Validation of tests of completeness of vagotomy in rats

The Vagus Nerve and Spleen: Influence on White Adipose Mass and Histology of Obese and Non-obese Rats

The vagus nerve (VN) and spleen represent a complex interface between neural and immunological functions, affecting both energy metabolism and white adipose tissue (WAT) content. Here, we evaluated whether vagal and splenic axis participates in WAT mass regulation in obese and non-obese male Wistar rats. High doses of monosodium glutamate (M; 4 g/Kg) were administered during the neonatal period to induce hypothalamic lesion and obesity (M-Obese rats). Non-obese or Control (CTL) rats received equimolar saline. At 60 days of life, M-Obese and CTL rats were randomly distributed into experimental subgroups according to the following surgical procedures: sham, subdiaphragmatic vagotomy (SV), splenectomy (SPL), and SV + SPL (n = 11 rats/group). At 150 days of life and after 12 h of fasting, rats were euthanized, blood was collected, and the plasma levels of glucose, triglycerides, cholesterol, insulin, and interleukin 10 (IL10) were analyzed. The visceral and subcutaneous WAT depots were excised, weighed, and histologically evaluated for number and size of adipocytes as well as IL10 protein expression. M-Obese rats showed higher adiposity, hyperinsulinemia, hypertriglyceridemia, and insulin resistance when compared with CTL groups (p < 0.05). In CTL and M-Obese rats, SV reduced body weight gain and triglycerides levels, diminishing adipocyte size without changes in IL10 expression in WAT (p< 0.05). The SV procedure resulted in high IL10 plasma levels in CTL rats, but not in the M-Obese group. The splenectomy prevented the SV anti-adiposity effects, as well as blocked the elevation of IL10 levels in plasma of CTL rats. In contrast, neither SV nor SPL surgeries modified the plasma levels of IL10 and IL10 protein expression in WAT from M-Obese rats. In conclusion, vagotomy promotes body weight and adiposity reduction, elevating IL10 plasma levels in non-obese animals, in a spleen-dependent manner. Under hypothalamic obesity conditions, VN ablation also reduces body weight gain and adiposity, improving insulin sensitivity without changes in IL10 protein expression in WAT or IL10 plasma levels, in a spleen-independent manner. Our findings indicate that the vagal-spleen axis influence the WAT mass in a health state, while this mechanism seems to be disturbed in hypothalamic obese animals.

Dissecting the Role of Subtypes of Gastrointestinal Vagal Afferents

Gastrointestinal (GI) vagal afferents convey sensory signals from the GI tract to the brain. Numerous subtypes of GI vagal afferent have been identified but their individual roles in gut function and feeding regulation are unclear. In the past decade, technical approaches to selectively target vagal afferent subtypes and to assess their function has significantly progressed. This review examines the classification of GI vagal afferent subtypes and discusses the current available techniques to study vagal afferents. Investigating the distribution of GI vagal afferent subtypes and understanding how to access and modulate individual populations are essential to dissect their fundamental roles in the gut-brain axis.

Withdrawal and restoration of central vagal afferents within the dorsal vagal complex following subdiaphragmatic vagotomy

Vagotomy, a severing of the peripheral axons of the vagus nerve, has been extensively utilized to determine the role of vagal afferents in viscerosensory signaling. Vagotomy is also an unavoidable component of some bariatric surgeries. Although it is known that peripheral axons of the vagus nerve degenerate and then regenerate to a limited extent following vagotomy, very little is known about the response of central vagal afferents in the dorsal vagal complex to this type of damage. We tested the hypothesis that vagotomy results in the transient withdrawal of central vagal afferent terminals from their primary central target, the nucleus of the solitary tract (NTS). Sprague-Dawley rats underwent bilateral subdiaphragmatic vagotomy and were sacrificed 10, 30, or 60 days later. Plastic changes in vagal afferent fibers and synapses were investigated at the morphological and functional levels by using a combination of an anterograde tracer, synapse-specific markers, and patch-clamp electrophysiology in horizontal brain sections. Morphological data revealed that numbers of vagal afferent fibers and synapses in the NTS were significantly reduced 10 days following vagotomy and were restored to control levels by 30 days and 60 days, respectively. Electrophysiology revealed transient decreases in spontaneous glutamate release, glutamate release probability, and the number of primary afferent inputs. Our results demonstrate that subdiaphragmatic vagotomy triggers transient withdrawal and remodeling of central vagal afferent terminals in the NTS. The observed vagotomy-induced plasticity within this key feeding center of the brain may be partially responsible for the response of bariatric patients following gastric bypass surgery.

