Somatostatin depresses long-term potentiation and Ca2+ signaling in mouse dentate gyrus

Somatostatin and the pathophysiology of Alzheimer's disease

Among the central features of Alzheimer's disease (AD) progression are altered levels of the neuropeptide somatostatin (SST), and the colocalisation of SST-positive interneurons (SST-INs) with amyloid-β plaques, leading to cell death. In this theoretical review, I propose a molecular model for the pathogenesis of AD based on SST-IN hypofunction and hyperactivity. Namely, hypofunctional and hyperactive SST-INs struggle to control hyperactivity in medial regions in early stages, leading to axonal Aβ production through excessive presynaptic GABAB inhibition, GABAB1a/APP complex downregulation and internalisation. Concomitantly, excessive SST-14 release accumulates near SST-INs in the form of amyloids, which bind to Aβ to form toxic mixed oligomers. This leads to differential SST-IN death through excitotoxicity, further disinhibition, SST deficits, and increased Aβ release, fibrillation and plaque formation. Aβ plaques, hyperactive networks and SST-IN distributions thereby tightly overlap in the brain. Conversely, chronic stimulation of postsynaptic SST2/4 on gulutamatergic neurons by hyperactive SST-INs promotes intense Mitogen-Activated Protein Kinase (MAPK) p38 activity, leading to somatodendritic p-tau staining and apoptosis/neurodegeneration - in agreement with a near complete overlap between p38 and neurofibrillary tangles. This model is suitable to explain some of the principal risk factors and markers of AD progression, including mitochondrial dysfunction, APOE4 genotype, sex-dependent vulnerability, overactive glial cells, dystrophic neurites, synaptic/spine losses, inter alia. Finally, the model can also shed light on qualitative aspects of AD neuropsychology, especially within the domains of spatial and declarative (episodic, semantic) memory, under an overlying pattern of contextual indiscrimination, ensemble instability, interference and generalisation.

Increasing NPYergic transmission in the hippocampus rescues aging-related deficits of long-term potentiation in the mouse dentate gyrus

Loss of neuropeptide Y (NPY)-expressing interneurons in the hippocampus and decaying cholinergic neuromodulation are thought to contribute to impaired cognitive function during aging. However, the interaction of these two neuromodulatory systems in maintaining hippocampal synaptic plasticity during healthy aging has not been explored so far. Here we report profound sex differences in the Neuropeptide-Y (NPY) levels in the dorsal dentate gyrus (DG) with higher NPY concentrations in the male mice compared to their female counterparts and a reduction of NPY levels during aging specifically in males. This change in aged males is accompanied by a deficit in theta burst-induced long-term potentiation (LTP) in the medial perforant path-to-dorsal DG (MPP-DG) synapse, which can be rescued by enhancing cholinergic activation with the acetylcholine esterase blocker, physostigmine. Importantly, NPYergic transmission is required for this rescue of LTP. Moreover, exogenous NPY application alone is sufficient to recover LTP induction in aged male mice, even in the absence of the cholinergic stimulator. Together, our results suggest that in male mice NPYergic neurotransmission is a critical factor for maintaining dorsal DG LTP during aging.

N-Type Ca Channel in Epileptic Syndromes and Epilepsy: A Systematic Review of Its Genetic Variants

N-type voltage-gated calcium channel controls the release of neurotransmitters from neurons. The association of other voltage-gated calcium channels with epilepsy is well-known. The association of N-type voltage-gated calcium channels and pain has also been established. However, the relationship between this type of calcium channel and epilepsy has not been specifically reviewed. Therefore, the present review systematically summarizes existing publications regarding the genetic associations between N-type voltage-dependent calcium channel and epilepsy.

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.

Somatostatin contributes to long-term potentiation at excitatory synapses onto hippocampal somatostatinergic interneurons

Somatostatin-expressing interneurons (SOM-INs) are a major subpopulation of GABAergic cells in CA1 hippocampus that receive excitation from pyramidal cells (PCs), and, in turn, provide feedback inhibition onto PC dendrites. Excitatory synapses onto SOM-INs show a Hebbian long-term potentiation (LTP) mediated by type 1a metabotropic glutamate receptors (mGluR1a) that is implicated in hippocampus-dependent learning. The neuropeptide somatostatin (SST) is also critical for hippocampal long-term synaptic plasticity, as well as learning and memory. SST effects on hippocampal PCs are well documented, but its actions on inhibitory interneurons remain largely undetermined. In the present work, we investigate the involvement of SST in long-term potentiation of CA1 SOM-IN excitatory synapses using pharmacological approaches targeting the somatostatinergic system and whole cell recordings in slices from transgenic mice expressing eYFP in SOM-INs. We report that application of exogenous SST₁₄ induces long-term potentiation of excitatory postsynaptic potentials in SOM-INs via somatostatin type 1–5 receptors (SST_1-5Rs) but does not affect synapses of PC or parvalbumin-expressing interneurons. Hebbian LTP in SOM-INs was prevented by inhibition of SSTRs and by depletion of SST by cysteamine treatment, suggesting a critical role of endogenous SST in LTP. LTP of SOM-IN excitatory synapses induced by SST₁₄ was independent of NMDAR and mGluR1a, activity-dependent, and prevented by blocking GABA_A receptor function. Our results indicate that endogenous SST may contribute to Hebbian LTP at excitatory synapses of SOM-INs by controlling GABA_A inhibition, uncovering a novel role for SST in regulating long-term synaptic plasticity in somatostatinergic cells that may be important for hippocampus-dependent memory processes.

Changes in Hippocampal Plasticity in Depression and Therapeutic Approaches Influencing These Changes

Depression is a common neurological disease that seriously affects human health. There are many hypotheses about the pathogenesis of depression, and the most widely recognized and applied is the monoamine hypothesis. However, no hypothesis can fully explain the pathogenesis of depression. At present, the brain-derived neurotrophic factor (BDNF) and neurogenesis hypotheses have highlighted the important role of plasticity in depression. The plasticity of neurons and glial cells plays a vital role in the transmission and integration of signals in the central nervous system. Plasticity is the adaptive change in the nervous system in response to changes in external signals. The hippocampus is an important anatomical area associated with depression. Studies have shown that some antidepressants can treat depression by changing the plasticity of the hippocampus. Furthermore, caloric restriction has also been shown to affect antidepressant and hippocampal plasticity changes. In this review, we summarize the latest research, focusing on changes in the plasticity of hippocampal neurons and glial cells in depression and the role of BDNF in the changes in hippocampal plasticity in depression, as well as caloric restriction and mitochondrial plasticity. This review may contribute to the development of antidepressant drugs and elucidating the mechanism of depression.

