CSF somatostatin in Alzheimer's disease and major depression: relationship to hypothalamic-pituitary-adrenal axis and clinical measures

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.

Down-regulation of habenular calcium-dependent secretion activator 2 induces despair-like behavior

Calcium-dependent secretion activator 2 (CAPS2) regulates the trafficking and exocytosis of neuropeptide-containing dense-core vesicles (DCVs). CAPS2 is prominently expressed in the medial habenula (MHb), which is related to depressive behavior; however, how MHb neurons cause depressive symptoms and the role of CAPS2 remains unclear. We hypothesized that dysfunction of MHb CAPS neurons might cause defects in neuropeptide secretion and the activity of monoaminergic centers, resulting in depressive-like behaviors. In this study, we examined (1) CAPS2 expression in the habenula of depression animal models and major depressive disorder patients and (2) the effects of down-regulation of MHb CAPS2 on the animal behaviors, synaptic transmission in the interpeduncular nucleus (IPN), and neuronal activity of monoamine centers. Habenular CAPS2 expression was decreased in the rat chronic restraint stress model, mouse learned helplessness model, and showed tendency to decrease in depression patients who died by suicide. Knockdown of CAPS2 in the mouse habenula evoked despair-like behavior and a reduction of the release of DCVs in the IPN. Neuronal activity of IPN and monoaminergic centers was also reduced. These results implicate MHb CAPS2 as playing a pivotal role in depressive behavior through the regulation of neuropeptide secretion of the MHb-IPN pathway and the activity of monoaminergic centers.

Somatostatin Ameliorates β-Amyloid-Induced Cytotoxicity via the Regulation of CRMP2 Phosphorylation and Calcium Homeostasis in SH-SY5Y Cells

Somatostatin is involved in the regulation of multiple signaling pathways and affords neuroprotection in response to neurotoxins. In the present study, we investigated the role of Somatostatin-14 (SST) in cell viability and the regulation of phosphorylation of Collapsin Response Mediator Protein 2 (CRMP2) (Ser522) via the blockade of Ca²⁺ accumulation, along with the inhibition of cyclin-dependent kinase 5 (CDK5) and Calpain activation in differentiated SH-SY5Y cells. Cell Viability and Caspase 3/7 assays suggest that the presence of SST ameliorates mitochondrial stability and cell survival pathways while augmenting pro-apoptotic pathways activated by Aβ. SST inhibits the phosphorylation of CRMP2 at Ser522 site, which is primarily activated by CDK5. Furthermore, SST effectively regulates Ca²⁺ influx in the presence of Aβ, directly affecting the activity of calpain in differentiated SH-SY5Y cells. We also demonstrated that SSTR2 mediates the protective effects of SST. In conclusion, our results highlight the regulatory role of SST in intracellular Ca²⁺ homeostasis. The neuroprotective role of SST via axonal regeneration and synaptic integrity is corroborated by regulating changes in CRMP2; however, SST-mediated changes in the blockade of Ca²⁺ influx, calpain expression, and toxicity did not correlate with CDK5 expression and p35/25 accumulation. To summarize, our findings suggest two independent mechanisms by which SST mediates neuroprotection and confirms the therapeutic implications of SST in AD as well as in other neurodegenerative diseases where the effective regulation of calcium homeostasis is required for a better prognosis.

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

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

The Corticotropin-Releasing Factor Family: Physiology of the Stress Response

The physiological stress response is responsible for the maintenance of homeostasis in the presence of real or perceived challenges. In this function, the brain activates adaptive responses that involve numerous neural circuits and effector molecules to adapt to the current and future demands. A maladaptive stress response has been linked to the etiology of a variety of disorders, such as anxiety and mood disorders, eating disorders, and the metabolic syndrome. The neuropeptide corticotropin-releasing factor (CRF) and its relatives, the urocortins 1–3, in concert with their receptors (CRFR1, CRFR2), have emerged as central components of the physiological stress response. This central peptidergic system impinges on a broad spectrum of physiological processes that are the basis for successful adaptation and concomitantly integrate autonomic, neuroendocrine, and behavioral stress responses. This review focuses on the physiology of CRF-related peptides and their cognate receptors with the aim of providing a comprehensive up-to-date overview of the field. We describe the major molecular features covering aspects of gene expression and regulation, structural properties, and molecular interactions, as well as mechanisms of signal transduction and their surveillance. In addition, we discuss the large body of published experimental studies focusing on state-of-the-art genetic approaches with high temporal and spatial precision, which collectively aimed to dissect the contribution of CRF-related ligands and receptors to different levels of the stress response. We discuss the controversies in the field and unravel knowledge gaps that might pave the way for future research directions and open up novel opportunities for therapeutic intervention.

Synaptic plasticity modulation by circulating peptides and metaplasticity: Involvement in Alzheimer's disease

Synaptic plasticity is a cellular process involved in learning and memory whose alteration in its two main forms (Long Term Depression (LTD) and Long Term Potentiation (LTP)), is observed in most brain pathologies, including neurodegenerative disorders such as Alzheimer's disease (AD). In humans, AD is associated at the cellular level with neuropathological lesions composed of extracellular deposits of β-amyloid (Aβ) protein aggregates and intracellular neurofibrillary tangles, cellular loss, neuroinflammation and a general brain homeostasis dysregulation. Thus, a dramatic synaptic environment perturbation is observed in AD patients, involving changes in brain neuropeptides, cytokines, growth factors or chemokines concentration and diffusion. Studies performed in animal models demonstrate that these circulating peptides strongly affect synaptic functions and in particular synaptic plasticity. Besides this neuromodulatory action of circulating peptides, other synaptic plasticity regulation mechanisms such as metaplasticity are altered in AD animal models. Here, we will review new insights into the study of synaptic plasticity regulatory/modulatory mechanisms which could influence the process of synaptic plasticity in the context of AD with a particular attention to the role of metaplasticity and peptide dependent neuromodulation.

