Intracellular recordings from isolated rabbit retinal Müller (glial) cells

ATP-mediated increase in H+ flux from retinal Müller cells: a role for Na+/H+ exchange

Small alterations in extracellular H⁺ can profoundly alter neurotransmitter release by neurons. We examined mechanisms by which extracellular ATP induces an extracellular H⁺ flux from Müller glial cells, which surround synaptic connections throughout the vertebrate retina. Müller glia were isolated from tiger salamander retinae and H⁺ fluxes examined using self-referencing H⁺-selective microelectrodes. Experiments were performed in 1 mM HEPES with no bicarbonate present. Replacement of extracellular sodium by choline decreased H⁺ efflux induced by 10 µM ATP by 75%. ATP-induced H⁺ efflux was also reduced by Na⁺/H⁺ exchange inhibitors. Amiloride reduced H⁺ efflux initiated by 10 µM ATP by 60%, while 10 µM cariporide decreased H⁺ flux by 37%, and 25 µM zoniporide reduced H⁺ flux by 32%. ATP-induced H⁺ fluxes were not significantly altered by the K⁺/H⁺ pump blockers SCH28080 or TAK438, and replacement of all extracellular chloride with gluconate was without effect on H⁺ fluxes. Recordings of ATP-induced H⁺ efflux from cells that were simultaneously whole cell voltage clamped revealed no effect of membrane potential from -70 mV to 0 mV. Restoration of extracellular potassium after cells were bathed in 0 mM potassium produced a transient alteration in ATP-dependent H⁺ efflux. The transient response to extracellular potassium occurred only when extracellular sodium was present and was abolished by 1 mM ouabain, suggesting that alterations in sodium gradients were mediated by Na⁺/K⁺-ATPase activity. Our data indicate that the majority of H⁺ efflux elicited by extracellular ATP from isolated Müller cells is mediated by Na⁺/H⁺ exchange.NEW & NOTEWORTHY Glial cells are known to regulate neuronal activity, but the exact mechanism(s) whereby these "support" cells modulate synaptic transmission remains unclear. Small changes in extracellular levels of acidity are known to be particularly powerful regulators of neurotransmitter release. Here, we show that extracellular ATP, known to be a potent activator of glial cells, induces H⁺ efflux from retinal Müller (glial) cells and that the bulk of the H⁺ efflux is mediated by Na⁺/H⁺ exchange.

Comparative electrophysiology of retinal Müller glial cells-A survey on vertebrate species

Müller cells are the dominant macroglial cells in the retina of all vertebrates. They fulfill a variety of functions important for retinal physiology, among them spatial buffering of K⁺ ions and uptake of glutamate and other neurotransmitters. To this end, Müller cells express inwardly rectifying K⁺ channels and electrogenic glutamate transporters. Moreover, a lot of voltage- and ligand-gated ion channels, aquaporin water channels, and electrogenic transporters are expressed in Müller cells, some of them in a species-specific manner. For example, voltage-dependent Na⁺ channels are found exclusively in some but not all mammalian species. Whereas a lot of data exist from amphibians and mammals, the results from other vertebrates are sparse. It is the aim of this review to present a survey on Müller cell electrophysiology covering all classes of vertebrates. The focus is on functional studies, mainly performed using the whole-cell patch-clamp technique. However, data about the expression of membrane channels and transporters from immunohistochemistry are also included. Possible functional roles of membrane channels and transporters are discussed. Obviously, electrophysiological properties involved in the main functions of Müller cells developed early in vertebrate evolution. GLIA 2017;65:533-568.

Short-circuiting effects of K+ currents on electrical responses of type-1-like astrocytes from mouse cerebral cortex

The membrane potential and membrane input resistance of cortical astrocytes from newborn mice were recorded with and without exposure to 1 mM barium. Barium treatment drastically decreased the membrane response to 0 and 35 mM K+. It also revealed an electrogenic component of the Na+,K(+)-ATPase as evident by a biphasic depolarization as a response to ouabain, which was monophasic without barium presence. Untreated mouse astrocytes reacted with small monophasic depolarizations to GABA and glutamate exposure. Barium-treated astrocytes exhibited additional transient responses to both transmitters, similar to those responses of rat astrocytes as found in the literature. The transmitter responses were not changed by exposure to uptake blockers for both transmitter substances. Thus, this electrophysiological study confirms earlier studies with radioactive K+ fluxes in showing that astrocytes derived from mouse brain are capable of short-circuiting electrogenic components and transmitter responses. This extreme high K+ permeability resembles the one reported for endfeet of retinal Muller cells and dissociated astrocytes from optic nerve.

Changes in [K+]0 at the vitreal surface compared with those around receptors in the isolated rabbit retina

Isolated rabbit retinas were superfused from the receptor side with a plasma-saline mixture kept at 35 degrees C. The vitreal side was exposed to an atmosphere of humidified warm oxygen. In one study the second-order neuronal activity was suppressed with aspartate and glutamate; in another study transmission was not blocked. When all neurons were active, [K+]0 around receptors was 4.5 +/- 0.4 mM in the dark. During a long (60s) exposure to light stimulus, [K+]0 dropped to 73% of the dark value and reaccumulated to 80%. At the vitreal surface, [K+]0 in the dark was 4.7 +/- 0.8 mM. During the 60s light stimulus, [K+]0 increased transiently, dropped to 83% of the dark value, then increased again to 91%. A continuous decrease of [K+]0 at the vitreal surface during long light stimuli concurrent with the increase of [K+]0 around receptors would indicate that the spatial buffering capability of the Müller cells contributes to the reaccumulation of potassium. Such a decrease, however, was not detected. After the blockage of transmission, [K+]0 values did not vary significantly from those after light stimulus in unblocked preparations. In the dark, [K+]0 was 5.2 +/- 0.9 mM at the vitreal surface and 4.6 +/- 0.4 mM around the receptors.

