Department of Physiology, Okayama University Medical School

Transcription

1 The Japanese Journal of Physiology 15, pp , 1965 Department of Physiology, Okayama University Medical School BAYLISS and STARLING 1) and others 6, 7, 9, 12, 14, 15) have reported that the stimulation of the peripheral end of the cervical or thoracic vagus produces inhibition, as well as excitation, of the movements of the small intestine. They not, however, investigate the course of the nerve fibers causing the inhibitory and excitatory effects and their synapses. HUKUHARA and co-workers 8) have confirmed that the intrinsic intestinal reflex (the local reflex) consists of the mucosal and muscular reflexes, and they have supposed that the reflex arc of the former contains the centrifugal inhibitory and excitatory neurones and that their cell bodies are located in Auerbach's plexus. It has been supposed by HILL5) that many of the ganglion cells in Auerbach's plexus are postsynaptic neurones of the vagus to the intestine. I have supposed that both the inhibitory and excitatory effects are produced by the stimulation of the vagus, because the presynaptic fibers of the vagus would form the synapses with both the inhibitory and excitatory neurones in relation to the intrinsic mucosal reflex in Auerbach's plexus. Therefore, I have carried out the vagal stimulation experiment as below and have consequently obtained concrete evidence in confirmation of the above-described hypothesis. Dogs were usually anesthetized with urethane (1g/kg) and morphine hydrochloride (3mg/kg); occasionally, pentobarbital sodium (25mg/kg) was injected intravenously, or only urethane was used. During artificial respiration, the stimulation was applied at the cervical vagus or at the dorsal root in the region just above the diaphragm in the thoracic cavity. In order to eliminate the reflex effects through the centripetal pathways, the cervical vagi and the great and small splanchnic nerves were cut bilaterally. Movements of the jejunal loop, which is about 15 cm in length and which was isolated from the other parts of the intestine, were recorded by way of a balloon introduced into the jejunal lumen, a water manometer and Marey's tambour. The electrical stimulus was usually repetitive square waves with a frequency of 20/sec, 4.5

2 msec in duration, and of varied intensities. n order to block the function I of the nervous elements in the intestinal wall, cocaine hydrochloride, tubocurarine hydrochloride, nicotine tartarate, hexamethonium bromide, and atropine sulfate were used. 1. Effects on jejunal movements of the stimulation of the vagus nerve. The relatively weaker stimuli produced in general only the excitatory response (FIG. 1A); on the other hand, the stronger stimuli elicited the inhibitory response (FIG. 1B, 4A and 6A), or one remarkable increase in amplitude of the contraction wave, followed by diminution of the movements in amplitude and tone (FIG. 2A, 3A and 5A). Immediately after cessation of the stimulation, the contraction waves became greater in amplitude and tone than before the stimulation. FIG. 1. Effects of stimulation of the thoracic vagus on the jejunal movements. A: Weaker stimulation produced the excitatory response. Parameter of the stimulus: 4.5msec duration, 10/sec frequency, 4.5 volts intensity. B: Stronger stimulation caused the inhibitory one. Parameter of the stimulus: 4.5msec duration, 50/sec frequency, 4.5 volts intensity. Time in 6 sec. 2. Effects of the stimulation of the vagus after application of various drugs. A. Effect of cocaine. When the vagal inhibitory effect described above was recorded, 10-15ml of 0.4% cocaine solution dissolved in normal saline was introduced into the intestinal lumen. Three to eight minutes after cocainization, the stimulation of the vagus produced excitatory effects, which had been inhibitory prior to cocainization (FIG. 2A and B). About one hour after washing and filling the intestinal lumen with Ringer solution, the inhibitory response of the vagus was again obtained (FIG. 2C), i. e. the function of cocaine was reversible. The effect of cocaine on the stimulation of the cervical vagus was essentially the same as that of the thoracic. It may be considered that

