Plasticity of cutaneous primary afferent projections to the spinal dorsal horn

Trigeminal Sensory Supply Is Essential for Motor Recovery after Facial Nerve Injury

Recovery of mimic function after facial nerve transection is poor. The successful regrowth of regenerating motor nerve fibers to reinnervate their targets is compromised by (i) poor axonal navigation and excessive collateral branching, (ii) abnormal exchange of nerve impulses between adjacent regrowing axons, namely axonal crosstalk, and (iii) insufficient synaptic input to the axotomized facial motoneurons. As a result, axotomized motoneurons become hyperexcitable but unable to discharge. We review our findings, which have addressed the poor return of mimic function after facial nerve injuries, by testing the hypothesized detrimental component, and we propose that intensifying the trigeminal sensory input to axotomized and electrophysiologically silent facial motoneurons improves the specificity of the reinnervation of appropriate targets. We compared behavioral, functional, and morphological parameters after single reconstructive surgery of the facial nerve (or its buccal branch) with those obtained after identical facial nerve surgery, but combined with direct or indirect stimulation of the ipsilateral infraorbital nerve. We found that both methods of trigeminal sensory stimulation, i.e., stimulation of the vibrissal hairs and manual stimulation of the whisker pad, were beneficial for the outcome through improvement of the quality of target reinnervation and recovery of vibrissal motor performance.

Large-scale functional ultrasound imaging of the spinal cord reveals in-depth spatiotemporal responses of spinal nociceptive circuits in both normal and inflammatory states

Despite a century of research on the physiology/pathophysiology of the spinal cord in chronic pain condition, the properties of the spinal cord were rarely studied at the large-scale level from a neurovascular point of view. This is mostly due to the limited spatial and/or temporal resolution of the available techniques. Functional ultrasound imaging (fUS) is an emerging neuroimaging approach that allows, through the measurement of cerebral blood volume, the study of brain functional connectivity or functional activations with excellent spatial (100 μm) and temporal (1 msec) resolutions and a high sensitivity. The aim of this study was to increase our understanding of the spinal cord physiology through the study of the properties of spinal hemodynamic response to the natural or electrical stimulation of afferent fibers. Using a combination of fUS and ultrasound localization microscopy, the first step of this study was the fine description of the vascular structures in the rat spinal cord. Then, using either natural or electrical stimulations of different categories of afferent fibers (Aβ, Aδ, and C fibers), we could define the characteristics of the typical hemodynamic response of the rat spinal cord experimentally. We showed that the responses are fiber-specific, located ipsilaterally in the dorsal horn, and that they follow the somatotopy of afferent fiber entries in the dorsal horn and that the C-fiber response is an N-methyl-D-aspartate receptor-dependent mechanism. Finally, fUS imaging of the mesoscopic hemodynamic response induced by natural tactile stimulations revealed a potentiated response in inflammatory condition, suggesting an enhanced response to allodynic stimulations.

Central Plasticity of Cutaneous Afferents Is Associated with Nociceptive Hyperreflexia after Spinal Cord Injury in Rats

Electrical stimulations of dorsal cutaneous nerves (DCNs) at each lumbothoracic spinal level produce the bilateral cutaneus trunci muscle (CTM) reflex responses which consist of two temporal components: an early and late responses purportedly mediated by Aδ and C fibers, respectively. We have previously reported central projections of DCN A and C fibers and demonstrated that different projection patterns of those afferent types contributed to the somatotopic organization of CTM reflex responses. Unilateral hemisection spinal cord injury (SCI) was made at T10 spinal segments to investigate the plasticity of early and late CTM responses 6 weeks after injury. Both early and late responses were drastically increased in response to both ipsi- and contralateral DCN stimulations both above (T6 and T8) and below (T12 and L1) the levels of injury demonstrating that nociceptive hyperreflexia developed at 6 weeks following hemisection SCI. We also found that DCN A and C fibers centrally sprouted, expanded their projection areas, and increased synaptic terminations in both T7 and T13, which correlated with the size of hemisection injury. These data demonstrate that central sprouting of cutaneous afferents away from the site of injury is closely associated with enhanced responses of intraspinal signal processing potentially contributing to nociceptive hyperreflexia following SCI.

Central expression of synaptophysin and synaptoporin in nociceptive afferent subtypes in the dorsal horn

Central sprouting of nociceptive afferents in response to neural injury enhances excitability of nociceptive pathways in the central nervous system, often causing pain. A reliable quantification of central projections of afferent subtypes and their synaptic terminations is essential for understanding neural plasticity in any pathological condition. We previously characterized central projections of cutaneous nociceptive A and C fibers, selectively labeled with cholera toxin subunit B (CTB) and Isolectin B4 (IB4) respectively, and found that they expressed a general synaptic molecule, synaptophysin, largely depending on afferent subtypes (A vs. C fibers) across thoracic dorsal horns. The current studies extended the central termination profiles of nociceptive afferents with synaptoporin, an isoform of synaptophysin, known to be preferentially expressed in C fibers in lumbar dorsal root ganglions. Our findings demonstrated that synaptophysin was predominantly expressed in both peptidergic and IB4-binding C fiber populations in superficial laminae of the thoracic dorsal horn. Cutaneous IB4-labeled C fibers showed comparable expression levels of both isoforms, while cutaneous CTB-labeled A fibers exclusively expressed synaptophysin. These data suggest that central expression of synaptophysin consistently represents synaptic terminations of projecting afferents, at least in part, including nociceptive A-delta and C fibers in the dorsal horn.

A touching Sight: EEG/ERP correlates for the vicarious processing of affectionate touch

Observers can simulate aspects of other people's tactile experiences. We asked whether they do so when faced with full-body social interactions, whether emerging representations go beyond basic sensorimotor mirroring, and whether they depend on processing goals and inclinations. In an EEG/ERP study, we presented line-drawn, dyadic interactions with and without affectionate touch. In an explicit and an implicit task, participants categorized images into touch versus no-touch and same versus opposite sex interactions, respectively. Modulations of central Rolandic rhythms implied that affectionate touch displays engaged sensorimotor mechanisms. Additionally, the late positive potential (LPP) being larger for images with as compared to without touch pointed to an involvement of higher order socio-affective mechanisms. Task and sex modulated touch perception. Sensorimotor responding, indexed by Rolandic rhythms, was fairly independent of the task but appeared less effortful in women than in men. Touch induced socio-affective responding, indexed by the LPP, declined from explicit to implicit processing in women and disappeared in men. In sum, this study provides first evidence that vicarious touch from full-body social interactions entails shared sensorimotor as well as socio-affective experiences. Yet, mental representations of touch at a socio-affective level are more likely when touch is goal relevant and observers are female. Together, these results outline the conditions under which touch in visual media may be usefully employed to socially engage observers.

