Decreased rhythmic GABAergic septal activity and memory-associated theta oscillations after hippocampal amyloid-beta pathology in the rat.

The memory deficits associated with Alzheimer's disease result to a great extent from hippocampal network dysfunction. The coordination of this network relies on theta (symbol) oscillations generated in the medial septum. Here, we investigated in rats the impact of hippocampal amyloid beta (Abeta) injections on the physiological and cognitive functions that depend on the septohippocampal system. Hippocampal Abeta injections progressively impaired behavioral performances, the associated hippocampal theta power, and theta frequency response in a visuospatial recognition test. These alterations were associated with a specific reduction in the firing of the identified rhythmic bursting GABAergic neurons responsible for the propagation of the theta rhythm to the hippocampus, but without loss of medial septal neurons. Such results indicate that hippocampal Abeta treatment leads to a specific functional depression of inhibitory projection neurons of the medial septum, resulting in the functional impairment of the temporal network.

Ghrelin and obestatin modulate growth hormone-releasing hormone release and synaptic inputs onto growth hormone-releasing hormone neurons.

Ghrelin, a natural ligand of the growth hormone secretagogue receptor (GHS-R), is synthesized in the stomach but may also be expressed in lesser quantity in the hypothalamus where the GHS-R is located on growth hormone-releasing hormone (GHRH) neurons. Obestatin, a peptide derived from the same precursor as ghrelin, is able to antagonize the ghrelin-induced increase of growth hormone (GH) secretion in vivo but not from pituitary explants in vitro. Thus, the blockade of ghrelin-induced GH release by obestatin could be mediated at the hypothalamic level by the neuronal network that controls pituitary GH secretion. Ghrelin increased GHRH and decreased somatostatin (somatotropin-releasing inhibitory factor) release from hypothalamic explants, whereas obestatin only reduced the ghrelin-induced increase of GHRH release, thus indicating that the effect of ghrelin and obestatin is targeted to GHRH neurons. Patch-clamp recordings on mouse GHRH-enhanced green fluorescent protein neurons indicated that ghrelin and obestatin had no significant effects on glutamatergic synaptic transmission. Ghrelin decreased GABAergic synaptic transmission in 44% of the recorded neurons, an effect blocked in the presence of the GHS-R antagonist BIM28163, and stimulated the firing rate of 78% of GHRH neurons. Obestatin blocked the effects of ghrelin by acting on a receptor different from the GHS-R. These data suggest that: (i) ghrelin increases GHRH neuron excitability by increasing their action potential firing rate and decreasing the strength of GABA inhibitory inputs, thereby leading to an enhanced GHRH release; and (ii) obestatin counteracts ghrelin actions. Such interactions on GHRH neurons probably participate in the control of GH secretion.

Role of VAMP7-Dependent Secretion of Reticulon 3 in Neurite Growth.

VAMP7 is involved in autophagy and in exocytosis-mediated neurite growth, two yet unconnected cellular pathways. Here, we find that nutrient restriction and activation of autophagy stimulate axonal growth, while autophagy inhibition leads to loss of neuronal polarity. VAMP7 knockout (KO) neuronal cells show impaired neurite growth, whereas this process is increased in autophagy-null ATG5 KO cells. We find that endoplasmic reticulum (ER)-phagy-related LC3-interacting-region-containing proteins Atlastin 3 and Reticulon 3 (RTN3) are more abundant in autophagy-related protein ATG5 KO and less abundant in VAMP7 KO secretomes. Treatment of neuronal cells with ATG5 or VAMP7 KO conditioned medium does not recapitulate the effect of these KOs on neurite growth. A nanobody directed against VAMP7 inhibits axonal overgrowth induced by nutrient restriction. Furthermore, expression of the inhibitory Longin domain of VAMP7 impairs the subcellular localization of RTN3 in neurons. We propose that VAMP7-dependent secretion of RTN3 regulates neurite growth.

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.

Relationships Between Lower Olfaction and Brain White Matter Lesions in Elderly Subjects with Mild Cognitive Impairment.

