Lower Mechanical Thresholds Research Articles

Electrophysiologic Characteristics of Large Neurons in Dorsal Root Ganglia during Development and after Hind Paw Incision in the Rat

Withdrawal thresholds in the paw are lower in younger animals, and incision further reduces these thresholds. The authors hypothesized that these differences result in part from changes in intrinsic electrophysiologic properties of large neurons. Using isolated whole dorsal root ganglion, current clamping was performed to determine the electrophysiologic properties of large neurons before and after incision in animals aged 1 and 4 weeks. Mechanical withdrawal thresholds were used to follow paw sensitivity. After paw incision, withdrawal thresholds decreased to a similar degree at both ages, but returned to control threshold at 72 h only in the 1-week-old animals. The resting membrane potential was less negative and the rheobase and the resistance of the membrane were lower at baseline in the 1-week-old animals (P < 0.05). After incision, the membrane potential became more depolarized and the rheobase was less in both ages. These changes remained 72 h after the incision in both ages. These findings suggest that lower mechanical thresholds in the younger animals may be partially attributed to the intrinsic electrophysiologic properties of the larger-diameter afferent neurons. The lack of resolution of the electrophysiologic changes in the young despite the resolution of the withdrawal response suggests that continued input from large fibers into the central nervous system may occur at this age despite the apparent resolution of behavioral changes. Further studies are needed to determine the etiology of these differences, their impact in the central nervous system, and whether theses changes can be prevented.

Identification of capsaicin-sensitive rectal mechanoreceptors activated by rectal distension in mice

Rodents detect visceral pain in response to noxious levels of rectal distension. However, the mechanoreceptors that innervate the rectum and respond to noxious levels of rectal distension have not been identified. Here, we have identified the mechanoreceptors of capsaicin-sensitive rectal afferents and characterized their properties in response to circumferential stretch of the rectal wall. We have also used the lethal spotted (ls/ls) mouse to determine whether rectal mechanoreceptors that respond to capsaicin and stretch may also develop in an aganglionic rectum that is congenitally devoid of enteric ganglia. In wild type (C57BL/6) mice, graded increases in circumferential stretch applied to isolated rectal segments activated a graded increase in firing of slowly-adapting rectal mechanoreceptors. Identical stimuli applied to the aganglionic rectum of ls/ls mice also activated similar graded increases in firing of stretch-sensitive rectal afferents. In both wild type and aganglionic rectal preparations, focal compression of the serosal surface using von Frey hairs identified mechanosensitive “hot spots,” that were associated with brief bursts of action potentials. Spritzing capsaicin (10 μM) selectively onto each identified mechanosensitive hot spot activated an all or none discharge of action potentials in 32 of 56 identified hot spots in wild type mice and 24 of 62 mechanosensitive hot spots in the aganglionic rectum of ls/ls mice. Each single unit activated by both capsaicin and circumferential stretch responded to low mechanical thresholds (1–2 g stretch). No high threshold rectal afferents were ever recorded in response to circumferential stretch. Anterograde labeling from recorded rectal afferents revealed two populations of capsaicin-sensitive mechanoreceptor that responded to stretch: one population terminated within myenteric ganglia, the other within the circular and longitudinal smooth muscle layers. In the aganglionic rectum of ls/ls mice, only the i.m. mechanoreceptors were identified. Both myenteric and i.m. mechanoreceptors could be identified by their immunoreactivity to the anti-TRPV1 antibody and the vesicular glutamate transporter, Vglut2. Myenteric mechanoreceptors had a unique morphology, consisting of smooth bulbous nodules that ramified within myenteric ganglia. In summary, the rectum of wild type mice is innervated by at least two populations of capsaicin-sensitive rectal mechanoreceptor, both of which respond to low mechanical thresholds within the innocuous range. These findings suggest that the visceral pain pathway activated by rectal distension is likely to involve low threshold rectal mechanoreceptors that are activated within the normal physiological range.

