Excitation of primate spinothalamic neurons by cutaneous C-fiber volleys

Thermosensory thalamus: parallel processing across model organisms

The thalamus acts as an interface between the periphery and the cortex, with nearly every sensory modality processing information in the thalamocortical circuit. Despite well-established thalamic nuclei for visual, auditory, and tactile modalities, the key thalamic nuclei responsible for innocuous thermosensation remains under debate. Thermosensory information is first transduced by thermoreceptors located in the skin and then processed in the spinal cord. Temperature information is then transmitted to the brain through multiple spinal projection pathways including the spinothalamic tract and the spinoparabrachial tract. While there are fundamental studies of thermal transduction via thermosensitive channels in primary sensory afferents, thermal representation in the spinal projection neurons, and encoding of temperature in the primary cortical targets, comparatively little is known about the intermediate stage of processing in the thalamus. Multiple thalamic nuclei have been implicated in thermal encoding, each with a corresponding cortical target, but without a consensus on the role of each pathway. Here, we review a combination of anatomy, physiology, and behavioral studies across multiple animal models to characterize the thalamic representation of temperature in two proposed thermosensory information streams.

Sodium-calcium exchanger-3 regulates pain "wind-up": From human psychophysics to spinal mechanisms

Repeated application of noxious stimuli leads to a progressively increased pain perception; this temporal summation is enhanced in and predictive of clinical pain disorders. Its electrophysiological correlate is "wind-up," in which dorsal horn spinal neurons increase their response to repeated nociceptor stimulation. To understand the genetic basis of temporal summation, we undertook a GWAS of wind-up in healthy human volunteers and found significant association with SLC8A3 encoding sodium-calcium exchanger type 3 (NCX3). NCX3 was expressed in mouse dorsal horn neurons, and mice lacking NCX3 showed normal, acute pain but hypersensitivity to the second phase of the formalin test and chronic constriction injury. Dorsal horn neurons lacking NCX3 showed increased intracellular calcium following repetitive stimulation, slowed calcium clearance, and increased wind-up. Moreover, virally mediated enhanced spinal expression of NCX3 reduced central sensitization. Our study highlights Ca2+ efflux as a pathway underlying temporal summation and persistent pain, which may be amenable to therapeutic targeting.

Convergence of peptidergic and non-peptidergic protein markers in the human dorsal root ganglion and spinal dorsal horn

Peripheral sensory neurons are characterized by their size, molecular profiles, and physiological responses to specific stimuli. In mouse, the peptidergic and non-peptidergic subsets of nociceptors are distinct and innervate different lamina of the spinal dorsal horn. The unique molecular signature and neuroanatomical organization of these neurons supports a labeled line theory for certain types of nociceptive stimuli. However, long-standing evidence supports the polymodal nature of nociceptors in many species. We have recently shown that the peptidergic marker, CGRP, and the non-peptidergic marker, P2X3R, show largely overlapping expression at the mRNA level in human dorsal root ganglion (DRG). Herein, our aim was to assess the protein distribution of nociceptor markers, including their central projections, in the human DRG and spinal cord. Using DRGs obtained from organ donors, we observed that CGRP and P2X3R were co-expressed by approximately 33% of human DRG neurons and TrpV1 was expressed in ~60% of human DRG neurons. In the dorsal spinal cord, CGRP, P2X3R, TrpV1, and Nav1.7 proteins stained the entirety of lamina 1-2, with only P2XR3 showing a gradient of expression. This was confirmed by measuring the size of the substantia gelatinosa using Hematoxylin and Eosin staining of adjacent sections. Our findings are consistent with the known polymodal nature of most primate nociceptors and indicate that the central projection patterns of nociceptors are different between mice and humans. Elucidating how human nociceptors connect to subsets of dorsal horn neurons will be important for understanding the physiological consequences of these species differences.

Spinal cord stimulation in chronic pain: evidence and theory for mechanisms of action

Well-established in the field of bioelectronic medicine, Spinal Cord Stimulation (SCS) offers an implantable, non-pharmacologic treatment for patients with intractable chronic pain conditions. Chronic pain is a widely heterogenous syndrome with regard to both pathophysiology and the resultant phenotype. Despite advances in our understanding of SCS-mediated antinociception, there still exists limited evidence clarifying the pathways recruited when patterned electric pulses are applied to the epidural space. The rapid clinical implementation of novel SCS methods including burst, high frequency and dorsal root ganglion SCS has provided the clinician with multiple options to treat refractory chronic pain. While compelling evidence for safety and efficacy exists in support of these novel paradigms, our understanding of their mechanisms of action (MOA) dramatically lags behind clinical data. In this review, we reconstruct the available basic science and clinical literature that offers support for mechanisms of both paresthesia spinal cord stimulation (P-SCS) and paresthesia-free spinal cord stimulation (PF-SCS). While P-SCS has been heavily examined since its inception, PF-SCS paradigms have recently been clinically approved with the support of limited preclinical research. Thus, wide knowledge gaps exist between their clinical efficacy and MOA. To close this gap, many rich investigative avenues for both P-SCS and PF-SCS are underway, which will further open the door for paradigm optimization, adjunctive therapies and new indications for SCS. As our understanding of these mechanisms evolves, clinicians will be empowered with the possibility of improving patient care using SCS to selectively target specific pathophysiological processes in chronic pain.

Neuronal networks and nociceptive processing in the dorsal horn of the spinal cord

The dorsal horn (DH) of the spinal cord receives a variety of sensory information arising from the inner and outer environment, as well as modulatory inputs from supraspinal centers. This information is integrated by the DH before being forwarded to brain areas where it may lead to pain perception. Spinal integration of this information relies on the interplay between different DH neurons forming complex and plastic neuronal networks. Elements of these networks are therefore potential targets for new analgesics and pain-relieving strategies. The present review aims at providing an overview of the current knowledge on these networks, with a special emphasis on those involving interlaminar communication in both physiological and pathological conditions.

The Periaqueductal Gray Orchestrates Sensory and Motor Circuits at Multiple Levels of the Neuraxis

The periaqueductal gray (PAG) coordinates behaviors essential to survival, including striking changes in movement and posture (e.g., escape behaviors in response to noxious stimuli vs freezing in response to fear-evoking stimuli). However, the neural circuits underlying the expression of these behaviors remain poorly understood. We demonstrate in vivo in rats that activation of the ventrolateral PAG (vlPAG) affects motor systems at multiple levels of the neuraxis through the following: (1) differential control of spinal neurons that forward sensory information to the cerebellum via spino-olivo-cerebellar pathways (nociceptive signals are reduced while proprioceptive signals are enhanced); (2) alterations in cerebellar nuclear output as revealed by changes in expression of Fos-like immunoreactivity; and (3) regulation of spinal reflex circuits, as shown by an increase in α-motoneuron excitability. The capacity to coordinate sensory and motor functions is demonstrated in awake, behaving rats, in which natural activation of the vlPAG in fear-conditioned animals reduced transmission in spino-olivo-cerebellar pathways during periods of freezing that were associated with increased muscle tone and thus motor outflow. The increase in spinal motor reflex excitability and reduction in transmission of ascending sensory signals via spino-olivo-cerebellar pathways occurred simultaneously. We suggest that the interactions revealed in the present study between the vlPAG and sensorimotor circuits could form the neural substrate for survival behaviors associated with vlPAG activation.

