C-fiber Activation Research Articles

Targeting afferent hyperexcitability for therapy of the painful bladder syndrome

The involvement of C-fiber afferent pathways in urinary frequency and pain associated with painful bladder syndrome raises the possibility of multiple targets for the treatment of this disease. Using an in vivo measurement of bladder activity as well as whole-cell patch clamp recording techniques to examine the properties of bladder afferent neurons in animal models of chronic cystitis, we have documented that tetrodotoxin-resistant sodium channels encoded by the Na v 1.8 (PN3/SNS) gene and nitric oxide acting via a cyclic guanosine monophosphate (cGMP)-dependent mechanism are important in modulating bladder pain responses. Thus, suppression of C-fiber afferent nerve activity by blocking specific sodium channels, elevating nitric oxide levels, or activating cGMP-dependent pathways might represent novel strategies for the treatment of symptoms in patients with painful bladder syndrome. Another treatment strategy is suppression of release or activity of proinflammatory agents that can cause normally unexcitable C-fiber afferents to become hyperactive or hyperexcitable. This approach to management of bladder pain was tested in patients with painful bladder syndrome by examining the effectiveness of the antiallergic agent suplatast tosilate (IPD-1151T), which suppresses urinary frequency in a rat model of cystitis. IPD-1151T is an immunoregulator that suppresses cytokine production in T-helper 2 cells and inhibits immunoglobulin E antibody formation and antigen-induced histamine release from mast cells. Preliminary data from an open-label clinical trial showed that 16 of 23 (70%) patients responded to treatment with IPD-1151T (300 mg/day orally for 12 months). The finding that expression of platelet-derived endothelial cell growth factor, which can activate mast cells, was lower in the bladder of responders than nonresponders indicates that bladder levels of platelet-derived endothelial cell growth factor may be a useful marker for this disease.

P/Q-Type Calcium-Channel Blockade in the Periaqueductal Gray Facilitates Trigeminal Nociception: A Functional Genetic Link for Migraine?

The discovery of mis-sense mutations in the alpha1A subunit of the P/Q-type calcium channel in patients with familial hemiplegic migraine indicates the potential involvement of dysfunctional ion channels in migraine. The periaqueductal gray (PAG) region of the brainstem modulates craniovascular nociception and, through its role in the descending pain modulation system, may contribute to migraine pathophysiology. In this study we sought to investigate the possible link between the genetic mutations found in migraineurs and the PAG as a modulator of craniovascular nociception. We microinjected the P/Q-type calcium-channel blocker omega-agatoxin IVA into the rat ventrolateral PAG (vlPAG). We examined its effect on the nociceptive transmission of second-order neurons recorded in the trigeminal nucleus caudalis and activated by stimulation of the parietal dura mater. After injection of agatoxin into the vlPAG (n = 20) responses to dural stimulation were facilitated by 143% (p < 0.0001) for Adelta-fiber activity and 180% for C-fiber activity (p < 0.05). Similarly, spontaneous background activity increased by 163% (p < 0.0001). These results demonstrate that P/Q-type calcium channels in the PAG play a role in modulating trigeminal nociception and suggest a role for dysfunctional P/Q-type calcium channels in migraine pathophysiology.

Induction of connexin 37 expression in a rat model of neuropathic pain

Activation of cutaneous C-fibers by capsaicin or sciatic nerve transection increases the number of astrocytic gap junctions as well as the levels of connexin 43 in the dorsal horn on the stimulated side. Changes in connexin 37 mRNA expression following nerve injury have not been previously documented. We examined the role of gap junction protein connexin 37 in neuropathic hypersensitivity following peripheral nerve injury. Study results showed ipsilaterally increased connexin 37 mRNA levels proximally and distally in rat sciatic nerves after injury and behavioral thermal hyperalgesia at 7 and 14 days. Proximal and distal connexin 37 mRNA levels returned to baseline by 21 days. Sciatic nerve connexin 37 mRNA increases were proportional to the extent of thermal hyperalgesia, but skin, muscle, and lumbar spinal cord connexin 37 mRNA showed no significant changes. Neuropathic pain relief correlated with downregulation of connexin 37 mRNA. Results indicate that upregulation of connexin 37 mRNA following sciatic nerve injury correlates with subsequent thermal hyperalgesia, which suggests that gap junctions (connexin 37) are responsible for the hyperexcitability following peripheral nerve injury.

