Estradiol Enhances the Development of Addiction-Like Features in a Female Rat Model of Opioid Use Disorder.

INTRODUCTION: Women are more vulnerable than men in many aspects of opioid use disorder (OUD); a major theory of sex differences in substance use disorders is that these differences are due to ovarian hormones with estradiol enhancing vulnerability in females. However, most of this evidence is for psychostimulants and alcohol; evidence with opioids is sparse. Thus, the goal of this study was to determine the impact of estradiol on vulnerability in females in a rat model of OUD. METHOD: Following self-administration training, ovariectomized (OVX) females with (E) or without (V) estradiol replacement were given extended (24 h/day), intermittent access (2, 5-min trials/h) to fentanyl for 10 days. Then, the development of three key features of OUD were assessed, including physical dependence, defined by the magnitude and time course of weight loss during withdrawal; an enhanced motivation for fentanyl, assessed using a progressive-ratio schedule; and relapse vulnerability, assessed using an extinction/cue-induced reinstatement procedure. These later two characteristics were examined following 14 days of withdrawal when the phenotypes are known to be highly expressed. RESULTS: OVX+E females self-administered markedly higher levels of fentanyl under extended, intermittent-access conditions and showed a longer time course of physical dependence, a greater increase in motivation for fentanyl, and an enhanced sensitivity to the reinstating effects of fentanyl-associated cues compared to OVX+V rats. Severe health complications were also observed in OVX+E, but not OVX+V females, during withdrawal. CONCLUSION: These results indicate that, as with findings with psychostimulants and alcohol, estradiol enhances vulnerability in females to developing opioid addiction-like features and serious opioid-related health complications.

Intermittent access training produces greater motivation for a non-drug reinforcer than long access training.

It has recently been proposed that the intermittent access (IntA) drug self-administration procedure better produces behavioral changes relevant to addiction than the long access (LgA) procedure. In this version of the IntA procedure, the drug is made available for a 5-min period during each half hour of a 6-h session. In contrast, on the LgA procedure, the drug is available continuously for 6 h. Previous studies have found that IntA drug self-administration produces greater drug motivation, measured by increased progressive ratio breakpoints, than LgA self-administration. It has been hypothesized that this effect is due to the rapid, "spiking" brain levels of the drug, and consequent neuroadaptations, experienced by rats during IntA sessions. However, no study has compared the effects of IntA versus LgA training on reinforcer motivation when using a non-drug reinforcer. The present study compared motivation for a saccharin reinforcer after IntA or LgA training. In Experiment 1, separate groups of rats lever-pressed for saccharin on the IntA or LgA procedures. In Experiment 2, a within-subjects design was used where rats pressed one lever on the IntA procedure and another lever on the LgA procedure for saccharin. In both experiments, IntA training produced greater breakpoints than LgA training. As no drug was used here, spiking drug levels could not have been responsible for the increased saccharin motivation observed after IntA training. Instead, it is proposed that differences in stimulus-reinforcer associations learned during IntA versus LgA training may be responsible for the effect. Future research is needed to determine the extent to which such learning factors may contribute to the increased motivation observed after IntA training with drug reinforcers.

Intermittent sucrose solution intake and its schedule of access modulate energy intake and weight gain in response to chronic variable stress in mice.

There is a strong relationship between stress and the intake of calorically-dense palatable food. Additionally, intake of sodas is an important contributory factor to obesity, and is often associated with palatable food consumption. We studied the effects of 2-h intermittent access to sucrose-sweetened water (SSW, 12.3%, soda-like) and its schedule of administration on the response to chronic variable stress in mice fed a high-fat, high-sugar diet. C57BL/6 mice (n = 64) had access to water or to both water and 2-h SSW during 5 weeks, in addition to their diet. After the first two weeks, half of the animals from each group were stressed daily using a chronic variable stress (CVS) paradigm, while the other half were kept undisturbed. During the CVS exposure period, 2-h SSW access was either scheduled randomly, right before the stressors or right after the stressors. The effects of SSW and its schedule of administration on dietary intake, stress hormones and adiposity were analyzed. Results showed a larger consumption of SSW and higher bodyweight gain in mice receiving SSW after the stressor. In addition, SSW consumption was shown to affect appetite regulation by reducing CCK sensitivity. The present study suggests that SSW leads to overconsumption and weight gain only if provided after exposure to stress. These findings may implicate a relation between exposure to stress, binge-drinking behaviors of sugar sweetened beverages that ensues, and weight gain in humans consuming a western diet.

