Malchow R P, Qian H H, Ripps H, Dowling J E
Department of Ophthalmology, University of Illinois College of Medicine, Illinois 60612.
J Gen Physiol. 1990 Jan;95(1):177-98. doi: 10.1085/jgp.95.1.177.
Two morphologically distinct types of horizontal cell have been identified in the all-rod skate retina by light- and electron-microscopy as well as after isolation by enzymatic dissociation. The external horizontal cell is more distally positioned in the retina and has a much larger cell body than does the internal horizontal cell. However, both external and internal horizontal cells extend processes to the photoreceptor terminals where they end as lateral elements adjacent to the synaptic ribbons within the terminal invaginations. Whole-cell voltage-clamp studies on isolated cells similar in appearance to those seen in situ showed that both types displayed five separate voltage-sensitive conductances: a TTX-sensitive sodium conductance, a calcium current, and three potassium-mediated conductances (an anomalous rectifier, a transient outward current resembling an A current, and a delayed rectifier). There was, however, a striking difference between external and internal horizontal cells in the magnitude of the current carried by the anomalous rectifier. Even after compensating for differences in the surface areas of the two cell types, the sustained inward current elicited by hyperpolarizing voltage steps was a significantly greater component of the current profile of external horizontal cells. A difference between external and internal horizontal cells was seen also in the magnitudes of their TEA-sensitive currents; larger currents were usually obtained in recordings from internal horizontal cells. However, the currents through these K+ channels were quite small, the TEA block was often judged to be incomplete, and except for depolarizing potentials greater than or equal to +20 mV (i.e., outside the normal operating range of horizontal cells), this current did not provide a reliable indicator of cell type. The fact that two classes of horizontal cell can be distinguished by their electrophysiological responses, as well as by their morphological appearance and spatial distribution in the retina, suggests that they may play different roles in the processing of visual information within the retina.
通过光学显微镜、电子显微镜以及酶解分离后，在全视杆鳐鱼视网膜中已鉴定出两种形态不同的水平细胞类型。外部水平细胞在视网膜中位置更靠外，其细胞体比内部水平细胞大得多。然而，外部和内部水平细胞都向光感受器终末延伸突起，它们在终末内陷处作为与突触带相邻的侧向元件终止。对外观与原位观察到的细胞相似的分离细胞进行的全细胞电压钳研究表明，两种类型的细胞都表现出五种独立的电压敏感电导：一种对河豚毒素（TTX）敏感的钠电导、一种钙电流以及三种钾介导的电导（一种反常整流器、一种类似于A电流的瞬时外向电流和一种延迟整流器）。然而，外部和内部水平细胞在由反常整流器携带的电流大小上存在显著差异。即使在补偿了两种细胞类型表面积的差异之后，超极化电压阶跃引发的持续内向电流在外部水平细胞的电流波形中仍是一个显著更大的成分。外部和内部水平细胞在其对四乙铵（TEA）敏感的电流大小上也存在差异；通常在内部水平细胞的记录中获得更大的电流。然而，通过这些钾通道产生的电流相当小，TEA阻断通常被判定为不完全，并且除了大于或等于 +20 mV的去极化电位（即超出水平细胞的正常工作范围）外，这种电流并不能可靠地指示细胞类型。两类水平细胞可通过其电生理反应以及形态外观和在视网膜中的空间分布来区分，这一事实表明它们在视网膜内视觉信息处理中可能发挥不同作用。
