Prolonged nicotine dependence associated with extended access to nicotine self-administration in rats

Rationale Most nicotine self-administration (NSA) studies in rats are performed under limited-access conditions. Few studies have examined the relationship between nicotine dependence and NSA.Objectives To determine how NSA access conditions affect NSA and the duration of nicotine dependence during abstinence, as reflected in somatic signs of withdrawal precipitated by administration of the nicotinic receptor antagonist mecamylamine.Methods The effects of different NSA access conditions (zero, 1 h/5 days, 1 h/7 days and 6 h/7 days per week) and non-contingent nicotine administration on NSA and somatic signs were examined.Results Daily NSA access (30 days) resulted in spontaneous and mecamylamine-induced somatic signs. Both daily access groups (1 h/day and 6 h/day, 7 days/week) exhibited spontaneous somatic signs on day 25 of NSA (17 h post-NSA) and sensitivity to mecamylamine up to 2 and 4 weeks of abstinence, respectively. In contrast, the 1 h/day, 5 days/week access group exhibited mecamylamine-induced somatic signs only up to 1 week of abstinence. NSA behavior was stable in rats with 1 h/day 5 days/week and 1 h/day 7 days/week access, but decreased from initially high rates in the 6 h/day 7 days/week access group, and decreased in rats receiving non-contingent nicotine. In contrast, extended cocaine self-administration access resulted in a gradual escalation in cocaine intake.Conclusion There was no escalation in nicotine intake with extended access conditions, unlike cocaine self-administration. Nevertheless, daily nicotine self-administration seven days per week, for either 1 or 6 h per day, was sufficient to induce long-lasting adaptations in nicotinic acetylcholine receptor activity reflected in spontaneous and antagonist-precipitated somatic signs of withdrawal, possibly reflecting aspects of nicotine dependence.     

Affective and somatic aspects of spontaneous and precipitated nicotine withdrawal in C57BL/6J and BALB/cByJ mice

The aversive aspects of nicotine withdrawal are powerful motivational forces contributing to the tobacco smoking habit. We evaluated measures of affective and somatic aspects of nicotine withdrawal in C57BL/6J and BALB/cByJ mice. Nicotine withdrawal was induced by termination of chronic nicotine delivery through osmotic minipumps or precipitated with the nicotinic acetylcholine receptor (nAChR) antagonists mecamylamine or dihydro-beta-erythroidine (DHbetaE). A rate-independent discrete-trial intracranial self-stimulation threshold procedure was used to assess brain reward function. Anxiety-like behavior and sensorimotor gating were assessed in the light-dark box and prepulse inhibition (PPI) tests, respectively. Acoustic startle response and somatic signs of withdrawal were also evaluated. Spontaneous nicotine withdrawal after 14-day exposure to 10-40 mg/kg/day nicotine induced no alterations in anxiety-like behavior, startle reactivity, PPI, or somatic signs in either strain, and no changes in thresholds in C57BL/6J mice. Extended 28-day exposure to 40 mg/kg/day nicotine induced threshold elevations, increased somatic signs, and anxiety-like behavior 24 h post-nicotine in C57BL/6J mice; thresholds returned to baseline levels by day 4 in nicotine-exposed mice. Mecamylamine or DHbetaE administration induced threshold elevations in nicotine-exposed C57BL/6J mice compared with saline-exposed mice. In conclusion, administration of relatively high nicotine doses over prolonged periods of time induces both the affective and somatic aspects of spontaneous nicotine withdrawal in the mouse, while exposure to nicotine for shorter periods of time is sufficient for nAChR antagonist-precipitated nicotine withdrawal. The current study is one of the first to demonstrate reward deficits associated with both spontaneous and nAChR antagonist-precipitated nicotine withdrawal in C57BL/6J mice.     

