Contralateral Afferent Input to Lumbar Lamina I Neurons as a Neural Substrate for Mirror-Image Pain.

Mirror-image pain arises from pathologic alterations in the nociceptive processing network that controls functional lateralization of the primary afferent input. Although a number of clinical syndromes related to dysfunction of the lumbar afferent system are associated with the mirror-image pain, its morphophysiological substrate and mechanism of induction remain poorly understood. Therefore, we used ex vivo spinal cord preparation of young rats of both sexes to study organization and processing of the contralateral afferent input to the neurons in the major spinal nociceptive projection area Lamina I. We show that decussating primary afferent branches reach contralateral Lamina I, where 27% of neurons, including projection neurons, receive monosynaptic and/or polysynaptic excitatory drive from the contralateral Aδ-fibers and C-fibers. All these neurons also received ipsilateral input, implying their involvement in the bilateral information processing. Our data further show that the contralateral Aδ-fiber and C-fiber input is under diverse forms of inhibitory control. Attenuation of the afferent-driven presynaptic inhibition and/or disinhibition of the dorsal horn network increased the contralateral excitatory drive to Lamina I neurons and its ability to evoke action potentials. Furthermore, the contralateral Aßδ-fibers presynaptically control ipsilateral C-fiber input to Lamina I neurons. Thus, these results show that some lumbar Lamina I neurons are wired to the contralateral afferent system whose input, under normal conditions, is subject to inhibitory control. A pathologic disinhibition of the decussating pathways can open a gate controlling contralateral information flow to the nociceptive projection neurons and, thus, contribute to induction of hypersensitivity and mirror-image pain.SIGNIFICANCE STATEMENT We show that contralateral Aδ-afferents and C-afferents supply lumbar Lamina I neurons. The contralateral input is under diverse forms of inhibitory control and itself controls the ipsilateral input. Disinhibition of decussating pathways increases nociceptive drive to Lamina I neurons and may cause induction of contralateral hypersensitivity and mirror-image pain.

Effects of inflammation on the properties of Nav1.8-ChR2-positive and Nav1.8-ChR2-negative afferent mechanoreceptors in the hindpaw glabrous skin of mice.

We recently used Nav1.8-ChR2 mice in which Nav1.8-expressing afferents were optogenetically tagged to classify mechanosensitive afferents into Nav1.8-ChR2-positive and Nav1.8-ChR2-negative mechanoreceptors. We found that the former were mainly high threshold mechanoreceptors (HTMRs), while the latter were low threshold mechanoreceptors (LTMRs). In the present study, we further investigated whether the properties of these mechanoreceptors were altered following tissue inflammation. Nav1.8-ChR2 mice received a subcutaneous injection of saline or Complete Freund's Adjuvant (CFA) in the hindpaws. Using the hind paw glabrous skin-tibial nerve preparation and the pressure-clamped single-fiber recordings, we found that CFA-induced hind paw inflammation lowered the mechanical threshold of many Nav1.8-ChR2-positive Aß-fiber mechanoreceptors but heightened the mechanical threshold of many Nav1.8-ChR2-negative Aß-fiber mechanoreceptors. Spontaneous action potential impulses were not observed in Nav1.8-ChR2-positive Aß-fiber mechanoreceptors but occurred in Nav1.8-ChR2-negative Aß-fiber mechanoreceptors with a lower mechanical threshold in the saline goup, and a higher mechanical threshold in the CFA group. No significant change was observed in the mechanical sensitivity of Nav1.8-ChR2-positive and Nav1.8-ChR2-negative Aδ-fiber mechanoreceptors and Nav1.8-ChR2-positive C-fiber mechanoreceptors following hind paw inflammation. Collectively, inflammation significantly altered the functional properties of both Nav1.8-ChR2-positive and Nav1.8-ChR2-negative Aß-fiber mechanoreceptors, which may contribute to mechanical allodynia during inflammation.

Presynaptic Interactions between Trigeminal and Cervical Nociceptive Afferents Supplying Upper Cervical Lamina I Neurons.

