H]muscimol binding returned to control values. Similar results were found after neonatal removal of all vibrissae. Neonatal lesion of selected rows of vibrissae, however, resulted in a decrease of [3H]muscimol binding (by about 26%) lasting up to 60 days in corresponding rows of barrels. This last result was accompanied by severe cytoarchitectonic malformation of the barrel field. The results support the hypothesis that a decrease of inhibition plays a facilitatory role in the plastic reorganization of cortical circuitry.

Regulation of gamma-aminobutyric acidB (GABAB) receptors in cerebral cortex during the estrous cycle

We examined binding of the GABAB receptor agonist baclofen to brain synaptic membranes as a function of the natural variations in gonadal steroids that occur during the estrous cycle of the adult rat. We found that the binding of baclofen to neocortical membranes varied systematically as a function of the estrous cycle, with the lowest binding occurring during the estrus stage. Binding to archicortical (hippocampal) and hypothalamic preparations also varied with the estrous cycle, except that the lowest level of binding in these latter cases occurred during the diestrus stage. The variation of [3H]baclofen binding during the estrous cycle was different with respect to the binding of [3H]muscimol, an agonist for GABAA receptors, and [3H]8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), an agonist for serotonin 5-HT1A receptors that shares similar G proteins and effectors with GABAB receptors. Saturation binding studies of cortical GABAB receptors showed that apparent receptor density (Bmax) rather than affinity (Kd) best accounted for the change in binding during the estrous cycle in that Bmax, like total specific binding, was at a minimum during the estrus stage. The robust regulation of GABAB receptors in neocortex was unexpected and its functional significance is at present unknown. However, the correlation of the menstrual cycle with mood and other behavioral changes, and the correlations of the estrous and menstrual cycles with seizure susceptibility, may somehow depend upon hormonal regulation of transmitter systems such as the one we have observed here.

Influence of flumazenil on the learning-enhancing effect of ambocarb in rats

The effects of flumazenil (Ro 15-1788) and a new beta-carboline, ambocarb (AMB), on learning were investigated using the multichoice maze. The drugs, administered either alone or simultaneously, were injected once a day before training for eight days. AMB, administered alone, improved the performance and decreased the working errors, whilst flumazenil had no effect on performance during its sole administration but weakly prevented the learning-improving effect of AMB. More significantly, flumazenil antagonized the motor activity depressed by AMB. In the study ex vivo, flumazenil decreased and AMB increased the apparent affinity of [3H]flunitrazepam to the central benzodiazepine receptors. Flumazenil reversed the action of AMB on the central benzodiazepine receptors, but failed to reduce significantly the modulative effects of AMB on [3H]muscimol and [35S]t-butylbicyclophosphorothionate ([35S]TBPS) binding. These data indicate that flumazenil, due to its action on the central benzodiazepine receptors, more effectively reverses the inhibition of motor activity than the performance-improving effect of AMB.