Differences in the connectivity of rat pudendal motor nuclei as revealed by retrograde transneuronal transport of wheat germ agglutinin

Bilateral coordinated activation of pudendal motoneurons is an essential component of penile reflexes in male rats. However, little is known about the intraspinal organization of these reflexes. In the present study, retrograde transneuronal transport of wheat germ agglutinin (WGA) was used to examine the organization of spinal motoneurons and putative interneurons mediating penile reflexes in adult male rats. Injection of WGA into the ventral bulbospongiosus muscle resulted in direct retrograde labeling of motoneurons in the ipsilateral dorsomedial (DM) nucleus and transneuronal labeling of ipsilateral and contralateral DM motoneurons. Motoneurons in the ipsilateral and contralateral dorsolateral (DL) nuclei were not labeled. WGA-labeled putative interneurons were observed bilaterally, primarily in the ventromedial spinal gray matter extending dorsally to the central canal and the dorsal gray commissure. The number of transneuronally labeled putative interneurons increased with longer survival times. Injection of WGA into the ischiocavernosus muscle resulted in direct retrograde labeling of motoneurons in the medial subdivision of the ipsilateral DL nucleus. However, no WGA labeling was detected in motoneurons in the lateral subdivision of the ipsilateral DL nucleus, the contralateral DL nucleus, or the DM nuclei at any of the survival times studied (1–7 days). Only a small number of transneuronally labeled putative interneurons was observed in the ventrolateral gray matter at longer survival times (3–7 days). Thus, marked differences were observed between the DM and DL nuclei with respect to the transneuronal transport of WGA. These results are discussed with respect to the organization of the spinal circuits that mediate pudendal motor reflexes. © 1995 Wiley-Liss, Inc.     

Alteration in axial motoneuronal morphology in the spinal cord injured spastic rat

Following spinal cord injury (SCI), exaggerated reflexes and muscle tone emerge that contribute to a general spastic syndrome in humans. At present, the underlying mechanisms involved with the development of spasticity following traumatic spinal cord injury, especially with regard to axial musculature, remains unclear. The purpose of the present study was to examine the temporal changes in sacrocaudal motoneuronal morphology following complete transection of the sacral spinal cord and to correlate these changes with the onset and progression of spasticity within the tail musculature. The spinal cords of rats were transected at the upper sacral (S(2)) level. Animals were behaviorally tested for the onset and progression of spasticity in the tail and at 1, 2, 4, or 12 weeks postinjury were sacrificed. At these time points, the animals demonstrated stage 1, 2, 3, or 4 spastic behavior, respectively. Sacrocaudal motoneurons innervating selected flexor muscles within the tail were retrogradely labeled with cholera toxin beta-subunit and neuronal morphology was analyzed using a combination of immunocytochemistry and standard microscopy. Initially over the first 2 weeks postinjury, a transient increase in the lengths of primary and secondary dendrites occurred. However, a progressive decrease in the overall number of dendritic branches was observed between 2 and 12 weeks postinjury, which parallels the time frame for the progressive increase in spastic behavior in the tail musculature. Following spinal cord injury, there is an alteration in the morphology of tail flexor motoneurons, which may be relevant to the development of spasticity within the tail.     

Contrasting neuropathology and functional recovery after spinal cord injury in developing and adult rats

Conflicting findings exist regarding the link between functional recovery and the regrowth of spinal tracts across the lesion leading to the restoration of functional contacts. In the present study, we investigated whether functional locomotor recovery was attributable to anatomical regeneration at postnatal day 1 (PN1), PN7, PN14 and in adult rats two months after transection injury at the tenth thoracic segment of the spinal cord. The Basso, Beattie, and Bresnahan scores showed that transection led to a failure of hindlimb locomotor function in PN14 and adult rats. However, PN1 and PN7 rats showed a significant level of stepping function after complete spinal cord transection. Unexpectedly, unlike the transected PN14 and adult rats in which the spinal cord underwent limited secondary degeneration and showed a scar at the lesion site, the rats transected at PN1 and PN7 showed massive secondary degeneration both anterograde and retrograde, leaving a >5-mm gap between the two stumps. Furthermore, retrograde tracing with fluorogold (FG) also showed that FG did not cross the transection site in PN1 and PN7 rats as in PN14 and adult rats, and re-transection of the cord caused no apparent loss in locomotor performance in the rats transected at PN1. Thus, these three lines of evidence strongly indicated that the functional recovery after transection in neonatal rats is independent of regrowth of spinal tracts across the lesion site. Our results support the notion that the recovery of locomotor function in developing rats may be due to intrinsic adaptations in the spinal circuitry below the lesion that control hindlimb locomotor activity rather than the regrowth of spinal tracts across the lesion. The difference in secondary degeneration between neonatal and adult rats remains to be explored.     

