In Vivo Characterization of 6β-Naltrexol, an Opioid Ligand with Less Inverse Agonist Activity Compared with Naltrexone and Naloxone in Opioid-Dependent Mice

Abstract

The -opioid receptor displays basal signaling activity, which seems to be enhanced by exposure to opioid agonists. This study assesses the in vivo pharmacology of the putative "neutral" antagonist 6␤-naltrexol in comparison to other ligands with varying efficacy, such as naloxone, an inverse agonist in the opioid-dependent state. ICR mice were used to generate full antagonist dose-response curves for naloxone, naltrexone, nalbuphine, and 6␤-naltrexol in blocking acute antinociceptive effects of morphine and precipitating opioid withdrawal in models of physical dependence. 6␤-Naltrexol was roughly equipotent to naloxone and between 4.5-and 10-fold less potent than naltrexone in blocking morphine-induced antinociception and locomotor activity, showing that 6␤-naltrexol enters the central nervous system. In contrast to naloxone and naltrexone, 6␤-naltrexol precipitated only minimal withdrawal at high doses in an acute dependence model and was ϳ77-and 30-fold less potent than naltrexone and naloxone, respectively, in precipitating withdrawal in a chronic dependence model. 6␤-Naltrexol reduced the inverse agonist effects of naloxone in vitro and in vivo, as expected for a neutral antagonist. Therefore, the pharmacological effects of 6␤-naltrexol differ markedly from those of naloxone and naltrexone in the opioid-dependent state. A reduction of withdrawal effects associated with neutral -opioid receptor antagonists may offer advantages in treating opioid overdose and addiction.Basal signaling/constitutive activity of G-protein coupled receptors is now firmly established largely on the basis of in vitro results (Kenakin, 2003(Kenakin, , 2004a, with receptor ligands displaying a range of efficacies from full agonists to full inverse agonists. However, for receptor systems that display constitutive activity, many questions remain to be resolved. 1) Do the in vitro observations translate into changes that can be measured in vivo? 2) Are there clinical applications for inverse agonists versus neutral antagonists? 3) How is basal receptor activity regulated, and how does disease and/or chronic drug exposure alter levels of basal signaling and ligand efficacy? Studies on ␤-adrenergic receptors, for example, suggest that these issues may contribute to patient outcomes in diseases such as congestive heart failure and asthma (Maack et al., 2000;Callaerts-Vegh et al., 2004).We and others have established that the -and ␦-opioid receptors display basal signaling, which is altered by exposure to opioid agonists (Costa and

Constitutively Active Mu Opioid Receptors Mediate the Enhanced Conditioned Aversive Effect of Naloxone in Morphine-Dependent Mice

“…The mu opioid receptor has been shown in vitro to couple to signaling mechanisms in the absence of ligand, and this level of mu receptor constitutive activity was enhanced following exposure to agonists such as morphine (Liu and Prather, 2001;Raehal et al, 2005;Wang et al, 2001Wang et al, , 2004Wang et al, , 1994. It has been hypothesized that this increase in constitutively active receptors following morphine administration plays a role in morphine dependence (Cruz et al, 1996;Wang et al, 1994).…”

“…Naloxone has been shown to act as an inverse agonist at the mu receptor in vitro, stimulating cAMP levels and inhibiting GTPgS binding in morphine-pretreated, but not untreated, tissue (Liu and Prather, 2001;Raehal et al, 2005;Wang et al, 2001Wang et al, , 2004Wang et al, , 1994. Such uncovering of the inverse agonist properties of naloxone by morphine pretreatment is predicted, since morphine would be expected to increase the number of constitutively active mu receptors (for a review, see Kenakin, 2004).…”

Comparison of the opioid receptor antagonist properties of naltrexone and 6β-naltrexol in morphine-naïve and morphine-dependent mice

“…Differences in the ability of naltrexone and 6β-naltrexol to precipitate withdrawal in morphine-dependent mice have been observed (Wang et al, 2004;Raehal et al, 2005). In contrast, naltrexone and 6β-naltrexol have similar affinities in vitro for the μ-opioid receptor expressed in HEK293 cells (Wang et al, 2001), suggesting a differential ability to displace μ-opioid receptor agonist bound to receptor may not be responsible for the observed differences to precipitate withdrawal.…”

The Therapeutic Potential of Nociceptin/Orphanin FQ Receptor Agonists as Analgesics without Abuse Liability

“…25 6β-Naltrexol was characterized as a neutral MOP antagonist, but naltrexone displayed an inverse MOP agonist action in morphine-dependent mice. 26 In contrast, 6β-naltrexol failed to block naltrexone-precipitated withdrawal in morphine-dependent monkeys; instead, it potentiated the effects of naltrexone when the dose of 6β-naltrexol was increased, indicating that both naltrexone and 6β-naltrexol have the same pharmacological actions only with large potency differences in primates. 27 These findings suggest that there might be difference between primates and rodents in terms of the MOP basal signaling activity changed by repeated MOP agonist administration.…”