Somatostatin receptors in the central nervous system

Somatostatin receptors (SSTR1-5) on inhibitory interneurons in the barrel cortex

Inhibitory interneurons in the cerebral cortex contain specific proteins or peptides characteristic for a certain interneuron subtype. In mice, three biochemical markers constitute non-overlapping interneuron populations, which account for 80–90% of all inhibitory cells. These interneurons express parvalbumin (PV), somatostatin (SST), or vasoactive intestinal peptide (VIP). SST is not only a marker of a specific interneuron subtype, but also an important neuropeptide that participates in numerous biochemical and signalling pathways in the brain via somatostatin receptors (SSTR1-5). In the nervous system, SST acts as a neuromodulator and neurotransmitter affecting, among others, memory, learning, and mood. In the sensory cortex, the co-localisation of GABA and SST is found in approximately 30% of interneurons. Considering the importance of interactions between inhibitory interneurons in cortical plasticity and the possible GABA and SST co-release, it seems important to investigate the localisation of different SSTRs on cortical interneurons. Here, we examined the distribution of SSTR1-5 on barrel cortex interneurons containing PV, SST, or VIP. Immunofluorescent staining using specific antibodies was performed on brain sections from transgenic mice that expressed red fluorescence in one specific interneuron subtype (PV-Ai14, SST-Ai14, and VIP-Ai14 mice). SSTRs expression on PV, SST, and VIP interneurons varied among the cortical layers and we found two patterns of SSTRs distribution in L4 of barrel cortex. We also demonstrated that, in contrast to other interneurons, PV cells did not express SSTR2, but expressed other SSTRs. SST interneurons, which were not found to make chemical synapses among themselves, expressed all five SSTR subtypes.

Cellular Mechanisms for Antinociception Produced by Oxytocin and Orexins in the Rat Spinal Lamina II-Comparison with Those of Other Endogenous Pain Modulators

Much evidence indicates that hypothalamus-derived neuropeptides, oxytocin, orexins A and B, inhibit nociceptive transmission in the rat spinal dorsal horn. In order to unveil cellular mechanisms for this antinociception, the effects of the neuropeptides on synaptic transmission were examined in spinal lamina II neurons that play a crucial role in antinociception produced by various analgesics by using the whole-cell patch-clamp technique and adult rat spinal cord slices. Oxytocin had no effect on glutamatergic excitatory transmission while producing a membrane depolarization, γ-aminobutyric acid (GABA)-ergic and glycinergic spontaneous inhibitory transmission enhancement. On the other hand, orexins A and B produced a membrane depolarization and/or a presynaptic spontaneous excitatory transmission enhancement. Like oxytocin, orexin A enhanced both GABAergic and glycinergic transmission, whereas orexin B facilitated glycinergic but not GABAergic transmission. These inhibitory transmission enhancements were due to action potential production. Oxytocin, orexins A and B activities were mediated by oxytocin, orexin-1 and orexin-2 receptors, respectively. This review article will mention cellular mechanisms for antinociception produced by oxytocin, orexins A and B, and discuss similarity and difference in antinociceptive mechanisms among the hypothalamic neuropeptides and other endogenous pain modulators (opioids, nociceptin, adenosine, adenosine 5’-triphosphate (ATP), noradrenaline, serotonin, dopamine, somatostatin, cannabinoids, galanin, substance P, bradykinin, neuropeptide Y and acetylcholine) exhibiting a change in membrane potential, excitatory or inhibitory transmission in the spinal lamina II neurons.

Sustained somatostatin gene expression reverses kindling-induced increases in the number of dividing Type-1 neural stem cells in the hippocampi of behaviorally responsive rats

Neurogenesis persists throughout life in the hippocampi of all mammals, including humans. In the healthy hippocampus, relatively quiescent Type-1 neural stem cells (NSCs) can give rise to more proliferative Type-2a neural progenitor cells (NPCs), which generate neuronal-committed Type-2b NPCs that mature into Type-3 neuroblasts. Many Type-3 neuroblasts survive and mature into functionally integrated granule neurons over several weeks. In kindling models of epilepsy, neurogenesis is drastically upregulated and many new neurons form aberrant connections that could support epileptogenesis and/or seizures. We have shown that sustained vector-mediated hippocampal somatostatin (SST) expression can both block epileptogenesis and reverse seizure susceptibility in fully kindled rats. Here we test whether adeno-associated virus (AAV) vector-mediated sustained SST expression modulates hippocampal neurogenesis and microglial activation in fully kindled rats. We found significantly more dividing Type-1 NSCs and a corresponding increased number of surviving new neurons in the hippocampi of kindled versus sham-kindled rats. Increased numbers of activated microglia were found in the granule cell layer and hilus of kindled rats at both time points. After intrahippocampal injection with either eGFP or SST-eGFP vector, we found similar numbers of dividing Type-1 NSCs and -2 NPCs and surviving BrdU⁺ neurons and glia in the hippocampi of kindled rats. Upon observed variability in responses to SST-eGFP (2/4 rats exhibited Grade 0 seizures in the test session), we conducted an additional experiment. We found significantly fewer dividing Type-1 NSCs in the hippocampi of SST-eGFP vector-treated responder rats (5/13 rats) relative to SST-eGFP vector-treated non-responders and eGFP vector-treated controls that exhibited high-grade seizures on the test session. The number of activated microglia was upregulated in the GCL and hilus of kindled rats, regardless of vector treatment. These data support the hypothesis that sustained SST expression exerts antiepileptic effects potentially through normalization of neurogenesis and suggests that abnormally high proliferating Type-1 NSC numbers may be a cellular mechanism of epilepsy.

The Differences in Local Translatome across Distinct Neuron Types Is Mediated by Both Baseline Cellular Differences and Post-transcriptional Mechanisms

Local translation in neurites is a phenomenon that enhances the spatial segregation of proteins and their functions away from the cell body, yet it is unclear how local translation varies across neuronal cell types. Further, it is unclear whether differences in local translation across cell types simply reflect differences in transcription or whether there is also a cell type-specific post-transcriptional regulation of the location and translation of specific mRNAs. Most of the mRNAs discovered as being locally translated have been identified from hippocampal neurons because their laminar organization facilitates neurite-specific dissection and microscopy methods. Given the diversity of neurons across the brain, studies have not yet analyzed how locally translated mRNAs differ across cell types. Here, we used the SynapTRAP method to harvest two broad cell types in the mouse forebrain: GABAergic neurons and layer 5 projection neurons. While some transcripts overlap, the majority of the local translatome is not shared across these cell types. In addition to differences driven by baseline expression levels, some transcripts also exhibit cell type-specific post-transcriptional regulation. Finally, we provide evidence that GABAergic neurons specifically localize mRNAs for peptide neurotransmitters, including somatostatin and cortistatin, suggesting localized production of these key signaling molecules in the neurites of GABAergic neurons. Overall, this work suggests that differences in local translation in neurites across neuronal cell types are poised to contribute substantially to the heterogeneity in neuronal phenotypes.

