NT-3 promotes growth of lesioned adult rat sensory axons ascending in the dorsal columns of the spinal cord

The regeneration capacity of spinal cord axons is severely limited. Recently, much attention has focused on promoting regeneration of descending spinal cord pathways, but little is known about the regenerative capacity of ascending axons. Here we have assessed the ability of neurotrophic factors to promote regeneration of sensory neurons whose central axons ascend in the dorsal columns. The dorsal columns of adult rats were crushed and either brain-derived neurotrophic factor (BDNF), glial cell line-derived neurotrophic factor (GDNF), neurotrophin-3 (NT-3) or a vehicle solution was delivered continuously to the lesion site for 4 weeks. Transganglionic labelling with cholera toxin beta subunit (CTB) was used to selectively label large myelinated Abeta fibres. In lesioned rats treated with vehicle, CTB-labelled fibres were observed ascending in the gracile fasciculus, but these stopped abruptly at the lesion site, with no evidence of sprouting or growth into lesioned tissue. No CTB-labelled terminals were observed in the gracile nucleus, indicating that the lesion successfully severed all ascending dorsal column axons. Treatment with BDNF did not promote axonal regeneration. In GDNF-treated rats fibres grew around cavities in caudal degenerated tissue but did not approach the lesion epicentre. NT-3, in contrast, had a striking effect on promoting growth of lesioned dorsal column axons with an abundance of fibre sprouting apparent at the lesion site, and many fibres extending into and beyond the lesion epicentre. Quantification of fibre growth confirmed that only in NT-3-treated rats did fibres grow into the crush site and beyond. No evidence of terminal staining in the gracile nucleus was apparent following any treatment. Thus, although NT-3 promotes extensive growth of lesioned axons, other factors may be required for complete regeneration of these long ascending projections back to the dorsal column nuclei. The intrathecal delivery of NT-3 or other neurotrophic molecules has obvious advantages in clinical applications, as we show for the first time that dorsal column axonal regeneration can be achieved without the use of graft implantation or nerve lesions.

Responses of medullary dorsal horn neurons to corneal stimulation by CO(2) pulses in the rat

Corneal-responsive neurons were recorded extracellularly in two regions of the spinal trigeminal nucleus, subnucleus interpolaris/caudalis (Vi/Vc) and subnucleus caudalis/upper cervical cord (Vc/C1) transition regions, from methohexital-anesthetized male rats. Thirty-nine Vi/Vc and 26 Vc/C1 neurons that responded to mechanical and electrical stimulation of the cornea were examined for convergent cutaneous receptive fields, responses to natural stimulation of the corneal surface by CO(2) pulses (0, 30, 60, 80, and 95%), effects of morphine, and projections to the contralateral thalamus. Forty-six percent of mechanically sensitive Vi/Vc neurons and 58% of Vc/C1 neurons were excited by CO(2) stimulation. The evoked activity of most cells occurred at 60% CO(2) after a delay of 7-22 s. At the Vi/Vc transition three response patterns were seen. Type I cells (n = 11) displayed an increase in activity with increasing CO(2) concentration. Type II cells (n = 7) displayed a biphasic response, an initial inhibition followed by excitation in which the magnitude of the excitatory phase was dependent on CO(2) concentration. A third category of Vi/Vc cells (type III, n = 3) responded to CO(2) pulses only after morphine administration (>1.0 mg/kg). At the Vc/C1 transition, all CO(2)-responsive cells (n = 15) displayed an increase in firing rates with greater CO(2) concentration, similar to the pattern of type I Vi/Vc cells. Comparisons of the effects of CO(2) pulses on Vi/Vc type I units, Vi/Vc type II units, and Vc/C1 corneal units revealed no significant differences in threshold intensity, stimulus encoding, or latency to sustained firing. Morphine (0.5-3.5 mg/kg iv) enhanced the CO(2)-evoked activity of 50% of Vi/Vc neurons tested, whereas all Vc/C1 cells were inhibited in a dose-dependent, naloxone-reversible manner. Stimulation of the contralateral posterior thalamic nucleus antidromically activated 37% of Vc/C1 corneal units; however, no effective sites were found within the ventral posteromedial thalamic nucleus or nucleus submedius. None of the Vi/Vc corneal units tested were antidromically activated from sites within these thalamic regions. Corneal-responsive neurons in the Vi/Vc and Vc/C1 regions likely serve different functions in ocular nociception, a conclusion reflected more by the difference in sensitivity to analgesic drugs and efferent projection targets than by the CO(2) stimulus intensity encoding functions. Collectively, the properties of Vc/C1 corneal neurons were consistent with a role in the sensory-discriminative aspects of ocular pain due to chemical irritation. The unique and heterogeneous properties of Vi/Vc corneal neurons suggested involvement in more specialized ocular functions such as reflex control of tear formation or eye blinks or recruitment of antinociceptive control pathways.

