RESUMO
3-tert-Butyl-7-(5-methylisoxazol-3-yl)-2-(1-methyl-1H-1,2,4-triazol-5-ylmethoxy)-pyrazolo[1,5-d][1,2,4]triazine (MRK-016) is a pyrazolotriazine with an affinity of between 0.8 and 1.5 nM for the benzodiazepine binding site of native rat brain and recombinant human alpha1-, alpha2-, alpha3-, and alpha5-containing GABA(A) receptors. It has inverse agonist efficacy selective for the alpha5 subtype, and this alpha5 inverse agonism is greater than that of the prototypic alpha5-selective compound 3-(5-methylisoxazol-3-yl)-6-[(1-methyl-1,2,3-triazol-4-hdyl)methyloxy]-1,2,4-triazolo[3,4-a]phthalazine (alpha5IA). Consistent with its greater alpha5 inverse agonism, MRK-016 increased long-term potentiation in mouse hippocampal slices to a greater extent than alpha5IA. MRK-016 gave good receptor occupancy after oral dosing in rats, with the dose required to produce 50% occupancy being 0.39 mg/kg and a corresponding rat plasma EC(50) value of 15 ng/ml that was similar to the rhesus monkey plasma EC(50) value of 21 ng/ml obtained using [(11)C]flumazenil positron emission tomography. In normal rats, MRK-016 enhanced cognitive performance in the delayed matching-to-position version of the Morris water maze but was not anxiogenic, and in mice it was not proconvulsant and did not produce kindling. MRK-016 had a short half-life in rat, dog, and rhesus monkey (0.3-0.5 h) but had a much lower rate of turnover in human compared with rat, dog, or rhesus monkey hepatocytes. Accordingly, in human, MRK-016 had a longer half-life than in preclinical species ( approximately 3.5 h). Although it was well tolerated in young males, with a maximal tolerated single dose of 5 mg corresponding to an estimated occupancy in the region of 75%, MRK-016 was poorly tolerated in elderly subjects, even at a dose of 0.5 mg, which, along with its variable human pharmacokinetics, precluded its further development.
Assuntos
Agonistas GABAérgicos/farmacologia , Agonistas de Receptores de GABA-A , Isoxazóis/farmacologia , Triazinas/farmacologia , Animais , Ansiedade/psicologia , Comportamento Animal/efeitos dos fármacos , Convulsivantes/farmacologia , Cães , Relação Dose-Resposta a Droga , Estimulação Elétrica , Eletrofisiologia , Potenciais Pós-Sinápticos Excitadores/efeitos dos fármacos , Fibroblastos , Flumazenil/metabolismo , Agonistas GABAérgicos/metabolismo , Agonistas GABAérgicos/farmacocinética , Moduladores GABAérgicos/metabolismo , Hepatócitos/metabolismo , Hipocampo/efeitos dos fármacos , Hipocampo/metabolismo , Humanos , Macaca mulatta , Masculino , Aprendizagem em Labirinto/efeitos dos fármacos , Camundongos , Técnicas de Patch-Clamp , Equilíbrio Postural/efeitos dos fármacos , Ratos , Ratos Sprague-Dawley , Receptores de GABA-A/metabolismo , Adulto JovemRESUMO
Benzodiazepines are sedative hypnotics that produce marked anterograde amnesia in humans. These pharmacological properties are thought to result from the potentiation of GABA-A receptor function and subsequent attenuation of long-term potentiation (LTP), however many reports have suggested this is not the case for triazolam. Using electrophysiological recordings in a cell line expressing recombinant GABA-A receptors, we confirm that triazolam is an efficacious positive allosteric modulator of GABA-A receptors. Triazolam also slowed the decay of spontaneous inhibitory synaptic currents, reduced the amplitude of fEPSPs elicited during a theta burst and reduced the magnitude of LTP in hippocampal CA1 neurones in vitro. These data show that triazolam modifies LTP induction consistent with an enhancement of GABA-A receptor function via activation of the allosteric benzodiazepine-site.
