Catherine Videau

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 Alzheimers 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.

Characterisation of [125I]-TyroDTrp8-somatostatin binding in sst1- to sst4- and SRIF-gene-invalidated mouse brain

Five somatostatin receptors (sst) have been cloned and mRNAs for the first four (sst1–4) are expressed in many brain regions. In the present work, we compared the distribution of the non-selective ligand [125I]-Tyr0-DTrp8-SRIF14 by autoradiography in 24 brain regions and pituitary in wild type, sst1- to sst4- or SRIF-gene invalidated (KO) mice. [125I]-Tyr0-DTrp8-SRIF14 binding was not significantly modified in sst1 KO mouse brain with the noticeable exception of the substantia nigra and only moderately decreased in pituitary. For sst2 KO mice, a general decrease (>75%) was observed in most regions, with the noticeable exception of the olfactory bulb and CA1 field of the hippocampus. SST3 KO brain displayed a decrease in binding in the external plexiform layer of the olfactory bulb only (−54%). For sst4 KO mice, [125I]-Tyr0-DTrp8-SRIF14 binding levels in the external plexiform (−35%) and glomerular (−39%) layers of the olfactory bulb as well as the hippocampus CA1 field (−68%) were significantly decreased. In SRIF KO mice, a significant increase in binding levels was observed in olfactory bulb, anterior olfactory nucleus, frontal cortex upper layers, lateral septum, CA1 field, zona incerta and lateral hypothalamus, substantia nigra, periaqueductal grey and parabrachial nucleus. Competition with selective ligands (CH275, octreotide or L-779,976, L-796,778, L-803,087, and octreotide or L-817,778, for sst1–5 receptors, respectively) was in accordance with these findings. Moreover, octreotide was still able to compete on residual [125I]-Tyr0-DTrp8-SRIF14 binding sites in sst2 KO pituitary. It is concluded that most [125I]-Tyr0-DTrp8-SRIF14 binding sites in mouse brain and pituitary belong to the sst2 subtype but for the olfactory bulb (sst3 and sst4 receptors), the CA1 of the hippocampus (sst4 receptors) and the pituitary (sst5 and sst1 receptors) in which other subtypes are also expressed. The overall increase in [125I]-Tyr0-DTrp8-SRIF14 binding in SRIF KO mice indicates that SRIF receptors, mostly from the sst2 subtype, are regulated by the endogenous ligand(s).

Somatostatin interneurons delineate the inner part of the external plexiform layer in the mouse main olfactory bulb.

Neuropeptides play a major role in the modulation of information processing in neural networks. Somatostatin, one of the most concentrated neuropeptides in the brain, is found in many sensory systems including the olfactory pathway. However, its cellular distribution in the mouse main olfactory bulb (MOB) is yet to be characterized. Here we show that ≈95% of mouse bulbar somatostatin‐immunoreactive (SRIF‐ir) cells describe a homogeneous population of interneurons. These are restricted to the inner lamina of the external plexiform layer (iEPL) with dendritic field strictly confined to the region. iEPL SRIF‐ir neurons share some morphological features of Van Gehuchten short‐axon cells, and always express glutamic acid decarboxylase, calretinin, and vasoactive intestinal peptide. One‐half of SRIF‐ir neurons are parvalbumin‐ir, revealing an atypical neurochemical profile when compared to SRIF‐ir interneurons of other forebrain regions such as cortex or hippocampus. Somatostatin is also present in fibers and in a few sparse presumptive deep short‐axon cells in the granule cell layer (GCL), which were previously reported in other mammalian species. The spatial distribution of somatostatin interneurons in the MOB iEPL clearly outlines the region where lateral dendrites of mitral cells interact with GCL inhibitory interneurons through dendrodendritic reciprocal synapses. Symmetrical and asymmetrical synaptic contacts occur between SRIF‐ir dendrites and mitral cell dendrites. Such restricted localization of somatostatin interneurons and connectivity in the bulbar synaptic network strongly suggest that the peptide plays a functional role in the modulation of olfactory processing. J. Comp. Neurol. 518:1976–1994, 2010.

