Testicular regression in pinealectomized Syrian hamsters following infusions of melatonin delivered on non-circadian schedules

Abstract

The aims of this study were to validate the use of a timed infusion protocol for restoring physiologically appropriate rhythms of melatonin in the circulation of pinealectomized hamsters and, using such infusions, to compare the relative importance of the parameters of the nocturnal melatonin signal-frequency, phase, and duration of the interval between signals-in the photoperiodic control of testicular function in male Syrian hamsters. Hamsters were pinealectomized and fitted with a chronic s.c. cannula enabling them to receive timed infusions of melatonin (50 ng/h) or saline vehicle (50 μl/h). In experiment 1, RIA of serum samples confirmed that s.c. infusions produced a pattern of melatonin in the blood equivalent in amplitude and duration to that observed previously in pineal-intact animals exposed to a short photoperiod. In experiment 2, we investigated the relative importance of the frequency of the melatonin signal and the duration of the interval between signals. Pinealectomized animals that received infusions of saline for 6 wk had large testes and high concentrations of LH in the serum. Animals that received a series of short-day-like infusions of melatonin of 14-h duration, separated by an interval of either 10 h or 6 h, underwent gonadal regression and had low serum concentrations of LH. Animals that received infusions of melatonin of 8-h duration, separated by intervals of 12 h, also exhibited full gonadal collapse. However, animals that received the same 8-h infusions separated by intervals of 8 h (i.e., once every 16 h) did not undergo gonadal regression and their circulating levels of LH remained high. These results demonstrate that the frequency at which melatonin signals are received, rather than the duration of the interval between them, is critical to interpretation of photoperiodic information. To confirm that the continuous duration of a melatonin signal is also important, another group of animals received two daily infusions of melatonin of 5-h duration, separated by a break of 4 h, giving an overall signal of 14 h. These infusions were ineffective, there being no difference in testicular size or serum LH titers between animals receiving melatonin and those receiving saline. In experiment 3, we tested whether the phase at which a signal is presented is important in order for animals to recognize a series of long signals and engage a short day-like response. Controls receiving infusions of melatonin of 8-h duration at the same phase every day (infusions terminated 1 h before lights-off) for 6 wk exhibited gonadal regression. Experimental animals received the same number of infusions, but these were delivered at one of three phases of the light:dark (LD) cycle, in an irregular order such that no infusion was predicted by the phase of its predecessor. After 6 wk, these animals exhibited significant gonadal atrophy, demonstrating that the phase of the LD cycle at which a melatonin signal is encountered does not matter and that a series of signals need not be encountered in an ordered fashion for a photoperiodic response to occur.