Neuroregulatory properties of substance P in the enteric nervous system

Substance P (SP) is a putative neurotransmitter in both central and peripheral nervous systems. Its presence in intrinsic neurons of the gut, combined with its potent biological effects on this tissue, suggest that endogenous SP may play a role in the physiological regulation of gastrointestinal function. SP elicits potent, atropine-resistant contractions of guinea-pig ileum which mimic the effects of high-frequency electrical field stimulation. In addition, SP-like immunoreactivity was found to be released from segments of guinea-pig ileum in a calcium-dependent fashion by electrical stimulation. A SP radioligand binding assay was developed in order to characterize SP receptors in the rat gut. 3H-SP binds with specificity and high-affinity to membranes of rat small intestine; Scatchard plots of saturation data are curved, indicating the presence of multiple binding sites. The K/sub D/ for the high-affinity site is 0.25 nM as determined by computerized non-linear least squares analysis. Specific binding is linear with protein, dependent on temperature, and reversible. The rate constants for association and dissociation of 0.5 nm 3H-SP are: value derived form these constants, 0.34nM, agrees well with K/sub D/ derived from Scatchard plots. The rank order of potency for various tachykinins in inhibiting 3H-SP binding indicates that the high-affinity site is a P-type tachykinin receptor. Specific 3H-SP binding is modulated in a dose-related fashion by guanine nucleotides; a reduction in binding is seen which can be largely attributed to an increase in the rate of dissociation of 3H-SP in the presence of GTP. This suggests that the binding site is a receptor linked to an effector system by a GTP-binding protein