Cutaneous afferent C-fibers regenerating along the distal nerve stump after crush lesion show two types of cold sensitivity

Local translation in primary afferents and its contribution to pain

ABSTRACT

Chronic pain remains a significant problem due to its prevalence, impact, and limited therapeutic options. Progress in addressing chronic pain is dependent on a better understanding of underlying mechanisms. Although the available evidence suggests that changes within the central nervous system contribute to the initiation and maintenance of chronic pain, it also suggests that the primary afferent plays a critical role in all phases of the manifestation of chronic pain in most of those who suffer. Most notable among the changes in primary afferents is an increase in excitability or sensitization. A number of mechanisms have been identified that contribute to primary afferent sensitization with evidence for both increases in pronociceptive signaling molecules, such as voltage-gated sodium channels, and decreases in antinociceptive signaling molecules, such as voltage-dependent or calcium-dependent potassium channels. Furthermore, these changes in signaling molecules seem to reflect changes in gene expression as well as posttranslational processing. A mechanism of sensitization that has received far less attention, however, is the local or axonal translation of these signaling molecules. A growing body of evidence indicates that this process not only is dynamically regulated but also contributes to the initiation and maintenance of chronic pain. Here, we review the biology of local translation in primary afferents and its relevance to pain pathobiology.

Electrophysiological characterization of ectopic spontaneous discharge in axotomized and intact fibers upon nerve transection: a role in spontaneous pain?

Many patients experience positive symptoms after traumatic nerve injury. Despite the increasing number of experimental studies in models of peripheral neuropathy and the knowledge acquired, most of these patients lack an effective treatment for their chronic pain. One possible explanation might be that most of the preclinical studies focused on the development of mechanical or thermal allodynia/hyperalgesia, neglecting that most of the patients with peripheral neuropathies complain mostly about spontaneous forms of pains. Here, we summarize the aberrant electrophysiological behavior of peripheral nerve fibers recorded in experimental models, the underlying pathophysiological mechanisms, and their relationship with the symptoms reported by patients. Upon nerve section, axotomized but also intact fibers develop ectopic spontaneous activity. Most interestingly, a proportion of axotomized fibers might present receptive fields in the skin far beyond the site of damage, indicative of a functional cross talk between neuromatose and intact fibers. All these features can be linked with some of the symptoms that neuropathic patients experience. Furthermore, we spotlight the consequence of primary afferents with different patterns of spontaneous discharge on the neural code and its relationship with chronic pain states. With this article, readers will be able to understand the pathophysiological mechanisms that might underlie some of the symptoms that experience neuropathic patients, with a special focus on spontaneous pain.

The Role of Cold-Sensitive Ion Channels in Peripheral Thermosensation

The detection of ambient cold is critical for mammals, who use this information to avoid tissue damage by cold and to maintain stable body temperature. The transduction of information about the environmental cold is mediated by cold-sensitive ion channels expressed in peripheral sensory nerve endings in the skin. Most transduction mechanisms for detecting temperature changes identified to date depend on transient receptor potential (TRP) ion channels. Mild cooling is detected by the menthol-sensitive TRPM8 ion channel, but how painful cold is detected remains unclear. The TRPA1 ion channel, which is activated by cold in expression systems, seemed to provide an answer to this question, but whether TRPA1 is activated by cold in neurons and contributes to the sensation of cold pain continues to be a matter of debate. Recent advances have been made in this area of investigation with the identification of several potential cold-sensitive ion channels in thermosensory neurons, including two-pore domain potassium channels (K2P), GluK2 glutamate receptors, and CNGA3 cyclic nucleotide-gated ion channels. This mini-review gives a brief overview of the way by which ion channels contribute to cold sensation, discusses the controversy around the cold-sensitivity of TRPA1, and provides an assessment of some recently-proposed novel cold-transduction mechanisms. Evidence for another unidentified cold-transduction mechanism is also presented.

