GPR88 in A2AR Neurons Enhances Anxiety-Like Behaviors

Molecular insights into orphan G protein-coupled receptors relevant to schizophrenia

Schizophrenia remains a sizable socio-economic burden that continues to be treated with therapeutics based on 70-year old science. All currently approved therapeutics primarily target the dopamine D2 receptor to achieve their efficacy. Whilst dopaminergic dysregulation is a key feature in this disorder, the targeting of dopaminergic machinery has yielded limited efficacy and an appreciable side effect burden. Over the recent decades, numerous drugs that engage non-dopaminergic G protein-coupled receptors (GPCRs) have yielded a promise of efficacy without the deleterious side effect profile, yet none have successfully completed clinical studies and progressed to the market. More recently, there has been increased attention around non-dopaminergic GPCR-targeting drugs, which demonstrated efficacy in some schizophrenia symptom domains. This provides renewed hope that effective schizophrenia treatment may lie outside of the dopaminergic space. Despite the potential for muscarinic receptor- (and other well-characterised GPCR families) targeting drugs to treat schizophrenia, they are often plagued with complications such as lack of receptor subtype selectivity and peripheral on-target side effects. Orphan GPCR studies have opened a new avenue of exploration with many demonstrating schizophrenia-relevant mechanisms and a favourable expression profile, thus offering potential for novel drug development. This review discusses centrally expressed orphan GPCRs: GPR3, GPR6, GPR12, GPR52, GPR85, GPR88 and GPR139 and their relationship to schizophrenia. We review their expression, signalling mechanisms and cellular function, in conjunction with small molecule development and structural insights. We seek to provide a snapshot of the growing evidence and development potential of new classes of schizophrenia therapeutics. LINKED ARTICLES: This article is part of a themed issue Therapeutic Targeting of G Protein-Coupled Receptors: hot topics from the Australasian Society of Clinical and Experimental Pharmacologists and Toxicologists 2021 Virtual Annual Scientific Meeting. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.14/issuetoc.

Orphan peptide and G protein-coupled receptor signalling in alcohol use disorder

Neuropeptides and G protein-coupled receptors (GPCRs) have long been, and continue to be, one of the most popular target classes for drug discovery in CNS disorders, including alcohol use disorder (AUD). Yet, orphaned neuropeptide systems and receptors (oGPCR), which have no known cognate receptor or ligand, remain understudied in drug discovery and development. Orphan neuropeptides and oGPCRs are abundantly expressed within the brain and represent an unprecedented opportunity to address brain function and may hold potential as novel treatments for disease. Here, we describe the current literature regarding orphaned neuropeptides and oGPCRs implicated in AUD. Specifically, in this review, we focus on the orphaned neuropeptide cocaine- and amphetamine-regulated transcript (CART), and several oGPCRs that have been directly implicated in AUD (GPR6, GPR26, GPR88, GPR139, GPR158) and discuss their potential and pitfalls as novel treatments, and progress in identifying their cognate receptors or ligands.

Chronic tianeptine induces tolerance in analgesia and hyperlocomotion via mu-opioid receptor activation in mice

Introduction

Tianeptine is approved in some countries to treat depression and anxiety. In addition to its activity on serotonin and glutamate neurotransmission, tianeptine has been proven to be a mu-opioid receptor (MOR) agonist, but only a few preclinical studies have characterized the opioid-like behavioral effects of tianeptine.

Methods

In this study, we tested tianeptine activity on G protein activation using the [S35] GTPγS binding assay in brain tissue from MOR+/+ and MOR-/- mice. Then, to determine whether tianeptine behavioral responses are MOR-dependent, we characterized the analgesic, locomotor, and rewarding responses of tianeptine in MOR+/+ and MOR-/- mice using tail immersion, hot plate, locomotor, and conditioned place preference tests.

Results

Using the [S35] GTPγS binding assay, we found that tianeptine signaling is mediated by MOR in the brain with properties similar to those of DAMGO (a classic MOR agonist). Furthermore, we found that the MOR is necessary for tianeptine's analgesic (tail immersion and hot plate), locomotor, and rewarding (conditioned place preference) effects. Indeed, these behavioral effects could only be measured in MOR+/+ mice but not in MOR-/- mice. Additionally, chronic administration of tianeptine induced tolerance to its analgesic and hyperlocomotor effects.

Discussion

These findings suggest that tianeptine's opioid-like effects require MOR and that chronic use could lead to tolerance.

Improvement of the Metabolic Stability of GPR88 Agonist RTI-13951-33: Design, Synthesis, and Biological Evaluation

GPR88 is an orphan G protein-coupled receptor mainly expressed in the brain, whose endogenous ligand has not yet been identified. To elucidate GPR88 functions, our group has developed RTI-13951-33 (1b) as the first in vivo active GPR88 agonist, but its poor metabolic stability and moderate brain permeability remain to be further optimized. Here, we report the design, synthesis, and pharmacological characterization of a new series of RTI-13951-33 analogues with the aim of improving pharmacokinetic properties. As a result, we identified a highly potent GPR88 agonist RTI-122 (30a) (cAMP EC50 = 11 nM) with good metabolic stability (half-life of 5.8 h) and brain permeability (brain/plasma ratio of >1) in mice. Notably, RTI-122 was more effective than RTI-13951-33 in attenuating the binge-like alcohol drinking behavior in the drinking-in-the-dark paradigm. Collectively, our findings suggest that RTI-122 is a promising lead compound for drug discovery research of GPR88 agonists.

