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Glucagon-like peptide-1 (7-36)amide (tGLP-1), a representative humoral incretin, released into the portal circulation in response to a meal ingestion, exerts insulinotropic action through binding to the tGLP-1 receptor known to be a single molecular form thus far. We previously reported that the hepatic vagal nerve is receptive to intraportal tGLP-1, but not to non-insulinotropic full-length GLP-1-(1-37), through a mechanism mediated by specific receptor to the hormone. In the present study, we aimed to examine how modification of the receptor function alters this neural reception of tGLP-1, by using the specific agonist, exendin-4, and the specific antagonist, exendin (9-39)amide, of the receptor at doses known to exert their effects on the insulinotropic action of tGLP-1. Intraportal injection of 0.2 or 4.0 pmol tGLP-1, a periphysiological and pharmacological dose, respectively, facilitated significantly the afferent impulse discharge rate of the hepatic vagus in anesthetized rats, as reported previously. However, unexpectedly, intraportal injection of exendin-4 at a dose of 0.2 or 4.0 pmol, or of even 40.0 pmol, did not facilitate the afferents at all. Moreover, intraportal injection of exendin (9-39)amide at 100 times or more molar dose to that of tGLP-1, either 5 min before or 10 min after injection of 0.2 or 4.0 pmol tGLP-1, failed to modify the tGLP-1-induced facilitation of the afferents. The present results suggest that the neural reception of tGLP-1 involves a receptor mechanism distinct from that in the well-known humoral insulinotropic action.

Paraganglia are clusters of cells containing catecholamines (CA), mainly norepinephrine (NE) and dopamine (DA). The presence of epinephrine (E), on the other hand, has only been determined by indirect methods in retroperitoneal paraganglia of newborn and aged rats. Because their location, paraganglia associated with the hepatic branch of the vagus nerve may be a possible source of CA for the liver. The main purposes of the present study were to determine CA levels and whether E can be found in the omentum minus which includes paraganglia associated with the hepatic branch of the vagus nerve, and then to study the effects of 6-hydroxydopamine and reserpine on their CA content. Twenty-four female Wistar rats were randomly ascribed to three groups receiving two intraperitoneal injections of either 6-hydroxydopamine, reserpine or saline. Twenty-four hours after the last administration the rats were anesthetized and a portion of the omentum minus was obtained. Left adrenal medulla and a liver fragment were also collected as controls. The samples were processed to be analyzed by high performance liquid chromatography and catecholamine histofluorescence. The results confirm previous reports about the presence of considerable amounts of norepinephrine and dopamine in paraganglia. Norepinephrine and dopamine in the omentum like the adrenal medulla were significantly depleted by reserpine but not by 6-hydroxydopamine treatment, suggesting that some other sources in addition to sympathetic terminals are responsible for CA in the omentum. On the contrary, both drugs reduced liver NE, consistent with the localization of this amine mainly to hepatic sympathetic terminals. Histofluorescence of the omentum revealed 2-4 paraganglia per tissue fragment. Paraganglia associated with the hepatic branch of the vagus nerve contain also E. The presence of perihepatic sources of extra-adrenal CA, and more specifically E, could be of physiological significance.

The effect of intraportal administrations of interleukin 1 beta (IL-1 beta) on the afferent activity of the hepatic branch of the vagus nerve was observed in urethane anesthetized rats. An intraportal injection of IL-1 beta in doses of 10 pg and 100 pg per animal (300-400 g body wt.) resulted in dose-dependent increase in the afferent activity, which lasted about 70-100 min. Further, intraportal injection of IL-1 beta (100 pg) induced reflex activation of efferent activity of the splenic (sympathetic) nerve and vagal thymic nerve. This reflex activation was not observed in hepatic vagotomized rat and after an administration of the same dose of IL-1 beta into the systemic vein in normal rat. The results suggest the existence of sensors for IL-1 beta which send information on IL-1 beta in the portal venous blood to the central nervous system through the hepatic branch of the vagus nerve and play some role in reflex regulation of immune function.

Peripheral administration of the fructose analogue 2,5-anhydro-D-mannitol (AM), that inhibits hepatic glucose release and ATP formation, stimulates food intake in rats. This effect is partly generated in the hepato-portal area and transmitted to the central nervous system by the common hepatic vagus branch because hepatic branch vagotomy eliminated the feeding response to AM. In the present study, we investigated if the pertinent signal to increase food intake changes the discharge rate in hepatic vagal afferents. An in vivo preparation was used to record afferent hepatic vagal activity following intraportal infusion of AM in anaesthetized rats. Fine nerve filaments were isolated from the distal cut end of the hepatic vagus branch. Nerve activity was recorded by a bipolar electrode and analyzed after conversion of raw data to standard pulses. Standard pulses were integrated into spike counts of 5 s duration and the mean number of spikes in a 50 s interval at baseline was compared to spike count 10, 30 and 50 min after infusion of AM (100 or 300 mg/kg) or saline (control). Saline infusion did not influence afferent hepatic vagal activity. Intraportal infusion of AM, however, dose-dependently increased afferent activity in the hepatic vagus branch. In conclusion, AM increased the afferent discharge rate in the common hepatic vagus branch at doses that have previously been shown to increase food intake. These findings agree with the proposed role of fuel metabolism in the hepato-portal area in the control of food intake and with the suggestion that fuel availability controls food intake by influencing the hepatic afferent discharge rate.

