Parallel Neural Pathways Mediate CO2 Avoidance Responses in Drosophila

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Science  14 Jun 2013:
Vol. 340, Issue 6138, pp. 1338-1341
DOI: 10.1126/science.1236693

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Too Much or Too Little

An important task of the nervous system is to distribute information appropriately throughout the brain. The olfactory and gustatory systems of Drosophila provide good models for understanding these processes and the underlying mechanisms (see the Perspective by Su and Carlson). Lin et al. (p. 1338) mapped out the circuit that detects carbon dioxide (CO2), an important environmental and communication signal for fruit flies. Two distinct classes of projection neurons mediate avoidance of high and low concentrations of CO2, while a third class, comprising inhibitory neurons, shuts down the low-concentration pathway at high concentrations. In contrast to other basic taste qualities, salt is innately attractive at low concentrations, but aversive at high concentrations. The mechanisms underlying salt detection are poorly understood in any species mainly because of a lack of specific molecular tools. Zhang et al. (p. 1334) discovered that Drosophila uses two types of gustatory receptor neurons to distinguish between high and low concentrations of salt. One type is activated maximally by low salt and induces attractive feeding behavior. The other class of receptors is activated primarily by high salt and leads to avoidance behavior.


Different stimulus intensities elicit distinct perceptions, implying that input signals are either conveyed through an overlapping but distinct subpopulation of sensory neurons or channeled into divergent brain circuits according to intensity. In Drosophila, carbon dioxide (CO2) is detected by a single type of olfactory sensory neuron, but information is conveyed to higher brain centers through second-order projection neurons (PNs). Two distinct pathways, PNv-1 and PNv-2, are necessary and sufficient for avoidance responses to low and high CO2 concentrations, respectively. Whereas low concentrations activate PNv-1, high concentrations activate both PNvs and GABAergic PNv-3, which may inhibit PNv-1 pathway-mediated avoidance behavior. Channeling a sensory input into distinct neural pathways allows the perception of an odor to be further modulated by both stimulus intensity and context.

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