This past winter semester I took a class about recent studies into complex behavior. The majority of the papers we read were about fruit flies’ mating habits and other model organisms, investigating the very basics of how flies decide who to mate with, for example. The last portion of the class we individually prepared presentations about papers often having to to with mammals and less fundamental research. I thought I’d share my presentation on a paper ( Gradinaru et al. 2009) whose goal was using optogenetics to determine why deep brain stimulation (DBS) actually works. So DBS stimulation, in essence, involves sticking an electrode down into the brain to the subthalamic nucleus (STN), a part of the brain involved in motor control. The electrode injects current at a high frequency and this often eliminates Parkinson’s symptoms, but nobody knows why.
Electrical stimulation affects all cells nearby, be they neuron bodies or glial cells or even axons passing near the electrode, so even though it is known where DBS occurs it’s harder to say which cells being affected by DBS are having a therapeutic effect. The leading hypotheses regarding the are that excitatory cells in the STN are inhibited by the electric stimulation, that astroglia (nearby support cells) are activated and secrete a chemical which inhibits excitatory cells in the STN, and that the cells of the STN are firing in the wrong pattern. To attempt to tease these different possibilities apart from one another the investigators used optogenetics in hemiparkinsonian rats. So firstly, hemiparkinsonian rats are rats that have half (hemi) Parkinson’s, which is achieved by applying a chemical to one side of their brains’ motor circuit, causing a lesion which leads to Parkinson’s symptoms only in one direction. One result of this is that the rats “rotate” or walk in circles sometimes. DBS can be given to these rats which eliminates their rotations and other Parkinson’s symptoms. Optogenetics is the introduction of light sensitive ion channels/pumps into cells using (in this case) viruses. The damaging parts of the viral DNA are removed and replaced with genes coding for these light-sensitive proteins and these genes are under the control of DNA regulatory elements called promoters which are only activated in certain cell types. The result is that you can inject a small amount of these viruses into an area that you’d like to stimulate, the viruses insert the DNA into all the cell bodies around that area (the viruses don’t tend to inject into axons passing through), but only cells capable of driving the promoter (cells that produce the necessary transcription factors) that has been injected will actually make the protein. The protein produced will find its way to the cell membrane where it is ready to activate or deactivate that cell upon application of light of the appropriate wavelength (color). I’ll go over one figure now and leave the rest for later. Here are the results from Figure 1:
CaMKIIα::eNpHR refers to the viral DNA setup used. It was Halorhodopsin (eNpHR) under the calmodulin-dependent kinase type II promoter (found specifically in excitatory cells (those that use glutamate as their neurotransmitter)). The black band across the middle is an electrical recording of the area being stimulated with light. Voltage is on the Y axis, time on the X. The yellow bar shows when yellow light is applied (halorhodopsin is most sensitive to yellow light). The graphs at the bottom show the same thing.
The chart on the left and the black bar in the right shows that the rotations were unaffected by the light. The red bar shows that electrical stimulation (DBS) was indeed effective (as was known and expected).
The conclusion from this is that deep brain stimulation is not effective (at least solely) because it inhibits the activity of excitatory neurons in the STN.