Nicotine withdrawal: a behavioral assessment using schedule controlled responding, locomotor activity, and sensorimotor reactivity

Central histaminergic transmission modulates the expression of chronic nicotine withdrawal induced anxiety-like and somatic behavior in mice

The present study investigated the plausible modulatory role of central histaminergic transmission on the expression of nicotine withdrawal induced anxiety and somatic behavior in mice. Abrupt cessation of chronic nicotine (2 mg/kg, i.p. × 3/day) treatment for 12 days to mice, expressed increased anxiety in light & dark test and total abstinence (somatic) score at 24 h post nicotine withdrawal time. The somatic signs includes a composite score of all behaviors such as grooming, rearing, jumping, body shakes, forelimb tremors, head shakes, abdominal constrictions, scratching, empty mouth chewing or teeth chattering, genital licking, tail licking. Mice exhibited higher expression to nicotine withdrawal induced anxiety in light & dark test at 24 h post-nicotine withdrawal time on pre-treatment centrally (i.c.v) with histaminergic agents like histamine (0.1, 50 μg/mouse), histamine H₃ receptor inverse agonist, thioperamide (2, 10 μg/mouse), histamine H₁ receptor agonist, FMPH (2, 6.5 μg/mouse) or H₂ receptor agonist amthamine (0.1, 0.5 μg/mouse) or intraperitoneally (i.p.) with histamine precursor, l-histidine (250, 500 mg/kg) as compared to control nicotine withdrawn animals. Furthermore, mice pre-treated with all these histaminergic agents except histamine H₁ receptor agonist, FMPH shows exacerbated expression to post-nicotine withdrawal induced total abstinence (somatic) score in mice. On the other hand, central injection of selective histamine H₁ receptor antagonist, cetirizine (0.1 μg/mouse, i.c.v.) or H₂ receptor antagonist, ranitidine (50 μg/mouse, i.c.v) to mice 10 min before 24 h post-nicotine withdrawal time completely alleviated the expression of nicotine withdrawal induced anxiety and somatic behavior. Thus, it can be contemplated that the blockade of central histamine H₁ or H₂ receptor during the nicotine withdrawal phase could be a novel approach to mitigate the nicotine withdrawal associated anxiety-like manifestations. Contribution of endogenous histamine via H₁ or H₂ receptor stimulation in the nicotine withdrawal induced anxiety and somatic behavior is proposed.

Corticotropin-releasing factor (CRF) and α 2 adrenergic receptors mediate heroin withdrawal-potentiated startle in rats

Anxiety is one of the early symptoms of opioid withdrawal and contributes to continued drug use and relapse. The acoustic startle response (ASR) is a component of anxiety that has been shown to increase during opioid withdrawal in both humans and animals. We investigated the role of corticotropin-releasing factor (CRF) and norepinephrine (NE), two key mediators of the brain stress system, on acute heroin withdrawal-potentiated ASR. Rats injected with heroin (2 mg/kg s.c.) displayed an increased ASR when tested 4 h after heroin treatment. A similar increase in ASR was found in rats 10-20 h into withdrawal from extended access (12 h) to i.v. heroin self-administration, a model that captures several aspects of heroin addiction in humans. Both the α 2 adrenergic receptor agonist clonidine (10 μg/kg s.c.) and CRF1 receptor antagonist N,N-bis(2-methoxyethyl)-3-(4-methoxy-2-methylphenyl)-2,5-dimethyl-pyrazolo[1,5-a] pyrimidin-7-amine (MPZP; 20 mg/kg s.c.) blocked heroin withdrawal-potentiated startle. To investigate the relationship between CRF1 and α 2 adrenergic receptors in the potentiation of the ASR, we tested the effect of MPZP on yohimbine (1.25 mg/kg s.c.)-potentiated startle and clonidine on CRF (2 μg i.c.v.)-potentiated startle. Clonidine blocked CRF-potentiated startle, whereas MPZP partially attenuated but did not reverse yohimbine-potentiated startle, suggesting that CRF may drive NE release to potentiate startle. These results suggest that CRF1 and α 2 receptors play an important role in the heightened anxiety-like behaviour observed during acute withdrawal from heroin, possibly via CRF inducing the release of NE in stress-related brain regions.

A mechanistic hypothesis of the factors that enhance vulnerability to nicotine use in females

Women are particularly more vulnerable to tobacco use than men. This review proposes a unifying hypothesis that females experience greater rewarding effects of nicotine and more intense stress produced by withdrawal than males. We also provide a neural framework whereby estrogen promotes greater rewarding effects of nicotine in females via enhanced dopamine release in the nucleus accumbens (NAcc). During withdrawal, we suggest that corticotropin-releasing factor (CRF) stress systems are sensitized and promote a greater suppression of dopamine release in the NAcc of females versus males. Taken together, females display enhanced nicotine reward via estrogen and amplified effects of withdrawal via stress systems. Although this framework focuses on sex differences in adult rats, it is also applied to adolescent females who display enhanced rewarding effects of nicotine, but reduced effects of withdrawal from this drug. Since females experience strong rewarding effects of nicotine, a clinical implication of our hypothesis is that specific strategies to prevent smoking initiation among females are critical. Also, anxiolytic medications may be more effective in females that experience intense stress during withdrawal. Furthermore, medications that target withdrawal should not be applied in a unilateral manner across age and sex, given that nicotine withdrawal is lower during adolescence. This review highlights key factors that promote nicotine use in females, and future studies on sex-dependent interactions of stress and reward systems are needed to test our mechanistic hypotheses. Future studies in this area will have important translational value toward reducing health disparities produced by nicotine use in females. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Tobacco addiction and the dysregulation of brain stress systems

Tobacco is a highly addictive drug and is one of the most widely abused drugs in the world. The first part of this review explores the role of stressors and stress-associated psychiatric disorders in the initiation of smoking, the maintenance of smoking, and relapse after a period of abstinence. The reviewed studies indicate that stressors facilitate the initiation of smoking, decrease the motivation to quit, and increase the risk for relapse. Furthermore, people with depression or an anxiety disorder are more likely to smoke than people without these disorders. The second part of this review describes animal studies that investigated the role of brain stress systems in nicotine addiction. These studies indicate that corticotropin-releasing factor, Neuropeptide Y, the hypocretins, and norepinephrine play a pivotal role in nicotine addiction. In conclusion, the reviewed studies indicate that smoking briefly decreases subjective stress levels but also leads to a further dysregulation of brain stress systems. Drugs that decrease the activity of brain stress systems may diminish nicotine withdrawal and improve smoking cessation rates.

Emotional reactivity to emotional and smoking cues during smoking abstinence: potentiated startle and P300 suppression

Negative affect is thought to be an important factor in the maintenance of cigarette smoking, and thus it is important to further develop objective measures of smoking-related emotional responses. Nonsmokers, non abstinent smokers, and abstinent smokers participated in a cue reactivity task where eyeblink startle amplitude and startle probe P300 (P3) suppression were measured during the presentation of emotional pictures.During unpleasant pictures, the amplitude of both measures was smaller in non abstinent smokers than in nonsmokers or abstinent smokers. P3 suppression, but not startle amplitude, was larger in abstinent smokers than in nonsmokers. Abstinence-induced increases in cigarette craving were associated with P3 suppression during tobacco-related pictures. Results suggest that tobacco abstinence increases emotional reactivity to unpleasant stimuli, which is consistent with negative reinforcement models of tobacco addiction.