In vivo microdialysis was applied to investigate the effects of hepatic nerve stimulation on glycogenolysis in rat liver under anesthesia. We analyzed the norepinephrine (NE) outflow and glucose output from the liver through the measurement of NE and glucose in the microdialysis dialyzate, as well as the plasma glucose level. Stimulation of the hepatic nerves (10 Hz, 20 V, 2 ms, 20 s every minute) increased NE outflow and glucose output from the liver. The blood glucose level increased by 1.5-1.6 times over the basal level at the end of the 10 min intermittent stimulation. Bilateral adrenalectomy and pancreatectomy did not abolish the glycogenolysis that was induced by the nerve stimulation. Phentolamine an alpha-antagonist, reduced the effects of nerve stimulation on the glucose output and the plasma glucose level. Phentolamine caused an increase in the NE outflow. Quinacline, an inhibitor of phospholipase A2, inhibited the glycogenolytic nerve effects without any inhibition of the NE outflow. These data show that hepatic nerve stimulation produces glycogenolysis via alpha-adrenergic mechanism and partly mediated by eicosanoids, and that microdialysis is a useful and simple method for the study of liver metabolism in physiological conditions.

To examine whether incretin hormones, truncated glucagon-like peptide-1 (tGLP-1) and glucose-dependent insulinotropic polypeptide (GIP), are recognized by the hepatic vagal nerve, changes of the impulse discharge rate in the afferent vagus upon their intraportal administrations were measured in situ in rats anesthetized with urethan and chloralose. One-min injection of tGLP-1 at a periphysiological dose of 0.2 pmol or a pharmacological dose of 4.0 pmol, but not of the vehicle, significantly facilitated the hepatic vagal afferents. However, the injection of GIP at either a physiological dose of 0.2 pmol, a periphysiological dose of 4.0 pmol, or an even much larger dose of 40.0 pmol did not change the afferents at all. The present results indicate that the hepatic vagus specifically recognizes an intraportal appearance of tGLP-1 in the hepatoportal area, suggesting that the vagal monitoring system for intraportal levels of the incretin hormone operates on ingestion of a mixed meal.

This study was designed to investigate the effects of hepatic denervation (HD) on NaCl balance and arterial blood pressure in conscious Wistar-Kyoto rats maintained on normal or high-NaCl food. When rats were fed normal NaCl food (0.45% NaCl), HD had no significant effect on NaCl balance and arterial pressure. However, when rats were fed high-NaCl food (8% NaCl for 10 days), NaCl balance became more positive in HD rats than in sham rats. Furthermore, mean arterial pressure in HD rats (120 +/- 2 mmHg) was higher than that in sham rats (108 +/- 3 mmHg). These results indicate that the hepatic nerves play a significant role in the long-term control of NaCl balance and arterial pressure during high-NaCl food intake.

Our previous results have indicated that the hepatic vagus is receptive to intraportal appearance of somatostatin-14 (SS), a gastroenteropancreatic peptide hormone, in rat. To examine the actual involvement of the SS receptor mechanism in the neural reception, the receptor function was modified with monoclonal antibody against rat neural SS receptor. Our monoclonal antibody of IgG1(kappa) isotype bound to the SS binding site of the receptor in competition with the ligand. Intraportal SS injection at a physiological dose (3.05 pmol) significantly facilitated the afferent firing discharge rate of the hepatic vagus in rats anesthetized with urethane and chloralose. The antibody ascites, but not the control ascites, administered intraportally 5 min prior to the SS injection abolished completely the SS-induced increase in discharge rate. However, when the antibody was administered more than 15 min after the SS injection, the SS-induced increase in discharge rate did appear, implying an unique post-receptor mechanism in reception. The present results indicate that the SS receptor mechanism operates in the neurochemoreception of SS, suggesting the existence of a hepatic vagal, receptor-mediated system for monitoring the hormone secreted from the splanchnic organs.

Jejunal electrolyte absorption was measured in the jejunal loop of anesthetized rats during an infusion of 9% NaCl solution via the portal vein. Net absorption of Na and Cl, and osmolality were significantly depressed by the portal 9% NaCl infusion. The decrease in net absorption was completely abolished by hepatic denervation. In addition, bilateral chemical inactivation of the nucleus tractus solitarius (NTS) by microinjection of kainic acid abolished the depressing effect of the portal venous infusion of 9% NaCl. These findings indicate that NTS is involved in the central pathway of the hepatojejunal reflex.

Hepatic pyruvate, lactate and glycogen concentrations were measured 40 min after gavage of 5 ml 35% fructose or glucose to rats with sham surgery or section of the hepatic branch of the vagus nerve. Other animals were treated with saline or the peripheral anticholinergic drug, atropine methyl nitrate, prior to gavage of fructose. The increase in pyruvate concentration produced by the carbohydrate loads was attenuated by both hepatic vagotomy and cholinergic blockade. The increase in lactate concentration after a fructose load was attenuated by hepatic branch vagotomy but not atropine injection. The increase in glycogen deposition after a fructose load was attenuated by atropine injection but not hepatic branch vagotomy. These results suggest that the hepatic metabolic response to fructose and glucose loads in rats depends on a functional parasympathetic nervous system.

