Compensatory changes in the hippocampus of somatostatin knockout mice: upregulation of somatostatin receptor 2 and its function in the control of bursting activity and synaptic transmission

Evaluation of a Novel DNA Vaccine Double Encoding Somatostatin and Cortistatin for Promoting the Growth of Mice

Animal growth traits are directly linked with the economics of livestock species. A somatostatin DNA vaccine has been developed to improve the growth of animals. However, the growth-promoting effect is still unsatisfying. The current study aimed to evaluate the effect of a novel eukaryotic dual expression vaccine known as pIRES-S/CST14-S/2SS, which encodes the genes obtained by fusing somatostatin (SS) and cortistatin (CST) into hepatitis B surface antigen (HBsAg). After transfection into GH3 cells with pIRES-S/CST14-S/2SS, green fluorescence signals were observed by fluorescence microscopy, suggesting the effective expression of CST and SS in GH3 cells using the IRES elements. Subsequently, both GH and PRL levels were found to be significantly lower in pIRES-S/CST14-S/2SS-treated cells as compared to the control group (p < 0.05). Furthermore, the antibody level, hormone secretion, and weight gain in the mice injected with novel recombinant plasmids were also evaluated. The anti-SS antibodies were detectable in all vaccine treated groups, resulting in significantly higher levels of GH secretion (p < 0.05). It is worth mentioning that pIRES-S/CST14-S/2SS (10 μg/100 μL) vaccinated mice exhibited a higher body weight gain in the second immunization period. This study increases the understanding of the relationship between somatostatin and cortistatin, and may help to develop an effective growth-promoting DNA vaccine in animals.

Somatostatin, a Presynaptic Modulator of Glutamatergic Signal in the Central Nervous System

Somatostatin is widely diffused in the central nervous system, where it participates to control the efficiency of synaptic transmission. This peptide mainly colocalizes with GABA, in inhibitory, GABA-containing interneurons from which it is actively released in a Ca²⁺ dependent manner upon application of depolarizing stimuli. Once released in the synaptic cleft, somatostatin acts locally, or it diffuses in the extracellular space through "volume diffusion", a mechanism(s) of distribution which mainly operates in the cerebrospinal fluid and that assures the progression of neuronal signalling from signal-secreting sender structures towards receptor-expressing targeted neurons located extrasynaptically, in a non-synaptic, inter-neuronal form of communication. Somatostatin controls the efficiency of central glutamate transmission by either modulating presynaptically the glutamate exocytosis or by metamodulating the activity of glutamate receptors colocalized and functionally coupled with somatostatin receptors in selected subpopulations of nerve terminals. Deciphering the role of somatostatin in the mechanisms of "volume diffusion" and in the "receptor-receptor interaction" unveils new perspectives in the central role of this fine tuner of synaptic strength, paving the road to new therapeutic approaches for the cure of central disorders.

Neuroprotective Peptides in Retinal Disease

In the pathogenesis of many disorders, neuronal death plays a key role. It is now assumed that neurodegeneration is caused by multiple and somewhat converging/overlapping death mechanisms, and that neurons are sensitive to unique death styles. In this respect, major advances in the knowledge of different types, mechanisms, and roles of neurodegeneration are crucial to restore the neuronal functions involved in neuroprotection. Several novel concepts have emerged recently, suggesting that the modulation of the neuropeptide system may provide an entirely new set of pharmacological approaches. Neuropeptides and their receptors are expressed widely in mammalian retinas, where they exert neuromodulatory functions including the processing of visual information. In multiple models of retinal diseases, different peptidergic substances play neuroprotective actions. Herein, we describe the novel advances on the protective roles of neuropeptides in the retina. In particular, we focus on the mechanisms by which peptides affect neuronal death/survival and the vascular lesions commonly associated with retinal neurodegenerative pathologies. The goal is to highlight the therapeutic potential of neuropeptide systems as neuroprotectants in retinal diseases.

