BINDING OF THE BENZODIAZEPINE LIGAND [H-3] RO-15-1788 TO BRAIN MEMBRANE OF THE SALTWATER FISH MULLUS SURMULETUS

The distribution and the pharmacological properties of the binding of the benzodiazepine receptor antagonist [H-3]-Ro 15-1788 fluoro-3-carbo ethoxy-5,6-dihydro-5-methyl-6-oxo-4H imidazol [1,5-a] 1,4 benzodiazepi ne) were compared in some brain membranes of the saltwater teleost fis h, Mullus surmuletus: only a single population of [H-3]-Ro 15-1788 bin ding sites was detected. The binding was saturable and reversible with a high affinity, revealing a significant population of binding sites (K-d value of 2.1 +/- 0.2 nM and B-max value of 1400-900 fmol mg(-1) o f protein, depending on fish length). The highest concentration of ben zodiazepine recognition sites labelled with [H-3]-Ro 15-1788 was prese nt in the optic lobe and the olfactory bulb and the lowest concentrati on was found in the medulla oblongata, cerebellum and spinal cord. In order to explore behavioural selectivity as a consequence of multiple receptor subtypes, six benzodiazepine receptor ligands, flunitrazepam ydro-1-methyl-7-nitro-2H-1,4-benzodiazepin-2-one), alpidem, (N,N-dipro pyl-6-chloro-2-(4-chlorophenyl)imidazo [1,2-a] pyridine-3-acetamide) z olpidem {N,N,6, trimethyl-2-(4-methyl-phenyl) imidazo [1,2-a] pyridine -3-acetamide hemitartrate}, methyl beta carboline-3-carboxylate (beta CCM), Ro 15-1788 and Ro 5-4864 (4'-chlorodiazepam), were tested in vit ro by binding of [H-3]-Ro 15-1788 to membrane preparations from Variou s brain areas of Mullus surmuletus. Displacement studies showed a simi lar rank order of efficacy of various unlabelled ligands. In all regio ns of the brain and in the spinal cord, GABA potentiate [H-3]-flunitra zepam binding in a similar order, suggesting that the BDZ recognition sites are part of the GABA(A) receptor structure. These results sugges t that central-type benzodiazepine receptors are present in one class of benzodiazepine binding sites in the saltwater teleost fish brain of Mullus surmuletus (type I-like). Here we report initial evidence of h omogeneity of subtypes of central benzodiazepine receptors in the spin al cord of the saltwater teleost fish, Mullus surmuletus. (C) 1997 Els evier Science Ltd. All rights reserved.