Acute effect of an incision on mechanosensitive afferents in the plantar rat hindpaw.

The purpose of this study was to examine which primary afferent fibers are sensitized to mechanical stimuli after an experimental surgical incision to the glabrous skin of the rat hindpaw. Afferent fibers teased from the L(5) dorsal root or the tibial nerve were recorded in anesthetized rats. The mechanical response properties of each fiber were characterized before and 45 min after an incision (or sham procedure) within the mechanical receptive field. Sensitization is characterized by an expansion of the mechanical receptive field, an increase in background activity, an increase in response magnitude, or a decrease in response threshold. After incision, the background activity and response properties of Abeta-fibers (n = 9) to mechanical stimuli were unchanged. Four of 13 mechanosensitive Adelta-fibers exhibited sensitization after the incision; response threshold decreased, response magnitude increased, or receptive field size increased. Background activity of Adelta-fibers was not increased by the incision. Sensitization was observed in 4 of 18 mechanosensitive C-fibers 45 min after the incision. Background activity of C-fibers was not increased by the incision. In a group of mechanically insensitive afferent fibers (MIAs), 3 of 7 Adelta-fibers and 4 of 10 C-fibers sensitized 45 min after incision. Response threshold was decreased in only 2 of 17 MIAs; receptive field size increased in 7 of 17 MIAs. Abeta-fibers did not sensitize after the incision, and only 8 of 31 (26%) mechanosensitive Adelta- and C-fibers gave evidence of sensitization. In a group of MIA Adelta- and C-fibers, a greater percentage of 17 fibers studied (41%) were sensitized after incision. In this model, the principal effect of an incision, when examined 45 min after the insult, is an increase in receptive field size of the afferents, particularly those characterized as MIAs. To the extent that the mechanical hyperalgesia characterized in the same model is initiated in the periphery, it would appear that spatial summation of modestly increased response magnitude is important to the development of hyperalgesia.

Conduction velocity of the spinothalamic tract following CO 2 laser stimulation of C-fibers in humans

Pain-related somatosensory-evoked potential following CO 2 laser stimulation (laser-evoked potential (LEP)) is now used not only for research objectives, but also for clinical applications. Estimating the conduction velocity (CV) of the spinothalamic tract (STT) by analyzing LEP following activation of Aδ-fibers (Aδ-CVSTT) by CO 2 laser stimulation has been performed previously, but estimating the CV of STT following activation of C-fibers (C-CVSTT) has not. This is the first report to estimate the C-CVSTT in humans; by using the novel method of CO 2 laser stimulation applied to tiny skin areas. The calculation method was based on that of Kakigi and Shibasaki (Electroenceph clin Neurophysiol 80 (1991) 39) who measured Aδ-CVSTT by conventional CO 2 laser stimulation. The C-CVSTT ranged between 1.4 and 4.0 m/s, and its mean±SD was 2.9±0.8 m/s. This C-CVSTT was significantly slower than the Aδ-CVSTT, which ranged approximately from 10 to 21 m/s. The nociceptive signal of the C-fibers in STT is probably conveyed by unmyelinated axons of projection neurons to reach the thalamus. Our findings provide the first physiological evidence of the signals ascending through unmyelinated axons in the spinal cord in humans. In addition, estimating C-CVSTT and Aδ-CVSTT combined with conventional methods to measure the CV of the posterior column using electrical stimulation should be useful and have important clinical applications, particularly in patients with spinal cord lesions showing various kinds of sensory disturbances.

Selective regulation of acid, pepsinogen, and bicarbonate secretion in the stomach by different C-fiber populations of vagus nerve.

Acute experiments on anesthetized rats showed that group B nerve fibers of the subphrenic portion of the vagus nerve do not participate in the regulation of gastric secretion. Gastric acid production is mainly controlled by fast C-fibers (2.11 0.09 m/sec), while secretion of pepsinogen and bicarbonates depends on activity of both fast and slow (0.95 0.11 m/sec) C-fibers. Some fast conduction C-fibers stimulating the release of bicarbonates in the stomach are capsaicin-sensitive afferents. The local effect of these afferents depends on cholinergic transmission and most probably it is mediated via its modulation.

