Intrinsic optical signals in the rat optic nerve: Role for K⁺ uptake via NKCC1 and swelling of astrocytes

Abstract

Measurements of extracellular space volume and imaging of intrinsic optical signals (IOSs) have shown that neuronal activity increases light transmittance by causing cellular swelling. However, the cellular mechanisms underlying these volume changes and the contribution of astrocyte swelling to the changes in tissue volume are unclear. In this study, we have investigated IOSs in optic nerves to analyze the mechanisms contributing to these signals in a system consisting of only axons and glial cells. We examined both intact optic nerves and enucleated optic nerves, which contained no axons and consisted primarily of astrocytes. Electrical stimulation of intact optic nerves evoked an increase in light transmittance, which was graded with increasing stimulation frequency and was mimicked by raising extracellular K(+) concentration ([K(+)](o)). The stimulation-induced IOS grew in amplitude and had a time course similar to extracellular space shrinkage. Tetrodotoxin (TTX) blocked the electrically induced but not the high K(+)-induced IOS. In enucleated nerves, light transmittance progressively increased in higher [K(+)](o). The high [K(+)](o)-induced IOSs were reversibly depressed by furosemide and bumetanide, antagonists for Na-K-2Cl cotransport, but were unaltered by TTX. We also used a monoclonal antibody to the NKCC1 form of the Na-K-2Cl cotransporter to show that NKCC1 is expressed in optic nerves as shown in Western blotting and is colocalized in GFAP immunopositive astrocytes. In summary, these results indicated that KCl uptake into astrocytes via an Na-K-2Cl cotransporter during raised [K(+)](o) contributes to the generation of cellular swelling and the intrinsic optical signals.

Glutamate-Evoked Alterations of Glial and Neuronal Cell Morphology in the Guinea Pig Retina

“…One of the possible mediators is neuronally released K ϩ ; however, in contrast to previously described swelling-inducing effects of high extracellular K ϩ on glial cells (Walz and Hinks, 1985;Syková et al, 1999;MacVicar et al, 2002), we observed a fast and transient decrease of the thickness of glial cell processes (Fig. 7 A, B).…”

“…It occurred after longer incubation time periods (at least 1 hr) and is thought to be mediated by the Na ϩ transport that drives the uptake of glutamate (Izumi et al, 1996). Likewise, elevated extracellular K ϩ has been shown to cause astrocytic cell swelling (Syková and Orkand, 1980;Walz and Hinks, 1985;Anderova et al, 2001;MacVicar et al, 2002). Rapid glutamate-or K ϩ -evoked morphological alterations of retinal glial cells (i.e., thinning and elongation of stem processes) have never been described.…”

“…Cell volume alterations are important, particularly under pathological conditions such as epilepsy and ischemia in which overexcited neurons release large amounts of glutamate and K ϩ (Adachi et al, 1972;Glass and Dragunow, 1995;Nicholson and Syková, 1998). Increased extracellular glutamate may evoke slow swelling of retinal glial cells resulting from the influx of Na ϩ , which drives the glial glutamate transporters (Izumi et al, 1996), and high K ϩ may cause swelling of astrocytes mediated by the Na-K-2Cl cotransport (Walz and Hinks, 1985;MacVicar et al, 2002). Spreading depression, a profound transient and progressing depolarization of neurons and glia (Somjen, 2001), is accompanied by cell swelling and can be blocked by antagonists of NMDA receptors, whereas blockers of AMPA-kainate receptors are essentially ineffective (Anderson and Andrew, 2002).…”

Evaluation of anxiolytic effect of furosemide in Albino rats

“…3 Specifically, furosemide antagonize the Na + -K + -2Cl -(NKCC1) cotransporter that is present on both neurons and glial cells, and the neuron-specific K + -Cl -(KCC2) cotransporter. 10,11 Furosemide has significantly greater affinity for NKCC1 over KCC2. 10 Hyperpolarizing inhibitory postsynaptic potentials in neurons are generated by the influx of anions (HCO3 -and Cl -) down their electrochemical gradients.…”

“…The probable mechanism of anxiolytic properties of Furosemide could be by enhancing GABA inhibition, causing antagonism of Na + -K + -2CL -and Na + -CL -cotransporters. [5][6][7][8][9][10] Further studies are required to explore the exact mechanism of action of anxiolytic effect of furosemide.…”