GABA RECEPTORS IN THE REGION OF THE DORSOMEDIAL HYPOTHALAMUS OF RATS ARE IMPLICATED IN THE CONTROL OF MELATONIN AND CORTICOSTERONE RELEASE

Recently, anatomical evidence was presented that the mammalian circadi an clock located in the suprachiasmatic nuclei (SCN) may utilize GABA to transmit diurnal information to the dorsomedial hypothalamus (DMH). The present study provides further physiological evidence for the inv olvement of this GABAergic projection ill the regulation of diurnal rh ythms. Infusion of the GABA agonist muscimol in the region of the DMH completely blocked the daily increase of plasma melatonin during darkn ess and reduced sympathetic output in the pineal gland resulting in lo wer pineal melatonin production, as measured with transpineal microdia lysis. Further experiments in SCN-lesioned animals indicated that the origin of this inhibitory input to the DMH is indeed the SCN. The resu lts of this study imply that the SCN can influence the sympathetic out flow of the hypothalamus through its GABA-containing projection. Furth ermore, the present results probably explain the previously reported s trong inhibitory effect of benzodiazepines on plasma melatonin in both animals and humans.