Binding of [ 3 H]DMCM, a Convulsive Benzodiazepine Ligand, to Rat Brain Membranes: Preliminary Studies

DMCM (methyl 6,7‐dimethoxy‐4‐ethyl‐β‐carbo‐line‐3‐carboxylate) produces convulsions in mice and rats, probably by interacting with benzodiazepine (BZ) receptors. Investigation of specific binding of [ 3 H]DMCM to rat hippocampus and cortex revealed polyphasic saturation curves, indicating a high‐affinity site ( K D = 0.5–0.8 n M ) and a site with lower affinity ( K D = 3–6 n M ). BZ receptor ligands of various chemical classes, but not other agents, displace [ 3 H]DMCM from specific binding sites—indicating that [ 3 H]DMCM binds to BZ receptors in rat brain. The regional distribution of [ 3 H]DMCM binding is complementary to that of the BZ 1 ‐ ‐ selective radioligand [ 3 H]PrCC. Specific binding of [ 3 H]DMCM (0.1 n M ) was reduced by γ‐aminobutyric acid (GABA) receptor agonist to ∼20% of the control value at 37°C in chloride‐containing buffers; the reduction was bicuculline methiodideand RU 5135‐sensitive. The effective concentrations of 10 GABA analogues in reducing [ 3 H]DMCM binding correlated closely to published values for their GABA receptor affinity. Specific binding of [ 3 H]DMCM is regulated by unknown factors; e.g., enhanced binding was found by Ag + treatment of membranes, in the presence of picrotoxinin, or by exposure to ultraviolet light in the presence of flunitrazepam. In conclusion, [ 3 H]DMCM appears to bind to high‐affinity brain BZ receptors, although the binding properties are different from those of [ 3 H]flunitrazepam and [ 3 H]PrCC. These differences might relate in part to subclass selectivity and in part to differences in efficacy of DMCM at BZ receptors.