Activation Of Primary Afferent Fibers Research Articles

Neuropathic Pain: How to Assess and Treat a Maladaptive Pain Response

Neuropathic pain is associated with primary lesion or dysfunction of the peripheral and central nerve systems, affecting up to 10% of the general population. Although both nociceptive pain and neuropathic pain utilize the same nervous system pathways, physiologic differences exist in the pathologic mechanism, clinical presentation, and treatments. Ectopic activity in primary afferent fibers, excitatory and inhibitory somatosensory signaling, nociceptive neuron alterations, and central pain modulation have been implicated in neuropathic pain. These neuropathic mechanisms are associated with the complexity of symptoms, difficult treatment decisions, and challenging poor outcomes. Treatment options include pharmacologic (e.g., anticonvulsants, antidepressants, lidocaine, N-methyl-D-aspartate antagonist, opioids), physical, psychological (e.g., cognitive behavioral therapy), or interventional management (e.g., peripheral or neuro-axial nerve blockade, spinal cord stimulators, intrathecal medications). Medication selection should be individualized, considering patients’ symptoms and potential beneficial or deleterious effects (side effects) on comorbidities. The interventional management of chronic neuropathic pain should be considered for patients who have not responded to pharmacologic and non-interventional treatments, as an integral component of a more comprehensive approach. This article presents an overview of physiological mechanisms, clinical presentation, and assessment of neuropathic pain, and discusses treatment options for neuropathic pain.

Orexin receptors mediate long-term depression of excitatory synaptic transmission in the spinal cord dorsal horn.

Neuropeptides orexin-A and -B are related to the regulation of sleep/wakefulness and feeding behaviors. Recently, the peptides have also been shown to yield antinociceptive effects in various pain models. However, it is not clear whether orexins are involved in forms of synaptic plasticity, such as long-term potentiation (LTP) and long-term depression (LTD), the increase and the decrease of synaptic efficacy, respectively. In the present study, we examined whether orexin receptor type 1 (OX1) and 2 (OX2) are involved in the induction or maintenance of LTD of excitatory synaptic transmission using transverse spinal cord slices of young rats. Repetitive electrical stimulation of Lissauer's tract zone at 2Hz for 5min (600 pulses), combined with a holding potential of -30mV, induced LTD of the amplitude of excitatory postsynaptic currents (EPSCs) which are evoked by the activation of primary afferent fibers. The maintenance of LTD was significantly prevented by bath application of SB674042 (1μM), an OX1 antagonist, or EMPA (1μM), an OX2 antagonist. In addition, LTD was dependent on the NMDA receptor, as the NMDA receptor antagonist D-AP5 blocked the maintenance of LTD. Our study suggests that orexins, via activation of both OX1 and OX2, play a significant role in the expression of NMDA-dependent LTD, thereby contributing to the spinal modulation of pain transmission.

Calcitonin gene-related peptide-expressing sensory neurons and spinal microglial reactivity contribute to pain states in collagen-induced arthritis.

ObjectiveTo evaluate the contribution of sensory neurons in ankle joints and adjacent tissue to the development of pain in collagen‐induced arthritis (CIA), and to determine the relationship between pain and the appearance of clinical signs.MethodsMechanical and heat hypersensitivity and hind paw swelling were assessed in Lewis rats before and until 18 days following collagen immunization. We examined the effect of intrathecal administration of a calcitonin gene‐related peptide (CGRP) antagonist (CGRP8–37) from day 11 to day 18 postimmunization on CIA‐induced hypersensitivity. During CIA development, CGRP and p‐ERK immunoreactivity was quantified in lumbar dorsal root ganglia in which sensory neurons innervating the ankle joint were identified by retrograde labeling with Fluoro‐Gold. Microgliosis in the lumbar dorsal horn was assessed by immunohistochemistry, and release of CGRP evoked by activity of primary afferent fibers was measured using a preparation of isolated dorsal horn with dorsal roots attached.ResultsCIA was associated with mechanical hypersensitivity that was evident before hind paw swelling and that was exacerbated with the development of swelling. Heat hyperalgesia developed along with swelling. Concomitant with the development of mechanical hypersensitivity, joint innervating neurons exhibited enhanced CGRP expression and an activated phenotype (increased p‐ERK expression), and significant microgliosis became evident in the dorsal horn; these peripheral and central changes were augmented further with disease progression. CGRP release evoked by dorsal root stimulation was higher in the dorsal horn on day 18 in rats with CIA compared to control rats. Prolonged intrathecal administration of CGRP8–37 attenuated established mechanical hypersensitivity and reduced spinal microgliosis.ConclusionSensory neuron–derived CGRP sustains mechanical hypersensitivity and spinal microglial reactivity in CIA, suggesting that central mechanisms play critical roles in chronic inflammatory pain. Blockade of these central events may provide pain relief in rheumatoid arthritis patients.

