Menthol-induced Ca2+ release from presynaptic Ca2+ stores potentiates sensory synaptic transmission

Menthol and many of its derivatives produce profound sensory and mental effects. The receptor for menthol has been cloned and named cold- and menthol-sensitive receptor-1 (CMR1) or transient receptor potential channel M8 (TRPM8) receptor. Using a dorsal root ganglion (DRG) and dorsal horn (DH) coculture system as a model for the first sensory synapse in the CNS, we studied menthol effects on sensory synaptic transmission and the underlying mechanisms. We found that menthol increased the frequency of miniature EPSCs (mEPSCs). The effects persisted under an extracellular Ca2+-free condition but were abolished by intracellular BAPTA and pretreatment with thapsigargin. Menthol-induced increases of mEPSC frequency were blocked by 2-aminoethoxydiphenylborane (2-APB) but not affected by the phospholipase C inhibitor U73122 [GenBank] or by the cADP receptor inhibitor 8-bromo-cADPR (8Br-cADPR). Double-patch recordings from DRG-DH pairs showed that menthol could potentiate evoked EPSCs (eEPSCs) and change the paired-pulse ratio of eEPSCs. A Ca2+ imaging study on DRG neurons demonstrated that menthol could directly release Ca2+ from intracellular Ca2+ stores. Menthol-induced Ca2+ release was abolished by 2-APB but not affected by U73122 [GenBank] or 8Br-cADPR. Taken together, our results indicate that menthol can act directly on presynaptic Ca2+ stores of sensory neurons to release Ca2+, resulting in a facilitation of glutamate release and a modulation of neuronal transmission at sensory synapses. Expression of TRPM8 receptor on presynaptic Ca2+ stores, a novel localization for this ligand-gated ion channel, is also strongly suggested.

NR2B containing NMDA receptor dependent windup of single spinal neurons

Windup, the frequency dependent build-up of spinal neuronal responses, is implicated in the development of central sensitization of nociceptive pathways. N-methyl-D-aspartate (NMDA) receptors have been shown to be involved in these processes but the role of various receptor subtypes at the spinal level is not fully understood. In our experiments, we compared the inhibitory effect of MK-801 (a nonselective NMDA receptor antagonist, 0.01-3 mg/kg i.v.) and CI-1041 (an NR2B subunit specific NMDA receptor antagonist, 0.3-10 mg/kg i.v.) on the formation of dorsal horn neuronal windup in spinalized rats, in vivo. Both types of antagonist blocked windup considerably at doses not affecting the normal synaptic transmission. These results are in agreement with the well-documented effectivity of NR2B subtype selective NMDA receptor antagonists in chronic pain models and give the first direct evidence that spinal mechanisms are involved in this effect.

Nociceptive response to innocuous mechanical stimulation is mediated via myelinated afferents and NK-1 receptor activation in a rat model of neuropathic pain

Peripheral nerve injury in humans can produce a persistent pain state characterized by spontaneous pain and painful responses to normally innocuous stimuli (allodynia). Here we attempt to identify some of the neurophysiological and neurochemical mechanisms underlying neuropathic pain using an animal model of peripheral neuropathy induced in male Sprague-Dawley rats by placing a 2-mm polyethylene cuff around the left sciatic nerve according to the method of Mosconi and Kruger. von Frey hair testing confirmed tactile allodynia in all cuff-implanted rats before electrophysiological testing. Rats were anesthetized and spinalized for extracellular recording from single spinal wide dynamic range neurons (L(3-4)). In neuropathic rats (days 11-14 and 42-52 after cuff implantation), ongoing discharge was greater and hind paw receptive field size was expanded compared to control rats. Activation of low-threshold sensory afferents by innocuous mechanical stimulation (0.2 N for 3 s) in the hind paw receptive field evoked the typical brief excitation in control rats. However, in neuropathic rats, innocuous stimulation also induced a nociceptive-like afterdischarge that persisted 2-3 min. This afterdischarge was never observed in control rats, and, in this model, is the distinguishing feature of the spinal neural correlate of tactile allodynia. Electrical stimulation of the sciatic nerve at 4 and at 20 Hz each produced an initial discharge that was identical in control and in neuropathic rats. This stimulation also produced an afterdischarge that was similar at the two frequencies in control rats. However, in neuropathic rats, the afterdischarge produced by 20-Hz stimulation was greater than that produced by 4-Hz stimulation. Given that acutely spinalized rats were studied, only peripheral and/or spinal mechanisms can account for the data obtained; as synaptic responses from C fibers begin to fail above approximately 5-Hz stimulation [Pain 46 (1991) 327], the afterdischarge in response to 20-Hz stimulation suggests a change mainly in myelinated afferents and a predominant role of these fibers in eliciting this afterdischarge. These data are consistent with the suggestion that peripheral neuropathy induces phenotypic changes predominantly in myelinated afferents, the sensory neurons that normally respond to mechanical stimulation. The NK-1 receptor antagonist, CP-99,994 (0.5 mg/kg, i.v.), depressed the innocuous pressure-evoked afterdischarge but not the brief initial discharge of wide dynamic range neurons, and decreased the elevated ongoing rate of discharge in neuropathic rats. These results support the concept that following peripheral neuropathy, myelinated afferents may now synthesize and release substance P. A result of this is that tonic release of substance P from the central terminals of these phenotypically altered neurons would lead to ongoing excitation of NK-1-expressing nociceptive spinal neurons. In addition, these spinal neurons would also exhibit exaggerated responses to innocuous pressure stimulation. The data in this study put forth a possible neurophysiological and neurochemical basis of neuropathic pain and identify substance P and the NK-1 receptor as potential neurochemical targets for its management.

