Role of GABAB receptors in intracellular Ca2+ homeostasis and possible interaction between GABAA and GABAB receptors in regulation of transmitter release in cerebellar granule neurons

GABAA and GABAB Receptors Mediate GABA-Induced Intracellular Ca2+ Signals in Human Brain Microvascular Endothelial Cells

Numerous studies recently showed that the inhibitory neurotransmitter, γ-aminobutyric acid (GABA), can stimulate cerebral angiogenesis and promote neurovascular coupling by activating the ionotropic GABA_A receptors on cerebrovascular endothelial cells, whereas the endothelial role of the metabotropic GABA_B receptors is still unknown. Preliminary evidence showed that GABA_A receptor stimulation can induce an increase in endothelial Ca²⁺ levels, but the underlying signaling pathway remains to be fully unraveled. In the present investigation, we found that GABA evoked a biphasic elevation in [Ca²⁺]_i that was initiated by inositol-1,4,5-trisphosphate- and nicotinic acid adenine dinucleotide phosphate-dependent Ca²⁺ release from neutral and acidic Ca²⁺ stores, respectively, and sustained by store-operated Ca²⁺ entry. GABA_A and GABA_B receptors were both required to trigger the endothelial Ca²⁺ response. Unexpectedly, we found that the GABA_A receptors signal in a flux-independent manner via the metabotropic GABA_B receptors. Likewise, the full Ca²⁺ response to GABA_B receptors requires functional GABA_A receptors. This study, therefore, sheds novel light on the molecular mechanisms by which GABA controls endothelial signaling at the neurovascular unit.

Characterization of the Functional Cross-Talk between Surface GABAA and Dopamine D5 Receptors

The γ-aminobutyric acid type A receptor (GABAAR) plays a major role in fast inhibitory synaptic transmission and is highly regulated by the neuromodulator dopamine. In this aspect, most of the attention has been focused on the classical intracellular signaling cascades following dopamine G-protein-coupled receptor activation. Interestingly, the GABAAR and dopamine D5 receptor (D5R) have been shown to physically interact in the hippocampus, but whether a functional cross-talk occurs is still debated. In the present study, we use a combination of imaging and single nanoparticle tracking in live hippocampal neurons to provide evidence that GABAARs and D5Rs form dynamic surface clusters. Disrupting the GABAAR-D5R interaction with a competing peptide leads to an increase in the diffusion coefficient and the explored area of both receptors, and a drop in immobile synaptic GABAARs. By means of patch-clamp recordings, we show that this fast lateral redistribution of surface GABAARs correlates with a robust depression in the evoked GABAergic currents. Strikingly, it also shifts in time the expression of long-term potentiation at glutamatergic synapses. Together, our data both set the plasma membrane as the primary stage of a functional interplay between GABAAR and D5R, and uncover a non-canonical role in regulating synaptic transmission.

Safety Challenges of Using High Dose Baclofen for Alcohol Use Disorder: A Focused Review

Since the early 2000s, the gamma-aminobutyric acid type B (GABA-B) receptor agonist baclofen has been extensively used for treating alcohol use disorder (AUD). In some countries, like France, Australia, or Germany, baclofen has been used at patient-tailored dose regimens, which can reach 300 mgpd or even more in some patients. The GABA-B-related pharmacology of baclofen expose patients to a specific profile of neuropsychiatric adverse drug reactions (ADRs), primarily some frequent sedative symptoms whose risk of occurrence and severity are both related to the absolute baclofen dosing and the kinetics of dose variations. Other frequent neuropsychiatric ADRs can occur, i.e., tinnitus, insomnia, or dizziness. More rarely, other serious ADRs have been reported, like seizures, manic symptoms, or sleep apnea. However, real-life AUD patients are also exposed to other sedative drugs, like alcohol of course, but also benzodiazepines, other drugs of abuse, or other sedative medications. Consequently, the occurrence of neuropsychiatric safety issues in these patients is essentially the result of a complex multifactorial exposure, in which baclofen causality is rarely obvious by itself. As a result, the decision of initiating baclofen, as well as the daily dose management should be patient-tailored, according the medical history but also the immediate clinical situation of the patient. The overall safety profile of baclofen, as well as the clinical context in which baclofen is used, have many similarities with the use of opiate substitution medications for opiate use disorder. This empirical statement has many implications on how baclofen should be managed and dosing should be adjusted. Moreover, this constant patient-tailored adjustment can be difficult to adapt in the design of clinical trials, which may explain inconsistent findings in baclofen-related literature on AUD.

