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. 2017 Feb 8;37(6):1428-1438.
doi: 10.1523/JNEUROSCI.2245-16.2016. Epub 2016 Dec 27.

Differences in Glomerular-Layer-Mediated Feedforward Inhibition Onto Mitral and Tufted Cells Lead to Distinct Modes of Intensity Coding

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Free PMC article

Differences in Glomerular-Layer-Mediated Feedforward Inhibition Onto Mitral and Tufted Cells Lead to Distinct Modes of Intensity Coding

Matthew Geramita et al. J Neurosci. .
Free PMC article

Abstract

Understanding how each of the many interneuron subtypes affects brain network activity is critical. In the mouse olfactory system, mitral cells (MCs) and tufted cells (TCs) comprise parallel pathways of olfactory bulb output that are thought to play distinct functional roles in odor coding. Here, in acute mouse olfactory bulb slices, we test how the two major classes of olfactory bulb interneurons differentially contribute to differences in MC versus TC response properties. We show that, whereas TCs respond to olfactory sensory neuron (OSN) stimulation with short latencies regardless of stimulation intensity, MC latencies correlate negatively with stimulation intensity. These differences between MCs and TCs are caused in part by weaker excitatory and stronger inhibitory currents onto MCs than onto TCs. These differences in inhibition between MCs and TCs are most pronounced during the first 150 ms after stimulation and are mediated by glomerular layer circuits. Therefore, blocking inhibition originating in the glomerular layer, but not granule-cell-mediated inhibition, reduces MC spike latency at weak stimulation intensities and distinct temporal patterns of odor-evoked responses in MCs and TCs emerge in part due to differences in glomerular-layer-mediated inhibition.SIGNIFICANCE STATEMENT Olfactory bulb mitral and tufted cells display different odor-evoked responses and are thought to form parallel channels of olfactory bulb output. Therefore, determining the circuit-level causes that drive these differences is vital. Here, we find that longer-latency responses in mitral cells, compared with tufted cells, are due to weaker excitation and stronger glomerular-layer-mediated inhibition.

Keywords: inhibition; olfaction; olfactory bulb.