SIGNIFICANCE STATEMENT

Neural circuits that coordinate survival behaviors remain poorly understood. We demonstrate in rats that the periaqueductal gray (PAG) affects motor systems at the following multiple levels of the neuraxis: (1) through altering transmission in spino-olivary pathways that forward sensory signals to the cerebellum, reducing and enhancing transmission of nociceptive and proprioceptive information, respectively; (2) by alterations in cerebellar output; and (3) through enhancement of spinal motor reflex pathways. The sensory and motor effects occurred at the same time and were present in both anesthetized animals and behavioral experiments in which fear conditioning naturally activated the PAG. The results provide insights into the neural circuits that enable an animal to be ready and able to react to danger, thus assisting in survival.

Neural correlates of temporal summation of second pain in the human brainstem and spinal cord

Temporal summation of second pain (TSSP) occurs when painful stimuli are presented repetitively (≥ 0.33 Hz) and results from a C-fibre evoked enhancement (or "wind-up") of the dorsal horn neurons. Based on electrophysiological studies in intact animals, windup is considered a purely central phenomenon. With advancements in functional MRI (fMRI), we can now probe the central mechanisms of this pain response in humans. The aim of this study is to characterize the fMRI responses in the healthy human brainstem and spinal cord that correspond to TSSP. Functional MRI of healthy female adults (N = 15) was conducted while brief, repetitive heat pain stimuli were applied to the right thenar eminence (C6 dermatome), and TSSP (0.33 Hz) and control (0.17 Hz) heat pain paradigms were employed. The stimulus intensity was adjusted to each participant's heat pain sensitivity. Data were analyzed by means of a general linear model, and region-of-interest analyses. As predicted, participants demonstrated significant behavioural summation of pain in the TSSP condition. FMRI results identified enhanced activity in the spinal cord dorsal horn at C6 in response to the TSSP condition. Additionally, multiple areas of the brainstem (RVM and PAG) showed greater responses with the TSSP condition. These results suggest that, in humans, increased pain perception in the TSSP condition is reflected by greater responses in the dorsal horn and in regions known to play a role in the descending modulation of pain, which may modulate the spinal cord response.

Spinal sensory projection neuron responses to spinal cord stimulation are mediated by circuits beyond gate control

Spinal cord stimulation (SCS) is a therapy used to treat intractable pain with a putative mechanism of action based on the Gate Control Theory. We hypothesized that sensory projection neuron responses to SCS would follow a single stereotyped response curve as a function of SCS frequency, as predicted by the Gate Control circuit. We recorded the responses of antidromically identified sensory projection neurons in the lumbar spinal cord during 1- to 150-Hz SCS in both healthy rats and neuropathic rats following chronic constriction injury (CCI). The relationship between SCS frequency and projection neuron activity predicted by the Gate Control circuit accounted for a subset of neuronal responses to SCS but could not account for the full range of observed responses. Heterogeneous responses were classifiable into three additional groups and were reproduced using computational models of spinal microcircuits representing other interactions between nociceptive and nonnociceptive sensory inputs. Intrathecal administration of bicuculline, a GABAA receptor antagonist, increased spontaneous and evoked activity in projection neurons, enhanced excitatory responses to SCS, and reduced inhibitory responses to SCS, suggesting that GABAA neurotransmission plays a broad role in regulating projection neuron activity. These in vivo and computational results challenge the Gate Control Theory as the only mechanism underlying SCS and refine our understanding of the effects of SCS on spinal sensory neurons within the framework of contemporary understanding of dorsal horn circuitry.

Immobilization stress sensitizes rat dorsal horn neurons having input from the low back

BACKGROUND

Stress is known to promote several forms of muscle pain including non-specific low back pain. However, the question if stress alone activates nociceptive central neurons has not been studied systematically. Here, we investigated the influence of repeated immobilization stress on dorsal horn neurons and behaviour in the rat.

METHODS

The stress consisted of immobilization in a narrow tube for 1 h on 12 days. Single dorsal horn neurons were recorded with microelectrodes introduced into the spinal segment L2. In this segment, about 14% of the neurons responded to mechanical stimulation of the subcutaneous soft tissues of the low back in naïve rats. The neurons often behaved like wide dynamic range cells in that they had a low mechanical threshold and showed graded responses to noxious stimuli.

RESULTS

The stress-induced changes in neuronal response behaviour were (1) appearance of new receptive fields in the deep tissues of the hindlimb, (2) increased input from deep soft tissues, but unchanged input from the skin and (3) significant increase in resting activity. Surprisingly, the pressure-pain threshold of the low back remained unchanged, although dorsal horn neurons were sensitized. In the open field test, the rats showed signs of increased anxiety.

CONCLUSIONS

This study shows that stress alone is sufficient to sensitize dorsal horn neurons. The data may explain the enhanced pain low back patients report when they are under stress. The increased resting discharge may lead to spontaneous pain.

Mechanisms and models of spinal cord stimulation for the treatment of neuropathic pain

Spinal cord stimulation (SCS) is an established and cost-effective therapy for treating severe chronic pain. However, despite over 40 years of clinical practice and the development of novel electrode designs and treatment protocols, increases in clinical success, defined as the proportion of patients that experience 50% or greater self-reported pain relief, have stalled. An incomplete knowledge of the neural circuits and systems underlying chronic pain and the interaction of SCS with these circuits may underlie this plateau in clinical efficacy. This review summarizes prior work and identifies gaps in our knowledge regarding the neural circuits related to pain and SCS in the dorsal horn, supraspinal structures, and the Pain Matrix. In addition, this review discusses and critiques current experimental and computational models used to investigate and optimize SCS. Further research into the interactions between SCS and pain pathways in the nervous system using animal and computational models is a fruitful approach to improve this promising therapy.

Epidermal neural crest stem cell (EPI-NCSC)--mediated recovery of sensory function in a mouse model of spinal cord injury

Here we show that epidermal neural crest stem cell (EPI-NCSC) transplants in the contused spinal cord caused a 24% improvement in sensory connectivity and a substantial recovery of touch perception. Furthermore we present a novel method for the ex vivo expansion of EPI-NCSC into millions of stem cells that takes advantage of the migratory ability of neural crest stem cells and is based on a new culture medium and the use of microcarriers. Functional improvement was shown by two independent methods, spinal somatosensory evoked potentials (SpSEP) and the Semmes-Weinstein touch test. Subsets of transplanted cells differentiated into myelinating oligodendrocytes. Unilateral injections of EPI-NCSC into the lesion of midline contused mouse spinal cords elicited bilateral improvements. Intraspinal EPI-NCSC did not migrate laterally in the spinal cord or invade the spinal roots and dorsal root ganglia, thus implicating diffusible factors. EPI-NCSC expressed neurotrophic factors, angiogenic factors, and metalloproteases. The strength of EPI-NCSC thus is that they can exert a combination of pertinent functions in the contused spinal cord, including cell replacement, neuroprotection, angiogenesis and modulation of scar formation. EPI-NCSC are uniquely qualified for cell-based therapy in spinal cord injury, as neural crest cells and neural tube stem cells share a higher order stem cell and are thus ontologically closely related.

