“Transplant Memories” Between Sea Snails在海蝸牛之間“移植記憶”

伊恩·桑普爾/文 陳薈/譯

Experiment shows some memories are encoded in molecules that form part of an organisms genetic machinery， researchers say.研究人員稱，實驗表明，一些記憶被編碼在構成生物遺傳機制的分子中。

Science may never know what wistful memories play on the mind of the California sea hare， a foot-long hermaphrodite marine snail， as it munches on algae in the shallow tide pools of the Pacific coast.

But in a new study， researchers claim to have made headway in understanding the simplest kind of memory a mollusc might form， and， with a swift injection， managed to transfer such a memory from one sea snail to another.

David Glanzman， a neurobiologist at the University of California in Los Angeles， believes the kinds of memories that trigger a defensive reflex in the snail are encoded not in the connections between brain cells， as many scientists would argue， but in RNA molecules that form part of an organisms genetic machinery.

In an experiment to test the idea， Glanzman implanted wires into the tails of California sea hares， or Aplysia californica， and gave them a series of electric shocks. The procedure sensitised the animals so that when they were prodded in a fleshy spout called a siphon， they contracted their gills in a robust defensive action. Glanzman likens the reaction to being jumpy in the moments after an earthquake： the memory of the event induces an involuntary reflex to any loud noise.

After sensitising the sea snails， Glanzman extracted RNA from the animals and injected it into other sea snails to see what happened. He found the recipient sea snails became sensitised， suggesting the “memory” of the electrical shocks had been transplanted. When Glanzman repeated the experiment with RNA from sea snails that had been hooked up to wires but not shocked， the reflex behaviour did not transfer.

According to the researchers， the experiments show how essential parts of the memory trace， or engram， that gives rise to sea hare sensitisation are held in RNA， rather than in the connectivity of brain cells as traditional neuroscience dictates.

“What we are talking about are very specific kinds of memories， not the sort that says what happened to me on my fifth birthday， or who is the president of the United States，” said Glanzman， whose study appears in the journal? ? eNeuro.

The work has not found widespread acceptance. “Obviously further work needs to be carried out to determine whether these changes are robust and what are the underlying mechanisms，” said Prof Seralynne Vann， who studies memory at Cardiff University. “While the Aplysia is a fantastic model for studying basic neuroscience， we must be very careful in drawing comparisons to human memory processes， which are much more complex.”

Tomás Ryan， who studies memory at Trinity College Dublin， is firmly unconvinced. “Its interesting， but I dont think theyve transferred a memory，” he said. “This work tells me that maybe the most basic behavioural responses involve some kind of switch in the animal and there is something in the soup that Glanzman extracts that is hitting that switch.”

But Ryan added that radical thinking about memory was sorely needed： “In a field like this which is so full of dogma， where we are waiting for people to retire so we can move on， we need as many new ideas as possible. This work takes us down an interesting road， but I have a huge amount of scepticism about it.”

科學可能永遠無從得知，加州海兔（一種1英尺長的雌雄同體海蝸牛）在太平洋海岸的淺潮池中津津有味地咀嚼海藻時，它的腦海中究竟呈現著怎樣依依不舍的記憶。

但是在一項新的研究中，研究人員聲稱，他們在理解軟體動物可能形成的最簡單的記憶方面已經取得了進展，并且通過快速注射，成功地將這一記憶從一只海蝸牛轉移到了另一只身上。

加利福尼亞大學洛杉磯分校的神經生物學家戴維·格蘭茨曼相信，觸發海蝸牛防御性反射的記憶類型并不像許多科學家認為的那樣被編碼在大腦細胞之間的連接結構中，而是在構成生物遺傳機制的核糖核酸分子中。

在驗證這一想法的實驗中，格蘭茨曼在加州海兔的尾部植入電線，并給予它們一系列電擊。這一過程會使這些動物變得敏感，以至于當海兔被稱為虹管的肉質噴水管受到刺激時，它們會收縮鰓部，進行強有力的防御。格蘭茨曼將這一反應比作在地震后不久的神經質：對地震的記憶引起對任何巨大噪音的無意識反射。

在使海蝸牛變得敏感之后，格蘭茨曼從這些動物體內提取了核糖核酸，并將其注入其他海蝸牛體內，觀察會發生什么。他發現，受體海蝸牛也變得敏感，這表明電擊的“記憶”已經被移植了。而另一組海蝸牛與電線相連但是未被電擊，當格蘭茨曼用這些海蝸牛的核糖核酸重復該實驗時，反射行為沒有發生轉移。

據研究人員稱，實驗展示了引起海兔敏感化的記憶痕跡（或稱記憶印記）的重要部分是存儲在核糖核酸中的，而不是像傳統神經科學斷定的那樣存儲在大腦細胞的連接部位中。

“我們在此討論的是非常特定的記憶類型，而不是我5歲生日時發生了什么，或者誰是美國總統的那種記憶。”格蘭茨曼說道，其研究發表在美國神經科學學會在線期刊eNeuro上。

然而這一成果并未被廣泛接受。“顯然，還需要展開進一步的工作來確定這些變化是否穩固以及這一現象的基本原理是什么。”卡迪夫大學研究記憶的塞拉林恩·范恩教授說道，“盡管海兔是研究基礎神經科學極好的模型，我們在與人類記憶過程進行對比時還是必須十分小心，這復雜得多。”

都柏林圣三一學院研究記憶的托馬斯·瑞安則堅決不信。“這項工作很有趣，但是我認為他們并沒有轉移記憶。”他說，“這項工作告訴我，也許最基本的行為反應涉及到動物體內的某種開關，而格蘭茨曼提取的液體中正好有按下這一開關的某些物質。”

但是瑞安補充道，記憶這一領域也十分需要全新的思維，“這個領域里思想僵化，我們要等到前人退休才能向前推進，在這樣一個領域，我們需要盡可能多的新想法。這項工作將我們帶上了一條有趣的道路，但我仍然對它深表懷疑。”