The Protective Effects of IGF-I against β-Amyloid-related Downregulation of Hippocampal Somatostatinergic System Involve Activation of Akt and Protein Kinase A

Somatostatin (SRIF), a neuropeptide highly distributed in the hippocampus and involved in learning and memory, is markedly reduced in the brain of Alzheimer's disease patients. The effects of insulin-like growth factor-I (IGF-I) against β amyloid (Aβ)-induced neuronal death and associated cognitive disorders have been extensively reported in experimental models of this disease. Here, we examined the effect of IGF-I on the hippocampal somatostatinergic system in Aβ-treated rats and the molecular mechanisms associated with changes in this peptidergic system. Intracerebroventricular Aβ25-35 administration during 14 days (300 pmol/day) to male rats increased Aβ25-35 levels and cell death and markedly reduced SRIF and SRIF receptor 2 levels in the hippocampus. These deleterious effects were associated with reduced Akt and cAMP response element-binding protein (CREB) phosphorylation and activation of c-Jun N-terminal kinase (JNK). Subcutaneous IGF-I co-administration (50 µg/kg/day) reduced hippocampal Aβ25-35 levels, cell death and JNK activation. In addition, IGF-I prevented the reduction in the components of the somatostatinergic system affected by Aβ infusion. Its co-administration also augmented protein kinase A (PKA) activity, as well as Akt and CREB phosphorylation. These results suggest that IGF-I co-administration may have protective effects on the hippocampal somatostatinergic system against Aβ insult through up-regulation of PKA activity and Akt and CREB phosphorylation.

Functional Amyloids and their Possible Influence on Alzheimer Disease

Amyloids play critical roles in human diseases but have increasingly been recognized to also exist naturally. Shared physicochemical characteristics of amyloids and of their smaller oligomeric building blocks offer the prospect of molecular interactions and crosstalk amongst these assemblies, including the propensity to mutually influence aggregation. A case in point might be the recent discovery of an interaction between the amyloid β peptide (Aβ) and somatostatin (SST). Whereas Aβ is best known for its role in Alzheimer disease (AD) as the main constituent of amyloid plaques, SST is intermittently stored in amyloid-form in dense core granules before its regulated release into the synaptic cleft. This review was written to introduce to readers a large body of literature that surrounds these two peptides. After introducing general concepts and recent progress related to our understanding of amyloids and their aggregation, the review focuses separately on the biogenesis and interactions of Aβ and SST, before attempting to assess the likelihood of encounters of the two peptides in the brain, and summarizing key observations linking SST to the pathobiology of AD. While the review focuses on Aβ and SST, it is to be anticipated that crosstalk amongst functional and disease-associated amyloids will emerge as a general theme with much broader significance in the etiology of dementias and other amyloidosis.

Decreased Numbers of Somatostatin-Expressing Neurons in the Amygdala of Subjects With Bipolar Disorder or Schizophrenia: Relationship to Circadian Rhythms

BACKGROUND

Growing evidence points to a key role for somatostatin (SST) in schizophrenia (SZ) and bipolar disorder (BD). In the amygdala, neurons expressing SST play an important role in the regulation of anxiety, which is often comorbid in these disorders. We tested the hypothesis that SST-immunoreactive (IR) neurons are decreased in the amygdala of subjects with SZ and BD. Evidence for circadian SST expression in the amygdala and disrupted circadian rhythms and rhythmic peaks of anxiety in BD suggest a disruption of rhythmic expression of SST in this disorder.

METHODS

Amygdala sections from 12 SZ, 15 BD, and 15 control subjects were processed for immunocytochemistry for SST and neuropeptide Y, a neuropeptide partially coexpressed in SST-IR neurons. Total numbers (Nt) of IR neurons were measured. Time of death was used to test associations with circadian rhythms.

RESULTS

SST-IR neurons were decreased in the lateral amygdala nucleus in BD (Nt, p = .003) and SZ (Nt, p = .02). In normal control subjects, Nt of SST-IR neurons varied according to time of death. This pattern was altered in BD subjects, characterized by decreases of SST-IR neurons selectively in subjects with time of death corresponding to the day (6:00 am to 5:59 pm). Numbers of neuropeptide Y-IR neurons were not affected.

CONCLUSIONS

Decreased SST-IR neurons in the amygdala of patients with SZ and BD, interpreted here as decreased SST expression, may disrupt responses to fear and anxiety regulation in these individuals. In BD, our findings raise the possibility that morning peaks of anxiety depend on a disruption of circadian regulation of SST expression in the amygdala.

Decrease in somatostatin-positive cell density in the amygdala of females with major depression

BACKGROUND

Somatostatin (SST) is a neuropeptide expressed in a subtype of gamma-aminobutyric acid (GABA) interneurons that target the dendrites of pyramidal neurons. We previously reported reduced levels of SST gene and protein expression in the postmortem amygdala of subjects with major depressive disorder (MDD). This reduction was specific to female subjects with MDD.

