Nicotine administration reduces neuropeptide Y and neuropeptide Y mRNA concentrations in the rat hypothalamus: NPY may mediate nicotine's effects on energy balance

Chronic nicotine induces growth retardation in neonatal rat pups

In the United State, 20% of pregnant women smoke. One of the most consistent adverse outcomes is reduced birth weight in the off-spring. Animal studies using chronic nicotine, the major psychoactive tobacco ingredient, have shown conflicting results, questioning the role of nicotine in growth retardation. To evaluate the direct effects of nicotine during a period equivalent to the human third trimester, we developed an oral gastric intubation model using neonatal rat pups. Nicotine (6 mg/kg/day) was dissolve in milk-formula and delivered during three feedings daily from postnatal day (P)1 to P7. Nicotine immediately and significantly [P<0.05] decreased weight gain per day (WGD) by 13.5% (+/-) 1 day after onset of treatment in both genders and throughout the treatment period. This resulted in significantly lower body weight at P4 and P5 in male and female pups, respectively. After nicotine withdrawal, WGD returned to control level within 1 day, whereas total body weight recovered by P18. There were no long-term consequences on body weight or growth pattern in either gender. The nicotinic acetylcholine receptor (nAChR) antagonist dihydro-beta-erythroidine (DHbetaE) reversed nicotine's effects on WGD suggesting an involvement of heteromeric alpha4beta2, whereas methyllycaconitine (MLA) an antagonist for the homomeric alpha7-type receptor was ineffective. The immediate decrease of growth in neonatal pups suggests that nicotine's effect on birth weight results from direct anorexic rather then indirect effects due to placental dysfunction or increased fetal hypoxia. The postnatal oral gastric intubation model seems to accurately reflect the direct effects of nicotine in neonates.

Meal patterns and body weight after nicotine in male rats as a function of chow or high-fat diet

Studies of the effects of nicotine (NIC) on meal patterns in rats often employ chow pellet diets that contain little fat, whereas humans using NIC commonly consume diets relatively rich in fat. The aim of the present study was therefore to compare the impact of NIC administration and NIC cessation on meal pattern in adult male rats offered a standard powdered chow (CHOW: 10.9% fat by calories) diet or a palatable high-fat (HIFAT: 58.3% fat by calories) diet. Computerized meal pattern analyses were conducted for male rats treated for 14 days with injections of either saline or 1.4 mg/kg/day of NIC (as the free base given in 5 equal amounts) during the dark phase and continued for 10 days after NIC cessation. The suppression of daily caloric intake by NIC was larger in HIFAT-NIC rats than in CHOW-NIC rats (p < .01), such that NIC induced a greater suppression of body weight in HIFAT-NIC rats, relative to CHOW-NIC rats (p < 0.02). NIC administration reduced MS in both CHOW and HIFAT rats. CHOW fed rats showed a gradual increase in meal number in response to NIC, whereas HIFAT fed rats showed a significant initial suppression of meal number, which returned to control levels by day 4 of the 14 day NIC treatment period. In addition, NIC increased water intake more in HIFAT fed rats than in CHOW rats. Cessation of NIC resulted in transient increases in daily caloric intake in CHOW and in HIFAT rats. The present study demonstrates that NIC actions on food intake suppression, meal patterns, and weight reduction differ depending on whether the rats are fed low- or high-fat diets.

Neuroprotective effect of nicotine against 3-nitropropionic acid (3-NP)-induced experimental Huntington's disease in rats

Nicotinic acetylcholine receptors (nAChRs) are regarded as potential therapeutic targets to control various neurodegenerative diseases. Owing to the relevance of cholinergic neurotransmission in the pathogenesis of Huntington's disease (HD) this investigation was aimed to study the effect of nicotine, a nAChR agonist, on 3-nitropropionic acid (3-NP)-induced neurodegeneration in female Wistar rats. Systemic administration of 3-NP in rats serves as an important model of HD. The animals received subcutaneous injections of nicotine (0, 0.25, 0.50 and 1.00 mg/kg) daily for 7 days. 3-NP (25 mg/kg, i.p.) was administered daily 30 min after nicotine for the same duration. One additional group of rats served as control (vehicle only). On day 8, the animals were observed for neurobehavioral performance (motor activity, inclined plane test, grip strength test, paw test and beam balance). Immediately after behavioral studies, the animals were transcardially perfused with neutral buffered formalin (10%) and brains were fixed for histological studies. Lesions in the striatal dopaminergic neurons were assessed by immunohistochemical method using tyrosine hydroxylase (TH) immunostaining. Treatment of rats with nicotine significantly and dose-dependently attenuated 3-NP-induced behavioral deficits. Administration of 3-NP alone caused significant depletion of striatal dopamine (DA) and glutathione (GSH), which was significantly and dose-dependently attenuated by nicotine. Preservation of striatal dopaminergic neurons by nicotine was also confirmed by immunohistochemical studies. These results clearly showed neuroprotective effect of nicotine in experimental model of HD. The clinical relevance of these findings in HD patients remains unclear and warrants further studies.