Identification of functional intramuscular rectal mechanoreceptors in aganglionic rectal smooth muscle from piebald lethal mice

The mechanosensitive endings of low-threshold, slowly adapting pelvic afferents that innervate the rectum have been previously identified as rectal intraganglionic laminar endings (rIGLEs) that lie within myenteric ganglia. We tested whether the aganglionic rectum of piebald-lethal (s(l)/s(l)) mice lacks rIGLEs and whether this could explain impaired distension-evoked reflexes from this region. Extracellular recordings were made from fine rectal nerves in C57BL/6 wild-type and s(l)/s(l) mice, combined with anterograde labeling. In C57BL/6 mice, graded circumferential stretch applied to the rectum activated graded increases in firing of slowly adapting rectal mechanoreceptors. In s(l)/s(l) mice, graded stretch of the aganglionic rectum activated similar graded increases in rectal afferent firing. Stretch-sensitive afferents responded at low mechanical thresholds and fired more intensely at noxious levels of stretch. They could also be activated by probing their receptive fields with von Frey hairs and by muscle contraction. Anterograde labeling from recorded rectal nerves identified the mechanoreceptors of muscular afferents in the aganglionic rectal smooth muscle. A population of afferents were also recorded in both C57BL/6 and s(l)/s(l) mice that were activated by von Frey hair probing, but not stretch. In summary, the aganglionic rectum is innervated by a population of stretch-sensitive rectal afferent mechanoreceptor which develops and functions in the absence of any enteric ganglia. These results suggest that in patients with Hirschsprung's disease the inability to activate extrinsic distension reflexes from the aganglionic rectum is unlikely to be due to the absence of stretch-sensitive extrinsic mechanoreceptors.

Measurement of Mechanical Allodynia and Local Anesthetic Efficacy in Patients With Irreversible Pulpitis and Acute Periradicular Periodontitis

An essential feature of symptomatic periradicular inflammation is mechanical allodynia, defined as reduced mechanical pain thresholds. A previous study evaluating a new digital force transducer showed that it reliably measures mechanical pain thresholds of teeth with normal periradicular tissues. In this study, we tested the hypothesis that the force transducer quantitatively measures mechanical allodynia in teeth with acute periradicular periodontitis (APP) and detects the effects of local anesthetic injection. Mechanical pain thresholds were measured in patients (n = 30) with irreversible pulpitis (IP) and APP and compared with their contralateral teeth. The results show that the mechanical pain thresholds of teeth with IP and APP were reduced by 77% compared with contralateral control teeth. The administration of local anesthesia reversed the mechanical allodynia by 62%, and significant sex-specific effects were observed. In addition, the normal teeth contralateral to the symptomatic teeth had lower mechanical thresholds than those observed in healthy volunteers, suggesting that central sensitization occurs during this type of odontalgia. Thus, we show that the mechanical pain thresholds are significantly reduced in teeth with IP and APP and that the force transducer has potential application as a diagnostic aid in measuring mechanical allodynia and as an outcome measure in endodontic clinical trials such as pharmacologic studies and mechanistic research.

Mechanically evoked sensory and motor responses to dynamic compression of the ulnar nerve

Mechanical deformation of a peripheral nerve can evoke action potentials in sensory and motor axons. The generation of these impulses with brief stimuli (<0.5 s) and their relationship to the deformation conditions have not been systematically studied in human subjects. Controlled compression stimuli over a range of amplitudes, durations, and loading rates were delivered to the ulnar nerve at the medial epicondyle in awake human subjects. Compound muscle action potentials were recorded from the first dorsal interosseous muscle. Subjects rated the magnitude of evoked paresthesias. Mechanically evoked motor and sensory responses varied linearly with the magnitude (P < 0.001) and rate of deformation (P < 0.01), but not the duration, and occurred only during the compression phase. Cutaneous axons had lower mechanical thresholds than motor axons. We relate these findings to the viscoelastic properties of peripheral nerves and differences in biophysical properties of cutaneous and motor axons.

Organizational and activational effects of testosterone on carrageenan-induced inflammatory pain and morphine analgesia

Men are typically reported to have higher pain thresholds than women. Gonadal hormones, particularly testosterone for males, may contribute to this effect. This study tested whether changes in the male hormonal milieu early or late in development alter the inflammatory pain induced by carrageenan (CARR, 3%, intraarticular). Male rats were either gonadectomized or sham gonadectomized neonatally. Once adults, the same rats underwent either sham surgery or gonadectomy, and received a testosterone or oil implant. After baseline testing two weeks later, animals were tested in response to mechanical and thermal stimulation following CARR-induced inflammation and injection of saline, 1 mg/kg, or 5 mg/kg morphine. Hormone alterations did not affect baseline responses, but CARR induced mechanical and thermal hyperalgesia in the injured limb. Gonadectomy in adult males injected with saline enhanced inflammation-induced sensitivity to mechanical but not thermal stimulation and the effect was reversed by testosterone. After inflammation, saline-treated males gonadectomized neonatally displayed lower mechanical threshold than control sham animals but this effect was not reversed by testosterone. Both doses of morphine increased mechanical and thermal thresholds. However, compared with the control group, 1 mg/kg morphine was equally effective in reducing mechanical hyperalgesia among groups of animals gonadectomized as adults, but less effective in males gonadectomized neonatally. The results suggest that in males: 1. the antihyperalgesic effect of testosterone (or its metabolites) in CARR-induced inflammation is established during development and maintained by circulating levels of testosterone in adulthood; 2. the nociception-related interaction between the opioid and gonadal systems influences the sensitivity to mechanical stimuli and is likely established during the period of sexual differentiation.

