Mechanically-evoked C-fiber activity in painful alcohol and AIDS therapy neuropathy in the rat

1744-8069-3-5 1744-8069 Research Mechanically-evoked C-fiber activity in painful alcohol and AIDS therapy neuropathy in the rat Chen Xiaojie xiaojie.chen@ucsf.edu Levine D Jon jon.levine@ucsf.edu

Departments of Anatomy, Medicine and Oral and Maxillofacial Surgery, Division of Neuroscience, NIH Pain Center, University of California, San Francisco, CA 94143, USA

Molecular Pain 1744-8069 2007 3 1 5 http://www.molecularpain.com/content/3/1/5 17319957 10.1186/1744-8069-3-5 12 1 2007 23 2 2007 23 2 2007 2007 Chen and Levine; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

While altered activities in sensory neurons were noticed in neuropathic pain, caused by highly diverse insults to the peripheral nervous system, such as diabetes, alcohol ingestion, cancer chemotherapy and drugs used to treat AIDS, other infections and autoimmune diseases, as well as trauma, our understanding of how these various peripheral neuropathies manifest as altered neuronal activity is still rudimentary. The recent development of models of several of those neuropathies has, however, now made it possible to address their impact on primary afferent nociceptor function. We compared changes in mechanically-evoked C-fiber activity, in models of painful peripheral neuropathy induced by drinking ethanol (alcohol) or administering 2',3'-dideoxycytidine (ddC), a nucleoside reverse transcriptase inhibitor for AIDS therapy, two co-morbid conditions in which pain is thought to be mediated by different second messenger signaling pathways. In C-fiber afferents, ddC decreased conduction velocity. In contrast, alcohol but not ddC caused enhanced response to mechanical stimulation (i.e., decrease in threshold and increase in response to sustained threshold and supra-threshold stimulation) and changes in pattern of evoked activity (interspike interval and action potential variability analyses). These marked differences in primary afferent nociceptor function, in two different forms of neuropathy that produce mechanical hyperalgesia of similar magnitude, suggest that optimal treatment of neuropathic pain may differ depending on the nature of the causative insult to the peripheral nervous system, and emphasize the value of studying co-morbid conditions that produce painful peripheral neuropathy by different mechanisms.

Background

The second messenger signaling pathways in primary afferent nociceptors that mediate hypersensitivity to mechanical stimuli differ between models of painful peripheral neuropathies 1. Two extreme examples of this are the neuropathies induced by chronic ethanol consumption, and by acquired immunodeficiency disease syndrome (AIDS) therapy (nucleoside reverse transcriptase inhibitors). In alcohol-induced neuropathy, protein kinase Cε(PKCε) has a major contribution to mechanical hyperalgesia 2, whereas in AIDS therapy neuropathy, Ca⁺⁺, caspase signaling and mitochondrial electron transport 345 but not PKCε or a number of other second messenger signaling pathways (i.e., protein kinase A, protein kinase G, extracellular signal-regulated kinases 1/2 or nitric oxide) contribute 3.

Enhanced activity in sensory neurons is thought to contribute to pain reported by patients with small-fiber peripheral neuropathies. Microneurography techniques have demonstrated pathological responses such as sensitization to mechanical stimuli, in patients with trigeminal neuralgia 6, traumatic nerve injury 7, entrapment neuropathy 8, phantom limb 9 and erythromelalgia 10. However, there are practical limitations in performing microneurography in patients, including inability to classify fiber functions fully, small numbers of fibers that can be evaluated in an individual patient and the potential for inducing further injury by introducing a microelectrode into an already damaged nerve. Furthermore, in spite of the fact that in most patients, metabolic abnormalities, toxins, drugs or infectious organisms are producing the neuropathic conditions, most microneurography studies have been done in patients with a traumatic nerve injury 789.

Single-fiber electrophysiology has been performed in animal models of metabolic and toxic, as well as traumatic nerve injury-associated painful peripheral neuropathy. Following traumatic nerve injury it has been reported that there is increased spontaneous activity occurring in irregular bursts 111213; in diabetic neuropathy, in addition to increased spontaneous activity, a decrease in threshold and increase in response to supra-threshold stimulation has been reported 141516171819; in models of cancer chemotherapy neuropathy, C-fibers have been reported to be hyperresponsive and to fire irregularly 120; in alcohol neuropathy, C-fibers also demonstrate a decrease in threshold and increased response to stimulation 2; and, in nucleoside reverse transcriptase inhibitor-induced AIDS-therapy neuropathy, a change in post-stimulus interspike interval (ISI) histogram, without change in threshold or number of action potentials in response to threshold or suprathreshold mechanical stimulus has been reported 3. In this study, we have performed a side-by-side comparison of evoked C-fiber activity in models of two frequently co-morbid forms of peripheral neuropathy, alcohol and AIDS therapy-induced painful peripheral neuropathy, which differ markedly in the nociceptor second messenger signaling pathways involved 23.

