AUTORADIOGRAPHIC EVIDENCE THAT THE GABA(A) RECEPTOR ANTAGONIST SR-95531 IS A SUBSTRATE INHIBITOR OF MAO-A IN THE RAT AND HUMAN LOCUS-COERULEUS

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 unde r the inhibitory control of gamma-aminobutyric acid (GABA) via ionotro pic GABA(A) receptors. However, to date, little is known of the recept or binding characteristics of these neurons. In the present investigat ion 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 [H- 3]SR 95531 (gabazine, a GABA(A) receptor antagonist for low affinity G ABA recognition sites). However, at the concentrations used, no bindin g sites in the LC were detectable for the benzodiazepine receptor anta gonist [H-3]flumazenil, the GABA(A) receptor agonist (for high affinit y sites) [H-3]muscimol or the ionophore ligand [S-35]t-butyl bicycloph osphorothionate (TBPS). Unexpectedly, the pharmacological specificity of [H-3]SR 95531 binding to the LC differed markedly from that to most brain regions (IC50 values for GABA and RU 5135 respectively in the L C were > 10(-2) and 10(-3) M; and, for example, in the dentate gyrus t he most labelled structure after the LC, 8 x 10(-7) and 1.8 x 10(-9) M ). These differences prompted the further characterization of [H-3]SR 95531 binding in the LC, revealing a significant affinity for monoamin e 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 [H-3 ]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 do ses of the MAO-A inhibitors moclobemide and Ro 41 - 1049 (but not the MAO-B inhibitor lazabemide) the in vitro binding of [H-3]SR 95531 was markedly reduced (by 77 and 82% of controls respectively). Moreover, e nzyme radioautography with [H-3]Ro 41 - 1049 revealed that SR 95531 ha s a significant affinity for MAO-A (IC50 values were 10(-5) and 4 x 10 (-6) M in the LC and dentate gyrus respectively) but not for MAO-B ([H -3]lazabemide binding). Altogether, these findings suggest that the hi gh-affinity binding of [H-3]SR 95531 to the LC mainly reflects its aff inity 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 o f LC neurons and facilitation of noradrenergic transmission) can be ex ploited as a principle in the development of new anxiolytics or antide pressants.