Hypoosmotic and glutamate‐induced swelling of bipolar cells in the rat retina: comparison with swelling of Müller glial cells

Abstract

Regulation of cellular volume is of great importance to avoid changes in neuronal excitability resulting from a decrease in the extracellular space volume. We compared the volume regulation of retinal glial (M€ uller) and neuronal (bipolar) cells under hypoosmotic and glutamate-stimulated conditions. Freshly isolated slices of the rat retina were superfused with a hypoosmotic solution (60% osmolarity; 4 min) or with a glutamate (1 mM)-containing isoosmotic solution (15 min), and the size changes of M€ uller and bipolar cell somata were recorded. Bipolar cell somata, but not M€ uller cell somata, swelled under hypoosmotic conditions and in the presence of glutamate. The hypoosmotic swelling of bipolar cell somata might be mediated by sodium flux into the cells, because it was not observed under extracellular sodium-free conditions, and was induced by activation of metabotropic glutamate receptors and sodium-dependent glutamate transporters. The

Endothelins Inhibit Osmotic Swelling of Rat Retinal Glial and Bipolar Cells by Activation of Growth Factor Signaling

“…I t has been shown that Müller cells, but not bipolar cells, possess endogenous signaling mechanisms that prevent cellular swelling under hypoosmotic conditions [42]. In agreement with this study, somata of freshly isolated bipolar cells (Fig.…”

“…In addition, the outer processes of both cell types display different morphologies, i.e., bipolar cells have a short and thick dendritic tree which contacts the synapses in the outer plexiform layer whereas Müller cells have one relatively thin outer stem process which draws to the outer nuclear layer. In previous experiments, the cellular identity was additionally confirmed by immunohistochemical staining of retinal slices for the Müller cell marker glutamine synthetase and the bipolar cell marker protein kinase Cα [13,42].…”

“…Müller and bipolar cell somata in retinal slices were identified as recently described [13,42]. Müller cell somata are localized to the central part of the inner nuclear layer, while bipolar cell bodies are localized to the outer part of this layer.…”

“…The data may suggest that the effect of ET-1 on the swelling of bipolar cells in retinal slices is indirectly mediated via an ET-1-induced release of growth factors from Müller cells. In contrast to ET-1, ET-2 cells does not depend on Kir channel-mediated potassium currents, but on glutamate receptor activation [42], swelling of bipolar cell somata was investigated in the absence of barium chloride.…”

“…I t has been shown that Müller cells, but not bipolar cells, possess endogenous volume-regulatory mechanisms that prevent cellular swelling when the extracellular solution becomes hypoosmotic [42]. The Müller cell volume homeostasis depends on passive potassium currents through inwardly rectifying potassium (Kir) channels [28] and on activation of a glutamatergicpurinergic signal transduction cascade resulting in the opening of potassium and chloride channels in the plasma membrane [41].…”