RESUMEN
The behavioral and functional significance of the extrasynaptic inhibitory GABA(A) receptors in the brain is still poorly known. We used a transgenic mouse line expressing the GABA(A) receptor alpha6 subunit gene in the forebrain under the Thy-1.2 promoter (Thy1alpha6) mice ectopically expressing alpha6 subunits especially in the hippocampus to study how extrasynaptically enriched alphabeta(gamma2)-type receptors alter animal behavior and receptor responses. In these mice extrasynaptic alpha6beta receptors make up about 10% of the hippocampal GABA(A) receptors resulting in imbalance between synaptic and extrasynaptic inhibition. The synthetic GABA-site competitive agonist gaboxadol (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol; 3 mg/kg) induced remarkable anxiolytic-like response in the light : dark exploration and elevated plus-maze tests in Thy1alpha6 mice, while being almost inactive in wild-type mice. The transgenic mice also lost quicker and for longer time their righting reflex after 25 mg/kg gaboxadol than wild-type mice. In hippocampal sections of Thy1alpha6 mice, the alpha6beta receptors could be visualized autoradiographically by interactions between gaboxadol and GABA via [(35)S]TBPS binding to the GABA(A) receptor ionophore. Gaboxadol inhibition of the binding could be partially prevented by GABA. Electrophysiology of recombinant GABA(A) receptors revealed that GABA was a partial agonist at alpha6beta3 and alpha6beta3delta receptors, but a full agonist at alpha6beta3gamma2 receptors when compared with gaboxadol. The results suggest strong behavioral effects via selective pharmacological activation of enriched extrasynaptic alphabeta GABA(A) receptors, and the mouse model represents an example of the functional consequences of altered balance between extrasynaptic and synaptic inhibition.
Asunto(s)
Conducta Animal/efectos de los fármacos , Agonistas del GABA/farmacología , Hipocampo/efectos de los fármacos , Hipocampo/metabolismo , Isoxazoles/farmacología , Receptores de GABA-A/metabolismo , Animales , Autorradiografía/métodos , Compuestos Bicíclicos Heterocíclicos con Puentes/farmacología , Adaptación a la Oscuridad/efectos de los fármacos , Relación Dosis-Respuesta a Droga , Conducta Exploratoria/efectos de los fármacos , Femenino , Regulación de la Expresión Génica/efectos de los fármacos , Masculino , Aprendizaje por Laberinto/efectos de los fármacos , Potenciales de la Membrana/efectos de los fármacos , Potenciales de la Membrana/genética , Potenciales de la Membrana/fisiología , Potenciales de la Membrana/efectos de la radiación , Ratones , Ratones Endogámicos C57BL , Ratones Transgénicos , Unión Proteica/efectos de los fármacos , Subunidades de Proteína/metabolismo , Tiempo de Reacción/efectos de los fármacos , Receptores de GABA-A/genética , Reflejo/efectos de los fármacos , Antígenos Thy-1/genética , Antígenos Thy-1/metabolismo , Transfección/métodos , Ácido gamma-Aminobutírico/farmacologíaRESUMEN
Mu opioid receptors are critical for heroin dependence, and A118G SNP of the mu opioid receptor gene (OPRM1) has been linked with heroin abuse. In our population of European Caucasians (n = 118), approximately 90% of 118G allelic carriers were heroin users. Postmortem brain analyses showed the OPRM1 genotype associated with transcription, translation, and processing of the human striatal opioid neuropeptide system. Whereas down-regulation of preproenkephalin and preprodynorphin genes was evident in all heroin users, the effects were exaggerated in 118G subjects and were most prominent for preproenkephalin in the nucleus accumbens shell. Reduced opioid neuropeptide transcription was accompanied by increased dynorphin and enkephalin peptide concentrations exclusively in 118G heroin subjects, suggesting that the peptide processing is associated with the OPRM1 genotype. Abnormal gene expression related to peptide convertase and ubiquitin/proteosome regulation was also evident in heroin users. Taken together, alterations in opioid neuropeptide systems might underlie enhanced opiate abuse vulnerability apparent in 118G individuals.