Prolactin-releasing activity of GHRP-5 (Momany peptide) on lactotrophs in vivo and in vitro.

In the present study the in vivo and in vitro effects of GHRP-5 on the PRL-releasing activity in correlation with the morphological changes of lactotroph cells and their transcriptional activity were evaluated. The in vivo treatment (12 micrograms/100 g BW/day for 3 days) of male rats with GHRP-5 does not induce any significant changes in serum PRL levels. In contrast, the addition of GHRP-5 to pituitary cell cultures increased significantly the release of PRL. This effect is enhanced in cell cultures of enriched lactotrophs, increasing significantly the secretion of PRL, the concentrations of which were 50% higher than that of untreated control cells. The administration of GHRP-5 provokes several changes in the fine structure of lactotrophs, compatible with an increased secretory activity. After the GHRP-5 treatment the different lactotroph subtypes persist but the subtype I displaying secretory granules of larger size (500-900 nm) and a significant development of the Golgi apparatus and RER were more frequently observed. These results can be correlated with a significant augmentation in PRL mRNA after the GHRP-5 treatment. In spite of that no variations in serum PRL levels were observed in vivo, following GHRP-5 treatment, the lactotroph population experienced evident fine structure modifications, concordant with an upsurge of PRL synthesis. These observations confirmed a direct action of GHRP-5 on receptors expressed by lactotrophs. The differential actions of GHRP-5 on in vivo and in vitro designs confirm a different effectiveness of this secretagogue to induce PRL secretion.