title: Heterogeneity of [3H]ethyl beta-carboline-3-carboxylate binding sites and [3H]gamma-aminobutyric acid binding sites.
abstract: Binding properties of [3H]flunitrazepam ([3H]FNZ), [3H]ethyl beta-carboline-3-carboxylate ([3H]beta CCE), [3H]muscimol ([3H]MUSC) and [3H]gamma-aminobutyric acid ([3H]GABA) to bovine cortical membranes and to their Triton extracts were studied. GABA, 1 X 10(-5) M, stimulated [3H]FNZ binding of frozen, thawed and washed membranes by an increase in affinity without alteration of the maximal number of binding sites, and this GABA stimulated [3H]FNZ binding can be inhibited by bicuculline methobromide. Freeze, thaw and wash with Triton X-100 removed the low affinity [3H]MUSC binding sites. The ratios of [3H]FNZ binding sites to [3H]beta CCE binding sites and [3H]MUSC binding sites to [3H]GABA binding sites were always found to be about 1:2 in both membranes and soluble extracts. The fact that [3H]beta CCE, after displaced by clonazepam, can be further displaced by unlabelled beta CCE from its binding sites and that a portion of [3H]beta CCE binding sites can be survived from FNZ-photolysis implied that there are at least two subclasses of beta CCE binding sites, one is sensitive to beta CCE only and the other is sensitive to both beta CCE and benzodiazepines (BZs). [3H]GABA, after displaced by MUSC, can be further displaced by unlabelled GABA from its binding sites. The results also support that there are two subclasses of BZ-related GABA binding sites, one is sensitive to GABA only and the other is sensitive to both GABA and MUSC. Furthermore, the decay rates of [3H]beta CCE and [3H]GABA binding activities exposed to various degree of electron bombardment are identical, which is evident that these two binding sites are believed to be functional associated in a macromolecular complex with molecular mass about 220,000 Mr.
mesh_terms: D000818:Animals; D001667:Binding, Competitive; D002243:Carbolines / Q000378:metabolism*; D002417:Cattle; D002540:Cerebral Cortex / Q000378:metabolism*; D003902:Detergents; D005445:Flunitrazepam / Q000378:metabolism; D007700:Kinetics; D008970:Molecular Weight; D009118:Muscimol / Q000378:metabolism; D010782:Photolysis; D011869:Radioligand Assay; D011963:Receptors, GABA-A / Q000145:classification / Q000378:metabolism; D017981:Receptors, Neurotransmitter / Q000145:classification / Q000378:metabolism*; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
chemical_list: D002243:Carbolines; D003902:Detergents; D011963:Receptors, GABA-A; D017981:Receptors, Neurotransmitter; D009118:Muscimol; D005680:gamma-Aminobutyric Acid; D005445:Flunitrazepam; C024915:beta-carboline-3-carboxylic acid ethyl ester