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

Abstract

We investigated the impact of frequency and pattern of melatonin signals on reproductive development in Siberian hamsters. Juvenile males gestated in short day lengths and housed in constant illumination to suppress melatonin secretion were infused with melatonin for 5 h either once or twice per day for 20 days. Melatonin infusions at either frequency produced equivalent increases in testes and body weights that exceeded those of animals infused with saline but were indistinguishable from those of hamsters transferred to long day lengths. The reproductive system appears to be maximally stimulated by a single short melatonin signal each day. Other animals kept from birth in a short photoperiod were treated 6 h after onset of darkness with the β-adrenergic receptor antagonistdl-propranolol to shorten melatonin secretion on the night of injection but not on subsequent nights. This permitted interpolation of short nightly melatonin signals of 4–5 h duration against a background of long melatonin signals of 10–12 h duration on other nights. Treatment regimes that maintained a 1:1 ratio of short to long melatonin signals for 8 wk stimulated reproductive development; a 1:2 signal ratio, in each of three different patterns, was uniformly ineffective. The number of successive short melatonin signals had little influence on the interval across which successive melatonin signals were summated to influence photoperiodic traits. The neuroendocrine axis appears more responsive to short melatonin signal frequency than pattern for development of the summer phenotype.

Temporal integration of melatonin infusion duration: signal averaging versus frequency dependence

“…The frequency‐dependent effects of intermittent short melatonin signals on gonadal growth reported in other experiments [22, 23] are shown here to be context specific. Prior work illustrated that a shortened melatonin signal provoked gonadal growth in photo‐inhibited hamsters if delivered every second night, but not if the short melatonin signal was presented less frequently.…”

Testicular and Somatic Growth in Siberian Hamsters Depend on the Melatonin‐Free Interval Between Twice Daily Melatonin Signals

“…The rate of testicular growth is not significantly increased in response to two daily 5‐h melatonin infusions compared to a single 5‐h melatonin infusion (5, 7). It is possible that the absence of accelerated gonadal growth in response to two short melatonin signals each day is due to refractoriness of target tissues to melatonin lasting several hours (11), beginning with the termination of the first melatonin signal.…”

“…Siberian hamsters infused with two 5‐h melatonin infusions each day separated by a melatonin‐free interval of 1 h retain small testes, apparently bridging this intersignal gap and responding as if to a single, long melatonin signal of 10 h duration (5, 7–10). Testicular growth occurs, however, if the interval between the two short signals is 2 h; the melatonin‐responsive neuroendocrine system is apparently unable to bridge this longer intersignal interval.…”

“…Testicular growth occurs, however, if the interval between the two short signals is 2 h; the melatonin‐responsive neuroendocrine system is apparently unable to bridge this longer intersignal interval. The amplitude of the short melatonin signal, time of day of infusion, and the number of short melatonin signals each day are without significant effect on the rate of gonadal growth (5, 7–10).…”

“…If this hypothesis is correct then animals may only perceive and respond to the first melatonin signal each day. Alternatively, stimulation of the hypothalamic‐pituitary‐gonadal (HPG) axis may be maximal after a single short signal, in which case the second short melatonin signal, even if detected by neuroendocrine target tissues, would not produce additional gonadal growth (7).…”