Membrane ultrastructure preservation and membrane potentials after isolation of rabbit retinal glial (Müller) cells by papain

Enzymatically isolated retinal glial (Müller) cells have been the subject of many electrophysiological studies. Local high membrane conductivities for potassium ions have been speculated to correspond with local occurrence of orthogonal arrays of intramembranous particles (OAP) observed in freeze-fracture replicas of retinal Müller cells in situ. We studied whether such OAP are preserved after enzymatic digestion of the retinal tissue which is necessary for isolation of living cells for electrophysiology. We found that strong papain digestion leads not only to disturbances in the cell's ultrastructure as seen in ultrathin sections but evokes both a redistribution of intramembranous particles and a disappearance of OAP as seen in the freeze-fracture replica. Furthermore, such isolated cells have low membrane potentials and lose their topographical specialization in K+ conductance. If, however, the retinae were exposed to papain as short as possible to get just some isolated cells, their cytoplasmic and membranous ultrastructure was preserved very well, and high resting membrane potentials were recorded in cells with marked regional specialization of membrane conductivity. Our results show that indeed sites of high K+ conductance may correspond with the occurrence of OAP, even in isolated cells.

Efficient K+ buffering by mammalian retinal glial cells is due to cooperation of specialized ion channels

Radial glial (Müller) cells were isolated from rabbit retinae by papaine and mechanical dissociation. Regional membrane properties of these cells were studied by using the patch-clamp technique. In the course of our experiments, we found three distinct types of large K+ conducting channels. The vitread process membrane was dominated by high conductance inwardly rectifying (HCR) channels which carried, in the open state, inward currents along a conductance of about 105 pS (symmetrical solutions with 140 mM K+) but almost no outward currents. In the membrane of the soma and the proximal distal process, we found low conductance inwardly rectifying (LCR) channels which had an open state-conductance of about 60 pS and showed rather weak rectification. The endfoot membrane, on the other hand, was found to contain non-rectifying very high conductance (VHC) channels with an open state-conductance of about 360 pS (same solutions). These results suggest that mammalian Müller cells express regional membrane specializations which are optimized to carry spatial buffering currents of excess K+ ions.

Cytotopographical specialization of enzymatically isolated rabbit retinal Müller (glial) cells: K+ conductivity of the cell membrane

Müller (radial glial) cells were isolated from rabbit retinae by means of papaine and mechanical dissociation. Regional membrane properties of these cells were studied by intracellular microelectrode recordings of potential responses to local application of high K+ solutions. When different parts of the cell membrane were exposed to high K+, the amplitude of the depolarizing responses varied greatly, indicating a strong regional specialization of the membrane properties. Using morphometrical data of isolated rabbit Müller cells, and a simple circuit model, we calculated the endfoot membrane to constitute more than 80% of the total K+ conductance of the cell; the specific resistivity of the endfoot membrane was about 400 omega cm2, i.e., more than 40 times less than that of the membrane of the vitread process, which is immediately adjacent. This kind of regional membrane specialization seems to be optimized in respect to the Müller cells' ability to carry spatial buffering K+ currents.

Spatial buffering of potassium by retinal Müller (glial) cells of various morphologies calculated by a model

In a previous study we found the morphometrical data of rabbit retinal Müller (radial glial) cells to vary greatly with their localization in various parts of the retina. The long cells of the central retina have thinner vitreal processes and smaller endfeet than the short cells of the retinal periphery. This configuration should impair the spatial buffering capacity of the central Müller cells for excess K+ ions. To test this hypothesis, we developed a simple modified model for the calculation of K+ clearance by spatial buffering, diffusion through the extracellular space, and co-operation of both processes. K+ clearance processes were demonstrated to depend greatly on the retinal geometry and Müller cell morphology in different parts of the retina. The efficiency of spatial buffering exhibited an obvious optimum for Müller cells of intermediate length, and decreased very steeply in longer cells. Some conclusions are drawn with respect to retinal physiology. In particular, it is suggested that very long and slender radial glia is unable to perform sufficient K+ clearance preventing long-lasting extracellular [K+] elevations after neuronal activity. Such [K+] elevations could depolarize these glial cells so as to enforce their mitotic division. This mechanism might lead to the perinatal transformation of embryonic radial glia into adult multipolar glia when neuronal activity commences in CNS tissues thicker than the maximal effective length of radial glial cells.

High Na+ affinity of the Na+,K+ pump in isolated rabbit retinal Müller (glial) cells

Rabbit retinal Müller (glial) cells were isolated by means of papain and mechanical dissociation. In a special perfusion chamber, the cells were penetrated with a recording microelectrode. Membrane potential changes were recorded in response to extracellular application of both high-K+ solutions and of ouabain, and that during perfusion with normal and Na+-free solutions, respectively. In other Müller cell preparations, Na+,K+-adenosine triphosphatase (ATPase) activity was measured using a radiochemical method, and its Na+ dependence was determined. All results strongly suggest that the Müller cell's Na+,K+ pump can be activated in the presence of extremely low amounts of Na+. This provides additional evidence for significant differences between the glial and the neuronal enzyme.