A spinal cord injury doesn't need to be complete to cause paralysis – even with some nerves left intact, messages from the brain still don't seem to get through. While investigating why that's the case, researchers at the F.M. Kirby Neurobiology Center of Boston Children's Hospital determined that a certain drug helps balance the Yin and Yang of the nervous system to restore limb movement.

In the past, some of the best results in restoring limb movement after spinal injuries have come from epidural stimulation, where electrical signals are applied below the damaged site. An implanted electrode array can read sensory information from the legs and stimulate the muscles in response, allowing voluntary movement again with some rehab training. In other cases, instructions from the brain bypass the injury site, making a detour (either wired or wirelessly) through a computer and then to an implant on the lower spine.

"Epidural stimulation seems to affect the excitability of neurons," says Zhigang He, lead researcher on the Boston Children's Hospital study. "However, in these studies, when you turn off the stimulation, the effect is gone. We tried to come up with a pharmacologic approach to mimic the stimulation and better understand how it works."

The new study tested a range of compounds that are known to excite neurons in mice with spinal cord injuries that were severe but incomplete. The team injected each compound into groups of 10 mice for eight to 10 weeks, as well as a control group that received placebos.

The best results came from one compound in particular. Known as CLP290, the drug restored the stepping ability in the paralyzed mice after four or five weeks of treatment, with few side effects. The hindlimb muscles were found to be active in electromyography scans, and even two weeks after the treatment ended, their "walking scores" remained higher than those of the control group.

So what makes this particular drug so successful? The team found that it effectively restores balance to the types of signals in the spinal circuits, which is lost as a result of the injury.

Basically, the brain and the spinal circuit are made up of two types of neurons – excitatory and inhibitory, which produce positive or negative signals, respectively. Combining both allows all kinds of complex patterns of activity to arise, but too much of one or the other will effectively shut down the system.

In an injured spinal cord, inhibitory neurons lose the ability to process inhibitory signals from the brain, and will only respond to excitatory signals. It's kind of a double-negative situation – the positive signals are telling the negative neurons to fire their negative signals, which results in the legs essentially being told not to move.

The team found that CLP290 was allowing inhibitory neurons to process inhibitory signals from the brain again. That means the negative neurons are told NOT to fire their negative signals, which allows more of the excitatory signals from other neurons to get through to the limbs, letting them move.

The key was a protein called KCC2. After a spinal cord injury, inhibitory neurons produce far less of this protein, which is why they lose their function. CLP290, meanwhile, restores the levels of KCC2 and, by extension, restores the normal function of these neurons.

"Restoring inhibition will allow the whole system to be excited more easily," says He. "Too much excitation is not good, and too much inhibition is not good either. You really need to get a balance. This hasn't been demonstrated in a rigorous way in spinal cord injury before."

With these promising results, the team plans to continue researching KCC2's role, and explore other compounds that might restore the protein.

The research was published in the journal Cell.

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