Nicotine self-administration and withdrawal: modulation of anxiety in the social interaction test in rats

RATIONALE: Most smokers report smoking has an anxiolytic effect, which may contribute to nicotine dependence. OBJECTIVE: To examine effects in the social interaction test (SI) of anxiety after 4 weeks' self-administered nicotine (15 infusions of 0.03 mg/kg, totalling 0.45 mg/kg per day), and after 24 and 72 h of withdrawal. The effect of exposure to the operant chamber on withdrawal responses was also examined. METHODS: Animals were trained to self-administer saline or nicotine and after 4 weeks they were tested in SI after their daily self-administration session. Animals were retested after 24 and 72 h withdrawal, when they were either taken directly from the home cage or were tested 5 min after a 30-min exposure to the operant chamber. RESULTS: Compared with the saline control group, the animals that had been self-administering nicotine for 4 weeks showed decreased social interaction with no decrease in locomotor activity, indicating a significant anxiogenic effect of the nicotine infusions. There was no change in social interaction after 24 and 72 h withdrawal from chronic nicotine, regardless of whether or not the rats were exposed to the operant chamber just prior to being tested. CONCLUSIONS: Nicotine self-administration is not maintained because of its anxiolytic effect, but despite, or because of, its anxiogenic effect. There was no evidence of an anxiogenic response after either 24 or 72 h of withdrawal and thus increased anxiety on withdrawal from nicotine does not seem to contribute to nicotine self-administration.     

The mGluR5 antagonist MPEP decreased nicotine self-administration in rats and mice

Rationale. Nicotine increases glutamate release in the ventral tegmental area and the nucleus accumbens, and thus enhances dopamine neurotransmission in the mesolimbic system that has been implicated in mediating the rewarding effects of drugs. Metabotropic glutamate receptors 5 (mGluR5) are found in the nucleus accumbens and may play a role in modulating the post-synaptic response to both glutamate and dopamine. Objectives. The present study investigated the effects of the mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) on intravenous nicotine self-administration in Wistar rats and DBA/2J mice. Methods. Rats were allowed to self-administer nicotine (0.01, 0.03 mg/kg per infusion) or respond for food on one of two fixed-ratio 5 schedules of reinforcement. Drug-naive mice were acutely exposed, in pairs, to nicotine (0, 0.016, 0.048, 0.16, 0.48 μg per infusion) self-administration under a fixed ratio 1 schedule of reinforcement, with one subject controlling the delivery of nicotine to both subjects in each pair. Results. MPEP (1–9 mg/kg) dose-dependently reduced nicotine self-administration with no effect on food-maintained responding in the rats. Self-administration of nicotine was obtained only at the 0.048 μg per infusion dose by the mice, and administration of MPEP (5–20 mg/kg) decreased nicotine self-administration response rates in the mice. Conclusions. These results indicate that blockade of mGluR5 decreased nicotine self-administration in both rats and mice, and are consistent with findings showing a role of mGluR5 in cocaine self-administration. It is postulated that mGluR5 plays an essential role in mediating the reinforcing effects of nicotine, possibly but not exclusively, via modulation of mesolimbic dopaminergic neurotransmission. Electronic Publication     

Effects of continuous nicotine infusion on nicotine self-administration in rats: relationship between continuously infused and self-administered nicotine doses and serum concentrations

Rationale The efficacy of nicotine replacement therapy (NRT) for smoking cessation is limited. One reason for this limited efficacy may be that typical serum nicotine concentrations provided by NRT do not match the peak arterial nicotine concentrations achieved from smoking.Objective The purpose of the present study was to determine whether continuous nicotine infusion at a rate producing serum nicotine concentrations that match the estimated peak arterial nicotine concentrations associated with nicotine self-administration (NSA) in rats produces greater suppression of NSA than lower infusion rates.Methods The effects of continuous nicotine infusion were studied by intravenously administering nicotine at various rates (1.0, 3.0, and 8.0 mg/kg per day) to rats concurrently self-administering nicotine (0.03 mg/kg per infusion) during 23-h sessions or cocaine (0.17 mg/kg per infusion) during 2-h sessions.Results Continuous nicotine infusion suppressed NSA in a rate-related fashion. NSA was suppressed by 17, 50, and 73% at infusion rates of 1.0, 3.0 and 8.0 mg/kg per day, respectively. The 8.0-mg/kg per day infusion rate, which provided venous serum nicotine concentrations equaling the peak arterial concentrations associated with NSA, suppressed NSA to a greater extent than lower infusion rates. The 8.0-mg/kg per day nicotine infusion rate had no effect on cocaine-maintained responding, demonstrating that its effects were specific for suppression of NSA. This infusion rate provided a mean percentage replacement of nicotine from NSA of more than 700%. Reacquisition of NSA after suppression by the two highest infusion rates was delayed compared with reacquisition after saline extinction.Conclusions Continuous nicotine infusion produced an infusion rate-related suppression of NSA that was greatest when the infusion provided nicotine doses and venous serum concentrations substantially higher than those typically associated with NRT in humans.     