Synthesis and pharmacological validation of a novel radioligand for the orphan GPR88 receptor

GPR88 is an orphan G protein-coupled receptor which has been implicated in a number of striatal-associated disorders. Herein we describe the synthesis and pharmacological characterization of the first GPR88 radioligand, [3H]RTI-33, derived from a synthetic agonist RTI-13951-33. [3H]RTI-33 has a specific activity of 83.4 Ci/mmol and showed one-site, saturable binding (KD of 85 nM) in membranes prepared from stable PPLS-HA-hGPR88-CHO cells. A competition binding assay was developed to determine binding affinities of several known GPR88 agonists. This radioligand represents a powerful tool for future mechanistic and cell-based ligand-receptor interaction studies of GPR88.

The orphan receptor GPR88 controls impulsivity and is a risk factor for Attention-Deficit/Hyperactivity Disorder

The neural orphan G protein coupled receptor GPR88 is predominant in the striatum and cortex of both rodents and humans, and considered a potential target for brain disorders. Previous studies have shown multiple behavioral phenotypes in Gpr88 knockout mice, and human genetic studies have reported association with psychosis. Here we tested the possibility that GPR88 contributes to Attention Deficit Hyperactivity Disorder (ADHD). In the mouse, we tested Gpr88 knockout mice in three behavioral paradigms, best translatable between rodents and humans, and found higher motor impulsivity and reduced attention together with the reported hyperactivity. Atomoxetine, a typical ADHD drug, reduced impulsivity in mutant mice. Conditional Gpr88 knockout mice in either D1R-type or D2R-type medium spiny neurons revealed distinct implications of the two receptor populations in waiting and stopping impulsivity. Thus, animal data demonstrate that deficient GPR88 activity causally promotes ADHD-like behaviors, and identify circuit mechanisms underlying GPR88-regulated impulsivity. In humans, we performed a family-based genetic study including 567 nuclear families with DSM-IV diagnosis of ADHD. There was a minor association for SNP rs2036212 with diagnosis, treatment response and cognition. A stronger association was found for SNP rs2809817 upon patient stratification, suggesting that the T allele is a risk factor when prenatal stress is involved. Human data therefore identify GPR88 variants associated with the disease, and highlight a potential role of life trajectories to modulate GPR88 function. Overall, animal and human data concur to suggest that GPR88 signaling should be considered a key factor for diagnostic and treatment of ADHD.

Systems spatiotemporal dynamics of traumatic brain injury at single-cell resolution reveals humanin as a therapeutic target

BACKGROUND

The etiology of mild traumatic brain injury (mTBI) remains elusive due to the tissue and cellular heterogeneity of the affected brain regions that underlie cognitive impairments and subsequent neurological disorders. This complexity is further exacerbated by disrupted circuits within and between cell populations across brain regions and the periphery, which occur at different timescales and in spatial domains.

METHODS

We profiled three tissues (hippocampus, frontal cortex, and blood leukocytes) at the acute (24-h) and subacute (7-day) phases of mTBI at single-cell resolution.

RESULTS

We demonstrated that the coordinated gene expression patterns across cell types were disrupted and re-organized by TBI at different timescales with distinct regional and cellular patterns. Gene expression-based network modeling implied astrocytes as a key regulator of the cell-cell coordination following mTBI in both hippocampus and frontal cortex across timepoints, and mt-Rnr2, which encodes the mitochondrial peptide humanin, as a potential target for intervention based on its broad regional and dynamic dysregulation following mTBI. Treatment of a murine mTBI model with humanin reversed cognitive impairment caused by mTBI through the restoration of metabolic pathways within astrocytes.

CONCLUSIONS

Our results offer a systems-level understanding of the dynamic and spatial regulation of gene programs by mTBI and pinpoint key target genes, pathways, and cell circuits that are amenable to therapeutics.

Characterization of Affective Behaviors and Motor Functions in Mice With a Striatal-Specific Deletion of Bmal1 and Per2

The expression of circadian clock genes, either centrally or in the periphery, has been shown to play an integral role in the control of behavior. Brain region-specific downregulation of clock genes revealed behavioral phenotypes associated with neuropsychiatric disorders and neurodegenerative disease. The specific function of the clock genes as well as the underlying mechanisms that contribute to the observed phenotypes, however, are not yet fully understood. We assessed anxiety- and depressive-like behavior and motor functions in male and female mice with a conditional ablation of Bmal1 or Per2 from medium spiny neurons (MSNs) of the striatum as well as mice lacking one copy of Gpr88. Whereas the conditional knockout of Bmal1 and Per2 had mild effects on affective behaviors, a pronounced effect on motor functions was found in Bmal1 knockout mice. Subsequent investigation revealed an attenuated response of Bmal1 knockout mice to dopamine receptor type 1 agonist treatment, independently of the expression of targets of the dopamine signaling pathway or mitochondrial respiration in MSNs. The study thus suggests a potential interaction of Bmal1 within the direct dopamine signaling pathway, which may provide the link to a shared, MSN-dependent mechanism regulating affective behavior and motor function in mice.

