Psychopharmacology

Genetic and other predisposing factors can influence the progression from initiation of drug intake to compulsive substance use through distinct biobehavioral processes. Operant cocaine self-administration studies in laboratory mice offer a powerful method to dissect the biology of this progression from initiation, dose-response, extinction, and cued reinstatement in a controlled, tractable system. However, many such studies encompass limited genetic diversity and rarely examine self-administration behaviors beyond the acquisition stage. Here, we study three high-diversity mouse populations - 50 strains from the Collaborative Cross (CC) reference panel, a large sample of Diversity Outbred (J:DO) population and their eight founder strains - to characterize the varied phenotypic manifestation of behaviors across multiple phases of cocaine intravenous self-administration (IVSA) in both sexes. We observed distinct strain differences among the founders and CC strains in all phases of self-administration, with heritability estimates ranging from 0 to 0.585 and many CC and J:DO phenotypic values exceeding the range of founders including the C57BL/6J strain. Sex differences were common across behaviors, some manifesting as main effects, others as strain interactions. Finally, by adopting a multi-stage design, we identified extreme strains for various cocaine intake and response traits and evaluated whether these strains exhibited differences in behavioral assays that model compulsive drug seeking. Together, these findings demonstrate the utility of extended self-administration protocols in advanced mouse populations for discovery and characterization of biological mechanisms of substance use traits and for preclinical studies in relevant, complex mouse models.

RationaleLearning the association between rewards and predictive cues is critical for appetitive behavioral responding. The mesolimbic dopamine system is thought to play an integral role in establishing these cue-reward associations. The dopamine response to cues can signal differences in reward value, though this emerges only after significant training. This suggests that the dopamine system may differentially regulate behavioral responding depending on the phase of training. ObjectivesThe purpose of this study was to determine whether antagonizing dopamine receptors elicited different effects on behavior depending on the phase of training or the type of Pavlovian task. MethodsSeparate groups of male rats were trained on Pavlovian tasks in which distinct audio cues signaled either differences in reward size or differences in reward rate. The dopamine receptor antagonist flupenthixol was systemically administered prior to either the first ten sessions of training (acquisition phase) or the second ten sessions of training (expression phase) and we monitored the effect of these manipulations for an additional ten training sessions. ResultsWe identified acute effects of dopamine receptor antagonism on conditioned responding, the latency to respond, and post-reward head entries in both Pavlovian tasks. Interestingly, dopamine receptor antagonism during the expression phase produced persistent deficits in behavioral responding only in rats trained on the reward size Pavlovian task. ConclusionsTogether, our results illustrate that dopamines control over behavior in Pavlovian tasks depends upon ones prior training experience and the information signaled by the cues.

Understanding the determinants of individual differences in cue-reactivity and drug sensitivity is critical to identifying neurobiological mechanisms underlying vulnerability to addiction. In this study, we examined the relationship between dopamine D1 and D2 receptor sensitivity and the attribution of incentive salience to reward cues and sensitivity to cocaine. Male Sprague Dawley rats were classified as having high or low sensitivity to the D2 receptor agonist quinpirole, and a subset was tested with the D1 receptor agonist SKF 82958. Cue-reactivity was assessed using a Pavlovian conditioned approach (PavCA) task, which distinguishes between sign-tracking (approach to a cue that predicts reward) and goal-tracking (approach to the site of reward delivery). Cocaine sensitivity was measured by locomotor activity and 50-kHz ultrasonic vocalizations (USVs), a putative measure of appetitive states. High D2 responders exhibited more sign-tracking and greater cocaine-induced USVs than low responders despite no difference in cocaine-induced locomotion. Sign-trackers also showed greater locomotor sensitivity to D1 receptor stimulation than goal-trackers and produced more cocaine-induced USVs. Rats with high sensitivity to both D1 and D2 receptor stimulation showed the strongest sign-tracking behavior and affective response to cocaine. These findings suggest that dopamine receptor sensitivity is associated with the propensity to attribute incentive salience to reward cues and potentially the appetitive effects of cocaine. This dopaminergic phenotype may reflect a mechanism contributing to both individual differences in cue-reactivity and drug responsiveness.

