Developmental plasticity of reticulospinal and vestibulospinal axons in the north American opossum, Didelphis virginiana

Raphe-spinal neurons display an age-dependent differential capacity for neurite outgrowth compared to other brainstem-spinal populations

Functional regeneration of brainstem-spinal pathways occurs in the developing chick when the spinal cord is severed prior to embryonic day (E) 13. Functional spinal cord regeneration is not observed in animals injured after E13. This developmental transition from a permissive to a restrictive repair period may be due to the formation of an extrinsic inhibitory environment preventing axonal growth, and/or an intrinsic inability of mature neurons to regenerate. Here, we investigated the capacity of specific populations of brainstem-spinal projection neurons to regrow neurites in vitro from young (E8) versus mature (E17) brainstem explants. A crystal of carbocyanine dye (DiI) was implanted in ovo into the E5 cervical spinal cord to retrogradely label brainstem-spinal projection neurons. Three or 12 days later, discrete regions of the brainstem containing DiI-labeled neurons were dissected to produce explant cultures grown in serum-free media on laminin substrates. The subsequent redistribution of DiI into regenerating processes permitted the study of in vitro neurite outgrowth from identified brainstem-spinal neurons. When explanted on E8, i.e., an age when brainstem-spinal neurons are normally elongating through the spinal cord and are capable of in vivo functional regeneration, robust neurite outgrowth was observed from all brainstem populations, including rubro-, reticulo-, vestibulo-, and raphe-spinal neurons. In contrast, when explanted on E17, robust neurite outgrowth was seen only from raphe-spinal neurons. Neurite outgrowth from raphe-spinal neurons was 5-hydroxy-tryptamine immunoreactive. This study demonstrates that in growth factor-free environments with permissive growth substrates, neurite outgrowth from brainstem-spinal neurons is dependent on both neuronal age and phenotype.

Developmental plasticity of ascending spinal axons studies using the North American opossum, Didelphis virginiana

The objectives of the present study were to determine if axons of all ascending tracts grow through the lesion after transection of the thoracic spinal cord during development in the North American opossum, and if so, whether they reach regions of the brain they normally innervate. Opossum pups were subjected to transection of the mid-thoracic cord at PD5, PD8, PD12, PD20, or PD26 and injections of Fast Blue (FB) into the lower thoracic or upper lumbar cord 30-40 days or 6 months later. In the PD5 transected cases, labeled axons were present in all of the supraspinal areas labeled by comparable injections in unlesioned, age-matched controls. In the experimental cases, however, labeled axons appeared to be fewer in number and in some areas more restricted in location than in the controls. When lesions were made at PD8, labeled axons were present in the brain of animals allowed to survive 30-40 days prior to FB injections but they were not observed in those allowed to survive 6 months. When lesions were made at PD12 or later, labeled axons were never found rostral to the lesion. It appears, therefore, that axons of all ascending spinal pathways grow though the lesion after transection of the thoracic cord in developing opossums and that they innervate appropriate areas of the brain. Interestingly, the critical period for such growth is shorter than that for most descending axons, suggesting that factors which influence loss of developmental plasticity are not the same for all axons.

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.

Developmental plasticity of selected spinocerebellar axons. Studies using the North American opossum, Didelphis virginiana

When the thoracic spinal cord of the opossum is hemisected at postnatal day 5 or 8, but not at day 12 or later ages, spinocerebellar axons which originate from spinal border cells, the sacral/coccygeal ventrolateral nucleus, and Stilling's nucleus grow through the lesion and reach the cerebellum. The critical period for such growth is comparable to that reported previously for spinocerebellar axons originating within Clarke's nucleus and for axons of the fasciculus gracilis, but shorter than that for most descending spinal axons. It appears, therefore, that differences exist in the ability of ascending and descending axons to traverse a lesion of their spinal pathway during development.

