AXONAL MIGRATION OF PROTEINS IN THE CENTRAL NERVOUS SYSTEM AND PERIPHERAL NERVES AS SHOWN BY RADIOAUTOGRAPHY

Routes of the thalamus through the history of neuroanatomy

The most distant roots of neuroanatomy trace back to antiquity, with the first human dissections, but no document which would identify the thalamus as a brain structure has reached us. Claudius Galenus (Galen) gave to the thalamus the name 'thalamus nervorum opticorum', but later on, other names were used (e.g., anchae, or buttocks-like). In 1543, Andreas Vesalius provided the first quality illustrations of the thalamus. During the 19th century, tissue staining techniques and ablative studies contributed to the breakdown of the thalamus into subregions and nuclei. The next step was taken using radiomarkers to identify connections in the absence of lesions. Anterograde and retrograde tracing methods arose in the late 1960s, supporting extension, revision, or confirmation of previously established knowledge. The use of the first viral tracers introduced a new methodological breakthrough in the mid-1970s. Another important step was supported by advances in neuroimaging of the thalamus in the 21th century. The current review follows the history of the thalamus through these technical revolutions from Antiquity to the present day.

Pharmacological Manipulation of Translation as a Therapeutic Target for Chronic Pain

Dysfunction in regulation of mRNA translation is an increasingly recognized characteristic of many diseases and disorders, including cancer, diabetes, autoimmunity, neurodegeneration, and chronic pain. Approximately 50 million adults in the United States experience chronic pain. This economic burden is greater than annual costs associated with heart disease, cancer, and diabetes combined. Treatment options for chronic pain are inadequately efficacious and riddled with adverse side effects. There is thus an urgent unmet need for novel approaches to treating chronic pain. Sensitization of neurons along the nociceptive pathway causes chronic pain states driving symptoms that include spontaneous pain and mechanical and thermal hypersensitivity. More than a decade of preclinical research demonstrates that translational mechanisms regulate the changes in gene expression that are required for ongoing sensitization of nociceptive sensory neurons. This review will describe how key translation regulation signaling pathways, including the integrated stress response, mammalian target of rapamycin, AMP-activated protein kinase (AMPK), and mitogen-activated protein kinase-interacting kinases, impact the translation of different subsets of mRNAs. We then place these mechanisms of translation regulation in the context of chronic pain states, evaluate currently available therapies, and examine the potential for developing novel drugs. Considering the large body of evidence now published in this area, we propose that pharmacologically manipulating specific aspects of the translational machinery may reverse key neuronal phenotypic changes causing different chronic pain conditions. Therapeutics targeting these pathways could eventually be first-line drugs used to treat chronic pain disorders. SIGNIFICANCE STATEMENT: Translational mechanisms regulating protein synthesis underlie phenotypic changes in the sensory nervous system that drive chronic pain states. This review highlights regulatory mechanisms that control translation initiation and how to exploit them in treating persistent pain conditions. We explore the role of mammalian/mechanistic target of rapamycin and mitogen-activated protein kinase-interacting kinase inhibitors and AMPK activators in alleviating pain hypersensitivity. Modulation of eukaryotic initiation factor 2α phosphorylation is also discussed as a potential therapy. Targeting specific translation regulation mechanisms may reverse changes in neuronal hyperexcitability associated with painful conditions.

Presynaptic Protein Synthesis Is Required for Long-Term Plasticity of GABA Release

Long-term changes of neurotransmitter release are critical for proper brain function. However, the molecular mechanisms underlying these changes are poorly understood. While protein synthesis is crucial for the consolidation of postsynaptic plasticity, whether and how protein synthesis regulates presynaptic plasticity in the mature mammalian brain remain unclear. Here, using paired whole-cell recordings in rodent hippocampal slices, we report that presynaptic protein synthesis is required for long-term, but not short-term, plasticity of GABA release from type 1 cannabinoid receptor (CB₁)-expressing axons. This long-term depression of inhibitory transmission (iLTD) involves cap-dependent protein synthesis in presynaptic interneuron axons, but not somata. Translation is required during the induction, but not maintenance, of iLTD. Mechanistically, CB₁ activation enhances protein synthesis via the mTOR pathway. Furthermore, using super-resolution STORM microscopy, we revealed eukaryotic ribosomes in CB₁-expressing axon terminals. These findings suggest that presynaptic local protein synthesis controls neurotransmitter release during long-term plasticity in the mature mammalian brain.

