Somatostatin sst2 receptor knock-out mice: localisation of sst1-5 receptor mRNA and binding in mouse brain by semi-quantitative RT-PCR, in situ hybridisation histochemistry and receptor autoradiography

Characterization of Neurons Expressing the Novel Analgesic Drug Target Somatostatin Receptor 4 in Mouse and Human Brains

Somatostatin is an important mood and pain-regulating neuropeptide, which exerts analgesic, anti-inflammatory, and antidepressant effects via its Gi protein-coupled receptor subtype 4 (SST₄) without endocrine actions. SST₄ is suggested to be a unique novel drug target for chronic neuropathic pain, and depression, as a common comorbidity. However, its neuronal expression and cellular mechanism are poorly understood. Therefore, our goals were (i) to elucidate the expression pattern of Sstr4/SSTR4 mRNA, (ii) to characterize neurochemically, and (iii) electrophysiologically the Sstr4/SSTR4-expressing neuronal populations in the mouse and human brains. Here, we describe SST₄ expression pattern in the nuclei of the mouse nociceptive and anti-nociceptive pathways as well as in human brain regions, and provide neurochemical and electrophysiological characterization of the SST₄-expressing neurons. Intense or moderate SST₄ expression was demonstrated predominantly in glutamatergic neurons in the major components of the pain matrix mostly also involved in mood regulation. The SST₄ agonist J-2156 significantly decreased the firing rate of layer V pyramidal neurons by augmenting the depolarization-activated, non-inactivating K⁺ current (M-current) leading to remarkable inhibition. These are the first translational results explaining the mechanisms of action of SST₄ agonists as novel analgesic and antidepressant candidates.

Characterization of agonist-dependent somatostatin receptor subtype 2 trafficking in neuroendocrine cells

BACKGROUND

Somatostatin (SOM) receptor subtype 2 (SSTR2) is the major receptor subtype mediating SOM effects throughout the neuraxis. We previously demonstrated that the non-selective agonist [D-Trp⁸]-SOM induces intracellular sequestration of SSTR2, whereas this receptor is maintained at the cell surface after treatment with the SSTR2-selective agonist L-779,976 in cells co-expressing SSTR2 and SSTR5.

METHODS AND RESULTS

In this study, we knocked-out SSTR5 in AtT20 cells endogenously expressing both SSTR2 and SSTR5 and used immuno-labeling and confocal microscopy to investigate the effect of SSTR5 on regulation of SSTR2 trafficking. Our results indicate that unlike [D-Trp⁸]-SOM-induced intracellular sequestration, L-779,976 stimulation results in the maintenance of SSTR2 at the cell surface regardless of whether SSTR5 is present or not. We then examined the trafficking pathways of SSTR2 upon stimulation by either agonist. We found that both [D-Trp⁸]-SOM and L-779,976 induce SSTR2 internalization via transferrin-positive vesicles. However, SSTR2 internalized upon L-779,976 treatment undergoes rapid recycling to the plasma membrane, whereas receptors internalized by [D-Trp⁸]-SOM recycle slowly after washout of the agonist. Furthermore, [D-Trp⁸]-SOM stimulation induces degradation of a fraction of internalized SSTR2 whereas L-779,976-dependent, rapid SSTR2 recycling appears to protect internalized SSTR2 from degradation. In addition, Octreotide which has preferential SSTR2 affinity, induced differential effects on both SSTR2 trafficking and degradation.

CONCLUSION

Our results indicate that the biased agonistic property of L-779,976 protects against SSTR2 surface depletion by rapidly initiating SSTR2 recycling while SSTR5 does not regulate L-779-976-dependent SSTR2 trafficking.

Somatostatin, Olfaction, and Neurodegeneration

Alzheimer's and Parkinson's diseases are the most prevalent neurodegenerative disorders in aging. Hyposmia has been described as an early symptom that can precede cognitive and motor deficits by decades. Certain regions within the olfactory system, such as the anterior olfactory nucleus, display the neuropathological markers tau and amyloid-β or α-synuclein from the earliest stages of disease progression in a preferential manner. Specific neuronal subpopulations, namely those expressing somatostatin (SST), are preferentially affected throughout the olfactory and limbic systems. SST is a neuropeptide present in a subpopulation of GABAergic interneurons throughout the brain and its main function is to inhibit principal neurons and/or other interneurons. It has been reported that SST expression is reduced by 50% in Alzheimer's disease and that it is related to the formation of Aβ oligomers. The mechanisms underlying the preferential vulnerability of SST-expressing neurons in Alzheimer's disease (and, to a minor extent, in Parkinson's disease) are not known but analysis of the available data could shed light on their etiology. This short review aims to update the knowledge of functional features of somatostatin within the olfactory system and its role in olfactory deficits during neurodegeneration.

Effect of Immune Activation during Early Gestation or Late Gestation on Inhibitory Markers in Adult Male Rats

People with schizophrenia exhibit deficits in inhibitory neurons and cognition. The timing of maternal immune activation (MIA) may present distinct schizophrenia-like phenotypes in progeny. We investigated whether early gestation [gestational day (GD) 10] or late gestation (GD19) MIA, via viral mimetic polyI:C, produces deficits in inhibitory neuron indices (GAD1, PVALB, SST, SSTR2 mRNAs) within cortical, striatal, and hippocampal subregions of male adult rat offspring. In situ hybridisation revealed that polyI:C offspring had: (1) SST mRNA reductions in the cingulate cortex and nucleus accumbens shell, regardless of MIA timing; (2) SSTR2 mRNA reductions in the cortex and striatum of GD19, but not GD10, MIA; (3) no alterations in cortical or striatal GAD1 mRNA of polyI:C offspring, but an expected reduction of PVALB mRNA in the infralimbic cortex, and; (4) no alterations in inhibitory markers in hippocampus. Maternal IL-6 response negatively correlated with adult offspring SST mRNA in cortex and striatum, but not hippocampus. These results show lasting inhibitory-related deficits in cortex and striatum in adult offspring from MIA. SST downregulation in specific cortical and striatal subregions, with additional deficits in somatostatin-related signalling through SSTR2, may contribute to some of the adult behavioural changes resulting from MIA and its timing.

A role for the neuropeptide somatostatin in the neurobiology of behaviors associated with substances abuse and affective disorders

In recent years, neuropeptides which display potent regulatory control of stress-related behaviors have been extensively demonstrated to play a critical role in regulating behaviors associated with substance abuse and affective disorders. Somatostatin (SST) is one neuropeptide known to significantly contribute to emotionality and stress behaviors. However, the role of SST in regulating behavior has received relatively little attention relative to other stress-involved peptides, such as neuropeptide Y or corticotrophin releasing factor. This review characterizes our current understanding of the role of SST and SST-expressing cells in general in modulating several behaviors intrinsically linked to substance abuse and affective disorders, specifically: anxiety and fear; stress and depression; feeding and drinking; and circadian rhythms. We further summarize evidence of a direct role for the SST system, and specifically somatostatin receptors 2 and 4, in substance abuse disorders. This article is part of the special issue on 'Neuropeptides'.

Central somatostatin signaling and regulation of food intake

The discovery of somatostatin (SST) in the hypothalamus implicated the peptide in the inhibition of growth hormone release. However, as observed for numerous neuropeptides, SST was neither restricted to this one brain site nor to this one function. Subsequent studies established a widespread but specific expression of SST in the central nervous system of rodents and humans along with the expression patterns of five receptors (sst_1-5 ). Among biological actions, the activation of central SST signaling induced a robust stimulation of food and water intake, which is mediated by the sst₂ as assessed using selective sst agonists. The past years have witnessed the identification of brain SST circuitries involved using chemogenetic and optogenetic approaches and further established a physiological orexigenic role of brain SST signaling. The present review will discuss these recent findings.

Regional and temporal regulation and role of somatostatin receptor subtypes in the mouse brain following systemic kainate-induced acute seizures

Somatostatin reduces neuronal excitability via somatostatin receptors (Sst₁-Sst₅) and inhibits seizure activity. However, the expression status of the Sst subtypes in epileptic mice and their role in the antiepileptic effects of somatostatin remain unclear. Here, we show that the Sst subtypes are regulated differently by epileptic neuronal activity in mice. Systemic kainate injection rapidly and transiently elevated the Sst₂ and Sst₃ mRNA and reduced Sst₁ and Sst₄ mRNA in the hippocampus; however, among all the subtypes, only Sst₂ mRNA was increased in the excitatory neurons of the basolateral amygdala, accompanied by a decrease in the level of Sst₂ protein. Following kainate administration, recovery from seizure was delayed by reduced expression of Sst₂ in the basolateral amygdala, but not in the dentate gyrus of the hippocampus; higher expression levels of Bdnf, a neuronal activity marker, were observed in both conditions. These results suggest that Sst₂ contributes to seizure termination by feedback inhibition in the amygdala. This could be a potential therapeutic target for acute seizures.

