Melatonin-independent Photoperiodic Entrainment of the Circannual TSH Rhythm in the Pars Tuberalis of the European Hamster

Apr 6, 2018Journal of biological rhythms

Seasonal timing of hormone rhythms in European hamsters set by day length without melatonin

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Abstract

Pinealectomized European hamsters exhibit a seasonal cycle in reproductive and physiological states despite the absence of melatonin.

Photoperiodic information appears to regulate TSHβ expression in the pars tuberalis and downstream hypothalamic pathways without the influence of melatonin.
Half of the pinealectomized hamsters maintained entrainment to an accelerated photoperiodic regime, while the other half followed only every second cycle.
Higher levels of PT TSHβ and hypothalamic dio2 expressions correlate with a summer physiological state compared to a winter state.
Reproductive animals showed higher expression of rfrp and lower expression of kiss1 genes, reflecting the integration of photoperiodic signals in the hypothalamus.
The results suggest that the suprachiasmatic nucleus constructs a melatonin-independent photoperiodic message sufficient for entraining circannual rhythms.

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Adaptation of biological rhythms to a seasonal environment in circannual mammals is achieved via the synchronization of intrinsic circannual rhythms to the external year by photoperiod. In mammals, the photoperiodic information is integrated to seasonal physiology via the pineal hormone melatonin regulation of pars tuberalis (PT) TSHβ expression and its downstream control of hypothalamic dio2 gene expression. In the circannual European hamster, however, photoperiodic entrainment of the circannual clock is possible in pinealectomized animals. The present study explores whether the TSHβ expression in the PT and the downstream hypothalamic pathways are regulated by photoperiod in European hamsters in the absence of melatonin. All animals were kept on an accelerated photoperiodic regime, which compressed the natural year to a 6-month cycle. Sham-operated European hamsters and half of the pinealectomized European hamsters entrained their annual cycle in reproduction, body weight, and activity pattern to this cycle, whereas the other half of the pinealectomized animals followed only each second cycle. In all animals, PT TSHβ and hypothalamic dio2 expressions were higher in hamsters displaying a summer physiological state than in those in winter state. Moreover, in agreement with their seasonal state, reproductive animals (summer state) showed higher expression of rfrp and lower expression of kiss1-genes encoding central regulators of the reproductive axis-than those animals in reproductive quiescence (winter state), indicating the hypothalamic integration of the photoperiodic signal even in pinealectomized animals. The appropriate occurrence of a well-characterized activity pattern indicative of a so-called sensitive phase to short photoperiod suggested that the SCN constructs the melatonin-independent photoperiodic message. This message is sufficient to entrain the circannual rhythm in TSHβ expression in the PT and the downstream hypothalamic neuroendocrine pathway through a yet unknown pathway. These results reinforce the hypothesis that the PT is the site for the integration of circannual and photoperiodic information.

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Melatonin-independent Photoperiodic Entrainment of the Circannual TSH Rhythm in the Pars Tuberalis of the European Hamster

Abstract

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