Central peptidergic neurons are hyperactive during collateral sprouting and inhibition of activity suppresses sprouting.

Little is known regarding the effect of chronic changes in neuronal activity on the extent of collateral sprouting by identified CNS neurons. We have investigated the relationship between activity and sprouting in oxytocin (OT) and vasopressin (VP) neurons of the hypothalamic magnocellular neurosecretory system (MNS). Uninjured MNS neurons undergo a robust collateral-sprouting response that restores the axon population of the neural lobe (NL) after a lesion of the contralateral MNS (). Simultaneously, lesioned rats develop chronic urinary hyperosmolality indicative of heightened neurosecretory activity. We therefore tested the hypothesis that sprouting MNS neurons are hyperactive by measuring changes in cell and nuclear diameters, OT and VP mRNA pools, and axonal cytochrome oxidase activity (COX). Each of these measures was significantly elevated during the period of most rapid axonal growth between 1 and 4 weeks after the lesion, confirming that both OT and VP neurons are hyperactive while undergoing collateral sprouting. In a second study the hypothesis that chronic inhibition of neuronal activity would interfere with the sprouting response was tested. Chronic hyponatremia (CH) was induced 3 d before the hypothalamic lesion and sustained for 4 weeks to suppress neurosecretory activity. CH abolished the lesion-induced increases in OT and VP mRNA pools and virtually eliminated measurable COX activity in MNS terminals. Counts of the total number of axon profiles in the NL revealed that CH also prevented axonal sprouting from occurring. These results are consistent with the hypothesis that increased neuronal activity is required for denervation-induced collateral sprouting to occur in the MNS.     

Expression of a glycosyl phosphatidylinositol-anchored adhesion molecule, the glycoprotein F3, in the adult rat hypothalamo-neurohypophysial system

The F3 cell surface glycoprotein consists of six immunoglobulin-like domains, four fibronectin type III repeats and a glycosyl-phosphatidylinositol anchor and is found in membrane-bound and soluble form. Until now, it has been localized mainly on axons of subsets of developing and postnatal neurons and has been implicated in axonal growth and synaptogenesis. We here examined its expression in the adult rat hypothalamo-neurohypophysial system composed of magnocellular neurons whose axons can undergo remodelling in adulthood in response to lesion or physiological stimulation. Immunoblot analyses demonstrated high levels of F3 immunoreactivity in the hypothalamic nuclei containing the somata of the neurons, in the median eminence, through which pass their axons and in the neurohypophysis, where they terminate. The amount of F3 detected in the latter was 2-fold that in the hypothalamus. In addition, soluble forms predominated in the neurohypophysis and GPI-linked forms in the hypothalamus. Immuncytochemistry revealed a strong F3 immunoreactivity throughout the neurohypophysis and internal layer of the median eminence, characterized by a punctate labeling of fibers and dense filling of dilatations. In the hypothalamic nuclei, staining of variable intensity was visible in the cytoplasm of some magnocellular somata. In contrast, in colchicine-treated rats, all magnocellular somata throughout the hypothalamus displayed intense labeling while staining in the neurohypophysis was greatly reduced. Our observations reveal that neurons of the adult hypothalamo-neurohypophysial system express high level of F3, even under normal conditions. In view of its distribution and the differing proportions of membrane-bound and soluble forms, we propose that, after synthesis in the hypothalamus, F3 is targeted to the neurohypophysis where it accumulates in neurosecretory terminals or is released into the extracellular space. It remains to be seen whether its expression is linked to the secretion of the neurohypophysial peptides and in particular, to the ability of these neurons to undergo structural remodelling in adulthood.     

