What are the neurophysiological mechanisms by which SCI promotes SA and OA in NA neurons? Two of the 3 intrinsic functional aspects of membrane potential that in principle can generate SA (and promote extrinsically driven OA) are prolonged depolarization of RMP and a hyperpolarizing shift in the voltage threshold for AP generation. Persistent SCI-induced depolarization of RMP was found previously in dissociated small DRG neurons,11 but AP voltage threshold was not measured, and whether either of these SA-promoting effects occurs in NA neurons after SCI has not been documented. Compared to NA neurons in the naive and sham groups, NA neurons in the SCI group showed significant depolarization of RMP and significant reduction in voltage threshold for AP generation (Table 2). No significant differences in these properties were found between the naive and sham groups. Three other measures also revealed significantly greater excitability in NA neurons in the SCI group vs the naive group: rheobase dropped by 50%, repetitive firing in response to currents twice the rheobase value nearly doubled, and membrane resistance increased by 30% (Table 2). Interestingly, rheobase and membrane resistance in the sham group were significantly different from values in the naive group, providing additional evidence for persistent hyperexcitability after sham surgery. No significant effects of SCI were found in RA neurons (Table 2). Fewer RA than NA neurons were examined, so it is possible that weak effects of SCI or sham treatment could be revealed by larger samples of RA neurons. These results show that 2 physiological alterations important for driving SA, persistent depolarization and reduction of AP voltage threshold, are induced in NA neurons by SCI. All the measures of hyperexcitability were especially prominent in spontaneously active NA neurons taken from SCI rats (Table 3), consistent with a hyperexcitable state being induced by SCI that functions to promote SA.11 In addition, sham surgery can also persistently increase excitability of NA neurons, expressed as lowered rheobase, but without substantial alteration of RMP or AP voltage threshold.
The third functional aspect of membrane potential that in principle can generate SA and promote extrinsically driven OA is an increase in the frequency of large DSFs that can reach AP threshold. Irregular SFs of membrane potential have long been evident in published whole-cell patch recordings from dissociated small- and medium-sized DRG neurons, but they have received remarkably little experimental attention. The most detailed study87 found no obvious association between fluctuation amplitude and SA in a rat chronic constriction injury model of neuropathic pain, but systematic quantitative measurements were not performed. We used 2 quantitative approaches to test whether SCI increases DSF amplitude and frequency in NA neurons. First, we asked whether total fluctuation amplitude (peak to peak) increased after SCI. Our preliminary results (not shown) indicated that, unlike the regular sinusoidal oscillations in large and medium-sized DRG neurons that are enhanced by axotomy,2,3,54 the irregular fluctuations in small DRG neurons lack large sinusoidal components that contribute significantly to OA generated at RMP negative to −40 mV (see also Ref. 3), which is the RMP range where we have investigated SA and OA. Thus, as an alternative to fast Fourier transform analysis, we simply compared the SD of all points (excluding APs and AHPs) in randomly selected 50-second samples in NA cells from each group. SD provides a symmetrical measure of dispersion of the fluctuations from the mean RMP. The SDs of fluctuation amplitudes were significantly larger in the SCI group (mean of the fluctuation SDs for each neuron, 3.0 ± 0.3 mV, 27 neurons) than in the naive group (1.2 ± 0.3 mV, 9 neurons) or sham group (1.1 ± 0.2 mV, 12 neurons) (Tukey multiple comparison P < 0.01 in each case). This result shows that SCI increases fluctuation amplitudes but does not distinguish between any differential effects of SCI on DSFs and HSFs.
Plotting all points in each trace relative to the median instead of the mean revealed a skew in the depolarizing direction, raising the possibility that SCI might selectively promote the generation of large DSFs in addition to (or instead of) enhancing oscillatory or hyperpolarizing fluctuations. This is important because HSFs as well as sinusoidal oscillations have been described in isolated DRG neurons.3,58 To rigorously test this prediction, we used a novel set of automated algorithms for measuring DSFs and HSFs, which were defined by reference to a sliding median of all points measured during 50-second samples (see Methods). An example of part of an analyzed trace is shown in Figure 4A. Note that DSFs are defined operationally and are unlikely to represent unitary events; indeed, there seems to be complex summation of smaller depolarizing (and possibly hyperpolarizing) events in many of the DSFs shown in this and the other illustrations in this article. Analysis of DSFs in NA neurons exhibiting SA (from naive, sham, and SCI groups) revealed that mean DSF amplitude was largest (∼5 mV) when RMP was between −45 and −40 mV (Fig. 4B). Given that the voltage threshold for AP generation after SCI ranged from −28 to −50 mV, and RMP ranged between −70 and −40 mV (see also Table 2), we predicted that relatively large DSFs (>5 mV) could reach AP threshold often enough to contribute significantly to observed SA. Analysis of NA neurons exhibiting SA showed that the frequency within each trace of DSFs with amplitudes >5 mV (most of which initiated APs; see below) and medium-sized DSFs with amplitudes of 3 to 5 mV (which almost never evoked APs) showed striking parallels to the frequency of APs in the same neurons plotted as a function of RMP (Fig. 4C). This close parallel provides strong evidence that large DSFs play an important role in triggering APs in NA neurons. Importantly, significantly more NA neurons in the SCI group had large DSFs (>5 mV) than did neurons in the naive or sham groups (Fig. 4D). Moreover, frequencies both of large DSFs and of APs within each recording were significantly greater in the SCI group (Fig. 4E).