Multiple firing patterns in deep dorsal horn neurons of the spinal cord: computational analysis of mechanisms and functional implications

Deep dorsal horn relay neurons (dDHNs) of the spinal cord are known to exhibit multiple firing patterns under the control of local metabotropic neuromodulation: tonic firing, plateau potential, and spontaneous oscillations. This work investigates the role of interactions between voltage-gated channels and the occurrence of different firing patterns and then correlates these two phenomena with their functional role in sensory information processing. We designed a conductance-based model using the NEURON software package, which successfully reproduced the classical features of plateau in dDHNs, including a wind-up of the neuronal response after repetitive stimulation. This modeling approach allowed us to systematically test the impact of conductance interactions on the firing patterns. We found that the expression of multiple firing patterns can be reproduced by changes in the balance between two currents (L-type calcium and potassium inward rectifier conductances). By investigating a possible generalization of the firing state switch, we found that the switch can also occur by varying the balance of any hyperpolarizing and depolarizing conductances. This result extends the control of the firing switch to neuromodulators or to network effects such as synaptic inhibition. We observed that the switch between the different firing patterns occurs as a continuous function in the model, revealing a particular intermediate state called the accelerating mode. To characterize the functional effect of a firing switch on information transfer, we used correlation analysis between a model of peripheral nociceptive afference and the dDHN model. The simulation results indicate that the accelerating mode was the optimal firing state for information transfer.

Injury-induced mechanical hypersensitivity requires C-low threshold mechanoreceptors

Mechanical pain contributes to the morbidity associated with inflammation and trauma, but primary sensory neurons that convey the sensation of acute and persistent mechanical pain have not been identified. Dorsal root ganglion (DRG) neurons transmit sensory information to the spinal cord using the excitatory transmitter glutamate, a process that depends on glutamate transport into synaptic vesicles for regulated exocytotic release. Here we report that a small subset of cells in the DRG expresses the low abundance vesicular glutamate transporter VGLUT3 (also known as SLC17A8). In the dorsal horn of the spinal cord, these afferents project to lamina I and the innermost layer of lamina II, which has previously been implicated in persistent pain caused by injury. Because the different VGLUT isoforms generally have a non-redundant pattern of expression, we used Vglut3 knockout mice to assess the role of VGLUT3(+) primary afferents in the behavioural response to somatosensory input. The loss of VGLUT3 specifically impairs mechanical pain sensation, and in particular the mechanical hypersensitivity to normally innocuous stimuli that accompanies inflammation, nerve injury and trauma. Direct recording from VGLUT3(+) neurons in the DRG further identifies them as a poorly understood population of unmyelinated, low threshold mechanoreceptors (C-LTMRs). The analysis of Vglut3(-/-) mice now indicates a critical role for C-LTMRs in the mechanical hypersensitivity caused by injury.

Transient receptor potential vanilloid type 1 receptor regulates glutamatergic synaptic inputs to the spinothalamic tract neurons of the spinal cord deep dorsal horn

The spinothalamic tract (STT) neurons in the spinal dorsal horn play an important role in transmission and processing of nociceptive sensory information. Although transient receptor potential vanilloid type 1 (TRPV1) receptors are present in the spinal cord dorsal horn, their physiological function is not fully elucidated. In this study, we examined the role of TRPV1 in modulating neuronal activity of the STT neurons through excitatory and inhibitory synaptic inputs. Whole-cell patch-clamp recordings were performed on STT neurons labeled by a retrograde fluorescent tracer injected into the ventral posterior lateral (VPL) nucleus of the thalamus. Capsaicin (1 microM) increased the frequency of miniature excitatory postsynaptic currents (mEPSC) without changing the amplitude or decay time constant of mEPSC. In contrast, capsaicin had no distinct effect on GABAergic miniature inhibitory postsynaptic currents (mIPSC). Capsazepine (10 microM), a TRPV1 receptor antagonist, abolished the effect of capsaicin on mEPSCs. Capsazepine itself did not affect the baseline amplitude and frequency of mEPSC. The effect of capsaicin on mEPSC was also abolished by removal of external Ca(2+), but not by treatment with Cd(2+). Furthermore, capsaicin increased the firing activity of the STT neurons and this increase in neuronal activity by capsaicin was abolished in the presence of non-N-methyl-d-aspartic acid (NMDA) and NMDA receptor antagonists, 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione (NBQX) and (R)-amino-5-phosphonovaleric acid (APV). These data suggest that activation of TRPV1 potentiates the glutamate release from excitatory terminals of primary afferent fibers and subsequently increases the neural activity of STT neurons of the rat spinal cord deep dorsal horn.

An inhibitory interaction of human cortical responses to stimuli preferentially exciting Adelta or C fibers

Finely myelinated (type Adelta) and unmyelinated (type C) fibers are the major afferent inputs to spinothalamic tract neurons mediating sensory and reflex responses to noxious and thermal stimuli. These two fiber types differ in their sensory and biophysical properties, raising questions about the interaction of their supraspinal responses. Therefore, we investigated the interaction of cortical responses to stimuli that preferentially excite these fibers in human subjects using evoked potential recordings in a paired conditioning stimulation (CS) and test stimulation (TS) paradigm. There were two experiments, one with Adelta as CS and C as TS (Adelta-C) and another with these stimuli reversed (C-Adelta). We used intra-epidermal electrical pulses applied to the dorsal left hand at 2x and 1x pinprick threshold (pp) for the preferential stimulation of Adelta fibers and 37-50 degrees C contact heat pulses applied to the left or right thenar and left hypothenar eminences for the preferential stimulation of C fibers. We found that the cortical response to preferential Adelta or C fiber stimulation was attenuated whenever either cortical response preceded the other. Standardized values of peak and integrated amplitudes were <1 in all pairing conditions and in all subjects in both experiments. The suppressive effect varied in magnitude with the intensity of the conditioning stimulus in both Adelta-C and C-Adelta experiments. Furthermore, intra-segmental interaction was differentially effective for Adelta conditioning (peak amplitude, P<0.008; analysis of variance). Our experiments provide the first neurophysiological evidence for a somatotopically distributed, mutually suppressive interaction between cortical responses to preferentially activated Adelta and C afferents in humans. This suppressive interaction of cortical responses suggests contrasting and possibly mutually exclusive sensorimotor functions mediated through the Adelta and C fiber afferent channels.

Brain activity related to temporal summation of C-fiber evoked pain

Temporal summation of "second pain" (TSSP) is considered to be the result of C-fiber-evoked responses of dorsal horn neurons, termed 'windup'. This phenomenon is dependent on stimulus frequency (0.33 Hz) and relevant for central sensitization and chronic pain. Previous brain imaging studies have only been used to characterize neural correlates of second pain but not its temporal summation. We utilized functional magnetic resonance imaging (fMRI) in healthy volunteers to measure brain responses associated with TSSP. Region of interest analysis was used to assess TSSP related brain activation. Eleven pain-free normal subjects underwent fMRI scanning during repetitive heat pulses to the right foot at 0.33 and 0.17 Hz. Stimulus intensities were adjusted to each individual's heat sensitivity to achieve comparable TSSP ratings of moderate pain in all subjects. As predicted, experimental pain ratings showed robust TSSP during 0.33 Hz but not 0.17 Hz stimuli. fMRI statistical maps identified several brain regions with stimulus and frequency dependent activation consistent with TSSP, including contralateral thalamus (THAL), S1, bilateral S2, anterior and posterior insula (INS), mid-anterior cingulate cortex (ACC), and supplemental motor areas (SMA). TSSP ratings were significantly correlated with brain activation in somatosensory areas (THAL, S1, left S2), anterior INS, and ACC. These results show that neural responses related to TSSP are evoked in somatosensory processing areas (THAL, S2), as well as in multiple areas that serve other functions related to pain, such as cognition (ACC, PFC), affect (INS, ACC, PAG), pre-motor activity (SMA, cerebellum), and pain modulation (rostral ACC).