Somatostatin receptor 5 is a prominent regulator of signaling pathways in cells with coexpression of Cannabinoid receptors 1

Endocannabinoids and somatostatin (SST) play critical roles in several pathophysiological conditions via binding to different receptor subtypes. Cannabinoid receptor 1 (CB1R) and somatostatin receptors (SSTRs) are expressed in several brain regions and share overlapping functions. Whether these two prominent members of G-protein-coupled receptor (GPCR) family interact with each other and constitute a functional receptor complex is not known. In the present study, we investigated the colocalization of CB1R and SSTR5 in rat brain, and studied receptor internalization, interaction and signal transduction pathways in HEK-293 cells cotransfected with human cannabinoid receptor 1 (hCB1R) and hSSTR5. Our results showed that CB1R and SSTR5 colocalized in rat brain cortex, striatum, and hippocampus. CB1R was expressed in SSTR5 immunoprecipitate prepared from the brain tissue lysate, indicating their association in a system where these receptors are endogenously expressed. In cotransfected HEK-293 cells, SSTR5 and CB1R existed in a constitutive heteromeric complex under basal condition, which was disrupted upon agonist treatments. Furthermore, concurrent receptor activation led to preferential formation of SSTR5 homodimer and dissociation of CB1R homodimer. We also discovered that second messenger cyclic adenosine monophosphate and downstream signaling pathways were modulated in a SSTR5-dominant and concentration-dependent manner in the presence of receptor-specific agonist. In conclusion, with predominant role of SSTR5, the functional consequences of crosstalk between SSTR5 and CB1R resulting in the regulation of receptor trafficking and signal transduction pathways open new therapeutic avenue in cancer biology and excitotoxicity.

Role of somatostatin and somatostatin receptor type 2 in postincisional nociception in rats

Somatostatin (SST) and the somatostatin receptor type 2 (sstr2) are expressed in the superficial part (Laminae I-III) of the dorsal horn of the spinal cord. Since the neurons in these laminae also receive nociceptive sensation from the periphery, it was hypothesized that both SST and sstr2 could be involved in the modulation of nociceptive transmission. To the best of knowledge, there are no studies on the involvement of SST and sstr2 in hind paw incision model in rats, which mimics postoperative pain in humans. Sprague-Dawley rats were subjected to hind paw incision under isoflurane anaesthesia and the resulting mechanical allodynia and thermal hyperalgesia were evaluated for 5 days. In another set of animals, the spinal cord was isolated at specified time intervals after incision and examined for SST and sstr2 expression using immunohistochemistry and immunoblotting procedures. Finally, nociceptive parameters were again evaluated in incised rats, which had received SST (400 µg/kg i.p. three times per day). Blood glucose level and locomotor activity were determined after SST treatment. Both allodynia and hyperalgesia were highest immediately after incision. Spinal SST expression increased at 2 h. A further increase was noted on day 3. Expression of sstr2 increased initially but decreased at day 1. These changes could be due to exocytosis of SST and internalization of the ligand-receptor complex. SST injection significantly attenuated mechanical allodynia but not thermal hyperalgesia. Significant change in blood glucose level or locomotor activity was absent. SST appears to contribute to postincisional pain. This finding could be of clinical relevance.

Neuropeptides in the posterodorsal medial amygdala modulate central cardiovascular reflex responses in awake male rats

The rat posterodorsal medial amygdala (MePD) links emotionally charged sensory stimuli to social behavior, and is part of the supramedullary control of the cardiovascular system. We studied the effects of microinjections of neuroactive peptides markedly found in the MePD, namely oxytocin (OT, 10 ng and 25 pg; n=6/group), somatostatin (SST, 1 and 0.05 μM; n=8 and 5, respectively), and angiotensin II (Ang II, 50 pmol and 50 fmol; n=7/group), on basal cardiovascular activity and on baroreflex- and chemoreflex-mediated responses in awake adult male rats. Power spectral and symbolic analyses were applied to pulse interval and systolic arterial pressure series to identify centrally mediated sympathetic/parasympathetic components in the heart rate variability (HRV) and arterial pressure variability (APV). No microinjected substance affected basal parameters. On the other hand, compared with the control data (saline, 0.3 µL; n=7), OT (10 ng) decreased mean AP (MAP50) after baroreflex stimulation and increased both the mean AP response after chemoreflex activation and the high-frequency component of the HRV. OT (25 pg) increased overall HRV but did not affect any parameter of the symbolic analysis. SST (1 μM) decreased MAP50, and SST (0.05 μM) enhanced the sympathovagal cardiac index. Both doses of SST increased HRV and its low-frequency component. Ang II (50 pmol) increased HRV and reduced the two unlike variations pattern of the symbolic analysis (P<0.05 in all cases). These results demonstrate neuropeptidergic actions in the MePD for both the increase in the range of the cardiovascular reflex responses and the involvement of the central sympathetic and parasympathetic systems on HRV and APV.

Somatostatin varicosities contain the vesicular GABA transporter and contact orexin neurons in the hypothalamus

Somatostatin (SST) is a neuropeptide with known inhibitory actions in the hypothalamus, where it inhibits release of growth hormone-releasing hormone (GHRH), while also influencing the sleep-wake cycle. Here we investigated in the rat whether SST neurons might additionally release GABA (gamma-aminobutyric acid) or glutamate in different regions and whether they might contact orexin neurons that play an important role in the maintenance of wakefulness. In dual-immunostained sections viewed by epifluorescence microscopy, we examined if SST varicosities were immunopositive for the vesicular transporter for GABA (VGAT) or glutamate (VGLUT2) in the posterolateral hypothalamus and neighboring arcuate nucleus and median eminence. Of the SST varicosities in the posterolateral hypothalamus, 18% were immunopositive for VGAT, whereas ≤ 1% were immunopositive for VGLUT2. In the arcuate and median eminence, 26 and 64% were VGAT+ and < 3% VGLUT2 + , respectively. In triple-immunostained sections viewed by epifluorescence and confocal microscopy, SST varicosities were seen in contact with orexin somata, and of these varicosities, a significant proportion (23%) contained VGAT along with synaptophysin, the presynaptic marker for small synaptic vesicles, and a similar proportion (25%) abutted puncta that were immunostained for gephyrin, the postsynaptic marker for GABAergic synapses. Our results indicate that a significant proportion of SST varicosities in the hypothalamus have the capacity to release GABA, to form inhibitory synapses upon orexin neurons, and accordingly through their peptide and/or amino acid, to inhibit orexin neurons, as well as GHRH neurons. Thus while regulating GHRH release, SST neurons could serve to attenuate arousal and permit progression through the sleep cycle.

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.

Anxiolytic-like effects of somatostatin isoforms SST 14 and SST 28 in two animal models (Rattus norvegicus) after intra-amygdalar and intra-septal microinfusions

RATIONALE AND OBJECTIVES

Somatostatin (SST) isoforms, SST 14 and SST 28, inhibit regulatory hormones in the periphery (e.g., growth hormone) and are widely distributed in the brain. In recent experiments, intracerebroventricular (ICV) SST produced anxiolytic-like effects in both behavioral and electrophysiological models. The sites of action of these anxiolytic effects in the brain, however, and the relative contributions of SST 14 and SST 28 to these effects are unknown.

MATERIALS AND METHODS

Anxiolytic effects were assessed in the plus-maze and shock-probe tests after (1) intra-amygdalar microinfusion of SST 14 (0.5 or 3 μg per hemisphere) or SST 28 (3 μg per hemisphere), (2) intra-septal microinfusion of SST 14 (0.5 or 1.5 μg per hemisphere) or SST 28 (1.5 μg per hemisphere), or (3) intra-striatal microinfusion of SST 14 (3 μg per hemisphere).