International Union of Basic and Clinical Pharmacology. CV. Somatostatin Receptors: Structure, Function, Ligands, and New Nomenclature

Somatostatin, also known as somatotropin-release inhibitory factor, is a cyclopeptide that exerts potent inhibitory actions on hormone secretion and neuronal excitability. Its physiologic functions are mediated by five G protein-coupled receptors (GPCRs) called somatostatin receptor (SST)1-5. These five receptors share common structural features and signaling mechanisms but differ in their cellular and subcellular localization and mode of regulation. SST₂ and SST₅ receptors have evolved as primary targets for pharmacological treatment of pituitary adenomas and neuroendocrine tumors. In addition, SST₂ is a prototypical GPCR for the development of peptide-based radiopharmaceuticals for diagnostic and therapeutic interventions. This review article summarizes findings published in the last 25 years on the physiology, pharmacology, and clinical applications related to SSTs. We also discuss potential future developments and propose a new nomenclature.

sst2-receptor gene deletion exacerbates chronic stress-induced deficits: Consequences for emotional and cognitive ageing

This study investigated whether sst2 gene deletion interacts with age and chronic stress exposure to produce exacerbated emotional and cognitive ageing. Middle-aged (10-12 month) sst2 knockout (sst2KO) and wild-type (WT) mice underwent an unpredictable chronic mild stress (UCMS) procedure for 6 weeks or no stress for control groups. This was followed by a battery of tests to assess emotional and cognitive functions and neuroendocrine status (CORT level). A re-evaluation was performed 6 months later (i.e. with 18-month-old mice). UCMS reproduced neuroendocrine and behavioral features of stress-related disorders such as elevated circulating CORT levels, physical deteriorations, increased anxiety- and depressive-like behaviors and working memory impairments. sst2KO mice displayed behavioral alterations which were similar to stressed WT and exhibited exacerbated changes following UCMS exposure. The evaluations performed in the older mice showed significant long-term effects of UCMS exposure. Old sst2KO mice previously exposed to UCMS exhibited spatial learning and memory accuracy impairments and high levels of anxiety-like behaviors which drastically added to the effects of normal ageing. Spatial abilities and emotionality scores (mean z-scores) measured both at the UCMS outcome and 6 months later were correlated with the initially measured CORT levels in middle-age. The present findings indicate that the deletion of the sst2 receptor gene produces chronic hypercorticosteronemia and exacerbates sensitivity to stressors which over time, have consequences on ageing brain function processes.

Differential plastic changes in synthesis and binding in the mouse somatostatin system after electroconvulsive stimulation

OBJECTIVE

Electroconvulsive therapy (ECT) is regularly used to treat patients with severe major depression, but the mechanisms underlying the beneficial effects remain uncertain. Electroconvulsive stimulation (ECS) regulates diverse neurotransmitter systems and induces anticonvulsant effects, properties implicated in mediating therapeutic effects of ECT. Somatostatin (SST) is a candidate for mediating these effects because it is upregulated by ECS and exerts seizure-suppressant effects. However, little is known about how ECS might affect the SST receptor system. The present study examined effects of single and repeated ECS on the synthesis of SST receptors (SSTR1-4) and SST, and SST receptor binding ([125I]LTT-SST28) in mouse hippocampal regions and piriform/parietal cortices.

RESULTS

A complex pattern of plastic changes was observed. In the dentate gyrus, SST and SSTR1 expression and the number of hilar SST immunoreactive cells were significantly increased at 1 week after repeated ECS while SSTR2 expression was downregulated by single ECS, and SSTR3 mRNA and SST binding were elevated 24 h after repeated ECS. In hippocampal CA1 and parietal/piriform cortices, we found elevated SST mRNA levels 1 week after repeated ECS and elevated SST binding after single ECS and 24 h after repeated ECS. In hippocampal CA3, repeated ECS increased SST expression 1 week after and SST binding 24 h after. In the parietal cortex, SSTR2 mRNA expression was downregulated after single ECS while SSTR4 mRNA expression was upregulated 24 h after repeated ECS.

CONCLUSION

Considering the known anticonvulsant effects of SST, it is likely that these ECS-induced neuroplastic changes in the SST system could participate in modulating neuronal excitability and potentially contribute to therapeutic effects of ECT.

Does somatostatin have a role to play in migraine headache?

Migraine is a condition without apparent pathology. Its cardinal symptom is the prolonged excruciating headache. Theories about this pain have posited pathologies which run the gamut from neural to vascular to neurovascular, but no observations have detected a plausible pathology. We believe that no pathology can be found for migraine headache because none exists. Migraine is not driven by pathology - it is driven by neural events produced by triggers - or simply by neural noise- noise that has crossed a critical threshold. If these ideas are true, how does the pain arise? We hypothesise that migraine headache is a consequence of withdrawal of descending pain control, produced by "noise" in the cerebral cortex. Nevertheless, there has to be a neural circuit to transform cortical noise to withdrawal of pain control. In our hypothesis, this neural circuit extends from the cortex, synapses in two brainstem nuclei (the periaqueductal gray matter and the raphe magnus nucleus) and ultimately reaches the first synapse of the trigeminal sensory system. The second stage of this circuit uses serotonin (5HT) as a neurotransmitter, but the neuronal projection from the cortex to the brainstem seems to involve relatively uncommon neurotransmitters. We believe that one of these is somatostatin (SST). Temporal changes in levels of circulating SST mirror the temporal changes in the incidence of migraine, particularly in women. The SST₂ receptor agonist octreotide has been used with some success in migraine and cluster headache. A cortical to PAG/NRM neural projection certainly exists and we briefly review the anatomical and neurophysiological evidence for it and provide preliminary evidence that SST may the critical neurotransmitter in this pathway. We therefore suggest that the withdrawal of descending tone in SST-containing neurons, might create a false pain signal and hence the headache of migraine.

Regulation of somatostatin receptor 2 in the context of antidepressant treatment response in chronic mild stress in rat

RATIONALE

The role of somatostatin and its receptors for the stress-related neuropsychiatric disorders has been widely raised. Recently, we have also demonstrated the involvement of somatostatin receptor type 2-sst2R and dopamine receptor type 2-D2R in stress.

OBJECTIVE

In this context, we decided to find if these receptors are involved in response to antidepressant treatment in animal model of depression-chronic mild stress (CMS).

METHODS

Here, we report data obtained following 7-week CMS procedure. The specific binding of [125I]Tyr3-Octreotide to sst2R and [3H]Domperidone to D2R was measured in the rat brain, using autoradiography. Additionally, the level of dopamine and metabolites was measured in the rat brain.

RESULTS

In the final baseline test after 7 weeks of stress, the reduced consumption of sucrose solution was observed (controls vs the stressed animals (6.25 0.16 vs. 10.39 0.41; p < 0.05). Imipramine was administered for the next 5 weeks, and it reversed anhedonia in majority of animals (imipramine-reactive); however, in some animals, it did not (imipramine-non-reactive). Two-way repeated measures ANOVA revealed significant effects of stress and treatment and time interaction [F(16, 168) = 3.72; p < 0.0001], n = 10 per groups. We observed decreased binding of [125I]Tyr3-Octreotide in most of rat brain regions in imipramine non-reactive groups of animals. The decrease of D2R after stress in striatum and nucleus accumbens and no effect of imipramine were observed. In the striatum and prefrontal cortex, the significant role of stress and imipramine in dopamine levels was observed.

CONCLUSIONS

The results obtained in binding assays, together with dopamine level, indicate the involvement of sst2R receptors for reaction to antidepressant treatment. Besides, the stress context itself changes the effect of antidepressant drug.

Activation of Brain Somatostatin Signaling Suppresses CRF Receptor-Mediated Stress Response

Corticotropin-releasing factor (CRF) is the hallmark brain peptide triggering the response to stress and mediates—in addition to the stimulation of the hypothalamus-pituitary-adrenal (HPA) axis—other hormonal, behavioral, autonomic and visceral components. Earlier reports indicate that somatostatin-28 injected intracerebroventricularly counteracts the acute stress-induced ACTH and catecholamine release. Mounting evidence now supports that activation of brain somatostatin signaling exerts a broader anti-stress effect by blunting the endocrine, autonomic, behavioral (with a focus on food intake) and visceral gastrointestinal motor responses through the involvement of distinct somatostatin receptor subtypes.

