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Imaging the brain

Researchers use the latest light techniques to advance brain research, offering the potential of state-of-the-art diagnostics and surgical techniques never before possible. Just a few of the techniques include: A near-infrared endoscope that identifies tumors early on; patch clamp technology that creates an on-off switch for neuronal activity; a single fiber sends electrical and optical signals to and from the brain; and a photonic photodetector that can be implanted deep within the brain and can act as a photoreceptor that can be controlled by light.

A new dye spots brain tumors

Researchers in the Perelman School of Medicine at the University of Pennsylvania developed an experimental near-infrared endoscopic tool that allows surgeons to identify pituitary adenomas tumor tissue. While these tumors rarely turn cancerous, they can cause blindness, hormonal disorders and even giantism. The tool uses OTL38, a targeted fluorescent dye, to light up the benign brain tumors. In a pilot study, which was published in the Journal of Neurosurgery, surgeons used a targeted, near-infrared dye—the first to be employed in brain tumor surgery. Other dyes are limited either by their fluorescent range being in the visible spectrum or by lack of specificity. The fluorescent dye consists of two parts: vitamin B9 (a necessary ingredient for cell growth), and a near infrared glowing dye. As tumors try to grow and proliferate, they overexpress folate receptors. This dye binds to these receptors and thus allows for the tumors to be identified.

A challenge in this type of brain surgery is making sure that the entire tumor is removed because when parts of the tumor are not removed, 20% of patients have a recurrence. The technique uses a VisionSense Iridium 4-mm endoscope system that can be used in the nasal cavity to illuminate the pituitary adenoma. Both the dye and the camera system are needed in order to perform the surgery successfully.

This fluorescent dye imaging tool may someday serve as a replacement for large MRI machines in the operating room. Over the past four years, Singhal, Lee and their colleagues have performed more than 400 surgeries using both nonspecific and targeted near infrared dyes. The breadth of tumor types includes lung, brain, bladder and breast. In a recent lung cancer trial used the OTL38 dye, surgeons were able to identify and remove a greater number of cancerous nodules from lung cancer patients with the dye using preoperative positron emission tomography (PET), scans. Penn’s imaging tool identified 60 of the 66 previously known lung nodules, or 91%. In addition, doctors used the tool to identify nine additional nodules that were undetected by the PET scan or by traditional intraoperative monitoring.

An on and off switch

Traditional tools such as patch clamp technology are still widely used because they provide superior data for many types of experiments. Researchers at the AstraZeneca-Tufts Laboratory for Basic and Translational Research are using patch clamp technology to study epileptic seizures.

When signals are transmitted along neurons, there is a spike in electrical potential along outer membranes, which is called “action potential.” Patch clamp technology helps researchers to study electrical potential changes across a cell membrane and requires the strategic placement of electrodes on either side of that membrane. Specialized circuitry is then used to maintain a set potential difference between the two electrodes, i.e., to clamp the voltage difference, hence the name “patch clamp.”

Tarek Deeb is Program Director in a large research group in the AstraZeneca-Tufts lab headed by Professor Stephen Moss of Tufts Dept of Neuroscience and Dr. Nick Brandon, Chief Scientist, Neuroscience, AstraZeneca. Deeb explains, “It is well-known in neurophysiology that healthy function of the vertebrate central nervous system depends on both synaptic excitation and synaptic inhibition. You can think of these as the “on” and “off” switches that define the signaling between neurons. Impaired neuronal inhibition leads to uncontrolled neuron activity (“firing”), which has long been associated with the increased probability of seizure occurrence and heightened seizure severity. My research goal is to develop novel strategies for the treatment of disorders linked to impaired inhibitory transmission in the brain. Specifically, I’m currently studying the impact of KCC2 inhibitors in this context.”

The principal synaptic transmission inhibitor in the brain is gamma-aminobutyric acid (GABA). Its fundamental role is underlined by the fact that GABA receptors are also critical drug targets for anti-convulsants, sedatives and anesthetics. These receptors are ligand-gated ion channels that allow chloride (Cl-) ion influx into neurons causing hyperpolarization of the neuron membrane and thereby preventing neuron firing.  Deeb notes that, “This fast-inhibitory mechanism critically depends on there being a low C- concentration in the neurons.  In adult mammalian brains, this low Cl- concentration is maintained by a membrane pump called K+/Cl- co-transporter-2, or KCC2 for short. So, normal synaptic inhibition in turn depends on the correct KCC2 function. Our research is looking at precisely how decreased KCC2 transport function impacts seizure event severity. We are conducting these studies at the cellular level using patch clamp electrodes to monitor the behavior of neurons in slices of murine brain tissue in continuously oxygenated artificial cerebrospinal fluid (ACSF). We use brain samples from normal mice and from a strain of genetically modified mice where a point-mutation is known to impair KCC2 action.”

Flexible fiber sends signals to and from the brain

A single fiber no wider than a human hair has been demonstrated by MIT graduate students to successfully deliver a combination of optical, electrical and chemical signals back and forth into the brain. The new approach could provide an improved way to learn about the functions and interconnections of different brain regions. The fibers mimic the softness and flexibility of brain tissue, which could make it possible to leave implants in place and have them retain their functions over longer periods than with stiff, metallic fibers. In tests with lab mice, the researchers were able to inject viral vectors that carried genes called opsins, which sensitize neurons to light, through one of two fluid channels in the fiber. Previous research efforts relied on separate devices: needles to inject viral vectors for optogenetics, optical fibers for light delivery and arrays of electrodes for recording. Now one single fiber can do it all or more than one can be used to probe different regions of the brain at the same time.

After much work, the team was able to engineer a composite of conductive polyethylene doped with graphite flakes. The polyethylene was initially formed into layers, sprinkled with graphite flakes, then compressed; then another pair of layers was added and compressed, and then another, and so on. That method increased the conductivity of the polymer by a factor of four or five. The researchers found that it takes about 11 days to produce effects.

Photonic probes in optogenetics

Stimulating neural circuits with very high precision light to control cells is key to advances in the study and mapping of the living brain. A group from Caltech, Baylor College of Medicine, and Stanford University, describe a solution published in the journal Neurophotonics. Patterned photostimulation via visible-wavelength photonic probes for deep brain optogenetics combines nanophotonics and microelectromechanical systems (MEMS) in an implantable, ultra-narrow, silicon-based photonic probe that when inserted, delivers light deep within brain tissues. Using techniques of optogenetics, a protein in the brain serves as a sensory photoreceptor and can be controlled by specific wavelengths of light.

These breakthroughs present widespread and promising applications for the neuroscience and neuromedical research communities. From characterizing the role of specific neurons and identifying neural circuits responsible for behavior to enabling new methods of operant conditioning through reward-induced circuit activations, optogenetics has become a new path for neuroscientists seeking advances in research capabilities.

Written by Anne Fischer, Managing Editor, Novus Light Technologies Today

Labels: brain,endoscope,spectroscopy,near-infrared,photonics,photodetector,photoreceptor,biomedical,imaging,

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