Molecular and Cellular Mechanisms Underlying Somatostatin-Based Signaling in Two Model Neural Networks, the Retina and the Hippocampus

Neural inhibition plays a key role in determining the specific computational tasks of different brain circuitries. This functional "braking" activity is provided by inhibitory interneurons that use different neurochemicals for signaling. One of these substances, somatostatin, is found in several neural networks, raising questions about the significance of its widespread occurrence and usage. Here, we address this issue by analyzing the somatostatinergic system in two regions of the central nervous system: the retina and the hippocampus. By comparing the available information on these structures, we identify common motifs in the action of somatostatin that may explain its involvement in such diverse circuitries. The emerging concept is that somatostatin-based signaling, through conserved molecular and cellular mechanisms, allows neural networks to operate correctly.

Roles of Hippocampal Somatostatin Receptor Subtypes in Stress Response and Emotionality

Altered brain somatostatin functions recently appeared as key elements for the pathogenesis of stress-related neuropsychiatric disorders. The hippocampus exerts an inhibitory feedback on stress but the mechanisms involved remain unclear. We investigated herein the role of hippocampal somatostatin receptor subtypes in both stress response and behavioral emotionality using C57BL/6, wild type and sst₂ or sst₄ knockout mice. Inhibitory effects of hippocampal infusions of somatostatin agonists on stress-induced hypothalamo-pituitary-adrenal axis (HPA) activity were tested by monitoring peripheral blood and local hippocampus corticosterone levels, the latter by using microdialysis. Anxiolytic and antidepressant-like effects were determined in the elevated-plus maze, open field, forced swimming, and stress-sensitive beam walking tests. Hippocampal injections of somatostatin analogs and sst₂ or sst_4, but not sst₁ or sst₃ receptor agonists produced rapid and sustained inhibition of HPA axis. sst₂ agonists selectively produced anxiolytic-like behaviors whereas both sst₂ and sst₄ agonists had antidepressant-like effects. Consistent with these findings, high corticosterone levels and anxiety were found in sst₂KO mice and depressive-like behaviors observed in both sst₂KO and sst₄KO strains. Both hippocampal sst₂ and sst₄ receptors selectively inhibit stress-induced HPA axis activation but mediate anxiolytic and antidepressive effects through distinct mechanisms. Such results are to be accounted for in development of pathway-specific somatostatin receptor agents in the treatment of hypercortisolism (Cushing's disease) and stress-related neuropsychiatric disorders.

Somatostatin, neuronal vulnerability and behavioral emotionality

Somatostatin (SST) deficits are common pathological features in depression and other neurological disorders with mood disturbances, but little is known about the contribution of SST deficits to mood symptoms or causes of these deficits. Here we show that mice lacking SST (Sst(KO)) exhibit elevated behavioral emotionality, high basal plasma corticosterone and reduced gene expression of Bdnf, Cortistatin and Gad67, together recapitulating behavioral, neuroendocrine and molecular features of human depression. Studies in Sst(KO) and heterozygous (Sst(HZ)) mice show that elevated corticosterone is not sufficient to reproduce the behavioral phenotype, suggesting a putative role for Sst cell-specific molecular changes. Using laser capture microdissection, we show that cortical SST-positive interneurons display significantly greater transcriptome deregulations after chronic stress compared with pyramidal neurons. Protein translation through eukaryotic initiation factor 2 (EIF2) signaling, a pathway previously implicated in neurodegenerative diseases, was most affected and suppressed in stress-exposed SST neurons. We then show that activating EIF2 signaling through EIF2 kinase inhibition mitigated stress-induced behavioral emotionality in mice. Taken together, our data suggest that (1) low SST has a causal role in mood-related phenotypes, (2) deregulated EIF2-mediated protein translation may represent a mechanism for vulnerability of SST neurons and (3) that global EIF2 signaling has antidepressant/anxiolytic potential.

