Incidence Of Spontaneous Activity Research Articles

Analgesic dorsal root ganglion field stimulation blocks both afferent and efferent spontaneous activity in sensory neurons of rats with monosodium iodoacetate-induced osteoarthritis

Chronic joint pain is common in patients with osteoarthritis (OA). Non-steroidal anti-inflammatory drugs and opioids are used to relieve OA pain, but they are often inadequately effective. Dorsal root ganglion field stimulation (GFS) is a clinically used neuromodulation approach, although it is not commonly employed for patients with OA pain. GFS showed analgesic effectiveness in our previous study using the monosodium iodoacetate (MIA) - induced OA rat pain model. This study was to evaluate the mechanism of GFS analgesia in this model. After osteoarthritis was induced by intra-articular injection of MIA, pain behavioral tests were performed. Effects of GFS on the spontaneous activity (SA) were tested with in vivo single-unit recordings from teased fiber saphenous nerve, sural nerve, and dorsal root. Two weeks after intra-articular MIA injection, rats developed pain-like behaviors. In vivo single unit recordings from bundles teased from the saphenous nerve and third lumbar (L3) dorsal root of MIA-OA rats showed a higher incidence of SA than those from saline-injected control rats. GFS at the L3 level blocked L3 dorsal root SA. MIA-OA reduced the punctate mechanical force threshold for inducing AP firing in bundles teased from the L4 dorsal root, which reversed to normal with GFS. After MIA-OA, there was increased retrograde SA (dorsal root reflex), which can be blocked by GFS. These results indicate that GFS produces analgesia in MIA-OA rats at least in part by producing blockade of afferent inputs, possibly also by blocking efferent activity from the dorsal horn.

Comprehensive phenotyping of cutaneous afferents reveals early-onset alterations in nociceptor response properties, release of CGRP, and hindpaw edema following spinal cord injury

Spinal cord injury (SCI) is a complex syndrome that has profound effects on patient well-being, including the development of medically-resistant chronic pain. The mechanisms underlying SCI pain have been the subject of thorough investigation but remain poorly understood. While the majority of the research has focused on changes occurring within and surrounding the site of injury in the spinal cord, there is now a consensus that alterations within the peripheral nervous system, namely sensitization of nociceptors, contribute to the development and maintenance of chronic SCI pain. Using an ex vivo skin/nerve/DRG/spinal cord preparation to characterize afferent response properties following SCI, we found that SCI increased mechanical and thermal responding, as well as the incidence of spontaneous activity (SA) and afterdischarge (AD), in below-level C-fiber nociceptors 24 hr following injury relative to naïve controls. Interestingly, the distribution of nociceptors that exhibit SA and AD are not identical, and the development of SA was observed more frequently in nociceptors with low heat thresholds, while AD was found more frequently in nociceptors with high heat thresholds. We also found that SCI resulted in hindpaw edema and elevated cutaneous calcitonin gene-related peptide (CGRP) concentration that were not observed in naïve mice. These results suggest that SCI causes a rapidly developing nociceptor sensitization and peripheral inflammation that may contribute to the early emergence and persistence of chronic SCI pain.

Quantitative electromyography: Normative data in paraspinal muscles.

Quantitative electromyography of paraspinal muscle is a valuable diagnostic tool, but normative data are lacking. Needle electromyography (EMG) was obtained in 65 healthy subjects (49% men, 51% women) aged 21 to 82 years at C7, Th10, and L5 segments bilaterally. The incidence of spontaneous activity; motor unit potential (MUP) amplitudes, durations, and the incidence of polyphasic potentials; and the recruitment pattern at maximal voluntary contraction (MVC) were evaluated. The incidence of fibrillation potentials was similar to limb muscles. The mean MUP duration and amplitude, and the amplitude at MVC increased caudally, while the incidence of polyphasic potentials was similar at all levels. EMG parameters did not correlate with sex or age. In contrast to limb muscles, EMG parameters did not change with age, while polyphasic potentials were more frequent in paraspinal muscle than in limb muscles. The EMG gradient suggests larger motor units at more caudal segments.

Cutaneous Aβ-Non-nociceptive, but Not C-Nociceptive, Dorsal Root Ganglion Neurons Exhibit Spontaneous Activity in the Streptozotocin Rat Model of Painful Diabetic Neuropathy in vivo.

