Expression of dopamine D-3 receptor dimers and tetramers in brain and in transfected cells

The expression and characteristics of the dopamine D-3 receptor protein were studied in brain and in stably transfected GH3 cells, Monoclonal antibodies were used for immunoprecipitation and immunoblot experiments. Immunoprecipitates obtained from primate and rodent brain tissues contain a low molecular weight D-3 protein and one or two larger protein species whose molecular mass are integral multiples of the low molecular weight protein and thus appear to have resulted from dimerization and tetramerization of a D-3 monomer, Whereas D-3 receptor multimers were found to be abundantly expressed in brain, the major D-3 immunoreactivity expressed in stable D-3-expressing rat GH3 cells was found to be a monomer, However, multimeric D-3 receptor species with electrophoretic mobilities similar to those expressed in brain were also seen in D-3-expressing GH3 cells when a truncated D-3-like protein (named D-3nf) was coexpressed in these cells, Furthermore, results from immunoprecipitation experiments with D-3- and D-3nf-specific antibodies show that the higher order D-3 proteins extracted from brain and D-3/D-3nf double transfectants also contain D-3nf immunoreactivity, and immunocytochemical studies show that the expression of D-3 and D-3nf immunoreactivities overlaps substantially in monkey and rat cortical neurons. Altogether, these data show oligomeric D-3 receptor protein expression in vivo and they suggest that at least some of these oligomers are heteroligomeric protein complexes containing D-3 and the truncated D-3nf protein.