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.

Hippocampal changes in mice lacking an active prohormone convertase 2

Prohormone convertase 2 (PC2) is essential for the biosynthesis of many neuropeptides, including several of them in hippocampus. In mouse brain, lacking an enzymatically active PC2 (PC2-null) causes accumulation of many neuropeptides in their precursor or intermediate forms. Little is known about how a PC2-null state may affect the function of the hippocampus. In this study, adult PC2-null mice and their wildtype (WT) littermates were subjected to three analyses to determine possible changes associated with PC2-null at physiological, behavioral, and molecular levels, respectively, under normal and stressed conditions. Electrophysiological recordings of hippocampal slices were performed to measure evoked field-excitatory postsynaptic potentials (EPSP), long-term potentiation (LTP), and paired-pulse facilitation (PPF). Morris water maze (MWM) testing was conducted to examine behavioral changes that are indicative of hippocampal integrity. Quantitative mass spectrometry analysis was used to determine changes in the hippocampal proteome in response to a focal cerebral ischemic insult. We found that there were no significant differences in the threshold of evoked EPSPs between PC2-null and WT animals. However, an increase in LTP in both triggering rate and amplitude was observed in PC2-null mice, suggesting that PC2 may be involved in regulating synaptic strength. The PPF, on the other hand, showed a decrease in PC2-null mice, suggesting a presynaptic mechanism. Consistent with changes in LTP, PC2-null mice displayed decreased latencies in finding the escape platform in the MWM test. Further, after distal focal cerebral ischemia, the hippocampal proteomes incurred changes in both WT and PC2-null mice, with a prominent change in proteins associated with neurotransmission, exocytosis, and transport processes seen in the PC2-null but not WT mice. Taken together, our results suggest that PC2 is involved in regulating hippocampal synaptic plasticity, learning, and memory behaviors, as well as the hippocampal response to stresses originating in other regions of the brain.

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

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

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.

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

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

Somatostatin depresses the excitability of subicular bursting cells: Roles of inward rectifier K+ channels, KCNQ channels and Epac

The hippocampus is a crucial component for cognitive and emotional processing. The subiculum provides much of the output for this structure but the modulation and function of this region is surprisingly under-studied. The neuromodulator somatostatin (SST) interacts with five subtypes of SST receptors (sst₁ to sst₅ ) and each of these SST receptor subtypes is coupled to Gi proteins resulting in inhibition of adenylyl cyclase (AC) and decreased level of intracellular cAMP. SST modulates many physiological functions including cognition, emotion, autonomic responses and locomotion. Whereas SST has been shown to depress neuronal excitability in the subiculum, the underlying cellular and molecular mechanisms have not yet been determined. Here, we show that SST hyperpolarized two classes of subicular neurons with a calculated EC₅₀ of 0.1 μM. Application of SST (1 μM) induced outward holding currents by primarily activating K⁺ channels including the G-protein-activated inwardly-rectifying potassium channels (GIRK) and KCNQ (M) channels, although inhibition of cation channels in some cells may also be implicated. SST-elicited hyperpolarization was mediated by activation of sst₂ receptors and required the function of G proteins. The SST-induced hyperpolarization resulted from decreased activity of AC and reduced levels of cAMP but did not require the activity of either PKA or PKC. Inhibition of Epac2, a guanine nucleotide exchange factor, partially blocked SST-mediated hyperpolarization of subicular neurons. Furthermore, application of SST resulted in a robust depression of subicular action potential firing and the SST-induced hyperpolarization was responsible for its inhibitory action on LTP at the CA1-subicilum synapses. Our results provide a novel cellular and molecular mechanism that may explain the roles of SST in modulation of subicular function and be relevant to SST-related physiological functions.

AMPA GluA1-flip targeted oligonucleotide therapy reduces neonatal seizures and hyperexcitability

Glutamate-activated α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPA-Rs) mediate the majority of excitatory neurotransmission in brain and thus are major drug targets for diseases associated with hyperexcitability or neurotoxicity. Due to the critical nature of AMPA-Rs in normal brain function, typical AMPA-R antagonists have deleterious effects on cognition and motor function, highlighting the need for more precise modulators. A dramatic increase in the flip isoform of alternatively spliced AMPA-R GluA1 subunits occurs post-seizure in humans and animal models. GluA1-flip produces higher gain AMPA channels than GluA1-flop, increasing network excitability and seizure susceptibility. Splice modulating oligonucleotides (SMOs) bind to pre-mRNA to influence alternative splicing, a strategy that can be exploited to develop more selective drugs across therapeutic areas. We developed a novel SMO, GR1, which potently and specifically decreased GluA1-flip expression throughout the brain of neonatal mice lasting at least 60 days after single intracerebroventricular injection. GR1 treatment reduced AMPA-R mediated excitatory postsynaptic currents at hippocampal CA1 synapses, without affecting long-term potentiation or long-term depression, cellular models of memory, or impairing GluA1-dependent cognition or motor function in mice. Importantly, GR1 demonstrated anti-seizure properties and reduced post-seizure hyperexcitability in neonatal mice, highlighting its drug candidate potential for treating epilepsies and other neurological diseases involving network hyperexcitability.

