Infusion of nicotine into the LHA enhances dopamine and 5-HT release and suppresses food intake

Neurobiological mechanisms of nicotine's effects on feeding and body weight

Nicotine, a neuroactive substance in tobacco products, has been widely studied for its effects on feeding and body weight, mostly focusing on the involvement of nervous system, metabolism, hormones, and gut microbiota. To elucidate the action mechanism of nicotine on feeding and body weight, especially the underlying neurobiological mechanisms, we reviewed the studies on nicotine's effects on feeding and body weight by the regulation of various nerve systems, energy expenditure, peripheral hormones, gut microbiota, etc. The role of neuronal signaling molecules such as AMP-activated protein kinase (AMPK) and kappa opioid receptor (κOR) were specialized in the nicotine-regulating energy expenditure. The energy homeostasis-related neurons, pro-opiomelanocortin (POMC), agouti-related peptide (AgRP), prolactin-releasing hormone (Prlh), etc, were discussed about the responsibility for nicotine's effects on feeding. Nicotine's actions on hypothalamus and its related neural circuits were described in view of peripheral nervous system, reward system, adipose browning, hormone secretion, and gut-brain axis. Elucidation of neurobiological mechanism of nicotine's actions on feeding and body weight will be of immense value to the therapeutic strategies of smoking, and advance the medicine research for the therapy of obesity.

Nicotinic Acetylcholine Receptor Signaling in the Hypothalamus: Mechanisms Related to Nicotine's Effects on Food Intake

Despite health risks associated with smoking, up to 20% of the US population persist in this behavior; many smoke to control body weight or appetite, and fear of post-cessation weight gain can motivate continued smoking. Nicotine and tobacco use is associated with lower body weight, and cessation yields an average weight gain of about 4 kg, which is thought to reflect a return to the body weight of a typical nonsmoker. Nicotine replacement therapies can delay this weight gain but do not prevent it altogether, and the underlying mechanism for how nicotine is able to reduce weight is not fully understood. In rodent models, nicotine reduces weight gain, reduces food consumption, and alters energy expenditure, but these effects vary with duration and route of nicotine administration. Nicotine, acting through nicotinic acetylcholine receptors (nAChRs), increases the firing rate of both orexigenic agouti-related peptide and anorexigenic proopiomelanocortin neurons in the arcuate nucleus of the hypothalamus (ARC). Manipulation of nAChR subunit expression within the ARC can block the ability of nicotine and the nicotinic agonist cytisine from decreasing food intake; however, it is unknown exactly how this reduces food intake. This review summarizes the clinical and preclinical work on nicotine, food intake, and weight gain, then explores the feeding circuitry of the ARC and how it is regulated by nicotine. Finally, we propose a novel hypothesis for how nicotine acts on this hypothalamic circuit to reduce food intake. Implications: This review provides a comprehensive and updated summary of the clinical and preclinical work examining nicotine and food intake, as well as a summary of recent work examining feeding circuits of the hypothalamus. Synthesis of these two topics has led to new understanding of how nAChR signaling regulates food intake circuits in the hypothalamus.

Dopamine modulates excitatory transmission to orexin neurons in a receptor subtype-specific manner

Dopamine (DA) can promote or inhibit consummatory and reward-related behaviors by activating different receptor subtypes in the lateral hypothalamus and perifornical area (LH/PF). Because orexin neurons are involved in reward and localized in the LH/PF, DA may modulate these neurons to influence reward-related behaviors. To determine the cellular mechanism underlying dopaminergic modulation of orexin neurons, the effect of DA on excitatory transmission to these neurons was investigated using in vitro electrophysiology on rat brain slices. We found that low concentrations (0.1-1 µM) of DA increased evoked excitatory postsynaptic current amplitude while decreasing paired-pulse ratio. In contrast, high concentrations (10-100 µM) of DA did the opposite. The excitatory effect of low DA was blocked by the D₁ receptor antagonist SCH-23390, whereas the inhibitory effect of high DA was blocked by the D₂ receptor antagonist sulpiride. These results indicate distinct roles of D₁ and D₂ receptors in bidirectional presynaptic modulation of excitatory transmission. DA had stronger effects on isolated synaptic activity than repetitive ones, suggesting that sensitivity to dopaminergic modulation depends on the level of network activity. In orexin neurons from high-fat diet-fed rats, a high concentration of DA was less effective in suppressing repetitive synaptic activity compared with chow controls. Therefore, in diet-induced obesity, intense synaptic inputs may preferentially reach orexin neurons while intermittent signals are inhibited by high DA levels. In summary, our study provides a cellular mechanism by which DA may exert opposite behavioral effects in the LH/PF through bidirectional modulation of orexin neurons via different DA receptors.

Protective Role of Crocin Against Nicotine-induced Damages on Male Mice Liver

Background:

Nicotine is a major pharmacologically active substance in cigarette smoke. It is mainly metabolized in liver and causes devastating effects. Crocin is the chemical ingredient primarily responsible for the color of saffron. It has different pharmacological effects such as antioxidant and anticancer. This study was designed to evaluate the protective role of crocin against nicotine on the liver of mice.