METHODS

Here, we used in situ hybridization to examine the regional and cellular patterns of reductions in SST expression in a cohort of female MDD subjects with known SST deficits in the amygdala (N = 10/group).

RESULTS

We report a significant reduction in the density of SST-labeled neurons in the lateral, basolateral, and basomedial nuclei of the amygdala of MDD subjects compared to controls. SST mRNA levels per neuron did not differ between MDD and control subjects in the lateral or basolateral nuclei, but were lower in the basomedial nucleus. There was no difference in cross-sectional density of total cells.

CONCLUSIONS

In summary, we report an MDD-related reduction in the density of detectable SST-positive neurons across several nuclei in the amygdala, with a reduction in SST mRNA per cell restricted to the basomedial nucleus. In the absence of changes in total cell density, these results suggest the possibility of a change in SST cell phenotype rather than cell death in the amygdala of female MDD subjects.

Chronic mild stress alters the somatostatin receptors in the rat brain

RATIONALE

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Somatostatin, neuronal vulnerability and behavioral emotionality

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

HPA axis and aging in depression: systematic review and meta-analysis

One of the most consistent findings in the biology of depression is an altered activity of the hypothalamic-pituitary-adrenal (HPA) axis. However, data concerning this issue have never been examined with a focus on the older population. Here we present a systematic review and meta-analysis, based on studies investigating levels of cortisol, adrenocorticotropic hormone (ACTH) and corticotropin-releasing hormone (CRH) in depressed participants older than 60 and compared with healthy controls. We found 20 studies, for a total of 43 comparisons on different indices of HPA axis functioning. Depression had a significant effect (Hedges' g) on basal cortisol levels measured in the morning (0.89), afternoon (0.83) and night (1.39), but a smaller effect on cortisol measured continuously (0.51). The effect of depression was even higher on post-dexamethasone cortisol levels (3.22), whereas it was non-significant on morning ACTH and CRH levels. Subgroup analyses indicated that various methodological and clinical factors can influence the study results. Overall, older participants suffering from depression show a high degree of dysregulation of HPA axis activity, with differences compared with younger adults. This might depend on several mechanisms, including physical illnesses, alterations in the CNS and immune-endocrinological alterations. Further studies are needed to clarify the implications of altered HPA axis activity in older patients suffering from depression. Novel pharmacological approaches might be effective in targeting this pathophysiological feature, thus improving the clinical outcomes.

Reduced brain somatostatin in mood disorders: a common pathophysiological substrate and drug target?

Our knowledge of the pathophysiology of affect dysregulation has progressively increased, but the pharmacological treatments remain inadequate. Here, we summarize the current literature on deficits in somatostatin, an inhibitory modulatory neuropeptide, in major depression and other neurological disorders that also include mood disturbances. We focus on direct evidence in the human postmortem brain, and review rodent genetic and pharmacological studies probing the role of the somatostatin system in relation to mood. We also briefly go over pharmacological developments targeting the somatostatin system in peripheral organs and discuss the challenges of targeting the brain somatostatin system. Finally, the fact that somatostatin deficits are frequently observed across neurological disorders suggests a selective cellular vulnerability of somatostatin-expressing neurons. Potential cell intrinsic factors mediating those changes are discussed, including nitric oxide induced oxidative stress, mitochondrial dysfunction, high inflammatory response, high demand for neurotrophic environment, and overall aging processes. Together, based on the co-localization of somatostatin with gamma-aminobutyric acid (GABA), its presence in dendritic-targeting GABA neuron subtypes, and its temporal-specific function, we discuss the possibility that deficits in somatostatin play a central role in cortical local inhibitory circuit deficits leading to abnormal corticolimbic network activity and clinical mood symptoms across neurological disorders.

Corticotropin-Releasing Hormone, Glutamate, and γ-Aminobutyric Acid in Depression

Stress response and depression have a significant impact on modern society. Although the symptoms are well characterized, the molecular mechanisms underlying depression are largely unknown. The monoamine hypothesis, which postulates dysfunctional noradrenergic and serotonergic systems as the underlying primary cause of depression, has been valuable for the development of conventional antidepressants, which can reverse these dysfunctional states to some degree. However, recent data from various neuroscience disciplines have questioned the major role of amines in the pathogenesis of depression. A considerable amount of evidence has accumulated that suggests that normalization of the hypothalamo—pituitary—adrenal (HPA) system might be the final step necessary for a remission of depression. In addition, an increasing body of clinical and postmortem evidence is pointing to a role played by γ-aminobutyric acid (GABA) and glutamate in the etiology of depression. This review examines the evidence, mainly obtained from clinical studies or from postmortem brain material, for a major role of the HPA axis, glutamatergic, and GABAergic systems in the pathogenesis of major and bipolar depression. The authors hope that these insights will stimulate further studies with the final aim of developing new types of antidepressants that combine increased efficacy with a shorter delay of the onset of action and reduced side-effect profiles.

Depression and antidepressants: molecular and cellular aspects

Clinical depression is viewed as a physical and psychic disease process having a neuropathological basis, although a clear understanding of its ethiopathology is still missing. The observation that depressive symptoms are influenced by pharmacological manipulation of monoamines led to the hypothesis that depression results from reduced availability or functional deficiency of monoaminergic transmitters in some cerebral regions. However, there are limitations to current monoamine theories related to mood disorders. Recently, a growing body of experimental data has showed that other classes of endogenous compounds, such as neuropeptides and amino acids, may play a significant role in the pathophysiology of affective disorders. With the development of neuroscience, neuronal networks and intracellular pathways have been identified and characterized, describing the existence of the interaction between monoamines and receptors in turn able to modulate the expression of intracellular proteins and neurotrophic factors, suggesting that depression/antidepressants may be intermingled with neurogenesis/neurodegenerative processes.

