Control of Proglucagon-Derived Peptide Synthesis and Secretion in Fetal Rat Hypothalamus

The hypothalamus has the highest concentration of proglucagon-derived peptides (Pgdp’s) in the brain, however, the control of the synthesis and secretion of these peptides is not understood. The goal of our studies was to examine in detail the regulation of synthesis and secretion of Pgdp’s in the hypothalamus. Hypothalamic cultures were prepared from fetal rats on day 19–21 of gestation and Pgdp’s in media and cells were determined by radioimmunoassay after treatment with test agents. Dibutyryl cyclic AMP or forskolin, activators of protein kinase A, markedly stimulated both Pgdp synthesis (by 5-fold) and secretion (by 10-fold) after 24 h of treatment (p < 0.05). The effects of protein kinase A stimulation on Pgdp’s in the hypothalamus were greater than seen in our previous studies with the Pgdp-producing pancreatic A and intestinal L cells. Therefore there are tissue-specific differences with regard to the magnitude of the response of Pgdp’s to protein kinase A stimulation. Consistent with an involvement of protein kinase A in hypothalamic Pgdp synthesis and secretion, somatostatin-14, an inhibitor of protein kinase A, was found to inhibit Pgdp synthesis and secretion in a dose-dependent fashion (p < 0.05). Phorbol myristate acetate (PMA), a stimulator of protein kinase C, did not significantly affect the synthesis or secretion of Pgdp’s at 6 h, but significantly stimulated Pgdp secretion after 24 h (p < 0.05). The inactive phorbol ester, phorbol triacetate was without effect on Pgdp synthesis or secretion after 24 h of incubation (p > 0.05). Protein kinase C stimulation was previously shown to increase Pgdp secretion, but not synthesis in the pancreas and intestine, an effect similar to that seen in the hypothalamus. However, the effects of PMA on Pgdp secretion were seen at 2 and 24 h in the intestine but only at 24 h in the hypothalamus. Gastrin-releasing peptide, a hypothalamic peptide that stimulates protein kinase C, had no effect on Pgdp production in the hypothalamus (p > 0.05). In conclusion, our studies have shown both similarities and differences in the regulation of Pgdp synthesis and secretion by protein kinase A and C in the hypothalamus, as compared to the pancreatic A and intestinal L cell.