Enteric Glia Are Targets of the Sympathetic Innervation of the Myenteric Plexus in the Guinea Pig Distal Colon

Astrocytes respond to synaptic activity in the CNS. Astrocytic responses are synapse specific and precisely regulate synaptic activity. Glia in the peripheral nervous system also respond to neuronal activity, but it is unknown whether glial responses are synapse specific. We addressed this issue by examining the activation of enteric glia by distinct neuronal subpopulations in the enteric nervous system. Enteric glia are unique peripheral glia that surround enteric neurons and respond to neuronally released ATP with increases in intracellular calcium ([Ca2+]i). Autonomic control of colonic function is mediated by intrinsic (enteric) and extrinsic (sympathetic, parasympathetic, primary afferent) neural pathways. Here we test the hypothesis that a defined population of neurons activates enteric glia using a variety of techniques to ablate or stimulate components of the autonomic innervation of the colon. Our findings demonstrate that, in the male guinea pig colon, activation of intrinsic neurons does not stimulate glial [Ca2+]i responses and fast enteric neurotransmission is not necessary to initiate glial responses. However, ablating extrinsic innervation significantly reduces glial responses to neuronal activation. Activation of primary afferent fibers does not activate glial [Ca2+]i responses. Selectively ablating sympathetic fibers reduces glial activation to a similar extent as total extrinsic denervation. Neuronal activation of glia follows the same frequency dependence as sympathetic neurotransmitter release, but the only sympathetic neurotransmitter that activates glial [Ca2+]i responses is ATP, suggesting that sympathetic fibers release ATP to activate enteric glia. Therefore, enteric glia discern activity in adjacent synaptic pathways and selectively respond to sympathetic activation.

Dynamic neuronal responses in cortical and thalamic areas during different phases of formalin test in rats

Although formalin-induced activity in primary afferent fibers and spinal dorsal horn is well described, the forebrain neural basis underlying each phase of behavior in formalin test has not yet been clarified. The present study was designed to investigate the cortical and thalamic neuronal responses and interactions among forebrain areas during different phases after subcutaneous injection of formalin. Formalin-induced neuronal activities were simultaneously recorded from primary somatosensory cortex (SI), anterior cingulate cortex (ACC) and medial dorsal (MD) and ventral posterior (VP) thalamus during different phases (i.e., first phase, interphase, second phase and third recovery phase starting from 70 min after injection) of formalin test, using a multi-channel, single-unit recording technique. Our results showed that, (i) unlike the responses in primary afferent fibers and spinal dorsal horn, many forebrain neurons displayed monophasic excitatory responses in the first hour after formalin injection, except a small portion of neurons which exhibited biphasic responses; (ii) the response patterns of many cortical and thalamic neurons changed from excitatory to inhibitory at the end of the second phase; (iii) the direction of information flow also changed dramatically, i.e., from cortex to thalamus and from the medial to the lateral pathway in the first hour, but reversed in phase 3. These results indicate that the changes of activity pattern in forebrain networks may underlie the emerging and subsiding of central sensitization-induced pain behavior in the second phase of formalin test.

