Recurrent axon collaterals of lumbar motor neurons in turtles

A combined morphophysiological study was made of connections between motoneurons on the superfused isolated lumbar spinal cord of Testudo horsfieldi. Postsynaptic potentials of motoneurons, followed by antidromic stimulations of ventral root filaments (VR-PSPs), were recorded intracellularly. Depolarizing VP-PSPs had short latencies (1.0-1.5 mc) and amplitudes in the range of 0.3-3.0 mV. At the constant stimulus intensity, the fluctuations of amplitudes were recorded. In some motoneurons, hyperpolarizing VP-PSRs with the latencies 2.5-3.0 mc were observed. A possible structural basis of VR-PSPs was studied by the horseradish peroxidase (HRP) method. After HRP application on thin ventral root filaments the retrograde staining of motoneurons revealed recurrent axon collaterals of labeled motoneurons. Three-dimensional computer reconstructions showed one to three collaterals given off by motoneuron axons. There were up to 19 points of branching in a single collateral. In some cases the full length of collateral trees reached 4.0 mm. The collateral branches had up to 72 "en passant" and terminal axon swellings. The swellings (presumed contacting boutons) were distributed in the ventral and intermedial gray matter and in the ventromedial while matter and revealed on motoneurons and inerneurons. These data suggest the participation of the motor axon collaterals in the motoneuron--motoneuron communication in the turtle spinal cord whereas only dendro-dendritic contacts had been discussed earlier.     

Differences in surface molecules of motor axon terminals correlated with cell-cell recognition

The cell-cell interactions leading to the formation of synaptic connections among cells in the nervous system may be mediated by cell surface macromolecules. In the cockroach the specific reformation of the original innervation pattern of a set of leg muscles during axonal regeneration indicates a significant contribution from cell-cell recognition. Macromolecules mediating such a process would be expected to be distributed differentially among the axon terminals of the various motor neurons. Monoclonal antibodies have been isolated that selectively bind to the surfaces of axon terminals of some motor neurons and not others. Preliminary biochemical characterization indicates that these antigens are glycoproteins and are good candidates for consideration as recognition macromolecules.     

Changes in synaptic vesicles of axon terminals of the cat motor cortex after stimulation

The relationship between the size of synaptic vesicles and their distance from the active zone of the synapse was investigated quantitatively in axon terminals on dendritic spines and branches of neurons of the cat motor cortex in a resting state (moderate barbiturate anesthesia) and after prolonged repetitive stimulation of somatosensory area SII. The dimensions of the vesicles belonging to each of the three layers distinguishable in transverse section through the terminals on electron micrographs were recorded as a diminishing variance series. They were characterized by the value of a special rank statistic. Predominance of vesicles of the smallest sizes in layer I, next to the active zone of the synapse, in both the control and the experimental material was established by statistical analysis (using, in particular, the criterion of signs and ²). After stimulation of cortico-cortical projections a significant gradient of decrease in the mean size of the vesicles from peripheral layer III to layer I developed in the terminals studied. Compared with the control, the size of the vesicles decreased both in layer I and in the intermediate layer II. The functional significance of the phenomenon is discussed.A. A. Zhdanov State University, Leningrad. Translated from Neirofiziologiya, Vol. 7, No. 6, pp. 639–646, November–December, 1975.     

Endogenous timing in competitive interactions among relatives

Most evolutionary game theory models solve for equilibrium levels of some behaviour on the restrictive assumptions that players choose their actions simultaneously, and that a player cannot change its action after observing that of its opponent. An alternative framework is provided by sequential or 'Stackelberg' games in which one player commits to a 'first move' and the other has an opportunity to observe this move before choosing its response. Recent interest in the economic literature has focused on Stackelberg games which exhibit 'endogenous timing', i.e. games in which a leader and a follower arise spontaneously as a consequence of each player attempting to maximize its reward. Here, we provide the first demonstration of endogenous timing in an evolutionary context using a simple model of resource competition (the 'tug-of-war' model). We show that whenever two related individuals compete for a share of communal resources, both do best to adopt distinct roles in a sequential game rather than engage in simultaneous competition. Somewhat counterintuitively, the stable solution is for the weaker individual to act as leader and commit to a first move, because this arrangement leads to a lower total effort invested in competition. Endogenous timing offers a new explanation for the spontaneous emergence of leaders and followers in social groups, and highlights the benefits of commitment in social interaction.     