Relaxin-3 inputs target hippocampal interneurons and deletion of hilar relaxin-3 receptors in "floxed-RXFP3" mice impairs spatial memory

Hippocampus is innervated by γ-aminobutyric acid (GABA) "projection" neurons of the nucleus incertus (NI), including a population expressing the neuropeptide, relaxin-3 (RLN3). In studies aimed at gaining an understanding of the role of RLN3 signaling in hippocampus via its G_i/o -protein-coupled receptor, RXFP3, we examined the distribution of RLN3-immunoreactive nerve fibres and RXFP3 mRNA-positive neurons in relation to hippocampal GABA neuron populations. RLN3-positive elements were detected in close-apposition with a substantial population of somatostatin (SST)- and GABA-immunoreactive neurons, and a smaller population of parvalbumin- and calretinin-immunoreactive neurons in different hippocampal areas, consistent with the relative distribution patterns of RXFP3 mRNA and these marker transcripts. In light of the functional importance of the dentate gyrus (DG) hilus in learning and memory, and our anatomical data, we examined the possible influence of RLN3/RXFP3 signaling in this region on spatial memory. Using viral-based Cre/LoxP recombination methods and adult mice with a floxed Rxfp3 gene, we deleted Rxfp3 from DG hilar neurons and assessed spatial memory performance and affective behaviors. Following infusions of an AAV^(1/2) -Cre-IRES-eGFP vector, Cre expression was observed in DG hilar neurons, including SST-positive cells, and in situ hybridization histochemistry for RXFP3 mRNA confirmed receptor depletion relative to levels in floxed-RXFP3 mice infused with an AAV^(1/2) -eGFP (control) vector. RXFP3 depletion within the DG hilus impaired spatial reference memory in an appetitive T-maze task reflected by a reduced percentage of correct choices and increased time to meet criteria, relative to control. In a continuous spontaneous alternation Y-maze task, RXFP3-depleted mice made fewer alternations in the first minute, suggesting impairment of spatial working memory. However, RXFP3-depleted and control mice displayed similar locomotor activity, anxiety-like behavior in light/dark box and elevated-plus maze tests, and learning and long-term memory retention in the Morris water maze. These data indicate endogenous RLN3/RXFP3 signaling can modulate hippocampal-dependent spatial reference and working memory via effects on SST interneurons, and further our knowledge of hippocampal cognitive processing. © 2017 Wiley Periodicals, Inc.

Adeno-associated viral vector-mediated preprosomatostatin expression suppresses induced seizures in kindled rats

Somatostatin is expressed widely in the hippocampus and notably in hilar GABAergic neurons that are vulnerable to seizure neuropathology in chronic temporal lobe epilepsy. We previously demonstrated that sustained bilateral preprosomatostatin (preproSST) expression in the hippocampus prevents the development of generalized seizures in the amygdala kindling model of temporal lobe epilepsy. Here we tested whether sustained preproSST expression is anticonvulsant in rats already kindled to high-grade seizures. Rats were kindled until they exhibited 3 consecutive Racine Grade 5 seizures before adeno-associated virus serotype 5 (AAV5) vector driving either eGFP (AAV5-CBa-eGFP) or preproSST and eGFP (AAV5-CBa-preproSST-eGFP) expression was injected bilaterally into the hippocampal dentate gyrus and CA1 region. Retested 3 weeks later, rats that received control vector (AAV5-CBa-eGFP) continued to exhibit high-grade seizures whereas 6/13 rats that received preproSST vector (AAV5-CBa-preproSST-eGFP) were seizure-free. Of these rats, 5/6 remained seizure-free after repeated stimulation sessions and when the stimulation current was increased. These results suggest that vector-mediated expression of preproSST may be a viable therapeutic strategy for temporal lobe epilepsy.

Protein phosphatase modulation of somatostatin receptor signaling in the mouse hippocampus

Many inhibitory interneurones in the hippocampus release the neuropeptide somatostatin (SST) which inhibits neuronal excitability through Gi/Go-coupled receptors. To investigate the signaling pathways underlying the SST inhibition of neuronal excitability in the hippocampus, we performed perforated patch-clamp recordings from CA1 pyramidal neurones in acute brain slices from P14-P18 mice. Bath application of 1 μM SST reversibly reduces the frequency of action potential firing in response to depolarising current steps, and is associated with neuronal hyperpolarisation and a reduction in membrane resistance. This effect is mediated by potassium channels with KCNK-like pharmacology. In addition, in slices that have been cultured in vitro for seven days or more, SST also produces a hyperpolarisation independent reduction in action potential firing, which can be also observed in acute slices when the Ser/Thr protein phosphatases PP2A and PP4 are inhibited selectively with fostriecin. This hyperpolarisation independent effect of SST appears to be mediated by G-protein-activated inwardly rectifying K+ (GIRK) channels. Knockdown of protein phosphatase 5, by Cre recombinase mediated deletion of the floxed Ppp5c gene, blocks the hyperpolarisation independent effect of SST, and reduces the hyperpolarisation dependent effect in a manner consistent with increased SST receptor desensitisation. Thus, reversible protein phosphorylation provides a mechanism to enhance or diminish the inhibitory effect of SST, which could allow system level regulation of circuit excitability in the hippocampus.

Inositol 1,4,5-trisphosphate 3-kinase A is a novel microtubule-associated protein: PKA-dependent phosphoregulation of microtubule binding affinity

Inositol 1,4,5-trisphosphate 3-kinase A (IP(3)K-A) is a brain specific and F-actin-binding protein. We recently demonstrated that IP(3)K-A modulates a structural reorganization of dendritic spines through F-actin remodeling, which is required for synaptic plasticity and memory formation in brain. However, detailed functions of IP(3)K-A and its regulatory mechanisms involved in the neuronal cytoskeletal dynamics still remain unknown. In the present study, we identified tubulin as a candidate of IP(3)K-A-binding protein through proteomic screening. By various in vitro and in vivo approaches, we demonstrated that IP(3)K-A was a novel microtubule-associated protein (MAP), and the N terminus of IP(3)K-A was a critical region for direct binding to tubulin in dendritic shaft of hippocampal neurons. Moreover, PKA phosphorylated Ser-119 within IP(3)K-A, leading to a significant reduction of microtubule binding affinity. These results suggest that PKA-dependent phosphorylation and microtubule binding of IP(3)K-A are involved in its regulatory mechanism for activity-dependent neuronal events such as local calcium signaling and its synaptic targeting.