Glial cell line-derived neurotrophic factor enhances axonal regeneration following sciatic nerve transection in adult rats

Adult rat sciatic nerve was transected and sutured with an entubulation technique. The nerve interstump gap was filled with either collagen gel (COL) or collagen gel mixed with glial cell line-derived neurotrophic factor (COL/GDNF). Four weeks after nerve transection, horseradish peroxidase (HRP)-labelled spinal cord motoneurons and the myelinated distal stump axons were quantified. Compared with the COL group, the percentages of labeled spinal somas and axon number were significantly increased after topically applied glial cell line-derived neurotrophic factor (GDNF). The functional recovery of the transected nerve was improved in COL/GDNF group. GAP-43 expression was also significantly higher in COL/GDNF group 1 and 2 weeks after sciatic nerve axotomy vs. COL group. These data provide strong evidence that GDNF could promote axonal regeneration in adult rats, suggesting the potential use of GDNF in therapeutic approaches to peripheral nerve injury and neuropathies.     

Cell death of motoneurons in the chick embryo spinal cord. III. The differentiation of motoneurons prior to their induced degeneration following limb-bud removal

The differentiation of motoneurons following early limb-bud ablation was studied in chick embryos from four days to hatching. Following the removal of the normal targets of these cells about 90% of the neurons in the lateral motor column (LMC) of segments 23–29 (lumbar) were found to disappear. By counting degenerating cells it was shown that virtually all of the cell loss could be accounted for by cell death, rather than impaired proliferation or enhanced migration away from the LMC. Quantitative comparisons of cell death between the peripherally deprived and the control, non-deprived side demonstrated that limb-bud removal not only enhanced the 50% natural cell death known to occur in this system, but also greatly accelerated the whole process. By stage 30 (6.5-7 days) 75% of the final cell loss had occurred on the deprived side, whereas only 40% of the final cell loss had occurred on the control side. In both cases, however, cell death was confined to the period of limb innervation. Axon counts of the peripherally deprived ventral root showed that all the deprived neurons initially had sent an axon out of the spinal cord. Most of these, however, became caught in a neuroma before reaching the site of limb attachment. Though no synapses were found in the neuroma the axons were shown to be able to transport HRP back to the spinal cord. Before they began to degenerate, the deprived LMC motoneurons developed dendritic processes and these were able to form synapses with axons in the prospective lateral white matter. In early stages, frequent axo-glial “synapses” were observed in the prospective lateral white matter of both deprived and control sides of the spinal cord. Since by stage 36 (day 10) these had virtually all disappeared, it was suggested that synapse formation in this region of the spinal cord may initially be under few constraints. In late stages (i.e., after day 8) it was noted that there were frequently signs of axonal degeneration in the lateral white matter on both sides of the spinal cord, suggesting a retrograde transneuronal degeneration initiated by the earlier cell death of motoneurons. Electron microscopic examination of the deprived LMC cells at different stages prior to degeneration failed to uncover any obvious differences between them and control cells on the non-deprived side of the spinal cord. By histochemical and neurochemical methods the cholinergic enzymes acetylcholinesterase and choline acetyltransferase were found to develop normally up until the onset of frank degeneration in the deprived motoneurons, on day 5 or 6. After this the enzymes decreased at a rate comparableto the morphological loss of motoneurons by cell death. On the basis of these various lines of evidence, it is argued that all the motoneurons in the LMC have a remarkable intrinsic capacity to initiate differentiation and that neurons experimentallydeprived of their normal target are no different in this respect.     

The orthograde flow of tritiated proline in corticospinal neurons at various ages and after spinal cord injury.

