Nicotinic acetylcholine receptors (nAChRs) on dopaminergic (DA) and GABAergic (Gaba) projection neurons of the substantia nigra (SN) and ventral tegmental area (VTA) are characterized by single-cell RT-PCR and patch-clamp recordings in slices of rat and wild-type, β2−/−, α4−/−, and α7−/− mice. The eight nAChR subunits expressed in these nuclei, α3–7 and β2–4, contribute to four different types of nAChR-mediated currents. Most DA neurons in the SN and VTA express two nAChR subtypes. One is inhibited by dihydro-β-erythroidine (2 μm), α-conotoxin MII (10 nm), and methyllycaconitine (1 nm) but does not contain the α7 subunit; it possesses a putative α4α6α5(β2)₂ composition. The other subtype is inhibited by dihydro-β-erythroidine (2 μm) and has a putative α4α5(β2)₂ composition. Gaba neurons in the VTA exhibit a third subtype with a putative (α4)₂(β2)₃ composition, whereas Gaba neurons in the SN have either the putative (α4)₂(β2)₃ oligomer or the putative α4α6α5(β2)₂ oligomer. The fourth subtype, a putative (α7)₅ homomer, is encountered in less than half of DA and Gaba neurons, in the SN as well as in the VTA. Neurons in the DA nuclei thus exhibit a diversity of nAChRs that might differentially modulate reinforcement and motor behavior.

Antagonistic and reciprocal interactions are known to exist between adenosine and dopamine receptors in the striatum. In the present study, double immunofluorescence experiments with confocal laser microscopy showed a high degree of colocalization of adenosine A_2A receptors (A_2AR) and dopamine D₂ receptors (D₂R) in cell membranes of SH-SY5Y human neuroblastoma cells stably transfected with human D₂R and in cultured striatal cells. A_2AR/D₂R heteromeric complexes were demonstrated in coimmunoprecipitation experiments in membrane preparations from D₂R-transfected SH-SY5Y cells and from mouse fibroblast Ltk⁻ cells stably transfected with human D₂R (long form) and transiently cotransfected with the A_2AR double-tagged with hemagglutinin. Long term exposure to A_2AR and D₂R agonists in D₂R-cotransfected SH-SY5Y cells resulted in coaggregation, cointernalization and codesensitization of A_2AR and D₂R. These results give a molecular basis for adenosine-dopamine antagonism at the membrane level and have implications for treatment of Parkinson's disease and schizophrenia, in which D₂R are involved.