Norepinephrine release in amygdala of rats during chronic nicotine self-administration: an in vivo microdialysis study

The essential role of the amygdala in learning and memory, including cue-associated learning, is influenced by local release of norepinephrine (NE). The current study investigated changes in amygdaloid NE secretion in rats learning to self-administer nicotine in an unlimited access model (23 h/day). In vivo microdialysis of NE was performed for 9 h intervals during three phases of nicotine self-administration: acquisition (day 1); early maintenance, when self-administration rates first stabilized (day 8.4+/-0.7); and later, during fully stable maintenance (day 17.6+/-1.0). On day 1, a greater number of self-administration episodes (SAEs) were associated with elevated NE levels in rats bar-pressing for nicotine (88% vs. 39% with saline). By early maintenance, such episodes increased threefold and overall NE levels were greater. During later maintenance, however, bar-pressing behavior was similar and NE was elevated by the first SAE of the day, but total daily NE levels were no longer elevated. In all the three phases, the enhanced NE release during the first daily SAE did not occur in the last SAE 9 h later. Thus, in an animal model of unlimited nicotine self-administration that approximates the human pattern of nicotine consumption via smoking, the amygdaloid NE response to nicotine diminishes over each day and with the stabilization of self-administration. The decline of amygdaloid NE secretion after long-term nicotine self-administration likely reflects desensitization to the pharmacological effects of nicotine. In addition, amygdaloid NE release, which enhances the consolidation of amygdala-dependent memory, may no longer be necessary once self-administration behavior has been established.     

Preadolescent tobacco smoke exposure leads to acute nicotine dependence but does not affect the rewarding effects of nicotine or nicotine withdrawal in adulthood in rats

Epidemiological studies indicate that parental smoking increases the risk for smoking in children. However, the underlying mechanisms by which parental smoking increases the risk for smoking are not known. The aim of these studies was to investigate if preadolescent tobacco smoke exposure, postnatal days 21-35, affects the rewarding effects of nicotine and nicotine withdrawal in adult rats. The rewarding effects of nicotine were investigated with the conditioned place preference procedure. Nicotine withdrawal was investigated with the conditioned place aversion procedure and intracranial self-stimulation (ICSS). Elevations in brain reward thresholds in the ICSS paradigm reflect a dysphoric state. Plasma nicotine and cotinine levels in the preadolescent rats immediately after smoke exposure were 188 ng/ml and 716 ng/ml, respectively. Preadolescent tobacco smoke exposure led to the development of nicotine dependence as indicated by an increased number of mecamylamine-precipitated somatic withdrawal signs in the preadolescent tobacco smoke exposed rats compared to the control rats. Nicotine induced a similar place preference in adult rats that had been exposed to tobacco smoke or air during preadolescence. Furthermore, mecamylamine induced place aversion in nicotine dependent rats but there was no effect of preadolescent tobacco smoke exposure. Finally, preadolescent tobacco smoke exposure did not affect the elevations in brain reward thresholds associated with precipitated or spontaneous nicotine withdrawal. These studies indicate that passive exposure to tobacco smoke during preadolescence leads to the development of nicotine dependence but preadolescent tobacco smoke exposure does not seem to affect the rewarding effects of nicotine or nicotine withdrawal in adulthood.     

Blockade of CRF1 receptors in the central nucleus of the amygdala attenuates the dysphoria associated with nicotine withdrawal in rats

