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Somatostatin receptor on the afferent nerve terminals in the rat hepatoportal area
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Neuroscience Letters 183 (1995) 46-49
N[m01K[ LLI([RI
Somatostatin receptor on the afferent nerve terminals in the rat hepatoportal area H a j i m e N a k a b a y a s h i a,*, K a t s u j i K o b a y a s h i b, I t s u k o O. N a k a b a y a s h i c, Y o r i a k i K u r a t a d aDepartment of lnternal Medicine(II), School of Medicine, Kanazawa University, 13-1 Takara-machi, Kanazawa 920, Japon bDepartment of Neuropsychiatry, School of Medicine, Kanazawa University, Kanazawa 920, Japan CDepartment of Immunobiology, Cancer Research Institute, Kanazawa University, Kânazawa 920, Japon dDepartment of Pathophysiology, Cancer Research Institute, Kanazawa University, Kanazawa 920, Japan Received 12 September 1994; revised version received 21 October 1994; accepted 21 October 1994
Abstract
To determine whether somatostatin receptor (SSR) actually exists on the nerve terminals in the rat hepatoportal area, the area was immunostained by the labeled streptoavidin-biotin complex method using a monoclonal antibody against rat brain SSR. The SSR staining revealed many fiber arborizations with terminal nodular swellings like the afferent nerve endings in the neural body, which was located beneath the endothelium of the large branches of the intrahepatic portal vein. The results indicate that SSR is expressed on the characteristic structure, adding further evidence for out previous observation which showed the hepatic vagal reception for intraportal somatostatin and the somatostatin-binding neural body as a relevant structure.
Keywords: Somatostatin receptor; Monoclonal antibody to somatostatin receptor; Terminal nerve endings; Neural body; Intrahepatic portal vein
The hepatic afferent vagus is known to be receptive to changes of metabolite concentration (e.g. glucose), osmolality and temperature in the portal vein blood as validated by behavioral and electrophysiological studies [7,8,10] (see also a review by Sawchenko and Friedman [11]). The vagal afferent nerve fibers are distributed to the hepatoportal area [11]. Studies with transmission electron microscopy and laser scanning confocal microscopy have recently demonstrated the terminal location of the fibers in the portal adventitia [1]. However, little attention has been paid to the vagal reception of gastrointestinal peptide hormone, a substance different from metabolites, circulating in the portal vein blood. Somatostatin (SS), one of the brain-gut hormones, is a cyclic tetradecapeptide widely distributed throughout the central and peripheral nervous systems as well as in the pancreas and gastrointestinal tract [9]. The splanchnic organs are known to be the major source of circulating somatostatin [9]. We previously demonstrated that injec* Corresponding author, Tel.: +81 762 62 8151, Ext. 3308; Fax: +81 762 61 2211.
0304-3940/95/$09.50
tion of SS at a physiological dose into the rat portal vein facilitated the afferent activity in the hepatic branch of the vagus nerve, suggesting another case of chemoreception by the hepatic vagus [2]. Moreover, we disclosed histologically the neural bodies which were located beneath the endothelium of the large branches of the rat portal vein [2]. (The neural body had been termed the neural corpuscle in the report.) The body was ellipsoid (2040/~m in diameter) with the long axis along the blood stream and contained the terminal nerve filaments in the core. The body did bind the exogenous SS injected into the portal vein, as revealed by immunostaining for SS. A similar electrophysiological phenomenon occurred in the pancreatic afferent vagus by SS injection into the rat pancreatic artery [3]. Furthermore, the glomus-like neurovascular body was found in the peripancreatic sinusoidal vein [3]. In this case, the body included many small corpuscles, and the corpuscle contained the afferent nerve endings. The structure preferentially bound exogenous SS, which was also revealed by immunostaining for SS. These electrophysiological and histological findings have indicated that the vagal nerve is receptive to SS in the
