Age-related differences in affective behaviors in mice: possible role of prefrontal cortical-hippocampal functional connectivity and metabolomic profiles

Introduction

The differential expression of emotional reactivity from early to late adulthood may involve maturation of prefrontal cortical responses to negative valence stimuli. In mice, age-related changes in affective behaviors have been reported, but the functional neural circuitry warrants further investigation.

Methods

We assessed age variations in affective behaviors and functional connectivity in male and female C57BL6/J mice. Mice aged 10, 30 and 60 weeks (wo) were tested over 8 weeks for open field activity, sucrose preference, social interactions, fear conditioning, and functional neuroimaging. Prefrontal cortical and hippocampal tissues were excised for metabolomics.

Results

Our results indicate that young and old mice differ significantly in affective behavioral, functional connectome and prefrontal cortical-hippocampal metabolome. Young mice show a greater responsivity to novel environmental and social stimuli compared to older mice. Conversely, late middle-aged mice (60wo group) display variable patterns of fear conditioning and during re-testing in a modified context. Functional connectivity between a temporal cortical/auditory cortex network and subregions of the anterior cingulate cortex and ventral hippocampus, and a greater network modularity and assortative mixing of nodes was stronger in young versus older adult mice. Metabolome analyses identified differences in several essential amino acids between 10wo mice and the other age groups.

Discussion

The results support differential expression of 'emotionality' across distinct stages of the mouse lifespan involving greater prefrontal-hippocampal connectivity and neurochemistry.

Mifepristone decreases nicotine intake in dependent and non-dependent adult rats

BACKGROUND

Addiction to tobacco and nicotine products has adverse health effects and afflicts more than a billion people worldwide. Therefore, there is an urgent need for new treatments to reduce tobacco and nicotine use. Glucocorticoid receptor blockade shows promise as a novel treatment for drug abuse and stress-related disorders.

AIM

These studies aim to investigate whether glucocorticoid receptor blockade with mifepristone diminishes the reinforcing properties of nicotine in rats with intermittent or daily long access to nicotine.

METHODS

The rats self-administered 0.06 mg/kg/inf of nicotine for 6 h per day, with either intermittent or daily access for 4 weeks before treatment with mifepristone. Daily nicotine self-administration models regular smoking, while intermittent nicotine self-administration models occasional smoking. To determine whether the rats were dependent, they were treated with the nicotinic acetylcholine receptor antagonist mecamylamine, and somatic signs were recorded.

RESULTS

The rats with intermittent access to nicotine had a higher level of nicotine intake per session than those with daily access but only the rats with daily access to nicotine showed signs of physical dependence. Furthermore, mecamylamine increased nicotine intake during the first hour of access in rats with daily access but not in those with intermittent access. Mifepristone decreased total nicotine intake in rats with intermittent and daily access to nicotine. Moreover, mifepristone decreased the distance traveled and rearing in the open field test and operant responding for food pellets.

CONCLUSION

These findings indicate that mifepristone decreases nicotine intake but this effect may be partially attributed to the sedative effects of mifepristone.

Age-Related Differences in Affective Behaviors in Mice: Possible Role of Prefrontal Cortical-Hippocampal Functional Connectivity and Metabolomic Profiles

The differential expression of emotional reactivity from early to late adulthood may involve maturation of prefrontal cortical responses to negative valence stimuli. In mice, age-related changes in affective behaviors have been reported, but the functional neural circuitry warrants further investigation. We assessed age variations in affective behaviors and functional connectivity in male and female C57BL6/J mice. Mice aged 10, 30 and 60 weeks (wo) were tested over 8 weeks for open field activity, sucrose preference, social interactions, fear conditioning, and functional neuroimaging. Prefrontal cortical and hippocampal tissues were excised for metabolomics. Our results indicate that young and old mice differ significantly in affective behavioral, functional connectome and prefrontal cortical-hippocampal metabolome. Young mice show a greater responsivity to novel environmental and social stimuli compared to older mice. Conversely, late middle-aged mice (60wo group) display variable patterns of fear conditioning and with re-testing with a modified context. Functional connectivity between a temporal cortical/auditory cortex network and subregions of the anterior cingulate cortex and ventral hippocampus, and a greater network modularity and assortative mixing of nodes was stronger in young versus older adult mice. Metabolome analyses identified differences in several essential amino acids between 10wo mice and the other age groups. The results support differential expression of 'emotionality' across distinct stages of the mouse lifespan involving greater prefrontal-hippocampal connectivity and neurochemistry.

Prevalence of Underreported Nicotine Exposure Among US Nonsmoking Adults: A Comparison of Self-Reported Exposure and Serum Cotinine Levels From NHANES 2013-2020

INTRODUCTION

Secondhand smoke (SHS) poses a significant health risk. However, individuals who do not smoke may be unaware of their exposure, thereby failing to take protective actions promptly.

AIMS AND METHODS

We assessed the prevalence of underreported nicotine exposure in a nationally representative sample of US nonsmoking adults using data from the US National Health and Examination Survey. Individuals with underreported nicotine exposure were defined as those who reported no exposure to all tobacco products (traditional tobacco, nicotine replacements, and e-cigarettes) or SHS, yet had detectable levels of serum cotinine (>0.015 ng/mL). We fitted logistic regression models to determine sociodemographic and chronic condition factors associated with underreported nicotine exposure.

RESULTS

Our analysis included 13 503 adults aged 18 years and older. Between 2013 and 2020, the prevalence of self-reported SHS exposure, serum cotinine-assessed nicotine exposure, and underreported nicotine exposure among US nonsmokers were 22.0%, 51.2%, and 34.6%, respectively. Remarkably, 67.6% with detectable serum cotinine reported no SHS exposure. Males, non-Hispanic blacks, individuals of other races (including Asian Americans, Native Americans, and Pacific Islanders), and those without cardiovascular diseases were more likely to underreport nicotine exposure than their counterparts. The median serum cotinine value was higher in respondents who reported SHS exposure (0.107 ng/mL) than in those who reported no exposure (0.035 ng/mL). We estimate that approximately 56 million US residents had underreported nicotine exposure.

CONCLUSIONS

Over a third of US nonsmokers underreport their nicotine exposure, underlining the urgent need for comprehensive public awareness campaigns and interventions. Further research into sociodemographic determinants influencing this underreporting is needed.

IMPLICATIONS

Understanding the extent of underreported nicotine exposure is crucial for developing effective public health strategies and interventions. It is imperative to bolster public consciousness about the risks associated with SHS. Additionally, surveillance tools should also incorporate measures of exposure to outdoor SHS and e-cigarette vapor to enhance the quality of data monitoring. Findings from this study can guide tobacco control initiatives and inform smoke-free air legislation.

Dopamine D1-like receptor activation decreases nicotine intake in rats with short or long access to nicotine

The use of nicotine and tobacco products is highly addictive. The dopaminergic system plays a key role in the initiation and maintenance of nicotine intake. Dopamine D1-like receptor blockade diminishes nicotine intake in rats with daily short (1 h) access to nicotine, but little is known about the effects of dopamine receptor antagonists or agonists on nicotine intake in rats with intermittent long (23 h) access. Because of the extended access conditions and high nicotine intake, the intermittent long access procedure might model smoking and vaping better than short access models. We investigated the effects of the dopamine D1-like receptor antagonist SCH 23390 and the D1-like receptor agonist A77636 on nicotine intake in male rats with intermittent short or long access to nicotine. The rats self-administered nicotine for 5 days (1 h/day) and were then given 15 intermittent short (1 h/day) or long (23 h/day) access sessions (3 sessions/week, 0.06 mg/kg/inf). The D1-like receptor antagonist SCH 23390 decreased nicotine intake to a similar degree in rats with short or long access to nicotine. The D1-like receptor agonist A77636 induced a greater decrease in nicotine intake in the rats with long access to nicotine than in rats with short access. Treatment with A77636 induced a prolonged decrease in nicotine intake that lasted throughout the dark and light phase in the long access rats. These findings indicate that blockade and stimulation of D1-like receptors decrease nicotine intake in an intermittent long access animal model that closely models human smoking and vaping.

An ethogram analysis of cutaneous thermal pain sensitivity and oxycodone reward-related behaviors in rats

Inter-relationships between pain sensitivity, drug reward, and drug misuse are of considerable interest given that many analgesics exhibit misuse potential. Here we studied rats as they underwent a series of pain- and reward-related tests: cutaneous thermal reflex pain, induction and extinction of conditioned place preference to oxycodone (0.56 mg/kg), and finally the impact of neuropathic pain on reflex pain and reinstatement of conditioned place preference. Oxycodone induced a significant conditioned place preference that extinguished throughout repeated testing. Correlations identified of particular interest included an association between reflex pain and oxycodone-induced behavioral sensitization, and between rates of behavioral sensitization and extinction of conditioned place preference. Multidimensional scaling analysis followed by k-clustering identified three clusters: (1) reflex pain, rate of behavioral sensitization and rate of extinction of conditioned place preference (2) basal locomotion, locomotor habituation, acute oxycodone-stimulated locomotion and rate of change in reflex pain during repeated testing, and (3) magnitude of conditioned place preference. Nerve constriction injury markedly enhanced reflex pain but did not reinstate conditioned place preference. These results suggest that high rates of behavioral sensitization predicts faster rates of extinction of oxycodone seeking/reward, and suggest that cutaneous thermal reflex pain may be predictive of both.

