Somatostatin inhibits potassium-evoked glutamate release by activation of the sst2 somatostatin receptor in the mouse retina

In the mammalian retina, somatostatin (SRIF-14) acts through distinct receptor subtypes (ss(1-5)). Among them, sst(2) has been localized to numerous retinal cells, including photoreceptors and rod bipolar cells (RBCs). The specific role Of sst(2) in the retina is largely undetermined. In this study, we characterized retinas of mice with targeted deletion Of sst(2) (sst(2) KO) and we investigated functions Of sst(2) in respect to its possible modulation of glutamate (GLU) release, as measured by HPLC. In contrast with wild-type (WT) mice, sst(2) mRNA and sst(2A) immunoreactivity were no longer detectable in the retina Of sst(2) KO mice. In retinal explants of WT mice, SRIF and its analogue octreotide that displays high selectivity for sst(2), similarly reduced the evoked release of GLU without affecting its basal level. In sst(2) KO retinas, SRIF or octreotide did not affect GLU release indicating that they act at sst(2). Unexpectedly, the compound CYN-154806, although introduced as the first potent sst(2) antagonist, reduced the evoked release of GLU with equipotency to SRIF and octreotide. Its inhibitory effect was no longer observed in sst(2) KO retinas, indicating that this substance acts at sst(2) receptors as an agonist. In conclusion, SRIF controls evoked release of GLU through sst(2) receptors and this control may represent part of a mechanism by which SRIF regulates GLU concentration in the retina.