The influence of hepatic branch vagotomy (HBV) in comparison to sham-vagotomy (SV) on the hyperphagia induced in rats by intraperitoneally injected 2-deoxy-D-glucose (2-DG, 250 mg/kg b.wt.) was investigated in nine experiments. Injections were given at various times of the 12-h light/12-h dark cycle in ad libitum fed rats (0 h, 1 h, 2 h, 4 h, 10 h after dark onset; 0 h and 6 h after onset of the bright phase). In two further experiments, 2-DG was injected 1 h after the onset of darkness or at the onset of light immediately after a meal (duration: 30 min) induced by short-term (2 h) food deprivation. 2-DG significantly increased food intake in SV-rats in all experiments, whereas in HBV rats there was no significant response to 2-DG in two experiments (injections 1 h after dark onset after ingestion of a meal, or 4 h after the onset of darkness). These findings indicate that an intact hepatic vagus branch is a prerequisite for "glucoprivic" feeding under certain conditions, and that hepatic vagal glucoreceptors participate in the control of feeding.

Exercise is known to increase hepatic glucose production. Previous studies have suggested that the sympathetic nerves only marginally contribute to this process. This study examined whether increased catecholamine response or increased adrenoceptor sensitivity might have affected previous results showing no effect of hepatic denervation on the increased hepatic glucose production during exercise. Hepatic sympathetic denervated rats, sham-operated rats and control rats were forced to swim against a counter current for 15 minutes. Denervations and sham operations were performed 9 days prior to swimming. The results show that denervation did not affect the changes in levels of blood glucose, plasma FFA, and catecholamines before, during and after swimming. Furthermore, hepatic adrenoceptor sensitivity was not altered in denervated rats, since intravenous infusions of epinephrine (20 ng/min) and norepinephrine (50 ng/min) similarly changed blood glucose and plasma FFA levels in liver-denervated, sham-operated and control rats. Thus, the increase in blood glucose levels during intravenous infusion of epinephrine and norepinephrine in the respective groups was 1.2 +/- 0.3 and 1.0 +/- 0.3 mmol/l (liver-denervated rats), 1.6 +/- 0.4 and 0.7 +/- 0.3 mmol/l (sham-operated rats) and 1.3 +/- 0.3 and 0.8 +/- 0.3 mmol/l (control rats), respectively. After adrenodemedullation, however, the rise of glucose levels during swimming in liver-denervated and control rats was completely abolished. Thus, the glucose response to swimming with and without adrenodemullation was 0.1 +/- 0.4 and 1.7 +/- 0.4 mmol/l in liver-denervated rats (P < 0.01) and -0.2 +/- 0.4 and 2.2 +/- 0.2 mmol/l in control rats (P < 0.001), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

To clarify the physiological role of vagal amino acid sensors in the liver, the effect of hepatic vagotomy and/or celiac vagotomy (sectioning of the hepatic branch and/or the celiac branches of the vagus nerve) on the secretion of insulin and glucagon after intraperitoneal injection of neutral (L-alanine, L-leucine, and L-phenylalanine), acidic (L-glutamate), or nonmetabolized (cycloleucine) acids, was examined in rats. Hepatic vagotomy enhanced both plasma glucose and glucagon concentrations after intraperitoneal injection of alanine more than those in sham-vagotomized (control) rats, while after intraperitoneal injection of leucine, hepatic vagotomy decreased plasma glucose concentrations and enhanced plasma insulin concentrations more than in control animals. These effects, following both alanine and leucine administration, were blocked by celiac vagotomy. Glutamate, phenylalanine, and cycloleucine stimulation in hepatic-vagotomized rats caused no significant differences in plasma glucose, insulin, or glucagon levels as compared to those in sham-vagotomized rats. Celiac vagotomy alone did not affect plasma glucose, insulin, or glucagon concentrations after stimulation by these five amino acids. The physiological role of alanine and leucine sensors may be to prevent amino acid-induced exaggerated pancreatic hormone secretion and to maintain blood glucose homeostasis, while glutamate, phenylalanine, and cycloleucine have no effect on this pancreatic neuroendocrine system.

Neurons in the suprachiasmatic nucleus (SCN) are suggested to be involved in the mechanism of glucose homeostasis. This mechanism was examined by studies on the effect of illumination on the activity of autonomic efferents to the adrenals, pancreas and liver. Exposure of one eye of anesthetized rats to light enhanced the efferent activity of the adrenal nerve and suppressed that of vagal pancreatic and hepatic nerves. No change in efferent activities of these nerves was observed on light-stimulation of rats with lesions that included the bilateral SCN. These findings indicate that light signals modulate visceral functions including metabolic processes through the retinohypothalamic tract probably via the SCN to autonomic efferent pathways innervating visceral organs.

We examined responses of renal nerve activity, urine flow, and urinary Na+ excretion to a hypertonic NaCl infusion in chronically instrumented conscious rabbits with unilateral renal denervation. The intravenous infusion of 20% NaCl, at 0.2 ml/min for 30 min, increased plasma osmolality by 27 +/- 5 mOsm/kg, and plasma Na+ by 16 +/- 3 mEq/l, and decreased hematocrit by 5 +/- 1%. These changes were accompanied by a marked decrease in renal nerve activity by 82 + 7%. Urine flow and urinary Na+ excretion increased gradually and peaked at the end of infusion. The innervated kidney excreted 23.3 +/- 3.3 ml urine and 5.5 +/- 0.7 mEq Na+ for the subsequent 60 min. However, the contralateral denervated kidney excreted only 9.5 +/- 2.0 ml urine and 2.2 +/- 0.6 mEq Na+; these values were significantly less than those of the innervated kidney. To examine the role of the sinoaortic and cardiopulmonary baroreceptors and the hepatic nerves in the response of renal nerve activity to the hypertonic NaCl infusion, renal nerve activity was examined in conscious rabbits with sinoaortic baroreceptor denervation (SAD) plus vagotomy and/or section of the anterior and posterior hepatic nerves (SAPH). In rabbits with SAD plus vagotomy or SAPH, the NaCl infusion also decreased renal nerve activity. After combining SAPH and SAD plus vagotomy, the decrease in renal nerve activity was completely blocked. These results indicate that hypertonic NaCl infusion elicits a marked decrease in renal nerve activity which is mediated predominantly by sinoaortic and cardiopulmonary baroreflexes and the hepatic nerves, and that the decrease in renal nerve activity plays an important role in the augmentation of renal function.

