In vivoStimulus-Induced Vasodilation Occurs without IP₃Receptor Activation and May Precede Astrocytic Calcium Increase

Abstract

Calcium-dependent release of vasoactive gliotransmitters is widely assumed to trigger vasodilation associated with rapid increases in neuronal activity. Inconsistent with this hypothesis, intact stimulus-induced vasodilation was observed in inositol 1,4,5-triphosphate (IP3) type-2 receptor (R2) knockout (KO) mice, in which the primary mechanism of astrocytic calcium increase – the release of calcium from intracellular stores following activation of an IP3-dependent pathway – is lacking. Further, our results in wild type (WT) mice indicate that in vivo onset of astrocytic calcium increase in response to sensory stimulus could be considerably delayed relative to the simultaneously measured onset of arteriolar dilation. Delayed calcium increases in WT mice were observed in both astrocytic cell bodies and perivascular endfeet. Thus, astrocytes may not play a role in the initiation of blood flow response, at least not via calcium-dependent mechanisms. Moreover, an increase in astrocytic intracellular calcium was not required for normal vasodilation in the IP3R2-KO animals.

Rapid stimulus-evoked astrocyte Ca 2+ elevations and hemodynamic responses in mouse somatosensory cortex in vivo

“…The Ca 2+ responses in the neuropil appear to have been larger and more frequent than fast astrocytic Ca 2+ responses as reported by others (34). This disparity raised the possibility that the fast Ca 2+ responses we assigned to astrocytes were in fact fluorescence from the neuropil as has recently been proposed (8). We addressed this question in three ways.…”

“…Astrocytes are ideally situated for controlling activity-dependent increases in CBF because they closely associate with synapses and contact blood vessels with their end-feet (1,6). Whether or not astrocytic Ca 2+ responses develop often or rapidly enough to account for vascular signals in vivo is still controversial (7)(8)(9)(10). Ca 2+ responses are of interest because intracellular Ca 2+ is a key messenger in astrocytic communication and because enzymes that synthesize the vasoactive substances responsible for neurovascular coupling are Ca 2+ -dependent (1,4).…”

“…Neuronal activity releases glutamate at synapses and activates metabotropic glutamate receptors on astrocytes, and this activation can be monitored by imaging cytosolic Ca 2+ changes (11). Astrocytic Ca 2+ responses are often reported to evolve on a slow (seconds) time scale, which is too slow to account for activity-dependent increases in CBF (8,10,12,13). Furthermore, uncaging of Ca 2+ in astrocytes triggers vascular responses in brain slices through specific Ca 2+ -dependent pathways with a protracted time course (14,15).…”

Enhanced Parenchymal Arteriole Tone and Astrocyte Signaling Protect Neurovascular Coupling Mediated Parenchymal Arteriole Vasodilation in the Spontaneously Hypertensive Rat

“…2,3 These studies demonstrate that cerebral vasodilation precedes elevations in astrocytic Ca 2+ when NVC is elicited. Further, hyperemic responses remain intact in the absence of inositol triphosphate receptor-mediated Ca 2+ signaling in astrocytes.…”

Long-term In Vivo Calcium Imaging of Astrocytes Reveals Distinct Cellular Compartment Responses to Sensory Stimulation

“…Previous studies have shown that inositol-1,4,5-trisphosphate (IP3)-dependent calcium release from the endoplasmic reticulum (ER) is important for astrocyte calcium signaling (Petravicz et al 2008;Nizar et al 2013;Takata et al 2013;Srinivasan et al 2015). We examined the involvement of this pathway in sensory stimulation-evoked astrocyte responses by expressing GCaMP6s in astrocytes from Ip3r2 -/-mice and wild-type (WT) littermate controls (Li et al 2005) and selecting sub-cellular ROIs in the same manner as previous experiments (manual selection for endfeet and somata ROIs and automated detection for processes).…”