Effects of pituitary adenylate cyclase-activating polypeptide isoforms in nucleus accumbens subregions on ethanol drinking.

While limited research has implicated the neuropeptide, pituitary adenylate cyclase-activating polypeptide (PACAP), in problematic alcohol use, the brain regions and isoforms involved in this effect remain to be determined. One region that has been found both to exhibit PACAP binding and, separately, to be involved in ethanol drinking is the nucleus accumbens (NAc). Thus, this study sought to characterize the effect of the PACAP isoforms in the NAc on ethanol drinking under the intermittent-access two-bottle-choice paradigm, in male and female Long-Evans rats. With microinjection into the medial NAc shell, PACAP-27 but not PACAP-38 was found to dose-dependently reduce binge-like ethanol drinking. In contrast, the PACAP receptor antagonist, PACAP (6-27), but not PACAP (6-38), enhanced ethanol drinking. This effect of PACAP was substance specific, as neither isoform in the NAc shell affected binge-like sucrose drinking. It was also anatomically specific, as PACAP-38 rather than PACAP-27 suppressed ethanol drinking when injected into the NAc core, and PACAP-27 instead enhanced drinking when injected into the caudal third of the medial NAc shell. Finally, while PACAP-38 in the NAc shell affected stress-related exploratory behavior, reducing time spent in the light chamber of a light-dark box, PACAP-27 did not significantly affect behavior in a light-dark box or open field. Together, these results, showing that PACAP-27 in the NAc shell attenuates binge-like ethanol drinking without affecting select stress-related behaviors, suggest that compounds related to this PACAP isoform should be investigated as potential novel therapeutics for the treatment of alcohol use disorder.

Sex differences in cocaine self-administration behaviour under long access versus intermittent access conditions.

Studies in humans suggest that women progress more rapidly from initial cocaine use to addiction. Similarly, female rats can show more incentive motivation for cocaine than male rats do. Most preclinical studies on this issue have used self-administration procedures that provide continuous cocaine access during each session ("long-access" or LgA and "short-access"). However, intermittent access (IntA) cocaine self-administration better models the intermittency of human cocaine use. Here, we compared cocaine use in female and male rats that received ten, daily 6-hour LgA or IntA sessions. Cocaine intake was greatest under LgA, and female LgA rats escalated their intake. Only IntA rats (both sexes) developed locomotor sensitization to self-administered cocaine, and sensitization was greatest in females. Five and 25 days after the last self-administration session, we quantified responding for cocaine (0.083-0.75 mg/kg/infusion) under a progressive ratio (PR) schedule, a measure of motivation for drug. Across conditions, females earned more cocaine infusions than males under the PR schedule. Across sexes, IntA rats earned more infusions than LgA rats, even though IntA rats had previously taken much less cocaine. Cumulative cocaine intake significantly predicted responding for cocaine under the PR schedule in male LgA rats only. In IntA rats, the extent of locomotor sensitization significantly predicted responding under the PR schedule. Thus, LgA might be appropriate to study sex differences in cocaine intake, whereas IntA might be best suited to study sex differences in sensitization-related neuroadaptations involved in cocaine addiction. This has implications for modelling distinct features of cocaine addiction in preclinical studies.

Incentive and dopamine sensitization produced by intermittent but not long access cocaine self-administration.

