Repeated cocaine treatment and withdrawal produces changes in brain function thought to be involved in relapse to drug use. Withdrawal from repeated cocaine reduced in vivo extracellular glutamate in the nucleus accumbens of rats by decreasing the exchange of extracellular cystine for intracellular glutamate. In vivo restoration of cystine/glutamate exchange by intracranial perfusion of cystine or systemically administered N-acetylcysteine normalized the levels of glutamate in cocaine-treated subjects. To determine if the reduction in nonvesicular glutamate release is a mediator of relapse, we examined cocaine-primed reinstatement of drug seeking after cocaine self-administration was stopped. Reinstatement was prevented by stimulating cystine/glutamate exchange with N-acetylcysteine and restoring extracellular glutamate. Thus, withdrawal from repeated cocaine increases susceptibility to relapse in part by reducing cystine/glutamate exchange, and restoring exchanger activity prevents cocaine-primed drug seeking.

Repeated cocaine causes enduring changes in dopamine and glutamate transmission in the nucleus accumbens, and dopamine and glutamate terminals synapse on GABAergic accumbens neurons. The present study demonstrates that there are changes in GABA transmission in the accumbens at 3 weeks after discontinuing daily cocaine injections. No-net flux microdialysis revealed a significant increase in the basal levels of extracellular GABA in the accumbens of cocaine-treated rats. The elevated extracellular GABA was normalized by blocking voltage-dependent Na+ channels and provided increased tone on GABA(B) presynaptic autoreceptors and heteroreceptors because blocking GABA(B) receptors produced a greater elevation in extracellular GABA, dopamine, and glutamate in cocaine-treated compared with control subjects. For many G-protein-coupled receptors, increased agonist can cause receptor desensitization. Consistent with GABA(B) receptor desensitization, baclofen-stimulated GTPgammaS binding was reduced, and the reduction in G-protein coupling was accompanied by reduced Ser phosphorylation of the GABA(B2) receptor subunit. No effect by repeated cocaine was found in the levels of total GABA(B1) or GABA(B2) protein. Together, these data demonstrate that withdrawal from repeated cocaine treatment produces an increase in the basal levels of extracellular GABA in the accumbens that depends on neuronal activity. The increase may be mediated in part by functional desensitization of GABA(B) receptors, likely the result of diminished Ser phosphorylation of the GABA(B2) receptor.

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Homer proteins form functional assemblies in the excitatory postsynaptic density, and withdrawal from repeated cocaine administration reduces the expression of Homer1b/c in the nucleus accumbens. To determine if the reduction in Homer1b/c may be contributing to cocaine-induced behavioural sensitization, antisense oligonucleotides were infused over two weeks into the nucleus accumbens of rats to reduce Homer1 gene expression by approximately 35%. Infusion of antisense sequences (AS1 and AS2) caused a sensitization-like augmentation in the motor response to acute cocaine administration in naive rats. One of the sequences (AS1) also prevented the development of sensitization to repeated cocaine treatment, while AS2 was without effect. A panel of immunoblots for other proteins in the excitatory postsynaptic density revealed that AS1, but not AS2 reduced the level of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunit GluR1 protein. This posed the possibility that altered AMPA signalling may mediate the inhibitory effect of AS1 on the development of sensitization. To examine this possibility, rats were pretreated in the accumbens with drugs to block AMPA/kainate, N-methyl-d-aspartate, group 1 metabotropic glutamate or dopamine receptors prior to each daily injection of cocaine. Only AMPA/kainate receptor blockade prevented the development of behavioural sensitization to cocaine. These data indicate that the expression of behavioural sensitization arises in part from a reduction in Homer1 gene products in the accumbens, while the development of sensitization requires stimulation of AMPA/kainate receptors.

Chronic cocaine administration reduces G protein signaling efficacy. Here, we report that the expression of AGS3, which binds to GialphaGDP and inhibits GDP dissociation, was upregulated in the prefrontal cortex (PFC) during late withdrawal from repeated cocaine administration. Increased AGS3 was mimicked in the PFC of drug-naive rats by microinjecting a peptide containing the Gialpha binding domain (GPR) of AGS3 fused to the cell permeability domain of HIV-Tat. Infusion of Tat-GPR mimicked the phenotype of chronic cocaine-treated rats by manifesting sensitized locomotor behavior and drug seeking and by increasing glutamate transmission in nucleus accumbens. By preventing cocaine withdrawal-induced AGS3 expression with antisense oligonucleotides, signaling through Gialpha was normalized, and both cocaine-induced relapse to drug seeking and locomotor sensitization were prevented. When antisense oligonucleotide infusion was discontinued, drug seeking and sensitization were restored. It is proposed that AGS3 gates the expression of cocaine-induced plasticity by regulating G protein signaling in the PFC.