A neurokinin-1 receptor antagonist reduced hypersalivation and gastric contractility related to emesis in dogs

Naohiro Furukawa1, Hiroyuki Fukuda1, Mizue Hatano1, Tomoshige Koga1, and Yasuteru Shiroshita2

1 Department of Physiology, Kawasaki Medical School, Kurashiki 701-01; and 2 Section I, Pharmacology Department, Research Division, Tsukuba Research Laboratories, Nippon Glaxo, Tsukuba 300-42, Japan

    ABSTRACT
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

The roles of tachykinin neurokinin-1 (NK1) receptors in the induction of fictive retching, hypersalivation, and gastric responses associated with emesis induced by abdominal vagal stimulation were studied in paralyzed, decerebrated dogs. Vagal stimulation induced gradual increases in salivary secretion and activity of the parasympathetic postganglionic fibers to the submandibular gland, relaxation of the gastric corpus and antrum, and fictive retching. However, hypersalivation and increased nerve activity were suppressed and antral contractility was enhanced during fictive retching. An NK1 receptor antagonist, GR-205171, abolished the enhancement of antral contractility and fictive retching but had no effect on corpus and antral relaxation. Hypersalivation and increased nerve activity were inhibited by GR-205171 but were not completely abolished. Reflex salivation by lingual nerve stimulation was unaffected. These results suggest that GR-205171 acts on the afferent pathway in the bulb and diminishes hypersalivation and antral contraction related to emesis as well as fictive retching but does not affect gastric relaxation or hypersalivation induced by the vagovagal, vagosalivary, and linguosalivary reflexes.

retching; substance P; chorda tympani; linguosalivary reflex

    INTRODUCTION
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

IT IS WELL KNOWN THAT abdominal vagal afferent nerves play an important role in the induction of emesis by visceral stimulation. 5-HT3 receptor antagonists have been shown to suppress emesis by blocking the stimulatory effects of the chemical and radiological treatment of cancers on peripheral vagal nerve terminals (6, 7). Recently, neurokinin-1 (NK1) receptors have attracted a great deal of attention from many investigators, because these receptors in the central nervous system are thought to be involved in the induction of emesis (1, 11-13, 25-27, 29). However, almost all of these studies have revealed the effects of NK1 antagonists only on somatomotor responses. Lang et al. (22) indicated that a series of neural mechanisms is involved in emesis: the first participates in gastrointestinal motor correlates, and the second induces somatomotor emetic responses. Furukawa and Okada (9) reported that canine salivary secretion was increased before fictive retching induced by emetic stimuli but was suppressed during fictive retching. Because the salivary center receives two opposite inputs in the preretching phase and in the retching phase, these workers suggested the existence of an afferent relay station for emesis that triggers the central pattern generator (CPG) for somatomotor emetic action (3, 4, 21) and simultaneously induces autonomic nervous responses associated with emesis. Therefore, in further research on mechanisms of emesis, it is necessary to investigate the effects of NK1 antagonists on autonomic nervous responses associated with emesis, as well as somatomotor responses. However, there has been no previous study of the effects of NK1 receptor antagonists on autonomic nervous responses associated with emesis. This study was undertaken to investigate the effects of NK1 receptor antagonist on the activity of the afferent relay station suggested by Furukawa and Okada (9) by observing the effects on autonomic responses after emetic stimulation.

Because salivary secretion from the submaxillary gland exactly corresponds to the activity of the innervating nerves and because quantitative analysis is easy, we used salivary secretion as an index of autonomic activity. Afferent stimulation of the vagus nerves is thought to be useful for studying emesis in acute experiments in dogs, because stable emetic responses can be induced repeatedly. Therefore, afferent vagal stimulation was used to induce emesis. However, vagal stimulation also induces other reflex responses. For example, esophageal and gastric distension and afferent stimulation of the gastric vagal branch elicit hypersalivation by the vagosalivary reflexes (9, 16, 19), and afferent stimulation of the gastric vagal branch elicits gastric relaxation by the vagovagal reflex (18). Salivary secretion is thought to be increased by vagal stimulation not only related to emesis but also via the vagosalivary reflexes. Qu et al. (23) reported that gastric antral contractility was inhibited by vagal stimulation via the vagovagal reflex but enhanced during fictive retching. Recording of gastric motility makes it possible to distinguish between the gastric response related to emesis induced by vagal stimulation and that evoked by the vagovagal reflex. For these reasons, in this study we investigated the effects of a nonpeptide NK1 receptor antagonist, GR-205171, on fictive somatomotor responses, gastric corpus and antral contractile responses, and salivary secretion associated with emesis induced by vagal stimulation. Furthermore, the effects of GR-205171 on salivary responses associated with emesis were compared with those on salivary secretion induced by the linguosalivary reflex (9, 15) to examine its effects on the efferent pathway to the salivary glands. Portions of the results have been reported elsewhere as an abstract (8).

