Evidence for Noncanonical Neurotransmitter Activation: Norepinephrine as a Dopamine D2-Like Receptor Agonist

Comparison of the function of two novel human dopamine D2 receptor variants identifies a likely mechanism for their pathogenicity

Two recently discovered DRD2 mutations, c.634A > T, p.Ile212Phe and c.1121T > G, p.Met374Arg, cause hyperkinetic movement disorders that have overlapping features but apparently differ in severity. The two known carriers of the Met374Arg variant had early childhood disease onset and more severe motor, cognitive, and neuropsychiatric deficits than any known carriers of the Ile212Phe variant, whose symptoms were first apparent in adolescence. Here, we evaluated if differences in the function of the two variants in cultured cells could explain differing pathogenicity. Both variants were expressed less abundantly than the wild type receptor and exhibited loss of agonist-induced arrestin binding, but differences in expression and arrestin binding between the variants were minor. Basal and agonist-induced activation of heterotrimeric Gi/o/z proteins, however, showed clear differences; agonists were generally more potent at Met374Arg than at the Ile212Phe or wild type variants. Furthermore, all Gα subtypes tested were constitutively activated more by Met374Arg than by Ile212Phe. Met374Arg produced greater constitutive inhibition of cyclic AMP accumulation than Ile212Phe or the wild type D2 receptor. Met374Arg and Ile212Phe were more sensitive to thermal inactivation than the wild type D2 receptor, as reported for other constitutively active receptors, but Ile212Phe was affected more than Met374Arg. Additional pharmacological characterization suggested that the mutations differentially affect the shape of the agonist binding pocket and the potency of dopamine, norepinephrine, and tyramine. Molecular dynamics simulations provided a structural rationale for enhanced constitutive activation and agonist potency. Enhanced constitutive and agonist-induced G protein-mediated signaling likely contributes to the pathogenicity of these novel variants.

Stress-induced stenotic vascular remodeling via reduction of plasma omega-3 fatty acid metabolite 4-oxoDHA by noradrenaline

Stress has garnered significant attention as a prominent risk factor for inflammation-related diseases, particularly cardiovascular diseases (CVDs). However, the precise mechanisms underlying stress-driven CVDs remain elusive, thereby impeding the development of preventive and therapeutic strategies. To explore the correlation between plasma lipid metabolites and human depressive states, liquid chromatography-mass spectrometry (LC/MS) based analysis of plasma and the self-rating depression (SDS) scale questionnaire were employed. We also used a mouse model with restraint stress to study its effects on plasma lipid metabolites and stenotic vascular remodeling following carotid ligation. In vitro functional and mechanistic studies were performed using macrophages, endothelial cells, and neutrophil cells. We revealed a significant association between depressive state and reduced plasma levels of 4-oxoDHA, a specific omega-3 fatty acid metabolite biosynthesized by 5-lipoxygenase (LO), mainly in neutrophils. In mice, restraint stress decreased plasma 4-oxoDHA levels and exacerbated stenotic vascular remodeling, ameliorated by 4-oxoDHA supplementation. 4-oxoDHA enhanced Nrf2-HO-1 pathways, exerting anti-inflammatory effects on endothelial cells and macrophages. One of the stress hormones, noradrenaline, reduced 4-oxoDHA and the degraded 5-LO in neutrophils through the proteasome system, facilitated by dopamine D2-like receptor activation. Our study proposed circulating 4-oxoDHA levels as a stress biomarker and supplementation of 4-oxoDHA as a novel therapeutic approach for controlling stress-related vascular inflammation.

Direct interrogation of context-dependent GPCR activity with a universal biosensor platform

G protein-coupled receptors (GPCRs) are the largest family of druggable proteins in the human genome, but progress in understanding and targeting them is hindered by the lack of tools to reliably measure their nuanced behavior in physiologically-relevant contexts. Here, we developed a collection of compact ONE vector G-protein Optical (ONE-GO) biosensor constructs as a scalable platform that can be conveniently deployed to measure G-protein activation by virtually any GPCR with high fidelity even when expressed endogenously in primary cells. By characterizing dozens of GPCRs across many cell types like primary cardiovascular cells or neurons, we revealed new insights into the molecular basis for G-protein coupling selectivity of GPCRs, pharmacogenomic profiles of anti-psychotics on naturally-occurring GPCR variants, and G-protein subtype signaling bias by endogenous GPCRs depending on cell type or upon inducing disease-like states. In summary, this open-source platform makes the direct interrogation of context-dependent GPCR activity broadly accessible.

Structure of the dopamine D3 receptor bound to a bitopic agonist reveals a new specificity site in an expanded allosteric pocket

Although aminergic GPCRs are the target for ~25% of approved drugs, developing subtype selective drugs is a major challenge due to the high sequence conservation at their orthosteric binding site. Bitopic ligands are covalently joined orthosteric and allosteric pharmacophores with the potential to boost receptor selectivity, driven by the binding of the secondary pharmacophore to non-conserved regions of the receptor. Although bitopic ligands have great potential to improve current medications by reducing off-target side effects, the lack of structural information on their binding mode impedes rational design. Here we determine the cryo-EM structure of the hD3R coupled to a GO heterotrimer and bound to the D3R selective bitopic agonist FOB02-04A. Structural, functional and computational analyses provide new insights into its binding mode and point to a new TM2-ECL1-TM1 region, which requires the N-terminal ordering of TM1, as a major determinant of subtype selectivity in aminergic GPCRs. This region is underexploited in drug development, expands the established secondary binding pocket in aminergic GPCRs and could potentially be used to design novel and subtype selective drugs.

Acute stress modulates noradrenergic signaling in the ventral tegmental area-amygdalar circuit

Noradrenergic neurotransmission is a critical mediator of stress responses. In turn, exposure to stress induces noradrenergic system adaptations, some of which are implicated in the etiology of stress-related disorders. Adrenergic receptors (ARs) in the ventral tegmental area (VTA) have been demonstrated to regulate phasic dopamine (DA) release in the forebrain, necessary for behavioral responses to conditional cues. However, the impact of stress on noradrenergic modulation of the VTA has not been previously explored. We demonstrate that ARs in the VTA regulate dopaminergic activity in the VTA-BLA (basolateral amygdala) circuit, a key system for processing stress-related stimuli; and that such control is altered by acute stress. We utilized fast-scan cyclic voltammetry to assess the effects of intra-VTA microinfusion of α₁ -AR and α₂ -AR antagonists (terazosin and RX-821002, respectively), on electrically evoked phasic DA release in the BLA in stress-naïve and stressed (unavoidable electric shocks - UES) anesthetized male Sprague-Dawley rats. In addition, we used western blotting to explore UES-induced alterations in AR protein level in the VTA. Intra-VTA terazosin or RX-821002 dose-dependently attenuated DA release in the BLA. Interestingly, UES decreased the effects of intra-VTA α₂ -AR blockade on DA release (24 h but not 7 days after stress), while the effects of terazosin were unchanged. Despite changes in α₂ -AR physiological function in the VTA, UES did not alter α₂ -AR protein levels in either intracellular or membrane fractions. These findings demonstrate that NA-ergic modulation of the VTA-BLA circuit undergoes significant alterations in response to acute stress, with α₂ -AR signaling indicated as a key target.