Accumulating Evidence for Axonal Translation in Neuronal Homeostasis

The specialized structure of the neuron requires that homeostasis is sustained over the meter or more that may separate a cell body from its axonal terminus. Given this impressive distance and an axonal volume that is many times that of the cell body, how is such a compartment grown during development, re-grown after injury, and maintained throughout adulthood? While early answers to these questions focused on the local environment or the cell soma as supplying the needs of the axon, it is now well-established that the axon has some unique needs that can only be met from within. Decades of research have revealed local translation as an indispensable mechanism of axonal homeostasis during development and regeneration in both invertebrates and vertebrates. In contrast, the extent to which the adult, mammalian axonal proteome is maintained through local translation remains unclear and controversial. This mini-review aims to highlight important experiments that have helped to shape the field of axonal translation, to discuss conceptual arguments and recent evidence that supports local translation as important to the maintenance of adult axons, and to suggest experimental approaches that have the potential to further illuminate the role of axonal translation in neuronal homeostasis.

Presynaptic Protein Synthesis Is Required for Long-Term Plasticity of GABA Release

Long-term changes of neurotransmitter release are critical for proper brain function. However, the molecular mechanisms underlying these changes are poorly understood. While protein synthesis is crucial for the consolidation of postsynaptic plasticity, whether and how protein synthesis regulates presynaptic plasticity in the mature mammalian brain remain unclear. Here, using paired whole-cell recordings in rodent hippocampal slices, we report that presynaptic protein synthesis is required for long-term, but not short-term, plasticity of GABA release from type 1 cannabinoid receptor (CB₁)-expressing axons. This long-term depression of inhibitory transmission (iLTD) involves cap-dependent protein synthesis in presynaptic interneuron axons, but not somata. Translation is required during the induction, but not maintenance, of iLTD. Mechanistically, CB₁ activation enhances protein synthesis via the mTOR pathway. Furthermore, using super-resolution STORM microscopy, we revealed eukaryotic ribosomes in CB₁-expressing axon terminals. These findings suggest that presynaptic local protein synthesis controls neurotransmitter release during long-term plasticity in the mature mammalian brain.

Review on Cowan WM, Gottlieb DI, Hendrickson AE, Price JL, Woolsey TA. 1972. The autoradiographic demonstration of axonal connections in the central nervous system. Brain Res 37: 21-51

Axoplasmically transported proteins synthesized in neuronal somata labeled by radioactively labeled amino acids (tritium), following local targeted injections for tracing of pathways in the central nervous system using autoradiography. Results from a number of neuronal systems, including: the rat olfactory bulb; cortico-thalamic projections in the mouse; commissural connections of the rat hippocampus; and retinal projections in the monkey and chick are documented. Pathway origins are clear, as the number and distribution of the labeled cells and the normal structure of the injection site is preserved. Light and electron microscopic autoradiography shows that proteins are transported, at two rates: rapid transport (>100mm/day) of fewer proteins accumulating in axon terminals; and, slow transport (1-5mm/day) of the bulk of labeled proteins distributed along the length of axons. Different survival times can be selected to evaluate terminal projection field(s) or pathways from origin to termination. The clarity of autoradiographic labeling of pathways and their terminations is comparable to other techniques (such as the Nauta-Gygax and the Fink-Heimer methods and the electron microscopy of terminal degeneration). Labeled amino acids do not label molecules in fibers of passage and there is no retrograde transport of labeled material from the axon terminals. The functional polarity of fiber pathways can be easily established. We summarize the merits of this technique is based upon an established physiological properties of neurons that are summarized in contrast to currently used techniques dependent upon pathological changes in neurons, axons, or axonal terminals.

ARTICLE ABSTRACT

This article considers a heavily cited Brain Research article that reported an extremely important turning point in the ability to demonstrate neuroanatomical pathways in the central nervous system. Using radioactive leucine microinjections into the brain, neurons synthesized proteins from this amino acid that were transported down their axons to the terminal synapses on the target neurons. Tracing the transport of the labeled protein by autoradiography permitted quantitative analysis of projections and pathways. As a result, pathway analysis was transformed from studying the degenerating processes of lesioned neurons to the study of intact pathways in non-manipulated brains. The classical protocol has since been widely applied and used to investigate countless brain circuits. This article is part of a Special Issue entitled SI:50th Anniversary Issue.

Axonal maintenance, glia, exosomes, and heat shock proteins

Of all cellular specializations, the axon is especially distinctive because it is a narrow cylinder of specialized cytoplasm called axoplasm with a length that may be orders of magnitude greater than the diameter of the cell body from which it originates. Thus, the volume of axoplasm can be much greater than the cytoplasm in the cell body. This fact raises a logistical problem with regard to axonal maintenance. Many of the components of axoplasm, such as soluble proteins and cytoskeleton, are slowly transported, taking weeks to months to travel the length of axons longer than a few millimeters after being synthesized in the cell body. Furthermore, this slow rate of supply suggests that the axon itself might not have the capacity to respond fast enough to compensate for damage to transported macromolecules. Such damage is likely in view of the mechanical fragility of an axon, especially those innervating the limbs, as rapid limb motion with high impact, like running, subjects the axons in the limbs to considerable mechanical force. Some researchers have suggested that local, intra-axonal protein synthesis is the answer to this problem. However, the translational state of axonal RNAs remains controversial. We suggest that glial cells, which envelop all axons, whether myelinated or not, are the local sources of replacement and repair macromolecules for long axons. The plausibility of this hypothesis is reinforced by reviewing several decades of work on glia-axon macromolecular transfer, together with recent investigations of exosomes and other extracellular vesicles, as vehicles for the transmission of membrane and cytoplasmic components from one cell to another.