Vesicular glutamate transporter isoforms: The essential players in the somatosensory systems

In nervous system, glutamate transmission is crucial for centripetal conveyance and cortical perception of sensory signals of different modalities, which necessitates vesicular glutamate transporters 1-3 (VGLUT 1-3), the three homologous membrane-bound protein isoforms, to load glutamate into the presysnaptic vesicles. These VGLUTs, especially VGLUT1 and VGLUT2, selectively label and define functionally distinct neuronal subpopulations at each relay level of the neural hierarchies comprising spinal and trigeminal sensory systems. In this review, by scrutinizing each structure of the organism's fundamental hierarchies including dorsal root/trigeminal ganglia, spinal dorsal horn/trigeminal sensory nuclear complex, somatosensory thalamic nuclei and primary somatosensory cortex, we summarize and characterize in detail within each relay the neuronal clusters expressing distinct VGLUT protein/transcript isoforms, with respect to their regional distribution features (complementary distribution in some structures), axonal terminations/peripheral innervations and physiological functions. Equally important, the distribution pattern and characteristics of VGLUT1/VGLUT2 axon terminals within these structures are also epitomized. Finally, the correlation of a particular VGLUT isoform and its physiological role, disclosed thus far largely via studying the peripheral receptors, is generalized by referring to reports on global and conditioned VGLUT-knockout mice. Also, researches on VGLUTs relating to future direction are tentatively proposed, such as unveiling the elusive differences between distinct VGLUTs in mechanism and/or pharmacokinetics at ionic/molecular level, and developing VGLUT-based pain killers.

Bone Trauma Causes Massive but Reversible Changes in Spinal Circuitry

Bone fracture with subsequent immobilization of the injured limb can cause complex regional pain syndrome (CRPS) in humans. Mechanisms of CRPS are still not completely understood but bone fracture with casting in mice leads to a similar post-traumatic inflammation as seen in humans and might therefore be an analog to human CRPS. In this article we report behavioral and spinal electrophysiological changes in mice that developed swelling of the paw, warming of the skin, and pain in the injured limb after bone fracture. The receptive field sizes of spinal neurons representing areas of the hind paws increased after trauma and recovered over time-as did the behavioral signs of inflammation and pain. Interestingly, both sides-the ipsi- and the contralateral limb-showed changes in mechanical sensitivity and neuronal network organization after the trauma. The characteristics of evoked neuronal responses recorded in the dorsal horn of the mice were similar between uninjured controls and fractured animals. However, we saw a caudal extension of the represented area of the hind paw in the spinal cord at the injured side and an occurrence of large receptive fields of wide dynamic range neurons. The findings in mice compare with human symptoms in CRPS with ipsi- and also contralateral allodynia and pain. In all mice tested, all signs subsided 12 weeks after trauma. Our data suggest a significant reorganization of spinal circuitry after limb trauma, in a degree more comprehensive than most models of neuropathies. This process seems to be reversible in the rodent.

PERSPECTIVE

The discovery of enlarged spinal neuronal receptive fields and caudal extension of the representation area of the injured body part, which subsides several weeks after a bone trauma in mice, might give hope to patients of CRPS if-in the future-we are able to translate the rodent recovery mechanisms to post-traumatic humans.

The dorsal spinal cord and hindbrain: From developmental mechanisms to functional circuits

Neurons of the dorsal hindbrain and spinal cord are central in receiving, processing and relaying sensory perception and participate in the coordination of sensory-motor output. Numerous cellular and molecular mechanisms that underlie neuronal development in both regions of the nervous system are shared. We discuss here the mechanisms that generate neuronal diversity in the dorsal spinal cord and hindbrain, and emphasize similarities in patterning and neuronal specification. Insight into the developmental mechanisms has provided tools that can help to assign functions to small subpopulations of neurons. Hence, novel information on how mechanosensory or pain sensation is encoded under normal and neuropathic conditions has already emerged. Such studies show that the complex neuronal circuits that control perception of somatosensory and viscerosensory stimuli are becoming amenable to investigations.

Rapid and persistent impairments of the forelimb motor representations following cervical deafferentation in rats

Skilled motor control is regulated by the convergence of somatic sensory and motor signals in brain and spinal motor circuits. Cervical deafferentation is known to diminish forelimb somatic sensory representations rapidly and to impair forelimb movements. Our focus was to determine what effect deafferentation has on the motor representations in motor cortex, knowledge of which could provide new insights into the locus of impairment following somatic sensory loss, such as after spinal cord injury or stroke. We hypothesized that somatic sensory information is important for cortical motor map topography. To investigate this we unilaterally transected the dorsal rootlets in adult rats from C4 to C8 and mapped the forelimb motor representations using intracortical microstimulation, immediately after rhizotomy and following a 2-week recovery period. Immediately after deafferentation we found that the size of the distal representation was reduced. However, despite this loss of input there were no changes in motor threshold. Two weeks after deafferentation, animals showed a further distal representation reduction, an expansion of the elbow representation, and a small elevation in distal movement threshold. These changes were specific to the forelimb map in the hemisphere contralateral to deafferentation; there were no changes in the hindlimb or intact-side forelimb representations. Degradation of the contralateral distal forelimb representation probably contributes to the motor control deficits after deafferentation. We propose that somatic sensory inputs are essential for the maintenance of the forelimb motor map in motor cortex and should be considered when rehabilitating patients with peripheral or spinal cord injuries or after stroke.

Non-peptidergic small diameter primary afferents expressing VGluT2 project to lamina I of mouse spinal dorsal horn

BACKGROUND

Unmyelinated primary afferent nociceptors are commonly classified into two main functional types: those expressing neuropeptides, and non-peptidergic fibers that bind the lectin IB4. However, many small diameter primary afferent neurons neither contain any known neuropeptides nor bind IB4. Most express high levels of vesicular glutamate transporter 2 (VGluT2) and are assumed to be glutamatergic nociceptors but their terminations within the spinal cord are unknown. We used in vitro anterograde axonal tracing with Neurobiotin to identify the central projections of these putative glutamatergic nociceptors. We also quantitatively characterised the spatial arrangement of these terminals with respect to those that expressed the neuropeptide, calcitonin gene-related peptide (CGRP).

RESULTS

Neurobiotin-labeled VGluT2-immunoreactive (IR) terminals were restricted to lamina I, with a medial-to-lateral distribution similar to CGRP-IR terminals. Most VGluT2-IR terminals in lateral lamina I were not labeled by Neurobiotin implying that they arose mainly from central neurons. 38 ± 4% of Neurobiotin-labeled VGluT2-IR terminals contained CGRP-IR. Conversely, only 17 ± 4% of Neurobiotin-labeled CGRP-IR terminals expressed detectable VGluT2-IR. Neurobiotin-labeled VGluT2-IR or CGRP-IR terminals often aggregated into small clusters or microdomains partially surrounding intrinsic lamina I neurons.

CONCLUSIONS

The central terminals of primary afferents which express high levels of VGluT2-IR but not CGRP-IR terminate mainly in lamina I. The spatial arrangement of VGluT2-IR and CGRP-IR terminals suggest that lamina I neurons receive convergent inputs from presumptive nociceptors that are primarily glutamatergic or peptidergic. This reveals a previously unrecognized level of organization in lamina I consistent with the presence of multiple nociceptive processing pathways.

