Structure of human GABAB receptor in an inactive state.

The human GABAB receptor-a member of the class C family of G-protein-coupled receptors (GPCRs)-mediates inhibitory neurotransmission and has been implicated in epilepsy, pain and addiction1. A unique GPCR that is known to require heterodimerization for function2-6, the GABAB receptor has two subunits, GABAB1 and GABAB2, that are structurally homologous but perform distinct and complementary functions. GABAB1 recognizes orthosteric ligands7,8, while GABAB2 couples with G proteins9-14. Each subunit is characterized by an extracellular Venus flytrap (VFT) module, a descending peptide linker, a seven-helix transmembrane domain and a cytoplasmic tail15. Although the VFT heterodimer structure has been resolved16, the structure of the full-length receptor and its transmembrane signalling mechanism remain unknown. Here we present a near full-length structure of the GABAB receptor, captured in an inactive state by cryo-electron microscopy. Our structure reveals several ligands that preassociate with the receptor, including two large endogenous phospholipids that are embedded within the transmembrane domains to maintain receptor integrity and modulate receptor function. We also identify a previously unknown heterodimer interface between transmembrane helices 3 and 5 of both subunits, which serves as a signature of the inactive conformation. A unique 'intersubunit latch' within this transmembrane interface maintains the inactive state, and its disruption leads to constitutive receptor activity.

Author Correction: Structure of human GABAB receptor in an inactive state.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Comparison of K+ Channel Families.

K+ channels enable potassium to flow across the membrane with great selectivity. There are four K+ channel families: voltage-gated K (Kv), calcium-activated (KCa), inwardly rectifying K (Kir), and two-pore domain potassium (K2P) channels. All four K+ channels are formed by subunits assembling into a classic tetrameric (4x1P = 4P for the Kv, KCa, and Kir channels) or tetramer-like (2x2P = 4P for the K2P channels) architecture. These subunits can either be the same (homomers) or different (heteromers), conferring great diversity to these channels. They share a highly conserved selectivity filter within the pore but show different gating mechanisms adapted for their function. K+ channels play essential roles in controlling neuronal excitability by shaping action potentials, influencing the resting membrane potential, and responding to diverse physicochemical stimuli, such as a voltage change (Kv), intracellular calcium oscillations (KCa), cellular mediators (Kir), or temperature (K2P).

Calcium modulates calmodulin/α-actinin 1 interaction with and agonist-dependent internalization of the adenosine A2A receptor.

Adenosine receptors are G protein-coupled receptors that sense extracellular adenosine to transmit intracellular signals. One of the four adenosine receptor subtypes, the adenosine A2A receptor (A2AR), has an exceptionally long intracellular C terminus (A2AR-ct) that mediates interactions with a large array of proteins, including calmodulin and α-actinin. Here, we aimed to ascertain the α-actinin 1/calmodulin interplay whilst binding to A2AR and the role of Ca2+ in this process. First, we studied the A2AR-α-actinin 1 interaction by means of native polyacrylamide gel electrophoresis, isothermal titration calorimetry, and surface plasmon resonance, using purified recombinant proteins. α-Actinin 1 binds the A2AR-ct through its distal calmodulin-like domain in a Ca2+-independent manner with a dissociation constant of 5-12µM, thus showing an ~100 times lower affinity compared to the A2AR-calmodulin/Ca2+ complex. Importantly, calmodulin displaced α-actinin 1 from the A2AR-ct in a Ca2+-dependent fashion, disrupting the A2AR-α-actinin 1 complex. Finally, we assessed the impact of Ca2+ on A2AR internalization in living cells, a function operated by the A2AR-α-actinin 1 complex. Interestingly, while Ca2+ influx did not affect constitutive A2AR endocytosis, it abolished agonist-dependent internalization. In addition, we demonstrated that the A2AR/α-actinin interaction plays a pivotal role in receptor internalization and function. Overall, our results suggest that the interplay of A2AR with calmodulin and α-actinin 1 is fine-tuned by Ca2+, a fact that might power agonist-mediated receptor internalization and function.

Photomodulation of G protein-coupled adenosine receptors by a novel light-switchable ligand.