    MATERIALS AND METHODS
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

General methods of animal preparation. All of the experiments were approved by the Animal Research Committee of the Kawasaki Medical School and conducted according to the Guide for the Care and Use of Laboratory Animals. Six mongrel dogs (6-11 kg) were fasted for 16 h and used for the present study. All of the dogs were anesthetized with ketamine (25 mg/kg im). Almost all of the animals became quite flaccid within 5 min. If the animal was not flaccid enough ~5 min after administration, a further 10 mg/kg of ketamine was added. Midcollicular decerebration was performed during the subsequent 10 min. The dogs were paralyzed with gallamine triethiodide (2 mg/kg iv) and artificially ventilated with a respirator. Body temperature was maintained near 37.5°C by a feedback system, using an infrared heating lamp. A microtip pressure transducer was inserted into the right femoral artery, and mean systemic arterial pressure was monitored using a digital pressure monitor (Camino, M 420). To record gastric circular contractilities, two force transducers were sewn onto the wall of the gastric corpus and antrum ~3 cm orad to the pyloric sphincter in the direction of the circular muscle. A polyethylene catheter (2 mm ID, 20 cm long) was inserted into the left Wharton's duct, and drops of saliva (each drop was ~0.02 ml) from the catheter were counted using a photoelectric drop counter.

Recording of nerve discharges. The left phrenic nerve and the nerve innervating the left rectus abdominis were isolated through incisions along the cervical midline and the dorsal edge of the obliquus externus abdominis, respectively. The postganglionic branch from the right submandibular ganglion was isolated after partial removal of the masseter muscle through a midline maxillary incision. These nerves were carefully separated from the surrounding connective tissues under a stereoscopic dissecting microscope and covered with liquid paraffin. Centrifugal discharge from these nerves was monitored via bipolar platinum wire electrodes. Neural discharge was converted into frequency histograms of 200-ms, 500-ms, or 1-s bins using spike counters (Dia Medical, DSE-325 A), and the histograms were recorded (NEC San-ei, OMNIACE RT2116A-08). Activities of these nerves were monitored with an oscilloscope (Nihon Kohden, VC 11). The details of these methods have been reported previously (9).

Stimulation of the lingual and vagal nerves and experimental design. The lingual nerve on the left side was isolated, and electrical afferent stimulation (1 ms, 10 V, 10 Hz) was applied via a bipolar silver wire electrode to induce reflex salivation from the left submaxillary gland. The dorsal vagal trunk was sectioned just above the diaphragm, and fictive retching was induced by electrical stimulation (1 ms, 20-25 V, 3-10 Hz) of the central part of the severed vagal trunk. Coactivating rhythmic volleys of the phrenic nerve and the nerve to the rectus abdominis were used as an index of fictive retching. For the control experiments, vagal stimulation of at least 90 s was applied as emetic stimulation. When fictive retching persisted 90 s after the initiation of vagal stimulation, the stimulation was discontinued after the cessation of fictive retching. In the control experiments, two to four applications of vagal stimulation and two or three applications of lingual nerve stimulation of a 15-s period were applied. The NK1 receptor antagonist GR-205171 [2-methoxy-5-(5-trifluoromethyl-tetrazol-l-yl-benzyl)-(2S-phenyl-piperidin-3S-yl)-amine dihydrochloride] (Glaxo Wellcome) was then administered (50 µg/kg iv). This dose of 50 µg/kg was comparable to that (0.1 mg/kg) used in dogs by Gardner et al. (12) and was effective enough to suppress fictive retching induced by vagal stimulation but did not produce any obvious changes in resting respiratory activity of the phrenic nerve or in blood pressure and heart rate in dogs. Five or ten minutes after the administration of GR-205171, vagal stimulation was again applied for a 110- to 140-s period and repeated two to four times at 10-min intervals. Lingual nerve stimulation was applied two to three times before or after vagal stimulation. Rest periods of more than 2 or 6 min were allowed after lingual and vagal nerve stimulation, respectively.