Differences in surface molecules of motor axon terminals correlated with cell–cell recognition

The cell–cell interactions leading to the formation of synaptic connections among cells in the nervous system may be mediated by cell surface macromolecules. In the cockroach the specific reformation of the original innervation pattern of a set of leg muscles during axonal regeneration indicates a significant contribution from cell–cell recognition. Macromolecules mediating such a process would be expected to be distributed differentially among the axon terminals of the various motor neurons. Monoclonal antibodies have been isolated that selectively bind to the surfaces of axon terminals of some motor neurons and not others. Preliminary biochemical characterization indicates that these antigens are glycoproteins and are good candidates for consideration as recognition macromolecules.     

Reciprocal interactions between perisynaptic Schwann cells and regenerating nerve terminals at the frog neuromuscular junction

The perisynaptic Schwann cell (PSC) has gained recent attention with respect to its roles in synaptic function, remodeling, and regeneration at the vertebrate neuromuscular junction (NMJ). Here we test the hypothesis that, following nerve injury, processes extended by PSCs guide regenerating nerve terminals (NTs) in vivo, and that the extension of sprouts by PSCs is triggered by the arrival of regenerating NTs. Frog NMJs were double-stained with a fluorescent dye, FM4-64, for NTs, and fluorescein isothiocyanate (FITC)-tagged peanut agglutinin (PNA) for PSCs. Identified NMJs were imaged in vivo repeatedly for several months after nerve injury. PSCs sprouted profusely beginning 3-4 weeks after nerve transection and, as reinnervation progressed, regenerating NTs closely followed the preceding PSC sprouts, which could extend tens to hundreds of microns beyond the original synaptic site. The pattern of reinnervation was dictated by PSC sprouts, which could form novel routes joining neighboring junctions or develop into new myelinated axonal pathways. In contrast to mammals, profuse PSC sprouting in frog muscles was not seen in response to axotomy alone, and did not occur at chronically denervated NMJs. Instead, sprouting coincided with the arrival of regenerating NTs. Immunofluorescent staining revealed that in muscle undergoing reinnervation 4 weeks after axotomy, 91% of NMJs bore PSC sprouts, compared to only 6% of NMJs in muscle that was chronically denervated for 4 weeks. These results suggest that reciprocal interactions between regenerating NTs and PSCs govern the process of reinnervation at frog NMJs: regenerating NTs induce PSCs to sprout, and PSC sprouts, in turn, lead and guide the elaboration of NTs.     

Frequency-dependence stabilizes competitive interactions among four annual plants

It is the combination of large fitness differences and strong stabilizing mechanisms that often constitute niche-based explanations for species abundance patterns. Despite the importance of this assumption to much of community ecology, empirical evidence is surprisingly limited. Empirical tests are critical because many abundance patterns are also consistent with neutral-based alternatives (that assume no fitness differences or stabilization). We quantified interactions of four annual grassland species in two-species mixtures at varying frequencies. We found evidence of strong negative frequency-dependent stabilization, where scaled population growth rates increased with decreasing frequency for all four species. There was also a consistent competitive hierarchy among these species indicative of strong fitness differences that, in most cases, suggested potential competitive exclusion despite the observed strong stabilization.     

Ultrastructure of synapses with different transmitter-releasing characteristics on motor axon terminals of a crab,Hyas areneas

Synaptic terminals on branches of an excitatory motor axon in a spider crab (Hyas areneas) were examined by electron microscopy to determine whether differences in size, structure, and number of synapses could be correlated with differences in transmitter release. Terminals releasing relatively large amounts of transmitter during low frequencies of nerve impulses ("high-output" terminals) had larger synapses, more prominent presynaptic dense bodies (active zones), and fewer synapses per unit length than terminals releasing relatively small amounts of transmitter ("low-output" terminals). Neither the difference in synaptic area, nor the quantitative differences in the active zones, were sufficient in themselves to explain the difference in synaptic efficacy, and it is postulated that a non-linear relationship may exist between structural features of the synapse and release of transmitter by a nerve impulse, and that differences other than those apparent from the ultrastructure could be involved. Greater facilitation at low-output terminals with high frequencies of nerve impulses may be due to greater reserves of "immediately available" transmitter, and to recruitment or activation of more individual synaptic contacts.     

