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 posterior pituitary (neurointermediate lobe) of the male Syrian hamster. After 3 weeks of melatonin administration, TH activity was significantly reduced in the posterior pituitary. This was associated with a significant decrease in norepinephrine (NE) content. After 5 weeks, TH activity and NE content were no longer significantly different from controls. Dopamine (DA) content of the posterior pituitary was decreased progressively by melatonin administration, with a reduction of greater than 50% after 5 weeks of treatment. These data provide evidence that melatonin has a potent inhibitory effect on the regulation of the dopaminergic system of the neurointermediate lobe--an effect that appears unrelated to changes in axonal TH.