Longer lasting performance - all in the brain...

From last month

Last month, I nearly bored anyone reading the article to tears with a heavy dose of math. This month, we mercifully move away from modelling and simulation and discuss a paper that many may find a little more palatable.

The paper for discussion this month is titled Evaluation of physically small p-phenylacetate-modified carbon electrodes against fouling during dopamine detection in vivo. This work was the second from my PhD candidature and explored the application of small electrodes (of the type discussed last month – click here) towards something exciting. 

The brain is an exciting environment where millions of chemical transformations occur every second. That is not an exaggeration. Chemical processes within the brain fluid, electrical pulses, even the release and uptake of several chemicals from the synapses into the synaptic gap (known as emotions and feelings, but it’s all chemistry really) all are a continuous maze of activities that are vital to a person’s being and survival.

An interest of psychologists is to be able to measure the dynamics (release, denaturation, uptake, metabolism) of one such important chemical, known as dopamine. Dopamine is a chemical messenger, known as part of a group called neurotransmitters. Dopamine is associated with the body’s reward mechanism, so happy feelings really, in conjunction with other neurotransmitters such as serotonin.

While dopamine presence leaves a feeling of satisfaction and contentment, loss of the chemical or irregular release amounts in the brain are associated with serious conditions like Parkinson’s and Alzheimer’s diseases. It is therefore important to be able to monitor dopamine in the brain, which is a niche scientific genre popular with psychologists for instance.

The challenges

It is fairly elementary that to measure the dopamine activity in the brain, the brain must be alive. A dead brain doesn’t do much thinking. Also in agreement is the fact that a brain cannot be harvested out of the head for such analyses, without significantly harming the individual, whether it be a human or a lab specimen like a rat. Unless, the measurement device is a very small sensor that is capable of being inserted in the brain without touching any of the components and be suspended in the synaptic gap where the (exocellular) brain fluid exists.

See where this is heading? Last month, I discussed the geometry of the electrodes I made in my PhD research, which were cylindrical with a pointy end about 2 microns wide in radius. Such a size is perfect for implanting in a live brain.

And that is exactly what I did in my study. I used Sprague Dawley rats (beautiful white, furry creatures), put them in a coma and inserted the electrodes from my study into their brain. The brain was then excited electrically using pulses and the excited brain released dopamine, which my electrodes measured. Straightforward.

Except, the problem with putting such electrodes, made of carbon coated surfaces is that they get fouled by hydrophilic proteins, peptides and lipids which stick to the electrode surface, sometimes permanently. This deactivates the surface, affecting measurement. Essentially, the electrode dies.

What did we do?

To prevent electrode fouling, we need to get creative. Hydrophilic fouling agents like proteins, peptides and lipids need to be kept away from the similarly-hydrophilic carbon surface of the electrode, allowing it to work normally without any hindrance. Remember, like attracts like (hydrophilic prefers hydrophilic).

In my work, we realised that it would be far easier to put a hydrophobic film on the pointy end of the electrode, without compromising its functionality. To do so, I prepared a film of a compound called p-phenylacetate which was then deposited on the electrode surface. The surface was rigorously tested in the lab, including in artificial solutions of the brain fluid to mimic the brain environment. Only those electrodes that were successful in getting modified and still worked were then put inside the rat brain.

?What did we find?

The results showed that the films were quite effective at keeping the nasties at bay, allowing the electrode to work for about 40 min unhindered where 70–95% of the dopamine signal remained. After the next 20 min (60 min in total), 50% signal remained. When compared with other electrode that were not modified, constant reduction in the dopamine signal was observed.

We were also keen on measuring the rate of degradation of the electrode surface, known as the fouling rate. An average electrode surface fouling rate of 0.54% min?1 was estimated at the modified carbon electrodes during the first 40 min of the experiments. The non-modified electrode degraded almost instantly. A 40-min timeframe is plenty for such work, considering that dopamine and other neurotransmitters are released with neuronal firing, which can be in timeframes as short as 50 milliseconds.

The rest of my work was to perfect the process so we could have longer lasting performance (sounds cheesy I know) from the electrodes, and for which we used other modifications. But, as always, that’s for another story, next month! J