Protein synthetic machinery in the dendrites of the magnocellular neurosecretory neurons of wild-type Long-Evans and homozygous Brattleboro rats

Measuring mRNA translation in neuronal processes and somata by tRNA-FRET

In neurons, the specific spatial and temporal localization of protein synthesis is of great importance for function and survival. Here, we visualized tRNA and protein synthesis events in fixed and live mouse primary cortical culture using fluorescently-labeled tRNAs. We were able to characterize the distribution and transport of tRNAs in different neuronal sub-compartments and to study their association with the ribosome. We found that tRNA mobility in neural processes is lower than in somata and corresponds to patterns of slow transport mechanisms, and that larger tRNA puncta co-localize with translational machinery components and are likely the functional fraction. Furthermore, chemical induction of long-term potentiation (LTP) in culture revealed up-regulation of mRNA translation with a similar effect in dendrites and somata, which appeared to be GluR-dependent 6 h post-activation. Importantly, measurement of protein synthesis in neurons with high resolutions offers new insights into neuronal function in health and disease states.

Detection of activity-dependent vasopressin release from neuronal dendrites and axon terminals using sniffer cells

Our understanding of neuropeptide function within neural networks would be improved by methods allowing dynamic detection of peptide release in living tissue. We examined the usefulness of sniffer cells as biosensors to detect endogenous vasopressin (VP) release in rat hypothalamic slices and from isolated neurohypophyses. Human embryonic kidney cells were transfected to express the human V1a VP receptor (V1aR) and the genetically encoded calcium indicator GCaMP6m. The V1aR couples to Gq₁₁, thus VP binding to this receptor causes an increase in intracellular [Ca²⁺] that can be detected by a rise in GCaMP6 fluorescence. Dose-response analysis showed that VP sniffer cells report ambient VP levels >10 pM (EC₅₀ = 2.6 nM), and this effect could be inhibited by the V1aR antagonist SR 49059. When placed over a coverslip coated with sniffer cells, electrical stimulation of the neurohypophysis provoked a reversible, reproducible, and dose-dependent increase in VP release using as few as 60 pulses delivered at 3 Hz. Suspended sniffer cells gently plated over a slice adhered to the preparation and allowed visualization of VP release in discrete regions. Electrical stimulation of VP neurons in the suprachiasmatic nucleus caused significant local release as well as VP secretion in distant target sites. Finally, action potentials evoked in a single magnocellular neurosecretory cell in the supraoptic nucleus provoked significant VP release from the somatodendritic compartment of the neuron. These results indicate that sniffer cells can be used for the study of VP secretion from various compartments of neurons in living tissue. NEW & NOTEWORTHY The specific functional roles of neuropeptides in neuronal networks are poorly understood due to the absence of methods allowing their real-time detection in living tissue. Here, we show that cultured "sniffer cells" can be engineered to detect endogenous release of vasopressin as an increase in fluorescence.

The Oxytocin Receptor: From Intracellular Signaling to Behavior

The many facets of the oxytocin (OXT) system of the brain and periphery elicited nearly 25,000 publications since 1930 (see FIGURE 1 , as listed in PubMed), which revealed central roles for OXT and its receptor (OXTR) in reproduction, and social and emotional behaviors in animal and human studies focusing on mental and physical health and disease. In this review, we discuss the mechanisms of OXT expression and release, expression and binding of the OXTR in brain and periphery, OXTR-coupled signaling cascades, and their involvement in behavioral outcomes to assemble a comprehensive picture of the central and peripheral OXT system. Traditionally known for its role in milk let-down and uterine contraction during labor, OXT also has implications in physiological, and also behavioral, aspects of reproduction, such as sexual and maternal behaviors and pair bonding, but also anxiety, trust, sociability, food intake, or even drug abuse. The many facets of OXT are, on a molecular basis, brought about by a single receptor. The OXTR, a 7-transmembrane G protein-coupled receptor capable of binding to either Gαior Gαqproteins, activates a set of signaling cascades, such as the MAPK, PKC, PLC, or CaMK pathways, which converge on transcription factors like CREB or MEF-2. The cellular response to OXT includes regulation of neurite outgrowth, cellular viability, and increased survival. OXTergic projections in the brain represent anxiety and stress-regulating circuits connecting the paraventricular nucleus of the hypothalamus, amygdala, bed nucleus of the stria terminalis, or the medial prefrontal cortex. Which OXT-induced patterns finally alter the behavior of an animal or a human being is still poorly understood, and studying those OXTR-coupled signaling cascades is one initial step toward a better understanding of the molecular background of those behavioral effects.