Dopamine-induced arrestin recruitment and desensitization of the dopamine D4 receptor is regulated by G protein-coupled receptor kinase-2

The dopamine D₄ receptor (D₄R) is expressed in the retina, prefrontal cortex, and autonomic nervous system and has been implicated in attention deficit hyperactivity disorder (ADHD), substance use disorders, and erectile dysfunction. D₄R has also been investigated as a target for antipsychotics due to its high affinity for clozapine. As opposed to the closely related dopamine D₂ receptor (D₂R), dopamine-induced arrestin recruitment and desensitization at the D₄R have not been studied in detail. Indeed, some earlier investigations could not detect arrestin recruitment and desensitization of this receptor upon its activation by agonist. Here, we used a novel nanoluciferase complementation assay to study dopamine-induced recruitment of β-arrestin2 (βarr2; also known as arrestin3) and G protein-coupled receptor kinase-2 (GRK2) to the D₄R in HEK293T cells. We also studied desensitization of D₄R-evoked G protein-coupled inward rectifier potassium (GIRK; also known as Kir3) current responses in Xenopus oocytes. Furthermore, the effect of coexpression of GRK2 on βarr2 recruitment and GIRK response desensitization was examined. The results suggest that coexpression of GRK2 enhanced the potency of dopamine to induce βarr2 recruitment to the D₄R and accelerated the rate of desensitization of D₄R-evoked GIRK responses. The present study reveals new details about the regulation of arrestin recruitment to the D₄R and thus increases our understanding of the signaling and desensitization of this receptor.

Dopaminergic regulation of hippocampal plasticity, learning, and memory

The hippocampus is responsible for encoding behavioral episodes into short-term and long-term memory. The circuits that mediate these processes are subject to neuromodulation, which involves regulation of synaptic plasticity and local neuronal excitability. In this review, we present evidence to demonstrate the influence of dopaminergic neuromodulation on hippocampus-dependent memory, and we address the controversy surrounding the source of dopamine innervation. First, we summarize historical and recent retrograde and anterograde anatomical tracing studies of direct dopaminergic projections from the ventral tegmental area and discuss dopamine release from the adrenergic locus coeruleus. Then, we present evidence of dopaminergic modulation of synaptic plasticity in the hippocampus. Plasticity mechanisms are examined in brain slices and in recordings from in vivo neuronal populations in freely moving rodents. Finally, we review pharmacological, genetic, and circuitry research that demonstrates the importance of dopamine release for learning and memory tasks while dissociating anatomically distinct populations of direct dopaminergic inputs.

Dopamine and norepinephrine are embracing their immune side and so should we

While the history of neuroimmunology is long, the explicit study of neuroimmune communication, and particularly the role of catecholamines in neuroimmunity, is still emerging. Recent studies have shown that catecholamines, norepinephrine, epinephrine, and dopamine, are central to multiple complex mechanisms regulating immune function. These studies show that catecholamines can be released from both the nervous system and directly from immune cells, mediating both autocrine and paracrine signaling. This commentary highlights the importance of catecholaminergic immunomodulation and discusses new considerations needed to study the role of catecholamines in immune homeostasis to best leverage their contribution to disease processes for the development of new therapeutic approaches.

Alpha-2A but not 2B/C noradrenergic receptors in ventral tegmental area regulate phasic dopamine release in nucleus accumbens core

Adrenergic receptors (AR) in the ventral tegmental area (VTA) modulate local neuronal activity and, as a consequence, dopamine (DA) release in the mesolimbic forebrain. Such modulation has functional significance: intra-VTA blockade of α₁-AR attenuates behavioral responses to salient environmental stimuli in rat models of drug seeking and conditioned fear as well as phasic DA release in the nucleus accumbens (NAc). In contrast, α₂-AR in the VTA has been suggested to act primarily as autoreceptors, limiting local noradrenergic input. The regulation of noradrenaline efflux by α₂-AR could be of clinical interest, as α₂-AR agonists are proposed as promising pharmacological tools in the treatment of PTSD and substance use disorder. Thus, the aim of our study was to determine the subtype-specificity of α₂-ARs in the VTA capable of modulating phasic DA release. We used fast scan cyclic voltammetry (FSCV) in anaesthetized male rats to measure DA release in the NAc after combined electrical stimulation and infusion of selected α₂-AR antagonists into the VTA. Intra-VTA microinfusion of idazoxan - a non-subtype-specific α₂-AR antagonist, as well as BRL-44408 - a selective α_2A-AR antagonist, attenuated electrically-evoked DA in the NAc. In contrast, local administration of JP-1302 or imiloxan (α_2B- and α_2C-AR antagonists, respectively) had no effect. The effect of BRL-44408 on DA release was attenuated by intra-VTA DA D₂ antagonist (raclopride) pre-administration. Finally, we confirmed the presence of α_2A-AR protein in the VTA using western blotting. In conclusion, these data specify α_2A-, but not α_2B- or α_2C-AR as the receptor subtype controlling NA release in the VTA.

Dual pancreatic adrenergic and dopaminergic signaling as a therapeutic target of bromocriptine

Bromocriptine is approved as a diabetes therapy, yet its therapeutic mechanisms remain unclear. Though bromocriptine's actions have been mainly attributed to the stimulation of brain dopamine D2 receptors (D2R), bromocriptine also targets the pancreas. Here, we employ bromocriptine as a tool to elucidate the roles of catecholamine signaling in regulating pancreatic hormone secretion. In β-cells, bromocriptine acts on D2R and α2A-adrenergic receptor (α2A-AR) to reduce glucose-stimulated insulin secretion (GSIS). Moreover, in α-cells, bromocriptine acts via D2R to reduce glucagon secretion. α2A-AR activation by bromocriptine recruits an ensemble of G proteins with no β-arrestin2 recruitment. In contrast, D2R recruits G proteins and β-arrestin2 upon bromocriptine stimulation, demonstrating receptor-specific signaling. Docking studies reveal distinct bromocriptine binding to α2A-AR versus D2R, providing a structural basis for bromocriptine's dual actions on β-cell α2A-AR and D2R. Together, joint dopaminergic and adrenergic receptor actions on α-cell and β-cell hormone release provide a new therapeutic mechanism to improve dysglycemia.

Catecholaminergic innervation and D2-like dopamine receptor-mediated modulation of brainstem nucleus incertus neurons in the rat

Nucleus incertus (NI) is a brainstem structure involved in the control of arousal, stress responses and locomotor activity. It was reported recently that NI neurons express the dopamine type 2 (D2) receptor that belongs to the D2-like receptor (D2R) family, and that D2R activation in the NI decreased locomotor activity. In this study, using multiplex in situ hybridization, we observed that GABAergic and glutamatergic NI neurons express D2 receptor mRNA, and that D2 receptor mRNA-positive neurons belong to partially overlapping relaxin-3- and cholecystokinin-positive NI neuronal populations. Our immunohistochemical and viral-based retrograde tract-tracing studies revealed a dense innervation of the NI area by fibers containing the catecholaminergic biosynthesis enzymes, tyrosine hydroxylase (TH) and dopamine β-hydroxylase (DBH), and indicated the major sources of the catecholaminergic innervation of the NI as the Darkschewitsch, raphe and hypothalamic A13 nuclei. Furthermore, using whole-cell patch clamp recordings, we demonstrated that D2R activation by quinpirole produced excitatory and inhibitory influences on neuronal activity in the NI, and that both effects were postsynaptic in nature. Moreover, the observed effects were cell-type specific, as type I NI neurons were either excited or inhibited, whereas type II NI neurons were mainly excited by D2R activation. Our results reveal that rat NI receives a strong catecholaminergic innervation and suggest that catecholamines acting within the NI are involved in the control of diverse processes, including locomotor activity, social interaction and nociceptive signaling. Our data also strengthen the hypothesis that the NI acts as a hub integrating arousal-related neuronal information.

