Relative contributions of rod and cone bipolar cell inputs to AII amacrine cell light responses in the mouse retina

AII amacrine cells (AIIACs) are crucial relay stations for rod-mediated signals in the mammalian retina and they receive synaptic inputs from depolarizing and hyperpolarizing bipolar cells (DBCs and HBCs) as well as from other amacrine cells. Using whole-cell voltage-clamp technique in conjunction with pharmacological tools, we found that the light-evoked current response of AIIACs in the mouse retina is almost completely mediated by two DBC synaptic inputs: a 6,7-dinitro-quinoxaline-2,3-dione (DNQX)-resistant component mediated by cone DBCs (DBC C s) through an electrical synapse, and a DNQX-sensitive component mediated by rod DBCs (DBC R s). This scheme is supported by AIIAC current responses recorded from two knockout mice. The dynamic range of the AIIAC light response in the Bhlhb4−/− mouse (which lacks DBC R s) resembles that of the DNQX-resistant component, and that of the connexin36 (Cx36)−/− mouse resembles the DNQX-sensitive component. By comparing the light responses of the DBC C s with the DNQX-resistant AIIAC component, and light responses of the DBC R s with the DNQX-sensitive AIIAC component, we obtained the input–output relations of the DBC C →AIIAC electrical synapse and the DBC R →AIIAC chemical synapse. Similar to other glutamatergic chemical synapses in the retina, the DBC R →AIIAC synapse is non-linear. Its highest voltage gain (approximately 5) is found near the dark membrane potential, and it saturates for presynaptic signals larger than 5.5 mV. The DBC C →AIIAC electrical synapse is approximately linear (voltage gain of 0.92), consistent with the linear junctional conductance found in retinal electrical synapses. Moreover, relative DBC R and DBC C contributions to the AIIAC response at various light intensity levels are determined.