Frequency coding of melatonin signals sufficient to induce testicular growth in photoregressed Siberian hamsters

Integration of melatonin signals over the course of several weeks was studied in male Siberian hamsters maintained in a short day length (10 h light/day) from birth (d 0). On d 35, hamsters with undeveloped testes were housed in constant light and received 5 h s.c. infusions of melatonin (MEL; 100 ng/infusion) or saline (SAL) over the next 30 days. Infusions were provided either every day (30 MEL), every other day (1 MEL/1 OFF), for 2 consecutive days followed by 2 days with no infusion (2 MEL/2 OFF), for 5 consecutive days followed by 5 days with no infusion (5 MEL/5 OFF), for 15 consecutive days followed by 15 days with no infusion (15 MEL/15 OFF), or saline every day (30 SAL). On d 65 testes weights of 30 MEL hamsters were greater than those from 30 SAL, 1 MEL/1 OFF, or 2 MEL/2 OFF groups, but did not differ significantly from those of 5 MEL/5 OFF and 15 MEL/15 OFF animals. Serum FSH concentrations of 30 MEL hamsters exceeded those of all other groups which did not differ among each other. Between 15 and 30 consecutive daily melatonin signals are necessary and sufficient to initiate and sustain maximal gonadotropic activity in juvenile male hamsters kept in constant light. The neuroendocrine system responsive to melatonin does not bridge intervals of more than a day in any of several combinations and apparently is frequency-coded for maximal responsiveness to daily signals.