Unexpected Loss of Sensitivity to the nAChR Antagonist Activity of Mecamylamine and DHβEin Nicotine-Tolerant C57BL/6J Mice
There has always been interest in developing nAChR antagonists as smoking cessation aids, to add to nAChR agonists (e.g., nicotine replacement) already used for that indication. Previous studies have demonstrated that daily nicotine treatment confers tolerance to some of the effects of nicotine, as well as cross-tolerance to other nAChR agonists. The current study assessed the extent to which antagonism of nicotine varies as a function of daily nicotine treatment. The rate-decreasing and hypothermic effects of nicotine, as well as antagonism of those effects, were examined in C57BL/6J mice before, during treatment with, and after discontinuation of three daily injections of 1.78 mg/kg nicotine. The nonselective nAChR antagonist mecamylamine and the {beta}2 nAChR antagonist DH{beta}E were studied in combination with nicotine. The ED50 values of nicotine to produce rate-decreasing and hypothermic effects were, respectively, 0.44 and 0.82 mg/kg prior, 1.6 and 3.2 mg/kg during, and 0.74 and 1.1 mg/kg after discontinuation of daily nicotine treatment. Prior to daily nicotine treatment, mecamylamine decreased response rate and rectal temperature; however, during daily nicotine, mecamylamine (up to 5.6 mg/kg) only decreased rectal temperature. DH{beta}E (up to 5.6 mg/kg) when studied prior to daily nicotine decreased rectal temperature, but that decrease was abolished during chronic nicotine treatment. Mecamylamine and DH{beta}E antagonized the rate-decreasing and hypothermic effects of nicotine before and after daily nicotine; however, during daily nicotine, mecamylamine and DH{beta}E antagonized only the hypothermic effects of nicotine. The differential antagonism of rate-decreasing and hypothermic effects implicates differential involvement of nAChR subtypes. The decreased capacity of mecamylamine and DH{beta}E to antagonize nicotine during chronic nicotine treatment may indicate that their effectiveness as smoking cessations might vary as a function of nicotine tolerance and dependence.