The role of GRK6 in animal models of Parkinson's disease and L-DOPA treatment

Dopamine-Depleted Dopamine Transporter Knockout (DDD) Mice: Dyskinesia with L-DOPA and Dopamine D1 Agonists

L-DOPA is the mainstay of treatment for Parkinson's disease (PD). However, over time this drug can produce dyskinesia. A useful acute PD model for screening novel compounds for anti-parkinsonian and L-DOPA-induced dyskinesia (LID) are dopamine-depleted dopamine-transporter KO (DDD) mice. Treatment with α-methyl-para-tyrosine rapidly depletes their brain stores of DA and renders them akinetic. During sensitization in the open field (OF), their locomotion declines as vertical activities increase and upon encountering a wall they stand on one leg or tail and engage in climbing behavior termed "three-paw dyskinesia". We have hypothesized that L-DOPA induces a stereotypic activation of locomotion in DDD mice, where they are unable to alter the course of their locomotion, and upon encountering walls engage in "three-paw dyskinesia" as reflected in vertical counts or beam-breaks. The purpose of our studies was to identify a valid index of LID in DDD mice that met three criteria: (a) sensitization with repeated L-DOPA administration, (b) insensitivity to a change in the test context, and (c) stimulatory or inhibitory responses to dopamine D1 receptor agonists (5 mg/kg SKF81297; 5 and 10 mg/kg MLM55-38, a novel compound) and amantadine (45 mg/kg), respectively. Responses were compared between the OF and a circular maze (CM) that did not hinder locomotion. We found vertical counts and climbing were specific for testing in the OF, while oral stereotypies were sensitized to L-DOPA in both the OF and CM and responded to D1R agonists and amantadine. Hence, in DDD mice oral stereotypies should be used as an index of LID in screening compounds for PD.

G Protein-Coupled Receptor Kinase 2 Selectively Enhances β-Arrestin Recruitment to the D2 Dopamine Receptor through Mechanisms That Are Independent of Receptor Phosphorylation

The D2 dopamine receptor (D2R) signals through both G proteins and β-arrestins to regulate important physiological processes, such as movement, reward circuitry, emotion, and cognition. β-arrestins are believed to interact with G protein-coupled receptors (GPCRs) at the phosphorylated C-terminal tail or intracellular loops. GPCR kinases (GRKs) are the primary drivers of GPCR phosphorylation, and for many receptors, receptor phosphorylation is indispensable for β-arrestin recruitment. However, GRK-mediated receptor phosphorylation is not required for β-arrestin recruitment to the D2R, and the role of GRKs in D2R-β-arrestin interactions remains largely unexplored. In this study, we used GRK knockout cells engineered using CRISPR-Cas9 technology to determine the extent to which β-arrestin recruitment to the D2R is GRK-dependent. Genetic elimination of all GRK expression decreased, but did not eliminate, agonist-stimulated β-arrestin recruitment to the D2R or its subsequent internalization. However, these processes were rescued upon the re-introduction of various GRK isoforms in the cells with GRK2/3 also enhancing dopamine potency. Further, treatment with compound 101, a pharmacological inhibitor of GRK2/3 isoforms, decreased β-arrestin recruitment and receptor internalization, highlighting the importance of this GRK subfamily for D2R-β-arrestin interactions. These results were recapitulated using a phosphorylation-deficient D2R mutant, emphasizing that GRKs can enhance β-arrestin recruitment and activation independently of receptor phosphorylation.

Inhibition of PDE10A in a New Rat Model of Severe Dopamine Depletion Suggests New Approach to Non-Dopamine Parkinson's Disease Therapy

