Perinatal activity of the hypothalamic-pituitary-gonadal axis in the lamb. III. LH, testosterone and prolactin secretory pattern in newborn lambs.

Crossbred male and female lambs from Suffolk rams and Dorset-Finnish ewes born in January and February were studied at 1, 3, 7, 21 and 28 days of age and female lambs at 14 days. Ultradian variations in plasma luteinizing hormone (LH), prolactin (PRL) and testosterone were studied at 30-min intervals during a 4-hour period. In female lambs LH secretory peaks were observed at 7 days of age the amplitude of LH peaks increasing 8-fold between 7 and 14 days and 2-fold between 21 and 28 days. The number of secretory peaks also increased with age. Ultradian variations in PRL were observed from 1 day of age, the amplitude of the PRL peaks increasing also with age. In male lambs, LH and testosterone peaks were observed at 3 days of age, the testosterone peak being preceded by or concomitant with LH peaks. The amplitude of LH peaks increased with age together with basal testosterone levels although the amplitude of testosterone peaks was unchanged. This study demonstrates pulsatile secretion of LH and PRL in lambs of both sexes and of testosterone in male lambs in the first week of postnatal life. In addition, since the ovine testis is responsive to endogenous gonadotropic stimulation at this period, it is suggested that the hypothalamic-pituitary-gonadal axis is already operative at this age. It can be concluded that the hypothalamic activity concerned with pulsatile LH and prolactin secretion is present very early in the lamb neonate.