Establishing preclinical withdrawal syndrome symptomatology following heroin self-administration in male and female rats

A rat model of operant negative reinforcement in opioid-dependent males and females

RATIONALE AND OBJECTIVE

Avoidance of opioid withdrawal plays a key role in human opioid addiction. Here, we present a procedure for studying operant negative reinforcement in rats that was inspired by primate procedures where opioid-dependent subjects lever-press to prevent naloxone infusions.

METHODS

In Experiment 1, we trained rats (n = 30, 15 females) to lever-press to escape and then avoid mild footshocks (0.13-0.27 mA) for 35 days (30 trials/d). Next, we catheterized them and implanted minipumps containing methadone (10 mg/kg/day) or saline. We then paired (4 times, single session) a light cue (20-s) with a naloxone infusion (20 µg/kg, i.v) that precipitated opioid withdrawal. Next, we trained the rats to escape naloxone injections for 10 days (30 trials/d). Each trial started with the onset of the opioid-withdrawal cue. After 20-s, the lever extended, and an infusion of naloxone (1 to 2.2 µg/kg/infusion) began; a lever-press during an 11-s window terminated the withdrawal-paired cue and the infusion. In Experiment 2, we trained rats (n = 34, 17 females) on the same procedure but decreased the footshock escape/avoidance training to 20 days.

RESULTS

All rats learned to lever-press to escape or avoid mild footshocks. In both experiments, a subset, 56% (10/18) and 33% (8/24) of methadone-dependent rats learned to lever-press to escape naloxone infusions.

CONCLUSIONS

We introduce an operant negative reinforcement procedure where a subset of opioid-dependent rats learned to lever-press to escape withdrawal-inducing naloxone infusions. The procedure can be used to study mechanisms of individual differences in opioid negative reinforcement-related behaviors in opioid-dependent rats.

Amelioration of morphine withdrawal syndrome by systemic and intranasal administration of mesenchymal stem cell-derived secretome in preclinical models of morphine dependence

BACKGROUND

Morphine is an opiate commonly used in the treatment of moderate to severe pain. However, prolonged administration can lead to physical dependence and strong withdrawal symptoms upon cessation of morphine use. These symptoms can include anxiety, irritability, increased heart rate, and muscle cramps, which strongly promote morphine use relapse. The morphine-induced increases in neuroinflammation, brain oxidative stress, and alteration of glutamate levels in the hippocampus and nucleus accumbens have been associated with morphine dependence and a higher severity of withdrawal symptoms. Due to its rich content in potent anti-inflammatory and antioxidant factors, secretome derived from human mesenchymal stem cells (hMSCs) is proposed as a preclinical therapeutic tool for the treatment of this complex neurological condition associated with neuroinflammation and brain oxidative stress.

METHODS

Two animal models of morphine dependence were used to evaluate the therapeutic efficacy of hMSC-derived secretome in reducing morphine withdrawal signs. In the first model, rats were implanted subcutaneously with mini-pumps which released morphine at a concentration of 10 mg/kg/day for seven days. Three days after pump implantation, animals were treated with a simultaneous intravenous and intranasal administration of hMSC-derived secretome or vehicle, and withdrawal signs were precipitated on day seven by i.p. naloxone administration. In this model, brain alterations associated with withdrawal were also analyzed before withdrawal precipitation. In the second animal model, rats voluntarily consuming morphine for three weeks were intravenously and intranasally treated with hMSC-derived secretome or vehicle, and withdrawal signs were induced by morphine deprivation.

RESULTS

In both animal models secretome administration induced a significant reduction of withdrawal signs, as shown by a reduction in a combined withdrawal score. Secretome administration also promoted a reduction in morphine-induced neuroinflammation in the hippocampus and nucleus accumbens, while no changes were observed in extracellular glutamate levels in the nucleus accumbens.

CONCLUSION

Data presented from two animal models of morphine dependence suggest that administration of secretome derived from hMSCs reduces the development of opioid withdrawal signs, which correlates with a reduction in neuroinflammation in the hippocampus and nucleus accumbens.

