EFFECTS OF DIFFERENT DOSES AND DURATIONS OF MELATONIN INFUSIONS ON PLASMA MELATONIN CONCENTRATIONS IN PINEALECTOMIZED SYRIAN-HAMSTERS - CONSEQUENCES AT THE LEVEL OF SEXUAL-ACTIVITY

The effect of different doses and durations of melatonin infusions on plasma melatonin concentrations has been studied in pinealectomized Syrian hamsters maintained under short photoperiod at either 7-degrees-C or 18-degrees-C. The effects of the infusions on plasma melatonin concentrations and on gonadal activity were compared. The results show that the minimal effective quantity of infused melatonin that induced gonadal atrophy was 40 ng/h at 7-degrees-C and 20 ng/h at 18-degrees-C. An infusion of 8 hr duration per day is necessary to inhibit sexual activity, while an infusion of 6 hr duration was ineffective. This finding suggests that the critical duration of melatonin infusion is between 6 and 8 hr. Despite the various doses of melatonin infused, plasma melatonin concentrations measured in the middle of the infusion period did not differ significantly from concentrations measured in intact animals. This finding suggests that the metabolism of infused melatonin increases as the dose of melatonin increases. Moreover, the different physiological effects observed after the various melatonin infusions cannot be explained by variations in plasma melatonin concentrations.