Somatostatin-like immunoreactivity and glycine high-affinity uptake colocalize to an interplexiform cell of the Xenopus laevis retina

Antibodies directed against somatostain have been used to label a population of interplexiform cells (IPCs) in the Xenopus laevis retina. These cells have spherical soma which lie in the inner nuclear layer (INL), adjacent to or one cell distal to the inner plexiform layer (IPL). Processes from these cells project throughout the IPL, with a fairly dense accumulation of labeled dendrites in the upper two‐fifths of the IPL and a dense, narrow band of labeled dendrites adjacent to the ganglion cell layer. These cells also have finer processes, originating at the cell body, that traverse the INL and ramify in the outer plexiform layer (OPL). Double label experiments show that all of the cells that contain somatostatin‐like immunoreactivity (SOMLI) in the INL are also labeled by high‐affinity uptake with 3H‐glycine. Immunocytochemistry of retinal whole mounts shows that these cells are evenly distributed across the retina at a density of 542 ± 65 cells/mm2. On the basis of the colocalization experiments and the morphological homogeneity of these cells, we suggest that they represent a single cell type. Interplexiform cell processes were further characterized by electron microscopy after immunocytochemistry or 3H‐glycine autoradiography. In the IPL, IPC processes are seen to be postsynaptic at both ribbon and conventional synapses. This input is found almost entirely in the distal two‐fifths of the IPL. Interplexiform cell processes are presynaptic to unlabeled processes in both the distal and proximal IPL. In the OPL, labeled processes are found near or contiguous with photoreceptor bases, and are often presynaptic to small‐diameter processes. The postsynaptic processes have been identified as bipolar cell dendrites in six cases. Interplexiform cell processes may also contact horizontal cell processes in the OPL.