Vagal afferent controls of feeding: a possible role for gastrointestinal BDNF

INTRODUCTION

Vagal gastrointestinal (GI) afferents do not appear to contribute to long-term controls of feeding, despite downstream connections that could support such a role. This view is largely attributable to a lack of evidence for long-term effects, especially the failure of vagal afferent lesions to produce hyperphagia or obesity.

AIMS

Here, the possibility is evaluated that "side effects" of vagal lesion methods resulting largely from complexities of vagal organization would probably suppress long-term effects. Criteria based on knowledge of vagal organization were utilized to critique and compare vagal lesion methods and to interpret their effects on GI function, feeding and body weight.

RESULTS AND CONCLUSIONS

This analysis suggested that it was premature to eliminate a long-term vagal GI afferent role based on the effects of these lesions and highlighted aspects of vagal organization that must be addressed to reduce the problematic side effects of vagal lesions. The potential of "genetic" lesions that alter vagal sensory development to address these aspects, examination of the feasibility of this approach, and the properties of brain-derived neurotrophic factor (BDNF) that made it an attractive candidate for application of this approach are described. BDNF knockout from GI smooth muscle unexpectedly demonstrated substantial overeating and weight gain associated with increased meal size and frequency. The decay of eating rate during a scheduled meal was also reduced. However, meal-induced c-Fos activation was increased in the dorsal motor nucleus of the vagus, suggesting that the effect on eating rate was due to augmentation of GI reflexes by vagal afferents or other neural systems.

Hepatic leptin signalling and subdiaphragmatic vagal efferents are not required for leptin-induced increases of plasma IGF binding protein-2 (IGFBP-2) in ob/ob mice

AIMS/HYPOTHESIS

The fat-derived hormone leptin plays a crucial role in the maintenance of normal body weight and energy expenditure as well as in glucose homeostasis. Recently, it was reported that the liver-derived protein, insulin-like growth factor binding protein-2 (IGFBP-2), is responsible for at least some of the glucose-normalising effects of leptin. However, the exact mechanism by which leptin upregulates IGFBP-2 production is unknown. Since it is believed that circulating IGFBP-2 is predominantly derived from the liver and leptin has been shown to have both direct and indirect actions on the liver, we hypothesised that leptin signalling in hepatocytes or via brain-liver vagal efferents may mediate leptin control of IGFBP-2 production.

METHODS

To address our hypothesis, we assessed leptin action on glucose homeostasis and plasma IGFBP-2 levels in both leptin-deficient ob/ob mice with a liver-specific loss of leptin signalling and ob/ob mice with a subdiaphragmatic vagotomy. We also examined whether restoring hepatic leptin signalling in leptin receptor-deficient db/db mice could increase plasma IGFBP-2 levels.

RESULTS

Continuous leptin administration increased plasma IGFBP-2 levels in a dose-dependent manner, in association with reduced plasma glucose and insulin levels. Interestingly, leptin was still able to increase plasma IGFBP-2 levels and improve glucose homeostasis in both ob/ob mouse models to the same extent as their littermate controls. Further, restoration of hepatic leptin signalling in db/db mice did not increase either hepatic or plasma IGFBP-2 levels.

CONCLUSIONS/INTERPRETATION

Taken together, these data indicate that hepatic leptin signalling and subdiaphragmatic vagal inputs are not required for leptin upregulation of plasma IGFBP-2 nor blood glucose lowering in ob/ob mice.

