Functional coupling of M(1) muscarinic acetylcholine receptor to Gα(q/11) in dorsolateral prefrontal cortex from patients with psychiatric disorders: a postmortem study

A growing understanding of the role of muscarinic receptors in the molecular pathology and treatment of schizophrenia

Pre-clinical models, postmortem and neuroimaging studies all support a role for muscarinic receptors in the molecular pathology of schizophrenia. From these data it was proposed that activation of the muscarinic M1 and/or M4 receptor would reduce the severity of the symptoms of schizophrenia. This hypothesis is now supported by results from two clinical trials which indicate that activating central muscarinic M1 and M4 receptors can reduce the severity of positive, negative and cognitive symptoms of the disorder. This review will provide an update on a growing body of evidence that argues the muscarinic M1 and M4 receptors have critical roles in CNS functions that are dysregulated by the pathophysiology of schizophrenia. This realization has been made possible, in part, by the growing ability to visualize and quantify muscarinic M1 and M4 receptors in the human CNS using molecular neuroimaging. We will discuss how these advances have provided evidence to support the notion that there is a sub-group of patients within the syndrome of schizophrenia that have a unique molecular pathology driven by a marked loss of muscarinic M1 receptors. This review is timely, as drugs targeting muscarinic receptors approach clinical use for the treatment of schizophrenia and here we outline the background biology that supported development of such drugs to treat the disorder.

5-HT2A receptor-mediated Gαq/11 activation in psychiatric disorders: A postmortem study

OBJECTIVES

Serotonin-2A (5-HT_2A) receptors play an important role in the regulation of many brain functions that are disturbed in patients with such psychiatric diseases as mood disorders and schizophrenia. The objective of this study was to evaluate 5-HT_2A receptor-mediated signalling pathway through Gα_q/11 activation in psychiatric patients by using post-mortem brain samples.

METHODS

Functional activation of Gα_q/11 proteins coupled to 5-HT_2A receptors was determined by means of [³⁵S]GTPγS binding/immunoprecipitation assay in post-mortem prefrontal cortex of psychiatric patients diagnosed as bipolar disorder (BP), major depressive disorder (MDD), and schizophrenia, and individually matched controls. The effects of antipsychotic treatment as well as suicide were also analysed.

RESULTS

There was no significant difference in maximum percent increase (%E_max) or slope factor among the four groups. The negative logarithm of concentration eliciting the half-maximal effect (pEC₅₀) was significantly reduced in BP and schizophrenia patients as compared to controls. These alterations were attributable to antipsychotic medication. The pEC₅₀ values in 'non-suicide' group of schizophrenia, but not in 'suicide' group, were significantly reduced as compared with controls.

CONCLUSIONS

Altered 5-HT_2A receptor-mediated signalling pathway through Gα_q/11 proteins in prefrontal cortex might be apparently involved in pathophysiology and pharmacotherapy of BP and schizophrenia. In schizophrenic patients, these alterations as a result of successful treatment with antipsychotic agents may help in prevention of suicidal behaviour.

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.

The Role of G Protein-Coupled Receptors (GPCRs) and Calcium Signaling in Schizophrenia. Focus on GPCRs Activated by Neurotransmitters and Chemokines

Schizophrenia is a common debilitating disease characterized by continuous or relapsing episodes of psychosis. Although the molecular mechanisms underlying this psychiatric illness remain incompletely understood, a growing body of clinical, pharmacological, and genetic evidence suggests that G protein-coupled receptors (GPCRs) play a critical role in disease development, progression, and treatment. This pivotal role is further highlighted by the fact that GPCRs are the most common targets for antipsychotic drugs. The GPCRs activation evokes slow synaptic transmission through several downstream pathways, many of them engaging intracellular Ca²⁺ mobilization. Dysfunctions of the neurotransmitter systems involving the action of GPCRs in the frontal and limbic-related regions are likely to underly the complex picture that includes the whole spectrum of positive and negative schizophrenia symptoms. Therefore, the progress in our understanding of GPCRs function in the control of brain cognitive functions is expected to open new avenues for selective drug development. In this paper, we review and synthesize the recent data regarding the contribution of neurotransmitter-GPCRs signaling to schizophrenia symptomology.

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.

5-HT2A receptor- and M1 muscarinic acetylcholine receptor-mediated activation of Gαq/11 in postmortem dorsolateral prefrontal cortex of opiate addicts

BACKGROUND

Chronic exposure to opiates causes the development of tolerance and physical dependence as well as persistent brain neuroplasticity. Despite a wealth of postmortem human studies for opiate addicts, little direct information regarding the functional status of serotonergic and cholinergic receptor-mediated signaling pathways in the human brain of opiate addicts is yet available.

METHODS

Functional activation of Gα_q/11 proteins coupled to 5-HT_2A and M₁ type muscarinic acetylcholine receptor (mAChR) was assessed by using the method named [³⁵S]GTPγS binding/immunoprecipitation in frontal cortical membrane preparations from postmortem human brains obtained from opiate addicts and matched controls.

RESULTS

Concentration-response curves for 5-HT and carbachol in individual subjects were analyzed according to a nonlinear regression model, which generated the values of maximum percent increase (%E_max), negative logarithm of the half-maximal effect (pEC₅₀) and slope factor. As for 5-HT_2A receptor-mediated Gα_q/11 activation, the %E_max values were reduced significantly and the pEC₅₀ values were decreased significantly in opiate addicts as compared to the control group. Regarding carbachol-induced Gα_q/11 activation, no significant difference in %E_max or pEC₅₀ values was detected between the both groups, whereas the slope factor was increased significantly in opiate addicts as compared to the control group.

CONCLUSION

Our data demonstrate that the signaling pathways mediated by Gα_q/11 proteins coupled with 5-HT_2A receptors and M₁ mAChRs in prefrontal cortex are functionally altered in opiate addicts in comparison with control subjects. These alterations may underpin some aspects of addictive behavior to opiate as well as neuropsychological consequences or comorbid mental disorders associated with opioid use.

Use of bladder antimuscarinics is associated with an increased risk of dementia: a retrospective population-based case-control study

The association between bladder antimuscarinic use and dementia development is unclear. We used data from the Taiwan National Health Insurance Research Database to determine the association between the exposure dose and duration of bladder antimuscarinics and the subsequent dementia risk. We enrolled participants aged 55 years or more and defined a dementia cohort (International Classification of Diseases, Ninth Revision, Clinical Modification codes 290, 294.1, and 331.0). We used a propensity score matching method, and randomly enrolled two controls without dementia. We evaluated dementia risk with respect to the exposure dose and duration of treatment with seven bladder antimuscarinics (oxybutynin, propiverine, tolterodine, solifenacin, trospium, darifenacin, and fesoterodine) used for at least 1 year before the index date, after adjusting for age, sex, comorbidities, and medications. The dementia risk was 2.46-fold (95% confidence interval: 2.22–2.73) higher in Taiwanese patients who used bladder antimuscarinics for ≥ 1 year than in those who were not exposed to this treatment. The risk proportionally increased with increasing doses of antimuscarinics for less than 4 years. Taiwanese patients aged 55 years or more on bladder antimuscarinics exhibited a higher risk of dementia. Additional studies in other countries are required to determine whether this result is valid worldwide.