BACKGROUND: Olfactory impairment is reported in mild cognitive impairment (MCI) and Alzheimer's disease (AD) and is associated with hippocampal atrophy. In elderly people, dementia with AD neuropathology and white matter lesions (WML) is common. In this context, olfactory impairment could also depend on the presence of WML. OBJECTIVE: To assess the cross-sectional relationship between olfaction and WML in elderly subjects with MCI. METHODS: Consecutive subjects, >65 years old, diagnosed as MCI after a comprehensive neuropsychological assessment in an expert memory center, with a brain MRI performed within a year and without major depressive state, were included. Olfaction was assessed by the Brief Smell Identification Test (BSIT). Two trained neuroradiologists, blind to cognitive and olfaction status, visually assessed hippocampal atrophy according to Scheltens' scale and WML according to Fazekas criteria. RESULTS: Seventy-five MCI subjects (mean age (SD) = 77.1 (6.2) years, 74.7% of women) were included. After adjustment for age and sex, factors associated with low BSIT scores were older age (p = 0.007), lower BMI (p = 0.08), lower MMSE score (p = 0.05), lower FCRST (p = 0.008), hippocampal atrophy (p = 0.04), periventricular WML (p = 0.007), and deep WML burden (p = 0.005). In multivariate analysis, severe deep WML (OR (95% CI) = 6.29 (1.4-35.13), p = 0.02) remained associated with low BSIT score independently from hippocampal atrophy. CONCLUSION: In elderly MCI subjects, low olfactory performances are associated with WML, whose progression may be slowed by vascular treatments. A longitudinal study to evaluate whether the progression of WML, hippocampal atrophy and low olfactory function, can predict accurately conversion from MCI to dementia is ongoing.

Firing properties of anatomically identified neurons in the medial septum of anesthetized and unanesthetized restrained rats.

Cholinergic and GABAergic neurons in the medial septum-diagonal band of Broca (MS-DB) project to the hippocampus where they are involved in generating theta rhythmicity. So far, the functional properties of neurochemically identified MS-DB neurons are not fully characterized. In this study, MS-DB neurons recorded in urethane anesthetized rats and in unanesthetized restrained rats were labeled with neurobiotin and processed for immunohistochemistry against glutamic acid decarboxylase (GAD), parvalbumin (PV), and choline acetyltransferase (ChAT). The majority of the 90 labeled neurons (75.5%) were GAD+. Among them, 34.0% were also PV+, but none were ChAT+. Only 8.8% of the labeled neurons were found ChAT+. Remaining neurons (15.5%) were not identified. In anesthetized rats, all of the PV/GAD+ and 65% of GAD+ neurons exhibited burst-firing activity at the theta frequency. PV/GAD+ neurons displayed higher discharge rate and longer burst duration compared with GAD+ neurons. At variance, all of the ChAT+ neurons were slow-firing. Cluster-firing and tonic-firing were observed in GAD+ and unidentified neurons. In unanesthetized rats, during wakefulness or rapid eye movement sleep with hippocampal theta, the bursting neurons were PV/GAD+ or GAD+, whereas all of the ChAT+ neurons were slow-firing. Across the sleep-wake cycle, the GABAergic component of the septohippocampal pathway was always more active than the cholinergic one. The fact that cholinergic MS-DB neurons do not display theta-related bursting or tonic activity but have a very low firing rate questions how acetylcholine exerts its activating role in the septohippocampal system.

Somatostatin-IRES-Cre Mice: Between Knockout and Wild-Type?

The neuropeptide somatostatin (SOM) is widely expressed in rodent brain and somatostatin-IRES-Cre (SOM-cre) mouse strains are increasingly used to unravel the physiology of SOM-containing neurons. However, while knock-in targeting strategy greatly improves Cre-Lox system accuracy, recent reports have shown that genomic insertion of Cre construct per se can markedly affect physiological function. We show that Cre transgene insertion into the 3'UTR of the somatostatin gene leads to the selective and massive depletion of endogenous SOM in all tested brain regions. It also strongly impacts SOM-related neuroendocrine responses in a similar manner to what has been reported for SST KO mice: increased corticosterone levels after 30-min restraint stress, decreased amplitude and regularity of ultradian growth hormone secretory patterns accompanied by changes in sexually dimorphic liver gene expression (serpina1, Cyp2b9, Cyp2a4, Cyp2d9, and Cyp7b1). In addition to demonstrating the need for examination of the consequences of Cre transgenesis, these results also reveal how this SOM-cre strain may be a useful tool in studying the functional consequences of moderate to low SOM levels as reported in neurological and psychiatric disorders.

Medial septal GABAergic neurons express the somatostatin sst2A receptor: functional consequences on unit firing and hippocampal theta.