Non-invasive stimulation of the vibrissal pad improves recovery of whisking function after simultaneous lesion of the facial and infraorbital nerves in rats

We have recently shown that manual stimulation of target muscles promotes functional recovery after transection and surgical repair to pure motor nerves (facial: whisking and blink reflex; hypoglossal: tongue position). However, following facial nerve repair, manual stimulation is detrimental if sensory afferent input is eliminated by, e.g., infraorbital nerve extirpation. To further understand the interplay between sensory input and motor recovery, we performed simultaneous cut-and-suture lesions on both the facial and the infraorbital nerves and examined whether stimulation of the sensory afferents from the vibrissae by a forced use would improve motor recovery. The efficacy of 3 treatment paradigms was assessed: removal of the contralateral vibrissae to ensure a maximal use of the ipsilateral ones (vibrissal stimulation; Group 2), manual stimulation of the ipsilateral vibrissal muscles (Group 3), and vibrissal stimulation followed by manual stimulation (Group 4). Data were compared to controls which underwent surgery but did not receive any treatment (Group 1). Four months after surgery, all three treatments significantly improved the amplitude of vibrissal whisking to 30° versus 11° in the controls of Group 1. The three treatments also reduced the degree of polyneuronal innervation of target muscle fibers to 37% versus 58% in Group 1. These findings indicate that forced vibrissal use and manual stimulation, either alone or sequentially, reduce target muscle polyinnervation and improve recovery of whisking function when both the sensory and the motor components of the trigemino-facial system regenerate.

Spinal reflexes in the long-tailed stingray, Himantura fai

We have exploited the segregation of motor and sensory axons into peripheral nerve sub-compartments to examine spinal reflex interactions in anaesthetized stingrays. Single, supra-maximal electrical stimuli delivered to segmental sensory nerves elicited compound action potentials in the motor nerves of the stimulated segment and in rostral and caudal segmental motor nerves. Compound action potentials elicited in segmental motor nerves by single stimuli delivered to sensory nerves were increased severalfold by prior stimulation of adjacent sensory nerves. This facilitation of the segmental reflex produced by intense conditioning stimuli decreased as it was applied to more remote segments, to approximately the same degree in up to seven segments in the rostral and caudal direction. In contrast, an asymmetric response was revealed when test and conditioning stimuli were delivered to different nerves, neither of which was of the same segment as the recorded motor nerve: in this configuration, conditioning volleys generally inhibited the responses of motoneurons to stimuli delivered to more caudally located sensory nerves. This suggests that circuitry subserving trans-segmental interactions between spinal afferents is present in stingrays and that interneuronal connections attenuate the influence that subsequent activity in caudal primary afferents can have on the motor elements.

Chondroitinase ABC-mediated plasticity of spinal sensory function

Experimental therapeutics designed to enhance recovery from spinal cord injury (SCI) primarily focus on augmenting the growth of damaged axons by elevating their intrinsic growth potential and/or by nullifying the influence of inhibitory proteins present in the mature CNS. However, these strategies may also influence the wiring of intact pathways. The direct contribution of such effects to functional restoration after injury has been mooted, but as yet not been described. Here, we provide evidence to support the hypothesis that reorganization of intact spinal circuitry enhances function after SCI. Adult rats that underwent unilateral cervical spared-root lesion (rhizotomy of C5, C6, C8, and T1, sparing C7) exhibited profound sensory deficits for 4 weeks after injury. Delivery of a focal intraspinal injection of the chondroitin sulfate proteoglycan-degrading enzyme chondroitinase ABC (ChABC) was sufficient to restore sensory function after lesion. In vivo electrophysiological recordings confirm that behavioral recovery observed in ChABC-treated rats was consequent on reorganization of intact C7 primary afferent terminals and not regeneration of rhizotomized afferents back into the spinal cord within adjacent segments. These data confirm that intact spinal circuits have a profound influence on functional restoration after SCI. Furthermore, comprehensive understanding of these targets may lead to therapeutic interventions that can be spatially tailored to specific circuitry, thereby reducing unwanted maladaptive axon growth of distal pathways.

Intraspinal sprouting of unmyelinated pelvic afferents after complete spinal cord injury is correlated with autonomic dysreflexia induced by visceral pain

Autonomic dysreflexia is a potentially life-threatening hypertensive syndrome following high thoracic (T) spinal cord injury (SCI). It is commonly triggered by noxious pelvic stimuli below the injury site that correlates with increased sprouting of primary afferent C-fibers into the lumbosacral (L/S) spinal cord. We have recently demonstrated that injury-induced plasticity of (L/S) propriospinal neurons, which relay pelvic visceral sensations to thoracolumbar sympathetic preganglionic neurons, is also correlated with the development of this syndrome. To determine the phenotype of pelvic afferent fiber sprouts after SCI, cholera toxin subunit beta (CTb) was injected into the distal colon 2 weeks post-T4 transection/sham to label colonic visceral afferents. After 1 week of transport, the (L/S) spinal cords were cryosectioned and immunohistochemically stained for CTb, the nociceptive-specific marker calcitonin gene-related peptide (CGRP), and the myelinated fiber marker RT97. Quantitative analysis showed that the density of CGRP(+) afferent fibers was significantly increased in the L/S dorsal horns of T4-transected versus sham rats, whereas RT97(+) afferent fiber density showed no change. Importantly, CTb-labeled pelvic afferent fibers were co-localized with CGRP(+) fibers, but not with RT97(+) fibers. These results suggest that the sprouting of unmyelinated nociceptive pelvic afferents following high thoracic SCI, but not myelinated fibers, contributes to hypertensive autonomic dysreflexia induced by pelvic visceral pain.

Neural plasticity of neonatal hypoglossal nerve for effective suckling

The adaptive movement of the tongue after unilateral lesion of the hypoglossal (XII) nerve during the early postnatal days is essential for recovery of milk intake. The present study investigated the basic mechanisms underlying such adaptation, focusing on the neural plasticity that allows effective suckling. After resection of the ipsilateral XII nerve on P1, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlolate (DiI), a postmortem neuronal tracer, was applied to the contralateral uninjured XII nerve on P4 and P7. DiI-labeled fibers were traced successfully within the tongue and showed gradually increased extension over the XII nerve-injured side in the central core portion of the denervated tongue between P4 and P7. Systematic neuroanatomic experiments showed that contralateral axonal sprouting occurred as early as 1 day after nerve injury (P2), and that such axonal sprouting occurred exclusively from the medial branch of the XII nerve responsible for tongue protrusion, an essential movement for suckling. These findings provide direct evidence of functional neural plasticity that allows effective suckling in XII nerve-injured newborns with suckling disturbance.

Neural plasticity after peripheral nerve injury and regeneration

Injuries to the peripheral nerves result in partial or total loss of motor, sensory and autonomic functions conveyed by the lesioned nerves to the denervated segments of the body, due to the interruption of axons continuity, degeneration of nerve fibers distal to the lesion and eventual death of axotomized neurons. Injuries to the peripheral nervous system may thus result in considerable disability. After axotomy, neuronal phenotype switches from a transmitter to a regenerative state, inducing the down- and up-regulation of numerous cellular components as well as the synthesis de novo of some molecules normally not expressed in adult neurons. These changes in gene expression activate and regulate the pathways responsible for neuronal survival and axonal regeneration. Functional deficits caused by nerve injuries can be compensated by three neural mechanisms: the reinnervation of denervated targets by regeneration of injured axons, the reinnervation by collateral branching of undamaged axons, and the remodeling of nervous system circuitry related to the lost functions. Plasticity of central connections may compensate functionally for the lack of specificity in target reinnervation; plasticity in human has, however, limited effects on disturbed sensory localization or fine motor control after injuries, and may even result in maladaptive changes, such as neuropathic pain, hyperreflexia and dystonia. Recent research has uncovered that peripheral nerve injuries induce a concurrent cascade of events, at the systemic, cellular and molecular levels, initiated by the nerve injury and progressing throughout plastic changes at the spinal cord, brainstem relay nuclei, thalamus and brain cortex. Mechanisms for these changes are ubiquitous in central substrates and include neurochemical changes, functional alterations of excitatory and inhibitory connections, atrophy and degeneration of normal substrates, sprouting of new connections, and reorganization of somatosensory and motor maps. An important direction for ongoing research is the development of therapeutic strategies that enhance axonal regeneration, promote selective target reinnervation, but are also able to modulate central nervous system reorganization, amplifying those positive adaptive changes that help to improve functional recovery but also diminishing undesirable consequences.

