Roles of Hippocampal Somatostatin Receptor Subtypes in Stress Response and Emotionality

Subregion-specific transcriptomic profiling of rat brain reveals sex-distinct gene expression impacted by adolescent stress

Stress during adolescence clearly impacts brain development and function. Sex differences in adolescent stress-induced or exacerbated emotional and metabolic vulnerabilities could be due to sex-distinct gene expression in hypothalamic, limbic, and prefrontal brain regions. However, adolescent stress-induced whole-genome expression changes in key subregions of these brain regions were unclear. In this study, female and male adolescent Sprague Dawley rats received one-hour restraint stress daily from postnatal day (PD) 32 to PD44. Corticosterone levels, body weights, food intake, body composition, and circulating adiposity and sex hormones were measured. On PD44, brain and blood samples were collected. Using RNA-sequencing, sex-specific differences in stress-induced differentially expressed (DE) genes were identified in subregions of the hypothalamus, limbic system, and prefrontal cortex. Canonical pathways reflected well-known sex-distinct maladies and diseases, substantiating the therapeutic potential of the DE genes found in the current study. Thus, we proposed specific sex distinct, adolescent stress-induced transcriptional changes found in the current study as examples of the molecular bases for sex differences witnessed in stress induced or exacerbated emotional and metabolic disorders. Future behavioral studies and single-cell studies are warranted to test the implications of the DE genes identified in this study in sex-distinct stress-induced susceptibilities.

Drug development and potential targets for Cushing's syndrome

Cushing's syndrome (CS) is a complex disorder characterized by the excessive secretion of cortisol, with Cushing's disease (CD), particularly associated with pituitary tumors, exhibiting heightened morbidity and mortality. Although transsphenoidal pituitary surgery (TSS) stands as the primary treatment for CD, there is a crucial need to optimize patient prognosis. Current medical therapy serves as an adjunctive measure due to its unsatisfactory efficacy and unpredictable side effects. In this comprehensive review, we delve into recent advances in understanding the pathogenesis of CS and explore therapeutic options by conducting a critical analysis of potential drug targets and candidates. Additionally, we provide an overview of the design strategy employed in previously reported candidates, along with a summary of structure-activity relationship (SAR) analyses and their biological efficacy. This review aims to contribute valuable insights to the evolving landscape of CS research, shedding light on potential avenues for therapeutic development.

Differential Serum Levels of CACNA1C, Circadian Rhythm and Stress Response Molecules in Subjects with Bipolar Disorder: Associations with Genetic and Clinical Factors

Background

Many patients with bipolar disorder (BD) do not respond to or have difficulties tolerating lithium and/or other mood stabilizing agents. There is a need for personalized treatments based on biomarkers in guiding treatment options. The calcium voltage-gated channel CACNA1C is a promising candidate for developing personalized treatments. CACNA1C is implicated in BD by genome-wide association studies and several lines of evidence suggest that targeting L-type calcium channels could be an effective treatment strategy. However, before such individualized treatments can be pursued, biomarkers predicting treatment response need to be developed.

Methods

As a first step in testing the hypothesis that CACNA1C genotype is associated with serum levels of CACNA1C, we conducted ELISA measures on serum samples from 100 subjects with BD and 100 control subjects.

Results

We observed significantly higher CACNA1C (p<0.01) protein levels in subjects with BD. The risk SNP (rs11062170) showed functional significance as subjects homozygous for the risk allele (CC) had significantly greater CACNA1C protein levels compared to subjects with one (p=0.013) or no copies (p=0.009). We observed higher somatostatin (SST) (p<0.003) protein levels and lower levels of the clock protein ARTNL (p<0.03) and stress signaling factor corticotrophin releasing hormone (CRH) (p<0.001) in BD. SST and PER2 protein levels were associated with both alcohol dependence and lithium response.

Conclusions

Our findings represent the first evidence for increased serum levels of CACNA1C in BD. Along with altered levels of SST, ARNTL, and CRH our findings suggest CACNA1C is associated with circadian rhythm and stress response disturbances in BD.

Somatostatin signaling modulates binge drinking behavior via the central nucleus of the amygdala

Somatostatin (SST) is a neuropeptide widely expressed in the central nervous system with dense expression in limbic regions such as the extended amygdala. It has recently gained attention for playing a role in modulating alcohol use disorders and co-morbid neuropsychiatric disorders. However, the role of SST in the central nucleus of the amygdala (CeA), a key region for neuropeptide regulation of alcohol and anxiety related behaviors, in alcohol consumption has not been assessed. In this work we perform an initial examination of the interaction between the CeA SST system and binge ethanol intake. Binge intake is a dangerous pattern of excessive ethanol consumption associated with health complications and the transition into alcohol dependence. We use the Drinking in the Dark (DID) model of binge intake in C57BL/6J male and female mice to examine: 1) the impact of 3 DID cycles on CeA SST expression; 2) the effect of intra-CeA SST injection on binge-like ethanol consumption; and 3) if the SST receptor 2 or 4 (SST2R or SST4R) mediate any effect on consumption. Our results show binge-like ethanol intake decreases SST expression in the CeA, but not neighboring basolateral amygdala. We further found intra-SST CeA administration reduces binge ethanol intake. This decrease was replicated by the administration of an SST4R agonist. These effects were not sex-dependent. Overall, this work lends further support for SST playing a role in alcohol related behaviors and as a potential therapeutic target.

Somatostatin peptide signaling dampens cortical circuits and promotes exploratory behavior

We sought to characterize the unique role of somatostatin (SST) in the prelimbic (PL) cortex in mice. We performed slice electrophysiology in pyramidal and GABAergic neurons to characterize the pharmacological mechanism of SST signaling and fiber photometry of GCaMP6f fluorescent calcium signals from SST neurons to characterize the activity profile of SST neurons during exploration of an elevated plus maze (EPM) and open field test (OFT). We used local delivery of a broad SST receptor (SSTR) agonist and antagonist to test causal effects of SST signaling. SSTR activation hyperpolarizes layer 2/3 pyramidal neurons, an effect that is recapitulated with optogenetic stimulation of SST neurons. SST neurons in PL are activated during EPM and OFT exploration, and SSTR agonist administration directly into the PL enhances open arm exploration in the EPM. This work describes a broad ability for SST peptide signaling to modulate microcircuits within the prefrontal cortex and related exploratory behaviors.

Bed nuclei of the stria terminalis: A key hub in the modulation of anxiety

The bed nuclei of the stria terminalis (BST) is recognised as a pivotal integrative centre for monitoring emotional valence. It is implicated in the regulation of diverse affective states and motivated behaviours, and decades of research have firmly established its critical role in anxiety-related behavioural processes. Researchers have recently intricately dissected the BST's dynamic activities, its connection patterns and its functions with respect to specific cell types using multiple techniques such as optogenetics, in vivo calcium imaging and transgenic tools to unmask the complex circuitry mechanisms that underlie anxiety. In this review, we principally focus on studies of anxiety-involved neuromodulators within the BST and provide a comprehensive architecture of the anxiety network-highlighting the BST as a key hub in orchestrating anxiety-like behaviour. We posit that these promising efforts will contribute to the identification of an accurate roadmap for future treatment of anxiety disorders.

