An illustrative picture of the Neuronal Positional System used for brain mapping.
(photo credit: DR. SHLOMO TSURIEL AND DR. ALEX BINSHTOK)
Adapting global positioning systems used by smartphones, scientists from the Hebrew University of Jerusalem and Harvard University in Massachusetts have succeeded in mapping the circuitry of the brain. Their description of the new method was published on Monday evening in the online advanced edition of the prestigious journal Nature Methods.
The innovative work on a Neuronal Positional System was carried out by Dr. Shlomo Tsuriel, a postdoctoral fellow, and student Sagi Gudes, under the guidance of Dr. Alex Binshtok at the HU Faculty of Medicine, and Dr. Jeff Lichtman at Harvard. Just as GPS mapping systems use data from three or more satellites to triangulate its position, they called the new brain mapping technique NPS.
Nerves look like trees, with axonal branches (“arbors”). The binational team managed to map many individual neurons simultaneously at the high resolution of individual axons. Thus, by “seeing” many axons in the same preparation, it became possible to understand how specific neurons in one region are wired to other neuronal types and other regions in the brain.
For more than a century, neuroscientists have tried to discover the structure of the brain’s neuronal circuits so they could better understand how the brain functions. These brain circuits – which perform functions such as processing information and triggering reflexes – are made up of nervous system cells called neurons that work together to carry out a specialized function. Neurons send the messages to other neurons or to target tissues such as skin and muscle that they innervate, via specialized wire-like structures called axons.
Just as one would need to know the exact wiring of an electrical circuit to understand how it works, it is necessary to map the axonal wiring of neuronal circuits to understand how they function. Therefore a fundamental goal of neuroscience research is to understand the structural and functional connections of the brain’s circuits.
While numerous scientific consortia have advanced our understanding of neuronal organization, the available mapping techniques remain imperfect. For example, serial electron microscope techniques are limited to the area they can map, and tracer-based techniques are limited in the detail resolution. This new approach makes it possible to learn about organizational principles of neuronal networks that would otherwise be nearly impossible to study.
Instead of trying to trace entire neurons all the way from the axon tips to the cell body, Tsuriel labeled only the cell body, but in a way that indicates the locations of its axonal branches. With that aim, he used multiple injections in overlapping regions of a target tissue, with three or more differently colored retrograde tracers.
At each point the tracer was injected in a high concentration and spread to the area between the injection points, such that each area in the target tissue had a different color combination depending on its distance from the injection site.
Axons leading to each area took up the dyes and transported them in small vesicles to the cell body, such that each vesicle had a color combination reflecting the area it was taken from.
A few hours after the injection, each neuronal cell body was filled with vesicles in a variety of colors reflecting the colors in the areas that these neurons innervate. Thus, based on the combinations and intensities of the colors in the individual vesicles transported to the cell, the projection sites of the axon can be outlined.
“The new method that we developed allows us to answer a ‘big question’ in neuroscience about the organizational principles of neuronal circuitism,” explained Binshtok.
“Using the NPS technique that maps many axons in the same tissue, we now can study what defines the routes along which the neurons will send their projections, as well as their targets. We can also learn how the wiring of the neuronal circuits changes during development and in a variety of pathological conditions. The answers to these questions will be the first step to comprehending how the information flows and is processed in the nervous system and how changes in the neuronal organization affect neuronal function. I believe many scientists will find the NPS approach useful to help them answer the question of how the brain works.”