Altered visceral sensation in response to somatic pain in the rat

BACKGROUND & AIMS

Patients with fibromyalgia commonly have symptoms of abdominal pain, suggesting that altered somatic afferent activity may influence visceral sensations. It is hypothesized that a noxious somatic stimulus increases input to the projection neurons in the dorsal horn, resulting in visceral hyperalgesia.

METHODS

Two injections (100 microL, pH 4.0) were given unilaterally in the gastrocnemius muscle 2 days apart in male Sprague-Dawley rats. Paw withdrawal reflex (PWR) was measured to assess somatic pain. The control group received pH 7.2 saline injections. Similar injections (pH 4.0) were given in the front leg in a different group. Electromyography (EMG) from the external oblique muscle was recorded to graded colorectal distention at different time intervals. NMDA receptor antagonist (CGS-19755, 20 nmol) or AMPA/kainate receptor antagonist (NBQX, 20 nmol) was injected intrathecally before low-pH injections.

RESULTS

A bilateral decrease in PWR threshold occurred 72 hours after the second low-pH injection. There was no decrease in the threshold in rats injected with pH 7.2 saline. A significant increase in EMG to colorectal distention (> or =30 mm Hg) occurred at 72 hours and 2 weeks in the pH 4.0 group. No change in EMG was observed following 2 unilateral low-pH injections in the front leg. Both the visceral hyperalgesia and the decrease in somatic pain thresholds were prevented by prior intrathecal CGS-19755 or NBQX injections.

CONCLUSIONS

Noxious somatic afferent input from the hind limb facilitates visceral hyperalgesia, which is due to viscerosomatic convergence in the lower spinal cord. This can be blocked by ionotropic glutamate receptor antagonists.

Gabapentin may inhibit synaptic transmission in the mouse spinal cord dorsal horn through a preferential block of P/Q-type Ca2+ channels

Gabapentin is a lipophilic analog of gamma-amino butyric acid (GABA) with therapeutic activity against certain forms of epilepsy and neuropathic pain. Despite its structural similarity to GABA, it does not bind GABAA or GABAB receptors and the mechanism, especially of its analgesic action, has remained elusive. Here, we have studied its effects on synaptic transmission mediated by the major spinal fast excitatory and inhibitory neurotransmitters, L-glutamate and glycine, in the superficial layers of the spinal cord dorsal horn, a CNS area, which is critically involved in nociception. Gabapentin reversibly reduced evoked excitatory postsynaptic currents mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA-EPSCs) and inhibitory postsynaptic currents mediated by glycine (gly-IPSCs). Inhibition of AMPA-EPSCs and gly-IPSCs occurred with similar potencies (approximately 10-50 nM) and by about the same degree (approximately 40% at 1 microM). Gabapentin did not affect membrane currents elicited by exogenously applied glutamate or glycine arguing against a postsynaptic site of action. Selective blockade of N-type Ca2+ channels with omega-conotoxin GVIA dramatically increased and blockade of P/Q-type channels with omega-agatoxin IVA strongly attenuated inhibition of evoked synaptic transmission by gabapentin. These results show that gabapentin affects both excitatory and inhibitory spinal neurotransmission via a presynaptic mechanism which preferentially involves P/Q-type Ca2+ channels.

Meperidine Suppresses the Excitability of Spinal Dorsal Horn Neurons

Background

In addition to local anesthetics, meperidine has been successfully used for local anesthesia. When applied intrathecally, the dorsal horn neurons of the superficial laminae are exposed to high concentrations of meperidine. These cells represent an important point for the transmission of pain information. This study investigated the blocking effects of meperidine on different ionic currents of spinal dorsal horn neurons and, in particular, its impact on the generation of action potentials.

Methods

Using a combination of the patch clamp technique and the entire soma isolation method, the action of meperidine on voltage-gated Na+ and K+ currents in spinal dorsal horn neurons of rats was described. Current clamp recordings from intact neurons showed the functional relevance of the ion current blockade for the generation of action potentials.

Results

Externally applied meperidine reversibly blocked voltage-gated Na+ currents with a half-maximum inhibiting concentration (IC50) of 112 microM. During repetitive stimulation, a slight phasic block occurred. In addition, A-type K+ currents and delayed-rectifier K+ currents were affected in a dose-dependent manner, with IC50 values of 102 and 52 microM, respectively. In the current clamp mode, single action potentials were suppressed by meperidine. The firing frequency was lowered to 54% at concentrations (100 microM) insufficient for the suppression of a single action potential.

