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PubMed1978

Categories of axons in mammalian rami communicantes. Part II.

The mammalian rami communicantes consist of preganglionic, postganglionic and sensory axons. The present study focusses on the number of sensory axons in the midthoracic rami communicantes in the cat. The conclusions are that the white rami contain approximately 250 myelinated and 200 unmyelinated sensory axons and the grey rami 4 to 5 myelinated and 20 to 25 unmyelinated sensory axons. A previous study determined the numbers of pre- and postganglionic fibers in these same rami. Thus the total number of axons in these rami are as follows: for the grey rami there are: (1) 3,000 to 5,000 unmyelinated postganglionic axons, (2) 25 myelinated postganglionic axons, (3) 20 to 25 unmyelinated sensory axons and; (4) 4 to 5 myelinated sensory axons; and for the white rami there are approximately: (1) 1,000 unmyelinated preganglionic axons, (2) 1,500 myelinated preganglionic axons, (3) 2,000 unmyelinated postganglionic axons, (4) 250 unmyelinated sensory axons and (5) 200 myelinated sensory axons. The implications of these findings are discussed.

Animals↗
PubMed1983

Computer-assisted analyses of barrel neuron axons and their putative synaptic contacts.

The "barrels" in layer IV of rodent SmI neocortex receive inputs from individual whiskers on the contralateral face. Previous analyses of neuronal morphology in mouse and rat barrel cortex, as revealed by Golgi impregnations, have focused on the dendritic patterns of the stellate cells. The cells can be classified into two groups: Class I cells with spiny dendrites and Class II cells with smooth, beaded dendrites. These classes can be subdivided further according to somal position and spatial distribution of dendrites with respect to barrel cytoarchitectonic boundaries. In the present study the axons of these cells were examined and the locations of close appositions to dendrites of other impregnated neurons were mapped. All data are taken from Golgi-Cox preparations, cut parallel to layer IV at 140 microns, counterstained with Nissl to reveal the barrels, and measured with a computer-microscope. Axons which had extensive branching within the section (present on 10% of all impregnated cells) were chosen for measurement. The analysis of the axons revealed: (1) Class I axons are thin and directed to the white matter with recurrent collaterals in the barrels, while Class II axons are thick, frequency beaded, and directed toward the pia before cascading down into the barrels; (2) in layer IV, the axons of both cell classes tend to be as restricted to a barrel as the dendrites of the same cell are (i.e., most axons are confined to one barrel); (3) within layer IV, the Class II cell axons have a total length about three times that of Class I cell axons, and about four times as many branch points. The analysis of the appositions of these axons to impregnated dendrites of other cells revealed: (1) A majority of "contacts" tended to be made by terminal branches of the axonal trees. (2) For the Class I neurons, a greater number of appositions occur near the distal ends of complete dendritic segments. As measured from the "contacted" cell soma, appositions are more or less uniformly distributed along dendritic trees. (3) No striking patterns are found, such as an obvious propensity for axons of one cell type to prefer or avoid another cell type. These results show that the axons of barrel cells of each class are as consistent and distinctive as their dendritic trees. Specifically, the cells in each class can be distinguished by their axonal patterns on purely numerical bases.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1987

Mode of growth of retinal axons within the tectum of Xenopus tadpoles, and implications in the ordered neuronal connection between the retina and the tectum.

Retinal axons of Xenopus tadpoles at various stages of larval development were filled with horseradish peroxidase (HRP), and their trajectories and the patterns of branching within the tectum were analyzed in wholemount preparations. To clarify temporal and spatial modes of growth of retinal axons during larval development, special attention was directed to labeling a restricted regional population of retinal axons with HRP, following reported procedures (H. Fujisawa, K. Watanabe, N. Tani, and Y. Ibata, Brain Res. 206:9-20, 1981; 206:21-26, 1981; H. Fujisawa, Dev. Growth Differ 26:545-553, 1984). In developing tadpoles, individual retinal axons arrived at the tectum, without clear sprouting. Axonal sprouting first began when growing tips of each retinal axon had arrived at the vicinity of its site of normal innervation within the tectum. Thus, the terminals of the newly added retinal axons were retinotopically aligned within the tectum. The retinotopic alignment of the terminals may be due to an active choice of topographically appropriate tectal regions by growth cones of individual retinal axons. The stereotyped alignment of the newly added retinal axons was followed by widespread axonal branching and preferential selection of those branches. Each retinal axon was sequentially bifurcated within the tectum, and old branches that had inevitably been left at ectopic parts of the tectum (owing to tectal growth) were retracted or degenerated in the following larval development. The above mode of axonal growth provides an adequate explanation of cellular mechanisms of terminal shifting of retinal axons within the tectum during development of retinotectal projection. Selection of appropriate branches may also lead to a reduction in the size of terminal arborization of retinal axons, resulting in a refinement in targeting.