Bidirectional actin transport is influenced by microtubule and actin stability

Local and long-distance transport of cytoskeletal proteins is vital to neuronal maintenance and growth. Though recent progress has provided insight into the movement of microtubules and neurofilaments, mechanisms underlying the movement of actin remain elusive, in large part due to rapid transitions between its filament states and its diverse cellular localization and function. In this work, we integrated live imaging of rat sensory neurons, image processing, multiple regression analysis, and mathematical modeling to perform the first quantitative, high-resolution investigation of GFP-actin identity and movement in individual axons. Our data revealed that filamentous actin densities arise along the length of the axon and move short but significant distances bidirectionally, with a net anterograde bias. We directly tested the role of actin and microtubules in this movement. We also confirmed a role for actin densities in extension of axonal filopodia, and demonstrated intermittent correlation of actin and mitochondrial movement. Our results support a novel mechanism underlying slow component axonal transport, in which the stability of both microtubule and actin cytoskeletal components influence the mobility of filamentous actin.

Myosin Va but Not nNOSα is Significantly Reduced in Jejunal Musculomotor Nerve Terminals in Diabetes Mellitus

Nitric oxide (NO) mediated slow inhibitory junction potential and mechanical relaxation after electrical field stimulation (EFS) is impaired in diabetes mellitus. Externally added NO donor restore nitrergic function, indicating that this reduction result from diminution of NO synthesis within the pre-junctional nerve terminals. The present study aimed to investigate two specific aims that may potentially provide pathophysiological insights into diabetic nitrergic neuropathy. Specifically, alteration in nNOSα contents within jejunal nerve terminals and a local subcortical transporter myosin Va was tested 16 weeks after induction of diabetes by low dose streptozotocin (STZ) in male Wistar rats. The results show that diabetic rats, in contrast to vehicle treated animals, have: (a) nearly absent myosin Va expression in nerve terminals of axons innervating smooth muscles and (b) significant decrease of myosin Va in neuronal soma of myenteric plexus. In contrast, nNOSα staining in diabetic jejunum neuromuscular strips showed near intact expression in neuronal cell bodies. The space occupancy of nitrergic nerve fibers was comparable between groups. Normal concentration of nNOSα was visualized within a majority of nitrergic terminals in diabetes, suggesting intact axonal transport of nNOSα to distant nerve terminals. These results reveal the dissociation between presences of nNOSα in the nerve terminals but deficiency of its transporter myosin Va in the jejunum of diabetic rats. This significant observation of reduced motor protein myosin Va within jejunal nerve terminals may potentially explain impairment of pre-junctional NO synthesis during EFS of diabetic gut neuromuscular strips despite presence of the nitrergic synthetic enzyme nNOSα.

Mathematical modeling of neuronal polarization during development

During development of the brain, morphogenesis of neurons is dynamically organized from a simple rounded shape to a highly polarized morphology consisting of soma, one axon, and dendrites, which is a basis for establishing the unidirectional transfer of electric signals between neurons. The mechanism of such polarization is thought to be "local activation-global inhibition"; however, globally diffusing inhibitor molecules have not been identified. In this chapter, we present a theoretical modeling approach of such neuronal development. We first summarize biological research on neuronal polarization and then develop a biophysical model. Through mathematical analysis, principles of local activation-global inhibition are illustrated based on active transport, protein degradation, and neurite growth, but not on globally diffusing inhibitor.

The suprachiasmatic nucleus and the circadian timing system

The circadian timing system (CTS) in mammals may be defined as a network of interconnected diencephalic structures that regulate the timing of physiological processes and behavioral state. The central feature of the CTS is the suprachiasmatic nucleus (SCN) of the hypothalamus, a self-sustaining circadian oscillator entrained by visual afferents, input from other brain and peripheral oscillators. The SCN was first noted as a distinct component of the hypothalamus during the late nineteenth century and recognized soon after as a uniform feature of the mammalian and lower vertebrate brain. But, as was true for so many brain components identified in that era, its function was unknown and remained so for nearly a century. In the latter half of the twentieth century, numerous tools for studying the brain were developed including neuroanatomical tracing methods, electrophysiological methods including long-term recording in vivo and in vitro, precise methods for producing localized lesions in the brain, and molecular neurobiology. Application of these methods provided a body of data strongly supporting the view that the SCN is a circadian pacemaker in the mammalian brain. This chapter presents an analysis of the functional organization of the SCN as a component of a neural network, the CTS. This network functions as a coordinator of hypothalamic regulatory systems imposing a temporal organization of physiological processes and behavioral state to promote environmental adaptation.