Circadian modulation of anxiety: a role for somatostatin in the amygdala

Pharmacological evidence suggests that the neuropeptide somatostatin (SST) exerts anxiolytic action via the amygdala, but findings concerning the putative role of endogenous SST in the regulation of emotional responses are contradictory. We hypothesized that an endogenous regulation of SST expression over the course of the day may determine its function and tested both SST gene expression and the behavior of SST knock out (SST^-/-) mice in different aversive tests in relation to circadian rhythm. In an open field and a light/dark avoidance test, SST^-/- mice showed significant hyperactivity and anxiety-like behavior during the second, but not during the first half of the active phase, failing to show the circadian modulation of behavior that was evident in their wild type littermates. Behavioral differences occurred independently of changes of intrinsically motivated activity in the home cage. A circadian regulation of SST mRNA and protein expression that was evident in the basolateral complex of the amygdala of wild type mice may provide a neuronal substrate for the observed behavior. However, fear memory towards auditory cue or the conditioning context displayed neither a time- nor genotype-dependent modulation. Together this indicates that SST, in a circadian manner and putatively via its regulation of expression in the amygdala, modulates behavior responding to mildly aversive conditions in mice.

Somatostatin receptor types 1 and 2 in the developing mammalian cochlea

The neuropeptide somatostatin (SST) exerts several important physiological actions in the adult central nervous system through interactions with membrane-bound receptors. Transient expression of SST and its receptors has been described in several brain areas during early ontogeny. It is therefore believed that SST may play a role in neural maturation. The present study provides the first evidence for the developmental expression of SST receptors in the mammalian cochlea, emphasizing their possible roles in cochlear maturation. In the developing mouse cochlea, cells immunoreactive to somatostatin receptor 1 (SSTR1) and somatostatin receptor 2 (SSTR2) were located in the embryonic cochlear duct on Kolliker's organ as early as embryonic day (E) 14 (E14). At E17, the expression of both receptors was high and already located at the hair cells and supporting cells along the length of the cochlear duct, which have become arranged into the characteristic pattern for the organ of Corti (OC) at this stage. At birth, SSTR1- and SSTR2-containing cells were only localized in the OC. In general, immunoreactivity for both receptors increased in the mouse cochlea from postnatal day (P) 0 (P0) to P10; the majority of immunostained cells were inner hair cells, outer hair cells, and supporting cells. Finally, a peak in the mRNA and protein expression of both receptors is present near the time when they respond to physiological hearing (i.e., hearing of airborne sound) at P14. At P21, SSTR1 and SSTR2 levels decrease dramatically. A similar developmental pattern was observed for SSTR1 and SSTR2 mRNA, suggesting that the expression of the SSTR1 and SSTR2 genes is controlled at the transcriptional level throughout development. In addition, we observed reduced levels of phospho-Akt and total Akt in SSTR1 knockout and SSTR1/SSTR2 double-knockout mice compared with wild-type mice. We know from previous studies that Akt is involved in hair cell survival. Taken together, the dynamic nature of SSTR1 and SSTR2 expression at a time of major developmental changes in the cochlea suggests that SSTR1 and SSTR2 (and possibly other members of this family) are involved in the maturation of the mammalian cochlea.

Somatostatin modulates generation of inspiratory rhythms and determines asphyxia survival

Breathing and the activity of its generator (the pre-Bötzinger complex; pre-BötC) are highly regulated functions. Among neuromodulators of breathing, somatostatin (SST) is unique: it is synthesized by a subset of glutamatergic pre-BötC neurons, but acts as an inhibitory neuromodulator. Moreover, SST regulates breathing both in normoxic and in hypoxic conditions. Although it has been implicated in the neuromodulation of breathing, neither the locus of SST modulation, nor the receptor subtypes involved have been identified. In this study, we aimed to fill in these blanks by characterizing the SST-induced regulation of inspiratory rhythm generation in vitro and in vivo. We found that both endogenous and exogenous SST depress all preBötC-generated rhythms. While SST abolishes sighs, it also decreases the frequency and increases the regularity of eupnea and gasping. Pharmacological experiments showed that SST modulates inspiratory rhythm generation by activating SST receptor type-2, whose mRNA is abundantly expressed in the pre-Bötzinger complex. In vivo, blockade of SST receptor type-2 reduces gasping amplitude and consequently, it precludes auto-resuscitation after asphyxia. Based on our findings, we suggest that SST functions as an inhibitory neuromodulator released by excitatory respiratory neurons when they become overactivated in order to stabilize breathing rhythmicity in normoxic and hypoxic conditions.