© 1995 Elsevier Science Ireland Ltd. Ail rights reserved
SSDI 0 3 0 4 - 3 9 4 0 ( 9 4 ) 1 1 1 1 1-R
H. Nakabayashi et al. I Neuroscience Letters 183 (1995) 46-49
portal blood, suggesting the involvement of the unique neural apparatus in the reception. As for the actual participation of the somatostatin receptor (SSR) system in the vagal reception of SS ih the portal blood, we bave recently demonstrated that a murine monoclonal antibody (MAb) against SSR uniquely modiries the reception [6]. In the study, we used the IgGl(x) MAb to rat neural S~R, which bound to the SS binding site of the receptor in competition with the ligand [4]. When the ascites containing the MAb was administered intraportally prior to I~S injection, the administration completely abolished the SS-induced increase in the hepatic vagal afferent discharge rate. On the other hand, the antibody-free eï3ritrol ascites did not exert any effect. In thé present immunohistochemical study, we further aimed to determine wh(«ther SSR actually existed in the neural bodies, which w~« previously found to be located beneath the endothelium of the intrahepatic portal vein. Immunostaining for SS1;', with the MAb di~èlosed thåt the neural body eontained a structure like afferent nerve endings on which SSR was expressed. Three maie Wistar (230-260 g) rats which were held in a conditioned colony room (12/12 h, light/dark cycle; 24 _+2°C) and fasted overnight were administered a lethal dose of sodium pentobm'bital (100 mg/kg, i.v.). The liver accompanying the main portal vein was extirpated immediately and perfused through the portal vein with normal saline and with 4% paraformaldehyde (pli 7.4). The tissues were stored furth(«r in the saine fixative for 12 h at 4°C, dehydrated, and embedded in paraffin. Serial sections, 4/zm thick, of the liver tissue near the porta were obtained along the axis of the portal vein and mounted on resin-coated glasses. Every tenth section was stained by hematoxylin-eosin (H-É) and examined. Immunostaining for SSR was performed using the labeled streptoavidinbiotin complex method (LSAB kit, DAKO Corp., Carpinteria, CA, USA) using the ascites containing the MAb (IgGl(x), SRA-17E) against rat neural SSR as primary antibody (the ascites was produced in female Balb/c mouse) [4]. Briefly, the deparaffinized sections were incubated with 3% hydrogen peroxide for 10 min. Some sections were pretreated with 1% trypsin (type 1, Sigma Chemical Company, St. Louis, MO, USA) for 10 min at room temperature. After incubation with normal goat serum for 30 rein, the sections were reacted with the MAb ascites containing 2 mg/ml of IgG at 1:50 dilution for 12 h at 4°C and visualized with 0.01% diaminobenzidine tetrahydrochloride and 0005% hydrogen peroxide. Sections incubated with the control ascites, prepared by injecting Sp 2/0 myeloma cells instead of the hybridoma cells, at 1:20 dilution, were also examined. Some of the immunostained sections were counterstained with hematoxylin. As shown in Fig. lA and reported previously by us [2], the encapsulated neural bodies (20-40 mm in diameter) were found at the beginniing of the large branches of the
47
portal veifi 0.9-1,5 mm apart from the porta of the liver. They lay beneath the gtldolh¢lium, protruded into the vessel lumen to various degrees, and appeared ellipsoid
C
Fig. 1. (A) The neural body (thick arrow) associated with the large branch (v) of the intrahepaticportal vein (P). Thin arrow indicates the neural body aeeompanied by the nerve bundle. H, hepatoeytes; B, peribiliary glands. Bar, 100pm. H-E staining. (B) A subendothelial ncural body in thc adjacent section (correspondingto the body indicated by the thick atrow in (A)) includes many fiber arborizationswith terminal nodular swellings. Immunostaining by the labeled streptoavidin-biotin method using the ascites containing the monoclonal antibody to somatostatin receptor (IgGl(x), SRA-17E; 1:50 dilution). Bar, 10pm. (C) Diagramshowing the fiber arborizationswith terminal nodular swellings (arrows)in the neural body associatedwith the large branch (V) ofthe intrahepaticportal vein seen in (B). Bar, lO/zm.