Diabetic peripheral neuropathic pain (DPNP) is the most devastating complication of diabetes mellitus. Unfortunately, successful therapy for DPNP remains a challenge because its pathogenesis is still elusive. However, DPNP is believed to be due partly to abnormal hyperexcitability of dorsal root ganglion (DRG) neurons, but the relative contributions of specific functional subtypes remain largely unknown. Here, using the strepotozotocin (STZ) rat model of DPNP induced by a STZ injection (60 mg/kg, i.p), and intracellular recordings of action potentials (APs) from DRG neurons in anesthetized rats, we examined electrophysiological changes in C-and Aβ-nociceptive and Aβ-low threshold mechanoreceptive (LTM) neurons that may contribute to DPNP. Compared with control, we found in STZ-rats with established pain hypersensitivity (5 weeks post-STZ) several significant changes including: (a) A 23% increase in the incidence of spontaneous activity (SA) in Aβ-LTMs (but not C-mechanosensitive nociceptors) that may cause dysesthesias/paresthesia suffered by DPNP patients, (b) membrane hyperpolarization and a ∼85% reduction in SA rate in Aβ-LTMs by Kv7 channel activation with retigabine (6 mg/kg, i.v.) suggesting that Kv7/M channels may be involved in mechanisms of SA generation in Aβ-LTMs, (c) decreases in AP duration and in duration and amplitude of afterhyperpolarization (AHP) in C-and/or Aβ-nociceptors. These faster AP and AHP kinetics may lead to repetitive firing and an increase in afferent input to the CNS and thereby contribute to DPNP development, and (d) a decrease in the electrical thresholds of Aβ-nociceptors that may contribute to their sensitization, and thus to the resulting hypersensitivity associated with DPNP.

Interleukin-10 resolves pain hypersensitivity induced by cisplatin by reversing sensory neuron hyperexcitability.

Understanding the mechanisms that drive transition from acute to chronic pain is essential to identify new therapeutic targets. The importance of endogenous resolution pathways acting as a "brake" to prevent development of chronic pain has been largely ignored. We examined the role of interleukin-10 (IL-10) in resolution of neuropathic pain induced by cisplatin. In search of an underlying mechanism, we studied the effect of cisplatin and IL-10 on spontaneous activity (SA) in dorsal root ganglia neurons. Cisplatin (2 mg/kg daily for 3 days) induced mechanical hypersensitivity that resolved within 3 weeks. In both sexes, resolution of mechanical hypersensitivity was delayed in Il10 mice, in WT mice treated intrathecally with neutralizing anti-IL-10 antibody, and in mice with cell-targeted deletion of IL-10R1 on advillin-positive sensory neurons. Electrophysiologically, small- to medium-sized dorsal root ganglia neurons from cisplatin-treated mice displayed an increase in the incidence of SA. Cisplatin treatment also depolarized the resting membrane potential, and decreased action potential voltage threshold and rheobase, while increasing ongoing activity at -45 mV and the amplitude of depolarizing spontaneous fluctuations. In vitro addition of IL-10 (10 ng/mL) reversed the effect of cisplatin on SA and on the depolarizing spontaneous fluctuation amplitudes, but unexpectedly had little effect on the other electrophysiological parameters affected by cisplatin. Collectively, our findings challenge the prevailing concept that IL-10 resolves pain solely by dampening neuroinflammation and demonstrate in a model of chemotherapy-induced neuropathic pain that endogenous IL-10 prevents transition to chronic pain by binding to IL-10 receptors on sensory neurons to regulate their activity.

Muscle velocity recovery cycles in neurogenic muscles

ObjectiveTo examine muscle membrane properties in neurogenic muscles using Muscle Velocity Recovery Cycles (MVRCs). MethodsForty-seven patients referred to Nerve Conduction Studies (NCS) and Electromyography (EMG) for peroneal nerve entrapment neuropathy were prospectively included. The patients were categorized as peroneal nerve entrapment neuropathy across knee (n = 22), L5-radiculapathy (n = 10), normal NCS/EMG (n = 9) and other disorders (n = 6) using NCS/EMG and neuroimaging results. Strength in anterior tibial muscle was measured by Medical Council Scale (MRC) and disease duration was recorded. In addition to conventional NCS/EMG, all subjects were examined with MVRCs in anterior tibial muscle. This provided parameters of muscle relative refractory period (MRRP) and early supernormality (ESN) and late supernormality (LSN). The results were compared with 29 age-matched healthy control subjects. ResultsMRRP was prolonged and ESN and LSN were reduced in neurogenic muscles. MRRP, ESN and LSN correlated to MRC and incidence of spontaneous activity but not to motor unit potential parameters or disease duration. ConclusionsMVRC changes provide in vivo evidence of depolarization in intact human muscle fibres that could underlie reduced muscle excitability and hence weakness in neurogenic muscles. SignificanceMVRCs appear to be a useful technique for revealing disease mechanism in a broad range of neuromuscular diseases.