Sciatic Chronic Constriction Injury Produces Cell-Type-Specific Changes in the Electrophysiological Properties of Rat Substantia Gelatinosa Neurons

Peripheral nerve injury increases spontaneous action potential discharge in spinal dorsal horn neurons and augments their response to peripheral stimulation. This "central hypersensitivity, " which relates to the onset and persistence of neuropathic pain, reflects spontaneous activity in primary afferent fibers as well as long-term changes in the intrinsic properties of the dorsal horn (centralization). To isolate and investigate cellular mechanisms underlying "centralization," sciatic nerves of 20-day-old rats were subjected to 13-25 days of chronic constriction injury (CCI; Mosconi-Kruger polyethylene cuff model). Spinal cord slices were then acutely prepared from sham-operated or CCI animals, and whole cell recording was used to compare the properties of five types of substantia gelatinosa neuron. These were defined as tonic, irregular, phasic, transient, or delay according to their discharge pattern in response to depolarizing current. CCI did not affect resting membrane potential, rheobase, or input resistance in any neuron type but increased the amplitude and frequency of spontaneous and miniature excitatory postsynaptic currents (EPSCs) in delay, transient, and irregular cells. These changes involved alterations in the action potential-independent neurotransmitter release machinery and possible increases in the postsynaptic effectiveness of glutamate. By contrast, in tonic cells, CCI reduced the amplitude and frequency of spontaneous and miniature EPSCs. Such changes may relate to the putative role of tonic cells as inhibitory GABAergic interneurons, whereas increased synaptic drive to delay cells may relate to their putative role as the excitatory output neurons of the substantia gelatinosa. Complementary changes in synaptic excitation of inhibitory and excitatory neurons may thus contribute to pain centralization.

Selective Activation of Primary Afferent Fibers Evaluated by Sine-Wave Electrical Stimulation

Transcutaneous sine-wave stimuli at frequencies of 2000, 250 and 5 Hz (Neurometer) are thought to selectively activate Aβ, Aδ and C afferent fibers, respectively. However, there are few reports to test the selectivity of these stimuli at the cellular level. In the present study, we analyzed action potentials (APs) generated by sine-wave stimuli applied to the dorsal root in acutely isolated rat dorsal root ganglion (DRG) preparations using intracellular recordings. We also measured excitatory synaptic responses evoked by transcutaneous stimuli in substantia gelatinosa (SG) neurons of the spinal dorsal horn, which receive inputs predominantly from C and Aδ fibers, using in vivo patch-clamp recordings. In behavioral studies, escape or vocalization behavior of rats was observed with both 250 and 5 Hz stimuli at intensity of ~0.8 mA (T5/ T250), whereas with 2000 Hz stimulation, much higher intensity (2.14 mA, T2000) was required. In DRG neurons, APs were generated at T5/T250 by 2000 Hz stimulation in Aβ, by 250 Hz stimulation both in Aβ and Aδ, and by 5 Hz stimulation in all three classes of DRG neurons. However, the AP frequencies elicited in Aβ and Aδ by 5 Hz stimulation were much less than those reported previously in physiological condition. With in vivo experiments large amplitude of EPSCs in SG neurons were elicited by 250 and 5 Hz stimuli at T5/ T250. These results suggest that 2000 Hz stimulation excites selectively Aβ fibers and 5 Hz stimulation activates noxious transmission mediated mainly through C fibers. Although 250 Hz stimulation activates both Aδ and Aβ fibers, tactile sensation would not be perceived when painful sensation is produced at the same time. Therefore, 250 Hz was effective stimulus frequency for activation of Aδ fibers initiating noxious sensation. Thus, the transcutaneous sine-wave stimulation can be applied to evaluate functional changes of sensory transmission by comparing thresholds with the three stimulus frequencies.

Effects of TAK-637 on NK 1 receptor-mediated mechanisms regulating colonic secretion

This study investigates the effect of a selective NK 1 receptor antagonist TAK-637 on enteric mechanisms involved in regulation of epithelial secretion in the colon. Mucosal sheets isolated from guinea-pig colon were placed in modified Ussing chambers and the net active transport of electrolytes was measured as short-circuit current (Isc). GR-73632, a selective NK 1 receptor agonist, induced an increase in basal Isc, which was inhibited by TAK-637 (IC 50 of 21 nM). The increase in Isc induced by GR-73632 was significantly attenuated by tetrodotoxin (TTX, 1 μM), indicating that TAK-637 inhibits neuronal NK 1 receptors. Moreover, TAK-637 reduced the TTX-resistant component of the response to GR-73632 suggesting that NK 1 receptors expressed by epithelial cells are inhibited by TAK-637. In separate experiments, TAK-637 partially inhibited the submaximal Isc induced by electrical field stimulation (EFS, 0.5 ms, 15 Hz) of enteric nerves or by activation of primary afferent fibers using capsaicin (50 μM). TAK-637 had no significant effect on the basal Isc or on responses induced by neurokinin A (NKA), senktide, or forskolin. The results imply that inhibition of peripheral NK 1 receptors may reduce autonomic epithelial secretion in response to activation of autonomic secretomotor pathways, while having no significant effect on basal epithelial transport.

