Journal of Neuroscience, Vol 3, 433-440, Copyright © 1983 by Society for Neuroscience
Photoaffinity labeling of benzodiazepine receptors causes altered agonist-antagonist interactions
JW Thomas and JF Tallman
Previous studies have shown that [3H]flunitrazepam forms irreversible
cross-links with brain tissue when exposed to ultraviolet irradiation.
Comparison of the amount of [3H]flunitrazepam irreversibly incorporated and
the number of benzodiazepine binding sites blocked after photolabeling has
indicated that several binding sites are inactivated for each molecule of
[3H]flunitrazepam incorporated. To learn the cause of this discrepancy,
binding to the benzodiazepine binding sites has been examined using several
radiolabeled benzodiazepine antagonists. Binding of a beta-carboline ester,
CGS-8216, and Ro 15-1788 was not altered by photolabeling; however,
displacement studies revealed that photolabeling converted a homogeneous
set of benzodiazepine binding sites into two subsets: one of high affinity
(unaltered sites) and one of low affinity. The low affinity sites could be
detected by displacement studies of antagonist binding by benzodiazepines,
and conversion to a low affinity form accounts for the discrepancy observed
after photolabeling using [3H]flunitrazepam as ligand.