The dopamine D1-D2 receptor heteromer localizes in dynorphin/enkephalin neurons: increased high affinity state following amphetamine and in schizophrenia

Cannabinoid regulation of angiotensin II-induced calcium signaling in striatal neurons

Calcium ion (Ca2+) homeostasis is crucial for neuron function and neurotransmission. This study focused on the actions mediated by the CB1 receptor (CB1R), the most abundant G protein-coupled receptor (GPCR) in central nervous system (CNS) neurons, over by the AT1R, which is one of the few G protein-coupled CNS receptors able to regulate cytoplasmic Ca2+ levels. A functional interaction suggesting a direct association between these receptors was detected. AT1-CB1 receptor heteromers (AT1CB1Hets) were identified in HEK-293T cells by bioluminescence resonance energy transfer (BRET2). Functional interactions within the AT1-CB1 complex and their potential relevance in Parkinson's disease (PD) were assessed. In situ proximity ligation assays (PLA) identified AT1CB1Hets in neurons, in which an important finding was that Ca2+ level increase upon AT1R activation was reduced in the presence of cannabinoids acting on CB1Rs. AT1CB1Het expression was quantified in samples from the 6-hydroxydopamine (6-OHDA) hemilesioned rat model of PD in which a lower expression of AT1CB1Hets was observed in striatal neurons from lesioned animals (versus non-lesioned). AT1CB1Het expression changed depending on both the lesion and the consequences of levodopa administration, i.e., dyskinesias versus lack of involuntary movements. A partial recovery in AT1CB1Het expression was detected in lesioned animals that developed levodopa-induced dyskinesias. These findings support the existence of a compensatory mechanism mediated by AT1CB1Hets that modulates susceptibility to levodopa-induced dyskinesias in PD. Therefore, cannabinoids may be useful in reducing calcium dyshomeostasis in dyskinesia.

Alamandine, a protective component of the renin-angiotensin system, reduces cellular proliferation and interleukin-6 secretion in human macrophages through MasR-MrgDR heteromerization

Alamandine (ALA) exerts protective effects similar to angiotensin (Ang) (1-7) through Mas-related G protein-coupled receptor type D receptor (MrgDR) activation, distinct from Mas receptor (MasR). ALA induces anti-inflammatory effects in mice but its impact in human macrophages remains unclear. We aimed to investigate the anti-inflammatory effects of ALA in human macrophages. Interleukin (IL)-6 and IL-1β were measured by ELISA in human THP-1 macrophages and human monocyte-derived macrophages exposed to lipopolysaccharide (LPS). Consequences of MasR-MrgDR heteromerization were investigated in transfected HEK293T cells. ALA decreased IL-6 and IL-1β secretion in LPS-activated THP-1 macrophages. The ALA-induced decrease in IL-6 but not in IL-1β was prevented by MasR blockade and MasR downregulation, suggesting MasR-MrgDR interaction. In human monocyte-derived M1 macrophages, ALA decreased IL-1β secretion independently of MasR. MasR-MrgDR interaction was confirmed in THP-1 macrophages, human monocyte-derived macrophages, and transfected HEK293T cells. MasR and MrgDR formed a constitutive heteromer that was not influenced by ALA. ALA promoted Akt and ERK1/2 activation only in cells expressing MasR-MrgDR heteromers, and this effect was prevented by MasR blockade. While Ang-(1-7) reduced cellular proliferation in MasR -but not MrgDR- expressing cells, ALA antiproliferative effect was elicited in cells expressing MasR-MrgDR heteromers. ALA also induced an antiproliferative response in THP-1 cells and this effect was abolished by MasR blockade, reinforcing MasR-MrgDR interaction. MasR-MrgDR heteromerization is crucial for ALA-induced anti-inflammatory and antiproliferative responses in human macrophages. This study broaden our knowledge of the protective axis of the RAS, thus enabling novel therapeutic approaches in inflammatory-associated diseases.

Integrative multi-dimensional characterization of striatal projection neuron heterogeneity in adult brain

Striatal projection neurons (SPNs) are traditionally segregated into two subpopulations expressing dopamine (DA) D1-like or D2-like receptors. However, this dichotomy is challenged by recent evidence. Functional and expression studies raise important questions: do SPNs co-express different DA receptors, and do these differences reflect unique striatal spatial distributions and expression profiles? Using RNAscope in mouse striatum, we report heterogenous SPN subpopulations distributed across dorsal-ventral and rostral-caudal axes. SPN subpopulations co-express multiple DA receptors, including D1 and D2 (D1/2R) and D1 and D3. Our integrative approach using single-nuclei multi-omics analyses provides a simple consensus to describe SPNs across diverse datasets, connecting it to complementary spatial mapping. Combining RNAscope and multi-omics shows D1/2R SPNs further separate into distinct subtypes according to spatial organization and conserved marker genes. Each SPN cell type contributes uniquely to genetic risk for neuropsychiatric diseases. Our results bridge anatomy and transcriptomics to offer new understandings of striatal neuron heterogeneity.

Δ9-Tetrahydrocannabinol does not upregulate an aversive dopamine receptor mechanism in adolescent brain unlike in adults

Earlier age of cannabis usage poses higher risk of Cannabis Use Disorder and adverse consequences, such as addiction, anxiety, dysphoria, psychosis, largely attributed to its principal psychoactive component, Δ9-tetrahydrocannabinol (THC) and altered dopaminergic function. As dopamine D1-D2 receptor heteromer activation causes anxiety and anhedonia, this signaling complex was postulated to contribute to THC-induced affective symptoms. To investigate this, we administered THC repeatedly to adolescent monkeys and adolescent or adult rats. Drug-naïve adolescent rat had lower striatal densities of D1-D2 heteromer compared to adult rat. Repeated administration of THC to adolescent rat or adolescent monkey did not alter D1-D2 heteromer expression in nucleus accumbens or dorsal striatum but upregulated it in adult rat. Behaviourally, THC-treated adult, but not adolescent rat manifested anxiety and anhedonia-like behaviour, with elevated composite negative emotionality scores that correlated with striatal D1-D2 density. THC modified downstream markers of D1-D2 activation in adult, but not adolescent striatum. THC administered with cannabidiol did not alter D1-D2 expression. In adult rat, co-administration of CB1 receptor (CB1R) inverse agonist with THC attenuated D1-D2 upregulation, implicating cannabinoids in the regulation of striatal D1-D2 heteromer expression. THC exposure revealed an adaptable age-specific, anxiogenic, anti-reward mechanism operant in adult striatum but deficient in adolescent rat and monkey striatum that may confer increased sensitivity to THC reward in adolescence while limiting its negative effects, thus promoting continued use and increasing vulnerability to long-term adverse cannabis effects.

The World of GPCR dimers - Mapping dopamine receptor D2 homodimers in different activation states and configuration arrangements

G protein-coupled receptors (GPCRs) are known to dimerize, but the molecular and structural basis of GPCR dimers is not well understood. In this study, we developed a computational framework to generate models of symmetric and asymmetric GPCR dimers using different monomer activation states and identified their most likely interfaces with molecular details. We chose the dopamine receptor D2 (D2R) homodimer as a case study because of its biological relevance and the availability of structural information. Our results showed that transmembrane domains 4 and 5 (TM4 and TM5) are mostly found at the dimer interface of the D2R dimer and that these interfaces have a subset of key residues that are mostly nonpolar from TM4 and TM5, which was in line with experimental studies. In addition, TM2 and TM3 appear to be relevant for D2R dimers. In some cases, the inactive configuration is unaffected by the partnered protomer, whereas in others, the active protomer adopts the properties of an inactive receptor. Additionally, the β-arrestin configuration displayed the properties of an active receptor in the absence of an agonist, suggesting that a switch to another meta-state during dimerization occurred. Our findings are consistent with the experimental data, and this method can be adapted to study heterodimers and potentially extended to include additional proteins such as G proteins or β-arrestins. In summary, this approach provides insight into the impact of the conformational status of partnered protomers on the overall quaternary GPCR macromolecular structure and dynamics.

