Stress-induced peptide release from rat intermediate pituitary. An ultrastructural analysis.

We tested the hypothesis that acute restraint stress results in ultrastructural evidence for enhanced release of alpha-melanocyte-stimulating hormone (alpha-MSH) and beta-endorphin from the intermediate lobe (IL) of the rat pituitary. Measurements of plasma alpha-MSH- and beta-endorphin-immunoreactivity (ir) were used to confirm ultrastructural findings. Plasma alpha-MSH-ir was elevated after 20 and 30 min of restraint while plasma beta-endorphin-ir peaked 10 min after the onset of restraint. Ultrastructural analysis revealed a decrease in the content of secretory granules within IL cells of stressed rats. Analysis of Golgi-related immature secretory granules in IL cells indicated that new peptide synthesis was not enhanced after 30 min of restraint. These results confirm previous studies showing an elevation of plasma beta-endorphin and alpha-MSH-ir during acute restraint. Furthermore, these results indicate that quantitative analysis at the ultrastructural level can be used to assess peptide release from IL secretory cells during stress.