Stereotypic wheel running decreases cortical activity in mice

Abstract

Prolonged wakefulness is thought to gradually increase 'sleep need' and influence subsequent sleep duration and intensity, but the role of specific waking behaviours remains unclear. Here we report the effect of voluntary wheel running during wakefulness on neuronal activity in the motor and somatosensory cortex in mice. We find that stereotypic wheel running is associated with a substantial reduction in firing rates among a large subpopulation of cortical neurons, especially at high speeds. Wheel running also has longer-term effects on spiking activity across periods of wakefulness. Specifically, cortical firing rates are significantly higher towards the end of a spontaneous prolonged waking period. However, this increase is abolished when wakefulness is dominated by running wheel activity. These findings indicate that wake-related changes in firing rates are determined not only by wake duration, but also by specific waking behaviours.

Figure 1. Cortical neuronal activity in the primary motor cortex (M1) during voluntary wheel running in freely behaving mice.

(a) A photograph of the custom-made cage providing continuous free access to a running wheel positioned at the rear. Scale bars, 5 cm. (b) From top to bottom: 12-h profile of EEG slow-wave activity (SWA, EEG power between 0.5–4.0 Hz, represented as % of 12-h mean) recorded in the frontal cortex, running-wheel (RW) activity (counts per second) and the distribution of sleep-wake stages (W=wakefulness, N=NREM sleep, R=REM sleep) from a representative mouse. (c) Schematic representation of the position of the primary motor cortex (M1, blue area drawn with reference to Paxinos & Franklin, 2001) shown on the dorsal surface of the mouse head, and the position of the 16-ch microwire array above M1 (dots indicate the position of individual wires within the array, scale bar, 1 mm). Traces on the right: top, representative local field potentials (LFP) recorded during NREM sleep from one row of microwires; bottom, raster plot of multiunit activity (MUA, each vertical line represents a spike) recorded from the same wires (scale bar, 500 μV). Note the close temporal relationship between positive LFP waves and periods of generalized neuronal silence. (d) Two MUA traces recorded from M1 in the same animal with corresponding RW-activity (bottom, each vertical bar represents a single wheel rung count, scale bars, amplitude 100 μV, time 1 s). Corresponding waveforms of the action potentials recorded extracellularly are shown on the right (scale bar, 0.5 ms). (e) The distribution of all putative single units recorded in n=11 mice as a function of the ratio of their average firing rates (FR) during running (wRUN) and non-running waking (nwRUN). Note that a smaller proportion of neurons increase firing during wheel running (red, RUN on neurons), while the majority decrease FR during running (blue, RUN off neurons). (f) Average FR in M1 during nwRUN waking, wRUN waking and NREM sleep. Mean values, s.e.m., n=11 (individual mice: grey symbols). Significant differences between vigilance states are depicted above the bars (obtained by two-tailed paired t-test following significant one-way ANOVA). (g) FR in M1 shown as function of running speed. Thick line: mean values, s.e.m., n=9 mice. Values from individual animals are shown as thin line plots.

Figure 2. Cortical neuronal activity in the somatosensory cortex (SCx) during voluntary wheel running.

(a) Schematic representation of the position of the 16-ch microwire array shown relative to the barrel cortex (drawn in reference to Paxinos & Franklin, 2001) shown on the dorsal surface of the mouse head (dots indicate the position of individual wires within the array, scale bar, 1 mm). (b) Individual representative examples depicting multiunit activity (MUA) in SCx during wheel running (scale bars: amplitude 100 μV, time 1 s). Corresponding running wheel (RW)-activity is shown below each trace (each vertical bar represents a single wheel rung count). (c) The distribution of all putative single units recorded in SCx (in n=5 mice) as a function of the ratio of their average firing rates (FR) during running (wRUN) and non-running waking (nwRUN). Note that a smaller proportion of neurons increase FR during running (red), while the majority decrease spiking activity (blue). (d) FR in SCx shown as a function of running speed (counts per second). Thick line: mean values, s.e.m., n=5 mice. Values from individual animals are shown as thin line plots.

Figure 3. The relationship between cortical neuronal activity and changes in wheel running speed.

