Hepato-vagal pathway associated with nicotine's anorectic effect in the rat

Influence of nicotine on choline-deficient, L-amino acid-defined diet-induced non-alcoholic steatohepatitis in rats

Nicotine, a major compound in cigarette smoke, decreases food intake and body weight gain in mammals; however, the influence of nicotine on the progression of non-alcoholic steatohepatitis (NASH) remains controversial. This study aimed to investigate the effect of nicotine on NASH in rat models. Male Wistar rats were fed choline-deficient, l-amino acid-defined (CDAA) diet and treated with nicotine or saline. Food intake, body weight gain, presence of hepatic steatosis, inflammation, and fibrosis were assessed 6 weeks after the rats were fed CDAA diet. Hepatic branch vagotomy was performed to elucidate the mechanism through which nicotine affected steatohepatitis. CDAA diet induced hepatic steatosis, inflammation, and fibrosis, as well as increased the expression of inflammation-related genes. Conversely, nicotine significantly attenuated food intake, body weight gain, and inhibited the CDAA-diet-induced hepatic steatosis, inflammation, and fibrosis, together with increased expression of inflammation-related genes. Hepatic branch vagotomy by itself decreased food intake, body weight gain, and attenuated the CDAA-diet-induced hepatic steatosis, but not inflammation. However, nicotine did not change the food intake, body weight gain, and CDAA diet-induced hepatic steatosis and inflammation in vagotomized rats. These results suggest that nicotine attenuates the CDAA-diet-induced hepatic steatosis and inflammation through the hepatic branch of the vagus nerve in rats.

Possible role of afferent autonomic signals in abdominal organs in anorexic and cardiovascular responses to nicotine injection in rats

Smoking generally causes an increase in nicotine levels in the blood, affecting the brain components, such as the hypothalamus (feeding-related area) or the brain stem (cardiovascular control area). In terms of nicotine transmission to the brain, a new insight that the afferent vagal nerve in the liver is important for sensing increased nicotine levels in the blood and informing the brain was reported in an experiment with rats. However, it has not been clarified whether the afferent autonomic nerve system is implicated in feeding and cardiovascular responses to nicotine. Here, we examined the possible role of afferent autonomic nerve transmission in rats in regulating feeding behavior and cardiovascular functions by nicotine. An intravenous injection of nicotine dose dependently increased the blood pressure (BP) in urethane-anesthetized rats; high nicotine doses also led to an increase in BP in conscious rats. Further, an intravenous injection of nicotine for 3 days reduced food intake and body weight gain in rats. The weight-reducing action of intravenous nicotine was abolished by blocking the afferent sympathetic signals in the abdominal organs, but not the vagal nerve signals. Moreover, the hypertensive action of nicotine was not abolished either by afferent sympathectomy or by vagotomy. Thus, these data suggest that nicotine injected into the vein acts on the afferent sympathetic nerve in the abdominal organs and transmits signals to the brain for reducing body weight, but not for suppressing appetite or increasing BP.

Animal models of nicotine exposure: relevance to second-hand smoking, electronic cigarette use, and compulsive smoking

Much evidence indicates that individuals use tobacco primarily to experience the psychopharmacological properties of nicotine and that a large proportion of smokers eventually become dependent on nicotine. In humans, nicotine acutely produces positive reinforcing effects, including mild euphoria, whereas a nicotine abstinence syndrome with both somatic and affective components is observed after chronic nicotine exposure. Animal models of nicotine self-administration and chronic exposure to nicotine have been critical in unveiling the neurobiological substrates that mediate the acute reinforcing effects of nicotine and emergence of a withdrawal syndrome during abstinence. However, important aspects of the transition from nicotine abuse to nicotine dependence, such as the emergence of increased motivation and compulsive nicotine intake following repeated exposure to the drug, have only recently begun to be modeled in animals. Thus, the neurobiological mechanisms that are involved in these important aspects of nicotine addiction remain largely unknown. In this review, we describe the different animal models available to date and discuss recent advances in animal models of nicotine exposure and nicotine dependence. This review demonstrates that novel animal models of nicotine vapor exposure and escalation of nicotine intake provide a unique opportunity to investigate the neurobiological effects of second-hand nicotine exposure, electronic cigarette use, and the mechanisms that underlie the transition from nicotine use to compulsive nicotine intake.

