Inhibitory Effect of the Ovaries on Neonatal Androgen Imprinting of Growth Hormone Secretion in Female Rats*

The effect of neonatal androgen treatment on the GH secretory pattern was examined in intact and ovariectomized adult female rats. Neonatal ovariectomy or sham operation was performed at 1-2 days of age; thereafter, the animals were immediately given testosterone propionate (250 micrograms) or vehicle. Other rats, also treated neonatally with testosterone, were ovariectomized 15-22 days before blood sampling. Plasma GH was measured in blood samples obtained from indwelling intraatrial cannulae every 20 min for 8 h when the animals were 100-140 days old. Plasma GH secretory patterns were analyzed by a pulse analysis computer program (PULSAR). Neonatal testosterone treatment did not affect the GH secretory pattern of female rats with intact ovaries. In contrast, neonatal androgen treatment enhanced GH pulse height as well as mean GH concentration in neonatally ovariectomized female rats to levels comparable to those in intact male rats. Neonatal testosterone administration also significantly increased GH pulse height and mean plasma GH concentration in female rats that were ovariectomized during adulthood. However, the GH secretory pattern of ovariectomized female rats given testosterone neonatally still differed markedly from that of normal males, in that GH pulses occurred less regularly and baseline levels were higher. Pituitary GH content and concentration in neonatally ovariectomized female rats were increased to levels indistinguishable from those in male rats by neonatal testosterone treatment. No significant effect of neonatal testosterone was observed in sham-operated females. Neonatal ovariectomy decreased basal plasma GH levels, but did not affect plasma GH pulse height or pituitary GH levels. The serum estradiol concentration was markedly decreased in ovariectomized female rats, but was unchanged in sham-operated rats given neonatal testosterone, raising the possibility that serum estradiol secretion mediated the antagonistic effect of the ovaries on neonatal androgen imprinting. These results indicate that the presence of ovaries can prevent the stimulatory effect of neonatal androgen exposure on GH storage and secretion in adult female rats.