Interleukin-1beta immunoreactivity in identified neurons of the rat magnocellular neurosecretory system: evidence for activity-dependent release

Interleukin-1beta has been demonstrated in neurons of the rat hypothalamus, including cells of the magnocellular neurosecretory system and tuberoinfundibular system (Lechan et al., [1990] Brain Res. 514:135-140). Despite its potential importance to regulation of neuroendocrine function, however, neither the specific cell types that express interleukin-1beta or the conditions that may result in its release have yet been described. Therefore, we utilized a combination of immunocytochemical and immunoelectron microscopic localization, in conjunction with Western blot analysis, on normonatremic, hypernatremic, and lactating rats to assess the site of synthesis and potential secretion characteristics of interleukin-1beta in the rat magnocellular neurosecretory system. Interleukin-1beta immunoreactivity was localized within both oxytocin and vasopressin neurons in the paraventricular, supraoptic, accessory and periventricular hypothalamic nuclei. Additionally, interleukin-1beta immunoreactive fibers were localized in the zona interna and zona externa of the median eminence and in the neurohypophysis. Immunoelectron microscopic analysis revealed that interleukin-1beta immunoreactivity is associated with small spherical structures, distinct from neurosecretory granules, in neurosecretory axons within the neurohypophysis. Furthermore, stimulation of heightened neurosecretory activity via chronic osmotic challenge and lactation resulted in a marked diminution in levels of interleukin-1beta immunoreactivity in the neurohypophysis with a subsequent return to normal levels after cessation of the stimuli. Western blot analysis confirmed the existence of interleukin-1beta protein in the neurohypophysis and provided further evidence for reduction in levels of IL-1beta immunoreactivity after stimulation of secretory activity. These results suggest an endogenous neuronal source of interleukin-1beta exists within the rat magnocellular neurosecretory system under normal physiological conditions. The potential for activity-dependent release of IL-1beta and implications for the involvement of interleukin-1beta in regulation of neurosecretory activity are discussed.     

Characterization and distribution of a new cell surface marker of neuronal precursors

In a screen for novel cell surface markers of neuronal progenitors, we recently identified mAb 2F7 that recognizes an epitope present on both progenitor cells and postmitotic neurons, in the developing CNS and PNS. In the embryonic rat telencephalon, the mAb 2F7 epitope is expressed by migratory and postmigratory neurons in the developing cerebral cortex, as well as by presumptive neuronal progenitor cells of the ventricular zone. In the neonatal forebrain mAb 2F7 labels postmitotic neurons, including those of the developing cerebral cortex and olfactory bulb, as well as the axons of the corpus callosum. While mAb 2F7 immunoreactivity is present on only a low density of the neuronal progenitor cells situated in the anterior part of the subventricular zone, a progressively higher proportion of cells forming the rostral migratory stream express this epitope. mAb 2F7 labels the surfaces of neurons and neuronal precursors, but not mature oligodendrocytes and astrocytes in primary cultures derived from the rat neural tube. In vivo, migrating neural crest cells, motor neurons, and axonal projections associated with the spinal cord express the mAb 2F7 epitope. Immunoblot analyses reveal that the mAb 2F7 epitope resides on several high-molecular-weight, membrane-associated proteins, and is likely to be composed of N-linked carbohydrate. These findings suggest that mAb 2F7 recognizes a novel epitope that is present on progenitor cells and postmitotic, differentiating neurons in the developing mammalian nervous system.     

Development of visual cortical axons: layer-specific effects of extrinsic influences and activity blockade

During normal cortical development, individual pyramidal neurons form intracortical axonal arbors that are specific for particular cortical layers. Pyramidal neurons within layer 6 are able to develop layer-specific projections in cultured slices of ferret visual cortex, indicating that extrinsic influences, including patterned visual activity, are not required (Dantzker and Callaway [1998] J Neurosci 18:4145-4154). However, when spontaneous activity is blocked in cultures with tetrodotoxin, layer 6 pyramidal neurons fail to preferentially target their axons to layer 4. To determine whether mechanisms that regulate the development of layer 6 pyramidal neuron arbors can be generalized to pyramidal neurons in other layers, we examined the development of layer 5 and layer 2/3 pyramidal neurons in cultured slices of ferret visual cortex prepared on postnatal day 14 or 15. Layer 5 pyramidal neurons developed layer-specific axonal arbors during 5-7 days in vitro. However, unlike layer 6 pyramidal neurons, layer 5 pyramidal neurons formed layer-specific axonal arbors in the presence of tetrodotoxin. In contrast to layer 5 and layer 6 pyramidal neurons, layer 2/3 pyramidal neurons did not form appropriate layer-specific projections during 5-7 days in vitro. Taken together, these data suggest that the development of layer-specific axons is regulated by different mechanisms for neurons in different layers and cannot be categorically classified as either activity-dependent or independent. Instead, the type of pyramidal neuron, the layers targeted, and the type of activity must be considered.     

