Psychopharmacology

Prevailing preclinical models of cocaine use have not resulted in an FDA-approved treatment for cocaine use disorder, potentially due to a focus on cocaine use in isolation, which may not translate well to polysubstance use in clinical populations. Clinically, nicotine has been shown to increase cocaines potency and reinforcing efficacy, but some preclinical studies suggest that non-contingent nicotine exposure is not sufficient to alter cocaine self-administration in rats; therefore, this experiment examined if the addition of nicotine to the cocaine solution would alter self-administration behavior. Male Sprague Dawley rats (N=7) were trained to self-administer cocaine (0.75mg/kg/inf), and tested on a long access, fixed ratio 1 schedule of reinforcement (6 hour sessions, unlimited inf, 5 days), for cocaine alone (0.75mg/kg/inf), followed by cocaine and nicotine (0.75mg/kg/inf cocaine+0.03mg/kg/inf nicotine). Finally, rats responded on a progressive ratio schedule for varied doses of cocaine with and without concurrent nicotine at a consistent dose (1.5, 0.75, 0.375, 0.19mg/kg/inf cocaine{+/-} 0.03mg/kg/inf nicotine). Unexpectedly, under long access conditions, rats self-administering cocaine and nicotine responded less than for cocaine alone, and did not escalate responding. However, under progressive ratio conditions, responding for cocaine and nicotine was greater than responding for cocaine alone across low and moderate cocaine doses, and decreased at high cocaine doses, indicating a leftward shift in the dose response curve. Together, these data highlight the importance of evaluating multiple outcome measures in nicotine + cocaine paradigms, and suggest that concurrent self-administration of cocaine and nicotine results in greater motivated responding than for cocaine alone.

The use of non-drug alternative reinforcers has long been utilized as a component of therapeutic interventions for the management of substance use disorder; however, the conditions under which alternative reinforcers are most effective are not well characterized. This study evaluated the impact of varying the magnitude of an alternative reinforcer on oxycodone self-administration and reinstatement in male and female squirrel monkeys. Subjects (n=4/sex) were trained under concurrent second-order schedules of reinforcement for intravenous oxycodone (0.001-0.1mg/kg/inj) on one lever, and sweetened condensed milk (5, 10, 20, 30% in water) on another. Oxycodone-primed reinstatement was evaluated by administering 0.32mg/kg oxycodone prior to sessions in which saline was available on the drug-paired lever. During oxycodone self-administration sessions, milk availability decreased oxycodone self-administration and preference in a concentration-dependent manner; low milk concentrations were more effective at decreasing oxycodones reinforcing potency in males. During reinstatement tests, milk significantly attenuated oxycodone-primed responding in both males and females; low milk concentrations were more effective at decreasing the priming effects of oxycodone in females. That alternative reinforcers differentially impacted self-administration and reinstatement in a sex-dependent manner suggests that treatment strategies that utilize alternative reinforcers may be more effective in males or females depending on when they are implemented.

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.

Tobacco use disorder is a chronic condition characterized by compulsive nicotine use, withdrawal, and relapse following abstinence. Impulsivity contributes to persistent nicotine use and poor cessation outcomes. This study examined whether nicotinic acetylcholine receptor (nAChR) modulators alter impulsive action in a nicotine self-administration Go/No-Go task in male and female rats. Rats acquired intravenous nicotine self-administration and were then trained in a Go/No-Go procedure in which active lever presses were reinforced during Go periods but not during No-Go periods. We then assessed the effects of varenicline (0.1-3 mg/kg), nicotine (0.1-0.6 mg/kg), and the nAChR antagonist mecamylamine (0.5-2 mg/kg) in the Go/No-Go procedure. Varenicline and nicotine pretreatment reduced active responding during both Go and No-Go periods, whereas mecamylamine selectively reduced responding during No-Go periods. Mecamylamine decreased the percentage of active responses during No-Go trials, indicating reduced bias toward the nicotine-associated lever. In contrast, nicotine and varenicline did not alter response allocation, suggesting that their effects reflected nonspecific reductions in responding rather than changes in impulsive action. No sex differences were observed. Substituting saline for nicotine during self-administration did not alter active responding during Go periods, but rats in the saline group had fewer active responses during No-Go periods than rats in the nicotine group. These results show that chronic nicotine self-administration increases impulsive action and that nAChR antagonism, but not agonism or partial agonism, reduces nicotine-related impulsive action. This work supports the utility of the Go/No-Go self-administration task for investigating nAChR-dependent mechanisms underlying nicotine-induced impulsivity.

