Effects of opioid receptor blockade on the renewal of alcohol seeking induced by context: relationship to c-fos mRNA expression.

Contextual stimuli associated with alcohol availability can induce alcohol seeking during abstinence. Using a renewal procedure, we tested the effect of opioid receptor blockade on context-induced alcohol seeking, and its relationship to the activity of brain sites involved in learning and reward. Thirty-six male Wistar rats were trained to lever press for a 12% (w/v) alcohol solution before undergoing extinction sessions (no alcohol delivery). Half of the rats underwent training, extinction and testing in a single context with a distinct set of olfactory, visual, auditory and tactile properties [training, extinction and test in Context A (AAA)]. The other half were trained and extinguished in different contexts and returned to the training context on the test day [training, extinction and test in Contexts A, B and A, respectively (ABA)]. On the test day, the rats from each condition were pre-treated with either saline or 1 mg/kg naltrexone (s.c.) and tested for alcohol seeking. Immediately following the test session, rats were killed and their brains were analysed for c-fos mRNA expression using in-situ hybridization. Re-exposure to the alcohol-associated context (ABA) significantly increased operant behaviour on the previously active lever relative to the AAA groups and this increased responding was associated with increased c-fos mRNA expression in the basal and lateral amygdala and the CA3 subregion of the hippocampus. Naltrexone pre-treatment attenuated context-induced alcohol seeking and inhibited c-fos mRNA expression in the lateral amygdala and CA3. Our findings point to a critical role for the basolateral amygdala and dorsal hippocampus in mediating context-induced renewal of alcohol seeking and suggest that opioidergic mechanisms mediate this effect.