Methods:

Forty-eight mice were equally divided into 8 groups; control (normal saline), nicotine (2.5 mg/kg), crocin (12.5, 25 and 50 mg/kg) and crocin plus nicotine treated groups. Saline, crocin, nicotine and crocin/nicotine (once a day) were intraperitoneally injected for 4 weeks. The liver weight and histology, aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP), and serum nitric oxide levels have been studied.

Results:

The results indicated that nicotine administration significantly decreased liver weight (48.37%) and increased the mean diameter of hepatocyte (239%), central hepatic vein (28.45%), liver enzymes level (ALP 29.43%, AST 21.81%, ALT 21.55%), and blood serum nitric oxide level (57.18%) compared to saline group (P < 0.05). However, crocin and crocin plus nicotine administration significantly boosted liver weight (49.54%) and decreased the mean diameter of hepatocyte (40.48%), central hepatic vein (15.44%), liver enzymes (ALP 22.02%, AST 19.05%, ALT 23.11%), and nitric oxide levels (35.80%) in all groups compared to nicotine group (percentages represent the maximum dose) (P < 0.05).

Conclusions:

Crocin showed its partly protective effect against nicotine-induced liver toxicity.

Nicotinic α4 Receptor-Mediated Cholinergic Influences on Food Intake and Activity Patterns in Hypothalamic Circuits

Nicotinic acetylcholine receptors (nAChRs) play an important role in regulating appetite and have been shown to do so by influencing neural activity in the hypothalamus. To shed light on the hypothalamic circuits governing acetylcholine's (ACh) regulation of appetite this study investigated the influence of hypothalamic nAChRs expressing the α4 subunit. We found that antagonizing the α4β2 nAChR locally in the lateral hypothalamus with di-hydro-ß-erythroidine (DHβE), an α4 nAChR antagonist with moderate affinity, caused an increase in food intake following free access to food after a 12 hour fast, compared to saline-infused animals. Immunocytochemical analysis revealed that orexin/hypocretin (HO), oxytocin, and tyrosine hydroxylase (TH)-containing neurons in the A13 and A12 of the hypothalamus expressed the nAChR α4 subunit in varying amounts (34%, 42%, 50%, and 51%, respectively) whereas melanin concentrating hormone (MCH) neurons did not, suggesting that DHβE-mediated increases in food intake may be due to a direct activation of specific hypothalamic circuits. Systemic DHβE (2 mg/kg) administration similarly increased food intake following a 12 hour fast. In these animals a subpopulation of orexin/hypocretin neurons showed elevated activity compared to control animals and MCH neuronal activity was overall lower as measured by expression of the immediate early gene marker for neuronal activity cFos. However, oxytocin neurons in the paraventricular hypothalamus and TH-containing neurons in the A13 and A12 did not show differential activity patterns. These results indicate that various neurochemically distinct hypothalamic populations are under the influence of α4β2 nAChRs and that cholinergic inputs to the lateral hypothalamus can affect satiety signals through activation of local α4β2 nAChR-mediated transmission.

The role of serotonin in drug use and addiction

The use of psychoactive drugs is a wide spread behaviour in human societies. The systematic use of a drug requires the establishment of different drug use-associated behaviours which need to be learned and controlled. However, controlled drug use may develop into compulsive drug use and addiction, a major psychiatric disorder with severe consequences for the individual and society. Here we review the role of the serotonergic (5-HT) system in the establishment of drug use-associated behaviours on the one hand and the transition and maintenance of addiction on the other hand for the drugs: cocaine, amphetamine, methamphetamine, MDMA (ecstasy), morphine/heroin, cannabis, alcohol, and nicotine. Results show a crucial, but distinct involvement of the 5-HT system in both processes with considerable overlap between psychostimulant and opioidergic drugs and alcohol. A new functional model suggests specific adaptations in the 5-HT system, which coincide with the establishment of controlled drug use-associated behaviours. These serotonergic adaptations render the nervous system susceptible to the transition to compulsive drug use behaviours and often overlap with genetic risk factors for addiction. Altogether we suggest a new trajectory by which serotonergic neuroadaptations induced by first drug exposure pave the way for the establishment of addiction.

Stimulation of nicotine reward and central cholinergic activity in Sprague-Dawley rats exposed perinatally to a fat-rich diet

RATIONALE

While clinical studies show maternal consumption of palatable fat-rich diets during pregnancy to negatively impact the children's behaviors and increase their vulnerability to drug abuse, the precise behavioral and neurochemical mechanisms mediating these phenomena have yet to be examined.

OBJECTIVE

The study examined in rats whether gestational exposure to a high-fat diet (HFD) can increase the offspring's propensity to use nicotine and whether disturbances in central nicotinic cholinergic signaling accompany this behavioral effect.

METHODS

Rat offspring exposed perinatally to a HFD or chow diet were characterized in terms of their nicotine self-administration behavior in a series of operant response experiments and the activity of acetylcholinesterase (AChE) and density of nicotinic ACh receptors (nAChRs) in different brain areas.

RESULT

Perinatal HFD compared to chow exposure increased nicotine-self administration behavior during fixed ratio and dose-response testing and caused an increase in breakpoint using progressive ratio testing, while nicotine seeking in response to nicotine prime-induced reinstatement was reduced. This behavioral change induced by the HFD was associated with a significant reduction in activity of AChE in the midbrain, hypothalamus, and striatum and increased density of β2-nAChRs in the ventral tegmental area and substantia nigra and of α7-nAChRs in the lateral and ventromedial hypothalamus.