RESULTS

Intra-amygdalar and intra-septal microinfusions of SST 14 and SST 28 produced robust anxiolytic-like effects in the behavioral tests, unlike intra-striatal microinfusions. The magnitude of the anxiolytic effects in the amygdala and septum were comparable to those found previously with ICV SST 14, ICV L-779976, an SST (sst2) receptor agonist, and ICV diazepam, a classical benzodiazepine anxiolytic.

CONCLUSIONS

SST receptors in the septum and amygdala are responsive to both SST 14 and SST 28, but not those in the striatum. Although no obvious differences in the anxiolytic-like effects of the isoforms were detected, quantitative or even qualitative differences in their specific anxiolytic effects may occur in different sub-regions of the septum and amygdala, as has been found for benzodiazepine anxiolytics.

Comparison of the anti-inflammatory and anti-nociceptive effects of cortistatin-14 and somatostatin-14 in distinct in vitro and in vivo model systems

We showed that somatostatin (SST) exerts anti-inflammatory and anti-nociceptive effects through somatostatin receptor subtypes 4 and 1 (sst(4)/sst(1)). Since cortistatin (CST) is a structurally similar peptide, we aimed at comparing the sst(1)- and sst(4)-binding and activating abilities, as well as the effects of SST-14 and CST-14 on inflammatory and nociceptive processes. CST-14 concentration-dependently displaced radiolabeled SST-14 binding, induced similar sst(1) and sst(4)-activation with a less potency, and exerted significantly greater inhibitory effect on endotoxin-stimulated interleukin (IL)-1β production of murine peritoneal macrophages. Capsaicin-induced calcitonin gene-related peptide release from peripheral sensory nerve terminals of isolated rat tracheae was significantly decreased by 2 μM CST and 100 nM SST, but concentration-response correlation was not found. Mustard oil-evoked acute neurogenic plasma protein extravasation in the rat hindpaw skin, carrageenan-induced mouse paw edema, mechanical hyperalgesia, and IL-1β, tumor necrosis factor-α production, as well as mild heat injury-evoked thermal hyperalgesia were similarly attenuated by both peptides. In the latter case, i.pl. and i.p. injections exerted equal inhibitory actions. CST-14 and SST-14 similarly diminish both acute neurogenic and cellular inflammatory processes, as well as mechanical and heat hyperalgesia, in which their inhibitory effect on sensory nerve endings is likely to be involved. However, CST-14 exerts remarkably greater inhibition on cytokine production.

A novel somatostatin-immunoreactive mossy fiber pathway associated with HSP25-immunoreactive purkinje cell stripes in the mouse cerebellum

Somatostatin 28 immunoreactivity (Sst28-ir) identifies a specific subset of mossy fiber terminals in the adult mouse cerebellum. By using double-labeling immunohistochemistry, we determined that Sst28-ir is associated with presynaptic mossy fiber terminal rosettes, and not Purkinje cells, Golgi cells, or unipolar brush cells. Sst28-ir mossy fibers are restricted to the central zone (lobules VI/VII) and nodular zone (lobules IX, X) of the vermis, and the paraflocculus and flocculus. Within each transverse zone the mossy fiber terminal fields form a reproducible array of parasagittal stripes. The boundaries of Sst28-ir stripes align with a specific array of Purkinje cell stripes revealed by using immunocytochemistry for the small heat shock protein HSP25. In the cerebellum of the homozygous weaver mouse, in which a subpopulation of HSP25-ir Purkinje cells are located ectopically, the corresponding Sst28-ir mossy fiber projection is also ectopic, suggesting a role for a specific Purkinje cell subset in afferent pattern formation. Likewise, in the scrambler mutant mouse, Sst28-ir mossy fibers show a very close association with HSP25-ir Purkinje cell clusters. HSP25 itself does not appear to be critical for normal patterning, however: in the KJR mouse, which does not express cerebellar HSP25, Sst28 expression appears to be normal. Likewise, the Purkinje cell patterning antigens zebrin II and HSP25 are expressed normally in both Sst- and Sst-receptor knockout mice, suggesting that somatostatinergic transmission is not necessary for Purkinje cell stripe formation.

Growth hormone-releasing hormone activates sleep regulatory neurons of the rat preoptic hypothalamus

We examined whether growth hormone-releasing hormone (GHRH) may promote non-rapid eye movement (NREM) sleep via activation of GABAergic neurons in the preoptic area. Male Sprague-Dawley rats were implanted with EEG, EMG electrodes and a unilateral intracerebroventricular cannula. Groups of rats received injections (3 microl icv) with gonadotropin-releasing hormone (GHRH) (0.1 nmol/100 g body wt) or equal volume of physiological saline at the onset of the dark period and were permitted spontaneous sleep for 90 min. Separate groups of rats were sleep deprived by gentle handling for 90 min, beginning at the time of GHRH or saline injection, at the onset of the dark period. Other groups of rats received intracerebroventricular octreotide (somatostatin analog OCT) injections, intracerebroventricular injection of one of two doses of competitive GHRH antagonist, or intracerebroventricular saline injection at light onset and were then permitted 90 min spontaneous sleep-waking. Rats were killed immediately after the 90-min sleep/wake monitoring period. Brain tissue was processed for immunohistochemistry for c-Fos protein and glutamic acid decarboxylase (GAD). Single c-Fos and dual Fos-GAD cell counts were determined in the median preoptic nucleus (MnPN), and in the core and the extended parts of the ventrolateral preoptic nucleus (cVLPO and exVLPO). Intracerebroventricular GHRH elicited a significant increase in NREM sleep amount. Double-labeled Fos+GAD cell counts were significantly elevated after GHRH injection in the MnPN and VLPO in both undisturbed and sleep-deprived groups. OCT and GHRH antagonist significantly decreased NREM sleep amount compared with control rats. OCT injection increased single c-Fos-labeled cell counts in the MnPN, but not in the VLPO. Double-labeled cell counts were significantly reduced after OCT and the high dose of GHRH antagonist injection in all areas examined. These findings identify GABAergic neurons in the MnPN and VLPO as potential targets of the sleep-regulatory actions of GHRH.

Expression of somatostatin and somatostatin receptor subtypes in Apolipoprotein D (ApoD) knockout mouse brain: An immunohistochemical analysis

Apolipoprotein D (ApoD) is widely distributed in central and peripheral nervous system. ApoD expression has been shown to increase in several neurodegenerative and neuropsychiatric disorders, as well as during regeneration in the nervous system. Like ApoD, in the central nervous system somatostatin (SST) is widely present and functions as neurotransmitter and neuromodulator. The biological effects of SST are mediated via binding to five high-affinity G-protein coupled receptors termed SSTR1-5. Mice lacking ApoD exhibit reduced SST labeling in cortex and hippocampus and increased expression in striatum and amygdala without any noticeable changes in substantia nigra. Changes in SSTRs expressions have been described in several neurodegenerative disorders including Alzheimer's, Parkinson's and Huntington's diseases. In the present study, using SSTR1-5 receptor-specific antibodies, we mapped their distribution in wild type (wt) and ApoD knockout (ApoD(-/-)) mouse brain. SSTR1-5 expression was observed both as membrane and cytoplasmic protein and display regions and receptor specific differences between wt and ApoD(-/-) mice brains. In cortex and hippocampus, SSTR subtypes like immunoreactivity are decreased in ApoD(-/-) mice brain. Unlike cortex and hippocampus, in the striatum of ApoD(-/-) mice, projection neurons showed increased SSTR immunoreactivity, as compared to wt. Higher SSTR subtypes immunoreactivity is seen in substantia nigra pars compacta (SNpc) whereas lower in substantia nigra pars reticulata (SNpr) of ApoD(-/-) mice brains as compared to wt. Whereas, amygdala displayed SSTR subtypes changes in different nuclei of ApoD(-/-) mice in comparison to wt mice brain. Taken together, our results describe receptor and region specific changes in SST and SSTR subtypes expression in ApoD(-/-) mice brain, which may be linked to specific neurological disorders.

