In vitro, animal and human studies are performed to elucidate mechanisms of opioid action and to develop new modalities for treating opioid addiction. Chronic morphine treatment produces dependence and tolerance, as demonstrated in electrophysiological assays of synaptic activity in the isolated spinal cord from neonatal rats. Chemically and electrically evoked responses of nociceptive neurons show opioid tolerance that is not prevented by co-treatment with dizocilpine. A series of extensive behavioral ex periments demonstrate that various nitric oxide synthase (NOS) inhibitors consistently produce dose-dependent reductions in certain signs of opioid withdrawal in the rat, and yield pharmacological profiles similar to that of clonidine. 7-Nitroindazole (7-NI), a NOS inhibitor specific for cerebral NOS, attenuates more withdrawal signs than other NOS inhibitors. Further, 7-NI lacks the vasopressor activity common to other NOS inhibitors, suggesting that 7-NI holds promise as a possible treatment for opioid addiction in humans. The increased firing rates of locus coeruleus (LC) neurons during opiate withdrawal suggest a functional role of the LC in this phenomenon. Our work in rats shows that behavioral signs of opioid withdrawal and widespread stimulation of cerebral glucose metabolism occur when methylnaloxonium, a quaternary opioid antagonist, is injected directly into the LC of morphine-dependent rats, whereas injections into the central amygdala produce minimal changes in metabolism. The findings demonstrate that increased metabolic activity in regions of the brain when withdrawal is precipitated by peripheral opioid antagonist administration is most likely due at least in part to activation of the LC.