Taurine immunoreactivity in the rat supraoptic nucleus: Prominent localization in glial cells

Taurine is an inhibitory amino acid that hyperpolarizes magnocellular neurosecretory neurons. To determine which cell types in the rat supraoptic nucleus contain taurine, we used a monoclonal antibody raised against a taurine conjugate. Preembedding immunocytochemistry was carried out at the light and electron microscopic levels using diaminobenzidine and gold-substituted silver-intensified peroxidase as markers. We report the presence of taurine in all cellular compartments of the supraoptic nucleus, except axons, with variable labeling intensities among the different compartments. Few cell bodies of magnocellular neurons were immunoreactive, but many distal dendrites and some proximal ones showed weak-to-moderate levels of immunoreactivity. Strong immunoreactivity was found over glial cell bodies and their processes, in particular in the ventral glial lamina of the supraoptic nucleus. Large astrocytic processes labeled with the taurine antibody included the endfeet participating in the glial limitans around capillaries and at the ventral surface of the hypothalamus. Other types of immunoreactive astropytic profiles were found scattered within the neuropil where these processes participated in different interactions with the neuronal elements of the supraoptic nucleus. Immunoreactive glial expansions, sometimes even the main process of the glial cell, engulfed axonal boutons. Other labeled glial processes were found between two magnocellular perikarya or closely apposed to the membrane of axonal boutons contacting the neuronal cell bodies. The frequent finding of closely apposed glial and dendritic elements bearing different levels of taurine-like immunoreactivity suggests that exchange of taurine between those two compartments may occur. We propose that taurine could be released from supraoptic glia by a small decrease in osmolarity or by receptor-mediated mechanisms during conditions of low hormonal (vasopressin and/or oxytocin) needs. Such released taurine could then act on presynaptic or postsynaptic sites, or both, to exert its neuromodulatory actions. © 1995 Wiley-Liss, Inc.     

Axon-axon interactions in neuronal circuit assembly: lessons from olfactory map formation

During the development of the nervous system, neurons often connect axons and dendrites over long distances, which are navigated by chemical cues. During the past few decades, studies on axon guidance have focused on chemical cues provided by the axonal target or intermediate target. However, recent studies have shed light on the roles and mechanisms underlying axon-axon interactions during neuronal circuit assembly. The roles of axon-axon interactions are best exemplified in recent studies on olfactory map formation in vertebrates. Pioneer-follower interaction is essential for the axonal pathfinding process. Pre-target axon sorting establishes the anterior-posterior map order. The temporal order of axonal projection is converted to dorsal-ventral topography with the aid of secreted molecules provided by early-arriving axons. An activity-dependent process to form a discrete map also depends on axon sorting. Thus, an emerging principle of olfactory map formation is the 'self-organisation' of axons rather than the 'lock and key' matching between axons and targets. In this review, we discuss how axon-axon interactions contribute to neuronal circuit assembly.     

Ferroportin in the postnatal rat brain: implications for axonal transport and neuronal export of iron

This study mapped the distribution of ferroportin in the developing rat brain using Wistar rats aged postnatal (P) days P7, P21, and P70 (adult). Ferroportin immunoreactivity was observed in neurons throughout the CNS regardless of the age of the animals studied. The neuronal labeling was detected in both perikarya, and axons and dendrites. The labeling intensity within the neurons varied among the different ages of the rats with an overall higher ferroportin immunoreactivity seen at P21, particularly in axons and white matter tracts. The neuronal labeling was high in the neocortex, striatum, hippocampus, brain stem nuclei, deep cerebellar nuclei, catecholaminergic neurons, and reticular nuclei, and particularly high in neurons of the mesencephalic trigeminal nucleus and medial habenular nucleus. In axonal tracts, ferroportin immunoreactivity was high in fibers of the internal capsule, fimbria, mamillothalamic tract and the habenulo-interpedunculo pathway. Slight ferroportin immunoreactivity was observed in oligodendrocytes and differentiating macrophages that invade the postnatal brain. The finding of a pronounced content of ferroportin in axons of the developing brain are in keeping with the idea of elevated axonal transport and export of iron possibly because of higher metabolic needs.     