The role(s) of somatostatin, structurally related peptides and somatostatin receptors in the gastrointestinal tract: a review

Extensive functional and morphological research has demonstrated the pivotal role of somatostatin (SOM) in the regulation of a wide variety of gastrointestinal activities. In addition to its profound inhibitory effects on gastrointestinal motility and exocrine and endocrine secretion processes along the entire gastrointestinal tract, SOM modulates several organ-specific activities. In contrast to these well-known SOM-dependent effects, knowledge on the SOM receptors (SSTR) involved in these effects is much less conclusive. Experimental data on the identities of the SSTRs, although species- and tissue-dependent, point towards the involvement of multiple receptor subtypes in the vast majority of gastrointestinal SOM-mediated effects. Recent evidence demonstrating the role of SOM in intestinal pathologies has extended the interest of gastrointestinal research in this peptide even further. More specifically, SOM is supposed to suppress intestinal inflammatory responses by interfering with the extensive bidirectional communication between mucosal mast cells and neurons. This way, SOM not only acts as a powerful inhibitor of the inflammatory cascade at the site of inflammation, but exerts a profound antinociceptive effect through the modulation of extrinsic afferent nerve fibres. The combination of these physiological and pathological activities opens up new opportunities to explore the potential of stable SOM analogues in the treatment of GI inflammatory pathologies.

Effect of cortistatin on tau phosphorylation at Ser262 site

The development of intraneuronal lesions as a result of the progressive deposition of hyperphosphorylated tau at specific brain regions (such as hippocampus and cortex) plays a key role in the pathological process of Alzheimer's disease. However, the mechanisms by which tau phosphorylation is regulated, mainly in the pathology found in the cortex, are still poorly understood. Here, we analyzed the effect of cortistatin, a cortical neuropeptide related to somatostatin, on tau phosphorylation at Ser262 in cultures of murine cortical neurons. Both somatostatin and cortistatin induce tau phosphorylation at Ser262, a site modified in Alzheimer's disease, although with different kinetics in cortex. The effect of cortistatin likely is mediated by heterodimeric receptors composed of somatostatin receptor subtypes 2 and 4 and also by protein kinase C signaling. Cortistatin-deficient mice show decreased tau phosphorylation at Ser262 in the cortex but not in other brain regions tested. Our results suggest an important role for cortistatin in the regulation of tau phosphorylation that may be associated with the pathophysiology of Alzheimer's disease in regions such as the cerebral cortex.

Activation of GIRK channels in substantia gelatinosa neurones of the adult rat spinal cord: a possible involvement of somatostatin

Recent studies have suggested that spinal G-protein-coupled, inwardly rectifying K(+) (GIRK) channels play an important role in thermal nociception and the analgesic actions of morphine and other agents. In this study, we show that spinal GIRK channels are activated by an endogenous neurotransmitter using whole-cell patch-clamp recordings from substantia gelatinosa (SG) neurones in adult rat spinal cord slices. Although repetitive stimuli applied to the dorsal root did not induce any slow responses, ones focally applied to the spinal dorsal horn produced slow inhibitory postsynaptic currents (IPSCs) at a holding potential of -50 mV in about 30% of the SG neurones recorded. The amplitude and duration of slow IPSCs increased with the number of stimuli and decreased with removal of Ca(2+) from the external Krebs solution. Slow IPSCs were associated with an increase in membrane conductance; their polarity was reversed at a potential close to the equilibrium potential for K(+), calculated from the Nernst equation. Slow IPSCs were blocked by addition of GDP-beta-S into the patch-pipette solution, reduced in amplitude in the presence of Ba(2+), and significantly suppressed in the presence of an antagonist of GIRK channels, tertiapin-Q. Somatostatin produced an outward current in a subpopulation of SG neurones and the slow IPSC was occluded during the somatostatin-induced outward current. Moreover, slow IPSCs were significantly inhibited by the somatostatin receptor antagonist cyclo-somatostatin. These results suggest that endogenously released somatostatin may induce slow IPSCs through the activation of GIRK channels in SG neurones; this slow synaptic transmission might play an important role in spinal antinociception.

Distribution of somatostatin immunoreactive neurons and fibres in the central nervous system of a chondrostean, the Siberian sturgeon (Acipenser baeri)

Somatostatin (SOM) is a neuropeptide that is widely distributed in the central nervous system of vertebrates. Two isoforms of somatostatin (SS1 and SS2) have been characterized in sturgeon and in situ hybridisation studies in the sturgeon brain have demonstrated that mRNAs of the two somatostatin precursors (PSS1 and PSS2) are differentially expressed in neurons [Trabucchi, M., Tostivint, H., Lihrmann, I., Sollars, C., Vallarino, M., Dores, R.M., Vaudry, H., 2002. Polygenic expression of somatostatin in the sturgeon Acipenser transmontanus: molecular cloning and distribution of the mRNAs encoding two somatostatin precursors. J. Comp. Neurol. 443, 332-345.]. However, neither the morphology of somatostatinergic neurons nor the patterns of innervation have yet been characterized. To gain further insight into the evolution of this system in primitive bony fishes, we studied the distribution of somatostatin-immunoreactive (SOM-ir) cells and fibres in the brain of the Siberian sturgeon (Acipenser baeri). Most SOM-ir cells were found in the preoptic area and hypothalamus and abundant SOM-ir fibres coursed along the hypothalamic floor towards the median eminence, suggesting a hypophysiotrophic role for SOM in sturgeon. In addition, SOM-ir cells and fibres were observed in extrahypothalamic regions such as the telencephalon thalamus, rhombencephalon and spinal cord, which also suggests neuromodulatory and/or neurotransmitter functions for this peptide. Overall there was a good correlation between the distribution of SOM-ir neurons throughout the brain of A. baeri and that of PSS1 mRNA in Acipenser transmontanus. Comparative analysis of the results with those obtained in other groups of fishes and tetrapods indicates that widespread distribution of this peptide in the brain is shared by early vertebrate lines and that the general organization of the somatostatinergic systems has been well-conserved during evolution.

Cerebellar granule cell migration and the effects of alcohol

In the developing brain the majority of neurons migrate from their birthplace to their final destination. This active movement is essential for the formation of cortical layers and nuclei. The impairment of migration does not affect the viability of neurons but often results in abnormal differentiation. The proper migration of neurons requires the orchestrated activities of multiple cellular and molecular events, such as pathway selection, the activation of specific receptors and channels, and the assembly and disassembly of cytoskeletal components. The migration of neurons is very vulnerable to exposure to environmental toxins, such as alcohol. In this article, we will focus on recent developments in the migration of cerebellar granule cells. First, we will describe when, where and how granule cells migrate through different cortical layers to reach their final destination. Second, we will present how internal programs control the sequential changes in granule cell migration. Third, we will review the roles of external guidance cues and transmembrane signals in granule cell migration. Finally, we will reveal mechanisms by which alcohol exposure impairs granule cell migration.

