Sample Preparation for Multicolor STED Microscopy

Stimulated emission depletion (STED) microscopy is one of the optical superresolution microscopy (SRM) techniques, more recently also referred to as nanoscopy, that have risen to popularity among biologists during the past decade. These techniques keep pushing the physical boundaries of optical resolution toward the molecular scale. Thereby, they enable biologists to image cellular and tissue structures at a level of almost molecular detail that was previously only achievable using electron microscopy. All the while, they retain the advantages of light microscopy, in particular with regards to sample preparation and flexibility of imaging. Commercially available SRM setups have become more and more available and also increasingly sophisticated, both in terms of optical performance and, importantly, ease of use. Institutional microscopy core facilities now offer widespread access to this type of systems. However, the field has grown so rapidly, and keeps growing, that biologists can be easily overwhelmed by the multitude of available techniques and approaches. From this vast array of SRM modalities, STED stands out in one respect: it is essentially an extension to an advanced confocal microscope. Most experienced users of confocal microscopy will find the transition to STED microscopy relatively easy as compared with some other SRM techniques. This also applies to STED sample preparation. Nonetheless, because resolution in STED microscopy does not only depend on the wavelength of the incident light and the numerical aperture of the objective, but crucially also on the square root of the intensity of the depletion laser and, in general, on the photochemical interaction of the fluorophore with the depletion laser, some additional considerations are necessary in STED sample preparation. Here we describe the single color staining of the somatostatin receptor subtype 2A (SSTR2A) and dual color staining of the trans-Golgi-network protein TGN 38 and the t-SNARE syntaxin-6 for STED in the endocrine cell line AtT20 and STED imaging of the samples, providing the protocols in as general a form as possible. The protocols in this chapter are used in this way in an institutional microscopy core facility.

MIROs and DRP1 drive mitochondrial-derived vesicle biogenesis and promote quality control

Mitochondrial-derived vesicles (MDVs) are implicated in diverse physiological processes-for example, mitochondrial quality control-and are linked to various neurodegenerative diseases. However, their specific cargo composition and complex molecular biogenesis are still unknown. Here we report the proteome and lipidome of steady-state TOMM20⁺ MDVs. We identified 107 high-confidence MDV cargoes, which include all β-barrel proteins and the TOM import complex. MDV cargoes are delivered as fully assembled complexes to lysosomes, thus representing a selective mitochondrial quality control mechanism for multi-subunit complexes, including the TOM machinery. Moreover, we define key biogenesis steps of phosphatidic acid-enriched MDVs starting with the MIRO1/2-dependent formation of thin membrane protrusions pulled along microtubule filaments, followed by MID49/MID51/MFF-dependent recruitment of the dynamin family GTPase DRP1 and finally DRP1-dependent scission. In summary, we define the function of MDVs in mitochondrial quality control and present a mechanistic model for global GTPase-driven MDV biogenesis.

Stereological investigation of 5-HT3 receptors in the substantia nigra and dorsal raphe nucleus in the rat

Serotonin (5-HT) is a common neurotransmitter in mammals, playing a central role in the regulation of various processes such as sleep, perception, cognitive and autonomic functions in the nervous system. Previous studies have demonstrated that 5-HT type 3 (5-HT₃) receptors are expressed in either or both the substantia nigra (SN) and the dorsal raphe nucleus (DRN) in humans, marmosets, rats and Syrian hamsters. Here, we quantify the distribution of 5-HT₃ receptors across these regions in the adult rat. Fluorescent immunohistochemistry was performed on sections of rat brain covering the entire rostro-caudal extent of the SN and DRN with antibodies specific to the 5-HT_3A receptor subunit, as well as others targeting the monoaminergic markers tyrosine hydroxylase (TH) and the 5-HT transporter (SERT). The number of 5-HT_3A receptor-positive, TH-positive (n = 28,428 ± 888, Gundersen's m = 1 coefficient of error [CE] = 0.05) and SERT-positive (n = 12,852 ± 462, CE = 0.06) cells were estimated in both the SN and the DRN using stereology. We found that 5-HT_3A receptor-positive cells are present in the SNr (n = 1250 ± 64, CE = 0.24), but they did not co-localise with TH-positive cells, nor were they present in the SNc. In contrast, no 5-HT_3A receptor-positive cells were found in the DRN. These results support the presence of 5-HT₃ receptors in the SN, but not in the DRN, and do not support their expression on monoaminergic cells within these two brain areas.

Characterization of agonist-dependent somatostatin receptor subtype 2 trafficking in neuroendocrine cells

BACKGROUND

Somatostatin (SOM) receptor subtype 2 (SSTR2) is the major receptor subtype mediating SOM effects throughout the neuraxis. We previously demonstrated that the non-selective agonist [D-Trp⁸]-SOM induces intracellular sequestration of SSTR2, whereas this receptor is maintained at the cell surface after treatment with the SSTR2-selective agonist L-779,976 in cells co-expressing SSTR2 and SSTR5.

METHODS AND RESULTS

In this study, we knocked-out SSTR5 in AtT20 cells endogenously expressing both SSTR2 and SSTR5 and used immuno-labeling and confocal microscopy to investigate the effect of SSTR5 on regulation of SSTR2 trafficking. Our results indicate that unlike [D-Trp⁸]-SOM-induced intracellular sequestration, L-779,976 stimulation results in the maintenance of SSTR2 at the cell surface regardless of whether SSTR5 is present or not. We then examined the trafficking pathways of SSTR2 upon stimulation by either agonist. We found that both [D-Trp⁸]-SOM and L-779,976 induce SSTR2 internalization via transferrin-positive vesicles. However, SSTR2 internalized upon L-779,976 treatment undergoes rapid recycling to the plasma membrane, whereas receptors internalized by [D-Trp⁸]-SOM recycle slowly after washout of the agonist. Furthermore, [D-Trp⁸]-SOM stimulation induces degradation of a fraction of internalized SSTR2 whereas L-779,976-dependent, rapid SSTR2 recycling appears to protect internalized SSTR2 from degradation. In addition, Octreotide which has preferential SSTR2 affinity, induced differential effects on both SSTR2 trafficking and degradation.

CONCLUSION

Our results indicate that the biased agonistic property of L-779,976 protects against SSTR2 surface depletion by rapidly initiating SSTR2 recycling while SSTR5 does not regulate L-779-976-dependent SSTR2 trafficking.

Regulated resurfacing of a somatostatin receptor storage compartment fine-tunes pituitary secretion

In pituitary cells, internalized somatostatin receptor is held in a GLUT4-like storage compartment. The receptor rapidly resurfaces in response to selective signaling pathways in a process that fine-tunes pituitary hormone release.

The surfacing of the glucose transporter GLUT4 driven by insulin receptor activation provides the prototypic example of a homeostasis response dependent on mobilization of an intracellular storage compartment. Here, we generalize this concept to a G protein–coupled receptor, somatostatin receptor subtype 2 (SSTR2), in pituitary cells. Following internalization in corticotropes, SSTR2 moves to a juxtanuclear syntaxin-6–positive compartment, where it remains until the corticotropes are stimulated with corticotropin releasing factor (CRF), whereupon SSTR2 exits the compartment on syntaxin-6–positive vesicular/tubular carriers that depend on Rab10 for their fusion with the plasma membrane. As SSTR2 activation antagonizes CRF-mediated hormone release, this storage/resurfacing mechanism may allow for a physiological homeostatic feedback system. In fact, we find that SSTR2 moves from an intracellular compartment to the cell surface in pituitary gland somatotropes, concomitant with increasing levels of serum growth hormone (GH) during natural GH cycles. Our data thus provide a mechanism by which signaling-mediated plasma membrane resurfacing of SSTR2 can fine-tune pituitary hormone release.

