Simple 'learning' in cell culture.
"We are beginning to get a system where long-term memory can be explored in cellular detail," Kadenl told a symposium at the new Fidia-Georgetown Institute for the Neurosciences at Georgetown University in Washington, D.C. In their recent experiments, Kandel, Samuel Schacher and their colleagues provided isolated cells with conditions that mimic a simple learning experience in the intact snail. The isolated cells responded with the same changes that underlie forms of both short-term and long-term memory in an animal.
The learning process under study is called sensitazation. It can cause an animal to pay attention to an innocuous stimulus, because that stimulus has previously accompanied a painful stimulus.
The marine snail Aplysia demonstrates sensitazation in the reflex that withdraws its gill and siphon, a small fleshy spout above the gill, when the siphon is touched. The neural circuit underlying this reflex (SN: 1/22/83, p. 58) includes a sensory nerve cell, a motor nerve cell and a few other nerve cells. When a touch to the siphon has been accompanied by an electric shock to the tail, the signal that directs the gill withdrawal response is enhanced. The enhanced response to a siphon touch last for a few minutes. But if the noxious shock is repeated several times, this sesitization can last weeks.
In previous work, Kandel and his colleagues determined the steps underlying sensitization. Some nerve cells of the reflex release neurotransmitters, one of which is called serotonin, that activate receptors on the sensory nerve cell. These receptors trigger a cascade of biochemical events that eventually increases the amount of neurotransmitter the sensory cell releases, enhancing its signal to the motor nerve cell.
Earlier experiments analyzed the effect of serotonin on the cell body, rather than on the thin extension, the axon, where the cell makes its contacts. More recent work indicates that serotonin also increases the nerve cell signal in growth cones, the developmental precursors to mature axons. Because this enhancement occurs even when growth cones have been separated from the cell body (the site of protein synthesis), no new protein is required.
Long-term sensitization appears to involve a more extensive change. When viewing nerve cell connections with an electron microscope, scientists can see active zones, sites where neurotransmitter is released. Long-term sensitization increases both the number and size of active zones in the sensory nerve cells.
The molecular basis of long-term sensitization is now being examined in sensory and motor cells maintained in laboratory culture. The scientists mimic a sensitizing stimulus by applying serotonin directly to the cells. The sensory cells respond with an enhanced signal. To produce long-term sensitization, the scientists apply serotonin to the cells five times at 15-minute intervals. A day or more later, the cells still show an enhanced signal.
Long-term sensitization appears to involve new protein production. During the repeated exposures to serotonin, the scientists have treated the cells with chemicals that block protein synthesis. These chemicals do not interfere with short-term sensitization, but they do block long-term enhancement. One of these drugs inhibits an early step in gene expression.
"This indicates that long-term memory involves an alteration in gene expression," Kandel says. "Now two questions remain: What is the signal and what are the genes involved?"
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|Title Annotation:||experiments on sensitization in simple animals|
|Author:||Miller, Julie Ann|
|Date:||Nov 16, 1985|
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