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Researcher Profile : May 2006Unlocking the mysteries of learning
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Dr. Dan Johnston, professor of neurobiology, leads the Center for Learning and Memory at The University of Texas at Austin. It will have a dozen researchers when fully staffed. |
| Photo: Matt Lankes |
“I’ve always been interested in how neurons process and store information,” said Johnston, director of the College of Natural Science’s Center for Learning and Memory and the Institute for Neuroscience.
Johnston works on neurons in the hippocampus, an important region of the brain for learning and short-term memory.
“The hippocampus is a fast learner,” he said. “It takes in information from everyday activities, holds it temporarily, gets rid of what isn’t important and helps slowly develop a long-term memory where appropriate.”
Without a hippocampus, you wouldn’t be able to learn or remember new information.
The hippocampus is also one of the first regions of the brain to suffer from Alzheimer’s disease and is prone to epileptic seizures.
“It turns out that the things that make the hippocampus a fast learner—the circuitry and the connections—also make it the most prone area of the brain to have seizures,” Johnston said.
When learning occurs, neurons in the hippocampus change, and much attention has focused on how learning changes synapses—the contact points where neurons connect. Synapses connect the axon end of one neuron to the dendrites of another. During learning, synapses go through molecular and physical changes that help information flow more easily between affected neurons, like widening a bridge allows more cars to travel across.
“When learning and memory events occur, changes in synapses increase the throughput [of information through] learned synapses,” Johnston said.
But synapses aren’t the only parts of a neuron affected by learning and memory.
Johnston and his colleagues are finding that learning changes neurons throughout their dendrites—the long, branchlike extensions of these cells.
“Originally we thought that dendrites were passive and that only synapses changed during learning,” Johnston said. “Our work strongly supports the idea that learning involves changes in dendrites.”
In a recent article published in Nature Neuroscience, Johnston showed that a kind of membrane channel, called an h-channel, increased in dendrites of rat hippocampal cells when they were exposed to a simulated learning task. He and his colleagues electrically stimulated neurons in a pattern that mimics the electrical pulses that occur during learning. They found that the neurons’ dendrites produced more h-channels and rapidly produced proteins used to build more h-channels.
The channels may keep neurons from over-firing when information is pouring into them from many synapses, Johnston said. A neuron’s dendrites serve as a kind of information switchboard, and a single hippocampal neuron can be connected to thousands of other neurons through synapses on its dendrites.
During learning, Johnston said the production of new h-channels in dendrites could help a neuron stay in normal firing range.
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Neurons in the hippocampus, the part of the brain where short-term memory occurs, are the focus of Johnston’s studies. |
“h-channel plasticity might keep the cell within an operating window in which it can continue to learn,” he said.
In a separate study published in Neuron, Johnston found that rats with low numbers of h-channels in their dendrites experienced epileptic seizures.
“We found that a decrease in h-channels allowed cells to be hyper-excited,” he said. “When there are no h-channels, the cells fire more and this leads to epilepsy.”
It’s not clear yet what mechanisms cause h-channels to decrease in dendrites, but Johnston and his colleagues did find that changes in other channels can be acquired through some injury to the brain, a process they reported in Science that they labeled “acquired channelopathy.”
The relationship between disease and learning and memory comes as no surprise to Johnston, who said that the fundamental mechanisms of learning and memory overlap in almost all areas of neuroscience.
“You start talking about schizophrenia, age-related memory problems, or mental health disorders,” he said, “and you start talking about the basic mechanisms of learning and memory: synaptic transmission and the plasticity and circuitry of neurons.”
To better understand the basic mechanisms of learning and memory, Johnston is bringing together top minds from diverse fields at the college’s new Center for Learning and Memory (CLM). Scientists from chemistry and psychology to medicine and computer science will find a home at the CLM, which promises to be at the epicenter of new discoveries in learning and memory.
The CLM will provide a wealth of research opportunities for undergraduate and graduate students, eventually housing a total of twelve research faculty in the recently completed Neural and Molecular Science building.
Already at the CLM is Dr. Helmut Koester, an innovator in the development of high-speed optical imaging of neurons, a method that allows him to view activity around a single synapse. Dr. Rick Aldrich, a renowned neuroscientist who studies the molecular mechanisms of ion channel function, will join the CLM and the Department of Neurobiology from Stanford University in Spring 2006.
“This is going to be an exciting place for doing neuroscience,” Johnston said.
Lee Clippard
This article originally appeared in the Spring 2006 issue of Focus on Science, the magazine for the College of Natural Sciences.
