Rubrospinal Research Articles

The role of some brain structures in the switching of the descending influences in operantly conditioned rats

A hypothesis was proposed according to which the switching of descending influences by the corticospinal and corticorubrospinal systems was associated with rubro-olivary projection involvement depending on the context of movement [Kennedy P. R. (1990) Trends Neurosci. 13, 474–479]. Our results confirmed and extended this hypothesis. It was shown that a preliminary transection of the dorsolateral funiculus (containing the rubrospinal tract) accelerated the compensatory rehabilitation process following lesions of the red nucleus and the ventrolateral thalamic nucleus in albino rats with learned instrumental reflexes on equilibrium. A preliminary lesion of the ventrolateral thalamic nucleus considerably hampered the switching process; nevertheless, performance of the reflexes suggested that the switching of cerebellar ascending influences to the cerebral cortex could be completed through other cerebellocortical pathways as well. Comparison of the results of electrolytic and chemical lesions of the red nucleus suggested a similar conclusion. It was established that the conditioning and recovery of already learned instrumental reflexes were impossible after complete neurotoxic destruction of the inferior olive. The data obtained emphasize the role of the inferior olive, ventrolateral thalamic nucleus and red nucleus in the switching of descending influences in operantly motor conditioned rats. Motor deficit and the compensatory rehabilitation process depended on the severity of inferior olive destruction combined with a high transection of the dorsolateral funiculus and a destroyed red nucleus. Long-lasting training improved compensation of motor deficit and stabilized instrumental reflexes to some extent in rats with incomplete destruction of the inferior olive. It has been suggested that these modifications occur because of collateral sprouting in the olivocerebellar system.

Immunological Myelin Disruption Does Not Alter Expression of Regeneration-Associated Genes in Intact or Axotomized Rubrospinal Neurons

The inability of axotomized neurons to regenerate within the CNS has been partially attributed to a number of inhibitory factors associated with CNS myelin that are extrinsic to the severed neurons. However, some neurons are capable of limited regeneration after injury and this ability has been shown to correlate with the expression of certain regeneration-associated genes (RAGs) intrinsic to injured neurons. It has therefore been postulated that neutralization of inhibitory factors, as well as the induction of an appropriate neuronal cell body response, would facilitate improved regrowth of injured CNS axons. In previous studies we have shown that immunological removal of myelin from the spinal cord facilitates axonal regeneration by rubrospinal neurons, as indicated by retrograde transport of a fluorescent dye placed distal to the site of injury. Here, we investigated whether the immunological focal removal of spinal cord myelin, following a thoracic spinal cord injury, concomitantly stimulated an increase in the expression of RAGs in rubrospinal neurons. In situ hybridization for Tα-1 tubulin and GAP-43 at days 7, 14, and 21 revealed no significant increase in gene expression in rubrospinal neurons following immunological demyelination. The ability of various neuronal populations to sprout or slowly regrow without expressing the previously characterized cell body response is reviewed. We conclude that the recently demonstrated regeneration of rubrospinal tract, after immunologically directed spinal cord demyelination, is the result of either axonal sprouting or slow axonal regrowth without the increased expression of RAGs characteristic for fast axon regeneration.

Effects of noxious skin heating on spontaneous cell activity in the magnocellular red nucleus of the cat.

Neurones were extracellularly recorded in the magnocellular red nucleus (RNm) of decerebrated cats and identified by their monosynaptic responses to stimulation of the contralateral brachium conjunctivum (BC), and by their antidromic responses after stimulation of the rubrospinal tract in the cord. After each cell was isolated, noxious stimulation was applied to the skin, by touch-free radiant heat. The large majority of the cells (91.5%) responded to noxious stimulation of the skin, even after the RNm was deprived of cerebellar input by sectioning the contralateral BC. About half of this cell group was activated by noxious stimulation and the other half was inhibited. To obtain responses in RNm cells, the stimulus had to be above the thermal pain threshold, and the stimulation area had to be inside the somatic receptive field of the cell for low-threshold afferents. Thus, the receptive fields for nociceptive and low-threshold afferents are similarly organized somatotopically. Noxious stimulation sometimes induced a rhythmic pattern of spike bursts in RNm cells, which could be a reflection of the locomotor-like activity generated in the spinal cord. The results indicate that nociceptive information participates in sensorimotor control via the spinorubrospinal loop, which may also transmit low-threshold cutaneous input and corollary discharge of the activity in spinal motor centres to the RNm.