The effects of noxious dental heating on the jaw-opening reflex and trigeminal Fos expression in the ferret

Previous studies have established that the activation of peripheral nociceptors alters the central processing of nociceptive stimuli. In this study, we examined whether noxious heating of the dental pulp enhances the nociceptive jaw-opening reflex (JOR) and the expression of the immediate early gene c-fos in chloral hydrate/pentobarbital-anesthetized ferrets. We hypothesized that the application of noxious heat to the dental pulp, a procedure that evokes a preferential activation of pulpal C-fibers, will enhance JOR responses to electrical stimulation of the tooth pulp and that this enhanced response will be associated with the expression of Fos protein in discrete regions of the trigeminal nucleus. Consistent with our predictions, we observed that noxious heat conditioning enhanced the JOR as indicated by an increase in the magnitude of the signal averaged digastric electromyogram response evoked by electrical stimuli applied to either a heat-conditioned maxillary canine or the contralateral nonconditioned canine. The enhancement in JOR responses was independent of temporal summation of the electrical stimulus for test stimuli delivered at either 1.0 or 0.1 Hz. Sensitization of the JOR was associated with an increase in the number of immunohistochemically identified Fos-positive nuclei in trigeminal caudalis (Vc) and the transition zone between trigeminal interpolaris and caudalis (Vi/Vc) ipsilateral to the site of stimulation compared with sham stimulated animals. These findings suggest that neuronal populations in Vc and Vi/Vc play a role in the enhanced reflex responses to tooth pulp stimulation and may contribute to the pain and hyperalgesia associated with a symptomatic pulpitis. © 2001 by the American Pain Society

Enhanced lung C-fiber responsiveness in sensitized adult guinea pigs exposed to chronic tobacco smoke.

Tobacco smoke (TS) exposure induces bronchoconstriction and increases airway secretions and plasma extravasation in certain sensitive individuals, particularly those with asthma. C-fiber activation also induces these effects. Although the mechanism by which chronic TS exposure induces airway dysfunction is not well understood, TS exposure may enhance C-fiber responsiveness. To investigate the effect of chronic TS exposure on C-fiber responsiveness to capsaicin and bradykinin, especially in atopic individuals, we exposed ovalbumin (OA)-sensitized guinea pigs to TS (5 mg/l air, 30 min/day for 7 days/wk) or to compressed air. Nonsensitized guinea pigs were also exposed to either compressed air or TS. Beginning after 120 days of exposure, C fibers and rapidly adapting receptors (RARs) were challenged with capsaicin and bradykinin. TS exposure enhanced sensory receptor and airway responsiveness to both intravenous capsaicin and bradykinin challenge. C-fiber, RAR, and airway responsiveness to capsaicin challenge was greatest in OA-sensitized guinea pigs exposed to TS. OA alone induced capsaicin hyperresponsiveness at 5 microg. Airway responsiveness to bradykinin was also greatest in OA-sensitized guinea pigs exposed to TS. OA alone enhanced C-fiber responsiveness to bradykinin at 5 and 10 microg. C-fiber activation by either agonist appeared direct, whereas RAR activation appeared indirect. Therefore, a mechanism of airway hyperirritability induced by the combination of OA sensitization and chronic TS exposure may include hyperirritability of lung C fibers.

High strength stimulation of the vagus nerve in awake humans: a lack of cardiorespiratory effects

Vagus nerve stimulation is used to reduce the frequency and intensity of seizures in patients with epilepsy. In the present study four such patients were studied while awake. We analyzed the physiological responses to vagus nerve stimulation over a broad range of tolerable stimulus parameters to identify vagal A-fiber threshold and to induce respiratory responses typical of C-fiber activation. A-fiber threshold was determined by increasing stimulation current until laryngeal motor A-fibers were excited (frequency=30 Hz). With A-fiber threshold established, C-fiber excitation was attempted with physiologically appropriate stimulus parameters (low frequency and high amplitude). Results: A-fiber thresholds were established in all patients, threshold currents ranged between 0.5 and 1.5 mA. Stimulation at lower frequency (2–10 Hz) and higher amplitudes (2.75–3.75 mA) did not produce cardiorespiratory effects consistent with C-fiber activation. It is possible that such effects were not observed because vagal C-fibers were not excited, because C-fiber effects were masked by the ‘wakeful drive’ to breathe, or because epilepsy or the associated therapy had altered central processing of the vagal afferent inputs.