The effects of microinjection of morphine into thalamic nucleus submedius on formalin-evoked nociceptive responses of neurons in the rat spinal dorsal horn

Previous studies have indicated that the thalamic nucleus submedius (Sm), as an ascending component, is involved in an endogenous analgesic system consisting of spinal cord-Sm-ventrolateral orbital cortex (VLO)-periaqueductal gray (PAG)-spinal cord loop. To investigate the action of opioid in this antinociception pathway, the effects of microinjection of morphine and naloxone into the Sm on the formalin-induced nociceptive responses of neurons in the spinal dorsal horn were determined in the anesthetized rat. Formalin (5%, 50 microl) subcutaneously injected into unilateral hindpaw produced a biphasic nociceptive response which was similar to that obtained from assessing the nociceptive behavior either in the relative magnitude of response or the time course. A unilateral microinjection of morphine (5 microg, 0.5 microl) into the Sm 15 min after formalin injection significantly depressed the second phasic responses of neurons induced by formalin, and this effect was significantly attenuated by pre-microinjection of opioid receptor antagonist naloxone (1 microg, 0.5 microl) into the same site. The results suggest that the Sm is involved in opioid receptor-mediated antinociceptive effect on the persistent nociception through depression of the nociceptive transmission at the spinal cord level.

Involvement of Dorsal Column Nucleus Neurons in Nociceptive Transmission in Aged Rats

To clarify the functional role of the dorsal column nucleus (DCN) in nociception in rats with advancing age, single neuronal activity and substance P–like immunoreactivity (SP-LI) of the gracile nucleus (GN) were studied in aged rats (29 to 34 mo old) and adult rats (9 to 12 mo old). A total of 122 neurons [aged: 34 wide-dynamic-range (WDR), two nociceptive-specific (NS), and 32 low-threshold mechanical (LTM) neurons; adult: 22 WDR and 32 LTM neurons] were recorded from GN. For WDR neurons, the latency to antidromic activation of the ventral posterior lateral nucleus of the thalamus showed no difference between the aged and adult rats. Sciatic nerve stimulation with C-fiber intensity induced responses of GN with significantly longer latency in aged rats than in adults, whereas there was no difference in the response latency to A-fiber intensity stimulation. Background activity and afterdischarges were significantly higher in the aged rats than those in the adult rats. Responses to noxious mechanical and thermal stimuli were significantly greater in the aged rats during application of graded stimuli. There were no significant differences in responses to nonnoxious mechanical stimulus, mechanical response threshold, and the size of the receptive fields between neurons in the aged and adult rats. The area occupied by SP-LI fibers in the GN and the size of SP-LI dorsal root ganglia neurons were significantly larger in aged rats than in adults. The present findings suggest that the hyperexcitability of GN neurons could be involved in abnormal noxious pain sensations with advancing age.

The effect of isoflurane and halothane on electroencephalographic activation elicited by repetitive noxious c-fiber stimulation

Windup is the progressive increase in neuronal response to a repetitive noxious stimulus. This response is most often observed in the spinal cord, but it is unclear how this response is manifested in supraspinal structures. We investigated the effects of isoflurane and halothane on electroencephalographic responses to repetitive noxious electrical stimuli (20 pulses at 0.1, 1 and 3 Hz) applied to the tail in rats. Halothane and isoflurane concentrations were adjusted to 0.8 and 1.2 minimum alveolar concentration (MAC), where MAC is the concentration needed to prevent gross and purposeful movement in 50% of animals. At 0.8 MAC halothane, the 3 Hz stimulus caused electroencephalographic (EEG) activation primarily by increasing the median edge frequency (MEF), while at 1.2 MAC halothane the spectral edge frequency (SEF) was increased by the 1 and 3 Hz stimuli, and the MEF was increased by the 3 Hz stimuli. At 0.8 MAC isoflurane, the 1 and 3 Hz stimuli evoked EEG activation by increasing the MEF and SEF, while at 1.2 MAC only the MEF was increased by the 1 Hz stimulus. No EEG activation occurred with the 0.1 Hz repetitive stimulus with either isoflurane or halothane. These data suggest that repetitive electrical stimulation normally associated with windup in spinal neurons can evoke EEG activation.

Dorsal horn neuron response patterns to graded heat stimuli in the rat

Sensory input from various receptors in the periphery first becomes integrated in the spinal cord dorsal horn. The response of the spinal cord dorsal horn neurons to mechanical stimuli are classified as low threshold, high threshold, and wide dynamic range neurons. However, the response pattern of deep dorsal horn cells to heat has not been well described. In this study, the response of the spinal cord dorsal horn neurons to graded heat stimuli were characterized in 147 neurons in rats by extracellular single cell recording. After a differentiable cell was identified, the Peltier heat stimulator was applied to the receptive field and the base temperature was set at 30 degrees C. The heat stimulus was delivered for 10 s from 37-51 degrees C in 2 degrees C increments, with an inter-stimulus interval of 30 s. Out of the 147 neurons, five statistically distinguishable response patterns were identified by latent class cluster analysis. It is concluded that variation of temperature may account for the observed results and indicate functionally different subsets of heat-responsive cells in the deep dorsal horn.

Somatotopic organization of human somatosensory cortices for pain: a single trial fMRI study

The ability to locate pain plays a pivotal role in immediate defense and withdrawal behavior. However, how the brain localizes nociceptive information without additional information from somatotopically organized mechano-receptive pathways is not well understood. To investigate the somatotopic organization of the nociceptive system, we applied Thulium-YAG-laser evoked pain stimuli, which have no concomitant tactile component, to the dorsum of the left hand and foot in randomized order. We used single-trial functional magnetic resonance imaging (fMRI) to assess differential hemodynamic responses to hand and foot stimulation for the group and in a single subject approach. The primary somatosensory cortex (SI) shows a clear somatotopic organization ipsi- and contralaterally to painful stimulation. Furthermore, a differential representation of hand and foot stimulation appeared within the contralateral opercular--insular region of the secondary somatosensory cortex (SII). This result provides evidence that both SI and SII encode spatial information of nociceptive stimuli without additional information from the tactile system and highlights the concept of a redundant representation of basic discriminative stimulus features in human somatosensory cortices, which seems adequate in view of the evolutionary importance of pain perception.

A critical review of the role of the proposed VMpo nucleus in pain

The evidence presented by Craig and his colleagues for an important projection from lamina I spinothalamic tract neurons to a renamed thalamic nucleus (the posterior part of the ventral medial nucleus or VMpo), as well as to the ventrocaudal medial dorsal and the ventral posterior inferior thalamic nuclei, is critically reviewed. Of particular concern is the denial of an important nociceptive lamina I projection to the ventrobasal complex. Contrary evidence is reviewed that strongly favors a role of spinothalamic projections from both lamina I and deep layers of the dorsal horn to the ventrobasal complex and other thalamic nuclei and from there to the SI and SII somatosensory cortices in the sensory-discriminative processing of pain and temperature information.

Electrophysiological changes in adult rat dorsal horn neurons after neonatal peripheral inflammation

Neonatal peripheral inflammation has been shown to produce profound anatomical changes in the dorsal horn of adult rats. In this study, we explored whether parallel physiological changes exist. Neonatal rats were injected with complete Freund's adjuvant (CFA) into the left hind paw. At 8-10 wk of age, single dorsal horn neurons were recorded in response to graded intensities of mechanical stimuli delivered to the receptive field. In addition, cord dorsum potentials, produced by electrical stimuli delivered to the left sciatic nerve at 2.5x threshold, were recorded bilaterally from L2 to S3. There were significant increases in background activity and responses to brush and pinch in neonatal rats that were treated with CFA, as compared with control rats. Further analysis showed similar significant changes when dorsal horn neurons were categorized into wide dynamic range (WDR), high-threshold (HT), and low-threshold (LT) groups. The receptive field was significantly larger in neonatally treated rats as compared with control rats. Additionally, there was a significant increase in the response to a 49 degrees C heat stimulus in neonatally treated rats as compared with control rats. There was also a trend for the amplitudes of N1, N2, and P waves of the cord dorsum potential to increase and latencies to decrease in neonatally treated rats, but no significant differences were detected between different levels of the spinal cord (L2 to S3). These data further support the notion that anatomical and physiological plasticity changes occurred in the spinal cord following early neonatal CFA treatment.