Stimulus-induced Epileptic Spike-Wave Discharges in Thalamocortical Model with Disinhibition

Epileptic absence seizure characterized by the typical 2–4 Hz spike-wave discharges (SWD) are known to arise due to the physiologically abnormal interactions within the thalamocortical network. By introducing a second inhibitory neuronal population in the cortical system, here we propose a modified thalamocortical field model to mathematically describe the occurrences and transitions of SWD under the mutual functions between cortex and thalamus, as well as the disinhibitory modulations of SWD mediated by the two different inhibitory interneuronal populations. We first show that stimulation can induce the recurrent seizures of SWD in the modified model. Also, we demonstrate the existence of various types of firing states including the SWD. Moreover, we can identify the bistable parametric regions where the SWD can be both induced and terminated by stimulation perturbations applied in the background resting state. Interestingly, in the absence of stimulation disinhibitory functions between the two different interneuronal populations can also both initiate and abate the SWD, which suggests that the mechanism of disinhibition is comparable to the effect of stimulation in initiating and terminating the epileptic SWD. Hopefully, the obtained results can provide theoretical evidences in exploring dynamical mechanism of epileptic seizures.

Identification of the transmitter and receptor mechanisms responsible for REM sleep paralysis

During REM sleep the CNS is intensely active, but the skeletal motor system is paradoxically forced into a state of muscle paralysis. The mechanisms that trigger REM sleep paralysis are a matter of intense debate. Two competing theories argue that it is caused by either active inhibition or reduced excitation of somatic motoneuron activity. Here, we identify the transmitter and receptor mechanisms that function to silence skeletal muscles during REM sleep. We used behavioral, electrophysiological, receptor pharmacology and neuroanatomical approaches to determine how trigeminal motoneurons and masseter muscles are switched off during REM sleep in rats. We show that a powerful GABA and glycine drive triggers REM paralysis by switching off motoneuron activity. This drive inhibits motoneurons by targeting both metabotropic GABA(B) and ionotropic GABA(A)/glycine receptors. REM paralysis is only reversed when motoneurons are cut off from GABA(B), GABA(A) and glycine receptor-mediated inhibition. Neither metabotropic nor ionotropic receptor mechanisms alone are sufficient for generating REM paralysis. These results demonstrate that multiple receptor mechanisms trigger REM sleep paralysis. Breakdown in normal REM inhibition may underlie common sleep motor pathologies such as REM sleep behavior disorder.

GABA(A) Receptors: Post-Synaptic Co-Localization and Cross-Talk with Other Receptors

γ-Aminobutyric acid type A receptors (GABA_ARs) are the major inhibitory neurotransmitter receptors in the central nervous system, and importantly contribute to the functional regulation of the nervous system. Several studies in the last few decades have convincingly shown that GABA can be co-localized with other neurotransmitters in the same synapse, and can be co-released with these neurotransmitters either from the same vesicles or from different vesicle pools. The co-released transmitters may act on post-synaptically co-localized receptors resulting in a simultaneous activation of both receptors. Most of the studies investigating such co-activation observed a reduced efficacy of GABA for activating GABA_ARs and thus, a reduced inhibition of the post-synaptic neuron. Similarly, in several cases activation of GABA_ARs has been reported to suppress the response of the associated receptors. Such a receptor cross-talk is either mediated via a direct coupling between the two receptors or via the activation of intracellular signaling pathways and is used for fine tuning of inhibition in the nervous system. Recently, it was demonstrated that a direct interaction of different receptors might already occur in intracellular compartments and might also be used to specifically target the receptors to the cell membrane. In this article, we provide an overview on such cross-talks between GABA_ARs and several other neurotransmitter receptors and briefly discuss their possible physiological and clinical importance.

Coactivation of GABA receptors inhibits the JNK3 apoptotic pathway via disassembly of GluR6-PSD-95-MLK3 signaling module in KA-induced seizure

PURPOSE

Past work has demonstrated that kainic acid (KA)-induced seizures could cause the enhancement of excitation and lead to neuronal death in rat hippocampus. To counteract such an imbalance between excitation and inhibition, we designed experiments by activating the inhibitory gamma-aminobutyric acid (GABA) receptor to investigate whether such activation suppresses the excitatory glutamate signaling induced by KA and to elucidate the underlying molecular mechanisms.

METHODS

Muscimol coapplied with baclofen was intraperitoneally administrated to the rats 40 min before KA injection by intracerebroventricular infusion. Subsequently we used a series of methods including immunoprecipitation, immunoblotting, histologic analysis, and immunohistochemistry to analyze the interaction, expression, and phosphorylation of relevant proteins as well as the survival of the CA1/CA3 pyramidal neurons.