P51-F Quantitative electromyography: Normative data in paraspinal muscles

Objectives Paraspinal muscle may be affected in neuromuscular disorders, and EMG may be crucial in the diagnosis. The aim of the study was to develop normative data at different spinal segments. Methods Sixty-five healthy subjects (49% men, 51% women) aged 21–82 years were studied. In each subject, 16–92 motor unit potentials (MUPs) were recorded bilaterally at C7, Th10 and L5 segments using concentric needle electrodes. At each segment, average MUP amplitudes, durations, incidence of polyphasic potentials, and the amplitude at maximal voluntary contraction (MVC) were measured. The incidence of spontaneous activity was examined. Results There was no relationship between the EMG parameters and age. The mean MUP duration and amplitude, and the amplitude at MVC increased caudally (ANOVA, P Conclusions In contrast to limb-muscles EMG parameters were unrelated to the age of the subject. The gradient of EMG parameters caudally from C7 suggesting larger motor units at more caudal segments. The proportion of polyphasic potentials may be due to the organization of muscle fibers. The incidence of fibrillation activity was similar to limb muscles. Significance The study describes normative data for EMG of paraspinal muscles according to the examined spinal segment. These data may be useful in quantitative EMG examination of paraspinal muscle in neuromuscular disease.

Platform Session – Electromyography: Two novel electrophysiological methods: In the evaluation of ALS patients – MScan MUNE and MVRC

Introduction Amyotrophic Lateral Sclerosis (ALS) is characterized by progressive loss of motor units, but electromyography (EMG) with quantitative motor unit potential (MUP) analysis does not directly quantify the denervation process. A more extensive clinical use of Motor Unit Estimation Methods: (MUNE), like MScan MUNE, could hence be of potentially great value. Muscle Velocity Recovery Cycle (MVRC) recordings can provide in vivo information about changes in muscle fiber membrane properties, and may enlighten the effect of the denervation process on the muscle fibers. The aim of this study was to examine the clinical utility of MScan MUNE and MVRC in ALS patients; which also contribute to a better understanding of disease pathophysiology. Methods 27 ALS patients and 23 age and sex-matched healthy control subjects were included. All participants were evaluated electrophysiologically by a standard quantitative EMG and by the two novel methods: MScan MUNE and MVRC, all in the anterior tibial muscle (TA). MScan MUNE, MVRC and EMG parameters were compared between ALS patients and healthy controls using parametric unpaired t-tests. ROC curves were used to test how these methods distinguish patients from healthy controls. MVRC parameters were correlated to EMG results and MScan parameters using linear regression analysis. Results The MScan MUNE value for ALS patients (55 (95% CI: 42–68)) was significantly lower than for the healthy controls (108 (95% CI: 98–117)) (p 5 sites) and controls. And there was no significant correlation between MVRC and MUP parameters or incidence of spontaneous activity. Conclusion The novel MScan MUNE method is a sensitive quantitative measure of loss of motor units in ALS patients in TA. The finding of no difference in MVRC for ALS patients compared to healthy controls indicate, that the muscle fiber membranes are not prominently affected by the denervation process. This may be explained by the compensatory process of collateral reinnervation, which initially is strong and ensures that the vast majority of muscle fibers are innervated and behave normally.