Somatostatinergic systems: an update on brain functions in normal and pathological aging

Somatostatin is highly expressed in mammalian brain and is involved in many brain functions such as motor activity, sleep, sensory, and cognitive processes. Five somatostatin receptors have been described: sst(1), sst(2) (A and B), sst(3), sst(4), and sst(5), all belonging to the G-protein-coupled receptor family. During the recent years, numerous studies contributed to clarify the role of somatostatin systems, especially long-range somatostatinergic interneurons, in several functions they have been previously involved in. New advances have also been made on the alterations of somatostatinergic systems in several brain diseases and on the potential therapeutic target they represent in these pathologies.

Endogenous opioid peptides contribute to associative LTP in the hippocampal CA3 region

The medial and lateral perforant path projections to the hippocampal CA3 region display distinct mechanisms of long-term potentiation (LTP) induction, N-methyl-d-aspartate (NMDA) and opioid receptor dependent, respectively. However, medial and lateral perforant path projections to the CA3 region display associative LTP with coactivation, suggesting that while they differ in receptors involved in LTP induction they may share common downstream mechanisms of LTP induction. Here we address this interaction of LTP induction mechanisms by evaluating the contribution of opioid receptors to the induction of associative LTP among the medial and lateral perforant path projections to the CA3 region in vivo. Local application of the opioid receptor antagonists naloxone or Cys2-Tyr3-Orn5-Pen7-amide (CTOP) normally block induction of lateral perforant path-CA3 LTP. However, these opioid receptor antagonists failed to block associative LTP in lateral perforant path-CA3 synapses when it was induced by strong coactivation of the medial perforant pathway which displays NMDAR-dependent LTP. Thus strong activation of non-opioidergic afferents can substitute for the opioid receptor activation required for lateral perforant path LTP induction. Conversely, medial perforant path-CA3 associative LTP was blocked by opioid receptor antagonists when induced by strong coactivation of the opioidergic lateral perforant path. These data indicate endogenous opioid peptides contribute to associative LTP at coactive synapses when induced by strong coactivation of an opioidergic afferent system. These data further suggest that associative LTP induction is regulated by the receptor mechanisms of the strongly stimulated pathway. Thus, while medial and lateral perforant path synapses differ in their mechanisms of LTP induction, associative LTP at these synapses share common downstream mechanisms of induction.

Lamina- and cell-specific alterations in cortical somatostatin receptor 2 mRNA expression in schizophrenia

Disturbed cortical γ-aminobutyric acid (GABA) neurotransmission in schizophrenia is evident from lamina- and cell type- specific alterations in presynaptic markers. In the dorsolateral prefrontal cortex (DLPFC), these alterations include lower transcript expression of glutamic acid decarboxylase (GAD67) and somatostatin (SST), a neuropeptide expressed in the Martinotti subpopulation of GABA neurons whose axons innervate the distal apical dendrites of pyramidal neurons. However, whether the alterations in SST-containing interneurons are associated with changes in post-synaptic receptors for SST has not been examined. Thus, we used in situ hybridization to quantify the mRNA expression levels of SST receptors subtype 1 (SSTR1) and subtype 2 (SSTR2) in DLPFC area 9 from 23 matched pairs of subjects with schizophrenia and normal comparison subjects. We also assessed the effects of potential confounding variables within the human subjects and in brain specimens from macaque monkeys with long term exposure to antipsychotic drugs. SSTR1 mRNA levels did not differ between subject groups. In contrast, mean cortical SSTR2 mRNA levels were significantly 19% lower in the subjects with schizophrenia. Laminar and cellular level analyses revealed that lower SSTR2 mRNA levels were localized to pyramidal cells in cortical layers 5-6. Expression of SSTR2 mRNA did not differ between monkeys exposed chronically to high doses of haloperidol or olanzapine and control animals, or between subjects with schizophrenia on or off antipsychotic medications at the time of death. However, levels of SSTR2 mRNA were significantly 37.6% lower in monkeys exposed chronically to low dose haloperidol, suggesting that the lower levels of SSTR2 mRNA selectively in pyramidal neurons in DLPFC layers 5-6 in schizophrenia should be interpreted with caution. In concert with prior findings of lower SST mRNA expression in the same subjects, the results of this study suggest the convergence of pre- and post-synaptic mechanisms to reduce inhibitory inputs to pyramidal neurons in the infragranular layers of the DLPFC.

STEP regulation of seizure thresholds in the hippocampus

PURPOSE

To investigate whether striatal enriched protein tyrosine phosphatase (STEP) influences ictogenesis.

METHODS

STEP knockout mice were compared to wild-type (WT) mice in pilocarpine-induced seizures. Hippocampal slices were also prepared from these two mouse populations, allowing the examination of ictal-like stimulation in these slices using calcium imaging and electrophysiologic recordings.

KEY FINDINGS

To examine seizure thresholds, increasing doses of pilocarpine were administered to adult mice and seizures were scored behaviorally. Significantly fewer STEP knockout mice developed seizures that progressed to the stage of status epilepticus compared to WT mice. To examine potential differences in neural circuits that might account for this finding, seizure-like activity was induced in hippocampal slices. Electrical stimulation of the hippocampal-entorhinal cortex pathway in STEP knockout mice resulted in less activation of the dentate gyrus granule cell layer (GCL), but greater activation of the hilus in STEP knockouts, compared with heterozygous slices.

SIGNIFICANCE

STEP deficiency is associated with higher seizure thresholds. The locus of these effects appears to include the dentate gyrus granule cell layer and hilus.

Somatostatin signaling in neuronal cilia is critical for object recognition memory

Most neurons possess a single, nonmotile cilium that projects out from the cell surface. These microtubule-based organelles are important in brain development and neurogenesis; however, their function in mature neurons is unknown. Cilia express a complement of proteins distinct from other neuronal compartments, one of which is the somatostatin receptor subtype SST(3). We show here that SST(3) is critical for object recognition memory in mice. sst3 knock-out mice are severely impaired in discriminating novel objects, whereas they retain normal memory for object location. Further, systemic injection of an SST(3) antagonist (ACQ090) disrupts recall of familiar objects in wild-type mice. To examine mechanisms of SST(3), we tested synaptic plasticity in CA1 hippocampus. Electrically evoked long-term potentiation (LTP) was normal in sst3 knock-out mice, while adenylyl cyclase/cAMP-mediated LTP was impaired. The SST(3) antagonist also disrupted cAMP-mediated LTP. Basal cAMP levels in hippocampal lysate were reduced in sst3 knock-out mice compared with wild-type mice, while the forskolin-induced increase in cAMP levels was normal. The SST(3) antagonist inhibited forskolin-stimulated cAMP increases, whereas the SST(3) agonist L-796,778 increased basal cAMP levels in hippocampal slices but not hippocampal lysate. Our results show that somatostatin signaling in neuronal cilia is critical for recognition memory and suggest that the cAMP pathway is a conserved signaling motif in cilia. Neuronal cilia therefore represent a novel nonsynaptic compartment crucial for signaling involved in a specific form of synaptic plasticity and in novelty detection.

