Use of the immobility reflex ("animal hypnosis") in neuropharmacological studies

The immobility reflex (IR), a reversible, involuntary, immobility response in certain species is advocated as a uniquely useful assay system for testing of psychoactive drugs. One of the two potential areas of application is that measures of IR duration or arousal threshold serve to screen drugs to help establish drug classification, relative potency, and degree of extrapyramidal side effects. Drugs can also be tested for their neural target sites and modes of action of recording electrographic responses in various brain areas during IR. Electrographic activity (EEG, averaged evoked responses, multiple-unit activity) is relatively stable, artifact-free, and less influenced by behavioral feedback and other variables that are problems with alternative experimental preparations. The reversibility of the IR offers the advantages of chronic studies (evaluation of long-term effects, replication of results, and dose-response testing in which each animal can serve as his own control). Results from both areas of application would ultimately need cross-checking by other methods to rule out interactions of IR and the independent variable being tested. Further possible interactions in long-term studies include potential interactions between the degree of tolerance developed to repeated IR trials and to repeateddrug administration.