Dopamine and Glutamate Crosstalk Worsen the Seizure Outcome in TLE-HS Patients

Temporal lobe epilepsy (TLE), accompanied by hippocampal sclerosis (HS), is the most common form of drug-resistant epilepsy (DRE). Nearly 20% of the patients showed seizure recurrence even after surgery, and the reasons are yet to be understood. Dysregulation of neurotransmitters is evident during seizures, which can induce excitotoxicity. The present study focused on understanding the molecular changes associated with Dopamine (DA) and glutamate signaling and their possible impact on the persistence of excitotoxicity and seizure recurrence in patients with drug-resistant TLE-HS who underwent surgery. According to the International League against Epilepsy (ILAE) suggested classification for seizure outcomes, the patients (n = 26) were classified as class 1 (no seizures) and class 2 (persistent seizures) using the latest post-surgery follow-up data to understand the prevalent molecular changes in seizure-free and seizure-recurrence patient groups. Our study uses thioflavin T assay, western blot analysis, immunofluorescence assays, and fluorescence resonance energy transfer (FRET) assays. We have observed a substantial increase in the DA and glutamate receptors that promote excitotoxicity. Patients who had seizure recurrence showed a significant increase in (pNR2B, p < 0.009; and pGluR1, p < 0.01), protein phosphatase1γ (PP1γ; p < 0.009), protein kinase A (PKAc; p < 0.001) and dopamine-cAMP regulated phospho protein32 (pDARPP32T34; p < 0.009) which are critical for long-term potentiation (LTP), excitotoxicity compared to seizure-free patients and controls. A significant increase in D1R downstream kinases like PKA (p < 0.001), pCAMKII (p < 0.009), and Fyn (p < 0.001) was observed in patient samples compared to controls. Anti-epileptic DA receptor D2R was found to be decreased in ILAE class 2 (p < 0.02) compared to class 1. Since upregulation of DA and glutamate signaling supports LTP and excitotoxicity, we believe it could impact seizure recurrence. Further studies about the impact of DA and glutamate signaling on the distribution of PP1γ at postsynaptic density and synaptic strength could help us understand the seizure microenvironment in patients. Dopamine, Glutamate signal crosstalk. Diagram representing the PP1γ regulation by NMDAR negative feedback inhibition signaling (green circle-left) and D1R signal (red circle-middle) domination over PP1γ though increased PKA, pDARPP32T34, and supports pGluR1, pNR2B in seizure recurrent patients. D1R-D2R hetero dimer activation (red circle-right) increases cellular Ca2+ and pCAMKIIα activation. All these events lead to calcium overload in HS patients and excitotoxicity, particularly in patients experiencing recurrent seizures.

The Neurobiological Underpinnings of Obsessive-Compulsive Symptoms in Psychosis, Translational Issues for Treatment-Resistant Schizophrenia

Almost 25% of schizophrenia patients suffer from obsessive-compulsive symptoms (OCS) considered a transdiagnostic clinical continuum. The presence of symptoms pertaining to both schizophrenia and obsessive-compulsive disorder (OCD) may complicate pharmacological treatment and could contribute to lack or poor response to the therapy. Despite the clinical relevance, no reviews have been recently published on the possible neurobiological underpinnings of this comorbidity, which is still unclear. An integrative view exploring this topic should take into account the following aspects: (i) the implication for glutamate, dopamine, and serotonin neurotransmission as demonstrated by genetic findings; (ii) the growing neuroimaging evidence of the common brain regions and dysfunctional circuits involved in both diseases; (iii) the pharmacological modulation of dopaminergic, serotoninergic, and glutamatergic systems as current therapeutic strategies in schizophrenia OCS; (iv) the recent discovery of midbrain dopamine neurons and dopamine D1- and D2-like receptors as orchestrating hubs in repetitive and psychotic behaviors; (v) the contribution of N-methyl-D-aspartate receptor subunits to both psychosis and OCD neurobiology. Finally, we discuss the potential role of the postsynaptic density as a structural and functional hub for multiple molecular signaling both in schizophrenia and OCD pathophysiology.

Neuroprotection afforded by targeting G protein-coupled receptors in heteromers and by heteromer-selective drugs

G protein-coupled receptors (GPCRs) are the target of hundreds of approved drugs. Although these drugs were designed to target individual receptors, it is becoming increasingly apparent that GPCRs interact with each other to form heteromers. Approved drug targets are often part of a GPCR heteromer, and therefore new drugs can be developed with heteromers in mind. This review presents several strategies to selectively target GPCRs in heteromeric contexts, namely, taking advantage of i) heteromer-mediated biased agonism/signalling, ii) discovery of drugs with higher affinity for the receptor if it is part of a heteromer (heteromer selective drugs), iii) allosteric compounds directed against the interacting transmembrane domains and, eventually, iv) antagonists that block both GPCRs in a heteromer. Heteromers provide unique allosteric sites that should help designing a new type of drug that by definition would be a heteromer selective drug. The review also provides examples of rhodopsin-like class A receptors in heteromers that could be targeted to neuroprotect and/or delay the progression of diseases such as Parkinson's and Alzheimer's. GPCRs in heteromers (GriH) with the potential to address dyskinesias, a common complication of dopaminergic replacement therapy in parkinsonian patients, are also described.

The adenosine A2A receptor in the basal ganglia: Expression, heteromerization, functional selectivity and signalling

Adenosine is a neuroregulatory nucleoside that acts through four G protein-coupled receptors (GPCRs), A1, A2A, A2B and A3, which are widely expressed in cells of the nervous system. The A2A receptor (A2AR), the GPCR with the highest expression in the striatum, has a similar role to that of receptors for dopamine, one of the main neurotransmitters. Neuronal and glial A2ARs participate in the modulation of dopaminergic transmission and act in almost any action in which the basal ganglia is involved. This chapter revisits the expression of the A2AR in the basal ganglia in health and disease, and describes the diversity of signalling depending on whether the receptors are expressed as monomer or as heteromer. The A2AR can interact with other receptors as adenosine A1, dopamine D2, or cannabinoid CB1 to form heteromers with relevant functions in the basal ganglia. Heteromerization, with these and other GPCRs, provides diversity to A2AR-mediated signalling and to the modulation of neurotransmission. Thus, selective A2AR antagonists have neuroprotective potential acting directly on neurons, but also through modulation of glial cell activation, for example, by decreasing neuroinflammatory events that accompany neurodegenerative diseases. In fact, A2AR antagonists are safe and their potential in the therapy of Parkinson's disease has already led to the approval of one of them, istradefylline, in Japan and United States. The receptor also has a key role in reward circuits and, again, heteromers with dopamine receptors, but also with cannabinoid CB1 receptors, participate in the events triggered by drugs of abuse.

Amelioration of Nicotine-Induced Conditioned Place Preference Behaviors in Mice by an FABP3 Inhibitor

We previously demonstrated that fatty acid-binding protein 3 null (FABP3−/−) mice exhibit resistance to nicotine-induced conditioned place preference (CPP). Here, we confirm that the FABP3 inhibitor, MF1 ((4-(2-(1-(2-chlorophenyl)-5-phenyl-1H-pyrazol-3-yl)phenoxy) butanoic acid), successfully reduces nicotine-induced CPP scores in mice. MF1 (0.3 or 1.0 mg/kg) was orally administered 30 min before nicotine, and CPP scores were assessed in the conditioning, withdrawal, and relapse phases. MF1 treatment decreased CPP scores in a dose-dependent manner. Failure of CPP induction by MF1 (1.0 mg/kg, p.o.) was associated with the inhibition of both CaMKII and ERK activation in the nucleus accumbens (NAc) and hippocampal CA1 regions. MF1 treatment reduced nicotine-induced increases in phosphorylated CaMKII and cAMP-response element-binding protein (CREB)-positive cells. Importantly, the increase in dopamine D2 receptor (D2R) levels following chronic nicotine exposure was inhibited by MF1 treatment. Moreover, the quinpirole (QNP)-induced increase in the level of CaMKII and ERK phosphorylation was significantly inhibited by MF1 treatment of cultured NAc slices from wild type (WT) mice; however, QNP treatment had no effect on CaMKII and ERK phosphorylation levels in the NAc of D2R null mice. Taken together, these results show that MF1 treatment suppressed D2R/FABP3 signaling, thereby preventing nicotine-induced CPP induction. Hence, MF1 can be used as a novel drug to block addiction to nicotine and other drugs by inhibiting the dopaminergic system.

