Involvement of Postnatal Gonads in the Maturation of Dopaminergic Regulation of Prolactin Secretion in Female Rats

ermanent effects of postnatal gonads on dopamine (DA)-mediated regulation of PRL secretion were examined in adult female rats of the Wistar-Imamichi strain. Experiments were uniformly done at the age of about 10 weeks. First, rats were bilaterally ovariectomized on varying postnatal days, i.e. 24 h after birth [neonatal castration (NC)] or at 1, 2, 3, 4, or 6 weeks of age. Intact diestrous females were also examined. All groups were injected ip with saline or sulpiride (1 mg/kg BW), and blood samples were collected by rapid decapitation 20 min later. Also, in saline control groups, the anterior pituitary (AP) PRL content was determined. In NC females, significant PRL release was not observed by either sulpiride or saline. However, stimulation of PRL release by sulpiride was observed consistently in rats ovariectomized on or after 1 week of age. Although there was not a complete correlation between the plasma PRL level attained after sulpiride treatment and the AP PRL content, the general trend was that the AP PRL content was increased as the postnatal age of the ovariectomy was delayed. Values in intact adult females exceeded those in all of the castrated groups in both parameters. Second, daily sc injection for 7 days with estradiol benzoate (E2; 2.5 Mg/kg BW) and/ or progesterone (P; 5 mg/kg BW) was given after NC to determine which ovarian secretion(s) is involved in the aforementioned findings. Sulpiride stimulated PRL release in NC females that had been treated neonatally (postnatal days 3–9) with either E2 or P. This effect was accompanied by an increase in the AP PRL content. An enhancement of both parameters was also observed when the same dose of E2 or P was given in the fourth week (postnatal days 22–28). However, neither parameter was further augmented by concurrent administration of E2 and P at either postnatal time. Characteristics of AP DA receptors were subsequently analyzed in some of the foregoing groups using [3H]spiperone. The number of binding sites, when expressed as femtomoles per pituitary, did not differ among all groups examined. However, a significant difference in the dissociation constant (Kd) was observed between NC females and females castrated at 6 weeks of age. The Kd in NC females was not altered by subsequent neonatal treatment with E2 and/or P. Nonetheless, the E2 effect on Kd was dependent on when E2 was given after NC. E2 treatment at 4 weeks of age caused a significantly smaller Kd value than neonatal E2 treatment. In conclusion, even neonatally the postnatal long term presence of E2 and P was demonstrated to be responsible for the feminization of PRL release induced by DA receptor blockade. The effects of neonatal E2 and/or P in this regard were not exerted by influence on AP DA receptors, but, rather, by influence on AP PRL content. However, achievement of adult levels of AP DA receptors seemed to require postnatal ovarian secretion. It is possible that E2 might be involved in this process in a postnatal agedependent manner. (Endocrinology120: 2212–2219,1987)