The segmental precision of the motor projection to the intercostal muscles in the developing chicken embryo. A differential labelling study using fluorescent tracers

Each skeletal muscle in the vertebrate is innervated by a group of motoneurons called a motoneuron pool. Retrograde labelling of single motoneuron pools has suggested that the arrangement of motoneuron pools innervating different limb muscles does not change during the embryonic period when more than 50% of the motoneurons die. In this study we retrogradely labelled neighbouring intercostal motoneuron pools differentially with latex microspheres or dextran amines coupled to fluorescent dyes. We then mapped the positions of the differentially labelled motoneurons in whole-mount preparations using a computer-aided drawing system. While the intercostal motoneuron pools are clearly segregated even at early stages, there is some intermingling at the rostral and caudal ends. We used a logistic regression to determine the extent of segmental overlap, and to facilitate a quantitative comparison of the overlap at different stages. Statistical analysis shows that the overlap (expressed as the percentage of the length of the overlapping motoneuron pools) decreases modestly during the period of motoneuron death. Computer simulations suggest that this decrease does not result from random motoneuron death alone; one alternative possibility is selective death of motoneurons in the overlap zone. Occasional "rogue" motoneurons, that is, motoneurons of one pool that scatter into the neighbouring pool, are still present at the end of the period of cell death, representing a potential source of "noise" in the establishment of segmental patterns of connectivity.     

Spatial and temporal features of recurrent facilitation among motoneurons innervating synergistic muscles of the cat

1. The temporal features and strength of recurrent facilitatory potentials were examined in pairs of lumbosacral motoneurons that were separated by a known distance and were identified by antidromic stimulation of muscle nerves. One motoneuron was stimulated by injecting depolarizing current pulses, and responses were recorded in the second motoneuron. The distance between motoneurons in pairs was also measured to assess the spatial distribution in strength of recurrent facilitation in motor pools. All motoneurons in these pairs innervated muscles that act as hip or ankle extensors. 2. Recurrent facilitatory potentials were found frequently among motoneurons innervating the hindlimb extensor muscles examined. Several categories of recurrent facilitatory responses were identified. One category was composed of facilitation responses that followed an inhibition response. A second category was composed of facilitation responses that were not preceded by a significant inhibition and consisted of a monophasic response. There was a considerable range of latencies in this category. 3. Responses in which recurrent facilitatory potentials were preceded by recurrent inhibitory postsynaptic potentials (RIPSPs) among close motoneuron pairs demonstrated an inverse correlation between the durations of the facilitatory and the inhibitory phases. In addition, the duration of inhibition responses without facilitation was longer on average, than the duration of inhibitory responses that were followed by facilitation. It was suggested that recurrent facilitation may restrict the time course of RIPSPs. 4. In contrast to the topographic distribution of RIPSPs described in the previous report, amplitudes of monophasic facilitations were directly correlated with the distance separating motoneurons in pairs, rather than inversely correlated as was the case for RIPSP amplitudes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dual mechanisms of twitching during sleep in neonatal rats.

Twitches of the limbs during REM sleep in adult mammals result from descending motor activation from the brainstem. In contrast, many spontaneous movements in embryos appear similar to REM-related twitches and result from the local firing of spinal motor neurons. To determine which mechanism produces twitches in neonates, we analyzed twitching in 5- and 8-day-old rat pups that had spinal cords transected in the lower thoracic region. This transection separated motor units controlling forelimb movements from motor units controlling hindlimb movements. Spinal transection did not significantly affect the amount of forelimb twitching. In contrast, the amount of hindlimb twitching in transected pups was reduced by only 35%–50%. Given that hindlimb twitching was not eliminated by spinal transection, it is concluded that there are 2 independent mechanisms producing twitches at these ages. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Hepatocyte growth factor (HGF/SF) is a muscle-derived survival factor for a subpopulation of embryonic motoneurons