Ultradian Secretion of Growth Hormone in Mice: Linking Physiology With Changes in Synapse Parameters Using Super-Resolution Microscopy

Neuroendocrine circuits are orchestrated by the pituitary gland in response to hypothalamic hormone-releasing and inhibiting factors to generate an ultradian and/or circadian rhythm of hormone secretion. However, mechanisms that govern this rhythmicity are not fully understood. It has been shown that synaptic transmission in the rodent hypothalamus undergoes cyclical changes in parallel with rhythmic hormone secretion and a growing body of evidence suggests that rapid rewiring of hypothalamic neurons may be the source of these changes. For decades, structural synaptic studies have been utilizing electron microscopy, which provides the resolution suitable for visualizing synapses. However, the small field of view, limited specificity and manual analysis susceptible to bias fuel the search for a more quantitative approach. Here, we apply the fluorescence super-resolution microscopy approach direct Stochastic Optical Reconstruction Microscopy (dSTORM) to quantify and structurally characterize excitatory and inhibitory synapses that contact growth hormone-releasing-hormone (GHRH) neurons during peak and trough values of growth hormone (GH) concentration in mice. This approach relies on a three-color immunofluorescence staining of GHRH and pre- and post-synaptic markers, and a quantitative analysis with a Density-Based Spatial Clustering of Applications with Noise (DBSCAN) algorithm. With this method we confirm our previous findings, using electron microscopy, of increased excitatory synaptic input to GHRH neurons during peak levels of GH. Additionally, we find a shift in synapse numbers during low GH levels, where more inhibitory synaptic inputs are detected. Lastly, we utilize dSTORM to study novel aspects of synaptic structure. We show that more excitatory (but not inhibitory) pre-synaptic clusters associate with excitatory post-synaptic clusters during peaks of GH secretion and that the numbers of post-synaptic clusters increase during high hormone levels. The results presented here provide an opportunity to highlight dSTORM as a valuable quantitative approach to study synaptic structure in the neuroendocrine circuit. Importantly, our analysis of GH circuitry sheds light on the potential mechanism that drives ultradian changes in synaptic transmission and possibly aids in GH pulse generation in mice.

[Pharyngeal acid load and different functional endoscopy findings]

The findings of functional endoscopy (upper esophageal sphincter insufficiency, cardia insufficiency, esophagitis, gastric heterotopia, axial sliding hernia, and visible aerosols) can be traced back to pharyngeal acid exposure by oropharyngeal pH measurement. Significantly increased pharyngeal acid loads are seen in gastric heterotopy and axial sliding hernia. For all measured statistics, the pharyngeal acid load is in the pathological or even very pathological range. The value of functional endoscopy in the context of laryngopharyngeal reflux diagnosis is clearly documented. The findings "heterotopic gastric mucosa" and "axial sliding hernia" may cause marked airway symptoms and a pathogenetic relationship with otorhinolaryngologic reflux-associated symptoms must be postulated for these entities.

Adult NG2-Glia Are Required for Median Eminence-Mediated Leptin Sensing and Body Weight Control

While leptin is a well-known regulator of body fat mass, it remains unclear how circulating leptin is sensed centrally to maintain energy homeostasis. Here we show that genetic and pharmacological ablation of adult NG2-glia (also known as oligodendrocyte precursors), but not microglia, leads to primary leptin resistance and obesity in mice. We reveal that NG2-glia contact the dendritic processes of arcuate nucleus leptin receptor (LepR) neurons in the median eminence (ME) and that these processes degenerate upon NG2-glia elimination, which explains the consequential attenuation of these neurons' molecular and electrical responses to leptin. Our data therefore indicate that LepR dendrites in the ME represent the principal conduits of leptin's anorexigenic action and that NG2-glia are essential for their maintenance. Given that ME-directed X-irradiation confirmed the pharmacological and genetically mediated ablation effects on body weight, our findings provide a rationale for the known obesity risk associated with cranial radiation therapy.

Activation of annexin A1 signalling in renal fibroblasts exerts antifibrotic effects

AIM

The anti-inflammatory protein annexin A1 (AnxA1) and its formyl peptide receptor 2 (FPR2) have protective effects in organ fibrosis. Their role in chronic kidney disease (CKD) has not yet been elucidated. Our aim was to characterize the AnxA1/FPR2 system in models of renal fibrosis.

METHODS

Rats were treated with angiotensin receptor antagonist during the nephrogenic period (ARAnp) to induce late-onset hypertensive nephropathy and fibrosis. Localization and regulation of AnxA1 and FPR2 were studied by quantitative real-time PCR and double labelling immunofluorescence. Biological effects of AnxA1 were studied in cultured renal fibroblasts from AnxA1(-/-) and wild-type mice.

RESULTS

Angiotensin receptor antagonist during the nephrogenic period kidneys displayed matrix foci containing CD73(+) fibroblasts, alpha-smooth muscle actin (a-SMA)(+) myofibroblasts and CD68(+) macrophages. TGF-β and AnxA1 mRNAs were ~threefold higher than in controls. AnxA1 was localized to macrophages and fibroblasts; myofibroblasts were negative. FPR2 was localized to fibroblasts, myofibroblasts, macrophages and endothelial cells. AnxA1 and FPR2 immunoreactive signals were increased in the foci, with fibroblasts and macrophages expressing both proteins. AnxA1(-/-) fibroblasts revealed higher α-SMA (sevenfold) and collagen 1A1 (Col1A1; 144-fold) mRNA levels than controls. Treatment of murine WT fibroblasts with TGF-β (22.5 ng mL 24 h(-1)) increased mRNA levels of α-SMA (9.3-fold) and Col1A1 (fourfold). These increases were greatly attenuated upon overexpression of AnxA1 (1.5- and 1.7-fold, respectively; P < 0.05). Human fibroblasts reacted similarly when receiving the FPR2 inhibitor WRW4.

CONCLUSION

Our results demonstrate that AnxA1 and FPR2 are abundantly expressed in the renal interstitium and modulate fibroblast phenotype and extracellular matrix synthesis activity.

Quantitative analysis of the myelin g-ratio from electron microscopy images of the macaque corpus callosum

We provide a detailed morphometric analysis of eight transmission electron micrographs (TEMs) obtained from the corpus callosum of one cynomolgus macaque. The raw TEM images are included in the article, along with the distributions of the axon caliber and the myelin g-ratio in each image. The distributions are analyzed to determine the relationship between axon caliber and g-ratio, and compared against the aggregate metrics (myelin volume fraction, fiber volume fraction, and the aggregate g-ratio), as defined in the accompanying research article entitled 'In vivo histology of the myelin g-ratio with magnetic resonance imaging' (Stikov et al., NeuroImage, 2015).

Correlating habenular subnuclei in rat and mouse by using topographic, morphological, and cytochemical criteria

The mammalian habenulae consist of medial (MHb) and lateral (LHb) nuclear complexes. Especially the LHb has received much interest because it has been recognized as the potential center of an "anti-reward system." Subnuclear organization and connectivity of the LHb are well known. In contrast, criteria to classify habenular neurons into distinct groups with potentially different biological functions are missing, most likely as a result of the lack of appropriate marker proteins. Actually, a huge amount of data concerning the localization of more than 20,000 mouse protein genes is provided in the Allen Brain Atlas. Unfortunately, the immediate use of this information is prohibited by the fact that the subnuclear organization of the habenular complexes in mouse is not known so far. The present report, therefore, uses topographic, structural, and cytochemical information from the rat to recognize corresponding areas within the mouse habenulae. Taking advantage of the fact that the Klüver-Barrera technique allows simultaneous observation of neuronal cell bodies and myelinated fibers, we were able to correlate subnuclear areas in the mouse habenula to subnuclei, which had been rigorously identified by several criteria in the rat. Our data suggest that the topographic localization of habenular subnuclei is rather similar between mouse and rat and that they may be homologous in these two species. Consequently, our data may allow using the Allen Brain Atlas as a source of basal information, which should be helpful to select candidate molecular markers for functionally different neurons in the mouse and potentially in higher mammalian species.

