Robert H. LaMotte

Central changes in processing of mechanoreceptive input in capsaicin‐induced secondary hyperalgesia in humans.

1. Capsaicin, the algesic substance in chilli peppers, was injected intradermally in healthy human subjects. A dose of 100 micrograms given in a volume of 10 microliters caused intense pain lasting for a few minutes after injection and resulted in a narrow area of hyperalgesia to heat and a wide surrounding area of hyperalgesia to mechanical stimuli (stroking) lasting for 1‐2 h. 2. Nerve compression experiments with selective block of impulse conduction in myelinated (A) but not in unmyelinated (C) fibres indicated that afferent signals in C fibres contributed to pain from capsaicin injection and to heat hyperalgesia, whereas conduction in afferent A fibres was necessary for the perception of mechanical hyperalgesia. 3. Electrical intraneural microstimulation normally eliciting non‐painful tactile sensations was accompanied by pain when the sensation was projected to skin areas within the region of mechanical hyperalgesia induced by capsaicin injection. 4. The threshold for pain evoked by intraneural microstimulation was reversibly lowered and pain from suprathreshold stimulation was exaggerated during the period of mechanical hyperalgesia, regardless of lidocaine anaesthesia of the cutaneous innervation territory of the stimulated fibres. 5. The results indicate that hyperalgesia to stroking on a skin area surrounding a painful intradermal injection of capsaicin is due to reversible changes in the central processing of mechanoreceptive input from myelinated fibres which normally evoke non‐painful tactile sensations.

Pain, hyperalgesia and activity in nociceptive C units in humans after intradermal injection of capsaicin

1. Capsaicin, the potent algesic substance in chilli peppers, was applied topically to, or injected intradermally into or outside, the receptive fields of 14 C mechanoheat (polymodal) nociceptor units in awake humans. The nociceptor discharges were recorded using microelectrodes inserted into the peroneal nerve. Simultaneously, the subjects estimated the magnitude of pain as a function of time during the first 1.5‐3 min after injection. Magnitude estimates of pain produced by heat and/or mechanical stimuli were also obtained before and after capsaicin in order to assess the magnitude of cutaneous hyperalgesia. 2. An injection within or adjacent to, but not greater than 4 mm outside, the receptive fields of C nociceptor units evoked discharges. The magnitude of pain and the mean discharge rate of the units were both maximal on injection, declining rapidly over the next 1‐3 min, which indicates that these nociceptors contribute to the magnitude and duration of pain evoked by capsaicin injection. 3. Reduced or abolished excitability in C nociceptors after capsaicin injection within the receptive fields correlated with analgesia at the injection site. 4. Capsaicin injection produced a wide surround area of mechanical hyperalgesia, i.e. pain on gently stroking the skin or abnormally intense pain on punctate stimulation. Nevertheless, the injections did not lower the thresholds or enhance the responses to such mechanical stimuli of C nociceptor units with their receptive fields in this hyperalgesic area. 5. Topical application of capsaicin evoked on‐going discharges in four units tested. Both nociceptor response thresholds and pain thresholds were lowered for heat from 45 to 35 degrees C. A newly developed weak response to stroking the skin in two units after capsaicin was accompanied by faint pain. 6. On‐going activity in sensitized C nociceptors and concomitant pain were effectively reduced by cooling the skin in the receptive area. 7. It is concluded that activity in C mechanoheat (polymodal) nociceptors contributes to the magnitude and duration of pain evoked by intradermal injection of capsaicin. The after‐effects of capsaicin on C nociceptor excitability depend on concentration: high concentration (by injection) leads to desensitization, whereas low concentration (by topical application) leads to sensitization. On‐going discharges and lowered response thresholds to heat in these units after topical application of capsaicin correlates with background pain as well as lowered pain thresholds to heat of the affected skin (primary hyperalgesia). The unchanged responsiveness of C nociceptors in the skin well outside the injection area indicates that central rather than peripheral sensitization accounts for the observed mechanical hyperalgesia in this region (secondary hyperalgesia).

