Dopamine-induced functional activation of Gαq mediated by dopamine D1-like receptor in rat cerebral cortical membranes

Activation of Multiple G Protein Pathways to Characterize the Five Dopamine Receptor Subtypes Using Bioluminescence Technology

G protein-coupled receptors show preference for G protein subtypes but can recruit multiple G proteins with various downstream signaling cascades. This functional selection can guide drug design. Dopamine receptors are both stimulatory (D1-like) and inhibitory (D2-like) with diffuse expression across the central nervous system. Functional selectivity of G protein subunits may help with dopamine receptor targeting and their downstream effects. Three bioluminescence-based assays were used to characterize G protein coupling and function with the five dopamine receptors. Most proximal to ligand binding was the miniG protein assay with split luciferase technology used to measure recruitment. For endogenous and selective ligands, the G-CASE bioluminescence resonance energy transfer (BRET) assay measured G protein activation and receptor selectivity. Downstream, the BRET-based CAMYEN assay quantified cyclic adenosine monophosphate (cAMP) changes. Several dopamine receptor agonists and antagonists were characterized for their G protein recruitment and cAMP effects. G protein selectivity with dopamine revealed potential Gq coupling at all five receptors, as well as the ability to activate subtypes with the "opposite" effects to canonical signaling. D1-like receptor agonist (+)-SKF-81297 and D2-like receptor agonist pramipexole showed selectivity at all receptors toward Gs or Gi/o/z activation, respectively. The five dopamine receptors show a wide range of potentials for G protein coupling and activation, reflected in their downstream cAMP signaling. Targeting these interactions can be achieved through drug design. This opens the door to pharmacological treatment with more selectivity options for inducing the correct physiological events.

Baihui (DU20), Shenmen (HT7) and Sanyinjiao (SP6) target the cAMP/CREB/BDNF and PI3K/Akt pathways to reduce central nervous system apoptosis in rats with insomnia

Insomnia can cause damage to function and other medical and mental illnesses, and it is also a risk factor for increasing medical care costs. Although simple behavior intervention is feasible in primary medical institutions, the lack of corresponding technical training has obviously restricted its use, patients' autonomy dependence is generally poor, and early missions have some difficulties. Relatively speaking, acupuncture in traditional therapy is more likely to be accepted, but the mechanism is still unclear. In this study, a model of insomnia was constructed using chlorophenylalanine (PCPA) in 6-week-old male SD rats. Electroacupuncture was used to stimulate Baihui (DU20), Shenmen (HT7), and Sanyinjiao (SP6), and the behavior, histopathology, cAMP/CREB/BDNF, PI3K/Akt pathways and the expression of sleep-related factors were observed. Our study showed that IL-1β, PGD2, MT, IL-10, IL-6, TNF-α, IFN-γ and CORT in rats could be regulated after electroacupuncture stimulation. The expression of TrkB, PI3K, Akt, P-TrkB, p-Akt, cAMP, CREB, and BDNF can also be up- or downregulated. Apoptosis-related Bax, Bad and Caspase-3, as well as the monoamine neurotransmitters 5-HT, DA, NE and EPI, were also modulated by electroacupuncture. Taken together, these data illustrate the potential of DU20, HT7 and SP6 as a multitargeted therapy for insomnia in rats. The novelty of the study lies in the description of the Traditional Chinese Medicine stimulation methods different from Chinese Herbs: electroacupuncture stimulates acupoints of sleep factors, cAMP/CREB/BDNF, PI3K/Akt pathways and the multipath and multitarget body response regulation mechanism of apoptosis.

Dopaminergic and nitric oxide systems interact to regulate the electrical activity of neurons in the medial septal nucleus in rats