A half century of experimental neuroanatomical tracing

Most of our current understanding of brain function and dysfunction has its firm base in what is so elegantly called the 'anatomical substrate', i.e. the anatomical, histological, and histochemical domains within the large knowledge envelope called 'neuroscience' that further includes physiological, pharmacological, neurochemical, behavioral, genetical and clinical domains. This review focuses mainly on the anatomical domain in neuroscience. To a large degree neuroanatomical tract-tracing methods have paved the way in this domain. Over the past few decades, a great number of neuroanatomical tracers have been added to the technical arsenal to fulfill almost any experimental demand. Despite this sophisticated arsenal, the decision which tracer is best suited for a given tracing experiment still represents a difficult choice. Although this review is obviously not intended to provide the last word in the tract-tracing field, we provide a survey of the available tracing methods including some of their roots. We further summarize our experience with neuroanatomical tracers, in an attempt to provide the novice user with some advice to help this person to select the most appropriate criteria to choose a tracer that best applies to a given experimental design.

Flexible search for single-axon morphology during neuronal spontaneous polarization

Polarization, a disruption of symmetry in cellular morphology, occurs spontaneously, even in symmetrical extracellular conditions. This process is regulated by intracellular chemical reactions and the active transport of proteins and it is accompanied by cellular morphological changes. To elucidate the general principles underlying polarization, we focused on developing neurons. Neuronal polarity is stably established; a neuron initially has several neurites of similar length, but only one elongates and is selected to develop into an axon. Polarization is flexibly controlled; when multiple neurites are selected, the selection is eventually reduced to yield a single axon. What is the system by which morphological information is decoded differently based on the presence of a single or multiple axons? How are stability and flexibility achieved? To answer these questions, we constructed a biophysical model with the active transport of proteins that regulate neurite growth. Our mathematical analysis and computer simulation revealed that, as neurites elongate, transported factors accumulate in the growth cone but are degraded during retrograde diffusion to the soma. Such a system effectively works as local activation-global inhibition mechanism, resulting in both stability and flexibility. Our model shows good accordance with a number of experimental observations.

Axonal mRNA transport and localized translational regulation of kappa-opioid receptor in primary neurons of dorsal root ganglia

kappa-opioid receptor (KOR) is detected pre- and postsynaptically, but the subcellular localization, translation, and regulation of kor mRNA in presynaptic compartments of sensory neurons remain elusive. In situ hybridization detected axonal distribution of kor mRNA in primary neurons of dorsal root ganglia (DRG). The MS2-fused GFP tracked kor mRNA transport from DRG neuronal soma to axons, requiring its 5' and 3' UTRs. In Campenot chambers, axonal translation of kor mRNA was demonstrated for DRG neurons, which depended on its 5' UTR and was stimulated by KCl depolarization. KCl depolarization of DRG neurons rendered redistribution of kor mRNA from the postpolysomal fraction to the translationally active polysomal fraction. This study provided evidence for mRNA transport and regulation of presynaptic protein synthesis of nonstructural proteins like KOR in primary sensory neurons and demonstrated a mechanism of KCl depolarization-stimulated axonal mRNA redistribution for localized translational regulation.

RNA translation in axons

The cell body has classically been considered the exclusive source of axonal proteins. However, significant evidence has accumulated recently to support the view that protein synthesis can occur in axons themselves, remote from the cell body. Indeed, local translation in axons may be integral to aspects of synaptogenesis, long-term facilitation, and memory storage in invertebrate axons, and for growth cone navigation in response to environmental stimuli in developing vertebrate axons. Here we review the evidence supporting mRNA translation in axons and discuss the potential roles that local protein synthesis may play during development and subsequent neuronal function. We advance the view that local translation provides a rapid supply of nascent proteins in restricted axonal compartments that can potentially underlie long-term responses to transient stimuli.

Neuroanatomical tracing at high resolution

Most techniques used for the study of the fiber connectivity in the central nervous system produce results which are visualized in the conventional light microscope or fluorescence microscope. Although in some cases this may be sufficient, often proof is necessary that fibers which enter a particular brain area indeed terminate here. Alternatively, it may be necessary to determine whether the axon terminals of traced fibers form synapses with specific processes of specific neurons. With the latter neurons all cellular elements are meant which can be labeled in some way. Evidence of synaptic connectivity necessitates visualization at a higher level of resolution, that is at the electron-microscopic level. In this contribution to the Special Issue we discuss several methods currently available to visualize individual tracers, and methods developed to visualize two different markers, that is one marker attached to a fiber or an axon terminal, and the second marker attached to a presumed pre- or postsynaptic neuronal element.