RationaleGlucagon-like peptide-1 (GLP-1) receptor agonists, such as semaglutide, are increasingly utilized in clinical practice due to their efficacy in promoting sustained weight loss following chronic administration. While acute treatment with GLP-1 receptor agonists has been shown to suppress food intake and reward-seeking behaviors in rodent models, the impact of prolonged exposure on preclinical measures of motivated behavior remains insufficiently characterized. ObjectivesThis study aimed to systematically evaluate the effects of chronic administration of semaglutide on both the acquisition and expression phases of Pavlovian conditioned approach (PavCA)--a behavioral paradigm used to assess the attribution of incentive salience to a food-paired cue. The influence of chronic semaglutide on the conditioned reinforcing properties of the food-associated cue, performance on a progressive ratio (PR) schedule for food reward, and ad libitum consumption of the food reward were also assessed. ResultsChronic semaglutide administration did not significantly alter either the acquisition or the expression of PavCA behavior. However, relative to vehicle-treated controls, semaglutide markedly enhanced responding for the food-associated cue during a conditioned reinforcement test and increased PR responding for the food reward. In contrast, semaglutide reduced both free consumption of the food reward and homecage chow intake. ConclusionsThese findings demonstrate that chronic semaglutide administration potentiates the incentive value of food-paired cues and increases motivation for food reward under restricted access conditions, yet attenuates overall food consumption when food is freely available. This dissociation highlights the nuanced effects of semaglutide on motivated behavior and suggests an amplification of the reinforcing properties of discrete, limited food rewards and associated cues.

Opioid Use Disorder (OUD) is an ongoing worldwide public health concern. Genetic factors contribute to multiple OUD-related phenotypes, such as opioid-induced analgesia, initiation of opioid use, and opioid dependence. Here, we present findings from a behavioral phenotyping protocol using male and female rats from 15 genetically diverse inbred strains from the Hybrid Rat Diversity Panel (HRDP). We used a self-administration paradigm to measure the acquisition of oxycodone intake during ten 2-hour sessions and escalation of oxycodone use during ten 12-hour sessions. During both the acquisition and escalation phases of self-administration, we observed that genetic background and sex influence oxycodone intake. The heritability of oxycodone intake phenotypes ranged between 0.26 to 0.54, indicating that genetic background plays a major role in the variability of oxycodone consumption. Genetic background and sex also influenced additional phenotypes recorded during oxycodone self-administration including lever discrimination and timeout responding. The genetic contribution to these traits was slightly more moderate, with heritability estimates ranging between 0.25 to 0.42. Measures of oxycodone intake were highly positively correlated between acquisition and escalation phases. Interestingly, the efficacy of oxycodone analgesia was positively correlated with oxycodone intake during the escalation phase, indicating that the initial behavioral responses to oxycodone may predict self-administration phenotypes. Together, these data demonstrate that sex and genetic background are major contributors to oxycodone self-administration phenotypes.

Although numerous studies have suggested that pharmacological alteration of the dopamine (DA) system modulates reward discounting, these studies have produced inconsistent findings. Here, we conducted a systematic review and pre-registered meta-analysis to evaluate DA drug-mediated effects on reward discounting of time, probability, and effort costs in studies of healthy rats. This produced a total of 1,343 articles to screen for inclusion/exclusion. From the literature, we identified 117 effects from approximately 1,549 individual rats. Using random-effects with maximum-likelihood estimation, we meta-analyzed placebo-controlled drug effects for (1) DA D1-like receptor agonists and (2) antagonists, (3) D2-like agonists and (4) antagonists, and (5) DA transporter-modulating drugs. Meta-analytic effects showed that DAT-modulating drugs decreased reward discounting. While D1-like and D2-like antagonists both increased discounting, agonist drugs for those receptors had no significant effect on discounting behavior. A number of these effects appear contingent on study design features like cost type, rat strain, and microinfusion location. These findings suggest a nuanced relationship between DA and discounting behavior and urge caution when drawing generalizations about the effects of pharmacologically manipulating dopamine on reward-based decision making.

