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

Abstract

In the striatum, histamine H3 receptors (H3Rs) are co-expressed with adenosine A2A receptors (A2ARs) 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 H3Rs and A2ARs to regulate the striatal GABAergic and glutamatergic transmission. However, whether these receptors interact to modulate the intra-striatal synaptic transmission has not yet been assessed. To test this hypothesis a heteromer-selective in vitro assay was used to detect functional complementation between a chimeric A2AR302-Gqi4 and wild-type H3Rs in transfected HEK-293 cells. H3R 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. This response was not observed with histamine, suggesting a RAMH bias for heteromers. Functional A2AR-H3R heteromers were confirmed by co-immunoprecipitation and observations of differential cAMP signaling when both receptors were co-expressed in the same cell. In membranes from rat striatal synaptosomes, H3R activation decreased A2AR affinity for the agonist CGS-21680 (pKi values 8.10 {+/-} 0.04 and 7.70 {+/-} 0.04). Moreover, H3Rs and A2ARs co-immunoprecipitated in protein extracts from striatal synaptosomes. These results support the existence of a H3R/A2AR heteromer, and reveal a new mechanism by which these receptors may modulate the unction of the striatum and the basal ganglia.