Ketoconazole suppresses food restriction-induced increases in heroin self-administration in rats: sex differences

Oral Fentanyl Consumption Alters Sleep Rhythms, Promotes Avoidance Behaviors, Impairs Fear Extinction Learning, and Alters Basolateral Amygdala Physiology in Male and Female Mice

The number of opioid overdose deaths has increased over the past several years, mainly driven by an increase in the availability of highly potent synthetic opioids, like fentanyl, in the illicit drug supply. While many previous studies on fentanyl and other opioids have focused on intravenous administration, other routes of administration remain relatively understudied. Here, we used a drinking in the dark (DiD) paradigm to model oral fentanyl self-administration using increasing fentanyl concentrations in male and female mice over 5 weeks. Fentanyl consumption peaked in both female and male mice at the 30 µg/mL dose, with female mice consuming significantly more fentanyl than male mice. Mice consumed sufficient fentanyl such that withdrawal was precipitated with naloxone, with males having more severe withdrawal symptoms, despite lower pharmacological exposure. Fentanyl consumption disrupted normal sleep rhythms in both male and female mice. We also performed behavioral assays to measure avoidance behavior and reward-seeking during fentanyl abstinence. Female mice displayed more avoidance behaviors in the open field assay, whereas male mice showed evidence of these behaviors in the light/dark box assay. Female mice also exhibited increased reward-seeking in the sucrose preference test. Fentanyl-consuming mice of both sexes showed impaired cued fear extinction learning following fear conditioning and increased excitatory synaptic drive and increased excitability of BLA principal neurons. Our experiments demonstrate that long-term oral fentanyl consumption results in wide-ranging physiological and behavioral disruptions. This model could be useful to further study fentanyl withdrawal syndrome, fentanyl seeking, and behaviors associated with protracted fentanyl withdrawal.

Estradiol and Mu opioid-mediated reward: The role of estrogen receptors in opioid use

Opioid use and opioid use disorder are characterized by sex and gender differences, and some of these differences may be mediated by differences in the hormonal milieu within and across individuals. This review focuses on the role of ovarian hormones, and particularly estradiol, on the endogenous mu opioid receptor system. There is an abundance of data indicating that estradiol influences the activity of endogenous mu opioid peptides, the activation of mu opioid receptors, and the internalization and desensitization of mu opioid receptors. These effects have functional consequences on behaviors mediated by endogenous mu opioid receptor activity and on sensitivity to mu opioid agonists and antagonists. Recent behavioral data suggest these consequences extend to mu opioid reward, and preclinical studies report that estradiol decreases self-administration of mu opioid receptor agonists across a range of experimental conditions. Data collected in human laboratory studies suggest that estradiol may have functionally similar effects in clinical populations, and thus estrogen receptors may be a potential target in the development of novel therapeutics. This review summarizes data from cellular assays to clinical trials to explore how estradiol influences mu opioid receptor activity, as well as potential ways in which estrogen receptors may be targeted to address the problems of opioid use.

Focus on fentanyl in females: Sex and gender differences in the physiological and behavioral effects of fentanyl

The prevalence of opioid use disorder and overdose continues to harm the U.S. population and is further exacerbated by the use of the synthetic opioid, fentanyl, and its analogs. Gender differences in the effects of fentanyl are not well understood. The present article reviews evidence for gender and sex differences in the physiological and behavioral effects of fentanyl in humans and animals. Biological sex seems to be a foundational driver in addiction vulnerability and affects mechanisms related to opioid use including fentanyl. Fentanyl has distinct pharmacodynamics and enhanced efficacy relative to other opioids that highlights the need to investigate how females may be uniquely altered by its use. Behavioral and physiological responses to fentanyl are found to differ by sex and gender in many cases, including outputs like affective symptoms, analgesia, tolerance, and withdrawal emphasizing the need for further research about the role of biological sex on fentanyl use.

Neuroinflammatory Response in Reward-Associated Psychostimulants and Opioids: A Review

Substance abuse is one of the significant problems in social and public health worldwide. Vast numbers of evidence illustrate that motivational and reinforcing impacts of addictive drugs are primarily attributed to their ability to change dopamine signaling in the reward circuit. However, the roles of classic neurotransmitters, especially dopamine and neuromodulators, monoamines, and neuropeptides, in reinforcing characteristics of abused drugs have been extensively investigated. It has recently been revealed that central immune signaling includes cascades of chemokines and proinflammatory cytokines released by neurons and glia via downstream intracellular signaling pathways that play a crucial role in mediating rewarding behavioral effects of drugs. More interestingly, inflammatory responses in the central nervous system modulate the mesolimbic dopamine signaling and glutamate-dependent currents induced by addictive drugs. This review summarized researches in the alterations of inflammatory responses accompanied by rewarding and reinforcing properties of addictive drugs, including cocaine, methamphetamine, and opioids that were evaluated by conditioned place preference and self-administration procedures as highly common behavioral tests to investigate the motivational and reinforcing impacts of addictive drugs. The neuroinflammatory responses affect the rewarding properties of psychostimulants and opioids.

Transcriptomics in the nucleus accumbens shell reveal sex- and reinforcer-specific signatures associated with morphine and sucrose craving

Incubation of craving is a well-documented phenomenon referring to the intensification of drug craving over extended abstinence. The neural adaptations that occur during forced abstinence following chronic drug taking have been a topic of intense study. However, little is known about the transcriptomic changes occurring throughout this window of time. To define gene expression changes associated with morphine consumption and extended abstinence, male and female rats underwent 10 days of morphine self-administration. Separate drug-naive rats self-administered sucrose in order to compare opioid-induced changes from those associated with natural, non-drug rewards. After one or 30 days of forced abstinence, rats were tested for craving, or nucleus accumbens shell tissue was dissected for RNA sequencing. Morphine consumption was predictive of drug seeking after extended (30 days) but not brief (1 day) abstinence in both sexes. Extended abstinence was also associated with robust sex- and reinforcer-specific changes in gene expression, suggesting sex differences underlying incubation of morphine and sucrose seeking respectively. Importantly, these changes in gene expression occurred without re-exposure to drug-paired cues, indicating that chronic morphine causes long-lasting changes in gene expression that prime the system for increased craving. These findings lay the groundwork for identifying specific therapeutic targets for curbing opioid craving without impacting the natural reward system in males and females.

Dose- and Sex-Dependent Bidirectional Relationship between Intravenous Fentanyl Self-Administration and Gut Microbiota

Gut bacteria influence neural circuits in addiction-related behaviors. Given the association between opioid use, gastrointestinal distress, and microbial dysbiosis in humans and mice, we test the hypothesis that interactions between gut bacteria and the brain mediate the rewarding and reinforcing properties of fentanyl. We implant rats with intravenous catheters in preparation for fentanyl intravenous self-administration (IVSA) on an escalating schedule of reinforcement to determine factors that influence fentanyl intake, including sex, dose, and gut microbiota. Our data show the impact of fentanyl IVSA on gut microbiota diversity, as well as the role of gut microbiota on fentanyl IVSA, in Sprague Dawley rats in a sex- and dose-dependent manner (n = 10–16/group). We found that the diversity of gut microbiota within females dose-dependently predicts progressive but not fixed ratio schedules of fentanyl IVSA. Depending on sex and fentanyl dose, alpha diversity (richness and evenness measured with Shannon index) is either increased or decreased following fentanyl IVSA and predicts progressive ratio breakpoint. Our findings collectively suggest a role of gut bacteria in drug-related behavior, including motivation and reinforcement. This work provides feasibility for an intravenous fentanyl self-administration model and uncovers potential factors mediating drug use, which may lead to the development of effective addiction interventions.

Escalation and reinstatement of fentanyl self-administration in male and female rats

RATIONALE

Escalation of drug intake and craving are two DSM-5 hallmark symptoms of opioid use disorder (OUD).

OBJECTIVES

This study determined if escalation of intake as modeled by long access (LgA) self-administration (SA) and craving measured by reinstatement are related.

METHODS

Adult male and female Sprague-Dawley rats were trained to self-administer fentanyl across 7 daily 1-h short access (ShA) sessions, followed by 21 SA sessions of either 1- or 6-h duration (ShA or LgA). Following 14 1-h extinction sessions, Experiment 1 assessed reinstatement induced by either fentanyl (10 or 30 µg/kg) or yohimbine (1 or 2 mg/kg), and Experiment 2 assessed reinstatement induced by a drug-associated cue light.

