Excitatory interactions between olfactory processing channels in the Drosophila antennal lobe - PubMed (original) (raw)
Excitatory interactions between olfactory processing channels in the Drosophila antennal lobe
Shawn R Olsen et al. Neuron. 2007.
Abstract
Each odorant receptor gene defines a unique type of olfactory receptor neuron (ORN) and a corresponding type of second-order neuron. Because each odor can activate multiple ORN types, information must ultimately be integrated across these processing channels to form a unified percept. Here, we show that, in Drosophila, integration begins at the level of second-order projection neurons (PNs). We genetically silence all the ORNs that normally express a particular odorant receptor and find that PNs postsynaptic to the silent glomerulus receive substantial lateral excitatory input from other glomeruli. Genetically confining odor-evoked ORN input to just one glomerulus reveals that most PNs postsynaptic to other glomeruli receive indirect excitatory input from the single ORN type that is active. Lateral connections between identified glomeruli vary in strength, and this pattern of connections is stereotyped across flies. Thus, a dense network of lateral connections distributes odor-evoked excitation between channels in the first brain region of the olfactory processing stream.
Figures
Figure 1. Olfactory stimuli trigger excitatory interactions among glomeruli
(A) Both the antennae and the maxillary palps project to the antennal lobes. Severing the antennal nerves removes direct ORN input to antennal glomeruli, but leaves input to palp glomeruli intact. Recording from a PN postsynaptic to a deafferented glomerulus reveals indirect input to that cell from palp ORNs. (B) Axon targeting of palp ORNs is not altered by removing antennal ORNs. Projections of confocal stacks through the antennal lobes (neuropil in magenta) show ORN axons labeled with CD8:GFP (green). Glomeruli targeted by palp ORNs are outlined. Arrow indicates maxillary nerve. Arrowheads indicate axons projecting to or from the midline; each ORN projects bilaterally. Scale bars=20 μm. (C) With antennal nerve intact, a recording from an antennal PN (in glomerulus DM3) shows spontaneous and odor-evoked activity. Period of odor delivery is indicated by gray bar (500 msec). (D) Recording from an antennal PN (in glomerulus DM3) after severing antennal nerves. Spontaneous activity is abolished, but a small odor-evoked depolarization remains. (E) Recording from an antennal PN (in glomerulus VC3) after severing antennal nerves shows a large odor-evoked depolarization. (F) Same as E, but averaged across 6 trials with the same odor, and low-pass filtered to remove spikes. (G) Odor-evoked depolarizations averaged across all experiments with severed antennal nerves (magenta). No depolarization is observed when both antennae and maxillary palps are removed (blue). Note that this analysis pools data from 6 PNs corresponding to different antennal glomeruli.
Figure 2. PNs postsynaptic to “silent” ORNs show reduced activity but normal morphology
(A) Schematic of experiments in Figs. 2–4: recording from PNs postsynaptic to ORNs lacking odorant receptors. (B) Odorant receptor mutations reduce spontaneous activity in both ORNs and their postsynaptic PNs. Bars show mean ± SEM averaged across experiments, with n values above each bar (p<0.05 for each wild type/mutant comparison, t-tests). (C) Spontaneous electrophysiological activity and morphology of VM2 PNs in wild-type flies (top) and Or43b1 mutant flies (bottom). Images on the right are projections of confocal stacks through one antennal lobe (neuropil in magenta) showing the primary dendrite of the biocytin-filled PN (biocytin/streptavidin in green). The cell body and axon of the recorded PN are not present in these _z_-sections. Scale bars=20 μm. (D) Spontaneous electrophysiological activity and morphology of DL1 PNs in wild-type flies (top) and Or10af03694 mutant flies (bottom).
Figure 3. Comparing odor responses in PNs postsynaptic to normal versus “silent” ORNs
Representative responses of PNs postsynaptic to wild-type ORNs (left column) and PNs postsynaptic to non-functional ORNs (right column). Most odors elicit a substantially larger response when presynaptic ORNs are functional (A1, B1). However, some odors elicit a similar response with or without functional presynaptic ORNs (A2, B2). Gray bars indicate the 500-ms period of odor stimulation. Transient stimulus artifacts from the olfactometer (at the end of the odor stimulus period) were blanked in some traces.
Figure 4. Odor tuning of PN responses postsynaptic to normal versus “silent” ORNs
(A–B) Tuning curves comparing odor responses of PNs postsynaptic to wild-type (magenta) and mutant ORNs (blue). PNs are postsynaptic to glomerulus VM2 (A) or DL1 (B). Each point represents firing rate over the 500-msec odor stimulus period, averaged across experiments (mean ± SEM). Note different _y_-scales for magenta and blue symbols. Odors are arranged so the smallest wild-type responses are on the left and the largest are on the right. See Methods for odor abbreviations. (C) Tuning curves comparing odor responses of VM2 and DL1 PNs postsynaptic to mutant ORNs. Note correlated but not identical odor tuning.