Spinal opioid analgesia: how critical is the regulation of substance P signaling?

Although opioids can reduce stimulus-evoked efflux of Substance P (SP) from nociceptive primary afferents, the consequences of this reduction on spinal cord nociceptive processing has not been studied. Rather than assaying SP release, in the present study we examined the effect of opioids on two postsynaptic measures of SP release, Fos expression and neurokinin-1 (NK-1) receptor internalization, in the rat. The functional significance of the latter was first established in in vitro studies that showed that SP-induced Ca(2+) mobilization is highly correlated with the magnitude of SP-induced NK-1 receptor internalization in dorsal horn neurons. Using an in vivo analysis, we found that morphine had little effect on noxious stimulus-evoked internalization of the NK-1 receptor in lamina I neurons. However, internalization was reduced when we coadministered morphine with a dose of an NK-1 receptor antagonist that by itself was without effect. Thus, although opioids may modulate SP release, the residual release is sufficient to exert maximal effects on the target NK-1 receptors. Morphine significantly reduced noxious stimulus-induced Fos expression in lamina I, but the Fos inhibition was less pronounced in neurons that expressed the NK-1 receptor. Taken together, these results suggest that opioid analgesia predominantly involves postsynaptic inhibitory mechanisms and/or presynaptic control of non-SP-containing primary afferent nociceptors.

Involvement of peripheral NMDA and non-NMDA receptors in development of persistent firing of spinal wide-dynamic-range neurons induced by subcutaneous bee venom injection in the cat

To study the roles of peripheral excitatory amino acids receptor subtypes N-methyl-D-aspartate (NMDA) and non-NMDA receptors in persistent nociception, extracellular single unit recording technique was used to assess the effects of a single dose NMDA and non-NMDA receptor antagonists, AP(5) (5-aminophosphonovaleric acid) and CNQX (6-cyano-7-nitroquinoxaline-2,3-dione) or DNQX (6,7-dinitroquinoxaline-2,3-dione), on s.c. bee venom-induced increase in firing of wide-dynamic-range (WDR) neurons in the spinal dorsal horn of the urethane-chloralose anesthetized cats. Subcutaneous bee venom injection into the cutaneous receptive field resulted in a single phase of increased firing of WDR neurons over the background activity for more than 1 h. Local pre-administration of AP(5) (200 microg/100 microl) or CNQX (8.3 microg/100 microl) into the bee venom injection site produced 94% (1.01+/-0.96 spikes/s, n=5) or 76% (2.97+/-0.58 spikes/s, n=4) suppression of the increased neuronal firing when compared with local saline (16.32+/-4.55 spikes/s, n=10) or dimethyl sulfoxide (DMSO) (12.37+/-6.36 spikes/s, n=4) pre-treated group, respectively. Local post-administration of the same dose of AP(5) produced a similar result to the pre-treatment group with a 67% inhibition of the mean firing rate, however, the same treatment with CNQX and even a higher dose of DNQX (100 microg/100 microl) did not produce any inhibition of the neuronal firing induced by s.c. bee venom injection (DNQX vs. DMSO: 23.91+/-0. 25 vs. 22.14+/-0.04 spikes/s, P=0.0298, n=5). In the control experiments, local pre-administration of the same dose of AP(5) or CNQX into a region on the contralateral hindpaw symmetrical to the bee venom injection site produced no significant influence on the increased firing of the WDR neurons [contralateral AP(5) vs. saline: 14.17+/-6.27 spikes/s (n=5) vs. 16.32+/-4.55 spikes/s (n=10), P0.05; contralateral CNQX vs. DMSO: 12.85+/-6.38 spikes/s (n=4) vs. 12. 37+/-6.36 spikes/s (n=4), P0.05], implicating that the suppressive action of local AP(5) or CNQX was not the result of systemic effects. The present results suggest that activation of the peripheral NMDA receptors is involved in both induction and maintenance, while activation of non-NMDA receptors is only involved in induction, but not in the maintenance of persistent firing of the dorsal horn WDR neurons induced by s.c. bee venom injection.