Survival and regeneration of neurons of the supraoptic nucleus following surgical transection of neurohypophysial axons depend on the existence of collateral projections of these neurons to the dorsolateral hypothalamus

The aim of the present study was to compare the postlesional responses of vasopressin-producing (VP) and oxytocin-producing (OT) neurons of the supraoptic nucleus (SON) to transection of neurohypophysial axons. At different times after sectioning the median eminence of adult rats, immunocytochemical staining of both types of neuronal cell bodies and axons indicated that: (1) the number of OT neurons detected within the SON was only slightly decreased as compared with controls (−20%), whereas the number of VP neurons was severely decreased (−60%); and (2) the large majority of axonal sprouts that regenerated into the external layer of the median eminence were OT neurohypophysial axons. The injection of a retrograde tracer into various areas surrounding the SON further showed that numerous SON neurons could be retrogradely labeled when the injection was centered in the lateral hypothalamus dorsal to the SON. The immunocytochemical identification of these retrogradely labeled neurons demonstrated that most of them were OT neurons. When animals were subjected to median eminence transection and to a unilateral surgical cut placed in the lateral hypothalamus above the SON, the survival of both OT and VP neurons was dramatically reduced in the SON ipsilateral to the hypothalamic lesion, as compared to the contralateral SON. Taken together, these data indicate that OT (and to a lesser extent VP) neurons of the SON display collateral projections towards the lateral hypothalamus that protect them from retrograde degeneration following the lesion of their neurohypophysial projections.     

Rho/Rho-kinase signaling pathway controls axon patterning of a specified subset of cranial motor neurons

Cranial motor neurons, which are divided into somatic motor (SM), branchiomotor (BM) and visceral motor (VM) neurons, form distinct axonal trajectories to innervate their synapse targets. Rho GTPase regulates various neuronal functions through one of the major effector proteins, Rho-kinase. Here, we addressed the in vivo role of the Rho/Rho-kinase signaling pathway in axon patterning of cranial motor neurons. We performed conditional expression of a dominant-negative mutant for RhoA or Rho-kinase in transgenic mice by using the Cre-loxP system to suppress the activity of these molecules in developing cranial motor neurons. Blockade of the Rho/Rho-kinase signaling pathway caused defects in the patterning of SM axons but not in that of BM/VM axons, in which defects were accompanied by reduced muscle innervation and reduced synapse formation by SM neurons. In addition, blockade of the signaling pathway shifted the trajectory of growing SM axons in explant cultures, whereas it did not appear to affect the rate of spontaneous axonal outgrowth. These results indicate that the Rho/Rho-kinase signaling pathway plays an essential role in the axon patterning of cranial SM neurons during development.     

Immunoreactivity of β-amyloid precursor protein in amyotrophic lateral sclerosis

We investigated the localization and extent of β-amyloid precursor protein (β-APP₆₉₅) immunoreactivity as a sensitive marker for impairment of fast axonal transport in the spinal cords of 21 patients with amyotrophic lateral sclerosis (ALS), paying special attention to anterior horn neurons. Specimens from 18 patients without neurological disease served as controls. Increased β-APP immunoreactivity was frequently recognized in the anterior horns of the ALS patients with short clinical courses or with mild depletion of anterior horn cells, while no β-APP immunoreactivity was demonstrated in those with severe depletion of anterior horn neurons or with long-standing clinical courses. Increased β-APP immunoreactivity in the anterior horn neurons was mainly confined to the perikarya and no immunoreactivity was recognized in the dendrites or proximal axons directly emanating from the somata, except some spheroids (proximal axonal swellings) which showed increased immunoreactivity of β-APP. Increased β-APP immunoreactivity was spotted or focally aggregated in the perikarya of normal-looking large anterior horn neurons, while it was frequently diffuse in that of degenerative neurons such as central chromatolytic cells and or those with simple atrophy. On the other hand, the controls showed no immunostaining with β-APP in the spinal cord. These findings suggest that increased immunoreactivity of β-APP in neuronal perikarya of the anterior horn cells and in some proximal axonal swellings is an early change of ALS, and may be a response of the increased synthesis of β-APP resulting from neuronal damage, or the impairment of fast axonal transport. Received: 27 August 1998 / Revised, accepted: 4 November 1998     

