Somatostatin-immunoreactive cells in the adult cat retina

Peptides have been found in the retinas of all mammalian species studied to date, but little is known about their localization and function in the cat. Using two mouse monoclonal antibodies directed to somatostatin 14, we have observed two sparse groups of somatostatin-immunoreactive neurons in the cat, both distributed preferentially in the inferior retina. The more numerous cell type is characterized by a small- to medium-sized soma (mean diameter = 16.3 +/- 9.0 microns; n = 186) with sparsely branching, far-reaching varicose processes that ramify mainly in the inner plexiform layer. The majority of these cells are located in the ganglion cell layer, with the remainder in the proximal inner nuclear layer and the inner plexiform layer. They are in especially high density at the retinal margin. In morphology and soma size, these cells resemble wide-field amacrine cells. The second cell type has a large, granular-staining soma (mean diameter = 29.7 +/- 14.8 microns; n = 145) with poorly stained primary processes and is found only in the ganglion cell layer. Cells of this type are most similar in their size and morphology to alpha ganglion cells. In contrast to the location of somatostatin-immunoreactive somata, a dense meshwork of immunoreactive processes was observed at all eccentricities within the inner plexiform layer, adjacent to the inner nuclear layer and to the ganglion cell layer. Labeled processes arising from the inner plexiform layer were also occasionally detected in the outer plexiform layer and the nerve fiber layer. Additional processes of unknown origin were observed in the nerve fiber layer and the optic nerve head. The extensive distribution of immunoreactive processes suggests that somatostatin-immunoreactive somata located preferentially in the inferior half of the retina have a widespread influence on neural activity.