RELATIONSHIP BETWEEN NITRIC-OXIDE AND VASOACTIVE INTESTINAL POLYPEPTIDE IN ENTERIC INHIBITORY NEUROTRANSMISSION

Although considerable evidence suggests that NO serves as a neurotrans mitter in gastrointestinal muscles, it is unlikely to be the only subs tance involved in enteric inhibitory neurotransmission. Vasoactive int estinal polypeptide (VIP) is known to be expressed by inhibitory motor neurons in the gut, and it appears to be co-localized with nitric oxi de synthase (NOS) in a subpopulation of enteric neurons. These data su ggest that NO and VIP may be parallel neurotransmitters. Others have s uggested that VIP is the primary inhibitory transmitter, and it stimul ates production of NO in smooth muscle cells. In this ''serial cascade '' model NO is a paracrine substance. We performed experiments on circ ular muscles and cells from the canine proximal colon to further test the idea that NO and VIP are parallel neurotransmitters and to determi ne the validity of the serial cascade model in these muscles. We found that NO-independent inhibitory effects were unmasked when excitatory and NO-dependent inhibitory responses were blocked. NO-independent inh ibitory effects were reduced by a-chymotrypsin and blocked by tetrodot oxin. NOS- and VIP-like immunoreactivities were co-localized in enteri c neurons and varicose fibers in the circular muscle layer. Similar to several other reports we found no evidence for a constitutive NOS in smooth muscle cells. Several aspects of the serial cascade model were not supported by our results: (i) the electrical and mechanical effect s of VIP did not depend upon NO synthesis; (ii) VIP-induced changes in [Ca2+](i) did not depend upon NO synthesis; and (iii) VIP did not cau se the release of NO from canine colonic muscles. These results are co nsistent with the hypothesis that NO and VIP are co-transmitters, rele ased in parallel from enteric inhibitory nerves.