PUBERTY IN THE MALE CHIMPANZEE - TIME-RELATED VARIATIONS IN CIRCULATING INHIBIN

Circulating immunoreactive inhibin concentrations were determined in m ale chimpanzees (n = 12) during a 36-mo period starting 18 mo before t he onset of pubertal testicular growth (Po). Serum levels of inhibin s tarted to increase, together with LH and testosterone, 6 mo before a r ise in FSH and Po. Before Po, inhibin was not related to gonadotropins but was related to testosterone levels (r = 0.263; p < 0.005; R2 = 7% ). After Po, inhibin remained related to testosterone (r = 0.289, p < 0.01) but became inversely related to gonadotropins; FSH was the best predictor (r = -0.382, p < 0.0005; R2 = 30% ) of this relationship. D uring the progressive rise in circulating inhibin, FSH increased and p lateaued during 9 mo, then dropped to prepubertal levels. These findin gs suggest that a specific closed-loop negative feedback system for th e secretion of FSH by inhibin is not functional before the onset of pu berty in the chimpanzee. This feedback system seems to fully operate s everal months after the LH rise and may be related to the complete mat uration of both Sertoli cells and spermatogenesis.