Brain activation following peripheral administration of the GLP-1 receptor agonist exendin-4

The aim of our study was to investigate the anorectic and brain stimulatory effects of various doses of exendin-4 (Ex-4) and to investigate the role of the vagus nerve in Ex-4-induced brain activation. A dose-related increase in c- fos mRNA expression was observed following Ex-4 administration (0.155–15.5 μg/kg). Doses of Ex-4 that caused anorexia without aversive effects (0.155, 0.775 μg/kg) induced c- fos expression in the hypothalamic arcuate and paraventricular (PVH; parvocellular) nuclei as well as in the limbic and brainstem structures. Doses of Ex-4 that caused aversion (1.55, 15.5 μg/kg) stimulated the same regions (in a more intense way) and additionally activated the magnocellular hypothalamic structures (supraoptic nucleus and PVH magnocellular). The brain c- fos pattern induced by Ex-4 showed both similarities and differences with that induced by refeeding. Subdiaphragmatic vagotomy significantly blunted the stimulation of c- fos mRNA expression induced by Ex-4 in the nodose ganglion, the medial part of nucleus of the solitary tract, and the parvocellular division of the PVH. Pretreatment with Ex-9-39 (330 μg/kg ip) impaired the neuronal activation evoked by Ex-4 in all brain regions and in the nodose ganglion. Effects of Ex-4 on hypothalamic-pituitary-adrenal axis activity were not altered by vagotomy. Results of this study demonstrate and relate the anorectic and brain stimulatory effects of aversive and nonaversive doses of Ex-4 and indicate that the activation of specific central regions induced by the peripheral administration of Ex-4 is, at least in part, dependent on the integrity of the vagus nerve.

Early life activation of toll-like receptor 4 reprograms neural anti-inflammatory pathways

A single postnatal exposure to the bacterial endotoxin, lipopolysaccharide (LPS), reduces the neuroimmune response to a subsequent LPS exposure in the adult rat. The attenuated fever and proinflammatory response is caused by a paradoxical, amplified, early corticosterone response to LPS. Here we identify the mechanisms underlying the heightened corticosterone response to LPS in adults after early life exposure to LPS. In postnatal LPS-treated rats, hypothalamic corticotrophin-releasing hormone mRNA, pituitary proopiomelanocortin mRNA, and circulating adrenocorticotrophic hormone were all increased after adult exposure to LPS without significant modification to hippocampal or hypothalamic glucocorticoid receptor mRNA or protein or vagally mediated afferent signaling to the brain. Postnatal LPS administration did cause a persistent upregulation of the LPS Toll-like receptor-4 (TLR4) mRNA in liver and spleen, but not in brain, pituitary, or adrenal gland. In addition, cyclooxygenase-2 (COX-2), which is a prostaglandin biosynthetic enzyme and is normally undetectable in most peripheral tissue, was constitutively expressed in the liver. Adult immune activation of the upregulated TLR4 and COX-2 caused a rapid, amplified rise in circulating, but not brain, prostaglandin E(2) that induced an early, enhanced activation of the hypothalamic-pituitary-adrenal (HPA) axis. Thus, postnatal LPS reprograms the neuroimmune axis by priming peripheral tissues to create a novel, prostaglandin-mediated activation of the HPA axis brought about by increased constitutive expression of TLR4 and COX-2.

The hepatic vagus nerve attenuates Fas-induced apoptosis in the mouse liver via alpha7 nicotinic acetylcholine receptor

BACKGROUND & AIMS

Although accumulating evidence has recently shown that the efferent vagus nerve attenuates systemic inflammation, it remains unclear whether or not the vagus nerve can affect Fas-induced liver apoptosis. We investigated the effect of the vagus nerve by using a selective hepatic vagotomy.

METHODS

We assessed the mortality and apoptosis in Fas-induced fulminant hepatitis in sham-operated and vagotomized male C57BL/6 mice. To determine how the nerve influences hepatocyte apoptosis, hepatitis was preceded by pretreatment with nicotine; PNU-282987, an alpha7 nicotinic acetylcholine receptor (AChR) agonist; liposome-encapsulated dichloromethylene diphosphonate (lipo-Cl(2)MDP), a macrophage eliminator; and Mn (III) tetrakis (4-benzoic acid) porphyrin chloride (MnTBAP), an oxidative inhibitor.

RESULTS

Mortality in the vagotomized mice was significantly higher than that in the sham-operated mice following intravenous administration with the anti-Fas antibody Jo-2. This result was also supported by the data from both terminal deoxynucleotidyl-transferase mediated dUTP nick-end labeling and caspase-3 assay, in which vagotomized livers showed a significant elevation in the number of apoptotic hepatocytes and increased caspase-3 activity following Jo-2 treatment compared with the sham-operated livers. Supplementation with nicotine and PNU-282987 dose dependently inhibited this detrimental effect of the vagotomy. Moreover, the vagotomy-triggered exacerbation of Fas-induced hepatitis was completely blocked by lipo-Cl(2)MDP. Similarly, pretreatment with MnTBAP also completely suppressed the vagotomy-triggered exacerbation.

