Evidence that clomethiazole interacts with the macromolecular GABA A-receptor complex in the central nervous system and in the anterior pituitary gland

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Abstract
Clomethiazole (CLOM) is known to be an anti-convulsant drug and has been also reported to decrease serum prolactin (PRL) in humans. Both effects may be mediated by an enhancement of gabaergic transmission. In order to determine if (CLOM) interacts with GABA metabolism and/or at the GABA receptor level, we studied its effect on PRL release and on the binding of various compounds that interact with the GABAA-benzodiazepine-receptor complex. Intraperitoneal (IP) administration of CLOM to rats significantly decreased PRL levels, and this effect was antagonized by IP administration of bicuculline, an antagonist of the GABAA receptor. In vitro, the inhibitory effect of muscimol on PRL release from rat hemiadenohypophysis was potentiated in a dose-dependent manner by preincubation with CLOM. This effect was antagonized by picrotoxin (10−6 M). On the other hand, CLOM had no effect on GABA metabolism and did not compete with GABAA, GABAB or benzodiazepine binding sites in cortical membranes. GLOM competed, however, with the picrotoxin binding site labelled with [35S]-butylbicyclophosphoro-thionate (TBPS), at an IC50 value of 1.2 × 10−4 M, which is in the same range as some barbiturates. These results concerning PRL release and binding experiments with cortical membranes suggest that GLOM interacts with the picrotoxin/barbiturate site of the GABAA-receptor-chloride channel complex.