The temporal pattern of drug use (pharmacokinetics) has a profound effect on the ability of self-administered cocaine to produce addiction-like behavior in rodents, and to change the brain. To further address this issue, we compared the effects of long access (LgA) cocaine self-administration, which is widely used to model the transition to addiction, with intermittent access (IntA), which is thought to better reflect the pattern of drug use in humans, on the ability of a single, self-administered injection of cocaine to increase dopamine (DA) overflow in the core of the nucleus accumbens (using in vivo microdialysis), and to produce addiction-like behavior. IntA experience was more effective than LgA in producing addiction-like behavior-a drug experience-dependent increase in motivation for cocaine assessed using behavioral economic procedures, and cue-induced reinstatement-despite much less total drug consumption. There were no group differences in basal levels of DA in dialysate [DA], but a single self-administered IV injection of cocaine increased [DA] in the core of the nucleus accumbens to a greater extent in rats with prior IntA experience than those with LgA or limited access experience, and the latter two groups did not differ. Furthermore, high motivation for cocaine was associated with a high [DA] response. Thus, IntA, but not LgA, produced both incentive and DA sensitization. This is consistent with the notion that a hyper-responsive dopaminergic system may contribute to the transition from casual patterns of drug use to the problematic patterns that define addiction.

Demand elasticity predicts addiction endophenotypes and the therapeutic efficacy of an orexin/hypocretin-1 receptor antagonist in rats.

Behavioral economics is a powerful, translational approach for measuring drug demand in both humans and animals. Here, we asked if demand for cocaine in rats with limited drug experience could be used to identify individuals most at risk of expressing an addiction phenotype following either long- or intermittent access self-administration schedules, both of which model the transition to uncontrolled drug-seeking. Because the orexin-1 receptor antagonist SB-334867 (SB) is particularly effective at reducing drug-seeking in highly motivated individuals, we also asked whether demand measured after prolonged drug experience could predict SB efficacy. Demand elasticity (α) measured immediately following acquisition of cocaine self-administration ('baseline α') was positively correlated with α assessed after 2w of long- or intermittent access. Baseline α also predicted the magnitude of compulsive responding for cocaine, drug-seeking in initial abstinence and cued reinstatement following long-, intermittent- or standard short access. When demand was measured after these differential access conditions, α predicted the same addiction endophenotypes predicted by baseline α, as well as primed reinstatement and the emergence of negative emotional mood behavior following abstinence. α also predicted the efficacy of SB, such that high demand rats showed greater reductions in motivation for cocaine following SB compared to low demand rats. Together, these findings indicate that α might serve as a behavioral biomarker to predict individuals most likely to progress from controlled to uncontrolled drug use, and to identify individuals most likely to benefit from orexin-based therapies for the treatment of addiction.

Neurotensin in the posterior thalamic paraventricular nucleus: inhibitor of pharmacologically relevant ethanol drinking.

Individuals prone to ethanol overconsumption may have preexisting neurochemical disturbances that contribute to their vulnerability. This study examined the paraventricular nucleus of the thalamus (PVT), a limbic structure recently shown to participate in ethanol intake. To identify individuals prone to ethanol overconsumption, we tested Long-Evans rats in behavioral paradigms and found high levels of vertical time (rearing behavior) in a novel activity chamber to be a consistent predictor of subsequent excessive 20 percent ethanol drinking under the intermittent access model. Examining neurochemicals in the PVT, we found before ethanol exposure that prone rats with high rearing, compared with non-prone rats, had significantly lower levels of neurotensin (NTS) mRNA and peptide in the posterior (pPVT) but not anterior (aPVT) subregion of the PVT. Our additional finding that ethanol intake has no significant impact on either rearing or NTS levels indicates that these measures, which are different in prone rats before ethanol consumption, remain stable after ethanol consumption. The possibility that NTS directly controls ethanol drinking is supported by our finding that NTS administration specifically suppresses ethanol drinking when injected into the pPVT but not aPVT, with this effect occurring exclusively in higher drinkers that presumably have lower endogenous levels of NTS. Further, an NTS antagonist in the pPVT augments intake in lower drinkers with presumably more endogenous NTS, while NTS in the pPVT inhibits novelty-induced rearing that predicts excessive drinking. Together, these results provide strong evidence that low endogenous levels of NTS in the pPVT contribute to an increased propensity toward excessive ethanol drinking.

