Benzodiazepines specifically modulate GABA-mediated postsynaptic inhibition in cultured mammalian neurones

Behavioural evidence for GABAergic activity of the benzodiazepine flurazepam

Following reports that unilateral intranigral injections of putative GABAergic drugs induce contralateral rotational behaviour in rats, the effects of similar injections of the benzodiazepine flurazepam have been studied. Flurazepam mimicked the effects of the GABA agonist muscimol and the GABA analogue baclofen by inducing a dose-related contralateral rotation. This response was anatomically associated with the GABA-rich zona reticulata of the substantia nigra and was attenuated by the GABA antagonist picrotoxin but not by the dopamine antagonist haloperidol or by destruction of the ipsilateral nigrostriatal dopamine pathway with 6-hydroxydopamine. These results suggest that in this behavioural model flurazepam may show GABAergic activity by indirectly enhancing GABA transmission at synapses with receptors located on nigral non-dopaminergic neurons controlling postural asymmetry.

Central actions of benzodiazepines: general introduction

After a brief review of the characteristic somatic and psychotropic effects of benzodiazepines evidence is presented which supports a specific facilitatory action of these drugs on GABA ergic synapses within the mammalian central nervous system. Benzodiazepines enhance presynaptic inhibition in the spinal cord and dorsal column nuclei as well as postsynaptic inhibition in dorsal column nuclei, hippocampus, hypothalamus, cerebral cortex, cerebellar cortex, which are all examples of recurrent and collateral inhibition mediated by GABA ergic intrinsic neurones. In addition, the compounds also enhance the inhibitory effect of GABA ergic long projection neurones in the substantia nigra and the lateral vestibular nucleus of Deiters. Several problems remain to be solved, such as the exact site at which benzodiazepines initiate their action (pre-synaptically at GABA ergic nerve endings or postsynaptically at the target cells) and the possible existence of endogenous ligands for the benzodiazepine receptor. Some suspected implications which studies on benzodiazepine binding sites could have for a deeper understanding of the mode of action of these drugs are discussed.

Molecular mechanisms mediating the action of diazepam on GABA receptors

Two types of crude synaptic membranes have been prepared which differ in their kinetic properties to bind 3H-GABA in a Na+-free medium. One type (B) has one type of receptor (affinity or KD about 0.2 micrometer), the other type (A) has two populations of receptors which have a high (0.16 micrometer) and a low ((.02 micrometer) KD for GABA binding. This difference is due to the presence of a selective endogenous inhibitor of the high affinity receptor for GABA, a thermostable protein of about 15,000 molecular weight. Inhibitor action is counteracted by diazepam (7 X 10(-7) M), competitively. Clonazepam is more active but chlordiazepoxide is less active than diazepam. Two enantiomers of a benzodiazepine were studied. One of them is endowed with anxiolytic activity and nullifies the action of the endogenous inhibitor on high affinity GABA binding, the other is devoid of anxiolytic activity and is inactive against the inhibitor. The endogenous protein inhibitor also competitively blocks the high affinity binding of 3H-diazepam to Type A membranes. It is concluded that the endogenous inhibitor and diazepam act on the same receptor; and suggested that the endogenous inhibitor may be the natural ligand for the brain receptors that bind benzodiazepines with high affinity.