The anxiogenic drug yohimbine reinstates methamphetamine seeking in a rat model of drug relapse

Oxytocin attenuates yohimbine-induced responding for oral oxycodone under a progressive ratio schedule in male and female rats

Opioid Use Disorder (OUD) has become an epidemic in the United States, with oxycodone (OXY) being one of the most widely misused opioids. Stress plays a key role in triggering opioid use, which can lead to addiction and relapse, underscoring the urgent need for effective therapeutic interventions. Recent studies suggest that the neuropeptide oxytocin (OXT) may reduce addiction-related behaviors and possess anxiolytic properties. The present study investigates the effect of peripheral administration of OXT on attenuating stress-induced motivation to seek oral OXY, as measured by progressive ratio (PR) responding in both male and female rats. Animals were first trained in an operant conditioning paradigm to orally self-administer a sucrose solution by pressing an active lever for access to the solution. As responding stabilized, subjects were switched to an OXY-sucrose solution, with sucrose concentration reduced overtime, until subjects were self-administering oral OXY alone. To test the effect of stress on OXY responding the pharmacological stressor, yohimbine (YOH), was administered prior to a progressive ratio test in which animals were required to produce increasingly higher responses to receive a single exposure to OXY. Through a within subjects design, when OXT was concurrently administered, this YOH-induced enhancement of OXY reward strength was attenuated in both male and female rats. These results suggest that OXT may serve as a potential therapeutic remedy to mitigate the deleterious effects of stress on OXY addiction in both sexes.

Antianxiety effects of dexmedetomidine: systematic review and meta-analysis

BACKGROUND

Despite numerous studies of the anxiolytic effects of dexmedetomidine compared with those of other drugs or saline, the results have been inconsistent. Here we report a systematic review and meta-analysis to comprehensively evaluate the evidence of the anxiolytic effects of dexmedetomidine.

METHODS

This research has been registered in the International Prospective Register of Systematic Reviews. The PubMed, Embase, Cochrane Library, Web of Science, and ClinicalTrials.gov database were searched for clinical trials that compared the anxiolytic effects of dexmedetomidine with those of a control group with valid anxiety scores from inception to December 29, 2024.

RESULTS

Overall, this systematic review and meta-analysis included 25 clinical studies with 2159 participants who underwent surgery. The primary outcome revealed that patients who were treated with dexmedetomidine had significantly lower anxiety scores than others did overall (MD = - 1.73, 95% CI = [ - 2.33, - 1.13], p < 0.00001, I2 = 86.5%). Dexmedetomidine was found to be more effective than benzodiazepines in relieving anxiety (MD = - 1.34, 95% CI = [ - 2.08, - 0.60], p = 0.0004, I2 = 83.3%). The secondary outcomes revealed no significant differences in satisfaction, pain level, sedation scores or the risk of postoperative nausea and vomiting between patients who were treated with dexmedetomidine and controls. However, the occurrence of bradycardia was more common in the dexmedetomidine groups.

CONCLUSIONS

Overall, this meta-analysis provided evidence of the potential of dexmedetomidine for relieving anxiety among patients who undergo surgery, with superior antianxiety effects compared with those of benzodiazepines.

Neurobiology of the incubation of drug craving: An update

Relapse to drug use is often preceded by drug craving. Clinical observations in the 1980s led clinical investigators to postulate that cue-induced cocaine craving may increase during abstinence. Over 2 decades ago, investigators identified an analogous phenomenon in rats of time-dependent increases in drug-seeking behavior during homecage abstinence and termed it incubation of cocaine craving. In 2011, we reviewed the first decade of studies on brain mechanisms of incubation of drug craving. In this review, we provide an update on incubation-related brain mechanisms from studies published since 2011. We first review studies using the standard method of incubation after homecage-forced abstinence from cocaine, methamphetamine, opioid drugs, and nicotine. Next, we review studies using newer methods to study incubation after voluntary abstinence in the drug environment. In these studies, abstinence is achieved by either providing rats alternative nondrug rewards in a choice setting or introducing rats to adverse consequences to drug seeking or taking. We then discuss translational human studies on incubation of cue-induced drug craving. We conclude by discussing several emerging topics, including sex differences in incubation of drug craving, role of sleep patterns, and similarities and differences in mechanisms of incubation of craving across drug classes and abstinence conditions. Our 2 main conclusions are as follows: (1) across drug classes, there are both similarities and differences in mechanisms of incubation of drug craving after forced abstinence, and (2) the method used to achieve abstinence (forced or voluntary) can influence the mechanisms controlling incubation of drug craving or its expression. SIGNIFICANCE STATEMENT: This article reviews results from preclinical and clinical studies published since 2011 on neurobiological mechanisms of incubation of drug craving after homecage-forced abstinence or voluntary abstinence in the drug environment. This article also reviews translational human studies on incubation of cue-induced subjective drug craving and brain response during abstinence. The results of the studies reviewed indicate that multiple brain mechanisms control incubation of drug craving after homecage-forced abstinence or voluntary abstinence.

Profiling gene alterations in striatonigral neurons associated with incubation of methamphetamine craving by cholera toxin subunit B-based fluorescence-activated cell sorting

Introduction

In both rats and humans, methamphetamine (Meth) seeking progressively increases during abstinence, a behavioral phenomenon termed "incubation of Meth craving". We previously demonstrated a critical role of dorsal striatum (DS) in this incubation in rats. However, circuit-specific molecular mechanisms in DS underlying this incubation are largely unknown. Here we combined a newly developed fluorescence-activated sorting (FACS) protocol with fluorescence-conjugated cholera toxin subunit B-647 (CTb-647, a retrograde tracer) to examine gene alterations in the direct-pathway (striatonigral) medium spiny neurons (MSNs) associated with incubation of Meth craving.

Methods

We injected CTb-647 bilaterally into substantia nigra before or after training rats to self-administer Meth or saline (control condition) for 10 days (6 h/d). On abstinence day 1 or day 28, we collected the DS tissue from both groups for subsequent FACS and examined gene expressions in CTb-positive (striatonigral MSNs) and CTb-negative (primarily non-striatonigral MSNs). Finally, we examined gene expressions in DS homogenates, to demonstrate cell-type specificity of gene alterations observed on abstinence day 28.

Results

On abstinence day 1, we found mRNA expression of Gabrb3 decreased only in CTb-positive (but not CTb-negative) neurons of Meth rats compared with saline rats, while mRNA expression of Usp7 decreased in all sorted DS neurons. On abstinence day 28, we found increased mRNA expression for Grm3, Opcml, and Usp9x in all sorted DS neurons, but not DS homogenate.

Discussion

Together, these data demonstrated that incubation of Meth craving was associated with time-dependent, circuit-specific, and cell type-specific gene alterations in DS involved in glutamatergic, GABAergic, opioidergic, and protein degradation signaling.

The anxiogenic drug yohimbine is a reinforcer in male and female rats

The indole alkaloid yohimbine is an anxiogenic drug that activates stress-responsive systems in the brain. However, because yohimbine also elicits approach behaviors, this study employed male and female Sprague-Dawley rats to explore its potential reinforcing effects. Thus, it was first determined if intravenous (IV) infusions of yohimbine (0.25 mg/kg/infusion) could maintain lever pressing, whether intake could be modulated by dose/infusion, and if lever pressing would persist in the absence of yohimbine or yohimbine-paired cues. Next, to assess yohimbine's effect on memory consolidation, 0.3, 1.25 or 3 mg/kg yohimbine was administered post-training using an object recognition memory task. Finally, place conditioning assessed whether doses of yohimbine that elevate blood serum corticosterone levels (1.25 or 3 mg/kg) could elicit a conditioned place preference. It was found that both sexes acquired yohimbine IV self-administration, that intake was modulated by dose/infusion, and that lever pressing persisted during extinction and in the absence of the yohimbine-paired cue. As well, post-training injections of 1.25 mg/kg yohimbine enhanced consolidation of object memory, and 1.25 and 3 mg/kg elevated corticosterone levels and elicited a place preference in both sexes. Finally, in behavioral tests of psychomotor functions, acute yohimbine increased lever pressing for a visual cue and elevated locomotor activity. These findings reveal a profile of yohimbine's behavioral effects that is consistent with that of psychostimulant reinforcing drugs.

