Neuroendocrine correlates of the critical day length response in the Soay sheep.

In mammals, melatonin is the hormone responsible for synchronisation of seasonal physiological cycles of physiology to the solar year. Melatonin is secreted by the pineal gland with a profile reflecting the duration of the night and acts via melatonin-responsive cells in the pituitary pars tuberalis (PT), which in turn modulate hypothalamic thyroid hormone status. Recent models suggest that the actions of melatonin in the PT depend critically on day length-dependent changes in the expression of eyes absent 3 (Eya3), which is a coactivator for thyrotrophin ß-subunit (Tshß) gene transcription. According to this model, short photoperiods suppress Eya3 and hence Tshß expression, whereas long photoperiods produce the inverse effect. Studies underpinning this model have relied on step changes in photoperiod (from 8 to 16 hours of light/24 hours) and have not compared the sensitive ranges of photoperiods for changes in Eya3 and Tshß expression with those for relevant downstream molecular and endocrine responses. We therefore performed a "critical day length" experiment in Soay sheep, in which animals acclimated to 8 hours of light/24 hours (SP) were exposed to a range of increased photoperiods spanning the range 11.75 to 16 hours (LP) and then responses at the level of the PT, hypothalamus and hormonal output were assessed. Although Eya3 and Tshß both showed the predicted SP vs LP differences, they responded quite differently to intermediate photoperiods within this range and, at the individual animal level, no clear Eya3-Tshß relationship could be seen. This result is inconsistent with a simple coactivator model for EYA3 action in the PT. Further downstream layers of nonlinearity were also seen in terms of the Tshß-dio2 and the dio2-testosterone relationships. We conclude that the transduction of progressive changes in photoperiod into transitions in endocrine output is an emergent property of a multistep signalling cascade within the mammalian neuroendocrine system.

Ancestral TSH mechanism signals summer in a photoperiodic mammal.

In mammals, day-length-sensitive (photoperiodic) seasonal breeding cycles depend on the pineal hormone melatonin, which modulates secretion of reproductive hormones by the anterior pituitary gland [1]. It is thought that melatonin acts in the hypothalamus to control reproduction through the release of neurosecretory signals into the pituitary portal blood supply, where they act on pituitary endocrine cells [2]. Contrastingly, we show here that during the reproductive response of Soay sheep exposed to summer day lengths, the reverse applies: Melatonin acts directly on anterior-pituitary cells, and these then relay the photoperiodic message back into the hypothalamus to control neuroendocrine output. The switch to long days causes melatonin-responsive cells in the pars tuberalis (PT) of the anterior pituitary to increase production of thyrotrophin (TSH). This acts locally on TSH-receptor-expressing cells in the adjacent mediobasal hypothalamus, leading to increased expression of type II thyroid hormone deiodinase (DIO2). DIO2 initiates the summer response by increasing hypothalamic tri-iodothyronine (T3) levels. These data and recent findings in quail [3] indicate that the TSH-expressing cells of the PT play an ancestral role in seasonal reproductive control in vertebrates. In mammals this provides the missing link between the pineal melatonin signal and thyroid-dependent seasonal biology.

Redefining the limits of day length responsiveness in a seasonal mammal.

At temperate latitudes, increases in day length in the spring promote the summer phenotype. In mammals, this long-day response is mediated by decreasing nightly duration of melatonin secretion by the pineal gland. This affects adenylate cyclase signal transduction and clock gene expression in melatonin-responsive cells in the pars tuberalis of the pituitary, which control seasonal prolactin secretion. To define the photoperiodic limits of the mammalian long day response, we transferred short day (8 h light per 24 h) acclimated Soay sheep to various longer photoperiods, simulating those occurring from spring to summer in their northerly habitat (57 degrees N). Locomotor activity and plasma melatonin rhythms remained synchronized to the light-dark cycle in all photoperiods. Surprisingly, transfer to 16-h light/day had a greater effect on prolactin secretion and oestrus activity than shorter (12 h) or longer (20 and 22 h) photoperiods. The 16-h photoperiod also had the largest effect on expression of circadian (per1) and neuroendocrine output (betaTSH) genes in the pars tuberalis and on kisspeptin gene expression in the arcuate nucleus of the hypothalamus, which modulates reproductive activity. This critical photoperiodic window of responsiveness to long days in mammals is predicted by a model wherein adenylate cyclase sensitization and clock gene phasing effects of melatonin combine to control neuroendocrine output. This adaptive mechanism may be related to the latitude of origin and the timing of the seasonal transitions.

Influence of photoperiod and gonadal status on food intake, adiposity, and gene expression of hypothalamic appetite regulators in a seasonal mammal.

We studied the effects of photoperiod on metabolic profiles, adiposity, and gene expression of hypothalamic appetite-regulating peptides in gonad-intact and castrated Soay rams. Groups of five to six animals were studied 6, 18, or 30 wk after switching from long photoperiod (LP: 16 h of light) to short photoperiod (SP: 8 h of light). Reproductive and metabolic indexes were measured in blood plasma. Expression of neuropeptide Y (NPY), proopiomelanocortin (POMC), and leptin receptor (ObRb) in the arcuate nucleus was measured using in situ hybridization. Testosterone levels of intact animals were low under LP, increased to a peak at 16 wk under SP, and then declined. Voluntary food intake (VFI) was high under LP in both intact and castrated animals, decreased to a nadir at 12-16 wk under SP, and then recovered, but only in intact rams as the reproductive axis became photorefractory to SP. NPY gene expression varied positively and POMC expression varied negatively with the cycle in VFI, with differences between intact and castrate rams in the refractory phase. ObRb expression decreased under SP, unrelated to changes in VFI. Visceral fat weight also varied between the intact and castrated animals across the cycle. We conclude that 1) photoperiodic changes in VFI reflect changes in NPY and POMC gene expression, 2) changes in ObRb gene expression are not necessarily determinants of changes in VFI, 3) gonadal status affects the pattern of VFI that changes with photoperiod, and 4) in the absence of gonadal factors, animals can eat less but gain adiposity.