Statistical analysis. The volume of salivary secretion, frequency of spikes of converted neural discharge of the parasympathetic nerve, and relative magnitudes of gastric contraction were statistically analyzed. In one dog, stable neural activity could not be obtained throughout the entire experiment, so the statistical analysis of neural activity was performed in the five remaining dogs. The analyses of salivary secretion and nerve discharge were performed by considering 10-s periods. The changes in salivary secretion induced by afferent vagal stimulation are thought to be composed of two different responses, as mentioned above (introduction), i.e., salivation related to emesis and salivation by the vagovagal reflex. Because the latencies to the start of fictive retching differed between individual dogs before GR-205171, and fictive retching disappeared after GR-205171, the data before GR-205171 were analyzed during the period from 70 s before to 160 s after the initiation of fictive retching, and the data after GR-205171 were analyzed during the period from 30 s before to 200 s after the initiation of vagal stimulation. The basal values before stimulation were determined by averaging the values obtained from 30 to 60 s before stimulation. The mean frequency of nerve activity from 90 to 100 s and the mean volumes of saliva from 120 to 130 s after the cessation of stimulation were also calculated. The values obtained from each dog were averaged (n = 2-4). The mean values obtained from all of the dogs were again averaged, and the results were expressed as means ± SE (n = number of dogs). The statistical significance of differences between the basal value before stimulation and the value of each 10-s period was analyzed by Student's t-test (paired). Furthermore, to compare the control responses in salivary secretion and parasympathetic nerve activity before GR-205171 with the test responses after GR-205171, the values during 10-s periods in the following three phases as well as the two basal phases were also analyzed. Before GR-205171, salivary secretion and nerve activity were gradually increased by vagal stimulation, decreased during fictive retching, and again increased after the cessation of retching. Therefore, the values of salivary secretion and nerve activity in each 10-s period immediately before fictive retching (preretching phase), immediately before the cessation of retching (late-retching phase), and immediately after the cessation of stimulation (postretching phase) in the control responses were compared with those at the corresponding times after the onset of vagal stimulation in the test responses after GR-205171. The mean values of the nerve activity from 40 to 50 s after the cessation of retching in the controls, and those at the corresponding times in the test cases after GR-205171, were also calculated to determine any difference in the time course of recovery. The relative magnitudes of gastric contraction were measured by calculating the area of the recorded contractility within a 30-s period before stimulation and within a 30-s period during the maximal antral contraction during fictive retching in the control experiments. After the application of NK1 antagonist, the area of the recorded contractility was calculated within a 30-s period before stimulation and within a 30-s period at the corresponding time as in the control. The statistically significant differences between the basal values before and ~10 min after the administration of NK1 antagonist and the values before and after stimulation in the corpus and antral contractilities were analyzed. Probability values of P <0.05 were considered significant.

    RESULTS
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

Effects of vagal stimulation on the parasympathetic nerve activity to the submandibular gland, salivary secretion, and gastric motility in the control experiments. Vagal stimulation induced fictive retching, with a mean latency of 41.3 ± 7.2 s (n = 6) in all of the dogs. Multiunit activity of the right chorda tympani nerve was slightly increased by vagal stimulation, although high-frequency activity was transiently exhibited at the onset of the vagal stimulation in some cases. This increased activity gradually increased further until fictive retching occurred. Subsequently, the excitatory effect was partially depressed in association with retching and again recovered after the cessation of retching. After the cessation of vagal stimulation, nerve activity gradually decreased and returned to the basal level ~1 min after the end of stimulation (Fig. 1A). The mean 10-s values of the activity immediately before retching (preretching phase) and immediately before (late-retching phase) and after (postretching phase) the cessation of retching and at 40-50 s after the cessation of retching were significantly larger than the basal value. There was no significant difference between the basal value and the value at 90-100 s after the cessation of retching (Fig. 2). Similar changes were observed in salivary secretion from the contralateral submandibular gland, although these changes were delayed by several seconds from the changes in nerve activity (Fig. 1A). The mean 10-s volumes at the preretching, late-retching, and postretching phases were significantly greater than the basal value. The mean volume at 120-130 s after the cessation of retching was not significantly different from the basal value before stimulation (Fig. 3).