Vesicle degradation in dendrites of magnocellular neurones of the rat supraoptic nucleus

The magnocellular neurones of the supraoptic nucleus (SON) and paraventricular nucleus release neuropeptide from their axon terminals and also from their dendrites. In the axon terminals, swellings known as Herring bodies are responsible for the degradation of aged, unreleased large dense-cored vesicles (LDCVs) by lysosomes. Dendrites of magnocellular neurones also contain a large number of LDCVs but specialised areas of vesicle degradation have yet to be discovered. Using immunofluorescence labelling for lysosomes in vasopressin-enhanced green fluorescent protein (vasopressin-eGFP) transgenic rats, we found that lysosomes are preferentially located in the centre of the dendrites where there was a high density of vasopressin-eGFP expression. These data suggest that there are local "hot spots", but not specific compartments for vesicle degradation in magnocellular dendrites.

Alpha2-adrenergic impact on hypothalamic magnocellular oxytocinergic neurons in long evans and brattleboro rats: effects of agonist and antagonists

We have previously demonstrated that alpha2-adrenoceptors regulate hypothalamic magnocellular oxytocinergic (OXY) neurons in Sprague Dawley rats. Here we investigated whether activation/inhibition of alpha2-adrenoceptors may similarly trigger/downregulate the activity of OXY neurons in control Long Evans (+/+) and permanently osmotically stressed Brattleboro (di/di) rats. The effect of alpha2-adrenoceptor agonist, xylazine (XYL) and alpha2-adrenoceptor antagonists, atipamezole (ATIP), and idazoxan (IDX) were evaluated in the supraoptic (SON) and paraventricular (PVN) hypothalamic nuclei. Saline (SAL, 0.1 ml/100 g), XYL (10 mg/kg), ATIP, (1 mg/kg), and IDX (10 mg/kg) and IDX or ATIP followed by XYL were applied intraperitoneally. Rats were sacrificed 90 min later and Fos/OXY co-labelings analyzed in microscope. In control +/+ rats no or few Fos/OXY co-labelings occurred in SON and PVN. XYL significantly increased Fos incidence in OXY neurons in both nuclei. ATIP significantly suppressed the effect of XYL in both nuclei and IDX only in SON. In di/di controls 81% of OXY neurons in SON and 44% in PVN revealed Fos presence and XYL did not further elevate Fos number in SON OXY neurons and slightly increased Fos number in PVN. ATIP or IDX only partially reduced Fos in SAL or XYL treated di/di rats. Our data indicate that: (1) XYL stimulation is not effective in di/di rats because of sustained upregulation of OXY neurons activity and (2) neither ATIP nor IDX reduced significantly the OXY activity in control di/di rats. These findings suggest that alpha2-adrenoceptors have only a limited impact in maintaining OXY cells activity upregulation in PVN and SON of di/di rats.

Response of substances co-expressed in hypothalamic magnocellular neurons to osmotic challenges in normal and Brattleboro rats

The intention of this review is to emphasize the current knowledge about the extent and importance of the substances co-localized with magnocellular arginine vasopressin (AVP) and oxytocin (OXY) as potential candidates for the gradual clarification of their actual role in the regulation of hydromineral homeostasis. Maintenance of the body hydromineral balance depends on the coordinated action of principal biologically active compounds, AVP and OXY, synthesized in the hypothalamic supraoptic and paraventricular nuclei. However, on the regulation of water-salt balance, other substances, co-localized with the principal neuropetides, participate. These can be classified as (1) peptides co-localized with AVP or OXY with unambiguous osmotic function, including angiotensin II, apelin, corticotropin releasing hormone, and galanin and (2) peptides co-localized with AVP or OXY with an unknown role in osmotic regulation, including cholecystokinin, chromogranin/secretogranin, dynorphin, endothelin-1, enkephalin, ferritin protein, interleukin 6, kininogen, neurokinin B, neuropeptide Y, vasoactive intestinal peptide, pituitary adenylate cyclase-activating polypeptide, TAFA5 protein, thyrotropin releasing hormone, tyrosine hydroxylase, and urocortin. In this brief review, also the responses of these substances to different hyperosmotic and hypoosmotic challenges are pointed out. Based on the literature data published recently, the functional implication of the majority of co-localized substances is still better understood in non-osmotic than osmotic functional circuits. Brattleboro strain of rats that does not express functional vasopressin was also included in this review. These animals suffer from chronic hypernatremia and hyperosmolality, accompanied by sustained increase in OXY mRNA in PVN and SON and OXY levels in plasma. They represent an important model of animals with constantly sustained osmolality, which in the future, will be utilizable for revealing the physiological importance of biologically active substances co-expressed with AVP and OXY, involved in the regulation of plasma osmolality.

