Benzodiazepines probably exert their anxiolytic, hypnotic, and anticonvulsant effects by interacting with brain-specific high-affinity benzodiazepine receptors. In searching for possible endogenous ligands for these receptors, we have purified a compound 107-fold from human urine by extractions, treatment with hot ethanol, and column chromatography. The compound was identified as ,-carboline-3-carboxylic acid ethyl ester (IIc) by mass spectrometry, NMR spectrometry, and synthesis; HIc was also isolated from brain tissues (20 ng/g) by similar procedures. Very small concentrations of TIc displaced [3H]diazepam completely from specific cerebral receptors, but not from liver andkidney binding sites; the concentration causing 50% inhibition of specific [3H]diazepam binding (IC50) was 4-7 nM compared to ca. 5 nM for the potent benzodiazepine lorazepam. Specific binding sites for quinuclidinyl benzilate, naloxone, spiroperidol, serotonin, muscimol, and WB 4101 were not affected by IIc. In contrast to benzodiazepines, I1c exhibits "mixed type" competitive inhibition of forebrain benzodiazepine receptors (negative cooperativity). We surmise that an endogenous ligand for benzodiazepine receptors may be a derivative of P-carboline-3-carboxylic acid.[3H]Diazepam and [3H]flunitrazepam bind specifically and with high affinity to brain membranes from all higher vertebrates, including man (1-5). Binding is brain specific (6), correlated to pharmacological potency (1, 7-10), located on neurons (11-17), distributed unevenly in the brain (6), and slightly sensitive to seizures and stressful conditions (18,19 5.5. Further impurities were removed by washing twice with 0.05 vol of borate buffer (100 mM, pH 9) and three times with 0.05 vol of aqueous NH3 (100 mM). All aqueous phases were washed twice in counter current fashion with ethyl acetate to decrease losses. The combined ethyl acetate phase was evaporated to dryness and the residue was dissolved in 300 ml of 0.18-0.25 M HCI and washed with 500 ml of diethyl ether, which removed large amounts of impurities. The active fraction was then extracted twice with 500 ml of diethyl ether at pH 5-5.5. After evaporation the brown-orange waxy residue was dissolved in 3 ml of 50% ethanol and chromatographed on a Sephadex LH-20 column [1.5 X 84 cm, elution volume (Ve) = 144 ml] in 50% ethanol. The active fraction (y-fraction) was eluted in a volume of [15][16][17][18][19][20][21][22][23][24][25]