IDENTIFICATION AND PHARMACOLOGICAL CHARACTERIZATION OF [I-125] L-750,667, A NOVEL RADIOLIGAND FOR THE DOPAMINE D-4 RECEPTOR

We identified a navel azaindole derivative, L-750,667, that has high a ffinity (K-i = 0.51 nM) and >2000-fold selectivity for D-4 dopamine re ceptors compared with its activity at D-2 and D-3 dopamine receptors. L-750,667 had little affinity for rat D-1/D-5 dopamine receptors, sigm a binding sites, or 5-hydroxytryptamine(1A) or 5-hydroxytryptamine, re ceptors. In functional studies, L-750,667 exhibited high affinity anta gonist activity at D-4 receptors, reversing dopamine (1 mu M)-induced inhibition of cAMP accumulation in human embryonic kidney (HEK) cells expressing the human D-4 receptor (hD4 HEK) with an EC(50) value of 80 nM. The radioiodinated form of L-750,667 bound specifically to the hu man dopamine D-4 receptor expressed in HEK cells and saturation analys is revealed a single high affinity binding site for [O-125]L-750,667 ( K-d = 0.16 +/- 0.06 nM). The maximum number of binding sites (B-max) e stimated using [I-125]L-750,667 in hD4 HEK cells was 251 +/- 71 fmol/m g, which correlated well with the Bmax value determined using [H-3]spi perone (227 +/- 83 fmol/mg) in the same membrane preparations. The pha rmacological profile of [I-125]L-750,667 binding to hD4 HEK cells was evaluated using known dopamine receptor agonists and antagonists. The rank order of potencies for dopamine receptor agonists was dopamine > quinpirole > 6,7-aminodihydroxytetralin > 5,6-aminodihydroxytetralin. Dopamine receptor antagonists also showed high affinity, with a rank o rder of haloperidol > chlorpromazine > domperidone > (+)-butaclamol > (-)-sulpiride = (+)-sulpiride > (+)-SCH23390 > (-)-butaclamol. [I-125] L-750,667 bound to D-4 receptors in a stereoselective manner with (+)- butaclamol showing higher activity than its respective enantiomer (-)- butaclamol. These results show that [I-125]L-750,667 is a novel, highl y selective radioligand for dopamine D-4 receptors and may be used to investigate the dopamine D-4 receptor population in the central nervou s system.