Basal cortisol level modulates stress-induced opioid-seeking behavior

In preclinical studies and our human laboratory, the α2-noradrenergic autoreceptor antagonist yohimbine was found to promote drug-seeking behavior. This study evaluated effects of dose-combinations of yohimbine and the glucocorticoid receptor agonist hydrocortisone to model intensity-dependent effects of stimulating each neurochemical system, alone and together, on stress-reactivity and opioid-seeking. Twelve regular heroin-using participants diagnosed with opioid use disorder (OUD) were stabilized on sublingual buprenorphine (8-mg/day), then passed a hydromorphone 18-mg vs. placebo intramuscular reinforcement screen. Across 9 experimental conditions (3 × 3 within-subject, randomized crossover, placebo-controlled, double-blind design) during inpatient buprenorphine maintenance, combinations of oral pretreatment doses of yohimbine (0, 27, 54-mg; t = 0 min) then hydrocortisone (0, 20, 40-mg; t = 45 min) were administered. In each condition, subjective drug and mood effects, cardiovascular responses, and saliva cortisol and α-amylase levels were assessed to evaluate stress-reactivity, and participants completed a 12-trial choice progressive ratio task during which they could earn units of hydromorphone (1.5-mg intramuscular) and/or money ($2.00). Yohimbine dose-dependently increased blood pressure, α-amylase, and anxiety scores, and decreased opioid agonist symptoms; hydrocortisone dose-dependently increased cortisol levels. Yohimbine/hydrocortisone dose-combinations significantly shifted within-session responding from money to opioid-seeking among participants with lower basal cortisol levels. These findings replicate yohimbine effects on stress biomarkers and demonstrate that noradrenergic/glucocorticoid-potentiated opioid-seeking is modulated by basal cortisol level. In persons with OUD stabilized on buprenorphine, basal HPA-axis activity and acute stressors can enhance opioid relative reinforcing efficacy. These factors may limit OUD treatment efficacy and highlight the need for novel interventions that prevent stress-induced opioid-seeking.

Pharmacological Treatments for Methamphetamine Use Disorder: Current Status and Future Targets

The illicit use of the psychostimulant methamphetamine (METH) is a major concern, with overdose deaths increasing substantially since the mid-2010s. One challenge to treating METH use disorder (MUD), as with other psychostimulant use disorders, is that there are no available pharmacotherapies that can reduce cravings and help individuals achieve abstinence. The purpose of the current review is to discuss the molecular targets that have been tested in assays measuring the physiological, the cognitive, and the reinforcing effects of METH in both animals and humans. Several drugs show promise as potential pharmacotherapies for MUD when tested in animals, but fail to produce long-term changes in METH use in dependent individuals (eg, modafinil, antipsychotic medications, baclofen). However, these drugs, plus medications like atomoxetine and varenicline, may be better served as treatments to ameliorate the psychotomimetic effects of METH or to reverse METH-induced cognitive deficits. Preclinical studies show that vesicular monoamine transporter 2 inhibitors, metabotropic glutamate receptor ligands, and trace amine-associated receptor agonists are efficacious in attenuating the reinforcing effects of METH; however, clinical studies are needed to determine if these drugs effectively treat MUD. In addition to screening these compounds in individuals with MUD, potential future directions include increased emphasis on sex differences in preclinical studies and utilization of pharmacogenetic approaches to determine if genetic variances are predictive of treatment outcomes. These future directions can help lead to better interventions for treating MUD.

The estrous cycle has no effect on incubation of methamphetamine craving and associated Fos expression in dorsomedial striatum and anterior intralaminar nucleus of thalamus

Relapse is a major challenge in treating drug addiction, and drug seeking progressively increases after abstinence, a phenomenon termed "incubation of drug craving". Previous studies demonstrated both sex differences and an effect of estrous cycle in female rats in incubation of cocaine craving. In contrast, while incubation of methamphetamine craving is similar across sexes, whether estrous cycle plays a role in this incubation has yet to be fully addressed. Moreover, whether neural mechanisms underlying incubation of methamphetamine craving differ across estrous cycles is largely unknown. To address these gaps, we first compared methamphetamine self-administration, and methamphetamine seeking on both abstinence days 1 and 28 between male rats and female rats across the estrous cycle. Next, we examined neuronal activation associated with incubated methamphetamine seeking in dorsomedial striatum (DMS) and lateral portion of the anterior intralaminar nucleus of thalamus (AIT-L), two brain areas previously implicated in incubation of methamphetamine craving. We found no effect of sex or estrous cycle on methamphetamine self-administration and methamphetamine seeking on abstinence days 1 and 28. We also found no effect of sex or estrous cycle on the number of Fos-expressing cells in DMS or AIT-L following methamphetamine seeking test. Taken together, our results showed that methamphetamine self-administration and incubation of methamphetamine craving was not dependent on sex or estrous cycles under our experimental condition, and the role of DMS and AIT-L in incubation of methamphetamine craving may be similar across sexes and across estrous cycles in female rats.

Enhanced Risky Choice in Male Rats Elicited by the Acute Pharmacological Stressor Yohimbine Involves Prefrontal Dopamine D1 Receptor Activation

BACKGROUND

Acute stress alters risk-based decision-making; however, the underlying neural and neurochemical substrates are underexplored. Given their well-documented stress-inducing effects in humans and laboratory animals, glucocorticoids such as cortisol and corticosterone and the α2-adrenoceptor antagonist yohimbine represent potent pharmacological tools to mimic some characteristics of acute stress.

METHODS

Here, we analyzed the effects of the pharmacological stressors corticosterone and yohimbine given systemically on risk-based decision-making in male rats. Moreover, we investigated whether pharmacological stressor effects on risk-based decision-making involve dopamine D1 receptor stimulation in the dorsal prelimbic cortex (PL). We used a risk discounting task that requires choosing between a certain/small reward lever that always delivered 1 pellet and a risky/large reward lever that delivered 4 pellets with a decreasing probability across subsequent trials.

RESULTS

Systemic administration of yohimbine increased the preference for the risky/large reward lever. By contrast, systemic single administration of corticosterone did not significantly promote risky choice. Moreover, co-administration of corticosterone did not enhance the effects of yohimbine on risky choice. The data further show that the increased preference for the risky/large reward lever under systemic yohimbine was lowered by a concurrent pharmacological blockade of dopamine D1 receptors in the PL.

CONCLUSIONS

Our rodent data provide causal evidence that stimulation of PL D1 receptors may represent a neurochemical mechanism by which the acute pharmacological stressor yohimbine, and possibly nonpharmacological stressors as well, promote risky choice.

Neurobiology of Stress-Induced Nicotine Relapse

Tobacco smoking is the leading cause of preventable death and disease. Although there are some FAD-approved medicines for controlling smoking, the relapse rate remains very high. Among the factors that could induce nicotine relapse, stress might be the most important one. In the last decades, preclinical studies have generated many new findings that lead to a better understanding of stress-induced relapse of nicotine-seeking. Several molecules such as α3β4 nicotinic acetylcholine receptor, α2-adrenergic receptors, cannabinoid receptor 1, trace amine-associated receptor 1, and neuropeptide systems (corticotropin-releasing factor and its receptors, dynorphine and kappa opioid receptor) have been linked to stress-induced nicotine relapse. In this review, we discuss recent advances in the neurobiology, treatment targets, and potential therapeutics of stress-induced nicotine relapse. We also discuss some factors that may influence stress-induced nicotine relapse and that should be considered in future studies. In the final section, a perspective on some research directions is provided. Further investigation on the neurobiology of stress-induced nicotine relapse will shed light on the development of new medicines for controlling smoking and will help us understand the interactions between the stress and reward systems in the brain.

Modeling methamphetamine use disorder and relapse in animals: short- and long-term epigenetic, transcriptional., and biochemical consequences in the rat brain

Methamphetamine use disorder (MUD) is a neuropsychiatric disorder characterized by binge drug taking episodes, intervals of abstinence, and relapses to drug use even during treatment. MUD has been modeled in rodents and investigators are attempting to identify its molecular bases. Preclinical experiments have shown that different schedules of methamphetamine self-administration can cause diverse transcriptional changes in the dorsal striatum of Sprague-Dawley rats. In the present review, we present data on differentially expressed genes (DEGs) identified in the rat striatum following methamphetamine intake. These include genes involved in transcription regulation, potassium channel function, and neuroinflammation. We then use the striatal data to discuss the potential significance of the molecular changes induced by methamphetamine by reviewing concordant or discordant data from the literature. This review identified potential molecular targets for pharmacological interventions. Nevertheless, there is a need for more research on methamphetamine-induced transcriptional consequences in various brain regions. These data should provide a more detailed neuroanatomical map of methamphetamine-induced changes and should better inform therapeutic interventions against MUD.

