VPAC₁ receptors regulate intestinal secretion and muscle contractility by activating cholinergic neurons in guinea pig jejunum

Abstract

In the gastrointestinal tract, vasoactive intestinal peptide (VIP) is found exclusively within neurons. VIP regulates intestinal motility via neurally mediated and direct actions on smooth muscle and secretion by a direct mucosal action, and via actions on submucosal neurons. VIP acts via VPAC₁ and VPAC₂ receptors; however, the subtype involved in its neural actions is unclear. The neural roles of VIP and VPAC₁ receptors (VPAC₁R) were investigated in intestinal motility and secretion in guinea pig jejunum. Expression of VIP receptors across the jejunal layers was examined using RT-PCR. Submucosal and myenteric neurons expressing VIP receptor subtype VPAC₁ and/or various neurochemical markers were identified immunohistochemically. Isotonic muscle contraction was measured in longitudinal muscle-myenteric plexus preparations. Electrogenic secretion across mucosa-submucosa preparations was measured in Ussing chambers by monitoring short-circuit current. Calretinin⁺ excitatory longitudinal muscle motor neurons expressed VPAC₁R. Most cholinergic submucosal neurons, notably NPY⁺ secretomotor neurons, expressed VPAC₁R. VIP (100 nM) induced longitudinal muscle contraction that was inhibited by TTX (1 μM), PG97-269 (VPAC₁ antagonist; 1 μM), and hyoscine (10 μM), but not by hexamethonium (200 μM). VIP (50 nM)-evoked secretion was depressed by hyoscine or PG97-269 and involved a small TTX-sensitive component. PG97-269 and TTX combined did not further depress the VIP response observed in the presence of PG97-269 alone. We conclude that VIP stimulates ACh-mediated longitudinal muscle contraction via VPAC₁R on cholinergic motor neurons. VIP induces Cl(-) secretion directly via epithelial VPAC₁R and indirectly via VPAC₁R on cholinergic secretomotor neurons. No evidence was obtained for involvement of other neural VIP receptors.