Photoperiod-melatonin relay in deer

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.