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

Abstract

Background: Alterations of dopamine D-1 (D1R) and D-2 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 [S-35]GTP gamma S-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 [S-35]GTP gamma S binding were analyzed in antipsychotic-free (n = 10) and antipsychotic-treated (n = 7) schizophrenia subjects and matched controls (n = 17).

Results: In caudate, [S-35]GTP gamma S-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. [S-35]GTP gamma S-binding activation by NPA in caudate displayed biphasic curves with similar profile in schizophrenia (EC50H = 7.94 nM; EC50L = 7.08 mu M) and control (EC50H = 7.24 nM; EC50L = 15.14 mu M) 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 [S-35]GTP gamma S-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.