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Fig. 1.   Effects of a neurokinin-1 (NK1) receptor antagonist, GR-205171, on salivary secretion and activity of parasympathetic nerve innervating the submandibular gland induced by emetic vagal stimulation. A: before administration of GR-205171 (control). B: after administration of GR-205171 (50 µg/kg iv). In B, vagal stimulation was applied 30 min after administration of GR-205171. Phrenic N., centrifugal activity of the phrenic nerve represented as frequency histograms with 200-ms bins; Abdominal M. N., centrifugal activity of an abdominal muscle branch of the L1 spinal nerve represented as frequency histograms with 200-ms bins; Saliva from L. Mand., salivary flow (1 drop was considered to be 0.02 ml) from the left submandibular gland; R. Parasympa. N., centrifugal activity of the parasympathetic nerve innervating the right submandibular gland represented as frequency histograms with 500-ms bins; imp, impulses; stim, stimulation. After GR-205171, fictive retching was not induced by vagal stimulation, and responses in salivary secretion and nerve activity were decreased but sustained. Note that salivary secretion during retching in the control experiments was equivalent to that induced by vagal stimulation after GR-205171.


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Fig. 2.   Changes in activity of parasympathetic nerve innervating the submandibular gland associated with emetic vagal stimulation before and after GR-205171. A: before GR-205171. B: after GR-205171 (50 µg/kg). C: comparison of responses in nerve activity before and after GR-205171. y-Axis, mean frequency of nerve activity in a 10-s period. Basal, mean values determined by averaging values obtained from 30 to 60 s before stimulation; 90-100 s after Off, mean values in 10-s period from 90 to 100 s after cessation of vagal stimulation; N, number of animals. In A and B, numbers in parentheses indicate total number of stimulations. In A, horizontal bar at top indicates duration of retching. Numbers above bar indicate number of incidences of retching sustained at corresponding time/total number of stimulations. In B, horizontal bar at top indicates duration of stimulation. Numbers above bar indicate number of stimulations applied at corresponding time/total number of stimulations. In C, Pre-Retch shows mean values during 10-s periods immediately before retching in control responses and at corresponding time after onset of stimulation after GR-205171. Late-retch, mean frequencies during 10-s periods immediately before cessation of retching in control and at corresponding time after GR-205171; Post-Retch, mean values during 10-s periods immediately after cessation of retching in control and at corresponding time after GR-205171; 40-50 s after Off, mean values in 10-s period from 40 to 50 s after cessation of vagal stimulation. * P < 0.05, ** P < 0.01 vs. basal values. @ P < 0.05, @@ P < 0.01 before vs. after GR-205171. NO, no significant difference (P > 0.05).


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Fig. 3.   Changes in salivary secretion associated with emetic vagal stimulation and salivary response to lingual nerve stimulation before and after GR-205171. A: before GR-205171. B: after GR-205171 (50 µg/kg). C: comparison of salivary responses to vagal stimulation before and after GR-205171. D: comparison of salivary responses to lingual nerve stimulation before and after GR-205171. C and D: crosshatched bars, before GR-205171. Hatched bars, after GR-205171. y-Axis shows mean volume of salivary secretion in a 10-s period. N., nerve. For further details, see legend to Fig. 2. * P < 0.05, ** P < 0.01 vs. basal values. @ P < 0.05 before vs. after GR-205171.

Gastric corpus contractility was continuously decreased by vagal stimulation in five dogs. In one dog only, this contractility did not change immediately after the initiation of vagal stimulation but rather at the same time as retching occurred (Fig. 4A). Gastric antral contractility was decreased immediately after the initiation of vagal stimulation, but was enhanced during retching in five dogs (Fig. 4A); contractility was only slightly enhanced during retching in the remaining dog. The mean value of the relative magnitude of gastric contractility was significantly decreased in the corpus and significantly increased in the antrum during fictive retching (Fig. 5).


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Fig. 4.   Effects of GR-205171 on gastric contractility induced by emetic vagal stimulation. A: before administration of GR-205171. B: after administration of GR-205171 (50 µg/kg iv). In B, vagal stimulation was applied 5 min after administration of GR-205171. In B, activities of phrenic and abdominal muscle nerves were facilitated by vagal stimulation, but fictive retching was not induced. Note that enhanced antral contraction observed during retching in A was completely abolished after GR-205171, whereas corpus and antral relaxations induced by vagal stimulation persisted. For abbreviations, see legend to Fig. 1.


