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

Behavioral recovery and spinal motoneuron remodeling after polyethylene glycol fusion repair of singly cut and ablated sciatic nerves

Polyethylene glycol repair (PEG-fusion) of severed sciatic axons restores their axoplasmic and membrane continuity, prevents Wallerian degeneration, maintains muscle fiber innervation, and greatly improves recovery of voluntary behaviors. We examined alterations in spinal connectivity and motoneuron dendritic morphology as one potential mechanism for improved behavioral function after PEG-fusion. At 2–112 days after a single-cut or allograft PEG-fusion repair of transected or ablated sciatic nerves, the number, size, location, and morphology of motoneurons projecting to the tibialis anterior muscle were assessed by retrograde labeling. For both lesion types, labeled motoneurons were found in the appropriate original spinal segment, but also in inappropriate segments, indicating mis-pairings of proximal-distal segments of PEG-fused motor axons. Although the number and somal size of motoneurons was unaffected, dendritic distributions were altered, indicating that PEG-fusion preserves spinal motoneurons but reorganizes their connectivity. This spinal reorganization may contribute to the remarkable behavioral recovery seen after PEG-fusion repair.

Neuroprotective Effects of Exercise on the Morphology of Somatic Motoneurons Following the Death of Neighboring Motoneurons

Background. Motoneuron loss is a severe medical problem that can result in loss of motor control and eventually death. We have previously demonstrated that partial motoneuron loss can result in dendritic atrophy and functional deficits in nearby surviving motoneurons, and that treatment with androgens can be neuroprotective against this dendritic atrophy. Exercise has also been shown to be protective following a variety of neural injury models and, in some cases, is dependent on androgen action. Objective. In this study, we explored whether exercise shows the same neuroprotective effect on induced dendritic atrophy as that seen with androgen treatment. Methods. Motoneurons innervating the vastus medialis muscles of adult male rats were selectively killed by intramuscular injection of cholera toxin-conjugated saporin. Following saporin injections, some animals were allowed free access to a running wheel attached to their home cages. Four weeks later, motoneurons innervating the ipsilateral vastus lateralis muscle were labeled with cholera toxin-conjugated horseradish peroxidase, and dendritic arbors were reconstructed in 3 dimensions. Results. Dendritic arbor lengths of animals allowed to exercise were significantly longer than those not allowed to exercise. Conclusions. These findings indicate that exercise following neural injury exerts a protective effect on motoneuron dendrites comparable to that seen with exogenous androgen treatment.

Neuroprotective Effects on the Morphology of Somatic Motoneurons Following the Death of Neighboring Motoneurons: A Role for Microglia?

Partial depletion of spinal motoneuron populations induces dendritic atrophy in neighboring motoneurons, and treatment with testosterone protects motoneurons from induced dendritic atrophy. We explored a potential mechanism for this induced atrophy and protection by testosterone, examining the microglial response to partial depletion of motoneurons. Motoneurons innervating the vastus medialis muscles of adult male rats were killed by intramuscular injection of cholera toxin-conjugated saporin; some saporin-injected rats were treated with testosterone. Microglia were later visualized via immunohistochemical staining, classified as monitoring or activated, and counted stereologically. Partial motoneuron depletion increased the number of activated microglia in the quadriceps motor pool, and this increase was attenuated with testosterone treatment. The attenuation in microglial response could reflect an effect of testosterone on suppressing microglia activation, potentially sparing motoneuron dendrites. Alternatively, testosterone could be neuroprotective, sparing motoneuron dendrites, secondarily resulting in reduced microglial activation. To discriminate between these hypotheses, following partial motoneuron depletion, rats were treated with minocycline to inhibit microglial activation. Motoneurons innervating the ipsilateral vastus lateralis muscle were later labeled with cholera toxin-conjugated horseradish peroxidase, and dendritic arbors were reconstructed. Reduction of microglial activation by minocycline did not prevent induced dendritic atrophy following partial motoneuron depletion. Further, reduction of microglial activation by minocycline treatment resulted in dendritic atrophy in intact animals. Together, these findings indicate that the neuroprotective effect of testosterone on dendrites following motoneuron death is not due to inhibiting microglial activation, and that microglial activity contributes to the normal maintenance of dendritic arbors.