The potent α2-adrenoceptor antagonist RS 79948 also inhibits dopamine D2 -receptors: Comparison with atipamezole and raclopride

Neurochemical, electrophysiological and behavioral evidence indicate that the potent α₂-adrenoceptor antagonist RS 79948 is also a dopamine (DA) D₂ receptor antagonist. Thus, results from ligand binding and adenylate cyclase activity indicate that RS 79948 binds to D₂ receptors and antagonized D₂ receptor-mediated inhibition of cAMP synthesis at nanomolar concentrations. RESULTS: from microdialysis indicated that RS 79948 shared with the selective α₂-adrenergic antagonist atipamezole the ability to increase the co-release of DA and norepinephrine (NE) from noradrenergic terminals in the medial prefrontal cortex (mPFC), except that RS 79948-induced DA release persisted after noradrenergic denervation, unlike atipamezole effect, indicating that RS 79948 releases DA from dopaminergic terminals as well. Similarly to the D₂ antagonist raclopride, but unlike atipamezole, RS 79948 increased extracellular DA and DOPAC in the caudate nucleus. Electrophysiological results indicate that RS 79948 shared with raclopride the ability to activate the firing of ventral tegmental area (VTA) DA neurons, while atipamezole was ineffective. RESULTS: from behavioral studies indicated that RS 79948 exerted effects mediated by independent, cooperative and contrasting inhibition of α₂-and D₂ receptors. Thus, RS 79948, but not atipamezole, prevented D₂-autoreceptor mediated hypomotility produced by a small dose of quinpirole. RS 79948 potentiated, more effectively than atipamezole, quinpirole-induced motor stimulation. RS 79948 antagonized, less effectively than atipamezole, raclopride-induced catalepsy. Future studies should clarify if the dual α₂-adrenoceptor- and D₂-receptor antagonistic action might endow RS 79948 with potential therapeutic relevance in the treatment of schizophrenia, drug dependence, depression and Parkinson's disease.

Effector membrane translocation biosensors reveal G protein and βarrestin coupling profiles of 100 therapeutically relevant GPCRs

The recognition that individual GPCRs can activate multiple signaling pathways has raised the possibility of developing drugs selectively targeting therapeutically relevant ones. This requires tools to determine which G proteins and βarrestins are activated by a given receptor. Here, we present a set of BRET sensors monitoring the activation of the 12 G protein subtypes based on the translocation of their effectors to the plasma membrane (EMTA). Unlike most of the existing detection systems, EMTA does not require modification of receptors or G proteins (except for G_s). EMTA was found to be suitable for the detection of constitutive activity, inverse agonism, biased signaling and polypharmacology. Profiling of 100 therapeutically relevant human GPCRs resulted in 1500 pathway-specific concentration-response curves and revealed a great diversity of coupling profiles ranging from exquisite selectivity to broad promiscuity. Overall, this work describes unique resources for studying the complexities underlying GPCR signaling and pharmacology.

Ligand-directed bias of G protein signaling at the dopamine D2 receptor

G-protein-coupled receptors (GPCRs) represent the largest family of drug targets. Upon activation, GPCRs signal primarily via a diverse set of heterotrimeric G proteins. Most GPCRs can couple to several different G protein subtypes. However, how drugs act at GPCRs contributing to the selectivity of G protein recognition is poorly understood. Here, we examined the G protein selectivity profile of the dopamine D₂ receptor (D₂), a GPCR targeted by antipsychotic drugs. We show that D₂ discriminates between six individual members of the G_i/o family, and its profile of functional selectivity is remarkably different across its ligands, which all engaged D₂ with a distinct G protein coupling pattern. Using structural modeling, receptor mutagenesis, and pharmacological evaluation, we identified residues in the D₂ binding pocket that shape these ligand-directed biases. We further provide pharmacogenomic evidence that natural variants in D₂ differentially affect its G protein biases in response to different ligands.

Functional and pharmacological role of the dopamine D4 receptor and its polymorphic variants

The functional and pharmacological significance of the dopamine D₄ receptor (D₄R) has remained the least well understood of all the dopamine receptor subtypes. Even more enigmatic has been the role of the very prevalent human DRD4 gene polymorphisms in the region that encodes the third intracellular loop of the receptor. The most common polymorphisms encode a D₄R with 4 or 7 repeats of a proline-rich sequence of 16 amino acids (D_4.4R and D_4.7R). DRD4 polymorphisms have been associated with individual differences linked to impulse control-related neuropsychiatric disorders, with the most consistent associations established between the gene encoding D_4.7R and attention-deficit hyperactivity disorder (ADHD) and substance use disorders. The function of D₄R and its polymorphic variants is being revealed by addressing the role of receptor heteromerization and the relatively avidity of norepinephrine for D₄R. We review the evidence conveying a significant and differential role of D_4.4R and D_4.7R in the dopaminergic and noradrenergic modulation of the frontal cortico-striatal pyramidal neuron, with implications for the moderation of constructs of impulsivity as personality traits. This differential role depends on their ability to confer different properties to adrenergic α_2A receptor (α_2AR)-D₄R heteromers and dopamine D₂ receptor (D₂R)-D₄R heteromers, preferentially localized in the perisomatic region of the frontal cortical pyramidal neuron and its striatal terminals, respectively. We also review the evidence to support the D₄R as a therapeutic target for ADHD and other impulse-control disorders, as well as for restless legs syndrome.

α-1 Adrenoceptor Activation in the Dorsal Raphe Nucleus Decreases Food Intake in Fasted Rats

The dorsal raphe (DR) nucleus is involved in a myriad of physiological functions, such as the control of sleep-wake cycle, motivation, pain, energy balance, and food intake. We have previously demonstrated that in ad libitum fed rats the intra-DR administration of phenylephrine, an α-1 receptor agonist, does not affect food intake, whereas clonidine, an α-2 receptor agonist, potently stimulates food intake. These results indicated that in fed rats an increased adrenergic tonus blocked food intake, since the activation of α-2 auto-receptors, which decreases pre-synaptic release of adrenaline/noradrenaline, affected food intake. Thus, in this study we assessed whether the response to adrenergic stimuli would differ after overnight fasting, a situation of low adrenergic activity in the DR. Intra-DR administration of adrenaline and noradrenaline blocked food intake evoked by overnight fasting. Similarly, phenylephrine administration decreased hunger-induced food intake. These changes in food intake were accompanied by changes in other behaviors, such as increased immobility time and feeding duration. On the other hand, intra-DR administration of clonidine did not affect food-intake or associated behaviors. These results further support the hypothesis that in fed animals, increased adrenergic tonus in DR neurons inhibiting feeding, while in fasted rats the adrenergic tonus decreases and favors food intake. These data indicate a possible mechanism through which adrenergic input to the DRN contributes to neurobiology of feeding.

Dopamine Receptor-Mediated Binding and Cellular Uptake of Polydopamine-Coated Nanoparticles

Polydopamine (PDA)-coated nanoparticles (NPs) are emerging carriers of therapeutic agents for nanomedicine applications due to their biocompatibility and abundant entry to various cell types, yet it remains unknown whether their cellular entry engages cell-surface receptors. As monomeric dopamine (DA) is an endogenous ligand of dopamine receptor and raw ingredient of PDA, we elucidate the interaction between polyethylene glycol-stabilized, PDA-coated gold NPs (Au@PDA@PEG NPs) and dopamine receptors, particularly D2 (D2DR). After proving the binding of Au@PDA@PEG NPs to recombinant and cellular D2DR, we employ antibody blocking, gene knockdown, and gene overexpression to establish the role of D2DR in the cellular uptake of Au@PDA@PEG NPs in vitro. By preparing a series of PEG-coated AuNPs that contain different structural analogues of DA (Au@PEG-X NPs), we demonstrate that catechol and amine groups collectively enhance the binding of NPs to D2DR and their cellular uptake. By intravenously injecting Au@PDA@PEG NPs to Balb/c mice, we reveal their in vivo binding to D2DR in the liver by competitive inhibition and immunohistochemistry together with their preferential association to D2DR-rich resident Kupffer cells by flow cytometry, a result consistent with the profuse expression of D2DR by resident Kupffer cells. Catechol and amine groups jointly contribute to the preferential association of NPs to D2DR-rich Kupffer cells. Our data highlight the importance of D2DR expression and DA-related functional groups in mediating the cell-nano interactions of PDA-based nanomedicines.