The adenosinergic system operates through G protein-coupled adenosine receptors, which have become promising therapeutic targets for a wide range of pathological conditions. However, the ubiquity of adenosine receptors and the eventual lack of selectivity of adenosine-based drugs have frequently diminished their therapeutic potential. Accordingly, here we aimed to develop a new generation of light-switchable adenosine receptor ligands that change their intrinsic activity upon irradiation, thus allowing the spatiotemporal control of receptor functioning (i.e., receptor activation/inactivation dependent on location and timing). Therefore, we synthesized an orthosteric, photoisomerizable, and nonselective adenosine receptor agonist, nucleoside derivative MRS5543 containing an aryl diazo linkage on the N(6) substituent, which in the dark (relaxed isomer) behaved as a full adenosine A3 receptor (A3R) and partial adenosine A2A receptor (A2AR) agonist. Conversely, upon photoisomerization with blue light (460 nm), it remained a full A3R agonist but became an A2AR antagonist. Interestingly, molecular modeling suggested that structural differences encountered within the third extracellular loop of each receptor could modulate the intrinsic, receptor subtype-dependent, activity. Overall, the development of adenosine receptor ligands with photoswitchable activity expands the pharmacological toolbox in support of research and possibly opens new pharmacotherapeutic opportunities.

Striatal adenosine A2A receptor expression is controlled by S-adenosyl-L-methionine-mediated methylation.

Adenosine A2A receptor (A2AR) is a G protein-coupled receptor enriched in the striatum for which an increased expression has been demonstrated in certain neurological diseases. Interestingly, previous in vitro studies demonstrated that A2AR expression levels are reduced after treatment with S-adenosyl-L-methionine (SAM), a methyl donor molecule involved in the methylation of important biological structures such as DNA, proteins, and lipids. However, the in vivo effects of SAM treatment on A2AR expression are still obscure. Here, we demonstrated that 2 weeks of SAM treatment produced a significant reduction in the rat striatal A2AR messenger RNA (mRNA) and protein content as well as A2AR-mediated signaling. Furthermore, when the content of 5-methylcytosine levels in the 5'UTR region of ADORA2A was analyzed, this was significantly increased in the striatum of SAM-treated animals; thus, an unambiguous correlation between SAM-mediated methylation and striatal A2AR expression could be established. Overall, we concluded that striatal A2AR functionality can be controlled by SAM treatment, an issue that might be relevant for the management of these neurological conditions that course with increased A2AR expression.

Visualizing G Protein-Coupled Receptor-Receptor Interactions in Brain Using Proximity Ligation In Situ Assay.

G protein-coupled receptors (GPCRs) constitute the largest family of plasma membrane receptors and the main drug targets in therapeutics. GPCRs can establish direct receptor-receptor interactions (oligomerization), which can also be considered as targets for drug development (GPCR oligomer-based drugs). However, prior to designing any novel GPCR oligomer-based drug development program, demonstrating the existence of a named GPCR oligomer in native tissues is needed as part of its target engagement definition. Here, we discuss the proximity ligation in situ assay (P-LISA), an experimental approach that reveals GPCR oligomerization in native tissues. We provide a detailed step-by-step protocol to perform P-LISA experiments and visualize GPCR oligomers in brain slices. We also provide instructions for slide observation, data acquisition, and quantification. Finally, we discuss the critical aspects determining the success of the technique, namely the fixation process and the validation of the primary antibodies used. Overall, this protocol may be used to straightforwardly visualize GPCR oligomers in the brain. © 2023 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol: Visualization of GPCR oligomers by proximity ligation in situ assay (P-LISA) Support Protocol: Slide observation, image acquisition, and quantification.

Optical control of neuronal activities with photoswitchable nanovesicles.

Precise modulation of neuronal activity by neuroactive molecules is essential for understanding brain circuits and behavior. However, tools for highly controllable molecular release are lacking. Here, we developed a photoswitchable nanovesicle with azobenzene-containing phosphatidylcholine (azo-PC), coined 'azosome', for neuromodulation. Irradiation with 365 nm light triggers the trans-to-cis isomerization of azo-PC, resulting in a disordered lipid bilayer with decreased thickness and cargo release. Irradiation with 455 nm light induces reverse isomerization and switches the release off. Real-time fluorescence imaging shows controllable and repeatable cargo release within seconds (< 3 s). Importantly, we demonstrate that SKF-81297, a dopamine D1-receptor agonist, can be repeatedly released from the azosome to activate cultures of primary striatal neurons. Azosome shows promise for precise optical control over the molecular release and can be a valuable tool for molecular neuroscience studies.