Muscle-derived factors are known to be important for the survival of developing spinal motoneurons, but the molecules involved have not been characterized. Hepatocyte growth factor/scatter factor (HGF/SF) plays an important role in muscle development and motoneuron axon outgrowth. We show that HGF/SF has potent neurotrophic activity (EC50=2 pM) for a subpopulation (40%) of purified embryonic rat motoneurons. Moreover, HGF/SF is an essential component of muscle-derived support for motoneurons, since blocking antibodies to HGF/SF specifically inhibited 65% of the trophic activity of media conditioned by C2/C7 skeletal myotubes, but did not inhibit the trophic activity secreted by Schwann cell lines. High levels of expression of the HGF/SF receptor c-Met in the spinal cord are restricted to subsets of motoneurons, mainly in limb-innervating segments. Consistent with this distribution, cultured motoneurons from limb-innervating brachial and lumbar segments showed a more potent response to HGF/SF than did thoracic motoneurons. By the end of the period of motoneuron cell death, levels of c-Met mRNA in motoneurons were markedly reduced, suggesting that the effects of HGF/SF may be limited to the period of motoneuron cell death. HGF/SF may play an important role during motoneuron development as a muscle-derived survival factor for a subpopulation of limb-innervating motoneurons.     

Physical and mental health in later life: the self-system as mediator

The distribution and morphology of motoneurons innervating specific types of muscle fibers in the levator scapulae superior (LSS) muscle complex of the bullfrog (Rana catesbeiana) and tiger salamander (Ambystoma tigrinum) were studied by retrograde labelling with cholera toxin-conjugated horseradish peroxidase (CT-HRP). The LSS muscle complex in both of these amphibians has a segregated pattern of muscle-fiber types (tonic; fast oxidative-glycolytic twitch [FOG]; fast glycolytic twitch [FG]) along an anteroposterior axis. The entire motor pool was labelled by injection of CT-HRP into the whole LSS muscle complex. The motoneurons innervating specific fiber types were labelled by injection of CT-HRP into certain muscle regions. The organization of the motoneuron pool of the LSS complex of both species was arranged in two columns--one ventrolateral and one medial. In bullfrogs, the ventrolateral column contains motoneurons innervating FG and tonic fiber types and the medial column contains motoneurons innervating FOG fiber types. In tiger salamanders, the ventrolateral column contains motoneurons innervating FG fiber types and the medial column contains motoneurons innervating FOG and tonic fiber types. The different motoneuron types also have different soma sizes and patterns of dendritic arborization. In both species, FG motoneurons are the largest, whereas FOG motoneurons are intermediate in size and tonic motoneurons are the smallest. In bullfrogs, the main dendrites of FG motoneurons extend into the dorsolateral and the ventrolateral gray matter of the spinal cord, whereas the dendrites of FOG motoneurons extend into the ventral and medial cord. In the tiger salamander, dendrites of FG motoneurons extend into the ventrolateral spinal cord and dendrites of the FOG motoneurons extend more generally into the ventral cord. Dendrites of tonic motoneurons in both amphibians were small and short, and difficult to observe. These results establish that motoneurons innervating different types of muscle fibers in the LSS muscle complex are segregated spatially and display consistent morphological differences.     

MyoD and Myf-5 differentially regulate the development of limb versus trunk skeletal muscle

The myogenic progenitors of epaxial (paraspinal and intercostal) and hypaxial (limb and abdominal wall) musculature are believed to originate in dorsal-medial and ventral-lateral domains, respectively, of the developing somite. To investigate the hypothesis that Myf-5 and MyoD have different roles in the development of epaxial and hypaxial musculature, we further characterized myogenesis in Myf-5- and MyoD-deficient embryos by several approaches. We examined expression of a MyoD-lacZ transgene in Myf-5 and MyoD mutant embryos to characterize the temporal-spatial patterns of myogenesis in mutant embryos. In addition, we performed immunohistochemistry on sectioned Myf-5 and MyoD mutant embryos with antibodies reactive with desmin, nestin, myosin heavy chain, sarcomeric actin, Myf-5, MyoD and myogenin. While MyoD(-/-) embryos displayed normal development of paraspinal and intercostal muscles in the body proper, muscle development in limb buds and brachial arches was delayed by about 2.5 days. By contrast, Myf-5(-/-) embryos displayed normal muscle development in limb buds and brachial arches, and markedly delayed development of paraspinal and intercostal muscles. Although MyoD mutant embryos exhibited delayed development of limb musculature, normal migration of Pax-3-expressing cells into the limb buds and normal subsequent induction of Myf-5 in myogenic precursors was observed. These results suggest that Myf-5 expression in the limb is insufficient for the normal progression of myogenic development. Taken together, these observations strongly support the hypothesis that Myf-5 and MyoD play unique roles in the development of epaxial and hypaxial muscle, respectively.     

