The Role Of β-arrestin2 In Cocaine Reward Related Behavior

In the physical states, brain function is under precise regulation, and abnormalities in brain structure and brain function are highly related to several neurological diseases and psychiatric disorders. Drug addiction is considered as a chronic and palindromic brain disease which is characterized by compulsive drug seeking behavior and drug taking regardless of severe adverse consequences. There are few effective clinical therapies for individual who is addicted to a drug of abuse, and most of them relapse within a short period of time. Increasing studies suggest that the abnormality of gene expression in the brain reward regions is one of the most critical mechanisms involved in a state of drug addiction, in which abnormal GPCR signaling occurs.G-protein-coupled receptors (GPCRs) represent the largest known family of proteins involved in signal transduction across cell membranes. They recognize a large diversity of ligands like hormones and neurotransmitters, as a consequence, regulate endogenesis hormone balance and neurotransmitter release. β-arrestins are originally described as negative regulators of traditional GPCR-stimulated G-protein signaling, which inhibit GPCR/G-protein coupling and promote desensitization of GPCRs. More recently, novel roles for P-arrestins have emerged and researches demonstrate that they can mediate intracellular signaling in a G-protein independent manner. Through their scaffolding property, β-arrestins lead to the assembly of intracellular molecules that can affect the function of various signaling cascades, such as the MAP kinase cascades. P-arrestins are widely distributed in the central nervous system, such as cortex, hippocampus, extended amygdala and hypothalamus, which suggests they are involved in mediating signal transduction of brain circuit.β-arrestins expressing in mammals include β-arrestinl and β-arrestin2. Studies demonstrate that β-arrestin2knockout mice experience enhanced morphine-induced or△9-tetrahydrocannabinol-stimulated CB1cannabinoid receptor-mediated analgesia, and are resistant to the development of chronic morphine-induced tolerance. Neurology and behavior studies suggest that κ-opioid receptor-mediated dysphoria involves GRK3and β-arrestin2. β-arrestin2scaffolded complex of Akt/β-arrestin2/proteinphosphatase2A, acting as a negtive mediator of Akt signaling, mediate amphetamine-induced hyper-locomotor activity and lithium-induced anti-depression effect. A common feature of clinically effective antipsychotics, acting at D2dopamine receptors, is their ability to inhibit D2-receptor-stimulated β-arrestin2recruitment. These studies indicate that β-arrestin might be involved in multiple psychiatric disorders, especially in drug addiction.In this study, we used the conditioned place preference (CPP) as test model mainly and systematically assessed cocaine-induced CPP acquisition, extinction, reinstatement and incubation respectively with β-arrestin2knockout mice and their littermate controls. We found that:1. Deletion of β-arrestin2potentiates conditioned place preference induced by moderate and high, but not low or ascending dose of cocaine. Both Arrb2+/+and Arrb2-/-mice showed preference to cocaine-paired side during test after3or4days’ conditioning by low(10mg/kg), moderate(20mg/kg), high(30mg/kg) and ascending(5、10、15、20mg/kg) dose of cocaine, which suggested that both Arrb2+/+and Arrb2-/-mice acquired CPP. Moreover, in the Arrb2-/-mice, moderate and high, but not low or ascending dose of cocaine, induced pronounced increases of CPP scores, which were higher than those in the Arrb2+/+mice. The above results indicated that β-arrestin2is involved in conditioned place preference.2. P-arrestin2is not necessary for extinction in the model of conditioned place preference. AfterArrb2+/+and Arrb2-1-mice gained cocaine-induced CPP(10mg/kg), two experiments of extinction were implemented. In experiment one, one day extinction training were given after CPP test, in which both original cocaine-paired and saline-paired sides were paired with saline. In the next day’s test after extinction, Arrb2+/+and Arrb2-/-mice showed no preference to any side and there is no significant difference between them in terms of CPP score. In experiment two, to assess the extinction curve, Arrb2+/+and Arrb2-/-mice were received five consecutive tests and there is no significant difference between them in terms of CPP score during every single test.3. P-arrestin2is not necessary for incubation in the model of conditioned place preference. Arrb2-/-mice showed same incubation effect in conditioned place preference induced by half dose of cocaine(5mg/kg) after one-month’s withdrawal in their homecages with acquisition of cocaine-induced CPP(10mg/kg), compared with Arrb2+/mice. The same phenomenon was seen with higher dose of cocaine-induced CPP(20mg/kg).4. Ablation of P-arrestin2impairs stress-induced, but not drug-primed reinstatement of extinguished cocaine-conditioned place preference.Arrb2-/-mice showed impaired reinstatement induced by forced swim test of extinguished cocaine-conditioned place preference after three days’ extinction conditioning. However, Arrb2-/-mice exhibited intact cocaine-primed reinstatement in the CPP test five days after stress-induced reinstatement. To confirm the effect of β-arrestin2on the stress-induce reinstatement, we used another mode of stress, restraint stress. Interestingly, restraint stress could not induce reinstatement in the Arrb2-/-mice, while Arrb2+/+mice reinstated high preference to the original cocaine-paired side. The results suggest that β-arrestin2takes part in stress-induced, but not drug-primed reinstatement in the model of CPP.5. Ablation of β-arrestin2increases enhanced locomotor activity induced by cocaine. To testify the effect of β-arrestin2on cocaine reward related behavior, we tested the locomotor activity, which is another standard measuring reward effect. Arrb2-/-mice exhibited increased hyper-locomotor activity induced by cocaine, while Arrb2-/-mice showed the same distance travelled during test with Arrb2++mice.6. Deletion of β-arrestin2boosts food seeking. VTA-NAc pathway and the other limbic region mediate positive emotional effect of natural rewards, such as food and sex. After observing the role of β-arrestin2in cocaine reward behaviour, we then detected the effect of β-arrestin2on natural reward in the model of food seeking. Arrb2-/-mice learn the correlation between nose pock and food giving more quickly and consumed more sugar pellets during two weeks’ test, which suggest that Arrb2-/-mice showed higher food seeking motivation.In the present study, we firstly demonstrated that P-arrestin2is required for formation of conditioned place preference induced by cocaine; ablation of β-arrestin2inhibits stress-induced, but not drug-primed reinstatement of extinguished cocaine-conditioned place preference; β-arrestin2positively mediates cocaine-induced locomotor activity; β-arrestin2is involved in food seeking. This study shows that β-arrestin2plays critical roles in cocaine and natural reward related behavior and offers new evidence to pharmacological mechanism of cocaine addiction.