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.

Methods for lymphatic vessel culture and gene transfection

OBJECTIVE

To develop the techniques needed for the specific gene/protein targeting transfection experiments in isolated lymphatic vessels, we completed two major tasks: 1) optimize the experimental conditions to maintain the viability of isolated rat lymphatic vessels in culture for sufficiently long periods of time to permit knockdown or overexpression of selected proteins/genes and 2) develop effective transfection protocols for lymphatic muscle and endothelial cells in intact lymphatic vessels without nonspecific impairment of lymphatic contractile function due to the transfection protocol itself.

METHODS

Experimental protocols were developed for the maintenance of isolated lymphatic vessels under nonpressurized and pressurized conditions for 3-12 days in culture and for adenoviral gene transfection of the lymphatic muscle and endothelial cells.

RESULTS

The data demonstrate the effectiveness of the newly developed experimental protocols for the maintenance of isolated rat mesenteric lymphatic vessels and thoracic duct in culture up to 3-12 days without significant impairment of the parameters of their pumping and effective adenoviral/GFP transfection of lymphatic endothelial and muscle cells in isolated rat mesenteric lymphatic vessels.

CONCLUSIONS

These experimental techniques will extend the set of the modern experimental tools available to researchers investigating the physiology of lymphatic function.

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.

Somatostatin inhibition of gonadotropin-releasing hormone neurons in female and male mice

Previous studies indicate that somatostatin regulates gonadotropin secretion. We investigated here whether somatostatin has direct effects on GnRH neurons in the adult male and female mice. Dual-labeling immunofluorescence experiments revealed the presence of somatostatin-immunoreactive fibers adjacent to GnRH neurons, and three-dimensional confocal reconstructions demonstrated apparent somatostatin fiber appositions with 50-60% of GnRH neurons located throughout the brain in both male and female mice. Perforated patch-clamp recordings from GnRH-green fluorescent protein neurons revealed that approximately 70% of GnRH neurons responded in a dose-dependent manner to 10-300 nm somatostatin with an acute membrane hyperpolarization and cessation of firing. This effect persisted in the presence of tetrodotoxin and amino acid receptor antagonists, indicating a direct postsynaptic site of action on the GnRH neuron. The identity of the somatostatin receptors underlying this action was assessed using GnRH neuron single-cell RT-PCR. Of the somatostatin receptor subtypes, the sstr2 transcript was the most prevalent and detected in both males and females. The expression of sstr2 by GnRH neurons was confirmed in the sstr2 knockout/LacZ knock-in mouse line. Electrophysiological studies demonstrated that the sstr2-selective agonist seglitide exerted acute hyperpolarizing actions on GnRH neurons identical to those of somatostatin. Together, these studies reveal somatostatin, acting through sstr2, to be one of the most potent inhibitors of electrical excitability of male and female GnRH neurons identified thus far.

Relative expression of mRNA for the somatostatin receptors in the caudate putamen of C57BL/6J and 129P3/J mice: strain and heroin effects

Using real time qPCR, we examined the expression of mRNAs for the five somatostatin receptors (SSTRs) in the caudate putamen of male C57BL/6J and 129P3/J mice. Animals were exposed to multiple injections of heroin, or saline, in the setting of a conditioned place preference study. The relative expression levels of the five SSTR mRNAs differed between the two strains. In both strains, SSTR-1 mRNA was expressed at the highest levels and SSTR-5 at the lowest. Interestingly, in 129P3/J mice SSTR-3 mRNA was not detected in the caudate putamen. We confirmed this finding in the frontal cortex, hypothalamus, nucleus accumbens and a region containing the substantia nigra and ventral tegmental area. We also found strain differences in the mRNA levels of SSTR-2 and -4. Intermittent heroin administration had a dose-dependent effect on the levels of SSTR-1 and -3 mRNAs. These results demonstrate strain differences in the expression of specific mRNAs and a heroin-induced dose-dependent elevation of SSTR-1 and -3 mRNAs in the mouse caudate putamen.