Large DSFs, similar to OA observed in vitro and in vivo in presumptive C-fiber nociceptors after SCI,11 occur randomly (Fig. 5C1) (see also Ref. 87). Stochastic DSF occurrence was also seen during the 2-second depolarizations used to measure rheobase and repetitive firing (Fig. 1). A striking finding was the much longer latency to the first AP generated in the rheobase tests in NA compared with RA neurons (Table 1). This is consistent with the AP at rheobase in NA neurons being triggered by infrequent, randomly occurring, large DSFs. If so, the increase in frequency of large DSFs after SCI should increase the likelihood that large DSFs occur early during depolarizing test pulses, and this should decrease the latency to the first AP. Confirming this prediction, the mean latency to the first AP generated in NA neurons during rheobase measurement in the SCI group was much shorter than the latency in the naive or sham groups (Table 2). Together, these findings show (1) that DSFs play a major role in generating the irregular SA found in NA neurons, and (2) that enhancement of DSF amplitude and large DSF frequency contributes to SCI-induced SA.
Is enhancement of DSFs and the consequent promotion of nociceptor activity solely a long-term phenomenon, perhaps unique to SCI, or can nociceptor DSFs also be enhanced acutely by extrinsic signals? In particular, could acute exposure to an inflammatory signal enhance DSFs and promote OA? To address this question, we used serotonin (5-HT), an inflammatory mediator that in the periphery can induce pain and hyperalgesia.78,84,89 5-HT is interesting because it has complex effects on nociceptors,44,53,77,79,90,118 one of which is to reduce AP voltage threshold.17 In contrast to nearly all other studies of 5-HT's actions on nociceptors, which used very high 5-HT concentrations (typically 10 μM), an early study showed that 10-nM 5-HT caused alterations in tetrodotoxin-resistant Na+ current that should lower AP threshold.34 An implication of this observation is that a 5-HT concentration that modulates but does not activate NA nociceptors could potentiate depolarization-dependent OA in NA nociceptors. Potentiation of such OA would be much more likely if the same concentration of 5-HT also enhances DSFs. We tested this possibility in NA nociceptors dissociated from naive rats.
Treatment of each dish with 100-nM 5-HT for 10 to 30 minutes before and during recording produced no hint of sustained depolarization (Table 4). Furthermore, 5-HT did not induce OA at RMP (Fig. 6A, left panel). When a prolonged extrinsic depolarizing input (modeled by constant current injection through the patch pipette to hold the membrane potential at ∼−45 mV for 30-60 seconds) was added to promote OA after vehicle treatment, no significant increase in the incidence of OA was found vs the incidence of SA at RMP (compare vehicle groups in left and right panels of Fig. 6A). By contrast, when 5-HT–treated nociceptors were depolarized to −45 mV, ∼80% showed OA (Fig. 6A, right panel). When depolarized to −45 mV, AP firing rates during OA and the corresponding large DSF frequencies were significantly greater in 5-HT–treated neurons than in vehicle-treated neurons (Fig. 6B). Amplitudes of DSFs ≥1.5 mV were also enhanced in 5-HT–treated neurons that were depolarized to −45 mV (Fig. 6C, left panel), and like the effects of SCI (Fig. 4B), the DSFs were largest in neurons with OA (Fig. 6C, right panel). Examples of DSFs and APs (OA) in NA neurons held at −45 mV with and without 5-HT treatment are shown in Figure 6D. Depolarizing SFs occurred randomly after either vehicle treatment or 5-HT treatment (Figs. 6D, 7A1 and 7B1). 5-HT increased the number of medium amplitude (3-5 mV) and large (>5 mV) DSFs during each recording (Figs. 7A2 and B2). The number of large DSFs paralleled the number of APs evoked during the same 30-second samples (Fig. 7B2). As predicted,17 5-HT treatment also significantly (and substantially) lowered the voltage threshold for AP generation (Table 4). This likely contributed to the increased percentage of DSFs 3 to 5 mV and especially >5 mV that triggered APs (Figs. 7A2 and B2). In addition, 5-HT treatment significantly decreased the rheobase (consistent with an increase in the frequency of large DSFs) (Table 4). In contrast to the effect of SCI on AP latency at rheobase, 5-HT did not decrease AP latency. However, because of the low frequency and stochastic occurrence of APs (and underlying DSFs), demonstrating possible effects on AP latency is likely to require a much larger sample size.
In the RA neurons, pretreatment with 5-HT did not induce OA at RMP in any of the neurons tested, and comparisons between vehicle- and 5-HT–treated RA neurons did not reveal significant changes in excitability, although there was a trend for depolarization of RMP (Table 4). This suggests that RA neurons, unlike NA neurons, are not sensitive to low concentrations of 5-HT.
These findings show that acute enhancement of DSFs along with reduction of AP threshold by an inflammatory signal can strongly potentiate depolarization-dependent OA in NA nociceptors under conditions where the inflammatory signal does not by itself cause depolarization or produce OA.
Formal definitions of neuronal OA and SA are lacking in the pain research field. We think a useful distinction is implicit in everyday meanings of “ongoing” and “spontaneous.” Thus, we define OA generally as continuing discharge of APs driven by any intrinsic and/or extrinsic sources of ongoing excitation. We define SA as a subclass of OA that is generated solely by alterations intrinsic to the active neuron, which can only be demonstrated conclusively if the active neuron is isolated from extrinsic drivers of activity. Ongoing activity observed in vivo may involve both extrinsically driven OA and intrinsic SA.