Chronic mild stress alters the somatostatin receptors in the rat brain

RATIONALE

The involvement of somatostatin (SST) and its receptors in the pathophysiology of depression and stress has been evidenced by numerous studies.

OBJECTIVES

The purpose of the present study was to find whether chronic mild stress (CMS), an animal model of depression, affects the SST receptors in the rat brain and pituitary, as well as the level of SST in plasma.

METHODS

In CMS model, rats were subjected to 2 weeks of stress and behaviorally characterized using the sucrose consumption test into differently reacting groups based on their response to stress, i.e., stress-reactive (anhedonic), stress-non-reactive (resilient), and invert-reactive rats (characterized by excessive sucrose intake). We measured specific binding of [125I]Tyr3-Octreotide, expression of mRNA encoding sst2R receptors in the rat brains, expression of SST and its receptors in rat pituitary, and the level of SST in the plasma.

RESULTS

The obtained results show decreases in binding of [125I]Tyr3-Octreotide in most of rat brain regions upon CMS and no significant differences between three stressed groups of animals, except for significant up-regulation of sst2 receptor in medial habenula (MHb) in the stress-reactive group. In the same group of animals, significant increase in plasma SST level was observed.

CONCLUSIONS

There are two particularly sensitive sites distinguishing the response to stress in CMS model. In the brain, it is MHb, while on the periphery this predictor is SST level in plasma. These changes may broaden an understanding of the mechanisms involved in the stress response and point to the intriguing role of MHb.

The role of brain somatostatin receptor 2 in the regulation of feeding and drinking behavior

Somatostatin was discovered four decades ago as hypothalamic factor inhibiting growth hormone release. Subsequently, somatostatin was found to be widely distributed throughout the brain and to exert pleiotropic actions via interaction with five somatostatin receptors (sst1-5) that are also widely expressed throughout the brain. Interestingly, in contrast to the predominantly inhibitory actions of peripheral somatostatin, the activation of brain sst2 signaling by intracerebroventricular injection of stable somatostatin agonists potently stimulates food intake and independently, drinking behavior in rodents. The orexigenic response involves downstream orexin-1, neuropeptide Y1 and μ receptor signaling while the dipsogenic effect is mediated through the activation of the brain angiotensin 1 receptor. Brain sst2 activation is part of mechanisms underlying the stimulation of feeding and more prominently water intake in the dark phase and is able to counteract the anorexic response to visceral stressors.

The inhibitory effect of somatostatin receptor activation on bee venom-evoked nociceptive behavior and pCREB expression in rats

The present study examined nociceptive behaviors and the expression of phosphorylated cAMP response element-binding protein (pCREB) in the dorsal horn of the lumbar spinal cord and the dorsal root ganglion (DRG) evoked by bee venom (BV). The effect of intraplantar preapplication of the somatostatin analog octreotide on nociceptive behaviors and pCREB expression was also examined. Subcutaneous injection of BV into the rat unilateral hindpaw pad induced significant spontaneous nociceptive behaviors, primary mechanical allodynia, primary thermal hyperalgesia, and mirror-thermal hyperalgesia, as well as an increase in pCREB expression in the lumbar spinal dorsal horn and DRG. Octreotide pretreatment significantly attenuated the BV-induced lifting/licking response and mechanical allodynia. Local injection of octreotide also significantly reduced pCREB expression in the lumbar spinal dorsal horn and DRG. Furthermore, pretreatment with cyclosomatostatin, a somatostatin receptor antagonist, reversed the octreotide-induced inhibition of the lifting/licking response, mechanical allodynia, and the expression of pCREB. These results suggest that BV can induce nociceptive responses and somatostatin receptors are involved in mediating the antinociception, which provides new evidence for peripheral analgesic action of somatostatin in an inflammatory pain state.

Modulation of the adaptive response to stress by brain activation of selective somatostatin receptor subtypes

Somatostatin-14 was discovered in 1973 in the hypothalamus as a peptide inhibiting growth hormone release. Somatostatin interacts with five receptor subtypes (sst(1-5)) which are widely distributed in the brain with a distinct, but overlapping, expression pattern. During the last few years, the development of highly selective peptide agonists and antagonists provided new insight to characterize the role of somatostatin receptor subtypes in the pleiotropic actions of somatostatin. Recent evidence in rodents indicates that the activation of selective somatostatin receptor subtypes in the brain blunts stress-corticotropin-releasing factor (CRF) related ACTH release (sst2/5), sympathetic-adrenal activaton (sst5), stimulation of colonic motility (sst1), delayed gastric emptying (sst5), suppression of food intake (sst2) and the anxiogenic-like (sst2) response. These findings suggest that brain somatostatin signaling pathways may play an important role in dampening CRF-mediated endocrine, sympathetic, behavioral and visceral responses to stress.

Somatostatin type-2 receptor activation inhibits glutamate release and prevents status epilepticus

Newer therapies are needed for the treatment of status epilepticus (SE) refractory to benzodiazepines. Enhanced glutamatergic neurotransmission leads to SE, and AMPA receptors are modified during SE. Reducing glutamate release during SE is a potential approach to terminate SE. The neuropeptide somatostatin (SST) is proposed to diminish presynaptic glutamate release by activating SST type-2 receptors (SST2R). SST exerts an anticonvulsant action in some experimental models of seizures. Here, we investigated the mechanism of action of SST on excitatory synaptic transmission at the Schaffer collateral-CA1 synapses and the ability of SST to treat SE in rats using patch-clamp electrophysiology and video-EEG monitoring of seizures. SST reduced action potential-dependent EPSCs (sEPSCs) at Schaffer collateral-CA1 synapses at concentrations up to 1μM; higher concentrations had no effect or increased the sEPSC frequency. SST also prevented paired-pulse facilitation of evoked EPSCs and did not alter action-potential-independent miniature EPSCs (mEPSCs). The effect of SST on EPSCs was inhibited by the SST2R antagonist cyanamid-154806 and was mimicked by the SST2R agonists, octreotide and lanreotide. Both SST and octreotide reduced the firing rate of CA1 pyramidal neurons. Intraventricular administration of SST, within a range of doses, either prevented or attenuated pilocarpine-induced SE or delayed the median time to the first grade 5 seizure by 11min. Similarly, octreotide or lanreotide prevented or attenuated SE in more than 65% of animals. Compared to the pilocarpine model, octreotide was highly potent in preventing or attenuating continuous hippocampal stimulation-induced SE in all animals within 60min of SE onset. Our results demonstrate that SST, through the activation of SST2Rs, diminishes presynaptic glutamate release and attenuates SE.

Targeting the somatostatin receptors as a therapeutic approach for the preservation and protection of the mammalian cochlea from excitotoxicity

The neuropeptide somatostatin (SST) is an important modulator of neurotransmission in the central nervous system (CNS) and binds to G-protein-coupled receptors (SSTR1-5) on target cells. Little is known about the expression and function of the somatostatinergic system in the mammalian cochlea. We analyzed the expression of SSTR1-SSTR5 in the immature mammalian cochlea. The peak in the expression of SSTR1 and SSTR2 at mRNA and protein level is around the onset of hearing to airborne sound, at postnatal day (P)14. This suggests their involvement in the maturation of the mammalian cochlea. We demonstrated that all five receptors are expressed in the inner hair cells (IHC) and outer hear cells (OHC) as well as in defined supporting cells of the organ of Corti (OC) in the adult mouse cochlea. A similar expression of the SSTRs in the IHC and OHC was found in cultivated P6 mouse OC explants as well as in neuroepithelial cell culture. In order to learn more about the regulation of SSTRs, we used mice with either a deletion of SSTR1, SSTR2 or SSTR1/SSTR2 double knock out (DKO). In DKO mice, SSTR5 was up-regulated and SSTR3 and SSTR4 were down regulated. These findings provide evidence of a compensatory regulation in the mammalian cochlea as a consequence of a receptor subtype deletion. In addition, we observed reduced levels of phospho-Akt and total-Akt in SSTR1 KO and DKO mice as compared to wild type (WT) mice. Akt is likely to be involved in hair cell survival. Most importantly, we found improved hair cell survival in somatostatin and octreotide treated OC explants that had been exposed to gentamicin compared to those explants exposed to gentamicin alone. These findings propose that the somatostatinergic system within the cochlea may have neuroprotective properties.

