Benzodiazepines produce most, if not all, of their numerous effects on the central nervous system (CNS) primarily by increasing the function of those chemical synapses that use gamma-amino butyric acid (GABA) as transmitter. This specific enhancing effect on GABAergic synaptic inhibition is initiated by the interaction of benzodiazepines with membrane proteins of certain central neurones, to which drugs of this chemical class bind with high affinity and specificity. The molecular processes triggered by the interaction of these drugs with central benzodiazepine receptors, and which result in facilitation of GABAergic transmission, are still incompletely understood. Theoretically, benzodiazepines could mimic the effect of hypothetical endogenous ligands for the benzodiazepine receptors, although there is no convincing evidence for their existence; in vitro studies indicate that benzodiazepines might compete with a modulatory peptide which is present in the supramolecular assembly formed by GABA receptor, chloride ionophore and benzodiazepine receptor and which reduces the affinity of the GABA receptor for its physiological ligand. The mechanisms of action of benzodiazepines at the molecular level are likely to be better understood following our recent discovery of benzodiazepine derivatives, whose unique pharmacological activity is to prevent or abolish in a highly selective manner at the receptor level all the characteristic centrally mediated effects of active benzodiazepines. Here, we describe the main properties of a representative of this novel class of specific benzodiazepine antagonists.

In neurological and behavioral studies in mice, rats, dogs and squirrel monkeys, the imidazobenzodiazepinone Ro 15-1788 acted as a potent benzodiazepine antagonist. The antagonistic activity was both preventive and curative and seen at doses at which no intrinsic effects were detected. It was highly selective in that it acted against CNS effects induced by benzodiazepines but not against those produced by other depressants, such as phenobarbitone, meprobamate, ethanol, and valproate. The onset of action was rapid even after oral administration. Depending on the animal species studied, the antagonistic effects lasted from a few hours to 1 day. The acute and subacute toxicity of Ro 15-1788 was found to be very low. Benzodiazepine-like effects were not seen.

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Basic aspects and recent advances in the understanding of the pharmacological mechanism of action of the clinically most used antiparkinson drugs are reviewed. Recent human and animal biochemical investigations clearly confirm and extend previous findings indicating that benserazide is much more potent than carbidopa as peripheral decarboxylase inhibitor. L-DOPA in combination with benserazide or carbidopa constitutes the best available therapy for Parkinson's disease (PD). To reduce peaks and rapid fluctuations of L-DOPA plasma levels (possibly responsible for peak-dose dyskinesias and end-of-dose deterioration) a slow-release formulation of L-DOPA in combination with benserazide or with benserazide plus catechol-O-methyltransferase inhibitors should be developed. In parkinsonian patients under long-term L-DOPA therapy monoamine oxidase inhibitors type B (MAO-B) e.g. (-)deprenyl and direct dopamine receptor agonists (bromocriptine, lisuride, pergolide etc.), due to their L-DOPA-sparing effects, alleviate in some cases L-DOPA-induced side-effects e.g. dyskinesias and on-off phenomena. However, since (-)deprenyl, due to its metabolism to (-)methamphetamine and (-)amphetamine, seem to have indirect sympathomimetic activity, new selective MAO-B inhibitors devoid of indirect sympathomimetic effects should be tested clinically to assess the functional role of pure MAO-B inhibition in the therapy of PD. The auxiliary therapy with direct dopamine receptor agonists of the D-2 subtype represents another valid approach which should be further investigated in order to find novel dopamine agonists, less expensive than bromocriptine, and strictly selective for D-2 receptor sites.

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