Role of spinal mu opioid receptors in the development of morphine tolerance and dependence.

We previously demonstrated that the spinal cord is a primary site for the development of morphine-induced tolerance and dependence. In the current investigation, we have determined the significance of mu opioid receptors in the spinal cord in tolerance and dependence induced by systemically administered morphine. Rats surgically implanted with intrathecal (i.t.) catheters were injected i.t. with various doses of beta-funaltrexamine (beta-FNA), a specific irreversible mu opioid receptor antagonist. beta-FNA (i.t.) dose-dependently antagonized morphine-induced analgesia (i.p.) with approximately one-half the potency of beta-chlornaltrexamine, a nonselective irreversible opioid receptor antagonist. Rats injected with saline i.t. 24 h before implanting morphine pellets developed a significant degree of tolerance and dependence 72 h after morphine administration. Tolerance did not develop in similarly treated animals that received i.t. injections of 4.5 nmol beta-FNA. Signs of naloxone-precipitated withdrawal were also significantly antagonized in all instances, except weight loss, in animals pretreated with beta-FNA (i.t.). We conclude that i.t. injections of beta-FNA significantly antagonized the development of tolerance and dependence induced by systemically administered morphine. Therefore, our results indicate that mu opioid receptors within the spinal cord, and probably throughout the central nervous system, play a primary role in morphine-induced tolerance and dependence.