Corrigendum: Striatal dopamine D2-muscarinic acetylcholine M1 receptor-receptor interaction in a model of movement disorders.

[This corrects the article DOI: 10.3389/fphar.2020.00194.].

The mGlu5 Receptor Protomer-Mediated Dopamine D2 Receptor Trans-Inhibition Is Dependent on the Adenosine A2A Receptor Protomer: Implications for Parkinson's Disease.

The adenosine A2A receptor (A2AR), dopamine D2 receptor (D2R) and metabotropic glutamate receptor type 5 (mGluR5) form A2AR-D2R-mGluR5 heteroreceptor complexes in living cells and in rat striatal neurons. In the current study, we present experimental data supporting the view that the A2AR protomer plays a major role in the inhibitory modulation of the density and the allosteric receptor-receptor interaction within the D2R-mGluR5 heteromeric component of the A2AR-D2R-mGluR5 complex in vitro and in vivo. The A2AR and mGluR5 protomers interact and modulate D2R protomer recognition and signalling upon forming a trimeric complex from these receptors. Expression of A2AR in HEK293T cells co-expressing D2R and mGluR5 resulted in a significant and marked increase in the formation of the D2R-mGluR5 heteromeric component in both bioluminescence resonance energy transfer and proximity ligation assays. A highly significant increase of the the high-affinity component of D2R (D2RKi High) values was found upon cotreatment with the mGluR5 and A2AR agonists in the cells expressing A2AR, D2R and mGluR5 with a significant effect observed also with the mGluR5 agonist alone compared to cells expressing only D2R and mGluR5. In cells co-expressing A2AR, D2R and mGluR5, stimulation of the cells with an mGluR5 agonist like or D2R antagonist fully counteracted the D2R agonist-induced inhibition of the cAMP levels which was not true in cells only expressing mGluR5 and D2R. In agreement, the mGluR5-negative allosteric modulator raseglurant significantly reduced the haloperidol-induced catalepsy in mice, and in A2AR knockout mice, the haloperidol action had almost disappeared, supporting a functional role for mGluR5 and A2AR in enhancing D2R blockade resulting in catalepsy. The results represent a relevant example of integrative activity within higher-order heteroreceptor complexes.

Striatal Dopamine D2-Muscarinic Acetylcholine M1 Receptor-Receptor Interaction in a Model of Movement Disorders.

Parkinson's disease (PD) is a neurodegenerative disorder characterized by motor control deficits, which is associated with the loss of striatal dopaminergic neurons from the substantia nigra. In parallel to dopaminergic denervation, there is an increase of acetylcholine within the striatum, resulting in a striatal dopaminergic-cholinergic neurotransmission imbalance. Currently, available PD pharmacotherapy (e.g., prodopaminergic drugs) does not reinstate the altered dopaminergic-cholinergic balance. In addition, it can eventually elicit cholinergic-related adverse effects. Here, we investigated the interplay between dopaminergic and cholinergic systems by assessing the physical and functional interaction of dopamine D2 and muscarinic acetylcholine M1 receptors (D2R and M1R, respectively), both expressed at striatopallidal medium spiny neurons. First, we provided evidence for the existence of D2R-M1R complexes via biochemical (i.e., co-immunoprecipitation) and biophysical (i.e., BRET1 and NanoBiT®) assays, performed in transiently transfected HEK293T cells. Subsequently, a D2R-M1R co-distribution in the mouse striatum was observed through double-immunofluorescence staining and AlphaLISA® immunoassay. Finally, we evaluated the functional interplay between both receptors via behavioral studies, by implementing the classical acute reserpine pharmacological animal model of experimental parkinsonism. Reserpinized mice were administered with a D2R-selective agonist (sumanirole) and/or an M1R-selective antagonist (VU0255035), and alterations in PD-related behavioral tasks (i.e., locomotor activity) were evaluated. Importantly, VU0255035 (10 mg/kg) potentiated the antiparkinsonian-like effects (i.e., increased locomotor activity and decreased catalepsy) of an ineffective sumanirole dose (3 mg/kg). Altogether, our data suggest the existence of putative striatal D2R/M1R heteromers, which might be a relevant target to manage PD motor impairments with fewer adverse effects.

Adenosine A2A-Cannabinoid CB1 Receptor Heteromers in the Hippocampus: Cannabidiol Blunts Δ9-Tetrahydrocannabinol-Induced Cognitive Impairment.