Animals↗
PubMed1992

Nerve growth factor-induced sprouting of mature, uninjured sympathetic axons.

The infusion of nerve growth factor (NGF) into the lateral ventricle of the mature rat brain elicits a sprouting response from axons associated with the intradural segment of the internal carotid artery. Using electron microscopic techniques, we observed a three-fold increase in the total number of perivascular axons. This NGF-elicited response is characterized by a dramatic reduction in glial cell ensheathment similar to that observed during development and by the presence of profiles devoid of organelles that may represent newly formed sprouts. In spite of the increase in axon number, no significant changes in the percentage of small, medium, or large axons were observed. The three-fold increase in the total number of axons was accompanied by an increase in the number of axons/fascicle but no change in the number of fascicles. This, along with the observation that a majority of sprouted axons were associated with other axons, supports the idea that the sprouted axons tend to associate preferentially with other axons. Bilateral superior cervical ganglionectomies following cytochrome C infusion indicate that approximately 60% of the axons associated with the internal carotid artery arise from the superior cervical ganglion and that the majority of axons contacting the smooth muscle layer arise from this ganglion. Sympathectomy following NGF infusion resulted in a 79% reduction in the total number of perivascular axons, demonstrating overwhelmingly that the majority of sprouted axons are sympathetic fibers. These results demonstrate that infusion of NGF into the mature rat brain results in the preferential sprouting of sympathetic axons associated with the internal carotid artery. These findings are consistent with the hypothesis that NGF normally plays a role in the regulation of autonomic cerebrovascular innervation in the adult animal and that mature, uninjured sympathetic neurons remain responsive to NGF.

Animals↗
PubMed1993

Distribution of transitory corpus callosum axons projecting to developing cat visual cortex revealed by DiI.

Functional studies of the development of the corpus callosum in the cat have shown that an intact callosum during postnatal month 1 is necessary for normal visual development. In vivo tracing techniques have not provided enough information on corpus callosum connectivity to fully evaluate the evidence for a morphological mechanism for the functional effects of neonatal callosum section. However, lipophilic in vitro membrane tracers permit a more detailed search for such evidence because the entire limit of many cells can be labeled simultaneously. To investigate the morphological basis for the observed functional results in cats, the corpus callosum was labeled in vitro with the carbocyanine dye, DiI. Crystals of DiI were placed in the midsagittal callosum in tissue from 2 to 277-day-old cats. Tissue was coronally sectioned 3-22 months later. Sections were photographed and reconstructed to show the overall distribution of corpus callosum projections, as well as the locations of individual corpus callosum axons and their presumed terminals. The distribution of corpus callosum projections, examined in cortical areas 17-19, 7, and posterior medial lateral suprasylvian cortex, changes significantly during development. During postnatal week 1, callosal axons extend throughout these cortical areas to layer I. Numerous varicosities on callosal axons are located en passant and at axon terminals in layer I. During postnatal week 2, the density of callosal projections is reduced in all cortical areas, although many axons still extend to layer I. By postnatal month 2, the callosal axons extending to layer I are predominantly near the border with adjacent cortical areas; in the nonborder regions of these areas, many axons extend to layer VI while a much smaller number of axons extend to layers II-V. By postnatal month 3, the callosal projections to supragranular layers are almost exclusively restricted to cytoarchitectonic border regions; in the remaining regions, including medial area 17, there are occasional axons extending to the supragranular layers and only a moderate number of axons extending to infragranular layers. Thus, a substantial number of elaborately formed transitory corpus callosum axons, distributed throughout visual cortex, exist for several weeks during postnatal development; in area 17, these axons are found in central through peripheral visual field representations. The transitory callosal axons appear to have axon terminals in layer I as well as en passant terminals while extending through layers II-VI. If some of these terminals were to form synapses, there would be extensive opportunities for the corpus callosum to provide input to layers I-VI throughout visual cortex during the period of development in which cortical microcircuitry is being established.