Spinal glial cell line-derived neurotrophic factor infusion reverses reduction of Kv4.1-mediated A-type potassium currents of injured myelinated primary afferent neurons in a neuropathic pain model

High frequency spontaneous activity in injured primary afferents has been proposed as a pathological mechanism of neuropathic pain following nerve injury. Although spinal infusion of glial cell line-derived neurotrophic factor reduces the activity of injured myelinated A-fiber neurons after fifth lumbar (L5) spinal nerve ligation in rats, the implicated molecular mechanism remains undetermined. The fast-inactivating transient A-type potassium current (I_A) is an important determinant of neuronal excitability, and five voltage-gated potassium channel (Kv) alpha-subunits, Kv1.4, Kv3.4, Kv4.1, Kv4.2, and Kv4.3, display I_A in heterologous expression systems. Here, we examined the effect of spinal glial cell line-derived neurotrophic factor infusion on I_A and the expression of these five Kv mRNAs in injured A-fiber neurons using the in vitro patch clamp technique and in situ hybridization histochemistry. Glial cell line-derived neurotrophic factor infusion reversed axotomy-induced reduction of the rheobase, elongation of first spike duration, and depolarization of the resting membrane potential. L5 spinal nerve ligation significantly reduced the current density of I_A and glial cell line-derived neurotrophic factor treatment reversed the reduction. Among the examined Kv mRNAs, only the change in Kv4.1-expression was parallel with the change in I_A after spinal nerve ligation and glial cell line-derived neurotrophic factor treatment. These findings suggest that glial cell line-derived neurotrophic factor should reduce the hyperexcitability of injured A-fiber primary afferents by I_A recurrence. Among the five I_A-related Kv channels, Kv4.1 should be a key channel, which account for this I_A recurrence.

Mechano- and thermosensitivity of injured muscle afferents 20 to 80 days after nerve injury

Chronic injury of limb nerves leading to neuropathic pain affects deep somatic nerves. Here the functional properties of injured afferent fibers in the lateral gastrocnemius-soleus nerve were investigated 20 and 80 days after suturing the central stump of this muscle nerve to the distal stump of the sural nerve in anesthetized rats. Neurophysiological recordings were made from afferent axons identified in either the sciatic nerve (87 A-, 63 C-fibers) or the dorsal root L4/L5 (52 A-, 26 C-fibers) by electrical stimulation of the injured nerve. About 70% of the functionally identified A-fibers had regenerated into skin by 80 days after nerve suture; the remaining A-fibers could be activated only from the injured nerve. In contrast, 93% of the functionally identified C-fibers could only be activated from the injured sural nerve after 80 days. Nearly half of the injured A- (45%) and C-fibers (44%) exhibited ongoing and/or mechanically or thermally evoked activity. Because ~50% of the A- and C-fibers are somatomotor or sympathetic postganglionic axons, respectively, probably all injured muscle afferent A- and C-fibers developed ectopic activity. Ongoing activity was present in 17% of the A- and 46% of the C-fibers. Mechanosensitivity was present in most injured A- (99%) and C-fibers (85%), whereas thermosensitivity was more common in C-fibers (cold 46%, heat 47%) than in A-fibers (cold 18%, heat 12%). Practically all thermosensitive A-fibers and C-fibers were also mechanosensitive. Thus, unlike cutaneous axons, almost all A- and C-fibers afferents in injured muscle nerves demonstrate ectopic activity, even chronically after nerve injury. NEW & NOTEWORTHY After chronic injury of a muscle nerve, allowing the nerve fibers to regenerate to the target tissue, 1) most afferent A-fibers are mechanosensitive and regenerate to the target tissue; 2) ectopic ongoing activity, cold sensitivity, and heat sensitivity significantly decrease with time after injury in A-afferents; 3) most afferent C-fibers do not regenerate to the target tissue; and 4) injured C-afferents maintain the patterns of ectopic discharge properties they already show soon after nerve injury.

