Substance P and opioid interaction on stimulated and non-stimulated guinea pig ileum

Mechanism of "substance P": on mesenteric blood flow; interactions with opioid peptides

The effects of close intra arterial injection of substance P (SP) and the interaction of SP with opioid peptides, have been studied on mesenteric blood flow in the anaesthetized dog. Injection of SP into the superior mesenteric artery caused a dose dependent increase in mesenteric blood flow. This effect was enhanced by pretreatment with the alpha 2 adrenergic antagonist yohimbine. The action of SP was significantly reduced by pretreatment with muscarinic and nicotinic cholinergic, dopamine and histamine (H1) receptor antagonists. Beta adrenergic, histamine (H2) and opiate receptor antagonists did not influence the action of SP. Pretreatment with tetrodotoxin partially inhibited the effect of SP, indicating that as well as a neurogenic action, it has a direct action on the vascular smooth muscle. The effect of SP was enhanced by D-Met2-NleS5-enkephalinamide, a delta opiate receptor agonist. The actions of D-Met2-NleS5-enkephalinamide were abolished by pretreatment with hexamethonium, partially abolished by naloxone but were unaffected by atropine. The mu opiate receptor agonist, D-Met2-Pro5-enkephalinamide, inhibited the effects of SP.

Prevention of stress-induced analgesia by substance P

It has been shown that a variety of stressful procedures, such as immobilization and footshock, can induce a significant degree of analgesia in mice. In addition, it has been shown that for some, but not all, stressful treatments, the analgesic effect is mediated via endogenous opioids. This report describes the effects of substance P, administered systemically, on both opioid-mediated immobilization-induced analgesia and non-opioid footshock-induced analgesia. Substance P completely blocked the opioid-mediated form of stress-induced analgesia while having no effect on the non-opioid form. Exogenous substance P appears to interact with endogenous opioid pain-suppressing systems.

[D-Met2, Pro5]enkephalinamide and dynorphin-(1-13) inhibit the cholinergic contraction induced in the guinea-pig ileum by substance P

In the isolated guinea-pig ileum, the sustained phase of the longitudinal contractile response to substance P is, unlike the initial peak response, mediated by stimulation of cholinergic neurons. This cholinergically mediated response to substance P was inhibited by the specific substance P antagonist, [D-Pro2,D-Trp7,9]substance P, which suggests that substance P stimulates enteric cholinergic neurons through a specific site of action. [D-Met2,Pro5]enkephalinamide and dynorphin-(1-13) also decreased whereas naloxone increased the sustained response to substance P. The results indicate that the enteric cholinergic neurons, which are stimulated by substance P, are also under the control of enkephalin and/or dynorphin neurons.

Evidence that the contractile response of the guinea-pig ileum to capsaicin is due to release of substance P

1. The possible roles of substance P and opioids in the contractile response of the isolated guinea-pig ileum to the sensory stimulant drug capsaicin were investigated, and the contractions were found to be inhibited by about 60% in preparations desensitized to substance P. 2. Contractions evoked by stimulation of the mesenteric nerves in the presence of the adrenergic blocking drug guanethidine were inhibited by about 75% after the ileum had been rendered insensitive to substance P. 3. Atropine partially inhibited the effect of capsaicin. The atropine-resistant component of the contractile response to capsaicin was inhibited by more than 85% in preparations desensitized to substance P and almost abolished by the substance P antagonist, (D-Pro2,D-Trp7,9)-substance P. 4. The opioid peptide (D-Met2, Pro5)-enkephalinamide inhibited, whereas the opiate antagonist naloxone enhanced the atropine-resistant contractions in response to capsaicin. 5. The results indicate that the contractile response of the guinea-pig ileum to capsaicin and mesenteric nerve stimulation is mediated by release of substance P, presumably from sensory nerve endings in the gut. Substance P appears to act on the smooth muscle both directly and indirectly via cholinergic neurones. It is proposed that opioids modulate the non-cholinergic response to capsaicin by inhibiting the release of substance P.

Indirect evidence for the inhibition of enteric substance P neurones by opiate agonists but not by capsaicin

There is good evidence indicating that hyoscine-resistant contractions of the guinea-pig ileum evoked by stimulation of the intramural nerves are mediated by substance P (SP). In the present experiments, non-cholinergic neurogenic ileum contractions to field stimulation (100 imp., 5-50 Hz) were inhibited by the opiate agonists morphine and [D-Met2,Pro5]enkephalinamide (10(-6) M) in a naloxone-reversible manner. Neither morphine nor naloxone influenced the musculodirect contracting effect of histamine. Capsaicin, a drug that has been shown to deplete SP from primary afferent neurones, exerted no long-lasting effect on non-cholinergic contractions to field stimulation. Repeated administration of long trains of stimuli (900 impulses) resulted in a progressive decrease of the contractions evoked. Addition of naloxone (3 X 10(-7) M) restored the original height of the responses. The above inhibitory action of opiate agonists and of repeated long-train stimulation was 3-6 times greater at 5 Hz than at 50 Hz. It is concluded that opiate agonists inhibit the release of SP from intramural neurones of the guinea-pit ileum. The decrease in responses to repeated long-train stimulations is mediated, at least in part, by the release of endogenous opioid substance(s).

Antagonism of cholecystokinin-like peptides by opioid peptides, morphine or tetrodotoxin

Morphine, beta-endorphin, Met-enkephalin, and Leu-enkephalin antagonized intestinal actions of cholecystokinin octapeptide (CCK-8), caerulein, and pentagastrin in a manner partly suggesting physiologically competitive antagonism. Further, these acidic peptides (CCK-8, caerulein, pentagastrin) were much more sensitive to the actions of opioids than was angiotensin. Tetrodotoxin also caused changes in the concentration-effect curves, but these were different from the shifts due to the opioids and differentiated between CCK-8, caerulein, and pentagastrin. Naloxone did not modify the response to CCK-8 and caerulein, but completely abolished the antagonistic influence of the opioids. The potencies of morphine and the opioid peptides as antagonists of CCK-8, were of nearly the same order of magnitude. This and the presence in gut and brain of both CCK-like and opioid peptides suggests the hypothesis that these two groups of peptides interact on both myenteric and central nervous system receptors, and thus are directly involved in the regulation of both intestinal motility and satiety.

Effects of intra-arterial bradykinin and substance P on isolated, blood-perfused small intestine of the rat

Experiments were conducted on rat isolated, small intestine perfused at a fixed flow rate through the superior mesenteric artery with arterial blood from a donor rat, to determine the responses of the ileum to different peptides. Drugs were closely injected into the superior mesenteric artery. Single injections of bradykinin produced monophasic fast contractions of the ileum preceded by an initial fall and a subsequent rise of tone. The fast contraction was abolished by tetrodotoxin (TTX), morphine and hexamethonium (C6), but was resistant to blockade by atropine or mepyramine. Changes in ileal tone induced by bradykinin remained evident even in the presence of these blocking agents, thereby suggesting a direct action on smooth muscle fibers of the ileum. The fast contraction in response to substance P was not influenced by either TTX, morphine, C6, atropine or mepyramine. The present results indicate that bradykinin induces the fast contraction of the ileum by excitation of myenteric neuronal elements involving cholinergic interneurons, while substance P produces the contraction by a direct stimulation of smooth muscle fibers of the ileal region.