It has been called the Kim Kardashian of neurotransmitters. Not for its networking skills (though it's good in that area too), but for its celebrity status among brain compounds. Dopamine's role as a reward chemical is well established, credited with the pleasure we derive from things like food, drugs and sex. Meanwhile, dysregulation of dopamine has been linked to conditions such as as depression and Parkinson's, but measuring its activity in the brain has proven a very difficult task indeed. MIT scientists have now come up with a technology they say could change the game: an electrode-array that allows for long-term dopamine tracking with much higher precision than was previously possible.
"We did not have multi-electrode arrays to measure dopamine previously," Helen Schwerdt, lead author of the research, explains to New Atlas. "All commercially available neural probes such as electrodes and arrays for measuring "electrical" activity from the brain were not configured to measure chemicals, because they did not have the proper electrochemical interface or the appropriate surface areas."
Neuroscientists have previously been able to measure dopamine in the brain, but the approach has had some serious limitations. Using carbon electrodes with a 100-micron diameter, they can gain readings on the chemical's activity, but there is only about a 50 percent chance that the single electrode will hit the spot where there is measurable dopamine. What's more, within a day it produces scar tissue that hampers the probe's ability to interact with the dopamine and produce accurate readings.
Looking to overcome these drawbacks, the team built arrays of eight electrodes that each measure just 10 microns. These slender and delicate electrodes were also wrapped in a rigid polymer called PEG, which keeps them straight and pointed during insertion but then quickly dissolves.
When it comes to measuring dopamine, these arrays work in the same way that the earlier, larger electrodes do. Scientists send an oscillating voltage along the electrodes, which, when that voltage hits a certain point, triggers an electrochemical reaction with any dopamine in the area. This in turn creates an electric current that can be measured to reveal the presence of dopamine.
But by inserting the electrodes into eight different spots rather than just one, they are not only more likely to find dopamine, but are able to track its levels in different areas at the same time. The team used the arrays to record dopamine at 16 sites in the brain region known as the striatum in rats. This is the area in which dopamine-producing cells are vital for habit formation and reward-reinforced learning in both rats and humans.
The researchers found that the dopamine levels varied greatly in the striatum, describing the results as "revealing a remarkable spatiotemporal contrast." This didn't surprise them, as they didn't believe the striatum contained an even spread of dopamine, but it did serve as a useful proof of concept for how varying dopamine levels can be measured at the same time.
Further to covering more ground in the brain, the array could also represent a big breakthrough in long-term tracking of dopamine. Whereas the previous electrodes are limited to one day of usage, the team reports that the array remained in working order for up to two months.
This raises the prospect of using extended sensing to monitor dopamine levels in response to long-term behahviors, such as learning a new skill or forming new habits. And for neurological conditions that are linked to the dysregulation of dopamine, such as Parkinson's, it could become a vital tool in monitoring the effectiveness of therapies.
"Right now deep brain stimulation is being used to treat Parkinson's disease, and we assume that that stimulation is somehow resupplying the brain with dopamine, but no one's really measured that," Schwerdt said in a statement accompanying the research.
From here, Schwerdt and her team are carrying out tests to see how long the electrode-array can maintain a measurable signal. Another focus will be making the probes longer for the purpose of reaching deep subcortical structures in primates.
The research was published in the journal Lab on a Chip.
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