In vivo activity of norhydrocodone: an active metabolite of hydrocodone.

Hydrocodone is primarily metabolized to hydromorphone and norhydrocodone. Although hydromorphone is a known active metabolite of hydrocodone, the in vivo activity of norhydrocodone is not well documented. In the current study, the pharmacodynamics of norhydrocodone were evaluated and compared with hydrocodone and hydromorphone. Binding studies established that norhydrocodone, similar to hydrocodone and hydromorphone, is a µ-selective opioid ligand. In vivo analgesia studies (tail flick) demonstrated that, following subcutaneous, intrathecal, and intracerebroventricular administration, norhydrocodone produced analgesia. Following subcutaneous administration, norhydrocodone was ∼70-fold less potent, and hydromorphone was ∼5.4-fold more potent than hydrocodone in producing analgesia. Following intrathecal administration, norhydrocodone produced a shallow analgesia dose-response curve and maximal effect of 15-45%, whereas hydrocodone and hydromorphone produced dose-dependent analgesia. Intrathecal hydromorphone was ∼174-fold more potent than intrathecal hydrocodone. Following intracerebroventricular administration, norhydrocodone had similar potency to hydrocodone in producing analgesia, while hydromorphone was ∼96-fold more potent than hydrocodone. Analgesia induced by the three drugs following subcutaneous, intrathecal, and intracerebroventricular administration was antagonized by subcutaneous naltrexone, confirming that it is opioid receptor-mediated. Subcutaneous norhydrocodone-induced analgesia was completely blocked by intracerebroventricular naltrexone, indicating that norhydrocodone-induced analgesia is likely a supraspinal effect. Seizure activity was observed following intrathecal administration of all three drugs. Norhydrocodone and hydromorphone were ∼3.7 to 4.6-fold more potent than hydrocodone in inducing seizure activity. Naltrexone did not antagonize opioid-induced seizure activity, suggesting that seizures were not opioid receptor-mediated. Taken together, norhydrocodone is an active metabolite of hydrocodone and may contribute to therapeutic and toxic effects following hydrocodone administration.

[(35)S]GTPγS binding and opioid tolerance and efficacy in mouse spinal cord.

The present study examined efficacy of a series of opioid agonists and then using chronic in vivo treatment protocols, determined tolerance to opioid agonist stimulated [(35)S]GTPγS (guanosine 5'-O-(3-[(35)S] thio)triphosphate) binding in mouse spinal cord membranes and compared it directly to spinal analgesic tolerance. The [(35)S]GTPγS binding assay was used to estimate efficacy (E(max) and τ; Operational Model of Agonism) of a series of opioid agonists for G-protein activation in mouse spinal cord. The rank order of opioid agonist efficacy determined in the [(35)S]GTPγS assay using the Operational Model and E(max) was similar. These efficacy estimates correlated with historical analgesic efficacy estimates. For tolerance studies, mice were continuously treated s.c. for 7days with morphine, oxycodone, hydromorphone, etorphine or fentanyl and [(35)S]GTPγS studies were conducted in spinal cord membranes. Other mice were tested in i.t. analgesia dose response studies (tailflick). Tolerance to DAMGO ([D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin) or morphine stimulated [(35)S]GTPγS binding (decrease in E(max)) was observed following etorphine and fentanyl treatment only. These treatment protocols downregulate µ-opioid receptor density whereas morphine, oxycodone and hydromorphone do not. Spinal analgesic tolerance was observed following all treatment protocols examined (morphine, oxycodone and etorphine). Opioid antagonist treatment that specifically upregulates (chronic naltrexone) or downregulates (clocinnamox) µ-opioid receptor density produced a corresponding change in opioid agonist stimulated [(35)S]GTPγS binding. Although receptor downregulation and G-protein uncoupling are among potential mechanisms of opioid tolerance, the present results suggest that uncoupling in mouse spinal cord plays a minor role and that the [(35)S]GTPγS assay is particularly responsive to changes in µ-opioid receptor density.

The role of opioid antagonist efficacy and constitutive opioid receptor activity in the opioid withdrawal syndrome in mice.

On the basis of efficacy, opioid antagonists are classified as inverse opioid agonists (e.g. naltrexone) or neutral opioid antagonists (e.g. 6ß-naltrexol). This study examined the interaction between naltrexone and 6ß-naltrexol in the precipitated opioid withdrawal syndrome in morphine dependent mice. Furthermore, the possible contribution of constitutive opioid receptor activity to precipitated withdrawal was evaluated using increasing levels of morphine dependence. In the first experiment, low doses of 6ß-naltrexol antagonized naltrexone precipitated withdrawal while high doses acted additively. All doses of naltrexone increased 6ß-naltrexol's potency to precipitate withdrawal. The next experiment examined changes in antagonist potency to precipitate withdrawal with increasing morphine dependence. Mice were exposed to morphine for 1-6 days and then withdrawal was precipitated. Naltrexone was more potent than 6ß-naltrexol at all the time points. The ED(50) of both drugs decreased at the same rate suggesting that increased dependence produced no change in constitutive opioid receptor activity. Taken together these results indicate that the functional efficacy of 6ß-naltrexol is dose-dependent and that constitutive opioid receptor activity did not change as opioid dependence increased from 1 to 6 days.