Early development and developmental plasticity of the fasciculus gracilis in the North American opossum (Didelphis virginiana)

The first objective of the present study was to ask when axons of the fasciculus gracilis reach the nucleus gracilis in the North American opossum (Didelphis virginiana). When Fast Blue (FB) was injected into the lumbar cord on postnatal day (PD) 1 and the pups were killed 2 days later, labeled axons were present within a distinct fasciculus gracilis at thoracic and cervical levels of the cord. When comparable injections were made at PD3 or 5 and the pups were allowed to survive for the same time period, a few labeled axons could be followed to the caudal medulla where they were located dorsal to the presumptive nucleus gracilis. In order to verify these observations and to determine if any of the axons which innervate the nucleus gracilis early in development originate within dorsal root ganglia, we also employed cholera toxin conjugated to horseradish peroxidase (CT-HRP) to label dorsal root axons transganglionically. When CT-HRP was injected into the hindlimb on PD1 and the pups were maintained for 1 day prior to death and HRP histochemistry, labeled axons were present within the fasciculus gracilis at thoracic and cervical levels, but they could not be traced into the medulla. When comparable injections were made on PD3, and the pups were maintained for 2 days, labeled axons were present within the caudal medulla. Our second objective was to determine whether axons of the fasciculus gracilis grow through a lesion of their spinal pathway during early development. In one group of animals, the thoracic cord was transected at PD5, 8, 12, 20 and 26 and bilateral injections of Fast Blue (FB) were made four segments caudal to the lesion 30-40 days later. After a 3-5 day survival, the pups were killed and perfused so that the spinal cord and brainstem could be removed and sectioned for fluorescence microscopy. In all of the cases lesioned at PD5, axons of the fasciculus gracilis were labeled rostral to the site of transection and they could be followed to the nucleus gracilis. Evidence for growth of fasciculus gracilis axons into the caudal medulla was also seen in cases lesioned at PD8. In contrast, labeled axons were not observed rostral to the lesion when it was made at PD12 or at later stages of development. In order to verify that some of the axons which crossed the lesion originated within dorsal root ganglia, the thoracic cord was transected at PD5 in another group of animals and 7 days later, injections of CT-HRP were made into one of the hindlimbs. After a 3 day survival, labeled axons could be traced through the lesion site and into the caudal medulla. We conclude that axons of the fasciculus gracilis reach the nucleus gracilis by at least PD5 in the opossum and that they grow through a lesion of their spinal pathway when it is made at the same age or shortly thereafter. The critical period for such growth appears to end between PD8 and PD12.

Attempts to facilitate dorsal column axonal regeneration in a neonatal spinal environment

The response to injury of ascending collaterals of dorsal root axons within the dorsal column (DC) was studied after neonatal spinal overhemisection (OH) made at different levels of the spinal cord. The transganglionic tracer, cholera toxin conjugated to horseradish peroxidase, and the anterograde tracer, biotinylated dextran amine, were used to label dorsal root ganglion cells with peripheral axons contributing to the sciatic nerve. There was no indication of a regenerative attempt by DC axons at acute survival times (3 days and later) after cervical injury, replicating previous work done at chronic survival periods (Lahr and Stelzner [1990] J. Comp. Neurol. 293:377-398). There was also no evidence of DC regeneration after lumbar OH injury even though immunohistochemical studies using the oligodendrocyte markers Rip and myelin basic protein showed few oligodendrocytes in the gracile fasciculus at lumbar levels at birth. Therefore, the lack of myelin in the dorsal funiculus at lumbar levels does not enhance the growth of neonatally axotomized DC axons. In addition, DC axons did not regenerate when presented with fetal spinal tissue implanted into thoracic OH lesions, even though positive control experiments showed that segmental dorsal root axons containing calcition gene-related peptide and corticospinal axons grew into these implants, replicating previous work of others. When a thoracic OH lesion, with or without a fetal spinal implant, was combined with sciatic nerve injury to attempt to stimulate an intracellular regenerative response of DRG neurons, again, no evidence of DC axonal regeneration was detected. Quantitative studies of the L4 and L5 dorsal root ganglia (DRG) showed that OH injury did not result in DRG neuronal loss. However, sciatic nerve injury did result in significant post-axotomy retrograde cell loss of DRG neurons, even in groups receiving thoracic embryonic spinal implants, and is one explanation for the minimal effect of sciatic nerve injury on DC regeneration. Although fetal tissue did not appear to rescue a significant number of DRG neurons, the quantitative analysis showed an enlargement of the largest class of DRG neuron, the class that contributes to the DC projection, in all groups receiving fetal tissue implants. This apparent trophic effect did not affect DC regeneration or neuronal survival after peripheral axotomy. Further studies are needed to determine why DC axons do not regenerate in a neonatal spinal environment or within fetal tissue implants, especially because previous work by others in both the developing and adult spinal cord shows that dorsal root axons will grow within the same type of fetal spinal implant.

