Scientists at the Massachusetts Institute of Technology (MIT) have developed a new type of probe for use with functional magnetic resonance imaging (fMRI) that can trace connections between neural populations and monitor how they interact with each other, which could help researchers to map the circuits behind behavior and perception.
fMRI imaging detects changes in the brain associated with blood flow. When a neuron receives a signal from another neuron, an influx of calcium is triggered that causes nitric oxide to be released. Nitric oxide acts as a vasodilator that causes an increase in blood flow. The increase in blood flow is an indicator of the neurons that are active.
fMRI scans allow researchers to image brain function in real-time, but as useful as the scans can be, they do not allow researchers to study specific populations of cells. It is possible to monitor calcium directly in the brain, which provides a more precise view of brain activity, but that technique requires the introduction of fluorescent chemicals and invasive procedures, and one of the main benefits of fMRI scans is they are non-invasive, require no chemical markers, surgery, or ionizing radiation.
“With regular fMRI, we see the action of all the gears at once. But with our new technique, we can pick up individual gears that are defined by their relationship to the other gears, and that’s critical for building up a picture of the mechanism of the brain,” said Alan Jasanoff, an MIT professor of biological engineering, brain and cognitive sciences, and nuclear science and engineering.
The researchers wanted to develop a new technique for using fMRI scans to measure brain activity more specifically, without losing the important benefits of fMRI. “The way that we chose to do that in this study was to essentially hijack the molecular basis of fMRI itself,” said Jasanoff.
The new technique involves genetically targeting the probe to view specific populations of cells. Viruses are used to deliver a genetic probe that codes for a protein – dubbed NOSTIC (nitric oxide synthase) – that sends a signal when a neuron is active. When calcium levels are elevated as a result of neuronal activity, nitric oxide is generated and that creates an artificial fMRI signal that can only arise from cells that contain NOSTIC.
The virus that delivers the genetic probe is injected into a specific site, then travels along the axons of the neurons that connect to that site, which allows the researchers to label all neural populations that feed to that particular location. fMRI scans are then performed of those cells, and the researchers can then measure what makes an input to that region of the brain take place and the types of input that arrive at that location.
There is a potential problem with this approach, and that is while the FMRI scans can pick up brain activity from the genetically altered neurons, the fMRI scan also picks up regular signals. To distinguish between the two, the researchers ran their experiments twice, once with the probe switched on and once with the probe deactivated by a drug. They were then able to filter out the normal activity and only view the activity they were looking to study.
In animal models, the researchers were able to identify specific populations of cells that responded to rewarding stimuli – deep brain stimulation of the lateral hypothalamus which is involved in appetite and motivation. The researchers also determined that several neural populations in the motor cortex and entorhinal cortex also sent input following deep brain stimulation in response to a rewarding stimulus.
“It’s not simply input from the site of the deep brain stimulation or from the cells that carry dopamine. There are these other components, both distally and locally, that shape the response, and we can put our finger on them because of the use of this probe,” said Jasanoff.
The researchers are now using their new “hemogenics” approach to study other networks in the brain. “One of the things that’s exciting about the approach that we’re introducing is that you can imagine applying the same tool at many sites in the brain and piecing together a network of interlocking gears, which consist of these input and output relationships,” said Jasanoff. “This can lead to a broad perspective on how the brain works as an integrated whole, at the level of neural populations.”