Effects and consequences of nerve injury on the electrical properties of sensory neurons.

Nociceptive pain alerts the body to potential or actual tissue damage. By contrast, neuropathic or "noninflammatory" pain, which results from injury to the nervous system, serves no useful purpose. It typically continues for years after the original injury has healed. Sciatic nerve lesions can invoke chronic neuropathic pain that is accompanied by persistent, spontaneous activity in primary afferent fibers. This activity, which reflects changes in the properties and functional expression of Na+, K+, and Ca2+ channels, initiates a further increase in the excitability of second-order sensory neurons in the dorsal horn. This change persists for many weeks. The source of origin of the pain thus moves from the peripheral to the central nervous system. We hypothesize that this centralization of pain involves the inappropriate release of peptidergic neuromodulators from primary afferent fibers. Peptides such as substance P, neuropeptide Y (NPY), calcitonin-gene-related peptide (CGRP), and brain-derived neurotrophic factor (BDNF) may promote enduring changes in excitability as a consequence of neurotrophic actions on ion channel expression in the dorsal horn. Findings that form the basis of this hypothesis are reviewed. Study of the neurotrophic control of ion channel expression by spinal peptides may thus provide new insights into the etiology of neuropathic pain.

Stability and Reliability of Detection Thresholds for Human A-Beta and A-Delta Sensory Afferents Determined by Cutaneous Electrical Stimulation

Activity in primary afferent fibers that usually mediate fine touch can evoke sensations of pain in conditions in which there is sensitization of central neurons. Input from these large diameter Aβ afferents may also sustain and exacerbate these central mechanisms. The role of these fibers in clinical pain syndromes can be evaluated by applications of electrical stimuli that preferentially activate Aβ axons. This study assessed the stability and reliability of a method of electrical stimulation (ES) useful for clinical evaluation. Monopolar constant-current rectangular pulses were delivered to 5 equi-spaced sites on the volar aspect of the left forearm along a transverse line 5 cm distal to the antecubital crease. Current intensity was gradually increased to determine detection threshold and pain detection threshold. This study determined: 1) Effect of pulse duration (1, 2, and 5 msec); 2) the variation of detection threshold and pain threshold over repeated stimulation; 3) the effect of electrode position with respect to distance from the trunk of underlying ulnar or median nerves; and 4) the effect of re-positioning the electrode on variability of detection threshold and pain threshold. There was no significant variability over time for either detection threshold (DT) or pain threshold (PT) at any of the 3 pulse durations tested. There was also no significant effect on variability of shifting the electrode between sites, nor was there a significant difference in variability between sites when placed either over or adjacent to peripheral nerves. Under simulated clinical conditions of electrode re-positioning, the mean detection threshold in 300 trials and ten subjects was 0.30 mA with an overall standard error of 0.007, standard errors of 0.014 over the 10 subjects, 0.003 over the 6 trials, and 0.012 over the 5 locations. Similarly, mean pain threshold in these 300 trials was 3.24 ± 0.093, with standard errors of 0.12 over the 10 subjects, 0.023 over the 6 trials, and 0.13 over the 5 locations. Mean ratio of pain threshold divided by detection threshold ratio was 10.9 ± 0.25 with a range of 2.0–28.3. Single pulse, constant current electrical stimulation of the skin at threshold levels is a quantifiable and reliable sensory method that is repeatable within and between testing sessions. Our results suggest that in skin unaffected by allodynia, a ratio of the two sensory thresholds (pain threshold and detection threshold) of less than 2.0 is uncommon. We propose that, in the presence of mechanical allodynia, a pain threshold/detection threshold of less than 2.0 suggests that altered central nervous system processing of Aβ input may contribute to allodynia.

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.