48
H. Nakabayashi et al. I Neuroscience Letters 183 (1995) 46--49
with their long axis along the blood stream. When the body was stained by the labeled streptoavidin-biotin complex method using anti-SSR MAb, fiber arborizations with terminal nodular swellings like the structure of afferent nerve endings, many in number, were seen within the body (Fig. lB and diagrammatically shown in Fig. 1C). Some of the arborizations were located just beneath the endothelium of the portal branch. The somewhat thick nerve fibers only near the terminal-like structures also showed SSR staining in some sections of the body. No terminal arborizations and fibers with SSR staining were observed in hepatic parenchyma. The sections stained with the antibody-free control ascites did not reveal such fiber arborizations with terminal swellings or the fibers (data not shown). As reported previously [2], we observed the arborizations of rather thick nerve fibers in the body, resembling those known as the afferent apparatus of Krause, when the body was silver-stained by the method of Holmes. In the present study, we have further disclosed that the body contained many thinner fiber arborizations with terminal nodular swellings. The structure was visualized only when the body was immunostained with the MAb to SSR. The structure seemed to be limited by a very thin membrane, although the thin membrane itself was not clearly immunostained with the MAb. These findings clearly show that SSR does exist almost exclusively on the tips of the arborizations of nerve fibers in the body. As for the fine terminal arborizations in the neural bodies, the size and structure of the terminal-like structures resemble those of the fine terminal arborizations in the corpuscles within the glomus-like neurovascular body observed in the rat peripancreatic sinusoidal vein, which have been reported by us [3]. In the aspect of the nature of SS binding, the neural body, as a whole, preferentially bound the exogenous SS injected into the portal vein, as shown by outstanding diffuse SS staining [2] However, only the terminal nerve structure in the corpuscle within the glomus-like body caught exogenous SS injected into the pancreatic artery, as clearly revealed by the immunostaining for SS. Thus, it is intriguing to speculate that the neural body attached to the portal vein itself traps SS at high blood flow, low SS concentration area, whereas the tiny nerve structure in the capillaryrich, 'soaking up' glomus-like body preferentially catches SS at low blood flow, high SS concentration region near the pancreas. The mechanism involved in the diffuse staining for SS in the neural body remains to be solved. In any event, the presence of SSR on the tip of the terminal arborizations in the SS-trapping neural body could provide an efficient means for SS reception in the portal vein. The present histological observations explain our recent observation indicating the actual involvement of a SSR mechanism in the vagal reception of SS in the hepatoportal area; prior administration of anti-SSR MAb into
the portal vein completely abolished the increase in the hepatic afferent vagal discharge rate otherwise induced by an intraportal SS administration [6]. Concerning the anatomical relation between the hepatic afferent vagus and the SSR-expressing terminal arborizations in the neural body, this still remains to be clarified. However, the combined results of these immunohistological and electrophysiological studies strongly suggest that a peptide hormone secreted from the splanchnic organs acts on its own specifïc receptor on the afferent vagal nerve in the hepatoportal area. This concept concerning the presence of a specific receptor on the afferent vagus was further supported by our recent observation in the reception of a glucagon-like peptide-1 (GLP-1) [5]. GLP-1 (7-36) amide, a truncated form of GLP-1 (1-37) known to have the most powerful incretin effect, facilitated hepatic afferent vagal activity when it Was injected into the portal vein at a physiological dose. However, GLP-1 (1-37), known to have no incretin effect, failed to do so even at a pharmacological dose, although both forms of GLP-1 are secreted from the intestinal L cells. Thus, the specific receptor for GLP-1, which can distinguish a difference of only six amino acids, may be operating on the afferent vagus in this case. Tbis is the first observation to substantiate the presence of SSR on a unique structure similar to afferent nerve terminals in the hepatoportal area. Such a receptormediated system may convert humoral information to neural information. Signais thus generated could contribute to regulation of appetite, eating behavior, and nutrient homeostasis.
