An endogenous substance from porcine brain antagonizes the anticonvulsant effect of diazepam

Copper signalling: causes and consequences

Copper-containing enzymes perform fundamental functions by activating dioxygen (O2) and therefore allowing chemical energy-transfer for aerobic metabolism. The copper-dependence of O2 transport, metabolism and production of signalling molecules are supported by molecular systems that regulate and preserve tightly-bound static and weakly-bound dynamic cellular copper pools. Disruption of the reducing intracellular environment, characterized by glutathione shortage and ambient Cu(II) abundance drives oxidative stress and interferes with the bidirectional, copper-dependent communication between neurons and astrocytes, eventually leading to various brain disease forms. A deeper understanding of of the regulatory effects of copper on neuro-glia coupling via polyamine metabolism may reveal novel copper signalling functions and new directions for therapeutic intervention in brain disorders associated with aberrant copper metabolism.

Nerve endings from rat brain tissue release copper upon depolarization. A possible role in regulating neuronal excitability

Membrane vesicles from rat cerebral cortex were prepared and the functional response of the GABAA receptor was followed by monitoring GABA-activated influx of the radiotracer 36Cl- ion. CuCl2 decreased GABA-activated 36Cl- influx into synaptosomal membrane vesicles. The effect of Cu2+ was concentration dependent (5-500 microM CuCl2) and occurred with saturating (1 mM) as well as low (30 microM) GABA concentrations. A similar inhibition of the responses to muscimol (30 microM) was also observed with 50 microM CuCl2. In addition, release of copper from cortical synaptosomes and median eminence was followed by atomic absorption technique. An increased release of copper into the extracellular space was observed upon depolarization with 50 mM K+. A minimal concentration of copper was estimated to be 100 microM in the synaptic cleft. These findings suggest that copper may play a role in regulating neuronal excitability.

Cu2+ is the active principle of an endogenous substance from porcine cerebral cortex which antagonizes the anticonvulsant effect of diazepam

A boiled extract of porcine cerebral cortex was fractionated on Sephadex G-75 and LH-20 followed by paper chromatography of the active fraction having inhibitory activity towards [3H]GABA binding to rat brain synaptic membranes. A ninhydrin-negative substance migrating more quickly than authentic GABA was identified as a copper-GABA complex. The complex inhibited specific [3H]GABA binding (IC50 approximately equal to 1 microM) and antagonized the anticonvulsant effect exerted by intraamygdaloid injection of diazepam. The effect of a synthetic copper-GABA complex was compared and found to be similar to the endogenous complex. Cu2+ alone has no affinity for the GABA recognition site but antagonizes the anticonvulsant effect of diazepam. Therefore, Cu2+ is suggested to be the pharmacologically active principle of the endogenous substance. Cu2+ does not seem to function via the recognition site of the GABA receptor.

Inhibition of [3H]GABA binding to rat brain synaptic membranes by bicuculline related alkaloids

The binding of 45 bicuculline related phthalideisoquinoline alkaloids to the GABAA receptor was studied using rat brain synaptic membranes prepared both in Tris-HCl and in Tyrode buffers. The IC50 values determined in Tyrode for phthalideisoquinolines are lower (by about one order of magnitude) than and correlate well (r2 = 0.95) with the IC50 data obtained by [3H]GABA displacement in Tris-HCl. Applying Tyrode, the activities of GABA agonists relative to Tris-HCl are decreased. It can be recognized that activities in receptor binding are dependent on the conformations phthalideisoquinolines prefer in solution. On the basis of systematic alterations in the phthalideisoquinoline molecule the main structural elements involved in the binding of phthalideisoquinoline alkaloids appear to be identical with those of GABA agonists, suggesting that the same binding conformation of the GABAA receptor may be implicated for both agonists and antagonists. The opposite shift in relative potencies of agonists and antagonists may be the consequence of an alteration in the "ionic status" rather than that in the conformation of the GABAA receptor.

New endogenous benzodiazepine receptor ligand in human urine: identity with endogenous monoamine oxidase inhibitor?

Normal human urine contains both monoamine oxidase-inhibiting and benzodiazepine receptor-binding material. Each was extracted into ethyl acetate at pH 1 and subjected to high performance liquid chromatography: they ran similarly, showing three major peaks. The correlation coefficient between the pattern of MAO inhibition and inhibition of 3H-flunitrazepam binding to benzodiazepine receptors in the second half of the elution process was 0.78 (p less than 0.001): most UV-absorbing material present was eluted earlier in the run. These results are compatible with, although they do not prove, the hypothesis that the endogenous MAO inhibitor, previously shown to be increased in stress, is also an endogenous inhibitor of 3H-flunitrazepam binding to the benzodiazepine receptor. This material is different from other putative endogenous ligands: it migrates more rapidly than the potent but artefactual beta-carboline-3-carboxylic acid ethyl ester previously isolated from human urine; nor can the effect we have identified derive from harmane, inosine, hypoxanthine or nicotinamide which fail to extract into ethyl acetate at pH 1.

Structure-activity relationships in kynurenine, diazepam and some putative endogenous ligands of the benzodiazepine receptors

Kynurenine, an endogenous cerebral and peripheral neuroactive metabolite of 1-tryptophan, exerts stimulant and convulsant effects in mice, rats and frogs. In mice it (intracerebroventricularly, ICV) antagonized the anticaffeine effect of diazepam and in smaller doses potentiated its sedative action. In rats 1-kynurenine (ICV) potentiated the convulsant action of caffeine. The effect of pentylenetetrazol was not altered in either species. The convulsant effect of 1-kynurenine is the most resistant among various convulsants towards the protective action of diazepam. The structure of 1-kynurenine is similar to benzophenones, metabolites of diazepam, and has four structural fragments common with diazepam. Putative endogenous and non-endogenous ligands of the benzodiazepine receptors have from one to three of these common fragments. Among the antagonists of diazepam exhibiting stimulant and convulsant action ethyl-beta-carboline-3-carboxylate has the same four fragments, Ro 5-3663 and Ro 15-1788 have three and caffeine two. The most striking dissimilarity is a diazo-moiety (N-C-C-N or N-C=C-C=N) absent in the structure of 1-kynurenine. This moiety seems to be the most important for the binding to the benzodiazepine receptors. A role of each fragment and their combinations as well as the stereoconfiguration for the pharmacological activity is considered. It is suggested that 1-kynurenine is a putative endogenous modulator or, less probably, ligand of the benzodiazepine receptor of either type (most probably that which mediates anxiolytic action of benzodiazepines) or a part of this receptor. The benzodiazepine receptor might be a phylogenetically transformed kynurenine receptor. Highly selective antagonism of purines to 1-kynurenine suggests that it can modulate the function of the benzodiazepine receptors via purinergic mechanisms. Stimulant and convulsant action of 1-kynurenine can be related to a moiety of succinic acid (O=C-C-C-C=O) which is typical of quinolinic acid, the strongest endogenous convulsant among kynurenines, and aspartic acid, an excitatory amino acid. 1-Kynurenine is suggested to be an anxiogenic and convulsigenic endogenous factor.