Visual Hallucinations and the Role of Medications in Parkinson's Disease: Triggers, Pathophysiology, and Management

Visual hallucinations, which are part of the syndrome of Parkinson's disease (PD) psychosis, affect patients' quality of life and increase the likelihood of residential aged-care placement. The association between visual hallucinations and dopaminergic and other medications that are necessary for the symptomatic management of motor and other symptoms of PD is a common clinical dilemma. While dopaminergic medications have long been associated with PD psychosis, a clear causal link has not been established, and other neurotransmitter systems, particularly noradrenaline, serotonin, and acetylcholine, are implicated and important. A diverse range of demographic and disease-related risk factors, some being modifiable, highlight the complexity of potential underlying pathophysiological processes but also broaden practical options for prevention and treatment that can be multifaceted and individualized. The investigators reviewed the clinical features and epidemiology of visual hallucinations and PD, explored the pathological evidence for dysfunction of multiple neurotransmitter systems that may be relevant to these phenomena, and addressed the potential of medications commonly used in PD to either trigger or treat these symptoms.

Heteromerization between α2A adrenoceptors and different polymorphic variants of the dopamine D4 receptor determines pharmacological and functional differences. Implications for impulsive-control disorders

Polymorphic alleles of the human dopamine D₄ receptor gene (DRD4) have been consistently associated with individual differences in personality traits and neuropsychiatric disorders, particularly between the gene encoding dopamine D_4.7 receptor variant and attention deficit hyperactivity disorder (ADHD). The α_2A adrenoceptor gene has also been associated with ADHD. In fact, drugs targeting the α_2A adrenoceptor (α_2AR), such as guanfacine, are commonly used in ADHD treatment. In view of the involvement of dopamine D₄ receptor (D₄R) and α_2AR in ADHD and impulsivity, their concurrent localization in cortical pyramidal neurons and the demonstrated ability of D₄R to form functional heteromers with other G protein-coupled receptors, in this study we evaluate whether the α_2AR forms functional heteromers with D₄R and weather these heteromers show different properties depending on the D₄R variant involved. Using cortical brain slices from hD_4.7R knock-in and wild-type mice, here, we demonstrate that α_2AR and D₄R heteromerize and constitute a significant functional population of cortical α_2AR and D₄R. Moreover, in cortical slices from wild-type mice and in cells transfected with α_2AR and D_4.4R, we detect a negative crosstalk within the heteromer. This negative crosstalk is lost in cortex from hD_4.7R knock-in mice and in cells expressing the D_4.7R polymorphic variant. We also show a lack of efficacy of D₄R ligands to promote G protein activation and signaling only within the α_2AR-D_4.7R heteromer. Taken together, our results suggest that α_2AR-D₄R heteromers play a pivotal role in catecholaminergic signaling in the brain cortex and are likely targets for ADHD pharmacotherapy.

Norepinephrine May Oppose Other Neuromodulators to Impact Alzheimer's Disease

While much of biomedical research since the middle of the twentieth century has focused on molecular pathways inside the cell, there is increasing evidence that extracellular signaling pathways are also critically important in health and disease. The neuromodulators norepinephrine (NE), serotonin (5-hydroxytryptamine, 5HT), dopamine (DA), acetylcholine (ACH), and melatonin (MT) are extracellular signaling molecules that are distributed throughout the brain and modulate many disease processes. The effects of these five neuromodulators on Alzheimer's disease (AD) are briefly examined in this paper, and it is hypothesized that each of the five molecules has a u-shaped (or Janus-faced) dose-response curve, wherein too little or too much signaling is pathological in AD and possibly other diseases. In particular it is suggested that NE is largely functionally opposed to 5HT, ACH, MT, and possibly DA in AD. In this scenario, physiological "balance" between the noradrenergic tone and that of the other three or four modulators is most healthy. If NE is largely functionally opposed to other prominent neuromodulators in AD, this may suggest novel combinations of pharmacological agents to counteract this disease. It is also suggested that the majority of cases of AD and possibly other diseases involve an excess of noradrenergic tone and a collective deficit of the other four modulators.

Ligands of Adrenergic Receptors: A Structural Point of View

Adrenergic receptors are G protein-coupled receptors for epinephrine and norepinephrine. They are targets of many drugs for various conditions, including treatment of hypertension, hypotension, and asthma. Adrenergic receptors are intensively studied in structural biology, displayed for binding poses of different types of ligands. Here, we summarized molecular mechanisms of ligand recognition and receptor activation exhibited by structure. We also reviewed recent advances in structure-based ligand discovery against adrenergic receptors.

Catecholaminergic Innervation of the Lateral Nucleus of the Cerebellum Modulates Cognitive Behaviors

The cerebellum processes neural signals related to rewarding and aversive stimuli, suggesting that the cerebellum supports nonmotor functions in cognitive and emotional domains. Catecholamines are a class of neuromodulatory neurotransmitters well known for encoding such salient stimuli. Catecholaminergic modulation of classical cerebellar functions have been demonstrated. However, a role for cerebellar catecholamines in modulating cerebellar nonmotor functions is unknown. Using biochemical methods in male mice, we comprehensively mapped TH+ fibers throughout the entire cerebellum and known precerebellar nuclei. Using electrochemical (fast scan cyclic voltammetry), and viral/genetic methods to selectively delete Th in fibers innervating the lateral cerebellar nucleus (LCN), we interrogated sources and functional roles of catecholamines innervating the LCN, which is known for its role in supporting cognition. The LCN has the most TH+ fibers in cerebellum, as well as the most change in rostrocaudal expression among the cerebellar nuclei. Norepinephrine is the major catecholamine measured in LCN. Distinct catecholaminergic projections to LCN arise only from locus coeruleus, and a subset of Purkinje cells that are positive for staining of TH. LC stimulation was sufficient to produce catecholamine release in LCN. Deletion of Th in fibers innervating LCN (LCN-Th-cKO) resulted in impaired sensorimotor integration, associative fear learning, response inhibition, and working memory in LCN-Th-cKO mice. Strikingly, selective inhibition of excitatory LCN output neurons with inhibitory designer receptor exclusively activated by designer drugs led to facilitation of learning on the same working memory task impaired in LCN-Th-cKO mice. Collectively, these data demonstrate a role for LCN catecholamines in cognitive behaviors.SIGNIFICANCE STATEMENT Here, we report on interrogating sources and functional roles of catecholamines innervating the lateral nucleus of the cerebellum (LCN). We map and quantify expression of TH, the rate-limiting enzyme in catecholamine synthesis, in the entire cerebellar system, including several precerebellar nuclei. We used cyclic voltammetry and pharmacology to demonstrate sufficiency of LC stimulation to produce catecholamine release in LCN. We used advanced viral techniques to map and selectively KO catecholaminergic neurotransmission to the LCN, and characterized significant cognitive deficits related to this manipulation. Finally, we show that inhibition of excitatory LCN neurons with designer receptor exclusively activated by designer drugs, designed to mimic Gi-coupled catecholamine GPCR signaling, results in facilitation of a working memory task impaired in LCN-specific TH KO mice.

Akathisia and Restless Legs Syndrome: Solving the Dopaminergic Paradox

Akathisia is an urgent need to move that is associated with treatment with dopamine receptor blocking agents (DRBAs) and with restless legs syndrome (RLS). The pathogenetic mechanism of akathisia has not been resolved. This article proposes that it involves an increased presynaptic dopaminergic transmission in the ventral striatum and concomitant strong activation of postsynaptic dopamine D₁ receptors, which form complexes (heteromers) with dopamine D₃ and adenosine A₁ receptors. It also proposes that in DRBA-induced akathisia, increased dopamine release depends on inactivation of autoreceptors, whereas in RLS it depends on a brain iron deficiency-induced down-regulation of striatal presynaptic A₁ receptors.

