Pericytes in capillaries are contractile in vivo, but arterioles mediate functional hyperemia in the mouse brain

Abstract

Modern functional imaging techniques of the brain measure local hemodynamic responses evoked by neuronal activity. Capillary pericytes recently were suggested to mediate neurovascular coupling in brain slices, but their role in vivo remains unexplored. We used two-photon microscopy to study in real time pericytes and the dynamic changes of capillary diameter and blood flow in the cortex of anesthetized mice, as well as in brain slices. The thromboxane A(2) analog, 9,11-dideoxy-9α,11α-methanoepoxy Prostaglandin F2α (U46619), induced constrictions in the vicinity of pericytes in a fraction of capillaries, whereas others dilated. The changes in vessel diameter resulted in changes in capillary red blood cell (RBC) flow. In contrast, during brief epochs of seizure activity elicited by local administration of the GABA(A) receptor antagonist, bicuculline, capillary RBC flow increased without pericyte-induced capillary diameter changes. Precapillary arterioles were the smallest vessels to dilate, together with penetrating and pial arterioles. Our results provide in vivo evidence that pericytes can modulate capillary blood flow in the brain, which may be important under pathological conditions. However, our data suggest that precapillary and penetrating arterioles, rather than pericytes in capillaries, are responsible for the blood flow increase induced by neural activity.

Fig. 1.

Pericytes can be visualized in the brains of GFP transgenic mice. (A) The small (∼10 μm) spindle-shaped cell bodies protruding from the capillary wall and their processes were positive for the pericyte marker APN (arrowhead). (B) Negative immunoreactivity of the protruding cell body associated to the vessel wall for PECAM-1, an endothelial cell marker (arrowhead). Insets in A–C show reconstructions of the z-sections at the planes marked by the dashed lines. (C) Immunoreactivity for laminin reveals the basal membrane, which completely encloses the capillary vascular GFP-positive structures. Spindle-shaped cells are separated by basal membrane from the underlying endothelium (Inset). (D) Protruding, spindle-shaped vascular cell bodies and their processes are positive for the pericyte marker PDGFR-β. (E) Some of these cells express also the contractile protein α-smooth muscle actin (α-SM actin). (F) Pericyte bodies are contacted by GFAP-positive astrocyte processes, but separated from them by the vascular basal membrane (Inset). (Scale bars: 10 μm; Insets, 5 μm.)

Fig. 2.

U46619 induces pericyte contraction in brain slice preparations. (A) TPLSM images of capillaries in slice, before or after superfusion with 100 nM U46619. Pericyte bodies (p) can be identified. Discrete constrictions along the vessel can be observed (arrowheads), and bulging of a pericyte cell body is evident (Inset). (Scale bars: 10 μm; Inset, 5 μm.) (B) Effect of different concentrations of U46619 on capillary diameters at the segments of maximal effect (*P < 0.05, **P < 0.001, n = 11; b, basal). (C) Effects of superfusion with vehicle (n = 7), 100 nM U46619 (n = 15), or 100 nM U46619 plus preincubation with 1 μM of SQ 29,548 (n = 5) (*P < 0.0001; **P < 0.00001). (D) Partial reversal of the U46619-induced constriction (100 nM) by 1 μM SQ 29,548 (n = 6). (E) The constrictions elicited by 100 nM U46619 at pericyte bodies are greater than constrictions at vessels halfway between pericyte bodies (connected circles represent mean constrictions for one experiment at either location; n = 19; *P < 0.005).

Fig. 3.

U46619 induces pericyte contraction in vivo, paralleled by RBC velocity and flux changes. (A) Capillary of the sensory cortex of anesthetized mouse, imaged before and after the application of 10 μM U46619. Plasma was labeled by TRITC-dextran (red). Dark stripes correspond to the passage of RBCs. Arrowheads point at constrictions in the vicinity of the pericyte body (p) (Scale bar: 10 μm.) (B) Box plots of the changes in diameter, RBC flux, and velocity (Top to Bottom, expressed as percentage of basal) after the superfusion of 10 μM U46619 (n = 37 capillaries, five animals) or in controls superfused with vehicle (n = 45 capillaries, five animals). Outliers are not represented in the box plot (one animal exhibited increases in velocity in capillaries of between 300 and 600% after treatment with U46619). (C) Scatter plots of the maximal absolute differences from basal after superfusion with U46619 (red dots) or in the control experiments (blue dots) expressed in micrometers, RBC·s⁻¹ or, mm·s⁻¹ for diameter, flux, and velocity differences (Top to Bottom). A strong positive linear correlation is found between the differences in RBC velocity and flux (two groups pooled). Positive correlations are also found for the pair diameter vs. RBC flux and diameter vs. RBC velocity. (D) The constrictions elicited by superfusion with U46619 at pericyte bodies were greater than constrictions halfway between pericyte bodies (connected circles represent mean constrictions for each capillary network, measured at either location; n = 18; *P < 0.0005).

Fig. 4.

Capillary dilatation does not partake in the hyperemic response induced by bicuculline. (A) Original LFP, diameter, and RBC velocity traces of different vascular segments. The LFP traces (blue) show typical bicuculline-induced activity bursts. (Upper) In the pial, penetrating, or precapillary arterioles, but not at pericytes in the capillaries, neuronal activity bursts are associated with increases in diameter. High burst frequency caused summation of the diameter or flow responses. (Lower) RBC velocity changes of the same vessels, demonstrating brief increases associated with increased neuronal activity. (B) Average traces ± SD (gray areas) of the pooled diameter (Upper) or RBC velocity responses (Lower) of the different vessel types, binned in 200-ms segments. The blue vertical lines at t = 0 represent the time point of the maximal neuronal depolarization during the spike bursts. Whereas an average dilatation is present in arteriolar segments, capillaries do not respond at all. Velocity increases are detectable in each vessel type studied. Because activity bursts recurred frequently, the fall before the burst reflects the decline of the preceding response. No RBC velocity data are available for penetrating arterioles. (C and D) Distribution of the basal diameters (C) and RBC velocities (D) of the different vessel types investigated (*P < 0.0001, see E for sample n). (E and F) Scatter plots of the observed absolute diameter changes in capillaries and the observed changes in the RBC velocity or flux, respectively; no significant correlations are found. (G) Scheme of the preparation: A micropipette filled with 10 mM bicuculline was inserted in the parietal cortex through a partially closed cranial window and served for LFP recordings. (H) Topology of cortical microvasculature. Pial arterioles run at the cortical surface and dive into the cortex to become penetrating arterioles. Precapillary arterioles arise from penetrating arterioles radially into the parenchyma, where they give rise to the capillary network provided with pericytes.

Fig. 5.

Capillaries dilate passively during CSD. (A) Scheme of the preparation. A bipolar stimulation electrode placed over the frontal cortex is used to elicit CSDs. TPLSM imaging, ECoG/DC, and LDF recording are performed through a closed parietal window. (B) Electrophysiological and LDF tracings of the passage of a CSD showing a transient decrease of ECoG activity, DC potential shift, and transient elevation of CBF (LDF trace). (C, Left) Cortical volumes were imaged containing parenchymal arterioles (art) and their downstream capillaries. p, pericytes. (C, Right) Diameter tracings at different vessel segments, populated or not by pericyte bodies, showing simultaneous dilatation of all vessel segments. (D) Paired maximal diameter changes measured either at pericyte bodies or halfway between pericyte bodies do not differ significantly.