鸟类亮度编码神经元参与光周期同步和半脑睡眠

Vision is the most important one of all sensory perceptions in humans and most animals, and it originates from the spatial and temporal distribution patterns of incident photons on retinal surface. Information about daily changes in ambient luminance is received by the brain through the eyes, resulting in photoentrainment of physiology and behaviors of animals to the day/night cycle, e.g. sleep/wake cycle. Birds are the only class of terrestrial vertebrates that has a sleep with one eye closed, i.e. unihemispheric sleep. Owing to that avian eyes are laterally positioned, the optic nerves completely cross to the contralateral hemisphere, and interocular cartilage is transparent, we would suggest that during unihemispheric sleep, ambient light first stimulates intrinsically photosensitive retinal ganglion cells and then rod/cone cells in the open eye, and the remaining light shines through interocular tissues to activate rod/cone cells in the closed eye, thereby photic information picked up by both eyes would play essential physiological roles via luminance encoding neurons in the brain. The present project would use the pigeon as a model, in which single cell recordings are made from the visual thalamus, the suprachiasmatic nucleus and the pretectal area while individual or both eyes are stimulated with light of various intensities or light/dark cycle stimuli, aiming at revealing the functional significance of luminance encoding neurons in photoentrainment and unihemispheric sleep. A combination of recording neuronal activity with an implanted electrode array and videography of behaviors of animals would provide behavioral significance of these cell-level findings. Preliminary experiments have shown that interocular light transmission would play essential roles in physiology, and the firing activity of luminance encoding neurons could be entrained to external light/dark cycle stimuli，and they can memorize and replay their own entrained firing patterns.

视觉是人和多数动物最重要的感知，它发端于光量子在视网膜上的时空分布图景。昼夜亮度信息通过眼睛进入脑内，促使动物的生理和行为与昼夜节律同步，例如睡眠/觉醒循环。鸟类是唯一单眼闭合即半脑睡眠的陆生脊椎动物。鉴于鸟类双眼侧置，视神经完全交叉，眼间骨透明，我们推测鸟类在半脑睡眠时，环境光首先刺激睁眼视网膜的本体感光神经节细胞，继而激发视杆/视锥细胞，剩余光线透过眼间骨作用于闭眼的视杆/视锥细胞，两眼接收的光能信息通过脑内亮度编码细胞发挥重要的生理作用。本课题以家鸽为模型，用不同强度的光或亮/暗循环分别或同时刺激两眼，在视觉丘脑、视交叉上核和顶盖前区做单细胞记录，以揭示亮度编码神经元在光周期同步和半脑睡眠中的作用；并将埋植电极记录与动物行为相结合进一步为单细胞反应提供行为学意义。预实验表明，眼间透射光有重要生理意义，亮度编码细胞的放电活动能与外界亮/暗周期刺激同步，并记忆和回放光周期同步的放电模式。

研究工作主要探讨鸟类脑内不同脑区的亮度编码神经元如何参与外界光周期的同步和双眼的瞳孔光反射。.动物使用以前经历的周期性事件的时间信息来指导他们未来的行为。视网膜和皮层已经证实参与了这种行为，但是对于将视觉信息从视网膜传递到皮层的核团-丘脑，我们在很大程度上并不清楚它如何处理周期性的视觉刺激。例如，外界周期性亮/暗光刺激的时间间隔是如何在丘脑被提取和编码。在麻醉家鸽上，我们发现，在背外侧丘脑的DLA核团中（the nucleus dorsolateralis anterior thalami）的亮度编码细胞的反应和周期性视觉刺激的亮暗变化同步，视觉刺激周期的时间间隔在数秒到数分钟。在经过多个周期的训练后，在稳定光照下，细胞仍然维持节律性的反应并且持续性的报告周期性事件要发生的时间。家鸽DLA的亮度细胞编码周期性视觉刺激时间间隔的能力，不受pallial wulst（相当于哺乳动物皮层）的反馈输入的影响。最后，通过数学模型对此现象的神经机制提出一种可能的解释。相反，虽然顶盖前区（pretectal area）（相当于哺乳动物的olivary pretectal nucleus）的亮暗细胞同样能够维持自身反应和周期性视觉刺激同步，但顶盖前区的亮度编码细胞是无法记忆和重放所经历的周期性视觉刺激。研究表明，大脑中依赖于经验的时间间隔的编码和检测可能并不局限于皮层，而是可能早在丘脑一级出现。.鸟类脑内亮暗编码细胞可能参与双眼的瞳孔光反射。鸟类的丘脑和顶盖前区均存在双眼细胞，即细胞对相对于记录位置同侧，对侧和双侧眼睛给予的光照刺激均有反应。鸟类的松果体可以和视网膜一样感知外界光照变化，当视网膜损伤时，鸟类的日夜节律行为不会受到影响。而我们发现，这些亮暗细胞的光亮反应不受松果体损伤的影响，说明细胞对亮暗的感知来自于视网膜。鸟类双眼间的眼间骨选择性透射波长较长的红光而阻断波长较短的蓝光，行为学实验将进一步证实是否有脑内神经回路参与双眼间的瞳孔光反射。