Effects of timed melatonin infusions on reproductive development in the Djungarian hamster (Phodopus sungorus).

In confirmation of earlier work, daily melatonin infusions of 9- or 12-h duration inhibited testicular development in pinealectomized juvenile Djungarian hamsters, while daily infusions of 6-h duration did not prevent gonadal growth. Two other methoxyindoles (5-methoxytryptophol and 5-methoxytryptamine) had less than 4% of the activity shown by melatonin. When pinealectomized hamsters received two short duration (i.e. 3, 5, or 6 h) melatonin infusions each day, gonadal development was not inhibited if the two daily infusions were separated by a period of 2 or 3 h without melatonin treatment. Thus, the effects of separate pulses of melatonin were not additive in this paradigm. When single, short duration daily infusions of melatonin (5-h) or isoproterenol (6-h) were administered to pineal-intact hamsters, gonadal inhibition occurred only when the infusions were given at times that would lead to an expected overlap with and extension of the endogenous nocturnal melatonin peak. Significant inhibition of testicular growth could be obtained with as few as 2-4 days of melatonin treatment. These observations further support the importance of melatonin peak duration and also indicate other interesting temporal aspects of melatonin action in the Djungarian hamster.