Monoclonal antibodies against the human somatostatin receptor subtypes 1-5: development and immunohistochemical application in neuroendocrine tumors

BACKGROUND

Activation of somatostatin receptors (sstr1-5) by somatostatin and its analogues exerts an inhibitory effect on hormone secretion and provides the basis for the treatment of a range of endocrine diseases such as acromegaly, Cushing's disease and neuroendocrine tumors (NET). The lack of well-characterized commercially available sstr subtype-specific antibodies prevents routine identification of the sstr expression profile in patients.

METHODS

We generated and characterized new mouse monoclonal antibodies (mAbs) targeting the five human sstr subtypes using ELISA and immunohistochemistry, and tested their suitability in formalin-fixed and paraffin-embedded (FFPE) human tissues and archival samples of normal pancreatic tissue and NET.

RESULTS

All mAbs were highly specific with no cross-reactivity. The sstr1-5 immunoreactivity in gastrointestinal NET (n=67) was correlated with clinicopathologic data. With the exception of sstr3, NET were highly positive for all receptor subtypes (42, 63, 6, 32 and 65% of tumors were positive for sstr1, sstr2a, sstr3, sstr4 and sstr5, respectively). sstr1, sstr2a and sstr5 were present at the plasma membrane and in the cytoplasm of tumor cells, whereas sstr3 and sstr4 were almost exclusively cytoplasmic. Immunoreactivity of sstr1, sstr2a and sstr4 tended to decrease as tumor aggressiveness increased. sstr5 showed an opposite pattern, with higher staining in well-differentiated carcinomas compared with well-differentiated tumors. sstr5 immunoreactivity was correlated with the presence of metastases and angioinvasion, suggesting a possible association with more aggressive behavior.

CONCLUSION

Determination of the sstr1-5 by immunohistochemistry using subtype-specific mAbs is feasible in FFPE tissue and may provide a tool for routine clinical practice.

Somatostatinergic systems: an update on brain functions in normal and pathological aging

Somatostatin is highly expressed in mammalian brain and is involved in many brain functions such as motor activity, sleep, sensory, and cognitive processes. Five somatostatin receptors have been described: sst(1), sst(2) (A and B), sst(3), sst(4), and sst(5), all belonging to the G-protein-coupled receptor family. During the recent years, numerous studies contributed to clarify the role of somatostatin systems, especially long-range somatostatinergic interneurons, in several functions they have been previously involved in. New advances have also been made on the alterations of somatostatinergic systems in several brain diseases and on the potential therapeutic target they represent in these pathologies.

Targeted entry via somatostatin receptors using a novel modified retrovirus glycoprotein that delivers genes at levels comparable to those of wild-type viral glycoproteins

Here we report a novel viral glycoprotein created by replacing a natural receptor-binding sequence of the ecotropic Moloney murine leukemia virus envelope glycoprotein with the peptide ligand somatostatin. This new chimeric glycoprotein, which has been named the Sst receptor binding site (Sst-RBS), gives targeted transduction based on three criteria: (i) a gain of the use of a new entry receptor not used by any known virus; (ii) targeted entry at levels comparable to gene delivery by wild-type ecotropic Moloney murine leukemia virus and vesicular stomatitis virus (VSV) G glycoproteins; and (iii) a loss of the use of the natural ecotropic virus receptor. Retroviral vectors coated with Sst-RBS gained the ability to bind and transduce human 293 cells expressing somatostatin receptors. Their infection was specific to target somatostatin receptors, since a synthetic somatostatin peptide inhibited infection in a dose-dependent manner and the ability to transduce mouse cells bearing the natural ecotropic receptor was effectively lost. Importantly, vectors coated with the Sst-RBS glycoprotein gave targeted entry of up to 1 × 10(6) transducing U/ml, a level comparable to that seen with infection of vectors coated with the parental wild-type ecotropic Moloney murine leukemia virus glycoprotein through the ecotropic receptor and approaching that of infection of VSV G-coated vectors through the VSV receptor. To our knowledge, this is the first example of a glycoprotein that gives targeted entry of retroviral vectors at levels comparable to the natural capacity of viral envelope glycoproteins.

The somatostatinergic system in the mammalian cochlea

BACKGROUND

Little is known about expression and function of the somatostatinergic system in the mammalian cochlea. We have previously shown that somatostatin administration may have a protective effect on gentamicin-induced hair cell loss. In this study, we have analyzed the cochlear expression of somatostatin receptor 1 (SST1) and somatostatin receptor 2 (SST2) at both the mRNA and the protein level in wild-type mice, as well as in SST1 and SST2 knock-out (KO) mice and in cultivated neurosensory cells.

RESULTS

We demonstrate that the somatostatin receptors SST1 and SST2 are specifically expressed in outer and inner hair cells (HCs) of the organ of Corti (OC), as well as in defined supporting cells. The expression of SST1 and SST2 receptors in cultivated P5 mouse OC explants was similar to their expression in inner and outer hair cells. Somatostatin itself was not expressed in the mammalian cochlea, suggesting that somatostatin reaches its receptors either through the blood-labyrinthine barrier from the systemic circulation or via the endolymphatic duct from the endolymphatic sac. We used mice with a deletion of either SST1 or SST2 to learn more about the regulation of SST1 and SST2 receptor expression. We demonstrate that in SST1 KO mice, SST2 was expressed in outer HCs and Deiters' cells, but not in pillar cells or inner HCs, as compared with wild-type mice. In contrast, in SST2 KO mice, the expression pattern of the SST1 receptor was not altered relative to wild-type mice.

CONCLUSIONS

These findings reveal that somatostatin receptors demonstrate specific expression in HCs and supporting cells of the mouse cochlea, and that absence of SST1 alters the expression of SST2. This specific expression pattern suggests that somatostatin receptors may have important functional roles in the inner ear.

Central somatostatin receptor 1 activation reverses acute stress-related alterations of gastric and colonic motor function in mice

BACKGROUND

Corticotropin-releasing factor (CRF) signaling induced by stress is well established to delay gastric emptying (GE) and stimulate colonic functions. The somatostatin receptor (sst(1-5) ) agonist, ODT8-SST acts in the brain to inhibit stress-induced adrenocorticotropic hormone and epinephrine secretion. We investigated whether ODT8-SST acts in the brain to influence stress-related alterations of gastric and colonic motor function and sst receptor subtype(s) involved.

METHODS

Peptides were injected intracerebroventricularly (i.c.v.) under short isoflurane anesthesia and GE, fecal pellet output (FPO) and distal colonic motility monitored in conscious mice.

KEY RESULTS

The stress of acute anesthesia/vehicle i.c.v. injection reduced GE by 67% and increased defecation by 99% compared to non-injected controls. Both responses were abolished by ODT8-SST (1μg= 0.75nmol) or sst(1) agonist (0.65-1.95nmol). The sst(1) agonist (1.95nmol) also prevented the abdominal surgery-induced delayed GE. Octreotide (sst(2)  >sst(5)  > sst(3) ) and the sst(2) or sst(4) agonists (1μg=0.78 or 0.70nmol, respectively) injected i.c.v. did not influence FPO while i.c.v. somatostatin-28 mimicked ODT8-SST's effect. The ODT8-SST-induced increased food intake was inhibited by i.c.v. sst(2) antagonist while the reduced FPO was unchanged. ODT8-SST i.c.v. reduced distal colonic motility in semi-restrained mice compared with vehicle and blocked water avoidance- and i.c.v. CRF (0.5μg=0.09nmol)-induced stimulated FPO while a similar colonic secretomotor response to i.p. 5-hydroxytryptophane (10mgkg(-1) =36.4μmol kg(-1) ) was unaltered. Conclusions & Inferences  ODT8-SST counteracts stress/i.c.v. CRF-related stimulation of colonic motor function and delayed GE which can be reproduced mainly by activation of sst(1) receptors. These data opens new insight to brain somatostatinergic signaling pathways interfering with brain circuitries involved in gut motor responses to acute stress.