We reported the existence of vagal arginine sensors in the liver which modulate arginine-induced pancreatic hormone secretion. The present study was carried out to examine the possible existence of other amino acid sensors such as L-alanine and L-leucine in the hepato-portal system in rats using an electrophysiological approach. Afferent discharges were recorded from fine filaments dissected from the peripheral cut end of the hepatic branch of the vagus nerve. Administration of 0.1, 1, and 10 mM L-alanine and L-leucine solution (0.1 ml) into the portal vein caused an increase in the discharge rate of hepatic vagal afferents in a dose-dependent manner. The results suggest the existence of vagal amino acid sensors which are sensitive to alanine and leucine in the hepato-portal system.

Electrophysiological evidence exists for sensitivity to many aspects of nutrient depletion and repletion at all levels of the viscera and beyond. Behavioral evidence suggests that feeding is sensitive to manipulations at all of these levels, while at the same time showing that no one site or nutrient is required for feeding regulation. This suggests that one must look at the sequence and interdependence of events occurring from ingestion to ultimate tissue distribution in order to understand the control of food intake.

The afferent and parasympathetic preganglionic innervations of the rat liver were investigated by the use of retrograde transport of horseradish peroxidase (HRP). Vagal nerve fibers reach the rat liver by way of the left and right hepatic nerves, which originate from the homonimous abdominal vagal trunks. Three different experimental protocols were used: (i) intraparenchymal HRP injections; (ii) retrograde HRP injection through the common bile duct; (iii) HRP application to the central end of the severed hepatic nerves. The technical problems inherent in these 3 methods were experimentally investigated, with regard to the possible leakage of HRP from the liver after retrograde injection. It is concluded that leakage of HRP occurs, but it is not sufficient to cause non-specific labeling. Neurons of the lower thoracic dorsal root ganglia (DRG) are bilaterally labeled following intraparenchymal and retrograde HRP injections. Bilateral labeling of the nodose ganglia (NGs) following retrograde injection is still observed after subdiaphragmatic section of the left abdominal vagus, whereas cervical transection prevents labeling of the ipsilateral NG. Labeling produced by exposure of the left hepatic nerve to HRP is prevented by subdiaphragmatic transection of the left abdominal vagus. Efferent neurons located bilaterally in the dorsal motor nucleus (DMN) and in the left nucleus ambiguus (NA) are labeled following retrograde HRP injection. Only ipsilateral labeling is observed after HRP application to the cut left hepatic nerve. HRP exposure of the left abdominal vagus yields bilateral labeling in both DMN and NA. It is concluded that: (i) the afferent innervation of the rat liver is provided by the lower thoracic DRG and by the NGs of both vagi, mostly by the left; partial crossing of vagal afferent fibers takes place at thoracic level; and (ii) the liver receives efferent fibers bilaterally from the DMN and from the left NA; the DMN neurons project to the liver via the homolateral hepatic nerves, and those of the NA via the left hepatic nerve.

Intraportal venous infusion of glucose resulted in a significant post-infusion decrease in the mean discharge rate in the adrenal nerve and the hepatic branch of the splanchnic nerve. Intraportal venous infusion of the same amount of glucose caused a significant increase in discharge rate of the pancreatic branch of the vagus nerve. After section of the hepatic branch of the vagus nerve, the same procedure induced no change in discharge rates in any of these nerves. Repetitive electrical stimulation of the central stump of the hepatic branch of the vagus nerve also caused reflex changes in efferent discharge rates in these three nerves, an increase in discharge rate in the hepatic branch of the splanchnic nerve and adrenal nerve, and a decrease in the pancreatic branch of the vagus nerve in the normal animals and also in midpontine-transected animals. The reflex network may consist of glucose-sensitive hepatic afferents, a reflex center in the hindbrain, and vagal pancreatic efferents, adrenal and splanchnic-hepatic efferents. This network contributes a neural regulatory mechanism to the control of blood glucose levels.

The contribution of the central nervous system to recovery from insulin hypoglycemia was studied in dogs. Animals with spinal transection just above the sympathetic outflow (to isolate it from encephalic influences), bilateral vagotomy or both types of lesion received an i.v. insulin injection. Also dogs in which the entire central nervous system had been inactivated by osmotic shock were submitted to this test. The results, compared to those obtained from control groups, showed that either spinal transection or severance of the vagi alone did not prevent full recovery from hypoglycemia. When both surgical procedures were combined or the entire central nervous system had been functionally destroyed normal blood glucose concentration was not regained.

