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Central orexin facilitates gastric relaxation and contractility in rats
Neuroscience Letters 332 (2002) 171–174 www.elsevier.com/locate/neulet
Central orexin facilitates gastric relaxation and contractility in rats Motoi Kobashi a,*, Yuichi Furudono b, Ryuji Matsuo a, Takashi Yamamoto b a
Department of Oral Physiology, Okayama University Graduate School of Medicine and Dentistry, Shikata-cho 2-5-1, Okayama 700-8525, Japan b Department of Behavioral Physiology, Graduate School of Human Sciences, Osaka University, Suita 565-0871, Japan Received 31 July 2002; received in revised form 21 August 2002; accepted 22 August 2002
Abstract The effects of the intracisternal administration of synthetic orexin-A (3 nmol) on gastric motility were examined in rats. The administration of orexin but not a vehicle induced relaxation of the proximal stomach lasting for more than 30 min. Phasic contractions in the distal stomach were facilitated in response to the administration of orexin but not a vehicle. Facilitation in the distal stomach was not observed in the animals which underwent the sectioning of the bilateral vagi at the subdiaphragmatic level. Relaxation of the proximal stomach was observed in vagotomized animals but the magnitude of relaxation was significantly smaller than that in intact animals. These results suggest that central orexin facilitates distal stomach motility and relaxation of the proximal stomach via vagi. q 2002 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Orexin; Stomach; Gastric; Vagus; Motility; Relaxation
Orexins (or hypocretins) are novel neuropeptides synthesized mainly in the lateral hypothalamic area (LHA) and are implicated in the regulation of food intake and arousal [3,15,16]. Since feeding is associated with gastric functions, orexins might regulate them. In fact, intracisternal administration of orexin-A induced gastric acid secretion [19]. No systematic study, however, has been reported on orexininducing gastric motility. Concerning gastric motor activities, feeding increases gastric motility [14], more precisely, relaxation in the proximal stomach and enhanced motility in the distal stomach are observed during feeding [1]. Neuropeptides such as neuropeptide Y, galanin or ghrelin facilitate food intake and potently regulate gastric motility [2,12,18]. Therefore, orexins might also be involved in the regulation of gastric motility. The present study was undertaken to examine whether central orexins affect the gastric motor activities. Since the stomach has different roles in motility depending on the part of the stomach, the effects of orexins on either proximal or distal stomach motility were separately examined. Animal care was in accordance with the guidelines of the Physiological Society of Japan. Twenty male Sprague– * Corresponding author. Tel.: 181-86-235-6641; fax: 181-86235-6641. E-mail address: [email protected] (M. Kobashi).
Dawley rats (280–320 g) housed on a 12:12 light:dark cycle were used. Each animal was fasted for 1 day prior to starting the experiment to empty the stomach. All experiments were conducted in a daytime. Each animal was anesthetized with an intraperitoneal injection of urethanechloralose (urethane, 0.8 g/kg; chloralose, 65 mg/kg body wt). An intragastric balloon was introduced into the proximal or the distal stomach as described previously [7,8]. When the distal stomach motility was recorded, the balloon was inflated with warm water at 378C to a volume of 0.3 ml. When the proximal stomach motility was recorded, the balloon was inflated with 0.3–0.6 ml water regulating the initial intragastric pressure became 0.6–0.7 kPa. Consequently, the basal level of intragastric pressure (IGP) before the administration of solutions was not different between groups (see Fig. 1Ab). In ten animals, bilateral sections of the vagi were made at the subdiaphragmatic level in addition to the installation of the gastric balloon. After the abdominal surgery, each animal was mounted on a stereotaxic apparatus. The neck muscles were removed, and the ligaments between the occipital bone and the atlas were carefully removed. A small hole was made through the dura matter to administer solutions. Orexin-A (Peptide Institute Inc., Osaka, Japan) dissolved in normal Ringer solution (1 mM) was prepared. After gastric balloon was inflated, a vehicle (3 ml) was administered through the hole of the dura
0304-3940/02/$ - see front matter q 2002 Elsevier Science Ireland Ltd. All rights reserved. PII: S03 04 - 394 0( 0 2) 00 95 8- 8
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M. Kobashi et al. / Neuroscience Letters 332 (2002) 171–174
Fig. 1. Effects of the intracisternal administration of orexin-A (3 nmol) on the gastric motor response in intact and vagotomized animals. Aa: response of proximal stomach motility to a vehicle or orexin administration in intact animal. Ab: mean intragastric pressure before (pre) and after (post) the administration of solutions in intact and vagotomized animals. Ba: response of distal stomach motility to a vehicle or orexin administration in intact animal. Bb: mean motility index before (pre) and after (post) administration of solutions in intact and vagotomized animals. In Ab and Bb: vehicle administration in intact animals (open circles), orexin administration in intact animals (filled circles) and orexin administration in vagotomized animals (filled triangles) are shown. Asterisks show significant difference compared with the mean value obtained from pre-administration of orexin in intact animals. Dagger shows significant difference compared with the value obtained from post-administration of orexin in vagotomized animals.
