The chemical transmitter of motor impulses to the stomach

Lesioning of TRPV1 expressing primary afferent neurons prevents PAR-2 induced motility, but not mechanical hypersensitivity in the rat colon

BACKGROUND

Proteinase activated receptor 2 (PAR-2) is expressed by many neurons in the colon, including primary afferent neurons that co-express transient receptor potential vanilloid 1 (TRPV1). Activation of PAR-2 receptors was previously found to enhance colonic motility, increase secretion and produce hypersensitivity to mechanical stimuli. This study examined the functional role of TRPV1/PAR-2 expressing neurons that innervate the colon by lesioning TRPV1 bearing neurons with the highly selective and potent TRPV1 agonist resiniferatoxin.

METHODS

Colonic motility in response to PAR-2 activation was evaluated in vitro using isolated segments of descending colon and in vivo using manometry. Colonic mechanical nociceptive thresholds were measured using colorectal distension. Transient receptor potential vanilloid 1 expressing neurons were selectively lesioned with resiniferatoxin.

KEY RESULTS

In vitro, the PAR-2 agonists, trypsin and SLIGRL did not alter contractions of colon segments when applied alone, however, the agents enhanced acetylcholine stimulated contraction. In vivo, PAR-2 agonists administered intraluminally induced contractions of the colon and produced hypersensitivity to colorectal distention. The PAR-2 agonist enhancement of colonic contraction was eliminated when TRPV1 expressing neurons were lesioned with resiniferatoxin, but the PAR-2 agonist induced hypersensitivity remained in the lesioned animals.

CONCLUSIONS & INFERENCES

Our findings indicate that TRPV1/PAR-2 expressing primary afferent neurons mediate an extrinsic motor reflex pathway in the colon. These data, coupled with our previous studies, also indicate that the recently described colospinal afferent neurons are nociceptive, suggesting that these neurons may be useful targets for the pharmacological control of pain in diseases such as irritable bowel syndrome.

Involvement of TRPV2 activation in intestinal movement through nitric oxide production in mice

Transient receptor potential channel vanilloid 2 (TRPV2) can detect various stimuli such as temperature (>52 °C), stretch, and chemicals, including 2-aminoethoxydiphenyl borate, probenecid, and lysophospholipids. Although expressed in many tissues, including sensory and motor neurons, TRPV2 expression and function in the gastrointestinal tract is poorly understood. Here, we show TRPV2 expression in the murine intestine and its involvement in intestinal function. Almost all mouse intestinal intrinsic sensory and inhibitory motor neurons, both cell bodies and nerve fibers, showed TRPV2 immunoreactivity. Several known TRPV2 activators increased cytosolic Ca²+ concentrations and evoked TRPV2-like current responses in dissociated myenteric neurons. Interestingly, mechanical stimuli activated inward currents in a strength-dependent manner, which were inhibited by a TRPV2 inhibitor tranilast. TRPV2 activation in isolated intestine inhibited spontaneous circular muscle contraction, which did not occur in the presence of the TRPV2 antagonist, tetrodotoxin or nitro oxide (NO) synthase pathway inhibitors. Also, increased intestinal NO production was observed in response to a TRPV2 agonist, and gastrointestinal transit in vivo was accelerated by TRPV2 agonists or an NO donor. In conclusion, TRPV2 may contribute to intestinal motility through NO production, and TRPV2 is a promising target for controlling intestinal movement.

Non-adrenergic non-cholinergic (NANC) transmission to smooth muscle: 35 years on

In 1963, two substances were thought to mediate all transmission between neurons, as well as between nerve and muscle in the peripheral nervous system, namely acetylcholine and noradrenaline. This paradigm primarily was due to the research of Dale, Loewi and von Euler in the first half of the century [Dale, 1937 (Transmission of nervous effects by acetylcholine, Harvey Lect. 32, pp. 229-245)]. However, in 1963, a series of experiments were carried out using recently introduced electrophysiological techniques, which showed unequivocally for the first time that the classical paradigm was not correct. Both inhibitory and excitatory junctions between nerves and smooth muscle cells were shown to exist in which transmission was mediated by non-adrenergic, non-cholinergic (NANC) transmitters. In the succeeding 35 years, identification of these NANC transmitters has been a major task of neuropharmacology, with nitric oxide, neuropeptides, and purines being isolated. This review presents an historical account of the developments this century of the classical paradigm, of how it was displaced, and of the progress made in identifying the neuromuscular transmitters of the autonomic nervous system.

Investigation of catecholamine effects on stomach and duodenum motor functions in unanaesthetized dogs

The aim of this study was to investigate the effects of catecholamines on motility of stomach and intestine in chronic experiments on dogs with fistulas in stomach and duodenum. The contractions of stomach and duodenum were registered by a balloon method. It was established that i.v. adrenaline and noradrenaline injections in dogs with intact vagi inhibited food-induced motility in stomach and duodenum and did not produce contractions in the empty and quiescent gastrointestinal tract. Beta-adrenoagonist isoprenaline on the contrary produced stomach and duodenum contractions in fed dogs during the quiescent period. In fed dogs isoprenaline caused a 3-phasic reaction: a phase of primary short-term suppression of stomach and duodenum motility, a phase of motility increase and a phase of secondary suppression of stomach and duodenum motility. The contractions evoked by isoprenaline in the stomach resemble in amplitude and shape, periodical activities which are typical for fasting dogs. Vagotomy intensified isoprenaline motor effects in hungry and fed dogs and eliminated the phase of primary suppression in fed dogs. The isoprenaline effects were blocked by propranolol, but not by phentolamine. They were decreased more effectively in the stomach by atropine and by hexomethonium in the duodenum.

Gastric and pyloric motor response to sympathetic nerve stimulation after chemical sympathectomy

This study evaluated the effect of electrical stimulation of the greater splanchnic nerve on gastric and gastroduodenal motility in 6-hydroxydopamine (6-OHDA)-pretreated anesthetized dogs. Biopsies from the stomach, pylorus and duodenum were obtained and analyzed by catecholamine fluorescence microscopy. Degeneration of adrenergic terminals in Auerbach's plexus was complete at 1 week. The majority of stimulations in control dogs or dogs treated with 6--OHDA produced contractions in the gastric body, antrum, and pylorus and relaxation of inhibition in the duodenum. The excitatory motor responses in the gastric body, antrum and pylorus were unaffected by the administration of hexamethonium. Following atropine administration, contractions due to splanchnic nerve stimulation virtually were abolished in the antrum, pylorus and duodenum and were reduced in number in the gastric body. Relaxation and inhibition were no longer observed in the duodenum after atropine. The greater splanchnic nerve appears to contain both excitatory are inhibitory cholinergic pathways to the musculature of the stomach, pylorus, and duodenum.

Ileus: the restoration of alimentary-tract motility by pharmacological means

The nature of ileus and the clinical problem it poses are discussed. A consideration of the previous multiplicity of treatments is presented. The concepts behind the present mode of treatment are outlined. This consists of removing the sympathetic inhibition of the alimentary tract in the condition and potentiating or supplementing tone. Case histories are presented to illustrate the use of the drugs suggested, and the indications for therapy are discussed. The mistaken concept that the gut is “paralysed” in ileus should be finally abandoned.