RESULTS

Coadministration of muscimol and baclofen exerted neuroprotection against neuron death induced by KA; inhibited the increased assembly of the GluR6-PSD-95-MLK3 module induced by KA; and suppressed the activation of MLK3, MKK7, and JNK3.

DISCUSSION

Taken together, we demonstrate that coactivation of the inhibitory GABA receptors can attenuate the excitatory JNK3 apoptotic signaling pathway via inhibiting the increased assembly of the GluR6-PSD-95-MLK3 signaling module induced by KA. This provides a new insight into the therapeutic approach to epileptic seizure.

Novel environment and GABA agonists alter event-related potentials in N-methyl-D-aspartate NR1 hypomorphic and wild-type mice

Clinical and experimental data suggest dysregulation of N-methyl-d-aspartate receptor (NMDAR)-mediated glutamatergic pathways in schizophrenia. The interaction between NMDAR-mediated abnormalities and the response to novel environment has not been studied. Mice expressing 5 to 10% of normal N-methyl-d-aspartate receptor subunit 1 (NR1) subunits [NR1(neo)(-/-)] were compared with wild-type littermates for positive deflection at 20 ms (P20) and negative deflection at 40 ms (N40) auditory event-related potentials (ERPs). Groups were tested for habituation within and across five testing sessions, with novel environment tested during a sixth session. Subsequently, we examined the effects of a GABA(A) positive allosteric modulator (chlordiazepoxide) and a GABA(B) receptor agonist (baclofen) as potential interventions to normalize aberrant responses. There was a reduction in P20, but not N40 amplitude within each habituation day. Although there was no amplitude or gating change across habituation days, there was a reduction in P20 and N40 amplitude and gating in the novel environment. There was no difference between genotypes for N40. Only NR1(neo)(-/-) mice had reduced P20 in the novel environment. Chlordiazepoxide increased N40 amplitude in wild-type mice, whereas baclofen increased P20 amplitude in NR1(neo)(-/-) mice. As noted in previous publications, the pattern of ERPs in NR1(neo)(-/-) mice does not recapitulate abnormalities in schizophrenia. In addition, reduced NR1 expression does not influence N40 habituation but does affect P20 in a novel environment. Thus, the pattern of P50 (positive deflection at 50 ms) but not N100 (negative deflection at 100 ms) in human studies may relate to subjects' reactions to unfamiliar environments. In addition, NR1 reduction decreased GABA(A) receptor-mediated effects on ERPs while causing increased GABA(B) receptor-mediated effects. Future studies will examine changes in GABA receptor subunits after reductions in NR1 expression.

Co-activation of GABA receptors inhibits the JNK3 apoptotic pathway via the disassembly of the GluR6-PSD95-MLK3 signaling module in cerebral ischemic-reperfusion

In this study, we investigated whether the increase of inhibitory gamma-amino butyric acid (GABA) signal suppresses the excitatory glutamate signal induced by cerebral ischemia and the underlying mechanisms. In global cerebral ischemia, focal cerebral ischemia and oxygen-glucose deprivation, application of muscimol and baclofen, agonists of GABA(A) receptor and GABA(B) receptor, exerted neuroprotection. The agonists inhibited the increased assembly of the GluR6-PSD-95-MLK3 module induced by cerebral ischemia and the activation of the MLK3-MKK4/7-JNK3 cascade. Our results suggest that stimulation of the inhibitory GABA receptors can attenuate the excitatory JNK3 apoptotic signaling pathway via inhibiting the increased assembly of the GluR6-PSD-95-MLK3 signaling module in cerebral ischemia.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Specific GABA(A) agonists and partial agonists

The GABA(A) receptor system is implicated in a number of neurological and psychiatric diseases, making GABA(A) receptor ligands interesting as potential therapeutic agents. Only a few different classes of structures are currently known as ligands for the GABA recognition site on the hetero-pentameric GABA(A) receptor complex, reflecting the very strict structural requirements for GABA(A) receptor recognition and activation. Within the series of compounds showing agonist activity at the GABA(A) receptor site that have been developed, most of the ligands are structurally derived from the GABA(A) agonists muscimol, THIP, or isoguvacine, which we developed in the initial stages of the project. Using recombinant GABA(A) receptors, functional selectivity was demonstrated for a number of compounds, including THIP, showing highly subunit-dependent potency and maximal response. In light of the interest in partial GABA(A) receptor agonists as potential therapeutics, structure-activity studies of a number of analogs of 4-PIOL, a low-efficacy partial GABA(A) agonist derived from THIP, have been performed. In this connection, a series of GABA(A) ligands has been developed that exhibit pharmacological profiles from moderately potent low-efficacy partial GABA(A) agonist activity to potent and selective antagonist effects. Very little information is available on direct-acting GABA(A) receptor agonists in clinical studies. However, the results of clinical studies on the effect of the partial GABA(A) agonist THIP on human sleep patterns show that the functional consequences of a direct-acting agonist are different from those seen after the administration of GABA(A) receptor modulators, such as benzodiazepines and barbiturates.

