Octopus brain tissue keeps synapses strong for 10 hours without new proteins; nitric oxide implicated
In vitro, octopus vertical lobe connections stayed strengthened with protein synthesis blocked. Researchers propose a nitric oxide loop as the mechanism.
Researchers at the Hebrew University of Jerusalem and the Ruppin Academic Center in Israel report that connections in octopus brain tissue stay strengthened for at least 10 hours even when protein production is blocked, according to Earth.com. The team of Binyamin Hochner worked with Tal Shomrat's group. Ana Luiza Turchetti-Maia ran most of the experiments on living tissue from common octopuses caught off Israel's Mediterranean coast. The study is published in The Journal of Neuroscience. earth.com
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Learning without new proteins
In mammals and in the sea slug Aplysia, lasting changes in nerve connections depend on new proteins. Blocking protein production in the mammalian hippocampus prevents the late, lasting phase of strengthening.
The octopus vertical lobe, which supports visual learning and plays a role similar to the hippocampus, behaved differently. Tissue was treated with anisomycin, a protein synthesis blocker, for two and a half hours, starting half an hour before stimulation. The connections still stayed stronger for at least 10 hours. A glowing marker for new proteins confirmed the drug worked: untreated tissue glowed brightly, treated tissue very little.
The nitric oxide loop
The researchers stimulated incoming nerve fibers with four rapid bursts of 20 pulses each. Blockers of nitric oxide synthase, the enzyme that makes the gas, were added after the connections had strengthened. Ten minutes later one drug reduced the response by an average of 63% and another by 82%. A mirror-image version that cannot block the enzyme reduced it by 23%, a drop that could be chance. Washing the active blockers out restored the strength, including when one blocker was added 10 minutes, 90 minutes, or four hours after stimulation. earth.com
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A brief dose of a nitric oxide-releasing chemical strengthened connections for only a few minutes. The team proposes that strong activity starts a loop in which the enzyme makes nitric oxide and nitric oxide keeps the enzyme active. In four lengthy experiments, blocking nitric oxide still reduced strength in anisomycin-treated tissue.
Among 11 blocker experiments, five showed fast responses, with an average 97% loss of added strength. In the other six, the blocker reduced strength by 72%, and further stimulation added almost nothing.
Limitations
The work was done on tissue in a dish, not on octopuses learning a task, and most comparisons used five or six brain slices per group. The authors doubt this switch alone explains lasting memories. Earlier work found that removing the vertical lobe after training did not stop octopuses from remembering learned tasks. It is also not yet known whether nitric oxide keeps its enzyme active directly or through another enzyme. The team now wants to identify the specific molecular changes involved. earth.com
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