The binding of somatostatin to the mouse retina is altered by the pearl mutation

Abstract

Pearl mutants have a night-blind phenotype and abnormal optokinetic nystagmus. Preliminary results from another study14 showed that the light responses of retinal ganglion cells of pearl mutant mice were affected by bathing the isolated retina with low (125I-[Tyr11]-somatostatin-14 and 125I-[Leu8, -Trp22, Tyr25]-somatostatin-28 to frozen, unfixed sections of eyes of wild-type (C57BL/6J +/+) and congenic pearl mutant (C57BL/6J-pin pepin/pepin) mice. As found previously for wild-type mice, specific binding occured in 3 maxima in pearl mutants: a broad band extending from the retinal ganglion cell to the inner nuclear layer, a narrow and inconstant band over the outer plexiform layer, and a band over the pigment epithelium and choroid. We quantified the label over the inner plexiform layer and found evidence for a single saturable site in both genotypes. However, several results indicate that somatostatin-14 binds more avidly to pearl mutant retinas than to wild-type retinas. In saturation binding studies, Kd for 125I-[Tyr11]-somatostatin-14 was 600 pM in pearl mutants vs 1.5 nM in wild-type; whereas, for 125I-[Leu8, -Trp22, Tyr25]-somatostatin-28, Kd was nearly equal in the two genotypes (500 and 625 pM, respectively). Bmax was nearly equal for both ligands in both genotypes (63-69 fmol/mg protein). Somatostatin-(14) was also a more potent competitor in pearl mutant retinas (against 125I-[Tyr11]-somatostatin-14, Kd was 250 pM for pearl mutants and 1.12 nM for wild-type; against 125I-[Leu8, -Trp22, Tyr25]-somatostatin-28 these values were 640 pM and 3.77 nM, respectively). However, somatostatin-28 compared nearly equally in both genotypes (against 125I-[Tyr11]-somatostatin-14, Kd was 510 pM for pearl mutants and 420 pM for wild-type; against 125I-[Leu8, -Trp22, Tyr25]-somatostatin-28, Kd was 1.28 and 1.20 nM, respectively). These results indicate that the greater affinity possessed by wild-type retinas for somatostatin-28 over somatostatin-14 is altered by the pearl mutation, elevating the affinity of the retina for somatostatin-14 is altered by the pearl mutation, elevating the affinity of the retina for somatostatin-14.