The influence of sex and gonadectomy on the hypothalamo–pituitary–adrenal axis of the sheep B J Canny, K A O’Farrell, I J Clarke 1 and A J Tilbrook Department of Physiology, Monash University, Clayton, Victoria, 3168 Australia 1 Prince Henry’s Institute of Medical Research, PO Box 5152, Clayton, Victoria, 3168, Australia (Requests for offprints should be addressed to B J Canny) Abstract There is a sex difference in the hypothalamo–pituitary– adrenal (HPA) axis of many species, although there are sparse data on the sheep. In the present study we have compared the HPA axes of intact and gonadectomised adult male and female sheep at the level of the median eminence, pituitary and adrenal glands using a variety of in vitro approaches. The concentration of arginine vasopressin (AVP) was higher (P<0·01) in the median eminence of male than female sheep, and was also elevated by gonadectomy of either sex (P<0·01). The concen- tration of corticotrophin-releasing factor (CRF) in the median eminence did not differ between the sexes, but was also elevated in both sexes following gonadectomy (P<0·01). Anterior pituitary pro-opiomelanocortin mRNA concentrations were higher (P<0·05) in intact male sheep than in intact females, with the levels in gonadectomised animals of both sexes being intermediate. In contrast to this finding, basal ACTH secretion from anterior pituitary cells was higher (P<0·05) in cultures derived from female sheep than those from males, but gonadectomy was without effect. There was no effect of sex or gonadectomy on in vitro ACTH secretion in response to AVP, CRF or the combination of AVP and CRF, and in all cases the combination of AVP and CRF generated greater (P<0·0001) ACTH secretion than AVP alone. AVP alone was more effective (P<0·01) than CRF alone as an ACTH secretagogue. The adrenal glands were larger (P<0·05) in female than male sheep, with no effect of gonadectomy. Basal cortisol production was greatest (P<0·05) in cultures of adrenal cells from intact male sheep, though ACTH- and 8BrcAMP-induced cortisol production was greater in the cultures of cells from females (P=0·05); there were no effects of gonadectomy. Cultures of adrenocortical cells from male sheep had greater (P<0·05) basal cAMP production, but ACTH-stimulated cAMP production did not differ between any of the groups of animals. These findings show a range of differences in the HPA axis of male and female sheep. Furthermore, they suggest that the heightened activity of the axis in the female occurs primarily due to differences at the level of the adrenal gland, and that greater adrenal responsiveness of female animals is due to differences in the latter stages of steroidogenesis, rather than an effect on ACTH signal transduction at its receptor. Journal of Endocrinology (1999) 162, 215–225 Introduction It is well established for various species that there are sex differences in the hypothalamo–pituitary–adrenal axis (HPA), and that these differences occur at virtually all levels of the axis. There are sex-related differences in: the levels of Type I and II glucocorticoid receptors in the hippocampus and hypothalamus (Turner & Weaver 1985, Carey et al. 1995, MacLusky et al. 1996); the expression of corticotrophin-releasing factor (CRF) and arginine vaso- pressin (AVP) in the hypothalamus (Vamvakopoulos & Chrousos 1993, Paulmyer-Lacroix et al. 1996, Viau & Meaney 1996); the secretion of adrenocorticotrophin (ACTH) from the anterior pituitary (Gallucci et al. 1993); and the size and steroidogenic potential of the adrenal glands (Kitay 1961, Gaskin & Kitay 1970, Roelfsma et al. 1993, El-Migdadi et al. 1995). It is thought that the differences observed between males and females reflect differential effects of the sex steroids on the HPA axis, as the activity of many proteins which regulate the axis are directly regulated by the steroids and sex steroid receptors are present at many levels of the axis (Hirst et al. 1992, Herbison 1995, Bethea et al. 1996, Madigou et al. 1996). In particular, oestrogen appears to have a stimulatory action on the HPA axis, and hence the axis of the female exhibits generally greater reactivity than that of the male (Handa et al. 1994, Young 1995). Precise physiological or pathological explanations for this increased activity in the female are, however, lacking, but it has been recently hypothesised that the differences in the 215 Journal of Endocrinology (1999) 162, 215–225 0022–0795/99/0162–215  1999 Society for Endocrinology Printed in Great Britain Online version via http://www.endocrinology.org