European researchers said they have found a way to motivate paralysed rats to learn to walk again through a combination of spinal cord stimulation and robotic-aided therapy.
The key to the method's success was how it engaged the rats to participate in their own rehabilitation, said Gregoire Courtine, lead author of the study published in the US journal Science.
"In the beginning... the animal is struggling and it is really difficult," said Courtine, chair of the International Paraplegic Foundation in Spinal Cord Repair at the Ecole Polytechnique Federale de Lausanne in Switzerland.
"Then the first time it happens, the animal is surprised. It looks at you like, 'Wow. I walked!'"
The rats also showed a massive three-fold increase in the connections between the brain and spinal cord after training, according to the research.
"The motor cortex developed new pathways to regain control of the area below the injury. This was really fascinating to see," Courtine said.
A chocolate reward
"What we observed was this extensive reorganisation of the central nervous system not only at the level of the injury but throughout the brain, brain stem and spinal cord."
The therapy combines an electrical-chemical stimulation of the spinal cord, mimicking the signals the brain would normally send to initiate movement in the limbs, and a rehabilitation device that helps the rats stay upright.
The rats were hoisted into a two-legged standing position with a robotic harness that did not propel them forward but stabilised them if they tottered sideways, so they could try to walk without falling.
A chocolate reward was placed in front of the rats. Soon, the animals succeeded in making a few steps.
Within five to six weeks, as their skills improved, the rats were voluntarily climbing stairs, dodging obstacle courses and even sprinting without any treat in view.
"We had a very high percentage of success with these animals. We always observed, in all of the animals we treated, recovery of voluntary movement," said Courtine, adding that more than 100 lab rats were tested.
Rats not able to walk independenlty
"In some animals it was weak. In some animals it was spectacular."
A similar therapy has been tried in one human subject, an American in his 20s named Rob Summers who was paralysed from the chest down after being hit by a car. His case was described in an article in British journal The Lancet last year.
That study provided the first proof-of-concept that such therapies may help restore some voluntary movement in humans.
Courtine said he hopes to begin trials in people using his team's technique in the next couple of years.
One challenge that remains is how to keep the electrochemical stimulation going so that the rats can perform.
"Our frustration, I would say, was that despite this major reorganisation, without the electrical and chemical stimulation, the rat would not be able to walk independently," he said.
No spinal cord cure
"So if you provide them with this electrochemical neuroprosthesis, they can climb on the stairs. But if you remove the electrochemical stimulation, they can push a little bit but they can't really walk."
Researchers at EPFL are coordinating a R93 million project, NeuWalk, with the goal of designing a fully operative spinal neuroprosthetic system for implanting into humans.
"We are not thinking this will cure spinal cord injury. We need to be very clear on this. This is not a cure," Courtine added.
"What we observed in rats – the plasticity and the extent of the recovery is very surprising – so now we need to optimise all these systems for humans and do our best to at least improve functional recovery."