3 VAGAL ACTION ON INTESTINAL MOVEMENTS FIG. 2. Effects of stimulation of the cervical vagus on the jejunal movements after cocainization. A: Before cocainization. B: 8 min after application of 0.4% cocaine solution to the intestinal lumen. C: 70 min after cocaine solution was replaced by Ringer solution. 5.5v indicates 5.5 volts intensity. Time in 6 sec. The explanations are applied to the subsequent figures. the reverse phenomenon after cocainization is due to a unique action of cocaine on the inhibitory nervous element in the intestinal wall through the mucosa. B. Effect of tubocurarine. After recording the inhibitory effect of the vagus (FIG. 3A), tubocurarine (0.1 mg/kg) was injected intravenously. The jejunal movements about 15 sec after the injection of tubocurarine decreased remarkably, accompanied by diminution in tone. In such a condition the vagal stimulation produced slightly excitatory response, but about 10 minutes after the injection, the excitatory response became more remarkable. The stimulation of the vagus after the injection of 0.2 mg/kg tubocurarine evoked no excitatory response for about 8 min; however, 9 min after injection of tubocurarine, the excitatory response was produced which became striking in the course of time (FIG. 3B). The same inhibitory response as prior to the application of tubocurarine was again produced more than 2 hours after the injection, but in some of our experiments the inhibitory effect was not produced again until the end of the experiment. C. Effect of nicotine. The intravenous injection of 3 mg/kg nicotine abolished the jejunal movements completely for 4-5 min, after which time they increased gradually. The stimulation of the vagus, however, caused no response on jejunal movements, but about one and half hours after the injection, the stimulation of the vagus produced the excitatory response converse to that

4 FIG. 3. Effects of stimulation of the thoracic vagus on the jejunal movements after administration of tubocurarine. A: Before administration of tubocurarine. B: 21min after intravenous injection of 0.2mg/kg tubocurarine. FIG. 4. Effects of stimulation of the thoracic vagus on the jejunal movements after administration of nicotine. A: Before administration of nicotine. B: 90min after intravenous injection of 3mg/kg nicotine. C: 2 hours after injection of nicotine,

5 VAGAL ACTION ON INTESTINAL MOVEMENTS before the injection (FIG. 4B). About two hours after the injection of nicotine, the jejunal movements exhibited effects similar to those of its normal state (FIG. 4C), i. e. the summation of the excitatory and inhibitory responses or the initial excitatory effect followed by the inhibitory one. In one experiment, in which 5 mg/kg of nicotine was intravenously injected, the vagal stimulation was not effective for 60 minutes, but 74 minutes after the injection the excitatory response was produced for 3 hours. However, no inhibitory effect could again be observed. D. Effect of hexamethonium bromide (C6). The effect of hexamethonium bromide was unlike that of cocaine, tubocurarine and nicotine in that it was irregular and indefinite. However, in one case, about 1.5 min after the intravenous injection of 2.5 mg/kg C6, the vagal stimulation produced only the excitatory response without the inhibitory (FIG. 5B) and the same was true again FIG. 5. Effects of stimulation of the thoracic vagus on the jejunal movements after injection of C6. A: Before administration of C6. B: After intravenous injection of C6. In H, 2.5 mg/kg C6 was intravenously injected. when the stimulation was applied two hours later. In another case, in which the inhibitory effect was produced by the stimulation of the vagus and was followed by the excitatory after cessation of the stimulation (FIG. 6A), the stimulation 1.5 min after the intravenous injection of 1.5 mg/kg C6 resulted in no response (FIG. 6B), but 13 min later, during the stimulation, a slight inhibition was evoked and was followed by notable excitation after cessation of the stimulation, similar to that before the application of C6 (FIG. 6C). E. Effect of atropine. In a case where the stimulation of the vagus resulted in the diphasic responses of the jejunum (FIG. 7A), after the intravenous injection of 0.5 mg/kg atropine the excitatory response of the vagus was reversed to the inhibitory one (FIG. 7B).