Review on chronic pelvic pain from a urological point of view

Chronic pelvic pain or interstitial cystitis (IC) is an enigmatic and frustrating condition to manage as physician and to cope with as patient. Patients report moderate to excruciating pain in the suprapubic and/or vaginal area, urgency and frequency (up to 50 micturitions per day), and disruption of their social life. Many patients are declared as drug addicted and neurotic. Social status and the number of sexual partners showed no correlation with incidence. Diagnosis of IC is made by exclusion. A number of pathophysiologic mechanisms have been proposed: changes in epithelial permeability, pelvic floor dysfunction, mastocytosis, activation of C-fibers, increase of nerve growth factors, and bradykinin. No single theory can explain IC.

Role of vagal C-fiber afferents in the bronchomotor response to lactic acid in the newborn dog.

We addressed the hypothesis that vagal C-fiber afferents and cyclooxygenase products are the mechanisms responsible for lactic acid (LA)-induced bronchoconstriction in the newborn dog. Perineural capsaicin and indomethacin were used to block conduction of vagal C fibers and production of cyclooxygenase products, respectively. Perineural capsaicin eliminated (85%) the increase in lung resistance (RL; 45 +/- 5.6%) due to capsaicin (25 microg/kg), whereas the increase in RL (54 +/- 6.9%) due to LA (0.4 mmol/kg) was only inhibited by 37 +/- 4.7% (P < 0.05). Atropine reduced LA-induced bronchoconstriction (42 +/- 2.1%) by an amount similar to that obtained with perineural capsaicin. However, inhibition was significantly increased when atropine was combined with indomethacin (61 +/- 2.7%; P < 0.05), implicating cyclooxygenase products in the LA-induced bronchoconstrictor response. We conclude that the mechanisms responsible for LA-induced bronchoconstriction in the newborn are 1) activation of vagal C-fibers, which, through projections to medullary respiratory centers, leads to activation of vagal cholinergic efferents; 2) production of cyclooxygenase products, which cause bronchoconstriction independent of medullary involvement; and 3) an unknown bronchoconstrictor mechanism, putatively tachykinin mediated. On the basis of our data, pharmaceutical targeting of pulmonary afferents would prevent multiple downstream mechanisms that lead to airway narrowing due to inflammatory lung disease.

Chronic tobacco smoke exposure increases cough to capsaicin in awake guinea pigs

Chronic exposure to irritants such as tobacco smoke (TS) can induce spontaneous and enhanced irritant-induced coughing, especially in asthma. To determine if the mechanism of enhanced coughing involves activation of capsaicin-sensitive sensory receptors (C-fibers), we exposed both non-sensitized (NS) and ovalbumin-sensitized guinea pigs to TS (5 mg/L air, 30 min exposure, and 7 days/week). Similar groups were exposed to compressed air. After 90 days of exposure, we challenged the airways with capsaicin, bradykinin, histamine and methacholine. Capsaicin induced coughing as well as bronchoconstriction in guinea pigs exposed to TS. In ovalbumin (OA) guinea pigs coughing and bronchoconstriction were enhanced. Tachykinin receptor antagonists attenuated coughing to both capsaicin and acute TS challenge. Bradykinin also induced coughing and bronchoconstriction in guinea pigs exposed to TS. There was no statistical separation between the two TS groups however. Histamine and methacholine induced similar bronchoconstriction but fewer coughs in all four experimental groups. In conclusion, chronic TS exposure induced coughing to capsaicin and bradykinin challenge. The effect of capsaicin was further enhanced in OA guinea pigs. Enhanced coughing induced by TS exposure likely involves activation of capsaicin-sensitive sensory C-fibers and neuropeptide release with possible subsequent activation of rapidly-adapting receptors.

Temporal facilitation of the flexor reflex induced by C-fiber activity: comparison between adult and aged rats

We investigated the wind-up phenomenon of the flexor reflex in adult and aged rats. The sural nerve was stimulated at C-fiber strength and reflex activity was recorded from the semitendinosus muscle. The wind-up rate, the increment rate of the C-fiber response (i.e. activity from 100 to 600 ms after stimulation) by successive stimuli (five train pulses), was decreased exponentially with increasing stimulus intervals from 3 to 20 s. The time constant of the decay for the aged rats was 9.2±3.2 s (mean±SD), which was significantly longer than for the adult rats (6.4±2.9 s). The findings indicate that the effects of C-fiber activation on the spinal nociceptive pathways attenuate more slowly in aged rats as compared with adult rats.