Regional and subcellular distribution of the receptor-targeting protein PIST in the rat central nervous system

Protein interacting specifically with Tc10, PIST, is a Golgi-associated sorting protein involved in regulating cell-surface targeting of plasma membrane receptors. The present study provides the first comprehensive description of PIST distribution in the mammalian central nervous system and of its subcellular localization by immunocytochemistry. PIST is distributed widely throughout the neuraxis, predominantly associated with neuronal cell bodies and dendrites. In hippocampal neurons, in vitro and in situ, PIST displayed a patchy subcellular distribution in an area surrounding the nucleus and extending into one of the major dendrites. By colocalization with the trans-Golgi marker TGN38, we were able to show that PIST is associated largely but not exclusively with the trans-Golgi network in central neurons. High or moderate to high levels of PIST-like immunoreactivity were found in cortical areas, in particular in layer V of the neocortex. The motor cortex was most strongly labeled. Also, the piriform and insular cortices displayed strong PIST labeling. In the hippocampus, CA2 but not CA1 or CA3 pyramidal cells displayed strong PIST-labeling, extending into their apical dendrites. In the thalamus, ventrolateral and laterodorsal nuclei were most strongly stained, whereas in the hypothalamus the supraoptic nucleus stood out with strong immunoreactivity. Strikingly, in the brainstem all cranial nerve motor nuclei were PIST-positive at varying levels, which is in keeping with the prominent expression of PIST in forebrain motor areas. This selective distribution of PIST suggests that the protein serves distinctive roles in specific neuronal populations, establishing functionally distinct zones, for instance, in the hippocampus.

Somatostatin varicosities contain the vesicular GABA transporter and contact orexin neurons in the hypothalamus

Somatostatin (SST) is a neuropeptide with known inhibitory actions in the hypothalamus, where it inhibits release of growth hormone-releasing hormone (GHRH), while also influencing the sleep-wake cycle. Here we investigated in the rat whether SST neurons might additionally release GABA (gamma-aminobutyric acid) or glutamate in different regions and whether they might contact orexin neurons that play an important role in the maintenance of wakefulness. In dual-immunostained sections viewed by epifluorescence microscopy, we examined if SST varicosities were immunopositive for the vesicular transporter for GABA (VGAT) or glutamate (VGLUT2) in the posterolateral hypothalamus and neighboring arcuate nucleus and median eminence. Of the SST varicosities in the posterolateral hypothalamus, 18% were immunopositive for VGAT, whereas ≤ 1% were immunopositive for VGLUT2. In the arcuate and median eminence, 26 and 64% were VGAT+ and < 3% VGLUT2 + , respectively. In triple-immunostained sections viewed by epifluorescence and confocal microscopy, SST varicosities were seen in contact with orexin somata, and of these varicosities, a significant proportion (23%) contained VGAT along with synaptophysin, the presynaptic marker for small synaptic vesicles, and a similar proportion (25%) abutted puncta that were immunostained for gephyrin, the postsynaptic marker for GABAergic synapses. Our results indicate that a significant proportion of SST varicosities in the hypothalamus have the capacity to release GABA, to form inhibitory synapses upon orexin neurons, and accordingly through their peptide and/or amino acid, to inhibit orexin neurons, as well as GHRH neurons. Thus while regulating GHRH release, SST neurons could serve to attenuate arousal and permit progression through the sleep cycle.

Peri-pubertal emergence of UNC-5 homologue expression by dopamine neurons in rodents

Puberty is a critical period in mesocorticolimbic dopamine (DA) system development, particularly for the medial prefrontal cortex (mPFC) projection which achieves maturity in early adulthood. The guidance cue netrin-1 organizes neuronal networks by attracting or repelling cellular processes through DCC (deleted in colorectal cancer) and UNC-5 homologue (UNC5H) receptors, respectively. We have shown that variations in netrin-1 receptor levels lead to selective reorganization of mPFC DA circuitry, and changes in DA-related behaviors, in transgenic mice and in rats. Significantly, these effects are only observed after puberty, suggesting that netrin-1 mediated effects on DA systems vary across development. Here we report on the normal expression of DCC and UNC5H in the ventral tegmental area (VTA) by DA neurons from embryonic life to adulthood, in both mice and rats. We show a dramatic and enduring pubertal change in the ratio of DCC:UNC5H receptors, reflecting a shift toward predominant UNC5H function. This shift in DCC:UNC5H ratio coincides with the pubertal emergence of UNC5H expression by VTA DA neurons. Although the distribution of DCC and UNC5H by VTA DA neurons changes during puberty, the pattern of netrin-1 immunoreactivity in these cells does not. Together, our findings suggest that DCC:UNC5H ratios in DA neurons at critical periods may have important consequences for the organization and function of mesocorticolimbic DA systems.

Depolarization recruits DCC to the plasma membrane of embryonic cortical neurons and enhances axon extension in response to netrin-1

The netrin-1 receptor Deleted in Colorectal Cancer (DCC) is required for the formation of major axonal projections by embryonic cortical neurons, including the corpus callosum, hippocampal commissure, and cortico-thalamic tracts. The presentation of DCC by axonal growth cones is tightly regulated, but the mechanisms regulating DCC trafficking within neurons are not well understood. Here, we investigated the mechanisms regulating DCC recruitment to the plasma membrane of embryonic cortical neurons. In embryonic spinal commissural neurons, protein kinase A (PKA) activation recruits DCC to the plasma membrane and enhances axon chemoattraction to netrin-1. We demonstrate that PKA activation similarly recruits DCC and increases embryonic cortical neuron axon extension, which, like spinal commissural neurons, respond to netrin-1 as a chemoattractant. We then determined if depolarization might recruit DCC to the plasma membrane. Neither netrin-1 induced axon extension, nor levels of plasma membrane DCC, were altered by depolarizing embryonic spinal commissural neurons with elevated levels of KCl. In contrast, depolarizing embryonic cortical neurons increased the amount of plasma membrane DCC, including at the growth cone, and increased axon outgrowth evoked by netrin-1. Inhibition of PKA, phosphatidylinositol-3-kinase, protein kinase C, or exocytosis blocked the depolarization-induced recruitment of DCC and suppressed axon outgrowth. Inhibiting protein synthesis did not affect DCC recruitment, nor were the distributions of trkB or neural cell adhesion molecule (NCAM) influenced by depolarization, consistent with selective mobilization of DCC. These findings identify a role for membrane depolarization modulating the response of axons to netrin-1 by regulating DCC recruitment to the plasma membrane.

Scyl1, mutated in a recessive form of spinocerebellar neurodegeneration, regulates COPI-mediated retrograde traffic

Scy1-like 1 (Scyl1), a member of the Scy1-like family of catalytically inactive protein kinases, was recently identified as the gene product altered in muscle-deficient mice, which suffer from motor neuron degeneration and cerebellar atrophy. To determine the function of Scyl1, we have now used a mass spectrometry-based screen to search for Scyl1-binding partners and identified components of coatomer I (COPI) coats. The interaction was confirmed in pull-down assays, and Scyl1 co-immunoprecipitates with betaCOP from brain lysates. Interestingly, and unique for a non-transmembrane domain protein, Scyl1 binds COPI coats using a C-terminal RKLD-COO(-) sequence, similar to the KKXX-COO(-) COPI-binding motif found in transmembrane endoplasmic reticulum (ER) proteins. Scyl1 co-localizes with betaCOP and is localized, in an Arf1-independent manner, to the ER-Golgi intermediate compartment and the cis-Golgi, sites of COPI-mediated membrane budding. The localization and binding properties of Scyl1 strongly suggest a function in COPI transport, and inhibitory RNA-mediated knock down of the protein disrupts COPI-mediated retrograde traffic of the KDEL receptor to the ER without affecting anterograde traffic from the ER. Our data demonstrate a function for Scyl1 as an accessory factor in COPI trafficking and suggest for the first time that alterations in the COPI pathway result in neurodegenerative disease.

Histone deacetylases: novel targets for prevention of colitis-associated cancer in mice

OBJECTIVE

Inhibition of histone deacetylases, well known for its antiproliferative efficacy in vivo, was recently shown to ameliorate inflammation in experimental colitis. Since inflammatory bowel disease is associated with an increased risk of developing colon cancer, here the combined anti-inflammatory and proapoptotic efficacy of histone deacetylase inhibitors was studied in mouse models.