The appetite-suppressant effect of nicotine is enhanced by caffeine

AIM

To test whether the anorectic effect of nicotine may be amplified by caffeine.

METHODS

Chewing gums with nicotine and caffeine were administered to 12 healthy young men of normal weight. Different combinations of 0, 1 or 2 mg of nicotine and 0, 50 or 100 mg of caffeine were applied during a 2-h period in a randomized, double blind, cross over design. Appetite sensations were measured using visual analogue scales.

RESULTS

Hunger and prospective food consumption were negatively associated with the increasing doses of nicotine, whereas satiety and fullness were positively associated with the increasing doses of nicotine (p < 0.05). Caffeine appeared to amplify the effects of nicotine on hunger and fullness as a caffeine x nicotine x time interaction was observed in these scores (p < 0.05). The 2-mg dose of nicotine in combination with the 100-mg dose of caffeine caused nausea in four of the non-smokers. However, the effects of nicotine and the caffeine x nicotine x time interaction persisted after the exclusion of these subjects.

CONCLUSION

Caffeine added to nicotine chewing gum appears to amplify its attenuating effects on appetite and the combinations of 1-mg of nicotine with caffeine seem to be well tolerated.

Nicotine stimulates prolactin-releasing peptide (PrRP) cells and non-PrRP cells in the solitary nucleus

Nicotine has been reported to regulate food intake and body weight. But the mechanisms underlying these roles have not been fully elucidated. In the present study, we showed that acute administration of nicotine (0.5 mg/kg s.c.) could activate prolactin-releasing peptide (PrRP)-bearing neurons in the A2 area of the NTS of rats, suggesting that PrRP may be associated with nicotine-induced effects in the central nervous system (CNS). We next treated rats with nicotine chronically (4 mg/kg/day for 7 days i.p.), and the results showed that the body weight was strongly reduced and food intake was greatly suppressed compared to the vehicle control group (p<0.01). Immunocytochemical studies revealed that PrRP-bearing neurons in the NTS were evidently activated after chronic administration of nicotine, suggesting that PrRP was involved in the regulation of nicotine-mediated body weight loss and food intake suppression in rats. We also found that acute/chronic administration of nicotine activated PrRP-negative neurons in the NTS, and the majority of these neurons were shown to be TH-negative, suggesting that noncatecholaminergic, PrRP-negative neurons in the NTS are associated with the roles of nicotine. Nicotine has also been shown to stimulate the secretion of ACTH, a stress responsive hormone. In the present study, rats received nicotine (0.5 mg/kg s.c.) or saline followed by restraint stress for 30 min. The immunocytochemical results showed that nicotine/stress and saline/stress both activated the majority of the PrRP neurons in the NTS, there being no significant difference between the two treatments (p>0.05). Nicotine/stress also greatly activated PrRP/TH-negative neurons in the NTS. Saline/stress, however, caused much lower effect on the activation of PrRP/TH-negative neurons. In addition, the activation effect of nicotine/stress on PrRP/TH-negative neurons was much stronger than that of nicotine alone (p<0.01). These results indicated that PrRP was associated with stress responses, but it had little effect on nicotine-mediated stress responses. On the other hand, nicotine and restraint stress may synergistically activate PrRP/TH-negative neurons in the NTS. Taken together, our data show that PrRP is involved in the nicotine-induced regulation of body weight and food intake, but may not be involved in the mediation of nicotine on stress responses. PrRP/TH-negative neurons in the NTS are also associated with the roles of nicotine in the CNS.

