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Study identifies impact of neural connections in learning process


Research validates decades of work, points to future avenues for understanding learning

Through a clever experimental design, Carnegie Mellon University neuroscientists have validated decades of experiments to show how learning and memory may be encoded in a living animal. The research, published in the March issue of Neuron, identifies for the first time the specific neural connections that strengthen as an animal’s brain responds to new experiences.

"We are very excited by this finding and the ability of researchers worldwide to build upon it," said the study’s principal investigator Alison Barth, assistant professor of biological sciences at the university’s Mellon College of Science.

According to Barth, the study is the first to verify "synaptic plasticity" in a living animal’s brain that has not been artificially altered to affect neural transmission. Synaptic plasticity is the process in which molecular changes modify a single neuron’s activity in a living animal.

"Verifying this principle of synaptic plasticity to how neurons function in vivo is critical to advancing our knowledge of the mechanisms that underlie learning and memory," Barth added.

Many neuroscientists believe that the cellular basis of learning and memory results from molecular-scale changes occurring at synapses, the communication junctions between neurons. Although great strides have been made in identifying how different patterns of neuronal activity can alter synapses in vitro and in revealing long-term or short-term synaptic plasticity, Barth said it has been unclear whether these findings hold true in a normal, unaltered brain.

Previous research has artificially stimulated neurons or genetically modified them so that they produce an abundance of AMPA receptors, molecules on the surface of some neurons that are implicated in learning and memory. But such experiments alter the native environment of the brain and may influence the normal activity at a single synapse, explains Barth.

"Evidence from in vitro studies and in vivo studies using viral over-expression systems supports the notion that production of certain AMPA receptors is increased and that they are transported to the synapse during learning. Neuroscientists have always thought implicitly that this phenomenon underlies learning and memory," said Barth. "Our study reveals for the first time in vivo the dynamic activity of these receptors within a single synapse and that these changes result in experience-dependent plasticity during a specific behavioral experience."

Barth’s strategy to reveal plasticity in a single synapse involved a two-step approach. First, she used a novel tool she created--a transgenic mouse that couples the green fluorescent protein (GFP) with the gene c-fos, which turns on when nerve cells are activated. Using this method, Barth was able to "light up" clusters of neurons in living brain tissue that were activated during a specific rearing condition --experiencing the world through one whisker. By locating such a cluster of glowing neurons, she could precisely identify the area of the brain involved in processing sensory input from that single whisker. Once these neurons were located, Barth examined how the inputs to these neurons had been modified by experience.

Barth and graduate student Roger Clem, the lead author on the study, found that this change in sensory experience causes a subtle, but very significant, change in AMPA receptor properties at a defined group of synapses in an area of the brain identified by fos-GFP labeling. Barth and Clem achieved this finding by using an electrophysiological technique called patch clamping to detect unique voltage "signatures" that characterize and differentiate in real-time AMPA receptors.

Why would detecting different AMPA receptors be important? Different subtypes of AMPA receptors are highly regulated within a cell. Each AMPA receptor is made from varying combinations of four subunits: GluR1, GluR2, GluR3 or GluR4. The different ratios of these subunits alter their voltage signatures.

The GluR1 subunit is delivered to synapses specifically when a neuron is activated, according to earlier studies by other researchers. Conversely, GluR2 production is constitutive, meaning it basically occurs at a steady state. So AMPA receptors comprised of GluR1 would implicate neural activation and should have a specific voltage signature -- in fact, what Barth and Clem found.

Barth’s work is especially provocative given that previous electrophysiological studies have not been able to distinguish the voltage signal of AMPA receptors comprised exclusively of GluR1 subunits. It provides the first evidence that the same mechanisms that have been observed in cultured nerve cells are actually used by the brain during normal behavior.

"In sum, our data indicate that enhanced sensory experience alters the properties and subunit composition of AMPA receptors," Barth said. "These patch-clamping studies are the first to effectively reveal that the up-regulation of GluR1-only receptors is related to learning."

Lauren Ward | EurekAlert!
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