BINDING OF 1,4-BENZODIAZEPINES TO A NOVEL [H-3] RO15-4513 BINDING-SITE IN THE RAT SPINAL-CORD

An alpidem-insensitive benzodiazepine binding site in the rat spinal c ord has recently been identified in our laboratory. We report here the binding of 23 1,4-benzodiazepines to this site using [H-3]Ro15-4513 e thyl-8-azido-6-dihydro-5-methyl-4H-imidazol:1,5-a ][1,4]benzodiazepine -3-carboxylate) in the presence of 65 mu M alpidem (6-chloro-2-(4-chlo rophenyl)-N,N-dipropy midazo[1,2-a]pyridine-3-acetamide). This binding site displays a wide affinity for 1,4-benzodiazepines, most of which show much higher affinity for benzodiazepine receptors in various brai n regions and transfected cell systems. The highest affinity ligands a re: brotizolam thieno[3,2-f][1,2,4]triazolo[4,3-a][1,4]diazepine) (4.3 nM), Ro15-4513 (5.0 nM), Ro42-8773 ro-5-methyl-6H-imidazo[1,5-a][1,4] benzodiazepin-6- one) (5.7 nM), Ro16-6028 (t-butyl xo-9H-imidazo[1,5-a ][1,4]benzodiazepine-1-carboxy- late) (5.9 nM) and triazolam thyl-4H-[ 1,2,4]triazolo[4,3-a][1,4]benzodiazepine) (7.9 nM). The structural fea ture common to these compounds is an imidazo- or triazolo-ring on the 1- and 2-position of the benzodiazepine. However, the presence of this feature does not guarantee high affinity binding as Ro15-1788 -methyl -6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine acid ethyl ester) (100 nM) and Ro23-0364 H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxamide) (360 nM) display much lower affinity for this site. Studies are currently underway to investigate the functional significance of this unusual be nzodiazepine binding site.