Effect of pulsed Nd:YAG laser radiation on action potential conduction in isolated mammalian spinal nerves

BACKGROUND AND OBJECTIVE

Dental lasers are claimed to produce analgesia, but the mechanisms and extent of any effects are uncertain. This study investigated the effects of lasing on nerve conduction in isolated nerves.

STUDY DESIGN/MATERIALS AND METHODS

Pulsed Nd:YAG laser energy was applied to spinal nerves in vitro and effects were measured as attenuation of the compound action potential (CAP) evoked by electrical stimulation.

RESULTS

Lasing for 1 minute at 0.3-3.0 W caused a dose-dependent attenuation of all components of the CAP (P < 0.03). With 0.3-1.0 W power, the CAP recovered to > 95% of the control levels 7 minutes after lasing; recovery was incomplete after lasing at > 2.0 W.

CONCLUSION

Isolated nerves were remarkably tolerant of lasing. The degree of nerve conduction block increased with laser power. The data indicate that lasing could diminish sensations, including pain, mediated by peripheral nerves in soft tissues.

Enlargement of the receptive field size to low intensity mechanical stimulation in the rat spinal nerve ligation model of neuropathy

One characteristic of plasticity after peripheral tissue or nerve damage is receptive field reorganization, and enlargement of receptive field size has been suggested to occur in certain models of neuropathic pain. The aim of the present study was to explore whether enlargement of neuronal receptive fields could contribute to the mechanical allodynia found on the ipsilateral paw in the spinal nerve ligation model of neuropathy. After ligation of L(5)-L(6) spinal nerves, all rats developed behavioral signs of mechanical allodynia, while the sham-operated control group displayed no such changes. The characteristics of the evoked responses of the neurones recorded in the dorsal horn of the rats were similar between the spinal nerve ligation, the sham operated control group, and the nonoperated control group, except for spontaneous activity, which was significantly increased in the spinal nerve ligation group. The mean size of the receptive field on the ipsilateral hindpaw, mapped using low-intensity stimulation with 9-g von Frey hair, was significantly increased in the spinal nerve ligation group, as compared to the sham-operated group. No significant difference was seen with 15- or 75-g von Frey hairs. The distribution of the receptive fields over the plantar surface of the paw was similar between the study groups. The enlargement of receptive field for non-noxious touch could be an indication of central sensitization in this model.

Fibroblast growth factor treatment produces differential effects on survival and neurite outgrowth from identified bulbospinal neurons in vitro

The in vivo application of appropriate trophic factors may enhance regeneration of bulbospinal projections after spinal cord injury. Currently, little is known about the sensitivities of specific bulbospinal neuron populations to the many identified trophic factors. We devised novel in vitro assays to study trophic effects on the survival and neurite outgrowth of identified bulbospinal neurons. Carbocyanine dye crystals implanted into the cervical spinal cord of embryonic day (E)5 chick embryos retrogradely labeled developing bulbospinal neurons. On E8, dissociated cultures containing labeled bulbospinal neurons were prepared. Fibroblast growth factor (FGF)-2 (but not FGF-1) promoted the survival of bulbospinal neurons. FGF receptor expression was widespread in the E8 brainstem, but not detected in young bulbospinal neurons, suggesting that nonneuronal cells mediated the FGF-stimulated survival response. Astrocytes synthesize a variety of trophic factors, and astrocyte-conditioned medium (ACM) also promoted the survival of bulbospinal neurons. As might be expected, FGF-2 function blocking antibodies did not suppress ACM-promoted survival, nor did an ELISA detect FGF-2 in ACM. This suggests that nonneuronal cells synthesize other factors in response to exogenous FGF-2 which promote the survival of bulbospinal neurons. Focusing on vestibulospinal neurons, dissociated (survival assay) or explant (neurite outgrowth assay) cultures were prepared. FGF-2 promoted both survival and neurite outgrowth of identified vestibulospinal neurons. Interestingly, FGF-1 promoted neurite outgrowth but not survival; the converse was true of FGF-9. Thus, differential effects of specific growth factors on survival or neurite outgrowth of bulbospinal neurons were distinguished.

Effect of motor unit recruitment on functional vasodilatation in hamster retractor muscle

1. The effect of motor unit recruitment on functional vasodilatation was investigated in hamster retractor muscle. Recruitment (i.e. peak tension) was controlled with voltage applied to the spinal accessory nerve (high = maximum tension; intermediate = approximately 50% maximum; low = approximately 25% maximum). Vasodilatory responses (diameter times time integral, DTI) to rhythmic contractions (1 per 2s for 65s) were evaluated in first, second and third order arterioles and in feed arteries. Reciprocal changes in duty cycle (range, 2.5-25%) effectively maintained the total active tension (tension times time integral, TTI) constant across recruitment levels. 2. With constant TTI and stimulation frequency (40 Hz), DTI in all vessels increased with motor unit recruitment. DTI increased from distal arterioles up through proximal feed arteries. 3. To determine whether the effect of recruitment on DTI was due to increased peak tension, the latter was controlled with stimulation frequency (15, 20 and 40 Hz) during maximum (high) recruitment. With constant TTI, DTI then decreased as peak tension increased. 4. To explore the interaction between recruitment and duty cycle on DTI, each recruitment level was applied at 2.5, 10 and 20 % duty cycle (at 40 Hz). For a given increase in TTI, recruitment had a greater effect on DTI than did duty cycle. 5. Functional vasodilatation in response to rhythmic contractions is facilitated by motor unit recruitment. Thus, vasodilatory responses are determined not only by the total tension produced, but also by the number of active motor units.