The majority of smokers relapse during the acute withdrawal phase when withdrawal symptoms are most severe. The goal of the present studies was to investigate the role of corticotropin-releasing factor (CRF) and noradrenergic transmission in the central nucleus of the amygdala (CeA) in the dysphoria associated with smoking cessation. It was investigated if blockade of CRF1 receptors, blockade of α1-adrenergic receptors, or stimulation of α2-adrenergic receptors in the CeA diminishes the deficit in brain reward function associated with nicotine withdrawal in rats. Nicotine dependence was induced by implanting minipumps that delivered a nicotine solution. Withdrawal was precipitated with the nicotinic acetylcholine receptor antagonist mecamylamine. A discrete-trial intracranial self-stimulation procedure was used to assess the negative affective aspects of nicotine withdrawal. Elevations in brain reward thresholds are indicative of a deficit in brain reward function. In all the experiments, mecamylamine elevated the brain reward thresholds of the rats chronically treated with nicotine and did not affect the brain reward thresholds of the saline-treated control rats. Intra-CeA administration of the CRF1 receptor antagonist R278995/CRA0450 completely prevented the mecamylamine-induced elevations in brain reward thresholds in the nicotine-treated rats and did not affect the brain reward thresholds of the saline-treated control rats. R278995/CRA0450 has also been shown to block sigma-1 receptors but there is no evidence that this could affect negative mood states. Intra-CeA administration of the α1-adrenergic receptor antagonist prazosin or the α2-adrenergic receptor agonist clonidine did not affect the brain reward thresholds of the nicotine or saline-treated rats. These studies suggest that CRF1 receptor antagonists may diminish the dysphoria associated with smoking cessation by blocking CRF1 receptors in the CeA.     

Effects of repeated withdrawal episodes,nicotine dose,and duration of nicotine exposure on the severity and duration of nicotine withdrawal in rats

Rationale The aversive aspects of nicotine withdrawal contribute to high relapse rates to tobacco smoking after cessation attempts. Objectives To investigate the influence of nicotine dose, duration of nicotine exposure, and withdrawal history on the severity of nicotine withdrawal in rats, as assessed by brain stimulation reward thresholds and somatic signs of withdrawal. Methods Repeated spontaneous and precipitated withdrawals were investigated through four successive removals of osmotic minipumps delivering nicotine/saline, or with daily injections of the nicotinic receptor antagonist dihydro-β-erythroidine during chronic nicotine/saline exposure, respectively. The effects of dose and duration of exposure were investigated using minipumps of varying duration delivering different nicotine doses. Results Increased duration of nicotine exposure: a) prolonged the duration but did not alter the magnitude of withdrawal-associated threshold elevations; b) increased somatic signs early during withdrawal. Increased total nicotine exposure (i.e. increased dose and exposure duration) increased the duration of threshold elevations (no effect on magnitude) but had no effect on somatic signs. Neither repeated spontaneous nor repeated precipitated withdrawals altered the magnitude of withdrawal significantly. Conclusions Increases in total nicotine dose resulted in increased severity of the affective aspects of withdrawal. Further, continuous drug exposure resulted in longer lasting withdrawal than intermittent administration even when the total nicotine dose was the same. There was no correlation between threshold elevations and somatic signs of withdrawal. In conclusion, the severity of nicotine withdrawal is mitigated by characteristics of the drug exposure regimen such as drug dose, duration of exposure and whether exposure is continuous or intermittent.     

Effects of a nicotine conjugate vaccine on the acquisition and maintenance of nicotine self-administration in rats

Rationale Immunization of rats against nicotine using a nicotine conjugate vaccine reduces the distribution of nicotine to brain in rats and attenuates some of nicotine's physiological and behavioral effects. It is not known whether such a vaccine can attenuate nicotine's reinforcing effects. Objective The present experiment was conducted to determine whether a nicotine conjugate vaccine could interfere with the acquisition and maintenance of nicotine self-administration (NSA) in rats given 23 h day⁻¹ access to nicotine. Methods To examine acquisition of NSA, rats were vaccinated with nicotine or control immunogen prior to being given access to a 0.01 mg kg⁻¹ infusion⁻¹ nicotine under a fixed-ratio(FR) 1 schedule for week 1, FR 2 for week 2, and FR 3 for week 3. Acquisition of cocaine self-administration (CSA) was similarly examined to determine the specificity of vaccination effects. To examine maintenance of NSA, rats were initially trained to self-administer nicotine under an FR 3 schedule, and then vaccinated with nicotine or control immunogen while NSA continued to be monitored. Results NSA was significantly lower in vaccinated rats compared to controls during the acquisition protocol, with a 38% decrease in the number of infusions during the last week of training. The percentage of rats meeting acquisition criteria in the vaccinated group was lower (36%) than that in the control group (70%), but this difference was not statistically significant. Vaccination did not affect acquisition of CSA, demonstrating its specificity for nicotine. Maintenance of NSA was significantly reduced in vaccinated rats as compared to controls after the final vaccine injection, with a mean reduction of 57%. There was no evidence in either protocol that vaccinated rats attempted to compensate for altered nicotine distribution by increasing nicotine intake. Conclusion These data suggest that vaccination against nicotine can reduce the reinforcing effects of nicotine in rats and may have therapeutic potential for the treatment of tobacco dependence.     