CONCLUSIONS

The hepatic vagus nerve appears to play an important role in attenuating Fas-induced hepatocyte apoptosis through alpha7 nicotinic AChR, perhaps by causing the Kupffer cells to reduce their generation of an excessive amount of reactive oxygen species.

Effects of perivagal administration of capsaicin on post-surgical food intake

The vagus nerve has been related to the short-term control of food intake. This involvement has previously been explored by examining the food intake of animals after recovery from a vagotomy or immediately after the intervention, among other methods. In the present work, a study was conducted on the impact of the perivagal application of capsaicin (a specific neurotoxic treatment that destroys most of the vagal afferent pathways) on the intake of water and solid (experiment 1) or liquid (experiment 2) food presented after the surgery The results of experiment 1 showed that lesioned animals consume significantly larger amounts of food and water compared with controls at 6, 12, and 24 h (but not at 48 or 72 h) after the surgical intervention. Likewise, experiment 2 revealed a greater intake of liquid food by capsaicin-treated animals at the first post-surgical sessions. These data are discussed in terms of the role played by vagal afferent fibers in the control of short-term food intake.

Signaling the brain in systemic inflammation: which vagal branch is involved in fever genesis?

Recent evidence has suggested a role of abdominal vagal afferents in the pathogenesis of the febrile response. The abdominal vagus consists of five main branches (viz., the anterior and posterior celiac branches, anterior and posterior gastric branches, and hepatic branch). The branch responsible for transducing a pyrogenic signal from the periphery to the brain has not as yet been identified. In the present study, we address this issue by testing the febrile responsiveness of male Wistar rats subjected to one of four selective vagotomies: celiac (CBV), gastric (GBV), hepatic (HBV), or sham (SV). In the case of CBV, GBV, and HBV, only the particular vagal branch(es) was cut; for SV, all branches were left intact. After the postsurgical recovery (26-29 days), the rats had a catheter implanted into the jugular vein. On days 29-32, their colonic temperature (Tc) responses to a low dose (1 microg/kg) of Escherichia coli lipopolysaccharide (LPS) were studied. Three days later, the animals were subjected to a 24-h food and water deprivation, and the effectiveness of the four vagotomies to induce gastric food retention, pancreatic hypertrophy, and impairment of the portorenal osmotic reflex was assessed by weighing the stomach and pancreas and measuring the specific gravity of bladder urine, respectively. Stomach mass, pancreas mass, and urine density successfully separated the four experimental groups into four distinct clusters, thus confirming that each type of vagotomy had a different effect on the indexes measured. The Tc responses of SV, CBV, and GBV rats to LPS did not differ and were characterized by a latency of approximately 40 min and a maximal rise of 0.7 +/- 0.1, 0.6 +/- 0.1, and 0.9 +/- 0.2 degrees C, respectively. The fever response of the HBV rats was different; practically no Tc rise occurred (0.1 +/- 0.2 degrees C). The HBV appeared to be the only selective abdominal vagotomy affecting the febrile responsiveness. We conclude, therefore, that the hepatic vagus plays an important role in the transduction of a pyrogenic signal from the periphery to the brain.

Lack of effects of an acute hepatic vagotomy on insulin and catecholamine responses in rats following exercise

The purpose of the present investigation was to evaluate the effects of an acute hepatic vagotomy on hormonal responses to hyperglycemic and hypoglycemic challenges in rats previously submitted to an exercise protocol. Two experiments were conducted. In a first experiment, 8-week trained (TR) and untrained (UNTR) rats, subdivided into acutely hepatic vagotomized (HV) and sham-operated (SHM) groups, were submitted to an intraperitoneal glucose tolerance test (0.5 g/kg) under anesthesia. Training was associated with a tendency (P = 0.07) for blood glucose levels to be less elevated (at time point 10 min), and with a significant (P < 0.01) lower glucose/insulin ratio following the glucose injection. The HV did not have any effects on these responses. In a second experiment, non-exercised rats and a group of rats submitted to an acute bout of exercise (treadmill, 60 min, 26 m/min, 5% slope) 24 h before the experiment, each one of these two groups being subdivided into acutely HV and SHM groups, were submitted to an insulin-induced hypoglycemia protocol, under anesthesia. Blood glucose concentrations were decreased significantly (P < 0.01) to approximately 40 mg/dl in all groups 60 and 80 min after the insulin injection. Plasma adrenaline and noradrenaline levels were increased significantly (P < 0.01) in all groups. The catecholamine increase was not influenced by the HV or the acute exercise bout. The present results do not indicate an implication of the hepatic vagus nerve on hormonal responses to hyper and hypoglycemia following exercise.