Results

Conduction velocity

Conduction velocity, a measure of axonal excitability, has been used extensively in the classification and diagnosis of peripheral neuropathies. The conduction velocity of individual C-fibers, whose mechanical receptive fields had been identified, was measured in sensory neurons innervating the dorsum of the hind paw of ethanol-consuming and ddC-treated rats that demonstrated mechanical hyperalgesia prior to electrophysiology study, and in control rats. While there was a decrease in conduction velocity in both ethanol (decrease 11.7%) and ddC (decrease 16.4%) treated rats, the decrease was statistically significant only in the AIDS therapy model (Figure 1, p < 0.05). Thus, as in patients with diverse forms of peripheral neuropathy who have a decrease in conduction velocity in myelinated fibers, a decrease in the conduction velocity in C-fibers of rats with peripheral neuropathy can also be shown. Since it is generally considered that slowed conduction velocity is a manifestation of alterations in axonal ionic conductance 21, our findings are compatible with changes in ionic conductance in C-fiber axons in AIDS therapy neuropathy. How such changes might contribute to symptoms associated with ddC peripheral neuropathy requires further studies.

Figure 1

Mean conduction velocities (0.86 ± 0.03, 0.76 ± 0.04, 0.72 ± 0.03 m/sec) of C-fibers from the three groups of rats (i.e., naive, ethanol (EtOH) and ddC, respectively) were significantly different (one way ANOVA, p < 0.05). The conduction velocity of C-fibers from the ddC group (n = 18) was significantly lower than that of control rats (n = 38, p < 0.05) while the conduction velocity was similar between EtOH (n = 15) and control groups (p > 0.05).

Response to mechanical stimulation

Threshold stimulus intensity

Alterations in primary afferent nociceptor function associated with enhanced pain are thought to be due to a decrease in threshold for nociceptor activation and an increase in number of action potentials fired. Therefore, the mechanical threshold and response at threshold of C-fiber nociceptors was determined in control as well as in alcohol-consuming and ddC-treated rats. In contrast to conduction velocity, changes in most of the other electrophysiological parameters occurred only in the rat model of alcohol-induced painful peripheral neuropathy. Thus, chronic ethanol ingestion but not ddC administration produces a decrease in average C-fiber mechanical threshold (43.5%; p < 0.05); in ddC-treated rats there was actually a small, not statistically significant, increase in mechanical threshold (Figure 2).

Figure 2

The mechanical threshold of C-fibers in the EtOH group (n = 15) was significantly lower that of control C-fibers (n = 38, p < 0.05, Mann Whitney test) while the mechanical thresholds between ddC (n = 18) and control groups were similar (p > 0.05, Mann Whitney test)

Response to sustained threshold and suprathreshold stimulus

Similar to their effects on C-fiber mechanical threshold, ethanol consumption but not ddC administration significantly enhanced the number of action potentials fired in response to sustained threshold and fixed suprathreshold (10 g) intensity mechanical stimulation (Figure 3). These results provide further support for the suggestion that enhanced C-fiber response contributes to the symptoms of alcohol-induced peripheral neuropathy and raises the question of how the enhanced nociception in AIDS therapy painful peripheral neuropathy is encoded. Thus, while our data support a role for enhanced C-fiber response contributing to pain associated with alcohol neuropathy, it does not appear to contribute in AIDS therapy neuropathy.

Figure 3

The mean responses to both sustained (60 sec) threshold and 10 g stimuli of ddC, EtOH and control C-fibers were significantly different (one way ANOVA, p < 0.01)

The mean responses to both sustained (60 sec) threshold and 10 g stimuli of ddC, EtOH and control C-fibers were significantly different (one way ANOVA, p < 0.01). The responses of EtOH group (n = 15) were significantly higher than those of control rats (n = 38, p < 0.01, Tukey's multiple comparison test) while the responses of C-fibers in the ddC group (n = 18) were similar to those of controls (p > 0.05, Tukey's multiple comparison test).

Activity pattern

ISI

In spite of the ability of activity pattern to signal, independent of average firing frequency 2022, changes in activity pattern produced by various forms of painful peripheral neuropathy has not been studied systematically. To analyze the changes in activity pattern generated in response to stimulation of C-fiber nociceptors in the mechanical receptive field, we first analyzed the ISI histograms for the response of C-fibers to sustained (60 sec) threshold and suprathreshold (10 g) mechanical stimulation, in ethanol-consuming, ddC-treated and control rats. In alcohol consuming rats, the proportion of short ISIs was significantly increased (p < 0.05, Figure 4B; p < 0.01, Figure 4E). A small, albeit statistically significant, increase in intermediate ISI was observed with threshold stimulus, in ddC-treated rats (Figure 4C). That there is an increase in the number of short ISIs in rats with alcohol neuropathy, suggests that temporal summation may play a role in the neuropathic symptoms associated with alcohol consumption but not ddC treatment.

