Corticotropin-releasing factor (CRF) and neurotransmitters modulate melanotropic peptide release from rat neurointermediate pituitary in vitro

Isolated pituitary neurointermediate lobes from adult male Sprague-Dawley rats were incubated for up to 90 minutes in media containing stimulatory or inhibitory agents, to study the potential effects on secretion of alpha-melanocyte stimulating hormone (alpha-MSH) from the pars intermedia. Dopamine (DA), as expected, inhibited the release of alpha-MSH, as measured by radioimmunoassay (RIA), while corticotropin-releasing factor (CRF) and serotonin (5-HT) stimulated peptide release to varying degrees over the incubation period. A combination of DA with CRF produced an inhibition of peptide release, suggesting that DA suppresses the stimulatory effects of the hypothalamic peptide. Immune staining of pituitary tissue for beta endorphin or alpha-MSH, and quantified for staining intensity, supported the specific effects of stimulation or inhibition of the respective treatments. In addition, electron microscopy of incubated tissues demonstrated alterations in secretory vesicles and membranous organelles which were consistent with peptide secretion patterns. Our in vitro findings suggest that neurotransmitters and hypothalamic peptide, which are present together in vivo in the pituitary intermediate lobe, interact at the level of the endocrine cells to modulate pro-opiomelanocortin peptide secretion.