Somatostatin in medium-sized aspiny interneurons of striatum is responsible for their preservation in quinolinic acid and N-methyl-D-asparate-induced neurotoxicity

Somatostatin (SST) is a multifunctional peptide and involves in several neurodegenerative diseases. N-Methyl-D-asparate (NMDA) receptor agonist quinolinic acid (QUIN)-induced neurotoxicity mimics an experimental model of Huntington's disease that is characterized by the selective preservation of medium-sized aspiny interneurons and degeneration of medium-sized spiny projection neurons in striatum. In QUIN- and NMDA-induced neurotoxicity, increased expression of SST and messenger RNA levels along with SST release in culture medium is generally observed. However, the molecular mechanisms and the functional consequences of increased SST are still obscure. In the present study, the role of SST was determined using immunoneutralization and immunoblockade of SST in cultured striatal neurons upon QUIN- and NMDA-induced neurotoxicity. The immunoblockade of SST with antisense oligonucleotides and immunoabsorption of released SST with specific antibodies potentiate QUIN- and NMDA-induced neuronal cell death. NADPH-diaphorase positive neurons that are selectively spared in several processes of neurodegeneration result in severe damage upon immunoblockade or immunoabsorption of SST. In addition, exogenous SST along with QUIN and NMDA provides selective preservation of projection neurons, which are selectively susceptible in excitotoxicity. Neuroprotective effect of SST is completely blocked by pertussis toxins, suggesting the role of somatostatin receptors. Taken together, these results provide first evidence that the presence of SST is a unique feature for the selective sparing of medium sized aspiny interneurons in excitotoxicity.

Somatostatin and Alzheimer's disease

Alzheimer's disease (AD) is characterized by the cerebral deposition of senile plaques that are mainly composed of a set of peptides referred to as amyloid beta-peptides (Abeta). Among the numerous neuropeptides produced in intrinsic cortical and hippocampal neurons, somatostatin (SRIF) has been found to be the most consistently reduced in the brain and cerebrospinal fluid of AD patients. SRIF receptors (SSTR), which mediate the neuromodulatory signals of SRIF, are also markedly depleted in the AD brain, there being subtype-selective alterations in cortical areas. In the rat temporal cortex, we have shown that intracerebroventricular infusion of Abeta25-35 results in a decrease in SRIF-like immunoreactivity and in SRIF receptor subtype 2 (SSTR2) mRNA and protein levels, in correlation with a decrease in SSTR functionality. Insulin-like growth factor-I prevents the reduction in these parameters induced by Abeta25-35. Abeta has recently been demonstrated to be degraded primarily by a neutral endopeptidase, neprilysin, in the brain. SRIF regulates brain Abeta levels via modulation of neprilysin activity. Because SRIF expression in the brain declines upon aging in various mammals, including rodents, apes and humans, the aging-dependent reduction of SRIF has been hypothesized to trigger accumulation of Abeta in the brain by suppressing neprilysin action. Here we present an overview of recent advances on the role of SRIF in AD and its relationship with Abeta peptides.

The role of corticotropin-releasing factor and noradrenaline in stress-related responses, and the inter-relationships between the two systems

Substantial evidence indicates that brain neurons containing and secreting noradrenaline and corticotropin-releasing factor (CRF) are activated during stress, and that physiological and behavioural responses observed during stress can be induced by exogenous administration of CRF and adrenoceptor agonists. This review focusses on the evidence for the involvement of these two factors in stress-related responses, and the inter-relationships between them. The possible abnormalities of these two systems in depressive illness are also discussed.

Somatostatin down-regulates the expression and release of endozepines from cultured rat astrocytes via distinct receptor subtypes

Endozepines, a family of regulatory peptides related to diazepam-binding inhibitor (DBI), are synthesized and released by astroglial cells. Because rat astrocytes express various subtypes of somatostatin receptors (sst), we have investigated the effect of somatostatin on DBI mRNA level and endozepine secretion in rat astrocytes in secondary culture. Somatostatin reduced in a concentration-dependent manner the level of DBI mRNA in cultured astrocytes. This inhibitory effect was mimicked by the selective sst4 receptor agonist L803-087 but not by the selective sst1, sst2 and sst3 receptor agonists L779-591, L779-976 and L797-778, respectively. Somatostatin was unable to further reduce DBI mRNA level in the presence of the MEK inhibitor U0126. Somatostatin and the sst1, sst2 and sst4 receptor agonists induced a concentration-dependent inhibition of endozepine release. Somatostatin and the sst1, sst2 and sst4 receptor agonists also inhibited cAMP formation dose-dependently. In addition, somatostatin reduced forskolin-induced endozepine release. H89 mimicked the inhibitory effect of somatostatin on endozepine secretion. In contrast the PLC inhibitor U73122, the PKC activator PMA and the PKC inhibitor calphostin C had no effect on somatostatin-induced inhibition of endozepine release. The present data demonstrate that somatostatin reduces DBI mRNA level mainly through activation of sst4 receptors negatively coupled to the MAPK pathway, and inhibits endozepine release through activation of sst1, sst2 and sst4 receptors negatively coupled to the adenylyl cyclase/PKA pathway.