Development of Dependence in Smokers and Rodents With Voluntary Nicotine Intake: Similarities and Differences

INTRODUCTION

Smoking and vaping throughout adolescence and early adulthood lead to nicotine dependence. Nicotine withdrawal is associated with somatic and affective withdrawal symptoms that contribute to smoking and relapse. Affective nicotine withdrawal symptoms in humans include craving for cigarettes, depression, anxiety, trouble sleeping, and cognitive deficits.

METHODS

Herein, we review clinical studies that investigated nicotine dependence in people who smoke or vape. We also discuss studies that investigated the development of dependence in animals with oral nicotine intake, nicotine aerosol self-administration, and intravenous nicotine self-administration.

RESULTS

Clinical studies report that adolescents who smoke daily develop nicotine dependence before those who smoke infrequently, but ultimately all smokers become dependent in adulthood. Preclinical studies indicate that rats that self-administer nicotine also become dependent. Rats that self-administer nicotine display somatic withdrawal signs and affective withdrawal signs, including increased anxiety and depressive-like behavior, cognitive deficits, and allodynia. Most nicotine withdrawal signs were observed in rodents with daily (7 days/week) or intermittent long access (23-hour) to nicotine. Clinical smoking studies report symptoms of nicotine dependence in adolescents of both sexes, but virtually all preclinical nicotine self-administration studies have been done with adult male rats.

CONCLUSIONS

The role of sex and age in the development of dependence in nicotine self-administration studies remains under-investigated. However, the role of sex and age in nicotine withdrawal has been thoroughly evaluated in studies in which nicotine was administered noncontingently. We discuss the need for volitional nicotine self-administration studies that explore the gradual development of dependence during adolescence and adulthood in rodents of both sexes.

IMPLICATIONS

The reviewed clinical studies investigated the development of nicotine dependence in male and female adolescent and young adult smokers and vapers. These studies indicate that most adolescent smokers and vapers gradually become nicotine dependent. Preclinical studies with rodents show that nicotine intake in widely used self-administration models also leads to dependence. However, almost all animal studies that investigated the development of nicotine dependence have been conducted with adult male rats. To better model smoking and vaping, it is important that nicotine intake in rats or mice starts during adolescence and that both sexes are included.

Opportunities and Challenges of Kava in Lung Cancer Prevention

Lung cancer is the leading cause of cancer-related deaths due to its high incidence, late diagnosis, and limited success in clinical treatment. Prevention therefore is critical to help improve lung cancer management. Although tobacco control and tobacco cessation are effective strategies for lung cancer prevention, the numbers of current and former smokers in the USA and globally are not expected to decrease significantly in the near future. Chemoprevention and interception are needed to help high-risk individuals reduce their lung cancer risk or delay lung cancer development. This article will review the epidemiological data, pre-clinical animal data, and limited clinical data that support the potential of kava in reducing human lung cancer risk via its holistic polypharmacological effects. To facilitate its future clinical translation, advanced knowledge is needed with respect to its mechanisms of action and the development of mechanism-based non-invasive biomarkers in addition to safety and efficacy in more clinically relevant animal models.

An ethogram analysis of cutaneous thermal pain sensitivity and oxycodone reward-related behaviors in rats

Inter-relationships between pain sensitivity, drug reward, and drug misuse are of considerable interest given that many analgesics exhibit misuse potential. Here we studied rats as they underwent a series of pain- and reward-related tests: cutaneous thermal reflex pain, induction and extinction of conditioned place preference to oxycodone (0.56 mg/kg), and finally the impact of neuropathic pain on reflex pain and reinstatement of conditioned place preference. Oxycodone induced a significant conditioned place preference that was extinguished throughout repeated testing. Correlations identified of particular interest included an association between reflex pain and oxycodone-induced behavioral sensitization, and between rates of behavioral sensitization and extinction of conditioned place preference. Multidimensional scaling analysis followed by k-clustering identified three clusters: (1) reflex pain and the rate of change in reflex pain response throughout repeated testing, (2) basal locomotion, locomotor habituation, and acute oxycodone-stimulated locomotion, and (3) behavioral sensitization, strength of conditioned place preference, and rate of extinction. Nerve constriction injury markedly enhanced reflex pain but did not reinstate conditioned place preference. These results support the notion that behavioral sensitization relates to the acquisition and extinction of oxycodone seeking/reward, but suggest that generally cutaneous thermal reflex pain poorly predicts oxycodone reward-related behaviors except for behavioral sensitization.

The D1/D2-like receptor antagonist flupentixol and the D2-like receptor antagonist L-741626 decrease operant responding for nicotine and food and locomotor activity in male and female rats

BACKGROUND

The reinforcing properties of nicotine play a critical role in smoking and vaping. There is a need for treatments that decrease the reinforcing properties of nicotine and thereby improve smoking and vaping rates. Dopamine plays a role in the reinforcing properties of nicotine, but little is known about the role of dopamine D2-like receptors in nicotine intake and whether there are sex differences in the effects of dopaminergic drugs on nicotine intake.

AIM

The goal of the present studies was to investigate the effects of the D1/D2-like receptor antagonist flupentixol and the D2-like receptor antagonist L-741626 on nicotine self-administration in male and female rats.

METHODS

The effects of flupentixol and L-741626 on operant responding for nicotine and food and locomotor activity in a small open field were investigated.

RESULTS

There were no sex differences in baseline nicotine intake. The D1/D2-like receptor antagonist flupentixol and the D2-like receptor antagonist L-741626 decreased operant responding for nicotine. Blockade of D1/D2-like receptors and blockade of D2-like receptors also decreased operant responding for food and decreased locomotor activity. Flupentixol induced a greater decrease in operant responding for food in males than females. However, in the other tests, there were no sex differences in the effects of the dopamine receptor antagonists.

CONCLUSIONS

Blockade of D1/D2-like receptors with flupentixol and D2-like receptors with L-741626 decreases nicotine and food intake in rats of both sexes. These compounds also decrease locomotor activity which might be indicative of a sedative effect.

Oxycodone decreases anxiety-like behavior in the elevated plus-maze test in male and female rats

The prescription opioid oxycodone is widely used for the treatment of pain in humans. Oxycodone misuse is more common among people with an anxiety disorder than those without one. Therefore, oxycodone might be misused for its anxiolytic properties. We investigated if oxycodone affects anxiety-like behavior in adult male and female rats. The rats were treated with oxycodone (0.178, 0.32, 0.56, or 1 mg/kg), and anxiety-like behavior was investigated in the elevated plus-maze test. Immediately after the elevated plus-maze test, a small open field test was conducted to determine the effects of oxycodone on locomotor activity. In the elevated plus-maze test, oxycodone increased the percentage of time spent on the open arms, the percentage of open arm entries, time on the open arms, open arm entries, and the distance traveled. The males treated with vehicle had a lower percentage of open arm entries than the females treated with vehicle, and oxycodone treatment led to a greater increase in the percentage of open arm entries in the males than females. Furthermore, the females spent more time on the open arms, made more open arm entries, spent less time in the closed arms, and traveled a greater distance than the males. In the small open field test, treatment with oxycodone did not affect locomotor activity or rearing. Sex differences were observed; the females traveled a greater distance and displayed more rearing than the males. In conclusion, oxycodone decreases anxiety-like behavior in rats, and oxycodone has a greater anxiolytic-like effect in males than females.

Dopamine D1-like receptor blockade and stimulation decreases operant responding for nicotine and food in male and female rats

Dopamine has been implicated in the reinforcing effects of smoking. However, there remains a need for a better understanding of the effects of dopamine D1-like receptor agonists on nicotine intake and the role of sex differences in the effects of dopaminergic drugs on behavior. This work studied the effects of D1-like receptor stimulation and blockade on operant responding for nicotine and food and locomotor activity in male and female rats. The effects of the D1-like receptor antagonist SCH 23390 (0.003, 0.01, 0.03 mg/kg) and the D1-like receptor agonist A77636 (0.1, 0.3, 1 mg/kg) on responding for nicotine and food, and locomotor activity were investigated. The effects of SCH 23390 were investigated 15 min and 24 h after treatment, and the effects of the long-acting drug A77636 were investigated 15 min, 24 h, and 48 h after treatment. Operant responding for nicotine and food and locomotor activity were decreased immediately after treatment with SCH 23390. Treatment with SCH 23390 did not have any long-term effects. Operant responding for nicotine was still decreased 48 h after treatment with A77636, and food responding was decreased up to 24 h after treatment. Treatment with A77636 only decreased locomotor activity at the 48 h time point. There were no sex differences in the effects of SCH 23390 or A77636. In conclusion, the D1-like receptor antagonist SCH 23390 reduces nicotine intake and causes sedation in rats. Stimulation of D1-like receptors with A77636 decreases nicotine intake at time points that the drug does not cause sedation.