Revisiting long-access versus short-access cocaine self-administration in rats: intermittent intake promotes addiction symptoms independent of session length.

In rats, continuous cocaine access during long self-administration sessions (6 versus 1-2 hours) promotes the development of behavioral symptoms of addiction. This has led to the assumption that taking large amounts of drug during extended daily bouts is necessary to develop an addiction phenotype. Recent work shows that within-session intermittent access (IntA) to cocaine produces much less drug intake than continuous-access procedures (i.e. long-access sessions) but evokes addiction symptoms more effectively. IntA-sessions are also long, typically lasting 6 hours. It is not known whether IntA-sessions must be extended to promote addiction-relevant changes in drug use over time. Here, we determined the influence of IntA-session length on patterns of cocaine use relevant to addiction. Two groups of male Wistar rats self-administered cocaine (0.25 mg/kg/injection, injected over 5 seconds) during 18 daily IntA-sessions. One group had long 6-hour sessions (Long-IntA), the other group had shorter, 2-hour sessions (Short-IntA). Only Long-IntA rats escalated their cocaine intake over sessions, but both groups developed a burst-like pattern of drug use over time and similar levels of psychomotor sensitization. The two groups also showed robust and similar levels of both responding for cocaine under a progressive ratio schedule of reinforcement and cocaine-induced reinstatement of extinguished drug-seeking behavior. In summary, long IntA-sessions lead to greater cocaine intake than shorter IntA-sessions, but the two conditions are equally effective in evoking the patterns of drug-taking and drug-seeking that define addiction. This suggests that chronic intermittent cocaine use, even during short daily bouts, is sufficient to promote addiction symptoms.

Pituitary Adenylate Cyclase-Activating Polypeptide-27 (PACAP-27) in the Thalamic Paraventricular Nucleus Is Stimulated by Ethanol Drinking.

BACKGROUND: The paraventricular nucleus of the thalamus (PVT) is a limbic brain structure that affects ethanol (EtOH) drinking, but the neurochemicals transcribed in this nucleus that may participate in this behavior have yet to be fully characterized. The neuropeptide, pituitary adenylate cyclase-activating polypeptide (PACAP), is known to be transcribed in other limbic areas and to be involved in many of the same behaviors as the PVT itself, possibly including EtOH drinking. It exists in 2 isoforms, PACAP-38 and PACAP-27, with the former expressed at higher levels in most brain regions. The purpose of this study was to characterize PACAP in the PVT and to assess its response to EtOH drinking. METHODS: First, EtOH-naïve, Sprague Dawley rats were examined using quantitative real-time polymerase chain reaction (qPCR) and immunohistochemistry, to characterize PACAP mRNA and peptide throughout the rostrocaudal axis of the PVT. Next, EtOH-naïve, vGLUT2-GFP transgenic mice were examined using immunohistochemistry, to identify the neurochemical phenotype of the PACAPergic cells in the PVT. Finally, Long Evans rats were trained to drink 20% EtOH under the intermittent-access paradigm and then examined with PCR and immunohistochemistry, to determine the effects of EtOH on endogenous PACAP in the PVT. RESULTS: Gene expression of PACAP was detected across the entire PVT, denser in the posterior than the anterior portion of this nucleus. The protein isoform, PACAP-27, was present in a high percentage of cell bodies in the PVT, again particularly in the posterior portion, while PACAP-38 was instead dense in fibers. All PACAP-27+ cells colabeled with glutamate, which itself was identified in the majority of PVT cells. EtOH drinking led to an increase in PACAP gene expression and in levels of PACAP-27 in individual cells of the PVT. CONCLUSIONS: This study characterizes the PVT neuropeptide, PACAP, and its understudied protein isoform, PACAP-27, and demonstrates that it is involved in pharmacologically relevant EtOH drinking. This indicates that PACAP-27 should be further investigated for its possible role in EtOH drinking.