The Reln gene encodes the extracellular glycoprotein Reelin that regulates synaptic plasticity and activity-dependent gene expression with implications in several neuropsychiatric disorders, including substance use disorder. While reduced Reln expression alters responses to psychostimulants and cannabinoid, its role in opioid-related behaviors remains unknown. Here, we examined whether Reln haploinsufficiency modifies behavioral and molecular responses to the synthetic opioid fentanyl. Heterozygous Reeler (Reln+/-) mice and wild-type littermates were assessed using using complementary contingent and non-contingent models of fentanyl exposure, including multi-phase fentanyl intravenous self-administration paradigm, conditioned place preference paradigm, locomotor assay, and dorsal striatal immediate early gene expression. Reln haploinsufficiency did not alter acquisition, extinction, or cue-induced reinstatement during self-administration, indicating stable opioid reinforcement and relapse-like behavior. Progressive ratio testing revealed a sex-dependent effect in which male Reln+/- mice showed reduced motivation for fentanyl compared to male wild-type mice. In contrast, following passive fentanyl exposure, Reln+/- mice exhibited enhanced fentanyl-induced locomotion and increased Fos immunoreactivity in the dorsal striatum, while CPP remained unchanged. Together, these findings demonstrate that Reln haploinsufficiency does not substantially modify opioid reinforcement or cue-driven drug seeking but enhances acute pharmacological sensitivity to fentanyl. These results identify Reln as a modulatory factor in opioid-responsive neural circuits that preferentially influences acute drug-evoked neuronal activation rather than the associative learning processes underlying opioid reinforcement.

Epigallocatechin-3-gallate (EGCG) and caffeine are the two primary compounds found in green tea. While EGCG has anxiolytic and anti-inflammatory effects, its acute effects on cognition are not well understood. Furthermore, despite widespread green tea consumption, little is known about how EGCG and caffeine co-administration impact behavior. Here, we investigated the effects of multiple doses of either EGCG or caffeine on a rat model of risk-taking. This was assessed using the risky decision-making task (RDT), in which rats choose between a small, safe reward and a large reward with escalating risk of mild footshock. Rats were tested in RDT after acute systemic administration of EGCG, caffeine, or joint EGCG and caffeine. EGCG caused a dose dependent reduction in risk-taking without affecting reward discrimination or task engagement. Caffeine did not impact risk taking, but elevated locomotor activity and reduced task engagement at high doses. Finally, exposure to both EGCG and caffeine had no effect on risk-taking, suggesting that low-dose caffeine is sufficient to mask the risk-aversion caused by EGCG. These data suggest EGCG as a potential therapeutic treatment for psychological disorders that induce compulsive risky decision-making.

Pleasant subjective effects of drugs (e.g., euphoria) have been demonstrated to contribute to their abuse potential. In humans, there is some evidence that acute pain states may decrease the positive subjective effects of opioids; however, no studies have directly tested the impact of a long-lasting pain state. Therefore, the goal of this study was to directly evaluate the discriminative stimulus of mu opioid receptor (MOR) agonist, fentanyl, or the non-opioid drug of abuse, cocaine, in the presence or absence of spared nerve injury (SNI) induced chronic neuropathic pain. Prior to surgery, MOR agonists (fentanyl, morphine, nalbuphine) dose-dependently increased % fentanyl-like responding, as expected; surprisingly, after surgery, we saw small, significant rightward shifts in the fentanyl and morphine dose response curves in both sham and SNI groups suggesting that the observed shifts were not due to chronic pain. In both sham and SNI groups, there was an increase in the generalization of nalbuphine to the fentanyl-discriminative stimulus. There was no change in the discriminative stimulus of cocaine (or amphetamine substitutions) over 4 months of SNI-induced chronic neuropathic pain or sham states, suggesting that the SNI model failed to alter the discriminative stimuli of fentanyl and cocaine. Following induction of chronic neuropathic pain, there was an observed increase in quinpirole-induced generalization to the cocaine discriminative stimulus. In the future, studies should directly examine the abuse potential of low efficacy MOR agonists and dopaminergic agonists in the presence and absence of chronic pain states. Significance StatementSubjective or interoceptive effects of drugs of abuse are known to contribute to the abuse potential. This study demonstrated that long-lasting neuropathic pain failed to alter the discriminative stimulus of mu opioid receptor agonists or cocaine; however, we observed an increase in quinpirole-induced generalization to the cocaine discriminative stimulus, suggesting the abuse potential of direct dopaminergic agonists should be further evaluated in the presence or absence of pain states.