Involvement of alpha1-adrenergic receptors in tranylcypromine enhancement of nicotine self-administration in rat

Rationale The mechanisms mediating tobacco addiction remain elusive. Nicotine, the psychoactive component in tobacco, is generally believed to be the main cause of reward and addiction. However, tobacco smoke contains thousands of constituents, some of which may interact with nicotine to enhance reward. It has previously been shown that monoamine oxidase (MAO) inhibition, known to result from smoking, can enhance nicotine self-administration. The aim of the present study was to evaluate the role of noradrenergic systems in mediating this enhancement of nicotine reward. Objective The objective of this study was to test the hypothesis that MAO inhibitor pretreatment enhances nicotine self-administration by activation of noradrenergic pathways that regulate dopamine release in the nucleus accumbens (NAc). Methods The effect of prazosin (0.0625–0.5 mg/kg, i.p.), a specific α1-adrenergic receptor antagonist, was examined on male rats pretreated with tranylcypromine (3 mg/kg), an irreversible inhibitor of MAO A and B. Acquisition of nicotine (10 μg kg⁻¹ inj⁻¹, i.v.) self-administration behavior was examined over a 5-day period. Nicotine (60 μg kg⁻¹ inj⁻¹, i.v.)-induced increase in NAc extracellular dopamine levels was examined by in vivo microdialysis in non-self-administering animals. Results We have shown that (1) tranylcypromine enhances nicotine self-administration, (2) prazosin pretreatment blocks both the acquisition and the expression of nicotine self-administration, and (3) prazosin pretreatment diminishes nicotine-induced dopamine release in the NAc. Conclusion These data indicate that the stimulation of α1-adrenergic receptors is critical for tranylcypromine enhancement of nicotine reward and suggest a critical interplay between the noradrenergic and dopaminergic systems in tobacco addiction.     

Active immunisation against nicotine blocks the reward facilitating effects of nicotine and partially prevents nicotine withdrawal in the rat as measured by dopamine output in the nucleus accumbens, brain reward thresholds and somatic signs

We recently showed that active immunisation with the nicotine immunoconjugate IP18–KLH reduces the nicotine-induced increase in dopamine (DA) output in the nucleus accumbens (NAC) and prevents reinstatement of nicotine-seeking behaviour in rats. These effects are mediated by altered distribution of nicotine, resulting in reduced amounts of nicotine reaching the brain, thereby interfering with the rewarding properties of the drug. The present study was designed to explore the effect of immunisation against nicotine on mecamylamine-precipitated nicotine withdrawal as assessed by the reduction in DA output in the NAC in rats. Measuring brain reward thresholds and somatic signs of nicotine withdrawal, the effects of immunisation were also tested during chronic nicotine treatment and after its withdrawal. Finally, we examined the effect of immunisation on challenge injections of nicotine on brain reward thresholds after the increases in somatic signs and reward thresholds associated with nicotine withdrawal had dissipated. The results show that immunisation with IP18–KLH prevented the decrease in DA output in the NAC associated with mecamylamine-precipitated nicotine withdrawal. Moreover, immunisation against nicotine did not precipitate a withdrawal syndrome, as measured by brain reward thresholds and somatic signs, in rats chronically exposed to nicotine. Furthermore, the withdrawal syndrome elicited after cessation of chronic nicotine administration was attenuated in immunised rats compared to that of mock-immunised rats. Finally, the lowering in reward thresholds after nicotine challenge injections was attenuated in both naïve and previously nicotine-exposed immunised rats. In conclusion, the present results show that immunisation with IP18–KLH did not precipitate nicotine withdrawal in rats. Thus, immunisation with IP18–KLH may not elicit nicotine withdrawal in smokers either. Furthermore, since the withdrawal syndrome in rats was attenuated by immunisation, the nicotine withdrawal in smokers should not be worsened but may even be ameliorated during a quit attempt.     

