Regulatory effects of dopamine, oxytocin, and thyrotropin-releasing hormone upon the release of prolactin variants from lactating rat adenohypophysis

Previous studies used western blotting to show that prolactin (PRL) released from the adenohypophysis (AP) of lactating rats in vitro contains size variants from 7–14 kDa to 70–97 kDa. These variants when eluted from sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) gels and incubated with AP lactotrophs from male rats and rats in other conditions, promoted the selective stimulation and/or inhibition of the in vitro release of PRL variants. In the present study, we determined the regulatory effects of dopamine (DA), thyrotropin-releasing hormone (TRH), and oxytocin (OT) on release of PRL variants from AP lactotrophs. Primary cultures of lactotrophs from lactating rats, which were non-suckled (NS) for 6 h or suckled (S) for 15 min after NS, were incubated with PRL variants that were electroeluted from SDS-PAGE gels along with different doses of DA (0.5, 1.0, 1.5 µM), TRH (0.1, 1.0, 10 µM), or OT (0.1, 1.0, 10 µM). The secretion of PRL from the lactotrophs was determined by enzyme-linked immunosorbant assay. The results showed stimulatory and/or inhibitory effects of DA, TRH, and OT on the release of PRL variants from AP lactotrophs both by the presence of PRL variants. These results indicate that PRL variants are released from AP lactotrophs, and, in concert with hypothalamic hormones, they regulate the release of PRL from lactating rat APs.