Somatostatin receptors (sst2) regulate cGMP production in rat retina

Relevance of Peptide Homeostasis in Metabolic Retinal Degenerative Disorders: Curative Potential in Genetically Modified Mice

The mammalian retina contains approximately 30 neuropeptides that are synthetized by different neuronal cell populations, glia, and the pigmented epithelium. The presence of these neuropeptides leaves a mark on normal retinal molecular processes and physiology, and they are also crucial in fighting various pathologies (e.g., diabetic retinopathy, ischemia, age-related pathologies, glaucoma) because of their protective abilities. Retinal pathologies of different origin (metabolic, genetic) are extensively investigated by genetically manipulated in vivo mouse models that help us gain a better understanding of the molecular background of these pathomechanisms. These models offer opportunities to manipulate gene expression in different cell types to help reveal their roles in the preservation of retinal health or identify malfunction during diseases. In order to assess the current status of transgenic technologies available, we have conducted a literature survey focused on retinal disorders of metabolic origin, zooming in on the role of retinal neuropeptides in diabetic retinopathy and ischemia. First, we identified those neuropeptides that are most relevant to retinal pathologies in humans and the two clinically most relevant models, mice and rats. Then we continued our analysis with metabolic disorders, examining neuropeptide-related pathways leading to systemic or cellular damage and rescue. Last but not least, we reviewed the available literature on genetically modified mouse strains to understand how the manipulation of a single element of any given pathway (e.g., signal molecules, receptors, intracellular signaling pathways) could lead either to the worsening of disease conditions or, more frequently, to substantial improvements in retinal health. Most attention was given to studies which reported successful intervention against specific disorders. For these experiments, a detailed evaluation will be given and the possible role of converging intracellular pathways will be discussed. Using these converging intracellular pathways, curative effects of peptides could potentially be utilized in fighting metabolic retinal disorders.

Activation of somatostatin receptor 5 suppresses T-type Ca2+ channels through NO/cGMP/PKG signaling pathway in rat retinal ganglion cells

Somatostatin has been shown to modulate a variety of neuronal functions by activating the five specific G-protein coupled receptors (sst₁-sst₅). Here, effects of sst₅ receptor activation on T-type Ca²⁺ channels in acutely isolated retinal ganglion cells (RGCs) of rats were investigated using whole-cell patch-clamp techniques. The sst₅ receptor specific agonist L-817,818 significantly and reversibly suppressed T-type Ca²⁺ currents, and shifted inactivation curve of the channels toward hyperpolarization direction. The effect of L-817,818 was in a dose-dependent manner, with an IC₅₀ being 8.8 μM. Pertussis toxin-sensitive G_i/o protein mediated intracellular nitric oxide (NO)/cGMP/protein kinase G (PKG) signaling cascade was involved in the L-817,818 effect on Ca²⁺ currents because pharmacological interference of each of these signaling molecules abolished the L-817,818 effect. In contrast, neither phospholipase C/protein kinase C nor cAMP/protein kinase A signal pathways seemed likely to be involved because the L-817,818 effect persisted when these signaling pathways were blocked by U73122, bisindolylmaleimide IV, chelerythrine chloride, and Rp-cAMP, respectively. These results suggest that activation of sst₅ receptors suppresses T-type Ca²⁺ currents in rat RGCs through intracellular NO/cGMP/PKG signaling pathway, which may provide a potential mechanism for protecting RGCs against injury.

Release kinetics of somatostatin from self-assembled nanostructured hydrogels

Somatostatin-14 is a native neuropeptide with widespread functions in the body. Self-assembly of somatostatin-14 into amyloid-like nanofibrils has been previously demonstrated in aqueous media. We here hypothesize that the somatostatin nanofibrils can form a stable depot that release monomers in a controlled manner. This study aims to investigate if somatostatin monomers are released from physical hydrogels formed in water and in the presence of electrolytes. The release kinetics of the somatostatin monomers is investigated for the first time. This is correlated with the rheological properties of the hydrogels formed. We demonstrate that at the concentrations tested, there is release of somatostatin monomers from the hydrogels following a novel hybrid model of zero-order and first-order release. In the presence of electrolytes, somatostatin hydrogels demonstrated higher elastic moduli (G') which correlates to the narrower and higher density of nanofibrils observed with TEM. The presence of electrolytes resulted in a slower release of the somatostatin monomers and in a lower cumulative percentage released over 48 hrs. It is questionable that the concentrations released will be therapeutically effective. However, self-assembled somatostatin hydrogels have the potential to act as a depot for ocular drug delivery.