Impulsive choice is the suboptimal preference for a smaller-sooner (SS, "impulsive") option over a larger-later (LL, "self-controlled") option. Fixed-interval (FI) training delivers delayed-reinforcement trials to increase LL choices and improve FI timing precision. While there are plenty of studies exploring the neurobiological factors underlying impulsive choice, it is unknown what neurobiological changes account for the FI training effects. The prelimbic cortex (PL) region is implicated in both impulsive choice and timing. To investigate the role of the PL, we used designer receptors exclusively activated by designer drugs (GiDREADDs) to reversibly inhibit the PL during either the FI training phase or the follow-up impulsive choice task in male and female Sprague-Dawley rats. Compared to a control group, the GiDREADDs rats showed reduced LL choices when CNO was administered during the FI training or impulsive choice tasks. GiDREADDS did not alter response rates or latency to choose. Overall, these data demonstrate that inhibition of the PL increases impulsive choice and may block the effect of the FI training to improve self-control.

Smoking addicts have deficits in cognition, in particular deficits in attention, even long after smoking cessation. It is not clear however, whether deficits are a cause or a consequence, or both, of chronic nicotine use. Here we set out a series of experiments in rats to address this question and, more specifically, to assess the long-term effects of exposure to and withdrawal from chronic nicotine self-administration on attentional performance. Animals were trained in a 5-choice serial reaction time task to probe individual attentional performance and, then, were given access to a fixed versus increasing dose of intravenous nicotine for self-administration, a differential dose procedure known to induce two between-session patterns of nicotine intake: a stable versus escalation pattern. Attentional performance was measured daily before, during and also 24-h after chronic access to the differential dose procedure of nicotine self-administration. We found that pre-existing individual variation in attentional performance predicts individual vulnerability to develop escalation of nicotine intake. Moreover, while chronic nicotine self-administration increases attention, withdrawal from nicotine intake escalation induces attentional deficits, a withdrawal effect that is dose-dependently reversed by acute nicotine. Together, these results suggest that pre-existing individual variation in attentional performance predicts individual vulnerability to develop escalation of nicotine intake, and that part of the motivation for using nicotine during escalation might be to alleviate withdrawal-induced attentional deficits.

BackgroundMost substance users are polysubstance users; however, little is known about how the combined use of different drugs affects the course of substance use disorder or effectiveness of treatment. Notably, co-use of cannabis and nicotine is very common, and we previously demonstrated that nicotine enhances the self-administration of a synthetic cannabinoid receptor agonist and the primary psychoactive phytocannabinoid in cannabis, {Delta}-9-tetrahydrocannabinol (THC). MethodsHere we aimed to further investigate the patterns of nicotine and THC self-administration when available in a concurrent choice model, and to determine the effects of nicotinic acetylcholine receptor (nAchR) antagonists on nicotine-enhanced THC self-administration in male and female rats. ResultsDuring concurrent choice, nicotine availability increased THC self-administration in females without affecting THC metabolism, while THC availability decreased nicotine self-administration and preference in females relative to when nicotine and saline were concurrently available. In females, THC self-administration was reduced by the {beta}2/4 subunit-containing nAchR antagonist dihydro-beta-erythroidine (DH{beta}E) in both nicotine and saline concurrent availability groups; while nicotine self-administration was reduced in both sexes by the 7nAchR antagonist methylylcaconitine (MLA), but only in rats that had concurrent access to THC. The nonspecific nAchR antagonist mecamylamine had minimal effects in the concurrent choice model, but it prevented nicotine-induced enhancement of THC self-administration when nicotine was given prior to a single choice THC only self-administration session. ConclusionsThus, behavioral regulation of self-administration is differentially influenced by nAchR subtypes depending on the availability of other substances, which has implications for the efficacy of treatments in the context of polysubstance use. Significance StatementPolysubstance use is extremely common, but very understudied in both clinical and preclinical research. The results presented here highlight that pharmacological modulators of drug reinforcement can differ when multiple drugs are available simultaneously, highlighting the importance of investigating potential treatments for substance use disorders in the context of polysubstance use.

