The actions of nicotine and cocaine in a mouse model of anxiety

Dynamic effects of ventral hippocampal NRG3/ERBB4 signaling on nicotine withdrawal-induced responses

Tobacco smoking remains a leading cause of preventable death in the United States, with approximately a 5% success rate for smokers attempting to quit. High relapse rates have been linked to several genetic factors, indicating that the mechanistic relationship between genes and drugs of abuse is a valuable avenue for the development of novel smoking cessation therapies. For example, various single nucleotide polymorphisms (SNPs) in the gene for neuregulin 3 (NRG3) and its cognate receptor, the receptor tyrosine-protein kinase erbB-4 (ERBB4), have been linked to nicotine addiction. Our lab has previously shown that ERBB4 plays a role in anxiety-like behavior during nicotine withdrawal (WD); however, the neuronal mechanisms and circuit-specific effects of NRG3-ERBB4 signaling during nicotine and WD are unknown. The present study utilizes genetic, biochemical, and functional approaches to examine the anxiety-related behavioral and functional role of NRG3-ERBB4 signaling, specifically in the ventral hippocampus (VH) of male and female mice. We report that 24hWD from nicotine is associated with altered synaptic expression of VH NRG3 and ERBB4, and genetic disruption of VH ErbB4 leads to an elimination of anxiety-like behaviors induced during 24hWD. Moreover, we observed attenuation of GABAergic transmission as well as alterations in Ca2+-dependent network activity in the ventral CA1 area of VH ErbB4 knock-down mice during 24hWD. Our findings further highlight contributions of the NRG3-ERBB4 signaling pathway to anxiety-related behaviors seen during nicotine WD.

Dynamic Effects of Ventral Hippocampal NRG3/ERBB4 Signaling on Nicotine Withdrawal-Induced Responses

Tobacco smoking remains a leading cause of preventable death in the United States, with a less than 5% success rate for smokers attempting to quit. High relapse rates have been linked to several genetic factors, indicating that the mechanistic relationship between genes and drugs of abuse is a valuable avenue for the development of novel smoking cessation therapies. For example, various single nucleotide polymorphisms (SNPs) in the gene for neuregulin 3 (NRG3) and its cognate receptor, the receptor tyrosine-protein kinase erbB-4 (ERBB4), have been linked to nicotine addiction. Our lab has previously shown that ERBB4 plays a role in anxiety-like behavior during nicotine withdrawal (WD); however, the neuronal mechanisms and circuit-specific effects of NRG3-ERBB4 signaling during nicotine and WD are unknown. The present study utilizes genetic, biochemical, and functional approaches to examine the anxiety-related behavioral and functional role of NRG3-ERBB4 signaling, specifically in the ventral hippocampus (VH). We report that 24hWD from nicotine is associated with altered synaptic expression of VH NRG3 and ERBB4, and genetic disruption of VH ErbB4 leads to an elimination of anxiety-like behaviors induced during 24hWD. Moreover, we observed attenuation of GABAergic transmission as well as alterations in Ca2+-dependent network activity in the ventral CA1 area of VH ErbB4 knock-down mice during 24hWD. Our findings further highlight contributions of the NRG3-ERBB4 signaling pathway to anxiety-related behaviors seen during nicotine WD.

Expression of stable and reliable preference and aversion phenotypes following place conditioning with psychostimulants

RATIONALE AND OBJECTIVES

Drug-seeking behavior occurs more readily in some individuals than others. This phenomenon is considered in studies of drug self-administration in which high drug-seeking/taking individuals can be identified. In contrast, studies of conditioned place preference (CPP) often involve a random sample of drug-naïve rodents that includes phenotypes not considered relevant to addiction. The main objective of the current studies was to determine if a priori identification of different conditioning phenotypes could improve the validity and sensitivity of CPP expression as a preclinical test for vulnerability to addiction.

METHODS AND RESULTS

Analysis of cocaine place conditioning data from 443 Swiss-Webster mice revealed a trimodal distribution with peaks corresponding to means of k = 3 clusters. The cluster means occurred at high, low, or negative preference scores, the latter suggesting a phenotype acquiring conditioned place aversion (CPA). The same clusters were identified in mice conditioned with methamphetamine, MDPV, or amphetamine, and these clusters remained stable and reliable during three additional expression tests spaced at 24 h. A meta-analysis of effect sizes obtained from CPP literature revealed a positively skewed distribution affected by sample size, consistent with the existence of a CPA phenotype within the populations tested. A dopamine receptor antagonist, flupentixol, blocked cocaine CPP expression in a group containing all phenotypes, but sensitivity improved markedly when CPA phenotypes were excluded from the dataset.

CONCLUSIONS

These studies suggest that taking phenotype into consideration when designing place conditioning studies will improve their application as a preclinical tool in addiction biology and drug discovery.

Brain systems in cocaine abstinence-induced anxiety-like behavior in rodents: A review

Cocaine use disorder (CUD) is a significant public health issue that generates substantial personal, familial, and economic burdens. Still, there are no FDA-approved pharmacotherapies for CUD. Cocaine-dependent individuals report anxiety during withdrawal, and alleviation of anxiety and other negative affective states may be critical for maintaining drug abstinence. However, the neurobiological mechanisms underlying abstinence-related anxiety in humans or anxiety-like behavior in rodents are not fully understood. This review summarizes investigations regarding anxiety-like behavior in mice and rats undergoing cocaine abstinence, as assessed using four of the most common anxiety-related assays: the elevated plus (or its derivative, the elevated zero) maze, open field test, light-dark transition test, and defensive burying task. We first summarize available evidence that cocaine abstinence generates anxiety-like behavior that persists throughout protracted abstinence. Then, we examine investigations concerning neuropeptide, neurotransmitter, and neuromodulator systems in cocaine abstinence-induced anxiety-like behavior. Throughout, we discuss how differences in sex, rodent strain, cocaine dose and dosing strategy and abstinence duration interact to generate anxiety-like behavior.

Neurotoxic Effects of Neonicotinoids on Mammals: What Is There beyond the Activation of Nicotinic Acetylcholine Receptors?-A Systematic Review

Neonicotinoids are a class of insecticides that exert their effect through a specific action on neuronal nicotinic acetylcholine receptors (nAChRs). The success of these insecticides is due to this mechanism of action, since they act as potent agonists of insect nAChRs, presenting low affinity for vertebrate nAChRs, which reduces potential toxic risk and increases safety for non-target species. However, although neonicotinoids are considered safe, their presence in the environment could increase the risk of exposure and toxicity. On the other hand, although neonicotinoids have low affinity for mammalian nAChRs, the large quantity, variety, and ubiquity of these receptors, combined with its diversity of functions, raises the question of what effects these insecticides can produce in non-target species. In the present systematic review, we investigate the available evidence on the biochemical and behavioral effects of neonicotinoids on the mammalian nervous system. In general, exposure to neonicotinoids at an early age alters the correct neuronal development, with decreases in neurogenesis and alterations in migration, and induces neuroinflammation. In adulthood, neonicotinoids induce neurobehavioral toxicity, these effects being associated with their modulating action on nAChRs, with consequent neurochemical alterations. These alterations include decreased expression of nAChRs, modifications in acetylcholinesterase activity, and significant changes in the function of the nigrostriatal dopaminergic system. All these effects can lead to the activation of a series of intracellular signaling pathways that generate oxidative stress, neuroinflammation and, finally, neuronal death. Neonicotinoid-induced changes in nAChR function could be responsible for most of the effects observed in the different studies.