Endocannabinoid regulation of acute and protracted nicotine withdrawal: effect of FAAH inhibition

Evidence shows that the endocannabinoid system modulates the addictive properties of nicotine. In the present study, we hypothesized that spontaneous withdrawal resulting from removal of chronically implanted transdermal nicotine patches is regulated by the endocannabinoid system. A 7-day nicotine dependence procedure (5.2 mg/rat/day) elicited occurrence of reliable nicotine abstinence symptoms in Wistar rats. Somatic and affective withdrawal signs were observed at 16 and 34 hours following removal of nicotine patches, respectively. Further behavioral manifestations including decrease in locomotor activity and increased weight gain also occurred during withdrawal. Expression of spontaneous nicotine withdrawal was accompanied by fluctuation in levels of the endocannabinoid anandamide (AEA) in several brain structures including the amygdala, the hippocampus, the hypothalamus and the prefrontal cortex. Conversely, levels of 2-arachidonoyl-sn-glycerol were not significantly altered. Pharmacological inhibition of fatty acid amide hydrolase (FAAH), the enzyme responsible for the intracellular degradation of AEA, by URB597 (0.1 and 0.3 mg/kg, i.p.), reduced withdrawal-induced anxiety as assessed by the elevated plus maze test and the shock-probe defensive burying paradigm, but did not prevent the occurrence of somatic signs. Together, the results indicate that pharmacological strategies aimed at enhancing endocannabinoid signaling may offer therapeutic advantages to treat the negative affective state produced by nicotine withdrawal, which is critical for the maintenance of tobacco use.

Translational research in addiction: toward a framework for the development of novel therapeutics

The development of novel substance use disorder (SUD) therapeutics is insufficient to meet the medical needs of a growing SUD patient population. The identification of translatable SUD models and tests is a crucial step in establishing a framework for SUD therapeutic development programs. The present review begins by identifying the clinical features of SUDs and highlights the narrow regulatory end-point required for approval of a novel SUD therapeutic. A conceptual overview of dependence is provided, followed by identification of potential intervention targets in the addiction cycle. The main components of the addiction cycle provide the framework for a discussion of preclinical models and their clinical analogs, all of which are focused on isolated behavioral end-points thought to be relevant to the persistence of compulsive drug use. Thus, the greatest obstacle to successful development is the gap between the multiplicity of preclinical and early clinical end-points and the regulatory end-point of sustained abstinence. This review proposes two pathways to bridging this gap: further development and validation of the preclinical extended access self-administration model; inclusion of secondary end-points comprising all of the measures highlighted in the present discussion in Phase 3 trials. Further, completion of the postdictive validation of analogous preclinical and clinical assays is of high priority. Ultimately, demonstration of the relevance and validity of a variety of end-points to the ultimate goal of abstinence will allow researchers to identify truly relevant therapeutic mechanisms and intervention targets, and establish a framework for SUD therapeutic development that allows optimal decision-making and resource allocation.

Potentiated startle as a measure of the negative affective consequences of repeated exposure to nicotine in rats

RATIONALE

Elevated acoustic startle amplitude has been used to measure anxiety-like effects of drug withdrawal in humans and animals. Withdrawal from a single opiate administration has been shown to produce robust elevations in startle amplitude ("withdrawal-potentiated startle") that escalate in severity with repeated exposure. Although anxiety is a clinical symptom of nicotine dependence, it is currently unknown whether anxiety-like behavior is elicited during the early stages of nicotine dependence in rodents.

OBJECTIVE

The objective of this study is to examine whether, as is the case with opiates, single or repeated exposure to nicotine can produce withdrawal-potentiated startle.

METHODS

Rats received daily nicotine injections for 14 days, and startle amplitude was tested during spontaneous withdrawal on injection days 1, 7, and 14.

RESULTS

Elevated startle responding was observed during nicotine withdrawal on days 7 and 14 but not on day 1, was greater at higher nicotine doses, and was reduced by a nicotine replacement injection given during an additional test session on day 15. Additional experiments demonstrated that nicotine withdrawal-potentiated startle was reduced by the alpha(2)-adrenergic agonist clonidine and that precipitated withdrawal-potentiated startle could not be induced by injection of the nicotinic acetylcholine receptor antagonist mecamylamine.

CONCLUSIONS

These results suggest that nicotine withdrawal escalates in severity across days, similar to the previously reported escalation of opiate withdrawal-potentiated startle. Potentiated startle may be a reliable measure of withdrawal from different classes of abused drugs and may be useful in the study of the early stages of drug dependence.

Removal of continuous nicotine infusion produces somatic but not behavioral signs of withdrawal in mice

The introduction of transgenic and knockout mice has shaped new interest in developing novel and modified behavioral methods for mice that evaluate the various manifestations of nicotine withdrawal syndromes. This study assessed the disruption of operant baselines during drug withdrawal, an established rat model of nicotine dependence, in mice. Subjects were trained to lever press for food reinforcement during daily operant sessions. After stable operant baselines were established, mice were implanted with osmotic minipumps containing 0 (saline), 6, 12, 24, or 48 mg/kg/day nicotine base. Operant responding was assessed for disruptions in daily sessions throughout the experiment. Somatic signs of withdrawal were assessed after the operant session on day 7, following administration of mecamylamine (1 mg/kg), and on days 12, 13, and 14, following spontaneous removal of nicotine. Spontaneous removal of nicotine increased somatic signs of withdrawal but did not disrupt operant responding. Mecamylamine failed to produce signs of precipitated withdrawal in either procedure. This study demonstrated nicotine dependence in mice during spontaneous removal of nicotine. Moreover, since signs of behavioral withdrawal (i.e. disruptions in operant response rates) were not observed, these findings suggest the importance of considering differences in the apparent manifestations of withdrawal syndromes while evaluating nicotine dependence.

Measurement of affective state during chronic nicotine treatment and withdrawal by affective taste reactivity in mice: the role of endocannabinoids

Despite tobacco being highly addictive, it is unclear if nicotine has significant affective properties. To address this, we studied taste reactions to gustatory stimuli, palatable sucrose and unpalatable quinine, which are believed to reflect ongoing affective state. Taste reactivity was assessed during chronic nicotine administration and spontaneous withdrawal and the role of the endogenous cannabinoids was also investigated. C57BL6J mice were implanted with intraoral fistula to allow passive administration of solutions. In the first study, taste reactivity was tracked throughout chronic vehicle or nicotine (12 mg/kg/day) infusion via osmotic minipumps and spontaneous withdrawal following removal of minipumps. In the second study, the endocannabinoid CB1-receptor antagonist AM251 (1, 3 and 10mg/kg, intraperitoneal) or vehicle was acutely administered before taste reactivity measurement during chronic nicotine administration. Chronic nicotine treatment and spontaneous withdrawal did not influence taste reactions to sucrose or quinine. AM251 decreased positive reactions to sucrose and increased negative reactions to quinine. The effects of AM251 were respectively attenuated and enhanced in nicotine infused mice. These results suggest chronic nicotine exposure and withdrawal has no apparent affective sequelae, as probed by taste reactivity, and thus may not explain the difficulty tobacco-users have in achieving abstinence. In contrast, endocannabinoids elevate affective state in drug-naïve animals and changes in endogenous endocannabinoid tone may underlie compensations in affective state during chronic nicotine exposure.