Anterior thalamic paraventricular nucleus is involved in intermittent access ethanol drinking: role of orexin receptor 2.

The paraventricular nucleus of the thalamus (PVT) has been shown to participate in hedonic feeding and is thought to influence drug seeking. This understudied nucleus contains anterior (aPVT) and posterior (pPVT) subregions, which receive dense projections from hypothalamic orexin/hypocretin (OX) but exhibit anatomical and functional differences. This study sought to characterize in Long-Evans rats the involvement of these PVT subregions and their OX receptor activity in consumption of the drug, ethanol. Compared with those maintained on water and chow only (water group), rats trained to drink pharmacologically relevant levels of ethanol (ethanol group) showed increased neuronal activation in the PVT, specifically the aPVT but not pPVT, as indicated by c-Fos immunoreactivity. Similar results were obtained in rats administered ethanol via oral gavage, indicating that this site-specific effect was due to ethanol exposure. In support of the involvement of OX, the ethanol group also showed increased mRNA levels of this neuropeptide in the hypothalamus and of OX 2 receptor (OX2R) but not OX 1 receptor (OX1R), again in the aPVT but not pPVT. Similarly, ethanol gavage increased double labeling of c-Fos with OX2R but not OX1R, specifically in the aPVT. Evidence directly supporting a role for aPVT OX2R in ethanol consumption was provided by results with local injections, showing ethanol intake to be enhanced by OX-A or OX-B in the aPVT but not pPVT and reduced by a local antagonist of OX2R but not OX1R. These results focus attention on the aPVT and specifically its OX2R in mediating a positive feedback relationship with ethanol intake.

Intermittent access ethanol consumption dysregulates CRF function in the hypothalamus and is attenuated by the CRF-R1 antagonist, CP-376395.

Corticotrophin-releasing factor (CRF) is a mediator of stress responses and a key modulator of ethanol-mediated behaviors. We report here that the CRF receptor 1 (CRF-R1) antagonist, CP-376395 reduces 20% ethanol consumption in animals trained to consume ethanol on an intermittent, but not a continuous, schedule. Furthermore, using [(35) S]GTPγS binding assays, we demonstrate that CRF-mediated G-protein signaling in the hypothalamus of the intermittent drinkers is decreased when compared to controls suggesting that the effects of CP-376395 are mediated by extrahypothalamic mechanisms. The present study provides further support for the use of CRF-R1 antagonists for the treatment of alcohol use disorders and suggests that ethanol consumption dysregulates CRF function in the hypothalamus.

A test of the role of stimulus-response and stimulus-outcome associations in the effects of intermittent-access training.

Increased reinforcer motivation in rats has been repeatedly demonstrated following intermittent-access (IntA) training, where the reinforcer is only available for brief periods during a session, compared to continuous-access (ContA) training where the reinforcer is available throughout the session. The present study investigated whether different associations learned during training on the two procedures contributes to the effect. Two experiments tested the importance of the stimulus-response (S-R) and stimulus-outcome (S-O) associations between the IntA availability cues and the training response and reinforcer, respectively. In Exp. 1, separate groups of rats were trained to lever press for saccharin on the IntA or ContA procedures. Increased motivation for saccharin was observed in the IntA group on a later progressive ratio test where nosepoking was the operant (but not when lever pressing was the operant). The outcome of the nosepoke test suggests that a potential S-R association formed during IntA training was not critical for the effect. In Exp. 2, increased saccharin motivation (on nosepoke tests) after IntA training (with lever pressing) was observed regardless of the presence or absence of IntA availability cues, indicating that the S-O association formed during training is not critical for the effect either. Overall, these results suggest that the elemental associations learned on IntA procedures may not be what drives increased motivation observed after IntA training.

Standard rodent diets differentially impact alcohol consumption and preference and gut microbiome diversity.