Nitric oxide modulates the temporal properties of the glutamate response in type 4 OFF bipolar cells

Nitric oxide (NO) is involved in retinal signal processing, but its cellular actions are only partly understood. An established source of retinal NO are NOACs, a group of nNOS-expressing amacrine cells which signal onto bipolar, other amacrine and ganglion cells in the inner plexiform layer. Here, we report that NO regulates glutamate responses in morphologically and electrophysiologically identified type 4 OFF cone bipolar cells through activation of the soluble guanylyl cyclase-cGMP-PKG pathway. The glutamate response of these cells consists of two components, a fast phasic current sensitive to kainate receptor agonists, and a secondary component with slow kinetics, inhibited by AMPA receptor antagonists. NO shortened the duration of the AMPA receptor-dependent component of the glutamate response, while the kainate receptor-dependent component remained unchanged. Application of 8-Br-cGMP mimicked this effect, while inhibition of soluble guanylate cyclase or protein kinase G prevented it, supporting a mechanism involving a cGMP signaling pathway. Notably, perfusion with a NOS-inhibitor prolonged the duration of the glutamate response, while the NO precursor L-arginine shortened it, in agreement with a modulation by endogenous NO. Furthermore, NO accelerated the response recovery during repeated stimulation of type 4 cone bipolar cells, suggesting that the temporal response properties of this OFF bipolar cell type are regulated by NO. These results reveal a novel cellular mechanism of NO signaling in the retina, and represent the first functional evidence of NO modulating OFF cone bipolar cells.

Somatostatin and diabetic retinopathy: current concepts and new therapeutic perspectives

Somatostatin (SST) is abundantly produced by the human retina, and the main source is the retinal pigment epithelium (RPE). SST exerts relevant functions in the retina (neuromodulation, angiostatic, and anti-permeability actions) by interacting with SST receptors (SSTR) that are also expressed in the retina. In the diabetic retina, a downregulation of SST production does exist. In this article, we give an overview of the mechanisms by which this deficit of SST participates in the main pathogenic mechanisms involved in diabetic retinopathy (DR): neurodegeneration, neovascularization, and vascular leakage. In view of the relevant SST functions in the retina and the reduction of SST production in the diabetic eye, SST replacement has been proposed as a new target for treatment of DR. This could be implemented by intravitreous injections of SST analogs or gene therapy, but this is an aggressive route for the early stages of DR. Since topical administration of SST has been effective in preventing retinal neurodegeneration in STZ-induced diabetic rats, it seems reasonable to test this new approach in humans. In this regard, the results of the ongoing clinical trial EUROCONDOR will provide useful information. In conclusion, SST is a natural neuroprotective and antiangiogenic factor synthesized by the retina which is downregulated in the diabetic eye and, therefore, its replacement seems a rational approach for treating DR. However, clinical trials will be needed to establish the exact position of targeting SST in the treatment of this disabling complication of diabetes.

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.

Peptide receptor targeting in cancer: the somatostatin paradigm

Peptide receptors involved in pathophysiological processes represent promising therapeutic targets. Neuropeptide somatostatin (SST) is produced by specialized cells in a large number of human organs and tissues. SST primarily acts as inhibitor of endocrine and exocrine secretion via the activation of five G-protein-coupled receptors, named sst1–5, while in central nervous system, SST acts as a neurotransmitter/neuromodulator, regulating locomotory and cognitive functions. Critical points of SST/SST receptor biology, such as signaling pathways of individual receptor subtypes, homo- and heterodimerization, trafficking, and cross-talk with growth factor receptors, have been extensively studied, although functions associated with several pathological conditions, including cancer, are still not completely unraveled. Importantly, SST exerts antiproliferative and antiangiogenic effects on cancer cells in vitro, and on experimental tumors in vivo. Moreover, SST agonists are clinically effective as antitumor agents for pituitary adenomas and gastro-pancreatic neuroendocrine tumors. However, SST receptors being expressed by tumor cells of various tumor histotypes, their pharmacological use is potentially extendible to other cancer types, although to date no significant results have been obtained. In this paper the most recent findings on the expression and functional roles of SST and SST receptors in tumor cells are discussed.

