Skaliora I, Scobey R P, Chalupa L M
Department of Psychology, University of California, Davis 95616.
J Neurosci. 1993 Jan;13(1):313-23. doi: 10.1523/JNEUROSCI.13-01-00313.1993.
The development of precise retinofugal projections is dependent on activity-mediated events, but as yet nothing is known about the ontogeny of excitable membrane properties in retinal ganglion cells (RGCs). In order to begin to understand how functional maturity is attained in these neurons, whole-cell patch-clamp recordings were obtained from acutely dissociated RGCs of fetal and postnatal timed-pregnant cats. Current-clamp recordings revealed a pronounced developmental increase in the proportion of RGCs capable of generating action potentials. At embryonic day 30 (E30), 5 weeks before birth and during a time when RGCs are still being generated, electrical stimulation elicited spikes in only a third of the cells. None of these neurons were capable of multiple discharges in response to maintained depolarization. The proportion of spiking neurons increased during ontogeny, such that by E55 all RGCs could be induced to generate action potentials, with the majority manifesting repetitive spiking patterns. Application of tetrodotoxin abolished spike activity of all fetal RGCs, indicating that sodium-mediated action potentials are present very early in development. At the same time, voltage-clamp recordings revealed significant ontogenetic modifications in several key properties of the sodium currents (INa). These were (1) a twofold increase in Na current densities; (2) a shift in the voltage dependence of both activation and steady state inactivation: with maturity, sodium currents activate at more negative potentials, while steady state inactivation of INa occurs at less negative potentials; and (3) a decrease in decay time constants of the Na current, at membrane potentials negative to -15 mV. These developmental changes were largely restricted to the period of axon ingrowth (E30-E38), suggesting that maturation of INa is not the limiting factor for the onset of activity-dependent restructuring of retinofugal projections.
精确的视网膜神经纤维投射的发育依赖于活动介导的事件，但目前对于视网膜神经节细胞（RGCs）中可兴奋膜特性的个体发生仍一无所知。为了开始理解这些神经元如何实现功能成熟，我们从胎猫和出生后不同时间点的孕猫急性分离的RGCs中进行了全细胞膜片钳记录。电流钳记录显示，能够产生动作电位的RGCs比例在发育过程中显著增加。在胚胎第30天（E30），即出生前5周且RGCs仍在产生的时期，电刺激仅能使三分之一的细胞产生动作电位。这些神经元中没有一个能够在持续去极化时进行多次放电。在个体发育过程中，产生动作电位的神经元比例增加，以至于到E55时所有RGCs都能被诱导产生动作电位，大多数表现出重复的放电模式。应用河豚毒素消除了所有胎儿RGCs的动作电位活动，表明钠介导的动作电位在发育早期就已存在。与此同时，电压钳记录显示钠电流（INa）的几个关键特性在个体发育过程中发生了显著变化。这些变化包括：（1）钠电流密度增加两倍；（2）激活和稳态失活的电压依赖性发生改变：随着成熟，钠电流在更负的电位下激活，而INa的稳态失活在较不那么负的电位下发生；（3）在膜电位负于 -15 mV时，钠电流的衰减时间常数减小。这些发育变化主要局限于轴突生长时期（E30 - E38），这表明INa的成熟不是视网膜神经纤维投射依赖活动的重组开始的限制因素。