Cooperation between hippocampal somatostatin receptor subtypes 4 and 2: functional relevance in interactive memory systems

The hippocampal somatostatin (sst) receptor subtype 4 (sst(4)) modulates memory formation by diminishing hippocampus-based spatial function while enhancing striatum-dependent behaviors. sst(4)-mediated regulations on neuronal activity in the hippocampus appear to depend on both competitive and cooperative interactions with sst receptor subtype 2 (sst(2)). Here, we investigated whether interactions with sst(2) receptors are required for sst(4)-mediated effects on hippocampus-dependent spatial memory and striatum-dependent cued memory in a water maze paradigm. Competition was assessed in mice by intrahippocampal injections of the sst(4) agonist L-803,087 alone or combined with sst(2) agonists (L-779,976 or octreotide). Effects of L-803,087 were also tested in sst(2) knockout mice to assess for receptor cooperation. Finally, sst(2a) and sst(4) localizations within hippocampal subregions were analyzed by immunohistochemistry and expression levels of sst(2a) and sst(2b) were quantified by real-time qPCR. Hippocampal injections of L-803,087 impaired spatial memory but enhanced cued memory. The latter effect was lost not only in sst(2) knockout mice but also in the presence of sst(2) agonists, whereas the former effect remained unaffected by sst(2) agonists or gene deletion. Octreotide and L-779,976 did not yield memory effects but reduced swim velocity throughout the acquisition trials suggesting that stimulation of sst(2) affected motivation and/or anxiety. sst(2a) and sst(4) were respectively detected in the dentate gyrus (DG) and the CA1 subfield suggesting that their functional interactions are not mediated by direct receptor coupling. Hippocampus sst(2a) expression was 36-fold higher than sst(2b). Possible neural mechanisms and functional significances for interaction between memory systems in relationship with stress-induced changes in hippocampal functions are discussed.

Functional effects of somatostatin receptor 1 activation on synaptic transmission in the mouse hippocampus

Somatostatin-14 (SRIF) co-localizes with GABA in the hippocampus and regulates neuronal excitability. A role of SRIF in the control of hippocampal activity has been proposed, although the exact contribution of each SRIF receptor (sst(1)-sst(5)) in mediating SRIF action requires some clarification. We used hippocampal slices of wild-type and sst(1) knockout (KO) mice and selective pharmacological tools to provide conclusive evidence for a role of sst(1) in mediating SRIF inhibition of synaptic transmission. With single- and double-label immunohistochemistry, we determined the distribution of sst(1) in hippocampal slices and we quantified sst(1) colocalization with SRIF. With electrophysiology, we found that sst(1) activation with CH-275 inhibited both the NMDA- and the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-mediated responses. Results from sst(1) KO slices confirmed the specificity of CH-275 effects; sst(1) activation did not affect the inhibitory transmission which was in contrast increased by sst(4) activation with L-803,087 in both wild-type and sst(1) KO slices. The AMPA-mediated responses were increased by L-803,087. Functional interaction between sst(1) and sst(4) is suggested by the finding that their combined activation prevented the CH-275-induced inhibition of AMPA transmission. The involvement of pre-synaptic mechanisms in mediating inhibitory effects of sst(1) on excitatory transmission was demonstrated by the finding that CH-275 (i) increased the paired-pulse facilitation ratio, (ii) did not influence the AMPA depolarization in the presence of tetrodotoxin, and (iii) inhibited glutamate release induced by epileptiform treatment. We conclude that SRIF control of excitatory transmission through an action at sst(1) may represent an important contribution to the regulation of hippocampal activity.

alpha-Tocopherol decreases the somatostatin receptor-effector system and increases the cyclic AMP/cyclic AMP response element binding protein pathway in the rat dentate gyrus

Neuronal survival has been shown to be enhanced by alpha-tocopherol and modulated by cyclic AMP (cAMP). Somatostatin (SST) receptors couple negatively to adenylyl cyclase (AC), thus leading to decreased cAMP levels. Whether alpha-tocopherol can stimulate neuronal survival via regulation of the somatostatinergic system, however, is unknown. The aim of this study was to investigate the effects of alpha-tocopherol on the SST signaling pathway in the rat dentate gyrus. To that end, 15-week-old male Sprague-Dawley rats were treated daily for 1 week with (+)-alpha-tocopherol or vehicle and sacrificed on the day following the last administration. No changes in either SST-like immunoreactivity (SST-LI) content or SST mRNA levels were detected in the dentate gyrus as a result of alpha-tocopherol treatment. A significant decrease in the density of the SST binding sites and an increase in the dissociation constant, however, were detected. The lower SST receptor density in the alpha-tocopherol-treated rats correlated with a significant decrease in the protein levels of the SST receptor subtypes SSTR1-SSTR4, whereas the corresponding mRNA levels were unaltered. G-protein-coupled-receptor kinase 2 expression was decreased by alpha-tocopherol treatment. This vitamin induced a significant increase in both basal and forskolin-stimulated AC activity, as well as a decrease in the inhibitory effect of SST on AC. Whereas the protein levels of AC type V/VI were not modified by alpha-tocopherol administration, ACVIII expression was significantly enhanced, suggesting it might account for the increase in AC activity. In addition, this treatment led to a reduction in Gialpha1-3 protein levels and in Gi functionality. alpha-Tocopherol did not affect the expression of the regulator of G-protein signaling 6/7 (RGS6/7). Finally, alpha-tocopherol induced an increase in the levels of phosphorylated cAMP response element binding protein (p-CREB) and total CREB in the dentate gyrus. Since CREB synthesis and phosphorylation promote the survival of many cells, including neurons, whereas SST inhibits the cAMP-PKA pathway, which is known to be involved in CREB phosphorylation, the alpha-tocopherol-induced reduction of SSTR observed here might possibly contribute, via increased cAMP levels and CREB activity, to the mechanism by which this vitamin promotes the survival of newborn neurons in the dentate gyrus.

Hippocampal SSTR4 somatostatin receptors control the selection of memory strategies

RATIONALE

Somatostatin (SS14) has been implicated in various cognitive disorders, and converging evidence from animal studies suggests that SS14 neurons differentially regulate hippocampal- and striatal-dependent memory formation. Four SS14 receptor subtypes (SSTR1-4) are expressed in the hippocampus, but their respective roles in memory processes remain to be determined.

OBJECTIVES

In the present study, effects of selective SSTR1-4 agonists on memory formation were assessed in a water-maze task which can engage either hippocampus-dependent "place" and/or striatum-dependent "cue" memory formation.

MATERIALS AND METHODS

Mice received an intrahippocampal injection of one of each of the selective agonists and were then trained to locate an escape platform based on either distal cues (place memory) or a visible proximal cue (cue memory). Retention was tested 24 h later on probe trials aimed at identifying which memory strategy was preferentially retained.

RESULTS

Both SS14 and the SSTR4 agonist (L-803,087) dramatically impaired place memory formation in a dose-dependent manner, whereas SSTR1 (L-797,591), SSTR2 (L-779,976), or SSTR3 (L-796,778) agonists did not yield any behavioral effects. However, unlike SS14, the SSTR4 agonist also dose-dependently enhanced cue-based memory formation. This effect was confirmed in another striatal-dependent memory task, the bar-pressing task, where L-803,087 improved memory of the instrumental response, whereas SS14 was once again ineffective.

CONCLUSIONS

These data suggest that hippocampal SSTR4 are selectively involved in the selection of memory strategies by switching from the use of hippocampus-based multiple associations to the use of simple dorsal striatum-based behavioral responses. Possible neural mechanisms and functional implications are discussed.

Unaltered D1, D2, D4, and D5 dopamine receptor mRNA expression and distribution in the spinal cord of the D3 receptor knockout mouse

Dopamine (DA) acts through five receptor subtypes (D1-D5). We compared expression levels and distribution patterns of all DA mRNA receptors in the spinal cord of wild-type (WT) and loss of function D3 receptor knockout (D3KO) animals. D3 mRNA expression was increased in D3KO, but no D3 receptor protein was associated with cell membranes, supporting the previously reported lack of function. In contrast, mRNA expression levels and distribution patterns of D1, D2, D4, and D5 receptors were similar between WT and D3KO animals. We conclude that D3KO spinal neurons do not compensate for the loss of function of the D3 receptor with changes in the other DA receptor subtypes. This supports use of D3KO animals as a model to provide insight into D3 receptor dysfunction in the spinal cord.