Cervical and trigeminal afferents innervate neighboring cranial territories, and their convergence on upper cervical dorsal horn neurons provides a potential substrate for pain referral in primary headache syndromes. Lamina I neurons are central to this mechanism, as they relay convergent nociceptive input to supraspinal pain centers. Unfortunately, little is known about the interactions between trigeminal and cervical afferents supplying Lamina I neurons. Here, we used rats of both sexes to show that cervical and trigeminal afferents interact via presynaptic inhibition, where monosynaptic inputs to Lamina I neurons undergo unidirectional as well as reciprocal presynaptic control. This means that afferent-driven presynaptic inhibition shapes the way trigeminal and cervical Aδ-fiber and C-fiber input reaches Lamina I projection neurons (PNs) and local-circuit neurons (LCNs). We propose that this inhibition provides a feedforward control of excitatory drive to Lamina I neurons that regulates their convergent and cervical-specific or trigeminal-specific processing modes. As a consequence, disruption of the trigeminal and cervical afferent-driven presynaptic inhibition may contribute to development of primary headache syndromes.SIGNIFICANCE STATEMENT Cervical and trigeminal afferents innervate neighboring cranial territories, and their convergence on upper cervical dorsal horn neurons provides a potential substrate for pain referral in primary headache syndromes. Lamina I neurons are central to this mechanism as they relay convergent nociceptive input to supraspinal pain centers. Here, we show that cervical and trigeminal afferents interact via presynaptic inhibition, where inputs to Lamina I neurons undergo unidirectional as well as reciprocal control. The afferent-driven presynaptic inhibition shapes the trigeminocervical Aδ-fiber and C-fiber input to Lamina I neurons. This inhibition provides control of excitatory drive to Lamina I neurons that regulates their convergent and cervical-specific or trigeminal-specific processing modes. Disruption of this control may contribute to development of primary headache syndromes.

A topographical and physiological exploration of C-tactile afferents and their response to menthol and histamine.

Unmyelinated tactile (C-tactile or CT) afferents are abundant in arm hairy skin and have been suggested to signal features of social affective touch. Here, we recorded from unmyelinated low-threshold mechanosensitive afferents in the peroneal and radial nerves. The most distal receptive fields were located on the proximal phalanx of the third finger for the superficial branch of the radial nerve and near the lateral malleolus for the peroneal nerve. We found that the physiological properties with regard to conduction velocity and mechanical threshold, as well as their tuning to brush velocity, were similar in CT units across the antebrachial (n = 27), radial (n = 8), and peroneal (n = 4) nerves. Moreover, we found that although CT afferents are readily found during microneurography of the arm nerves, they appear to be much more sparse in the lower leg compared with C-nociceptors. We continued to explore CT afferents with regard to their chemical sensitivity and found that they could not be activated by topical application to their receptive field of either the cooling agent menthol or the pruritogen histamine. In light of previous studies showing the combined effects that temperature and mechanical stimuli have on these neurons, these findings add to the growing body of research suggesting that CT afferents constitute a unique class of sensory afferents with highly specialized mechanisms for transducing gentle touch.NEW & NOTEWORHY Unmyelinated tactile (CT) afferents are abundant in arm hairy skin and are thought to signal features of social affective touch. We show that CTs are also present but are relatively sparse in the lower leg compared with C-nociceptors. CTs display similar physiological properties across the arm and leg nerves. Furthermore, CT afferents do not respond to the cooling agent menthol or the pruritogen histamine, and their mechanical response properties are not altered by these chemicals.

Molecular/Ionic Basis of Vagal Bronchopulmonary C-Fiber Activation by Inflammatory Mediators.

Stimulation of bronchopulmonary vagal afferent C fibers by inflammatory mediators can lead to coughing, chest tightness, and changes in breathing pattern, as well as reflex bronchoconstriction and secretions. These responses serve a defensive function in healthy lungs but likely contribute to many of the signs and symptoms of inflammatory airway diseases. A better understanding of the mechanisms underlying the activation of bronchopulmonary C-fiber terminals may lead to novel therapeutics that would work in an additive or synergic manner with existing anti-inflammatory strategies.

Excitation properties of computational models of unmyelinated peripheral axons.