METHODS

The novel histone deacetylase inhibitor ITF2357 was compared with suberoylanilide hydroxamic acid in models of experimental colitis. Effects on tumour growth were studied after treatment of mice with azoxymethane and dextran sulphate sodium, and in interleukin 10 (IL10) knockout mice, respectively. Possible underlying mechanisms involving apoptosis and nuclear factor (NF)-kappaB activation were addressed by flow cytometry and western blot.

RESULTS

In dextran sulphate sodium- and trinitrobenzene sulphonic acid-induced colitis, treatment with ITF2357 was superior to suberoylanilide hydroxamic acid as shown by macroscopic and histological amelioration of inflammation, by reduced production of interferon gamma (IFN gamma) and by increased production of IL10. In both models of inflammation-mediated tumourigenesis, inhibition of histone deacetylases resulted in a significant suppression of tumour growth in terms of size and number, along with reduced signs of inflammation. As for potential mechanisms of ITF2357 action, increased acetylation of histone 3, reduced production of IFN gamma and enhanced apoptosis in lamina propria mononuclear cells were found to accompany a histone deacetylase-dependent activation of NF-kappaB.

CONCLUSIONS

These results indicate that inhibition of histone deactylases can attenuate inflammation-mediated tumour growth, which is paralled by an inhibition of NF-kappaB. Thus histone deacetylase inhibitors provide a promising strategy that combines anti-inflammatory and proapoptotic modes of action.

Embryonic and postnatal development of the serotonergic raphe system and its target regions in 5-HT1A receptor deletion or overexpressing mouse mutants

The neurotransmitter 5-HT regulates early developmental processes in the CNS. In the present study we followed the embryonic and postnatal development of serotonergic raphe neurons and catecholaminergic target systems in the brain of 5-HT1A receptor knockout (KO) and overexpressing (OE) in comparison with wild-type (WT) mice from embryonic day (E) 12.5 to postnatal day (P) 15.5. Up to P15.5 no differences were apparent in the differentiation and distribution of serotonergic neurons in the raphe area as revealed by the equal number of serotonergic neurons in the dorsal raphe in all three genotypes. However, the establishment of serotonergic projections to the mesencephalic tegmentum and hypothalamus was delayed at E12.5 in KO and OE animals and projections to the cerebral cortex between E16.5 and E18.5 were delayed in OE mice. This delay was only transient and did not occur in other brain areas including septum, hippocampus and striatum. Moreover, OE mice caught up with WT and KO animals postnatally such that at P1.5 serotonergic innervation of the cortex was more extensive in the OE than in KO and WT mice. Tissue levels of 5-HT and of its main metabolite 5-hydroxyindoleacetic acid as well as 5-HT turnover were considerably higher in brains of OE mice and slightly elevated in KO mice in comparison with the WT, starting at E16.5 through P15.5. The initial differentiation of dopaminergic neurons and fibers in the substantia nigra at E12.5 was transiently delayed in KO and OE mice as compared with WT mice, but no abnormalities in noradrenergic development were apparent in later stages. The present data indicate that 5-HT1A receptor deficiency or overexpression is associated with increased 5-HT synthesis and turnover in the early postnatal period. However, they also show that effects of 5-HT1A KO or OE on the structural development of the serotonergic system are at best subtle and transient. They may nonetheless contribute to the establishment of increased or reduced anxiety-like behavior, respectively, in adult mice.

Coexpression of somatostatin receptor subtype 5 affects internalization and trafficking of somatostatin receptor subtype 2

The somatostatin [somatotropin release-inhibiting factor (SRIF)] receptor subtypes sst(2A) and sst(5) are frequently coexpressed in SRIF-responsive cells, including endocrine pituitary cells. We previously demonstrated that sst(2A) and sst(5) exhibit different subcellular localizations and regulation of cell surface expression, although they have similar signaling properties. We investigated here whether sst(2A) and sst(5) functionally interact in cells coexpressing the two receptor subtypes. We stimulated both transfected cells stably expressing sst(2A) alone (CHO-sst(2A)) or together with sst(5) (CHO-sst(2A+5)) and the pituitary cell line AtT20, which endogenously expresses the two receptor subtypes, with either the nonselective agonist [D-Trp(8)]-SRIF-14 or the sst(2)-selective agonist L-779,976. In CHO-sst(2A) cells, stimulation with either ligand resulted in the loss of approximately 75% of cell surface SRIF binding sites and massive internalization of sst(2A) receptors. The cells were desensitized to subsequent stimulation with [D-Trp(8)]-SRIF-14, which failed to inhibit forskolin-evoked cAMP accumulation. Similarly, in CHO-sst(2A+5) and AtT20 cells, [D-Trp(8)]-SRIF-14 induced the loss of 60-70% of SRIF binding sites as well as massive sst(2A) endocytosis. By contrast, in cells expressing both sst(2A) and sst(5), selective stimulation of sst(2A) with L-779,976 resulted in only 20-40% loss of cell surface binding and markedly reduced sst(2A) internalization. Consequently, whereas CHO-sst(2A+5) and AtT20 cells stimulated with [D-Trp(8)]-SRIF-14 were desensitized to a second stimulation with the same agonist, cells prestimulated with L-779,976 were not desensitized to subsequent [D-Trp(8)]-SRIF-14 stimulation. These findings indicate that the presence of sst(5) in the same cells modulates trafficking and cell surface regulation of sst(2A) and cellular desensitization to the effects of SRIF.

Morphine priming in rats with chronic inflammation reveals a dichotomy between antihyperalgesic and antinociceptive properties of deltorphin

We previously showed that prolonged morphine treatment and chronic inflammation both enhanced delta opioid receptor (deltaOR) cell surface density in lumbar spinal cord neurons. Here, we sought to determine whether administration of morphine to rats with chronic inflammation would further increase the bio-availability of deltaOR, and thereby the analgesic properties of the deltaOR agonist deltorphin, over that produced by inflammation alone. We found that chronic inflammation produced by injection of complete Freund's adjuvant (CFA) into the hind paw resulted in a bilateral increase in the binding and internalization of fluorescent deltorphin in neurons of the lumbar spinal cord as did prolonged morphine treatment [Morinville A, Cahill CM, Aibak H, Rymar VV, Pradhan A, Hoffert C, Mennicken F, Stroh T, Sadikot AF, O'Donnell D, Clarke PB, Collier B, Henry JL, Vincent JP, Beaudet A (2004a) Morphine-induced changes in delta opioid receptor trafficking are linked to somatosensory processing in the rat spinal cord. J Neurosci 24:5549-5559]. This effect was accompanied by an increase in the antinociceptive efficacy of intrathecal deltorphin as measured using the tail-flick test. Treatment of CFA-injected rats with morphine decreased the cell surface availability of deltaOR in neurons of the dorsal horn of the lumbar spinal cord as compared with treatment with CFA alone. Behaviorally, it significantly enhanced the antihyperalgesic effects of deltorphin (plantar test; % maximum possible antihyperalgesic effect (MPAHE)=113.5%+/-32.4% versus 26.1%+/-11.6% in rats injected with CFA alone) but strongly reduced the antinociceptive efficacy of the drug (tail-flick test; % maximum possible antinociceptive effect (MPE)=29.6%+/-3.6% versus 66.6%+/-6.3% in rats injected with CFA alone) suggesting that the latter, but not the former, is linked to the deltaOR trafficking events observed neuroanatomically. These results demonstrate that in chronic inflammation, the antihyperalgesic effects of deltaOR agonists may be enhanced by morphine pre-treatment. They also reveal a dichotomy between mechanisms underlying antihyperalgesic and antinociceptive effects of deltaOR agonists.

