Abstract

The experiments reported here examined the time course of the development and loss of tolerance to nicotine as well as the time course for the up-regulation and return to basal levels of brain nicotinic receptors. Nicotine was administered by continuous i.v. infusion through cannulae implanted in the right jugular veins of DBA mice. Tolerance to the effects of nicotine on Y-maze activity and rears, body temperature and heart rate was seen accompanying the infusion of 4 mg/kg/hr of nicotine. Maximal tolerance was obtained in 4 days and was paralleled by increases in the number of brain nicotine binding sites. The binding of alpha-bungarotoxin was also increased by chronic nicotine treatment but increases in binding preceded the development of detectable tolerance to the challenge dose of nicotine. Chronic nicotine treatment had no effect on quinuclidinyl benzilate binding. Tolerance to the effects of nicotine was lost at different rates for the various tests in that base-line response was attained for the Y-maze activity and rearings test in 8 days, whereas 12 to 16 days were required to regain control response for the body temperature test. Tolerance for the heart rate test persisted throughout the 20-day withdrawal period. Brain nicotine binding sites had returned to control levels by 8 days after treatment whereas the alpha-bungarotoxin binding sites were at control levels at the earliest time of postinfusion testing (4 days). Brain quinuclidinyl benzilate binding sites were unaffected by nicotine treatment and did not change during the withdrawal period. The changes in nicotine binding in the various brain regions during the onset and offset experiments were virtually identical. These changes in binding correlated highly with the acquisition and loss of tolerance to the effects of nicotine on the Y-maze locomotor activity and body temperature tests. The correlations between changes in nicotine binding and the Y-maze rearings and the heart rate tests were lower but some association is suggested. Although changes in nicotine binding may explain some of the tolerance observed, other explanations for tolerance must be sought as well.