Biophysically based computational models of nerve fibers are important tools for designing electrical stimulation therapies, investigating drugs that affect ion channels, and studying diseases that affect neurons. Although peripheral nerves are primarily composed of unmyelinated axons (i.e., C-fibers), most modeling efforts focused on myelinated axons. We implemented the single-compartment model of vagal afferents from Schild et al. (1994) (Schild JH, Clark JW, Hay M, Mendelowitz D, Andresen MC, Kunze DL. J Neurophysiol 71: 2338-2358, 1994) and extended the model into a multicompartment axon, presenting the first cable model of a C-fiber vagal afferent. We also implemented the updated parameters from the Schild and Kunze (1997) model (Schild JH, Kunze DL. J Neurophysiol 78: 3198-3209, 1997). We compared the responses of these novel models with those of three published models of unmyelinated axons (Rattay F, Aberham M. IEEE Trans Biomed Eng 40: 1201-1209, 1993; Sundt D, Gamper N, Jaffe DB. J Neurophysiol 114: 3140-3153, 2015; Tigerholm J, Petersson ME, Obreja O, Lampert A, Carr R, Schmelz M, Fransén E. J Neurophysiol 111: 1721-1735, 2014) and with experimental data from single-fiber recordings. Comparing the two models by Schild et al. (1994, 1997) revealed that differences in rest potential and action potential shape were driven by changes in maximum conductances rather than changes in sodium channel dynamics. Comparing the five model axons, the conduction speeds and strength-duration responses were largely within expected ranges, but none of the models captured the experimental threshold recovery cycle-including a complete absence of late subnormality in the models-and their action potential shapes varied dramatically. The Tigerholm et al. (2014) model best reproduced the experimental data, but these modeling efforts make clear that additional data are needed to parameterize and validate future models of autonomic C-fibers.NEW & NOTEWORTHY Peripheral nerves are primarily composed of unmyelinated axons, and there is growing interest in electrical stimulation of the autonomic nervous system to treat various diseases. We present the first cable model of an unmyelinated vagal nerve fiber and compare its ion channel isoforms and conduction responses with other published models of unmyelinated axons, establishing important tools for advancing modeling of autonomic nerves.

Different sensitivity of action potential generation to the rate of depolarization in vagal afferent A-fiber versus C-fiber neurons.

This study demonstrates that the action potential discharge in vagal afferent A-fiber neurons is about 20 times more sensitive to the rate of membrane depolarization compared to C-fiber neurons. The sensitivity of action potential generation to the depolarization rate in vagal sensory neurons is independent of the intensity of current stimuli but nearly abrogated by inhibiting the D-type potassium channel. These findings help better understand the mechanisms that control the activation of vagal afferent nerves.

Spinal GABAergic neurons are under feed-forward inhibitory control driven by Aδ and C fibers in Gad2 td-Tomato mice.

BACKGROUND: Spinal GABAergic neurons act as a critical modulator in sensory transmission like pain or itch. The monosynaptic or polysynaptic primary afferent inputs onto GABAergic neurons, along with other interneurons or projection neurons make up the direct and feed-forward inhibitory neural circuits. Previous research indicates that spinal GABAergic neurons mainly receive excitatory inputs from Aδ and C fibers. However, whether they are controlled by other inhibitory sending signals is not well understood. METHODS: We applied a transgenic mouse line in which neurons co-expressed the GABA-synthesizing enzyme Gad65 and the enhanced red fluorescence (td-Tomato) to characterize the features of morphology and electrophysiology of GABAergic neurons. Patch-clamp whole cell recordings were used to record the evoked postsynaptic potentials of fluorescent neurons in spinal slices in response to dorsal root stimulation. RESULTS: We demonstrated that GABAergic neurons not only received excitatory drive from peripheral Aß, Aδ and C fibers, but also received inhibitory inputs driven by Aδ and C fibers. The evoked inhibitory postsynaptic potentials (eIPSPs) mediated by C fibers were mainly Glycinergic (66.7%) as well as GABAergic mixed with Glycinergic (33.3%), whereas the inhibition mediated by Aδ fibers was predominately both GABA and Glycine-dominant (57.1%), and the rest of which was purely Glycine-dominant (42.9%). CONCLUSION: These results indicated that spinal GABAergic inhibitory neurons are under feedforward inhibitory control driven by primary C and Aδ fibers, suggesting that this feed-forward inhibitory pathway may play an important role in balancing the excitability of GABAergic neurons in spinal dorsal horn.

Deoxycholic acid activates and sensitizes vagal nociceptive afferent C-fibers in guinea pig esophagus.

