Testicular and somatic growth in Siberian hamsters depend on the melatonin-free interval between twice daily melatonin signals

MT1 melatonin receptors mediate somatic, behavioral, and reproductive neuroendocrine responses to photoperiod and melatonin in Siberian hamsters (Phodopus sungorus)

Environmental day length drives nocturnal pineal melatonin secretion, which in turn generates or entrains seasonal cycles of physiology, reproduction, and behavior. In mammals, melatonin (MEL) binds to a number of receptor subtypes including high-affinity (MT1 and MT2) and low-affinity (MT3, nuclear orphan receptors) binding sites, which are distributed throughout the central nervous system and periphery. The MEL receptors that mediate photoperiodic reproductive and behavioral responses to MEL have not been identified in a reproductively photoperiodic species. Here I tested the hypothesis that MT1 receptors are necessary and sufficient to engage photoperiodic responses by challenging male Siberian hamsters (Phodopus sungorus), a species that does not express functional MT2 receptors, with ramelteon (RAM), a specific MT1/MT2 receptor agonist. In hamsters housed in a long-day photoperiod, late-afternoon RAM treatment inhibited gonadotropin secretion, induced gonadal regression, and suppressed food intake and body mass, mimicking effects of MEL. In addition, chronic (24 h/d) RAM infusions were sufficient to obscure endogenous MEL signaling, and these treatments attenuated gonadal regression in short days. Together, the outcomes indicate that signaling at the MT1 receptor is sufficient and necessary to mediate the effects of photoperiod-driven changes in MEL on behavior and reproductive function in a reproductively photoperiodic mammal.

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

Day length affects somatic and reproductive physiology of Siberian hamsters via regulation of the duration of nocturnal pineal melatonin secretion. Nightly 'long' (e.g. 12 hr) or 'short' (e.g. 6 hr) melatonin signals inhibit or stimulate gonadal growth, respectively. When long and short signals are presented in combination, however, neuroendocrine mechanisms exhibit a frequency-dependent response, stimulating gonadal growth only if short signals are presented every second night or more frequently. The present experiments further assessed formal models for the temporal integration of melatonin signals changing abruptly in duration from night to night. Photo-inhibited Siberian hamsters were housed in constant light and infused subcutaneously with various combinations of nightly short or long melatonin signals according to one of the several regimes that varied the frequency of short melatonin signal occurrence, average duration of the nightly melatonin signal, or both. Six weeks of nightly alternating short and long signals yielded different gonadal responses depending on the average melatonin signal duration. Moreover, when average melatonin signal duration was held constant between groups, gonadal stimulation was independent of the frequency of the constituent melatonin signals except when the duration of the short signal was reduced to 3 hr. Thus, neuroendocrine mechanisms do not solely categorize melatonin signals as either long or short but attend also to the duration of each component signal. In the majority (six of seven) of infusion regimes, reproductive responses to chimeric patterns of long and short melatonin signals were compatible with a simple signal-averaging mechanism.