Xylazine suppresses fentanyl consumption during self-administration and induces a unique sex-specific withdrawal syndrome that is not altered by naloxone in rats

Prescription and illicit opioid use are a public health crisis, with the landscape shifting to fentanyl use. Since fentanyl is 100-fold more potent than morphine, its use is associated with a higher risk of fatal overdose that can be remediated through naloxone (Narcan) administration. However, recent reports indicate that xylazine, an anesthetic, is increasingly detected in accidental fentanyl overdose deaths. Anecdotal reports suggest that xylazine may prolong the fentanyl "high," alter the onset of fentanyl withdrawal, and increase resistance to naloxone-induced reversal of overdose. To date, no preclinical studies have evaluated the impacts of xylazine on fentanyl self-administration (SA; 2.5 μg/kg/infusion) or withdrawal to our knowledge. We established a rat model of xylazine/fentanyl co-SA and withdrawal and evaluated outcomes as a function of biological sex. When administered alone, chronic xylazine (2.5 mg/kg, intraperitoneal) induced unique sex-specific withdrawal symptomatology, whereby females showed delayed onset of signs and a possible enhancement of sensitivity to the motor-suppressing effects of xylazine. Xylazine reduced fentanyl consumption in both male and female rats regardless of whether it was experimenter-administered or added to the intravenous fentanyl product (0.05, 0.10, and 0.5 mg/kg/infusion) when compared to fentanyl SA alone. Interestingly, this effect was dose-dependent when self-administered intravenously. Naloxone (0.1 mg/kg, subcutaneous injection) did not increase somatic signs of fentanyl withdrawal, regardless of the inclusion of xylazine in the fentanyl infusion in either sex; however, somatic signs of withdrawal were higher across time points in females after xylazine/fentanyl co-SA regardless of naloxone exposure as compared to females following fentanyl SA alone. Together, these results indicate that xylazine/fentanyl co-SA dose-dependently suppressed fentanyl intake in both sexes and induced a unique withdrawal syndrome in females that was not altered by acute naloxone treatment. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

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.

Voluntary alcohol intake alters the motivation to seek intravenous oxycodone and neuronal activation during the reinstatement of oxycodone and sucrose seeking

Opioid-alcohol polysubstance use is prevalent and worsens treatment outcomes. Here we assessed whether co-consumption of oxycodone and alcohol influence the intake of one another, demand for oxycodone, and the neurocircuitry underlying cue-primed reinstatement of oxycodone-seeking. Male and female rats underwent oxycodone intravenous self-administration (IVSA) with homecage access to alcohol (20% v/v) and/or water immediately after the IVSA session. Next, economic demand for intravenous oxycodone was assessed while access to alcohol and/or water continued. Control rats self-administered sucrose followed by access to alcohol and/or water. Rats underwent a cue-primed reinstatement test and brains were processed for c-fos mRNA expression. While both sexes decreased oxycodone intake if they had access to alcohol, and decreased alcohol intake if they had access to oxycodone, only female oxycodone + alcohol rats exhibited decreased demand elasticity and increased cue-primed reinstatement. Alcohol consumption increased the number of basolateral and central amygdala neurons activated during sucrose and oxycodone reinstatement and the number of ventral and dorsal striatum neurons engaged by sucrose reinstatement. Nucleus accumbens shell dopamine 1 receptor expressing neurons displayed activation patterns consistent with oxycodone reinstatement. Thus, alcohol alters the motivation to seek oxycodone in a sex-dependent manner and the neural circuitry engaged by cue-primed reinstatement of sucrose and oxycodone-seeking.

Voluntary alcohol intake alters the motivation to seek intravenous oxycodone and neuronal activation during the reinstatement of oxycodone and sucrose seeking