Effect of short-term cigarette smoke exposure on body weight, appetite and brain neuropeptide Y in mice

Although nicotinic receptors have been demonstrated in hypothalamic appetite-regulating areas and nicotine administration alters food intake and body weight in both animals and humans, the mechanisms underlying the effects of smoking on appetite circuits remain unclear. Conflicting effects of nicotine on the major orexigenic peptide, neuropeptide Y (NPY), have been observed in the brain, but the effects of smoking are unknown. Thus, we aimed to investigate how cigarette smoking affects body weight, food intake, plasma leptin concentration, hypothalamic NPY peptide, adipose mass and mRNA expression of uncoupling proteins (UCP), and tumor necrosis factor (TNF) alpha. Balb/C mice (8 weeks) were exposed to cigarette smoke (three cigarettes, three times a day for 4 consecutive days) or sham exposed. Body weight and food intake were recorded. Plasma leptin and brain NPY were measured by radioimmunoassay. UCPs and TNF alpha mRNA were measured by real-time PCR. Food intake dropped significantly from the first day of smoking, and weight loss became evident within 2 days. Brown fat and retroperitoneal white fat masses were significantly reduced, and plasma leptin concentration was decreased by 34%, in line with the decreased fat mass. NPY concentrations in hypothalamic subregions were similar between two groups. UCP1 mRNA was decreased in white fat and UCP3 mRNA increased in brown fat in smoking group. Short-term cigarette smoke exposure led to reduced body weight, food intake, and fat mass. The reduction in plasma leptin concentration may have been too modest to increase NPY production; alternatively, change in NPY or its function might have been offset by nicotine or other elements in cigarette smoke.

The therapeutic potential of nicotine and nicotinic agonists for weight control

Transdermal nicotine patches have been successfully introduced as a safe and powerful aid to smoking cessation; this has contributed to the rising interest in additional therapeutic applications for nicotine and synthetic nicotinic agonists. Nicotine and nicotinic agonists may have a therapeutic potential for a variety of disorders, including Alzheimer's and Parkinson's diseases, depression, attention deficit disorder, Tourette's syndrome and ulcerative colitis. These interests are partially fuelled by the urgent need of the tobacco industry to find new niches for nicotine in a world bound eventually to retire from cigarette smoking. At the same time, there is an increased interest in developing drugs for fighting obesity, a growing affliction of industrialised nations. This review presents data on the potential of nicotine, and in particular synthetic nicotinic agonists, for controlling body weight. Nicotinic agonists may become relatively safe, effective and inexpensive alternatives for several optional drugs currently being developed for treating human obesity, including beta-3-adrenergic agonists, leptin and its agonists, and neuropeptide Y antagonists.

Physiologic estradiol levels enhance hypothalamic expression of the long form of the leptin receptor in intact rats

Estradiol is a potent hypophagic agent that reduces food intake and body weight without a concomitant fall in plasma leptin levels. We investigated whether the hypophagic effect of estradiol is mediated by stimulating POMC and/or inhibiting NPY neuronal pathways in the hypothalamus, which respectively inhibit and stimulate feeding. We examined hypothalamic gene expression of Ob-Rb, NPY, POMC, MC4-R, and AgRP in intact Wistar rats treated with estradiol for 48 hours. Food intake and body weight were reduced in estradiol-treated rats but fat mass was unchanged; plasma leptin and insulin levels were not significantly different from untreated, freely fed controls. In untreated rats that were pair-fed to match the estradiol-treated group, body weight was also reduced without changes in fat mass, although leptin and insulin levels decreased significantly. Ob-Rb expression was increased in both hypophagic groups despite serum leptin were only decreased in pair-fed animals, suggesting an estradiol-stimulating effect on Ob-Rb expression. No significant differences were found in POMC, AgRP, or MC4-R expression among any of the experimental groups. A significant but small decrease in NPY expression was also found in both hypophagic groups; this was explained by the combined effect of both surgery and reduced food intake. These results indicate that estradiol mediated hypophagia in intact rats could be brought about by an enhanced hypothalamic leptin sensitivity but is unlikely to be driven by changes in NPY or melanocortin system.

