The effect of morphine tolerance and abstinence on the binding of [3H]naltrexone to discrete brain regions and spinal cord of the rat was determined. 2. Male Sprague-Dawley rats were implanted s.c. under light ether anesthesia with six morphine pellets for a 7-day period. Each pellet contained 75 mg of morphine base. Rats implanted with six placebo pellets each served as controls. 3. This procedure resulted in the development of tolerance to morphine as evidenced by decreased analgesic response to various doses of morphine. 4. The binding characteristics (Bmax or Kd values) of [3H]naltrexone, an opiate receptor antagonist, were determined in various tissues of morphine tolerant and abstinent rats. In the tolerant rats, the pellets were left in place at the time of sacrificing, whereas in the abstinent rats, the pellets were removed 18 hr prior to sacrificing. 5. The binding of [3H]naltrexone to opiate receptors on membranes prepared from brain regions (hypothalamus, hippocampus, cortex, pons and medulla, midbrain, corpus striatum and amygdala) and spinal cord of rats from various treatment groups was determined. 6. [3H]Naltrexone bound to tissue membranes at a single high affinity binding sites. The Bmax values of [3H]naltrexone to bind to opiate receptors on the membranes of amygdala and striatum were increased significantly in morphine tolerant rats when compared to the placebo controls, but the Kd values did not differ. 7. The Bmax and Kd values of [3H]naltrexone did not differ in any other brain region or spinal cord of morphine tolerant rats and their placebo controls. The binding constants of [3H]naltrexone were unaffected in morphine abstinent rats. 8. Previously we had shown that the binding of [3H]D-Ala2, MePhe4, Gly-ol5 enkephalin (DAMGO), a highly specific agonist for mu-opiate receptors was decreased in cortex, pons and medulla and spinal cord of morphine tolerant but not in the abstinent rats. In addition, delta and kappa receptors are unaffected in morphine tolerant and abstinent rats. 9. The results suggest that direction of change, as well as, the brain areas for mu-agonist and -antagonist opiate binding sites are affected differentially in morphine tolerant rats.
