Growth, nutrition and the neuroendocrine control of puberty in female sheep

Abstract

Puberty in the female lamb may be defined as the time when a preovulatory follicle develops within the ovary, and secretes sufficient oestradiol as to activate the LH surge mechanism, and thus, trigger its own ovulation. The development of the ovarian follicle results from a marked increase in the frequency of episodic luteinisincg hormone (LH) secretion from the pituitary gland (1). This probably reflects increased secretion of gonadotrophin-releasing hormone (GnRH) from the hypothalamus; studies in adult female sheep have demonstrated that pulses of LH in the peripheral circulation correlate with peaks of GnRH in the hypothalamic portal /circulation (2). In the Suffolk sheep, a very seasonal breed, changes in photoperiod provide a precise time cue to initiate reproduction (3). Under natural conditions, the autumnal decrease in daylength triggers puberty at ~30 weeks of age (4). In other breeds, the introduction of mature rams may also be important in triggering puberty. However, photoperiod and social cues can only induce puberty if appropriate growth has occurred (1, 4). Traditionally, a critical body weight or composition has been considered necessary for puberty. Although changes in body weight and composition may be good correlates of puberty, this notion neglects to consider the actual mechansim by which the brain percieves somatic changes. An alternative viewpoint is that the brain monitors and responds to changing concentrations of blood-borne metabolites or metabolic hormones or some other aspect of metabolism during development, and uses this information to trigger enhanced GnRH secretion. To investigate the mechanisms by which the central nervous system monitors somatic development, we have developed an experimental model in which growth is retarded by means of dietary restriction (4,5). Suffolk lambs are placed on low nutrition at weaning at ~8 weeks of age. The diet provides 25-50% of the caloric intake for a normally growing lamb at 20 kg, with water, minerals and vitamins ad libitum. Although the lambs remain active and healthy, growth is slowed, and eventually plateaud at ~20 kg. Puberty is markedly delayed beyond the normal 30 weeks of age in such lambs. If the lambs are ovariectomised (OVX), LH secretion remains low, indicating that the hypogonadotrophic state does not arise from enhanced responsiveness to ovarian steroid negative feedback (4, 5). Exogenous GnRH at physiological doses (2.5 ng/kg BW) can induce LH and FSH secretion, indicating that the pituitary gland is capable of adequate function (5).