The research, published online Oct. 23 and in the November issue of the journal Nature Neuroscience, shows that ion channels distributed in the dendritic membrane change during a simulated learning task and that this requires the rapid production of new proteins.
"Our new work strongly supports the idea that learning involves changes in dendrites," says Dr. Daniel Johnston, director of the Center for Learning and Memory and professor in the Institute for Neuroscience.
The finding could also lead to advances in understanding conditions like epilepsy and age-related memory loss and could point to potential treatment opportunities for such conditions in the future.
Dendrites--the thin branch-like extensions of a neuron cell--receive many inputs from other neurons that transmit information through contact points called synapses. Much attention has been focused on the role that changes at synapses play in learning. They change in ways that make it easier for connected neurons to pass information.
Johnston and his colleagues show that learning and memory are likely to not only involve changes at synapses, but also in dendrites. They found that h-channels, which are distributed throughout the dendrite membrane and allow the passage of potassium and sodium ions into and out of the neuron, are altered during learning.
"The h-channels undergo plasticity, not near the synapse but probably throughout the dendritic tree," says Johnston.
To record the changes during learning, cells from the rat hippocampus (an important area of the brain for short-term memory) were electrically stimulated using a high frequency pattern called theta-bursts. Theta-bursts mimic the electrical stimulus that shoots through neurons when animals perform a learning task. The researchers found that when stimulated with theta-bursts, hippocampus neurons showed h-channel plasticity and a rapid increase in the synthesis of h-channel proteins.
The proteins were produced in the rat hippocampal neurons within 10 minutes, which is pretty rapid for cells, says Johnston.
"This really pushes the envelope with respect to how fast a neuron can produce new proteins important for learning," he says.
Learning and memory researchers know that protein synthesis in neurons is related to long-term memory, because protein synthesis inhibitors block long-term memory in animals.
Johnston says it's possible that the new proteins are being used by the neuron to build more h-channels in the dendrite membrane. He has a working hypothesis that h-channels may help buffer receiving neurons from being barraged and over-stimulated by inputs coming from information transmitting neurons.
"The h-channel plasticity alters the way the entire dendritic tree responds to the synaptic inputs," he says.
H-channel plasticity may normalize the firing rate of the cell.
"If cells aren't kept in a normal operating regime, learning would not be as effective," Johnston says. "H-channel plasticity might keep the cell within an operating window in which it can continue to learn."
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