Michael H. Cain

Binding of [3H]ethyl-β-carboline-3-carboxylate to brain benzodiazepine receptors: Effect of drugs and anions

Ethyl&carboline-3-carboxylate (P-CCE) has been isolated from brain and urine extracts and found to potently inhibit the binding of [3H]benzodiazepines to specific receptor sites in the brain [ 11. Although /3-CCE (or a closely related derivative) was initially postulated to be an endogenous ligand of the benzodiazepine receptor [ 11, subsequent studies strongly suggest [2] that this compound (as well as its methyl ester and 3-carboxylic acid derivatives) is formed artifactually during the extraction and isolation procedures. Nonetheless, the extremely high affinity of @-CCE and related compounds in displacing [3H]benzodiazepines from receptor sites in the central nervous system [3,4] coupled with recent reports that P-CCE and related /3-carbolines are specific antagonists of many of the pharmacologic actions of benzodiazepines [S91, suggests that these compounds may be valuable tools for studying the regulation of the benzodiazepine receptor. cimol) markedly enhanced the apparent affinity of [3H]benzodiazepines for the benzodiazepine receptor [ 111. These observations prompted us to examine the effect of other drugs and anions known to enhance the apparent affinity of [3H]benzodiazepines for benzodiazepine receptors on the binding of /3-[3H]CCE. We now report that in contrast to the changes in affinity of [ 3H] benzodiazepines elicited by halide ions [ 121, barbiturates [ 13151, and pyrazolopyridines [16,17], the apparent affinity of p-[3H]CCE is unaffected by these agents. Furthermore, Scatchard analysis of /3-[3H]CCE binding to cerebral cortical and cerebellar membranes revealed a significantly greater number of binding sites than was observed with either [3H]diazepam or [3H]flunitrazepam, suggesting that at low concentrations benzodiazepines selectively label a subpopulation of the receptors labelled with fl[3H] CCE. Alternatively, p[ 3H] CCE may bind to sites that are distinct from those labelled with [3H]benzodiazepines.