Two-photon imaging enables deeper understanding of neuron function

Researchers are exploring the fact that neurons can function in many different ways: Some signals that dendrites receive do not continue to the next neuron, as they instead seem to change the way that the neuron handles the subsequent signals. This could help neurons function as part of a large network, but researchers still have many questions. So Dr. Sigita Augustinaite, a researcher in the Optical Neuroimaging Unit at the Okinawa Institute of Science and Technology (OIST) Graduate University in Japan, suggested using two-photon imaging to explain how neurons help the network function.

Related: Imaging distributed function and networks in the human brain

Augustinaite studies the visual pathway, where signals from the retina are sent to the visual cortex, which interprets signals from the eye. Between the eye and the visual cortex, the signals must pass through through thalamocortical (TC) neurons, which can switch between a "sleeping" state and a "waking" state depending on input they receive from neurons and other brain areas. When an animal is awake, TC neurons transmit the incoming retinal signals on to the cortex, but when the animal is asleep, the neurons block retinal signals.

A thalamocortical (TC) neuron labeled with fluorescent dye, as used in Dr. Augustinaite's study. The image shows a voltage recording device, at bottom left, entering the yellow cell body, and a stimulation device, at top, reaching the dendrites. Color in this image shows the depth in the slice.

The visual cortex also sends a massive input back to TC neurons to control retinal signals traveling through the thalamus. But Augustinaite says that the suggested mechanisms of this control bring more questions than answers. To understand more, she conducted experiments in acute brain slices, which are small pieces of brain tissue where neurons stay alive and maintain their physiological properties. She added glutamate to dendrites far from the cell body to emulate a feedback signal from the visual cortex. Then, she measured the neuron's response, shown as a voltage difference between inside and outside of the membrane.

Augustinaite found that stimulating the neurons in this way depolarizes their membranes, creating N-methyl-D-aspartate (NMDA) spike/plateau potentials. If strong enough, depolarization can cause a neuron to fire an action potential, which travels through the axon to activate other neurons. Action potentials look like a sharp, 1 ms increase in membrane voltage, and they transmit signals from retina to cortex. But if NMDA spike/plateaus induce action potentials, signals from the cortex and signals from the retina would be indistinguishable. With her experiments, Augustinaite showed that the NMDA spike/plateau potentials in TC neurons do not trigger action potentials. Instead, they lift the voltage of the membrane, changing the neuron's properties for a few-hundred milliseconds and creating conditions for reliable signal transmission from retina to cortex.

"The research gives, for the first time, a clear view on what dendritic potentials are good for," explains Prof. Bernd Kuhn, who leads the lab where Augustinaite works. "It points directly to the mechanism," he concluded.

Showing how dendritic plateaus function is just one important step toward understanding how neurons function as a network. "This mechanism could also be used in many other neuronal circuits, where one input regulates how another input moves through the network," Augustinaite says. "This mechanism is an exciting logical element in the neuronal network, but just the start of putting the puzzle together."

Full details of the work appear in the Journal of Neuroscience; for more information, please visit http://www.jneurosci.org/content/34/33/10892.

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