The role of the sensory input involved in the control of the food intake is briefly reviewed. Signals from mouth and throat, as well as signals from the gastrointestinal tract and the liver, are necessary but none of them are sufficient to induce satiation. The oral cues initiated by the food intake have a dual action on the feeding behavior. They first stimulate the intake in the hungry subjects. They sustain it until the amount of food ingested reaches the level necessary to equilibrate the energy balance. Finally, they participate, with the sensory input from the gastrointestinal tract, in the process inhibiting the food intake. The hedonic hypothesis of the food intake control is an attempt to explain this dual action of the peripheral alimentary analyzer in this homeostatic behavior. It was proposed that the alimentary pleasure or displeasure brought by alimentary stimuli acts as a drive for the food intake behavior. Indeed, in normal human subjects, the affective component of the alimentary sensations is not constant, but depends on the subjects' internal state. From pleasant, in subjects requiring some energy supply for long-term or short-term energy regulation, alimentary sensations become less pleasant or unpleasant in satiated subjects or in overfed subjects. This change of the affective component of a peripheral sensation, according to the internal state, was called: negative alimentary alliesthesia. Alliesthesia is a specific and reproducible phenomenon. It has been shown that the internal signal which induces negative alimentary alliesthesia depends on the carbohydrate concentration into the gastrointestinal tract, mainly into the duodenum. This signal may provide information related to the energy content of the ingested food. The neural or humoral nature of the transmission of this information from the gut to the brain is discussed. A neural link possibly involves the enteric glucidoreceptors recently described. In conclusion, the validity and the limits of the hedonic hypothesis of the food intake is briefly discussed.

Afferent discharges were recorded from nerve filaments dissected from the hepatic branch of the vagus nerve in the guinea-pig in experiments on isolated and perfused liver preparations and on those done in vivo. A significant decrease in discharge rate was observed following injection of D-glucose into the portal vein but not after similar injections of D-mannose, D-fructose, D-galactose, L-glucose, D-xylose or D-arabinose. The mean discharge rate of these fibers was inversely related to the concentration of glucose in the portal vein. The administration of a glucose analogue, 2-deoxy-D-glucose (2-DG), caused an increase in discharge rate. The results suggest that the inhibitory effect of D-glucose on the rate of firing and facilitatory effect of 2-DG on firing are related to the facilitation or inhibition of glucose metabolism in the glucose sensors in the liver.

The effects of selective hepatic vagotomy on compensatory adjustments of feeding in response to experimentally induced changes in body weight were investigated in rats. Repeated injections of slow-acting insulin increased food intake and body weight in vagotomized as well as in sham-vagotomized rats. When the treatment was stopped, the rats of both surgical groups displayed a compensatory hypophagia of similar magnitude and body weight returned to preinjection levels. In turn, after 7 days of restricted feeding (5 g/day) with a concomitant loss of body weight, rats became hyperphagic and body weight approached the normal level. Again, the feeding responses or body weight changes did not differ between vagotomized and sham-vagotomized rats. The results demonstrate that selective hepatic vagotomy does not disrupt the putative feedback-loop between body weight and feeding, and raise questions concerning the role of the liver in the lipostatic control of food intake.

To test the possibility that vagally mediated signals derived from hepatic fatty acid oxidation affect feeding, we investigated the influence of selective hepatic vagotomy on the acute hyperphagic effect of the fatty acid oxidation inhibitor 2-mercaptoacetate (MA) in rats kept on a medium fat diet (18% fat). An i.p. injection of MA (400 mumol/kg b.wt.) stimulated feeding in sham-vagotomized rats clearly more than in vagotomized rats. A high dose of MA (800 mumol/kg b.wt.) initially increased food intake in sham-vagotomized but not in vagotomized rats and later decreased food intake similarly in both surgical groups. MA retained its potency to stimulate feeding after an i.p. injection of atropine methylnitrate (5 mg/kg b.wt.). These results indicate that hepatic fatty acid oxidation provides a satiety signal that is mediated by vagal afferents.

The hepatic branch of the vagus nerve has been implicated as an important source of afferent input controlling both physiological and behavioral homeostasis. In addition, it is clear that parasympathetic efferents to the liver can significantly alter hepatic functions. In order to begin physiological studies on the nature of hepatic afferent and efferent relations, it will be necessary to understand the central anatomical organization of the components of this small visceral nerve. By carefully exposing and dissecting the hepatic branch of the vagus and applying crystalline horseradish peroxidase (HRP) to it, we were able to elucidate a predominant pattern of afferent terminations within the left subnucleus gelatinosus, the medial division of the left solitary nucleus and the left lateral edge of the area postrema. Efferent nuclei were concentrated in the left dorsal motor nucleus of the vagus (DMN) with a few scattered neurons located in the right DMN as well as the left anterior nucleus ambiguous.

Cats were prepared with chronically implanted catheters in the portal and femoral vein for administration of glucose. A femoral arterial catheter was used for blood sampling. After at least 5 days following the implantations, cats received an infusion of 3 mg/kg/min into the portal vein and the insulin and glucagon responses were compared with a similar infusion into the femoral vein. The hypothesis predicted that glucose receptors in the liver would result in greater pancreatic hormonal responses than would be seen with the femoral infusions of glucose. This did not occur. There was no statistically significant difference between the rise in insulin and the decrease in glucagon elicited by the glucose infusion into the two alternate infusion sites. A sham-denervated series showed similar responses. A group of cats that underwent hepatic denervation using topical phenol application to the hepatic plexus near the hilum of the liver was tested. The responses of the denervated group were similar to those for the control and sham-operated groups, confirming that hepatic glucose receptors do not serve as an afferent limb of a hepato-pancreatic reflex controlling insulin and glucagon release in conscious cats. In all experiments, a bolus of glucose (500 mg) was injected i.v. to represent a tolerance test. The responses of blood glucose levels, pancreatic insulin and glucagon and recovery from the stimulus were similar in all groups which suggests that hepatic nerves are not essential for a normal hepatic response to intravenously infused glucose. It is suggested, however, that hepatic nerves may be important in producing normal hepatic responses to oral glucose loads.