Conclusions

Meperidine inhibits the complex mechanism of generating action potentials in spinal dorsal horn neurons by the blockade of voltage-gated Na+ and K+ channels. This can contribute to the local anesthetic effect of meperidine during spinal anesthesia.

Comparison of responses of primate spinothalamic tract neurons to pruritic and algogenic stimuli

We investigated the role of mechanosensitive spinothalamic tract (STT) neurons in mediating 1) the itch evoked by intradermal injection of histamine, 2) the enhanced sense of itch evoked by innocuous stroking (alloknesis), and 3) the enhanced pain evoked by punctate stimulation (hyperalgesia) of the skin surrounding the injection site. Responses to intradermal injections of histamine and capsaicin were compared in STT neurons recorded in either the superficial or the deep dorsal horn of the anesthetized monkey. Each neuron was identified by antidromic activation from the ventral posterior lateral nucleus of thalamus and classified by its initial responses to mechanical stimuli as wide dynamic range (WDR) or high-threshold (HT). Approximately half of the WDRs and one of the HTs responded weakly to histamine, some with a duration > 5 min, the maximal time allotted. WDRs but not HTs exhibited a significant increase in response to punctate stimulation after histamine consistent with their possible role in mediating histamine-induced hyperalgesia. Neither type of neuron exhibited significant changes in response to stroking, consistent with their unlikely role in mediating alloknesis. Furthermore, nearly all STT neurons exhibited vigorous and persistent responses to capsaicin, after which they became sensitized to stroking and to punctate stimulation. We conclude that the STT neurons in our sample are more likely to contribute to pain, allodynia, and hyperalgesia than to itch and alloknesis.

[Role of phospho-calcium/ calmodulin-dependent protein kinase II in the induction and maintenance of long-term potentiation of C-fiber-evoked field potentials in spinal dorsal horn of the rat]

Our previous studies have shown that long-term potentiation (LTP) of C-fiber-evoked field potentials in the spinal dorsal horn is NMDA receptor dependent. It is known that elevation of Ca(2+) in the postsynaptic neurons through NMDA receptor channels during high-frequency stimulation of the afferent fibers is crucial for LTP induction, but how this leads to a prolonged potentiation of synaptic transmission in the spinal dorsal horn is not clear. In the hippocampus, a rise of Ca(2+) activates calcium/calmodulin-dependent protein kinase II (CaMK II) through autophosphorylation. Once this occurs, the kinase remains active, even when Ca(2+) concentration returns to baseline level. Phosphorylated CaMK II potentiates synaptic transmission by enhancement of AMPA receptor channel function via phosphorylation of GluR1 subunit of the receptor and the addition of AMPA receptors to synapses. Up to now, the role of CaMK II in the induction and maintenance of LTP of the C-fiber-evoked field potentials in spinal dorsal horn has not been evaluated. In the present study, we examined the expression of CaMK II and phospho-CaMK II in the lumbar segments (L4-L6) of the rat spinal dorsal horn at 30 min and 3 h after the establishment of LTP induced by tetanic electrical stimulation of the sciatic nerve (40 V, 0.5 ms pulses at 100 Hz for 1 s repeated four times at 10 s intervals) by using Western blot and electrophysiological techniques. To determine the role of the phospho-CaMK II in the induction and maintenance of the spinal LTP, a selective CaMK II inhibitor KN-93 (100 micromol/L) was applied directly onto the spinal cord at the recording segments before and after LTP induction. We found that (1) the protein level of phospho-CaMKII increased at both 30 min and 3 h after LTP induction, while the total protein level of CaMK II increased at 3 h but not at 30 min after LTP induction. (2) Spinal application of KN-93 at 30 min prior to the tetanus blocked both LTP induction and the increase in phospho-CaMK II. (3) 30 min after LTP induction, spinal application of KN-93 depressed LTP and the level of phospho-CaMK II (n=3). (4) Spinal application of KN-93 at 3 h after LTP, however, affected neither the amplitude of the spinal LTP nor the level of phospho-CaMK II in the spinal dorsal horn. These results suggest that activation of CaMK II is probably crucial for the induction and the early-phase maintenance of LTP of C-fiber-evoked field potentials in the spinal dorsal horn.