NTS2 modulates the intracellular distribution and trafficking of NTS1 via heterodimerization

Neurotensin (NT) receptors NTS1 and NTS2 are known to display considerable distributional overlap in mammalian central nervous system (CNS). Using co-immunoprecipitation approaches, we demonstrated here that NTS1 forms constitutive heterodimers with NTS2 in transfected COS-7 cells. We also showed that co-expression of NTS2 with NTS1 markedly decreases the cell surface density of NTS1 without affecting ERK1/2 MAPK activity or NT-induced NTS1 internalization. However, radioligand-binding studies indicated that upon prolonged NT stimulation, cell surface NTS1 receptors are more resistant to down-regulation in cells co-expressing NTS1 and NTS2 than in cells expressing NTS1 alone. Taken together, these data suggest that NTS1/NTS2 heterodimerization affects the intracellular distribution and trafficking of NTS1 by making it more similar to that of NTS2 as witnessed in cells expressing NTS2 alone. NTS1/NTS2 heterodimerization might therefore represent an additional mechanism in the regulation of NT-triggered responses mediated by NTS1 and NTS2 receptors.

Toll-like receptor expression and response to specific stimulation in adipocytes and preadipocytes: on the role of fat in inflammation

Data in this study indicate that both adipocytes and preadipocytes express abroad set of TLRs and they also respond to specific stimulation by cytokine production. The may be of relevance to Crohn's disease and a suggests a closer link between adipose tissue and innate immunity.

Sustained neurotensin exposure promotes cell surface recruitment of NTS2 receptors

In this study, we investigated whether persistent agonist stimulation of NTS2 receptors gives rise to down-regulation, in light of reports that their activation induced long-lasting effects. To address this issue, we incubated COS-7 cells expressing the rat NTS2 with neurotensin (NT) for up to 24 h and measured resultant cell surface [125I]-NT binding. We found that NTS2-expressing cells retained the same surface receptor density despite efficient internalization mechanisms. This preservation was neither due to NTS2 neosynthesis nor recycling since it was not blocked by cycloheximide or monensin. However, it appeared to involve translocation of spare receptors from internal stores, as NT induced NTS2 migration from trans-Golgi network to endosome-like structures. This stimulation-induced regulation of cell surface NTS2 receptors was even more striking in rat spinal cord neurons. Taken together, these results suggest that sustained NTS2 activation promotes recruitment of intracellular receptors to the cell surface, thereby preventing functional desensitization.

Potent spinal analgesia elicited through stimulation of NTS2 neurotensin receptors

Intrathecal administration of the neuropeptide neurotensin (NT) was shown previously to exert antinociceptive effects in a variety of acute spinal pain paradigms including hotplate, tail-flick, and writhing tests. In the present study, we sought to determine whether some of these antinociceptive effects might be elicited via stimulation of low-affinity NTS2 receptors. We first established, using immunoblotting and immunohistochemical techniques, that NTS2 receptors were extensively associated with putative spinal nociceptive pathways, both at the level of the dorsal root ganglia and of the superficial layers of the dorsal horn of the spinal cord. We then examined the effects of intrathecal administration of NT or selective NTS2 agonists on acute thermal pain. Both NT and NTS2 agonists, levocabastine and Boc-Arg-Arg-Pro-Tyrpsi(CH2NH)Ile-Leu-OH (JMV-431), induced dose-dependent antinociceptive responses in the tail-flick test. The effects of levocabastine and of JMV-431 were unaffected by coadministration of the NTS1-specific antagonist 2-[(1-(7-chloro-4-quinolinyl)-5-(2,6-dimethoxy-phenyl)pyrazol-3-yl)carboxylamino]tricyclo)3.3.1.1.(3.7))-decan-2-carboxylic acid (SR48692), confirming that they were NTS2 mediated. In contrast, the antinociceptive effects of NT were partly abolished by coadministration of SR48692, indicating that NTS1 and NTS2 receptors were both involved. These results suggest that NTS2 receptors play a role in the regulation of spinal nociceptive inputs and that selective NTS2 agonists may offer new avenues for the treatment of acute pain.

Immunohistochemical Distribution and Subcellular Localization of the Somatostatin Receptor Subtype 1 (sst1) in the Rat Hypothalamus

The aim of the present study was to examine the cellular and sub-cellular distribution of the somatostatin (SRIF) receptor subtype sst1 in the rat hypothalamus. Receptors were immunolabeled using an antibody directed against an antigenic sequence in the N-terminus of the receptor. Immunopositive neuronal cell bodies and dendrites were observed throughout the mediobasal hypothalamus, including the medial preoptic area, paraventricular, periventricular, and arcuate nuclei. Immunoreactive axons and axon terminals were also observed in the median eminence, suggesting that sst1 is also located pre-synaptically. Electron microscopic examination of the arcuate nucleus revealed a predominant association of immunoreactive sst1 with perikarya and dendrites. Most immunoreactive receptors were intracellular and localized to tubulovesicular compartments and organelles such as the Golgi apparatus, but 14% were associated with the plasma membrane. Of the latter, 47% were apposed to abbuting afferent axon terminals and 20% localized immediately adjacent to an active synaptic zone. These results demonstrate a widespread distribution of sst1 receptors in rat hypothalamus. They also show that somatodendritic sst1 receptors in the arcuate nucleus are ideally poised to mediate SRIF's modulation of afferent synaptic inputs, including central SRIF effects on growth hormone-releasing hormone neurons documented in this area.

Morphine and pain-related stimuli enhance cell surface availability of somatic delta-opioid receptors in rat dorsal root ganglia

The present study demonstrates that perikaryaldelta-opioid receptors (deltaORs) in rat dorsal root ganglion (DRG) neurons bind and internalize opioid ligands circulating in the CSF. Using confocal and electron microscopy, we found that prolonged morphine treatment increased the cell surface density of these perikaryal deltaORs and, by way of consequence, receptor-mediated internalization of the fluorescent deltorphin (DLT) analog omega-Bodipy 576/589 deltorphin-I 5-aminopentylamide (Fluo-DLT) in all three types of DRG neurons (small, medium, and large). In contrast, chronic inflammatory pain induced by the injection of complete Freund's adjuvant (CFA) into one hindpaw selectively increased Fluo-DLT internalization in small and medium-sized DRG neurons ipsilateral to the inflammation. Based on our previous studies in the spinal cord of mu-opioid receptor (muOR) knock-out mice, it may be assumed that the enhanced membrane recruitment of deltaORs observed after sustained morphine is attributable to stimulation of muORs. However, the selectivity of the effect induced by inflammatory pain suggests that it involves a different mechanism, namely a modality-specific and pain-related activation of C and Adelta fibers. Indeed, stimulation by capsaicin of transient receptor potential vanilloid 1 receptors, which are selectively expressed by small diameter (< 600 microm2) DRG neurons, increased Fluo-DLT internalization exclusively in this cell population. The present results, therefore, demonstrate that DRG neurons express perikaryal deltaORs accessible to CSF-circulating ligands and that the density and, hence, presumably also the responsiveness, of these receptors may be modulated by both pain-related stimuli and sustained exposure to muOR agonists.

Cell-type-specific pathways of neurotensin endocytosis

The neurotensin receptor subtype 1 (NTS1) is a G-protein-coupled receptor (GPCR) mediating a large number of central and peripheral effects of neurotensin. Upon stimulation, NTS1 is rapidly internalized and targeted to lysosomes. This process depends on the interaction of the phosphorylated receptor with beta-arrestin. Little is known about other accessory endocytic proteins potentially involved. Here, we investigated the involvement of dynamin, amphiphysin, and intersectin in the internalization of NTS1 receptor-ligand complexes in transfected COS-7 and HEK 293 cells, by using the transferrin receptor as an internal control for the constitutive endocytic pathway. We found that NTS1 endocytosis was not only arrestin-dependent, but also dynamin-dependent in both COS-7 and HEK 293 cells, whereas internalization of the transferrin receptor was independent of arrestin but required dynamin. Overexpression of the SH3 domain of amphiphysin II had no effect on receptor internalization in either cell type. By contrast, overexpression of full-length intersectin or of its SH3 domain (but not of its EH domain) inhibited NTS1 internalization in COS-7 but not in HEK 293 cells. This difference between COS-7 and HEK 293 cells was not attributable to differences in endogenous intersectin levels between the two cell lines. Indeed, the same constructs inhibited transferrin endocytosis equally well in COS-7 and HEK 293 cells. However, immunogold electron microscopy revealed that internalized NTS1 receptors were associated with clathrin-coated pits in COS-7 cells but with smooth vesicles in HEK 293 cells, suggesting that NTS1 internalization proceeds via different endocytic pathways in these two cell types.

