Probing the brain’s chemistry
October 19, 2012
They use voltammetry, a method of electrochemical scanning where voltage is applied to, and current is collected from, a carbon fiber microelectrode that is about 10 times smaller than a human hair. The resulting data is in the shape of a graph called a voltammogram.
The size of the graph indicates how much of a particular chemical is present and the shape tells the researchers which chemical it is. So they can “see” and measure specific chemicals accurately, without worrying about interference from all of the other chemicals in our inundated brains.
Some of the chemicals Sombers is interested in measuring — like glucose, for instance — are normally invisible to electrochemical measuring techniques like voltammetry. So to make it work, Sombers attaches an enzyme to the electrode that reacts with glucose. The glucose molecule reacts with the enzyme and produces hydrogen peroxide, which oxidizes as an electrical potential is applied to the electrode.
The resulting current gets measured, and that data is captured in the voltammogram. When the scientists see the hydrogen peroxide in their voltammogram, they know they’ve found glucose.
The most commonly used method of electrochemical scanning takes about 10 to 20 seconds per scan, but Sombers and Lugo-Morales can make 10 full scans per second, which gives them information about chemical fluctuations in the brain in real time.
“Speed is the key,” Sombers says. “We use carbon for our electrode, which is biologically friendly and inexpensive, but it isn’t really a catalyst. By ramping the voltage up and down at 400 volts per second, we build an oxygen layer on the surface of the electrode. The oxygen acts as a catalyst to help oxidize the hydrogen peroxide generated by the enzyme attached to the probe, and we can measure the molecules we are looking for and how they fluctuate in real time.”
Lugo-Morales has already used the probe to make real-time measurements of glucose fluctuations at different locations in a rodent brain. She found that the amounts differed depending upon where the probe was located and that they fluctuated quite a bit over very short times — sub-seconds — which is how quickly our neurons work. This suggests that our brains depend on some pretty precise regulation of glucose in order to function.
“A lot of people want to understand glucose dynamics in the brain,” Sombers says. “Sixty to 70 percent of diabetics show neuronal dysfunction, plus glucose has been linked to diseases like schizophrenia and Alzheimer’s. If we can understand how glucose is used by the brain we can create better treatments for these diseases.”