Dose-dependent pain and mechanical hyperalgesia in humans after intradermal injection of capsaicin

&NA; Psychophysical measurements of pain and mechanical hyperalgesia were obtained following different doses of capsaicin injected intradermally into the forearms of human subjects. Each subject received a 10 &mgr;1 injection of the vehicle and capsaicin doses of 0.01, 0.1, 1, 10 and 100&mgr;g. The relationship between capsaicin dose and the magnitude and duration of pain was determined using the method of magnitude estimation. In addition to pain, capsaicin produced a flare and mechanical hyperalgesia. The area of flare and the area and time course of mechanical hyperalgesia were measured as a function of the dose of capsaicin. The magnitude and duration of pain, based on averaged responses of all subjects, increased as a negatively accelerating function of dose. The lowest dose of capsaicin to produce more pain than the vehicle was 0.1 &mgr;g. The area and duration of mechanical hyperalgesia also increased as a negatively accelerating function of dose. The lowest dose of capsaicin to produce an area of mechanical hyperalgesia was 0.1&mgr;g. An area of hyperalgesia was present within seconds following injection. For doses of 10 and 100 &mgr;g, the area of hyperalgesia grew to reach a maximum within 5 and 7 min following the injection and gradually decreased, disappearing within 15 and 137 min, respectively. Capsaicin doses of 1, 10 and 100&mgr;g produced successively greater areas of flare. The results demonstrate that humans can scale the magnitude of pain produced by capsaicin in a dose‐dependent fashion. Further, the duration of pain, the area and duration of mechanical hyperalgesia, and the area of flare are dose‐dependent. It is concluded that intradermal injection of capsaicin will provide a useful method of carrying out parallel psychophysical and neurophysiological studies of pain and hyperalgesia.

Plasticity of sodium channel expression in DRG neurons in the chronic constriction injury model of neuropathic pain

Previous studies have shown that transection of the sciatic nerve induces dramatic changes in sodium currents of axotomized dorsal root ganglion (DRG) neurons, which are paralleled by significant changes in the levels of transcripts of several sodium channels expressed in these neurons. Sodium currents that are resistant to tetrodotoxin (TTX-R) and the transcripts of two TTX-R sodium channels are significantly attenuated, while a rapidly repriming tetrodotoxin-sensitive (TTX-S) current emerges and the transcripts of alpha-III sodium channel, which produce a TTX-S current when expressed in oocytes, are up-regulated. We report here on changes in sodium currents and sodium channel transcripts in DRG neurons in the chronic constriction injury (CCI) model of neuropathic pain. CCI-induced changes in DRG neurons, 14 days post-surgery, mirror those of axotomy. Transcripts of NaN and SNS, two sensory neuron-specific TTX-R sodium channels, are significantly down-regulated as is the TTX-R sodium current, while transcripts of the TTX-S alpha-III sodium channel and a rapidly repriming TTX-S Na current are up-regulated in small diameter DRG neurons. These changes may provide at least a partial basis for the hyperexcitablity of DRG neurons that contributes to hyperalgesia in this model.

A subpopulation of nociceptors specifically linked to itch.

Itch-specific neurons have been sought for decades. The existence of such neurons has been doubted recently as a result of the observation that itch-mediating neurons also respond to painful stimuli. We genetically labeled and manipulated MrgprA3+ neurons in the dorsal root ganglion (DRG) and found that they exclusively innervated the epidermis of the skin and responded to multiple pruritogens. Ablation of MrgprA3+ neurons led to substantial reductions in scratching evoked by multiple pruritogens and occurring spontaneously under chronic itch conditions, whereas pain sensitivity remained intact. Notably, mice in which TRPV1 was exclusively expressed in MrgprA3+ neurons exhibited itch, but not pain, behavior in response to capsaicin. Although MrgprA3+ neurons were sensitive to noxious heat, activation of TRPV1 in these neurons by noxious heat did not alter pain behavior. These data suggest that MrgprA3 defines a specific subpopulation of DRG neurons mediating itch. Our study opens new avenues for studying itch and developing anti-pruritic therapies.

Behavioral differentiation between itch and pain in mouse.