Medication-related pharmacological manipulations of nicotine self-administration in the rat maintained on fixed- and progressive-ratio schedules of reinforcement

Rationale  The use of animal models to study existing medications for smoking cessation can elucidate the mechanism(s) of action of cessation agents and further validate the models for medication development. Objective  The objective of the study was to evaluate the response of nicotine self-administration (NSA) to pharmacological agents related to the smoking cessation medication bupropion and to nicotine dosing mimicking nicotine replacement on fixed-ratio (FR) and progressive-ratio (PR) schedules of reinforcement. Materials and methods  NSA was maintained at a nicotine dose of 30 μg/kg/infusion i.v. in rats trained on FR5 and PR40% schedules. Pharmacological manipulations related to bupropion were examined by treating animals with a dopamine reuptake inhibitor [GBR 12909 (GBR)], a norepinephrine reuptake inhibitor [nisoxetine (NIS)], and a nicotinic antagonist [dihydro-β-erythroidine (DHβE)]. The effect of nicotine replacement was examined on the PR schedule by chronic dosing with osmotic minipumps. Results  Significant treatment effects occurred with NIS and combinations of NIS–DHβE and with GBR on response rates. Chronic nicotine dosing reduced self-administration. The two schedules yielded different results with some treatments. Conclusions  Noradrenergic–nicotinic cholinergic interactions and enhanced responding consequent to dopamine reuptake inhibition may be part of the complex behavioral pharmacology of bupropion-like compounds. Observation of differential results with the two schedules has implication for the use of self-administration techniques to elaborate the mechanisms of dependence as well as drug discovery.     

Pharmacological manipulations of the pedunculopontine tegmental nucleus in the rat reduce self-administration of both nicotine and cocaine

RATIONALE: The pedunculopontine tegmental nucleus (PPTg) has been implicated in the self-administration of drugs, particularly nicotine, which acts directly through the PPTg in addition to targeting midbrain dopamine neurons. The direct action of nicotine in PPTg may be through GABAergic mechanisms that have been shown to influence nicotine self-administration preferentially compared to cocaine. OBJECTIVE: The purpose of these experiments was to examine several pharmacological manipulations that alter neuronal activity in the PPTg for their specificity or generality in nicotine versus cocaine reinforcement. METHODS AND RESULTS: Rats trained to self-administer nicotine or cocaine intravenously were prepared with brain microcannulae directed to the PPTg. Intra-PPTg microinfusions of the muscarinic agonist carbachol (0.1-1.0 microg), the micro opioid agonist DAMGO (0.005 and 0.05 microg), tetrodotoxin (5 ng) and neostigmine (0.5 nmol) each reduced the self-administration of nicotine and cocaine maintained on an FR5 schedule of reinforcement. The muscarinic antagonist scopolamine (0.1-1.0 microg) and the opioid antagonist CTOP (1 microg) did not affect self-administration, but reversed the effects of the respective agonist when co-administered with it. Carbachol and DAMGO were also tested in self-administration maintained on a progressive-ratio schedule; each agonist again reduced both nicotine and cocaine self-administration. CONCLUSIONS: PPTg manipulations are able to alter established self-administration of nicotine, which acts at the level of the ventral tegmental area and the PPTg itself, and cocaine, which acts through the mesolimbic dopamine system. These data suggest that the PPTg is an important substrate in drug dependence.     

Rate-dependent effects of bupropion on nicotine self-administration and food-maintained responding in rats