Distribution and expression levels of somatostatin and somatostatin receptors in the ileum of normal and acutely Schistosoma mansoni-infected SSTR2 knockout/lacZ knockin mice

We recently described the widespread expression of somatostatin (SOM) receptors (SSTRs) in the non-inflamed and inflamed murine ileum. Surprisingly, no significant changes were observed in the SSTR2 expression during intestinal inflammation. These data, combined with several recent independent lines of investigation, raised some question about the long presumed central role of SSTR2 in the SOM-mediated effects in the physiological and pathological activity of the gastrointestinal (GI) tract. To further unravel the role of SSTR2 in GI physiology, we studied the expression of SOM and SSTRs in the normal and inflamed SSTR2 knockout/lacZ knockin (SSTR2(-/-)) ileum. The SSTR2(-/-) ileum was characterized by a widespread distribution of multiple SSTR subtypes in non-inflamed and inflamed conditions. Moreover, the absence of SSTR2 did not induce any compensatory effect in the distribution pattern or expression level of any of the other SSTR subtypes. In contrast, the amount of SOM mRNA was significantly lower in SSTR2(-/-) ileum than that in wild type animals. Quantitative analysis revealed a decreased number of SOM-expressing neurons in both enteric plexuses of the knockout animals, implying a possible link between the number of SOM-expressing enteric neurons and the expression of SSTR2 in the enteric nervous system. In conclusion, these data show that a reconsideration of the role of SSTR2 in the GI somatostatinergic effects is in order and further corroborate recent data on the role of other SSTR subtypes in the inflammatory effects of SOM during intestinal inflammation.

Null mutant mouse models of somatostatin and cortistatin, and their receptors

Somatostatin (somatotropin release inhibitory factor, SRIF) and the related cortistatin (CST) are multifunctional peptide molecules attributed with neurohormone, neurotransmitter/modulator, and autocrine/paracrine actions. The physiological responses of SRIF and CST are mediated by five widely distributed G protein-coupled receptors (sst1-5) which have been implicated in regulating numerous biological processes. Much of the information on the effects of somatostatin has been gained through pharmacological studies with analogs and antagonists. The possibility of targeted mutagenesis in the mouse has resulted, over the last 10 years, in the generation of mouse models which genetically lack somatostatin ligands or receptors. We will review here the mouse models generated, the studies undertaken with them, and what has been learned so far.

Amyloid precursor protein knockdown by siRNA impairs spontaneous alternation in adult mice

The cleavage-product of amyloid precursor protein (APP) constitutes the core component of plaques found in the brains of Alzheimer's disease (AD) patients. APP is ubiquitously expressed and its precise physiological functions remain unclear. This protein has been proposed to regulate synaptic function and processes underlying learning and memory. While APP knockout mice show behavioral impairments, these may occur due to early changes during development and/or due to abolition of APP function in adult. To investigate the acute effects of APP knockdown without involving developmental processes, APP expression was reduced using RNA interference in adult mouse brain. Small interfering RNAs (siRNAs) that down-regulated mouse APP protein levels (APP-siRNA) were identified using an APP plasmid-siRNA co-transfection assay in mouse NIH/3T3 fibroblast cells. Infusion of APP-siRNAs into the ventricular system for 2 weeks also down-regulated APP mRNA in mouse brain. Highest knockdown of APP mRNA levels was found in the CA2-CA3 regions of the hippocampus. Mice treated with the most active APP-siRNA showed a significant reduction in spontaneous alternation rate in the Y-maze, without effects on forelimb grip strength or locomotor activity. These data suggest that acute knockdown of APP in adult mouse brain impairs hippocampus-dependent spatial working memory.

The somatostatin sst1 receptor: an autoreceptor for somatostatin in brain and retina?

The sst1 receptor was the first of the 5 somatostatin receptors to be cloned by homology with the glucagon receptor 13 years ago. It is a 7-transmembrane domain G-protein-coupled receptor that is negatively coupled to adenylyl cyclase, but can also trigger other transduction pathways. The distribution of sst1 mRNA, immunolabeling, and radioligand binding are not entirely overlapping, but the recent availability of knockout (KO) mice and a (still limited) number of selective agonists/antagonists has increased our knowledge about this receptor. These new tools have helped to reveal a role for the sst1 receptor in hippocampal, hypothalamic, basal ganglia, and retinal functions. In at least the latter 3 structures, the sst1 receptor appears to act as an inhibitory autoreceptor located on somatostatin neurons, whereas in the hippocampus such a role is still based on circumstantial evidence.

Binding and functional properties of the novel somatostatin analogue KE 108 at native mouse somatostatin receptors

Clinically used somatostatin (SRIF) analogs, octreotide and lanreotide, act primarily by binding to SRIF receptor subtype 2 (sst2). In contrast, the recently described multiligand SOM230 binds with high affinity to sst(1-3) and sst5 and KE 108 is characterised as a high affinity ligand for all five SRIF receptors. In tumoural mouse corticotrophs (AtT-20 cells) and in mouse hippocampus, binding and functional features of KE 108 were examined and compared to SRIF-14, octreotide and SOM230. In AtT-20 cells, KE 108 bound with high affinity at [125I]LTT-SRIF-28-labelled sites similarly to SRIF-14, octreotide and SOM230. At the functional level, all four ligands increased guanosine-5'-O-(3-[35S]thio)-triphosphate binding and decreased cAMP accumulation or intracellular Ca2+ concentration through G(i/o) proteins. In hippocampal slices, KE 108, octreotide and SOM230 also bound with high affinity at [125I]LTT-SRIF-28-labelled sites similarly to SRIF-14, but KE 108, octreotide or SOM230 did not influence spontaneous epileptiform activity which was, in contrast, inhibited by SRIF-14. In conclusion, this study demonstrates that KE 108 has high affinity for native mouse SRIF receptors. Functionally, KE 108 mediates SRIF action at sst(2/5) in corticotrophs whereas it does not mimic the SRIF-induced inhibition of hippocampal excitation suggesting that the high potency and efficacy of a synthetic ligand to all known SRIF receptors may not reproduce entirely the effects of the natural SRIF.

Somatostatin receptors differentially affect spontaneous epileptiform activity in mouse hippocampal slices

Somatostatin-14 [somatotropin release-inhibiting factor (SRIF)] reduces hippocampal epileptiform activity but the contribution of its specific receptors (sst1-5) is poorly understood. We have focused on the role of sst1 and sst2 in mediating SRIF modulation of epilepsy using hippocampal slices of wild-type (WT) and sst1 or sst2 knockout (KO) mice. Recordings of epileptiform discharge induced by Mg2+ -free medium with 4-aminopyridine were performed from the CA3 region before and after the application of SRIF compounds. In WT mice, SRIF and the sst1 agonist CH-275 reduce epilepsy whereas sst1 blockade with its antagonist SRA-880 increases the bursting discharge. Activation of sst2 does not affect the bursting frequency unless its agonist octreotide is applied with SRA-880, indicating that sst1 masks sst2-mediated modulation of epilepsy. In sst1 KO mice: (i) the bursting frequency is lower than in WT; (ii) SRIF, CH-275 and SRA-880 are ineffective on epilepsy and (iii) octreotide is also devoid of effects, whereas blockade of sst2 with the antagonist D-Tyr8 Cyn 154806 increases the bursting frequency. In sst2 KO mice, the SRIF ligand effects are similar to those in WT. In the whole hippocampus of sst1 KO mice, sst2 mRNA, protein and binding are higher than in WT and reverse transcription-polymerase chain reaction of the CA3 subarea confirms an increase of the sst2 messenger. We conclude that sst1 mediates inhibitory actions of SRIF and that interactions between sst1 and sst2 may prevent sst2 modulation of epilepsy. We suggest that, in sst1 KO mice, activation of over-expressed sst2 reduces the bursting frequency, indicating that sst2 density represents the rate-limiting factor for ss(2-mediated modulation of epilepsy.

Cortistatin radioligand binding in wild-type and somatostatin receptor-deficient mouse brain

Cortistatin-14 (CST-14) is a recently discovered member of the somatostatin family of neuropeptides. It shares 11 of its 14 amino acids with somatostatin-14 (SRIF-14). In the present study, binding sites for cortistatin-14 in the mouse brain were examined and compared to those for somatostatin using iodinated cortistatin-14 and iodinated somatostatin-14. By in vitro receptor autoradiography, high densities of cortistatin-14 and somatostatin-14 specific binding sites were detected in the cortex, hippocampal formation, basolateral amygdala and medial habenula. Unlabeled 100 nM cortistatin-14 inhibited iodinated somatostatin-14 binding in the hippocampus, but not in the cortex or amygdaloid nuclei. In somatostatin receptor subtype-2 knock-out (KO) mice, autoradiographic iodinated somatostatin-14 binding was observed in the hippocampus and habenula but was removed in the cortex and amygdaloid nuclei, specific iodinated cortistatin-14 binding sites were found in the hippocampus, habenula and throughout the cortex. We conclude that the somatostatin receptor subtype-2 is responsible for somatostatin binding in cortical and amygdaloid regions and that cortistatin predominantly interacts with the same receptors as somatostatin.

