Melatonin: effects on dopaminergic and serotonergic neurons of the caudate nucleus of the striatum of male Syrian hamsters.

The effects of daily late afternoon administration of the indoleamine, melatonin, on the in situ activity of tyrosine hydroxylase (TH) and tryptophan hydroxylase (TPH) were examined in the caudate nuclei of the striatum of male Syrian hamsters. TH and TPH activities were determined in tissue extracts by measuring the accumulation of L-Dopa and 5-HTP respectively, following the administration of the aromatic L-amino acid decarboxylase inhibitor, NSD-1015. Animals were sacrificed at 4 time points over the 24 light/dark cycle after 9.5 weeks of melatonin treatment. TH activity was significantly increased by melatonin during the early part of the dark phase of the light/dark cycle. While no significant effects of melatonin on TPH was observed, melatonin significantly increased 5-HT concentrations, suggesting a melatonin-induced inhibition of 5-HT release. The data suggest that the striatum may be a region in which dopaminergic neurons are subject to significant regulation by melatonin, either directly or through serotonergic neurons which synapse on dopaminergic neurons in the striatum.