The hemo-neural hypothesis : effects of vasodilation on astrocytes in mammalian neocortex

Abstract

Astrocytes play an important role in regulating neuronal activity and local brain states, in part by serving as intermediaries between neurons and vasculature. We postulate that neurons and astrocytes are sensitive to biophysical conditions in their local environment, in addition to their participation in traditional signaling networks with other neurons. Mechanically sensitive astrocytic endfeet ensheathe cerebral blood vessels, which change size in order to regulate blood flow. We found that changes in local biophysical state caused by mechanical perturbations exerted through blood vessels can depolarize astrocytes and some neurons in slice. To test the hemoneural hypothesis in vivo, we developed a means of inducing dilation using the SUR2B receptor agonist pinacidil, which is specific to vascular smooth muscle. It was important to ascertain that pinacidil had no direct effect on astrocytes or neurons, and we confirmed this in whole cell recordings in cortical slices. We then used two-photon imaging to visualize astrocytic calcium dynamics in vivo while manipulating vasodilation in vivo. Pinacidil caused a 10-20% dilation in most vessels, a degree of dilation of similar magnitude to those naturally evoked by persistent sensory stimulation (e.g. in fMRI studies). We found that increases in pial arteriole diameter could occasionally evoke traveling calcium waves in astrocytes. We also saw consistently slow increases (which took tens of seconds to onset, and persisted for minutes) in astrocytic calcium levels at both endfeet and soma in cortical layer 1, corresponding to vessel dilation. When vessels partially reconstricted due to pinacidil washout, calcium levels also showed a relative decrease. At short time scales (from 0.5 - 5 seconds) we saw strong correlations (>0.5) between small fluctuations in astrocytic calcium levels (1-3%) and vessel diameter (1-3%). Fluctuations in vessel diameter predicted similar fluctuations in astrocytic calcium, as often and as strongly as the reverse, suggesting feedback regulation between vascular diameter and astrocytic calcium activation levels.