Serotonin (5-HT) 2A receptors (5-HT2AR) modulate corticolimbic circuits regulating fear extinction. Although activation of these receptors has been shown to facilitate fear extinction, the behavioral consequences of 5-HT2AR antagonism during extinction is not well defined. Here, we examined the systemic effects of two 5-HT2A receptor antagonists, the mixed 5-HT2A/2C antagonist MDL 11,939 (Glemanserin) and the selective 5-HT2A antagonist MDL 100,907 (Volinanserin) on fear extinction in adult C57BL/6J mice. Prior to drug administration, mice assigned to future treatment groups acquired comparable conditioned freezing responses during delay fear conditioning. Twenty-four hours later, acute administration of MDL 11,939 (1.0 mg/kg) or MDL 100,907 (0.01 mg/kg) increased freezing to the first conditioned stimulus (CS) presentation on Extinction Day 1, indicating enhanced expression of conditioned fear. However, acquisition of fear extinction differed between the respective cohorts of mice treated with the two 5-HT2AR antagonists. Repeated administration of MDL 11,939 significantly impaired extinction, as evidenced by increased freezing across extinction trials and an increased number of trials required to reach extinction criterion. In contrast, MDL 100,907 has reported affinity for did not significantly alter extinction under either acute or repeated dosing conditions. Because MDL 11,939 has reported affinity for 5-HT2C receptors, we tested potential contributions of 5-HT2C receptor antagonism in a separate cohort of mice using two doses of the selective 5-HT2C antagonist, SB 242084. Neither dose affected conditioned fear expression, extinction learning, or trials required to reach extinction criterion. Together, these findings demonstrate ligand-specific and dose-dependent effects of 5-HT2AR antagonism on fear extinction and suggest that distinct intracellular receptor signaling pathways may differentially regulate extinction-related behavior.

Opioid Use Disorder (OUD) and opioid-related deaths remain a major public health concern in the United States. Both environmental and genetic factors influence risk for OUD. We previously identified Hnrnph1 as a quantitative trait gene underlying the stimulant, rewarding, and reinforcing properties of methamphetamine. Prior work demonstrates that hnRNP H1, the RNA-binding protein encoded by Hnrnph1, post-transcriptionally regulates Oprm1 (mu opioid receptor gene) - the primary molecular target for the therapeutic and addictive properties of opioids. Because genetic variants can exert pleiotropic effects on behaviors induced by multiple drugs of abuse, in the current study, we tested the hypothesis that Hnrnph1 mutants would show reduced behavioral sensitivity to the mu opioid receptor agonist fentanyl. Hnrnph1 mutants showed reduced sensitivity to fentanyl-induced locomotor activity, along with a female-specific reduction in, and a male-specific induction of, locomotor sensitization following three, daily injections (0.2 mg/kg, i.p.). Hnrnph1 mutants also required a higher dose of fentanyl to exhibit opioid reward as measured via conditioned place preference. Male Hnrnph1 mutants showed reduced fentanyl reinforcement. Hnrnph1 mutants also showed reduced sucrose motivation, suggesting a reward deficit. No genotypic differences were observed in baseline thermal nociception, fentanyl-induced antinociception, physical or negative affective signs of opioid dependence, or in sensorimotor gating. In the context of our prior work, these findings suggest that Hnrnph1 dysfunction exerts a selective role in reducing the addiction liability to drugs of abuse (opioids and psychostimulants), which could provide new biological pathways to improve their therapeutic profiles.

