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

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.