Evidence for age-dependent changes in motoneuron dendritic morphology following neonatal nerve-crush in the rat

Effects of limb exercise after spinal cord injury on motor neuron dendrite structure

An integration center subserving locomotor leg movements resides in the upper lumbar spinal cord. If this neuronal network is preserved after a spinal cord injury, it is possible to stimulate this circuitry to initiate and promote walking. The several effective approaches (electrical stimulation, pharmacologic agents, physical therapy training programs) may all share a common modus operandi of altering synaptic activity within segmental spinal cord. To understand the neural substrate for the use-dependent behavioral improvement, we studied the dendritic architecture of spinal motor neurons. In the first experiment, we compared three groups of animals: animals with an intact spinal cord, animals that had a complete spinal cord transection (SCT) and animals with SCT who engaged in a daily exercise program of actively moving paralyzed hindlimbs through the motions of walking. When compared with animals with an intact spinal cord, the motor neurons from animals with SCT displayed marked atrophy, with loss of dendritic membrane and elimination of branching throughout the visible tree within transverse tissue slices. None of these regressive changes were found in the motor neurons from SCT animals that underwent exercise. In a second experiment, we inquired whether exercise of animals with an intact spinal cord influenced dendrite structure. Increased exercise had very modest effects on dendrite morphology, indicating an upper limit of use-dependent dendrite growth. Our findings suggest that the dendritic tree of motor neurons deprived of descending influences is rapidly pruned, and this finding is not observed in motor neurons after SCT if hindlimbs are exercised. The functional benefits of exercise after SCT injury may be subserved, in part, by stabilizing or remodeling the dendritic tree of motor neurons below the injury site.

Repeated injury to the sciatic nerve in immature rats causes motoneuron death and impairs muscle recovery

Injury to the sciatic nerve of newborn rats causes motoneuron death, while the same insult inflicted 5 days later does not. In this study the effects of prolonging the period of target deprivation and axonal regeneration were investigated by inflicting a second nerve crush 6 days after the first, just before reinnervation of the muscle occurred. Two to 4 months later the number of motoneurons supplying soleus, tibialis anterior, and extensor digitorum longus muscles was established by retrograde labeling with horseradish peroxidase injected into the muscle. After nerve injury at 5 days there was no significant loss of motoneurons to any muscle. However, when the injury was repeated, the number of labeled motoneurons was reduced, suggesting that a significant proportion had died. Motoneurons to soleus were affected more than those to the fast muscles, reflecting their lesser maturity. Moreover, motoneurons to soleus that survived both injuries to their axon failed to grow to their full size. The relative impairment of recovery of the muscles, indicated by weight and maximal tetanic tension, mirrored the loss of motoneurons in each case. Previous studies have suggested that repeated nerve injuries in adult animals can enhance reinnervation. However, the present results along with those of other recent studies suggest that immature motoneurons that are repeatedly induced to support growth of their axons are at greater risk of death and can result in poorer reinnervation of the muscles.

The effects of neonatal dorsal root section on the survival and dendritic development of lumbar motoneurons in the rat

Peripheral nerve crush during the early neonatal period results in the death of a large proportion of affected motoneurons and abnormal dendritic development in those which survive. The present study reports the effects of neonatal dorsal root section on motoneurons supplying the extensor digitorum longus muscle of the rat. This lesion did not result in motoneuron death, but did disrupt subsequent dendritic development. In cells retrogradely labelled with cholera toxin subunit B conjugated to horseradish peroxidase, there was little change in adult dendritic morphology in the transverse plane, where abnormalities associated with loss of efferent contact and cell death have been found. However, there was a caudal expansion of the dendritic field, an effect seen following nerve crush but not after blockade of neuromuscular transmission alone. The results show that disruption of dorsal root sensory inputs alone can affect the dendritic development of motoneurons but does not cause their death. In conjunction with our earlier findings, it is clear that both afferent and efferent connections are required for normal dendritic development, and disruption of either has a characteristic effect on survival and dendritic morphology.

Magnesium ions reduce motoneuron death following nerve injury or exposure to N-methyl-D-aspartate in the developing rat

Developing motoneurons can be induced to die by target deprivation and there is evidence that this cell death involves the excitotoxic effects of N-methyl-D-aspartate. Treatment with dizocilpine maleate, an antagonist of this receptor, has been shown to rescue a proportion of those motoneurons destined to die following nerve injury at birth. However, this is a relatively toxic compound. In this study we examined whether systemic treatment with magnesium sulphate, a non-competitive antagonist of the N-methyl-D-aspartate receptor which is better tolerated than dizocilpine maleate, could prevent motoneuron death. Motoneurons were induced to die either by sciatic nerve injury at birth or by nerve injury at five days followed by exposure to N-methyl-D-aspartate. The number of surviving motoneurons reinnervating the tibialis anterior and extensor digitorum longus muscles were counted using retrograde labelling. Following nerve injury at birth and treatment with magnesium sulphate, there was a small increase in the survival of injured motoneurons, although this improvement was not significant. Nerve injury at five days does not result in motoneuron death, but when followed by treatment with N-methyl-D-aspartate, only 42 +/- 2.9% of motoneurons to these flexor muscles survived. Treatment with magnesium sulphate prior to injection of N-methyl-D-aspartate significantly increased motoneuron survival, so that 67 +/- 5.8% of motoneurons survived. Thus, systemic treatment with magnesium can prevent the death of motoneurons rendered susceptible to the excitotoxic effects of N-methyl-D-aspartate by nerve injury.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrical properties of embryonic rat brainstem motoneurones in organotypic slice culture