matter intracisternally using 10 ml of Hamilton syringe. More than 30 min after the administration of the vehicle, orexin (3 nmol, 3 ml) was administered. Gastric responses were stored in a personal computer using the Power Lab system (AD-Instruments). To analyze relaxation of the proximal stomach, the minimum value of the contraction wave was measured every minute. The mean of these values for 10 min before the administration of solutions and that for 15–25 min after the administration of solutions were used for statistical analyses. To analyze contractile response of the distal stomach, the area under the contraction wave above minimum point was calculated every minute. The mean of these values for 10 min before the administration of solutions and that for 5–15 min after the administration of solutions were used for statistical analyses as motility indexes (MI). All data were expressed by mean ^ SEM. Statistical analyses were made between pre-stimulus values and post-stimulus values using the pared t-test (P , 0:05 for significance). Student’s t-tests were used for analyzing post-stimulus value between intact and the vagotomized groups.
The administration of orexin-A but not a vehicle induced relaxation in the proximal stomach (N ¼ 5) (Fig. 1Aa). The IGP decreased gradually after orexin administration. The maximal fall of the IGP was observed 8–16 min after the orexin administration. The mean time to peak was 14.3 ^ 1.73 min. The approximate duration of effects varied from 47 to 110 min. The mean IGP after orexin administration (0.56 ^ 0.02 kPa) was significantly smaller (t ¼ 8:19, P , 0:05) than that before administration (0.66 ^ 0.02 kPa). The mean IGP after the administration of the vehicle (0.66 ^ 0.02) was not different (t ¼ 1:32, NS) from that before administration (0.66 ^ 0.02). The administration of orexin but not a vehicle facilitated gastric contractility in the distal stomach (N ¼ 5) (Fig. 1Ba). Phasic contractions gradually increased in magnitude after orexin administration. The maximal increase in motility was observed 7–12 min after orexin administration. The mean time to peak was 9.0 ^ 0.86 min. The approximate duration of effects varied from 37 to 134 min. The mean MI after orexin administration (0.40 ^ 0.06 kPa £ min) was significantly larger (t ¼ 3:62, P , 0:05) than that before administration
M. Kobashi et al. / Neuroscience Letters 332 (2002) 171–174
(0.14 ^ 0.05 kPa £ min). The mean MI after the administration of the vehicle (0.14 ^ 0.07 kPa £ min) was not different (t ¼ 0:085, NS) from that before administration (0.14 ^ 0.05 kPa £ min). A weak relaxation of the proximal stomach induced by orexin administration was observed in the animals which underwent bilateral sectioning of the vagi (N ¼ 5). In the vagotomized animals, the mean IGP after orexin administration (0.63 ^ 0.02 kPa) was significantly smaller (t ¼ 3:24, P , 0:05) than that before administration (0.66 ^ 0.03 kPa) in the proximal stomach (Fig. 1Ab). However, the magnitude of relaxation was rather smaller than that in intact animals. The mean IGP after orexin administration was significantly larger (t ¼ 2:32, P , 0:05) than that obtained from intact animals. The mean decrease in IGP induced by orexin in vagotomized animals (0.03 ^ 0.02 kPa) was significantly smaller (t ¼ 4:33, P , 0:05) than that in intact animals (0.09 ^ 0.03 kPa). Orexin administration did not induce any change in distal stomach motility in the vagotomized animals (N ¼ 5). The mean MI after orexin administration (0.05 ^ 0.02 kPa £ min) was not different (t ¼ 5:43, P , 0:05) from that before administration (0.05 ^ 0.01 kPa £ min) (Fig. 1Bb). Orexin-A is more potent to induce feeding, drinking and gastric acid secretion than