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. Based upon studies in slices of rat brain, that show inhibition of nitric oxide synthase (NOS) diminishes the development of tolerance to morphine in noradrenergic neurons of the locus ceruleus (LC) of adult morphine-treated rats, new studies in intact anesthetized rats investigate the role of LC as a cellular substrate for the involvement of nitric oxide in opioid withdrawal. New behavioral experiments demonstrate that various NOS inhibitors consistently reduce certain withdrawal signs 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. Other 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 LC of morphine-dependent rats. 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 stimulation of the LC. In drug-naive rats, buprenorphine mimics the effect of morphine on cell firing, but in morphine-dependent rats buprenorphine mimics naloxone, consistent with the hypothesis that the actions of buprenorphine, a partial opioid agonist, vary with the state of opioid dependence. An assessment in human volunteers of the interactions of the calcium channel antagonist verapamil with morphine shows that verapamil antagonizes the positive affective changes produced by morphine and potentiates the analgesic effects of morphine.
