Somatostatin and somatostatin subtype 2A expression in the mammalian retina

This review discusses the expression and cellular localization of the neuro peptide somatostatin (SRIF) and one of the SRIF subtype (sst) receptors, ss t, in the mammalian retina. SRIF immunoreactivity is predominantly localize d to a sparse population of amacrine and displaced amacrine cells in the ga nglion cell layer in several mammalian retinas including the rat, rabbit, c at, and primate. These cells, characterized by multiple processes, form a s parse network in the inner plexiform layer (IPL) in all retinal regions. Ve ry few processes are also in the outer plexiform layer. In contrast to the predominant distribution of SRIF processes to the IPL, there is a widesprea d distribution of sst(2A) immunoreactivity to both the inner and outer reti na in all mammalian retinas studied to date. In rabbit retina, sst(2A) immu noreactivity is predominant in rod bipolar cells and in sparse wide-field a macrine cells. In the rat retina, sst(2A) immunoreactivity is localized to several neuronal cell types-cone photoreceptors, horizontal cells, rod and cone bipolar cells, and amacrine cells. Reverse-transcriptase-polymerase ch ain reaction analysis found that set, mRNA is expressed in the rat retina, while sst(2A) mRNA is not detected. Finally, in the primate retina sst(2) i mmunoreactivity is predominant in cone photoreceptors, with additional immu nostained cell bodies and processes in the inner retina. These findings ind icate that SRIF may modulate several neuronal cell types in the retina, and that it has a broad influence on both scotopic and photopic visual pathway s. (C) 2000 Wiley-Liss. Inc.