Optogenetics, two-photon microscopy observe neuronal transmission in live mouse brain

Using optogenetics, scientists at École Polytechnique Fédérale de Lausanne (EPFL; Lausanne, Switzerland) have observed and measured synaptic transmission in a live animal for the first time. Synaptic transmission is critical for the brain and the spinal cord to quickly process the huge amount of incoming stimuli and generate outgoing signals. However, studying synaptic transmission in living animals is very difficult, and researchers normally have to use artificial conditions that don't capture the real-life environment of neurons.

Related: Fast optogenetic method can sense neuronal activity with red light

Optogenetics works by inserting the gene of a light-sensitive protein into live neurons, from a single cell to an entire family of them. The genetically modified neurons then produce a light-sensitive protein, which sits on their outside—the membrane. When light is shone on the neuron, the channel opens up and allows electrical ions to flow into the cell; a bit like a battery being charged by a solar cell.

The addition of electrical ions changes the voltage balance of the neuron and if the optogenetic stimulus is sufficiently strong, it generates an explosive electrical signal in the neuron. And that is the impact of optogenetics: controlling neuronal activity by switching a light on and off.

Aurélie Pala of EPFL's Brain Mind Institute used optogenetics to stimulate single neurons of anesthetized mice to see if this approach could be used to record synaptic transmissions. The neurons she targeted were located in a part of the mouse's brain called the barrel cortex, which processes sensory information from the mouse's whiskers.

When Pala shone blue light on the neurons that contained the light-sensitive protein, the neurons activated and fired signals. At the same time, she measured electrical signals in neighboring neurons using microelectrodes that can record small voltage changes across a neuron's membrane.

Using these approaches, the researchers looked at how the light-sensitive neurons connected to some of their neighbors: small, connector neurons called "interneurons." In the brain, interneurons are usually inhibitory: when they receive a signal, they make the next neuron down the line less likely to continue the transmission.

The researchers recorded and analyzed synaptic transmissions from light-sensitive neurons to interneurons. In addition, they used two-photon microscopy that allowed them to look deep into the brain of the live mouse and identify the type of each interneuron they were studying. The data showed that the neuronal transmissions from the light-sensitive neurons differed depending on the type of interneuron on the receiving end.

"This is a proof-of-concept study," says Pala, who received her PhD for this work. "Nonetheless, we think that we can use optogenetics to put together a larger picture of connectivity between other types of neurons in other areas of the brain."

The scientists are now aiming to explore other neuronal connections in the mouse barrel cortex. They also want to try this technique on awake mice, to see how switching neuronal activity on and off with a light can affect higher brain functions.

Full details of the work appear in the journal Neuron; for more information, please visit http://dx.doi.org/10.1016/j.neuron.2014.11.025.

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