orexin-B [4,10,19], suggesting that orexin-A is closely related with the regulation of food intake and gastric function. Therefore, we used orexin-A in the present study. The dose used in the present study is comparable with the studies to induce feeding and gastric acid secretion [4,10,19]. The results showed that the intracisternal administration of orexin-A relaxed the proximal stomach and enhanced distal stomach motility. These motor responses obtained in the present study might be anticipatory response to accommodate the intake of food. Relaxation in the proximal stomach facilitates reservoir function and enhanced motility of the distal stomach facilitates stirring and emptying gastric contents. Facilitation in distal stomach motility and the greater part of relaxation in the proximal stomach induced by orexin were abolished by vagotomy. These results indicate that the vagi have a substantial role in motor responses of the stomach induced by the central administration of orexin. A relative smaller relaxation was observed after orexin administration in the vagotomized animals, suggesting neural elements except for vagi such as sympathetic nerve may contribute to relaxation induced by orexin. Since central orexin increased blood pressure [17], adrenergic system may be involved in relaxation induced by orexin. The brain sites where orexin affects to induce gastric motor responses were not clarified in the present study. The orexin-A immunoreactive neurons project to the dorsal medulla [5]. The orexin receptors messenger RNA were observed in the nucleus tractus solitarius (NTS) and the dorsal motor nucleus of the vagus (DMV) [11]. Orexin excites the certain part of DMV neurons which project to
173
the abdominal viscera [6]. Direct action of orexin on the dorsal vagal complex possibly explains the gastric motor responses. Recently, Krowicki et al. [9] reported that the injection of orexin-A into the DMV induced marked increase in antral motility. This observation indicates a possibility that orexin directly affects on the DMV neurons to induce facilitation of gastric contractility. They also mentioned that the intragastric pressure was increased by the injection of orexin into the DMV. This result seems to differ from ours obtained in the present study. Considering the facts that orexin receptors are found in the NTS [11], and the connection between the NTS and the DMV is mainly inhibitory [20], we can suggest a possibility that orexin may act on the NTS neurons, which in turn inhibit the DMV neurons. The proximal stomach relaxed in response to stimuli associated with food intake, i.e. swallowing, esophageal distension and gastric distension [13]. Therefore, we speculate that relaxation of the proximal stomach is essential when orexins are released from the LHA. This study was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (Nos: 13671937 and 14370593). [1] Azpiroz, F. and Malagelada, J.-R., Physiological variations in canine gastric tone measured by an electronic barostat, Am. J. Physiol. Gastrointest. Liver Physiol., 248 (1985) G229–G237. [2] Bauer, F.E., Zintel, A., Kenny, M.J., Calder, D., Ghatei, M.A. and Bloom, S.R., Inhibitory effect of galanin on postprandial gastrointestinal motility and gut hormone release in humans, Gastroenterology, 97 (1989) 260–264. [3] de Lecea, L., Kilduff, T.S., Peyron, C., Gao, X., Foye, P.E., Danielson, P.E., Fukuhara, C., Battenberg, E.L., Gautvik, V.T., Bartlett 2nd, F.S., Frankel, W.N., van den Pol, A.N., Bloom, F.E., Gautvik, K.M. and Sutcliffe, J.G., The hypocretins: hypothalamus-specific peptides with neuroexcitatory activity, Proc. Natl. Acad. Sci. USA, 95 (1998) 322–327. [4] Edwards, C.M., Abusnana, S., Sunter, D., Murphy, K.G., Ghatei, M.A. and