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.

There is now evidence from multiple Phase II clinical trials that psychedelic drugs can exert long-lasting anxiolytic, anti-depressant, and anti-drug abuse (nicotine and ethanol) effects in patients. Despite these benefits, the hallucinogenic actions of these drugs at the serotonin 2A receptor (5-HT2AR) limit their clinical use in diverse settings. Activation of the 5-HT2AR can stimulate both G protein and {beta}-arrestin ({beta}Arr) -mediated signaling. Lisuride is a G protein biased agonist at the 5-HT2AR and, unlike the structurally-related LSD, the drug does not typically produce hallucinations in normal subjects at routine doses. Here, we examined behavioral responses to lisuride, in wild-type (WT), {beta}Arr1-KO, and {beta}Arr2-KO mice. In the open field, lisuride reduced locomotor and rearing activities, but produced a U-shaped function for stereotypies in both {beta}Arr lines of mice. Locomotion was decreased overall in {beta}Arr1-KOs and {beta}Arr2-KOs, relative to WT controls. Incidences of head twitches and retrograde walking to lisuride were low in all genotypes. Grooming was depressed in {beta}Arr1 mice, but was increased then decreased in {beta}Arr2 animals with lisuride. Prepulse inhibition (PPI) was unaffected in {beta}Arr2 mice, whereas 0.5 mg/kg lisuride disrupted PPI in {beta}Arr1 animals. The 5-HT2AR antagonist MDL100907 failed to restore PPI in {beta}Arr1 mice, whereas the dopamine D2/D3 antagonist raclopride normalized PPI in WTs but not in {beta}Arr1-KOs. Using vesicular monoamine transporter 2 mice, lisuride reduced immobility times in tail suspension and promoted a preference for sucrose that lasted up to 2 days. Together, it appears {beta}Arr1 and {beta}Arr2 play minor roles in lisurides actions on many behaviors, while this drug exerts anti-depressant drug-like responses without hallucinogenic-like activities.

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.

BackgroundCannabis may reduce the nonmedical use of prescription opioids. Causality of polydrug use is difficult to establish from epidemiological data, and thus controlled laboratory models can test whether cannabinoid co-use with opioids can modulate opioid intake. MethodsMale and female rats were trained to intravenously self-administer (IVSA) oxycodone (0.15 mg/kg/infusion) during 6 h sessions. Separate groups were injected with the vehicle or with THC (5 mg/kg, i.p.; N=10) 30 minutes before sessions for the first three weeks. Treatments were swapped in the fourth week. One male group was trained in the intracranial self-stimulation (ICSS) procedure and assessed for brain reward thresholds prior to each IVSA session. ResultsTHC treated animals self-administered less oxycodone during acquisition, with a larger differential expressed in the female group. Tolerance to the THC effect developed over the initial weeks, and increasing the dose of THC (10 mg/kg, i.p.) prolonged the suppressing effect on IVSA. While ICSS thresholds increased with sequential IVSA sessions, no differences between THC- and Vehicle-treated groups were observed. Oxycodone IVSA was increased following the first 60 h abstinence interval in THC-treated, but not vehicle-treated, rats. Acute injection of THC, when all animals had been THC abstinent for several weeks, increased breakpoints in a Progressive Ratio procedure. ConclusionThese data support the interpretation that THC enhances the reinforcing efficacy of a given dose of oxycodone and may therefore increase the addiction liability.