Ang II and Ang IV: unraveling the mechanism of action on synaptic plasticity, memory, and epilepsy

The central angiotensin system plays a crucial role in cardiovascular regulation. More recently, angiotensin peptides have been implicated in stress, anxiety, depression, cognition, and epilepsy. Angiotensin II (Ang II) exerts its actions through AT(1) and AT(2) receptors, while most actions of its metabolite Ang IV were believed to be independent of AT(1) or AT(2) receptor activation. A specific binding site with high affinity for Ang IV was discovered and denominated "AT(4) receptor". The beneficiary effects of AT(4) ligands in animal models for cognitive impairment and epileptic seizures initiated the search for their mechanism of action. This proved to be a challenging task, and after 20 years of research, the nature of the "AT(4) receptor" remains controversial. Insulin-regulated aminopeptidase (IRAP) was first identified as the high-affinity binding site for AT(4) ligands. Recently, the hepatocyte growth factor receptor c-MET was also proposed as a receptor for AT(4) ligands. The present review focuses on the effects of Ang II and Ang IV on synaptic transmission and plasticity, learning, memory, and epileptic seizure activity. Possible interactions of Ang IV with the classical AT(1) and AT(2) receptor subtypes are evaluated, and other potential mechanisms by which AT(4) ligands may exert their effects are discussed. Identification of these mechanisms may provide a valuable target in the development in novel drugs for the treatment of cognitive disorders and epilepsy.

The role of kisspeptin and GPR54 in the hippocampus

The granule cells of the dentate gyrus form the input stage of the hippocampal trisynaptic circuit and their function is strongly influenced by peptidergic systems. GPR54 is highly and discretely expressed in these cells. We have found that activation of GPR54 with kisspeptin-10 causes a rapid and large increase in the amplitude of excitatory synaptic responses in granule cells, without changing membrane properties. The effect was suppressed by the G-protein inhibitor GDP-beta-S and the calcium chelator BAPTA, and analysis of miniature EPSCs revealed an increase in mean amplitude but not event frequency, indicating that GPR54 and the mechanisms for enhancing EPSCs are postsynaptic, possibly involving changes in AMPA receptor number or conductance. The kisspeptin-induced synaptic potentiation was abolished by inhibitors of ERK1/2, tyrosine kinase, and CaMKII. RT-PCR experiments showed that KiSS-1 is expressed in the dentate gyrus. KiSS-1 mRNA was significantly increased by seizure activity in rats and when neuronal activity in organotypic hippocampal slice cultures was enhanced by kainate or picrotoxin, while mRNA for GPR54 remained essentially unchanged. These results suggest that kisspeptin may be locally synthesized and act as an autocrine factor. In separate experiments, hippocampal KiSS-1 mRNA in male rats was increased after gonadectomy. In summary, kisspeptin is a novel endogenous factor which is dynamically regulated by neuronal activity and which, in marked distinction from other neuropeptides, increases synaptic transmission in dentate granule cells through signaling cascades possibly linked to the MAP kinase system. This novel peptide system may play a role in cognition and in the pathogenesis of epilepsy.

In vivo modulation of O-GlcNAc levels regulates hippocampal synaptic plasticity through interplay with phosphorylation

O-Linked N-acetylglucosamine (O-GlcNAc) is a cytosolic and nuclear carbohydrate post-translational modification most abundant in brain. We recently reported uniquely extensive O-GlcNAc modification of proteins that function in synaptic vesicle release and post-synaptic signal transduction. Here we examined potential roles for O-GlcNAc in mouse hippocampal synaptic transmission and plasticity. O-GlcNAc modifications and the enzyme catalyzing their addition (O-GlcNAc transferase) were enriched in hippocampal synaptosomes. Pharmacological elevation or reduction of O-GlcNAc levels had no effect on Schaffer collateral CA1 basal hippocampal synaptic transmission. However, in vivo elevation of O-GlcNAc levels enhanced long term potentiation (LTP), an electrophysiological correlate to some forms of learning/memory. Reciprocally, pharmacological reduction of O-GlcNAc levels blocked LTP. Additionally, elevated O-GlcNAc led to reduced paired-pulse facilitation, a form of short term plasticity attributed to presynaptic mechanisms. Synapsin I and II are presynaptic proteins that increase synaptic vesicle availability for release when phosphorylated, thus contributing to hippocampal synaptic plasticity. Synapsins are among the most extensively O-GlcNAc-modified proteins known. Elevating O-GlcNAc levels increased phosphorylation of Synapsin I/II at serine 9 (cAMP-dependent protein kinase substrate site), serine 62/67 (Erk 1/2 (MAPK 1/2) substrate site), and serine 603 (calmodulin kinase II site). Activation-specific phosphorylation events on Erk 1/2 and calmodulin kinase II, two proteins required for CA1 hippocampal LTP establishment, were increased in response to elevation of O-GlcNAc levels. Thus, O-GlcNAc is a novel regulatory signaling component of excitatory synapses, with specific roles in synaptic plasticity that involve interplay with phosphorylation.

Somatostatin: an endogenous antiepileptic

The neuropeptide somatostatin (SST) is highly expressed in brain regions associated with seizures. In hippocampus, SST expression and release is regulated by seizures, and SST-containing neurons within the hilus of the dentate gyrus are sensitive to seizure-induced death. In vivo and in vitro studies suggest that the loss of SST function in the dentate could contribute to epileptogenesis and seizure susceptibility. SST also has inhibitory actions in the CA1 and CA3 hippocampus indicating this peptide is an important homeostatic regulator throughout the hippocampus. In vivo studies show SST has robust antiepileptic properties with the major site of action being hippocampus. In rodents, somatostatin receptor subtype 2 (SST(2)) and SST(4) appear to mediate the majority of the antiepileptic actions of SST, with SST(2) predominate in rat and SST(4) in mouse. Thus SST receptors may be appropriate targets for new antiepileptic drugs (AEDs), although validation in human tissue is lacking.