Opioid-alcohol polysubstance use is prevalent and worsens treatment outcomes. Here we assessed whether co-consumption of oxycodone and alcohol would influence intake of one another, demand for oxycodone, and the neurocircuitry underlying cue-primed reinstatement of oxycodone-seeking. Male and female rats underwent oxycodone intravenous self-administration (IVSA) with access to either alcohol (20% v/v) and water or only water immediately after the IVSA session. Next, economic demand for intravenous oxycodone was assessed while access to alcohol and/or water continued. Control rats self-administered sucrose followed by access to alcohol and/or water. Rats underwent extinction training and brains were processed for c-fos mRNA expression immediately following a cue-primed reinstatement test. While both sexes decreased oxycodone intake if they had access to alcohol, and decreased alcohol intake if they had access to oxycodone, female oxycodone+alcohol rats exhibited decreased demand elasticity for intravenous oxycodone and increased cue-primed reinstatement while male rats did not. Spontaneous withdrawal signs were correlated with oxycodone intake while alcohol intake was correlated with anxiety-like behavior. Alcohol consumption increased the number of basolateral and central amygdala neurons activated during sucrose and oxycodone reinstatement and the number of ventral and dorsal striatum neurons engaged by sucrose reinstatement. Nucleus accumbens shell dopamine 1 receptor containing neurons displayed activation patterns consistent with oxycodone reinstatement. Thus, alcohol alters the motivation to seek oxycodone in a sex-dependent manner and alters the neural circuitry engaged by cue-primed reinstatement of sucrose and oxycodone-seeking.

Advances in the characterization of negative affect caused by acute and protracted opioid withdrawal using animal models

Opioid use disorder (OUD) is a chronic brain disease which originates from long-term neuroadaptations that develop after repeated opioid consumption and withdrawal episodes. These neuroadaptations lead among other things to the development of a negative affect, which includes loss of motivation for natural rewards, higher anxiety, social deficits, heightened stress reactivity, an inability to identify and describe emotions, physical and/or emotional pain, malaise, dysphoria, sleep disorders and chronic irritability. The urge for relief from this negative affect is one of major causes of relapse, and thus represents a critical challenge for treatment and relapse prevention. Animal models of negative affect induced by opioid withdrawal have recapitulated the development of a negative emotional state with signs such as anhedonia, increased anxiety responses, increased despair-like behaviour and deficits in social interaction. This research has been critical to determine neurocircuitry adaptations during chronic opioid administration or upon withdrawal. In this review, we summarize the recent literature of rodent models of (i) acute withdrawal, (ii) protracted abstinence from passive administration of opioids, (iii) withdrawal or protracted abstinence from opioid self-administration. Finally, we describe neurocircuitry involved in acute withdrawal and protracted abstinence. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

Estradiol administration increases anxiety-like behavior following chronic escalating morphine administration in hormone-replaced ovariectomized female rats

Withdrawal from opioids can induce a state of anxiety and irritability. This negative state can facilitate continued drug taking, as the administration of opioids can alleviate unpleasant symptoms associated with acute and protracted withdrawal. It is, therefore, of interest to investigate factors that can contribute to the severity of anxiety during periods of abstinence. One such factor is the fluctuation of ovarian hormones. Evidence from a non-opioid drug indicates that estradiol increases, while progesterone decreases anxiety during withdrawal. However, no work has yet studied how ovarian hormones might influence the severity of anxiety during withdrawal from opioids. To explore this, we ovariectomized female rats and provided a four-day repeating cycle of ovarian hormone administration (Day 1: estradiol, Day 2: estradiol, Day 3: progesterone, Day 4: peanut oil). Male rats were given sham surgeries and administered peanut oil daily in lieu of hormone replacement. All rats received twice daily injections of morphine (or 0.9 % saline) for 10 days total at a dose that doubled every two days (2.5 mg/kg, 5 mg/kg, 10 m/kg, 20 mg/kg, 40 mg/kg). Rats underwent spontaneous withdrawal and were tested for anxiety-like behaviors 12 and 108 h after the last morphine treatment. At 12 h, morphine-withdrawn female rats treated with estradiol on the day of testing displayed significantly more anxiety-like behavior in light-dark box testing than female morphine-withdrawn and (marginally) male morphine-withdrawn rats receiving vehicle that day. Somatic withdrawal behaviors (wet dog shakes, head shakes, writhing) were also taken every 12 h through 108 h. We found no meaningful contribution of sex or hormone for these measures. This study is the first of its kind to provide evidence that ovarian hormones influence anxiety-like behavior during morphine withdrawal.