Nicotine sensitization (Part 2): Time spent in the centre of an open field sensitizes to repeated nicotine into the drug-free state in female rats

RATIONALE

Nicotine is initially anxiogenic and becomes anxiolytic after prolonged exposure. The mechanisms that facilitate the shift in anxiety-like behaviour produced by nicotine are unclear.

OBJECTIVE

We investigated the change in time spent in the centre of an open field (as a measure anxiety-like behaviour) produced by three intermittent injections of nicotine as part of experiments of locomotor sensitization to nicotine.

METHODS

Rats were injected with nicotine (0.4 mg/kg) or saline and immediately placed in the open field arena for 1 h on two consecutive days and again 9 days later.

RESULTS

When given saline, time spent in the centre of the arena did not change, whereas repeated nicotine injections increased in time spent in the centre beyond the increase produced by an acute injection of nicotine. Repeated nicotine (and not acute nicotine) also increased time in the centre in a drug-free state when tested 24 h after the last injection.

CONCLUSION

Repeated nicotine sensitized the time spent in the centre of an open field with the long-lasting sensitization of this measure of anxiety-like behaviour evident in a drug-free state, in contrast to locomotor sensitization which does not persist in the drug-free state. The results suggest independence of the mechanisms of sensitization that underlie locomotor and anxiolytic effects.

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.

Opposing effects of acute and repeated nicotine exposure on boldness in zebrafish

Nicotine is an addictive compound that activates neuronal nicotinic acetylcholine receptors (nAChRs) and causes behavioural effects that vary with dose, schedule of administration, and animal model. In zebrafish (Danio rerio), acute doses of nicotine have been consistently found to have anxiolytic properties, whereas, chronic exposure elicits anxiogenic effects. To date, however, studies on repeated nicotine administration and the effects of nicotine withdrawal have not been well explored using this model. In this study, we administered nicotine with three different dosing regimens: 1. Single exposures of a "high" dose (25, 50, 100, or 400 mg/L) for 3 minutes. 2. Single exposures to a "low" dose (2.5, 5, or 20 mg/L) for one hour. 3. Repeated one-hour exposure to a "low" dose (2.5, 5, or 20 mg/L) for 21 days. The novel object approach test was used to examine boldness based on the tendency of the fish to explore a novel object. Acutely, nicotine significantly increased the time spent approaching the object with both three-minute and onehour durations of exposure, indicating increased boldness. Conversely, after repeated nicotine exposure for 21 days, fish spent less time approaching the object suggesting a decrease in boldness. Distance moved was unaffected one hour after repeated nicotine exposure, yet decreased after a two-day withdrawal period. Our work suggests that nicotine can have opposing effects on boldness that vary based on dosage and schedule of exposure.

Pharmacological intervention and abstinence in smokers undergoing cessation treatment: A psychophysiological study

As a composite concept, negative affect comprises various aversive emotional experiences, such as irritability and nervousness. It is a critical motivational factor that helps maintain smoking behavior, and contributes significantly to smoking cessation failure as a core withdrawal symptom. Prior research has indicated an important role of nicotinic mechanisms in negative affect processing. The most effective smoking cessation medication, varenicline, targets nicotinic acetylcholine receptors (nAChRs) as a partial agonist, while another first-line cessation medication, bupropion, has shown antagonistic effects on nAChRs. Therefore, it is possible that both medications work to reduce smoking behavior through modulating negative affect processing. To evaluate this hypothesis, we examined the impact of varenicline tartrate and bupropion hydrochloride sustained-release on electrophysiological responses to affective, cigarette-related, and neutral cues before and during smoking cessation treatment in a randomized placebo-controlled clinical trial. The participants were 206 smokers, a subset of 294 participants that were enrolled in a larger smoking cessation clinical trial who were randomly assigned to one medication group for 12weeks. Orbicularis oculi (startle eyeblink response) and corrugator supercilii facial electromyographic (EMG) reactivity toward emotional pictures (i.e., pleasant and unpleasant) in a picture-viewing task were measured before treatment and 2 and 6weeks after treatment was started. The startle and corrugator EMG activities increase with the exposure to unpleasant cues, and served as indices for negative emotional reactivity (NER). We found that after 6weeks, drug reduced startle-related NER in the varenicline group, but not in the bupropion or placebo group. Independent of medication treatment, lower baseline NER, as measured by the corrugator EMG activity, predicted a higher likelihood of smoking abstinence 1 and 3months after quitting smoking. These findings indicate the important roles of varenicline in negative affect processing and negative emotional reactivity in the course of smoking cessation.

Alteration of adolescent aversive nicotine response and anxiety-like behavior in nicotine-exposed rats during late lactation period

Early nicotine exposure is an important cause of further habitual tobacco smoking. Although nicotine has not only rewarding but also aversive properties, the effects of early nicotine exposure on the distinct properties of nicotine are not well known. To reveal the effects of early adolescent nicotine exposure on further persistent tobacco smoking, we demonstrated developmental changes in nicotine-related appetitive and aversive behaviors of rats exposed to nicotine during the late lactation period. Sprague-Dawley rats were injected with saline or nicotine (2, 6 and 12mg/kg). We performed a two bottle free-choice test using escalating doses of nicotine (25, 50 and 100μg/ml), saccharin and quinine and the open field test in both adolescent and adult rats. The rats' aversive response to nicotine was increased according to the increase in nicotine concentration. Adolescent rats showed higher nicotine preference and consumption behaviors than did adult rats at an aversive dose of nicotine. Nicotine-exposed rats increased adolescent nicotine consumption when the nicotine concentration was 12mg/kg. We observed significant increases in anxious behaviors in adolescent nicotine-injected rats compared to saline-injected rats, but there were no alterations in adult rats. In both adolescent and adult rats, saccharin and quinine intake were not significantly different between groups. Taken together, it suggests that repeated nicotine exposure in late lactation period affect changes in aversive nicotine responses and anxious behaviors during adolescence but there is no difference in adults.

Distinct Roles of CREB Within the Ventral and Dorsal Hippocampus in Mediating Nicotine Withdrawal Phenotypes

Addiction to nicotine and the inability to quit smoking are influenced by genetic factors, emphasizing the importance of understanding how genes and drugs of abuse mechanistically impact each other. One well-characterized protein responsible for regulating both response to drugs and gene expression is the transcription factor CREB (cAMP-responsive element binding protein). Previous work indicates that hippocampal-specific alterations in CREB signaling and synaptic plasticity may underlie certain nicotine withdrawal phenotypes. However, the structure of the hippocampus possesses dorsal and ventral subregions, each differing in behavioral, anatomic and gene expression characteristics. This study examines the effects of CREB deletion specifically in the ventral or dorsal hippocampus of animals chronically treated with saline, nicotine, or undergoing 24 h withdrawal. After region-specific viral injections of AAV-GFP or AAV-CRE in CREB^loxP/loxP animals, behavioral testing measured anxiety levels, using the Novelty-Induced Hypophagia test, and cognition, using a contextual fear conditioning paradigm. Deletion of CREB in the ventral, but not dorsal, hippocampus resulted in amelioration of nicotine withdrawal-induced anxiety-like behavior in the Novelty-Induced Hypophagia test. In contrast, CREB deletion in the dorsal hippocampus resulted in learning and memory deficits in fear conditioning, whereas CREB deletion in the ventral hippocampus showed an enhancement in learning. Gene expression analysis showed differential treatment- and region-dependent alterations of several CREB target genes that are well-known markers of neuroplasticity within the hippocampus. Collectively, these data provide persuasive evidence towards the distinct roles of CREB within the dorsal and ventral hippocampus separately in mediating select nicotine withdrawal phenotypes.

