The olfactory circuit of the fruit fly Drosophila melanogaster

The functional logic of odor information processing in the Drosophila antennal lobe

Recent advances in molecular transduction of odorants in the Olfactory Sensory Neurons (OSNs) of the Drosophila Antenna have shown that the odorant object identity is multiplicatively coupled with the odorant concentration waveform. The resulting combinatorial neural code is a confounding representation of odorant semantic information (identity) and syntactic information (concentration). To distill the functional logic of odor information processing in the Antennal Lobe (AL) a number of challenges need to be addressed including 1) how is the odorant semantic information decoupled from the syntactic information at the level of the AL, 2) how are these two information streams processed by the diverse AL Local Neurons (LNs) and 3) what is the end-to-end functional logic of the AL? By analyzing single-channel physiology recordings at the output of the AL, we found that the Projection Neuron responses can be decomposed into a concentration-invariant component, and two transient components boosting the positive/negative concentration contrast that indicate onset/offset timing information of the odorant object. We hypothesized that the concentration-invariant component, in the multi-channel context, is the recovered odorant identity vector presented between onset/offset timing events. We developed a model of LN pathways in the Antennal Lobe termed the differential Divisive Normalization Processors (DNPs), which robustly extract the semantics (the identity of the odorant object) and the ON/OFF semantic timing events indicating the presence/absence of an odorant object. For real-time processing with spiking PN models, we showed that the phase-space of the biological spike generator of the PN offers an intuit perspective for the representation of recovered odorant semantics and examined the dynamics induced by the odorant semantic timing events. Finally, we provided theoretical and computational evidence for the functional logic of the AL as a robust ON-OFF odorant object identity recovery processor across odorant identities, concentration amplitudes and waveform profiles.

Gut cytokines modulate olfaction through metabolic reprogramming of glia

Infection-induced aversion against enteropathogens is a conserved sickness behaviour that can promote host survival^1,2. The aetiology of this behaviour remains poorly understood, but studies in Drosophila have linked olfactory and gustatory perception to avoidance behaviours against toxic microorganisms^3-5. Whether and how enteric infections directly influence sensory perception to induce or modulate such behaviours remains unknown. Here we show that enteropathogen infection in Drosophila can modulate olfaction through metabolic reprogramming of ensheathing glia of the antennal lobe. Infection-induced unpaired cytokine expression in the intestine activates JAK-STAT signalling in ensheathing glia, inducing the expression of glial monocarboxylate transporters and the apolipoprotein glial lazarillo (GLaz), and affecting metabolic coupling of glia and neurons at the antennal lobe. This modulates olfactory discrimination, promotes the avoidance of bacteria-laced food and increases fly survival. Although transient in young flies, gut-induced metabolic reprogramming of ensheathing glia becomes constitutive in old flies owing to age-related intestinal inflammation, which contributes to an age-related decline in olfactory discrimination. Our findings identify adaptive glial metabolic reprogramming by gut-derived cytokines as a mechanism that causes lasting changes in a sensory system in ageing flies.

Seven Questions on the Chemical Ecology and Neurogenetics of Resource-Mediated Speciation

Adaptation to different environments can result in reproductive isolation between populations and the formation of new species. Food resources are among the most important environmental factors shaping local adaptation. The chemosensory system, the most ubiquitous sensory channel in the animal kingdom, not only detects food resources and their chemical composition, but also mediates sexual communication and reproductive isolation in many taxa. Chemosensory divergence may thus play a crucial role in resource-mediated adaptation and speciation. Understanding how the chemosensory system can facilitate resource-mediated ecological speciation requires integrating mechanistic studies of the chemosensory system with ecological studies, to link the genetics and physiology of chemosensory properties to divergent adaptation. In this review, we use examples of insect research to present seven key questions that can be used to understand how the chemosensory system can facilitate resource-mediated ecological speciation in consumer populations.

Odor-evoked inhibition of olfactory sensory neurons drives olfactory perception in Drosophila

Inhibitory response occurs throughout the nervous system, including the peripheral olfactory system. While odor-evoked excitation in peripheral olfactory cells is known to encode odor information, the molecular mechanism and functional roles of odor-evoked inhibition remain largely unknown. Here, we examined Drosophila olfactory sensory neurons and found that inhibitory odors triggered outward receptor currents by reducing the constitutive activities of odorant receptors, inhibiting the basal spike firing in olfactory sensory neurons. Remarkably, this odor-evoked inhibition of olfactory sensory neurons elicited by itself a full range of olfactory behaviors from attraction to avoidance, as did odor-evoked olfactory sensory neuron excitation. These results indicated that peripheral inhibition is comparable to excitation in encoding sensory signals rather than merely regulating excitation. Furthermore, we demonstrated that a bidirectional code with both odor-evoked inhibition and excitation in single olfactory sensory neurons increases the odor-coding capacity, providing a means of efficient sensory encoding.