Survival of newly postmitotic motoneurons is transiently independent of exogenous trophic support

We compared the survival requirements of early- and late-born motoneurons from E5 chicken spinal cord. Density gradient centrifugation followed by immunopanning using SC1 antibody allowed us to purify two size classes of motoneuron. Large motoneurons retained by 6.8% metrizamide were shown by BrdU labeling in ovo to be born on average 1.5 d earlier than the small motoneurons recovered from the metrizamide pellet. Large motoneurons were both biochemically and functionally more mature: they expressed higher levels of choline acetyltransferase and low-affinity neurotrophin receptor, and had an acute requirement for trophic support from muscle-derived factors. After 24 hr in culture in basal medium, all early-born motoneurons died, whereas 60% of late-born motoneurons survived. Small motoneurons can develop into large motoneurons in ovo, suggesting that they represent a general transitional stage in motoneuron development. Our results suggest that a defined period elapses between birth of a motoneuron and its acquisition of trophic dependence, possibly corresponding to the time required for target innervation. This property may have important consequences for the timing and regulation of developmental motoneuron death.     

High-affinity aptamers selectively inhibit human nonpancreatic secretory phospholipase A2 (hnps-PLA2)

Evidence is growing that reactive oxygen species (ROS), by-products of (normal) cellular aerobic metabolism, are involved in the pathogenesis of neurodegenerative diseases. One of these diseases is amyotrophic lateral sclerosis (ALS), in which motoneurons die, leading to paralysis and death. It remains uncertain whether ROS are the cause of (apoptotic) motoneuron death in ALS. To further understand the role of ROS in motoneuron death, we investigated the effects of ROS on isolated spinal rat motoneurons in culture. ROS were generated with a combination of iron(III) and ascorbate, or with hydrogen peroxide. Both toxic treatments resulted in a dose-dependent motoneuron death. Iron(III)/ascorbate toxicity was completely prevented with the hydrogen peroxide detoxifying enzyme catalase and partially prevented with the antioxidant vitamin E. SOD1, the enzyme that removes superoxide, did not protect against iron(III)/ascorbate toxicity. ROS treatment caused apoptotic motoneuron death: low doses of iron(III)/ ascorbate or hydrogen peroxide resulted in complete apoptosis ending in nuclear fragmentation, while high doses of ROS resulted in incomplete apoptosis (nuclear condensation). Thus, depending on the dose of ROS, the motoneurons complete the apoptotic pathway (low dose) or are stopped somewhere during this route (high dose).     

Gait analysis of adult paraplegic rats after spinal cord repair

This study presents a novel detailed method of analysis of rat gait and uses this method to demonstrate recovery of forward locomotion patterns in adult rats made paraplegic by surgical spinal cord transection and subjected to a novel strategy for spinal cord repair. Six normal rats were compared to five animals in which the cord was transected at T8-T9, and a 5-mm segment of the spinal cord removed, and to seven animals in which, following spinal cord transection and removal of a spinal cord segment, multiple intercostal peripheral nerve bridges were implanted, rerouting pathways from white to gray matter in both directions. The implanted area was filled with fibrin glue containing acidic fibroblast growth factor. Details of the repair strategy have been published (H. Cheng, Y. Cao, and L. Olson, 1996, Science 273: 510-513). Gait analysis was carried out 3 and 4 months after surgery and once in the normal animals. Animals were allowed to walk across a runway with a transparent floor. Each test consisted of five trials, and each trial was videorecorded from underneath. Using frame-by-frame playback, individual footprints were then recorded regarding location and order of limb use, as well as step quality (degree of weight bearing, etc.). These data allowed measuring runway transit time, five different measures of step numbers, all possible temporal patterns of limb use, stride length, and base of support. Transected controls remained paralyzed in the hindlimbs with only occasional reflex hindlimb movements without weight bearing. Animals subjected to the full repair procedure were significantly faster than the controls, used their hindlimbs for 25-30% of the movements, and regained several of the specific limb recruitment patterns used by normal rats. Taken together, the gait analysis data demonstrate remarkable recovery of coordinated gait in the repaired animals, which was significantly better than controls for all relevant parameters, while at the same time clearly inferior to normal rats for most of the examined parameters. We conclude that normal rats use a multitude of interchangeable step sequence patterns, and that our spinal cord repair strategy leads to recovery of some of these patterns following complete spinal cord transection. These data suggest functionally relevant neuronal communication across the lesion.     