Protective Effects of Estradiol and Dihydrotestosterone following Spinal Cord Injury

Spinal cord injury (SCI) results in lesions that destroy tissue and disrupt spinal tracts, producing deficits in locomotor and autonomic function. We previously demonstrated that motoneurons and the muscles they innervate show pronounced atrophy after SCI, and these changes are prevented by treatment with testosterone. Here, we assessed whether the testosterone active metabolites estradiol and dihydrotestosterone have similar protective effects after SCI. Young adult female rats received either sham or T9 spinal cord contusion injuries and were treated with estradiol, dihydrotestosterone, both, or nothing via Silastic capsules. Basso-Beattie-Bresnahan locomotor testing was performed weekly and voiding behavior was assessed at 3 weeks post-injury. Four weeks after SCI, lesion volume and tissue sparing, quadriceps muscle fiber cross-sectional area, and motoneuron dendritic morphology were assessed. Spontaneous locomotor behavior improved after SCI, but hormone treatments had no effect. Voiding behavior was disrupted after SCI, but was significantly improved by treatment with either estradiol or dihydrotestosterone; combined treatment was maximally effective. Treatment with estradiol reduced lesion volume, but dihydrotestosterone alone and estradiol combined with dihydrotestosterone were ineffective. SCI-induced decreases in motoneuron dendritic length were attenuated by all hormone treatments. SCI-induced reductions in muscle fiber cross-sectional areas were prevented by treatment with either dihydrotestosterone or estradiol combined with dihydrotestosterone, but estradiol treatment was ineffective. These findings suggest that deficits in micturition and regressive changes in motoneuron and muscle morphology seen after SCI are ameliorated by treatment with estradiol or dihydrotestosterone, further supporting a role for steroid hormones as neurotherapeutic agents in the injured nervous system.

Neuroprotective effects of testosterone metabolites and dependency on receptor action on the morphology of somatic motoneurons following the death of neighboring motoneurons

Partial depletion of spinal motoneuron populations induces dendritic atrophy in neighboring motoneurons, and treatment with testosterone is neuroprotective, attenuating induced dendritic atrophy. In this study we examined whether the protective effects of testosterone could be mediated via its androgenic or estrogenic metabolites. Furthermore, to assess whether these neuroprotective effects were mediated through steroid hormone receptors, we used receptor antagonists to attempt to prevent the neuroprotective effects of hormones after partial motoneuron depletion. Motoneurons innervating the vastus medialis muscles of adult male rats were selectively killed by intramuscular injection of cholera toxin-conjugated saporin. Simultaneously, some saporin-injected rats were treated with either dihydrotestosterone or estradiol, alone or in combination with their respective receptor antagonists, or left untreated. Four weeks later, motoneurons innervating the ipsilateral vastus lateralis muscle were labeled with cholera toxin-conjugated horseradish peroxidase, and dendritic arbors were reconstructed in three dimensions. Compared with intact normal animals, partial motoneuron depletion resulted in decreased dendritic length in remaining quadriceps motoneurons. Dendritic atrophy was attenuated with both dihydrotestosterone and estradiol treatment to a degree similar to that seen with testosterone, and attenuation of atrophy was prevented by receptor blockade. Together, these findings suggest that neuroprotective effects on motoneurons can be mediated by either androgenic or estrogenic hormones and require action via steroid hormone receptors, further supporting a role for hormones as neurotherapeutic agents in the injured nervous system. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 691-707, 2017.