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Fig. 5.   Changes in gastric contractilities associated with emetic vagal stimulation before and after GR-205171. A: corpus. B: antrum. y-Axis, mean value of relative magnitude of gastric contractility during a 30-s period. Basal, mean value determined by averaging values obtained from 0 to 30 s before stimulation. Retching, mean value during 30-s period at maximal antral contractions during retching in control experiments and at corresponding time after onset of stimulation after GR-205171. Before GR-205171, corpus contractility was significantly decreased, and antral contractility was significantly enhanced during retching. GR-205171 abolished excitatory response in antrum but did not change inhibitory response in corpus. * P < 0.05 vs. basal value.

Effects of vagal stimulation on activity in the parasympathetic nerve innervating the submandibular gland and salivary secretion after GR-205171. After the intravenous administration of 50 µg/kg of GR-205171, no fictive retching was induced in any of the dogs, although facilitatory effects on the phrenic and abdominal muscle nerves activities were sometimes observed (Fig. 4B). Vagal stimulation induced a small, but sustained, increase in parasympathetic nerve activity and salivary secretion, and the responses rapidly diminished after the cessation of stimulation (Fig. 2B). The mean values of salivary secretion and nerve activity during vagal stimulation after GR-205171 were similar to those during the retching phase in the control experiments (Figs. 2 and 3C, late retch). All of the mean 10-s values during and just after vagal stimulation (preretch, late retch, and postretch) were significantly higher than the basal value, but the facilitation in nerve activity at 40-50 s after the cessation of stimulation observed before GR-205171 was abolished. The mean 10-s value of nerve activity after GR-205171 was significantly decreased at the postretching phase and at 40-50 s after the cessation of stimulation compared with the control experiment but was not significantly changed in the preretching and late-retching phases (Fig. 2C). Similar changes were observed in salivary secretion, although these were delayed after the changes in nerve activity by several seconds (Fig. 3B). The mean 10-s value of salivary secretion after GR-205171 was significantly decreased in the preretch and postretch phases compared with that in the control experiment but was not significantly changed in the late-retch phase or at 120-130 s after the cessation of stimulation (Fig. 2C). To summarize, after GR-205171, salivary secretion was increased by vagal stimulation, but the marked increases in the preretching and postretching phases observed in the control experiments were diminished, and a small increase of the same magnitude as that observed during fictive retching in the control experiments was sustained.

Effects of vagal stimulation on gastric motility after GR-205171. GR-205171 did not change basal gastric contractilities. The mean relative magnitudes of gastric contractility in the 30-s periods just before and ~10 min after GR-205171 were 4.31 ± 0.39 and 4.02 ± 0.41 in the corpus and 4.33 ± 0.48 and 4.20 ± 0.55 in the antrum, respectively. There was no significant difference between the basal values before and after GR-205171. After GR-205171, antral contractility was slightly inhibited by vagal stimulation in five dogs (Fig. 4B) and was not changed in the remaining dog. The significant increase in the mean value of antral contractility during fictive retching observed in the control experiments was abolished by GR-205171 (Fig. 5). The mean value of the relative magnitude of antral contractility in the 30-s period just after vagal stimulation was significantly decreased to 3.81 ± 0.52 from the basal value of 4.05 ± 0.55. Corpus contractility was immediately inhibited by vagal stimulation in all six dogs (Fig. 4B). A significant decrease in the mean value of the relative magnitude of corpus contractility during fictive retching persisted even after GR-205171 (Fig. 5).

Salivary secretion induced by lingual nerve stimulation before and after GR-205171. Salivary secretion was significantly increased by afferent stimulation of the lingual nerve (10 V, 10 Hz) in the control experiments (Fig. 3D). The salivary response to lingual nerve stimulation was not significantly changed by GR-205171. Salivary secretion was also significantly increased by lingual nerve stimulation after GR-205171. There were no significant differences in the basal values or in the stimulation response values before or after GR-205171 (Fig. 3D).