Habenula as a Possible Target for Treatment-Resistant Depression Phenotype in Wistar Kyoto Rats

The mechanisms of treatment-resistant depression (TRD) are not clear and are difficult to study. An animal model resembling human TRD is the Wistar Kyoto rat strain. In the present study, we focused on selecting miRNAs that differentiate rats of the WKY strain from Wistar Han (WIS) rats in two divisions of the habenula, the lateral and medial (LHb and MHb, respectively). Based on our preliminary study and literature survey, we identified 32 miRNAs that could be potentially regulated in the habenula. Six miRNAs significantly differentiated WKY rats from WIS rats within the MHb, and three significantly differentiated WKY from WIS rats within the LHb. Then, we selected relevant transcripts regulated by those miRNAs, and their expression in the habenular nuclei was investigated. For mRNAs that differentiated WKY rats from WIS rats in the MHb (Cdkn1c, Htr7, Kcnj9, and Slc12a5), their lower expression correlated with a higher level of relevant miRNAs. In the LHb, eight mRNAs significantly differentiated WKY from WIS rats (upregulated Htr4, Drd2, Kcnj5, and Sstr4 and downregulated Htr2a, Htr7, Elk4, and Slc12a5). These data indicate that several important miRNAs are expressed in the habenula, which differentiates WKY rats from WIS rats and in turn correlates with alterations in the expression of target transcripts. Of particular note are two genes whose expression is altered in WKY rats in both LHb and MHb: Slc12a5 and Htr7. Regulation of KCC2 via the 5-HT7 receptor may be a potential target for the treatment of TRD.

Exposure to di (2-ethylhexyl) phthalate causes locomotor increase and anxiety-like behavior via induction of oxidative stress in brain

Di (2-ethylhexyl) phthalate (DEHP) is one of the most prevalent xenoestrogen endocrine disruptor in daily life. A growing number of studies showed that DEHP could exhibit long-term adverse health effects on the human body, particularly in the liver, kidneys, heart and reproductive systems. However, the impact of oral intake of DEHP on the nervous system is extremely limited. In the present study, the adult C57BL/6J male mice were intragastrically administered with two dosages of DEHP for 35 days. The behavioral parameters were assessed using the elevated plus maze and open-field test. The mRNA expression levels of neuropeptides and the oxidative stress-associated proteins were detected by qPCR and western blot seperately. The histopathologic alterations of the brain were observed by H&E and Nissl staining. The results demonstrated that DEHP exposure could result in neurobehavioral impairments such as locomotor increase and anxiety-like behavior. Furthermore, pathological damages were clearly observed in the cerebral cortex and hippocampus, accompanied by a decrease in neuropeptides and an increase in oxidative stress, which were all positively correlated with the dose of DEHP. Together, these findings provide valuable clues into the DEHP-induced neurotoxicity.

Lateral Septum Somatostatin Neurons are Activated by Diverse Stressors

The lateral septum (LS) is a forebrain structure that has been implicated in a wide range of behavioral and physiological responses to stress. However, the specific populations of neurons in the LS that mediate stress responses remain incompletely understood. Here, we show that neurons in the dorsal lateral septum (LSd) that express the somatostatin gene (hereafter, LSd^Sst neurons) are activated by diverse stressors. Retrograde tracing from LSd^Sst neurons revealed that these neurons are directly innervated by neurons in the locus coeruleus (LC), the primary source of norepinephrine well-known to mediate diverse stress-related functions in the brain. Consistently, we found that norepinephrine increased excitatory synaptic transmission onto LSd^Sst neurons, suggesting the functional connectivity between LSd^Sst neurons and LC noradrenergic neurons. However, optogenetic stimulation of LSd^Sst neurons did not affect stress-related behaviors or autonomic functions, likely owing to the functional heterogeneity within this population. Together, our findings show that LSd^Sst neurons are activated by diverse stressors and suggest that norepinephrine released from the LC may modulate the activity of LSd^Sst neurons under stressful circumstances.

Altered expression of somatostatin signaling molecules and clock genes in the hippocampus of subjects with substance use disorder

Substance use disorders are a debilitating group of psychiatric disorders with a high degree of comorbidity with major depressive disorder. Sleep and circadian rhythm disturbances are commonly reported in people with substance use disorder and major depression and associated with increased risk of relapse. Hippocampal somatostatin signaling is involved in encoding and consolidation of contextual memories which contribute to relapse in substance use disorder. Somatostatin and clock genes also have been implicated in depression, suggesting that these molecules may represent key converging pathways involved in contextual memory processing in substance use and major depression. We used hippocampal tissue from a cohort of subjects with substance use disorder (n = 20), subjects with major depression (n = 20), subjects with comorbid substance use disorder and major depression (n = 24) and psychiatrically normal control subjects (n = 20) to test the hypothesis that expression of genes involved in somatostatin signaling and clock genes is altered in subjects with substance use disorder. We identified decreased expression of somatostatin in subjects with substance use disorder and in subjects with major depression. We also observed increased somatostatin receptor 2 expression in subjects with substance use disorder with alcohol in the blood at death and decreased expression in subjects with major depression. Expression of the clock genes Arntl, Nr1d1, Per2 and Cry2 was increased in subjects with substance use disorder. Arntl and Nr1d1 expression in comparison was decreased in subjects with major depression. We observed decreased expression of Gsk3β in subjects with substance use disorder. Subjects with comorbid substance use disorder and major depression displayed minimal changes across all outcome measures. Furthermore, we observed a significant increase in history of sleep disturbances in subjects with substance use disorder. Our findings represent the first evidence for altered somatostatin and clock gene expression in the hippocampus of subjects with substance use disorder and subjects with major depression. Altered expression of these molecules may impact memory consolidation and contribute to relapse risk.

Mu opioid receptors on hippocampal GABAergic interneurons are critical for the antidepressant effects of tianeptine

Tianeptine is an atypical antidepressant used in Europe to treat patients who respond poorly to selective serotonin reuptake inhibitors (SSRIs). The recent discovery that tianeptine is a mu opioid receptor (MOR) agonist has provided a potential avenue for expanding our understanding of antidepressant treatment beyond the monoamine hypothesis. Thus, our studies aim to understand the neural circuits underlying tianeptine's antidepressant effects. We show that tianeptine induces rapid antidepressant-like effects in mice after as little as one week of treatment. Critically, we also demonstrate that tianeptine's mechanism of action is distinct from fluoxetine in two important aspects: (1) tianeptine requires MORs for its chronic antidepressant-like effect, while fluoxetine does not, and (2) unlike fluoxetine, tianeptine does not promote hippocampal neurogenesis. Using cell-type specific MOR knockouts we further show that MOR expression on GABAergic cells-specifically somatostatin-positive neurons-is necessary for the acute and chronic antidepressant-like responses to tianeptine. Using central infusion of tianeptine, we also implicate the ventral hippocampus as a potential site of antidepressant action. Moreover, we show a dissociation between the antidepressant-like phenotype and other opioid-like phenotypes resulting from acute tianeptine administration such as analgesia, conditioned place preference, and hyperlocomotion. Taken together, these results suggest a novel entry point for understanding what circuit dysregulations may occur in depression, as well as possible targets for the development of new classes of antidepressant drugs.