Alteration of the somatostatinergic system in the striatum of rats with acute experimental autoimmune encephalomyelitis

To date, the neurochemical basis underlying the motor and cognitive deficits described in patients with multiple sclerosis (MS) is unclear. Since the neuropeptide somatostatin (SRIF) and the striatum have been implicated in movement control and implicit memory, the aim of this study was to analyze the striatal somatostatinergic system in an animal model of MS, experimental autoimmune encephalomyelitis (EAE). Female Lewis rats were immunized with an emulsion containing myelin basic protein (MBP) in complete Freund's adjuvant to induce the disease. The animals were decapitated when limp tail (grade 1) or severe hind limb paralysis (grade 3) was observed. Acute EAE in grade 3 did not modify striatal somatostatin-like immunoreactivity (SRIF-LI) content but decreased the overall SRIF receptor density, without affecting the apparent affinity, in the rat striatal membranes. A selective reduction in the protein levels of the SRIF receptor subtype sst2, analyzed by Western blotting, was detected in the EAE rats, which correlated with decreased sst2 mRNA levels. The expression of the receptor subtypes sst1, sst3 or sst4 was unaltered by the disease. The decrease in the SRIF receptor density was accompanied by an attenuated capacity of SRIF to inhibit both basal and forskolin-stimulated adenylyl cyclase activity. No significant changes, however, were found in the protein levels of Gi proteins (G(ialpha1), G(ialpha2) or G(ialpha3)) nor in those of the G-protein-coupled receptor kinase subtypes GRK2, GRK5 or GRK6. Acute EAE in grade 1 did not modify any of the parameters studied. In conclusion, these data demonstrate that acute EAE, in grade 3, disrupts the rat striatal SRIF receptor-effector system. These findings provide new insight into the molecular basis of EAE which might contribute to a better understanding of multiple sclerosis in humans.

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.

Expression and localisation of somatostatin receptor subtypes sst1-sst5 in areas of the rat medulla oblongata involved in autonomic regulation

Somatostatin is known to modulate the activity of neurones of the medulla oblongata involved in autonomic regulation, mediated through five subtypes of G protein-coupled receptors, sst1-sst5. This study utilises reverse transcription polymerase chain reaction and immunohistochemistry to investigate the expression of sst1-sst5, including the sst2(A)/sst2(B) isoforms, in the main autonomic centres of the rat medulla oblongata: nucleus of the solitary tract (NTS), dorsal motor vagal nucleus (DVN) and ventrolateral medulla (VLM). In tissue from the cerebral cortex, hippocampus and cerebellum all subtype mRNAs were detected, but sst5 signals were weak, and the distribution of sst1-sst5 immunoreactivities was consistent with previous reports. In the medulla, all sst mRNAs gave clear amplicons and subtype-specific antibodies produced characteristic patterns of immunolabelling, frequently in areas of somatostatinergic innervation. Anti-sst1 labelled beaded fibres, sst2(A), sst2(B), sst4 and sst5 gave somatodendritic labelling and sst3 labelled presumptive neuronal cilia. In NTS tissue, sst1, sst2(A), sst4 and sst5 mRNAs were strongly expressed, while in VLM tissue sst1, sst2(A), sst2(B) and sst4 predominated. In both areas of the medulla, neurones with intense somatodendritic sst2(A) immunoreactivity were principally catecholaminergic in phenotype, being double labelled for tyrosine hydroxylase (TH) and phenylethanolamine-N-methyl-transferase (PNMT). Some TH/PNMT positive neurones were also sst2(B) and sst4 immunoreactive. Cholinergic parasympathetic neurones in the DVN were immunoreactive for the sst2(A), sst2(B), sst4 and sst5 subtypes. These observations are consistent with the proposal that multiple somatostatin receptor subtypes, possibly combining as heterodimers, are involved in mediating the modulatory effects of somatostatin on autonomic function, including cardiovascular, respiratory and gastrointestinal reflex activity.

The Hypothalamic Orexinergic System: Pain and Primary Headaches

The primary headaches are a group of distinct individually characterized attack forms, which although varying in presentation, share some common anatomical basis responsible for the pain component of the attack. The hypothalamus is known to modulate a multitude of functions and has been shown to be involved in the pathophysiology of a variety of primary headaches including cluster headache and chronic migraine. It seems likely that it may be involved in other primary headache disorders due to their episodic nature and may underlie many of their diverse symptoms. We discuss the hypothalamic involvement in the modulation of trigeminovascular processing and examine the involvement of the hypothalamic orexinergic system as a key regulator of this function.

Inhibitory effect of somatostatin on inflammation and nociception

The present review focuses on promising new opportunities for anti-inflammatory and analgesic therapy. The theoretical background is an original observation based on our own experimental results. These data demonstrate that somatostatin is released from capsaicin-sensitive, peptidergic sensory nerve endings in response to noxious heat and chemical stimuli such as vanilloids, protons or lipoxygenase products. It reaches distant parts of the body via the circulation and exerts systemic anti-inflammatory and analgesic effects. Somatostatin binds to G-protein-coupled membrane receptors (sst(1)-sst(5)) and diminishes neurogenic inflammation by prejunctional action on sensory-efferent nerve terminals, as well as by postjunctional mechanisms on target cells. It decreases the release of pro-inflammatory neuropeptides from sensory nerve endings and also acts on receptors of vascular endothelial, inflammatory and immune cells. Analgesic effect is mediated by an inhibitory action on peripheral terminals of nociceptive neurons, since circulating somatostatin cannot exert central action. Somatostatin itself is not suitable for drug development because of its broad spectrum and short elimination half life, stable, receptor-selective agonists have been synthesized and investigated. The present overview is aimed at summarizing the physiological importance of somatostatin and sst receptors, pharmacological significance of synthetic agonists and their potential in the development of novel anti-inflammatory and analgesic drugs. These compounds might provide novel perspectives in the pharmacotherapy of acute and chronic painful inflammatory diseases, as well as neuropathic conditions.

Inhibition of nociceptive dural input in the trigeminal nucleus caudalis by somatostatin receptor blockade in the posterior hypothalamus

Somatostatin is a neuromodulator in the central nervous system and is involved in the regulation of metabolic and neuroendocrine functions. Recent experimental and clinical findings point to a role for somatostatin in the central processing of nociception. We studied the effects of somatostatin receptor modulation in the posterior hypothalamic area (PH) of the rat on dural nociceptive input. Somatostatin (10 microg/microl) and the somatostatin antagonist cyclo-somatostatin (50 microg/microl) were microinjected into the PH and the effects on responses of neurons in the trigeminal subnucleus caudalis studied. Injection of somatostatin (n=11) did not affect A- and C-fibre responses to dural electrical stimulation, nor was spontaneous activity altered (P>0.05). Injection of cyclo-somatostatin (n=10) into the PH reduced A-(-35.5+/-5.8%) and C-fibre (-43.1+/-7.5%) responses to dural stimulation and resulted in decreased spontaneous activity (-38.1+/-7.3%, P<0.05). Responses to facial thermal stimulation were decreased by 51.2+/-5.8% (n=5). Control injections had no significant effect (n=9). Blockade of somatostatin receptors in the PH has an anti-nociceptive effect on dural and facial input, probably mediated via GABAergic mechanisms. As somatostatin is also involved in hypothalamic regulation of metabolic, neuroendocrine and autonomic functions, somatostatin receptor mechanisms in the PH may play a role in the pathophysiology of primary headache disorders, such as migraine or cluster headache.

