Somatostatin and SMS 201-995 reverse the impairment of cognitive functions induced by cysteamine depletion of brain somatostatin

Somatostatin: Linking Cognition and Alzheimer Disease to Therapeutic Targeting

Over 4 decades of research support the link between Alzheimer disease (AD) and somatostatin [somatotropin-releasing inhibitory factor (SRIF)]. SRIF and SRIF-expressing neurons play an essential role in brain function, modulating hippocampal activity and memory formation. Loss of SRIF and SRIF-expressing neurons in the brain rests at the center of a series of interdependent pathological events driven by amyloid-β peptide (Aβ), culminating in cognitive decline and dementia. The connection between the SRIF and AD further extends to the neuropsychiatric symptoms, seizure activity, and inflammation, whereas preclinical AD investigations show SRIF or SRIF receptor agonist administration capable of enhancing cognition. SRIF receptor subtype-4 activation in particular presents unique attributes, with the potential to mitigate learning and memory decline, reduce comorbid symptoms, and enhance enzymatic degradation of Aβ in the brain. Here, we review the links between SRIF and AD along with the therapeutic implications. SIGNIFICANCE STATEMENT: Somatostatin and somatostatin-expressing neurons in the brain are extensively involved in cognition. Loss of somatostatin and somatostatin-expressing neurons in Alzheimer disease rests at the center of a series of interdependent pathological events contributing to cognitive decline and dementia. Targeting somatostatin-mediated processes has significant therapeutic potential for the treatment of Alzheimer disease.

Neuroprotective Effects of Vapreotide on Tau Transfection-Induced Neurodegeneration

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by loss of neurons and synapses. The aim of this study was to investigate the effect of somatostatin analogue Vapreotide in an in vitro Alzheimer's model and its effects based on the relationship between somatostatinergic transmission and neurodegenerative functions. In this study, tau transfection was performed using the MAPT gene cloned into the pcDNA3.1 vector and transfection reagent into the SH-SY5Y cell line. In viability experiments using 10 µM Memantine as a positive control, it was observed that Vapreotide at 50 µM (p < 0.0001) and 100 µM (p < 0.05) had a protective effect on cell viability, 100 µM CYN154806 was found to decrease (p < 0.05) cell viability. It was determined that Vapreotide, decreased the expression levels (50 µM-p < 0.001; 100 µM-p < 0.001; 200 µM-p < 0.0001) and phosphorylation of Tau and p-Tau proteins by western blots. With the qRT-PCR method, it was found that Vapreotide, decreased the BAX/BCL2 (50 µM-p < 0.001; 100 µM-p < 0.01; 200 µM-p < 0.001) expression level and decreased the expression level (50 µM-p < 0.01; 100 µM-p < 0.01; 200 µM-p < 0.001) of the APOE4 gene, which constitutes a genetic risk for AD. This study demonstrates a potential therapeutic role for a somatostatin analogue Vapreotide in Alzheimer's disease.

Synthesis and structure-activity relationships of 3,4,5-trisubstituted-1,2,4-triazoles: high affinity and selective somatostatin receptor-4 agonists for Alzheimer's disease treatment

Somatostatin receptor-4 (SST₄) is highly expressed in brain regions affiliated with learning and memory. SST₄ agonist treatment may act to mitigate Alzheimer's disease (AD) pathology. An integrated approach to SST₄ agonist lead optimization is presented herein. High affinity and selective agonists with biological efficacy were identified through iterative cycles of a structure-based design strategy encompassing computational methods, chemistry, and preclinical pharmacology. 1,2,4-Triazole derivatives of our previously reported hit (4) showed enhanced SST₄ binding affinity, activity, and selectivity. Thirty-five compounds showed low nanomolar range SST₄ binding affinity, 12 having a K _i < 1 nM. These compounds showed >500-fold affinity for SST₄ as compared to SST_2A. SST₄ activities were consistent with the respective SST₄ binding affinities (EC₅₀ < 10 nM for 34 compounds). Compound 208 (SST₄ K _i = 0.7 nM; EC₅₀ = 2.5 nM; >600-fold selectivity over SST_2A) display a favorable physiochemical profile, and was advanced to learning and memory behavior evaluations in the senescence accelerated mouse-prone 8 model of AD-related cognitive decline. Chronic administration enhanced learning with i.p. dosing (1 mg kg^-1) compared to vehicle. Chronic administration enhanced memory with both i.p. (0.01, 0.1, 1 mg kg^-1) and oral (0.01, 10 mg kg^-1) dosing compared to vehicle. This study identified a novel series of SST₄ agonists with high affinity, selectivity, and biological activity that may be useful in the treatment of AD.

Biological and Biochemical Basis of the Differential Efficacy of First and Second Generation Somatostatin Receptor Ligands in Neuroendocrine Neoplasms

Endogenous somatostatin shows anti-secretory effects in both physiological and pathological settings, as well as inhibitory activity on cell growth. Since somatostatin is not suitable for clinical practice, researchers developed synthetic somatostatin receptor ligands (SRLs) to overcome this limitation. Currently, SRLs represent pivotal tools in the treatment algorithm of neuroendocrine tumors (NETs). Octreotide and lanreotide are the first-generation SRLs developed and show a preferential binding affinity to somatostatin receptor (SST) subtype 2, while pasireotide, which is a second-generation SRL, has high affinity for multiple SSTs (SST₅ > SST₂ > SST₃ > SST₁). A number of studies demonstrated that first-generation and second-generation SRLs show distinct functional properties, besides the mere receptor affinity. Therefore, the aim of the present review is to critically review the current evidence on the biological effects of SRLs in pituitary adenomas and neuroendocrine tumors, by mainly focusing on the differences between first-generation and second-generation ligands.