Peripheral thermoreceptors in innocuous temperature detection

The mammalian skin is innervated by cold-sensitive afferent neurons. These neurons exhibit ongoing activity at temperatures between ~10 and 42°C, are activated by innocuous cold stimuli, inhibited by warm stimuli and are mechanoinsensitive. Their axons are small-diameter myelinated (Aδ-) fibers in primates and unmyelinated (C-) fibers in nonprimate mammals. The mammalian skin is innervated by warm-sensitive afferent neurons. The density of innervation by these neurons is lower than that by cold-sensitive afferents. They exhibit ongoing activity between ~38 and 48°C, are activated by warm stimuli, inhibited by cold stimuli, and are mechanoinsensitive. Their axons are unmyelinated (C-) fibers. Cold-sensitive unmyelinated afferent neurons exhibit prominent cold sensitivity of their axons (in rats). The discharge pattern of the cutaneous cold-sensitive afferent neurons is fully preserved after nerve injury. Ongoing impulse activity and cold-evoked impulses originate ectopically at the nerve injury site. Deep somatic tissues and viscera are innervated by thermosensitive afferent neurons. Most are warm-sensitive and mechanoinsensitive and have unmyelinated axons. These afferent neurons have only rarely and incompletely been studied, e.g., in the upper gastrointestinal tract, the liver (both vagal afferents), the dorsal abdominal wall, and the skeletal muscle. Spinal cord warm sensitivity may be mediated by cutaneous afferent neurons with unmyelinated axons that are excited by spinal cord warming.

Effects of menthol application on the skin during prolonged immersion in cool and cold water

The aim of the study was to compare the effect of skin surface menthol application on rectal temperature (Tre) during prolonged immersion in cool and cold water. We hypothesized that menthol application would lead to a slower Tre decline due to the reduced heat loss as a consequence of the menthol-induced vasoconstriction and that this effect would be attenuated during cold-water immersion. Six male subjects were immersed for 55 minutes in stirred cool (24°C) or cold (14°C) water immediately after attaining a Tre of 38°C by cycling at 60% of maximum heart rate on two occasions: without (ΝM) and with (M) whole-body skin application of menthol cream. Tre, the proximal-distal skin temperature gradient, and oxygen uptake were continuously measured. ANOVA with repeated measures was employed to detect differences among variables. Significance level was set at 0.05. The area under the curve for Tre was calculated and was greater in 24°C M (-1.81 ± 8.22 a.u) compared to 24°C NM (-27.09 ± 19.09 a.u., P = .03, r = .90), 14°C NM (-18.08 ± 10.85 a.u., P = .03, r = .90), and 14°C M (-11.71 ± 12.58 a.u, P = .05, r = .81). In cool water, oxygen uptake and local vasoconstriction were increased (P ≤ .05) by 39 ± 25% and 56 ± 37%, respectively, with menthol compared to ΝM, while no differences were observed in cold water. Menthol application on the skin before prolonged immersion reduces heat loss resulting in a blunted Tre decline. However, such a response is less obvious at 14°C water immersion, possibly because high-threshold cold-sensitive fibers are already maximally recruited and the majority of cold receptors saturated.

De novo expression of Nav1.7 in injured putative proprioceptive afferents: Multiple tetrodotoxin-sensitive sodium channels are retained in the rat dorsal root after spinal nerve ligation

Tetrodotoxin-sensitive (TTX-s) spontaneous activity is recorded from the dorsal roots after peripheral nerve injury. Primary sensory neurons in the dorsal root ganglion (DRG) express multiple TTX-s voltage-gated sodium channel α-subunits (Navs). Since Nav1.3 increases, whereas all other Navs decrease, in the DRG neurons after peripheral nerve lesion, Nav1.3 is proposed to be critical for the generation of these spontaneous discharges and the contributions of other Navs have been ignored. Here, we re-evaluate the changes in expression of three other TTX-s Navs, Nav1.1, Nav1.6 and Nav1.7, in the injured 5th lumbar (L5) primary afferent components following L5 spinal nerve ligation (SNL) using in situ hybridization histochemistry and immunohistochemistry. While the overall signal intensities for these Nav mRNAs decreased, many injured DRG neurons still expressed these transcripts at clearly detectable levels. All these Nav proteins accumulated at the proximal stump of the ligated L5 spinal nerve. The immunostaining patterns of Nav1.6 and Nav1.7 associated with the nodes of Ranvier were maintained in the ipsilateral L5 dorsal root. Interestingly, putative proprioceptive neurons characterized by α3 Na+/K+ ATPase-immunostaining specifically lacked Nav1.7 mRNA in naïve DRG but displayed de novo expression of this transcript following SNL. Nav1.7-immunoreactive fibers were significantly increased in the ipsilateral gracile nucleus where central axonal branches of the injured A-fiber afferents terminated. These data indicate that multiple TTX-s channel subunits could contribute to the generation and propagation of the spontaneous discharges in the injured primary afferents. Specifically, Nav1.7 may cause some functional changes in sensory processing in the gracile nucleus after peripheral nerve injury.