Effects of neutralizing antibodies to tumor necrosis factor-alpha on nucleus pulposus-induced abnormal nociresponses in rat dorsal horn neurons

STUDY DESIGN

The effect of an anti-tumor necrosis factor alpha (anti-TNFalpha) antibody on abnormal discharges caused by application of nucleus pulposus to the nerve root was investigated in an electrophysiologic study.

OBJECTIVES

To assess whether inhibition of TNFalpha can reduce nucleus pulposus-induced abnormal discharges.

SUMMARY OF BACKGROUND DATA

It has been shown that TNFalpha, a proinflammatory cytokine, is a key pathogenic factor in the development of nucleus pulposus-induced abnormal discharges as a pain sensation. However, the electrophysiologic mechanisms involved in sciatica after disc herniation still have not been elucidated.

METHODS

Extracellular activities of wide-dynamic-range neurons were assessed in 21 rats. Autologous nucleus pulposus harvested from the tail was applied to the L5 nerve root. The animals were simultaneously treated with antibodies to TNFalpha (anti-TNF + nucleus pulposus group) and with phosphate-buffered saline (nucleus pulposus group). As a control (control group), a similar volume of muscle was applied to the nerve root with phosphate-buffered saline. Responses of wide-dynamic-range neurons to noxious and innocuous stimuli were examined for 2 hours.

RESULTS

Discharges evoked during noxious stimulation and discharges after withdrawal of stimulation in the nucleus pulposus group were significantly higher than those in the control group (P < 0.05). In the anti-TNF + nucleus pulposus group, discharges after withdrawal of stimulation were remarkably inhibited, as compared with those of the nucleus pulposus group (P < 0.05). However, evoked discharges during stimulation apparently were not inhibited. Responses to innocuous stimulation did not change throughout the measurements.

CONCLUSIONS

These data indicate that application of TNFalpha antibodies to the nerve root partially prevents the nucleus pulposus-induced abnormal nociresponses. Therefore, anti-TNFalpha treatment may have a therapeutic effect on sciatica after lumbar disc herniation.

Changes of wide dynamic range neuronal responses to mechanical cutaneous stimuli following acute compression of the rat sciatic nerve

To investigate the changes in central neuronal activity in response to cutaneous stimulation following acute nerve compression, the responses of wide dynamic range dorsal horn neurons (n = 13) to graded mechanical cutaneous stimuli (brush, press, pinch, and squeeze) were recorded in anesthetized male Sprague-Dawley rats before and during compression, and after the release of the compression. A small vascular clip (pinching force, 120 g) was applied to compress the sciatic nerve for 30 min. In 12 neurons, background activities increased transiently for 1.5 to 4 min after nerve compression. Similarly, after the release of the compression, 7 neurons showed a transient increase in background activities. The responses to brush, press, and pinch were inhibited significantly 10 min after the nerve compression. The responses to press and pinch recovered gradually after the release of the compression. Responses to squeeze were little affected throughout the experimental period. Acute compression of the rat sciatic nerve can cause short periods of continuous increased spontaneous firing and can attenuate the dorsal horn neuronal responses to cutaneous stimuli. In particular, the neuronal responses to innocuous stimuli were more susceptible to compression of the nerve than the responses to noxious stimuli.

A comparison of modality-specific somatosensory changes during menstruation in dysmenorrheic and nondysmenorrheic women

OBJECTIVE

The objective was to evaluate somatosensory thresholds to a multimodality stimulation regimen applied both within and outside areas of referred menstrual pain in dysmenorrheic women, over four phases of confirmed ovulatory cycles, and to compare them with thresholds in nondysmenorrheic women during menstruation.

DESIGN

Twenty dysmenorrheic women with menstrual pain scoring 5.45 +/- 0.39 cm (mean +/- standard error of mean) on a visual analog scale (10 cm) participated. Fifteen nondysmenorrheic women with a menstrual pain score of 0.4 +/- 0.2 cm participated as controls. Ovulation was confirmed by an enzyme-multiplied immunoassay technique. Menstrual pain was described with the McGill Pain Questionnaire. Areas within menstrual pain referral were two abdominal sites and the midline of the low back, and the arm and thigh were the control areas. The pressure pain threshold (PPT) and pinch pain threshold were determined by a hand-held electronic pressure algometer, the heat pain threshold (HPT) by a contact thermode, and the tactile threshold with von Frey hairs.

RESULTS

In dysmenorrheic women the McGill Pain Questionnaire showed a larger sensory and affective component of pain than the evaluative and miscellaneous groups. The HPT and PPT were lower in the menstrual phase than in the ovulatory, luteal, and premenstrual phases, both within and outside areas of referred menstrual pain (p <0.01), with a more pronounced decrease at the referral pain areas. The pinch pain threshold was lower in the menstrual phase than in the ovulatory phase (p <0.02), and the tactile threshold did not differ significantly across the menstrual phases or within any site. Dysmenorrheic women had a lower HPT at the control sites and a lower PPT at the abdomen, back, and control sites, than in those of nondysmenorrheic women in the menstrual phase.

CONCLUSIONS

The results show reduced somatosensory pain thresholds during menstruation to heat and pressure stimulation, both within and outside areas of referred menstrual pain in dysmenorrheic women. Dysmenorrheic women showed a lower HPT at the control sites and a lower PPT at all the sites than those for nondysmenorrheic women in the menstrual phase. The altered somatosensory thresholds may be dependent on a spinal mechanism of central hyperexcitability, induced by recurrent moderate to severe menstrual pain.

Modular headache theory: a new approach

Many people experience headaches that do not fulfill the International Headache Society's criteria for a specific headache disorder, yet behave biologically like that disorder. Others fulfill criteria for one headache disorder but have features of another. To explain these observations, we propose that groups of neurons, called modules, become activated to produce each symptom of a primary headache disorder, and that each module is linked to other modules that together produce an individual's headache. Headaches develop phenotypic stability through a process referred to as learned stereotypy. This theory has implications for the classification, research, and treatment of primary and secondary headache patients.

Non-noxious A fiber afferent input enhances capsaicin-induced mechanical hyperalgesia in the rat

Intradermal injection of capsaicin induces primary hyperalgesia at the injection site and secondary hyperalgesia in the surrounding undamaged skin. The secondary hyperalgesia is thought to be due to central sensitization of the dorsal horn neurons while primary hyperalgesia is caused by sensitization of nociceptors in the damaged skin. In this study, we asked if additional non-noxious afferent input from the undamaged skin influences the already developed secondary hyperalgesia, which follows an intradermal injection of capsaicin. Capsaicin dissolved in olive oil was injected into the middle of the hind paw of male Sprague-Dawley rats (250-300 g) under gaseous anesthesia. This produced a decrease in the mechanical threshold at the base of the toes for hind limb withdrawals lasting for 1-2h, thus showing a short-lasting (hours) secondary hyperalgesia. When the capsaicin injection was immediately followed by repeated non-noxious mechanical stimuli or weak electrical stimuli (A fiber strength) applied to the area of secondary hyperalgesia (toes) for 30 min, the reduction of the mechanical threshold lasted longer than 24h. These results suggest that non-noxious A fiber afferent input can powerfully modulate central sensitization in the spinal dorsal horn, causing the duration of the secondary hyperalgesia to be greatly extended.

