There are two types of neurons in the brain: excitatory and inhibitory neurons.  They do exactly what you think they would.  Excitatory neurons release chemical messengers, which activate other neurons, which may eventually lead to some sort of perception or action.  Inhibitory neurons release chemicals that silence other neurons.  Why would you want inhibitory neurons in your brain?  Well, if all your neurons were excitatory and interconnected, all your neurons would be active all the time and the signals would be meaningless.  In fact, this sort of overactivation in the brain can lead to seizures.  It’s been shown in numerous cases of epilepsy that there is some sort of dysfunction of the inhibitory neurons.  The excitatory neurons have free reign and go crazy, leading to a seizure.
How is epilepsy treated?  Medications that potentiate the inhibitory neurons can help, but they activate all inhibitory neurons throughout the brain, when maybe the problem is more localized to one spot.  Just as all excitatory neurons is a bad thing, too much inhibition is also bad and can lead to cognitive side effects.  Another treatment is to open up the patient’s head, try to find the overactive area and cut it out or zap those neurons with a laser.  Destroying brain cells is always a last resort, though.
In a recent paper published in Nature Neuroscience by Hunt et al., the authors propose another potential treatment: adding new inhibitory neurons into the epileptic brain.  Like all new medical ideas, the story starts with mice.  They can create a model of human epilepsy in these mice by treating them with a potent drug.  These epileptic mice have seizures just like humans do.
Where do you get new inhibitory neurons?
The researchers obtained progenitor cells from mice embryos.  In other words, these weren’t inhibitory neurons yet, but they were destined to turn into them as the mice developed.  They grafted these progenitors into adult epileptic mice in the hippocampal region of the brain (a common area for seizures).   Amazingly, these pre-neurons migrated throughout the brain region, as far as 1.5 mm (that’s a lot… think about how small a mouse brain is).  Then the progenitors differentiated into inhibitory neurons, as if they were in a normal developing brain.  One week later, the epileptic mice with extra inhibitory neurons had hardly any seizures, whereas the untreated mice were having about 2 a day.  Not only that, but the treated mice showed cognitive improvements compared to the untreated epileptic mice. 
So they seemed to “cure” the epileptic mice by giving them some new inhibitory neurons that were able to make functional connections with the existing neurons.  This isn’t as invasive as brain surgery and it’s much more localized than medication.  If the epilepsy were focused in a different part of the brain, then they could transplant the cells there instead.
Is this possible to try in humans?  Maybe so, but the first problem is that we can’t take inhibitory progenitor cells from human embryos.  There are some ethical issues with growing clones to harvest parts from them.  However, you could use embryonic stem cells, or induced pluripotent stem cells.  Pluri-what?  Recent technology allows researchers to take a skin biopsy, do some genetic engineering to these cells and push them back in developmental time to a stem cell.  Pluripotent means that these stem cells have the potential to become any type of cell, like an inhibitory neuron.  All it takes is turning on the right genes in these cells to push them to a particular fate, and if that isn’t already known for inhibitory neurons, I bet it’s not too far off.  Plus there’s the benefit that the transplanted cells will have the same genome as all the patient’s other cells, because they originated from their skin cells.  Just wait, regenerative medicine is moving ahead at lightning speed.

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