Cocaine dose and self-administration history, but not initial cocaine locomotor responsiveness, affects sensitization to the motivational effects of cocaine in rats

Expression of stable and reliable preference and aversion phenotypes following place conditioning with psychostimulants

RATIONALE AND OBJECTIVES

Drug-seeking behavior occurs more readily in some individuals than others. This phenomenon is considered in studies of drug self-administration in which high drug-seeking/taking individuals can be identified. In contrast, studies of conditioned place preference (CPP) often involve a random sample of drug-naïve rodents that includes phenotypes not considered relevant to addiction. The main objective of the current studies was to determine if a priori identification of different conditioning phenotypes could improve the validity and sensitivity of CPP expression as a preclinical test for vulnerability to addiction.

METHODS AND RESULTS

Analysis of cocaine place conditioning data from 443 Swiss-Webster mice revealed a trimodal distribution with peaks corresponding to means of k = 3 clusters. The cluster means occurred at high, low, or negative preference scores, the latter suggesting a phenotype acquiring conditioned place aversion (CPA). The same clusters were identified in mice conditioned with methamphetamine, MDPV, or amphetamine, and these clusters remained stable and reliable during three additional expression tests spaced at 24 h. A meta-analysis of effect sizes obtained from CPP literature revealed a positively skewed distribution affected by sample size, consistent with the existence of a CPA phenotype within the populations tested. A dopamine receptor antagonist, flupentixol, blocked cocaine CPP expression in a group containing all phenotypes, but sensitivity improved markedly when CPA phenotypes were excluded from the dataset.

CONCLUSIONS

These studies suggest that taking phenotype into consideration when designing place conditioning studies will improve their application as a preclinical tool in addiction biology and drug discovery.

Tropisetron Facilitates Footshock Suppression of Compulsive Cocaine Seeking

BACKGROUND

The hallmark characteristics of the murine model of drug addiction include the escalation of cocaine consumption and compulsive punishment-resistant drug seeking. In this study, we evaluated the motivation for drug seeking in cocaine self-administering rats exposed to an escalated dosing regimen that endeavored to mimic the characteristic of escalating drug intake in human addicts. Tropisetron is a 5-HT3 receptor antagonist and α7-nicotinic receptor partial agonist. Utilizing rats trained on the escalated-dosing regimen, we examined the effects of tropisetron on control over compulsive drug-seeking behavior that was defined as footshock-resistant lever pressing.

METHODS

Rats were trained to self-administer cocaine with incremental-infusion doses (from 0.6 to 2.4 mg/kg/infusion) across training sessions (3 h/session) or with a long-access paradigm (i.e., 0.6 mg/kg/infusion, 6 h/d training session). The drug-seeking motivations of 2 groups were estimated by the patterns of drug intake and progressive-ratio schedule. The compulsivity for drug seeking of the group with an escalated dose was further evaluated using the footshock-associated seeking-taking chain task.

RESULTS

The rats trained on the dose-escalated protocol achieved the same levels of motivated drug seeking as those subjected to a long-access paradigm, as indicated by cocaine intake per training session and breakpoints on a progressive ratio schedule. Tropisetron attenuated compulsive behavior of rats when pressing of the seeking lever potentially led to footshock. Intriguingly, tropisetron did not change the motivation to seek cocaine when footshock was absent. Tropisetron had no effect on locomotor activities or saccharin self-administration.

CONCLUSIONS

These results demonstrate that tropisetron restored control over compulsive cocaine seeking, and they indicate that 5-HT3/α7-nicotinic receptors may be potential therapeutic targets for relieving compulsive drug seeking.

Opioid Self-Administration is Attenuated by Early-Life Experience and Gene Therapy for Anti-Inflammatory IL-10 in the Nucleus Accumbens of Male Rats

Early-life conditions can contribute to the propensity for developing neuropsychiatric disease, including substance abuse disorders. However, the long-lasting mechanisms that shape risk or resilience for drug addiction remain unclear. Previous work has shown that a neonatal handling procedure in rats (which promotes enriched maternal care) attenuates morphine conditioning, reduces morphine-induced glial activation, and increases microglial expression of the anti-inflammatory cytokine interleukin-10 (IL-10). We thus hypothesized that anti-inflammatory signaling may underlie the effects of early-life experience on later-life opioid drug-taking. Here we demonstrate that neonatal handling attenuates intravenous self-administration of the opioid remifentanil in a drug-concentration-dependent manner. Transcriptional profiling of the nucleus accumbens (NAc) from handled rats following repeated exposure to remifentanil reveals a suppression of pro-inflammatory cytokine and chemokine gene expression, consistent with an anti-inflammatory phenotype. To determine if anti-inflammatory signaling alters drug-taking behavior, we administered intracranial injections of plasmid DNA encoding IL-10 (pDNA-IL-10) into the NAc of non-handled rats. We discovered that pDNA-IL-10 treatment reduces remifentanil self-administration in a drug-concentration-dependent manner, similar to the effect of handling. In contrast, neither handling nor pDNA-IL-10 treatment alters self-administration of food or sucrose rewards. These collective observations suggest that neuroimmune signaling mechanisms in the NAc are shaped by early-life experience and may modify motivated behaviors for opioid drugs. Moreover, manipulation of the IL-10 signaling pathway represents a novel approach for influencing opioid reinforcement.