Evidence for growth of supraspinal axons through the lesion after transection of the thoracic spinal cord in the developing opossum Didelphis virginiana

In the present study, we asked whether supraspinal axons grow through a complete transection of the spinal cord in the developing opossum Didelphis virginiana. When the thoracic cord was transected at postnatal day (PD) 5 and bilateral injections of Fast Blue (FB) were made four segments caudal to the lesion 30-40 days later, FB-containing neurons were found in each of the supraspinal nuclei labeled by comparable injections in age-matched unlesioned controls. Continuity between the cut ends of the cord was obviously gross when the animals were killed, and histologically recognizable spinal cord was present at the lesion site. When the same procedure was followed on pups subjected to transection at PD12, FB-containing neurons were still present at supraspinal levels, but they appeared to be fewer in number than in the PD5 cases or the age-matched controls, and none were found within the medial pontine reticular and lateral vestibular nuclei. When the lesion was made at PD20, labeled neurons were even fewer in number, and when it was made at PD26, they were restricted to the medullary raphe and the red nuclei. There was no evidence for growth of supraspinal axons across lesions made at PD33. We conclude that supraspinal axons grow through the lesion after transection of the spinal cord in neonatal opossums and that the critical period for growth of reticulospinal and vestibulospinal axons through the lesion ends earlier than that for comparable growth of raphespinal and rubrospinal axons.

Growth of dorsal spinocerebellar axons through a lesion of their spinal pathway during early development in the North American opossum, Didelphis virginiana

Supraspinal axons grow around or through lesions of their spinal pathway during specific critical periods of mammalian development, but comparable plasticity has not been documented for axons which form ascending tracts. In the present study, we asked whether axons of the dorsal spinocerebellar tract (DSCT) are capable of such growth. The spinal cord of the North American opossum, Didelphis virginiana, was hemisected at mid-thoracic levels between postnatal day (PD) 5 and 68 and after varying survival times, bilateral injections of Fluoro-Gold or Fast Blue were made into the anterior lobe of the cerebellum, the major target of DSCT axons. Seven days later, the pups were sacrificed and their spinal cord processed for fluorescence microscopy. In animals lesioned between PD5 and 9, and allowed to survive for 37-269 days, neurons were labeled bilaterally in Clarke's nucleus (CN) caudal to the lesion, but they were fewest in number and smallest in size on the lesioned side. Since the DSCT originates almost entirely within CN on the ipsilateral side, we conclude that the neurons labeled ipsilateral and caudal to the lesion supported axons which grew around or through it. Histological examination revealed that recognizable spinal cord was present at the lesion site and that labeled spinocerebellar axons were located in their normal position ipsilateral to the lesion. It appears, therefore, that growth occurred through the lesion. In animals lesioned between PD13 and 68, labeled neurons were not found in CN caudal and ipsilateral to the lesion although they were present on the contralateral (control) side. We conclude that DSCT axons, like axons which form descending tracts, grow through a lesion of their spinal pathway if it is made early in development.

Myelin-associated neurite growth-inhibitory proteins and suppression of regeneration of immature mammalian spinal cord in culture

Neurite outgrowth across spinal cord lesions in vitro is rapid in preparations isolated from the neonatal opossum Monodelphis domestica up to the age of 12 days. At this age oligodendrocytes, myelin, and astrocytes develop and regeneration ceases to occur. The role of myelin-associated neurite growth-inhibitory proteins, which increase in concentration at 10-13 days, was investigated in culture by applying the antibody IN-1, which blocks their effects. In the presence of IN-1, 22 out of 39 preparations from animals aged 13-17 days showed clear outgrowth of processes into crushes. When 34 preparations from 13-day-old animals were crushed and cultured without antibody, no axons grew into the lesion. The success rate with IN-1 was comparable to that seen in younger animals but the outgrowth was less profuse. IN-1 was shown by immunocytochemistry to penetrate the spinal cord. Other antibodies which penetrated the 13-day cord failed to promote fiber outgrowth. To distinguish between regeneration by cut neurites and outgrowth by developing uncut neurites, fibers in the ventral fasciculus were prelabeled with carbocyanine dyes and subsequently injured. The presence of labeled fibers in the lesion indicated that IN-1 promoted regeneration. These results show that the development of myelin-associated growth-inhibitory proteins contributes to the loss of regeneration as the mammalian central nervous system matures. The definition of a critical period for regeneration, coupled with the ability to apply trophic as well as inhibitory molecules to the culture, can permit quantitative assessment of molecular interactions that promote spinal cord regeneration.