Rewarding Effects of Nicotine Self-administration Increase Over Time in Male and Female Rats

INTRODUCTION

Smoking and the use of other nicotine-containing products is rewarding in humans. The self-administration of nicotine is also rewarding in male rats. However, it is unknown if there are sex differences in the reward-enhancing effects of nicotine self-administration and if the rewarding effects of nicotine change over time.

METHODS

Rats were prepared with catheters and intracranial self-stimulation (ICSS) electrodes to investigate the effects of nicotine and saline self-administration on reward function. A decrease in thresholds in the ICSS procedure reflects an enhancement of reward function. The ICSS parameters were determined before and after the self-administration sessions from days 1 to 10, and after the self-administration sessions from days 11 to 15.

RESULTS

During the first 10 days, there was no sex difference in nicotine intake, but during the last 5 days, the females took more nicotine than the males. During the first 10 days, nicotine self-administration did not lower the brain reward thresholds but decreased the response latencies. During the last 5 days, nicotine lowered the reward thresholds and decreased the response latencies. An analysis with the 5-day averages (days 1-5, 6-10, and 11-15) showed that the reward enhancing and stimulatory effects of nicotine increased over time. There were no sex differences in the reward-enhancing and stimulatory effects of nicotine. The nicotinic receptor antagonist mecamylamine diminished the reward-enhancing and stimulatory effects of nicotine.

CONCLUSION

These findings indicate that the rewarding effects of nicotine self-administration increase over time, and there are no sex differences in the reward-enhancing effects of nicotine self-administration in rats.

IMPLICATIONS

This study investigated the rewarding effect of nicotine and saline self-administration in male and female rats. The self-administration of nicotine, but not saline, enhanced brain reward function and had stimulatory effects. The rewarding effects of nicotine increased over time in the males and the females. Despite that the females had a higher level of nicotine intake than the males, the reward-enhancing effects of nicotine self-administration were the same. These findings suggest that in new tobacco and e-cigarette users, nicotine's rewarding effects might increase quickly, and a higher level of nicotine use in females might not translate into greater rewarding effects.

Rodent models for nicotine withdrawal

BACKGROUND

Animal models are critical to improve our understanding of the neuronal mechanisms underlying nicotine withdrawal. Nicotine dependence in rodents can be established by repeated nicotine injections, chronic nicotine infusion via osmotic minipumps, oral nicotine intake, tobacco smoke exposure, nicotine vapor exposure, and e-cigarette aerosol exposure. The time course of nicotine withdrawal symptoms associated with these methods has not been reviewed in the literature.

AIM

The goal of this review is to discuss nicotine withdrawal symptoms associated with the cessation of nicotine, tobacco smoke, nicotine vapor, and e-cigarette aerosol exposure in rats and mice. Furthermore, age and sex differences in nicotine withdrawal symptoms are reviewed.

RESULTS

Cessation of nicotine, tobacco smoke, nicotine vapor, and e-cigarette aerosol exposure leads to nicotine withdrawal symptoms such as somatic withdrawal signs, changes in locomotor activity, anxiety- and depressive-like behavior, learning and memory deficits, attention deficits, hyperalgesia, and dysphoria. These withdrawal symptoms are most pronounced within the first week after cessation of nicotine exposure. Anxiety- and depressive-like behavior, and deficits in learning and memory may persist for several months. Adolescent (4-6 weeks old) rats and mice display fewer nicotine withdrawal symptoms than adults (>8 weeks old). In adult rats and mice, females show fewer nicotine withdrawal symptoms than males. The smoking cessation drugs bupropion and varenicline reduce nicotine withdrawal symptoms in rodents.

CONCLUSION

The nicotine withdrawal symptoms that are observed in rodents are similar to those observed in humans. Tobacco smoke and e-cigarette aerosol contain chemicals and added flavors that enhance the reinforcing properties of nicotine. Therefore, more valid animal models of tobacco and e-cigarette use need to be developed by using tobacco smoke and e-cigarette aerosol exposure methods to induce dependence.

Effects of repeated adolescent exposure to cannabis smoke on cognitive outcomes in adulthood

BACKGROUND

Cannabis (marijuana) is the most widely used illicit drug in the USA, and consumption among adolescents is rising. Some animal studies show that adolescent exposure to delta 9-tetrahydrocannabinol or synthetic cannabinoid receptor 1 agonists causes alterations in affect and cognition that can persist into adulthood. It is less clear, however, whether similar alterations result from exposure to cannabis via smoke inhalation, which remains the most frequent route of administration in humans.

AIMS

To begin to address these questions, a rat model was used to determine how cannabis smoke exposure during adolescence affects behavioral and cognitive outcomes in adulthood.

METHODS

Adolescent male Long-Evans rats were assigned to clean air, placebo smoke, or cannabis smoke groups. Clean air or smoke exposure sessions were conducted daily during adolescence (from P29-P49 days of age ) for a total of 21 days, and behavioral testing began on P70.

RESULTS

Compared to clean air and placebo smoke conditions, cannabis smoke significantly attenuated the normal developmental increase in body weight, but had no effects on several measures of either affect/motivation (open field activity, elevated plus maze, instrumental responding under a progressive ratio schedule of reinforcement) or cognition (set shifting, reversal learning, intertemporal choice). Surprisingly, however, in comparison to clean air controls rats exposed to either cannabis or placebo smoke in adolescence exhibited enhanced performance on a delayed response working memory task.

CONCLUSIONS

These findings are consistent with a growing body of evidence for limited long-term adverse cognitive and affective consequences of adolescent exposure to relatively low levels of cannabinoids.

Tobacco smoke exposure enhances reward sensitivity in male and female rats

RATIONALE

Systemic administration of the tobacco smoke constituent nicotine stimulates brain reward function in rats. However, it is unknown if the inhalation of tobacco smoke affects brain reward function.

OBJECTIVES

These experiments investigated if exposure to smoke from high-nicotine SPECTRUM research cigarettes increases reward function and affects the rewarding effects of nicotine in adult male and female Wistar rats.

METHODS

Reward function after smoke or nicotine exposure was investigated using the intracranial self-stimulation (ICSS) procedure. A decrease in reward thresholds reflects an increase in reward function. In the first experiment, the rats were exposed to tobacco smoke for 40 min/day for 9 days, and the rewarding effects of nicotine (0.03-0.6 mg/kg) were investigated 3 weeks later. In the second experiment, the dose effects of tobacco smoke exposure (40-min sessions, 1-4 cigarettes burnt simultaneously) on reward function were investigated.

RESULTS

Tobacco smoke exposure did not affect the nicotine-induced decrease in reward thresholds or response latencies in male and female rats. Smoke exposure lowered the brain reward thresholds to a similar degree in males and females and caused a greater decrease in latencies in females. There was a positive relationship between plasma nicotine and cotinine levels and the nicotine content of the SPECTRUM research cigarettes. Similar smoke exposure conditions led to higher plasma nicotine and cotinine levels in female than male rats.

CONCLUSION

These findings indicate that tobacco smoke exposure enhances brain reward function but does not potentiate the rewarding effects of nicotine in male and female rats.

Evaluation of Sex Differences in the Elasticity of Demand for Nicotine and Food in Rats

INTRODUCTION

Animal studies can inform policy regarding nicotine levels in tobacco products and e-cigarette solutions. Increasing the price of nicotine-containing products decreases their use, but it is unknown how the relationship between price and consumption is affected by both sex and nicotine dose.

METHODS

A behavioral economics procedure was used to determine the demand elasticity for nicotine in male and female rats. Demand elasticity describes the relationship between price and consumption. A high level of elasticity indicates that consumption is relatively sensitive to increases in price. The rats self-administered a low dose (0.01 mg/kg/inf) or a standard dose (0.03 mg/kg/inf) of nicotine for 9 days under a fixed-ratio (FR) 1 schedule. Then the price (FR schedule) of nicotine was increased, and a demand analysis was conducted. A similar study was conducted with palatable food pellets.

RESULTS

There were no sex differences in nicotine or food intake under the FR1 schedule. However, demand for 0.03 mg/kg/inf of nicotine was more elastic in females than males. Demand for 0.01 mg/kg/inf of nicotine and food was more elastic in males than females.

CONCLUSIONS

These findings indicate that there are no differences in nicotine and food intake between males and females when the price is low. When the price of nicotine or food is increased, males maintain their old level of intake longer than females when they have access to a standard dose of nicotine, and females maintain their intake longer when they have access to a low dose of nicotine or food.