Nitric oxide signaling in the retina: what have we learned in two decades?

Two decades after its first detection in the retina, nitric oxide (NO) continues to puzzle visual neuroscientists. While its liberation by photoreceptors remains controversial, recent evidence supports three subtypes of amacrine cells as main sources of NO in the inner retina. NO synthesis was shown to depend on light stimulation, and mounting evidence suggests that NO is a regulator of visual adaptation at different signal processing levels. NO modulates light responses in all retinal neuron classes, and specific ion conductances are activated by NO in rods, cones, bipolar and ganglion cells. Light-dependent gap junction coupling in the inner and outer plexiform layers is also affected by NO. The vast majority of these effects were shown to be mediated by activation of the NO receptor soluble guanylate cyclase and resultant cGMP elevation. This review analyzes the current state of knowledge on physiological NO signaling in the retina.

The synergetic modulation of the excitability of central gray matter by a neuropeptide: two protocols using excitation waves in chick retina

The isolated chick retina provides an in vitro tissue model, in which two protocols were developed to verify the efficacy of a peptide in the excitability control of the central gray matter. In the first, extra-cellular potassium homeostasis is challenged at long intervals and in the second, a wave is trapped in a ring of tissue causing the system to be under self-sustained challenge. Within the neuropil, the extra-cellular potassium transient observed in the first protocol was affected from the initial rising phase to the final concentration at the end of the five-minute pulse. There was no change in the concomitants of excitation waves elicited by the extra-cellular rise of potassium. However, there was an increase on the elicited waves latency and/or a rise in the threshold potassium concentration for these waves to appear. In the second protocol, the wave concomitants and the propagation velocity were affected by the peptide. The results suggest a synergetic action of the peptide on glial and synaptic membranes: by accelerating the glial Na/KATPase and changing the kinetics of the glial potassium channels, with glia tending to accumulate KCl. At the same time, there is an increase in potassium currents through nerve terminals.

Modulation of the neuronal response to ischaemia by somatostatin analogues in wild-type and knock-out mouse retinas

Somatostatin acts at five G protein-coupled receptors, sst(1)-sst(5). In mouse ischaemic retinas, the over-expression of sst(2) (as in sst(1) knock-out mice) results in the reduction of cell death and glutamate release. In this study, we reported that, in wild-type retinas, somatostatin, the multireceptor ligand pasireotide and the sst(2) agonist octreotide decreased ischaemia-induced cell death and that octreotide also decreased glutamate release. In contrast, cell death was increased by blocking sst(2) with cyanamide. In sst(2) over-expressing ischaemic retinas, somatostatin analogues increased cell death, and octreotide also increased glutamate release. To explain this reversal of the anti-ischaemic effect of somatostatin agonists in the presence of sst(2) over-expression, we tested sst(2) desensitisation because of internalisation or altered receptor function. We observed that (i) sst(2) was not internalised, (ii) among G protein-coupled receptor kinases (GRKs) and regulators of G protein signalling (RGSs), GRK1 and RGS1 expression increased following ischaemia, (iii) both GRK1 and RGS1 were down-regulated by octreotide in wild-type ischaemic retinas, (iv) octreotide down-regulated GRK1 but not RGS1 in sst(2) over-expressing ischaemic retinas. These results demonstrate that sst(2) activation protects against retinal ischaemia. However, in the presence of sst(2) over-expression sst(2) is functionally desensitised by agonists, possibly because of sustained RGS1 levels.