Protein synthesis in axons and terminals: significance for maintenance, plasticity and regulation of phenotype. With a critique of slow transport theory

This article focuses on local protein synthesis as a basis for maintaining axoplasmic mass, and expression of plasticity in axons and terminals. Recent evidence of discrete ribosomal domains, subjacent to the axolemma, which are distributed at intermittent intervals along axons, are described. Studies of locally synthesized proteins, and proteins encoded by RNA transcripts in axons indicate that the latter comprise constituents of the so-called slow transport rate groups. A comprehensive review and analysis of published data on synaptosomes and identified presynaptic terminals warrants the conclusion that a cytoribosomal machinery is present, and that protein synthesis could play a role in long-term changes of modifiable synapses. The concept that all axonal proteins are supplied by slow transport after synthesis in the perikaryon is challenged because the underlying assumptions of the model are discordant with known metabolic principles. The flawed slow transport model is supplanted by a metabolic model that is supported by evidence of local synthesis and turnover of proteins in axons. A comparison of the relative strengths of the two models shows that, unlike the local synthesis model, the slow transport model fails as a credible theoretical construct to account for axons and terminals as we know them. Evidence for a dynamic anatomy of axons is presented. It is proposed that a distributed "sprouting program," which governs local plasticity of axons, is regulated by environmental cues, and ultimately depends on local synthesis. In this respect, nerve regeneration is treated as a special case of the sprouting program. The term merotrophism is proposed to denote a class of phenomena, in which regional phenotype changes are regulated locally without specific involvement of the neuronal nucleus.

Cerebral glucose utilization and glucose transporter expression: response to water deprivation and restoration

The relationship between local rates of cerebral glucose utilization (ICMRglc) and glucose transporter expression was examined during physiologic activation of the hypothalamoneurohypophysial system. Three days of water deprivation, which is known to activate the hypothalamoneurohypophysial system, resulted in increased ICMRglc and increased concentrations of GLUT1 and GLUT3 in the neurohypophysis; mRNA levels of GLUT1 and GLUT3 were decreased and increased, respectively. Water deprivation also increased ICMRglc in the hypothalamic supraoptic and paraventricular nuclei; mRNA levels of GLUT1 and GLUT3 appeared to increase in these nuclei, but the changes did not achieve statistical significance. Restoration of water for 3 to 7 days reversed all observed changes in GLUT expression (protein and mRNA): restoration of water also reversed changes in ICMRglc in both the neurohypophysis and the hypothalamic nuclei. These results indicate that under conditions of neural activation and recovery, changes in ICMRglc and the levels of GLUT1 and GLUT3 are temporally correlated in the neurohypophysis and raise the possibility that GLUT1 and GLUT3 transporter expression may be regulated by chronic changes in functional activity. In addition, increases in the expression of GLUT5 mRNA in the neurohypophysis after dehydration provide evidence for involvement of microglial activation.

Differential compartmentalization of vasopressin messenger RNA and neuropeptide within the rat hypothalamo-neurohypophysial axonal tracts: light and electron microscopic evidence

Arginine vasopressin messenger RNA is axonally transported in the rat hypothalamo-neurohypophysial system [for review see Mohr et al. (1993) In Vasopressin (eds Gross P., Richter D. and Robertson C. L.), pp. 119-129, John Libbey Eurotext]. Upon chronic dehydration (2% saline-feeding for seven days), vasopressin messenger RNA within this axonal compartment is dramatically increased and appears aggregated in a selected subset of axonal swellings confined to the median eminence and posterior pituitary. In this study, we analysed the axonal distribution of the vasopressin messenger RNA within the hypothalamo-neurohypophysial tracts of control and saline-fed animals, and compared this distribution to that of the vasopressin peptide. Our data further support a selective aggregation of the vasopressin messenger RNA in a subset of distal axonal swellings and/or terminals of the median eminence and posterior pituitary. The selective aggregation is observed not only in saline-fed animals, but also in control animals. Although the osmotic stimulus dramatically enhances the axonal transport of vasopressin messenger RNA, the consequent general distribution pattern of the messenger RNA in the hypothalamo-neurohypophysial system is not changed. However, the physiological perturbation does increase the number of vasopressin messenger RNA-containing swellings within the median eminence and the posterior pituitary. In both saline-fed and control animals, the level of messenger RNA label within individual swellings appeared roughly similar to that found in the perikaryal cytoplasm of extra-hypothalamic vasopressinergic neurons. A detailed comparison of the axonal compartmentalization of vasopressin messenger RNA and vasopressin peptide demonstrates that the axonal distribution of vasopressin messenger RNA does not precisely overlap that of vasopressin peptide along the hypothalamo-neurohypophysial tract. In seven-day saline-fed animals, the majority of the messenger RNA-containing swellings of the median eminence also contain detectable vasopressin peptide; however in the same animals, nearly all the messenger RNA-containing swellings of the posterior pituitary appear devoid of vasopressin peptide. Therefore, our work strongly suggests that at least in the posterior pituitary, the vasopressin messenger RNA might be selectively targeted and aggregated in a selected subset of axonal swellings containing little if any vasopressin, and hence very few neurosecretory granules. Given this evidence that vasopressin messenger RNA and neuropeptide are differentially compartmentalized in axons of magnocellular neurons, we propose that vasopressin messenger RNA and peptide probably rely on different intracellular transport systems with respect to packaging, transport and/or aggregation within these selected axonal locations.