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

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

Somatostatin receptor subtype 4 couples to the M-current to regulate seizures

The K(+) M-current (I(M), Kv7) is an important regulator of cortical excitability, and mutations in these channels cause a seizure disorder in humans. The neuropeptide somatostatin (SST), which has antiepileptic properties, augments I(M) in hippocampal CA1 pyramidal neurons. We used SST receptor knock-out mice and subtype-selective ligands to investigate the receptor subtype that couples to I(M) and mediates the antiepileptic effects of SST. Using pentylenetetrazole as a chemoconvulsant, SST(2), SST(3), and SST(4) receptor knock-out mice all had shorter latencies to different seizure stages and increased seizure severity when compared with wild-type mice. However, the most robust differences were observed in the SST(4) knock-outs. When seizures were induced by systemic injection of kainate, only SST(4) knock-outs showed an increase in seizure sensitivity. We next examined the action of SST and subtype-selective SST agonists on electrophysiological parameters in hippocampal slices of wild-type and receptor knock-out mice. SST(2) and SST(4) appear to mediate the majority of SST inhibition of epileptiform activity in CA1. SST lacked presynaptic effects in mouse CA1, in contrast to our previous findings in rat. SST increased I(M) in CA1 pyramidal neurons of wild-type and SST(2) knock-out mice, but not SST(4) knock-out mice. Using M-channel blockers, we found that SST(4) coupling to M-channels is critical to its inhibition of epileptiform activity. This is the first demonstration of an endogenous enhancer of I(M) that is important in controlling seizure activity. SST(4) receptors could therefore be an important novel target for developing new antiepileptic and antiepileptogenic drugs.

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

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

An update on somatostatin receptor signaling in native systems and new insights on their pathophysiology

The peptide somatostatin (SRIF) has important physiological effects, mostly inhibitory, which have formed the basis for the clinical use of SRIF compounds. SRIF binding to its 5 guanine nucleotide-binding proteins-coupled receptors leads to the modulation of multiple transduction pathways. However, our current understanding of signaling exerted by receptors endogenously expressed in different cells/tissues reflects a rather complicated picture. On the other hand, the complexity of SRIF receptor signaling in pathologies, including pituitary and nervous system diseases, may be studied not only as alternative intervention points for the modulation of SRIF function but also to exploit new chemical space for drug-like molecules.

Presynaptic inhibition of glutamate release by neuropeptides: use-dependent synaptic modification

Neuropeptides are signaling molecules that interact with G-protein coupled receptors located both pre- and postsynaptically. Presynaptically, these receptors are localized in axons and terminals away from presynaptic specializations. Neuropeptides are stored in dense core vesicles that are distinct from the clear synaptic vesicles containing classic neurotransmitters such as glutamate and GABA. Because they require a stronger Ca(2+) signal than synaptic vesicles, dense core vesicles do not release neuropeptides with single action potentials but rather require high-frequency trains. Thus, neuropeptides only modulate strongly stimulated synapses, providing negative or positive feedback. Many neuropeptides have been found to inhibit glutamate release from presynaptic terminals, and the major mechanism is likely direct interaction of betagamma G-protein subunits with presynaptic proteins such as SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor). The use of mouse genetic models and specific receptor antagonists are beginning to unravel the function of inhibitory neuropeptides. The opioid receptors kappa and mu, which are activated by endogenous opioid peptides such as dynorphin, enkephalin, and possibly the endomorphins, are important in modulating pain transmission. Dynorphin, nociceptin/orphanin FQ, and somatostatin and its related peptide cortistatin appear to play a role in modulation of learning and memory. Neuropeptide Y has important functions in ingestive behavior and also in entraining circadian rhythms. The existence of neuropeptides greatly expands the computational ability of the brain by providing additional levels of modulation.

Somatostatin in the dentate gyrus

The neuropeptide somatostatin (SST) is expressed in a discrete population of interneurons in the dentate gyrus. These interneurons have their soma in the hilus and project to the outer molecular layer onto dendrites of dentate granule cells, adjacent to perforant path input. SST-containing interneurons are very sensitive to excitotoxicty, and thus are vulnerable to a variety of neurological diseases and insults, including epilepsy, Alzheimer's disease, traumatic brain injury, and ischemia. The SST gene contains a prototypical cyclic AMP response element (CRE) site. Such a regulatory site confers activity-dependence to the gene, such that it is turned on when neuronal activity is high. Thus SST expression is increased by pathological conditions such as seizures and by natural stimulation such as environmental enrichment. SST may play an important role in cognition by modulating the response of neurons to synaptic input. In the dentate, SST and the related peptide cortistatin (CST) reduce the likelihood of generating long-term potentiation, a cellular process involved in learning and memory. Thus these neuropeptides would increase the threshold of input required for acquisition of new memories, increasing "signal to noise" to filter out irrelevant environmental cues. The major mechanism through which SST inhibits LTP is likely through inhibition of voltage-gated Ca(2+) channels on dentate granule cell dendrites. Transgenic overexpression of CST in the dentate leads to profound deficits in spatial learning and memory, validating its role in cognitive processing. A reduction of synaptic potentiation by SST and CST in dentate may also contribute to the well-characterized antiepileptic properties of these neuropeptides. Thus SST and CST are important neuromodulators in the dentate gyrus, and disruption of this signaling system may have major impact on hippocampal function.

