Localization of a memory trace in the mammalian brain

The localization of sites of memory formation within the brain has proven to be a formidable task even for simple forms of learning and memory. In order to localize a particular site of memory formation within the brain, the rabbit eyeblink response was classically conditioned while regions of the cerebellum or red nucleus were temporarily inactivated by microinfusions of the gamma-aminobutyric acid agonist muscimol. Cerebellar inactivation completely blocked learning but had no effect on subsequent learning after inactivation, whereas red nucleus inactivation did not prevent learning but did block the expression of conditioned responses. The site of memory formation for this learned response thus appears to be localized within the cerebellum.

The dorsal tegmental noradrenergic projection: an analysis of its role in maze learning

The hypothesis that the noradrenergic projection from the locus coeruleus (LC) to the cerebral cortex and hippocampus is an important neural substrate for learning was evaluated. Maze performance was studied in rats receiving either electrolytic lesions of LC or 6-hydroxydopamine (6-OHDA) lesions of the dorsal tegmental noradrenergic projection. The LC lesions did not disrupt the acquisition of a running response for food reinforcement in an L-shaped runway, even though hippocampal-cortical norepinephrine (NE) was reduced to 29%. Greater telencephalic NE depletions (to 6% of control levels) produced by 6-OHDA also failed to disrupt the acquisition of this behavior or to impair the acquisition of a food-reinforced position habit in a T-maze. Neither locomotor activity nor habituation to a novel environment was affected by the 6-OHDA lesions. Rats with such lesions were, however, found to be significantly more distractible than were controls during the performance of a previously trained response. The hypothesis that telencephalic NE is of fundamental importance in learning was not supported. The data suggest that this system may participate in attentional mechanisms.

Pharmacological inactivation in the analysis of the central control of movement

In this review, we describe how pharmacological inactivation can be used to elucidate the central control of skilled limb movement. Local anesthetics and tetrodotoxin block neuronal cell bodies and passing fibers while gamma-aminobutyric acid (GABA) and muscimol only block cell bodies. Blockade induction time is short (several minutes) for all the agents. Blockade duration produced by local anesthetics and GABA is 15-60 min, while that of tetrodotoxin and muscimol is up to several days. We describe our drug injection system, with an integrated microelectrode and a viewing port for visually monitoring drug flow into the injection cannula. We used glucose metabolism to assess the extent of inactivation. Intracortical lidocaine or muscimol injection produces a central core of maximal hypometabolism (1 mm radius), which could be due to drug spread, surrounded by an extensive region (several millimeters) of reduced hypometabolism, possibly due to reduced synaptic activity of neurons receiving projections from the core region. Drug injection only depresses neuronal activity, which contrasts with cooling, where there can be neuronal hyperexcitability at the periphery of the inactivation site. Our experiments in behaving animals show how pharmacological inactivation is an effective analytical tool for dissecting the differential functional contributions of subcortical and cortical forelimb representations to limb movement control.

Quantitative Susceptibility Mapping in Parkinson's Disease

BACKGROUND

Quantitative susceptibility mapping (QSM) and R2* relaxation rate mapping have demonstrated increased iron deposition in the substantia nigra of patients with idiopathic Parkinson's disease (PD). However, the findings in other subcortical deep gray matter nuclei are converse and the sensitivity of QSM and R2* for morphological changes and their relation to clinical measures of disease severity has so far been investigated only sparsely.

METHODS

The local ethics committee approved this study and all subjects gave written informed consent. 66 patients with idiopathic Parkinson's disease and 58 control subjects underwent quantitative MRI at 3T. Susceptibility and R2* maps were reconstructed from a spoiled multi-echo 3D gradient echo sequence. Mean susceptibilities and R2* rates were measured in subcortical deep gray matter nuclei and compared between patients with PD and controls as well as related to clinical variables.

RESULTS

Compared to control subjects, patients with PD had increased R2* values in the substantia nigra. QSM also showed higher susceptibilities in patients with PD in substantia nigra, in the nucleus ruber, thalamus, and globus pallidus. Magnetic susceptibility of several of these structures was correlated with the levodopa-equivalent daily dose (LEDD) and clinical markers of motor and non-motor disease severity (total MDS-UPDRS, MDS-UPDRS-I and II). Disease severity as assessed by the Hoehn & Yahr scale was correlated with magnetic susceptibility in the substantia nigra.