Modular headache theory

Many people experience headaches that do not fulfil the International Headache Society's criteria for a specific headache disorder yet behave biologically like that disorder. Others fulfil criteria for one headache disorder and yet have features of another disorder. To explain these observations, we propose that groups of neurones called modules become activated to produce each symptom of a primary headache disorder, and that each module is linked to other modules that together produce an individual's headache. This theory has implications for the classification, research and treatment of primary and secondary headache patients.

Sustained L-type calcium currents in dissociated deep dorsal horn neurons of the rat: characteristics and modulation

Deep dorsal horn neurons present plateau properties involved in non-linear integration of nociceptive inputs, in the windup of the discharge, and in the expression of long-lasting afterdischarges. In vitro experiments using intracellular recordings in a slice preparation of the rat spinal cord have established that they are supported in part by voltage-dependent calcium currents, and positively modulated by metabotropic glutamate receptor activation. In the present study, whole-cell patch-clamp recordings in acutely isolated soma of dorsal horn neurons (n=48) were used to analyse the voltage-dependent calcium currents involved.Deep dorsal horn neurons expressed both inactivating and non-inactivating calcium currents with Ca(2+) or Ba(2+) used as a charge carrier. The non-inactivating component activated at intermediate threshold (-55mV), and was blocked mostly by nifedipine (61+/-6%). Although voltage-dependent facilitation of whole-cell calcium currents could be obtained by prepulses to +100mV, repetitive depolarization at potentials compatible with the plateau (-45mV and -10mV) failed to induce facilitation of calcium currents. No direct modulation of somatic calcium currents by application of (S)-3,5-dihydroxyphenylglycine, a selective group I metabotropic glutamate receptor agonist and 1S,3R-1-amino-1,3-cyclopentanedicarboxylic acid, a group I and II metabotropic glutamate receptor agonist, was found, while application of the metabotropic GABA(B) receptor agonist baclofen induced a significant decrease in calcium currents.Thus, the present voltage-clamp study shows that rat deep dorsal horn neurons express a non-inactivating, nifedipine sensitive, intermediate threshold (-55mV) calcium current which could provide the depolarizing drive to generate plateau potentials near threshold. Our results also indicate that calcium currents are not sensitized following repetitive stimulation, and not modulated by metabotropic glutamate receptor activation. They provide, however, the first evidence for a direct modulation of voltage-gated calcium channels in dorsal horn neurons by GABA(B) receptor activation, which may contribute to the mechanism of baclofen's antinociceptive activity.

Serotonin Increases the Incidence of Primary Afferent-Evoked Long-Term Depression in Rat Deep Dorsal Horn Neurons

5-hydroxytryptamine (5-HT) is released in spinal cord by descending systems that modulate somatosensory transmission and can potently depress primary afferent-evoked synaptic responses in dorsal horn neurons. Since primary afferent activity-induced long-term potentiation (LTP) may contribute to central sensitization of nociception, we studied the effects of 5-HT on the expression of sensory-evoked LTP and long-term depression (LTD) in deep dorsal horn (DDH) neurons. Whole cell, predominantly current clamp, recordings were obtained from DDH neurons in transverse slices of neonatal rat lumbar spinal cord. The effect of 5-HT on dorsal-root stimulation-evoked synaptic responses was tested before, during, or after high-frequency conditioning stimulation (CS). In most cells (80%), 5-HT caused a depression of the naı̈ve synaptic response. Even though 5-HT depressed evoked responses, CS in the presence of 5-HT was not only still capable of inducing LTD but also increased its incidence from 54% in controls to 88% ( P < 0.001). Activation of ligands selective for 5-HT1A/1B and 5-HT1B, but not 5-HT2A/2C or 5-HT3receptors, best reproduced these actions. 5-HT also potently depressed postconditioning synaptic responses regardless of whether the induced plasticity was LTP or LTD. Our results demonstrate that in addition to depressing the amplitude of evoked sensory input, 5-HT can also control the direction of its long-term modifiability, favoring the expression of LTD. These findings demonstrate cellular mechanisms that may contribute to the descending serotonergic control of nociception.

Projections from the marginal zone and deep dorsal horn to the ventrobasal nuclei of the primate thalamus

It has been concluded recently that if a projection from the marginal zone to the ventral posterior lateral (VPL) nucleus exists, it is sparse. Given the importance of the marginal zone in nociception, this conclusion has raised doubts about the significance of the role of the ventrobasal complex in nociception. We have reexamined this projection using injections of the retrograde tracer, cholera toxin subunit B, into one side of the lateral thalamus in macaque monkeys. The injections were confined to the ventrobasal complex (with minimal spread to adjacent nuclei that do not receive spinal projections) in two animals. Many retrogradely labeled neurons were found in lamina I (as well as in lamina V) of the contralateral spinal and medullary dorsal horn. The results are consistent with the view that neurons in the marginal zone contribute prominently to the spinothalamic and trigeminothalamic projections to the VPL and ventral posterior medial (VPM) nuclei. This pathway is likely to be important for the sensory-discriminative processing of nociceptive information with respect to the location and intensity of painful stimuli.

Wind-up of spinal cord neurones and pain sensation: much ado about something?

Wind-up is a frequency-dependent increase in the excitability of spinal cord neurones, evoked by electrical stimulation of afferent C-fibres. Although it has been studied over the past thirty years, there are still uncertainties about its physiological meaning. Glutamate (NMDA) and tachykinin NK1 receptors are required to generate wind-up and therefore a positive modulation between these two receptor types has been suggested by some authors. However, most drugs capable of reducing the excitability of spinal cord neurones, including opioids and NSAIDs, can also reduce or even abolish wind-up. Thus, other theories involving synaptic efficacy, potassium channels, calcium channels, etc. have also been proposed for the generation of this phenomenon. Whatever the mechanisms involved in its generation, wind-up has been interpreted as a system for the amplification in the spinal cord of the nociceptive message that arrives from peripheral nociceptors connected to C-fibres. This probably reflects the physiological system activated in the spinal cord after an intense or persistent barrage of afferent nociceptive impulses. On the other hand, wind-up, central sensitisation and hyperalgesia are not the same phenomena, although they may share common properties. Wind-up can be an important tool to study the processing of nociceptive information in the spinal cord, and the central effects of drugs that modulate the nociceptive system. This paper reviews the physiological and pharmacological data on wind-up of spinal cord neurones, and the perceptual correlates of wind-up in human subjects, in the context of its possible relation to the triggering of hyperalgesic states, and also the multiple factors which contribute to the generation of wind-up.

