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Radioautographic Evidence that the GABA A Receptor Antagonist SR 95531 is a Substrate Inhibitor of MAO‐A in the Rat and Human Locus Coeruleus
Author(s) -
Luque J. M.,
Erat R.,
Kettler R.,
Cesura A.,
Prada M. Da,
Richards J. G.
Publication year - 1994
Publication title -
european journal of neuroscience
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.346
H-Index - 206
eISSN - 1460-9568
pISSN - 0953-816X
DOI - 10.1111/j.1460-9568.1994.tb00598.x
Subject(s) - locus coeruleus , antagonist , neuroscience , substrate (aquarium) , locus (genetics) , chemistry , pharmacology , receptor , biology , central nervous system , biochemistry , ecology , gene
The locus coeruleus (LC), a major noradrenergic nucleus in the brain, probably has a functional role in the regulation of anxiety states as well as vigilance, attention, learning and memory. LC neurons are under the inhibitory control of γ‐aminobutyric acid (GABA) via ionotropic GABA A receptors. However, to date, little is known of the receptor binding characteristics of these neurons. In the present investigation we therefore examined by receptor radioautography the localization of the binding sites for different components of the GABA A receptor complex in the rat and human LC. Both rat and human LC neurons have a high density of binding sites for the pyridazinyl‐GABA derivative [ 3 H]SR 95531 (gabazine, a GABA A receptor antagonist for low affinity GABA recognition sites). However, at the concentrations used, no binding sites in the LC were detectable for the benzodiazepine receptor antagonist [ 3 H]flumazenil, the GABA A receptor agonist (for high affinity sites) [ 3 H]muscimol or the ionophore ligand [ 35 S] t ‐butyl bicyclophosphorothionate (TBPS). Unexpectedly, the pharmacological specificity of [ 3 H]SR 95531 binding to the LC differed markedly from that to most brain regions (IC 50 values for GABA and RU 5135 respectively in the LC were > 10 ‐2 and 10 ‐3 M; and, for example, in the dentate gyrus the most labelled structure after the LC, 8 × 10 ‐7 and 1.8 × 10 ‐9 M). These differences prompted the further characterization of [ 3 H]SR 95531 binding in the LC, revealing a significant affinity for monoamine oxidase type A (MAO‐A), which is highly concentrated in this nucleus. In a competition binding study, a reduction of up to 25% of the [ 3 H]SR 95531 binding was observed with MAO‐A but not MAO‐B inhibitors, at concentrations which produce maximum but selective enzyme inhibition. Correspondingly, 2 h after the oral administration of supramaximal doses of the MAO‐A inhibitors moclobemide and Ro 41 ‐1049 (but not the MAO‐B inhibitor lazabemide) the in vitro binding of [ 3 H]SR 95531 was markedly reduced (by 77 and 82% of controls respectively). Moreover, enzyme radioautography with [ 3 H]Ro 41 ‐1049 revealed that SR 95531 has a significant affinity for MAO‐A (ICgo values were 10 ‐5 and 4x 10 ‐6 M in the LC and dentate gyrus respectively) but not for MAO‐B ([ 3 H]lazabemide binding). Altogether, these findings suggest that the high‐affinity binding of [ 3 H]SR 95531 to the LC mainly reflects its affinity for MAO‐A, which questions its utility as a selective ligand for low‐affinity GABA recognition sites in the CNS. It remains to be seen whether the dual pharmacological profile of SR 95531 (desinhibition of LC neurons and facilitation of noradrenergic transmission) can be exploited as a principle in the development of new anxiolytics or antidepressants.