A Bright Idea
An illuminating symposium chaired by Mark Schnitzer, Ph.D., assistant professor of biology and of applied physics at Stanford University, explored “New Technologies for Probing Brain Disease with Light: From Super-Resolution and in vivo Imaging to Optical Control of Circuits.” Light energy is helping researchers explore the healthy and diseased nervous system with technologies such as optogenetics, super-resolution imaging, and microscopy in live unanesthetized animals.
Guoping Feng, Ph.D., associate professor of neurobiology at Duke University’s Institute for Brain Sciences, described the development of cell type-specific optogenetic mice for use in probing neural circuitry and dysfunction. Xiaowei Zhuang, Ph.D., professor of chemistry and chemical biology and of physics at Harvard University, gave a presentation on the application of nanoscopic fluorescence imaging to study neurons and brain tissue.
Maiken Nedergaard, from the University of Rochester, described her work using in vivo two-photon imaging in astrocyte cell cultures containing luciferase to study the role these cells play in neuronal function and communication.
Dr. Schnitzer’s work using minimally invasive imaging in awake mice to study the brain’s “cellular orchestra”, aims to define large-scale patterns that underlie brain function. Light-scattering imaging techniques that rely on confocal and two-photon microscopy are limited by how deeply in the brain they are able to look. Dr. Schnitzer described an alternative technique called fluorescence microendoscopy that uses micro-optical probes to target specific brain regions. By introducing fluorescein into the plasma of mice and inserting probes into the hippocampal region, he demonstrated the ability to image capillary blood flow. “At the highest resolution you can see individual red blood cells.”
An important advantage of this relatively low-cost technology is the ability to re-image the same site in the same animal over time. He described a range of clinical applications in which microendoscopy could be relevant; in muscular dystrophy, for example, the probes can be inserted directly into the muscle to image the contractile behavior of individual muscle fibers. Using laser scanning just once can image the dynamics of sarcomere contractility over time.
Karl Deisseroth, M.D., Ph.D., associate professor in the departments of bioengineering and of psychiatry and behavioral sciences at Stanford University, showed how light-responsive proteins called opsins, derived from microorganisms such as algae, respond to specific wavelengths of light and can be used to control individual cell types in neural circuits, leaving others unaffected.
Channelrhodopsin-2 (ChR2), a positive stimulator (or “on switch”) for neurons that was isolated from Chlamydomonas reinhardtii, is an example of an opsin. Fiberoptic delivery of pulses of blue light at 470 nm into the brains of ChR2-expressing transgenic mice results in optogenetic stimulation, causing the light-gated ion channels to open and allowing sodium ions to flow across the membranes of ChR2-containing cells.
Feng Zhang, a graduate student in Dr. Deisseroth’s laboratory described the ongoing search for light-responsive proteins in microorganisms across diverse ecological environments, with the hope of finding defined populations of molecules that he described as “microbial antennae,” which will allow for targeted light stimulation of neurons to activate or deactivate specific neuronal circuits and synaptic connections.
Herbert Covington III, a postdoctoral fellow, described research performed in Eric Nestler, M.D., Ph.D.’s laboratory of molecular psychiatry at Mount Sinai School of Medicine, in which directed laser light was used to stimulate Ch2 in the medial prefrontal cortex of mice with a depression-like phenotype. This region of the brain exhibits altered function in humans suffering from major depression. Optogenetic stimulation of Ch2 was associated with antidepressant effects in the mice.
At the University of California, San Francisco, Garret Stuber, Ph.D., and colleagues are exploring the use of optogenetic control of brain reward circuitry to modulate addictive behaviors. They are using Ch2 and light stimulation in the amygdala to determine which synapses in the brain have reinforcing effects similar to drugs of abuse.
Preliminary results of studies on optical stimulation-driven memory recall performed by Michael Häusser, Ph.D., and co-workers at University College London show that Ch2 activation to provoke memory recall stimulates neural activity and plasticity only in brain cells initially involved in memory formation. The researchers used a mouse model of memory based on fear conditioning, in which the mice were conditioned to respond to a tone. They demonstrated that in mice injected in the hippocampus with the Ch2 construct, light activation of Ch2 could evoke fear memory recall and the same behavior the mice exhibit after hearing the tone.