Furukawa Kei, Inoshita Takuma, Kawaguchi Shin-Ya
Department of Biophysics, Graduate School of Science, Kyoto University, Kyoto, Japan.
J Physiol. 2024 Jul 25. doi: 10.1113/JP286668.
All-or-none signalling by action potentials (APs) in neuronal axons is pivotal for the precisely timed and identical size of outputs to multiple distant targets. However, technical limitations with respect to measuring the signalling in small intact axons have hindered the evaluation of high-fidelity signal propagation. Here, using direct recordings from axonal trunks and/or terminals of cerebellar Purkinje cells in slice and culture, we demonstrate that the timing and amplitude of axonal outputs are gradually modulated by cAMP depending on the length of axon. During the propagation in long axon, APs were attenuated and slowed in conduction by cAMP via specifically decreasing axonal Na currents. Consequently, the Ca influx and transmitter release at distal boutons are reduced by cAMP, counteracting its direct facilitating effect on release machinery as observed at various CNS synapses. Together, our tour de force functional dissection has unveiled the axonal distance-dependent graded control of output timing and strength by intracellular signalling. KEY POINTS: The information processing in the nervous system has been classically thought to rely on the axonal faithful and high-speed conduction of action potentials (APs). We demonstrate that the strength and timing of axonal outputs are weakened and delayed, respectively, by cytoplasmic cAMP depending on the axonal length in cerebellar Purkinje cells (PCs). Direct axonal patch clamp recordings uncovered axon-specific attenuation of APs by cAMP through reduction of axonal Na currents. cAMP directly augments transmitter release at PC terminals without changing presynaptic Ca influx or readily releasable pool of vesicles, although the extent is weaker compared to other CNS synapses. Two opposite actions of cAMP on PC axons, AP attenuation and release augmentation, together give rise to graded control of synaptic outputs in a manner dependent on the axonal length.
神经元轴突中动作电位（APs）的全或无信号传导对于精确计时以及向多个远处靶标输出相同大小的信号至关重要。然而，测量完整小轴突中信号传导的技术限制阻碍了对高保真信号传播的评估。在这里，我们通过对切片和培养物中小脑浦肯野细胞的轴突主干和/或终末进行直接记录，证明轴突输出的时间和幅度会根据轴突长度受到cAMP的逐渐调节。在长轴突中传播时，cAMP通过特异性降低轴突钠电流使动作电位衰减并减慢传导速度。因此，cAMP减少了远端突触小体处的钙内流和递质释放，抵消了其在各种中枢神经系统突触中观察到的对释放机制的直接促进作用。总之，我们的这项功能剖析工作揭示了细胞内信号传导对轴突输出时间和强度的距离依赖性分级控制。要点：经典观点认为神经系统中的信息处理依赖于动作电位（APs）在轴突中的忠实且高速传导。我们证明，在小脑浦肯野细胞（PCs）中，根据轴突长度，细胞质cAMP分别削弱和延迟了轴突输出的强度和时间。直接的轴突膜片钳记录发现，cAMP通过减少轴突钠电流对动作电位产生轴突特异性衰减。尽管与其他中枢神经系统突触相比程度较弱，但cAMP直接增强了PC终末的递质释放，而不改变突触前钙内流或囊泡的易释放池。cAMP对PC轴突的两种相反作用，即动作电位衰减和释放增强，共同以依赖于轴突长度的方式对突触输出产生分级控制。