Excitatory and modulatory effects of inflammatory cytokines and neurotrophins on mechanosensitive group IV muscle afferents in the rat

In inflamed tissue – including skeletal muscle – the concentrations of cytokines and neurotrophins are known to increase. However, nothing is known about a possible contribution of these agents to muscle pain and hyperalgesia. The present study investigated acute effects of cytokines and neurotrophins on response properties of slowly conducting muscle afferents. In anaesthetised rats, the impulse activity of single mechanosensitive group IV fibres innervating the gastrocnemius–soleus muscle was recorded and tumour necrosis factor-α (TNF-α), interleukin-6 (IL-6), nerve growth factor (NGF), or brain-derived neurotrophic factor (BDNF) were injected into the muscle. Changes in the mechanosensibility of the endings following administration of the agents were tested with repeated pressure stimuli of defined forces. A low mechanical threshold in the innocuous range was found in 44.4% of the units tested, 55.6% required strong, potentially tissue-damaging pressure stimuli for activation. NGF excited only units that had a high mechanical threshold, while IL-6 was a stimulant for low-threshold mechanosensitive units only. TNF-α and BDNF did not excite group IV units but had a desensitising action: after TNF-α or BDNF, the response magnitudes to pressure stimuli decreased significantly. The data indicate that cytokines and neurotrophins influence the impulse activity and mechanosensitivity of group IV muscle afferent units. These effects could be of functional significance when the agents are released from muscle cells under pathophysiological circumstances.

Fibromyalgia: Where are we a decade after the American College of Rheumatology classification criteria were developed?

The American College of Rheumatology (ACR) published classification criteria for fibromyalgia (FM) in 1990 (1). These criteria allow investigators around the world to investigate mechanisms of symptom expression in FM with assurance that research subjects share the presence of chronic widespread pain and evidence of altered pain processing, as demonstrated by the presence of tender points. With publication of the classification criteria, the ACR assumed a leadership position in research into basic mechanisms underlying FM symptoms and management of patients with FM. What progress has been made in the decade since the ACR classification criteria were developed? A strength of the criteria from a research standpoint is that FM patients meeting the ACR classification criteria are at the extreme end of the spectrum of pain and tenderness. As a result, a number of studies have demonstrated important alterations in pain processing in FM. These studies use subjective ratings of experimental pain paradigms as well as objective demonstration of an altered central representation of painful stimuli. Petzke et al demonstrated that patients with FM had low mechanical and thermal pain thresholds regardless of whether the stimulus was presented in a predictable, ascending manner (as with tender point determinations or dolorimetery) or randomly (2). These data suggest that, compared with healthy individuals, FM patients do not rate pain stimuli higher because of psychological hypervigilance or “expectancy.” Staud et al showed that patients with FM exhibit altered temporal summation of pain stimuli administered as a thermal stimulus to skin or as a mechanical stimulus to muscle (3,4). The results of these studies suggest a parallel between the human condition of FM and the “wind-up” phenomenon that leads to hyperalgesia and has been extensively studied in animal models (5). Other data corroborating the veracity of FM patients’ descriptions of pain have been collected using paradigms that are not dependent on subjective reports by patients. For example, Lorenz et al demonstrated altered laser-evoked potentials as objective representation of the central nervous system (CNS) response to cutaneous stimulation (6). Patients with FM were shown by Mountz et al and by Cianfrini et al to have reduced thalamic blood flow under resting conditions (a finding observed in other chronic pain states) and altered responses to mechanical pain stimuli (7,8). Finally, Grant et al demonstrated that increased blood flow to brain regions known to receive pain input correlates with the intensity of perceived pain rather than the absolute level of the stimulus (9). All these data arguably complement the most consistent biologic finding supporting aberrant central pain transmission in FM—the 3-fold higher concentrations of substance P in cerebrospinal fluid (CSF) of FM patients compared with those in the CSF of normal controls (10–13). Taken together, the data on pain processing in FM demonstrate that the central representation of pain correlates with patient reports of pain, and that purely behavioral or psychological factors are not primarily responsible for the pain and tenderness seen in FM. The ACR classification criteria focus only on pain, however, and disregard other important FM symptoms, including fatigue, cognitive disturbance, sleep disturbance, and psychological distress. If one takes a more general view of FM that includes these symptoms and commonly associated syndromes (e.g., chronic fatigue syndrome, irritable bowel syndrome, and depression), then the ACR criteria fail to capture the essence of the FM syndrome. When research subjects are identified using the ACR criteria, there is greater variability Leslie J. Crofford, MD: University of Michigan, Ann Arbor; Daniel J. Clauw, MD: University of Michigan, Ann Arbor, and Georgetown University, Washington, DC. Address correspondence and reprint requests to Leslie J. Crofford, MD, Room 5510A, MSRB-I, 1150 West Medical Center Drive, Ann Arbor, MI 48109-0680. E-mail: crofford@umich.edu. Submitted for publication November 27, 2001; accepted December 5, 2001.

