Cortical feedback control of olfactory bulb circuits

Abstract

Olfactory cortex pyramidal cells integrate sensory input from olfactory bulb mitral and tufted (M/T) cells and project axons back to the bulb. However, the impact of cortical feedback projections on olfactory bulb circuits is unclear. Here, we selectively express channelrhodopsin-2 in olfactory cortex pyramidal cells and show that cortical feedback projections excite diverse populations of bulb interneurons. Activation of cortical fibers directly excites GABAergic granule cells, which in turn inhibit M/T cells. However, we show that cortical inputs preferentially target short axon cells that drive feedforward inhibition of granule cells. In vivo, activation of olfactory cortex that only weakly affects spontaneous M/T cell firing strongly gates odor-evoked M/T cell responses: cortical activity suppresses odor-evoked excitation and enhances odor-evoked inhibition. Together, these results indicate that although cortical projections have diverse actions on olfactory bulb microcircuits, the net effect of cortical feedback on M/T cells is an amplification of odor-evoked inhibition.

Figure 1. Conditional Expression of ChR2 in Piriform Cortex Pyramidal Cells Reveals Cortical Feedback Projections to the Olfactory Bulb

(A₁) Overlay of bright-field and fluorescence (red) image of a horizontal section (300 μm) of forebrain from an Ntsr1-cre mouse showing ChR2-mCherry expression in olfactory cortex. APC, anterior piri-form cortex; LOT, lateral olfactory tract; OB, olfactory bulb. (A₂) Blow-up of region in (A₁) indicating expression of ChR2 in layer 2/3 pyramidal cells. (A₃) ChR2 is expressed in the ipsilateral, but not contralateral OB from the same mouse. GCL, granule cell layer. (A₄) Overlay of fluorescence and bright field images of the bulbs. (B) Two-photon image (30 μm z-projection) of ChR2-mCherry in the GCL of a slice counter-stained with DAPI. (C) Left: gray-scale fluorescence image of ChR2-mCherry (black) in a slice (300 μm) indicating few fibers in the mitral cell layer (MCL) and external plexiform layer (EPL), but numerous fibers and varicosities surrounding glomeruli (yellow circles). Right: average fluorescence intensity (F, arbitrary units) along the vertical axis of the image. GL, glomerular layer; IPL, internal plexiform layer.

Figure 2. Cortical Feedback Drives Disynaptic Inhibition of Mitral Cells

(A₁) Anatomical reconstruction (top) and voltage clamp recording (bottom) of a mitral cell. Brief LED flashes (4 ms, black bar) evoke IPSCs at 0 mV (gray, individual trials; black, average). Inset: recording schematic. (A₂) Light-evoked IPSCs are blocked by gabazine (GBZ, 10 μM). (A₃) Light-evoked IPSCs are driven by AMPARs. Left: time course of one experiment (bottom) and traces (top) showing that the IPSC is unaffected by APV (100 μM) but abolished following subsequent application of NBQX (10 μM). Right: summary of the effects of APV alone (n = 4) and coapplication of APV and NBQX (n = 11) on IPSCs. (B₁) Disynaptic IPSPs can reduce mitral cell firing. Current clamp recordings of a mitral cell depolarized to fire APs under control conditions (top) and on interleaved trials with a train of LED flashes (5 pulses, 20 Hz). Ten trials superimposed for each condition. (B₂) Summary of the effects of light-evoked IPSPs (LED AP Rate) on firing (Control AP rate, n = 9 cells). Each point represents one cell. Error bars represent SEM. See also Figure S1.

