Odor-Induced Multi-Level Inhibitory Maps in Drosophila

Optical imaging of intracellular Ca2+ influx as a correlate of neuronal excitation represents a standard technique for visualizing spatiotemporal activity of neuronal networks. However, the information-processing properties of single neurons and neuronal circuits likewise involve inhibition of neuronal membrane potential. Here, we report spatially resolved optical imaging of odor-evoked inhibitory patterns in the olfactory circuitry of Drosophila using a genetically encoded fluorescent Cl- sensor. In combination with the excitatory component reflected by intracellular Ca2+ dynamics, we present a comprehensive functional map of both odor-evoked neuronal activation and inhibition at different levels of olfactory processing. We demonstrate that odor-evoked inhibition carried by Cl- influx is present both in sensory neurons and second-order projection neurons (PNs), and is characterized by stereotypic, odor-specific patterns. Cl--mediated inhibition features distinct dynamics in different neuronal populations. Our data support a dual role of inhibitory neurons in the olfactory system: global gain control across the neuronal circuitry and glomerulus-specific inhibition to enhance neuronal information processing

Odor-Induced Multi-Level Inhibitory Maps in Drosophila

Grabe V, Schubert M, Strube-Bloss M, et al. Odor-Induced Multi-Level Inhibitory Maps in Drosophila. eNeuro. 2019;7(1): ENEURO.0213-19.2019

Insect odorant response sensitivity is tuned by metabotropically autoregulated olfactory receptors.

Insects possess one of the most exquisitely sensitive olfactory systems in the animal kingdom, consisting of three different types of chemosensory receptors: ionotropic glutamate-like receptors (IRs), gustatory receptors (GRs) and odorant receptors (ORs). Both insect ORs and IRs are ligand-gated ion channels, but ORs possess a unique configuration composed of an odorant-specific protein OrX and a ubiquitous coreceptor (Orco). In addition, these two ionotropic receptors confer different tuning properties for the neurons in which they are expressed. Unlike IRs, neurons expressing ORs are more sensitive and can also be sensitized by sub-threshold concentrations of stimuli. What is the mechanistic basis for these differences in tuning? We show that intrinsic regulation of Orco enhances neuronal response to odorants and sensitizes the ORs. We also demonstrate that inhibition of metabotropic regulation prevents receptor sensitization. Our results indicate that Orco-mediated regulation of OR sensitivity provides tunable ionotropic receptors capable of detecting odors over a wider range of concentrations, providing broadened sensitivity over IRs themselves

Elucidating the Neuronal Architecture of Olfactory Glomeruli in the Drosophila Antennal Lobe

Olfactory glomeruli are morphologically conserved spherical compartments of the olfactory system, distinguishable solely by their chemosensory repertoire, anatomical position, and volume. Little is known, however, about their numerical neuronal composition. We therefore characterized their neuronal architecture and correlated these anatomical features with their functional properties in Drosophila melanogaster. We quantitatively mapped all olfactory sensory neurons (OSNs) innervating each glomerulus, including sexually dimorphic distributions. Our data reveal the impact of OSN number on glomerular dimensions and demonstrate yet unknown sex-specific differences in several glomeruli. Moreover, we quantified uniglomerular projection neurons for each glomerulus, which unraveled a glomerulus-specific numerical innervation. Correlation between morphological features and functional specificity showed that glomeruli innervated by narrowly tuned OSNs seem to possess a larger number of projection neurons and are involved in less lateral processing than glomeruli targeted by broadly tuned OSNs. Our study demonstrates that the neuronal architecture of each glomerulus encoding crucial odors is unique

Regulation of OR response by cAMP signaling is intrinsic.

<p><b>A</b>, Orco (left), Orco mut (middle) and Or22a (right) proteins visualized in adult antennal sections with specific antibodies (red). The proteins show expression in cell bodies (arrowhead) and dendrites (arrow). Or22a-expressing cells are housed in few sensilla opposite to arista (a). Scale bar 50 mm. <b>B</b>, Normalized ab3A neuron spike frequency (f_norm) upon Etb stimulation wild type flies (Orco, <i>n</i> = 12), for Orco null mutants (no Orco, <i>n</i> = 15), and mutants rescued with Orco mut (“Orco mut flies”; <i>n</i> = 14; <i>P</i> = 0.016 vs. Control, Mann-Whitney U test). <b>C</b>, f_norm as in <b>B</b> upon Etb stimulation in Orco mut flies (<i>n</i> = 17) before (Control) and after forskolin injection.</p

Manipulation of cAMP signalling in <i>Drosophila</i> ab3 sensilla affects the odorant response.

