Abstract
Abstract: :
Purpose: Somatostatin (SRIF) is a potent inhibitory peptide that is localized to sparsely occurring wide field amacrine cells in the inner nuclear layer (INL) and displaced amacrine cells in the ganglion cell layer (GCL). These cells have overlapping processes that mainly ramify in laminae 1 and 5 of the IPL. The cellular actions of SRIF in mammals are mediated through a family of five G–protein–coupled receptors, sst1–5. We characterized the expression and cellular localization of the sst1 receptor in mouse retina to better understand the functional role of SRIF in the retina. Methods: Wild type and a thy1–CFP mouse line (#23) having CFP–containing ganglion cells were used to evaluate the cellular localization of sst1 receptor immunoreactivity. Mouse retinas were fixed with 4% paraformaldehyde, and retinal sections or whole mounts were processed by immunohistochemistry using an affinity–purified antibody to the C–terminus of the sst1 receptor. Antibody specificity was evaluated by immunostaining with the sst1 receptor antibody preadsorbed with 10–6M C–terminus sst1 receptor peptide. A second group of double label immunohistochemical studies used antibodies to VGluT–1 and VGluT–3 to evaluate bipolar and amacrine cell contacts, respectively on sst1 receptor immunoreactive cells. Results: Specific sst1 receptor immunoreactivity was localized to small cell bodies in the GCL that usually have one or two primary dendrites that ramify and form a bistratified plexus in lamina 3 of the inner plexiform layer (IPL). In thy1–CFP mouse retinas, sst1 receptor immunoreactivity was only localized to CFP–containing ganglion cells. Most sst1 receptor immunoreactive cells measured between 11 and 13µ in diameter. Immunoreactive cell bodies are characterized by a thin rim of immunoreactivity along the plasma membrane and well stained secondary dendrites with numerous varicosities in lamina 3 of the IPL. Axons were not immunoreactive. Bipolar cell axonal terminals and amacrine cell processes expressing VGluT–1 and VGluT–3, respectively were in close apposition to sst1 receptor immunoreactive dendrites in lamina 3 of the IPL. Conclusions: The sst1 receptor is expressed by a moderate population of small bistratified ganglion cells, which are likely to have ON/OFF response properties on the basis of the distribution of their dendrites in lamina 3 of the IPL. These findings indicate a widespread action for SRIF in the retina, and a direct action by SRIF on a distinct population of ganglion cells.
Keywords: retina: proximal (bipolar, amacrine, and ganglion cells) • neuropeptides • neurotransmitters/neurotransmitter systems