Activated Somatostatin Type 2 Receptors Traffic In Vivo in Central Neurons from Dendrites to the Trans Golgi Before Recycling

Understanding the trafficking of G‐protein‐coupled receptors (GPCRs) is of particular importance, especially when modifications of the neurochemic environment occur as in pathological or therapeutic circumstances. In the central nervous system, although some GPCRs were reported to internalize in vivo, little is known about their trafficking downstream of the endocytic event. To address this issue, distribution and expression pattern of the major somatostatin receptor subtype, the somatostatin type 2 (sst2), was monitored in the hippocampus using immunofluorescence, autoradiographic and immunogold experiments from 10 minutes to 7 days after in vivo injection of the receptor agonist octreotide. We then analyzed whether postendocytic trafficking of the receptor was dependent upon integrity of the microtubule network using colchicine‐injected animals. Together, our results suggest that upon agonist stimulation, dendritic receptors are retrogradely transported through a microtubule‐dependent mechanism to a trans Golgi domain enriched in the t‐SNARE syntaxin 6 and trans Golgi network 38 proteins, before recycling. Because we show that the exit rate from the trans Golgi apparatus back to the plasma membrane (hours) is slower than the entry rate (minutes), the neuronal postendocytic trafficking of sst2 receptor is likely to have functional consequences in several neurological diseases in which an increase in somatostatin release occurs.

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.

Estradiol Regulation of Somatostatin Receptors in the Arcuate Nucleus of the Female Rat

Somatostatin receptors on lactotroph cells of the anterior pituitary are positively regulated by estradiol. In the present work, we investigated whether estradiol regulation of somatostatin receptors also occurred in the female rat brain. 125I-Tyr0-DTrp8-somatostatin (125I-SRIF: 780 Ci/mM) was used as a ligand. Female adult rats were ovariectomized and treated or not with estradiol benzoate (20 micrograms/day for 1 or 8 days). In female brains, 125I-SRIF binding, as assessed by film radioautography, was high in the basolateral amygdala, CA1 field and dentate gyrus of the hippocampus and locus coeruleus, moderate in the median habenula and deep layers all through the cortex. Castration or estradiol treatment did not modify 125I-SRIF binding in these regions. By light-microscopic radioautography, a subpopulation of 125I-SRIF-labeled cells was localized in the ventrolateral portion of the arcuate nucleus. Ovariectomy alone did not significantly affect the number and binding density of 125I-SRIF-labeled cells in the arcuate nucleus. However, estradiol treatment in ovariectomized animals significantly increased both parameters. Along the estrus cycle, the number of 125I-SRIF-labeled cells was not significantly modified but 125I-SRIF binding density was significantly higher in proestrus as compared to diestrus I, diestrus II and estrus. These results demonstrate that brain 125I-SRIF binding sites are positively regulated by estradiol only in the arcuate nucleus of the hypothalamus.

Distribution and pharmacological characterization of somatostatin receptor binding sites in the sheep brain

Somatostatin binding sites have been localized and quantified in the sheep brain using 125I-Tyr0-DTrp8-somatostatin, by quantitative high resolution light microscopic autoradiography. Sections were analyzed by densitometry on radioautographic film, and subsequently on slides coated with photoemulsion. Specific somatostatin binding sites were concentrated in the medial habenula, superior colliculus, dorsal motor nucleus of the vagus nerve, inferior olive, spinal trigeminal nucleus, and cerebellum. In competition experiments, octreotide, a sst2/sst3/sst5 selective agonist only partially displaced 125I-Tyr0-DTrp8-somatostatin in the three cerebellar layers while it was fully active as compared to somatostatin 14 and 28 in the deeper layers of the parietal cortex. Moderate to low somatostatin receptor densities were present in the mesencephalic periaqueductal gray, dorsal raphe, thalamic paraventricular nucleus, interpeduncular nucleus, pineal gland, dorsal tegmental, dorsolateral tegmental and parabrachial nuclei, nucleus of the solitary tract. The distribution of somatostatin binding sites generally correlates with the data obtained on slides dipped in photoemulsion which provided better resolution and more precise localization. In most of the labeled areas, 125I-Tyr0-DTrp8-somatostatin receptor binding was distributed between both neuropil and perikarya. Perikarya bearing 125I-Tyr0-DTrp8-somatostatin receptors were observed in areas which did not display detectable binding sites on film such as the preoptic-anterior hypothalamic complex and arcuate nucleus and in the locus coeruleus. In conclusion, the distribution of 125I-Tyr0-DTrp8-somatostatin binding sites in sheep brain is very reminiscent of other mammals being closer to the human than to rodents.