Parkinson's disease is the second most common neurodegenerative pathology. Due to the limitations of existing therapeutic approaches, novel anti-parkinsonian medicines with non-dopamine mechanisms of action are clearly needed. One of the promising pharmacological targets for anti-Parkinson drug development is phosphodiesterase (PDE) 10A. The stimulating motor effects of PDE10A inhibition were detected only under the conditions of partial dopamine depletion. The results raise the question of whether PDE10A inhibitors are able to restore locomotor activity when dopamine levels are very low. To address this issue, we (1) developed and validated the rat model of acute severe dopamine deficiency and (2) tested the action of PDE10A inhibitor MP-10 in this model. All experiments were performed in dopamine transporter knockout (DAT-KO) rats. A tyrosine hydroxylase inhibitor, α-Methyl-DL-tyrosine (αMPT), was used as an agent to cause extreme dopamine deficiency. In vivo tests included estimation of locomotor activity and catalepsy levels in the bar test. Additionally, we evaluated the tissue content of dopamine in brain samples by HPLC analysis. The acute administration of αMPT to DAT-KO rats caused severe depletion of dopamine, immobility, and catalepsy (Dopamine-Deficient DAT-KO (DDD) rats). As expected, treatment with the L-DOPA and carbidopa combination restored the motor functions of DDD rats. Strikingly, administration of MP-10 also fully reversed immobility and catalepsy in DDD rats. According to neurochemical studies, the action of MP-10, in contrast to L-DOPA + carbidopa, seems to be dopamine-independent. These observations indicate that targeting PDE10A may represent a new promising approach in the development of non-dopamine therapies for Parkinson's disease.

β-arrestin 2 is essential for fluoxetine-mediated promotion of hippocampal neurogenesis in a mouse model of depression

Over the last decade, the roles of β-arrestins in the treatment of neuropsychological diseases have become increasingly appreciated. Fluoxetine is the first selective serotonin reuptake inhibitor developed and is approved for the clinical treatment of depression. Emerging evidence suggests that fluoxetine can directly combine with the 5-HT receptor, which is a member of the G protein-coupled receptor (GPCR) family, in addition to suppressing the serotonin transporter. In this study, we prepared a chronic mild stress (CMS)-induced depression model with β-arrestin2-/- mice and cultured adult neural stem cells (ANSCs) to investigate the involvement of the 5-HT receptor-β-arrestin axis in the pathogenesis of depression and in the therapeutic effect of fluoxetine. We found that β-arrestin2 deletion abolished the fluoxetine-mediated improvement in depression-like behaviors and monoamine neurotransmitter levels, although β-arrestin2 knockout did not aggravate CMS-induced changes in mouse behaviors and neurotransmitters. Notably, the β-arrestin2-/- mice had a shortened dendritic length and reduced dendritic spine density, as well as decreased neural precursor cells, compared to the WT mice under both basal and CMS conditions. We further found that β-arrestin2 knockout decreased the number of proliferating cells in the hippocampal dentate gyrus and suppressed the proliferative capability of ANSCs in vitro. Moreover, β-arrestin2 knockout aggravated the impairment of cell proliferation induced by corticosterone and further blocked the fluoxetine-mediated promotion of mouse hippocampal neurogenesis. Mechanistically, we found that the 5-HT2BR-β-arrestin2-PI3K/Akt axis is essential to maintain the modulation of hippocampal neurogenesis in depressed mice. Our study may provide a promising target for the development of new antidepressant drugs.

Normal Ethanol Sensitivity and Rapid Tolerance Require the G Protein Receptor Kinase 2 in Ellipsoid Body Neurons in Drosophila

BACKGROUND

G protein signaling pathways are key neuromodulatory mechanisms for behaviors and neurological functions that affect the impact of ethanol (EtOH) on locomotion, arousal, and synaptic plasticity. Here, we report a novel role for the Drosophila G protein-coupled receptor kinase 2 (GPRK2) as a member of the GRK4/5/6 subfamily in modulating EtOH-induced behaviors.

METHODS

We studied the requirement of Drosophila Gprk2 for naïve sensitivity to EtOH sedation and ability of the fly to develop rapid tolerance after a single exposure to EtOH, using the loss of righting reflex (LORR) and fly group activity monitor (FlyGrAM) assays.

RESULTS

Loss-of-function Gprk2 mutants demonstrate an increase in alcohol-induced hyperactivity, reduced sensitivity to the sedative effects of EtOH, and diminished rapid tolerance after a single intoxicating exposure. The requirement for Gprk2 in EtOH sedation and rapid tolerance maps to ellipsoid body neurons within the Drosophila brain, suggesting that wild-type Gprk2 is required for modulation of locomotion and alertness. However, even though Gprk2 loss of function leads to decreased and fragmented sleep, this change in the sleep state does not depend on Gprk2 expression in the ellipsoid body.

CONCLUSION

Our work on GPRK2 has established a role for this GRK4/5/6 subfamily member in EtOH sensitivity and rapid tolerance.