Spinal nerve function in five volunteers experiencing transient neurologic symptoms after lidocaine subarachnoid anesthesia

The etiology of transient neurologic symptoms (TNS) after 5% lidocaine spinal anesthesia remains undetermined. Previous case reports have shown that patients acutely experiencing TNS have no abnormalities on neurologic examination or magnetic resonance imaging. The aim of our study was to determine whether volunteers with TNS would exhibit abnormalities in spinal nerve electrophysiology. Twelve volunteers with no history of back pain or neurologic disease underwent baseline electromyography (EMG), nerve conduction studies, and somatosensory-evoked potential (SSEP) testing. Then, the volunteers were administered 50 mg of 5% hyperbaric lidocaine spinal anesthesia and were placed in a low lithotomy position (legs on four pillows). The next day, all volunteers underwent follow-up EMG, nerve conduction, and SSEP testing and were questioned and examined for the presence of complications including TNS (defined as pain or dysthesia in one or both buttocks or legs occurring within 24 h of spinal anesthesia). Volunteers who had TNS underwent additional EMG testing 4-6 wk later. Five of the 12 volunteers reported TNS. No volunteer had an abnormal EMG, nerve conduction study, or SSEP at 24 h follow up, nor were there any changes in EMG studies at delayed testing in the five volunteers experiencing TNS. On statistical analysis, the right peroneal and the right tibial nerve differed significantly for all volunteers from pre- to postspinal testing. When comparing pre- and postspinal testing of the TNS and non-TNS volunteers, statistically significant changes occurred in the nerve conduction tests of the right peroneal and left tibial nerve. There was no difference in measurements of F response, H reflex latency, amplitude, or velocity for either leg. Multivariate analysis of variance showed no significant difference between TNS and non-TNS volunteers for the changes in the nine nerve conduction tests when considered together (P = 0.4). We conclude that acute TNS after lidocaine spinal anesthesia did not result in consistent abnormalities detectable by EMG, nerve conduction studies, or SSEP in five volunteers.

IMPLICATIONS

Electrophysiologic testing in volunteers experiencing transient neurologic symptoms is not abnormal.

Assessment of the infraspinatus spinal stretch reflex in the normal, athletic, and multidirectionally unstable shoulder

To examine neural aspects of motor control in the glenohumeral joint, this study evaluates utilization of an innate spinal segmental pathway, the spinal stretch reflex, as an investigational tool that reflects neural circuitry. The purpose of this study was to determine if this reflex could be evoked from the infraspinatus muscle, if the testing apparatus and protocol for elicitation were reliable, and if the reflex response varies between groups of subjects and therefore could be useful clinically. These reflex characteristics were evaluated in the infraspinatus muscle, since rotator cuff muscle activity in subjects with glenohumeral instability exhibits differences in electromyographic activity and coordination patterns, implicating its role in dynamic stability. Normal shoulders were compared with athletic shoulders and shoulders with multidirectional instability. The spinal stretch reflex was elicited in a controlled and reliable manner. Shoulders with multidirectional instability exhibited a more-prominent spinal stretch reflex response than normal shoulders, whereas athletic shoulders exhibited a more-quiescent spinal stretch reflex response. As the spinal stretch reflex probably plays a role in motor control, variation in this reflex profile may reflect some differences in development that contribute to the variable expression of dynamic glenohumeral stability. This study suggests that the spinal stretch reflex profile may be a useful clinical tool to assist in discriminating between the normal and pathologic state. This information may also be useful in the evaluation of new treatment approaches exploiting spinal cord plasticity and spinal stretch reflex mutability through neuromuscular training.

The contribution of lumbar sympathetic neurones activity to rat's skin blood flow oscillations

Skin blood flow on the rat's paws using laser Doppler flowmeter, electrical activity of the heart (ECG) and respiration were measured simultaneously. The signals were recorded for 20 minutes, both before and after denervation, at core temperature 37 degrees C and 38.5 degrees C, that was maintained constant during the recordings. Spinal nerve fibres, at the level L3-L4, were transected. Experiments were performed on 15 adult Wistar rats under general anaesthesia. The oscillations in the measured signals were analysed in the time-frequency domain using wavelet transform. On the frequency region from 0.7 Hz to 5 Hz two characteristic peaks were observed in the skin blood flow spectrum. They correspond to the main peaks in the spectra of the ECG (around 3.3 Hz) and respiration (around 1.3 Hz). Several additional peaks were observed in the low frequency region, from 0.01 to 0.7 Hz, in all measured signals. In this frequency region the relative energy contribution of the blood flow oscillations decreased after denervation only in the denervated left hind paw. This difference was not statistically significant at 37 degrees C (p=0.098, Kruskal-Wallis test) but became statistically significant at 38.5 degrees C (p=0.017). Relative energy contribution of the low frequency region, from 0.01 to 0.7 Hz, decreased 2.5-fold in the blood flow of the denervated paw. Within this region the relative energy contribution decreased significantly in two intervals, from 0.01 to 0.08 Hz and from 0.08 to 0.2 Hz (p=0.023). In the higher frequency region, from 0.7 to 5 Hz, o statistically significant differences were obtained in any paws when compared before and after denervation at the same core temperature. We conclude that the activity of lumbar sympathetic neurones contributes to low frequency skin blood flow oscillations.

Electrical stimulation of the rat lumbar spine induces reflex action potentials in the nerves to the lower abdomen

STUDY DESIGN

The distribution of the nerve action potentials reflexively elicited by electrical stimulation of the lumbar spine was investigated in rats.

OBJECTIVES

To elucidate the relation between the lumbar spine and other body regions that compose the spinal reflex.

SUMMARY OF BACKGROUND DATA

The hypothesis was that the ventral portion of the L5-L6 disc spatially corresponds to the groin.

METHODS

In Experiments 1 and 2, wire electrodes were placed 1) in the ventral and dorsal portions of the disc, facet joint, and muscle fascia at L5-L6, and 2) in the ventral portions of L3-L4, L4-L5, L5-L6, and L6-S discs. A needle electrode was inserted in the L5-L6 disc by 0.4-mm increments, and action potentials were serially recorded from the genitofemoral nerve.

RESULTS

Experiments 1 and 2: Reflex action potentials were elicited in the iliohypogastric (T13 and L1), ilioinguinal (L1), and genitofemoral (L2) nerves. Experiment 1: Stimulation of the disc induced reflex discharges significantly more frequently than stimulation of the facet joint and muscle fascia. Experiment 2: The more cranial the disc stimulated, the more frequently the reflex discharge was induced in the iliohypogastric nerve. Experiment 3: The depth of stimulation did not influence the size of the reflex action potential.