Study of Cutaneous Reflex Compensation During Locomotion After Nerve Section in the Cat

In the cat, section of all cutaneous nerves of the hindfeet except the tibial (Tib) nerve supplying the plantar surface results in a long-lasting decrease in the intensity of Tib stimulation needed for a threshold response in flexor muscles and an increase in the amplitude of the phase-dependent responses recorded in various muscles during locomotion. Stimulating through chronically implanted nerve cuffs ensured a stable stimulation over time. The increase in reflex amplitude was well above the small increase in the amplitude of the locomotor bursts themselves that results from the denervation. Short latency responses (P1) were seen in flexor muscles, especially at the knee (semitendinosus) and ankle (tibialis anterior and extensor digitorum longus), with stimuli applied in the swing phase and also to a lesser degree in the later part of the cycle. Longer latency responses (P2) were increased in hip, knee, and ankle flexors, as well as in a contralateral extensor (vastus lateralis) when applied in late stance. Responses evoked from stimulating the proximal end of sectioned nerves were not larger than before neurectomy. This suggests that the increased responsiveness to Tib stimulation is not simply caused by an increase in motoneuron excitability, because this would have resulted in a nonspecific increase of responses to stimulation of any nerve. It is concluded that the adult locomotor system is capable of central reorganization to enhance specific remaining cutaneous reflex pathways after a partial cutaneous denervation of the paw.

Local and diffuse mechanisms of primary afferent depolarization and presynaptic inhibition in the rat spinal cord

Two types of dorsal root potential (DRP) were found in the spinal cord of urethane-anaesthetized rats. Local DRPs with short latency-to-onset were evoked on roots close to the point of entry of an afferent volley. Diffuse DRPs with a longer latency-to-onset were seen on more distant roots up to 17 segments from the volley entry zone. The switch to long latency-to-onset occurred abruptly as a function of distance along the cord and could not be explained by conduction delays within the dorsal columns. Long-latency DRPs were also present and superimposed on the short-latency DRPs on nearby roots. Both local and diffuse DRPs were evoked by light mechanical stimuli: von Frey hair thresholds were Collapse

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MESH Headings

• Action Potentials/physiology
• Animals
• Electrophysiology
• Evoked Potentials/physiology
• GABA Antagonists/pharmacology
• Male
• Neural Inhibition/physiology
• Neurons, Afferent/physiology
• Picrotoxin/pharmacology
• Presynaptic Terminals/physiology
• Rats
• Rats, Sprague-Dawley
• Spinal Cord/physiology
• Spinal Nerve Roots/physiology
• Sural Nerve/drug effects
• Sural Nerve/physiology
• Synaptic Transmission/physiology

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Grants

• Wellcome Trust
• 052639 Wellcome Trust

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Affiliation(s)

Malcolm Lidierth King's College London, Hodgkin Building, Guy's Hospital Campus, London SE1 1UL, UK.

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Transplantation of neuronal and glial restricted precursors into contused spinal cord improves bladder and motor functions, decreases thermal hypersensitivity, and modifies intraspinal circuitry

Transplanting neuronal and glial restricted precursors (NRP/GRP) into a midthoracic injury 9 d after contusion improved bladder and motor function, diminished thermal hypersensitivity, and modified lumbosacral circuitry compared with operated controls (OP-controls). Histological analysis showed that NRP/GRP survived, filled the lesion site, differentiated into neurons and glia, and migrated selectively. Volume of spinal cord spared was increased in NRP/GRP recipients, suggesting local protection. Bladder areflexia developed in both operated groups, but NRP/GRP recipients exhibited an accelerated recovery, with decreased micturition pressure and fewer episodes of detrusor hyperreflexia. Because noradrenergic receptors proliferate after spinal injury and descending noradrenergic pathways contribute to regulation of bladder control, we examined the effects of administering an alpha-1A-adrenergic antagonist, Tamsulosin, on urodynamics. This improved all cystometric parameters in both operated groups, and micturition pressure in NRP/GRP rats recovered to normal levels. Both operated groups initially showed increased sensitivity to a thermal stimulus applied to the tail; the NRP/GRP rats showed significant improvement over time. NRP/GRP grafts also produced greater recovery of hindlimb function in several tests, although both groups showed persistent and similar deficits in locomotion on a grid. Because bladder, hindlimb, and tail sensory and motor functions are organized through lumbosacral cord, we examined descending and primary afferent projections at L6-S1. The density of serotonergic, noradrenergic, and corticotrophin releasing factor-positive fibers increased in the NRP/GRP group compared with OP-controls, suggesting some sparing and/or sprouting of these modulatory pathways. Immunocytochemical staining density of dorsal root axons in the dorsal horn increased in the OP-controls but appeared normal in the NRP/GRP group. Synaptophysin immunoreactivity in the lumbosacral dorsal horn was similar among groups, consistent with restoration of synaptic density in both groups of operated animals but by different pathways. We suggest that local protection provided by NRP/GRP resulted in increased sparing/sprouting of descending pathways, which prevented sprouting by dorsal root axons, and that this modification in lumbosacral circuitry contributes to the recovery of function.

Transplants of fibroblasts expressing BDNF and NT-3 promote recovery of bladder and hindlimb function following spinal contusion injury in rats

We examined whether fibroblasts, genetically modified to express BDNF and NT-3 (Fb-BDNF/NT3) and transplanted into a thoracic spinal injury site, would enhance recovery of bladder function and whether this treatment would be associated with reorganization of lumbosacral spinal circuits implicated in bladder function. Rats received modified-moderate contusion injuries at T8/9, and 9 days later, Fb-BDNF/NT3 or unmodified fibroblasts (OP-controls) were delivered into the cord. Fb-BDNF/NT3 rats recovered from areflexic bladder earlier, showed decreased micturition pressure and fewer episodes of detrusor hyperreflexia, compared to OP-controls. There were also improvements in hindlimb function in the Fb-BDNF/NT3 group although locomotion on a more challenging substrate (grid) and tail withdrawal latency in response to a thermal stimulus showed persisting deficits, little recovery, and no differences between the groups. Immunocytochemistry at L6-S1 revealed changes in density of afferent and descending projections to L6-S1 cord. The density of small dorsal root axons increased in the superficial layers of the dorsal horn in OP-controls but not in Fb-BDNF/NT3, suggesting sprouting of primary afferents following injury that was inhibited by Fb-BDNF/NT-3. In contrast, the trophic factor secreting transplants stimulated sprouting and/or sparing of descending modulatory pathways projecting to the lumbosacral spinal cord. No differences in synaptophysin immunoreactivity were seen in the dorsal horn which suggested that synaptic density was similar but achieved by sprouting of different systems in the two operated groups. Fb-BDNF/NT3 transplanted into injured spinal cord thus improved both bladder and hindlimb function, and this was associated with reorganization of spinal circuitry.