Most people sample addictive drugs, but use becomes disordered in only a small minority. Two important factors that influence susceptibility to addiction are individual differences in personality traits and biological sex. The influence of traits on addiction-like behaviour is well characterized in preclinical models of cocaine self-administration, but less is understood in regards to opioids. How biological sex influences trait susceptibility to opioid self-administration is likewise less studied than psychostimulants. Thus, we sought to elucidate how biological sex and several addiction-relevant traits interact with the propensity to self-administer the opioid remifentanil. We first screened female (n=19) and male (n=19) rats for four addiction-relevant traits: impulsivity, novelty place-preference, anxiety-like behaviour, and attribution of incentive value to reward cues. Rats were then trained to self-administer remifentanil in a "conflict model" of drug self-administration. Rats had to endure a mild electric shock to access the response manipulandum that triggered an intravenous infusion of remifentanil. In male rats, high anxiety-like behaviour was positively correlated with the number of drug infusions if the shock level was low or completely absent. In females, sign-tracking was predictive of greater resistance to punishment during drug seeking; an effect that was mediated by anxiety-like behaviour. Females consumed more remifentanil under all conditions, and their drug seeking persisted in the face of significantly greater current than males. These findings demonstrate that the influence of behavioural traits over the propensity to self-administer opioids is dependent upon biological sex.

Chronic psychostimulant use can cause long lasting changes to neural and cognitive function that persist even after long periods of abstinence. As cocaine users transition from drug use to abstinence, a parallel transition from hyperactivity to hypoactivity has been found in orbitofrontal-striatal glucose metabolism, and striatal D2/D3 receptor activity. Targeting these changes pharmacologically, using highly selective dopamine D3 receptor (D3R) antagonists and partial agonists, has shown significant promise in reducing drug-taking, and attenuating relapse in animal models of cocaine and opioid use disorder. However, much less attention has been focused on treating inflexible and potentially maladaptive non-drug behaviors following chronic psychostimulant use. Here we tested the selective D3R antagonist VK4-116 as a treatment for the long-term behavioral inflexibility in abstinent male and female rats with a prior history of chronic cocaine use. Rats were first trained to self-administer cocaine (0.75 mg/kg/reinforcer) or a sucrose liquid (10%, .04 mL/reinforcer) for 2 weeks (FR1 schedule, max 60 reinforcers in 3 hrs/ day), followed by 4 weeks of abstinence. Cognitive and behavioral flexibilities were then assessed using a sensory preconditioning (SPC) learning paradigm. Rats were given an VK4-116 (15 mg/kg, i.p.) or vehicle 30 mins prior to each SPC training session, thus creating four drug-treatment groups: sucrose-vehicle, sucrose-VK4-116, cocaine-vehicle, cocaine-VK4-116. The control groups (sucrose-vehicle, sucrose-VK4-116) demonstrated significant evidence of flexible SPC behavior, whereas cocaine use (cocaine-vehicle) disrupted SPC behavior. Remarkably, the D3R antagonist VK4-116 mitigated this cocaine deficit in the cocaine-VK4-116 group, demonstrating flexible SPC to levels comparable to the control groups. These preclinical findings demonstrate that highly selective dopamine D3R antagonists, particularly VK4-116, show significant promise as a pharmacological treatment for the long-term negative behavioral consequences of cocaine use disorder.

RationaleReward-associated cues can acquire incentive motivational properties and invigorate reward-seeking actions via Pavlovian-to-instrumental transfer (PIT). Glutamatergic neurotransmission mediates the appetitive effects of reward-associated cues. We characterized the expression of PIT and its mediation by metabotropic group II glutamate (mGlu2/3) receptor activity in female and male rats. ObjectivesAcross the sexes, we used PIT procedures to determine i) cue-triggered increases in incentive motivation for water reward (Experiment 1), ii) the respective influences of the mGlu2/3 receptor agonist LY379268 and reward devaluation by satiation on this effect (Experiment 2). MethodsWater-restricted male and female Sprague-Dawley rats learned to lever press for water. Separately, they learned that one of two auditory stimuli predicts free water (CS+ vs CS-). On PIT test days, the CS+ and CS- were presented non-contingently, and we measured effects on lever pressing under extinction (no water). In Experiment 1, we characterized PIT across the sexes. In Experiment 2, we measured PIT after systemic LY379268 administration (0, 0.3 and 1 mg/kg), and water satiation, respectively. ResultsFemale and male rats showed similar PIT, with CS+ but not CS- presentations potentiating water-seeking behaviour. LY379268 (1 mg/kg) attenuated CS+ evoked increases in both water-associated lever pressing and conditioned approach to the water port. Reward devaluation attenuated both water-seeking and CS+ evoked conditioned approach behaviour. ConclusionsThe sexes show similar cue-triggered increases in reward wanting, and water devaluation suppresses both water seeking and cue-triggered anticipation of water reward. Finally, across the sexes, mGlu2/3 receptor activity mediates cue-triggered increases in reward wanting.