Somatostatin receptor type 2 undergoes plastic changes in the human epileptic dentate gyrus

Temporal lobe epilepsy (TLE) is characterized by hippocampal sclerosis together with profound losses and phenotypic changes of different classes of interneurons, including those expressing somatostatin (SRIF). To understand the functional significance of the plasticity of SRIF transmission in TLE, unraveling the status of SRIF receptors is, however, a prerequisite. To address this issue, we characterized expression and distribution of the major SRIF receptor, the sst2 subtype, in hippocampal tissue resected in patients with TLE using complementary neuroanatomic approaches. In patients with hippocampal sclerosis, the number of cells expressing sst2 receptor mRNA as well as sst2 receptor-binding sites and immunoreactivity decreased significantly in the CA1-3, reflecting neuronal loss. By contrast, in the dentate gyrus, sst2 receptor mRNA expression was strongly increased in the granule cell layer, and sst2 receptor-binding sites and immunoreactivity was preserved in the inner but decreased significantly in the outer molecular layer. In this latter region, pronounced changes in SRIF terminal fields were observed. Decreased receptor density in the distal dendrites of granule cells is likely to reflect downregulation of sst2 receptors in response to physiopathologic release of SRIF. Because sst2 receptors have anticonvulsant and antiepileptogenic properties, this phenomenon may contribute to the etiology of TLE seizures.

The endocrine disruptor atrazine accounts for a dimorphic somatostatinergic neuronal expression pattern in mice

It has now been established that a large number of man-made and natural chemicals are capable of interfering with the action of natural hormones. In this category "endocrine disruptors" such as the herbicide atrazine, when administered at ecological low doses (1 or 100 microg/kg per day) from gestational day 14 to postnatal day 21, provided a clear dimorphic neurodegenerative pattern in some brain areas of the domestic mouse (Mus musculus). Indeed, the high concentration (100 microg/kg per day) with respect to the low concentration (1 microg/kg per day) induced relevant neuronal damage in extrahypothalamic sites, such as the cortical and striatal areas in both sexes. Marked alterations in other areas, including the hippocampal and hypothalamic nuclei, were mostly typical of the female. At the neuronal level, the neuropeptide somatostatin, specific for the secretion of growth hormone, seemed to be a major target of atrazine effects, as demonstrated by evident subtype2,3,5 receptor mRNA differences of this neuropeptide, at least for the first two subtypes. In particular, a very strong (p < 0.001) upregulation of subtype2 expressing neurons was detected in female hypothalamic areas, specifically the suprachiasmatic nucleus, whereas a similar downregulatory trend was reported for some extrahypothalamic areas such as the striatum. Interestingly, very strong upregulatory and downregulatory actions were detected for neurons expressing subtype3 in male hypothalamic and amygdalar regions and in the cortical and hippocampal areas, respectively. Overall, it appears that these first neurotoxicological effects of atrazine are very likely linked to dimorphic expression patterns of specific somatostatin subtypes in discrete but key hypothalamic and extrahypothalamic areas of Mus musculus.

Metabolism of amyloid-beta peptide and Alzheimer's disease

The accumulation of amyloid-beta peptide (Abeta), a physiological peptide, in the brain is a triggering event leading to the pathological cascade of Alzheimer's disease (AD) and appears to be caused by an increase in the anabolic activity, as seen in familial AD cases or by a decrease in catabolic activity. Neprilysin is a rate-limiting peptidase involved in the physiological degradation of Abeta in the brain. As demonstrated by reverse genetics studies, disruption of the neprilysin gene causes elevation of endogenous Abeta levels in mouse brain in a gene-dose-dependent manner. Thus, the reduction of neprilysin activity will contribute to Abeta accumulation and consequently to AD development. Evidence that neprilysin in the hippocampus and cerebral cortex is down-regulated with aging and from an early stage of AD development supports a close association of neprilysin with the etiology and pathogenesis of AD. Therefore, the up-regulation of neprilysin represents a promising strategy for therapy and prevention. Recently, somatostatin, which acts via a G-protein-coupled receptor (GPCR), has been identified as a modulator that increases brain neprilysin activity, resulting in a decrease of Abeta levels. Thus, it may be possible to pharmacologically control brain Abeta levels with somatostatin receptor agonists.

Mechanisms of somatostatin-evoked responses in neurons of the rat lateral amygdala

The effects of somatostatin in the rat lateral amygdala (LA) in vitro were investigated through whole cell recording techniques. Somatostatin induced an inwardly rectifying K+ current in approximately 98% of LA projection neurons. Half-maximal effects were obtained by 189 nM somatostatin. The effects of somatostatin were insensitive to tetrodotoxin, reduced by Ba2+, occluded or abolished by the presence of nonhydrolysable GTP or GDP analogues, respectively, and blocked or mimicked by a somatostatin receptor type 2 antagonist (BIM-23627) or somatostatin receptor type 2 agonist (L-779,976), respectively, while somatostatin receptor type 1, 3 and 4 agonists were ineffective (L-797,591, L-796,778, L-803,087). Responses to somatostatin were associated with membrane hyperpolarization and decrease in input resistance, resulting in a dampening of cell excitability. It is suggested that these cellular mechanisms contribute to the role of somatostatin in decreasing anxiety behaviour as well as to anticonvulsant and antiepileptogenic actions of somatostatin or somatostatin agonists in the amygdala.

Expression of somatostatin in the adult and developing mouse kidney

BACKGROUND

Somatostatin (somatotropin release inhibiting factor) (SRIF) has potent antiproliferative and antisecretory actions. In the adult kidney, somatostatin alters renal blood flow, ion transport, and water permeability. While some evidence suggests that SRIF may be produced by adult kidney tubular cells, the specific tubules generating SRIF are unknown. Somatostatin has also been detected in a variety of embryonic tissues, although it has not been described in the developing kidney. Our objective was to determine the expression pattern of SRIF in both the adult and embryonic mouse kidney.

METHODS

We performed reverse transcriptase-polymerase chain reaction (RT-PCR) and immunofluorescence for SRIF in developing and adult mouse kidney tissues. We localized SRIF by dual or serial labeling immunofluorescence with specific markers.

RESULTS

Somatostatin mRNA was present in kidneys throughout embryogenesis and into adulthood. Starting at embryonic day (E) 12.5, SRIF was strongly expressed at the interface of the metanephric mesenchymal cells and the basolateral surfaces of ureteric bud trunks. Starting at E16.5, the staining at the interface was confined to the peripheral ureteric bud trunks and the clefts of newly dividing ureteric bud ampullae. In older embryos, SRIF also appeared in medullary tubules that appeared to be maturing thin descending limbs of Henle. In the adult kidney, SRIF proteins localized exclusively to medullary thin descending limbs of the Henle loop.

CONCLUSION

In embryonic kidneys, SRIF is expressed first at the interface of the metanephric mesenchyme and basolateral ureteric bud and later in maturing thin descending limbs of Henle. Expression in the thin descending limb persists in the adult kidney.

