Ionic basis of neuronal hyperexcitability

在本财年，该项目最重要的成就是实施了物理学文献中的控制理论策略，以抑制碱化鱿鱼巨型轴突的节律性放电。鱿鱼轴突是一种强大的起搏器细胞，当细胞内pH升高到7.8或更高时，以25赫兹的频率连续放电数小时。它们在-55 mV有一个所谓的“固定点”，当电池在电压钳条件下保持在该电位时，膜净电流为零。然而，电压钳方法与脑深部刺激无关。此外，不动点是不稳定的。电压钳位关闭后，立即恢复有节奏的触发。我们用一种被称为动态钳位的技术抑制了触发，在这种技术中，电压波形被实时监控，与电压成比例的电流信号被延迟注入电池。延迟的参考点是最近动作电位的最大超调时间。当延迟在未受干扰的起搏器周期的60%点，并且反馈增益处于适当的水平时，放电立即停止。电池接近其固定点，因此反馈电路注入的电流立即降至零，或接近零。由于固定点不稳定，膜电位会偏离固定点，但反馈电路会施加电流以防止膜电位偏离固定点超过几mV。这是一种抑制过度兴奋的机制，一旦放电停止，只需要很少的电流。平坦的基线永远不可能实现，但动作电位的激发就会被消除。我们目前正在研究这些结果与脑深部刺激方法的相关性。

The most significant accomplishment of this project during the current fiscal year has been the implementation of a control theory strategy from the physics literature for suppressing rhythmic firing in alkalinized squid giant axons. Squid axons are robust pacemaker cells firing at 25 Hz for hours on end when the intracellular pH is elevated to 7.8, or higher. They do have what is known as a "fixed point" at -55 mV where the net membrane current is zero when the cell is held at that potential in voltage clamp conditions. However, voltage clamp methodology is not relevant to deep brain stimulation. Moreover, the fixed point is unstable. Rhythmic firing resumes immediately after the voltage clamp is turned off. We have suppressed firing with a technique knows as a dynamic clamp in which the voltage waveform is monitored in real time and a current signal proportional to the voltage is injected back into the cell with a delay. The reference point for the delay is the time of the maximum overshoot of the most recent action potential. When the delay is at the 60% point of the unperturbed pacemaker cycle and the feedback gain is at the appropriate level, firing stops immediately. The cell is close to its fixed point and so the current injected by the feedback circuit immediately drops to zero, or close to zero. The membrane potential moves away from the fixed point since this point is unstable, but the feedback circuit applies current to keep it from moving more than a few mV away from that point. This is a mechanism for suppressing hyperexcitability in which very little current is required once firing has ceased. A flat baseline can never be achieved, but firing of action potentials is eliminated. We are currently investigating the relevance of these results for deep brain stimulation methodologies.