Somatostatin and Somatostatin-Containing Neurons in Shaping Neuronal Activity and Plasticity

Since its discovery over four decades ago, somatostatin (SOM) receives growing scientific and clinical interest. Being localized in the nervous system in a subset of interneurons somatostatin acts as a neurotransmitter or neuromodulator and its role in the fine-tuning of neuronal activity and involvement in synaptic plasticity and memory formation are widely recognized in the recent literature. Combining transgenic animals with electrophysiological, anatomical and molecular methods allowed to characterize several subpopulations of somatostatin-containing interneurons possessing specific anatomical and physiological features engaged in controlling the output of cortical excitatory neurons. Special characteristic and connectivity of somatostatin-containing neurons set them up as significant players in shaping activity and plasticity of the nervous system. However, somatostatin is not just a marker of particular interneuronal subpopulation. Somatostatin itself acts pre- and postsynaptically, modulating excitability and neuronal responses. In the present review, we combine the knowledge regarding somatostatin and somatostatin-containing interneurons, trying to incorporate it into the current view concerning the role of the somatostatinergic system in cortical plasticity.

Peptide receptor targeting in cancer: the somatostatin paradigm

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

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.

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.

Cyclic insulin-regulated aminopeptidase (IRAP)/AT4 receptor ligands

The angiotensin IV receptor (AT4 receptor) is the insulin-regulated aminopeptidase enzyme (IRAP, EC 3.4.11.3). This membrane-spanning enzyme belongs to the M1 family of zinc-dependent metallo-peptidases. It has been proposed that AT4 receptor ligands exert their physiological effects by binding to the active site of IRAP and thereby inhibiting the catalytic activity of the enzyme. The biological activity of a large series of linear angiotensin IV analogs was previously disclosed. Herein, the synthesis and biological evaluation of a series of angiotensin IV analogs, encompassing macrocyclic ring systems of different sizes, are presented. It is demonstrated that disulfide cyclizations of angiotensin IV can deliver ligands with high IRAP/AT4 receptor affinity. One ligand, with an 11-membered ring system (4), inhibited human IRAP and aminopeptidase N (AP-N) activity with similar potency as angiotensin IV but was considerably more stable than angiotensin IV toward enzymatic degradation. The compound provides a promising starting point for further optimization toward more drug-like derivatives. The cyclic constrained analogs allowed us to propose a tentative bioactive conformation of angiotensin IV and it seems that the peptide adopts an inverse gamma-turn at the C-terminal.

Somatostatin and Alzheimer's disease

One of the most consistent neurochemical deficits in Alzheimer's disease is a reduction in cortical somatostatin concentrations. The probability of a predominant regulatory change is heightened by the finding that 90% of somatostatin positive nonpyramidal neurons are also positive for NADPH, and NADPH neurons are 'protected' in Alzheimer's disease and do not appear to be lost. The first evidence that somatostatin influences learning and memory processes in experimental animals was published more than a decade ago. These reports of somatostatin effects on cognitive functions in rats were later confirmed by several other studies. The somatostatin depleting substance cysteamine inhibited the learning and memory performance of rats in active and passive avoidance behavior tests. Post-mortem human studies suggest that although somatostatin concentration is reduced, the somatostatin receptors are less affected in the brain in Alzheimer's disease. These findings may be of importance for possible therapeutic approaches using somatostatin-receptor-influencing compounds.

Spatial learning and synaptic hippocampal plasticity in type 2 somatostatin receptor knock-out mice

Somatostatin is implicated in a number of physiological functions in the CNS. These effects are elicited through the activation of at least five receptor subtypes. Among them, sst2 receptors appear the most widely expressed in the cortex and hippocampal region. However, the specific role of this somatostatin receptor subtype in these regions is largely undetermined. In this study, we investigated the role of the sst2 receptor in the hippocampus using mice invalidated for the sst2 gene (sst2 KO mice). Complementary experimental approaches were used. First, mice were tested in behavioral tests to explore the consequences of the gene deletion on learning and memory. Spatial discrimination learning in the radial maze was facilitated in sst2 KO mice, while operant learning of a bar-pressing task was slightly altered. Mice were then processed for electrophysiological study using the ex vivo hippocampal slice preparation. Extracellular recordings in the CA1 area showed an enhancement in glutamatergic (AMPA and NMDA) responses in sst2 KO mice which displayed an increase in the magnitude of the short-term potentiation and long-term depression. In contrast, long-term potentiation was not significantly altered. Taken together, these data demonstrate that somatostatin, acting via sst2 hippocampal receptors, may contribute to a global decrease in glutamate efficiency and consequently alter glutamate-dependent plasticity and spatial learning.