O99 Muscle velocity recovery cycles in neurogenic muscles

Muscle Velocity Recovery Cycles (MVRC) enable assessment of muscle membrane properties. MVRC has been studied in some myopathies, however, the method has never been tested in neurogenic muscles. We aimed in this study to examine whether MVRC differ in neurogenic and normal muscles and how MVRC variables correlate to the incidence of spontaneous activity (SA) in electromyography (EMG). Twenty-five patients with confirmed peroneal neuropathy (twenty) or L5 root lesion (five) were prospectively included. In addition to EMG, MVRC was done in anterior tibial muscle using Qtrac-software. MVRC parameters comprised muscle relative refractory period (MRRP) and early, late and residual supernormality after 1, 2 or 5 conditioning stimuli. Results were compared to 16 age- and sex-matched healthy subjects (HS). There was significant correlation between the incidence of SA and MRRP (r = 0.54) and early and late supernormality most pronounced in early supernormality after 5 conditioning stimulation (r = 0.64) (p < 0.001). The same MVRC parameters differed between patients and HS (t-test, p < 0.05). In contrast, residual supernormality did not differ between groups and there was no correlation to SA. Significantly prolonged MRRP and pronounced slowing in propagation velocity in early supernormal period suggest lower depolarization threshold in the denervated fibers. This is supported by significant correlation to incidence of SA in EMG. MVRC may be a potential diagnostic tool for electrodiagnosis of neuromuscular disorders and may provide information in understanding muscle pathophysiology. Future studies are required with larger patient groups with comparison of different nerve and muscle disorders.

TRPV1 channels make major contributions to behavioral hypersensitivity and spontaneous activity in nociceptors after spinal cord injury

Chronic neuropathic pain is often a severe and inadequately treated consequence of spinal cord injury (SCI). Recent findings suggest that SCI pain is promoted by spontaneous activity (SA) generated chronically in cell bodies of primary nociceptors in dorsal root ganglia (DRG). Many nociceptors express transient receptor potential V1 (TRPV1) channels, and in a preceding study most dissociated DRG neurons exhibiting SA were excited by the TRPV1 activator, capsaicin. The present study investigated roles of TRPV1 channels in behavioral hypersensitivity and nociceptor SA after SCI. Contusive SCI at thoracic segment T10 increased expression of TRPV1 protein in lumbar DRG 1month after injury and enhanced capsaicin-evoked ion currents and Ca2+ responses in dissociated small DRG neurons. A major role for TRPV1 channels in pain-related behavior was indicated by the ability of a specific TRPV1 antagonist, AMG9810, to reverse SCI-induced hypersensitivity of hind limb withdrawal responses to mechanical and thermal stimuli at a dose that did not block detection of noxious heat. Similar reversal of behavioral hypersensitivity was induced by intrathecal oligodeoxynucleotides antisense to TRPV1, which knocked down TRPV1 protein and reduced capsaicin-evoked currents. TRPV1 knockdown also decreased the incidence of SA in dissociated nociceptors after SCI. Prolonged application of very low concentrations of capsaicin produced nondesensitizing firing similar to SA, and this effect was enhanced by prior SCI. These results show that TRPV1 makes important contributions to pain-related hypersensitivity long after SCI, and suggest a role for TRPV1-dependent enhancement of nociceptor SA that offers a promising target for treating chronic pain after SCI.

Chronic spontaneous activity generated in the somata of primary nociceptors is associated with pain-related behavior after spinal cord injury.

Mechanisms underlying chronic pain that develops after spinal cord injury (SCI) are incompletely understood. Most research on SCI pain mechanisms has focused on neuronal alterations within pain pathways at spinal and supraspinal levels associated with inflammation and glial activation. These events might also impact central processes of primary sensory neurons, triggering in nociceptors a hyperexcitable state and spontaneous activity (SA) that drive behavioral hypersensitivity and pain. SCI can sensitize peripheral fibers of nociceptors and promote peripheral SA, but whether these effects are driven by extrinsic alterations in surrounding tissue or are intrinsic to the nociceptor, and whether similar SA occurs in nociceptors in vivo are unknown. We show that small DRG neurons from rats (Rattus norvegicus) receiving thoracic spinal injury 3 d to 8 months earlier and recorded 1 d after dissociation exhibit an elevated incidence of SA coupled with soma hyperexcitability compared with untreated and sham-treated groups. SA incidence was greatest in lumbar DRG neurons (57%) and least in cervical neurons (28%), and failed to decline over 8 months. Many sampled SA neurons were capsaicin sensitive and/or bound the nociceptive marker, isolectin B4. This intrinsic SA state was correlated with increased behavioral responsiveness to mechanical and thermal stimulation of sites below and above the injury level. Recordings from C- and Aδ-fibers revealed SCI-induced SA generated in or near the somata of the neurons in vivo. SCI promotes the entry of primary nociceptors into a chronic hyperexcitable-SA state that may provide a useful therapeutic target in some forms of persistent pain.