Characterization of weight loss and weight regain mechanisms after Roux-en-Y gastric bypass in rats

Roux-en-Y gastric bypass (RYGB) is the most effective therapy for morbid obesity, but it has a ∼20% failure rate. To test our hypothesis that outcome depends on differential modifications of several energy-related systems, we used our established RYGB model in Sprague-Dawley diet-induced obese (DIO) rats to determine mechanisms contributing to successful (RGYB-S) or failed (RYGB-F) RYGB. DIO rats were randomized to RYGB, sham-operated Obese, and sham-operated obese pair-fed linked to RYGB (PF) groups. Body weight (BW), caloric intake (CI), and fecal output (FO) were recorded daily for 90 days, food efficiency (FE) was calculated, and morphological changes were determined. d-Xylose and fat absorption were studied. Glucose-stimulated vagal efferent nerve firing rates of stomach were recorded. Gut, adipose, and thyroid hormones were measured in plasma. Mitochondrial respiratory complexes in skeletal muscle and expression of energy-related hypothalamic and fat peptides, receptors, and enzymes were quantified. A 25% failure rate occurred. RYGB-S, RYGB-F, and PF rats showed rapid BW decrease vs. Obese rats, followed by sustained BW loss in RYGB-S rats. RYGB-F and PF rats gradually increased BW. BW loss in RYGB-S rats is achieved not only by RYGB-induced decreased CI and increased FO, but also via sympathetic nervous system activation, driven by increased peptide YY, CRF, and orexin signaling, decreasing FE and energy storage, demonstrated by reduced fat mass associated with the upregulation of mitochondrial uncoupling protein-2 in fat. These events override the compensatory response to the drop in leptin levels aimed at conserving energy.

Separation of hepatic and gastrointestinal signals from the common “hepatic” branch of the vagus

Anatomic studies show that the common hepatic branch (CHB) of the vagus contains afferent fibers that innervate sites outside the hepatoportal region, primarily in the gastrointestinal tract. In the current experiments on the anesthetized rat, the source of signals from the CHB was determined by recording CHB neurophysiological responses before and after transection of the gastroduodenal branch (GDB) of the CHB. Serotonin [5-hydroxytryptamine (5-HT)] and CCK-8 were used as probes to stimulate the CHB. Most of the CHB afferent fibers were 5-HT sensitive (56%), and 35% of these were also sensitive to CCK-8. Portal vein vs. jugular vein infusion of 5-HT and CCK-8 and GDB transection showed that 5-HT- and CCK-sensitive fibers innervate the hepatoportal region and areas outside the hepatic hilus (e.g., the gastrointestinal tract). Suppression of basal nerve activity by a 5-HT3 receptor antagonist (Y-25130) suggests that ∼50% of CHB afferent fibers contain 5-HT3 receptors, but none of these fibers appears to be in the hepatoportal region because only in rats with an intact GDB did Y-25130 reduce nerve activity. In summary, these data are in close agreement with anatomic observations on the distribution of the CHB fibers and indicate that neurophysiological studies of the CHB must be carefully evaluated given the prominent role of nonhepatoportal afferent signals recorded from the CHB.

Cholecystokinin and prostaglandins inhibit responses of vagal afferent activity to systemic administration of nicotine in anesthetized rats

Systemic administration of nicotine suppresses food intake. Since gastric vagal afferents convey satiation signals to the hypothalamus in response to cholecystokinin, we investigated the possibility that nicotine increases afferent activity of the gastric vagal nerves by stimulating release of cholecystokinin. Furthermore, involvement of prostaglandins in the responses of gastric vagal afferents to nicotine was also investigated because prostaglandins stimulate gastric vagal afferent activity. Experiments were performed in urethane-anesthetized rats. Intravenous administration of 300 microg/kg but not 3 or 30 microg/kg nicotine produced biphasic increases in afferent activity. The maximum of the first increase was reached within 1 min, while that of the second increase was reached 10-15 min after nicotine injection. Pretreatment with MK-329, a type A cholecystokinin receptor antagonist, significantly reduced the first increase, without influencing the second increase. Pretreatment with indomethacin, a cyclooxygenase inhibitor, further reduced the first increase and abolished the second increase. These results suggest that nicotine can exert its anorexic effect via an increase in gastric vagal afferent activity which is caused by enhanced release of both cholecystokinin and prostaglandins.