Proteostasis and resilience: on the interphase between individual's and intracellular stress

A long proportion of the population is resilient to the negative consequences of stress. Glucocorticoids resulting from endocrine responses to stress are essential adaptive mediators, but also drive alterations to brain function, negatively impacting neuronal connectivity, synaptic plasticity, and memory-related processes. Recent evidence has indicated that organelle function and cellular stress responses are relevant determinant of vulnerability and resistance to environmental stress. At the molecular level, a fundamental mechanism of cellular stress adaptation is the maintenance of proteostasis, which also have key roles in sustaining basal neuronal function. Here, we discuss recent evidence suggesting that proteostasis unbalance at the level of the endoplasmic reticulum, the main site for protein folding in the cell, represents a possible mechanistic link between individuals and cellular stress.

Molecular origin of somatostatin-positive neuron vulnerability

Reduced somatostatin (SST) and dysfunction of SST-positive (SST+) neurons are hallmarks of neurological disorders and associated with mood disturbances, but the molecular origin of SST+ neuron vulnerability is unknown. Using chronic psychosocial stress as a paradigm to induce elevated behavioral emotionality in rodents, we report a selective vulnerability of SST+ neurons through exacerbated unfolded protein response (UPR) of the endoplasmic reticulum (ER), or ER stress, in the prefrontal cortex. We next show that genetically suppressing ER stress in SST+ neurons, but not in pyramidal neurons, normalized behavioral emotionality induced by psychosocial stress. In search for intrinsic factors mediating SST+ neuron vulnerability, we found that the forced expression of the SST precursor protein (preproSST) in SST+ neurons, mimicking psychosocial stress-induced early proteomic changes, induces ER stress, whereas mature SST or processing-incompetent preproSST does not. Biochemical analyses further show that psychosocial stress induces SST protein aggregation under elevated ER stress conditions. These results demonstrate that SST processing in the ER is a SST+ neuron-intrinsic vulnerability factor under conditions of sustained or over-activated UPR, hence negatively impacting SST+ neuron functions. Combined with observations in major medical illness, such as diabetes, where excess ER processing of preproinsulin similarly causes ER stress and β cell dysfunction, this suggests a universal mechanism for proteinopathy that is induced by excess processing of native endogenous proteins, playing critical pathophysiological roles that extend to neuropsychiatric disorders.

Somatostatin and Somatostatin-Containing Interneurons—From Plasticity to Pathology

Despite the obvious differences in the pathophysiology of distinct neuropsychiatric diseases or neurodegenerative disorders, some of them share some general but pivotal mechanisms, one of which is the disruption of excitation/inhibition balance. Such an imbalance can be generated by changes in the inhibitory system, very often mediated by somatostatin-containing interneurons (SOM-INs). In physiology, this group of inhibitory interneurons, as well as somatostatin itself, profoundly shapes the brain activity, thus influencing the behavior and plasticity; however, the changes in the number, density and activity of SOM-INs or levels of somatostatin are found throughout many neuropsychiatric and neurological conditions, both in patients and animal models. Here, we (1) briefly describe the brain somatostatinergic system, characterizing the neuropeptide somatostatin itself, its receptors and functions, as well the physiology and circuitry of SOM-INs; and (2) summarize the effects of the activity of somatostatin and SOM-INs in both physiological brain processes and pathological brain conditions, focusing primarily on learning-induced plasticity and encompassing selected neuropsychological and neurodegenerative disorders, respectively. The presented data indicate the somatostatinergic-system-mediated inhibition as a substantial factor in the mechanisms of neuroplasticity, often disrupted in a plethora of brain pathologies.

Recent advances in neuropeptide-related omics and gene editing: Spotlight on NPY and somatostatin and their roles in growth and food intake of fish

Feeding and growth are two closely related and important physiological processes in living organisms. Studies in mammals have provided us with a series of characterizations of neuropeptides and their receptors as well as their roles in appetite control and growth. The central nervous system, especially the hypothalamus, plays an important role in the regulation of appetite. Based on their role in the regulation of feeding, neuropeptides can be classified as orexigenic peptide and anorexigenic peptide. To date, the regulation mechanism of neuropeptide on feeding and growth has been explored mainly from mammalian models, however, as a lower and diverse vertebrate, little is known in fish regarding the knowledge of regulatory roles of neuropeptides and their receptors. In recent years, the development of omics and gene editing technology has accelerated the speed and depth of research on neuropeptides and their receptors. These powerful techniques and tools allow a more precise and comprehensive perspective to explore the functional mechanisms of neuropeptides. This paper reviews the recent advance of omics and gene editing technologies in neuropeptides and receptors and their progresses in the regulation of feeding and growth of fish. The purpose of this review is to contribute to a comparative understanding of the functional mechanisms of neuropeptides in non-mammalians, especially fish.

Sirtuin 5-Mediated Lysine Desuccinylation Protects Mitochondrial Metabolism Following Subarachnoid Hemorrhage in Mice

BACKGROUND AND PURPOSE

Sirt5 (Sirtuin 5) desuccinylates multiple metabolic enzymes and plays an important role in maintaining energy homeostasis. The goal of this study was to determine whether Sirt5-mediated desuccinylation restores the energy metabolism and protects brain against subarachnoid hemorrhage (SAH).

METHODS

Male C57BL/6 or Sirt5^-/- mice were used. The endovascular perforation SAH model was applied. Protein lysine succinylation in the brain cortex was examined using liquid chromatography-tandem mass spectrometry analysis. The brain metabolism was evaluated by measurement of brain pH as well as ATP and reactive oxygen species level. Neuronal cell death and neurobehavioral deficits were assessed 24 hours after SAH. The expression and desuccinylation activity of Sirt5, lysine succinylation of citrate synthase and ATP synthase subunits were investigated by Western blot, immunohistochemistry, and ELISA in SAH mice and patients. Furthermore, the benefits of resveratrol-mediated Sirt5 activation were investigated.