Neuroprotective effects of testosterone on motoneuron and muscle morphology following spinal cord injury

Treatment with testosterone is neuroprotective/neurotherapeutic after a variety of motoneuron injuries. Here we assessed whether testosterone might have similar beneficial effects after spinal cord injury (SCI). Young adult female rats received either sham or T9 spinal cord contusion injuries and were implanted with blank or testosterone-filled Silastic capsules. Four weeks later, motoneurons innervating the vastus lateralis muscle of the quadriceps were labeled with cholera toxin-conjugated horseradish peroxidase, and dendritic arbors were reconstructed in three dimensions. Soma volume, motoneuron number, lesion volume, and tissue sparing were also assessed, as were muscle weight, fiber cross-sectional area, and motor endplate size and density. Contusion injury resulted in large lesions, with no significant differences in lesion volume, percent total volume of lesion, or spared white or gray matter between SCI groups. SCI with or without testosterone treatment also had no effect on the number or soma volume of quadriceps motoneurons. However, SCI resulted in a decrease in dendritic length of quadriceps motoneurons in untreated animals, and this decrease was completely prevented by treatment with testosterone. Similarly, the vastus lateralis muscle weights and fiber cross-sectional areas of untreated SCI animals were smaller than those of sham-surgery controls, and these reductions were both prevented by testosterone treatment. No effects on motor endplate area or density were observed across treatment groups. These findings suggest that regressive changes in motoneuron and muscle morphology seen after SCI can be prevented by testosterone treatment, further supporting a role for testosterone as a neurotherapeutic agent in the injured nervous system.

Overexpression of androgen receptors in target musculature confers androgen sensitivity to motoneuron dendrites

The dendritic arbors of spinal motoneurons are dynamically regulated by a variety of factors, and several lines of evidence indicate that trophic interactions with the target musculature are of central importance. In highly androgen-sensitive motoneuron populations, androgens are thought to regulate motoneuron dendrites through their action at the receptor-enriched target musculature. Using rats transgenically modified to overexpress androgen receptor (AR) in skeletal muscle, we directly tested the hypothesis that the enhanced expression of AR in the target musculature can underlie the androgenic regulation of motoneuron dendritic morphology. The morphology of motoneurons innervating the quadriceps muscle was examined in wild-type (WT) rats as well as in rats that had been transgenically modified to overexpress ARs in their skeletal musculature. Motoneurons innervating the vastus lateralis muscle of the quadriceps in gonadally intact male rats, and castrated males with or without androgen replacement, were labeled with cholera toxin-conjugated horseradish peroxidase, and dendritic arbors were reconstructed in three dimensions. In WT rats, quadriceps motoneuron dendrites were insensitive to hormonal manipulation. In contrast, quadriceps motoneuron dendrites in gonadally intact transgenic males were larger than those of WT males. Furthermore, overexpression of ARs in the quadriceps muscle resulted in androgen sensitivity in dendrites, with substantial reductions in dendritic length occurring after castration; this reduction was prevented with testosterone replacement. Thus, it appears that the androgen sensitivity of motoneuron dendrites is conferred indirectly via the enrichment of ARs in the musculature.

Neuroprotective effects of testosterone on dendritic morphology following partial motoneuron depletion: efficacy in female rats

Motoneuron loss is a significant medical problem, capable of causing severe movement disorders and even death. We have previously demonstrated that partial depletion of motoneurons induces dendritic atrophy in remaining motoneurons, with a concomitant reduction in motor activation. Treatment of male rats with testosterone attenuates the regressive changes following partial motoneuron depletion. To test whether testosterone has similar effects in females, we examined potential neuroprotective effects in motoneurons innervating muscles of the quadriceps of female rats. Motoneurons were selectively killed by intramuscular injection of cholera toxin-conjugated saporin. Simultaneously, some saporin-injected rats were given implants containing testosterone or left untreated. Four weeks later, surviving motoneurons were labeled with cholera toxin-conjugated HRP, and dendritic arbors were reconstructed in three dimensions. Compared to normal females, partial motoneuron depletion resulted in decreased dendritic length in remaining quadriceps motoneurons, and this atrophy was greatly attenuated by testosterone treatment. These findings suggest that testosterone has neuroprotective effects on morphology in both males and females, further supporting a role for testosterone as a neurotherapeutic agent in the injured nervous system.

