The effect of 5-methoxytryptamine on golden hamster gonads is not a consequence of its acetylation into melatonin.

Radioimmunoassay and high performance liquid chromatography were used to determine if the gonadal atrophy induced by late afternoon injections of 5-methoxytryptamine (5-MT) in golden hamsters kept under long photoperiod could be due to the acetylation of this compound into melatonin. An increase in plasma concentrations of melatonin (10-13 nmol/l) was detected 15 min after injection of 130 nmol 5-MT. An injection of 4.3 nmol melatonin generated a similar plasma concentration of melatonin. 5-MT (130 nmol) and melatonin (4.3 nmol) were then injected daily in the late afternoon to golden hamsters kept under long photoperiod. After 8 weeks, 5-MT induced total testicular regression, while melatonin induced partial atrophy only. Thus under these experimental conditions, 5-MT had a physiological activity independent of that of melatonin.