Figure 3. Cortical Feedback Directly Excites GCs

(A₁) Anatomical reconstruction (left) and voltage clamp recording (V_m = −70 mV) (right) of a GC receiving light-evoked EPSCs. Bar, LED illumination. MCL, mitral cell layer. Inset: recording schematic. (A₂) Light-evoked GC EPSCs are blocked by TTX (1 μM) and partially recovered by application of 4-AP (1 mM). (A₃) EPSCs have both AMPAR and NMDAR components. In the presence of picrotoxin (100 μM), depolarization to +40 mV reveals a slow current blocked by APV (100 μM). Subsequent addition of NBQX (20 μM) abolishes the fast EPSC. (A₄) Current-voltage relationship of the AMPAR EPSC (in the presence of APV) is linear (n = 5 cells). Inset: EPSCs from a representative cell. (B₁) Simultaneous voltage clamp recording of a mitral cell (0 mV) and cell-attached recording of a GC show that light-evoked GC APs overlap with mitral cell inhibition. (B₂) Simultaneous voltage clamp recording of a mitral (0 mV) and granule (−70 mV) cell reveal that the onset of cortically-driven EPSCs precedes that of mitral cell IPSCs. Bottom, IPSC-EPSC latencies (n = 7 cell pairs). (C₁) Recording schematic (top) and light-evoked responses from a mitral cell (bottom, 0 mV) when illumination was directed to the GC layer (GCL) or the glomerular layer (GL). (C₂) Summary data (n = 6 cells) shows that shifting photostimulation to the glomerular layer reduces the amplitude of light-evoked IPSCs. Error bars represent SEM.

Figure 4. Cortical Feedback Drives Disynaptic Feedforward Inhibition of GCs

(A₁) GC voltage clamp recording illustrating a light-evoked EPSC (−80 mV) and short-latency IPSC (0 mV). Inset: latency between the IPSC and EPSC onset (circles, n = 14 cells; bar, mean). (A₂) Light-evoked IPSCs (V_m = 0 mV) in GCs are abolished by gabazine (10 μM, GBZ). (A₃) IPSCs are abolished by glutamate receptor antagonists (NBQX, 10 μM and APV, 100 μM, n = 8). Inset: responses from a representative cell (V_m = 0 mV). (B) Current clamp recording of a GC (−60 mV) showing that disynaptic inhibition limits the time course, but not the amplitude of the cortically-evoked EPSP. (C) Left: average inhibitory (GI) and excitatory (GE) conductances in the same cells (n = 42) evoked by photostimulation. Right: GI relative to total conductance was highly variable across individual cells (circles: individual cells; black diamond: mean ± SEM). Only cells with GI or GE >0.5 nS are included. (D₁) Photoactivation of cortical fibers can lead to net increases or decreases in excitability in neighboring GCs. Traces show superimposed responses (five consecutive trials) to depolarizing current injection (50 pA) on interleaved trials with (+LED) and without (Control) a train of 5 light pulses at 20 Hz (arrowheads). Rasters show APs for 20 trials. Cell 1 responded to activation of cortical fibers with an increase in firing during the LED train whereas Cell 2 showed a decrease in firing. (D₂) AP rate of cells (n = 12) measured during the train of LED stimuli show that cortical input increased firing in the majority of cells whereas some cells were inhibited. Cells shown in (D₁) are indicated. Error bars represent SEM.

Figure 5. dSACs Mediate Disynaptic Inhibition of GCs and Receive a Higher Convergence of Cortical Feedback Projections

