Functional histamine H3 and adenosine A2A receptor heteromers in recombinant cells and rat striatum

In the striatum, histamine H-3 receptors (H(3)Rs) are co-expressed with adenosine A(2A) receptors (A(2A)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 H(3)Rs and A(2A)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 A(2A) R-302-G alpha(qi4) and wild-type H(3)Rs in transfected HEK-293T cells. H3R activation with the agonist RAMH resulted in Ca2+ mobilization (pEC(50) 7.31 +/- 0.23; maximal stimulation, Emax 449 +/- 25% of basal) indicative of receptor heterodimerization. Functional H3R-A(2A)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, H3R activation decreased A(2A)R affinity for the agonist CGS-21680 (pKi values 8.10 +/- 0.04 and 7.70 +/- 0.04). Moreover, H(3)Rs and A(2A)Rs co-immunoprecipitated in protein extracts from striatal synaptosomes. These results support the existence of a H3R-A(2A)R heteromer with possible physiological implications for the modulation of the intra-striatal transmission. (C) 2017 Elsevier Ltd. All rights reserved.