The selective effects of somatostatin- and GABA-mediated transmissions on voltage gated Ca2+ channel immunoreactivity in the gerbil hippocampus

To identify whether altered expressions of voltage gated Ca(2+) channel (VGCC) are linked to inhibitory transmission abnormalities in the gerbil hippocampus, we investigated the effects of GABA receptor or somatostatin receptor (SST) antagonists/agonists on VGCC immunoreactivity in vivo. VGCC immunoreactivities in the hippocampus were significantly higher in seizure sensitive (SS) gerbils than in seizure resistant (SR) gerbils. P/Q-type VGCC immunoreactivity in the gerbil hippocampus was reduced by enhancement in GABA(A) and GABA(B) receptor-mediated transmission, but not by SST-mediated transmission. N-type VGCC immunoreactivity was reduced only by a SST agonist, whereas L-type (alpha1C) VGCC immunoreactivity was reduced only by a GABA(A) receptor agonist, and L-type (alpha1D) VGCC immunoreactivity was modulated by the GABA(B) receptor acting drugs. These findings provide a comprehensive description of the differential responses of VGCC subunits to alteration in GABAergic or somatostatinergic transmission. These findings also suggest that up-regulated VGCC immunoreactivity may be consequence of the neuronal excitability caused by a reduction in inhibitory neurotransmission in the gerbil hippocampus.

Postsynaptic targets of somatostatin-containing interneurons in the rat basolateral amygdala

The basolateral amygdala contains several subpopulations of inhibitory interneurons that can be distinguished on the basis of their content of calcium-binding proteins or peptides. Although previous studies have shown that interneuronal subpopulations containing parvalbumin (PV) or vasoactive intestinal peptide (VIP) innervate distinct postsynaptic domains of pyramidal cells as well as other interneurons, very little is known about the synaptic outputs of the interneuronal subpopulation that expresses somatostatin (SOM). The present study utilized dual-labeling immunocytochemical techniques at the light and electron microscopic levels to analyze the innervation of pyramidal cells, PV+ interneurons, and VIP+ interneurons in the anterior basolateral amygdalar nucleus (BLa) by SOM+ axon terminals. Pyramidal cell somata and dendrites were selectively labeled with antibodies to calcium/calmodulin-dependent protein kinase II (CaMK); previous studies have shown that the vast majority of dendritic spines, whether CAMK+ or not, arise from pyramidal cells. Almost all SOM+ axon terminals formed symmetrical synapses. The main postsynaptic targets of SOM+ terminals were small-caliber CaMK+ dendrites and dendritic spines, some of which were CaMK+. These SOM+ synapses with dendrites were often in close proximity to asymmetrical (excitatory) synapses to these same structures formed by unlabeled terminals. Few SOM+ terminals formed synapses with CaMK+ pyramidal cell somata or large-caliber (proximal) dendrites. Likewise, only 15% of SOM+ terminals formed synapses with PV+, VIP+, or SOM+ interneurons. These findings suggest that inhibitory inputs from SOM+ interneurons may interact with excitatory inputs to pyramidal cell distal dendrites in the BLa. These interactions might affect synaptic plasticity related to emotional learning.

Cortistatin overexpression in transgenic mice produces deficits in synaptic plasticity and learning

Cortistatin-14 (CST) is a neuropeptide expressed in cortical and hippocampal interneurons that shares 11 of 14 residues with somatostatin. In contrast to somatostatin, infusion of CST decreases locomotor activity and selectively enhances slow wave sleep. Here, we show that transgenic mice that overexpress cortistatin under the control of neuron-specific enolase promoter do not express long-term potentiation in the dentate gyrus. This blockade of dentate LTP correlates with profound impairment of hippocampal-dependent spatial learning. Exogenously applied CST to slices of wild-type mice also blocked induction of LTP in the dentate gyrus. Our findings implicate cortistatin in the modulation of synaptic plasticity and cognitive function. Thus, increases in hippocampal cortistatin expression during aging could have an impact on age-related cognitive deficits.

Role of galanin receptor 1 and galanin receptor 2 activation in synaptic plasticity associated with 3',5'-cyclic AMP response element-binding protein phosphorylation in the dentate gyrus: studies with a galanin receptor 2 agonist and galanin receptor 1 knockout mice

The neuropeptide galanin was shown to impair cognitive performance and reduce hippocampal CA1 long-term potentiation (LTP) in rodents. However, the contribution of the two main galanin receptors; GalR1 and GalR2, present in the hippocampus to these effects is not known. In the present study, we determined the protein expression levels of GalR1 and GalR2 in the mouse dentate gyrus (DG) and used galanin (2-11), a recently introduced GalR2 agonist, and GalR1 knockout mice to examine the contribution of GalR1 and GalR2 to the modulation of LTP and 3',5'-cyclic AMP response element-binding protein (CREB)-dependent signaling cascades. In the DG, 57+/-5% of the galanin binding sites were GalR2, and the remaining population corresponded to GalR1. In hippocampal slices, galanin (2-11) fully blocked the induction of DG LTP, whereas galanin (1-29), a high affinity agonist for both GalR1 and GalR2, strongly but not fully attenuated the late phase of LTP by 80+/-1.5%. Application of galanin (1-29) or galanin (2-11) after LTP induction caused a transient reduction in the maintenance phase of LTP, with the larger effect displayed by superfusion of galanin (2-11). The induction and maintenance of DG LTP was not altered in the GalR1 knockout mice. Superfusion of galanin (1-29) or galanin (2-11) blocked the LTP induction to the same degree indicating a role for GalR2 in the induction phase of DG LTP. Furthermore, we analyzed the effects of GalR1 and/or GalR2 activation on DG LTP-induced CREB phosphorylation, associated with the late transcriptional effects of LTP. In the lateral part of the granule cell layer, high-frequency trains stimulation caused a significant increase in the level of CREB phosphorylation, which was significantly reduced by application of either galanin (1-29) or galanin (2-11), indicating that both GalR1 and/or GalR2 can mediate some of their effects on LTP through inhibition of CREB-related signaling cascades.

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

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

Homeostatic activity-dependent paradigm for neurotransmitter specification

Calcium-signaling plays a central role in specification of the chemical transmitters neurons express, adjusting the numbers of cells that express excitatory and inhibitory transmitters as if to achieve homeostatic regulation of excitability. Here we review the extent to which this activity-dependent regulation is observed for a range of different transmitters. Strikingly the homeostatic paradigm is observed both for classical and for peptide transmitters and in mature as well as in embryonic nervous systems. Transmitter homeostasis adds another dimension to homeostatic regulation of function in the nervous system that includes regulation of levels of voltage-gated ion channels, densities of neurotransmitter receptors, and synapse numbers and strength.