Update on GPCR-based targets for the development of novel antidepressants

Traditional antidepressants largely interfere with monoaminergic transport or degradation systems, taking several weeks to have their therapeutic actions. Moreover, a large proportion of depressed patients are resistant to these therapies. Several atypical antidepressants have been developed which interact with G protein coupled receptors (GPCRs) instead, as direct targeting of receptors may achieve more efficacious and faster antidepressant actions. The focus of this review is to provide an update on how distinct GPCRs mediate antidepressant actions and discuss recent insights into how GPCRs regulate the pathophysiology of Major Depressive Disorder (MDD). We also discuss the therapeutic potential of novel GPCR targets, which are appealing due to their ligand selectivity, expression pattern, or pharmacological profiles. Finally, we highlight recent advances in understanding GPCR pharmacology and structure, and how they may provide new avenues for drug development.

Evaluation of Amide Bioisosteres Leading to 1,2,3-Triazole Containing Compounds as GPR88 Agonists: Design, Synthesis, and Structure-Activity Relationship Studies

The orphan receptor GPR88 has been implicated in a number of striatal-associated disorders, yet its endogenous ligand has not been discovered. We have previously reported that the amine functionality in the 2-AMPP-derived GPR88 agonists can be replaced with an amide (e.g., 4) without losing activity. Later, we have found that the amide can be replaced with a bioisosteric 1,3,4-oxadiazole with improved potency. Here, we report a further study of amide bioisosteric replacement with a variety of azoles containing three heteroatoms, followed by a focused structure-activity relationship study, leading to the discovery of a series of novel 1,4-disubstituted 1H-1,2,3-triazoles as GPR88 agonists. Collectively, our medicinal chemistry efforts have resulted in a potent, efficacious, and brain-penetrant GPR88 agonist 53 (cAMP EC50 = 14 nM), which is a suitable probe to study GPR88 functions in the brain.

Design, Synthesis, and Structure-Activity Relationship Studies of (4-Alkoxyphenyl)glycinamides and Bioisosteric 1,3,4-Oxadiazoles as GPR88 Agonists

Increasing evidence implicates the orphan G protein-coupled receptor 88 (GPR88) in a number of striatal-associated disorders. In this study, we report the design and synthesis of a series of novel (4-alkoxyphenyl)glycinamides (e.g., 31) and the corresponding 1,3,4-oxadiazole bioisosteres derived from the 2-AMPP scaffold (1) as GPR88 agonists. The 5-amino-1,3,4-oxadiazole derivatives (84, 88-90) had significantly improved potency and lower lipophilicity compared to 2-AMPP. Compound 84 had an EC₅₀ of 59 nM in the GPR88 overexpressing cell-based cAMP assay. In addition, 84 had an EC₅₀ of 942 nM in the [³⁵S]GTPγS binding assay using mouse striatal membranes but was inactive in membranes from GPR88 knockout mice, even at a concentration of 100 μM. In vivo pharmacokinetic testing of 90 in rats revealed that the 5-amino-1,3,4-oxadiazole analogues may have limited brain permeability. Taken together, these results provide the basis for further optimization to develop a suitable agonist to probe GPR88 functions in the brain.

Orphan G Protein Coupled Receptors in Affective Disorders

G protein coupled receptors (GPCRs) are the main mediators of signal transduction in the central nervous system. Therefore, it is not surprising that many GPCRs have long been investigated for their role in the development of anxiety and mood disorders, as well as in the mechanism of action of antidepressant therapies. Importantly, the endogenous ligands for a large group of GPCRs have not yet been identified and are therefore known as orphan GPCRs (oGPCRs). Nonetheless, growing evidence from animal studies, together with genome wide association studies (GWAS) and post-mortem transcriptomic analysis in patients, pointed at many oGPCRs as potential pharmacological targets. Among these discoveries, we summarize in this review how emotional behaviors are modulated by the following oGPCRs: ADGRB2 (BAI2), ADGRG1 (GPR56), GPR3, GPR26, GPR37, GPR50, GPR52, GPR61, GPR62, GPR88, GPR135, GPR158, and GPRC5B.

Control of aversion by glycine-gated GluN1/GluN3A NMDA receptors in the adult medial habenula

The unconventional N-methyl-d-aspartate (NMDA) receptor subunits GluN3A and GluN3B can, when associated with the other glycine-binding subunit GluN1, generate excitatory conductances purely activated by glycine. However, functional GluN1/GluN3 receptors have not been identified in native adult tissues. We discovered that GluN1/GluN3A receptors are operational in neurons of the mouse adult medial habenula (MHb), an epithalamic area controlling aversive physiological states. In the absence of glycinergic neuronal specializations in the MHb, glial cells tuned neuronal activity via GluN1/GluN3A receptors. Reducing GluN1/GluN3A receptor levels in the MHb prevented place-aversion conditioning. Our study extends the physiological and behavioral implications of glycine by demonstrating its control of negatively valued emotional associations via excitatory glycinergic NMDA receptors.

