The Role of NMDA Receptors in Extinction of Cocaine Self-Administration

Relapse is highly prevalent among recovering addicts, and can be triggered by associations made between the rewarding effects of the drug and cues, such as drug paraphernalia or contexts. Inhibiting these associations, through new extinction learning, could help reduce relapse rates. Extinction is formed in phases, like other types of memory. The memory first is acquired in short-term memory, then is consolidated into long-term storage from which it can be retrieved at a later time (Quirk & Mueller, 2008). NMDA receptors are necessary for extinction in other paradigms (Santini, Muller, & Quirk, 2001), and we previously found that blocking NMDA receptors before four 45-minute extinction sessions disrupts retention when tested during a subsequent full extinction session. However, it is unclear which learning phase was disrupted. To determine how NMDA receptors affect extinction of cocaine self-administration, rats were trained to lever press for i.v. infusions of cocaine. Following training, rats underwent four 45-minute extinction sessions and received post-extinction session systemic injections of either the NMDA receptor antagonist CPP or the NMDA receptor coagonist D-serine. I hypothesized that CPP would disrupt extinction learning, and D-serine would facilitate extinction learning. Post-extinction session injections of CPP did not disrupt the consolidation of extinction, but instead appears to facilitate extinction. Alternative explanations such as pharmacological side effects or disrupted reconsolidation could explain these results. Post-session injections of D-serine facilitated consolidation of extinction compared to controls. These studies indicate that NMDA receptors are necessary for acquisition and maybe consolidation of extinction, and potentiating NMDA receptors will facilitate extinction learning. Additionally, western blotting was conducted on ventral medial prefrontal cortex (vmPFC) and nucleus accumbens (NAc) tissue to determine glutamate receptor expression following extinction of cocaine self-administration, withdrawal from cocaine, or extinction of sucrose reinforcement. Results indicated a trend for increased NR2B-containing NMDA receptor expression in the vmPFC after extinction of sucrose reinforcement and increased expression of NR2A- and NR2B-containing NMDA receptors in the NAc following cocaine withdrawal compared to expression following extinction of sucrose reinforcement. Thus, these studies indicate that cocaine use and extinction learning can induce changes with NMDA receptors in the NAc and possibly vmPFC

The Mechanisms Underlying Cocaine-Induced Overexpression of Basic Fibroblast Growth Factor (bFGF, FGF2), an Effect Reversed By Extinction

Drug addiction is characterized by compulsive drug use and chronic relapse despite negative consequences. Drug-induced structural and functional changes in the brain are thought to underlie these characteristics. One mechanism that may mediate these characteristics are growth factors, such as basic fibroblast growth factor (bFGF or FGF2), as they are necessary for cellular growth, survival, differentiation, and have roles in memory, mood, and anxiety disorders. bFGF mRNA and protein expression is increased following stimulant administration and is necessary for stimulant-induced changes in dendrites and behavioral sensitization. Moreover, addiction is maintained by cues associated with the drug, as they can can evoke craving and promote relapse. Therefore, reducing cue reactivity, such as with extinction, could reduce relapse rates. Inhibiting bFGF in the infralimbic medial prefrontal cortex (IL-mPFC), following self-administration, facilitates extinction. Extinction of drug seeking can reduce bFGF expression in IL-mPFC, nucleus accumbens (NAc), and dorsal hippocampus (dHipp), indicating that bFGF may mediate drug-associated learning. However, the circuitry and mechanisms underlying extinction and the role of bFGF is unknown. Therefore, the current experiments investigated if drug-induced plasticity was altered by extinction and if bFGF had a role, in brain regions associated with learning or addiction (i.e., IL-mPFC, NAc, and dHipp). We found that cocaine self-administration induced changes in plasticity-related protein expression, such as ARC and pGSK3beta, in each brain region, and extinction could ameliorate some of that plasticity. Moreover, we found that neutralizing bFGF in NAc prior to four 30-min extinction sessions disrupted initial extinction retention. However, if bFGF was neutralized without four 30-min extinction sessions, subsequent extinction was enhanced. In dHipp, neutralizing bFGF, with or without four 30-min extinction sessions, facilitated subsequent extinction over days. Overall, these results suggest that blocking the biological function of bFGF in a number of reward- and learning-related brain regions can facilitate subsequent extinction. Understanding the neuronal mechanisms by which bFGF regulates extinction at systems and molecular levels will allow for development of new pharmacotherapeutics to enhance extinction-based therapies for addiction

Infralimbic NR2A-containing NMDA receptors are necessary for the reconsolidation of cocaine self-administration memory.

Addiction is characterized by high susceptibility to relapse, which can be triggered by drug-associated cues. Cue presentation results in retrieval of the original drug-cue memory that becomes labile and must be reconsolidated back into long-term storage. Repeated unpaired cue presentation, however, induces extinction. Thus, cue-reactivity can be reduced by either blocking reconsolidation or facilitating extinction. Previous research revealed that systemic blockade of NMDA receptors (NMDArs) can disrupt reconsolidation of drug-cue associations in a modified self-administration paradigm (Milton et al., 2008) or extinction in a standard self-administration paradigm (Hafenbreidel et al., 2014). To further characterize these processes, we examined the effects of post-extinction injections of an NMDAr antagonist on drug seeking following self-administration. Rats acquired cocaine self-administration (0.25 mg/inf, i.v., 90 min/day) followed by extinction. Extinction consisted of four 45-min extinction sessions in which rats were administered the NMDAr antagonist CPP (10 mg/kg, i.p.) immediately after each session. Extinction retention was then tested during a subsequent 90-min session. CPP treatment decreased lever pressing during subsequent extinction sessions, suggesting either disrupted reconsolidation or facilitated extinction consolidation. We next targeted the infralimbic medial prefrontal cortex (IL-mPFC), a structure implicated in extinction (Quirk & Mueller, 2008). Using the same procedure, CPP infusions (36 µg/ 0.3 µL) before or after four brief extinction sessions resulted in a similar reduction in lever pressing across subsequent days. To determine the NMDAr subtype involved, we infused either the NR2A-selective antagonist NVP (1 mg/ 0.3 mL) or the NR2B-selective antagonist Ro25 (2 mg/ 0.3 mL) after four 45-min extinction sessions. Similar to the effects of nonspecific NMDAr blockade, blocking NR2A- but not NR2B-containing NMDArs reduced lever pressing across subsequent days. Finally, to dissociate if blocking NR2A-containing NMDArs disrupts reconsolidation or facilitates extinction consolidation, NVP was infused into the IL-mPFC after four 10-min reactivation trials or in the absence of behavioral testing. Memory retention was then tested during a subsequent 90-min session, revealing that blocking NR2A-containing NMDArs after memory reactivation results in reduced lever pressing. Overall, these results indicate that blocking NR2A-containing NMDArs in the IL-mPFC disrupts reconsolidation following reactivation of the original drug-cue memory rather than facilitates extinction consolidation. Support by DA027870 and the University of Wisconsin-Milwaukee Research Growth Initiativ