Organotypic co-cultures of embryonic E18-19 rat brainstem slices and tongue muscle maintained in vitro for more than 16 days were used to study the electrogenic properties of developing embryonic brainstem motoneurones derived from oculomotor, facial or hypoglossal nuclei. This preparation offers a unique opportunity to study the development of prenatal mammalian embryonic motoneurones. Our results show that embryonic rat brainstem motoneurones grown in organotypic culture with tongue muscle develop electrogenic membrane properties that can be compared with those described in newborn and adult animals in slice preparation. These motoneurones displayed a variety of sodium and calcium conductances. Two types of sodium conductances were present in all the recorded motoneurones. An Hodgkin-Huxley TTX-sensitive sodium conductance was involved in the spike potential, whereas a TTX-insensitive high threshold sodium conductance was uncovered when potassium and calcium currents were suppressed. Prominent calcium potentials contributed to the large delayed depolarizations following the spike potentials. High threshold calcium potentials were triggered with a slightly lower threshold than the sodium spike and contributed to the control of the pattern of discharge of the cell in response to slight shifts of membrane potentials. Low threshold calcium potentials were seen in 16% of the recorded motoneurones in hyperpolarized conditions. These calcium currents underlie the triggering of doublet spikes and rebound responses. Brainstem motoneurones in culture did not display differences that could be correlated with the origin of the motoneurone pool explanted and retained undifferentiated features suggesting that development of electrophysiological properties specific of each motoneurone pool are determined by presynaptic networks and target properties.

Nerve injury in adult rats causes abnormalities in the motoneuron dendritic field that differ from those seen following neonatal nerve injury

Disruption of neuromuscular contact by nerve-crush during the early postnatal period causes increased activity and abnormal reflex responses in affected motoneurons, but such changes are not found after nerve-crush in adult animals. We found previously that neonatally lesioned cells develop an abnormal dendritic field, which may explain the functional changes. Here we have studied the dendritic morphology of the same motoneuron pool after nerve-crush at maturity in order to correlate the observed alterations in morphology with physiological findings. One to two months after sciatic nerve-crush in adult animals, motoneurons supplying the extensor hallucis longus muscles of the rat were retrogradely labelled with cholera toxin subunit-B conjugated to horseradish peroxidase. The dendritic tree of labelled cells was then analysed. Following adult nerve-crush, the dendritic tree of the motoneurons was smaller but did not display the localised increase in dendritic density seen after neonatal nerve-crush. These findings support the view that such specific morphological changes contribute to the physiological abnormalities seen only after neonatal nerve injury.

Transient muscle paralysis in neonatal rats renders motoneurons susceptible to N-methyl-D-aspartate-induced neurotoxicity

Paralysis of the soleus muscle in newborn rats causes a large proportion of motoneurons to die by 10 weeks of age. However, all of these neurons are still present at three to four weeks of age. We have previously shown that although nerve injury at five days does not result in any motoneuron death, it does render these neurons susceptible to the toxic effects of the glutamate agonist N-methyl-D-aspartate. Using retrograde labelling of soleus motoneurons, in this study we show that an increased susceptibility to glutamate also plays a role in the eventual death of those motoneurons which survive for three weeks after interruption of neuromuscular transmission at birth but die by 10 weeks. Treatment with dizocilpine maleate an antagonist of the N-methyl-D-aspartate receptor increased the survival of motoneurons to alpha-bungarotoxin-treated soleus muscles. By 10 weeks of age the size of motoneurons to alpha-bungarotoxin-treated soleus muscles is smaller than that of controls, but after treatment with dizocilpine maleate the sizes of motoneurons to control and treated muscles are similar. Moreover, only 55 +/- 2.7% of motoneurons to the soleus muscle paralysed at birth with alpha-bungarotoxin survive for three weeks after a single injection of N-methyl-D-aspartate at 12 days of age. This motoneuron death is due to the application of N-methyl-D-aspartate since treatment with alpha-bungarotoxin alone causes no loss of neurons at this age.(ABSTRACT TRUNCATED AT 250 WORDS)

Intrinsic programmes of growth and survival in developing vertebrate neurons

Neurons become dependent on a supply of target-derived neurotrophins for survival when their axons reach their targets in development. Because the distance axons have to grow varies from one population of neurons to another, the timing of dependence on neurotrophins likewise varies. Although it would be expected that the simplest way of getting the timing right would be for the target to provide a suitable signal to the arriving axons, detailed studies on developing cranial sensory neurons suggest that these neurons are programmed, before differentiation, to acquire dependence at the correct time independently of external signals. Neurons not only partly compensate for different target distances by extending axons more rapidly the further they have to grow, but possess an intrinsic clock that switches on dependence at the right time in accordance with the time it normally takes their axons to reach their targets. Here the experimental evidence for these intrinsic programmes of growth and survival is reviewed, the rationale that might have favoured their evolution is discussed, and parallels are drawn with other developing neuronal systems.