Integral-differential calculus computations by short-term potentiation and depression in NTS-pontine pathways

Neural integrators are a class of neural networks that afford extended temporal summation of neural activity or, equivalently, low-pass frequency filtering of neurotransmission [1]. In the respiratory system, a biphasic neural integrator in the carotid chemoreflex response is demonstrated by the classic short-term potentiation (STP) and afterdischarge of phrenic activity [2]. Recently, a new class of neural networks called neural differentiators, which afford high-pass frequency filtering of afferent inputs through short-term depression (STD) of neurotransmission, has been identified in the vagal-carotid control of the expiratory off-switch [3]. The differentiator response is characterized by: 1) a habituation-like adaptation to the vagal Hering-Breuer (HB) reflex lengthening or carotid chemore-flex shortening of expiratory duration; 2) secondary adaptive effects in the form of pontine-mediated and NMDA receptor-dependent post-stimulus rebound and recovery of these vagal or carotid [4](or hypoxia [5]) mediated responses. To elucidate the mechanism of the habituation of the HB reflex, we obtained extracellular field potentials (FPs) in the medial and commissural regions of the nucleus tractus solitarius (NTS) evoked by electrical vagal stimulation (1 min) in urethane-anesthetized, paralyzed and mechanically ventilated rats (n =17) in vivo. FPs from vagal A-fibers (stimulus current, 20–70 μA, response latency, 4–5 ms) revealed a stimulus frequency-dependent (20–80 Hz) STD similar to the synaptic accommodation observed in vitro [6] and the habituation of HB reflex in vivo [4]. FPs from C-fibers (150–350 μA, 20–45 ms) also demonstrated a frequency-dependent (5–20 Hz) STD during afferent activation but their amplitude remained depressed for >50 s post-stimulation. Microinjection of the NMDA receptor antagonist D-APV abolished the short-term memory following C-fiber activation but not the frequency-dependent STD with A- or C-fibers. The characteristics of the vagal A-and C-fiber mediated responses are reminiscent of the type I and type II NTS neurons found in vitro [7]. To elucidate the mechanism of the pontine-mediated adaptation of the HB reflex, we obtained extracellular recordings of pontine neuronal activity simultaneously with phrenic nerve recording during electrical vagal stimulation (1.5×T, 80 Hz, 1 min). Neurons with tonic and respiratory modulated activity were found within or near the parabrachialis (PB) and Kolliker-Fuse (KF) complex. Most tonic neurons in KF (n = 11) were depressed during and/or shortly after vagal stimulation that elicited the characteristic biphasic adaptation of the HB reflex in the phrenic activity, whereas most tonic neurons in medial PB (n = 22) were not affected. Activity of respiratory modulated neurons (n = 4) recorded in KF remained in phase with phrenic activity during vagal stimulation. Results showed that vagal input induced STD of tonic neurotransmission in KF. These findings support a dual-process nonassociative learning model of integral-differential calculus computations in the brain [8] through activity-dependent STP and STD of neurotransmission in primary and secondary afferent pathways.

Capsaicin induces a slow inward current which is not mediated by substance P in substantia gelatinosa neurons of the rat spinal cord

