Dopaminergic neurons (DANs) signal punishment and reward during associative learning. In mammals, DANs show associative plasticity that correlates with the discrepancy between predicted and actual reinforcement (prediction error) during classical conditioning. Also in insects, such as Drosophila, DANs show associative plasticity that is, however, less understood. Here, we study associative plasticity in DANs and their synaptic partners, the Kenyon cells (KCs) in the mushroom bodies (MBs), while training Drosophila to associate an odorant with a temporally separated electric shock (trace conditioning). In most MB compartments DANs strengthened their responses to the conditioned odorant relative to untrained animals. This response plasticity preserved the initial degree of similarity between the odorant- and the shock-induced spatial response patterns, which decreased in untrained animals. Contrary to DANs, KCs (α'/β'-type) decreased their responses to the conditioned odorant relative to untrained animals. We found no evidence for prediction error coding by DANs during conditioning. Rather, our data supports the hypothesis that DAN plasticity encodes conditioning-induced changes in the odorant's predictive power.

Animals encounter fine-scale temporal patterns of odorant mixtures that contain information about the distance and number of odorant sources. To study the role of such temporal cues for odorant detection and source localization, one needs odorant delivery devices that are capable of mimicking the temporal stimulus statistics of natural odor plumes. However, current odorant delivery devices either lack temporal resolution or are limited to a single odorant channel. Here, we present an olfactory stimulator that features precise control of high-bandwidth stimulus dynamics, which allows generating arbitrary fluctuating binary odorant mixtures. We provide a comprehensive characterization of the stimulator's performance and use it to demonstrate that odor background affects the temporal resolution of insect olfactory receptor neurons, and we present a hitherto unknown odor pulse-tracking capability of up to 60 Hz in Kenyon cells, which are higher order olfactory neurons of the insect brain. This stimulator might help investigating whether and how animals use temporal stimulus cues for odor detection and source localization. Because the stimulator is easy to replicate it can facilitate generating the same odor stimulus dynamics at different experimental setups and across different labs.

Animals can form associations between temporally separated stimuli. To do so, the nervous system has to retain a neural representation of the first stimulus until the second stimulus appears. The neural substrate of such sensory stimulus memories is unknown. Here, we search for a sensory odor memory in the insect olfactory system and characterize odorant-evoked Ca2+ activity at three consecutive layers of the olfactory system in Drosophila: in olfactory receptor neurons (ORNs) and projection neurons (PNs) in the antennal lobe, and in Kenyon cells (KCs) in the mushroom body. We show that the post-stimulus responses in ORN axons, PN dendrites, PN somata, and KC dendrites are odor-specific, but they are not predictive of the chemical identity of past olfactory stimuli. However, the post-stimulus responses in KC somata carry information about the identity of previous olfactory stimuli. These findings show that the Ca2+ dynamics in KC somata could encode a sensory memory of odorant identity and thus might serve as a basis for associations between temporally separated stimuli.

SUMMARY Anopheles mosquitoes are the sole vectors of malaria. Although adult females are directly responsible for disease transmission and accordingly have been extensively studied, the survival of pre-adult larval stages is vital. Mosquito larvae utilize a spectrum of chemosensory and other cues to navigate their aquatic habitats to avoid predators and search for food. Here we examine larval olfactory responses, in which the peripheral components are associated with the antennal sensory cone. Larval behavior and sensory cone responses to volatile stimuli in Anopheles coluzzii demonstrate the sensory cone is particularly tuned to alcohols, thiazoles, and heterocyclics, and these responses can be assigned to discrete groups of sensory cone neurons with distinctive profiles. These studies reveal that the anopheline larvae actively sample volatile odors above their aquatic habitats via a highly sophisticated olfactory system that is sensitive to a broad range of compounds with significant behavioral relevance.

SeqPeelSort is a spike sorting algorithm for single sensillum recordings (SSR). Single sensillum recordings are a standard technique in insect neuroscience. In such electrophysiological recordings, a single electrode is inserted into a sensory sensillum and the transepithelial potential is recorded. This technique is extensively (but not only) used in Drosophila olfaction studies (see for example Hallem, Ho, & Carlson (2004) or Lin & Potter (2015) and citations therein).