Endogenous opioid system: a promising target for future smoking cessation medications

BACKGROUND

Nicotine addiction continues to be a health challenge across the world. Despite several approved medications, smokers continue to relapse. Several human and animal studies have evaluated the role of the endogenous opioid system as a potential target for smoking cessation medications.

METHODS

In this review, studies that have elucidated the role of the mu (MORs), delta (DORs), and kappa (KORs) opioid receptors in nicotine reward, nicotine withdrawal, and reinstatement of nicotine seeking will be discussed. Additionally, the review will discuss discrepancies in the literature and therapeutic potential of the endogenous opioid system, and suggest studies to address gaps in knowledge with respect to the role of the opioid receptors in nicotine dependence.

RESULTS

Data available till date suggest that blockade of the MORs and DORs decreased the rewarding effects of nicotine, while activation of the MORs and DORs decreased nicotine withdrawal-induced aversive effects. In contrast, activation of the KORs decreased the rewarding effects of nicotine, while blockade of the KORs decreased nicotine withdrawal-induced aversive effects. Interestingly, blockade of the MORs and KORs attenuated reinstatement of nicotine seeking. In humans, MOR antagonists have shown benefits in select subpopulations of smokers and further investigation is required to realize their full therapeutic potential.

CONCLUSION

Future work must assess the influence of polymorphisms in opioid receptor-linked genes in nicotine dependence, which will help in both identifying individuals vulnerable to nicotine addiction and the development of opioid-based smoking cessation medications. Overall, the endogenous opioid system continues to be a promising target for future smoking cessation medications.

CRF antagonism within the ventral tegmental area but not the extended amygdala attenuates the anxiogenic effects of cocaine in rats

In addition to its initial rewarding effects, cocaine has been shown to produce profound negative/anxiogenic actions. Recent work on the anxiogenic effects of cocaine has examined the role of corticotropin releasing factor (CRF), with particular attention paid to the CRF cell bodies resident to the extended amygdala (i.e., the central nucleus of the amygdala [CeA] and the bed nucleus of the stria terminalis [BNST]) and the interconnections within and projections outside the region (e.g., to the ventral tegmental area [VTA]). In the current study, localized CRF receptor antagonism was produced by intra-BNST, intra-CeA or intra-VTA application of the CRF antagonists, D-Phe CRF(12-41) or astressin-B. The effect of these treatments were examined in a runway model of i.v. cocaine self-administration that has been shown to be sensitive to both the initial rewarding and delayed anxiogenic effects of the drug in the same animal on the same trial. These dual actions of cocaine are reflected in the development of an approach-avoidance conflict ("retreat behaviors") about goal box entry that stems from the mixed associations that subjects form about the goal. CRF antagonism within the VTA, but not the CeA or BNST, significantly reduced the frequency of approach-avoidance retreat behaviors while leaving start latencies (an index of the positive incentive properties of cocaine) unaffected. These results suggest that the critical CRF receptors contributing to the anxiogenic state associated with acute cocaine administration may lie outside the extended amygdala, and likely involve CRF projections to the VTA.

Increased conditioned place preference for cocaine in high anxiety related behavior (HAB) mice is associated with an increased activation in the accumbens corridor

Anxiety disorders and substance use disorders are strongly associated in humans. Accordingly, a widely held but controversial concept in the addiction field, the so-called “self-medication hypothesis,” posits that anxious individuals are more vulnerable for drug dependence because they use drugs of abuse to alleviate their anxiety. We tested this hypothesis under controlled experimental conditions by quantifying the conditioned place preference (CPP) to 15 mg/kg i.p. cocaine given contingently (COCAINE) in CD1 mice selectively bred for high anxiety-related behavior (HAB) vs. normal anxiety-related behavior (NAB). Cocaine was conditioned to the initially non-preferred compartment in an alternate day design (cocaine vs. saline, four pairings each). HAB and NAB mice were also tested for the effects of non-contingent (NONCONT) cocaine administration. HAB mice showed a slightly higher bias for one of the conditioning compartments during the pretest than NAB mice that became statistically significant (p = 0.045) only after pooling COCAINE and NONCONT groups. Cocaine CPP was higher (p = 0.0035) in HAB compared to NAB mice. The increased cocaine CPP was associated with an increased expression of the immediate early genes (IEGs) c-Fos and Early Growth Related Protein 1 (EGR1) in the accumbens corridor, i.e., a region stretching from the anterior commissure to the interhemispheric border and comprising the medial nucleus accumbens core and shell, the major island of Calleja and intermediate part of the lateral septum, as well as the vertical limb of the diagonal band and medial septum. The cocaine CPP-induced EGR1 expression was only observed in D1- and D2-medium spiny neurons, whereas other types of neurons or glial cells were not involved. With respect to the activation by contingent vs. non-contingent cocaine EGR1 seemed to be a more sensitive marker than c-Fos. Our findings suggest that cocaine may be more rewarding in high anxiety individuals, plausibly due to an anxiolytic effect.

Prior extended daily access to cocaine elevates the reward threshold in a conditioned place preference test

We have previously shown that extended-access subjects exhibit heightened motivation for cocaine in the runway model, as reflected by reduced number of retreats. This heightened motivation could reflect either an increase in cocaine-induced reward or a decrease in cocaine-induced aversion. The current experiment was therefore devised to assess the cocaine-induced reward and aversion in extended-access rats using a place conditioning test. Rats trained to lever press for intravenous (IV) cocaine (0.25 mg/infusion) were provided 6-hour daily access to the drug over 10 days. Lever pressing in control subjects produced IV infusions of saline. Following drug self-administration, subjects underwent place conditioning for the immediate or delayed effects of cocaine (1.0 or 2.5 mg/kg, IV). In control subjects, the immediate effects of the low dose of cocaine produced conditioned places preferences (CPPs), while the delayed effects produced conditioned place aversions (CPAs). In contrast, the animals receiving low cocaine dose for 6 hours, exhibited place aversions but not preferences; an effect that was reversed when the dose of cocaine was increased. Additionally, in the 6-hour group, delayed conditioning was associated with a reduction in zif268 immunoreactivity in the medial prefrontal cortex and nucleus accumbens shell while immediate conditioning was associated with an increase in zif268-positive cells in the central nucleus of the amygdala. Collectively, these data suggest that extended daily access to cocaine produces a shift in the subject's perceived reward threshold that is paralleled by alterations in the activity of both the reward and stress pathways.

The combination of metyrapone and oxazepam for the treatment of cocaine and other drug addictions

Although scientists have been investigating the neurobiology of psychomotor stimulant reward for many decades, there is still no FDA-approved treatment for cocaine or methamphetamine abuse. Research in our laboratory has focused on the relationship between stress, the subsequent activation of the hypothalamic-pituitary-adrenal (HPA) axis, and psychomotor stimulant reinforcement for almost 30 years. This research has led to the development of a combination of low doses of the cortisol synthesis inhibitor, metyrapone, and the benzodiazepine, oxazepam, as a potential pharmacological treatment for cocaine and other substance use disorders. In fact, we have conducted a pilot clinical trial that demonstrated that this combination can reduce cocaine craving and cocaine use. Our initial hypothesis underlying this effect was that the combination of metyrapone and oxazepam reduced cocaine seeking and taking by decreasing activity within the HPA axis. Even so, doses of the metyrapone and oxazepam combination that consistently reduced cocaine taking and seeking did not reliably alter plasma corticosterone (or cortisol in the pilot clinical trial). Furthermore, subsequent research has demonstrated that this drug combination is effective in adrenalectomized rats, suggesting that these effects must be mediated above the level of the adrenal gland. Our evolving hypothesis is that the combination of metyrapone and oxazepam produces its effects by increasing the levels of neuroactive steroids, most notably tetrahydrodeoxycorticosterone, in the medial prefrontal cortex and amygdala. Additional research will be necessary to confirm this hypothesis and may lead to the development of improved and specific pharmacotherapies for the treatment of psychomotor stimulant use.