Alterations in dorsal horn neurones in a rat model of cancer-induced bone pain

Cancer-induced bone pain is a major clinical problem. A rat model based on intra-tibial injection of MRMT-1 mammary tumour cells was used to mimic progressive cancer-induced bone pain. At the time of stable behavioural changes (decreased thresholds to mechanical and cold stimuli) and bone destruction, in vivo electrophysiology was used to characterize natural (mechanical, thermal, and cold) and electrical-evoked responses of superficial and deep dorsal horn neurones in halothane-anaesthetized rats. Receptive field size was significantly enlarged for superficial neurones in the MRMT-1 animals. Superficial cells were characterised as either nociceptive specific (NS) or wide dynamic range (WDR). The ratio of WDR to NS cells was substantially different between sham operated (growth media alone) (26:74%) and MRMT-1 injected rats (47:53%). NS cells showed no significant difference in their neuronal responses in MRMT-1-injected compared to sham rats. However, superficial WDR neurones in MRMT-1-injected rats had significantly increased responses to mechanical, thermal and electrical (A beta-, C fibre-, and post-discharge evoked response) stimuli. Deep WDR neurones showed less pronounced changes to the superficial dorsal horn, however, the response to thermal and electrical stimuli, but not mechanical, were significantly increased in the MRMT-1-injected rats. In conclusion, the spinal cord is significantly hyperexcitable with previously superficial NS cells becoming responsive to wide-dynamic range stimuli possibly driving this plasticity via ascending and descending facilitatory pathways. The alterations in superficial dorsal horn neurones have not been reported in neuropathy or inflammation adding to the evidence for cancer-induced bone pain reflecting a unique pain state.

Interferon-gamma induces characteristics of central sensitization in spinal dorsal horn neurons in vitro

Hyperexcitability of spinal dorsal horn neurons, also known as 'central sensitization', is a component of pain associated with pathological conditions in the nervous system. The aim of the present study was to analyze if the pro-inflammatory cytokine, interferon-gamma (IFN-gamma), which can be released for extended periods of time in the nervous system during inflammatory and infectious events, can alter synaptic activity in dorsal horn neurons and thereby contribute to such hyperexcitability. Treatment of cultured dorsal horn neurons with IFN-gamma for 2 weeks resulted in a significantly reduced clustering of alpha-amino-3-hydroxy-5-methylisoxazole (AMPA) receptor subunit 1 (GluR1) that was dependent on nitric oxide. The neurons displayed an increased frequency and amplitude of excitatory postsynaptic currents (EPSCs) upon IFN-gamma treatment. Treated dorsal horn neurons also exhibited increased responsiveness to stimulation of dorsal root ganglia (DRG) axons in a two-compartment model. Furthermore, disinhibition by the GABA(A) receptor antagonist picrotoxin (PTX) significantly increased EPSC frequency and induced bursting in untreated cultures but did not significantly increase the frequency in treated neurons, which displayed bursting even without PTX. GABA(A) agonists reduced activity more strongly in treated cultures and immunochemical staining for GABA(A) receptors showed no difference from controls. Since GluR1-containing AMPA receptors (AMPARs) occur predominantly on inhibitory neurons in the dorsal horn, we suggest that the IFN-gamma-mediated increase in spontaneous activity and responsiveness to DRG axon stimulation, decrease in sensitivity to PTX and tendency for EPSC bursting result from a reduced expression of GluR1 on these neurons and not from a reduction in active GABA(A) receptors in the network. IFN-gamma thereby likely causes disinhibition of synaptic activity and primary afferent input in the dorsal horn, which consequently results in central sensitization.

Spinal and cortical spreading depression enhance spinal cord activity

Cortical spreading depression (CSD) has been suggested to underlie some neurological disorders such as migraine. Despite the intensity with which many investigators have studied SD in the brain, only a few studies have aimed to identify SD in the spinal cord. Here we described the main characteristic features of SD in the spinal cord induced by different methods including various spinal cord injury models and demonstrated that SD enhances the spinal cord activity following a transient suppressive period. These findings suggest that SD may play a role in the mechanisms of spinal neurogenic shock, spinal cord injury, and pain. Furthermore, we studied the effect of CSD on the neuronal activity of the spinal cord. CSD was induced via cortical pinprick injury or KCl injection in the somatosensory cortex. CSD did not propagate into the cervical spinal cord. However, intracellular recordings of the neurons in the dorsal horn of C2 segment, ipsilateral to the hemisphere in which CSD was evoked, showed a transient suppression of spontaneous burst discharges, followed by a significant enhancement of the neuronal activity. This indicates a link between a putative cause of the neurological symptoms and the subsequent pain of migraine.

Plasticity of spinal nicotinic acetylcholine receptors following spinal nerve ligation

The nicotinic cholinergic system is known to be important in the processing of nociceptive information. In the spinal cord, nicotinic receptors are expressed on primary afferent terminals, inhibitory interneurons and descending noradrenergic and serotoninergic fibers. Following peripheral nerve injury, the expression of numerous receptors involved in nociceptive processing is altered in the superficial dorsal horn of the spinal cord. However, the expression of nicotinic acetylcholine receptor subunits in the lumbar spinal cord following peripheral nerve injury has not been investigated. We examined the expression of the alpha3, alpha4, alpha5, alpha7, beta2, beta3 and beta4 nicotinic subunits in the spinal cord of normal and spinal nerve ligated rats using immunocytochemistry. Two nicotinic subunits were found to have an increased expression following spinal nerve ligation. The number of cells expressing the alpha3 subunit in the dorsal horn increased bilaterally following spinal nerve injury. Also, the number of alpha5 immunoreactive fibers increased significantly ipsilateral to ligation. The expression of the alpha4, alpha7, beta2, beta3 and beta4 subunits was unchanged. We propose that the increased expression of the alpha3 and alpha5 nicotinic subunits may contribute to the mechanical hypersensitivity observed following spinal nerve ligation.