CONCLUSION

The established finding of higher R2* rates in the substantia nigra was extended by QSM showing superior sensitivity for PD-related tissue changes in nigrostriatal dopaminergic pathways. QSM additionally reflected the levodopa-dosage and disease severity. These results suggest a more widespread pathologic involvement and QSM as a novel means for its investigation, more sensitive than current MRI techniques.

Reorganization of descending motor tracts in the rat spinal cord

Following lesion of the central nervous system (CNS), reinnervation of denervated areas may occur via two distinct processes: regeneration of the lesioned fibres or/and sprouting from adjacent intact fibres into the deafferented zone. Both regeneration and axonal sprouting are very limited in the fully mature CNS of higher vertebrates, but can be enhanced by neutralizing the neurite outgrowth inhibitory protein Nogo-A. This study takes advantage of the distinct spinal projection pattern of two descending tracts, the corticospinal tract (CST) and the rubrospinal tract (RST), to investigate if re-innervation of denervated targets can occur by sprouting of anatomically separate, undamaged tracts in the adult rat spinal cord. The CST was transected bilaterally at its entry into the pyramidal decussation. Anatomical studies of the RST in IN-1 antibody-treated rats showed a reorganization of the RST projection pattern after neutralization of the myelin associated neurite growth inhibitor Nogo-A. The terminal arborizations of the rubrospinal fibres, which are normally restricted to the intermediate layers of the spinal cord, invaded the ventral horn but not the dorsal horn of the cervical spinal cord. Moreover, new close appositions were observed, in the ventral horn, onto motoneurons normally receiving CST projections. Red nucleus microstimulation experiments confirmed the reorganization of the RST system. These observations indicate that mature descending motor tracts are capable of significant intraspinal reorganization following lesion and suggests the expression of cues guiding and/or stabilizing newly formed sprouts in the adult, denervated spinal cord.

Survival effects of BDNF and NT-3 on axotomized rubrospinal neurons depend on the temporal pattern of neurotrophin administration

This study shows that both BDNF and NT-3 can prevent cell death in axotomized adult rat rubrospinal neurons (RSNs), but that the efficacy of neuroprotection depends on the temporal pattern of treatment. At 8 weeks after cervical spinal cord injury, 51% of the RSNs had died. Subarachnoidal BDNF infusion into the cisterna magna for 4 weeks resulted in neuronal hypertrophy and 71% survival. Continuous infusion for 8 weeks into the lumbar subarachnoidal space with either BDNF or NT-3 gave similar survival rates, while a combination of BDNF and NT-3 resulted in 96% survival, although the cells were atrophic. When administration of either BDNF or NT-3 was delayed and performed during postoperative weeks 5-8, the number of surviving neurons was increased compared to early treatment. Delayed treatment with a combination of BDNF and NT-3 resulted in complete survival and a reduction in neuronal atrophy. A decreased expression of TrkB receptors and microtubule-associated protein-2 in the RSNs after axotomy was counteracted by BDNF and NT-3. Microglial activity remained increased even when complete cell survival was achieved. Thus, the combination of neurotrophins as well as the temporal pattern of treatment need to be adequately defined to optimize survival of injured spinal tract neurons.

Neurotrophic factors prevent the death of CNS neurons after spinal cord lesions in newborn rats

The aim of this study was to determine if the exogenous administration of neurotrophic factors can rescue immature axotomized CNS neurons in vivo. After spinal cord hemisection in newborn rats, the exogenous administration of neurotrophic factors brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and nerve growth factor (NGF), prevents the retrograde cell death of axotomized red nucleus neurons (and other brain stem spinal neurons) in vivo. Rescue of red nucleus neurons was maintained in the presence of BDNF, but only transiently maintained by NT-3 and NGF. Neurons within the nucleus dorsalis (Clarke's nucleus) of the spinal cord are also axotomized by this lesion. The application of exogenous NT-3, but not NGF or BDNF, rescued Clarke's nucleus neurons. These observations indicate that neurotrophic factors play a crucial role in the survival of CNS neurons in vivo during development and after injury. Furthermore, these results indicate that particular populations of neurons are dependent upon specific neurotrophic support after injury.