Identification of crucial genes associated with Parkinson's disease using microarray data

The present study aimed to examine potential crucial genes associated with Parkinson's disease (PD) in addition to the interactions and regulators of these genes. The chip data (GSE7621) were obtained from the Gene Expression Omnibus and standardized using the robust multi‑array average in the Affy package of R software. The differentially expressed genes (DEGs) were then screened using the Samr package with a false discovery rate (FDR) <0.05 and |log2 fold change (FC)|>1. Crucial PD‑associated genes were predicted using the Genetic Association Database in the Database for Annotation, Visualization and Integrated Discovery and sequence alignment. Furthermore, transcription factors (TFs) of the crucial PD‑associated genes were predicted, and protein‑protein interactions (PPIs) between the crucial PD‑associated genes were analyzed using the Search Tool for the Retrieval of Interacting Genes/Proteins. Additionally, another dataset of PD was used to validate the expression of crucial PD‑associated genes. A total of 670 DEGs (398 upregulated and 272 downregulated genes) were identified in the PD samples. Of these, 10 DEGs enriched in pathways associated with the nervous system were predicted to be crucial in PD, including C‑X‑C chemokine receptor type 4 (CXCR4), deleted in colorectal cancer (DCC) and NCL adaptor protein 2 (NCK2). All 10 genes were associated with neuron development and differentiation. They were simultaneously modulated by multiple TFs, including GATA, E2F and E4 promoter‑binding protein 4. The PPI networks showed that DCC and CXCR4 were hub proteins. The DCC‑netrin 1‑roundabout guidance receptor 2‑slit guidance ligand 1 interaction pathway, and several genes, including TOX high mobility group box family member 4, kinase insert domain receptor and zymogen granule protein 16B, which interacted with CXCR4, were novel findings. Additionally, CXCR4 and NCK2 were upregulated in another dataset (GSE8397) of PD. These genes, interactions of proteins and TFs may be important in the progression of PD.

Progress towards a public chemogenomic set for protein kinases and a call for contributions

Protein kinases are highly tractable targets for drug discovery. However, the biological function and therapeutic potential of the majority of the 500+ human protein kinases remains unknown. We have developed physical and virtual collections of small molecule inhibitors, which we call chemogenomic sets, that are designed to inhibit the catalytic function of almost half the human protein kinases. In this manuscript we share our progress towards generation of a comprehensive kinase chemogenomic set (KCGS), release kinome profiling data of a large inhibitor set (Published Kinase Inhibitor Set 2 (PKIS2)), and outline a process through which the community can openly collaborate to create a KCGS that probes the full complement of human protein kinases.

G protein-coupled receptor kinases as regulators of dopamine receptor functions

Actions of the neurotransmitter dopamine in the brain are mediated by dopamine receptors that belong to the superfamily of G protein-coupled receptors (GPCRs). Mammals have five dopamine receptor subtypes, D1 through D5. D1 and D5 couple to Gs/olf and activate adenylyl cyclase, whereas D2, D3, and D4 couple to Gi/o and inhibit it. Most GPCRs upon activation by an agonist are phosphorylated by GPCR kinases (GRKs). The GRK phosphorylation makes receptors high-affinity binding partners for arrestin proteins. Arrestin binding to active phosphorylated receptors stops further G protein activation and promotes receptor internalization, recycling or degradation, thereby regulating their signaling and trafficking. Four non- visual GRKs are expressed in striatal neurons. Here we describe known effects of individual GRKs on dopamine receptors in cell culture and in the two in vivo models of dopamine-mediated signaling: behavioral response to psychostimulants and L-DOPA- induced dyskinesia. Dyskinesia, associated with dopamine super-sensitivity of striatal neurons, is a debilitating side effect of L-DOPA therapy in Parkinson's disease. In vivo, GRK subtypes show greater receptor specificity than in vitro or in cultured cells. Overexpression, knockdown, and knockout of individual GRKs, particularly GRK2 and GRK6, have differential effects on signaling of dopamine receptor subtypes in the brain. Furthermore, deletion of GRK isoforms in select striatal neuronal types differentially affects psychostimulant-induced behaviors. In addition, anti-dyskinetic effect of GRK3 does not require its kinase activity: it is mediated by the binding of its RGS-like domain to Gαq/11, which suppresses Gq/11 signaling. The data demonstrate that the dopamine signaling in defined neuronal types in vivo is regulated by specific and finely orchestrated actions of GRK isoforms.