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.

Constitutive somatostatin receptor activity determines tonic pituitary cell response

Somatostatin (SRIF) binds G protein-coupled SRIF receptor subtypes (SST1, -2, -3, -4, and -5) to regulate cell secretion and proliferation. Hypothalamic SRIF inhibits pituitary growth hormone, thyroid stimulating hormone, and ACTH secretion. We tested SRIF-independent constitutive SST activity in AtT20 mouse pituitary corticotroph cells in which ACTH secretion is highly sensitive to SRIF action. Stable transfectants expressing SST2 or SST5 were sensitized to selective agonist action, and constitutive SST receptor activity was demonstrated by forskolin and pertussis toxin cAMP cell responses. Persistent constitutive SST activity decreased cell ACTH responses to CRH through decreased expression of CRH receptor subtype 1. Decreased dopamine receptor type 1 expression was associated with attenuated dopamine agonist action, whereas responses to isoproterenol were enhanced through increased beta2-adrenoreceptor expression. Thus, integrated pituitary cell ACTH regulation is determined both by phasic SRIF action, as well as by tonic constitutive SST activity, independently of SRIF.

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.

The expression of somatostatin receptors in the hippocampus of pilocarpine-induced rat epilepsy model

During the course of this study, we sought examine whether the expression of somatostatin receptors (SSTRs) is altered in the hippocampus following pilocarpine-induced status epilepticus (SE) in order to understand the role/function of SSTRs in the hippocampus after epileptogenic insults. SSTR1 and SSTR4 immunoreactivities were increased in the hippocampus at 1 week after SE. At 4 weeks after SE, SRIF1-family (SSTR 2A, SSTR2B, and SSTR5) immunoreactivity was increased only in neuropil. Both SSTR2A and 2B immunoreactivities were increased in CA2-3 pyramidal cells. However, SSTR3 and SSTR4 immunoreactivities were reduced in the CA1 pyramidal cells of epileptic rat due to neuronal loss. In addition, SSTR5 immunoreactivity was reduced in CA2 pyramidal cells and various interneurons. Both SSTR2B and SSTR4 immunoreactivities were increased within microglia following SE. Our findings suggest that increases in neuron-glial SSTR expressions may be closely related to the enhanced inhibition of the dentate gyrus and regulation of reactive microgliosis in the hippocampus of a pilocarpine model of temporal lobe epilepsy.

Involvement of the cAMP-dependent pathway in the reduction of epileptiform bursting caused by somatostatin in the mouse hippocampus

The cyclic AMP pathway is major signal transduction system involved in hippocampal neurotransmission. Recently, the peptide somatostatin-14 (SRIF) has emerged as a key signal that, by activating its receptors, inhibits epileptiform bursting in the mouse hippocampus. Little is known on transduction mechanisms, which may mediate SRIF function in native cell/tissues. Using a well-established model of epileptiform activity induced by Mg(2+)-free medium with 4-aminopyridine [0 Mg(2+)/4-aminopyridine (4-AP)] in mouse hippocampal slices, we demonstrated that protein kinase A (PKA)-related signaling is upregulated by hippocampal bursting and that treatment with SRIF normalizes this upregulation. We also demonstrated that the SRIF-induced inhibition of PKA impairs phosphorylation of the NMDA receptor subunit NR1. Extracellular recordings of the 0 Mg(2+)/4-AP-induced hippocampal discharge from the CA3 region demonstrated that treating slices with compounds, which interfere with PKA activity, prevent SRIF inhibition of epileptiform bursting. Our results suggest that SRIF modulation of hippocampal activity may involve PKA-related signaling.

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.

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.

