Taurine Regulation of Glutamate Currents Through Activation of a New Receptor

Abstract

Purpose: : To understand the role of taurine in retina, we studied regulation of taurine in glutamate currents and elucidated the underlying mechanisms for the action of taurine in retinal neurons. Although retinas are taurine-rich tissues, less is known about the function of taurine in retina. Our previous immunocytochemical studies have shown taurine to be in high concentrations in both photoreceptors and OFF-bipolar cells in larval salamander (Ambystoma tigrinum) retinas. We used salamander retina as a model tissue to study taurine’s effect in the inner retina.

Methods: : Whole-cell patch clamp recordings were made on isolated third-order neurons from larval tiger salamander retinas. In Ca²⁺ imaging experiments, cells were loaded with Fluo4-AM for 45 minutes. Images were captured using a Qimaging Rolera-MGiPlus camera and analyzed using IP Lab 4.0 software. All tested compounds were locally puffed on the neurons using the DAD-VM system (ALA Scientific Co).

Results: : Previous study demonstrates that taurine regulates glutamate currents by a biphasic manner, enhancing weak glutamate current response and suppressing strong glutamate current response in the third-order neurons. The mechanism for the regulation of glutamate currents was studied in intracellular Ca²⁺ in these neurons. Application of taurine substantially increased intracellular Ca²⁺ levels in the third-order neurons. 2µM strychnine, a glycine receptor antagonist, could block approximately 70-80% of the intracellular Ca²⁺ increase by taurine. However, in the same cell, glycine did not increase intracellular Ca²⁺, suggesting that taurine increased intracellular Ca²⁺ through activating receptors other than glycine receptors, although the effect was strychnine-sensitive. In whole-cell recording, taurine reduced glutamate currents in the third-order neurons, which was also blocked by strychnine. Glycine did not repeat taurine’s effect in regulation of glutamate currents.

Conclusions: : Taurine is possibly released from OFF-bipolar cells. The effects of taurine enhance intracellular Ca²⁺ levels and reduced glutamate currents in the third-order neurons. Taurine produced effects are not repeated by glycine, although the effect of taurine is sensitive to strychnine. These suggest that taurine might activate a new receptor that possibly increases intracellular Ca²⁺.

Keywords: taurine • retina: proximal (bipolar, amacrine, and ganglion cells) • neurotransmitters/neurotransmitter systems