CONCLUSIONS

Perinatal exposure to a HFD increases the vulnerability of the offspring to excessive nicotine use by enhancing its reward potential, and these behavioral changes are accompanied by a stimulation of nicotinic cholinergic signaling in mesostriatal and hypothalamic brain areas important for reinforcement and consummatory behavior.

Nicotine alters food-cue reactivity via networks extending from the hypothalamus

Obesity and smoking constitute two of the main causes of preventable deaths in the developed countries today. Many smokers motivate consumption as a means to control their body weight because smoking cessation increases the risk to gain weight. Although it is well established that nicotine reduces feeding in animals and that smoking is associated with reduced body weight in quasi-experimental studies of humans, acute nicotine effects are mixed and little is known about the brain networks supporting these effects. Thus, we investigated 26 normal-weighted never-smokers who received either nicotine (2 mg) or placebo gums following a double-blinded randomized cross-over design. We used functional magnetic resonance imaging (fMRI) to investigate reactivity to palatable food cues after both overnight fasting and following a standardized caloric intake (75 g oral glucose tolerance test (OGTT)). Participants viewed food or low-level control pictures in a block design and rated their current appetite after each block. Nicotine had a small- to medium-sized effect on subjective appetite and significantly altered food-cue reactivity in a region sensitive to caloric intake that extended from the right hypothalamus to the basal ganglia. During placebo sessions, the OGTT reduced functional coupling of this region with a 'salience network' (ie, amygdala, ventromedial prefrontal cortex) in processing of food pictures. Furthermore, nicotine reduced coupling with the nucleus accumbens and the OGTT reduced coupling with an 'interoceptive network' (ie, insula, operculum) instead. We conclude that locally restricted acute effects of nicotine in the hypothalamic area have profound effects on food-processing networks.

Nicotinic regulation of energy homeostasis

INTRODUCTION

The ability of nicotine, the primary psychoactive substance in tobacco smoke, to regulate appetite and body weight is one of the factors cited by smokers that prevents them from quitting and is the primary reason for smoking initiation in teenage girls. The regulation of feeding and metabolism by nicotine is complex, and recent studies have begun to identify nicotinic acetylcholine receptor (nAChR) subtypes and circuits or cell types involved in this regulation.

DISCUSSION

We will briefly describe the primary anatomical and functional features of the input, output, and central integration structures of the neuroendocrine systems that regulate energy homeostasis. Then, we will describe the nAChR subtypes expressed in these structures in mammals to identify the possible molecular targets for nicotine. Finally, we will review the effects of nicotine and its withdrawal on feeding and energy metabolism and attribute them to potential central and peripheral cellular targets.

Involvement of cholinergic mechanisms in the behavioral effects of dietary fat consumption

Clinical reports suggest a positive association between fat consumption and the incidence of hyperactivity, impulsivity and cognitive abnormalities. To investigate possible mechanisms underlying these disturbances under short-term conditions, we examined in Sprague-Dawley rats the influence of 7-day consumption of a high-fat diet (HFD) compared to chow on anxiety, novelty-seeking and exploratory behaviors and also on acetylcholine (ACh) neurotransmission that may mediate these behaviors. The HFD consumption, which elevated circulating fatty acids but produced no change in caloric intake or body weight, stimulated novelty-seeking and exploration in an open field, while reducing anxiety in an elevated plus maze. Using the Ellman assay to measure ACh esterase (AChE) activity that breaks down ACh, the second experiment showed HFD consumption to significantly reduce AChE activity in the frontal cortex, hypothalamus and midbrain. With measurements of [¹²⁵I]-epibatidine or [¹²⁵I]-bungarotoxin binding to nicotinic ACh receptors (nAChRs) containing β2 or α7 subunits, respectively, the results also showed HFD consumption to increase both β2-nAChR binding in the medial prefrontal cortex and substantia nigra and α7-nAChR binding in the lateral and ventromedial hypothalamus. When treated with an acute dose of the nicotinic antagonist, mecamylamine (0.5 mg/kg, sc), the HFD animals responded with significantly reduced exploratory and novelty-seeking behaviors, whereas the chow-consuming rats exhibited no response. These findings suggest that the exploratory and novelty-seeking behaviors induced by dietary fat may be mediated by enhanced nicotinic cholinergic activity, which is accompanied by increased density of β2-nAChRs in cortical and midbrain regions associated with impulsivity and locomotor activity and of α7-nAChRs in hypothalamic regions associated with arousal and energy balance.