Whole-cell voltage-clamp techniques were employed to investigate a capsaicin-induced current in substantia gelatinosa (SG) neurons in the dorsal horn of adult rat spinal cord slices. Bath-applied capsaicin (2 μM) for 30 s activated a slow excitatory current having an amplitude of 21.3±6.3 pA and a duration of 93±13 s ( n=10; V H=−70 mV). This capsaicin current was compared in amplitude under various conditions among different SG neurons. After either neonatal capsaicin treatment or sciatic-nerve transection, by which C-afferent fibers are known to degenerate, this capsaicin current was reduced in amplitude to 5.0±3.5 pA ( n=8) or 4.5±2.3 pA ( n=6), respectively. A non-N-methyl-D-aspartate (NMDA)-receptor antagonist, CNQX (10 μM), depressed greatly the capsaicin current to 4.0±1.3 pA ( n=9). On the other hand, this current had an amplitude of 14.4±2.7 pA ( n=10) in the presence of an NMDA-receptor antagonist, AP-5 (50 μM); this value was not significantly different from that in the control ( P>0.05). Substance P (SP; 1–2 μM) superfused for 2 min had no detectable effect on all SG neurons examined ( n=7). After SP washout, however, these cells exhibited a capsaicin current (22.8±12.1 pA); this current persisted in the presence of a neurokinin-1 receptor antagonist, L-732,138 (1 μM; 19.8±3.5 pA, n=9). The capsaicin current was not abolished by an intracellular dialysis with GDP-β-S (1 mM; 20.2±2.4 pA, n=9) which inhibited a baclofen (10 μM) response mediated by the G-protein-coupled GABA B receptor. These results indicate that the capsaicin-induced current is mediated through the activation of C-fibers by non-NMDA receptors. This mechanism in SG neurons is different from that known in neurons in other laminae of the dorsal horn that is thought to be a direct action of SP released from C-fibers. This current in SG neurons would contribute to the pain sensation caused by capsaicin.

NEUROMODULATION REDUCES C-FOS GENE EXPRESSION IN SPINALIZED RATS: A DOUBLE-BLIND RANDOMIZED STUDY

Neuromodulation of the sacral nerve roots is effective to treat various voiding dysfunctions, but the underlying mechanism of neuromodulation is not known. The objective of this study is to evaluate whether inhibition of afferent c-fiber activity is the underlying mechanism of sacral nerve root neuromodulation. Twenty-nine female Sprague-Dawley rats weighing 220 to 250 gm. were divided into 4 groups: normal control (normal rats without any procedure; n = 5), sham with saline (spinalized rats at T9 with saline bladder instillation; n = 7), sham with acetic acid (spinalized rats at T9 with acetic acid bladder instillation; n = 8) and stimulation group (spinalized rats at T9 with acetic acid bladder instillation plus electrical stimulation; n = 9). A cystometrogram was performed 10 days after spinal cord transection to confirm the development of bladder hyperreflexia. Bilateral electrode wires were implanted into S1 dorsal foramina and electrical stimulation was performed 8 hours a day for three weeks. The rats were perfused with 4% paraformaldehyde and an immunocytochemical method was used to stain fos-protein that was encoded by c-fos gene. A double-blind method was used in counting fos-protein positive neurons. Bladder hyperreflexia developed in all spinalized rats 10 days after spinal cord transection. Peak bladder pressure was found significantly reduced after neuromodulation (30.4 +/- 4.2 cm. water) compared with the same rats before neuromodulation (82.4 +/- 10.2 cm. water; p = 0. 007). The number of fos-protein positive neurons in the L6 spinal cord segment in the neuromodulation group (93.2 +/- 13.3 cells/section) decreased significantly when compared with the sham with acetic acid group (160.6 +/- 25.0 cells/section; p = 0.02). There was no significant difference in c-fos expression between the sham with saline group (90.5 +/- 15.6 cells/section) and the neuromodulation group (p = 0.92). Sacral dorsal root neuromodulation reduces c-fos gene expression and bladder hyperreflexia in spinalized rats, through inhibition of afferent c-fiber activity.

Interstitial cystitis: the painful bladder syndrome.

Interstitial cystitis (IC) is an enigmatic and frustrating condition to manage as a physician and to cope with as a patient. Many patients are declared as drug addicted and neurotic. Social status and number of sexual partners showed no correlation with incidence. Patients report moderate to excruciating pain in the suprapubic or vaginal area, urgency and frequency (up to 50 micturitions/d), and disruption of their social life. Diagnosis of IC is made by exclusion. Different pathophysiologic mechanisms have been proposed: changes in epithelial permeability, pelvic floor dysfunction, mastocytosis, activation of c-fibers, and increase of nerve growth factors and bradykinin. No single theory can explain IC.

