Opiate effect on the threat display in the crab Carcinus mediterraneus

The well-known defense response of a crab (laterus merus display, LMD) was easily evoked in Carcinus mediterraneus by striking the cephalothoraxic protogastric region between the eyestalks. Following a program aimed at investigating the regulatory action of diverse neuromodulators on the LMD of this crab, a study on the role of opioids was started by testing the effect of morphine administration. Injection of morphine HC1 (MP) (40, 50, 60, 70, or 100 micrograms/g) produced a dose-dependent reduction of the LMD so elicited that dissipated with the postinjection time. Only MP doses higher than 50 micrograms/g were effective 30 min after drug administration. The MP-induced inhibition of LMD was blocked by a 4.8-micrograms naloxone HC1/g dose injected 10 min before MP. These results and those previously obtained as the action of GABA on the LMD of this crab are discussed in connection with results reporting a similar effect of these drugs on another agonistic item of behavior in the crab Chasmagnatus granulatus. The possibility of demonstrating habituation of the LMD to an iterated stimulation in C. mediterraneus and of using such a process to elucidate the acting paths of the drugs is discussed.