Occupation of brain receptors by benzodiazepines and β‐carbolines: Multiple mechanisms and responses

High affinity for the brain benzodiazepine receptor can no longer be considered predictive of benzodiazepine‐like pharmacology in vivo. Studies of 3 H‐diazepam ( 3 H‐DZ) and 3 H‐propyl‐β‐carboline‐3‐carboxylate ( 3 H‐PrCC) binding were performed to investigate how occupation of the benzodiazepine receptor by these two high‐affinity ligands leads to their significantly different pharmacological effects. In mouse hypothalamus, there are four times as many 3 H‐DZ binding sites as 3 H‐PrCC binding sites (B max values = 1,025 and 265 fmol/mg protein, respectively) and both types of site form a distinct anterior‐to‐posterior gradient in this tissue. Physiological Ca +2 concentrations do not regulate 3 H‐ligand binding in Ca +2 ‐depleted whole mouse brain membranes, but in the presence of increasing concentrations of guanosine 5′‐triphosphate (GTP), Ca +2 stimulates 3 H‐DZ binding up to 25%. In contrast, 3 H‐PrCC binding is unaffected by Ca +2 and GTP. Diazepam (DZ) and ethyl β‐carboline‐3‐carboxylate (βCCE) both stimulate 3 H‐muscimol binding to GABA receptors in whole mouse brain membranes (DZ = +57%, βCCE = +27%), and βCCE partially blocks the effect of the benzodiazepine. Moreover, alkyl β‐carboline‐3‐carboxylates prevent a temperature‐induced thermodynamic transition in the benzodiazepine receptor that occurs in the presence of GABA. Studies of 8‐anilino‐1‐naphthalene sulfonic acid fluorescence (ANS) perturbation have also suggested that occupation of the receptor by a benzodiazepine results in a conformational change. Although benzodiazepines and β‐carbolines are high‐affinity ligands for the benzodiazepine receptor, these results suggest that their different pharmacological profiles may result from significant differences in binding mechanisms and receptor responses.