Sex-related difference in substance P level in rat anterior pituitary: a model of neonatal imprinting by testosterone.

There was a distinct sex-related difference in the anterior pituitary level of substance P-like immunoreactivity (SP-LI) in adult rats; the anterior pituitaries of male rats contained a higher concentration of SP-LI than those of females. Neonatal castration (day 1) resulted in a marked decrease of the SP-LI level in adult males, and this decrease in the SP-LI level was restored by neonatal testosterone replacement (days 2, 4, 6) but not by androgen replacement in adulthood. On the other hand, testosterone injection of neonatal females (days 2, 4, 6) caused a significant elevation of the SP-LI level after maturation. Ovariectomy of adult females failed to alter the SP-LI level. These results indicate that neonatal exposure to testosterone plays a critical role in the sexual differentiation of the substance P level in rat anterior pituitary.