Cooling of the urinary bladder activates neurons in the dorsal horn of the spinal cord

Although visceral innocuous cold receptors have been documented, the central termination of their afferents is unknown. We used menthol solution (0.6 mM) to obtain selective activation of cold receptors in the urinary bladder of rats. Innocuous cold stimulation induced Fos expression in a population of neurons in the superficial dorsal horn of L6-S1 segments of the spinal cord. Neurons in other regions of the spinal cord, e.g. the lumbar parasympathetic nucleus or the dorsal commissure region, were activated to a similar degree by menthol and control infusions, indicating a response to bladder filling. Our results are consistent with the proposal that subsets of modality-specific dorsal horn neurons convey specific information regarding the exteroceptive and interoceptive state of the animal.

Muscle stretch-induced modulation of noxiously activated dorsal horn neurons of feline spinal cord

The present work was designed to check for the possibility of interactions between mechanical innocuous and chemically induced noxious muscle afferent inputs on discharge behavior of nociceptive superficial dorsal horn neurons (SDHNs) of the spinal cord in decerebrated cats. The innocuous and noxious stimuli were applied separately and in combination, so that the effects of the innocuous stimulus on nociceptive processing could be evaluated. The innocuous stimulus consisted of ramp-and-hold stretches of the gastrocnemius muscles, whereas the noxious stimulus consisted of i.a. injections of bradykinin (BK; 0.5-1 ml, 50 microg/ml) into the arterial circulation of same muscles. Only neurons up to approximately 1mm depth and those that responded to noxious pinch of the gastrocnemius muscles were selected for further analysis. The activity of 16 dorsal horn neurons was recorded extracellularly with high-impedance glass microelectrodes, out of which seven responded to stretch, while 12 neurons responded to bradykinin injections. The bradykinin injections induced three types of responses: excitatory, inhibitory and mixed. The majority of the neurons that showed excitatory and mixed responses to bradykinin were also influenced by stretches applied directly after the bradykinin injection. In these neurons, the stretch usually counteracted the bradykinin-induced response, i.e. shortening and reducing bradykinin-induced excitation and re-exciting the cells after bradykinin-induced inhibition. The mechanism of the stretch modulation is proposed to reside in a segmental spinal control of the nociceptive transmission.

The signaling components of sensory fiber transmission involved in the activation of ERK MAP kinase in the mouse dorsal horn

The stimulation of C-fiber sensory neurons is known to induce activation of the ERK MAP kinase signaling pathway in the spinal cord dorsal horn. In this study we have elucidated some of the signaling components of C-fiber transmission responsible for ERK activation. Using an in vitro slice preparation of the mouse spinal cord dorsal horn, we compared the release of substance P (SP) and BDNF with the activation of ERK in postsynaptic neurons. We observed that primary afferent stimulation recruiting C-fibers was required for both SP and BDNF release and ERK activation in post-synaptic dorsal horn neurons. Glutamate transmission via NMDA and mGluR1 but not AMPA receptors was critical to this ERK activation. BDNF signaling via TrkB receptors but not SP signaling via NK(1) were also involved in ERK recruitment. In conclusion, glutamate and BDNF are the important C-fiber signaling components for ERK activation in dorsal horn neurons.

Cellular GDNF delivery promotes growth of motor and dorsal column sensory axons after partial and complete spinal cord transections and induces remyelination

Glial cell line-derived neurotrophic factor (GDNF) is the prototypical member of a growth factor family that signals via the cognate receptors ret and GDNF-receptor alpha-1. The latter receptors are expressed on a variety of neurons that project into the spinal cord, including supraspinal neurons, dorsal root ganglia, and local neurons. Although effects of GDNF on neuronal survival in the brain have previously been reported, GDNF effects on injured axons of the adult spinal cord have not been investigated. Using an ex vivo gene delivery approach that provides both trophic support and a cellular substrate for axonal growth, we implanted primary fibroblasts genetically modified to secrete GDNF into complete and partial mid-thoracic spinal cord transection sites. Compared to recipients of control grafts expressing a reporter gene, GDNF-expressing grafts promoted significant regeneration of several spinal systems, including dorsal column sensory, regionally projecting propriospinal, and local motor axons. Local GDNF expression also induced Schwann cell migration to the lesion site, leading to remyelination of regenerating axons. Thus, GDNF exerts tropic effects on adult spinal axons and Schwann cells that contribute to axon growth after injury.

