Cervetto L, Demontis G C, Gargini C
Dipartimento di Fisiologia e Biochimica G. Moruzzi, Università di Pisa, Pisa, Italy.
Br J Pharmacol. 2007 Feb;150(4):383-90. doi: 10.1038/sj.bjp.0706998. Epub 2007 Jan 8.
Visual sensations evoked by stimuli other than luminance changes are called phosphenes. Phosphenes may be an early symptom in a variety of diseases of the retina or of the visual pathways, but healthy individuals may perceive them as well. Phosphene-like phenomena are perhaps the most common side effect reported in clinical pharmacology. Ivabradine, a novel anti-anginal drug that reduces heart-rate by inhibiting the hyperpolarization activated current expressed in cardiac sinoatrial node cells (I(f)) induces phosphenes in some patients. One hypothesis is that ivabradine interacts with the visual system by inhibiting hyperpolarization-activated current in retinal cells (Ih). An Ih current with properties similar to cardiac I(f) has been reported in retinal neurones. Under normal circumstances most of the random fluctuations generated within the retinal circuits do not reach the level of conscious perception because they are filtered out. Presumably, filtering occurs mostly within the retina and one serious candidate for this action is the ability of Ih to act as a negative-feedback mechanism. Ih activation in the membrane of visual cells causes dampening of responses to slow noisy inputs thus tuning the visual system to perceptually more relevant signals of higher frequency. Ih inhibition, by altering at the retinal synapses the filtering of signals generated by thermal breakdown of rhodopsin or other fluctuations, is expected to increase the probability of phosphene occurrence. It is the purpose of the present paper to outline and discuss the features of the visual system and the pharmacological conditions relevant to phosphene perception.
由亮度变化以外的刺激所诱发的视觉感觉被称为光幻视。光幻视可能是视网膜或视觉通路多种疾病的早期症状，但健康个体也可能会感觉到它们。类光幻视现象或许是临床药理学中报告的最常见的副作用。伊伐布雷定是一种新型抗心绞痛药物，通过抑制心脏窦房结细胞中表达的超极化激活电流（I(f)）来降低心率，在一些患者中会诱发光幻视。一种假说认为，伊伐布雷定通过抑制视网膜细胞中的超极化激活电流（Ih）与视觉系统相互作用。在视网膜神经元中已报道存在具有与心脏I(f)相似特性的Ih电流。在正常情况下，视网膜回路内产生的大多数随机波动不会达到意识感知水平，因为它们被过滤掉了。据推测，过滤主要发生在视网膜内，而这种作用的一个重要候选机制是Ih作为负反馈机制的能力。视觉细胞细胞膜中的Ih激活会减弱对缓慢噪声输入的反应，从而使视觉系统能够感知频率更高、更相关的信号。通过改变视网膜突触处对视紫红质热分解或其他波动产生的信号的过滤，Ih抑制预计会增加光幻视出现的概率。本文的目的是概述和讨论与光幻视感知相关的视觉系统特征和药理学条件。