We found that SCI changed each of the 3 intrinsic functional aspects of membrane potential that in principle can drive SA and promote extrinsically driven OA. Spinal cord injury promoted an OA state characterized by: (1) prolonged depolarization of RMP, (2) a hyperpolarizing shift in the voltage threshold for AP generation, and (3) an increase in the incidence of large transient DSFs (Fig. 8). Acute 5-HT treatment was found to produce the latter 2 changes. Ongoing activity and all 3 physiological alterations promoting it were found only in NA neurons. Extensive examination of RMP, voltage threshold, and DSFs in RA neurons after SCI showed no hint of an effect on RMP or DSFs. However, a trend for reduction of AP threshold was found in RA neurons, which may have contributed to the significant reduction in rheobase of RA neurons after SCI. If reduction in RA and/or NA neuron AP threshold occurs near sensory terminals, this could contribute to the peripheral hypersensitivity observed after SCI that is likely to promote evoked pain.11,18
Under our in vitro conditions, NA neurons were twice as common as RA neurons. Despite dramatically different responses in firing evoked by depolarization and differences in RMP and membrane time constant, the 2 types shared many properties. No significant differences were found in soma size or membrane capacitance, although a trend for RA neurons to be slightly larger suggests this class may include some Aδ neurons. About 70% of sampled NA and RA neurons exhibited capsaicin sensitivity and/or IB4 binding, indicating that each type is largely nociceptive. Both types may also include non-nociceptive neurons and some nociceptors that could appear mechanically insensitive in vivo (as do many human C-fiber nociceptors displaying OA).46,64,82 The similarities of NA and RA types raise the possibility that the 2 types could be different functional states rather than stable phenotypes. Indeed, there may have been a weak trend for SCI to increase the ratio of NA to RA neurons. It will be important to map these electrophysiologically defined types onto the large number of functionally and molecularly defined classes of nociceptors,31,92 and to test systematically the possibility that transitions can occur between NA and RA types.
A possibility that must be considered is that the NA type is enriched by the stressful procedures involved in DRG removal, neuronal dissociation, and whole-cell recording—either because NA neurons are more likely to survive these procedures or because injury-related stresses promote a transition to the NA state. Dissociation can increase repetitive firing and/or OA in invertebrate and mammalian nociceptors.52,120 However, the naive group in this study and our other SCI studies showed much lower SA incidence in vitro than did the SCI group. Moreover, a high incidence of OA after SCI was also found both in vitro and in vivo—generated by C fibers in or near the DRG.11 This suggests that OA mechanisms defined in dissociated nociceptors are sufficiently similar to those operating in vivo to provide detailed insights that can guide in vivo investigations. Our unexpected finding that sham surgery (which injures deep tissues, including muscle and bone) moderately enhanced SA and hyperexcitability is consistent with the idea that diverse injury-related stresses can be detected by nociceptors to promote entry into a hyperactive state, with the probability of entry into this state increasing with the severity of injury.97
A significant discovery was that enhancement of nociceptor DSFs can be a major contributor to OA. Large, randomly occurring fluctuations of RMP have long been noticed in small- and medium-sized DRG neurons, and they were proposed to explain the irregular firing patterns characteristic of nociceptor OA more than 2 decades ago,87 which our observations confirm. However, that study attributed the generation of OA after chronic constriction injury entirely to the observed reduction in AP threshold.87 By contrast, our finding that SCI caused an increase in frequency of large DSFs that closely paralleled the frequency of APs during SA shows that enhancement of DSFs during a neuropathic pain condition plays a major role in OA. The biophysical and cell signaling mechanisms that generate and enhance DSFs have not been described and are under investigation. These mechanisms are likely to be complex, in part because they interact with complementary mechanisms that reduce AP threshold and depolarize RMP.8 Identification of DSF mechanisms may help in the development of more targeted treatments for ongoing pain.
Our findings indicate that nociceptor specializations for OA are likely to contribute to forms of ongoing pain other than SCI pain. In particular, 5-HT in combination with extrinsic depolarization acutely potentiated OA by inducing 2 of the 3 physiological changes that can drive SA. This finding indicates that intrinsic mechanisms that generate SA chronically after SCI can also promote acute OA when engaged by extrinsic input. It is likely that combinations of all 3 physiological changes drive nociceptor OA in many ongoing pain conditions. Nociceptors are modulated by numerous inflammatory mediators, extrinsic damage-associated molecular patterns, and neuromodulators, some of which cause multiple sensitizing or excitatory effects.33,41,59,98,99 Given the plethora of signals released for long periods during significant injury and/or inflammation, it seems likely that depolarization, lowering of AP threshold, and enhancement of DSFs are produced simultaneously by combined actions of extrinsic signals and intrinsic alterations to drive persistent OA in NA nociceptors and consequent ongoing pain in many conditions.
The selective expression within NA nociceptors of multiple physiological processes that intrinsically promote OA and synergize with extrinsic inflammatory inputs suggests that nociceptor OA in neuropathic conditions need not be a purely pathological side effect of neuropathy, as is sometimes suggested.21 Nociceptor OA may represent a natural function mediated by complementary specializations within a distinct NA type (or NA state) that maintains OA under injury- and inflammation-related conditions. These specializations may also promote somally generated OA when the soma is disconnected traumatically from peripheral terminals. Although purely pathological forms of nociceptor OA certainly exist,47,62,67,76 these are likely to engage OA mechanisms that evolved for adaptive functions. A plausible adaptive function for OA is to maintain continuous protective awareness of severely injured (and thus highly vulnerable) tissue.22,96,97 Thus, persistent nociceptor OA, by continuously driving vigilance and guarding behavior, would complement the protection afforded by sensitization and allodynia.