The effects of chronic nicotine on meal patterns, food intake, metabolism and body weight of male rats

It is unclear what contribution food intake and metabolism have in causing weight loss after administering a dose of nicotine equivalent to smoking one to three packs of cigarettes per day because previous studies have been of a very short duration. To address this question, male Sprague Dawley rats were housed in computerized food intake modules and fed 45 mg pellets: Group 1 [nicotine injected with 1.4 mg/kg/day (free base), fed ad libitum]; and Group 2 [saline injected and pair-fed by computer with Group 2]; and Group 3 [saline injected (i.p.), fed ad libitum]. The rats received 4 equally spaced injections over the dark phase. Treatment consisted of: Phase 1 (nicotine or saline for 14 days), Phase 2 (all rats saline for 8 days and Phase 3 (pair-fed group "unyoked" for 6 days)). Nicotine inhibited food intake over the first 6 days. On termination of nicotine, there was no compensatory hyperphagia in either Groups 1 or 2; and their body weight was reduced starting on day 5 until day 28. In another study, rats were housed in an indirect calorimetry system. Saline or nicotine was injected for 14 days, as noted above; then all rats were injected with saline for 4 days and then no injections for 10 days to follow changes in body weight. Energy expenditure (Kcal/Kg(0.75)) was measured for 18 days. Nicotine significantly reduced food intake on 7 of 14 days of nicotine injections. The body weight of the nicotine injected rats was significantly reduced starting on day 3 until day 25. There were no differences in energy expenditures of the groups, which suggested that a decrease in food intake and not an increase in metabolism was the reason the rats lost weight after administering nicotine.

Nicotine's attenuation of body weight involves the perifornical hypothalamus

Previously we showed that intermittent administration of nicotine (NIC) in the dark phase decreased food intake and body weight and this could be blocked when the NIC receptor antagonist mecamylamine was infused into the fourth ventricle. Catecholaminergic neurons adjacent to the fourth ventricle contain NIC receptors and directly innervate the perifornical hypothalamus (PFH) which has been shown to be involved in regulation of feeding. This study explored whether NIC regulates feeding behavior by modulating catecholaminergic input to the PFH. Epinephrine and norepinephrine neuronal input was ablated within the PFH by infusion of 6-hydroxydopamine hydrobromide (6-OHDA), while bupropion was infused to protect dopaminergic neurons. After recovery of body weights to pre-surgery levels, food intake, meal size, meal number and body weight were measured after intermittent NIC injections. The results showed the PFH lesioned animals did not exhibit the typical prolonged drop in food intake, meal size and body weight normally associated with NIC administration. High performance liquid chromatography analyses demonstrated that compared to control rats, 6-OHDA administration significantly reduced PFH norepinephrine and epinephrine levels, but not dopamine levels. These results are consistent with NIC reducing food intake in part by acting through catecholaminergic neurons within or extending through the PFH.

Nicotine regulates mRNA expression of feeding peptides in the arcuate nucleus in neonatal rat pups

Maternal smoking results in low birth weight. Using a neonatal gastric intubation model corresponding to the third trimester in humans, nicotine, the major psychoactive ingredient in tobacco, causes growth retardation in rat pups. Here, we wanted to determine the underlying mechanisms of nicotine's anorexic effects. In adults, body weight and energy expenditure are regulated by the adiposity hormone leptin and the orexigenic peptides neuropeptide Y (NPY) and agouti-related peptide (AgRP) and anorexic peptides proopiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART) expressed in the hypothalamic arcuate (Arc) nucleus. Activation of nicotinic acetylcholine receptors (nAChRs) could regulate leptin release and/or peptide expression in the Arc. Neonatal rat pups were treated twice daily with nicotine (0.25, 1.5, and 3 mg/kg) from postnatal day 1 to 8 (P1-8). This resulted in an upregulation of heteromeric nAChR binding sites in the ventromedial nucleus of the hypothalamus and Arc. Nicotine at all three doses significantly reduced body weight gain and increased mRNA expression of NPY, AgRP, and POMC effects, which were blocked by dihydro-beta-erythroidine (DHbetaE), an alpha4beta2* nAChR antagonist, but CART expression was unaffected. In contrast, serum leptin levels were significantly increased only by 3 and 1.5 mg/kg, and the increase was only partially blocked by DHbetaE. These data suggest that in neonates chronic nicotine regulates body weight gain independent from serum leptin levels by a central mechanism involving alpha4beta2* heteromeric nAChRs and stimulated increased expression of the anorexic peptide POMC. Whereas, increased NPY and AgRP expression could be a secondary response to reduction in weight gain.

Neurotransmitters in key neurons of the hypothalamus that regulate feeding behavior and body weight

During the last two decades attention has been focussed on the role of different neuropeptides in hypothalamic control of feeding behavior. Several hypothalamic peptides that participate in the control of ingestive behavior are produced in neuronal cell bodies of the arcuate nucleus and/or the lateral hypothalamic area. Apart from producing orexigenic or anorexigenic compounds of peptidergic nature, these neurons also produce excitatory and inhibitory amino acid neurotransmitters. The role of GABA and glutamate in regulating energy balance has received less attention in comparison to neuropeptides. The arcuate nucleus-median eminence area, a region with a weak blood-brain barrier, contains at least two neuronal cell populations that exert opposing actions on energy balance. The majority of the neurons located in the ventromedial aspect of the arcuate nucleus, which produce the orexigenic peptides neuropeptide Y (NPY) and agouti-related peptide (AGRP), contain in addition the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD) and the vesicular GABA transporter (VGAT), thereby supporting their GABAergic nature. Some neurons producing pro-opiomelanocortin (POMC)- and cocaine- and amphetamine-regulated transcript (CART), located in the ventrolateral division of the arcuate nucleus have recently been reported to contain the vesicular glutamate transporter 2 (VGLUT2), a marker for glutamatergic neurons, and the acetylcholine (ACh) synthesizing enzyme choline acetyltransferase (ChAT) as well as the vesicular ACh transporter (VAChT), supporting also a cholinergic phenotype. In the lateral hypothalamic area, hypocretin/orexin neurons express VGLUT1 or VGLUT2, but not GAD, whereas some melanin-concentrating hormone (MCH) cells contain GAD. These observations support the view that several classical transmitters, relatively neglected feeding transmitters candidates, are present in key neurons that regulate body weight and consequently may represent important orexigenic/anorexigenic mediators that convey information to other neurons within the hypothalamus as well as from the hypothalamus to other brain regions that participate in regulation of energy balance.