The authors thank Ryoyu Takeda, MD, Morinobu Takahashi, MD and Fumihiko Muramori, MD for supporting the study, and also Teruo Ikeda and Motoji Hisatomi for their technical assistance. [1] Berthoud,H.R., Kressel,M. and Neuhuber, W.L., An anterograde tracing study of the vagal innervation of rat liver, portal rein and biliary system,Anat. Embryoi., 186 (1992) 431--442. [2] Nakabayashi, H., Niijima, A., Kurata, Y., Usukura, N. and Takeda, R., Somatostatin-sensitive neural system in the liver, Neurosci. Lett., 67 (1986) 78-81. [3] Nakabayashi,H., Niijima, A., Knrata, Y., Jiang, Z.-Y., Usukura, N. and Takeda, R., Pancreatic vagal nerve is receptive to somatostatin in rats, Am. J. Physiol., 253 (1987) R200--R203. [4] Nakabayashi,I.O. and Nakabayashi,H., Monoelonal antibodies to somatostatin receptor of rat brain, Hybridoma, 11 (1992) 789794. [5] Nakabayashi,H., Nishizawa, M., Takeda, R. and Niijima, A., A novel role of truncated glucagon-likepeptide-I in the enteroinsular axis, Diabetes,41 (snppl. 1) (1992) 368A. [6] Nakabayashi,H., Niijima, A., Nishizawa, M., Nakabayashi, I.O. and Takeda, R., A unique receptor-mediatedmechanismin vagal chernoreception of somatostaanin the hepatoportalarea, J. Auton. Nerv. Syst., 50 (1994) 45-50. [7] Niijima, A., Afferent discharges from osrnoreceptors in the liver of the guinea pig, Science, 166 (1969) 1519-1520.
H. Nakabayashi et al. I Neuroscience Letters 183 (1995) 46-49
[8] Niijima, A., Glucose-sensitive afferent nerve fibers in the hepatic braneh of the vagus nerve in the guinea pig, J. Physiol. (London), 332 (1982) 315-323. [9] Reiehlin, S., Somatostatin, N. Engl. J. Med., 309 (1983) 14951501; 1556-1563.
49
[10] Russek, M., Participation of hepatic glucoreeeptors in the eontrol of food, Nature (London), 197 (1963) 79-80. [I1] Sawchenko, P.E. and Friedman, M.I., Sensory funefions of the liver: a review, Ara. J. Physiol., 236 (1979) R5-R20.
Neuroscience Letters 183 (1995) 46-49
N[m01K[ LLI([RI
Somatostatin receptor on the afferent nerve terminals in the rat hepatoportal area H a j i m e N a k a b a y a s h i a,*, K a t s u j i K o b a y a s h i b, I t s u k o O. N a k a b a y a s h i c, Y o r i a k i K u r a t a d aDepartment of lnternal Medicine(II), School of Medicine, Kanazawa University, 13-1 Takara-machi, Kanazawa 920, Japon bDepartment of Neuropsychiatry, School of Medicine, Kanazawa University, Kanazawa 920, Japan CDepartment of Immunobiology, Cancer Research Institute, Kanazawa University, Kânazawa 920, Japon dDepartment of Pathophysiology, Cancer Research Institute, Kanazawa University, Kanazawa 920, Japan Received 12 September 1994; revised version received 21 October 1994; accepted 21 October 1994
Abstract
To determine whether somatostatin receptor (SSR) actually exists on the nerve terminals in the rat hepatoportal area, the area was immunostained by the labeled streptoavidin-biotin complex method using a monoclonal antibody against rat brain SSR. The SSR staining revealed many fiber arborizations with terminal nodular swellings like the afferent nerve endings in the neural body, which was located beneath the endothelium of the large branches of the intrahepatic portal vein. The results indicate that SSR is expressed on the characteristic structure, adding further evidence for out previous observation which showed the hepatic vagal reception for intraportal somatostatin and the somatostatin-binding neural body as a relevant structure.