Continued drug-taking despite adverse consequences is hypothesized to be an insidious behavioral hallmark of drug addiction. Although most preclinical research has focused on drug self-administration in the presence of positive punishment, another source of potential adverse consequences is behavioral allocation away from negative reinforcers (i.e., escape/avoid electric shock) and towards drug reinforcers. The goals of the present study were to establish a discrete-trial cocaine-vs-negative reinforcer choice procedure in male and female rats and determine sensitivity of choice behavior to environmental and pharmacological manipulations. Rats could make up to nine discrete choices between an intravenous cocaine infusion (0.32 - 1.8 mg/kg/inf) under a fixed-ratio (FR) 3 schedule and a negative reinforcer (escape or avoidance of electric shock, 0.1 - 0.7 mA) under an FR1 schedule. The negative reinforcer was consistently chosen over all cocaine doses. Lowering shock magnitude decreased negative reinforcer trials, increased omitted trials, and failed to promote behavioral reallocation towards cocaine. Increasing the negative reinforcement response requirement between sessions only increased omitted trials. Introduction of 12-hr extended access cocaine self-administration sessions across two weeks resulted in high daily cocaine intakes but failed to significantly increase cocaine choice. Acute diazepam pretreatment also did not impact choice behavior up to doses that produced behavioral depression. Overall, the lack of behavioral allocation between cocaine infusions and a negative reinforcer suggests these two reinforcers may be economic independents. Additionally, the failure of extended cocaine access to increase cocaine choice highlights the importance of alternative reinforcers and environmental context in preclinical models of drug addiction.

Stress is a significant contributor to the development and progression of substance use disorders (SUDs) and is problematic as it is unavoidable in daily life. Therefore, it is important to understand the neurobiological mechanisms that underlie the influence of stress on drug use. We have previously developed a model to examine the contribution of stress to drug-related behavior by administering a stressor, electric footshock stress, daily at the time of cocaine self-administration in rats resulting in an escalation of cocaine intake. This stress-induced escalation of cocaine intake involves neurobiological mediators of stress and reward such as cannabinoid signaling. However, all of this work has been conducted in male rats. Here we test the hypothesis that repeated daily stress can produce an escalation of cocaine in both male and female rats. We further hypothesize that cannabinoid receptor 1 (CB1R) signaling is recruited by repeated stress to influence cocaine intake in both male and female rats. Male and female Sprague-Dawley rats self-administered cocaine (0.5 mg/kg/inf, i.v.) during a modified short-access paradigm wherein the 2-hr access was separated into 4-30 min self-administration blocks separated by 4-5 min drug free period. Footshock stress produced a significant escalation of cocaine intake similarly in both male and female rats. Female stress-escalated rats did display greater time-out non-reinforced responding and greater "front-loading" behavior. In males, systemic administration of a CB1R inverse agonist/antagonist Rimonabant only attenuated cocaine intake in rats with a history of combined repeated stress and cocaine self-administration. However, in females, Rimonabant attenuated cocaine intake in the no stress control group but only at the highest dose of Rimonabant (3 mg/kg, i.p.) suggesting that females show a greater sensitivity to CB1R antagonism. However, female rats with a history of stress showed even greater sensitivity to CB1R antagonism as both doses of Rimonabant (1, 3 mg/kg) attenuated cocaine intake in stress-escalated rats similar to males. Altogether these data demonstrate that stress can produce significant changes in cocaine self-administration and suggests that repeated stress at the time of cocaine self-administration recruits CB1Rs to regulate cocaine-taking behavior across sexes.

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.