Bloom, S.R., The effect of the orexins on food intake: comparison with neuropeptide Y, melaninconcentrating hormone and galanin, J. Endocrinol., 160 (1999) R7–R12. [5] Harrison, T.A., Chen, C.T., Dun, N.J. and Chang, J.K., Hypothalamic orexin A-immunoreactive neurons project to the rat dorsal medulla, Neurosci. Lett., 273 (1999) 17–20. [6] Hwang, L.L., Chen, C.T. and Dun, N.J., Mechanisms of orexin-induced depolarizations in rat dorsal motor nucleus of vagus neurones in vitro, J. Physiol. (Lond.), 537 (2001) 511–520. [7] Kobashi, M., Koga, T., Mizutani, M. and Matsuo, R., Suppression of vagal motor activities evokes laryngeal afferentmediated inhibition of gastric motility, Am. J. Physiol. Regul. Integr. Comp. Physiol., 282 (2002) R818–R827. [8] Kobashi, M., Mizutani, M. and Matsuo, R., Water stimulation of the posterior oral cavity induces inhibition of gastric motility, Am. J. Physiol. Regul. Integr. Comp. Physiol., 279 (2000) R778–R786. [9] Krowicki, Z.K., Berthoud, H.R., Burmeister, M.A. and Hornby, P.J., Orexin-A in the dorsal motor nucleus of the vagus potently increases gastric motor function via orexin 1 recep-
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M. Kobashi et al. / Neuroscience Letters 332 (2002) 171–174 tor in rats, Gasrotroenterology, 120 (Suppl. 1) (2001) A113 (Abstract). Kunii, K., Yamanaka, A., Nambu, T., Matsuzaki, I., Goto, K. and Sakurai, T., Orexins/hypocretins regulate drinking behaviour, Brain Res., 842 (1999) 256–261. Marcus, J.N., Aschkenasi, C.J., Lee, C.E., Chemelli, R.M., Saper, C.B., Yanagisawa, M. and Elmquist, J.K., Differential expression of orexin receptors 1 and 2 in the rat brain, J. Comp. Neurol., 435 (2001) 6–25. Masuda, Y., Tanaka, T., Inomata, N., Ohnuma, N., Tanaka, S., Itoh, Z., Hosoda, H., Kojima, M. and Kangawa, K., Ghrelin stimulates gastric acid secretion and motility in rats, Biochem. Biophys. Res. Commun., 276 (2000) 905–908. Meyer, E.A., The physiology of gastric storage and emptying, In L.R. Johnson (Ed.), Physiology of the Gastrointestinal Tract, 3rd Edition, Raven Press, New York, 1994, pp. 929–976. Miolan, J.P. and Roman, C., Discharge of efferent vagal fibers supplying gastric antrum: indirect study by nerve suture technique, Am. J. Physiol. Endocrinol. Metab. Gastrointest. Physiol., 235 (1978) E366–E373. Mondal, M.S., Nakazato, M. and Matsukura, S., Orexins (hypocretins): novel hypothalamic peptides with divergent functions, Biochem. Cell Biol., 78 (2000) 299–305.
[16] Sakurai, T., Amemiya, A., Ishii, M., Matsuzaki, I., Chemelli, R.M., Tanaka, H., Williams, S.C., Richarson, J.A., Kozlowski, G.P., Wilson, S., Arch, J.R., Buckingham, R.E., Haynes, A.C., Carr, S.A., Annan, R.S., McNulty, D.E., Liu, W.S., Terrett, J.A., Elshourbagy, N.A., Bergsma, D.J. and Yanagisawa, M., Orexins and orexin receptors: a family of hypothalamic neuropeptides and G protein-coupled receptors that regulate feeding behavior, Cell, 92 (1998) 573–585. [17] Samson, W.K., Gosnell, B., Chang, J.K., Resch, Z.T. and Murphy, T.C., Cardiovascular regulatory actions of the hypocretins in brain, Brain Res., 831 (1999) 248–253. [18] Sheikh, S.P., Neuropeptide Y peptide YY: major modulators of gastrointestinal blood flow and function, Am. J. Physiol., 261 (1991) G701–G715. [19] Takahashi, N., Okumura, T., Yamada, H. and Kohgo, Y., Stimulation of gastric acid secretion by centrally administered orexin-A in conscious rats, Biochem. Biophys. Res. Commun., 254 (1999) 623–627. [20] Travagli, R.A. and Rogers, R.C., Receptors and transmission in the brain-gut axis: potential for novel therapies. V. Fast and slow extrinsic modulation of dorsal vagal complex circuits, Am. J. Physiol. Gastrointest. Liver Physiol., 281 (2001) G595–G601.