RESULTS

A total of 211 lysine succinylation sites were differentially expressed on 170 proteins in mice brain after SAH. Thirty-nine percent of these succinylated proteins were localized in mitochondria and they are related to energy metabolism. SAH caused a decrease of Sirt5 expression and succinylated citrate synthase as well as the subunits of ATP synthase, subsequently lowered brain pH, reduced ATP and increased reactive oxygen species production, leading to neuronal cell death, and neurological deficits. Knockdown of Sirt5 aggravated SAH-induced effects, mentioned above. Administration of resveratrol resulted in activation of Sirt5. The activation was accompanied both with restoration of the mitochondrial metabolism and alleviation of early brain injury as well as with desuccinylating citrate synthase and ATP synthase.

CONCLUSIONS

Protein lysine succinylation is a biochemical hallmark of metabolic crisis after SAH, and disruption of lysine succinylation through activation of Sirt5 might be a promising therapeutic strategy for the treatment of SAH.

Somatostatin receptor 4 agonism normalizes stress-related excessive amygdala glutamate release and Pavlovian aversion learning and memory in rodents

Background

Excessive processing of aversive life events is a major pathology in stress-related anxiety and depressive disorders. Current pharmacological treatments have rather nonspecific mechanisms of action. Somatostatin is synthesized and released as an inhibitory co-neurotransmitter by specific GABA (gamma-aminobutyric acid) interneurons, and one of its receptors, SSTR4 (somatostatin receptor 4), is localized in brain regions involved in adaptive aversion processing and implicated in negative valence neuropathology, including the amygdala.

Methods

Rat and mouse experiments were conducted to investigate effects of specific SSTR4 agonism on neurobehavioral aversion processing, including any normalization of stress-related hyperresponsiveness. A mouse experiment to investigate stress and SSTR4 agonism effects on reward processing was also conducted.

Results

In male rats (n = 5–10/group) fitted with glutamate biosensors in basolateral amygdala, SSTR4 agonism attenuated glutamate release to restraint stress in control rats and particularly in rats previously exposed to chronic corticosterone. In male mice (n = 10–18/group), SSTR4 agonism dose-dependently attenuated Pavlovian tone/footshock learning and memory measured as freezing behavior, in both control mice and mice exposed to chronic social stress, which induces excessive Pavlovian aversion learning and memory. Specificity of SSTR4 agonism effects to aversion learning/memory was demonstrated by absence of effects on discriminative reward (sucrose) learning/memory in both control mice and mice exposed to chronic social stress; SSTR4 agonism did increase reward-to-effort valuation in a dose-dependent manner and in both control mice and mice exposed to chronic social stress, which attenuates reward motivation.

Conclusions

These neuropsychopharmacological findings add substantially to the preclinical proof-of-concept evidence for SSTR4 agonism as a treatment in anxiety and depressive disorders.

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.

Hippocampal Somatostatin Interneurons, Long-Term Synaptic Plasticity and Memory

A distinctive feature of the hippocampal structure is the diversity of inhibitory interneurons. These complex inhibitory interconnections largely contribute to the tight modulation of hippocampal circuitry, as well as to the formation and coordination of neuronal assemblies underlying learning and memory. Inhibitory interneurons provide more than a simple transitory inhibition of hippocampal principal cells (PCs). The synaptic plasticity of inhibitory neurons provides long-lasting changes in the hippocampal network and is a key component of memory formation. The dendrite targeting interneurons expressing the peptide somatostatin (SOM) are particularly interesting in this regard because they display unique long-lasting synaptic changes leading to metaplastic regulation of hippocampal networks. In this article, we examine the actions of the neuropeptide SOM on hippocampal cells, synaptic plasticity, learning, and memory. We address the different subtypes of hippocampal SOM interneurons. We describe the long-term synaptic plasticity that takes place at the excitatory synapses of SOM interneurons, its singular induction and expression mechanisms, as well as the consequences of these changes on the hippocampal network, learning, and memory. We also review evidence that astrocytes provide cell-specific dynamic regulation of inhibition of PC dendrites by SOM interneurons. Finally, we cover how, in mouse models of Alzheimer’s disease (AD), dysfunction of plasticity of SOM interneuron excitatory synapses may also contribute to cognitive impairments in brain disorders.

Postnatal Fluoxetine Treatment Alters Perineuronal Net Formation and Maintenance in the Hippocampus

Elevation of serotonin via postnatal fluoxetine (PNFlx) treatment during critical temporal windows is hypothesized to perturb the development of limbic circuits thus establishing a substratum for persistent disruption of mood-related behavior. We examined the impact of PNFlx treatment on the formation and maintenance of perineuronal nets (PNNs), extracellular matrix (ECM) structures that deposit primarily around inhibitory interneurons, and mark the closure of critical period plasticity. PNFlx treatment evoked a significant decline in PNN number, with a robust reduction in PNNs deposited around parvalbumin (PV) interneurons, within the CA1 and CA3 hippocampal subfields at postnatal day (P)21 in Sprague Dawley rat pups. While the reduction in CA1 subfield PNN number was still observed in adulthood, we observed no change in colocalization of PV-positive interneurons with PNNs in the hippocampi of adult PNFlx animals. PNFlx treatment did not alter hippocampal PV, calretinin (CalR), or Reelin-positive neuron numbers in PNFlx animals at P21 or in adulthood. We did observe a small, but significant increase in somatostatin (SST)-positive interneurons in the DG subfield of PNFlx-treated animals in adulthood. This was accompanied by altered GABA-A receptor subunit composition, increased dendritic complexity of apical dendrites of CA1 pyramidal neurons, and enhanced neuronal activation revealed by increased c-Fos-positive cell numbers within hippocampi of PNFlx-treated animals in adulthood. These results indicate that PNFlx treatment alters the formation of PNNs within the hippocampus, raising the possibility of a disruption of excitation-inhibition (E/I) balance within this key limbic brain region.

GABAergic deficits in absence of LPA1 receptor, associated anxiety-like and coping behaviors, and amelioration by interneuron precursor transplants into the dorsal hippocampus

Defects in GABAergic function can cause anxiety- and depression-like behaviors among other neuropsychiatric disorders. Therapeutic strategies using the transplantation of GABAergic interneuron progenitors derived from the medial ganglionic eminence (MGE) into the adult hippocampus reversed the symptomatology in multiple rodent models of interneuron-related pathologies. In turn, the lysophosphatidic acid receptor LPA₁ has been reported to be essential for hippocampal function. Converging evidence suggests that deficits in LPA₁ receptor signaling represent a core feature underlying comparable hippocampal dysfunction and behaviors manifested in common neuropsychiatric conditions. Here, we first analyzed the GABAergic interneurons in the hippocampus of wild-type and maLPA₁-null mice, lacking the LPA₁ receptor. Our data revealed a reduction in the number of neurons expressing GABA, calcium-binding proteins, and neuropeptides such as somatostatin and neuropeptide Y in the hippocampus of maLPA₁-null mice. Then, we used interneuron precursor transplants to test links between hippocampal GABAergic interneuron deficit, cell-based therapy, and LPA₁ receptor-dependent psychiatric disease-like phenotypes. For this purpose, we transplanted MGE-derived interneuron precursors into the adult hippocampus of maLPA₁-null mice, to test their effects on GABAergic deficit and behavioral symptoms associated with the absence of the LPA₁ receptor. Transplant studies in maLPA₁-null mice showed that grafted cells were able to restore the hippocampal host environment, decrease the anxiety-like behaviors and neutralize passive coping, with no abnormal effects on motor activity. Furthermore, grafted MGE-derived cells maintained their normal differentiation program. These findings reinforce the use of cell-based strategies for brain disorders and suggest that the LPA₁ receptor represents a potential target for interneuron-related neuropsychiatric disorders.