Somatostatinergic systems in brain: networks and functions

Somatostatin is abundantly expressed in mammalian brain. The peptide binds with high affinity to six somatostatin receptors, sst1, sst2A and B, sst3 to 5, all belonging to the G-protein-coupled receptor family. Recent advances in the neuroanatomy of somatostatin neurons and cellular distribution of sst receptors shed light on their functional roles in the neuronal network. Beside their initially described neuroendocrine role, somatostatin systems subserve neuromodulatory roles in the brain, influencing motor activity, sleep, sensory processes and cognitive functions, and are altered in brain diseases like affective disorders, epilepsia and Alzheimer's disease.

Role of endogenous somatostatin in regulating GH output under basal conditions and in response to metabolic extremes

Somatostatin (SST) was first described over 30 years ago as a hypothalamic neuropeptide which inhibits GH release. Since that time a large body of literature has accumulated describing how endogenous SST mediates its effects on GH-axis function under normal conditions and in response to metabolic extremes. This review serves to summarize the key findings in this field with a focus on recent progress, much of which has been made possible by the availability of genetically engineered mouse models and SST receptor-specific agonists.

Endocrine actions of cortistatin: in vivo studies

Cortistatin (CST) shares high structural homology with somatostatin (SST) and binds all SST-receptors (SST-R) subtypes with similar affinity. However, CST actions, tissue expression patterns and regulation do not fully overlap with those of SST, and, moreover, CST, but not SST, also binds and activates proadrenomedullin N-terminal peptide receptor (MrgX2) and shows binding affinity to ghrelin receptor (GHS-R1a). Several studies performed to clarify the endocrine actions of CST, compared with SST, showed that, in humans, CST and SST share the same endocrine actions, i.e. inhibition of GH and insulin secretion in physiological conditions and in acromegaly. A similar inhibitory effect on PRL and ACTH secretion was shown in acromegaly, prolactinoma or in Cushing's disease. This identity of endocrine actions by CST and SST suggests that SST-R activation by CST overrides any other independent action of this peptide mediated by other receptors. Thus, in terms of endocrine actions, CST can well be considered a natural alternative to SST.

Role of the somatostatin system in contextual fear memory and hippocampal synaptic plasticity

Somatostatin has been implicated in various cognitive and emotional functions, but its precise role is still poorly understood. Here, we have made use of mice with somatostatin deficiency, based upon genetic invalidation or pharmacologically induced depletion, and Pavlovian fear conditioning in order to address the contribution of the somatostatin system to associative fear memory. The results demonstrate an impairment of foreground and background contextual but not tone fear conditioning in mice with targeted ablation of the somatostatin gene. These deficits were associated with a decrease in long-term potentiation in the CA1 area of the hippocampus. Both the behavioral and the electrophysiological phenotypes were mimicked in wild-type mice through application of the somatostatin-depleting substance cysteamine prior to fear training, whereas no further deficits were observed upon application in the somatostatin null mutants. These results suggest that the somatostatin system plays a critical role in the acquisition of contextual fear memory, but not tone fear learning, and further highlights the role of hippocampal synaptic plasticity for information processing concerning contextual information.

Use of preproenkephalin knockout mice and selective inhibitors of enkephalinases to investigate the role of enkephalins in various behaviours

RATIONALE

The most simple and efficient method to study the physiological role of enkephalins is to increase the lifetime of these endogenous opioid peptides by inhibiting their inactivating enzymes. Enkephalins are degraded by the concomitant action of two metallopeptidases: neutral endopeptidase (NEP, EC3.4.21.11) and aminopeptidase N (APN, EC3.4.11.2), both enzymes releasing inactive metabolites.

OBJECTIVES

Potent dual inhibitors have been developed, such as RB101. However, NEP and APN have a broad specificity and can cleave various peptides in vitro. Therefore, it was essential to investigate the specific involvement of enkephalins in the various pharmacological responses induced by dual inhibitors.

MATERIALS AND METHODS

We compared the pharmacological responses induced by RB101 in wild-type and preproenkephalin-deficient mice (Penk1-/-) using several behavioural assays.

RESULTS

In all the tests used (hot plate test, force swim test, castor-oil-induced diarrhoea), RB101 induced strong effects in wild-type animals, whereas slight effects were observed in Penk1-/- animals. These residual effects are blocked by pre-administration of the opioid antagonist naloxone, supporting the involvement of the opioid receptors in the responses observed.