Figure 4

The ISI distributions of both EtOH (n = 15) group of C-fibers in responses to sustained (60 sec) threshold and 10 g stimuli were significantly changed

The ISI distributions of both EtOH (n = 15) group of C-fibers in responses to sustained (60 sec) threshold and 10 g stimuli were significantly changed. A&D, the ISI distributions of control C-fibers (n = 38) in responses to sustained threshold and 10 g stimuli, respectively. B&C, ISI 0.1–0.2 s of responses in EtOH group was significantly higher than that of controls (p < 0.05, t-test) and ISI 0.3–0.4 s of responses in ddC group (n = 18) was significantly higher than that of controls (p < 0.05, t-test). E&F, the ISI distributions of EtOH group in responses to 10 g stimulation were significantly changed while the ISI distributions of ddC group were similar to those of controls. ISI 0–0.2 s of responses in EtOH group was significantly higher than those of control animals (p < 0.01, t-test)

Co-efficient of variability (Cv2)

Finally, changes in activity pattern, generated in response to mechanical stimuli, was analyzed by determining the coefficient of variability (Cv2) distribution for the response to sustained threshold and suprathreshold mechanical stimulation in C-fibers from ethanol fed, ddC treated or control rats. The plots of Cv2 versus number of spikes, in response to mechanical stimulation, for high-firing fibers in alcohol fed rats were different from that of low-firing and control fibers (Figure 5). In these high-firing fibers, the maximum Cv2 values were less, such that there were almost no occurrences of Cv2 values greater than 1.1, unlike the distribution of Cv2 values in low-firing and control C-fibers. The variability of Cv2 values was also smaller. These changes contrast with those observed in vincristine and diabetic neuropathy 1620, where high and variable Cv2 values were observed in high-firing fibers. In ddC-treated rats there were no hi-firing C-fibers; the Cv2 distribution for C-fibers in ddC-treated rats was similar to that for C-fibers from controls rats (Figure 5A&D).

Figure 5

The Cv2 distribution of high firing fibers (B, n = 7) for EtOH group is markedly different from low firing fibers (C, n = 8) for EtOH group and control animals (A, n = 38) while they were similar between low firing fibers for EtOH group and controls

Discussion

While it is generally accepted that enhanced activity in primary afferent nociceptors plays an important role in the pain experienced by patients with peripheral neuropathy 678910, changes in activity in primary afferent nociceptors have received little attention, including direct comparisons between changes in primary afferent nociceptor function in different forms of painful peripheral neuropathy. In this study, we have compared mechanically evoked C-fiber activity in rat models of alcohol and AIDS therapy-induced peripheral neuropathy, for which enhanced nociception has been shown to be dependent on different second messenger signaling pathways.

While the mechanical hyperalgesia observed in these two models of painful peripheral neuropathy are of similar magnitude 23, the changes in C-fiber function differ markedly, being fairly well restricted to a decrease in conduction velocity for AIDS therapy, while many aspects of mechanically-evoked activity were effected by alcohol. Although clinical studies show slowed conduction velocity in many types of peripheral neuropathy 21232425, the mechanism underlying slowing of conduction velocity remains to be established. Available data suggest that changes in ionic currents, most especially for voltage-gated ion channels, contribute to conduction velocity abnormalities 212627. The most well studied model with respect to mechanisms involved in changes in conduction velocity is diabetic neuropathy 21, in which I_Na+, I_K+and I_hhave been shown to decrease 2627 and I_Ca2+to increase 282930. However, depending on the composition of other ion channels in the membrane of the sensory neuron, one may observe either enhanced or attenuated sensation 21. Since slowing of nerve conduction velocity is the major change in C-fiber function in the rat model of ddC-induced painful peripheral neuropathy, direct analysis of ionic currents in dorsal root ganglion neurons treated with AIDS therapy could provide important insights into the mechanisms involved in the pain associated with this class of neuropathies.

