Functional histamine H 3 and adenosine A 2A receptor heteromers in recombinant cells and rat striatum.

In the striatum, histamine H3 receptors (H3 Rs) are co-expressed with adenosine A2A receptors (A2A Rs) in the cortico-striatal glutamatergic afferents and the GABAergic medium-sized spiny neurons that originate the indirect pathway of the basal ganglia. This location allows H3 Rs and A2A Rs to regulate the striatal GABAergic and glutamatergic transmission. However, whether these receptors can physically interact has not yet been assessed. To test this hypothesis, a heteromer-selective in vitro assay was used to detect functional complementation between a chimeric A2A R302 -Gαqi4 and wild-type H3 Rs in transfected HEK-293T cells. H3 R activation with the agonist RAMH resulted in Ca2+ mobilization (pEC50 7.31 ± 0.23; maximal stimulation, Emax 449 ± 25% of basal) indicative of receptor heterodimerization. Functional H3 R-A2A R heteromers were confirmed by co-immunoprecipitation and observations of differential cAMP signaling when both receptors were co-expressed in the same cells. In membranes from rat striatal synaptosomes, H3 R activation decreased A2A R affinity for the agonist CGS-21680 (pKi values 8.10 ± 0.04 and 7.70 ± 0.04). Moreover, H3 Rs and A2A Rs co-immunoprecipitated in protein extracts from striatal synaptosomes. These results support the existence of a H3 R-A2A R heteromer with possible physiological implications for the modulation of the intra-striatal transmission.