[3H]dopamine labeling of D3 dopaminergic sites in human, rat, and calf brain

The binding of [3H]dopamine to brain regions of calf, rat, and human was investigated. The calf caudate contained the highest density of [3H]dopamine binding sites, with a Bmax value of 185 fmol/mg protein, whereas rat and human striatum contained one-third this number of sites. The KD values for [3H]dopamine in all tissues were 2-3 nM. Dopaminergic catecholamines (dopamine, apomorphine, 6,7-dihydroxy-2-aminotetralin, and N-propylnorapomorphine) inhibited the binding of [3H]dopamine in all three species, at low concentrations, with IC50 values of 1.5 to 6 nM. Neuroleptics, in contrast, inhibited the binding at high concentrations (with IC50 values of 200 to 40,000 nM). The [3H]dopamine binding sites were saturable, heat-labile, and detectable only in dopamine-rich brain regions; these sites differed from D2 dopamine sites (labeled by [3H]butyrophenone neuroleptics), and from D1 dopamine sites (labeled by [3H]thioxanthene neuroleptics) associated with the dopamine-stimulated adenylate cyclase. We have, therefore, called these high-affinity [3H]dopamine binding sites D3 sites. [3H]Apomorphine and [3H]ADTN also appeared to label D3 sites. These ligands however, were less selective than [3H]dopamine, and labeled sites other than D3 as well. Assay conditions were important in determining the parameters of [3H]dopamine binding. The optimum conditions for selective labeling of the D3 dopaminergic sites, using [3H]dopamine, required the presence of EDTA and ascorbate.