This study was designed to determine the reflex effects of renal nerve activity during acutely altered portal venous pressure in anesthetized dogs. Following inflation of a balloon in the main portal vein of animals with the neuraxis intact, mean blood pressure (MBP) fell significantly from 127 +/- 8 mm Hg to 101 +/- 7 mm Hg, while renal nerve activity (RNA) increased to 189 +/- 11%. This significant increase in RNA during inflation of the balloon was not entirely eliminated by either carotid sinus nerve (CSN) denervation (163 +/- 9% of the control level) or vagotomy (161 +/- 11% of the control level). The reflex increase in RNA observed with the combined denervation of the carotid sinus and vagal nerves (CSN + Va denervation) was 138 +/- 2% of the control level, and this level of RNA increase was significantly smaller compared to those in the other 3 groups (intact, CSN or Va denervated). Even when the portal venous pressure below caudal to the balloon was controlled at a pre-distension level by means of opening an artificial bypass constructed between the mesenteric vein and the femoral vein in the CSN + Va denervated group, the increased RNA response during inflation of the balloon remained. After hepatic denervation in the CSN + Va denervated group with the opening of the bypass, however, inflation of a balloon caused no significant changes in RNA, even when more profound hypotension existed during inflation of the balloon.(ABSTRACT TRUNCATED AT 250 WORDS)

Afferent discharges were recorded from the hepatic branch of the vagus nerve. It was revealed that some afferent fibers were sensitive to an increase in liver temperature and that their Q10 values lay between 4 and 16. These thermosensitive fibers failed to respond to mechanical stimulations applied to the liver. The other types of afferent fibers in this nerve such as an osmosensitive one did not respond to thermal stimulation. These data support the existence of a thermosensitive structure in the liver which may possibly contribute to maintain thermal homeostasis.

The hepatic vagus nerve contains various thermosensitive afferent fibers which are widely varied in their sensitivity. Their Q10 values lie between 4 and 16. The discharge rate is positively correlated with increase of liver temperature (warm fiber type). The result supports the existence of a thermosensitive structure in the liver which may possibly contribute to maintain thermal homeostasis. Neural responses to the osmotic changes in the perfusion solution have been analyzed. It was found that two different types of osmosensitive afferent fibers exist in the hepatic vagus; one is characterized by increasing the frequency of spike discharges in response to higher osmotic pressure, while the other shows the same response to lowered levels. Behavioral changes caused by hepatic vagotomy were observed. These results provide evidence for the existence of an osmoreceptor mechanism. The role of these hepatic afferent nerves in homeostasis are briefly discussed.

Changes in acid outputs from the stomach were examined after portal injections of D-glucose and its optical anomers in the bilaterally adrenalectomized rats with insulin hypoglycaemia. Significant decrease in gastric acid outputs was noted after portal injections of alpha-D-glucose, optically equilibrated D-glucose (OEDG) consisting of 36% alpha-anomer and 64% beta-anomer and beta-D-glucose. The effect of beta-D-glucose was most potent in reducing the acid outputs and the inhibitory response was entirely prevented by prior vagotomy at the hepatic level. The injections of isotonic NaCl solution, however, produced no change in the acid outputs. Results suggest that changes in glucose levels in the portal vein may modulate gastric acid secretion through hepatic vagal afferents and gastric vagal efferents and suggest that activation of hepatic glucosensitive mechanisms may be dependent on the anomeric stereospecificity of D-glucose in the blood.

Afferent neurons in the solitary tract nucleus (NTS) which innervate the liver were identified by electrical stimulation of the hepatic branch of the vagus. This nerve branch projects predominantly into the left side of the medulla. A collision test was employed to discriminate the afferent unit discharges from the efferent ones. These afferent neurons are characterized by no cancellation of the evoked unit discharges by the collision. Latencies were widely scattered and lay between 73.2 and 222.5 ms. A variable latency in each response was always recognized. Excitability curves obtained by applications of conditioning and test stimuli revealed that there exist two different types of units; one is characterized by a higher safety factor for synaptic transmission, while the other shows a lower safety factor than the former. It is concluded that the neurons in the NTS activated by the hepatic vagal afferent nerve locate predominantly in the left side of the medulla and also these nerve fibers are mostly non-medullated fibers in which conduction velocities are slower than 1 m/s. The variable latency recognized in each neuron and low safety factors for synaptic transmission recognized in some neurons indicate integrative action within the NTS as well as a relay of the afferent signals from the liver.

In a series of combined electrophysiological and peroxidase histochemical studies, we have established that first-order hepatic and gustatory afferents project to separate regions of the solitary nucleus (NST) and do not converge upon neurons within this structure. Additionally, we have found that hepatic vagal afferent fibers travel in the left cervical vagal trunk and project to the postero-medial division of the left NST. Very small (100-200 micron diameter) iontophoretic injections of horseradish peroxidase (HRP) into physiologically identified zones of the NST reveal that the immediately subjacent parvocellular reticular formation sends efferents to and receives afferents from both the hepatic and gustatory divisions of this nucleus. Further, hepatic and gustatory-NST regions send projections throughout the parabrachial nucleus (PBN). Axons from both the hepatic and gustatory regions of the NST terminate heavily in the posterior dorsomedial part of the PBN. At anterior levels, the gustatory and visceral NST projections are distinctly separate; the medial PBN receives input from gustatory-NST, while the lateral region received input from the hepatic-NST. These results provide anatomical support for our previous findings that single neurons within the PBN can be activated by both gustatory and hepatic afferent activation. Overlapping projections from hepatic (vagal) and gustatory regions of the NST within both the immediately subjacent parvocellular reticular formation as well as the postero-medial PBN may explain in part the decerebrate rat's ability to not only discriminate gustatory stimuli, but also switch its response to a constant stimulus dependent on its nutritive state.