Differentiation in the short- and long-term effects of smoking on plasma total ghrelin concentrations between male nonsmokers and habitual smokers

To explore the association between the anorexigenic effects of nicotine and the orexigenic properties of ghrelin, plasma total ghrelin levels were measured in nonsmokers and habitual smokers before and after short-term exposure to cigarette smoke. Thirty-one male smokers and 23 nonsmoking volunteers were matched for age and body mass index. After an overnight fast and abstinence from smoking, they all smoked 2 cigarettes consecutively (same brand, rate of inhalation, and duration of smoking). Total ghrelin concentrations were measured by radioimmunoassay before smoking (baseline), immediately afterward, and 30, 60, and 90 minutes after the second cigarette. Baseline ghrelin levels were not different between smokers and nonsmokers. Smoking did not have an immediate influence on ghrelin concentrations in smokers (analysis of variance for repeated measurements, P=0.74), whereas there was a progressive decline in nonsmokers, reaching statistical significance at 30 minutes (P=.04) and a nadir at 60 minutes (P=.04) after smoking. Moreover, the area under the curve for the changes of ghrelin over time after smoking was lower in nonsmokers than in smokers (-287.2+/-167.1 vs 29.2+/-125.3 ng.min/L, P=.03). In conclusion, fasting plasma total ghrelin concentrations are not different between male smokers and nonsmokers. Smoking does not provoke any short-term change in ghrelin levels in smokers, but induces a decline in nonsmokers. If the anorectic effect of smoking is ghrelin induced, this effect may be present only in people not habituated to smoke exposure. In habitual smokers, ghrelin suppression by short-term smoking could be blunted as a result of desensitization due to prolonged nicotine exposure.

Hypothalamic proopiomelanocortin (POMC) neurons have a cholinergic phenotype

Neuronal networks originating in the hypothalamic arcuate nucleus play fundamental roles in the control of energy balance. Neuropeptide Y (NPY)-producing neurons in the arcuate nucleus stimulate food intake, whereas arcuate nucleus neurons that release the proopiomelanocortin (POMC)-derived peptide alpha-melanocyte-stimulating hormone (alpha-MSH) potently reduce food intake. Relatively little attention has been focused on classical neurotransmitters in regulation of food intake. Here, we have investigated the potential presence of acetylcholine (ACh) in NPY- and POMC-containing neuronal populations of the arcuate nucleus. Antisera to proteins required for cholinergic neurotransmission, including choline acetyltransferase (ChAT) and the vesicular acetylcholine transporter (VAChT), were employed in double-labeling immunohistochemical experiments. In colchicine-treated rats, ChAT- and VAChT-immunopositive cell bodies were located in the ventral aspect of the arcuate nucleus. ChAT and VAChT immunoreactivities were demonstrated in alpha-MSH- and cocaine- and amphetamine-regulated transcript (CART)-containing cell bodies of the arcuate nucleus, whereas cell bodies containing NPY or agouti-related peptide (AGRP) were distinct from VAChT-immunoreactive neuronal perikarya. VAChT immunoreactivity was also present in a large number of alpha-MSH-containing nerve fiber varicosities throughout the central nervous system. In the commissural part of the nucleus tractus solitarius, no alpha-MSH-containing cell bodies were found to have ChAT or VAChT immunoreactivity. The presence of markers for cholinergic neurotransmission in a subpopulation of hypothalamic POMC/CART neurons suggests co-release of ACh with peptides derived from the POMC precursor and CART. The results indicate a role for ACh in control of energy balance, mediating the effects of peripheral hormones such as leptin and insulin.

Exposure to smoke during development: fetal programming of adult disease

It is well established that smoking has potent effects on a number of parameters including food intake, body weight, metabolism, and blood pressure. For example, it is well documented that 1) there is an inverse relationship between smoking and body weight, and 2) smoking cessation is associated with weight gain. However, there is increasing evidence that smoking can exert deleterious effects on energy balance through maternal exposure during fetal development. Specifically, there appears to be an increased incidence of metabolic disease (including obesity), and cardiovascular disease in children and adults that were exposed to smoke during fetal development. The present review will examine the relationship between maternal smoke and adult disease in offspring. The epidemiological studies highlighting this relationship will be reviewed as well as the experimental animal models that point to potential mechanisms underlying this relationship. A better understanding of how smoking effects changes in energy balance may lead to treatments to ameliorate the long-lasting effects of perinatal exposure to smoke as well as increasing the health benefits associated with smoking cessation.

