Over the last ten years, optogenetics has opened up a whole new world of possibilities within neuroscience research, highlighting the use of ion channels as physiological tools in the process.
Optogenetics combines the focal use of light with the targeted expression of light-sensitive proteins to manipulate and monitor the function of defined populations of cells1. These proteins are currently divided into two groups: the ‘actuator’ proteins, which can transduce a light stimulus into a neuronal signal, and the ‘indicator’ proteins, which report neuronal signals by the emission of light. Here, the focus is on the former. In a recent article in Nature Biotechnology (Dec 2015)2, Park and colleagues used the prokaryotic channelrhodopsin protein (a cation channel responsive to blue light) to reversibly stimulate peripheral and spinal pain pathways in mice which expressed this protein in all, or specific sub-populations, of sensory neurons. By using pulses of blue light, they were able to alter the membrane potential of the cells in which channelrhodopsins were expressed, resulting in neuronal activation, with observable effects on the animals’ behaviour. This in principle is nothing new; what is novel, and possibly revolutionary within optogenetics, in the method developed by this research group to deliver and control the light stimulus.
An improved, reversible pain model that enables natural behaviour.
The in vivo use of optogenetics to target discrete cell populations has several important advantages over lesion-based or genetic knockout animal models: spatial and temporal control over the stimulation or inhibition of the cell population, and more importantly, reversibility of the stimulus. In order to study pain, animals no longer have to be subject to constant pain. However, as with all developing technologies, the earlier incarnations of this technique have been far from optimal. Focal light stimulation has been achieved using fibre-optic cables, which during the course of an experiment, requires an animal to be effectively tethered by their light-source. This impedes natural behaviour and raises a number of both experimental and welfare questions, especially when monitoring behavioural changes is often one of the key read-outs of the experiment.
The research published by Park et al describes an elegant solution to this problem. They have developed a soft, stretchable wireless implant, smaller than a fingertip, which contains radio-frequency-powered LEDs capable of activation 30 cm from transmitter source. Having overcome the problems of heat generation and long-term durability, the implants have been used for reliable, reversible nerve stimulation for up to 6 months without impeding the normal movement or behaviour of animals when not in use (figure 1, used from reference 2 with permission).
Using this technology, implanted either above the sciatic nerve or lumbar spine, coupled with cre-recombinase-based transgenic expression of channelrhodopsin in the nociceptive neurones in mice, the group were able to observe robust, reproducible nocifensive responses on activation of the implanted blue LEDs, which reverted to normal behaviour on deactivation and were absent in cre-negative littermates with implants. Whole-cell voltage- and current-clamp of isolated DRG neurones from these mice confirmed frequency-matched depolarisation and neuronal firing in response to blue light stimulation.
This refined pain model appears to effectively mimic the symptoms of neuropathic pain, and could become an invaluable tool for the investigation of neuropathic pain relief, and possibly other peripheral neurological disorders. As an alternative approach, using optogenetic ion channels to stabilize membrane potential in order to reduce neuronal activity is also possible1. Indeed, the authors conclude with a reference to gene therapy approaches delivering optogenetic channels to human cells already undergoing clinical trial. With these implants, perhaps controlling chronic pain conditions may become as simple as flicking on a light switch?
Blog written by Sarah Lilley.
- Fenno L, Yizhar O & Deisseroth K (2011). The Development and Application of Optogenetics. Annu. Rev. Neurosci. 2011. 34:389–412
- Park S et al (2015). Soft, stretchable, fully implantable miniaturised optoelectronic systems for wireless optogenetics. Nature Biotechnology 33(12):1280-1286 (doi:10.1038/nbt for methods)
Figure used by permission from Macmillan Publishers Ltd: Nature Biotechnology (citation 2), copyright 2015.