​A deeper dive into paired stimulation, and the importance of replication

Posted by Amanda Zimmerman in Research News on April 13, 2020 # Research

A couple of months ago (before a lot of research labs temporarily closed down due to COVID-19), I had the opportunity to visit the Carmel lab at Columbia Medical School. I wanted to take the time this week to do a deeper dive into one of their studies, a replication study from last year on paired electrical stimulation of the spinal cord and brain as a way to improve function after injury.

First, a word on replication studies. As noted, 17 years ago in forming the FORE-SCI replication program, there have been numerous papers reporting some intervention, drug, or biologic leading to functional improvement after SCI in rodent models, and then fizzling out before even making it into clinical trials. There are obviously many hurdles to bring a promising preclinical therapeutic to clinical trials, from toxicology and bioavailability to translating rodent findings into humans to basic commercialization problems. Yet one nagging possibility has always been a lack of robustness of the initial findings. In order to rule out the latter possibility and increase the likelihood of success if a therapy makes it into the clinic, one can have additional labs replicate the initial findings. If independent labs can wind up at the same result, we have much more confidence in the strength of the initial finding, and hopefully, its potential for human use.

As the initial FORE-SCI replication program reported, failure to replicate can be due to differences in methodology and not just lack of robust findings. In order to avoid this, the first part of the study was focused on method consistency with the original Zareen et al paper from the Martin lab at CUNY School of Medicine. Did the two labs deliver similar contusion injuries to the rats? Did the type and placement of electrodes on the cortex result in similar evoked movements in the arms and hands? Was there reliability in scoring behavior between scorers and labs? By working in collaboration with the original lab, the Carmel lab could be sure their methods were similar. Then, they stopped talking, and blinded the scientists in the lab doing behavioral training, testing, scoring and anatomical analysis. In short, this is exactly what we need more of in the field of SCI.

Yet, I didn’t just choose to highlight this study for its replication rigor. Both this study, and the one it replicated, have a relatively unique take on electrical stimulation, and both forms of stimulation can be applied non-invasively in humans (but haven’t yet been tested in this manner). The researchers studied the effects of pairing stimulation of the cortex (where voluntary movement is initiated) with DC transcutaneous stimulation. Here C4 contused rats were delivered paired stimulation for 27 minutes every day for 10 days, starting 11 days after injury. This is the equivalent of the subacute phase in human SCI. They tested the rats on two behavioral tasks that require a high degree of manual dexterity: a horizontal ladder and a food manipulation task.Paired Stimulation

So, does it work? Both the Carmel and Martin labs showed robust improvement on both of these behavioral metrics, with a multi-week lag between when the stimulus pairing was last applied and when functional improvements were seen (e.g. the one figure replicated below, measuring the food manipulation task performance). Both groups noted increased sprouting of corticospinal neurons in the stimulated group, suggesting that the plasticity of this brain-> spinal cord tract might underlie the functional differences seen. Yet this doesn’t rule out stimulation induced plasticity of other pathways that indirectly connect the brain to the spinal cord or modify/strengthen connections within the spinal cord itself.

While I appreciate these studies for their reproducibility and rigor (and if you’re a quantitative person like me, you may want to spend some time looking at their finite element modeling to assess optimal placement of spinal cord stimulation electrodes), there are clearly some outstanding questions. Below are 4 questions I had, as well as some responses from the senior author (Jason Carmel):

(1) Does this work in more chronic stages, especially if plasticity is triggered (e.g. pharmacologically or with acute intermittent hypoxia)?

We have not tried this particular approach beyond 10 days after injury, but we did a study a few years ago showing efficacy of brain only stimulation initiated 8 weeks after injury that showed positive effects. This would be considered subacute in people, but researchers consider this a more chronic time point in rats.

(2) Can these findings be replicated in larger animals with more similar anatomy to humans? This is particularly important with electrical stimulation, as the current flow depends a lot on anatomical differences.

Yes, we have a translational grant from New York State to look at the same model in rats, cats and people. This is a collaboration between 3 labs. My lab did the rat study. Jack Martin, PhD, who designed and tested the approach originally in rats, is doing the cat work in his lab. The human part of the translational grant is being done in Noam Harel’s lab at the Bronx VA. A computer model of how current flows in the body is designed to make the dose of current similar in each of the three species.

(3) While the invasive brain stimulation paradigm used here has been replicated using transcranial magnetic stimulation (TMS) non-invasively in other cases, does TMS replicate the functional improvements seen here?

So far, we are looking only at the immediate effects of stimulation, which show promising effects in humans. We have been working with a company to make neck stimulation more tolerable for people undergoing stimulation. Our hope is that we can find a stimulation method that delivers a similar dose of stimulation in people compared to animals but is also tolerable.

And (4) Can we do better with stimulation parameters? These studies used patterned stimulation of the brain, paired with direct current (think constant) stimulation of the spinal cord. If the goal is to guide meaningful plasticity, what happens if researchers can mimic signaling more similar to that which naturally occurs in a non-injured individual?

Good question. For our current stimulation pattern of motor cortex and cervical spinal cord stimulation, we did an engineering experiment to look at timing between pulses, intensity, duration, timing between pairs, and various burst frequencies. We are using a stimulation intensity and pattern that have been optimized for these parameters. But there are very many stimulation parameters, and our goal in the future is to use data science to improve stimulation further.

I’m looking forward to seeing where this research goes next.