Physiological properties of the lamina I spinoparabrachial neurons in the rat

Single-unit extracellular recordings of spino-parabrachial (spino-PB) neurons (n = 53) antidromically driven from the contralateral parabrachial (PB) area were performed in the lumbar cord in anesthetized rats. All the spino-PB neurons were located in the lamina I of the dorsal horn. Their axons exhibited conduction velocities between 2.8 and 27.8 m/s, in the thin myelinated fibers range. They had an extremely low spontaneous activity (median = 0. 064 Hz) and a small excitatory receptive field (</=2 toes or pads). They were all activated by both peripheral A (mainly Adelta) and C fibers after intense transcutaneous electrical stimulation. Their discharge always increased in response to noxious natural stimuli of increasing intensities. The great majority (75%) of spino-PB neurons were nociceptive specific, i.e., they were excited only by noxious stimuli. The remaining (25%) still were excited primarily by noxious stimuli but also responded moderately to innocuous stimuli. Almost all spino-PB neurons (92%, 49/53) were activated by both mechanical and heat noxious stimuli. Among them, 35% were in addition moderately activated by noxious cold (thresholds between +20 and -10 degrees C). Only (8%, 4/53) responded exclusively to noxious heat. Spino-PB neurons clearly encoded the intensity of mechanical (n = 39) and thermal (n = 38) stimuli in the noxious range, and most of the individual stimulus-response functions were monotonic and positive up to 40/60 N. cm(-2) and 50 degrees C, respectively. For the mechanical modality, the mean threshold was 11.5 +/- 1.25 N. cm(-2) (mean +/- SE), the response increased almost linearly with the logarithm of the pressure between 10 and 60 N. cm(-2), the mean p(50) (pressure evoking 50% of the maximum response) and the maximum responsiveness were: 30 +/- 2.4 N. cm(-2) and 40.5 +/- 5 Hz, respectively. For the thermal modality, the mean threshold was 43.6 +/- 0.5 degrees C, the mean curve had a general sigmoid aspect, the steepest portion being in the 46-48 degrees C interval, the mean t(50) and the maximum responsiveness were: 47.4 +/- 0.3 degrees C and 40 +/- 4.4 Hz, respectively. Most of the spino-PB neurons tested (13/16) had their noxiously evoked responses clearly inhibited by heterotopic noxious stimuli. The mean response to noxious stimuli during heterotopic stimuli was 31.7 +/- 6.1% of the control response. We conclude that the nociceptive properties of the lamina I spino-PB neurons are reflected largely by those of PB neurons that were suggested to be involved in autonomic and emotional/aversive aspects of pain.

The effect of morphine on responses of nucleus ventroposterolateralis neurons to colorectal distension in the rat

In 71 halothane-anesthetized rats, we characterized the responses of single neurons in the nucleus ventroposterolateralis (VPL) of the thalamus to a noxious visceral stimulus (colorectal balloon distension; CRD) and studied the effects of intravenous morphine on these responses using standard extracellular microelectrode recording techniques. One hundred nine neurons were isolated on the basis of spontaneous activity. Sixty-four (59%) responded to CRD, of which 52 (81 %) had excitatory and 12 (19%) had inhibitory responses. Neurons showed graded responses to graded CRD pressures (20-100 mmHg), with maximum excitation or inhibition occurring at 80 mmHg. Responses to noxious (pinch, heat) and innocuous (brush, tap) cutaneous stimuli were studied in 95 of the VPL neurons isolated. Eighty-three of these neurons (48 CRD responsive and 35 CRD nonresponsive) (87%) had cutaneous receptive fields, of which 96% were small and contralateral and 4% were large and contralateral or bilateral. Ninety-four percent of these neurons responded to both noxious and innocuous cutaneous stimulation, and 6% responded to only noxious stimulation. No neurons responded solely to innocuous stimulation. Cumulative doses of morphine (0.125, 0.25, 0.5, 1, and 2 mg/kg, i.v) produced statistically significant dose-dependent attenuation of neuronal responses to CRD. Naloxone (0.4 mg/ kg, i.v.) reversed the effects of morphine. Morphine and naloxone had no significant effects on spontaneous activity. These data support the involvement of VPL neurons in visceral nociception and are consistent with a role of VPL in sensory-discriminative aspects of nociception.

Branching and/or collateral projections of spinal dorsal horn neurons

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

C fiber responses of wide dynamic range neurons in the spinal dorsal horn

Responses of wide dynamic range neurons in the spinal dorsal horn to input from C fibers with various conduction velocities were analyzed. The wide dynamic range neurons studied were located in the laminae IV-VI of the spinal dorsal horn. The C fiber response to stimulation of the superficial peroneal nerve consisted of three components: early, middle, and late. The separation into three components was found to be caused by asynchronous volleys in three different classes of C fibers in the superficial peroneal nerve. The phenomenon of windup was observed to occur always in the late component, frequently in the middle component, and to a far lesser extent in the early component. The early component was augmented by sciatic nerve compression, indicating that the initial part of the C fiber response is suppressed within the spinal cord by an inhibitory effect of A fiber afferent volleys. An intravenous administration of a specific N-methyl-D-aspartate receptor antagonist, MK-801 hydrogen maleate, significantly suppressed the middle and late components of the C fiber response, although the effect on the early component was insignificant. Thus, one can infer that the hyperexcitability of wide dynamic range neurons to input from early components of cutaneous C fiber stimuli will yield hyperpathic symptoms such as paresthesia, hyperalgesia, and allodynia associated with damaged peripheral nerves, and that learning how to inhibit the hyperexcitability of wide dynamic range neurons to input from early components of C fiber stimuli will lead to the curative treatment for neuropathic pain.

The effects of strychnine, bicuculline, and ketamine on 'immersion-inhibited' dorsal horn convergent neurons in intact and spinalized rats

In both intact and spinalized rats, this study examined the effects of strychnine (a glycine antagonist), bicuculline (a GABAA antagonist), and ketamine (a non-competitive NMDA receptor antagonist) on one particular class of lumbar dorsal horn convergent neurons. This group of convergent neurons are inhibited when a rat's entire ipsilateral hindpaw is immersed in 50 degrees C water and has a strong afterdischarge as soon as the paw is removed from the water. Strychnine (2 mg/kg, iv) increased ongoing activity and blocked the 'inhibition phase' in both intact and spinalized rats demonstrating that a spinal-related glycine mechanism was involved in the inhibition. However, only in intact rats did the firing rate of the 'afterdischarge phase' increase significantly from pre-drug levels, suggesting that supraspinal sites may be involved in modulating this phase. Ketamine (15 mg/kg, iv) depressed ongoing activity and the firing rate in the afterdischarge phase of these neurons. Additionally, ketamine reversed the strychnine-induced increase in ongoing activity. Bicuculline (2 mg/kg, iv) had no effect on the activity of this cell class. As shown previously, and replicated here, these 'immersion-inhibited' neurons invariably have both inhibitory and excitatory mechano-receptive fields on the ipsilateral hindpaw. Thus, the response of this class of convergent neurons to noxious stimulation may be a function of relative inputs of glycine and EAA's, each possibly triggered by the stimulation of different receptive fields/regions on the same paw. Furthermore, when both fields are co-stimulated during noxious immersion of the entire paw, glycine has a stronger influence on activity than does the EAA's.

Neuroanatomy of the pain system and of the pathways that modulate pain

We review many of the recent findings concerning mechanisms and pathways for pain and its modulation, emphasizing sensitization and the modulation of nociceptors and of dorsal horn nociceptive neurons. We describe the organization of several ascending nociceptive pathways, including the spinothalamic, spinomesencephalic, spinoreticular, spinolimbic, spinocervical, and postsynaptic dorsal column pathways in some detail and discuss nociceptive processing in the thalamus and cerebral cortex. Structures involved in the descending analgesia systems, including the periaqueductal gray, locus ceruleus, and parabrachial area, nucleus raphe magnus, reticular formation, anterior pretectal nucleus, thalamus and cerebral cortex, and several components of the limbic system are described and the pathways and neurotransmitters utilized are mentioned. Finally, we speculate on possible fruitful lines of research that might lead to improvements in therapy for pain.