Improved motor function of denervated rat hindlimb muscles induced by embryonic spinal cord grafts

Loss of motoneurons results in a decrease in force production by skeletal muscles and paralysis. Although it has been shown that missing motoneurons of rats can be replaced by embryonic homotopic neurons, attempts to guide their axons to their target muscles that have lost their innervation have been unsuccessful. In this study attempts were made to guide axons from grafted embryonic motoneurons to their target via a reimplanted ventral root. Adult hosts that received an embryonic graft prelabelled with 5-bromo-2'-deoxyuridine had their L4 ventral root avulsed and reimplanted into the spinal cord. Three to six months later, neurons that had their axons in the L4 ventral ramus were retrogradely labelled with fast blue and diamidino yellow. In five animals that had received an embryonic graft 116 +/- 16 cells were retrogradely labelled, and of these at least 15% were of graft origin, since they were positive for 5-bromo-2'-deoxyuridine. In five animals that had their L4 ventral root reimplanted but did not receive a graft, only 12 +/- 1.3 cells were retrogradely labelled. However, meaningful functional recovery could be achieved only if the regenerating axons of embryonic motoneurons found in the L4 ventral ramus were able to reverse the loss of force of muscles that had lost their innervation. This study shows that axons of embryonic motoneurons grafted into an adult rat spinal cord, as well as some axons of host origin, can be guided to denervated hindlimb muscles via reimplanted lumbar ventral roots. In normal rats approximately 30 motor axons innervated the extensor digitorum longus and 60 innervated the tibialis anterior via the L4 ventral root. In rats that did not receive a graft only 3.7 +/- 1.2 axons reached the extensor digitorum longus and 3.5 +/- 0.4 reached the tibialis anterior muscle via the implanted L4 ventral root. In animals that had an embryonic graft, 7.6 +/- 0.5 axons innervated the extensor digitorum longus and 8.5 +/- 0.5 reached the tibialis anterior muscle via the implanted root. In rats without a transplant the maximum tetanic tension elicited by stimulating the implanted L4 root was 16 +/- 7 g for the extensor digitorum longus and 53 +/- 36 g for the tibialis anterior muscle, whereas the corresponding muscles in animals that had an embryonic graft developed 82 +/- 16 and 281 +/- 95 g respectively. Thus it appears that the grafted motoneurons contributed to the innervation and functional recovery of the denervated muscles.     

Common soccer injuries. Diagnosis, treatment and rehabilitation

Motoneurons that supply the clearnose skate's hypobranchial musculature, via the occipital nerve and first seven ventral spinal nerve roots, are located within a column that extends from a level just caudal to the obex through the corresponding rostral spinal cord segments. Individual muscle motoneuron pools within the column are considerably intermingled and overlap. Comparisons with tetrapods, particularly mammals, where the hypobranchial musculature is greatly modified, reveal general conserved features. The motor column's multisegmental organization is retained although, in mammals, the column begins rostrally at medullary levels, where hypobranchial muscle motoneurons are intimately associated with motoneurons to lingual muscles, and it is restricted caudally to fewer spinal cord segments. In addition, despite an intermingling of motoneurons that supply individual hypobranchial muscles there is a shared rostrocaudal sequence of the motor pools. Rostral most hypobranchial motoneurons supply the most ventral and anterior muscles (i.e., m. coracomandibularis, and likely m. coracohyoideus, of skate and the suprahyoid musculature, m. geniohyoideus, of tetrapods). Caudal hypobranchial motoneurons supply the skate's mm. coracohyomandibularis, coracoarcualis communis and coracobranchialis and the tetrapod's entire infrahyoid muscle complex. The intermingling of multisegmental motoneuron populations innervating different hypobranchial muscles might be attributed to intermixing of premuscle mesoderm derived from several postotic somites but the musculotopic organization along the rostrocaudal axis indicates that pre- and posthyoid muscle mesoderm may partially keep its identity during its migration to the floor of the pharynx and oral cavity.     