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7 FIG. 7. Effect of stimulation of the thoracic vagus on the jejunal movements after admininistration of atropine. A: Before injection of atropine. B: After intravenous injection of 0.5mg/kg atropine. According to HUKUHARA and co-workers 8), there are at least two kinds of the effector neurones in the myenteric plexus of the intestine with regard to the intrinsic intestinal reflex: the excitatory and the inhibitory neurones. AMBACHE 1) and AMBACHE and EDWARDS 2) have reported that the excitatory effect of nicotine on the small intestine in vitro was reversed to the inhibitory one after the administration of botulinum toxin or atropine, and they therefore have assumed that there are two kinds of functionally distinct ganglion cells in the myenteric plexus: one cholinergic, the other adrenergic. Assuming that the excitatory as well as inhibitory neurones connect with the preganglionic fibers in the vagus synaptically, the results described above can easily be understood. The vagal inhibitory response after atropinization was observed by BAYLISS and STARLING 3) in the small intestine of the cat and by YAMAGAMIl 7) in the stomach of the dog. It is generally accepted 9) that

8 the administration of atropine diminishes the excitatory response of the vagus on the small intestine, but does not completely abolish it. Therefore it may be considered that after atropinization the vagal inhibitory response overcomes the excitatory one, because the postganglionic cholinergic fibers in the myenteric plexus are affected markedly by atropine, while the inhibitory ones are not. It may also be supposed that in the myenteric plexus, nicotine and tubocurarine block the synapses on the inhibitory neurones more selectively than those on the excitatory ones and that C6 has little such selective action. In the previous paper 13) we have confirmed that the application of 0.1% cocaine solution to the intestinal lumen anesthetizes only the sensory receptors in the mucosa and 1% cocaine permeates the mucosa and submucosa and anesthetizes all the nervous elements in the myenteric plexus. Because in another experiment we have also confirmed that 0.4% cocaine solution has never anesthetized the inhibitory nerve fiber in the splanchnic nerve, which has no synapse in the myenteric plexus, it may be concluded that a small amount of cocaine through the mucosa affects the synapses on the inhibitory neurones more strongly than the excitatory ones in the myenteric plexus. It is accepted that cocaine anesthetizes the sensory nerve in the mucosa and accentuates the effect of adrenaline. Because adrenaline usually inhibits the intestinal movements and, in the present experiment, of ter the application of cocaine into the jejunal lumen the vagal stimulation caused the excitatory response, it cannot be said that the reverse phenomenon is due to an accentuation of the adrenaline effect caused by cocaine. It is known that the stimulation of the vagus often results in the inhibition of the gastric movements similar to that of the movements of the small intestine (VEACH 16), MCSWINEY and WADGE 10), CARLSON, BOYD and PEARCY 4), MCSWINEY 11). MCSWINY and WADGE 10) and CARLSON et al. 4) found that under conditions of low tone, the stimulation of the vagus with low or high frequencies or intensities causes the contraction of the stomach and the increase in tone. Under conditions of high tone, the stimulation of the vagus with low or high frequencies or intensities causes relaxation. However, in one of our experiments in which only urethane was used as narcotica, even under conditions of low tone, the stimulation of the vagus resulted in the inhibition of the jejunal movements (FIG. 6). In another dog, anesthetized with the intravenous injection of pentobarbital sodium, the vagal stimulation with a frequency of 10/sec produced the excitatory response and with a frequency of 50/sec, the inhibitory one. These results are similar to those in the stomach found by- VEACH 16) who considered that the inhibitory effects are the results of a phenomenon linked to the.wedensky inhibition. However, of ter the application of cocaine into the jejunal lumen, the stimulation with a frequency of 50/sec resulted in the excitatory effect only. Therefore, the Wedensky inhibition cannot be applied in such an inhibitory phenomenon.