RESULTS

Females acquired fentanyl SA faster than males. When shifted to LgA sessions, LgA rats escalated fentanyl intake, but ShA rats did not; no reliable sex difference in the rate of escalation was observed. In extinction, compared to ShA rats, LgA rats initially responded less and showed less decay of responding across sessions. A priming injection of fentanyl induced reinstatement, with LgA rats reinstating more than ShA rats at the 30 µg/kg dose. Yohimbine (1 mg/kg) also induced reinstatement, but there was no effect of access group or sex. With cue-induced reinstatement, LgA females reinstated less than LgA males and ShA females.

CONCLUSION

Among the different reinstatement tests assessed, escalation of fentanyl SA increased only drug-primed reinstatement, suggesting a limited relationship between escalation of drug intake and craving (reinstatement) for OUD.

Studying Sex Differences in Rodent Models of Addictive Behavior

Animal models of addictive behaviors are useful for uncovering neural mechanisms involved in the development of dependence and for identifying risk factors for drug abuse. One such risk factor is biological sex, which strongly moderates drug self-administration behavior in rodents. Female rodents are more likely to acquire drug self-administration behaviors, consume higher amounts of drug, and reinstate drug-seeking behavior more readily. Despite this female vulnerability, preclinical addiction research has largely been done in male animals. The study of sex differences in rodent models of addictive behavior is increasing, however, as more investigators are choosing to include both male and female animals in experiments. This commentary is meant to serve as an introductory guide for preclinical investigators new to the study of sex differences in addiction. We provide an overview of self-administration models, a broad view of female versus male self-administration behaviors, and suggestions for study design and implementation. Inclusion of female subjects in preclinical addiction research is timely, as problem drug and alcohol use in women is increasing. With proper attention, design, and analysis, the study of sex differences in addiction has the potential to uncover novel neural mechanisms and lead to greater translational success for addiction research. © 2021 Wiley Periodicals LLC.

A buprenorphine-validated rat model of opioid use disorder optimized to study sex differences in vulnerability to relapse

RATIONALE

Opioid use disorder (OUD) is a major epidemic in the USA. Despite evidence indicating that OUD may be particularly severe for women, preclinical models have yet to establish sex as a major factor in OUD.

OBJECTIVES

Here, we examined sex differences in vulnerability to relapse following intermittent access fentanyl self-administration and protracted abstinence and used buprenorphine, the FDA-approved treatment for OUD, to test the validity of our model.

METHODS

Following acquisition of fentanyl self-administration under one of two training conditions, male and female rats were given extended, 24-h/day access to fentanyl (0.25 μg/kg/infusion, 10 days) using an intermittent access procedure. Vulnerability to relapse was assessed using an extinction/cue-induced reinstatement procedure following 14 days of abstinence; buprenorphine (0 or 3 mg/kg/day) was administered throughout abstinence.

RESULTS

Levels of drug-seeking were high following extended-access fentanyl self-administration and abstinence; buprenorphine markedly decreased drug-seeking supporting the validity of our relapse model. Females self-administered more fentanyl and responded at higher levels during subsequent extinction testing. Buprenorphine was effective in both sexes and eliminated sex and estrous phase differences in drug-seeking. Interestingly, the inclusion of a time-out during training had a major impact on later fentanyl self-administration in females, but not males, indicating that the initial exposure conditions can persistently impact vulnerability in females.

CONCLUSIONS

These findings demonstrate the utility of this rat model for determining sex and hormonal influences on the development and treatment of OUD.

Relapse-like behavior in a mouse model of the OPRM1 (mu-opioid receptor) A118G polymorphism: Examination with intravenous oxycodone self-administration

The widely abused prescription opioid oxycodone is a mu-opioid receptor (MOP-r) agonist and addiction to such opioids is a relapsing disorder. The human MOP-r gene (OPRM1) has an important functional single nucleotide polymorphism (SNP), A118G, which affects risk of severe opioid use disorders. A112G (G/G) knock-in mice are models of human A118G carriers. We examined oxycodone self-administration (SA) in male and female G/G versus wild type (A/A) mice in SA sessions and in relapse-like behavior. Adult male and female G/G and A/A mice self-administered oxycodone (0.25 mg/kg/infusion, FR1) for 10 consecutive days. Following 10-day home cage drug free withdrawal, the mice were re-exposed to oxycodone SA for a further 10 days. MOP-r receptor mRNA in various brain regions were examined immediately after the last re-exposure session. We found that G/G mice had greater oxycodone SA than A/A mice in the initial and in re-exposure sessions. Mice of both genotypes had greater oxycodone intake during the re-exposure period than during the initial exposure. We also detected differences in MOP-r gene expression due to genotype, sex and oxycodone SA history in the dorsal striatum, hippocampus, and prefrontal cortex. These studies may improve our understanding of MOP-r-agonist self-exposure and relapse in human carriers of the A118G SNP.

Sex and Gender Differences in the Effects of Novel Psychoactive Substances

Sex and gender deeply affect the subjective effects and pharmaco-toxicological responses to drugs. Men are more likely than women to use almost all types of illicit drugs and to present to emergency departments for serious or fatal intoxications. However, women are just as likely as men to develop substance use disorders, and may be more susceptible to craving and relapse. Clinical and preclinical studies have shown important differences between males and females after administration of “classic” drugs of abuse (e.g., Δ9-tetrahydrocannabinol (THC), morphine, cocaine). This scenario has become enormously complicated in the last decade with the overbearing appearance of the new psychoactive substances (NPS) that have emerged as alternatives to regulated drugs. To date, more than 900 NPS have been identified, and can be catalogued in different pharmacological categories including synthetic cannabinoids, synthetic stimulants (cathinones and amphetamine-like), hallucinogenic phenethylamines, synthetic opioids (fentanyls and non-fentanyls), new benzodiazepines and dissociative anesthetics (i.e., methoxetamine and phencyclidine-derivatives). This work collects the little knowledge reached so far on the effects of NPS in male and female animal and human subjects, highlighting how much sex and gender differences in the effects of NPS has yet to be studied and understood.

Review of addiction risk potential associated with adolescent opioid use

Adolescence is a critical period of development with robust behavioral, morphological, hormonal, and neurochemical changes including changes in brain regions implicated in the reinforcing effects of drugs such as opioids. Here we examine the preclinical and, where appropriate complementary clinical literature, for the behavioral and neurological changes induced by adolescent opioid exposure/use and their long-term consequences during adulthood. Adolescent opioid exposure results in a widened biphasic shift in reinforcement with increased impact of positive rewarding aspects during initial use and profound negative reinforcement during adulthood. Females may have enhanced vulnerability due to fast onset of antinociceptive tolerance and reduced severity of somatic withdrawal symptoms during adolescence. Overall, adolescent opioid exposure, be it legally prescribed protracted intake or illicit consumption, results in significant and prolonged consequences of increased opioid reward concomitant with reduced analgesic efficacy and exacerbated somatic withdrawal severity during opioid use/exposure in adulthood. These findings are highly relevant to physicians, parents, law makers, and the general public as adolescent opioid exposure/misuse results in heightened risk for substance use disorders.

