Variations in hypothalamic luteinizing hormone releasing hormone content and release in vitro and plasma concentrations of luteinizing hormone and testosterone in developing cockerels

Abstract

An in-vitro superfusion system was used to study age-dependent changes in the functional activity of LH releasing hormone (LHRH) neurones terminating in the mediobasal hypothalamus (MBH) of the cockerel. Fragments of MBH tissue were obtained from cockerels killed at 8, 12, 16 and 20 weeks of age and both the rate of release of LHRH in vitro and the residual content of LHRH were determined by radioimmunoassay. Blood was collected from a similar group of cockerels of the same age for determination of plasma LH and testosterone concentrations. Superfused cockerel MBH showed both basal and depolarization-induced release of LHRH and the calcium-dependency of the release process was demonstrated. The viability of MBH in vitro was indicated by the observation that the rate of CO2 production by the tissue remained constant during a 3-h period of superfusion. Both the basal rate of release of LHRH (7·24 ±0·63 pg/2MBH per h; mean ± s.e.m., n = 4) and the residual content of LHRH (1·10 ± 0·24 ng/2MBH) were lowest in the 8-week-old cockerel and increased progressively to reach levels three (P<0·001) and 13 (P<0·001) times greater, respectively, by 20 weeks of age. Concentrations of LH and testosterone in plasma did not increase significantly until 16 weeks of age when the respective values were 5·86 ±0·37 μg/l and 1·88 ± 0·31 nmol/l (n = 11). Whereas plasma testosterone increased further to 5·76±0·42 nmol/l (n = 11) at 20 weeks of age, plasma LH fell significantly (P<0·02) to a concentration of 4·08 ± 0·41 μg/l (n = 11). These observations support the conclusion that the onset of sexual maturation in the cockerel is primarily determined by a central neural mechanism which results in an increased activity of LHRH neurones in the hypothalamus.