Modulation of dorsal spinocerebellar responses to limb movement. I. Effect of serotonin

Spinocerebellar neurons (DSCT) receive converging sensory information from various sensory receptors in the hindlimbs and lower trunk. Previous studies have shown that sensory processing by DSCT neurons results in a representation of global hindlimb kinematic parameters such as the length and the orientation of the limb axis. In addition to the sensory input, the DSCT circuitry also receives a descending input from the raphe nuclei in the brain stem. Recent studies have demonstrated that the raphe serotonergic terminals synapse directly on DSCT neurons and exert a differential modulatory influence on their sensory inputs. We examined the role of serotonergic modulation on the DSCT representation of hindlimb kinematic parameters by recording DSCT activity during passive hindlimb movements before and after perturbing serotonergic transmission. We used two types of perturbation: electrical stimulation of the raphe areas in the brain stem to release serotonin in the spinal cord (42 neurons) and intravenous administration of serotonergic agonists or antagonists, mostly the 5HTP2 antagonist ketanserin (30 neurons). We found that movement responses were altered in approximately 70% of the DSCT units studied with each protocol. Changes could include shifts in mean firing rate, increases or decreases in response amplitude, and changes in response waveform. We used a principal component analysis (PCA) to examine waveform components and to determine how they contributed to the response waveform changes caused by serotonin perturbation. Such changes could be explained by new or different response components that might indicate a modification in the data processing or by a different weighting of existing components that might indicate a modification of synaptic weighting. The results were consistent with the second alternative. We found that the same underlying response components could account for both control responses and those altered by serotonin perturbations. The observed changes in waveform could be entirely accounted for by a re-weighting of response components. In particular, the changes observed after raphe stimulation could be accounted for by selective changes in the weighting of the first principal component (PC) with only minor changes of the weighting of the second PC. Because these response components were shown previously to correlate with the limb axis orientation and length trajectories respectively, the finding is consistent with the idea that limb axis length and orientation information are processed separately within the spinal circuitry.

Modulation of dorsal spinocerebellar responses to limb movement. II. Effect of sensory input

Dorsal spinocerebellar tract (DSCT) neurons receive converging sensory inputs from muscle, skin, and joint receptors and their cerebellar projection is a product of the spinal sensory processing of movement-related information. We concluded earlier that DSCT activity relates to global rather than to local parameters of hindlimb postures and movement, specifically to a kinematic representation of the limb endpoint. The waveforms of principal components (PCs) derived from an ensemble of DSCT movement responses were found to correlate with either the waveform of the limb axis length or orientation trajectories. It was not clear, however, whether these global representations resulted from neural processing or from biomechanical factors. In this study, we perturbed the limb biomechanical factors by decoupling limb geometry from endpoint position during passively applied limb trajectories patterned after a step cycle. We used two types of perturbations: mechanical constraints that limited joint rotations and electrical stimulation of hindlimb muscles. We found that about half of the 89 cells studied showed statistically different response patterns during the perturbations. We compared the PCs of the altered responses with the PCs of the control responses, and found two basic results. With the joint constraints, >85% of the total variance in both control and changed responses was accounted for by the same five PCs that were also observed in the earlier study. The differences between altered and control responses could be fully accounted for by changes in the PC weighting, suggesting a modulation of global response components rather than an explicit representation of local parameters. With the muscle stimulation, only the first and third PCs were the same for the control and altered responses. The second PC was modified, and additional PCs were also required to account for the altered responses. This suggests that the stimulus parameters were specifically represented in the responses. The changes induced by both types of perturbation affected primarily the weighting or waveform of the second PC, which relates to the limb axis length trajectory. The results are consistent with the suggestion that information about limb orientation and length may be separately modulated.

Characterization of wide dynamic range neurons in the deep dorsal horn of the spinal cord in preprotachykinin-a null mice in vivo

We previously reported that mice with a deletion of the preprotachykinin-A (pptA) gene, from which substance P (SP) and neurokinin A (NKA) are derived, exhibit reduced behavioral responses to intense stimuli, but that behavioral hypersensitivity after injury is unaltered. To understand the contribution of SP and NKA to nociceptive transmission in the spinal cord, we recorded single-unit activity from wide dynamic range neurons in the lamina V region of the lumbar dorsal horn of urethane-anesthetized wild-type and ppt-A null mutant (-/-) mice. We found that intensity coding to thermal stimuli was largely preserved in the ppt-A -/- mice. Neither the peak stimulus-evoked firing nor the neuronal activity during the initial phase (0-4 s) of the 41-49 degrees C thermal stimuli differed between the genotypes. However, electrophysiological responses during the late phase of the stimulus (5-10 s) and poststimulus (11-25 s) were significantly reduced in ppt-A -/- mice. To activate C-fibers and to sensitize the dorsal horn neurons we applied mustard oil (MO) topically to the hindpaw. We found that neither total MO-evoked activity nor sensitization to subsequent stimuli differed between the wild-type and ppt-A -/- mice. However, the time course of the sensitization and the magnitude of the poststimulus discharges were reduced in ppt-A -/- mice. We conclude that SP and/or NKA are not required for intensity coding or sensitization of nociresponsive neurons in the spinal cord, but that these peptides prolong thermal stimulus-evoked responses. Thus whereas behavioral hypersensitivity after injury is preserved in ppt-A -/- mice, our results suggest that the magnitude and duration of these behavioral responses would be reduced in the absence of SP and/or NKA.