Repeated subcutaneous administration of PT150 has dose-dependent effects on sign tracking in male Japanese quail

A devastating feature of drug dependence is the susceptibility of relapse (40-60%) after stretches of abstinence. In both animal and human research, it has been demonstrated that cues (e.g., levers, paraphernalia) associated with drug reward can instigate renewed drug taking. Research has shown animals that attend to a cue that predicts reward more than the location of reward delivery when the cue is present (sign trackers) have an increase in corticosterone (CORT), a primary stress hormone when compared with animals that do not sign track. This interaction of sign tracking and CORT implicate CORT's effects as a possible pharmacological target for cue-induced relapse behaviors. PT150 is a novel glucocorticoid receptor antagonist that reduces the effects of CORT. Previous research has shown that oral administration of 40 mg/kg PT150 reduced sign tracking. To better understand dose-dependent effects and to control for more accurate doses, the current experiment hypothesized that PT150 (20/40/60 mg/kg) given by subcutaneous (SC) injection to male quail would reduce sign tracking to a keylight conditional stimulus that predicts a grain unconditioned stimulus dose dependently. Results showed that SC injection of 20 mg/kg PT150 reduced sign tracking, but 40 or 60 mg/kg did not. The main findings from the current study are that the glucocorticoid receptor antagonist PT150 reduces sign tracking behavior dose dependently, and SC administration may provide better bioavailability compared with our previous study that used an oral route of administration. The current findings support previous literature by suggesting that the glucocorticoid receptor may be a potential pharmacological target for reducing relapse-like behaviors. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Sex differences in opioid reinforcement under a fentanyl vs. food choice procedure in rats

Clinical evidence suggest that men are more sensitive than women to the abuse-related effects of mu-opioid agonists. In contrast, preclinical studies suggest the opposite sex difference. The aim of the present study was to clarify this discrepancy using a fentanyl vs. diluted Ensure^® choice procedure to assess sex differences in opioid reinforcement. Sex differences in intravenous (IV) fentanyl self-administration were examined under a fixed-ratio (FR5) schedule, a multi-day progressive-ratio (PR) schedule for behavioral economic analysis, and a concurrent (choice) schedule of fentanyl and diluted Ensure^® reinforcement in Sprague-Dawley male and female rats. The fentanyl dose-effect function under the FR5 schedule was significantly shifted upward in females compared to males. Similarly, the reinforcing effectiveness of both fentanyl (3.2 and 10 µg/kg per injection, IV) and diluted Ensure^® (18 and 56%) were greater in females than in males as assessed using behavioral economic analysis, irrespective of dose or concentration. However, under a fentanyl vs. foodchoice procedure, males chose 3.2 µg/kg per injection fentanyl injections over 18%, but not 56%, diluted Ensure^® at a higher percentage compared to females. Overall, these results suggest that the expression of sex differences in opioid reinforcement depends upon the schedule of reinforcement and that preclinical opioid vs. food choice procedures provide a translationally relevant measure (i.e., behavioral allocation) consistent with the direction of sex differences reported in the clinical literature.

Incubation of Methamphetamine but not Heroin Craving After Voluntary Abstinence in Male and Female Rats

We recently introduced an animal model of incubation of methamphetamine craving after choice-based voluntary abstinence in male rats. Here we studied the generality of this phenomenon to (1) female rats, and (2) male and female rats with a history of heroin self-administration. We first trained rats to self-administer palatable food pellets for 6 days (6 h per day) for either methamphetamine (0.1 mg/kg/infusion) or heroin (0.1 mg/kg/infusion) for 12 days (6 h/day). We then assessed relapse to drug seeking under extinction conditions after 1 and 21 abstinence days. Between tests, the rats underwent either voluntary abstinence (achieved via a discrete choice procedure between drug and palatable food; 20 trials/day) or home-cage forced abstinence. We found no sex differences in methamphetamine self-administration or in the strong preference for the palatable food over methamphetamine during the choice-based voluntary abstinence. In both sexes, methamphetamine seeking in the relapse tests was higher after 21 days of either voluntary or forced abstinence than after 1 day (incubation of methamphetamine craving). We also found no sex differences in heroin self-administration or the strong preference for the palatable food over heroin during the choice-based voluntary abstinence. However, male and female rats with a history of heroin self-administration showed incubation of heroin craving after forced but not voluntary abstinence. Our results show that incubation of methamphetamine craving after voluntary abstinence generalizes to female rats. Unexpectedly, prolonged voluntary abstinence prevented the emergence of incubation of heroin craving in both sexes.

Oxycodone self-administration in male and female rats

RATIONALE

Oxycodone is one of the most widely prescribed painkillers in the USA. However, its use is complicated by high abuse potential. As sex differences have been described in drug addiction, the present study tested for sex differences in intravenous oxycodone self-administration in rats.

METHODS

Male and female Sprague-Dawley rats were implanted with jugular vein catheters and trained to self-administer oxycodone (0.03 mg/kg/infusion) under fixed ratio 1 (FR1), FR2, and FR5 schedules of reinforcement followed by a dose-response study to assess sensitivity to the reinforcing effects of oxycodone. In separate rats, sucrose pellet self-administration was assessed under an FR1 schedule to determine whether sex differences in oxycodone self-administration could be generalized across reinforcers. In separate rats, oxycodone distribution to plasma and brain was measured after intravenous drug delivery.

RESULTS

In the first 3 trials under an FR1 schedule of reinforcement, male rats self-administered more oxycodone than females. In contrast, females self-administered more sucrose pellets. Under FR2 and FR5 schedules, no significant sex differences in oxycodone intake were observed, although female rats had significantly more inactive lever presses. Male and female rats showed similar inverted U-shaped dose-effect functions, with females tending to self-administer more oxycodone than males at higher doses. No significant sex differences were observed in plasma or brain oxycodone levels, suggesting that sex differences in oxycodone self-administration behavior were not due to pharmacokinetics.

CONCLUSION

Our results suggest subtle sex differences in oxycodone self-administration, which may influence the abuse liability of oxycodone and have ramifications for prescription opioid addiction treatment.

Effects of sex and remifentanil dose on rats' acquisition of responding for a remifentanil-conditioned reinforcer

Opioid-conditioned reinforcement is thought to exacerbate opioid abuse and dependence. Sex/gender can influence opioid abuse behaviors, but the effects of sex/gender on opioid-conditioned reinforcement, specifically, are unclear. In this study, we compared new-response acquisition with opioid-conditioned reinforcement in male and female rats. First, separate groups received response-independent remifentanil injections (0.0-32.0 μg/kg, intravenous) and presentations of a light-noise stimulus. In the experimental groups, injections and stimulus presentations always co-occurred [paired Pavlovian conditioning (PAV)]; in the control groups, the two occurred independently of each other (random PAV). Next, in the instrumental acquisition (ACQ) sessions, two novel nose-poke manipulanda were introduced. All animals (regardless of sex, dose, and PAV type) could respond in the active nose-poke, which produced the stimulus alone, or in the inactive nose-poke. Both males and females dose-dependently acquired nose-poke responding (active>inactive) after paired PAV, but not after random PAV. Therefore, the stimulus was a conditioned reinforcer. We identified three sex differences. First, only females acquired responding after paired PAV with 32.0 μg/kg remifentanil. Second, using a progressive ratio schedule for ACQ, both sexes acquired responding, but females made significantly more active responses. Third, when a single session of PAV was conducted, only males acquired responding. Thus, rats' sex interacts with pharmacological and environmental factors to determine opioid-conditioned reinforcement.

Effects of paclitaxel on mechanical sensitivity and morphine reward in male and female C57Bl6 mice

This study evaluated the hypothesis that a paclitaxel treatment regimen sufficient to produce mechanical allodynia would alter sensitivities of male and female mice to the conditioned rewarding and reinforcing effects of morphine. Saline or paclitaxel were administered on Days 1, 3, 5, and 7 in male and female C57Bl/6 mice to induce morphine-reversible mechanical allodynia as measured by the Von Frey filament test. Paclitaxel treatment did not change sensitivity to morphine conditioned place preference (CPP) relative to saline treatment in either male or female mice. Morphine produced peak self-administration under a fixed ratio-1 (FR1) schedule of reinforcement for 0.03 mg/kg morphine per infusion in female mice and 0.1 mg/kg morphine per infusion in male mice. During the progressive ratio experiments, saline treatment in male mice decreased the number of morphine infusions for 12 days whereas the paclitaxel-treated male mice maintained responding for morphine similar to baseline levels during the same time period. However, paclitaxel did not have an overall effect on the reinforcing efficacy of morphine assessed over a limited dose range during the course of the repeated self-administration. These results suggest that the reward-related behavioral effects of morphine are overall not robustly altered by the presence of paclitaxel treatment under the current dosing regimen, with the exception of maintaining a small yet significant higher baseline than saline treatment during the development of allodynia in male mice. (PsycINFO Database Record

The role of dose and restriction state on morphine-, cocaine-, and LiCl-induced suppression of saccharin intake: A comprehensive analysis

Rats avoid intake of a taste cue when paired with a drug of abuse or with the illness-inducing agent, lithium chloride (LiCl). Although progress has been made, it is difficult to compare the suppressive effects of abused agents and LiCl on intake of a gustatory conditioned stimulus (CS) because of the cross-laboratory use of different CSs, different unconditioned stimuli (USs), and different doses of the drugs, different conditioning regimens, and different restriction states. Here we have attempted to unify these variables by comparing the suppressive effects of a range of doses of morphine, cocaine, and LiCl on intake of a saccharin CS using a common regimen in non-restricted, food restricted, or water restricted male Sprague-Dawley rats. The results showed that, while the putatively aversive agent, LiCl, was effective in suppressing intake of the taste cue across nearly all doses, regardless of restriction state, the suppressive effects of both morphine and cocaine were greatly reduced when evaluated in either food or water restricted rats. Greater sensitivity to drug was revealed, at very low doses, when testing occurred in the absence of need (i.e., when the rats were non-restricted). Together, these results provide the first uniform and comprehensive analysis of the suppressive effects of morphine, cocaine, and LiCl as a function of dose and restriction state. In the present case, the suppressive effects of morphine and cocaine are found to differ from those of LiCl and, in some respects, from one another as well.