Neuroprotective actions of androgens on motoneurons

Androgens have a variety of protective and therapeutic effects in both the central and peripheral nervous systems. Here we review these effects as they related specifically to spinal and cranial motoneurons. Early in development, androgens are critical for the formation of important neuromuscular sex differences, decreasing the magnitude of normally occurring cell death in select motoneuron populations. Throughout the lifespan, androgens also protect against motoneuron death caused by axonal injury. Surviving motoneurons also display regressive changes to their neurites as a result of both direct axonal injury and loss of neighboring motoneurons. Androgen treatment enhances the ability of motoneurons to recover from these regressive changes and regenerate both axons and dendrites, restoring normal neuromuscular function. Androgens exert these protective effects by acting through a variety of molecular pathways. Recent work has begun to examine how androgen treatment can interact with other treatment strategies in promoting recovery from motoneuron injury.

Neuroprotective effects of testosterone on the morphology and function of somatic motoneurons following the death of neighboring motoneurons

Motoneuron loss is a significant medical problem, capable of causing severe movement disorders or even death. We have previously shown that partial depletion of motoneurons from sexually dimorphic, highly androgen-sensitive spinal motor populations induces dendritic atrophy in remaining motoneurons, and this atrophy is attenuated by treatment with testosterone. To test whether testosterone has similar effects in more typical motoneurons, we examined potential neuroprotective effects in motoneurons innervating muscles of the quadriceps. Motoneurons innervating the vastus medialis muscle were selectively killed by intramuscular injection of cholera toxin-conjugated saporin. Simultaneously, some saporin-injected rats were given implants containing testosterone or left untreated. Four weeks later, motoneurons innervating the ipsilateral vastus lateralis muscle were labeled with cholera toxin-conjugated horseradish peroxidase, and dendritic arbors were reconstructed in three dimensions. Compared with intact normal males, partial motoneuron depletion resulted in decreased dendritic length in remaining quadriceps motoneurons, and this atrophy was attenuated by testosterone treatment. To examine the functional consequences of the induced dendritic atrophy, and its attenuation with testosterone treatment, the activation of remaining quadriceps motoneurons was assessed using peripheral nerve recording. Partial motoneuron depletion resulted in decreased amplitudes of motor nerve activity, and these changes were attenuated by treatment with testosterone, providing a functional correlate to the neuroprotective effects of testosterone treatment on quadriceps motoneuron morphology. Together these findings suggest that testosterone has neuroprotective effects on morphology and function in both highly androgen-sensitive as well as more typical motoneuron populations, further supporting a role for testosterone as a neurotherapeutic agent in the injured nervous system.

Synaptic remodeling in the nucleus ambiguus following vagal–hypoglossal nerve anastomosis in the cat

We reported recently the occurrence of a massive and selective elimination of synaptic boutons on motoneurons in the dorsal motor nucleus of the vagus (DMV) in the cat following vagal-hypoglossal nerve anastomosis (VHA) [J. Comp. Neurol. 458 (2003) 195]. This study was aimed to explore the synaptic reorganization in the other major nucleus associated with the vagus, namely, the nucleus ambiguus (NA) following the same treatment. In view of the tremendous difference in function, the NA and DMV are considered to be two ideal nuclei for explanatory studies seeking to elucidate how VHA could induce different plasticity of brainstem neurons influenced by the newly reestablished neural pathway. The present results showed that the vagal efferent neurons in the NA had responded to VHA in a different manner compared with those in the DMV. Firstly, the numbers of axon terminals containing round (R), round with dense-cored (R+D), pleomorphic (P) or flattened (F) synaptic vesicles contacting the NA motoneurons were markedly increased at 500-day postoperation, the longest reinnervation interval. The percent increases in the synapse frequency for R, R+D, P and F boutons were 8.6%, 274.4%, 238.3% and 400.0%, respectively. Secondly, the formation of astroglial ensheathment around the motoneurons in the DMV following VHA was not evident in the NA. Another striking difference was the extensive dendritic sprouting of the NA neurons as opposed to the dendritic retraction of the DMV neurons as shown by a significant increase in distal dendrites of NA motoneurons. The different modes of neural remodeling between NA and DMV may be attributed to the unique nature of the two nuclei to structures they normally supply and their different compatibility with the newly innervated target, viz. tongue skeletal musculature.

