Prepubertal Ontogeny of Luteinizing Hormone-Releasing Hormone Immunoreactivity in Developing Pig Brain
Luteinizing hormone-releasing hormone (LHRH, GnRH) is a 10-amino acid peptide produced in the brain that regulates the release of LH from the pituitary gland. LH is crucial for initiating the successful ovulation of mature ovarian follicles (Graafian follicles) and transforming the ovulated follicle into a steroid-secreting corpus luteum. In the male, LH causes Leydig cells in the testis to secrete testosterone, a hormone essential for male sexual behavior secretory activity of accessory glands of the reproductive tract, muscle accretion, and spermatogenesis. The focus of this study was to determine the prepubertal ontogeny of LHRH-like immunoreactivity (LHRH-IR) in the male Chinese Meishan pig. The Meishan breed is known for reproductive traits, including increased litter size and precocious puberty, but slow growth and obesity. Brains of animals from gestational day (g) 30, 50, 70, 90, and 110 and postnatal day (pn) 1, 10, 20, and 50 (duration of pregnancy averages 114 days) were processed by a standard immunohistochemical technique utilizing a commercially available rabbit anti-LHRH antibody. Coronal sections of the brain revealed LHRH-IR in cell bodies and fibers at g30 entering the brain via the terminal nerve and in the septal region of the basal telencephalon. The numbers of LHRH-IR cells increased at g50 and cells were localized to the septum, organum vasculosum of the lamina terminalis, optic area, and lateral hypothalamus whereas immunoreactive fibers were present throughout the septum and hypothalamus and had reached the median eminence, the lowest connecting link to the anterior pituitary gland. Results from this study indicate that LHRH may be present in the Meishan brain earlier during development and fibers containing LHRH-IR appear in the median eminence (lower part of the hypothalamus near the pituitary gland) earlier than previously reported for the domestic pig. These results suggest a breed difference in the ontogeny of reproductive control systems in the pig. An understanding of the LHRH neuronal network within the brain and hormones and exteroceptive factors that affect its secretion of LHRH that, in turn, causes gonadotropin (luteinizing hormone and follicle stimulating hormone) secretion by the pituitary gland will reveal neuroendocrine mechanisms regulating reproduction in the pig.