Cholinergic modulation of appetite-related synapses in mouse lateral hypothalamic slice

Nicotine administration reduces appetite and alters feeding patterns; a major deterrent to smoking cessation is hyperphagia and resultant weight gain. We demonstrate here that lateral hypothalamic (LH) circuits involving melanin-concentrating hormone (MCH) neurons are subject to cholinergic modulation that may be related to the effects of nicotine on appetite control. Cholinergic input to the perifornical LH area of the mouse is confirmed by examination of immunostaining for vesicular acetylcholine (ACh) transporter (VAT) in conjunction with antibodies to MCH and the vesicular GABA transporter (vGABAT). vAChT-positive neurons border the LH, and VAT-positive projections are detected throughout the perifornical area. MCH-positive dendrites appear studded with vGABAT-positive contacts, consistent with recordings of GABAergic inputs to LH/MCH neurons identified by their location, morphology, electrophysiological profile, and MCH expression. Activation of presynaptic nicotinic ACh receptors (nAChRs) enhances GABAergic transmission. GABAergic transmission is potentiated by (1) direct nicotine application, (2) increasing local ACh concentration, and (3) stimulation of cholinergic projections. Based on pharmacological studies and comparisons of wild-type versus alpha7 nAChR subunit mutant mice, we propose that alpha7*-nAChRs are required for the modulation of GABAergic inputs in LH. Prenatal exposure to nicotine elicits a persistent elevation of GABAergic transmission in the LH of postnatal pups. Furthermore, GABAergic inputs to LH of prenatal nicotine-exposed pups are insensitive to subsequent nicotine challenge. Our studies support the hypothesis that nicotine administration or elevated cholinergic tone enhance inhibition of perifonical LH/MCH neurons via activation of presynaptic alpha7*-nAChRs.

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.

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 infusion into rat ventromedial nuclei and effects on monoaminergic system

Nicotine increases satiety and reduces food intake (FI). We hypothesize that nicotine influences FI via alteration of serotonin (5HT) and dopamine (DA) concentration in ventromedial nucleus (VMN) and lateral hypothalamic area (LHA). Microdialysis cannulas were implanted into ipsilateral VMN and contralateral LHA. Nicotine or vehicle was infused for 60 min into VMN of overnight food-deprived rats, followed by ad lib food for 40 min. Hypothalamic changes in 5HT and DA concentrations were measured every 20 min. Intra-VMN nicotine induced a long-lasting increase in 5HT concentration and an increase in DA for a short duration in the VMN, associated with an increase in 5HT in the LHA. Our data suggest that the nicotine-induced hypophagia correlates with VMN and LHA monoaminergic changes.

Alpha-lactalbumin combined with a regular diet increases plasma Trp-LNAA ratio

Brain serotonin influences food intake and mood. It is synthesised from tryptophan (Trp) of which uptake in the brain is dependent on plasma ratio of tryptophan to the sum of other large neutral amino acids (Trp-LNAA). A carbohydrate-rich diet increases this ratio, whereas a protein-rich diet decreases it. Yet, if the protein source is alpha-lactalbumin the ratio increases. It is, however, unknown whether this also happens in the context of a regular diet (15% protein). We studied the effect of an alpha-lactalbumin supplement combined with regular diet on plasma Trp-LNAA ratio, serum prolactin (marker of serotonin synthesis), food intake, appetite, macronutrient preference and mood. Eighteen healthy males participated in a double-blind, randomised, placebo-controlled, crossover study. One hour after breakfast they received a drink containing alpha-lactalbumin and carbohydrates (AS) or carbohydrates (PS) only. Plasma Trp-LNAA ratio, serum prolactin, food intake, appetite, macronutrient preference and mood were assessed before and 90 min after consumption of the supplement. Changes of plasma Trp-LNAA ratio differed (P<.001) between both supplements, increasing by 16% after AS and decreasing by 17% after PS. Decrease of serum prolactin was slightly smaller after AS than after PS (P=.083). Appetite, food intake, macronutrient preference or mood did not differ between supplements. We conclude that an alpha-lactalbumin-enriched supplement combined with a regular diet increases plasma Trp-LNAA ratio and may influence serum prolactin, but we could not demonstrate effects on appetite, food intake, macronutrient preference and mood.

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.

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.

Distribution of serotonin 5-hydroxytriptamine 1B (5-HT(1B)) receptors in the normal rat hypothalamus

This is the study first attempting to evaluate distribution of neurons expressing serotonin 5-hydroxytriptamine 1B (5-HT(1B)) receptors in hypothalamus by using immunocytochemistry. The 5-HT(1B)-immunoreactive neurons were widely distributed in hypothalamus. Accumulations of 5-HT(1B) neurons occurred in magnocellular nuclei, supraoptic nucleus, paraventricular nucleus (dorsolateral part) and accessory perifornical, circular and retrochiasmatic nuclei. Magnocellular neurons manifested an intense immunostaining suggesting a high level of 5-HT(1B) receptors. Large and middle-sized neurons with different 5-HT(1B) staining patterns were scattered throughout lateral hypothalamus, periventricular nucleus and lateral preoptic area. Immunofluorescent double-labeling revealed a great overlapping of the distribution 5-HT(1B) neurons and dense network of neuropeptide Y-immunoreactive fibers in paraventricular, supraoptic and arcuate nuclei. The potential functional significance of 5-HT(1B) receptors in the 5-HT control of endocrine functions and feeding are discussed.