Physiology and pathology of somatostatin in the mammalian retina: a current view

In the retina, peptidergic signalling participates in multiple circuits of visual information processing. The neuropeptide somatostatin (SRIF) is localised to amacrine cells and, in some instances, in a subset of ganglion cells. The variegated expression patterns of SRIF receptors (sst(1)-sst(5)) and the variety of signalling mechanisms activated by retinal SRIF suggest that this peptide may exert multiple actions on retinal neurons and on retinal physiology, although our current understanding reflects a rather complicated picture. SRIF, mostly through sst(2), may act as a positive factor in the retina by regulating retinal homeostasis and protecting neurons against damage. In this respect, SRIF analogues seem to constitute a promising therapeutic arsenal to cure different retinal diseases, as for instance, ischemic and diabetic retinopathies. However, further investigations are needed not only to fully understand the functional role of the SRIF system in the retina but also to exploit new chemical space for drug-like molecules.

Novel signals mediating the functions of somatostatin: the emerging role of NO/cGMP

The neuropeptide somatostatin is a cyclic tetradecapeptide, which is widely distributed in the peripheral and central nervous system. It mediates a plethora of physiological actions and functions as a neurotransmitter, neuromodulator or trophic factor. Somatostatin activates six receptor subtypes that are expressed differentially in different tissues and are coupled to diverse signalling pathways. In order to elucidate the functional role of the individual receptor subtypes, many investigations focused on the assignment of each receptor to a particular signalling pathway. Signalling pathways involving enzyme (adenylate cyclase, phospholipases, phosphatases) and ion channel systems in native and recombinant receptor systems have been extensively studied. A one to one situation (receptor/pathway) has yet to be established, thus justifying the diverse actions of somatostatin. Recently, a NO/cGMP pathway has been shown to mediate the functions of somatostatin and its receptors. This review will present the findings that support the emerging role of NO/cGMP as a novel signal in SRIF's actions in retinal physiology and somatotroph release.

Lower somatostatin expression is an early event in diabetic retinopathy and is associated with retinal neurodegeneration

OBJECTIVE

To test the hypothesis that a reduction of somatostatin (SST) in the retina exists in patients without clinically detectable diabetic retinopathy and that it is associated with retinal neurodegeneration.

RESEARCH DESIGN AND METHODS

Human diabetic postmortem eyes (n = 10) without clinically detectable retinopathy were compared with eyes (n = 10) from nondiabetic donors. SST mRNA (RT-PCR) and SST-28 immunoreactivity (confocal laser) were measured separately in neuroretina and retinal pigment epithelium (RPE). In addition, SST-28 (radioimmunoassay) was measured in the vitreous fluid. Glial fibrillar acidic protein (GFAP) was assessed by immunofluorescence and Western blot. Apoptotic cells were quantified using transferase-mediated dUTP nick-end labeling.

RESULTS

A higher expression of SST was detected in RPE than neuroretina in both groups. SST mRNA levels and SST-28 immunoreactivity were significantly lower in both RPE and the neuroretina from diabetic donors compared with nondiabetic donors. These results were in agreement with those obtained by measuring SST-28 in the vitreous fluid of the same donors. Increased GFAP and a higher degree of apoptosis were observed in diabetic retinas compared with nondiabetic retinas. These changes were most evident in patients with the higher deficit of SST.

CONCLUSIONS

Underproduction of SST is an early event in the eyes of diabetic patients and is associated with glial activation and neural death. In addition, our results suggest that RPE is an important source of SST in the human eye. The possible role of the lower production of SST in the pathogenesis of diabetic retinopathy requires further investigation.

