Ovarian steroids modulate responsiveness to dopamine and expression of G-proteins in lactotropes

The effects of chronic ovarian steroid treatment on the secretory activity of individual lactotropes and the mechanisms modulating their responsiveness to dopamine (DA) were studied. Female rats were ovariectomized (OVX) and implanted with Silastic capsules containing progesterone (P-4), 17 beta), 17 beta-estradiol (E-2) or both E-2 (E-2+P-4). Ten days after surgery, anterior pituitaries were enzymatically dispersed and the reverse hemolytic plaque assay (RHPA) was performed to assess the release of prolactin (PRL) from individual lactotropes. RHPA was combined with immunocytochemistry (ICC) for PRL, G(alpha s) or G(i alpha 3)/ G(alpha o) proteins. E-2 treatment alone or in combination with P-4 increased the percentage of immunoreactive lactotropes among anterior pituitary cells. Incidence of active (plaque-forming) lactotropes, however, was increased both in P-4-, and E-2-treated rats and E-2+P-4 treatment increased it even further. While P-4 treatment did not affect the frequency distribution of lactotropes, both E-2 and E-2+P-4 treatments increased the large plaque-forming lactotrope population. This increase was reflected by the significantly greater mean plaque areas of lactotropes from E-2- and E-2+P-4-treated rats compared to OVX or P-4-treated animals. The responsiveness of lactotropes to DA from P-4-treated rats did not differ from that of OVX rats: thus challenge with 1 mu M DA inhibited the release of PRL, while 100 pM DA had no effect. E-2 and E-2+P-4 treatments, however, profoundly changed the lactotrope's responsiveness: challenge with 1 mu M DA had no effect and 100 pM DA resulted in moderate stimulation of PRL release in E-2+P-4 rats. Double-label ICC revealed that ovarian steroid treatments did not affect the expression of G(alpha s) in lactotropes. The incidence of G(i alpha 3)/G(alpha o)-immunoreactive lactotropes, however, decreased after E-2 treatment, alone or in combination with P-4. Although expression of G(alpha s) was similar in all plaque-forming cells regardless of plaque size, lactotropes expressing G(i alpha 3)/G(alpha o) were more likely to form small plaques in all treatment groups. These data suggest: (1) ovarian steroid treatment recruits quiescent lactotropes to release PRL; (2) E-2 treatment alone or in combination with P-4 increases the amount of PRL released by individual lactotropes; (3) E-2-induced alterations in the frequency distribution and lactotrope responsiveness to DA may be due in part to a decreased expression of G(i alpha 3)/G(alpha o).