Effect of morphine and naloxone on shock avoidance learning in headless cockroaches (Periplaneta americana L.).

Systemic administration of high doses of morphine (56 micrograms morphine/g body weight) or of naloxone (54 micrograms naloxone/g body weight) results in a significant improvement of shock avoidance behavior in headless cockroaches. In both cases the learning parameter stimulation time (time during which an animal receives shocks) is significantly decreased. The behavioral parameters, stimulation rate (activity) and mean stimulation duration (shock responsiveness), as the two factors of the stimulation time considered individually, do not significantly change with morphine. Only the responses of both parameters together give the significant decrease of stimulation time (improvement of shock avoidance behavior) as mentioned above. However, the administration of naloxone causes a significant shortening of the mean stimulation duration indicating that these animals avoid the shocks by learning to lift their legs more quickly out of electrified saline in order to terminate the shocks (escape learning). Combined administration of both drugs together also causes a significant improvement of the leg-lifting response even at lower doses (morphine: 0.56 microgram plus naloxone: 0.54 microgram/g; morphine: 0.0056 microgram/g plus naloxone: 0.0054 microgram/g). Although the various effective doses of these drugs administered in combination have similar effects on the stimulation time, they affect the behavioral parameters in different ways. These different actions on activity and shock responsiveness as well as the efficacy of doses smaller than those known from vertebrates are discussed as behavioral evidence of opiate receptors in the cockroach.