Morphometric ultrastructural evaluation of the axonal endings in the neuromuscular junctions of pigeons after long lasting limitation of movement

To analyze the discharge patterns of the reticulospinal (R-S) neurons associated with four-limb movement, we recorded the unit spikes of 108 R-S neurons in 18 thalamic cats. (1) Unit spikes of R-S neurons exhibited alternating firings during leg movements, not only stepping on the treadmill but also upon passive flexion and extension movement by the experimenter's hand. (2) R-S neurons manifested firing patterns associated with diagonal, reciprocal and quadrupedal leg movements. About half of the neurons showed reciprocal patterns upon bilateral forelimb movements; spikes were increased when the ipsilateral forelimb was in a backward position; they were decreased when that leg was in a forward position. In contrast, the spikes were increased when the contralateral forelimb was placed forward and decreased when it was backward. About 15% of the R-S neurons showed discharge patterns correlated with quadrupedal leg movements. Firing increased when the left forelimb and right hindlimb were placed backward and the left hindlimb and right forelimb were forward. In contrast, when the position of all 4 limbs was reversed, firing rates decreased. (3) When brief touch stimulation was applied to the skin around the leg, bursting spikes were obtained; these were suppressed upon touching the skin of the contralateral limb. Even after transection of the muscle nerves, alternating firings were observed. (4) Local anesthesia to the shoulder joint resulted in a marked reduction of spontaneous discharges and alternating firings. (5) Our results indicate that afferents of joints and of cutaneous origins in individual limbs ascend to the brainstem reticular formation, that integrative action is organized as pattern generation in that region, and that this patterned information is sent to the spinal cord via the reticulospinal tracts.     

Polysynaptic neuronal pathways from group I and group II afferents innervating tail muscles to hindlimb motoneurons in the cat

Postsynaptic potentials evoked in motoneurons innervating m. posterior biceps and semitendinosus (PBSt) and m. triceps surae (GS) by low threshold afferents from various tail muscles located at the level of the second-third caudal vertebrae were investigated in the non-anesthetized and spinalized cat. Afferent inputs from tail muscles on both sides predominantly evoked depolarizing potential in PBSt motoneurons and hyperpolarizing potential in GS motoneurons. The findings suggest that in general, tail muscle afferents facilitate flexor and inhibit extensor hindlimb motoneurons through polysynaptic pathways, so that the pelvic girdle is kept in a low position to maintain the stability of the body irrespective of different movements or posture of the tail.     

Growth cone choices of Drosophila motoneurons in response to muscle fiber mismatch

In Drosophila embryos, each motoneuron is accurately matched to one or more singly identifiable muscle fibers. In this article we altered the number and pattern of the embryonic muscle fibers using genetic, heat shock, and laser ablation methods to test whether motoneuron growth cones are able to recognize specific targets. The choices made by two motoneurons were assayed using both intracellular dye fills and immunocytochemistry. The motoneurons RP1 and RP3 have nearly identical central and peripheral axonal trajectories. However, RP3 innervates the two most ventral longitudinal muscle fibers, 7 and 6, while RP1 grows past these fibers to innervate only muscle fiber 13. In rhomboid mutants muscle fiber 7 does not develop. Despite the loss of one of its targets, RP3 faithfully innervated the remaining muscle fiber 6 in over 80% of the observed cases. Furthermore, neuron RP1 accurately innervated muscle fiber 13, although it traversed one fiber fewer to reach it. Laser ablation of muscle fiber 7 confirmed the target choices shown by the motoneurons. In numb mutants, multiple muscle fibers, including 7, 13, and 12, fail to develop. This allowed us to test whether fibers distal to the target are involved in muscle fiber recognition, possibly by halting the growth cone advance. In mutant embryos, RP3 innervated muscle fiber 6 at the same frequency regardless of the absence of the distal muscle fiber 13. By contrast, RP1, which had lost its target entirely, frequently failed to innervate any muscle fiber during the period examined. Finally, muscle fiber 13 can be duplicated in wild-type embryos by means of a brief heat pulse during myogenesis. Presented with two targets, RP1 innervated both fibers in each case examined, while RP3 synapsed with muscle fibers 7 and 6 normally. Neuron-specific antibodies revealed that the embryonic growth cone choices were not transient, but persisted into the larval neuromuscular projections. These results indicate that each motoneuron growth cone has a primary target preference, which is retained even when the numbers of the muscle fibers, and therefore their relative positions, are altered. We therefore suggest that synaptic recognition by Drosophila motoneuron growth cones relies on unique features of the individual muscle fibers.     

