Presynaptic Ca2+ stores contribute to odor-induced responses in Drosophila olfactory receptor neurons

In both vertebrates and invertebrates, olfactory receptor neurons (ORNs) respond to several odors. They also adapt to stimulus variations, and this is considered to be a simple form of non-associative learning and neuronal plasticity. Different mechanisms have been described to support neuronal and/or synaptic plasticity. For example in vertebrates, presynaptic Ca2+ stores relying on either the ryanodine receptor (RyR) or the inositol (1,4,5)-trisphosphate receptor (InsP(3)R) have been reported to participate in synaptic transmission, in hippocampal pyramidal neurons, and in basket cell-Purkinje cell synapses. However, in invertebrates, especially in sensory neurons such as ORNs, similar mechanisms have not yet been detected. In this study, using Drosophila and taking advantage of an in vivo bioluminescence Ca2+-imaging technique in combination with genetic and pharmacological tools, first we show that the GFP-aequorin Ca2+ sensor is sensitive enough to detect odor-induced responses of various durations. Second, we show that for a relatively long (5 s) odor application, odor-induced Ca2+ responses occurring in the axon terminals of ORNs involve intracellular Ca2+ stores. This response is decreased by specifically targeting InsP(3)R or RyR by RNAi, or application of the specific blockers thapsigargin or ryanodine, suggesting that Ca2+ stores serve to amplify the presynaptic signal. Furthermore, we show that disrupting the intracellular Ca2+ stores in the ORNs has functional consequences since InsP(3)R- or RyR-RNAi expressing flies were defective in olfactory behavior. Altogether, our results indicate that for long odor applications in Drosophila, the olfactory response depends on intracellular Ca2+ stores within the axon terminals of the ORNs.