Single photon emission tomography measurement of benzodiazepine receptor number and affinity in primate brain: a constant infusion paradigm with [ 123I]iomazenil

Benzodiazepine receptor number and affinity were measured in vivo with single photon emission tomography (SPECT). Following an initial bolus injection, the radiotracer [ 123I]iomazenil was infused at a constant rate for 5 to 8 h. This procedure induced a state of sustained equilibrium at the receptor level. Nondisplaceable activity was measured after injection of a receptor saturating dose of flumazenil. Experiments performed at high and low specific permitted estimation of an equilibrium binding affinity constant of 0.47 nM and a maximum binding capacity of 127 nM in occipital cortex.