Sex Differences in Behavioral Dyscontrol: Role in Drug Addiction and Novel Treatments

The purpose of this review is to discuss recent findings related to sex differences in behavioral dyscontrol that lead to drug addiction, and clinical implications for humans are discussed. This review includes research conducted in animals and humans that reveals fundamental aspects of behavioral dyscontrol. The importance of sex differences in aspects of behavioral dyscontrol, such as impulsivity and compulsivity, is discussed as major determinants of drug addiction. Behavioral dyscontrol during adolescence is also an important consideration, as this is the time of onset for drug addiction. These vulnerability factors additively increase drug-abuse vulnerability, and they are integral aspects of addiction that covary and interact with sex differences. Sex differences in treatments for drug addiction are also reviewed in terms of their ability to modify the behavioral dyscontrol that underlies addictive behavior. Customized treatments to reduce behavioral dyscontrol are discussed, such as (1) using natural consequences such as non-drug rewards (e.g., exercise) to maintain abstinence, or using punishment as a consequence for drug use, (2) targeting factors that underlie behavioral dyscontrol, such as impulsivity or anxiety, by repurposing medications to relieve these underlying conditions, and (3) combining two or more novel behavioral or pharmacological treatments to produce additive reductions in drug seeking. Recent published work has indicated that factors contributing to behavioral dyscontrol are an important target for advancing our knowledge on the etiology of drug abuse, intervening with the drug addiction process and developing novel treatments.

Stress is a principal factor that promotes tobacco use in females

Tobacco use is a major economic and health problem. It is particularly concerning that women consume more tobacco products, have a more difficult time quitting smoking, and are less likely to benefit from smoking cessation therapy than men. As a result, women are at higher risk of developing tobacco-related diseases. Clinical evidence suggests that women are more susceptible to anxiety disorders, and are more likely to smoke in order to cope with stress than men. During smoking abstinence, women experience more intense anxiety than men and report that the anxiety-reducing effects of smoking are the main reason for their continued tobacco use and relapse. Consistent with this, pre-clinical studies using rodent models suggest that females display more intense stress during nicotine withdrawal than males. This review posits that in women, stress is a principal factor that promotes the initiation of tobacco use and relapse behavior during abstinence. Studies are reviewed at both the clinical and pre-clinical levels to provide support for our hypothesis that stress plays a central role in promoting tobacco use vulnerability in females. The clinical implications of this work are also considered with regard to treatment approaches and the need for more research to help reduce health disparities produced by tobacco use in women.

Sex differences in behavior and neural development and their role in adolescent vulnerability to substance use

Adolescents are especially prone to risky behavior and to the emergence of psychological disorders like substance abuse, anxiety and depression. However, there is a sex (or gender) difference in this vulnerability, with females being more prone to developing internalizing disorders and males being more likely to engage in risky behavior and drug use. While several researchers have proposed that there is a relationship between corticolimbic circuit development and adolescent vulnerability, the current proposed models do not take sex differences into account. In this review, we explore recent findings from both human and rodent studies of sex differences during adolescence. In particular, we consider epidemiological studies on the factors that contribute to the development of substance abuse and internalizing disorders, laboratory studies on reward-related and decision-making behavior, and neuroanatomical studies on the development of several structures in the corticolimbic circuit (i.e., prefrontal cortex [PFC], amygdala and striatum). We then integrate these recent findings into models of adolescent vulnerability to substance use that have previously not addressed sex differences. Lastly, we discuss methodological considerations for the interpretation and design of studies on sex (or gender) differences during adolescence while highlighting some opportunities for future investigations.

Food restriction-induced augmentation of heroin seeking in female rats: manipulations of ovarian hormones

RATIONALE

Food restriction augments heroin seeking in chronically food-restricted male rats under withdrawal, an effect not yet examined in female rats. Importantly, women and female rats possess an increased vulnerability to drugs of abuse, which may be mediated by fluctuations in ovarian hormones.

OBJECTIVES

We investigated the role of estradiol and progesterone in augmented heroin seeking in chronically food-restricted female rats, under withdrawal.

METHODS

Female rats self-administered heroin for 10-12 days and were then allowed unrestricted (sated) or restricted access to food (FDR; ∼10 % reduction in body weight) for 14 days. On day 14, rats underwent a heroin-seeking test. Exp. 1: Rats underwent ovariectomy or sham surgery and were treated with a low dose of estradiol (5.0 % in cholesterol; subcutaneous capsule). Exp. 2: Rats underwent ovariectomy and were administered with a high dose of estradiol (0.5 mg/kg; subcutaneous) for 8 days before testing. Exp. 3: Progesterone injections (2.0 mg/kg; subcutaneous) were administered 24 h and 2 h before testing.

RESULTS

Food restriction resulted in augmented heroin seeking, compared to sated controls. While ovariectomy had no effect, estradiol replacement attenuated the food restriction effect. Injections of progesterone had no effect on heroin seeking in either the sated or FDR groups.

CONCLUSIONS

The effect of food restriction on heroin seeking in female rats under withdrawal is as robust as previously found in males. Interestingly, estradiol replacement, but not progesterone, attenuates the food restriction effect in the ovariectomized rats, possibly due to its anorexic properties.

Intracranial self-stimulation reward thresholds during morphine withdrawal in rats bred for high (HiS) and low (LoS) saccharin intake

RATIONALE

Sweet preference is a marker of vulnerability to substance use disorders, and rats selectively bred for high (HiS) vs. low saccharin (LoS) intake display potentiated drug-seeking behaviors. Recent work indicated that LoS rats were more responsive to the negative effects of drugs in several assays.

OBJECTIVE

The current study used the intracranial self-stimulation (ICSS) procedure to investigate the anhedonic component of morphine withdrawal in male HiS and LoS rats.

METHODS

Rats were administered morphine (10mg/kg) or saline for 8 days. To evaluate withdrawal effects, reward thresholds were measured 24 and 28h following the 8th morphine injection (spontaneous withdrawal) and again for 4 days following daily acute morphine and naloxone (1mg/kg) administration (precipitated withdrawal).

RESULTS

24h following the final morphine injection, reward thresholds in LoS rats were significantly elevated compared to reward thresholds in LoS controls, indicating spontaneous withdrawal. This effect was not observed in HiS rats. LoS rats also showed greater elevations of reward thresholds on several days during naloxone-precipitated withdrawal compared to their HiS counterparts.

CONCLUSIONS

LoS rats were more sensitive to morphine withdrawal-mediated elevations in ICSS thresholds than HiS rats. While these differences were generally modest, our data suggest that severity of the negative affective component of opiate withdrawal may be influenced by genotypes related to addiction vulnerability.

Cocaine-, caffeine-, and stress-evoked cocaine reinstatement in high vs. low impulsive rats: treatment with allopregnanolone

BACKGROUND

Previous research indicates that individual differences in traits such as impulsivity, avidity for sweets, and novelty reactivity are predictors of several aspects of drug addiction. Specifically, rats that rank high on these behavioral measures are more likely than their low drug-seeking counterparts to exhibit several characteristics of drug-seeking behavior. In contrast, initial work suggests that the low drug-seeking animals are more reactive to negative events (e.g., punishment and anxiogenic stimuli). The goal of this study was to compare high and low impulsive rats on reinstatement of cocaine-seeking behavior elicited by cocaine (COC) and by negative stimuli such as the stress-inducing agent yohimbine (YOH) or a high dose of caffeine (CAFF). An additional goal was to determine whether treatment with allopregnanolone (ALLO) would reduce reinstatement (or relapse) of cocaine-seeking behavior under these priming conditions.