Robust suppression of afferent-induced excitation in the rat spinal dorsal horn after conditioning low-frequency stimulation

The neuronal plasticity in the spinal dorsal horn induced after conditioning low-frequency stimulation of afferent A fibers, and its relationship with spinal inhibitory networks, was investigated with an optical-imaging method that detects neuronal excitation. High-intensity single-pulse stimulation of the dorsal root activating both A and C fibers evoked an optical response in the dorsal horn in transverse slices of 12- to 25-day-old rat spinal cords stained with a voltage-sensitive dye, RH-482. The optical response, reflecting the net excitation of neuronal elements along the thickness of each slice, was suppressed after a conditioning low-frequency stimulation (0.2-1 Hz for 10 min) to A fibers in the dorsal root. The degree of suppression was largest in the lamina II of the dorsal horn (48% reduction), where the majority of C fibers terminate, and much less in the deeper dorsal horn (5% reduction in laminae III-IV). The onset of suppression was somewhat slow; after the low-frequency stimulation, the magnitude of excitation gradually decreased, reached the maximum effect 30 min after the conditioning, and remained at the suppressed level for >1 h. Suppression was not observed when the low-frequency stimulation was given during a 20-min perfusion with a solution containing an NMDA-receptor antagonist, DL-2-amino-5-phosphonovaleric acid (30 microM). A brief application of an opioid-receptor antagonist, naloxone (0.5 microM), inhibited the induction, but not the maintenance, of low-frequency stimulus-induced suppression. However, treatments with the GABA(A) receptor antagonist bicuculline (1 microM) and the glycine receptor antagonist strychnine (0.3 microM) did not affect suppression induction and maintenance. In conclusion, conditioning low-frequency stimulation to A fibers interferes with the afferent-induced excitation in the dorsal horn. The low-frequency stimulation-induced suppression is maintained by a reduction of glutamatergic excitatory transmissions in the dorsal horn, not by an enhanced inhibition. Activation of the spinal opioid-mediated system by low-frequency stimulation, but not the inhibitory amino acid-mediated system, is necessary to initiate robust suppression. The long-term depression of afferent synaptic efficacy onto excitatory interneurons likely takes the primary role in the robust suppression of neuronal excitation in the dorsal horn.

Axotomy results in major changes in BDNF expression by dorsal root ganglion cells: BDNF expression in large trkB and trkC cells, in pericellular baskets, and in projections to deep dorsal horn and dorsal column nuclei

Brain derived neurotrophic factor (BDNF) is normally expressed by a small number of predominantly trkA-expressing dorsal root ganglion cells. Using immunocytochemistry and in situ hybridization, we have examined the effect of sciatic nerve section on the expression of BDNF in the adult rat. Following axotomy there was a long lasting (4-week) increase in BDNF mRNA and protein in large-diameter, trkB- and trkC-expressing dorsal root ganglion cells. By 2 days postaxotomy, expression of BDNF mRNA had increased from 2% of trkB cells to 50%, and from 18% of trkC cells to 56%. In contrast, BDNF expression in most trkA cells was unchanged, although was increased in the small population of medium- and large-sized trkA cells. Following axotomy, BDNF-immunoreactive terminals appeared in the central axonal projections of large-diameter cells, including the deep dorsal horn and gracile nucleus. Neuropeptide Y was also upregulated following axotomy and was coexpressed with BDNF in the cell bodies and central terminals of the large cells. Ultrastructural analysis in lamina IV of the spinal cord revealed that BDNF terminals in these central projections establish synaptic contacts. Immunoreactivity at 4 weeks was also observed in pericellular baskets that contained calcitonin gene-related peptide (CGRP) and surrounded trkA- and trkB-expressing cells in L4 and L5 lumbar ganglia. These baskets are likely to arise from local, highly immunoreactive, BDNF/CGRP/trkA-expressing cells. Our results identify several novel targets for BDNF and imply that it acts locally in both autocrine and paracrine modes, as well as centrally in a synaptic mode, to modulate the response of somatosensory pathways in nerve injury.

The role of non-N-methyl-D-aspartate ionotropic glutamate receptors in the spinal transmission of nociception in normal animals and animals with carrageenan inflammation

