| Abstract: | An in-vitro superfusion technique was used to study basal and depolarization-induced (32 mmol K+/l) release of LHRH from the mediobasal hypothalamus (MBH) of pullets at 8-25 weeks of age. Plasma LH concentrations and the incremental change (delta LH) after an i.v. injection of 1 or 15 micrograms synthetic ovine LHRH/kg body weight were also determined. Between 8 and 25 weeks of age, significant (P less than 0.01) increases in basal and depolarization-induced release of LHRH (93 and 330%, respectively) were accompanied by a significant (P less than 0.01) rise in the residual LHRH content of MBH tissue (152%), observations which suggest that the ability of the hypothalamus to synthesize and secrete LHRH increases as sexual maturation proceeds. However, plasma LH, which reached a maximum concentration of 2.05 +/- 0.43 micrograms/l at 15 weeks, fell significantly (P less than 0.05) to 1.14 +/- 0.05 micrograms/l at 25 weeks. Since delta LH in response to exogenous LHRH showed a marked and progressive decline between 12 and 20 weeks of age, the low plasma concentration of LH typical of the mature hen is probably attributable to a direct negative-feedback action of ovarian steroids on the anterior pituitary gland rather than to an impaired secretion of LHRH from the median eminence. It is suggested that a dramatic increase in the responsiveness of LHRH nerve terminals in the MBH to depolarization by 32 mmol K+/l between 20 and 25 weeks of age (mean age at onset of lay 21.9 weeks; range 19-25 weeks) may reflect the development of hypothalamic responsiveness to the positive feedback action of progesterone. |
