Tolerance to the effects of carbon monoxide on operant behavior in rats: Repeated brief exposures

Three rats were trained under a fixed-consecutive-number schedule of food reinforcement to make sequences of 20 or more consecutive responses on one lever followed by a single response on a second lever. During a 75-min exposure to 700 ppm carbon monoxide (CO), response rates for all subjects were severely decreased. Over five successive daily sessions of exposure, rates recovered completely. However, performing the behavioral task during CO exposure was not necessary for the development of tolerance. A control experiment in which four 75-min postsession exposures to CO preceded a single CO session showed that mere exposure to CO was sufficient for the development of tolerance to the behavioral effects of CO. Tolerance was not permanent since, after a CO-free period greater than two weeks, performance again was sensitive to CO.