Nogo-66 receptor antagonist peptide (NEP1-40) administration promotes functional recovery and axonal growth after lateral funiculus injury in the adult rat

OBJECTIVE

The myelin protein Nogo inhibits axon regeneration by binding to its receptor (NgR) on axons. Intrathecal delivery of an NgR antagonist (NEP1-40) promotes growth of injured corticospinal axons and recovery of motor function following a dorsal hemisection. The authors used a similar design to examine recovery and repair after a lesion that interrupts the rubrospinal tract (RST).

METHODS

Rats received a lateral funiculotomy at C4 and NEP1-40 or vehicle was delivered to the cervical spinal cord for 4 weeks. Outcome measures included motor and sensory tests and immunohistochemistry.

RESULTS

Gait analysis showed recovery in the NEP1-40-treated group compared to operated controls, and a test of forelimb usage also showed a beneficial effect. The density of labeled RST axons increased ipsilaterally in the NEP1-40 group in the lateral funiculus rostral to the lesion and contralaterally in both gray and white matter. Thus, rubrospinal axons exhibited diminished dieback and/or growth up to the lesion site. This was accompanied by greater density of 5HT and calcitonin gene-related peptide axons adjacent to and into the lesion/matrix site in the NEP1-40 group.

CONCLUSIONS

NgR blockade after RST injury is associated with axonal growth and/or diminished dieback of severed RST axons up to but not into or beyond the lesion/matrix site, and growth of serotonergic and dorsal root axons adjacent to and into the lesion/matrix site. NgR blockade also supported partial recovery of function. The authors' results indicate that severed rubrospinal axons respond to NEP1-40 treatment but less robustly than corticospinal, raphe-spinal, or dorsal root axons.

Survival of chronically-injured neurons can be prolonged by treatment with neurotrophic factors

Axonal regeneration by chronically-injured supraspinal neurons can be enhanced by neurotrophic factor treatment at the site of injury, although the number of regenerating neurons decreases as the interval between spinal cord injury and treatment increases. This study investigated whether this decline in regenerative response could be due to continued loss of neurons during the post-injury period. Adult rats received a cervical hemisection lesion and axotomized neurons were labeled by retrograde transport of True Blue from the lesion site. Animals were killed one, four or eight weeks after injury and surviving neurons (True Blue-labeled) were counted in the red nucleus and lateral vestibular nucleus. The neuron number in the lateral vestibular nucleus was stable for eight weeks after spinal cord injury, while survival in the red nucleus decreased by 25% between four and eight weeks. To test how neurons respond to a second injury with or without trophic factor treatment, at four, eight, 14 or 22 weeks after injury the lesion cavity was enlarged by 0.5 mm in a rostral direction. Gel foam saturated with ciliary neurotrophic factor, brain-derived neurotrophic factor or basic fibroblast growth factor was placed into the cavity. Animals were killed four weeks later. Re-injury of the spinal cord caused a significant decrease in neuron survival in both the red nucleus and lateral vestibular nucleus, the effects of which were lessened by treatment with ciliary neurotrophic factor or brain-derived neurotrophic factor for the red nucleus and with ciliary neurotrophic factor for the lateral vestibular nucleus, when re-injured at four or eight weeks. Basic fibroblast growth factor did not affect neuron survival at any time post-injury. Ciliary neurotrophic factor was not effective with longer delays (14 or 22 weeks) between the initial injury and re-injury. These results indicate a delayed pattern of secondary neuronal cell loss after spinal cord injury that is exaggerated by re-injury, but which can be ameliorated by treatment with neurotrophic factors.

Drug specificity of pharmacological dystonia

Three (+)-benzomorphans that bind to sigma receptors produced dystonia in a dose-related manner when microinjected into the red nucleus of rats. Two lines of evidence suggest that these effects were related to the sigma-binding properties of the compounds. First, the behavioral potency of the (+)-benzomorphans and other active sigma compounds correlated highly with their affinities for [3H]1,3-di-o-tolylguanidine-labelled sigma receptors in the rat brain (r = .94). Second, similar intrarubral injections of non-sigma ligands were without effect: various vehicles, a structurally related (+)-opiate with no affinity for sigma receptors, and selective dopaminergic and serotonergic compounds failed to significantly alter the normal posture of rats. The only ligand in this study that binds with high affinity to sigma receptors, but failed to elicit torsional head movements was (+)-[3-(3-hydroxyphenyl)-N-(1-propyl)piperidine] [(+)-3PPP], a ligand with mixed activity at sigma and dopamine receptors. Since (+)-3PPP failed to produce an effect on its own and also failed to attenuate the dystonia produced by another sigma ligand (DTG), it may interact with a non-sigma mechanism or with a different sigma receptor type from the other compounds.