Interactions with PDZ Domain Proteins PIST/GOPC and PDZK1 Regulate Intracellular Sorting of the Somatostatin Receptor Subtype 5

By yeast two-hybrid screening we have identified interaction partners for the intracellular C-terminal tail of the human and rodent somatostatin receptor subtype 5 (SSTR5). Interactions with the PDZ domain-containing proteins PIST and PDZK1 are mediated by the PDZ ligand motif at the C terminus of the receptor; in case of the human and mouse (but not the rat) receptors, a slight sequence variation of this motif also allows for binding of the peroxisomal receptor PEX5. PIST is Golgi-associated and retains SSTR5 in the Golgi apparatus when coexpressed with the receptor; PDZK1 on the other hand associates with the SSTR5 at the plasma membrane. Endogenous SSTR5 in the neuroendocrine AtT-20 tumor cell line is colocalized with PIST in the Golgi apparatus. On a functional level, removal of the PDZ ligand motif of the receptor does not interfere with agonist-dependent internalization of the receptor or its targeting to a Golgi-associated compartment; however, recycling of the receptor to the plasma membrane after washout of the agonist is inhibited, suggesting that the PDZ-mediated interaction of SSTR5 is required for postendocytic sorting.

Prolonged morphine treatment selectively increases membrane recruitment of delta-opioid receptors in mouse basal ganglia

In recent years, we demonstrated that prolonged (48-h) treatment of rats or mice with selective m-opioid receptor ((mu)OR) agonists induced a translocation of delta-opioid receptors ((delta)ORs) from intracellular compartments to neuronal plasma membranes in the dorsal horn of the spinal cord. It remained to be determined whether this phenomenon also occurred in the brain. To resolve this issue, we analyzed by immunogold histochemistry the subcellular distribution of (delta)ORs in the nucleus accumbens, dorsal neostriatum, and frontal cortex in mice treated or not with morphine (48 h). We observed that prolonged treatment with morphine induced a translocation of (delta)ORs from intracellular to subplasmalemmal and membrane compartments in dendrites from both the nucleus accumbens and the dorsal neostriatum but not from the frontal cortex. We propose that this (mu)OR-(delta)OR interaction might prolong and modulate the sensitivity of neurons to opiates in specific target regions.

Effects of brain-derived neurotrophic factor (BDNF) on glial cells and serotonergic neurones during development

Serotonergic neurones are among the first to develop in the central nervous system. Their survival and maturation is promoted by a variety of factors, including serotonin itself, brain-derived neurotrophic factor (BDNF) and S100beta, an astrocyte-specific Ca(2+) binding protein. Here, we used BDNF-deficient mice and cell cultures of embryonic raphe neurones to determine whether or not BDNF effects on developing serotonergic raphe neurones are influenced by its action on glial cells. In BDNF-/- mice, the number of serotonin-immunoreactive neuronal somata, the amount of the serotonin transporter, the serotonin content in the striatum and the hippocampus, and the content of 5-hydroxyindoleacetic acid in all brain regions analysed were increased. By contrast, reduced immunoreactivity was found for myelin basic protein (MBP) in all brain areas including the raphe and its target region, the hippocampus. Exogenously applied BDNF increased the number of MBP-immunopositive cells in the respective culture systems. The raphe area displayed selectively reduced immunoreactivity for S100beta. Accordingly, S100beta was increased in primary cultures of pure astrocytes by exogenous BDNF. In glia-free neuronal cultures prepared from the embryonic mouse raphe, addition of BDNF supported the survival of serotonergic neurones and increased the number of axon collaterals and primary dendrites. The latter effect was inhibited by the simultaneous addition of S100beta. These results suggest that the presence of BDNF is not a requirement for the survival and maturation of serotonergic neurones in vivo. BDNF is, however, required for the local expression of S100beta and production of MBP. Therefore BDNF might indirectly influence the development of the serotonergic system by stimulating the expression of S100beta in astrocytes and the production MBP in oligodendrocytes.

Area-specific effects of brain-derived neurotrophic factor (BDNF) genetic ablation on various neuronal subtypes of the mouse brain

The effects of brain-derived neurotrophic factor (BDNF) on the development of presynaptic terminals and of neuronal subtypes in various brain areas were studied in BDNF-knockout (BDNF-/-) mice at postnatal days 15-17. Western analysis revealed no changes in the overall amount of a variety of synaptic proteins in BDNF-/- mice as compared to wild type mice. In addition, the complex between the vesicular proteins, synaptophysin and synaptobrevin, as well as their respective homodimers were unaltered. Moreover, no changes in the density of neurons were found in, e.g., the CA3 region of the hippocampus and the nucleus nervi facialis of BDNF-/- mice. However, cholinergic cells were reduced by 20% in the medial septum of BDNF-/- mice associated with a decrease in the activity of choline acetyltransferase and protein levels of nerve growth factor in the hippocampus by 16% and 44%, respectively. In the striatum, however, the total number of cholinergic cells were comparable in both groups, although the activity of choline acetyltransferase was decreased by 46%. In GABAergic interneurons, the expression of neuropeptides in various brain areas was differentially affected by BDNF deletion as revealed by immunohistochemistry. In the hippocampus and cortex of BDNF-/- mice, the density of neuropeptide Y-, somatostatin-, and parvalbumin-immunoreactive cells was drastically reduced, whereas the density of calretinin-positive cells was increased. The extent of these changes in neuropeptide-containing cells varied among hippocampal subregions. In the striatum, only the density of parvalbumin-immunoreactive cells was decreased by approximately 45%. In conclusion, BDNF deficiency is accompanied by a differential dysregulation in the expression of neuropeptides and calcium-binding proteins in otherwise intact GABAergic and glutamatergic neurons in a region-specific manner.

Reduced anxiety-related behaviour in transgenic mice overexpressing serotonin 1A receptors

Serotonergic neurons play a major role in the modulation of emotion and behaviour. Especially knockout studies have revealed a role for the serotonin(1A) (5-HT(1A)) receptor in anxiety related behaviour. Mutant animals exhibit enhanced anxiety-like responses, possibly resulting from impaired autoinhibitory control of midbrain serotonergic neurons. To further elucidate the role of the 5-HT(1A) receptors in affective behaviour, a complementary approach has been used and transgenic mice overexpressing this receptor subtype have been generated. The expression of the active 5-HT(1A) receptor protein as indicated by autoradiography was transiently increased during early postnatal development (P1.5) as compared to wild-type mice. Within the next 2 weeks, the increase in receptor binding vanished and was also not apparent in adult animals indicating adaptive changes in the regulation of 5-HT(1A) receptor expression. Although no evidence for increased receptor binding in the brains of adult homozygous mice was found by autoradiography, typical phenotypic changes indicative of 5-HT(1A) receptor overactivity were apparent. Transgenic mice revealed a reduced molar ratio of 5-hydroxyindoleacetic acid to serotonin in several brain areas and elevated serotonin values in the hippocampus and striatum. Moreover, anxiety-like behaviour was decreased in male and female transgenic mice and body temperature was lowered in male transgenic mice in comparison with heterozygous and wild-type mice. These findings further underline the pivotal role of 5-HT(1A) receptors in the homeostasis of anxiety-like behaviour and the crucial importance of stimulation of the 5-HT(1A) receptor during the early postnatal development for normal anxiety-like behaviour throughout life.

