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

Abstract

Antibodies directed against somatostatin 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 (SOM-LI) 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.

Immunocytochemical study of glycine receptors in the retina of the frog Xenopus laevis

“…The glycine receptor immunoreactivity revealed in this layer can be regarded to for the synaptic contacts which the glycinergic interplexiform cells make here, as have been described for X. laevis and fish retinae (Smiley and Basinger 1988;Wu and Maple 1998;Connaughton et al 1999).…”

Photoreceptor classes and transmission at the photoreceptor synapse in the retina of the clawed frog,Xenopus laevis

“…The most potent agent in this regard is glycine, which selectively and completely removes the depolarizing component. In the Xenopus retina, glycine is brought to the outer retina by a glycinergic interplexiform (IPC) cell (Smiley and Basinger, 1988). The same class of IPC co-localizes glycine.…”

Glycine input induces the synaptic facilitation in salamander rod photoreceptors

“…Glycinergic synapses in the outer retinal neurons are solely conducted by a group of centrifugal neurons, glycinergic interplexiform cells. These neurons receive glutamatergic input in the IPL and release glycine to the OPL, creating a long range feedback loop in retinas [7][8][9]. The function of this feedback from the network is unclear.…”