Modulation of inflammatory paw oedema by cysteamine in the rat

Cysteamine, a potent somatostatin depletor, was used in the present study to investigate the role of endogenous somatostatin in acute peripheral inflammation. The acute inflammation was induced by intraplantar injection of carrageenan (1%), histamine (5 micromol), or formalin (2.5%) in the rat hind paw. The induced inflammation and the formation of oedema were determined by measurement of the paw thickness. Given subcutaneously (s.c.) 1 h before carrageenan, cysteamine caused significant, dose-dependent and long-lasting inhibition of rat paw oedema induced by carrageenan. At doses of 12.5, 25, 50 or 100 mg kg (-1), cysteamine significantly inhibited the carrageenan-induced paw oedema at 4 h by 52.3, 40, 40.7 or 26.3%. Cysteamine given at 300 mg kg (-1), a dose well known to deplete tissue somatostatin, reduced oedema by only 16.2% vs control values. Significant inhibition of the carrageenan-induced rat paw oedema was still evident 24 h post-injection at cysteamine doses of 12.5, 25, 50 or 100 mg kg (-1). Given s.c. at 300 mg kg (-1), 4 h prior to carrageenan, cysteamine decreased rat paw oedema at 4 h by 14.9%. Cysteamine (300 mg kg (-1)), 4 h beforehand, had little modulatory effect on the oedema induced by formalin (2.5%) but reduced that caused by intraplantar histamine (5 micromol). The anti-oedematogenic effect of indomethacin, but not that of the selective COX-2 inhibitor celecoxib, was less marked in rats pre-treated with cysteamine at 300 mg kg (-1). Cysteamine (0.3 microg- 0.3 mg paw (-1)) co-administered with carrageenan was devoid of anti-inflammatory effect and even promoted inflammation at low concentrations. Cysteamine given locally alone induced slight paw oedema. These data indicate that systemic cysteamine possesses potent and long-lasting anti-inflammatory effects and modulates the anti-inflammatory effect of cyclooxygenase inhibitors in a model of peripheral inflammation in the rat. The effect of cysteamine is likely to be mediated via central action.

A bicyclic and hsst2 selective somatostatin analogue: design, synthesis, conformational analysis and binding

A backbone bridged and disulfide bridged bicyclic somatostatin analogue, compound 1 (PTR-3205), was designed and synthesized by solid-phase methodology. The binding of compound 1 to the five different somatostatin receptors, expressed in CHO or COS-7 cells, indicate a high degree of selectivity towards hsstr2. The three-dimensional structure of this compound has been determined in DMSO-d(6) and in water by 1H NMR and by molecular dynamics simulations. Similar backbone conformations were observed in both solvents. We have established direct evidence that the backbone of this bicyclic somatostatin analogue assumes a 'folded' conformation in solution, where the lactam ring extends roughly in the plane of the beta-turn. The pharmacophoric region Phe-(D)-Trp-Lys-Thr of compound 1 is in accord with that of both the Veber compound L-363,301 (Merck) and sandostatin. We believe that the enhanced selectivity towards the hsst2 receptor, in comparison with other analogues, is due to its large hydrophobic region, composed of the lactam ring and the Phe side chains at positions 1 and 8.

Novel proline endopeptidase inhibitors do not modify Abeta40/42 formation and degradation by human cells expressing wild-type and swedish mutated beta-amyloid precursor protein

Previous studies have suggested that proline endopeptidase (PE) could participate to the catabolism of the beta-amyloid peptide (Abeta) or to the physiopathological maturation of the beta-amyloid protein precursor (betaAPP). We have examined the putative ability of human purified PE to catabolize Abeta40 and Abeta42 and the possible contribution of this enzyme to the generation of Abeta40 and Abeta42 in human HEK293 cells. We show first that purified human PE does not degrade synthetic Abeta40 and Abeta42, in vitro. We establish that HEK293 cell homogenates exhibit a Z-Gly-Pro-7AMC-cleaving enzyme, the activity of which is inhibited by Z-Pro-Prolinal and S17092 and S19825, two novel PE inhibitors, with affinities similar to those displayed on the purified human PE. These inhibitors also penetrate cells and achieve a full inhibition of endogenous proline endopeptidase in human cells. By means of selective antibodies directed towards the C-terminal of Abeta40 and Abeta42, we assessed the effect of PE inhibitors on the recovery of both Abeta species. This was examined in HEK293 cells stably overexpressing the wild-type and the familial Alzheimer's disease-related Swedish mutated beta-APP. We establish that none of these inhibitors affected Abeta40 or Abeta42 production in these transfected cells. Overall, our study indicates that human PE does not degrade Abeta40 and Abeta42. Furthermore, PE does not contribute to Abeta40 and Abeta42 formation in HEK293 cells. Therefore, PE does not appear to contribute to the Abeta-related aetiology of Alzheimer's disease.

Somatostatin and behaviour: the need for genetically engineered models

Somatostatin was originally characterised as a hypothalamic neurohormone responsible for the inhibition of pituitary Growth Hormone secretion. In mammals two genes encode for somatostatin-related peptides, somatostatin 14 and 28, and cortistatins, respectively. All peptides bind with similar affinities to the five cloned somatostatin receptors (sst), which belong to the GPCR family. Despite numerous studies, no clear behavioural function has yet been attributed to somatostatin-related peptides. This is due to the lack of good pharmacological tools (selective antagonists) and animal models. This review will focus on the recent development of such tools.

Intrahippocampal injections of cysteamine improve the retention of a bar-pressing task in mice

Cysteamine was used as a tool aimed at investigating the role of central somatostatin (SS-14) and was shown to modulate learning in a task-dependent manner. However, direct arguments have not yet been provided to support the hypothesis that impairments and facilitation of learning produced by cysteamine are both mediated by the hippocampus. Mice were given daily intrahippocampal injections of artificial cerebrospinal fluid (CSF) or cysteamine at doses of either 2.5 microg/0.2 microl or 25 microg/0.2 microl 1 h prior to each learning session of a bar-pressing task, for which the acquisition was previously shown to be improved by systemic injections. The results showed that, with respect to CSF, the mice injected with cysteamine learned the bar pressing task faster whereas no evidence of changes in locomotor activity was provided. Moreover, the results showed that retention was specifically increased in the two groups injected with cysteamine. It is argued that the action of cysteamine on the hippocampus is sufficient to modulate specifically learning-memory processes in a task-dependent manner. In conclusion, the blockade of some hippocampal information processing function by cysteamine is discussed to understand the bidirectional effects of drugs on learning and memory.

