Reproductive experience and expression of dopamine D2 receptor mRNA:: A possible mechanism for reduced prolactin secretion in primiparous rats

Reproductive experience (i.e. pregnancy and lactation) leads to reduced levels of circulating prolactin in both women and rats. Stimulation of prolactin secretion by dopamine antagonists is also blunted following reproductive experience in both species. Whereas a parity-induced reduction in haloperidol-stimulated prolactin secretion is evident in ovariectomised rats, it is unknown whether a similar attenuation of prolactin secretion is present in reproductively experienced, cycling pro-oestrous rats. The present study examined this possibility. Moreover, to determine possible mechanisms involved in parity-mediated changes in prolactin secretion, both dopamine utilisation within the arcuate nucleus/median eminence and expression of dopamine D-2 receptor mRNA (short and long forms) in the anterior pituitary were measured across the afternoon of pro-oestrous in reproductively experience and inexperienced females. Prolactin secretion was lower on the afternoon of pro-oestrous in primiparous females compared to age-matched, nulliparous controls. In addition, haloperidol-stimulated prolactin secretion was reduced in ovariectomised, reproductively experienced females. Although no differences in dopamine utilisation were observed as a function of reproductive experience, parity did affect the expression of both forms of D-2 receptor mRNA in the anterior pituitary. Compared with nulliparous controls, primiparous females had increased D-2 long mRNA expression at 12.00 h on pro-oestrous as well as increased D-2 short mRNA expression at 14.00 h. Because the ratio of D-2 long/D-2 short can significantly effect lactotroph proliferation and prolactin secretion, a shift in relative expression of the two D-2 receptor isoforms within the anterior pituitary of parous females may help account for the reduction in prolactin secretion that occurs following reproductive experience.