Background: Rats with extended daily cocaine access show escalating cocaine self-administration and behavioral signs of dependence. Regulation of glutamatergic transmission by metabotropic glutamate receptors has emerged as a mechanism in the addictive actions of drugs of abuse. We examined here whether neuroadaptive dysregulation of metabotropic glutamate receptor function is a factor in escalating cocaine self-administration.
Methods: Rats with 1 hour daily cocaine access (short access [ShA]) versus 6-hour access (long access [LgA]) were tested for differences in the effects of the metabotropic glutamate receptor 2/3 (mGluR2/3) agonist (-)-2-oxa-4-aminobicylco(3.1.0)hexane-4,6-dicarboxylic acid (LY379268) and the metabotropic glutamate receptor 5 (mGluR5) antagonist 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]-pyridine (MTEP) on cocaine-reinforced progressive-ratio responding and differences in expression levels and functional activity of mGluR2/3 and mGluR5.
Results: The LgA groups showed higher progressive-ratio breakpoints than ShA groups. LY379268 (0-3 mg/kg subcutaneous) dose-dependently lowered breakpoints in the LgA group but reduced breakpoints only at 3 mg/kg in the ShA group. Consistent with this behavioral effect, functional mGluR2/3 activity was significantly elevated following LgA cocaine exposure. MTEP (0-3 mg/kg intraperitoneal) reduced breakpoints in the ShA group only. Long access cocaine exposure was associated with decreased mGluR5 expression, accompanied by reduced functional mGluR5 activity in the nucleus accumbens. A downward trend developed in mGluR5 protein expression in the medial prefrontal cortex and hippocampus.
Conclusions: Functional upregulation of mGluR2/3 and downregulation of mGluR5 are likely factors in the transition to cocaine dependence. The differential behavioral effects of LY379268 and MTEP in rats with a history of long access to cocaine have implications for the treatment target potential of mGluR2/3 and mGluR5.
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