It is well established that odor-evoked excitation in olfactory sensory neurons (OSNs) encodes odor information. Here the authors report that odor-evoked inhibition in OSNs of Drosophila also encodes odor identity, and can in itself drive both attraction and avoidance behaviors.

A Population of Projection Neurons that Inhibits the Lateral Horn but Excites the Antennal Lobe through Chemical Synapses in Drosophila

In the insect olfactory system, odor information is transferred from the antennal lobe (AL) to higher brain areas by projection neurons (PNs) in multiple AL tracts (ALTs). In several species, one of the ALTs, the mediolateral ALT (mlALT), contains some GABAergic PNs; in the Drosophila brain, the great majority of ventral PNs (vPNs) are GABAergic and project through this tract to the lateral horn (LH). Most excitatory PNs (ePNs), project through the medial ALT (mALT) to the mushroom body (MB) and the LH. Recent studies have shown that GABAergic vPNs play inhibitory roles at their axon terminals in the LH. However, little is known about the properties and functions of vPNs at their dendritic branches in the AL. Here, we used optogenetic and patch clamp techniques to investigate the functional roles of vPNs in the AL. Surprisingly, our results show that specific activation of vPNs reliably elicits strong excitatory postsynaptic potentials (EPSPs) in ePNs. Moreover, the connections between vPNs and ePNs are mediated by direct chemical synapses. Neither pulses of GABA, nor pharmagological, or genetic blockade of GABAergic transmission gave results consistent with the involvement of GABA in vPN-ePN excitatory transmission. These unexpected results suggest new roles for the vPN population in olfactory information processing.

A Single Pair of Serotonergic Neurons Counteracts Serotonergic Inhibition of Ethanol Attraction in Drosophila

Attraction to ethanol is common in both flies and humans, but the neuromodulatory mechanisms underlying this innate attraction are not well understood. Here, we dissect the function of the key regulator of serotonin signaling—the serotonin transporter–in innate olfactory attraction to ethanol in Drosophila melanogaster. We generated a mutated version of the serotonin transporter that prolongs serotonin signaling in the synaptic cleft and is targeted via the Gal4 system to different sets of serotonergic neurons. We identified four serotonergic neurons that inhibit the olfactory attraction to ethanol and two additional neurons that counteract this inhibition by strengthening olfactory information. Our results reveal that compensation can occur on the circuit level and that serotonin has a bidirectional function in modulating the innate attraction to ethanol. Given the evolutionarily conserved nature of the serotonin transporter and serotonin, the bidirectional serotonergic mechanisms delineate a basic principle for how random behavior is switched into targeted approach behavior.

Synaptic organization of the Drosophila antennal lobe and its regulation by the Teneurins

Understanding information flow through neuronal circuits requires knowledge of their synaptic organization. In this study, we utilized fluorescent pre- and postsynaptic markers to map synaptic organization in the Drosophila antennal lobe, the first olfactory processing center. Olfactory receptor neurons (ORNs) produce a constant synaptic density across different glomeruli. Each ORN within a class contributes nearly identical active zone number. Active zones from ORNs, projection neurons (PNs), and local interneurons have distinct subglomerular and subcellular distributions. The correct number of ORN active zones and PN acetylcholine receptor clusters requires the Teneurins, conserved transmembrane proteins involved in neuromuscular synapse organization and synaptic partner matching. Ten-a acts in ORNs to organize presynaptic active zones via the spectrin cytoskeleton. Ten-m acts in PNs autonomously to regulate acetylcholine receptor cluster number and transsynaptically to regulate ORN active zone number. These studies advanced our ability to assess synaptic architecture in complex CNS circuits and their underlying molecular mechanisms.

Olfactory habituation in Drosophila-odor encoding and its plasticity in the antennal lobe

A ubiquitous feature of an animal's response to an odorant is that it declines when the odorant is frequently or continuously encountered. This decline in olfactory response, termed olfactory habituation, can have temporally or mechanistically different forms. The neural circuitry of the fruit fly Drosophila melanogaster's olfactory system is well defined in terms of component cells, which are readily accessible to functional studies and genetic manipulation. This makes it a particularly useful preparation for the investigation of olfactory habituation. In addition, the insect olfactory system shares many architectural and functional similarities with mammalian olfactory systems, suggesting that olfactory mechanisms in insects may be broadly relevant. In this chapter, we discuss the likely mechanisms of olfactory habituation in context of the participating cell types, their connectivity, and their roles in sensory processing. We overview the structure and function of key cell types, the mechanisms that stimulate them, and how they transduce and process odor signals. We then consider how each stage of olfactory processing could potentially contribute to behavioral habituation. After this, we overview a variety of recent mechanistic studies that point to an important role for potentiation of inhibitory synapses in the primary olfactory processing center, the antennal lobe, in driving the reduced response to familiar odorants. Following the discussion of mechanisms for short- and long-term olfactory habituation, we end by considering how these mechanisms may be regulated by neuromodulators, which likely play key roles in the induction, gating, or suppression of habituated behavior, and speculate on the relevance of these processes for other forms of learning and memory.