Evidence against cholera toxin B subunit as a reliable tracer for sprouting of primary afferents following peripheral nerve injury

In order to investigate whether cholera toxin B subunit (CTb) is transported by unmyelinated primary afferents following nerve injury, we transected the sciatic nerves of six rats, and injected the transected nerves (and in three cases also the intact contralateral nerves) with CTb, 2 weeks later. The relationship between CTb and two neuropeptides, vasoactive intestinal peptide (VIP) and neuropeptide Y (NPY), was then examined in neurons in the ipsilateral L4 and L5 dorsal root ganglia, using immunofluorescence staining and confocal microscopy. We also immunostained sections of spinal cord and caudal medulla for CTb, NPY and VIP. Following nerve section, VIP immunoreactivity was increased in laminae I-II of the spinal cord while NPY immunoreactivity was increased in laminae III-IV of the spinal cord and in the gracile nucleus. On the contralateral side, CTb labelling was detected in laminae I and III-V of the dorsal horn of the L4 and L5 spinal segments, as well as in the gracile nucleus. CTb labelling was seen in the same areas on the lesioned side, but with a dramatic increase in lamina II. No VIP or NPY immunoreactivity was observed in L4 and L5 dorsal root ganglia on the side of the intact nerve, but on the lesioned side VIP was detected in many small neurons and NPY in numerous large neurons. In agreement with the report by Tong et al. [J. Comp. Neurol. 404 (1999) 143], we found that while CTb labelling in the dorsal root ganglion on the side of the intact nerve was mainly in large neurons, on the lesioned side CTb was present in dorsal root ganglion neurons of all sizes. The main finding of the present study was that almost all of the VIP- (96%) and NPY- (98%) positive neurons in the dorsal root ganglia on the lesioned side were also CTb-labelled. After nerve injury VIP is upregulated in fine afferents that terminate in laminae I and II, and most of these probably have unmyelinated axons. Since the cell bodies of these neurons were labelled with CTb that had been injected into the transected sciatic nerve, this suggests that many of these fine afferents, which do not normally transport CTb, are capable of doing so after injury.

Focal peripheral nerve injury induces leukocyte trafficking into the central nervous system: potential relationship to neuropathic pain

The present study was undertaken to determine whether leukocytes are recruited into the spinal cord following a peripheral L5 spinal nerve transection that results in mechanical allodynia (increased tactile sensitivity behavior correlates with neuropathic pain). In rats subjected to bone marrow irradiation, donor-specific major histocompatibility complex (MHC) class I (I1-69) positive peripheral immune cells trafficked to the L5 spinal cord in response to an L5 spinal nerve injury. The number of I1-69 positive cell profiles increased over time and correlated with increased mechanical allodynia. At early time points following injury, I1-69 positive immune cells co-regionalized with the expression of the macrophage marker ED2. At later time points following injury, some of the infiltrating immune cells did not co-regionalize with the macrophage marker ED2. At no time did the infiltrating cells co-regionalize with the neuronal marker (NeuN). Both macrophage-like morphology and T cell-like morphology were observed in the I1-69 positive cellular infiltrate. Conversely, animals that underwent sham surgery demonstrated little mechanical allodynia and a minimal number of infiltrating peripheral immune cells. In a separate group of rats, infiltration of CD3+ T-lymphocytes was confirmed at 14 days post-nerve transection. This study demonstrates trafficking of leukocytes into the lumbar spinal cord at time points that correlate with mechanical allodynia suggesting a role of central neuroinflammation in persistent neuropathic pain.

Central control of information transmission through the intraspinal arborizations of sensory fibers examined 100 years after Ramón y Cajal

About 100 years ago, Santiago Ramón y Cajal reported that sensory fibers entering the spinal cord have ascending and descending branches, and that each of them sends collaterals to the gray matter where they have profuse ramifications. To him this was a fundamental discovery and proposed that the intraspinal branches of the sensory fibers were "centripetal conductors by which sensory excitation is propagated to the various neurons in the gray matter". In addition, he assumed that "conduction of excitation within the intraspinal arborizations of the afferent fibers would be proportional to the diameters of the conductors", and that excitation would preferentially flow through the coarsest branches. The invariability of some elementary reflexes such as the knee jerk would be the result of a long history of plastic adaptations and natural selection of the safest neuronal organizations. There is now evidence suggesting that in the adult cat, the intraspinal branches of sensory fibers are not hard wired routes that diverge excitation to spinal neurons in an invariable manner, but rather dynamic pathways where excitation flow can be centrally addressed to reach specific neuronal targets. This central control of information flow is achieved by means of specific sets of GABAergic interneurons that produce primary afferent depolarization (PAD) via axo-axonic synapses and reduce transmitter release (presynaptic inhibition). The PAD produced by single, or by small groups of GABAergic interneurons in group I muscle afferents, can remain confined to some sets of intraspinal arborizations of the afferent fibers and not spread to nearby collaterals. In muscle spindle afferents this local character of PAD allows cutaneous and descending inputs to differentially inhibit the PAD in segmental and ascending collaterals of individual fibers, which may be an effective way to decouple the information flow arising from common sensory inputs. This feature appears to play an important role in the selection of information flow in muscle spindles that occurs at the onset of voluntary contractions in humans.

Labeling of central projections of primary afferents in adult rats: a comparison between biotinylated dextran amine, neurobiotin and Phaseolus vulgaris-leucoagglutinin

The efficacy of anterograde labeling of the central projections of primary afferent fibers were compared between biotinylated dextran amine (BDA), neurobiotin (NB) and Phaseolus vulgaris-leucoagglutinin (PHA-L) after injections into the L5 or T13 dorsal root ganglia (DRGs) of adult rats. Excellent labeling was obtained with BDA, which visualized fibers with fine terminal boutons in the L5 and T13 spinal cord segments, Clarke's nucleus and the gracile nucleus. Rarely observed crossed projections to the gracile nucleus and L5 ventral horn of the contralateral side could also be distinguished. Even in the most successful experiments, however, BDA labeled only about one-third of the axons originating from the injected dorsal root ganglion. BDA was also efficient as transganglionic tracer after application to the transected sciatic nerve. NB produced no significant labeling of the L5 primary afferents, and was only moderately effective on the T13 level. PHA injections resulted in sparse terminal labeling of the T13 and L5 afferents. Thus, BDA is an effective tracer for long-range labeling of primary afferent projections in the spinal cord and brain stem. Since not all stem fibers become labeled, however, the method does not allow quantification of all axon branches and terminals arising from the injected DRGs.