Cytoplasmic mechanisms of axonal and dendritic growth in neurons

The structural mechanisms responsible for the gradual elaboration of the cytoplasmic elongation of neurons are reviewed. In addition to discussing recent work, important older work is included to inform newcomers to the field how the current perspective arose. The highly specialized axon and the less exaggerated dendrite both result from the advance of the motile growth cone. In the area of physiology, studies in the last decade have directly confirmed the classic model of the growth cone pulling forward and the axon elongating from this tension. Particularly in the case of the axon, cytoplasmic elongation is closely linked to the formation of an axial microtubule bundle from behind the advancing growth cone. Substantial progress has been made in understanding the expression of microtubule-associated proteins during neuronal differentiation to stiffen and stabilize axonal microtubules, providing specialized structural support. Studies of membrane organelle transport along the axonal microtubules produced an explosion of knowledge about ATPase molecules serving as motors driving material along microtubule rails. However, most aspects of the cytoplasmic mechanisms responsible for neurogenesis remain poorly understood. There is little agreement on mechanisms for the addition of new plasma membrane or the addition of new cytoskeletal filaments in the growing axon. Also poorly understood are the mechanisms that couple the promiscuous motility of the growth cone to the addition of cytoplasmic elements.

BC1 RNA and vasopressin mRNA in rat neurohypophysis: axonal compartmentalization and differential regulation during dehydration and rehydration

Brain cytoplasmic 1 (BC1) RNA is a small non-translated RNA polymerase III transcript. Because this RNA can be detected in the rat posterior pituitary with 35S in situ hybridization autoradiography, it has been hypothesized that this RNA might be transported in the axons of hypothalamo-neurohypophyseal neurons. In the present study, we aimed to determine the cellular localization of BC1 more precisely by using non-radioactive in situ hybridization of BC1 RNA at both the light and electron microscopic levels. Our studies revealed that BC1 RNA was indeed located intra-axonally. Furthermore, BC1 RNA was abundant within a subset of axonal swellings and/or terminals, and was also found in discrete cytoplasmic domains of undilated axonal segments. Using a semiquantitative in situ hybridization approach, we have measured and compared the changes in BC1 RNA and arginine vasopressin (AVP) mRNA during dehydration (chronic salt-loading) and rehydration. Chronic salt-loading significantly increased both BC1 RNA and AVP mRNA. The increase in BC1 RNA labelling (2.5-fold), however, was modest and somewhat less enduring than the increase in AVP mRNA labelling (13-fold). Upon rehydration, both the BC1 and vasopressin transcripts in the posterior pituitary rapidly returned to control values. In conclusion, like vasopressin mRNA, BC1 RNA is transported in axons of the hypothalamo-neurohypophyseal system where it aggregates in a subset of axonal swellings, and its axonal transport is similarly regulated. Therefore, we propose that BC1 RNA might be involved in the axonal targeting, docking and/or transport of AVP or other axonal mRNAs.

Rapid changes in protein synthesis and cell size in the cochlear nucleus following eighth nerve activity blockade or cochlea ablation

Destruction of the cochlea causes secondary changes in the central auditory pathway through transynaptic regulation. These changes appear to be mediated by an activity-dependent process and can be detected in the avian auditory system as early as 30 minutes after deafferentation. We compared the early changes in cochlear nucleus neurons following deafferentation by cochlea ablation with those seen following activity deprivation by perilymphatic tetrodotoxin (TTX) exposure. Protein synthesis and size of large spherical cells in the anteroventral cochlear nucleus (AVCN) of 14-day-old gerbils were measured during the first 48 hours after the manipulations. Both cochlea ablation and TTX produced a reliable decrease in protein synthesis by AVCN neurons (30-40%) by 1 hour. The magnitude of change in tritiated leucine incorporation was similar at all survival times, in both experimental groups. In contrast to the rapid changes in protein synthesis, the decrease in cell size was first evident 18 hours after TTX exposure and 48 hours after cochlea ablation. There was no significant change in protein synthesis or cell size in control groups at any of the survival times. These findings are consistent with changes in the avian auditory system in response to deafferentation and TTX exposure. Cochlea ablation and TTX exposure induced similar transneuronal changes, supporting the hypotheses that activity of auditory afferents in young mammals plays a regulatory role in the metabolism and morphology of their target neurons in the central auditory pathway, and that early changes following destruction of the peripheral receptor are due to reduction of activity-dependent interactions of presynaptic and postsynaptic cells.