Central anatomy of individual rapidly adapting low‐threshold mechanoreceptors innervating the “hairy” skin of newborn mice: Early maturation of hair follicle afferents

Adult skin sensory neurons exhibit characteristic projection patterns in the dorsal horn of the spinal gray matter that are tightly correlated with modality. However, little is known about how these patterns come about during the ontogeny of the distinct subclasses of skin sensory neurons. To this end, we have developed an intact ex vivo somatosensory system preparation in neonatal mice, allowing single, physiologically identified cutaneous afferents to be iontophoretically injected with Neurobiotin for subsequent histological analyses. The present report, centered on rapidly adapting mechanoreceptors, represents the first study of the central projections of identified skin sensory neurons in neonatal animals. Cutaneous afferents exhibiting rapidly adapting responses to sustained natural stimuli were encountered as early as recordings were made. Well-stained representatives of coarse (tylotrich and guard) and fine-diameter (down) hair follicle afferents, along with a putative Pacinian corpuscle afferent, were recovered from 2–7-day-old neonates. All were characterized by narrow, uninflected somal action potentials and generally low mechanical thresholds, and many could be activated via deflection of recently erupted hairs. The central collaterals of hair follicle afferents formed recurrent, flame-shaped arbors that were essentially miniaturized replicas of their adult counterparts, with identical laminar terminations. The terminal arbors of down hair afferents, previously undescribed in rodents, were distinct and consistently occupied a more superficial position than tylotrich and guard hair afferents. Nevertheless, the former extended no higher than the middle of the incipient substantia gelatinosa, leaving a clear gap more dorsally. In all major respects, therefore, hair follicle afferents display the same laminar specificity in neonates as they do in adults. The widely held misperception that their collaterals extend exuberant projections into pain-specific regions of the dorsal horn during early postnatal life is shown to have multiple, deep-rooted underpinnings. J. Comp. Neurol. 436:304–323, 2001. © 2001 Wiley-Liss, Inc.

Central anatomy of individual rapidly adapting low‐threshold mechanoreceptors innervating the “hairy” skin of newborn mice: Early maturation of hair follicle afferents

Adult skin sensory neurons exhibit characteristic projection patterns in the dorsal horn of the spinal gray matter that are tightly correlated with modality. However, little is known about how these patterns come about during the ontogeny of the distinct subclasses of skin sensory neurons. To this end, we have developed an intact ex vivo somatosensory system preparation in neonatal mice, allowing single, physiologically identified cutaneous afferents to be iontophoretically injected with Neurobiotin for subsequent histological analyses. The present report, centered on rapidly adapting mechanoreceptors, represents the first study of the central projections of identified skin sensory neurons in neonatal animals. Cutaneous afferents exhibiting rapidly adapting responses to sustained natural stimuli were encountered as early as recordings were made. Well-stained representatives of coarse (tylotrich and guard) and fine-diameter (down) hair follicle afferents, along with a putative Pacinian corpuscle afferent, were recovered from 2-7-day-old neonates. All were characterized by narrow, uninflected somal action potentials and generally low mechanical thresholds, and many could be activated via deflection of recently erupted hairs. The central collaterals of hair follicle afferents formed recurrent, flame-shaped arbors that were essentially miniaturized replicas of their adult counterparts, with identical laminar terminations. The terminal arbors of down hair afferents, previously undescribed in rodents, were distinct and consistently occupied a more superficial position than tylotrich and guard hair afferents. Nevertheless, the former extended no higher than the middle of the incipient substantia gelatinosa, leaving a clear gap more dorsally. In all major respects, therefore, hair follicle afferents display the same laminar specificity in neonates as they do in adults. The widely held misperception that their collaterals extend exuberant projections into pain-specific regions of the dorsal horn during early postnatal life is shown to have multiple, deep-rooted underpinnings.