Less is more: prolonged intermittent access cocaine self-administration produces incentive-sensitization and addiction-like behavior

RATIONALE

Contemporary animal models of cocaine addiction focus on increasing the amount of drug consumption to produce addiction-like behavior. However, another critical factor is the temporal pattern of consumption, which in humans is characterized by intermittency, both within and between bouts of use.

OBJECTIVE

To model this, we combined prolonged access to cocaine (∼70 days in total) with an intermittent access (IntA) self-administration procedure and used behavioral economic indicators to quantify changes in motivation for cocaine.

RESULTS

IntA produced escalation of intake, a progressive increase in cocaine demand (incentive-sensitization), and robust drug- and cue-induced reinstatement of drug-seeking behavior. We also asked whether rats that vary in their propensity to attribute incentive salience to reward cues (sign-trackers [STs] vs. goal-trackers [GTs]) vary in the development of addiction-like behavior. Although STs were more motivated to take cocaine after limited drug experience, after IntA, STs and GTs no longer differed on any measure of addiction-like behavior.

CONCLUSIONS

Exposure to large quantities of cocaine is not necessary for escalation of intake, incentive-sensitization, or other addiction-like behaviors (IntA results in far less total cocaine consumption than 'long access' procedures). Also, the ST phenotype may increase susceptibility to addiction, not because STs are inherently susceptible to incentive-sensitization (perhaps all individuals are at risk), but because this phenotype promotes continued drug use, subjecting them to incentive-sensitization. Thus, the pharmacokinetics associated with the IntA procedure are especially effective in producing a number of addiction-like behaviors and may be valuable for studying associated neuroadaptations and for assessing individual variation in vulnerability.

Escalation of cocaine consumption in short and long access self-administration procedures

BACKGROUND

Escalation of consumption is a hallmark of cocaine addiction. Many animal models reveal escalation by increasing the duration of drug access (e.g., 6-24 h/day) after longer histories of self-administration. We recently developed a method that reveals escalation early post-acquisition under shorter access conditions. However, whether or not rats will escalate cocaine consumption both early post-acquisition under short access (2 h/day) conditions, and later under long access (6 h/day) conditions, has not been demonstrated.

METHODS

All rats acquired cocaine self-administration (0.8 mg/kg, i.v.) under 2 h conditions, and then continued 2h self-administration for an additional 13 sessions. Then, rats were assigned either to 2 or 6h conditions, and self-administered cocaine (0.8 mg/kg, i.v.) for an additional 19 sessions. In addition, four cocaine-induced locomotor activity measurements were taken for each rat: before cocaine exposure, after non-contingent cocaine administration, and after escalation in the short and long access experimental phases.

RESULTS

Following acquisition, rats displayed a robust escalation of intake during 2 h sessions. Rats that self-administered cocaine in continued 2h sessions exhibited stable intake, whereas rats that self-administered cocaine in 6h sessions further escalated intake. Despite the second escalation in 6h rats, cocaine-induced locomotor activity did not differ between 2 and 6h rats.

CONCLUSIONS

Escalation of cocaine self-administration can occur in the same rats both early post-acquisition, and later under long access conditions. Importantly, this early post-acquisition period provides a new opportunity to determine the mechanisms first involved in the escalation phenomenon.

Continuous exposure to dizocilpine facilitates the acquisition and escalation of cocaine consumption in male Sprague-Dawley rats

BACKGROUND

Blocking N-methyl-d-aspartate (NMDA) glutamate receptors (NMDARs) prevents cocaine locomotor sensitization, but facilitates escalation of cocaine self-administration and produces ambiguous effects on acquisition of cocaine self-administration. This study used a recently described model of acquisition and escalation to test the hypothesis that continuous NMDAR antagonism functionally increases the effects of a given dose of cocaine.

METHODS

We assessed acquisition of cocaine self-administration (0.6 mg/kg/infusion) in rats treated continuously with either vehicle or the NMDAR antagonist dizocilpine (0.4 mg/kg/day) for 14 consecutive 2h fixed ratio 1 (FR1) sessions. In a separate experiment that assessed the effect of dizocilpine treatment on escalation of cocaine self-administration, rats acquired cocaine self-administration (0.6 mg/kg/infusion) prior to vehicle or dizocilpine treatment. Then, immediately post-acquisition, rats were treated continuously with either vehicle or dizocilpine and allowed to self-administer either 0.6 or 1.2mg/kg/infusion cocaine for an additional seven consecutive 2h FR1 sessions.

RESULTS

Relative to vehicle-treated rats, a significantly greater percentage of dizocilpine-treated rats acquired cocaine self-administration. During the escalation experiment, both vehicle- and dizocilpine-treated rats escalated intake of 1.2mg/kg/infusion cocaine. Whereas vehicle-treated rats exhibited stable intake of 0.6 mg/kg/infusion cocaine, dizocilpine-treated rats escalated intake of this moderate cocaine dose to levels indistinguishable from intake levels produced by self-administration of the high cocaine dose (i.e., 1.2mg/kg/infusion).

