PHOTOPERIOD PINEAL HYPOTHALAMIC RELAY IN SHEEP

Mammals from cold and temperate climates, including many farmed species, utilise the annual cycle in daylength to time the seasonal changes in reproduction, moulting, fattening and other characteristics. These animals have a photoperiodic time-measuring system that encodes information regarding daylength into a hormonal signal, which acts within the brain to coordinate the diverse changes in physiology and behaviour. The neural relay involves the eyes, the suprachiasmatic nuclei (SCN) of the rostral hypothalamus and the pineal gland. The SCN functions as the internal biological clock. It contains neurones expressing endogenous circadian rhythms in electrophysiological activity that are entrained by the periodic photic signals from the 24-h light-dark cycle. The SCN is linked by a sympathetic innervation to the pineal gland, and via this neural pathway regulates the endogenous rhythm in secretion of melatonin. Light acts to entrain the circadian rhythm system and to inhibit the production of melatonin. Melatonin is thus released into the peripheral circulation only at night, and the duration of secretion provides an endocrine index of nightlength and thus daylength. In sheep, timed infusions of melatonin in pinealectomised animals induce biological responses according to the duration of continuous exposure to melatonin. Daily 8-h infusions of melatonin induce long-day responses, while 16-h infusions induce short-day responses. The time of day when these treatments are given has no influence on the response. Exposure of intact animals to artificial photoperiods (non 24-h light-dark cycles or night-interruption protocols) induce effects consistent with the duration hypothesis. The melatonin signal is thought to mediate the multiple effects of photoperiod by affecting the secretion of hormones from the pituitary gland. Placement of micro-implants of melatonin in the mediobasal hypothalamus (MBH) in rams induce a complete spectrum of short-day responses. Two models are presented for the actions of melatonin. One proposes that melatonin acts on dopaminergic and/or opioidergic neuronal pathways within the MBH to influence the release of the peptides and neurotransmitters which normally regulate the pituitary gland. The other proposes that melatonin acts on the adjacent pars tuberalis of the pituitary gland, a tissue with the highest concentration of melatonin receptors, to induce the multiple responses.