Different somatostatin receptor subtypes are operating in the brain of the teleost fish,Coris julis

Characterization of somatostatinergic (sst) neuronal activity through the application of nonpeptidyl agonists L-779,976 and L-817,818 which are highly specific for the sst receptors (sstr) sstr(2) and sstr(5), respectively, shows for the first time that sstr2, 5-like subtypes are the two major sstr subtypes operating in the brain of the teleost sea wrasse, Coris julis. A somewhat high but heterogeneous distribution pattern (> 30 < 180 fmol/mg wet tissue weight) of neurons expressing sstr2, 5 was reported in the different diencephalic regions plus in mesencephalon and telencephalon while low values were obtained in the cerebellum. Application of the above nonpeptidyl agonists permitted us to identify sstr2-like as the predominant subtype in telencephalic areas such as the entopeduncular nucleus (E) and postcommissural nucleus of the ventral telencephalon (Vp) as well as in hypothalamic and thalamic areas. At the same time high levels of neurons expressing sstr5-like, that greatly overlap those of sstr2-like in the diencephalic areas such as the anteroventral part of the preoptic nucleus (NPOav), the dorsal habenular nucleus (NHd) and the ventrolateral thalamic nucleus (VL), indicate that sstr2-like is very likely not the only sstr subtype acting in this fish brain. The predominance of sstr5-like in other brain areas is confirmed by the high quantities of this subtype in mesencephalic areas such as the torus longitudinalis (TLo). Overall, the discriminately differing densities of neurons expressing both subtypes seem to point to this system as a key molecular basis accounting for the distinct neurophysiological and behavioral sst-dependent activities in Coris julis.

Octreotide is not effective in the acute treatment of migraine

The aim of this study was to determine whether subcutaneous octreotide is effective for the treatment of acute migraine. Patients with migraine with and without aura as classified by the International Headache Society were recruited to a double-blind placebo-controlled crossover study. Patients were instructed to treat two attacks of at least moderate pain severity, with at least a 7 day interval, using subcutaneous 100 microg octreotide or matching placebo. The primary endpoint was the headache response defined as: severe or moderate pain becomes mild or nil, at 2 h. The primary endpoint was analysed using a Multilevel Analysis approach. Secondary end-points included associated symptoms and a four-point functional disability score. The study was powered to detect a 30% difference at an alpha of 0.05 and a beta of 0.8. A total of 51 patients were recruited, of whom 42 provided efficacy data on an attack treated with octreotide and 41 with placebo. Modelling the headache response as a binomial determined by treatment, using the patient as the level 2 variable, and considering a possible period effect, and sex and migraine type as other variables of interest, subcutaneous octreotide was not significantly superior to placebo. The two hour headache response rates were 20% for placebo and 14% for octreotide, whilst the two hour pain free rates were 7% and 2%, respectively. Subcutaneous octreotide 100 microg is not effective in the acute treatment of migraine when compared to placebo.

The expression and localization of somatostatin receptors in dorsal root ganglion neurons of normal and monoarthritic rats

Somatostatin has antinociceptive effects by acting on somatostatin (sst) receptors in primary afferent neurons. Five sst receptor subtypes (sst(1-5)) have been identified. In the present study we assessed the expression and localization of the sst receptor subtypes in lumbar dorsal root ganglia of normal rats and of rats with unilateral antigen-induced arthritis (AIA) in the knee joint. We used polymerase chain reaction (PCR) of material from dorsal root ganglia and immunohistochemistry in dorsal root ganglion paraffin sections. PCR data show that sst(1), sst(2(a)), sst(2(b)), sst(3), and sst(4) receptors are expressed in lumbar dorsal root ganglia of the rat. The sst(5) receptor was expressed in a few samples. Available antibodies revealed sst(2(a)) and sst(2(b)) receptor-like immunoreactivity in the vast majority of neurons, and sst(4) receptor-like immunoreactivity in about 40% of the dorsal root ganglion neurons and in some satellite cells. Real time PCR at 3, 10 and 21 days after induction of AIA did not reveal changes in receptor expression. Immunohistochemistry showed that a similar high proportion of neuronal profiles expressed sst(2(b)) receptor-like IR in control and AIA rats, but the proportion of neuronal profiles with sst(2(a)) receptor-like IR was significantly lower in acute and chronic AIA rats than in control rats. Although the proportion of neuronal profiles with sst(4) receptor-like IR was significantly higher at 21 days than at 3 days values at 3 or at 21 days were not significantly different from control. These data show that the majority of dorsal root ganglion neurons exhibit somatostatin receptor-like IR thus suggesting a high potential for inhibition by somatostatin. The reduction in the proportion of neuronal profiles with sst(2(a)) immunoreactivity suggests that inhibition of neuronal activity by somatostatin is reduced during painful arthritis.

Artifactual insulin release from differentiated embryonic stem cells

Several recent reports claim the generation of insulin-producing cells from embryonic stem cells via the differentiation of progenitors that express nestin. Here, we investigate further the properties of these insulin-containing cells. We find that although differentiated cells contain immunoreactive insulin, they do not contain proinsulin-derived C-peptide. Furthermore, we find variable insulin release from these cells upon glucose addition, but C-peptide release is never detected. In addition, many of the insulin-immunoreactive cells are undergoing apoptosis or necrosis. We further show that cells cultured in the presence of a phosphoinositide 3-kinase inhibitor, which previously was reported to facilitate the differentiation of insulin(+) cells, are not C-peptide immunoreactive but take up fluorescein isothiocyanate-labeled insulin from the culture medium. Together, these data suggest that nestin(+) progenitor cells give rise to a population of cells that contain insulin, not as a result of biosynthesis but from the uptake of exogenous insulin. We conclude that C-peptide biosynthesis and secretion should be demonstrated to claim insulin production from embryonic stem cell progeny.

Centrally Applied Somatostatin Inhibits the Estrogen-Induced Luteinizing Hormone Surge via Hypothalamic Gonadotropin-Releasing Hormone Cell Activation in Female Rats1

Overexpression of growth hormone (GH) as well as GH-deficiency dramatically impairs reproductive function. Decreased reproductive function as a result of altered GH release is, at least partially, due to changes at the hypothalamic-pituitary level. We hypothesize that hypothalamic somatostatin (SOM), the inhibiting factor of GH release from the pituitary, may play a central role in the "crosstalk" between the somatotropic and gonadotropic axes. In the present study we investigated the possible effects of a centrally applied SOM analog on the LH surge and the concurrent activation of hypothalamic GnRH neurons in female rats. To this end, female rats were treated with estradiol 2 wk after ovariectomy and were given a single central injection with either the SOM analog, octreotide, or saline just prior to surge onset, after which hourly blood samples were taken to measure LH. Two weeks later, the experimental setup was randomly repeated to collect brains during the anticipated ascending phase of the LH surge. Vibratome sections were subsequently double-stained for GnRH and cFos peptide. Following octreotide treatment, LH surges were significantly attenuated compared to those in saline-treated control females. Also, octreotide treatment significantly decreased the activation of hypothalamic GnRH neurons. These results clearly demonstrate that SOM is able to inhibit LH release, at least in part by decreasing the activation of GnRH neurons. Based on these results, we hypothesize that hypothalamic SOM may be critically involved in the physiological regulation of the proestrus LH surge.