Abstract The standard rodent model of itch uses scratching with the hind limb as a behavioral response to pruritic stimuli applied to the nape of the neck. The assumption is that scratching is an indicator of the sensation of itch. But because only one type of site‐directed behavior is available, one cannot be certain that scratching is not a response to nociceptive or other qualities of sensations in addition to, or instead of, itch. To extend the model, we administered chemical stimuli to the cheek of the mouse and counted scratching with the hind limb as an indicator of itch and wiping with the forelimb as an indicator of pain. An intradermal injection of histamine and capsaicin, known to evoke predominantly itch and pain, respectively, in humans, each elicited hind limb scratching behavior when injected into the nape of the neck of the mouse. In contrast, when the same chemicals were injected into the cheek of the mouse, there were two site‐directed behaviors: histamine again elicited scratching with the hind limb, but capsaicin evoked wiping with the forelimb. We conclude that the “cheek model of itch” in the mouse provides a behavioral differentiation of chemicals that elicit predominantly itch in humans from those that evoke nociceptive sensations. That is, the model provides a behavioral differentiation between itch and pain in the mouse.

Cowhage-Evoked Itch Is Mediated by a Novel Cysteine Protease: A Ligand of Protease-Activated Receptors

Cowhage spicules provide an important model for histamine-independent itch. We determined that the active component of cowhage, termed mucunain, is a novel cysteine protease. We isolated mucunain and demonstrate that both native and recombinant mucunain evoke the same quality of itch in humans. We also show that mucunain is a ligand for protease-activated receptors two and four. These results support and expand the relationship between proteases, protease-activated receptors, and itch.

Latency to detection of first pain

The latency to detection of heat stimuli applied to the distal forearm and thenar eminence was measured in 3 subjects in order to determine whether short latency responses correlated with perception of first pain. Only one temperature was used in a given run and stimuli ranged from 39 to 51 degrees C. In addition, subjects were interviewed at the end of each run regarding the quality of sensations experienced. In one series of experiments the quality of the first sensation evoked by each stimulus rather than latency was recorded. The median response latency decreased exponentially from 1100 ms to 400 ms for the distal arm and 1100 ms to 700 ms for the hand. The higher temperatures elicited a double pain sensation on the arm, but not on the glabrous hand. Warmth was always the first sensation felt on the hand. It is concluded that short latencies (less than 450 ms) reliably denote the presence of first pain, and that at least some portion of the primary afferents that signal first pain must have conduction velocities greater than 6 m/s.

Psychophysical and Physiological Evidence for Parallel Afferent Pathways Mediating the Sensation of Itch

The neuronal pathways for itch have been characterized mainly based on responses to histamine. Intracutaneous application of histamine produces intense itch and a large area of axon-reflexive vasodilation (“flare”) around the application site. Both phenomena are thought to be mediated through neuronal activity in itch-specific, mechanoinsensitive C-fiber afferents (CMi). However, mechanical and electrical stimuli that do not activate CMi fibers can cause the sensation of itch, and itch may occur without flare, suggesting that other neuronal itch pathways exist. Because cutaneous application of spicules from the plant Mucuna pruriens (cowhage) has been anecdotally reported to produce itch without flare, we performed psychophysical experiments to investigate whether the mechanisms underlying cowhage- and histamine-induced itch differ. Although histamine and cowhage produced itch of similar magnitude, the itch to cowhage was not correlated with the itch to histamine; some subjects had intense itch to cowhage and little itch to histamine and visa versa. Laser Doppler measurements of blood flow revealed that histamine led to a large area of vasodilation, whereas cowhage produced vasodilation restricted to the application site. Pretreatment of the skin with an antihistamine blocked the itch produced by histamine but did not prevent cowhage-induced itch. Desensitization of the skin with topical capsaicin abolished cowhage-induced itch but did not significantly alter histamine-induced itch. These findings indicate that cowhage itch is signaled through a population of capsaicin-sensitive afferent nerve fibers that is distinct from CMi fibers mediating histamine-induced itch. Cowhage may be useful to investigate the neural pathway mediating nonhistaminergic itch.

Response properties of high-threshold cutaneous cold receptors in the primate.

Cutaneous high-threshold cold receptors (HCRs) in the monkey were identified as sensitive only to cold temperatures below 27 degrees C and not responsive to mechanical or heat noxious stimulation. Some HCRs had axons conducting in the low A-delta range while others had C fibers. The response properties of HCRs were contrasted with those of mechanothermal nociceptors, the latter believed to contribute to the sense of cold pain. HCRs with A delta fibers may contribute to the sense of innocuous cold below temperatures to which low-threshold cold receptors are maximally responsive.