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Characterization of [ 3 H]‐imidazenil binding to rat brain membranes
Author(s) -
Lipartiti Maria,
Arban Roberto,
Fadda Emanuela,
Zanotti Adriano,
Giusti Pietro
Publication year - 1995
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1995.tb13328.x
Subject(s) - picrotoxin , binding site , chemistry , population , dissociation constant , flumazenil , gabaa receptor , membrane , ligand (biochemistry) , cerebellum , receptor , batrachotoxin , biophysics , stereochemistry , biochemistry , endocrinology , biology , sodium , sodium channel , demography , organic chemistry , sociology
1 The binding of [ 3 H]‐imidazenil, an imidazobenzodiazepine carboxamide, to rat cerebellar membranes was characterized at different temperatures. 2 Specific binding was linear with tissue concentrations and reached maximum after 90, 30 and 5 min incubation at 0, 21 and 37°, respectively. The binding was of high affinity, specific and saturable; non linear regression and Scatchard analysis of the data was compatible with the presence of a single population of receptor sites with B max of 0.74 ± 0.020, 0.90 ± 0.011 and 1.0 ± 0.036 pmol mg −1 protein at 0, 21 and 27°, respectively. Binding affinity decreased with increasing temperature: K d were 0.29 ± 0.051 nM (0°), 1.0 ± 0.080 nM (21°) and 2.4 ± 0.38 nM (37°). 3 At all tested temperatures, [ 3 H]‐imidazenil binding was reversible and the K d calculated from the dissociation and association rate constants approximated the equilibrium K d . 4 In the presence of γ‐aminobutyric acid (GABA), K d increased 4 fold at 0°, whereas B max increased, albeit slightly, at all temperatures. 5 Benzodiazepines (BZDs), imidazopyridines and methyl‐β‐carboline‐3‐carboxylate (βCCM) were effective inhibitors of [ 3 H]‐imidazenil binding. Conversely, GABA A antagonists, barbiturates, picrotoxin and peripheral BZD receptor ligands were devoid of any activity. 6 Comparing [ 3 H]‐imidazenil to [ 3 H]‐flumazenil binding in various brain areas, similar densities of recognition sites as well as like regional differences in the distribution of binding sites for both radioligands were observed (cortex=striatum>cerebellum>spinal cord). 7 The present results indicate that [ 3 H]‐imidazenil specifically binds to the BZD sites of GABA A receptors. Furthermore, the effects of GABA and temperature differentiate imidazenil from classical BZDs. It is suggested that the characteristics of imidazenil binding may be relevant to the in vivo pharmacology of the drug.