<p><b>A</b>, Recordings of neuronal activity (large action potentials, Or22a neuron; small action potentials, Or85b neuron) before and after Etb stimulation (−5 v/v; 0.5 s, shaded area) in the presence of indicated compounds. While 8-br-cAMP enhances the Etb response, inhibition of adenylyl cyclase with SQ22536 attenuates it. <b>B</b>, Normalized spike frequency (f_norm) of ab3A upon Etb stimulation (0 to 0.5 s, shaded area) at indicated dilution after injection of saline solution (Control; <i>n</i> = 11), of 8-bromo-cAMP (<i>n</i> = 11; <i>P</i><0.05, Mann-Whitney U test) and of the adenylyl cyclase inhibitor SQ22536 (<i>n</i> = 17; <i>P</i><0.01, U test). <b>C</b>, Concentration dependence of the maximum frequency f_max of f_norm to Etb stimulation after saline, forskolin and SQ22536 injection (**<i>P</i><0.01, ***<i>P</i><0.001, ANOVA). <b>D</b>, f_norm as described in (B) after injection of saline solution (Control; <i>n</i> = 11), U73122 plus 8-br-cAMP (<i>n</i> = 10; <i>P</i> = 0.18, U test), and Gö6976 plus 8-br-cAMP (<i>n</i> = 17; <i>P</i> = 0.16, U test). In the presence of the PLC or PKC inhibitors 8-br-cAMP fails to enhance the odor response. <b>E</b>, Comparison of treatment effects on Etb response before and after microinjection. f_norm on Etb stimulation (0.5 s) as determined from area under the curve measurements of the total response (1.35 s). Responses to Etb were measured 20 s after commencement of recording (before injection) and 200 s after injection (after injection) of the control (<i>n</i> = 11), SQ22536 (<i>n</i> = 17), 8-br-cAMP (<i>n</i> = 11), forskolin (<i>n</i> = 9; data from Olsson et al., 2011), cholera toxin (CTX; <i>n</i> = 12), 8-br-cAMP plus U73122 (<i>n</i> = 10), and 8-br-cAMP plus Gö6976 (<i>n</i> = 17). Error bars represent s.e.m. Asterisks indicate significant differences (<i>P</i><0.05, Paired Wilcoxon Signed-Rank Test).</p

Repeated subthreshold stimulation does not sensitize ionotropic receptors (IRs).

<p><b>A</b>, Recordings of neuronal activity from ac3 sensilla (large action potentials, Ir75abc neuron; small action potentials, Or35a neuron) upon before and after 20 s repeated butyric acid (Ba) stimulation (−7 v/v; 0.5 s, shaded area). Both stimulations fail to elicit a response. <b>B</b>, Dependence of normalized Ir75abc neuron spike frequency (f_norm) during 1^st and 2^nd subthreshold Etb stimulation (−7 v/v; 0.5 s) on the interval between stimulations (<i>n</i> = 12). <b>C</b>, Time course of f_norm for 1^st and 2^nd stimulation (interval 20 s, <i>n</i> = 12). <b>D–F</b>, Mean f_norm for Ir75abc (<b>D</b>), Ir41a (<b>E</b>) and Ir84a (<b>F</b>) neuron to repetitive subthreshold Ba (<b>D</b>), Dab (<b>E</b>) and Paa (<b>F</b>) stimulations (interval 20 s, <i>n</i> = 12). N.s.; Paired Wilcoxon Signed Ranks test.</p

Repeated subthreshold stimulation sensitizes odorant receptors.

<p><b>A</b>, Recordings of neuronal activity from ab3 sensilla (large action potentials, ab3A neuron expressing Or22a; small action potentials, ab3B neuron expressing Or85b) upon before and after 20 s repeated ethyl butyrate (Etb) stimulation (−10 v/v; 0.5 s, shaded area). The first stimulation fails to elicit a response while the second does so. <b>B</b>, Dependence of normalized ab3A neuron spike frequency (f_norm) upon 1^st and 2^nd subthreshold Etb stimulation (−10 v/v; 0.5 s) on the interval between stimulations (<i>n</i> = 12). <b>C</b>, Time course of f_norm for 1^st and 2^nd stimulation (interval 20 s, <i>n</i> = 12). <b>D–F</b>, Mean f_norm for ab3A (<b>D</b>), ac3B (<b>E</b>) and ab2A (<b>F</b>) neuron to repetitive subthreshold Etb (<b>D</b>), ethyl acetate (Eta, <b>E</b>) and methyl acetate (Mea, <b>F</b>) stimulations (interval 20 s, <i>n</i> = 12). *<i>P</i><0.05, **<i>P</i><0.01, ***<i>P</i><0.001; Paired Wilcoxon Signed Ranks test.</p

Inverse resource allocation between vision and olfaction across the genus Drosophila

Neural architecture may be shaped by selection, but is likely also constrained by development. Here, Keesey and colleagues find an inverse relationship between allocation towards visual and olfactory sensory systems across the genus Drosophila, which may reflect a developmental trade-off