The amount of radioactive proline which reaches the cervical cord by axoplasmic flow after intracortical injection of label is higher in rapidly growing 3 to 6 week old rats but becomes relatively constant in unoperated control rats beyond age 10 weeks. In adult rats with spinal cord transection at T-8, however, the amount of tritiated proline detected in the cervical cord above the site of transection is markedly increased five weeks after surgery, falls to more normal levels by 14 weeks after surgery, and is significantly below normal at 25 weeks after surgery. These findings are consistent with abortive attempts to regenerate axons at five weeks after injury. Twenty-five weeks after injury neuronal death and loss of both cells and axons which would normally project to the caudal cord through the site of spinal cord transection result in a decrease in the axon label found in the cervical region. Recognition of this variability in the amount of radioactivity that reaches the cervical region after spinal cord injury forced a reconsideration of previously reported evidence for regeneration in spinal cord transected animals receiving no specific postoperative therapy. There is no evidence for regeneration in such untreated transected rats.     

Influence of the paraventricular nucleus and oxytocin on the retrograde stain of pubococcygeus muscle motoneurons in male rats

Lumbosacral cord motoneurons innervating the pubococcygeus muscle (Pcm) at the pelvic floor of male rats were analyzed. We showed previously that these motoneurons participate in sexual functions and are sensitive to fluctuations of systemic androgen and estrogen. Though estrogen receptors have not been identified in Lamina IX at these spinal areas, the release of oxytocin from the paraventricular nucleus of the hypothalamus (PvN) has been found to control pelvic sexual physiology. We therefore worked on the hypothesis that steroid hormones in the PvN induce the release of oxytocin at the lumbosacral level to modulate the function of Pcm motoneurons. Four experiments were developed, and results were observed with the retrograde staining of motoneurons with horseradish peroxidase. Data indicated that morphometric parameters of Pcm motoneurons were significantly reduced after castration or blocking of the steroids at the PvN site, or following complete transection of the spinal cord at the T8 level. In each case, the reduction of the stain was recovered after intrathecal treatment with oxytocin. Thus, present results show that Pcm motoneurons respond to spinal oxytocin. The conclusive model that we propose is that steroids stimulate the PvN, causing the nucleus to release oxytocin at the level of the lumbosacral spinal cord, and the release of the peptide regulates the spread of the stain of Pcm motoneurons. This work also shows that motoneurons distal to a transected area in the spinal cord could respond to exogenous oxytocin, an important finding for the research of spinal cord lesioned subjects.     

Ascending tract neurons survive spinal cord transection in the neonatal rat

Retrograde axonal transport was used to determine which ascending nerve tracts from the lumbosacral spinal cord are present in the cervical spinal cord of the newborn rat and if their cell bodies survive axotomy. A pledget of true blue was applied to a low cervical spinal transection in the newborn rat (N = 4). After a 5-day survival period, neurons were labeled in the laminae of origin of all ascending nerve tracts throughout the lumbosacral spinal cord. Neurons labeled in the same way survived for at least 1 month postoperatively when the spinal cord was transected at a midthoracic level at 5 days of age (N = 4). No neurons in the lumbosacral spinal cord were labeled if the midthoracic spinal cord was transected at the same time as application of the dye to cervical spinal cord (N = 2). Therefore, neurons labeled with true blue from cervical spinal cord during the neonatal period are likely to have been axotomized by thoracic injury made at 5 days of age. Three months after midthoracic spinal transection of newborn rats, HRP was injected or a pledget was applied to the first spinal segment caudal to this lesion (N = 8). The same population of neurons was labeled as in adult rats receiving application of HRP to an acute midthoracic spinal transection (N = 4). Neurons were seldom labeled in adult rats in which HRP was injected and ascending nerve tract axons not damaged (N = 4). These results suggest that most ascending nerve tract axons are present in cervical spinal cord during the neonatal period (by 4 to 5 days of age).(ABSTRACT TRUNCATED AT 250 WORDS)     

Axonal regeneration of descending brain neurons in larval lamprey demonstrated by retrograde double labeling

