Rapid whisker movements in sleeping newborn rats

Abstract

Spontaneous activity in the sensory periphery drives infant brain activity and is thought to contribute to the formation of retinotopic and somatotopic maps. In infant rats during active (or REM) sleep, brainstem-generated spontaneous activity triggers hundreds of thousands of skeletal muscle twitches each day; sensory feedback from the resulting limb movements is a primary activator of forebrain activity. The rodent whisker system, with its precise isomorphic mapping of individual whiskers to discrete brain areas, has been a key contributor to our understanding of somatotopic maps and developmental plasticity. But although whisker movements are controlled by dedicated skeletal muscles, spontaneous whisker activity has not been entertained as a contributing factor to the development of this system. Here we report in 3- to 6-day-old rats that whiskers twitch rapidly and asynchronously during active sleep; furthermore, neurons in whisker thalamus exhibit bursts of activity that are tightly associated with twitches but occur infrequently during waking. Finally, we observed barrel-specific cortical activity during periods of twitching. This is the first report of self-generated, sleep-related twitches in the developing whisker system, a sensorimotor system that is unique for the precision with which it can be experimentally manipulated. The discovery of whisker twitching will allow us to attain a better understanding of the contributions of peripheral sensory activity to somatosensory integration and plasticity in the developing nervous system.

Figure 1

Diverse patterns of whisker movements during active sleep in 4–6-day-old rats. (A) Left: One side of the snout, highlighting 11 marked whiskers (boxed area). Right: Labels and relative locations of the 11 marked whiskers (black circles); also shown are the 2 markings on the skin surface of the mystacial pad (red circles). (B) Quiver plots depicting 4 types of whisker and mystacial pad movements observed using high-speed videography. The locations of the whiskers (black circles) and the mystacial pad markings (red circles) correspond to those in (A). The lines emanating from the circles are proportional to the whisker or pad displacement over the previous 25 ms (5 frames); the direction of movement is also indicated. The 4 patterns depicted correspond to Movie S1. See Figure S1 for additional examples of the diversity of whisker trajectories.

Figure 2

Extrinsic whisker muscles twitch during active sleep. (A) Locations of EMG electrodes (black asterisks) in m. maxillolabialis and m. nasolabialis (also referred to as m. levator labii superioris in [8], from which this drawing is adapted). (B) Raw EMG record depicting twitching in the two extrinsic muscles as well as the nuchal muscle. Sharp spikes in the EMG records indicate myoclonic twitches. (C) Cross-correlograms showing highly correlated bouts of twitch activity for each pair of muscles. The horizontal dashed line indicates statistical significance.

Figure 3

VPM neurons fire in bursts in close proximity to sleep-related twitching. (A) Coronal sections through the thalamus to show the locations recording sites. VPM, ventral posteromedial thalamus; VPL, ventral posterolateral thalamus; Po, posterior thalamus. (B) Representative data depicting multiunit activity (MUA), single unit activity, and nuchal EMG. Twitch and wake movements (black and red asterisks, respectively) are also shown. Recording site is the yellow circle in (A). (C) Perievent histogram for the VPM unit. Vertical line at 0 ms denotes time of nuchal muscle twitch and horizontal line indicates statistical significance. The twitch-following properties of another VPM unit were further investigated by anesthetizing the whisker pad (see Figure S2). (D) Frequency histogram of interspike intervals (ISI) for the same VPM unit. (E) VPM unit firing rate (spikes/5 s), unit activity, and bursts activity over a 15-min recording session. (F) Mean (+SE) bursts/min during active sleep and wake across all 7 subjects. * significant difference from wake, P<0.001.

Figure 4

Voltage-sensitive dye (VSD) imaging of barrel cortex activity during active sleep in 4-day-old rats. (A) The experimental set-up for VSD imaging, highlighting the 4 × 4 mm cranial window (black box) to expose the right barrel cortical field. The 2.5 × 2.5 mm region of interest (ROI) is outlined by the red box. b, bregma. See Figure S3 for methods pertaining to the VSDI procedure. (B) Three representative samples of barrel cortex activity, from isolated activity within 1 or 2 barrels (top two rows) to global activity across the barrel field (bottom row). Each image corresponds to the ROI in (A). The number beneath each frame denotes time from the initiation of a nuchal muscle twitch. Color bars at the right of each sequence indicate range of values of dF/F₀. For clarity of presentation, obvious instances of noise were removed manually from some VSD images.