Ovarian steroids differentially regulate the expression of PRL-R in neuroendocrine dopaminergic neuron populations: a double label confocal microscopic study.

The aims of this study were (1) to identify the possible hypothalamic targets for a short prolactin (PRL) feedback in the adult female rat by identifying DAergic neuron populations expressing PRL receptor (PRL-R); (2) to describe the effect of ovarian steroids on the expression of PRL-R and (3) to compare the distribution of both the extracellular (EC) and ligand binding (LB) domains of the PRL-R on the hypothalamic dopaminergic neurons by applying double label immunocytochemistry for the different domains of PRL-R and for tyrosine hydroxylase (TH). Five- to six-month-old female rats were ovariectomized (OVX) and implanted with either 17 beta-estradiol (E2), progesterone (P4) or received an E2 and a P4 implant (E2 + P4) at the same time. In the periventricular nucleus and in the dorsomedial portion of the middle arcuate nucleus, a dramatic increase in PRL-REC immunoreactivity was observed in E2 implanted rats. This increase was attenuated in E2 + P4 rats, but P4 treatment alone had no effect. Changes in PRL-REC expression were paralleled by changes in serum PRL levels. Interestingly, PRL-REC expression in the rostral arcuate nucleus decreased in P4 implanted rats, however, P4 did not attenuate the E2-induced increase in PRL-REC density. PRL-REC immunostaining was observed on the membrane, in the cytoplasm and in the nucleus. PRL-RLB immunoreactivity was also detectable in the TH positive neurons, but no nuclear staining was observed with this antibody. However, we found a strong PRL-RLB immunostaining in the ependymal lining of the 3rd ventricle and in the processes of tanycytes projecting to the median eminence. These data indicate that (1) all neuroendocrine DAergic cells can be targets for PRL, (2) expression of PRL-R is differentially affected by ovarian steroids in the different TH cell populations, (3) PRL-RLB domain may be involved in trafficking PRL in the median eminence.