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.