Somatostatin inhibits interleukin 6 release from rat cortical type I astrocytes via the inhibition of adenylyl cyclase

Interleukin 6 is a pleiotropic cytokine produced in the central nervous system (CNS) that has been involved in both direct neurotrophic activities and in the regulation of the production of acute phase proteins both at peripheral and central levels. In rat cortical type I astrocytes, interleukin 6 release is under the control of cAMP-protein kinase A and calcium-phospholipids-protein kinase C systems. Somatostatin is a neuropeptide, acting as a neurotransmitter, highly concentrated within the CNS, where it has been involved in the modulation of learning and memory processes. The aim of this study was to characterize the effects of somatostatin on the release of interleukin 6 from rat cortical type I astrocytes and the intracellular mechanisms involved in this activity. Our results show that somatostatin, in a concentration-dependent manner, inhibited basal and forskolin-stimulated interleukin 6 release from rat cortical type I astrocytes in culture. The EC50 of the inhibitory action was calculated to be approximately 10 nM. Furthermore, this effect of somatostatin was completely abolished by pretreating cortical astrocytes with pertussis toxin that, uncoupling, by ADP-rybosylating, the inhibitory GTP-binding protein from the receptors, prevents the activation of the intracellular effectors such as the adenylyl cyclase enzyme. To identify the intracellular mechanism mediating the effects of somatostatin on the interleukin 6 release, we evaluated the peptide modulation of basal and stimulated intracellular accumulation of cAMP. In our experimental conditions somatostatin significantly inhibited both basal and forskolin-stimulated cAMP accumulation. Conversely, somatostatin did not affect the increase of interleukin 6 release induced by dibutyryl-cAMP, a nonhydrolizable cAMP analog that, bypassing the effects of somatostatin on adenylyl cyclase activity, directly activated protein kinase A. These observations support the hypothesis that somatostatin inhibitory activity on interleukin 6 release is mediated by its effects on cAMP production. Somatostatin analog SMS 201-995 did not affect interleukin 6 production either in basal or stimulated conditions. Since, SMS 201-995 was reported to bind with high affinity only to somatostatin receptors type 2, 3 and 5, the lack of effect of this compound on interleukin 6 release suggests that the inhibitory action of somatostatin could be mediated by the activation of either type 1 or type 4 somatostatin receptors. In conclusion, our data demonstrate that the release of interleukin 6 from rat cortical type I astrocytes is inhibited by somatostatin through the activation of a somatostatin receptor coupled to the inhibition of adenylyl cyclase via a G-protein sensitive to pertussis toxin.

Examination of the role of endopeptidase 3.4.24.15 in A beta secretion by human transfected cells

1. We have taken advantage of our recent development of highly potent and specific phosphinic inhibitors of endopeptidase 3.4.24.15 to examine the putative contribution of the enzyme in the secretion of A beta by HK293 transfected cells overexpressing the wild type and the Swedish (Sw) double mutated form of beta APP751. 2. First, we showed that HK293 cells contain a peptidase activity, the inhibition profile of which fully matches that of purified endopeptidase 3.4.24.15. Second, we established that the treatment of HK293 cells with specific phosphinic inhibitors leads to about 80% inhibition of intracellular endopeptidase 3.4.24.15 activity, indicating that these inhibitors penetrate the cells. 3. Metabolic labelling of wild type and Sw beta APP751-expressing cells, followed by immunoprecipitation of A beta-containing peptides, revealed the secretion of A beta and the intracellular formation of an A beta-containing 12 kDa product. 4. A beta secretion by Sw beta APP751 transfected cells was drastically enhanced when compared to cells expressing wild type beta APP751. This production was not affected by endopeptidase 3.4.24.15 inhibitors in either cell type. This correlates well with the observation that endopeptidase 3.4.24.15 does not cleave recombinant baculoviral Sw beta APP751, in vitro. 5. Our previous data indicated that endopeptidase 3.4.24.15 activity was reduced in the parietal cortex of Alzheimer's disease affected brains and that the enzyme probably participated, in this brain area, to the catabolism of somatostatin 1-14. However, the present work indicates that endopeptidase 3.4.24.15 does not seem to behave as a beta-secretase in HK293 transfected cells. Therefore, it is suggested that endopeptidase 3.4.24.15 could participate in the symptomatology, but probably not in the aetiology of Alzheimer's disease.

Alterations of peptide metabolism and neuropeptidase activity in senile dementia of the Alzheimer's type

Work in our laboratory has shown that in addition to previously characterized changes in the level of neuropeptides in SDAT brain, the activity of degradative enzymes responsible for peptide metabolism is also affected. In addition to other reported alterations in peptide metabolism, we have observed that SS-28 degradation is increased in Brodmann area 22 whereas substance P degradation is increased in temporal cortex. Changes in the degradation of these neuropeptides known to be affected in SDAT correlate well with alterations in the activity of specific neuropeptidases. Trypsin-like serine protease activity is increased in SDAT Brodmann area 22 which parallels the increased degradation of SS-28. The activity of MEP 24.15 is decreased in temporal cortex which corresponds to the decreased degradation of substance P. Changes in the activity of these degradative enzymes in SDAT brain can potentially affect the action of other neuropeptide substrates because the neuropeptidases discussed here terminate the action of several neuropeptides. As more neuropeptide and degradative peptidase alterations are discovered in SDAT, greater emphasis may be placed on the role that peptides and neuropeptidases play in the progression of SDAT.