IMPLICATIONS

This behavioral economics analysis indicates that there is no sex difference in nicotine intake when the price of nicotine is low. Increasing the price of nicotine decreases nicotine intake in a dose- and sex-specific manner. Males maintain their old level of intake longer when they have access to a standard dose of nicotine and females when they have access to a low dose. This has implications for tobacco regulatory policy. In a regulatory environment where only low nicotine-containing products are allowed, increasing the price of nicotine products may lead to a greater decrease in nicotine use in males than females.

Evaluation of the rewarding effects of mitragynine and 7-hydroxymitragynine in an intracranial self-stimulation procedure in male and female rats

BACKGROUND

Kratom (Mitragyna speciosa Korth.) has been used in Southeast Asia for hundreds of years to increase energy, for relaxation, and to diminish opioid withdrawal. Kratom use has recently spread to Western countries. Kratom could potentially be used for the treatment of opioid withdrawal and pain, but more insight is needed into its abuse potential. Therefore, we investigated the rewarding properties of the primary kratom alkaloid mitragynine and its active metabolite 7-hydroxymitragynine, and morphine as a reference drug in male and female rats. These compounds have agonist activity at mu-opioid receptors.

METHODS

The compounds were tested in an intracranial self-stimulation (ICSS) procedure, which allows for the evaluation of the rewarding/aversive and sedative effects of drugs. Rewarding doses of drugs decrease the brain reward thresholds, and aversive drug doses have the opposite effect.

RESULTS

Mitragynine, 7-hydroxymitragynine, and morphine affected the brain reward thresholds. A high dose of 7-hydroxymitragynine (3.2 mg/kg) increased the brain reward thresholds, whereas an intermediate dose of morphine (10 mg/kg) decreased the reward thresholds. 7-Hydroxymitragynine and morphine affected the response latencies. Five mg/kg of morphine increased response latencies. 7-Hydroxymitragynine tended to increase the response latencies, but the post hoc analyses did not reveal a significant effect. There were no sex differences in the effects of mitragynine, 7-hydroxymitragynine, and morphine on the reward thresholds and the response latencies.

CONCLUSIONS

These initial findings indicate that mitragynine and 7-hydroxymitragynine are not rewarding in the ICSS procedure. The present results suggest that these kratom alkaloids do not have abuse potential.

Relationship Between Nicotine Intake and Reward Function in Rats With Intermittent Short Versus Long Access to Nicotine

INTRODUCTION

Tobacco use improves mood states and smoking cessation leads to anhedonia, which contributes to relapse. Animal studies have shown that noncontingent nicotine administration enhances brain reward function and leads to dependence. However, little is known about the effects of nicotine self-administration on the state of the reward system.

METHODS

To investigate the relationship between nicotine self-administration and reward function, rats were prepared with intracranial self-stimulation electrodes and intravenous catheters. The rats were trained on the intracranial self-stimulation procedure and allowed to self-administer 0.03 mg/kg/infusion of nicotine. All rats self-administered nicotine daily for 10 days (1 hour/day) and were then switched to an intermittent short access (ShA, 1 hour/day) or long access (LgA, 23 hour/day) schedule (2 days/week, 5 weeks).

RESULTS

During the first 10 daily, 1-hour sessions, nicotine self-administration decreased the reward thresholds, which indicates that nicotine potentiates reward function. After switching to the intermittent LgA or ShA schedule, nicotine intake was lower in the ShA rats than the LgA rats. The LgA rats increased their nicotine intake over time and they gradually consumed a higher percentage of their nicotine during the light phase. The nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine induced a larger increase in reward thresholds (ie, anhedonia) in the LgA rats than the ShA rats. In the LgA rats, nAChR blockade with mecamylamine decreased nicotine intake for 2 hours and this was followed by a rebound increase in nicotine intake.

CONCLUSIONS

A brief period of nicotine self-administration enhances reward function and a high level of nicotine intake leads to dependence.

IMPLICATIONS

These animal studies indicate that there is a strong relationship between the level of nicotine intake and brain reward function. A high level of nicotine intake was more rewarding than a low level of nicotine intake and nicotine dependence was observed after long, but not short, access to nicotine. This powerful combination of nicotine reward and withdrawal makes it difficult to quit smoking. Blockade of nAChRs temporarily decreased nicotine intake, but this was followed by a large rebound increase in nicotine intake. Therefore, nAChR blockade might not decrease the use of combustible cigarettes or electronic cigarettes.

Rewarding Effects of Nicotine in Adolescent and Adult Male and Female Rats as Measured Using Intracranial Self-stimulation

INTRODUCTION

Tobacco is highly addictive, and after the development of dependence, it is difficult to quit smoking. Therefore, it is important to understand the factors that play a role in the initiation of smoking. The rewarding effects of nicotine play a role in the initiation of smoking and the goal of the present study was to determine the rewarding effects of nicotine in adolescent and adult male and female rats.

METHODS

Male and female Wistar rats were prepared with intracranial self-stimulation (ICSS) electrodes between postnatal day (P) 23 and 33. They were then trained on the ICSS procedure and the effect of nicotine (0, 0.03, 0.1, 0.3 mg/kg) on the reward thresholds and response latencies was investigated during adolescence (P40-59) or adulthood (>P75).

RESULTS

Nicotine lowered the brain reward thresholds of the adult and adolescent male and female rats. The nicotine-induced decrease in the reward thresholds was the same in the adult male and adult female rats. However, nicotine induced a greater decrease in the reward thresholds of the adolescent female rats than the adolescent male rats. Nicotine decreased the response latencies of all groups and there was no effect of age or sex.

CONCLUSIONS

Nicotine enhances reward function and psychomotor performance in adolescent and adult male and female rats. Adolescent female rats are more sensitive to the acute rewarding effects of nicotine than adolescent male rats. Therefore, the rewarding effects of nicotine might play a greater role in the initiation of smoking in adolescent females than in adolescent males.

IMPLICATIONS

The great majority of people start smoking during adolescence. The present studies suggest that during this period female rats are more sensitive to the acute rewarding effects of low and intermediate doses of nicotine than male rats. The rewarding properties of nicotine play a role in the initiation of smoking and establishing habitual smoking. Therefore, the present findings might explain why adolescent females are at a higher risk for becoming nicotine dependent than adolescent males.

Adolescent nicotine and tobacco smoke exposure enhances nicotine self-administration in female rats

Most people start experimenting with tobacco products or e-cigarettes in early adolescence and become habitual smokers in late adolescence or adulthood. These studies investigated if exposure to tobacco smoke or nicotine during early and mid-adolescence affects nicotine intake in late adolescence and early adulthood. Male and female rats were exposed to tobacco smoke from low- and high-nicotine SPECTRUM cigarettes or nicotine (0.3 mg/kg, twice a day) from postnatal day (P) 24-42. The self-administration sessions started at P55. The rats self-administered nicotine for 14-15 days under a fixed-ratio 1 schedule, and on the first day, the maximum number of infusions was twenty. Exposure to smoke from high, but not low, nicotine cigarettes during adolescence increased nicotine self-administration in female but not male rats. Adolescent treatment with nicotine facilitated nicotine self-administration. On the first day of nicotine self-administration, nicotine-treated rats reached the maximum number of infusions before the saline-treated control rats. Nicotine intake was also higher in the nicotine-treated female rats than in the saline-treated females. There was no sex difference in nicotine intake in controls when the data from the studies were combined. Smoke exposure led to a dose-dependent increase in plasma nicotine and cotinine levels in adolescent rats. Exposure to smoke from high-nicotine cigarettes and 0.3 mg/kg of nicotine led to plasma nicotine and cotinine levels that are similar to those in tobacco users. These findings indicate that in females, but not males, exposure to nicotine during adolescence may facilitate smoking and e-cigarette use later in life.

Exposure to smoke from high- but not low-nicotine cigarettes leads to signs of dependence in male rats and potentiates the effects of nicotine in female rats

Nicotine is only mildly rewarding, but after becoming dependent, it is difficult to quit smoking. The goal of these studies was to determine if low-nicotine cigarettes are less likely to cause dependence and enhance the reinforcing effects of nicotine than regular high-nicotine cigarettes. Male and female rats were exposed to tobacco smoke with a low or high nicotine level for 35 days. It was investigated if smoke exposure affects the development of dependence, anxiety- and depressive-like behavior, and nicotine-induced behavioral sensitization. Smoke exposure did not affect locomotor activity in a small open field or sucrose preference. Mecamylamine precipitated somatic withdrawal signs in male rats exposed to smoke with a high level of nicotine, but not in male rats exposed to smoke with a low level of nicotine or in females. After cessation of smoke exposure, there was a small decrease in sucrose preference in the male rats, which was not observed in the females. Cessation of smoke exposure did not affect anxiety-like behavior in the large open field or the elevated plus maze test. Female rats displayed less anxiety-like behavior in both these tests. Repeated treatment with nicotine increased locomotor activity, rearing, and stereotypies. Prior exposure to smoke with a high level of nicotine increased nicotine-induced rearing in the females. These findings indicate that exposure to smoke with a low level of nicotine does not lead to dependence and does not potentiate the effects of nicotine. Exposure to smoke with a high level of nicotine differently affects males and females.