METHODS

Female rats were selected as high (HiI) or low (LoI) impulsive using a delay-discounting task. After selection, they were allowed to self-administer cocaine for 12 days. Cocaine was then replaced with saline, and rats extinguished lever responding over 16 days. Subsequently, rats were pretreated with either vehicle control or ALLO, and cocaine seeking was reinstated by injections of COC, CAFF, or YOH.

RESULTS

While there were no phenotype differences in maintenance and extinction of cocaine self-administration or reinstatement under control treatment conditions, ALLO attenuated COC- and CAFF-primed reinstatement in LoI but not HiI rats.

CONCLUSIONS

Overall, the present findings suggest that individual differences in impulsive behavior may influence efficacy of interventions aimed to reduce drug-seeking behavior.

Effects of combined exercise and progesterone treatments on cocaine seeking in male and female rats

BACKGROUND

Individually, both treatment with progesterone and concurrent access to an exercise wheel reduce cocaine self-administration under long-access conditions and suppress cocaine-primed reinstatement in female rats. In the present study, wheel running and progesterone (alone and combined) were assessed for their effects on reinstatement of cocaine-seeking primed by yohimbine, cocaine, and cocaine-paired cues.

METHODS

Male and female rats were implanted with an intravenous catheter and allowed to self-administer cocaine (0.4 mg/kg/inf, iv) during 6-h sessions for 10 days. Subsequently, the groups of male and female rats were each divided into two groups that were given concurrent access to either a locked or unlocked running wheel under extinction conditions for 14 days. Next, all four groups were tested in a within-subjects design for reinstatement of cocaine-seeking precipitated by separate administration of cocaine-paired stimuli, yohimbine, or cocaine or the combination of yohimbine + cocaine-paired stimuli or cocaine + cocaine-paired stimuli. These priming conditions were tested in the presence of concurrent wheel access (W), pretreatment with progesterone (P), or both (W + P).

RESULTS

In agreement with previous results, females responded more for cocaine than males during maintenance. Additionally, concurrent wheel running attenuated extinction responses and cocaine-primed reinstatement in females but not in males. Across all priming conditions, W + P reduced reinstatement compared to control conditions, and for cocaine-primed reinstatement in male rats, the combined W + P treatment was more effective than W or P alone.

CONCLUSION

Under certain conditions, combined behavioral (exercise) and pharmacological (progesterone) interventions were more successful at reducing cocaine-seeking behavior than either intervention alone.

Cocaine self-administration punished by i.v. histamine in rat models of high and low drug abuse vulnerability: effects of saccharin preference, impulsivity, and sex

A key feature of substance use disorders is continued drug consumption despite aversive consequences. This has been modeled in the animal laboratory by pairing drug self-administration with electric shock, thereby punishing drug intake (Deroche-Gamonet et al. 2004). In the present experiments, we examined the effects of punishment on i.v. cocaine self-administration by adding histamine to the cocaine solution with three different animal models of high and low vulnerability to drug abuse: rats selectively bred for high (HiS) and low (LoS) saccharin consumption, rats selected for high (HiI) and low (LoI) impulsivity, and sex differences. Animals were allowed to self-administer cocaine (0.4 mg/kg/infusion) to establish a baseline of operant responding. Histamine (4.0mg/kg/infusion) was then added directly into the cocaine solution and its consequent effects on self-administration were compared to baseline. The histamine+cocaine solution was then replaced with a cocaine-only solution, and the rats' operant responding was again compared to baseline. Concurrent histamine exposure was effective in reducing cocaine consumption in all groups of rats; however, LoS and female rats took longer to return to baseline levels of cocaine consumption after histamine was removed compared to HiS and male rats. These data suggest that the reduction of drug self-administration by aversive consequences may differ in groups that vary in drug use vulnerability . Such results may inform pharmacological strategies that enhance the negative aspects of drug consumption.

Maternally administered sustained-release naltrexone in rats affects offspring neurochemistry and behaviour in adulthood

Naltrexone is not recommended during pregnancy. However, sustained-release naltrexone implant use in humans has resulted in cases of inadvertent foetal exposure. Here, we used clinically relevant dosing to examine the effects of maternally administered sustained-release naltrexone on the rat brain by examining offspring at birth and in adulthood. Maternal treatment (naltrexone or placebo implant) started before conception and ceased during gestation, birth or weaning. Morphometry was assessed in offspring at birth and adulthood. Adult offspring were evaluated for differences in locomotor behaviour (basal and morphine-induced, 10 mg/kg, s.c.) and opioid neurochemistry, propensity to self-administer morphine and cue-induced drug-seeking after abstinence. Blood analysis confirmed offspring exposure to naltrexone during gestation, birth and weaning. Naltrexone exposure increased litter size and reduced offspring birth-weight but did not alter brain morphometry. Compared to placebo, basal motor activity of naltrexone-exposed adult offspring was lower, yet they showed enhanced development of psychomotor sensitization to morphine. Developmental naltrexone exposure was associated with resistance to morphine-induced down-regulation of striatal preproenkephalin mRNA expression in adulthood. Adult offspring also exhibited greater operant responding for morphine and, in addition, cue-induced drug-seeking was enhanced. Collectively, these data show pronounced effects of developmental naltrexone exposure, some of which persist into adulthood, highlighting the need for follow up of humans that were exposed to naltrexone in utero.

Exercise to reduce the escalation of cocaine self-administration in adolescent and adult rats

RATIONALE

Concurrent access to an exercise wheel decreases cocaine self-administration under short access (5 h/day for 5 days) conditions and suppresses cocaine-primed reinstatement in adult rats.

OBJECTIVE

The effect of exercise (wheel running) on the escalation of cocaine intake during long access (LgA, 6 h/day for 26 days) conditions was evaluated.

METHODS

Adolescent and adult female rats acquired wheel running, and behavior was allowed to stabilize for 3 days. They were then implanted with an iv catheter and allowed to self-administer cocaine (0.4 mg/kg, iv) during 6-h daily sessions for 16 days with concurrent access to either an unlocked or a locked running wheel. Subsequently, for ten additional sessions, wheel access conditions during cocaine self-administration sessions were reversed (i.e., locked wheels became unlocked and vice versa).

RESULTS

In the adolescents, concurrent access to the unlocked exercise wheel decreased responding for cocaine and attenuated escalation of cocaine intake irrespective of whether the locked or unlocked condition came first. However, cocaine intake increased when the wheel was subsequently locked for the adolescents that had initial access to an unlocked wheel. Concurrent wheel access either before or after the locked wheel access did not reduce cocaine intake in adults.

CONCLUSIONS

Wheel running reduced cocaine intake during LgA conditions in adolescent but not adult rats, and concurrent access to the running wheel was necessary. These results suggest that exercise prevents cocaine seeking and that this effect is more pronounced in adolescents than adults.

Considering gender in cannabinoid research: a step towards personalized treatment of marijuana addicts

Addiction is a complex disorder with interacting factors, including environmental factors, drug-induced neurobiological changes, comorbidity, personality traits and stress responsivity. Numerous genetic variants that affect these factors may work in concert to affect vulnerability and severity of addiction. Traditionally, abuse of illicit drugs, including cannabis, was considered to be primarily a problem specific to men but the recent focus on drug addiction in women has brought attention to numerous sex differences in the central effects of these drugs, epidemiology of abuse-related disorders, etiologic considerations and psychiatric comorbidity. Gender is now recognised as a major factor in the modulation of the pharmacological effects of drugs of abuse, and sex differences have been reported in various phases of the addiction cycle in both humans and animals. Recently, important gender-dependent differences have been detected in the rates of initiation of marijuana smoking and in the frequency of use. Several animal studies, and in particular self-administration studies, confirmed the crucial role played by sex and gonadal hormones in determining higher sensitivity to marijuana's rewarding properties and vulnerability to cannabis addiction in females than in males. In general, women also show higher rate of relapse to drug use than men, likely due to divergent withdrawal experiences and increased reactivity to internal (emotional) and external (drug-associated) cues. According with this, craving for marijuana smoking and propensity to re-use cannabis after abstinence may also develop differently between the sexes and require distinct treatment approaches, thus highlighting the urgent need for gender-tailored prevention strategies and detoxification treatments.

