Friday, September 26, 2008

Scientific American Mind - Neural Light Show: Scientists Use Genetics to Map and Control Brain Functions

A very cool article on using new brain scan technology to see how the mind operates at the deepest levels (brain circuits and genes). It's important to keep clear that consciousness is the reducible to brain areas lighting up, but it is still quite interesting.

We are beginning to see the objective manifestation of subjective states, which can only help us to make more sense of the complexity that is consciousness.

Neural Light Show: Scientists Use Genetics to Map and Control Brain Functions

A clever combination of optics and genetics is allowing neuroscientists to identify and control brain circuits with unprecedented precision

By Gero Miesenböck

GUIDING LIGHT: New methods that employ light to reveal and control neural activity are enabling researchers to study individual circuits in animals—work that should also lead to a better understanding of how the human brain functions. Alfred T. Kamajian

Key Concepts

  • Neuroscientists have traditionally studied the function of the brain by stimulating and recording the activity of single nerve cells with elec­trodes. But this method is indirect, making analyses of specific neurons very difficult.
  • The emerging field of optogenetics, which combines genetic engineering with light to observe and control groups of neurons, is allowing researchers to scrutinize individual neural circuits—an approach that will revolutionize the study of brain function.

In 1937 the great neuroscientist Sir Charles Scott Sherrington of the University of Oxford laid out what would become a classic description of the brain at work. He imagined points of light signaling the activity of nerve cells and their connections. During deep sleep, he proposed, only a few remote parts of the brain would twinkle, giving the organ the appearance of a starry night sky. But at awakening, “it is as if the Milky Way entered upon some cosmic dance,” Sherrington reflected. “Swiftly the head-mass becomes an enchanted loom where millions of flashing shuttles weave a dissolving pattern, always a meaningful pattern though never an abiding one; a shifting harmony of subpatterns.”

Although Sherrington probably did not realize it at the time, his poetic metaphor contained an important scientific idea: that of the brain revealing its inner workings optically. Understanding how neurons work together to generate thoughts and behavior remains one of the most difficult open problems in all of biology, largely because scientists generally cannot see whole neural circuits in action. The standard approach of probing one or two neurons with electrodes reveals only tiny fragments of a much bigger puzzle, with too many pieces missing to guess the full picture. But if one could watch neurons communicate, one might be able to deduce how brain circuits are laid out and how they function. This alluring notion has inspired neuroscientists to attempt to realize Sherrington’s vision.

Their efforts have given rise to a nascent field called optogenetics, which combines genetic engineering with optics to study specific cell types. Already investigators have succeeded in visualizing the functions of various groups of neurons. Furthermore, the approach has enabled them to actually control the neurons remotely—simply by toggling a light switch. These achievements raise the prospect that optogenetics might one day lay open the brain’s circuitry to neuroscientists and perhaps even help physicians to treat certain medical disorders.

Enchanting the Loom
Attempts to turn Sherrington’s vision into reality began in earnest in the 1970s. Like digital computers, nervous systems run on electricity; neurons encode information in electrical signals, or action potentials. These impulses, which typically involve voltages less than a tenth of those of a single AA battery, induce a nerve cell to release neurotransmitter molecules that then activate or inhibit connected cells in a circuit. In an effort to make these electrical signals visible, Lawrence B. Cohen of Yale University tested a large number of fluorescent dyes for their ability to respond to voltage changes with changes in color or intensity. He found that some dyes indeed had voltage-sensitive optical properties. By staining neurons with these dyes, Cohen could observe their activity under a microscope.

Dyes can also reveal neural firing by reacting not to voltage changes but to the flow of specific charged atoms, or ions. When a neuron generates an action potential, membrane channels open and admit calcium ions into the cell. This calcium influx stimulates the release of neurotransmitters. In 1980 Roger Y. Tsien, now at the University of California, San Diego, began to synthesize dyes that could indicate shifts in calcium concentration by changing how brightly they fluoresced. These optical reporters have proved extraordinarily valuable, opening new windows on information processing in single neurons and small networks.