Extinction and reinstatement of methamphetamine-induced conditioned place preference in zebrafish

Conditioned place preference (CPP) paradigm in zebrafish has been used to measure drug reward, but there is limited research on CPP reinstatement to determine relapse vulnerability. The present study aimed to investigate extinction and reinstatement of methamphetamine (MA)-induced CPP in zebrafish and evaluate the model's predictive validity. Zebrafish received different doses of MA (0-60 mg/kg) during CPP training. The preferred dose of MA at 40 mg/kg was used for extinction via either confined or nonconfined procedures. The extinguished CPP was reinstated by administering a priming dose of MA (20 mg/kg) or various stressors. To assess persistent susceptibility to reinstatement, MA CPP and reinstatement were retested following 14 days of abstinence. In addition, the effects of SCH23390, naltrexone, and clonidine on MA CPP during acquisition, expression, or reinstatement phases were monitored. MA induced CPP in a dose-dependent manner. Both nonconfined and confined extinction procedures time-dependently reduced the time spent on the MA-paired side. A priming dose of MA, chasing stress, or yohimbine reinstated the extinguished CPP. After 14 days of abstinence, the MA CPP remained extinguished and was significantly reinstated by MA priming or chasing stress. Similar to the observations in rodents, SCH23390 suppressed the acquisition of MA CPP, naltrexone reduced the expression and MA priming-induced reinstatement, while clonidine prevented stress-induced reinstatement of MA CPP. This work expanded the zebrafish CPP paradigm to include extinction and reinstatement phases, demonstrating predictive validity and highlighting its potential as a valuable tool for exploring drug relapse.

Disrupting astrocyte-neuron lactate transport prevents cocaine seeking after prolonged withdrawal

Energy supply, especially the transfer of lactate from astrocytes to neurons, is critical for neuronal plasticity. However, its role in the incubation of cocaine craving remains largely unknown. Using an extended-access self-administration model and in vivo 1H-magnetic resonance spectroscopy, we found that lactate synthesis in the central amygdala (CeA) is required for the intensified cocaine craving after prolonged withdrawal. Furthermore, incubated cocaine seeking was associated with a selective increase in monocarboxylate transporter 2 (MCT2) and MCT4 expression levels. Down-regulation of astrocytic MCT4 or neuronal MCT2 using targeted antisense oligonucleotides or cell type-specific shRNA attenuated cocaine craving and reduced the expression of plasticity-related proteins and excitatory synaptic transmission. Meanwhile, lactate administration rescued MCT4 but not MCT2 disruption-induced behavioral changes due to the inability of lactate to be transported into neurons. Together, our study highlights the critical role of astrocyte-neuron lactate transport in the CeA in the incubation of cocaine craving and suggests a potential therapeutic target for drug addiction.

Oxytocin Attenuates Yohimbine-Induced Reinstatement of Alcohol-Seeking in Female Rats via the Central Amygdala

Alcohol use disorder is a significant public health concern, further exacerbated by an increased risk of relapse due to stress. In addition, factors such as biological sex may contribute to the progression of addiction, as females are especially susceptible to stress-induced relapse. While there have been many studies surrounding potential pharmacological interventions for male stress-induced ethanol reinstatement, research regarding females is scarce. Recently, the neuropeptide oxytocin has gained interest as a possible pharmacological intervention for relapse. The present study examines how oxytocin affects yohimbine-induced reinstatement of ethanol-seeking in female rats using a self-administration paradigm. Adult female rats were trained to press a lever to access ethanol in daily self-administration sessions. Rats then underwent extinction training before a yohimbine-induced reinstatement test. Rats administered with yohimbine demonstrated significantly higher lever response indicating a reinstatement of ethanol-seeking behavior. Oxytocin administration, both systemically and directly into the central amygdala, attenuated the effect of yohimbine-induced reinstatement of ethanol-seeking behavior. The findings from this study establish that oxytocin is effective at attenuating alcohol-relapse behavior mediated by the pharmacological stressor yohimbine and that this effect is modulated by the central amygdala in females. This provides valuable insight regarding oxytocin's potential therapeutic effect in female stress-induced alcohol relapse.

Acute stress persistently alters instrumental motivation without affecting appetitive Pavlovian conditioning, extinction, or contextual renewal

In two experiments, we adapted the stress-enhanced fear learning approach to evaluate the persistent effects of acute stress on appetitive learning and motivation in adult male Long Evans rats. In Experiment 1, we found that exposure to a battery of footshocks in one context had no effect on the acquisition, extinction, or contextual renewal of an appetitive Pavlovian discrimination in different contexts. However, when rats were subsequently trained to respond on a progressive ratio instrumental schedule, rats with a history of shock showed lower response rates and progressive ratio break points. Extinction of the shock-associated context had little effect on progressive ratio responding. In Experiment 2, we replicated this instrumental responding deficit with a continuous reinforcement schedule when the Pavlovian phases did not intervene in the time between shock and instrumental testing. Our findings here demonstrate that highly stressful acute experiences produce long-lasting deficits in instrumental motivation for food in male rats.

Amphetamine-induced prolonged disturbances in tissue levels of dopamine and serotonin in the rat brain

BACKGROUND

A hallmark of psychostimulants is the persistence of neurobiological changes they produce. The difficulty in reversing long-time effects of psychostimulants use is why addiction therapy is so ineffective. This study aimed to look for such drug-induced changes that can be detected even after many weeks of abstinence.

METHODS

Rats were given 12 doses of amphetamine (Amph) at 1.5 mg/kg. The rewarding effect of Amph was assessed using ultrasonic vocalization. After 14 and 28 days of abstinence, tissue levels of dopamine (DA), serotonin (5-HT), and their metabolites were measured in the prefrontal cortex (PFC), nucleus accumbens (Acb), dorsomedial (CPuM), and dorsolateral (CPuL) striatum.

RESULTS

After 28 days of abstinence, DA levels were increased in the dorsal striatum while 5-HT levels were decreased in all brain regions studied. The opposite direction of changes in DA and 5-HT tissue levels observed in the dorsal striatum may be related to the changes in the emotional state during abstinence and may contribute to the incubation of craving and relapses. Tissue levels of 5-HT and DA showed intra- and inter-structural correlations, most pronounced after 14 days of abstinence. Most of them were absent in the control group (ctrl), which may indicate that their appearance was related to the changes induced by earlier Amph administration. We did not find any associations between reward sensitivity and the persistence of Amph-induced neurochemical disturbances.

CONCLUSIONS

Administration of 12 moderate doses of Amph causes prolonged changes in DA and 5-HT tissue levels. The direction and severity of the changes are dependent on the brain region and the neurotransmitter studied.

Persistent Neuroadaptations in the Nucleus Accumbens Core Accompany Incubation of Methamphetamine Craving in Male and Female Rats

Relapse is a major problem in treating methamphetamine use disorder. "Incubation of craving" during abstinence is a rat model for persistence of vulnerability to craving and relapse. While methamphetamine incubation has previously been demonstrated in male and female rats, it has not been demonstrated after withdrawal periods greater than 51 d and most mechanistic work used males. Here, we address both gaps. First, although methamphetamine intake was higher in males during self-administration training (6 h/d × 10 d), incubation was similar in males and females, with "incubated" craving persisting through withdrawal day (WD)100. Second, using whole-cell patch-clamp recordings in medium spiny neurons (MSNs) of the nucleus accumbens (NAc) core, we assessed synaptic levels of calcium-permeable AMPA receptors (CP-AMPARs), as their elevation is required for expression of incubation in males. In both sexes, compared with saline-self-administering controls, CP-AMPAR levels were significantly higher in methamphetamine rats across withdrawal, although this was less pronounced in WD100-135 rats than WD15-35 or WD40-75 methamphetamine rats. We also examined membrane properties and NMDA receptor (NMDAR) transmission. In saline controls, MSNs from males exhibited lower excitability than females. This difference was eliminated after incubation because of increased excitability of MSNs from males. NMDAR transmission did not differ between sexes and was not altered after incubation. In conclusion, incubation persists for longer than previously described and equally persistent CP-AMPAR plasticity in NAc core occurs in both sexes. Thus, abstinence-related synaptic plasticity in NAc is similar in males and females although other methamphetamine-related behaviors and neuroadaptations show differences.

Acute glucagon-like peptide-1 receptor agonist liraglutide prevents cue-, stress-, and drug-induced heroin-seeking in rats

Substance use disorder is challenging to treat due to its relapsing nature. In the last decade, opioid use disorder has been a threat to public health, being declared an epidemic by the Centers for Disease Control and Prevention. This is a tragic situation, considering there currently are only three effective, yet not ideal, treatments to prevent relapse to opioids. Recent research has shown that hormones that modulate hunger and satiety also can modulate motivated behavior for drugs of abuse. For example, the short-acting analog of glucagon-like peptide-1 (GLP-1), an incretin hormone that regulates homeostatic feeding, has been shown to reduce responding for rewarding stimuli such as food, cocaine, heroin, and nicotine when administered over several days or weeks. This may serve as an effective adjuvant during treatment; however, whether it would be effective when used acutely to bridge a patient between cessation of use and onset of medication for the treatment of an opioid addiction is unknown. Here, we tested the acute effects of the longer acting GLP-1 analog, liraglutide, on heroin-seeking. In rats with heroin self-administration experience, we found that subcutaneous administration of an acute dose of 0.3-mg/kg liraglutide was effective in preventing drug-seeking after exposure to three major precipitators: drug-associated cues, stress (yohimbine-induced), and the drug itself. Finally, we confirmed that the reduction in drug-seeking is not due to a locomotor impairment, as liraglutide did not significantly alter performance in a rotarod test. As such, acute use of GLP-1 analogs may serve as a new and effective nonopioid bridge to treatment.