Rodent models of nicotine withdrawal syndrome

Simple, rapid and inexpensive rodent models of nicotine physical dependence and withdrawal syndrome have proved useful for preliminary screening of smoking cessation treatments. They have led to an exponential increase of knowledge regarding the underlying neurobiological mechanisms of dependence and withdrawal syndrome. The human nicotine withdrawal syndrome in smoking cessation is variable and multidimensional, involving irritability, anxiety, depression, cognitive and attentional impairments, weight gain, sleep disturbances, and craving for nicotine. Aside from sleep disturbances, analogous phenomena have been seen in rodent models using different measures of withdrawal intensity. It appears likely that different withdrawal phenomena may involve some partially divergent mechanisms. For example, depression-like phenomena may involve alterations in mechanisms such as the mesolimbic dopamine pathway from the ventral tegmental area to the nucleus accumbens. Irritability and anxiety may involve alterations in endogenous opioid systems and other regions, such as the amygdala. This chapter reviews many additional anatomical, neurochemical, and developmental elements that impact nicotine physical dependence.

Spontaneous nicotine withdrawal potentiates the effects of stress in rats

Anxiety is a common symptom of nicotine withdrawal in humans, and may predict an inability to abstain from cigarette smoking. It is not clear if self-reports of anxiety during abstinence reflect increased baseline anxiety and/or increased responses to exogenous stressors. We hypothesized that nicotine withdrawal selectively exacerbates reactivity to aversive stimuli in rodents. Here, we investigated the effect of withdrawal from chronic nicotine administration (3.16 mg/kg per day base, delivered via subcutaneous osmotic minipumps) in the light-enhanced startle (LES) test in Wistar rats. In this procedure, baseline startle responding in the dark is compared to startle responding when the chamber is brightly lit. Bright illumination is aversive for rats and potentiates the startle response. Hence, this procedure allows comparisons of withdrawal effects on startle reactivity between relatively neutral and stressful contexts. We found that spontaneous nicotine withdrawal (24 h post-pump removal) did not influence baseline startle responding, but produced a selective increase in LES. Precipitated nicotine withdrawal through injections of one of two nicotinic acetylcholine receptor (nAChR) antagonists, dihydro-beta-erythroidine hydrobromide (DHbetaE: 0, 1.5, 3, or 6 mg/kg) or mecamylamine (0, 1, 2, or 4 mg/kg), did not influence baseline startle responding or LES. These results suggest that spontaneous nicotine withdrawal selectively potentiates responses to anxiogenic stimuli, but does not by itself produce a strong anxiogenic effect. These findings support the hypothesis that nicotine withdrawal exacerbates stress responding, and indicate LES may be a useful model to examine withdrawal effects on anxiety.

Nicotine administration and withdrawal affect survival in systemic inflammation models

How different regimens of nicotine administration and withdrawal affect systemic inflammation is largely unknown. We studied the effects of chronic and acute nicotine administration and of nicotine withdrawal on the outcome of aseptic and septic systemic inflammation. Male C57BL/6 mice were implanted with subcutaneous osmotic pumps (to deliver nicotine) and intrabrain telemetry probes (to measure temperature). Aseptic inflammation was induced by lipopolysaccharide (40 mg/kg ip); sepsis was induced by cecal ligation and puncture. The chronic nicotine administration group received nicotine (28 mg.kg(-1).day(-1)) for 2 wk before the induction of inflammation and continued receiving nicotine until the end of the experiment; the acute nicotine administration group received saline for 2 wk and nicotine thereafter; the nicotine withdrawal group received nicotine for 2 wk and saline thereafter; and the no-nicotine group was infused with saline throughout the experiment. Compared with no nicotine, the chronic nicotine administration did not affect survival in either model of inflammation, possibly due to the development of nicotine tolerance. The acute nicotine administration increased the survival rate in aseptic inflammation from 11 to 33% (possibly by suppressing inflammation) but worsened the outcome of sepsis (possibly because the suppression of inflammation promoted microbial proliferation). Oppositely to acute nicotine, nicotine withdrawal increased the survival rate in sepsis from 18 to 40%. The effects on survival were not due to changes in body temperature. We conclude that acute nicotine administration and nicotine withdrawal affect survival in systemic inflammation and that these effects strongly depend on whether inflammation is aseptic or septic.

The role of NMDA receptor antagonists in nicotine tolerance, sensitization, and physical dependence: a preclinical review

Nicotine, the primary psychoactive component of tobacco products, produces diverse neurophysiological, motivational, and behavioral effects through several brain regions and neurochemical pathways. Various neurotransmitter systems have been explored to understand the mechanisms behind nicotine tolerance, dependence, and withdrawal. Recent evidence suggests that glutamate neurotransmission has an important role in this phenomenon. The aim of the present review is to discuss preclinical findings concerning the role of N-methyl-D-aspartate (NMDA) receptor neurotransmission in mediating the behavioral effects of nicotine, tolerance, sensitization, dependence, and withdrawal. Based on preclinical findings, it is hypothesized that NMDA receptors mediate the common adaptive processes that are involved in the development, maintenance, and expression of nicotine addiction. Modulation of glutamatergic neurotransmission with NMDA receptor antagonists may prove to be useful in alleviating the symptoms of nicotine abstinence and facilitate tobacco-smoking cessation.

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.

Analysis of the chronic intake of and withdrawal from diazepam on emotional reactivity and sensory information processing in rats

It has been demonstrated that, on abrupt withdrawal, patients with chronic exposure can experience a number of symptoms indicative of a dependent state. In clinical patients, the earliest to arise and most persistent signal of withdrawal from chronic benzodiazepine (Bzp) treatment is anxiety. In laboratory animals, anxiety-like effects following abrupt interruption of chronic Bzp treatment can also be reproduced. In fact, signs that oscillate from irritability to extreme fear behaviours and seizures have been described already. As anxiety remains one of the most important symptoms of Bzp withdrawal, in this study we evaluated the anxiety levels of rats withdrawn from diazepam. Also studied were the effects on the motor performance and preattentive sensory gating process of rats under diazepam chronic treatment and upon 48-h withdrawal on three animal models of anxiety, the elevated plus-maze (EPM), ultrasonic vocalizations (USV) and startle+prepulse inhibition tests. Data obtained showed an anxiolytic- and anxiogenic-like profile of the chronic intake of and withdrawal from diazepam regimen in the EPM test, 22-KHz USV and startle reflex. Diazepam chronic effects or its withdrawal were ineffective in promoting any alteration in the prepulse inhibition (PPI). However, an increase of PPI was achieved in both sucrose and diazepam pretreated rats on 48-h withdrawal, suggesting a procedural rather than a specific effect of withdrawal on sensory gating processes. It is also possible that the prepulse can function as a conditioned stimulus to informing the delivery of an aversive event, as the auditory startling-eliciting stimulus. All these findings are indicative of a sensitization of the neural substrates of aversion in diazepam-withdrawn animals without concomitant changes on the processing of sensory information.

Nicotine withdrawal disrupts both foreground and background contextual fear conditioning but not pre-pulse inhibition of the acoustic startle response in C57BL/6 mice

Nicotine withdrawal is associated with multiple symptoms such as anxiety, increased appetite, and disrupted cognition in humans. Although animal models have provided insights into the somatic and affective symptoms of nicotine withdrawal, less research has focused on the effects of nicotine withdrawal on cognition. Therefore, in this study, C57BL/6J mice were used to test the effects of withdrawal from chronic nicotine on foreground and background contextual fear conditioning, which present the context as a primary or secondary stimulus, respectively. Mice withdrawn from 12 days of chronic nicotine (6.3mg/kg/day) or saline were trained and tested in either foreground or background contextual fear conditioning; nicotine withdrawal-associated deficits in contextual fear conditioning were observed in both conditions. Mice were also tested for the effects of withdrawal on pre-pulse inhibition of the acoustic startle reflex (PPI), a measure of sensory gating, and on the acoustic startle reflex. Mice withdrawn from 12 days of chronic nicotine (6.3 or 12.6 mg/kg/day) or saline underwent one 30-min PPI and startle session; no effect of withdrawal from chronic nicotine on PPI or startle was observed for either dose at 24h after nicotine removal. Therefore, mice were tested at different time points following withdrawal from 12.6 mg/kg/day chronic nicotine (8, 24, and 48 h after nicotine removal). No effect of withdrawal from chronic nicotine was observed at any time point for PPI. Overall, these results demonstrate that nicotine withdrawal disrupts two methods of contextual learning but not sensory gating in C57BL/6J mice.