The histamine H3 receptor modulates dopamine D2 receptor-dependent signaling pathways and mouse behaviors

The histamine H3 receptor (H3R) is highly enriched in the spiny projection neurons (SPNs) of the striatum, in both the D1 receptor (D1R)-expressing and D2 receptor (D2R)-expressing populations. A crossantagonistic interaction between H3R and D1R has been demonstrated in mice, both at the behavioral level and at the biochemical level. Although interactive behavioral effects have been described upon coactivation of H3R and D2R, the molecular mechanisms underlying this interaction are poorly understood. Here, we show that activation of H3R with the selective agonist R-(-)-α-methylhistamine dihydrobromide mitigates D2R agonist-induced locomotor activity and stereotypic behavior. Using biochemical approaches and the proximity ligation assay, we demonstrated the existence of an H3R-D2R complex in the mouse striatum. In addition, we examined consequences of simultaneous H3R-D2R agonism on the phosphorylation levels of several signaling molecules using immunohistochemistry. H3R agonist treatment modulated Akt (serine/threonine PKB)-glycogen synthase kinase 3 beta signaling in response to D2R activation via a β-arrestin 2-dependent mechanism in D2R-SPNs but not in D1R-SPNs. Phosphorylation of mitogen- and stress-activated protein kinase 1 and rpS6 (ribosomal protein S6) was largely unchanged under these conditions. As Akt-glycogen synthase kinase 3 beta signaling has been implicated in several neuropsychiatric disorders, this work may help clarify the role of H3R in modulating D2R function, leading to a better understanding of pathophysiology involving the interaction between histamine and dopamine systems.

Canonical and Non-Canonical Antipsychotics' Dopamine-Related Mechanisms of Present and Next Generation Molecules: A Systematic Review on Translational Highlights for Treatment Response and Treatment-Resistant Schizophrenia

Schizophrenia is a severe psychiatric illness affecting almost 25 million people worldwide and is conceptualized as a disorder of synaptic plasticity and brain connectivity. Antipsychotics are the primary pharmacological treatment after more than sixty years after their introduction in therapy. Two findings hold true for all presently available antipsychotics. First, all antipsychotics occupy the dopamine D2 receptor (D2R) as an antagonist or partial agonist, even if with different affinity; second, D2R occupancy is the necessary and probably the sufficient mechanism for antipsychotic effect despite the complexity of antipsychotics' receptor profile. D2R occupancy is followed by coincident or divergent intracellular mechanisms, implying the contribution of cAMP regulation, β-arrestin recruitment, and phospholipase A activation, to quote some of the mechanisms considered canonical. However, in recent years, novel mechanisms related to dopamine function beyond or together with D2R occupancy have emerged. Among these potentially non-canonical mechanisms, the role of Na²⁺ channels at the dopamine at the presynaptic site, dopamine transporter (DAT) involvement as the main regulator of dopamine concentration at synaptic clefts, and the putative role of antipsychotics as chaperones for intracellular D2R sequestration, should be included. These mechanisms expand the fundamental role of dopamine in schizophrenia therapy and may have relevance to considering putatively new strategies for treatment-resistant schizophrenia (TRS), an extremely severe condition epidemiologically relevant and affecting almost 30% of schizophrenia patients. Here, we performed a critical evaluation of the role of antipsychotics in synaptic plasticity, focusing on their canonical and non-canonical mechanisms of action relevant to the treatment of schizophrenia and their subsequent implication for the pathophysiology and potential therapy of TRS.

Dopamine Dynamics and Neurobiology of Non-Response to Antipsychotics, Relevance for Treatment Resistant Schizophrenia: A Systematic Review and Critical Appraisal

Treatment resistant schizophrenia (TRS) is characterized by a lack of, or suboptimal response to, antipsychotic agents. The biological underpinnings of this clinical condition are still scarcely understood. Since all antipsychotics block dopamine D2 receptors (D2R), dopamine-related mechanisms should be considered the main candidates in the neurobiology of antipsychotic non-response, although other neurotransmitter systems play a role. The aims of this review are: (i) to recapitulate and critically appraise the relevant literature on dopamine-related mechanisms of TRS; (ii) to discuss the methodological limitations of the studies so far conducted and delineate a theoretical framework on dopamine mechanisms of TRS; and (iii) to highlight future perspectives of research and unmet needs. Dopamine-related neurobiological mechanisms of TRS may be multiple and putatively subdivided into three biological points: (1) D2R-related, including increased D2R levels; increased density of D2Rs in the high-affinity state; aberrant D2R dimer or heteromer formation; imbalance between D2R short and long variants; extrastriatal D2Rs; (2) presynaptic dopamine, including low or normal dopamine synthesis and/or release compared to responder patients; and (3) exaggerated postsynaptic D2R-mediated neurotransmission. Future points to be addressed are: (i) a more neurobiologically-oriented phenotypic categorization of TRS; (ii) implementation of neurobiological studies by directly comparing treatment resistant vs. treatment responder patients; (iii) development of a reliable animal model of non-response to antipsychotics.

G Protein-Dependent Activation of the PKA-Erk1/2 Pathway by the Striatal Dopamine D1/D3 Receptor Heteromer Involves Beta-Arrestin and the Tyrosine Phosphatase Shp-2

The heteromer composed of dopamine D1 and D3 receptors (D1R-D3R) has been defined as a structure able to trigger Erk1/2 and Akt signaling in a G protein-independent, beta-arrestin 1-dependent way that is physiologically expressed in the ventral striatum and is likely involved in the control of locomotor activity. Indeed, abnormal levels of D1R-D3R heteromer in the dorsal striatum have been correlated with the development of L-DOPA-induced dyskinesia (LID) in Parkinson's disease patients, a motor complication associated with striatal D1R signaling, thus requiring Gs protein and PKA activity to activate Erk1/2. Therefore, to clarify the role of the D1R/D3R heteromer in LID, we investigated the signaling pathway induced by the heteromer using transfected cells and primary mouse striatal neurons. Collectively, we found that in both the cell models, D1R/D3R heteromer-induced activation of Erk1/2 exclusively required the D1R molecular effectors, such as Gs protein and PKA, with the contribution of the phosphatase Shp-2 and beta-arrestins, indicating that heterodimerization with the D3R abolishes the specific D3R-mediated signaling but strongly allows D1R signals. Therefore, while in physiological conditions the D1R/D3R heteromer could represent a mechanism that strengthens the D1R activity, its pathological expression may contribute to the abnormal PKA-Shp-2-Erk1/2 pathway connected with LID.

Lithium engages autophagy for neuroprotection and neuroplasticity: translational evidence for therapy

Here an overview is provided on therapeutic/neuroprotective effects of Lithium (Li⁺) in neurodegenerative and psychiatric disorders focusing on the conspicuous action of Li⁺ through autophagy. The effects on the autophagy machinery remain the key molecular mechanisms to explain the protective effects of Li⁺ for neurodegenerative diseases, offering potential therapeutic strategies for the treatment of neuropsychiatric disorders and emphasizes a crossroad linking autophagy, neurodegenerative disorders, and mood stabilization. Sensitization by psychostimulants points to several mechanisms involved in psychopathology, most also crucial in neurodegenerative disorders. Evidence shows the involvement of autophagy and metabotropic Glutamate receptors-5 (mGluR5) in neurodegeneration due to methamphetamine neurotoxicity as well as in neuroprotection, both in vitro and in vivo models. More recently, Li⁺ was shown to modulate autophagy through its action on mGluR5, thus pointing to an additional way of autophagy engagement by Li⁺ and to a substantial role of mGluR5 in neuroprotection related to neural e neuropsychiatry diseases. We propose Li⁺ engagement of autophagy through the canonical mechanisms of autophagy machinery and through the intermediary of mGluR5.

Roles of Pancreatic Islet Catecholamine Neurotransmitters in Glycemic Control and in Antipsychotic Drug-Induced Dysglycemia

Catecholamine neurotransmitters dopamine (DA) and norepinephrine (NE) are essential for a myriad of functions throughout the central nervous system, including metabolic regulation. These molecules are also present in the pancreas, and their study may shed light on the effects of peripheral neurotransmission on glycemic control. Though sympathetic innervation to islets provides NE that signals at local α-cell and β-cell adrenergic receptors to modify hormone secretion, α-cells and β-cells also synthesize catecholamines locally. We propose a model where α-cells and β-cells take up catecholamine precursors in response to postprandial availability, preferentially synthesizing DA. The newly synthesized DA signals in an autocrine/paracrine manner to regulate insulin and glucagon secretion and maintain glycemic control. This enables islets to couple local catecholamine signaling to changes in nutritional state. We also contend that the DA receptors expressed by α-cells and β-cells are targeted by antipsychotic drugs (APDs)-some of the most widely prescribed medications today. Blockade of local DA signaling contributes significantly to APD-induced dysglycemia, a major contributor to treatment discontinuation and development of diabetes. Thus, elucidating the peripheral actions of catecholamines will provide new insights into the regulation of metabolic pathways and may lead to novel, more effective strategies to tune metabolism and treat diabetes.