Regulation of activity-dependent dendritic vasopressin release from rat supraoptic neurones

Magnocellular neurones of the hypothalamus release vasopressin and oxytocin from their dendrites and soma. Using a combination of electrophysiology, microdialysis, in vitro explants, and radioimmunoassay we assessed the involvement of intracellular Ca(2+) stores in the regulation of dendritic vasopressin release. Thapsigargin and cyclopiazonic acid, which mobilize Ca(2+) from intracellular stores of the endoplasmic reticulum, evoked vasopressin release from dendrites and somata of magnocellular neurones in the supraoptic nucleus. Thapsigargin also produced a dramatic potentiation of dendritic vasopressin release evoked by osmotic or high potassium stimulation. This effect is long lasting, time dependent, and specific to thapsigargin as caffeine and ryanodine had no effect. Furthermore, antidromic activation of electrical activity in the cell bodies released vasopressin from dendrites only after thapsigargin pretreatment. Thus, exposure to Ca(2+) mobilizers such as thapsigargin or cyclopiazonic acid primes the releasable pool of vasopressin in the dendrites, so that release can subsequently be evoked by electrical and depolarization-dependent activation. Vasopressin itself is effective in inducing dendritic vasopressin release, but it is ineffective in producing priming.

ERp29, a general endoplasmic reticulum marker, is highly expressed throughout the brain

ERp29 is a recently discovered resident of the endoplasmic reticulum (ER) that is abundant in brain and most other mammalian tissues. Investigations of nonneural secretory tissues have implicated ERp29 in a major role producing export proteins, but a molecular activity remains wanting for this functional orphan. Intriguingly, ERp29 appears to be heavily utilized in the cerebellum, a brain region not conventionally regarded as neurosecretory. To elucidate this functional quandary, we used immunochemical approaches to characterize the regional, cellular, and subcellular distributions of ERp29 in rat brain. Immunohistochemistry revealed ubiquitous expression in neuronal and nonneuronal cells, with a distinctive variation in somatic ERp29 levels. Highly expressing cells were found in diverse locations, implying that ERp29 is not biased towards the cerebellum functionally. Using immunolocalization data mined from the literature, a proteomic profile was developed to assess the functional significance of ERp29's characteristic expression pattern. Surprisingly, ERp29 correlated poorly with classical markers of neurosecretion, but strongly with a variety of major membrane proteins. Together with immunogold localization of ERp29 to somatic ER, these observations led to a novel hypothesis that ERp29 is involved primarily in production of endomembrane proteins rather than proteins destined for export. This study establishes ERp29 as a general ER marker for brain cells and provides a stimulating clue about ERp29's enigmatic function. ERp29 appears to have broad significance for neural pathophysiology, given its ubiquitous distribution and prominence in brain over classical ER residents like BiP and protein disulfide isomerase.

Endoplasmic reticulum export site formation and function in dendrites

The elongated and polarized characteristics of neurons render targeting of receptors to the plasma membrane of distal axonal projections and dendritic branches a major sorting task. Although the majority of biosynthetic cargo synthesis, transport, and sorting are believed to occur in the soma, local membrane protein translation and sorting has been reported recently to take place in dendrites and axons. We investigated where endoplasmic reticulum (ER) export occurs in dendrites using an in vitro permeabilized neuron system that enables us to specifically control the assembly of ER export sites. We show that ER export sites are assembled regularly throughout the entire dendritic tree by the regulated sequential recruitment of Sar1 and COPII (coat protein complex II). Moreover, activation of metabotropic glutamate receptors leads to the recruitment of the NMDA receptor subunit NR1 to remodeled ER export sites. We propose that regulation of receptor assembly and export from the ER in dendrites plays an important role in modulating receptor surface expression and neuronal function.