Red nucleus lesions impair overground locomotion in rats: a kinetic analysis.

The red nucleus is a prominent brainstem nucleus in mammals which is thought to be involved in production of skilled limb movements. The presence of the red nucleus and associated rubrospinal tract in animals that do not produce skilled limb movements, however, suggests that these structures might also be involved in control of more general limb actions, such as those occurring during locomotion. The present study investigates this question by measuring the three-dimensional ground reaction forces produced by locomoting rats with unilateral excitotoxic lesions of the red nucleus. Twenty-four to 48 h after the lesion, rats moved with an asymmetric gait during which abnormal braking and propulsive forces were produced during the dual contact time of the forelimb contralateral to the lesion and the ipsilateral hindlimb. Rats did not recover normal symmetrical locomotion within the 55-day duration of the study. The persistent asymmetry produced by red nucleus ablation provides the first unequivocal demonstration that the red nucleus plays a role in ongoing overground locomotion in the rat. Species differences in phylogeny and connectivity of the red nucleus are discussed, as well as the possibility that there is a general compensatory response to unilateral CNS injury in the rat.

Rescue of axotomized rubrospinal neurons by brain-derived neurotrophic factor (BDNF) in the developing opossum, Didelphis virginiana

Many rubrospinal neurons die in developing opossums when their axon is cut at thoracic levels of the spinal cord and in the present study we asked whether they can be rescued by brain-derived neurotrophic factor (BDNF). Bilateral injections of Fast Blue (FB) were made into the rostral lumbar cord to prelabel rubrospinal neurons and 5 days later the rubrospinal tract was cut unilaterally by hemisecting the thoracic cord. Immediately after hemisection, BDNF-soaked gelfoam was placed into the lesion cavity. Since pilot data indicated that one application of BDNF was not sufficient to produce a rescue effect, a second application was made 7 days later. Seven days after the second application the pups were killed by an overdose of anesthetic so that the red nucleus contralateral and ipsilateral to the lesion site could be examined for labeled neurons. The rubrospinal tract is almost entirely crossed, so the red nucleus contralateral to the lesion contained many axotomized neurons, whereas the red nucleus ipsilateral to it did not. Age-matched controls were subjected to the same procedures, but the gelfoam applied to the lesion site in the experimental animals was soaked only in the vehicle used to deliver BDNF. In all cases, labeled neurons were fewer in number in the red nucleus contralateral to the lesion than ipsilateral to it. It was of particular interest, however, that labeled neurons contralateral to the lesion were more numerous in the animals treated with BDNF than in the controls. We conclude that BDNF rescues at least some rubrospinal neurons from axotomy-induced cell death in developing opossums suggesting that loss of access to BDNF, and perhaps other neurotrophins, contributes to failure of rubrospinal neurons to survive axotomy.

Influence of the axotomy to cell body distance in rat rubrospinal and spinal motoneurons: differential regulation of GAP-43, tubulins, and neurofilament-M.

