Franceschetti S, Taverna S, Sancini G, Panzica F, Lombardi R, Avanzini G
Department of Neurophysiology, Istituto Nazionale Neurologico Carlo Besta, Via Celoria 11, 20133 Milano, Italy.
J Physiol. 2000 Oct 15;528 Pt 2(Pt 2):291-304. doi: 10.1111/j.1469-7793.2000.00291.x.
The effect of the protein kinase C (PKC) activator 1-oleoyl-2-acetyl-sn-glycerol (OAG) on TTX-sensitive Na+ currents in neocortical pyramidal neurones was evaluated using voltage-clamp and intracellular current-clamp recordings. In pyramid-shaped dissociated neurones, the addition of OAG to the superfusing medium consistently led to a 30% reduction in the maximal peak amplitude of the transient sodium current (I(Na,T)) evoked from a holding potential of -70 mV. We attributed this inhibitory effect to a significant negative shift of the voltage dependence of steady-state channel inactivation (of approximately 14 mV). The inhibitory effect was completely prevented by hyperpolarising prepulses to potentials that were more negative than -80 mV. A small but significant leftward shift of INa,T activation was also observed, resulting in a slight increase of the currents evoked by test pulses at potentials more negative then -35 mV. In the presence of OAG, the activation of the persistent fraction of the Na+ current (INa,P) evoked by means of slow ramp depolarisations was consistently shifted in the negative direction by 3.9+/-0.5 mV, while the peak amplitude of the current was unaffected. In slice experiments, the OAG perfusion enhanced a subthreshold depolarising rectification affecting the membrane response to the injection of positive current pulses, and thus led the neurones to fire in response to significantly lower depolarising stimuli than those needed under control conditions. This effect was attributed to an OAG-induced enhancement of INa,P, since it was observed in the same range of potentials over which I(Na,P) activates and was completely abolished by TTX. The qualitative firing characteristics of both the intrinsically bursting and regular spiking neurones were unaffected when OAG was added to the physiological perfusing medium, but their firing frequency increased in response to slight suprathreshold depolarisations. The obtained results suggest that physiopathological events working through PKC activation can increase neuronal excitability by directly amplifying the I(Na,P)-dependent subthreshold depolarisation, and that this facilitating effect may override the expected reduction in neuronal excitability deriving from OAG-induced inhibition of the maximal INa, T peak amplitude.
利用电压钳和细胞内电流钳记录技术，评估了蛋白激酶C（PKC）激活剂1-油酰基-2-乙酰基-sn-甘油（OAG）对新皮质锥体神经元中河豚毒素敏感的Na⁺电流的影响。在锥形解离神经元中，向灌流介质中添加OAG始终导致从-70 mV的钳制电位诱发的瞬时钠电流（I(Na,T)）的最大峰值幅度降低30%。我们将这种抑制作用归因于稳态通道失活的电压依赖性显著负移（约14 mV）。通过将预脉冲超极化到比-80 mV更负的电位，可完全防止这种抑制作用。还观察到I(Na,T)激活有小但显著的向左移位，导致在比-35 mV更负的电位下测试脉冲诱发的电流略有增加。在存在OAG的情况下，通过缓慢斜坡去极化诱发的Na⁺电流（I(Na,P)）的持续性部分的激活始终向负方向移位3.9±0.5 mV，而电流的峰值幅度不受影响。在脑片实验中，OAG灌流增强了影响膜对正电流脉冲注入反应的阈下去极化整流，从而使神经元在比对照条件下所需的去极化刺激显著更低时就产生动作电位。这种效应归因于OAG诱导的I(Na,P)增强，因为它在I(Na,P)激活的相同电位范围内观察到，并且被河豚毒素完全消除。当将OAG添加到生理灌流介质中时，内在爆发性神经元和规则发放神经元的定性放电特性均未受影响，但它们的放电频率在轻微阈上刺激时增加。所得结果表明，通过PKC激活起作用的生理病理事件可通过直接放大依赖I(Na,P)的阈下去极化来增加神经元兴奋性，并且这种促进作用可能会抵消因OAG诱导的最大I(Na,T)峰值幅度抑制而预期的神经元兴奋性降低。