Tactile allodynia in the absence of C-fiber activation: altered firing properties of DRG neurons following spinal nerve injury

We examined the relation between ectopic afferent firing and tactile allodynia in the Chung model of neuropathic pain. Transection of the L5 spinal nerve in rats triggered a sharp, four- to six-fold increase in the spontaneous ectopic discharge recorded in vivo in sensory axons in the ipsilateral L5 dorsal root (DR). The increase, which was not yet apparent 16 h postoperatively, was complete by 24 h. This indicates rapid modification of the electrical properties of the neurons. Only A-neurons, primarily rapidly conducting A-neurons, contributed to the discharge. No spontaneously active C-neurons were encountered. Tactile allodynia in hindlimb skin emerged during precisely the same time window after spinal nerve section as the ectopia, suggesting that ectopic activity in injured myelinated afferents can trigger central sensitization, the mechanism believed to be responsible for tactile allodynia in the Chung model. Most of the spike activity originated in the somata of axotomized DRG neurons; the spinal nerve end neuroma accounted for only a quarter of the overall ectopic barrage. Intracellular recordings from afferent neuron somata in excised DRGs in vitro revealed changes in excitability that closely paralleled those seen in the DR axon recordings in vivo. Corresponding changes in biophysical characteristics of the axotomized neurons were catalogued. Axotomy carried out at a distance from the DRG, in the mid-portion of the sciatic nerve, also triggered increased afferent excitability. However, this increase occurred at a later time following axotomy, and the relative contribution of DRG neuronal somata, as opposed to neuroma endings, was smaller. Axotomy triggers a wide variety of changes in the neurochemistry and physiology of primary afferent neurons. Investigators studying DRG neurons in culture need to be alert to the rapidity with which axotomy, an inevitable consequence of DRG excision and dissociation, alters key properties of these neurons. Our identification of a specific population of neurons whose firing properties change suddenly and synchronously following axotomy, and whose activity is associated with tactile allodynia, provides a powerful vehicle for defining the specific cascade of cellular and molecular events that underlie neuropathic pain.

Response of feline intradental nerve fibers to tooth cutting by Er:YAG laser.

The aim of this study was to investigate the response of intradental A- and C-fibers during tooth cutting by Er:YAG laser. Bipolar electrical stimulation was applied to the cat's canine to identify functional single nerve fibers of the inferior alveolar nerve. The tip of the canine tooth was cut in 0.5-mm steps until the pulp was exposed. Teeth were alternately cut by using Er:YAG laser (50 mJ, 5 pps) and micromotor under water cooling. The nerve response recorded from the single nerve fibers during laser cutting was compared with that during micromotor cutting. All 26 A-fibers responded to laser cutting with high frequency of nerve firings. The nerve firing rate was significantly higher during laser cutting compared with that during micromotor cutting of superficial dentin (Chi(2) test, P < 0.05) but was not significantly different at deep dentin (P > or = 0. 05). Nine of 11 C-fibers responded to laser cutting when the deep dentin was cut. Among those nine nerve fibers, three also showed a low frequency response to laser cutting of the superficial dentin. During the tooth cutting, Er:YAG laser was more effective in activating intradental A-fibers compared with micromotor and also caused the activation of intradental C-fibers.

Ventromedial Thalamic Neurons Convey Nociceptive Signals from the Whole Body Surface to the Dorsolateral Neocortex

The somatosensory properties of ventromedial (VM) thalamic neurons were investigated in anesthetized rats by examining their responses to calibrated cutaneous stimuli. A population of neurons within the lateral part of the ventromedial thalamus (VMl) showed two peaks of activation after percutaneous electrical stimuli, regardless of which part of the body was stimulated. The early and late peaks were elicited by Adelta- and C-fiber activities with mean conduction velocities of 12.9 +/- 0.9 and 1 +/- 0.2 m/sec, respectively. These responses were strongly depressed or blocked after microinjections within the medullary subnucleus reticularis dorsalis of xylocaine or the NMDA antagonist MK-801. None of the VMl neurons responded to innocuous cutaneous or proprioceptive stimuli. In contrast, all these neurons responded to noxious mechanical and thermal stimulation of the limbs and showed monotonic increases in their discharges to increasingly strong noxious cutaneous stimuli. In addition, some VMl neurons were antidromically activated by stimulation in layer I of the dorsolateral frontal cortex. These findings suggest that the rat VMl conveys and encodes cutaneous nociceptive inputs from any part of the body surface to layer I of the dorsolateral neocortex. This reticulo-thalamo-cortical network may allow any signal of pain to gain access to widespread areas of the neocortex and thus help prime the cortex for attentional reactions and/or the coordination of motor responses.