The reinforcing and psychomotor effects of morphine involve opiate stimulation of the dopaminergic system via activation of mu-opioid receptors (muOR). Both mu-opioid and dopamine receptors are members of the G-protein-coupled receptor (GPCR) family of proteins. GPCRs are known to undergo desensitization involving phosphorylation of the receptor and the subsequent binding of beta(arrestins), which prevents further receptor-G-protein coupling. Mice lacking beta(arrestin)-2 (beta(arr2)) display enhanced sensitivity to morphine in tests of pain perception attributable to impaired desensitization of muOR. However, whether abrogating muOR desensitization affects the reinforcing and psychomotor properties of morphine has remained unexplored. In the present study, we examined this question by assessing the effects of morphine and cocaine on locomotor activity, behavioral sensitization, conditioned place preference, and striatal dopamine release in beta(arr2) knock-out (beta(arr2)-KO) mice and their wild-type (WT) controls. Cocaine treatment resulted in very similar neurochemical and behavioral responses between the genotypes. However, in the beta(arr2)-KO mice, morphine induced more pronounced increases in striatal extracellular dopamine than in WT mice. Moreover, the rewarding properties of morphine in the conditioned place preference test were greater in the beta(arr2)-KO mice when compared with the WT mice. Thus, beta(arr2) appears to play a more important role in the dopaminergic effects mediated by morphine than those induced by cocaine.

G protein-coupled receptor regulation by G protein-coupled receptor kinases and ␤-arrestins can lead to desensitization and subsequent internalization of the receptor. In in vitro and cellular systems, ␤-arrestins do not seem to play a major role in regulating opioid receptor (OR) responsiveness. Removal of the ␤arrestin2 (␤arr2) gene in mice leads paradoxically to enhanced and prolonged OR-mediated antinociception. The ␤arr2 knockout (␤arr2-KO) mice also fail to develop morphine antinociceptive tolerance in the hot-plate test, further indicating that the ␤arr2 protein plays an essential role in OR regulation in vivo. In this study, the contribution of ␤arr2 to the regulation of the OR was examined in both human embryonic kidney 293 cells and in ␤arr2-KO mice after treatment with several opiate agonists. A green fluorescent protein tagged ␤arr2 was used to assess receptor-␤arr2 interactions in living cells. Opiate agonists that induced robust ␤arr2-green fluorescent protein translocation produced similar analgesia profiles in wild-type and ␤arr2-KO mice, whereas those that do not promote robust ␤arr2 recruitment, such as morphine and heroin, produce enhanced analgesia in vivo. In this report, we present a rationale to explain the seemingly paradoxical relationship between ␤-arrestins and OR regulation wherein morphine-like agonists fail to promote efficient internalization and resensitization of the receptor.

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It is well established that Cannabis sativa can increase appetite, particularly for sweet and palatable foods. In laboratory animals, cannabinoid CB1 receptor antagonism decreases motivation for palatable foods, and most recently, the CB1 receptor antagonist SR141716A, or rimonabant (Acomplia), was reported to produce weight loss in obese human subjects. Indeed, the endocannabinoid system plays a select role in the rewarding properties of palatable foods, and this is well characterized in laboratory animals with sweet sucrose solutions. In the present study, CB1 knockout mice (CB1 KO) and wild-type littermate mice (WT) were trained to respond for a complex sweet as well as a pure fat reinforcer under a progressive ratio (PR) schedule, to determine whether motivation to consume different palatable foods is tonically regulated by CB1 receptors. To assess sweet reinforcement, several concentrations of the liquid nutritional drink, Ensure, were presented under the PR schedule. For fat reinforcement, several concentrations of corn oil (emulsified in 3% xanthan gum) were made available. Additionally, to compare the result of genetic invalidation of the CB1 receptor to antagonism of the CB1 receptor system, the effect of SR141716A (3.0 mg/kg) on responding for Ensure and corn oil were also assessed using the PR schedule. We also assessed the effect of the CB1 agonist CP-55940 (30 microg/kg) on responding for Ensure and corn oil. CB1 KOs took significantly longer to acquire operant responding maintained by Ensure, and responding for Ensure under the PR schedule was significantly reduced in CB1 KOs as well as in WTs pretreated with SR141716A, as compared to WT controls. Additionally, pretreatment with the CB1 agonist CP-55940 increased responding for Ensure. In contrast, responding for corn oil during acquisition and under the PR schedule was not significantly different in CB1 KOs versus wild-type mice. However, SR141716A did reduce responding for corn oil in WTs, and CP-55940 significantly increased responding for corn oil. Taken together, these results suggest that CB1 receptors are preferentially involved in the reinforcing effects of a complex sweet, as compared to a pure fat, reinforcer. These data also suggest, however, that antagonism of CB1 receptors with SR141716A is sufficient to attenuate the reinforcing effect of Ensure and corn oil, while activation of the central CB1 system is sufficient to enhance Ensure and corn oil reinforcement.

NMDA receptor-mediated glutamate transmission is required for several forms of neuronal plasticity. Its role in the neuronal responses to addictive drugs is an ongoing subject of investigation. We report here that the acute locomotor-stimulating effect of cocaine is absent in NMDA receptor-deficient mice (NR1-KD). In contrast, their acute responses to amphetamine and to direct dopamine receptor agonists are not significantly altered. The striking attenuation of cocaine's acute effects is not likely explained by alterations in the dopaminergic system of NR1-KD mice, since most parameters of pre-and postsynaptic dopamine function are unchanged. Consistent with the behavioral findings, cocaine induces less c-Fos expression in the striatum of these mice, while amphetamine-induced c-Fos expression is intact. Furthermore, chronic cocaine-induced sensitization and conditioned place preference are attenuated and develop more slowly in mutant animals, but amphetamine's effects are not altered significantly. Our results highlight the importance of NMDA receptor-mediated glutamatergic transmission specifically in cocaine actions, and support a hypothesis that cocaine and amphetamine elicit their effects through differential actions on signaling pathways.