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

Self-administration acquisition latency predicts locomotor sensitivity to cocaine in male rats

Individual differences in drug use emerge soon after initial exposure, and only a fraction of individuals who initiate drug use go on to develop a substance use disorder. Variability in vulnerability to establishing drug self-administration behavior is also evident in preclinical rodent models. Latent characteristics that underlie this variability and the relationship between early drug use patterns and later use remain unclear. Here, we attempt to determine whether propensity to establish cocaine self-administration is related to subsequent cocaine self-administration behavior in male Sprague-Dawley rats (n = 14). Prior to initiating training, we evaluated basal locomotor and anxiety-like behavior in a novel open field test. We then trained rats to self-administer cocaine in daily 3 h cocaine (0.75 mg/kg/infusion) self-administration sessions until acquisition criteria (≥30 active lever presses with ≥70 % responding on the active lever in one session) was met and divided rats into Early and Late groups by median-split analysis based on their latency to meet acquisition criteria. After each rat met acquisition criteria, we gave them 10 additional daily cocaine self-administration sessions. We then conducted a progressive ratio, cocaine-induced locomotor sensitivity test, and non-reinforced cocaine seeking test after two weeks of forced abstinence. Early Learners exhibited significantly less locomotion after an acute injection of cocaine, but the groups did not differ in any other behavioral parameter examined. These results indicate that cocaine self-administration acquisition latency is not predictive of subsequent drug-taking behavior, but may be linked to physiological factors like drug sensitivity that can predispose rats to learn the operant task.

Inhibition of a cortico-thalamic circuit attenuates cue-induced reinstatement of drug-seeking behavior in "relapse prone" male rats

RATIONALE

Relapse often occurs when individuals are exposed to stimuli or cues previously associated with the drug-taking experience. The ability of drug cues to trigger relapse is believed to be a consequence of incentive salience attribution, a process by which the incentive value of reward is transferred to the reward-paired cue. Sign-tracker (ST) rats that attribute enhanced incentive value to reward cues are more prone to relapse compared to goal-tracker (GT) rats that primarily attribute predictive value to such cues.

OBJECTIVES

The neurobiological mechanisms underlying this individual variation in relapse propensity remains largely unexplored. The paraventricular nucleus of the thalamus (PVT) has been identified as a critical node in the regulation of cue-elicited behaviors in STs and GTs, including cue-induced reinstatement of drug-seeking behavior. Here we used a chemogenetic approach to assess whether "top-down" cortical input from the prelimbic cortex (PrL) to the PVT plays a role in mediating individual differences in relapse propensity.

RESULTS

Chemogenetic inhibition of the PrL-PVT pathway selectively decreased cue-induced reinstatement of drug-seeking behavior in STs, without affecting behavior in GTs. In contrast, cocaine-primed drug-seeking behavior was not affected in either phenotype. Furthermore, when rats were characterized based on a different behavioral phenotype-locomotor response to novelty-inhibition of the PrL-PVT pathway had no effect on either cue- or drug-induced reinstatement.

CONCLUSIONS

These results highlight an important role for the PrL-PVT pathway in vulnerability to relapse that is consequent to individual differences in the propensity to attribute incentive salience to discrete reward cues.

Safety pharmacology assessment of Ethanamizuril, a novel triazines coccidiostat

In the current study, to support the safety pharmacology assessment of Ethanamizuril as a new potent anticoccidial agent of triazine compounds, the effects of Ethanamizuril on the central nervous system, cardiovascular system and respiratory system were investigated. Using locomotor activity test, climbing behavior test and nembutal subthreshold hypnotic test at each time point after oral administration of Ethanamizuril to mice, the effects on the central nervous system were evaluated. An assessment of Ethanamizuril effects on the cardiovascular and respiratory system were performed by the use of a telemetry system in conscious beagle dogs. The results showed that the treatment of Ethanamizuril had no effects on motor activity, behavioral changes, coordination, and sensory/motor reflex responses in mice. There were also no changes in heart rate, blood pressure, and electrocardiogram at all doses and each time points in beagle dogs. Our data suggested that Ethanamizuril showed no adverse effects on the central nervous system, cardiovascular system, and respiratory system.

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.

Tolerance to the locomotor-activating effects of 3,4-methylenedioxymethamphetamine (MDMA) predicts escalation of MDMA self-administration and cue-induced reinstatement of MDMA seeking in rats

Pre-clinical studies of individual differences in addiction vulnerability have been increasing over recent years, but the amphetamine derivative 3,4-methylenedioxymethamphetamine (MDMA; ecstasy) has received relatively little attention in this regard. Previously, we reported large individual differences both in rats' initial behavioral response to experimenter-administered MDMA and their degree of behavioral sensitization to repeated administration. To determine whether these differences could predict subsequent patterns of MDMA-taking or -seeking behaviors we used the self-administration-extinction-reinstatement model to examine addiction-like behavior (i.e., escalation of MDMA self-administration and cue-induced reinstatement of MDMA seeking) in rats a priori characterized for either locomotor sensitization or tolerance to MDMA. Rats that developed tolerance to the locomotor-activating effects of MDMA had a significantly larger locomotor response to the first MDMA injection relative to rats that developed sensitization. Importantly, rats that developed tolerance subsequently displayed an escalation of MDMA self-administration over days, as well as clear cue-induced reinstatement of MDMA seeking following extinction. Conversely, rats that developed locomotor sensitization to MDMA subsequently maintained relatively stable levels of MDMA self-administration over days and showed no cue-induced reinstatement of MDMA seeking. These results show that differences in the level of psychomotor activation following acute and repeated MDMA administration can reliably predict two important addiction-like behaviors in rats, which may have implications in the prediction of compulsive MDMA use in humans.

Adrenaline rush: the role of adrenergic receptors in stimulant-induced behaviors

Psychostimulants, such as cocaine and amphetamines, act primarily through the monoamine neurotransmitters dopamine (DA), norepinephrine, and serotonin. Although stimulant addiction research has largely focused on DA, medication development efforts targeting the dopaminergic system have thus far been unsuccessful, leading to alternative strategies aimed at abating stimulant abuse. Noradrenergic compounds have shown promise in altering the behavioral effects of stimulants in rodents, nonhuman primates, and humans. In this review, we discuss the contribution of each adrenergic receptor (AR) subtype (α1, α2, and β) to five stimulant-induced behaviors relevant to addiction: locomotor activity, conditioned place preference, anxiety, discrimination, and self-administration. AR manipulation has diverse effects on these behaviors; each subtype profoundly influences outcomes in some paradigms but is inconsequential in others. The functional neuroanatomy and intracellular signaling mechanisms underlying the impact of AR activation/blockade on these behaviors remain largely unknown, presenting a new frontier for research on psychostimulant-AR interactions.