Bile acid reflux in the esophagus plays a role in the pathogenesis of certain esophageal disorders, where it can induce esophageal pain and heartburn. The present study aimed to determine whether bile acid, deoxycholic acid (DCA), directly activates and sensitizes esophageal vagal nociceptive afferent C-fiber subtypes. DCA-elicited effects on vagal nodose and jugular neurons were studied by calcium imaging. Its effects on esophageal-labeled nodose and jugular neurons were then determined by patch-clamp recording. At nodose and jugular C-fiber nerve endings in the esophagus, DCA-evoked action potentials (APs) were compared by extracellular single-unit recordings in ex vivo esophageal-vagal preparations. DCA application induced calcium influxes in nodose and jugular neurons and elicited inward currents in esophageal-labeled nodose and jugular neurons. In the presence of DCA, the current densities elicited by capsaicin were enhanced in those labeled neurons. Consistently, DCA perfusion at nerve terminals in the esophagus evoked APs in about 50% of esophageal nodose and jugular C-fibers. In DCA-sensitive C-fibers, DCA perfusion also sensitized the fibers such that the subsequent response to capsaicin was amplified. Collectively, these results provide new evidence that DCA directly activates and sensitizes nociceptive nodose and jugular C-fibers in the esophagus. Such activation and sensitization effects may contribute to bile acid-induced esophageal nociceptive symptoms that are refractory to proton-pump inhibitor therapy.NEW & NOTEWORTHY Bile acid reflux in the esophagus can induce pain and heartburn in certain esophageal disorders, but the underlying neuronal mechanism is still unclear. The present study demonstrated that bile acid, deoxycholic acid (DCA), directly activates esophageal vagal afferent nodose and jugular nociceptive C-fibers and sensitizes their response to capsaicin. Such effects may contribute to bile acid-induced esophageal nociceptive symptoms that refractory to proton-pump inhibitors (PPIs) therapy.

Stomach region stimulated determines effects on duodenal motility in rats.

Gastric electrical stimulation (GES) is used clinically to promote proximal GI emptying and motility. In acute experiments, we measured duodenal motor responses elicited by GES applied at 141 randomly chosen electrode sites on the stomach serosal surface. Overnight-fasted (H2O available) anesthetized male rats (n = 81) received intermittent biphasic GES for 5 min (20-s-on/40-s-off cycles; I = 0.3 mA; pw = 0.2 ms; 10 Hz). A strain gauge on the serosal surface of the proximal duodenum of each animal was used to evaluate baseline motor activity and the effect of GES. Using ratios of time blocks compared with a 15-min prestimulation baseline, we evaluated the effects of the 5-min stimulation on concurrent activity, on the 10 min immediately after the stimulation, and on the 15-min period beginning with the onset of stimulation. We mapped the magnitude of the duodenal response (three different motility indices) elicited from the 141 stomach sites. Post hoc electrode site maps associated with duodenal responses suggested three zones similar to the classic regions of forestomach, corpus, and antrum. Maximal excitatory duodenal motor responses were elicited from forestomach sites, whereas inhibitory responses occurred with stimulation of the corpus. Moderate excitatory duodenal responses occurred with stimulation of the antrum. Complex, weak inhibitory/excitatory responses were produced by stimulation at boundaries between stomach regions. Patterns of GES efficacies coincided with distributions of previously mapped vagal afferents, suggesting that excitation of the duodenum is strongest when GES electrodes are situated over stomach concentrations of vagal intramuscular arrays, putative stretch receptors in the muscle wall.

Low mechano-afferent fibers reduce thermal pain but not pain intensity in CRPS.