Chronic stress has different immediate and delayed effects on hippocampal calretinin- and somatostatin-positive cells

Past studies find that chronic stress alters inhibitory, GABAergic circuitry of neurons in distinct hippocampal subregions. Less clear is whether these effects persist weeks after chronic stress ends, and whether these effects involve changes in the total number of hippocampal GABAergic neurons or modulates the function of specific GABAergic subtypes. A transgenic mouse line (VGAT:Cre Ai9) containing an indelible marker for GABAergic neurons (tdTomato) throughout the brain was used to determine whether chronic stress alters total GABAergic neuronal number or the expression of two key GABAergic cell subtypes, calretinin expressing (CR+) and somatostatin expressing (SOM+) neurons, and whether these changes endure weeks later. Male and female mice were chronically stressed in wire mesh restrainers for 6h/d/21d (Str) or not (Con), and then allowed a 3 week rest period (Str-Rest) and compared to those without a rest period (Str-NoRest). Epifluorescent microscope images of immunohistochemistry-processed brains were quantified to estimate the total number of fluorescently-labeled hippocampal GABAergic neurons and the proportion that were CR+ or SOM+. Neither chronic stress nor sex altered the total number of GABAergic cells. In contrast, chronic stress reduced the expression of CR+ in the CA3 region of the hippocampus in both males and females, with robust reductions in the DG region of males, but not females, and these changes reversed following a rest period. Chronic stress also reduced the proportion of hippocampal SOM+ neurons and this reduction persisted even with a rest period. These results show chronic stress dynamically reduced CR expression without changing total inhibitory neuronal number and point to CR as a potential new lead to understand mechanisms by which chronic stress alters hippocampal function.

Behavioral Deficits Induced by Somatostatin-Positive GABA Neuron Silencing Are Rescued by Alpha 5 GABA-A Receptor Potentiation

INTRODUCTION

Deficits in somatostatin-positive gamma-aminobutyric acid interneurons (SST+ GABA cells) are commonly reported in human studies of mood and anxiety disorder patients. A causal link between SST+ cell dysfunction and symptom-related behaviors has been proposed based on rodent studies showing that chronic stress, a major risk factor for mood and anxiety disorders, induces a low SST+ GABA cellular phenotype across corticolimbic brain regions; that lowering Sst, SST+ cell, or GABA functions induces depressive-/anxiety-like behaviors (a rodent behavioral construct collectively defined as "behavioral emotionality"); and that disinhibiting SST+ cells has antidepressant-like effects. Recent studies found that compounds preferentially potentiating receptors mediating SST+ cell functions, α5-GABAA receptor positive allosteric modulators (α5-PAMs), achieved antidepressant-like effects. Together, the evidence suggests that SST+ cells regulate mood and cognitive functions that are disrupted in mood disorders and that rescuing SST+ cell function via α5-PAM may represent a targeted therapeutic strategy.

METHODS

We developed a mouse model allowing chemogenetic manipulation of brain-wide SST+ cells and employed behavioral characterization 30 minutes after repeated acute silencing to identify contributions to symptom-related behaviors. We then assessed whether an α5-PAM, GL-II-73, could rescue behavioral deficits.

RESULTS

Brain-wide SST+ cell silencing induced features of stress-related illnesses, including elevated neuronal activity and plasma corticosterone levels, increased anxiety- and anhedonia-like behaviors, and impaired short-term memory. GL-II-73 led to antidepressant- and anxiolytic-like improvements among behavioral deficits induced by brain-wide SST+ cell silencing.

CONCLUSION

Our data validate SST+ cells as regulators of mood and cognitive functions and demonstrate that bypassing low SST+ cell function via α5-PAM represents a targeted therapeutic strategy.

Versatile Functions of Somatostatin and Somatostatin Receptors in the Gastrointestinal System

Somatostatin (SST) and somatostatin receptors (SSTRs) play an important role in the brain and gastrointestinal (GI) system. SST is produced in various organs and cells, and the inhibitory function of somatostatin-containing cells is involved in a range of physiological functions and pathological modifications. The GI system is the largest endocrine organ for digestion and absorption, SST-endocrine cells and neurons in the GI system are a critical effecter to maintain homeostasis via SSTRs 1-5 and co-receptors, while SST-SSTRs are involved in chemo-sensory, mucus, and hormone secretion, motility, inflammation response, itch, and pain via the autocrine, paracrine, endocrine, and exoendocrine pathways. It is also a power inhibitor for tumor cell proliferation, severe inflammation, and post-operation complications, and is a first-line anti-cancer drug in clinical practice. This mini review focuses on the current function of producing SST endocrine cells and local neurons SST-SSTRs in the GI system, discusses new development prognostic markers, phosphate-specific antibodies, and molecular imaging emerging in diagnostics and therapy, and summarizes the mechanism of the SST family in basic research and clinical practice. Understanding of endocrines and neuroendocrines in SST-SSTRs in GI will provide an insight into advanced medicine in basic and clinical research.

Altered GABA-mediated information processing and cognitive dysfunctions in depression and other brain disorders

Cognitive dysfunctions, including impaired attention, learning, memory, planning and problem solving, occur in depressive episodes, often persist during remission, predict relapse, worsen with recurrent episodes, and are not treated by current antidepressants or other medications. Cognitive symptoms are also present in other psychiatric disorders, are a hallmark of aging, and define several late-life disorders, including Alzheimer's disease. This pervasive occurrence suggests either a non-specific outcome of a diseased brain, or a shared underlying pathology contributing to this symptom dimension. Recent findings suggest a role for altered GABAergic inhibition in cognitive symptoms. Cellular, molecular and biochemical studies in human subjects report changes affecting the gamma-amino butyric acid (GABA) system, specifically somatostatin-expressing (SST+) GABAergic interneurons, across brain disorders and during aging. SST+ neurons gate excitatory input onto pyramidal neurons within cortical microcircuits. Experimentally reducing the function of these neurons affects excitatory signal-to-noise ratio, reduces synchronized cellular and neural activity, and leads to cognitive dysfunctions. Conversely, augmenting SST+ cell post-synaptic α5-GABA-A receptor activity has pro-cognitive efficacy in stress and aging models. Together, this suggests that reduced signaling of the SST+ neuron/α5-GABA-A receptor pathway contributes to cognitive dysfunctions, and that it represents a novel therapeutic target for remediating mood and cognitive symptoms in depression, other psychiatric disorders and during aging.

