Udin S B, Grant S
Department of Physiology and Biophysics, State University of New York, Buffalo 14214, USA.
Prog Neurobiol. 1999 Oct;59(2):81-106. doi: 10.1016/s0301-0082(98)00096-3.
Xenopus frogs exhibit dramatic changes in the binocular projections to the tectum during a critical period of development. Their eyes change position in the head, moving from lateral to dorsal and creating an increasing region of binocular overlap. There is a corresponding shift of binocular projections to the tectum that keeps the two eyes' maps in register with each other throughout this period. The ipsilateral input is relayed via the nucleus isthmi. Two factors bring the ipsilateral projection into register with the contralateral projection. First, chemoaffinity cues establish a crude topographic map beginning when the shift of eye position begins. Approximately 1 month later, visual cues bring the ipsilateral map into register with the contralateral map. The role of visual input is demonstrated by the ability of the axons that bring the ipsilateral eye's map to the tectum to reorganize in response to a surgical rotation of one eye and to come into register with the contralateral eye's map. This plasticity can be blocked by NMDA receptor antagonists during the critical period. In normal adults, reorganization is minimal. Eye rotation fails to induce reorganization of the ipsilateral map. However, plasticity persists indefinitely in animals that are reared in the dark, and plasticity can be restored in normally-reared animals by treatment with NMDA. The working model to explain this plasticity posits that correlated input from the two eyes triggers opening of NMDA receptor channels and initiates events that stabilize appropriately-located isthmotectal connections. Specific tests of this model are discussed.
非洲爪蟾在发育的关键时期，其向顶盖的双眼投射会发生显著变化。它们的眼睛在头部的位置发生改变，从侧面移至背部，从而使双眼重叠区域不断增加。与此同时，向顶盖的双眼投射也相应地发生移位，在此期间始终保持两只眼睛的图谱相互对齐。同侧输入通过峡核进行中继。有两个因素使同侧投射与对侧投射对齐。首先，化学亲和性线索在眼睛位置开始移动时就建立起一个粗略的地形图。大约1个月后，视觉线索使同侧图谱与对侧图谱对齐。将同侧眼的图谱带到顶盖的轴突能够响应一只眼睛的手术旋转而重新组织，并与对侧眼的图谱对齐，这证明了视觉输入的作用。在关键时期，这种可塑性可被NMDA受体拮抗剂阻断。在正常成年动物中，重组极少发生。眼睛旋转不会诱导同侧图谱的重组。然而，在黑暗中饲养的动物中，可塑性会无限期持续，并且通过用NMDA处理，正常饲养的动物也可恢复可塑性。解释这种可塑性的工作模型假定，来自两只眼睛的相关输入会触发NMDA受体通道的开放，并启动一些事件来稳定位置适当的峡核 - 顶盖连接。文中还讨论了对该模型的具体测试。
