Reversal of nicotine action on the intestine by atropine

Postnatal changes in vagal control of esophageal muscle contractions in rats

AIMS

Replacement of smooth muscles by striated muscles occurs in the esophagus during the early postnatal period. The aim of this study was to clarify postnatal changes in vagal control of esophageal muscle contractions in rats.

MAIN METHODS

An isolated segment of the neonatal rat esophagus was placed in an organ bath and the contractile responses were recorded using a force transducer.

KEY FINDINGS

Electrical stimulation of the vagus trunk evoked a biphasic contractile response in the neonatal esophageal segment. The first and second components of the contractions were inhibited by α-bungarotoxin and atropine, respectively. Ganglion blockers, hexamethonium and mecamylamine, did not affect vagally mediated contractions. The first component gradually enlarged with age in days, whereas the second component declined during the first week after birth. Application of d-tubocurarine or acetylcholine caused an apparent contraction in the esophageal striated muscle at postnatal day 0, but responses to these drugs were not observed at 1 week after birth. The neonatal esophagus expressed the γ-subunit of nicotinic acetylcholine receptors. In contrast, the ε-subunit was dominantly expressed in the adult esophagus.

SIGNIFICANCE

The vagus nerves directly innervate both the esophageal striated muscles and smooth muscles in the early neonatal period. During the process of muscle rearrangement, the property of the striated muscles is altered substantially. The specific features of striated muscles in the neonatal rat esophagus might compensate for immature formation of neuromuscular junctions. Unsuccessful conversion of the striated muscle property during postnatal muscle rearrangement would be related to disorders of esophageal motility.

Nicotine and the effect of antisympathomimetic agents on the aorta of the rabbit

The responses of strips of rabbit aorta to almost maximal doses of nicotine were less readily antagonized by five antisympathomimetic agents than were comparable responses to noradrenaline. The effect was most marked with dibenamine, ergotamine, and tolazoline: approximately twice the dose of noradrenaline was required to match the test dose of nicotine after treatment with the antagonists. Dose/response curves for nicotine before and after phentolamine 10(-7) indicate that the phenomenon may be reversed with low doses of nicotine and that the release of noradrenaline by nicotine within the tissues is probably a graded response. The pattern of nicotine/phentolamine antagonism in this preparation is consistent with the view that nicotine acts indirectly by releasing a noradrenaline-like substance, and the difficulty found in antagonizing responses to nicotine with antisympathomimetic agents is probably similar to that responsible for failure of atropine to block some parasympathomimetic responses to nicotine.

Action of nicotine on the rabbit muscular organ (ileo-colic sphincter)

The effects of nicotine on the longitudinal muscle of the rabbit muscular organ (ileo-colic sphincter) have been studied. Nicotine in doses from 1 to 10 mug/ml. inhibits the pendular movements of the muscle. This inhibition can also be produced by acetylcholine in the presence of atropine. In both instances, the inhibitory effect can be blocked by ephedrine, hexamethonium and bretylium. Doses of hexamethonium which block the inhibitory action of nicotine reveal an excitatory action, which itself can be blocked by higher doses of hexamethonium. Bretylium blocks both actions of nicotine at the same dose level. It is suggested that nicotine produces inhibition by releasing catechol amines, the site of action being a cholinergic junction between sympathetic nerve and catechol amine store, and further that bretylium blocks the action of acetylcholine and nicotine at this site as well as at the parasympathetic ganglia.

Nitric oxide and the non-adrenergic non-cholinergic neurotransmission

In the early 1960s, the first evidence was reported demonstrating neurally mediated responses in the presence of adrenergic and cholinergic antagonists, leading to the introduction of the concept of non-adrenergic non-cholinergic neurotransmission. The inhibitory component of this part of the autonomic nervous system has been illustrated in numerous organ systems mediating a wide range of physiological events. Since the discovery of these nerves, several substances have been proposed as putative neurotransmitter, with ATP and vasoactive intestinal polypeptide as main candidates. Finally, the ongoing research on the nature of the substance released by these nerves has generated the nitrergic theory proposing nitric oxide as putative neurotransmitter. By now, increasing evidence is reported to support the idea that inhibitory neurons release more neurotransmitters, interacting with each other at pre- and/or postsynaptic levels.

