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Artificial electrical stimulation of the nervous system is one of the foundations of neurotechnology. However, the use of electrical currents to stimulate the nervous system presents several challenges. It is difficult to achieve selective stimulation of only the targeted neurons without activating neighboring neurons. Further, electrochemical reactions at the electrode-tissue interface may lead to electrode dissolution or tissue damage.
As an alternative, magnetic stimulation can be used to stimulate non-invasively. However, the power requirements for magnetic stimulation are high and the resulting stimulation is non-selective. Two recent publications may be harbingers of a future where light?another form of electromagnetic energy?is used to stimulate the nervous system.
Hirase and colleagues from Rafael Yuste's lab at Columbia University applied laser illumination to depolarize and excite single neurons in vitro. This work follows on the classic work of Fork who, in the early 1970s, demonstrated that laser illumination could produce excitation of molluscan neurons through a reversible, but unknown mechanism. Hirase et al., reporting in the Journal of Neurobiology, used modern two-photon techniques that enabled the laser light to be focused much more precisely than the technology used by Fork. They demonstrated that excitation of pyramidal neurons in brain slices from mouse visual cortex required the illumination to be applied tangential to the membrane of the cell, and that excitation was ineffective if the laser was focused below or within the cell.
Although not entirely clear, the experiments suggested two mechanisms. First, the data support that light-induced membrane depolarization resulted from a photochemical reaction that produced reactive oxygen species adjacent to the cell. The second mechanism was a transient perforation of the membrane that quickly re-sealed after the light was discontinued.
Illumination was able to excite neurons at short latency and the probability of excitation was modulated by both the intensity and wavelength of illumination. Thus, two-photon laser illumination provides a selective and controllable method to excite selectively single neurons. This will provide a powerful tool to understand processing within networks of neurons, and lays a foundation for further work developing light-based methods for directly stimulating neurons.
While Hirase et al. used focused illumination to achieve selective stimulation of single neurons, a report in Neuron by Zemelan and colleagues from Gero Miesenbock's lab at the Memorial Sloan-Kettering Cancer Center demonstrated genetic manipulation to make only certain neurons responsive to illumination. They expressed in cultured hippocampal neurons genes coding for elements of the invertebrate retina. The retinal elements produced a light-controlled source of excitatory current in the effected cells, as they would in the native retina.
When exposed to light the neurons that were transfected with the retinal elements depolarized and generated action potentials at latencies between less than one second and several tens of seconds. The pattern of firing ranged from single spikes to bursts of spikes, as would be observed during conventional intracellular recording, and the firing frequency could be increased by increasing the light intensity. The variable nature of genetic transfection was presumably the cause of the variability in responsiveness across neurons.
Alternating periods of dark and light demonstrated that there was hysteresis in the neuronal response, that the latencies were long and variable, and that there was apparent continued excitation after the illumination was turned off. These effects were presumably the result of expressing only the minimal subset of the retinal elements needed to produce light responsiveness, rather than the full complement of retinal regulatory proteins. Application of this technique in vivo would provide a means to selectively excite only a specific class or classes of neurons that were transfected using cell-specific methods.
Optical technology has had a huge impact on everything from entertainment to communications. Within the realm of neurotechnology modern optical methods have greatly increased our ability to see into the nervous system with such things as multi-photon microscopy and chemical-dependent and voltage -dependent fluorescent dyes. The recent advances demonstrating optical stimulation of the nervous system provide powerful new tools to study neural function. They are perhaps telling of a day when neurotechnology will follow in the footsteps of communications technology and move from electrically-based devices to optically-based devices.
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