EFFECT OF NALOXONE PRETREATMENT ON HABITUATION IN THE CRAB CHASMAGNATHUS-GRANULATUS

On sudden presentation of a passing shadow, the crab Chasmagnathus granulatus reacts with an escape response that habituates after repeating the same stimulus. In a first series of experiments, a range of naloxone (NX) doses (0.8, 2.4, 3.2, and 6.2 μg/g) was injected into crabs 15 min before one 15-trial habituation session. An enhancing effect of 3.2 μg NX/g on responsiveness appeared over trials, that cannot be explained either by a ceiling effect or by a delay in peak drug action. Two doses below 3.2 μg/g and one dose above had no significant effect. Results from a second series of experiments showed that the 3.2 μg NX/g effect vanishes after 15 trials (1 h after injection). The hypothesis that crab's habitution involves the action of an endogenous opioid mechanism is put forward to account for the naloxone pretreatment effect. © 1978 Academic Press, Inc.