Supersensitivity to opioids following the chronic blockade of endorphin action by naloxone.

Chronic treatment of guinea pigs with the narcotic antagonist naloxone (pellet implantation technique) caused an increased sensitivity to opioids of the electrically stimulated longitudinal muscle-myenteric plexus preparation of the ileum. This supersensitivity is associated with an elevation of the total number of opiate receptor binding sites in both the peripheral and the central nervous system.