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Has the presence of distinct types of muscarine receptors in the LES been detected in man as in animals ?
J.R. Crist (Boston)
There is good evidence to suggest that intramural cholinergic neurons present in the LES cause contraction of the LES through activation of muscarine receptors on circular smooth muscle celles (figure /) [2, 5]. This is evidenced by a number of findings including: 1) muscarine agents cause contraction of the LES in man and various other species, 2) binding studies in the cat LES demonstrate muscarinic cholinergic receptors and, 3) intramural cholinergic neurons have been identified in the opossum LES using acetylcholinesterase and choline acetyltransferase histochemical techniques [4]. These cholinergic neurons exert their influence on LES tone by releasing the neurotransmitter acetylcholine which stimulate muscarinic receptors on the LES circular smooth muscle cells. There is continued controversy as to what role continuous release of acetylcholine from these intramural nerves might play in resting LES tone in man. In most studies, the muscarinic antagonist atropine causes a small decrease in resting LES tone.
Figure 1. Cholinergic motorneuron innervating circular smooth muscle of the LES. Acetylcholine is released at the neuromuscular junction resulting in activatin of muscarinic receptors and contraction of the smooth muscle.
There is also good evidence to suggest that muscarinic receptors are present on the soma or dendrites of noncholinergic inhibitory neurons in the LES (figure 2) [2, 5]. Activation of these muscarinic receptors cause release of an unknown inhibitory neurotransmitter at the neuromuscular junction resulting in relaxation of the LES [ 1 ]. Since the relaxation produced by activation of muscarinic receptors on inhibitory neurons is nerve-mediated, it is blocked by the nerve blocker tetrodotoxin in addition to the muscarinic antagonist atropine.
Until recently, all muscarinic receptors were believed to be alike. However, it has become clear from studies in a variety of tissues that at least three different types can be identified on a pharmacological basis and at least five different types can be identified on a molecular basis. Recent remarkable advances in the characterization of different muscarinic subtypes has resulted in some confusing non-standardized nomenclature of subtypes. However, nomenclature based on pharmacological studies has defined three muscarinic receptor subtypes known as Ml (which has a high affinity for the antagonist pirenzepine); M2 (which has a high affinity for the antagonist gallamine), and M3 (which has a high affinity for the antagonist 4-DAMP) [3].
Studies by Gilbert et al. [2] in the opossum were the first to demonstrate that muscarinic receptors located on inhibitory neurons in the LES differed from those on the circular smooth muscle cells of the LES [2]. These studies involved monitoring LES pressures in anesthetized animals and infusion of muscarinic agonists McNA-343 and bethanechol into the arterial supply. McNA-343 caused
Figure 2. Inhibitory motorneuron innervating circular smooth muscle of the LES. Muscarinic receptors are located on the soma of the neuron. Their activation by acetylcholine results in the release of an unknown inhibitory neurotransmitter at the neuromuscular junction causing relaxation of the LES.
relaxation of the LES after a brief contraction and bethanechol caused a prolonged contraction.
The Ml antagonist pirenzepine antagonized the McNA-343 induced relaxation but did not modify the contraction occurring with either McNA-343 or bethanechol. The M3 muscarinic antagonist 4-DAMP on the other hand did not modify the McNA-343 induced relaxation but antagonized the sphincter contraction caused by McNA-343 or bethanechol. These findings provided evidence that Ml muscarinic receptors are present on intramural inhibitory neurons causing relaxation of the sphincter and M3 muscarinic receptors are present on sphincter muscle causing sphincter contraction. Stimulation of M1 muscarinic receptors on neurons and M3 muscarinic receptors on smooth muscle have been shown to cause excitation through entirely different mechanisms. Stimulation of M1 muscarinic receptors on neurons cause excitation through a decrease in potassium ion conductance. Three types of K+ channels have been shown to be involved including voltage-independent, M-current, and calcium-dependent K+ channels. Stimulation of M3 muscarinic receptors on smooth muscle cause excitation through an increase in phophatidylinositol turnover resulting in mobilization of intracellular calcium and an influx of extracellular calcium causing muscle contraction [3].
Presently, there are no agonists with high selectivity for the various muscarinic subtypes. The experimental compound McNA-343 is somewhat selective for Ml receptors. Carbachol is an agonist for all subtypes with somewhat greater binding to M3 receptors than Ml receptors. This lack of selective agonists limits our ability to pharmacologically manipulate muscarinic receptors in the LES.
Nonetheless, bethanechol or carbachol are used to improve LES tone in man most likely through their actions on M3 receptors. Moreover, prokinetic agents such as metoclopramide and cisapride appear to increase LES tone through the release of acetylcholine from intramural cholinergic nerves which activate M3 muscle receptors causing LES muscle contraction. One can expect that the development of selective muscarinic agonists will have an important role in the treatment of decreased LES tone seen in some patients with gastroesophageal reflux disease.
At present, unwanted side effects due to the lack of muscarinic subtype selectivity severely limits the use of muscarinic agents in the pharmacological manipulation of LES tone [3].
References
1. Christie MJ, North RA (1988) Control of ion conductances by muscarinic receptors. Trends Pharmacol Sci [Supplement]: 9 : 30-34.
2. Gilbert R, Rattan S, Goyal RK (1984) Pharmacologic identification, activation and antagonism of two muscarine receptor subtypes in the LES. J Pharmacol Exp Ther 230 : 284-291.
3. Goyal RK (1988) Identification, localization and classification of muscarinic receptor subtypes in the gut. Life Sci 43 : 2209-2220.
4. Goyal RK, Paterson WG. (1989) Esophageal Motility. In: Handbook of Physiology - The Gastrointestinal System I. Edited by JD Wood. Bethesda, MD. American Physiological Society, Chapter 22, p. 865-908.
5. Rattan S, Goyal RK (1984) Identification of Ml and M2 muscarinic receptor subtypes in the control of the LES in the opossum. Trends Pharmacol Sci [Suppl]: 78-81.
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