Prolactin-releasing peptides do not stimulate prolactin release in vivo

The prolactin (PRL)-releasing activity of the novel prolactin-releasing pep tides (PrRPs) was studied in vivo using male and lactating female rats. Whe reas thyrotropin-releasing hormone effectively stimulated PRL and thyrotrop in release as expected, PrRP in both animal models neither stimulated PRL s ecretion nor affected the release of other pituitary hormones. At the anter ior pituitary level, in situ hybridization (ISH) histochemistry and Norther n blot analysis revealed significantly higher expression levels of PrRP rec eptor (UHR-1) transcripts in female compared to male rats but not between l actating and nonlactating animals. By ISH, expression of UHR-1 mRNA was als o detected in the intermediate lobe but not in the posterior pituitary. UHR -1 transcripts were also readily detectable in various hypothalamic brain a reas whereas expression of PrRP mRNA was restricted to the ventral part of the dorsomedial hypothalamic nucleus but was not detected in neuroendocrine hypothalamic nuclei (e.g. PVN, SON). We thus assume that in the central ne rvous system, PrRP may likely have functions as a neuromodulator. However, together with the detailed cytochemical studies of various investigators th at failed to detect PrRP-immunopositive nerve endings in the me dian eminen ce, our results strongly suggest that the hypothalamic PrRPs cannot be clas sified as hypophysiotrophic factors. Copyright (C) 2000 S. Karser AG, Basel .