Activation of ganglionic tyrosine hydroxylase by peptides of the secretin-glucagon family: structure-function studies

The hydroxylation of tyrosine to dopa is the rate-limiting reaction in catecholamine biosynthesis. It has been previously reported that secretin, vasoactive intestinal peptide and peptide histidine isoleucine amide, all members of the secretin-glucagon family of peptides, increase dopa synthesis in superior cervical ganglia in vitro. We report here that two other members of this peptide family, rat growth hormone-releasing factor and helodermin H38, a component of Gila monster venom, also increase the rate of dopa synthesis, while glucagon-like peptides I and II and a number of other peptides tested produce no effect. Since analogs of cAMP also increase dopa synthesis, it is of particular interest that all of the peptides that increase catechol synthesis also raise the levels of this cyclic nucleotide in the superior cervical ganglion. Helodermin H38 stimulated the rate of dopa synthesis and the level of cAMP with similar potencies (EC50S of approximately 10 nM) and with maximal effects of two- and two-fold, respectively. By either measure, rat growth hormone-releasing factor produced a two-fold increase at 10 microM and a three- to four-fold increase at 30 microM. Analogs of peptides of the secretin-glucagon family with a deletion or modification of the N-terminal histidine were much less effective in these assays at the concentrations tested than were their parent compounds, demonstrating an important role for this amino acid in conferring activity on these peptides. In addition to increasing dopa synthesis in intact tissue, incubation of ganglia with rat growth hormone-releasing factor, secretin, vasoactive intestinal peptide or peptide histidine isoleucine amide also increased the activity of tyrosine hydroxylase measured subsequently in ganglion homogenates. Thus, the peptidergic stimulation of dopa synthesis observed in the intact superior cervical ganglion appears to be due, at least in part, to the activation of tyrosine hydroxylase. Together with previous studies, these findings support the hypothesis that certain members of the secretin-glucagon family increase catecholamine synthesis in sympathetic neurons by a cAMP-dependent activation of tyrosine hydroxylase.