Development of somatostatin immunoreactive neurons in rat retina.

Levels of somatostatin-like immunoreactivity (SLI) were measured in the rat retina by radioimmunoassay during prenatal and postnatal development. SLI-containing cells were visualized by immunocytochemistry throughout this time period. SLI is present as early as embryonic day 15 (E15) and by E16 at concentrations 10-fold (4.4 +/- 1.4 pg/micrograms protein) higher than the adult retina, and the SLI is confined to cells in the inner neuroblastic layer. As the inner plexiform layer (IPL) develops (E17-18), the level of SLI falls (2.0 pg/micrograms protein) and distinct SLI neurites are seen throughout the inner neuroblastic layer. At birth (P0), the SLI level continues to decline at a time when cells are migrating. Few immunoreactive cells are seen in the developing ganglion cell layer (GCL), presumably as a result of this migration. At postnatal day 4 (P4), the amount of SLI continues to decline, and very few immunoreactive cells are noted. At P8, distinct cells contain SLI both in the GCL and inner nuclear layer (INL). SLI then increases as the retina matures postnatally. After eye opening, the amount of SLI approaches adult levels and the immunocytochemical pattern is similar to that reported for the adult rat retina; i.e. SLI is confined to two subpopulations of cells, one in the INL and one in the GCL. The early prenatal appearance of SLI suggests that it may play a role in retinal development.