Colocalization of neuronal NO synthase with urotensins I and II in the caudal neurosecretory neurons and the urophysis of the teleost oreochromis niloticus: a gold immunoelectron microscopic study

The intracellular distribution of neuronal nitric oxide synthase (nNOS) was studied in the caudal neurosecretory system of a teleost, Oreochromis niloticus (Cichlids), by means of post-embedding immunogold labeling with a polyclonal antibody directed against nNOS of human origin. Ultrastructural examination demonstrated that neuronal NOS-like molecules are distributed within the Dahlgren cell perikarya, the neurosecretory axons, and the urophysial axon terminals. In the neurosecretory somata, gold particles for nNOS were mainly cytosolic, whereas in the neurosecretory axons and axon terminals they were associated with the membrane and/or the dense core of neurosecretory granules. Double immunogold labelings for nNOS/urotensin I (UI) and nNOS/urotensin II (UII) demonstrated that nNOS-like molecules are colocalized with UI and/or UII in the neurosecretory granules contained within the urophysial terminals. The present findings suggest that both a soluble cytosolic and a particulate neuronal NOS are expressed in the caudal neurosecretory neurons. They confirm previous biochemical data on the same species.     

Palmitoyl protein thioesterase 1 is targeted to the axons in neurons

Palmitoyl protein thioesterase 1 (PPT1) is a depalmitoylating enzyme whose deficiency leads to infantile neuronal ceroid lipofuscinosis. The disease is characterized by early loss of vision and massive neuronal death. Although PPT1 is expressed in many tissues, a deficiency of PPT1 damages neurons only in the cerebral and cerebellar cortexes and retina; other cell types remain relatively unaffected. We previously demonstrated that PPT1 is present in the synaptosomes and synaptic vesicles of neurons. To understand the crucial role of PPT1 for neuronal cells, we further investigated the expression and targeting of PPT1 in retinal, hippocampal, and cortical neurons during their maturation in culture. We found that PPT1 activity increases by neuronal maturation and is highest in retinal neuron cultures. In retinal neurons the expression of PPT1 precedes that of the synaptic vesicle protein 2 and synaptophysin, indicating a significant role for PPT1 in the early development of neuronal cells. We also found by quantitative confocal immunofluorescence microscopy that PPT1 is targeted preferably to axons in mature neurons, as indicated by its colocalization with the axonal marker microtubule-associated protein 1. In axons PPT1 is targeted specifically to axonal varicosities and presynaptic terminals, as indicated by its significant colocalization with growth-associated protein 43 and synaptophysin. Axonal localization of PPT1 was confirmed by double labeling with synaptophysin and postembedding immunoelectron microscopy. The polarized axonal targeting of PPT1 may well indicate a role for PPT1 in the exocytotic pathway of neurons.     

Robo2-Slit and Dcc-Netrin1 Coordinate Neuron Axonal Pathfinding within the Embryonic Axon Tracts

In the embryonic vertebrate brain, early born neurons establish highly stereotyped embryonic axonal tracts along which the neuronal interconnections form. To understand the mechanism underlying neuron axonal pathfinding within the embryonic scaffold of axon tracts, we studied zebrafish anterior dorsal telencephalic (ADt) neuron development. While previous studies suggest the ADt neuronal axons extend along a commissural tract [anterior commissure (AC)] and a descending ipsilateral tract [supraoptic tract (SOT)], it is unclear whether individual ADt neuronal axons choose specific projection paths at the intersection between the AC and the SOT. We labeled individual ADt neurons using a forebrain-specific promoter to drive expression of fluorescent proteins. We found the ADt axonal projection patterns were heterogeneous and correlated with their soma positions. Our results suggest that cell intrinsic differences along the dorsal ventral axis of the telencephalon regulate the axonal projection choices. Next, we determined that the guidance receptors roundabout2 (Robo2) and deleted in colorectal cancer (Dcc) were differentially expressed in the ADt neurons. We showed that knocking down Robo2 function by injecting antisense morpholino oligonucleotides abolished the ipsilateral SOT originating from the ADt neurons. Knocking down Dcc function did not prevent formation of the AC and the SOT. In contrast, the AC was specifically reduced when Netrin1 function was knocked down. Further mechanistic studies suggested that Robo2 responded to the repellent Slit signals and suppressed the attractive Netrin signals. These findings demonstrate how Robo2-Slit and Dcc-Netrin coordinate the axonal projection choices of the developing neurons in the vertebrate forebrain.     