Research characterizing the neuronal substrate of anxiety has implicated different brain areas, including the medial septal nucleus (m-SEPT). Previous reports indicated a role of dopamine and nitric oxide (NO) in anxiety-related behaviors. In this study, the extracellular single-unit recording was performed from the m-SEPT in adult male albino Wistar rats. Baseline activity was recorded for 5 min, and the post-injection recording was performed for another 5 min after the microinjection of each drug. The results showed that (1) both D1- and D2-like receptor agonists (SKF-38393 and quinpirole) enhanced the firing rate of m-SEPT neurons; (2) both D1- and D2-like antagonists (SCH-23390 and sulpiride) attenuated the firing rate of m-SEPT neurons; (3) L-arginine (NO precursor) increased the firing rate of m-SEPT neurons, but a non-specific NOS inhibitor, L-NAME, elicited no significant alterations; (4) the non-specific NOS inhibitor reversed the enhanced firing rate produced by SKF-38393 and quinpirole; (5) neither of the dopaminergic antagonists changed the enhanced activity resulted from the application of the NO precursor. These results contribute to our understanding of the complex neurotransmitter interactions in the m-SEPT and showed that both dopaminergic and NO neurotransmission are involved in the modulation of the firing rate of neurons in the m-SEPT.

Characterization of dopamine D2 receptor coupling to G proteins in postmortem brain of subjects with schizophrenia

Background

Alterations of dopamine D₁ (D1R) and D₂ receptor (D2R) are proposed in schizophrenia but brain neuroimaging and postmortem studies have shown controversial results in relation to D1R and D2R density. Besides, scarce information on the functionality of brain D1R and D2R is available. The present study characterized G-protein activation by D1R and D2R agonists in postmortem human brain. Furthermore, D2R functional status was compared between schizophrenia and control subjects.

Methods

G-protein receptor coupling was assessed in control caudate nucleus and frontal cortex by [³⁵S]GTPγS-binding stimulation induced by increasing concentrations (10^–10–10^–3 M) of dopamine, and the selective dopaminergic agonists SKF38393 (D1R) and NPA (D2R). Concentration–response curves to NPA stimulation of [³⁵S]GTPγS binding were analyzed in antipsychotic-free (n = 10) and antipsychotic-treated (n = 7) schizophrenia subjects and matched controls (n = 17).

Results

In caudate, [³⁵S]GTPγS-binding responses to agonists were compatible with the existence of functional D2R. In contrast, stimulations in cortex showed responses that did not correspond to D1R or D2R. [³⁵S]GTPγS-binding activation by NPA in caudate displayed biphasic curves with similar profile in schizophrenia (EC_50H = 7.94 nM; EC_50L = 7.08 μM) and control (EC_50H = 7.24 nM; EC_50L = 15.14 μM) subjects. The presence or absence of antipsychotic medication did not influence the pharmacological parameters.

Conclusions

Feasibility of functional evaluation of dopamine receptors in postmortem human brain by conventional [³⁵S]GTPγS-binding assays appears to be restricted to signalling through inhibitory G_i/o proteins. These findings provide functional information about brain D2R status in subjects with schizophrenia and do not support the existence of D2R supersensitive in this mental disorder.

Supplementary Information

The online version contains supplementary material available at 10.1007/s43440-021-00305-4.

Functional approaches to the study of G-protein-coupled receptors in postmortem brain tissue: [35S]GTPγS binding assays combined with immunoprecipitation

G-protein-coupled receptors (GPCRs) have an enormous biochemical importance as they bind to diverse extracellular ligands and regulate a variety of physiological and pathological responses. G-protein activation measures the functional consequence of receptor occupancy at one of the earliest receptor-mediated events. Receptor coupling to G-proteins promotes the GDP/GTP exchange on Gα subunits. Thus, modulation of the binding of the poorly hydrolysable GTP analog [³⁵S]GTPγS to the Gα-protein subunit can be used as a functional approach to quantify GPCR interaction with agonist, antagonist or inverse agonist drugs. In order to determine receptor-mediated selective activation of the different Gα-proteins, [³⁵S]GTPγS binding assays combined with immunodetection by specific antibodies have been developed and applied to physiological and pathological brain conditions. Currently, immunoprecipitation with magnetic beads and scintillation proximity assays are the most habitual techniques for this purpose. The present review summarizes the different procedures, advantages and limitations of the [³⁵S]GTPγS binding assays combined with selective Gα-protein sequestration methods. Experience of functional coupling of several GPCRs to different Gα-proteins and recommendations for optimal performance in brain membranes are described. One of the biggest opportunities opened by these techniques is that they enable evaluation of biased agonism in the native tissue, which results in high interest in drug discovery. The available results derived from application of these functional methodologies to study GPCR dysfunctions in neuro-psychiatric disorders are also described. In conclusion, [³⁵S]GTPγS binding combined with antibody-mediated immunodetection represents an useful method to separately evaluate the functional activity of drugs acting on GPCRs over each Gα-protein subtype.