Axotomized motoneurons regenerate their axons regardless of whether axotomy occurs proximally or distally from their cell bodies. In contrast, regeneration of rubrospinal axons into peripheral nerve grafts has been detected after cervical but not after thoracic injury of the rubrospinal tract. By using in situ hybridization (ISH) combined with reliable retrograde tracing methods, we compared regeneration-associated gene expression after proximal and distal axotomy in spinal motoneurons versus rubrospinal neurons. Regardless of whether they were axotomized at the iliac crest (proximal) or popliteal fossa (distal), sciatic motoneurons underwent highly pronounced changes in ISH signals for Growth Associated Protein 43 (GAP-43) (10-20x increase) and neurofilament M (60-85% decrease). In contrast, tubulin ISH signals substantially increased only after proximal axotomy (3-5x increase). To compare these changes in gene expression with those of axotomized rubrospinal neurons, the rubrospinal tract was transected at the cervical (proximal) or thoracic (distal) levels of the spinal cord. Cervically axotomized rubrospinal neurons showed three- to fivefold increases in ISH signals for GAP-43 and tubulins (only transient) and a 75% decrease for neurofilament-M. In sharp contrast, thoracic axotomy had only marginal effects. After implantation of peripheral nerve transplants into the spinal cord injury sites, retrograde labeling with the sensitive retrograde tracer Fluoro-Gold identified regenerating rubrospinal neurons only after cervical axotomy. Furthermore, rubrospinal neurons specifically regenerating into the transplants were hypertrophied and expressed high levels of GAP-43 and tubulins. Taken together, these data support the concept that, even if central nervous system (CNS) axons are presented with a permissive/supportive environment, appropriate cell body responses to injury are a prerequisite for CNS axonal regeneration.

The effect of rubrospinal tractotomy on a conditioned limb response in the cat

Five cats were trained to perform a forelimb conditioned response to a paired tone CS/shock UCS. Rubrospinal tract section ipsilateral to the trained limb was carried out following criterion CR performance. Lesion sites were identified histologically and further confirmed by observation of cellular changes in the red nucleus contralateral to the trained limb. Tractotomy resulted in total or near-total loss of the CR. Prolonged postoperative training resulted in no increase in CR performance levels. The UCR remained unaffected, as did limb placing, accuracy of striking at moving objects, grooming, running and walking. Training of the opposite limb in two subjects resulted in mean scores of 90 and 85% within three sessions. Control lesions in those subjects resulted in no changes in CR performance scores. The red nucleus receives a substantial input from sensorimotor cortex and cerebellum, both of which have been shown to represent essential parts of the brain circuitry involved in associative learning and memory. Since pyramidotomy has no effect on limb CR performance [Vonedia TJ. Exp Neurol 1976;19:483–493], the possible role of the red nucleus/rubrospinal tract is discussed in terms of a critical trigger area for the expression of a learned motor response.

Recovery of food-taking in cats after lesions of the cortico- and rubrospinal tracts.

Recovery of food-taking in cats after lesions of the cortico- and rubrospinal tracts.

Disinhibition of lower midbrain neurons enhances non-shivering thermogenesis in anesthetized rats

The hypothesis that the lower midbrain, specifically in and around the retrorubral field (RRF) and/or rubrospinal tract (rs), contains a tonic inhibitory mechanism on non-shivering thermogenesis (NST) was examined in urethane-anesthetized rats. It has been proposed that removal of the tonic inhibitory mechanism in the lower midbrain causes body temperature increase through disinhibition-induced activation of the central sympathetic nervous system. The present experiments were carried out to examine whether and where the proposed midbrain region contains cell bodies that tonically inhibit the NST, and if so, whether they receive any influence from the hypothalamus. Male Wistar rats were anesthetized with urethane (1.0–1.2 g/kg, i.p.), and various agents were microinjected into the RRF and rs areas of one side before and after knife-cut in the other side of the lower midbrain or isolation of the hypothalamus from the midbrain. Changes in interscapular brown adipose tissue (IBAT), rectum, and tail skin temperatures were monitored. Results: (1) unilateral midbrain procaine increased IBAT and rectal temperatures only after un-injected side of the midbrain had been pre-transected. (2) Effective midbrain sites for procaine to increase IBAT and rectal temperatures was laterally extended. (3) Tetrodotoxin microinjected into the midbrain site where procaine increased IBAT and rectal temperatures also raised both temperatures. (4) l-glutamate decreased IBAT and rectal temperatures when microinjected into one of the most inner midbrain area of procaine-sensitive sites without affecting tail skin temperature. (5) Isolation of the hypothalamus from the lower midbrain did not affect midbrain procaine-induced IBAT and rectal temperature increases. These results suggest that neurons that tonically inhibit the NST are located in the area close to the midline adjacent to the RRF and rs, and that the integrity of the neurons to tonically inhibit the NST is not affected by disconnecting the hypothalamus from the midbrain.