Structural plasticity of axon terminals in the adult

There is now conclusive evidence for widespread ongoing structural plasticity of presynaptic boutons and axon side-branches in the adult brain. The plasticity complements that of postsynaptic spines, but axonal plasticity samples larger volumes of neuropil, and has a larger impact on circuit remodeling. Axons from distinct neurons exhibit unique ratios of stable (t1/2>9 months) and dynamic (t1/2 5-20 days) boutons, which persist as spatially intermingled subgroups along terminal arbors. In addition, phases of side-branch dynamics mediate larger scale remodeling guided by synaptogenesis. The plasticity is most pronounced during critical periods; its patterns and outcome are controlled by Hebbian mechanisms and intrinsic neuronal factors. Novel experience, skill learning, life-style, and age can persistently modify local circuit structure through axonal structural plasticity.     

Collapsin response mediator protein-2 accelerates axon regeneration of nerve-injured motor neurons of rat

The rat collapsin response mediator protein-2 (CRMP-2) is a member of CRMP family (CRMP-1-5). The functional consequence of CRMP-2 during embryonic development, particularly in neurite elongation, is relatively understood; however, the role in nerve regeneration is unclear. Here we examined the role of CRMP-2 during nerve regeneration using rat hypoglossal nerve injury model. Among the members, CRMP-1, CRMP-2, CRMP-5 mRNA expressions increased after nerve injury, whereas CRMP-3 and CRMP-4 mRNA did not show any significant change. In the N1E-115 cells, CRMP-2 has the most potent neurite elongation activity among the CRMP family members. In dorsal root ganglion (DRG) organ culture, CRMP-2 overexpression by adenoviral vector demonstrated substantial neurite elongation. On the other hand, CRMP-2 (DeltaC381), which acts as a dominant negative form of CRMP-2, inhibited neurite formation. Collectively, it would be plausible that CRMP-2 has potent nerve regeneration activity after nerve injury. We therefore examined whether CRMP-2 overexpression in the injured hypoglossal motor neurons accelerates nerve regeneration. A retrograde-tracer, Fluoro-Gold (FG), was used to evaluate the number of reprojecting motor neurons after nerve injury. CRMP-2-overexpressing motor neurons demonstrated the accelerated reprojection. The present study suggests that CRMP-2 has potent neurite elongation activity in nerve regeneration in vivo.     

Computerized quantification of immunofluorescence-labeled axon terminals and analysis of co-localization of neurochemicals in axon terminals with a confocal scanning laser microscope

The confocal scanning laser microscope (CSLM) offers improved optical resolution and contrast, high photometric precision, and the ability to make optical sections. These benefits were explored for use in quantitative analysis of immunofluorescence-labeled axon terminals. Guidelines were obtained for adjustments of the CSLM parameters. In the present applications, bleaching of the fluorescence did not represent a serious obstacle to analysis with the CSLM. A method was developed to distinguish the background fluorescence from the specific fluorescence labeling. This procedure made way for the development of automated quantification of immunolabeled axon terminals. The automated procedures substantially reduced the man-hour expenditure for analysis and provided highly reproducible quantifications compared with manual methods. The increased resolution and contrast of the CSLM allowed measurements of the fluorescence signal strength of individual axon terminals. The CSLM also allowed detection of co-localized neurochemicals in axon terminals.     

Distribution of motor cortical neuron synaptic terminals on monkey parvocellular red nucleus neurons.

We determined the location of 54 horseradish peroxidase (HRP)-labeled motor cortical neuron synaptic terminals on 17 parvocellular neurons in the monkey red nucleus. Synaptic terminals and their postsynaptic elements were identified and reconstructed, using light- and electron-microscopic techniques, from serial thick and thin sections. Terminals were found on proximal and distal dendrites of small and medium-sized parvocellular neurons, where they formed excitatory synapses. Some were 180 microns from cell somata. Approximately half of the labeled terminals, aside from those located at dendritic origins, were situated strategically at or near dendritic branch points. Since monkey parvocellular neurons show little activity during movement, the obvious next question is this: How and in what way does motor cortex influence these cells?     

Experimental small-scale grassland fragmentation alters competitive interactions among ant species

Different species may respond differently to habitat fragmentation. Theory predicts that abundant generalist species should be less affected by fragmentation than specialist species. In ant communities, the most abundant species is often behaviourally dominant. Thus, habitat fragmentation could alter competitive interactions between the dominant ant species and the other species. We tested this hypothesis in a long-term grassland fragmentation experiment. Fragments of different size (20.25 and 2.25 m²) were isolated by a 5-m wide strip of frequently mown vegetation. Control plots were situated in adjacent undisturbed grassland. Ant density and species composition were assessed 3 and 6 years after initiation of the experimental fragmentation. The effect of the dominant ant species on the resource use of the other species was examined at natural sugar resources (aphids and extrafloral nectaries) and at artificial sugar baits. Lasius paralienus was the most abundant ant species (72% of nests) in the grasslands examined. Species richness and forager density in the other species decreased with increasing density of L. paralienus in fragments but not in control plots. The overall forager density of the other species was positively related to their habitat niche overlap with L. paralienus. The density of foragers of the other species at sugar resources was not affected by L. paralienus forager density. The experimental fragmentation resulted in an increase in natural sugar resources in fragments. This may have reduced the intensity of interspecific competition for sugar resources. Our study shows that the grassland fragmentation altered interactions between the dominant L. paralienus and the other ant species.Electronic Supplementary Material Supplementary material is available for this article at     