The orphan receptor GPR88 blunts the signaling of opioid receptors and multiple striatal GPCRs

GPR88 is an orphan G protein-coupled receptor (GPCR) considered as a promising therapeutic target for neuropsychiatric disorders; its pharmacology, however, remains scarcely understood. Based on our previous report of increased delta opioid receptor activity in Gpr88 null mice, we investigated the impact of GPR88 co-expression on the signaling of opioid receptors in vitro and revealed that GPR88 inhibits the activation of both their G protein- and β-arrestin-dependent signaling pathways. In Gpr88 knockout mice, morphine-induced locomotor sensitization, withdrawal and supra-spinal analgesia were facilitated, consistent with a tonic inhibitory action of GPR88 on µOR signaling. We then explored GPR88 interactions with more striatal versus non-neuronal GPCRs, and revealed that GPR88 can decrease the G protein-dependent signaling of most receptors in close proximity, but impedes β-arrestin recruitment by all receptors tested. Our study unravels an unsuspected buffering role of GPR88 expression on GPCR signaling, with intriguing consequences for opioid and striatal functions.

Genetic deletion of GPR88 enhances the locomotor response to L-DOPA in experimental parkinsonism while counteracting the induction of dyskinesia

Parkinson's disease (PD) is characterized by progressive loss of midbrain dopaminergic neurons and treated with the dopamine precursor, 3,4-dihydroxy-l-phenylalanine (L-DOPA). Prolonged L-DOPA treatment is however associated with waning efficacy and the induction of L-DOPA induced dyskinesia (LID). GPR88 is an orphan G-protein Coupled Receptor (GPCR) expressed in dopaminoceptive striatal medium spiny neurons (MSNs) and their afferent corticostriatal glutamatergic neurons. Here, we studied the role of GPR88 in experimental parkinsonism and LID. Chronic L-DOPA administration to male GPR88 KO mice, subjected to unilateral 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle, resulted in more rotations than in their WT counterparts. Conversely, GPR88 KO mice had a lower abnormal involuntary movements (AIMs) score. These behavioral responses were accompanied by altered transcription of L-DOPA upregulated genes in lesioned GPR88 KO compared to WT striata. In accordance with a role for serotonin neurons in LID development, WT but not GPR88 KO striata exhibited 5-hydroxytryptamine displacement upon repeated L-DOPA treatment. Intact male GPR88 KO mice showed diminished tacrine-induced PD-like tremor and spontaneous hyperlocomotion. Dopamine and its metabolites were not increased in male GPR88 KO mice, but biosensor recordings revealed increased spontaneous/basal and evoked glutamate release in striata of male GPR88 KO mice. In conclusion, genetic deletion of GPR88 promotes l-DOPA-induced rotation and spontaneous locomotion yet suppresses the induction of LIDs and also reduces tremor. These data provide behavioral, neurochemical and molecular support that GPR88 antagonism may favour motor relief in PD patients without aggravating the induction of motor side effects.

Endometriosis alters brain electrophysiology, gene expression and increases pain sensitization, anxiety, and depression in female mice

Endometriosis is an estrogen-dependent inflammatory disorder among reproductive-aged women associated with pelvic pain, anxiety, and depression. Pain is characterized by central sensitization; however, it is not clear if endometriosis leads to increased pain perception or if women with the disease are more sensitive to pain, increasing the detection of endometriosis. Endometriosis was induced in mice and changes in behavior including pain perception, brain electrophysiology, and gene expression were characterized. Behavioral tests revealed that mice with endometriosis were more depressed, anxious and sensitive to pain compared to sham controls. Microarray analyses confirmed by qPCR identified differential gene expression in several regions of brain in mice with endometriosis. In these mice, genes such as Gpr88, Glra3 in insula, Chrnb4, Npas4 in the hippocampus, and Lcn2 in the amygdala were upregulated while Lct, Serpina3n (insula), and Nptx2 (amygdala) were downregulated. These genes are involved in anxiety, locomotion, and pain. Patch clamp recordings in the amygdala were altered in endometriosis mice demonstrating an effect of endometriosis on brain electrophysiology. Endometriosis induced pain sensitization, anxiety, and depression by modulating brain gene expression and electrophysiology; the effect of endometriosis on the brain may underlie pain sensitization and mood disorders reported in women with the disease.

GPR88 in D1R-Type and D2R-Type Medium Spiny Neurons Differentially Regulates Affective and Motor Behavior

The orphan receptor GPR88 is highly expressed in D1 receptor (D1R)- and D2R-medium spiny neurons (MSNs) and has been associated to striatum-dependent functions in rodents. The total deletion of Gpr88 in mice was shown to decrease anxiety-like behaviors, increase stereotypies and locomotion, and impair motor coordination and motor learning. Knowing the opposing role of D1R- and D2R-MSNs, we here investigated the respective roles of GPR88 in the two MSN subtypes for these behaviors. To do so, we compared effects of a conditional Gpr88 gene knock-out (KO) in D1R-MSNs (D1R-Gpr88 mice) or D2R-MSNs (A_2AR-Gpr88 mice) with effects of the total Gpr88 KO (CMV-Gpr88 mice). Overall, most phenotypes of CMV-Gpr88 mice were recapitulated in A_2AR-Gpr88 mice, including reduced marble burying, increased social interactions, increased locomotor activity and stereotypies in the open field, and reduced motor coordination in the rotarod. Exceptions were the reduced habituation to the open field and reduced motor skill learning, which were observed in CMV-Gpr88 and D1R-Gpr88 mice, but not in A_2AR-Gpr88 mice. D1R-Gpr88 mice otherwise showed no other phenotype in this study. Our data together show that GPR88 modulates the function of both D1R- and D2R-MSNs, and that GPR88 activity in these two neuron populations has very different and dissociable impacts on behavior. We suggest that GPR88 in D2R-MSNs shapes defensive and social behavior and contributes in maintaining the inhibition of basal ganglia outputs to control locomotion, stereotypies and motor coordination, while GPR88 in D1R-MSNs promotes novelty habituation and motor learning.

