Barbara Lykke Lind

Rapid stimulus-evoked astrocyte Ca2+ elevations and hemodynamic responses in mouse somatosensory cortex in vivo

Significance The morphology of astrocytes places them as likely contributors to communication between nerve cells and blood vessels. They are reported to respond with few and slow Ca2+ elevations, which exclude them as possible participants in initiation of blood flow responses or synapse communication. We establish that astrocytes have fast responses in addition to the slow. These rapid, brief Ca2+ responses were present in a large proportion of astrocytes. We were able to observe these changes due to a signal enhancement analysis, which is useful when responses are small compared with baseline activity. Our findings indicate a higher sensitivity than generally believed of astrocytes. Increased neuron and astrocyte activity triggers increased brain blood flow, but controversy exists over whether stimulation-induced changes in astrocyte activity are rapid and widespread enough to contribute to brain blood flow control. Here, we provide evidence for stimulus-evoked Ca2+ elevations with rapid onset and short duration in a large proportion of cortical astrocytes in the adult mouse somatosensory cortex. Our improved detection of the fast Ca2+ signals is due to a signal-enhancing analysis of the Ca2+ activity. The rapid stimulation-evoked Ca2+ increases identified in astrocyte somas, processes, and end-feet preceded local vasodilatation. Fast Ca2+ responses in both neurons and astrocytes correlated with synaptic activity, but only the astrocytic responses correlated with the hemodynamic shifts. These data establish that a large proportion of cortical astrocytes have brief Ca2+ responses with a rapid onset in vivo, fast enough to initiate hemodynamic responses or influence synaptic activity.

GABAA Receptor-Mediated Bidirectional Control of Synaptic Activity, Intracellular Ca2+, Cerebral Blood Flow, and Oxygen Consumption in Mouse Somatosensory Cortex In Vivo

Neural activity regulates local increases in cerebral blood flow (ΔCBF) and the cortical metabolic rate of oxygen (ΔCMRO2) that constitutes the basis of BOLD functional neuroimaging signals. Glutamate signaling plays a key role in brain vascular and metabolic control; however, the modulatory effect of GABA is incompletely understood. Here we performed in vivo studies in mice to investigate how THIP (which tonically activates extrasynaptic GABAARs) and Zolpidem (a positive allosteric modulator of synaptic GABAARs) impact stimulation-induced ΔCBF, ΔCMRO2, local field potentials (LFPs), and fluorescent cytosolic Ca(2+) transients in neurons and astrocytes. Low concentrations of THIP increased ΔCBF and ΔCMRO2 at low stimulation frequencies. These responses were coupled to increased synaptic activity as indicated by LFP responses, and to Ca(2+) activities in neurons and astrocytes. Intermediate and high concentrations of THIP suppressed ΔCBF and ΔCMRO2 at high stimulation frequencies. Zolpidem had similar but less-pronounced effects, with similar dependence on drug concentration and stimulation frequency. Our present findings suggest that slight increases in both synaptic and extrasynaptic GABAAR activity might selectively gate and amplify transient low-frequency somatosensory inputs, filter out high-frequency inputs, and enhance vascular and metabolic responses that are likely to be reflected in BOLD functional neuroimaging signals.

Interneuron Deficit Associates Attenuated Network Synchronization to Mismatch of Energy Supply and Demand in Aging Mouse Brains

Abstract Higher cognitive functions depend critically on synchronized network activity in the gamma range (30‐100 Hz), which results from activity of fast‐spiking parvalbumin‐positive (PV) interneurons. Here, we examined synaptic activity in the gamma band in relation to PV interneuron activity, stimulation‐induced calcium activity in neurons and astrocytes, and cerebral blood flow and oxygen responses in the somatosensory cortex of young adult and old adult mice in vivo using electrical whisker pad stimulation. Gamma activity was reduced in old adult mice, and associated with reduced calcium activity of PV interneurons, whereas the overall responses of neurons and astrocytes were unchanged. Hemodynamic responses were highly correlated to the power of synaptic activity in both young adult and old adult mice, but the hemodynamic response amplitude attained was lower in old adult mice. In comparison, the work‐dependent rise in O2 use, that is, the rise in the cerebral metabolic rate of oxygen (CMRO2) evoked by excitatory postsynaptic currents almost doubled in old adult mice. We conclude that PV interneuron function and gamma activity are particularly affected in old adult mice. Alterations in neurovascular coupling and CMRO2 responses may contribute to increased frailty and risk of cognitive decline in aged brains.