In urethan-chloralose anesthetized rats, units in the nucleus tractus solitarius (NTS) which antidromically responded to electrical stimulation of the parabrachial nucleus (PB) were investigated for their responses to hepatic-vagal afferent signals. Among 63 NTS units examined, 25 (40%) were excited, 17 (27%) inhibited and the remaining 21 (33%) unaffected by single shock electrical stimulation of the hepatic branch of the vagus nerve. Topical application of Na+ produced an increase in discharge rate of 9 units and a decrease in 5 units. Portal infusion of hypertonic saline produced an increase in discharge rate of 3 units and a decrease in 5 units. Furthermore, 3 units responded to both topical application of Na+ and portal infusion of hypertonic saline.

In order to provide a detailed surgical anatomy of the rat abdominal vagus, we examined pyridine silver-stained tissue from one group of normal animals and a second group that survived 9 months after vagotomy. In the normal sample, as has been established for man, there was considerable variability in the levels at which each of the vagal branches separated from the main trunks. Contrary to reports from dissection studies, most of the branches were not single fiber bundles but rather consisted of two or more separate bundles. At the extreme, the posterior gastric and coeliac branches each consisted of as many as 15 individual bundles. Even the main trunks of the subdiaphragmatic vagus were occasionally observed to have multiple components (anterior trunk, 13% of the cases; posterior, 25%). In addition to the classically recognized hepatic, anterior gastric, coeliac, and posterior gastric branches, we also observed an accessory coeliac branch of the anterior trunk in all animals. This accessory coeliac division originated just caudal to the hepatic branching and extended first laterally and then dorsally while running caudally to exit from the esophagus just before the separation of the coeliac branch from the posterior trunk. The vagi were observed to contain paraganglia consisting of islands of glomus cells, neurons, and extensive capillary beds, all situated within the perineurium. The paraganglia occurred in greatest frequency at the sites where the hepatic and coeliac branches divide from their respective trunks. Paraganglia were also observed peripherally within vagal branches; there they were most numerous within the coeliac branch and least numerous in the accessory coeliac. Other studies yielded evidence that regeneration had occurred after complete vagotomy. First, stumps of the branches distal to the resection scar contained axons. Central to the scar, axons grew out in all directions from the neuroma; some of them appeared to cross the scar and to reinnervate the distal stumps. Secondly, 30% of the animals in which regeneration was thought to be possible increased their insulin secretion in response to electrical stimulation of the cervical vagus. The implications of the above findings for experiments that involve manipulation or recording of the vagus are discussed.

The effect of section of the rostral hepatic nerve (RNH), branch of the left abdominal vagus, on the circadian rhythm of tyrosine transaminase (TAT) activity in rat liver has been investigated. It has been shown that the midnight peak of TAT activity is suppressed after section of the rostral hepatic nerve and by subdiaphragmatic vagotomy, but is unaffected by sham-operation and section of the ventral abdominal vagus caudal to the emergence of the rostral hepatic nerve. It is concluded that the rhythm of TAT activity is under neural control, either directly or reflexely.

Glucose-sensitive neural elements exist in the hypothalamus, the nucleus of the solitary tract (NTS) and autonomic afferents from visceral organs such as liver and gastrointestinal tract. Glucose affects neural activity through these central and peripheral chemosensors. Glucose is generally suppressive in the liver, the NTS and the lateral hypothalamic area (LHA), and generally excitatory in the small intestine and ventromedial hypothalamic nucleus (VMH). The hypothalamus is involved in the control of pancreatic hormone secretion through autonomic efferent nerves. Stimulation or lesion of the hypothalamus induces various changes in pancreatic autonomic nerve activity. The VMH, the dorsomedial hypothalamic nucleus and the paraventricular nucleus have inhibitory effects on vagal nerve activity and excitatory effects on splanchnic nerve activity. The LHA is excitatory to the vagal nerve, and both excitatory and inhibitory to the splanchnic nerve. These findings suggest that the neural network of the glucose monitoring system, which also analyzes and integrates information concerning other metabolites and peptides in the blood and cerebrospinal fluid, contributes to regulation of peripheral metabolism and endocrine activity as well as feeding behavior. The physiological function and input-output organization of this network are discussed.

The discharge rates of glucose-sensitive hepatic vagal afferents and glucose concentrations in the portal vein showed an inverse relationship in experiments performed using isolated and perfused liver preparations and in in vivo experiments on the guinea pig. The rate of discharge of these afferents in the rat was facilitated following administration of insulin and inhibited after application of glucagon and CCK. Hepato-pancreatic, -adrenal and -hepatic reflexes initiated through vagal hepatic afferents were demonstrated in the rabbit, guinea pig or rat. The results obtained suggest that glucose-sensitive vagal afferents from the liver play an important role in the control of food intake as well as in the control of blood glucose levels.