Most research on pain-related OA in rodent sensory neurons has measured allodynia rather than ongoing pain and has focused on allodynic consequences of OA generated in Aβ fibers that are usually non-nociceptive.24,28,55,71,88,112 By contrast, studies of OA in human sensory neurons have largely focused on nociceptor OA. Microneurographic recordings of intact peripheral nerves have shown strong associations between self-reports of ongoing pain and OA in C-fiber units in patients with peripheral neuropathy (including diabetic, chemotherapy-induced, systemic lupus erythematosus, and idiopathic neuropathy) or fibromyalgia.46,64,67,81,82 Although the low signal-to-noise ratio of this technique has not allowed for precise determination of the rates and patterns of firing, the human OA appears to be of low frequency and irregular pattern, and it resembles C-fiber OA measured with the same methods in rat neuropathy models.81 These and other rodent neuropathy models have revealed low-frequency, irregular C-fiber OA.1,13,14,26,27,35,45,101,105,107,109,113 Evidence that this OA drives ongoing pain in rodents comes from correlations between nociceptor OA and spontaneous foot lifting after inflammation or nerve injury,27 from conditioned preference for a place (CPP) paired with blockade of afferent activity generated at sites of injury or inflammation,19,39,65,66 and from blocking CPP after SCI by knockdown of a sensory neuron–specific Na+ channel that is expressed primarily in nociceptors.117 Some of the human and rodent nociceptor OA may arise within somata in DRG, as we have found in our rat SCI model.11 Suggestive evidence that injury-related ongoing pain in humans can be driven by nociceptor OA generated within DRG has come from relief of amputation pain in patients produced by local delivery of lidocaine to the DRG.93 Thus, mechanisms found to drive OA in dissociated rodent nociceptor somata may provide insight into mechanisms that promote ongoing pain in humans.
The authors have no conflict of interest to declare.
This work was supported by National Institute of Neurological Diseases and Stroke Grant NS091759 to C.W. Dessauer and E.T. Walters, US Army Medical Research Grant W81XWH-12-1-0504 to E.T. Walters, and a Zilkha Family Fellowship to M.A. Odem.
The authors thank Guo-Ying Xu, Kendra Wicks, and Tamara McGhee for technical assistance.
Supplemental video content
Video content associated with this article can be found online at http://links.lww.com/PAIN/A629.
. Ali Z, Ringkamp M, Hartke TV, Chien HF, Flavahan NA, Campbell JN, Meyer RA. Uninjured C-fiber nociceptors develop spontaneous activity
and alpha-adrenergic sensitivity following L6 spinal nerve ligation in monkey. J Neurophysiol 1999;81:455–66.
. Amir R, Liu CN, Kocsis JD, Devor M. Oscillatory mechanism in primary sensory neurones. Brain 2002;125:421–35.
. Amir R, Michaelis M, Devor M. Membrane potential oscillations in dorsal root ganglion neurons: role in normal electrogenesis and neuropathic pain. J Neurosci 1999;19:8589–96.
. Amir R, Michaelis M, Devor M. Burst discharge in primary sensory neurons: triggered by subthreshold oscillations, maintained by depolarizing afterpotentials. J Neurosci 2002;22:1187–98.
. Baccaglini PI, Hogan PG. Some rat sensory neurons in culture express characteristics of differentiated pain sensory cells. Proc Natl Acad Sci U S A 1983;80:594–8.
. Baron R, Hans G, Dickenson AH. Peripheral input and its importance for central sensitization. Ann Neurol 2013;74:630–6.
. Basso DM, Beattie MS, Bresnahan JC. A sensitive and reliable locomotor rating scale for open field testing in rats. J Neurotrauma 1995;12:1–21.
. Bavencoffe A, Li Y, Wu Z, Yang Q, Herrera J, Kennedy EJ, Walters ET, Dessauer CW. Persistent electrical activity in primary nociceptors after spinal cord injury
is maintained by scaffolded adenylyl cyclase and protein kinase A and is associated with altered adenylyl cyclase regulation. J Neurosci 2016;36:1660–8.
. Beaudry H, Daou I, Ase AR, Ribeiro-da-Silva A, Séguéla P. Distinct behavioral responses evoked by selective optogenetic stimulation of the major TRPV1+ and MrgD+ subsets of C-fibers. PAIN 2017;158:2329–39.
. Bedi SS, Lago MT, Masha LI, Crook RJ, Grill RJ, Walters ET. Spinal cord injury
triggers an intrinsic growth-promoting state in nociceptors. J Neurotrauma 2012;29:925–35.
. Bedi SS, Yang Q, Crook RJ, Du J, Wu Z, Fishman HM, Grill RJ, Carlton SM, Walters ET. Chronic spontaneous activity
generated in the somata of primary nociceptors is associated with pain-related behavior after spinal cord injury
. J Neurosci 2010;30:14870–82.
. Bennett GJ. What is spontaneous pain
and who has it. J Pain 2012;13:921–9.
. Bernal L, Lopez-Garcia JA, Roza C. Spontaneous activity
in C-fibres after partial damage to the saphenous nerve in mice: effects of retigabine. Eur J Pain 2016;20:1335–45.
. Bove GM. Focal nerve inflammation induces neuronal signs consistent with symptoms of early complex regional pain syndromes. Exp Neurol 2009;219:223–7.
. Bromm B, Treede RD. Nerve fibre discharges, cerebral potentials and sensations induced by CO2 laser stimulation. Hum Neurobiol 1984;3:33–40.
. Burchiel KJ. Effects of electrical and mechanical stimulation on two foci of spontaneous activity
which develop in primary afferent neurons after peripheral axotomy. PAIN 1984;18:249–65.
. Cardenas LM, Cardenas CG, Scroggs RS. 5HT increases excitability of nociceptor-like rat dorsal root ganglion neurons via cAMP-coupled TTX-resistant Na(+) channels. J Neurophysiol 2001;86:241–8.
. Carlton SM, Du J, Tan HY, Nesic O, Hargett GL, Bopp AC, Yamani A, Lin Q, Willis WD, Hulsebosch CE. Peripheral and central sensitization in remote spinal cord regions contribute to central neuropathic pain after spinal cord injury
. PAIN 2009;147:265–76.