Expression of somatostatin receptor subtypes (SSTR1–5) in Alzheimer’s disease brain: An immunohistochemical analysis

Somatostatin, widely distributed in human cortical brain regions, acts through specific high affinity somatostatin receptors (SSTR1-5) to exert profound effects on motor, sensory, behavioral, cognitive and autonomic functions. Somatostatin levels are consistently decreased in the cortex of Alzheimer's disease (AD) brain and in cerebrospinal fluid, and have become reproducible markers of this disease. In the present study, the distributional pattern of SSTR1-5 antigens in the frontal cortex of AD and age-matched control brains was studied using antipeptide polyclonal rabbit antibodies directed against the five human somatostatin receptor subtypes. All five SSTRs were differentially expressed as membrane and cytoplasmic proteins in cortical neurons with significant variations in control vs. AD brain. In AD cortical brain region, somatostatin and neuropeptide-Y-positive neurons decreased (>70%), and glial fibrillary acidic protein-positive astrocytes significantly increased (>130%) in comparison to control brain. SSTR2 and 4 were the predominant subtypes followed by SSTR1, 3 and 5. AD cortex showed a marked reduction in neuronal expression of SSTR4 and 5 and a modest decrease in SSTR2-like immunoreactivity without any changes in SSTR1 immunoreactive neurons. In contrast, SSTR3 was the only receptor subtype that increased in AD cortex. In AD cortex, SSTR1-, 3- and 4-like immunoreactivities were strongly expressed in glial cells but not SSTR2 and 5. These findings suggest the differential loss of immunoreactivity of SSTR2, 4 and 5 but not SSTR1, and increased SSTR3 in frontal cortex of AD brain as well as subtype-selective glial expression in AD brain. In summary, subtype-selective changes in the expression of SSTRs at protein levels in AD cortical regions suggest that somatostatin and SSTR-containing neurons are pathologically involved in AD and could possibly be used as markers of this disease.

Neuroendocrinology of neurodegenerative diseases. Insights from transgenic mouse models

The nervous system plays a key role in the regulation of neuroendocrine axes and, in turn, the released neurohormones modulate the activity of different brain regions. Neurodegenerative diseases, which are known to affect specific neuronal populations, may provoke neuroendocrine dysfunctions that alter the intimate relationship between both systems. In addition, these modifications may influence the progression of the neurodegenerative process. In the present review, we summarise some of the endocrine changes characterising three major neurodegenerative diseases: Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis. Special attention is focused on the contribution of disease transgenic models to elucidate such alterations.

Low CSF somatostatin associated with response to nimodipine in patents with affective illness

BACKGROUND

In patients with depression, treatment with nimodipine has been shown to increase cerebrospinal fluid (CSF) somatostatin (SRIF) and ameliorate baseline global cerebral hypometabolism. This study was conducted to assess whether a low baseline level of CSF SRIF was associated with response to nimodipine treatment.

METHODS

Twenty-one depressed patients underwent lumbar puncture for analysis of CSF somatostatin-like immunoreactivity (SRIF-LI) during a medication-free period and after at least 6 weeks of nimodipine monotherapy. Twenty-five healthy control subjects were utilized as a comparison group. Clinical improvement was assessed using the Clinical Global Impression Scale for Bipolar Illness.

RESULTS

As predicted, baseline CSF SRIF-LI was significantly lower in eventual nimodipine responders (33.1 +/- 2.8 pg/mol) compared to eventual nonresponders [41.9 +/- 2.6 pg/mL; t(19) = 1.98, p =.03, one-tailed].

CONCLUSIONS

Low baseline CSF somatostatin in depression may be associated with response to nimodipine, which in turn may be related to the ability of nimodipine to increase CSF somatostatin.

Deficit of somatostatin-like immunoreactivity in the vitreous fluid of diabetic patients: possible role in the development of proliferative diabetic retinopathy

OBJECTIVE

To evaluate the vitreous levels of somatostatin-like immunoreactivity (SLI) in patients with proliferative diabetic retinopathy (PDR).

RESEARCH DESIGN AND METHODS

A total of 14 diabetic patients with PDR, in whom a vitrectomy was performed, were included in the study. Sixteen nondiabetic patients, with other conditions requiring vitrectomy, served as a control group. Both venous blood and vitreous samples were collected at the time of vitreoretinal surgery. Patients in whom intravitreous hemoglobin was detectable were excluded. In addition, a correction for plasma levels of SLI and intravitreal proteins was performed. SLI was measured by radioimmunoassay and vitreous hemoglobin by spectrophotometry.

RESULTS

SLI in the vitreous fluid was significantly lower in diabetic patients than in the control group (68 +/- 18.7 vs. 193.6 +/- 30.8 pg/ml, P < 0.01). The vitreous SLI-to-plasma SLI ratio was strikingly higher in nondiabetic subjects than in diabetic patients with PDR (5.3 [1.2-71.1] vs. 0.6 [0.03-4.1], P < 0.01). After correcting for total vitreous protein concentration, SLI (pg/mg of proteins) remained significantly higher in nondiabetic control subjects than in diabetic patients with PDR (186 [51-463] vs. 7.5 [0.8-82], P < 0.0001). Remarkably, intravitreous levels of SLI were higher than those obtained in plasma in nondiabetic control subjects (193.6 +/- 30.8 vs. 43.5 +/- 10.7 pg/ml, P < 0.0001). Finally, a lack of relationship between plasma and vitreous levels of SLI was observed in both diabetic patients with PDR and nondiabetic control subjects.

