Wednesday, July 23, 2008

Chlorpromazine competes for [3H]-LSD binding site

Radioactive LSD binds to cerebral cortex and subcortex of rat brain.

LSD binding at receptors can be measured in studies with drugs that emit radioactive particles. First [3H]-LSD is mixed with rat brain homogenate, and the binding reaction is allowed to take place. Next the homogenates are washed extensively with water and other buffers, to remove any [3H]-LSD that is nonspecifically bound. Then brain homogenates are exposed to a special plate for counting radioactivity. The amount of radioactivity is quantified and converted into membrane affinity. In this way, it is possible to obtain an approximate value of the binding affinity of [3H]-LSD to receptors, and of the ability of certain inhibitor drugs to act as antagonists to [3H]-LSD binding.

In 1974, Bennett and Aghajanian found that co-incubation of [3H]-LSD with certain inhibitor drugs suppressed the amount of radioactive LSD bound (Table I).


The most effective inhibitor of [3H]-LSD binding was D-LSD itself, which prevented binding at a concentration 6x10E-9 M. D-LSD was followed by methysergide, 5-HT, DMT, and promethazine. Chlorpromazine at a rather low concentration, 5x10E-7 M, displaced [3H]-LSD in rat brain membranes, suggesting that chlorpromazine might bind the same brain receptor as LSD. The psychologically inert stereoisomer L-LSD was approximately 10,000 times less effective than D-LSD at competing for the [3H]-LSD binding site, showing the stereoselectivity of biological receptors.

The EC50 values (table above) obtained by Bennett and Aghajanian have been used by Domelsmith in correlational analysis. The average of the first and second ionization potentials of dopamine, mescaline, DOM, dimethyltryptamine, promethazine, chlorpromazine and LSD are 8.54, 8.16, 8.15, 7.80, 7.73, 7.71, and 7.65 eV while the membrane affinity (-log ED50) of these compounds are 3.52, 4.40, 5.30, 6.52, 7.00, 7.00, and 8.22 respectively. Thus, there is a linear correlation between the ionization potential and drug potency at displacing [3H]-LSD binding to rat brain homogenates. Increasing ability of drugs to displace LSD from rat brain homogenates was paralleled by decreasing ionization potential.


Reference

Bennett J. L. and G. K. Aghajanian (1974). d-LSD binding to brain homogenates: possible relationship to serotonin receptors. Life Sciences 15, 1935-1944. doi:10.1016/0024-3205(74)90044-7