Intracellular calcium reduces light-induced excitatory post-synaptic responses in salamander retinal ganglion cells

摘要

class="enumerated" style="list-style-type:decimal">The whole-cell patch clamp technique was used to study the effect of intracellular Ca²⁺ on light-evoked EPSCs in on-off ganglion cells in salamander retinal slices. Both AMPA and NMDA receptors contributed to the light-evoked responses.In the presence of strychnine and picrotoxin, ganglion cells responded to light onset and offset with transient inward currents at -70 mV. These currents were reduced by 35 ± 3 % when the light stimulus was preceded by a depolarizing step from -70 to 0 mV.The inhibitory effect of depolarization on light-evoked EPSCs was strongly reduced in the presence of 10 mm BAPTA.The degree of EPSC inhibition by the prepulse holding potential followed the current-voltage relationship of the Ca²⁺ current found in the ganglion cell.In the presence of the NMDA receptor antagonist AP-7, glutamate-dependent current was nearly abolished when high Ca²⁺ was substituted for high Na⁺ solution.The release of Ca²⁺ from internal stores by caffeine or inositol trisphosphate reduced the EPSCs by 36 ± 5 and 38 ± 11 %, respectively, and abolished the inhibitory effect of depolarization.The inhibitory effect of depolarization on EPSCs was reduced 5-fold in the presence of AP-7, but was not reduced by the AMPA receptor antagonist CNQX.Neither inhibition of Ca²⁺-calmodulin-dependent enzymes, nor inhibition of protein kinase A or C had any significant effect on the depolarization-induced inhibition of EPSCs.Our data suggest that elevation of [Ca²⁺]i, through voltage-gated channels or by release from intracellular stores, reduced primarily the NMDA component of the light-evoked EPSCs.