de Curtis M, Manfridi A, Biella G
Department of Experimental Neurophysiology, Istituto Nazionale Neurologico, 20133 Milan, Italy.
J Neurosci. 1998 Sep 15;18(18):7543-51. doi: 10.1523/JNEUROSCI.18-18-07543.1998.
The mechanisms that control the periodicity of spontaneous epileptiform cortical potentials were investigated in the in vitro isolated guinea pig brain preparation. A brief intracortical application of bicuculline in the piriform cortex induced spontaneous interictal spikes (sISs) that recurred with high periodicity (8.5 +/- 3.1 sec, mean +/- SD). Intracellular recordings from principal neurons showed that the early phase of the inter-sIS period is caused by a GABAb receptor-mediated inhibitory potential. The late component of the interspike period correlated to a slowly decaying depolarization abolished at membrane potentials positive to -32.1 +/- 5.3 mV and was not associated with membrane conductance changes. Specific pharmacological tests excluded the contribution of synaptic and intrinsic conductances to the late inter-sIS interval. Recordings with ion-sensitive electrodes demonstrated that sISs determined both a rapid increase in extracellular K+ concentration (0.5-1 mM) and an extracellular alkalinization (0.05-0.08 pH units) that slowly decayed during the inter-sIS period and returned to control values just before a subsequent sIS was generated. These observations were not congruous with the presence of a silent period, because both extracellular increase in K+ and alkalinization are commonly associated with an increase in neuronal excitability. Extracellular alkalinization could be correlated to an sIS-induced intracellular acidification, a phenomenon that reduces cell coupling by impairing gap junction function. When intracellular acidification was transiently prevented by arterial perfusion with NH4Cl (10-20 mM), spontaneous ictal-like epileptiform discharges were induced. In addition, the gap junction blockers octanol (0.2-2 mM) and 18-alpha-glycyrrethinic acid (20 microM) applied either via the arterial system or locally in the cortex completely and reversibly abolished the sIS. The results reported here suggest that the massive cell discharge associated with an sIS induce a strong inhibition, possibly secondary to a pH-dependent uncoupling of gap junctions, that regulates sIS periodicity.
在体外分离的豚鼠脑标本中研究了控制自发性癫痫样皮质电位周期性的机制。在梨状皮质短暂皮层内应用荷包牡丹碱可诱发自发性发作间期棘波（sISs），其以高周期性（8.5±3.1秒，平均值±标准差）反复出现。对主要神经元的细胞内记录显示，发作间期棘波间期的早期阶段是由GABAb受体介导的抑制电位引起的。棘波间期的后期成分与在膜电位高于-32.1±5.3 mV时消失的缓慢衰减的去极化相关，且与膜电导变化无关。特定的药理学测试排除了突触和内在电导对发作间期棘波后期间隔的影响。用离子敏感电极记录表明，sISs既导致细胞外K+浓度快速升高（0.5 - 1 mM），又导致细胞外碱化（0.05 - 0.08 pH单位），在发作间期棘波间期缓慢衰减，并在随后的sIS产生前恢复到对照值。这些观察结果与静息期的存在不一致，因为细胞外K+增加和碱化通常都与神经元兴奋性增加有关。细胞外碱化可能与sIS诱导的细胞内酸化相关，这一现象通过损害缝隙连接功能来减少细胞间耦合。当用NH4Cl（10 - 20 mM）动脉灌注短暂阻止细胞内酸化时，可诱发自发性癫痫样放电。此外，通过动脉系统或局部在皮层应用缝隙连接阻滞剂辛醇（0.2 - 2 mM）和18-α-甘草次酸（20 μM）可完全且可逆地消除sIS。此处报道的结果表明，与sIS相关的大量细胞放电诱导了一种强烈的抑制作用，可能继发于缝隙连接的pH依赖性解偶联，从而调节sIS的周期性。