The somatostatin receptor (sst1) modulates the release of somatostatin in the nucleus accumbens of the rat

The aim of the present study was to examine the function of the somatostatin receptor (sst(1)) in the nucleus accumbens (NAc) of the basal ganglia. Radioligand binding studies were performed in rats to assess the presence of the receptor, while in vivo microdialysis studies were performed to examine its role in somatostatin release. CH-275, which is selective for sst(1), MK-678, selective for sst(2) and L-803,087, selective for sst(4) receptors displaced [(125)I]-Tyr(11)-somatostatin specific binding in a concentration-dependent manner with IC(50) values of 75, 0.21 and 11 nM, respectively. Infusion of CH-275 (10(-5), 10(-6) or 10(-7) M) in the NAc of freely moving rats resulted in a decrease in somatostatin levels only at the concentration of 10(-5) M. This effect was reversed by 10(-5) M of the selective sst(1) antagonist SRA-880. The sst(1) agonist L-797,591 (10(-5) M) mimicked the effect of CH-275, while MK-678 and L-803,087 at the same concentration were unable to influence somatostatin levels. These results provide functional evidence to demonstrate that the sst(1) receptor modulates somatostatin release in the basal ganglia.

Somatostatin inhibits glutamate release from mouse cerebrocortical nerve endings through presynaptic sst2 receptors linked to the adenylyl cyclase-protein kinase A pathway

The effects of somatostatin (SRIF, somatotropin release inhibiting factor) on the release of glutamate have been investigated using superfused mouse cerebrocortical synaptosomes. SRIF-14 inhibited the K+ (12 mM)-evoked overflow of preaccumulated [3H]D-aspartate as well as that of endogenous glutamate. Cyanamid 154806, a selective sst2 receptor antagonist, but not BIM-23056, an antagonist at sst5 receptors, prevented the SRIF-14 effect. Octreotide and L779976, selective agonists at sst2 receptors, mimicked SRIF-14, whereas L797591, L796778, L803087 and L362855, selective agonists at sst1, sst3, sst4 and sst5 receptor subtypes, were inactive. Activation of sst2 receptors seems to involve inhibition of the adenylyl cyclase-protein kinase A pathway present in glutamatergic terminals since the adenylyl cyclase inhibitor MDL-12,330A and the protein kinase A inhibitor H89 prevented the K+-evoked [3H]D-aspartate overflow. Consistent with the involvement of adenylyl cyclase, depolarization with 12 mM K+ increased synaptosomal cyclic AMP (cAMP) content, while forskolin, an adenylyl cyclase activator, potentiated basal [3H]D-aspartate release in an octreotide-, MDL-12,330A- and H89-sensitive manner. To conclude, glutamatergic cerebrocortical nerve endings possess release-inhibiting sst2 receptors which represent potential targets for new drugs able to mitigate the effects of excessive glutamate transmission.

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.

Characterisation of [125I]-Tyr0DTrp8-somatostatin binding in sst1- to sst4- and SRIF-gene-invalidated mouse brain

Five somatostatin receptors (sst) have been cloned and mRNAs for the first four (sst1-4) are expressed in many brain regions. In the present work, we compared the distribution of the non-selective ligand [125I]-Tyr0-DTrp8-SRIF14 by autoradiography in 24 brain regions and pituitary in wild type, sst1- to sst4- or SRIF-gene invalidated (KO) mice. [125I]-Tyr0-DTrp8-SRIF14 binding was not significantly modified in sst1 KO mouse brain with the noticeable exception of the substantia nigra and only moderately decreased in pituitary. For sst2 KO mice, a general decrease (>75%) was observed in most regions, with the noticeable exception of the olfactory bulb and CA1 field of the hippocampus. SST3 KO brain displayed a decrease in binding in the external plexiform layer of the olfactory bulb only (-54%). For sst4 KO mice, [125I]-Tyr0-DTrp8-SRIF14 binding levels in the external plexiform (-35%) and glomerular (-39%) layers of the olfactory bulb as well as the hippocampus CA1 field (-68%) were significantly decreased. In SRIF KO mice, a significant increase in binding levels was observed in olfactory bulb, anterior olfactory nucleus, frontal cortex upper layers, lateral septum, CA1 field, zona incerta and lateral hypothalamus, substantia nigra, periaqueductal grey and parabrachial nucleus. Competition with selective ligands (CH275, octreotide or L-779,976, L-796,778, L-803,087, and octreotide or L-817,778, for sst1-5 receptors, respectively) was in accordance with these findings. Moreover, octreotide was still able to compete on residual [125I]-Tyr0-DTrp8-SRIF14 binding sites in sst2 KO pituitary. It is concluded that most [125I]-Tyr0-DTrp8-SRIF14 binding sites in mouse brain and pituitary belong to the sst2 subtype but for the olfactory bulb (sst3 and sst4 receptors), the CA1 of the hippocampus (sst4 receptors) and the pituitary (sst5 and sst1 receptors) in which other subtypes are also expressed. The overall increase in [125I]-Tyr0-DTrp8-SRIF14 binding in SRIF KO mice indicates that SRIF receptors, mostly from the sst2 subtype, are regulated by the endogenous ligand(s).

Pharmacological characterisation of native somatostatin receptors in AtT-20 mouse tumour corticotrophs

1. The mouse corticotroph tumour cell line AtT-20 is a useful model to investigate the physiological role of native somatostatin (SRIF, Somatotropin release inhibitory factor) receptor subtypes (sst(1) - sst(5)). The objective of this study was to characterise the pharmacological features and the functional effects of SRIF receptors expressed by AtT-20 cells using radioligand binding and cAMP accumulation. 2. [(125)I]LTT-SRIF-28, [(125)I]CGP 23996, [(125)I]Tyr(10)-cortistatin-14 and [(125)I]Tyr(3)-octreotide labelled SRIF receptor binding sites with high affinity and in a saturable manner (B(max)=315, 274, 239 and 206 fmol mg(-1), respectively). [(125)I]LTT-SRIF-28 labels significantly more sites than [(125)I]Tyr(10) -cortistatin-14 and [(125)I]Tyr(3) -octreotide as seen previously in cells expressing pure populations of sst(2) or sst(5) receptors. 3. SRIF analogues displaced the binding of the four radioligands. sst(2/5) receptor-selective ligands showed much higher affinity than sst(1/3/4) receptor-selective ligands. The binding profile of [(125)I]Tyr(3)-octreotide was different from that of [(125)I]LTT-SRIF-28, [(125)I]CGP 23996 and [(125)I]Tyr(10)-cortistatin-14. The sst(5/1) receptor-selective ligand L-817,818 identified two binding sites, one with subnanomolar affinity (sst(5) receptors) and one with micromolar affinity (sst(2) receptors); however, the proportions were different: 70 - 80% high affinity with [(125)I]LTT-SRIF-28, [(125)I]CGP 23996, [(125)I]Tyr(10)-cortistatin-14, but only 20% with [(125)I]Tyr(3)-octreotide. 4. SRIF analogues inhibited the forskolin-stimulated cAMP levels depending on concentration. sst(2/5) receptor-selective ligands were highly potent, whereas sst(1/3/4) receptor-selective ligands had no significant effects. The sst(2) receptor antagonist D-Tyr(8)-CYN 154806 competitively antagonised the effects of SRIF-14 and sst(2) receptor-preferring agonists, but not those of L-817,818. 5. The complex binding properties of SRIF receptor analogues indicate that sst(2) and sst(5) receptors are the predominant SRIF receptors expressed on AtT-20 cell membranes with no or only negligible presence of sst(1), sst(3) and sst(4) receptors. In the functional studies using cAMP accumulation, only sst(2) and sst(5) receptors appear to play a role. However, the "predominant" receptor appears to be the sst(2) receptor, although sst(5) receptors can also mediate the effect, when the ligand is not able to activate sst(2) receptors. This clearly adds flexibility to SRIF-mediated functional effects and suggests that the physiological role of SRIF and its analogues may be mediated preferentially via one subtype over another.

