The purpose is to study the mechanism of development of tolerance to narcotic analgesic agents. Our initial finding was that mice made tolerant to morphine were highly tolerant to subcutaneous morphine but not cross tolerant to heroin or etorphine. Furthermore, when the central nervous system sensitivity to morphine was assessed by direct injection intracerebroventricularly (ICV) or intrathecally (IT), neither of these sites manifested tolerance to morphine. The present approach rests on the finding that the mechanism of tolerance to morphine involved a decrease in the synergistic interaction between supraspinal and spinal sites of action of systemic morphine. First, the aim is to see what receptor system(s) are involved in the synergism in control mice. Our experiments will show that morphine given ICV interacts synergistically with IT administered morphine through a delta opioid agonist action at the spinal cord. The latter receptor specificity will be established by D-Pen2-D-Pen5-enkephalin, a delta agonist, synergizing with the ICV morphine. Mu receptor agonists will be shown not to do so. IT administered kappa agonists will be investigated similarly since preliminary experiments indicate synergism here also. Second, we postulate that supraspinal morphine activates norepinephrine release at the spinal cord which then interacts with the effect of the delta opioid agonist. This situation will be mimicked by administering norepinephrine IT to show similarities in synergism to the morphine activated release of norepinephrine. To assess receptor specificity, both the irreversible opiate mu receptor antagonist, Beta-funaltrexamine, and adrenergic antagonists like phentolamine, prazosin and yohimbine will be used. T. Beta-Funaltrexamine being a mu receptor antagonist should not affect the synergism of delta agonist with norepinephrine while alpha adrenergic receptor blockade should decrease such synergism. In morphine (pellet implanted) tolerant mice we will show that these redundant descending spinopetal pathways for antinociception decrease in their synergistic interaction. The effect of supraspinal morphine on spinal opioid receptors will be shown by changes in vivo binding of 3H-etorphine given IT. This approach appears feasible because the morphine is applied at a remote location and 3H-etorphine binding is assessed in the presence of synergistic factors such as norepinephrine in the spinal cord.