Effect of hepatic portal injection of ouabain on the hepato-sympathoadrenal reflex

The purpose of the present investigation was to evaluate the effects of an intraportal injection of ouabain (2 mg/kg), an inhibitor of the sodium-potassium pump, on plasma catecholamine response in unrestrained normally fed rats with and without an intact hepatic vagus nerve. Three groups of rats were submitted to two injection conditions each. Hepatic vagotomized (HV) rats were randomly injected with ouabain or saline (0.9%) in the portal vein. Sham-operated rats were either injected with ouabain or saline in the portal or jugular vein. Ouabain or saline were injected at 0 min and again at 20 min. Plasma catecholamines were measured before the first injection and 15 min after each injection. Blood glucose concentrations were significantly (p < 0.01) increased by the ouabain injection as compared with basal values and saline-injected groups. The hyperglycemic effect of ouabain was not affected by the hepatic vagotomy or the site of infusion. The injection of ouabain, either into the portal or the jugular vein and either after HV or the sham operation, resulted in a significant (p < 0.01) increase in epinephrine levels as compared with saline-infused rats. Plasma norepinephrine levels were significantly (p < 0.05) increased after the second intraportal injection of ouabain in both HV and sham-operated groups. However, the injection of ouabain into the jugular vein did not change the plasma norepinephrine levels. The latter observation indicates a specific action of ouabain in the liver on the sympathetic activity.

Arginine-induced pancreatic hormone secretion during exercise in rats

The aim of the present investigation was to 1) determine whether arginine-induced pancreatic hormone secretion can be modified during an exercise bout, and 2) verify whether the sectioning of the hepatic branch of the vagus nerve can alter the arginine-induced insulin and glucagon secretion during exercise in rats. To this end, we studied the effects of an intraperitoneal injection of arginine (1 g/kg body mass) during an exercise bout (30 min, 26 m/min, 0% grade) on the pancreatic hormone responses. These effects were determined in one group of sham-operated exercising rats and compared with three control groups: one group of resting rats, one group of saline-injected exercising rats, and one group of hepatic-vagotomized exercising rats. Five minutes after the injection of arginine, significant (P < 0.05) increases in insulin, glucagon, and C-peptide concentrations were observed in exercising as well as in resting rats. These responses were not, however, altered by the hepatic vagotomy and/or by the exercise bout. It is concluded that arginine is a potent stimulus of pancreatic hormone secretion during exercise, even though the sympathoadrenal system is activated. These results also indicate that a hepatic vagotomy does not seem to influence arginine-induced hormonal pancreatic responses and question the role of the putative hepatic arginoreceptors in the control of the pancreatic hormone secretion during exercise.

Effect of inhibition of gluconeogenesis on arginine-induced insulin secretion

It is well known that several amino acids, such as arginine, are potent stimuli for insulin and glucagon secretion from the pancreas. Recently, vagal arginine sensors, which modulate arginine-induced pancreatic hormone secretion, have been reported to exist in the liver. The present investigation was designed to evaluate the role played by gluconeogenesis in this hepatic influence. To this end, we studied the effects of an intraperitoneal injection of 3-mercaptopicolinic acid (3-MPA), a gluconeogenic inhibitor, on the pancreatic hormonal response induced by intraperitoneal administration of arginine (1 g/kg body mass) to hepatic vagotomized and sham vagotomized rats. Fifteen min following the injection of arginine, the increases in glucose and insulin concentrations were significantly lower in rats with an inhibited gluconeogenesis than in rats with an intact capacity for gluconeogenesis. There were no effects of the hepatic vagotomy on the arginine-induced hormonal responses either with or without the 3-MPA injection. The results suggest that gluconeogenesis is implicated in the hepatic modulation of arginine-induced pancreatic hormone secretion.