Heterogeneity of ventral tegmental area neurons: single-unit recording and iontophoresis in awake, unrestrained rats

Single-unit recording combined with iontophoresis of dopamine, GABA, and glutamate was used in awake, unrestrained rats to characterize the electrophysiological and receptor properties of neurons in the ventral tegmental area under naturally occurring behavioural conditions. All isolated ventral tegmental area units (n=90) were analysed and compared with cells (n=58) recorded from dorsally adjacent areas of the pre-rubral area and red nucleus. Two distinct neuronal groups were identified in the ventral tegmental area: units with triphasic, long-duration spikes (78/90) and units with biphasic, short-duration spikes (12/90). Although all long-spike units discharged in an irregular, bursting pattern with varying degrees of within-burst decrements in spike amplitude, they could be further subdivided into at least three distinct subgroups. Type I long-spike units (36/78) discharged at a relatively slow and stable rate (mean: 6.03 imp/s; range: 0.42-15.78) with no evident fluctuations during movement. These cells were inhibited by dopamine and GABA and responded to glutamate with a low-magnitude excitation accompanied by a pronounced decrement in spike amplitude and a powerful rebound inhibition. Type II long-spike units (23/78) had relatively high and unstable discharge rates (mean: 22.82 imp/s; range: 4.42-59.67) and showed movement-related phasic activations frequently followed by partial or complete cessation of firing. Some Type II cells (4/9) were inhibited by dopamine, but all were excited by glutamate at very low currents (0-10 nA). With an increase in current, the glutamate-induced excitation often (18/22) progressed into a cessation of firing. All these cells were inhibited by GABA followed by a strong rebound excitation (8/9), which also frequently (6/8) resulted in cessation of firing. Type III long-spike units (19/78) had properties that differed from either Type I or Type II cells, including a lack of spontaneous firing (5/19). Short-spike ventral tegmental area units were either silent (4/12) and unresponsive to dopamine and GABA or spontaneously active (range: 0.89-34.13 imp/s) and inhibited by GABA and, in some cases (2/8). by dopamine; all were phasically activated during movement and glutamate iontophoresis. It appears that ventral tegmental area neurons, including those with long-duration spikes, do not comprise a uniform population in awake, unrestrained rats. Type I, long-spike units match the characteristics of histochemically-identified dopamine neurons, and they appear to express dopamine autoreceptors, which may explain the relatively slow, stable rate of activity and the limited responsiveness to excitatory inputs. Although the nature of the other long-spike units in our sample is unclear, they may include dopamine neurons without autoreceptors as well as non-dopamine cells. The heterogeneity of ventral tegmental area neurons is an important consideration for further attempts to assess the role of the mesocorticolimbic dopamine system in motivated behaviour.

FK506 increases the regeneration of spinal cord axons in a predegenerated peripheral nerve autograft

The authors examined the ability of FK506 to accelerate axonal regeneration of rat spinal cord axons in a peripheral nerve (PN) graft. Predegenerated autografts were produced by transecting the left tibial nerve 1 week prior to spinal cord implantation into the lumbar (L-3-L-4) spinal cord. Rats were given daily injections of either FK506 (5 mg/kg, subcutaneous) or vehicle for 21 days. The PN grafts from FK506-treated rats contained larger sized regenerating axons compared with vehicle-treated controls, and mean axonal areas increased by 25% at 7.5 mm along the PN graft. Fluoro-Gold retrograde labeling confirmed that the regenerating axons originated from the central nervous system. Unexpectedly, the majority (>50%) of neurons in the red nucleus were retrogradely labeled in the FK506-treated animals only. The results indicate that FK506 not only accelerates the elongation of spinal cord axons but also promotes regeneration of rubrospinal neurons.