Figures

Figure 1.
Figure 1.
MCs and TCs encode the intensity of olfactory sensory neuron stimulation differently. a, b, Schematic (a) and examples (b) of experimental setup. Spiking responses to electrical stimulation (10 μs) of OSNs at five intensities was recorded in either mitral (b, top) or tufted (b, bottom) cells. c, Example of spike rasters (top), PSTHs (middle) and spike time reliability plots (bottom) for a MC (black) and TC (red) across 5 stimulation intensities (see Materials and Methods). Plots of spike time reliability depict the percentage of trials in which the cell fired an action potential during each 10 ms time bin after OSN stimulation (“v” represents the first time bin after OSN stimulation during which spike timing was >80% reliable). d, MCs and TCs respond to OSN stimulation with similar numbers of spikes (p = 0.911). e, TCs, however, respond with higher firing rates, measured as the peak of the PSTH, than MCs (p = 8.8e-16). fj, TCs respond to low-intensity OSN stimulation at shorter latencies than MCs. f, g, Latency to PSTH peak is shorter in TCs than in MCs when plotted vs stimulation intensity (f; p = 1.58e-9) or PSTH peak (g; p = 1.15e-5). h, I, First spike latency (h; p = 0.007) and time to first reliable spike (i; p = 0.0083) are also shorter in TCs than MCs at low stimulation intensities. j, At low stimulation intensities, a larger percentage of TCs than MCs show reliable spike timing. Data were taken from 14 MCs and 20 TCs. Significance was assessed in df using two-way ANOVA and in gi as unpaired t test comparing τ's derived from exponential fit from data in each cell. Asterisks (**) in dj indicate significant (p < 0.05) differences between MCs and TCs.
Figure 2.
Figure 2.
Optical activation of single glomeruli in olfactory bulb sections from OMP-ChR2:EYFP mice. a, Schematic of experimental setup analogous to the one used in Figure 1. Spiking in single MCs or TCs in response to photostimulation (10 ms) of a single glomerulus at five intensities. MCs and TCs respond to OSN photostimulation with similar numbers of spikes (b; p = 0.4); however, TCs responded with higher firing rates than MCs (c; p = 1.83e-10). In addition, TCs responded to low-intensity OSN photostimulation with shorter latencies than MCs as assessed by the time to PSTH peak (d; p = 0.0045), first spike latency (e; p = 0.0023), and time to first reliable spike (f; p = 0.004). g, A larger percentage of TCs than MCs display reliable spike timing. Data were taken from five MCs and five TCs. Significance was assessed in b and c using two-way ANOVA and in df as unpaired t test comparing τ's derived from exponential fit from data in each cell. Asterisks (**) in bg indicate significant (p < 0.05) differences between MCs and TCs.
Figure 3.
Figure 3.
TCs respond to electrical stimulation of OSNs with stronger excitatory and weaker inhibitory currents than MCs. a, b, Excitatory (Vh= −70 mV) and inhibitory (Vh = 0 mV) currents were measured in single MCs or TCs after electrical stimulation of OSNs at five intensities. b, Examples of inhibitory (top) and excitatory (bottom) currents at each of the five stimulation intensities in one example MC (black) and TC (red). ce, Comparisons of inhibitory (top) and excitatory current peak amplitude (c), charge in 1 s after stimulation (d), and charge in first 150 ms after stimulation (e) between MCs and TCs. Compared with TCs, inhibitory currents in MCs have larger peak amplitudes (p = 5.36e-18) and charge transferred in the 1 s after stimulation (p = 3.16e-8) and the first 150 ms after stimulation (p = 7.56e-10). However, excitatory currents in MCs have larger peak currents (p = 4.58e-14), but similar charge transferred in the 1 s after stimulation (p = 0.47) and the first 150 ms after stimulation (p = 0.18) compared with TCs. f, Ratio of excitatory to inhibitory currents is larger in TCs than in MCs when calculated as the ratio of charge transferred during 1 s after stimulation (top; p = 4.2e-7) or during the first 150 ms after stimulation (bottom; p = 0.0005). g, h, Latency to the peak of inhibitory currents does not differ between MCs and TCs (g; p = 0.40); however, the latency to the peak of excitatory currents differs between MCs and TCs (h; p = 0.001). i, At all five intensities, the peak of excitatory currents precedes inhibition in TCs. However, in MCs, excitation lags inhibition at the weakest two intensities, but leads inhibition at the strongest three intensities. Data were taken from eight MCs and eight TCs and are plotted as mean ± SEM. Significance was assessed using two-way ANOVA. Asterisks (**) in ci indicate significant (p < 0.05) differences between MCs and TCs.
Figure 4.
Figure 4.
MCs receive stronger PGC-mediated inhibition than TCs. Inhibitory (a,b) and excitatory (c,d) currents in MCs and TCs were measured before and after limiting GC-mediated inhibition by blocking mGluRs (LY36785, 100 μm) and NMDARs (APV, 25 μm). Currents were evoked using photostimulation in OMP-ChR2-YFP mice at minimal stimulation intensities. e, Peak amplitude of PGC-mediated inhibition is larger in MCs than in TCs (p = 1.40e-6). f, g, Latency to peak of inhibition (f; p = 0.56) and the duration of inhibition (g; p = 0.89; comparing exponential decay constants of currents) did not differ between MCs and TCs. h, Peak amplitude of excitatory currents was larger in TCs than in MCs (p = 5.15e-4). i, Latency to the peak of excitatory currents is longer in MCs than in TCs (p = 0.006). Data were taken from nine MCs and nine TCs and are plotted as mean ± SEM. Significance was assessed using paired t tests.
Figure 5.
Figure 5.
Blocking GC-mediated inhibition does not alter firing rates or spike latencies in MCs or TCs. a, b, Example spike rasters after electrical stimulation of OSNs at three intensities in a MC (a) and a TC (b) before (black/red) and after (gray/pink) limiting GC-mediated inhibition by blocking mGluRs (LY36785) and NMDARs (APV). c, GC-mediated inhibition does not affect the number of spikes in either MCs (minimum, p = 0.72; Mid – p = 0.97; maximum, p = 0.57) or TCs (minimum, p = 0.73; middle, p = 0.29; maximum, p = 0.64). d, GC-mediated inhibition does not affect peak firing rates (PSTH peak) in either MCs (minimum, p = 0.49; middle, p = 0.91; maximum, p = 0.81) or TCs (minimum, p = 0.56; middle, p = 0.67; maximum, p = 0.62). e, f, GC-mediated inhibition does not affect response latency as measured by the time to PSTH peak (e) in either MCs (minimum, p = 0.93; middle, p = 0.97; maximum, p = 0.58) or TCs (minimum, p = 0.44; middle, p = 0.79; maximum, p = 0.53) or the time to reliable spiking (f) in either MCs (minimum, NA; middle, p = 0.67; maximum, p = 0.29) or TCs (minimum, p = 0.38; middle, p = 0.50; maximum, p = 0.73). Data were taken from four MCs and four TCs. Significance was assessed using paired t tests.
Figure 6.
Figure 6.
Blocking glomerular-layer-mediated inhibition alters firing rates and spike latencies in MCs and TCs. a, b, Spikes were measured before and after puffing gabazine in the recorded cell's home glomerulus to limit glomerular-layer-mediated inhibition. c, d, Spike rasters in a MC (c) and TC (d) before (black/red), during (gray/pink), and after (blue) blocking glomerular-layer-mediated inhibition. e, Blocking glomerular-layer-mediated inhibition increased the number of spikes in MCs at all three stimulation intensities (minimum, p = 0.003; middle, p = 0.005; maximum, p = 0.0075) and in TCs at the weakest intensity (minimum, p = 0.0078; middle, p = 0.91; maximum, p = 0.12). f, Blocking glomerular-layer-mediated inhibition increased the firing rate, as measured by the peak of the PSTH, at all three intensities in both MCs (minimum, p = 0.0039; middle, p = 0.011; maximum, p = 0.012) and TCs (minimum, p = 0.0027; middle, p = 0.037; maximum, p = 0.0023). g, Firing rates, however, were more strongly affected by blocking glomerular-layer-mediated inhibition in MCs than in TCs at the strongest two intensities (minimum, p = 0.22; middle, p = 0.014; maximum, p = 0.0039). h, Blocking glomerular-layer-mediated inhibition reduced the latency to the PSTH peak in MCs at the weakest two intensities (minimum, p = 0.007; middle, p = 0.018; maximum, p = 0.56), but did not affect the latency in TCs (minimum, p = 0.43; middle, p = 0.98; maximum, p = 0.48). i, Latency to reliable spiking did not change in TCs after glomerular-layer-mediated inhibition blockade (minimum, p = 0.09; middle, p = 0.13; maximum, p = 0.57); however, in MCs, the timing of responses in all four MCs became reliable at the weakest intensity and latency was reduced at the middle (p = 0.03), but not the maximum (p = 0.75), intensity. j, Spontaneous firing rates of both MCs (p = 0.0034) and TCs (p = 0.0054) increased after puffing gabazine. Data were taken from four MCs and four TCs. Significance was assessed using paired t tests. Asterisks (**) in ei indicate significant (p < 0.05) differences between MCs and TCs.

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