Anomalies in prefrontal cortex (PFC) function are posited to underpin difficulties in learning to suppress drug-seeking behavior during abstinence. As Group1 metabotropic glutamate receptors (mGluRs) regulate drug-related learning, we assayed the consequences of extended access to intravenous cocaine (6 hrs/day; 0.25 mg/infusion for 10 days) upon the PFC expression of Group1 mGluRs and the relevance of observed changes for cocaine-seeking. Following protracted withdrawal, cocaine-experienced animals exhibited a time-dependent intensification of cue-induced cocaine-seeking behavior and an impaired extinction of this behavior. These behavioral phenomena were associated with a time-dependent reduction in mGluR1/5 expression within ventromedial PFC (vmPFC) of cocaine-experienced animals exposed to extinction testing, but not in untested ones. Interestingly, pharmacological manipulations of vmPFC mGluR1/5 produced no immediate effects upon cue-induced cocaine-seeking behavior, but produced residual effects on a subsequent test for cocaine-seeking. At 3 days withdrawal, cocaine-experienced rats infused intra-vmPFC with mGluR1/5 antagonists, either before or after an initial test for cocaine-seeking, persisted in their cocaine-seeking akin to cocaine-experienced rats in protracted withdrawal. Conversely, cocaine-experienced rats infused with an mGluR1/5 agonist before the initial test for cocaine-seeking at 30 days withdrawal exhibited a facilitation of extinction learning. These data indicate that cue-elicited deficits in vmPFC Group1 mGluR function mediate resistance to extinction during protracted withdrawal from a history of extensive cocaine self-administration and pose pharmacological stimulation of these receptors as a potential approach to facilitate learned suppression of drug-seeking behavior which may aid drug abstinence.

Methamphetamine is a highly addictive psychomotor stimulant yet the neurobiological consequences of methamphetamine self-administration remain under-characterized. Thus, we employed microdialysis in rats trained to self-administer intravenous (IV) infusions of methamphetamine (METH-SA) or saline (SAL) and a group of rats receiving non-contingent IV infusions of methamphetamine (METH-NC) at 1 or 21 days withdrawal to determine the dopamine and glutamate responses in the nucleus accumbens (NAC) to a 2 mg/kg methamphetamine intraperitoneal challenge. Furthermore, basal NAC extracellular glutamate content was assessed employing no net-flux procedures in these three groups at both time points. At both 1-and 21-day withdrawal points, methamphetamine elicited a rise in extracellular dopamine in SAL animals and this effect was sensitized in METH-NC rats. However, METH-SA animals showed a much greater sensitized dopamine response to the drug challenge compared with the other groups. Additionally, acute methamphetamine decreased extracellular glutamate in both SAL and METH-NC animals at both time-points. In contrast, METH-SA rats exhibited a modest and delayed rise in glutamate at 1-day withdrawal and this rise was sensitized at 21 days withdrawal. Finally, no net-flux microdialysis revealed elevated basal glutamate and increased extraction fraction at both withdrawal time-points in METH-SA rats. Although METH-NC rats exhibited no change in the glutamate extraction fraction, they exhibited a time-dependent elevation in basal glutamate levels. These data illustrate for the first time that a history of methamphetamine self-administration produces enduring changes in NAC neurotransmission and that non-pharmacological factors have a critical role in the expression of these methamphetamine-induced neurochemical adaptations.