G protein-coupled receptor kinase 6/β-arrestin 2 system in a rat model of dopamine supersensitivity psychosis

In humans, long-term antipsychotic treatment is known to induce movement disorders and a psychosis, called dopamine supersensitivity psychosis (DSP). The mechanism by which chronic administration of antipsychotic(s) causes DSP may be the treatment-induced up-regulation of dopamine D2 receptors (DRD2). G protein-coupled receptor kinase 6 (GRK6) and beta-arrestin 2 (ARRB2) play important roles in the trafficking of DRD2 by phosphorylation and internalization. We investigated the effects of chronic continuous treatment with mini-pump-administered haloperidol (HAL) on the sensitivity of Wistar rats to dopamine, as measured by the locomotor response to methamphetamine (MAP) and the density of striatal DRD2. Chronic continuous treatment with HAL resulted in significantly higher locomotor response to MAP and significantly higher striatal DRD2 density compared with those in rats administered vehicle (VEH). Enzyme-linked immunosorbent assays revealed that striatal ARRB2 in DSP model rats tended to decrease in comparison with that in the VEH group. In addition, the ratio of GRK6/ARRB2 in DSP model rats was significantly higher than that in controls. Our results suggest that alterations of the GRK6 and ARRB2 system could induce both DRD2 up-regulation and impairment of the dopamine signaling pathway, resulting potentially in the development of DSP.

Pathophysiology of L-dopa-induced motor and non-motor complications in Parkinson's disease

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa (L-dopa) therapy for Parkinson's disease (PD). L-dopa-induced dyskinesia (LID) are ultimately experienced by the vast majority of patients. In addition, psychiatric conditions often manifested as compulsive behaviours, are emerging as a serious problem in the management of L-dopa therapy. The present review attempts to provide an overview of our current understanding of dyskinesia and other L-dopa-induced dysfunctions, a field that dramatically evolved in the past twenty years. In view of the extensive literature on LID, there appeared a critical need to re-frame the concepts, to highlight the most suitable models, to review the central nervous system (CNS) circuitry that may be involved, and to propose a pathophysiological framework was timely and necessary. An updated review to clarify our understanding of LID and other L-dopa-related side effects was therefore timely and necessary. This review should help in the development of novel therapeutic strategies aimed at preventing the generation of dyskinetic symptoms.

Molecular mechanisms underlying β-adrenergic receptor-mediated cross-talk between sympathetic neurons and immune cells

Cross-talk between the sympathetic nervous system (SNS) and immune system is vital for health and well-being. Infection, tissue injury and inflammation raise firing rates of sympathetic nerves, increasing their release of norepinephrine (NE) in lymphoid organs and tissues. NE stimulation of β2-adrenergic receptors (ARs) in immune cells activates the cAMP-protein kinase A (PKA) intracellular signaling pathway, a pathway that interfaces with other signaling pathways that regulate proliferation, differentiation, maturation and effector functions in immune cells. Immune-SNS cross-talk is required to maintain homeostasis under normal conditions, to develop an immune response of appropriate magnitude after injury or immune challenge, and subsequently restore homeostasis. Typically, β2-AR-induced cAMP is immunosuppressive. However, many studies report actions of β2-AR stimulation in immune cells that are inconsistent with typical cAMP-PKA signal transduction. Research during the last decade in non-immune organs, has unveiled novel alternative signaling mechanisms induced by β2-AR activation, such as a signaling switch from cAMP-PKA to mitogen-activated protein kinase (MAPK) pathways. If alternative signaling occurs in immune cells, it may explain inconsistent findings of sympathetic regulation of immune function. Here, we review β2-AR signaling, assess the available evidence for alternative signaling in immune cells, and provide insight into the circumstances necessary for "signal switching" in immune cells.