Dissecting the association between psychiatric disorders and neurological proteins: a genetic correlation and two-sample bidirectional Mendelian randomization study

OBJECTIVES

The role of neurological proteins in the development of bipolar disorder (BD) and schizophrenia (SCZ) remains elusive now. The current study aims to explore the potential genetic correlations of plasma neurological proteins with BD and SCZ.

METHODS

By using the latest genome-wide association study (GWAS) summary data of BD and SCZ (including 41,917 BD cases, 11,260 SCZ cases, and 396,091 controls) derived from the Psychiatric GWAS Consortium website (PGC) and a recently released GWAS of neurological proteins (including 750 individuals), we performed a linkage disequilibrium score regression (LDSC) analysis to detect the potential genetic correlations between the two common psychiatric disorders and each of the 92 neurological proteins. Two-sample Mendelian randomisation (MR) analysis was then applied to assess the bidirectional causal relationship between the neurological proteins identified by LDSC, BD and SCZ.

RESULTS

LDSC analysis identified one neurological protein, NEP, which shows suggestive genetic correlation signals for both BD (coefficient = -0.165, p value = 0.035) and SCZ (coefficient = -0.235, p value = 0.020). However, those association did not remain significant after strict Bonferroni correction. Two sample MR analysis found that there was an association between genetically predicted level of NEP protein, BD (odd ratio [OR] = 0.87, p value = 1.61 × 10^-6) and SCZ (OR = 0.90, p value = 4.04 × 10^-6). However, in the opposite direction, there is no genetically predicted association between BD, SCZ, and NEP protein level.

CONCLUSION

This study provided novel clues for understanding the genetic effects of neurological proteins on BD and SCZ.

Class A and C GPCR Dimers in Neurodegenerative Diseases

Neurodegenerative diseases affect over 30 million people worldwide with an ascending trend. Most individuals suffering from these irreversible brain damages belong to the elderly population, with onset between 50 and 60 years. Although the pathophysiology of such diseases is partially known, it remains unclear upon which point a disease turns degenerative. Moreover, current therapeutics can treat some of the symptoms but often have severe side effects and become less effective in long-term treatment. For many neurodegenerative diseases, the involvement of G proteincoupled receptors (GPCRs), which are key players of neuronal transmission and plasticity, has become clearer and holds great promise in elucidating their biological mechanism. With this review, we introduce and summarize class A and class C GPCRs, known to form heterodimers or oligomers to increase their signalling repertoire. Additionally, the examples discussed here were shown to display relevant alterations in brain signalling and had already been associated with the pathophysiology of certain neurodegenerative diseases. Lastly, we classified the heterodimers into two categories of crosstalk, positive or negative, for which there is known evidence.

The Interrelation between Interleukin-2 and Schizophrenia

Interleukin-2 (IL-2) is a growth factor that regulates T-cell autocrine secretion and has long been considered to be closely related to immune response. With the advance in neuroinflammation theory and immunology research on schizophrenia, it is interesting and meaningful to discuss the possible role of IL-2 in schizophrenia. Here, we reviewed a series of studies published from the 1990s and found that IL-2 was closely associated with schizophrenia. For example, IL-2 is responsible for mediating toxic reactions, which are the causes of schizophrenia symptoms in patients, and such symptoms resolve after discontinuation of the drug. In addition, we focused on the changes of IL-2 in the onset, progression and treatment of schizophrenia and the possible mechanisms by which IL-2 affects schizophrenia. Our review suggests that IL-2 is associated with schizophrenia and plays a role in its pathogenesis, and progression IL-2 and sIL-2R could serve as potential biomarkers of schizophrenia.

Dopamine Receptors: Is It Possible to Become a Therapeutic Target for Depression?

Dopamine and its receptors are currently recognized targets for the treatment of several neuropsychiatric disorders, including Parkinson’s disease, schizophrenia, some drug use addictions, as well as depression. Dopamine receptors are widely distributed in various regions of the brain, but their role and exact contribution to neuropsychiatric diseases has not yet been thoroughly studied. Based on the types of dopamine receptors and their distribution in different brain regions, this paper reviews the current research status of the molecular, cellular and circuit mechanisms of dopamine and its receptors involved in depression. Multiple lines of investigation of these mechanisms provide a new future direction for understanding the etiology and treatment of depression and potential new targets for antidepressant treatments.

Clozapine's multiple cellular mechanisms: What do we know after more than fifty years? A systematic review and critical assessment of translational mechanisms relevant for innovative strategies in treatment-resistant schizophrenia

Almost fifty years after its first introduction into clinical care, clozapine remains the only evidence-based pharmacological option for treatment-resistant schizophrenia (TRS), which affects approximately 30% of patients with schizophrenia. Despite the long-time experience with clozapine, the specific mechanism of action (MOA) responsible for its superior efficacy among antipsychotics is still elusive, both at the receptor and intracellular signaling level. This systematic review is aimed at critically assessing the role and specific relevance of clozapine's multimodal actions, dissecting those mechanisms that under a translational perspective could shed light on molecular targets worth to be considered for further innovative antipsychotic development. In vivo and in vitro preclinical findings, supported by innovative techniques and methods, together with pharmacogenomic and in vivo functional studies, point to multiple and possibly overlapping MOAs. To better explore this crucial issue, the specific affinity for 5-HT₂R, D1R, α_2c, and muscarinic receptors, the relatively low occupancy at dopamine D2R, the interaction with receptor dimers, as well as the potential confounder effects resulting in biased ligand action, and lastly, the role of the moiety responsible for lipophilic and alkaline features of clozapine are highlighted. Finally, the role of transcription and protein changes at the synaptic level, and the possibility that clozapine can directly impact synaptic architecture are addressed. Although clozapine's exact MOAs that contribute to its unique efficacy and some of its severe adverse effects have not been fully understood, relevant information can be gleaned from recent mechanistic understandings that may help design much needed additional therapeutic strategies for TRS.

The Signaling and Pharmacology of the Dopamine D1 Receptor

The dopamine D1 receptor (D1R) is a Gα_s/olf-coupled GPCR that is expressed in the midbrain and forebrain, regulating motor behavior, reward, motivational states, and cognitive processes. Although the D1R was initially identified as a promising drug target almost 40 years ago, the development of clinically useful ligands has until recently been hampered by a lack of suitable candidate molecules. The emergence of new non-catechol D1R agonists, biased agonists, and allosteric modulators has renewed clinical interest in drugs targeting this receptor, specifically for the treatment of motor impairment in Parkinson's Disease, and cognitive impairment in neuropsychiatric disorders. To develop better therapeutics, advances in ligand chemistry must be matched by an expanded understanding of D1R signaling across cell populations in the brain, and in disease states. Depending on the brain region, the D1R couples primarily to either Gα_s or Gα_olf through which it activates a cAMP/PKA-dependent signaling cascade that can regulate neuronal excitability, stimulate gene expression, and facilitate synaptic plasticity. However, like many GPCRs, the D1R can signal through multiple downstream pathways, and specific signaling signatures may differ between cell types or be altered in disease. To guide development of improved D1R ligands, it is important to understand how signaling unfolds in specific target cells, and how this signaling affects circuit function and behavior. In this review, we provide a summary of D1R-directed signaling in various neuronal populations and describe how specific pathways have been linked to physiological and behavioral outcomes. In addition, we address the current state of D1R drug development, including the pharmacology of newly developed non-catecholamine ligands, and discuss the potential utility of D1R-agonists in Parkinson's Disease and cognitive impairment.

