Vasoactive intestinal peptide which coexists with acetylcholine decreases acetylcholine turnover in mouse salivary...

Acetylcholine (ACh) and vasoactive intestinal peptide (VIP) probably coexist in cholinergic neurons of rodent salivary glands. In this tissue, cholinergic drugs regulate release of both ACh and VIP from postganglionic cholinergic neurons. In the present study we investigated whether VIP could modulate the metabolism of ACh in mouse submandibular gland cholinergic neurons using ACh turnover rate (TRACh) as a parameter. The TRACh was estimated via measurement of the formation of [3H]ACh during constant rate infusion of [3H]choline. Choline and ACh were separated by reverse phase high-performance liquid chromatography and were detected electrochemically after enzymatic postcolumn reaction. We calculated that the TRACh was about 3 nmol/mg of protein per hr. Pilocarpine, a muscarinic agonist decreased the TRACh about 5-fold whereas atropine methyl Br, a muscarinic antagonist, caused a large increase in turnover. Turnover, therefore, appears to be regulated by a feedback mechanism triggered by occupancy of postsynaptic receptors. VIP infused i.v. (40 micrograms/kg/min) decreased the TRACh by about 50%. Atropine completely prevented the inhibition of the TRACh induced by VIP. These results suggest that, by changing postsynaptic or presynaptic muscarinic receptor function, VIP may participate in the control of ACh metabolism. Parasympathetic decentralization of salivary glands did not prevent the effect of either atropine or VIP on TRACh. This finding suggests that the central afferent input to the ganglionic cells is not required for the regulation of ACh metabolism and, therefore, the feedback loop probably acts via a postganglionic mechanism which is not elucidated by present experiments.