Morphine Withdrawal-Induced Hyperalgesia in Models of Acute and Extended Withdrawal Is Attenuated by l-Tetrahydropalmatine

Effective pain control is an underappreciated aspect of managing opioid withdrawal, and its absence presents a significant barrier to successful opioid detoxification. Accordingly, there is an urgent need for effective non-opioid treatments to facilitate opioid detoxification. l-Tetrahydropalmatine (l-THP) possesses powerful analgesic properties and is an active ingredient in botanical formulations used in Vietnam for the treatment of opioid withdrawal syndrome. In this study, rats receiving morphine (15 mg/kg, i.p.) for 5 days per week displayed a progressive increase in pain thresholds during acute 23 h withdrawal as assessed by an automated Von Frey test. A single dose of l-THP (5 or 7.5 mg/kg, p.o.) administered during the 4th and 5th weeks of morphine treatment significantly improves pain tolerance scores. A 7-day course of l-THP treatment in animals experiencing extended withdrawal significantly attenuates hyperalgesia and reduces the number of days to recovery to baseline pain thresholds by 61% when compared to vehicle-treated controls. This indicates that the efficacy of l-THP on pain perception extends beyond its half-life. As a non-opioid treatment for reversing a significant hyperalgesic state during withdrawal, l-THP may be a valuable addition to the currently limited arsenal of opioid detoxification treatments.

Sex-differences in anxiety, neuroinflammatory markers, and enhanced fear learning following chronic heroin withdrawal

Post-traumatic stress disorder (PTSD) and opioid use disorder (OUD) are comorbid in clinical populations. However, both pre-clinical and clinical studies of these co-occurring disorders have disproportionately represented male subjects, limiting the applicability of these findings. Our previous work has identified chronic escalating heroin administration and withdrawal can produce enhanced fear learning. This behavior is associated with an increase in dorsal hippocampal (DH) interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), and glial fibrillary acidic protein (GFAP) immunoreactivity. Further, we have shown that these increases in IL-1β and TNF-α are mechanistically necessary for the development of enhanced fear learning. Although these are exciting findings, this paradigm has only been studied in males. The current studies aim to examine sex differences in the behavioral and neuroimmune effects of chronic heroin withdrawal and future enhanced fear learning. In turn, we determined that chronic escalating heroin administration can produce withdrawal in female rats comparable to male rats. Subsequently, we examined the consequence of heroin withdrawal on future enhanced fear learning and IL-1β, TNF-α, and GFAP immunoreactivity. Strikingly, we identified sex differences in these neuroimmune measures, as chronic heroin administration and withdrawal does not produce enhanced fear learning or immunoreactivity changes in females. Moreover, we determined whether heroin withdrawal produces short-term and long-term anxiety behaviors in both female and males. Collectively, these novel experiments are the first to test whether heroin withdrawal can sensitize future fear learning, produce neurobiological changes, and cause short-term and long-term anxiety behaviors in female rats.

Machine learning and deep learning frameworks for the automated analysis of pain and opioid withdrawal behaviors

The automation of behavioral tracking and analysis in preclinical research can serve to advance the rate of research outcomes, increase experimental scalability, and challenge the scientific reproducibility crisis. Recent advances in the efficiency, accuracy, and accessibility of deep learning (DL) and machine learning (ML) frameworks are enabling this automation. As the ongoing opioid epidemic continues to worsen alongside increasing rates of chronic pain, there are ever-growing needs to understand opioid use disorders (OUDs) and identify non-opioid therapeutic options for pain. In this review, we examine how these related needs can be advanced by the development and validation of DL and ML resources for automated pain and withdrawal behavioral tracking. We aim to emphasize the utility of these tools for automated behavioral analysis, and we argue that currently developed models should be deployed to address novel questions in the fields of pain and OUD research.

