Astrocyte Ca2+ Waves and Subsequent Non-Synchronized Ca2+ Oscillations Coincide with Arteriole Diameter Changes in Response to Spreading Depolarization

Spreading depolarization (SD) is a wave of mass depolarization that causes profound perfusion changes in acute cerebrovascular diseases. Although the astrocyte response is secondary to the neuronal depolarization with SD, it remains to be explored how glial activity is altered after the passage of SD. Here, we describe post-SD high frequency astrocyte Ca2+ oscillations in the mouse somatosensory cortex. The intracellular Ca2+ changes of SR101 labeled astrocytes and the SD-related arteriole diameter variations were simultaneously visualized by multiphoton microscopy in anesthetized mice. Post-SD astrocyte Ca2+ oscillations were identified as Ca2+ events non-synchronized among astrocytes in the field of view. Ca2+ oscillations occurred minutes after the Ca2+ wave of SD. Furthermore, fewer astrocytes were involved in Ca2+ oscillations at a given time, compared to Ca2+ waves, engaging all astrocytes in the field of view simultaneously. Finally, our data confirm that astrocyte Ca2+ waves coincide with arteriolar constriction, while post-SD Ca2+ oscillations occur with the peak of the SD-related vasodilation. This is the first in vivo study to present the post-SD astrocyte Ca2+ oscillations. Our results provide novel insight into the spatio-temporal correlation between glial reactivity and cerebral arteriole diameter changes behind the SD wavefront.

Glutamate-induced calcium signals stimulate CO production in piglet astrocytes

Glutamate-stimulated, astrocyte-derived carbon monoxide (CO) causes cerebral arteriole dilation by activating smooth muscle cell large-conductance Ca2+-activated K+ channels. Here, we examined the hypothesis that glutamate activates heme oxygenase (HO)-2 and CO production via the intracellular Ca2+ concentration ([Ca2+]i)/Ca2+-calmodulin signaling pathway in newborn pig astrocytes. The major findings are: 1) glutamate stimulated Ca2+ transients and increased steady-state [Ca2+]i in cerebral cortical astrocytes in primary culture, 2) in astrocytes permeabilized with ionomycin, elevation of [Ca2+]i concentration-dependently increased CO production, 3) glutamate did not affect CO production at any [Ca2+]i when the [Ca2+]i was held constant, 4) thapsigargin, a sarco/endoplasmic reticulum Ca2+-ATPase blocker, decreased basal CO production and blocked glutamate-induced increases in CO, and 5) calmidazolium, a calmodulin inhibitor, blocked CO production induced by glutamate and by [Ca2+]i elevation. Taken together, our data are consistent with the hypothesis that glutamate elevates [Ca2+]i in astrocytes, leading to Ca2+- and calmodulin-dependent HO-2 activation, and CO production.

Hydrogen sulfide dilates cerebral arterioles by activating smooth muscle cell plasma membrane KATP channels

Hydrogen sulfide (H2S) is a gaseous signaling molecule that appears to contribute to the regulation of vascular tone and blood pressure. Multiple potential mechanisms of vascular regulation by H2S exist. Here, we tested the hypothesis that piglet cerebral arteriole smooth muscle cells generate ATP-sensitive K+ (KATP) currents and that H2S induces vasodilation by activating KATP currents. Gas chromatography/mass spectrometry data demonstrated that after placing Na2S, an H2S donor, in solution, it rapidly (1 min) converts to H2S. Patch-clamp electrophysiology indicated that pinacidil (a KATP channel activator), Na2S, and NaHS (another H2S donor) activated K+ currents at physiological steady-state voltage (−50 mV) in isolated cerebral arteriole smooth muscle cells. Glibenclamide, a selective KATP channel inhibitor, fully reversed pinacidil-induced K+ currents and partially reversed (∼58%) H2S-induced K+ currents. Western blot analysis indicated that piglet arterioles expressed inwardly rectifying K+ 6.1 (Kir6.1) channel and sulfonylurea receptor 2B (SUR2B) KATP channel subunits. Pinacidil dilated pressurized (40 mmHg) piglet arterioles, and glibenclamide fully reversed this effect. Na2S also induced reversible and repeatable vasodilation with an EC50 of ∼30 μM, and this effect was partially reversed (∼55%) by glibenclamide. Vasoregulation by H2S was also studied in pressurized resistance-size cerebral arteries of mice with a genetic deletion in the gene encoding SUR2 (SUR2 null). Pinacidil- and H2S-induced vasodilations were smaller in arterioles of SUR2 null mice than in wild-type controls. These data indicate that smooth muscle cell KATP currents control newborn cerebral arteriole contractility and that H2S dilates cerebral arterioles by activating smooth muscle cell KATP channels containing SUR2 subunits.

Time-dependent action of carbon monoxide on the newborn cerebrovascular circulation

Carbon monoxide (CO) causes cerebral arteriolar dilation in newborn pigs by the activation of large-conductance Ca2+-activated K+ channels. In adult rat cerebral and skeletal muscle arterioles, CO has been reported to produce constriction caused by the inhibition of nitric oxide (NO) synthase (NOS). We hypothesized that, in contrast to dilation to acute CO, more prolonged exposure of newborn cerebral arterioles to elevated CO produces constriction by reducing NO. In piglets with closed cranial windows, pial arteriolar responses to isoproterenol (10−6 M), sodium nitroprusside (SNP; 10−7 and 3 × 10−7 M), and l-arginine ethyl ester (l-Arg; 10−5 and 10−4 M) were determined before and after 2 h of treatment with CO. CO (10−7 M) caused transient dilation and had no further effects. CO (2 × 10−7 and 10−6 M) initially caused vasodilation, but over the 2-h exposure, pial arterioles constricted and removal of the CO caused dilation. Exposure to elevated CO (2 h) did not alter dilation to SNP or isoproterenol. Conversely, the NOS substrate l-Arg caused dilation before CO that was progressively lost over 90 min of elevated CO. If NO was held constant, CO caused dilation that was sustained for 2 h. We conclude that in neonates, cerebral arteriole responses to CO are biphasic: dilation to acute elevation with subsequent constriction from NOS inhibition after more prolonged exposure. As a result, short episodic production of CO allows function as a dilator gasotransmitter, whereas prolonged elevation can reduce NO to elevate cerebrovascular tone. The interaction between heme oxygenase/CO and NOS/NO could form a negative feedback system in the control of cerebral vascular tone.