Orphan Receptor GPR88 as an Emerging Neurotherapeutic Target

Although G protein-coupled receptors (GPCRs) are recognized as pivotal drug targets involved in multiple physiological and pathological processes, the majority of GPCRs including orphan GPCRs (oGPCRs) are unexploited. GPR88, a brain-specific oGPCR with particularly robust expression in the striatum, regulates diverse brain and behavioral functions, including cognition, mood, movement control, and reward-based learning, and is thus emerging as a novel drug target for central nervous system disorders including schizophrenia, Parkinson's disease, anxiety, and addiction. Nevertheless, no effective GPR88 synthetic ligands have yet entered into clinical trials, and GPR88 endogenous ligands remain unknown. Despite the recent discovery and early stage study of several GPR88 agonists, such as 2-PCCA, RTI-13951-33, and phenylglycinol derivatives, further research into GPR88 pharmacology, medicinal chemistry, and chemical biology is urgently needed to yield structurally diversified GPR88-specific ligands. Drug-like pharmacological tool function and relevant signaling elucidation will also accelerate the evaluation of this receptor as a viable neurotherapeutic target.

Inactivation of the Mouse L-Proline Transporter PROT Alters Glutamatergic Synapse Biochemistry and Perturbs Behaviors Required to Respond to Environmental Changes

The endogenous neutral amino acid L-proline exhibits a variety of physiological and behavioral actions in the nervous system, highlighting the importance of accurately regulating its extracellular abundance. The L-proline transporter PROT (Slc6A7) is believed to control the spatial and temporal distribution of L-proline at glutamatergic synapses by rapid uptake of this amino acid into presynaptic terminals. Despite the importance of members of the Slc6 transporter family regulating neurotransmitter signaling and homeostasis in brain, evidence that PROT dysfunction supports risk for mental illness is lacking. Here we report the disruption of the PROT gene by homologous recombination. Mice defective in PROT displayed altered expression of glutamate transmission-related synaptic proteins in cortex and thalamus. PROT deficiency perturbed mouse behavior, such as reduced locomotor activity, decreased approach motivation and impaired memory extinction. Thus, our study demonstrates that PROT regulates behaviors that are needed to respond to environmental changes in vivo and suggests that PROT dysfunctions might contribute to mental disorders showing altered response choice following task contingency changes.

Discovery of a Potent, Selective, and Brain-Penetrant Small Molecule that Activates the Orphan Receptor GPR88 and Reduces Alcohol Intake

The orphan G-protein-coupled receptor GPR88 is highly expressed in the striatum. Studies using GPR88 knockout mice have suggested that the receptor is implicated in alcohol seeking and drinking behaviors. To date, the biological effects of GPR88 activation are still unknown due to the lack of a potent and selective agonist appropriate for in vivo investigation. In this study, we report the discovery of the first potent, selective, and brain-penetrant GPR88 agonist RTI-13951-33 (6). RTI-13951-33 exhibited an EC50 of 25 nM in an in vitro cAMP functional assay and had no significant off-target activity at 38 GPCRs, ion channels, and neurotransmitter transporters that were tested. RTI-13951-33 displayed enhanced aqueous solubility compared to (1 R,2 R)-2-PCCA (2) and had favorable pharmacokinetic properties for behavioral assessment. Finally, RTI-13951-33 significantly reduced alcohol self-administration and alcohol intake in a dose-dependent manner without effects on locomotion and sucrose self-administration in rats when administered intraperitoneally.

Expression map of 78 brain-expressed mouse orphan GPCRs provides a translational resource for neuropsychiatric research

Orphan G-protein-coupled receptors (oGPCRs) possess untapped potential for drug discovery. In the brain, oGPCRs are generally expressed at low abundance and their function is understudied. Expression profiling is an essential step to position oGPCRs in brain function and disease, however public databases provide only partial information. Here, we fine-map expression of 78 brain-oGPCRs in the mouse, using customized probes in both standard and supersensitive in situ hybridization. Images are available at http://ogpcr-neuromap.douglas.qc.ca. This searchable database contains over 8000 coronal brain sections across 1350 slides, providing the first public mapping resource dedicated to oGPCRs. Analysis with public mouse (60 oGPCRs) and human (56 oGPCRs) genome-wide datasets identifies 25 oGPCRs with potential to address emotional and/or cognitive dimensions of psychiatric conditions. We probe their expression in postmortem human brains using nanoString, and included data in the resource. Correlating human with mouse datasets reveals excellent suitability of mouse models for oGPCRs in neuropsychiatric research.

Aliza Ehrlich et al. report the fine-mapping of orphan GPCR (oGPCR) transcripts in the mouse brain using in situ hybridization and provide a public resource for data mining. The authors also mapped 25 selected oGPCRs in human brains, identifying oGPCRs with high correlation between species and potential roles in neuropsychiatric disorders.

Increased Alcohol Seeking in Mice Lacking Gpr88 Involves Dysfunctional Mesocorticolimbic Networks

BACKGOUND

Alcohol use disorder (AUD) is devastating and poorly treated, and innovative targets are actively sought for prevention and treatment. The orphan G protein-coupled receptor GPR88 is enriched in mesocorticolimbic pathways, and Gpr88 knockout mice show hyperactivity and risk-taking behavior, but a potential role for this receptor in drug abuse has not been examined.

