Diversity of Functional Astroglial Properties in the Respiratory Network

Abstract

A population of neurons in the caudal medulla generates the rhythmic activity underlying breathing movements. Although this neuronal network has attracted great attention for studying neuronal aspects of synaptic transmission, functions of glial cells supporting this neuronal activity remain unclear. To investigate the role of astrocytes in the respiratory network, we applied electrophysiological and immunohistochemical techniques to characterize astrocytes in regions involved in the generation and transmission of rhythmic activity. In the ventral respiratory group and the hypoglossal nucleus (XII) of acutely isolated brainstem slices, we analyzed fluorescently labeled astrocytes obtained from TgN(GFAP-EGFP) transgenic mice with the whole-cell voltage-clamp technique. Three subpopulations of astrocytes could be discerned by their distinct membrane current profiles. A first group of astrocytes was characterized by nonrectifying, symmetrical and voltage-independent potassium currents and a robust glutamate transporter response to D-aspartate. A second group of astrocytes showed additional A-type potassium currents, whereas a third group, identified by immunolabeling for the glial progenitor marker NG2, expressed outwardly rectifying potassium currents, smaller potassium inward currents, and only minimal D-aspartateinduced transporter currents. Astrocytes of all groups showed kainate-induced inward currents.We conclude that most of the astrocytes serve as a buffer system of excess extracellular glutamate and potassium; however, a distinct cell population (NG2-positive, A-type potassium currents) may play an important role for network plasticity.

NMDA Receptors Mediate Neuron-to-Glia Signaling in Mouse Cortical Astrocytes

“…The identity of astrocytes was further confirmed by their characteristic electrophysiology. In contrast to experiments performed on hippocampal and brainstem slices, which found several distinct population of astrocytes, different in their voltage-gated conductances and expression of glutamate receptors and glutamate transporters (Matthias et al, 2003;Grass et al, 2004;Wallraff et al, 2004), we failed to observe a significant heterogeneity in electrophysiological properties of cortical astroglial cells in both isolation and in acute slices. This may represent intrinsic regional differences as well as the fact that the line GFEC shows a higher percentage of bright versus low fluorescent astroglial cells (Kirchhoff, unpublished observation).…”

“…Experiments were performed on transgenic mice expressing enhanced green fluorescent protein (EGFP) under the control of the human glial fibrillary acidic protein (GFAP) promoter [line TgN(GFAP-EGFP) GFEC-FKi]. This line has been generated similarly to the line [now designated TgN(GFAP-EGFP)GFEA-FKi] described the first time by Nolte et al (2001) and further studied by Schipke et al (2001), Malatesta et al (2003), Matthias et al (2003), Grass et al (2004), and Hirrlinger et al (2004). The most significant difference of the GFEC line is the higher percentage of so-called classic or protoplasmic astrocytes expressing high levels of EGFP in the cortex, hippocampus, and brainstem (F. Kirchhoff, unpublished observation).…”

Glia Contribute to the Purinergic Modulation of Inspiratory Rhythm-Generating Networks

“…pH was adjusted to 7.25-7.30 by 5 M KOH. Glia were targeted based on published anatomical features (Grass et al, 2004), such as being half the diameter of neurons in the region (ϳ10 m), ovoid, with small processes, and close to blood vessels. Glia were distinguished from neurons electrophysiologically under current clamp by delivering an incrementing series of rectangular current steps (8 steps, 1 s) from a holding potential of Ϫ60 mV until the cell fired action potentials or membrane potential was depolarized above Ϫ10 mV.…”

“…Since neurons and glia in this region cannot be unequivocally distinguished based on visual inspection, cells showing any detectable change in fluorescence in response to ATP application were outlined as ROIs and divided into three groups based on cross-sectional area (group 1, Յ50 m 2 ; group 2, 50 -100 m 2 ; group 3, Ն100 m 2 ). Based on published whole-cell recording data (Grass et al, 2004) and our own whole-cell recordings from nonrhythmic, 300-m-thick slices (see Fig. 5), nonexcitable glia are Յ10 m in diameter (ϳ80 m 2 ); therefore, groups 1 and 2 were considered most likely to comprise glia.…”

“…Thus, to further test whether glia can respond directly to ATP, we used whole-cell recording to target nonexcitable cells (that could not fire action potentials) with morphology similar to that described previously (Grass et al, 2004). ATP (0.1 mM, 30 s) or the P2Y 1 R agonist (0.1 mM MRS 2365, 30 s) was applied to 8 cells (Ͻ10 m diameter) in total (ATP to 7 cells and MRS 2365 to 5 cells).…”