CONCLUSIONS

Electrical stimulation of the lumbar disc and facet joint induced reflex discharges in the nerves to the lower abdominal regions. It was postulated that the reflex discharges are related to muscle contraction resulting in referred pain in the loin and groin.

Synergistic effect between intrathecal non-NMDA antagonist and gabapentin on allodynia induced by spinal nerve ligation in rats

BACKGROUND

Glutamate and non-N-methyl-D-aspartate (NMDA) receptors have been implicated in the development of neuroplasticity in the spinal cord in neuropathic pain. The spinal cord has been identified as one of the sites of the analgesic action of gabapentin. In the current study, the authors determined the antiallodynic effect of intrathecal 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) in a rat model of neuropathic pain. Also tested was a hypothesis that intrathecal injection of CNQX and gabapentin produces a synergistic effect on allodynia in neuropathic rats.

METHODS

Allodynia was produced in rats by ligation of the left L5 and L6 spinal nerves. Allodynia was determined by application of von Frey filaments to the left hind paw. Through an implanted intrathecal catheter, 10-100 microg gabapentin or 0.5-8 microg CNQX disodium (a water-soluble formulation of CNQX) was injected in conscious rats. Isobolographic analysis was performed comparing the interaction of intrathecal gabapentin and CNQX using the ED50 dose ratio of 15:1.

RESULTS

Intrathecal treatment with gabapentin or CNQX produced a dose-dependent increase in the withdrawal threshold to mechanical stimulation. The ED50 for gabapentin and CNQX was 45.9+/-4.65 and 3.4+/-0.22 microg, respectively. Intrathecal injection of a combination of CNQX and gabapentin produced a strong synergistic antiallodynic effect in neuropathic rats.

CONCLUSIONS

This study shows that intrathecal administration of CNQX exhibits an antiallodynic effect in this rat model of neuropathic pain. Furthermore, CNQX and gabapentin, when combined intrathecally, produce a potent synergistic antiallodynic effect on neuropathic pain in spinal nerve-ligated rats.

Projection linkage from spinal neurons to both lateral cervical nucleus and solitary tract nucleus in the cat

Intracellular recordings from the lumbosacral dorsal horn were made to identify the axonal projection and the afferent innervation of the lateral cervical nucleus (LCN) and solitary tract nucleus (STN) on the spinal neurons of chloralose-anesthetized cats. A total of 49 neurons from laminae III-V in the spinal dorsal horn responded to stimulation of both the LCN and STN. Of these, 28 and 21 neurons responded antidromically and orthodromically to stimulation of the LCN and STN, respectively. Seven of the 28 antidromically activated neurons were followed by one or more responses synaptically driven from the LCN and/or STN. The diameter of these ascending or descending fibers was in the range of A delta fibers. The results indicate that (1) some spinal neurons, namely spinocervical tract-spinosolitary tract (SCT-SST) neurons, issue branched axons of A delta-fibers and dually project to both LCN and STN; (2) some SCT-SST neurons receive innervation from both the LCN and STN; (3) some spinal neurons and interneurons are dually innervated by descending fibers originating from both the LCN and STN, and (4) the convergence and integration between somatic and visceral sensory inputs might occur in the SCT-SST neurons.

Neuropilin-2 is required in vivo for selective axon guidance responses to secreted semaphorins

Neuropilins are receptors for class 3 secreted semaphorins, most of which can function as potent repulsive axon guidance cues. We have generated mice with a targeted deletion in the neuropilin-2 (Npn-2) locus. Many Npn-2 mutant mice are viable into adulthood, allowing us to assess the role of Npn-2 in axon guidance events throughout neural development. Npn-2 is required for the organization and fasciculation of several cranial nerves and spinal nerves. In addition, several major fiber tracts in the brains of adult mutant mice are either severely disorganized or missing. Our results show that Npn-2 is a selective receptor for class 3 semaphorins in vivo and that Npn-1 and Npn-2 are required for development of an overlapping but distinct set of CNS and PNS projections.

Abnormal substance P release from the spinal cord following injury to primary sensory neurons

The neuropeptide substance P (SP) modulates nociceptive transmission within the spinal cord. Normally, SP is uniquely contained in a subpopulation of small-calibre axons (Adelta- and C-fibres) within primary afferent nerve. However, it has been shown that after nerve transection, besides being downregulated in small axons, SP is expressed de novo in large myelinated Abeta-fibres. In this study we investigated whether, following peripheral nerve injury, SP was released de novo from the spinal cord after selective activation of Abeta-fibres. Spinal cords with dorsal roots attached were isolated in vitro from rats 2 weeks following distal sciatic axotomy or proximal spinal nerve lesion (SNL). The ipsilateral dorsal roots were electrically stimulated for two consecutive periods at low- or high-threshold fibre strength, spinal cord superfusates were collected and SP content was determined by radioimmunoassay. SNL, but not axotomized or control rat cords, released significant amounts of SP after selective activation of Abeta-fibres. Not only do these data support the idea that Abeta myelinated fibres contribute to neuropathic pain by releasing SP, they also illustrate the importance of the proximity of the lesion to the cell body.

Ascending projections from the area around the spinal cord central canal: A Phaseolus vulgaris leucoagglutinin study in rats

A single small iontophoretic injection of Phaseolus vulgaris leucoagglutinin labels projections from the area surrounding the spinal cord central canal at midthoracic (T6-T9) or lumbosacral (L6-S1) segments of the spinal cord. The projections from the midthoracic or lumbosacral level of the medial spinal cord are found: 1) ascending ipsilaterally in the dorsal column near the dorsal intermediate septum or the midline of the gracile fasciculus, respectively; 2) terminating primarily in the dorsal, lateral rim of the gracile nucleus and the medial rim of the cuneate nucleus or the dorsomedial rim of the gracile nucleus, respectively; and 3) ascending bilaterally with slight contralateral predominance in the ventrolateral quadrant of the spinal cord and terminating in the ventral and medial medullary reticular formation. Other less dense projections are to the pons, midbrain, thalamus, hypothalamus, and other forebrain structures. Projections arising from the lumbosacral level are also found in Barrington's nucleus. The results of the present study support previous retrograde tract tracing and physiological studies from our group demonstrating that the neurons in the area adjacent to the central canal of the midthoracic or lumbosacral level of the spinal cord send long ascending projections to the dorsal column nucleus that are important in the transmission of second-order afferent information for visceral nociception. Thus, the axonal projections through both the dorsal and the ventrolateral white matter from the CC region terminate in many regions of the brain providing spinal input for sensory integration, autonomic regulation, motor and emotional responses, and limbic activation.

