Prolactin-releasing factor: cellular origin in the intermediate lobe of the pituitary

Our laboratory has provided substantial evidence for the presence of PRL-releasing factor (PRF) in the posterior pituitary. The objectives of this study were 1) to determine the distribution of PRF activity between the neural and intermediate lobes, and 2) to assess the PRF activity of cultured posterior pituitary cells. Posterior pituitaries from adult male rats were dispersed with trypsin and cultured for 1-7 days. Cultured cells or intact posterior pituitaries were extracted with acid and lyophilized. PRF activity was determined by the ability of reconstituted extracts to increase PRL release from cultured anterior pituitary cells. Upon dissection of the posterior pituitary, PRF activity was primarily present in the intermediate lobe. There was minimal contamination between the two lobes, as indicated by the localization of 90% of the total oxytocin in the neural lobe and 95% of alpha MSH in the intermediate lobe. Extracts from intact posterior pituitaries and posterior pituitary cells cultured for 4 days stimulated PRL secretion in a similar dose-dependent manner. Cultured liver and cerebral cortex cells had very low PRF activity. Both oxytocin and dopamine, two neuronal markers, were reduced to less than 5% of their original values within 1 week of cell culture. There was also a significant reduction in the cell content of alpha MSH. On the other hand, PRF activity was relatively stable during culture. Incubation of posterior pituitary cells for 4 days with either cycloheximide or PRL caused a 55-60% reduction of the PRF activity of the cells. We conclude the following. 1) PRF is localized, almost exclusively, in the intermediate lobe of the pituitary. 2) PRF activity is present within nonneuronal cells, either melanotrophs or a small subpopulation of nonopioid-producing cells. 3) PRF is tissue specific, and its presence in cultured posterior pituitary cells depends at least in part on de novo synthesis. 4) The synthesis and/or release of PRF may be subjected to short loop negative feedback regulation by PRL.