Lincoln G A
MRC Reproductive Biology Unit, Centre for Reproductive Biology, Edinburgh, UK.
Acta Vet Hung. 1998;46(3):341-56.
Long-lived mammals from cold and temperate climates, including many species of deer, express overt cycles in reproduction, moulting, fattening and other characteristics. These cycles persist under constant conditions, but are normally induced and entrained by the annual cycle in daylength. The photoperiod-relay involves the eyes, the suprachiasmatic nuclei (SCN) of the hypothalamus and the pineal gland which secretes melatonin only at night. The duration of daily melatonin secretion varies with daylength and provides an internal endocrine signal for the time-of-year. In deer, treatments with melatonin induce phase-shifts in all overt seasonal rhythms. Melatonin is thought to act on specific target cells in the brain and pituitary gland which express high affinity melatonin receptors. In sheep, micro-implants of melatonin placed in the mediobasal hypothalamus (MBH) induce a complete spectrum of short-day responses, while surgical disconnection of the pituitary gland blocks all photoperiodic responses except for the regulation of prolactin. These observations support the 'dual-site hypothesis' that melatonin acts primarily in the MBH to control gonadotrophin secretion and the reproductive axis, but acts primarily in the pituitary gland via the pars tuberalis, to control prolactin secretion and the pelage axis. This differential regulation helps explains how prolactin can be 'the hormone of summer' in all photoperiodic ungulates irrespective of their seasonal breeding characteristics.
来自寒冷和温带气候的长寿哺乳动物，包括许多鹿种，在繁殖、换毛、育肥和其他特征方面表现出明显的周期性。这些周期在恒定条件下持续存在，但通常由日照长度的年度周期诱导和调节。光周期传递涉及眼睛、下丘脑的视交叉上核（SCN）和仅在夜间分泌褪黑素的松果体。每日褪黑素分泌的持续时间随日照长度而变化，并为一年中的时间提供内部内分泌信号。在鹿中，褪黑素处理会诱导所有明显的季节性节律发生相位变化。褪黑素被认为作用于大脑和垂体中表达高亲和力褪黑素受体的特定靶细胞。在绵羊中，将褪黑素微植入中基底下丘脑（MBH）会诱导一系列完整的短日照反应，而垂体的手术切断会阻断除催乳素调节之外的所有光周期反应。这些观察结果支持“双位点假说”，即褪黑素主要在MBH中起作用以控制促性腺激素分泌和生殖轴，但主要通过结节部在垂体中起作用，以控制催乳素分泌和被毛轴。这种差异调节有助于解释催乳素如何在所有光周期有蹄类动物中成为“夏季激素”，而不论它们的季节性繁殖特征如何。