Background and PurposeTo remain abstinent represents one of the major challenges for the treatment of cocaine use disorder. Cocaine seeking elicited by drug-associated cues progressively intensifies during abstinence in a process termed incubation of craving, representing an aggravating factor for relapse. Cannabidiol is a phytocannabinoid that exerts protecting effects upon cocaine-seeking behaviour, although its effects on cocaine-craving incubation have never been elucidated. Experimental ApproachWe developed a mouse model of behavioural economic analysis of demand curves and incubation of cue-induced cocaine craving. Changes in the protein expression of AMPAR subunits and ERK1/2 phosphorylation were analysed. We also assessed the effects of cannabidiol (20 mg{middle dot}kg-1) administered either during acquisition of cocaine self-administration or abstinence. Key ResultsMice efficiently performed the demand task and incubation of cocaine craving. Besides, changes in GluA1 and GluA2 protein levels were found along the abstinence in prelimbic cortex, ventral striatum and amygdala, as well as a decrease in ERK1/2 phosphorylation in ventral striatum. Cannabidiol reduced ongoing cocaine intake when administered during the acquisition phase of the self-administration, but failed to alter the subsequent demand task performance and incubation of cocaine craving. No effects were found when cannabidiol was administered during the abstinence period. Conclusion and ImplicationsWe provide here a novel model of behavioural economic analysis of demand curves and cue-induced incubation of cocaine-seeking behaviour for mice. Moreover, we show that cannabidiol exerts differential effects on the current model depending on the self-administration phase in which it was administered. What is already knownO_LIBehavioural economics and incubation of cocaine craving are well-stablished paradigms to evaluate cocaine seeking in rats. C_LIO_LICBD reduces cocaine-seeking and cocaine-taking behaviours. C_LI What this study addsO_LIA mouse model of behavioural economic analysis of demand curves and incubation of cue-induced cocaine craving. C_LIO_LICBD reduces cocaine self-administration and has no effect over demand task and cocaine-craving incubation. C_LI Clinical significanceO_LIA new behavioural model for studying cocaine addiction in mice. C_LIO_LICBD exerts differential effects depending on when it was administered in the addictive process. C_LI TO_SCPLOWABLESC_SCPLOWO_SCPCAP C_SCPCAPO_SCPLOWOFC_SCPLOW LO_SCPLOWINKSC_SCPLOW O_TBL View this table: org.highwire.dtl.DTLVardef@654e3aorg.highwire.dtl.DTLVardef@f2d37aorg.highwire.dtl.DTLVardef@1a0396org.highwire.dtl.DTLVardef@1e19d1dorg.highwire.dtl.DTLVardef@1121b7b_HPS_FORMAT_FIGEXP M_TBL C_TBL

RationaleMescaline is a classical psychedelic compound with a phenylethylamine structure that primarily acts on serotonin 5-HT2A/C receptors, but also binds to 5-HT1A and 5-HT2B receptors. Despite being the first psychedelic ever isolated and synthesized, the precise role of different serotonin receptor subtypes in its behavioral pharmacology is not fully understood. ObjectivesIn this study, we aimed to investigate how selective antagonists of 5-HT2A, 5-HT2B, 5-HT2C, and 5-HT1A receptors affect the behavioral changes induced by subcutaneous administration of mescaline (at doses of 10, 20, and 100 mg/kg) in rats. MethodsWe used adult male Wistar rats in all our experiments. We evaluated locomotor activity using the open field test, and assessed sensorimotor gating deficits by measuring prepulse inhibition (PPI) of acoustic startle reaction (ASR). ResultsWhile the highest dose of mescaline induced hyperlocomotion, which almost all the other antagonists reversed, the PPI deficits were selectively normalized by the 5-HT2A antagonist. The 5-HT2C antagonist partially reversed the small decrease in locomotor activity induced by lower doses of mescaline. ConclusionOur findings suggest that mescaline-induced changes in behavior are primarily mediated by the 5-HT2A receptor subtype, with less pronounced contributions from the 5-HT2C receptor. The other antagonists had limited effects.

