Extrinsic desensitization requiring synaptic transmission. (A) Illustration of insect olfactory neuron anatomy. Olfactory neurons project axons into the antenna lobe (equivalent of the olfactory bulb in vertebrates) where they activate second-order PNs that project to higher brain centers. Olfactory neurons also activate LNs that are local feedback neurons. (B) Normal signal transmission between ORNs and PNs in the glomeruli. ORN action potentials result in an influx of Ca2+ that triggers release of acetylcholine (ACh) that activates the downstream PNs. Activity in the ORNs and PNs is highly correlated.24 The ORN axon also synapses with GABAergic and peptidergic Drosophila tachykinin (DTK) LNs. ORNs express both GABAA and GABAB receptors as well as tachykinin receptors (DTKRs) that provide negative feedback to the ORN. (C) Sustained activity in the olfactory receptor neurons suppresses ORN activity through GABA and tachykinin-mediated presynaptic inhibition. Ionotropic GABAA receptors are chloride channels that hyperpolarize the neuron and mediate rapid adaptation, whereas metabotropic GABAB receptors mediate long-term adaptation.23 TKRs also modulate the sensitivity of ORNs.30 GABA, γ-aminobutyric acid; LNs, local neurons; ORN, olfactory receptor neuron; PNs, projection neurons; TKRs, tachykinin receptors.