GABAA antagonists

Advantages of an antagonist: bicuculline and other GABA antagonists

The convulsant alkaloid bicuculline continues to be investigated more than 40 years after the first publication of its action as an antagonist of receptors for the inhibitory neurotransmitter GABA. This historical perspective highlights key aspects of the discovery of bicuculline as a GABA antagonist and the sustained interest in this and other GABA antagonists. The exciting advances in the molecular biology, pharmacology and physiology of GABA receptors provide a continuing stimulus for the discovery of new antagonists with increasing selectivity for the myriad of GABA receptor subclasses. Interesting GABA antagonists not structurally related to bicuculline include gabazine, salicylidene salicylhydrazide, RU5135 and 4-(3-biphenyl-5-(4-piperidyl)-3-isoxazole. Bicuculline became the benchmark antagonist for what became known as GABAA receptors, but not all ionotropic GABA receptors are susceptible to bicuculline. In addition, not all GABAA receptor antagonists are convulsants. Thus there are still surprises in store as the study of GABA receptors evolves.

Participation of GABAA chloride channels in the anxiolytic-like effects of a fatty acid mixture

Human amniotic fluid and a mixture of eight fatty acids (FAT-M) identified in this maternal fluid (C12:0, lauric acid, 0.9 μg%; C14:0, myristic acid, 6.9 μg%; C16:0, palmitic acid, 35.3 μg%; C16:1, palmitoleic acid, 16.4 μg%; C18:0, stearic acid, 8.5 μg%; C18:1cis, oleic acid, 18.4 μg%; C18:1trans, elaidic acid, 3.5 μg%; C18:2, linoleic acid, 10.1 μg%) produce anxiolytic-like effects that are comparable to diazepam in Wistar rats, suggesting the involvement of γ-aminobutyric acid-A (GABA_A) receptors, a possibility not yet explored. Wistar rats were subjected to the defensive burying test, elevated plus maze, and open field test. In different groups, three GABA_A receptor antagonists were administered 30 min before FAT-M administration, including the competitive GABA binding antagonist bicuculline (1 mg/kg), GABA_A benzodiazepine antagonist flumazenil (5 mg/kg), and noncompetitive GABA_A chloride channel antagonist picrotoxin (1 mg/kg). The FAT-M exerted anxiolytic-like effects in the defensive burying test and elevated plus maze, without affecting locomotor activity in the open field test. The GABA_A antagonists alone did not produce significant changes in the behavioral tests. Picrotoxin but not bicuculline or flumazenil blocked the anxiolytic-like effect of the FAT-M. Based on the specific blocking action of picrotoxin on the effects of the FAT-M, we conclude that the FAT-M exerted its anxiolytic-like effects through GABA_A receptor chloride channels.

Protection from neuronal damage evoked by a motivational excitation is a driving force of intentional actions

Motivation may be understood as an organism's subjective attitude to its current physiological state, which somehow modulates generation of actions until the organism attains an optimal state. How does this subjective attitude arise and how does it modulate generation of actions? Diverse lines of evidence suggest that elemental motivational states (hunger, thirst, fear, drug-dependence, etc.) arise as the result of metabolic disturbances and are related to transient injury, while rewards (food, water, avoidance, drugs, etc.) are associated with the recovery of specific neurons. Just as motivation and the very life of an organism depend on homeostasis, i.e., maintenance of optimum performance, so a neuron's behavior depends on neuronal (i.e., ion) homeostasis. During motivational excitation, the conventional properties of a neuron, such as maintenance of membrane potential and spike generation, are disturbed. Instrumental actions may originate as a consequence of the compensational recovery of neuronal excitability after the excitotoxic damage induced by a motivation. When the extent of neuronal actions is proportional to a metabolic disturbance, the neuron theoretically may choose a beneficial behavior even, if at each instant, it acts by chance. Homeostasis supposedly may be directed to anticipating compensation of the factors that lead to a disturbance of the homeostasis and, as a result, participates in the plasticity of motivational behavior. Following this line of thought, I suggest that voluntary actions arise from the interaction between endogenous compensational mechanisms and excitotoxic damage of specific neurons, and thus anticipate the exogenous compensation evoked by a reward.