Transplants of Fibroblasts Genetically Modified to Express BDNF Promote Regeneration of Adult Rat Rubrospinal Axons and Recovery of Forelimb Function

Adult mammalian CNS neurons do not normally regenerate their severed axons. This failure has been attributed to scar tissue and inhibitory molecules at the injury site that block the regenerating axons, a lack of trophic support for the axotomized neurons, and intrinsic neuronal changes that follow axotomy, including cell atrophy and death. We studied whether transplants of fibroblasts genetically engineered to produce brain-derived neurotrophic factor (BDNF) would promote rubrospinal tract (RST) regeneration in adult rats. Primary fibroblasts were modified by retroviral-mediated transfer of a DNA construct encoding the human BDNF gene, an internal ribosomal entry site, and a fusion gene of lacZ and neomycin resistance genes. The modified fibroblasts produce biologically active BDNF in vitro. These cells were grafted into a partial cervical hemisection cavity that completely interrupted one RST. One and two months after lesion and transplantation, RST regeneration was demonstrated with retrograde and anterograde tracing techniques. Retrograde tracing with fluorogold showed that approximately 7% of RST neurons regenerated axons at least three to four segments caudal to the transplants. Anterograde tracing with biotinylated dextran amine revealed that the RST axons regenerated through and around the transplants, grew for long distances within white matter caudal to the transplant, and terminated in spinal cord gray matter regions that are the normal targets of RST axons. Transplants of unmodified primary fibroblasts or Gelfoam alone did not elicit regeneration. Behavioral tests demonstrated that recipients of BDNF-producing fibroblasts showed significant recovery of forelimb usage, which was abolished by a second lesion that transected the regenerated axons.

Clusterin Upregulation Following Rubrospinal Tract Lesion in the Adult Rat

We have examined the expression of the multifunctional protein clusterin in the axotomized red nucleus, at the lesion site in the lateral funiculus of C3, as well as along the Wallerian degeneration in the lateral funiculus of T1. There was a marked increase in clusterin-immunoreactivity (IR) and clusterin mRNA in red nucleus nerve cell bodies. An early, transient occurrence of large, heavily clusterin-IR globules were found in axons in the spinal cord at the lesion site in C3 as well as a marked upregulation of mRNA for clusterin, presumably associated with reactive astrocytes and oligodendrocytes from 1 to 4 weeks postoperatively. Clusterin-IR and its mRNA were markedly increased in the zone of Wallerian degeneration at T1, where some strongly expressing cells were identified as oligodendrocytes. Taken together with previous changes in clusterin expression following peripheral nerve and dorsal root injury, we suggest that this protein is involved in regenerative as well as degenerative neural responses.

Chapter 19 Do Lengthening Contractions Represent a Case of Reversal in Recruitment Order?

Publisher Summary The central nervous system (CNS) recruits spinal motoneurons in an orderly fashion under most of the tested experimental paradigms. Experimental observations suggesting selective excitation/recruitment of larger motor units during the electrical stimulation of cutaneous nerves, electrical stimulation of the rubrospinal tract, and lengthening contractions have been reported. The results utilizing synchronous electrical stimulation of cutaneous nerves, or the red nucleus, have to be interpreted as nonphysiological. Instead, under physiological activation of these pathways, the preferential inhibition of small motoneurons by these pathways would change the slope of the recruitment curve instead of causing selective recruitment of large motoneurons. The implications and mechanisms for the selective recruitment reported during lengthening contractions have to be reconsidered. This chapter presents the data obtained with two different experimental paradigms, both of which result in lengthening of active muscles. These observations do not support selective recruitment of fast twitch motor units during lengthening contractions. Two main papers where single motor unit data have been produced and interpreted to suggest selective recruitment of large motor units are central to the discussion in the chapter.