Formalin-evoked activity in identified primary afferent fibers: systemic lidocaine suppresses phase-2 activity

Formalin injected subcutaneously into the paw is a frequently used pain assay; it evokes an initial period of licking and flinching followed by a period of quiescence and last by a second period of intense and protracted licking and flinching. The prominent second phase is believed to reflect the development of a central (spinal cord) facilitation. This conclusion is based on the assumption that formalin evokes an initial burst of activity in fine afferent fibers, followed by prolonged low levels of activity in C fibers. Detailed reports substantiating this essential assumption have not been published. Thus, we recorded in situ from single sural nerve fibers, identified by their conduction velocity and modality, in the barbiturate anesthetized rat. Following formalin (2.5%, 50 μl) injection into their receptive fields, phase-1 activity was prominent in Aβ and Aδ fibers as well as in high-threshold C nociceptive afferent fibers. Phase-2 activity was observed in Aδ fibers with receptive fields in hairy skin and in all mechanically sensitive C fibers. Mean phase-2 activity in these fibers was 1 2 – 2 3 of the magnitude achieved in phase 1. Mechanically insensitive fibers and Aδ and C fibers with receptive field centers outside of the injection zone began firing 15 min or more post-injection and would contribute to phase-2, but not phase-1, behavioral activity. Intravenous infusion of low doses of lidocaine yielding plasma levels of 3.6–7.9 μg/ml administered during phase 2 blocked formalin-evoked activity in primary afferent fibers in a dose-related fashion without blocking either electrically or mechanically evoked activity. Effective plasma doses were comparable to those found to relieve neuropathic pain. These data indicate that phase 2 in the formalin test is more closely related to ongoing afferent input than had previously been thought.

Negative feedback neuroendocrine control of the inflammatory response in rats

We describe that an ongoing inflammatory response at one site (produced by complete Freund's adjuvant injection in the hindpaw) produces a negative feedback inhibition on plasma extravasation, produced by perfusion of the inflammatory mediator, bradykinin (160 nM), at a second site (the knee joint). This negative feedback process is abolished in rats that have been neonatally treated with capsaicin to deplete most of their unmyelinated primary afferent fibers, which suggests that this negative feedback process is mediated by activation of primary afferent fibers. Electrical stimulation of the hindpaw at intensities that excite C-fibers also inhibited bradykinin-induced plasma extravasation. Stimulation at intensities that only excite A-fibers had no effect on bradykinin-induced plasma extravasation. Platelet activating factor-induced plasma extravasation, which is not dependent on the innervation of the joint, was not inhibited by stimulation of C-fibers from the hindpaw. Acute surgical interruption of the lumbar sympathetic outflow to the hind limb (the paravertebral ganglia between L2 and L4) did not attenuate the depression of bradykinin-induced plasma extravasation produced by C-fiber stimulation. This indicates that the depression is not mediated by activity in the sympathetic outflow. Transection of the spinal cord, hypophysectomy, inhibition of corticosterone synthesis, and adrenalectomy (but not adrenal medullectomy) all prevented the inhibition of BK-induced plasma extravasation by electrical simulation, indicating that the negative feedback inhibition on plasma extravasation is dependent on an intact neuraxis and an intact hypothalamic-pituitary-adrenocortical axis. In summary, our data demonstrate a negative feedback inhibition of an inflammatory process, which is elicited by stimulation of C-fiber afferents.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotransmitters of the olivocochlear lateral efferent system: with an emphasis on dopamine.

The olivocochlear lateral efferent system (OLES) of the adult mammalian cochlea uses variety of neuroactive substances, such as acetyl choline, GABA, dopamine (DA), enkephalins, dynorphins and CGRP. These neuroactive substances have been located within the efferent, small and dense matrix, fibers and terminals of the inner spiral and tunnel bundles. However, some of these neuroactive substances have also been found outside the OLES. Acetyl choline and CGRP, for instance, appear within the olivocochlear medial efferent fibers, and DA and CGRP may also be present in the perivascular innervation. A special case is GABA innervation at the apical coil, where nerve fibers containing GABA also make synapses with OHCs bodies. All these substances of the OLES could be involved in a highly selective filter modulating the activity of primary afferent fibers. For instance, sound stimulation results in an increase of cochlear DA turnover, indicating the release of DA from OLES fibers. DA probably acts on D-2 receptors since the administration of piribedil, a D-2 agonist, results in blocking of noise effects, while D-1 receptor stimulation does not modify cochlear DA turnover induced by noise. Therefore, DA could play an important role in the modulation and noise-protection of cochlear primary afferents. During cochlear development, all the aforementioned neuroactive substances appear a long time before the onset of hearing (evidenced by the recording of cochlear compound action potential and microphonic potentials). Thus, they may act during development on the late reorganization and plasticity on the afferent and efferent fibers. Moreover, the positive neurotrophic effect observed in cultured cochlear neurons, with GABA or glutamate, add new support to that hypothesis.