Alcohol Use Disorder (AUD) is a complex and widespread disease with limited pharmacotherapies. Preclinical animal models of AUD use a variety of voluntary alcohol consumption procedures to recapitulate different phases of AUD including binge alcohol consumption and dependence. However, voluntary alcohol consumption in mice is widely variable rendering it difficult to reproduce results across labs. Accumulating evidence indicates that different brands of commercially available rodent chow can profoundly influence alcohol intake. In this study, we investigated the effects of three commercially available and widely used rodent diet formulations on alcohol consumption and preference in C57BL/6J mice using the 24h intermittent access procedure. The three brands of chow tested were LabDiet 5001 (LD 5001), LabDiet 5053 (LD 5053), and Teklad 2019S (TL2019S) from two companies (Research Diets and Envigo respectively). Mice fed LD5001 displayed the highest levels of alcohol consumption and preference followed by LD5053 and TL2019S. We also found that alcohol consumption and preference could be rapidly switched by changing the diet 48h prior to alcohol administration. Sucrose, saccharin, and quinine preference were not altered suggesting that the diets did not alter taste perception. We also found that mice fed LD5001 displayed increased quinine-resistant alcohol intake compared to mice fed TL2019S, suggesting that diets could influence the development of "compulsive" like alcohol consumption. We profiled the gut microbiome of water and alcohol drinking mice that were maintained on different diets and found significant differences in bacterial alpha and beta diversity, which could impact gut-brain axis signaling and alcohol consumption.

Effects of access condition on substance use disorder-like phenotypes in male and female rats self-administering MDPV or cocaine.

Substance use disorder (SUD) is a heterogeneous disorder, where severity, symptoms, and patterns of use vary across individuals. Yet, when rats self-administer cocaine under short-access conditions, their behavior tends to be well-regulated, though individual differences can emerge with long- or intermittent-access. In contrast, significant individual differences emerge when rats self-administer 3,4-methylenedioxypyrovalerone (MDPV), even under short-access conditions, wherein ~30 % of rats exhibit high levels of drug-taking. This study assessed SUD-like phenotypes of male and female rats self-administering MDPV or cocaine by comparing level of drug intake, responding during periods of signaled drug unavailability, and sensitivity to footshock punishment to determine whether: (1) under short-access conditions, rats that self-administer MDPV will exhibit a more robust SUD-like phenotype than rats that self-administer cocaine; (2) female rats will have a more severe phenotype than male rats; and (3) compared to short-access, long- and intermittent-access to MDPV or cocaine self-administration will result in a more robust SUD-like phenotype. Compared to cocaine, rats that self-administered MDPV exhibited a more severe phenotype, even under short-access conditions. Long- and intermittent-access to cocaine and MDPV temporarily altered drug-taking patterns but did not systematically change SUD-like phenotypes. Behavioral and quantitative autoradiography studies suggest phenotypic differences are not due to expression of dopamine transporter, dopamine D2 or D3 receptors, or 5-HT1B, 5-HT2A, or 5-HT2C receptors. This study suggests individuals who use synthetic cathinones may be at greater risk for developing a SUD, and short-access MDPV self-administration may provide a useful method to study the transition to disordered substance use in humans.

Effects of voluntary chronic intermittent access to ethanol on the behavioral performance in adult C57BL/6 J mice.

BACKGROUND: Chronic alcohol drinking increases the risk of alcohol use disorders, causing various neurological disorders. However, the impact of different ethanol levels on a spectrum of behaviors during chronic drinking remains unclear. In this study, we established an intermittent access to ethanol in a two-bottle choice (IA2BC) procedure to explore the dose-dependent effects of ethanol on the behavioral performance of C57BL/6 J mice. METHODS: Adult male C57BL/6 J mice were provided voluntary access to different ethanol concentrations (0 %, 5 %, 10 %, and 20 % ethanol) under a 12-week IA2BC paradigm. A battery of behavioral tests was administered to assess alterations in pain threshold, anxiety-like behaviors, locomotor activity, motor coordination, and cognition. Ethanol consumption and preference were monitored during each session. Moreover, the liver, heart, and lung tissues were examined using pathological microscopy. RESULTS: The average (standard deviation) ethanol consumption of mice under the IA2BC paradigm increased dose-dependently to 5.1 (0.2), 8.7 (0.7), and 15.9 (0.8) g/kg/24 h with 5 %, 10 %, and 20 % ethanol, respectively. However, there is no significant difference in ethanol preference among all the ethanol groups. Chronic ethanol drinking caused hyperalgesia, cognitive impairment, and motor incoordination, but caused no changes in body temperature, locomotor activity, or anxiety-like behaviors. Minor histopathological alterations in the liver were detected; however, no major abnormal pathology was observed in the heart or lungs. CONCLUSION: These findings clarify the link between ethanol dosage and behavioral changes in mice over a 12-week IA2BC paradigm, thereby bridging the knowledge gap regarding the effects of chronic ethanol drinking on neurological disorders.