While the relatively small change in single fiber electrophysiological properties of primary afferent nociceptors observed in rats with ddC neuropathy might suggest that changes in the peripheral terminal of sensory neurons make a minor contribution to AIDS therapy-induced pain, we have previously shown that peripheral administration, at the site of nociceptive testing, of antagonists of intracellular calcium 3, caspase signaling 4 and the mitochondrial electron transport chain 5, which in control animals have no effect on mechanical nociceptive threshold, reverses ddC-induced mechanical hyperalgesia. Taken together these findings provide support for the suggestion that changes in primary afferent nociceptor function, not tested for in the present study, may play a role in the decreased behavioral mechanical nociceptive threshold in the ddC-induced painful peripheral neuropathy. Alternatively, since the mechanism of action of nucleoside reverse transcriptase inhibitor-induced neurotoxicity is via their effects on mitochondrial function 3132, it may be that a fraction of mitochondria are affected in most neurons, leading to a smaller change in function in a larger percentage of sensory neurons. In contrast, in alcohol, diabetic 1617 and vincristine 20 peripheral neuropathy, the toxic insult appears to produce an all-or-none change in activity, in a subset of neurons (i.e., the high-firing fibers) not observed in AIDS therapy neuropathy.

Decrease in mechanical threshold and increase in number of action potentials elicited by the same intensity stimulus contribute to inflammatory pain 3334, which is characterized by mechanical hyperalgesia. In the present study we found a decrease in mechanical threshold and increase in number of action potentials produced by threshold and suprathreshold stimulation in rats consuming alcohol, but not in ddC-treated rats. The increase in number of short ISIs, in response to both threshold and suprathreshold mechanical stimulation, in alcohol fed rats, will increase temporal summation in postsynaptic spinal dorsal horn neurons; increasing the range of ISIs, near 100 ms, as observed in rats consuming alcohol, causes greater temporal summation of C-fiber-evoked excitatory postsynaptic currents in dorsal horn neurons 35, and in the same range of ISIs, temporal summation of afferent activity appears to be an important factor in human pain perception 363738394041.

While pattern of activity in a presynaptic neuron can dramatically affect activity in its postsynaptic neurons 4243444546, much less attention has been given to the importance of the pattern of activity elicited by mechanical stimulation of primary afferents in the pain associated with peripheral neuropathy. In previous studies of painful peripheral neuropathy we have observed that changes in primary afferent nociceptor function occur in an all-or-nothing fashion. Thus, in models of diabetic 1617 and vincristine 20 neuropathy, we found enhanced activity restricted to a subpopulation of C-fibers (i.e., high-firing fibers), the function of the remaining C-fibers being similar to those in control rats. In alcohol-induced neuropathy this dichotomy was also present. Therefore, in our analysis of variability in action potential timing we also separately evaluated the change in activity pattern in high- and low-firing C-fibers. Marked alteration in the distribution of Cv2 values was observed in high-firing C-fibers in alcohol-induced painful peripheral neuropathy; however, this change was different from that in high-firing C-fibers in diabetic and vincristine-treated rats, in that there was a marked decrease in maximum Cv2 in rats with alcohol-induced neuropathy. While the mechanism underlying these changes is unknown, the lower Cv2 value can be generated by a repetitively bursting pattern of activity 42. The functional significance of variability in neuronal discharge patterns has been the focus of study in somatosensory cortex and other sensory areas 42434446. It has been suggested that such "variability may not be so much a flaw as a feature that the brain puts to good use" to "provide the dynamic range for rapid modulation of synaptic efficacy" 45. This may be relevant to the function of nociceptors as afferent activity-dependent plasticity in spinal nociceptive pathways is thought to be a crucial feature of pain signaling 47, and may contribute to the progressive increase in pain during a prolonged stimulus, even while adaptation decreases the mean firing frequency of nociceptive nerve fibers 48.

In summary, in two models of painful peripheral neuropathies that differ markedly based on the involvement of second messenger signaling mechanisms in primary afferents, we have found marked differences in C-fiber activity. Our findings raise the question; does activity in sensory neurons from different forms of peripheral neuropathy have unique signatures? Since alcohol consumption and AIDS are common co-morbid conditions 495051, the possibility that they produce painful peripheral neuropathy by different mechanisms raises the question are symptoms more severe in AIDS patients who chronically consume alcohol? One step in developing an understanding of the importance of these mechanisms would be to directly activate individual second messengers in primary afferent nociceptors, to determine their effect on mechanically-evoked nociceptor activity, and then to study specific ion channels in dorsal root ganglion neurons, in vitro, to determine the ionic basis of these differences. In vitro studies of the effect of ddC on specific ionic conductance may be especially important in furthering our understanding of the functional alterations in AIDS therapy neuropathy, which does not appear to markedly alter function of individual primary nociceptors.

Conclusion

Our results demonstrated that only ddC decreased conduction velocity of C-fiber afferents. In contrast, alcohol but not ddC caused enhanced response to mechanical stimulation and changes in pattern of evoked activity. Our data also support the suggestion that different therapies are likely to be needed to effectively manage symptoms in different forms of peripheral neuropathy.