GABAergic projection neurons route selective olfactory inputs to specific higher-order neurons

We characterize an inhibitory circuit motif in the Drosophila olfactory system, parallel inhibition, which differs from feedforward or feedback inhibition. Excitatory and GABAergic inhibitory projection neurons (ePNs and iPNs) each receive input from antennal lobe glomeruli and send parallel output to the lateral horn, a higher center implicated in regulating innate olfactory behavior. Ca(2+) imaging of specific lateral horn neurons as an olfactory readout revealed that iPNs selectively suppressed food-related odor responses, but spared signal transmission from pheromone channels. Coapplying food odorant did not affect pheromone signal transmission, suggesting that the differential effects likely result from connection specificity of iPNs, rather than a generalized inhibitory tone. Ca(2+) responses in the ePN axon terminals show no detectable suppression by iPNs, arguing against presynaptic inhibition as a primary mechanism. The parallel inhibition motif may provide specificity in inhibition to funnel specific olfactory information, such as food and pheromone, into distinct downstream circuits.

Functional identification of spike-processing neural circuits

We introduce a novel approach for a complete functional identification of biophysical spike-processing neural circuits. The circuits considered accept multidimensional spike trains as their input and comprise a multitude of temporal receptive fields and conductance-based models of action potential generation. Each temporal receptive field describes the spatiotemporal contribution of all synapses between any two neurons and incorporates the (passive) processing carried out by the dendritic tree. The aggregate dendritic current produced by a multitude of temporal receptive fields is encoded into a sequence of action potentials by a spike generator modeled as a nonlinear dynamical system. Our approach builds on the observation that during any experiment, an entire neural circuit, including its receptive fields and biophysical spike generators, is projected onto the space of stimuli used to identify the circuit. Employing the reproducing kernel Hilbert space (RKHS) of trigonometric polynomials to describe input stimuli, we quantitatively describe the relationship between underlying circuit parameters and their projections. We also derive experimental conditions under which these projections converge to the true parameters. In doing so, we achieve the mathematical tractability needed to characterize the biophysical spike generator and identify the multitude of receptive fields. The algorithms obviate the need to repeat experiments in order to compute the neurons' rate of response, rendering our methodology of interest to both experimental and theoretical neuroscientists.

The Smell of Blue Light: A New Approach toward Understanding an Olfactory Neuronal Network

Olfaction is one of the most important senses throughout the animal kingdom. It enables animals to discriminate between a wide variety of attractive and repulsive odorants and often plays a decisive role in species specific communication. In recent years the analysis of olfactory systems both invertebrates and invertebrates has attracted much scientific interest. In this context a pivotal question is how the properties and connectivities of individual neurons contribute to a functioning neuronal network that mediates odor-guided behavior. As a novel approach to analyze the role of individual neurons within a circuitry, techniques have been established that make use of light-sensitive proteins. In this review we introduce a non-invasive, optogenetic technique which was used to manipulate the activity of individual neurons in the olfactory system of Drosophila melanogaster larvae. Both channelrhodopsin-2 and the photosensitive adenylyl cyclase PAC α in individual olfactory receptor neurons (ORNs) of the olfactory system of Drosophila larvae allows stimulating individual receptor neurons by light. Depending on which particular ORN is optogenetically activated, repulsion or attraction behavior can be induced, indicating which sensory neurons underlie which type of behavior.

A novel wide-field neuron with branches in the lamina of the Drosophila visual system expresses myoinhibitory peptide and may be associated with the clock

Although neuropeptides are widespread throughout the central nervous system of the fruifly Drosophila, no records exist of peptidergic neurons in the first synaptic region of the visual system, the lamina. Here, we describe a novel type of neuron that has wide-field tangential arborizations just distal to the lamina neuropil and that expresses myoinhibitory peptide (MIP). The cell bodies of these neurons, designated lateral MIP-immunoreactive optic lobe (LMIo) neurons, lie anteriorly at the base of the medulla of the optic lobe. The LMIo neurons also arborize in several layers of the medulla and in the dorso-lateral and lateral protocerebrum. Since the LMIo resemble LN(v) clock neurons, we have investigated the relationships between these two sets of neurons by combining MIP-immunolabeling with markers for two of the clock genes, viz., Cryptochrome and Timeless, or with antisera to two peptides expressed in clock neurons, viz., pigment-dispersing factor and ion transport peptide. LMIo neurons do not co-express any of these clock neuron markers. However, branches of LMIo and clock neurons overlap in several regions. Furthermore, the varicose lamina branches of LMIo neurons superimpose those of two large bilateral serotonergic neurons. The close apposition of the terminations of MIP- and serotonin-producing neurons distal to the lamina suggests that they have the same peripheral targets. Our data indicate that the LMIo neurons are not bona fide clock neurons, but they may be associated with the clock system and regulate signaling peripherally in the visual system.