GABAergic septohippocampal neurons play a major role in the generation of hippocampal theta rhythm, but modulatory factors intervening in this function are poorly documented. The neuropeptide somatostatin (SST) may be one of these factors, because nearly all hippocampal GABAergic neurons projecting to the medial septum/diagonal band of Broca (MS-DB) express SST. In this study, we took advantage of the high and selective expression of the SST receptor sst2A in MS-DB to examine its possible role on theta-related activity. Immunohistochemical experiments demonstrated that sst2A receptors were selectively targeted to the somatodendritic domain of neurons expressing the GABAergic marker GAD67 but were not expressed by cholinergic neurons. In addition, a subpopulation of GABAergic septohippocampal projecting neurons expressing parvalbumin (PV) also displayed sst2A receptors. Using in vivo juxtacellular recording and labeling with neurobiotin, we showed that a number of bursting and nonbursting neurons exhibiting high discharge rates and brief spikes were immunoreactive for PV or GAD67 and expressed the sst2A receptor. Microiontophoresis applications of SST and the sst2A agonist octreotide (OCT) showed that sst2A receptor activation decreased the discharge rate of both nonbursting and bursting MS-DB neurons and lessened the rhythmic activity of the latter. Finally, intraseptal injections of OCT and SST in freely moving rats reduced the power of hippocampal EEG in the theta band. Together, these in vivo experiments suggest that SST action on MS-DB GABAergic neurons, through sst2A receptors, represents an important modulatory mechanism in the control of theta activity.

Deletion of CCK2 receptor in mice results in an upregulation of the endogenous opioid system.

Stimulation of the brain CCK2 receptor by the C-terminal octapeptide CCK8 of cholecystokinin (CCK) negatively modulates opioid responses. This suggests the existence of physiologically relevant interactions between endogenous CCK and opioid peptides, opening new perspectives particularly in the treatment of pain or drug addiction. CCK2 receptor-deficient mice were used to analyze the incidence of this gene invalidation on opioid system. Compared with wild-type mice, mutants exhibited the following: (1) a hypersensitivity to the locomotor activity induced by inhibitors of enkephalin catabolism or by morphine; (2) a spontaneous hyperalgesia to thermal nociceptive stimulus, which was reversed by previous administration of the NMDA antagonist MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate], and a large reduction in analgesic effects of endogenous or exogenous opioids; and (3) a more severe withdrawal syndrome after chronic morphine treatment. As expected, stimulation of mu, delta, and D2 receptors on brain tissue of wild-type animals induced a dose-dependent decrease in adenylate cyclase activity, whereas a striking mirror effect was observed in mutants. All of these results suggest that the absence, in knock-out mice, of the negative feedback control on the opioid system, normally performed out by CCK2 receptor stimulation, results in an upregulation of this system. These biochemical and pharmacological results demonstrate the critical role played by CCK2 receptors in opioid-dependent responses.

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

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

Aging, but not tau pathology, impacts olfactory performances and somatostatin systems in THY-Tau22 mice.

Somatostatin (SOM) cortical levels decline in Alzheimer's disease (AD) in correlation with cognitive impairment severity, the latter being closely related to the presence of neurofibrillary tangles. Impaired olfaction is another hallmark of AD tightly related to tau pathology in the olfactory pathways. Recent studies showed that SOM modulates olfactory processing, suggesting that alterations in SOM levels participate to olfactory deficits in AD. Herein, we first observed that human olfactory peduncle and cortex are enriched in SOM cells and fibers, in aged postmortem brains. Then, the possible link between SOM alterations and olfactory deficits was evaluated by exploring the impact of age and tau hyperphosphorylation on olfactory SOM networks and behavioral performances in THY-Tau22 mice, a tauopathy transgenic model. Distinct molecular repertoires of SOM peptide and receptors were associated to sensory or cortical olfactory processing structures. Aging mainly affected SOM neurotransmission in piriform and entorhinal cortex in wild-type mice, although olfactory performances decreased. However, no further olfactory impairment was evidenced in THY-Tau22 mice until 12 months although tau pathology early affected olfactory cortical structures. Thus, tau hyperphosphorylation per se has a limited impact on olfactory performances in THY-Tau22 mice.

Targeting sst2A receptor-expressing cells in the rat hypothalamus through in vivo agonist stimulation: neuroanatomical evidence for a major role of this subtype in mediating somatostatin functions.

Numerous physiological studies as well as in situ hybridization and PCR experiments concur in reporting a role for the sst2A receptor in transducing somatostatin (SRIF) actions in the rat hypothalamus. However, the distribution of this receptor protein is not known within this structure. Regional and cellular localization of the sst2A receptor was therefore examined in the rat hypothalamus using highly sensitive immunohistochemical techniques. In close correspondence with the distribution of SRIF-immunoreactive fibers, numerous hypothalamic areas displayed sst2A receptor immunoreactivity. Receptor labeling was, however, diffusely distributed over the tissue, and few immunopositive cells were apparent. Unraveling the distribution of receptor-expressing cells was achieved through acute in vivo agonist stimulation and subsequent receptor internalization. At the cellular level, double-immunolabeling experiments with synaptophysin and microtubule-associated protein 2 demonstrated that sst2A receptors were predominantly internalized in perikarya and dendrites. Double-labeling experiments with SRIF revealed that 93% of arcuate, but only 18% of periventricular, SRIF-positive neurons expressed internalized receptors. Taken together, these results demonstrate for the first time that the sst2A receptor protein is widely, but selectively, distributed in the hypothalamus, and that postsynaptic sst2A auto- and heteroreceptors are well poised to play an important role in the somatostatinergic regulation of hypothalamic endocrine and metabolic processes.