Overexpression of corticotropin-releasing factor in the nucleus accumbens enhances the reinforcing effects of nicotine in intact female versus male and ovariectomized female rats

This study assessed the role of stress systems in the nucleus accumbens (NAc) in promoting sex differences in the reinforcing effects of nicotine. Intravenous self-administration (IVSA) of various doses of nicotine was compared following overexpression of corticotropin-releasing factor (CRF) in the NAc of female and male rats. Ovariectomized (OVX) females were also included to assess the role of ovarian hormones in promoting nicotine reinforcement. Rats received intra-NAc administration of an adeno-associated vector that overexpressed CRF (AAV2/5-CRF) or green fluorescent protein (AAV2/5-GFP). All rats were then given extended access (23 h/day) to an inactive and an active lever that delivered nicotine. Separate groups of rats received intra-NAc AAV2/5-CRF and saline IVSA. Rats were also allowed to nose-poke for food and water during IVSA testing. At the end of the study, the NAc was dissected and rt-qPCR methods were used to estimate CRF overexpression and changes in CRF receptors (CRFr1, CRFr2) and the CRF receptor internalizing protein, β-arrestin2 (Arrb2). Overexpression of CRF in the NAc increased nicotine IVSA to a larger extent in intact female versus male and OVX females. Food intake was increased to a larger extent in intact and OVX females as compared to males. The increase in CRF gene expression was similar across all groups; however, in females, overexpression of CRF resulted in a larger increase in CRFr1 and CRFr2 relative to males. In males, overexpression of CRF produced a larger increase in Arrb2 than females, suggesting greater CRF receptor internalization. Our results suggest that stress systems in the NAc promote the reinforcing effectiveness of nicotine in female rats in an ovarian hormone-dependent manner.

Pharmacokinetic and Pharmacodynamic Characterization of Tetrahydrocannabinol-Induced Cannabinoid Dependence After Chronic Passive Cannabis Smoke Exposure in Rats

Introduction: Cannabis is the most widely used illicit drug in the US, and cannabis use among young adults continues to rise. Previous studies have shown that chronic administration of delta 9-tetrahydrocannabinol (THC), the main psychoactive component of cannabis, induces dependence in animal models. Because smoking is the most frequent route of THC self-administration, it is critical to investigate the effects of cannabis smoke inhalation. The goal of the current study was to develop a rat model to characterize the pharmacokinetics (PKs) of THC after cannabis smoke inhalation, and to determine if chronic cannabis smoke inhalation leads to the development of cannabis dependence. Materials and Methods: For the PK study, male Wistar rats were administered THC intravenously (1 mg/kg) or exposed to smoke from 5 or 10 sequentially smoked cannabis cigarettes (5.3% THC) in an automated smoking machine. Plasma samples were collected from 10 min to 10 hours post smoke exposure (or intravenous administration) and analyzed using liquid chromatography-mass spectrometry to characterize the PK of THC. A three-compartment PK model was used to characterize the PKs. In a separate study, three groups of male Wistar rats were trained in an intracranial self-stimulation (ICSS) procedure, and exposed to smoke from burning 5 or 10 cannabis cigarettes (or clean air control conditions), 5 days/week for 4 weeks. Discussion and Conclusions: Across exposure days, the change from baseline in ICSS thresholds for cannabis smoke-exposed groups was significantly lower and response latencies were significantly faster in the cannabis smoke-exposed groups compared to controls, suggesting that chronic cannabis smoke exposure has rewarding properties. Acute administration of the CB1 receptor antagonist rimonabant (0.3, 1.0, 3.0 mg/kg) induced a dose-dependent increase in ICSS thresholds in the smoke-exposed rats, suggestive of dependence and withdrawal. Finally, an effect compartment PK-pharmacodynamic model was used to describe the relationship between THC concentrations and changes in ICSS thresholds after cannabis smoke exposure.

Sex differences in the reward deficit and somatic signs associated with precipitated nicotine withdrawal in rats

Female smokers are more likely to relapse than male smokers, but little is known about sex differences in nicotine withdrawal. Therefore, male and female rats were prepared with minipumps that contained nicotine or saline and sex differences in precipitated and spontaneous nicotine withdrawal were investigated. The intracranial self-stimulation (ICSS) procedure was used to assess mood states. Elevations in brain reward thresholds reflect a deficit in reward function. Anxiety-like behavior was investigated after the acute nicotine withdrawal phase in a large open field and the elevated plus maze test. The nicotinic receptor antagonist mecamylamine elevated the brain reward thresholds of the nicotine-treated rats but did not affect those of the saline-treated control rats. A low dose of mecamylamine elevated the brain reward thresholds of the nicotine-treated male rats but not those of the females. Mecamylamine also precipitated more somatic withdrawal signs in the nicotine-treated male than female rats. Minipump removal elevated the brain reward thresholds of the nicotine-treated rats for about 36 h but did not affect those of the saline-treated rats. There was no sex difference in the reward deficit during spontaneous nicotine withdrawal. In addition, the nicotine-treated male and female rats did not display increased anxiety-like behavior three to four days after minipump removal. In conclusion, these studies suggest that relatively low doses of a nicotinic receptor antagonist induce a greater reward deficit and more somatic withdrawal signs in male than female rats, but there is no sex difference in the reward deficit during spontaneous withdrawal.

Effects in rats of adolescent exposure to cannabis smoke or THC on emotional behavior and cognitive function in adulthood

RATIONALE

Cannabis use is common among adolescents and some research suggests that adolescent cannabis use increases the risk for depression, anxiety, and cognitive impairments in adulthood. In human studies, however, confounds may affect the association between cannabis use and the development of brain disorders.

OBJECTIVES

These experiments investigated the effects of adolescent exposure to either cannabis smoke or THC on anxiety- and depressive-like behavior and cognitive performance in adulthood in Long-Evans rats.

METHODS

Adolescent rats of both sexes were exposed to either cannabis smoke from postnatal days (P) 29-49 or ascending doses of THC from P35-45. When the rats reached adulthood (P70), anxiety-like behavior was investigated in the large open field and elevated plus maze, depressive-like behavior in the sucrose preference and forced swim tests, and cognitive function in the novel object recognition test.

RESULTS

Despite sex differences on some measures in the open field, elevated plus maze, forced swim, and novel object recognition tests, there were no effects of either adolescent cannabis smoke or THC exposure, and only relatively subtle interactions between exposure conditions and sex, such that sex differences on some performance measures were slightly attenuated.

CONCLUSION

Neither cannabis smoke nor THC exposure during adolescence produced robust alterations in adult behavior after a period of abstinence, suggesting that adverse effects associated with adolescent cannabis use might be due to non-cannabinoid concomitants of cannabis use.

Simultaneous quantification of cannabinoids tetrahydrocannabinol, cannabidiol and CB1 receptor antagonist in rat plasma: An application to characterize pharmacokinetics after passive cannabis smoke inhalation and co-administration of rimonabant

A highly sensitive and selective liquid chromatography-tandem mass spectrometry method for the determination of tetrahydrocannabinol (THC), cannabidiol, and rimonabant in rat plasma was developed. Analytes and the internal standard were extracted from plasma using a combination of protein precipitation followed by liquid-liquid extraction. Chromatographic separation was done using Waters Symmetry C18, 4.6 × 150 mm, 5 um column using 10 mm ammonium formate buffer and methanol. The total run time was 6 min, and separation was achieved using isocratic elution at a flow rate of 1 mL/min using a 10:90 (aqueous: organic) ratio. The ionization of the analytes was optimized using electrospray ionization in positive mode, and multiple reaction mode was used for this analysis. This method showed linearity from 0.1 to 100 ng/ml for all the analytes and was validated according to FDA Bioanalytical Method Validation Guidance in terms of accuracy, precession, linearity, stability, matrix effect, recovery, and stability. This method was successfully applied to characterize the pharmacokinetics of THC in rats after continuous passive smoke exposure for 50 min when rimonabant was co-administered with cannabis smoke. Maximum concentration (C_max) for THC was observed immediately after rats were removed from the exposure chamber (10 min post completion) which declined with a terminal half-life of 3.7 h and clearance was calculated to be 1.1 (L/h). Rimonabant (i.p) at a dose of 3 mg/kg was rapidly absorbed and maximum concentration (Cmax) was seen at 11 min which declined with a terminal half-life of 5.4 h and clearance was calculated to be 2.0 (L/h). Exposure AUC_inf (h* μg/L) for THC and rimonabant were 13.9 and 457.6 respectively. As this method was highly sensitive and required only 50 μL of plasma, it is applicable in rodent models that assess the exposure-response relationships of these drugs.

Video-assisted topographical measurement of cigarette smoking: Exploration of an objective approach to evaluate nicotine dependence

INTRODUCTION

Addressing nicotine dependence is key to effective smoking cessation. While self-report measures of nicotine dependence are often challenged for their reliability and validity, there is a lack of non-invasive and inexpensive objective measures of nicotine dependence. In this study, we aimed to explore the potential of using video-assisted smoking topography to derive objective measures indicative of nicotine dependence with the guidance of nicotine-receptor-based self-titration theory.