Systemic Antiinflammatory Corticosteroid Reduces Mechanical Pain Behavior, Sympathetic Sprouting, and Elevation of Proinflammatory Cytokines in a Rat Model of Neuropathic Pain

Chronic pain models are commonly defined as either nerve-injury or inflammation models, but recent work suggests inflammatory processes are important in nerve injury-induced pain. In the rat spinal nerve ligation model, the authors examined effects of systemic corticosteroid triamcinolone acetonide (TA) on the cytokine protein profile and sympathetic sprouting in the axotomized sensory ganglia, excitability of sensory neurons, and mechanical sensitivity. By postoperative day 3, marked increases (5- to 16-fold) in monocyte chemoattractant protein-1, growth-related oncogene (GRO/KC or CXCL1), and interleukin (IL)-6 were observed, whereas IL-4 and IL-2 levels fell more than fourfold. The increased cytokines and number of sympathetic basket formations in the sensory ganglia were reduced toward normal values by TA given starting at the time of injury. Interleukin-4 and IL-2 levels were not restored by TA. Systemic TA also reduced the firing rate and incidence of bursting activity, but not the overall incidence of spontaneous activity, in large- and medium-sized neurons. Mechanical hypersensitivity on postoperative day 3 was reduced by TA, and some effect could still be observed 4 days after cessation of TA. However, starting TA at day 7 was ineffective. Several components of the spinal nerve injury model are responsive to corticosteroid, suggesting inflammatory processes are important in the development of neuropathic pain. The observation that TA was effective when given starting at the time of injury suggests that steroid treatment might alter the development of chronic pain after surgical procedures that involve nerve injury, such as amputation or hernia repair.

P22.16 Incidence of spontaneous activity in the fourth dorsal interosseus pedis muscle

P22.16 Incidence of spontaneous activity in the fourth dorsal interosseus pedis muscle

Enhanced excitability of dissociated primary sensory neurons after chronic compression of the dorsal root ganglion in the rat

A chronic compression of the dorsal root ganglion (CCD) produces ipsilateral cutaneous hyperalgesia and allodynia in rats. Intracellular electrophysiological recordings from formerly compressed neurons in the intact dorsal root ganglion (DRG) reveal lower than normal current thresholds (CTs) and abnormal spontaneous activity (SA) (Zhang JM, Song XJ, LaMotte RH. Enhanced excitability of sensory neurons in rats with cutaneous hyperalgesia produced by chronic compression of the dorsal root ganglion. J Neurophysiol 1999;82:3359–66). To determine if the neuronal hyperexcitability is intrinsic to the soma, L4 and L5 DRG neurons from rats that had prior CCD surgery or those that did not (controls) were dissociated, and intracellular recordings obtained 3–8 h (acute) or 24–30 h (1d) after culture. The CTs of large- (>45 μm diameter) and medium- (30∼45 μm) sized neurons from control rats after acute or 1d culture were similar to those formerly recorded from the intact DRG and significantly lower for CCD than for control rats. However, the CTs of small- (≤30 μm) sized neurons from control rats were significantly lower in acute or 1d culture groups than they were in the intact DRG and not significantly different from those of dissociated small neurons from CCD rats. The overall incidence of SA was higher for CCD than for control neurons after 1d culture (10.3 vs. 1.8%) and similar to that obtained in the intact DRG. We conclude that the CCD-induced hyperexcitability of medium- and large-sized neurons remains after dissociation and is intrinsic to the soma. For small-sized neurons, the effects of CCD observed in the intact DRG are less apparent after dissociation possibly due to the hyperexcitability produced by the dissociation process itself.

Estrogen and Inflammation Increase the Excitability of Rat Temporomandibular Joint Afferent Neurons

Several painful conditions, including temporomandibular disorders (TMD), are more prevalent and more severe in women than in men. Although the physiological basis for this sex difference remains to be determined, it is likely that estrogen is an underlying factor. The present study was performed to test the hypotheses that estrogen increases the excitability of rat temporomandibular joint (TMJ) afferents and exacerbates the inflammation-induced sensitization of these sensory neurons. Retrogradely labeled TMJ neurons from ovariectomized rats and ovariectomized rats receiving chronic estrogen replacement were studied using whole cell patch-clamp techniques three days after injecting the TMJ with either saline or Complete Freund's Adjuvant to induce inflammation. Excitability was assessed with depolarizing current injection to determine action potential threshold, rheobase, and the response to suprathreshold stimuli. Spontaneous activity was also assessed. Both inflammation and estrogen increased the excitability of TMJ neurons as reflected by decreases in action potential threshold and rheobase and increases in the incidence of spontaneous activity. The effects were additive with neurons from rats receiving both estrogen and inflammation being the most excitable. The increases in excitability were associated with changes in passive properties and action potential waveform, suggesting that estrogen and inflammation affect the expression and/or properties of ion channels in TMJ neurons. Importantly, the influence of estrogen on both baseline and inflammation-induced changes in TMJ neuronal excitability may help explain the profound sex difference observed in TMD as well as suggest a novel target for the treatment of this pain condition.