Central orexin facilitates gastric relaxation and contractility in rats Motoi Kobashi a,*, Yuichi Furudono b, Ryuji Matsuo a, Takashi Yamamoto b a
Department of Oral Physiology, Okayama University Graduate School of Medicine and Dentistry, Shikata-cho 2-5-1, Okayama 700-8525, Japan b Department of Behavioral Physiology, Graduate School of Human Sciences, Osaka University, Suita 565-0871, Japan Received 31 July 2002; received in revised form 21 August 2002; accepted 22 August 2002
Abstract The effects of the intracisternal administration of synthetic orexin-A (3 nmol) on gastric motility were examined in rats. The administration of orexin but not a vehicle induced relaxation of the proximal stomach lasting for more than 30 min. Phasic contractions in the distal stomach were facilitated in response to the administration of orexin but not a vehicle. Facilitation in the distal stomach was not observed in the animals which underwent the sectioning of the bilateral vagi at the subdiaphragmatic level. Relaxation of the proximal stomach was observed in vagotomized animals but the magnitude of relaxation was significantly smaller than that in intact animals. These results suggest that central orexin facilitates distal stomach motility and relaxation of the proximal stomach via vagi. q 2002 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Orexin; Stomach; Gastric; Vagus; Motility; Relaxation
Orexins (or hypocretins) are novel neuropeptides synthesized mainly in the lateral hypothalamic area (LHA) and are implicated in the regulation of food intake and arousal [3,15,16]. Since feeding is associated with gastric functions, orexins might regulate them. In fact, intracisternal administration of orexin-A induced gastric acid secretion [19]. No systematic study, however, has been reported on orexininducing gastric motility. Concerning gastric motor activities, feeding increases gastric motility [14], more precisely, relaxation in the proximal stomach and enhanced motility in the distal stomach are observed during feeding [1]. Neuropeptides such as neuropeptide Y, galanin or ghrelin facilitate food intake and potently regulate gastric motility [2,12,18]. Therefore, orexins might also be involved in the regulation of gastric motility. The present study was undertaken to examine whether central orexins affect the gastric motor activities. Since the stomach has different roles in motility depending on the part of the stomach, the effects of orexins on either proximal or distal stomach motility were separately examined. Animal care was in accordance with the guidelines of the Physiological Society of Japan. Twenty male Sprague– * Corresponding author. Tel.: 181-86-235-6641; fax: 181-86235-6641. E-mail address: [email protected] (M. Kobashi).
Dawley rats (280–320 g) housed on a 12:12 light:dark cycle were used. Each animal was fasted for 1 day prior to starting the experiment to empty the stomach. All experiments were conducted in a daytime. Each animal was anesthetized with an intraperitoneal injection of urethanechloralose (urethane, 0.8 g/kg; chloralose, 65 mg/kg body wt). An intragastric balloon was introduced into the proximal or the distal stomach as described previously [7,8]. When the distal stomach motility was recorded, the balloon was inflated with warm water at 378C to a volume of 0.3 ml. When the proximal stomach motility was recorded, the balloon was inflated with 0.3–0.6 ml water regulating the initial intragastric pressure became 0.6–0.7 kPa. Consequently, the basal level of intragastric pressure (IGP) before the administration of solutions was not different between groups (see Fig. 1Ab). In ten animals, bilateral sections of the vagi were made at the subdiaphragmatic level in addition to the installation of the gastric balloon. After the abdominal surgery, each animal was mounted on a stereotaxic apparatus. The neck muscles were removed, and the ligaments between the occipital bone and the atlas were carefully removed. A small hole was made through the dura matter to administer solutions. Orexin-A (Peptide Institute Inc., Osaka, Japan) dissolved in normal Ringer solution (1 mM) was prepared. After gastric balloon was inflated, a vehicle (3 ml) was administered through the hole of the dura
0304-3940/02/$ - see front matter q 2002 Elsevier Science Ireland Ltd. All rights reserved. PII: S03 04 - 394 0( 0 2) 00 95 8- 8
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M. Kobashi et al. / Neuroscience Letters 332 (2002) 171–174
Fig. 1. Effects of the intracisternal administration of orexin-A (3 nmol) on the gastric motor response in intact and vagotomized animals. Aa: response of proximal stomach motility to a vehicle or orexin administration in intact animal. Ab: mean intragastric pressure before (pre) and after (post) the administration of solutions in intact and vagotomized animals. Ba: response of distal stomach motility to a vehicle or orexin administration in intact animal. Bb: mean motility index before (pre) and after (post) administration of solutions in intact and vagotomized animals. In Ab and Bb: vehicle administration in intact animals (open circles), orexin administration in intact animals (filled circles) and orexin administration in vagotomized animals (filled triangles) are shown. Asterisks show significant difference compared with the mean value obtained from pre-administration of orexin in intact animals. Dagger shows significant difference compared with the value obtained from post-administration of orexin in vagotomized animals.
