Sernagor E, Grzywacz N M
Department of Child Health, Medical School, University of Newcastle upon Tyne, UK.
Curr Biol. 1996 Nov 1;6(11):1503-8. doi: 10.1016/s0960-9822(96)00755-5.
The role played by early neural activity in shaping retinal functions has not yet been established. In the developing vertebrate retina, ganglion cells fire spontaneous bursts of action potentials before the onset of visual experience. This spontaneous bursting disappears shortly after birth or eye opening. In the present study, we have investigated whether the outgrowth of receptive fields in turtle retinal ganglion cells is affected by early spontaneous bursting or by early visual experience.
Ganglion cells normally stop bursting spontaneously 2-4 weeks post-hatching, the time when receptive-field areas reach adult size. When turtles are reared in the dark, the spontaneous bursting persists. Concomitantly, receptive-field areas expand to more than twice those observed in normal adults. To test whether chronic blockade of spontaneous bursting inhibits the expansion of developing receptive-field areas, we have exposed the retina to curare, a nicotinic cholinergic antagonist, because spontaneous bursting by ganglion cells requires acetylcholine. Curare was released from Elvax, a slow-release polymer that was implanted in the eye. When spontaneous bursting was chronically blocked with curare in hatchlings, dark-induced expansion of receptive fields was abolished. Moreover, receptive fields of ganglion cells exposed to curare in hatchlings reared in normal light and dark cycles were smaller than normal.
These results strongly suggest that early, acetylcholine-dependent spontaneous bursts of activity control the outgrowth of receptive-field areas in retinal ganglion cells. The onset of visual experience induces the disappearance of the immature spontaneous bursts, resulting in the stabilization of receptive-field areas to their mature size.
早期神经活动在塑造视网膜功能中所起的作用尚未明确。在发育中的脊椎动物视网膜中，神经节细胞在视觉经验开始之前就会自发发放动作电位。这种自发发放的现象在出生后或睁眼后不久就会消失。在本研究中，我们探究了龟视网膜神经节细胞感受野的生长是否受早期自发发放或早期视觉经验的影响。
神经节细胞通常在孵化后2 - 4周停止自发发放，此时感受野面积达到成年大小。当龟在黑暗中饲养时，自发发放持续存在。与此同时，感受野面积扩展到正常成年龟的两倍以上。为了测试对自发发放的长期阻断是否会抑制发育中感受野面积的扩展，我们将视网膜暴露于筒箭毒碱（一种烟碱型胆碱能拮抗剂）中，因为神经节细胞的自发发放需要乙酰胆碱。筒箭毒碱由植入眼中的缓释聚合物聚醋酸乙烯酯（Elvax）释放。当用筒箭毒碱对幼龟的自发发放进行长期阻断时，黑暗诱导的感受野扩展被消除。此外，在正常明暗周期饲养的幼龟中，暴露于筒箭毒碱的神经节细胞的感受野比正常的小。
这些结果强烈表明，早期依赖乙酰胆碱的自发活动控制着视网膜神经节细胞感受野面积的生长。视觉经验的开始导致未成熟自发发放的消失，从而使感受野面积稳定到其成熟大小。