Noradrenergic Signaling Disengages Feedforward Transmission in the Nucleus Accumbens Shell

The nucleus accumbens shell (NAcSh) receives extensive monoaminergic input from multiple midbrain structures. However, little is known how norepinephrine (NE) modulates NAc circuit dynamics. Using a dynamic electrophysiological approach with optogenetics, pharmacology, and drugs acutely restricted by tethering (DART), we explored microcircuit-specific neuromodulatory mechanisms recruited by NE signaling in the NAcSh of parvalbumin (PV)-specific reporter mice. Surprisingly, NE had little direct effect on modulation of synaptic input at medium spiny projection neurons (MSNs). In contrast, we report that NE transmission selectively modulates glutamatergic synapses onto PV-expressing fast-spiking interneurons (PV-INs) by recruiting postsynaptically-localized α₂-adrenergic receptors (ARs). The synaptic effects of α₂-AR activity decrease PV-IN-dependent feedforward inhibition onto MSNs evoked via optogenetic stimulation of cortical afferents to the NAcSh. These findings provide insight into a new circuit motif in which NE has a privileged line of communication to tune feedforward inhibition in the NAcSh.SIGNIFICANCE STATEMENT The nucleus accumbens (NAc) directs reward-related motivational output by integrating glutamatergic input with diverse neuromodulatory input from monoamine centers. The present study reveals a synapse-specific regulatory mechanism recruited by norepinephrine (NE) signaling within parvalbumin-expressing interneuron (PV-IN) feedforward inhibitory microcircuits. PV-IN-mediated feedforward inhibition in the NAc is instrumental in coordinating NAc output by synchronizing the activity of medium spiny projection neurons (MSNs). By negatively regulating glutamatergic transmission onto PV-INs via α₂-adrenergic receptors (ARs), NE diminishes feedforward inhibition onto MSNs to promote NAc output. These findings elucidate previously unknown microcircuit mechanisms recruited by the historically overlooked NE system in the NAc.

Structural insights into the human D1 and D2 dopamine receptor signaling complexes

The D1- and D2-dopamine receptors (D1R and D2R), which signal through Gs and Gi, respectively, represent the principal stimulatory and inhibitory dopamine receptors in the central nervous system. D1R and D2R also represent the main therapeutic targets for Parkinson's disease, schizophrenia, and many other neuropsychiatric disorders, and insight into their signaling is essential for understanding both therapeutic and side effects of dopaminergic drugs. Here, we report four cryoelectron microscopy (cryo-EM) structures of D1R-Gs and D2R-Gi signaling complexes with selective and non-selective dopamine agonists, including two currently used anti-Parkinson's disease drugs, apomorphine and bromocriptine. These structures, together with mutagenesis studies, reveal the conserved binding mode of dopamine agonists, the unique pocket topology underlying ligand selectivity, the conformational changes in receptor activation, and potential structural determinants for G protein-coupling selectivity. These results provide both a molecular understanding of dopamine signaling and multiple structural templates for drug design targeting the dopaminergic system.

Dopamine regulates pancreatic glucagon and insulin secretion via adrenergic and dopaminergic receptors

Dopamine (DA) and norepinephrine (NE) are catecholamines primarily studied in the central nervous system that also act in the pancreas as peripheral regulators of metabolism. Pancreatic catecholamine signaling has also been increasingly implicated as a mechanism responsible for the metabolic disturbances produced by antipsychotic drugs (APDs). Critically, however, the mechanisms by which catecholamines modulate pancreatic hormone release are not completely understood. We show that human and mouse pancreatic α- and β-cells express the catecholamine biosynthetic and signaling machinery, and that α-cells synthesize DA de novo. This locally-produced pancreatic DA signals via both α- and β-cell adrenergic and dopaminergic receptors with different affinities to regulate glucagon and insulin release. Significantly, we show DA functions as a biased agonist at α_2A-adrenergic receptors, preferentially signaling via the canonical G protein-mediated pathway. Our findings highlight the interplay between DA and NE signaling as a novel form of regulation to modulate pancreatic hormone release. Lastly, pharmacological blockade of DA D₂-like receptors in human islets with APDs significantly raises insulin and glucagon release. This offers a new mechanism where APDs act directly on islet α- and β-cell targets to produce metabolic disturbances.

Droxidopa alters dopamine neuron and prefrontal cortex activity and improves attention-deficit/hyperactivity disorder-like behaviors in rats

Finding alternative treatments for attention-deficit/hyperactivity disorder (ADHD) is crucial given the safety and efficacy problems of current ADHD medications. Droxidopa, also known as L-threo-dihydroxyphenylserine (L-DOPS), is a norepinephrine prodrug that enhances brain norepinephrine and dopamine levels. In this study, we used electrophysiological tests to examine effects of L-DOPS on the prefrontal cortex (PFC) and dopamine neurons in the ventral tegmental area. We also conducted behavioral tests to assess L-DOPS' effects on ADHD-like behaviors in rats. In chloral hydrate-anesthetized rats, PFC local field potentials oscillated between the active, depolarized UP state and the hyperpolarized DOWN state. Mimicking the effect of d-amphetamine, L-DOPS, given after the peripheral amino acid decarboxylase inhibitor, benserazide (BZ), increased the amount of time the PFC spent in the UP state, indicating an excitatory effect of L-DOPS on PFC neurons. Like d-amphetamine, L-DOPS also inhibited dopamine neurons, an effect significantly reversed by the D2-like receptor antagonist raclopride. In the behavioral tests, BZ + L-DOPS improved hyperactivity, inattention and impulsive action of the adolescent spontaneously hypertensive rat (SHR/NCrl), well-validated animal model of the combined type of ADHD. BZ + L-DOPS also reduced impulsive choice and impulsive action of Wistar rats, but did not ameliorate the inattentiveness of Wistar Kyoto rats (WKY/NCrl), proposed model of the ADHD-predominantly inattentive type. In conclusion, L-DOPS produced effects on the PFC and dopamine neurons characteristic of drugs used to treat ADHD. BZ + L-DOPS ameliorated ADHD-like behaviors in rats suggesting its potential as an alternative ADHD treatment.

Delayed retinal vein recovery responses indicate both non-adaptation to stress as well as increased risk for stroke: the SABPA study

OBJECTIVES

Low or high sympatho-adrenal-medullary axis (SAM) and hypothalamic-pituitary-adrenal axis (HPA) dysregulation reflect chronic stress. Retinal vessel dynamics may relate to SAM, HPA activity and stroke risk. Our objectives were therefore to assess the relationships between retinal vessel, SAM and HPA responses, and to determine stroke risk.

METHODS

A prospective bi-ethnic gender cohort (n = 275, 45 ± 9 years) was included. Urine/serum/saliva samples for SAM [norepinephrine:creatinine ratio (u-NE)] and HPA [adrenocorticotrophic hormone (ACTH), cortisol] were obtained at baseline, three-year follow up and upon flicker light-induced provocation. Diastolic ocular perfusion pressure was measured as a marker of hypo-perfusion. Retinal arterial narrowing and venous widening calibres were quantified from digital images in the mydriatic eye. A validated stress and stroke risk score was applied.

RESULTS

An interaction term was fitted for venous dilation in u-NE tertiles (p ≤ 0.05) and not in u-NE median/quartiles/quintiles. Independent of race or gender, tertile 1 (low u-NE) had a 112% increase in u-NE, decreases in cortisol, and no changes in ACTH over three years (positive feedback). Tertile 3 (high u-NE) contradictorily had decreases in u-NE and cortisol, and increases in ACTH (negative feedback). In tertile 1, reduced arterial dilation, and faster arterial vasoconstriction and narrowing were related to higher SAM activity and hypo-perfusion (p ≤ 0.05), whereas delayed venous dilation, recovery and widening were related to cortisol hypo-secretion (p ≤ 0.05). In tertile 1, delayed venous recovery responses predicted stress and stroke risk [odds ratio 4.8 (1.2-19.6); p = 0.03]. These associations were not found in u-NE tertiles 2 and 3.

CONCLUSIONS

In response to low norepinephrine, a reflex increase in SAM activity occurred, enhancing arterial vasoconstriction and hypo-perfusion. Concomitant HPA dysregulation attenuated retinal vein vasoactivity and tone, reflecting delayed vein recovery responses and non-adaptation to stress. These constrained vein recovery responses are indicative of increased chronic stress and stroke risk.

