Patterns of ethanol intake in male rats with partial dopamine transporter deficiency

Mesolimbic dopamine signaling plays a major role in alcohol and substance use disorders as well as comorbidities such as anxiety and depression. Growing evidence suggests that alcohol drinking is modulated by the function of the dopamine transporter (DAT), which tightly regulates extracellular dopamine concentrations. Adult male rats on a Wistar Han background (DAT+/+) and rats with a partial DAT deletion (DAT+/-) were used in this study. First, using fast-scan cyclic voltammetry in brain slices containing the nucleus accumbens core from ethanol-naïve subjects, we measured greater evoked dopamine concentrations and slower dopamine reuptake in DAT+/- rats, consistent with increased dopamine signaling. Next, we measured ethanol drinking using the intermittent access two-bottle choice paradigm (20% v/v ethanol vs. water) across 5 weeks. DAT+/- rats voluntarily consumed less ethanol during its initial availability (the first 30 min), especially after longer periods of deprivation. In addition, DAT+/- males consumed less ethanol that was adulterated with the bitter tastant quinine. These findings suggest that partial DAT blockade and concomitant increase in brain dopamine levels has potential to reduce drinking and ameliorate alcohol use disorder (AUD).

Escalation of alcohol intake is associated with regionally decreased insular cortex activity but not changes in taste quality

BACKGROUND

Intermittent access to ethanol drives persistent escalation of intake and rapid transition from moderate to compulsive-like drinking. Intermittent ethanol drinking may facilitate escalation of intake in part by altering aversion-sensitive neural substrates, such as the insular cortex (IC), thus driving greater approach toward stimuli previously treated as aversive.

METHODS

We conducted a series of experiments in rats to examine behavioral and neural responses associated with escalation of ethanol intake. First, taste reactivity analyses quantified the degree to which intermittent brief-access ethanol exposure (BAEE) alters sensitivity to the aversive properties of ethanol. Next, we determined whether pharmacological IC inhibition facilitated ethanol escalation. Finally, given that the IC is primary gustatory cortex, we employed psychophysical paradigms to assess whether escalation of ethanol intake induced changes in ethanol taste. These paradigms measured changes in sensitivity to the intensity of ethanol taste and whether escalation in intake shifts the salient taste quality of ethanol by measuring the degree to which the taste of ethanol generalized to a sucrose-like ("sweet") or quinine-like ("bitter") percept.

RESULTS

We found a near-complete loss of aversive oromotor responses in ethanol-exposed relative to ethanol-naïve rats. Additionally, we observed significantly lower expression of ethanol-induced c-Fos expression in the posterior IC in exposed rats relative to naïve rats. Inhibition of the IC resulted in a modest, but statistically reliable increase in the acceptance of higher ethanol concentrations in naïve rats. Finally, we found no evidence of changes in the psychophysical assessment of the taste of ethanol in exposed, relative to naïve, rats.

CONCLUSIONS

Our results demonstrate that neural activity within the IC adapts following repeated presentations of ethanol in a manner that correlates with reduced sensitivity to the aversive hedonic properties of ethanol. These data help to establish that alterations in IC activity may be driving exposure-induced escalations in ethanol intake.

Increased heroin intake and relapse vulnerability in intermittent relative to continuous self-administration: Sex differences in rats

BACKGROUND AND PURPOSE

Studies using intermittent-access drug self-administration show increased motivation to take and seek cocaine and fentanyl, relative to continuous access. In this study, we examined the effects of intermittent- and continuous-access self-administration on heroin intake, patterns of self-administration and cue-induced heroin-seeking, after forced or voluntary abstinence, in male and female rats. We also modelled brain levels of heroin and its active metabolites.

EXPERIMENTAL APPROACH

Rats were trained to self-administer a palatable solution and then heroin (0.075 mg·kg^-1 per inf) either continuously (6 h·day^-1 ; 10 days) or intermittently (6 h·day^-1 ; 5-min access every 30-min; 10 days). Brain levels of heroin and its metabolites were modelled using a pharmacokinetic software. Next, heroin-seeking was assessed after 1 or 21 abstinence days. Between tests, rats underwent either forced or voluntary abstinence. The oestrous cycle was measured using a vaginal smear test.

KEY RESULTS

Intermittent access exacerbated heroin self-administration and was characterized by a burst-like intake, yielding higher brain peaks of heroin and 6-monoacetylmorphine concentrations. Moreover, intermittent access increased cue-induced heroin-seeking during early, but not late abstinence. Heroin-seeking was higher in females after intermittent, but not continuous access, and this effect was independent of the oestrous cycle.