Rapid changes in 2-deoxyglucose uptake and amino acid incorporation following unilateral odor deprivation: a laminar analysis

Unilateral naris occlusion in neonatal rats results in large alterations in the olfactory bulb, including substantial changes in laminar volume and enhanced cell death. These gross changes are undoubtedly the result of a cascade of more basic cellular regulatory events. The present study assesses the possibility of rapid post-deprivation changes in two such processes: glucose metabolism and protein synthesis. On the day after the day of birth rat pups underwent unilateral naris occlusion or sham surgery. In one study, either 1, 12, 24 or 48 h following surgery [3H]2-deoxyglucose [( 3H]2-DG) was administered to gauge patterns of glucose uptake. In a second study, [3H]leucine was injected to assess patterns of protein synthesis. Autoradiographs were then subjected to quantitative analyses. As early as 1 h following occlusion reduced 2-DG uptake was observed in many bulb regions. By 24 h, leucine incorporation was also uniformly diminished. While 2-DG uptake remained suppressed 48 h after deprivation, levels of amino acid incorporation returned to normal patterns in most laminae, with the exception of the mitral cell layer, where increased uptake was encountered. To evaluate whether the effects were developmental by nature a group of P40-P45 animals treated similarly were also examined. While 24 h of deprivation impaired 2-DG uptake in older animals, no alterations in amino acid incorporation were observed. The results indicate that early odor deprivation has rapid and specific effects on cellular functioning within the developing olfactory bulb.

Melanocortins and fast axonal transport in intact and regenerating sciatic nerve

The effect of ACTH/MSH peptides on fast axonal transport along intact or regenerating sciatic nerve was examined following injection of tritiated leucine into the rat lumbar spinal cord. The rate of fast axonal transport was not significantly changed by treatment with ACTH/MSH(4-10), the ACTH(4-9) analog ORG 2766, hypophysectomy, or adrenalectomy. Fast axonal transport was unchanged in regenerating nerves and in regenerating, ACTH(4-10)-treated nerves. However, treatment with ORG 2766 in dosages of either 1 or 10 micrograms/kg/day IP for seven days significantly reduced (62% and 64%, respectively) the crest height of the fast axonal transport curve of intact sciatic nerve. The results suggest that the reported peptide-induced enhancement of nerve regeneration is not due to changes in the rate of fast axonal transport.

Decreased protein synthesis in cochlear nucleus following developmental auditory deprivation. Use of vascular saline perfusion to improve small tissue sample analysis

The incorporation of tritiated leucine was used as an index of protein synthesis in the cochlear nucleus (CN) of mice unilaterally (right side) hearing deprived throughout the period of hearing development. Right-left differences in radiolabel concentration were measured by scintillation counting of whole tissue homogenates. To improve upon the detection of small differences in radiolabel incorporation, the brain was perfused with saline prior to removal of CN tissue and the results compared with the standard nonperfusion method of tissue collection. Statistical analyses demonstrated the perfusion significantly reduced the acid soluble (unbound) label in CN without affecting the amount of protein bound label. Furthermore, a significant right side decrease in leucine incorporation was seen with the perfusion treatment, but not in the nonperfused treatment. This demonstrated that developmental auditory deprivation led to a decrease in protein synthesis at maturity. The results also demonstrated that mechanisms for leucine uptake were not impaired and the decrease in protein synthesis was not due to reduced availability of precursor amino acid. Thus, the use of saline perfusion prior to tissue collection facilitated the identification of protein synthesis differences that were unidentified by the traditional method.

