Seasonal and photoperiod-induced changes in the secretion of α-melanocyte-stimulating hormone in Soay sheep: temporal relationships with changes in β-endorphin, prolactin, follicle-stimulating hormone, activity of the gonads and growth of wool and horns

Blood plasma concentrations of α-melanocyte-stimulating hormone (α-MSH), β-endorphin (β-END), prolactin and follicle-stimulating hormone (FSH), and associated changes in the size of the testes, and growth of the horns and pelage were measured in male (n=8), castrated male (n=5) and female (n=9) Soay sheep. The animals were born in April and kept outdoors near Edinburgh (56 o N) during the first two years of life. In all groups there was a close association between the weekly changes in the plasma concentrations of α-MSH and β-END; the molar ratio in mean concentrations was close to 1:1. The blood plasma concentrations of both hormones varied markedly with season with a 3- to 10-fold increase in concentrations from the minimum in winter to the maximum in autumn. The seasonal peak occurred in September in the first year of life as juveniles, and between july (males) and September (females) in the second year when the animals were sexually mature. The plasma concentrations of ACTH did not vary in parallel with the seasonal changes in the concentrations of α-MSH (measured only in males); the molar ratio for the concentrations of α-MSH:ACTH was 1:0.12. The seasonal increase in the concentrations of α-MSH occurred 1-3 months after the seasonal increase in the concentrations of prolactin and the associated growth in horns and pelage, and slightly before, 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) housed indoors under an artificial lighting regimen of alternating 16-week 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 photoperiod-induced cycle in the plasma concentrations of α-MSH with a 25-fold increase from a minimum under long days to a maximum under short days. The concentrations of β-END varied in close parallel with the changes in α-MSH, and the temporal associations with the changes in the other pituitary hormones were similar to those observed in animals housed outdoors. Overall, the results support the view that α-MSH is co-secreted with β-END from the melanotrophs in the pars intermedia of the pituitary gland, and that the secretory activity of the melanotrophs changes markedly with season, increasing in summer and autumn, and decreasing in winter and spring. The annual cycle in daylength is likely to be the principle environmental cue timing the cycle in α-MSH and β-END. α-MSH is known to have biological effects 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 seasonal increase in the secretion of α-MSH may regulate multiple physiological changes in autumn in preparation for winter