没有大脑的生物,也需要睡觉吗?

新发现,欧洲科学人文杂志第一品牌

尽管没有大脑,水母仍能睡觉——这一元祖级睡眠形态的发现对我们人类具有重要的启示意义。


水母晚上都在干什么?时至今日,几乎没有人严肃地思考过这个问题……这种明胶状的透明生物似乎亘古不变,它们从不追随大千世界的节奏,慵懒地漫游在海中,随性起舞。

然而美国加州理工学院(Caltech)三位年轻研究人员却发现,当夜晚来临,这种古老的生物也会进入深度睡眠,就像你我一样。

这一结果看似无关紧要,但实际上这是目前所发现的最远古的睡眠形式。“我们的研究首次揭示没有中枢神经系统的动物也能入眠。”Caltech 的分子遗传学家拉维·纳特(Ravi Nath)指出。

古老的、无需大脑的睡眠,或将解答现代神经生物学最难以证明的谜题之一:为什么要睡觉?为什么要耗费三分之一的生命在这不合时宜的休息上,占用我们社交和进食的时间,还使我们暴露在致命的危险中?

答案可能正隐藏在水母的无脑睡眠中。“这项试验具有创新性及可靠性,找到了所有定义睡眠的基本指标。”美国宾夕法尼亚大学的神经内科副教授大卫·雷曾(David Raizen)赞赏道。

仙后水母(Cassiopea)

Caltech 的研究人员选取仙后水母(Cassiopea)的热带品种为研究对象,通过为期六天六夜的实验,确认了这类所谓的“颠倒水母”具备睡眠的三个关键特征:显著活动较少,但睡眠状态下的迅速可逆性足以证明其并非处于昏迷或麻痹状态;对于刺激的反应性低下;剥夺睡眠后白天活动减少。

诚然,科学家在众多哺乳动物、鸟类甚至是像蚯蚓一样简单的无脊椎动物身上发现过这些特性,但水母要比其他动物古老得多。

“秀丽隐杆线虫虽然只有 302 个神经元,但它身上还是存在相对集中化的脑部。”大卫·雷曾说明道,“而水母没有集中、成型的神经系统,其神经元呈弥散分布。水母的身体也并非两侧对称,不同于脊椎动物、线虫、节肢动物和软体动物……从这点上来说,相比果蝇或秀丽隐杆线虫,水母和人类的亲缘关系更远。”

事实上,作为最早拥有特化细胞(神经细胞、肌肉细胞等)的生物之一,水母在至少 6 亿年前便与两侧对称动物分道扬镳了——它们显然是能入眠生物中最古老的!

“他们的试验还证实水母对褪黑素(调节睡眠的激素)有反应,和目前研究过的其他动物一样。”大卫·雷曾补充道,“这喻示睡眠的机制是十分古老的。”“越来越多对于不同种系发生的研究使人相信,睡眠是具有共同起源的,并非是平行演化的结果。”拉维·纳特赞同道。

这一古老睡眠所揭示的当然远不止于此。美国加利福尼亚州立大学的生物化学家谢立尔·范巴斯科克(Cheryl Van Buskirk)就赞叹道:“这显然驳斥了只有集中化的神经系统才会产生睡眠需求的教条,我很喜欢!”

不管怎样,“水母睡眠巩固了一个假说,即睡眠的出现与高度组织化的复杂神经回路无关,后者普遍被认为用于处理和储存大量信息。”法国里昂神经科学研究中心的洛朗·瑟涅(Laurent Seugnet)补充道。


积极的副作用

换言之,我们需要睡眠的真正原因很可能与记忆的巩固或其他高级学习功能毫无瓜葛。

“睡眠状态随着演化越来越复杂,与此同时有各种不同的功能被加入进来。”按拉维·纳特的观点,个中价值对于个体机能和脑容积提升而言只有好处没有坏处。看来需要睡眠的真正原因得在别处寻找了。

没有大脑的水母构造

去哪里找呢? “像水母一样拥有简单神经系统的动物也睡觉,说明睡眠与神经元的基本特性有关。”拉维·纳特表示。

谜团的关键在皮质细胞层面,且相比整体更需着眼于局部。近些年展开的多项研究都是朝这个方向在进行。

例如,美国威斯康星大学的一个团队发现在保持清醒的大鼠脑中有一些孤立的神经元产生了慢波睡眠的波相——都是些频率仅为 1 赫兹左右的高度局部化的睡眠。

更令人吃惊的是,瑞士洛桑大学的神经学家证实,经试管培养的简单神经细胞网络会产生一种现象,这种现象具备睡眠的所有特性(变更调节基因的表达、慢波等)!

Enhancement of sleep slow waves: underlying mechanisms and practical consequences

“睡眠的出现是为了保护神经元。”最早发现这一试管睡眠的迈赫迪·塔夫蒂(Mehdi Tafti)指出,“我们注意到,在睡眠剥夺期间,与细胞内钙含量升高有关的 Homer1 基因被激活,而这可能会对细胞产生毒性。我们还观测到在清醒时,溶血脂类的含量出现大幅上升,它们可能会破坏细胞膜的完整性。”

真是惊人,但关于这一行为起源的争论还远未结束。睡眠的好处如此众多——对大脑、免疫系统、心血管系统等都有益——使我们不禁想反问,为什么人类要醒着?

 

水母构造极为简单,因而十分适合成为探索这一问题的研究对象。一如大卫·雷曾所说,“对它们整个睡眠周期和睡眠剥夺期间的行为进行更详尽的研究有助于启迪我们睡眠的真正功能”。

了解水母的漫漫长夜已然意义非凡。但或许我们以后还会把目光投向海绵,甚至植物。

撰文 Vincent Nouyrigat
编译 沈一蕊

新发现公众号 id:sciencevie

发帖时间: news

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