Gonadotrophin-releasing hormone and GnRH-associated peptide neurobiology from the rearing period until puberty in the female sheep

Abstract

Using morphological criteria we describe the postnatal ontogeny of gonadotrophin-releasing hormone (GnRH) and GnRH-associated peptide (GAP) of the GnRH prohormone in the ovine preoptic area (POA)-hypothalamus. The immunohistochemical GnRH- and GAP-neuroanatomy was investigated in infantile (12-week-old weanlings), prepubertal (15-, 22-week-old juveniles), peripubertal (30-, 52-week-old during maturational quiescence) and pubertal (63-week-old) females born after the summer solstice, thus undergoing delayed puberty. The changes observed in the medial POA included a postinfantile increase in the number of multipolar GnRH-immunoreactive (ir) perikarya until late prepuberty ( p < 0.05). Whereas in peripubertal lambs were observed both an increase in the number of GnRH-ir and GAP-ir cell clusters in the medial POA and an increase in the mediobasal hypothalamus (MBH) subpopulation of GnRH-ir and GAP-ir neurons ( p < 0.05). The GnRH-ir perikarya, nerve fibers and terminals showed a progressive increase in numbers until peripuberty. Conversely, the acute postinfantile alterations of decrease and then increase in the population of GAP-ir nerve elements were observed in the POA-hypothalamus until late prepuberty ( p < 0.05). The preoptico-hypothalamic ir GAP and GnRH were of comparable abundance in both peripubertal groups and scarce during puberty. In conclusion, the infantile/prepubertal transition period changes GnRH posttranslational biosynthesis and GnRH–GAP-neuroanatomy from the infantile to the pubertal pattern, which allows the inauguration of the breeding season. The postinfantile MBH distribution of GnRH-containing neurons and increase in the POA population of GnRH–GAP neurons and their somatic appositions seem important for maturation. The nature of GnRH–GAP neurobiology during inauguration of puberty seems morphotic and in female sheep is seasonal.