In larval lamprey, the large, identified descending brain neurons (Müller and Mauthner cells) are capable of axonal regeneration. However, smaller, unidentified descending brain neurons, such as many of the reticulospinal (RS) neurons, probably initiate locomotion, and it is not known whether the majority of these neurons regenerate their axons after spinal cord transection. In the present study, this issue was addressed by using double labeling of descending brain neurons. In double-label control animals, in which Fluoro-Gold (FG) was applied to the spinal cord at 40% body length (BL; measured from anterior to posterior from tip of head) and Texas red dextran amine (TRDA) was applied later to the spinal cord at 20% BL, an average of 98% of descending brain neurons were double labeled. In double-label experimental animals, FG was applied to the spinal cord at 40% BL; two weeks later the spinal cord was transected at 10% BL; and, eight weeks or 16 weeks after spinal cord transection, TRDA was applied to the spinal cord at 20% BL. At eight weeks and 16 weeks after spinal cord transection, an average of 49% and 68%, respectively, of descending brain neurons, including many unidentified RS neurons, were double labeled. These results in larval lamprey are the first to demonstrate that the majority of descending brain neurons, including small, unidentified RS neurons, regenerate their axons after spinal cord transection. Therefore, in spinal cord-transected lamprey, axonal regeneration of descending brain neurons probably contributes significantly to the recovery of locomotor function.     

L1 CAM expression is increased surrounding the lesion site in rats with complete spinal cord transection as neonates

L1 is a cell adhesion molecule associated with axonal outgrowth, fasciculation, and guidance during development and injury. In this study, we examined the long-term effects of spinal cord injury with and without exercise on the re-expression of L1 throughout the rat spinal cord. Spinal cords from control rats were compared to those from rats receiving complete mid-thoracic spinal cord transections at postnatal day 5, daily treadmill step training for up to 8 weeks, or both transection and step training. Three months after spinal cord transection, we observed substantially higher levels of L1 expression by both Western blot analysis and immunocytochemistry in rats with and without step training. Higher expression levels of L1 were seen in the dorsal gray matter and in the dorsal lateral funiculus both above and below the lesion site. In addition, L1 was re-expressed on the descending fibers of the corticospinal tract above the lesion. L1-labeled axons also expressed GAP-43, a protein associated with axon outgrowth and regeneration. Treadmill step training had no effect on L1 expression in either control or transected rats despite the fact that spinal transected rats displayed improved stepping patterns indicative of spinal learning. Thus, spinal cord transection at an early age induced substantial L1 expression on axons near the lesion site, but was not additionally augmented by exercise.     

Localization of the spinal accessory motoneurons in the cervical cord in connection with the phrenic nucleus: an HRP study in cats

The localization of the spinal accessory motoneurons (SAMNs) that innervate the accessory respiratory muscles, the sternocleidomastoid (SCM) and trapezius (TP) muscles, was identified in the cat using the horseradish peroxidase (HRP) method. In the cases of HRP bathing of the transected spinal accessory nerve (SAN), HRP-labeled motoneurons were observed ipsilaterally from the C1 to the rostral C6 segments of the spinal cord. Labeled neurons were located principally in the medial and central regions of the dorsomedial cell column of the ventral horn in the C1 segment, in the lateral region of the ventrolateral cell column in the C2-C4 segments, between the ventrolateral and ventromedial cell columns in the C5 segment and in the lateral region of the ventromedial cell column in the C6 segment. In the cases of HRP injection into either SCM or TP muscles, labeled SCM motoneurons were found in the C1-C3 segments of the spinal cord and labeled TP motoneurons were chiefly localized more caudally within the spinal accessory nucleus. The present study revealed that, in the C5 and C6 segments, the SAMNs have a very similar topographic localization to the phrenic nucleus in the ventral horn. This finding implicated the functional linkage of the SAMNs with the phrenic motoneurons in particular types of respiration.     

Retrograde labeling of phrenic motoneurons by intrapleural injection

Studies of motoneuron plasticity during development or in response to injury or disease rely on the ability to correctly identify motoneurons innervating specific muscle groups. Commonly, injections of retrograde tracer molecules into a target muscle or into a transected nerve are used to label specific motoneuron pools. However, intramuscular injection does not consistently label all motoneurons in the target pool; and either injection site may involve additional surgical procedures and muscle or nerve perturbations. For instance, retrograde labeling of phrenic motoneurons by injection into the diaphragm muscle is commonly employed in studies of plasticity in respiratory motor control. Diaphragm intramuscular injection involves a laparotomy, and this additional surgery may limit the ability to conduct labeling studies particularly in small animals. In the present study, we provide validation of a novel method for phrenic motoneuron labeling using intrapleural injection of Alexa 488-conjugated cholera toxin subunit B. Only phrenic motoneurons were labeled in the cervical spinal cord as verified by co-staining with rhodamine-conjugated dextran injected into the diaphragm muscle or applied via phrenic nerve dip. Thoracic intercostal motoneurons and some dorsal root ganglia neurons were also labeled by intrapleural injection, but there was no evidence of trans-synaptic labeling. Phrenic motoneuron labeling was not present if the phrenic nerve was transected prior to intrapleural injection. This novel method is ideally suited for morphological studies and analyses of mRNA expression in isolated phrenic motoneurons using techniques such as laser capture microdissection.     