Methods

Animal model

Male Sprague-Dawley Rats (280–420 g) from Bantin and Kingman (Fremont, CA, USA) were used in these experiments. Animal care and use conformed to National Institutes of Health (NIH) guidelines and was approved by the University of California at San Francisco Committee on Animal Research.

Alcohol-induced painful neuropathy

The rats used in these experiments housed one per cage were fed Lieber-DeCarli liquid diet (Dyets Inc., Bethlem, PA) containing ethanol (6.5% ethanol) 2525354 for 12 weeks. In this protocol, alcohol-induced hyperalgesia is well established by the end of the seventh week and maximal between 8–12 weeks 2. All rats demonstrated mechanical hyperalgesia prior to electrophysiology study.

2',3'-dideoxycytidine-induced neuropathy

The nucleoside reverse transcriptase inhibitor for AIDS therapies induces a painful peripheral neuropathy in the rat 3. A single dose of the AIDS therapy drug, 2',3'-dideoxycytidine (ddC, 50 mg/kg i.v.), produces a significant reduction in nociceptive threshold from day 1 after its administration, which persisted for more than 20 days 3. Since the model produced mechanical hyperalgesia in 100% of animals 3, we did not perform behavioral studies for each of the animals used in the electrophysiology experiments. Of note, this would only have the potential to underestimate the effect of neuropathy on sensory neuron function.

Electrophysiology

In vivo single-fiber electrophysiology was performed, as previously described 161820. Briefly, rats were anesthetized with sodium pentobarbital (initially 50 mg/kg, i.p., with additional doses given throughout the experiment to maintain areflexia). At the end of the experiment the rat was euthanized by pentobarbital overdose followed by bilateral thoracotomy. Recordings were made from the saphenous nerve, which innervates the dorsal surface of the hind paw. Bipolar stimulating electrodes were placed under the nerve at a site distal to the recording site. The nerve was crushed proximal to the recording site to prevent flexor reflexes during electrical stimulation of the nerve. Fine fascicles of axons were dissected from the nerve and placed on a silver-wire recording electrode. Single units were first detected by electrical stimulation of the nerve. Receptive fields of identified C-fibers were located using a mechanical search stimulus, either a blunt probe with smooth tip or a 60 g von Frey hair (VFH). Each fiber's conduction velocity was calculated by dividing the distance between the stimulating and recording electrodes by the latency of the electrically-evoked action potential. Fibers that conducted slower than 2 m/s were classified as C-fibers 5556. The fiber was determined to be cutaneous if it was activated by lifting and stimulating the skin and/or by moving skin with respect to its subcutaneous tissue. All C-fibers employed in the present experiment had cutaneous receptive fields. The electrically evoked action potential corresponding to the C-fiber whose receptive field had been identified was verified by the latency delay technique, in which electrically evoked spikes resulted in longer latency when the receptive field of the same fiber was stimulated mechanically 57. Mechanical threshold was determined with calibrated VFH and defined as the lowest force that elicited ≥2 spikes within 1 s, in at least 50% of trials.

Sustained threshold mechanical stimulation was performed using a calibrated VFH that was manually placed on the receptive field for 60 s. Sustained (60 s) suprathreshold (10 g) mechanical stimulation was accomplished by use of a mechanical stimulator consisting of a force-measuring transducer (Entran, Fairfield, NJ, USA) mounted in series with interchangeable VFH filaments. Neural activity was stored using an IBM compatible computer with micro 1401 interface (CED, Cambridge, UK) and further analyzed off-line with Spike2 software (CED).

ISI analysis

ISI analysis was used to evaluate the temporal characteristics of the response of C-fiber nociceptors to sustained mechanical stimulation, which was adopted from our study of nociceptor activity in the rat model of vincristine-induced painful neuropathy 20. The ISIs for each C-fiber's response was grouped into 100 ms bins between 0 and 499 ms; ISIs greater than or equal to 500 ms were not analyzed 20. This bin width also allows our data to be compared with that in other studies 355859. The number of intervals occurring in each bin was expressed as the percentage of the total number of ISIs in the trial. This trial-by-trial normalization procedure allowed the distribution of ISIs from several fibers to be averaged together.

Action potential firing variability (Cv2)

The coefficient of variability (Cv2) was calculated to compare the relative difference between adjacent ISIs 42. Cv2 is defined as the square root of 2 multiplied by the standard deviation of two ISIs divided by their mean 42. Thus, it is a dimensionless value that is independent of absolute firing rate. Based on our previous studies in rat models of vincristine- and diabetes-induced peripheral neuropathy 1620, the response of each C-fiber during the 1 min duration of the stimulus was divided into six consecutive 10 s periods, and the average Cv2 for all fibers in each corresponding 10 s period was calculated. Based on our similar finding in rat models of vincristine and diabetes-induced painful peripheral neuropathy 1620, fibers were divided into two groups, "low-firing" fibers which fired <100 spikes and "high-firing" fibers which fired >100 spikes, so that the firing pattern of C-fiber activity from the peripheral neuropathy models can be compared. The high-firing C-fibers had approximately 2.5-fold higher responses to sustained threshold and suprathreshold mechanical stimulation compared with control fibers during the 60 s stimulus while the low-firing frequency C-fibers had responses similar to those of controls.