Multi-neuronal recordings reveal a differential effect of thapsigargin on bicuculline- or gabazine-induced epileptiform excitability in rat hippocampal neuronal networks

The present study was performed to investigate the effects of depleting intracellular Ca(2+) stores on bicuculline- or gabazine-induced epileptiform excitability. Studies were performed on monolayer rat hippocampal neuronal networks utilising a system that allowed simultaneous multiple extracellular single-unit recordings of neuronal activity. Hippocampal neuronal networks were prepared from enzymatically dissociated hippocampi from 18-day-old fetal Wistar rats. The cells were cultured in Neurobasal medium with B27 serum-free supplements directly onto the surface of planar multiple microelectrode arrays with a central recording array of 64 (4 x 16) indium-tin thin-film recording electrodes. All cells recorded at 21 days-in-vitro exhibited spontaneous discharge activity with firing rates between 0.3-30.7 Hz. gamma-aminobutyric acid (GABA) produced a concentration-dependent decrease in firing (EC(50)=9.1 microM) which could be blocked by pre-application of bicuculline methobromide (10 microM). Addition of the GABA(A)-receptor antagonists gabazine (10 microM) or bicuculline (10 microM) resulted in the rapid generation of synchronised bursting within all the cells recorded. Bicuculline exhibited heterogeneity of action on firing rate, whereas gabazine always increased firing. Pre-incubation with thapsigargin, which depletes intracellular calcium stores, resulted in a decrease in the amount of neuronal excitation produced by bicuculline, but not by gabazine, suggesting that bicuculline-induced neuronal excitation requires release of Ca(2+) from intracellular stores.

Differential paired-pulse effects of gabazine and bicuculline in rat hippocampal CA3 area

Field potentials were evoked in hippocampal area CA3 of anaesthetised rats by commissural stimulation, in order to study the effect of the prototypic gamma-aminobutyric acid (GABA)A antagonists gabazine (SR-95531; GBZ) and bicuculline methiodide (BMI) on paired-pulse interaction. Prominent paired-pulse inhibition of the orthodromic population spike (PS2) was observed when the interpulse interval (IPI) was < or = 40 ms, while facilitation occurred at IPIs >100 ms. Paired-pulse facilitation was lost at 500 ms. The antidromic population spike (PS1) presented paired-pulse facilitation at low-IPI, which decayed exponentially at increasing IPI. When the recording micropipettes contained millimolar concentrations of either GBZ, or BMI, single stimuli evoked repetitive (epileptiform) orthodromic PS2, of higher amplitude, while the antidromic PS1 was only weakly influenced. BMI reduced, but GBZ enhanced the low-IPI paired-pulse inhibition of the orthodromic PS2. Furthermore, BMI blunted paired-pulse facilitation of the antidromic PS1 at low-IPI, while GBZ caused strong paired-pulse inhibition of PS1 at IPI < or = 60 ms. The differential effects of GBZ and BMI on paired-pulse interaction might reflect different mechanisms of action of these compounds.

Morphofunctional evidence for mature synaptic contacts on the Mauthner cell of 52-hour-old zebrafish larvae

In a previous study, miniature inhibitory synaptic events recorded in the Mauthner cell of the 52-hour-old zebrafish larvae (Brachydanio rerio) were found to be mainly glycinergic. Their amplitude distribution was not Gaussian and it was proposed that their large amplitude variation might reflect the activation of immature synapses. However, ultrastructural studies of the synaptic contacts over the M-cell soma of 52 h larvae described here, revealed that numerous synaptic contacts on this neuron are already mature at this developmental stage and that most of them already contain a single active zone. As in the adult goldfish, immunohistochemistry indicates the presence of both glycine- and GABA-immunoreactive boutons which establish synaptic contacts. We also found that, in addition to the predominant glycinergic postsynaptic inhibitory currents, some postsynaptic currents are also GABAergic since they are specifically inhibited by bicuculline (20 microM). GABAergic miniature events (time to peak close to 0.8 ms and decay time-constant close to 45 ms) were only detected in the presence of 11.5 mM [KCl]o. Their amplitude distributions were well fitted by one, or at most two, Gaussian curves. Outside-out recordings showed one class of GABA receptors with a main conductance state of 23 pS. This indicates that the smallest GABAergic miniature inhibitory synaptic events correspond to the opening of 14-20 chloride channels Pre- and postsynaptic factors which contribute to the predominance of glycinergic synaptic currents over GABAergic ones in untreated preparations and to the striking differences between their frequencies and their respective amplitude distribution histograms are discussed with reference to the morphological characteristics of the mature synaptic endings impinging on this still developing neuron.