Developmental alterations in locomotor and anxiety-like behavior as a function of D1 and D2 mRNA expression

The majority of smokers start smoking in adolescence, beginning a potentially lifelong struggle with nicotine use and abuse. In rodent models of the effects of nicotine, the drug has been shown to elicit both locomotor and anxiety-like behavioral effects. Research suggests that these behavioral effects may be due in part to dopamine (DA) receptors D1 and D2 in the mesolimbic system, specifically the nucleus accumbens (NAc). We examined early adolescent (P28), late adolescent (P45), and adult (P80) male Long-Evans rats in the elevated plus maze (EPM) under normal conditions and the open field (OF) post-nicotine in order to test locomotor and anxiety-like behavior. These behavioral findings were then correlated with expression of DA D1 and D2 mRNA levels as determined via in situ hybridization. Nicotine-induced locomotor behavior was found to be significantly different between age groups. After a single injection of nicotine, early adolescents exhibited increases in locomotor behavior, whereas both late adolescents and adults responded with decreases in locomotor behavior. In addition, it was found that among, early adolescents, open arm and center time in the EPM were negatively correlated with D2 mRNA expression. In contrast, among adults, distance traveled in the center and center time in the OF were negatively correlated with D2 mRNA expression. This study suggests that DA D2 receptors play a role in anxiety-like behavior and that the relationship between observed anxiety-like behaviors and D2 receptor expression changes through the lifespan.

Enhancement of behavioral sensitization, anxiety-like behavior, and hippocampal and frontal cortical CREB levels following cocaine abstinence in mice exposed to cocaine during adolescence

Adolescence has been linked to greater risk-taking and novelty-seeking behavior and a higher prevalence of drug abuse and risk of relapse. Decreases in cyclic adenosine monophosphate response element binding protein (CREB) and phosphorylated CREB (pCREB) have been reported after repeated cocaine administration in animal models. We compared the behavioral effects of cocaine and abstinence in adolescent and adult mice and investigated possible age-related differences in CREB and pCREB levels. Adolescent and adult male Swiss mice received one daily injection of saline or cocaine (10 mg/kg, i.p.) for 8 days. On day 9, the mice received a saline injection to evaluate possible environmental conditioning. After 9 days of withdrawal, the mice were tested in the elevated plus maze to evaluate anxiety-like behavior. Twelve days after the last saline/cocaine injection, the mice received a challenge injection of either cocaine or saline, and locomotor activity was assessed. One hour after the last injection, the brains were extracted, and CREB and pCREB levels were evaluated using Western blot in the prefrontal cortex (PFC) and hippocampus. The cocaine-pretreated mice during adolescence exhibited a greater magnitude of the expression of behavioral sensitization and greater cocaine withdrawal-induced anxiety-like behavior compared with the control group. Significant increases in CREB levels in the PFC and hippocampus and pCREB in the hippocampus were observed in cocaine-abstinent animals compared with the animals treated with cocaine in adulthood. Interestingly, significant negative correlations were observed between cocaine sensitization and CREB levels in both regions. These results suggest that the behavioral and neurochemical consequences of psychoactive substances in a still-developing nervous system can be more severe than in an already mature nervous system.

The dopamine antagonist cis-flupenthixol blocks the expression of the conditioned positive but not the negative effects of cocaine in rats

Human cocaine users report that the initial "high" produced by cocaine administration is followed by an anxiogenic "crash". Given that cocaine has such robust and opposing properties, it is likely that both positive and negative effects of cocaine contribute to an individual's motivation to administer the drug. Despite this likelihood, the neurobiology underlying cocaine's dual processes remains unclear. While much literature supports a role for dopamine (DA) in cocaine reward, it is uncertain if DA also contributes to the drug's negative effects. Our laboratory has extensively utilized a modified conditioned place test to explore cocaine's opponent processes. In this paradigm rats develop conditioned place preferences (CPPs) for an environment paired with the immediate/positive effects of cocaine, and conditioned place aversions (CPAs) for an environment paired with the delayed/negative effects present 15-min after i.v. injection. In the current study rats were conditioned to associate an environment with either the immediate or delayed effects of i.v. cocaine (1mg/kg/0.1ml) 3h after i.p. pre-treatment with either the DA D1/D2 receptor antagonist cis-flupenthixol (0.5mg/kg/ml) or saline vehicle. As expected, vehicle-treated control animals developed the normal pattern of CPPs for cocaine's immediate effects or CPAs for the delayed effects of cocaine. However, while DA receptor antagonism prevented the expression of cocaine CPPs it did not alter the expression of cocaine-induced CPAs. These data confirm a role for DA transmission in cocaine reward but suggest that different neural pathways mediate the drug's negative/anxiogenic properties.

On the persistence of cocaine-induced place preferences and aversions in rats

RATIONALE

Rats develop preferences for places associated with the immediate rewarding effects of cocaine and aversions for places paired with the drug's delayed negative effects. The motivation to seek cocaine should therefore depend upon the relative magnitude of these two opposing effects of the drug.

OBJECTIVE

The current study tested this notion by assessing the relative persistence of the positive and negative associations formed between environmental cues and the immediate or delayed effects of cocaine.

METHODS

Rats were administered 1.0 mg/kg intravenous cocaine and placed into a distinctive environment either immediately or 15-min after injection, alternating daily with pairings of a second environment with saline. After four drug-place and four saline-place pairings, rats were returned to their home cages for 1, 7, or 21 days after which a 15-min place preference test was conducted. In a second experiment, the effectiveness of a single reconditioning session (one drug-place and one saline-place pairing) to reactivate learned cocaine-place associations was assessed after 1 or 3 weeks of drug abstinence.

RESULTS

Places associated with the immediate effects of cocaine were preferred (CPP), while places associated with the delayed effects of cocaine were avoided (CPA). The persistence of these effects differed with CPP remaining viable at 3 weeks of withdrawal, while CPA was no longer present after 1 week. Reconditioning with an additional cocaine-place pairing failed to reinstate the CPA.

CONCLUSIONS

Cue-induced "relapse" of cocaine-seeking behavior may be fueled in part by an increased persistence of positive relative to negative associations with drug-paired stimuli.