. Chen J, Winston JH, Fu Y, Guptarak J, Jensen KL, Shi XZ, Green TA, Sarna SK. Genesis of anxiety, depression, and ongoing abdominal discomfort in ulcerative colitis-like colon inflammation. Am J Physiol Regul Integr Comp Physiol 2015;308:R18–27.
. Choi JS, Dib-Hajj SD, Waxman SG. Differential slow inactivation and use-dependent inhibition of Nav1.8 channels contribute to distinct firing properties in IB4+ and IB4-DRG neurons. J Neurophysiol 2007;97:1258–65.
. Costigan M, Scholz J, Woolf CJ. Neuropathic pain: a maladaptive response of the nervous system to damage. Annu Rev Neurosci 2009;32:1–32.
. Crook RJ, Hanlon RT, Walters ET. Squid have nociceptors that display widespread long-term sensitization and spontaneous activity
after bodily injury. J Neurosci 2013;33:10021–6.
. Daou I, Tuttle AH, Longo G, Wieskopf JS, Bonin RP, Ase AR, Wood JN, De Koninck Y, Ribeiro-da-Silva A, Mogil JS, Seguela P. Remote optogenetic activation and sensitization of pain pathways in freely moving mice. J Neurosci 2013;33:18631–40.
. Devor M. Ectopic discharge in Abeta afferents as a source of neuropathic pain. Exp Brain Res 2009;196:115–28.
. Djouhri L. Electrophysiological evidence for the existence of a rare population of C-fiber low threshold mechanoreceptive (C-LTM) neurons in glabrous skin of the rat hindpaw. Neurosci Lett 2016;613:25–9.
. Djouhri L, Fang X, Koutsikou S, Lawson SN. Partial nerve injury induces electrophysiological changes in conducting (uninjured) nociceptive and nonnociceptive DRG neurons: possible relationships to aspects of peripheral neuropathic pain and paresthesias. PAIN 2012;153:1824–36.
. Djouhri L, Koutsikou S, Fang X, McMullan S, Lawson SN. Spontaneous pain
, both neuropathic and inflammatory, is related to frequency of spontaneous firing in intact C-fiber nociceptors. J Neurosci 2006;26:1281–92.
. Djouhri L, Lawson SN. Abeta-fiber nociceptive primary afferent neurons: a review of incidence and properties in relation to other afferent A-fiber neurons in mammals. Brain Res Brain Res Rev 2004;46:131–45.
. Dong H, Fan YH, Wang YY, Wang WT, Hu SJ. Lidocaine suppresses subthreshold oscillations by inhibiting persistent Na(+) current in injured dorsal root ganglion neurons. Physiol Res 2008;57:639–45.
. Douglas DH, Peucker TK. Algorithms for the reduction of the number of points required to represent a digitized line or its caricature. Cartographica Int J Geogr Inf Geovisualization 1973;10:112–22.
. Fang X, McMullan S, Lawson SN, Djouhri L. Electrophysiological differences between nociceptive and non-nociceptive dorsal root ganglion neurones in the rat in vivo. J Physiol 2005;565:927–43.
. Gold MS, Dastmalchi S, Levine JD. Co-expression of nociceptor properties in dorsal root ganglion neurons from the adult rat in vitro. Neuroscience 1996;71:265–75.
. Gold MS, Gebhart GF. Nociceptor sensitization in pain pathogenesis. Nat Med 2010;16:1248–57.
. Gold MS, Reichling DB, Shuster MJ, Levine JD. Hyperalgesic agents increase a tetrodotoxin-resistant Na+ current in nociceptors. Proc Natl Acad Sci U S A 1996;93:1108–12.
. Gorodetskaya N, Constantin C, Janig W. Ectopic activity in cutaneous regenerating afferent nerve fibers following nerve lesion in the rat. Eur J Neurosci 2003;18:2487–97.
. Gracely RH, Lynch SA, Bennett GJ. Painful neuropathy: altered central processing maintained dynamically by peripheral input. PAIN 1992;51:175–94.
. Haroutounian S, Nikolajsen L, Bendtsen TF, Finnerup NB, Kristensen AD, Hasselstrom JB, Jensen TS. Primary afferent input critical for maintaining spontaneous pain
in peripheral neuropathy. PAIN 2014;155:1272–9.
. Harper AA. Similarities between some properties of the soma and sensory receptors of primary afferent neurones. Exp Physiol 1991;76:369–77.
. Havelin J, Imbert I, Cormier J, Allen J, Porreca F, King T. Central sensitization and neuropathic features of ongoing pain in a rat model of advanced osteoarthritis. J Pain 2016;17:374–82.
. Hu J, Lewin GR. Mechanosensitive currents in the neurites of cultured mouse sensory neurones. J Physiol 2006;577:815–28.
. Ji RR, Chamessian A, Zhang YQ. Pain regulation by non-neuronal cells and inflammation. Science 2016;354:572–7.
. Kajander KC, Wakisaka S, Bennett GJ. Spontaneous discharge originates in the dorsal root ganglion at the onset of a painful peripheral neuropathy in the rat. Neurosci Lett 1992;138:225–8.
. Kelly S, Dunham JP, Murray F, Read S, Donaldson LF, Lawson SN. Spontaneous firing in C-fibers and increased mechanical sensitivity in A-fibers of knee joint-associated mechanoreceptive primary afferent neurones during MIA-induced osteoarthritis in the rat. Osteoarthritis Cartilage 2012;20:305–13.
. Khomula EV, Ferrari LF, Araldi D, Levine JD. Sexual dimorphism in a reciprocal interaction of ryanodine and IP3 receptors in the induction of hyperalgesic priming. J Neurosci 2017;37:2032–44.