Effect of nicotine on the expression of leptin and forebrain leptin receptors in the rat

We have previously reported that chronic nicotine administration (4.0 mg/kg/day by i.p. injection over 14 days) up-regulates orexin/hypocretin and neuropeptide Y (NPY) mRNA expression and peptide levels within the hypothalamus. Since there exists a coregulation between these neuropeptides and the protein leptin, the present study was undertaken to determine whether nicotine has a regulatory effect on leptin signaling. Under the same experimental regimen used previously, we found that nicotine down-regulates plasma leptin concentration by 48.8% (P<0.001) and leptin RNA level by 11.4% and 12.4%, respectively, in the perirenal and epididymal white adipose tissue (PWAT, EWAT) compared to the saline controls. We also measured an approximately 20% decrease in white and brown adipose tissue (BAT) by weight in nicotine-treated animals relative to saline controls (P<0.05). On the other hand, we found that chronic nicotine administration increased the expression levels of OB-Rb mRNA by 12% and OB-R mRNA by 25% in the medial basal hypothalamus compared to control rats. Subsequent radioligand binding assays indicated that nicotine also significantly increased leptin binding in ventromedial hypothalamic area (VMA), medial basal hypothalamic area (MBA), arcuate nucleus/median eminence, paraventricular nuclei and piriform cortex. Taken together, our results revealed that nicotine is involved in the regulation of leptin signaling, suggesting that leptin and its receptor play a role in the anorectic effects of nicotine on food intake and body weight in rats.

Effect of i.c.v. infusion of the alpha-MSH agonist MTII on meal patterns in male rats following nicotine withdrawal

The present study explored the role of endogenous alpha-MSH in the alteration of meal patterns induced by nicotine (NIC) withdrawal. Male Sprague Dawley rats bearing third ventricle cannulas were placed in computerized food intake monitors. On days 1-21, the rats were given 4 mg/kg/day of NIC or saline (SAL) in four equal i.p. doses during the dark period. NIC suppressed (P < 0.05) food intake only during the first week. The normalization of food intake occurred when the reduced meal size of the NIC injected rats was countered by an increase in meal number. Despite the normalization of 24-h food intake, body weight in NIC rats was decreased (P < 0.05) for 21 days. On day 22, the rats were divided into 4 groups (n's = 7-8 each) and injected into the third ventricle with various doses of the alpha-MSH agonist MTII or artificial cerebrospinal fluid (aCSF): SAL + aCSF, SAL + MTII, NIC + aCSF, NIC + MTII. Infusion of MTII (30 ng/rat) suppressed (P < 0.01) dark phase food intake in both groups, but the NIC + MTII group ate (P < 0.05) more than the SAL + MTII group. Meal number during the dark phase was suppressed by MTII, but the NIC + MTII group took significantly more meals that the SAL + MTII group. Infusion of MTII suppressed meal size in SAL and NIC treated rats, but this effect was attenuated in NIC treated rats. All meal parameters normalized by the day after i.c.v. infusion. These data indicate that NIC treatment differentially affects the neural controls of meal number and meal size and attenuates the suppression by MTII of meal number and meal size.

Nicotine and its withdrawal modify dorsal raphe 8-hydroxy-2-(di-n-propylamino) tetralin feeding

Nicotine (NIC) and its withdrawal modify dorsal raphe (DR) serotonin (5-HT) neurotransmission in ways that may contribute to the body weight loss vs. gain associated with cigarette smoking vs. cessation, respectively. Modifications in feeding to DR infusions of the 5-HT-1A receptor agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), were used to characterize these potential relationships in the DR-5-HT system during NIC administration vs. withdrawal. Two groups of female rats (total N=45) were implanted for 14 days with subcutaneous Alzet minipumps containing NIC (6 mg/kg/day) or saline. Mid-light cycle (1300-1500 h) 8-OH-DPAT feeding tests occurred three times: (1) 2 days after pump implant, (2) 12 days after pump implant, and (3) 2 days after pump removal. Each feeding test consisted of a 1-h measure of pre-feeding, then a 1-h measure of feeding after DR injection of 8-OH-DPAT (0.6 nmol) or 0.4 microl saline. NIC administration produced acute hypophagia, weight loss, and attenuated 8-OH-DPAT-induced feeding. NIC withdrawal produced acute hyperphagia, weight gain, and a transient increase in 8-OH-DPAT feeding. These findings provide behavioral evidence that systemic NIC modifies the DR 5-HT system in ways that may contribute to NIC's ability to alter feeding and body weight.

