MELATONIN REDUCES DOPAMINE CONTENT IN THE NEUROINTERMEDIATE LOBE OF MALE SYRIAN-HAMSTERS

The effect of daily late afternoon administration of melatonin on the in situ activity of tyrosine hydroxylase (TH) was studied in the poste rior pituitary (neurointermediate lobe) of the male Syrian hamster. Af ter 3 weeks of melatonin administration, TH activity was significantly reduced in the posterior pituitary. This was associated with a signif icant decrease in norepinephrine (NE) content. After 5 weeks, TH activ ity and NE content were no longer significantly different from control s. Dopamine (DA) content of the posterior pituitary was decreased prog ressively by melatonin administration, with a reduction of greater tha n 50% after 5 weeks of treatment. These data provide evidence that mel atonin has a potent inhibitory effect on the regulation of the dopamin ergic system of the neurointermediate lobe-an effect that appears unre lated to changes in axonal TH.