Researchers have found a new way to track dopamime in brain, much more precisely than previously possible, allowing scientists to gain insight into dopamine's roles in learning, memory and emotion.
The study was published in the journal Lab on a Chip. Dopamine is one of the many neurotransmitters that neurons in the brain use to communicate with each other. Previous systems for measuring these neurotransmitters have been limited in how long they provide accurate readings and how much of the brain they can cover. The new MIT device, an array of tiny carbon electrodes, overcomes both of those obstacles.
"Nobody has really measured neurotransmitter behavior at this spatial scale and timescale. Having a tool like this will allow us to explore potentially any neurotransmitter-related disease," says Michael Cima, the David H. Koch Professor of Engineering in the Department of Materials Science and Engineering, a member of MIT's Koch Institute for Integrative Cancer Research, and the senior author of the study. Furthermore, because the array is so tiny, it has the potential to eventually be adapted for use in humans, to monitor whether therapies aimed at boosting dopamine levels are succeeding.
Dopamine-producing cells within the striatum are critical for habit formation and reward-reinforced learning. Until now, neuroscientists have used carbon electrodes with a shaft diameter of about 100 microns to measure dopamine in the brain. However, these can only be used reliably for about a day because they produce scar tissue that interferes with the electrodes' ability to interact with dopamine, and other types of interfering films can also form on the electrode surface over time. The MIT team designed electrodes that are only 10 microns in diameter and combined them into arrays of eight electrodes.
These delicate electrodes are then wrapped in a rigid polymer called PEG, which protects them and keeps them from deflecting as they enter the brain tissue. These tiny electrodes measure dopamine in the same way that the larger versions do. The researchers apply an oscillating voltage through the electrodes, and when the voltage is at a certain point, any dopamine in the vicinity undergoes an electrochemical reaction that produces a measurable electric current. Using this technique, dopamine's presence can be monitored at millisecond timescales.
"What motivated us to pursue this high-density array was the fact that now we have a better chance to measure dopamine in the striatum, because now we have eight or 16 probes in the striatum, rather than just one," Schwerdt says. The researchers are now conducting tests to see how long these electrodes can continue giving a measurable signal, and so far the device has kept working for up to two months.
"We and other people have struggled with getting good long-term readings," says Graybiel, who is a member of MIT's McGovern Institute for Brain Research. "We need to be able to find out what happens to dopamine in mouse models of brain disorders, for example, or what happens to dopamine when animals learn something". This study is part of a larger collaboration between Cima's and Graybiel's labs that also includes efforts to develop injectable drug-delivery devices to treat brain disorders.