Smoking, stress, and negative affect: correlation, causation, and context across stages of smoking

This transdisciplinary review of the literature addresses the questions, Do stress and negative affect (NA) promote smoking? and Does smoking genuinely relieve stress and NA? Drawing on both human and animal literatures, the authors examine these questions across three developmental stages of smoking--initiation, maintenance, and relapse. Methodological and conceptual distinctions relating to within- and between-subjects levels of analyses are emphasized throughout the review. Potential mechanisms underlying links between stress and NA and smoking are also reviewed. Relative to direct-effect explanations, the authors argue that contextual mediator-moderator approaches hold greater potential for elucidating complex associations between NA and stress and smoking. The authors conclude with recommendations for research initiatives that draw on more sophisticated theories and methodologies.

Nicotine administration decreases neuropeptide Y expression and increases leptin receptor expression in the hypothalamus of food-deprived rats

The effects of nicotine on the expressions of neuropeptide Y (NPY) and leptin receptor in the rat hypothalamus were investigated via immunohistochemistry. The results show that NPY expression is not affected in the arcuate nucleus (ARN) and is increased only slightly in the paraventricular nucleus (PVN) by nicotine administration under normal (i.e. fed) conditions and that leptin receptor expression is decreased slightly in the ARN and not affected in the PVN following nicotine treatment under the same conditions. Food deprivation enhanced NPY and suppressed leptin receptor expression in the ARN and PVN of the hypothalamus. Nicotine administration resulted in decreased NPY and increased leptin receptor levels.

The effects of corticotropin-releasing factor on the cortical EEG are reduced following adolescent nicotine exposure

Although smoking is highly prevalent among adolescents, relatively little is known about the lasting neurobehavioral consequences associated with adolescent nicotine exposure. Prior studies from our laboratory suggest that adolescent nicotine exposure induces an anxiogenic profile in adult rats. Corticotropin-releasing factor (CRF) systems are important modulators of anxiety and response to stress. Since acute nicotine administration has been shown to stimulate central CRF activity, the purpose of this study was to examine the effects of adolescent nicotine exposure on CRFs modulation of the cortical and hippocampal EEG in adult rats. Male Sprague-Dawley rats were exposed to nicotine (5 mg/kg/day) between postnatal days 35-40 using transdermal nicotine patches. Six weeks after nicotine exposure ended, the effects of intracerebroventricular administration of CRF (0.01-1.0 microg/5 microl) on EEG activity in the cortex and hippocampus were assessed in nicotine-exposed rats and age-matched control rats. The overall effects CRF were consistent with previous reports. CRF decreased low to moderate frequency EEG activity (1-32 Hz) and increased high frequency EEG activity (32-50 Hz). However, in nicotine-exposed rats the effects of CRF on the frontal and parietal cortical EEG were blunted by 30-50% compared to control rats. A similar pattern of decreased response to CRF was not observed in the hippocampus. These blunted effects of CRF on the cortical EEG suggest that long-term changes in systems responsive to CRF result from adolescent nicotine exposure. Given the role of CRF systems in behavioral responses to stress and anxiety, these data suggest that adolescent nicotine exposure may produce long-term decreases in neurophysiological responses to stress.

Meal patterns in male rats during and after intermittent nicotine administration

Continuous administration of nicotine (NIC) reduces food intake (FI) and body weight (BW), whereas rebound eating and BW gain occur after NIC cessation. However, generalizations derived from prior studies on meal patterns in rats using continuous 24-h NIC administration are limited, because human smokers use NIC intermittently during their active period. In the present study, computerized meal pattern analyses (MPA) were conducted for adult male rats treated for 14 days with either saline or 2 or 4 mg/kg/day of NIC spread over five equal amounts during the dark phase. MPA analyses continued for 14 days after cessation of NIC. Only the 4 mg/kg/day NIC dose caused consistent changes in meal patterns and only that dose is reported herein. Dark period FI was reduced, whereas light period FI was unchanged in the NIC-treated group; thus, there was no rebound eating during the 12-h nontreatment phase. MPA analyses revealed the FI reduction on Day 1 of NIC administration was caused by a persistent decrease in dark phase meal size. On Day 5 of NIC, the rats compensated by significantly increasing the number of meals they took, which tended to normalize dark phase FI. Congruently, dark phase intermeal interval was decreased. Importantly, these changes in meal patterns persisted for 2 weeks after termination of NIC. Upon NIC cessation, the NIC group had a transient elevated FI. The NIC-treated group's BW was significantly suppressed by Day 6 of NIC and after stoppage these rats slowly, but incompletely, regained lost BW over the next 14 days. These results document that administration of NIC during the dark phase resulted in a reorganization of the microstructure of FI in male rats and that long-lasting alterations in the microstructure of FI (e.g., meal size and meal number) were noted for up to 2 weeks after cessation of NIC. These results differ from studies in which NIC was given continuously 24-h/day and indicate that dark phase NIC administration in rats may represent an appropriate model to study the impact of NIC on meal patterns.

