Evidence for an endogenous per1 ‐ and ICER ‐independent seasonal timer in the hamster pituitary gland

Abstract

Most mammals use changing annual day-length cycles to regulate pineal melatonin secretion and thereby drive many physiological rhythms including reproduction, metabolism, immune function, and pelage. Prolonged exposure to short winter day lengths results in refractoriness, a spontaneous reversion to long-day physiological status. Despite its critical role in the timing of seasonal rhythms, refractoriness remains poorly understood. The aim of this study was therefore to describe cellular and molecular mechanisms driving the seasonal secretion of a key hormone, prolactin, in refractory Syrian hamsters. We used recently developed single cell hybridization and reporter assays to show that this process is initiated by timed reactivation of endocrine signaling from the pars tuberalis (PT) region of the pituitary gland, a well-defined melatonin target site, causing renewed activation of prolactin gene expression. This timed signaling is independent of per1 clock gene expression in the suprachiasmatic nuclei and PT and of melatonin secretion, which continue to track day length. Within the PT, there is also a continued short day-like profile of ICER expression, suggesting that the change in hormone secretion is independent of cAMP signaling. Our data thus identify the PT as a key anatomical structure involved in endogenous seasonal timing mechanisms, which breaks from prevailing day length-induced gene expression.

Circadian Clock Gene Per2 Is Not Necessary for the Photoperiodic Response in Mice

“…Although the timings of the Per1 and Cry1 peaks were consistent with those in earlier reports in C3H mice [51], [58], those timings were slightly different from the results of hamsters [59], [60]. Per is believed to be expressed at dawn in response to declines in melatonin signal.…”

Tracking the seasons: the internal calendars of vertebrates

“…Prolactin mRNA and protein content of the pars distalis decrease in short-day-housed hamsters but rebound to approximate long-day levels in short-day refractory animals, indicating neuroendocrine refractory state of these cells (Bockers et al 1997;Johnston et al 2003). The endocrine cells of the PT of Soay sheep and Siberian hamsters also become refractory as assessed by the a-glycoprotein hormone subunit (Bockers et al 1997;Lincoln et al 2005).…”

“…Yet mRNA rhythms of such genes continue to faithfully represent the ambient photoperiod and melatonin profile in short-day refractory Syrian hamsters and long-day refractory sheep (Johnston et al 2003;Lincoln et al 2005).…”

Biological Rhythms

“…A number of studies suggest a role for clock genes in the control of seasonality (Hofman, 2004;Johnston et al, 2003;Lincoln, Andersson, & Loudon, 2003). In Syrian and Siberian hamsters, photoperiod alters the duration of clock and clock -controlled gene expression, while the amplitude of gene expression is influenced by photoperiod in the pars tuberalis (Johnston et al, 2003;Messager, Hazlerigg, Mercer, & Morgan, 2000).…”

“…In Syrian and Siberian hamsters, photoperiod alters the duration of clock and clock -controlled gene expression, while the amplitude of gene expression is influenced by photoperiod in the pars tuberalis (Johnston et al, 2003;Messager, Hazlerigg, Mercer, & Morgan, 2000). In sheep, however, the relative timing of clock genes is altered by photoperiod in the pars tuberalis, providing a mechanism of temporal encoding and downstream control (Hazlerigg, Andersson, Johnston, & Lincoln, 2004;Lincoln et al, 2003;Lincoln, Johnston, Andersson, Wagner, & Hazlerigg, 2005;Lincoln, Messager, Andersson, & Hazlerigg, 2002).…”