Oxytocin as an adolescent treatment for methamphetamine addiction after early life stress in male and female rats

Early life stress (ELS) is associated with perturbed neural development and augmented vulnerability to mental health disorders, including addiction. How ELS changes the brain to increase addiction risk is poorly understood, and there are no therapies which target this ELS-induced vulnerability. ELS disrupts the oxytocin system, which can modulate addiction susceptibility, suggesting that targeting the oxytocin system may be therapeutic in this ELS-addiction comorbidity. Therefore, we determined whether adolescent oxytocin treatment after ELS could: (1) reduce vulnerability to anxiety, social deficits, and methamphetamine-taking and reinstatement; and (2) restore hypothalamic oxytocin and corticotropin-releasing factor expressing neurons and peripheral oxytocin and corticosterone levels. Long Evans pups underwent maternal separation (MS) for either 15 min or 360 min on postnatal days (PND) 1-21. During adolescence (PNDs 28-42), rats received a daily injection of either oxytocin or saline. In Experiment 1, adult rats were assessed using the elevated plus-maze, social interaction procedure, and methamphetamine self-administration procedure, including extinction, and cue-, methamphetamine- and yohimbine-induced reinstatement. In Experiment 2, plasma for enzyme immunoassays and brain tissue for immunofluorescence were collected from adult rats after acute stress exposure. Adolescent oxytocin treatment ameliorated ELS-induced anxiety and reduced methamphetamine- and yohimbine-induced reinstatement in both sexes, and suppressed methamphetamine intake and facilitated extinction in males only. Additionally, adolescent oxytocin treatment after ELS restored oxytocin-immunoreactive cells and stress-induced oxytocin levels in males, and attenuated stress-induced corticosterone levels in both sexes. Adolescent oxytocin treatment reverses some of the ELS effects on later-life psychopathology and vulnerability to addiction.

Involvement of the ghrelin system in the maintenance and reinstatement of cocaine-motivated behaviors: a role of adrenergic action at peripheral β1 receptors

Cocaine addiction is a significant medical and public concern. Despite decades of research effort, development of pharmacotherapy for cocaine use disorder remains largely unsuccessful. This may be partially due to insufficient understanding of the complex biological mechanisms involved in the pathophysiology of this disorder. In the present study, we show that: (1) elevation of ghrelin by cocaine plays a critical role in maintenance of cocaine self-administration and cocaine-seeking motivated by cocaine-conditioned stimuli; (2) acquisition of cocaine-taking behavior is associated with the acquisition of stimulatory effects of cocaine by cocaine-conditioned stimuli on ghrelin secretion, and with an upregulation of ghrelin receptor mRNA levels in the ventral tegmental area (VTA); (3) blockade of ghrelin signaling by pretreatment with JMV2959, a selective ghrelin receptor antagonist, dose-dependently inhibits reinstatement of cocaine-seeking triggered by either cocaine or yohimbine in behaviorally extinguished animals with a history of cocaine self-administration; (4) JMV2959 pretreatment also inhibits brain stimulation reward (BSR) and cocaine-potentiated BSR maintained by optogenetic stimulation of VTA dopamine neurons in DAT-Cre mice; (5) blockade of peripheral adrenergic β1 receptors by atenolol potently attenuates the elevation in circulating ghrelin induced by cocaine and inhibits cocaine self-administration and cocaine reinstatement triggered by cocaine. These findings demonstrate that the endogenous ghrelin system plays an important role in cocaine-related addictive behaviors and suggest that manipulating and targeting this system may be viable for mitigating cocaine use disorder.

Roflumilast treatment during forced abstinence reduces relapse to methamphetamine seeking and taking

Methamphetamine (METH) is a psychostimulant with high abuse potential. Currently, there are no pharmacological treatments specific for METH abuse or stimulant use disorder generally. Although phosphodiesterase inhibitors have shown some promise, current animal models have not examined their use in abstinence from stimulant abuse. We employed a METH self-administration model in the rat followed by a forced abstinence period during which roflumilast, a phosphodiesterase 4 inhibitor, was administered. A detailed behavioral analysis of chronic treatment with roflumilast during 7 days of forced abstinence showed that roflumilast reduced METH seeking and METH taking upon subsequent relapse test. Roflumilast treatment during 7 days of forced abstinence did not affect sucrose seeking and sucrose taking behaviors. These data suggest that roflumilast may be a treatment for METH use disorder that is effective when administered only during abstinence.

Sex Differences in Opioid and Psychostimulant Craving and Relapse: A Critical Review

A widely held dogma in the preclinical addiction field is that females are more vulnerable than males to drug craving and relapse. Here, we first review clinical studies on sex differences in psychostimulant and opioid craving and relapse. Next, we review preclinical studies on sex differences in psychostimulant and opioid reinstatement of drug seeking after extinction of drug self-administration, and incubation of drug craving (time-dependent increase in drug seeking during abstinence). We also discuss ovarian hormones' role in relapse and craving in humans and animal models and speculate on brain mechanisms underlying their role in cocaine craving and relapse in rodent models. Finally, we discuss imaging studies on brain responses to cocaine cues and stress in men and women.The results of the clinical studies reviewed do not appear to support the notion that women are more vulnerable to psychostimulant and opioid craving and relapse. However, this conclusion is tentative because most of the studies reviewed were correlational, not sufficiently powered, and not a priori designed to detect sex differences. Additionally, imaging studies suggest sex differences in brain responses to cocaine cues and stress. The results of the preclinical studies reviewed provide evidence for sex differences in stress-induced reinstatement and incubation of cocaine craving but not cue- or cocaine-induced reinstatement of cocaine seeking. These sex differences are modulated in part by ovarian hormones. In contrast, the available data do not support the notion of sex differences in craving and relapse/reinstatement for methamphetamine or opioids in rodent models. SIGNIFICANCE STATEMENT: This systematic review summarizes clinical and preclinical studies on sex differences in psychostimulant and opioid craving and relapse. Results of the clinical studies reviewed do not appear to support the notion that women are more vulnerable to psychostimulant and opioid craving and relapse. Results of preclinical studies reviewed provide evidence for sex differences in reinstatement and incubation of cocaine seeking but not for reinstatement or incubation of methamphetamine or opioid seeking.

Effects of heroin self-administration and forced withdrawal on the expression of genes related to the mTOR network in the basolateral complex of the amygdala of male Lewis rats

RATIONALE

The development of substance use disorders involves long-lasting adaptations in specific brain areas that result in an elevated risk of relapse. Some of these adaptations are regulated by the mTOR network, a signalling system that integrates extracellular and intracellular stimuli and modulates several processes related to plasticity. While the role of the mTOR network in cocaine- and alcohol-related disorders is well established, little is known about its participation in opiate use disorders.

OBJECTIVES

To use a heroin self-administration and a withdrawal protocol that induce incubation of heroin-seeking in male rats and study the associated effects on the expression of several genes related to the mTOR system and, in the specific case of Rictor, its respective translated protein and phosphorylation.

RESULTS

We found that heroin self-administration elicited an increase in the expression of the genes Igf1r, Igf2r, Akt2 and Gsk3a in the basolateral complex of the amygdala, which was not as evident at 30 days of withdrawal. We also found an increase in the expression of Rictor (a protein of the mTOR complex 2) after heroin self-administration compared to the saline group, which was occluded at the 30-day withdrawal period. The activation levels of Rictor, measured by the phosphorylation rate, were also reduced after heroin self-administration, an effect that seemed more apparent in the protracted withdrawal group.

CONCLUSIONS

These results suggest that heroin self-administration under extended access conditions modifies the expression profile of activators and components of the mTOR complexes and show a putative irresponsive mTOR complex 2 after withdrawal from heroin use.

The effect of adolescent social isolation on vulnerability for methamphetamine addiction behaviours in female rats

RATIONALE

Stress exposure during adolescence contributes to developing a methamphetamine (METH) use disorder. However, most of the studies investigating addiction-related behaviours include only male rodents, despite METH addiction rates being higher in females. Furthermore, animal studies investigating the effects of stress on methamphetamine addiction have used only basic self-administration models which may not be sensitive to the effects of stress.

OBJECTIVES

This project explored whether adolescent isolation stress exposure increases the incidence of four key addiction-related behaviours in female rats.

METHODS

Thirty-two female rat pups were caged in groups of four or individually during adolescence from postnatal (PND) day 22, with the latter being re-socialised in groups of four on PND 43. In adulthood, rats were tested for addiction-like behaviours in a METH self-administration paradigm modelling motivation to take METH, persistence in drug-seeking behaviour when METH was not available, resistance to extinction, and propensity to reinstate after a period of withdrawal.

