Chronic exposure to either somatostatin (SS) or octreotide, a long-lasting SS analogue, affects SS expression in the postnatal visual cortex of the rat

Altered morphology of rod bipolar cell axonal terminals in the retinas of mice carrying genetic deletion of somatostatin subtype receptor 1 or 2

Somatostatin (SRIF), similar to other neuropeptides, is likely to influence the morpho-functional characteristics of neurons. We studied possible morphological alterations of mouse retinal neurons following genetic deletion of SRIF subtype receptor 1 [sst1 knockout (KO)] or 2 (sst2 KO). In sst1 KO retinas, axonal terminals of rod bipolar cells (RBCs), identified with protein kinase C immunoreactivity, were 25% larger than in controls. In contrast, in sst2 KO retinas, RBC axonal terminals were significantly smaller (-14%). No major ultrastructural differences were observed between control and KO RBCs. In sst2 KO retinas, SRIF levels decreased by about 35%, while both sst1 receptor mRNA and protein increased by about 170% and 100%, respectively. This compares to previous results reporting an increase of both retinal SRIF and sst2 receptors following sst1 receptor deletion. Together, these findings suggest that, on the one hand, sst1 receptor deletion induces over-expression of sst2 receptors, and vice versa; on the other hand, that an imbalance in sst1 and sst2 receptor expression and/or changes in the levels of retinal SRIF induced by sst1 or sst2 receptor deletion are responsible for the morphological changes in RBC axonal terminals. Similar alterations of RBC terminals were observed in KO retinas at 2 weeks of age (eye opening). In addition, reverse transcription-polymerase chain reaction analysis of the expression of sst2 and sst1 receptors in developing sst1 and sst2 KO retinas, respectively, demonstrated that these receptors are up-regulated at or near eye opening. These findings suggest that the integrity of the somatostatinergic system during development is necessary for proper RBC maturation.

Identification of multiple somatostatin receptors in the rat somatosensory cortex during development

The present study was aimed at identifying somatostatin receptor subtypes on the basis of their ligand-binding properties in the rat somatosensory cortex during fetal and postnatal development. Characterization of somatostatin-binding sites was performed in individual cortical layers by using three radioligands and eight competitors with known selectivities for the five somatostatin receptor subtypes. Binding sites sensitive to sst2-selective ligands were detected with high densities in the intermediate zone of the fetal cortex. From embryonic day 21 to 21 days postnatal (P21), mixed populations of receptors were detected in the cortical plate and emerging layers I-VI. Putative sst2 receptors were detected throughout the entire period but displayed different affinities for somatostatin and analogs, and a different sensitivity to GTP, depending on the developmental stage and the cortical layer considered. High densities of binding sites exhibiting characteristics of the sst1, sst3/5, and sst4 receptor subtypes were observed from P4 to P7, P7 to P14, and P7 to P21, respectively. In addition, each type of site exhibited a particular distribution pattern across the cortical layers that varied during the development. In the adult cortex, binding sites with sst1 and sst2 receptor characteristics were predominant. This study provides evidences of developmental expression windows of four sst receptor subtypes in selected areas of the rat cerebral cortex.