Paw and limb use in skilled and spontaneous reaching after pyramidal tract, red nucleus and combined lesions in the rat: behavioral and anatomical dissociations

The pyramidal tract and red nucleus send prominent projections to the spinal cord and are thought to co-operate in producing skilled movements. In the present study, skilled reaching for food located on a shelf and spontaneous grasping, handling and eating pieces of pasta were video-recorded and analyzed in control rats, rats with unilateral ibotenic acid lesions of the red nucleus (RN), unilateral pyramidal tract lesions (PT) and combined lesions. The behavioral results suggest that skilled movements are organized as action patterns, easily recognizable and distinctive for each task. In both skilled and spontaneous reaching, PT lesions reduced success more than RN lesions, suggesting a greater role for the PT in guiding limb movements. Both lesions impaired rotatory movements including limb aiming, pronation and supination. RN lesions additionally abolished the arpeggio movement by which the paw is oriented for searching and grasping. Combined lesions were additive as rats lost both rotatory movements and arpeggio. Nevertheless, even after combined lesions, the rats were able to advance the limb, grasp food and withdraw the limb. The sparing following combined lesions suggests that other neural systems as well as compensatory adjustments assist the impaired limb. The results are discussed in relation to the possible distinctive contributions of the rubrospinal and corticospinal tract to the action patterns that comprise skilled movements in rats.

Indications for coupling between feline spinocervical tract neurones and midlumbar interneurones.

The possibility of collateral segmental actions of spinocervical tract (SCT) neurones upon interneurones with input from cutaneous and group II muscle afferents was investigated in deeply anaesthetized cats. To this end, intracellular and/or extracellular recordings were made from 35 dorsal horn and 15 intermediate zone interneurones in midlumbar segments of the spinal cord and effects of stimulation of the ipsilateral dorso-lateral funiculus (DLF) at C3 and C1 levels, i.e. below and above the lateral cervical nucleus where axons of SCT cells terminate, were compared. The stimuli applied at the C3 segment were within the range of stimuli (50-100 microA) required for antidromic activation of SCT neurones in the same experiment. Those applied at the C segment (200-500 microA) were at least 3 times stronger than C3 stimuli. Under the same experimental conditions, long ascending and descending tract neurones (dorsal spino-cerebellar and rubro-spinal tract neurones) with axons in the DLF were activated at similar thresholds from the C and C3 segments. Intracellular recordings were made from 29 interneurones of which 19 (65%) were dorsal horn and 10 (35%) were intermediate zone interneurones. Excitatory postsynaptic potentials (EPSPs) evoked by single stimuli applied at the C3 segment, but not the C segment, were found in 14 (48%) of those interneurones; their latencies (3.0-5.7 ms) and frequency following with only minimal temporal facilitation were as required for potentials being evoked monosynaptically by the fastest conducting SCT neurones. Extracellular recordings were made from 30 interneurones (24 dorsal horn and 6 intermediate zone interneurones), and in these neurones spike potentials induced from the C3, but not from the C segment, were evoked only by short trains of stimuli. However, their latencies from the first effective stimulus (4.3-5.4 ms) were compatible with mono- or oligosynaptically mediated collateral actions of SCT neurones. They were found in 10 (33%) of the 30 investigated interneurones. Similar effects of C3 stimuli were found in similar proportions of dorsal horn interneurones and intermediate zone interneurones. Indications were also found for synaptic actions evoked by C3 stimuli that could not be attributed to direct collateral actions of SCT neurones. In some intracellularly recorded dorsal horn interneurones, short-latency EPSPs were evoked from the C3 segment by the 2nd or 3rd stimulus in the train, but not by single stimuli. In other dorsal horn and intermediate zone interneurones, inhibitory postsynaptic potentials (IPSPs) were evoked from the C3 segment at minimal latencies (2.7-3.2 ms), which might be too short to allow their mediation via SCT neurones. We conclude that SCT neurones might be used to forward information from muscle group II and cutaneous afferents not only to neurones in the lateral cervical nucleus and via them to thalamus and cerebral cortex but also to interneurones in spinal reflex pathways. Thereby reflex actions evoked from group II and cutaneous afferents might be co-ordinated with responses mediated by supraspinal neurones. We conclude also that dorsal horn and intermediate zone mid-lumbar interneurones might contribute to the previously reported di-and poly-synaptic excitation or inhibition of postsynaptic dorsal column (PSDC), spinothalamic tract (STT) and spinomesencephalic tract (SMT) neurones by collateral actions of SCT cells. Thereby these interneurones might contribute to the co-ordination of responses mediated by various populations of supraspinal neurones.

