The astroglial syncytium

Abstract

Small aqueous pores, the so-called gap junction channels, couple astrocytes into an extensive syncytium-like organisation. Astroglial gap junctions are mainly composed of connexin-43 proteins. They provide a pathway for intercellular diffusion of ions and small ((1000 Da) molecules, such as second messengers and metabolites. The organisation into multicellular functional units is probably a prerequisite for the participation of astroglial cells in the control of extracellular homeostasis. Waves of increased intracellular Ca2+ concentration can propagate between astrocytes. This is of particular interest, as cytosolic Ca2+ is a second messenger that affects ion channels, carriers, and enzymes and thereby mediates short-, intermediate-, and long-term effects on astroglial function. The aim of this thesis was to study the modulation of gap junction communication and intra- and intercellular Ca2+ signalling induced by various neuroactive substances. Differences in connexin-43 expression, gap junction communication, and Ca2+ signalling in various brain regions were investigated. Enriched astroglial and mixed astroglial-neuronal primary cultures were used as model systems.Stimulation of astrocytic 5-HT2A receptors increased intracellular free Ca2+ concentrations through the release of stored Ca2+ and influx across the plasma membrane. Not only the release of stored Ca2+ but also the influx was dependent on intact Ca2+ stores. The results indicate the opening of depletion-operated Ca2+ channels. The study of intercellular Ca2+ signalling involved developing methods for quantification of Ca2+ wave variables. The amplitude of the Ca2+ increase in cells and the velocity between cells participating in the Ca2+ waves initially showed fast decreases and then stabilised with lower rates of decrease. The areas of Ca2+ wave propagation were increased by glutamate and decreased by 5-HT, noradrenaline, and endothelins in hippocampal astrocytes. In some cases, these modulations varied among astrocytes derived from different brain regions. A relationship was observed between changes in gap junction permeability and changes in the extent of intercellular Ca2+ signalling. Endothelins completely blocked gap junctions and abolished intercellular Ca2+ waves. The levels of Connexin-43 mRNA and protein and the permeability of gap junctions varied among astrocytes derived from different brain regions, but they varied together, such that if levels of mRNA were high in one region, protein expression and gap junction permeability were also high in that region. The extent of Ca2+ signalling was partially related to these three variables.Several lines of data indicate that astrocytes are involved in the regulation of the interstitial fluid and thereby of neuronal physiology. Gap junction coupling of astrocytes into functional networks has been suggested to play a role in spatial buffering of K+, in nutrient supply to neurons, and in dissipation of cell volume changes. Intercellular astroglial Ca2+ waves might have a synchronising role in the regulation of astroglial syncytial functions. The results presented in this thesis indicate that endogenous compounds dynamically modulate astroglial gap junction communications. Increased knowledge about the control of gap junction-mediated astroglial cell-to-cell signalling provides new dimensions in the understanding of neuro-glial interactions in physiological and pathophysiological