Differential effects of [3H]nemonapride and [3H]spiperone binding on human dopamine D4 receptors

We compared some binding parameters of [3H]nemonapride and [3H]spiperone in human dopamine D4 (hD4) receptors with three different numbers of tandem repeat units. Although both of the radioligands showed similar affinity constants for each hD4 receptor variant, the maximal number of binding sites labeled by [3H]nemonapride was approximately 1.35-fold higher than that by [3H]-spiperone for all variants. Estimated Ki values for the inhibition of [3H]nemonapride binding by a series of dopaminergic ligands were highly correlated to respective values obtained for the inhibition of [3H]spiperone binding to each hD4 receptor. These results suggest that the hD4 receptor, as shown for the D2 receptor, may exist in multiple molecular forms as a monomer-dimer equilibria, and that [3H]spiperone may discriminate in the multiple molecular forms.