CONCLUSIONS

These findings suggest that chronic NMDAR blockade potentiates, rather than attenuates, cocaine's effects and argue for reconsideration of the role of NMDARs in cocaine "addiction-like" behavior.

Continuous exposure to dizocilpine facilitates escalation of cocaine consumption in male Sprague-Dawley rats

BACKGROUND

Although the escalation of cocaine consumption is a hallmark of cocaine dependence, the neurobiological mechanisms that underlie this change in behavior are not well understood.

METHODS

This study used an extended access version of the drug self-administration procedure to explore how N-methyl-d-aspartate (NMDA) receptors are involved in escalation of cocaine consumption. Male Sprague-Dawley rats (n=59) were first trained to self-administer cocaine (0.33 mg/infusion, i.v.) under a fixed-ratio 1 (FR1) schedule of reinforcement. After training, rats were implanted with subcutaneous osmotic minipumps filled with vehicle or the non-competitive NMDAR antagonist, dizocilpine (0.2 or 0.4 mg/kg/d), and subsequently allowed to self-administer cocaine in 2h or 6h self-administration sessions.

RESULTS

In the 6h groups, vehicle-treated rats escalated cocaine self-administration across 15 self-administration sessions; rats treated with dizocilpine escalated cocaine self-administration at a greater rate and to a greater degree. Rats that self-administered cocaine during 2h sessions did not escalate consumption of cocaine under any treatment condition. Discontinuation of dizocilpine treatment in the 6h access condition led to a substantial decrease in cocaine consumption, down to pre-escalation levels, and then control rates of escalation thereafter. Despite large differences in intake under the FR1 schedule, post-escalation break point under a progressive ratio schedule of reinforcement did not differ between groups.

CONCLUSION

These data suggest that glutamate tone through NMDA receptors can play a dynamic role in regulating cocaine intake and escalation of consumption.

Rats classified as low or high cocaine locomotor responders: a unique model involving striatal dopamine transporters that predicts cocaine addiction-like behaviors

Individual differences are a hallmark of drug addiction. Here, we describe a rat model based on differential initial responsiveness to low dose cocaine. Despite similar brain cocaine levels, individual outbred Sprague-Dawley rats exhibit markedly different magnitudes of acute cocaine-induced locomotor activity and, thereby, can be classified as low or high cocaine responders (LCRs or HCRs). LCRs and HCRs differ in drug-induced, but not novelty-associated, hyperactivity. LCRs have higher basal numbers of striatal dopamine transporters (DATs) than HCRs and exhibit marginal cocaine inhibition of in vivo DAT activity and cocaine-induced increases in extracellular DA. Importantly, lower initial cocaine response predicts greater locomotor sensitization, conditioned place preference and greater motivation to self-administer cocaine following low dose acquisition. Further, outbred Long-Evans rats classified as LCRs, versus HCRs, are more sensitive to cocaine's discriminative stimulus effects. Overall, results to date with the LCR/HCR model underscore the contribution of striatal DATs to individual differences in initial cocaine responsiveness and the value of assessing the influence of initial drug response on subsequent expression of addiction-like behaviors.

Low- and high-cocaine locomotor responding rats differ in reinstatement of cocaine seeking and striatal mGluR5 protein expression

Behavioral responsiveness to initial cocaine use varies among individuals and may contribute to differential vulnerability to cocaine addiction. Rats also exhibit individual differences in cocaine's effects and can be classified as low or high cocaine responders (LCRs or HCRs, respectively), based on their initial cocaine-induced locomotor activity (10 mg/kg, i.p.). Here, we used the extinction/reinstatement model to address whether or not LCRs and HCRs differ in (i) extinction/reinstatement of cocaine self-administration behavior and (ii) levels of metabotropic glutamate receptors (mGluRs) following these behaviors. During the earliest acquisition sessions, LCRs exhibited significantly greater cocaine intake (0.8 mg/kg/infusion) and cocaine-paired lever responding than HCRs, but intake and lever responding converged by the end of the cocaine self-administration portion of the study. LCRs and HCRs did not differ in cocaine seeking during the first extinction session and extinguished cocaine seeking similarly. HCRs exhibited greater reinstatement than LCRs to lower (2.5 and 5 mg/kg), but not higher (10 mg/kg), i.p. priming doses of cocaine. The effect of drug-paired cues on reinstatement following extinction was complex, with HCRs and LCRs showing the greater effect of cue depending on the order in which cue- and drug-primed tests were given. Western blot analysis revealed that mGluR5 heteromers were significantly higher in the dorsal striatum of HCRs than LCRs following reinstatement testing. Although our previous findings with the LCR/HCR model have uniformly supported the idea that lower initial cocaine-induced activation predicts more ready development of cocaine addiction-like behaviors, here, we show a more complex relationship with cocaine reinstatement.