Statistics

Group data are expressed as mean ± S.E.M. Statistical analyses were done using analysis of variance (ANOVA) followed by Tukey's multiple comparison post hoc test or unpaired t-test and Mann Whitney U test, as appropriate. Differences were considered significant at P < 0.05.

List of abbreviations

ddC : 2',3'-dideoxycytidine

ISI : interspike interval

AIDS : Acquired Immunodeficiency Disease Syndrome

PKCε : protein kinase Cε

VFH : von Frey hair

Cv2 : coefficient of variability

EtOH : ethanol

Authors' contributions

XC participated in the design of the study, carried out all the experiment, performed the statistical analysis and drafted the manuscript. JDL participated in the design of the study and drafted the manuscript. All authors read and approved the final manuscript.

Acknowledgements

This research was supported by NIH grant NS21647. We thank Drs. Yinka Dina and Elizabeth Joseph provided ethanol- and ddC- treated rats, respectively.

Role of protein kinase Cepsilon and protein kinase A in a model of paclitaxel-induced painful peripheral neuropathy in the rat Dina OA Chen X Reichling D Levine JD Neuroscience 2001 108 507 515 10.1016/S0306-4522(01)00425-0 11738263 Key role for the epsilon isoform of protein kinase C in painful alcoholic neuropathy in the rat Dina OA Barletta J Chen X Mutero A Martin A Messing RO Levine JD J Neurosci 2000 20 8614 8619 11069970 Novel mechanism of enhanced nociception in a model of AIDS therapy-induced painful peripheral neuropathy in the rat Joseph EK Chen X Khasar SG Levine JD Pain 2004 107 147 158 10.1016/j.pain.2003.10.010 14715401 Caspase signalling in neuropathic and inflammatory pain in the rat Joseph EK Levine JD Eur J Neurosci 2004 20 2896 2902 10.1111/j.1460-9568.2004.03750.x 15579143 Mitochondrial electron transport in models of neuropathic and inflammatory pain Joseph EK Levine JD Pain 2006 121 105 114 10.1016/j.pain.2005.12.010 16472913 Pathophysiology of trigeminal neuralgia: new evidence from a trigeminal ganglion intraoperative microneurographic recording. Case report Burchiel KJ Baumann TK J Neurosurg 2004 101 872 873 15540931 Chronic hyperalgesia and skin warming caused by sensitized C nociceptors Cline MA Ochoa J Torebjork HE Brain 1989 112 ( Pt 3) 621 647 10.1093/brain/112.3.621 2731024 Ectopic sensory discharges and paresthesiae in patients with disorders of peripheral nerves, dorsal roots and dorsal columns Nordin M Nystrom B Wallin U Hagbarth KE Pain 1984 20 231 245 10.1016/0304-3959(84)90013-7 6096790 Microelectrode recordings from transected nerves in amputees with phantom limb pain Nystrom B Hagbarth KE Neurosci Lett 1981 27 211 216 10.1016/0304-3940(81)90270-6 7322453 Pathological C-fibres in patients with a chronic painful condition Orstavik K Weidner C Schmidt R Schmelz M Hilliges M Jorum E Handwerker H Torebjork E Brain 2003 126 567 578 10.1093/brain/awg060 12566278 Abnormal discharge originates at the site of nerve injury in experimental constriction neuropathy (CCI) in the rat Tal M Eliav E Pain 1996 64 511 518 10.1016/0304-3959(95)00175-1 8783316 Electrophysiological evidence for hyperalgesia in the peripheral neuropathy Xie YK Xiao WH Sci China B 1990 33 663 672 2168712 Afterdischarge and interactions among fibers in damaged peripheral nerve in the rat Lisney SJ Devor M Brain Res 1987 415 122 136 10.1016/0006-8993(87)90275-7 3620940 Lowered response threshold and increased responsiveness to mechanical stimulation of cutaneous nociceptive fibers in streptozotocin-diabetic rat skin in vitro--correlates of mechanical allodynia and hyperalgesia observed in the early stage of diabetes Suzuki Y Sato J Kawanishi M Mizumura K Neurosci Res 2002 43 171 178 10.1016/S0168-0102(02)00033-0 12067753 Conduction of neural impulses in diabetic neuropathy Mackel R Brink E Clin Neurophysiol 2003 114 248 255 10.1016/S1388-2457(02)00320-6 12559231 Altered temporal pattern of mechanically evoked C-fiber activity