Animal models and treatments for addiction and depression co-morbidity

The high rates of co-morbidity of drug addiction with depression may be attributable to shared neurobiology. Here, we discuss shared neurobiological substrates in drug withdrawal and depression, with an emphasis on changes in brain reward circuitry that may underlie anhedonia, a core symptom of depression and drug withdrawal. We explored experimentally whether clinical antidepressant medications or other treatments would reverse the anhedonia observed in rats undergoing spontaneous nicotine or amphetamine withdrawal, defined operationally as elevated brain reward thresholds. The co-administration of selective serotonin reuptake inhibitors with a serotonin-1A receptor antagonist, or the tricyclic antidepressant desipramine, or the atypical antidepressant bupropion ameliorated nicotine or amphetamine withdrawal in rats. Thus, increases in monoaminergic neurotransmission, or neuroadaptations induced by increased monoaminergic neurotransmission, ameliorated depression-like aspects of drug withdrawal. Further, chronic pretreatment with the atypical antipsychotic clozapine, that has some efficacy in the treatment of the depression-like symptoms of schizophrenia, attenuated nicotine and amphetamine withdrawal. Finally, a metabotropic glutamate 2/3 receptor antagonist reversed threshold elevations associated with nicotine withdrawal. The effects of these pharmacological manipulations are consistent with the altered neurobiology observed in drug withdrawal and depression. Thus, these data support the hypothesis of common substrates mediating the depressive symptoms of drug withdrawal and those seen in psychiatric patients. Accordingly, the anhedonic state associated with drug withdrawal can be used to study the neurobiology of anhedonia, and thus contribute to the identification of novel targets for the treatment of depression-like symptoms seen in various psychiatric and neurological disorders.

Withdrawal from chronic nicotine in adolescent and adult rats

The purpose of the present experiment is to assess potential differences in nicotine withdrawal in both adolescent and adult rats. Nicotine dependence was induced via osmotic minipump in adolescent rats (releasing 22.2 mg/kg/day on Postnatal Day 28) and adults (release rate of 18.4 mg/kg/day on Postnatal Day 60); differential initial release rates were used across age to compensate for the more rapid weight gain of adolescents. On Day 7 of nicotine exposure, withdrawal was induced via the administration of a nicotinic antagonist, mecamylamine (1.0 mg/kg i.p.), and withdrawal-induced anxiogenesis assessed on the elevated plus maze. On Days 1 and 4 after pump removal, animals were examined for startle responses and prepulse inhibition in an acoustic startle chamber. Adult animals exhibited a nicotine withdrawal-induced increase in anxiety, while adolescents did not. One day following the removal of minipumps, only nicotine dependent adolescent animals exhibited a disruption in prepulse inhibition. Nicotine withdrawal failed to produce an alteration in acoustic startle response in either group. Together these data suggest that ontogenic differences in nicotine withdrawal are dependent on the withdrawal measure examined, with adolescents being less sensitive than adults to anxiety-like symptoms, while being more sensitive to withdrawal-induced cognitive disruption.

Sex differences in negative affective response during nicotine withdrawal

This study examined physiological indicants of the neurobiological mediators of negative affect during acute nicotine withdrawal. Eighty subjects (41 male) were assigned to one of four groups (24-h deprived or nondeprived dependent smokers, occasional smokers, and nonsmokers) and participated in an instructed fear conditioning paradigm involving cued administration of electric shock. Negative affective response was measured with fear-potentiated startle during cues that signaled electric shock and during the postcue offset recovery period. Salivary cortisol and self-report measures were also collected. Fear-potentiated startle results indicated that affective recovery postcue offset was delayed in nicotine-deprived women. Nicotine-deprived women also displayed elevated cortisol levels throughout the fear conditioning procedure.

Dose, duration, and pattern of nicotine administration as determinants of behavioral dependence in rats

RATIONALE

Relatively little is known about the role of dose, duration, and pattern of nicotine exposure in the development of dependence. Disruption of learned behavior during antagonist-precipitated withdrawal can be a sensitive, quantitative measure of behavioral dependence.

OBJECTIVES

The present study sought to determine whether behavioral dependence upon nicotine could be induced in rats and, if so, what exposure conditions were essential for inducing it. Our primary focus was on whether continuous exposure over several days was necessary to produce dependence.

METHODS

Male Sprague-Dawley rats were trained to lever press under fixed-ratio 10 schedules of food reinforcement during daily, 15-min experimental sessions. Nicotine was then administered s.c. via osmotic minipumps that delivered various nicotine dosage regimens, some including 24-h nicotine-free periods, to manipulate pattern of exposure. The presence of dependence was tested with challenges with the nicotinic acetylcholine receptor antagonist, mecamylamine, or during spontaneous withdrawal.

RESULTS

After 7 days of 3, 6, and 12 mg kg(-1) day(-1) nicotine administration, response rates were significantly reduced in nicotinized, but not in saline-treated rats following mecamylamine challenges. Subsequent studies demonstrated that 4 days, but not 3 days, of cumulative 3 mg kg(-1) day(-1) nicotine administration was sufficient to induce dependence. The induction of dependence could be prevented by imposing a nicotine-free period between the first and second days during these 4-day regimens but not at other times.

CONCLUSION

Behavioral dependence upon nicotine can be induced in the rat, and its induction is dependent upon its cumulative duration and pattern of exposure suggesting that tobacco dependencies could be controlled by similar determinants.

Nicotine as a typical drug of abuse in experimental animals and humans

RATIONALE AND BACKGROUND

Tobacco use through cigarette smoking is the leading preventable cause of death in the developed world. Nicotine, a psychoactive component of tobacco, appears to play a major role in tobacco dependence, but reinforcing effects of nicotine often are difficult to demonstrate directly in controlled laboratory studies with animal or human subjects.

OBJECTIVE

To review the major findings obtained with various procedures developed to study dependence-related behavioral effects of nicotine in experimental animals and humans, i.e., drug self-administration, conditioned place preference, subjective reports of nicotine effects and nicotine discrimination, withdrawal signs, and ratings of drug withdrawal.

RESULTS

Nicotine can function as an effective reinforcer of drug-seeking and drug-taking behavior both in experimental animals and humans under appropriate conditions. Interruption of chronic nicotine exposure produces withdrawal symptoms that may contribute to relapse. Difficulties encountered in demonstrating reinforcing effects of nicotine under some conditions, relative to other drugs of abuse, may be due to weaker primary reinforcing effects of nicotine or to a more critical contribution of environmental stimuli to the maintenance of drug-seeking and drug-taking behavior with nicotine than with other drugs of abuse. Further experiments are also needed to delineate the role other chemical substances inhaled along with nicotine in tobacco smoke play in sustaining smoking behavior.

CONCLUSION

Nicotine acts as a typical drug of abuse in experimental animals and humans.

Enhanced prepulse inhibition following adolescent ethanol exposure in Sprague-Dawley rats

OBJECTIVES

Recent studies have demonstrated that ethanol exposure differentially affects adolescents and adults. The current studies were designed to compare the effects of 2-week exposure to ethanol during adolescence or adulthood on the acoustic startle response (ASR) and prepulse inhibition (PPI) METHODS: Male Sprague-Dawley rats were exposed to ethanol vapor 12 hr/d (on from 6 pm to 6 am) for 14 days during adolescence or adulthood. Six days after the cessation of ethanol vapor exposure, the ASR and PPI were assessed.

