Hetero-dimerization of serotonin 5-HT_2A and dopamine D₂ receptors

Abstract

In the present study, detailed information is presented on the hetero-dimerization of the serotonin 5-HT_2A receptor and the dopamine D₂ receptor. Biophysical approaches (fluorescence spectroscopy as well as fluorescence lifetime microscopy) were used to determine the degree of fluorescence resonance energy transfer (FRET) between cyan and yellow fluorescent protein labeled receptor variants co-expressed in human embryonic kidney 293 cells (HEK293). Recorded data demonstrate the existence of energy transfer between the wild-type forms of 5-HT_2AR and D₂R, pointing toward the formation of hetero-5-HT_2AR/D₂R dimers and homo-5-HT_2AR/5-HT_2AR dimers. Moreover, the present study investigates the role of specific motifs (one containing adjacent arginine residues (217RRRRKR222) in the third intracellular loop (ic3) of D₂R, and the other consisting of acidic glutamate residues (454EE455) in the C-tail of (5-HT_2AR) in the formation of noncovalent complexes between these receptors. Our results suggest that these regions of 5-HT_2AR and D₂R may be involved in the interaction between these two proteins. On the other hand, the above-mentioned motifs do not play an important role in the homo-dimerization of these receptors. Furthermore, we estimated the influence of specific receptor ligands on the dimerization processes. Agonists (DOI and quinpirole) and antagonists (ketanserin and butaclamol) cause different effects on FRET efficiency depending on whether homo- or hetero-complexes are present. These data may have therapeutic implications, since (using the immunofluorescence double labeling protocols) the co-localization of these two receptors was demonstrated in the medial prefrontal cortex and pars reticulate of the substantia nigra of the rat brain.