CONCLUSIONS AND IMPLICATIONS

Intermittent heroin access in rats resembles critical features of heroin use disorder: a self-administration pattern characterized by repeated large doses of heroin and higher relapse vulnerability during early abstinence. This has significant implications for refining animal models of substance use disorder and for better understanding of the neuroadaptations responsible for this disorder.

LINKED ARTICLES

This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Tauopathy and alcohol consumption interact to alter locus coeruleus excitatory transmission and excitability in male and female mice

Alcohol use disorder is a major public health concern in the United States. Recent work has suggested a link between chronic alcohol consumption and the development of tauopathy disorders, such as Alzheimer's disease and frontotemporal dementia. However, relatively little work has investigated changes in neural circuitry involved in both tauopathy disorders and alcohol use disorder. The locus coeruleus (LC) is the major noradrenergic nucleus in the brain and is one of the earliest sites to be affected by tau lesions. The LC is also implicated in the rewarding effects of ethanol and alcohol withdrawal. In this study we assessed effects of long-term ethanol consumption and tauopathy on the physiology of LC neurons. Male and female P301S mice, a humanized transgenic mouse model of tauopathy, underwent 16 weeks of intermittent access to 20% ethanol from 3 to 7 months of age. We observed higher total alcohol consumption in female mice regardless of genotype. Male P301S mice consumed more ethanol and had a greater preference for ethanol than wild-type (WT) males. At the end of the drinking study, LC function was assessed using ex vivo whole cell electrophysiology. We found significant changes in excitatory inputs to the LC due to both ethanol and genotype. We found significantly increased excitability of the LC due to ethanol with greater effects in female P301S mice than in female WT mice. Our study identifies significant changes in the LC due to interactions between tauopathy and long-term ethanol use. These findings could have important implications regarding LC activity and changes in behavior due to both ethanol- and tauopathy-related dementia.

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.

Taking Rapid and Intermittent Cocaine Infusions Enhances Both Incentive Motivation for the Drug and Cocaine-induced Gene Regulation in Corticostriatal Regions

A goal in addiction research is to distinguish forms of neuroplasticity that are involved in the transition to addiction from those involved in mere drug taking. Animal models of drug self-administration are essential in this context. Here, we compared in male rats two cocaine self-administration procedures that differ in the extent to which they evoke addiction-like behaviours. We measured both incentive motivation for cocaine using progressive ratio procedures, and cocaine-induced c-fos mRNA expression, a marker of neuronal activity. Rats self-administered intravenous cocaine (0.25 mg/kg/infusion) for seven daily 6-hour sessions. One group had intermittent access (IntA; 6 minutes ON, 26 min OFF × 12) to rapid infusions (delivered over 5 s). This models the temporal kinetics of human cocaine use and produces robust addiction-like behaviour. The other group had Long access (LgA) to slower infusions (90 s). This produces high levels of intake without promoting robust addiction-like behaviour. LgA-90 s rats took twice as much cocaine as IntA-5 s rats did, but IntA-5 s rats showed greater incentive motivation for the drug. Following a final self-administration session, we quantified c-fos mRNA expression in corticostriatal regions. Compared to LgA-90 s rats, IntA-5 s rats had more cocaine-induced c-fos mRNA in the orbitofrontal and prelimbic cortices and the caudate-putamen. Thus, a cocaine self-administration procedure (intermittent intake of rapid infusions) that promotes increased incentive motivation for the drug also enhances cocaine-induced gene regulation in corticostriatal regions. This suggests that increased drug-induced recruitment of these regions could contribute to the neural and behavioural plasticity underlying the transition to addiction.

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.

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.

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.

Aggression and increased glutamate in the mPFC during withdrawal from intermittent alcohol in outbred mice

RATIONALE

Disrupted social behavior, including occasional aggressive outbursts, is characteristic of withdrawal from long-term alcohol (EtOH) use. Heavy EtOH use and exaggerated responses during withdrawal may be treated using glutamatergic N-methyl-D-aspartate receptor (NMDAR) antagonists.

OBJECTIVES

The current experiments explore aggression and medial prefrontal cortex (mPFC) glutamate as consequences of withdrawal from intermittent access to EtOH and changes in aggression and mPFC glutamate caused by NMDAR antagonists memantine and ketamine.

METHODS

Swiss male mice underwent withdrawal following 1-8 weeks of intermittent access to 20 % EtOH. Aggressive and nonaggressive behaviors with a conspecific were measured 6-8 h into EtOH withdrawal after memantine or ketamine (0-30 mg/kg, i.p.) administration. In separate mice, extracellular mPFC glutamate after memantine was measured during withdrawal using in vivo microdialysis.