Rewarding brain stimulation induces only sparse Fos-like immunoreactivity in dopaminergic neurons

In this study, c-fos immunohistochemistry was used to identify the brain regions activated by rewarding brain stimulation in rats. Rats had monopolar electrodes implanted in the medial forebrain bundle and were allocated to either a self-stimulation (n=4), yoked stimulation (n=4) or no stimulation (n=6) group. In a single 1h test session, each rat in the self-stimulation group made 1 000 nose poke responses with each response followed by a 0.5s train of brain stimulation. Rats in the yoked-stimulation group were paired with a partner in the self-stimulation group and received brain stimulation whenever their partner did. However, their nose poke responses did not trigger stimulation. This yoked procedure was thus used to identify any Fos-like immunoreactivity due to operant responding. Rats in the no stimulation group were placed in the same apparatus as the other rats but received no brain stimulation and were thus used to assess baseline Fos-like immunoreactivity. Results showed that stimulation increased Fos-like immunoreactivity in many areas of the brain in both the self-stimulation and yoked groups. The areas with the highest Fos-like immunoreactivity were ipsilateral to the electrode site and included the medial prefrontal cortex, lateral septum, nucleus accumbens (shell), the medial and lateral preoptic areas, bed nucleus of the stria terminalis, central amygdala, lateral habenula, dorsomedial hypothalamus, lateral hypothalamus and the anterior ventral tegmental area. Bilateral Fos-like immunoreactivity was evident in the nucleus accumbens core, paraventricular nucleus of the hypothalamus, the retrorubral fields and the locus coeruleus. A double-labelling procedure identifying both Fos and tyrosine hydroxylase was used to show that very few (<5%) of the A10 dopamine cell bodies in the ventral tegmental area expressed Fos following brain stimulation. In contrast, most of the noradrenergic neurons of the locus coeruleus (A6), rubrospinal tract (A5) and pontine tegmental area (A7) were Fos positive.Overall, the results show that rewarding brain stimulation induces Fos-like immunoreactivity in many forebrain regions but only sparsely in mesolimbic and mesocortical dopamine neurons. The similar patterns of Fos-like immunoreactivity seen in the self-stimulation and yoked-stimulation groups suggests that the operant responding for brain stimulation causes minimal Fos expression in itself.

Robust growth of chronically injured spinal cord axons induced by grafts of genetically modified NGF-secreting cells.

Little spontaneous regeneration of axons occurs after acute and chronic injury to the CNS. Previously we have shown that the continuous local delivery of neurotrophic factors to the acutely injured spinal cord induces robust growth of spinal and supraspinal axons. In the present study we examined whether chronically injured axons also demonstrate significant neurotrophin responsiveness. Adult rats underwent bilateral dorsal hemisection lesions that axotomize descending supraspinal pathways, including the corticospinal, rubrospinal, and cerulospinal tracts, and ascending dorsal spinal sensory projections. One to three months later, injured rats received grafts of syngenic fibroblasts genetically modified to produce nerve growth factor (NGF). Control subjects received unmodified cell grafts or cells transduced to express the reporter gene beta-galactosidase. Three to five months after grafting, animals that received NGF-secreting grafts showed dense growth of putative cerulospinal axons and primary sensory axons of the dorsolateral fasciculus into the grafted lesion site. Growth from corticospinal, raphaespinal, and local motor axons was not detected. Thus, robust growth of defined populations of supraspinal and spinal axons can be elicited in chronic stages after spinal cord injury by localized, continuous transgenic delivery of neurotrophic factors.