Synthetic dyes suffer from a serious drawback, however. Neural tissue is composed of many different cell types. Estimates suggest that the brain of a mouse, for example, houses many hundreds of types of neurons plus numerous kinds of support cells. Because interactions between specific types of neurons form the basis of neural information processing, someone who wants to understand how a particular circuit works must be able to identify and monitor the individual players and pinpoint when they turn on (fire an action potential) and off. But because synthetic dyes stain all cell types indiscriminately, it is generally impossible to trace the optical signals back to specific types of cells.

Genes and Photons
Optogenetics emerged from the realization that genetic manipulation might be the key to solving this problem of indiscriminate staining. An individual’s cells all contain the same genes, but what makes two cells different from each other is that different mixes of genes get turned on or off in them. Neurons that release the neurotransmitter dopamine when they fire, for instance, need the enzymatic machinery for making and packaging dopamine. The genes encoding the protein components of this machinery are thus switched on in dopamine-producing (dopaminergic) neurons but stay off in other, nondopaminergic neurons.

In theory, if a biological switch that turned a dopamine-making gene on was linked to a gene encoding a dye and if the switch-and-dye unit were engineered into the cells of an animal, the animal would make the dye only in dopaminergic cells. If researchers could peer into the brains of these creatures (as is indeed possible), they could see dopaminergic cells functioning in virtual isolation from other cell types. Furthermore, they could observe these cells in the intact, living brain. Synthetic dyes cannot perform this type of magic, because their production is not controlled by genetic switches that flip to on exclusively in certain kinds of cells. The trick works only when a dye is encoded by a gene—that is, when the dye is a protein.

The first demonstrations that genetically encoded dyes could report on neural activity came a decade ago, from teams led independently by Tsien, Ehud Y. Isacoff of the University of California, Berkeley, and me, with James E. Rothman, now at Yale University. In all cases, the gene for the dye was borrowed from a luminescent marine organism, typically a jellyfish that makes the so-called green fluorescent protein. We tweaked the gene so that its protein product could detect and reveal the changes in voltage or calcium that underlie signaling within a cell, as well as the release of neurotransmitters that enable signaling between cells.

Armed with these genetically encoded activity sensors, we and others bred animals in which the genes encoding the sensors would turn on only in precisely defined sets of neurons. Many favorite organisms of geneticists—including worms, zebra fish and mice—have now been analyzed in this way, but fruit flies have proved particularly willing to spill their secrets under the combined assault of optics and genetics. Their brains are compact and visible through a microscope, so entire circuits can be seen in a single field of view. Furthermore, flies are easily modified genetically, and a century of research has identified many of the genetic on-off switches necessary for targeting specific groups of neurons. Indeed, it was in flies that Minna Ng, Robert D. Roorda and I, all of us then at Memorial Sloan-Kettering Cancer Center in New York City, recorded the first images of information flow between defined sets of neurons in an intact brain. We have since discovered new circuit layouts and new operating principles. For example, last year we found neurons in the fly’s scent-processing circuitry that appear to inject “background noise” into the system. We speculate that the added buzz amplifies faint inputs, thus heightening the animal’s sensitivity to smells—all the better for finding food.

The sensors provided us with a powerful tool for observing communication among neurons. But back in the late 1990s we still had a problem. Most experiments probing the function of the nervous system are rather indirect. Investigators stimulate a response in the brain by exposing an animal to an image, a tone or a scent, and they try to work out the resulting signaling pathway by inserting electrodes at downstream sites and measuring the electrical signals picked up at these positions. Unfortunately, sensory inputs undergo extensive reformatting as they travel. Consequently, knowing exactly which signals underlie responses recorded at some distance from the eye, ear or nose becomes harder the farther one moves from these organs. And, of course, for the many circuits in the brain that are not devoted to sensory processing but rather to movement, thought or emotion, the approach fails outright: there is no direct way of activating these circuits with sensory stimuli.

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