Synaptic currents generating the inhibitory surround of ganglion cells in the mammalian retina

The receptive field (RF) of retinal ganglion cells (RGCs) consists of an ex citatory central region, the RF center, and an inhibitory peripheral region , the RF surround. It is still unknown in detail which inhibitory interneur ons (horizontal or amacrine cells) and which inhibitory circuits (presynapt ic or postsynaptic) generate the RF surround. To study surround inhibition, light-evoked whole-cell currents were recorde d from RGCs of the isolated, intact rabbit retina. The RFs were stimulated with light or dark spots of increasing diameters and with annular light sti muli. Direct inhibitory currents could be isolated by voltage clamping ganglion c ells close to the Na+/K+ reversal potential. They mostly represent an input from GABAergic amacrine cells that contribute to the inhibitory surround o f ganglion cells. This direct inhibitory input and its physiological functi on were also investigated by recording light-evoked action potentials of RG Cs in the current-clamp mode and by changing the intracellular Cl- concentr ation. The excitatory input of the ganglion cells could be isolated by voltage cla mping ganglion cells at the Cl- reversal potential. Large light spots and a nnular light stimuli caused a strong attenuation of the excitatory input. B oth GABA(A) receptors and GABA(C) receptors contributed to this inhibition, and picrotoxinin was able to completely block it. Together, these results show that the RF surround of retinal ganglion cells is mediated by a combination of direct inhibitory synapses and presynaptic surround inhibition.