Paralyzed Rats Learn to Walk Again — But What Does this Mean for Humans?
Dr. Pam Osenkowski, NAVS Science Advisor
One of the top stories that made headlines last week was about paralyzed rats that regained their ability to walk. Video clips showed the rats, whose hind legs were completely paralyzed following a lab-induced injury, taking voluntary steps, after several weeks of electrical and chemical stimulation to the brain and spine. The rats looked human-like, standing upright on their hind feet, harnessed in little black vests. Scientists boasted that indeed there might be a human connection, that the results of this study might have implications for treatment of human spinal cord injuries.
Trust me when I say that I will be overjoyed the day that humans, paralyzed from spinal cord injuries, are able to walk again. I have a number of close friends and former students who would benefit tremendously from such a treatment. However, my major concern is that animal models have long been used in spinal cord injury research, but time and time again, we have seen that experiments in animals do not accurately predict what happens in humans.
There are a number of reasons why scientists must be cautious when translating results from animal experiments to humans. Regarding the study mentioned above, one important thing to consider is that there are major differences between how spinal cord injuries are induced in laboratory animals and how they actually occur in people. In this study, the rats’ spinal cord injuries resulted from two direct cuts through the spinal cord, with neither cut completely severing it. In humans, most spinal cord injuries result instead from compression or bruising, and it is not clear if results from this study could be extrapolated to humans due to the different natures of the injuries. Another interesting thing to consider is how much bigger the spinal cord is in humans than rats. The human spinal cord is four times as long as the whole rat central nervous system, which also include the brain and spinal cord. This makes it hard to translate results to humans because, in rodents, much less nerve regeneration is needed to restore function compared to a human.
Still, there are other considerations. Human spinal cord injuries tend to be much more complex than those in experimental animals, whose induced injuries are very controlled in a specific spinal cord location. In humans, some of the main causes of spinal cord injuries are traumatic accidents and gunshot wounds, which involve more areas of the body than just the spinal cord. When doctors treat patients for these more systemic problems, it is possible that the treatment will influence how well a specific spinal cord treatment would work.
Another consideration is the difference in time between when the spinal cord injury occurs and when treatment begins. Will the electro-chemical stimulation used in this study be effective on people who have had damaged spinal cords for many years? Will the results be different if scar tissue has formed, or if many nerve cells died as a result of the injury?
In order for researchers to be able to help humans with spinal cord injuries, scientists need to develop more human-relevant approaches. Although animals are still heavily used in spinal cord research studies, there are a number of promising non-animal models as well. Human neurons from the brain and spinal cord can be studied in vitro in conditions that mimic a post-injury environment. Additionally, a number of computer-based models are available to study spinal cord injury. And imaging studies with individuals with spinal cord injuries as well as post-mortem studies with human cadavers can teach us a great deal about the human spinal cord. NAVS is optimistic that a treatment for spinal cord injuries is on the horizon, but to be able to find it, we need to work with systems that mimic more human-like conditions.
For more information on the study, see MSNBC.com
Akjtar, AZ., et al. “Animal models in spinal cord injury: a review.” Rev Neurosci. 2008. 19(1):47-60.
Van den Brans, R., et al. “Restoring voluntary control of locomotion after paralyzing spinal cord injury.” Science. 2012. 336: 1182-5.