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

In the striatum, histamine H receptors (HRs) are co-expressed with adenosine A receptors (ARs) 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 HRs and ARs 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 AR-Gα and wild-type HRs in transfected HEK-293T cells. HR activation with the agonist RAMH resulted in Ca mobilization (pEC 7.31 ± 0.23; maximal stimulation, Emax 449 ± 25% of basal) indicative of receptor heterodimerization. Functional HR-AR 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, HR activation decreased AR affinity for the agonist CGS-21680 (pKi values 8.10 ± 0.04 and 7.70 ± 0.04). Moreover, HRs and ARs co-immunoprecipitated in protein extracts from striatal synaptosomes. These results support the existence of a HR-AR heteromer with possible physiological implications for the modulation of the intra-striatal transmission.