Lateral presynaptic inhibition mediates gain control in an olfactory circuit

Abstract

Olfactory signals are transduced by a large family of odorant receptor proteins, each of which corresponds to a unique glomerulus in the first olfactory relay of the brain. Crosstalk between glomeruli has been proposed to be important in olfactory processing, but it is not clear how these interactions shape the odour responses of second-order neurons. In the Drosophila antennal lobe (a region analogous to the vertebrate olfactory bulb), we selectively removed most interglomerular input to genetically identified second-order olfactory neurons. Here we show that this broadens the odour tuning of these neurons, implying that interglomerular inhibition dominates over interglomerular excitation. The strength of this inhibitory signal scales with total feedforward input to the entire antennal lobe, and has similar tuning in different glomeruli. A substantial portion of this interglomerular inhibition acts at a presynaptic locus, and our results imply that this is mediated by both ionotropic and metabotropic receptors on the same nerve terminal.

Figure 1. Removing lateral input disinhibits PNs

a, Drosophila olfactory organs. b, Experimental configuration. c, Spiking responses of ORNs and PNs for the palp glomerulus VM7 (n = 4–12 for each response). PNs are shown with and without lateral input from antennal glomeruli. Throughout, black bars represent the 500-msec odor stimulus period. d, Input-output functions for palp glomeruli VM7 and VC1. Each point represents the average PN response to an odor versus the response of the cognate ORNs. PNs are shown with antennae intact (magenta) or removed (blue). Responses to 18 of 20 odors are significantly disinhibited in VM7; 13 of 20 in VC1 (p < 0.05, t-tests). e, Odor selectivity of ORNs (green) and cognate PNs with antennae intact (magenta) or removed (blue). Lifetime sparseness = 0 for an unselective cell, 1 for a maximally selective cell. All within-glomerulus comparisons are significant (p < 0.002, Mann-Whitney U-test), except ORNs versus antennae-intact-PNs for VC1.

Figure 2. Lateral inhibition suppresses spontaneous EPSCs and scales with total ORN input

a, Left: experimental configuration. Right: recording from a PN in glomerulus VM7. Olfactory stimulation evokes depolarization. Spontaneous EPSPs are absent. b, Left: palps are shielded from odors. Right: recording from a PN in glomerulus VM7. Olfactory stimulation suppresses spontaneous EPSPs. c, Average VM7 PN responses as in a (green) or b (black), n = 6–7 PNs for each condition. When ORNs are shielded (black), inhibition dominates. When ORNs are absent (green), excitation dominates throughout the stimulus period. (Off-inhibition likely reflects lateral postsynaptic inhibition, see Supplementary Fig. 5.) d, Lateral input to VM7 (as in b) is correlated with the disinhibition in VM7 PNs after antennal removal (see Fig. 1b). Each point represents a different odor. Lateral input is measured as the time-integrated change in membrane potential. e, Lateral input to VM7 is correlated with total antennal ORN spiking activity evoked by each odor. f, Lateral input to VM7 is correlated with the disinhibition in VC1 PNs after antennal removal.

Figure 3. Lateral GABAergic suppression of ORN-PN synapses

a, Experimental configuration. b, Electrical stimulation of the antennal nerve (arrowheads) evokes EPSCs in a PN (average of 20 trials). Olfactory stimulation (500 ms) inhibits EPSCs. Note that odor also evokes a transient inward current reflecting lateral postsynaptic excitation; this is resistant to GABA antagonists (Supplementary Fig. 6). c, Inhibition is blocked by the GABA_B antagonist CGP54626 together with the GABA_A antagonist picrotoxin (control n = 12, PCT n = 5, CGP n = 5, CGP+PCT n = 5). All pairwise comparisons are significantly different except control versus PCT (p < 0.05, t-tests). d, Experimental configuration, substituting GABA iontophoresis for olfactory stimulation. e, As in b for GABA iontophoresis (asterisk). Note that GABA also evokes an outward current. f, As in c for GABA iontophoresis (control n = 13, PCT n = 5, CGP n = 6, CGP+PCT n = 7). All pairwise comparisons between conditions are significantly different except control versus PCT (p < 0.05, t-tests).

Figure 4. Genetic evidence that GABA_B receptors inhibit ORN-PN synapses at a presynaptic locus

a, Experimental configuration. b, When pertussis toxin is specifically expressed in ORNs, GABAergic inhibition of EPSCs (solid line, n = 11) is more transient than in control flies (dotted line, n = 13, reproduced from Fig. 3f). c, Pertussis toxin expression in ORNs renders the GABAergic inhibition of EPSCs completely insensitive to CGP, and completely sensitive to PCT (black trace reproduced from a, CGP n = 5, PCT n = 6, CGP+PCT n = 5). Compare Fig. 3f.

Figure 5. GABA receptor antagonists mimic removal of lateral input to a PN

a, Rasters show spiking responses to pentyl acetate (gray) in a VM7 ORN and a VM7 PN. With antennae intact, both antagonists are required to mimic the effect of antennal removal on PNs. (Note some off-inhibition in PNs persists after antennal removal or in the presence of antagonists; this probably reflects the off-inhibition in VM7 ORNs.) b, Average spike rates during odor stimulus period, minus baseline spike rates (n = 5–6 PNs for each condition). c, Average membrane potential responses to pentyl acetate in VM7 PNs (n = 5–6 PNs for each).