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Joseph P. Guzzone, Harris Ripps, Wen Shen; Glycine Positive Control of the Distal Retinal Signals by a NKCC-Mediated Mechanism. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4114.
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NKCC is a major Cl- uptake transporter, preferentially expressed in the distal retina, e.g., photoreceptor terminals, horizontal cells and the dendrites of On-bipolar cells. NKCC accumulates intracellular Cl- and raises the Cl- reversal potential. This results in a glycine-mediated depolarizing response. The results of the present study demonstrate the effect NKCC on modulation of the glycine feedback response at the photoreceptor-bipolar cell synapse.
Electroretinograph (ERG) recordings were performed on dark-adapted tiger salamander retinal eyecups with drug perfusion. Both green and red LEDs were used to evoke light responses from the retinas with a 5s/30s on/off rate. 10µM strychnine was used to block endogenous glycine response. 100µM bumetanide was applied to specifically block Cl- uptake by NKCC1. The effects of strychnine and bumetanide were evaluated on the b, slow PIII (sPIII) and d waves of the ERG. The dark membrane potentials and glycine reversal potentials in photoreceptors, On-bipolar cells and horizontal cells were studied in gramicidin-perforated patch whole-cell recording.
Strychnine blocked endogenous glycine inputs in the distal retina and suppressed the b-wave response by 35% compared to control. It had a lesser effect on the sPIII and d-wave, indicating that endogenous glycine input enhances the signals between photoreceptors and On-bipolar cells. Application of bumetanide to block Cl- uptake increased the b-wave. In the presence of bumetanide, strychnine hardly affected the b-wave or sPIII, suggesting that inhibiting Cl- uptake by bumetanide reduces the effect of endogenous glycine on photoreceptor to On-bipolar cell signaling. The efficacy of bumetanide in reducing Cl- reversal potentials was verified in rod-dominated On-bipolar cells and horizontal cells in the retinal slice preparation. On average, bumetanide could lower glycine-evoked reversal potentials by 5-20mV in horizontal and ON-bipolar cells, whereas in photoreceptors, it reduced the dark membrane potential.
Glycine input in the distal retina is via a group of interplexiform cells, and exerts a positive control on the electrical responses of distal neurons, i.e., a greater response from photoreceptors and On-bipolar cells due to the higher Cl- reversal potential in the dendrites of the bipolar cells and the photoreceptor terminals. Reducing the Cl- reversal potential by inhibiting NKCC uptake, therefore, causes suppression of the glycine positive effect in the distal retina.
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