Dopamine (D1) receptor activation and nitrinergic agents influence somatostatin levels in rat retina

Somatostatin (SRIF) influences the release of two important neuromodulators of retinal circuitry, dopamine (DA) and nitric oxide (NO). The aim of the present study was to examine whether DA and NO modulate SRIF release in rat retina, and the mechanisms involved in their actions. Retinas of adult female Sprague--Dawley rats (250--300 g) were mechanically detached from the eyecup and ex vivo experiments were performed. Retinal explants were incubated in the presence of dopaminergic [DA (10 microM, 100 microM and 200 microM), apomorphine (nonselective D1/D2 agonist, 0.50 mM, 1.0 microM and 10 microM), A68930 (D1 selective agonist, 0.50 microM, 1.0 microM and 10 microM), quinpirole (D2 selective agonist, 0.50 microM, 1.0 microM and 10 microM), SCH 23390 (D1 selective antagonist, 250 nM and 500 nM) and sulpiride (D2 selective antagonist, 100 microM and 200 microM)], and nitrinergic agents [arginine (62.5 microM--5mM), SIN-1 (50 microM, 100 microM and 500 microM) and 8-Br-cGMP (50 microM, 250 microM and 500 microM)]. SRIF levels were quantified using radioimmunoassay (RIA). Dopamine had no effect on SRIF levels. Apomorphine produced a concentration dependent decrease and increase in SRIF levels, suggestive of pre- and postsynaptic effects. A68930 (10 microM) and SCH 23390 (250 nM and 500 nM) mimicked and reversed apomorphine's postsynaptic actions, respectively. Quinpirole had no effect, but blockade of D2 autoreceptors by sulpiride (200 microM) afforded an increase in SRIF levels. Arginine and SIN-1 increased, and 8-Br-cGMP attenuated SRIF levels. These results show that dopamine D1 receptors, and NO/peroxynitrite agents modulate SRIF release in the retina suggesting that the triad SRIF--DA--NO have reciprocal interactions via which they regulate retinal circuitry and vision transduction.

An update on somatostatin receptor signaling in native systems and new insights on their pathophysiology

The peptide somatostatin (SRIF) has important physiological effects, mostly inhibitory, which have formed the basis for the clinical use of SRIF compounds. SRIF binding to its 5 guanine nucleotide-binding proteins-coupled receptors leads to the modulation of multiple transduction pathways. However, our current understanding of signaling exerted by receptors endogenously expressed in different cells/tissues reflects a rather complicated picture. On the other hand, the complexity of SRIF receptor signaling in pathologies, including pituitary and nervous system diseases, may be studied not only as alternative intervention points for the modulation of SRIF function but also to exploit new chemical space for drug-like molecules.

Somatostatin presynaptically inhibits both GABA and glutamate release onto rat basal forebrain cholinergic neurons

A whole cell patch-clamp study was carried out in slices obtained from young rat brain to elucidate the roles of somatostatin in the modulation of synaptic transmission onto cholinergic neurons in the basal forebrain (BF), a region that contains cholinergic and GABAergic corticopetal neurons and somatostatin (SS)-containing local circuit neurons. Cholinergic neurons within the BF were identified by in vivo prelabeling with Cy3 IgG. Because in many cases SS is contained in GABAergic neurons in the CNS, we investigated whether exogenously applied SS can influence GABAergic transmission onto cholinergic neurons. Bath application of somatostatin (1 muM) reduced the amplitude of the evoked GABAergic inhibitory presynaptic currents (IPSCs) in cholinergic neurons. SS also reduced the frequency of miniature IPSCs (mIPSCs) without affecting their amplitude distribution. SS-induced effect on the mIPSC frequency was significantly larger in the solution containing 7.2 mM Ca(2+) than in the standard (2.4 mM Ca(2+)) external solution. Similar effects were observed in the case of non-NMDA glutamatergic excitatory postsynaptic currents (EPSCs). SS inhibited the amplitude of evoked EPSCs and reduced the frequency of miniature EPSCs dependent on the external Ca(2+) concentration with no effect on their amplitude distribution. Pharmacological analyses using SS-receptor subtype-specific drugs suggest that SS-induced action of the IPSCs is mediated mostly by the sst(2) subtype, whereas sst subtypes mediating SS-induced inhibition of EPSCs are mainly sst(1) or sst(4). These findings suggest that SS presynaptically inhibits both GABA and glutamate release onto BF cholinergic neurons in a Ca(2+)-dependent way, and that SS-induced effect on IPSCs and EPSCs are mediated by different sst subtypes.