Antinociception induced by chronic exposure of rats to cigarette smoke.

To investigate if chronic exposure to cigarette smoke induces analgesia, rats were exposed to concentrated cigarette smoke in an environmental chamber over four successive 5-day blocks (6 h/day), with 2 smoke-free days between blocks. A control group was exposed to room air. Tail flick latencies increased significantly (analgesia) during each smoke exposure block, with a relative decline in analgesia across blocks (tolerance) and a return to control levels during the first three smoke-free interludes while remaining higher after the conclusion of the 4-week exposure period. Mechanical (von Frey) withdrawal thresholds declined over time in smoke-exposed and control groups, with the smoke-exposed group showing significantly lower thresholds. Plasma nicotine reached 95.4 +/- 32 (S.D.) ng/ml at the end of weekly smoke exposure and declined to 44.9 +/- 10.6 ng/ml 24 h after withdrawal. Rats lost weight during smoke exposure and quickly regained weight during smoke-free interludes and at the cessation of smoke exposure. Analgesia may contribute to the initiation of smoking, and rapid reversal of the analgesic effect following acute exposure may contribute to the difficulty in quitting smoking.