Differential effects of spinal cord gray and white matter on process outgrowth from grafted human NTERA2 neurons (NT2N, hNT)

To investigate host effects on grafts of pure, postmitotic, human neurons, we assessed the morphologic and molecular phenotype of purified NTera2N (NT2N, hNT) neurons implanted into the spinal cord of athymic nude mice. NT2N neurons were implanted into both spinal cord gray matter and white matter of neonatal, adolescent, and adult mice and were evaluated at postimplantation times up to 15 months. NT2N neurons remained at the implantation site and showed process integration into all host areas, and each graft exhibited similar phenotypic features regardless of location or host age at implantation. Evidence of host oligodendrocyte ensheathment of NT2N neuronal processes was seen, and grafted NT2N neurons acquired and maintained the morphologic and molecular phenotype of mature neurons. The microenvironments of host gray matter and white matter appear to exert differential effects on implanted neuronal processes, because consistent differences were noted in the morphologies of graft processes extending into white matter versus gray matter. NT2N processes extended for long distances (>2 cm) within white matter, whereas NT2N processes located within gray matter had shorter trajectories. This suggests that NT2N neurons integrate similarly into spinal cord gray matter and white matter, but they extend processes that respond differentially to gray matter and white matter cues. Further studies of the model system described here may identify the host molecular signals that support and direct integration of grafted human neurons as well as the outgrowth of their processes in the nervous system.

Ectopic discharges and adrenergic sensitivity of sensory neurons after spinal nerve injury

At various times after spinal nerve injury, dorsal root ganglia (DRGs) from injured segments were removed with attached dorsal roots and spinal nerves. In an in vitro recording chamber, spontaneously active units were recorded from teased dorsal root fascicles. Sustained spontaneous activity could first be recorded at 13 h after the ligation, but adrenergic sensitivity did not develop until 24 h after the injury. Almost all recorded activity originated from the DRG. Thus, the DRG is the most common site for ectopic discharge generation after spinal nerve injury and separate mechanisms seem to be involved in the development of ectopic discharges and adrenergic sensitivity.

Inhibition of spinal protein kinase C reduces nerve injury-induced tactile allodynia in neuropathic rats

We investigated the effects of inhibiting spinal protein kinases including PKC, PKA and PKG on tactile allodynia in rats with a unilateral tight ligation on L5/L6 spinal nerves (Chung model). The intrathecal (IT) delivery of GF109203X, a PKC inhibitor, produced a potent and long lasting anti-allodynic effect. The effect was dose-dependent and stereospecific. Bisindolymaleimide V, an inactive homologue of GF, had no effect. Additionally, two other PKC inhibitors, PKC19-31 and chelerythrine, displayed significant anti-allodynic action. Spinal PKA, but not PKG, is likely involved in Chung tactile allodynia, since H89 (a PKA inhibitor) showed anti-allodynic activity, while KT5823 (a PKG inhibitor) had only a minor effect. These data emphasize that spinal PKC plays an important role in nerve injury-induced tactile allodynia. Other protein kinases such as PKA may also contribute to this phenomenon.

Antinociceptive properties of fenfluramine, a serotonin reuptake inhibitor, in a rat model of neuropathy

Fenfluramine is an indirect agonist of 5-hydroxytryptamine (5-HT) receptors that acts by evoking 5-HT release and blocking 5-HT reuptake in neuronal cells. The current study compared the antinociceptive properties of fenfluramine with those of the tricyclic antidepressants amitriptyline and desipramine in rat models of acute, persistent, and neuropathic pain. In a rat model of neuropathic pain produced by tight ligation of the L(5)/L(6) spinal nerves, i.v. bolus injection of fenfluramine resulted in a dose-dependent and long-lasting (>4 h) blockade of mechanical allodynia (ED(50) = 3.5 mg/kg; 95% confidence interval, 2.2-5.4 mg/kg) and cold allodynia (ED(50) = 2.4 mg/kg; 95% confidence range, 1.2-4.6 mg/kg). Fenfluramine also prevented tonic pain evoked by the s.c. injection of dilute (5%) formaldehyde solution (formalin), into the dorsal hindpaw. The i.v. administration of amitriptyline (4.7 mg/kg) or desipramine (13.5 mg/kg) at maximum tolerated doses did not block either allodynia in rats with spinal nerve ligation-induced painful neuropathy or tonic pain in the formalin test. Fenfluramine had differential effects on acute behavioral responses to noxious thermal (heat), chemical (5% formaldehyde solution), and mechanical stimuli; it completely inhibited nociceptive behavior in the acute phase of the formaldehyde solution test and partially inhibited licking and jumping responses in the hot-plate test but did not alter nociceptive thresholds in either the paw pressure test or the tail immersion test. Intracerebroventricular bolus injection of 240 microg of fenfluramine significantly increased mechanical allodynia thresholds; however, the same dose administered spinally by intrathecal bolus injection was ineffective. The inhibitory effects of fenfluramine on mechanical allodynia (and tonic pain behavior in the formaldehyde solution test) were prevented by pretreatment with 10 mg/kg metergoline, a selective antagonist of 5-HT receptors, but not with the mu-opioid receptor antagonist naloxone. These results suggest that fenfluramine produces analgesia in the formaldehyde solution test and the spinal nerve ligation model of neuropathic pain by potentiating, at least in part, supraspinal 5-HT mediated processes.