Responses of intact and injured sural nerve fibers to cooling and menthol

Intact and injured cutaneous C-fibers in the rat sural nerve are cold sensitive, heat sensitive, and/or mechanosensitive. Cold-sensitive fibers are either low-threshold type 1 cold sensitive or high-threshold type 2 cold sensitive. The hypothesis was tested, in intact and injured afferent nerve fibers, that low-threshold cold-sensitive afferent nerve fibers are activated by the transient receptor potential melastatin 8 (TRPM8) agonist menthol, whereas high-threshold cold-sensitive C-fibers and cold-insensitive afferent nerve fibers are menthol insensitive. In anesthetized rats, activity was recorded from afferent nerve fibers in strands isolated from the sural nerve, which was either intact or crushed 6-12 days before the experiment distal to the recording site. In all, 77 functionally identified afferent C-fibers (30 intact fibers, 47 injured fibers) and 34 functionally characterized A-fibers (11 intact fibers, 23 injured fibers) were tested for their responses to menthol applied to their receptive fields either in the skin (10 or 20%) or in the nerve (4 or 8 mM). Menthol activated all intact (n = 12) and 90% of injured (n = 20/22) type 1 cold-sensitive C-fibers; it activated no intact type 2 cold-sensitive C-fibers (n = 7) and 1/11 injured type 2 cold-sensitive C-fibers. Neither intact nor injured heat- and/or mechanosensitive cold-insensitive C-fibers (n = 25) and almost no A-fibers (n = 2/34) were activated by menthol. These results strongly argue that cutaneous type 1 cold-sensitive afferent fibers are nonnociceptive cold fibers that use the TRPM8 transduction channel.

Reflex inhibition of cutaneous and muscle vasoconstrictor neurons during stimulation of cutaneous and muscle nociceptors

Cutaneous (CVC) and muscle (MVC) vasoconstrictor neurons exhibit typical reflex patterns to physiological stimulation of somatic and visceral afferent neurons. Here we tested the hypothesis that CVC neurons are inhibited by stimulation of cutaneous nociceptors but not of muscle nociceptors and that MVC neurons are inhibited by stimulation of muscle nociceptors but not of cutaneous nociceptors. Activity in the vasoconstrictor neurons was recorded from postganglionic axons isolated from the sural nerve or the lateral gastrocnemius-soleus nerve in anesthetized rats. The nociceptive afferents were excited by mechanical stimulation of the toes of the ipsilateral hindpaw (skin), by hypertonic saline injected into the ipsi- or contralateral gastrocnemius-soleus muscle, or by heat or noxious cold stimuli applied to the axons in the common peroneal nerve or tibial nerve. The results show that CVC neurons are inhibited by noxious stimulation of skin but not by noxious stimulation of skeletal muscle and that MVC neurons are inhibited by noxious stimulation of skeletal muscle but not by noxious stimulation of skin. These inhibitory reflexes are mostly lateralized and are most likely organized in the spinal cord. Stimulation of nociceptive cold-sensitive afferents does not elicit inhibitory or excitatory reflexes in CVC or MVC neurons. The reflex inhibition of activity in CVC or MVC neurons generated by stimulation of nociceptive cutaneous or muscle afferents during tissue injury leads to local increase of blood flow, resulting in an increase of transport of immunocompetent cells, proteins, and oxygen to the site of injury and enhancing the processes of healing.