The role of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and kainate receptors in spinal nociceptive transmission in both normal animals and animals with carrageenan inflammation was investigated using the AMPA/kainate receptor antagonist 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione (NBQX) and the selective GluR5 kainate receptor antagonist LY382884 [3S,4aR,6S,8aR-6-(4-carboxyphenyl)methyl-1,2,3,4,4a,5,6,7,8, 8a-deca-hydroisoquinoline-3-carboxylic acid]. In normal animals, spinal administration of 100 microg of LY382884 produced a significant inhibition of both the C-fibre-evoked response and post-discharge of dorsal horn neurons, with the wind-up of the neurons being reduced by both 50 and 100 microg of LY382884. The spinal actions of LY382884 were enhanced following 3 h of carrageenan inflammation, such that doses of 20 microg and above were able to produce significant inhibitions of the noxious-evoked response of the neurons. Spinal administration of NBQX in normal animals (5-50 microg) inhibited the C-fibre-evoked response of the dorsal horn neurons, but only 50 microg of NBQX was able to inhibit the wind-up and post-discharge of the neurons. Following 3 h of carrageenan inflammation, the ability of NBQX to inhibit the wind-up and post-discharge of the neurons was markedly enhanced. These data suggest that both AMPA and kainate GluR5 receptors play an enhanced role in spinal nociceptive processing following the development of peripheral inflammation, as antagonists at both receptors are more effective against nociceptive responses, including wind-up under these inflammatory conditions.

Oxytocinergic innervation of autonomic nuclei controlling penile erection in the rat

In the rat, spinal autonomic neurons controlling penile erection receive descending pathways that modulate their activity. The paraventricular nucleus of the hypothalamus contributes oxytocinergic fibers to the dorsal horn and preganglionic sympathetic and parasympathetic cell columns. We used retrograde tracing techniques with pseudorabies virus combined with immunohistochemistry against oxytocin and radioligand binding detection of oxytocinergic receptors to evidence the oxytocinergic innervation of thoracolumbar and lumbosacral spinal neurons controlling penile erection. Spinal neurons labelled with pseudo-rabies virus transsynaptically transported from the corpus cavernosum were present in the intermediolateral cell column and the dorsal gray commissure of the thoracolumbar and lumbosacral spinal cord. Confocal laser scanning microscopic observation of the same preparations revealed close appositions between oxytocinergic varicosities and pseudorabies virus-infected neurons, suggesting strongly the presence of synaptic contacts. Electron microscopy confirmed this hypothesis. Oxytocin binding sites were present in the superficial layers of the dorsal horn, the dorsal gray commissure and the intermediolateral cell column in both the thoracolumbar and lumbosacral segments. In rats, stimulation of the paraventricular nucleus induces penile erection, but the link between the nucleus and penile innervation remains unknown. Our findings support the hypothesis that oxytocin, released by descending paraventriculo-spinal pathways, activates proerectile spinal neurons.

Release of endomorphin-2 like substances from the rat spinal cord

Release of endomorphin (ENDO)-2 like substances from the dorsal horn of the isolated rat spinal cord was measured by the immobilized-antibody microprobe technique. Spinal cords were removed from anesthetized 4-6 week old rats and superfused with oxygenated Krebs solution at room temperature. Glass microprobes coated with ENDO-2 antibodies were inserted into the dorsal horn of the lumbar spinal cord 1.5 mm lateral to the midline to a depth 2.5 mm below the dorsal surface of the cord. Each probe remained in situ for 10 min periods before, during and after electrical stimulation applied to the dorsal root entry zone of the same spinal segment. There was no detectable basal release of immunoreactive endomorphin-2 like substance (irENDO) from the dorsal horns during the pre-stimulation, nor following the stimulation period. A significant release of irENDO was measured during the electrical stimulation. These results provide the first evidence of a irEndo release that is correlated spatially with the dorsal horn laminae I and II where ENDO-2-immunoreactive fibers are concentrated in the dorsal horn in response to electrical activation of primary afferent fibers.

Evidence for subnucleus interpolaris in craniofacial muscle pain mechanisms demonstrated by intramuscular injections with hypertonic saline

The subnucleus interpolaris (Vi) has been identified as a major recipient for trigeminal ganglionic input from jaw muscles, and contains neurons with nociceptive properties similar to those in the subnucleus caudalis (Vc). Therefore, Vi may be another important site for processing craniofacial muscle nociception. The aims of present study were to define functional properties of Vi neurons that receive input from masseter muscle afferents by characterizing their responses to electrical, mechanical, and to chemical stimulation of the muscle. Ninety cells were identified as masseter muscle units in 11 adult cats. Most of these units (79%) received additional inputs from orofacial skin. Following the intramuscular injection of 5% hypertonic saline, 49% of the cells showed a significant modulation of either the resting discharge and/or responses to innocuous mechanical stimulation on their cutaneous receptive fields (RFs). The most common response to saline injection was an induction or facilitation of resting discharge which declined as an exponential decay function, returning to pre-injection level within 3-4 min. Forty-five percent of the muscle units that were tested with mechanical stimulation (13/29) showed a prolonged inhibition of mechanically-evoked responses. In most cases (8/13), the inhibitory response was accompanied by initial facilitation. The observations that Vi contained a population of neurons that receive small diameter muscle afferent inputs, responded to noxious mechanical stimulation on the muscle and to a chemical irritant that is known to produce pain in humans provide compelling evidence for the involvement of Vi in craniofacial muscle pain mechanisms.