Evidence that transient nicotine lowers the body weight set point

OBJECTIVE

Smokers usually gain weight when they quit smoking. The present work explores the hypothesis according to which such a rise is a behavioral response to a raised body weight set point taking place when nicotine is eliminated from the body.

RESEARCH METHODS AND PROCEDURES

The human body weight set point was assessed with classical behavioral and psychophysical methods, from the delay to experience negative alliesthesia when repeatedly ingesting sweet stimuli. Seven habitual smokers were tested once before lunch, after smoking (nonabstinent) as usual and once again after refraining from smoking (abstinent). Three additional nicotine-naive subjects were tested under the same procedure after receiving at 0730 h in the morning a transdermal nicotine patch (14 mg) or a placebo patch. Two of the subjects also received nicotine (7 mg) for a third session.

RESULTS

Oral and transdermal administration of nicotine did not decrease the initial pleasure or modify the initial palatability of eating sweet stimuli, but significantly accelerated the following onset of self-reported displeasure (negative alliesthesia) aroused by repeated ingestion of sweet stimuli.

DISCUSSION

These results are understood as an acute lowering of the body weight set point by nicotine. The body weight gain taking place after quitting smoking may, therefore, be explained by the removal of the lowering of the body weight set point induced by nicotine.

Effects of nicotine on alcohol intake in a rat model of depression

Clinical studies suggest that depression facilitates alcohol abuse. Depressed individuals also have increased rates of smoking, and it has been suggested that nicotine may improve depression. It is therefore possible that nicotine may reduce alcohol use in depression. To investigate this potential relationship, we evaluated alcohol intake in an animal model of depression, which consists of administering clomipramine (CLI), a preferential serotonin reuptake inhibitor, to neonatal rats. This pharmacological manipulation produces adult depression-like behaviors, such as reduced aggressiveness, decreased pleasure seeking, diminished sexual activity, increased locomotor activity and increased REM sleep. In this study, we found that CLI rats exhibited significantly higher locomotor activity, lower aggressiveness and higher alcohol intake than control rats. Chronic administration of a low dose of nicotine (0.25 mg/kg/day) or a sham operation did not modify these behaviors. However, chronic administration of nicotine at a higher dose (1.5 mg/kg/day) significantly increased aggressive behavior and reduced alcohol intake in CLI rats. The effect of nicotine on alcohol intake lasted at least 1 month after cessation of nicotine administration. These results indicate that nicotine reverted some depression signs and reduced alcohol self-administration in the CLI model of depression.

Nicotinic--serotonergic interactions in brain and behaviour

This review focuses on nicotinic--serotonergic interactions in the central nervous system (CNS). Nicotine increases 5-hydroxytryptamine (5-HT) release in the cortex, striatum, hippocampus, dorsal raphé nucleus (DRN), hypothalamus, and spinal cord. As yet, there is little firm evidence for nicotinic receptors on serotonergic terminals and thus nicotine's effects on 5-HT may not necessarily be directly mediated, but there is strong evidence that the 5-HT tone plays a permissive role in nicotine's effects. The effects in the cortex, hippocampus, and DRN involve stimulation of 5-HT(1A) receptors, and in the striatum, 5-HT(3) receptors. The 5-HT(1A) receptors in the DRN play a role in mediating the anxiolytic effects of nicotine and the 5-HT(1A) receptors in the dorsal hippocampus and lateral septum mediate its anxiogenic effects. The increased startle and anxiety during nicotine withdrawal is mediated by 5-HT(1A) and 5-HT(3) receptors. The locomotor stimulant effect of acute nicotine is mediated by 5-HT(1A) receptors and 5-HT(2) receptors may play a role in the expression of a sensitised response after chronic nicotine treatment. Unfortunately, the role of 5-HT(1A) receptors in mediating nicotine seeking has not yet been investigated and would seem an important area for future research. There is also evidence for nicotinic--serotonergic interactions in the acquisition of the water maze, passive avoidance, and impulsivity in the five-choice serial reaction task.

Genotype by smoking interaction for leptin levels in the San Antonio Family Heart Study

Recent studies reported a marked inverse effect of smoking on serum levels of leptin (an adipocyte derived protein), offering a possible explanation for variation in body weight between smokers and non-smokers. The goal of this study was to examine the genetic architecture of the response to smoking in leptin levels using data from the San Antonio Family Heart Study. We employed a variance decomposition analysis using maximum likelihood methods to model genotype by smoking interactions for leptin levels, examined the impact of the exclusion of smokers in a subsequent linkage analysis, and incorporated the QTL identified in the linkage analysis in a model of genotype by smoking interaction. We found significant evidence (P = 0.001) for a genotype by smoking status interaction for serum leptin levels. In the subsequent linkage analysis with smokers excluded, we obtained a maximum LOD score of 3.1 (P = 0.00008) near D8S1102. Using this QTL in a model of genotype by smoking status interaction, we identified significant evidence for an interaction at this specific locus (P = 0.04). Given these results, we hypothesize that a quantitative trait locus in this vicinity of chromosome 8 may have a differential effect on the expression of leptin in smokers versus non-smokers.

Depression and nicotine: preclinical and clinical evidence for common mechanisms

Updated findings on the relationship between nicotine and depression are presented. Clinical and preclinical research on nicotine use and depression suggests that nicotine may have some properties in common with antidepressants. Updated findings involve the comorbidity of smoking and major depressive disorder (MDD), the influence of depression during withdrawal on failure to quit smoking, the course of MDD without nicotine and the neurobiology of smoking and depression.