Divergent functional effects of sazetidine-a and varenicline during nicotine withdrawal

Smoking is the largest preventable cause of death in the United States. Furthermore, a recent study found that <10% of quit attempts resulted in continuous abstinence for 1 year. With the introduction of pharmacotherapies like Chantix (varenicline), a selective α4β2 nicotinic partial agonist, successful quit attempts have significantly increased. Therefore, novel subtype-specific nicotinic drugs, such as sazetidine-A, present a rich area for investigation of therapeutic potential in smoking cessation. The present studies examine the anxiety-related behavioral and functional effects of the nicotinic partial agonists varenicline and sazetidine-A during withdrawal from chronic nicotine in mice. Our studies indicate that ventral hippocampal-specific infusions of sazetidine-A, but not varenicline, are efficacious in reducing nicotine withdrawal-related anxiety-like phenotypes in the novelty-induced hypophagia (NIH) paradigm. To further investigate functional differences between these partial agonists, we utilized voltage-sensitive dye imaging (VSDi) in ventral hippocampal slices to determine the effects of sazetidine-A and varenicline in animals chronically treated with saline, nicotine, or undergoing 24 h withdrawal. These studies demonstrate a functional dissociation of varenicline and sazetidine-A on hippocampal network activity, which is directly related to previous drug exposure. Furthermore, the effects of the nicotinic partial agonists in VSDi assays are significantly correlated with their behavioral effects in the NIH test. These findings highlight the importance of drug history in understanding the mechanisms through which nicotinic compounds may be aiding smoking cessation in individuals experiencing withdrawal-associated anxiety.

The role of the glucocorticoids in developing resilience to stress and addiction

There is emerging evidence that individuals have the capacity to learn to be resilient by developing protective mechanisms that prevent them from the maladaptive effects of stress that can contribute to addiction. The emerging field of the neuroscience of resilience is beginning to uncover the circuits and molecules that protect against stress-related neuropsychiatric diseases, such as addiction. Glucocorticoids (GCs) are important regulators of basal and stress-related homeostasis in all higher organisms and influence a wide array of genes in almost every organ and tissue. GCs, therefore, are ideally situated to either promote or prevent adaptation to stress. In this review, we will focus on the role of GCs in the hypothalamic-pituitary adrenocortical axis and extra-hypothalamic regions in regulating basal and chronic stress responses. GCs interact with a large number of neurotransmitter and neuropeptide systems that are associated with the development of addiction. Additionally, the review will focus on the orexinergic and cholinergic pathways and highlight their role in stress and addiction. GCs play a key role in promoting the development of resilience or susceptibility and represent important pharmacotherapeutic targets that can reduce the impact of a maladapted stress system for the treatment of stress-induced addiction.

Nicotine induces a conditioned place preference in male Japanese quail (Coturnix japonica)

Visual stimuli may play an important role in the development and maintenance of addiction in humans. Research with a visually-oriented animal model such as Japanese quail (Coturnix japonica) may provide insight into how visual cues contribute to the addiction process. The aim of the current study was to investigate the rewarding properties of nicotine in male Japanese quail using a biased conditioned place preference (CPP) procedure. Adult male quail (N=30) were allowed to freely explore the entire CPP apparatus during a place preference pre-test and time spent in each chamber was measured. During nicotine conditioning sessions, quail were administered nicotine (0.5, 1.0, or 2.0mg/kg) or saline and were then confined to their initially least preferred chamber. On alternating days, all quail received saline and were confined to their initially preferred chamber. Locomotor activity was assessed in both chambers. The conditioning chambers had yellow or green walls to enhance the visual salience of each context. Following 8 conditioning sessions (4 nicotine; 4 saline), quail were allowed to explore the entire apparatus during a CPP post-test and time spent in each chamber was measured. The results indicated that quail treated with 0.5 and 1.0mg/kg nicotine significantly increased the amount of time they spent in the nicotine-paired chamber compared to saline controls, suggesting that nicotine produced a CPP. Furthermore, quail treated with 0.5mg/kg nicotine showed a significant increase in locomotor activity with repeated treatments. The current findings suggest that nicotine may have a rewarding effect in quail and may tentatively suggest that the neuropharmacological mechanisms that mediate CPP for nicotine are conserved in birds.

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.

Stress-induced activation of the dynorphin/κ-opioid receptor system in the amygdala potentiates nicotine conditioned place preference

Many smokers describe the anxiolytic and stress-reducing effects of nicotine, the primary addictive component of tobacco, as a principal motivation for continued drug use. Recent evidence suggests that activation of the stress circuits, including the dynorphin/κ-opioid receptor system, modulates the rewarding effects of addictive drugs. In the present study, we find that nicotine produced dose-dependent conditioned place preference (CPP) in mice. κ-receptor activation, either by repeated forced swim stress or U50,488 (5 or 10 mg/kg, i.p.) administration, significantly potentiated the magnitude of nicotine CPP. The increase in nicotine CPP was blocked by the κ-receptor antagonist norbinaltorphimine (norBNI) either systemically (10 mg/kg, i.p.) or by local injection in the amygdala (2.5 μg) without affecting nicotine reward in the absence of stress. U50,488 (5 mg/kg, i.p.) produced anxiety-like behaviors in the elevated-plus maze and novel object exploration assays, and the anxiety-like behaviors were attenuated both by systemic nicotine (0.5 mg/kg, s.c.) and local injection of norBNI into the amygdala. Local norBNI injection in the ventral posterior thalamic nucleus (an adjacent brain region) did not block the potentiation of nicotine CPP or the anxiogenic-like effects of κ-receptor activation. These results suggest that the rewarding effects of nicotine may include a reduction in the stress-induced anxiety responses caused by activation of the dynorphin/κ-opioid system. Together, these data implicate the amygdala as a key region modulating the appetitive properties of nicotine, and suggest that κ-opioid antagonists may be useful therapeutic tools to reduce stress-induced nicotine craving.

Weakening of negative relative to positive associations with cocaine-paired cues contributes to cue-induced responding after drug removal

Cocaine has been shown to have initial positive (euphoric) and delayed negative (anxiogenic) effects in both humans and animals. Cocaine-paired cues are consequently imbued with mixed positive and negative associations. The current study examines the relative roles of these dual associations in the enhanced drug-seeking observed upon presentation of cocaine-paired cues. Rats ran a straight alley once/day for a single i.v. injection of cocaine (1.0 mg/kg/inj) in the presence of a distinctive olfactory cue (scented cotton swabs placed under the apparatus). An alternate scent was presented in a separate cage 2-h prior to runway testing. After 15 trials/days, the scents and cocaine reinforcer were removed and a series of extinction trials (lasting for 1 or 3 weeks) was initiated. Immediately following extinction, runway responding was tested during a single trial in the presence of the cocaine-paired or non-paired cue. As previously reported, while subjects initiated responding faster over trials (reduced latencies to leave the start box), they exhibited a progressive increase in approach-avoidance conflict behavior ("retreats") regarding goal-box entry, reflecting cocaine's dual positive+negative effects. Once established, retreat behaviors persisted over the course of 1 or 3 weeks days of extinction. However, both run times and retreats decreased in response to presentation of the cocaine-paired but not the non-paired scent. These data suggest that, after reinforcer removal, cue-induced cocaine-seeking stems in part from a reduction in approach-avoidance conflict; i.e., a greater weakening of the negative relative to the positive associations that animals form with cocaine-paired stimuli.

Mechanistic insights into nicotine withdrawal

Smoking is responsible for over 400,000 premature deaths in the United States every year, making it the leading cause of preventable death. In addition, smoking-related illness leads to billions of dollars in healthcare expenditures and lost productivity annually. The public is increasingly aware that successfully abstaining from smoking at any age can add years to one's life and reduce many of the harmful effects of smoking. Although the majority of smokers desire to quit, only a small fraction of attempts to quit are actually successful. The symptoms associated with nicotine withdrawal are a primary deterrent to cessation and they need to be quelled to avoid early relapse. This review will focus on the neuroadaptations caused by chronic nicotine exposure and discuss how those changes lead to a withdrawal syndrome upon smoking cessation. Besides examining how nicotine usurps the endogenous reward system, we will discuss how the habenula is part of a circuit that plays a critical role in the aversive effects of high nicotine doses and nicotine withdrawal. We will also provide an updated summary of the role of various nicotinic receptor subtypes in the mechanisms of withdrawal. This growing knowledge provides mechanistic insights into current and future smoking cessation therapies.

