NONANESTHETICS CAN SUPPRESS LEARNING

Nonanesthetic gases or vapors do not abolish movement in response to n oxious stimuli despite partial pressures and affinities for lipids tha t would, according to the Meyer-Overton hypothesis, predict such aboli tion. We investigated whether nonanesthetics depress learning and memo ry (i.e., provide amnesia). To define learning, we used a ''fear-poten tiated startle paradigm'': rats trained to associate light with a noxi ous stimulus (footshock) will startle more, as measured by an accelero meter, when a startle-eliciting stimulus (e.g., a noise) is paired wit h light than when the startle-eliciting stimulus is presented alone. W e imposed light-shock pairings on 98 rats under three conditions: no a nesthesia (control); 0.20, 0.29, and 0.38 times the minimum alveolar a nesthetic concentration (MAC) of desflurane; or two nonanesthetics (1, 2-dichloroperfluorocyclobutane and perfluoropentane) at partial pressu res predicted from their lipid solubilities to be between 0.2 and 1 MA C. Desflurane produced a dose-related depression of learning with abol ition of learning at 0.28 MAC. Perfluoropentane at 0.2-predicted MAC h ad the same effect as 0.28 MAC desflurane. 1,2-Dichloroperfluorocyclob utane at 0.5- to 1-predicted MAC abolished learning. Because nonanesth etics suppress learning but not movement (the two critical components of anesthesia), they may prove useful in discriminating between mechan isms and sites of action of anesthetics.