Aging↗
PubMed1994

Development of specificity in corticospinal connections by axon collaterals branching selectively into appropriate spinal targets.

Corticospinal projections in adult rodents arise exclusively from layer V neurons in the sensorimotor cortex. These neurons are topographically organized in their connections to spinal cord targets. Previous studies in rodents have shown that the mature distribution pattern of corticospinal neurons develops during the first 2 weeks postnatal from an initial widespread pattern that includes the visual cortex to a distribution restricted to the sensorimotor cortex. To determine whether specificity in corticospinal connections also emerges from an initially diffuse set of projections, we have studied the outgrowth of corticospinal axons and the formation of terminal arbors in developing hamsters. The sensitive fluorescent tracer 1,1',dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) was used to label corticospinal axons from the visual cortex or from small regions of the forelimb or hindlimb sensorimotor cortex in living animals at 4-17 days postnatal. Initially axon outgrowth was imprecise. Some visual cortical axons extended transiently beyond their permanent targets in the pontine nuclei, by growing through the pyramidal decussation and in some cases extending as far caudally as the lumbar enlargement. Forelimb sensorimotor axons also extended past their targets in the cervical enlargement, in many cases growing in the corticospinal tract to lumbar levels of the cord. By about 17 days postnatal these misdirected axons or axon segments were withdrawn from the tract. Despite these errors in axon trajectories within the corticospinal tract, terminal arbors branching into targets in the spinal gray matter were topographically appropriate from the earliest stages of innervation. Thus visual cortical axons never formed connections in the spinal cord, forelimb sensorimotor axons arborized only in the cervical enlargement, and hindlimb cortical axons terminated only in the lumbar cord at all stages of development examined. Corticospinal arbors formed from collaterals that extended at right angles from the shafts of primary axons, most likely by the process of interstitial branching after the primary growth cone had extended past the target. Once collaterals extended into the spinal gray matter, highly branched terminal arbors formed within 2-4 days, beginning at about 4 and 8 days postnatal for the cervical and lumbar enlargements, respectively. These results show that specificity in corticospinal connectivity is achieved by selective growth of axon collaterals into appropriate spinal targets from the beginning and not by the later remodeling of initially diffuse connections. In contrast, errors occur in the initial outgrowth of axons in the corticospinal tract, which are subsequently corrected.

Animals↗
PubMed1995

Regenerative axonal sprouting in the cat trochlear nerve.

Following peripheral trochlear nerve axotomy in the cat, the normal number of myelinated axons is restored despite significant motor neuron death, suggesting regulation of the number of myelinated axons in the regenerated nerve. In this study we used light and electron microscopy to examine the production and maintenance of axonal sprouts at different locations in the nerve and at different postoperative intervals. Despite proliferative sprouting and an overproduction of nonmyelinated axons in the regenerating trochlear nerve, the number of myelinated axons was strictly regulated. Only approximately 1,000 regenerated axons were eventually remyelinated, but many nonmyelinated axons were still present 6-8 months postaxotomy. Regenerated axons were remyelinated in a proximal-to-distal direction between 3 and 4 weeks postaxotomy. We also examined the maturation of regenerated myelinated axons by measuring axon diameter and myelin index (an expression of myelin thickness). Mean myelinated axon diameter remained significantly below normal in long-term regenerated nerves. Mean myelin index was not different from normal at 4 weeks postaxotomy but was significantly decreased at long postoperative intervals, reflecting a slightly thicker myelin sheath relative to the axon diameter. This relative increase in mean myelin thickness could serve to restore normal conduction velocity despite the decrease in mean axon diameter. We suggest that the regulation of the number of myelinated axons at the normal number despite cell death and the increase in mean myelin thickness may both be compensatory mechanisms that function to restore preoperative conditions and maximize functional recovery.

Animals↗
PubMed2001

Axon guidance at the midline choice point.