Dopamine Supersensitivity: A Novel Hypothesis of Opioid-Induced Neurobiological Mechanisms Underlying Opioid-Stimulant Co-use and Opioid Relapse

Emergent harms presented by the co-use of opioids and methamphetamine highlight the broader public health challenge of preventing and treating opioid and stimulant co-use. Development of effective therapeutics requires an understanding of the physiological mechanisms that may be driving co-use patterns, specifically the underlying neurobiology of co-use and how they may facilitate (or be leveraged to prevent) continued use patterns. This narrative review summarizes largely preclinical data that demonstrate clinically-meaningful relationships between the dopamine and opioid systems with direct implications for opioid and stimulant co-use. Synthesized conclusions of this body of research include evidence that changes in the dopamine system occur only once physical dependence to opioids develops, that the chronicity of opioid exposure is associated with the severity of changes, and that withdrawal leaves the organism in a state of substantive dopamine deficit that persists long after the somatic or observed signs of opioid withdrawal appear to have resolved. Evidence also suggests that dopamine supersensitivity develops soon after opioid abstinence and results in increased response to dopamine agonists that increases in magnitude as the abstinence period continues and is evident several weeks into protracted withdrawal. Mechanistically, this supersensitivity appears to be mediated by changes in the sensitivity, not quantity, of dopamine D2 receptors. Here we propose a neural circuit mechanism unique to withdrawal from opioid use with implications for increased stimulant sensitivity in previously stimulant-naïve or inexperienced populations. These hypothesized effects collectively delineate a mechanism by which stimulants would be uniquely reinforcing to persons with opioid physical dependence, would contribute to the acute opioid withdrawal syndrome, and could manifest subjectively as craving and/or motivation to use that could prompt opioid relapse during acute and protracted withdrawal. Preclinical research is needed to directly test these hypothesized mechanisms. Human laboratory and clinical trial research is needed to explore these clinical predictions and to advance the goal of developing treatments for opioid-stimulant co-use and/or opioid relapse prevention and withdrawal remediation.

The Role of Mesostriatal Dopamine System and Corticostriatal Glutamatergic Transmission in Chronic Pain

There is increasing recognition of the involvement of the nigrostriatal and mesolimbic dopamine systems in the modulation of chronic pain. The first part of the present article reviews the evidence indicating that dopamine exerts analgesic effects during persistent pain by stimulating the D2 receptors in the dorsal striatum and nucleus accumbens (NAc). Thereby, dopamine inhibits striatal output via the D2 receptor-expressing medium spiny neurons (D2-MSN). Dopaminergic neurotransmission in the mesostriatal pathways is hampered in chronic pain states and this alteration maintains and exacerbates pain. The second part of this article focuses on the glutamatergic inputs from the medial prefrontal cortex to the NAc, their activity changes in chronic pain, and their role in pain modulation. Finally, interactions between dopaminergic and glutamatergic inputs to the D2-MSN are considered in the context of persistent pain. Studies using novel techniques indicate that pain is regulated oppositely by two independent dopaminergic circuits linking separate parts of the ventral tegmental area and of the NAc, which also interact with distinct regions of the medial prefrontal cortex.

Sex Differences in Dopamine Receptors and Relevance to Neuropsychiatric Disorders

Dopamine is an important neurotransmitter that plays a key role in neuropsychiatric illness. Sex differences in dopaminergic signaling have been acknowledged for decades and have been linked to sex-specific heterogeneity in both dopamine-related behaviours as well as in various neuropsychiatric disorders. However, the overall number of studies that have evaluated sex differences in dopamine signaling, both in health and in these disorders, is low. This review will bring together what is known regarding sex differences in innate dopamine receptor expression and function, as well as highlight the known sex-specific roles of dopamine in addiction, depression, anxiety, schizophrenia, and attention deficit hyperactivity disorder. Due to differences in prognosis, diagnosis, and symptomatology between male and female subjects in disorders that involve dopamine signaling, or in responses that utilize pharmacological interventions that target dopamine receptors, understanding the fundamental sex differences in dopamine receptors is of vital importance for the personalization of therapeutic treatment strategies.

Translational evidence for lithium-induced brain plasticity and neuroprotection in the treatment of neuropsychiatric disorders

Increasing evidence indicates lithium (Li+) efficacy in neuropsychiatry, pointing to overlapping mechanisms that occur within distinct neuronal populations. In fact, the same pathway depending on which circuitry operates may fall in the psychiatric and/or neurological domains. Li+ restores both neurotransmission and brain structure unveiling that psychiatric and neurological disorders share common dysfunctional molecular and morphological mechanisms, which may involve distinct brain circuitries. Here an overview is provided concerning the therapeutic/neuroprotective effects of Li+ in different neuropsychiatric disorders to highlight common molecular mechanisms through which Li+ produces its mood-stabilizing effects and to what extent these overlap with plasticity in distinct brain circuitries. Li+ mood-stabilizing effects are evident in typical bipolar disorder (BD) characterized by a cyclic course of mania or hypomania followed by depressive episodes, while its efficacy is weaker in the opposite pattern. We focus here on neural adaptations that may underlie psychostimulant-induced psychotic development and to dissect, through the sensitization process, which features are shared in BD and other psychiatric disorders, including schizophrenia. The multiple functions of Li+ highlighted here prove its exceptional pharmacology, which may help to elucidate its mechanisms of action. These may serve as a guide toward a multi-drug strategy. We propose that the onset of sensitization in a specific BD subtype may predict the therapeutic efficacy of Li+. This model may help to infer in BD which molecular mechanisms are relevant to the therapeutic efficacy of Li+.

Autophagy as a gateway for the effects of methamphetamine: From neurotransmitter release and synaptic plasticity to psychiatric and neurodegenerative disorders

As a major eukaryotic cell clearing machinery, autophagy grants cell proteostasis, which is key for neurotransmitter release, synaptic plasticity, and neuronal survival. In line with this, besides neuropathological events, autophagy dysfunctions are bound to synaptic alterations that occur in mental disorders, and early on, in neurodegenerative diseases. This is also the case of methamphetamine (METH) abuse, which leads to psychiatric disturbances and neurotoxicity. While consistently altering the autophagy machinery, METH produces behavioral and neurotoxic effects through molecular and biochemical events that can be recapitulated by autophagy blockade. These consist of altered physiological dopamine (DA) release, abnormal stimulation of DA and glutamate receptors, as well as oxidative, excitotoxic, and neuroinflammatory events. Recent molecular insights suggest that METH early impairs the autophagy machinery, though its functional significance remains to be investigated. Here we discuss evidence suggesting that alterations of DA transmission and autophagy are intermingled within a chain of events underlying behavioral alterations and neurodegenerative phenomena produced by METH. Understanding how METH alters the autophagy machinery is expected to provide novel insights into the neurobiology of METH addiction sharing some features with psychiatric disorders and parkinsonism.

Task-Dependent Effects of SKF83959 on Operant Behaviors Associated With Distinct Changes of CaMKII Signaling in Striatal Subareas

BACKGROUND

SKF83959, an atypical dopamine (DA) D1 receptor agonist, has been used to test the functions of DA-related receptor complexes in vitro, but little is known about its impact on conditioned behavior. The present study examined the effects of SKF83959 on operant behaviors and assayed the neurochemical mechanisms involved.

METHODS

Male rats were trained and maintained on either a fixed-interval 30-second (FI30) schedule or a differential reinforcement of low-rate response 10-second (DRL10) schedule of reinforcement. After drug treatment tests, western blotting assayed the protein expressions of the calcium-/calmodulin-dependent protein kinase II (CaMKII) and the transcription factor cyclic AMP response element binding protein (CREB) in tissues collected from 4 selected DA-related areas.

RESULTS

SKF83959 disrupted the performance of FI30 and DRL10 behaviors in a dose-dependent manner by reducing the total number of responses in varying magnitudes. Moreover, the distinct profiles of the behavior altered by the drug were manifested by analyzing qualitative and quantitative measures on both tasks. Western-blot results showed that phospho-CaMKII levels decreased in the nucleus accumbens and the dorsal striatum of the drug-treated FI30 and DRL10 subjects, respectively, compared with their vehicle controls. The phospho-CREB levels decreased in the nucleus accumbens and the hippocampus of drug-treated FI30 subjects but increased in the nucleus accumbens of drug-treated DRL10 subjects.

CONCLUSIONS

Our results provide important insight into the neuropsychopharmacology of SKF83959, indicating that the drug-altered operant behavior is task dependent and related to regional-dependent changes of CaMKII-CREB signaling in the mesocorticolimbic DA systems.