Nitric oxide system alteration at spinal cord as a result of perinatal asphyxia is involved in behavioral disabilities: Hypothermia as preventive treatment

Perinatal asphyxia (PA) is able to induce sequelae such as spinal spasticity. Previously, we demonstrated hypothermia as a neuroprotective treatment against cell degeneration triggered by increased nitric oxide (NO) release. Because spinal motoneurons are implicated in spasticity, our aim was to analyze the involvement of NO system at cervical and lumbar motoneurons after PA as well as the application of hypothermia as treatment. PA was performed by immersion of both uterine horns containing full‐term fetuses in a water bath at 37°C for 19 or 20 min (PA19 or PA20) or at 15°C for 20 min (hypothermia during PA‐HYP). Some randomly chosen PA20 rats were immediately exposed for 5 min over grain ice (hypothermia after PA‐HPA). Full‐term vaginally delivered rats were used as control (CTL). We analyzed NO synthase (NOS) activity, expression and localization by nicotinamide adenine dinucleotide phosphate diaphorase (NADPH‐d) reactivity, inducible and neuronal NOS (iNOS and nNOS) by immunohistochemistry, and protein nitrotyrosilation state. We observed an increased NOS activity at cervical spinal cord of 60‐day‐old PA20 rats, with increased NADPH‐d, iNOS, and nitrotyrosine expression in cervical motoneurons and increased NADPH‐d in neurons of layer X. Lumbar neurons were not altered. Hypothermia was able to maintain CTL values. Also, we observed decreased forelimb motor potency in the PA20 group, which could be attributed to changes at cervical motoneurons. This study shows that PA can induce spasticity produced by alterations in the NO system of the cervical spinal cord. Moreover, this situation can be prevented by perinatal hypothermia. © 2008 Wiley‐Liss, Inc.     

A comparison of the effect of mid-thoracic spinal hemisection in the neonatal or weanling rat on the distribution and density of dorsal root axons in the lumbosacral spinal cord of the adult

Transecting the thoracic spinal cord of the rat has markedly different effects on behavioral responses of the hindlimbs if the lesion is made at the neonatal or weanling stage of development. The present investigation tested the possibility that the behavioral differences were related to a difference in the distribution or density of dorsal root connections in the lumbosacral spinal cord. In order to use each animal as its own control the distribution and density of dorsal root axons was compared on the two sides of the L5-S1 segments of the lumbosacral spinal cord in adult rats given a mid-thoracic spinal hemisection at the neonatal or weanling stage of development. Comparing the experimental (initially hemisected side) and control sides of the cord, we found no evidence for a change in the distribution of dorsal root axons. The distribution of Fink-Heimer stained degeneration 4--6 days after bilateral spinal root section was virtually identical on the two sides of the cord from animals hemisected at either stage. However, in rats spinally hemisected at the neonatal stage (n = 8), a significantly greater density of dorsal root degeneration was found within the intermediate nucleus of Cajal (INC) on the experimental side using coded material and a blind analysis. No difference in the density of dorsal root degeneration was detected in the group of rats spinally hemisected at the weanling stage (n = 6). Controls indicated that the increased density of degeneration was not due to compression resulting from shrinkage of the INC or to degeneration remaining from the initial hemisection. We conclude that the increased amount of argyrophilia within the INC of neonatally hemisected rats is due to an increased density of dorsal root axons in this zone. This result supports the hypothesis that the behavioral differences found when comparing animals transected at the neonatal or weanling stages of development are related to an increased number of dorsal root connections within the lumbosacral spinal cord.     

Motoneuron death in normal and spinal muscular atrophy-affected human fetuses

The onset and sequence of motoneuron death in the lumbar part of spinal cord of seven control and five affected fetuses carrying deletion of exon 7 in the SMN gene were studied by light and electron microscopy. Naturally occurring motoneuron death in control fetuses was detected between 8 and 13 weeks of gestation. In affected fetuses motoneuron death was prolonged and also observed at 16 and 20 weeks of gestation. In addition, motoneurons in the affected fetuses displayed nuclear abnormalities as early as 16 weeks of gestation and the changes progressed as the affected fetuses developed and can be considered first signs of motoneuron degeneration.     

