Lateral presynaptic inhibition mediates gain control in an olfactory circuit

Olfactory signals are transduced by a large family of odorant receptor proteins, each of which corresponds to a unique glomerulus in the first olfactory relay of the brain. Crosstalk between glomeruli has been proposed to be important in olfactory processing, but it is not clear how these interactions shape the odour responses of second-order neurons. In the Drosophila antennal lobe (a region analogous to the vertebrate olfactory bulb), we selectively removed most interglomerular input to genetically identified second-order olfactory neurons. Here we show that this broadens the odour tuning of these neurons, implying that interglomerular inhibition dominates over interglomerular excitation. The strength of this inhibitory signal scales with total feedforward input to the entire antennal lobe, and has similar tuning in different glomeruli. A substantial portion of this interglomerular inhibition acts at a presynaptic locus, and our results imply that this is mediated by both ionotropic and metabotropic receptors on the same nerve terminal. Odours in the balance The fruit fly is increasingly being recognized as an excellent system in which to study neural circuit function. In experiments combining in vivo systems neuroscience with synaptic physiology and Drosophila genetics, Shawn Olsen and Rachel Wilson have identified a presynaptic form of lateral inhibition in the olfactory system. This can promote coding efficiency when stimuli are strong and unambiguous, and maximize sensitivity when stimuli are weak and ambiguous. The results could shed light on vertebrate neuronal circuits such as those of the olfactory bulb or visual cortex. A study that combines in vivo systems neuroscience with synaptic physiology and Drosophila genetics identifies a presynaptic form of lateral inhibition in the olfactory system. The mechanism allows for a flexible form of gain control, which promotes coding efficiency when stimuli are strong and unambiguous, but maximizes sensitivity when stimuli are weak and ambiguous.