Heteromerization of dopaminergic receptors in the brain: Pharmacological implications

Dopamine exerts its physiological effects through two subtypes of receptors, i.e. the receptors of the D₁ family (D_1R and D_5R) and the D₂ family (D_2R, D_3R, and D_4R), which differ in their pattern of distribution, affinity, and signaling. The D₁-like subfamily (D_1R and D_5R) are coupled to Gα_s/olf proteins to activate adenylyl cyclase whereas the D₂-like receptors are coupled to Gα_i/o subunits and suppress the activity of adenylyl cyclase. Dopamine receptors are capable of forming homodimers, heterodimers, and higher-order oligomeric complexes, resulting in a change in the individual protomers' recognition, signaling, and pharmacology. Heteromerization has the potential to modify the canonical pharmacological features of individual monomeric units such as ligand affinity, activation, signaling, and cellular trafficking through allosteric interactions, reviving the field and introducing a new pharmacological target. Since heteromers are expressed and formed in a tissue-specific manner, they could provide the framework to design selective and effective drug candidates, such as brain-penetrant heterobivalent drugs and interfering peptides, with limited side effects. Therefore, heteromerization could be a promising area of pharmacology research, as it could contribute to the development of novel pharmacological interventions for dopamine dysregulated brain disorders such as addiction, schizophrenia, cognition, Parkinson's disease, and other motor-related disorders. This review is articulated based on the three criteria established by the International Union of Basic and Clinical Pharmacology for GPCR heterodimers (IUPHAR): evidence of co-localization and physical interactions in native or primary tissue, presence of a new physiological and functional property than the individual protomers, and loss of interaction and functional fingerprints upon heterodimer disruption.

The Role of CaMKII and ERK Signaling in Addiction

Nicotine is the predominant addictive compound of tobacco and causes the acquisition of dependence through its interactions with nicotinic acetylcholine receptors and various neurotransmitter releases in the central nervous system. The Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) and extracellular signal-regulated kinase (ERK) play a pivotal role in synaptic plasticity in the hippocampus. CaMKII is involved in long-term potentiation induction, which underlies the consolidation of learning and memory; however, the roles of CaMKII in nicotine and other psychostimulant-induced addiction still require further investigation. This article reviews the molecular mechanisms and crucial roles of CaMKII and ERK in nicotine and other stimulant drug-induced addiction. We also discuss dopamine (DA) receptor signaling involved in nicotine-induced addiction in the brain reward circuitry. In the last section, we introduce the association of polyunsaturated fatty acids and cellular chaperones of fatty acid-binding protein 3 in the context of nicotine-induced addiction in the mouse nucleus accumbens and provide a novel target for the treatment of drug abuse affecting dopaminergic systems.

The Distinct Functions of Dopaminergic Receptors on Pain Modulation: A Narrative Review

Chronic pain is considered an economic burden on society as it often results in disability, job loss, and early retirement. Opioids are the most common analgesics prescribed for the management of moderate to severe pain. However, chronic exposure to these drugs can result in opioid tolerance and opioid-induced hyperalgesia. On pain modulation strategies, exploiting the multitarget drugs with the ability of the superadditive or synergistic interactions attracts more attention. In the present report, we have reviewed the analgesic effects of different dopamine receptors, particularly D1 and D2 receptors, in different regions of the central nervous system, including the spinal cord, striatum, nucleus accumbens (NAc), and periaqueductal gray (PAG). According to the evidence, these regions are not only involved in pain modulation but also express a high density of DA receptors. The findings can be categorized as follows: (1) D2-like receptors may exert a higher analgesic potency, but D1-like receptors act in different manners across several mechanisms in the mentioned regions; (2) in the spinal cord and striatum, antinociception of DA is mainly mediated by D2-like receptors, while in the NAc and PAG, both D1- and D2-like receptors are involved as analgesic targets; and (3) D2-like receptor agonists can act as adjuvants of μ-opioid receptor agonists to potentiate analgesic effects and provide a better approach to pain relief.

Impaired Acquisition of Nicotine-Induced Conditioned Place Preference in Fatty Acid-Binding Protein 3 Null Mice

Nicotine causes psychological dependence through its interactions with nicotinic acetylcholine receptors in the brain. We previously demonstrated that fatty acid-binding protein 3 (FABP3) colocalizes with dopamine D2 receptors (D2Rs) in the dorsal striatum, and FABP3 deficiency leads to impaired D2R function. Moreover, D2R null mice do not exhibit increased nicotine-induced conditioned place preference (CPP) following chronic nicotine administration. To investigate the role of FABP3 in nicotine-induced CPP, FABP3 knockout (FABP3^-/-) mice were evaluated using a CPP apparatus following consecutive nicotine administration (0.5 mg/kg) for 14 days. Importantly, nicotine-induced CPP was suppressed in the conditioning, withdrawal, and relapse phases in FABP3^-/- mice. To resolve the mechanisms underlying impaired nicotine-induced CPP in these mice, we assessed c-Fos expression and Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) and extracellular signal-regulated kinase (ERK) signaling in both dopamine D1 receptor (D1R)- and D2R-positive neurons in the nucleus accumbens (NAc). Notably, 64% of dopamine receptor-positive neurons in the mouse NAc expressed both D1R and D2R. Impaired nicotine-induced CPP was correlated with lack of responsiveness of both CaMKII and ERK phosphorylation. The number of D2R-positive neurons was increased in FABP3^-/- mice, while the number of D1R-positive neurons and the responsiveness of c-Fos expression to nicotine were decreased. The aberrant c-Fos expression was closely correlated with CaMKII but not ERK phosphorylation levels in the NAc of FABP3^-/- mice. Taken together, these results indicate that impaired D2R signaling due to lack of FABP3 may affect D1R and c-Fos signaling and underlie nicotine-induced CPP behaviors.

A dynamic role for dopamine receptors in the control of mammalian spinal networks

Dopamine is well known to regulate movement through the differential control of direct and indirect pathways in the striatum that express D₁ and D₂ receptors respectively. The spinal cord also expresses all dopamine receptors; however, how the specific receptors regulate spinal network output in mammals is poorly understood. We explore the receptor-specific mechanisms that underlie dopaminergic control of spinal network output of neonatal mice during changes in spinal network excitability. During spontaneous activity, which is a characteristic of developing spinal networks operating in a low excitability state, we found that dopamine is primarily inhibitory. We uncover an excitatory D₁-mediated effect of dopamine on motoneurons and network output that also involves co-activation with D₂ receptors. Critically, these excitatory actions require higher concentrations of dopamine; however, analysis of dopamine concentrations of neonates indicates that endogenous levels of spinal dopamine are low. Because endogenous levels of spinal dopamine are low, this excitatory dopaminergic pathway is likely physiologically-silent at this stage in development. In contrast, the inhibitory effect of dopamine, at low physiological concentrations is mediated by parallel activation of D₂, D₃, D₄ and α₂ receptors which is reproduced when endogenous dopamine levels are increased by blocking dopamine reuptake and metabolism. We provide evidence in support of dedicated spinal network components that are controlled by excitatory D₁ and inhibitory D₂ receptors that is reminiscent of the classic dopaminergic indirect and direct pathway within the striatum. These results indicate that network state is an important factor that dictates receptor-specific and therefore dose-dependent control of neuromodulators on spinal network output and advances our understanding of how neuromodulators regulate neural networks under dynamically changing excitability.

Dopamine D1-D2 receptor heteromer expression in key brain regions of rat and higher species: Upregulation in rat striatum after cocaine administration

BACKGROUND

Dopamine receptors interact with other receptors to form heterooligomers. One such complex, the D1-D2 heteromer, demonstrated in cultured striatal neurons and rat striatum has been linked to drug addiction, Parkinson's disease, schizophrenia, depression and anhedonia.

METHODS

D1-D2 heteromer expression was evaluated using in situ proximity ligation assay, in parallel with cellular colocalization of D1 and D2 mRNA using in situ hybridization in 19 different key rat brain regions. Expression in higher species and changes in rat striatum after repeated cocaine administration were evaluated.

