Abstract
Nocturnal secretion of melatonin from the pineal gland may affect central and peripheral timing, in addition to its well-known involvement in the control of seasonal physiology. The Syrian hamster is a photoperiodic species, which displays gonadal atrophy and increased adiposity when adapted to short (winter-like) photoperiods. Here we investigated whether pineal melatonin secreted at night can impact daily rhythmicity of metabolic hormones and glucose in that seasonal species. For that purpose, daily variations of plasma leptin, cortisol, insulin and glucose were analyzed in pinealectomized hamsters, as compared to sham-operated controls kept under very long (16 h light/08 h dark) or short photoperiods (08 h light/16 h dark). Daily rhythms of leptin under both long and short photoperiods were blunted by pinealectomy. Furthermore, the phase of cortisol rhythm under a short photoperiod was advanced by 5.6 h after pinealectomy. Neither plasma insulin, nor blood glucose displays robust daily rhythmicity, even in sham-operated hamsters. Pinealectomy, however, totally reversed the decreased levels of insulin under short days and the photoperiodic variations in mean levels of blood glucose (i.e., reduction and increase in long and short days, respectively). Together, these findings in Syrian hamsters show that circulating melatonin at night drives the daily rhythmicity of plasma leptin, participates in the phase control of cortisol rhythm and modulates glucose homeostasis according to photoperiod-dependent metabolic state.
Highlights
Photoperiodic animals, such as Syrian hamsters, are species whose physiology is and reversibly regulated on a seasonal basis
The main findings of this study are that pineal melatonin acts as a time-giver for hormonal rhythms, and affects glucose homeostasis in a photoperiod-dependent way
The first conclusion is supported by the fact that pinealectomy in Syrian hamsters suppresses the daily rhythmicity of plasma leptin and can lead to shifts of the daily rhythm of plasma cortisol
Summary
Photoperiodic animals, such as Syrian hamsters, are species whose physiology is and reversibly regulated on a seasonal basis. They display seasonal changes in gonadal activity. When Syrian hamsters are exposed to long, summer-like photoperiods, gonadal function is active and their adiposity is decreased. Melatonin can be synthesized by several organs, including the pineal gland, retina and gastrointestinal tract (Pevet, 2003; Hardeland et al, 2011; Tosini et al, 2012). The daily duration of nocturnal melatonin transduces photoperiodic cues into neuroendocrine changes that modulate seasonal physiology, highlighting the pivotal role of pineal melatonin in the integration of seasonal changes in day length (Malpaux et al, 2001; Pevet, 2003)
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