. Kirillova I, Rausch VH, Tode J, Baron R, Janig W. Mechano- and thermosensitivity of injured muscle afferents. J Neurophysiol 2011;105:2058–73.
. Kleggetveit IP, Namer B, Schmidt R, Helås T, Rückel M, Ørstavik K, Schmelz M, Jørum E. High spontaneous activity
of C-nociceptors in painful polyneuropathy. PAIN 2012;153:2040–7.
. Kleggetveit IP, Schmidt R, Namer B, Salter H, Helås T, Schmelz M, Jørum E. Pathological nociceptors in two patients with erythromelalgia-like symptoms and rare genetic Nav 1.9 variants. Brain Behav 2016;6:e00528.
. Kovalsky Y, Amir R, Devor M. Subthreshold oscillations facilitate neuropathic spike discharge by overcoming membrane accommodation. Exp Neurol 2008;210:194–206.
. Kovalsky Y, Amir R, Devor M. Simulation in sensory neurons reveals a key role for delayed Na+ current in subthreshold oscillations and ectopic discharge: implications for neuropathic pain. J Neurophysiol 2009;102:1430–42.
. Kuner R, Flor H. Structural plasticity and reorganisation in chronic pain. Nat Rev Neurosci 2016;18:20–30.
. Lawson SN, McCarthy PW, Prabhakar E. Electrophysiological properties of neurones with CGRP-like immunoreactivity in rat dorsal root ganglia. J Comp Neurol 1996;365:355–66.
. Liao X, Gunstream JD, Lewin MR, Ambron RT, Walters ET. Activation of protein kinase A contributes to the expression but not the induction of long-term hyperexcitability caused by axotomy of Aplysia sensory neurons. J Neurosci 1999;19:1247–56.
. Lin SY, Chang WJ, Lin CS, Huang CY, Wang HF, Sun WH. Serotonin
receptor 5-HT2B mediates serotonin
-induced mechanical hyperalgesia. J Neurosci 2011;31:1410–18.
. Liu CN, Devor M, Waxman SG, Kocsis JD. Subthreshold oscillations induced by spinal nerve injury in dissociated muscle and cutaneous afferents of mouse DRG. J Neurophysiol 2002;87:2009–17.
. Liu CN, Wall PD, Ben-Dor E, Michaelis M, Amir R, Devor M. Tactile allodynia in the absence of C-fiber activation: altered firing properties of DRG neurons following spinal nerve injury. PAIN 2000;85:503–21.
. Lundberg LE, Jørum E, Holm E, Torebjörk HE. Intra-neural electrical stimulation of cutaneous nociceptive fibres in humans: effects of different pulse patterns on magnitude of pain. Acta Physiol Scand 1992;146:41–8.
. Marchettini P, Simone DA, Caputi G, Ochoa JL. Pain from excitation of identified muscle nociceptors in humans. Brain Res 1996;740:109–16.
. Mathers DA, Barker JL. Spontaneous voltage and current fluctuations in tissue cultured mouse dorsal root ganglion cells. Brain Res 1984;293:35–47.
. McMahon SB, La Russa F, Bennett DL. Crosstalk between the nociceptive and immune systems in host defence and disease. Nat Rev Neurosci 2015;16:389–402.
. Meyer RA, Raja SN, Campbell JN, Mackinnon SE, Dellon AL. Neural activity originating from a neuroma in the baboon. Brain Res 1985;325:255–60.
. Michaelis M, Blenk KH, Janig W, Vogel C. Development of spontaneous activity
and mechanosensitivity in axotomized afferent nerve fibers during the first hours after nerve transection in rats. J Neurophysiol 1995;74:1020–7.
. Namer B, Ørstavik K, Schmidt R, Kleggetveit IP, Weidner C, Mørk C, Kvernebo MS, Kvernebo K, Salter H, Carr TH, Segerdahl M, Quiding H, Waxman SG, Handwerker HO, Torebjörk HE, Jørum E, Schmelz M. Specific changes in conduction velocity recovery cycles of single nociceptors in a patient with erythromelalgia with the I848T gain-of-function mutation of Nav1.7. PAIN 2015;156:1637–46.
. Ochoa J, Torebjörk E. Sensations evoked by intraneural microstimulation of C nociceptor fibres in human skin nerves. J Physiol 1989;415:583–99.
. Ochoa JL, Campero M, Serra J, Bostock H. Hyperexcitable polymodal and insensitive nociceptors in painful human neuropathy. Muscle Nerve 2005;32:459–72.
. Okun A, DeFelice M, Eyde N, Ren J, Mercado R, King T, Porreca F. Transient inflammation-induced ongoing pain is driven by TRPV1 sensitive afferents. Mol Pain 2011;7:4.
. Okun A, Liu P, Davis P, Ren J, Remeniuk B, Brion T, Ossipov MH, Xie J, Dussor GO, King T, Porreca F. Afferent drive elicits ongoing pain in a model of advanced osteoarthritis. PAIN 2012;153:924–33.
. Orstavik K, Namer B, Schmidt R, Schmelz M, Hilliges M, Weidner C, Carr RW, Handwerker H, Jorum E, Torebjork HE. Abnormal function of C-fibers in patients with diabetic neuropathy. J Neurosci 2006;26:11287–94.
. Patrick Harty T, Waxman SG. Inactivation properties of sodium channel Nav1.8 maintain action potential amplitude in small DRG neurons in the context of depolarization. Mol Pain 2007;3:12.
. Petruska JC, Napaporn J, Johnson RD, Cooper BY. Chemical responsiveness and histochemical phenotype of electrophysiologically classified cells of the adult rat dorsal root ganglion. Neuroscience 2002;115:15–30.
