Background and Purpose. Heroin and cocaine users tailor their dosage, frequency, and method of administration, to maximize the drugs’ effects or prevent withdrawal symptoms. Counterintuitively, preclinical self-administration and choice experiments employ fixed unit-doses and timeouts (after doses) largely resulting in uniform drug-taking patterns. The application of these procedures also disregards the distinct pharmacokinetic properties of heroin and cocaine. This uniformity contrasts with the significantly different ways humans tailor their dosage and frequency of heroin and cocaine use. Here, by combining behavioral and pharmacokinetics assessments we revealed that self-administration procedures lacking the timeout may overcomes this limitation. Experimental Approach. We analyzed heroin and cocaine taking- and seeking-patterns and estimated drug-brain levels in the presence or absence of timeout. We further assessed how absence of timeout and the availability of drug or social peer (access time to the two rewards) affect drug preference in choice procedures. Key Results. Removing the timeout had a profound effect on heroin-taking patterns and seeking, promoting the emergence of burst-like intake, yielding higher brain peak concentrations of heroin. Timeout removal had marginal impact on cocaine taking patterns and seeking. Increasing the access time to the drug in the discrete choice procedure resulted in higher drug intake and increased preference for heroin, but it did not alter cocaine preference. Conclusion and Implications. Removing timeout during self-administration revealed distinct heroin and cocaine taking patterns. Self-administration without timeout and access to high heroin doses during choice more closely mimic human heroin taking patterns and related behaviors, including maladaptive choices.

Environmental enrichment (EE) has been shown to produce beneficial effects in addiction disorders; however, due to its configurational complexity, the underlying mechanisms are not yet fully elucidated. Recent evidence suggests that EE, acting as a metaplastic agent, may affect glutamatergic mechanisms underlying appetitive memory and, in turn, modulate reward-seeking behaviors: here, we have investigated such possibility following a brief EE exposure. Adult male Sprague-Dawley rats were exposed to EE for 22h and the expression of critical elements of the glutamate synapse was measured 2h after the end of EE in the medial prefrontal cortex (mPFC), nucleus accumbens (NAc), and hippocampus (Hipp) brain areas, which are critical for reward and memory. We focused our investigation on the expression of NMDA and AMPA receptor subunits, their scaffolding proteins SAP102 and SAP97, vesicular and membrane glutamate transporters vGluT1 and GLT-1, and critical structural components such as proteins involved in morphology and function of glutamatergic synapses, PSD95 and Arc/Arg3.1. Our findings demonstrate that a brief EE exposure induces metaplastic changes in glutamatergic mPFC, NAc, and Hipp. Such changes are area-specific and involve postsynaptic NMDA/AMPA receptor subunit composition, as well as changes in the expression of their main scaffolding proteins, thus influencing the retention of such receptors at synaptic sites. Our data indicate that brief EE exposure is sufficient to dynamically modulate the glutamatergic synapses in mPFC-NAc-Hipp circuits, which may modulate rewarding and memory processes.

Background and purpose: Amphetamine use disorder is a serious health concern, but surprisingly little is known about the vulnerability to the moderate and compulsive use of this psychostimulant and its underlying mechanisms. Previous research showed that inherited serotonin transporter (SERT) down-regulation increases the motor response to cocaine, as well as moderate and compulsive intake of this psychostimulant. Here we sought to investigate whether these findings generalize to amphetamine and the underlying mechanisms in the nucleus accumbens. Experimental Approach: In serotonin transporter knockout (SERT−/−) and wild-type control (SERT+/+) rats we assessed the locomotor response to acute amphetamine (AMPH) and intravenous AMPH self-administration under short access (ShA: 1 hr daily sessions) and long access (LgA: 6 hr daily sessions) conditions. 24 hrs after AMPH self-administration we analysed the expression of glutamate system components in the nucleus accumbens shell and core. Key results: We found that SERT−/− animals displayed an increased AMPH-induced locomotor response and increased AMPH self-administration under LgA, but not ShA conditions. Further, we observed changes in the vesicular and glial glutamate transporters, NMDA and AMPA receptor subunits and their respective postsynaptic scaffolding proteins as function of serotonin transporter genotype, AMPH exposure (baseline, ShA and LgA) and nucleus accumbens sub region. Conclusion and implications: We demonstrate that SERT gene deletion increases the psychomotor and reinforcing effects of AMPH, and that the latter is potentially mediated, at least in part, by homeostatic changes in the glutamatergic synapse of the nucleus accumbens shell and/or core.