Hetero-oligomerization between GABAA and GABAB Receptors Regulates GABAB Receptor Trafficking

The neurotransmitter gamma-aminobutyric acid (GABA) mediates inhibitory signaling in the brain via stimulation of both GABA(A) receptors (GABA(A)R), which are chloride-permeant ion channels, and GABA(B) receptors (GABA(B)R), which signal through coupling to G proteins. Here we report physical interactions between these two different classes of GABA receptor. Association of the GABA(B) receptor 1 (GABA(B)R1) with the GABA(A) receptor gamma2S subunit robustly promotes cell surface expression of GABA(B)R1 in the absence of GABA(B)R2, a closely related GABA(B) receptor that is usually required for efficient trafficking of GABA(B)R1 to the cell surface. The GABA(B)R1/gamma2S complex is not detectably functional when expressed alone, as assessed in both ERK activation assays and physiological analyses in oocytes. However, the gamma2S subunit associates not only with GABA(B)R1 alone but also with the functional GABA(B)R1/GABA(B)R2 heterodimer to markedly enhance GABA(B) receptor internalization in response to agonist stimulation. These findings reveal that the GABA(B)R1/gamma2S interaction results in the regulation of multiple aspects of GABA(B) receptor trafficking, allowing for cross-talk between these two distinct classes of GABA receptor.

Calcium signaling dysfunction in schizophrenia: a unifying approach

The present paper demonstrates a remarkable pervasiveness of underlying Ca(2+) signaling motifs among the available biochemical findings in schizophrenic patients and among the major molecular hypotheses of this disease. In addition, the paper reviews the findings suggesting that Ca(2+) is capable of inducing structural and cognitive deficits seen in schizophrenia. The evidence of the ability of antipsychotic drugs to affect Ca(2+) signaling is also presented. Based on these data, it is proposed that altered Ca(2+) signaling may constitute the central unifying molecular pathology in schizophrenia. According to this hypothesis schizophrenia can result from alterations in multiple proteins and other molecules as long as these alterations lead to abnormalities in certain key aspects of intracellular Ca(2+) signaling cascades.

Inherent desensitisation-preventing properties of a novel, subtype-selective AMPA receptor agonist, (S)-CPW 399, as a possible explanation for its excitotoxic action in cultured cerebellar granule cells

The synthesis and pharmacological characterisation of (S)-CPW 399 as a novel, potent and subtype-selective agonist of the AMPA receptor was recently reported. Studies have been extended to investigate its excitotoxic action in primary cultures of mouse cerebellar granule cells. (S)-CPW 399 induced neuronal cell death in a time- and concentration-dependent manner (EC(50) approximately 70 microM) at 24-h exposure. (S)-CPW-induced neuronal death could be prevented by co-administration with either of the AMPA/kainate selective receptor antagonists 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX) and 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione (NBQX) or by the 2,3-benzodiazepine, GYKI 53655 (a selective AMPA receptor antagonist); while no protection was afforded by either the NMDA receptor antagonist D,L(+/-)-2-amino-5-phosphonopentanoate (APV) or by nifedipine (an L-type calcium channel antagonist) when used alone or in combination. Cyclothiazide, which blocks AMPA receptor desensitisation, caused minimal potentiation of (S)-CPW 399-induced neuronal death, supporting accumulating evidence that (S)-CPW 399 is a full AMPA receptor agonist that markedly prevents a receptor desensitised conformation. (S)-AMPA, (S)-willardiine (a naturally-occurring heterocyclic excitatory amino acid) and its halogenated derivative, (S)-5-fluorowillardiine, had no deleterious effect on neuronal viability when used alone but each, in the presence of cyclothiazide, induced a concentration-dependent excitotoxic cell death with a rank order of potency (fluorowillardiine>>AMPA=willardiine). (S)-CPW 399 stimulated an increase in intracellular free-calcium levels ([Ca(2+)](i)) in a concentration-dependent fashion (EC(50) approximately 5 microM) attaining a value of six-fold that of 'resting' cells at maximum stimulation; achieved at approximately 100 microM (S)-CPW 399. The (S)-CPW 399-stimulated increase in [Ca(2+)](i) was virtually abolished by GYKI 53655, NBQX, CNQX and by cobalt ions; markedly inhibited by nifedipine and marginally affected by D-APV. These results suggest that (S)-CPW 399 may be used as a pharmacological tool to aid in the investigation of the role of AMPA receptors in excitotoxicity and their molecular mechanisms of desensitisation.

