PHOTO-NEUROENDOCRINE CONTROL OF SEASONAL CYCLES IN BODY-WEIGHT, PELAGE GROWTH AND REPRODUCTION - LESSONS FROM THE HPD SHEEP MODEL

This chapter summarises the results of a recent study which investigat ed the role of the hypothalamo-pituitary relay system in mediating the effects of photoperiod on seasonal cycles in: (a) body weight; (b) pe lage growth; and (c) reproduction in Soay rams. Hypothalamo-pituitary disconnected (HPD) and the control rams were housed indoors under an a rtificial lighting regimen of alternating Ig-weekly periods. These per iods consisted of long (16L:8D) and short days (8L:16D) and lasted for more than 2 years. The: (i) body weight; (ii) voluntary food intake; (iii) pelage and horn growth; and (iv) variations in testicular diamet er were measured routinely every 2-4 weeks. Twice-weekly blood samples were collected to monitor long-term changes in the blood concentratio ns of: (I) pituitary; (2) metabolic. and (3) reproductive hormones (pr olactin, GH, alpha-MSH, beta-endorphin, ACTH, TSH, LH, FSH, cortisol, insulin, IGF, and testosterone). In control rams there were clearly de fined photoperiod-induced cycles in blood concentrations of prolactin, alpha-MSH, beta-endorphin, LH, FSH, insulin and testosterone and asso ciated morphological changes consistent with causal relationships (e.g . prolactin versus wool and horn growth, alpha-MSH, beta-endorphin and insulin versus body weight/food intake, LH and FSH versus testis size ). In the HPD rams there were no photoperiod-induced cycles in the con centrations of any of the pituitary hormones with the exception of pro lactin which varied as in controls (10-fold higher under long days). T here was a permanent increase in blood concentrations of alpha-MSH, be ta-endorphin and insulin in the HPD animals and a decrease in the conc entrations of GH (loss of pulsatility) and IGF(1). These changes were associated with the development of obesity. The reproductive axis was inactivated (basal LH, FSH and testosterone) although there was residu al cyclicity in the size of the testis associated with the changes in prolactin secretion. Overall, the results support the view that the me latonin signal which encodes photoperiod, acts in the hypothalamus to regulate some photoperiodic responses (alpha-MSH and beta-endorphin-bo dy weight axis, gonadotrophin-gonadal axis) but acts in the pituitary gland to regulate other responses (prolactin-pelage axis). However, a functional hypothalamus is required to generate normal seasonal cycles in: (a) body weight; (b) food intake; (c) growth; (d) fattening, and (e) reproduction, to provide the internal coordination between differe nt systems and to facilitate the temporal entrainment to environmental cues. (C) 1998 Elsevier Science Inc. All rights reserved.