At present, clinical interest in the plant-derived cannabinoid compound cannabidiol (CBD) is rising exponentially, since it displays multiple therapeutic properties. In addition, CBD can counteract the undesirable effects of the psychoactive cannabinoid Δ9-tetrahydrocannabinol (Δ9-THC) that hinder clinical development of cannabis-based therapies. Despite this attention, the mechanisms of CBD action and its interaction with Δ9-THC are still not completely elucidated. Here, by combining in vivo and complementary molecular techniques, we demonstrate for the first time that CBD blunts the Δ9-THC-induced cognitive impairment in an adenosine A2A receptor (A2AR)-dependent manner. Furthermore, we reveal the existence of A2AR and cannabinoid CB1 receptor (CB1R) heteromers at the presynaptic level in CA1 neurons in the hippocampus. Interestingly, our findings support a brain region-dependent A2AR-CB1R functional interplay; indeed, CBD was not capable of modifying motor functions presumably regulated by striatal A2AR/CB1R complexes, nor anxiety responses related to other brain regions. Overall, these data provide new evidence regarding the mechanisms of action of CBD and the nature of A2AR-CB1R interactions in the brain.

Effects of the Dopamine Stabilizer, Pridopidine, on Basal and Phencyclidine-Induced Locomotion: Role of Dopamine D2 and Sigma-1 Receptors.

BACKGROUND: Pridopidine, a compound in clinical trials for Huntington's disease treatment, was originally synthesized as a dopamine D2 receptor (D2R) ligand, but later found to possess higher affinity for the sigma-1 receptor (S1R). However, the putative contributions of D2R and S1R to the behavioral profile of acutely administered pridopidine have not been investigated. OBJECTIVE: The present study sought to compare the effects of acute pridopidine on wild-type vs. D2R and S1R knockout mice, at high (60 mg/kg) and low (6 mg/kg) doses. METHOD: Pridopidine effects on basal and phencyclidine-induced locomotor activity was measured in the open field test. Additionally, the actions of pridopidine on prepulse inhibition was measured in animals treated with saline or phencyclidine. RESULTS: Whereas inhibition of spontaneous and phencyclidine-induced locomotion was readily observed at 60 mg/kg pridopidine, neither locomotor stimulation in habituated mice, nor any effects on prepulse inhibition were detected upon pridopidine treatment. Surprisingly, inhibition of spontaneous locomotion was unaffected by both D2R and S1R deletion. CONCLUSION: The present results suggest the involvement of additional targets, besides D2R and S1R, in mediating locomotor inhibition by pridopidine.

PBF509, an Adenosine A2A Receptor Antagonist With Efficacy in Rodent Models of Movement Disorders.

Adenosine A2A receptor (A2AR) antagonists have emerged as complementary non-dopaminergic drugs to alleviate Parkinson's disease (PD) symptomatology. Here, we characterize a novel non-xhantine non-furan A2AR antagonist, PBF509, as a potential pro-dopaminergic drug for PD management. First, PBF509 was shown to be a highly potent ligand at the human A2AR, since it antagonized A2AR agonist-mediated cAMP accumulation and impedance responses with KB values of 72.8 ± 17.4 and 8.2 ± 4.2 nM, respectively. Notably, these results validated our new A2AR-based label-free assay as a robust and sensitive approach to characterize A2AR ligands. Next, we evaluated the efficacy of PBF509 reversing motor impairments in several rat models of movement disorders, including catalepsy, tremor, and hemiparkinsonism. Thus, PBF509 (orally) antagonized haloperidol-mediated catalepsy, reduced pilocarpine-induced tremulous jaw movements and potentiated the number of contralateral rotations induced by L-3,4-dihydroxyphenylalanine (L-DOPA) in unilaterally 6-OHDA-lesioned rats. Moreover, PBF509 (3 mg/kg) inhibited L-DOPA-induced dyskinesia (LID), showing not only its efficacy on reversing parkinsonian motor impairments but also acting as antidyskinetic agent. Overall, here we describe a new orally selective A2AR antagonist with potential utility for PD treatment, and for some of the side effects associated to the current pharmacotherapy (i.e., dyskinesia).

Antipsychotic-Like Efficacy of Dopamine D2 Receptor-Biased Ligands is Dependent on Adenosine A2A Receptor Expression.

