Functional role of vasoactive intestinal polypeptide in inhibitory motor innervation in the mouse internal anal sphincter

There is evidence that vasoactive intestinal polypeptide (VIP) participates in inhibitory neuromuscular transmission (NMT) in the internal anal sphincter (IAS). However, specific details concerning VIP-ergic NMT are limited, largely because of difficulties in selectively blocking other inhibitory neural pathways. The present study used the selective P2Y1 receptor antagonist MRS2500 (1m) and the nitric oxide synthase inhibitor NG-nitro-l-arginine (l-NNA; 100m) to block purinergic and nitrergic NMT to characterize non-purinergic, non-nitrergic (NNNP) inhibitory NMT and the role of VIP in this response. Nerves were stimulated with electrical field stimulation (0.120Hz, 460s) and the associated changes in contractile and electrical activity measured in non-adrenergic, non-cholinergic conditions in the IAS of wild-type and VIP/ mice. Electrical field stimulation gave rise to frequency-dependent relaxation and hyperpolarization that was blocked by tetrodotoxin. Responses during brief trains of stimuli (4s) were mediated by purinergic and nitrergic NMT. During longer stimulus trains, an NNNP relaxation and hyperpolarization developed slowly and persisted for several minutes beyond the end of the stimulus train. The NNNP NMT was abolished by VIP628 (30m), absent in the VIP/ mouse and mimicked by exogenous VIP (1100nm). Immunoreactivity for VIP was co-localized with neuronal nitric oxide synthase in varicose intramuscular fibres but was not detected in the VIP/ mouse IAS. In conclusion, this study identified an ultraslow component of inhibitory NMT in the IAS mediated by VIP. In vivo, this pathway may be activated with larger rectal distensions, leading to a more prolonged period of anal relaxation.