Metabotropic glutamate receptor agonists regulate phosphorylation on synapsin I in visceral sensory neurons

Metabotropic glutamate receptors (mGluRs) have been hypothesized to serve as presynaptic autoreceptors which may regulate neurotransmitter release in glutamatergic neurons. The mechanisms involved in this inhibition of transmitter release are unknown. Synapsin I is a nerve terminal associated protein which, when phosphorylated, is known to potentiate the release of neurotransmitter. Recent studies in our laboratory have identified the presence of synapsin I protein in the terminal regions of aortic baroreceptors maintained in culture. This study tested the hypothesis that mGluRs may modulate neurotransmitter release in visceral sensory neurons by regulating the phosphorylation of snyapsin I. Rat nodose-ganglia dissociated cultures were maintained in serum-free media for 2 weeks. Cells were incubated in[³²P]orthophosphate and depolarized by addition of 60 mM KCl. Depolarization of the nodose neurons with 60 mM KCl increased phosphorylation of synapsin I relative to control by nearly 40%. To test the effects of activation of metabotropic receptors on this response, a cocktail mGluR ligands designed to activate multiple subtypes of mGluRs was used and included 1.0 mM t-ACPD and 1.0 mM L-AP4. mGluR ligands were added 15 min. prior to depolarization. Addition of the mGluR ligands markedly reduced the observed phosphorylation (or increased the dephosphorylation) in KCl depolarized cells. These results suggest the mGluR presynaptic regulation of neurotransmitter release may involve the modulation of synapsin I phosphorylation.