Motor behavior activates Bergmann glial networks

Abstract

Although it is firmly established that neuronal activity is a prime determinant of animal behavior, relationships between astrocytic excitation and animal behavior have remained opaque. Cerebellar Bergmann glia are radial astrocytes that are implicated in motor behavior and exhibit Ca(2+) excitation. However, Ca(2+) excitation in these cells has not previously been studied in behaving animals. Using two-photon microscopy we found that Bergmann glia exhibit three forms of Ca(2+) excitation in awake, behaving mice. Two of these are ongoing within the cerebellar vermis. During locomotor performance concerted Ca(2+) excitation arises in networks of at least hundreds of Bergmann glia extending across several hundred microns or more. Concerted Ca(2+) excitation was abolished by anesthesia or blockade of either neural activity or glutamatergic transmission. Thus, large networks of Bergmann glia can be activated by specific animal behaviors and undergo excitation of sufficient magnitude to potentially initiate macroscopic changes in brain dynamics or blood flow.

Figure 1. Two-Photon Microscopy in Cerebellar Cortex of Behaving Mice

(A) Two-photon microscopy in awake, head-restrained mice using an exercise ball. Mouse locomotion was tracked by an optical encoder on the ball, which provided a sensitive, quantitative, and repeatable readout of running onset/offset, speed and direction. In addition, mouse behavior was recorded on video (Supplemental Data). (B) Areas of lobules V and VI of the cerebellar vermis examined in this study by Ca²⁺-imaging (pink). Scale bar: 1 mm. (C) Probability densities of image displacements in dorso-ventral, z (blue), medio-lateral, x (green), and rostro-caudal, y (red), dimensions from a set of 7 example experiments. Insets: Colored traces show x (green) and y (red) image displacements (left inset), or x (green) and z (blue) image displacements (right inset), during two examples of mouse locomotion (black trace). x and y displacements were corrected by image registration prior to data analysis. Scale bars: Upper, 50 mm/s; Lower, 10 s and 5 μm.

Figure 2. Palisades and Somata of Bergmann Glia Were Identified Based on Anatomical Features and Sulforhodamine 101 (SR101) Co-staining

(A) The red fluorescent astrocyte marker SR101 stains Bergmann glia in the cerebellar cortex of adult mice. Localization of SR101 to Bergmann glia was verified in the Kosmos mouse line expressing EGFP under the control of an S100β promoter, an established marker of astrocytes. Left, EGFP expression in Bergmann glia of the Kosmos line. Center, SR101 staining pattern in the same mouse. Right, Image overlays showing that SR101 stains Bergmann glia somata and palisade fibers (yellow, open arrow heads) in the Purkinje cell (bottom) and molecular layers (top), respectively. Molecular layer interneurons and Purkinje cells (closed arrow heads) are labeled neither in the Kosmos line nor by SR101. (B) SR101 co-staining was used to identify Bergmann glial palisades and somata in OGB-1-AM fluorescence images. Left, OGB-1-AM staining pattern in the molecular (top) and Purkinje cell layer (bottom). Center, SR101 staining in the same mouse. Right, Overlay images, in which Bergmann glia fibers, end foot processes and somata are indicated (open arrow heads) (See also Figures S1 and S2). Molecular layer interneurons and Purkinje cells (closed arrowheads in top and bottom rows, respectively) are stained by OGB-1-AM but not SR101. Scale bars: 15 μm.

Figure 3. Sparkles Are Ongoing Ca²⁺ Transients That Appear in Individual Palisade Fibers

(A) OGB-1-AM image showing cellular structures in a superficial region of the cerebellar molecular layer. Ca²⁺ dynamics were analyzed in regions of interest (ROI) where palisade fibers (yellow) had been identified by SR101 co-staining. Example traces of relative increases in OGB-1-AM fluorescence, ΔF(t)/F, from 10 ROIs marked red are shown in (B). Scale bar: 50 μm. (B) Traces of Ca²⁺ signals from individual palisade fiber ROIs show examples of sparkles, defined as events of >10% ΔF(t)/F (marked by gray dots) for purposes of analysis. Traces are arranged by sparkle onset time. Scale bars: 5 s, 10% ΔF(t)/F. (C) High magnification OGB-1-AM ΔF(t)/F images showing a Ca²⁺ sparkle in one out of two neighboring palisade fibers, whose perimeters (yellow outlines) were determined from the simultaneously acquired SR101 images. Scale bar: 3 μm. (D) Overlay of 45 individual sparkle time courses (black) and an averaged trace (red) for sparkles that occurred within the yellow ROIs in (A). Scale bars: 5 s, 10% ΔF(t)/F.

