Purchase this article with an account.
D. Wu, D.R. Copenhagen; Glycine–Receptor–Mediated Regulation of Calcium in Bipolar Cell Synaptic Terminals . Invest. Ophthalmol. Vis. Sci. 2004;45(13):1331.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
Purpose: Glycinergic amacrine cells synapse onto bipolar cell axon terminals, a pathway believed to control release of glutamate from these cells. Furthermore, glycine receptors desensitize during prolonged agonist application (Pan et al. 1995). Cui et al. (2003) demonstrated that glycine can regulate calcium levels in rat rod bipolar cell synaptic terminals. We sought to understand the kinetics and underlying mechanisms of glycine suppression of [Ca2+]i. Methods: Intracellular calcium was monitored in enzymatically dissociated rat rod bipolar cells. Cells were loaded with the ratiometric calcium indicator Fura–2 AM. The effects of glycine were determined by assessing the suppression of potassium–evoked calcium increases. Results: High K+ evoked a sustained, reversible increase in [Ca2+]i mainly at the axon terminals of bipolar cells. This increase could be blocked by 5–10 µM nimodipine, indicating that calcium increases resulted from calcium influx through L–type calcium channels in the synaptic terminals. Glycine suppressed the high K+–induced calcium response in a dose–dependent manner; IC50 was close to 50 µM comparable to the affinity of glycine receptors. The inhibitory effects could be completely reversed by 2–10 µM strychnine, but not 200–500 µM picrotoxin, indicating a strychnine–sensitive and picrotoxin–insensitive glycine receptor. In low [Cl–]o solution, glycine did not exert any inhibitory effect on high K+–evoked calcium response, which suggested that high K+–induced depolarization was antagonized by Cl– influx via glycine receptors. The suppressive action of glycine on K+–evoked calcium response decreased during prolonged application, consistent with desensitization of glycine receptors. Conclusions:These results imply a functional role for glycinergic input to rat retinal bipolar cells in regulating synaptic calcium concentration and neurotransmission. It is the first report to show that glycine could dynamically regulate synaptic calcium responses via desensitizing glycine receptors.
This PDF is available to Subscribers Only