Modern molecular diagnostics and the management of haematological malignancies

5-Aza-2'-deoxycytidine (d-AZA) inhibits methylation of DNA, a process that serves as an epigenetic regulator of gene expression. We have shown that d-AZA causes temporally related defects in mice. Gestational day (GD) 10 treatment induced severe long-bone defects of the hindlimb but not the forelimb. Exposure of younger embryos (GD 8 or 9) does not induce similar defects in forelimbs. This limb-dependent response suggests that methylation alterations in genes specific for fore- or hindlimbs may contribute to the observed pattern of defects. Subtraction hybridization (SH) studies were conducted to identify differential expression of DNA subsequent to the administration of d-AZA to mice on GD 10. Hindlimb buds collected from both treated and untreated embryos at 4, 12, and 24 hours post-treatment were used. A clone isolated from the untreated sample (down-regulation in treated tissue) was identified as a member of the murine B1 family of repetitive sequences. The two other clones isolated from the treated tissue (up-regulation) were homologous to avian myogenic regulatory protein mRNA and activin receptor type II gene. Both species are active during embryogenesis. These findings suggest that the isolated clones may have roles in abnormal embryonic development when inappropriately expressed.     

Thrombin perturbs neurite outgrowth and induces apoptotic cell death in enriched chick spinal motoneuron cultures through caspase activation

Increasing evidence indicates several roles for thrombin-like serine proteases and their cognate inhibitors (serpins) in normal development and/or pathology of the nervous system. In addition to its prominent role in thrombosis and/or hemostasis, thrombin inhibits neurite outgrowth in neuroblastoma and primary neuronal cells in vitro, prevents stellation of glial cells, and induces cell death in glial and neuronal cell cultures. Thrombin is known to act via a cell surface protease-activated receptor (PAR-1), and recent evidence suggests that rodent neurons express PAR-1. Previously, we have shown that the thrombin inhibitor, protease nexin-1, significantly prevents neuronal cell death both in vitro and in vivo. Here we have examined the effects of human alpha-thrombin and the presence and/or activation of PAR-1 on the survival and differentiation of highly enriched cultures of embryonic chick spinal motoneurons. We show that thrombin significantly decreased the mean neurite length, prevented neurite branching, and induced motoneuron death by an apoptosis-like mechanism in a dose-dependent manner. These effects were prevented by cotreatment with hirudin, a specific thrombin inhibitor. Treatment of the cultures with a synthetic thrombin receptor-activating peptide (SFLLRNP) mimicked the deleterious effects of thrombin on motoneurons. Furthermore, cotreatment of the cultures with inhibitors of caspase activities completely prevented the death of motoneurons induced by either thrombin or SFLLRNP. These findings indicate that (1) embryonic avian spinal motoneurons express functional PAR-1 and (2) activation of this receptor induces neuronal cell degeneration and death via stimulation of caspases. Together with previous reports, our results suggest that thrombin, its receptor(s), and endogenous thrombin inhibitors may be important regulators of neuronal cell fate during development, after injury, and in pathology of the nervous system.     