METHODS

Videotaped topography data were collected for one episode of smoking a whole cigarette the first time of the day from 10 cigarette smokers with diverse racial backgrounds (4 males and 6 females, mean age = 27 years, SD = 7.2). Temporal patterns of individual topographic measures (i.e. puff interval, puff speed, puff duration, inhalation duration, and rest duration) were measured and plotted against time. Levels of nicotine dependence were evaluated using three standard scales, including the 14-item DSM-IV scale, the 6-item ICD-10 scale, and the 6-item Fagerström Test of Nicotine Dependence. Both linear and non-linear fold catastrophe dynamic models were used to fit the data.

RESULTS

Compared with a linear model (R² from 0.003 to 0.74), the non-linear model more adequately captured the temporal pattern of topographic measures (R² from 0.11 to 0.99), especially puff speed. The indicators derived from the fitted fold catastrophe curve (e.g. average puff speed) were significantly associated with nicotine dependence scores, especially DSM-IV scale scores (r from 0.64 to 0.93).

CONCLUSIONS

Study findings suggest the potential to objectively and non-invasively measure nicotine dependence using video-assisted smoking topography.

Functional connectivity, behavioral and dopaminergic alterations 24 hours following acute exposure to synthetic bath salt drug methylenedioxypyrovalerone

Among cathinone drugs known as bath salts, methylenedioxypyrovalerone (MDPV) exerts its potent actions via the dopamine (DA) system, and at intoxicating doses may produce adverse behavioral effects. Previous work by our group suggests that prolonged alterations in correlated neural activity between cortical and striatal areas could underlie, at least in part, the adverse reactions to this bath salt drug. In the present study, we assessed the effect of acute MDPV administration on brain functional connectivity at 1 and 24 h in rats. Using graph theory metrics to assess in vivo brain functional network organization we observed that 24 h after MDPV administration there was an increased clustering coefficient, rich club index, and average path length. Increases in these metrics suggests that MDPV produces a prolonged pattern of correlated activity characterized by greater interactions between subsets of high degree nodes but a reduced interaction with regions outside this core subset. Further analysis revealed that the core set of nodes include prefrontal cortical, amygdala, hypothalamic, somatosensory and striatal areas. At the molecular level, MDPV downregulated the dopamine transporter (DAT) in striatum and produced a shift in its subcellular distribution, an effect likely to involve rapid internalization at the membrane. These new findings suggest that potent binding of MDPV to DAT may trigger internalization and a prolonged alteration in homeostatic regulation of DA and functional brain network reorganization. We propose that the observed MDPV-induced network reorganization and DAergic changes may contribute to previously reported adverse behavioral responses to MDPV.

Pros and Cons of Medical Cannabis use by People with Chronic Brain Disorders

BACKGROUND

Cannabis is the most widely used illicit drug in the world and there is growing concern about the mental health effects of cannabis use. These concerns are at least partly due to the strong increase in recreational and medical cannabis use and the rise in tetrahydrocannabinol (THC) levels. Cannabis is widely used to self-medicate by older people and people with brain disorders such as amyotrophic lateral sclerosis (ALS), multiple sclerosis (MS), Alzheimer's disease (AD), Parkinson's disease (PD), bipolar disorder, and schizophrenia.

OBJECTIVE

This review provides an overview of the perceived benefits and adverse mental health effects of cannabis use in people with ALS, MS, AD, PD, bipolar disorder, and schizophrenia.

RESULTS

The reviewed studies indicate that cannabis use diminishes some symptoms associated with these disorders. Cannabis use decreases pain and spasticity in people with MS, decreases tremor, rigidity, and pain in people with PD, and improves the quality of life of ALS patients by improving appetite, and decreasing pain and spasticity. Cannabis use is more common among people with schizophrenia than healthy controls. Cannabis use is a risk factor for schizophrenia which increases positive symptoms in schizophrenia patients and diminishes negative symptoms. Cannabis use worsens bipolar disorder and there is no evidence that bipolar patients derive any benefit from cannabis. In late stage Alzheimer's patients, cannabis products may improve food intake, sleep quality, and diminish agitation.

CONCLUSION

Cannabis use diminishes some of the adverse effects of neurological and psychiatric disorders. However, chronic cannabis use may lead to cognitive impairments and dependence.

The sigma-1 receptor modulates methamphetamine dysregulation of dopamine neurotransmission

Dopamine neurotransmission is highly dysregulated by the psychostimulant methamphetamine, a substrate for the dopamine transporter (DAT). Through interactions with DAT, methamphetamine increases extracellular dopamine levels in the brain, leading to its rewarding and addictive properties. Methamphetamine also interacts with the sigma-1 receptor (σ1R), an inter-organelle signaling modulator. Using complementary strategies, we identified a novel mechanism for σ1R regulation of dopamine neurotransmission in response to methamphetamine. We found that σ1R activation prevents methamphetamine-induced, DAT-mediated increases in firing activity of dopamine neurons. In vitro and in vivo amperometric measurements revealed that σ1R activation decreases methamphetamine-stimulated dopamine efflux without affecting basal dopamine neurotransmission. Consistent with these findings, σ1R activation decreases methamphetamine-induced locomotion, motivated behavior, and enhancement of brain reward function. Notably, we revealed that the σ1R interacts with DAT at or near the plasma membrane and decreases methamphetamine-induced Ca2+ signaling, providing potential mechanisms. Broadly, these data provide evidence for σ1R regulation of dopamine neurotransmission and support the σ1R as a putative target for the treatment of methamphetamine addiction.

Neuropeptide systems and new treatments for nicotine addiction

RATIONALE

The mildly euphoric and cognitive enhancing effects of nicotine play a role in the initiation of smoking, while dysphoria and anxiety associated with smoking cessation contribute to relapse. After the acute withdrawal phase, smoking cues, a few cigarettes (i.e., lapse), and stressors can cause relapse. Human and animal studies have shown that neuropeptides play a critical role in nicotine addiction.

OBJECTIVES

The goal of this paper is to describe the role of neuropeptide systems in the initiation of nicotine intake, nicotine withdrawal, and the reinstatement of extinguished nicotine seeking.

RESULTS

The reviewed studies indicate that several drugs that target neuropeptide systems diminish the rewarding effects of nicotine by preventing the activation of dopaminergic systems. Other peptide-based drugs diminish the hyperactivity of brain stress systems and diminish withdrawal-associated symptom severity. Blockade of hypocretin-1 and nociceptin receptors and stimulation of galanin and neurotensin receptors diminishes the rewarding effects of nicotine. Both corticotropin-releasing factor type 1 and kappa-opioid receptor antagonists diminish dysphoria and anxiety-like behavior associated with nicotine withdrawal and inhibit stress-induced reinstatement of nicotine seeking. Furthermore, blockade of vasopressin 1b receptors diminishes dysphoria during nicotine withdrawal, and melanocortin 4 receptor blockade prevents stress-induced reinstatement of nicotine seeking. The role of neuropeptide systems in nicotine-primed and cue-induced reinstatement is largely unexplored, but there is evidence for a role of hypocretin-1 receptors in cue-induced reinstatement of nicotine seeking.

CONCLUSION

Drugs that target neuropeptide systems might decrease the euphoric effects of smoking and improve relapse rates by diminishing withdrawal symptoms and improving stress resilience.

Effect of Second-Hand Tobacco Smoke on the Nitration of Brain Proteins: A Systems Biology and Bioinformatics Approach

Second-hand smoke (SHS) exposure leads to the death of approximately 48,000 nonsmokers per year in the United States alone. SHS exposure has been associated with cardiovascular, respiratory, and neurodegenerative diseases. While cardiac function abnormalities and lung cancer due to SHS have been well characterized, brain injury due to SHS has not undergone a full systematic evaluation. Oxidative stress and nitration have been associated with smoking and SHS exposure. Animal studies suggest that exposure to tobacco smoke increases oxidative stress. Oxidative stress is characterized by an increase in reactive oxygen and nitrogen species (ROS/RNS). Among the oxidative mechanisms affecting protein functionality is the posttranslational modification (PTM)-mediated tyrosine nitration. Protein tyrosine nitration, a covalent posttranslational modification, is commonly used as a marker of cellular oxidative stress associated with the pathogenesis of several neurodegenerative diseases. In our previous published work, the utility of a targeted proteomic approach has been evaluated to identify two brain abundant proteins in an in vivo SHS rat model namely the GAPDH and UCH-L1. In this current study, mass spectrometric-based proteomic and complementary biochemical methods were used to characterize the SHS-induced brain nitroproteome followed by bioinformatics/systems biology approach analysis to characterize protein interaction map. Sprague Dawley rats were exposed to SHS for 5 weeks and then cortical tissues were collected. Nitroprotein enrichment was performed via 3-Nitro tyrosine (3-NT) immunoprecipitation of brain lysates proteins. Protein nitration was validated via Western blotting to confirm the presence of nitroproteins complemented by gel-free neuroproteomic analysis by data-dependent LC-MS/MS. We identified 29 differentially expressed proteins in the 3-NT-enriched samples; seven of these proteins were unique to SHS exposure. Network analysis revealed an association of the proteins to different cellular processes including oxidative stress, ROS generation, and cell death-related pathway. This confirms the association of oxidative stress mechanisms with SHS which may contribute to neuronal injury, an area that has not been well studied in the area smoking.