Withdrawal from a history of extended access to self-administered cocaine produces a time-dependent intensification of drug-seeking, which might relate to a cocaine-induced imbalance in the relative expression of constitutively expressed Homer1 versus Homer2 isoforms within the ventromedial aspect of the prefrontal cortex (vmPFC). Thus, we employed immunoblotting to examine the relation between cue-reinforced lever-pressing at 3 versus 30 days withdrawal from a 10-day history of extended access (6 hrs/day) to intravenous cocaine (0.25 mg/infusion) or saline (Sal6h) and the expression of Homer1b/c and Homer2a/b within the vmPFC versus the more dorsomedial aspect of this structure (dmPFC). Behavioral studies employed adeno-associated viral vectors (AAVs) to reverse cocaine-elicited changes in the relative expression of Homer1 vs. Homer2 isoforms and tested animals for cocaine prime-, and cue-induced responding following extinction training. Cocaine self-administration elevated both Homer1b/c and Homer2a/b levels within the vmPFC at 3 days withdrawal and the rise in Homer2a/b persisted for at least 30 days. dmPFC Homer levels did not change as a function of self-administration history. Reversing the relative increase in Homer2 versus Homer1 expression via Homer1c over-expression or Homer2b knock-down failed to influence cue-reinforced lever-pressing when animals were tested in a drug-free state, but both AAV treatments prevented cocaine-primed reinstatement of lever-pressing behavior. These data suggest that a cocaine-elicited imbalance in the relative expression of constitutively expressed Homer2 versus Homer1 within the vmPFC is necessary for the capacity of cocaine to reinstate drug-seeking behavior, posing drug-induced changes in vmPFC Homer expression as a molecular trigger contributing to drug-elicited relapse.

In individuals with a history of drug-taking, the capacity of drug-associated cues to elicit indices of drug-craving intensifies or incubates with the passage of time during drug abstinence. This incubation of cocaine-craving, as well as difficulties with learning to suppress drug-seeking behavior during protracted withdrawal, are associated with a time-dependent deregulation of ventromedial prefrontal cortex (vmPFC) function. As the molecular bases for cocaine-related vmPFC deregulation remain elusive, the present study assayed the consequences of extended access to intravenous cocaine (6 h/d; 0.25 mg/infusion for 10 d) on the activational state of protein kinase C epsilon (PKCε), an enzyme highly implicated in drug-induced neuroplasticity. The opportunity to engage in cocaine-seeking during cocaine abstinence time-dependently altered PKCε phosphorylation within vmPFC, with reduced and increased p-PKCε expression observed in early (3 days) and protracted (30 days) withdrawal, respectively. This effect was more robust within the ventromedial versus dorsomedial PFC, was not observed in comparable cocaine-experienced rats not tested for drug-seeking behavior and was distinct from the rise in phosphorylated extracellular signal-regulated kinase (ERK) observed in cocaine-seeking rats. Further, the impact of inhibiting PKCε translocation within the vmPFC using TAT infusion proteins upon cue-elicited responding was determined and inhibition coinciding with the period of testing attenuated cocaine-seeking behavior, with an effect also apparent the next day. In contrast, inhibitor pretreatment prior to testing during early withdrawal was without effect. Thus, a history of excessive cocaine-taking influences the cue-reactivity of important intracellular signaling molecules within the vmPFC, with PKCε playing a critical role in the manifestation of cue-elicited cocaine-seeking during protracted drug withdrawal.

Previous studies have shown that brief access to cocaine yields an increase in D2 receptor binding in the medial prefrontal cortex (mPFC), but that extended access to cocaine results in normalized binding of D2 receptors (i.e. the D2 binding returned to control levels). Extended access conditions have also been shown to produce increased expression of the NR2 subunit of the NMDA receptor in the mPFC. These results implicate disrupted glutamate and dopamine function within this area. Therefore, in the present study, we monitored glutamate and dopamine content within the mPFC during, or 24 hrs after, cocaine self-administration, in animals that experienced various amounts of exposure to the drug. Naïve subjects showed decreased glutamate, and increased dopamine, levels within the mPFC during cocaine self-administration. Exposure to 7 lhr daily cocaine self-administration sessions did not alter the response to self-administered cocaine, but resulted in decreased basal dopamine levels. While exposure to 17 lhr sessions also resulted in reduced basal dopamine levels, these animals showed increased dopaminergic, but completely diminished glutamatergic, response to self-administered cocaine. Finally, exposure to 17 cocaine self-administration sessions, the last ten of which being 6h sessions, resulted in diminished glutamatergic response to self-administered cocaine and reduced basal glutamate levels within the mPFC, while normalizing (i.e. causing a return to control levels) both the dopaminergic response to self-administered cocaine as well as basal dopamine levels within this area. These data demonstrate directly that the transition to escalated cocaine use involves progressive changes in dopamine and glutamate function within the mPFC.