Modulation of excitatory synaptic transmission by GABA(C) receptor-mediated feedback in the mouse inner retina

In many vertebrate CNS synapses, the neurotransmitter glutamate activates postsynaptic non-N-methyl-D-aspartate (NMDA) and NMDA receptors. Since their biophysical properties are quite different, the time course of excitatory postsynaptic currents (EPSCs) depends largely on the relative contribution of their activation. To investigate whether the activation of the two receptor subtypes is affected by the synaptic interaction in the inner plexiform layer (IPL) of the mouse retina, we analyzed the properties of the light-evoked responses of ON-cone bipolar cells and ON-transient amacrine cells in a retinal slice preparation. ON-transient amacrine cells were whole cell voltage-clamped, and the glutamatergic synaptic input from bipolar cells was isolated by a cocktail of pharmacological agents (bicuculline, strychnine, curare, and atropine). Direct puff application of NMDA revealed the presence of functional NMDA receptors. However, the light-evoked EPSC was not significantly affected by D(-)-2-amino-5-phosphonopentanoic acid (D-AP5), but suppressed by 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX) or 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI 52466). These results indicate that the light-evoked EPSC is mediated mainly by AMPA receptors under this condition. Since bipolar cells have GABA(C) receptors at their terminals, it has been suggested that bipolar cells receive feedback inhibition from amacrine cells. Application of (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA), a specific blocker of GABA(C) receptors, suppressed both the GABA-induced current and the light-evoked feedback inhibition observed in ON-cone bipolar cells and enhanced the light-evoked EPSC of ON-transient amacrine cells. In the presence of TPMPA, the light-evoked EPSC of amacrine cells was composed of AMPA and NMDA receptor-mediated components. Our results suggest that photoresponses of ON-transient amacrine cells in the mouse retina are modified by the activation of presynaptic GABA(C) receptors, which may control the extent of glutamate spillover.