Nicotinic receptor-mediated effects on appetite and food intake

It is well known, although not well understood, that smoking and eating just do not go together. Smoking is associated with decreased food intake and lower body weight. Nicotine, administered either by smoking or by smokeless routes, is considered the major appetite-suppressing component of tobacco. Perhaps the most renowned example of nicotine's influence on appetite and feeding behavior is the significant weight gain associated with smoking cessation. This article presents an overview of the literature at, or near, the interface of nicotinic receptors and appetite regulation. We first consider some of the possible sites of nicotine's action along the complex network of neural and non-neural regulators of feeding. We then present the hypothesis that the lateral hypothalamus is a particularly important locus of the anorectic effects of nicotine. Finally, we discuss the potential role of endogenous cholinergic systems in motivational feeding, focusing on cholinergic pathways in the lateral hypothalamus.

Nicotine treatment decreases food intake and body weight via a leptin-independent pathway in Psammomys obesus

It has been reported previously that leptin may be involved in nicotine's ability to reduce body weight. Our aim was to investigate whether the anorexic action of nicotine is related to the actions of leptin by utilizing lean leptin-sensitive and obese leptin-resistant Psammomys obesus. Lean and obese P. obesus were assigned to receive nicotine sulphate at 6, 9 or 12 mg/day or saline (control) for 9 days (n = 6-10 in each group), administered using mini-osmotic pumps. Food intake, body weight, plasma leptin concentrations, plasma insulin and blood glucose were measured at baseline and throughout the study period. Nicotine treatment reduced food intake by up to 40% in lean and obese P. obesus. Plasma leptin levels fell significantly only in lean nicotine-treated animals, whereas no changes were observed in obese nicotine-treated animals. However, both lean and obese nicotine-treated animals had similar reductions in body weight. Our results show that nicotine has dramatic effects on food intake and body weight, however, these changes appear to be independent of the leptin signalling pathway.

Nicotine administration decreases nitric oxide synthase expression in the hypothalamus of food-deprived rats

Effect of nicotine on nitric oxide synthase (NOS) expression in various hypothalamic regions was investigated in rats via nicotineamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry. Sprague-Dawley rats were divided into the fed group, the fed and nicotine-treated group, the food-deprived group, and the food-deprived and nicotine-treated group. The fed groups received abundant food and water, while food was withheld from the food-deprived groups for 48 h. The nicotine-treated groups were injected with nicotine. Following food deprivation, enhanced NAPDH-d expression was detected in the paraventricular nucleus, ventromedial hypothalamic nucleus, and lateral hypothalamic area of the hypothalamus. Nicotine administration to the food-deprived rats resulted in decreased NADPH-d positivity. The present results indicate that nicotine administration is effective in limiting the enhancement in NOS expression following food restriction.

Nicotine infusion alters leptin and uncoupling protein 1 mRNA expression in adipose tissues of rats

We attempted to clarify whether leptin and uncoupling protein 1 (UCP1) are involved in the action of nicotine on the energy balance. Male Wistar rats were infused subcutaneously with nicotine (12 mg x kg(-1) x day(-1)) for 4 or 14 days. At the end of the 4-day period, the plasma concentrations of leptin of the nicotine-treated and pair-fed rats were lower than those of the freely fed rats, although the levels of leptin mRNA expression in various white adipose tissues did not differ among the three groups. At the end of the 14-day nicotine infusion period, plasma concentrations of leptin were higher, and leptin mRNA expression in the omentum and epididymal and retroperitoneal adipose tissues was stronger in the nicotine-treated rats than in the pair-fed and freely fed rats. UCP1 mRNA expression in the brown adipose tissue of nicotine-treated was stronger than that of the pair-fed rats. These results suggest that continuous nicotine infusion differentially affects the synthesis and secretion of leptin according to the duration of infusion and stimulates UCP1 mRNA expression, probably in a manner independent of leptin.

