Clock Genes and the Long‐Term Regulation of Prolactin Secretion: Evidence for a Photoperiod/Circannual Timer in the Pars Tuberalis

Mar 8, 2003Journal of neuroendocrinology

Clock genes and long-term control of prolactin release: signs of a yearly light-based timer in the brain's hormone-regulating area

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Abstract

Prolactin secretion in sheep is regulated by changes in melatonin profiles associated with photoperiod, indicating an intrinsic mechanism in the pars tuberalis.

Prolactin secretion varies significantly in hypothalamic-pituitary disconnected sheep in response to changes in photoperiod and melatonin levels.
Sheep display long-term cycles in prolactin secretion that are specific to photoperiod, even when prolactin secretion is temporarily blocked.
Cells in the pars tuberalis, but not lactotrophs, have high levels of melatonin receptors and respond to melatonin with an inhibitory effect.
A full set of clock genes is expressed in the pars tuberalis, showing a 24-hour cyclical pattern that is consistent with circadian rhythms.
The phasing of Per and Cry gene expression changes with photoperiod, which may explain how melatonin signals are interpreted in relation to seasonal rhythms.

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Prolactin secretion is regulated by photoperiod through changes in the 24-h melatonin profile and displays circannual rhythmicity under constant photoperiod. These two processes appear to occur principally within the pituitary gland, controlled by the pars tuberalis. This is evident because: (i) hypothalamic-pituitary disconnected (HPD) sheep show marked changes in prolactin secretion in response to switches in photoperiod and manipulations of melatonin, similar to brain-intact controls; (ii) HPD sheep also show photoperiod-specific, long-term cycles in prolactin secretion under constant long or short days, with the timing maintained even when prolactin secretion is blocked for 2-3 months; and (iii) pars tuberalis cells, but not lactotrophs, express high concentrations of melatonin (MT1) receptor, and exhibit a duration-sensitive, cAMP-dependant, inhibitory response to physiological concentrations of melatonin. This suggests the existence of an intrinsic, reversible photoperiod-circannual timer in pars tuberalis cells. A full complement of clock genes (Bmal1, Clock, Per1, Per2, Cry1 and Cry2) are expressed in the ovine pars tuberalis, and undergo 24-h cyclical expression as observed in a cell autonomous, circadian clock. Activation of Per genes occurs in the early day (melatonin off-set), while activation of Cry genes occurs in the early night (melatonin on-set). This temporal association is evident under both long and short days, thus the Per-Cry interval varies directly with photoperiod. Because, PER : CRY, protein : protein interactions affect stability, nuclear entry and gene transcription based on rodent data, the change in phasing of Per/Cry expression provides a potential mechanism for decoding the long day/short day melatonin signal. A speculative, but testable, extension of this hypothesis is that intrinsically regulated changes in the phase of Per/Cry rhythms, regulates both photorefractoriness and the generation of circannual rhythms in prolactin secretion.

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Clock Genes and the Long‐Term Regulation of Prolactin Secretion: Evidence for a Photoperiod/Circannual Timer in the Pars Tuberalis

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