eNeuro
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Individuals with chronic pain may be driven to smoke more because the analgesic efficacy of nicotine diminishes. To determine whether persistent pain diminishes the actions of a nicotinic acetylcholine receptor (nAChR) agonist in pain modulatory pathways, we examined the effects of epibatidine in the rostral ventromedial medulla (RVM) of rats with and without inflammatory injury induced by intraplantar injection of complete Freund's adjuvant (CFA). In uninjured rats, epibatidine produced a dose-dependent antinociception that was completely blocked by dihydro-β-erythroidine (DHβE; α4β2 antagonist) and partially blocked by methyllycaconitine (MLA; α7 antagonist). ⋯ Neither antagonist alone altered paw withdrawal latency in uninjured or CFA-treated rats, suggesting that neither α4β2 nor α7 nAChRs are tonically active in the RVM. The Bmax and Kd of α4β2 nAChRs in the RVM were unchanged after CFA treatment. These observations provide the first evidence of pharmacological plasticity of the actions of α4β2 nAChR agonists in a critical brainstem pain modulatory pathway and may in part explain why people with chronic pain smoke more than the general population.