CONCLUSIONS

The significantly higher SLI in the vitreous fluid than in plasma detected in nondiabetic control subjects supports the concept that somatostatin plays a relevant role in retinal homeostasis. In addition, the intravitreous deficit of SLI observed in diabetic patients with PDR suggests that it might contribute to the process of retinal neovascularization.

Neuropeptides involved in the pathophysiology of schizophrenia and major depression

The present review summarizes the findings on the role of neuropeptides in the pathophysiology of schizophrenia and major depression. Several neuropeptides as vasopressin and endorphins in particular, beta-endorphin and gamma-type endorphins, cholecystokinin (CCK), neurotensin, somatostatin and Neuropeptide Y have been implicated in schizophrenia. During the last decade, however, few attempts to explore the significance of most of these and other neuropeptides in the pathophysiology of the disease or their therapeutic potential are found in the literature. An exception is neurotensin, which exerts neuroleptic-like effects in animal studies, while CSF, brain and blood studies are inconclusive. Things are different in major depression. Here much attention is paid to the endocrine abnormalities found in this disorder in particular the increased activity of the hypothalamic-pituitary-adrenal (HPA) axis. Neuropeptides as corticotropin-releasing hormone (CRH), vasopressin and corticosteroids are implicated in the symptomatology of this disorder. As a consequence much work is going on investigating the influence of CRH and corticosteroid antagonists or inhibitors of the synthesis of corticosteroids as potential therapeutic agents. This review emphasizes the role of vasopressin in the increased activity of the HPA axis in major depression and suggests exploration of the influence of the now available non-peptidergic vasopressin orally active V1 antagonists.

Processing of neuropeptide Y, galanin, and somatostatin in the cerebrospinal fluid of patients with Alzheimer's disease and frontotemporal dementia

Alzheimer's disease (AD) and frontotemporal dementia (FTD) are two prevalent neurodegenerative disorders for which the causes are unknown, except in rare familial cases. Several changes in neuropeptide levels as measured by radioimmunoassay (RIA) have been observed in these illnesses. Somatostatin (SOM) levels in cerebrospinal fluid (CSF) are consistently decreased in AD and FTD. Neuropeptide Y (NPY) levels are decreased in AD, but normal in FTD. Galanin (GAL) levels increase with the duration of illness in AD patients. The majority of studies of neuropeptides in CSF have not been verified by HPLC. The observed decrease in a neuropeptide level as measured by RIA may therefore reflect an altered synthesis or extracellular processing, resulting in neuropeptide fragments that may or may not be detected by RIA. Matrix-assisted laser desorption time-of-flight mass spectrometry (MALDI-MS) has been shown to be a powerful technique in the analysis of biological materials without any pre-treatment, by detecting peptides and proteins at a specific mass-to-charge (m/z) ratio. We studied the processing of the neuropeptides NPY, NPY, SOM and GAL in the cerebrospinal fluid of patients with AD (n = 3), FTD (n = 3) and controls (n = 2) using MALDI-MS. We found that considerable inter-individual variability exists in the rate of neuropeptide metabolism in CSF, as well as the number of peptide fragments formed. Certain patients showed differences in the processing of specific neuropeptides, relative to other patients and controls. This analysis of the metabolic processing of neuropeptides in CSF yielded a large amount of data for each individual studied. Further studies are required to determine the changes in neuropeptide processing that can be associated with AD and FTD. With further investigations using MALDI-MS analysis, it may be possible to identify a neuropeptide fragment or processing enzyme that can be correlated to these disease states.

Corticotropin-releasing hormone in depression and post-traumatic stress disorder

Corticotropin-releasing hormone (CRH) has been implicated in the regulation of a wide range of behaviors including arousal, motor function, feeding, and reproduction. Because depressed patients are often hypercortisolemic and intracerebroventricular administration of CRH to experimental animals produces a syndrome reminiscent of depression, dysregulation of this compound has been suggested to be involved in the pathogenesis of depressive and anxiety disorders. Studies of cerebrospinal fluid CRH levels and clinical neuroendocrine tests in patients with anxiety and affective disorders have supported this hypothesis. This review discusses these neuroendocrine findings in melancholic and atypical depression as well as post-traumatic stress disorder (PTSD). Overall, the data suggest that melancholic depression is characterized by hyperactive central CRH systems with overactivity of the pituitary-adrenal (HPA) axis. On the other hand, atypical depression is characterized by hypoactive central CRH systems and accompanying underactivity of the hypothalamic-pituitary-adrenal axis. Furthermore, the neuroendocrinology of PTSD appears to be unique, in that patients have hyperactive central CRH systems with underactivity of the pituitary-adrenal axis.