3.6 kb genomic sequence from Takifugu capable of promoting axon growth-associated gene expression in developing and regenerating zebrafish neurons

Unlike mammals, fish have the capacity for functional adult CNS regeneration, which is due, in part, to their ability to express axon growth-related genes in response to nerve injury. One such axon growth-associated gene is gap43, which is expressed during periods of developmental and regenerative axon growth, but is not expressed in CNS neurons that do not regenerate in adult mammals. We previously demonstrated that cis-regulatory elements of gap43 that are sufficient for developmental expression are not sufficient for regenerative expression in the zebrafish. Here we have identified a 3.6kb genomic sequence from Fugu rubripes that can promote reporter gene expression in the nervous system during both development and regeneration in zebrafish. This compact sequence is advantageous for functional dissection of regions important for axon growth-associated gene expression during development and/or regeneration. In addition, this sequence will also be useful for targeting gene expression to neurons during periods of growth and plasticity.     

Network interactions among sensory neurons in the leech

Interactions among mechanosensory neurons, sensitive to touch, pressure and nociceptive stimuli in the leech nervous system were studied in isolated ganglia and in body-wall preparations. Pairs of touch-pressure, touch-nociceptive and pressure-nociceptive neurons were tested by suprathreshold stimulation of one neuron while recording the response of the other, in both directions. Pressure and nociceptive stimulation evoked depolarizing and hyperpolarizing responses in touch cells, mediated by interneurons. The relative expression of these responses depended on the stimulus duration. One or two pressure cell spikes produced, predominantly, a depolarization of the touch cells, and increasing number of spikes evoked a hyperpolarization. Nociceptive cells produced primarily the hyperpolarization of touch cells at any stimulus duration. When touch cells were induced to fire by injection of positive current into the soma, stimulation of pressure cells inhibited touch cell activity. However, when touch cells were induced to fire by peripheral stimulation, pressure cell activation failed to inhibit touch cell firing. The results suggest that excitation of pressure and nociceptive cells would not limit the responses of touch cells to peripheral stimuli, but would inhibit the firing of touch cells evoked by their central connectivity network.     

Vasopressinergic axon collaterals and axon terminals in the magnocellular neurosecretory nuclei of the rat hypothalamus

Axon collaterals emerging from the vasopressinergic neurons of the supraoptic (SON) and paraventricular (PVN) nuclei and recurving back towards their respective nuclei have been previously reported. Since such axon collaterals can play a role in the neuromodulation of SON and PVN, these nuclei have been further investigated immunohistochemically under the light and electron microscope. The PAP technique, using a commercial antibody, was employed. Vasopressin-positive axon collaterals were seen to recurve towards their nuclei of origin. In the latter, vasopressinergic intrinsic neurons were also observed. Under the electron microscope, axon terminals containing vasopressin-immunoreactive neurosecretory granules were noted. Such terminals presumably arise from the vasopressin-positive recurrent axon collaterals or from the intrinsic neurons for the purpose of neuromodulation within the SON and PVN.     

Displacement of synaptic terminals from regenerating motoneurons by microglial cells

Summary Axonal reaction of motoneurons has been shown to be usually accompanied by an early and brisk proliferation of perineuronal microgliacytes. In order to clarify the real nature of such newly formed microglial satellites and their fine structural relationships to the regenerating nerve cells, facial nuclei from bilateral preparations were examined by light and electron microscopy 4 days after cutting the right facial nerve in rats. On the transected side, microgliacytes could often be observed closely adjoining motoneuron perikarya and main dendrites over long distances, and thereby removing morphologically intact synaptic terminals from the neuronal surface membranes. This displacement of boutons by microglial cells is probably preceded by a loosening of the synaptic contacts due to some unknown membrane changes in the regenerating motoneurons. The functional significance of this considerable deafferentation process could not be entirely elucidated.