RESULTS

Adolescent social isolation resulted in lower METH intake during acquisition; however, the paradigm modelling drug-seeking when the drug was unavailable engendered intermittent METH bingeing in all rats, abolishing the group differences in intake during this phase. Adolescent social isolation also accelerated extinction of non-reinforced lever pressing, and increased stress-primed reinstatement, compared to the group-housed rats.

CONCLUSIONS

Adolescent social isolation stress alters various methamphetamine addiction-like behaviours in female rats.

Factors modulating the incubation of drug and non-drug craving and their clinical implications

It was suggested in 1986 that cue-induced cocaine craving increases progressively during early abstinence and remains high during extended periods of time. Clinical evidence now supports this hypothesis and that this increase is not specific to cocaine but rather generalize across several drugs of abuse. Investigators have identified an analogous incubation phenomenon in rodents, in which time-dependent increases in cue-induced drug seeking are observed after abstinence from intravenous drug or palatable food self-administration. Incubation of craving is susceptible to variation in magnitude as a function of biological and/or the environmental circumstances surrounding the individual. During the last decade, the neurobiological correlates of the modulatory role of biological (sex, age, genetic factors) and environmental factors (environmental enrichment and physical exercise, sleep architecture, acute and chronic stress, abstinence reinforcement procedures) on incubation of drug craving has been investigated. In this review, we summarized the behavioral procedures adopted, the key underlying neurobiological correlates and clinical implications of these studies.

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.

Role of nucleus accumbens microRNA-181a and MeCP2 in incubation of heroin craving in male rats

RATIONALE

Epigenetic regulation has been implicated in the incubation of drug craving (the time-dependent increase in drug seeking after prolonged withdrawal from drug self-administration). There is little information available on the role of microRNAs in incubation of heroin craving.

OBJECTIVE

This study aimed to investigate the roles and mechanisms of miR-181a and methyl CpG binding protein 2 (MeCP2) in the nucleus accumbens (NAc) in incubation of heroin seeking.

METHODS

MiRNA sequencing was used to predict potential miRNAs, and miRNA profiles were performed in the NAc after 1 day or 14 days after withdrawal from heroin self-administration. Following 14 days of heroin self-administration, rats were injected of lentiviral vectors into the NAc and evaluated for the effects of overexpression of miR-181a or knockdown of MeCP2 on non-reinforced heroin seeking after 14 withdrawal days.

RESULTS

Lever presses during the heroin-seeking tests were higher after 14 withdrawal days than after 1 day (incubation of heroin craving). miR-181a expression in NAc was lower after 14 withdrawal days than after 1 day, and meCP2 expression in NAc was higher after 14 days than after 1 day. Luciferase activity assay showed that the 3'UTR of MeCP2 is directly regulated by miR-181a. Overexpression of miR-181a in NAc decreased heroin seeking after 14 withdrawal days and decreased MeCP2 mRNA and protein expression. Knockdown of MeCP2 expression in NAc by LV-siRNA-MeCP2 also decreased heroin seeking after 14 withdrawal days.

CONCLUSIONS

Results indicate that incubation of heroin craving is mediated in part by time-dependent decreases in NAc miR181a expression that leads to time-dependent increases in MeCP2 expression. Our data suggest that NAc miR-181a and MeCP2 contribute to incubation of heroin craving.

Incubation of Oxycodone Craving Following Adult-Onset and Adolescent-Onset Oxycodone Self-Administration in Male Rats

Relapse is a major obstacle to curb the ongoing epidemic of prescription opioid abuse. We and others previously demonstrated that oxycodone seeking in adult rats progressively increases after abstinence from oxycodone self-administration (incubation of oxycodone craving). In humans, the onset of oxycodone use in adolescents may increase individuals' vulnerability to later opioid addiction. However, little is known about incubation of oxycodone craving after adolescent-onset oxycodone self-administration in rats. In the first study, we trained single-housed adolescent (postnatal day 35 at start) and adult (postnatal day 77 at start) male Sprague-Dawley rats to self-administer oxycodone (0.1 mg/kg/infusion, 6 h/day for 10 days) and then tested oxycodone relapse on both abstinence day 1 and day 15. Given that social experience is critical for neurobehavioral development in adolescents, we performed the second study using group-housed adolescent and adult rats. In both studies, we observed no age differences in oxycodone self-administration and incubated oxycodone seeking on abstinence day 15. However, on abstinence day 1, we observed decreased oxycodone seeking in adolescents compared with adults. This pattern of data led to elevated incubation slopes in adolescent rats compared with adult rats. Finally, group-housed rats exhibited attenuated oxycodone seeking compared with single-housed rats on abstinence day 15, but not on day 1. Taken together, these data suggest that adolescents may be resistant to oxycodone relapse during early abstinence, but this resistance dissipates quickly during the transition between adolescent and young adulthood. In addition, group-housing plays a protective role against incubated oxycodone craving.

Voluntary exercise as a treatment for incubated and expanded drug craving leading to relapse to addiction: Animal models

Drug addiction is a chronic relapsing disorder, as more than 80% of former drug users relapse within a year after quit attempts have ended. This review examines incubated craving that develops over long periods of weeks to months after addictive drug use ends, when rats are given a small priming exposure to the formerly used drug, and a large amount of drug seeking occurs, reflecting large increases in craving over time. Expanded craving occurs when not only the recently-used drug, but other related or unrelated drugs of abuse elicit drug seeking that leads to relapse behavior, including common drugs like caffeine or nicotine, Thus, expanded craving is an increase in the conditions that elicit relapse, such as, a variety of drugs, and it persists weeks after drug use ends. Incubated and expanded craving occur with several drugs of abuse, and these forms of craving, can last for weeks to months and end in relapse. Voluntary physical exercise, blocked incubated cocaine craving, and expanded heroin craving elicited by multiple conditions was reduced in female and male rats. This review examines voluntary physical exercise as a long-term, self-initiated, and self-sustainable treatment that reduces long-term drug craving leading to relapse.

Neural Substrates and Circuits of Drug Addiction

Drug addiction is a chronic relapsing disorder, and a significant amount of research has been devoted to understand the factors that contribute to the development, loss of control, and persistence of compulsive addictive behaviors. In this review, we provide an overview of various theories of addiction to drugs of abuse and the neurobiology involved in elements of the addiction cycle. Specific focus is devoted to the role of the mesolimbic pathway in acute drug reinforcement and occasional drug use, the role of the mesocortical pathway and associated areas (e.g., the dorsal striatum) in escalation/dependence, and the contribution of these pathways and associated circuits to conditioned responses, drug craving, and loss of behavioral control that may underlie drug relapse. By enhancing the understanding of the neurobiological factors that mediate drug addiction, continued preclinical and clinical research will aid in the development of novel therapeutic interventions that can serve as effective long-term treatment strategies for drug-dependent individuals.

Role of orbitofrontal cortex in incubation of oxycodone craving in male rats

One of the main challenges in treating opioid-use disorders is relapse during abstinence, triggered by re-exposure to drug-associated cues. Previous studies have demonstrated that drug-seeking in rats progressively increases over time during withdrawal (incubation of drug craving). Here, we used male rats and examined neural mechanisms underlying incubation of craving to oxycodone, a commonly abused prescription opioid, and we focused on orbitofrontal cortex (OFC), a brain region previously implicated in incubation of heroin craving. We first used neuronal activity marker Fos and measured neuronal activation in OFC (ventral and lateral OFC) associated with day-1 and day-15 relapse tests. Next, we determined the effect of pharmacological reversible inactivation of OFC on incubated oxycodone seeking on withdrawal day 15. Finally, we determined the effect of reversible inactivation of OFC on nonincubated oxycodone seeking on withdrawal day 1. We found that lever presses during relapse tests were higher on withdrawal day 15 than on withdrawal day 1 (incubation of oxycodone craving). Incubation of oxycodone craving is accompanied with a time-dependent increase of Fos protein expression in both ventral and lateral OFC. Lastly, OFC inactivation decreased oxycodone seeking on withdrawal day 15 but had no effect on withdrawal day 1. Together with the previous heroin study, results here show that OFC plays a critical role in incubation of opioid craving.

