Synaptic and voltage-gated currents in interplexiform cells of the tiger salamander retina

We have correlated the membrane properties and synaptic inputs of interplexiform cells (IPCs) with their morphology using whole-cell patch-clamp and Lucifer yellow staining in retinal slices. Three morphological types were identified: (a) a bistratified IPC with descending processes ramifying in both sublaminas a and b of the inner plexiform layer (IPL), and an ascending process that branched in the outer plexiform layer (OPL) and originated from the soma, (b) another bistratified IPC with descending processes ramifying in both sublaminas a and b, and an ascending process that branched in the OPL and originated directly from IPC processes in the IPL, and (c) a monostratified IPC with a descending process ramifying over large lateral extents within the most distal stratum of the IPL, and sending an ascending process to the OPL with little branching. Similar voltage-gated currents were measured in all three types including: (a) a transient inward sodium current, (b) an outward potassium current, and (c) an L-type calcium current. All cells generated multiple spikes with frequency increasing monotonically with the magnitude of injected current. The IPCs that send their descending processes into both sublaminas of the IPL (bistratified) receive excitatory synaptic inputs at both light ON and OFF that decay with a time constant of approximately 1.3 s. Slowly decaying excitation at both ON and OFF suggests that bistratified IPCs may spike continuously in the presence of a dynamic visual environment.