Gastric vagotomy blocks opioid analgesia enhancement produced by placenta ingestion

Ingestion of amniotic fluid or placenta by rats has been shown to enhance opioid-mediated analgesia induced by morphine injection, footshock, vaginal/cervical stimulation, or late pregnancy. This enhancement by ingestion appears to be specific to the central actions of opioids. The present study was designed to examine the possibility that information traveling via the vagus nerve might be involved in mediating this effect. Rats that had undergone either selective gastric vagotomy or sham vagotomy were injected with either morphine sulfate or vehicle and fed either placenta or a meat control. Enhancement was observed in rats that had undergone sham vagotomy but not in those that had undergone gastric vagotomy. These results support an interpretation of vagal involvement in the enhancement of opioid-mediated analgesia by placenta.

Influence of hepatic vagus nerve on pancreatic hormone secretion during exercise

There has been recent evidence that the liver through the hepatic vagus nerve may influence the resting levels of plasma insulin in adrenalectomized rats. The present investigation was designed to evaluate whether such a relationship exists during physical exercise. To this end, the effects of a selective hepatic vagotomy on portal and peripheral insulin and on peripheral glucagon concentrations were studied after a 30-min treadmill run (26 m/min, 0% grade) in adrenodemelludated rats. Hepatic vagotomy was associated with small but significantly higher (P less than 0.05) levels of liver glycogen and blood glucose at rest and after exercise. No significant differences were observed between hepatic-vagotomized and sham-operated rats in resting insulin, glucagon, and plasma norepinephrine concentrations. Peripheral plasma insulin levels after exercise were significantly higher (P less than 0.01) in hepatic-vagotomized than in sham-operated rats [172 +/- 20 vs. 108 +/- 10 (SE) pmol/l]. Exercise was also associated with a significantly lower peripheral glucagon (P less than 0.01) and norepinephrine (P less than 0.05) levels in hepatic-vagotomized compared with sham-operated rats. These results indicate a role for the hepatic vagus nerve in the regulation of pancreatic islet secretion during exercise, possibly by contributing to the increase in sympathetic activity.

CCK inhibits real and sham feeding in gastric vagotomized rats

We tested both sham feeding and real feeding in gastric vagotomized rats equipped with chronic gastric cannulas, either without pretest food deprivation or after 18 hr food deprivation. In each condition, 0.3-4 micrograms/kg CCK inhibited food intake similarly in control and vagotomized rats. Behavioral observations indicated the presence of normal postprandial satiety after CCK. We then tested real feeding in noncannulated rats after either total abdominal vagotomy, gastric vagotomy, or control operation. Doses of 0.5-4 micrograms/kg CCK had no effect on food intake after total vagotomy, but again inhibited feeding with equal potency in gastric vagotomized and control rats. The inhibitory effect of 6 micrograms/kg CCK was attenuated but not blocked by total vagotomy. Finally, we tested rats with gastric cannulas after gastric plus celiac vagotomy. CCK also inhibited both real and sham feeding after this lesion. These data confirm previous findings that abdominal vagal fibers mediate the satiety effect of moderate intraperitoneal doses of CCK, but fail to support the hypothesis that gastric branch fibers are the necessary vagal contribution.

Specific postoperative syndromes after total and selective vagotomies in the rat

Male Sprague-Dawley rats which survived bilateral subdiaphragmatic vagotomy (with hepatic branch intact) exhibited an acute syndrome of hypophagia, hypodipsia and severe loss of body weight when maintained on solid food and water for 14 days after vagotomy. This postvagotomy syndrome was attenuated when rats were maintained on a liquid diet (116EC) chosen to minimize postvagotomy dysphagia and abnormal gastric retention of food; vagotomized rats were hypophagic and lost body weight, but the degree of weight loss was not so severe as for vagotomized rats eating solid food. When rats with total subdiaphragramatic vagotomy were maintained on palatable sweet milk food, the acute postvagotomy syndrome was abolished; these vagotomized rats ate and drank as much as rats with sham vagotomy and they did not lose weight. When rats that underwent selective hepatic, gastric or coeliac vagotomy were maintained on the sweet milk diet, three different postoperative syndromes occurred: after selective hepatic vagotomy, rats were hyperphagic, hyperdipsic and gained body weight at a greater than normal rate; after selective gastric vagotomy, rats lost weight despite relatively normal food and water intakes; and after selective coeliac vagotomy, there was no change in food or water intakes or body weight. These results demonstrate that a sweet milk diet abolishes the anorexia, hypodipsia and weight loss that usually occur in vagotomized rats maintained on pellets and water. Use of this sweet milk diet revealed different acute syndromes after bilateral and selective vagotomies. The differences among the syndromes suggest that hepatic, gastric and coeliac vagal branches serve different functions in the control of food and water intake and body weight.