Sympathetic postganglionic cholinergic fibres. 1959

When the postganglionic fibres to the nictitating membrane are stimulated in a cat treated with reserpine, the membrane contracts. The contraction is increased after the injection of eserine and is abolished with atropine. Thus the fibres stimulated appear to be cholinergic. When the splenic nerve is stimulated in a cat treated with reserpine there is similar evidence of the presence of cholinergic fibres; the spleen dilates, the dilatation is greater in the presence of eserine and is abolished by atropine. The hypogastric nerves when stimulated cause inhibition of the uterus of the virgin cat. When the cat is treated with reserpine, stimulation then causes contraction which is increased in the presence of eserine and abolished by atropine. There is also evidence of cholinergic fibres in the postganglionic supply to the vessels of the rabbit's ear. From this and other evidence it is suggested that a form of adrenergic mechanism may exist such that cholinergic fibres in the postganglionic sympathetic supply liberate acetylcholine; this in turn liberates noradrenaline from a store around the sympathetic nerve ending.

The relaxing response of the isolated rat duodenum to nicotine

1. The relaxing responses of the atropine-treated isolated rat duodenum to 12 microM nicotine (N) and transmural electrical stimulation (S) have been compared. 2. The effects of both N and S were unaffected by 3-30 microM guanethidine or previous reserpinization (1 mg/kg, i.p. during 5 days). 3. The effects of N and S were completely blocked by 0.5-2.0 mM procaine, 0.1 mM butacaine or 0.3-1.2 microM tetrodotoxin. 4. The relaxing response to N but not that to S was blocked by 0.3 mM hexamethonium or 3 microM mecamylamine and absent in the nicotine-desensitized preparation. 5. The effects of both N and S were unaffected in the adenosine-5'-triphosphate (ATP)-desensitized preparation. 6. It is suggested that the nonadrenergic and noncholinergic (NANC) relaxation of the rat duodenum in response to N and S is also nonpurinergic.

Neural inhibition of gastrointestinal smooth muscle motility in guinea-pigs

The neural inhibition of the gastrointestinal (GI) smooth muscle motility was studied by means of electrical transmural nerve stimulation (ETNS) in guinea-pigs. In untreated muscle strips, ETNS induced four types of response, consisting of three basic components, i.e., contraction and relaxation during ETNS, and after-contraction. Following atropinization, all the responses changed to only one type, i.e., relaxation during ETNS followed by after-contraction. The relaxation of the muscle strips induced by ETNS after atropinization was not significantly reduced under superimposing guanethidine treatment in any site of the GI tract. The maximal relaxation of the muscle strips induced by ETNS after atropine and guanethidine treatments was not uniform throughout the GI tract. The relaxation of the gastric body and colon was greater than that of the jejunum and ileum. The extent of the relaxation was significantly different even in the colon. Theophylline and phentolamine did not reduce the ETNS-induced relaxation following atropine and guanethidine treatments in the distal colon and taenia coli. The findings suggest the following: the non-adrenergic inhibitory (NAI) nerve is the main postganglionic nerve to inhibit the GI smooth muscle motility; and the role of adrenergic nerve is of little importance in respect to postganglionic inhibition. The innervation of the NAI nerve seems not to be uniform throughout the GI tract. Theophylline and phentolamine are not the specific antagonists of the NAI nerve.

The actions of some cholinomimetic drugs on the isolated taenia of the guinea-pig caecum

1. The actions on the taenia of 4-(m-chlorophenylcarbamoyloxy)-2-butynyl-trimethylammonium chloride (McN-A-343), N-benzyl-3-pyrrolidyl acetate methobromide (AHR-602), tetramethylammonium (TMA) and choline phenyl ether have been examined and compared with the actions of acetylcholine, nicotine and 1,1-dimethyl-4-phenylpiperazinium (DMPP).2. Responses of the taenia to these agonists are quantitatively and often qualitatively dependent on the tone of the preparation. A method is described which makes allowance for the effect of tone on heights of contractions.3. Acetylcholine, McN-A-343 and AHR-602 produced only contractions; TMA produced contractions or biphasic responses; and choline phenyl ether, nicotine and DMPP produced contractions, relaxations or biphasic responses.4. The mode of action of these compounds has been analysed by means of hyoscine, ganglion-blocking drugs, tetrodotoxin and local anaesthetics.5. It is concluded that acetylcholine, McN-A-343, AHR-602, TMA and choline phenyl ether act on muscarinic receptors in the smooth muscle. Choline phenyl ether has an additional action on nicotinic receptors of cholinergic neurones. Nicotine and DMPP act on nicotinic receptors of cholinergic neurones and of inhibitory neurones. An action on the latter is sometimes also seen with TMA and choline phenyl ether.6. With nicotine or DMPP, and TMA or choline phenyl ether in the presence of hyoscine, part of the contraction phase of biphasic responses (in which a contraction follows relaxation) is best explained as being triggered by the initial relaxation-that is, as a "rebound contraction".7. None of the compounds tested appeared to exert an atropine-sensitive action on neurones.8. In the presence of hyoscine high concentrations of agonists can act on sites not involved with lower concentrations.