Recent findings leading to the discovery of selective dopamine D4 receptor ligands for the treatment of widespread diseases

Since its discovery, the dopamine D₄ receptor (D₄R) has been suggested to be an attractive target for the treatment of neuropsychiatric diseases. Novel findings have renewed the interest in such a receptor as an emerging target for the management of different diseases, including cancer, Parkinson's disease, alcohol or substance use disorders, eating disorders, erectile dysfunction and cognitive deficits. The recently resolved crystal structures of D₄R in complexes with the potent ligands nemonapride and L-745870 strongly improved the knowledge on the molecular mechanisms involving the D₄R functions and may help medicinal chemists in drug design. This review is focused on the recent development of the subtype selective D₄R ligands belonging to classical or new chemotypes. Moreover, ligands showing functional selectivity toward G protein activation or β-arrestin recruitment and the effects of selective D₄R ligands on the above-mentioned diseases are discussed.

Treating hallucinations in Parkinson's disease

Introduction: Hallucinations in Parkinson's disease are common, can complicate medication management and significantly impact upon the quality of life of patients and their carers.Areas covered: This review aims to examine current evidence for the management of hallucinations in Parkinson's disease.Expert opinion: Treatment of hallucinations in Parkinson's disease should be both individualized and multifaceted. Screening, education, medication review and the avoidance of common triggers are important. For well-formed visual hallucinations, acetylcholinesterase inhibitors are recommended first-line. Refractory or severe symptoms may require the cautious use of atypical antipsychotics. Antidepressants may be beneficial in the appropriate setting. Unfortunately, current therapies for hallucinations offer only limited benefits and future research efforts are desperately required to improve the management of these challenging symptoms.

Dopamine receptor D1- and D2-agonists do not spark brown adipose tissue thermogenesis in mice

Brown adipose tissue (BAT) thermogenesis is considered a potential target for treatment of obesity and diabetes. In vitro data suggest dopamine receptor signaling as a promising approach; however, the biological relevance of dopamine receptors in the direct activation of BAT thermogenesis in vivo remains unclear. We investigated BAT thermogenesis in vivo in mice using peripheral administration of D1-agonist SKF38393 or D2-agonist Sumanirole, infrared thermography, and in-depth molecular analyses of potential target tissues; and ex vivo in BAT explants to identify direct effects on key thermogenic markers. Acute in vivo treatment with the D1- or D2-agonist caused a short spike or brief decrease in BAT temperature, respectively. However, repeated daily administration did not induce lasting effects on BAT thermogenesis. Likewise, neither agonist directly affected Ucp1 or Dio2 mRNA expression in BAT explants. Taken together, the investigated agonists do not seem to exert lasting and physiologically relevant effects on BAT thermogenesis after peripheral administration, demonstrating that D1- and D2-receptors in iBAT are unlikely to constitute targets for obesity treatment via BAT activation.

The role of norepinephrine in the pathophysiology of schizophrenia

Several lines of evidence have suggested for decades a role for norepinephrine (NE) in the pathophysiology and treatment of schizophrenia. Recent experimental findings reveal anatomical and physiological properties of the locus coeruleus-norepinephrine (LC-NE) system and its involvement in brain function and cognition. Here, we integrate these two lines of evidence. First, we review the functional and structural properties of the LC-NE system and its impact on functional brain networks, cognition, and stress, with special emphasis on recent experimental and theoretical advances. Subsequently, we present an update about the role of LC-associated functions for the pathophysiology of schizophrenia, focusing on the cognitive and motivational deficits. We propose that schizophrenia phenomenology, in particular cognitive symptoms, may be explained by an abnormal interaction between genetic susceptibility and stress-initiated LC-NE dysfunction. This in turn, leads to imbalance between LC activity modes, dysfunctional regulation of brain network integration and neural gain, and deficits in cognitive functions. Finally, we suggest how recent development of experimental approaches can be used to characterize LC function in schizophrenia.

Discovery, Optimization, and Characterization of ML417: A Novel and Highly Selective D3 Dopamine Receptor Agonist

To identify novel D₃ dopamine receptor (D3R) agonists, we conducted a high-throughput screen using a β-arrestin recruitment assay. Counterscreening of the hit compounds provided an assessment of their selectivity, efficacy, and potency. The most promising scaffold was optimized through medicinal chemistry resulting in enhanced potency and selectivity. The optimized compound, ML417 (20), potently promotes D3R-mediated β-arrestin translocation, G protein activation, and ERK1/2 phosphorylation (pERK) while lacking activity at other dopamine receptors. Screening of ML417 against multiple G protein-coupled receptors revealed exceptional global selectivity. Molecular modeling suggests that ML417 interacts with the D3R in a unique manner, possibly explaining its remarkable selectivity. ML417 was also found to protect against neurodegeneration of dopaminergic neurons derived from iPSCs. Together with promising pharmacokinetics and toxicology profiles, these results suggest that ML417 is a novel and uniquely selective D3R agonist that may serve as both a research tool and a therapeutic lead for the treatment of neuropsychiatric disorders.

Dopamine D3 receptor: A neglected participant in Parkinson Disease pathogenesis and treatment?

Parkinson disease (PD) is a neurodegenerative disorder characterized by motor and non-motor symptoms which relentlessly and progressively lead to substantial disability and economic burden. Pathologically, these symptoms follow the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) associated with abnormal α-synuclein (α-Syn) deposition as cytoplasmic inclusions called Lewy bodies in pigmented brainstem nuclei, and in dystrophic neurons in striatal and cortical regions (Lewy neurites). Pharmacotherapy for PD focuses on improving quality of life and primarily targets dopaminergic pathways. Dopamine acts through two families of receptors, dopamine D1-like and dopamine D2-like; dopamine D3 receptors (D3R) belong to dopamine D2 receptor (D2R) family. Although D3R's precise role in the pathophysiology and treatment of PD has not been determined, we present evidence suggesting an important role for D3R in the early development and occurrence of PD. Agonist activation of D3R increases dopamine concentration, decreases α-Syn accumulation, enhances secretion of brain derived neurotrophic factors (BDNF), ameliorates neuroinflammation, alleviates oxidative stress, promotes neurogenesis in the nigrostriatal pathway, interacts with D1R to reduce PD associated motor symptoms and ameliorates side effects of levodopa (L-DOPA) treatment. Furthermore, D3R mutations can predict PD age of onset and prognosis of PD treatment. The role of D3R in PD merits further research. This review elucidates the potential role of D3R in PD pathogenesis and therapy.

HCN channel-mediated neuromodulation can control action potential velocity and fidelity in central axons

Hyperpolarization-activated cyclic-nucleotide-gated (HCN) channels control electrical rhythmicity and excitability in the heart and brain, but the function of HCN channels at the subcellular level in axons remains poorly understood. Here, we show that the action potential conduction velocity in both myelinated and unmyelinated central axons can be bidirectionally modulated by a HCN channel blocker, cyclic adenosine monophosphate (cAMP), and neuromodulators. Recordings from mouse cerebellar mossy fiber boutons show that HCN channels ensure reliable high-frequency firing and are strongly modulated by cAMP (EC₅₀ 40 µM; estimated endogenous cAMP concentration 13 µM). In addition, immunogold-electron microscopy revealed HCN2 as the dominating subunit in cerebellar mossy fibers. Computational modeling indicated that HCN2 channels control conduction velocity primarily by altering the resting membrane potential and are associated with significant metabolic costs. These results suggest that the cAMP-HCN pathway provides neuromodulators with an opportunity to finely tune energy consumption and temporal delays across axons in the brain.

Luciferase complementation based-detection of G-protein-coupled receptor activity

Protein complementation assays (PCA) are used as pharmacological tools, enabling a wide array of applications, ranging from studies of protein-protein interactions to second messenger effects. Methods to detect activities of G protein-coupled receptors (GPCRs) have particular relevance for drug screening. Recent development of an engineered luciferase NanoLuc created the possibility of generating a novel PCA, which in turn could open a new avenue for developing drug screening assays. Here we identified a novel split position for NanoLuc and demonstrated its use in a series of fusion constructs to detect the activity of GPCRs. The split construct can be applied to a variety of pharmacological screening systems.

