Reversal of AMPA receptor plasticity underlying incubation of cocaine craving

Rosalind Franklin Univ Of Medicine & Sci, North Chicago IL

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Abstract

There is an urgent need for pharmacological agents that can prevent cue-induced relapse in abstinent cocaine users. In several rat models, cocaine seeking requires glutamate-mediated excitation of medium spiny neurons (MSN) of the nucleus accumbens (NAc). In rats that are drug-naïve or have had limited cocaine exposure, this excitation is mediated primarily by Ca2+-impermeable AMPA receptors (CI-AMPARs). After extended access cocaine self-administration (SA) and withdrawal, cue-induced craving intensifies (“incubates”). We have shown that the expression of incubated cue-induced craving is mediated by Ca2+-permeable AMPARs (CP-AMPARs), which have higher conductance than CI-AMPARs and therefore enable stronger excitation of MSN. The objective of this proposal is to determine if group I metabotropic glutamate receptors (mGluRs), which are expressed postsynaptically in MSN, can be targeted to remove CP-AMPARs from NAc synapses of “incubated rats”. The central hypothesis is that group I mGluRs normally exert an inhibitory influence on CP-AMPAR levels in NAc synapses, and that decreased group I mGluR signaling during withdrawal permits CP-AMPAR accumulation, whereas stimulating group I mGluRs will remove CP-AMPARs from the NAc of “incubated rats” and thus reduce craving. This hypothesis, formulated based on preliminary data showing that group I mGluR stimulation removes CP-AMPARs from NAc synapses in slices and cultures, will be tested by pursuing 3 Aims: 1) Determine the effect of group I mGluR activation and blockade on CP-AMPAR levels in NAc MSN in primary culture. Cultured MSN express CP-AMPARs, so this model system can be used to study acute and tonic regulation of CP-AMPAR trafficking by group I mGluRs. 2) Determine the effect of acute group I mGluR activation on AMPAR transmission in the NAc and the expression of cue-induced cocaine seeking. Slice recordings and behavioral studies in “incubated rats” will determine if acute group I mGluR stimulation removes CP-AMPARs from NAc synapses and reduces craving. 3) Measure group I mGluR surface expression and Homer levels in the NAc after cocaine withdrawal and determine if increasing group I mGluR signaling during withdrawal prevents CP-AMPAR accumulation and incubation. Homers are scaffolding proteins that can mediate group 1 mGluR signaling. Biochemical and behavioral experiments will test the hypothesis that group I mGluR surface expression, levels of long Homer isoforms, and/or mGluR-Homer interactions decrease in the NAc after extended access cocaine SA, permitting accumulation of CP-AMPARs, and that restoring group I mGluR signaling during withdrawal will prevent CP-AMPAR accumulation and incubation of cocaine craving. Our research plan is innovative because it challenges dogma by exploring the therapeutic utility, for addiction, of activating rather than blocking group I mGluRs and because it explores a novel form of synaptic plasticity in the NAc. The significance of our studies is that we expect them to identify a target that can be attacked with existing drugs (group I mGluR positive allosteric modulators) to reduce the risk of relapse during abstinence.