Movement and VIP Interneuron Activation Differentially Modulate Encoding in Mouse Auditory Cortex

Abstract

Information processing in sensory cortex is highly sensitive to nonsensory variables such as anesthetic state, arousal, and task engagement. Recent work in mouse visual cortex suggests that evoked firing rates, stimulus-response mutual information, and encoding efficiency increase when animals are engaged in movement. A disinhibitory circuit appears central to this change: inhibitory neurons expressing vasoactive intestinal peptide (VIP) are activated during movement and disinhibit pyramidal cells by suppressing other inhibitory interneurons. Paradoxically, although movement activates a similar disinhibitory circuit in auditory cortex (ACtx), most ACtx studies report reduced spiking during movement. It is unclear whether the resulting changes in spike rates result in corresponding changes in stimulus-response mutual information. We examined ACtx responses evoked by tone cloud stimuli, in awake mice of both sexes, during spontaneous movement and still conditions. VIP⁺ cells were optogenetically activated on half of trials, permitting independent analysis of the consequences of movement and VIP activation, as well as their intersection. Movement decreased stimulus-related spike rates as well as mutual information and encoding efficiency. VIP interneuron activation tended to increase stimulus-evoked spike rates but not stimulus-response mutual information, thus reducing encoding efficiency. The intersection of movement and VIP activation was largely consistent with a linear combination of these main effects: VIP activation recovered movement-induced reduction in spike rates, but not information transfer.

Keywords: information; interneurons; locomotion; optogenetics; vasoactive intestinal peptide.

Graphical abstract

Figure 1.

Extracellular recording and optogenetic manipulation of ACtx neurons in freely moving mice. A, Mice are head fixed atop a spherical treadmill to permit movement but constrain head position for extracellular physiology. A window in the headbar provides access to ACtx of the right hemisphere for extracellular recording with linear multielectrode arrays and optogenetic manipulation via LED fiber optic cables. Sounds are presented to the contralateral ear through an electrostatic speaker. B, Optogenetic activation of VIP⁺ inhibitory interneurons in ACtx. Left, Schematic of prominent connections among cortical inhibitory interneurons and pyramidal neurons. Sst⁺ and Pvalb⁺ interneurons predominantly connect to pyramidal dendrites and somata, respectively. VIP⁺ interneurons most prominently connect to Sst⁺ interneurons. Thus, VIP⁺ interneuron activation by blue light (right) thus tends to disinhibit pyramidal neurons by reducing Sst⁺ interneuron-mediated inhibition. Right, Control of VIP⁺ interneurons via blue light was accomplished by crossing VIP-Cre mice with Ai32 mice to express channelrhodopsin-2 in Cre-expressing VIP interneurons. Histology figure from ACtx in a representative Ai32/VIP-Cre mouse expressing eYFP-tagged ChR2 (green) in VIP⁺ interneurons. Scale bar, 100 μm. C, Stimuli comprised 500 ms tone clouds spanning 4–64 kHz presented in pseudorandom order. Optogenetic stimulation was delivered for half of all trials in pseudorandom order. Simultaneous recording from 2–15 neurons was permitted by 32-channel linear multielectrode arrays. Treadmill velocity was stored for off-line analysis, yielding both still and movement periods. D, Each unit was characterized by its peak/trough ratio (peak height divided by trough depth) and trough-to-peak time. A sharply bimodal distribution of trough-to-peak times permitted straightforward identification of NS (putative inhibitory) and BS (putative excitatory) neurons. Inset plots show example NS and BS waveforms (mean ± SD).

Figure 2.

Movement reduces stimulus-related spike rates, information content, and encoding efficiency in ACtx. A, Example unit exhibiting suppressed firing during movement. a, Raster plots depicting responses to tone clouds in the absence of (Still) and during movement (Mvmt). Each tick represents an action potential; each raster line reflects activity on a single trial. Unit waveform samples (∼1.8 ms) from each condition (mean ± SD) appear above the rasters. The stimulus period is represented by gray shading in the raster plots and by the sine waveform above the plots. b, Frequency–tuning curve plots for each condition (mean ± SEM), including spontaneous (Spon) firing rates. B, Example unit exhibiting elevated firing during movement. Subplot organization and labeling as in A. C–E, Scatterplot summaries of stimulus-evoked firing rates, stimulus information, and encoding efficiency during Still and Mvmt conditions (n = 71 BS units, n = 25 NS units). A difference box plot (Δ) appears to the right of each scatterplot representing Mvmt values subtracted from Still values. Box plot mid lines indicate medians, outer lines indicate 25th and 75th percentiles, and whiskers indicate the range of data points excluding outliers. The consequences of movement were heterogeneous for our unit samples. On balance, movement was associated with reduced spike rates in excess of spontaneous rates for both BS and NS units, but the difference only reached statistical significance for BS units. Stimulus information (bits) as well as encoding efficiency (bits/spike) were significantly reduced in BS units, but remained unchanged for NS units. *p < 0.05, signed-rank tests.

