Downregulation of long-form prolactin receptor mRNA during prolactin-induced luteal regression

Prolactin is capable of both trophic and lytic actions in rat corpora lutea. In corpora lutea responding to a trophic prolactin signal, the long form of the prolactin receptor is the dominant form and is upregulated by prolactin. We investigated whether mRNA for the short form of the prolactin receptor was dominant in corpora lutea responding to a lytic prolactin signal, and whether the relative concentrations of the mRNAs for both forms of the prolactin receptor were changed during this response.

Immature rats were ovulated by injection of 5 IU equine chorionic gonadotrophin and 5 IU human chorionic gonadotrophin, and were hypophysectomized shortly after ovulation. Nine days after hypophysectomy, rats were injected with prolactin (500 microg/day) or vehicle for 24 (n=6, n=6) or 72 h (n=13, n=5). Total RNA was isolated from corpora lutea and mRNA for both types of prolactin receptor were analyzed by semiquantitative RT-PCR using the ribosomal protein S16 as the internal control.

The intensities of the long- and short-form prolactin receptor signals were normalized to the S16 internal control and expressed as relative densitometric units. The normalized values at 24h for prolactin-treated vs vehicle-treated rats were 0.23 +/- 0.05 vs 0.49 +/- 0.15 (P>0.05) for the short form and 4.04 +/- 0.8 vs 4.23 +/- 0. 6 (P>0.05) for the long form. The values for 72 h were 0.30 +/- 0.05 vs 0.24 +/- 0.05 (P>0.05) for the short form and 2.76 +/- 0.4 vs 5. 53 +/- 0.3 (P<0.01) for the long form respectively.

The long form of the prolactin receptor is the dominant form at both time-points; however, the concentration of mRNA for this receptor isoform was specifically downregulated by prolactin treatment. Our results suggest that the short form of the prolactin receptor alone is unlikely to mediate the luteolytic action of prolactin, but that luteolytic events may be influenced via a change in the ratio of the two receptor isoforms.