The Significance of Chirality in Drug Design and Synthesis of Bitopic Ligands as D3 Receptor (D3R) Selective Agonists

Because of the large degree of homology among dopamine D₂-like receptors, discovering ligands capable of discriminating between the D₂, D₃, and D₄ receptor subtypes remains a significant challenge. Previous work has exemplified the use of bitopic ligands as a powerful strategy in achieving subtype selectivity for agonists and antagonists alike. Inspired by the potential for chemical modification of the D₃ preferential agonists (+)-PD128,907 (1) and PF592,379 (2), we synthesized bitopic structures to further improve their D₃R selectivity. We found that the (2S,5S) conformation of scaffold 2 resulted in a privileged architecture with increased affinity and selectivity for the D₃R. In addition, a cyclopropyl moiety incorporated into the linker and full resolution of the chiral centers resulted in lead compound 53 and eutomer 53a that demonstrate significantly higher D₃R binding selectivities than the reference compounds. Moreover, the favorable metabolic stability in rat liver microsomes supports future studies in in vivo models of dopamine system dysregulation.

Differential regulation of phasic dopamine release in the forebrain by the VTA noradrenergic receptor signaling

Phasic dopamine (DA) release from the ventral tegmental area (VTA) into forebrain structures is implicated in associative learning and conditional stimulus (CS)-evoked behavioral responses. Mounting evidence points to noradrenaline signaling in the VTA as an important regulatory input. Accordingly, adrenergic receptor (AR) blockade in the VTA has been shown to modulate CS-dependent behaviors. Here, we hypothesized that α₁ - and α₂ -AR (but not β-AR) activity preferentially modulates phasic, in contrast to tonic, DA release. In addition, these effects could differ between forebrain targets. We used fast-scan cyclic voltammetric measurements in rats to assess the effects of intra-VTA microinfusion of terazosin, a selective α₁ -AR antagonist, on electrically evoked phasic DA release in the nucleus accumbens (NAc) core and medial prefrontal cortex (mPFC). Terazosin dose-dependently attenuated phasic, but not tonic, DA release in the NAc core, but not in the mPFC. Next, we measured the effects of intra-VTA administration of the α₂ -AR selective antagonist RX-821002 on evoked DA in the NAc core. Similar to the effects of α₁ -AR blockade, intra-VTA α₂ -AR blockade with RX-0821002 strongly and dose-dependently attenuated phasic, but not tonic, DA release. In contrast, no regulation by RX-821002 was observed in the mPFC. This effect was sensitive to intra-VTA blockade of D2 receptors with raclopride. Finally, the β-AR antagonist propranolol ineffectively modulated DA release in the NAc core. These findings revealed both α₁ - and α₂ -ARs in the VTA as selective regulators of phasic DA release. Importantly, we demonstrated that AR blockade modulated mesolimbic, in contrast to mesocortical, DA release in previously unstudied heterogeneity in AR regulation of forebrain phasic DA.

Dopamine D4 Receptor-Selective Compounds Reveal Structure-Activity Relationships that Engender Agonist Efficacy

The dopamine D4 receptor (D4R) plays important roles in cognition, attention, and decision making. Novel D4R-selective ligands have promise in medication development for neuropsychiatric conditions, including Alzheimer's disease and substance use disorders. To identify new D4R-selective ligands, and to understand the molecular determinants of agonist efficacy at D4R, we report a series of eighteen novel ligands based on the classical D4R agonist A-412997 (1, 2-(4-(pyridin-2-yl)piperidin-1-yl)- N-( m-tolyl)acetamide). Compounds were profiled using radioligand binding displacement assays, β-arrestin recruitment assays, cyclic AMP inhibition assays, and molecular dynamics computational modeling. We identified several novel D4R-selective ( Ki ≤ 4.3 nM and >100-fold vs other D2-like receptors) compounds with diverse partial agonist and antagonist profiles, falling into three structural groups. These compounds highlight receptor-ligand interactions that control efficacy at D2-like receptors and may provide insights into targeted drug discovery, leading to a better understanding of the role of D4Rs in neuropsychiatric disorders.

Deciphering biased inverse agonism of cangrelor and ticagrelor at P2Y12 receptor

P2Y12 receptor (P2Y12-R) is one of the major targets for drug inhibiting platelet aggregation in the treatment/prevention of arterial thrombosis. However, the clinical use of P2Y12-R antagonists faces some limitations, such as a delayed onset of action (clopidogrel) or adverse effect profile (ticagrelor, cangrelor), justifying the development of a new generation of P2Y12-R antagonists with a better clinical benefit-risk balance. Although the recent concept of biased agonism offers the possibility to alleviate undesirable adverse effects while preserving therapeutic outcomes, it has never been explored at P2Y12-R. For the first time, using highly sensitive BRET2-based probes, we accurately delineated biased ligand efficacy at P2Y12-R in living HEK293T cells on G protein activation and downstream effectors. We demonstrated that P2Y12-R displayed constitutive Gi/o-dependent signaling that is impaired by the R122C mutation, previously associated with a bleeding disorder. More importantly, we reported the biased inverse agonist efficacy of cangrelor and ticagrelor that could underlie their clinical efficacy. Our study points out that constitutive P2Y12-R signaling is a normal feature of the receptor that might be essential for platelets to respond faster to a vessel injury. From a therapeutic standpoint, our data suggest that the beneficial advantages of antiplatelet drugs might be more related to inverse agonism at P2Y12-R than to antagonism of ADP-mediated signaling. In the future, deciphering P2Y12-R constitutive activity should allow the discovery of more selective biased P2Y12-R blockers demonstrating therapeutic advantages over classical antiplatelet drugs by improving therapeutic outcomes and concomitantly relieving undesirable adverse effects.

Novel and Potent Dopamine D2 Receptor Go-Protein Biased Agonists

The discovery of functionally biased and physiologically beneficial ligands directed toward G-protein coupled receptors (GPCRs) has provided the impetus to design dopamine D₂ receptor (D₂R) targeted molecules that may be therapeutically advantageous for the treatment of certain neuropsychiatric or basal ganglia related disorders. Here we describe the synthesis of a novel series of D₂R agonists linking the D₂R unbiased agonist sumanirole with privileged secondary molecular fragments. The resulting ligands demonstrate improved D₂R affinity and selectivity over sumanirole. Extensive in vitro functional studies and bias factor analysis led to the identification of a novel class of highly potent Go-protein biased full D₂R agonists with more than 10-fold and 1000-fold bias selectivity toward activation of specific G-protein subtypes and β-arrestin, respectively. Intracellular electrophysiological recordings from midbrain dopamine neurons demonstrated that Go-protein selective agonists can elicit prolonged ligand-induced GIRK activity via D₂Rs, which may be beneficial in the treatment of dyskinesias associated with dopamine system dysfunction.

Temperoammonic Stimulation Depotentiates Schaffer Collateral LTP via p38 MAPK Downstream of Adenosine A1 Receptors

We previously found that low-frequency stimulation of direct temperoammonic (TA) inputs to hippocampal area CA1 depotentiates previously established long-term potentiation in the Schaffer collateral (SC) pathway through complex signaling involving dopamine, endocannabinoids, neuregulin-1, GABA, and adenosine, with adenosine being the most distal modulator identified to date. In the present studies, we examined mechanisms contributing to the effects of adenosine in hippocampal slices from male albino rats. We found that extracellular conversion of ATP to adenosine via an ectonucleotidase contributes significantly to TA-mediated SC depotentiation and the depotentiation resulting from block of adenosine transport. Adenosine-mediated SC depotentiation does not involve activation of c-Jun N-terminal protein kinase, serine phosphatases, or nitric oxide synthase, unlike homosynaptic SC depotentiation. Rather, adenosine-induced depotentiation is inhibited by specific antagonists of p38 MAPK, but not by a structural analog that does not inhibit p38. Additionally, using antagonists with relative selectivity for p38 subtypes, it appears that TA-induced SC depotentiation most likely involves p38 MAPK β. These findings have implications for understanding the role of adenosine and other extrahippocampal and intrahippocampal modulators in regulating SC synaptic function and the contributions of these modulators to the cognitive dysfunction associated with neuropsychiatric illnesses.SIGNIFICANCE STATEMENT Low-frequency stimulation of temperoammonic (TA) inputs to stratum lacunosum moleculare of hippocampal area CA1 heterosynaptically depotentiates long-term potentiation of Schaffer collateral (SC) synapses. TA-induced SC depotentiation involves complex signaling including dopamine, endocannabinoids, GABA, and adenosine, with adenosine serving as the most downstream messenger in the cascade identified to date. The present results indicate that TA-induced depotentiation requires intact inputs from entorhinal cortex and that adenosine ultimately drives depotentiation via activation of p38 MAPK. These studies have implications for understanding the cognitive dysfunction of psychiatric illnesses and certain abused drugs.

