Firing rate homeostasis in visual cortex of freely behaving rodents

Abstract

It has been postulated that homeostatic mechanisms maintain stable circuit function by keeping neuronal firing within a set point range, but such firing rate homeostasis has never been demonstrated in vivo. Here we use chronic multielectrode recordings to monitor firing rates in visual cortex of freely behaving rats during chronic monocular visual deprivation (MD). Firing rates in V1 were suppressed over the first 2 day of MD but then rebounded to baseline over the next 2-3 days despite continued MD. This drop and rebound in firing was accompanied by bidirectional changes in mEPSC amplitude measured ex vivo. The rebound in firing was independent of sleep-wake state but was cell type specific, as putative FS and regular spiking neurons responded to MD with different time courses. These data establish that homeostatic mechanisms within the intact CNS act to stabilize neuronal firing rates in the face of sustained sensory perturbations.

Figure 1. Chronic multiunit recording from V1 of freely behaving rats

(A) Location of implanted microwires (arrowheads), overlaid with diagram of coronal section of rat V1m (modified from Paxinos and Watson, 1997). (B) Average LFP response from layer 2/3 to 50×50 msec light pulses delivered at 1 Hz (gray bar). (C) Raw traces collected on a single wire originating from two units. (D) Example of principal components clustering of units in C. Individual spikes are represented as points in eigenspace defined by the first four principal components. The clustering algorithm identifies discrete clusters (pink and green). (E) Plot of spike trough-to-peak vs slope between 0.25–0.57 ms after the spike trough revealed a bimodal distribution that corresponds to pFS cells (pink) and RSUs (green). Inset: mean and peak firing rates of the RSU and pFS populations. (F) Heat map of 150 minutes of firing from 5 neurons recorded simultaneously on a single array. All error bars indicate ±SEM.

Figure 2. Homeostatic Regulation of RSU Firing During MD

(A) Experimental design. (B) Example heat maps of all recorded well-isolated RSUs from a single animal on baseline 3, MD2, and MD6. (C) Average RSU firing rates in the non-deprived (control) hemisphere, and (D) in the deprived hemisphere; here and below baseline is blue, MD is grey. Number of neurons contributing to each mean indicated in white. (E) Distribution of mean RSU firing rates on Basline3 (BL3), MD2, and MD6. (F) Cumulative distribution of ISIs for BL3, MD2, and MD6; inset plots CV of ISIs, calculated for each cell and averaged. * = significantly different from BL3. All error bars indicate ±SEM.

Figure 3. Layer and Cell type Specificity of Firing Rate Homeostasis

(A) Top: example mEPSCs recorded ex vivo from L2/3 pyramidal neurons. Bar plot: mEPSC amplitude on MD, 4, and 6 expressed as % of non-deprived hemisphere control values. * = significantly different from control. (B) Firing rates from RSUs in layers 2–4 for control (blue) and MD (grey). * = significantly different from BL3. (C) ISI distribution from pFS cells for BL3, MD1, and MD6. Inset shows CV of ISI by day. (D) A comparison of RSUs and pFS normalized firing rates during baseline (blue bar) and MD (grey bar). * = significant difference between RSU and pFS. All error bars indicate ±SEM.

Figure 4. Firing Rate Homeostasis is Expressed Across Sleep-Wake States

(A) LFP delta (black trace, 1–4 Hz) and (B) gamma “high” band powers during epochs of sleep (light green), quiet waking (yellow) and active wake (light blue). (C) Heat map of firing during the sleep-wake transition illustrated in A,B. (D) RSUs (D) and pFS (E) firing rates during epochs of active wake (blue bars) and sleep (green bars) for baseline (dark blue horizontal bar) and 6 days of MD (gray horizontal bar). All error bars indicate ±SEM.