The effect of morphine dependence and abstinence on opioid receptors in the cerebral cortex and spinal cord of the rat.

The characteristics of cerebral cortical and spinal cord opiate mu, delta and kappa receptors were investigated by measuring of the specific binding of [3H]naloxone, [3H]DADLE and [3H]bremazocine in drug-free rats and in rats rendered tolerant to morphine and in those in which morphine abstinence syndrome was precipitated with naloxone. The opiate receptors in the spinal cord were less dense than those in the cortex; the difference was most pronounced in the case of delta receptors. The characteristics of the investigated opioid receptors did not change in the state of tolerance or abstinence.