Altered Accumbal Dopamine Terminal Dynamics Following Chronic Heroin Self-Administration

Administration of heroin results in the engagement of multiple brain regions and the rewarding and addictive effects are mediated, at least partially, through activation of the mesolimbic dopamine system. However, less is known about dopamine system function following chronic exposure to heroin. Withdrawal from chronic heroin exposure is likely to drive a state of low dopamine in the nucleus accumbens (NAc), as previously observed during withdrawal from other drug classes. Thus, we aimed to investigate alterations in NAc dopamine terminal function following chronic heroin self-administration to identify a mechanism for dopaminergic adaptations. Adult male Long Evans rats were trained to self-administer heroin (0.05 mg/kg/inf, IV) and then placed on a long access (FR1, 6-h, unlimited inf, 0.05 mg/kg/inf) protocol to induce escalation of intake. Following heroin self-administration, rats had decreased basal extracellular levels of dopamine and blunted dopamine response following a heroin challenge (0.1 mg/kg/inf, IV) in the NAc compared to saline controls. FSCV revealed that heroin-exposed rats exhibited reduced stimulated dopamine release during tonic-like, single-pulse stimulations, but increased phasic-like dopamine release during multi-pulse stimulation trains (5 pulses, 5–100 Hz) in addition to an altered dynamic range of release stimulation intensities when compared to controls. Further, we found that presynaptic D3 autoreceptor and kappa-opioid receptor agonist responsivity were increased following heroin self-administration. These results reveal a marked low dopamine state following heroin exposure and suggest the combination of altered dopamine release dynamics may contribute to increased heroin seeking.

A Glitch in the Matrix: The Role of Extracellular Matrix Remodeling in Opioid Use Disorder

Opioid use disorder (OUD) and deaths from drug overdoses have reached unprecedented levels. Given the enormous impact of the opioid crisis on public health, a more thorough, in-depth understanding of the consequences of opioids on the brain is required to develop novel interventions and pharmacological therapeutics. In the brain, the effects of opioids are far reaching, from genes to cells, synapses, circuits, and ultimately behavior. Accumulating evidence implicates a primary role for the extracellular matrix (ECM) in opioid-induced plasticity of synapses and circuits, and the development of dependence and addiction to opioids. As a network of proteins and polysaccharides, including cell adhesion molecules, proteases, and perineuronal nets, the ECM is intimately involved in both the formation and structural support of synapses. In the human brain, recent findings support an association between altered ECM signaling and OUD, particularly within the cortical and striatal circuits involved in cognition, reward, and craving. Furthermore, the ECM signaling proteins, including matrix metalloproteinases and proteoglycans, are directly involved in opioid seeking, craving, and relapse behaviors in rodent opioid models. Both the impact of opioids on the ECM and the role of ECM signaling proteins in opioid use disorder, may, in part, depend on biological sex. Here, we highlight the current evidence supporting sex-specific roles for ECM signaling proteins in the brain and their associations with OUD. We emphasize knowledge gaps and future directions to further investigate the potential of the ECM as a therapeutic target for the treatment of OUD.

Opioid and Sucrose Craving Are Accompanied by Unique Behavioral and Affective Profiles after Extended Abstinence in Male and Female Rats

Incubation of craving refers to the intensification of drug-seeking behavior in response to reward-paired cues over the course of abstinence. In rodents, craving and drug-seeking behaviors have been measured by an increase in lever pressing in the absence of reinforcer availability in response to cue presentations. However, craving in rodents is difficult to define and little is known about the behavioral signatures that accompany increased drug-seeking behavior measured by lever pressing. The affective components of relapse are also important, but understudied in rodents. Hormonal fluctuations influence craving for psychostimulants, but little is known about the impact of the estrous cycle on opioid-seeking behavior. This study sought to delineate the behavioral and affective signatures associated with craving, and to examine the influence of the female estrous cycle on craving. Male and female rats underwent 10 d of intravenous opioid self-administration. Separate cohorts of control rats self-administered oral sucrose, a natural nondrug reward. Cue-induced seeking tests were conducted after 1 or 30d of forced abstinence. These sessions were recorded and scored for overall locomotion, instances of sniffing, grooming, or hyperactivity. Ultrasonic vocalizations (USVs) were also recorded to determine affective profiles that accompany opioid seeking. Although active lever presses and overall locomotion increased unanimously over extended abstinence from heroin and sucrose, a sex- and reinforcer-specific behavioral and affective signature of craving emerged. Furthermore, although the female estrous cycle did not affect taking or seeking, it appears to influence more granular behaviors.