(A₁) Anatomical reconstruction of a dSAC (dendrites in black, axon in red, MCL, mitral cell layer). Inset: recording schematic. (A₂) Light-evoked EPSCs (V_m = −70 mV) from the cell in A₁ before (Control) and after application of NBQX and APV. (B₁) Simultaneous recording of a connected dSAC-GC pair. A current step triggers an AP in the dSAC (bottom) and a short-latency IPSC in the voltage-clamped GC (V_m = −50 mV). (B₂) Recording from another connected dSAC-GC pair shows GC IPSCs (top, V_m = 0 mV) and dSAC membrane potential (bottom) in response to a train of light flashes (arrowheads). Traces show responses under control conditions (black) and on interleaved trials when hyperpolarizing current was applied to prevent light-evoked APs in the dSAC (red). (B₃) Light-evoked GC IPSCs are consistently smaller when the connected dSAC does not fire spikes (−dSAC APs, n = 6 pairs). (C) Top: anatomical reconstruction of a simultaneously recorded dSAC and GC. Bottom: light-evoked EPSCs (V_m = −70 mV) are larger in the dSAC than the GC. (D) Summary showing that light-evoked EPSCs are larger in dSACs than GCs from the same slices. Circles, pairs from Ntsr1-cre animals; triangles, pairs from wild-type animals expressing ChR2 unconditionally. Mean ± SEM shown in red. (E₁) Minimal optical stimulation of a GC (top) and dSAC (bottom) reveal clear distinction between failures and single-fiber EPSCs (20 traces superimposed, V_m = −70 mV). (E₂) The average single fiber EPSC amplitude (red bar) is similar between the cell types, suggesting dSACs receive a higher convergence of cortical inputs. Error bars represent SEM.

Figure 6. Cortical Feedback Projections Activate Circuits in the Glomerular Layer

(A) Cortical feedback projections drive disynaptic inhibition of external tufted (ET) cells. (A₁) Anatomical reconstruction of an ET cell. (A₂) Voltage clamp recording from the cell in A₁ at −70 mV and 0 mV reveal that activation of cortical fibers elicits IPSCs. (B) Cortical feedback projections provide direct excitatory input to superficial short axon cells (sSACs). (B₁) Reconstruction of an sSAC. (B₂) Light-evoked EPSCs (−70 mV) from the same cell. (C) Cortical feedback inputs provide direct excitation to periglomerular (PG) cells. (C₁) Reconstruction of a PG cell. (C₂) Light-evoked EPSCs recorded in voltage clamp (−70 mV) from the same cell. (D) Recordings from sSAC-PG cell pairs reveal that sSACs receive stronger excitation than PG cells. Bars, mean amplitudes of EPSCs. (E) Circuit diagram illustrating OB neurons receiving direct excitation (red) and disynaptic inhibition (blue) elicited by cortical feedback projections. Dashed blue line is putative inhibitory connection. GC, granule cell; MC, mitral cell. Error bars represent SEM.

Figure 7. Photoactivation of Pyramidal Neurons Expressing ChR2 In Vivo Drives a Sustained Increase in γ-Synchronized Firing in Piriform Cortex

(A) Recording schematic and circuit diagram. A subset of layer 2/3 neurons express ChR2 (red). (B) Photoactivation generates γ oscillations. Top: LFP trace during LED illumination. Bottom: average spectrogram (20 trials) from the same experiment. The trapezoid and white box indicate the period of LED illumination. White bar, period from which the trace was derived. (C) Photoactivation drives γ frequency firing of pyramidal cells. (C₁) Raster plot of light-evoked firing of a single unit. (C₂) Spike-LFP coherence for the same unit during baseline (gray) and LED (red) periods. Shaded regions, 95% confidence intervals. Inset: spike-triggered average LFP. (D) Summary results show that photostimulation causes a marked increase in AP firing. Pooled histogram of multi-unit activity (MUA), gray shading indicates ±SEM. (E) Photoinduced cortical γ activity abolishes odor-evoked β oscillations. Representative experiment showing the effects of odor and combined odor + LED stimulation on LFP activity. Example LFP traces (top) and average spectrograms (bottom). White boxes indicate the odor or odor + LED period and white bars the extent of the example LFP traces. Trapezoid and rectangle indicate LED and odor timing, respectively. (E₁) Average spectrogram of LFP activity during odor application alone. (E₂) Average spectrogram of LFP activity during interleaved trials with odor + LED. (F) Summary results of the effect of photoactivation on firing activity in cortex during odor stimulation. Pooled MUA histogram averaged across odors (n = 3 mice). Normalized activity in response to odors alone (gray) and odors + LED (red) plotted with shaded regions representing ±SEM. (G) MUA scatter plot of normalized firing rates for odors alone versus odors + LED periods show that photoactivation increases AP output. Symbol shapes correspond to distinct odors (n = 15 odor/animal pairs). Diagonal represents unity line.