Figure 5. Two strategies for stimulating a single ORN type
(A) In experiment 1, only one maxillary palp ORN type (VA71) has functional odorant receptors. Antennal nerves are severed. Recordings are performed from PNs postsynaptic to antennal glomeruli. (B) Genetic strategy to limit functional odor receptors to a single type of ORN. Top: single-sensillum recording from wild-type ORN projecting to glomerulus VA71. (The second ORN in this sensillum has been killed with diphtheria toxin to show only the VA71 ORN spikes; see Supplemental Table S1). Middle: in the Or83b2 mutant, all spontaneous and odor-evoked activity is eliminated from maxillary palp ORNs. Bottom: odor response is rescued in VA71 ORNs by selective expression of Or83b in these neurons. Gray bar = 500 msec puff of methyl salicylate. (C) Both ORNs and PNs show correct glomerular targeting in flies with “rescued” VA71 ORNs. Top: projection of a confocal stack through antennal lobes of a fly with rescued VA71 ORNs. VA71 ORNs are labeled with CD8:GFP (green). A neighboring glomerulus (VA1v) is outlined in magenta. Bottom: biocytin-fill of a VA1v PN (magenta) recorded in a fly with rescued VA71 ORNs where all other ORNs are non-functional. This PN does not invade glomerulus VA71 (green). Scale bars=20 μm. (D) Experiment 2 uses a restricted odor set to selectively stimulate VM7 ORNs. Antennal nerves are severed, and the Δ85 mutation is used to reduce ORN activity in two maxillary palp ORN types. Recordings are performed from PNs postsynaptic to antennal glomeruli. (E) A restricted odor set activates only VM7 ORNs. Local field potential recordings from the maxillary palp of flies with either functional (blue; genotype +/+;Δ85) or non-functional VM7 ORNs (magenta; genotype Or42af04305; Δ85). Responses are only present when VM7 ORNs are functional. As a positive control, we confirmed that 4-methyl phenol elicits a robust response in both genotypes; this odor activates several maxillary palp ORNs that are functional in both genotypes. Traces are mean ± SEM, averaged across experiments (n=3 for each genotype).
Figure 6. Odor stimulation of one ORN type evokes lateral input to many PNs
(A) Experiment 1. Top: peristimulus-time histograms show odor responses of VA71 ORNs (n=5). Bottom: average depolarizations recorded in PNs postsynaptic to glomeruli lacking direct ORN input (n=72). (B) Experiment 2. Top: peristimulus-time histograms showing odor responses of VM7 ORNs (n=6). Bottom: average depolarizations recorded in PNs postsynaptic to glomeruli lacking direct ORN input (n=15). (C) Experiment 1. Average depolarization area plotted versus VA71 ORN firing rate. Each point represents a different odor. Curve is an exponential fit (only excitatory ORN responses are included in fit). (D) Experiment 2. Average depolarization area plotted versus VM7 ORN firing rate for each odor (solid symbols). Curve is an exponential fit. In flies lacking functional VM7 ORNs (open symbols), odor-evoked depolarizations are virtually absent (Or42af04305;Δ85; n=3). All panels: values are mean ± SEM, averaged across experiments.
Figure 7. Lateral excitatory circuits sub-linearly summate inputs from multiple ORN types
(A) Average depolarization evoked in antennal PNs by selective stimulation of VM7 ORNs with 2-butanone (10−5 dilution) in Δ85 flies. (B) Average depolarization evoked in antennal PNs by selective stimulation of VA71 ORNs with methyl salicylate (10−2 dilution) in Δ85 flies, recorded from the same PNs as in (A). (C) Comparing the average depolarization evoked by simultaneous stimulation of both ORN types (green), versus the predicted linear sum of stimulating each ORN type alone (black). (D) Depolarization quantified as the area under the membrane potential deflection. All panels: values are mean ± SEM, averaged across 8 PNs recorded in different flies. All PNs were postsynaptic to glomeruli DL5, DM6, or VM2. Vertical scaling is the same in panels A–C.
Figure 8. The strength of lateral connections is heterogeneous and stereotyped across flies
(A) Experiment 1: stimulation of VA71 ORNs evokes different levels of lateral depolarization in PNs postsynaptic to two different glomeruli (magenta=VC4, blue=VA1v). (B) Depolarization area in these PNs is plotted versus VA71 ORN firing rate for each odor. Curves are exponential fits (only excitatory ORN responses are included in fit). (C) Comparing lateral depolarization in all PN types that we recorded from at least 3 times (except DM6 where n = 2, gray dots). Graph shows average response to 4-methyl phenol in these PNs. (D) Experiment 2: stimulation of VM7 ORNs evokes different levels of lateral depolarization in PNs postsynaptic to two different glomeruli (magenta=DL5, blue=DM6). (E) Depolarization area in these PNs is plotted versus VM7 ORN firing rate for each odor. Curves are exponential fits. (F) Comparing lateral depolarization area in three different glomeruli (n≥3 for each). Graph shows average response to 2-pentanone for each glomerulus. All panels: values are mean ± SEM, averaged across experiments.
Figure 9. Lateral excitation is broadly distributed throughout the antennal lobe
(A) Schematic representation of all glomeruli in the antennal lobe, represented as three sections through the fly’s right lobe (modified from Laissue et al., 1999). Color indicates average relative depolarization measured in PNs postsynaptic to each glomerulus during selective stimulation of VA71 ORNs. Asterisk marks glomerulus VA71. We did not sample PNs postsynaptic to white glomeruli. (B) Glomeruli postsynaptic to different morphological types of sensilla receive similar levels of lateral input. Graph compares average depolarization area (± SEM) evoked by 4-methyl phenol in glomeruli targeted by ORNs in basiconic sensilla (n=14 glomeruli), coeloconic sensilla (n=3 glomeruli), and trichoid sensilla (n=6 glomeruli). Data on morphological types was taken from Couto et al. (2005). (C) There is no relationship between odor tuning and strength of lateral input. Comprehensive odor tuning data for 16 ORN types was taken from Hallem and Carlson (2006). For every possible pair-wise combination of 16 glomeruli, the difference in the average lateral depolarization evoked by 4-methyl phenol in these two PN types is plotted versus the correlation between the odor tuning of the ORN inputs to those glomeruli.
Comment in
References
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