Activity-dependent spinal cord plasticity in health and disease

Activity-dependent plasticity occurs in the spinal cord throughout life. Driven by input from the periphery and the brain, this plasticity plays an important role in the acquisition and maintenance of motor skills and in the effects of spinal cord injury and other central nervous system disorders. The responses of the isolated spinal cord to sensory input display sensitization, long-term potentiation, and related phenomena that contribute to chronic pain syndromes; they can also be modified by both classical and operant conditioning protocols. In animals with transected spinal cords and in humans with spinal cord injuries, treadmill training gradually modifies the spinal cord so as to improve performance. These adaptations by the isolated spinal cord are specific to the training regimen and underlie new approaches to restoring function after spinal cord injury. Descending inputs from the brain that occur during normal development, as a result of supraspinal trauma, and during skill acquisition change the spinal cord. The early development of adult spinal cord reflex patterns is driven by descending activity; disorders that disrupt descending activity later in life gradually change spinal cord reflexes. Athletic training, such as that undertaken by ballet dancers, is associated with gradual alterations in spinal reflexes that appear to contribute to skill acquisition. Operant conditioning protocols in animals and humans can produce comparable reflex changes and are associated with functional and structural plasticity in the spinal cord, including changes in motoneuron firing threshold and axonal conduction velocity, and in synaptic terminals on motoneurons. The corticospinal tract has a key role in producing this plasticity. Behavioral changes produced by practice or injury reflect the combination of plasticity at multiple spinal cord and supraspinal sites. Plasticity at multiple sites is both necessary-to insure continued performance of previously acquired behaviors-and inevitable-due to the ubiquity of the capacity for activity-dependent plasticity in the central nervous system. Appropriate induction and guidance of activity-dependent plasticity in the spinal cord is an essential component of new therapeutic approaches aimed at maximizing function after spinal cord injury or restoring function to a newly regenerated spinal cord. Because plasticity in the spinal cord contributes to skill acquisition and because the spinal cord is relatively simple and accessible, this plasticity is a logical and practical starting point for studying the acquisition and maintenance of skilled behaviors.

The paired homeodomain protein DRG11 is required for the projection of cutaneous sensory afferent fibers to the dorsal spinal cord

Cutaneous sensory neurons that detect noxious stimuli project to the dorsal horn of the spinal cord, while those innervating muscle stretch receptors project to the ventral horn. DRG11, a paired homeodomain transcription factor, is expressed in both the developing dorsal horn and in sensory neurons, but not in the ventral spinal cord. Mouse embryos deficient in DRG11 display abnormalities in the spatio-temporal patterning of cutaneous sensory afferent fiber projections to the dorsal, but not the ventral spinal cord, as well as defects in dorsal horn morphogenesis. These early developmental abnormalities lead, in adults, to significantly attenuated sensitivity to noxious stimuli. In contrast, locomotion and sensori-motor functions appear normal. Drg11 is thus required for the formation of spatio-temporally appropriate projections from nociceptive sensory neurons to their central targets in the dorsal horn of the spinal cord.

Effect of neonatal capsaicin treatment on neural activity in the medullary dorsal horn of neonatal rats evoked by electrical stimulation to the trigeminal afferents: an optical, electrophysiological, and quantitative study

To elucidate which glutamate receptors, NMDA or non-NMDA, have the main role in synaptic transmission via unmyelinated afferents in the trigeminal subnucleus caudalis (the medullary dorsal horn), and to examine the early functional effects of neonatal capsaicin treatment to the subnucleus caudalis, optical recording, field potential recording, and quantitative study using electron micrographs were employed. A medulla oblongata isolated from a rat 5--7 days old was sectioned horizontally 400-microm thick or parasagittally and stained with a voltage-sensitive dye, RH482 or RH795. Single-pulse stimulation with high intensity to the trigeminal afferents evoked optical responses mainly in the subnucleus caudalis. The optical signals were composed of two phases, a fast component followed by a long-lasting component. The spatiotemporal properties of the optical signals were well correlated to those of the field potentials recorded simultaneously. The fast component was eliminated by 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX; 10 microM), while the long-lasting component was not. The latter increased in amplitude under a condition of low Mg(2+) but was significantly reduced by DL-2-amino-5-phosphonovaleric acid (AP5; 30 microM). Neonatal capsaicin treatment also reduced the long-lasting component markedly. In addition, the decreases in the ratio of unmyelinated axons to myelinated axons and in the ratio of unmyelinated axons to Schwann cell subunits of trigeminal nerve roots both showed significant differences (P<0.05, Student's t-test) between the control group and the neonatal capsaicin treatment group. This line of evidence indirectly suggests that synaptic transmission via unmyelinated afferents in the subnucleus caudalis is mediated substantially by NMDA glutamate receptors and documented that neonatal capsaicin treatment induced a functional alteration of the neural transmission in the subnucleus caudalis as well as a morphological alteration of primary afferents within several days after the treatment.

Plasticity of motor systems after incomplete spinal cord injury

Although spontaneous regeneration of lesioned fibres is limited in the adult central nervous system, many people that suffer from incomplete spinal cord injuries show significant functional recovery. This recovery process can go on for several years after the injury and probably depends on the reorganization of circuits that have been spared by the lesion. Synaptic plasticity in pre-existing pathways and the formation of new circuits through collateral sprouting of lesioned and unlesioned fibres are important components of this recovery process. These reorganization processes might occur in cortical and subcortical motor centres, in the spinal cord below the lesion, and in the spared fibre tracts that connect these centres. Functional and anatomical evidence exists that spontaneous plasticity can be potentiated by activity, as well as by specific experimental manipulations. These studies prepare the way to a better understanding of rehabilitation treatments and to the development of new approaches to treat spinal cord injury.

Confocal microscopic analysis reveals sprouting of primary afferent fibres in rat dorsal horn after spinal cord injury

Following high thoracic spinal cord transection (SCT) in rats, abnormal changes in arterial pressure in response to sensory stimulation (autonomic dysreflexia) are correlated with changes in neural circuitry in the injured spinal cord. Anterograde transport of wheat germ agglutinin conjugated to Texas Red (WGATR) and confocal microscopy were used to characterize the increased arbourization of Adelta and Abeta fibre populations in laminae III-V of the dorsal horn. In cord-injured animals, significantly greater areas of WGATR-labeled fibres were found in the deeper laminae of the dorsal horn than in control rats. This increased area likely reflects sprouting of the Adelta, Abeta, and possibly C fibre populations. The time course of sprouting matches the onset of autonomic dysreflexia, indicating a possible functional correlation between the two phenomena.

Cellular mechanisms of hyperalgesia and spontaneous pain in a spinalized rat model of peripheral neuropathy: changes in myelinated afferent inputs implicated

Various hypotheses have been proposed to account for the mechanical hyperalgesia and spontaneous pain seen in animal models of peripheral neuropathy. The purpose of the present study was to determine whether there exists a spinal neuronal correlate to these properties. An experimental neuropathy was induced in male Sprague-Dawley rats by placing a 2-mm PE-90 polyethylene cuff around the sciatic nerve. All rats were subsequently confirmed to exhibit mechanical allodynia in the von Frey test. After induction of anaesthesia with pentobarbital and acute spinalization at T9, electrophysiological experiments were performed, recording extracellular single unit activity from ipsi- and contralateral wide dynamic range dorsal horn neurons in spinal segments L1-4. On-going activity was greater in short-term (11-22 days after cuff implantation) and long-term (42-52 days) cuff-implanted rats; 38 spikes/s in short-term versus 19 spikes/s in controls; 29 spikes/s in long-term ipsi- and contralateral neurons. Receptive fields in controls were always restricted, but in almost all cuff-implanted rats extended over the whole hind paw. Responses to noxious mechanical (pinch) and noxious heat stimulation of the cutaneous receptive field in controls consisted of the typical fast initial discharge followed by an afterdischarge. In all neurons from cuff-implanted rats the initial discharge resembled that in controls. However, the afterdischarge, particularly that in response to noxious pinch, was markedly greater in both magnitude and duration. It is suggested that the greater on-going discharge is the cellular correlate of spontaneous pain, and the potentiation of the afterdischarge in response to noxious stimulation is the correlate of hyperalgesia. Given that acutely spinalized rats were tested, only peripheral and/or spinal mechanisms can be considered to explain these data. Considering all the data, it can be concluded that there is a greater change in fibres mediating noxious mechanical than noxious thermal inputs. Among different hypotheses, the one with which the present data are most compatible is that which proposes that chronic nerve injury or inflammation induces phenotypic changes predominantly in myelinated afferents. There may be a redistribution of membrane-bound ion channels, predominantly sodium channels, which leads to ectopic activity and thus spontaneous discharge of dorsal horn neurons. With regard to mechanical stimulation-evoked synaptic input, the central terminals of myelinated afferents expand into regions of the spinal cord which normally receive their predominant input from unmyelinated nociceptive afferents. This may be coupled with a change in these myelinated afferents so that they now synthesize and release peptides, primarily substance P, from their central terminals with the result that the effects of their chemical mediators of synaptic transmission add to the effects of nociceptive inputs leading to exaggerated responses to painful stimuli, thus the basis of clinical hyperalgesia.