The effects of vagotomy on compensatory ovarian hypertrophy and follicular activation after unilateral ovariectomy

Following unilateral ovariectomy in the rat, the remaining ovary undergoes rapid compensatory changes including an increase in the number of antral follicles (follicular activation) and an increase in ovarian weight (compensatory ovarian hypertrophy). The ovary is innervated by the vagus nerve (Burden et al., 1983). In the present study, the effects of right and left cervical vagotomy and abdominal vagotomy on follicular activation and compensatory ovarian hypertrophy in the remaining right or left ovary were compared 15 days after unilateral ovariectomy. Neither right nor left cervical vagotomy affected compensatory ovarian hypertrophy of the right or left ovaries but abdominal vagotomy depressed compensatory ovarian hypertrophy in both the right and left ovaries. Left cervical vagotomy did not inhibit follicular activation, but right cervical vagotomy prevented follicular activation in the right but not left ovary. Also, abdominal vagotomy inhibited follicular activation in the right but not the left ovary. In animals with both ovaries which were subjected to the left or right cervical vagotomy or abdominal vagotomy follicular counts in both right and left ovaries were similar. Collectively, these data indicate that the vagus nerve participates in follicular activation after unilateral ovariectomy. The data also indicate that the right ovary is more dependent on vagal influences for follicular activation than the left ovary.

Mechanisms of radiation-induced conditioned taste aversion learning

The literature on taste aversion learning is reviewed and discussed, with particular emphasis on those studies that have used exposure to ionizing radiation as an unconditioned stimulus to produce a conditioned taste aversion. The primary aim of the review is to attempt to define the mechanisms that lead to the initiation of the taste aversion response following exposure to ionizing radiation. Studies using drug treatments to produce a taste aversion have been included to the extent that they are relevant to understanding the mechanisms by which exposure to ionizing radiation can affect the behavior of the organism.

Intragastric copper sulfate produces a more reliable conditioned taste aversion in vagotomized rats than in intact rats

Although copper sulfate is an emetic stimulus, preliminary experiments failed to obtain a taste aversion in intact rats following intragastric administration as had been previously reported in the literature. Several experiments were therefore run to further investigate the capacity of intragastric copper sulfate to function as an unconditioned stimulus for taste aversion learning and the role of the vagus in mediating that learning. The results of the first series of experiments showed that intragastric administration of copper sulfate (5 mg/kg X 5H2O) was more effective in reliably producing a taste aversion in vagotomized rats than in sham-operated control rats. The second experiment examined the effects of area postrema lesions on the acquisition of a taste aversion produced by intragastrically administered copper sulfate in vagotomized rats. The results indicated that the taste aversion observed following treatment with intragastric copper sulfate in vagotomized rats could be prevented by lesions of the area postrema. The present results indicate that intragastric administration of copper sulfate is a more reliable unconditioned stimulus for taste aversion learning in vagotomized rats than in intact rats. It is not certain what factors might account for the discrepant results between the present experiments and previously published research.

Brainstem origins and projections of the cervical and abdominal vagus in the golden hamster: a horseradish peroxidase study

Vagal afferent projections, and preganglionic parasympathetic neurones contributing to the vagus nerve in golden hamsters were traced following application of horseradish peroxidase (HRP) to the proximal end of the cervical or abdominal nerve stump. Efferents in the cervical vagus were traced to their perikarya or origin in the dorsal motor nucleus (DMN) of the vagus, the commissural gray of the cervical spinal cord, the nucleus ambiguus, the nucleus of the accessory spinal nerve (NASN), and in the ventral horn dorsolateral to the NASN. Perikarya in the NASN and the region dorsolateral to it did not contribute efferent fibres to the abdominal vagus. In the remaining cell groups, fewer labelled perikarya were labelled in the abdominal cases than in the cervical cases. Extraperikaryal labelling (presumptive terminals) in the cervical cases was seen primarily in the nucleus of the solitary tract. A modest distribution of extraperikaryal grains was also noted along the inner rim of the area postrema and the ventral border of the DMN. Anterograde labelling was sparser and had a more restricted distribution in the abdominal cases. A detailed description of brainstem pathways of vagal efferent and afferent fibres is provided, as is a comparison of the present observations with those in similar studies of other species.