Glutathione monoethyl ester improves functional recovery, enhances neuron survival, and stabilizes spinal cord blood flow after spinal cord injury in rats

Secondary damage after spinal cord (SC) injury remains without a clinically effective drug treatment. To explore the neuroprotective effects of cell-permeable reduced glutathione monoethyl ester (GSHE), rats subjected to SC contusion using the New York University impactor were randomly assigned to receive intraperitoneally GSHE (total dose of 12 mg/kg), methylprednisolone sodium succinate (total dose of 120 mg/kg), or saline solution as vehicle. Motor function, assessed using the Basso-Beattie-Bresnahan scale for 8 weeks, was significantly better in GSHE (11.2+/-0.6, mean+/-S.E.M., n=8, at 8 weeks) than methylprednisolone (9.3+/-0.6) and vehicle (9.4+/-0.7) groups. The number of neurons in the red nuclei labeled with FluoroRuby placed caudally to the injury site was significantly higher in GSHE (158+/-9.3 mean+/-S.E.M., n=4) compared with methylprednisolone (53+/-14.7) and vehicle (46+/-16.4) groups. Differences in the amount of spared SC tissue at the epicenter and neighboring areas were not significant among experimental groups. In a second series of experiments, using similar treatment groups (n=6), regional changes in microvascular SC blood flow were evaluated for 100 min by laser-Doppler flowmetry after clip compression injury. SC blood flow fell in vehicle-treated rats 20% below baseline and increased significantly with methylprednisolone approximately 12% above baseline; changes were not greater than 5% in rats given GSHE. In conclusion, GSHE given to rats early after moderate SC contusion/compression improves functional outcome and red nuclei neuron survival significantly better than methylprednisolone and vehicle, and stabilizes SC blood flow. These results support further investigation of reduced glutathione supplementation after acute SC injury for future clinical application.

Delayed treatment with chondroitinase ABC reverses chronic atrophy of rubrospinal neurons following spinal cord injury

Degradation of extracellular matrix chondroitin sulphate proteoglycans (CSPGs) using Chondroitinase ABC (ChABC) is a promising strategy for the treatment of spinal cord injury, with potent effects on promoting functional recovery and anatomical repair in spinal injured animals. We have previously demonstrated that ChABC treatment prevents atrophy of corticospinal projection neurons following spinal injury in adult YFP-H mice. Here, we investigate whether ChABC-mediated repair of the cell body extends to rubrospinal projection neurons (RSNs), whether neuroprotective effects can be sustained long-term and importantly, whether delayed treatment with ChABC can reverse chronic atrophy. Adult YFP-H mice underwent unilateral rubrospinal tract transection and were treated with ChABC or a control enzyme, delivered either acutely post-injury or after a one month delay. Eight weeks following injury and control treatment, RSNs in the injured red nucleus, identified by YFP label and NeuN immunoreactivity, showed severe atrophy, with ~40% loss of mean cell area compared to uninjured neurons in the contralateral red nucleus. Both acute and delayed treatment with ChABC promoted a significant rescue of injured RSNs, restoring cell area to ~80% and ~70%, respectively, of that in uninjured neurons. Thus, we demonstrate for the first time that CSPG degradation in the injured spinal cord not only promotes sustained rescue of cell atrophy when delivered acutely but can also reverse chronic atrophy in descending projection neurons. Thus, modulation of the extracellular matrix can mediate neuroprotective effects both early and late after spinal cord injury.

Degenerative processes after punctate intracerebral administration of 6-hydroxydopamine

Various doses of the neurotoxin 6-hydroxydopamine (6-OHDA) were infused into the substantia nigra-ventromedial tegmental area, caudateputamen nucleus, and red nucleus. A maximum of four successively occurring zones of neuropathology could by detected at all injection sites: (1) A zone of complete absence of neural, glial, and vascular elements due to tissue displacement by the cannula, (2) an area of glial cells, developing over time, which surrounded the cannula tract, (3) a region exhibiting virtually complete loss of neuronal elements, and (4) a zone of selective neuropathological reaction or cellular loss in which some neurons were affected and others were not. All investigators who have histochemically and/or histologically evaluated the effects of intracerebrally administered 6-OHDA agree on the existence and genesis, some non-selective process, of the first three zones. Although some scientists maintain that the fourth zone is where 6-OHDA operates selectively, data are presented in this report that other, well-established processes can account for neuropathology beyond zone 3. Prominent among these are retrograde and anterograde degeneration, ischemia as a function of interruption of blood supply, and nonselective traumatization. In addition, the topography of the injected site (e.g., morphology of affected neurons, degree of myelination) was found to be as important a determinant of the locus of tissue damage as the presumed selectivity of 6-OHDA's action. Furthermore, monoamine oxidase inhibition by nialamide did not appear to increase the neurotoxicity of 6-OHDA. Similarly, 1-(2,5-dihydroxy-4-methylphenyl)-2-aminopropane was not a more potent cytotoxin than 6-OHDA even though this new neurotoxin has a propane side chain which renders it immune to monoamine oxidase. These observations, taken together, suggest that considerable cautioon should be exercised in interpreting data from experiments in which 6-OHDA, or related neurotoxins, are used to uncover catecholaminergic mechanisms of behavior and other functional processes.