Cellular actions of somatostatin on rat periaqueductal grey neurons in vitro

Functional studies indicate that the midbrain periaqueductal grey (PAG) is involved in the analgesic actions of somatostatin; however, the cellular actions of somatostatin in this brain region are unknown. In the present study, whole-cell patch clamp recordings were made from rat PAG neurons in vitro. In 93% of acutely isolated neurons, somatostatin inhibited Ca(2+)-channel currents. This effect was mimicked by the sst-2 selective agonist BIM-23027, but not by the sst-1 and sst-5 selective agonists CH-275 and L-362855. In brain slices, 81% of neurons responded to somatostatin (300 nm) with an increase in K(+) conductance that reversed polarity at -114 mV. A greater proportion of somatostatin-sensitive neurons (93%) than somatostatin-insensitive neurons (53%) responded to the opioid agonist met-enkephalin (10 microm). Somatostatin also reduced the amplitude of evoked GABA(A)-mediated inhibitory postsynaptic currents (IPSCs). The actions of somatostatin in brain slices were mimicked by BIM-23027, but not by CH-275. Somatostatin had a variable effect on the rate of spontaneous miniature IPSCs in normal external potassium solutions. In high external potassium solutions, somatostatin reduced the rate of miniature IPSCs in all neurons, and this inhibition was abolished by addition of Cd(2+) (30 microm). Somatostatin had no effect on the amplitude of miniature IPSCs. These results indicate that somatostatin acts via sst-2 receptors to directly inhibit a subpopulation of PAG neurons by activating a potassium conductance and inhibits GABA release within PAG via a presynaptic Ca(2+)-dependent mechanism. Thus, like opioids, somatostatin has the potential to exert pre- and postsynaptic disinhibitory effects within the PAG.

Brain mapping of three somatostatin encoding genes in the goldfish

In the present study the brain distribution of three somatostatin (SRIF)-encoding genes, PSS-I, PSS-II, and PSS-III, was analyzed by in situ hybridization (ISH) in the goldfish. The PSS-I mRNA showed the widest distribution throughout the brain, whereas PSS-II transcripts were restricted to some hypothalamic nuclei. On the other hand, PSS-III presents an intermediate distribution pattern. All SRIF encoding genes are expressed in hypophysiotropic nuclei supporting the idea that, in addition to SRIF-14, [Pro(2)] SRIF-14, and gSRIF-28 have pituitary-controlling functions. Moreover, each of the genes is expressed in nuclei directly associated with feeding behavior, suggesting a role for SRIF peptides in the central control of food intake and energy balance. Alternatively, they might have a role in processing sensory information related with feeding behavior, since PSS genes are expressed in the main gustatory, olfactory, and visual centers, which project to the hypothalamic feeding center in teleost fish.

Differential effects of somatostatin on exploratory behavior after unilateral injections in to rat neostriatum

The effects of somatostatin (SRIF) microinjected unilaterally (left or right) at a dose of 10, 50, and 100 ng into the neostriatum of male Wistar rats on exploratory behavior were studied. Unilateral injections of SRIF suppressed dose-related the exploratory activity as decreased the number of horizontal and vertical movements compared to the respective controls. The effect was more pronounced when SRIF was microinjected into the right neostriatum as compared to the left neostriatum. These findings suggest some asymmetric effects of SRIF, depending on the dose and the microinjected hemisphere.

Physiological release of striatal acetylcholine (in vivo): effect of somatostatin on dopaminergic–cholinergic interaction

The effects of somatostatin (SOM) on the release of acetylcholine (ACh) and dopamine (DA) from striatum of freely moving rats were studied by transversal microdialysis. Acetylcholine (ACh) and dopamine (DA) were detected by high performance liquid chromatography (HPLC) with electrochemical detection. Somatostatin (0.1, 0.5 and 1 microM), administered locally through the microdialysis probe to the striatum, was able to release dose-dependently ACh from the cholinergic neurons of the striatum. The increase in the extracellular levels of ACh produced by 1 microM SOM in the striatum reached a maximum of 200%. ACh-releasing effect of SOM was completely inhibited by tetrodotoxin indicating that neuronal firing is involved in its effect. Local infusion of sulpiride, 10 microM, D(2) receptor antagonist, potentiated (about 100%) the SOM (1 microM)-induced release of ACh. SOM, 1 microM, was more effective in enhancing the release of ACh in the striatum (two-fold increase) after degeneration of the nigrostriatal DA pathway with 6-hydroxydopamine (6-OHDA) (250 microg/animal, i.c.v.). The D(2) receptor agonists bromcriptine, 10 microM, or apomorphine, 10 microM, completely antagonize SOM-induced release. SOM, 1 microM, enhanced the release of DA (about 400%). These findings indicate that SOM is capable of releasing both ACh and DA in the striatum, however, its effect on ACh release is partially masked unless the D(2) receptor-mediated tonic inhibitory effect of released DA from the nigro-striatal pathway is attenuated.

Generation of the melatonin endocrine message in mammals: a review of the complex regulation of melatonin synthesis by norepinephrine, peptides, and other pineal transmitters

Melatonin, the major hormone produced by the pineal gland, displays characteristic daily and seasonal patterns of secretion. These robust and predictable rhythms in circulating melatonin are strong synchronizers for the expression of numerous physiological processes in photoperiodic species. In mammals, the nighttime production of melatonin is mainly driven by the circadian clock, situated in the suprachiasmatic nucleus of the hypothalamus, which controls the release of norepinephrine from the dense pineal sympathetic afferents. The pivotal role of norepinephrine in the nocturnal stimulation of melatonin synthesis has been extensively dissected at the cellular and molecular levels. Besides the noradrenergic input, the presence of numerous other transmitters originating from various sources has been reported in the pineal gland. Many of these are neuropeptides and appear to contribute to the regulation of melatonin synthesis by modulating the effects of norepinephrine on pineal biochemistry. The aim of this review is firstly to update our knowledge of the cellular and molecular events underlying the noradrenergic control of melatonin synthesis; and secondly to gather together early and recent data on the effects of the nonadrenergic transmitters on modulation of melatonin synthesis. This information reveals the variety of inputs that can be integrated by the pineal gland; what elements are crucial to deliver the very precise timing information to the organism. This also clarifies the role of these various inputs in the seasonal variation of melatonin synthesis and their subsequent physiological function.

Neuroendocrine facets of human puberty

The unfolding of pubertal growth and maturation entails multisystem collaboration. Most notably, the outflow of gonadotropins and growth hormone (GH) proceeds both independently and jointly. The current update highlights this unique dependency in the human.

Somatostatin and somatostatin receptor physiology

Since the discovery of somatostatin (SST) over three decades ago, its ubiquitous distribution and manifold functions are still being documented. SST is synthesized in the hypothalamus and transported to the anterior pituitary gland where it tonicaly inhibits GH and TSH secretion as well as being responsible for GH pulsatile release. Several internal feedback loops, sleep, exercise, and chemical agents control and influence SST release. SST also impacts the function of a wide variety of cells and organ systems throughout the body. Knowledge of the structures of the SSTs has resulted in recognition of the essential four core conserved residues responsible for their actions. The SSTs act through six separate SST cell surface receptors (SSTRs), members of the family of G protein-coupled receptors. Receptor ligand binding (SST/SSTR) results in cellular activities specific for each receptor, or receptor combinations, and their tissue/cell localization. Understanding the structure/function relationship of the SSTs and their receptors, including the internalization of SST/SSTR complexes, has facilitated the development of a variety of novel pharmacologic agents for the diagnosis and treatment of neuroendocrine tumors and unfolding new applications.