The influence of hepatic nerves on glucagon-stimulated hyperglycaemia and hyperinsulinaemia was studied in rats. Using microsurgical techniques, hepatic sympathectomy (resulting in approximately 80% reduction of liver noradrenaline content) or hepatic vagotomy was performed. Glucagon (10 micrograms/kg b.wt.) was injected i.v. one week after the operative procedure. Plasma levels of glucose and insulin rose more rapidly and to a higher level in rats subjected to hepatic sympathectomy than in sham-operated or non-operated control animals. In contrast, hepatic vagotomy did not influence the plasma glucose or insulin response to glucagon. In conclusion, our findings suggest a role for the hepatic sympathetic nerves in glucagon-stimulated glycogenolysis and insulin secretion in the rat.

Changes in water intake after hepatic vagotomy were examined in the body weights of 3 different rats (about 100, 200 and 280 g). The vagotomy reduced water intake only in the 100-g rats.

Osmoreceptors are sensory organs of paramount importance in water and electrolyte balance, yet the mechanisms for their activation are virtually unknown. Peripheral osmoreceptors have been localised in the hepatic portal vein area of rats. We thus superfused the portal adventitia with 0.2 ml of 4% NaCl before and after various pharmacological pretreatments (0.4 ml of 1 mM solutions) of the portal area, while monitoring the neural activity of the hypothalamo-neurohypophysial system. Portal superfusion with verapamil, to reduce Ca-influx, reversibly inhibited the response to osmotic stimuli by up to 50% (P less than 0.0005). Such inhibition (58%; P less than 0.0005) was also seen with portal superfusion by atropine. Atropine did not affect hypothalamo-neurohypophysial responses to stimulation of portal bradykinin receptors with 0.2 ml 1 muM bradykinin, and portal superfusion with acetylcholine activated the hypothalamo-neurohypophysial system. The results thus support the hypothesis of a cholinergic neurotransmission linking portal osmoreceptive structures and afferent nerve endings. Diamide, which inhibits water efflux in frog skin, also reversibly inhibited responses to osmotic stimuli by 38% (P less than 0.0005). Pretreatments with trifluoperazine, a calmodulin inhibitor, and cordycepin, an adenylate cyclase inhibitor, diminished responses to osmotic stimuli by 30-45% (P less than 0.005), while cAMP and theophilline potentiated them by 38% (P less than 0.0005). Responses to bradykinin superfusion were reduced 20-30% (P less than 0.05) by both cordycepin and cAMP. The results suggest that portal osmoreceptors release acetylcholine to excite afferent nerves when exposed to an osmotic gradient. The mechanism of this release may be mediated by an efflux of water and an increase of intracellular calcium activity and cAMP.

Either the left or right cervical vagus was electrically stimulated in anesthetized rats before and after selective transection of either the coeliac, gastric and hepatic abdominal branches in order to evaluate the contribution of these branches to vagal controlled insulin secretion. Changes of insulin secretion were estimated on the basis of insulin concentration in venous plasma, sampled by indwelling jugular catheters. Plasma glucose concentration in overnight food-deprived rats was clamped between 130 and 160 mg/dl by means of continuous i.v. glucose infusion, and surgical stress-induced sympathetic activity was blocked by concomitant i.v. infusion of phentolamine and propranolol. Before transection of any abdominal branch, both right and left cervical vagal stimulation induced a 3- to 4-fold increase of plasma insulin concentration and significant increases of plasma glucose concentration, while the heart rate decreased rapidly and significantly. The right cervical vagal stimulation-induced insulin response (integrated incremental area) was significantly decreased by either bilateral coeliac (-37%) or bilateral gastric (-57%), but not by hepatic (-5%) vagotomy. The left cervical vagal stimulation-induced insulin response was significantly decreased (-41%) by hepatic vagotomy. The concomitant rises of plasma glucose concentration may have contributed more than 50% to the vagal stimulation-induced insulin responses. However, calculating the purely neural components revealed that the right cervical vagal stimulation-induced insulin response was still decreased by coeliac (-48%) or gastric (-84%) and not decreased (+24%) by hepatic vagotomy, and the left cervical vagal stimulation-induced insulin response was decreased (-52%) by hepatic branch vagotomy. We conclude that cervical vagal stimulation-induced insulin-secreting activity reaches the pancreas via all 3 abdominal divisions of the vagus nerve, and suggest that pancreatic beta-cells are innervated through all 3 abdominal divisions.

Previous investigations concerning the function of hepatic sodium and osmoreceptors indicated that the activation of these chemoreceptive structures significantly affected physiological and behavioral mechanisms related to water and electrolyte homeostasis. Though anatomical studies predicted that such information should follow the previously described gustatory afferent pathy, until recently, the precise course taken by higher order hepatic afferents was unknown. The studies described herein verify anatomical predictions with regard to the central course of vagal-visceral afferents, as hepatically activated neurons were localized to two areas known to relay gustatory input. Further, horseradish peroxidase histochemical studies verified that a path between the nucleus of the solitary tract, the parabrachial nucleus and the ventrobasal complex certainly exists. In the light of recent findings regarding projections from the ventrobasal complex to the supraoptic nuclei, from the parabrachial nucleus to the supraoptic nuclei and from the nucleus of the solitary tract to the paraventricular and supraoptic nuclei it seems likely that the viscero-gustatory path which carries hepatic afferent information is the pathway responsible for the physiological and perhaps the behavioral consequences of hepatic sodium or osmotic stimulation.