BACKGROUND: Human hairy (not glabrous skin) is equipped with a subgroup of C-fibers, the C-tactile (CT) fibers. Those do not mediate pain but affective aspects of touch. CT-fiber-activation reduces experimental pain if they are intact. In this pilot study we investigated pain modulating capacities of CT-afferents in CRPS. METHODS: 10 CRPS-patients (mean age 33 years, SEM 3.3) and 11 healthy controls (mean age 43.2 years, SEM 3.9) participated. CT-targeted-touch (brush stroking, velocity: 3 cm/s) was applied on hairy and glabrous skin on the affected and contralateral limb. Patients rated pleasantness of CT-targeted-touch (anchors: 1 "not pleasant"-4 "very pleasant") twice daily on 10 days. Pain intensity (NRS: 0 "no pain" - 10 "worst pain imaginable") was assessed before, 0, 30, 60 and 120 min after each CT-stimulation. To assess sensory changes, quantitative-sensory-testing was performed at the beginning and the end of the trial period. RESULTS: CT-targeted-touch was felt more pleasant on the healthy compared to the affected limb on hairy (p < 0.001) and glabrous skin (p 0.002), independent of allodynia. In contrast to healthy controls patients felt no difference between stimulating glabrous and hairy skin on the affected limb. Thermal pain thresholds increased after CT-stimulation on the affected limb (cold-pain-threshold: p 0.016; heat-pain-threshold: p 0.033). CONCLUSIONS: CT-stimulation normalizes thermal pain thresholds but has no effect on the overall pain in CRPS. Therefore, pain modulating properties of CT-fibers might be too weak to alter chronic pain in CRPS. Moreover, CT-fibers appear to lose their ability to mediate pleasant aspects of touch in CRPS.

Mutation Carriers with Reduced C-Afferent Density Reveal Cortical Dynamics of Pain-Action Relationship during Acute Pain.

The evidence that action shapes perception has become widely accepted, for example, in the domain of vision. However, the manner in which action-relevant factors might influence the neural dynamics of acute pain processing has remained underexplored, particularly the functional roles of anterior insula (AI) and midanterior cingulate cortex (mid-ACC), which are frequently implicated in acute pain. To address this, we examined a unique group of heterozygous carriers of the rare R221W mutation on the nerve growth factor (NGF) gene. R221W carriers show a congenitally reduced density of C-nociceptor afferent nerves in the periphery, but can nonetheless distinguish between painful and nonpainful stimulations. Despite this, carriers display a tendency to underreact to acute pain behaviorally, thus exposing a potential functional gap in the pain-action relationship and allowing closer investigation of how the brain integrates pain and action information. Heterozygous R221W carriers and matched controls performed a functional magnetic resonance imaging (fMRI) task designed to dissociate stimulus type (painful or innocuous) from current behavioral relevance (relevant or irrelevant), by instructing participants to either press or refrain from pressing a button during thermal stimulation. Carriers' subjective pain thresholds did not differ from controls', but the carrier group showed decreased task accuracy. Hemodynamic activation in AI covaried with task performance, revealing a functional role in pain-action integration with increased responses for task-relevant painful stimulation ("signal," requiring button-press execution) over task-irrelevant stimulation ("noise," requiring button-press suppression). As predicted, mid-ACC activation was associated with action execution regardless of pain. Functional connectivity between AI and mid-ACC increased as a function of reported urge to withdraw from the stimulus, suggesting a joint role for these regions in motivated action during pain. The carrier group showed greater activation of primary sensorimotor cortices-but not the AI and mid-ACC regions-during pain and action, suggesting compensatory processing. These findings indicate a critical role for the AI-mid-ACC axis in supporting a flexible, adaptive action selection during pain, alongside the accompanying subjective experience of an urge to escape the pain.

Thermal block of mammalian unmyelinated C fibers by local cooling to 15-25°C after a brief heating at 45°C.

The purpose of this study was to examine the changes in cold block of unmyelinated C fibers in the tibial nerve by preconditioning with heating and to develop a safe method for thermal block of C-fiber conduction. In seven cats under α-chloralose anesthesia, C-fiber-evoked potentials elicited by electrical stimulation were recorded on the tibial nerve during block of axonal conduction induced by exposing a small segment (9 mm) of the nerve to cooling (from 35°C to ≤5°C) or heating (45°C). Before heating, partial, reproducible, and reversible cold block was first detected at a threshold cold block temperature of 15°C and complete cold block occurred at a temperature of ≤5°C. After the nerve was heated at 45°C for 5-35 min, the threshold cold block temperature significantly (P < 0.05) increased from 15°C to 25°C and the complete cold block temperature significantly (P < 0.05) increased from ≤5°C to 15°C on average. The increased cold block temperatures persisted for the duration of the experiments (30-100 min) while the amplitude of the C-fiber-evoked potential measured at 35°C recovered significantly (P < 0.05) to ~80% of control. This study discovered a novel thermal method to block mammalian C fibers at an elevated temperature (15-25°C), providing the opportunity to develop a thermal nerve block technology to suppress chronic pain of peripheral origin. The interaction between heating and cooling effects on C-fiber conduction indicates a possible interaction between different temperature-sensitive channels known to be present in the mammalian C fibers.NEW & NOTEWORTHY Our study discovered that the temperature range for producing a partial to complete cold block of mammalian C-fiber axons can be increased from 5-15°C to 15-25°C on average after a preheating at 45°C. This discovery raises many basic scientific questions about the influence of temperature on nerve conduction and block. It also raises the possibility of developing a novel implantable nerve block device to treat many chronic diseases including chronic pain.