Differential mechanisms of glutamate-stimulated perturbations in the kinetics of c-fos mRNA induction are associated with maturation of cerebellar granule cells in primary culture

In further exploring proposals for the measurement of early gene (c-fos mRNA) levels as a predictive index for in vitro excitotoxicity, this study, using immature (2 days in vitro) cultures of mouse cerebellar granule cells as an experimental model system, was undertaken to determine the effect of glutamate (Glu) i) in stimulating increases in intracellular free-calcium ([Ca2+]), ii) on cell viability and iii) on induction of steady-state c-fos mRNA levels. In parallel experiments the action of agents (viz. 55 mM KCl and the calcium ionophore, A23187) that mediate Ca2+ entry into cells via different routes was also evaluated. Glu was unable to induce excitotoxicity in granule cells at this stage of development in culture, but did stimulate a concentration-dependent and marked increase in [Ca2+], levels while also mediating a dramatic concentration-dependent perturbation in the kinetics of c-fos mRNA induction that appeared to arise solely from NMDA receptor-mediated Ca2+ influx. The results are presented in comparison to the actions of KCI and A23187 and considered in relation to earlier studies undertaken using mature (7 days in vitro) cultures of cerebellar granule cells.

Neurotoxicity of lindane and picrotoxin: neurochemical and electrophysiological correlates in the rat hippocampus in vivo

In the present study, we compared in vivo changes of extracellular amino acid levels and nucleotide derivatives to a single ip dose of lindane (10-60 mg/kg) and picrotoxin (5 mg/kg) in the hippocampus of halothane anaesthetized rat by microdialysis-coupled HPLC analysis. Brain activity was monitored by EEG. The effects of lindane and picrotoxin on EEG pattern of rats as well as on hippocampal amino acid and nucleotide status were studied in 0-50 min, 50-100 min and 100-150 min periods post-dosing. Significant decreases in Glu and Asp were found after picrotoxin treatment. After 50-100 min post-dosing, hippocampal hypoxanthine and inosine levels increased to both lindane (10 mg/kg) and picrotoxin whereas xanthine and uridine levels increased to picrotoxin, only. Lindane elicited a dose-dependent occurrence of negative spikes accompanied with rhythmic activity at 4-5 Hz. The picrotoxin-induced 4-5 Hz activity did not display negative sharp waves and was accompanied by 10 Hz oscillations.

Possible role for the FosB/JunD AP-1 transcription factor complex in glutamate-mediated excitotoxicity in cultured cerebellar granule cells

The potent excitatory and neurotoxic actions of glutamate are known to influence the expression of a variety of genes, including those encoding the AP-1 transcription factor, which comprises proteins belonging to the Fos and Jun families. However, the precise role of Fos- and Jun-like transcription factors in these events remains elusive. Here we demonstrate, using primary cultures of mouse brain cerebellar granule cells as an in vitro model system, a possible involvement of the FosB/JunD heterodimer in excitotoxicity. Granule cells were grown for either 2 or 7 days in vitro (DIV) before exposure to varying concentrations (1-3000 microM) of the excitotoxin glutamate. In 7-DIV cells, glutamate induced a concentration-dependent neuronal death, whereas, in 2-DIV cells, no glutamate-induced neuronal damage was seen. We were particularly interested in comparing the protein composition of the AP-1 transcription factor complex in cells exposed to excitotoxic and to nontoxic conditions. AP-1 DNA binding activity was demonstrated by gel shift analysis in nuclear extracts derived from 7-DIV cells following exposure to either a nontoxic (10 microM) or an excitotoxic (250 microM) dose of glutamate and was similarly observed in extracts of 2-DIV cells exposed to the same levels of glutamate. Gel supershift analysis using antibodies against the different Fos and Jun family members allowed differentiation between AP-1 DNA binding in nuclear extracts as a function of both 1) viability status and 2) the stage of development. Of major significance was the finding that FosB could be detected as a component of AP-1 in 7-DIV cells only under excitotoxic conditions, whereas c-Fos, Fra-2, and JunD proteins were detectable under both excitotoxic and nontoxic conditions in cells of this age. In 2-DIV cells (in which glutamate is nontoxic), AP-1 comprised combinations of only Fra-1, Fra-2, c-Jun, and JunD. Because Fos family members are unable to form homodimers, this finding raises the possibility that the FosB/JunD heterodimer may have special significance in the mechanism of excitotoxic neuronal death.

