1-Methyl-?-carboline (Harmane), a potent endogenous inhibitor of benzodiazepine receptor binding

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Abstract
The interaction of several β-carbolines with specific [3H]-flunitrazepam binding to benzodiazepine receptors in rat brain membranes was investigated. Out of the investigated compounds, harmane and norharmane were the most potent inhibitors of specific [3H]-flunitrazepam binding, with IC50-values in the micromolar range. All other derivatives, including harmine, harmaline, and several tetrahydroderivatives were at least ten times less potent. Harmane has been previously found in rat brain and human urine, so it is the most potent endogenous inhibitor of specific [3H]-flunitrazepam binding known so far, with a several hundred fold higher affinity for the benodiazepine receptor than inosine and hypoxanthine. Thus, we suggest that harmane or other related β-carbolines could be potential candidates as endogenous ligands of the benzodiazepine receptor.