Long-term changes in sciatic-evoked A-fiber dorsal horn field potentials accompany loose ligation of the sciatic nerve in rats

The goal of the present study was to examine whether loose ligation of the sciatic nerve was associated with long-term changes in neuronal excitability in the spinal dorsal horn in urethane-anesthetized rats. The sciatic nerve was stimulated with 0. 1 ms long pulses at 1 stimulus/5 min, and the evoked dorsal horn field potentials remained stable in the absence of tetanic stimulation. In one set of control and ligated animals, high-frequency tetanic stimulation was applied to the nerve at 50 Hz (one 400 ms train of twenty 0.1 ms pulses), and the field potentials were recorded again (1 stimulus/5 min) for up to 4 h post-tetanus. In control animals, this protocol produced significant increases in field potential amplitudes at 15, 30 and 60 min post-tetanus. Interestingly, after this time the evoked field potentials began to decrease, and attained less than 50% of their pre-tetanic values at 240 min post-tetanus. In contrast, in ligated rats the pattern of post-tetanic potentiation was significantly different as the increases in amplitude persisted, and at 240 min post-tetanus the field potentials were almost twice their baseline values. In another set of control and ligated animals, low-frequency tetanic stimulation was applied at 5 Hz (one 400 ms train of two 0.1 ms pulses). Again a differential pattern of post-tetanic responses between control and ligated rats was seen. In control animals, a significant decrease in amplitude was evident within 30 min, and the depression became progressively more pronounced as the field potentials attained about a quarter of their baseline values at 180 min, and remained at these low levels at 240 min post-tetanus. On the other hand, in ligated animals, the depression was not significant, and at 240 min post-tetanus the field potentials were still at about 80% of their baseline values. These data demonstrate that long-term changes in spinal dorsal horn neuronal excitability accompany sciatic ligation to perhaps contribute to the development of neuropathic pain. These changes may result from a lessening of normally strong inhibitory processes in the spinal dorsal horn to generate conditions which favor post-tetanic potentiation over depression of dorsal horn neuronal responses.

Altered taste responses in adult NST after neonatal chorda tympani denervation

Anatomic and behavioral changes have been observed in the taste system after peripheral deafferentation, but their physiological consequences remain unknown. Interestingly, a recent behavioral study suggested that peripheral denervation could induce central plasticity. After neonatal chorda tympani (CT) transection, adult rats demonstrated a marked preference for a normally avoided salt, NH(4)Cl. In the present study, taste responses were recorded from the nucleus of the solitary tract (NST) in similarly CT-denervated rats to investigate a physiological basis for this behavioral phenomenon. We hypothesized that alterations in functional connectivity of remaining afferent nerves might underlie the behavioral change. Specifically, if NST neurons formerly activated by sodium-selective CT fibers were instead driven by more broadly tuned glossopharyngeal (GL) afferents, neural coding of salt responses would be altered. Such a change should be accompanied by a shift in orotopic representation and increased NH(4)Cl responses. This hypothesis was not supported. After CT denervation, orotopy was unaltered, NH(4)Cl responsiveness declined, and no other changes occurred that could simply explain the behavioral effects. Indeed, the most pronounced consequence of CT denervation was a 68% reduction in NaCl responses, supporting previous evidence for a critical role of this nerve in coding sodium salts. In addition, we found "reorganizational" changes similar to, albeit smaller than, those observed in other sensory systems after deafferentation. There was a trend for increased responses elicited by stimulation of receptor subpopulations innervated by the GL and greater superficial petrosal nerves. In addition, the spontaneous rate of nasoincisor duct-responsive cells increased significantly. This effect on spontaneous rate is opposite to that produced by CT anesthesia, suggesting that acute versus chronic denervation may affect central taste neurons differently. In conclusion, the taste system at the medullary level seems more resistant to large-scale plasticity than other sensory systems, but nevertheless reacts to lost afferent input. Because the most robust plastic changes have been documented at cortical levels in other sensory pathways, the substrate for the behavioral effect of neonatal CT transection may be located more centrally in the gustatory system.

Spinal cord injury in small animals. 1. Mechanisms of spontaneous recovery

Spinal cord injury causes obvious clinical deficits early in the course of lesion evolution, but it is commonly observed that recovery can occur spontaneously during a period of days, weeks or even months afterwards. Spinal cord dysfunction arises after injury because of a combination of reversible alterations in the concentration of intra- and extracellular ionic constituents and irreversible tissue destruction. Recovery can therefore occur through re-establishment of the normal microenvironment of the spinal cord, which occurs soon after injury induction, and also by formation of new patterns of central nervous system circuitry. Alterations in circuitry, termed 'plasticity', can occur during the immediate period after injury but apparently continue for many weeks or months. There are differences in the extent and nature of recovery between complete and incomplete experimental spinal cord injuries that illustrate the roles played by reorganisation of intra- and suprasegmental circuitry. Information that is available on mechanisms of spontaneous recovery may aid development of novel therapies for clinical spinal cord injury.

Contralateral trigeminal nerve lesion reduces polyneuronal muscle innervation after facial nerve repair in rats

Functional recovery after facial nerve surgery is poor. Axotomized motoneurons (hyperexcitable upon intracellular current injections, but unable to discharge upon afferent stimulation) outgrow supernumerary branches which are misrouted towards improper muscles. We hypothesized that alterations in the trigeminal input to axotomized electrophysiologically silent facial motoneurons might improve specificity of reinnervation. To test this we compared, in the rat, behavioural, electrophysiological, and morphological parameters after transection and suture of the buccal facial nerve (buccal-buccal anastomosis, BBA) with those after BBA plus excision of the ipsi- or contralateral infraorbital nerve (ION). After BBA, the mystacial vibrissae dropped and remained motionless until 18-21 days post operation (days PO). After BBA plus ipsilateral ION excision, there was no recovery of vibrissae whisking at all. Following BBA plus contralateral ION excision, full restoration of whisking occurred at 7-10 days PO. Electromyography of whiskerpad muscles showed normal waveform and amplitude was also most rapidly restored after BBA plus contralateral ION excision. Neuron counts after retrograde tracing showed that the intact buccal nerve contained axons of the superior (91%) and inferior (9%) buccolabial nerves. After BBA, the superior nerve comprised 56%, the inferior 21%, and 23% of the motoneurons projected within both nerves. After BBA plus ipsilateral ION excision, misdirection worsened and values changed to 48, 39 and 13%, respectively. After BBA plus contralateral ION excision, portions improved to 69, 23 and 8%. We conclude that, by reducing the redundant axon branching, lesion of contralateral ION provides the best conditions for recovery of vibrissae rhythmical whisking after reconstructive surgery on the facial nerve.