[D-Pen2-D-Pen5]enkephalin, a delta opioid agonist, given intracerebroventricularly in the mouse produces antinociception through medication of spinal GABA receptors

Intracerebroventricular (ICV) administration of [D-Pen2-D-Pen5]enkephalin (DPDPE), a delta opioid receptor agonist, activates a descending antinociceptive pathway that inhibits the tail-flick response in mice. Involvement of spinal GABA receptors in this response was studied by giving GABA antagonist intrathecally. First, antinociception produced by intrathecally administered isoguvacine, a GABAA agonist, was inhibited by intrathecal bicuculline (GABA receptor antagonist) or picrotoxin (chloride channel antagonist). Then, antinociception induced by ICV DPDPE was antagonized by intrathecal picrotoxin and bicuculline in a dose-and time-dependent manner. Second, intrathecal administration of 2-hydroxysaclofen, a GABAB antagonist (which inhibited antinociception induced by a GABAB agonist, baclofen, given IT), produced a shift of the dose-response curve for ICV DPDPE to the right. GABAA agonist, baclofen, given IT), produced a shift of the dose-response curve for ICV DPDPE to the right. GABAA and B antagonists given together intrathecally produced a greater than additive antagonistic effect against ICV DPDPE-induced antinociception. Thus, the delta agonist action of DPDPE in the brain leads to activation of descending spinal pathways which involve mediation by spinal GABAA and GABAB receptors in the antinociceptive response.

Glycinergic inhibitory synaptic currents and related receptor channels in the zebrafish brain

To extend our study of the inhibitory synaptic network we have developed an isolated whole-brain preparation of the 52-h-old zebrafish (Brachydanio rerio) in which the structural and functional integrity of the brain is preserved. We report the characterization of quantal inhibitory events and the correlation of their properties with those of the underlying activated channels. During whole-cell recordings of the Mauthner cells, applications of 10(-6) M tetrodotoxin greatly reduced the frequency and amplitude of the spontaneously occurring synaptic events, which were dominated by Cl--dependent inhibitory postsynaptic currents (IPSCs). Lowering Ca2+ and adding Mg2+ to tetrodotoxin-containing solutions resulted in a further decrease in amplitude of the recorded synaptic currents, the remaining ones being considered as miniature IPSCs (mIPSCs). Applications of 0.5-1 microM strychnine in the presence of tetrodotoxin eliminated > 90% of the inhibitory currents in the preparation. The amplitude histograms of these mIPSCs exhibited two initial equally spaced peaks, followed by a skewed distribution for higher values. The first two components were well fitted by the sum of two Gaussian curves, giving a mean quantal amplitude of 35.7 pA (at a holding potential of -50 mV) and a coefficient of variation of 0.25 for the first peak. Outside-out recordings showed at least two classes of glycine receptor channels, one having multiple conductance levels with a main state of 81-86 pS and another displaying only one opening level of 41-43 pS. These two mean conductance states had similar mean open times, of 0.6-1 and 4.5-6 ms respectively. In addition, three mean closed times were observed for the 41-43 pS level. The shortest group (0.6-1 ms) was considered as representing gaps within bursts. Burst analysis revealed three mean burst durations, of 0.6, 4 and 35 ms. Comparisons of the amplitude of the first class of mIPSCs and of the open channel conductances indicated that one quantum opens 14-22 channels. Moreover, the correspondence between the mean decay time of mIPSCs and the mean open time or medium burst duration (4-5 ms) suggests that glycine-activated channels open only once in response to a single exocytosis. The pre- and postsynaptic origins of mIPSCs amplitude fluctuations are discussed in the context of multivesicular release versus the hypothesis of postsynaptic receptor saturation.

Sleep disorders and anxiety: biochemical antecedents and pharmacological consequences

Evidence is presented that the most widely used and effective drugs used in the treatment of anxiety and insomnia act by indirectly activating GABA-A receptors in limbic regions of the brain. Since the discovery of the benzodiazepines, different classes of benzodiazepine receptor ligands (such as the cyclopyrroliones and imidazopyridines) have been developed which alleviate anxiety and insomnia by activating different sites on the benzodiazepine-GABA receptor complex to those activated by the 'classical' benzodiazepines as exemplified by temazepam and diazepam. There is evidence that natural ligands also exist in the mammalian brain which can modulate the benzodiazepine-GABA receptor complex. This raises the possibility that insomnia and anxiety states may arise as a consequence of a deficit in the availability of endogenous ligands that act as agonists at these sites.

Commentary on the mode of action of benzodiazepines

Evidence is presented showing that the benzodiazepines produce their variety of pharmacological effects by activating GABA A receptors in the mammalian brain. Different classes of benzodiazepine receptor ligands have been developed which can cause or alleviate anxiety according to the nature of their interaction with the GABA A receptor. There is now evidence that natural ligands also exist in the brain which can modulate GABA A receptor function. The changes in the responsiveness of the GABA A receptor to chronic benzodiazepine treatment is discussed with reference to the phenomenon of tolerance dependence and withdrawal.