Heterogeneity of ?-Aminobutyric Acid/Benzodiazepine/?-Carboline Receptor Complex in Rat Spinal Cord

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Abstract
The properties of muscimol, .beta.-carboline (BC), and benzodiazepine (BZD) binding to crude synaptic membranes were studied in the spinal cord and cerebellum of rats. In cerebellar membranes, the density of high-affinity [3H]muscimol and [3H]6,7-dimethoxy-4-ethyl-.beta.-carboline ([3H]BCCM) binding sites is almost identical to that of [3H]flunitrazepam ([3H]FLU) or [3H]flumazenil (Ro15-1788; ethyl-8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazol[1,5-a][1-4]benzodiazepine-3-carboxylate). In contrast to the cerebellum, the number of muscimol and BC binding sites in rat spinal cord is .apprx. 20-25% of the number of FLU or flumazenil binding sites. Moreover, in spinal cord membranes, BC recognition site ligands displace [3H]-flumazenil bound to those sites, with low affinity and a Hill slope significantly <1; the potency of the different BCs in displacing [3H]flumazenil is 25-50-fold lower in the spinal cord than in the cerebellum. [3H]Flumazenil is not displaced from spinal cord membranes by the peripheral BZD ligand Ro 5-4864 (4''-chlorodiazepam), whereas it is displaced with low affinity and a Hill slope of <1 (nH = 0.4) by CL 218,872 {3-methyl-6-(3-trifluoromethylphenyl)-1,2,4-triazolol[4,3-b]pyridazine}. These data suggest that a large number of BZD binding sites in spinal cord (.apprx.80%) are of the central-type, BZD2 subclass, whereas the BZD binding sites in cerebellum are predominately of the central-type, BZD1 subclass. In both cerebellar and spinal cord membranes, micromolar .gamma.-aminobutyric acid (GABA) enhanced the binding of [3H]FLU; however, this effect is less efficacious and less potent in the spinal cord, observations indicating two possibilities: (a) that in spinal cord some of the BZD2 binding sites are not coupled to the GABAA binding sites, or (b) that they are coupled in a GABAA/BZD2 receptor complex containing a large proportion of BZD2 binding sites associated with a relatively small number of GABAA binding sites.