Central neuronal changes after nerve injury: neuroplastic influences of injury and aging

Peripheral nerve injury produces a hyperexcitability of primary afferents and neurons in the spinal cord that is considered important in the development of nerve injury-induced pain. The authors recently developed a nerve injury model in the trigeminal region of the rat to study the neuronal mechanism of neuropathic pain in the trigeminal system. The escape thresholds to mechanical stimulation applied to the whisker pad area were significantly lower in rats with an inferior alveolar nerve (IAN) transection than those evoked from the contralateral, sham-operated whisker pad. Also, background activity and mechanically evoked responses in infraorbital nerve (ION) afferents and hyperpolarization-activated current (Ih) in trigeminal ganglion ION neurons were increased following IAN transection. Background activity and mechanically evoked responses of wide dynamic range (WDR) neurons in trigeminal subnucleus caudalis on the ipsilateral side relative to the transection were also significantly increased after the operation. A large number of cells expressed c-Fos-like immunoreactivity in the caudal medulla and upper cervical spinal cord following non-noxious mechanical stimulation of the faces of rats with IAN transection. The effect of aging on spinal dorsal horn neurons and the involvement of nerve injury in producing abnormal pain sensation in rats with advancing age were also studied. The incidence of licking behavior in response to noxious radiant heat stimulation of the hind paw was lower in the aged rats than in younger adults, but paw withdrawal latency was shorter and the activities of spinal dorsal horn neurons were higher in the aged rats. Furthermore, the descending inhibitory systems were impaired in the aged rats. These observations suggest that the changes in neuronal activity in the aged rats likely corresponded to the changes observed in the rat model of peripheral nerve injury.

Cellular neuroplasticity mechanisms mediating pain persistence

Transmission of noxious-stimulus-evoked inputs in the spinal and trigeminal systems is mediated primarily through excitatory glutamatergic synapses using alpha amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA), kainate and N-methyl-D-aspartate (NMDA) subtypes of glutamate receptors. Glutamatergic synapses exhibit multiple forms of short-lasting and long-lasting synaptic plasticity. Persistent enhancement of nociceptive transmission, known as "central sensitization," is a form of lasting plasticity that is similar mechanistically to long-term potentiation of glutamatergic transmission in other regions of the central nervous system. This potentiation of AMPA/kainate transmission is dependent upon the activity of NMDA receptors, which become enhanced following noxious peripheral stimulation as a result of several convergent mechanisms. Central sensitization is thus an expression of increased synaptic gain at glutamatergic synapses in central nociceptive-transmission neurons and thereby contributes importantly to pain hypersensitivity. In addition, recent evidence has revealed a new player in the mechanisms underlying pain hypersensitivity following nerve injury--microglia. Understanding of the roles of microglia may lead to new strategies for the diagnosis and management of neuropathic pain.

Future basic science directions into mechanisms of neuropathic pain

The aim of this article is to outline mechanisms underlying generation and maintenance of pain arising from trauma to peripheral nerve fibers and to present an overview of our recent studies of animal models of peripheral neuropathic pain and pain of temporomandibular disorders (TMD). The former model was induced by placing a polyethylene cuff around the sciatic nerve of the Sprague-Dawley rat and the TMD model was induced by injection of complete Freund's adjuvant into the rat's temporomandibular joint. In cuff-implanted rats, ongoing activity of dorsal horn neurons was greater than in controls, the cutaneous receptive field size of the neurons was greater, and both noxious and innocuous mechanical stimuli to the receptive field elicited an excitatory response during stimulation but also a marked afterdischarge that lasted up to 30 minutes; this afterdischarge was never observed in control rats in response to innocuous stimulation. The model of TMD was characterized by joint space narrowing, bone remodeling, infiltration of immune cells, loss in the range of jaw opening, and signs of nociception. Alterations in the neural substrate of nociception in animal models, and therefore also possibly in humans, appear to include changes in peripheral as well as central neurons. In the periphery, changes include alterations in the phenotype and central projections of large-diameter sensory nerve fibers. At the level of the trigeminal brainstem and spinal cord, there appear to be several types of change. One type is an increased efficacy of synaptic transmission onto second-order neurons. Another type of change is a reduction in inhibitory mechanisms, including a shift of gamma-amino butyric acid (GABAA) receptor activation to excitation. There is a need for further studies to focus on mechanisms for either the generation or the maintenance, or both, of neuropathic pain.

Do central terminals of intact myelinated primary afferents sprout into the superficial dorsal horn of rat spinal cord after injury to a neighboring peripheral nerve?