Onset and recovery of hyperalgesia and hyperexcitability of sensory neurons following intervertebral foramen volume reduction and restoration

Objective To investigate the relationships between L4 and L5 intervertebral foramen (IVF) stenosis (IVFS), as well as the restoration and onset and recovery of behavioral hyperalgesia and alterations in primary sensory neuron excitability. Methods IVFS was produced by surgically implanting stainless steel rods unilaterally into the intervertebral foramen at L4 and L5. The insertion of a stainless steel rod in the IVF caused IVF volume reduction, which mimics IVFS. The rods were kept for up to 14 weeks in 16 rats and 2 to 4 weeks in another 32 rats. Rod withdrawal was expected to restore the IVF volume. The rods were withdrawn in 20 rats on the 7th day and in another 20 rats on the 14th day, postoperatively. Two additional groups of control rats received no surgery or sham operation. Behavioral hyperalgesia was evidenced by the significantly decreased threshold and shortened latency of foot withdrawal to mechanical and thermal stimulation of the plantar surface. Electrophysiological intracellular recordings were obtained in vitro from L4 and/or L5 dorsal root ganglia (DRG). Results The IVFS rats exhibited a rapid-onset (≤1 day), long-lasting (10-11 weeks), mechanical, and thermal hyperalgesia. DRG neurons in each category, large-sized, medium-sized, and small-sized, from IVFS rats were more excitable than those from control rats, evaluated by the significantly decreased threshold current and action potential threshold and increased number of discharges evoked by depolarizing current and incidence of spontaneous activity. IVF volume restoration significantly reduced behavioral hyperalgesia and the increased excitability of DRG neurons. In contrast, sham surgery produced no behavioral or electrophysiological changes in the ganglion neurons. Conclusion The present study demonstrates that hyperalgesia and hyperexcitability of the primary sensory neurons can be induced following the IVF volume reduction produced by insertion of a stainless steel rod and mostly relieved by the rod withdrawal. The recovery of excitability of DRG cells to normal levels is associated with the abatement of hyperalgesia. These results support the hypothesis that increased excitability of DRG neurons is associated with the generation and maintenance of hyperalgesia and suggest that relief of the IVF stenosis, which could compress all of the normal constituents within the IVF (ie, DRG, nerve root, blood and lymph vessels, adipose, etc.), may help to alleviate chronic pain in humans.

Similar electrophysiological changes in axotomized and neighboring intact dorsal root ganglion neurons.

We investigated electrophysiological changes in chronically axotomized and neighboring intact dorsal root ganglion (DRG) neurons in rats after either a peripheral axotomy consisting of an L5 spinal nerve ligation (SNL) or a central axotomy produced by an L5 partial rhizotomy (PR). SNL produced lasting hyperalgesia to punctate indentation and tactile allodynia to innocuous stroking of the foot ipsilateral to the injury. PR produced ipsilateral hyperalgesia without allodynia with recovery by day 10. Intracellular recordings were obtained in vivo from the cell bodies (somata) of axotomized and intact DRG neurons, some with functionally identified peripheral receptive fields. PR produced only minor electrophysiological changes in both axotomized and intact somata in L5 DRG. In contrast, extensive changes were observed after SNL in large- and medium-sized, but not small-sized, somata of intact (L4) as well as axotomized (L5) DRG neurons. These changes included (in relation to sham values) higher input resistance, lower current and voltage thresholds, and action potentials with longer durations and slower rising and falling rates. The incidence of spontaneous activity, recorded extracellularly from dorsal root fibers in vitro, was significantly higher (in relation to sham) after SNL but not after PR, and occurred in myelinated but not unmyelinated fibers from both L4 (9.1%) and L5 (16.7%) DRGs. We hypothesize that the changes in the electrophysiological properties of axotomized and intact DRG neurons after SNL are produced by a mechanism associated with Wallerian degeneration and that the hyperexcitability of intact neurons may contribute to SNL-induced hyperalgesia and allodynia.