Sensory thresholds and the antinociceptive effects of GABA receptor agonists in mice lacking the beta3 subunit of the GABA(A) receptor

A line of mice was recently created in which the gabrb3 gene, which encodes the beta3 subunit of the GABA(A) receptor, was inactivated by gene-targeting. The existence of mice with a significantly reduced population of GABA(A) receptors in the CNS enabled an investigation of the role of GABA and GABA(A) receptors in nociception. The present study examined the sensory thresholds of these mice, as well as the antinociceptive effects of subcutaneously or intrathecally administered GABA(A) and GABA(B) receptor agonists. Homozygous null (beta3-/-) mice displayed enhanced responsiveness to low-intensity thermal stimuli in the tail-flick and hot-plate test compared to C57BL/6J and 129/SvJ progenitor strain mice, and their wild-type (beta3+/+) and heterozygous (beta3+/-) littermates. The beta3-/- mice also exhibited enhanced responsiveness to innocuous tactile stimuli compared to C57BL/6J, 129/SvJ and to their beta3+/+ littermates as assessed by von Frey filaments. The presence of thermal hyperalgesia and tactile allodynia in beta3-/- mice is consistent with a loss of inhibition mediated by presynaptic and postsynaptic GABA(A) receptors in the spinal cord. As expected, subcutaneous administration of the GABA(A) receptor agonist 4,5,6,7-tetrahydroisoxazolo-(5,4-c)pyridin-3-ol did not produce antinociception in beta3-/- mice, whereas it produced a dose-dependent increase in hot-plate latency in C57BL/6J, 129/SvJ, beta3+/+ and beta3+/- mice. However, the antinociceptive effect of the GABA(B) receptor agonist baclofen in the tail-flick and hot-plate tests was also reduced in beta3-/- mice compared to the progenitor strains, beta3+/+ or beta3+/- mice after either subcutaneous or intrathecal administration. This finding was unexpected and suggests that a reduction in GABA(A) receptors can affect the production of antinociception by other analgesic drugs as well.

Possible involvement of GABA(A) and GABA(B) receptors in the inhibitory action of lindane on transmitter release from cerebellar granule neurons

Cerebellar granule cells in culture express receptors for GABA belonging to the GABA(A) and GABA(B) classes. In order to characterize the ability of the insecticide lindane to interact with these receptors cells were grown in either plain culture media or media containing 150 microM THIP as this is known to influence the properties of both GABA(A) and GABA(B) receptors. It was found that lindane regardless of the culture condition inhibited evoked (40 mM K+) release of neurotransmitter ([3H]D-aspartate as label for glutamate). In naive cells both GABA(A) and GABA(B) receptor active drugs prevented the inhibitory action of lindane but in THIP treated cultures none of the GABA(A) and GABA(B) receptor active drugs had any effect on the inhibitory action of lindane. This lack of effect was not due to inability of baclofen itself to inhibit transmitter release. It is concluded that lindane dependent on the state of the GABA(A) and GABA(B) receptors is able to indirectly interfere with both GABA(A) and GABA(B) receptors. In case of the latter receptors it was shown using [3H]baclofen to label the receptors that lindane could not displace the ligand confirming that lindane is likely to exert its action at a site different from the agonist binding site.

Recent advances in GABA research

In this article I throw attention on to this GABA issue by outlining several aspects of current interest in the field of GABA research. The theme was selected in association with the Pharmacology and Therapeutical Potential of the GABA System symposium of the Second European Congress of Pharmacology held in July 1999 in Budapest, Hungary. A wide range of topics relating to the GABA system were outlined, including new members of the GABAA receptor gene family, subunit composition of native GABA(A) receptors, surface expression and clustering of GABA(A) receptor subunits, allosteric modulation of GABA(A) receptors, localization of agonist binding sites, GABA release, GABA(A)-GABA(B) receptor crosstalk, GABA(A) and GABA(B) receptor functions in different brain areas, altered transport and GABA(A) receptor pattern in different models of epilepsy.