Angiotensin II receptor type 2 activation is required for cutaneous sensory hyperinnervation and hypersensitivity in a rat hind paw model of inflammatory pain

Many pain syndromes are associated with abnormal proliferation of peripheral sensory fibers. We showed previously that angiotensin II, acting through its type 2 receptor (AT2), stimulates axon outgrowth by cultured dorsal root ganglion neurons. In this study, we assessed whether AT2 mediates nociceptor hyperinnervation in the rodent hind paw model of inflammatory pain. Plantar injection of complete Freund's adjuvant (CFA), but not saline, produced marked thermal and mechanical hypersensitivity through 7 days. This was accompanied by proliferation of dermal and epidermal PGP9.5-immunoreactive (ir) and calcitonin gene-related peptide-immunoreactive (CGRP-ir) axons, and dermal axons immunoreactive for GFRα2 but not tyrosine hydroxylase or neurofilament H. Continuous infusion of the AT2 antagonist PD123319 beginning with CFA injection completely prevented hyperinnervation as well as hypersensitivity over a 7-day period. A single PD123319 injection 7 days after CFA also reversed thermal hypersensitivity and partially reversed mechanical hypersensitivity 3 hours later, without affecting cutaneous innervation. Angiotensin II-synthesizing proteins renin and angiotensinogen were largely absent after saline but abundant in T cells and macrophages in CFA-injected paws with or without PD123319. Thus, emigrant cells at the site of inflammation apparently establish a renin-angiotensin system, and AT2 activation elicits nociceptor sprouting and heightened thermal and mechanical sensitivity.

PERSPECTIVE

Short-term AT2 activation is a potent contributor to thermal hypersensitivity, whereas long-term effects (such as hyperinnervation) also contribute to mechanical hypersensitivity. Pharmacologic blockade of AT2 signaling represents a potential therapeutic strategy aimed at biologic mechanisms underlying chronic inflammatory pain.

Quantifying nerve architecture in murine and human airways using three-dimensional computational mapping

The quantitative histological analysis of airway innervation using tissue sections is challenging because of the sparse and patchy distribution of nerves. Here we demonstrate a method using a computational approach to measure airway nerve architecture that will allow for more complete nerve quantification and the measurement of structural peripheral neuroplasticity in lung development and disease. We demonstrate how our computer analysis outperforms manual scoring in quantifying three-dimensional nerve branchpoints and lengths. In murine lungs, we detected airway epithelial nerves that have not been previously identified because of their patchy distribution, and we quantified their three-dimensional morphology using our computer mapping approach. Furthermore, we show the utility of this approach in bronchoscopic forceps biopsies of human airways, as well as the esophagus, colon, and skin.

Hypersensitivity and hyperinnervation of the rat hind paw following carrageenan-induced inflammation

Studies of human tissue show that many chronic pain syndromes are accompanied by abnormal increases in numbers of peripheral sensory nerve fibers. It is not known if sensory nerve sprouting occurs as a result of inflammation present in these conditions, or other factors such as infection or extensive tissue damage. In the present study, we used a well established model of inflammation to examine cutaneous innervation density in relation to mechanical and thermal hypersensitivity. Adult female rats were ovariectomized to eliminate fluctuations in female reproductive hormones and one week later, a hind paw was injected with carrageenan or saline vehicle. Behavioral testing showed that saline vehicle injection did not alter thermal or mechanical thresholds compared to pre-injection baselines. Carrageenan injections resulted in markedly reduced paw withdrawal thresholds at 24 and 72 h after injection; this was accompanied by increased mechanical sensitivity of the contralateral paw at 72 h. Analysis of innervation density using PGP9.5 as a pan-neuronal marker at 72 h showed that inflammation resulted in a 2-fold increase in cutaneous innervation density. We conclude that inflammation alone is sufficient to induce sprouting of sensory cutaneous axon endings leading local tissue hyperinnervation, which may contribute to hypersensitivity that occurs in painful inflammatory conditions.