In order to investigate whether normal myelinated primary afferent axons sprout into the territories of adjacent injured peripheral nerve fibers in the superficial dorsal horn of the spinal cord, adult rats underwent either sectioning of the saphenous or femoral nerves on one side, or else unilateral denervation of the skin of the posterior thigh. Two weeks later cholera toxin B subunit (CTb), which is normally transported selectively by myelinated somatic primary afferents, was injected into the ipsilateral (intact) sciatic nerve. The relationship between CTb, vasoactive intestinal peptide (VIP), and binding of Bandeiraea simplicifolia isolectin B4 (IB4) was then examined in the ipsilateral dorsal horn of the second to fifth lumbar spinal segments (L2-L5). Sectioning of the femoral or saphenous nerves resulted in a reduction of IB4 binding in laminae I-II in the medial third of the dorsal horn of L2, L3, and the upper part of L4. VIP-immunoreactivity was upregulated in exactly the same regions in which IB4-binding was reduced. These correspond to the areas that were previously innervated by unmyelinated afferents in the sectioned nerves. CTb-labeling was detected in regions known to receive input from myelinated sciatic afferents: lamina I and a band extending from the inner part of lamina II (IIi) to lamina V in the L3-5 segments, and the deepest part of the dorsal horn in L2. Importantly, no CTb-labeling was detected in the outer part of lamina II (IIo) in the denervated areas. Sectioning of branches of the posterior cutaneous nerve of the thigh resulted in a reduction of IB4-binding and upregulation of VIP-immunoreactivity in the lateral part of the superficial dorsal horn of caudal L4 and L5. Again, CTb-immunoreactivity showed the normal sciatic pattern in L4-L5, with no labeling detected in lamina IIo in the denervated region. These results do not support the suggestion that the central terminals of intact myelinated afferents sprout into regions of lamina II occupied by adjacent nerves that have been axotomized peripherally.

Synaptic transmission of chaotic spike trains between primary afferent fiber and spinal dorsal horn neuron in the rat

Primary sensory neurons can generate irregular burst firings in which the existence of significant deterministic behaviors of chaotic dynamics has been proved with nonlinear time series analysis. But how well the deterministic characteristics and neural information of presynaptic chaotic spike trains were transmitted into postsynaptic spike trains is still an open question. Here we investigated the synaptic transmission of chaotic spike trains between primary Adelta afferent fiber and spinal dorsal horn neuron. Two kinds of basic stimulus unit, brief burst and single pulse, were employed by us to comprise chaotic stimulus trains. For time series analysis, we defined "events" as the longest sequences of spikes with all interspike intervals less than or equal to a certain threshold and extracted the interevent intervals (IEIs) from spike trains. Return map analysis of the IEI series showed that the main temporal structure of chaotic input trains could be detected in postsynaptic output trains, especially under brief-burst stimulation. Using correlation dimension and nonlinear prediction methods, we found that synaptic transmission could influence the nonlinear characteristics of chaotic trains, such as fractal dimension and short-term predictability, with greater influence made under single-pulse stimulation. By calculating the mutual information between input and output trains, we found the information carried by presynaptic spike trains could not be completely transmitted at primary afferent synapses, and that brief bursts could more reliably transmit the information carried by chaotic input trains across synapses. These results indicate that although unreliability exists during synaptic transmission, the main deterministic characteristics of chaotic burst trains can be transmitted across primary afferent synapses. Moreover, brief bursts that come from the periphery can more reliably transmit neural information between primary afferent fibers and spinal dorsal horn neurons.

Brainstem mechanisms of persistent pain following injury

Nerve signals arising from sites of tissue or nerve injury lead to long-term changes in the central nervous system and contribute to hyperalgesia and the amplification and persistence of pain. These nociceptor activity-dependent changes are referred to as central sensitization. Central sensitization involves an increase in the excitability of medullary dorsal horn (subnucleus caudalis) and spinal dorsal horn neurons brought about by a series of events including neuronal depolarization; removal of the voltage-dependent magnesium block of the N-methyl-D-aspartate (NMDA) receptor; release of calcium from intracellular stores; phosphorylation of the NMDA, alpha amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA), and neurokinin (NK) 1 receptors via activation of protein kineses; a change in the neuron's excitability; and an increase in synaptic strength. Central sensitization occurs in trigeminal nociceptive pathways, and more robust neuronal hyperexcitability occurs following deep tissue stimulation than cutaneous stimulation. Utilizing Fos protein immunocytochemistry, it has been found that 2 distinct regions are activated in the trigeminal brainstem sensory nuclei, the subnuclei interpolaris/caudalis transition zone (Vi/Vc) and the caudal part of the subnucleus caudalis. The latter is very similar to the spinal dorsal horn and is involved in the sensory discriminative aspects of pain. In contrast, the ventral pole of the Vi/Vc is unique. In addition to its role in the nociceptive sensory processing of deep tissues, it is involved bilaterally in somatovisceral and somatoautonomic processing, activation of the pituitary-adrenal axis, and descending modulatory control. The findings support our overall hypothesis that the ventral pole of Vi/Vc is involved in the coordination of bilateral sensorimotor functions of the trigeminal system associated with the response to deep tissue injury.