Synaptic contribution of Ca2+-permeable and Ca2+-impermeable AMPA receptors on isolated carp retinal horizontal cells and their modulation by Zn2+

Ca2+-permeable and Ca2+-impermeable AMPA receptors are co-expressed on carp retinal horizontal cells. In the present study, we examined the synaptic contribution and Zn2+ modulatory effect of these two AMPA receptor subtypes using whole-cell patch clamp technique. Specific Ca2+-permeable AMPA receptor antagonist (1-naphthyl acetyl spermine, NAS) and selective Ca2+-impermeable AMPA receptor blocker (pentobarbital, PB) were used to separate the glutamate-response in isolated H1 horizontal cell mediated by these two subtypes of AMPA receptors respectively. Application of 100 μM NAS substantially suppressed the current elicited by 3 mM glutamate and the remaining NAS-insensitive component was completely blocked by application of 100 μM PB. In addition, Zn2+ had dual effects on Ca2+-permeable AMPA receptor-mediated current: at low concentration (10 μM), Zn2+ potentiated the current, but at higher concentrations (100 and 1000 μM), Zn2+ reduced the current in a dose-dependent manner. However, Zn2+ (10, 100 and 1000 μM) failed to modulate the NAS-insensitive current mediated by Ca2+-impermeable AMPA receptors. Overall, our results suggest that Ca2+-permeable AMPA receptors contribute more to the cell's glutamate-response than Ca2+-impermeable AMPA receptors. Furthermore, Zn2+ has dual effects on the Ca2+-permeable AMPA receptor activity without affecting Ca2+-impermeable AMPA receptors.