Overexpression of CRF in the BNST diminishes dysphoria but not anxiety-like behavior in nicotine withdrawing rats

Smoking cessation leads to dysphoria and anxiety, which both increase the risk for relapse. This negative affective state is partly mediated by an increase in activity in brain stress systems. Recent studies indicate that prolonged viral vector-mediated overexpression of stress peptides diminishes stress sensitivity. Here we investigated whether the overexpression of corticotropin-releasing factor (CRF) in the bed nucleus of the stria terminalis (BNST) diminishes nicotine withdrawal symptoms in rats. The effect of nicotine withdrawal on brain reward function was investigated with an intracranial self-stimulation (ICSS) procedure. Anxiety-like behavior was investigated in the elevated plus maze test and a large open field. An adeno-associated virus (AAV) pseudotype 2/5 vector was used to overexpress CRF in the lateral BNST and nicotine dependence was induced using minipumps. Administration of the nicotinic receptor antagonist mecamylamine and cessation of nicotine administration led to a dysphoria-like state, which was prevented by the overexpression of CRF in the BNST. Nicotine withdrawal also increased anxiety-like behavior in the elevated plus maze test and large open field test and slightly decreased locomotor activity in the open field. The overexpression of CRF in the BNST did not prevent the increase in anxiety-like behavior or decrease in locomotor activity. The overexpression of CRF increased CRF1 and CRF2 receptor gene expression and increased the CRF2/CRF1 receptor ratio. In conclusion, the overexpression of CRF in the BNST prevents the dysphoria-like state associated with nicotine withdrawal and increases the CRF2/CRF1 receptor ratio, which may diminish the negative effects of CRF on mood.

The Psychoactive Designer Drug and Bath Salt Constituent MDPV Causes Widespread Disruption of Brain Functional Connectivity

The abuse of 'bath salts' has raised concerns because of their adverse effects, which include delirium, violent behavior, and suicide ideation in severe cases. The bath salt constituent 3,4-methylenedioxypyrovalerone (MDPV) has been closely linked to these and other adverse effects. The abnormal behavioral pattern produced by acute high-dose MDPV intake suggests possible disruptions of neural communication between brain regions. Therefore, we determined if MDPV exerts disruptive effects on brain functional connectivity, particularly in areas of the prefrontal cortex. Male rats were imaged following administration of a single dose of MDPV (0.3, 1.0, or 3.0 mg/kg) or saline. Resting state brain blood oxygenation level-dependent (BOLD) images were acquired at 4.7 T. To determine the role of dopamine transmission in MDPV-induced changes in functional connectivity, a group of rats received the dopamine D1/D2 receptor antagonist cis-flupenthixol (0.5 mg/kg) 30 min before MDPV. MDPV dose-dependently reduced functional connectivity. Detailed analysis of its effects revealed that connectivity between frontal cortical and striatal areas was reduced. This included connectivity between the prelimbic prefrontal cortex and other areas of the frontal cortex and the insular cortex with hypothalamic, ventral, and dorsal striatal areas. Although the reduced connectivity appeared widespread, connectivity between these regions and somatosensory cortex was not as severely affected. Dopamine receptor blockade did not prevent the MDPV-induced decrease in functional connectivity. The results provide a novel signature of MDPV's in vivo mechanism of action. Reduced brain functional connectivity has been reported in patients suffering from psychosis and has been linked to cognitive dysfunction, audiovisual hallucinations, and negative affective states akin to those reported for MDPV-induced intoxication. The present results suggest that disruption of functional connectivity networks involving frontal cortical and striatal regions could contribute to the adverse effects of MDPV.

Behavioral Characterization of the Effects of Cannabis Smoke and Anandamide in Rats

Cannabis is the most widely used illicit drug in the world. Delta-9-tetrahydrocannabinol (Δ9-THC) is the main psychoactive component of cannabis and its effects have been well-studied. However, cannabis contains many other cannabinoids that affect brain function. Therefore, these studies investigated the effect of cannabis smoke exposure on locomotor activity, rearing, anxiety-like behavior, and the development of dependence in rats. It was also investigated if cannabis smoke exposure leads to tolerance to the locomotor-suppressant effects of the endogenous cannabinoid anandamide. Cannabis smoke was generated by burning 5.7% Δ9-THC cannabis cigarettes in a smoking machine. The effect of cannabis smoke on the behavior of rats in a small and large open field and an elevated plus maze was evaluated. Cannabis smoke exposure induced a brief increase in locomotor activity followed by a prolonged decrease in locomotor activity and rearing in the 30-min small open field test. The cannabinoid receptor type 1 (CB1) receptor antagonist rimonabant increased locomotor activity and prevented the smoke-induced decrease in rearing. Smoke exposure also increased locomotor activity in the 5-min large open field test and the elevated plus maze test. The smoke exposed rats spent more time in the center zone of the large open field, which is indicative of a decrease in anxiety-like behavior. A high dose of anandamide decreased locomotor activity and rearing in the small open field and this was not prevented by rimonabant or pre-exposure to cannabis smoke. Serum Δ9-THC levels were 225 ng/ml after smoke exposure, which is similar to levels in humans after smoking cannabis. Exposure to cannabis smoke led to dependence as indicated by more rimonabant-precipitated somatic withdrawal signs in the cannabis smoke exposed rats than in the air-control rats. In conclusion, chronic cannabis smoke exposure in rats leads to clinically relevant Δ9-THC levels, dependence, and has a biphasic effect on locomotor activity.

Chronic treatment with the vasopressin 1b receptor antagonist SSR149415 prevents the dysphoria associated with nicotine withdrawal in rats

Nicotine addiction is a chronic brain disorder that is characterized by dysphoria upon smoking cessation and relapse after brief periods of abstinence. It has been hypothesized that the negative mood state associated with nicotine withdrawal is partly mediated by a heightened activity of brain stress systems. Animal studies suggest that blockade of vasopressin 1b (V1b) receptors diminishes high levels of drug intake in dependent animals and attenuates the emotional response to stressors. The goal of the present studies was to investigate the effect of acute and chronic treatment with the V1b receptor antagonist SSR149415 on the negative mood state associated with nicotine withdrawal in rats. An intracranial self-stimulation (ICSS) procedure was used to assess mood states and nicotine dependence was induced using minipumps. The nicotinic receptor antagonist mecamylamine was used to precipitate withdrawal. Mecamylamine elevated the brain reward thresholds of the nicotine dependent rats, which reflects a negative mood state. Mecamylamine did not affect the brain reward thresholds of the saline-treated control rats. Chronic treatment with SSR149415 completely prevented the elevations in brain reward thresholds associated with nicotine withdrawal while acute treatment only partly prevented nicotine withdrawal. These data suggest that chronic treatment with V1b receptor antagonists may prevent the dysphoria associated with smoking cessation and thereby improve relapse rates.

Methamphetamine- and trauma-induced brain injuries: comparative cellular and molecular neurobiological substrates

The use of methamphetamine (METH) is a growing public health problem, because its abuse is associated with long-term biochemical and structural effects on the human brain. Neurodegeneration is often observed in humans, because of mechanical injuries (e.g., traumatic brain injury [TBI]) and ischemic damage (strokes). In this review, we discuss recent findings documenting the fact that the psychostimulant drug METH can cause neuronal damage in several brain regions. The accumulated evidence from our laboratories and those of other investigators indicates that acute administration of METH leads to activation of calpain and caspase proteolytic systems. These systems are also involved in causing neuronal damage secondary to traumatic and ischemic brain injuries. Protease activation is accompanied by proteolysis of endogenous neuronal structural proteins (alphaII-spectrin protein and microtubule-associated protein-tau), evidenced by the appearance of their breakdown products after these injuries. When taken together, these observations suggest that METH exposure, like TBI, can cause substantial damage to the brain by causing both apoptotic and necrotic cell death in the brains of METH addicts who use large doses of the drug during their lifetimes. Finally, because METH abuse is accompanied by functional and structural changes in the brain similar to those in TBI, METH addicts might experience greater benefit if their treatment involved greater emphasis on rehabilitation in conjunction with potential neuroprotective pharmacological agents such as calpain and caspase inhibitors similar to those used in TBI.