Local circuit neurons showing calbindin D28k-immunoreactivity in the substantia gelatinosa of the medullary dorsal horn of the rat. An immunohistochemical study combined with intracellular staining in slice preparation

Calbindin D28k (CB)-immunoreactive local circuit neurons in the substantia gelatinosa of the rat medullary dorsal horn were studied in slice preparations; intracellular injection of biocytin was followed by histochemical visualization of biocytin, then CB immunohistochemistry was performed. In six neurons so far examined, overall morphology, including those of dendritic fields, axonal fields, and arborization patterns, was common.

Intrathecal treatment with MK-801 suppresses thermal nociceptive responses and prevents c-fos immunoreactivity induced in rat lumbar spinal cord neurons

Sensitization of the second order neurons in the spinal dorsal horn after somatic noxious stimuli is partly mediated by the N-methyl-D-aspartate (NMDA) subtype of the glutamate receptor. These neurons also express c-Fos immunoreactivity in response to the somatic noxious stimuli. The present study assessed the influence of intrathecal pre-treatment with MK-801, a non-competitive antagonist of NMDA receptor, on thermal sensitization following peripheral noxious heat stimulation. In addition, the influence of MK-801 on c-Fos immunoreactivity in the rat lumbar spinal cord neurons after the peripheral noxious heat was examined. Sprague-Dawley rats were subject to intrathecal catheterization and administration of MK-801 or saline before and after noxious heat (52 degrees C) stimulation of rat hindpaws. Thermal sensitization was tested after MK-801 (0.1 mumol 10 microliters-1). Fos-like immunoreactivity was evaluated 2 h after noxious stimulation in a separate group of animals. MK-801 significantly increased the thermal withdrawal threshold by 60% following noxious heat stimulation and reduced c-Fos immunoreactivity in the second order neurons by 70% in the dorsal horn. The study suggests that glutamate plays a pivotal role in the thermal nociceptive pathway and indicates that the NMDA receptor is necessary to maintain normal thermal sensitization, possibly by regulating c-fos gene expression in second order neurons.

Functional circuitry for the induction of prolonged excitation in the rat spinal dorsal horn

The neuronal circuitry through which prolonged excitation is generated in the spinal dorsal horn was investigated using optical imaging of neuronal excitation in transverse slices of rat spinal cords. It is known that tetanic stimulation (20 Hz for 1 s) of the dorsal root that activates both A and C primary afferent fibres elicits slow intrinsic optical signals (IOS) in the dorsal horn, seen most intensely in the substantia gelatinosa (SG), lamina II, and that IOS expresses in part the slow synaptic response recorded intracellularly in dorsal horn neurons. We here report that the slow IOS within the SG were completely abolished after an incision was made at the border between the SG and the deeper laminae, but not after an incision within the deeper dorsal horn of the laminae III-V. The result demonstrates directly that, in order to generate prolonged excitation in the SG, the neuronal elements in the deeper dorsal horn must be intact. Thus, the afferent information might be received first by the deeper elements and then transmitted to the SG region, and/or collaboration between the SG and deeper elements is necessary to maintain prolonged excitation in the SG.

Calcitonin gene-related peptide 8-37 inhibits the evoked discharge frequency of wide dynamic range neurons in dorsal horn of the spinal cord in rats

The present study was performed to explore the effect of calcitonin gene-related peptide 8-37 (CGRP8-37) on the electrical stimulation-evoked discharge frequency of wide dynamic range (WDR) neurons in the dorsal horn of the spinal cord in rats. The discharge frequencies of WDR neurons were evoked by transdermic electrical stimulation applied on the ipsilateral hindpaw. CGRP8-37 was applied directly on the dorsal surface of the L3 to L5 spinal cord. After the administration of 3 nmol of CGRP8-37, the evoked discharge frequency of WDR neurons decreased significantly, an effect lasting more than 30 min. The results indicate that CGRP receptors play an important role in the transmission of presumed nociceptive information in the dorsal horn of the spinal cord.

What are the mechanisms of regional musculoskeletal pain?

Pain is a subjective experience that is unique to the individual. Although pain is usually associated with tissue injury or inflammation, it is strongly influenced by changes within the nervous system as well as by psychological and social factors. This review focuses on those 'nociceptive' mechanisms which contribute to pain and demonstates how an appreciation of underlying molecular and cellular mechanisms can lead to improved diagnosis and treatment.