3月5日，在英國(guó)倫敦生物醫(yī)學(xué)中心（BioMed Central）的《BMC-生物學(xué)》（BMC-Biology）上刊登的一項(xiàng)研究成果稱(chēng),，加利福尼亞大學(xué)博士大衛(wèi)普拉希茨基（David Plachetzki）探索研究一種天生缺失眼睛的淡水大頭水螅,，更神奇的是它卻能對(duì)光亮做出反應(yīng)。在生活習(xí)性上,，這種水螅每天都會(huì)出來(lái)活動(dòng)和尋找食物,，處于不停運(yùn)動(dòng)之中，并根據(jù)光亮做出反應(yīng)以此不斷地進(jìn)行生理與行為循環(huán),。
淡水水螅的圖片
研究發(fā)現(xiàn)特殊水螅的觸須刺囊細(xì)胞具有保護(hù)以及捕獲獵物的功能,，并通過(guò)一個(gè)簡(jiǎn)單的神經(jīng)系統(tǒng)與光敏細(xì)胞相連接,，形成了光反應(yīng)與獵取食物的反射弧，共同控制著水螅的獵食行為,。圖2中將藍(lán)色表示為具有觸須狀的視覺(jué)蛋白,，紅色則為神經(jīng)元細(xì)胞。而水螅是徑向?qū)ΨQ(chēng)生物大家庭中的一個(gè)成員,，屬于刺胞動(dòng)物門(mén),，顧名思義該生物通過(guò)專(zhuān)門(mén)的刺囊細(xì)胞來(lái)捕獲食物。屬于這個(gè)大家庭的生物還包括一些耳熟能詳?shù)纳钤谒械膭?dòng)物,，比如水母,、珊瑚等等，珊瑚亦屬于動(dòng)物界腔腸動(dòng)物門(mén),。它們都有一個(gè)共同的特征,，便是通過(guò)簡(jiǎn)單的結(jié)構(gòu)形成一個(gè)由刺囊細(xì)胞包裹起來(lái)的嘴。
水螅視覺(jué)蛋白與神經(jīng)元細(xì)胞
水螅的觸須向前延伸的同時(shí)還潛藏著刺,，刺中獵物的同時(shí)還能分泌毒素,，這樣可以使獵物眩暈。水螅的獵物包括水蚤等,，然而水螅在進(jìn)食它的獵物時(shí),，還會(huì)使用自己的觸須保護(hù)自己，免受其他動(dòng)物的攻擊,。加利福尼亞大學(xué)研究人員大衛(wèi)普拉希茨基（David Plachetzki）博士發(fā)現(xiàn)一種被稱(chēng)為光敏蛋白的視蛋白存在于水螅感覺(jué)細(xì)胞中,，可控制調(diào)節(jié)魚(yú)叉狀的刺囊細(xì)胞在捕食的時(shí)候向四周伸縮等動(dòng)作。
這些對(duì)光亮敏感的神經(jīng)元還被發(fā)現(xiàn)集中在水螅所有的“捕獵工具”中,，其中包括刺囊細(xì)胞,、卷刺絲囊細(xì)胞。水螅通過(guò)這些細(xì)胞演化形成了“捕食工具”,，在抓住獵物的同時(shí)還可以粘住獵物,。刺囊細(xì)胞與卷刺絲囊細(xì)胞不僅是水螅的捕獵工具，同時(shí)也控制著水螅的運(yùn)動(dòng),，在全天的捕食時(shí)間內(nèi)可使水螅通過(guò)不斷翻筋斗前進(jìn),。
研究人員發(fā)現(xiàn)連接到刺囊細(xì)胞的視蛋白解釋了水螅在缺失眼睛的情況下還能對(duì)光亮做出反應(yīng)。大衛(wèi)普拉希茨基博士還描述了在水螅感覺(jué)細(xì)胞中還可能存在著其他蛋白質(zhì)控制著光亮傳導(dǎo)信號(hào),，他認(rèn)為：“我們不僅發(fā)現(xiàn)了水螅感覺(jué)光亮的視蛋白連接,，也發(fā)現(xiàn)了在刺囊細(xì)胞中控制光信號(hào)傳導(dǎo)的其他細(xì)胞”,。其中包括環(huán)核苷酸控制的離子通道,，該通道負(fù)責(zé)傳遞反應(yīng)信號(hào)和控制水螅的抑制蛋白體，這種抑制蛋白的作用體現(xiàn)在刺囊細(xì)胞發(fā)生第一次光信號(hào)傳導(dǎo)之后,，可“清除”神經(jīng)回路上的興奮信號(hào),，為第二次光信號(hào)的傳遞提供干凈的通道,。
此外，大衛(wèi)普拉希茨基博士還證明了刺囊細(xì)胞還可能被光亮所影響,，而產(chǎn)生反作用,。水螅遇到這種情況時(shí)便會(huì)關(guān)閉光傳導(dǎo)的路徑。眾所周知,，腔腸動(dòng)物門(mén)中的多個(gè)物種在地球上已經(jīng)生活了超過(guò)6億年左右,，但是水螅卻還是通過(guò)最簡(jiǎn)單的光敏方法來(lái)生存和找機(jī)會(huì)利用光傳導(dǎo)信號(hào)捕捉獵物，而人類(lèi)也有著類(lèi)似的視覺(jué)傳導(dǎo)神經(jīng)興奮路徑,，這也暗示了我們可能有一個(gè)共同的祖先,。（生物谷 Bioon.com）
doi:10.1186/1741-7007-10-17
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Cnidocyte discharge is regulated by light and opsin-mediated phototransduction
David C Plachetzki, Caitlin R Fong and Todd H Oakley
Background
Cnidocytes, the eponymous cell type of the Cnidaria, facilitate both sensory and secretory functions and are among the most complex animal cell types known. In addition to their structural complexity, cnidocytes display complex sensory attributes, integrating both chemical and mechanical cues from the environment into their discharge behavior. Despite more than a century of work aimed at understanding the sensory biology of cnidocytes, the specific sensory receptor genes that regulate their function remain unknown.
Results
Here we report that light also regulates cnidocyte function. We show that non-cnidocyte neurons located in battery complexes of the freshwater polyp Hydra magnipapillata specifically express opsin, cyclic nucleotide gated (CNG) ion channel and arrestin, which are all known components of bilaterian phototransduction cascades. We infer from behavioral trials that different light intensities elicit significant effects on cnidocyte discharge propensity. Harpoon-like stenotele cnidocytes show a pronounced diminution of discharge behavior under bright light conditions as compared to dim light. Further, we show that suppression of firing by bright light is ablated by cis-diltiazem, a specific inhibitor of CNG ion channels.
Conclusions
Our results implicate an ancient opsin-mediated phototransduction pathway and a previously unknown layer of sensory complexity in the control of cnidocyte discharge. These findings also suggest a molecular mechanism for the regulation of other cnidarian behaviors that involve both photosensitivity and cnidocyte function, including diurnal feeding repertoires and/or substrate based locomotion. More broadly, our findings highlight one novel, non-visual function for opsin-mediated phototransduction in a cnidarian, the origins of which might have preceded the evolution of cnidarian eyes.
