Dopamine Receptors in Subcellular Fractions from Bovine Caudate: Enrichment of [3H]Spiperone Binding in a Postsynaptic Membrane Fraction

Abstract: Specific binding of tritiated dopamine, spiperone, and N-propylnorapomorphine was examined in subcellular fractions from bovine caudate nucleus. All fractions contained at least two sets of specific binding sites for [³H]spiperone (K_{D 1}^aPP= 0.2 nM, K_{D 2}^aPP= 2.2 nM), the higher affinity sites accounting for one-third to one-eighth of the total. [³H]Spiperone binding was slightly enriched over the total particulate fraction in P₂, P₃, SPM, and a crude fraction of synaptic mitochondria. A microsomal subfraction (P₃B₂) exhibited the highest specific binding capacity obtained, representing a fourfold enrichment over the total particulate fraction. [³H]Dopamine exhibited apparent binding to a single class of high-affinity sites in all fractions examined (K_D^aPP= 4.0 nM). A greater than twofold enrichment was observed in all fractions except myelin and P₃, with a fivefold enrichment in SPM and P₃B₂. At least two classes of receptors were labeled by [³H]-N-propylnorapomorphine (K_{D 1}^aPP= 0.55 nM, K_{D 2}^aPP= 20 nM), using 50 nM-spiperone together with 100 nM-dopamine to define nonspecific binding. Although binding to the higher affinity site was displaced by spiperone, and lower affinity binding by dopamine, comparison of receptor densities with values obtained by using [³H]spiperone and [³H]dopamine directly suggested that [³H]-N-propylnorapomorphine labeled additional sites. We have also examined a postsynaptic membrane (PSM) fraction obtained from SPM by successive extraction with salt and EGTA followed by sonication and separation on a density gradient. [³H]Spiperone binding in PSM was enriched two- to threefold over unfractionated SPM with a concomitant decrease in [³H]dopamine binding. The enrichment in spiperone receptors was almost entirely due to an increase in the number of lower affinity binding sites, suggesting that these sites may be associated with the postsynaptic membrane.