Rubrospinal neurons fail to respond to brain-derived neurotrophic factor applied to the spinal cord injury site 2 months after cervical axotomy

Numerous experimental therapies to promote axonal regeneration have shown promise in animal models of acute spinal cord injury, but their effectiveness is often found to diminish with a delay in administration. We evaluated whether brain-derived neurotrophic factor (BDNF) application to the spinal cord injury site 2 months after cervical axotomy could promote a regenerative response in chronically axotomized rubrospinal neurons. BDNF was applied to the spinal cord in three different concentrations 2 months after cervical axotomy of the rubrospinal tract. The red nucleus was examined for reversal of neuronal atrophy, GAP43 and Talpha1 tubulin mRNA expression, and trkB receptor immunoreactivity. A peripheral nerve transplant paradigm was used to measure axonal regeneration into peripheral nerve transplants. Rubrospinal axons were anterogradely traced and trkB receptor immunohistochemistry performed on the injured spinal cord. We found that BDNF treatment did not reverse rubrospinal neuronal atrophy, nor promote GAP-43 and Talpha1 tubulin mRNA expression, nor promote axonal regeneration into peripheral nerve transplants. TrkB receptor immunohistochemistry demonstrated immunoreactivity on the neuronal cell bodies, but not on anterogradely labeled rubrospinal axons at the injury site. These findings suggest that the poor response of rubrospinal neurons to BDNF applied to the spinal cord injury site 2 months after cervical axotomy is not related to the dose of BDNF administered, but rather to the loss of trkB receptors on the injured axons over time. Such obstacles to axonal regeneration will be important to identify in the development of therapeutic strategies for chronically injured individuals.     

The actin-binding protein profilin I is localized at synaptic sites in an activity-regulated manner

Morphological changes at synaptic specializations have been implicated in regulating synaptic strength. Actin turnover at dendritic spines is regulated by neuronal activity and contributes to spine size, shape and motility. The reorganization of actin filaments requires profilins, which stimulate actin polymerization. Neurons express two independent gene products - profilin I and profilin II. A role for profilin II in activity-dependent mechanisms at spine synapses has recently been described. Although profilin I interacts with synaptic proteins, little is known about its cellular and subcellular localization in neurons. Here, we investigated the subcellular distribution of this protein in brain neurons as well as in hippocampal cultures. Our results indicate that the expression of profilin I varies in different brain regions. Thus, in cerebral cortex and hippocampus profilin I immunostaining was associated predominantly with dendrites and was present in a subset of dendritic spines. In contrast, profilin I in cerebellum was associated primarily with presynaptic structures. Profilin I immunoreactivity was partially colocalized with the synaptic molecules synaptophysin, PSD-95 and gephyrin in cultured hippocampal neurons, indicating that profilin I is present in only a subset of synapses. At dendritic spine structures, profilin I was found primarily in protrusions, which were in apposition to presynaptic terminal boutons. Remarkably, depolarization with KCl caused a moderate but significant increase in the number of synapses containing profilin I. These results show that profilin I can be present at both pre- and postsynaptic sites and suggest a role for this actin-binding protein in activity-dependent remodelling of synaptic structure.     