RESULTS

During ethanol treatment, overall blood alcohol levels averaged 230 to 250 mg/dl in the adolescent and adult treatment groups. Assessment of the ASR revealed that latency to startle was more rapid in adolescents than in adults, but ASR latency was not altered by ethanol exposure. In addition, ASR magnitude was lower in adolescents and was decreased in ethanol-exposed rats on startle trials. Ethanol exposure significantly enhanced PPI, but only after adolescent exposure

CONCLUSIONS

These data further demonstrate a differential sensitivity of adolescents and adults to the effects of ethanol exposure. Specifically, a 2-week period of ethanol exposure during adolescence selectively enhanced PPI, a neurobehavioral index of sensorimotor gating. However, ASR magnitude was decreased by ethanol exposure regardless of age. On the basis of previous studies, the effects of ethanol exposure on PPI data could indicate that adolescent rats exposed to ethanol are more likely to exhibit behavioral inflexibility and that ethanol exposure acts as a more potent physical stressor in adolescent rats.

Mild anxiogenic effects of nicotine withdrawal in mice

Increased anxiety is one of the symptoms of nicotine withdrawal that may lead to relapse. Previous studies have shown that nicotine withdrawal affects anxiety-like behavior in different tests of anxiety in humans and rats. However, relatively few studies have focused on the anxiogenic effect of nicotine withdrawal in mice. The present study investigated the effect of nicotine withdrawal on anxiety-like behavior in DBA/2J and C57BL/6J mouse strains in the light-dark box, acoustic startle response, and prepulse inhibition tests. An initial experiment showed that nicotine administration of 12 or 24 mg/kg/day (free base) for 14 days did not result in significant effects during withdrawal in startle, prepulse inhibition, or light-dark box, but there was a trend towards an anxiogenic effect in the light-dark box 24 h, but not 1 or 4 h, after cessation of nicotine administration. A subsequent study was therefore performed, with minipumps delivering saline, 24 mg/kg/day nicotine, or 48 mg/kg/day nicotine (free base), for 14 days. The pumps were removed, and the mice were tested 24 h after cessation of nicotine administration. Cessation of administration of 48 mg/kg/day nicotine free base in C57BL/6J mice resulted in increased anxiety-like behavior in the light-dark box, while the behavior of DBA/2J mice was unaffected. The acoustic startle response and prepulse inhibition were also unaffected in both strains. In conclusion, the present data show that nicotine withdrawal is mildly anxiogenic in C57BL/6J mice under the conditions used in the present experiments.

Deficits in a sustained attention task following nicotine withdrawal in rats

OBJECTIVE

Behavioural consequences of spontaneous and antagonist-precipitated withdrawal from nicotine upon performance of rats were compared alongside non-nicotinic antagonists in the 5-choice serial reaction time task (5-CSRTT).

METHODS

Male hooded Lister rats were trained to detect and respond to brief flashes of light presented every 15 s in one of five holes until a stable level of performance was achieved.

RESULTS

Surgical removal of osmotic minipumps from rats having received nicotine (3.16 mg/kg per day base SC) chronically for 7 days produced marked deficits in performance. Compared to saline-treated controls, deficits were apparent 10 h and 16 h following nicotine abstinence; the percentage of omission errors increased concomitantly with modest decreases in response accuracy. Tests conducted 34 h and 106 h post-withdrawal indicated a progressive and complete recovery in attention performance, respectively. In another experiment, following the exposure to the same nicotine regime, administration of the competitive nicotine receptor antagonist dihydro-beta-erythroidine precipitated immediate deficits in performance that were greater than those observed in saline-treated subjects. Methyllycaconitine, an alpha(7) nicotinic receptor antagonist failed to precipitate attention deficits in nicotine-treated rats. Tests with SCH23390 and raclopride produced impairments that were similar in profile to nicotine withdrawal contrasting with non-specific effects of dizocilpine.

CONCLUSIONS

These results provide evidence of a cognitive impairment resulting from nicotine deprivation in rodents. Specifically, blockade of D(1) receptors by SCH23390 produced decrements in performance that were qualitatively similar but greater in magnitude to the alterations observed following nicotine withdrawal. Overall, assessing nicotine withdrawal in the 5-CSRTT presents an animal model that exhibits robust construct and face validity.

Measuring the emergence of tobacco dependence: the contribution of negative reinforcement models

This review of negative reinforcement models of drug dependence is part of a series that takes the position that a complete understanding of current concepts of dependence will facilitate the development of reliable and valid measures of the emergence of tobacco dependence. Other reviews within the series consider models that emphasize positive reinforcement and social learning/cognitive models. This review summarizes negative reinforcement in general and then presents four current negative reinforcement models that emphasize withdrawal, classical conditioning, self-medication and opponent-processes. For each model, the paper outlines central aspects of dependence, conceptualization of dependence development and influences that the model might have on current and future measures of dependence. Understanding how drug dependence develops will be an important part of future successful tobacco dependence measurement, prevention and treatment strategies.

Elevated startle during withdrawal from acute morphine: a model of opiate withdrawal and anxiety

RATIONALE

An elevated startle response has been observed in humans and animals during withdrawal from multiple substances of abuse, a phenomenon thought to reflect the anxiogenic effects of withdrawal. Although anxiety is a common symptom of opiate withdrawal, few studies have examined the effects of morphine withdrawal on acoustic startle.

OBJECTIVE

To develop a procedure for assessing opiate dependence through measurement of the startle reflex in rats.

METHODS

The effects of opiate withdrawal on startle were evaluated using both spontaneous and naloxone-precipitated withdrawal from an acute dose of morphine. The ability of the treatment drugs clonidine and chlordiazepoxide to block withdrawal-induced increases in startle was also tested.

RESULTS

Spontaneous withdrawal from an injection of morphine sulfate produced a significant increase in acoustic startle 2 h (3.2 mg/kg) or 4 h (10 mg/kg) after drug administration. Morphine withdrawal (10 mg/kg morphine sulfate) precipitated by the opiate antagonist naloxone (2.5 mg/kg) also produced a significant increase in startle magnitude. This elevation of startle was blocked by both clonidine (35 microg/kg) and chlordiazepoxide (10 mg/kg).

CONCLUSIONS

These data demonstrate that both spontaneous and precipitated withdrawal from an acutely administered opiate produce anxiety-like effects on acoustic startle. This paradigm may be useful in the study of anxiety and the early mechanisms of drug dependence.

Characterization of spontaneous and precipitated nicotine withdrawal in the mouse

The nicotine withdrawal syndrome was validated and characterized in the mouse using both somatic and affective measures after infusion with nicotine daily via subcutaneous minipumps. The influence of dose, duration of infusion, and repeated withdrawal as well as the contribution of genetic factors were investigated. We then characterized the contribution of nicotinic receptor and site mechanisms to withdrawal signs using various nicotinic antagonists. Our results showed that spontaneous nicotine withdrawal increased the number of somatic signs, decreased the time spent in open arms of the plus-maze test, and induced hyperalgesia. The effect was dose-dependent in all measures with no significant changes at the lowest dose of nicotine (6 mg/kg/day). Withdrawal signs were prominent shortly after pump removal and remained prominent through day 3 or 4. The results with the different antagonists (mecamylamine, dihydro-beta-erythroidine, and methyllycaconitine) suggest the involvement of several nicotinic subtypes such as alpha3beta4*, alpha4beta2*, and alpha7 in nicotine withdrawal. Increasing the duration of nicotine exposure (from 7 to 60 days) and the total nicotine exposure (increasing doses of infusing) augmented the severity of nicotine withdrawal signs. The withdrawal severity of nicotine differs between C57/BL and 129/SvEv inbred mice with nicotine withdrawal in C57 being more severe than in the 129 strain. In summary, our present results suggest that withdrawal from nicotine can be modulated by genetic factors, daily nicotine intake, duration of nicotine exposure, and withdrawal history. The present study demonstrates that our mouse nicotine withdrawal model will be useful for studying the pharmacological, biochemical, and genetic mechanisms involved in nicotine dependence.

