Hypothalamic GnRH-like bioactivity and immunoreactivity in prepubertal and adult male rats

In prepubertal (21-days old) and adult (50-days old) male rats, the immunological and biological properties of GnRH-like material extracted from the hypothalamus were studied. At both ages, hypothalamic material and synthetic GnRH resulted in a parallel inhibition of the binding of labelled GnRH to 2 different anti-GnRH antisera (As I and As II). Using both antisera, a similar amount of immunoreactivity was measured in several extracts from 50-day hypothalami. In contrast, hypothalamic extracts obtained at 21 days contained a greater immunoreactivity using As II than using As I. This discrepancy was only observed with the hypothalamic content whereas the immunoreactivity released in vitro was similar with the two antisera at both ages studied. Filtration of hypothalamic extracts on biogel P2 revealed two immunoreactive fractions, the major one being eluted as the synthetic decapeptide and showing a similar immunoreactivity using both antisera. A high molecular weight fraction was proportionally predominant in 21-day extracts and showed a greater immunoreactivity using As II than using As I. The biopotency of the hypothalamic extracts upon rat pituitary cells in vitro was similar at the two ages but around 30 times higher than the bioactivity expected for the immunoreactivity. We conclude that the heterogeneous physicochemical and immunological nature of hypothalamic GnRH is different before and after sexual maturation in the male rat, whereas the bioactivity, although much greater than expected, is similar at both ages.