METHODS

We tested Gpr88 knockout mice for alcohol-drinking and -seeking behaviors. To gain system-level understanding of their alcohol endophenotype, we also analyzed whole-brain functional connectivity in naïve mice using resting-state functional magnetic resonance imaging.

RESULTS

Gpr88 knockout mice showed increased voluntary alcohol drinking at both moderate and excessive levels, with intact alcohol sedation and metabolism. Mutant mice also showed increased operant responding and motivation for alcohol, while food and chocolate operant self-administration were unchanged. Alcohol place conditioning and alcohol-induced dopamine release in the nucleus accumbens were decreased, suggesting reduced alcohol reward in mutant mice that may partly explain enhanced alcohol drinking. Seed-based voxelwise functional connectivity analysis revealed significant remodeling of mesocorticolimbic centers, whose hallmark was predominant weakening of prefrontal cortex, ventral tegmental area, and amygdala connectional patterns. Also, effective connectivity from the ventral tegmental area to the nucleus accumbens and amygdala was reduced.

CONCLUSIONS

Gpr88 deletion disrupts executive, reward, and emotional networks in a configuration that reduces alcohol reward and promotes alcohol seeking and drinking. The functional connectivity signature is reminiscent of alterations observed in individuals at risk for AUD. The Gpr88 gene, therefore, may represent a vulnerability/resilience factor for AUD, and a potential drug target for AUD treatment.

Remodeling of Sensorimotor Brain Connectivity in Gpr88-Deficient Mice

Recent studies have demonstrated that orchestrated gene activity and expression support synchronous activity of brain networks. However, there is a paucity of information on the consequences of single gene function on overall brain functional organization and connectivity and how this translates at the behavioral level. In this study, we combined mouse mutagenesis with functional and structural magnetic resonance imaging (MRI) to determine whether targeted inactivation of a single gene would modify whole-brain connectivity in live animals. The targeted gene encodes GPR88 (G protein-coupled receptor 88), an orphan G protein-coupled receptor enriched in the striatum and previously linked to behavioral traits relevant to neuropsychiatric disorders. Connectivity analysis of Gpr88-deficient mice revealed extensive remodeling of intracortical and cortico-subcortical networks. Most prominent modifications were observed at the level of retrosplenial cortex connectivity, central to the default mode network (DMN) whose alteration is considered a hallmark of many psychiatric conditions. Next, somatosensory and motor cortical networks were most affected. These modifications directly relate to sensorimotor gating deficiency reported in mutant animals and also likely underlie their hyperactivity phenotype. Finally, we identified alterations within hippocampal and dorsal striatum functional connectivity, most relevant to a specific learning deficit that we previously reported in Gpr88^-/- animals. In addition, amygdala connectivity with cortex and striatum was weakened, perhaps underlying the risk-taking behavior of these animals. This is the first evidence demonstrating that GPR88 activity shapes the mouse brain functional and structural connectome. The concordance between connectivity alterations and behavior deficits observed in Gpr88-deficient mice suggests a role for GPR88 in brain communication.

Lack of anticipatory behavior in Gpr88 knockout mice showed by automatized home cage phenotyping

Mouse models are widely used to understand genetic bases of behavior. Traditional testing typically requires multiple experimental settings, captures only snapshots of behavior and involves human intervention. The recent development of automated home cage monitoring offers an alternative method to study mouse behavior in their familiar and social environment, and over weeks. Here, we used the IntelliCage system to test this approach for mouse phenotyping, and studied mice lacking Gpr88 that have been extensively studied using standard testing. We monitored mouse behavior over 22 days in 4 different phases. In the free adaptation phase, Gpr88 ^-/- mice showed delayed habituation to the home cage, and increased frequency of same corner returns behavior in their alternation pattern. In the following nose-poke adaptation phase, non-habituation continued, however, mutant mice acquired nose-poke conditioning similar to controls. In the place learning and reversal phase, Gpr88^-/- mice developed preference for the water/sucrose corner with some delay, but did not differ from controls for reversal. Finally, in a fixed schedule-drinking phase, control animals showed higher activity during the hour preceding water accessibility, and reduced activity after access to water was terminated. Mutant mice did not show this behavior, showing lack of anticipatory behavior. Our data therefore confirm hyperactivity, non-habituation and altered exploratory behaviors that were reported previously. Learning deficits described in other settings were barely detectable, and a novel phenotype was discovered. Home cage monitoring therefore extends previous findings and shows yet another facet of GPR88 function that deserves further investigation.