Bupropion has been found to be a useful pharmaceutical agent in furthering smoking abstinence. Preclinical research investigating the effects of bupropion on nicotine self-administration has indicated bupropion has selective effects on nicotine self-administration. However since response rates maintained by nicotine were significantly lower than rates of response maintained by the non-drug reinforcers, bupropion may have resulted in rate-dependent effects. The current experiments attempted to decrease the high response rate maintained through non-drug reinforcers in order to have more comparable control rates when investigating the selectivity of bupropion for nicotine self-administration. The effects of bupropion on nicotine self-administration (0.03 mg/kg/inf) were compared to food-maintained responding at two levels of food deprivation (deprived and satiated). Rats were satiated prior to the experimental session in order to decrease the overall response rates maintained through food reinforcement. Bupropion increased nicotine intake, but dose-dependently decreased food intake, when rats were food-deprived. However, when more comparable rates of behavior in the food-satiated group were investigated, bupropion had similar effects on nicotine and food-maintained responding. The data indicate that the effects of bupropion can be influenced by the control rate of responding. The results from these experiments also indicate that bupropion may not exert a selective effect on nicotine self-administration, since low rates of food and drug maintained responding were increased by the drug. These results indicate the importance of controlling for differences in response rates when attempting to assess the effects of drugs on responding maintained by different reinforcers. Furthermore, the results from the present study suggest that motivational variables (i.e. food deprivation) may be used to control for response rate differences maintained by drug and non-drug reinforcers.     

Deficit in brain reward function and acute and protracted anxiety-like behavior after discontinuation of a chronic alcohol liquid diet in rats

Rationale  Discontinuation of chronic and excessive alcohol consumption leads to a dysphoric state in humans. It is not known if there are changes in brain reward function after the discontinuation of an alcohol liquid in rats. Objectives  The aim of these studies was to investigate the effect of withdrawal from an alcohol liquid diet on brain reward function and acute and protracted anxiety-like behavior. Materials and methods  The intracranial self-stimulation procedure was used to assess brain reward function, and the elevated plus maze test was used to assess anxiety-like behavior. Results  Discontinuation of chronic, 12 weeks, exposure to a 6.2% v/v alcohol liquid diet lead to a minor deficit in brain reward function and did not increase anxiety-like behavior. Discontinuation of chronic, 12 weeks, exposure to a 10% v/v alcohol liquid diet lead to a pronounced deficit in brain reward function and increased anxiety-like behavior. Two weeks after discontinuation of the 10% v/v alcohol liquid diet, the rats with a history of alcohol dependence did not display increased anxiety-like behavior. Restraint stress increased anxiety-like behavior in the rats with a history of alcohol dependence, but not in the control rats. Brain reward thresholds were assessed during the chronic 10% v/v alcohol exposure period. During this period, there were no differences between the brain rewards thresholds of the alcohol and control rats. Conclusion  These findings indicate that withdrawal from a 10% v/v alcohol liquid diet leads to a pronounced deficit in brain reward function and acute and protracted anxiety-like behavior in rats.     

The cannabinoid antagonist AM251 attenuates nicotine self-administration and nicotine-seeking behaviour in rats

The cannabinoid receptor subtype (CB1) antagonist rimonabant (SR141716) has been shown to decrease nicotine self-administration and attenuate nicotine-evoked dopamine release in the nucleus accumbens; effects that support recent findings on its clinical efficacy as a smoking cessation aid. The present experiments aim to advance our understanding on the role of CB1 receptors in rodent models of nicotine dependence. AM251, a selective antagonist at CB1 receptors dose-dependently (1, 3 and 10mg/kg IP) suppressed intravenous nicotine (0.03mg/kg per infusion) self-administration in rats during three successive days of pre-treatment. This reduction was short lasting since behaviour was reinstated by suspending AM251 pre-treatment. This was relatively specific to nicotine self-administration since the profile of these reductions produced by AM251 was significantly different from the responses maintained by food pellets. In a model of nicotine-seeking behaviour, rats that had been extinguished by removal of nicotine and associated cues, and presented with a priming dose of nicotine (0.2mg/kg SC) with the cues, showed robustly reinstated responses to nicotine-seeking behaviour. Acute pre-treatment with AM251 (1-10mg/kg IP) dose-dependently attenuated the reinstatement effects produced by nicotine and the contingently presented cues. These preclinical findings support the use of rimonabant as a smoking cessation aid and highlight the CB1 receptor as a viable target to control intake of nicotine and prevent relapse.     