Identification and structure-function analysis of subfamily selective G protein-coupled receptor kinase inhibitors

Selective inhibitors of individual subfamilies of G protein-coupled receptor kinases (GRKs) would serve as useful chemical probes as well as leads for therapeutic applications ranging from heart failure to Parkinson's disease. To identify such inhibitors, differential scanning fluorimetry was used to screen a collection of known protein kinase inhibitors that could increase the melting points of the two most ubiquitously expressed GRKs: GRK2 and GRK5. Enzymatic assays on 14 of the most stabilizing hits revealed that three exhibit nanomolar potency of inhibition for individual GRKs, some of which exhibiting orders of magnitude selectivity. Most of the identified compounds can be clustered into two chemical classes: indazole/dihydropyrimidine-containing compounds that are selective for GRK2 and pyrrolopyrimidine-containing compounds that potently inhibit GRK1 and GRK5 but with more modest selectivity. The two most potent inhibitors representing each class, GSK180736A and GSK2163632A, were cocrystallized with GRK2 and GRK1, and their atomic structures were determined to 2.6 and 1.85 Å spacings, respectively. GSK180736A, developed as a Rho-associated, coiled-coil-containing protein kinase inhibitor, binds to GRK2 in a manner analogous to that of paroxetine, whereas GSK2163632A, developed as an insulin-like growth factor 1 receptor inhibitor, occupies a novel region of the GRK active site cleft that could likely be exploited to achieve more selectivity. However, neither compound inhibits GRKs more potently than their initial targets. This data provides the foundation for future efforts to rationally design even more potent and selective GRK inhibitors.

Genetic association between G protein-coupled receptor kinase 6/β-arrestin 2 and dopamine supersensitivity psychosis in schizophrenia

BACKGROUND/AIM

Dopamine supersensitivity psychosis (DSP), clinically characterized by unstable and severe psychosis or tardive dyskinesia and often categorized as treatment-resistant schizophrenia, is promoted by long-term antipsychotic treatment. An upregulation of the dopamine D2 receptor caused by antipsychotic(s) is involved in the development of DSP. The present study explored the potential roles of G protein-coupled receptor kinase 6 (GRK6) and β-arrestin 2 (ARRB2) that are involved in the trafficking of DRD2 in patients with DSP.

METHODS

We conducted a genetic association study of GRK6/ARRB2 between the patients with DSP episodes [DSP(+) group: N=108] and the patients without DSP(-) episodes [DSP(-) group: N=169] from the total group of patients (N=333). Based on the patients' treatment history, a DSP episode was defined as withdrawal psychosis, developed tolerance to antipsychotic effect, and tardive dyskinesia (the remaining 56 patients were excluded due to insufficient information).

RESULTS

The results revealed that none of the allelic or genotyping distributions of five single nucleotide polymorphisms (SNPs) of GRK6 and three SNPs of ARRB2 showed any significant difference between the DSP(+) and DSP(-) groups.

CONCLUSION

The results suggest that the SNP analyses of these two molecules fail to classify patients into the potential clinical subtype of DSP(+) or DSP(-) group. However, since GRK6 and ARRB2 are surely involved in dopamine D2 receptor metabolism, further studies based on prospective observations of the onset of DSP under specific antipsychotic treatments are needed.

D1 dopamine receptor coupling to PLCβ regulates forward locomotion in mice

Several studies have reported the coupling of dopamine signaling to phospholipase C β (PLCβ) both in vitro and in vivo. However, the precise physiological relevance of this signaling pathway in mediating dopamine behaviors is still unclear. Here we report that stimulation of dopamine receptor signaling in vivo with systemic administration of apomorphine, amphetamine, and cocaine leads to increased production of inositol triphosphate (IP3) in the mouse striatum. Using selective antagonists and dopamine D1 and D2 receptor knock-out animals, we show that the production of IP3 is mediated by the D1 receptor, but not the D2 receptor. A selective blocker of PLCβ, U73122, was used to assess the physiological relevance of D1-mediated IP3 production. We show that U73122 inhibits the locomotor-stimulating effects of apomorphine, amphetamine, cocaine, and SKF81297. Furthermore, U73122 also suppresses the spontaneous hyperactivity exhibited by dopamine transporter knock-out mice. Importantly, the effects of U73122 are selective to dopamine-mediated hyperactivity, as this compound does not affect hyperactivity induced by the glutamate NMDA receptor antagonist MK801. Finally, we present evidence showing that an imbalance of D1- and D2-mediated signaling following U73122 treatment modifies the locomotor output of animals from horizontal locomotor activity to vertical activity, further highlighting the importance of the PLCβ pathway in the regulation of forward locomotion via dopamine receptors.

NMDA receptor regulation of levodopa-induced behavior and changes in striatal G protein-coupled receptor kinase 6 and β-arrestin-1 expression in parkinsonian rats

BACKGROUND

Parkinson's disease is a neurodegenerative disorder caused by loss of dopaminergic neurons in the substantia nigra. The dopamine precursor, levodopa, remains the most effective and common treatment for this disorder. However, long-term administration of levodopa is known to induce characteristic dyskinesia, and molecular mechanisms underlying dyskinesia are poorly understood.