Mapping GPR88-Venus illuminates a novel role for GPR88 in sensory processing

GPR88 is an orphan G-protein coupled receptor originally characterized as a striatal-enriched transcript and is a potential target for neuropsychiatric disorders. At present, gene knockout studies in the mouse have essentially focused on striatal-related functions and a comprehensive knowledge of GPR88 protein distribution and function in the brain is still lacking. Here, we first created Gpr88-Venus knock-in mice expressing a functional fluorescent receptor to fine-map GPR88 localization in the brain. The receptor protein was detected in neuronal soma, fibers and primary cilia depending on the brain region, and remarkably, whole-brain mapping revealed a yet unreported layer-4 cortical lamination pattern specifically in sensory processing areas. The unique GPR88 barrel pattern in L4 of the somatosensory cortex appeared 3 days after birth and persisted into adulthood, suggesting a potential function for GPR88 in sensory integration. We next examined Gpr88 knockout mice for cortical structure and behavioral responses in sensory tasks. Magnetic resonance imaging of live mice revealed abnormally high fractional anisotropy, predominant in somatosensory cortex and caudate putamen, indicating significant microstructural alterations in these GPR88-enriched areas. Further, behavioral analysis showed delayed responses in somatosensory-, visual- and olfactory-dependent tasks, demonstrating a role for GPR88 in the integration rather than perception of sensory stimuli. In conclusion, our data show for the first time a prominent role for GPR88 in multisensory processing. Because sensory integration is disrupted in many psychiatric diseases, our study definitely positions GPR88 as a target to treat mental disorders perhaps via activity on cortical sensory networks.

Orphan G protein-coupled receptors: The role in CNS disorders

There are various types of receptors in the central nervous system (CNS). G protein-coupled receptors (GPCRs) have the highest expression with a wide range of physiological functions. A newer sub group of these receptors namely orphan GPCRs have been discovered. GPR3, GPR6, GPR17, GPR26, GPR37, GPR39, GPR40, GPR50, GPR52, GPR54, GPR55, GPR85, GPR88, GPR103, and GPR139 are the selected orphan GPCRs for this article. Their roles in the central nervous system have not been understood well so far. However, recent studies show that they may have very important functions in the CNS. Hence, in the present study, we reviewed most recent findings regarding the physiological roles of the selected orphan GPCRs in the CNS. After a brief presentation of each receptor, considering the results from genetic and pharmacological manipulation of the receptors, their roles in the pathophysiology of different diseases and disorders including anxiety, depression, schizophrenia, epilepsy, Alzheimer's disease, Parkinson's disease, and substance abuse will be discussed. At present, our knowledge regarding the role of GPCRs in the brain is very limited. However, previous limited studies show that orphan GPCRs have an important place in psychopharmacology and these receptors are potential new targets for the treatment of major CNS diseases.

GPR88 is a critical regulator of feeding and body composition in mice

GPR88 is an orphan G-protein-coupled receptor with predominant expression in reward-related areas in the brain. While the lack of GPR88 has been demonstrated to induce behavioral deficits, the potential function of the receptor in the control of food intake and energy balance remains unexplored. In this work, the role of GPR88 in energy homeostasis was investigated in Gpr88^−/− mice fed either standard chow or high fat diet (HFD). Gpr88^−/− mice showed significantly reduced adiposity accompanied with suppressed spontaneous food intake, particularly pronounced under HFD treatment. While energy expenditure was likewise lower in Gpr88^−/− mice, body weight gain remained unchanged. Furthermore, deregulation in glucose tolerance and insulin responsiveness in response to HFD was attenuated in Gpr88^−/− mice. On the molecular level, distinct changes in the hypothalamic mRNA levels of cocaine-and amphetamine-regulated transcript (Cartpt), a neuropeptide involved in the control of feeding and reward, were observed in Gpr88^−/− mice. In addition, GPR88 deficiency was associated with altered expressions of the anorectic Pomc and the orexigenic Npy in the arcuate nucleus, especially under HFD condition. Together, our results indicate that GPR88 signalling is not only important for reward processes, but also plays a role in the central regulatory circuits for energy homeostasis.

GPR88 in A2A receptor-expressing neurons modulates locomotor response to dopamine agonists but not sensorimotor gating

The orphan receptor, GPR88, is emerging as a key player in the pathophysiology of several neuropsychiatric diseases, including psychotic disorders. Knockout (KO) mice lacking GPR88 throughout the brain exhibit many abnormalities relevant to schizophrenia including locomotor hyperactivity, behavioural hypersensitivity to dopaminergic psychostimulants and deficient sensorimotor gating. Here, we used conditional knockout (cKO) mice lacking GPR88 selectively in striatal medium spiny neurons expressing A_2A receptor to determine neuronal circuits underlying these phenotypes. We first studied locomotor responses of A_2A R-Gpr88 KO mice and their control littermates to psychotomimetic, amphetamine, and to selective D1 and D2 receptor agonists, SKF-81297 and quinpirole, respectively. To assess sensorimotor gating performance, mice were submitted to acoustic and visual prepulse inhibition (PPI) paradigms. Total knockout GPR88 mice were also studied for comparison. Like total GPR88 KO mice, A_2A R-Gpr88 KO mice displayed a heightened sensitivity to locomotor stimulant effects of amphetamine and SKF-81297. They also exhibited enhanced locomotor activity to quinpirole, which tended to suppress locomotion in control mice. By contrast, they had normal acoustic and visual PPI, unlike total GPR88 KO mice that show impairments across different sensory modalities. Finally, none of the genetic manipulations altered central auditory temporal processing assessed by gap-PPI. Together, these findings support the role of GPR88 in the pathophysiology of schizophrenia and show that GPR88 in A_2A receptor-expressing neurons modulates psychomotor behaviour but not sensorimotor gating.