Dissociating the primary reinforcing and reinforcement-enhancing effects of nicotine using a rat self-administration paradigm with concurrently available drug and environmental reinforcers

Rationale Nicotine has two effects on reinforcement in traditional self-administration paradigms. It serves as a primary reinforcer by increasing the probability of behaviors that result in nicotine delivery. However, nicotine also potently enhances behaviors that result in the delivery of nonpharmacological reinforcers. Objectives The present study sought to dissociate these two effects of nicotine on reinforcement. Methods For one group of rats (2 lever), a nonpharmacological reinforcer [visual stimulus (VS)] was available for pressing one lever. Nicotine infusions were available for pressing a different lever. A second group (NIC + VS) received more traditional self-administration training; both the VS and nicotine were delivered for pressing a single active lever. Control groups received either nicotine infusions (NIC only) or VS presentations (VS only) for pressing the active lever. Results Nicotine alone was a weak reinforcer; the VS alone was slightly more reinforcing than nicotine. When these two reinforcers were combined (NIC + VS), response rates were synergistically increased. For the 2-lever group, responding on the nicotine lever was weak, matching the response rates of rats receiving nicotine alone. However, responding on the VS lever was potently enhanced in this group; equaling the response rates for rats receiving both reinforcers for making a single response (NIC + VS). Conclusions These data indicate that the reinforcement-enhancing effects of nicotine are very potent even when only moderate quantities of the drug are self-administered. Moreover, they provide the first demonstration that the reinforcement-enhancing and primary reinforcing effects of nicotine can be dissociated behaviorally.     

Effects of pregnancy on nicotine self-administration and nicotine pharmacokinetics in rats

Rationale Because of the adverse effects of smoking during pregnancy, understanding the factors that influence maternal smoking may help in developing better treatments to help women quit smoking during pregnancy. Animal models could be useful for this purpose. Objective The purpose of the present study was to begin the development of an animal model of smoking during pregnancy by initially characterizing nicotine self-administration (NSA) in pregnant rats. Another purpose was to begin to explore the effects of pregnancy on nicotine pharmacokinetics in rats. Materials and methods In experiment 1, female rats self-administering nicotine during 23-h sessions were examined throughout gestation and lactation. In experiment 2, locomotor activity was measured during pregnancy to assess further potential motor effects of pregnancy. Experiments 3 and 4 compared the single-dose pharmacokinetics of nicotine in male, nonpregnant female, and pregnant females in the first and third trimester of pregnancy and the first week of lactation. Results NSA decreased over the course of pregnancy with NSA significantly lower in the third trimester compared to nonpregnant controls. NSA remained suppressed for up to 10 days into lactation. Locomotor behavior was also significantly suppressed during the second and third trimesters and throughout lactation. Nicotine elimination was slower in pregnant females compared to nonpregnant females only in the third trimester. Conclusions NSA, locomotor behavior, and nicotine elimination in rats are decreased during late pregnancy. The present study is the first to characterize NSA during pregnancy in animals, providing a potential model of maternal smoking in humans.     

Effects of nicotine and mecamylamine-induced withdrawal on extracellular dopamine and acetylcholine in the rat nucleus accumbens

RATIONALE: Prior research suggests that high levels of acetylcholine (ACh) in the nucleus accumbens (NAc) are associated with aversive states such as morphine withdrawal, but this has not been tested for nicotine withdrawal. OBJECTIVES: The goal was to test the hypothesis that acute nicotine decreases extracellular ACh and increases extracellular dopamine (DA) in the NAc, while withdrawal from nicotine causes an opposite neurochemical imbalance with high extracellular ACh and low DA. METHODS: Rats were prepared with a microdialysis probe in the NAc (primarily the shell region). They received one injection of nicotine (0.5 mg/kg, s.c.) or chronic nicotine (9 mg/kg per day via osmotic minipump). RESULTS: Naive animals receiving acute nicotine showed a mild, significant increase in both ACh (122% of baseline) and DA (124%). After chronic nicotine administration for 7 days, the nicotinic antagonist mecamylamine (1.0 mg/kg, s.c.) precipitated withdrawal with the appearance of somatic signs (teeth chattering and shakes/tremors) and a significant increase in extracellular ACh to 125% of baseline, while extracellular DA decreased to 65%. Control groups receiving saline in place of nicotine or mecamylamine did not show these effects. CONCLUSIONS: Earlier work suggests that the observed release of accumbens ACh and DA in response to acute nicotine administration may be a factor in nicotine-induced suppression of appetite. ACh release during withdrawal, coupled with the decrease in extracellular DA may play a role in the aversive aspects of nicotine withdrawal that contribute to dependency.