Departamento de Biología, Universidad Nacional de Colombia, Bogotá.
J Gen Physiol. 2012 Jan;139(1):19-30. doi: 10.1085/jgp.201110717.
Two types of microvillar photoreceptors in the neural tube of amphioxus, an early chordate, sense light via melanopsin, the same photopigment as in "circadian" light detectors of higher vertebrates. Because in amphioxus melanopsin activates a G(q)/phospholipase C cascade, like phototransduction in arthropods and mollusks, possible commonalities in the photoconductance were investigated. Unlike other microvillar photoreceptors, reversal of the photocurrent can only be attained upon replacement of extracellular Na(+). In addition to Na(+), Ca(2+) is also permeant, as indicated by the fact that (a) in normal ionic conditions the photocurrent remains inward at V(m) > E(Na); (b) in Na-free solution a small residual inward photocurrent persists at V(m) near resting level, provided that Ca is present; and (c) V(rev) exhibits a modest shift with Ca manipulations. The unusual reversal is accounted for by an uncommonly low permeability of the light-dependent channels to K(+), as K only marginally affects the photocurrent amplitude and its reversal. Lanthanum and ruthenium red (RuR), two TRP channel antagonists, reversibly suppress the response to photostimulation of moderate intensity; therefore, the melanopsin-initiated cascade may recruit ion channels of the same family as those of rhabdomeric photoreceptors. With brighter lights, blockage declines, so that both La(3+) and RuR induce a right shift in the sensitivity curve without a reduction of its asymptote. Nonetheless, an effect on the transduction cascade, rather than the channels, was ruled out on the basis of the voltage dependency of the blockade and the lack of effects of intracellular application of the same substances. The mechanisms of action of these antagonists thus entail a state-dependent blockade, with a higher affinity for the channel in the closed conformation. Collectively, the results indicate a kinship of the light-sensitive channels of amphioxus with those of invertebrate rhabdomeric visual cells and support the representation of this lineage of photoreceptors among chordates.
文昌鱼神经管中的两种微绒毛光感受器通过黑视素感受光线，黑视素与高等脊椎动物“生物钟”光探测器中的光色素相同。由于文昌鱼中的黑视素激活 G(q)/磷脂酶 C 级联反应，就像节肢动物和软体动物的光传导一样，因此研究了可能存在的共同光电导。与其他微绒毛光感受器不同，只有在外液 Na(+)被取代的情况下，光电流才能反向。除了 Na(+)，Ca(2+)也可以通透，这一点可以从以下事实中得到证明：(a) 在正常离子条件下，光电流在 V(m) > E(Na)时仍保持内向；(b) 在无 Na 溶液中，只要存在 Ca，V(m)接近静息水平时，仍会持续存在小的残余内向光电流；(c) V(rev)随Ca的变化而适度变化。这种不寻常的反转是由于光依赖性通道对 K(+)的通透性异常低，因为K仅略微影响光电流幅度及其反转。镧和钌红（RuR），两种 TRP 通道拮抗剂，可逆地抑制对中等强度光刺激的反应；因此，黑视素引发的级联可能会募集与视杆状光感受器相同的离子通道。用更亮的光照射时，阻断作用会下降，因此 La(3+)和 RuR 都会导致灵敏度曲线向右移动，而不会降低其渐近值。尽管如此，基于阻断的电压依赖性和相同物质的细胞内应用没有效果，排除了对转导级联的作用而不是通道的作用。因此，这些拮抗剂的作用机制涉及到一种状态依赖性阻断，对关闭构象中的通道具有更高的亲和力。总的来说，这些结果表明文昌鱼的光敏感通道与无脊椎动物视杆状视觉细胞的通道具有亲缘关系，并支持该感光细胞谱系在脊索动物中的存在。