Interactions between excitatory amino acids and tachykinins in the rat spinal dorsal horn

Whole-cell patch-clamp technique of freshly isolated rat spinal dorsal horn (DH) neurons, intracellular recording from DH neurons in a slice preparation, and high performance liquid chromatography with fluorimetric detection of release of endogenous glutamate and aspartate from spinal cord slice following activation of primary afferent fibers were employed to investigate interactions between excitatory amino acids (EAA) and tachykinins [substance P (SP) and neurokinin A (NKA)]. Potentiation of N-methyl-D-aspartate (NMDA)-, quisqualate (QA)- and α-amino 3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-, but not kainate-induced currents by SP and NKA was found. Spantide II, a claimed novel nonselective tachykinin antagonist, effectively blocked the SP (2 n M )-induced potentiation of the responses of DH neurons to NMDA. In the presence of glycine (0.1 μM), the SP-evoked increase of the NMDA-induced current was prevented. However, 7-chlorokynurenic acid (2 μM), a competitive antagonist at the glycine allosteric site of the NMDA receptor, led to the reestablishment of the SP effect. Brief high frequency electrical stimulation of primary afferent fibers produced a longlasting potentiation of presumed monosynaptic and polysynaptic excitatory postsynaptic potentials and sustained enhanced release of endogenous glutamate (218.3± 66.1 %) and aspartate (286.3 ± 58.0%). Possible functional implications of the observed phenomena are discussed in relation to transmission and integration of sensory information, including pain.

C-fos Induction in the Spinal Cord after Peripheral Nerve Lesion.

Immunocytochemical localization of a product of the proto-oncogene c-fos, Fos protein, was used to map the activity of a subset of rat spinal neurons at 3 days, 3 weeks and 3 months following section of the sciatic nerve. In a well-established experimental paradigm, the gene was induced by activation of primary afferent fibres with brief noxious sensory stimulation under anaesthetic. Central sciatic projections were demonstrated with isolectin B4 counterstain and GAP-43 immunocytochemistry. In Rexed's lamina II of the spinal cord, in which there is somatotopic organization of afferent terminals, Fos-positive neurons were largely restricted to the projection area of intact peripheral nerves. Three days after a sciatic nerve lesion, the number of Fos-positive neurons in a cord region innervated by the saphenous nerve was similar to control levels, but was markedly increased by 3 weeks, remaining elevated at 3 months. Three weeks after sciatic nerve section the lectin stain in the area of sciatic representation had almost completely disappeared, and conversely GAP-43 staining had greatly intensified. There was no evidence of invasion by Fos-immunoreactive cells of the area of sciatic representation. After 3 months both the size and the intensity of the lectin gap, and of the corresponding area of increased GAP-43 immunoreactivity, appeared reduced. Thus a peripheral nerve lesion was followed by a delayed increase in excitability of the spinal cord as assessed by c-fos expression, so that greater numbers of second-order neurons were activated by sensory stimulation of an adjacent intact nerve. These changes may be related to the sensory abnormalities which follow nerve damage.