RESULTS

Differences in D1-D2 heteromer expression in striatal subregions are documented in higher species with nonhuman primate and human demonstrating higher density of heteromer-expressing neurons compared to rodents. All species had higher density of D1-D2 neurons in nucleus accumbens compared to dorsal striatum. Multiple other brain regions are identified where D1-D2 heteromer is expressed, prominently in cerebral cortical subregions including piriform, medial prefrontal, orbitofrontal and others; subcortical regions such as claustrum, amygdala and lateral habenula. Three categories of regions are identified: D1-D2 heteromer expressed despite little to no observed D1/D2 mRNA colocalization, likely representing heteromer on neuronal projections from other brain regions; D1-D2 heteromer originating locally with the density of neurons expressing heteromer matching neurons with colocalized D1/D2 mRNA; regions with both a local origin and targeted inputs projecting from other regions. Repeated cocaine administration significantly increased density of neurons expressing D1-D2 heteromer and D1/D2 mRNA colocalization in rat striatum, with changes in both direct and indirect pathway neurons.

CONCLUSION

The dopamine D1-D2 heteromer is expressed in key brain cortical and subcortical regions of all species examined. Species differences in striatum revealed greater abundance in human>nonhuman-primate>rat>mouse, suggesting an evolutionary biologic role for the D1-D2 heteromer in higher CNS function. Its upregulation in rat striatum following cocaine points to regulatory significance with possible relevance for clinical disorders such as drug addiction. The dopamine D1-D2 receptor heteromer may represent a potential target for neuropsychiatric and neurodegenerative disorders, given its distribution in highly relevant brain regions.

Aberrant Dopamine Receptor Signaling Plays Critical Role in the Impairment of Striatal Neurons in Experimental Cerebral Malaria

One-fourth survivors of cerebral malaria (CM) retain long-term cognitive and behavioral deficits. Structural abnormalities in striatum are reported in 80% of children with CM. Dopamine receptors (D1 and D2) are widely expressed in striatal medium spiny neurons (MSNs) that regulate critical physiological functions related to behavior and cognition. Dysregulation of dopamine receptors alters the expression of downstream proteins such as dopamine- and cAMP-regulated phosphoprotein (DARPP), Ca²⁺/calmodulin-dependent protein kinase II alpha (CaMKIIα), and p25/cyclin-dependent kinase 5 (cdk5). However, the role of dopamine receptor signaling dysfunction on the outcome of striatal neuron degeneration is unknown underlying the pathophysiology of CM. Using experimental CM (ECM), the present study attempted to understand the role of aberrant dopamine receptor signaling and its possible relation in causing MSNs morphological impairment. The effect of antimalarial drug artemether (ARM) rescue therapy was also assessed after ECM on the outcome of dopamine receptors downstream signaling. ECM was induced in C57BL/6 mice (male and female) infecting with Plasmodium berghei ANKA (PbA) parasite that reiterates the clinical setting of CM. We demonstrated that ECM caused a significant increase in the expression of D1, D2 receptors, phosphorylated DARPP, p25, cdk5, CaMKIIα, and D1-D2 heteromers. A substantial increase in neuronal damage observed in the dorsolateral striatum region of ECM brains (particularly in MSNs) as revealed by increased Fluoro-Jade C staining, reduced dendritic spine density, and impaired dendritic arborization with varicosities. While the ARM rescue therapy significantly altered the effects of ECM induced dopamine receptor signaling dysfunction and neurodegeneration. Overall, our data suggest that dysregulation of dopamine receptor signaling plays an important role in the degeneration of MSNs, and the ARM rescue therapy might provide better insights to develop effective therapeutic strategies for CM.

Profiling of behavioral effects evoked by ketamine and the role of 5HT2 and D2 receptors in ketamine-induced locomotor sensitization in mice

Ketamine has addictive potential, a troublesome fact due to its promising use as a therapeutic drug. An important phenomenon associated with drug addiction is behavioral sensitization, usually characterized as augmented locomotion. However, other behaviors may also be susceptible to sensitization, and/or interfere with locomotor activity. Thus, this study drew a comprehensive behavioral 'profiling' in an animal model of repeated administration of ketamine. Adult Swiss mice received single daily ketamine injections (30 or 50 mg/Kg, i.p.), which were followed by open field testing for 7 days (acquisition period, ACQ). A ketamine challenge (sensitization test, ST) was carried out after a 5-day withdrawal. Locomotion, rearing, grooming, rotation and falling were assessed during ACQ and ST. All behaviors were affected from the first ACQ day onwards, with no indication of competition between locomotion and the other behaviors. Only locomotion in response to 30 mg/Kg of ketamine both escalated during ACQ and expressed increased levels at ST, evidencing development and expression of locomotor sensitization. Considering the involvement of serotonin 5HT₍₂₎ and dopamine D₍₂₎ receptors on addiction mechanisms, we further tested the involvement of these receptors in ketamine-induced sensitization. Ketanserin (5HT₂ antagonist, 3 mg/Kg, s.c.) prevented ketamine-evoked development of locomotor sensitization. However, ketanserin pretreatment during ACQ failed to inhibit its expression during ST. Raclopride (D₂ antagonist, 0.5 mg/Kg, s.c.) evoked less robust reductions in locomotion but prevented the development of ketamine-evoked sensitization. Pretreatment during ACQ further inhibited the expression of sensitization during ST. These results indicate that a partial overlap in serotonergic and dopaminergic mechanisms underlies ketamine-induced locomotor sensitization.

Where Is Dopamine and how do Immune Cells See it?: Dopamine-Mediated Immune Cell Function in Health and Disease

Dopamine is well recognized as a neurotransmitter in the brain, and regulates critical functions in a variety of peripheral systems. Growing research has also shown that dopamine acts as an important regulator of immune function. Many immune cells express dopamine receptors and other dopamine related proteins, enabling them to actively respond to dopamine and suggesting that dopaminergic immunoregulation is an important part of proper immune function. A detailed understanding of the physiological concentrations of dopamine in specific regions of the human body, particularly in peripheral systems, is critical to the development of hypotheses and experiments examining the effects of physiologically relevant dopamine concentrations on immune cells. Unfortunately, the dopamine concentrations to which these immune cells would be exposed in different anatomical regions are not clear. To address this issue, this comprehensive review details the current information regarding concentrations of dopamine found in both the central nervous system and in many regions of the periphery. In addition, we discuss the immune cells present in each region, and how these could interact with dopamine in each compartment described. Finally, the review briefly addresses how changes in these dopamine concentrations could influence immune cell dysfunction in several disease states including Parkinson's disease, multiple sclerosis, rheumatoid arthritis, inflammatory bowel disease, as well as the collection of pathologies, cognitive and motor symptoms associated with HIV infection in the central nervous system, known as NeuroHIV. These data will improve our understanding of the interactions between the dopaminergic and immune systems during both homeostatic function and in disease, clarify the effects of existing dopaminergic drugs and promote the creation of new therapeutic strategies based on manipulating immune function through dopaminergic signaling. Graphical Abstract.

Sex difference in dopamine D1-D2 receptor complex expression and signaling affects depression- and anxiety-like behaviors

Depression and anxiety are more common among females than males and represent a leading cause of disease-related disability in women. Since the dopamine D1-D2 heteromer is involved in depression- and anxiety-like behavior, the possibility that the receptor complex may have a role in mediating sex differences in such behaviors and related biochemical signaling was explored.

In non-human primate caudate nucleus and in rat striatum, females expressed higher density of D1-D2 heteromer complexes and a greater number of D1-D2 expressing neurons compared to males. In rat, the sex difference in D1-D2 expression levels occurred even though D1 receptor expression was lower in female than in male with no difference in D2 receptor expression. In behavioral tests, female rats showed faster latency to depressive-like behavior and a greater susceptibility to the pro-depressive and anxiogenic-like effects of D1-D2 heteromer activation by low doses of SKF 83959, all of which were ameliorated by the selective heteromer disrupting peptide, TAT-D1. The sex difference observed in the anxiety test correlated with differences in low-frequency delta and theta oscillations in the nucleus accumbens. Analysis of signaling pathways revealed that the sex difference in D1-D2 heteromer expression led to differences in basal and heteromer-stimulated activities of two important signaling pathways, BDNF/TrkB and Akt/GSK3/β-catenin.

These results suggest that the higher D1-D2 heteromer expression in female may significantly increase predisposition to depressive-like and anxiety-like behavior in female animals.

