Expression of somatostatin receptor subtype 5 in rat retinal amacrine cells.

Somatostatin (SRIF), as a neuroactive peptide in the CNS, exerts its actions via five subtypes of specific receptors (ssts). In this work, the localization of sst(5) was studied immunocytochemically in rat retinal amacrine cells (ACs). Labeling for sst(5) was diffusely distributed throughout the full thickness of the inner plexiform layer (IPL) and formed two distinct fluorescence bands in the distal part of the IPL. Double labeling experiments showed that sst(5) was expressed in GABAergic ACs. It was further shown that labeling for sst(5) was observed in both dopaminergic and cholinergic ACs, stained by tyrosine hydroxylase (TH) and choline acetyltransferase (ChAT), respectively. The immunostaining appeared mainly on the cell membranes and somatodendritic compartments of these ACs. For the cholinergic ACs, weak sst(5)-immunoreactivity was also observed in the processes terminating in the IPL. In contrast, no sst(5)-immunoreactivity was found in glycinergic AII ACs, stained by parvalbumin (PV). Furthermore, labeling for SRIF was co-localized with sst(5) in both dopaminergic and cholinergic ACs. These results suggest that sst(5) may serve as an autoreceptor or conventional receptor in retinal ACs.