Prolonged antinociception following carbon dioxide anesthesia in the laboratory rat

In the laboratory rat, inhalation (30 s) of high (> 70%) CO 2 concentrations resulted in short-term (1–3 min) anesthesia, followed by a prolonged (up to 60 mn) mild antinociception. Exposure to 100% CO 2 resulted in significant thermal (hot-plate, 52°, and tail-flick) and mechanical (tail-pinch, 886 g force) antinociception. Control animals, placed in the same chamber filled with air, showed no such effects. Rats exposed to 70% CO 2 exhibited effects on the hot plate comparable to those seen after inhalation of 100% CO 2, indicating that the response is not due to CO 2-induced hypoxia. Additionally, recovery from halothane-induced anesthesia of comparable duration did not result in antinociception, confirming that anesthesia alone is not sufficient to produce the effect. Pretreatment with the opiate antagonist naltrexone (0.1–10 mg/kg i.p.) did not diminish the CO 2-induced antinociception, suggesting that endogenous opioids are not obligatory in the mechanism of this response. Furthermore, hypophysectomy abolished hot-plate antinociception in animals exposed to 100% CO 2 while sham-treated controls exhibited a pattern of hot-plate responses similar to that reported above. Taken together, these findings show that: (1) recovery from CO 2-induced anesthesia results in a prolonged mild antinociception, detectable with thermal and mechanical nociceptive tests; and (2) this response may represent a novel form of environmentally induced antinociception, mediated by a non-opiate hormonal substance.