Testosterone manipulation protects motoneurons from dendritic atrophy after contralateral motoneuron depletion

Dendritic morphology is reactive to many kinds of injuries, including axotomy and deafferentation. In this study, we examined the response of motoneurons in the spinal nucleus of the bulbocavernosus (SNB), an androgen-dependent population of motoneurons in the lumbar spinal cord of the rat, to partial motoneuron depletion. We depleted SNB motoneurons on one side only of the spinal cord by unilateral intramuscular injection of a retrogradely transported form of saporin, and examined the morphology of contralateral SNB motoneurons. Motoneuron morphology was assessed in normal control males, gonadally intact saporin-treated males, and saporin-treated males who had been castrated 6 weeks previously and given testosterone replacement beginning at the time of saporin injection. Untreated castrated males served as an additional control group. Four weeks after saporin treatment, SNB motoneurons contralateral to the saporin injection were retrogradely labeled with horseradish peroxidase conjugated to the cholera toxin B subunit and reconstructed in three dimensions. In gonadally intact males, unilateral motoneuron depletion caused regressive changes in contralateral SNB motoneurons: Soma size and dendritic length were both decreased. However, testosterone manipulation (i.e., castration followed by testosterone replacement) completely prevented the dendritic retraction. These data suggest a therapeutic role for testosterone in preventing, or accelerating recovery from, dendritic atrophy induced by motoneuron injury.

Fate of the soma and dendrites of cord-projection central neurons after proximal and distal spinal axotomy: an intracellular dye injection study

We used rat rubrospinal neurons as a model to study the soma-dendritic morphology of cord-projection neurons following spinal axonal injury. We examined lumbar-projection neurons following both upper cervical and lower thoracic axotomy to find out whether changes were dependent on the proximity of the lesion to the cell body. Axotomized neurons were marked with retrograde tracer and studied 4 and 8 weeks later with intracellular dye injection technique. Axotomy resulted in prominent shrinkage of their soma and relatively minor reduction of their dendritic spreads. The degree of soma shrinkage depended on both the duration of survival and the proximity of lesion. In addition, dendritic modification peaked 4 weeks following proximal lesion, which was also achieved 8 weeks following distal axotomy. Tractotomy at upper cervical and lower thoracic levels also allowed us to compare the effect of distal axotomy on cervical and lumbar-projection neurons. Results show that although cervical-projection neurons responded more quickly than lumbar-projecting ones, they however showed a similar degree of alteration in both their soma and dendrites 8 weeks following distal axotomy. In summary, cord-projection neurons survived 8 weeks following either upper cervical or lower thoracic axotomy with relatively intact dendritic features. Taken together, our data thus far suggest that cord-projection central neurons continue to integrate inputs and control supraspinal targets following spinal axotomy. The minor dendritic shrinkage within two months of spinal axotomy rejuvenates hopes for functional recovery if regeneration of their spinal axons can be achieved at least within this time frame.