. Petruska JC, Napaporn J, Johnson RD, Gu JG, Cooper BY. Subclassified acutely dissociated cells of rat DRG: histochemistry and patterns of capsaicin-, proton-, and ATP-activated currents. J Neurophysiol 2000;84:2365–79.
. Pitcher GM, Henry JL. Cellular mechanisms of hyperalgesia and spontaneous pain
in a spinalized rat model of peripheral neuropathy: changes in myelinated afferent inputs implicated. Eur J Neurosci 2000;12:2006–20.
. Pitcher GM, Henry JL. Governing role of primary afferent drive in increased excitation of spinal nociceptive neurons in a model of sciatic neuropathy. Exp Neurol 2008;214:219–28.
. Pogatzki EM, Gebhart GF, Brennan TJ. Characterization of Adelta- and C-fibers innervating the plantar rat hindpaw one day after an incision. J Neurophysiol 2002;87:721–31.
. Ramer U. An iterative procedure for the polygonal approximation of plane curves. Comput Graph Image Process 1972;1:244–56.
. Ratté S, Zhu Y, Lee KY, Prescott SA. Criticality and degeneracy in injury-induced changes in primary afferent excitability and the implications for neuropathic pain. Elife 2014;3:e02370.
. Sagafos D, Kleggetveit IP, Helas T, Schmidt R, Minde J, Namer B, Schmelz M, Jorum E. Single-fiber recordings of nociceptive fibers in patients with HSAN type V with congenital insensitivity to pain. Clin J Pain 2016;32:636–42.
. Salzer I, Gantumur E, Yousuf A, Boehm S. Control of sensory neuron excitability by serotonin
involves 5HT2C receptors and Ca(2+)-activated chloride channels. Neuropharmacology 2016;110:277–86.
. Schmelz M, Schmidt R, Weidner C, Hilliges M, Torebjork HE, Handwerker HO. Chemical response pattern of different classes of C-nociceptors to pruritogens and algogens. J Neurophysiol 2003;89:2441–8.
. Scroggs RS. Up-regulation of low-threshold tetrodotoxin-resistant Na+ current via activation of a cyclic AMP/protein kinase A pathway in nociceptor-like rat dorsal root ganglion cells. Neuroscience 2011;186:13–20.
. Sekerli M, Del Negro CA, Lee RH, Butera RJ. Estimating action potential thresholds from neuronal time-series: new metrics and evaluation of methodologies. IEEE Trans Biomed Eng 2004;51:1665–72.
. Serra J, Bostock H, Sola R, Aleu J, Garcia E, Cokic B, Navarro X, Quiles C. Microneurographic identification of spontaneous activity
in C-nociceptors in neuropathic pain states in humans and rats. PAIN 2012;153:42–55.
. Serra J, Collado A, Sola R, Antonelli F, Torres X, Salgueiro M, Quiles C, Bostock H. Hyperexcitable C nociceptors in fibromyalgia. Ann Neurol 2014;75:196–208.
. Shin DS, Kim EH, Song KY, Hong HJ, Kong MH, Hwang SJ. Neurochemical characterization of the TRPV1-positive nociceptive primary afferents innervating skeletal muscles in the rats. J Korean Neurosurg Soc 2008;43:97–104.
. Sommer C. Serotonin
in pain and analgesia: actions in the periphery. Mol Neurobiol 2004;30:117–25.
. Song XJ, Zhang JM, Hu SJ, LaMotte RH. Somata of nerve-injured sensory neurons exhibit enhanced responses to inflammatory mediators. PAIN 2003;104:701–9.
. Stucky CL, Lewin GR. Isolectin B(4)-positive and -negative nociceptors are functionally distinct. J Neurosci 1999;19:6497–505.
. Study RE, Kral MG. Spontaneous action potential activity in isolated dorsal root ganglion neurons from rats with a painful neuropathy. PAIN 1996;65:235–42.
. Suter MR, Berta T, Gao YJ, Decosterd I, Ji RR. Large A-fiber activity is required for microglial proliferation and p38 MAPK activation in the spinal cord: different effects of resiniferatoxin and bupivacaine on spinal microglial changes after spared nerve injury. Mol Pain 2009;5:53.
. Taiwo YO, Levine JD. Serotonin
is a directly-acting hyperalgesic agent in the rat. Neuroscience 1992;48:485–90.
. Tappe-Theodor A, Constantin CE, Tegeder I, Lechner SG, Langeslag M, Lepcynzsky P, Wirotanseng RI, Kurejova M, Agarwal N, Nagy G, Todd A, Wettschureck N, Offermanns S, Kress M, Lewin GR, Kuner R. Gα(q/11) signaling tonically modulates nociceptor function and contributes to activity-dependent sensitization. PAIN 2012;153:184–96.
. Treede RD. Transduction and transmission properties of primary nociceptive afferents. Ross Fiziol Zh Im I M Sechenova 1999;85:205–11.
. Usoskin D, Furlan A, Islam S, Abdo H, Lönnerberg P, Lou D, Hjerling-Leffler J, Haeggström J, Kharchenko O, Kharchenko PV, Linnarsson S, Ernfors P. Unbiased classification of sensory neuron types by large-scale single-cell RNA sequencing. Nat Neurosci 2015;18:145–53.
. Vaso A, Adahan HM, Gjika A, Zahaj S, Zhurda T, Vyshka G, Devor M. Peripheral nervous system origin of phantom limb pain. PAIN 2014;155:1384–91.
. Wall PD, Devor M. Sensory afferent impulses originate from dorsal root ganglia as well as from the periphery in normal and nerve injured rats. PAIN 1983;17:321–39.
. Wall PD, Gutnick M. Properties of afferent nerve impulses originating from a neuroma. Nature 1974;248:740–3.