Nicotine up-regulates expression of orexin and its receptors in rat brain

Orexins are two recently discovered neuropeptides that can stimulate food intake. As the chronic use of tobacco typically leads to a reduction in body weight, it is of interest to determine whether nicotine, the major biologically active tobacco ingredient, has an effect on orexin metabolism in the brain. Using a semiquantitative RT-PCR technique, the levels of messenger RNA (mRNA) for prepro-orexin, orexin A (OX1-R) and orexin B (OX2-R) receptors were 20-50% higher in rats receiving nicotine for 14 days at the level of 2-4 mg/kg day compared with rats receiving saline solvent alone. In animals treated with nicotine at 4 mg/kg x day, the expression levels of mRNA for prepro-orexin, OX1-R, and OX2-R were significantly higher compared with those in either the free-feeding control or pair-fed saline control rats. RIA data indicated that both orexin A and orexin B peptide levels were significantly elevated (45-54%; P < 0.01) in the dorsomedial nucleus (DMH) of the nicotine-treated rats compared with either solvent-only or pair-fed controls. Additionally, orexin B was significantly elevated (83%; P < 0.01), over levels in both types of the control animals, in the paraventricular nucleus (PVN) region. In summary, we demonstrated that an inverse association between nicotine and food intake as well as body weight held with doses comparable to those consumed by average human smokers. Moreover, our data indicated that chronic exposure to nicotine can induce a long-term increase in the expression levels of prepro-orexin and their receptor mRNA in the rat hypothalamus and in the levels of orexin A in the DMH and orexin B in the DMH and PVN among the six hypothalamic regions that we examined.

Hypothalamic dopamine and serotonin in the regulation of food intake

Because daily food intake is the product of the size of a meal and the frequency of meals ingested, the characteristic of meal size to meal number during a 24-h light-dark cycle constitutes an identifiable pattern specific to normal states and obesity and that occurs during early cancer anorexia. An understanding of simultaneous changes in meal size and meal number (constituting a change in feeding patterns) as opposed to an understanding of only food intake provides a more insightful dynamic picture reflecting integrated behavior. We have correlated this to simultaneous changes in dopamine and serotonin concentrations and to their postsynaptic receptors, focusing simultaneously on two discrete hypothalamic food-intake-related nuclei, in response to the ingestion of food. The relation between concentrations of dopamine and serotonin limited to the lateral hypothalamic area (LHA) and the ventromedial nucleus (VMN) as they relate to the influence of meal size and meal number during the hyperphagia of obesity and anorexia of cancer as measured in our experiments are discussed. Based on these data, conceptual models are proposed concerning: 1) an "afferent-efferent neurotransmitter unit," with facilitatory or inhibitory neuropeptide properties to generate an appropriate neuroendocrine and neuronal response that ultimately modifies food intake; 2) initiation and termination of a meal, thereby determining the number and size of a meal under normal conditions; and 3) a schema integrating the onset mechanism of cancer anorexia. Nicotine is used as a tool to further explore the relation of meal size to meal number, with a focus on simultaneous changes in dopamine and serotonin concentrations in the LHA and VMN with the onset of acute anorexia of nicotine infusion and acute hyperphagia of nicotine cessation. Data concerning the role of sex-related hormones on dopamine and serotonin with regard to the LHA and VMN in relation to the modulation of food intake are also presented.

Regulation of feeding-associated peptides and receptors by nicotine

Although numerous epidemiological studies have provided convincing evidence for the inverse association between tobacco smoking and body weight, the molecular mechanisms underlying this relationship are not well-understood. Nicotine, as a potent secretagogue, could be expected to influence the levels and expression of many classes of neurotransmitters, as well as of cell-membrane constituents linked to neurotransmission, including signal transducers and related effectors. A potentially major group of candidate molecules that could be involved in feeding-related actions of nicotine are the numerous neuropeptides and peptide hormones shown in the past two decades to regulate food intake and energy expenditure. These could include neuropeptide Y (NPY), orexins, leptins, and uncoupling proteins (UCPs). Some of these peptides were already shown to respond to nicotine treatment in terms of regulation of levels and of activity at the level of cell-membrane receptors. The primary objective of this review is to summarize our current understanding of the regulatory effects of nicotine on the food intake and energy expenditure as related to the expression levels of leptin, NPY, orexin, uncoupling proteins, and of NPY and orexin receptors.