Effect of spinal cord lesions on forelimb target-reaching and on visually guided switching of target-reaching in the cat

Cats were trained to reach to an illuminated tube placed horizontally at shoulder level and retrieve food with the forepaw. The trajectory of an infrared light emitting diode, taped to the wrist dorsum, was recorded with a SELSPOT-like recording system. Movement paths and velocity profiles were compared before and after lesions: (1) in dorsal C5, transecting cortico- and rubrospinal pathways to the forelimb segments so that the cats could only use the C3–C4 propriospinal neurones (PNs) to command reaching, (2) in the ventral part of the lateral funicle in C5, transecting the axons of C3–C4 PNs so that the cats had to use circuitry in the forelimb segments to command reaching. Comparison of trajectories and velocity profiles before and after lesion 1 did not reveal any major qualitative change. After lesion 2, the last third of the movement was fragmented with separate lifting and protraction. Switching of target-reaching occurred when illumination was shifted to another tube during the ongoing movement. The switching latency measured from the time of illumination shift to the earliest change in movement trajectory had a minimal value of 50–60 ms. Short latencies were present after lesion 1 as well as lesion 2 which suggest that fast switching mediated by the C3–C4 PNs and the interneuronal system in the forelimb segments is controlled in parallel by the brain. In order to test a hypothesis that fast switching depends on the tectospinal and tecto-reticulospinal pathways (the tecto-reticulo-spinal system) a ventral lesion was made in C2 aiming at interrupting these pathways. Large ventral C2 lesions tended to block conduction in the more dorsally located rubrospinal (less in corticospinal) axons probably due to compression during surgery. When conduction in the rubrospinal tract was completely interrupted by a ventral C2 lesion which also completely transected the axons of the tecto-reticulo-spinal system, then there was a prolongation of the switching latency with 10–20 ms. After a similar large ventral lesion with remaining conduction in the rubrospinal tract the switching latencies were unchanged. It is postulated that fast visually governed switching does not depend on the tecto-reticulo-spinal system alone but on more dorsally located pathways, presumably the rubrospinal tract, either acting alone or together with the tecto-reticulo-spinal system. It is further postulated that the delayed switching after interruption of conduction both in the rubrospinal tract and the tecto-reticulo-spinal system depends on the corticospinal tract. Visual control of rubrospinal and of corticospinal neurones is considered. It is postulated that target-reaching normally depends on signals in the cortico- and rubrospinal tracts and mechanisms for co-ordination of activity in them as required during switching is discussed in view of the findings now reported.

Axotomized rubrospinal neurons rescued by fetal spinal cord transplants maintain axon collaterals to rostral CNS targets.

Neurons that maintain extensive axon collaterals proximal to the site of axotomy may be better able to survive injury. Early lesions of the rubrospinal tract lead to retrograde cell death of the majority of axotomized immature neurons. Transplants of fetal spinal cord tissue rescue axotomized rubrospinal neurons and promote their axonal regeneration. Rubrospinal neurons develop many of their axon collaterals postnatally. The present study tests the hypothesis that the axotomized rubrospinal neurons that are rescued by transplants and regenerate their axons are those neurons that have established axon collaterals to targets rostral to the lesion. Neonatal rats received a transplant of fetal spinal cord tissue placed into a midthoracic spinal cord hemisection. One month after transplantation, the retrogradely transported fluorescent tracers fast blue (FB) and diamidino yellow (DY) were used to identify rubrospinal neurons with collaterals to particular targets. FB was injected either into the interpositus nucleus of the cerebellum or into the gray matter of the cervical enlargement to identify collaterals to these targets, and DY was injected into the spinal cord approximately 5 mm caudal to the transplant and lesion site to label retrogradely the neurons that regenerated their axons. Double labeling was observed in the axotomized neurons of the red nucleus after tracer injections into the cervical spinal cord but not after injections into the cerebellum. This labeling pattern indicates that axotomized rubrospinal neurons that are rescued and regenerate axons caudal to the transplant maintain axon collaterals at cervical spinal cord levels. Cerebellar collaterals do not appear to play a role in the survival and regrowth of axotomized rubrospinal neurons.