Relapse after intermittent access to cocaine: Discriminative cues more effectively trigger drug seeking than do conditioned cues.

RATIONALE: When people with drug addiction encounter cues associated with drug use, this can trigger cravings and relapse. These cues can include conditioned stimuli (CSs) signaling drug delivery and discriminative stimuli (DSs) signaling drug availability. Compared to CS effects, DS effects are less explored in preclinical studies on cue-induced relapse. OBJECTIVE: We compared CS and DS effects on reward seeking following abstinence from intermittent-access cocaine (or sucrose) self-administration. METHODS: During 15-20 intermittent-access sessions, rats self-administered i.v. cocaine or sucrose pellets paired with a light-tone CS. Cocaine/sucrose was available for 5-min (signalled by a light; DS+) and unavailable for 25 min (signalled by different lighting conditions; DS-), and this cycled for 4 h/session. Following abstinence, we measured cocaine/sucrose seeking under extinction triggered by CS and DS presentation, and instrumental responding reinforced by these cues. RESULTS: Across intermittent-access sessions, rats increased lever pressing for cocaine or sucrose during DS+ periods and decreased responding during DS- periods. On days 2 and 21 of abstinence, only presentation of the DS+ or DS+ and CS combined elicited increased cocaine/sucrose-seeking behaviour (i.e., increased active lever presses). Presenting the DS- alongside the DS+ suppressed the increased cocaine-seeking behaviour otherwise produced by the DS+ . Finally, on day 21 of abstinence, rats showed equivalent levels of lever pressing reinforced by the DS+ , CS and by the DS+ and CS combined, suggesting comparable conditioned reinforcing value. CONCLUSIONS: After intermittent self-administration, cocaine-associated DSs and CSs acquire similar conditioned reinforcing properties, but DSs more effectively trigger increases in drug seeking.

Intermittent nicotine access is as effective as continuous access in promoting nicotine seeking and taking in rats.

RATIONALE: Nicotine is a principal psychoactive agent in tobacco, contributing to tobacco's addictive potential. Preclinical studies on the effects of voluntary nicotine intake typically use self-administration procedures that provide continuous nicotine access during each self-administration session. However, many smokers consume cigarettes intermittently rather than continuously throughout each day. For drugs including cocaine and opioids, research in laboratory rats shows that intermittent intake can be more effective than continuous intake in producing patterns of drug use relevant to addiction. OBJECTIVE: We determined how intermittent versus continuous nicotine self-administration influences nicotine seeking and taking behaviours. METHODS: Female and male rats had continuous (i.e., Long Access; LgA, 6 h/day) or intermittent (IntA; 12 min ON, 60 min OFF, for 6 h/day) access to intravenous nicotine (15 µg/kg/infusion), for 12 daily sessions. We then assessed intake, responding for nicotine under a progressive ratio schedule of drug reinforcement and cue- and nicotine-induced reinstatement of drug seeking. We also estimated nicotine pharmacokinetic parameters during LgA and IntA self-administration. RESULTS: Overall, LgA rats took twice more nicotine than did IntA rats, yielding more sustained increases in estimated brain concentrations of the drug. However, the two groups showed similar motivation to seek and take nicotine, as measured using reinstatement and progressive ratio procedures, respectively. CONCLUSIONS: Intermittent nicotine use is just as effective as continuous use in producing addiction-relevant behaviours, despite significantly less nicotine exposure. This has implications for modeling nicotine self-administration patterns in human smokers and resulting effects on brain and behaviour.