Hippocampal GABAergic interneurons and their co-localized neuropeptides in stress vulnerability and resilience

Studies in humans and rodents suggest a critical role for the hippocampal formation in cognition and emotion, but also in the adaptation to stressful events. Successful stress adaptation promotes resilience, while its failure may lead to stress-induced psychopathologies such as depression and anxiety disorders. Hippocampal architecture and physiology is shaped by its strong control of activity via diverse classes of inhibitory interneurons that express typical calcium binding proteins and neuropeptides. Celltype-specific opto- and chemogenetic intervention strategies that take advantage of these biochemical markers have bolstered our understanding of the distinct role of different interneurons in anxiety, fear and stress adaptation. Moreover, some of the signature proteins of GABAergic interneurons have a potent impact on emotion and cognition on their own, making them attractive targets for interventions. In particular, neuropeptide Y is a promising endogenous agent for mediating resilience against severe stress. In this review, we evaluate the role of the major types of interneurons across hippocampal subregions in the adaptation to chronic and acute stress and to emotional memory formation.

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

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

Genetic Perturbation of TIA1 Reveals a Physiological Role in Fear Memory

TIA1 is a prion-related RNA-binding protein whose capacity to form various types of intracellular aggregates has been implicated in neurodegenerative disease. However, its role in normal brain function is poorly understood. Here, we show that TIA1 bidirectionally modulates stress-dependent synaptic plasticity in the hippocampus, a brain region involved in fear memory and olfactory discrimination learning. At the behavioral level, conditioned odor avoidance is potentiated by TIA1 deletion, whereas overexpression of TIA1 in the ventral hippocampus inhibits both contextual fear memory and avoidance. However, the latter genetic manipulations have little impact on other hippocampus-dependent tasks. Transcriptional profiling indicates that TIA1 presides over a large network of immune system genes with modulatory roles in synaptic plasticity and long-term memory. Our results uncover a physiological and partly sex-dependent function for TIA1 in fear memory and may provide molecular insight into stress-related psychiatric conditions, such as post-traumatic stress disorder (PTSD) and anxiety.

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'.

Emerging roles of the unfolded protein response (UPR) in the nervous system: A link with adaptive behavior to environmental stress?

Stressors elicit a neuroendocrine response leading to increased levels of glucocorticoids, allowing the organism to adapt to environmental changes and maintain homeostasis. Glucocorticoids have a broad effect in the body, modifying the activity of the immune system, metabolism, and behavior through the activation of receptors in the limbic system. Chronic exposition to stressors operates as a risk factor for psychiatric diseases such as depression and posttraumatic stress disorder. Among the cellular alterations observed as a consequence of environmental stress, alterations to organelle function at the level of mitochondria and endoplasmic reticulum (ER) are emerging as possible factors contributing to neuronal dysfunction. ER proteostasis alterations elicit the unfolded protein response (UPR), a conserved signaling network that re-establish protein homeostasis. In addition, in the context of brain function, the UPR has been associated to neurodevelopment, synaptic plasticity and neuronal connectivity. Recent studies suggest a role of the UPR in the adaptive behavior to stress, suggesting a mechanistic link between environmental and cellular stress. Here, we revise recent evidence supporting an evolutionary connection between the neuroendocrine system and the UPR to modulate behavioral adaptive responses.

The interoceptive hippocampus: Mouse brain endocrine receptor expression highlights a dentate gyrus (DG)-cornu ammonis (CA) challenge-sufficiency axis

The primeval function of the mammalian hippocampus (HPC) remains uncertain. Implicated in learning and memory, spatial navigation, and neuropsychological disorders, evolutionary theory suggests that the HPC evolved from a primeval chemosensory epithelium. Deficits in sensing of internal body status ('interoception') in patients with HPC lesions argue that internal sensing may be conserved in higher vertebrates. We studied the expression patterns in mouse brain of 250 endocrine receptors that respond to blood-borne ligands. Key findings are (i) the proportions and levels of endocrine receptor expression in the HPC are significantly higher than in all other comparable brain regions. (ii) Surprisingly, the distribution of endocrine receptor expression within mouse HPC was found to be highly structured: receptors signaling 'challenge' are segregated in dentate gyrus (DG), whereas those signaling 'sufficiency' are principally found in cornu ammonis (CA) regions. Selective expression of endocrine receptors in the HPC argues that interoception remains a core feature of hippocampal function. Further, we report that ligands of DG receptors predominantly inhibit both synaptic potentiation and neurogenesis, whereas CA receptor ligands conversely promote both synaptic potentiation and neurogenesis. These findings suggest that the hippocampus acts as an integrator of body status, extending its role in context-dependent memory encoding from 'where' and 'when' to 'how I feel'. Implications for anxiety and depression are discussed.

Prefrontal cortex interneurons display dynamic sex-specific stress-induced transcriptomes

γ-aminobutyric acid (GABA) inhibitory interneurons play a key role in efferent and afferent control of principle neuron activity in the prefrontal cortex (PFC), thereby regulating signal integrity of cognitive and behavioral processes. Recent evidence suggests that specific subtypes of interneurons in the PFC mediate stress-induced depressive-like behaviors. Abnormalities of GABA interneurons, particularly the somatostatin (human, SST; mouse, Sst) subtype, have been reported in postmortem brains of depressed subjects and include sex differences that could explain the increased incidence of depression in women. Here, we analyze the transcriptional profiles and the effects of chronic stress in males vs. females on GABA interneuron subtypes in the PFC. Using Sst- and Parvalbumin-fluorescence tagged reporter mice and fluorescence-activated cell sorting (FACS) combined with RNA sequencing, we identify distinct transcriptome profiles for these interneuron subtypes in the medial PFC. Based on evidence that SST interneurons are altered in depression, we then determined the effects of chronic stress on this interneuron subtype. Chronic stress causes significant dysregulation of several key pathways, including sex-specific differences in the Sst interneuron profiles. The transcriptional pathways altered by chronic stress in males overlap with enriched pathways in non-stressed females. These changes occurred predominantly in decreased expression of elongation initiation factor 2 (EIF2) signaling, suggesting that dysfunction of the translational machinery of SST interneurons could be critical to the development of depressive-like behaviors in males. In addition, SST interneurons from females exposed to chronic stress show dysregulation of different, growth factor signaling pathways.