Keywords: Somatostatin receptor; Monoclonal antibody to somatostatin receptor; Terminal nerve endings; Neural body; Intrahepatic portal vein
The hepatic afferent vagus is known to be receptive to changes of metabolite concentration (e.g. glucose), osmolality and temperature in the portal vein blood as validated by behavioral and electrophysiological studies [7,8,10] (see also a review by Sawchenko and Friedman [11]). The vagal afferent nerve fibers are distributed to the hepatoportal area [11]. Studies with transmission electron microscopy and laser scanning confocal microscopy have recently demonstrated the terminal location of the fibers in the portal adventitia [1]. However, little attention has been paid to the vagal reception of gastrointestinal peptide hormone, a substance different from metabolites, circulating in the portal vein blood. Somatostatin (SS), one of the brain-gut hormones, is a cyclic tetradecapeptide widely distributed throughout the central and peripheral nervous systems as well as in the pancreas and gastrointestinal tract [9]. The splanchnic organs are known to be the major source of circulating somatostatin [9]. We previously demonstrated that injec* Corresponding author, Tel.: +81 762 62 8151, Ext. 3308; Fax: +81 762 61 2211.
0304-3940/95/$09.50
tion of SS at a physiological dose into the rat portal vein facilitated the afferent activity in the hepatic branch of the vagus nerve, suggesting another case of chemoreception by the hepatic vagus [2]. Moreover, we disclosed histologically the neural bodies which were located beneath the endothelium of the large branches of the rat portal vein [2]. (The neural body had been termed the neural corpuscle in the report.) The body was ellipsoid (2040/~m in diameter) with the long axis along the blood stream and contained the terminal nerve filaments in the core. The body did bind the exogenous SS injected into the portal vein, as revealed by immunostaining for SS. A similar electrophysiological phenomenon occurred in the pancreatic afferent vagus by SS injection into the rat pancreatic artery [3]. Furthermore, the glomus-like neurovascular body was found in the peripancreatic sinusoidal vein [3]. In this case, the body included many small corpuscles, and the corpuscle contained the afferent nerve endings. The structure preferentially bound exogenous SS, which was also revealed by immunostaining for SS. These electrophysiological and histological findings have indicated that the vagal nerve is receptive to SS in the
© 1995 Elsevier Science Ireland Ltd. Ail rights reserved
SSDI 0 3 0 4 - 3 9 4 0 ( 9 4 ) 1 1 1 1 1-R
H. Nakabayashi et al. I Neuroscience Letters 183 (1995) 46-49
portal blood, suggesting the involvement of the unique neural apparatus in the reception. As for the actual participation of the somatostatin receptor (SSR) system in the vagal reception of SS ih the portal blood, we bave recently demonstrated that a murine monoclonal antibody (MAb) against SSR uniquely modiries the reception [6]. In the study, we used the IgGl(x) MAb to rat neural S~R, which bound to the SS binding site of the receptor in competition with the ligand [4]. When the ascites containing the MAb was administered intraportally prior to I~S injection, the administration completely abolished the SS-induced increase in the hepatic vagal afferent discharge rate. On the other hand, the antibody-free eï3ritrol ascites did not exert any effect. In thé present immunohistochemical study, we further aimed to determine wh(«ther SSR actually existed in the neural bodies, which w~« previously found to be located beneath the endothelium of the intrahepatic portal vein. Immunostaining for SS1;', with the MAb di~èlosed thåt the neural body eontained a structure like afferent nerve endings on which SSR was expressed. Three maie Wistar (230-260 g) rats which were held in a conditioned colony room (12/12 h, light/dark cycle; 24 _+2°C) and fasted overnight were administered a lethal dose of sodium pentobm'bital (100 mg/kg, i.v.). The liver accompanying the main portal vein was extirpated immediately and perfused through the portal vein with normal saline and with 4% paraformaldehyde (pli 7.4). The tissues were stored furth(«r in the saine fixative for 12 h at 4°C, dehydrated, and embedded in paraffin. Serial sections, 4/zm thick, of the liver tissue near the porta were obtained along the axis of the portal vein and mounted on