Abstract
According to the adenosine hypothesis of schizophrenia, the classically associated hyperdopaminergic state may be secondary to a loss of function of the adenosinergic system. Such a hypoadenosinergic state might either be due to a reduction of the extracellular levels of adenosine or alterations in the density of adenosine A2A receptors (A2ARs) or their degree of functional heteromerization with dopamine D2 receptors (D2R). In the present study, we provide preclinical and clinical evidences for this latter mechanism. Two animal models for the study of schizophrenia endophenotypes, namely the phencyclidine (PCP) mouse model and the A2AR knockout mice, were used to establish correlations between behavioural and molecular studies. In addition, a new AlphaLISA-based method was implemented to detect native A2AR-D2R heteromers in mouse and human brain. First, we observed a reduction of prepulse inhibition in A2AR knockout mice, similar to that observed in the PCP animal model of sensory gating impairment of schizophrenia, as well as a significant upregulation of striatal D2R without changes in A2AR expression in PCP-treated animals. In addition, PCP-treated animals showed a significant reduction of striatal A2AR-D2R heteromers, as demonstrated by the AlphaLISA-based method. A significant and pronounced reduction of A2AR-D2R heteromers was next demonstrated in postmortem caudate nucleus from schizophrenic subjects, even though both D2R and A2AR were upregulated. Finally, in PCP-treated animals, sub-chronic administration of haloperidol or clozapine counteracted the reduction of striatal A2AR-D2R heteromers. The degree of A2AR-D2R heteromer formation in schizophrenia might constitute a hallmark of the illness, which indeed should be further studied to establish possible correlations with chronic antipsychotic treatments.