A review of the modulation of the startle reflex by affective states and its application in psychiatry

OBJECTIVE

To provide an overview of startle reflex methodologies applied to the examination of emotional and motivational states in humans and to review the findings in different forms of psychopathology.

METHODS

Pertinent articles were searched mostly via MEDLINE and PsycINFO.

RESULTS

The startle reflex is a non-invasive translational tool of research that bridges the gap between animal and human investigations. Startle is used to study fear and anxiety, affective disturbances, sensitization, motivational states, and homeostasis.

CONCLUSIONS

The startle reflex is highly sensitive to various factors that are of interest in the studies of emotional disorders and has promoted new areas of investigations in psychiatry. However, research in psychiatry is still in its infancy and most findings await replication. Future progress will benefit from the development of innovative and powerful designs tailored to investigate specific disorders.

SIGNIFICANCE

The startle reflex has utility as a research tool to examine trauma-related disorders, fear learning, drug addiction, and to contrast affective states and emotional processing across diagnostic groups, but its usefulness as a diagnostic tool is limited.

Decreased prepulse inhibition during nicotine withdrawal in DBA/2J mice is reversed by nicotine self-administration

We characterized spontaneous and mecamylamine-precipitated nicotine withdrawal using intravenous nicotine self-administration, the acoustic startle response, prepulse inhibition and somatic signs of withdrawal in DBA/2J mice. Nicotine dependence was induced by continuous nicotine infusion through osmotic minipumps. Nicotine self-administration was studied before and after the induction of dependence. The initial test revealed significant nicotine self-administration at the 0.048 microg/infusion dose. During the second self-administration test, saline-treated mice exhibited increased aversiveness of response-contingent infusions of high nicotine doses; these changes were not seen in the nicotine-treated animals reflecting tolerance to nicotine's effects. Neither mecamylamine administration nor spontaneous withdrawal affected the expression of somatic signs, except that increases in jumping were observed during spontaneous withdrawal. Finally, nicotine withdrawal increased general activity in the startle chambers when no stimuli were presented, possibly reflecting increased body tremor and/or agitation, and decreased prepulse inhibition reflecting a sensorimotor gating deficit; the last two effects were reversed by nicotine self-administration. Thus, nicotine withdrawal results in modest, but yet detectable, changes in the behavior of mice.

Impact of nicotine withdrawal on novelty reward and related behaviors

The authors tested the decreased reward function hypothesis of nicotine withdrawal using a novel-object place conditioning task. A conditioned place preference was evident in controls and in rats that had experienced 4 nicotine withdrawal days, but not in rats that had experienced 1-3 withdrawal days. This implies that the rewarding properties of interacting with novel objects were not readily associated with the environment in which they were paired. Follow-up experiments eliminated other explanations based on withdrawal-induced failures to process object or environment information. Also, expression of conditioning was not affected, indicating that withdrawal likely altered acquisition. Further investigation into the neurochemical and behavioral changes that accompany nicotine withdrawal will lead to a better understanding of the withdrawal syndrome.

Effects of nicotine and stress on locomotion in Sprague-Dawley and Long-Evans male and female rats

Locomotor activity is widely used to study nicotine effects, including genotypic differences, in rodents. In rats, chronic nicotine's (administered via osmotic minipump) effects on locomotion may differ based on animal strain, with Long-Evans rats more sensitive than Sprague-Dawley rats. Males and females also may differ in sensitivity. No studies, however, have compared males and females of the two strains. In addition, stress relief is a frequently cited reason for smoking, but the behavioral consequences of nicotine-stress interactions have rarely been examined. This experiment evaluated locomotor responses of male and female Sprague-Dawley and Long-Evans rats to 0, 6, or 12 mg/kg/day nicotine administered by minipump. Half of the animals in each drug condition were exposed to 20 min/day of immobilization stress to examine nicotine-stress interactions. Horizontal and vertical activities were measured on Drug Days 4 and 10. Stress effects were minimal and stress did not alter effects of nicotine. Nicotine (6 mg/kg/day) increased horizontal activity among Long-Evans but not among Sprague-Dawleys, with greater effects in Long-Evans females. Nicotine (6 mg/kg/day) increased vertical activity of all groups and 12 mg/kg/day decreased vertical activity of all groups except for Sprague-Dawley males. Results indicate that genotype and sex are relevant to understand nicotine's behavioral actions.

Nicotine withdrawal syndrome: behavioural distress and selective up-regulation of the cyclic AMP pathway in the amygdala

Nicotine addiction is a major public health issue. The use of laboratory animal models is a crucial tool in research aiming at understanding the pathophysiological mechanisms of nicotine dependence and at proposing new therapies. In rodents, cessation of nicotine exposure or administration of the nicotinic antagonist mecamylamine induces a nicotine withdrawal syndrome. Antagonist-precipitated withdrawal from other abused drugs such as opiates or cannabinoids has been associated with region-specific modifications of the activity of the cyclic AMP pathway. Here we show that mecamylamine-precipitated nicotine withdrawal in the rat is characterized by an increase in thigmotaxis (time spent in the periphery of an open field) that may be indicative of behavioural distress and can be associated with a selective up-regulation of adenylyl cyclase activity in the amygdala, a region implicated in the regulation of negative affect in response to aversive stimuli, including withdrawal. Adenylyl cyclase activity that is increased during precipitated nicotine withdrawal is stimulated by calcium/calmodulin, as is also the case for opioid and cannabinoid abstinence. This suggests that directly or indirectly mediated increases in the activity of the cyclic AMP pathway could constitute a possible common molecular mechanism underlying neuroadaptive changes following abstinence from different abused drugs.

Drug trace discrimination with nicotine and morphine in rats

In typical drug discrimination experiments, subjects are exposed to psychoactive substances both prior to and during training sessions. The present experiments aimed to determine whether pre-session effects of drugs could serve as discriminative stimuli. Rats were trained in a two-lever discrimination procedure with food reinforcers presented on a tandem variable interval-fixed ratio (VI-FR) schedule. Injections of nicotine (0.6mg/kg 20 min pre-session) or saline were followed by administration of the nicotine antagonist mecamylamine (1.0 mg/kg 10 min pre-session) to block effects of nicotine during training sessions. Similarly, the action of morphine (10 mg/kg 30 min pre-session) was terminated by administering naloxone (0.1 mg/kg 10 min pre-session). These drug discriminations were acquired slowly to an accuracy of only 70-75% (n=10-12). Extinction tests confirmed stimulus control by nicotine in the presence of mecamylamine and by morphine in the presence of naloxone. The antagonists attenuated the response-rate reducing effects of the training doses of their respective agonists. The results are interpreted in terms of stimulus control by pre-session effects of the training drugs, but other explanations are considered. Stimulus control by pre-session drug states may be weak due to the time elapsed between termination of drug effects and training (trace conditioning).