Degeneration of Myelinated Efferent Fibers Induces Spontaneous Activity in Uninjured C-Fiber Afferents

We demonstrated recently that uninjured C-fiber nociceptors in the L4 spinal nerve develop spontaneous activity after transection of the L5 spinal nerve. We postulated that Wallerian degeneration leads to an alteration in the properties of the neighboring, uninjured afferents from adjacent spinal nerves. To explore the role of degeneration of myelinated versus unmyelinated fibers, we investigated the effects of an L5 ventral rhizotomy in rat. This lesion leads to degeneration predominantly in myelinated fibers. Mechanical paw-withdrawal thresholds were assessed with von Frey hairs, and teased-fiber techniques were used to record from single C-fiber afferents in the L4 spinal nerve. Behavioral and electrophysiological data were collected in a blinded manner. Seven days after surgery, a marked decrease in withdrawal thresholds was observed after the ventral rhizotomy but not after the sham operation. Single fiber recordings revealed low-frequency spontaneous activity in 25% of the C-fiber afferents 8-10 d after the lesion compared with only 11% after sham operation. Paw-withdrawal thresholds were inversely correlated with the incidence of spontaneous activity in high-threshold C-fiber afferents. In normal animals, low-frequency electrocutaneous stimulation at C-fiber, but not A-fiber, strength produced behavioral signs of secondary mechanical hyperalgesia on the paw. These results suggest that degeneration in myelinated efferent fibers is sufficient to induce spontaneous activity in C-fiber afferents and behavioral signs of mechanical hyperalgesia. Ectopic spontaneous activity from injured afferents was not required for the development of the neuropathic pain behavior. These results provide additional evidence for a role of Wallerian degeneration in neuropathic pain.

Neuronal thermosensitivity and survival of rat hypothalamic slices in recording chambers

Several studies have examined the activity of neurons in hypothalamic tissue slices. The present experiments studied relationships between neuronal activity (firing rate and thermosensitivity) and tissue survival as a function of time and slice thickness. Rat hypothalamic tissue slices were sectioned at different thicknesses (350, 450, and 600 μm) and maintained in an oxygenated interface chamber which was perfused with artificial cerebrospinal fluid (ACSF). Electron and light microscopy were used to examine tissue morphology at different depths from the slice surfaces, and extracellular recordings were used to measure each cell's spontaneous activity and response to changes in temperature. Tissue damage was most evident at tissue layers nearest the gas-exposed surface. At 9 h in the chamber, 350 μm thick slices showed subtle changes in morphology with little difference between the gas-exposed and ACSF-exposed surfaces. In the 450 and 600 μm thick slices, tissue degeneration became more evident with increased damage at the gas-exposed surface. This damage extended fully into the tissue of the 600 μm section. There were no differences in firing rate or thermosensitivity between 350 and 450 μm slices; but in 600 μm slices, there were fewer spontaneously active neurons, although these neurons had a higher mean thermosensitivity. Based on the incidence of spontaneous activity and morphological integrity, the results suggest that electrophysiological experiments using 350 μm slices are preferable to experiments using thicker slices.

Age and the effects of 2- d,l-amino-5-phosphonovalerate in an area of the chick forebrain which is essential for early learning

The intermediate, medial part of the hyperstriatum ventrale (IMHV) is a region of the avian forebrain which is known to be essential for early learning in the domestic chick. The IMHV in an in vitro slice preparation displays two forms of synaptic plasticity. The incidence of both varies with age and is maximal between 3 and 5 days post-hatch. Since NMDA receptors are critical for at least one of these plasticities, we have investigated the relationship between age and the contribution of NMDA receptors to the field response evoked by local, low-frequency stimulation and have found that the magnitude of the NMDA-dependent component of the response varies with age, peaking between 3 and 5 days post-hatch. Spontaneous neural activity, recorded intracellularly, can be completely and reversibly silenced by NMDA receptor blockade and the incidence of spontaneous activity also varies with age, peaking between 3 and 5 days. These results suggest that the IMHV contains NMDA receptors which can be activated near resting membrane potential. Either the efficiency or the numbers of these receptors is maximal at a specific point in development and their peak activity coincides with a peak in synaptic plasticity. These characteristics are similar to those reported for young mammals.