Role of neurotransmitter receptors in mediating light-evoked responses in retinal interplexiform cells

Interplexiform (IP) cells are a long-neglected group of retinal neurons the function of which is yet to be determined. Anatomical study indicates that IP cells are located in the inner nuclear layer, juxtaposed with the third-order neurons. However, the synaptic transmission of IP cells in the inner retina is poorly understood. Using whole cell patch-clamp and pharmacological techniques, we extensively studied synaptic receptors in IP cells. The IP cells in amphibian retinal slices were identified by electrical and morphological properties with voltage-clamp recording and Lucifer yellow dialysis. We find that light-evoked excitatory postsynaptic currents (L-EPSCs) are mediated by AMPA and N-methyl-d-aspartate receptors in IP cells. Although both receptors contributed to the amplitude and kinetics of L-EPSCs, AMPA receptor desensitization substantially shaped L-EPSCs in the neurons, similar to those found in the third-order neurons. The light-evoked inhibitory postsynaptic currents (L-IPSCs) in IP cells were primarily mediated by strychnine-sensitive glycine receptors with a small component of GABA(C) receptors. GABA(C) receptor rho2 subunits were detected in IP cells with single-cell RT-PCR assays. Expression of GABA(C) receptors is one of the special features for IP cells, distinct from most of the third-order neurons that depend on GABA(A) and glycine receptors to relay the inhibitory signals. However, GABA(A) receptors in IP cells acted like nonsynaptic receptors that were activated by exogenous GABA application. Furthermore, L-IPSCs in IP cells were inhibited by the serial inhibitions between amacrine cells in the inner retina. In addition, application of neurotransmitters on the axon terminals of IP cells had no significant current generated in the cells, indicating that the synaptic inputs of IP cells are mainly from the inner retina. This study demonstrates the important role that light signals are encoded by both experiment of inhibitory receptors in IP cells.