Assuntos
Hipocampo/efeitos dos fármacos , Potenciação de Longa Duração/efeitos dos fármacos , Triazolam/farmacologia , Animais , Relação Dose-Resposta a Droga , Antagonistas de Aminoácidos Excitatórios/farmacologia , Agonistas de Receptores de GABA-A , Hipocampo/fisiologia , Humanos , Potenciação de Longa Duração/fisiologia , Masculino , Camundongos , Camundongos Endogâmicos C57BL , Ratos , Ratos Sprague-Dawley , Receptores de GABA-A/fisiologiaRESUMO
In pursuit of a GABA(A) alpha5-subtype-selective inverse agonist to enhance cognition, a series of 6,7-dihydro-2-benzothiophen-4(5H)-ones has been identified as a novel class of GABA(A) receptor ligands. These thiophenes have higher binding affinity for the GABA(A) alpha5 receptor subtype compared to the GABA(A) alpha1, alpha2, and alpha3 subtypes, and several analogues exhibit high GABA(A) alpha5 receptor inverse agonism. 6,6-Dimethyl-3-(2-hydroxyethyl)thio-1-(thiazol-2-yl)-6,7-dihydro-2-benzothiophen-4(5H)-one (43) has been identified as a full inverse agonist at the GABA(A) alpha5 receptor and is functionally selective over the other major GABA(A) receptor subtypes. 43 readily penetrates into the CNS to give selective occupancy of GABA(A) alpha5 receptors. In addition, 43 enhances cognitive performance in rats in the delayed 'matching-to-place' Morris water maze test-a hippocampal-dependent memory task-without the convulsant or proconvulsant activity associated with nonselective, GABA(A) receptor inverse agonists.
Assuntos
Cognição/efeitos dos fármacos , Agonistas GABAérgicos/síntese química , Nootrópicos/síntese química , Receptores de GABA-A/efeitos dos fármacos , Tiazóis/síntese química , Tiofenos/síntese química , Animais , Encéfalo/metabolismo , Linhagem Celular , Feminino , Agonistas GABAérgicos/efeitos adversos , Agonistas GABAérgicos/farmacologia , Hipocampo/fisiologia , Humanos , Técnicas In Vitro , Ligantes , Masculino , Aprendizagem em Labirinto/efeitos dos fármacos , Memória/efeitos dos fármacos , Camundongos , Nootrópicos/efeitos adversos , Nootrópicos/farmacologia , Oócitos/efeitos dos fármacos , Oócitos/fisiologia , Técnicas de Patch-Clamp , Subunidades Proteicas , Ensaio Radioligante , Ratos , Convulsões/induzido quimicamente , Relação Estrutura-Atividade , Tiazóis/efeitos adversos , Tiazóis/farmacologia , Tiofenos/efeitos adversos , Tiofenos/farmacologia , Xenopus laevisRESUMO
The alpha5 subunit of the GABA(A) receptor is localized mainly to the hippocampus of the mammalian brain. The significance of this rather distinct localization and the function of alpha5-containing GABA(A) receptors has been explored by targeted disruption of the alpha5 gene in mice. The alpha5 -/- mice showed a significantly improved performance in a water maze model of spatial learning, whereas the performance in non-hippocampal-dependent learning and in anxiety tasks were unaltered in comparison with wild-type controls. In the CA1 region of hippocampal brain slices from alpha5 -/- mice, the amplitude of the IPSCs was decreased, and paired-pulse facilitation of field EPSP (fEPSP) amplitudes was enhanced. These data suggest that alpha5-containing GABA(A) receptors play a key role in cognitive processes by controlling a component of synaptic transmission in the CA1 region of the hippocampus.
Assuntos
Hipocampo/fisiologia , Aprendizagem , Memória , Receptores de GABA-A/fisiologia , Transmissão Sináptica , Animais , Aprendizagem da Esquiva , Comportamento Animal , Condutividade Elétrica , Potenciais Pós-Sinápticos Excitadores , Feminino , Cinética , Potenciação de Longa Duração , Masculino , Aprendizagem em Labirinto , Camundongos , Camundongos Knockout , Subunidades Proteicas , Receptores de GABA-A/genética , Ácido gama-Aminobutírico/metabolismoRESUMO
Nonselective inverse agonists at the benzodiazepine binding site on the GABA-A chloride ion channel enhance cognitive performance in animals but cannot be used in the treatment of cognitive disorders because of anxiogenic and convulsant side effects. We have identified a novel series of GABA-A alpha5 receptor ligands during our search for alpha5 receptor inverse agonists as potential cognition enhancers. In particular, 6,6-dimethyl-3-(2-hydroxyethyl)thio-1-(thiazol-2-yl)-6,7-dihydro-2-benzothiophen-4(5H)-one (26) has been identified as a functionally selective GABA-A alpha5 inverse agonist.