Chemosensitivity of nociceptive, mechanosensitive afferent nerve fibres in the guinea-pig ureter.

The mechanosensitivity and chemosensitivity of afferent fibres were investigated in an in vitro preparation of the guinea-pig ureter. Electrophysiological recordings were obtained from 5 U-1 (low mechanical threshold, contraction-sensitive) and 74 U-2 units (high threshold). U-2 units had significant higher levels of spontaneous activity, lower conduction velocities, higher mechanical thresholds (U-1: 7 mmHg; U-2: 39 mmHg), less pronounced phasic responses and longer latencies in the response to distensions than the U-1 units. For chemical stimulation, guinea-pig urine (> 800 mosmol/L), bradykinin and capsaicin were applied intraluminally. The responses of U-1 units mainly corresponded to the contractions induced by the chemical stimulation. The vast majority of the U-2 units were excited by urine, bradykinin (threshold: 0.1-1 microM) and capsaicin (threshold: 0.03-0.3 microM). The responses to urine could be mimicked by high concentrations of potassium ions (> 200 mM), but not by an equiosmolar solution of NaCl, urea and mannitol. Chemical stimulation could also result in a transient sensitization of the U-2 units to mechanical stimuli. In the anaesthetized guinea-pig, pseudo-affective responses could be evoked by ureteric distension (threshold: 30-60 mmHg) and serosal application of capsaicin. Intraluminal application of urine in vivo did not evoke any reactions, suggesting that the responses of the U-2 units to urine might be due to an impaired barrier function of the urothelium in vitro. The data are in agreement with the hypothesis that U-2 units are visceral polymodal nociceptors. Since the U-1 units were also able to encode at least noxious mechanical stimuli, their involvement in visceral nociception cannot be excluded.

Interganglionic segregation of distinct vagal afferent fibre phenotypes in guinea-pig airways.

1. The present study addressed the hypothesis that jugular and nodose vagal ganglia contain the somata of functionally and anatomically distinct airway afferent fibres. 2. Anatomical investigations were performed by injecting guinea-pig airways with the neuronal tracer Fast Blue. The animals were killed 7 days later, and the ganglia were removed and immunostained with antisera against substance P (SP) and neurofilament protein (NF). In the nodose ganglion, NF-immunoreactive neurones accounted for about 98% of the Fast Blue-labelled cells while in the jugular ganglion they accounted for approximately 48%. SP and NF immunoreactivity was never (n = 100) observed in the same cell suggesting that the antisera labelled distinct populations. 3. Electrophysiological investigations were performed using an in vitro guinea-pig tracheal and bronchial preparation with intact afferent vagal pathways, including nodose and jugular ganglia. Action potentials arriving from single airway afferent nerve endings were monitored extracellularly using a glass microelectrode positioned near neuronal cell bodies in either ganglion. 4. The nodose ganglion contained the somata of mainly fast-conducting tracheal A delta fibres whereas the jugular ganglion contained equal numbers of C fibre and A delta fibre tracheal afferent somata. The nodose A delta neurones adapted rapidly to mechanical stimulation, had relatively low mechanical thresholds, were not activated by capsaicin and adapted rapidly to a hyperosmotic stimulus. By contrast, jugular A delta and C fibres adapted slowly to mechanical stimulation, were often activated by capsaicin, had higher mechanical thresholds and displayed a slow adaptation to a hyperosmotic stimulus. 5. The anatomical, physiological and pharmacological data provide evidence to support the contention that the vagal ganglionic source of the fibre supplying the airways ultimately dictates its neurochemical and physiological phenotype.

