Optogenetics is a recently developed technique based on microbial proteins called channelrhodopsins (ChRs), which render neurons sensitive to light when inserted into them, thus enabling researchers to manipulate the activity of the cells using laser pulses.
Although still very new – the first ChR protein was isolated from a species of green algae in 2002 – optogenetics has already proven to be extremely powerful – it can be used to switch neurons on or off in an extremely precise manner and so to control simple behaviours in small organisms such as the nematode worm.
Earlier this year, ChR was used to restore vision to blind mice lacking the light-sensitive photoreceptor cells in the retina. And now researchers from Case Western Reserve University in Cleveland, Ohio have used the technique to restore motor function in rats paralysed by spinal cord injuries.
Among the most common types of spinal cord injuries in humans are those involving damage to the cervical area of the spine, which begins at the base of the skull and extends down through the neck. The cervical spinal cord gives rise to the nerves which control the head, neck, arms, upper body and diaphragm, so serious damage to this area leads to complete paralysis, and is often fatal, due to paralysis of the muscles required for breathing.
Jerry Silver and his colleagues used an animal model of cervical spinal cord injury in their study. They sectioned one half of the cord at the upper cervical level, leaving their rats paralysed down on side of the body, and with breathing difficulties because of the half-paralysed diaphragm. During the procedure, they also injected a modified Sindbis virus, containing the genes encoding channelrhodopsin and green fluorescent protein, directly into the ventral grey matter of the spinal cord, where the cell bodies of motor neurons are located. Some of these motor neurons form the phrenic nerve, which innervates the diaphragm and controls breathing
Four days later, the animals’ spinal cords were exposed again. The motor neurons were stimulated with blue light from a fibre optic cable, and the electrical activity of the diaphragm recorded. Brief episodic bursts of light were found to induce first erratic and then rhythmic activity, which became synchronised with the respiratory activity recorded from the other side of the diaphragm, aso that the rats could breathe normally. Remarkably, a longer repeating patteren of light was found to induce recovery of breathing which persisted for 24 hours.
Somehow, the spinal cord circuitry had adapted so that it could continue to generate the rhythmic patterns of activity required for respiratory movements long after the light stimulation had ceased. Analysis of the GFP expression pattern in the spinal cord showed that an average of 650 neurons in each experimental animal had been successfully infected with the virus carrying the channelrhodopsin gene. It also revealed that both motor neurons and interneurons had taken up the virus, and that some of the infected cells had processes which extended across to the opposite side of the spinal cord.
The researchers suggest that swiching on the infected motor neurons with the pulses of light had initiated a novel form of synaptic plasticity in the “crossed phrenic pathway”. This pathway contains the projections of motor neurons which cross the midline and form connections with their mirror image counterparts on the other side of the spinal cord. Normally, these synapses are weak, so motor neuron activity on one side does not elicit activity on the other., but the strengthening of these crossed pathways seems to have been sufficient to generate rhythmic respiratory activity.
This and other research could one day lead to the development of optical neuroprosthetic devices consisting of remote-controlled light sources (perhaps light-emitting diodes) implanted into the body. Eventually though, optogenetics-based treatments for various neurological diseases seems plausible. The ability to use ChR proteins to both excite and inhibit neurons would have advantages – activating spinal motor neurons or neurons in the brain would be benficial in conditions such as amyotrophic lateral sclerosis or stroke and Parkinson’s Disease, respectively, while inhibiting the activity of spinal neurons could prove to be an effective treatment for chronic pain.
Such treatments would of course involve first infecting patients with a genetically engineered virus, which poses major problems. In the meantime, Silver and his colleagues are using the same method to try to restore bladder function to the paralysed rats. They are also looking for ways to prolong ChR expression in the spinal motor neurons, and planning similar experiments in monkeys.
- Neuronal light switches
- Using light to control neuronal activity
- Channelrhodopsin restores vision in blind mice
- Control of rodent motor cortex with an optical neural interface
Alilain, W. J. et al (2008). Light-Induced Rescue of Breathing after Spinal Cord Injury. J. Neurosci. 28: 11862-11870. DOI: 0.1523/JNEUROSCI.3378-08.2008.