Neural connections between the hypothalamus and the liver

After receiving information from afferent nerves, the hypothalamus sends signals to peripheral organs, including the liver, to keep homeostasis. There are two ways for the hypothalamus to signal to the peripheral organs: by stimulating the autonomic nerves and by releasing hormones from the pituitary gland. In order to reveal the involvement of the autonomic nervous system in liver function, we focus in this study on autonomic nerves and neuroendocrine connections between the hypothalamus and the liver. The hypothalamus consists of three major areas: lateral, medial, and periventricular. Each area has some nuclei. There are two important nuclei and one area in the hypothalamus that send out the neural autonomic information to the peripheral organs: the ventromedial hypothalamic nucleus (VMH) in the medial area, the lateral hypothalamic area (LHA), and the periventricular hypothalamic nucleus (PVN) in the periventricular area. VMH sends sympathetic signals to the liver via the celiac ganglia, the LHA sends parasympathetic signals to the liver via the vagal nerve, and the PVN integrates information from other areas of the hypothalamus and sends both autonomic signals to the liver. As for the afferent nerves, there are two pathways: a vagal afferent and a dorsal afferent nerve pathway. Vagal afferent nerves are thought to play a role as sensors in the peripheral organs and to send signals to the brain, including the hypothalamus, via nodosa ganglia of the vagal nerve. On the other hand, dorsal afferent nerves are primary sensory nerves that send signals to the brain via lower thoracic dorsal root ganglia. In the liver, many nerves contain classical neurotransmitters (noradrenaline and acetylcholine) and neuropeptides (substance P, calcitonin gene-related peptide, neuropeptide Y, vasoactive intestinal polypeptide, somatostatin, glucagon, glucagon-like peptide, neurotensin, serotonin, and galanin). Their distribution in the liver is species-dependent. Some of these nerves are thought to be involved in the regulation of hepatic function as well as of hemodynamics. In addition to direct neural connections, the hypothalamus can affect metabolic functions by neuroendocrine connections: the hypothalamus-pancreas axis, the hypothalamus-adrenal axis, and the hypothalamus-pituitary axis. In the hypothalamus-pancreas axis, autonomic nerves release glucagon and insulin, which directly enter the liver and affect liver metabolism. In the hypothalamus-adrenal axis, autonomic nerves release catecholamines such as adrenaline and noradrenaline from the adrenal medulla, which also affects liver metabolism. In the hypothalamus-pituitary axis, release of glucocorticoids and thyroid hormones is stimulated by pituitary hormones. Both groups of hormones modulate hepatic metabolism. Taken together, the hypothalamus controls liver functions by neural and neuroendocrine connections.

Attentional effects of nicotine and amphetamine in rats at different levels of motivation

RATIONALE

The effects of drugs on performance of tasks used to assess attention might be confounded with changes in motivation. Few studies have investigated the role of motivational factors in such situations.

OBJECTIVES

To determine how changes in motivation for food influence performance of the 5-choice serial reaction time task and whether the effects of nicotine and amphetamine can be explained by motivational changes.

METHODS

Male hooded Lister rats were trained to respond to a 1-s light stimulus presented randomly in one of five apertures in order to obtain food reinforcers. For three groups of rats (n=9-10), access to food was restricted to maintain body weights at 80, 90 or 95% of control weights. Saline and nicotine (0.025-0.2 mg/kg) were tested in each group, with and without pre-feeding (5 g). In a second experiment, saline and amphetamine (0.03-0.9 mg/kg s.c.) were tested without pre-feeding.

RESULTS

High levels of motivation for food were associated with increases in anticipatory responses, fewer omission errors, shorter response latencies and completion of more trials, without change in accuracy. Nicotine, but not amphetamine, increased accuracy and the number of trials completed; whereas amphetamine, but not nicotine, increased omission errors. Both drugs decreased anticipatory responding at the largest doses tested. There were few interactions of motivational level with drug effects.

CONCLUSIONS

The improvements in performance produced by nicotine did not resemble the effect of increased motivation, but some effects of amphetamine resembled those of reducing the level of motivation for food. Motivational levels did not confound assessments of the attentional effects of the drugs in terms of response accuracy.

Sensory neurone responses to mucosal noxae in the upper gut: relevance to mucosal integrity and gastrointestinal pain

The digestive tract is supplied by extrinsic and intrinsic sensory neurones that, together with endocrine and immune cells, form a surveillance network that is essential to gut function. This article focuses on the responses of extrinsic afferent neurones to chemical insults of the gastrointestinal mucosa and their pathophysiological relevance to mucosal integrity and abdominal pain. Within the gastroduodenal region, spinal afferents subserve an emergency function because, in case of alarm by influxing acid, they stimulate mechanisms of mucosal protection via an efferent-like release of transmitters. Other sensory neurones signal chemical noxae to the brain, a task that is not confined to spinal afferents because vagal afferents communicate gastric acid and peripheral immune challenges to the brainstem and in this way elicit autonomic, endocrine, affective and behavioural reactions. Emerging evidence indicates that hypersensitivity of extrinsic afferent pathways to mechanical and chemical stimuli makes an important contribution to the abdominal hyperalgesia seen in functional dyspepsia and irritable bowel syndrome. Sensitization may be brought about by inflammatory processes that lead to up-regulation and functional alterations of receptors and ion channels on sensory neurones. Such sensory neurone-specific molecules, which include vanilloid (capsaicin) receptors, may represent important targets for novel drugs to treat abdominal pain.