Standard rodent diets differentially impact alcohol consumption, preference, and gut microbiome diversity.

Alcohol use disorder (AUD) is a complex and widespread disease with limited pharmacotherapies. Preclinical animal models of AUD use a variety of voluntary alcohol consumption procedures to recapitulate different phases of AUD, including binge alcohol consumption and dependence. However, voluntary alcohol consumption in mice is widely variable, making it difficult to reproduce results across labs. Accumulating evidence indicates that different brands of commercially available rodent chow can profoundly influence alcohol intake. In this study, we investigated the effects of three commercially available and widely used rodent diet formulations on alcohol consumption and preference in C57BL/6 J mice using the 24 h intermittent access procedure. The three brands of chow tested were LabDiet 5,001 (LD5001), LabDiet 5,053 (LD5053), and Teklad 2019S (TL2019S) from two companies (Research Diets and Envigo, respectively). Mice fed LD5001 and LD5053 displayed higher levels of alcohol consumption and preference compared to mice fed TL2019S. We also found that alcohol consumption and preference could be rapidly switched by changing the diet 48 h prior to alcohol administration. Sucrose, saccharin, and quinine preferences were not altered, suggesting that the diets did not alter sweet and bitter taste perception. We also found that mice fed LD5001 displayed increased quinine-resistant alcohol intake compared to mice fed TL2019S, suggesting that diets could influence the development of compulsive behaviors such as alcohol consumption. We profiled the gut microbiome of water- and alcohol-drinking mice that were maintained on different diets and found significant differences in bacterial alpha- and beta-diversities, which could impact the gut-brain axis signaling and alcohol consumption.

Sex Differences in the Development of an Opioid Addiction-Like Phenotype: A Focus on the Telescoping Effect.

Background: Women develop addiction and drug-related health consequences after fewer years of drug use than men; this accelerated time course, or telescoping effect, has been observed clinically for multiple drugs, including opioids. Preclinical studies indicate that this is a biologically based phenomenon; however, these studies have focused exclusively on cocaine, and none have considered health effects. Methods: In this study, we used a rat (Sprague Dawley) model to determine sex differences in the time course for the development of an opioid addiction-like phenotype, as defined by the development of physical dependence (withdrawal-induced weight loss) and an increase in motivation for fentanyl (under a progressive-ratio schedule). Effects were determined following either 10 days (optimized, experiment 1) or 3 days (threshold, experiment 2) of extended-access fentanyl self-administration (24 hours/day, fixed ratio 1, 2- to 5-minute trials/hour) or following short-access fentanyl self-administration (subthreshold, experiment 3; fixed ratio 1, up to 40 infusions/day). Opioid-related adverse health effects were also determined (experiment 4). Results: Motivation for fentanyl was similarly increased in males and females following 10 days of extended-access self-administration (experiment 1), was transiently increased in females, but not males, following 3 days of extended-access self-administration (experiment 2) and was not increased in either sex following short-access self-administration (experiment 3). Females developed fentanyl-associated adverse health effects more readily than males (experiment 4), with particularly robust differences during extended-access self-administration and withdrawal. Conclusions: As with findings in humans, female rats developed opioid addiction-like features and adverse health consequences more readily than male rats. These data provide support for a biologically based telescoping effect in females for opioids, particularly for opioid-related adverse health consequences.

In this issue, we explore how female rats develop signs of opioid addiction and related health issues faster than male rats, a phenomenon known as the telescoping effect. This study expands on previous research by using a rat model to assess addiction-like behaviors and health consequences following different withdrawal period and durations of fentanyl self-administration. The findings underline the biological underpinnings of sex differences in addiction trajectories, previously demonstrated in humans but not yet studied in opioids until now.