Changes in responses of wide dynamic range neurons in the spinal dorsal horn after dorsal root or dorsal root ganglion compression

STUDY DESIGN

The electrophysiologic responses of wide dynamic range neurons in the spinal dorsal horn by compression of the dorsal root or of the dorsal root ganglion were investigated.

OBJECTIVES

This study identified differences between the compression of the dorsal root against the compression of the dorsal root ganglion by examining the responses of wide dynamic range neurons.

SUMMARY OF BACKGROUND DATA

The wide dynamic range neurons studied were known to be excited by primary afferent fibers, not only combined A delta and C-nociceptive fibers but also low threshold mechanoreceptive A beta fibers and A delta fibers of down hairs. Thus, the wide dynamic range neurons are classified as nociceptive neurons.

METHODS

Extracellular activities of 32 wide dynamic range neurons were recorded from the laminae 4-6 of the seventh lumbar cord in anesthetized cats. A microvessel clip (40 g) was applied to compress the dorsal root or the dorsal root ganglion.

RESULTS

Dorsal root compression produced only an initial burst (about 10-40 seconds). Prolonged repetitive firings were rarely maintained. In contrast, dorsal root ganglion compression resulted in a maintained repetitive firing throughout the period of compression. After release of compression of the dorsal root and the dorsal root ganglion, responses to brushing were facilitated, and the low threshold center of the receptive field expanded.

CONCLUSIONS

These findings are consistent with the previous report that the radicular pain associated with a herniated intervertebral disc initially results from compression of the dorsal root ganglion.

Multisensory convergence and integration in the neostriatum and globus pallidus of the rat

The extracellular response properties of neurons in the caudate-putamen (CPu), globus pallidus (GP) and lateral amygdaloid nucleus (La) evoked by auditory and somatosensory stimuli were investigated. A total of 61 neurons in these areas responded either singly to somatosensory stimulation (unisensory), or to both somatosensory and auditory stimulation (multisensory). Higher rates of somatosensory stimulation reduced the response magnitude of CPu neurons more than that of GP neurons. In multisensory neurons, combined somatosensory and auditory stimulation compared to unisensory stimulation resulted in three characteristic response patterns: enhancement, depression or interaction. Temporal misalignment of the peak frequency latencies evoked by auditory and somatosensory stimulation altered the response magnitude in the majority of neurons. The response properties and anatomical connectivity of CPu and GP neurons suggest that the observed multisensory integrative effects may be used to facilitate motor responses to low intensity stimuli.

Intravenous midazolam suppresses noxiously evoked activity of spinal wide dynamic range neurons in cats

The effects of intravenously (i.v.) administered midazolam on noxiously evoked activity of spinal wide dynamic range (WDR) neurons were investigated in decerebrate, spinal-cord-transected cats. Extracellular, single-unit recordings were measured during stimulation by pinching the receptive field on the hind paw and the effect of midazolam at doses of 0.25, 0.5, 1, 2, and 4 mg/kg were measured. Two series of experiments were performed to characterize the analgesic effects of midazolam. In the first, dose-response experiments (n = 59) demonstrated a dose-dependent suppression of the noxiously evoked activity of spinal WDR neurons after midazolam administration. This effect of midazolam was maximal at a dose of 1 mg/kg i.v.. The second series of experiments (n = 14) demonstrated that a benzodiazepine antagonist, flumazenil (n = 8), promptly reversed the effect of midazolam, while an opioid antagonist, naloxone (n = 6), had no effect on the effect of midazolam. The present study demonstrates that i.v. administered midazolam suppresses noxiously evoked activity of spinal WDR neurons that is reversible by a benzodiazepine antagonist. This is consistent with an analgesic action of midazolam.

The effect of trans-ACPD, a metabotropic excitatory amino acid receptor agonist, on the responses of primate spinothalamic tract neurons

The responses of primate spinothalamic tract (STT) neurons to innocuous and noxious mechanical stimuli applied to the skin can be enhanced for more than an hour following prolonged noxious stimulation. This increased responsiveness is thought to reflect sensitization of dorsal horn neurons and may help account for secondary hyperalgesia and mechanical allodynia. The proposal that central sensitization is due to the activation of second messenger system was tested in this study by examining the effect of trans-ACPD (trans-D,L-1-amino-1,3-cyclopentanedicarboxylic acid), an agonist of metabotropic excitatory amino acid (EAA) receptors, introduced into the dorsal horn by microdialysis. A low dose of trans-ACPD resulted in an increase in the responses of STT cells to an innocuous mechanical stimulus (BRUSH), but no increase in the responses to noxious mechanical and thermal stimuli or in the excitation produced by iontophoretically applied EAAs. A high dose of trans-ACPD caused a transient increase in background activity, but no change in the responsiveness of spinothalamic cells to any of the test stimuli. It is concluded that low doses of trans-ACPD can selectively enhance transmission through interneuronal pathways mediating tactile inputs to spinothalamic cells.

Chronic effects of topical application of capsaicin to the sciatic nerve on responses of primate spinothalamic neurons

The responses of 144 spinothalamic tract (STT) cells were recorded in 15 anesthetized macaque monkeys (Macaca fascicularis). Three to 4 weeks prior to the acute experiment, the sciatic nerve was surgically exposed on one or both sides so that capsaicin or vehicle could be applied. Responses of STT cells recorded in 3 experimental groups were compared: untreated (21 cells), vehicle-treated (40 cells), and capsaicin-treated (83 cells). The background activity of cells in the vehicle- and capsaicin-treated groups was the same as in the untreated group (that is, cells on the side contralateral to surgery). Responses to innocuous (BRUSH) and noxious (PINCH) mechanical stimuli were unchanged by vehicle or by capsaicin treatment. However, responses to other noxious (PRESSURE and SQUEEZE) mechanical stimuli were significantly increased in the vehicle-treated group. Compared with a large reference population, all experimental groups showed a significant increase in overall responsiveness to mechanical stimuli (as determined by cluster analysis), greatest in the vehicle-treated group. Responses to noxious heat stimuli were significantly reduced in the capsaicin-treated group for 45 degrees C and 47 degrees C stimuli. Volleys in A fibers, probably A delta fibers, evoked prolonged responses in many STT cells of all treatment groups. Electron microscopic counts of axons in the sciatic nerves of animals treated with capsaicin showed a reduced number of C fibers but no appreciable loss of myelinated axons. This loss of unmyelinated sensory fibers was presumably responsible for the reduction in the responses of the STT cells to noxious heat stimuli. Increased responses to some noxious mechanical stimuli and to A fiber volleys may have been the consequence of several factors, including surgical manipulation, a chemical action of vehicle and a contralateral action of capsaicin treatment.

Contribution of central neuroplasticity to pathological pain: review of clinical and experimental evidence

Peripheral tissue damage or nerve injury often leads to pathological pain processes, such as spontaneous pain, hyperalgesia and allodynia, that persist for years or decades after all possible tissue healing has occurred. Although peripheral neural mechanisms, such as nociceptor sensitization and neuroma formation, contribute to these pathological pain processes, recent evidence indicates that changes in central neural function may also play a significant role. In this review, we examine the clinical and experimental evidence which points to a contribution of central neural plasticity to the development of pathological pain. We also assess the physiological, biochemical, cellular and molecular mechanisms that underlie plasticity induced in the central nervous system (CNS) in response to noxious peripheral stimulation. Finally, we examine theories which have been proposed to explain how injury or noxious stimulation lead to alterations in CNS function which influence subsequent pain experience.