A critical role for astrocytes in hypercapnic vasodilation in brain

Cerebral blood flow (CBF) is controlled by arterial blood pressure, arterial CO2, arterial O2, and brain activity and is largely constant in the awake state. Although small changes in arterial CO2 are particularly potent to change CBF (1 mmHg variation in arterial CO2 changes CBF by 3%–4%), the coupling mechanism is incompletely understood. We tested the hypothesis that astrocytic prostaglandin E2 (PgE2) plays a key role for cerebrovascular CO2 reactivity, and that preserved synthesis of glutathione is essential for the full development of this response. We combined two-photon imaging microscopy in brain slices with in vivo work in rats and C57BL/6J mice to examine the hemodynamic responses to CO2 and somatosensory stimulation before and after inhibition of astrocytic glutathione and PgE2 synthesis. We demonstrate that hypercapnia (increased CO2) evokes an increase in astrocyte [Ca2+]i and stimulates COX-1 activity. The enzyme downstream of COX-1 that synthesizes PgE2 (microsomal prostaglandin E synthase-1) depends critically for its vasodilator activity on the level of glutathione in the brain. We show that, when glutathione levels are reduced, astrocyte calcium-evoked release of PgE2 is decreased and vasodilation triggered by increased astrocyte [Ca2+]i in vitro and by hypercapnia in vivo is inhibited. Astrocyte synthetic pathways, dependent on glutathione, are involved in cerebrovascular reactivity to CO2. Reductions in glutathione levels in aging, stroke, or schizophrenia could lead to dysfunctional regulation of CBF and subsequent neuronal damage. SIGNIFICANCE STATEMENT Neuronal activity leads to the generation of CO2, which has previously been shown to evoke cerebral blood flow (CBF) increases via the release of the vasodilator PgE2. We demonstrate that hypercapnia (increased CO2) evokes increases in astrocyte calcium signaling, which in turn stimulates COX-1 activity and generates downstream PgE2 production. We demonstrate that astrocyte calcium-evoked production of the vasodilator PgE2 is critically dependent on brain levels of the antioxidant glutathione. These data suggest a novel role for astrocytes in the regulation of CO2-evoked CBF responses. Furthermore, these results suggest that depleted glutathione levels, which occur in aging and stroke, will give rise to dysfunctional CBF regulation and may result in subsequent neuronal damage.

Activity‐dependent calcium, oxygen, and vascular responses in a mouse model of familial hemiplegic migraine type 1

Familial hemiplegic migraine type 1 (FHM1) is a subtype of migraine with aura caused by a gain‐of‐function mutation in the pore‐forming α1 subunit of CaV2.1 (P/Q‐type) calcium channels. However, the mechanisms underlying how the disease is brought about and the prolonged aura remain incompletely understood.

Fast Ca2+ responses in astrocyte end‐feet and neurovascular coupling in mice

Cerebral blood flow (CBF) is regulated by the activity of neurons and astrocytes. Understanding how these cells control activity‐dependent increases in CBF is crucial to interpreting functional neuroimaging signals. The relative importance of neurons and astrocytes is debated, as are the functional implications of fast Ca2+ changes in astrocytes versus neurons. Here, we used two‐photon microscopy to assess Ca2+ changes in neuropil, astrocyte processes, and astrocyte end‐feet in response to whisker pad stimulation in mice. We also developed a pixel‐based analysis to improve the detection of rapid Ca2+ signals in the subcellular compartments of astrocytes. Fast Ca2+ responses were observed using both chemical and genetically encoded Ca2+ indicators in astrocyte end‐feet prior to dilation of arterioles and capillaries. A low dose of the NMDA receptor antagonist (5R,10s)‐(+)‐5‐methyl‐10,11‐dihydro‐5H‐dibenzo[a,d]cyclohepten‐5,10‐imine‐hydrogen‐maleate (MK801) attenuated fast Ca2+ responses in the neuropil and astrocyte processes, but not in astrocyte end‐feet, and the evoked CBF response was preserved. In addition, a low dose of 4,5,6,7‐tetrahydroisoxazolo[5,4‐c]pyridin‐3‐ol (THIP), an agonist for the extrasynaptic GABAA receptor (GABAAR), increased CBF responses and the fast Ca2+ response in astrocyte end‐feet but did not affect Ca2+ responses in astrocyte processes and neuropil. These results suggest that fast Ca2+ increases in the neuropil and astrocyte processes are not necessary for an evoked CBF response. In contrast, as local fast Ca2+ responses in astrocyte end‐feet are unaffected by MK801 but increase via GABAAR‐dependent mechanisms that also increased CBF responses, we hypothesize that the fast Ca2+ increases in end‐feet adjust CBF during synaptic activity.

Spinal dorsal horn astrocytes release GABA in response to synaptic activation

GABA is an essential molecule for sensory information processing. It is usually assumed to be released by neurons. Here we show that in the dorsal horn of the spinal cord, astrocytes respond to glutamate by releasing GABA. Our findings suggest a novel role for astrocytes in somatosensory information processing.

Multiscale vision model for event detection and reconstruction in two-photon imaging data

Reliable detection of calcium waves in multiphoton imaging data is challenging because of the low signal-to-noise ratio and because of the unpredictability of the time and location of these spontaneous events. This paper describes our approach to calcium wave detection and reconstruction based on a modified multiscale vision model, an object detection framework based on the thresholding of wavelet coefficients and hierarchical trees of significant coefficients followed by nonlinear iterative partial object reconstruction, for the analysis of two-photon calcium imaging data. The framework is discussed in the context of detection and reconstruction of intercellular glial calcium waves. We extend the framework by a different decomposition algorithm and iterative reconstruction of the detected objects. Comparison with several popular state-of-the-art image denoising methods shows that performance of the multiscale vision model is similar in the denoising, but provides a better segmenation of the image into meaningful objects, whereas other methods need to be combined with dedicated thresholding and segmentation utilities.