Facilitatory role of somatostatin via muscarinic cholinergic system in the generation of long-term potentiation in the rat dentate gyrus in vivo

We investigated whether somatostain modulates the generation of long-term potentiation (LTP) in rat perforant path-dentate gyrus synapse in vivo. When somatostatin was injected intracerebroventricularly (i.c.v.) 20 min prior to the tetanus, the intensity of LTP increased dose dependently. Synaptic potential evoked by a low-frequency test stimulation, however, was not altered by somatostatin. We next tested whether the LTP-augmenting effect of somatostain is mediated by cholinergic activation, because somatostatin was demonstrated to promote acetylcholine release in rat hippocampal slice. Pirenzepine (50 nmol/rat), a muscarinic M1 receptor antagonist, did not affect the tetanus-induced LTP by itself. But when it was co-applicated with the somatostatin (50 ng/rat) 20 min before tetanus, it completely abolished the LTP-augmenting effect of somatostatin. Then we examined the effect of octreotide, a potent agonist specifically binding to somatostatin receptor subtypes 2 and 4, on the generation of LTP. Octreotide (500 ng/rat) also facilitated the intensity of LTP. These results suggest that somatostatin facilitates the generation of perforant path-dentate gyrus granule cell LTP by activating the muscarinic cholinergic receptor and the effect of somatostatin is induced, at least partly, by somatostatin receptor subtypes 2 and 4 in vivo.

Correlations between water maze performance and cortical somatostatin mRNA and high-affinity binding sites during ageing in rats

Somatostatin levels and high-affinity (somatostatin-1) binding sites are decreased in post-mortem cortical samples of Alzheimer's disease patients but the relationships between such modifications and the cognitive deficits remain to be established. We investigated these relationships in the ageing rat. Three age groups (3-4, 14-15 and 26-27 months) were tested in a modified version of the Morris water maze. Somatostatin mRNA levels were quantified by in situ hybridization and somatostatin binding sites by radioautography using the selective agonist octreotide (SMS 201995) as a competing drug to evaluate high-affinity (somatostatin-1) and low-affinity (somatostatin-2) binding sites. The number of somatostatin mRNA-containing cells was not modified with age or memory performance in cortical, hippocampal and hypothalamic regions, but somatostatin mRNA densities were significantly decreased with age and with memory performance in the frontal and parietal cortex. In the frontal cortex somatostatin mRNA densities were already decreased in 14- to 15-month-old rats, whereas the decrease was observed only in 26- to 27-month-old rats in the parietal cortex. A decrease in somatostatin-1 binding was observed with memory performance, independently of age, in the basolateral amygdala only, while somatostatin-2 binding sites were not affected. In the frontal and parietal cortex, a significant correlation occurred between the latency to find the invisible platform in the water maze and somatostatin mRNA (r = -0.54 and 0.59 respectively, P < 0.02). These results indicate that ageing rats with memory impairments display some of the features of the somatostatinergic deficits observed in Alzheimer's disease.

Neuropeptide changes in cortical and deep gray structures in Alzheimer's disease

The alteration of certain neuropeptide levels is a dramatic and consistent finding in the brains of AD patients. Levels of SS, which is normally present in high concentrations in cerebral cortex /75/, are consistently decreased in the neocortex, hippocampus and CSF of AD patients. In addition, decreased levels of SS correlate regionally with the distribution of neurofibrillary tangles in AD /47/. Most available evidence suggests that the subset of SS-containing neurons which lack NADPH diaphorase may be relatively vulnerable to degeneration in AD. CRF is another neuropeptide with frequently observed changes in AD. Levels of CRF, which is normally present in low concentrations in cortical structures /75/, are decreased in the neocortex and hippocampus of AD patients. However, levels of CRF in the CSF of AD patients are not consistently reduced, but this is likely a reflection of the relatively low levels of CRF normally present in cerebral cortex. Studies of deep gray structures in AD brains reveal elevated levels of GAL in the nucleus basalis. The ability of GAL to inhibit cholinergic neurotransmission has generated considerable interest, since degeneration of cholinergic neurons in the basal forebrain consistently occurs in AD. In addition, the presence of NADPH diaphorase in GAL-containing neurons may underlie the relative resistance of these neurons to degeneration. From the aforementioned studies, it appears that the neurons which are relatively resistant to neurodegeneration in AD contain NADPH diaphorase. It is hypothesized that the presence of NADPH diaphorase protects these neurons from neurotoxicity mediated by glutamate or nitric oxide. Although one recent study /147/ has reported an elevation of the microtubule-associated protein tau in the CSF of AD patients (and this could become a useful antemortem diagnostic tool for AD), no similar CSF abnormality has been found for any of the neuropeptides. Thus, the measurement of CSF neuropeptide levels presently remains unhelpful in the diagnosis and treatment of AD. Future research on neuropeptides and their potential roles in the pathogenesis, diagnosis, and treatment of AD will likely involve further development of pharmacological modulators of neuropeptide systems, together with the further study of brain neuropeptidases.

Facilitatory effects of somatostatin on reduced uptake of 2-deoxyglucose in cerebral cortical and hippocampal slices from aged rats

The aim of the present study was to determine whether or not the reduction of 2-deoxyglucose uptake by the cerebral cortical slices in aged rats (22-23 months old) was attenuated by somatostatin or carbachol. In 8-week-old rats, somatostatin and carbachol produced concentration-dependent increases in 2-deoxyglucose uptake. 2-Deoxyglucose uptake of the cortical slices in 22-23-month-old rats was significantly facilitated by treatment with 0.1-1 microM somatostatin or 1-100 microM carbachol. Metabolic responses to somatostatin or carbachol were quite similar in young and aged rats. The present results demonstrated that 2-deoxyglucose uptake by the cerebral cortex was facilitated by somatostatin and carbachol in both young and old rats.