BackgroundIn male rats, physical contexts that are associated with alcohol can invigorate responding to a discrete, alcohol-predictive conditioned stimulus (CS), and amplify priming-induced reinstatement. Here, we examined these effects as a function of biological sex. MethodsMale and female Long-Evans rats were acclimated to drinking ethanol (EtOH, 15% v/v) in their home cages. Next, they were trained to associate an auditory CS (10 s; white noise; 15 trials per session) with EtOH delivery (0.2 ml per CS; 3.0 ml per session) into a fluid port for oral intake. Training occurred in a distinctive context containing specific visual, olfactory, and tactile stimuli. During alternating sessions rats were exposed to a second context where they did not receive EtOH. At test, CS presentations occurred in both contexts without EtOH delivery. Rats then underwent extinction using repeated unreinforced presentations of the CS in both contexts. An alcohol-primed reinstatement test was then conducted, in which 0.2 ml of EtOH was presented both at the start of the session and during the first CS presentation, after which no EtOH was delivered for the remainder of the session. ResultsAt both test and reinstatement, male rats made significantly more CS port-entries in the context associated with alcohol delivery than in the context in which alcohol was never experienced. Unlike males, female rats made a similar number of CS port-entries at test in both the alcohol context and the neutral context. The reinstatement observed in female rats was not affected by context. ConclusionsThese findings identify novel sex differences in the capacity of an alcohol-associated context to modulate responding to a discrete, alcohol-predictive cue.

Nicotine addiction is characterized by escalated drug use, craving and a high relapse rate after abstinence. However, because of difficulties in demonstrating escalation of nicotine use in rats, its relation to other addiction-related phenomena is currently unknown. Recently, we showed that, compared to rats with a fixed moderate dose of nicotine, rats with access to increasing high doses of nicotine for self-administration progressively escalated their nicotine intake. Whether these animals with escalating patterns of nicotine self-administration also develop other behavioral signs of addiction remains to be investigated. Here we report that after escalation of nicotine intake, animals have a greater difficulty of abstaining from seeking the drug, a greater responsiveness to nicotine-induced craving-like behavior, and an increased vulnerability to re-escalate nicotine intake post-extinction than rats with stable patterns of nicotine intake. No substantial sex differences in the development of these different addiction-related phenomena were observed. Finally, after escalation, nicotine intake also became primarily dependent on nicotine reinforcement and less so on the nicotine-paired cue. Overall, this study shows that most of the post-escalation behavioral changes previously seen with other drugs of abuse are generalizable to nicotine intake escalation.

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.

Background and PurposeThe use of "Bath Salts" drug preparations has been associated with high rates of toxicity and death. Preparations often contain mixtures of drugs including multiple synthetic cathinones or synthetic cathinones and caffeine; however, little is known about whether interactions among "Bath Salts" constituents contribute to the adverse effects often reported in users. Experimental ApproachThis study used adult male Sprague-Dawley rats to characterize the cardiovascular effects, locomotor effects, and pharmacokinetics of methylone, MDPV, and caffeine, administered alone and as binary mixtures. Dose-addition analyses were used to determine the effect levels predicted for a strictly additive interaction for each dose pair. Key ResultsMethylone, MDPV, and caffeine increased heart rate and locomotion, with methylone producing the largest increase in heart rate, MDPV producing the largest increase in locomotor activity, and caffeine being the least effective in stimulating heart rate and locomotor activity. MDPV and caffeine increased mean arterial pressure, with caffeine being more effective than MDPV. The nature of the interactions between methylone and MDPV tended toward sub-additivity for all endpoints, whereas interactions between MDPV or methylone and caffeine tended to be additive or sub-additive for cardiovascular endpoints, and additive or supra-additive for increases in locomotion. No pharmacokinetic interactions were observed between individual constituents, but methylone displayed non-linear pharmacokinetics at the largest dose evaluated. Conclusion and ImplicationsThese findings demonstrate that the composition of "Bath Salts" preparations can impact both cardiovascular and locomotor effects and suggest that such interactions among constituent drugs could contribute to the "Bath Salts" toxidrome reported by human users. What is already known"Bath Salts" preparations are associated with a sympathomimetic toxidrome in human users. What this study addsCharacterization of both pharmacokinetic and pharmacodynamic interactions between common "Bath Salts" constituents with regard to cardiovascular and locomotor effects. Clinical SignificanceThe vast majority of drug overdose deaths involve more than one substance. Though these studies focused on combinations of stimulant drugs, they provide direct evidence that the toxidrome resulting from multi-drug overdoses can be significantly different than would be expected for a single drug.