Changes in activity of spinal cells with muscular input in rats with referred muscular hyperalgesia from ureteral calculosis

In rats with hyperalgesia of the obliquus externus muscle (OE) from artificial calculosis of the ipsilateral upper ureter, changes in cell activity were studied in the ipsilateral spinal cord (T11–T12) versus control animals. In cases of hyperalgesia of high degree, in the dorsal horn (0–900 μm) the following were found: significantly higher percentages of cells driven by OE stimulation ( P < 0.03) and of spontaneously active cells with OE input ( P < 0.02); significantly higher frequency of background discharge of cells with OE input ( P < 0.002); among cells driven by OE stimulation, significantly higher percentages of neurons with exclusively deep input ( P < 0.0006) and of neurons with low mechanical threshold of activation ( P < 0.03). In the intermediate region of the cord (900–1600 μm), a significantly higher percentage was found of spontaneously active cells with OE input ( P < 0.009) while in the ventral horn (1600–2300 μm) no changes were detected. The results indicate that referred muscle hyperalgesia of high degree is accompanied by a state of central sensitization probably triggered by the abnormal afferent input from the visceral focus

Substance P, neurofilament, peripherin and SSEA4 immunocytochemistry of human dorsal root ganglion neurons obtained from post-mortem tissue: a quantitative morphometric analysis.

Immunocytochemical studies on lumbar dorsal root ganglia obtained at routine postmortem 24-36 h after death were carried out, and neuronal cross-sectional areas measured. The subjects were elderly (76-81 years), of both sexes, had died from heart attack or haemorrhage, and had no clinical evidence of clinical neuropathy or of disease known to be associated with neuropathy. The data were consistent between ganglia from the three subjects. There were striking similarities with data from other species. Two populations of cell profiles with overlapping size distributions were distinguished with an anti-neurofilament antibody, neurofilament-rich (45% of cell profiles) with a large mean area and neurofilament-poor with a smaller mean area. Anti-substance P and anti-peripherin antibodies both labelled a population with a small mean area, with extensive co-localization between them. There were also some differences between these human dorsal root ganglia and dorsal root ganglia from some other species. More neuronal profiles were labelled for substance P in humans (44%) than in rat (20%). More neuronal profiles were labelled for SSEA4 (stage specific embryonic antigen 4) in human (40.5%) than in rat dorsal root ganglia (10%), and the SSEA4-positive profiles were relatively smaller in human than in rat. No selective accumulation of lipofusin in profiles of large cells was apparent. This study also shows that quantitative morphometric analysis of immunocytochemically labelled dorsal root ganglion neuronal profiles can be carried out successfully on human sensory ganglia obtained at post-mortem. This is the first demonstration of the two main subgroups of dorsal root ganglia neurones with neurofilament-rich and poor somata in human tissue. The size distributions of neurons with neurofilament, substance P and peripherin are consistent with these neuronal populations having similar functional properties to those described in other species. From the known sensory and fibre loss with aging, it is speculated that the loss of some large diameter neurones with myelinated fibres and low mechanical thresholds, might account for the high percentage of neurones expressing substance P.

Mitochondria in fine afferent nerve fibres of the knee joint in the cat: a quantitative electron-microscopical examination.

The distribution of mitochondria, their content and concentration (expressed as the ratio of the mean volume of mitochondria and the surface of the sensory axon) were determined in group-III and -IV nerve fibres innervating the knee joint capsule in the cat. Mitochondria mainly accumulated in axonal swellings ("beads") and end bulbs of the terminal branches. Between single nerve fibres, marked differences in the content and the concentration of mitochondria were obtained in proximal portions (inside of the perineurium) and in distal portions (unmyelinated sensory endings). In group-III nerve fibres, the mitochondrial concentration ranged from 0.005 to 0.030 microns 3/microns 2 (proximal portion) and from 0.016 to 0.080 microns 3/microns 2 (distal portion). In unmyelinated group-IV nerve fibres, the values also showed a broad variation ranging from 0.001 to 0.011 microns 3/microns 2 (proximal portion) and from 0.003 to 0.019 microns 3/microns 2 (distal portion). The wide range of mitochondrial concentrations may reflect different energy consumption during receptive processes: nerve fibres with a low mechanical threshold and a high probability of excitatory events may be rich in mitochondria, whereas fibres with a high mechanical threshold and a low probability of excitatory events may be poor in mitochondria.