Neurochemical characterization of receptor-expressing cell populations by in vivo agonist-induced internalization: insights from the somatostatin sst2A receptor.

Characterization of both neurochemical phenotype of G protein-coupled receptor (GPCR)-expressing cells and receptor compartmentalization is a prerequisite for the elucidation of receptor functions in the central nervous system. However, it is often prevented by the diffuse and homogeneous distribution of receptor immunoreactivity. This is particularly true for the somatostatin (SRIF) sst2A receptor, which is largely distributed in the mammalian brain. By using this receptor as a model, we investigated whether receptor internalization, a biochemical property shared by numerous GPCRs, would reveal sst2A-expressing cell populations in the rat dorsolateral septum (LSD), a region in which SRIF might play an important modulatory role. Thirty minutes to 1 hour after intracerebroventricular injection of the sst2A receptor agonist octreotide, numerous sst2A-immunoreactive neurons and processes became apparent due to intracytoplasmic accumulation of intensely stained granules. Double-immunolabeling experiments with synaptophysin and MAP2 provided evidence that internalized sst2A receptors are predominantly localized in the somatodendritic compartment. Revealing sst2A receptor-expressing cell bodies permitted to analyze their neurotransmitter content. Quantitative analysis demonstrated an extensive overlap (approximately 85%) between SRIF- and sst2A-expressing neuronal populations. Additionally, numerous SRIF-immunoreactive axon-like terminals were found in close apposition with sst2A-positive cell bodies and dendrites. Taken together, these data suggest that the sst2A receptor is predominantly expressed in LSD neurons as a postsynaptic autoreceptor, thus providing novel neuroanatomic clues to elucidate SRIF neurotransmission in this region. More generally, in vivo agonist-induced internalization appears as a rapid and powerful tool for the neurochemical characterization of GPCR-expressing cell populations in the mammalian brain.

Role of the ghrelin/obestatin balance in the regulation of neuroendocrine circuits controlling body composition and energy homeostasis.

Ghrelin and obestatin are two peptides isolated from the gastrointestinal tract and encoded by the same preproghrelin gene. They convey to the central nervous system informations concerning the nutritional status and/or the energy stores. Ghrelin, mostly acting through the GH secretagogue receptor GHS-R, is a potent GH secretagogue, an orexigenic peptide and a long-term regulator of energy homeostasis. Obestatin was initially described for its anorexigenic effects and its binding to the G protein-coupled receptor 39 (GPR39). However, the role of obestatin is still controversial and the nature of the obestatin receptor remains an open question. This review is focussed on the possible implication of the ghrelin/obestatin system in psychiatric diseases with particular emphasis on eating disorders.

Somatostatinergic systems in brain: networks and functions.

Somatostatin is abundantly expressed in mammalian brain. The peptide binds with high affinity to six somatostatin receptors, sst1, sst2A and B, sst3 to 5, all belonging to the G-protein-coupled receptor family. Recent advances in the neuroanatomy of somatostatin neurons and cellular distribution of sst receptors shed light on their functional roles in the neuronal network. Beside their initially described neuroendocrine role, somatostatin systems subserve neuromodulatory roles in the brain, influencing motor activity, sleep, sensory processes and cognitive functions, and are altered in brain diseases like affective disorders, epilepsia and Alzheimer's disease.

Loss of VGLUT1 and VGLUT2 in the prefrontal cortex is correlated with cognitive decline in Alzheimer disease.

Several lines of evidence suggest that the glutamatergic system is severely impaired in Alzheimer disease (AD). Here, we assessed the status of glutamatergic terminals in AD using the first available specific markers, the vesicular glutamate transporters VGLUT1 and VGLUT2. We quantified VGLUT1 and VGLUT2 in the prefrontal dorsolateral cortex (Brodmann area 9) of controls and AD patients using specific antiserums. A dramatic decrease in VGLUT1 and VGLUT2 was observed in AD using Western blot. Similar decreases were observed in an independent group of subjects using immunoautoradiography. The VGLUT1 reduction was highly correlated with the degree of cognitive impairment, assessed with the clinical dementia rating (CDR) score. A significant albeit weaker correlation was also observed with VGLUT2. These findings provide evidence indicating that glutamatergic systems are severely impaired in the A9 region of AD patients and that this impairment is strongly correlated with the progression of cognitive decline. Our results suggest that VGLUT1 expression in the prefrontal cortex could be used as a valuable neurochemical marker of dementia in AD.