RationaleAppetitive Pavlovian cues can drive maladaptive reward seeking via stimulus-reward memories. Disrupting memory reconsolidation offers a potential strategy to reduce their influence, but evidence for {beta}-adrenergic blockade with propranolol is inconsistent across behavioural paradigms, particularly relative to NMDA receptor antagonism. ObjectivesWe tested whether propranolol disrupts reconsolidation of appetitive sucrose memories in a discriminative goal-tracking paradigm, and compared its effects with those of the most commonly used NMDA receptor antagonist, MK-801. MethodsAdult Lister hooded rats underwent discriminative Pavlovian conditioning. Thirty minutes before a brief memory reminder (non-reinforced or reinforced), rats received systemic drug treatment or saline control. In study 1, MK-801 (0.1 mg/kg) was administered to male rats. In study 2, propranolol (10 mg/kg) was administered to equal numbers of male and female rats. Goal-tracking was tested drug-free at 1 and 8 days. ResultsIn study 1, MK-801 impaired subsequent discriminated responding at test. These effects were observed not only when reminder was non-reinforced as in previous successful demonstrations, but also with reinforced reminder. In study 2, Propranolol also impaired subsequent goal-tracking, regardless of reminder type, and the effects were consistent across sexes. ConclusionsPropranolol can disrupt reconsolidation of appetitive goal-tracking memories to a similar extent as MK-801 under conditions that promote memory destabilisation. These findings demonstrate that {beta}-adrenergic blockade can impair appetitive memory reconsolidation in a goal-tracking paradigm, challenging prior null findings and revitalising the potential for propranolol-based interventions in maladaptive reward-seeking behaviours.

Xylazine is a veterinary sedative and widespread adulterant of illicit opioids, where it is commonly combined with the highly potent synthetic {micro} opioid receptor (MOR) agonist fentanyl. Xylazine adulteration of fentanyl is associated with increased risk of lethal overdose and decreased efficacy of reversal by the MOR antagonist naloxone. Here we use whole body plethysmography in mice to show that xylazine produces profound respiratory depression at subanesthetic doses. Xylazine rapidly and dose-dependently suppressed minute ventilation, tidal volume, and respiratory frequency. These effects were dependent on -2 adrenergic receptors and were fully blocked by coadministration of the -2 adrenergic antagonist atipamezole. Atipamezole, administered alone, produced only modest reversal of fentanyl-induced respiratory depression. Xylazine, when combined with a dose of fentanyl with modest respiratory effects, suppressed breathing with greater efficacy than when administered alone. Strikingly, doses of naloxone sufficient to completely reverse fentanyl-induced respiratory depression were ineffective in reversing the respiratory suppression induced by xylazine-adulterated fentanyl. By contrast, combinations of naloxone with atipamezole rapidly and fully reversed the suppression of breathing induced by xylazine-adulterated fentanyl. Our results show that xylazine suppresses breathing via activation of -2 receptors, an effect enhanced by coadministration with the MOR agonist fentanyl. Respiratory suppression inflicted by the mixture of xylazine and fentanyl resisted reversal by naloxone but was fully reversible by subsequent coadministration of both naloxone and atipamezole. These observations have profound implications for the current opioid epidemic.

