Donald A. Simone

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.

Topical capsaicin in humans: parallel loss of epidermal nerve fibers and pain sensation.

Capsaicin applied topically to human skin produces itching, pricking and burning sensations due to excitation of nociceptors. With repeated application, these positive sensory responses are followed by a prolonged period of hypalgesia that is usually referred to as desensitization, or nociceptor inactivation. Consequently, capsaicin has been recommended as a treatment for a variety of painful syndromes. The precise mechanisms that account for nociceptor desensitization and hypalgesia are unclear. The present study was performed to determine if morphological changes of intracutaneous nerve fibers contribute to desensitization and hypalgesia. Capsaicin (0.075%) was applied topically to the volar forearm four times daily for 3 weeks. At various time intervals tactile, cold, mechanical and heat pain sensations were assessed in the treated and in contralateral untreated areas. Skin blisters and skin biopsies were collected and immunostained for protein gene product (PGP) 9.5 to assess the morphology of cutaneous nerves and to quantify the number of epidermal nerve fibers (ENFs). Capsaicin resulted in reduced sensitivity to all cutaneous stimuli, particularly to noxious heat and mechanical stimuli. This hypalgesia was accompanied by degeneration of epidermal nerve fibers as evidenced by loss of PGP 9.5 immunoreactivity. As early as 3 days following capsaicin application, there was a 74% decrease in the number of nerve fibers in blister specimens. After 3 weeks of capsaicin treatment, the reduction was 79% in blisters and 82% in biopsies. Discontinuation of capsaicin was followed by reinnervation of the epidermis over a 6-week period with a return of all sensations, except cold, to normal levels. We conclude that degeneration of epidermal nerve fibers contributes to the analgesia accredited to capsaicin. Furthermore, our data demonstrate that ENFs contribute to the painful sensations evoked by noxious thermal and mechanical stimuli.

Psychophysical studies of the itch sensation and itchy skin ("alloknesis") produced by intracutaneous injection of histamine.

Psychophysical measurements of itch and itchy skin (alloknesis--itch produced by innocuous mechanical stimulation) were obtained in human volunteers following intracutaneous or subcutaneous injections of histamine or papain into the volar forearm. Histamine and papain were given in doses of 0.1, 1, or 10 micrograms in 10 microliters of saline. The effects of the depth of injection and of skin temperature on the latency, magnitude, and duration of itch were examined. Also, dose-response functions were obtained for the area of alloknesis produced by intracutaneous injections of histamine. Finally, the neural mechanisms underlying the spread of alloknesis were investigated via local anesthesia of the skin. Intracutaneous and subcutaneous injections of histamine, but not papain, produced a sensation of itch without pain. The latency of itch was shorter after an intracutanous than after a subcutaneous injection of histamine. The mean latencies of itch produced by a 1-microgram dose were 9.5 and 23.0 sec for intracutaneous and subcutaneous injections, respectively. No differences were observed in the magnitude or duration of itch. Similarly, the latency of itch was increased when the skin temperature at injection site was lowered to 15 degrees C, whereas the magnitude and duration of itch were unaffected. Intracutaneous and subcutaneous injections of histamine produced similar areas of alloknesis. However, the magnitude and duration of alloknesis were dependent on dose. The mean maximum areas of alloknesis produced by intracutaneous injections of 0.1, 1, and 10 micrograms of histamine were 28.3, 47.2, and 43.8 cm2, respectively. Alloknesis was present at 2 min after injection, increased to a maximum area without 10 min, and then gradually decreased during the next 25-40 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Sensitization of cat dorsal horn neurons to innocuous mechanical stimulation after intradermal injection of capsaicin

Intradermal injection of 100 micrograms of capsaicin in cats excited dorsal horn spinal neurons classified as either high threshold or wide dynamic range. Capsaicin enhanced their responses to innocuous mechanical stimulation. A role for these neurons in mechanical hyperalgesia is suggested.

Hyperalgesia to heat after intradermal injection of capsaicin

Capsaicin injected intradermally into the human forearm lowered the pain threshold for heat at the injection site. Both the magnitude and duration of hyperalgesia were dose dependent over the range of 0.1-100 micrograms, given in a constant volume of 10 microliter. Thus, capsaicin may be a useful tool in studies of the neural mechanisms of hyperalgesia.

The Magnitude and Duration of Itch Produced by Intracutaneous Injections of Histamine

The magnitude and duration of itch sensation produced by intracutaneous injection of histamine were determined for humans with the procedure of magnitude estimation scaling. Thirteen subjects received a 10-microliter intracutaneous injection of histamine at doses of 0.0001, 0.001, 0.01, 0.1, 1, and 10 micrograms into the volar forearm; eight of these subjects also received a 100-microgram dose. One subject received multiple injections over several weeks to determine the reliability of the magnitude estimates of itch. Following each injection, the area of flare and duration of itch were also determined. Intracutaneous injection of histamine produced a pure sensation of itch, without pain. The magnitude of itch increased in a dose-dependent fashion. The lowest histamine dose that produced itch greater than the itch produced by vehicle was 0.01 micrograms. The greatest itch was produced by the 100-microgram dose. A power function fitted to the mean magnitude estimates had an exponent of 0.17, indicating a negatively accelerating relation between the magnitude of itch and histamine dose. The one subject who received histamine over several weeks gave fairly reproducible estimates of itch magnitude. The duration of itch and the area of flare also increased in a dose-dependent fashion. The lowest dose of histamine that produced a duration of itch longer than the itch produced by the vehicle was 0.1 microgram, while the 100-microgram dose produced the longest duration of itch. Although the area of flare increased with each increase in dose from 0.1 to 10 micrograms, the areas of flare produced by 10 and 100 micrograms of histamine did not differ. These results indicate that humans can scale the magnitude of itch produced by histamine in a dose-dependent manner. In addition, the duration of itch and the area of flare produced by histamine are dose-dependent, confirming results of previous investigators. Intracutaneous histamine is easily quantifiable and may thus be a useful stimulus in neurophysiological studies of the peripheral neural mechanisms of itch.

Activation of peripheral cannabinoid receptors attenuates cutaneous hyperalgesia produced by a heat injury.

&NA; Accumulating evidence suggests that cannabinoids can produce antinociception through peripheral mechanisms. In the present study, we determined whether cannabinoids attenuated existing hyperalgesia produced by a mild heat injury to the glabrous hindpaw and whether the antihyperalgesia was receptor‐mediated. Anesthetized rats received a mild heat injury (55 °C for 30 s) to one hindpaw. Fifteen minutes after injury, animals exhibited hyperalgesia as evidenced by lowered withdrawal latency to radiant heat and increased withdrawal frequency to a von Frey monofilament (200 mN force) delivered to the injured hindpaw. