H2O2 sensitivity of afferent splanchnic C fiber units in vitro

1996 ◽  
Vol 76 (1) ◽  
pp. 371-380 ◽  
Author(s):  
D. W. Adelson ◽  
J. Y. Wei ◽  
L. Kruger
Keyword(s):  
Receptive Fields ◽  
Splanchnic Nerve ◽  
Mechanical Stimuli ◽  
H2o2 Treatment ◽  
Response Latencies ◽  
C Fibers ◽  
Afferent Fibers ◽  
Unit Impulse ◽  
Species Production

1. Single-unit impulse activity evoked by transient, focal application of hydrogen peroxide (H2O2) to identified visceral receptive fields has been characterized in an in vitro rat splanchnic nerve-mesentery preparation. In addition to H2O2 responsiveness, units were characterized in terms of sensitivity to mechanical stimuli, warming, and bradykinin. 2. Mesenteric receptive fields of single splanchnic afferent C fibers in vitro were located with the use of warm (approximately 45 degrees C saline) or mechanical search stimuli. After delimitation of the warm-sensitive and/or mechanosensitive receptive field, units were tested for responsiveness to transient, focal application of H2O2. Microliter volumes (usually 1 microliter) of H2O2 (88-880 mM) evoked responses in 25 of 42 (60%) units with identified warm-sensitive and/or mechanosensitive receptive fields, and in an additional 10 units for which H2O2 was the only effective stimulus. 3. Tachyphylaxis to repeated H2O2 stimulation was observed with interstimulus intervals <30 min, but did not indicate irreversible inactivation of the terminal, because 1) during this period warm and mechanical stimuli elicited responses equal to or greater than those before H2O2 treatment, and 2) H2O2 sensitivity was restored after units were allowed to recover. 4. Eight units unresponsive to an initial dose of H2O2 responded vigorously to a repeated application at the same site, suggesting a potentiating effect of prior H2O2 exposure. 5. Sixty-two percent (8 of 13) of H2O2-responsive units, but no (0 of 6) H2O2-unresponsive units responded to transient, focal bradykinin (9-90 nM) application. 6. An indirect mode of H2O2-evoked afferent excitation in some units was suggested by several observations, including the prolonged (up to 8 min) duration of the response of some units to transient H2O2 application, and the occasionally long (>2 min) response latencies to focal application of H2O2 to defined receptive fields. 7. Excitation of splanchnic neurons by H2O2 may be relevant to the modulation of reactive oxygen species production by immunocompetent cells, because sensory neuropeptides contained in these afferent fibers are known to influence the respiratory burst of macrophages and neutrophils.

Afferent fibers supplying the uterus in the rat

1988 ◽  
Vol 59 (1) ◽  
pp. 142-163 ◽  
Author(s):  
K. J. Berkley ◽  
A. Robbins ◽  
Y. Sato
Keyword(s):  
Receptive Fields ◽  
Chemical Stimulation ◽  
Future Research ◽  
Mechanical Stimuli ◽  
Response Characteristics ◽  
Afferent Fibers ◽  
Dose Dependent Fashion ◽  
Uterine Body

1. In the present three-part study electrophysiological techniques were used to characterize responses of afferent fibers in the rat hypogastric nerve to mechanical or chemical stimulation of the uterus, and anatomical techniques were used to identify the spinal segments through which uterine afferent fibers enter the spinal cord. 2. In an in vivo barbiturate-anesthetized preparation, hypogastric afferent fibers responded in a time-locked manner to mechanical stimulation confined to restricted regions of the uterus and adjacent ligament. Receptive fields were most often located on the uterine body, particularly over the cervix. The few located on the uterine horn were usually near regions irritated during preparative surgery. Effective mechanical stimuli (pressure, stretching, squeezing, probing, rarely contractions) were typically greater than 5 g and simultaneously accompanied by transient ischemia around the probe or contracted area. Distension, unless extreme, was not an effective stimulus. Retrospective analysis of the data indicated that fibers may be more sensitive to uterine stimulation when rats are in vaginal estrus/proestrus than in diestrus/metestrus. 3. In an in vitro preparation, hypogastric afferent fibers responded in a dose-dependent fashion to injections into the uterine artery of the algesic chemicals bradykinin, 5-hydroxytryptamine, and KCl. They also responded to high doses of CO2 (in saline) and NaCN, but rarely to lower doses. Nearly all fibers responded to more than one chemical with response characteristics unique to each chemical (e.g., latency, duration, peak rate). 4. Injections of horseradish peroxidase into the uterine body and small portions of the adjacent horns in rats in vaginal estrus consistently labeled a small number of cells in the L1-S1 dorsal root ganglia, with peaks at L2 and L6. Virtually no cells were labeled in rats whose estrous cycle had been disrupted (by inadvertently keeping them in constant light conditions for several weeks). 5. These results indicate that uterine afferent fibers travel to the central nervous system through both the hypogastric (e.g., L1-L4 ganglia) and pelvic (e.g., L5-S1 ganglia) nerves in the rat, and that hypogastric fibers are capable of conveying fairly precise information about temporal and spatial aspects of uterine mechanical and chemical stimulation. The results also encourage future research into the possibility that the responses of these fibers vary as a function of estrous stage or other aspects of the condition of the uterus (e.g., its irritation).(ABSTRACT TRUNCATED AT 400 WORDS)

Functional Properties of Cutaneous A- and C-Fibers 1–15 Months After a Nerve Lesion

2009 ◽  
Vol 102 (6) ◽  
pp. 3129-3141 ◽  
Author(s):  
Natalia Gorodetskaya ◽  
Lydia Grossmann ◽  
Cristina Constantin ◽  
Wilfrid Jänig
Keyword(s):  
Functional Properties ◽  
Receptive Fields ◽  
Nerve Trunk ◽  
Nerve Lesion ◽  
Target Tissue ◽  
Mechanical Stimuli ◽  
Ongoing Activity ◽  
Nerve Crush ◽  
C Fibers ◽  
Almost All

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.

scholarly journals Vagal and spinal mechanosensors in the rat stomach and colon have multiple receptive fields

2001 ◽  
Vol 280 (5) ◽  
pp. R1371-R1381 ◽  
Author(s):  
Hans-Rudolf Berthoud ◽  
Penny A. Lynn ◽  
L. Ashley Blackshaw
Keyword(s):  
Receptive Fields ◽  
Splanchnic Nerve ◽  
Distal Colon ◽  
Chemical Information ◽  
Visceral Sensation ◽  
Motor Patterns ◽  
Vagal Nerves ◽  
The Central Nervous System ◽  
Perifusion System

Mechano- and chemosensitive extrinsic primary afferents innervating the gastrointestinal tract convey important information regarding the state of ingested nutrients and specific motor patterns to the central nervous system via splanchnic and vagal nerves. Little is known about the organization of peripheral receptive sites of afferents and their correspondence to morphologically identified terminal structures. Mechano- and chemosensory characteristics and receptive fields of single vagal fibers innervating the stomach as well as lumbar splanchnic nerves innervating the distal colon were identified using an in vitro perifusion system. Twenty-three (17%) of one-hundred thirty-six vagal units identified were found to have multiple, punctate receptive fields, up to 35 mm apart, and were distributed throughout the stomach. Evidence was based on similarity of generated spike forms, occlusion, and latency determinations. Most responded with brief bursts of activity to mucosal stroking with von Frey hairs (10–200 mg) but not to stretch, and 32% responded to capsaicin (10−5M). They were classified as rapidly adapting mucosal receptors. Four (8%) of fifty-three single units recorded from the lumbar splanchnic nerve had more than one, punctate receptive field in the distal colon, up to 40 mm apart. They responded to blunt probing, particularly from the serosal side, and variously to chemical stimulation with 5-hydroxytryptamine and capsaicin. We conclude that a proportion of gastrointestinal mechanosensors has multiple receptive fields and suggest that they integrate mechanical and chemical information from an entire organ, constituting the generalists in visceral sensation.

Acute Effect of an Incision on Mechanosensitive Afferents in the Plantar Rat Hindpaw

2002 ◽  
Vol 87 (2) ◽  
pp. 712-720 ◽  
Author(s):  
Minna M. Hämäläinen ◽  
G. F. Gebhart ◽  
Timothy J. Brennan
Keyword(s):  
Receptive Field ◽  
Background Activity ◽  
Response Magnitude ◽  
Field Size ◽  
Response Threshold ◽  
Mechanical Stimuli ◽  
Receptive Field Size ◽  
C Fibers ◽  
Afferent Fibers ◽  
Aβ Fibers

The purpose of this study was to examine which primary afferent fibers are sensitized to mechanical stimuli after an experimental surgical incision to the glabrous skin of the rat hindpaw. Afferent fibers teased from the L5dorsal root or the tibial nerve were recorded in anesthetized rats. The mechanical response properties of each fiber were characterized before and 45 min after an incision (or sham procedure) within the mechanical receptive field. Sensitization is characterized by an expansion of the mechanical receptive field, an increase in background activity, an increase in response magnitude, or a decrease in response threshold. After incision, the background activity and response properties of Aβ-fibers ( n = 9) to mechanical stimuli were unchanged. Four of 13 mechanosensitive Aδ-fibers exhibited sensitization after the incision; response threshold decreased, response magnitude increased, or receptive field size increased. Background activity of Aδ-fibers was not increased by the incision. Sensitization was observed in 4 of 18 mechanosensitive C-fibers 45 min after the incision. Background activity of C-fibers was not increased by the incision. In a group of mechanically insensitive afferent fibers (MIAs), 3 of 7 Aδ-fibers and 4 of 10 C-fibers sensitized 45 min after incision. Response threshold was decreased in only 2 of 17 MIAs; receptive field size increased in 7 of 17 MIAs. Aβ-fibers did not sensitize after the incision, and only 8 of 31 (26%) mechanosensitive Aδ- and C-fibers gave evidence of sensitization. In a group of MIA Aδ- and C-fibers, a greater percentage of 17 fibers studied (41%) were sensitized after incision. In this model, the principal effect of an incision, when examined 45 min after the insult, is an increase in receptive field size of the afferents, particularly those characterized as MIAs. To the extent that the mechanical hyperalgesia characterized in the same model is initiated in the periphery, it would appear that spatial summation of modestly increased response magnitude is important to the development of hyperalgesia.

Electrophysiological characteristics of primate spinothalamic neurons with renal and somatic inputs

1989 ◽  
Vol 61 (6) ◽  
pp. 1121-1130 ◽  
Author(s):  
W. S. Ammons
Keyword(s):  
Nerve Stimulation ◽  
Receptive Fields ◽  
High Threshold ◽  
Spinothalamic Tract ◽  
Renal Nerve ◽  
C Fibers ◽  
Afferent Fibers ◽  
Renal Nerve Stimulation ◽  
Somatic Receptive Fields ◽  
Stimulation Of

1. Spinothalamic tract (STT) neurons in the T10-L3 segments were studied for responses to renal and somatic stimuli. A total of 90 neurons was studied in 25 alpha-chloralose anesthetized monkeys (Macaca fascicularis). All neurons were antidromically activated from the ventral posterior lateral nucleus of the thalamus. 2. Sixty-two cells were excited by renal nerve stimulation and six inhibited. Probability of locating cells with renal input was greatest in T11-L1. Cells were located in laminae I and IV-VII; however, most were located in laminae V-VII. Antidromic latencies averaged 4.61 +/- 0.32 (SE) ms, whereas antidromic conduction velocities averaged 43.23 +/- 9.03 m/s. 3. Cells with excitatory renal input received A delta input only (36 cells) or A delta- and C-fiber inputs (26 cells). Stimulation of A delta renal afferent fibers evoked bursts of 1-10 spikes/stimulus [mean 3.6 +/- 0.9 spikes/stimulus] with onset latencies of 10.7 +/- 0.5 ms. Stimulation of C-fibers evoked 1.3 +/- 0.5 spikes/stimulus with onset latencies of 61.7 +/- 11.1 ms. Magnitude of responses to A delta-fiber stimulation was greatest in T12 and decreased both rostrally and caudally. Inhibitory responses to renal nerve stimulation required activation of renal C-fibers. 4. All cells that responded to stimulation of renal afferent fibers received convergent inputs from somatic structures. Forty-four cells were classified as wide dynamic range, 10 were high threshold, 12 were high-threshold cells with inhibitory input from hair, 2 were deep, and 2 were low threshold. Somatic receptive fields were large and located on the flank and abdomen and/or the upper hindlimb. Fourteen cells had inhibitory receptive fields located on the contralateral hindlimb or one of the forearms. 5. It is concluded that T11-L1 STT cells in the monkey respond reliably to renal nerve stimulation. Thoracolumbar STT cells may thus play a role in pain that results from renal disease. The locations of the somatic receptive fields of the cells suggest that they are responsible for the referral of renal pain to the flank and abdomen.

scholarly journals Response Properties of Whisker-Associated Trigeminothalamic Neurons in Rat Nucleus Principalis

2003 ◽  
Vol 89 (1) ◽  
pp. 40-56 ◽  
Author(s):  
Brandon S. Minnery ◽  
Daniel J. Simons
Keyword(s):  
Receptive Fields ◽  
Stimulus Onset ◽  
Response Latencies ◽  
Primary Afferent ◽  
Medial Nucleus ◽  
Afferent Fibers ◽  
Highly Sensitive ◽  
Primary Afferent Fibers ◽  
Stimulus Features ◽  
Nucleus Principalis

Nucleus principalis (PrV) of the brain stem trigeminal complex mediates the processing and transfer of low-threshold mechanoreceptor input en route to the ventroposterior medial nucleus of the thalamus (VPM). In rats, this includes tactile information relayed from the large facial whiskers via primary afferent fibers originating in the trigeminal ganglion (NV). Here we describe the responses of antidromically identified VPM-projecting PrV neurons ( n = 72) to controlled ramp-and-hold deflections of whiskers. For comparison, we also recorded the responses of 64 NV neurons under identical experimental and stimulus conditions. Both PrV and NV neurons responded transiently to stimulus onset (on) and offset (off), and the majority of both populations also displayed sustained, or tonic, responses throughout the plateau phase of the stimulus (75% of NV cells and 93% of PrV cells). Averageon and off response magnitudes were similar between the two populations. In both NV and PrV, cells were highly sensitive to the direction of whisker deflection. Directional tuning was slightly but significantly greater in NV, suggesting that PrV neurons integrate inputs from NV cells differing in their preferred directions. Receptive fields of PrV neurons were typically dominated by a “principal” whisker (PW), whose evoked responses were on average threefold larger than those elicited by any given adjacent whisker (AW; n = 197). However, of the 65 PrV cells for which data from at least two AWs were obtained, most (89%) displayed statistically significanton responses to deflections of one or more AWs. AW response latencies were 2.7 ± 3.8 (SD) ms longer than those of their corresponding PWs, with an inner quartile latency difference of 1–4 ms (±25% of median). The range in latency differences suggests that some adjacent whisker responses arise within PrV itself, whereas others have a longer, multi-synaptic origin, possibly via the spinal trigeminal nucleus. Overall, our findings reveal that the stimulus features encoded by primary afferent neurons are reflected in the responses of VPM-projecting PrV neurons, and that significant convergence of information from multiple whiskers occurs at the first synaptic station in the whisker-to-barrel pathway.

Receptive Properties of Mouse Sensory Neurons Innervating Hairy Skin

1997 ◽  
Vol 78 (4) ◽  
pp. 1841-1850 ◽  
Author(s):  
Martin Koltzenburg ◽  
Cheryl L. Stucky ◽  
Gary R. Lewin
Keyword(s):  
Sensory Neurons ◽  
Action Potentials ◽  
Hair Follicles ◽  
High Threshold ◽  
Mechanical Stimuli ◽  
Stimulus Response ◽  
Myelinated Fibers ◽  
C Fibers ◽  
Hairy Skin ◽  
Afferent Fibers

Koltzenburg, Martin, Cheryl L. Stucky, and Gary R. Lewin. Receptive properties of mouse sensory neurons innervating hairy skin. J. Neurophysiol. 78: 1841–1850, 1997. Using an in vitro nerve skin preparation and controlled mechanical or thermal stimuli, we analyzed the receptive properties of 277 mechanosensitive single primary afferents with myelinated ( n = 251) or unmyelinated ( n = 26) axons innervating the hairy skin in adult or 2-wk-old mice. Afferents were recorded from small filaments of either sural or saphenous nerves in an outbred mice strain or in the inbred Balb/c strain. On the basis of their receptive properties and conduction velocity, several receptor types could be distinguished. In adult animals (>6 wk old), 54% of the large myelinated fibers (Aβ, n = 83) showed rapidly adapting (RA) discharges to constant force stimuli and probably innervated hair follicles, whereas 46% displayed a slowly adapting (SA) response and probably innervated Merkel cells in touch domes. Among thin myelinated fibers (Aδ, n = 91), 34% were sensitive D hair receptors and 66% were high-threshold mechanoreceptors (AM fibers). Unmyelinated fibers had high mechanical thresholds and nociceptive functions. All receptor types had characteristic stimulus-response functions to suprathreshold force stimuli. Noxious heat stimuli (15-s ramp from 32 to 47°C measured at the corium side of the skin) excited 26% (5 of 19) of AM fibers with a threshold of 42.5 ± 1.4°C (mean ± SE) and an average discharge of 15.8 ± 9.7 action potentials and 41% (7 of 17) C fibers with a mean threshold of 37.6 ± 1.9°C and an average discharge of 22.0 ± 6.0 action potentials. Noxious cold stimuli activated 1 of 10 AM fibers and 3 of 10 C fibers. One of 10 C units responded to both heat and cold stimuli. All types of afferent fibers present in adult mice could readily be recognized in mice at postnatal day 14. However, fibers had reduced conduction velocities and the stimulus-response function to mechanical stimuli was more shallow in all fibers except for the D hairs. In juvenile mice, 22% of RA units also displayed an SA response at high stimulus intensities; these units were termed RA/SA units. We conclude that all types of cutaneous afferent fibers are already committed to their phenotype 2 wk after birth but undergo some maturation over the following weeks. This preparation has great potential for the study of transgenic mice with targeted mutations of genes that code factors that are involved in the specification of sensory neuron phenotypes.

Warm-Sensitive Afferent Splanchnic C-Fiber Units In Vitro

1997 ◽  
Vol 77 (6) ◽  
pp. 2989-3002 ◽  
Author(s):  
David W. Adelson ◽  
Jen Yu Wei ◽  
Lawrence Kruger
Keyword(s):  
Receptive Fields ◽  
Sense Organ ◽  
Cardiovascular Responses ◽  
Peak Temperature ◽  
Mechanical Stimuli ◽  
Tonic Component ◽  
Brown Adipose ◽  
C Fiber

Adelson, David W., Jen Yu Wei, and Lawrence Kruger. Warm-sensitive afferent splanchnic C-fiber units in vitro. J. Neurophysiol. 77: 2989–3002, 1997. Receptive fields of 41 slowly conducting sensory fibers were located using a thermal (warm) search stimulus in an in vitro splanchnic nerve-mesentery preparation. Warm-sensitive receptive fields were punctate and were densest in the region surrounding the prevertebral ganglia, an area with prominent deposits of brown adipose tissue, where the abdominal aorta branches into the major trunks supplying the abdominal viscera. Impulse activity was recorded while applying a warm stimulus to identified receptive fields (RFs). The warm stimulus consisted of a warming ramp (10–15°C in 1–2 s to a 42–49°C peak temperature) followed by a 10- to 30-s period during which the RF was maintained at this peak temperature (plateau phase). Eighty percent (33/41) of warm-sensitive units responded to warming with discharge comprising both a phasic and a tonic component (slowly adapting warm-sensitive, or SA-W, units). The remainder (8/41) responded with only phasic discharge (rapidly adapting warm-sensitive, or RA-W, units). Units' adaptation characteristics were consistent from trial to trial and when applying stimuli from different positions. Fifty percent of SA-W units (8/16) and 17% of RA-W units (1/6) were activated by transient exposure to 9–90 nM bradykinin (BK). Twenty-seven percent (9/33) of SA-W units and 12%(1/8) of RA-W units were activated by probing their RF with von Frey hairs with bending forces <10 mN (∼1 g equivalent mass). An additional five SA-W units tested were activated by strong mechanical stimuli (compression with a metal probe or firm stretching). No BK-responsive warm-sensitive units were activated by von Frey probing <10 mN, but two (both SA-W) responded to strong mechanical stimuli. In six SA-W units and one RA-W unit, the number of impulses evoked by warming ∼5 min after exposure to BK was >2 SD greater than the mean pre-BK response, indicating sensitization. This sensitization was transient, the response to warming returning to within one standard deviation of the pretrial mean or less over the course of the next 5–10 min. Changes in background activity, mechanical sensitivity, BK sensitivity, and BK-induced sensitization were noted in various splanchnic units over the course of prolonged observations, suggesting that these indices may not reliably distinguish unit type, but instead may indicate the functional state of the sense organ. Splanchnic neurons responsive to the intense warming used in the present in vitro experiments may participate in the cardiovascular responses observed in vivo in heat-stressed rats. The dense distribution of warm-receptive fields in the vicinity of the celiac-superior mesenteric ganglionic complex is consistent with the localization of splanchnic thermosensitive units previously noted in vivo in the rabbit.

Activity-dependent sensory signal processing in mechanically responsive slowly conducting meningeal afferents

2014 ◽  
Vol 112 (12) ◽  
pp. 3077-3085 ◽  
Author(s):  
Michael Uebner ◽  
Richard W. Carr ◽  
Karl Messlinger ◽  
Roberto De Col
Keyword(s):  
Action Potential ◽  
Conduction Velocity ◽  
Receptive Fields ◽  
Low Frequency ◽  
Mechanical Stimulus ◽  
Mechanical Stimuli ◽  
Action Potential Firing ◽  
Afferent Fibers ◽  
Custom Made ◽  
Activity Dependent

Activity-dependent processes in slowly conducting afferents have been shown to modulate conduction and receptive properties, but it is not known how the frequency of action potential firing determines the responses of such fibers to mechanical stimulation. We examined the responses of slowly conducting meningeal afferents to mechanical stimuli and the influence of preceding action potential activity. In hemisected rat heads with adhering cranial dura mater, recordings were made from meningeal nerves. Dural receptive fields of mechanically sensitive afferent fibers were stimulated with a custom-made electromechanostimulator. Sinusoidal mechanical stimuli of different stimulus durations and amplitudes were applied to produce either high-frequency (phasic) or low-frequency (tonic) discharges. Most fibers showed slowing of their axonal conduction velocity on electrically evoked activity at ≥2 Hz. In this state, the peak firing frequency of phasic responses to a 250-ms mechanical stimulus was significantly reduced compared with control. In contrast, the frequency of tonic responses induced by mechanical stimuli of >500 ms did not change. In a rare subtype of afferents, which showed conduction velocity speeding during activity, an increase in the phasic responses to mechanical stimuli was observed. Depending on the axonal properties of the afferent fibers, encoding of phasic components of mechanical stimuli is altered according to the immediate firing history. Preceding activity in mechanoreceptors slowing their conduction velocity seems to provide a form of low-pass filtering of action potential discharges predominantly reducing the phasic component. This may improve discrimination between harmless and potentially harmful mechanical stimuli in normal tissue.

Characterization of Aδ- and C-Fibers Innervating the Plantar Rat Hindpaw One Day After an Incision

2002 ◽  
Vol 87 (2) ◽  
pp. 721-731 ◽  
Author(s):  
Esther M. Pogatzki ◽  
G. F. Gebhart ◽  
Timothy J. Brennan
Keyword(s):  
Spontaneous Activity ◽  
Mechanical Response ◽  
Tissue Injury ◽  
Mechanical Stimulus ◽  
Response Threshold ◽  
Mechanical Stimuli ◽  
Median Response ◽  
Sham Procedure ◽  
C Fibers ◽  
Afferent Fibers

Primary hyperalgesia after tissue injury is suggested to result from sensitization of primary afferent fibers, but sensitization to mechanical stimuli has been difficult to demonstrate. In the companion study, sensitization of mechano-responsive Aδ- and C-fibers did not explain pain behaviors 45 min after an incision in the rat hindpaw. In the present study, we examined mechanical response properties of Aδ- and C-fibers innervating the glabrous skin of the plantar hindpaw in rats 1 day after an incision or sham procedure. In behavioral experiments, median withdrawal thresholds to von Frey filaments were reduced from 522 mN before to 61 mN 2 and 20 h after incision; median withdrawal thresholds after sham procedure were stable (522 mN). Responses to a nonpunctate mechanical stimulus were increased after incision. In neurophysiological experiments in these same rats, 67 single afferent fibers were characterized from the left tibial nerve 1 day after sham procedure ( n = 39) or incision ( n = 28); electrical stimulation was used as the search stimulus to identify a representative population of Aδ- and C-fibers. In the incision group, 11 fibers (39%) had spontaneous activity with frequencies ranging from 0.03 to 39.3 imp/s; none were present in the sham group. The median response threshold of Aδ-fibers was less in the incision (56 mN, n = 13) compared with sham (251 mN, n = 26) group, mainly because the proportion of mechanically insensitive afferents (MIAs) was less (8 vs. 54% after sham procedure). Median C-fiber response thresholds were similar in incised (28 mN, n = 15) and sham rats (56 mN, n = 13). Responsiveness to monofilaments was significantly enhanced in Aδ-fibers 1 day after incision; stimulus response functions of C-fibers after incision and after sham procedure did not differ significantly. Only Aδ-fibers but not C-fibers sensitized to the nonpunctate mechanical stimulus. The size of receptive fields was increased in Aδ- and C-fibers 1 day after incision. The results indicate that sensitization of Aδ- and C-fibers is apparent 1 day after incision. Because sensitization of afferent fibers to mechanical stimuli correlated with behavioral results, sensitization may contribute to the reduced withdrawal threshold after incision. Spontaneous activity in Aδ- and C-fibers may account for nonevoked pain behavior and may also contribute to mechanical hyperalgesia by amplifying responses centrally.

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