Morphine catalepsy as an adaptive reflex state in rats.

These experiments demonstrate that morphine-induced catalepsy consists of two complementary, but opposite, behavioral extremes (rigid immobility and sudden locomotor bursts), each of which can be controlled by distinct classes of external stimuli. When stimuli that involve pain and/or nonnociceptive skin pressure are tonic (continuous), morphine-induced electroencephalographic (EEG) deactivation and behavioral immobility are potentiated, even to the extent that a stimulation-bound reversible coma results. In contrast, phasic (discrete) stimulation produces behavioral and/or EEG activation. EEG and behavioral rebound effects are observed following stressful (intense, prolonged) stimuli. On the basis of the observed stimulus controls, sensorimotor characteristics, and EEG reactions, it is suggested that similarities may exist between morphine-induced catalepsy and defensive reactions of immobility and escape in drug-free animals (i.e., the adaptive death- feigning reflex).