Atropine-resistant longitudinal muscle spasms due to excitation of non-cholinergic neurones in Auerbach's plexus

1. In accordance with the dual histology of Auerbach's plexus (Dogiel, 1899; Hill, 1927) two types of neurone can be shown to be humorally active in plexus-containing preparations of longitudinal muscle from guinea-pig ileum, taken at measured distances up to 95 cm above the ileocaecal valve, when such preparations are stimulated electrically under different conditions.2. The rapid twitch, lasting 3-8 sec, which is elicited by single shocks of 0.1 or 0.2 msec pulse width, and the effect of 5-15 ng doses of acetylcholine which matched this twitch, were both extinguished equally effectively and completely by atropine sulphate (0.4-1 x 10(-8) g/ml.) or by hyoscine hydrobromide. This twitch-response is therefore caused by an excitation of cholinergic motor neurones of normal susceptibility to atropine. These are believed to be the Dogiel (1899) Type II cells of Auerbach's plexus, as suggested by Hill (1927).3. After extinction of the twitch by the invariably effective atropineblock, a second type of muscle response was revealed by tetanic stimulation with 1 sec trains of 50 pulses of the same voltage and of pulse width preferably 0.2 msec. The tetanic responses consisted of spasms of longer delay and duration (20-60 sec). These spasms could be matched by doses of acetylcholine of the order of 200 ng. However, if the atropine concentration was now raised to 10(-7), or even 10(-6) g/ml., the effect of 200-1000 ng of acetylcholine was abolished, but the tetanic spasms persisted without decrease in amplitude. In other experiments the height of the spasms remained constant as the concentration of atropine sulphate was raised from 10(-8) to 10(-6) g/ml. and was only slightly decreased by 10(-5) g/ml. Hence, these tetanic contractions are not due to a surmounting of the atropine-block by the increased release of acetylcholine following the 50 pulses.4. The tetanic spasms originate from excitation of non-cholinergic neurones, perhaps the associative Dogiel Type I cells of Auerbach's plexus (Hill, 1927), since the spasms were abolished reversibly by tetrodotoxin 2 x 10(-7) g/ml. and were absent from plexus-free, nicotine-insensitive preparations of the longitudinal muscle, both before and after atropinization.5. The tetanic spasms were not reduced by ganglion-block with paralysing doses of nicotine, with (+)-tubocurarine or with hexamethonium.6. The tetanic spasms are not mediated by a release of 5-hydroxytryptamine (5-HT) or of histamine, since they persisted in concentrations of methysergide and mepyramine adequate to block matching doses of histamine or 5-HT, or multiples thereof. Catecholamines were also excluded.7. The tetanic spasms are not mediated by a release of a prostaglandin, because they were not reduced by 0.5-2 x 10(-6) g/ml. of patulin (Ambache, 1957), which blocked the contractions evoked by matching doses of prostaglandins PGE(2) or PGF(2alpha); even after this block, PGE(2) still potentiated subsequent tetanic responses.8. The tetanic spasms were reduced or virtually abolished by strychnine in concentrations which did not depress the twitch.

Pharmacological responses of the isolated innervated intestine and rectal caecum of the chick

Images

The inhibitory innervation of the taenia of the guinea-pig caecum

1. The inhibitory innervation of the taenia of the guinea-pig caecum has been studied, after blocking the responses to stimulation of excitatory cholinergic nerves with atropine.2. Stimulation of the perivascular nerves supplying the taenia caused relaxations. These nerves had properties which were typical of sympathetic post-ganglionic adrenergic nerves. The relaxations caused by stimulation were maximal at frequencies of stimulation above 30 pulses/sec and they were abolished by bretylium, guanethidine and 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP).3. The taenia is also innervated by intramural inhibitory nerves with their cell bodies in Auerbach's plexus. These nerves can be excited by electrical stimulation of the taenia or by the application of ganglion-stimulating drugs.4. The intramural inhibitory nerves have different properties from sympathetic adrenergic nerves. Relaxations in response to stimulation were maximal with frequencies of stimulation of about 5 pulses/sec and they were not blocked by bretylium, guanethidine or DMPP.5. Preganglionic cholinergic fibres in the caecal wall make synaptic connexions with the intramural inhibitory neurones.6. The role of the intramural inhibitory neurones in intestinal activity and their possible connexions with the central nervous system have been discussed.

The reactions of plexus-free circular muscle of cat jejunum to drugs

Images