Impact of circulating testosterone on iodomelatonin binding sites in the male rat brain

The effects of castration and subsequent testosterone and estradiol treatment and of a single injection of ethylene-1,2-dimethanesulphonate (EDS) on the distribution of [2-¹²⁵I]iodomelatonin ([¹²⁵I]melatonin) binding sites in the male rat brain were investigated. Castration produced a marked testosterone-reversible decrease in [¹²⁵I]melatonin binding in the male rat brain, particularly in the hypothalamus and hippocampus. In contrast, [¹²⁵I]melatonin binding in the parietal cortex, medulla-pons and cerebellum was generally unaffected by castration. Estradiol did not reverse the effect of castration on [¹²⁵I]melatonin binding. A single injection of EDS which causes the destruction of Leydig cells led to a marked decrease in [¹²⁵I]melatonin binding in the brain of the rats between 3 and 7 days after treatment. This decrease correlated with the decline in serum concentrations of testosterone. Specific [¹²⁵I]melatonin binding and serum concentrations of testosterone subsequently increased to control levels within 37 days after treatment in accord with the repopulation of the Leydig cells. The results clearly show that testosterone regulates the density of melatonin receptors in the hypothalamus and hippocampus of the male rat.