Light-evoked increases in extracellular K+ in the plexiform layers of amphibian retinas.

Abstract

Recordings of light-evoked changes in extracellular K+ concentration (A[K+]o) were obtained in the retinas of frog and mudpuppy . In eyecup preparations, various recording approaches were used and provided evidence for a K increase near the outer plexiform layer (distal K increase). This distal K increase could be pharmacologically dissociated from the well-known, large K increase in the proximal retina by the application of ethanol and -y-aminobutyric acid . The distal K increase also often showed surround antagonism . A retinal slice preparation was used to permit electrode placement into the desired retinal layers under direct visual control and without the risk of electrode damage to adjacent layers . In the slice, a distinct distal K increase was found in the outer plexiform layer, in addition to the prominent K increase in the inner plexiform layer. Compared with eyecups, only weak K increases were found in the nuclear layers of the slice. This suggests that the K responses observed in the nuclear layers of eyecups may be generated by K' diffusing along the electrode track from the plexiform layers. In the context of current models of ERG b-wave generation, the magnitude of the recorded distal K increase, compared with the proximal K increase, seems too small to give rise to the bwave . However, the distal K increase may be differentially depressed by electrode dead space. It is also possible that if certain aspects of the models of bwave generation were modified, then the observed distal K increase could give rise to the b-wave .

Astrocyte Regulation of Blood Flow in the Brain

“…Potassium-induced vasodilation is mediated by an increase in the conductance of inwardly rectifying K þ channels on vascular smooth muscle cells (Filosa et al 2006;Haddy et al 2006) and by activation of the Na þ -K þ ATPase on the smooth muscle cells (Bunger et al 1976 (Singer and Lux 1975;Connors et al 1979) and in the synaptic layers of the cat and frog retina (Dick et al 1985;Karwoski et al 1985 Paulson and Newman (1987) proposed that neurovascular coupling is mediated by a feedforward glial cell K þ siphoning mechanism . According to this model, the diffusion of K þ through the extracellular space from neurons to blood vessels is enhanced by a K þ current flow through astrocytes.…”

Functional Hyperemia and Mechanisms of Neurovascular Coupling in the Retinal Vasculature

“…[K þ ] o and flow vary similarly as the luminance and flicker frequency of the stimulus is changed. 27 In both mammalian 65 and amphibian 66,67 retinas, A number of years ago, it was proposed that neurovascular coupling is mediated by a feedforward glial cell K þ siphoning mechanism. 68 According to this model, 69 the diffusion of K þ from neurons to blood vessels is enhanced by a K þ current flow through Mü ller cells.…”

Aquaporin-4 Water Channel Protein in the Rat Retina and Optic Nerve: Polarized Expression in Müller Cells and Fibrous Astrocytes

“…In this study we investigated whether the expression of AQP4 in the retina and optic nerve is compatible with such a hypothesis, which would presuppose a heterogeneous cellular and subcellular expression of this aquaporin. Detailed analyses of the activity dependent ion fluxes and water redistribution have been performed in the retina and optic nerve (Steinberg et al, 1980;Connors et al, 1982;Karwoski et al, 1985Karwoski et al, , 1989Ransom et al, 1985;Coles, 1986;Coles et al, 1986;Nilius and Reichenbach, 1988;Reichenbach, 1991;Frishman et al, 1992;Reichenbach et al, 1992;Li et al, 1994a;Newman, 1996;Ransom and Orkand, 1996), allowing correlations to be made with aquaporin distribution. A more direct testing of AQP4 f unction will have to await the development of specific inhibitors.…”