Nicotine administration enhances NPY expression in the rat hypothalamus

Epidemiological studies have shown an inverse relationship between cigarette smoking and body weight. In rodents, a negative correlation between nicotine and body weight has been reported, but this observation was largely derived from studies where relatively high doses of nicotine ( approximately 12 mg/kg/day) were used. In the current study, we showed that a negative relationship also holds for low doses of nicotine that are comparable to that consumed by average human smokers (<6 mg/kg/day). We also demonstrated that 14 days of nicotine administration (4 mg/kg/day) reduced average daily food intake by 19.5% (P<0.01) in the free-feeding nicotine-treated group compared to saline controls. No significant differences in body weight were detected between the nicotine-treated and pair-fed groups. To determine whether the effects of nicotine on food intake and body weight were related to neuropeptide Y (NPY) expression, semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) and radioimmunoassay were utilized to measure NPY mRNA and peptide levels in various regions of the hypothalamus. Significantly higher levels of NPY mRNA (ca. 20-50%) and peptide (ca. 24-69%) were only detected in the nicotine-treated groups. In addition, significantly higher NPY contents were also obtained in two hypothalamic areas of pair-fed control animals. In summary, our data suggest that the pharmacological effects of nicotine on food intake and body weight may be mediated by changes in hypothalamic NPY levels, a neuropeptide that is pivotal to the hypothalamic regulation of food intake.

Effect of 6-hydroxydopamine on neuropeptide Y and corticotropin-releasing factor expression in rat amygdala

The influence of dopaminergic denervation on neuropeptide Y and corticotropin-releasing factor-containing neurons in the amygdala was investigated in rats by examining the effects of a selective, unilateral 6-hydroxydopamine lesion of mesencephalic dopaminergic neurons in both the substantia nigra and the ventral tegmental area on these peptides and their messenger RNA expression, observed eight to 10 days after the lesion. The studies were conducted by immunocytochemical and in situ hybridization methods. Neuropeptide Y or corticotropin-releasing factor-immunoreactive neurons were counted in sections of the amygdala under a microscope, and the messenger RNA expression was measured as optical density units in autoradiograms. A significant increase in both neuropeptide Y and corticotropin-releasing factor messenger RNA expression was found in the amygdala on the lesioned side in comparison with the contralateral one, as well as with the ipsilateral side of vehicle-injected controls. Immunohistochemical studies showed that the number of neuropeptide Y-immunoreactive neurons increased in the whole amygdala on the lesioned side. At the same time, the number of corticotropin-releasing factor-immunoreactive neurons grouped in the central amygdaloid nucleus declined, and so did the staining intensity. The obtained results indicate that dopaminergic denervation stimulates the synthesis of neuropeptide Y and corticotropin-releasing factor in rat amygdala, but the peptide levels are differently regulated, which points to a diverse release of these peptides.

Effects of cigarette smoking and its cessation on body weight and plasma leptin levels

Smokers weigh less than age-matched nonsmokers, and most smokers gain weight after smoking cessation due to an increase in food intake and a decrease in energy expenditure. Leptin is an endocrine signal thought to regulate body fat stores through hypothalamic control of energy intake and expenditure. To determine whether the "weight-reducing" effects of smoking may be mediated by leptin, we measured plasma leptin concentrations in 22 middle-aged and older male smokers (body mass index [BMI], 28 +/- 1 kg/m2, mean +/- SEM) and 22 nonsmokers matched to the smokers for age (64 +/- 1 years) and BMI (28 +/- 1 kg/m2). The body weight and leptin concentration were remeasured at 3 and 6 months in 13 of the smokers who successfully stopped smoking. The leptin concentration correlated positively with the BMI in both smokers (r = .74, P < .001) and nonsmokers (r = .76, P < .001). However, the intercept of the regression line was higher for smokers versus nonsmokers (P < .05), with no difference in the slope. Thus, male smokers have a higher leptin level for a given BMI than nonsmokers. Following 6 months of smoking cessation, body weight increased by 7% (6.0 +/- 0.1 kg, n = 13, P < .01). Despite this weight gain, the mean leptin concentration did not increase with smoking cessation. On average, leptin levels were 25% lower than would be expected for the amount of weight gained after smoking cessation. These findings suggest that cigarette smoking directly elevates circulating plasma leptin concentrations, and this increase may be one mechanism for the lower body weight of smokers compared with nonsmokers.