Effects of opiate ligands on intraspecific aggression in crickets.

In the cricket Gryllus bimaculatus, the opiate antagonist naloxone, 30 microg in 50 microl into hemolymph per animal, caused a release of intraspecific aggression in male-losers and in females. Naloxone had no significant effect on the aggression of winners and isolated males. The agonist of mu-opiate receptors DAGO, 45 microg, suppressed aggression in male winners and decreased the percentage and duration of contact fights between previously isolated males. The results suggest that, during social contacts, the activity of the endogenous opioid system may contribute to suppressing aggression in subordinate males, as well as in females.