Possible involvement of brain somatostatin in the memory formation of rats and the cognitive enhancing action of FR121196 in passive avoidance task

Using the passive avoidance learning task in rats, the role of brain somatostatin in cognitive function was investigated with special reference to that of the brain cholinergic system. In addition, the involvement of both the brain somatostatinergic and cholinergic systems in the anti-amnesic action of a newly introduced cognitive enhancer, FR121196 [N-(4-acetyl-1-piperazinyl)-4-fluorobenzenesulfonamide], was examined. Treatment with cysteamine (50, 100, 200 mg/kg, s.c.), a depletor of somatostatin, significantly and dose-dependently reduced the retention of single trial passive avoidance task. Similar memory impairments were found in rats which received central cholinergic blockade either by scopolamine (0.1-1 mg/kg) or by lesioning of the nucleus basalis magnocellularis (NBM). Intracerebroventricurally (i.c.v.) administered somatostatin (1-14) (10-1000 ng/rat) significantly ameliorated the memory impairments induced not only by cysteamine (200 mg/kg) but also by scopolamine (1 mg/kg) and NBM-lesioning. Although physostigmine (0.01-1 mg/kg) also ameliorated the memory impairments induced by cysteamine and scopolamine, it failed to affect the memory impairment seen in the NBM-lesioned rats. Administration of FR121196 (0.1-10 mg/kg) significantly ameliorated the memory deficits produced by scopolamine and NBM lesioning but not that induced by cysteamine.(ABSTRACT TRUNCATED AT 250 WORDS)

Environmental influence on somatostatin levels and gene expression in the rat brain

In the present study we have quantified preprosomatostatin-mRNA and somatostatin levels in rat brain following environmental stimulation. Animals were housed for 30 days in an enriched or impoverished environment prior to analysis. After 30 days of housing half of the rats from each environment were behaviourally tested for 3 days. Housing in enriched environment improved performance in a spatial learning situation. The open-field behaviour of these animals was characterized by initially higher rearing scores and a more rapid habituation to novel environment as measured by spontaneous locomotor activity. We found significantly elevated somatostatin levels in the cortex following enriched environment, compared with impoverished environment. Exposure to behavioural testing of impoverished animals led to increased cortical somatostatin levels. Hypothalamic somatostatin levels increased significantly after housing in enriched environment, while the testing procedure had no influence. Our data shows that the somatostatin system in the rat brain was activated in association with cognitive changes, that were induced by housing in an enriched environment.

Quantitative autoradiography of somatostatin receptors in the rat limbic system

The distribution of somatostatin receptors (SRIF-R) was analyzed in the limbic system of the adult rat by in vitro autoradiography with [125I-Tyr0,DTrp 8]S14 as a radioligand. Precise quantification of the density of binding sites, at 0.2 mm intervals throughout the different areas revealed a marked heterogeneity of labeling in most structures. In particular, SRIF-R were concentrated in the basal (104.4 +/- 3.3 fmol/mg proteins) and basolateral amygdaloid nuclei (94.8 +/- 4.3 fmol/mg proteins), and in the nucleus of the lateral olfactory tract (121.6 +/- 2.4 fmol/mg proteins), whereas moderate densities were detected in the amygdalo-hippocampal nucleus (76.4 +/- 2.8 fmol/mg proteins). The medial (41.3 +/- 1.9 fmol/mg proteins) and the central (24.0 +/- 1.4 fmol/mg proteins) amygdaloid nuclei contained lower SRIF-R concentrations. It appears from these observations, in the light of the anatomical pathways of the amygdala, that intra-amygdalian SRIF-containing neurons project to the amygdalo-hippocampal nucleus, and that SRIF-R in the basolateral complex are the target of afferents from limbic cortical areas. SRIF-R were detected at different levels of the hippocampal formation but their distribution was more restricted than that of SRIF-containing fibers. The maximal density of sites was detected in the ventral and dorsal parts of the subiculum (115.0 +/- 3.4 and 87.0 +/- 2.8 fmol/mg proteins, respectively) and in the parasubiculum (100.1 +/- 5.4 fmol/mg proteins). In Ammon's horn, the stratum oriens and stratum radiatum of the CA1 field were the only sites enriched in SRIF-R (74.1 +/- 2.0 and 74.6 +/- 1.9 fmol/mg proteins, respectively). The apparent lack of receptors in the pyramidal cell layer indicated that, in Ammon's horn, SRIF is involved in intra-hippocampal communication. Low levels of receptors were found in the hippocampal CA2 and CA3 fields. SRIF-R in the dentate gyrus were mainly concentrated in the molecular layer (57.3 +/- 1.2 fmol/mg proteins). A very high density of sites was also observed in the entorhinal cortex (up to 123.1 +/- 1.5 fmol/mg proteins). A clear mismatch between SRIF and SRIF-R was detected in the septum and the habenula. In the profound layers of the cingulum and retrosplenial cortex, a heterogeneous distribution of SRIF-R was observed. High concentrations of sites were detected in the rostral zone of the cingulate cortex (93.4 +/- 2.0 fmol/mg proteins) while the posterior cingulate only exhibited moderate concentrations of sites (66.5 +/- 0.7 fmol/mg proteins).(ABSTRACT TRUNCATED AT 400 WORDS)

Developmental changes in expression of a 60-kDa somatostatin receptor immunoreactivity in the rat brain