Eating high fat chow enhances the locomotor-stimulating effects of cocaine in adolescent and adult female rats

RATIONALE

Dopamine systems vary through development in a manner that can impact drugs acting on those systems. Dietary factors can also impact the effects of drugs acting on dopamine systems.

OBJECTIVES

This study examined whether eating high fat chow alters locomotor effects of cocaine (1-56 mg/kg) in adolescent and adult female rats.

METHODS

Cocaine was studied in rats (n = 6/group) with free access to standard (5.7% fat) or high fat (34.3%) chow or restricted access to high fat chow (body weight matched to rats eating standard chow).

RESULTS

After 1 week of eating high fat chow (free or restricted access), sensitivity to cocaine was significantly increased in adolescent and adult rats, compared with rats eating standard chow. Sensitivity to cocaine was also increased in adolescent rats with restricted, but not free, access to high fat chow for 4 weeks. When adolescent and adult rats that previously ate high fat chow ate standard chow, sensitivity to cocaine returned to normal. In adolescent and adult female rats eating high fat chow, but not those eating standard chow, sensitivity to cocaine increased progressively over once weekly tests with cocaine (i.e., sensitization) in a manner that was not statistically different between adolescents and adults.

CONCLUSIONS

These results show that eating high fat chow alters sensitivity of female rats to acutely administered cocaine and also facilitates the development of sensitization to cocaine. That the type of food consumed can increase drug effects might have relevance to vulnerability to abuse cocaine in the female population.

You are what you eat: influence of type and amount of food consumed on central dopamine systems and the behavioral effects of direct- and indirect-acting dopamine receptor agonists

The important role of dopamine (DA) in mediating feeding behavior and the positive reinforcing effects of some drugs is well recognized. Less widely studied is how feeding conditions might impact the sensitivity of drugs acting on DA systems. Food restriction, for example, has often been the focus of aging and longevity studies; however, other studies have demonstrated that mild food restriction markedly increases sensitivity to direct- and indirect-acting DA receptor agonists. Moreover, it is becoming clear that not only the amount of food, but the type of food, is an important factor in modifying the effects of drugs. Given the increased consumption of high fat and sugary foods, studies are exploring how consumption of highly palatable food impacts DA neurochemistry and the effects of drugs acting on these systems. For example, eating high fat chow increases sensitivity to some behavioral effects of direct- as well as indirect-acting DA receptor agonists. A compelling mechanistic possibility is that central DA pathways that mediate the effects of some drugs are regulated by one or more of the endocrine hormones (e.g. insulin) that undergo marked changes during food restriction or after consuming high fat or sugary foods. Although traditionally recognized as an important signaling molecule in regulating energy homeostasis, insulin can also regulate DA neurochemistry. Because direct- and indirect-acting DA receptor drugs are used therapeutically and some are abused, a better understanding of how food intake impacts response to these drugs would likely facilitate improved treatment of clinical disorders and provide information that would be relevant to the causes of vulnerability to abuse drugs. This article is part of a Special Issue entitled 'Central Control of Food Intake'.

Sex differences and ovarian hormones in animal models of drug dependence

Increasing evidence indicates the presence of sex differences in many aspects of drug abuse. Most studies reveal that females exceed males during the initiation, escalation, extinction, and reinstatement (relapse) of drug-seeking behavior, but males are more sensitive than females to the aversive effects of drugs such as drug withdrawal. Findings from human and animal research indicate that circulating levels of ovarian steroid hormones account for these sex differences. Estrogen (E) facilitates drug-seeking behavior, while progesterone (P) and its metabolite, allopregnanalone (ALLO), counteract the effects of E and reduce drug seeking. Estrogen and P influence other behaviors that are affiliated with drug abuse such as drug-induced locomotor sensitization and conditioned place preference. The enhanced vulnerability to drug seeking in females vs. males is also additive with the other risk factors for drug abuse (e.g., adolescence, sweet preference, novelty reactivity, and impulsivity). Finally, treatment studies using behavioral or pharmacological interventions, including P and ALLO, also indicate that females show greater treatment effectiveness during several phases of the addiction process. The neurobiological basis of sex differences in drug abuse appears to be genetic and involves the influence of ovarian hormones and their metabolites, the hypothalamic pituitary adrenal (HPA) axis, dopamine (DA), and gamma-hydroxy-butyric acid (GABA). Overall, sex and hormonal status along with other biological risk factors account for a continuum of addiction-prone and -resistant animal models that are valuable for studying drug abuse prevention and treatment strategies.

Sex differences in the self-administration of cannabinoids and other drugs of abuse

Many studies have provided evidence for important sex-dependent differences in the origins, outcomes and treatment of drug abuse and dependence. Preclinical studies typically have employed animal models of addiction, such as oral or intravenous self-administration, to untangle the environmental, neurobiological and genetic factors that contribute to the shift from occasional, recreational use to compulsive, uncontrolled intake of drugs. Craving and relapse of drug seeking in abstinent individuals have also been found to differ between men and women. Identification of the neurobiological basis of craving and drug dependence continues to pose a challenge to addiction research. Significant sex differences are emerging in substance-abuse-related behavior, which has increased the demand for research on how drug consumption may have different causes, progression and consequences in men and women. In keeping with epidemiological data in humans, differences between the two sexes in drug seeking and intake have been well-documented in animal studies, with most recent findings related to abuse of cannabinoids. Clinical and preclinical findings indicate that sex and gonadal hormones may account for individual differences in susceptibility to the reinforcing effects of addictive substances, and that differences in vulnerability to drug abuse may be mediated by the same biological mechanisms. This review focuses on the differences between males and females in relation to drug self-administration and how such behavior may be affected by hormonal status.

Sex differences in tolerance to the locomotor depressant effects of lobeline in periadolescent rats

Lobeline is being tested in clinical trials as a pharmacotherapy for methamphetamine abuse and attention deficit hyperactivity disorder. Preclinical research demonstrates that lobeline produces locomotor hypoactivity apart from its therapeutic effects; however, the hypothesis that there are sex differences in hypoactivity or in the development of tolerance to its locomotor depressant effects has not been investigated. Periadolescent rats were injected with saline to determine baseline locomotor activity. Animals received saline or lobeline (1.0-10mg/kg) daily for 7 consecutive days (post natal days 29-35), and were challenged with saline 24h later to assess baseline activity. Lobeline produced hypoactivity in total horizontal activity and center distance travelled. Tolerance developed to the lobeline-induced hypoactivity and sex differences in lobeline tolerance were observed on both measures. Females acquired tolerance to lobeline 5.6 mg/kg at a slower rate than males. Saline challenge revealed a linear dose-dependent trend of hyperactivity on both measures, which indicates that rats exhibited altered locomotor behavior 24h after the final lobeline treatment. These findings demonstrate sex differences in the hypoactive response to lobeline prior to puberty and suggest that females may experience more locomotor depressant effects than males. Chronic lobeline may induce hyperactivity following cessation of treatment.

Impulsivity as a behavioral measure of withdrawal of orally delivered PCP and nondrug rewards in male and female monkeys

BACKGROUND

Withdrawal of phencyclidine (PCP), ethanol (ETOH), and other drugs reduces operant responding maintained by food.

OBJECTIVES

Experiment 1 examined the effects of withdrawing daily short access (2 h) to drug on impulsivity for saccharin (SACC) using a delay discounting task and comparing male and female rhesus monkeys. Experiment 2 examined the effects of withdrawing a nondrug substance (e.g., food or SACC) on impulsivity for PCP.

MATERIALS AND METHODS

In experiment 1, either PCP or ETOH was available daily with water for 2 h under a fixed ratio 16 (FR 16) or FR 8 schedule, respectively. In a second component, SACC was available for 45 min under a delay discounting schedule. Next, water was substituted, and drug access was then restored. In experiment 2, PCP was available under a delay discounting schedule during food satiation or restriction or during concurrent SACC vs water access.

