Melatonin increases the ratio of 5-hydroxyindoleacetic acid to serotonin in the hypothalamus and brainstem concurrently with gonadal involution in male Syrian hamsters

Gonadal involution, with reduced serum levels of luteinizing hormone and prolactin, was induced in male Syrian hamsters by daily injections of melatonin, given in the evening. Highly significant increases in daytime concentrations of 5-hydroxyindoleacetic acid (5-HIAA) and in the ratio of 5-HIAA to serotonin (5-HT) were noted in extracts of mediobasal hypothalamus of melatonin-treated hamsters. An increase in the 5-HIAA/5-HT ratio, with decreased 5-HT concentrations, was found in brainstem extracts of hamsters treated with melatonin. The increase in the 5-HIAA/5-HT ratio was not prevented by inhibition of catecholamine synthesis with α-methyl-p-tyrosine, which suggests that the melatonin-induced increase in the 5-HIAA/5-HT ratio was not due to changes in catecholamine turnover. These results are consistent with the view that serotonergic neurons terminating in the hypothalamus mediate the melatonin-induced inhibition of luteinizing hormone, prolactin, and thyroid hormones accompanying gonadal involution.