Exposure to chronic mild stress alters thresholds for lateral hypothalamic stimulation reward and subsequent responsiveness to amphetamine

Chronic mild stress in rodents has been proposed to model some of the environmental factors that contribute to the induction of depressive disorders in humans. This model is based on the hypothesis that chronic mild stress induces a change in brain reward function that resembles the symptomatology of major depression, namely, a decrease in responsiveness to rewarding stimuli. The purpose of the first experiment was to investigate whether chronic mild stress affects brain reward function as measured by alterations in lateral hypothalamic self-stimulation behavior in rats. Exposure to chronic mild stress induces a reduction in body weight which might affect brain reward function on its own. Therefore, the potential contribution of a reduction in body weight to the chronic mild stress-induced alterations in brain reward function was examined in a separate group of food-restricted rats. Thresholds for lateral hypothalamic self-stimulation were slightly but significantly lowered in animals exposed to chronic mild stress, indicating an enhancement of stimulation reward efficacy. Food restriction had no effect on brain reward function. The second experiment examined the interaction between prior exposure to chronic mild stress or food restriction and responsiveness to a pharmacological challenge, amphetamine, that enhances brain reward function. Acute administration of amphetamine produced a greater enhancement of lateral hypothalamic self-stimulation reward in animals exposed to chronic stress relative to non-stressed and food-restricted animals. Taken together, the present findings indicate that chronic mild stress sensitizes the neural substrates that mediate both lateral hypothalamic stimulation and psychostimulant drug reward. These findings support the hypothesis that prior exposure to stress affects the vulnerability for drug-taking behavior by increasing the positive reinforcing properties of drug of abuse.

Effect of a benzodiazepine receptor agonist and corticotropin-releasing hormone receptor antagonists on long-term foot-shock-induced increase in defensive withdrawal behavior

RATIONALE

Traumatic life events can induce long-term alterations in neuronal substrates, which may ultimately lead to the development of anxiety disorders. It has been postulated that corticotropin-releasing hormone (CRH) plays an important role in anxiety-like behavior.

OBJECTIVES

(1) To study the long-term effects of a single foot-shock experience on defensive withdrawal (DW) behavior in rats. (2) To examine the effects of the benzodiazepine anxiolytic drug chlordiazepoxide on the behavior of preshocked and control rats in the DW test. (3) To study the role of endogenous CRH in the long-term stress-induced increase in DW behavior.

METHODS

(1) Rats were exposed to a single session of foot shocks or exposed to the grid cage without receiving any shocks. Two, six and ten weeks later, rats were tested in the DW tests (2, 3). In subsequent experiments, rats were exposed to foot shocks or exposed to the grid cage without receiving any shocks, and 2 weeks later the effect of pharmacological treatments on the behavioral response in the DW test was investigated. Chlordiazepoxide (1, 5, 10 mg/kg BW, i.p.) and the CRH antagonists D-Phe CRH(12-41) (0.2, 1, 5 microg per rat, i.c.v.) and alpha-helical CRH(9-41) (5 microg per rat, i.c.v.) were injected 30 min before the test.

RESULTS

A single session of foot shocks induced a long-term increase in DW behavior, which persisted after repeated testing for at least 10 weeks. Chlordiazepoxide decreased the latency but did not affect time spent in light, distance moved, or the number of entries in the open field. D-Phe CRH(12-41) had no behavioral effects. alpha-Helical CRH(9-41) increased the time spent outside the box, primarily as a result of effects of alpha-helical CRH(9-41) in controls.

CONCLUSION

These data demonstrate that preshocked rats display long-term increased anxiety-like behavior in the DW test but that CRH is unlikely to be involved in its expression.

LY354740 attenuates the expression of long-term behavioral sensitization induced by a single session of foot shocks

Exposure of rats to a single session of foot shocks sensitizes behavioral responses to novel stimuli. There is evidence that metabotropic glutamate (mGlu) receptors play a role in sensitization processes. In the present study, we investigated the role of mGlu(2/3) receptors in the long-term (14 days) increase in defensive withdrawal behavior after a single session of foot shocks. Exposure to foot shocks increased defensive withdrawal behavior. The mGlu(2/3) receptor agonist LY354740 ((1S,2S,5R,6S)-(+)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid, 0.1 mg/kg, i.p.) normalized the increased latency and the decreased time in the light of the preshocked rats. We conclude that activation of mGlu(2/3) receptors attenuates the foot shock-induced expression of behavioral sensitization.

Stress-induced sensitization of CRH-ir but not P-CREB-ir responsivity in the rat central nervous system

There is some evidence that a traumatic life event can induce long-term alterations in corticotropin-releasing hormone (CRH) producing neurons in humans, which may play a role in the pathophysiology of anxiety disorders, including post-traumatic stress disorder (PTSD). To study the long-term effects of a traumatic event on brain CRH-immunoreactivity (CRH-ir) and phospho-cAMP response element binding protein-immunoreactivity (P-CREB-ir), rats were exposed to a single session of foot shocks (preshocked) or no shocks (control). Two weeks later half of the control rats and half of the preshocked rats received an electrified prod in the home cage for 15 min and behavior was recorded. Fifteen minutes after the removal of the prod rats were perfused and brain sections were stained for CRH-ir and P-CREB-ir. There was no basal difference between preshocked and control rats in brain CRH-ir and P-CREB-ir. Exposure to the electrified prod induced a significant increase in CRH-ir in the paraventricular nucleus of the hypothalamus, the median eminence and the central amygdala in preshocked rats, but not in control rats. The electrified prod increased the number of P-CREB-ir neurons in the paraventricular nucleus of the hypothalamus and the locus coeruleus, but the preshock experience did not affect this response. In an additional experiment with a similar design plasma hormone levels were measured 14 days after the foot shocks. The preshock experience sensitized the shock prod-induced ACTH and corticosterone response. No behavioral differences between preshocked and control rats were found during the shock prod tests. We suggest that long-term stress-induced changes in neuropeptide dynamics of CRH-ir neurons may play a role in long-term stress-induced neuroendocrine sensitization.

Long-term sensitization of cardiovascular stress responses after a single stressful experience

There is evidence that the experience of traumatic events may play a role in the pathogenesis of somatic diseases, including cardiovascular disorders. In this study, telemetry was used to investigate the long-term effects of a single stressful experience on cardiovascular and behavioral responses to novel challenges 2 weeks later. Rats were exposed to footshocks and tested for sensitization using the following challenges: novel cylinder (Day 14); shock prod acquisition test (Day 15); and shock prod retention test (Day 16). No difference in basal somatomotor activity (SA), heart rate (HR) and blood pressure between preshocked rats and control rats was found. However, preshocked rats displayed an enhanced blood pressure response compared to controls during the shock prod acquisition test and the shock prod retention test. No differential increase in HR response between both groups was found. During the novel cylinder test, the preshocked rats displayed less SA while no behavioral differences were found in the shock prod acquisition test and the shock prod retention test. We conclude that a single stressful experience induces long-term sensitization of blood pressure responses to novel challenges that are not necessarily linked to sensitized behavioral responses. The footshock model may be a useful model to study autonomic hyperresponsivity found in posttraumatic stress disorder (PTSD).

Long-lasting stress sensitisation

Stressful experiences in humans can result in a spectrum of long-term changes in behavioural, autonomic and hormonal responsivity. An extreme form of such alterations is found in patients with post-traumatic stress disorder (PTSD). A number of animal models has been developed in which intense stressful experiences (shocks, social confrontations) result in longterm altered responsivity of behavioural, autonomic and hormonal responses to aversive challenges which mimic many of the changes seen in PTSD. These models of stress-induced sensitisation are beginning to generate a better understanding of the vulnerability factors, time-course and underlying neuronal substrates of the long-term disturbances experienced by humans as a result of stressful life events.

Sex differences in long-term stress-induced colonic, behavioural and hormonal disturbances

Functional bowel disorders are more prevalent in women than in men, but the reason for this is unclear. Stressful experiences can increase the risk for or precipitate intestinal dysfunction. Using a model for long-term stress-induced sensitisation in rats, it was investigated whether male and female rats differ in susceptibility for long-term colonic, behavioural and hormonal disturbances following brief but intense stress. Male and female Wistar rats were fitted with chronic electrodes on proximal colon and given either a 15-minute session of foot shocks or no shocks. Two weeks later, rats were exposed to two different novel stressful challenges in the home cage: an electrified prod (day 14) and an 85 dB noise stressor (day 15). Digitalised colonic myoelectric spike burst activity was quantified automatically. Behaviour during prod and noise exposure was scored blindly from videotape. Resting plasma hormone concentrations at the end of the study were determined by radio-immuno assay. Following prod stress on day 14, both male and female preshocked rats showed a greater increase in colonic spike burst frequency than controls, but similar behaviour, and the dynamics of colonic motility differed between sexes. Following noise stress on day 15, only a small change in burst frequency was seen in all rats, but preshocked rats showed less self-grooming behaviour and there was a tendency for preshocked females to show increased noise-induced immobility. Preshocked rats also had lower levels of plasma free thyroxine. While both male and female rats show long-term stress-induced colonic sensitisation and hormonal changes, females show a different activation pattern of colonic motility, and may be more vulnerable for altered behavioural reactivity, following stress.