The central nervous system (CNS) of higher organisms is bilaterally-symmetric. The transfer of information between the two sides of the nervous system occurs through commissures formed by neurons that project axons across the midline to the contralateral side of the CNS. Interestingly, these axons cross the midline only once. Other neurons extend axons that never cross the midline; they project exclusively on their own (ipsilateral) side of the CNS. Thus, the midline is an important choice point for several classes of pathfinding axons. Recent studies demonstrate that specialized midline cells play critical roles in regulating the guidance of both crossing and non-crossing axons at the ventral midline of the developing vertebrate spinal cord and the Drosophila ventral nerve cord. For example, these cells secrete attractive cues that guide commissural axons over long distances to the midline of the CNS. Furthermore, short-range interactions between guidance cues present on the surfaces of midline cells, and their receptors expressed on the surfaces of pathfinding axons, allow commissural axons to cross the midline only once and prevent ipsilaterally-projecting axons from entering the midline. Remarkably, the molecular composition of commissural axon surfaces is dynamically-altered as they cross the midline. Consequently, commissural axons become responsive to repulsive midline guidance cues that they had previously ignored on the ipsilateral side of the midline. Concomitantly, commissural axons lose responsiveness to attractive guidance cues that had initially attracted them to the midline. Thus, these exquisitely regulated guidance systems prevent commissural axons from lingering within the confines of the midline and allow them to pioneer an appropriate pathway on the contralateral side of the CNS. Many aspects of midline guidance are controlled by mechanistically and evolutionarily-conserved ligand-receptor systems. Strikingly, recent studies demonstrate that these receptors are modular; the ectodomains determine ligand recognition and the cytoplasmic domains specify the response of an axon to a given guidance cue. Despite rapid and dramatic progress in elucidating the molecular mechanisms that control midline guidance, many questions remain.

Animals↗
PubMed2006

Zebrafish bashful/laminin-alpha 1 mutants exhibit multiple axon guidance defects.

Laminin is known to provide a highly permissive substratum and in some cases directional information for axon outgrowth in vitro. However, there is still little known about laminin function in guiding axons in vivo. We investigated the axon guidance role of laminin-alpha1 in the developing zebrafish nervous system. Analysis of zebrafish bashful (bal)/laminin-a1 mutants revealed multiple functions for laminin-alpha1 in the outgrowth and guidance of central nervous system (CNS) axons. Most CNS axon pathways are defective in bal embryos. Some axon types, including retinal ganglion cell axons, early forebrain axons, and hindbrain reticulospinal axons, make specific pathfinding errors, suggesting laminin-alpha1 is required for directional decisions. Other axon tracts are defasciculated or not fully extended in bal embryos, suggesting a function for laminin-alpha1 in regulating adhesion or providing a permissive substratum for growth. In addition, some neurons have excessively branched axons in bal, indicating a potential role for laminin-alpha1 in branching. In contrast to CNS axons, most peripheral axons appear normal in bal mutants. Our results, thus, reveal important and diverse functions for laminin-alpha1 in guiding developing axons in vivo.

Alleles↗
PubMed1990

The vocal motor neurons of Xenopus laevis: development of sex differences in axon number.

Sex differences in the number of muscle fibers in the larynx of clawed frogs (Xenopus laevis) develop after metamorphosis. In order to examine possible contributions of neural innervation to this process, we prepared sections of the laryngeal nerve from tadpole stage 56, when the sexes can first be distinguished, through adulthood, and counted axons on electron micrographs. The adult number of axons is achieved by a sexually differentiated pattern of axonal addition and loss. Axon numbers are high at tadpole stage 56 and equal for males and females; thereafter, males have more axons. Sex differences are most pronounced at tadpole stage 62 because between stages 59 and 62 the number of laryngeal axons in males increases by an average of 119 per nerve. Ultrastructural evidence is congruent with the hypothesis that new axons are added to the laryngeal nerve between tadpole stages 56 and 62. The loss of axons from the laryngeal nerve is greater for females than for males. Between tadpole stages 56 and adulthood, overall axon number decreases by 47% in males and by 64% in females. Signs of axonal degeneration are present in both sexes before metamorphosis but are rare at juvenile or adult stages. The numbers of axons in juvenile frogs do not differ from those in adults and continue to be greater in males than in females. In contrast to the amount of axon addition and loss, the timing of axon loss and the percentage of myelinated axons is the same for males and females throughout development. Thus sex differences in the innervation of laryngeal muscle originate before metamorphosis and could contribute to the marked sex differences in muscle fiber addition that occur thereafter.

