The development of the hypothalamo–pituitary axis in the neonatal rat: sexual maturation in male and female rats as assessed by hypothalamic LHRH and pituitary and serum LH and FSH concentrations

Abstract

Using specific radioimmunoassay techniques, we have measured hypothalamic LHRH concentration and pituitary and serum LH and FSH concentrations in neonatal rats from 0 to 60 days after birth. There were no sex differences demonstrable for hypothalamic LHRH concentration, which rose from minimal values at 2 days to peak concentrations at 22–28 days and declined significantly with the approach of puberty. Pituitary LH concentration in the two sexes rose to peak values at 19–25 days, with significantly higher values observed in females (p < 0.01). Serum LH concentration was high in both sexes at birth, the values in females being significantly higher than those observed in males (p < 0.01). While the serum LH concentration remained relatively stable throughout the study period in males, it declined rapidly to a nadir at 28 days in females. Pituitary FSH concentration was low at birth in both sexes. Females demonstrated a marked rise to peak concentrations at 16 days while males achieved significantly lower peak prepuberal values (p < 0.01) at 28 days. Serum FSH concentration was significantly higher in females (p < 0.01) at birth and rose further to peak values at 16 days. Males attained peak serum FSH concentrations at 35 days. These data demonstrate that the maturation of the hypothalamo–pituitary–gonadal axis develops in the neonatal period in the rat and confirm the presence of significant differences in hypothalamic sexual differentiation and control. The possibility of this developmental process in the neonatal rat as a model for the study of the midgestational hypothalamic maturation of the human foetus is discussed.