Interactions of benzodiazepines and ?-carbolines with a histidine residue of the benzodiazepine receptor.

ABSTRACT

The involvement of a histidine residue in benzodiazepine binding is suggested by the decrease of Ro 15-1788 and flunitrazepam binding when pH is changed from 7.5 to 5.5. Diethyl pyrocarbonate inactivates the benzodiazepine binding site by modifying a histidine residue, as shown by its faster action at pH 7.5 than at pH 5.5. After a partial diethyl pyrocarbonate inactivation, the remaining Ro 15-1788 binding sites still have the same sensitivity to acid pH. No difference in the pH sensitivity of the Ro 15-1788 binding is observed between the cerebellum and the cerebral cortex. It is thus proposed that the histidine residue is present at every benzodiazepine binding site, be it either subtype I or II. The pure antagonists Ro 15-1788 and propyl-?-carboline-3-carboxylate protect, respectively, 36.2 and 1% of the benzodiazepine binding sites from diethyl pyrocarbonate inactivation, the full agonists flunitrazepam and diazepam provide, respectively, a 98.5 and a 46.5% protection and the full inverse agonists methyl-6,7-dimethoxy-?-carboline-3-carboxylate and methyl-?-carboline-3-carboxylate provide respectively, a 60 and an 11% protection. Data suggest that the histidine residue is absent from the propyl-?-carboline-3-carboxylate binding site, and that the ability of the compounds to protect the Bdz binding sites is not related to their agonist or inverse agonist potencies. The protections observed may be due to allosteric interactions between the Bdz and ?-carboline binding sites and the histidine residue.