Ketamine rapidly reverses stress-induced impairments in GABAergic transmission in the prefrontal cortex in male rodents

Dysfunction of medial prefrontal cortex (mPFC) in association with imbalance of inhibitory and excitatory neurotransmission has been implicated in depression. However, the precise cellular mechanisms underlying this imbalance, particularly for GABAergic transmission in the mPFC, and the link with the rapid acting antidepressant ketamine remains poorly understood. Here we determined the influence of chronic unpredictable stress (CUS), an ethologically validated model of depression, on synaptic markers of GABA neurotransmission, and the influence of a single dose of ketamine on CUS-induced synaptic deficits in mPFC of male rodents. The results demonstrate that CUS decreases GABAergic proteins and the frequency of inhibitory post synaptic currents (IPSCs) of layer V mPFC pyramidal neurons, concomitant with depression-like behaviors. In contrast, a single dose of ketamine can reverse CUS-induced deficits of GABA markers, in conjunction with reversal of CUS-induced depressive-like behaviors. These findings provide further evidence of impairments of GABAergic synapses as key determinants of depressive behavior and highlight ketamine-induced synaptic responses that restore GABA inhibitory, as well as glutamate neurotransmission.

Neuroendocrine drivers of risk and resilience: The influence of metabolism & mitochondria

The manifestation of risk versus resilience has been considered from varying perspectives including genetics, epigenetics, early life experiences, and type and intensity of the challenge with which the organism is faced. Although all of these factors are central to determining risk and resilience, the current review focuses on what may be a final common pathway: metabolism. When an organism is faced with a perturbation to the environment, whether internal or external, appropriate energy allocation is essential to resolving the divergence from equilibrium. This review examines the potential role of metabolism in the manifestation of stress-induced neural compromise. In addition, this review details the current state of knowledge on neuroendocrine factors which are poised to set the tone of the metabolic response to a systemic challenge. The goal is to provide an essential framework for understanding stress in a metabolic context and appreciation for key neuroendocrine signals.

Somatostatin Serves a Modulatory Role in the Mouse Olfactory Bulb: Neuroanatomical and Behavioral Evidence

Somatostatin (SOM) and somatostatin receptors (SSTR1-4) are present in all olfactory structures, including the olfactory bulb (OB), where SOM modulates physiological gamma rhythms and olfactory discrimination responses. In this work, histological, viral tracing and transgenic approaches were used to characterize SOM cellular targets in the murine OB. We demonstrate that SOM targets all levels of mitral dendritic processes in the OB with somatostatin receptor 2 (SSTR2) detected in the dendrites of previously uncharacterized mitral-like cells. We show that inhibitory interneurons of the glomerular layer (GL) express SSTR4 while SSTR3 is confined to the granule cell layer (GCL). Furthermore, SOM cells in the OB receive synaptic inputs from olfactory cortical afferents. Behavioral studies demonstrate that genetic deletion of SSTR4, SSTR2 or SOM differentially affects olfactory performance. SOM or SSTR4 deletion have no major effect on olfactory behavioral performances while SSTR2 deletion impacts olfactory detection and discrimination behaviors. Altogether, these results describe novel anatomical and behavioral contributions of SOM, SSTR2 and SSTR4 receptors in olfactory processing.

Cortical GABAergic Dysfunction in Stress and Depression: New Insights for Therapeutic Interventions

Major depressive disorder (MDD) is a debilitating illness characterized by neuroanatomical and functional alterations in limbic structures, notably the prefrontal cortex (PFC), that can be precipitated by exposure to chronic stress. For decades, the monoaminergic deficit hypothesis of depression provided the conceptual framework to understand the pathophysiology of MDD. However, accumulating evidence suggests that MDD and chronic stress are associated with an imbalance of excitation-inhibition (E:I) within the PFC, generated by a deficit of inhibitory synaptic transmission onto principal glutamatergic neurons. MDD patients and chronically stressed animals show a reduction in GABA and GAD67 levels in the brain, decreased expression of GABAergic interneuron markers, and alterations in GABA_A and GABA_B receptor levels. Moreover, genetically modified animals with deletion of specific GABA receptors subunits or interneuron function show depressive-like behaviors. Here, we provide further evidence supporting the role of cortical GABAergic interneurons, mainly somatostatin- and parvalbumin-expressing cells, required for the optimal E:I balance in the PFC and discuss how the malfunction of these cells can result in depression-related behaviors. Finally, considering the relatively low efficacy of current available medications, we review new fast-acting pharmacological approaches that target the GABAergic system to treat MDD. We conclude that deficits in cortical inhibitory neurotransmission and interneuron function resulting from chronic stress exposure can compromise the integrity of neurocircuits and result in the development of MDD and other stress-related disorders. Drugs that can establish a new E:I balance in the PFC by targeting the glutamatergic and GABAergic systems show promising as fast-acting antidepressants and represent breakthrough strategies for the treatment of depression.

Exploratory and locomotor activity, learning and memory functions in somatostatin receptor subtype 4 gene-deficient mice in relation to aging and sex

The inhibitory neuropeptide somatostatin regulates several functions in the nervous system including memory. Its concentrations decrease by age leading to functional alterations, but there are little known about the receptorial mechanism. We discovered that somatostatin receptor 4 (sst₄) mediates analgesic, anti-depressant, and anti-inflammatory effects without endocrine actions, and it is a unique target for drug development. We investigated the exploratory and locomotor activities and learning and memory functions of male and female sst₄gene-deficient mice compared with their wild-types (WT) at ages of 3, 12, 17 months in the Y-maze test, open field test (OFT), radial-arm maze (RAM) test and novel object recognition (NOR) test. Young sst₄ gene-deficient females visited, repeated, and missed significantly less arms than the WTs in the RAM; males showed decreased exploration in the NOR. Young mice moved significantly more, spend longer time in OFT center, and visited more arms in the Y-maze than older ones. Young WT females spend significantly longer time in the OFT center, visited, missed and repeated more arms of the RAM than males. Old males found more rewards than females. Young males explored longer the novel object than young females and older males in the NOR; the recognition index was smaller in females. We conclude that aging and sex are important factors of behavioral parameters that should be focused on in such studies. Sst₄ is likely to influence locomotion and exploratory behavior only in young mice, but not during normal aging, which is a beneficial feature of a good drug target focusing on the elderly.

Cortical GABAergic Dysfunction in Stress and Depression: New Insights for Therapeutic Interventions

Major depressive disorder (MDD) is a debilitating illness characterized by neuroanatomical and functional alterations in limbic structures, notably the prefrontal cortex (PFC), that can be precipitated by exposure to chronic stress. For decades, the monoaminergic deficit hypothesis of depression provided the conceptual framework to understand the pathophysiology of MDD. However, accumulating evidence suggests that MDD and chronic stress are associated with an imbalance of excitation-inhibition (E:I) within the PFC, generated by a deficit of inhibitory synaptic transmission onto principal glutamatergic neurons. MDD patients and chronically stressed animals show a reduction in GABA and GAD67 levels in the brain, decreased expression of GABAergic interneuron markers, and alterations in GABA_A and GABA_B receptor levels. Moreover, genetically modified animals with deletion of specific GABA receptors subunits or interneuron function show depressive-like behaviors. Here, we provide further evidence supporting the role of cortical GABAergic interneurons, mainly somatostatin- and parvalbumin-expressing cells, required for the optimal E:I balance in the PFC and discuss how the malfunction of these cells can result in depression-related behaviors. Finally, considering the relatively low efficacy of current available medications, we review new fast-acting pharmacological approaches that target the GABAergic system to treat MDD. We conclude that deficits in cortical inhibitory neurotransmission and interneuron function resulting from chronic stress exposure can compromise the integrity of neurocircuits and result in the development of MDD and other stress-related disorders. Drugs that can establish a new E:I balance in the PFC by targeting the glutamatergic and GABAergic systems show promising as fast-acting antidepressants and represent breakthrough strategies for the treatment of depression.