Stability and plasticity of primary afferent projections following nerve regeneration and central degeneration

Sensory neurons of the dorsal root ganglia (DRG) regenerate their peripheral axons with relative ease following a nerve lesion. The capacity for central regeneration appears more limited. However, after nerve lesion, some DRG neurons gain a regenerative advantage to sprout centrally. We developed a lesion model in the rat to test whether, after prior lesion of their peripheral axons, subsets of cutaneous afferents benefit differently in their ability to sprout into adjacent spinal segments denervated by dorsal rhizotomy. We found that under identical circumstances, myelinated sensory neurons, small-diameter peptidergic sensory neurons containing calcitonin gene related peptide (CGRP), and small-diameter nonpeptidergic neurons that bind the lectin from the plant Griffonia simplificolia, isolectin B4 (IB4) differ dramatically in their ability to regenerate centrally. Myelinated afferent terminals labelled transganglionically with cholera-toxin beta-subunit gain a small advantage in collaterally sprouting into the adjacent denervated neuropil in lamina III after prior peripheral nerve lesion. This central regenerative response was not mimicked by experimentally induced inflammation of sensory neuron cell bodies. Intact and unlesioned sensory neurons positive for CGRP sprout vigorously into segments denervated by rhizotomy in a nonsomatotopic manner. In contrast, IB4-positive sensory neurons maintain a somatotopic distribution centrally, which is not altered by prior nerve lesion. These data reveal a remarkably heterogeneous response to regeneration-promoting stimuli amongst three different types of cutaneous sensory neurons. In particular, the divergent responses of peptidergic and nonpeptidergic sensory neurons suggests profound functional differences between these neurochemically distinct neurons.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Branching and/or collateral projections of spinal dorsal horn neurons

Branching and/or collateral projections of spinal dorsal horn neurons is a common phenomenon. Evidence is presented for the existence of STTm/STTl, STTc/STTi, STT/SMT, STT/SRT, SCT/DCPS, SST/DCPS, SCT/SST, STT/SHT, STeT/SHT, STeTs and other doubly or multiply projecting spinal neurons that have been anatomically and physiologically identified and named based on the locations of the cells of origin and their terminations in the brain. These newly discovered spinal projection neurons are characterized by a single cell body and branched axons and/or collaterals that project to two or more target areas in the brain. These novel populations of neurons seem to be a fuzzy set of spinal projection neurons that function as an intersection set of the corresponding single projection spinal neurons and to be at an intermediate stage phylogenetically. Identification strategies are discussed, and general concluding remarks are made in this review.

Sprouting of primary afferent fibers after spinal cord transection in the rat

After spinal cord injury, hyper-reflexia can lead to episodic hypertension, muscle spasticity and urinary bladder dyssynergia. This condition may be caused by primary afferent fiber sprouting providing new input to partially denervated spinal interneurons, autonomic neurons and motor neurons. However, conflicting reports concerning afferent neurite sprouting after cord injury do not provide adequate information to associate sprouting with hyper-reflexia. Therefore, we studied the effect of mid-thoracic spinal cord transection on central projections of sensory neurons, quantified by area measurements. The area of myelinated afferent arbors, immunolabeled by cholera toxin B, was greater in laminae I-V in lumbar, but not thoracic cord, by one week after cord transection. Changes in small sensory neurons and their unmyelinated fibers, immunolabeled for calcitonin gene-related peptide, were assessed in the cord and in dorsal root ganglia. The area of calcitonin gene-related peptide-immunoreactive fibers in laminae III-V increased in all cord segments at two weeks after cord transection, but not at one week. Numbers of sensory neurons immunoreactive for calcitonin gene-related peptide were unchanged, suggesting that the increased area of immunoreactivity reflected sprouting rather than peptide up-regulation. Immunoreactive fibers in the lateral horn increased only above the lesion and in lumbar segments at two weeks after cord transection. They were not continuous with dorsal horn fibers, suggesting that they were not primary afferent fibers. Using the fluorescent tracer DiI to label afferent fibers, an increase in area could be seen in Clarke's nucleus caudal to the injury two weeks after transection. In conclusion, site- and time-dependent sprouting of myelinated and unmyelinated primary afferent fibers, and possibly interneurons, occurred after spinal cord transection. Afferent fiber sprouting did not reach autonomic or motor neurons directly, but may cause hyper-reflexia by increasing inputs to interneurons.

Enlargement of motoneuron peripheral field following partial denervation with or without dorsal rhizotomy

In partially denervated skeletal muscle, spared motor fibres sprout, enlarging motor unit size. Neuritogenesis and sprouting are known to depend on the synaptic input to the neurons. This suggests that spared motoneuron reaction to partial muscle denervation might be controlled by primary sensory neurons which directly or indirectly project to motoneurons. In two groups of rats, different surgical procedures were carried out: partial denervation of the extensor digitorum longus muscle without or with homolateral dorsal rhizotomy. Spared motoneuron peripheral field was evaluated by nerve-evoked tension measures. Following partial muscle denervation, spared motoneurons enlarged their projection peripheral field five to six times, innervating most of the denervated portion of the muscle. When dorsal rhizotomy was carried out together with partial denervation, the enlargement of the motoneuron's peripheral field occurred later; however, the peripheral field size was the same or greater than that found in partially denervated muscles without dorsal rhizotomy in the long term. Excitatory postsynaptic potential recordings at neuromuscular junctions consistently showed that innervation of denervated muscle cells by spared motoneurons was impaired when the dorsal roots were cut. Finally, in both groups of operated rats an increase in motor unit number occurred early after surgery, anticipating a process normally occurring in the same age range. These findings are consistent with the idea that sensory input trans-synaptically controls motoneuron peripheral field size.

Sensitization of pain pathways in the spinal cord: cellular mechanisms

Sensitization is manifested as an increased response of neurones to a variety of inputs following intense or noxious stimuli. It is one of the simplest forms of learning and synaptic plasticity and it represents an important feature of nociception. In the spinal cord, repeated stimulation (at constant strength) of dorsal root afferents including nociceptive C fibres can elicit a progressive increase in the number of action potentials generated by motoneurones and interneurones. This phenomenon is termed "action potential windup" and is used as a cellular model of pain sensitization developing at the level of the central nervous system. Understanding the mechanisms responsible for windup generation might allow clarification of the cellular mechanisms of pain signalling and development of new strategies for pain treatment. Action potential windup is observed in a minority of cells only, indicating that certain cell-specific mechanisms are responsible for its generation. The most reliable index to predict windup generation is the rate at which the membrane potential is depolarized during repetitive stimulation. This phenomenon has been proposed to be due to gradual recruitment of NMDA receptor activity, to summation of slow excitatory potentials mediated by substance P (and related peptides) or to facilitation of slow calcium channels by metabotropic glutamate receptors. Little is known about the role of synaptic inhibition in windup, although it should not be underestimated. Each theory per se is unable to account for all the experimental observations. Since NMDA receptors are involved in many forms of synaptic plasticity, additional mechanisms such as summation of slow peptidergic potentials, facilitation of slow Ca2+ currents and disinhibition are proposed as necessary to impart specificity to pain-induced sensitization. These additional mechanisms might be species specific and change during development or chronic pain states.