Role of neuropeptides and amino acids in controlling secretion of hormones from the anterior pituitary gland in pigs.

All reproductive processes involve one or more of the protein hormones secreted from the anterior pituitary gland: LH, FSH, prolactin, growth hormone, ACTH and thyroid-stimulating hormone (TSH). Primary hormones of reproduction, such as LH and FSH, directly regulate a reproductive activity. For example, LH and FSH stimulate follicular growth and the associated secretion of oestradiol in sows. In contrast, secondary hormones of reproduction such as TSH are permissive and regulate other physiological systems that indirectly, but profoundly, influence reproduction. Reproduction in pigs can be enhanced by developing strategies to alter and control secretion of hormones from the anterior pituitary gland. However, the successful manipulation of adenohypophysial hormone secretion will require a sound understanding of the mechanisms controlling the function of the hypothalamic-pituitary axis. Hypothalamic hormones including GnRH, dopamine, growth hormone-releasing hormone (GHRH), somatostatin, corticotrophin-releasing hormone (CRH) and thyrotrophin-releasing hormone (TRH) are synthesized in perikarya that possess axons that terminate at the median eminence. These hormones are released into the hypothalamo-hypophysial portal vasculature, travel to the anterior pituitary gland and stimulate or inhibit secretion of adenohypophysial hormones. Secretion of hypothalamic hormones is ultimately controlled by a variety of neurotransmitters and neuropeptides, the most studied in swine being the endogenous opioid peptides (EOP) and more recently, the excitatory amino acids (ExAA). In general, EOP inhibit GnRH and hence LH secretion, and this effect involves the central catecholaminergic system. A definitive role for EOP in the modulation of FSH release remains to be determined. EOP stimulate secretion of GHRH and thus growth hormone release, and depending on the animal model studied, EOP exert either stimulatory or inhibitory influences on prolactin secretion. ExAA, working via N-methyl-D-aspartate (NMDA) receptors at the central nervous system, stimulate secretion of LH, FSH, growth hormone and prolactin in appropriate animal models. However, in certain situations, an inhibitory effect of ExAA on LH secretion has been demonstrated. The modulation of growth hormone and prolactin secretion by ExAA involves EOP. Research investigating the function of ExAA and EOP in the physiological control of swine reproduction warrants further scrutiny.