SUPERSENSITIVITY OF SPINAL OPIOID RECEPTORS TO ANTAGONISTS IN INTRATHECAL BUTORPHANOL AND MORPHINE-DEPENDENCE

The purpose of this investigation was to evaluate changes in the sensi tivity of spinal opioid receptors to selective antagonists in rats ren dered dependent on intrathecal (i.t.) butorphanol and morphine. Using quantitative autoradiography, competitive binding assays with selectiv e opioid antagonists were performed in the spinal cord sections of i.t . butorphanol- and morphine-dependent rats in which withdrawal was pre cipitated by i.t. naloxone. In butorphanol-dependent rats, the spinal kappa-opioid receptor developed a greater degree of antagonist superse nsitivity than the spinal delta- and mu-opioid receptors did. In contr ast, the spinal mu-opioid receptor became more sensitive than the delt a-opioid receptor in morphine-dependent rats. These results indicate t hat differential supersensitivity of spinal opioid receptors was induc ed after chronic i.t. infusions of butorphanol and morphine.