Ca～(2+) Dynamics And Synaptic Plasticity In Carp Retinal Horizontal Cells

It was found in our laboratory that the efficacy of synaptic transmissionbetween retinal cone systems and luminosity-type horizontal cell (LHC) wasactivity-dependent: repetitive red flashes gradually increased the LHC'shyperpolarizing response to red light. To investigate the possible cellular or synapticmechanism(s) in these phenomena, intracellular recording, calcium imaging,mathematical modeling and patch clamping methods were applied. Loweringintracellular Ca²⁺ in the LHC by intracellular injection of Ca²⁺ chelator EGTAprevented the development of red-flickering-induced response enhancement. Theresponse enhancement could also be eliminated when a potent antagonist ofCa²⁺-permeable AMPA receptor (Synthetic joro spider toxin, JSTX-3) was applied.Besides, the administration of caffeine or ryanodine also attenuated the phenomenon.Furthermore, fura-2 fluorescent calcium imaging was applied to measure [Ca²⁺]_i infreshly dissociated horizontal cells, and a model containing endoplasmic reticulum(ER) membrane processes and plasma membrane processes was constructed forquantitative analyses of the AMPA-triggered calcium dynamics. The results suggestthat postsynaptic CICR triggered by calcium influx though Ca²⁺-permeable AMPAreceptor might be related to the red-flickering-induced response enhancement. Onthe other hand, we found that repetitive green flashes caused little changes in thecell's response to green light. Our hypothesis is that the reason why repetitive greenflashes do not induced response enhancement to green light might be the lacking ofCa²⁺-permeable AMPA receptor in the postsynaptic membrane opposite green cones.Therefore, fura-2 fluorescent calcium imaging and patch clamping were used foranalyzing the subtype of AMPA receptors in freshly dissociated horizontal cells. Thenifedipine-resistant [Ca2+]i increase was completely eliminated by JSTX-3. Theapplication of pentobarbital which blocked Ca2+-impermeable AMPA receptorscould also partly inhibit the AMPA-induced increase of [Ca2+]i andglutamate-induced current. These results suggest that Ca2+-permeable andCa2+-impermeable AMPA receptors are co-expressed on horizontal cells.Intracellular recording results also suggested that the subtype of AMPA receptor maybe expressed differently between red-cone pathway and green-cone pathway. Thus,the calcium permeability of AMPA receptor may determine the ability of synapticplasticity of horizontal cells.