Expression, localization, and functional coupling of the somatostatin receptor subtype 2 in a mouse model of oxygen-induced retinopathy.

Purpose. In the mouse model of oxygen-induced retinopathy (OIR), somatostatin-14 (SRIF) acting at the SRIF receptor subtype 2 (sst(2)) inhibits angiogenic responses to hypoxia through a downregulation of vascular endothelial growth factor. Information about where SRIF-sst(2) interactions take place is lacking, and downstream effectors mediating SRIF-sst(2) antiangiogenic actions are unknown. Methods. In the OIR model, retinal expression of SRIF was evaluated with RT-PCR and radioimmunoassay. The bindings of [(125)I]LTT-SRIF-28 and [(125)I]Tyr(3)-octreotide were measured in coronal sections of the eye. With Western blot analysis, the authors evaluated the levels of sst(2A) and the expression and activity of the signal transducer and activator of transcription (STAT)3. The analysis of STAT3 was performed in hypoxic mice treated with the sst(2) agonist octreotide or with the sst(2) antagonist D-Tyr(8) cyanamid 154806 (CYN). Retinal localization of sst(2A) was assessed by single and double immunohistochemistry with an endothelial cell marker. Results. In the hypoxic retina, both SRIF and sst(2) levels as well as [(125)I]Tyr(3)-octreotide binding were downregulated. In addition, sst(2A) immunostaining was decreased in the neuroretina but was increased in capillaries. Hypoxia increased both the expression and the activity of STAT3. This increase was inhibited by octreotide but was strengthened by CYN. Conclusions. These data suggest that sst(2) expressed by capillaries may be responsible for the antiangiogenic effects of SRIF and that downstream effectors in this action include the transcription factor STAT3. These results support the possibility of using sst(2)-selective ligands in the treatment of proliferative retinopathies and indicate STAT3 as an additional target for a novel therapeutic approach.

Overexpression of the V3 Vasopressin Receptor in Transgenic Mice Corticotropes Leads to Increased Basal Corticosterone

The vasopressin V3 receptor (V3) is specifically expressed in pituitary corticotropes and mediates the stimulatory effect of vasopressin on adrenocorticotropic hormone (ACTH) release. The V3 gene is overexpressed in corticotrope pituitary tumours compared to normal pituitaries. We hypothesized that V3 overexpression might induce changes in corticotrope function and alter the regulation of the hypothalamic‐pituitary‐adrenal axis. Thus, we generated transgenic mice (POMV3) expressing the human V3 receptor in the pituitary under the control of rat pro‐opiomelanocortin (POMC) promoter sequences. The transgene was efficiently transcribed and vasopressin binding was increased in both corticotropes and melanotropes. In‐vitro ACTH release and inositol phosphate formation were unchanged in POMV3 pituitaries, but the responses to vasopressin were significatively increased. In vivo, basal circulating concentrations of ACTH in POMV3 mice were similar to those of controls but corticosterone concentrations were moderately increased. In addition, the levels of POMC mRNA in the transgenic pituitaries were comparable to those of control mice. Finally, POMV3 mice responded with a similar maximal increase of ACTH and corticosterone to a 20‐min acute restraint stress. Together, these results show that hypophyseal V3 overexpression led to increased basal concentrations of corticosterone and suggest that the negative glucocorticoid feedback may be altered at the pituitary level.