BackgroundIntravenous self-administration (IVSA) of opioids by rats has been shown frequently to exhibit no sex differences, in many cases a higher intake of females, and only rarely higher rates in males. A diversity of methodological parameters (opioid identity, training doses, rat strain, session duration) makes it difficult to identify consistent contributions to these outcomes. ObjectiveTo determine if Heterogeneous Stock (HS) rats derived from 8 founder strains differ by sex in the IVSA of opioids. MethodsMale and female Heterogeneous Stock (N=7-8 per sex) rats were permitted to self-administer heroin (20 {micro}g/kg/infusion) in 2 hour sessions under a Fixed Ratio 1 schedule of reinforcement. After acquisition, animals completed sessions in which different infusion doses of heroin (0, 15, 30, 60, 120 {micro}g/kg/infusion), oxycodone (0, 30, 60, 150, 300 {micro}g/kg/infusion) and fentanyl (0, 0.625, 1.25, 2.5, 5.0 {micro}g/kg/infusion) were assessed. Next, animals were evaluated on doses of heroin (15, 30, 60, 120 {micro}g/kg/infusion), oxycodone (30, 60, 150, 300 {micro}g/kg/infusion) and fentanyl (0.625, 1.25, 2.5, 5.0 {micro}g/kg/infusion) under a Progressive Ratio schedule. Anti-nociceptive effects of heroin (0.56-2.4 mg/kg, s.c.) were examined with a warm water tail-withdrawal assay. ResultsFemale HS rats consistently self-administered more infusions of opioids, including heroin during acquisition, all three opioids during FR-1 dose substitution and of oxycodone and fentanyl in the PR procedure. Male rats were moderately more sensitive to the anti-nociceptive effects of heroin. ConclusionsFemale rats drawn at random from a genetically diverse population self-administer opioids at higher rates than their male counterparts.

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 g/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, IP) 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 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, SC) 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 timepoints 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 which was not altered by acute naloxone treatment.

After a history of intermittent cocaine intake, rats develop patterns of drug use characteristic of addiction. The dorsal striatum is involved in the increased pursuit of cocaine after intermittent drug self-administration experience. Within the dorsal striatum, chronic cocaine use changes metabotropic glutamate type II receptor (mGlu2/3) density and function. We examined the extent to which activity at these receptors mediates responding for cocaine after intermittent cocaine use. In Experiment 1, male Wistar rats (n = 11) self-administered 0.25 mg/kg/infusion cocaine during 10 daily intermittent access (IntA) sessions (5 min ON/25 min OFF, for 5 h/session). We then examined the effects of intra-dorsal striatum infusions of the mGlu2/3 receptor agonist LY379268 (0, 1, and 3 g/hemisphere) on cocaine self-administration under a progressive ratio schedule of reinforcement. We observed a non-significant tendency for LY379268 to reduce responding for cocaine. In Experiment 2, we used a larger sample of male (n = 11) and female (n = 10) rats. Across 10 IntA sessions, the sexes showed similar levels of cocaine intake. Across the sexes, locomotion significantly increased over sessions, suggesting that rats developed psychomotor sensitization to self-administered cocaine. After 10 IntA sessions, intra-dorsal striatum LY379268 significantly reduced breakpoints achieved for cocaine, active lever presses, and cocaine infusions earned under progressive ratio. LY379268 had no effects on locomotion or inactive lever presses, indicating no motor effects. These results suggest that mGlu2/3 receptor activation in the dorsal striatum suppresses incentive motivation for cocaine, and this holds promise for anti-addiction therapeutics.

Clinical trials suggest that a single dose of psilocybin is an effective treatment for substance use disorders (SUDs). Choice impulsivity is a value-based decision-making bias that predicts drug-intake escalation and is commonly associated with SUDs. The dorsomedial prefrontal cortex (dmPFC) regulates choice impulsivity and is enriched with 5-HT2A receptors that mediate effects of psilocybin. We hypothesized that psilocybin has long-term ([≥]48 hours) effects on choice impulsivity in association with dmPFC inhibitory interneurons with perineuronal nets (PNNs). Male Long Evans rats were trained in a delay discounting task (DDT) where rats chose between delayed large rewards (LR) and immediate small rewards (SR). 48 hours after psilocybin or vehicle injections, DDT was assessed, and rats brains processed for microscopy analysis of extracellular matrix (PNNs) together with inhibitory parvalbumin (PV) interneurons and c-fos as a marker of neuronal activity. Psilocybin acutely increased head-twitch responses. Psilocybin decreased LR choices and increased the latency to LR choices 48 hours after administration. These effects were independent of delay and therefore not consistent with changes in impulsivity. Psilocybin also increased the density of PNN+PV+cFos triple-labeled neurons in the dmPFC. These results suggest that psilocybin decreases reward seeking through the increased activation of dmPFC PV interneurons with PNNs.