The use of the immobility reflex (animal hypnosis) as a possible procedure for detecting sedative activity

The immobility reflex was induced in rabbits and the amount of electrical current necessary to interrupt this state was determined before and after drug administration. Morphine and the major tranquilizers, chlorpromazine, haloperidol, and reserpine, elevated the arousal threshold over a wide dose range while the minor tranquilizers, chlordiazepoxide and meprobamate, were active over a considerably narrower range. The sedative-hypnotics, phenobarbital, pentobarbital, ethanol, chloral hydrate, and ectylurea demonstrated approximately the same degree of activity as the minor tranquilizers but resulted in a loss of righting reflex at the higher doses. D-Amphetamine significantly lowered the arousal threshold. Imipramine, desmethylimipramine, nialamide, aspirin, diphenylhydantoin, chlorpheniramine, and diphenhydramine possessed little, if any, activity. Thus, the drug-induced changes in arousal threshold in rabbits exhibiting the immobility reflex can be used to classify compounds possessing major or minor tranquilizing or sedative-hypnotic activity.