Functional corticospinal projections are established prenatally in the human foetus permitting involvement in the development of spinal motor centres

From studies of subhuman primates it has been assumed that functional corticospinal innervation occurs post-natally in man. We report a post-mortem morphological study of human spinal cord, and neurophysiological and behavioural studies in preterm and term neonates and infants. From morphological studies it was demonstrated that corticospinal axons reach the lower cervical spinal cord by 24 weeks post-conceptional age (PCA) at the latest. Following a waiting period of up to a few weeks, it appears they progressively innervate the grey matter such that there is extensive innervation of spinal neurons, including motor neurons, prior to birth. Functional monosynaptic corticomotoneuronal projections were demonstrated neurophysiologically from term, but are also likely to be present from as early as 26 weeks PCA. At term, direct corticospinal projections to Group Ia inhibitory interneurons were also confirmed. Independent finger movements developed much later, between 6 and 12 months post-natally. These data do not support the proposal that in man, establishment of functional corticomotoneuronal projections occurs immediately prior to and provides the capacity for the expression of fine finger movement control. We propose instead that such early corticospinal innervation occurs to permit cortical involvement in activity dependent maturation of spinal motor centres during a critical period of perinatal development. Spastic cerebral palsy from perinatal damage to the corticospinal pathway secondarily involves disrupted development of spinal motor centres. Corticospinal axons retain a high degree of plasticity during axon growth and synaptic development. The possibility therefore exists to promote regeneration of disrupted corticospinal projections during the perinatal period with the double benefit of restoring corticospinal connectivity and normal development of spinal motor centres.

Expansion of the dendritic tree of motoneurons innervating neck muscles of the adult cat after permanent axotomy

The morphologic characteristics of neck motoneurons with intact axons were compared with those of neck motoneurons that had been permanently axotomized for 11 to 17 weeks. Motoneurons were identified antidromically, intracellularly stained with horseradish peroxidase (HRP) and examined after reconstructions of their entire dendritic tree. Axotomized motoneurons differed qualitatively and quantitatively from motoneurons with intact axons. The distal branches of axotomized motoneurons exhibited two novel features: some gave rise to tangled appendages that exhibited growth cone-like specializations resembling lamellipodia and filopodia; others followed a meandering path and had unusually large diameters. These branches showed a discontinuous pattern of staining that was similar to the appearance of myelinated axons stained intra-axonally with HRP. A quantitative analysis of the dendritic trees of 13 completely reconstructed dendritic trees (five axotomized motoneurons and eight motoneurons with intact axons) showed that total dendritic surface area, total dendritic length, and total number of branches increased 38, 34, and 215%, respectively, after axotomy. These measurements were confirmed by comparing the sizes of a larger number of motoneurons (16 axotomized and 21 intact), calculated on the basis of correlations between dendritic tree size and proximal dendritic diameter. We conclude, therefore, that neck motoneurons, in contrast to other types of motoneurons, expand their dendritic trees after axotomy. It is suggested that this expansion is a consequence of two mechanisms: one involves dendritic growth, possibly leading to new synaptic connections; the other causes a conversion of some dendrites into axons.

A differential and time-dependent decrease in AMPA-type glutamate receptor subunits in spinal motoneurons after sciatic nerve injury

After sciatic transection a strong decrease in immunoreactivity occurred, starting at 2 days. After 6, 10, 14, and 20 days survival only 5% of the sciatic motoneurons were strongly labeled for GluR2/3 against 80% in the control situation. From Day 20, GluR2/3 labeling started to increase again, reaching near normal levels at Day 80 after sciatic transection. In contrast, after sciatic crush, the decrease in GluR2/3 labeling in motoneurons was less pronounced and returned to normal in 30 days. In all animals, the GluR1 and GluR4 labeling of motoneurons remained unchanged after sciatic transection or crush. It is concluded that sciatic nerve injury leads to a strong, time-dependent decrease in the expression of GluR2 and 3 subunits in the corresponding motoneurons. As a consequence, AMPA receptors with a different subunit composition may be assembled, leading to a change in the functional properties of these receptors. Moreover, if they lack the GluR2 subunit, they may become calcium permeable.

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.