Projections of sympathetic non-noradrenergic neurons to skeletal muscle arteries in guinea-pig limbs vary with the metabolic character of muscles

This study set out to examine in detail the distribution of axons of sympathetic non-noradrenergic neurons innervating the arterial bed in skeletal muscles of the forelimb and hindlimb of guinea-pigs. The distribution of non-noradrenergic axons with immunoreactivity to vasoactive intestinal peptide (VIP) was examined in limb muscles of different histochemical character. The immunohistochemical demonstration of myosin heavy chain from fast-twitch muscle, and the histochemical demonstration of adenosine triphosphatase and succinic dehydrogenase, were used to determine the muscle fibre profile of 6 different limb muscles. Muscles included the oxidative type I muscle fibre-rich accessory semimembranosus muscle, the predominantly glycolytic type II muscle fibre-rich cranial gracilis and biceps brachii muscles and the plantaris, gastrocnemius medial head and triceps brachii long head of mixed muscle fibre composition. The frequency with which the VIP-immunoreactive (VIP-IR) axons innervated intramuscular arterial vessels was compared between categories of muscles defined by their muscle fibre profile. This study demonstrated that the projection of non-noradrenergic sympathetic neurons to skeletal muscle vasculature was widespread in guinea-pig limb muscles, but that it was not uniform. VIP-IR axons were more likely to innervate the arterial vasculature of muscles with a high proportion of type I and/or oxidative muscle fibres than of muscles with a large proportion of type IIb muscle fibres. This relationship between the distribution of sympathetic non-noradrenergic axons and the metabolic characteristics of muscle suggests that these presumed vasodilator neurons have an important role in matching blood flow to the particular metabolic demands of different limb muscles.     

Epstein-Barr virus infects and induces apoptosis in human neutrophils

Tibialis anterior and extensor digitorum longus muscles were partially denervated by cutting the L4 spinal nerve in three-day-old rats. The ultrastructure of the intact axons to these muscles in the L5 spinal nerve was examined in nine-day-old rats. In the control L5 spinal nerve, myelinated and unmyelinated axons were intermingled throughout the cross-section of the nerve, while on the operated side the nerve contained areas with predominantly small unmyelinated immature axons. The number of motoneurons innervating the partially denervated muscles was established by retrograde labelling with Diamidino Yellow. In nine- and 21-day-old rats, the number of labelled motoneurons on the partially denervated side, expressed as a percentage of the control side, was 26.1 +/- 5.5% and 20.7 +/- 3.0%, respectively. The response of these uninjured motoneurons to axotomy was tested. The axons of the motoneurons to the partially denervated muscles were crushed at nine days and the numbers of labelled motoneurons in the spinal cord of these rats counted at 21 days of age. Only 4.9 +/- 2.0% labelled motoneurons were seen on the operated side, as opposed to 20.7 +/- 3.0% present in animals without sciatic nerve injury. In normal animals, nerve injury at nine days does not cause motoneuron death. Thus, motoneurons to partially denervated muscles (i) have axons with several immature features and (ii) remain susceptible to axotomy-induced death for much longer than normal.     

Adult opossums (Didelphis virginiana) demonstrate near normal locomotion after spinal cord transection as neonates

When the thoracic spinal cord of the North American opossum (Didelphis virginiana) is transected on postnatal day (PD) 5, the site of injury becomes bridged by histologically recognizable spinal cord and axons which form major long tracts grow through the lesion. In the present study we asked whether opossums lesioned on PD5 have normal use of the hindlimbs as adults and, if so, whether that use is dependent upon axons which grow through the lesion site. The thoracic spinal cord was transected on PD5 and 6 months later, hindlimb function was evaluated using the Basso, Beattie, and Bresnahan (BBB) locomotor scale. All animals supported their weight with the hindlimbs and used their hindlimbs normally during overground locomotion. In some cases, the spinal cord was retransected at the original lesion site or just caudal to it 6 months after the original transection and paralysis of the hindlimbs ensued. Surprisingly, however, these animals gradually recovered some ability to support their weight and to step with the hindlimbs. Similar recovery was not seen in animals transected only as adults. In order to verify that descending axons which grew through the lesion during development were still present in the adult animal, opossums subjected to transection of the thoracic cord on PD5 were reoperated and Fast blue was injected several segments caudal to the lesion. In all cases, neurons were labeled rostral to the lesion in each of the spinal and supraspinal nuclei labeled by comparable injections in unlesioned, age-matched controls. The results of orthograde tracing studies indicated that axons which grew through the lesion innervated areas that were appropriate for them.