Neurobiology of the nicotine withdrawal syndrome

The aversive aspects of withdrawal from chronic nicotine exposure are thought to be an important motivational factor contributing to the maintenance of the tobacco habit in human smokers. Much emphasis has been placed on delineating the underlying neurobiological mechanisms mediating different components of the nicotine withdrawal syndrome. Recent studies have shown that both central and peripheral populations of nicotinic acetylcholine receptors (nAChRs) are involved in mediating somatic signs of nicotine withdrawal as measured by the rodent nicotine abstinence scale. However, only central populations of nAChRs are involved in mediating affective aspects of nicotine withdrawal, as measured by elevations in brain-stimulation reward thresholds and conditioned place aversion. Nicotine interacts with several neurotransmitter systems, including acetylcholine, dopamine, opioid peptides, serotonin, and glutamate systems. Evidence so far suggests that these neurotransmitters play a role in nicotine dependence and withdrawal processes. The available evidence also suggests that different underlying neurochemical deficits mediate somatic and affective components of nicotine withdrawal. The aim of the present review is to discuss preclinical findings concerning the neuroanatomical and neurochemical substrates involved in these different aspects of nicotine withdrawal.

Nicotine dependence: studies with a laboratory model

Simple, rapid preclinical models of nicotine physical dependence and abstinence syndrome are needed to identify underlying neurobiological mechanisms and screen potential therapies. One such model induces dependence by 7 days of continuous subcutaneous nicotine infusion in the rat. Abstinence is initiated through termination of infusion or injection of nicotinic antagonist drugs. The result is an abstinence syndrome involving a pattern of behaviors somewhat resembling opiate abstinence in the rat as well as weight gain and depressed locomotor activity. The model has met a number of validity criteria and its essential features have been replicated in several laboratories. Several research groups have modified or extended the model by measuring emotional/motivational changes associated with nicotine abstinence such as conditioned aversion, intracranial self-stimulation (ICSS) thresholds and the startle response. Dependence models have been used to identify neurobiological systems that contribute to nicotine dependence, particularly endogenous opiate systems and the mesolimbic dopamine pathway. It is hypothesized that these different systems contribute to different behavioral aspects of nicotine abstinence syndrome. Increasingly used as a preclinical screening tool, the model has proved sensitive to various abstinence-alleviating therapeutic approaches, including some with already demonstrated clinical effectiveness.

Tolerance to nicotine's effects in the elevated plus-maze and increased anxiety during withdrawal

In the elevated plus-maze test of anxiety, nicotine (0.1 mg/kg sc; 30 min after injection) had a significant anxiogenic effect, shown by specific decreases in the percentage of time spent on the open arms and in the percentage of open-arm entries. Tolerance developed to this anxiogenic effect after 7 days of nicotine treatment (0.1 mg/kg/day). Five minutes after an acute injection, nicotine (0.1 mg/kg) was ineffective, but after 7 days of treatment a significant anxiolytic effect, shown by specific increases in the percentage of time spent on the open arms and in the percentage of open-arm entries, emerged. After 14 days of nicotine treatment, tolerance developed to this anxiolytic effect. There was a complete dissociation between the effects of nicotine on the measures of anxiety, and on the locomotor activity as measured by closed-arm entries. No changes in closed-arm entries were found after acute administration of nicotine, but rats tested 30 min after their 7th injection made significantly fewer, and those tested 5 min after their 14th injection made significantly more, entries than their respective controls. Rats that were tested after 24 h withdrawal from six daily nicotine injections showed a significant anxiogenic effect. A low dose of nicotine (5 ng) injected into the dorsal hippocampus was without effect in vehicle pretreated rats, but it was able to reverse the anxiogenic effect found after 24 h of withdrawal from 6 days of nicotine treatment.

Moxonidine versus placebo as an aid in smoking cessation

This study assessed the effects of moxonidine as an aid in smoking cessation in 166 heavily addicted smokers who were motivated to quit smoking completely. Recruitment was via advertisement. Patients were randomly allocated to receive double-blind placebo or moxonidine (0.1 mg once or twice daily) for 6 weeks. Brief counseling was provided. An encouragement letter was sent prior to the quit date. Success was defined as not smoking any cigarettes during weeks 3 - 6, an expired carbon monoxide level of < 10 ppm, and a plasma cotinine level of < 25 ng/ml. The study failed to demonstrate a statistically significant effect for moxonidine on either nicotine withdrawal symptoms or smoking cessation. Reported side effects were not different with moxonidine than with placebo, however. Copyright 2000 John Wiley & Sons, Ltd.

Nicotine abstinence in the mouse

We present evidence that intermittent administration of nicotine, 2 mg/kg s.c., four times daily to mice for 14 days produces a somatic abstinence syndrome after discontinuing treatment. The nicotine abstinence was mild and protracted, lasting more than 92 h. The constellation of abstinence signs was characterized by rearing, jumping, shakes, abdominal constrictions, chewing, facial tremor and scratching. No autonomic symptomatology was observed. Nicotine abstinence was attenuated with a single dose of nicotine administered at 24 or 48 h into withdrawal. The nicotinic antagonist mecamylamine, 3 mg/kg, induced a small increase in the total abstinence score when given 60 min after the last nicotine injection. Nicotine-abstinent mice displayed reduced locomotor activity. Finally, mice lost weight during the nicotine treatment which was not recovered during the withdrawal. Along with the rat nicotine abstinence model, the mouse model of intermittent nicotine administration and abstinence might be useful for studying the pharmacological and biochemical mechanisms of nicotine addiction and tobacco use.

Effects of chronic nicotine administration on locomotion depend on rat sex and housing condition

Nicotine's behavioral actions in the human smoker by self-report depend, in part, on the individual's gender and environment. The purpose of the present experiment was to determine whether effects of nicotine on unconditioned behaviors of rats also depend on sex and environmental conditions. Long-Evans rats (96 males and 96 females) living in individual or grouped housing were administered saline or 12 mg/kg/day nicotine via osmotic minipump for 14 days. Horizontal activity (a measure of overall activity and arousal), vertical activity (a measure of exploratory behavior), and center time (a possible index of anxiety) were measured on Day 10 of drug administration and on Day 2 of nicotine cessation. Group housing decreased horizontal and vertical activity and center time, with effects occurring sooner in females. Nicotine's effects depended on housing and sex. For males, nicotine altered indices of arousal and exploration, increasing these variables for group-housed males but decreasing them for individually housed males. For females, nicotine altered possible indices of anxiety, reducing anxiety for group-housed females. In cessation, housing effects continued in females and appeared more robustly in males. Results indicate that nicotine's chronic effects depend on subjects' sex and living environment.

Effects of nicotine and stress on startle amplitude and sensory gating depend on rat strain and sex

We recently reported that 14 days of nicotine administration (12 mg/kg/day) reduced acoustic startle reflex amplitude and impaired prepulse inhibition (PPI) of startle in male and female Long-Evans rats. These findings contrasted with reports of nicotine-induced enhancement of startle and PPI in Sprague-Dawley (a different strain) male rats. The present experiment administered 0, 6, or 12 mg/kg/day nicotine via osmotic minipump for 14 days to 120 Sprague-Dawley rats (male and female) and to 120 Long-Evans rats (male and female) and examined ASR and PPI. Half of the subjects also were stressed by immobilization once each day to examine nicotine-stress interactions. Nicotine enhanced ASR and PPI responses of Sprague-Dawley rats but impaired these responses in Long-Evans rats, regardless of sex. Effects of stress were complex and depended on strain, sex, and drug dose. These findings indicate that effects of nicotine on measures of reactivity (ASR) and sensory gating (PPI) depend on genotype and that nicotine stress interactions depend on genotype, sex, and nicotine dosage.