Dopamine D2 receptor (D2R) activation triggers both G protein- and ß-arrestin-dependent signaling. Biased D2R ligands activating ß-arrestin pathway have been proposed as potential antipsychotics. The ability of D2R to heteromerize with adenosine A2A receptor (A2AR) has been associated to D2R agonist-induced ß-arrestin recruitment. Accordingly, here we aimed to demonstrate the A2AR dependence of D2R/ß-arrestin signaling. By combining bioluminescence resonance energy transfer (BRET) between ß-arrestin-2 tagged with yellow fluorescent protein and bimolecular luminescence complementation (BiLC) of D2R/A2AR homomers and heteromers, we demonstrated that the D2R agonists quinpirole and UNC9994 could promote ß-arrestin-2 recruitment only when A2AR/D2R heteromers were expressed. Subsequently, the role of A2AR in the antipsychotic-like activity of UNC9994 was assessed in wild-type and A2AR-/- mice administered with phencyclidine (PCP) or amphetamine (AMPH). Interestingly, while UNC9994 reduced hyperlocomotion in wild-type animals treated either with PCP or AMPH, in A2AR-/- mice, it failed to reduce PCP-induced hyperlocomotion or produced only a moderate reduction of AMPH-mediated hyperlocomotion. Overall, the results presented here reinforce the notion that D2R/A2AR heteromerization facilitates D2R ß-arrestin recruitment, and furthermore, reveal a pivotal role for A2AR in the antipsychotic-like activity of the ß-arrestin-biased D2R ligand, UNC9994.

Remote control of movement disorders using a photoactive adenosine A2A receptor antagonist.

G protein-coupled adenosine receptors are promising therapeutic targets for a wide range of neuropathological conditions, including Parkinson's disease (PD). However, the ubiquity of adenosine receptors and the ultimate lack of selectivity of certain adenosine-based drugs have frequently diminished their therapeutic use. Photopharmacology is a novel approach that allows the spatiotemporal control of receptor function, thus circumventing some of these limitations. Here, we aimed to develop a light-sensitive caged adenosine A2A receptor (A2AR) antagonist to photocontrol movement disorders. We synthesized MRS7145 by blocking with coumarin the 5-amino position of the selective A2AR antagonist SCH442416, which could be photoreleased upon violet light illumination (405 nm). First, the light-dependent pharmacological profile of MRS7145 was determined in A2AR-expressing cells. Upon photoactivation, MRS7145 precluded A2AR ligand binding and agonist-induced cAMP accumulation. Next, the ability of MRS7145 to block A2AR in a light-dependent manner was assessed in vivo. To this end, A2AR antagonist-mediated locomotor activity potentiation was evaluated in brain (striatum) fiber-optic implanted mice. Upon irradiation (405 nm) of the dorsal striatum, MRS7145 induced significant hyperlocomotion and counteracted haloperidol-induced catalepsy and pilocarpine-induced tremor. Finally, its efficacy in reversing motor impairment was evaluated in a PD animal model, namely the hemiparkinsonian 6-hydroxydopamine (6-OHDA)-lesioned mouse. Photo-activated MRS7145 was able to potentiate the number of contralateral rotations induced by L-3,4-dihydroxyphenylalanine (l-DOPA). Overall, MRS7145 is a new light-operated A2AR antagonist with potential utility to manage movement disorders, including PD.

Essential Control of the Function of the Striatopallidal Neuron by Pre-coupled Complexes of Adenosine A2A-Dopamine D2 Receptor Heterotetramers and Adenylyl Cyclase.