Salt sensitivity and hydration behavior of the toad, Bufo marinus

Toads, Bufo marinus, were placed on laboratory tissue saturated with water or with hyperosmotic (250 or 500 mM NaCl or KCl) solutions, and their behavior was observed for 5 min. Toads placed on water initially allowed their ventral skin to touch the surface without abducting the hind limbs. During this "seat patch down" (SPD) behavior toads appeared to be evaluating the suitability of a hydration source prior to initiating "water absorption response" (WR) behavior with the hind limbs fully abducted and the ventral skin pressed to the moist surface. Toads dehydrated by more than 10% showed significantly shorter periods of SPD behavior and initiated WR behavior more frequently than did hydrated toads. Dehydrated toads placed on 250 mM NaCl initiated WR behavior in only 18% of the trials, but spent significantly more time showing SPD behavior than they did on water, indicating that this concentration is marginally acceptable to them. Recordings from spinal nerve #6 showed an increase in activity when 250 mM NaCl or KCl solutions were perfused over the outer surface of the ventral skin. The response to KCl was significantly greater than NaCl. The addition of 10 microM amiloride to 250 mM NaCl resulted in a higher frequency of WR behavior and reversibly inhibited the neural response to 250 mM NaCl. These results suggest that epithelial Na+ channels in the skin serve a sensory function in this species. Neither the hydrated nor dehydrated toads initiated WR behavior on 250 or 500 mM KCl solutions, indicating that toads have a lower tolerance of K+ than of Na+ salts.

Neuronal nitric oxide synthase mRNA upregulation in rat sensory neurons after spinal nerve ligation: lack of a role in allodynia development

Pharmacological evidence suggests a functional role for spinal nitric oxide (NO) in the modulation of thermal and/or inflammatory hyperalgesia. To assess the role of NO in nerve injury-induced tactile allodynia, we examined neuronal NO synthase (nNOS) expression in the spinal cord and dorsal root ganglia (DRG) of rats with tactile allodynia because of either tight ligation of the left fifth and sixth lumbar spinal nerves or streptozotocin-induced diabetic neuropathy. RNase protection assays indicated that nNOS mRNA (1) was upregulated in DRG, but not spinal cord, neurons on the injury side beginning 1 d after nerve ligation, (2) peaked (approximately 10-fold increase) at 2 d, and (3) remained elevated for at least 13 weeks. A corresponding increase in DRG nNOS protein was also observed and localized principally to small and occasionally medium-size sensory neurons. In rats with diabetic neuropathy, there was no significant change in DRG nNOS mRNA. However, similar increases in DRG nNOS mRNA were observed in rats that did not develop allodynia after nerve ligation and in rats fully recovered from allodynia 3 months after the nerve ligation. Systemic treatment with a specific pharmacological inhibitor of nNOS failed to prevent or reverse allodynia in nerve-injured rats. Thus, regulation of nNOS may contribute to the development of neuronal plasticity after specific types of peripheral nerve injury. However, upregulation of nNOS is not responsible for the development and/or maintenance of allodynia after nerve injury.

Termination of upper limb movement by cutaneous afferents

We tested whether cutaneous afferents from the skin field close to an upper limb muscular region would carry information to spinal neurons at the onset or at the offset of a voluntary elbow extension movement lasting 1 s. We detected a depression of EMG activity both at onset and at the offset of the reaching movement but in the latter case depression was significantly larger and immediate. The marked depression of EMG activity suggests an inhibition, via spinal neurons, of the descending excitation to the motoneurones supplying the triceps brachii. This spinal control might be a very efficient mechanism for the termination of voluntary movement.

Sex differences in cholinergic analgesia II: differing mechanisms in two models of allodynia

BACKGROUND

Cholinergic agents reduce allodynia after nerve injury in animals and may be useful in the treatment of neuropathic pain. Intrathecally administered neostigmine and neuronal nicotinic agonists are more potent in female than in male rats against acute thermal noxious stimuli. The purpose of this study was to determine whether there is also a sex difference in the antiallodynic effects of intrathecal cholinomimetic agents in two models of allodynia and to test their pharmacologic mechanisms.

METHODS

Male and female rats with indwelling intrathecal catheters received injections of neostigmine, bethanechol (muscarinic agonist), RJR-2403 (neuronal nicotinic agonist) alone or with atropine (muscarinic antagonist), mecamylamine (nicotinic antagonist), phentolamine (alpha-adrenergic antagonist), or saline control. The effect of these agents was determined on mechanical allodynia produced by either intraplantar injection of capsaicin or ligation of spinal nerves.

RESULTS

Neostigmine and RJR-2403 but not bethanechol were more potent in female than in male rats in reducing allodynia after nerve injury, and antagonist studies were also consistent with a nicotinic component to explain this sex difference. Phentolamine did not reverse neostigmine's effect. In contrast, for capsaicin-induced allodynia, neostigmine plus mecamylamine but not neostigmine or RJR-2403 was more potent in female than in male rats.

CONCLUSIONS

These data demonstrate a sex difference of intrathecal neostigmine after nerve injury-induced allodynia similar to that observed in normal animals that received acute noxious thermal stimulation. However, this sex difference is not universal to all pain models because it was not present after intradermal capsaicin injection, nor is its interaction with spinal noradrenergic mechanisms consistent in all models.

The effect of perineural colchicine on nerve injury-induced spinal glial activation and neuropathic pain behavior

Factors transported centrally from the site of a peripheral nerve injury are known to provide cellular activation signals to the dorsal root ganglion and spinal cord. Yamamoto and Yaksh [35] were able to use colchicine disruption of axonal transport to abolish thermal hyperalgesia after sciatic chronic constriction in the rat. The current study set out to ascertain whether this observation could be reproduced by applying the same pharmacologic paradigm to a complete, segmentally specific, spinal nerve tight ligation (SPTL) and assessing the impact of this treatment on mechanical allodynia and central, spinal glial activation. Mechanical allodynia of the ipsilateral (lesion side) hind paw was measured at 1, 3, 5, 7, 10, and 14 days following SPTL. Spinal astrocytic and microglial activation were assessed immunohistochemically at 5 and 14 days. Colchicine was unable to prevent mechanical allodynia or spinal glial activation when applied perineurally just proximal to the site of SPTL. Administered alone, colchicine (without SPTL) induced both astrocytic and microglial activation, but not mechanical allodynia. Colchicine applied distal to the site of SPTL did not alter mechanical allodynia or glial responses to SPTL. Neuronal tracing experiments were performed to verify segmental disruption of axonal transport by either SPTL or colchicine treatment. Neuronal tracer injected into the sciatic nerve could not be found at the L5 spinal level following perineural colchicine treatment or tight ligation of the L5 spinal nerve, however, tracer was present at the unobstructed L4 spinal level. These results suggest that central astrocytic and microglial responses may be triggered by disruption of transported signals from the periphery, because they are induced by either colchicine or tight ligation. Conversely, axonally transported factors, either from the site of nerve injury or from the periphery, do not appear to be critical for the development of mechanical allodynia.