Dependence on smoking and the acoustic startle response in healthy smokers

Recent data from nicotine-dependent rats (14) and healthy smokers (18) would suggest that nicotine withdrawal modulates the acoustic startle reflex in a way similar to that of fear (5,20). We examined this directly using nonsmokers and healthy smokers who had no deprivation, brief deprivation (2-3 h), or prolonged deprivation (15 h). Groups differences in heart rate (HR), alveolar carbon monoxide (CO) levels, and desire and craving for cigarettes confirmed the presence of smoking withdrawal. However, there were no significant differences in the magnitude of the baseline startle response among the differently deprived smokers or between the smokers and the nonsmokers. Subsequent startle tests were carried out in the smokers during repeated sequences of preparing a cigarette for smoking (smoking cues) and then smoking. Whereas we did find statistically significant interactions of smoking deprivation with smoking cues and with renewed smoking, there was only weak confirmation of a priori predictions of motivational effects of smoking cues or of smoking. We conclude that smoking dependence may not affect the acoustic startle response itself; modulation seems to occur, but only after experience with the test situation. Discussed were possible mechanisms of this modulation in both humans and animals and further application of the startle response for providing interdisciplinary assessments of the motivational effects of nicotine withdrawal.

Reduced dopamine output in the nucleus accumbens but not in the medial prefrontal cortex in rats displaying a mecamylamine-precipitated nicotine withdrawal syndrome

Mesolimbocortical dopamine (DA) neurotransmission is important in the mediation of the dependence-producing actions of nicotine and other drugs of abuse. Withdrawal from chronic treatment with various types of addictive drugs, including amphetamine, cocaine, ethanol and morphine is associated with a decrease in dopaminergic output in the nucleus accumbens (NAC), whereas the effects of withdrawal from these drugs on dopaminergic output in the medial prefrontal cortex (PFC), as yet, remain largely unknown. This study examined putative changes in the extracellular levels of dopamine and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the NAC and in the PFC of rats displaying behavioral signs of nicotine withdrawal. Rats were infused for 7 days with nicotine via subcutaneously implanted minipumps, whereas control animals carried saline-containing pumps. On the fifth day of infusion a microdialysis probe was implanted in the NAC or the PFC of the rats. Forty-eight hours later the levels of DA and the monoamine metabolites were assessed in the dialysate. The behavioral and biochemical effects of a saline injection and a subsequent challenge with the nicotinic receptor antagonist mecamylamine (1 mg/kg s.c.) were determined. Following mecamylamine challenge in nicotine-treated animals, the levels of DA, DOPAC and HVA in the NAC, but not in the PFC, decreased below pre-injection levels and in relation to control animals. The score of abstinence signs increased in the nicotine-treated rats, as compared both to the score after saline and to that in control animals. The decreased DA output in the NAC in animals displaying nicotine withdrawal signs is similar to that seen after withdrawal of several other drugs of abuse, and may have bearing on motivational deficits associated with the abstinence reactions.

Prenatal exposure to nicotine: effects on prepulse inhibition and central nicotinic receptors

The present experiment examined effects of prenatal nicotine exposure (6 mg/kg/day via osmotic minipump) throughout gestation on prepulse inhibition of the acoustic startle response (PPI) and on the density of nicotinic acetylcholine receptors (nAchRs) in the brains of 5-week-old Sprague-Dawley rats. A total of 117 male and 103 female offspring were used. Prenatal nicotine reduced subsequent percent PPI to a 98 dB stimulus in female but not in male offspring. There was an inverse correlation between the percent of PPI and nAchR density in the cortex of male rats and the striatum of female rats.

LY354740: a metabotropic glutamate receptor agonist which ameliorates symptoms of nicotine withdrawal in rats

LY354740 is a conformationally constrained analog of glutamate with high selectivity and nanomolar agonist activity at Group II metabotropic glutamate receptors (mGluRs). This orally active compound is a new drug candidate which is being developed for the treatment of anxiety. In this study, LY354740 was investigated in a model of nicotine withdrawal using the acoustic startle reflex (sensorimotor reactivity) in rats. Nicotine (6 mg/kg/day) was administered for 12 days subcutaneously by osmotic minipumps. After 12 days the pumps were removed and the animals were allowed to go through spontaneous withdrawal. Cessation of chronic nicotine exposure led to increased startle responding for 4 days following withdrawal. Treatment with LY354740 (0.0001-0.1 mg/kg, i.p.; 0.03-3 mg/kg, oral) produced a dose-dependent attenuation of the enhanced auditory startle responding following withdrawal of nicotine with intraperitoneal and oral ED50 values of 0.003 mg/kg and 0.7 mg/kg, respectively. These effects were stereoselective since the (-)-enantiomer of LY354740, LY366563, was without effect in this model. LY354740 produced no changes in the sensorimotor reactivity of rats not exposed to nicotine at oral doses up to 10 mg/kg. These data support the functional role of mGluR agonists in nicotine withdrawal and indicate that LY354740 may be efficacious in reducing the symptoms associated with nicotine withdrawal during smoking cessation in humans.

Serotonin-1A antagonists attenuate the effects of nicotine withdrawal on the auditory startle response

Withdrawal from the chronic administration of nicotine has previously been shown to lead to an enhanced auditory startle response in rats. In order to explore the neuropharmacology and neurophysiology underlying this phenomenon, we examined the effects of various 5-hydroxytryptamine (5-HT)-1A antagonists and agonists on the nicotine-withdrawal-enhanced auditory startle response in male rats. Animals were treated with nicotine (6 mg/kg/day nicotine base, via subcutaneously implanted osmotic minipumps) for 12 days. After 12 days the pumps were removed and the animals allowed to undergo spontaneous withdrawal for several days. In agreement with previous results, nicotine withdrawal led to a significant elevation of the auditory startle response. Pretreatment with the 5-HT-1A agonists (+)8-OH-DPAT (0.001-0.1 mg/kg) and LY274600 (0.3-3.0 mg/kg) either had no affect or exacerbated the nicotine-withdrawal-enhanced startle response. Pretreatment with the 5-HT-1A antagonists NAN-190 (1-3 mg/kg), LY206130 (1-10 mg/kg), or WAY-100635 (0.1-1.0 mg/kg) blocked the increase in the startle response caused by nicotine withdrawal at doses that had no effect on baseline startle responses. These data indicate that 5-HT-1A receptors play a role in the neurophysiology of nicotine withdrawal. In addition, 5-HT-1A antagonists may be able to relieve some nicotine withdrawal symptoms in man and may represent a novel pharmacotherapy for smoking cessation.

Effects of nicotine withdrawal on central dopaminergic systems

The behavioral and neurochemical manifestations in rats 24 h following the cessation of 14-day nicotine administration were investigated. Animals were implanted subcutaneously with osmotic minipumps which continuously released either saline or nicotine (1.5 mg/kg/day or 3.0 mg/kg/day) for 14 days. After the animals were withdrawn from nicotine for 24 h, we observed a significant decrease of locomotor activities and a reduction of dopamine contents in the striatum and nucleus accumbens. Nicotine withdrawal did not affect the body weight, food, or water consumption, and no deficit in the acquisition of a conditioned avoidance task was found in these animals. In addition, nicotine withdrawal did not alter the density or the binding affinity (Kd) of ligands to D1 and D2 receptors in the striatum. Although nicotine withdrawal did not alter the density or binding affinity of ligands to D1 receptors in the nucleus accumbens, the maximum number of D2 receptor sites were reduced by nicotine treatment. These results offer possible neurochemical mechanisms for changes of locomotor activity which occurred in rats during nicotine abstinence.