Environmental influences on the development of tolerance to the effects of physostigmine on schedule-controlled...

The influence of environmental variables on the development of tolerance to physostigmine's effects in rats was examined using multiple fixed-ratio, extinction schedules of food presentation. Initial administration of physostigmine (0.4 mg/kg) produced nearly maximal decreases in the number of food pellets delivered, running response rate, and overall response rate, under multiple FR 10, EXT and multiple FR 50, EXT schedules. With repeated administration, tolerance to physostigmine's effects was observed when 10 responses were required to produce reinforcement but was not observed when 50 responses were required to produce reinforcement. Tolerance under the multiple FR 10, EXT schedule of reinforcement was also observed when physostigmine was administered post-session. When tolerance was acquired, it was retained for up to 25 drug-free days. These results suggest that tolerance to physostigmine's effects on schedule-controlled behavior is strongly influenced by response requirement, independently of physostigmine-induced reinforcement loss. Additionally, tolerance is not dependent on experience with the schedule while under the effects of physostigmine, and is retained for a substantial period of time in the absence of continued physostigmine administration.