The major objective of this proposed renewal is to elucidate the function of intracerebral arterioles in the context of the neurovascular unit - arterioles/astrocytes/neurons. In the brain, increased neuronal activity is accompanied by an increase in local cerebral blood flow that serves to satisfy enhanced glucose and oxygen demand. Although this process has been exploited clinically to map brain function, the mechanisms by which increased synaptic activity is communicated to the cerebral vasculature to cause vasodilation are poorly understood. Our central hypothesis is that Ca2+ signals arising in the astrocyte as the result of neuronal stimulation propagate to the endfeet (which encase the arterioles), leading to activation of large conductance, Ca2+-sensitive potassium (BK) channels in the endfeet and potassium (K+) release into the astrocyte-arteriolar space. This localized K+ elevation is proposed to act on arteriolar inward rectifier K (Kir) channels to hyperpolarize the smooth muscle cell (SMC) membrane, lower arteriolar Ca2+ and thereby cause vasodilation. A companion hypothesis posits a central role for SMC BK channels as a target of arachidonic acid (AA) metabolites (EETs, PGE2, 20-HETE), which are known vasomodulators, recently implicated in neurovascular coupling. An additional mechanism to be explored is that astrocytic AA metabolites may act on endfoot BK channels to modulate K+ release. To elucidate these mechanisms, Aim 1 will elucidate the roles of Ca2+ signaling, BK and Kir channels, and their modulation by external K+ and AA metabolites in isolated intracerebral arterioles and single arteriolar myocytes.
Aim 2 will provide an unparalleled view of Ca2+ dynamics and BK channel modulation in astrocytic endfeet which encase the arterioles.
Aim 3 will integrate the information from Aims 1 and 2 to elucidate key elements in signaling from active neurons, via astrocytes, to arterioles in brain slices. A novel combination of high speed calcium imaging, electrophysiology, and arteriolar diameter measurements in conjunction with unique mouse models lacking key elements of arteriolar smooth muscle and astrocytic BK and Kir channels will be exploited in myocytes, isolated arterioles, and the intact neurovascular unit. The cerebral microcirculation has a major role in a number of diseases including stroke, migraine, Parkinson's, Alzheimer's and early onset dementia. Thus, elucidating the mechanisms by which neuronal and astrocyte activity regulates intracerebral arterioles is critical to the development of new targets for effective therapies of pathological conditions associated with the cerebral vasculature.
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