BYHWD rescues axotomized neurons and promotes functional recovery after spinal cord injury in rats

ETHNOPHARMACOLOGICAL RELEVANCE

Buyang Huanwu Decoction (BYHWD), a Chinese prescription that has been used for hundreds of years to treat paralysis, has gained attention recently due to its significant neuroprotective properties.

AIM OF THE STUDY

This study was to investigate whether BYHWD treatment protected axotomized rubrospinal neurons (RN) following spinal cord injury (SCI) in rats.

MATERIALS AND METHODS

Adult rats received a right lateral funiculus transection at the level between C3 and C4, and were either treated with BYHWD or with distilled water (DW) via gastrogavage. Effects on RN viability and atrophy were determined by Nissl staining, axon regeneration was examined by biotinylated dextran amine (BDA) tracing techniques and functional recovery was studied by a test of forelimb usage during spontaneous vertical exploration.

RESULTS

RN cell number and mean somal size were 20% and 29% higher, respectively, in BYHWD-treated rats relative to DW-treated rats. There were a small number of BDA-labeled axons in the caudal of injury site in BYHWD-treated rats, whereas no caudal axonal regeneration was detected in DW-treated rats. BYHWD-treated rats used the injured forelimb more often than rats treated with DW.

CONCLUSIONS

These results indicate that administration of BYHWD following SCI protects injured neurons, promotes regeneration, and enhances functional recovery.

Cytological and cytochemical (RNA) studies on rubral neurons after unilateral rubrospinal tractotomy: the impact of GM1 ganglioside administration

The rubrospinal tract (RST) was cut unilaterally at C2-3 segment in 21 rats that were killed 3, 7, 10, 14, 28, 60, and 90 days later. Additionally, 14 rats, killed 14 or 28 days after lesioning, were treated postoperatively by daily intraperitoneal injections of GM1 ganglioside. Six unoperated, untreated rats served as controls. In untreated animals, axotomized neurons of the magnocellular division of the red nucleus (RN) exhibited cytoplasmic, nuclear, and nucleolar atrophy 7-10 days postoperatively. Atrophy progressed through the 90th postoperative day. Regression analyses disclosed a bimodal pattern to cytoplasmic and nucleolar atrophy, with an initial rapid phase changing to a slower but progressive mode from 14 days postoperatively. Nuclear atrophy proceeded in a unimodal manner. GM1 treatment did not affect these atrophic processes. Neuronal loss did not occur in the axotomized RN through the 60th postoperative day. Axotomized neurons of untreated rats showed significant and progressive reductions in mean somal (cytoplasmic) and nucleolar RNA from, respectively, the 7th and 14th postoperative day. GM1 partly prevented these RNA losses. Both in treated and untreated rats, spinal cord lesions contained many axonal sprouts 2 to 4 weeks after surgery, but newly generated axons did not traverse the rostro-caudal extent of any lesion.

The differential effects of baclofen on segmental and descending excitation of spinal interneurones in the cat

Intravenous baclofen (1-6.25 mg kg-1) substantially reduced the monosynaptic excitation of neurones in the intermediate nucleus of the cat spinal cord by impulses in group I extensor muscle primary afferent fibres, but had little or no effect on excitation by stimulating fibres of the ipsilateral dorsolateral funiculus or the contralateral red nucleus. Relatively low concentrations of baclofen thus appear not to influence the release of excitatory transmitter from the terminals of rubrospinal, corticospinal and long descending propriospinal fibres, in contrast to the reduction of the release of primary afferent transmitters.