Somatostatin receptor 2 knockout/lacZ knockin mice show impaired motor coordination and reveal sites of somatostatin action within the striatum

The peptide somatostatin can modulate the functional output of the basal ganglia. The exact sites and mechanisms of this action, however, are poorly understood, and the physiological context in which somatostatin acts is unknown. Somatostatin acts as a neuromodulator via a family of five 7-transmembrane G protein-coupled receptors, SSTR1-5, one of which, SSTR2, is known to be functional in the striatum. We have investigated the role of SSTR2 in basal ganglia function using mice in which Sstr2 has been inactivated and replaced by the lacZ reporter gene. Analysis of Sstr2lacZ expression in the brain by beta-galactosidase histochemistry demonstrated a widespread pattern of expression. By comparison to previously published in situ hybridization and immunohistochemical data, Sstr2lacZ expression was shown to accurately recapitulate that of Sstr2 and thus provided a highly sensitive model to investigate cell-type-specific expression of Sstr2. In the striatum, Sstr2 expression was identified in medium spiny projection neurons restricted to the matrix compartment and in cholinergic interneurons. Sstr2 expression was not detected in any other nuclei of the basal ganglia except for a sparse number of nondopaminergic neurons in the substantia nigra. Microdialysis in the striatum showed Sstr2-null mice were selectively refractory to somatostatin-induced dopamine and glutamate release. In behavioural tests, Sstr2-null mice showed normal levels of locomotor activity and normal coordination in undemanding tasks. However, in beam-walking, a test of fine motor control, Sstr2-null mice were severely impaired. Together these data implicate an important neuromodulatory role for SSTR2 in the striatum.

Somatostatin binds to murine macrophages through two distinct subsets of receptors

Somatostatin (SRIF) exerts anti-inflammatory effects, in part by deactivating monocytes/macrophages. Thus, the objective of this study was to characterize specific receptors for SRIF on these cells. Macrophages isolated from mouse peritoneal cells bound [125I]Tyr(0), D-Trp(8) SRIF(14) specifically. Scatchard analysis of saturation binding data revealed two classes of binding sites with an affinity of 0.44+/-0.13 and 2.58+/-0.56 nM, respectively. By sensitive and specific RT-PCR, the mRNAs for the five SRIF receptors (SSTR1 to SSTR5) could be detected. Evidence for the involvement of SSTR1 and SSTR2 in the binding of SRIF to the high and low affinity sites, respectively, was obtained by the demonstration that (1) only SSTR1 and SSTR2 subtype-specific agonists were active in competing for [125I]Tyr(0), D-Trp(8) SRIF(14) binding to high and low affinity sites, respectively, and (2) [125I]Tyr(0), D-Trp(8) SRIF(14) bound to high but not low affinity sites on macrophages isolated from SSTR2 knock-out mice. In conclusion, we have identified and characterized two different SRIF receptor subtypes in murine macrophages.

Biological activity of somatostatin receptors in GC rat tumour somatotrophs: evidence with sst1-sst5 receptor-selective nonpeptidyl agonists

The physiological actions of somatostatin-14 (SRIF: somatotrophin release inhibitory factor) receptor subtypes (sst(1)-sst(5)), which are endogenously expressed in growth cells (GC cells), have not yet been elucidated, although there is evidence that sst(2) receptors are negatively coupled to cytosolic free Ca(2+) concentration ([Ca(2+)](i)) and adenosine 3,5'-cyclic monophosphate (cAMP) accumulation. In addition, both sst(1) and sst(2) receptors are negatively coupled to growth hormone (GH) secretion in GC cells. Here we report on studies concerning the expression, the pharmacology and the functional role of native SRIF receptors in GC cells with the use of five nonpeptidyl agonists, highly selective for each of the SRIF receptors. Radioligand binding studies show that sst(2) and sst(5) receptors are present at different relative densities, while the presence of sst(3) and sst(4) receptors appears to be negligible. The absence of sst(1) receptor binding was unexpected in view of sst(1) receptor functional effects on GH secretion. This suggests very efficient receptor-effector coupling of a low-density population of sst(1) receptors. Functionally, only sst(2) receptors are coupled to the inhibition of [Ca(2+)](i) and cAMP accumulation and the selective activation of sst(5) receptors facilitates the stimulation of adenylyl cyclase activity through G(i/o) proteins. This effect was not observed when sst(2) and sst(5) receptors were simultaneously activated, suggesting that there is a functional interaction between sst(2) and sst(5) receptors. In addition, sst(1), sst(2) and sst(5) receptor activation inhibits GH release, further indicating that SRIF can modulate GH secretion in GC cells through mechanisms both dependent and independent on [Ca(2+)](i) and cAMP-dependent pathways. The present data suggest SRIF-mediated functional effects in GC cells to be very diverse and provides compelling arguments to propose that multiple native SRIF receptors expressed in the same cells are not simply redundant, but contribute to marked signalling diversity.

Somatostatin receptor subtypes 2 and 4 affect seizure susceptibility and hippocampal excitatory neurotransmission in mice

We have investigated the role of somatostatin receptor subtypes sst2 and sst4 in limbic seizures and glutamate-mediated neurotransmission in mouse hippocampus. As compared to wild-type littermates, homozygous mice lacking sst2 receptors showed a 52% reduction in EEG ictal activity induced by intrahippocampal injection of 30 ng kainic acid (P < 0.05). The number of behavioural tonic-clonic seizures was reduced by 50% (P < 0.01) and the time to onset of seizures was doubled on average (P < 0.05). Seizure-associated neurodegeneration was found in the injected hippocampus (CA1, CA3 and hilar interneurons) and sporadically in the ipsilateral latero-dorsal thalamus. This occurred to a similar extent in wild-type and sst2 knock-out mice. Intrahippocampal injection of three selective sst2 receptor agonists in wild-type mice (Octreotide, BIM 23120 and L-779976, 1.5-6.0 nmol) did not affect kainate seizures while the same compounds significantly reduced seizures in rats. L-803087 (5 nmol), a selective sst4 receptor agonist, doubled seizure activity in wild-type mice on average. Interestingly, this effect was blocked by 3 nmol octreotide. It was determined, in both radioligand binding and cAMP accumulation, that octreotide had no direct agonist or antagonist action at mouse sst4 receptors expressed in CCl39 cells, up to micromolar concentrations. In hippocampal slices from wild-type mice, octreotide (2 micro m) did not modify AMPA-mediated synaptic responses while facilitation occurred with L-803087 (2 micro m). Similarly to what was observed in seizures, the effect of L-803087 was reduced by octreotide. In hippocampal slices from sst2 knock-out mice, both octreotide and L-803087 were ineffective on synaptic responses. Our findings show that, unlike in rats, sst2 receptors in mice do not mediate anticonvulsant effects. Moreover, stimulation of sst4 receptors in the hippocampus of wild-type mice induced excitatory effects which appeared to depend on the presence of sst2 subtypes, suggesting these receptors are functionally coupled.

Somatostatin sst2 receptor knock-out mice: localisation of sst1-5 receptor mRNA and binding in mouse brain by semi-quantitative RT-PCR, in situ hybridisation histochemistry and receptor autoradiography

The peptide hormone/neurotransmitter somatostatin (somatotropin release inhibiting factor; SRIF) and its receptors (sst(1)-sst(5)) appear to regulate many physiological functions in the CNS. Semi-quantitative analysis of the densities of mRNA expression for sst(1-5) receptors and SRIF receptor binding sites were established in sst(2) receptor knock-out (KO) mice. Patterns of sst(1-5) receptor mRNA expression were largely conserved for sst(1,3,4) and sst(5) selective oligonucleotide probes; whereas sst(2) signals were completely absent in KO mouse brain. Autoradiographic analysis demonstrated [(125)I]LTT SRIF(28), [(125)I]CGP 23996 (two radioligands known to label all five recombinant SRIF receptors) and [(125)I]Tyr(3)-octreotide (sst(2) and sst(5) receptor selective) binding in wild type (WT) mouse brain sections; yet no specific binding of [(125)I]Tyr(3)-octreotide in KO mice. In contrast, [(125)I]LTT SRIF(28) and [(125)I]CGP 23996 binding was still present in a number of brain areas in KO mice, although to a lesser degree than in those regions where [(125)I]Tyr(3)-octreotide binding was found, in WT animals. The present data suggest first, that both sst(2) receptor protein and mRNA were completely absent in the brain of these KO animals. Second, there was little evidence of compensatory regulation, at the mRNA level, of the other SRIF receptors as a consequence of the sst(2) KO. Third, the absence of any [(125)I]Tyr(3)-octreotide binding, in KO mice, suggests that this particular ligand is selective for the sst(2) receptor subtype (under the conditions utilised); or that sst(5) receptors are only marginally expressed in brain. Fourth, there were regions where the binding of [(125)I]LTT SRIF(28) and [(125)I]CGP 23996 were moderately affected by the sst(2) KO, suggesting that additional SRIF receptors may well contribute to the binding of the aforementioned radioligands. Finally, since the relative distribution of these two ligands were not entirely superimposable, it suggests that their respective selectivity profiles towards the different SRIF receptor subtypes in situ are not identical.