Figure 3.

VIP⁺ interneuron activation increases spike rates but not stimulus–response mutual information in ACtx. A, Example unit exhibiting elevated firing in response to VIP⁺ interneuron activation. a, Raster plots depicting responses to tone clouds without (Still) and with (Still + VIP) optogenetic activation of VIP interneurons. Only trials in which subjects were not moving were included to isolate the consequences of VIP activation from those of movement. Each tick represents an action potential; each raster line reflects activity on a single trial. Unit waveform samples (∼1.8 ms) from each condition (mean ± SD) appear above the rasters. The stimulus period is represented by gray shading in the raster plots and by the sine waveform above the plots. The optogenetic activation period, including onset ramp, is indicated by the blue bar above the rasters. b, Frequency–tuning curve plots for each condition (mean ± SEM), including spontaneous (Spon) firing rates. B, Example unit exhibiting suppressed firing in response to VIP interneuron activation. Subplot organization and labeling as in A. C–E, Scatterplot summaries of stimulus-evoked firing rates, stimulus information, and encoding efficiency during Still and Still + VIP conditions (n = 71 BS units, n = 25 NS units). A difference box plot (Δ) appears to the right of each scatterplot representing Still + VIP values subtracted from Still values. Box plot mid lines indicate medians, outer lines indicate 25th and 75th percentiles, and whiskers indicate the range of data points excluding outliers. VIP⁺ interneuron activation elevated stimulus-evoked firing rates for the majority of both neuron classes but did not significantly alter information (bits) about the stimulus. Consequently, encoding efficiency (bits/spike) decreased for both unit types. *p < 0.05, signed-rank tests.

Figure 4.

VIP⁺ interneuron activation recovers evoked spike rates, but not stimulus–response mutual information lost during movement. A, Example unit for which baseline-subtracted spike rates and frequency–response functions are similar between Still and Mvmt + VIP conditions. a, Raster plots depicting responses to tone clouds in the absence of (Still) and with movement and VIP⁺ interneuron activation (Mvmt + VIP). Each tick represents an action potential; each raster line reflects activity on a single trial. Unit waveform samples (∼1.8 ms) from each condition (mean ± SD) appear above the rasters. The stimulus period is represented by gray shading in the raster plots and by the sine waveform above the plots. The optogenetic activation period, including onset ramp, is indicated by the blue bar above the rasters. b, Frequency–tuning curve plots for each condition (mean ± SEM), including spontaneous (Spon) firing rates. B, Example unit for which baseline-subtracted spike rates are similar between Still and Mvmt + VIP conditions, despite a relatively flattened frequency–response function in the Mvmt + VIP condition. Subplot organization and labeling as in A. C–E, Scatterplot summaries of stimulus-evoked firing rates, stimulus information, and encoding efficiency during Still and Mvmt + VIP conditions (n = 71 BS units, n = 25 NS units). A difference box plot (Δ) appears to the right of each scatterplot representing Mvmt + VIP values subtracted from Still values. Box plot mid lines indicate medians, outer lines indicate 25th and 75th percentiles, and whiskers indicate the range of data points excluding outliers. C, On average, movement and VIP⁺ interneuron activation had opposite influences on stimulus-related spike rates (Figs. 2, 3), which cancelled out when these respective influences occurred simultaneously. D, E, Whereas stimulus information and encoding efficiency decreased during movement for BS units, no changes were observed during VIP⁺ activation, and thus information remained low when both occurred at the same time. No significant changes were observed for NS units. *p < 0.05, signed-rank tests.

Figure 5.

Coordinated network variability decreases in ACtx are during movement but is unaffected by VIP⁺ interneuron activation. A, B, Scatterplot summaries of signal and noise correlations for Still and Mvmt conditions (n = 441 neuron pairs). A difference box plot (Δ) appears to the right of each scatterplot representing Mvmt values subtracted from Still values. Box plot mid lines indicate medians, outer lines indicate 25th and 75th percentiles, and whiskers indicate the range of data points excluding outliers. Movement was associated with a small but significant reduction in signal correlations (mean decrease, 0.006), which reflect similarity in frequency tuning. A more substantial reduction was observed for noise correlations (mean decrease, 0.0245), which reflect coordinated activity resulting from factors other than the stimulus. C–F, Scatterplot summaries of signal and noise correlations for Still and Still + VIP conditions (C, D) and for Still and Mvmt + VIP conditions (E, F). No significant differences from Still were observed during either VIP activation condition. *p < 0.05, signed-rank tests.

Figure 6.

Summary of the influences of movement and VIP⁺ interneuron activation in visual and auditory cortices. Relative to Still: ↑, increase; ↓, decrease; ≈, no change; blank entries, presently unknown. Note that these changes are intended to represent outcomes on balance, despite heterogeneity among individual neurons.