Female rats display higher methamphetamine-primed reinstatement and c-Fos immunoreactivity than male rats

Methamphetamine (meth) dependence is often characterized by persistent and chronic relapse (i.e., return to drug use). Previous work suggests females may be at greater risk to relapse. In this study, we extended this limited evidence and identified sex-dependent neural substrates related to meth-triggered reinstatement. Male and female Sprague-Dawley rats were implanted with indwelling jugular catheters. Half of the rats were then trained to self-administer meth (0.05 mg/kg/inf); the other half self-administered saline during 21 daily sessions (2 h). Rats were then given 12 extinction sessions. Twenty-four hours after the last extinction session, rats received reinstatement testing. Half of the rats received a meth-prime (0.3 mg/kg, IP) injection and the remaining rats received a saline injection. This design resulted in 4 separate groups for each sex, allowing for careful investigation of brain regions related to meth-triggered reinstatement. Brains were harvested following the reinstatement session and c-Fos immunoreactivity was measured in multiple brain regions. Meth triggered reinstatement in both sexes and this effect was more robust in females compared to males. Significant sex differences were detected. Females showed greater c-Fos immunoreactivity in the cingulate cortex area 1, lateral orbitofrontal cortex, prelimbic cortex, caudate-putamen, nucleus accumbens core and shell, and central nucleus of the amygdala following meth-primed reinstatement.

mGlu5 function in the nucleus accumbens core during the incubation of methamphetamine craving

Many studies have demonstrated that negative allosteric modulators (NAM) of metabotropic glutamate receptor 5 (mGlu5) reduce cocaine and methamphetamine seeking in extinction-reinstatement animal models of addiction. Less is known about effects of mGlu5 NAMs in abstinence models, particularly for methamphetamine. We used the incubation of drug craving model, in which cue-induced craving progressively intensifies after withdrawal from drug self-administration, to conduct the first studies of the following aspects of mGlu5 function in the rat nucleus accumbens (NAc) core during abstinence from methamphetamine self-administration: 1) functionality of the major form of synaptic depression in NAc medium spiny neurons, which is induced postsynaptically via mGlu5 and expressed presynaptically via cannabinoid type 1 receptors (CB1Rs), 2) mGlu5 surface expression and physical associations between mGlu5, Homer proteins, and diacylglycerol lipase-α, and 3) the effect of systemic and intra-NAc core administration of the mGlu5 NAM 3-((2-methyl-4-)ethynyl)pyridine (MTEP) on expression of incubated methamphetamine craving. We found that mGlu5/CB1R-dependent synaptic depression was lost during the rising phase of methamphetamine incubation but then recovered, in contrast to its persistent impairment during the plateau phase of incubation of cocaine craving. Furthermore, whereas the cocaine-induced impairment was accompanied by reduced mGlu5 levels and mGlu5-Homer associations, this was not the case for methamphetamine. Systemic MTEP reduced incubated methamphetamine seeking, but also reduced inactive hole nose-pokes and locomotion, while intra-NAc core MTEP had no significant effects. These findings provide the first insight into the role of mGlu5 in the incubation of methamphetamine craving and reveal differences from incubation of cocaine craving.

Methamphetamine seeking after prolonged abstinence is associated with activated projections from anterior intralaminar nucleus of thalamus to dorsolateral striatum in female rats

Methamphetamine (Meth) seeking progressively increases after cessation from drug self-administration (incubation of Meth craving). We have previously shown that both dorsomedial and dorsolateral striatum (DMS and DLS) play critical roles in this incubation in male rats. Moreover, our recent anatomical tracing study examined afferent projections into DMS and demonstrated a novel role of projections from anterior intralaminar nucleus of thalamus (AIT) to DMS in incubation of Meth craving in male rats. Here we investigated projection-specific activation of afferent glutamate projections into DLS associated with incubated Meth seeking in female rats. We trained female rats to self-administer Meth (6-h/d for 10 d). On abstinence day 12, we injected cholera toxin subunit B (CTb, a retrograde tracer) unilaterally into DLS. On abstinence day 26, we tested rats for relapse to Meth seeking and measured Fos (a neuronal activity marker), and double-labeling of CTb and Fos in anterior cingulate cortex, anterior insula cortex, orbitofrontal cortex, basolateral amygdala, AIT, and parafascicular nuclei of thalamus. We observed neuronal activation in both cortical and thalamic regions associated with incubated Meth seeking. At the circuit level, AIT➔DLS projections were strongly activated, followed by other corticostriatal projections. Overall our results suggest that AIT to DLS may play a role in Meth seeking after prolonged abstinence in female rats.

The pharmacological stressor yohimbine, but not U50,488, increases responding for conditioned reinforcers paired with ethanol or sucrose

RATIONALE

Environmental stimuli paired with alcohol can function as conditioned reinforcers (CRfs) and trigger relapse to alcohol-seeking. In animal models, pharmacological stressors can enhance alcohol consumption and reinstate alcohol-seeking, but it is unknown whether stress can potentiate the conditioned reinforcing properties of alcohol-paired stimuli.

OBJECTIVES

We examined whether the pharmacological stressors, the α-2 adrenoreceptor antagonist yohimbine (vehicle, 1.25, 2.5 mg/kg; IP) and the K-opioid receptor agonist U50,488 (vehicle, 1.25, 2.5 mg/kg; SC), increase responding for conditioned reinforcement, and if their effects interact with the nature of the reward (alcohol vs. sucrose). We subsequently examined the effects of yohimbine (vehicle, 1.25, 2.5 mg/kg; IP) on responding for sensory reinforcement.

METHODS

Male Long-Evans underwent Pavlovian conditioning, wherein a tone-light conditioned stimulus (CS) was repeatedly paired with 12% EtOH or 21.7% sucrose. Next, tests of responding for a CRf were conducted. Responding on the CRf lever delivered the CS, whereas responding on the other lever had no consequences. In a separate cohort of rats, the effects of yohimbine on responding just for the sensory reinforcer were examined.

RESULTS

Both doses of yohimbine, but not U50,488, increased responding for conditioned reinforcement. This enhancement occurred independently of the nature of the reward used during Pavlovian conditioning. Yohimbine-enhanced responding for a CRf was reproducible and stable over five tests; it even persisted when the CS was omitted. Both doses of yohimbine also increased responding for sensory reinforcement.

CONCLUSIONS

Yohimbine increases operant responding for a variety of sensory and conditioned reinforcers. This enhancement may be independent of its stress-like effects.

Neural mechanisms underlying incubation of methamphetamine craving: A mini-review

Cue-induced drug craving and seeking progressively increases during abstinence. This "incubation of drug craving" phenomenon has been observed in both laboratory animals and humans. Preclinical studies identified several neural mechanisms underlying incubation of drug craving after forced abstinence, primarily focusing on cocaine. Recently, studies started focusing on another powerful psychostimulant, methamphetamine (Meth), and developed new incubation procedures (choice-induced and punishment-induced abstinence). Here, we review mechanistic studies at the circuit, synaptic and molecular levels on incubation of Meth craving. First, we provide an overview of neural adaptations associated with prolonged forced abstinence after extended-access Meth self-administration. Next, we review studies examining the causal roles of discrete brain regions and associated circuits, glutamate transmission, histone deacetylase 5 and oxytocin in incubation of Meth craving after forced abstinence. Lastly, we review causal and correlational studies examining the mechanisms underlying incubation of Meth craving after choice-induced voluntary abstinence and punishment-induced abstinence, respectively. We conclude by discussing the translational potential of these mechanistic studies in Meth relapse prevention in human drug users.

Pharmacology of Herbal Sexual Enhancers: A Review of Psychiatric and Neurological Adverse Effects

Sexual enhancers increase sexual potency, sexual pleasure, or libido. Substances increasing libido alter the concentrations of specific neurotransmitters or sex hormones in the central nervous system. Interestingly, the same pathways are involved in the mechanisms underlying many psychiatric and neurological disorders, and adverse reactions associated with the use of aphrodisiacs are strongly expected. However, sexual enhancers of plant origin have gained popularity over recent years, as natural substances are often regarded as a safer alternative to modern medications and are easily acquired without prescription. We reviewed the psychiatric and neurological adverse effects associated with the consumption of herbal aphrodisiacs Areca catechu L., Argemone Mexicana L., Citrus aurantium L., Eurycoma longifolia Jack., Lepidium meyenii Walp., Mitragyna speciosa Korth., Panax ginseng C. A. Mey, Panax quinquefolius L., Pausinystalia johimbe (K. Schum.) Pierre ex Beille, Piper methysticum G. Forst., Ptychopetalum olacoides Benth., Sceletium tortuosum (L.) N. E. Brown, Turnera diffusa Willd. ex. Schult., Voacanga africana Stapf ex Scott-Elliot, and Withania somnifera (L.) Dunal. A literature search was conducted on the PubMed, Scopus, and Web of Science databases with the aim of identifying all the relevant articles published on the issue up to June 2020. Most of the selected sexual enhancers appeared to be safe at therapeutic doses, although mild to severe adverse effects may occur in cases of overdosing or self-medication with unstandardized products. Drug interactions are more concerning, considering that herbal aphrodisiacs are likely used together with other plant extracts and/or pharmaceuticals. However, few data are available on the side effects of several plants included in this review, and more clinical studies with controlled administrations should be conducted to address this issue.