Regulation of cutaneous C-fiber heat nociceptors by nerve growth factor in the developing rat

1. Previous work has indicated that nerve growth factor (NGF) is an important factor for the development of nociceptive A-delta fibers. Here we asked if the availability of NGF during the first 2 wk of development can influence the phenotypic development of unmyelinated afferent C-fibers. 2. To do this, we treated newborn rats with antibodies to NGF from postnatal day (PND) 2-14, a treatment known not to lead to cell death, or with exogenous NGF (PND 0-14). Untreated litter mates served as controls. When the animals were mature (5-20 wk later), they were anesthetized (urethan, 1.25 g/kg ip) and single-unit recordings were made from sural nerve C-fibers in dorsal root filaments. To obtain an unbiased estimate of the physiological types present, an electrocutaneous search technique was employed to isolate the location of the putative receptive field of the C-fiber being studied within the sural nerve territory. This technique enabled us to acquire a sample of C-fibers that was unbiased by natural search stimuli which are often damaging to skin. 3. We found that the proportion of C-fibers that could be driven by noxious heat and mechanical stimuli (C-mechanoheat fibers, C-MH) was reduced from 28% (11/40 fibers) of the sample in controls to 10% (3/31 fibers) in anti-NGF treated animals. The C-MH fibers appeared to have been replaced by a novel type of pressure receptor with an unusually low mechanical threshold (mean 0.86 +/- 0.58 compared with 13.0 +/- 8.3 g for control mechanonociceptors).(ABSTRACT TRUNCATED AT 250 WORDS)

An in vitro study of the properties of single vagal afferents innervating guinea‐pig airways.

1. A novel preparation of the trachea and main bronchi with attached vagus nerve from the guinea-pig maintained in vitro was used to study the properties of single vagal afferent nerve fibres with identified receptive fields. 2. Recordings were made from twenty-eight C fibres with a mean conduction velocity of 0.9 +/- 0.1 m s-1 and twenty-four A delta fibres with a mean conduction velocity of 8.4 +/- 1.3 m s-1. Receptive fields for C and A delta fibres were of small diameter, distributed throughout the trachea and right bronchus and possessed very low mechanical thresholds of 2.2 +/- 0.4 and 1.1 +/- 0.3 mN respectively. 3. The chemosensitivity of isolated afferents was studied by applying drugs directly onto identified receptive fields. A delta fibres were insensitive to capsaicin (up to 3 microM), bradykinin (3 microM), histamine (10 microM) and 5-hydroxytryptamine (5-HT; 10 microM) applied for up to 1 min. Histamine (10 microM), 5-HT (10 microM) and m-chlorophenylbiguanide (10 microM) were also ineffective in exciting C fibres. 4. Capsaicin, at concentrations ranging from 30 nM to 3 microM, evoked a sustained firing of all C fibres tested when applied for a period of 30 s directly onto receptive fields. Bradykinin (0.1-1 microM) also potently excited C fibres in a concentration-related manner. The effect of bradykinin appeared to be mediated by a B2 receptor since it was not mimicked by the selective B1 receptor agonist [des-Arg9]-bradykinin (3 microM) and was abolished by prior application of the selective B2 receptor antagonist D-Arg-[Hyp3,Thi5,D-Tic7,Oic8]-bradykinin (HOE 140; 0.1 microM). HOE 140 was without effect against capsaicin-evoked discharge of C fibres. 5. Capsaicin- and bradykinin-evoked discharge of C fibres was present to a similar degree in preparations pretreated with ibuprofen (1 microM), indicating that it was not dependent on, or influenced by, endogenous prostaglandin production. 6. These data demonstrate that single vagal afferents may be studied in vitro and provide the first examination of the properties of sensory fibres innervating guinea-pig airways. C and A delta fibres both exhibit low threshold mechanical sensitivity but show marked differences in terms of their chemosensitivity.

Ankle joint (artc. intertarsalis) receptors in the domestic fowl

The physiological responses of joint capsule sensory receptors in the ankle joint of the chicken were studied by recording the electrical activity from single sensory afferent nerve fibres dissected from the parafibular nerve. All units included in this study were sensitive to mechanical stimulation of the joint capsule and were classified with respect to nerve conduction velocity, receptive field size and response threshold. Rapidly adapting mechanoreceptors formed 23% of the receptors present in the sample and responded to the mechanical probe by giving a single response or a short burst of activity at the onset of stimulation. The majority of units identified showed a slowly adapting response and on the basis of conduction velocity were divided into group IV (CV 2.5 m/s), group III (CV 2.5–20 m/s) and those units conducting over 20 m/s. Group IV units had single spot-like receptive fields and mechanical thresholds ranging from 0.6 to 60 g. Group III units could be divided into two groups based on receptive field size. One group had spot-like receptive fields 1–2 mm in diameter, whereas in the other group the receptive fields were larger (over 2×3 mm). The large receptive field units had significantly faster conduction velocities, lower mechanical thresholds and steeper stimulus-response curves than small receptive field units. In response to movement of the joint very few of the receptors, whether rapidly or slowly adapting, were found to be excited by moving the joint by hand in the middle of its physiological range. The physiological properties of these avian mechanoreceptive fine afferent units suggest that while some are activated by normal joint movement and non-noxious local mechanical stimulation of the joint capsule, others have nociceptive functions. The receptors present in the ankle joint were correlated with the anatomical structures found in the avian joints and their physiological properties compared and contrasted to the joint receptors found in mammals.

