Patch-clamp recording from Müller (glial) cell endfeet in the intact isolated retina and acutely isolated Müller cells of mouse and guinea-pig.

Müller cells span through the entire retina and terminate with the formation of endfeet at the vitreous body. These endfeet are thought to be specialized for maintaining the K+ homeostasis in the retina based on the assumption that voltage signals can passively spread from the cell body to the endfeet. We employed the patch-clamp technique to study the physiological properties of these endfeet in a retinal wholemount preparation from guinea-pig or mouse. After assessing one endfoot with the patch pipette and establishing the whole cell recording configuration, a membrane area which approximately matched the size of one endfoot and proximal process could be voltage-clamped. This morphological correlation could be established by filling the cytoplasm with the fluorescent dye Lucifer Yellow via the patch-pipette. The morphological, immunocytochemical and ultrastructural inspection of the recorded cells revealed that mouse Müller cell endfeet were connected by only a thin stalk to the proximal process. In contrast, guinea-pig endfeet were connected by thick stalks. The endfoot current in the mouse was dominated by a voltage and time-independent K+ conductance. In contrast, in some of the recordings from guinea-pig, delayed and inwardly rectifying K+ currents were observed. These voltage-gated currents were more frequently observed or were facilitated when the membrane area under voltage clamp was increased, blocking the passive K+ currents by Ba2+ in both, mouse and guinea-pig. We thus assume that the voltage-gated currents were not in the endfeet membrane, but rather in the proximal process and could thus be better activated in the guinea-pig with its thicker stalk or after increasing the membrane area under voltage clamp control. Similar results were obtained in freshly isolated Müller cells; in contrast to the cells from the wholemount the voltage-gated currents were more frequently observed. These studies demonstrate that the Müller cell endfoot of the mouse with its vascularized retina is an electrically isolated unit and that voltage signals do not spread to the proximal process. Such a property would, however, be required for the redistribution of K+ via spatial buffer currents. In contrast, guinea-pig Müller glial cells with their stout morphological connection between endfoot and proximal process are better suited to fulfil this task.