Contextual cues associated with nicotine administration increase arc mRNA expression in corticolimbic areas of the rat brain

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Abstract
Conditioned responses to cues associated with the administration of drugs of misuse are an impediment to continued abstinence for drug‐free addicted individuals. In order to study the neuroanatomical and cellular response of the brain to cues associated with nicotine administration, we conditioned Sprague–Dawley rats to receive an ascending dose regimen of nicotine over 14 days in two distinct non‐home cage environments and assessed expression of the early response gene arc in corticolimbic areas in response to the nicotine‐associated context. All of the rats received the same dose regimen of nicotine. Three days after the last training day, the rats were exposed to the test environment. The rats that had previously received nicotine exhibited increased motor activity compared with the rats that had received saline in the test environment. After 45 min in the test environment, brains were taken for Northern blotting and in situ hybridization analysis, which revealed an increase in levels of activity‐regulated, dendritically localized mRNA for arc in a variety of brain regions (medial and lateral prefrontal cortices, cingulate cortex, primary sensory cortex, sensorimotor cortex, ventral striatum and amygdala). Plasma corticosterone levels were not different between the groups, suggesting that exposure to nicotine cues is insufficient to activate the hypothalamo‐pituitary‐adrenal axis. Given that Arc plays a direct role in neuronal plasticity and memory consolidation, its induction by nicotine‐associated cues in brain regions critical for cognitive and emotional processing suggests that rats may be learning that these cues are no longer necessarily predictive of nicotine administration. Further work will be needed in order to assess the role of arc expression in the extinction of conditioned responses to drug‐paired cues.