Elevated voltage-gated Ca2+ channel immunoreactivities in the hippocampus of seizure-prone gerbil

In present study, we investigated voltage-gated Ca2+ channel (VGCC) expressions in the hippocampus of the Mongolian gerbil and its association with different sequelae of spontaneous seizures, in an effort to identify the epileptogenesis in this animal. In the hippocampus of pre-seizure seizure sensitive (SS) gerbils, VGCC subunit expressions were significantly elevated, as compared with seizure-resistant (SR) gerbils. In 3 h postictal group, the alteration of VGCC expressions showed regional- and neuronal-specific manners; VGCC immunoreactivities in principal neurons were markedly decreased; however, their immunoreactivities in interneurons were significantly elevated. These results are the first comprehensive description of the distribution of VGCC immunoreactivities in the normal and epileptic hippocampus of gerbils, and suggest that these alterations in the hippocampus of the SS gerbil may be related with tissue excitability and have a role in modulating recurrent excitation following seizures.

Profiling neurotransmitter receptor expression in mouse gonadotropin-releasing hormone neurons using green fluorescent protein-promoter transgenics and microarrays

The definition of neurotransmitter receptors expressed by individual neuronal phenotypes is essential for our understanding of integrated neural regulation. We report here a single-neuron strategy using green fluorescent protein (GFP)-promoter transgenic mice and oligonucleotide microarrays that has enabled us to provide a qualitative profile of the neurotransmitter receptors expressed by the gonadotropin- releasing hormone (GnRH) neurons, critical for the neural regulation of fertility. Acute brain slices were prepared from adult female GnRH-GFP transgenic mice and single GnRH neurons identified and patched. The contents of GnRH neurons underwent reverse transcription and cDNA amplification using the switch mechanism at the 5' end of RNA templates system, and hybridization to mouse gene oligonucleotide arrays. Fifty different neurotransmitter receptor subunit mRNAs were detected in GnRH neurons. Many of the classical amino acid and aminergic receptors were present in addition to 14 distinct, and in most cases novel, neuropeptidergic receptor signaling families. Four of the latter were selected for functional validation with gramicidin-perforated patch-clamp electrophysiology. Galanin, GnRH and neuromedin B were all found to exert direct depolarizing actions upon GnRH neurons whereas somatostatin induced a potent hyperpolarizing response. These studies demonstrate a relatively straightforward approach for transcriptome profiling of specific neuronal phenotypes. The stimulatory actions of GnRH and galanin upon GnRH neurons found here indicate that positive ultrashort feedback loops exist among the GnRH neuronal population.

Interneuron Diversity series: Interneuronal neuropeptides--endogenous regulators of neuronal excitability

Interneurons are often classified according to neuropeptide content. However, it is becoming increasingly clear that neuropeptides are more than convenient neurochemical markers and can act as important modulators of neuronal activity. Recent advances in understanding neuropeptide release and physiological actions suggest that the interneuronal system of neuropeptides is crucial for maintaining appropriate brain function under normal and pathophysiological conditions. In particular, interneuronal neuropeptides appear to play roles in cognition and as endogenous anti-epileptic agents. This article describes current understanding of the conditions under which neuropeptides are released from interneurons, their specific effects on neuronal excitability and synaptic transmission, and the consequences of their loss of function.

An “eye” in the gut: the appendix as a sentinel sensory organ of the immune intelligence network

Neural systems are the traditional model of intelligence. Their complex interconnected network of wired neurons acquires, processes, and responds to environmental cues. We propose that the immune system is a parallel system of intelligence in which the gut, including the appendix, plays a prominent role in data acquisition. The immune system is essentially a virtual unwired network of interacting cells that acquires, processes, and responds to environmental data. The data is typically acquired by antigen-presenting cells (APCs) that gather antigenic information from the environment. The APCs chemically digest large antigens and deconstruct them into smaller data packets for sampling by other cells. The gut performs the same function on a larger scale. Morsels of environmental content that enter the gut are sequentially deconstructed by physical and chemical digestion. In addition to providing nutrients, the componentized contents offer environmental data to APCs in mucosa-associated lymphoid tissues (MALT) that relay the sampled information to the immune intelligence network. In this framework, positioning of the appendix immediately after the ileocecal valve is strategic: it is ideally positioned to sample environmental data in its maximally deconstructed state after small bowel digestion. For single-celled organisms, digestion of the environment has been the primary way to sample the surroundings. Prior to the emergence of complex sensory systems such as the eye, even multi-cellular organisms may have relied heavily on digestion to acquire environmental information. While the relative value of immune intelligence has diminished since the emergence of neural intelligence, organisms still use information from both systems in integrated fashion to respond appropriately to ecologic opportunities and challenges. Appendicitis may represent a momentary maladaptation in the evolutionary transition of sensory leadership from the gut to the eye. Relationships between immune dysfunctions and cognition are explored.

Gene expression of alpha-endosulfine in the rat brain: correlative changes with aging, learning and stress

Endosulfine (EDSF) belongs to a highly conserved cAMP-regulated phosphoprotein (ARPP) family and was first isolated from ovine brain as a possible endogenous ligand for sulfonylurea receptors. To explore its involvement in brain functions, we investigated regional distribution of alpha-EDSF gene expression in the rat brain, and its regulation under physiological and pathological conditions. The majority of alpha-EDSF gene was expressed in the pyramidal neurons, which represent the principal excitatory neurons in various brain regions. Down-regulation of alpha-EDSF mRNA was detected in the rat hippocampus during long-term memory consolidation following a spatial learning experience, whereas swimming-related stress caused persistent up-regulation of alpha-EDSF gene expression in several brain regions. These changes, however, were absent from brains of diabetic rats that were subjected to the same behavioral treatments. Intracerebroventricular injection of streptozocin with a toxic dose induced severe learning deficits and brain structure alteration accompanied by a massive increase of alpha-EDSF mRNA in the somatosensory cortex. These results suggest that alpha-EDSF gene expression is differentially regulated by distinct brain processes involving excitatory neuronal activities.