Exploring the involvement of Tac2 in the mouse hippocampal stress response through gene networking

Tachykinin 2 (Tac2) is expressed in a number of areas throughout the brain, including the hippocampus. However, knowledge about its function has been only well explored in the hypothalamus in the context of reproductive health. In this study, we identified and validated increased hippocampal Tac2 mRNA expression in response to chronic mild stress in mice. Expression quantitative trait locus (eQTL) analysis showed Tac2 is cis-regulated in the hippocampus. Using a systems genetics approach, we constructed a Tac2 co-expression network to better understand the relationship between Tac2 and the hippocampal stress response. Our network identified 69 total genes associated with Tac2, several of which encode major neuropeptides involved in hippocampal stress signaling as well as critical genes for producing neural plasticity, indicating that Tac2 is involved in these processes. Pathway analysis for the member of Tac2 gene network revealed a strong connection between Tac2 and neuroactive ligand-receptor interaction, calcium signaling pathway, as well as cardiac muscle contraction. In addition, we also identified 46 stress-related phenotypes, specifically fear conditioning response, that were significantly correlated with Tac2 expression. Our results provide evidence for Tac2 as a strong candidate gene who likely plays a role in hippocampal stress processing and neural plasticity.

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

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

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

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

The Somatostatin Receptor-4 Agonist J-2156 Alleviates Mechanical Hypersensitivity in a Rat Model of Breast Cancer Induced Bone Pain

In the majority of patients with breast cancer in the advanced stages, skeletal metastases are common, which may cause excruciating pain. Currently available drug treatments for relief of breast cancer-induced bone pain (BCIBP) include non-steroidal anti-inflammatory drugs and strong opioid analgesics along with inhibitors of osteoclast activity such as bisphosphonates and monoclonal antibodies such as denosumab. However, these medications often lack efficacy and/or they may produce serious dose-limiting side effects. In the present study, we show that J-2156, a somatostatin receptor type 4 (SST4 receptor) selective agonist, reverses pain-like behaviors in a rat model of BCIBP induced by unilateral intra-tibial injection of Walker 256 breast cancer cells. Following intraperitoneal administration, the ED₅₀ of J-2156 for the relief of mechanical allodynia and mechanical hyperalgesia in the ipsilateral hindpaws was 3.7 and 8.0 mg/kg, respectively. Importantly, the vast majority of somatosensory neurons in the dorsal root ganglia including small diameter C-fibers and medium-large diameter fibers, that play a crucial role in cancer pain hypersensitivities, expressed the SST4 receptor. J-2156 mediated pain relief in BCIBP-rats was confirmed by observations of a reduction in the levels of phosphorylated extracellular signal-regulated kinase (pERK), a protein essential for central sensitization and persistent pain, in the spinal dorsal horn. Our results demonstrate the potential of the SST4 receptor as a pharmacological target for relief of BCIBP and we anticipate the present work to be a starting point for further mechanism-based studies.

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

RATIONALE

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Somatostatin-Positive Gamma-Aminobutyric Acid Interneuron Deficits in Depression: Cortical Microcircuit and Therapeutic Perspectives

The functional integration of external and internal signals forms the basis of information processing and is essential for higher cognitive functions. This occurs in finely tuned cortical microcircuits whose functions are balanced at the cellular level by excitatory glutamatergic pyramidal neurons and inhibitory gamma-aminobutyric acidergic (GABAergic) interneurons. The balance of excitation and inhibition, from cellular processes to neural network activity, is characteristically disrupted in multiple neuropsychiatric disorders, including major depressive disorder (MDD), bipolar disorder, anxiety disorders, and schizophrenia. Specifically, nearly 3 decades of research demonstrate a role for reduced inhibitory GABA level and function across disorders. In MDD, recent evidence from human postmortem and animal studies suggests a selective vulnerability of GABAergic interneurons that coexpress the neuropeptide somatostatin (SST). Advances in cell type-specific molecular genetics have now helped to elucidate several important roles for SST interneurons in cortical processing (regulation of pyramidal cell excitatory input) and behavioral control (mood and cognition). Here, we review evidence for altered inhibitory function arising from GABAergic deficits across disorders and specifically in MDD. We then focus on properties of the cortical microcircuit, where SST-positive GABAergic interneuron deficits may disrupt functioning in several ways. Finally, we discuss the putative origins of SST cell deficits, as informed by recent research, and implications for therapeutic approaches. We conclude that deficits in SST interneurons represent a contributing cellular pathology and therefore a promising target for normalizing altered inhibitory function in MDD and other disorders with reduced SST cell and GABA functions.

Characterization of GABAergic marker expression in the chronic unpredictable stress model of depression

Evidence continues to build suggesting that the GABAergic neurotransmitter system is altered in brains of patients with major depressive disorder. However, there is little information available related to the extent of these changes or the potential mechanisms associated with these alterations. As stress is a well-established precipitant to depressive episodes, we sought to explore the impact of chronic stress on GABAergic interneurons. Using western blot analyses and quantitative real-time PCR (qPCR) we assessed the effects of five-weeks of chronic unpredictable stress (CUS) exposure on the expression of GABA-synthesizing enzymes (GAD₆₅ and GAD₆₇), calcium-binding proteins (calbindin (CB), parvalbumin (PV) and calretinin (CR)), and neuropeptides co-expressed in GABAergic neurons (somatostatin (SST), neuropeptide Y (NPY), vasoactive intestinal peptide (VIP) and cholecystokinin (CCK)) in the prefrontal cortex (PFC) and hippocampus (HPC) of rats. We also investigated the effects of corticosterone (CORT) and dexamethasone (DEX) exposure on these markers in vitro in primary cortical and hippocampal cultures. We found that CUS induced significant reductions of GAD₆₇ protein levels in both the PFC and HPC of CUS-exposed rats, but did not detect changes in GAD₆₅ protein expression. Similar protein expression changes were found in vitro in cortical neurons. In addition, our results provide clear evidence of reduced markers of interneuron population(s), namely SST and NPY, in the PFC, suggesting these cell types may be selectively vulnerable to chronic stress. Together, this work highlights that chronic stress induces regional and cell type-selective effects on GABAergic interneurons in rats. These findings provide additional supporting evidence that stress-induced GABA neuron dysfunction and cell vulnerability play critical roles in the pathophysiology of stress-related illnesses, including major depressive disorder.

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

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