Effects of post-trial injection of beta-endorphin on shock-induced fighting are dependent on baseline of fighting

The effects of repeated post-trial administration of 10 micrograms/kg beta-endorphin on the development of mutual fighting in pairs of rats submitted to various intensities of electric shock were investigated. beta-Endorphin blocked the development of fighting responses when a low footshock intensity was used, but facilitated it when a high shock intensity was delivered. A detailed analysis of the relationship between shock intensity, baseline of fighting, and effects of beta-endorphin showed that the effects of beta-endorphin were dependent on the behavioral baseline rather than on shock intensity per se.