RESULTS

In experiment 1, withdrawal of 0.5 mg/ml PCP increased impulsivity for SACC, but not SACC intake, in males and females. During 16% ETOH access, impulsivity for SACC was elevated compared to baseline water access, and it returned to baseline levels during ETOH withdrawal. In experiment 2, food restriction resulted in increased PCP intake in males and females and increases in impulsivity for PCP that were greater in males than in females. SACC withdrawal had no effect on impulsivity for PCP or PCP intake.

CONCLUSIONS

Withdrawal of PCP and reduced food access increased impulsivity for SACC or PCP, respectively. Impulsivity is a sensitive indicator of drug dependence.

Impulsivity predicts the escalation of cocaine self-administration in rats

Impulsivity, as measured by the delay-discounting task, predicts the acquisition of cocaine self-administration and reinstatement of cocaine seeking in rats. The purpose of this study was to extend these results to the escalation phase of drug self-administration. Female rats were initially screened for high (HiI) or low (LoI) impulsivity for food reinforcement using a delay-discounting procedure. They were then implanted with i.v. catheters and trained to lever press for cocaine infusions (0.8 mg/kg). Once cocaine intake stabilized, rats were allowed to self-administer cocaine (0.4 mg/kg) under a fixed-ratio 1 (FR 1) schedule during three, 2 h short-access sessions. Subsequently, performance was briefly assessed under a progressive ratio (PR) schedule for 3 doses of cocaine (0.2, 0.8, and 3.2 mg/kg). Following PR testing, the cocaine dose was then changed to 0.4 mg/kg. Session length was then extended to 6 h for 21 days (extended access), and 0.4 mg/kg cocaine was available under a FR 1 schedule. After the 21-day extended access phase, responses and infusions under the short access FR and PR dose-response conditions were reassessed. The results indicated that HiI rats escalated cocaine-reinforced responding during the extended access condition, but LoI rats did not. HiI rats also earned significantly more infusions than LoI rats under the post-escalation short access FR condition. However, HiI and LoI rats did not differ under the pre- and post-extended access PR conditions. This study suggests that individual differences in impulsivity predict escalation of cocaine self-administration in female rats, which may have implications in the prediction of binge-like patterns of cocaine intake in women.

Effects of allopregnanolone on the reinstatement of cocaine-seeking behavior in male and female rats

RATIONALE

Previous research indicates that progesterone (PROG) decreased cocaine-seeking behavior in female rats. This effect of PROG may be in part due to its metabolite allopregnanolone (ALLO), which has been shown to decrease the sensitizing effects of cocaine and reduce lethality associated with cocaine overdose in mice.

OBJECTIVE

The purpose of the present study was to examine the effects of ALLO on the reinstatement of cocaine-seeking behavior in female and male rats.

METHODS

Rats were trained to lever press for i.v. infusions of cocaine (0.4 mg/kg per infusion) during 2-h sessions, and once acquisition criteria were met, cocaine self-administration continued for 14 days. Cocaine was then replaced with saline, and lever pressing was allowed to extinguish over 21 days. After the extinction phase, rats received s.c. ALLO (15 or 30 mg/kg), PROG (0.5 mg/kg), PROG (0.5 mg/kg) plus the 5-alpha reductase inhibitor finasteride (25 mg/kg), or vehicle pretreatment for 3 days. Rats were then tested during reinstatement with three doses of cocaine (5, 10, and 15 mg/kg, i.p. in mixed order).

RESULTS

PROG, and to a greater extent ALLO, decreased cocaine-primed reinstatement in females, while finasteride blocked the attenuating effects of PROG on reinstatement. ALLO had no effect on cocaine-primed reinstatement in males.

CONCLUSION

These findings suggest that ALLO may explain part of PROG's inhibitory effect on cocaine-primed reinstatement, and it may serve as a novel approach for preventing relapse in female cocaine abusers.

Brain regional Fos expression elicited by the activation of mu- but not delta-opioid receptors of the ventral tegmental area: evidence for an implication of the ventral thalamus in opiate reward

Both mu-opioid receptors (MORs) and delta-opioid receptors (DORs) are expressed in the ventral tegmental area (VTA) and are thought to be involved in the addictive properties of opiates. However, their respective contributions to opiate reward remain unclear. We used intracranial self-administration (ICSA) to study the rewarding effects of morphine microinjections into the VTA of male and female MOR-/- and DOR-/- mice. In brains of mice tested for intra-VTA morphine self-administration, we analyzed regional Fos protein expression to investigate the neural circuitry underlying this behavior. Male and female WT and DOR-/- mice exhibited similar self-administration performances, whereas knockout of the MOR gene abolished intra-VTA morphine self-administration at all doses tested. Naloxone (4 mg/kg) disrupted this behavior in WT and DOR mutants, without triggering physical signs of withdrawal. Morphine ICSA was associated with an increase in Fos within the nucleus accumbens, striatum, limbic cortices, amygdala, hippocampus, the lateral mammillary nucleus (LM), and the ventral posteromedial thalamus (VPM). This latter structure was found to express high levels of Fos exclusively in self-administering WT and DOR-/- mice. Abolition of morphine reward in MOR-/- mice was associated with a decrease in Fos-positive neurons in the mesocorticolimbic dopamine system, amygdala, hippocampus (CA1), LM, and a complete absence within the VPM. We conclude that (i) VTA MORs, but not DORs, are critical for morphine reward and (ii) the role of VTA-thalamic projections in opiate reward deserves to be further explored.

Modeling the role of environment in addiction

The aim of this review is to provide an overview of the main types of animal models used to investigate the modulatory role of environment on drug addiction. The environment can alter the responsiveness to addictive drugs in at least three major ways. First, adverse life experiences can make an individual more vulnerable to develop drug addiction or to relapse into drug seeking. Second, neutral environmental cues can acquire, through Pavlovian conditioning, the ability to trigger drug seeking even after long periods of abstinence. Third, the environment immediately surrounding drug taking can alter the behavioral, subjective, and rewarding effects of a given drug, thus influencing the propensity to use the same drug again. We have focused in particular on the results obtained using an animal model we have developed to study the latter type of drug-environment interaction.

Estradiol-induced conditioned place preference may require actions at estrogen receptors in the nucleus accumbens

Intrinsic rewarding effects of estradiol (E(2)) may underlie some of the sex differences that emerge postpuberty for the prevalence of drug use and behavioral responses to drugs, but the effects and mechanisms of E(2) for reward have not been well characterized. Conditioned place preference (CPP), as measured by the time spent on the nonpreferred/drug-associated side of the chamber, was utilized as a functional assay to investigate the effects and mechanisms of E(2) in the nucleus accumbens for reward. To determine whether intracellular estrogen receptors (ERs) are important for E(2)-induced CPP, rats were administered E(2) (10 microg; subcutaneously (s.c.)), which produced CPP in each experiment, and/or ER blockers, such as tamoxifen (Experiment 1), ICI 182,780 (Experiment 2), or antisense oligonucleotides targeted to ERs (Experiment 3). Experiment 1: E(2) significantly increased the time spent on the originally nonpreferred side of the chamber. Coadministration of tamoxifen (10 mg/kg; s.c.) attenuated effects of E(2) to produce a CPP, but tamoxifen alone, increased time spent on the nonpreferred side. Experiment 2: coadministration of ICI 182,780 (10 microg/microl) to the nucleus accumbens attenuated effects of E(2) to enhance CPP and did not produce a CPP when administered alone. Experiment 3: coadministration of s.c. E(2) with ER antisense oligonucleotides to the nucleus accumbens significantly decreased time spent on the nonpreferred side and expression of ERs in the nucleus accumbens compared to scrambled antisense oligonucleotides or saline vehicle administration. Thus, E(2)'s rewarding effects may involve actions at ERs in the nucleus accumbens.