Animals↗
PubMed1996

Intraretinal grafting reveals growth requirements and guidance cues for optic axons in the developing avian retina.

To study environmental factors controlling the growth and navigation of optic axons in the eye, grafts of retinal, optic disc, optic tectum, and floor plate tissue were transplanted into organ-cultured embryonic chick or quail eyes. The growth of axons into and out of the graft was studied in cross sections of the cultured eyes and by DiI tracing in retinal whole mounts. Based on the location and trajectory of axons and based on the quantity of axons that entered and exited the grafts, several requirements for axonal navigation were established: (1) Axonal growth is restricted to an approximately 10-microm-thick layer at the vitreal surface of the retina. (2) The retinal neuroepithelium prior to axogenesis is nonpermissive for neurite outgrowth. This nonpermissive quality is transient and recedes peripherally as the differentiation of the retina progresses. (3) Embryonic axons are able to grow into neonatal and adult retinal grafts, demonstrating that older retina remains permissive for axonal growth. (4) The trajectory of axons into and from retinal grafts that had been rotated in their peripheral-central orientation showed that the retina has an inherent polarity that permits axon growth toward and away from the optic disc, but does not allow axon growth perpendicular to this direction. This centroperipheral cue operates locally rather than by long distance. (5) The optic disc provides an exit for the axons from the retina, but has no detectable neurotropic activity. Finally, optic axons from the host retina readily enter grafts of their target tissue, the optic tectum, but few axons are able to leave tectal transplants.

Animals↗
PubMed1990

Dual projections of secondary vestibular axons in the medial longitudinal fasciculus to extraocular motor nuclei and the spinal cord of the squirrel monkey.

Recordings were made from secondary vestibular axons in the medial longitudinal fasciculus (MLF) of barbiturate-anesthetized squirrel monkeys. Antidromic stimulation techniques were used to identify the axons as belonging to one of three classes of neurons: vestibulo-oculo-collic (VOC) neurons project both to the extraocular motor nuclei and to the spinal cord; vestibulo-ocular (VO) neurons do not have a spinal projection; and vestibulocollic (VC) neurons do not have an oculo-motor projection. Galvanic stimulation was used to show that axons of all three classes received excitatory inputs from one labyrinth and inhibitory inputs from the other. VOC axons were confined to the MLF contralateral to the labyrinth from which they were excited. They made up more than half of the vestibular axons descending in the contralateral medial vestibulospinal tract (MVST), but less than one-quarter of those ascending in the contralateral MLF to the level of the oculomotor nucleus. Spinal projections were restricted to cervical segments with about half of the axons reaching segment C6. Conduction velocities, measured for C6-projecting axons, were similar for VOC and VC axons and were typically 25-50 m/s. Unlike the situation in the rabbit (Akaike et al. 1973) and cat (Akaike 1983), none of the MVST axons had conduction velocities greater than 75 m/s. The morphology of VOC neurons was studied by injection of horseradish peroxidase (HRP) into 60 physiologically identified axons in the MLF. Since individual axons were only stained for short distances, it was not possible to ascertain their complete branching patterns. Labeled fibers could be traced to an origin in and around the ventral lateral vestibular nucleus. This localization was confirmed by comparing the distributions within the vestibular nuclei of neurons retrogradely labeled from the upper cervical spinal cord (this study) and from the oculomotor nucleus (McCrea et al. 1987a; Highstein and McCrea 1988). VOC axons reached the contralateral MLF at the level of the abducens nucleus and immediately divided into an ascending and a descending, usually thicker, branch. Seven VOC axons could be traced to the extraocular motor nuclei; three terminated in the medial aspect of the oculomotor nucleus bilaterally and four terminated in the medial aspect of the contralateral abducens nucleus. The former axons may be part of a crossed, excitatory anterior-canal pathway; the latter, part of a similar horizontal-canal pathway. There were no terminations in the trochlear nucleus even though 12 labeled fibers passed close to it.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1975

Degeneration of sensory and motor axons in transplanted segments of a crustacean peripheral nerve.

