GABA receptor agonists: Pharmacological spectrum and therapeutic actions

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