RESUMO
Persistent activation of GABAA receptors triggers compensatory changes in receptor function that are relevant to physiological, pathological and pharmacological conditions. Chronic treatment of cultured neurons with GABA for 48h has been shown to produce a down-regulation of receptor number and an uncoupling of GABA/benzodiazepine site interactions with a half-time of 24-25h. Down-regulation is the result of a transcriptional repression of GABAA receptor subunit genes and depends on activation of L-type voltage-gated calcium channels. The mechanism of this uncoupling is currently unknown. We have previously demonstrated that a single brief exposure of rat primary neocortical cultures to GABA for 5-10min (t½=3min) initiates a process that results in uncoupling hours later (t½=12h) without a change in receptor number. Uncoupling is contingent upon GABAA receptor activation and independent of voltage-gated calcium influx. This process is accompanied by a selective decrease in subunit mRNA levels. Here, we report that the brief GABA exposure induces a decrease in the percentage of α3-containing receptors, a receptor subtype that exhibits a high degree of coupling between GABA and benzodiazepine binding sites. Initiation of GABA-induced uncoupling is prevented by co-incubation of GABA with high concentrations of sucrose suggesting that it is dependent on a receptor internalization step. Moreover, results from immunocytochemical and biochemical experiments indicate that GABA exposure causes an increase in GABAA receptor endocytosis. Together, these data suggest that the uncoupling mechanism involves an initial increase in receptor internalization followed by activation of a signaling cascade that leads to selective changes in receptor subunit levels. These changes might result in the assembly of receptors with altered subunit compositions that display a lower degree of coupling between GABA and benzodiazepine sites. Uncoupling might represent a homeostatic mechanism that negatively regulates GABAergic transmission under physiological conditions in which synaptic GABAA receptors are transiently activated for several minutes.
Assuntos
Benzodiazepinas/farmacologia , Neurônios/efeitos dos fármacos , Receptores de GABA-A/química , Receptores de GABA-A/metabolismo , Ácido gama-Aminobutírico/farmacologia , Animais , Benzodiazepinas/metabolismo , Sítios de Ligação/efeitos dos fármacos , Células Cultivadas , Córtex Cerebral/citologia , Embrião de Mamíferos , Flunitrazepam/farmacocinética , Antagonistas GABAérgicos/farmacologia , Moduladores GABAérgicos/farmacocinética , Regulação da Expressão Gênica/efeitos dos fármacos , Picrotoxina/farmacologia , Ligação Proteica/efeitos dos fármacos , Transporte Proteico/efeitos dos fármacos , Ratos , Ratos Sprague-Dawley , Receptores de GABA-A/genética , Fatores de Tempo , Trítio/farmacocinéticaRESUMO
Several studies have shown that sleep deprivation produces deficits in learning tasks, but mechanisms underlying these effects remain unclear. Other lines of evidence indicate an involvement of brain GABA systems in cognitive processes. Here, we investigated the possibility that alterations in GABA(A) or benzodiazepine (BDZ) receptor binding might underlie avoidance deficits induced by sleep deprivation. Rats were deprived of sleep for 96 h using the platform method and then trained in a step-through inhibitory avoidance task, or allowed to recover sleep for 24 h before training (sleep rebound group). Thirty minutes after training, animals were given a retention test. Both sleep-deprived and sleep-recovered animals showed a significant impairment in avoidance responding compared to cage controls, and the sleep-deprived group performed significant worse than the sleep-recovered group. A separate group of animals was sacrificed either immediately after 96 h of sleep deprivation or after 96 h of sleep deprivation followed by 24 h of sleep recovery. [(3)H]muscimol and [(3)H]flunitrazepam binding were examined by quantitative autoradiography in 42 brain regions, including areas involved in cognitive processes. No significant differences among groups were found in any brain region, except for a reduction in [(3)H]flunitrazepam binding in the frontal cortex of sleep-recovered animals. These results confirm the deleterious effects of sleep loss on inhibitory avoidance learning, but suggest that such deficits cannot be attributed to altered GABA(A) or BDZ binding in brain.
Assuntos
Flunitrazepam/farmacocinética , Agonistas GABAérgicos/farmacocinética , Moduladores GABAérgicos/farmacocinética , Deficiências da Aprendizagem/metabolismo , Deficiências da Aprendizagem/psicologia , Muscimol/farmacocinética , Privação do Sono/metabolismo , Privação do Sono/psicologia , Animais , Autorradiografia , Aprendizagem da Esquiva , Encéfalo/metabolismo , Deficiências da Aprendizagem/etiologia , Masculino , Ratos , Ratos WistarRESUMO
We have recently reported the presence of the anxiolytic flavone 6-methylapigenin (MA) and of the sedative and sleep-enhancing flavanone glycoside 2S (-) hesperidin (HN) in Valeriana officinalis and Valeriana wallichii. MA, in turn, was able to potentiate the sleep-inducing properties of HN. The present paper reports the identification in V. officinalis of the flavone glycoside linarin (LN) and the discovery that it has, like HN, sedative and sleep-enhancing properties that are potentiated by simultaneous administration of valerenic acid (VA). These effects should be taken into account when considering the pharmacological actions of valeriana extracts.