Localization of somatostatin receptors at the light and electron microscopial level by using antibodies raised against fusion proteins

Somatostatin receptor-mediated suppression of gabaergic synaptic transmission in cultured rat retinal amacrine cells

Somatostatin (SRIF) modulates neurotransmitter release by activating the specific receptors (sst1-sst5). Our previous study showed that sst5 receptors are expressed in rat retinal GABAergic amacrine cells. Here, we investigated modulation of GABA release by SRIF in cultured amacrine cells, using patch-clamp techniques. The frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) in the amacrine cells was significantly reduced by SRIF, which was partially reversed by BIM 23056, an sst5 receptor antagonist, and was further rescued by addition of CYN-154806, an sst2 receptor antagonist. Both nimodipine, an L-type Ca2+ channel blocker, and ω-conotoxin GVIA, an N-type Ca2+ channel blocker, suppressed the sIPSC frequency, and in the presence of nimodipine and ω-conotoxin GVIA, SRIF failed to further suppress the sIPSC frequency. Extracellular application of forskolin, an activator of adenylate cyclase, increased the sIPSC frequency, while the membrane permeable protein kinase A (PKA) inhibitor Rp-cAMP reduced it, and in the presence of Rp-cAMP, SRIF did not change sIPSCs. However, SRIF persisted to suppress the sIPSCs in the presence of KT5823, a protein kinase G (PKG) inhibitor. Moreover, pre-incubation with Bis IV, a protein kinase C (PKC) inhibitor, or pre-application of xestospongin C, an inositol 1,4,5-trisphosphate receptor (IP3R) inhibitor, SRIF still suppressed the sIPSC frequency. All these results suggest that SRIF suppresses GABA release from the amacrine cells by inhibiting presynaptic Ca2+ channels, in part through activating sst5/sst2 receptors, a process that is mediated by the intracellular cAMP-PKA signaling pathway.

Detection of amino-terminal extracellular domain of somatostatin receptor 2 by specific monoclonal antibodies and quantification of receptor density in medulloblastoma

Somatostatin receptor 2 (SSTR2) is expressed by most medulloblastomas (MEDs). We isolated monoclonal antibodies (MAbs) to the 12-mer (33)QTEPYYDLTSNA(44), which resides in the extracellular domain of the SSTR2 amino terminus, screened the peptide-bound MAbs by fluorescence microassay on D341 and D283 MED cells, and demonstrated homogeneous cell-surface binding, indicating that all cells expressed cell surface-detectable epitopes. Five radiolabeled MAbs were tested for immunoreactive fraction (IRF), affinity (KA) (Scatchard analysis vs. D341 MED cells), and internalization by MED cells. One IgG(3) MAb exhibited a 50-100% IRF, but low KA. Four IgG(2a) MAbs had 46-94% IRFs and modest KAs versus intact cells (0.21-1.2 x 10(8) M(-1)). Following binding of radiolabeled MAbs to D341 MED at 4 degrees C, no significant internalization was observed, which is consistent with results obtained in the absence of ligand. However, all MAbs exhibited long-term association with the cells; binding at 37 degrees C after 2 h was 65-66%, and after 24 h, 52-64%. In tests with MAbs C10 and H5, the number of cell surface receptors per cell, estimated by Scatchard and quantitative FACS analyses, was 3.9 x 10(4) for the "glial" phenotype DAOY MED cell line and 0.6-8.8 x 10(5) for four neuronal phenotype MED cell lines. Our results indicate a potential immunotherapeutic application for these MAbs.

Genetic deletion of somatostatin receptor 1 alters somatostatinergic transmission in the mouse retina

In the mammalian retina, sparse amacrine cells contain somatostatin-14 (SRIF) which acts at multiple levels of neuronal circuitry through distinct SRIF receptors (sst(1-5)). Among them, the sst1 receptor has been localised to SRIF-containing amacrine cells in the rat and rabbit retina. Little is known about sst1 receptor localisation and function in the mouse retina. We have addressed this question in the retina of mice with deletion of sst1 receptors (sst1 KO mice). In the retina of wild type (WT) mice, sst1 receptors are localised to SRIF-containing amacrine cells, whereas in the retina of sst1 KO mice, sst1 receptors are absent. sst1 receptor loss causes a significant increase in retinal levels of SRIF, whereas it does not affect SRIF messenger RNA indicating that sst1 receptors play a role in limiting retinal SRIF at the post-transcriptional level. As another consequence of sst1 receptor loss, levels of expression of sst2 receptors are significantly higher than in control retinas. Together, these findings provide the first demonstration of prominent compensatory regulation in the mouse retina as a consequence of a distinct SRIF receptor deletion. The fact that in the absence of the sst1 receptor, retinal SRIF increases in concomitance with an increase in sst2 receptors suggests that SRIF may regulate sst2 receptor expression and that this regulatory process is controlled upstream by the sst1 receptor. This finding can be important in the design of drugs affecting SRIF function, not only in the retina, but also elsewhere in the brain.

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.

The ovine somatostatin receptor subtype 1 (osst1): partial cloning and tissue distribution

The sheep is a valuable model to study GH neuroregulation since its GH secretion pattern is close to that in human. Somatostatin receptor subtype 1 (sst1) appears to be important in central regulation of GH but ovine sst1 (osst1) has not yet been cloned. We report here the cloning of the major part of sst1 in that species. Using human primers from transmembrane domain 2 and 7, we amplified from sheep tissue by RT-PCR a 700 bp fragment. By screening a cDNA sheep library with this fragment, we isolated a 1.4 kb cDNA which contained the major part of the coding cDNA of osst1. The partial predicted protein consists of 347 amino acids exhibiting a putative seven transmembrane domain topology typical of G protein-coupled receptors. Nucleotide sequence comparisons with that of other species showed that osst1 displays 88% homology with human sst1, 84% with rat sst1 and 87% with mouse sst1. Southern blot analysis of ovine cortex DNA demonstrated that osst1 is encoded by a single gene. Northern blot studies evidenced a 3.9 kb transcript highly expressed in the cortex and the hippocampus. This transcript was also present in hypothalamus, striatum, cerebellum, olfactory bulb, spinal cord, brain stem, the lung, kidney, liver, adrenal glands and at a low level in the pituitary gland. No signal was noticeable in the pineal gland. The sequence homology, the tissue distribution, the length of the transcript link this cDNA to the somatostatin receptor family and particularly to sst1.