Hypothalamic control of gonadotrophin and prolactin secretion in pigs.

Interoceptive and exteroceptive stimuli detected by the central nervous system (CNS) are translated by the neuroendocrine system into signals which alter pituitary gland hormone function as depicted in Fig. 1 for gonadotrophin and prolactin secretion. The organization of this system has been thoroughly reviewed by Page (1988). Table 1 illustrates that components of the neuroendocrine— ovarian axis of the pig are functional before the onset of puberty at approximately 210 days of age and before the onset of oestrus that occurs 4-7 days after weaning. Post-partum sows in a state of prolonged anoestrus also respond to appropriate stimuli. Mechanisms which bring these components into the proper temporal relationships to generate oestrous cycles beginning at puberty and after weaning in sows reside in the CNS. The pattern of luteinizing hormone (LH) secretion, reflected by frequency, amplitude and duration of episodic release, is an important regulator of follicle development and ovulation in the primate, rat and sheep (Greenwald & Terranova, 1988). It is well established, at least in rats, that the pulsatile secretion of LH is controlled by a "pulse generator" of the hypothalamus and the preovulatory LH surge by a "surge generator" of the hypothalamus (Weiner el aL, 1988). These patterns of LH secretion presumably reflect the pattern of gonadotrophin-releasing hormone (Gn RH) released by neurosecretory neurones into the hypothalamo—hypophysial portal blood system (Goodman, 1988). Gonadal steroids and signals from other neurones modulate the frequency and amplitude of GnRH release. Prolactin secretion is also controlled by hypothalamic factors such as dopamine and thyroid-stimulating hormone-releasing hormone (TRH). This review will present evidence for control of gonadotrophin secretion by hypothalamic pulse and surge generators, and the role of ovarian steroids, various neurotransmitters and the endogenous opioid peptides (EOP; neural peptides with morphine-like biological activity) in modulating gonadotrophin and prolactin secretion during various physiological states in the pig.