Low‐threshold mechanoreceptive and nociceptive units with unmyelinated (C) fibres in the human supraorbital nerve.

1. In recordings from the human supraorbital nerve with tungsten microelectrodes, eleven afferent units with unmyelinated (C) axons were identified on the basis of their conduction velocities (0.6-1.4 m/s). 2. Eight units had low mechanical thresholds (less than or equal to 0.23 g) and could be activated up to their maximal firing rates of about 100 impulses/s by weak tactile stimuli, whereas three units had higher thresholds (5.5 g) and responded vigorously to noxious stimuli only. 3. During a skin indentation the low-threshold units adapted to an irregular low-frequency discharge, and release of the stimulus elicited a prominent off-response often ending with an after-discharge. Slow stroking was a particularly effective stimulus, even when done with cotton wool, whereas rapid stroking reduced the response. All types of stroking stimuli were occasionally followed by after-discharges. Repeated mechanical stimulation at short intervals resulted in a decline of the response, indicating receptor fatigue. For two units a response to skin cooling was observed. 4. The above low-threshold C units have all the main characteristics of the C mechanoreceptors known from the cat and primates but not previously proven to exist in man. The high-threshold C units are similar to the polymodal nociceptors found in other human skin areas.

Tonic descending influences on receptive-field properties of nociceptive dorsal horn neurons in sacral spinal cord of rat

1. Single-unit electrical activity has been recorded from 34 dorsal horn neurons in the sacral segments (S1-2) of the spinal cord in halothane-anesthetized rats. All of the neurons had cutaneous receptive fields (RFs) on the rat's tail. The neurons were classified according to their responses to both innocuous and noxious mechanical stimulation of their RFs. Twenty-five cells were driven by both innocuous and noxious skin stimulation (multireceptive or class 2), and 9 neurons were driven only by noxious skin stimulation (nocireceptive or class 3). 2. The RF size, mechanical threshold, and afferent input properties of these neurons were determined in the intact anesthetized and spinalized states. Reversible spinalization was achieved by cooling the cervical spinal cord to 4 degrees C. 3. The class 2 neurons had a mean RF size of 919.8 +/- 112.0 (SE) mm2 in the intact animal. Fourteen of the 25 class 2 cells had larger RFs in the spinal state (mean increase = 330.0 mm2, SE = 79.2) and so were under tonic descending inhibition. Five neurons, all with C-fiber input, had smaller RFs (mean decrease = 247.6 mm2, SE = 136.6) and higher mechanical thresholds in the spinal state and so were under tonic descending excitation. Six neurons were unaffected by spinalization. 4. Five class 3 neurons recorded in the superficial dorsal horn had small RFs in the intact animal (mean = 201.0 mm2, SE = 48.8) and showed little or no change in RF size on spinalization (mean increase = 33.4 mm2, SE = 16.7), but their mechanical thresholds did decrease, indicating weak tonic descending inhibition. In contrast, four class 3 neurons recorded in the deep dorsal horn had larger RFs in the intact animal (mean = 566.8 mm2, SE = 156.8), and were under strong tonic descending inhibition, because they had much larger RFs (mean increase = 461.0 mm2, SE = 68.3), lower mechanical thresholds, and stronger C-fiber afferent input in the spinal state. 5. We conclude that the majority of nociceptive dorsal horn neurons are subject to a net tonic descending control of their RF properties. The class 2 neurons in the deep dorsal horn appear to be a heterogeneous population, some cells being under tonic descending excitation and others under tonic descending inhibition. Class 3 cells can be separated into those located in the superficial dorsal horn, whose RF properties show very little change on spinalization, and those in the deep dorsal horn, whose RF properties change markedly on spinalization.(ABSTRACT TRUNCATED AT 400 WORDS)