GABAB receptors in the dorsomedial hypothalamus and heart rate in anesthetized rats

Previous studies have shown that: (1) activation of neurons in the dorsomedial hypothalamus (DMH) of the rat by blockade of local GABAA receptors with bicuculline methiodide (BMI) elicits cardiovascular changes resembling those seen in experimental stress, including marked sympathetically-mediated tachycardia, and (2) inhibition of neurons in the same region by local microinjection of the GABAA receptor agonist muscimol can virtually abolish stress-induced tachycardia. This study examined the possibility that GABAB receptors exist in the neural circuitry of the DMH, and that stimulation of these receptors might suppress the cardiovascular response to local disinhibition with BMI. Microinjection of BMI 10 pmol into the DMH in urethane-anesthetized rats resulted in marked tachycardia with little or no effect on arterial pressure. Simultaneous injection of the GABAB receptor agonist baclofen at doses of 2.5, 5.0 and 10 pmol produced dose-related suppression of BMI induced tachycardia. Coinjection of the GABAB receptor antagonist 2-hydroxysaclofen 100 or 200 pmol had no significant effect on the heart rate response to BMI, but reversed the suppression elicited in the presence of baclofen. These findings indicate that (1) functional GABAB receptors exist in the DMH, and (2) stimulation of these receptors inhibits the tachycardia resulting from blockade of local GABAA receptors.

Differential effects of nipecotic acid and 4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol on extracellular gamma-aminobutyrate levels in rat thalamus

Using the microdialysis technique and HPLC (high-performance liquid chromatography) determination of amino acids, the extracellular concentrations of gamma-aminobutyrate (GABA), glutamate, aspartate and a number of other amino acids were determined in rat thalamus during infusion through the microdialysis tubing of the GABA transport inhibitors 4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol (THPO) and nipecotic acid. Administration of 5.0 mM THPO led to a 200% increase in the extracellular GABA concentration. Simultaneous infusion of THPO and GABA (50 microM) increased the extracellular GABA concentration to 1200% of the basal level whereas GABA alone was found to increase the GABA level to 500%. If nipecotic acid (0.5 mM) was administered together with GABA (50 microM) the extracellular concentration of GABA was not increased further. While administration of GABA alone or GABA together with nipecotic acid had no effect on the extracellular levels of glutamate and aspartate it was found that GABA plus THPO increased the extracellular concentration of these amino acids. GABA administered alone or together with nipecotic acid or THPO led to relatively small but significant increases in the extracellular concentrations of the amino acids glycine, glutamine, serine and threonine. The results demonstrate that THPO, which preferentially inhibits glial GABA uptake and which is not a substrate for the GABA carriers, was more efficient increasing the extracellular concentration of GABA than nipocotic acid which is a substrate and an inhibitor of both neuronal and glial GABA uptake. This indicates that GABA uptake inhibitors that are not substrates for the carrier and which preferentially inhibit glial GABA uptake may constitute a group of drugs by which the efficacy of GABAergic neurotransmission may be enhanced.

NMDA receptor-mediated cGMP synthesis in primary cultures of mouse cerebellar granule cells appears to involve neuron-astrocyte communication with NO operating as the intercellular messenger

The possibility that neuron-astrocyte communication may be responsible for glutamate (Glu)-stimulated cGMP formation even in relatively homogeneous primary cultures of mouse cerebellar granule cells (7 days in vitro) was investigated. Pharmacological analysis using selective excitatory amino acid (EAA) receptor antagonists showed that cGMP production, stimulated in these cultures by Glu and a variety of endogenous EAAs structurally-related to Glu (namely, L-aspartate, L-cysteine sulphinate, L-homocysteate, S-sulpho-L-cysteine), was mediated wholly by N-methyl-D-aspartate (NMDA) receptor activation. Moreover, EAA-induced responses were dependent on the presence of extracellular calcium but unaffected by addition of the L-type voltage-sensitive calcium channel blockers nifedipine (10 microM) or verapamil (5 microM). The mode of calcium entry was also shown to be important since the calcium ionophore, A23187 (10 microM), was unable to stimulate cGMP levels above basal. cGMP formation was blocked by the competitive nitric oxide synthase inhibitor, L-NG-nitroarginine (100 microM), consistent with a role of nitric oxide (NO) in this signalling pathway. In the presence of added haemoglobin (1 microM), acting as a membrane-impermeable NO scavenger, Glu-stimulated cGMP formation was abolished implying that NO must act as an intercellular messenger. When the neuronal population was destroyed following a 24 hr exposure to the excitotoxin, S-sulpho-L-cysteine (200 microM), Glu-stimulated cGMP formation was abolished; whereas responses to the NO donor, sodium nitroprusside (SNP), although markedly reduced were still double that stimulated by Glu in the absence of the excitotoxin, suggesting the presence of non-neuronal cells that can generate cGMP if supplied directly with NO. Consistent with this suggestion, low levels of the glial specific enzyme, glutamine synthetase, were detected in granule cell cultures. Furthermore, omission or delayed addition of the antimitotic agent, cytosine arabinoside (20 microM), to the growth medium caused a significant increase in the level of Glu-stimulated cGMP formation.

