**7. Summary**

In summary, IEM provided direct evidence for the existence of homo- and hetero-oligomers of A1R and P2Y2R, not only in co-transfected cultured cells, but also *in situ* on the surface of neurons in various brain regions. The molecular mechanisms responsible for the control of A1R and P2YR monomer/homo-oligomer/hetero-oligomer ratios remain to be elucidated. Future investigation of GPCR oligomer formation is indispensable for revealing the elaborate mechanisms of cellular function.

The importance of these novel experimental procedures using IEM is to provide information concerning crosstalk between small molecules with high angle views of whole cells, although these methods do require a high level of technical skill. The development of ingenious histochemical and immunoelectronmicroscopic methods has made it possible to visuallize crosstalk and provide specific insight into the nature of hetero-oligomers of not only GPCRs, but also various proteins expressed by cells.