Pharmacological modulation of anxiety-like phenotypes in adult zebrafish behavioral models

Zebrafish (Danio rerio) are becoming increasingly popular in neurobehavioral research. Here, we summarize recent data on behavioral responses of adult zebrafish to a wide spectrum of putative anxiolytic and anxiogenic agents. Using the novel tank test as a sensitive and efficient behavioral assay, zebrafish anxiety-like behavior can be bi-directionally modulated by drugs affecting the gamma-aminobutyric acid, monoaminergic, cholinergic, glutamatergic and opioidergic systems. Complementing human and rodent data, zebrafish drug-evoked phenotypes obtained in this test support this species as a useful model for neurobehavioral and psychopharmacological research.

Nicotine-induced plasma corticosterone is attenuated by social interactions in male and female adolescent rats

Most smokers begin smoking during adolescence, a period during which social reward is highly influential. Initial exposure to nicotine can produce anxiogenic effects that may be influenced by social context. This study examined play behavior and plasma corticosterone following nicotine administration (0.6 mg/kg, s.c.) in both male and female adolescent (PND39) Sprague-Dawley rats in either isolate or social contexts. In blood samples collected immediately following the 15-min test session, nicotine increased plasma corticosterone relative to saline in both male and female isolate rats, but failed to do so in both males and females placed together in same-sex pairs. Nicotine also attenuated several indices of play behavior including nape attacks, pins and social contact. In isolate rats, nicotine selectively increased locomotor activity in females; however, when administered to social pairs, nicotine decreased locomotion in both sexes. These findings suggest that the presence of a social partner may decrease the initial negative, stress-activating effects of nicotine, perhaps leading to increased nicotine reward.

Desensitization of δ-opioid receptors in nucleus accumbens during nicotine withdrawal

RATIONALE

The synthesis and release of met-enkephalin and β-endorphin, endogenous ligands for δ-opioid peptide receptors (DOPrs), are altered following nicotine administration and may play a role in nicotine addiction.

OBJECTIVES

To investigate the consequences of altered opioidergic activity on DOPr expression, coupling, and function in striatum during early nicotine withdrawal.

METHODS

Mice received nicotine-free base, 2 mg/kg, or saline, subcutaneously (s.c.), four times daily for 14 days and experiments performed at 24, 48, and 72 h after drug discontinuation. DOPr binding and mRNA were evaluated by [³H]naltrindole autoradiography and in situ hybridization. DOPr coupling and function were investigated by agonist pCl-DPDPE-stimulated [³⁵S]GTPγS binding autoradiography and inhibition of adenylyl cyclase activity.

RESULTS

During nicotine withdrawal DOPr binding was unaltered in caudate/putamen (CPu) and nucleus accumbens (NAc) shell and core. Receptor mRNA was slightly increased in the shell at 72 h, but significant elevations were observed in prefrontal cortex and hippocampus. pCl-DPDPE-stimulated [³⁵S]GTPγS binding was attenuated in NAc, but not CPu. In the shell, binding was decreased by 48 h and remained decreased over 72 h; while in the core, significant reduction was seen at 72 h. Basal adenylyl cyclase activity was suppressed in striatum at 24 h, but recovered by 48 h. DOPr stimulation with pCl-DPDPE failed to inhibit adenylyl cyclase activity at 24 h and produced attenuated responses at 48 and 72 h.

CONCLUSIONS

These observations suggest that DOPr coupling and function are impaired in the NAc during nicotine withdrawal. DOPr desensitization might be involved in the affective component of nicotine withdrawal.

Inactivation of the dorsal raphé nucleus reduces the anxiogenic response of rats running an alley for intravenous cocaine

Rats traversing a straight alley once a day for delivery of a single i.v. injection of cocaine develop over trials an ambivalence about entering the goal box. This ambivalence is characterized by the increasing occurrence of "retreat behaviors" where animals leave the start box and run quickly to the goal box, but then stop at the entry point and "retreat" back toward the start box. This unique pattern of retreat behavior has been shown to reflect a form of "approach-avoidance conflict" that stems from the animals' concurrent positive (cocaine reward) and negative (cocaine-induced anxiety) associations with the goal box. Cocaine blocks reuptake of the serotonergic (5-HT) transporter and serotonin has been implicated in the modulation of anxiety. It was therefore of interest to determine whether inactivation of the serotonergic cell bodies residing in the dorsal raphé nucleus (DRN) and projecting to brain areas critical for the modulation of anxiety, would alter the anxiogenic state exhibited by rats running an alley for single daily i.v. injections of 1.0mg/kg cocaine. Reversible inactivation of the DRN was accomplished by intracranial application of a mixed solution of the GABA agonists baclofen and muscimol. While DRN inactivation had no impact on the subjects' motivation to initiate responding (i.e., latencies to leave the start box were unaffected) it reliably reduced the frequency of approach-avoidance retreat behaviors (conflict behavior). These data suggest that inactivation of the dorsal raphé reduces the conflict/anxiety otherwise present in experienced cocaine-seeking animals.

Nicotine acts in the anterior cingulate, but not dorsal or ventral hippocampus, to reverse ethanol-induced learning impairments in the plus-maze discriminative avoidance task

The current study examines the role of the dorsal and ventral hippocampus, and anterior cingulate in the interactive effects of ethanol and nicotine on learning, anxiety and locomotion in the plus-maze discriminative avoidance task, which allows dissociation of drug effects on each behaviour. At training, time spent in each of the arms of the elevated plus-maze was recorded for 5 minutes. Each time that the mouse entered the aversive enclosed arm, a light and white noise were turned on. At testing, no cues were turned on and time spent in each arm was recorded for 3 minutes. The effects of systemic ethanol (1.0 or 1.4 g/kg) and nicotine (0.35 µg/0.50 µl/side) infused into the anterior cingulate, dorsal and ventral hippocampus were examined, as were the interactive effects of systemic ethanol (1.0 g/kg) and nicotine (0.09 mg/kg) with the high-affinity nicotinic receptor antagonist dihydro-beta-erythroidine (DHβE) (18.0 µg/0.50 µl/side) infused into the anterior cingulate. Ethanol dose dependently decreased anxiety, increased locomotion, and decreased learning. Anterior cingulate-infused nicotine decreased anxiety and reversed ethanol-associated learning deficits. Anterior cingulate-infused DHβE blocked reversal of ethanol-induced learning deficits by systemic nicotine. Dorsal hippocampus-infused nicotine reversed ethanol-induced anxiolysis and hyper-locomotion (1.4 g/kg) but produced no behavioural changes in ethanol-naïve mice. Ventral hippocampus-infused nicotine enhanced anxiolysis associated with 1.4 g/kg ethanol, but had no other effects. The anterior cingulate is necessary and sufficient for nicotine reversal of ethanol-induced learning deficits. In addition, the anterior cingulate, dorsal hippocampus and ventral hippocampus may mediate drug-induced changes in anxiety.

