CHARACTERIZATION OF BINDING OF [H-3]PD-128907, A SELECTIVE DOPAMINE D-3 RECEPTOR AGONIST LIGAND, TO CHO-K1 CELLS

PD 128907 [4a R, 10 b R-(+)-trans-3, 4, 4a, 10 b-tetrahydro-4-n-propyl 2 H,5H-[1]benzop-yrano[4,3-b] 1,4-oxazin-9-ol.], a selective dopamine (DA) D3 receptor agonist ligand exhibits about a 1000-fold selectivity for human D3 receptors (K-i, 1 nM) versus human D-2 receptors (K-i, 1 183 nM) and a 10000-fold selectivity versus human D-4 receptors (K-i, 7000 nM) using [H-3]spiperone as the radioligand in CHO-K1-cells. Stud ies with [H-3]PD 128907, showed saturable, high affinity binding to hu man D-4 receptors expressed in CHO-K1 cells (CHO-K1-D-3) with an equil ibrium dissociation constant (K-d) of 0.99 nM and a binding density (B -max) of 475 fmol/mg protein. Under the same conditions, there was no significant specific binding in CHO-K1-cells expressing human D-2 rece ptors (CHO-K1-D-2). The rank order of potency for inhibition of [H-3]P D 128907 binding with reference DA agents was consistent with reported values for D-3 receptors. These results indicate that [H-3]PD 128907 is a new, highly selective D-3 receptor ligand with high specific acti vity, high specific binding and low non-specific binding and therefore should be useful for further characterizing the DAD(3) receptors.