Stimulus intensity-dependent recruitment of NaV1 subunits in action potential initiation in nerve terminals of vagal C-fibers innervating the esophagus.

We investigated voltage-gated sodium channel (NaV1) subunits that regulate action potential initiation in the nerve terminals of vagal nodose C-fibers innervating the esophagus. Extracellular single fiber recordings were made from the nodose C-fibers, with mechanically sensitive nerve terminals in the isolated innervated guinea pig esophagus. NaV1 inhibitors were selectively delivered to the tissue-containing nerve terminals. Graded esophageal distention was used for mechanical stimulation. The NaV1.7 inhibitor PF-05089771 nearly abolished action potential initiation in response to low levels of esophageal distention but only partially inhibited the response to higher levels of esophageal distention. The PF-05089771-insensitive component of the response progressively increased (up to ≈50%) with increasing esophageal distention and was abolished by tetrodotoxin (TTX). In addition to NaV1.7, nodose C-fiber [transient receptor potential channel-vanilloid subfamily member 1 (TRPV1)-positive] neurons retrogradely labeled from the esophagus expressed mRNA for multiple TTX-sensitive NaV1s. The group NaV1.1, NaV1.2, and NaV1.3 inhibitor ICA-121431 inhibited but did not abolish the PF-05089771-insensitive component of the response to high level of esophageal distention. However, combination of ICA-121431 with compound 801, which also inhibits NaV1.7 and NaV1.6, nearly abolished the response to the high level of esophageal distention. Our data indicate that the action potential initiation in esophageal nodose C-fibers evoked by low (innocuous) levels of esophageal distention is mediated by NaV1.7. However, the response evoked by higher (noxious) levels of esophageal distention has a progressively increasing NaV1.7-independent component that involves multiple TTX-sensitive NaV1s. The stimulus intensity-dependent recruitment of NaV1s may offer novel opportunities for strategic targeting of NaV1 subunits for inhibition of nociceptive signaling in visceral C-fibers.NEW & NOTEWORTHY We report that pharmacologically distinguishable voltage-gated sodium channels (NaV1) mediate action potential initiation at low (innocuous) versus high (noxious) intensity of esophageal distention in nerve terminals of vagal nodose C-fibers. Action potential initiation at low intensity is entirely dependent on NaV1.7; however, additional tetrodotoxin (TTX)-sensitive NaV1s are recruited at higher intensity of distention. This is the first demonstration that NaV1s underlying action potential initiation in visceral C-fibers depend on the intensity of the stimulus.

The Effect of Pain Resilience on Experimental Pain Experience Across Different Stimuli.

OBJECTIVE: Pain resilience, one's ability to maintain behavioral engagement and adaptively regulate cognitions and emotions despite intense or prolonged pain, has been shown to protect against negative pain-related outcomes in experimental settings. A weakness of this research, and much of experimental pain research in general, has been the lack of rationale behind the selection of noxious stimuli, which can activate different nociceptive fibers. The present study sought to determine if the relationship between pain resilience and pain ratings differed across stimuli based on the stimulated nociceptors. METHODS: Healthy undergraduate students (N = 100; mean [SD] age = 19.4 [1.2] years; 60% female) completed the Pain Resilience Scale and provided continuous pain ratings during exposure to three different tasks, each selected based on their ability to stimulate specific combinations of nociceptive fibers: pinprick (Aδ fibers), cold water immersion (Aδ and C fibers), and ischemic tourniquet (C fibers). RESULTS: Participants with high pain resilience reported lower pain ratings over time during cold water immersion than did those with low pain resilience (F(1, 39) = 8.526, p = .006); however, there was no relationship between pain resilience and pain ratings during either of the pinprick or ischemic tourniquet stimuli. CONCLUSIONS: This study provides further support for the use of multiple pain stimuli for pain assessment given their unique characteristics and concludes that outcome variables aside from pain ratings may provide additional insight into the role of resilience on pain adaptation.