in a model of diabetic neuropathy in the rat Chen X Levine JD Neuroscience 2003 121 1007 1015 10.1016/S0306-4522(03)00486-X 14580951 Hyper-responsivity in a subset of C-fiber nociceptors in a model of painful diabetic neuropathy in the rat Chen X Levine JD Neuroscience 2001 102 185 192 10.1016/S0306-4522(00)00454-1 11226682 C-fiber mechanical stimulus-response functions are different in inflammatory versus neuropathic hyperalgesia in the rat Ahlgren SC Wang JF Levine JD Neuroscience 1997 76 285 290 10.1016/S0306-4522(96)00290-4 8971778 Increased responsiveness of sensory neurons in the saphenous nerve of the streptozotocin-diabetic rat Ahlgren SC White DM Levine JD J Neurophysiol 1992 68 2077 2085 1491258 Altered temporal pattern of evoked afferent activity in a rat model of vincristine-induced painful peripheral neuropathy Tanner KD Reichling DB Gear RW Paul SM Levine JD Neuroscience 2003 118 809 817 10.1016/S0306-4522(03)00023-X 12710988 The role of axonal ion conductances in diabetic neuropathy: a review Quasthoff S Muscle Nerve 1998 21 1246 1255 10.1002/(SICI)1097-4598(199810)21:10<1246::AID-MUS2>3.0.CO;2-B 9736052 Nociceptor hyper-responsiveness during vincristine-induced painful peripheral neuropathy in the rat Tanner KD Reichling DB Levine JD J Neurosci 1998 18 6480 6491 9698336 Neurography: late responses Bischoff C Muscle Nerve 2002 Suppl 11 S59 65 10.1002/mus.10149 12116287 Pitfalls in electrodiagnosis Krarup C J Neurol 1999 246 1115 1126 10.1007/s004150050529 10653302 Electrophysiological studies in the critical care unit: investigating polyneuropathies Chen R Can J Neurol Sci 1998 25 S32 5 9532295 Reversible and irreversible nodal dysfunction in diabetic neuropathy Brismar T Sima AA Greene DA Ann Neurol 1987 21 504 507 10.1002/ana.410210515 2438993 Nodal Na(+)-channel displacement is associated with nerve-conduction slowing in the chronically diabetic BB/W rat: prevention by aldose reductase inhibition Cherian PV Kamijo M Angelides KJ Sima AA J Diabetes Complications 1996 10 192 200 10.1016/1056-8727(95)00084-4 8835918 Increased number of myocardial voltage-gated Ca2+ channels and unchanged total beta-receptor number in long-term streptozotocin-diabetic rats Gotzsche LB Rosenqvist N Gronbaek H Flyvbjerg A Gotzsche O Eur J Endocrinol 1996 134 107 113 8590944 Voltage-dependent calcium currents are enhanced in dorsal root ganglion neurones from the Bio Bred/Worchester diabetic rat Hall KE Sima AA Wiley JW J Physiol 1995 486 ( Pt 2) 313 322 1156523 7473199 Calcium signal prolongation in sensory neurones of mice with experimental diabetes Kostyuk E Pronchuk N Shmigol A Neuroreport 1995 6 1010 1012 7632883 Adverse effects of reverse transcriptase inhibitors: mitochondrial toxicity as common pathway Brinkman K ter Hofstede HJ Burger DM Smeitink JA Koopmans PP Aids 1998 12 1735 1744 10.1097/00002030-199814000-00004 9792373 Mitochondrial oxidative stress in human hepatoma cells exposed to stavudine Velsor LW Kovacevic M Goldstein M Leitner HM Lewis W Day BJ Toxicol Appl Pharmacol 2004 199 10 19 10.1016/j.taap.2004.03.005 15289086 Properties of high-threshold mechanoreceptors in the goat oral mucosa. II. Dynamic and static reactivity in carrageenan-inflamed mucosa Cooper B Ahlquist M Friedman RM Labanc J J Neurophysiol 1991 66 1280 1290 1761984 Peripheral mechanisms of cutaneous nociception Meyer RA Ringkamp M Campbell JN Raja SN Wall and Melzack's Textbook of Pain London, Elsevier/Churchill Livingstone McMahon SB and Koltzenburg M 2006 3 34 Glutamate-mediated slow synaptic currents in neonatal rat deep dorsal horn neurons in vitro Miller BA Woolf CJ J Neurophysiol 1996 76 1465 1476 8890267 Non-pain and pain sensations evoked by tooth pulp stimulation McGrath PA Gracely RH Dubner R Heft MW Pain 1983 15 377 388 10.1016/0304-3959(83)90073-8 6866536 The effect of temporal parameters on subjective sensations evoked by electrical tooth stimulation Virtanen AS Huopaniemi