matter intracisternally using 10 ml of Hamilton syringe. More than 30 min after the administration of the vehicle, orexin (3 nmol, 3 ml) was administered. Gastric responses were stored in a personal computer using the Power Lab system (AD-Instruments). To analyze relaxation of the proximal stomach, the minimum value of the contraction wave was measured every minute. The mean of these values for 10 min before the administration of solutions and that for 15–25 min after the administration of solutions were used for statistical analyses. To analyze contractile response of the distal stomach, the area under the contraction wave above minimum point was calculated every minute. The mean of these values for 10 min before the administration of solutions and that for 5–15 min after the administration of solutions were used for statistical analyses as motility indexes (MI). All data were expressed by mean ^ SEM. Statistical analyses were made between pre-stimulus values and post-stimulus values using the pared t-test (P , 0:05 for significance). Student’s t-tests were used for analyzing post-stimulus value between intact and the vagotomized groups.
The administration of orexin-A but not a vehicle induced relaxation in the proximal stomach (N ¼ 5) (Fig. 1Aa). The IGP decreased gradually after orexin administration. The maximal fall of the IGP was observed 8–16 min after the orexin administration. The mean time to peak was 14.3 ^ 1.73 min. The approximate duration of effects varied from 47 to 110 min. The mean IGP after orexin administration (0.56 ^ 0.02 kPa) was significantly smaller (t ¼ 8:19, P , 0:05) than that before administration (0.66 ^ 0.02 kPa). The mean IGP after the administration of the vehicle (0.66 ^ 0.02) was not different (t ¼ 1:32, NS) from that before administration (0.66 ^ 0.02). The administration of orexin but not a vehicle facilitated gastric contractility in the distal stomach (N ¼ 5) (Fig. 1Ba). Phasic contractions gradually increased in magnitude after orexin administration. The maximal increase in motility was observed 7–12 min after orexin administration. The mean time to peak was 9.0 ^ 0.86 min. The approximate duration of effects varied from 37 to 134 min. The mean MI after orexin administration (0.40 ^ 0.06 kPa £ min) was significantly larger (t ¼ 3:62, P , 0:05) than that before administration
M. Kobashi et al. / Neuroscience Letters 332 (2002) 171–174
(0.14 ^ 0.05 kPa £ min). The mean MI after the administration of the vehicle (0.14 ^ 0.07 kPa £ min) was not different (t ¼ 0:085, NS) from that before administration (0.14 ^ 0.05 kPa £ min). A weak relaxation of the proximal stomach induced by orexin administration was observed in the animals which underwent bilateral sectioning of the vagi (N ¼ 5). In the vagotomized animals, the mean IGP after orexin administration (0.63 ^ 0.02 kPa) was significantly smaller (t ¼ 3:24, P , 0:05) than that before administration (0.66 ^ 0.03 kPa) in the proximal stomach (Fig. 1Ab). However, the magnitude of relaxation was rather smaller than that in intact animals. The mean IGP after orexin administration was significantly larger (t ¼ 2:32, P , 0:05) than that obtained from intact animals. The mean decrease in IGP induced by orexin in vagotomized animals (0.03 ^ 0.02 kPa) was significantly smaller (t ¼ 4:33, P , 0:05) than that in intact animals (0.09 ^ 0.03 kPa). Orexin administration did not induce any change in distal stomach motility in the vagotomized animals (N ¼ 5). The mean MI after orexin administration (0.05 ^ 0.02 kPa £ min) was not different (t ¼ 5:43, P , 0:05) from that before administration (0.05 ^ 0.01 kPa £ min) (Fig. 1Bb). Orexin-A is more potent to induce feeding, drinking and gastric acid secretion than orexin-B [4,10,19], suggesting that orexin-A is closely related with the regulation of food intake and gastric function. Therefore, we used orexin-A in the present study. The dose used in the present study is comparable with the studies to induce feeding and gastric acid secretion [4,10,19]. The results showed that the intracisternal administration of orexin-A relaxed the proximal stomach and enhanced distal stomach motility. These motor responses obtained in the present study might be anticipatory response to accommodate the intake of food. Relaxation in the proximal stomach facilitates reservoir function and enhanced motility of