Chronic food restriction: enhancing effects on drug reward and striatal cell signaling

Chronic food restriction (FR) increases behavioral sensitivity to drugs of abuse in animal models and is associated with binge eating, which shares comorbidity with drug abuse, in clinical populations. Behavioral, biochemical and molecular studies conducted in this laboratory to elucidate the functional and mechanistic bases of these phenomena are briefly reviewed. Results obtained to date indicate that FR increases the reward magnitude and locomotor-activating effects of abused drugs, and direct dopamine (DA) receptor agonists, as a result of neuroadaptations rather than changes in drug disposition. Changes in striatal DA dynamics, and postsynaptic cell signaling and gene expression in response to D-1 DA receptor stimulation have been observed. Of particular interest is an upregulation of NMDA receptor-dependent MAP kinase and CaM Kinase II signaling, CREB phosphorylation, and immediate-early and neuropeptide gene expression in nucleus accumbens (NAc) which may facilitate reward-related learning, but also play a role in the genesis of maladaptive goal-directed behaviors. Covariation of altered drug reward sensitivity with body weight loss and recovery suggests a triggering role for one of the endocrine adiposity hormones. However, neither acute nor chronic central infusions of leptin or the melanocortin 3/4 receptor agonist, MTII, have attenuated d-amphetamine reward or locomotor activation in FR rats. Interestingly, chronic intracerebroventricular leptin infusion in ad libitum fed (AL) rats produced a sustained decrease in food intake and body weight that was accompanied by a reversible potentiation of rewarding and locomotor-activating effects of d-amphetamine. This raises the interesting possibility that rapid progressive weight loss is sufficient to increase behavioral sensitivity to drugs of abuse. Whether weight loss produced by leptin infusion produces the same neuroadaptations as experimenter-imposed FR, and whether any of the observed neuroadaptations are necessary for expression of increased behavioral responsiveness to acute drug challenge remain to be investigated.

From Malthus to motive: how the HPA axis engineers the phenotype, yoking needs to wants

The hypothalamo-pituitary-adrenal (HPA) axis is the critical mediator of the vertebrate stress response system, responding to environmental stressors by maintaining internal homeostasis and coupling the needs of the body to the wants of the mind. The HPA axis has numerous complex drivers and highly flexible operating characterisitics. Major drivers include two circadian drivers, two extra-hypothalamic networks controlling top-down (psychogenic) and bottom-up (systemic) threats, and two intra-hypothalamic networks coordinating behavioral, autonomic, and neuroendocrine outflows. These various networks jointly and flexibly control HPA axis output of periodic (oscillatory) functions and a range of adventitious systemic or psychological threats, including predictable daily cycles of energy flow, actual metabolic deficits over many time scales, predicted metabolic deficits, and the state-dependent management of post-prandial responses to feeding. Evidence is provided that reparation of metabolic derangement by either food or glucocorticoids results in a metabolic signal that inhibits HPA activity. In short, the HPA axis is intimately involved in managing and remodeling peripheral energy fluxes, which appear to provide an unidentified metabolic inhibitory feedback signal to the HPA axis via glucocorticoids. In a complementary and perhaps a less appreciated role, adrenocortical hormones also act on brain to provide not only feedback, but feedforward control over the HPA axis itself and its various drivers, as well as coordinating behavioral and autonomic outflows, and mounting central incentive and memorial networks that are adaptive in both appetitive and aversive motivational modes. By centrally remodeling the phenotype, the HPA axis provides ballistic and predictive control over motor outflows relevant to the type of stressor. Evidence is examined concerning the global hypothesis that the HPA axis comprehensively induces integrative phenotypic plasticity, thus remodeling the body and its governor, the brain, to yoke the needs of the body to the wants of the mind. Adverse side effects of this yoking under conditions of glucocorticoid excess are discussed.

Effects of central leptin infusion on the reward-potentiating effect of D-amphetamine

It was previously reported that chronic food restriction and maintenance of rats at 75-80% of initial body weight enhanced the reward-potentiating effect of D-amphetamine in the lateral hypothalamic self-stimulation (LHSS) paradigm. Moreover, the enhancement reversed in parallel with body weight recovery when ad libitum access to food was reinstated. The present study tested the hypothesis that hypoleptinemia during food restriction is necessary for expression of enhanced drug reward. In Experiment 1, intracerebroventricular (i.c.v.) infusion of leptin (0.5 microg/0.5 microl/hr for 8 days) in food-restricted rats did not alter the rewarding effect of D-amphetamine (0.5 mg/kg, i.p.). Considering that i.c.v. leptin may not diffuse into deep brain regions where direct effects on drug reward sensitivity may be exerted, effects of acute bilateral microinjection of leptin (0.5 microg) in ventral tegmental area and nucleus accumbens were tested in Experiment 2 and found to have no effect. In Experiment 3, chronic i.c.v. leptin infusion in ad libitum fed rats decreased food intake and body weight and enhanced the rewarding effect of D-amphetamine. Sensitivity to D-amphetamine returned to normal as body weight recovered following cessation of leptin infusion. This result suggests that weight loss, whether from hormone-induced appetite suppression or experimenter-imposed food restriction, is sufficient to enhance drug reward sensitivity. Experiment 4 tested whether food restriction in the absence of body weight loss alters drug reward sensitivity. Rats received chronic i.c.v. infusion of the orexigenic melanocortin receptor antagonist, SHU9119 (0.02 microg/0.5 microl/hr for 12 days), and a subset were pair-fed to vehicle-infused controls. Although these subjects ingested approximately 50% of the amount of food ingested by free-feeding SHU9119-infused rats, they displayed no weight loss and no change in sensitivity to D-amphetamine. Together, results of this study support the importance of weight loss, but not leptin, in the enhancement of drug reward sensitivity.

Effect of short- vs. long-term estrogen on reinstatement of cocaine-seeking behavior in female rats

Estrogen effects on cocaine-induced reinstatement of lever responding were examined in sham-operated, vehicle-treated (SH+VEH), ovariectomized (OVX+VEH), and OVX female Wistar rats with estrogen replacement (OVX+EB). The effect of long- (64+/-1.56 days) and short-term (9 days) EB treatment on reinstatement of cocaine-seeking behavior was compared in Experiment 1 and 2, respectively, in order to compare the effect of EB when it was present during the development vs. expression of reinstatement of cocaine-seeking behavior. Rats were trained to self-administer 0.4 mg/kg/inf cocaine. After the acquisition criteria were met, rats continued to respond for cocaine for 2 h/day for a 14-day maintenance period. Cocaine was then replaced with saline and the 21-day extinction period commenced. Subsequently, rats were tested for reinstatement of lever responding on the previously drug-paired lever after alternating daily injections of saline or cocaine. In both experiments, there were no differences between groups in self-administration behavior during training, maintenance, or extinction. In Experiment 1, SH+VEH and chronically treated OVX+EB rats had greater cocaine-induced reinstatement than OVX+VEH rats. In Experiment 2, short-term treated OVX+EB rats also showed enhanced cocaine-induced reinstatement compared to OVX+VEH rats. The results indicate that EB-mediated enhancement of cocaine-induced reinstatement is dependent on EB presence during the expression of reinstatement but not during the formation of stimulus-reward associations during the development of cocaine-reinforced behavior.

Calorie restriction increases cigarette use in adult smokers

RATIONALE

Cigarette smokers weigh less than nonsmokers, and smokers often gain weight when they quit. This is a major barrier to smoking cessation, especially among women. However, strict dieting is not recommended during smoking cessation out of concern that it might promote relapse. This concern derives, in part, from the observation that calorie restriction increases self-administration of drugs of abuse in animals. This relationship has never been experimentally demonstrated in humans.

OBJECTIVES

To evaluate whether calorie restriction increases cigarette smoking in humans.

METHODS

Seventeen (nine males, eight females) healthy, normal-weight smokers not attempting to quit were cycled in partially counterbalanced order, double-blind, through four diets-normal calorie (2,000-2,800 kcal/day), low calorie (700 kcal/day deficit), low-carbohydrate (CHO)/normal-calorie, and low-CHO/low-calorie-for 6 days per diet in an inpatient research ward. Smoking was assessed by cigarette counts, breath carbon monoxide (CO) levels, and cigarette craving.

RESULTS

Compared with the normal-calorie diet, while on the low-calorie diet, subjects smoked 8% more cigarettes (P<0.02) and had 11% higher breath CO levels (P<0.01). The low-CHO/normal-calorie diet showed no significant effect on either variable, but there was a 15% increase in breath CO levels (P<0.05) on the low-CHO/low-calorie diet. There were no changes in self-reported cigarette craving or mood.

CONCLUSIONS

Consistent with animal studies, moderate calorie restriction was associated with a small but statistically significant increase in cigarette smoking, with no independent effect of CHO deprivation. These findings suggest that dieting may increase smoking behavior and could impede smoking-cessation attempts.