. Walters ET. Injury-related behavior and neuronal plasticity: an evolutionary perspective on sensitization, hyperalgesia, and analgesia. Int Rev Neurobiol 1994;36:325–427.
. Walters ET. Nociceptors as chronic drivers of pain and hyperreflexia after spinal cord injury
: an adaptive-maladaptive hyperfunctional state hypothesis. Front Physiol 2012;3:309.
. Walters ET. Neuroinflammatory contributions to pain after SCI: roles for central glial mechanisms and nociceptor-mediated host defense. Exp Neurol 2014;258:48–61.
. Walters ET. How is chronic pain related to sympathetic dysfunction and autonomic dysreflexia following spinal cord injury
. Auton Neurosci 2018;209:79–89.
. Wang S, Lim J, Joseph J, Wang S, Wei F, Ro JY, Chung MK. Spontaneous and bite-evoked muscle pain are mediated by a common nociceptive pathway with differential contribution by TRPV1. J Pain 2017;18:1333–45.
. Wang T, Hurwitz O, Shimada SG, Qu L, Fu K, Zhang P, Ma C, LaMotte RH. Chronic compression of the dorsal root ganglion enhances mechanically evoked pain behavior and the activity of cutaneous nociceptors in mice. PLoS One 2015;10:e0137512.
. Wiesenfeld-Hallin Z, Hallin RG, Persson A. Do large diameter cutaneous afferents have a role in the transmission of nociceptive messages? Brain Res 1984;311:375–9.
. Woolf CJ. Central sensitization: implications for the diagnosis and treatment of pain. PAIN 2011;152:S2–15.
. Woolf CJ, Ma Q. Nociceptors–noxious stimulus detectors. Neuron 2007;55:353–64.
. Wu G, Ringkamp M, Hartke TV, Murinson BB, Campbell JN, Griffin JW, Meyer RA. Early onset of spontaneous activity
in uninjured C-fiber nociceptors after injury to neighboring nerve fibers. J Neurosci 2001;21:RC140.
. Wu Z, Li L, Xie F, Du J, Zuo Y, Frost JA, Carlton SM, Walters ET, Yang Q. Activation of KCNQ channels suppresses spontaneous activity
in dorsal root ganglion neurons and reduces chronic pain after spinal cord injury
. J Neurotrauma 2017;34:1260–70.
. Wu Z, Yang Q, Crook RJ, O'Neil RG, Walters ET. TRPV1 channels make major contributions to behavioral hypersensitivity and spontaneous activity
in nociceptors after spinal cord injury
. PAIN 2013;154:2130–41.
. Xiao WH, Bennett GJ. Persistent low-frequency spontaneous discharge in A-fiber and C-fiber primary afferent neurons during an inflammatory pain condition. Anesthesiology 2007;107:813–21.
. Xiao WH, Bennett GJ. Chemotherapy-evoked neuropathic pain: abnormal spontaneous discharge in A-fiber and C-fiber primary afferent neurons and its suppression by acetyl-L-carnitine. PAIN 2008;135:262–70.
. Xie W, Strong JA, Kim D, Shahrestani S, Zhang JM. Bursting activity in myelinated sensory neurons plays a key role in pain behavior induced by localized inflammation of the rat sensory ganglion. Neuroscience 2012;206:212–23.
. Xie W, Strong JA, Ye L, Mao JX, Zhang JM. Knockdown of sodium channel NaV1.6 blocks mechanical pain and abnormal bursting activity of afferent neurons in inflamed sensory ganglia. PAIN 2013;154:1170–80.
. Xie W, Strong JA, Zhang JM. Local knockdown of the NaV1.6 sodium channel reduces pain behaviors, sensory neuron excitability, and sympathetic sprouting in rat models of neuropathic pain. Neuroscience 2015;291:317–30.
. Xie Y, Zhang J, Petersen M, LaMotte RH. Functional changes in dorsal root ganglion cells after chronic nerve constriction in the rat. J Neurophysiol 1995;73:1811–20.
. Xing JL, Hu SJ, Jian Z, Duan JH. Subthreshold membrane potential oscillation mediates the excitatory effect of norepinephrine in chronically compressed dorsal root ganglion neurons in the rat. PAIN 2003;105:177–83.
. Xu J, Brennan TJ. Guarding pain and spontaneous activity
of nociceptors after skin versus skin plus deep tissue incision. Anesthesiology 2010;112:153–64.
. Xu Q, Yaksh TL. A brief comparison of the pathophysiology of inflammatory versus neuropathic pain. Curr Opin Anaesthesiol 2011;24:400–7.
. Yang Q, Wu Z, Hadden JK, Odem MA, Zuo Y, Crook RJ, Frost JA, Walters ET. Persistent pain after spinal cord injury
is maintained by primary afferent activity. J Neurosci 2014;34:10765–9.
. Zeitz KP, Guy N, Malmberg AB, Dirajlal S, Martin WJ, Sun L, Bonhaus DW, Stucky CL, Julius D, Basbaum AI. The 5-HT3 subtype of serotonin
receptor contributes to nociceptive processing via a novel subset of myelinated and unmyelinated nociceptors. J Neurosci 2002;22:1010–19.
. Zhang H, Dougherty PM. Enhanced excitability of primary sensory neurons and altered gene expression of neuronal ion channels in dorsal root ganglion in paclitaxel-induced peripheral neuropathy. Anesthesiology 2014;120:1463–75.
. Zheng JH, Walters ET, Song XJ. Dissociation of dorsal root ganglion neurons induces hyperexcitability that is maintained by increased responsiveness to cAMP and cGMP. J Neurophysiol 2007;97:15–25.
. Zhu YF, Henry JL. Excitability of Aβ sensory neurons is altered in an animal model of peripheral neuropathy. BMC Neurosci 2012;13:15.