Optogenetics studies the fly olfactory system to understand neural circuits

Researchers at the European Molecular Biology Laboratory (EMBL)'s Centre for Genomic Regulation (Barcelona, Spain) used optogenetics to explore how complex behaviors, such as chemotaxis (the movement of an organism in response to a chemical stimulus), are controlled by the activity of neural circuits. The study, conducted by the Sensory Systems and Behavior laboratory led by Matthieu Louis, used the fruit fly Drosophila melanogaster to make their observations.

Related: Bioimaging, optical stimulation enable real-time observation of brain circuits

To identify the neural circuits involved in chemotaxis, the research team decided to concentrate on the fruit fly larva, which comprises 10,000 neurons—10 times less than adult flies and 10 million times less than humans. They screened over 1100 fly strains where the function of a small subset of neurons of the brain could be genetically turned off. "At the beginning of this project, we had the feeling to be looking for a needle in a haystack. We knew about the 21 olfactory neurons in the head of the larva and the motor neurons in the equivalent of spinal cord in the larva. In contrast, we had virtually no clue about the identity of the neurons in between, the synapses responsible for the processing of the olfactory information and its conversion into navigational decisions," Louis explains.

From this screening, the research team was drawn to a handful of neurons located in a region traditionally associated with reflexive taste behavior. When the function of the identified neurons was silenced, larvae became unable to make accurate decision to navigate odor gradients. Using optogenetics, a method that exploits light to control and monitor neurons, Ibrahim Tastekin, one of the co-first authors of the work, was able to activate individual neurons. To his astonishment, he found that brief excitations were sufficient to trigger a change in orientation. "This was like magic: optogenetics gave us a means to remote control an elementary form of decision making. The fly genetic toolkit creates unprecedented possibilities to probe the function of individual neurons in a semi high-throughput manner. Here we proved the necessity and sufficiency of a couple of neurons to control a fundamental aspect of chemotaxis: the conversion of sensory information into behavior.” The team went further by demonstrating that the neurons identified in this zone are involved in the processing of odor, light, and temperature. “We are very excited to define how these neurons operate in concert with the rest of the circuitry in charge of chemotaxis,” Louis says.

Subesophageal ganglion (SOG) neurons.

Beyond identifying which brain regions control individual feature of a behavior of interest, the aim of Louis' lab is to understand how neurons form neural circuits, how circuits work together to achieve computations, and how complex behaviors emerge from brain computations.

Little is known about the neural circuits connecting the sensory neurons to the motor system. By adopting a systems neuroscience approach, Louis' lab hopes to understand how a relatively small nervous system of 10,000 neurons represents and integrates changes in sensory signals to direct navigational decisions, and how learning affects the function of neural circuits.

Full details of the work appear in the journal Current Biology; for more information, please visit http://dx.doi.org/10.1016/j.cub.2015.04.016.

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