Cerebellar Purkinje cell microcircuits are essential for tremor

Background

Tremors are an unfortunate condition that can have a number of causes, ranging from neurological disorders like Parkinson’s disease to side effects from drugs. The neuronal basis for tremors is a bit of a mystery in the field ¹. Some studies showed that the cerebellum, a region in the brain that controls muscle movement, is a promising candidate in this regard ². The cerebellum exercises its control through a class of neurons called Purkinje cells, which are known to be responsible for motor control.

 

Despite several investigations into the neuronal origins of tremors, the mechanisms behind their generation has still not been solved. This is partly because it is notoriously difficult to record from Purkinje cells in awake animals. Current findings indicate that the communication between Purkinje cells and the immediately downstream cerebellar nuclei mediate some aspects of tremor ³. This preprint follows up on this evidence and uses mice as a model system to outline the possible circuitry that causes tremors.

 

Key Findings

The scientists made use of two popular tools to study tremors in mice. With genetic manipulation in mice, they conditionally deleted the vesicular GABA transporter specifically in Purkinje cells to block their communication with other downstream neurons, which are mainly the cerebellar nuclei. The second tool they used was a drug called Harmaline, which induces tremors in mice without causing any other adverse effects in the brain. This combination of pharmacological and genetic manipulation gave the researchers a controlled setting in which to study the role of Purkinje cells in tremor generation.

 

They first administered Harmaline to both control and the GABA transporter mutant mice. The control mice experienced tremors, which was an expected outcome of the treatment with Harmaline. The mutant mice, on the other hand, appeared to be immune to the effect of the drug and did not show any signs of tremor. This was the first indication that just blocking outputs from Purkinje cells, but not completely deleting them, could reduce tremors.

 

The researchers then followed up by monitoring firing patterns during drug-induced tremors in awake, behaving mice. They looked at two types of spikes – simple and complex, each of which depend on specific neuronal inputs to the Purkinje cells. When tremors were induced, the simple spikes were of lower frequency than normal. They were also more “bursty”, meaning the spikes were spaced out in a more irregular manner. The complex spikes, however, showed the opposite trend; they occurred at higher frequency and the regularity of spiking increased. These changes in Purkinje cell activity were the second indication of their involvement in tremors. They also conveyed another important message – Purkinje cells are not the ones that generate tremors, but rather facilitate their propagation.

 

The authors then turned to optogenetics, a method of selectively stimulating neurons using light, to bypass the Purkinje cells and directly stimulate the cerebellar nuclei. They saw that stimulation at certain burst frequencies could induce corresponding tremors in the mice. This experiment helped them work out the connection between Purkinje cells and cerebellar nuclei during tremors, confirming that this circuit was a key propagator of the condition.

 

In their final move, the scientists decided to put their findings to the test by implementing a deep brain stimulation (DBS) set-up to target the erroneous circuitry during tremors. They built the set-up to monitor tremors in mice and, as soon as the tremors crossed a certain threshold, the cerebellar nuclei would be stimulated at frequencies known to reduce tremors. They first treated the mice with Harmaline and placed them in the DBS set-up. As soon as the tremors passed the threshold, the DBS was triggered. As long as the DBS continued, the tremors were suppressed. The mice returned almost to normal, despite having Harmaline in their systems. The minute the DBS stopped, tremors resumed. This was a clear demonstration that stimulating the cerebellar nuclei could alleviate the tremors caused by miscommunication between these cells and Purkinje cells.

 

What I Like About This Preprint

I chose this preprint for a few reasons. The first being that the issue of tremor origin has been a long-standing question in the field. Most studies that try to address it end up pointing to one or the other brain region, but very rarely do they lay out some aspect of the circuitry. In brains, things usually happen in concert, involving two or more brain regions. In my opinion, the authors of this preprint systematically broke down the problem and successfully answered at least a part of it.

 

The second reason I like the preprint is that they validated their results by devising a DBS therapy for tremors. They not only worked out some of the circuitry behind tremors, but applied their findings to a therapeutic method and tested them out further.

 

I am a fan of any study that uses multiple approaches to address a problem, which is exactly what this one did. The researchers used pharmacological, genetic and electrophysiological methods to ensure complementarity in their experiments.

 

Questions for the Authors

1. The inferior olive appears to be a promising candidate for signalling to the Purkinje cells during tremor. The region showed c-Fos activation upon Harmaline treatment that did not go down upon Purkinje cell silencing. Do you plan to tease out the circuitry upstream of Purkinje cells in this regard?
2. Different frequencies of cerebellar nuclei bursting cause different types of tremors. What could be factors modulating the bursting patterns?

 

References

1. Elias, W.J. and Shah, B.B., 2014. Tremor. Jama, 311(9), pp.948-954.
2. Filip, P., Lungu, O.V., Manto, M.U. and Bareš, M., 2016. Linking essential tremor to the cerebellum: physiological evidence. The Cerebellum, 15(6), pp.774-780.
3. Choe, M., Cortés, E., Vonsattel, J.P.G., Kuo, S.H., Faust, P.L. and Louis, E.D., 2016. Purkinje cell loss in essential tremor: random sampling quantification and nearest neighbor analysis. Movement Disorders, 31(3), pp.393-401.

Tags: brain, cerebellum, connectivity, mouse, tremor

doi: https://doi.org/10.1242/prelights.14266

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