SEASONAL AND PHOTOPERIOD-INDUCED CHANGES IN THE SECRETION OF ALPHA-MELANOCYTE-STIMULATING HORMONE IN SOAY SHEEP - TEMPORAL RELATIONSHIPS WITH CHANGES IN BETA-ENDORPHIN, PROLACTIN, FOLLICLE-STIMULATING-HORMONE,ACTIVITY OF THE GONADS AND GROWTH OF WOOL AND HORNS

Blood plasma concentrations of alpha-melanocyte-stimulating hormone (a lpha-MSH), beta-endorphin (beta-END), prolactin and follicle-stimulati ng hormone (FSH), and associated changes in the size of the testes, an d growth of the hems and pelage were measured in male (n=8), castrated male (n=5) and female (n=9) Soay sheep. The animals were born in Apri l and kept outdoors near Edinburgh (56 degrees N) during the first two years of life. In all groups there was a close association between th e weekly changes in the plasma concentrations of alpha-MSH and beta-EN D; the molar ratio in mean concentrations was close to 1:1. The blood plasma concentrations of both hormones varied markedly with season wit h a 3- to 10-fold increase in concentrations fr om the minimum in wint er to the maximum in autumn. The seasonal peak occurred in September i n the first year of life as juveniles, and between July (males) and Se ptember (females) in the second year when the animals were sexually ma ture. The plasma concentrations of ACTH did not vary in parallel with the seasonal changes in the concentrations of alpha-MSH (measured only in males); the molar ratio for the concentrations of alpha-MSH:ACTH w as 1:012. The seasonal increase in the concentrations of alpha-MSH occ urred 1-3 months after the seasonal increase in the concentrations of prolactin and the associated growth in hems and pelage, and slightly b efore, or coincident with the seasonal increase in the concentrations of FSH and the growth in the testes. In a second experiment, the same parameters were measured in a group of adult male Soay sheep (n=8) hou sed indoors under an artificial lighting regimen of alternating 16-wee k periods of long (16 h light:8 h darkness) and short days (8 h light: 16 h darkness). In this situation, there was a clearly defined photope riod-induced cycle in the plasma concentrations of alpha-MSH with a 25 -fold increase from a minimum under long days to a maximum under short days. The concentrations of beta-END varied in close parallel with th e changes in alpha-MSH, and the temporal associations with the changes in the other pituitary hormones were similar to those observed in ani mals housed outdoors. Overall, the results support the view that alpha -MSH is co-secreted with beta-END from the melanotrophs in the pars in termedia of the pituitary gland, and that the secretory activity of th e melanotrophs changes markedly with season, increasing in summer and autumn, and decreasing in winter and spring. The annual cycle in dayle ngth is likely to be the principle environmental cue timing the cycle in alpha-MSH and beta-END. alpha-MSH is known to have biological effec ts in the brain (neuroendocrine control of prolactin and FSH), and in the adrenal gland (secretion of glucocorticoids), adipose tissue (fat storage), and skin (melanogenesis and secretion of sebum). Thus the se asonal increase in the secretion of alpha-MSH may regulate multiple ph ysiological changes in autumn in preparation for winter.