the distal stomach facilitates stirring and emptying gastric contents. Facilitation in distal stomach motility and the greater part of relaxation in the proximal stomach induced by orexin were abolished by vagotomy. These results indicate that the vagi have a substantial role in motor responses of the stomach induced by the central administration of orexin. A relative smaller relaxation was observed after orexin administration in the vagotomized animals, suggesting neural elements except for vagi such as sympathetic nerve may contribute to relaxation induced by orexin. Since central orexin increased blood pressure [17], adrenergic system may be involved in relaxation induced by orexin. The brain sites where orexin affects to induce gastric motor responses were not clarified in the present study. The orexin-A immunoreactive neurons project to the dorsal medulla [5]. The orexin receptors messenger RNA were observed in the nucleus tractus solitarius (NTS) and the dorsal motor nucleus of the vagus (DMV) [11]. Orexin excites the certain part of DMV neurons which project to
173
the abdominal viscera [6]. Direct action of orexin on the dorsal vagal complex possibly explains the gastric motor responses. Recently, Krowicki et al. [9] reported that the injection of orexin-A into the DMV induced marked increase in antral motility. This observation indicates a possibility that orexin directly affects on the DMV neurons to induce facilitation of gastric contractility. They also mentioned that the intragastric pressure was increased by the injection of orexin into the DMV. This result seems to differ from ours obtained in the present study. Considering the facts that orexin receptors are found in the NTS [11], and the connection between the NTS and the DMV is mainly inhibitory [20], we can suggest a possibility that orexin may act on the NTS neurons, which in turn inhibit the DMV neurons. The proximal stomach relaxed in response to stimuli associated with food intake, i.e. swallowing, esophageal distension and gastric distension [13]. Therefore, we speculate that relaxation of the proximal stomach is essential when orexins are released from the LHA. This study was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (Nos: 13671937 and 14370593). [1] Azpiroz, F. and Malagelada, J.-R., Physiological variations in canine gastric tone measured by an electronic barostat, Am. J. Physiol. Gastrointest. Liver Physiol., 248 (1985) G229–G237. [2] Bauer, F.E., Zintel, A., Kenny, M.J., Calder, D., Ghatei, M.A. and Bloom, S.R., Inhibitory effect of galanin on postprandial gastrointestinal motility and gut hormone release in humans, Gastroenterology, 97 (1989) 260–264. [3] de Lecea, L., Kilduff, T.S., Peyron, C., Gao, X., Foye, P.E., Danielson, P.E., Fukuhara, C., Battenberg, E.L., Gautvik, V.T., Bartlett 2nd, F.S., Frankel, W.N., van den Pol, A.N., Bloom, F.E., Gautvik, K.M. and Sutcliffe, J.G., The hypocretins: hypothalamus-specific peptides with neuroexcitatory activity, Proc. Natl. Acad. Sci. USA, 95 (1998) 322–327. [4] Edwards, C.M., Abusnana, S., Sunter, D., Murphy, K.G., Ghatei, M.A. and Bloom, S.R., The effect of the orexins on food intake: comparison with neuropeptide Y, melaninconcentrating hormone and galanin, J. Endocrinol., 160 (1999) R7–R12. [5] Harrison, T.A., Chen, C.T., Dun, N.J. and Chang, J.K., Hypothalamic orexin A-immunoreactive neurons project to the rat dorsal medulla, Neurosci. Lett., 273 (1999) 17–20. [6] Hwang, L.L., Chen, C.T. and Dun, N.J., Mechanisms of orexin-induced depolarizations in rat dorsal motor nucleus of vagus neurones in vitro, J. Physiol. (Lond.), 537 (2001) 511–520. [7] Kobashi, M., Koga, T., Mizutani, M. and Matsuo, R., Suppression of vagal motor activities evokes laryngeal afferentmediated inhibition of gastric motility, Am. J. Physiol. Regul. Integr. Comp. Physiol., 282 (2002) R818–R827. [8] Kobashi, M., Mizutani, M. and Matsuo, R., Water stimulation of the posterior oral cavity induces inhibition of gastric motility, Am. J. Physiol. Regul. Integr. Comp. Physiol., 279 (2000) R778–R786. [9] Krowicki, Z.K., Berthoud, H.R., Burmeister, M.A. and Hornby, P.J., Orexin-A in the dorsal motor nucleus of the vagus potently increases gastric motor function via orexin 1 recep-
174
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[11]
[12]
[13]
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