A follow up study of suicide attempters: increase of CSF-somatostatin but no change in CSF-CRH

Concentrations of somatostatin and corticotrophin releasing hormone (CRH), measured in cerebrospinal fluid (CSF) have been reported to be low in suicidal patients with major depressive disorder (MDD). Often have MDD patients in general, high CSF-CRH and low CSF-somatostatin concentrations, which both seem to normalise with clinical recovery. The present study was designed to look for CSF-CRH and CSF-somatostatin alterations along with clinical changes in patients studied repeatedly after a suicide attempt. Sixteen patients with different diagnoses, initially inpatients after a suicide attempt (baseline), participated. Lumbar punctures and ratings according to the Suicidal Assessment Scale (SUAS) and the Montgomery-Asberg Depression Rating Scale (MADRS) were performed while patients were drug-free (baseline) and after a median of 7 (5 to 9) months. At follow up MADRS- and SUAS-scores were significantly decreased (P<0.05), whereas CSF-somatostatin was significantly increased (P=0.013) and CSF-CRH had not changed significantly. Thus, the patients showed long-lasting low CSF-CRH concentrations, in spite of changed CSF-somatostatin concentrations and clinical amelioration.

Proteomics in molecular medicine: applications in central nervous systems disorders

Bodily fluids such as cerebrospinal fluid (CSF) and serum can be analysed at the time of presentation and throughout the course of the disease. Changes in the protein composition of CSF may be indicative of altered CNS protein expression pattern with a causative or diagnostic disease link. These findings can be strengthened through subsequent proteomic analysis of specific brain areas implicated in the pathology. New isolation strategies of clinically relevant cellular material such as laser capture microdissection, protein enrichment procedures and proteomic approaches to neuropeptide and neurotransmitter analysis give us the opportunity to map out complex cellular interaction at an unprecedented level of detail. In neurological disorders multiple underlying pathogenic mechanisms as well as an acute and a chronic CNS disease components may require a selective repertoire of molecular targets and biomarkers rather than an individual protein to better define a complex disease. The resulting proteome database bypasses many ambiguities of experimental models and may facilitate pre- and clinical development of more specific disease markers and new selective fast acting therapeutics.

Stress system alterations and mood disorders in suicidal patients. A review

Stress system alterations, in particular HPA axis hyperactivations, are fairly well documented in suicide victims and in depressed suicide attempters who subsequently complete suicide. In suicide attempters with no documented completed suicide, the results are more inconsistent. This may depend on differences in studies due to diagnostic heterogeneity among suicide attempters, and the time between the suicide attempt and the examination. Recent data suggests differences in the stress system between depressed patients with a recent suicide attempt and depressed patients in general. The field merits further examination, with thorough examinations of genotypes, actual suicide attempts and stress in combination with examinations of the stress system.

A novel role for receptor-associated protein in somatostatin modulation: implications for Alzheimer's disease

Receptor-associated protein appears to play an important role in low-density lipoprotein receptor-related protein trafficking. Since ligands for the low-density lipoprotein receptor-related protein have been implicated in Alzheimer's disease and normal functioning of this protein is indispensable for central nervous system development, deficient receptor-associated protein expression may result in central nervous system alterations. In this study, receptor-associated protein knockout mice were behaviorally tested and nervous system integrity was assessed via in situ hybridization and immunocytochemical/laser confocal microscopy methods. Receptor-associated protein knockout mice were found to be cognitively impaired in the Morris water maze compared to controls. In wild-type mice, the receptor-associated protein was found to be highly co-expressed with somatostatin in hippocampal and neocortical inhibitory neurons. Receptor-associated protein knockout mice, however, showed a significant decrease in number of somatostatin-expressing neurons of the CA1 region and somatostatin expression within these neurons. The decreased number of somatostatin neurons significantly correlated with cognitive impairment observed in the receptor-associated protein knockout mice. These results suggest a novel role for receptor-associated protein in modulating the functioning of somatostatin-producing neurons. Furthermore, this has implications for Alzheimer's disease pathogenesis, in which altered regulation of both somatostatin and the known low-density lipoprotein receptor-related protein ligands are a consistent finding.

The rationale for corticotropin-releasing hormone receptor (CRH-R) antagonists to treat depression and anxiety

Neuroendocrine studies strongly suggest that dysregulation of the hypothalamic pituitary-adrenocortical (HPA) system plays a causal role in the development and course of depression. Whereas the initial mechanism resulting in HPA hyperdrive remains to be elucidated, evidence has emerged that corticosteroid receptor function is impaired in many patients with depression and in many healthy individuals at increased genetic risk for an depressive disorder. Assuming such impaired receptor function, then central secretion of CRH would be enhanced in many brain areas, which would account for a variety of depressive symptoms. As shown in rats and also in transgenic mice with impaired glucocorticoid receptor function, antidepressants enhance the signaling through corticosteroid receptors. This mechanism of action can be amplified through blocking central mechanisms that drive the HPA system. Animal experiments using antisense oligodeoxynucleotides directed against the mRNA of both CRH receptor subtypes identified the CRH1 receptor as the mediator of the anxiogenic effects of CRH. Studies in mouse mutants in which this receptor subtype had been deleted extended these findings as the animals were less anxious than wild-type mice when experimentally stressed. Thus, patients with clinical conditions that are causally related to HPA hyperactivity may profit from treatment with a CRH1 receptor antagonist.

Biological markers for the diagnosis of Alzheimer's disease

A diagnostic test for Alzheimer's disease (AD) based on biochemical markers in the cerebrospinal fluid can help improve diagnostic accuracy, which currently is approximately 90%, leaving every tenth AD patient undiagnosed or falsely diagnosed as having the disease. From all biochemical abnormalities described in AD patients, those related to the hallmark neuropathologic lesions, deposition of amyloid and formation of paired helical filaments mainly consisting of abnormally phosphorylated tau protein, are the most promising and the best documented, even though other markers bear some potential and remain to be further studied. Determining an increase of tau and a reduction of A beta 42 bears satisfactory, even though not absolute specificity for AD and represents a true aid for clinicians in diagnosing AD during the patients lifetime. It remains open if these markers will be helpful for the most challenging goal, diagnosing AD in the preclinical phase, when, according to morphological data, high amounts of these pathological proteins are already deposited in the brain tissue.