METHODS

In this study, we investigated the effect of 6-hydroxydopamine lesions in dopaminergic neurons and chronic treatment with levodopa on expression of G protein-coupled receptor kinase 6 and β-arrestin-1, two key regulators of G protein-coupled receptors, in the rat striatum.

RESULTS

We found that a unilateral 6-hydroxydopamine lesion reduced expression of G protein-coupled receptor kinase 6 and β-arrestin-1 protein in the lesioned striatum. Reduction of these two proteins persisted in 6-hydroxydopamine-lesioned rats on chronic levodopa treatment for 23 days. In addition, coadministration of the N-methyl-D-aspartate receptor antagonist, MK-801, and levodopa reversed the reduction of G protein-coupled receptor kinase 6 and β-arrestin-1 in the striatum. MK-801 also attenuated levodopa-induced dyskinetic behavior.

CONCLUSION

These data indicate that G protein-coupled receptor kinase 6 and β-arrestin-1 in striatal neurons are sensitive to dopamine depletion and are downregulated in rats with Parkinson's disease and in levodopa-treated rats with the disease. This downregulation seems to require activation of N-methyl-D-aspartate glutamate receptors.

AGC protein kinases: from structural mechanism of regulation to allosteric drug development for the treatment of human diseases

The group of AGC protein kinases includes more than 60 protein kinases in the human genome, classified into 14 families: PDK1, AKT/PKB, SGK, PKA, PKG, PKC, PKN/PRK, RSK, NDR, MAST, YANK, DMPK, GRK and SGK494. This group is also widely represented in other eukaryotes, including causative organisms of human infectious diseases. AGC kinases are involved in diverse cellular functions and are potential targets for the treatment of human diseases such as cancer, diabetes, obesity, neurological disorders, inflammation and viral infections. Small molecule inhibitors of AGC kinases may also have potential as novel therapeutic approaches against infectious organisms. Fundamental in the regulation of many AGC kinases is a regulatory site termed the "PIF-pocket" that serves as a docking site for substrates of PDK1. This site is also essential to the mechanism of activation of AGC kinases by phosphorylation and is involved in the allosteric regulation of N-terminal domains of several AGC kinases, such as PKN/PRKs and atypical PKCs. In addition, the C-terminal tail and its interaction with the PIF-pocket are involved in the dimerization of the DMPK family of kinases and may explain the molecular mechanism of allosteric activation of GRKs by GPCR substrates. In this review, we briefly introduce the AGC kinases and their known roles in physiology and disease and the discovery of the PIF-pocket as a regulatory site in AGC kinases. Finally, we summarize the current status and future therapeutic potential of small molecules directed to the PIF-pocket; these molecules can allosterically activate or inhibit the kinase as well as act as substrate-selective inhibitors. This article is part of a Special Issue entitled: Inhibitors of Protein Kinases (2012).

Elimination of GRK2 from cholinergic neurons reduces behavioral sensitivity to muscarinic receptor activation

Although G-protein-coupled receptor kinase 2 (GRK2) is the most widely studied member of a family of kinases that has been shown to exert powerful influences on a variety of G-protein-coupled receptors, its role in the brain remains largely unknown. Here we report the localization of GRK2 in the mouse brain and generate novel conditional knock-out (KO) mice to assess the physiological importance of this kinase in cholinergic neurons. Mice with the selective deletion of GRK2 in this cell population (ChAT(IRES-cre)Grk2(f/f) KO mice) exhibit reduced behavioral responsiveness to challenge with oxotremorine-M (Oxo-M), a nonselective muscarinic acetylcholine receptor agonist. Specifically, Oxo-M-induced hypothermia, hypolocomotion, and salivation were markedly reduced in these animals, while analgesic responses were unaltered. In contrast, we found that GRK2 deficiency in cholinergic neurons does not alter cocaine-induced psychomotor activation, behavioral sensitization, or conditioned place preference. These results demonstrate that the elimination of GRK2 in cholinergic neurons reduces sensitivity to select muscarinic-mediated behaviors, while dopaminergic effects remain intact and further suggests that GRK2 may selectively impair muscarinic acetylcholine receptor-mediated function in vivo.