Localization and dynamic changes of neuregulin-1 at C-type synaptic boutons in association with motor neuron injury and repair.

C-type synaptic boutons (C-boutons) provide cholinergic afferent input to spinal cord motor neurons (MNs), which display an endoplasmic reticulum (ER)-related subsurface cistern (SSC) adjacent to their postsynaptic membrane. A constellation of postsynaptic proteins is clustered at C-boutons, including M2 muscarinic receptors, potassium channels, and σ-1 receptors. In addition, we previously found that neuregulin (NRG)1 is associated with C-boutons at postsynaptic SSCs, whereas its ErbB receptors are located in the presynaptic compartment. C-bouton-mediated regulation of MN excitability has been implicated in MN disease, but NRG1-mediated functions and the impact of various pathologic conditions on C-bouton integrity have not been studied in detail. Here, we investigated changes in C-boutons after electrical stimulation, pharmacological treatment, and peripheral nerve axotomy. SSC-linked NRG1 clusters were severely disrupted in acutely stressed MNs and after tunicamycin-induced ER stress. In axotomized MNs, C-bouton loss occurred in concomitance with microglial recruitment and was prevented by the ER stress inhibitor salubrinal. Activated microglia displayed a positive chemotaxis to C-boutons. Analysis of transgenic mice overexpressing NRG1 type I and type III isoforms in MNs indicated that NRG1 type III acts as an organizer of SSC-like structures, whereas NRG1 type I promotes synaptogenesis of presynaptic cholinergic terminals. Moreover, MN-derived NRG1 signals may regulate the activity of perineuronal microglial cells. Together, these data provide new insights into the molecular and cellular pathology of C-boutons in MN injury and suggest that distinct NRG1 isoform-mediated signaling functions regulate the complex matching between pre- and postsynaptic C-bouton elements.-Salvany, S., Casanovas, A., Tarabal, O., Piedrafita, L., Hernández, S., Santafé, M., Soto-Bernardini, M. C., Calderó, J., Schwab, M. H., Esquerda, J. E. Localization and dynamic changes of neuregulin-1 at C-type synaptic boutons in association with motor neuron injury and repair.

Prolongation of bronchopulmonary C-fiber-mediated apnea by prenatal nicotinic exposure in rat pups: role of 5-HT3 receptors.

Prenatal nicotinic exposure (PNE) reportedly sensitizes bronchopulmonary C-fibers (PCFs) and prolongs PCF-mediated apnea in rat pups, contributing to the pathogenesis of sudden infant death syndrome. Serotonin, or 5-hydroxytryptamine (5-HT), induces apnea via acting on 5-HT receptor 3 (5-HT3R) in PCFs, and among the 5-HT3R subunits, 5-HT3B is responsible for shortening the decay time of 5-HT3R-mediated currents. We examined whether PNE would promote pulmonary 5-HT secretion and prolong the apnea mediated by 5-HT3Rs in PCFs via affecting the 5-HT3B subunit. To this end, the following variables were compared between the control and PNE rat pups: 1) the 5-HT content in bronchoalveolar lavage fluid, 2) the apneic response to the right atrial bolus injection of phenylbiguanide (a 5-HT3R agonist) before and after PCF inactivation, 3) 5-HT3R currents and the stimulus threshold of the action currents of vagal pulmonary C-neurons, and 4) the immunoreactivity (IR) and mRNA expression of 5-HT3A and 5-HT3B in these neurons. Our results showed that PNE up-regulated the pulmonary 5-HT concentration and strengthened the PCF 5-HT3R-mediated apnea. PNE significantly facilitated neural excitability by shortening the decay time of 5-HT3R currents, lowering the stimulus threshold, and increasing 5-HT3B IR. In summary, PNE prolongs the apnea mediated by 5-HT3Rs in PCFs, likely by increasing 5-HT3B subunits to enhance the excitability of 5-HT3 channels.-Zhao, L., Gao, X., Zhuang, J., Wallen, M., Leng, S., Xu, F. Prolongation of bronchopulmonary C-fiber-mediated apnea by prenatal nicotinic exposure in rat pups: role of 5-HT3 receptors.