The neuropeptide somatostatin (SRIF) exerts several important physiological actions in the adult CNS through interactions with membrane-bound receptors. SRIF expression is developmentally regulated and this regulation is most apparent in the cerebellum, where SRIF immunoreactivity is expressed at early postnatal ages and then disappears toward adulthood. The transitory nature of SRIF expression at a time of major changes in cerebellum suggests that this peptide may have a role in cerebellar development. To further investigate the role of the SRIF transmitter system during development, we have examined the levels of expression of SRIF receptors in the developing rat brain by immunoblotting using antiserum selective for a 60-kDa brain SRIF receptor. In whole rat brain, SRIF receptor immunoreactivity first appears at embryonic day 13 (E13), is elevated at E16, increases at birth, peaks at early postnatal ages, and then gradually declines with age. No apparent changes in size of the receptor occur with age. No consistent changes in levels of SRIF receptor immunoreactivity are detected from early postnatal ages to adulthood in the hippocampus, cerebral cortex, and striatum, but levels gradually decline in the hypothalamus. In contrast, SRIF receptor immunoreactivity is expressed transiently in cerebellum. SRIF receptor immunoreactivity is detectable in cerebellum at E16, increases in levels at birth, is apparent from postnatal day 3 to postnatal day 8, and then disappears. The transitory nature of SRIF receptor expression in cerebellum is unique and parallels the expression of SRIF immunoreactivity in this brain region. These findings support the hypothesis that SRIF has a role in cerebellar development.

Deficits in the somatostatin SS1 receptor sub-type in frontal and temporal cortices in Alzheimer's disease

The aim of this study was to evaluate the possible differential alterations of somatostatin (SRIF) receptor sub-types in Alzheimer's disease (AD). Consequently the binding profile of cortical SRIF receptors were examined in normal and AD brains using non-selective ([125I]Tyr0, D-Trp8-SRIF14) and SS1 receptor sub-type-selective ([125I]SMS204-090) radioligands. Maximal binding capacities, but not affinities, were reduced for both ligands in the temporal cortex. In contrast, only the maximal binding capacity of [125I]SMS204-090 was significantly reduced (68%) in the frontal cortex; no alterations were detected using the non-selective probe. This reveals that while the maximal binding capacity of the SS1 receptor sub-type is altered in frontal and temporal cortices in AD, other putative cortical SRIF receptor classes (such as SS2 sites) are not as broadly affected. This could be of significance for eventual therapeutic approaches using SRIF-related analogues.

Somatostatin replacement therapy for Alzheimer dementia

Somatostatin is consistently diminished in brains of patients with Alzheimer's disease. To evaluate whether pharmacological restoration of this transmitter deficit has therapeutic value, the synthetic analogue octreotide was administered intravenously to 14 Alzheimer patients under double-blind, placebo-controlled conditions. At the highest dose administered, spinal fluid concentrations approximated those found in brains of experimental animals receiving behaviorally effective amounts of the drug. Neuropsychological testing, however, showed no clinically significant improvement. Coadministration of octreotide and physostigmine to 1 patient also failed to improve cognition. Positron emission tomographic studies in 6 patients revealed a generalized decrease in glucose metabolism as a result of octreotide infusion. These findings suggest that stimulation of the somatostatin system has no value in the symptomatic treatment of Alzheimer dementia.

Age-related alterations of somatostatin gene expression in different rat brain areas

Numerous experimental and clinical studies have demonstrated that brain somatostatinergic neurotransmission plays an important role in the modulation of several brain functions, including learning and memory processes. Due to the gradual decline of cognitive performances occurring during aging, we evaluated whether an age-related modification of brain somatostatin gene activity occurred in discrete rat brain areas. Our study demonstrates that a significant reduction of pre-prosomatostatin mRNA levels occurred in aged animals (25 months) in the frontal cortex (-49%), in the parietal cortex (-80%) and in the striatum (-69%), despite the absence of changes in beta-actin gene expression. Conversely, no statistical differences were observed in the pre-prosomatostatin mRNA content of old animals in the hypothalamus. These results demonstrate that age-related alterations in somatostatin gene expression occur in the rat, and suggest that such alterations may be involved in the behavioral and cognitive impairments that occur during the aging process.

Somatostatin in neurodegenerative illnesses

Somatostatin may play a role in several neurodegenerative diseases. Somatostatin concentrations are depleted in cerebral cortex in both Alzheimer's disease and in the dementia that accompanies Parkinson's disease. Somatostatin neurons in both illnesses are markedly dystrophic and may be reduced in number. In Huntington's disease, somatostatin concentrations are increased in the basal ganglia, as is the density of somatostatin neurons. The precise role of somatostatin changes in the pathophysiology of these illnesses requires further study.

Neurosecretory and trophic action on fetal rat neuroblasts induced by an amino acid mixture

The effects of a synthetically obtained mixture of amino acids (FACE) were investigated on the trophic and neurosecretory activity of in vitro cultures of fetal rat neuronal cells. The addition of 10(-6) M FACE to the culture medium significantly increased cell DNA content. Secretions of IR-SRIF, IR-VIP, and IR-GRF were also augmented in different proportions by the presence of FACE. Time studies demonstrated that IR-SRIF was significantly increased after 48 (P less than 0.05) and 72 (P less than 0.01) hr of exposure to FACE, and IR-VIP secretion was potentiated after only 24 hr of culture. Dose-response experiments with 10(-7) to 10(-4) M FACE indicated that concentrations of 10(-5) and 10(-4) M significantly increased both somatostatin released to the medium and cell content of IR-SRIF. FACE concentrations as low as 10(-10) M augmented the secretion of IR-GRF, and there was a dose-response correlation between 10(-10) and 10(-5) M FACE. The release and cell content of IR-VIP were also increased by FACE, with a dose-response relation at concentrations of 10(-9) to 10(-6) M. It can thus be concluded that FACE has a powerful effect on the multiplication and survival of fetal cerebrocortical cells and is also an important potentiator of IR-SRIF, IR-VIP, and IR-GRF secretion.