    DISCUSSION
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

Salivary and gastric responses induced by vagal stimulation. Previously, Furukawa and Okada (9) reported in chloralose-anesthetized dogs that salivary secretion and the activity of the parasympathetic nerve innervating the submandibular gland were facilitated before fictive retching induced by intravenous apomorphine or intragastric copper sulfate and were suppressed during retching. In the present study, similar results were obtained by vagal stimulation in decerebrated dogs, although small increases in salivary secretion and nerve activity still remained during retching. It was suggested in a previous study that an afferent relay station in the bulb that drives the CPG for the somatomotor emetic act (3, 4, 21) may simultaneously excite the salivary secretory center, and outputs from the CPG may inhibit the salivary secretory center as well as elicit somatomotor responses (Fig. 6). The difference in the degree of inhibition during retching is thought to be due to the different methods used to induce retching. Vagal stimulation elicits the vagosalivary reflex to a greater extent than intravenous apomorphine, so, in the present study, inhibitory outputs from the CPG might suppress the excitatory effects from the relay station relating to emesis, but not the vagosalivary reflex.


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Fig. 6.   Diagrammatic representation of possible mechanisms that elicit retching, and antral and salivary responses associated with emesis induced by vagal stimulation. Abd. Vagal N., abdominal vagal afferent nerves; Abdominal M., abdominal muscles; Afferent Relay S., afferent relay station suggested by Furukawa and Okada (9); Autonomic N. Responses, autonomic nervous responses; CPG, central pattern generator for retching and expulsion suggested by Fukuda and Koga (3); DMV, dorsal motor nucleus of the vagus; Lingual N., lingual nerve; NK1 R., NK1 receptor; NTS, nucleus of the solitary tract. Antral motility is inhibited by vagovagal reflex, and salivary secretion is facilitated by linguo- and vagosalivary reflexes via route indicated by broken lines and arrows. Emetic signals from vagus nerve reach medial solitary nucleus (20) and subsequently excite afferent relay station. Outputs from afferent relay station facilitate antral motility and salivary secretion and drive CPG (solid lines and arrows). Activation of CPG elicits somatomotor retching activity and simultaneously inhibits afferent relay station (dotted lines and arrows). NK1 receptor antagonist, GR-205171, abolishes salivary and gastric responses related to emesis, as well as fictive retching (solid lines and arrows), but does not change the reflex responses in salivary secretion and antral motility (broken lines and arrows). Thus NK1 receptor may exist on neurons in afferent relay station. CPG is thought to exist in the area dorsal to the retrofacial nucleus (3, 20), and the afferent relay station may be situated at the medullar area medial to the most rostral part of the nucleus ambiguus (5).

Gastric contractility was generally inhibited by vagal stimulation, but antral contractility was enhanced during fictive retching in the control experiments, as reported by Qu et al. (23). They suggested that this inhibitory effect in the antrum was induced by the vagovagal reflex, whereas the excitatory effect was related to emesis, because high-frequency vagal stimulation strongly inhibited antral contractility without fictive retching or antral contraction.

Effects of NK1 antagonist on somatomotor emetic acts induced by vagal stimulation. The selective nonpeptide NK1 receptor antagonist GR-205171 abolished fictive retching in all of the dogs in the present study. GR-205171 and other nonpeptide and peptide NK1 receptor antagonists, e.g., CP-99,994, GR-203040, and GR-82334, have been shown to inhibit somatomotor emetic responses induced by various stimuli, i.e., afferent vagal stimulation, cisplatin, and copper sulfate, in the ferret, dog, and Suncus murinus (1, 11-13, 25-27). Therefore, it is clear that NK1 receptors play an important role in the induction of somatomotor emetic responses by various stimuli.

Effects of an NK1 antagonist on changes in gastric contractility induced by vagal stimulation. In the present study, GR-205171 abolished the excitatory response in the antrum as well as fictive retching, whereas the inhibitory responses in the corpus and antrum remained unaffected. Because the excitatory response and inhibitory responses are thought to be related to emesis and the vagovagal reflex, respectively, as mentioned above, GR-205171 may suppress not only retching activity but also the accompanying phenomena associated with emesis, without inhibiting other physiological reflexes. Similar results obtained for salivary secretion seem to support this hypothesis, as mentioned below.