The central adenosine system and adenosine receptors play a fundamental role in the modulation of dopaminergic neurotransmission. This is mostly achieved by the strategic co-localization of different adenosine and dopamine receptor subtypes in the two populations of striatal efferent neurons, striatonigral and striatopallidal, that give rise to the direct and indirect striatal efferent pathways, respectively. With optogenetic techniques it has been possible to dissect a differential role of the direct and indirect pathways in mediating "Go" responses upon exposure to reward-related stimuli and "NoGo" responses upon exposure to non-rewarded or aversive-related stimuli, respectively, which depends on their different connecting output structures and their differential expression of dopamine and adenosine receptor subtypes. The striatopallidal neuron selectively expresses dopamine D2 receptors (D2R) and adenosine A2A receptors (A2AR), and numerous experiments using multiple genetic and pharmacological in vitro, in situ and in vivo approaches, demonstrate they can form A2AR-D2R heteromers. It was initially assumed that different pharmacological interactions between dopamine and adenosine receptor ligands indicated the existence of different subpopulations of A2AR and D2R in the striatopallidal neuron. However, as elaborated in the present essay, most evidence now indicates that all interactions can be explained with a predominant population of striatal A2AR-D2R heteromers forming complexes with adenylyl cyclase subtype 5 (AC5). The A2AR-D2R heteromer has a tetrameric structure, with two homodimers, which allows not only multiple allosteric interactions between different orthosteric ligands, agonists, and antagonists, but also the canonical Gs-Gi antagonistic interaction at the level of AC5. We present a model of the function of the A2AR-D2R heterotetramer-AC5 complex, which acts as an integrative device of adenosine and dopamine signals that determine the excitability and gene expression of the striatopallidal neurons. The model can explain most behavioral effects of A2AR and D2R ligands, including the psychostimulant effects of caffeine. The model is also discussed in the context of different functional striatal compartments, mainly the dorsal and the ventral striatum. The current accumulated knowledge of the biochemical properties of the A2AR-D2R heterotetramer-AC5 complex offers new therapeutic possibilities for Parkinson's disease, schizophrenia, SUD and other neuropsychiatric disorders with dysfunction of dorsal or ventral striatopallidal neurons.

Visualizing G Protein-Coupled Receptor-Receptor Interactions in Brain Using Proximity Ligation In Situ Assay.

G protein-coupled receptors (GPCRs) constitute the largest family of plasma membrane receptors, thus representing the more investigated drug targets in the design of new therapeutic strategies. The existence of receptor-receptor interactions has revolutionized the field, since GPCR oligomerization might confer new intervention opportunities in pharmacotherapy. However, demonstrating the existence of such receptor-receptor interactions in native tissue has been a bottleneck in GPCR pharmacology. Here, we discuss an experimental approach, the proximity ligation in situ assay (P-LISA), which provides enough sensitivity to evaluate a receptor's close proximity within a named GPCR oligomer. Indeed, we provide a detailed step-by-step protocol for P-LISA experiments visualizing receptor-receptor interactions in brain slices. Additionally, we provide instructions for slide observation, data acquisition and quantification. Finally, we also discuss these critical aspects determining the success of the technique, namely the fixation process and the validation of the primary antibodies used. Overall, the P-LISA is a powerful and straightforward technique to visualize receptor-receptor interactions when performed under optimal conditions.

Untangling dopamine-adenosine receptor-receptor assembly in experimental parkinsonism in rats.

Parkinson's disease (PD) is a dopaminergic-related pathology in which functioning of the basal ganglia is altered. It has been postulated that a direct receptor-receptor interaction - i.e. of dopamine D2 receptor (D2R) with adenosine A2A receptor (A2AR) (forming D2R-A2AR oligomers) - finely regulates this brain area. Accordingly, elucidating whether the pathology prompts changes to these complexes could provide valuable information for the design of new PD therapies. Here, we first resolved a long-standing question concerning whether D2R-A2AR assembly occurs in native tissue: by means of different complementary experimental approaches (i.e. immunoelectron microscopy, proximity ligation assay and TR-FRET), we unambiguously identified native D2R-A2AR oligomers in rat striatum. Subsequently, we determined that, under pathological conditions (i.e. in a rat PD model), D2R-A2AR interaction was impaired. Collectively, these results provide definitive evidence for alteration of native D2R-A2AR oligomers in experimental parkinsonism, thus conferring the rationale for appropriate oligomer-based PD treatments.

Uncovering caffeine's adenosine A2A receptor inverse agonism in experimental parkinsonism.

Caffeine, the most consumed psychoactive substance worldwide, may have beneficial effects on Parkinson's disease (PD) therapy. The mechanism by which caffeine contributes to its antiparkinsonian effects by acting as either an adenosine A2A receptor (A2AR) neutral antagonist or an inverse agonist is unresolved. Here we show that caffeine is an A2AR inverse agonist in cell-based functional studies and in experimental parkinsonism. Thus, we observed that caffeine triggers a distinct mode, opposite to A2AR agonist, of the receptor's activation switch leading to suppression of its spontaneous activity. These inverse agonist-related effects were also determined in the striatum of a mouse model of PD, correlating well with increased caffeine-mediated motor effects. Overall, caffeine A2AR inverse agonism may be behind some of the well-known physiological effects of this substance both in health and disease. This information might have a critical mechanistic impact for PD pharmacotherapeutic design.