Segments of sensory and motor axons 0.3-0.5 mm in length were taken from crayfish peripheral limb nerves and transplanted into the abdominal cavity of the same animal. Transplanted sensory axons showed relatively few ultra-structural changes after one week, many had undergone complete lysis within two weeks, and almost all degenerated within three weeks. Transplanted motor axons appeared normal after one week, except for some hypertrophy of their surrounding glial sheaths. After two weeks, glial sheaths were grossly hypertrophied around motor axons; axonal mitochondria had increased in number and many had migrated from the periphery to the centre of the axon. The axonal membranes of all motor axons were still intact after three weeks, although most were no longer continuous after four weeks. By five weeks, all axonal material had completely disintegrated. These data suggest that axonal synthetic processes in crayfish sensory (and presumably motor) axons can maintain the axons relatively intact for 7-14 days and that transfer of substances form hypertrophied glial cells to motor axons may account for the longer survival times of transplanted motor axons.

Animals↗
PubMed1986

Dorsal root axonal regeneration in the adult frog spinal cord. A model of vertebrate CNS regeneration.

The frog dorsal root provides a useful model for the study of axonal regeneration in an adult vertebrate CNS. We have used the model to compare the regeneration of two very different types of axons within the same CNS environment and have found that regenerating dorsal root, as well as rerouted motoneuron axons, display similar growth patterns in the spinal cord. Both sensory and motor axons grow preferentially in some regions and not in others. They both regenerate effectively longitudinally as well as radially within the dorsolateral fasciculus (DLF). By contrast, fewer sensory and motor axons regenerate longitudinally or radially in the dorsal funiculus (DF). This similar preferential growth of two very different populations of axons suggests that the growth patterns reflect regional differences in the cellular environment of the cord. The DLF has fascicles of unmyelinated axons separated by radial glial processes and, after dorsal root injury, is mildly gliotic. By contrast, DF has very large myelinated axons, which widely separate the radial glial processes that traverse the region. After dorsal root injury, this region is markedly gliotic and contains myelin, debris and oligodendroglia, and microglial macrophages. Our data suggest that unmyelinated axons and radial glial processes are more preferred substrates for axonal growth than myelin debris, oligodendroglia and macrophages. It is not surprising, then, that regions of the adult mammalian CNS that are characterized by large myelinated axons fail to support axonal growth. Moreover, there is some evidence that regions of the adult mammalian CNS that are characterized by unmyelinated axons support axonal growth.

Animals↗
PubMed1998

Transient axonal side branches in the developing mammalian optic nerve.

Optic axons were labelled with horseradish peroxidase to establish the presence of side branches and examine their distribution and morphology in the developing optic nerve of the quokka wallaby, Setonix brachyurus, the cat and rat at stages when axon numbers are at their peak. In each species, three quarters of the axons were essentially straight and lacked side branches. The remaining axons took significantly longer paths and bore side branches, mostly at points where axons undulated or changed direction. Side branches occurred at intervals of 28-43 micron, had lengths of 2-3 micron and were usually simple rather than branched. A minority (1%) of the axons crossed diagonally between fascicles and two thirds of these had more side branches (interval: 10-18 microm) on the interfascicular portion than were found on the forward-directed axons. A small number of axons (0.01%) doubled back to grow retrogradely towards the eye, these axons also bore relatively more side branches (interval: 8-22 micron), especially at points where the axons changed direction. Ultrastructural reconstruction showed that side branches resembled small axonal profiles and constituted 2% of the total axon number. It is suggested that side branches are involved in the fine-tuning of growth cone navigation. Most side branches are lost by adulthood, indicating their transient nature. The absence of retrogradely-directed axons from adults suggests that cells with such axons are removed by naturally occurring cell death.

Age Factors↗
PubMed1987

Increased dopamine-beta-hydroxylase-like immunoreactivity in non-noradrenergic axons supplying the guinea-pig uterine artery after 6-hydroxydopamine treatment.

