Basal Signaling Activity of μ Opioid Receptor in Mouse Brain: Role in Narcotic Dependence

Abstract

Narcotic analgesics cause addiction by poorly understood mechanisms, involving opioid receptor (MOR). Previous cell culture studies have demonstrated significant basal, spontaneous MOR signaling activity, but its relevance to narcotic addiction remained unclear. In this study, we tested basal MORsignaling activity in brain tissue from untreated and morphinepretreated mice, in comparison to antagonist-induced withdrawal in morphine-dependent mice. Using guanosine 5Ј- O-(3-[35 S]thio)triphosphate ([ 35 S]GTP␥S) binding and adenylyl cyclase activity assay in brain homogenates, we demonstrated that morphine pretreatment of mice enhanced basal MOR signaling in mouse brain homogenates and, moreover, caused persistent changes in the effects of naloxone and naltrexone, antagonists that elicit severe withdrawal in dependent subjects. Naloxone and naltrexone suppressed basal [35 S]GTP␥S binding (acting as "inverse agonists") only after morphine pretreatment, but not in drug-naive animals. Moreover, naloxone and naltrexone stimulated adenylyl cyclase activity in striatum homogenates only after morphine pretreatment, by reversing the inhibitory effects of basal MOR activity. After cessation of morphine treatment, the time course of inverse naloxone effects on basal MOR signaling was similar to the time course of naltrexone-stimulated narcotic withdrawal over several days. The neutral antagonist 6␤-naltrexol blocked MOR activation without affecting basal signaling (G protein coupling and adenylyl cyclase regulation) and also elicited substantially less severe withdrawal. These results demonstrate long-lasting regulation of basal MOR signaling as a potential factor in narcotic dependence.

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).…”

“…Such activity might result in less CPA following morphine pretreatment by substituting for morphine and alleviating morphine withdrawal. However, 6-beta-naloxol was reported to have no such partial agonist activity (Chatterjie and Inturrisi, 1975) and naloxone itself has been reported to be a weak agonist (Fukuda et al, 1998;Liu and Prather, 2001;Wang et al, 2004) making such an explanation unlikely. Since the receptor binding profiles of 6-alphaand 6-beta-naloxol are incomplete, it is possible that these drugs interact, differentially from naloxone, with other, non-mu opioid, receptor systems resulting in counteraction of the morphine enhancement of both CPA and withdrawal jumping.…”

“…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).…”

“…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).…”

Attenuation of methamphetamine-induced behavioral sensitization in mice by systemic administration of naltrexone

“…A previous study in mice reported that the concentrations of NAT in plasma and brain tissue 60 min after i.p. injection (1 mg/kg) were 4 ± 1 ng/ml and 9 ± 1 ng/g, respectively [53]. In order to ensure the complete blockade of all opioid receptors while METH was administered, higher doses (i.e.…”