Effects of an NK1 antagonist on changes in salivary secretion induced by vagal stimulation. In the control experiments, salivary secretion and the activity of the parasympathetic nerve innervating the submandibular gland were greatly increased before retching (preretching phase) and immediately after the cessation of retching (postretching phase). After GR-205171, salivary secretion at the corresponding times as in the control was significantly reduced, whereas the mean value during the retching phase (late-retching phase) in the control was equivalent to that at the corresponding time after the NK1 antagonist. We previously reported that the frequency of parasympathetic nerve activity was related to the volume of salivary secretion (9). In the present study, the difference between the discharge rates before and after GR-205171 in the preretch phase was not significant. However, this seems to be due to the large variation in discharge rate among the individual dogs. Because the salivary secretion in the preretching phase was significantly decreased after GR-205171, parasympathetic nerve activity may also be decreased by GR-205171. It has been reported that substance P induces salivary secretion in the rat and ferret (2, 30), and Giuliani et al. (14) reported that a selective NK1 receptor agonist stimulated rat salivary secretion more potently than substance P itself. In the present study, however, salivary secretion induced by lingual nerve stimulation was not significantly different before and after GR-205171. Therefore, GR-205171 seems to have little or no effect on the efferent pathway of the salivary response induced by emetic stimulation under our experimental conditions in dogs. Because salivary secretion and parasympathetic nerve activity were strongly suppressed during retching induced by apomorphine (9), as mentioned above, the following conclusions should be considered. 1) The increased response in salivary secretion and nerve activity in the control experiments may reflect the sum of the response to the vagosalivary reflex and that related to emesis. 2) During the retching phase in the control experiments, only the response to the vagosalivary reflex may be present. 3) GR-205171 may diminish only the response related to emesis but leave the response to the vagosalivary reflex unaltered, which appeared immediately after the initiation of vagal stimulation with constant magnitude.

Possible location of NK1 receptors, and schema of the central mechanisms for induction of the salivary and gastric responses related to emesis. Gardner et al. (11) reported that a peptide NK1 receptor antagonist, GR-82334, inhibited cisplatin-induced emesis in the ferret after hindbrain administration but not when given peripherally. It has been shown that some vagal C fibers contain immunoreactive substance P (28) and are sensitive to capsaicin (10, 17). Koga and Fukuda (20) suggested that the medial part of the nucleus of the solitary tract (NTS) mediates emetic signals via the abdominal vagus to the CPG. Recently, Shiroshita et al. (24) reported in dogs that capsaicin or resinferatoxin, an ultrapotent capsaicin analog, administered into the fourth ventricle abolished fictive retching induced by vagal afferent stimulation but did not abolish the retching induced by stimulation of the medial NTS. These results suggested that substance P from capsaicin-sensitive vagal afferent nerves mediates visceral emetic signals to the medial NTS. However, very recently, Fukuda et al. (5) suggested that microinjection of GR-205171 at the medullar area medial to the most rostral part of the nucleus ambiguus not only abolished fictive retching but also decreased hypersalivation before retching. This area is situated between the medial NTS and the CPG, which may exist in the area dorsal to the retrofacial nucleus (3, 21). Therefore, it seems that vagal afferent fibers that mediate emetic signals are capsaicin sensitive, but the relevant transmitter may not be substance P, although further studies are required to clarify the transmitter from the vagal afferent fibers.

On the basis of the results of the present study and previous work, the following conclusions are postulated (Fig. 6). 1) Emetic vagal afferent nerves excite the neurons in the NTS, but NK1 receptors may not play a major role in this excitation. 2) The outputs from the neurons in the NTS excite an afferent relay station that drives the CPG and simultaneously facilitates salivary secretion and antral contractility. 3) NK1 receptors may exist on the neurons in the relay station. 4) The outputs from the CPG inhibit the salivary secretory centers and elicit somatomotor responses. This inhibition may be induced via the relay station.

In conclusion, an NK1 receptor antagonist may abolish salivary and gastric responses related to emesis as well as fictive retching but does not change the responses induced by the linguosalivary reflex and vagovagal gastric reflexes.

    ACKNOWLEDGEMENTS

We thank Dr. Kiyoko Fukai for help with the statistical analysis and Kiwamu Aoki and Rie Ohtomo for their assistance with the quantitative analysis.

    FOOTNOTES

This study was supported in part by a Research Project Grant (no. 9-711) from Kawasaki Medical School.

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Address reprint requests to N. Furukawa.

Received 8 April 1998; accepted in final form 15 July 1998.

    REFERENCES
Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References

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Am J Physiol Gastroint Liver Physiol 275(5):G1193-G1201
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