PLoS ONE：候鸟能用眼睛“看”方向

xielirong

候鸟长途迁徙中如何准确辨认出方向？德国科学家近日研究证实，候鸟眼中的分子与大脑引导飞行方向的区域有关。由此可以说，候鸟是用眼睛“看”地球磁场，辨认出正确的方向。相关研究论文9月26日发表在《公共科学图书馆·综合》（PLoS ONE）上。  

  在最新研究中，德国奥尔登堡大学的Dominik Heyers和同事分别向园莺（Sylvia borin）的前脑区和视网膜中注射了两种不同的“追踪器”（tracer）。该前脑区称为Cluster N，被认为是在鸟类确定方向时唯一有活性的区域。注射的追踪器能够与神经信号一道，沿着神经纤维移动。

  结果发现，在园莺经历过迁徙的渴望后，这两种不同的追踪器都停在了丘脑负责视觉的区域。研究人员表示，这种解剖学上的联系强有力地表明，候鸟很可能是用视觉感受到磁场的。  

  此前的德国马克斯普朗克鸟类研究中心的Claudia Mettke-Hofmann和Eberhard Gwinner追踪了超过一百只的花园莺（Sylvia borin）和萨丁尼亚莺（Sylvia melanocephala momus），并且利用解剖学方法研究发现鸟儿们在路途中增长不少见识。花园莺从非洲返回中欧时就带着个较大的海马回－－脑中处理空间学习的区域。而非迁徒性的萨丁尼亚莺则无该变化。然而迁徒能增长鸟儿记忆力的证据还是缺乏的。之前的研究表明，候鸟眼中的某种蛋白分子——蓝光受体（cryptochromes）可能在确定飞行方向中发挥了重要作用。此次研究为此提供了支持。Heyers表示，当候鸟专注于某一个方向时，磁北（magnetic north）看起来就像一个黑点。  

  英国牛津大学研究鸟类迁徙的Miriam Liedvogel认为，此次研究很好地显示了视网膜与Cluster N之间的联系。不过她并不认为据此就能证明鸟类能够“看见”磁场。她表示，希望将来有实验能够证明，改变磁场会使鸟类丘脑的神经活动发生变化。  

  此次研究也并不意味着鸟类飞行方向研究的终结，其他一些工作已经表明，候鸟的喙也与它们的飞行方向有关。Heyers认为，喙和视觉可能是互为补充的，喙用来测量磁场的强度，而蓝光受体则发挥着指南针的作用。   

原始出处:

A Visual Pathway Links Brain Structures Active during Magnetic Compass Orientation in Migratory Birds

Dominik Heyers1*, Martina Manns2, Harald Luksch3, Onur Güntürkün2, Henrik Mouritsen1

1 AG Neurosensorik, Institute of Biology, University of Oldenburg, Oldenburg, Germany, 2 Department of Biopsychology, Institute for Cognitive Neuroscience, Ruhr-University Bochum, Bochum, Germany, 3 Chair of Zoology, Department of Zoology, Technical University Munich, Freising-Weihenstephan, Germany

Abstract
The magnetic compass of migratory birds has been suggested to be light-dependent. Retinal cryptochrome-expressing neurons and a forebrain region, “Cluster N”, show high neuronal activity when night-migratory songbirds perform magnetic compass orientation. By combining neuronal tracing with behavioral experiments leading to sensory-driven gene expression of the neuronal activity marker ZENK during magnetic compass orientation, we demonstrate a functional neuronal connection between the retinal neurons and Cluster N via the visual thalamus. Thus, the two areas of the central nervous system being most active during magnetic compass orientation are part of an ascending visual processing stream, the thalamofugal pathway. Furthermore, Cluster N seems to be a specialized part of the visual wulst. These findings strongly support the hypothesis that migratory birds use their visual system to perceive the reference compass direction of the geomagnetic field and that migratory birds “see” the reference compass direction provided by the geomagnetic field.

Received: April 10, 2007; Accepted: September 5, 2007; Published: September 26, 2007



Figure 1. Neuronal tracing reveals that Cluster N receives input through the thalamofugal visual pathway.

A: Schematic side view of the bird's brain indicating the locations of tracer application. Retrograde tracer (BDA, shown in green) was iontophoretically applied into Cluster N (shown in magenta). Anterograde tracer (CtB, shown in red) was injected into the vitreous of the contralateral eye. B: Double-labeling of ZENK and the retrograde tracer BDA in sagittal brain sections at the level of Cluster N proves the correct placement of tracer into Cluster N: arrows point to examples of neurons displaying ZENK-immunoreactivity (shown in magenta) in the nucleus together with BDA (shown in green) in the somata. Scale bar: 25 祄. C: Tracer distribution in frontal brain sections at the level of the thalamic Gld. Anterogradely labeled fibers from the retina (shown in red) project upon all substructures of the Gld, i.e. LdOPT, SpRt and lateral/ventral parts of the DLL. Retrogradely labeled neurons projecting upon Cluster N (visualised green) mainly originate within the DLL, with few additional connections from the LdOPT and SpRt. Scale bar: 50 祄. D: Confocal 3D-stacks in the thalamic Gld at high magnification indicate direct contact (arrows) between retinofugal fibers (shown in red) and somata/proximal dendrites retrogradely labeled from Cluster N (shown in green). Scale bar: 4 祄. Abbreviations: DLL, Nucleus dorsolateralis anterior thalami, pars lateralis; Gld, dorsolateral geniculate complex; LdOPT, Nucleus lateralis dorsalis nuclei optici principalis thalami; Rt, Nucleus rotundus; SPC, Nervus superficialis parvocellularis; SpRt, Nucleus suprarotundus; TSM, Tractus septomesencephalicus.