Effects of the α-2 Adrenergic Receptor Agonists Lofexidine and Guanfacine on Food-Cocaine Choice in Socially Housed Cynomolgus Monkeys

Although norepinephrine (NE) does not appear to play a prominent role in mediating the abuse-related effects of cocaine, studies have indicated that NE α-2 receptor agonists can attenuate reinstatement of extinguished cocaine self-administration in rats and monkeys and can decrease cocaine craving in humans. In the present studies, we examined the effects of two α-2 receptor agonists, lofexidine and guanfacine, on choice between food and cocaine (0.0-0.1 mg/kg per injection) in cynomolgus monkeys. Male and female subjects were housed in stable same-sex social groups of four; social rank did not influence the effects of lofexidine and guanfacine. When administered acutely, lofexidine (0.03-3.0 mg/kg, i.v.) significantly decreased cocaine choice in females (n = 7) but not males (n = 8). However, in males, the same lofexidine doses produced dose-dependent decreases in core body temperature (n = 7), and acute guanfacine (0.003-1.0 mg/kg, i.v.) significantly decreased cocaine choice (n = 11). When lofexidine was administered for five consecutive days to a subset of the monkeys in whom lofexidine acutely decreased cocaine choice, tolerance to this effect developed to varying degrees of completeness in three of three males and two of four females. Taken together, these data suggest that α-2 receptor agonists can produce small decreases in the reinforcing strength of cocaine relative to food and that, even when efficacy is observed after acute administration, tolerance to the decreases in cocaine choice are apparent and more likely in males compared with females. SIGNIFICANCE STATEMENT: Cocaine use disorder remains a significant public health problem with no US Food and Drug Administration-approved treatments. Although cocaine elevates dopamine, serotonin, and norepinephrine (NE), the latter target has received less research. The present study noted modest effects of NE agonists on the relative reinforcing strength of cocaine with greater efficacy in female compared with male monkeys.

Comparative analysis of the modulation of perineuronal nets in the prefrontal cortex of rats during protracted withdrawal from cocaine, heroin and sucrose self-administration

Relapse into drug use is a significant problem for people recovering from addiction. The ability that conditioned cues have to reinstate and reinvigorate drug-seeking is potentiated over time (incubation of seeking), posing an additional difficulty for maintaining abstinence. While the prefrontal cortex has been involved in the incubation phenomenon and the extracellular matrix, perineuronal nets (PNNs) in particular, may play a vital role in brain plasticity associated to drug relapse, there are no comparative analyses between different drug classes and natural reinforcers. Here, we compare the effects of early (1 day) and protracted (30 days) withdrawal from to cocaine, heroin and sucrose self-administration on the total density and density per intensity range of PNNs of different territories of the prefrontal cortex of male Lewis rats. Our results show that cocaine self-administration increases the density of PNNs in the dorsal prelimbic, infralimbic and ventral orbitofrontal cortices, while protracted withdrawal reversesthis effect in the dorsal prelimbic cortex. Also, heroin self-administration increases the density of PNNs in the infralimbic cortex and ventral orbitofrontal cortices, but this effect is lost after 30 days of withdrawal in the infralimbic cortex. Finally, the self-administration of sucrose-sweetened water or the protracted withdrawal from this powerful reinforcer does not affect any of the PNN parameters analysed. Our results show that two different drugs of abuse (but not a natural reward) with specific pharmacological and physiological actions, differentially modulate PNNs in specific areas of the rodent prefrontal cortex with potential implications for the incubation of seeking phenomenon.

Higher anhedonia during withdrawal from initial opioid exposure is protective against subsequent opioid self-administration in rats

RATIONALE

Understanding factors contributing to individual differences in vulnerability to opioid addiction is essential for developing more effective preventions and treatments, yet few reliable behavioral predictors of subsequent opioid self-administration have been identified in rodents. Sensitivity to the acute effects of initial drug exposure predicts later addiction vulnerability in both humans and animals, but the relationship between sensitivity to withdrawal from initial drug exposure and later drug use vulnerability is unclear.

OBJECTIVE

The goal of the current study was to evaluate whether the degree of anhedonia experienced during withdrawal from early opioid exposure predicts subsequent vulnerability to opioid self-administration.

METHODS

Rats were first tested for withdrawal sensitivity following acute injections of morphine (i.e., "acute dependence"), measured as elevations in intracranial self-stimulation (ICSS) thresholds (anhedonia-like behavior) during naloxone-precipitated and spontaneous withdrawal. Rats were then tested for addiction-like behavior using various measures of i.v. morphine self-administration (MSA) including acquisition, demand, extinction, and reinstatement induced by morphine, stress, and/or drug-associated cues.

RESULTS

Greater naloxone-precipitated withdrawal across repeated morphine injections and greater peak spontaneous withdrawal severity following a single morphine injection were associated with lower addiction-like behavior on multiple MSA measures. Withdrawal-induced anhedonia predicted a wider range of MSA measures than did any individual measure of MSA itself.

CONCLUSIONS

Our data establish WIA as one of the first behavioral measures to predict individual differences in opioid SA in rodents. This model promises to be useful for furthering our understanding of behavioral and neurobiological mechanisms underlying vulnerability to opioid addiction.

The Molecular-Container Calabadion-2 Prevents Methamphetamine-Induced Reinstatement in Rats: A Potential Approach to Relapse Prevention?

BACKGROUND

Reexposure to methamphetamine with a single "priming dose" can trigger intense cravings and precipitate relapse in methamphetamine-dependent individuals. The acyclic cucurbit[n]uril "molecular container" calabadion-2 shows a high affinity to bind and sequester methamphetamine in vitro and attenuates its locomotor-stimulating effect in rats. The present study investigates whether pretreatment with calabadion-2 is sufficient to prevent the reinstatement of drug seeking by a priming dose of methamphetamine in rats.

METHODS

Male Long-Evans rats were trained to self-administer i.v. methamphetamine (0.06 mg/kg/infusion). Following 10 days of stable self-administration, rats underwent extinction training and were subsequently tested on a multi-phase reinstatement procedure. Drug-primed reinstatement sessions (0.3 mg/kg methamphetamine, i.v.) were preceded by either saline or calabadion-2 (130 mg/kg). Additional reinstatement tests were conducted after administration of yohimbine (1.0 mg/kg, i.v.) to define the pharmacological specificity of calabadion-2.

RESULTS

Pretreatment with calabadion-2 significantly attenuated methamphetamine-induced reinstatement of responding. Cal2 did not affect drug-seeking behavior stimulated by the pharmacological stressor yohimbine, indicating a mechanism of action specific to methamphetamine.

CONCLUSIONS

These results demonstrate the effectiveness of calabadion-2 in a preclinical model relapse-like behavior. With further structural optimization, molecular containers may provide a novel and efficacious pharmacokinetic approach to relapse prevention for methamphetamine-dependent individuals.

TAAR1 agonists attenuate extended-access cocaine self-administration and yohimbine-induced reinstatement of cocaine-seeking

BACKGROUND AND PURPOSE

The trace amine-associated receptor 1 (TAAR1) negatively modulates dopamine transmission. Our previous studies demonstrated that TAAR1 agonists attenuated cue- and drug-induced cocaine-seeking and increased the elasticity of the cocaine demand curve, in the short-access cocaine self-administration model. Compulsive use of cocaine, which is an essential criterion of cocaine use disorder, can be induced by extended access to cocaine self-administration.

EXPERIMENTAL APPROACH

To characterize the role of TAAR1 in compulsive cocaine use, we evaluated the effects of activation of TAAR1 on cocaine intake, cocaine binge and cue-induced cocaine-seeking using the extended-access cocaine self-administration model in adult male Sprague-Dawley rats. We also investigated the role of TAAR1 in stress-triggered cocaine relapse by using the α₂ -adrenoceptor antagonist yohimbine-induced reinstatement of cocaine-seeking.

KEY RESULTS

The selective TAAR1 partial agonist RO5263397 attenuated cocaine intake and did not develop tolerance during the 10-day extended-access cocaine self-administration. RO5263397 reduced a 12-h binge intake of cocaine after forced abstinence. RO5263397 also decreased cue-induced cocaine-seeking after prolonged abstinence from extended-access cocaine self-administration. Furthermore, RO5263397 and the selective TAAR1 full agonist RO5166017 reduced yohimbine-induced reinstatement of cocaine-seeking behaviour.

CONCLUSION AND IMPLICATIONS

Activation of TAAR1 attenuated extended-access cocaine self-administration and stress-induced cocaine reinstatement. These results suggest that TAAR1 agonists are promising pharmacological interventions to treat cocaine use disorder and relapse.