Figure 8. Activation of Piriform Cortex In Vivo Amplifies Odor-Evoked Inhibition in the Olfactory Bulb

(A) Recording schematic and circuit diagram. (B) Photo-induced γ activity in cortex propagates to the bulb and disrupts β oscillations. On interleaved trials, cortical photostimulation preceded (Trial A) or was coincident (Trial B) with odor application. (B₁₎ Top: LFP trace during odor application. Bottom: average spectrogram when cortical LED illumination (blue trapezoid) preceded odor application (gray box) for Trial A. (B₂) Top: LFP trace during coincident cortical photostimulation and odor application. Bottom: average spectrogram when LED illumination was coincident with odor application for Trial B. White bounding boxes represent periods over which spontaneous and odor-evoked LFP activity are calculated and white bars indicate periods used for LFP traces. (B₃) Summary results showing that cortical photoinduced γ activity reduces odor-evoked β oscillations. Spectral power in the β (10–30 Hz) and γ (40–70 Hz) bands for the effects of photostimulation alone (Spont. + LED, Trial A), odor alone (Odor, Trial A), and Odor + LED (Trial B). LFP power is normalized to the 4 s period in Trial B when LED and odor stimulation was absent (*p < 0.05 relative to control, #p < 0.05 for pairwise comparisons). (C) Cortical photostimulation reduces odor-evoked M/T cell firing but has minimal effects on spontaneous activity. (C₁) Example of multi-unit activity from one experiment. Peristimulus time histograms (PSTHs) of firing rate under control conditions (black) and with LED illumination (red) reveal a selective suppression of odor-evoked but not spontaneous firing. (C₂) Summary data of multi-unit activity showing that cortical photoactivation had variable effects on spontaneous firing but significantly suppressed odor-evoked activity (*p < 0.001). (D) Recordings from M/T cell single units reveal that cortical photostimulation reduces odor-evoked increases in firing and enhances odor-evoked inhibition. (D₁) Odor-activated unit. Top: raster of AP firing on interleaved trials (Trial A, Trial B) are segmented to show spontaneous (left) and odor-evoked activity (right) under control conditions (black) and during photostimulation (red). Trapezoids and gray shading indicating timing of LED and odor stimulation, respectively. Bottom: PSTHs under control conditions (black) and with LED illumination (red). (D₂) Odor-inhibited unit. (E) Raw AP rates of single units with (LED on) and without (LED off) photostimulation (n = 40 units). (E₁) LED activation of cortex has variable effects on spontaneous firing. (E₂) Cortical activation consistently reduces firing during odor-evoked responses. (F) Cortical activation suppresses firing rate independent of whether odor-evoked responses were excitatory or inhibitory. Scatter plot of odor modulation index (OMI) for single units. Odor-inhibited (blue, OMI < 0) (n = 18) and excited (green, OMI > 0) (n = 22) units are both affected by cortical stimulation. (G) Little correlation between the effects of cortical activation on spontaneous and odor-evoked activity. Scatter plot of light modulation index (LMI), calculated for spontaneous and odor periods, quantifying the effect of the LED on firing rates for inhibited (blue) (n = 18) and excited (green) (n = 22) units. Horizontal and vertical gray lines and labels define quadrants categorically. (H) Suppression of M/T cell responses by cortical activation is sensitive to odor concentration. Average LMI across single units (n = 30) when concentrations of 2, 10, and 50 ppm of a single odor were presented on interleaved trials. (*p < 0.05 relative to control, # p < 0.05 for pairwise comparisons, error bars represent ± SEM). See also Figure S2.