Complex Interactions Between Sex and Stress on Heroin Seeking

Rationale: Stress plays a dual role in substance use disorders as a precursor to drug intake and a relapse precipitant. With heroin use at epidemic proportions in the United States, understanding interactions between stress disorders and opioid use disorder is vital and will aid in treatment of these frequently comorbid conditions. Objectives: Here, we combine assays of stress and contingent heroin self-administration (SA) to study behavioral adaptations in response to stress and heroin associated cues in male and female rats. Methods: Rats underwent acute restraint stress paired with an odor stimulus and heroin SA for subsequent analysis of stress and heroin cue reactivity. Lofexidine was administered during heroin SA and reinstatement testing to evaluate its therapeutic potential. Rats also underwent tests on the elevated plus maze, locomotor activity in a novel environment, and object recognition memory following stress and/or heroin. Results: A history of stress and heroin resulted in disrupted behavior on multiple levels. Stress rats avoided the stress conditioned stimulus and reinstated heroin seeking in response to it, with males reinstating to a greater extent than females. Lofexidine decreased heroin intake, reinstatement, and motor activity. Previous heroin exposure increased time spent in the closed arms of an elevated plus maze, activity in a round novel field, and resulted in object recognition memory deficits. Discussion: These studies report that a history of stress and heroin results in maladaptive coping strategies and suggests a need for future studies seeking to understand circuits recruited in this pathology and eventually help develop therapeutic approaches.

Opioid withdrawal produces sex-specific effects on fentanyl-vs.-food choice and mesolimbic transcription

Background

Opioid withdrawal is a key driver of opioid addiction and an obstacle to recovery. However, withdrawal effects on opioid reinforcement and mesolimbic neuroadaptation are understudied and the role of sex is largely unknown.

Methods

Male (n=13) and female (n=12) rats responded under a fentanyl-vs.-food "choice" procedure during daily 2h sessions. In addition to the daily choice sessions, rats were provided extended access to fentanyl during 12h self-administration sessions. After two weeks of this self-administration regimen, the nucleus accumbens (NAc) and ventral tegmental area (VTA) of a subset of rats were subjected to RNA sequencing. In the remaining rats, a third week of this self-administration regimen was conducted, during which methadone effects on fentanyl-vs.-food choice were determined.

Results

Prior to opioid dependence, male and female rats similarly allocated responding between fentanyl and food. Abstinence from extended fentanyl access elicited similar increases in somatic withdrawal signs in both sexes. Despite similar withdrawal signs and extended access fentanyl intake, opioid withdrawal was accompanied by a maladaptive increase in fentanyl choice in males, but not females. Behavioral sex differences corresponded with a greater number of differentially expressed genes in the NAc and VTA of opioid-withdrawn females relative to males. Methadone blocked withdrawal-associated increases in fentanyl choice in males, but failed to further decrease fentanyl choice in females.

Conclusions

These results provide foundational evidence of sex-specific neuroadaptations to opioid withdrawal, which may be relevant to the female-specific resilience to withdrawal-associated increases in opioid choice and aid in the identification of novel therapeutic targets.

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.

Acquisition of remifentanil self-administration: Enhanced in female rats but no effect of adolescent stress exposure

Animal models of acquisition have been vital in shaping our understanding of vulnerability factors that influence susceptibility to drugs of abuse. Decades of research substantiates a number of biological, environmental, and behavioral factors that predict vulnerability - many of which have been important in the development of early intervention efforts in humans. The goal of the present study was to examine the acquisition of a synthetic opioid derivative in 66 adult male and female Long-Evans rats following histories of stress exposure during adolescence. Stress-exposed rats were subjected to a mild stress paradigm, which included alternating exposure to synthetic fox feces and physical restraint for eight days. Following stress induction and assessment, all rats were implanted with intravenous catheters in order to self-administer remifentanil (1 μm/kg/infusion) with no prior operant training. Acquisition of remifentanil self-administration was measured over 15 days. Findings indicate that regardless of stress condition, female rats acquired remifentanil self-administration sooner and emitted more active lever presses than males. Stress exposed animals exhibited increased anxiety-like response compared to the control group following exposure to stress, operationalized as decreased exploratory behavior on an Elevated Plus Maze. However, these effects were not expressed as significant differences in self-administration by stress. Together, these findings indicate that sex differences are evident in remifentanil self-administration.