T Narhi MV Pertovaara A Wallgren K Pain 1987 30 361 371 10.1016/0304-3959(87)90024-8 3670881 Stimulus pattern related plasticity of synapses between cones and horizontal cells in carp retina Hu JF Liu Y Liang PJ Brain Res 2000 857 321 326 10.1016/S0006-8993(99)02472-5 10700586 Synaptic plasticity in the hippocampus during afferent activation reproducing the pattern of the theta rhythm (theta plasticity) Kleshchevnikov AM Neurosci Behav Physiol 1999 29 185 196 10.1007/BF02465325 10432508 Patterned electrical activity modulates sodium channel expression in sensory neurons Klein JP Tendi EA Dib-Hajj SD Fields RD Waxman SG J Neurosci Res 2003 74 192 198 10.1002/jnr.10768 14515348 Regulation of gene expression by action potentials: dependence on complexity in cellular information processing Fields RD Eshete F Dudek S Ozsarac N Stevens B Novartis Found Symp 2001 239 160 72; discussion 172-6, 234-40 11529310 Comparison of discharge variability in vitro and in vivo in cat visual cortex neurons Holt GR Softky WR Koch C Douglas RJ J Neurophysiol 1996 75 1806 1814 8734581 Fractal features of dark, maintained, and driven neural discharges in the cat visual system Lowen SB Ozaki T Kaplan E Saleh BE Teich MC Methods 2001 24 377 394 10.1006/meth.2001.1207 11466002 Spike interval distributions for neurons and random walks with drift to a fluctuating threshold Wise ME Statistical distributions in scientific work Boston, Reidel Taillie CEA 1981 6 211 231 Synaptic transmission: noisy synapses and noisy neurons Smetters DK Zador A Curr Biol 1996 6 1217 1218 10.1016/S0960-9822(96)00699-9 8939560 Negative interspike interval correlations increase the neuronal capacity for encoding time-dependent stimuli Chacron MJ Longtin A Maler L J Neurosci 2001 21 5328 5343 11438609 Sensitization of pain pathways in the spinal cord: cellular mechanisms Baranauskas G Nistri A Prog Neurobiol 1998 54 349 365 10.1016/S0301-0082(97)00067-1 9481803 Nociceptor discharges and sensations due to prolonged noxious mechanical stimulation--a paradox Adriaensen H Gybels J Handwerker HO Van Hees J Hum Neurobiol 1984 3 53 58 6330012 Effect of chronic alcohol abuse on the CNS morbidity of HIV disease Fein G Fletcher DJ Di Sclafani V Alcohol Clin Exp Res 1998 22 196S 200S 10.1111/j.1530-0277.1998.tb04000.x 9727634 Correlates of alcohol and/or drug use among HIV-infected individuals Welch KJ AIDS Patient Care STDS 2000 14 317 323 10.1089/10872910050046340 10897504 HIV and alcohol use: consequences of comorbidity Bean P Am Clin Lab 2001 20 13 16 Experimental methods of ethanol administration Lieber CS DeCarli LM Sorrell MF Hepatology 1989 10 501 510 10.1002/hep.1840100417 2673971 Recommended amounts of nutrients do not abate the toxic effects of an alcohol dose that sustains significant blood levels of ethanol Lieber CS DeCarli LM J Nutr 1989 119 2038 2040 2621495 Liquid diet technique of ethanol administration: 1989 update Lieber CS DeCarli LM Alcohol Alcohol 1989 24 197 211 2667528 Primary afferent units from the hairy skin of the rat hind limb Lynn B Carpenter SE Brain Res 1982 238 29 43 10.1016/0006-8993(82)90768-5 6282398 Nociceptive pathways: anatomy and physiology of nociceptive ascending pathways Willis WD Philos Trans R Soc Lond B Biol Sci 1985 308 253 270 2858882 The electrophysiological identification of single nerve fibres, with particular reference to the slowest-conducting vagal afferent fibres in the cat Iggo A J Physiol 1958 142 110 126 1356697 13564422 Differential release of endogenous 5-hydroxytryptamine, substance P, and neurokinin A from rat ventral spinal cord in response to electrical stimulation Franck J Brodin E Fried G J Neurochem 1993 61 704 711 7687660 Facilitation of the withdrawal reflex by repeated transcutaneous electrical stimulation: an experimental study on central integration in humans Arendt-Nielsen L Sonnenborg FA Andersen OK Eur J Appl Physiol 2000 81 165 173 10.1007/s004210050026 10638373