Peripheral Vasodilatation after Spinal Cord Stimulation: Animal Studies of Putative Effector Mechanisms

Spinal cord stimulation has been reported to relieve ischemic pain and to enhance peripheral circulation. To elucidate the still unknown mechanisms behind these effects, changes in the peripheral blood flow in the skin of the hind paw after stimulation applied to the dorsal column was studied in the normal rat. A substantial flow increase, monitored by laser Doppler technique, was observed in response to stimulation with an intensity comparable to that used clinically in man, recruiting only low-threshold neuronal elements. The response remained after transection of the dorsal roots, as well as after spinal cord section rostrally to the stimulating electrode. Furthermore, monitoring of antidromic compound action potentials in a peripheral nerve indicated that the stimulus intensity applied to dorsal columns recruited solely low-threshold, large-diameter fibers. Stimulation failed to produce an increase in blood flow in addition to that produced by guanethidine and hexamethonium, but high-intensity dorsal column or dorsal root stimulation still was effective. The results indicate that spinal mechanisms are essential and that antidromic activation of primary afferent fibers is unlikely to account for the peripheral vasodilatation induced by low-intensity spinal cord stimulation. Our observations suggest a transitory inhibition of sympathetic vasoconstriction as the principal underlying mechanism.

Mechanisms mediating the brain stem control of somatosensory transmission in the dorsal horn of the cat's spinal cord: an intracellular analysis.

The effect of brainstem stimulation was studied on neurones recorded intracellularly in the superficial and deeper laminae of the lumbosacral dorsal horn of the spinal cord in anaesthetised cats. Stimulation in the nucleus locus coeruleus (LC) produced a hyperpolarization in 4/13 multireceptive neurones and produced a biphasic action consisting of a hyperpolarization which was followed by a depolarization in 3/13 neurones. These actions were produced irrespective of whether the multireceptive neurone was located in the superficial or deeper laminae of the dorsal horn. Stimulation failed to produced postsynaptic potentials in the remaining 6/13 multireceptive neurones. The amplitude of hyperpolarization was increased by the passage of depolarising pulses through the recording microelectrode and decreased by hyperpolarizing pulses. Stimulation in other brainstem areas such as, the lateral (FTL), paralemniscal (FTP) and central (FTC) divisions of the tegmental field and the nuclei raphe magnus (NRM) and reticularis magnocellularis (RMc) also hyperpolarized neurones in the dorsal horn. The polarity of hyperpolarization evoked from some brainstem areas (FTP, FTC, RMc) could be reversed to depolarisation by the passive diffusion of ions from the recording microelectrode containing 3M-KCl. Brainstem (LC, NRM, FTP, FTL) stimulation generated long lasting (700 ms) hyperpolarization on 4/4 selectively nocireceptive neurones of lamina I. There was, however, no effect on the activity of 5/5 neurones recorded in laminae I/II which in addition to receiving excitatory cutaneous inputs were inhibited by heat stimuli. Stimulation in LC also produced dorsal root potentials (DRPs) and reduced the amplitude of simultaneously recorded excitatory postsynaptic potentials (EPSPs) generated by the activation of primary afferent fibres in 3 multireceptive neurones. It is concluded that inhibition of nociceptive transmission in the spinal cord from LC and other brainstem areas may involve both pre- and postsynaptic mechanisms.

Spontaneous antidromic activity in primary afferent fibers after chronic axotomy

Spontaneous antidromic activity in primary afferent fibers after chronic axotomy

Electrophysiological properties of rat spinal dorsal horn neurones in vitro: calcium-dependent action potentials.

1. The electrophysiological properties of dorsal horn neurones have been investigated in the immature rat in vitro spinal cord slice preparation. 2. Intracellular recordings from dorsal horn neurones show that direct or orthodromic stimulation generates action potentials followed by a brief after-hyperpolarization. Synaptic potentials were elicited by the activation of primary afferent fibres in the dorsal root. 3. Input resistance for dorsal horn neurones ranged from 48 to 267 M omega, and the membrane time constant was in the range of 4-19 ms. 4. In response to strong depolarizing currents dorsal horn neurones perfused with TTX and TEA frequently exhibit a slow regenerative depolarizing potential followed by a slow after-hyperpolarization. The depolarizing potential probably results from an influx of Ca. It is blocked by low concentration Ca, Co or Mn, and enhanced by high levels of extracellular Ca. 5. There is, in addition, a low-threshold Ca-dependent response which is activated at membrane potentials more negative than -65 mV and has a maximum rate of rise at the polarization level of about -80 mV. 6. The addition of Ba or TEA to the perfusing medium provided support for the Ca-dependence of the low- and high-threshold responses, and the lack of fast inactivation of the high-threshold Ca potential.