Ischemic axonal injury and its recovery after focal cerebral ischemia

After focal cerebral infarction by occluding the middle cerebral artery (MCA) of the rat, the neuronal death occurred in the ipsilateral thalamic neurons, because axons of the thalamic neurons were injured by infarction and retrograde degeneration occurred in the thalamic neurons. However, cortical neurons adjacent to the infarction survived despite their axons injured by ischemia. We employed immunohistochemical staining for 200 kilodalton (kD) neurofilament (NF), in order to study those responses of cortical and thalamic neurons against axonal injury caused by focal cerebral infarction. In the sham operated rats the immunoreactivity to the anti-200 kD NF antibody was only detected in the axon but not in the cell bodies and dendrites. At 3 days after MCA occlusion, axonal swelling proximal to the site of ischemic injury was found in the caudoputamen and internal capsule of the ipsilateral side. At 7 days after occlusion, cell bodies and dendrites of the neurons in the ipsilateral cortex and thalamus were strongly stained with anti-NF antibodies. At 2 weeks after occlusion these responses disappeared in the cortex, but lasted in the thalamus. These phenomena are caused by stasis of the slow axonal transport, because the NF is transported by slow axonal transport. In the cortical neurons impairment of slow axonal transport recovered in the early phase after injury, but in the thalamic neurons the impairment prolonged up to 3 weeks after occlusion. The early recovery of axonal transport from ischemia seemed to be essential for survival of neurons after ischemic axonal injury.     

c-Jun expression after axotomy of corneal trigeminal ganglion neurons is dependent on the site of injury

The proto-oncogene c-Jun has been implicated in the control of neuronal responses to injury and in axonal growth during regenerative processes. We have investigated the expression of c-Jun during normal terminal remodelling in trigeminal ganglion neurons innervating the cornea and after acute injury of epithelial nerve terminals or parent axons. Remodelling and rearrangement, or damage limited to corneal epithelium endings, was not a trigger for activation of c-Jun expression. However, injury of parent axons in the stroma or in the orbital ciliary nerves induced c-Jun expression in 50% of the population of corneal neurons, which included all of the large myelinated and 20% of the small neuropeptide-containing corneal neurons. This suggests that c-Jun expression in trigeminal ganglion neurons is not associated with normal remodelling or regeneration of peripheral nerve terminals, and that it takes place only when parent axons are injured. A substantial number of damaged neurons do not express c-Jun, indicating that in primary sensory neurons, injury and regeneration may not always be coupled to the expression of this proto-oncogene.     

Ascorbate transport by primary cultured neurons and its role in neuronal function and protection against excitotoxicity

Neurons maintain relatively high intracellular concentrations of ascorbic acid, which is achieved primarily by the activity of the sodium-dependent vitamin C transporter SVCT2. In this work, we studied the mechanisms by which neuronal cells in culture transport and maintain ascorbate as well as whether this system contributes to maturation of neuronal function and cellular defense against oxidative stress and excitotoxic injury. We found that the SVCT2 helps to maintain high intracellular ascorbate levels, normal ascorbate transport kinetics, and activity-dependent ascorbate recycling. Immunocytochemistry studies revealed that SVCT2 is expressed primarily in the axons of mature hippocampal neurons in culture. In the absence of SVCT2, hippocampal neurons exhibited stunted neurite outgrowth, less glutamate receptor clustering, and reduced spontaneous neuronal activity. Finally, hippocampal cultures from SVCT2-deficient mice showed increased susceptibility to oxidative damage and N-methyl-D-aspartate-induced excitotoxicity. Our results revealed that maintenance of intracellular ascorbate as a result of SVCT2 activity is crucial for neuronal development, functional maturation, and antioxidant responses.     

Activity-dependent modulation of nerve terminal excitation in a mammalian peptidergic system.

Transmitter release at nerve terminals is triggered by voltage-gated calcium influx upon arrival of an action potential. Changes in coupling between axonal impulse discharge and depolarization of the nerve ending might therefore regulate exocytotic secretion. In the magnocellular neurosecretory system of the rat, the pattern of electrical activity can modify the duration of action potentials within individual nerve endings, and alter the spatial spread of excitation into the branching terminal field of neurohypophysial axons. The observed effects are consistent with the effects of firing rate and pattern on peptide release, indicating that activity-dependent changes in axon-terminal coupling may modulate secretion in this neuroendocrine system.