January 24, 1994
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Researchers at Johns Hopkins and the University of Maryland have developed a method that may let scientists 'see' learning take place at its most basic level in brain cells.
This ability could provide a clear and basic model for studying learning, and for pinpointing what improves or damages the process.
Using a technique called calcium Hopkins neuroscientists Jay Baraban, M.D., Ph.D., and Timothy Murphy, Ph.D., with W. Gal Weir and Lothar Blatter of the University of Maryland, have evidence that a single nerve cell from a rat brain can change the way it responds following basic changes in the environment, such as frequent stimulation.
It's the neurological equivalent of someone at an information desk quicker, giving snappier answers after getting the same question 15 times.
Plasticity is what neuroscientists call this capacity of a nerve cell to alter in response to the environment. 'We assume," says Baraban, "that plasticity is the key property of nerve cells which allows learning to take place."
The researchers have focused on single nerve cells in culture, and specifically
at the spots where they interact with surrounding nerve cells. This smallest
site of communication, the synapse, "is the most basic level critical for
learning and memory,' says Baraban.
'Even though each nerve cell in the brain can have up to 10,000 synapses with other neurons, with this technique, we can pinpoint activity at just one," says Murphy.
The researchers' study appears in this weeks issue of Science. Working with a microscope, they supplied brain cells with a dye that emits light when synapses are active. The dye travels from the body of the cell to the treelike branches, the dendrites, that field the bulk of information from other cells. Each branch of a dendrite is dotted with tiny nubs, site of the synapses the team studied.
With the dye, the photos of a dendrite show that active synapses "lit up" like towns along a road in a nighttime aerial map. In an active, fully stimulated nerve cell, says Murphy, the entire dendrite can be alight. In a resting nerve cell, only some synapses light up as a result of a small, occasional background stimulation.
"In a resting cell," says Baraban, "you'd expect to see random points of light along the nerve cell where random synapses were active. But you don't. Some are active repeatedly.'
The researchers believe this repetition is a possible sign that some synapses were 'preferred." "What we may be seeing" says Baraban, "is the result of plasticity."
Subsequent tests showed that in nerve cells kept inactive for hours, "all the synapses have an equal chance of being active," says Baraban. "It however, we stimulated a nerve cell beforehand in a natural way, certain synapses were much more active than others."
"We still don't know why some synapses are preferred,' says Murphy, 'but we intend to use calcium imaging coupled with other techniques to find out."
Funding for the research was by the National Institutes of Health and the American Heart Association.