We have reinvestigated the immunohistochemistry of autonomic axons supplying the guinea-pig uterine artery to determine whether non-noradrenergic paracervical ganglion neurons projecting to the artery contain immunoreactivity to dopamine-beta-hydroxylase (DBH) or somatostatin (SOM) in addition to neuropeptide Y (NPY) and vasoactive intestinal peptide (VIP). In untreated arteries no VIP axons had immunoreactivity to tyrosine hydroxylase (TH), although 9% had immunoreactivity to DBH. Somatostatin immunoreactivity was detected in 25% of non-noradrenergic axons containing NPY and VIP. After in vivo treatment with 6-hydroxydopamine (6-OHDA), noradrenergic axons containing immunoreactivity to NPY, DBH and TH were absent from the adventitia-medial junction. However, 65-70% of the non-noradrenergic axons with NPY and VIP showed DBH immunoreactivity after 6-OHDA. These axons did not show catecholamine fluorescence after incubation with pargyline together with noradrenaline, dopamine or L-DOPA. The number of axons with SOM immunoreactivity increased by 44% after 6-OHDA treatment, but only 24% of SOM axons had DBH immunoreactivity. Surgical destruction of the non-noradrenergic autonomic axons in 6-OHDA-treated animals led to the loss of all DBH immunoreactivity. These results demonstrate that DBH immunoreactivity can be detected in a small proportion of non-noradrenergic axons supplying uterine arteries from untreated animals. After chemical sympathectomy with 6-OHDA, the levels of DBH immunoreactivity in axons of non-noradrenergic neurons increased, and more axons with DBH immunoreactivity were detected. DBH immunoreactivity seemed to increase preferentially in axons with NPY and VIP, but not SOM. The number of NPY, VIP axons containing SOM also increased after 6-OHDA. These findings demonstrate that peripheral neurons containing several different potential neurotransmitters can change their levels of neuropeptides and transmitter-synthesizing enzymes in response to local environmental changes.

Animals↗
PubMed1990

Localized calcium influx orients axon formation in embryonic hippocampal pyramidal neurons.

A fundamental property of neurons is their polarization into distinct axonal and dendritic compartments which have characteristic structural and functional properties. The mechanisms regulating the formation of neuronal polarity are unknown. We used cultured embryonic rat hippocampal pyramidal neurons to test the hypothesis that a localized calcium influx can orient axon formation, and thereby direct the establishment of neuronal polarity. Transection of an initial axon, or focal application of A23187 or K+ to the initial axon, caused a new axon to form at a site distant from the initial axon. Fura-2 measurements of intracellular calcium revealed a localized calcium influx at the site of axon transection or focal application of A23187 or K+, and a calcium gradient spreading into the soma. New axon formation was inhibited when axons were transected in medium lacking calcium or containing calcium-elevating agents (conditions which prevented the formation of a calcium gradient). When calcium ionophore A23187 was applied focally to neurons which had not yet established an axon, the axon always formed at a site distant from the site of ionophore application; bath exposure to A23187 prevented axon formation. Taken together, these data demonstrate that a localized influx of calcium can suppress axon formation at the site of influx, and can thereby influence where the axon forms. These data suggest that gradients of intracellular calcium may be involved in orienting neuronal polarity.

Animals↗
PubMed1990

Target recognition and dynamics of axonal growth in the retinotectal system of fish.

Embryonic and regenerating retinal axons in fish are able to seek out their retinotopic target sites in the tectum. Neither a specific preordering of axons in the retinotectal pathway nor activity-dependent axon-target interactions are required for appropriate axonal targeting. Axon-target recognition appears to be predominantly mediated by positional cell surface markers. The discrimination of position-dependent differences by retinal axons in a special in vitro assay is consistent with this concept. To understand retinal axonal regeneration we have analyzed the glial cells of the fish optic nerve and the expression of growth-associated cell surface molecules on the regenerating axons. The surfaces of the glial cells identified as oligodendrocytes are excellent substrates for the elongation of regenerating axons. Raising monoclonal antibodies we have found 3 cell surface proteins specific for growing axons. In the normal adult goldfish optic nerve, these proteins are only expressed by the few new axons from the newborn ganglion cells at the retinal margin. They are re-expressed on all axons during regeneration. A known cell surface molecule, NCAM, is expressed in a similar, specific spatiotemporal pattern on the fish retinal axons and may--in normal nerves--contribute to the establishment of the age-related fiber association. Whether the re-expression of NCAM and the antigens detected by the novel monoclonal antibodies are functionally involved in axonal growth and regeneration remains to be investigated.

Animals↗