Rubrospinal neurons fail to respond to brain-derived neurotrophic factor applied to the spinal cord injury site 2 months after cervical axotomy

Numerous experimental therapies to promote axonal regeneration have shown promise in animal models of acute spinal cord injury, but their effectiveness is often found to diminish with a delay in administration. We evaluated whether brain-derived neurotrophic factor (BDNF) application to the spinal cord injury site 2 months after cervical axotomy could promote a regenerative response in chronically axotomized rubrospinal neurons. BDNF was applied to the spinal cord in three different concentrations 2 months after cervical axotomy of the rubrospinal tract. The red nucleus was examined for reversal of neuronal atrophy, GAP43 and Talpha1 tubulin mRNA expression, and trkB receptor immunoreactivity. A peripheral nerve transplant paradigm was used to measure axonal regeneration into peripheral nerve transplants. Rubrospinal axons were anterogradely traced and trkB receptor immunohistochemistry performed on the injured spinal cord. We found that BDNF treatment did not reverse rubrospinal neuronal atrophy, nor promote GAP-43 and Talpha1 tubulin mRNA expression, nor promote axonal regeneration into peripheral nerve transplants. TrkB receptor immunohistochemistry demonstrated immunoreactivity on the neuronal cell bodies, but not on anterogradely labeled rubrospinal axons at the injury site. These findings suggest that the poor response of rubrospinal neurons to BDNF applied to the spinal cord injury site 2 months after cervical axotomy is not related to the dose of BDNF administered, but rather to the loss of trkB receptors on the injured axons over time. Such obstacles to axonal regeneration will be important to identify in the development of therapeutic strategies for chronically injured individuals.

Inhibition of rubral neurons by a specific ligand for sigma receptors

1,3-Di-ortho-tolylguanidine (DTG), a compound with marked selectivity for sigma receptors was iontophoretically applied to neurons in the red nucleus and nearby mesencephalic reticular formation. Spontaneous firing of rubral neurons was reduced by DTG in most cases. However, in the reticular formation, less than half of the cells tested showed inhibition; the remaining cells showed either an increase in rate or no consistent response to DTG. This corresponds to the greater density of sigma receptors in the red nucleus and supports previous work which suggests that sigma receptors play a role in the regulation of movement and posture.

Effects of GABA, glycine, picrotoxin and bicuculline methochloride on rubrospinal neurones in cats

A microelectrophoretic study was performed of the actions of GABA and glycine, and of their antagonists on rubrospinal neurones. GABA and glycine depressed antidromic and synaptic potentials, hyperpolarized the cell membrane and increased the membrane conductance. GABA was more effective than glycine. Corticorubral IPSPs and the GABA-induced membrane hyperpolarization could be reversed by intracellular application of hyperpolarizing currents. Picrotoxin and bicuculline methochloride antagonized inhibitory GABA actions and the corticorubral IPSP or the corresponding extracellular positive field potential. The results support the hypothesis that GABA is the transmitter of the corticorubral inhibition in the cat.

Neuroimmunophilin ligands improve functional recovery and increase axonal growth after spinal cord hemisection in rats

We have previously shown that FK506 accelerates the rate of nerve regeneration in the peripheral nervous system (PNS) and increases regeneration of central nervous system (CNS) axons into a peripheral nerve graft. In the present study, we examined whether FK506 and a nonimmunosuppressive derivative (FK1706) improve functional recovery and long distance regeneration following a hemisection lesion of spinal cord at T10/T11. Rats were given daily subcutaneous injections of either FK506 (2 mg/kg/day), FK1706 (2 mg/kg/day), an equivalent volume of saline or 30% DMSO as vehicle, respectively. Functional recovery was assessed using a modified Tarlov/Klinger scale, walking along progressively narrower wooden beams (7.7-1.7 cm widths), and analysis of footprints obtained during walking. Compared to both control groups, FK506 and FK1706-treated animals demonstrated significant functional recovery 4 days (beam walking), 2 weeks (footprints), and 4 weeks (Tarlov/Klinger scale). By 11 weeks, FK506-treated and FK1706-treated animals were able to walk, albeit poorly, along even the narrowest (1.7 cm) beam. At 11 weeks, the spinal cords were re-exposed and a small piece of gel foam-soaked Fluoro-Gold was placed on the injured side 2-cm caudal to the first injury. Five days later, the animals were perfused and tissues prepared for fluorescence microscopy. FK506-treated and FK1706-treated rats demonstrate a significantly greater number of retrogradely labeled neurons in the red nucleus. The results implicate a nonimmunosuppressant mechanism in FK506's action and suggest that FK506 or a nonimmunosuppressant derivative may be useful for treatment of spinal cord injuries.