resin-coated glasses. Every tenth section was stained by hematoxylin-eosin (H-É) and examined. Immunostaining for SSR was performed using the labeled streptoavidinbiotin complex method (LSAB kit, DAKO Corp., Carpinteria, CA, USA) using the ascites containing the MAb (IgGl(x), SRA-17E) against rat neural SSR as primary antibody (the ascites was produced in female Balb/c mouse) [4]. Briefly, the deparaffinized sections were incubated with 3% hydrogen peroxide for 10 min. Some sections were pretreated with 1% trypsin (type 1, Sigma Chemical Company, St. Louis, MO, USA) for 10 min at room temperature. After incubation with normal goat serum for 30 rein, the sections were reacted with the MAb ascites containing 2 mg/ml of IgG at 1:50 dilution for 12 h at 4°C and visualized with 0.01% diaminobenzidine tetrahydrochloride and 0005% hydrogen peroxide. Sections incubated with the control ascites, prepared by injecting Sp 2/0 myeloma cells instead of the hybridoma cells, at 1:20 dilution, were also examined. Some of the immunostained sections were counterstained with hematoxylin. As shown in Fig. lA and reported previously by us [2], the encapsulated neural bodies (20-40 mm in diameter) were found at the beginniing of the large branches of the
47
portal veifi 0.9-1,5 mm apart from the porta of the liver. They lay beneath the gtldolh¢lium, protruded into the vessel lumen to various degrees, and appeared ellipsoid
C
Fig. 1. (A) The neural body (thick arrow) associated with the large branch (v) of the intrahepaticportal vein (P). Thin arrow indicates the neural body aeeompanied by the nerve bundle. H, hepatoeytes; B, peribiliary glands. Bar, 100pm. H-E staining. (B) A subendothelial ncural body in thc adjacent section (correspondingto the body indicated by the thick atrow in (A)) includes many fiber arborizationswith terminal nodular swellings. Immunostaining by the labeled streptoavidin-biotin method using the ascites containing the monoclonal antibody to somatostatin receptor (IgGl(x), SRA-17E; 1:50 dilution). Bar, 10pm. (C) Diagramshowing the fiber arborizationswith terminal nodular swellings (arrows)in the neural body associatedwith the large branch (V) ofthe intrahepaticportal vein seen in (B). Bar, lO/zm.
48
H. Nakabayashi et al. I Neuroscience Letters 183 (1995) 46--49
with their long axis along the blood stream. When the body was stained by the labeled streptoavidin-biotin complex method using anti-SSR MAb, fiber arborizations with terminal nodular swellings like the structure of afferent nerve endings, many in number, were seen within the body (Fig. lB and diagrammatically shown in Fig. 1C). Some of the arborizations were located just beneath the endothelium of the portal branch. The somewhat thick nerve fibers only near the terminal-like structures also showed SSR staining in some sections of the body. No terminal arborizations and fibers with SSR staining were observed in hepatic parenchyma. The sections stained with the antibody-free control ascites did not reveal such fiber arborizations with terminal swellings or the fibers (data not shown). As reported previously [2], we observed the arborizations of rather thick nerve fibers in the body, resembling those known as the afferent apparatus of Krause, when the body was silver-stained by the method of Holmes. In the present study, we have further disclosed that the body contained many thinner fiber arborizations with terminal nodular swellings. The structure was visualized only when the body was immunostained with the MAb to SSR. The structure seemed to be limited by a very thin membrane, although the thin membrane itself was not clearly immunostained with the MAb. These findings clearly show that SSR does exist almost exclusively on the tips of the arborizations of nerve fibers in the body. As for the fine terminal arborizations in the neural bodies, the size and structure of the terminal-like structures resemble those of the fine terminal arborizations in the corpuscles within the glomus-like neurovascular body observed in the rat peripancreatic sinusoidal vein, which have been reported by us [3]. In the aspect of the nature of SS binding, the neural body, as a whole, preferentially bound the exogenous SS injected into the portal vein, as shown by outstanding diffuse SS staining [2] However, only the terminal nerve structure in the corpuscle within the glomus-like body caught exogenous SS injected into the pancreatic