EPFL (Switzerland) scientists have used a cutting-edge method to
stimulate neurons with light. They have successfully recorded synaptic
transmission between neurons in a live animal for the first time.
Neurons, the cells of the nervous system,
communicate by transmitting chemical signals to each other through
junctions called synapses. This “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 have to use artificial conditions that don’t capture the
real-life environment of neurons. Now, EPFL scientists have observed and
measured synaptic transmission in a live animal for the first time,
using a new approach that combines genetics with the physics of light.
Their breakthrough work is published in Neuron.
Aurélie
Pala and Carl Petersen at EPFL’s Brain Mind Institute used a novel
technique, “optogenetics”, that has been making significant inroads in
the field of neuroscience in the past ten years. This method uses light
to precisely control the activity of specific neurons in living, even
moving, animals in real time. Such precision is critical in being able
to study the hundreds of different neuron types, and understand higher
brain functions such as thought, behavior, language, memory – or even
mental disorders.
Activating neurons with 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 the light-sensitive protein, which sits
on their outside, the membrane. There, it acts as an electrical channel –
something like a gate. 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.
Recording neuronal transmissions
Pala
used optogenetics to stimulate single neurons of anesthetized mice and
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 an advanced imaging technique (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 Aurélie 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.
Reference
Pala A, Petersen CCH. In Vivo Measurement of Cell-Type-Specific Synaptic Connectivity and Synaptic Transmission in Layer 2/3 Mouse Barrel Cortex. Neuron (2015) http://dx.doi.org/10.1016/j.neuron.2014.11.025