Figure 4. Bursts Are Ongoing Ca²⁺ Transients Spreading as 3D Radial Waves Over Multiple Bergmann Glia

(A) Example 3D imaging data showing ΔF(t)/F optical cross-sections of OGB-1-AM fluorescence at five time points. Indicated depth values are relative to the pial surface. Scale bar: 50 μm. (B) Top, 6 individual (black) ΔF(t)/F time traces for bursts and the mean time course (green) averaged over 96 bursts in 11 animals, measured over fixed areas that covered the bursts’ centers in awake resting mice. Bottom, averaged and normalized ΔF(t)/F traces under four conditions: isoflurane-anesthetized (blue; n = 8 events, 5 mice), awake resting (green; n = 96 events, 11 mice), TTX- (red; n = 9 events, 5 mice) and γDGG-treated (orange; n = 7 events, 2 mice). Different animals were used for different drugs. Scale bar: 5 s. (C) Cross-sectional ΔF(t)/F fluorescence image of a burst at a fixed depth and time. Yellow and peach lines respectively indicate directions parallel and perpendicular to the cerebellar folium. (D) ΔF(t)/F time courses along the yellow and peach-colored lines in (C). Note that bursts have larger diameter in the direction parallel to the folium and spread outwards from a central initiation site. (E) Top, locations and volumetric profiles of 7 bursts detected by 3D imaging during an 8.2 min period. Colored outlines show extent of Ca²⁺ spread for each burst at various depths. Bottom, z-projection image. (F) Population data showing bursts have greater extent parallel to the long axis of the cerebellar folium, which lies perpendicular to the parasagittal planes of cerebellar cortex. Colored data points denote individual bursts recorded under the four conditions noted in (B).

Figure 5. Motor Behavior Evokes Concerted Ca²⁺-Excitation Across Bergmann Glial Networks

(A) Encoder trace of the exercise ball’s rotational speed, showing multiple running bouts during an example 9.8 min period. Scale bars: 50 s and 20 mm/s; the former also applies to (B). (B) Top, SR101 ΔF(t)/F traces from 291 palisade fiber regions of interest (ROI) during the same time interval as in (A). Bottom, the same palisade fiber ROIs in OGB-1-AM images reliably exhibit concerted ΔF(t)/F increases, flares, during running. Signals of similar magnitude and time course are absent in ΔF(t)/F traces of non-functional indicator SR101. ΔF(t)/F scale bar in E also applies to B-D. ROIs were sorted by x coordinate. (C) OGB-1-AM ΔF(t)/F traces over the interval indicated in (B) by dashed lines. Flares are marked blue and are interleaved in time with sparkles (red) and bursts (green). Scale bar: 50 s. (D) Image frames of OGB-1-AM ΔF(t)/F increases relative to movement onset during the first flare in (C) illustrating the broad spatial extent of flares. Scale bar: 100 μm. (E) Images from a 3D recording of ΔF(t)/F increases during a flare. Indicated depth values were measured relative to the pial surface.

Figure 6. Locomotor Evoked Ca²⁺ Flares Depend on Neuronal Activity, Have Limited Duration, and Exhibit a Refractory Period

(A) Example traces revealing Ca²⁺ flares (bottom) as a function of locomotor activity (black), under four conditions: awake-untreated (green; averaged signal from 194 ROIs), PPADS- (purple; 231 ROIs), γDGG- (orange; 192 ROIs) and TTX-treated (red; 191 ROIs). Data for each drug condition are from a different individual mouse. Concerted Ca²⁺ activity was quantified by the rate of palisade fiber activation per unit area using a threshold criterion of 10% ΔF(t)/F for each individual fiber. Upper scale bars, 50 s and 150 mm/s. Lower scale bars, 1 kHz/mm². (B) Population data showing paired traces of running speed (left) and concurrent Ca²⁺-activity (right) in untreated (n = 187; 15 mice), PPADS- (n = 20; 3 mice), γDGG- (n = 45; 4 mice), and TTX-treated mice (n = 44; 5 mice). Red lines indicate movement onset. For each condition, traces are ordered by the pause duration after the prior movement. (C) Normalized mean time courses of flare activation triggered on movement onset (red line), for all (solid), brief (dotted), and extended movements (dashed) showing that flares have limited duration. Flares in panel B with peak amplitudes of at least 20 Hz/mm² were included for averaging. (D) The probability of observing a flare at locomotor initiation versus the pause duration between movements, computed across the 187 movements shown in B, showing flares have a refractory period (compare to B).

Figure 7. Temporal Relationships of Ca²⁺ Flares to Locomotor Evoked Blood Perfusion Increases

(A) Example traces showing blood perfusion increases (bottom) as a function of locomotor activity (top), under four conditions, awake-untreated (green), PPADS-(purple), γDGG- (orange) and TTX-treated (red). Data for each drug condition are from a different individual mouse. Upper scale bars, 50 s and 150 mm/s. Lower scale bar, 20 %. (B) Population data showing overlay of average blood perfusion increases in untreated (green; n = 134; 11 mice), PPADS- (purple; n = 47; 3 mice), γDGG- (orange; n = 45; 4 mice), and TTX-treated mice (red; n = 35; 3 mice) triggered upon movement onset for brief (left), extended (center), and all movements (right). Traces from each trial were normalized by the peak running speed in that trial, prior to averaging. Note that TTX and γDGG reduce the amplitude of blood perfusion increases and reduce their persistence during extended running periods. Scale bar, 10 %. (C) Overlay of average Ca²⁺ flare activation (black) and blood perfusion increase (green), triggered on movement onset (vertical lines), reveals similar kinetics during locomotor initiation in the untreated condition. Flares are not sustained during extended movements, but blood perfusion increases are. Flare traces are based on a set of 187 running bouts with peak running speeds >100 mm/s in 15 mice. (D) Sets of paired traces of running speed (left) and corresponding blood perfusion increases (right) in untreated (n = 134; 11 mice), PPADS- (n = 47; 3 mice), γDGG- (n = 45; 4 mice), and TTX-treated mice (n = 35; 3 mice). Dashed lines indicate movement onset. For each condition, traces are ordered by the pause duration after the prior movement. TTX and γDGG particularly reduce the persistence of perfusion increases.