Revisiting the Functional Role of Dopamine D4 Receptor Gene Polymorphisms: Heteromerization-Dependent Gain of Function of the D4.7 Receptor Variant

The two most common polymorphisms of the human DRD4 gene encode a dopamine D₄ receptor (D4R) with four or seven repeats of a proline-rich sequence of 16 amino acids (D4.4R or D4.7R). Although the seven-repeat polymorphism has been repeatedly associated with attention-deficit hyperactivity disorder and substance use disorders, the differential functional properties between D4.4R and D4.7R remained enigmatic until recent electrophysiological and optogenetic-microdialysis experiments indicated a gain of function of D4.7R. Since no clear differences in the biochemical properties of individual D4.4R and D4.7R have been reported, it was previously suggested that those differences emerge upon heteromerization with dopamine D₂ receptor (D2R), which co-localizes with D4R in the brain. However, contrary to a gain of function, experiments in mammalian transfected cells suggested that heteromerization with D2R results in lower MAPK signaling by D4.7R as compared to D4.4R. In the present study, we readdressed the question of functional differences of D4.4R and D4.7R forming homomers or heteromers with the short isoform of D2R (D2SR), using a functional bioluminescence resonance energy transfer (BRET) assay that allows the measurement of ligand-induced changes in the interaction between G protein-coupled receptors (GPCRs) forming homomers or heteromers with their cognate G protein. Significant functional and pharmacological differences between D4.4R and D4.7R were only evident upon heteromerization with the short isoform of D2R (D2SR). The most dramatic finding was a significant increase and decrease in the constitutive activity of D2S upon heteromerization with D4.7R and D4.4R, respectively, providing the first clear mechanism for a functional difference between both products of polymorphic variants and for a gain of function of the D4.7R.

α2A- and α2C-Adrenoceptors as Potential Targets for Dopamine and Dopamine Receptor Ligands

The poor norepinephrine innervation and high density of Gi/o-coupled α_2A- and α_2C-adrenoceptors in the striatum and the dense striatal dopamine innervation have prompted the possibility that dopamine could be an effective adrenoceptor ligand. Nevertheless, the reported adrenoceptor agonistic properties of dopamine are still inconclusive. In this study, we analyzed the binding of norepinephrine, dopamine, and several compounds reported as selective dopamine D₂-like receptor ligands, such as the D₃ receptor agonist 7-OH-PIPAT and the D₄ receptor agonist RO-105824, to α₂-adrenoceptors in cortical and striatal tissue, which express α_2A-adrenoceptors and both α_2A- and α_2C-adrenoceptors, respectively. The affinity of dopamine for α₂-adrenoceptors was found to be similar to that for D₁-like and D₂-like receptors. Moreover, the exogenous dopamine receptor ligands also showed high affinity for α_2A- and α_2C-adrenoceptors. Their ability to activate Gi/o proteins through α_2A- and α_2C-adrenoceptors was also analyzed in transfected cells with bioluminescent resonance energy transfer techniques. The relative ligand potencies and efficacies were dependent on the Gi/o protein subtype. Furthermore, dopamine binding to α₂-adrenoceptors was functional, inducing changes in dynamic mass redistribution, adenylyl cyclase activity, and ERK1/2 phosphorylation. Binding events were further studied with computer modeling of ligand docking. Docking of dopamine at α_2A- and α_2C-adrenoceptors was nearly identical to its binding to the crystallized D₃ receptor. Therefore, we provide conclusive evidence that α_2A- and α_2C-adrenoceptors are functional receptors for norepinephrine, dopamine, and other previously assumed selective D₂-like receptor ligands, which calls for revisiting previous studies with those ligands.

[18F]fallypride characterization of striatal and extrastriatal D2/3 receptors in Parkinson's disease

Parkinson's disease (PD) is characterized by widespread degeneration of monoaminergic (especially dopaminergic) networks, manifesting with a number of both motor and non-motor symptoms. Regional alterations to dopamine D2/3 receptors in PD patients are documented in striatal and some extrastriatal areas, and medications that target D2/3 receptors can improve motor and non-motor symptoms. However, data regarding the combined pattern of D2/3 receptor binding in both striatal and extrastriatal regions in PD are limited. We studied 35 PD patients off-medication and 31 age- and sex-matched healthy controls (HCs) using PET imaging with [18F]fallypride, a high affinity D2/3 receptor ligand, to measure striatal and extrastriatal D2/3 nondisplaceable binding potential (BPND). PD patients completed PET imaging in the off medication state, and motor severity was concurrently assessed. Voxel-wise evaluation between groups revealed significant BPND reductions in PD patients in striatal and several extrastriatal regions, including the locus coeruleus and mesotemporal cortex. A region-of-interest (ROI) based approach quantified differences in dopamine D2/3 receptors, where reduced BPND was noted in the globus pallidus, caudate, amygdala, hippocampus, ventral midbrain, and thalamus of PD patients relative to HC subjects. Motor severity positively correlated with D2/3 receptor density in the putamen and globus pallidus. These findings support the hypothesis that abnormal D2/3 expression occurs in regions related to both the motor and non-motor symptoms of PD, including areas richly invested with noradrenergic neurons.

Gs- versus Golf-dependent functional selectivity mediated by the dopamine D1 receptor

The two highly homologous subtypes of stimulatory G proteins Gαs (Gs) and Gαolf (Golf) display contrasting expression patterns in the brain. Golf is predominant in the striatum, while Gs is predominant in the cortex. Yet, little is known about their functional distinctions. The dopamine D₁ receptor (D1R) couples to Gs/olf and is highly expressed in cortical and striatal areas, making it an important therapeutic target for neuropsychiatric disorders. Using novel drug screening methods that allow analysis of specific G-protein subtype coupling, we found that, relative to dopamine, dihydrexidine and N-propyl-apomorphine behave as full D1R agonists when coupled to Gs, but as partial D1R agonists when coupled to Golf. The Gs/Golf-dependent biased agonism by dihydrexidine was consistently observed at the levels of cellular signaling, neuronal function, and behavior. Our findings of Gs/Golf-dependent functional selectivity in D1R ligands open a new avenue for the treatment of cortex-specific or striatum-specific neuropsychiatric dysfunction.

D1-like dopamine receptors are coupled to Golf proteins in the dorsal striatum but Gs in cortical and other areas. Here, the authors demonstrate selective agonism of Gs-coupled versus Golf-coupled D1 receptors.

Bioluminescence Resonance Energy Transfer Assay to Characterize Gi-Like G Protein Subtype-Dependent Functional Selectivity

G protein-coupled receptors (GPCRs) comprise the single most targeted protein class in pharmacology. G protein signaling transduces extracellular stimuli such as neurotransmitters into cellular responses. Although preference for a specific GPCR among different G protein families (e.g., Gs-, Gi-, or Gq-like proteins) is often well studied, preference for a specific G protein subtype (e.g., Gi1, Gi2, Gi3, Go1, and Go2) has received little attention. Due to tissue expression differences and potentially exploitable functional differences, G protein subtype-dependent functional selectivity is an attractive framework to expand GPCR drug development. Herein we present a bioluminescence resonance energy transfer (BRET)-based method to characterize functional selectivity among Gi-like protein subtypes. © 2017 by John Wiley & Sons, Inc.