Physiological analyses of voltage-gated ion channels and their molecular identifications in sensory neurons.

所有脊椎动物的光感受器，包括人类的光感受器，对光刺激的反应都是渐变的超极化，通常被认为是非尖峰神经元。为了研究人类光感受器是否能引发动作电位，我们使用全细胞膜片钳记录技术研究了手术切除的人类视网膜。在手术过程中，从视网膜脱离患者身上切除一小块（直径小于1mm）视网膜以重新附着视网膜。所有实验均按照《赫尔辛基宣言》的指导原则进行，并经我院伦理委员会批准。我们发现光感受器表达电压门控的Na^+通道，并在膜去极化时产生Na^+动作电位。这表明，钠峰可能是在强光刺激终止时引起的。Na动作电位可以放大强光突然偏移时的神经递质释放，从而选择性地放大光信号的“OFF”部分。我们还研究了人类杆状光感受器是否表达超极化激活阳离子电流（Ih）。超极化诱导的缓慢向内电流通过溶液中应用3 mM的铯离子（Ih的阻滞剂）显著降低。3毫米铯离子的镀液也阻挡了稳定的向内电流。我们的研究结果表明，Ih在明亮照明产生的大超极化期间被激活，并可能改变光电压的波形。人类的视觉非常灵敏，很少有视觉闪烁的现象。在电流箝位条件下，Cs^+、Tl^+和ZD7288阻断h电流，使视锥细胞和视杆细胞的静息电位超极化~ 10 ~ 15 mV，并产生自发动作电位。我们认为h电流可能通过抑制人类视锥细胞和视杆细胞中自发Na^+峰值的产生而有助于防止视觉闪烁。

All vertebrate photoreceptors, including those of humans, respond to light stimuli with a graded hyperpolarization, and are commonly thought to be non-spiking neurons. To examine whether human photoreceptors can elicit action potentials, we studied surgically excised human retina using the whole-cell patch-clamp recording technique. A small piece (diameter less than 1 mm) of retina was excised from patients with retinal detachment during the surgical procedure to re-attach the retina. All experiments were performed in compliance with the guideline of the Declaration of Helsinki and approved by the ethics committee of our institute. We found that photoreceptors express voltage-gated Na^+ channels, and generate Na^+ action potentials in response to membrane de polarization. This suggests that Na spikes might be elicited at the termination of a bright light stimulus. The Na action potentials would serve to amplify the release of neurotransmitter at the sudden offset of a bright light, and thus selectively amplify the "OFF" part of the light signal.We also examined whether human rod photoreceptors express hyperpolarization-activated cationic currents (Ih). The slow inward current induced by hyperpolarization was markedly reduced by bath application of 3 mM Cs^+, a blocker of Ih,. Bath application of 3 mM Cs^+ also blocked the steady inward current. Our findings suggest that Ih becomes activated during large hyperpolarizations generated by bright illumination and may modify the waveform of the photovoltage.The sense of vision in humans is quite robust and visual flickering is rarely experienced. Under current-clamp conditions, blockage by Cs^+, Tl^+ and ZD7288 of an h current hyperpolarized the resting potentials of cones and rods by 〜10-15 mV, and surprisingly generated spontaneous action potentials. We suggest that the h current may contribute to preventing visual flickering by inhibiting the generation of spontaneous Na^+ spikes in human cones and rods.