Lagrange Andre H, Botzolakis Emmanuel J, Macdonald Robert L
Department of Neurology, Vanderbilt University Medical Centre, 6140 Medical Research Building III, 465 21st Ave, South, Nashville, TN 37232-8552, USA.
J Physiol. 2007 Feb 1;578(Pt 3):655-76. doi: 10.1113/jphysiol.2006.122135. Epub 2006 Nov 23.
Up-regulation of the GABAA receptor alpha4 subunit subtype has been consistently shown in multiple animal models of chronic epilepsy. This isoform is expressed in both thalamus and hippocampus and is likely to play a significant role in regulating corticothalamic and hippocampal rhythms. However, little is known about its physiological properties, thus limiting understanding of the role of alpha4 subtype-containing GABAA receptors in normal and abnormal physiology. We used rapid GABA application to recombinant GABAA receptors expressed in HEK293T cells to compare the macroscopic kinetic properties of alpha4beta3gamma2L receptors to those of the more widely distributed alpha1beta3gamma2L receptors. These receptor currents had similar peak current amplitudes and GABA EC50 values. However, alpha4beta3gamma2L currents activated more slowly when exposed to submaximal GABA concentrations, had more fast desensitization (tau = 15-100 ms), and had less residual current during long GABA applications. In addition, alpha4beta3gamma2L currents deactivated more slowly than alpha1beta3gamma2L currents. Peak currents evoked by repetitive, brief GABA applications were more strongly attenuated for alpha4beta3gamma2L currents than alpha1beta3gamma2L currents. Moreover, the time required to recover from desensitization was prolonged in alpha4beta3gamma2L currents compared to alpha1beta3gamma2L currents. We also found that exposure to prolonged low levels of GABA, similar to those that might be present in the extrasynaptic space, greatly suppressed the response of alpha4beta3gamma2L currents to higher concentrations of GABA, while alpha1beta3gamma2L currents were less affected by exposure to low levels of GABA. Taken together, these data suggest that alpha4beta3gamma2L receptors have unique kinetic properties that limit the range of GABA applications to which they can respond maximally. While similar to alpha1beta3gamma2L receptors in their ability to respond to brief and low frequency synaptic inputs, alpha4beta3gamma2L receptors are less efficacious when exposed to prolonged tonic GABA or during repetitive stimulation, as may occur during learning and seizures.
在多种慢性癫痫动物模型中，一直显示GABAA受体α4亚基亚型上调。该亚型在丘脑和海马中均有表达，可能在调节皮质丘脑和海马节律中起重要作用。然而，对其生理特性了解甚少，因此限制了对含α4亚型的GABAA受体在正常和异常生理学中作用的理解。我们通过快速施加GABA于在HEK293T细胞中表达的重组GABAA受体，比较α4β3γ2L受体与分布更广泛的α1β3γ2L受体的宏观动力学特性。这些受体电流具有相似的峰值电流幅度和GABA EC50值。然而，当暴露于亚最大GABA浓度时，α4β3γ2L电流激活更慢，具有更多快速脱敏（τ = 15 - 100毫秒），并且在长时间GABA施加期间残留电流更少。此外，α4β3γ2L电流失活比α1β3γ2L电流更慢。重复性短暂施加GABA诱发的峰值电流，α4β3γ2L电流比α1β3γ2L电流衰减更强。而且，与α1β3γ2L电流相比，α4β3γ2L电流从脱敏恢复所需的时间延长。我们还发现，暴露于长时间低水平的GABA（类似于突触外空间中可能存在的水平），极大地抑制了α4β3γ2L电流对更高浓度GABA的反应，而α1β3γ2L电流受低水平GABA暴露的影响较小。综上所述，这些数据表明α4β3γ2L受体具有独特的动力学特性，限制了它们能最大程度响应的GABA应用范围。虽然在对短暂和低频突触输入的反应能力上与α1β3γ2L受体相似，但α4β3γ2L受体在暴露于长时间强直GABA或重复刺激时（如在学习和癫痫发作期间可能发生的情况）效果较差。
