THE RELATIONSHIP BETWEEN SHOCK SEVERITY AND THE FORM OF THE ANTINOCICEPTION OBSERVED IN PENTOBARBITAL-ANESTHETIZED RATS

Three experiments are reported that explore the opioid/nonopioid natur e of environmentally induced antinociception in pentobarbital-anesthet ized rats. In Experiment 1, the minimum shock intensity and duration t hat elicits antinociception was established and two alternative descri ptions of the tradeoff between shock intensity and duration were evalu ated: the coulometric relationship (duration X intensity) and an exten sion of Stevens's power law (duration X intensity(3.5)). The results s upport the power law. In Experiments 2 and 3, the relationship between shock severity and the form of the antinociception elicited was exami ned. It was found that the least severe shock schedules produced a nal trexone-insensitive ''nonopioid'' antinociception. Increasing shock se verity elicited an ''opioid'' antinociception that was attenuated by n altrexone. When shock severity was increased further, the antinocicept ion again became naltrexone insensitive. Implications of the results a re discussed.