Understanding the Binding Specificity of G-Protein Coupled Receptors toward G-Proteins and Arrestins: Application to the Dopamine Receptor Family

G-Protein coupled receptors (GPCRs) are involved in a myriad of pathways key for human physiology through the formation of complexes with intracellular partners such as G-proteins and arrestins (Arrs). However, the structural and dynamical determinants of these complexes are still largely unknown. Herein, we developed a computational big-data pipeline that enables the structural characterization of GPCR complexes with no available structure. This pipeline was used to study a well-known group of catecholamine receptors, the human dopamine receptor (DXR) family and its complexes, producing novel insights into the physiological properties of these important drug targets. A detailed description of the protein interfaces of all members of the DXR family (D1R, D2R, D3R, D4R, and D5R) and the corresponding protein interfaces of their binding partners (Arrs: Arr2 and Arr3; G-proteins: Gi1, Gi2, Gi3, Go, Gob, Gq, Gslo, Gssh, Gt2, and Gz) was generated. To produce reliable structures of the DXR family in complex with either G-proteins or Arrs, we performed homology modeling using as templates the structures of the β2-adrenergic receptor (β2AR) bound to Gs, the rhodopsin bound to Gi, and the recently acquired neurotensin receptor-1 (NTSR1) and muscarinic 2 receptor (M2R) bound to arrestin (Arr). Among others, the work demonstrated that the three partner groups, Arrs and Gs- and Gi-proteins, are all structurally and dynamically distinct. Additionally, it was revealed the involvement of different structural motifs in G-protein selective coupling between D1- and D2-like receptors. Having constructed and analyzed 50 models involving DXR, this work represents an unprecedented large-scale analysis of GPCR-intracellular partner interface determinants. All data is available at www.moreiralab.com/resources/dxr.

Pimavanserin exhibits serotonin 5-HT2A receptor inverse agonism for Gαi1- and neutral antagonism for Gαq/11-proteins in human brain cortex

Pimavanserin is claimed as the first antipsychotic drug that shows selectivity for serotonin 5-HT₂ receptors (5-HT₂Rs) and lacks of affinity for dopamine D₂ receptors (D₂Rs). Cell-based functional assays suggest that pimavanserin and antipsychotics with D₂R/5-HT₂R affinity could act as inverse agonists of 5-HT_2ARs. However, there is not evidence of such pharmacological profile in native brain tissue. 5-HT_2ARs are able to engage both canonical G_αq/11- and non-canonical G_αi1-proteins. In the present study, the response to pimavanserin of the 5-HT_2AR coupling to G_αq/11- and G_αi1-proteins was measured in membranes of postmortem human prefrontal cortex by antibody-capture [³⁵S]GTPγS binding scintillation proximity assays. Pimavanserin promoted a concentration-dependant inhibition of the 5-HT_2AR coupling to G_αi1-proteins whereas the response of G_αq/11-proteins was unaltered, suggesting inverse agonism and neutral antagonism properties, respectively. The inhibition was abolished in the presence of the selective 5-HT_2AR antagonist MDL-11,939 and was absent in brain cortex of 5-HT_2AR knock-out mice when compared to respective 5-HT_2AR wild-type animals. In conclusion, the results demonstrate the existence of constitutive 5-HT_2AR activity in human brain for the signalling pathway mediated by G_αi1-proteins. Pimavanserin demonstrates 5-HT_2AR functional selectivity and exhibits inverse agonist profile towards G_αi1-proteins, which is considered the effector pathway promoting hallucinogenic responses. In contrast, pimavanserin behaves as neutral antagonist on the 5-HT_2AR coupling to the canonical G_αq/11-protein pathway. The results strengthen the relevance of inverse agonism as potential mechanism of antipsychotic activity. Moreover, the existence of functional selectivity of 5-HT_2ARs for different G_α-proteins could contribute to better design of 5-HT_2AR-related antipsychotic drugs.