Genotype influences the development of tolerance to nicotine in the mouse

Previous studies have demonstrated that chronic infusion of DBA/2 inbred mice with nicotine results in a dose- and time-dependent tolerance to many of the effects elicited by a challenge dose of nicotine. This tolerance was paralleled by increases in the number of brain nicotinic receptors measured with L-[3H]nicotine and alpha-[125I]bungarotoxin binding, suggesting that receptor up-regulation may underlie tolerance to nicotine. The studies reported here assessed the development of tolerance to nicotine in five inbred mouse strains that differ markedly in sensitivity to acute doses of nicotine. Mice were infused with nicotine doses ranging between 0 and 6 mg/kg/hr for 10 days and were tested for sensitivity to nicotine using several tests 2 hr after infusion was stopped. Some mouse strains, such as the C57BL/6, developed tolerance to nicotine, as measured by shifts to the right of dose-response curves, at the lowest infusion doses, whereas other strains, such as the C3H/2 and BUB, did not develop measurable tolerance until the highest infusion doses were used. A correlation between sensitivity to acute nicotine treatment and the threshold for tolerance development was observed, suggesting that tolerance develops only after a physiological effect is elicited. All of the mouse strains exhibited a dose-dependent increase in nicotine binding in all brain regions; no marked strain differences were seen in up-regulation of this binding site. Chronic nicotine infusion also evoked increases in alpha-bungarotoxin binding, but higher doses were required to elicit this effect. Changes in this binding site were observed after treatment with nicotine doses that elicited tolerance in those mouse strains that are more resistant to an acute dose of nicotine. These results indicate that the relationship between tolerance development and brain nicotinic receptor up-regulation may not be simple.