Mechano- and thermosensitivity of injured muscle afferents

Injury of limb nerves leading to neuropathic pain mostly affects deep somatic nerves including muscle nerves. Here, we investigated the functional properties of injured afferent fibers innervating the lateral gastrocnemius-soleus muscle 4-13 h [time period (TP) I] and 4-7 days (TP II) after nerve crush in anesthetized rats using neurophysiological recordings from either the sciatic nerve (165 A-, 137 C-fibers) or the dorsal root L(5) (43 A-, 28 C-fibers). Ongoing activity and responses to mechanical or thermal stimulation of the injury site of the nerve were studied quantitatively. Of the electrically identified A- and C-fibers, 5 and 38% exhibited ectopic activity, respectively, in TP I and 51 and 61%, respectively, in TP II. Thus all afferent fibers in an injured muscle nerve developed ectopic activity since ∼ 50% of the fibers in a muscle nerve are somatomotor or sympathetic postganglionic. Ongoing activity was present in 50% of the afferent A-fibers (TP II) and in 53-56% of the afferent C-fibers (TP I and II). In TP II, mechanical, cold, and heat sensitivity were present in 91, 63, and 52% of the afferent A-fibers and in 50, 40, and 66% of the afferent C-fibers. The cold and heat activation thresholds were 5-27 and 35-48°C, respectively, covering the noxious and innocuous range. Most afferent fibers showed combinations of these sensitivities. Mechano- and cold sensitivity had a significantly higher representation in A- than in C-fibers, but heat sensitivity had a significantly higher representation in C- than in A-fibers. These functional differences between A- and C-fibers applied to large- as well as small-diameter A-fibers. Comparing the functional properties of injured muscle A- and C-afferents with those of injured cutaneous A- and C-afferents shows that both populations of injured afferent neurons behave differently in several aspects.

Axonal thermosensitivity and mechanosensitivity of cutaneous afferent neurons

We hypothesized that cutaneous afferent myelinated fibers (A-fibers) and afferent unmyelinated fibers (C-fibers) respond to the same natural stimuli applied to their axons as to their terminals in the skin. In anesthetized rats, activity was recorded from afferent axons in strands isolated proximally from the sural nerve. Mechanical, cold or heat stimuli were applied to the skin or along a 15-mm length of the distal sural nerve. One-hundred and eighteen A-fibers and 109 C-fibers were characterized by their conduction velocity and/or shape of their action potentials, and by their responses to natural stimulation of the skin. Then, these fibers were tested for their responses to the same stimuli applied to the nerve. In some cases, the nerve was crushed distally after the nerve fibers had been characterized by their responses to physiological stimulation of the skin, and the responses to stimuli applied to the nerve proximal to the lesion were tested again. Almost all non-nociceptive cold-sensitive (type 1) C-fibers (97%) could be activated by cold stimuli applied to the nerve. Of nociceptive cold-sensitive (type 2) C-fibers, 39% were activated by cold stimuli applied to the nerve. Furthermore, 34% of heat-sensitive C-fibers could be activated by heating the nerve. In contrast, only 2-4% of mechanosensitive A-fibers and C-fibers responded to mechanical stimuli applied to the nerve. In conclusion, cold and heat sensitivity of cutaneous afferent neurons is not restricted to their terminals in the skin, but often extends along the axons in the nerve. Mechanosensitivity is restricted to the afferent endings in the skin.

Contribution of primary afferent channels to neuropathic pain

Neuropathic pain remains a serious medical problem because of patient morbidity and the absence of effective therapeutic interventions. Recent evidence suggests that this type of pain may be particularly difficult to manage because underlying mechanisms are influenced by a variety of factors, including type of injury, site of injury, and time after injury. This situation is exacerbated by the fact that different mechanisms may contribute to unique aspects of neuropathic pain, including ongoing pain as well as mechanical and thermal hypersensitivity. The different ion channels present in primary afferent neurons implicated in each of these aspects of neuropathic pain are reviewed.

NGF induces non-inflammatory localized and lasting mechanical and thermal hypersensitivity in human skin