Using a direct current electrical field to promote spinal-cord regeneration

The authors used a direct current electrical field to promote spinal-cord regeneration in a canine model. Thirty-two dogs were randomly divided into four groups. Complete spinal-cord injury was induced, and electrical stimulators were then placed in the animals. Group 1 served as controls; Groups 2 to 4 were experimental groups, with varying stimulator voltages: 0V in Group 1, 12V in Groups 2 and 4, and 6V in Group 3, with the stimulator implanted 6 hr after spinal-cord injury in Group 4. Functional, electrophysiologic and morphometric assessments were carried out 1 to 3 months postoperatively. Results showed that spinal-cord function, cortical somatosensory evoked potentials, number of neurons, sectional area of neurons, and Nissl body density in the experimental groups were much better than those in the control group. In addition, all the indices in Group 2 were better than those in Groups 3 and 4. This indicated that direct current electrical stimulation could effectively promote spinal-cord regeneration and functional recovery in this model. The 12V voltage was safe for the animals. The stimulator was not rejected by the host for a relatively long period of time.

Spinal systems and pain processing: development of novel analgesic drugs with mechanistically defined models

Much research has been undertaken in the field of pain in an attempt to find an effective treatment. Insights into the underlying mechanisms of pain have been gained from studies using preclinical animal models (acute stimuli, post-tissue injury and peripheral nerve injury) and evaluating their similarity with the human condition. In this article, these pain models are summarized and the mechanisms of pain discussed in relation to spinal processing. In the context of this research the therapeutic potential of novel analgesics is highlighted as the future looks forward to the many possibilities that the targeted spinal delivery of drugs can bring.

Characterization of antiallodynic actions of ALE-0540, a novel nerve growth factor receptor antagonist, in the rat

There is growing evidence that nerve growth factor (NGF) may function as a mediator of persistent pain states. We have identified a novel nonpeptidic molecule, ALE-0540, that inhibits the binding of NGF to tyrosine kinase (Trk) A or both p75 and TrkA (IC50 5.88 +/- 1. 87 microM, 3.72 +/- 1.3 microM, respectively), as well as signal transduction and biological responses mediated by TrkA receptors. ALE-0540 was tested in models of neuropathic pain and thermally-induced inflammatory pain, using two routes of administration, a systemic i.p. and a spinal intrathecal (i.th.) route. Morphine was also tested for comparison in the antiallodynia model using mechanical stimuli. We show that either i.p. or i.th. administration of ALE-0540 in rats produced antiallodynia in the L5/L6 ligation model of neuropathic pain. The calculated A50 values (and 95% confidence intervals) for ALE-0540 administered i.p. and i. th. were 38 (17.5-83) mg/kg and 34.6 (17.3-69.4) microgram, respectively. ALE-0540 given i.th., at doses of 30 and 60 microgram, also blocked tactile allodynia in the thermal sensitization model. Although morphine displayed greater potency [A50 value of 7.1 (5.6-8. 8) mg/kg] than ALE-0540 in anti-allodynic effect when given i.p. to L5/L6-ligated rats, it was not active when administered i.th. These data suggest that a blockade of NGF bioactivity using a NGF receptor antagonist is capable of blocking neuropathic and inflammatory pain and further support the hypothesis that NGF is involved in signaling pathways associated with these pain states. ALE-0540 represents a nonpeptidic small molecule which can be used to examine mechanisms leading to the development of agents for the treatment of pain.

Desert toads discriminate salt taste with chemosensory function of the ventral skin

Toads obtain water by absorption across their skin. When dehydrated, desert toads exhibit stereotyped hydration behavior in which they press their ventral skin onto a moist surface. However, dehydrated toads avoid surfaces moistened with hyperosmotic NaCl and KCl solutions (Hoff KvS, Hillyard SD. 1993. J. Exp. Biol. 183:347-351). We have studied neural mechanisms for this avoidance with physiologic, behavioral, and morphologic approaches. Spinal nerves innervating the ventral skin could be stimulated by exposure to a hyperosmotic NaCl solution applied to the outer surface of the skin. This neural response occurred with much longer latency than to mechanical stimulation and could be reduced by amiloride, a blocker for Na+ channels known to be responsible for epithelial ion transport and salt taste transduction. In behavioral experiments, avoidance of a NaCl solution was also reduced by adding amiloride to the solution, suggesting involvement of amiloride-sensitive Na+ channels for detecting the hyperosmotic salt solution. Neural tracing with fluorescent dye revealed spinal nerve endings and connections to putative receptor cells, both located in the deeper layer of the epidermis. Either of these or both may be associated with the transduction of Na+ flowing into the skin. The ability of toads to detect hyperosmotic salt solutions in their environment reveals a previously unknown chemosensory function for spinal nerves in anuran amphibians.

Changes in the action potential in sensory neurones after peripheral axotomy in vivo

Following nerve injury, modified somatic ion channels may underlie ectopic activity in axotomized A-type neurones in dorsal root ganglia (DRGs) leading to abnormal pain signalling. Using intracellular microelectrodes both in vivo and in vitro, action potentials (APs) were recorded in rat DRG neurones classified by axonal conduction velocity. After lesions to L5 spinal or sciatic nerves, APs in both A alpha/beta and A delta cells were wider, and those in A alpha/beta neurones more frequently showed inflections during repolarization, than APs in cells in undamaged ganglia. AP amplitudes and dV/dt(max) were not significantly altered by axotomy. These results confirm previous observations in intact ganglia in vitro but differ from those reported for dissociated neurones using patch recording techniques.