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

Nicotine reduces appetite and body weight. Because the hepato-portal area senses different types of nutrients that transmit signals via vagal afferent nerves to the hypothalamus to modify food intake and feeding pattern, we investigated the effect of nicotine on a hepato-vagal-hypothalamic pathway. Low doses of nicotine (< 10 ng) injected into portal vein (i.p.v.) decreased, while high doses of nicotine increased (> or = 10 ng) electrophysiological activity of hepatic vagal afferents. Stimulatory effect of high dose of nicotine on vagal hepatic afferents was blocked by a prior i.p.v. injection of curare (30 microg) or hexamethonium (1 mg). Furthermore, activities of gastric vagal and adrenal sympathetic efferents were suppressed by low-dose, but stimulated by high-dose i.p.v. nicotine. These reflex effects did not occur in hepatic vagotomized rats. Results of experiments demonstrate that in addition to nicotine's anorectic effect being mediated via a direct central action, nicotine also acts peripherally via hepatic vagal afferents from sensors of nicotine in the hepato-portal region.

Nicotine alters the usual reciprocity between meal size and meal number in female rat

Tobacco smoking reduces appetite and body weight (BW). Cessation of smoking leads to hyperphagia and weight gain. Daily food intake (FI) is a function of meal number (MN) and meal size (MZ), i.e., FI=MNxMZ. Under normal conditions, the female Fischer rat has a periodic reciprocal fluctuation between MZ and MN corresponding to phase of estrous cycle. Wide fluctuations between MZ and MN compensate each other to keep FI constant. Nicotine (5 mg/kg BW/day) was infused via osmotic minipump for 7 days. Controls received saline. FI, MZ, and MN were measured by an Automated Computerized Rat Eater Meter. Nicotine significantly decreased BW and FI via a decrease in MZ without compensatory increase of MN. Nicotine cessation led to hyperphagia, normalizing BW loss via an increase in MZ, which exceeded a compensatory decrease in MN. Nicotine significantly prolonged the estrous cycle by an extension of proestrous phase. Nicotine significantly lengthened the intermeal interval (IMI), delaying the start of the next meal and simultaneously decreasing subsequent MZ. Stopping nicotine led to normalization of IMI and MZ. Data show that nicotine alters the usual reciprocal regulation between MZ and MN and leads to a prolongation of the estrous cycle.

Role of hypothalamic monoamines in nicotine-induced anorexia in menopausal rats

BACKGROUND

Nicotine reduces body weight by reducing appetite. Estradiol modulates food intake. Menopause or ovariectomy (Ovx) increases food intake and body weight. Nicotine and estradiol individually influence hypothalamic dopamine (DA) and serotonin (5-HT), whose interaction influences food intake and body weight. We investigated whether lower weight gain in menopausal smokers is mediated via changes in hypothalamic DA/5-HT.

METHODS

Ovx or sham-operated female rats had 2 microdialysis guide cannulas simultaneously implanted in ipsilateral ventromedial nucleus of hypothalamus (VMN) and contralateral lateral hypothalamic area (LHA). Rats were divided into 4 groups and received a continuous subcutaneous infusion of nicotine or saline Ovx and sham. DA and 5-HT in LHA and VMN were measured by in vivo microdialysis.

RESULTS

Nicotine infusion decreased food intake and body weight in Ovx and sham groups. Increase in LHA-DA and VMN-5-HT in sham group occurred with nicotine, whereas an increase in VMN-DA in Ovx groups with and without nicotine and VMN-5-HT in Ovx group with nicotine was observed.

CONCLUSIONS

In the presence of estradiol (ovary intact sham-operated rats), nicotine lowers food intake and body weight via increased LHA-DA and VMN-5-HT. In menopause (Ovx rats), nicotine lowers food intake and body weight only via increased VMN-DA and 5-HT. Data show that lower weight gain is mediated via changes in hypothalamic monoamines, primarily via ventromedial hypothalamus.

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.

Genetic dissection of nicotine-related behaviour: a review of animal studies

Nicotine has a broad spectrum of behavioural effects. A considerable body of data has emerged indicating genetic factors regulate the behavioural effects of nicotine. Experimental genetic techniques have been invaluable in generating knowledge on the interrelationship of genetic factors and behavioural responsiveness to nicotine. Three different approaches have been invoked to explore the relationship of genetic factors to response to nicotine. Firstly, the classical genetic tool of inbred lines has been exploited to delineate genetic influences in the effects of nicotine. Secondly, the use of selectively bred lines has been profitably employed to reveal genetic differences in behavioural responses, such as cognition and exploration, to nicotine. These approaches have also provided useful information on the contribution of genetic factors influencing nicotinic receptors function. Finally the molecular genetic technique of gene targetting to create mice with null mutations of specific genes in the central nervous system, which is having a tremendous impact in drug addiction research, has also been employed to gain insight into the molecular and cellular basis of nicotine action. These techniques are proving to be invaluable in dissecting the role of different subunits of the nicotinic acetylcholine receptors on behaviour. This paper provides a survey of the animal studies that have used the above mentioned techniques to gain insight into genetic basis of the behavioural effects of nicotine.

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.