RESULTS

At 6-8 h withdrawal from EtOH, mice exhibited more convulsions and aggression and decreased social contact compared to age-matched water controls. Memantine, but not ketamine, increased withdrawal aggression at the 5-mg/kg dose in mice with a history of 8 weeks of EtOH but not 1 or 4 weeks of EtOH or in water drinkers. Tonic mPFC glutamate was higher during withdrawal after 8 weeks of EtOH compared to 1 week of EtOH or 8 weeks of water. Five milligrams per kilogram of memantine increased glutamate in 8-week EtOH mice, but also in 1-week EtOH and water drinkers.

CONCLUSIONS

These studies reveal aggressive behavior as a novel symptom of EtOH withdrawal in outbred mice and confirm a role of NMDARs during withdrawal aggression and for disrupted social behavior.

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.

Is the rodent maternal separation model a valid and effective model for studies on the early-life impact on ethanol consumption?

RATIONALE

Early-life events can cause long-term neurobiological and behavioural changes with a resultant effect upon reward and addiction processes that enhance risk to develop alcohol use disorders. Maternal separation (MS) is used to study the mediating mechanisms of early-life influences in rodents. In MS studies, the pups are exposed to maternal absence during the first postnatal weeks. The outcome of MS experiments exhibits considerable variation and questions have been raised about the validity of MS models.

OBJECTIVES

This short review aims to provide information about experimental conditions that are important to consider when assessing the impact of early-life environment on voluntary ethanol consumption.

RESULTS

The results from currently used MS protocols are not uniform. However, studies consistently show that longer separations of intact litters predispose for higher ethanol consumption and/or preference in adult male rats as compared to shorter periods of MS. Studies using individual pup MS paradigms, other controls, low ethanol concentrations, adult females or examining adolescent consumption reported no differences or inconsistent results.

CONCLUSIONS

There is no "a rodent MS model", there are several models and they generate different results. The compiled literature shows that MS is a model of choice for analysis of early-life effects on voluntary ethanol consumption but there are examples of MS paradigms that are not suitable. These studies emphasize the importance to carefully designed MS experiments to supply the optimal conditions to definitely test the research hypothesis and to be particulate in the interpretation of the outcome.

Delta opioid receptors in brain function and diseases

Evidence that the delta opioid receptor (DOR) is an attractive target for the treatment of brain disorders has strengthened in recent years. This receptor is broadly expressed in the brain, binds endogenous opioid peptides, and shows as functional profile highly distinct from those of mu and kappa opioid receptors. Our knowledge of DOR function has enormously progressed from in vivo studies using pharmacological tools and genetic approaches. The important role of this receptor in reducing chronic pain has been extensively overviewed; therefore this review focuses on facets of delta receptor activity relevant to psychiatric and other neurological disorders. Beneficial effects of DOR agonists are now well established in the context of emotional responses and mood disorders. DOR activation also regulates drug reward, inhibitory controls and learning processes, but whether delta compounds may represent useful drugs in the treatment of drug abuse remains open. Epileptogenic and locomotor-stimulating effects of delta agonists appear drug-dependent, and the possibility of biased agonism at DOR for these effects is worthwhile further investigations to increase benefit/risk ratio of delta therapies. Neuroprotective effects of DOR activity represent a forthcoming research area. Future developments in DOR research will benefit from in-depth investigations of DOR function at cellular and circuit levels.

Context-induced relapse to alcohol seeking after punishment in a rat model

BACKGROUND

Rat studies have demonstrated that exposure to environments associated with alcohol intake reinstates alcohol seeking after extinction of alcohol-reinforced responding in a different context. However, extinction is limited as an abstinence model, because humans typically abstain because of negative consequences associated with excessive drinking. It is currently unknown whether alcohol-associated contexts can provoke relapse to alcohol seeking after alcohol-taking behavior is suppressed by adverse consequences in a different context.

METHODS

Alcohol-preferring P rats were first given home-cage access to 20% ethanol. Next, they were trained to self-administer 20% ethanol in one context (context A). Subsequently, all rats continued to self-administer alcohol in a different context (context B). For one group, 50% of alcohol-reinforced responses were punished by mild footshock; two other groups either received noncontingent shocks or no shock. A fourth group was given extinction training in context B. All rats were then tested for relapse to alcohol seeking under extinction conditions in contexts A and B.

RESULTS

In Context B, alcohol-taking behavior was suppressed by contingent shock (punishment) and extinction training but not by noncontingent shock. In Context A, relapse to alcohol seeking was reliably observed in the punished and extinction groups; a context switch had no effect on alcohol seeking in the no-shock or noncontingent shock groups.

CONCLUSIONS

Our data indicate that punishment-induced suppression of alcohol-taking behavior is context-dependent. We propose that our procedure can be used to explore mechanisms of context-induced relapse to alcohol seeking after alcohol-taking behavior is suppressed by adverse consequences.