The anorectic action of peripherally administered 5-HT is enhanced by vagotomy

Peripherally administered 5-HT produced a greater suppression of food intake in rats with subdiaphragmatic vagotomy than in sham-operated controls. The enhanced anorexia to 5-HT in vagotomised rats and the anorexia in sham-operated controls were reversed by methysergide, indicating the involvement of 5-HT receptors in the observed anorexia in both groups of animals. Thus the increased suppression of food intake in vagotomised rats cannot be explained in terms of non-specific effects of 5-HT. Both vagotomised and sham-operated rats showed an equivalent degree of anorexia when treated with fenfluramine suggesting that the receptor mechanism responsible for the anorectic action of 5-HT plays little or no part in the action of fenfluramine. Systemic administration of 5-HT was found to slow the rate of gastric clearance. Unlike the anorexia induced by 5-HT this effect was not reversed by methysergide. Thus it appears that peripherally administered 5-HT interacts with the vagus nerve but the mechanism responsible for 5-HT anorexia is independent of any action on gastric clearance.

Metabolic and feeding patterns: role of sympathetic and parasympathetic efferent pathways

The continuous transfer of fuel from the blood to tissues, and its adjustment to the rate of energy metabolism involve close relationships between the control of endogenous stores and that of the periodic intake of food. Neural and blood-borne signals to the brain and efferent sympathetic and parasympathetic pathways seem responsible for three different metabolic and feeding cycles. The matching of input and output of glucose to and from the blood, i.e. the regulation of the blood glucose level proper, is achieved by a short-term feedback mechanism which involves the action of insulin and glucagon on hepatic glucose production and peripheral glucose utilization. Oscillations in level of blood glucose, plasma insulin and glucagon in 8-11 min cycles, as observed in certain species, presumably reflect the play of this short-term regulation of the blood glucose level. The respective role of a pancreatic feedback and of a neural loop in this short-term regulation is discussed. Signals to brain targets and efferent pathways determine the periodic onset of meals after the energy ingested in the preceding meal has been exhausted in free-fed and briefly deprived rats. New evidence has been provided that the fall in blood glucose level exerts a decisive and direct action on the brain to determine meal onset and/or meal size. Superimposed on this prandial periodicity, a lipostatic mechanism modulates the feeding pattern in a dark-light periodicity by the alternation of fat synthesis and fat mobilization. The effects of VMH lesions and vagotomy, either combined or separately, on daily metabolic and feeding patterns suggest that the neural input to the pancreas and adipose tissues plays a primary role in this endogenous cycle and is also involved in the regulation of a constant fat body mass.

Altered endocrine pancreatic function following vagotomy: possible behavioral and metabolic bases for assessing completeness of vagotomy

Total vagotomy induces multiple alterations in the function of the endocrine pancreas. The temporal evolution of selected behavioral and pancreatic alterations following subdiaphragmatic vagotomy was investigated. Sustained modifications were observed in the circadian rhythm of plasma insulin, beta-cell responses to glucose and acetylcholine, whole system response to oral, but not intravenous, glucose and meal frequency. Transient alterations occurred in plasma insulin concentrations, peak glucose response to oral glucose, body weight, food intake, dark-phase meal size and light/dark food intake ratio. Since a major methodological problem in studies of the effects of vagotomy remains the assessment of the completeness of nerve section, the efficiency of several validation tests based on pancreatic alterations were investigated. Of these possible tests, the marked changes in the time-course of blood glucose following oral glucose administration and the in vivo assessment of acetylcholine sensitivity appear to be the most promising. Among other tests investigated, the water intake/100 g body weight and the stomach weight/body weight ratio had high efficiencies. It is recommended that a combination of objective tests, appropriate to the specific study, with varying degrees of invasiveness be used to assess the completeness of vagotomy.