In Siberian hamsters, day length is encoded by the duration of the nocturnal melatonin signal; short and long melatonin signals over the course of several weeks stimulate and inhibit somatic and gonadal development, respectively, in prepubertal males. We sought to determine whether juvenile male Siberian hamsters respond to multiple melatonin signals each day and the manner in. which the sequence of melatonin signals and the duration of the melatonin-free interval between signals affects development. Twenty-one day old male Siberian hamsters, gestated and maintained in a short-day photoperiod of 10 h light/day (10 L), were transferred to constant light to suppress endogenous melatonin secretion and received s.c. infusions of melatonin or saline for 12 days. Hamsters Infused with saline retained small testes, whereas one short melatonin infusion each. day resulted in significant testicular growth. Other hamsters were provided with two melatonin signals each day, one long (9 h) and one short (4 or 5 h); the order in which these signals was administered. and the duration of the melatonin-free interval after each signal varied between groups. In asymmetrical melatonin infusions, the first. and second daily infusions were followed by 3-h and 7-h melatonin-free intervals, respectively, whereas in symmetrical infusions, each melatonin signal was followed by a 5-h melatonin-free interval. In the asymmetrical sequence, the melatonin signal that immediately preceded the longer melatonin-free interval determined the rate gonadal growth. Equal melatonin-free intervals after each of the long and short daily melatonin infusions produced. intermediate increases in gonadal and somatic development. The hypothalamic-pituitary-gonadal axis of Siberian hamsters can respond to multiple melatonin signals each day, with. the rate of testicular growth determined primarily by the duration of the melatonin-free interval following each infusion.
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