CONCLUSIONS

The pharmacological effects induced by dual inhibitors acting on both NEP and APN are mainly due to the protection of the endogenous enkephalins at supraspinal and peripheral levels. It could be speculated that the residual effects observed in Penk1-/- mice after RB101 administration could be due to the direct action of other opioid peptides or through an indirect effect involving the protection of other peptide substrates of NEP or APN, as substance P or angiotensin.

Brain-gut communication: cortistatin, somatostatin and ghrelin

Although cortistatin (CST) shares great structural homology with somatostatin (SST) and binds to all SST receptor subtypes with similar affinity, these neurohormones have divergent biological roles, as evidenced by their different patterns of tissue expression and biological actions. Moreover, CST, but not SST, can bind to the proadrenomedullin N-terminal peptide (PAMP) receptor MrgX2 and type 1a growth hormone secretagogue (GHS) receptor (GHSR-1a), also known as the 'ghrelin' receptor. These findings suggest that CST-specific actions could be mediated by the GHSR-1a and CST might represent a link between the ghrelin and the SST systems. Here, we review the data leading to this working hypothesis and discuss the in vitro, in vivo and clinical implications of potential SST-receptor-independent, GHSR-1a-mediated neuroendocrine and metabolic effects of CST.

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.

Conditional brain-specific knockdown of MAPK using Cre/loxP regulated RNA interference

In the last years, RNA interference (RNAi)-mediated gene knockdown has developed into a routine method to assess gene function in cultured mammalian cells in a fast and easy manner. For the use of this technique in developing or adult mice, short hairpin (sh)RNA vectors expressed stably from the genome are a faster alternative to conventional knockout approaches. Here we describe an advanced strategy for conditional gene knockdown in mice, where we used the Cre/loxP system to activate RNAi in a time and tissue dependent manner in the adult mouse brain. By placing conditional RNAi constructs into the defined genomic Rosa26 locus and by using recombinase mediated cassette exchange (RMCE) instead of laborious homologous recombination, we developed a fast, easy and reproducible approach to assess gene function in adult mice. We applied this technique to three genes of the MAPK signaling pathway—Braf, Mek1 and Mek2—and demonstrate here the potential of this new tool in mouse mutagenesis.

Expression, pharmacology, and functional role of somatostatin receptor subtypes 1 and 2 in human macrophages

Somatostatin (SRIF)-14 is recognized as an important mediator between the nervous and the immune system, although the functional role of its receptors (sst(1)-sst(5)) is poorly understood in humans. In our study, we demonstrate that human macrophages, differentiated from PBMC-derived monocytes, express sst(1) and sst(2) mRNAs. sst(1) and sst(2) are mostly localized at the cell surface and display active binding sites. In particular, sst(1)/sst(2) activation results in a weak internalization of sst(1), and the sst(2) internalization appears more efficient. At the functional level, the activation of SRIF receptors by the multiligand analogs SOM230 and KE108, but not by SRIF-14 or cortistatin-14, reduces macrophage viability. Their effects are mimicked by the selective activation of sst(1) and sst(2) using CH-275 and SMS 201-995/L-779,976, respectively. Further, sst(1)- and sst(2)-mediated effects are reversed by the sst(1) antagonist SRA-880 or the sst(2) antagonist CYN 154806, respectively. CH-275, SMS 201-995, and L-779,976, but not SRIF-14, decrease mRNA expression and secretion of the MCP-1. In addition, SRIF-14, CH-275, SMS 201-995, and L-779,976 decrease IL-8 secretion, and they do not affect IL-8 mRNA expression. In contrast, SRIF-14 and sst(1)/sst(2) agonists do not affect the secretion of matrix metalloproteinase-9. Collectively, our results suggest that the SRIF system, through sst(1) and sst(2), exerts mainly an immunosuppressive effect in human macrophages and may, therefore, represent a therapeutic window that can be exploited for the development of new strategies in pharmacological therapy of inflammation.