Modulation of the escape response by [D-Ala2]Met-enkephalin in the crab Chasmagnathus.

The synthetic opioid analog [D-Ala2]Met-enkephalin (DAME) significantly reduces the escape response to a danger stimulus in the crab Chasmagnathus granulatus when administered within the dose range 0.01-1.0 micrograms/g. There is no reduction with lower or higher doses, thus suggesting a U-shaped dose-response curve. A 0.1-microgram/g dose of naloxone has no effect per se on the response, but when it is administered together with DAME, it completely blocks the decremental effect of this drug and an escape response generally higher than that of control is observed. An explanation for these results in terms of a possible dual action of DAME is offered. In addition, we present evidence for different degrees of opiate sensitivity among crabs of the same population in relation to their different degrees of reactivity to the visual danger stimulus.