Inverse agonists and neutral antagonists at µ opioid receptor (MOR): possible role of basal receptor signaling in narcotic dependence

Abstract

The m opioid receptor, MOR, displays spontaneous agonistindependent (basal) G protein coupling in vitro. To determine whether basal MOR signaling contributes to narcotic dependence, antagonists were tested for intrinsic effects on basal MOR signaling in vitro and in vivo, before and after morphine pretreatment. Intrinsic effects of MOR ligands were tested by measuring GTPgS binding to cell membranes and cAMP levels in intact cells. b-CNA, C-CAM, BNTX, and nalmefene were identi®ed as inverse agonists (suppressing basal MOR signaling). Naloxone and naltrexone were neutral antagonists (not affecting basal signaling) in untreated cells, whereas inverse agonistic effects became apparent only after morphine pretreatment. In contrast, 6a-and 6b-naltrexol and -naloxol, and 6b-naltrexamine were neutral antagonists regardless of morphine pretreatment. In an acute and chronic mouse model of morphine-induced dependence, 6b-naltrexol caused signi®cantly reduced withdrawal jumping compared to naloxone and naltrexone, at doses effective in blocking morphine antinociception. This supports the hypothesis that naloxoneinduced withdrawal symptoms result at least in part from suppression of basal signaling activity of MOR in morphinedependent animals. Neutral antagonists have promise in treatment of narcotic addiction.

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).…”

“…It has been proposed that the increase in constitutively active mu receptors induced by morphine treatment is important in the development of physical dependence to morphine (Cruz et al, 1996;Wang et al, 1994). In support of this, inverse mu agonists were shown to produce greater withdrawal jumping compared to neutral antagonists (Wang et al, 2001(Wang et al, , 2004, and a neutral antagonist attenuated the withdrawal jumping produced by the inverse agonist naloxone (Bilsky et al, 1996;Wang et al, 1994). Such physical withdrawal symptoms are one component of what is now recognized as a more complex opiate dependence syndrome including disruption of affective state (Schulteis and Koob, 1996).…”

“…Exposure to agonists, such as morphine, has been shown to produce an increase in the number of constitutively active mu receptors that outlasts agonist treatment and that is most likely due to a change in receptor phosphorylation (Cruz et al, 1996;Wang et al, 2001Wang et al, , 1994Wang et al, , 1996Wang et al, , 1999.…”

“…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).…”

“…6-Alpha-and 6-beta-naloxol were previously shown to be neutral antagonists at the mu receptor in vitro, with no affect on cAMP levels or GTPgS binding, regardless of morphine pretreatment (Wang et al, 2001). and were chosen as the neutral antagonists for this study because they are also structural analogues of naloxone, with similar affinity for the mu receptor, and are systemically active (Wang et al, 2001).…”