Identification and Functional Characterization of a 5-Transmembrane Domain Variant Isoform of the NTS2 Neurotensin Receptor in Rat Central Nervous System

The present study demonstrated that alternative splicing of the rat nts2 receptor gene generates a 5-transmembrane domain variant isoform (vNTS2) that is co-expressed with the full-length NTS2 receptor throughout the brain and spinal cord, as evidenced by reverse transcription-PCR. The vNTS2 polypeptide is 281 amino acids in length, which is 135 amino acids shorter than the full-length isoform. Immunohistochemical and radioligand binding studies revealed that the HA-tagged recombinant vNTS2 receptor is poorly targeted to plasma membranes in transfected COS-7 cells. Binding studies also showed that the truncated receptor displayed a 5000-fold lower affinity for neurotensin (NT) than its full-length counterpart (IC(50) of 10 mum and 2 nm, respectively). Yet NT binding induced efficient internalization of receptor-ligand complexes in vNTS2-transfected cells. Furthermore, it produced a rapid (<5 min) activation of the mitogen-activated protein kinases (ERK1/2) pathway, indicating functional coupling of the variant receptor. This activation is sustained (>1 h) and is also produced by the NTS2 agonist levocabastine. Western blotting experiments suggested that vNTS2 is not expressed in monomeric form in the rat central nervous system. However, it does appear to form a variety of multimeric complexes, including homodimers and heterodimers, with the full-length NTS2. Indeed, co-immunoprecipitation studies in dually transfected cells demonstrated that the two receptor isoforms can form stable associations. Taken together, the present results indicated that the rat vNTS2 is a functional receptor that may play a role in NT signaling in mammalian central nervous system.

Morphine-induced changes in delta opioid receptor trafficking are linked to somatosensory processing in the rat spinal cord

An in vivo fluorescent deltorphin (Fluo-DLT) internalization assay was used to assess the distribution and regulation of pharmacologically available delta opioid receptors (deltaORs) in the rat lumbar (L4-5) spinal cord. Under basal conditions, intrathecal injection of Fluo-DLT resulted in the labeling of numerous deltaOR-internalizing neurons throughout dorsal and ventral horns. The distribution and number of Fluo-DLT-labeled perikaryal profiles were consistent with that of deltaOR-expressing neurons, as revealed by in situ hybridization and immunohistochemistry, suggesting that a large proportion of these cells was responsive to intrathecally administered deltaOR agonists. Pretreatment of rats with morphine for 48 hr resulted in a selective increase in Fluo-DLT-labeled perikaryal profiles within the dorsal horn. These changes were not accompanied by corresponding augmentations in either deltaOR mRNA or (125)I-deltorphin-II binding levels, suggesting that they were attributable to higher densities of cell surface deltaOR available for internalization rather than to enhanced production of the receptor. Unilateral dorsal rhizotomy also resulted in increased Fluo-DLT internalization in the ipsilateral dorsal horn when compared with the side contralateral to the deafferentation or to non-deafferented controls, suggesting that deltaOR trafficking in dorsal horn neurons may be regulated by afferent inputs. Furthermore, morphine treatment no longer increased Fluo-DLT internalization on either side of the spinal cord after unilateral dorsal rhizotomy, indicating that microOR-induced changes in the cell surface availability of deltaOR depend on the integrity of primary afferent inputs. Together, these results suggest that regulation of deltaOR responsiveness through microOR activation in this region is linked to somatosensory information processing.

Distribution of NTS3 receptor/sortilin mRNA and protein in the rat central nervous system

The neurotensin (NT) receptor, NTS3, originally identified as the intracellular sorting protein sortilin, is a member of a recently discovered family of receptors characterized by a single transmembrane domain. The present study provides the first comprehensive description of the distribution of NTS3/sortilin mRNA and protein in adult rat brain using in situ hybridization and immunocytochemistry. Both NTS3/sortilin mRNA and immunoreactivity displayed a widespread distribution throughout the brain. High levels of NTS3/sortilin expression and immunoreactivity were found in neuronal cell bodies and dendrites of allocortical areas such as the piriform cortex and hippocampus. Regions expressing both high levels of NTS3/sortilin mRNA and protein also included several neocortical areas, the islands of Calleja, medial and lateral septal nuclei, amygdaloid nuclei, thalamic nuclei, the supraoptic nucleus, the substantia nigra, and the Purkinje cell layer of the cerebellar cortex. In the brainstem, all cranial nerve motor nuclei were strongly labeled. NTS3/sortilin mRNA and immunoreactivity were also detected over oligodendrocytes in major fiber tracts. Subcellularly, NTS3/sortilin was predominantly concentrated over intracytoplasmic membrane-bound organelles. Many of the areas exhibiting high levels of NTS3/sortilin (e.g., olfactory cortex, medial septum, and periaqueductal gray) have been documented to contain high concentrations of NT nerve cell bodies and axons, supporting the concept that NTS3/sortilin may play a role in NT sorting and/or signaling. Other areas (e.g., hippocampal CA fields, cerebellar cortex, and cranial nerve motor nuclei), however, are NT-negative, suggesting that NTS3/sortilin also exerts functions unrelated to NT signaling.

Immunohistochemical distribution of NTS2 neurotensin receptors in the rat central nervous system

In the present study, we localized the levocabastine-sensitive neurotensin receptor (NTS2) protein in adult rat brain by using an N-terminally-directed antibody. NTS2-like immunoreactivity was broadly distributed throughout the rat brain. At the cellular level, the reaction product was exclusively associated with neurons and predominantly, although not exclusively, with their dendritic arbors. No NTS2 signal was observed over astrocytes, as confirmed by dual confocal microscopic immunofluorescence studies using the astrocytic marker S100beta. High densities of NTS2-like immunoreactive nerve cell bodies and/or processes were detected in many regions documented to receive a dense neurotensinergic innervation, such as the olfactory bulb, bed nucleus of the stria terminalis, magnocellular preoptic nucleus, amygdaloid complex, anterodorsal thalamic nucleus, substantia nigra, ventral tegmental area, and several brainstem nuclei. Most conspicuous among the latter were structures implicated in the descending control of nociceptive inputs (e.g., the periaqueductal gray, dorsal raphe, gigantocellular reticular nucleus, pars alpha, lateral paragigantocellular, and raphe magnus), in keeping with the postulated role of NTS2 receptors in the mediation of neurotensin's supraspinal antinociceptive actions. However, the distribution of NTS2-like immunoreactivity largely exceeded that of neurotensin terminal fields, and some of the highest concentrations of the receptor were found in areas devoid of neurotensinergic inputs such as the cerebral cortex, the hippocampus, and the cerebellum, suggesting that neurotensin may not be the exclusive endogenous ligand for this receptor subtype.

Specific visualization of precipitated cerium by energy-filtered transmission electron microscopy for detection of alkaline phosphatase in immunoenzymatic double labeling of tyrosine hydroxylase and serotonin in the rat olfactory bulb

To understand in detail the functional morphology of neuronal circuits it is important to identify at the ultrastructural level the incoming axon, its target neuron, and members of the signaling cascades involved. This, however, represents a formidable task, requiring highly sophisticated electron microscopic multiple-labeling techniques. To extend available double-labeling procedures such as combinations of immunogold and peroxidase methods, an additional, gold- and peroxidase-independent procedure would represent a considerable advantage. The present investigation therefore aimed to use alkaline phosphatase as the immunoenzymatic label at the electron microscopic level via cerium phosphate precipitates. To our surprise we found that available techniques, which are well established for the visualization of endogenous enzymes in sections from various tissues, are not suitable for application to immunocytochemistry. Careful characterization of the individual reaction conditions, however, resulted in an optimized procedure with largely increased sensitivity. The novel technique yields cerium-containing precipitates which are massive enough to allow the detection of the immunoenzymatic reaction product in the electron microscope. Using the rat olfactory bulb as the model system we showed further that our technique allows the combination with the peroxidase/diaminobenzidine system for ultrastructural double labeling. For this purpose, the alkaline phosphatase product is identified by its cerium content via energy-filtered transmission electron microscopy and thereby differentiated from cerium-free peroxidase-derived precipitates. Doing so, we found that ascending serotoninergic fibers do not establish synapses with dopaminergic periglomerular cells in the rat olfactory bulb.