CNS cell groups involved in the control of the ischiocavernosus and bulbospongiosus muscles: A transneuronal tracing study using pseudorabies virus

Transneuronal tracing techniques were used to identify spinal and brainstem neurons involved in the control of perineal muscles in the male rat. Two penile muscles, the bulbospongiosus and ischiocavernosus muscles, were injected with Bartha's strain of pseudorabies virus. After survival periods of 2, 4, and 5 days, the rats were killed and viral labeled neurons identified by immunohistochemistry. After a 2 day survival period, only pudendal motoneurons were labeled. More spinal and brainstem neurons were labeled at longer survival times. Putative spinal interneurons were found from T13 to S1. Large numbers of neurons were found in the lateral horn of the T13-L2 and L6-S1 segments which contain sympathetic and parasympathetic preganglionic neurons, respectively. However, retrograde labeling experiments verified that very few of the viral neurons were preganglionic neurons. Other labeled neurons were found in the intermediate cord, especially around the central canal. Relatively few labeled neurons were seen in the dorsal or ventral horn. In the brainstem, consistent labeling was seen in the ventrolateral medulla, raphe pallidus, and magnus, the A5 and locus ceruleus noradrenergic cell groups, Barrington's nucleus in the pontine tegmentum, the periaqueductal gray, and the paraventricular nucleus of the hypothalamus. The transneuronal labeling was consistent with what is currently known of the central nervous system (CNS) control of the perineal muscles. © 1996 Wiley-Liss, Inc.     

Prelabeled red nucleus and sensorimotor cortex neurons of the rat survive 10 and 20 weeks after spinal cord transection

To demonstrate definitively the fate of the somata of rubrospinal and corticospinal neurons axotomized by a complete spinal cord transection at T-9, in young adult rats we prelabeled the neurons by injection into the lumbar enlargement of a retrogradely transported fluorescent dye, Fluoro-Gold, and four days later transected the cord. We found no loss in cell number ten or 20 weeks after axotomy. The average size of the neurons in each case is slightly but significantly reduced. These findings unequivocally demonstrate that the somata of long tract neurons of the rubrospinal and corticospinal systems persist in an atrophic and presumably inactive state for at least 20 weeks, and raise the possibility that treatment of spinal cord injury may normalize cell activity and allow long tract regeneration.     

Treadmill training induces plasticity in spinal motoneurons and sciatic nerve after sensorimotor restriction during early postnatal period: new insights into the clinical approach for children with cerebral palsy

The aim of the present study was to investigate whether locomotor stimulation training could have beneficial effects on the morphometric alterations of spinal cord and sciatic nerve consequent to sensorimotor restriction (SR). Male Wistar rats were exposed to SR from postnatal day 2 (P2) to P28. Control and experimental rats underwent locomotor stimulation training in a treadmill for three weeks (from P31 to P52). The cross-sectional area (CSA) of spinal motoneurons innervating hind limb muscles was determined. Both fiber and axonal CSA of myelinated fibers were also assessed. The growth-related increase in CSA of motoneurons in the SR group was less than controls. After SR, the mean motoneuron soma size was reduced with an increase in the proportion of motoneurons with a soma size of between 0 and 800 μm(2). The changes in soma size of motoneurons were accompanied by a reduction in the mean fiber and axon CSA of sciatic nerve. The soma size of motoneurons was reestablished at the end of the training period reaching controls level. Our results suggest that SR during early postnatal life retards the growth-related increase in the cell body size of motoneurons in spinal cord and the development of sciatic nerve. Additionally, three weeks of locomotor stimulation using a treadmill seems to have a beneficial effect on motoneurons' soma size.     

Motoneuron loss in the abducens nucleus of wobbler mice

The 'wobbler' mutant mouse can be recognized at about 4 weeks of age by its tremor and atrophy of forelimb muscles. In addition to degeneration of spinal motoneurons, especially in cervical spinal cord, selected bulbar motoneurons have also been reported to degenerate in the mutant. We examined a cranial motor nucleus and found a 31% loss of abducens motoneurons in 4-5-week-old wobbler mice as compared to age-matched control mice.