9 1. The stimulation of the thoracic or cervical vagus usually results in excitation of the jejunal movements followed by the inhibitory effect. In the present experiments the cause of the inhibitory response was investigated by the application of various agents affecting the nervous structures in the intestinal wall. 2. The movements of the jejunum, 15cm long and severed from the rest of the intestine, were recorded using the balloon method on dogs anesthetized with urethane and morphine or pentobarbital sodium. 3. The weak stimulation of the vagus caused, in general, the excitatory response only; on the other hand, the strong stimulation produced an initial excitation followed by inhibition. 4. After the application of 0.4% cocaine solution into the jejunal lumen for 3min, the response to the strong stimulation of the vagus reversed to excitatory. It may be considered that cocaine could paralyse the inhibitory neurone in the myenteric plexus of the intestinal wall. 5. The reverse phenomenon similar to that elicited by the application of cocaine was obtained also after the intravenous injection of tubocurarine or nicotine, but it was difficult with the intravenous injection of hexamethonium bromide. It may be supposed that ganglion-blocking agents such as nicotine and tubocurarine could affect the synapses of the inhibitory neurones more powerfully than those of the excitatory ones. 6. After atropinization the excitatory response of the vagus reversed to the inhibitory one. 7. Provided that there are at least two kinds of the effector neurones in the myenteric plexus, the excitatory and inhibitory neurones, and that these neurones connect synaptically with the presynaptic fibers in the vagus, it may be considered that the results described above could be easily explained. 1) AMBACHE, N. Unmasking, after cholinergic paralysis by botulinum toxin, of a reversed action of nicotine on the mammalian intestine, revealing the probable presence of local inhibitory ganglion cells in the enteric plexuses. Brit. J. Pharmacol., 6: 51-67, ) AMBACHE, N. AND EDWARDS, J. Reversal of nicotine action on the intestine by atropine. Brit. J. Pharmacol., 6: , ) BAYLISS, W. M. AND STARLING, E. H. The movements and innervation of the small intestine. J. Physiol., 24: , ) CARLSON, A. J., BOYD, T. E. AND PERCY, J. F. Studies on the visceral sensory nervous system. XIII The innervation of the cardia and the lower end of the esophagus in mammals. Amer. J. Physiol., 61: 14-41, ) HILL, C. J. A contribution to our knowledge of the enteric plexus. Phil. Trans.

10 Roy. Soc. (B), Lond. 215: , ) HUKUHARA, T. Innervation of the small intestine (with abdominal window method nd moving picture). Nissinigaku, 21 (No. 2): 1-26, a 7) HUKUHARA, T., TAKAGI, T. AND YAMAGAMI, M. Effects of stimulation of the vagus on the intestinal movements. J. Jap. Physiol. Soc., 16: , ) HUKUHARA, T., YAMAGAMI, M. AND NAKAYAMA, S. On the intestinal intrinsic reflexes. Jap. J. Physiol., 8: 9-20, ) MAGEE, D. F. Gastro-intestinal physiology. Illinois, Charles C Thomas, ) MCSWINEY, B. A. AND WADGE, W. J. Effects of variations in intensity and frequency on the contractions of the stomach obtained by stimulation of the vagus nerve. J. Physiol., 65: , ) MCSWINEY, B. A. Innervation of the stomach. Physiol. Rev., 11: , ) MELIK-MEGRABOV, A. M. Zur Physiologie der motorischen und sekretorischen Funktion des Diinndarms. I. Mitteilung. Der Einfluss der Vagusreizung auf die motorische Funktionen des Diinndarms. PjUgers Arch., 213 : , ) NAKAYAMA, S. Movements of the small intestine in transport of intraluminal contents. Jap. J. Physiol., 12: , ) TOURNACK, A. ET CHABROL, M. Effect sur la motoricite intestinale de l'excitation centrifuge du vague. C. r. Soc. Biol., 94: , ) VAN HARN, G. L. Responses of muscles of cat small intestine to autonomic nerve stimulation. Amer. J. Physiol., 204: , ) VEACH, H. O., SCHWARTZ, L. L. AND WEINSTEIN, M. Studies on the innervation of smooth muscle. On the relation of vagal gastric effects to Wedensky inhibition. Amer. J. Physiol, 92: , ) YAMAGAMI, M. The vagus innervation of the pyloric sphincter. J. Physiol. Soc. Jap., 17: , 1955.