Processing of neuropeptide Y and somatostatin in human cerebrospinal fluid as monitored by radioimmunoassay and mass spectrometry

The processing of four neuropeptides, neuropeptide Y (NPY) 1-36, NPY (18-36), somatostatin (SOM) 1-28, and SOM (15-28) was studied in human cerebrospinal fluid (CSF) by using a novel combination of methods that included radioimmunoassay (RIA) and mass spectrometry. Untreated CSF samples were chromatographed using reversed-phase high pressure liquid chromatography (RP-HPLC) followed by NPY-RIA or SOM-RIA. These results were compared with those obtained by incubating CSF with exogenous synthetic peptides and directly detecting peptide fragments by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-MS). Using this combination of methods, we were able to determine the probable identities of peptides/peptide fragments recognized in radioimmunoassays. The most important NPY-immunoreactive components in CSF were found to be NPY (1-36) and NPY (3-36). Metabolic products of SOM (15-28) were found to contribute to SOM-like immunoreactivity (SOM-LI) in CSF, but SOM (1-28) only to a lesser degree. Differences in the rate of neuropeptide processing were observed. These differences depended more on the length of the peptide than its sequence. NPY (18-36) and SOM (15-28) were rapidly and extensively processed, whereas NPV (1-36) and SOM (1-28) were processed much more slowly in CSF. The production of SOM (15-28) from SOM (1-28) by enzymes in CSF was not observed. Also, the presence of a disulfide bond in the somatostatins appeared to stabilize them against enzymatic digestion of the ring structure. The results detailed in this report confirm MALDI-MS important role in studies of neuropeptide processing in CSF.

Peripheral markers in testing pathophysiological hypotheses and diagnosing Alzheimer's disease

Alterations in amyloid precursor protein (APP) metabolism, calcium regulation, oxidative metabolism, and transduction systems have been implicated in Alzheimer's disease (AD). Limitations to the use of postmortem brain for examining molecular mechanisms underscore the need to develop a human tissue model representative of the pathophysiological processes that characterize AD. The use of peripheral tissues, particularly of cultured skin fibroblasts derived from AD patients, could complement studies of autopsy samples and provide a useful tool with which to investigate such dynamic processes as signal transduction systems, ionic homeostasis, oxidative metabolism, and APP processing. Peripheral cells as well as body fluids (i.e., plasma and CSF) could also provide peripheral biological markers for the diagnosis of AD. The criteria required for a definite diagnosis of AD presently include clinical criteria in association with histopathologic evidence obtained from biopsy or autopsy. Thus, the use of peripheral markers as a diagnostic tool, either to predict or at least to confirm a diagnosis, may be of great importance.

Low cerebrospinal fluid corticotropin-releasing hormone concentrations in eucortisolemic depression

Hypersecretion of corticotropin-releasing hormone (CRH) and resulting hypercortisolism have been implicated in the pathogenesis of major depression. To test this CRH hypersecretion hypothesis, cerebrospinal fluid (CSF) was continuously withdrawn from 11:00 AM to 5:00 PM via an indwelling subarachnoid catheter (placed at 8:00 AM), and immunoreactive CRH concentrations were determined at 10-min intervals in 10 depressed patients, the majority of whom exhibited at least one "atypical" symptom, and in 15 normal volunteers. CSF CRH was low, plasma adrenocorticotropin (ACTH) tended to be low, and plasma cortisol was normal in the depressed patients. Also, tobacco smokers had lower CSF CRH than nonsmokers. CRH increased acutely in response to lumbar puncture, had a brief half-life, showed rapid variability in concentration over time, and displayed a diurnal concentration rhythm that was preserved in fasting individuals and in most depressed patients. CSF CRH did not correlate with plasma ACTH or cortisol; this and its rapidly fluctuating levels suggest a primarily extrahypothalamic origin of lumbar CSF CRH.

Enhanced adrenal sensitivity to adrenocorticotrophic hormone (ACTH) is evidence of HPA axis hyperactivity in Alzheimer's disease

Adrenal sensitivity was assessed in 16 non-depressed patients with NINCDS/ADRDA Alzheimer's disease (AD) and 18 control subjects by measuring cortisol response to low dose (0.05 microgram/kg i.v.) exogenous adrenocorticotrophic hormone (ACTH). Controlling for sex and medication, both peak cortisol level (peak-baseline) and area under cortisol response curve (AUC above baseline) were significantly greater in AD subjects. This shows that HPA axis hyperactivity, as demonstrated by enhanced adrenal sensitivity to ACTH, occurs in AD. Similar findings have been reported to occur in depression. Among AD subjects, AUC cortisol response correlated with current age (r = 0.70, P = 0.001) and age at onset of dementia (r = 0.73, P = 0.001) and an inverse correlation was seen between cortisol AUC and cognitive test (CAMCOG) score (r = -0.51, P = 0.044). Our findings suggest that HPA axis hyperactivity in AD is associated with advancing age and cognitive dysfunction. Such changes may be cause, or consequence, of neuronal loss.

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.