Effects of changing dosage and urinary pH in rats self-administering nicotine on a food delivery schedule

The effects of different available dosage and of acidic and alkaline urinary pH have been investigated on the rates of self-administration of nicotine by rats on an FT60 food delivery schedule. Different groups of rats initially received one of 3 doses of nicotine (0.05, 0.1 and 0.25 mg/kg/infusion) contingent upon bar-pressing. The self-administration rates during an initial 6-day period of the 3 groups of rats were significantly different from each other, 26.1 ± 3.2 SEM), 15.4 ± 1.5 and 9.5 ± 0.9 at doses of 0.05, 0.1 and 0.25 mg/kg/infusion, respectively. However, once the rates of responding were established during the initial period, no significant changes occured when the doses were changed in all 3 groups after each subsequent 6-day period. These rates of self-administration decreased when saline replaced the available nicotine solution after Day 18. The urinary pH of groups of rats was maintained alkaline (pH9.0), acidic (pH 5.9) or normal (pH 6.7) by allowing them to drink sodium bicarbonate solution, ammonium chloride solution or water, respectively. The self-administration rates during the initial periods of these 3 groups of rats were also significantly different from each other 4.7 ± 0.66, 17.0 ± 0.76 and 9.4 ± 1.11, respectively). In contrast, however, when the rates of responding were established at normal urinary pH during the initial period when water was available, no significant changes occurred when urinary pH was subsequently changed in either an acidic or alkaline direction. The results suggest that the bar-pressing rates are dependent on the amount of nicotine available or present in plasma during the acquisition phase. Nevertheless, once the rate of bar-pressing is established on a food delivery schedule, it seems that the schedule exerts too powerful an effect on behavior for subsequent changes in nicotine levels to modify responding over the period of these experiments.