Biological basis of tobacco addiction: Implications for smoking-cessation treatment

Tobacco use became common all over the world after discovery of Americas. Tobacco, a plant carries in its leaves an alkaloid called nicotine, which is responsible not only for several pathophysiological changes in the body but also develops tolerance to its own action with repeated use. Studies suggest that the alpha-4 beta-2 nicotine acetylcholine receptor subtype is the main receptor that mediates nicotine dependence. Nicotine acts on these receptors to facilitate neurotransmitter release (dopamine and others), producing pleasure and mood modulation. Repeated exposure to nicotine develops neuroadaptation of the receptors, resulting in tolerance to many of the effects of nicotine. Withdrawal symptoms appear on stoppage of tobacco use, which are characterized by irritability, anxiety, increased eating, dysphoria, and hedonic dysregulation, among others. Smoking is also reinforced by conditioning. Pharmacotherapies for smoking cessation should reduce withdrawal symptoms and block the reinforcing effects of nicotine obtained from smoking without causing excessive adverse effects.

Age-related effects of acute nicotine on behavioural and neuronal measures of anxiety

Adolescence is considered a period of enhanced vulnerability to the initiation of tobacco use. Biological differences in the sensitivity of adolescents and adults to the anxiogenic and anxiolytic effects of nicotine may contribute to this heightened vulnerability. Here, we investigated the age-dependency of the effects of acute nicotine on anxiety-related behaviour and neurotransmission. In Experiment 1, male adolescent (P33-37) and adult (P65-69) Long-Evans rats received nicotine (0, 0.4 or 0.8 mg/kg,s.c.) prior to testing using two measures of anxiety, the elevated plus maze (EPM) and light-dark (LD) transition box. In Experiment 2, in situ hybridization was used to assess, in different male adolescent and adult rats, CRF mRNA expression in the BNST, PVN and CeA in response to acute nicotine. In the EPM, adult rats displayed less anxious behaviour than adolescents. Nicotine (0.4, 0.8 mg/kg) increased open and closed arm entries in adolescent rats, suggesting increased general activity, but it did not affect behaviour in the LD box. CRF mRNA expression was elevated in PVN of adolescent rats, relative to adults. Nicotine, however, had no effect on CRF mRNA expression in the BNST, PVN or CeA. The present findings suggest that adolescents are more sensitive to the general activational, rather than anxiety-related, effects of nicotine, and that CRF mRNA expression in stress-related brain regions does not correlate with these effects. This work further characterizes the age-related differences in the anxiety-related effects of nicotine, and provides insight into potential factors influencing vulnerability to tobacco abuse.

The M5 muscarinic receptor as possible target for treatment of drug abuse

Two reports published in the latter 1980s are generally given credit for being the first to announce the discovery of a new subtype of muscarinic acetylcholine receptor (mAChR), designated m5 or M5, and now officially M(5) (1). Both identifications were assigned using molecular biology techniques. Then - as now - no selective high-affinity ligands or toxins were available. In situ hybridization and reverse-transcriptase PCR have found M(5) AChR expression in brain to be distinct from that of the four other G protein-coupled mAChR subtypes and primarily localized to the substantia nigra, ventral tegmental area, hippocampus (CA1 and CA2 subfields), cerebral cortex (outermost layer) and striatum (caudate putamen). M(5) AChR brain region localization and involvement in the regulation of striatal dopamine release and in rewarding brain stimulation suggests a possible role for M(5) AChR as a target for novel therapy to treat excess hedonic drive, including drug abuse.

Dietary restriction mitigates cocaine-induced alterations of olfactory bulb cellular plasticity and gene expression, and behavior

Because the olfactory system plays a major role in food consumption, and because 'food addiction' and associated morbidities have reached epidemic proportions, we tested the hypothesis that dietary energy restriction can modify adverse effects of cocaine on behavior and olfactory cellular and molecular plasticity. Mice maintained on an alternate day fasting (ADF) diet exhibited increased baseline locomotion and increased cocaine-sensitized locomotion during cocaine conditioning, despite no change in cocaine conditioned place preference, compared with mice fed ad libitum. Levels of dopamine and its metabolites in the olfactory bulb (OB) were suppressed in mice on the ADF diet compared with mice on the control diet, independent of acute or chronic cocaine treatment. The expression of several enzymes involved in dopamine metabolism including tyrosine hydroxylase, monoamine oxidases A and B, and catechol-O-methyltransferase were significantly reduced in OBs of mice on the ADF diet. Both acute and chronic administration of cocaine suppressed the production of new OB cells, and this effect of cocaine was attenuated in mice on the ADF diet. Cocaine administration to mice on the control diet resulted in up-regulation of OB genes involved in mitochondrial energy metabolism, synaptic plasticity, cellular stress responses, and calcium- and cAMP-mediated signaling, whereas multiple olfactory receptor genes were down-regulated by cocaine treatment. ADF abolished many of the effects of cocaine on OB gene expression. Our findings reveal that dietary energy intake modifies the neural substrates underlying some of the behavioral and physiological responses to repeated cocaine treatment, and also suggest novel roles for the olfactory system in addiction. The data further suggest that modification of dietary energy intake could provide a novel potential approach to addiction treatments.

GABAA-benzodiazepine receptor availability in smokers and nonsmokers: relationship to subsyndromal anxiety and depression

Many smokers experience subsyndromal anxiety symptoms while smoking and during acute abstinence, which may contribute to relapse. We hypothesized that cortical gamma aminobutyric acid(A)-benzodiazepine receptor (GABA(A)-BZR) availability in smokers and nonsmokers might be related to the expression of subsyndromal anxiety, depressive, and pain symptoms. Cortical GABA(A)-BZRs were imaged in 15 smokers (8 men and 7 women), and 15 healthy age and sex-matched nonsmokers, and 4 abstinent tobacco smokers (3 men; 1 woman) using [(123)I]iomazenil and single photon emission computed tomography (SPECT). Anxiety and depressive symptoms were measured using the Spielberger's State-Trait Anxiety Index (STAI) and the Center for Epidemiology Scale for Depressive Symptoms (CES-D). The cold pressor task was administered to assess pain tolerance and sensitivity. The relationship between cortical GABA(A)-BZR availability, smoking status, and subsyndromal depression and anxiety symptoms, as well as pain tolerance and sensitivity, were evaluated. Surprisingly, there were no statistically significant differences in overall GABA(A)-BZR availability between smokers and nonsmokers or between active and abstinent smokers; however, cortical GABA(A)-BZR availability negatively correlated with subsyndromal state anxiety symptoms in nonsmokers but not in smokers. In nonsmokers, the correlation was seen across many brain areas with state anxiety [parietal (r = -0.47, P = 0.03), frontal (r = -0.46, P = 0.03), anterior cingulate (r = -0.47, P = 0.04), temporal (r = -0.47, P = 0.03), occipital (r = -0.43, P = 0.05) cortices, and cerebellum (r = -0.46, P = 0.04)], trait anxiety [parietal (r = -0.72, P = 0.02), frontal (r = -0.72, P = 0.02), and occipital (r = -0.65, P = 0.04) cortices] and depressive symptoms [parietal (r = -0.68; P = 0.02), frontal (r = -0.65; P = 0.03), anterior cingulate (r = -0.61; P = 0.04), and temporal (r = -0.66; P = 0.02) cortices]. The finding that a similar relationship between GABA(A)-BZR availability and anxiety symptoms was not observed in smokers suggests that there is a difference in GABA(A)-BZR function, but not number, in smokers. Thus, while subsyndromal anxiety and depressive symptoms in nonsmokers may be determined in part by GABA(A)-BZR availability, smoking disrupts this relationship. Aberrant regulation of GABA(A)-BZR function in vulnerable smokers may explain why some smokers experience subsyndromal anxiety and depression.

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.

