ANTINOCICEPTION FROM THE ADMINISTRATION OF BETA-ENDORPHIN INTO THE PERIAQUEDUCTAL GRAY OF RAT IS ENHANCED WHILE THAT OF MORPHINE IS INHIBITED BY BARBITURATE ANESTHESIA

Pentobarbital anesthesia causes about a 10-fold increase in the antinociceptive potency of beta-endorphin microinjected into the periaqueductal gray (PAG) region of the rat brain. The antinociceptive response to PAG morphine was markedly attenuated during anesthesia, but returned as the rats regained consciousness. As they recovered from anesthesia, muscular rigidity and body stiffness (catalepsy) also occurred in the pentobarbital treated animals receiving morphine. These results are consistent with the activation of separate and distinct descending pain inhibitory neuronal systems by these two opioid agonists, and the differential modulation of the systems by pentobarbital, They also suggest that the mechanism underlying muscular responses to morphine is sensitive to pentobarbital, and is not shared by beta-endorphin.