RECEPTOR IMAGING TECHNIQUE WITH C-11 LABELED RECEPTOR LIGANDS IN LIVING BRAIN-SLICES - ITS APPLICATION TO TIME-RESOLVED IMAGING AND SATURATION ANALYSIS OF BENZODIAZEPINE RECEPTOR USING [C-11] RO15-1788

Recently we developed a novel imaging technique using positron emitter -labeled compounds as probes and a storage phosphor screen as a detect or. This approach makes it possible to follow a variety of biochemical processes with spatial information in living brain slices. Further te chnical development is reported here in terms of time-resolved imaging and receptor characterization in a real equilibrium state. The method was validated by use of [C-11]Ro15-1788, a benzodiazepine receptor an tagonist. Fresh brain slices were incubated with [C-11]Ro15-1788 in ox ygenated Krebs-Ringer solution at 37 degrees C, in a specially designe d chamber. By placing the chamber on a storage phosphor screen, we cou ld obtain two-dimensional images of radioactivity in the slices. Time- resolved imaging was made at 5 min intervals, revealing that it took 6 0 min to reach equilibrium binding. The dissociation process was obser ved by adding an excess amount of unlabeled Ro15-1788 to the chamber, 25 min was required for the full dissociation. In the equilibrium stat e, i.e. in the presence of free radio-ligand, Scatchard plot analysis was performed on the cerebral cortex (K-d = 7.4 nM, B-max = 146 fmol/m g tissue) and striatum (K-d = 7.5 nM, B-max = 107 fmol/mg tissue), sug gesting the presence of a single component of binding site in these tw o regions. The present method, for the first time, made it possible to study a ligand-receptor interaction in living brain slices with tempo ral and spatial resolutions. This technique should prove useful for st udies of receptor function under physiological conditions.