The effect of chronic oxytocin treatment during abstinence from methamphetamine self-administration on incubation of craving, reinstatement, and anxiety

Methamphetamine (METH) abuse is characterised by chronic relapse and anxiety, for which there are no effective pharmacotherapies. Acute treatment with the neuropeptide oxytocin has shown therapeutic potential for METH addiction and has social and anxiolytic effects in METH-naïve rats. However, the effects of chronic oxytocin treatment in METH-experienced rats are unknown. This study investigated the effects of repeated oxytocin treatment during abstinence from METH self-administration on incubation of cue-induced relapse, yohimbine- and METH-induced reinstatement, trait anxiety, and social interaction. Male and female Sprague-Dawley rats self-administered intravenous METH for 2 h/day (12 days) and then on short-access (2 h/day; ShA) or long-access (6 h/day; LgA) sessions (10 days). Rats underwent 30 days of drug abstinence, during which they received 15 days of intraperitoneal oxytocin (1 mg/kg) or saline (days 6-20) injections. Anxiety and social interaction were tested on days 25-28, and incubation was assessed by testing cue-induced relapse on days 2 and 30. Rats underwent extinction after the final cue-relapse test, followed by yohimbine- and METH-primed reinstatement. LgA, but not ShA rats exhibited incubation of METH-craving and enhanced METH-primed reinstatement in both sexes, and enhanced yohimbine-induced reinstatement in females. Importantly, chronic oxytocin attenuated incubation and METH-primed reinstatement in both sexes, and yohimbine-induced reinstatement in females, although only in LgA rats. LgA produced a heightened anxiety phenotype, which was partially rescued by chronic oxytocin treatment. Using a translatable addiction model, these findings demonstrate the therapeutic efficacy of chronic oxytocin after METH self-administration and supports the clinical utility of oxytocin for METH addiction in both sexes.

Different periods of forced abstinence after instrumental learning for food reward of different macronutrient value on responding for conditioned cues and AMPAr subunit levels

Food craving can be viewed as an intense desire for a specific food that propagates seeking and consuming behavior. Prolonged forced abstinence from rewarding foods can result in escalated food-seeking behavior as measured via elevated responding for food-paired cues in the absence of the primary reward. Palatable food consumption and food-seeking is associated with changes in the abundance and composition of AMPA receptors in the nucleus accumbens (NAc) but differing results have been reported. The present study examined whether different food types could produce escalated food-seeking behavior after various abstinence periods and whether this was associated with changes in AMPA receptor protein levels. Rats were trained for 10 days to bar press for purified, sucrose, or chocolate-flavored sucrose pellets. Rats were tested at 24 hrs, 7 d or 14 d whereby bar pressing resulted in presentation of cues paired with food but no food reward was delivered. Western blotting was used to determine protein levels of GluR1, GluR1pSer845, and GluR2 in the NAc. Three separate groups were assessed: 1) a group that was trained on the operant task and tested for conditioned responding (tested group); 2) a group that was trained on the operant task but not tested (non-tested group); 3) a group that was neither trained nor tested (control). The purified food group showed a time-dependent elevation in conditioned bar pressing over the 3 abstinence periods. GluR1 AMPAr subunit levels were higher in the purified and sucrose groups tested at 24 hours compared to the non-tested and control values. GluR1 levels subsequently declined at the 7- and 14-day abstinence periods in the purified and sucrose tested and non-tested groups compared to control values. GluR2 and pSer845 Glur1 levels were similar across all groups and abstinence periods. These results show that food-seeking behavior associated with forced abstinence from different food rewards may depend on the macronutrient composition of the food reward or the food type given during the abstinence period. A clear link with AMPAr subunit levels in this model was not established.

Trial-based Discrimination Procedure for Studying Drug Relapse in Rats

In abstinent drug addicts, cues formerly associated with drug-taking experiences gain relapse-inducing potency ('incubate') over time. Animal models of incubation may help in developing treatments for relapse prevention. However, these models have primarily focused on the role of conditioned stimuli (CSs) signaling drug delivery and not on discriminative stimuli (DSs), which signal drug availability and are also known to play a major role in drug relapse. We recently showed that DS-controlled cocaine seeking in rats also incubates during abstinence and persists up to 300 days. We used a trial-based procedure to train male and female rats to discriminate between two light cues: one light cue (DS+) signaled the availability of cocaine reward and the second light cue (DS-) signaled the absence of reward. Rats learned to press a central retractable lever during trials in which the DS+ cue was presented and to suppress responding when the DS- cue was presented. Here, we provide a detailed protocol for the behavioral procedure used in our study. The trial-based design of this behavior lends itself well to time-locked in vivo recording and manipulation approaches that can be used to identify neurobiological mechanisms underlying the contributions of DSs to drug relapse.

Understanding Addiction Using Animal Models

Drug addiction is a neuropsychiatric disorder with grave personal consequences that has an extraordinary global economic impact. Despite decades of research, the options available to treat addiction are often ineffective because our rudimentary understanding of drug-induced pathology in brain circuits and synaptic physiology inhibits the rational design of successful therapies. This understanding will arise first from animal models of addiction where experimentation at the level of circuits and molecular biology is possible. We will review the most common preclinical models of addictive behavior and discuss the advantages and disadvantages of each. This includes non-contingent models in which animals are passively exposed to rewarding substances, as well as widely used contingent models such as drug self-administration and relapse. For the latter, we elaborate on the different ways of mimicking craving and relapse, which include using acute stress, drug administration or exposure to cues and contexts previously paired with drug self-administration. We further describe paradigms where drug-taking is challenged by alternative rewards, such as appetitive foods or social interaction. In an attempt to better model the individual vulnerability to drug abuse that characterizes human addiction, the field has also established preclinical paradigms in which drug-induced behaviors are ranked by various criteria of drug use in the presence of negative consequences. Separation of more vulnerable animals according to these criteria, along with other innate predispositions including goal- or sign-tracking, sensation-seeking behavior or impulsivity, has established individual genetic susceptibilities to developing drug addiction and relapse vulnerability. We further examine current models of behavioral addictions such as gambling, a disorder included in the DSM-5, and exercise, mentioned in the DSM-5 but not included yet due to insufficient peer-reviewed evidence. Finally, after reviewing the face validity of the aforementioned models, we consider the most common standardized tests used by pharmaceutical companies to assess the addictive potential of a drug during clinical trials.

Therapeutic efficacy of environmental enrichment for substance use disorders

Addiction to drug and alcohol is regarded as a major health problem worldwide for which available treatments show limited effectiveness. The biggest challenge remains to enhance the capacities of interventions to reduce craving, prevent relapse and promote long-term recovery. New strategies to meet these challenges are being explored. Findings from preclinical work suggest that environmental enrichment (EE) holds therapeutic potential for the treatment of substance use disorders, as demonstrated in a number of animal models of drug abuse. The EE intervention introduced after drug exposure leads to attenuation of compulsive drug taking, attenuation of the rewarding (and reinforcing) effects of drugs, reductions in control of behavior by drug cues, and, very importantly, relapse prevention. Clinical work also suggests that multidimensional EE interventions (involving physical activity, social interaction, vocational training, recreational and community involvement) might produce similar therapeutic effects, if implemented continuously and rigorously. In this review we survey preclinical and clinical studies assessing the efficacy of EE as a behavioral intervention for substance use disorders and address related challenges. We also review work providing empirical evidence for EE-induced neuroplasticity within the mesocorticolimbic system that is believed to contribute to the seemingly therapeutic effects of EE on drug and alcohol-related behaviors.

Distinct gene alterations between Fos-expressing striatal and thalamic neurons after withdrawal from methamphetamine self-administration

BACKGROUND

Methamphetamine (Meth) seeking progressively increases after withdrawal (incubation of Meth craving). We previously demonstrated a role of anterior intralaminar nucleus of thalamus (AIT) to dorsomedial striatum (DMS) projections in this incubation. Here, we examined molecular alterations in DMS and AIT neurons activated (identified by neuronal activity marker Fos) during "incubated" Meth-seeking relapse test after prolonged withdrawal.

METHODS

We trained male rats to self-administer Meth or saline (control condition) for 10 days (6 hr/day). Using fluorescence-activated cell sorting, we examined gene expression in Fos-positive (activated during a 2-hr relapse test) and Fos-negative (nonactivated) DMS and AIT neurons.

RESULTS

In DMS, we found increased mRNA expressions of immediate early genes (IEGs) (Arc, Egr1, Npas4, Fosb), Trkb, glutamate receptors subunits (Gria3, Grin1, Grin2b, Grm1), and epigenetic enzymes (Hdac3, Hdac5, Crebbp) in Fos-positive neurons, compared with Fos-negative neurons. In AIT, we found that fewer genes (Egr1, Fosb, TrkB, Grin1, and Hdac5) exhibited increased mRNA expression in Fos-positive neurons. Unexpectedly, in both brain regions, gene alterations described above also occurred in drug-naïve saline self-administration control rats.

CONCLUSIONS

These results demonstrated that transcriptional regulations in Fos-positive neurons activated during the relapse tests are brain region-specific but are not uniquely associated with drug exposure during the self-administration training.