Striatal GPR88 Modulates Foraging Efficiency

The striatum is anatomically and behaviorally implicated in behaviors that promote efficient foraging. To investigate this function, we studied instrumental choice behavior in mice lacking GPR88, a striatum-enriched orphan G-protein-coupled receptor that modulates striatal medium spiny neuron excitability. Our results reveal that hungry mice lacking GPR88 (KO mice) were slow to acquire food-reinforced lever press but could lever press similar to controls on a progressive ratio schedule. Both WT and KO mice discriminated between reward and no-reward levers; however, KO mice failed to discriminate based on relative quantity-reward (1 vs 3 food pellets) or effort (3 vs 9 lever presses). We also demonstrate preference for the high-reward (3 pellet) lever was selectively reestablished when GPR88 expression was restored to the striatum. We propose that GPR88 expression within the striatum is integral to efficient action-selection during foraging.SIGNIFICANCE STATEMENT Evolutionary pressure driving energy homeostasis favored detection and comparison of caloric value. In wild and laboratory settings, neural systems involved in energy homeostasis bias foraging to maximize energy efficiency. This is observed when foraging behaviors are guided by superior nutritional density or minimized caloric expenditure. The striatum is anatomically and functionally well placed to perform the sensory and motor integration necessary for efficient action selection during foraging. However, few studies have examined this behavioral phenomenon or elucidated underlying molecular mechanisms. Both humans and mice with nonfunctional GPR88 have been shown to present striatal dysfunctions and impaired learning. We demonstrate that GPR88 expression is necessary to efficiently integrate effort and energy density information guiding instrumental choice.

Neuro-psychopharmacological perspective of Orphan receptors of Rhodopsin (class A) family of G protein-coupled receptors

BACKGROUND

In the central nervous system (CNS), G protein-coupled receptors (GPCRs) are the most fruitful targets for neuropsychopharmacological drug development. Rhodopsin (class A) is the most studied class of GPCR and includes orphan receptors for which the endogenous ligand is not known or is unclear. Characterization of orphan GPCRs has proven to be challenging, and the production pace of GPCR-based drugs has been incredibly slow.

OBJECTIVE

Determination of the functions of these receptors may provide unexpected insight into physiological and neuropathological processes. Advances in various methods and techniques to investigate orphan receptors including in situ hybridization and knockdown/knockout (KD/KO) showed extensive expression of these receptors in the mammalian brain and unmasked their physiological and neuropathological roles. Due to these rapid progress and development, orphan GPCRs are rising as a new and promising class of drug targets for neurodegenerative diseases and psychiatric disorders.

CONCLUSION

This review presents a neuropsychopharmacological perspective of 26 orphan receptors of rhodopsin (class A) family, namely GPR3, GPR6, GPR12, GPR17, GPR26, GPR35, GPR39, GPR48, GPR49, GPR50, GPR52, GPR55, GPR61, GPR62, GPR63, GPR68, GPR75, GPR78, GPR83, GPR84, GPR85, GPR88, GPR153, GPR162, GPR171, and TAAR6. We discussed the expression of these receptors in mammalian brain and their physiological roles. Furthermore, we have briefly highlighted their roles in neurodegenerative diseases and psychiatric disorders including Alzheimer's disease, Parkinson's disease, neuroinflammation, inflammatory pain, bipolar and schizophrenic disorders, epilepsy, anxiety, and depression.

A novel herbal treatment reduces depressive-like behaviors and increases brain-derived neurotrophic factor levels in the brain of type 2 diabetic rats

BACKGROUND

Radix Puerariae and hawthorn fruit have been demonstrated to treat diabetes. They offer potential benefits for preventing depression in diabetes.

OBJECTIVE

The aim of this study was to investigate whether the combination of Radix Puerariae and hawthorn fruit (CRPHF) could prevent depression in a diabetic rat model generated by feeding the rats with a high-fat diet and a low-dose streptozotocin (STZ).

METHODS

The CRPHF was provided by the Shanghai Chinese Traditional Medical University. Twenty-four rats were randomly divided into four groups: normal control, normal-given-CRPHF (NC), diabetic control, and diabetic-given-CRPHF (DC) groups. The type 2 diabetic model was created by feeding the rats with a high-fat diet for 4 weeks followed by injection of 25 mg/kg STZ. CRPHF was given at 2 g/kg/d to the rats of NC and DC groups by intragastric gavage daily for 4 weeks after the type 2 diabetic model was successfully created. Body weight, random blood glucose (RBG), oral glucose tolerance test, total cholesterol (TC), triglyceride (TG), high-density lipoprotein cholesterol (HDL-C), and low-density lipoprotein cholesterol (LDL-C) were measured during the study. Depressive-like behavior was evaluated at the end of the treatment by using the open field test (OFT), the elevated plus-maze test (EPMT), locomotor activity test (LAT), and forced swimming test (FST). Levels of extracellular signal-regulated protein kinase (ERK) and brain-derived neurotrophic factor (BDNF) in the prefrontal cortex were evaluated by using Western blot.

RESULTS

1) CRPHF reduced RBG and improved glucose tolerance in diabetic rats; 2) CRPHF reduced TC and TG but did not significantly change HDL-C or LDL-C in diabetic rats; 3) CRPHF reversed the loss in body weights observed in diabetic rats; 4) CRPHF reduced depressive-like behavior as measured by OFT, EPMT, LAT, and FST; 5) BDNF was upregulated, and ERK was activated in the prefrontal cortex of diabetic rats treated with CRPHF.

CONCLUSION

CRPHF has the potential of preventing depression in patients with diabetes.