Thapsigargin-induced mobilization of dendritic dense-cored vesicles in rat supraoptic neurons

Dense-cored vesicles (DCVs) containing oxytocin or vasopressin are secreted from both the nerve terminals in the posterior pituitary and dendrites in the hypothalamus of magnocellular supraoptic neurons. Dendritic secretion can be enhanced (primed) by pretreatment with either thapsigargin or oxytocin for subsequent activity-dependent release. Here, we determined whether priming involves a translocation of DCV closer to the dendritic membrane. To reduce total vesicle content, rats were salt-loaded for 24 h before application of thapsigargin or vehicle onto the ventrally exposed surface of the supraoptic nucleus in vivo. Tissues were then prepared for quantitative electron microscopic analysis of the total incidence of DCVs within supraoptic dendritic cross-sections, and of the incidence and distance (within a 500-nm margin) of each DCV to the dendritic plasma membrane. Salt loading per se did not alter the frequency distribution or average proportion of DCVs found in the 500-nm margin but significantly decreased the average incidence of DCVs per dendrite by 30% (P < 0.05). However, thapsigargin treatment resulted in a significant increase in the total incidence of DCVs within the 500-nm margins and a higher incidence of DCVs within the first 200 nm of the plasma membrane (P < 0.05), indicating that the thapsigargin-induced priming involves a relocation of DCVs closer to sites of secretion.

Local synthesis of the rat Vasopressin precursor in dendrites of in vitro cultured nerve cells

Vasopressin (VP) mRNA is subject to dendritic targeting both in vivo and in primary cultured neurons microinjected with an appropriate expression vector. We have constructed a vector encoding the mutant Brattleboro rat VP precursor which is non-diffusable, because it cannot leave the site of its synthesis, the rough endoplasmic reticulum. Expression of this construct in cultured nerve cells shows that the mutant protein is readily detectable in dendrites when mRNA transport has occurred, while dendrites devoid of the mRNA lack the protein. These results demonstrate that neurons have the capacity to locally synthesize secretory proteins in the dendritic compartment.

Talking back: dendritic neurotransmitter release

Classical transmitters and neuropeptides can be released from the dendrites of many neuronal populations, to act as retrograde signals that modulate synaptic transmission, electrical activity and, in some cases, morphology of the cell of origin. For the hypothalamic neuroendocrine cells that release vasopressin and oxytocin, the stimuli, mechanisms and physiological functions of dendritic release have been revealed in detail that is not yet available for other neurons. The regulation of dendritic transmitter release is complex and at least partially independent from axon terminal release. Here, we provide an overview of recent findings on the mechanisms and physiological consequences of dendritic neuropeptide release and place this in the context of discoveries of dendritic neurotransmitter release in other brain regions.

Molecular determinants and physiological relevance of extrasomatic RNA localization in neurons

Specific sorting of mRNA molecules to subcellular microdomains is an evolutionarily conserved mechanism by which the polarized nature of eukayotic cells may be established and maintained. The molecular composition of the RNA localization machinery is complex. Sequence motifs within RNA molecules to be transported, called cis-acting elements, and proteins, referred to as trans-acting factors, are essential components. Transport of the resulting ribonucleoprotein complexes to distinct cytoplasmic regions occurs along the cytoskeletal network. The pathway is observed in organisms as diverse as yeast and human and it plays a critical role in development and cell differentiation. Moreover, RNA localization takes place in differentiated cell types including neurons. There is ample evidence to suggest that sorting of defined mRNA species to the neurites of nerve cells and on-site translation has an impact on various aspects of nerve cell biology.

Regional differences in processing of locally translated prohormone in peptidergic neurons of Aplysia californica

Earlier work showed that cell bodies and neurites of the peptidergic bag cell neurons of Aplysia californica contain mRNA for egg-laying hormone. The purpose of the present study was to determine if egg-laying hormone synthesis and prohormone processing is similar in the pleurovisceral connective nerves (containing neurites of bag cell neurons) and the bag cell neuron clusters (containing both cell bodies and neurites of bag cell neurons). Initial experiments confirmed by RT-PCR and sequencing that egg-laying hormone mRNA was present in the pleurovisceral connective nerves. To investigate possible regional differences in translation of mRNA and prohormone processing, clusters were separated from connective nerves and newly synthesized egg-laying hormone-immunoreactive proteins were analyzed. Results showed that synthesis and processing of prohormone occurred in both the clusters and isolated connective nerves; however, the relative abundance of prohormone, processing intermediates, and egg-laying hormone was different. Pulse-chase experiments showed that prohormone was processed more slowly in the connective nerves than in the clusters. These results show that mRNA in isolated neural processes of neuroendocrine cells can be translated, and that the cellular machinery for protein synthesis is present, but processing of the ELH prohormone is significantly compromised.