Effect of K+- and kainate-mediated depolarization on survival and functional maturation of GABAergic and glutamatergic neurons in cultures of dissociated mouse cerebellum

The effect of the depolarizing agents, an elevated potassium concentration (25 mM) or kainic acid (50 microM) on neuronal survival and differentiation was investigated in cultures of dissociated neurons from cerebella of 7-day-old mice. When maintained in the presence of an antimitotic agent such cultures consist primarily of glutamatergic and GABAergic neurons. Cell survival was monitored by measurement of DNA, and differentiation by determining uptake and depolarization coupled release of glutamate (D-aspartate as label) and GABA. The depolarizing agents were added separately or together either from the start of the culture period (7-8 days) or at day 5 in culture. The main findings are that K+ depolarization is important for differentiation of glutamatergic neurons but not for GABAergic neurons. This depolarizing signal is important during the early phase of development in culture. For glutamatergic neurons, kainate may replace K+ as a depolarizing signal whereas in case of the GABAergic neurons, kainate was toxic particularly during the late phase of development. It was further observed that the glutamatergic neurons when maintained in a medium with 5 mM K+ during the first 5 days in culture became sensitive to kainate toxicity when this amino acid was added at day 5. This was not the case when the medium contained 25 mM K+ from the start of the culture period.

Differential distribution of GABAA receptor subunits in soma and processes of cerebellar granule cells: effects of maturation and a GABA agonist

Quantitative analysis of the density of alpha 1 and beta 2/3 GABAA receptor subunits was performed at the electron microscope level after indirect pre-embedding immunogold labeling with subunit-specific antibodies of rat cerebellar granule cell cultures grown for 4 or 8 days and in the presence or absence of the GABAA receptor agonist 4,5,6,7-tetrahydroisoxazolo[5,4c]pyridin-3-ol (THIP). THIP (150 microM) induced a 2-fold increase in the number of alpha 1 and beta 2/3 subunits in both cell bodies and processes in 4-day-old cultures. Extending the culture period to 8 days led to a polarization of the receptor expression, since the increase in the number of subunits selectively was observed in the processes. Moreover, a general subcellular differentiation of the receptor population was observed in all culture conditions, since the ratio between the two subunits (beta 2/3; alpha 1) was four times higher in cell bodies compared to processes. A detailed analysis of the less mature (4-day-old) cultures revealed the existence of two populations of neurons exhibiting differences in the average number of receptors. During maturation neurons with few receptors developed into cells with a higher density of receptors resulting in a single population of the latter neurons, a process enhanced by exposure to THIP. This may indicate that receptor development is a discontinuous process with individual neurons following different temporal patterns. In double-labeling experiments, a spatially close association of the alpha 1 and beta 2/3 subunits could be seen, but the subunits were more frequently found separated from each other. In spite of the fact that exposure of the neurons to THIP increased the total number of receptor subunits, its presence apparently prevented formation of receptors with this subunit composition. Interestingly, receptor subunit clusters, consisting of alpha 1 alone, were more frequently observed than composite (alpha 1; beta 2/3) clusters. This substantiates the view that receptors not having alpha 1 and beta 2/3 subunits in the same complex may exist.

Modulatory actions of gamma aminobutyric acid (GABA) on GABA type A receptor subunit expression and function

Gamma aminobutyric acid (GABA) is present in the central nervous system (CNS) during very early embryogenesis. It is therefore likely to play a role not only as a neurotransmitter but also as a signal molecule for neuronal differentiation, growth, and development. It has been firmly established that formation of synapses is strengthened by GABA, and the expression of certain subunits of the GABA type A (GABAA) receptor complex is clearly promoted by GABA. This latter effect of GABA may have profound implications for the functional activity of GABAergic synapses since the pharmacological properties of GABAA receptors are governed by the subunit composition of the receptor complex. Dynamic changes in GABAA receptor expression and diversity during development and differentiation may therefore play important roles for the inhibitory potential of the CNS during mature stages.