Nerve growth factor (NGF) modulates sensitivity and sprouting of nociceptors. We explored the spatial and temporal sensitization induced by NGF injection (1 microg) in human skin. Hyperalgesia was investigated in 16 volunteers (36+/-9 years) at day 1, 3, 7, 21, and 49. Areas of mechanical (brush, pin-prick) and heat (43 degrees C) sensitization were mapped and thermal (heat and cold) pain thresholds, mechanical (impact stimulation) and electrically evoked pain, and axon reflex flare were assessed. No spontaneous pain or local inflammation was recorded upon NGF injection and during 49 days. Sensitization to heat was maximum at day 3 and lasted 21 days. Hyperalgesia to cold was recorded at day 7 and 21. Hypersensitivity to mechanical impact stimuli developed delayed, reached maximum at day 21, and persisted throughout 49 days. Fifty percent of all volunteers reported a static allodynia to tonic pressure until day 21. Electrical stimulation at 7.5 mA was more painful at the NGF site at day 21, which correlated significantly to maximum impact pain. Axon reflex flare was unaffected by NGF. Sensitization was limited to the NGF injection site, no touch- or pin-prick evoked secondary hyperalgesia was observed. Spatially restricted hyperalgesia indicates a peripheral rather than central mechanism. The temporal profile of lasting nociceptor sensitization suggests an altered peripheral axonal expression of sensory proteins specifically leading to mechanical and thermal sensitization. Intradermal NGF administration provokes a pattern of sensitization that can be used as experimental model for neuropathic pain.

Functional properties of cutaneous A- and C-fibers 1-15 months after a nerve lesion

The functional properties of cutaneous afferent fibers were investigated 1-15 mo after nerve lesions, which allowed regeneration into denervated skin. After crushing or transection and resuturing the rat sural nerve, ongoing activity and responses to cold, heat, and mechanical stimuli presented to the denervated skin or to the nerve distal to the lesion were examined in 273 A-fibers and 211 C-fibers. Reinnervation of skin by A-fibers was largely complete by 1-4 mo after crushing but incomplete after transection and resuturing. A few A-fibers could be activated from the nerve trunk, even after 10-15 mo. Almost all regenerated A-fibers were mechanosensitive and about 6% were cold- or heat-sensitive. A few A-fibers had ongoing activity after nerve crush. Only 15-35% of C-fibers could be activated at 1-4 mo, but 60% were excited from the skin at 10-15 mo, when many also had receptive fields within the lesioned nerve. The remaining C-fibers had receptive fields only within the nerve trunk. Responses of both intraneural and intradermal endings of C-fibers could be classified into functional groups similar to those of C-fibers in control nerves to cutaneous stimuli. The frequency of afferent C-fibers with ongoing activity that were not highly cold sensitive was 45%. We conclude that the functional characteristics of afferent A- and C-fibers are expressed by regenerating nerve endings, even when they do not reinnervate their target tissue. The reinnervation of skin by afferent C-fibers is extremely slow and may never recover to normal.

Mechano- and thermosensitivity of regenerating cutaneous afferent nerve fibers

Crush lesion of a skin nerve is followed by sprouting of myelinated (A) and unmyelinated (C) afferent fibers into the distal nerve stump. Here, we investigate quantitatively both ongoing activity and activity evoked by mechanical or thermal stimulation of the nerve in 43 A- and 135 C-fibers after crush lesion of the sural nerve using neurophysiological recordings in anesthetized rats. The discharge patterns in the injured afferent nerve fibers and in intact (control) afferent nerve fibers were compared. (1) Almost all (98%) A-fibers were mechanosensitive, some of them exhibited additionally weak cold/heat sensitivity; 7% had ongoing activity. (2) Three patterns of physiologically evoked activity were present in the lesioned C-fibers: (a) C-fibers with type 1 cold sensitivity (low cold threshold, inhibition on heating, high level of ongoing and cold-evoked activity; 23%): almost all of them were mechanoinsensitive and 40% of them were additionally heat-sensitive; (b) C-fibers with type 2 cold sensitivity (high cold threshold, low level of ongoing and cold-evoked activity; 23%). All of them were excited by mechanical and/or heat stimuli; (c) cold-insensitive C-fibers (54%), which were heat- and/or mechanosensitive. (3) The proportions of C-fibers exhibiting these three patterns of discharge to physiological stimuli were almost identical in the population of injured C-fibers and in a population of 91 intact cutaneous C-fibers. 4. Ongoing activity was present in 56% of the lesioned C-fibers. Incidence and rate of ongoing activity were the same in the populations of lesioned and intact type 1 cold-sensitive C-fibers. The incidence (but not rate) of ongoing activity was significantly higher in lesioned type 2 cold-sensitive and cold insensitive C-fibers than in the corresponding populations of intact C-fibers (42/93 fibers vs. 11/72 fibers).