Neuropathic Pain Dysregulates Gene Expression of the Forebrain Opioid and Dopamine Systems

Disturbances in the function of the mesostriatal dopamine system may contribute to the development and maintenance of chronic pain, including its sensory and emotional/cognitive aspects. In the present study, we assessed the influence of chronic constriction injury (CCI) of the sciatic nerve on the expression of genes coding for dopamine and opioid receptors as well as opioid propeptides in the mouse mesostriatal system, particularly in the nucleus accumbens. We demonstrated bilateral increases in mRNA levels of the dopamine D1 and D2 receptors (the latter accompanied by elevated protein level), opioid propeptides proenkephalin and prodynorphin, as well as delta and kappa (but not mu) opioid receptors in the nucleus accumbens at 7 to 14 days after CCI. These results show that CCI-induced neuropathic pain is accompanied by a major transcriptional dysregulation of molecules involved in dopaminergic and opioidergic signaling in the striatum/nucleus accumbens. Possible functional consequences of these changes include opposite effects of upregulated enkephalin/delta opioid receptor signaling vs. dynorphin/kappa opioid receptor signaling, with the former most likely having an analgesic effect and the latter exacerbating pain and contributing to pain-related negative emotional states.

The globus pallidus as a target for neuropeptides and endocannabinoids participating in central activities

The globus pallidus in the basal ganglia plays an important role in movement regulation. Neuropeptides and endocannabinoids are neuronal signalling molecules that influence the functions of the whole brain. Endocannabinoids, enkephalin, substance P, neurotensin, orexin, somatostatin and pituitary adenylate cyclase-activating polypeptides are richly concentrated in the globus pallidus. Neuropeptides and endocannabinoids exert excitatory or inhibitory effects in the globus pallidus mainly by modulating GABAergic, glutamatergic and dopaminergic neurotransmission, as well as many ionic mechanisms. Pallidal neuropeptides and endocannabinoids are associated with the pathophysiology of a number of neurological disorders, such as Parkinson's disease, Huntington's disease, schizophrenia, and depression. The levels of neuropeptides and endocannabinoids and their receptors in the globus pallidus change in neurological diseases. It has been demonstrated that spontaneous firing activity of globus pallidus neurons is closely related to the manifestations of Parkinson's disease. Therefore, the neuropeptides and endocannabinoids in the globus pallidus may function as potential targets for treatment in some neurological diseases. In this review, we highlight the morphology and function of neuropeptides and endocannabinoids in the globus pallidus and their involvement in neurological diseases.

Δ-Tetrahydrocannabinol Increases Dopamine D1-D2 Receptor Heteromer and Elicits Phenotypic Reprogramming in Adult Primate Striatal Neurons

Long-term cannabis users manifest deficits in dopaminergic functions, reflecting Δ9-tetrahydrocannabinol (THC)-induced neuroadaptive dysfunctional dopamine signaling, similar to those observed upon dopamine D1-D2 heteromer activation. The molecular mechanisms remain largely unknown. We show evolutionary and regional differences in D1-D2 heteromer abundance in mammalian striatum. Importantly, chronic THC increased the number of D1-D2 heteromer-expressing neurons, and the number of heteromers within individual neurons in adult monkey striatum. The majority of these neurons displayed a phenotype co-expressing the characteristic markers of both striatonigral and striatopallidal neurons. Furthermore, THC increased D1-D2-linked calcium signaling markers (pCaMKIIα, pThr75-DARPP-32, BDNF/pTrkB) and inhibited cyclic AMP signaling (pThr34-DARPP-32, pERK1/2, pS845-GluA1, pGSK3). Cannabidiol attenuated most but not all of these THC-induced neuroadaptations. Targeted pathway analyses linked these changes to neurological and psychological disorders. These data underline the importance of the D1-D2 receptor heteromer in cannabis use-related disorders, with THC-induced changes likely responsible for the reported adverse effects observed in heavy long-term users.

Expression of Melatonin and Dopamine D3 Receptor Heteromers in Eye Ciliary Body Epithelial Cells and Negative Correlation with Ocular Hypertension

BACKGROUND

Experiments in the late nineties showed an inverse relationship in the eye levels of melatonin and dopamine, thereby constituting an example of eye parameters that are prone to circadian variations. The underlying mechanisms are not known but these relevant molecules act via specific cell surface dopamine and melatonin receptors. This study investigated whether these receptors formed heteromers whose function impact on eye physiology. We performed biophysical assays to identify interactions in heterologous systems. Particular heteromer functionality was detected using Gi coupling, MAPK activation, and label-free assays. The expression of the heteroreceptor complexes was assessed using proximity ligation assays in cells producing the aqueous humor and human eye samples. Dopamine D₃ receptors (D₃Rs) were identified in eye ciliary body epithelial cells. We discovered heteromers formed by D₃R and either MT₁ (MT₁R) or MT₂ (MT₂R) melatonin receptors. Heteromerization led to the blockade of D₃R-Gi coupling and regulation of signaling to the MAPK pathway. Heteromer expression was negatively correlated with intraocular hypertension.

CONCLUSIONS

Heteromers likely mediate melatonin and dopamine actions in structures regulating intraocular pressure. Significant expression of D₃R-MT₁R and D₃R-MT₁R was associated with normotensive conditions, whereas expression diminished in a cell model of hypertension. A clear trend of expression reduction was observed in samples from glaucoma cases. The trend was marked but no statistical analysis was possible as the number of available eyes was 2.

Recent advances in the biosensing of neurotransmitters: material and method overviews towards the biomedical analysis of psychiatric disorders

Neurotransmitters are the most important messengers of the nervous system, and any changes in their balances and activities can cause serious neurological, psychiatric and cognitive disorders such as schizophrenia, Alzheimer's disease and Parkinson's disease.

Genetic variants in dopamine receptors influence on heterodimerization in the context of antipsychotic drug action

Human dopamine D2 receptor (D₂R) gene has polymorphic variants, three of them alter its amino acid sequence: Val96Ala, Pro310Ser and Ser311Cys. Their functional role never became the object of extensive studies, even though there are some evidence that they correlate with schizophrenia. The present work reviews data indicating that these mutations play a role in dimer formation with dopamine D1 receptor (D₁R), with the strongest effect observed for Ser311Cys variant. Similarly, the affinity for antipsychotic drugs of this genetic variant depends on whether it is expressed together with D₁R or not. Better understanding of altered ability of genetic variants of D₂R to form dimers with D₁R, as well as of altered affinity for antipsychotic drugs, depending on the absence or presence of the second dopamine receptor is of great importance-since these two receptors are not always co-expressed in the same cell. It may well be that targeting new compounds toward the D₁R-D₂R dimers, which the most probably form under conditions of excessive dopamine release, will result in antipsychotic drugs devoid of serious side effects.

Bipolar oscillations between positive and negative mood states in a computational model of Basal Ganglia

Bipolar disorder is characterized by mood swings-oscillations between manic and depressive states. The swings (oscillations) mark the length of an episode in a patient's mood cycle (period), and can vary from hours to years. The proposed modeling study uses decision making framework to investigate the role of basal ganglia network in generating bipolar oscillations. In this model, the basal ganglia system performs a two-arm bandit task in which one of the arms (action responses) leads to a positive outcome, while the other leads to a negative outcome. We explore the dynamics of key reward and risk related parameters in the system while the model agent receives various outcomes. Particularly, we study the system using a model that represents the fast dynamics of decision making, and a module to capture the slow dynamics that describe the variation of some meta-parameters of fast dynamics over long time scales. The model is cast at three levels of abstraction: (1) a two-dimensional dynamical system model, that is a simple two variable model capable of showing bistability for rewarding and punitive outcomes; (2) a phenomenological basal ganglia model, to extend the implications from the reduced model to a cortico-basal ganglia setup; (3) a detailed network model of basal ganglia, that incorporates detailed cellular level models for a more realistic understanding. In healthy conditions, the model chooses positive action and avoids negative one, whereas under bipolar conditions, the model exhibits slow oscillations in its choice of positive or negative outcomes, reminiscent of bipolar oscillations. Phase-plane analyses on the simple reduced dynamical system with two variables reveal the essential parameters that generate pathological 'bipolar-like' oscillations. Phenomenological and network models of the basal ganglia extend that logic, and interpret bipolar oscillations in terms of the activity of dopaminergic and serotonergic projections on the cortico-basal ganglia network dynamics. The network's dysfunction, specifically in terms of reward and risk sensitivity, is shown to be responsible for the pathological bipolar oscillations. The study proposes a computational model that explores the effects of impaired serotonergic neuromodulation on the dynamics of the cortico basal ganglia network, and relates this impairment to abstract mood states (manic and depressive episodes) and oscillations of bipolar disorder.