Receptor-mediated internalization of somatostatin in rat cortical and hippocampal neurons

Binding of neuropeptides to their receptors usually results in internalization of receptor-ligand complexes. This process serves a crucial role in receptor downregulation, resensitization, and transmembrane signaling. It has mainly been investigated in cells ectopically expressing recombinant receptors. In the present study, we investigated whether rat central neurons and astrocytes naturally expressing somatostatin (SRIF) receptors internalized this neuropeptide. We demonstrated that 29% of cortical and 45% of hippocampal neurons in culture expressed the SRIF receptor sst(2A) and that 40-50% of the neurons internalized fluorescent SRIF. Similarly, an important proportion of astrocytes expressed sst(2A) (up to 60% in cortical cultures) and internalized fluo-SRIF. Competition experiments using the sst(2)/sst(5)-preferring agonist SMS 201-995 (octreotide) showed that a subpopulation of neurons internalized fluo-SRIF via sst(2) and/or sst(5) receptors, but that others also did so via other subtypes. Fluo-SRIF labeling was barely competed for by the sst(1)-selective agonist CH-275, indicating that sst(1) was unlikely to be mediating SRIF internalization in hippocampal and cortical neurons. Given the paucity of sst(5) receptors in cerebral cortex and hippocampus and the poor yield of sst(4) internalization in transfected cells, we conclude that sst(2) and sst(3) subtypes are the most likely to be responsible for SRIF internalization in our culture systems.

Intracellular dynamics of sst5 receptors in transfected COS-7 cells: maintenance of cell surface receptors during ligand-induced endocytosis

Internalization of G protein-coupled receptors is crucial for resensitization of phosphorylation-desensitized receptors, but also for their long term desensitization through sequestration. To elucidate the mechanisms regulating cell surface availability of the somatostatin (SRIF) receptor subtype sst5, we characterized its internalization properties in transfected COS-7 cells using biochemical, confocal microscopic, and electron microscopic techniques. Our results demonstrated rapid and efficient sequestration of specifically bound [125I]Tyr0-D-Trp8-SRIF (up to 45% of bound radioactivity). Combined immunocytochemical detection of sst5 and visualization of a fluorescent SRIF analog by confocal microscopy revealed that whereas the internalized ligand progressively clustered toward the cell center with time, immunoreactive receptors remained predominantly associated with the plasma membrane. The preservation of cell surface receptors was confirmed by binding experiments on whole cells revealing a lack of saturability of [125I]Tyr0-D-Trp8-SRIF binding at 37 C. Binding was rendered saturable by the drug monensin, showing that receptor recycling played a key role in the preservation of cell surface receptors. Electron microscopy demonstrated that in addition to receptor recycling, internalization of receptor-ligand complexes triggered a massive recruitment of sst5 receptor molecules from intracellular stores to the membrane. This combination of recycling and recruitment of spare receptors may protect sst5 from long term down-regulation through sequestration and, therefore, facilitate extended SRIF signaling.

Immunohistochemical distribution of the somatostatin receptor subtype 5 in the adult rat brain: predominant expression in the basal forebrain

Somatostatin exerts its actions by means of a family of G protein-coupled receptors, five of which have so far been cloned. Whereas mRNAs for receptor subtypes sst(1)-sst(4) have been unequivocally localized in the brain, the data concerning the fifth subtype, sst(5), are contradictory. Moreover, whereas sst(1) and sst(2A) receptor proteins have been localized by immunohistochemistry, the distribution of sst(3)-sst(5) receptor proteins and/or subtype-specific binding remains to be determined in the central nervous system. In the present study, we investigated the distribution of immunoreactive sst(5) in adult rat brain and pituitary and demonstrated the presence of this receptor protein in the central nervous system by using an affinity-purified antibody generated against the C-terminus of the receptor. The specificity of the antibody for sst(5) was established by immunoblotting experiments on membranes prepared from cells transfected with cDNA encoding different somatotropin release inhibiting (SRIF) receptor subtypes as well as from anterior pituitary. In both systems, the antibody specifically recognized a band at approximately 50 kDa molecular mass, corresponding well to the reported size of the cloned receptor (48 kDa). Immunofluorescence in COS-7 cells transfected with individual SRIF receptor subtypes as well as in sections of rat pituitary demonstrated the antibody's applicability to the immunohistochemical detection of sst(5) receptors. In rat brain sections, sst(5) immunoreactivity was predominantly associated with neuronal perikarya and primary dendrites. Immunolabeling was most prominent in the olfactory tubercle, islands of Calleja, diagonal band of Broca, substantia innominata, and magnocellular preoptic nucleus of the basal forebrain as well as in the reticular nucleus of the thalamus. Other, less intensely labeled areas included the cerebral cortex, hippocampus, amygdala, preoptic area as well as the lateroanterior nucleus of the hypothalamus. The present findings provide the first characterization of the anatomic distribution of sst(5) receptors in the rat brain. They demonstrate a prominent expression of these receptors in the basal forebrain, suggesting that they may be involved in the mediation of somatostatin effects on the sleep-wake cycle through their association with cortically projecting subcortical systems.

Fluorescent ligands for studying neuropeptide receptors by confocal microscopy

This paper reviews the use of confocal microscopy as it pertains to the identification of G-protein coupled receptors and the study of their dynamic properties in cell cultures and in mammalian brain following their tagging with specific fluorescent ligands. Principles that should guide the choice of suitable ligands and fluorophores are discussed. Examples are provided from the work carried out in the authors' laboratory using custom synthetized fluoresceinylated or BODIPY-tagged bioactive peptides. The results show that confocal microscopic detection of specifically bound fluorescent ligands permits high resolution appraisal of neuropeptide receptor distribution both in cell culture and in brain sections. Within the framework of time course experiments, it also allows for a dynamic assessment of the internalization and subsequent intracellular trafficking of bound fluorescent molecules. Thus, it was found that neurotensin, somatostatin and mu- and delta-selective opioid peptides are internalized in a receptor-dependent fashion and according to receptor-specific patterns into their target cells. In the case of neurotensin, this internalization process was found to be clathrin-mediated, to proceed through classical endosomal pathways and, in neurons, to result in a mobilization of newly formed endosomes from neural processes to nerve cell bodies and from the periphery of cell bodies towards the perinuclear zone. These mechanisms are likely to play an important role for ligand inactivation, receptor regulation and perhaps also transmembrane signaling.

Expression of somatostatin in neurons of the central posterior/prepacemaker nucleus projecting to the preglomerular nucleus: immunohistochemical evidence for a non-synaptic function

In the diencephalon of the weakly electric gymnotiform fish Apteronous leptorhynchus, part of the central posterior/prepacemaker nucleus innervates the preglomerular nucleus. A minor population of these neurons expresses immunoreactivity against somatostatin, as has been shown by combining peptide immunohistochemistry with an in vitro tract-tracing technique. In contrast to the expectation, however, this neuropeptide does not appear to be transported along the axons to the projection site, as somatostatin-like immunoreactivity could not be detected in the preglomerular nucleus. It is, therefore, likely that somatostatin expressed in these neurons exerts a non-synaptic function in the region of the central posterior/prepacemaker nucleus itself.

The postembryonic development of somatostatin immunoreactivity in the central posterior/prepacemaker nucleus of weakly electric fish, Apteronotus leptorhynchus: a double-labelling study

The neuropeptide somatostatin (SS) is widely distributed in both the central and peripheral nervous system of vertebrates. Its widespread distribution is paralleled by a large variety of diverse functions. While embryonic and perinatal development of SS-like immunoreactivity have been well examined, little is known about the postnatal development of this neuropeptide. Since, in teleosts, neurogenesis persists in many brain regions during adulthood, these vertebrates are well suited to investigate this phenomenon. In the present study, we have, therefore, examined the development of somatostatinergic cells born during adulthood in the central posterior/prepacemaker nucleus (CP/PPn) of Apteronotus leptorhynchus, a weakly electric gymnotiform fish. This was achieved by labelling proliferating cells with the thymidine analogue 5-bromo-2'-deoxyuridine (BrdU) and by simultaneous immunocytochemical detection of SS-like immunoreactivity. SS-like immunoreactivity is adopted in a period between 2 days and 3.5 days after birth. While the number of BrdU-labelled cells in the CP/PPn decreases 10 days after birth, the percentage of double-labelled cells among the BrdU-labelled cells remains with 1.0-7.6% in the period between 3.5 days and 100 days after birth rather constant. This percentage matches well the fraction of SS-positive cells in the total population of cells present in the CP/PPn.