In a previous work, we have shown that GABA inhibits the release of alpha-melanocyte-stimulating hormone (alpha-melanotropin) from hypothalamic neurons through activation of GABAA receptors [Delbende et al. (1989) Brain Res. 497, 86-93]. Since GABA-gated channel activity can be allosterically modulated by a variety of compounds including benzodiazepines, we have investigated the effect of benzodiazepines in the control of alpha-melanotropin release by the rat basal hypothalamus. This study was conducted in vitro using perifused rat hypothalamic slices and the amount of alpha-melanotropin release was monitored with a sensitive and highly specific radioimmunoassay. Infusion of clonazepam (50 microM), a selective agonist for central-type benzodiazepine binding sites, induced an inhibition of KCl (50 mM)-evoked alpha-melanotropin release. The inhibitory effect of clonazepam was rapid and reversible. Administration of Ro 15-1788 (100 microM), a specific antagonist for central-type benzodiazepine receptors or SR 95531, a GABAA receptor antagonist, completely reversed the inhibitory effect of clonazepam. In addition, Ro 15-1788 and SR 95531 both enhanced the amplitude of the response observed during prolonged KCl infusion on alpha-melanotropin neurons, suggesting the existence of a tonic inhibitory effect of endogenous GABA and/or benzodiazepines in the release of alpha-melanotropin by hypothalamic neurons. To investigate further the effect of benzodiazepines in the regulation of alpha-melanotropin neurons, rats were treated in vivo with clonazepam (5 mg/kg) or the non-selective benzodiazepine receptor agonist diazepam (3 mg/kg). Both compounds caused a significant increase in the content of alpha-melanotropin and beta-endorphin in the rat hypothalamus within 3 h.(ABSTRACT TRUNCATED AT 250 WORDS)