Uninjured C-fiber nociceptors develop spontaneous activity and alpha-adrenergic sensitivity following L6 spinal nerve ligation in monkey

We investigated whether uninjured cutaneous C-fiber nociceptors in primates develop abnormal responses after partial denervation of the skin. Partial denervation was induced by tightly ligating spinal nerve L6 that innervates the dorsum of the foot. Using an in vitro skin-nerve preparation, we recorded from uninjured single afferent nerve fibers in the superficial peroneal nerve. Recordings were made from 32 C-fiber nociceptors 2-3 wk after ligation and from 29 C-fiber nociceptors in control animals. Phenylephrine, a selective alpha1-adrenergic agonist, and UK14304 (UK), a selective alpha2-adrenergic agonist, were applied to the receptive field for 5 min in increasing concentrations from 0.1 to 100 microM. Nociceptors from in vitro control experiments were not significantly different from nociceptors recorded by us previously in in vivo experiments. In comparison to in vitro control animals, the afferents found in lesioned animals had 1) a significantly higher incidence of spontaneous activity, 2) a significantly higher incidence of response to phenylephrine, and 3) a higher incidence of response to UK. In lesioned animals, the peak response to phenylephrine was significantly greater than to UK, and the mechanical threshold of phenylephrine-sensitive afferents was significantly lower than for phenylephrine-insensitive afferents. Staining with protein gene product 9.5 revealed an approximately 55% reduction in the number of unmyelinated terminals in the epidermis of the lesioned limb compared with the contralateral limb. Thus uninjured cutaneous C-fiber nociceptors that innervate skin partially denervated by ligation of a spinal nerve acquire two abnormal properties: spontaneous activity and alpha-adrenergic sensitivity. These abnormalities in nociceptor function may contribute to neuropathic pain.

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.

Oxidant treatment causes a dose-dependent phenotype of apoptosis in cultured motoneurons

Evidence is growing that reactive oxygen species (ROS), by-products of (normal) cellular aerobic metabolism, are involved in the pathogenesis of neurodegenerative diseases. One of these diseases is amyotrophic lateral sclerosis (ALS), in which motoneurons die, leading to paralysis and death. It remains uncertain whether ROS are the cause of (apoptotic) motoneuron death in ALS. To further understand the role of ROS in motoneuron death, we investigated the effects of ROS on isolated spinal rat motoneurons in culture. ROS were generated with a combination of iron(III) and ascorbate, or with hydrogen peroxide. Both toxic treatments resulted in a dose-dependent motoneuron death. Iron(III)/ascorbate toxicity was completely prevented with the hydrogen peroxide detoxifying enzyme catalase and partially prevented with the antioxidant vitamin E. SOD1, the enzyme that removes superoxide, did not protect against iron(III)/ascorbate toxicity. ROS treatment caused apoptotic motoneuron death: low doses of iron(III)/ ascorbate or hydrogen peroxide resulted in complete apoptosis ending in nuclear fragmentation, while high doses of ROS resulted in incomplete apoptosis (nuclear condensation). Thus, depending on the dose of ROS, the motoneurons complete the apoptotic pathway (low dose) or are stopped somewhere during this route (high dose).

Suppression of diaphragmatic activity during spontaneous ponto-geniculo-occipital waves in cat

It has been reported that spontaneous ponto-geniculo-occipital (PGO) waves, which occur during REM sleep in the cat, are associated with a brief inhibition of diaphragmatic activity (Orem, 1980). This report was preliminary and not supported by a detailed analysis. We report here analysis of the relationship between PGO waves and diaphragmatic activity based on 3073 PGO waves recorded simultaneously with diaphragmatic activity. The results show that there is indeed an inhibition of diaphragmatic activity during PGO waves. This inhibition has an amplitude up to 20% of background, and a duration (approximately 80 ms) approximately coinciding with the temporal duration of the PGO wave. In addition, we analyzed the relationships among the activity of medullary respiratory neurons, PGO waves, and diaphragmatic activity. Two neurons were observed whose relationships to diaphragmatic activity and PGO waves were consistent with the idea that they mediated the PGO-associated inhibition of diaphragmatic activity. However, the number of PGO waves involved in the analysis of the interaction between medullary respiratory neuronal activity and diaphragmatic activity was small and, although suggestive, was not conclusive.

Neurophysiological and anatomical variability of the greater auricular nerve

OBJECTIVE

To study the normal variation in greater auricular nerve (GAN) conduction and anatomical course.

MATERIALS AND METHODS

GAN nerve conduction was studied in 77 healthy adults while 17 GANs were dissected in 10 cadavers.

RESULTS

The largest sensory nerve action potentials (SNAP) were recorded when the stimulator was placed 6 or 7 cm from the external acoustic meatus (EAM). Amplitude decreased significantly with age (P = 0.001). The dissected GANs entered the subcutaneous layer between 55 and 105 mm (median 68 mm) from EAM. The postauricular branch was found in 13 of 17 cases.

CONCLUSION

A large variation was observed both anatomically and by way of electrical stimulation. SNAP variation does probably reflect the anatomical variation in GAN course, branching, and terminal distribution. The best sites for stimulation were found 6 and 7 cm from EAM at the posterior sternocleiodomastoid border. The maximal amplitude, right:left amplitude ratio, and distal latency may be useful variables in GAN conduction studies.

Reflex responses of bladder motility after stimulation of interspinous tissues in the anesthetized rat

OBJECTIVES

To determine the effects of somatic stimulation, including noxious chemical stimulation of interspinous tissues, on bladder motility in the anesthetized rat.

METHODS

Changes in pressure in the previously quiescent bladder were measured in anesthetized adult female Wistar rats after various forms of noxious and innocuous somatic stimulation, including injection of the thoracic and lumbar interspinous tissues with capsaicin. Measurements were taken in both central nervous system-intact and spinalized animals, as well as in animals in whom the pelvic nerves had been transected bilaterally. Changes in bladder pressure were also measured in response to electrical stimulation of the primary dorsal ramus of lumbar spinal nerves.

