In vitro binding of [(11)C]raclopride with ultrahigh specific activity in rat brain determined by homogenate assay and autoradiography.
OBJECTIVE
The aim of this study was to characterize the in vitro binding of [(11)C]raclopride with ultrahigh specific activity (SA) in the striatum and cerebral cortex of rat brain.


METHODS
[(11)C]Raclopride, a dopamine D(2) receptor ligand, with an ultrahigh SA of 4880+/-2360 GBq/micromol (132+/-64 Ci/micromol, n=25) was synthesized. In vitro binding experiment was performed using brain homogenate assay and autoradiography (ARG).


RESULTS
In vitro homogenate assay demonstrated that high SA [(11)C]raclopride (2520-6340 GBq/micromol; 68-171 Ci/micromol) had two-affinity (high and low) binding sites in the striatum and cerebral cortex of rat brain. In the striatum, K(d,high) and B(max,high) values were 0.005+/-0.002 nM and 0.19+/-0.04 fmol/mg tissue, respectively, while K(d,low) and B(max,low) values were 2.2+/-1.0 nM and 35.8+/-16.4 fmol/mg tissue, respectively. In the cerebral cortex, K(d,high) and B(max,high) values were 0.061+/-0.087 nM and 0.2+/-0.2 fmol/mg tissue, respectively, while K(d,low) and B(max,low) values were 2.5+/-3.2 nM and 5.5+/-4.8 fmol/mg tissue, respectively. On the other hand, only one binding site was found in the striatum and no binding site was identified in the cerebral cortex using low SA [(11)C]raclopride (44 GBq/micromol; 1.2 Ci/micromol). In vitro ARG for the rat brain using high SA [(11)C]raclopride (6212 GBq/micromol; 168 Ci/micromol) gave a coronal image of the striatum and cerebral cortex with a higher signal/noise ratio than using low SA [(11)C]raclopride (40 GBq/micromol; 1.1 Ci/micromol).


CONCLUSIONS
Using ultrahigh SA [(11)C]raclopride for the in vitro homogenate assay, we succeeded in detecting two-affinity binding sites of [(11)C]raclopride, not only in the striatum but also in the cerebral cortex of rat brain.
Noguchi
Junko
J
Radiochemistry Section, Department of Molecular Probes, Molecular Imaging Center, National Institute of Radiological Sciences, 4-9-1 Anagawa, Chiba 263-8555, Japan.
Zhang
Ming-Rong
MR
Yanamoto
Kazuhiko
K
Nakao
Ryuji
R
Suzuki
Kazutoshi
K
D000818:Animals; D001345:Autoradiography; D002250:Carbon Radioisotopes; D002540:Cerebral Cortex / Q000378:metabolism*; D003342:Corpus Striatum / Q000378:metabolism*; D008297:Male; D011759:Pyrrolidines / Q000378:metabolism; D020891:Raclopride / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D012457:Salicylamides / Q000378:metabolism
D002250:Carbon Radioisotopes; D011759:Pyrrolidines; D012457:Salicylamides; C065065:FLB 457; D020891:Raclopride