Melatonin increases the in situ activity of tyrosine hydroxylase in the mediobasal hypothalamus of male Syrian hamsters

The effects of daily late afternoon injections of melatonin on the in situ activity of tyrosine hydroxylase (TH) were examined in the median eminence/arcuate region of the mediobasal hypothalamus (MBH) and the neurointermediate lobe (NIL) of the male Syrian hamster. TH activity was determined in tissue extracts by measuring the accumulation of L-DOPA following administration of the dopa decarboxylase inhibitor, NSD-1015. After 9 weeks of melatonin treatment, highly significant increases in the activity of MBH TH were demonstrated over a 24 hr period, compared to saline-treated controls. Melatonin-induced elevations in TH occurred concomitantly with decreases in tuberoinfundibular dopamine (TIDA) and tuberohypophyseal dopamine (THDA) concentrations. Similar findings were observed in castrated hamsters, indicating that the melatonin-induced increase in TH was not secondary to melatonin-induced changes in circulating levels of gonadal hormones. These data led to the interpretation that melatonin treatment elevated TIDA synthesis either through a direct action on the arcuate nuclei or on neurons impinging on these nuclei.