The impact of mPFC glutamate on methamphetamine extinction and VTA dopamine neurons

Drug abuse accounts for a serious social problem in modern society, characterized by frequent reinstatement even patients accomplish rehabilitation program. Though the development of effective extinction program to eliminate the adverse memory trace is attempted recently, in particular the PTSD, the progress on management of abused drug relapse is severely hampered. To understand the molecular mechanism of methamphetamine (METH) extinction, we established an animal model of conditioned place preference (CPP) paired with METH (2 mg/kg) and demonstrated mice could distinguish METH-CPP after extinction training and reinstate CPP after stress. Among these behaviorally distinguished stages, we found that turn-over rate of dopamine (DOPAC/DA) in the nucleus accumbens enhanced during acquisition and relapse, but decreased at extinction. Similarly, levels of accumbal phosphor-TH/Ser40 increased in acquisition, decreased in extinction and re-activated after stress-induced drug relapse. As comparison, levels of phospho-TH/Ser19 and total amount of TH in the nucleus accumbens remained unchanged across those behavioral stages. On the other hand, ERK1/2 signals in ventral tegmental area (VTA) displayed a different pattern with a significant increase in phospho-ERK1/2 during extinction. In addition, it appears that branches/numbers of dendrite in the VTA neurons increased after METH-CPP extinction, as compared to acquisition and control mice. Since glutamate projection from prefrontal cortex to amygdala plays a significant role in PTSD extinction, we intend to test if prefrontal cortex that innervates the VTA would determine the METH-CPP extinction. To this end, experiments of infralimbic cortex lesion on development of METH-CPP extinction are under investigation. Further, due to VTA innervated massively the glutamate projections from prefrontal cortex, hippocampus and amygdala, the amount of glutamate before and after the lesion will be monitored after extinction training. Introduction Methamphetamine Methamphetamine is a psychostimulant with structure relates to amphetamine differing in a methyl group. The special public health concern with methamphetamine is this compound can cause severe loss of social welfares than other abused drugs, because abusers can rapidly build up a high body concentration that leads to high incidence of drug addiction and toxicities. Methamphetamine is a broad-action addictive drug; it could influence various neurotransmitter systems in both brain and peripheral tissues. The primary target of methamphetamine is the dopamine transporter; but also affects the norepinephrine and serotonin transporters . Methamphetamine could get through dopamine transporter to enter into dopamine terminals and force dopamine to evacuate from the vehicle, eventually increase the synaptic dopamine concentration. Long-term and escalated synaptic dopamine increase could target post-synaptic dopamine receptors, in turns induce behavioral sensitization and drug-craving . Relapse and animal model Drug abuse can be considered as a chronic brain disease since the synaptic plasticity would be reformed after chronic drug-taking, and it is known that subjects once reinforced to drug-taking is hardly withdrawal. However, drug withdrawal is not a fundamental solution to treat drug-abuse. The most severe follow-up problem of addiction is drug relapse, since neuronal circuitry has already been modified by the abused drug that makes drug reinstatement easily to be triggered. To date, due to the limit of research on drug addicts, most researches apply various animal models to mimic the drug-seeking and craving behaviors, in addition to study the reinstatement. In literature reviews, there are two major experimental paradigms to measure drug-relapse directly, i.e. the self-administration and the conditioned place preference. Of the essential operation parameters, establishing the connection between drug-produced euphoria and conditioned stimuli is pre-requisite in both animal models. Hence, animals will be first trained to connect non-contingent stimuli (cue, such as noise, light or smell) or contingent stimuli (specific environment or behavior, such as press level in self-administration) with unconditioned stimuli (drug-induced euphoria). After training, successfully learned animals would display ability to associate the reward (euphoria) with conditioned stimuli. Even when memory eliminated after subsequent extinction training, the contingent stimuli would still be a reliable trigger to induce relapse behavior. The phenomenon of reinstatement of learned behaviors after extinction is described in the early classical and operant conditioning studies of Pavlov and Skinner in the last century. Pavlov described the “priming effect” of re-exposure to the unconditioned stimulus after extinction in dogs trained in a classical conditioning study, and wrote that “the restoration of an extinguished reflex is greatly accelerated by a fresh application of the unconditioned stimulus’’(reviewed in Shaham et al., 2003). Using an operant paradigm, Skinner and others reported reinstatement of lever pressing in rats after extinction training by non-contingent presentations of food or water. Recently, several laboratories have developed a reinstatement procedure that is based on the conditioned place preference (CPP) model. In this procedure, animals are initially trained to associate one compartment of a choice apparatus with drug injections and a second compartment with injections of the vehicle. Following daily training, rats are given a choice between the two compartments similar to the first drug-free testing day, and will display an altered preference in the drug-paired compartment (defined as a conditioned place preference). Then, during an extinction phase, the acquired preference for the drug-paired chamber is extinguished by pairing injections of saline with both compartments (drugand vehicle-paired). Following a test for extinction, reinstatement of the CPP will appear after a drug injection or exposure to non-drug stimuli.