(a) Individual examples depicting cortical firing rates (FR) in the motor cortex (M1) and somatosensory cortex (SCx) modulated by running wheel behaviour. From top to bottom: MUA recorded in one representative channel of the 16-channel array, and running-wheel (RW)-activity (each vertical bar represents a single wheel rung count, scale bars, amplitude 50 μV, time 1 s). Note the irregular pattern of spiking associated with variability in running speed. (b) The distribution of 1-s epochs as a function of firing rates during wRUN and nwRUN waking. Four individual putative single units are shown. Blue background relates to data recorded in M1, pink background relates to data recorded in SCx. Note that running is associated with a substantial shift in the distribution of firing rates. Insets: corresponding spike wave forms (scale bars, amplitude 50 μV, time 0.5 ms). (c) The distribution of all putative single units recorded in M1 (n=9 mice) and SCx (n=5 mice) as a function of the ratio of the width of FR distribution during wRUN and nwRUN waking. Note that the majority of neurons have narrower distribution of FRs during wRUN waking in both M1 and SCx. (d) The effect of wheel running speed on the width of FRs distribution. Left and right panels: mean values for M1 and SCx, respectively. The curves in the middle schematically depict the procedure for calculating the width of FRs distributions at ½ of their height. Values above depict P values of paired t-test (after Bonferroni's correction). (e) Cortical FR in M1 (blue) and SCx (pink) shown as a function of the wheel acceleration–deceleration index. All 1-s epochs were subdivided into ten 10% deciles, each consisting of the same number of epochs. These were sorted as a function of the change in wheel running speed within an epoch from fast acceleration to fast deceleration (schematically shown as arrows above), and corresponding average FR were calculated before averaging between animals. Mean values, s.e.m., M1: n=9 mice, SCx: n=5. Mean values are shown as bar plots, the individual values from individual animals are shown as thin line plots.

Figure 4. Cortical activity during voluntary wheel running.

(a) Top traces show representative local field potentials (LFP, 0.1–100 Hz) in eight channels of the 16-channel microwire array placed in the primary motor cortex (M1). Middle: raster plot of multiunit activity (MUA) recorded in the same eight channels (each vertical line represents a spike). Bottom: corresponding running-wheel (RW)-activity (each bar represents a single wheel rung count). Note that a positive LFP wave is accompanied by reduced MUA in corresponding channels (scale bars, amplitude 500 μV, time 0.5 s). (b) Individual representative example of a positive LFP slow (2–6 Hz) wave recorded with one individual wire of a 16-channel microwire array during wheel running. Bar plot below shows corresponding MUA (scale-bar: 200 μV). (c) The distribution of interspike intervals (ISIs) as a function of wheel running behaviour. The number of ISIs calculated as a function of logarithmically increasing ISI duration bins, plotted against their lower limits. For each putative single unit ISIs were calculated during wRUN and nwRUN waking, and the difference between the two states was calculated separately for RUN on and RUN off neurons. Mean values, s.e.m., n=9 mice (red diamonds, P<0.05, paired t-test). Note a relative predominance of short ISIs during running among RUN off neurons, compared with nwRUN waking and RUN on neurons. (d,e,f) The same analysis for SCx. Mean values, s.e.m. (n=5 mice).

Figure 5. Longer-term effects of wheel running on cortical firing rates.

(a) The relationship between the proportion of time spent running during a period of spontaneous waking and the duration of the corresponding waking period. Each dot corresponds to an individual waking period. The data are shown separately from animals implanted in M1 (n=11) and SCx (n=5). Note that a higher proportion of running is associated with longer waking periods. (b) Sleep-wake stages (W=wakefulness, N=NREM sleep, R=REM sleep) from one representative mouse showing two consolidated spontaneous waking periods. Shaded areas indicate the 15-min time intervals used for comparison between the beginning (W1) and end (W2) of waking periods >40 min. Scale bar, 1 h. (c) Average firing rates (FR) during W1 and W2 represented as % of the mean value between W1 and W2 for each putative single unit. Note that on average spontaneous cortical FR increased significantly (triangles, P<0.05, paired t-test) from W1 to W2 in both M1 and SCx. Mean values, s.e.m. (n=11 and n=5 animals for M1 and SCx, respectively). (d) Average FR during W1 and W2 shown separately for waking periods with a high or low amount of running (top and bottom 50% of the distribution of all waking periods, respectively = ‘high'—red, and ‘low'—blue). FR during W2 are shown as % of corresponding values during W1. Note that FR increased significantly only in the course of waking periods with low RW-activity. (e) Average FR during W2 shown as % of W1 shown separately for RUN on neurons (top) and RUN off neurons (bottom). As in c, blue bars correspond to waking periods with low RW-activity, and red bars correspond to waking periods with high RW-activity. NS, not significant.