The effects of opioid and FMRF-amide peptides on thermal behavior in the snail.

Administration of methionine-enkephalin, beta-endorphin or, as previously shown, the opiate agonist, morphine sulfate (0.10-10.0 micrograms per snail), resulted in significant dose-dependent increases in the latency of thermal (40 degrees C hot plate) avoidance behavior of the terrestrial snail, Cepaea nemoralis. The analgesic effects could be blocked by the opiate antagonist, naloxone, as well as by the non-opioid peptides, FMRF-amide and YGG-FMRF-amide. When administered by themselves the FMRF-amide peptides had significant bimodal effects either decreasing (0.10 and 10.0 micrograms) or increasing (1.0 micrograms) the latency of the response to the thermal stimulus. These results indicate that opioid and FMRE-amide peptides may be involved in the determination of thermal behavior in the snail. They also suggest that FMRF-amide peptides may function as endogenous modulators of opioid activity.