Interactive effects of ethanol and nicotine on learning, anxiety, and locomotion in C57BL/6 mice in the plus-maze discriminative avoidance task

INTRODUCTION

Alcohol and nicotine both alter learning, locomotion, and anxiety, yet no study has directly examined the interactive effects of these drugs across these behaviors within subjects. Such a comparison would determine if the drugs produce independent effects on each behavior. The plus-maze discriminative avoidance task (PMDAT) allows within-subject measurement of these behaviors.

METHODS

For training, each mouse explored the elevated plus-maze for 5 min and each time a mouse entered the aversive enclosed arm, a light and white noise were turned on. For testing, each mouse was returned to the center of the maze and, for 3 min, the time in each arm or in the center area was recorded. No cues were turned on during testing. The effects of ethanol (0.6-2.6 g/kg 15 min before training) and nicotine (0.045-0.18 mg/kg 5 min before training), alone or in combination, on behavior were examined.

RESULTS

Ethanol dose-dependently decreased anxiety, increased locomotion, and decreased learning but different doses altered each behavior. Nicotine dose-dependently increased anxiety and locomotion and decreased learning but different doses altered each behavior. Nicotine (0.09 mg/kg) reversed ethanol-associated changes in learning (1.0 and 1.4 g/kg), locomotion (1.4 g/kg), and anxiety (1.4 g/kg).

CONCLUSIONS

The effects of nicotine or ethanol on learning occurred at different doses than those that altered anxiety or locomotion, suggesting that the drug effects on learning are independent of the effects on anxiety and locomotion. With combined administration, nicotine reduced ethanol-associated deficits in learning and changes in anxiety and locomotion.

Anxiolytic effects of nicotine in a rodent test of approach-avoidance conflict

RATIONALE

Nicotine has been reported to produce both anxiolytic and/or anxiogenic effects in humans and animals.

OBJECTIVES

This study examined whether pretreatment with nicotine would alter anxiety in a unique runway model of approach-avoidance conflict.

MATERIALS AND METHODS

Food-restricted rats were trained to run a straight alley once a day to obtain food upon goal-box entry. Beginning on trial 11, food reward was followed by a series of five foot shocks (0.3-0.4 mA, 0.5 s) in the goal box. Non-shocked control rats continued to run for food only. The resulting association of the goal box with both a positive (food) and negative (foot shock) stimulus produced an approach-avoidance conflict (subjects exhibited "retreat behaviors" in which they would approach the goal box, stop, and then retreat back towards the start box). Once retreats were established, their sensitivity to nicotine pretreatment (0.0, 0.03, 0.045, 0.06, or 0.075 mg/kg, i.v.) was compared to saline. In subsequent tests, the effects of nicotine (0.06 or 0.03 mg/kg) were examined on spontaneous activity (locomotion) and center-square entries in an open field (anxiety).

RESULTS

Doses of 0.06 and 0.075 mg/kg, but not lower doses of nicotine, reduced the number of runway retreats, and 0.06 mg/kg nicotine increased the number of open-field center entries relative to saline. No effects on locomotion were observed.

CONCLUSIONS

Nicotine reduced approach-avoidance conflict and increased the rats' willingness to enter the center of an open field, suggesting that the drug can produce anxiolytic properties and that such effects may serve as an important factor in the persistence of smoking behavior.

Subtypes of nicotinic acetylcholine receptors in nicotine reward, dependence, and withdrawal: evidence from genetically modified mice

Neuronal nicotinic acetylcholine receptors (nAChRs) can regulate the activity of many neurotransmitter pathways throughout the central nervous system and are considered to be important modulators of cognition and emotion. nAChRs are also the primary site of action in the brain for nicotine, the major addictive component of tobacco smoke. nAChRs consist of five membrane-spanning subunits (alpha and beta isoforms) that can associate in various combinations to form functional nAChR ion channels. Owing to a dearth of nAChR subtype-selective ligands, the precise subunit composition of the nAChRs that regulate the rewarding effects of nicotine and the development of nicotine dependence are unknown. The advent of mice with genetic nAChR subunit modifications, however, has provided a useful experimental approach to assess the contribution of individual subunits in vivo. Here, we review data generated from nAChR subunit knockout and genetically modified mice supporting a role for discrete nAChR subunits in nicotine reinforcement and dependence processes. Importantly, the rates of tobacco dependence are far higher in patients suffering from comorbid psychiatric illnesses compared with the general population, which may at least partly reflect disease-associated alterations in nAChR signaling. An understanding of the role of nAChRs in psychiatric disorders associated with high rates of tobacco addiction, therefore, may reveal novel insights into mechanisms of nicotine dependence. Thus, we also briefly review data generated from genetically modified mice to support a role for discrete nAChR subunits in anxiety disorders, depression, and schizophrenia.

Effects of bupropion, alone or coadministered with nicotine, on social behavior in mice

Bupropion, administered alone or combined with nicotine, is presently used to treat nicotine dependence. Despite experimental evidence of the complex behavioral actions of this drug, there have been little data reported about its effects on social behavior. Our main aim was to investigate the effects of acute administration of bupropion, alone or plus nicotine, on social interaction in mice. OF1 group-housed male mice were confronted in a neutral cage with an anosmic opponent during a 10 minutes encounter. Time allocated to body care and digging was reduced by administration of bupropion (40 mg/kg) both when administered alone and with nicotine (1 and 0.5 mg/kg). The lowest dose of bupropion (10 mg/kg) also reduced digging when combined with 1 mg/kg of nicotine. Time spent on non-social exploration and exploration from a distance was significantly higher in mice treated with bupropion (40 mg/kg) alone or combined with nicotine (1 and 0.5 mg/kg). The lowest dose of bupropion (10 mg/kg) increased non-social exploration when combined with 0.5 mg/kg of nicotine and exploration from a distance when combined with 1 mg/kg of nicotine. Ethopharmacological assessment of the behavior of groups of mice treated with different combinations of the two drugs indicates that nicotine can potentiate some of the behavioral effects of low doses of bupropion. Results also indicate that bupropion, either alone or combined with nicotine, has no significant effects on social investigation, suggesting that this drug does not induce a clear anxiolytic profile in OF1 mice.

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.

Behavioural and neurotoxic long-lasting effects of MDMA plus cocaine in adolescent mice

The poly-drug pattern is the most common among MDMA users, with cocaine being a frequently associated drug. The aim of the present work was to evaluate the behavioural and neurotoxic long-term effects of exposure during adolescence to MDMA alone or plus cocaine. Mice of 28 to 30 days of age received a treatment of two daily injections of an identical dose of MDMA (5, 10 or 20 mg/kg), alone or plus cocaine (25 mg/kg), for 3 days (6 administrations). Three weeks after receiving MDMA, an increase in the time dedicated by the animals to social contacts with their conspecifics was observed, whilst their behaviour in the elevated plus maze showed no differences from that of non-treated mice. After being exposed to MDMA plus cocaine, mice spent more time in social contacts during the interaction test, as well as exhibiting an anxiolytic profile in the elevated plus maze, with an increase in the time and number of entries in the open arms. The activity of mice treated with cocaine alone or plus MDMA remained constant; the decrease observed among the rest of the animals after the second hour was absent in their case. The level of dopamine in the striatum was diminished in mice treated with 20 mg/kg of MDMA, but this neurotransmitter was not affected in animals exposed to the same dose plus cocaine. The present results highlight pronounced alterations in the behaviour of adult mice after exposure to MDMA and cocaine during adolescence, and demonstrate that these long-term effects can occur without the dopaminergic system becoming affected.