RESULTS

Noxious and innocuous stimulation of the fore- and hindpaws and the skin overlying the sacrum generally failed to elicit discernible changes in bladder pressure. However, capsaicin injection of thoracic and lumbar interspinous tissues produced profound and long-lasting increases in bladder pressure. There were no significant differences in the responses to thoracic, as opposed to lumbar, stimulation. Spinalization above the level of stimulation abolished the response to capsaicin injection, as did bilateral transection of the pelvic nerves.

CONCLUSION

In general, pressure in the quiescent bladder was relatively insensitive to somatic stimulation. However, noxious chemical stimulation of the interspinous tissues produced a nonsegmentally organized, supraspinal, parasympathetically mediated reflex increase in bladder tone.

Differential arterial baroreflex regulation of renal, lumbar, and adrenal sympathetic nerve activity in the rat

Lumbar (LSNA), renal (RSNA), or adrenal sympathetic nerve activity (ASNA) is most commonly used as an index of sympathetic nerve activity in investigations of arterial baroreflex control in the rat. Although differential regulation of sympathetic outputs to different organs has been extensively studied, no direct and simultaneous comparisons of the full range of baroreflex reactivity have been described for these sympathetic outputs. Therefore, we compared steady-state sigmoidal baroreflex stimulus-response curves (via phenylephrine-nitroprusside infusion) for RSNA recorded simultaneously with LSNA or ASNA in urethan-chloralose-anesthetized male Sprague-Dawley rats. Characteristics of the baroreflex curves differed significantly between all three sympathetic outputs. ASNA exhibited the greatest range of baroreflex regulation, the highest upper level of activity, and the widest distribution of the gain over a broad range of mean arterial pressure (MAP). RSNA exhibited greater gain than LSNA. LSNA showed the smallest range and maximal inhibition in comparison to other sympathetic outputs. However, all three nerves responded similarly to baroreflex stimulation and unloading in the range in MAP close to the operating point. We conclude that baroreflex regulation of sympathetic activity shows wide regional variability in gain, range, and maximal inhibition. Therefore, the entire stimulus-response relationship should be considered in comparing regional sympathetic responses.

Neurogenesis of patterns of automatic ventilatory activity

Normal respiration, termed eupnea, is characterized by periodic filling and emptying of the lungs. Eupnea can occur 'automatically' without conscious effort. Such automatic ventilation is controlled by the brainstem respiratory centers of pons and medulla. Following removal of the pons, eupnea is replaced by gasping, marked by brief but maximal inspiratory efforts. The mechanisms by which the respiratory rhythms are generated have been examined intensively. Evidence is discussed that ventilatory activity can be generated in multiple regions of pons and medulla. Eupnea and gasping represent fundamentally different ventilatory patterns. Only for gasping has a critical region for neurogenesis been identified, in the rostral medulla. Gasping may be generated by the discharge of 'pacemaker' neurons. In eupnea, this pacemaker activity is suppressed and incorporated into the pontile and medullary neuronal circuit responsible for the neurogenesis of eupnea. Evidence for ventilatory neurogenesis which has been obtained from a number of in vitro preparations is discussed. A much-used preparation is that of a 'superfused' brainstem of the neonatal rat. However, activities of this preparation differ greatly from those of eupnea, as recorded in vitro or in arterially perfused in vitro preparations. Activities of this 'superfused' preparation are identical with gasping and, hence, results must be reinterpreted accordingly. The possibility is present that mechanisms responsible for generating respiratory rhythms may differ from those responsible for shaping respiratory-modulated discharge patterns of cranial and spinal nerves. The importance of pontile mechanisms in the neurogenesis and control of eupnea is reemphasized.

Responses of axotomized afferents to blockade of nitric oxide synthesis after spinal nerve lesion in the rat

Lesioned afferents were tested for their responses to blockade of nitric oxide synthesis in the spinal nerve L5 lesion model for neuropathic pain in Wistar rats. Seven single fibers with spontaneous activity split from dorsal root L5 showed no response after non-selective blockade of nitric oxide synthesis with N(G)-nitro-L-arginine methyl ester whereas five were excited after 5-7 min. Three previously silent units were recruited. Blood flow in the dorsal root ganglion decreased. None of fifteen axotomized afferents tested responded to selective blockade of neuronal nitric oxide synthesis with 7-nitroindazole. It is concluded that neuronal nitric oxide is not involved in the generation of spontaneous activity in axotomized afferent neurons in this model. We suggest that the vasoconstriction induced by blockade of endothelial nitric oxide may be responsible for the excitatory responses.

A novel spinal action of mexiletine in spinal somatosensory transmission of nerve injured rats

Mexiletine is widely used for the treatment of neuropathic pain although its site(s) of action remain unclear. Here we have studied the effect of spinal administration of mexiletine (10-1000 microg) on the spontaneous and peripherally evoked responses of spinal neurones of nerve injured (selective ligation of spinal nerves L5-L6; SNL) rats. Sham controls for the surgical intervention were performed. A high proportion of the spinal neurones of SNL rats exhibited de novo spontaneous activity (mean frequency of firing 4+/-1 Hz), this activity was highly sensitive to spinal mexiletine (F5,55 = 2.5, P < or = 0.05). The spinal neurones of the sham operated rats exhibited negligible spontaneous activity. The electrically evoked Abeta-fibre neuronal responses of SNL and sham operated rats were not significantly influenced by spinal mexiletine. In contrast, the Adelta-fibre and C-fibre evoked neuronal responses of the SNL rats, but not sham operated rats, were significantly reduced by spinal mexiletine (F5.52 = 4.9, P < or = 0.001 and F5,48 = 12, P < or = 0.0001, respectively). In addition, the mechanical punctate von Frey 9 and 50 g evoked neuronal responses of the SNL rats, but not sham operated rats, were significantly reduced by spinal mexiletine (F5,57 = 4.3, P < or = 0.002 and F5,52 = 6.1, P < or = 0.001). This pharmacological study suggests that following nerve injury there is a novel mexiletine sensitive spinal substrate which contributes to Adelta-fibre and C-fibre, but not Abeta-fibre, somatosensory transmission. This central action may underlie some of the clinical efficacy of mexiletine in the treatment of neuropathic pain states.