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

Abstract

Naloxone administration produces a robust conditioned place aversion (CPA) in opiate-naive rodents by blocking the action of enkephalins at mu opioid receptors. This aversive response is potentiated by prior exposure to morphine. In vitro studies indicate that morphine treatment may promote constitutive activity of mu opioid receptors. We hypothesized that such enhanced constitutive activity in vivo may underlie the increased aversive property of naloxone by uncovering the inverse agonist property of this drug. The CPA produced by naloxone was compared with that produced by the neutral antagonists 6-alpha-and 6-beta-naloxol in mice with and without prior morphine exposure. While all three drugs produced CPA, only naloxone CPA was enhanced by morphine given 20 h prior to each naloxone injection. Furthermore, only naloxone produced withdrawal jumping when given 20 h after morphine, even though 6-alpha-naloxol was able to produce jumping when given 4 h after morphine. These data suggest that morphine may enhance naloxone CPA by increasing levels of constitutively active mu receptors and further support the role of such constitutive activity in mediating naloxone-precipitated physical withdrawal. Such long-term changes in constitutive activity of the mu receptor induced by exogenous opiate exposure may thus be an important factor in hedonic homeostatic dysregulation proposed to underlie the addictive process.

Morphine Withdrawal Enhances Constitutive μ-Opioid Receptor Activity in the Ventral Tegmental Area

“…Withdrawal from chronic morphine enhances inverse agonistic effects at the MOR During morphine withdrawal, MOR inverse agonists evoke stronger withdrawal symptoms (e.g., withdrawal jumping, paw tremors, and conditioned place aversion) than MOR neutral antagonists (Wang et al, 1994;Bilsky et al, 1996;Raehal et al, 2005;Shoblock and Maidment, 2006). This suggests that MOR constitutive activity becomes more important during morphine withdrawal, especially in regions involved in the expression of withdrawal symptoms, like the VTA (Baumeister et al, 1989;Le Moal, 1997, 2001;AstonJones and Harris, 2004;Radke et al, 2011).…”

“…Although MOR neutral antagonists and inverse agonists can both produce opiate withdrawal effects (e.g., jumping, tremors, and place aversion), there is now strong evidence that neutral MOR antagonists do so to a lesser extent than inverse agonists (Wang et al, 1994;Bilsky et al, 1996;Raehal et al, 2005;Shoblock and Maidment, 2006). MOR neutral antagonists therefore have the potential to be safer therapeutic agents to treat both opiate addiction and overdose.…”

“…Since our finding suggests that enhanced MOR activity by an agonist can increase MOR constitutive activity, we cannot entirely rule out the possibility that the magnitude of constitutive MOR activity in our slice preparation was relatively heightened by the release of opiates during euthanasia. However, it is important to note that strong evidence suggests that MOR constitutive activity also occurs in freely moving animals (Wang et al, 1994;Bilsky et al, 1996;Raehal et al, 2005;Shoblock and Maidment, 2006), supporting its physiological relevance.…”

“…Constitutive MOR activity has been demonstrated in artificial cell systems (Wang et al, , 2001Burford et al, 2000;Sally et al, 2010), and in some native tissues (Wang et al, 2004;Heinzen et al, 2005;Raehal et al, 2005;Walwyn et al, 2007). Intriguingly, there is evidence that reduction of constitutive MOR activity exacerbates the expression of physical and psychological opioid withdrawal symptoms (Wang et al, 1994;Bilsky et al, 1996;Raehal et al, 2005;Shoblock and Maidment, 2006;Lam et al, 2011). Part of the neuroanatomical substrate for such adverse effects is likely a hypofunctional mesolimbic dopamine system (Acquas et al, 1991;Diana et al, 1995Diana et al, , 1999Le Moal, 1997, 2001;Aston-Jones and Harris, 2004;Radke et al, 2011).…”

Some implications of receptor theory for in vivo assessment of agonists, antagonists and inverse agonists

“…induction of effects opposite to those of an agonist) may reflect the presence of endogenous agonist rather than constitutive receptor activity. Distinguishing these different sources of tone and evaluating their relative contributions to organismal states and drug effects are likely to be important topics for pharmacological research in the coming years [16,18]. The theoretical models presented in this commentary suggest that intermediate efficacy ligands may be especially useful for investigating tone due to constitutive activity versus endogenous agonists.…”