artery, as clearly revealed by the immunostaining for SS. Thus, it is intriguing to speculate that the neural body attached to the portal vein itself traps SS at high blood flow, low SS concentration area, whereas the tiny nerve structure in the capillaryrich, 'soaking up' glomus-like body preferentially catches SS at low blood flow, high SS concentration region near the pancreas. The mechanism involved in the diffuse staining for SS in the neural body remains to be solved. In any event, the presence of SSR on the tip of the terminal arborizations in the SS-trapping neural body could provide an efficient means for SS reception in the portal vein. The present histological observations explain our recent observation indicating the actual involvement of a SSR mechanism in the vagal reception of SS in the hepatoportal area; prior administration of anti-SSR MAb into
the portal vein completely abolished the increase in the hepatic afferent vagal discharge rate otherwise induced by an intraportal SS administration [6]. Concerning the anatomical relation between the hepatic afferent vagus and the SSR-expressing terminal arborizations in the neural body, this still remains to be clarified. However, the combined results of these immunohistological and electrophysiological studies strongly suggest that a peptide hormone secreted from the splanchnic organs acts on its own specifïc receptor on the afferent vagal nerve in the hepatoportal area. This concept concerning the presence of a specific receptor on the afferent vagus was further supported by our recent observation in the reception of a glucagon-like peptide-1 (GLP-1) [5]. GLP-1 (7-36) amide, a truncated form of GLP-1 (1-37) known to have the most powerful incretin effect, facilitated hepatic afferent vagal activity when it Was injected into the portal vein at a physiological dose. However, GLP-1 (1-37), known to have no incretin effect, failed to do so even at a pharmacological dose, although both forms of GLP-1 are secreted from the intestinal L cells. Thus, the specific receptor for GLP-1, which can distinguish a difference of only six amino acids, may be operating on the afferent vagus in this case. Tbis is the first observation to substantiate the presence of SSR on a unique structure similar to afferent nerve terminals in the hepatoportal area. Such a receptormediated system may convert humoral information to neural information. Signais thus generated could contribute to regulation of appetite, eating behavior, and nutrient homeostasis.
The authors thank Ryoyu Takeda, MD, Morinobu Takahashi, MD and Fumihiko Muramori, MD for supporting the study, and also Teruo Ikeda and Motoji Hisatomi for their technical assistance. [1] Berthoud,H.R., Kressel,M. and Neuhuber, W.L., An anterograde tracing study of the vagal innervation of rat liver, portal rein and biliary system,Anat. Embryoi., 186 (1992) 431--442. [2] Nakabayashi, H., Niijima, A., Kurata, Y., Usukura, N. and Takeda, R., Somatostatin-sensitive neural system in the liver, Neurosci. Lett., 67 (1986) 78-81. [3] Nakabayashi,H., Niijima, A., Knrata, Y., Jiang, Z.-Y., Usukura, N. and Takeda, R., Pancreatic vagal nerve is receptive to somatostatin in rats, Am. J. Physiol., 253 (1987) R200--R203. [4] Nakabayashi,I.O. and Nakabayashi,H., Monoelonal antibodies to somatostatin receptor of rat brain, Hybridoma, 11 (1992) 789794. [5] Nakabayashi,H., Nishizawa, M., Takeda, R. and Niijima, A., A novel role of truncated glucagon-likepeptide-I in the enteroinsular axis, Diabetes,41 (snppl. 1) (1992) 368A. [6] Nakabayashi,H., Niijima, A., Nishizawa, M., Nakabayashi, I.O. and Takeda, R., A unique receptor-mediatedmechanismin vagal chernoreception of somatostaanin the hepatoportalarea, J. Auton. Nerv. Syst., 50 (1994) 45-50. [7] Niijima, A., Afferent discharges from osrnoreceptors in the liver of the guinea pig, Science, 166 (1969) 1519-1520.
H. Nakabayashi et al. I Neuroscience Letters 183 (1995) 46-49
[8] Niijima, A., Glucose-sensitive afferent nerve fibers in the hepatic braneh of the vagus nerve in the guinea pig, J. Physiol. (London), 332 (1982) 315-323. [9] Reiehlin, S., Somatostatin, N. Engl. J. Med., 309 (1983) 14951501; 1556-1563.
49
[10] Russek, M., Participation of hepatic glucoreeeptors in the eontrol of food, Nature (London), 197 (1963) 79-80. [I1] Sawchenko, P.E. and Friedman, M.I., Sensory funefions of the liver: a review, Ara. J. Physiol., 236 (1979) R5-R20.