Impaired mechanical, heat, and cold nociception in a murine model of genetic TACE/ADAM17 knockdown.

TNF-α-converting enzyme, a member of the ADAM (A disintegrin and metalloproteinase) protease family and also known as ADAM17, regulates inflammation and regeneration in health and disease. ADAM17 targets are involved in pain development and hypersensitivity in animal models of inflammatory and neuropathic pain. However, the role of ADAM17 in the pain pathway is largely unknown. Therefore, we used the hypomorphic ADAM17 (ADAM17ex/ex) mouse model to investigate the importance of ADAM17 in nociceptive behavior, morphology, and function of primary afferent nociceptors. ADAM17ex/ex mice were hyposensitive to noxious stimulation, showing elevated mechanical thresholds as well as impaired heat and cold sensitivity. Despite these differences, skin thickness and innervation were comparable to controls. Although dorsal root ganglia of ADAM17ex/ex mice exhibited normal morphology of peptidergic and nonpeptidergic neurons, a small but significant reduction in the number of isolectin ß-4-positive neurons was observed. Functional electrical properties of unmyelinated nociceptors showed differences in resting membrane potential, afterhyperpolarization, and firing patterns in specific subpopulations of sensory neurons in ADAM17ex/ex mice. However, spinal cord morphology and microglia activity in ADAM17ex/ex mice were not altered. Our data suggest that ADAM17 contributes to the processing of painful stimuli, with a complex mode of action orchestrating the function of neurons along the pain pathway.-Quarta, S., Mitric, M., Kalpachidou, T., Mair, N., Schiefermeier-Mach, N., Andratsch, M., Qi, Y., Langeslag, M., Malsch, P., Rose-John, S., Kress, M. Impaired mechanical, heat, and cold nociception in a murine model of genetic TACE/ADAM17 knockdown.

C-tactile touch perception in migraineurs - a case-control study.

BACKGROUND: Migraine is characterized by sensory hypersensitivity and habituation deficits. Slow brushing over the skin activates C-tactile nerve fibers, which mediate pleasant touch and analgesic effects in healthy subjects. As this function is altered in painful conditions, we aimed to examine whether the C-tactile processing is disrupted in migraines. METHODS: To psychophysically assess C-tactile function, we applied optimal and suboptimal C-tactile stroking stimuli on the dorsal forearm (body reference area) and the trigeminally innervated skin of 52 interictal migraineurs and 52 matched healthy controls. For habituation testing, 60 repeated C-tactile optimal stimuli were presented in both test areas. The participants rated each stimulus on a visual analogue scale by intensity, pleasantness, and painfulness. RESULTS: Regarding C-tactile function, migraineurs showed unphysiological rating patterns but no significantly different pleasantness ratings than controls. During repeated stimulation, controls showed stable pleasantness ratings while migraineurs' ratings decreased, especially in those experiencing tactile allodynia during headaches. Migraineurs taking triptans responded like controls. CONCLUSION: The C-tactile function of migraineurs is subclinically altered. Repeated C-tactile stimulation leads to altered habituation but differs from previous work by the direction of the changes. Although the pathophysiology remains unknown, causative mechanisms could include central and peripheral neuronal sensitization, tactile allodynia and hedonic stimulus attributions.

New wave-type mechanism of saltatory conduction in myelinated axons and micro-saltatory conduction in C fibres.

We present a new wave-type model of saltatory conduction in myelinated axons. Poor conductivity in the neuron cytosol limits electrical current signal velocity according to cable theory, to 1-3 m/s, whereas saltatory conduction occurs with a velocity of 100-300 m/s. We propose a wave-type mechanism for saltatory conduction in the form of the kinetics of an ionic plasmon-polariton being the hybrid of the electro-magnetic wave and of the synchronized ionic plasma oscillations in myelinated segments along an axon. The model agrees with observations and allows for description of the regulatory role of myelin. It explains also the mechanism of conduction deficiency in demyelination syndromes such as multiple sclerosis. The recently observed micro-saltatory conduction in ultrathin unmyelinated C fibers with periodic ion gate clusters is also explained.