Gonadal Responses of the Male Syrian Hamster to Programmed Infusions of Melatonin are Sensitive to Signal Duration and Frequency but Not to Signal Phase Nor to Lesions of The Suprachiasmatic Nuclei1

Abstract

This study investigated the roles of the melatonin signal and the circadian system in the induction of photoperiodic responses in the male Syrian hamster. Pinealectomized animals received programmed s.c. infusions of saline or melatonin. Saline infusions for 10 h or melatonin for 4 h during the night had no effect on the reproductive axis whereas nightly 10-h infusions of melatonin induced gonadal atrophy. Animals that received 10-h infusions of melatonin arranged such that consecutive daily signals were delivered alternately during the day and night also exhibited gonadal atrophy, whereas melatonin signals delivered every 48 h, exclusively during either the day or night, were without effect. These results demonstrate that the brain is able to read melatonin signals delivered at different phases of the circadian cycle and to use them in combination to generate an appropriate photoperiodic response. Melatonin signals lasting 10 h delivered to pinealectomized (PX) animals every 24 h induced gonadal regression. Melatonin delivered at periodicities of 20 h, 23 h, and 25 h also caused gonadal regression whereas infusions every 28 h were without effect, demonstrating that the systems responsive to melatonin are sensitive to signal frequency but do not need to receive the signal on a strictly circadian basis. These results are discussed in the context of the significance of the melatonin-free interval. PX animals that received sham or bilateral lesions of the suprachiasmatic nuclei (SCN) were infused nightly for 10 h with saline or melatonin. Melatonin infusions were equally effective at inducing gonadal atrophy and lowering serum testosterone levels in both sham- and SCN-lesioned animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma melatonin profiles of Romney Marsh sheep in natural photoperiod and in acutely extended darkness

“…Just how melatonin performs this function is unclear. In sheep and some other species the secretion of melatonin has generally been reported to follow the duration of the dark period (Rollag & Niswender, 1976;Kennaway et al, 1977;Lincoln & Short, 1980;Arendt et al, 1981) and this mechanism is believed to underlie the seasonal repro¬ ductive response given the longer period of melatonin secretion in winter than summer (Maywood et al, 1990). However, another major hypothesis is centred around the possibility that there is a phase of the circadian cycle sensitive to the presence of melatonin (Tamarkin et al, 1976;Rollag et al, 1978) …”

What is the role of melatonin within the anterior pituitary?

“…Removal of the pineal gland or ablation of circadian input to the pineal abolish both the normal pattern of melatonin synthesis and seasonal responses (Lincoln et al 1989). Conversely, exogenous melatonin delivered by injection or infusion can mimic the seasonal effects of changing photoperiod (Elliott et al 1989, Maywood et al 1990, Bartness et al 1993, Tamarkin et al 1977.…”

Testicular development in Siberian hamsters depends on frequency and pattern of melatonin signals

“…Likewise, long-duration melatonin infusions of 10-to 12-h duration induce gonadal regression if provided at 24-but not 36-or 48-h intervals (9). In Syrian hamsters, long-duration melatonin infusions given at 24-or 20-h intervals provoke gonadal regression, but frequencies that deviate substantially from 24 h are less effective (25). Neuroendocrine responsiveness to melatonin may be frequency tuned such that signals that recur at the endogenous frequency of once every day most effectively produce appropriate physiological responses.…”

“…." (25); i.e., the melatonin-free interval between successive signals may be a major determinant of substrate responsiveness to melatonin. Elliott and colleagues (9) also raised this issue in speculating that there may be a refractory period after each melatonin signal, during which the system is temporarily unresponsive to the hormone while it resets in preparation for the next measurement of melatonin duration.…”