Expression of β-calcitonin gene-related peptide in axotomized rubrospinal neurons and the effect of brain derived neurotrophic factor

The mRNA levels for α- and β-calcitonin gene-related peptide (CGRP) in rat rubrospinal neurons were studied by in situ hybridization 3, 7, 14, 28 and 56 days following cervical spinal hemisection. CGRP-like immunoreactivity (LI) in the rubrospinal neurons and the rubrospinal tract in cervical spinal cords were examined using immunohistochemistry. There was almost no signal for α- and β-CGRP mRNAs and undetectable level of CGRP-LI in the rubrospinal neurons ipsilateral to cervical spinal hemisection (control side). Fourteen days after spinal hemisection, the rubrospinal neurons contralateral to cervical hemisection (axotomized side) showed CGRP-LI in their cell bodies, and CGRP containing fibers were observed in the lateral funiculi just proximal, but not distal, to the injury sites. In situ hybridization showed upregulation of β-CGRP mRNA in a subpopulation of the rubrospinal neurons on the axotomized side. The proportion of β-CGRP mRNA-expressing neurons reached its maximum (approximately 19%) 4 days following axotomy and slowly decreased to about 5% 56 days after axotomy. The percentage of α-CGRP mRNA-expressing neurons was much lower than that of β-CGRP mRNA (maximum about 2.6% 4 days after axotomy) and not significantly different from the control side throughout the time period studied. These data indicate that axotomy induces de novo synthesis of the CGRP β-subtype in rubrospinal neurons and that the β-CGRP is transported to the injury site through the rubrospinal tract. In addition, we studied the effect of the intracerebral injections of brain derived neurotrophic factor (BDNF). BDNF treatment fully reversed the severe cell atrophy that followed axotomy and increased the number of neurons labeled for β-CGRP mRNA, but did not increase the percentage of rubrospinal neurons expressing β-CGRP mRNA. Thus, topical application of BDNF does not have direct modulatory effect on CGRP induction in axotomized neurons in the red nucleus.

Dorsal column but not lateral column transection prevents down-conditioning of H reflex in rats.

Operant conditioning of the H reflex, the electrical analogue of the spinal stretch reflex, in freely moving rats is a relatively simple model for studying long-term supraspinal control over spinal cord function. Motivated by food reward, rats can gradually increase or decrease the soleus H reflex. This study is the first effort to determine which spinal cord pathways convey the descending influence from supraspinal structures that changes the H reflex. In anesthetized Sprague-Dawley rats, the entire dorsal column (DC), which includes the main corticospinal tract, or the right lateral column (LC) was transected by electrocautery. Animals recovered quickly and the minimal transient effects of transection on the right soleus H reflex disappeared within 16 days. Beginning at least 18 days after transection, 12 rats were exposed to the HRdown-conditioning mode, in which reward was given when the H reflex of the right soleus muscle was below a criterion value. In seven LC rats exposed to the HRdown mode, the H reflex fell to 71 +/- 8% (mean +/- SE) of its initial value. In six of the seven, conditioning was successful (i.e., decrease to < or = 80%). These results were comparable with those previously obtained from normal rats. In contrast, in five DC rats exposed to the HRdown mode, the H reflex at the end of exposure was 106 +/- 12% of its initial value. In none of these rats was HRdown-conditioning successful. DC rats differed significantly from normal and LC rats in both final H reflex values and number successful. In five DC and three LC rats that continued under control conditions over 30-78 days, the H reflex at the end of the period was 98 +/- 4% and 100 +/- 8%, respectively, of its initial value, indicating that DC or LC transection itself did not lead to gradual increase or decrease in the H reflex. The results indicate that the DC, containing the main corticospinal tract, is essential for HRdown-conditioning, whereas the ipsilateral LC, containing the main rubrospinal, vestibulospinal, and reticulospinal tracts, is not essential. Combined with the known muscular specificity of conditioning, these results suggest that the main corticospinal tract is essential for HRdown-conditioning. The DC ascending tract might also be necessary. The respective roles of the DC descending and ascending tracts, and transection effects on HRup-conditioning and on the maintenance of both HRup- and HRdown-conditioning after they have occurred, remain to be defined.