Protein synthesis by rat hippocampal slices maintained in vitro

The present study evaluates protein synthesis in rat hippocampal slices maintained in vitro. Transverse slices of hippocampus were prepared from both adult rats and rat pups during postnatal development and incubated in a gassed (95% O2/5% CO2) balanced salt medium containing 5 nM 3H-leucine. The time course of 3H-leucine incorporation into TCA-precipitable protein was determined using slices removed from the media after 5, 10, 20, 30, 40, 60, and 120 min of incubation. The pattern of 3H-amino acid incorporation was evaluated by fixing slices with paraformaldehyde, embedding the slices in plastic, and sectioning the slices end on and en face for autoradiographic analysis. Biochemical analysis of 300 and 400 micron slices revealed that incorporation of leucine into protein proceeds at a constant rate. The autoradiographic analysis revealed that in adult hippocampal slices of 300-600 micron thickness there was complete penetration of 3H-leucine with no indication of a gradient in the extent of incorporation throughout the slice. The pattern of grain density within 300-600 micron slices matches that previously reported after in vivo injections of radiolabeled amino acid, where grain density is highest over neuronal cell bodies and lower over the laminae that contain dendritic processes and axons (Phillips et al: Mol Brain Res 2:251-261, 1987). Hippocampal slices of 200, 800, and 1,000 micron thickness showed irregular labeling. Slices of 200 micron were filled with pyknotic nuclei and vacuoles and exhibited patchy labeling. In 800 micron slices there were isolated areas of good preservation within the slice core, but these areas exhibited little incorporation. Relative to the 300-600 micron slices, there was a higher number of pyknotic nuclei and a much deeper layer of necrosis along the cut edges. Slices of 1,000 micron thickness showed poor preservation throughout and low levels of incorporation. Biochemical studies revealed a much higher rate of incorporation in the slices prepared from postnatal animals. Autoradiography of the slices from developing rats revealed that penetration was excellent and incorporation appeared to be greater as judged by an overall higher grain density. We believe that rat hippocampal slices provide a good in vitro model of protein metabolism that will be useful for studies of protein synthesis in isolated cell body and dendritic laminae and for the evaluation of whether protein synthesis in particular laminae is regulated by synaptic activity.

Autoradiographic study of the distribution of [3H]gamma-aminobutyrate-accumulating neural elements in guinea-pig intestine: evidence for a transmitter function of gamma-aminobutyrate

High affinity uptake, and the distribution of 3H-radiolabelled gamma-aminobutyrate (GABA), cis-3-aminocyclohexanecarboxylic acid, beta-alanine, proline, and leucine have been examined autoradiographically in laminar preparations of the myenteric plexus from the guinea-pig intestine. Following labelling with [3H]proline and [3H]leucine, which are incorporated into neurons, silver grains were concentrated over recognisable perikarya in the ganglia and meshworks of the plexus, whilst [3H]GABA labelled a smaller proportion of neurons and their processes. Specificity of labelling in the sites of [3H]GABA-uptake was established using combinations of labelled and unlabelled GABA, beta-alanine, and cis-3-aminocyclohexanecarboxylic acid, substrates for glial or neuronal high affinity GABA uptake systems. Only myenteric neurons and their processes were labelled significantly by [3H]GABA and its analogue cis-3-[3H]aminocyclohexanecarboxylic acid. Using autoradiographs of laminar preparations and paraffin sections, [3H]GABA labelling was found over nerve fibre bundles that could be traced from their ganglionic origins through the interconnecting meshworks of the myenteric plexus into the innervation of the deep muscular plexus of the circular muscle layer where GABA is evidently concerned with prejunctional modulation of transmitter release. The extensive but selective distribution of [3H]GABA high affinity uptake sites in neural elements of the guinea-pig myenteric plexus is consistent with GABA being an enteric neurotransmitter.

Growth cones, dying axons, and developmental fluctuations in the fiber population of the cat's optic nerve

We have studied the rise and fall in the number of axons in the optic nerve of fetal and neonatal cats in relation to changes in the ultrastructure of fibers, and in particular, to the characteristics and spatiotemporal distribution of growth cones and necrotic axons. Axons of retinal ganglion cells start to grow through the optic nerve on the 19th day of embryonic development (E-19). As early as E-23 there are 8,000 fibers in the nerve close to the eye. Fibers are added to the nerve at a rate of approximately 50,000 per day from E-28 until E-39--the age at which the peak population of 600,000-700,000 axons is reached. Thereafter, the number decreases rapidly: About 400,000 axons are lost between E-39 and E-53. In contrast, from E-56 until the second week after birth the number of axons decreases at a slow rate. Even as late as postnatal day 12 (P-12) the nerve contains an excess of up to 100,000 fibers. The final number of fibers--140,000-165,000--is reached by the sixth week after birth. Growth cones of retinal ganglion cells are present in the optic nerve from E-19 until E-39. At E-19 and E-23 they have comparatively simple shapes but in older fetuses they are larger and their shapes are more elaborate. As early as E-28 many growth cones have lamellipodia that extend outward from the core region as far as 10 microns. These sheetlike processes are insinuated between bundles of axons and commonly contact 10 to 20 neighboring fibers in single transverse sections. At E-28 growth cones make up 2.0% of the fiber population; at E-33 they make up about 1.0%; from E-36 to E-39 they make up only 0.3% of the population. Virtually none are present in the midorbital part of the nerve on or after E-44. At all ages growth cones are more common at the periphery of the nerve than at its center. This central-to-peripheral gradient increases with age: at E-28 the density of growth cones is two times greater at the edge than at the center but by E-39 the density is four to five times greater. Necrotic fibers are observed as early as E-28 in all parts of the nerve. Their axoplasm is dark and mottled and often contains dense vesiculated structures.(ABSTRACT TRUNCATED AT 400 WORDS)