Quantitative radioautographic study of somatostatin receptors heterogeneity in the rat extrahypothalamic brain

The possible heterogeneity of extrahypothalamic somatostatin receptors was studied in rat brain by quantitative radioautography. The respective distribution and relative proportion of two somatostatin receptor sub-types (SS1 and SS2) were assessed by using two radioligands, the non-selective probe [125I]Tyr3-D-Trp8-somatostatin14 and the SS1 selective analogue [125I]Tyr3-SMS 201-995. For both ligands, adjacent brain sections were processed in the presence of micromolar concentrations of either a non-discriminative competitor (somatostatin14) or SS1-selective analogue (SMS 201-995). The comparative analysis of the specific binding remaining in the presence of each non-radioactive competitor permitted a semi-quantitative analysis of the proportion of SS1 and SS2 receptor sub-types in each brain region examined. Data obtained correlate well with homogenate binding results reported previously [Reubi J. C. (1984) Neurosci. Lett. 49, 259-263]. Although the distribution patterns obtained with both radioligands were similar, [125I]Tyr3-SMS 201-995 labelled only a fraction of [125I]Tyr0-D-Trp8-somatostatin14-labelled sites in certain brain regions. For example, both superficial and deep cortical laminae, as well as the basolateral amygdaloid nucleus and CA1 hippocampal area exhibited different binding densities with [125I]Tyr0-D-Trp8-somatostatin14 depending on the competitor used in the assay (somatostatin14 or SMS 201-995). On the other hand, [125I]Tyr3-SMS 201-995 binding was eliminated in an identical fashion by either competitor in these very same brain areas. This suggests the existence of SS1 and SS2 somatostatin receptor sub-types in these regions. In all other brain areas examined, somatostatin receptor sites are apparently of the SS1 sub-type. The heterogeneity of somatostatin receptors observed in certain regions may have relevance for the various biological effects induced by somatostatin in the central nervous system.

Preclinical and clinical studies with cysteamine and pantethine related to the central nervous system

1. Cysteamine is formed by degradation of coenzyme A (CoA) and causes somatostatin (SS), prolactin and noradrenaline depletion in the brain and peripheral tissues. 2. Cysteamine influences several behavioral processes, like active and passive avoidance behavior, open-field activity, kindled seizures, pain perception and SS-induced barrel rotation. 3. Cysteamine has several established (cystinosis, radioprotection, acetaminophen poisoning) and theoretical (Huntington's disease, prolactin-secreting adenomas) indications in clinical practice. 4. Pantethine is a naturally occurring compound which is metabolized to cysteamine. 5. Pantethine depletes SS, prolactin and noradrenaline with lower efficacy compared to that of cysteamine. 6. Pantethine is well tolerated by patients and has been suggested to treatment of atherosclerosis. The other possible clinical indications (alcoholism, Parkinson's disease, instead of cysteamine) are discussed.

Comparative studies of somatostatin-14 and some of its fragments on passive avoidance behavior, open field activity and on barrel rotation phenomenon in rats

Behavioral effects of somatostatin-14, and some of its fragments [somatostatin(3-8), somatostatin(9-14), somatostatin(7-10)] after intracerebroventricular (ICV) administration have been investigated in male rats. In a passive avoidance learning test, somatostatin-14 (0.6 nM) given immediately after the learning session increased the avoidance latency at 24 hr after the injection, when compared to a somatostatin(3-8) (0.6 nM)-treated group. However, compared to a saline-treated group, the peptides did not significantly influence the avoidance latency. Somatostatin-14 administered in higher dose (6.0 nM) decreased the avoidance latency compared to the saline-treated group, while its fragments did not influence it. In an open field behavioral test, immediately after the 24-hr passive avoidance test, 6 nM of somatostatin-14 decreased the rearing activity, while the fragments did not influence this behavior. Somatostatin-14 produced barrel rotation in a dose-related manner, but after the injection of a high dose of the peptide (12 nM) all of the animals died in cardiorespiratory failure (apnea, pulmonary oedema). The fragments did not produce barrel rotation.

Somatostatin inhibition of adenylate cyclase activity in different brain areas

Somatostatin receptors have been identified in different brain areas but the characterization of their postreceptor effect is still lacking. In this study we analyze the somatostatin effect on adenylate cyclase activity in different brain regions, namely frontal cortex, striatum, hypothalamus and hippocampus. Somatostatin inhibited basal adenylate cyclase activity in all brain areas, being maximally effective in the frontal cortex (-42%). Moreover, somatostatin inhibited both dopamine- and norepinephrine-stimulated adenylate cyclase activity in the examined cerebral regions showing a higher effectiveness than in basal conditions. VIP stimulation of adenylate cyclase was also reduced by somatostatin. The peptide inhibited by 50% forskolin-stimulated (10 nM to 10 microM) enzyme activity in frontal cortex and hypothalamus (in hippocampus the inhibition reached only -25%) showing a non-competitive pattern of inhibition. Finally, pertussis toxin pretreatment abolished the somatostatin inhibition of forskolin-stimulated frontal cortex adenylate cyclase activity. These results show that brain somatostatin receptors are coupled in an inhibitory way with adenylate cyclase enzyme that may represent one of the postreceptor mechanisms mediating the somatostatin modulation of brain functions.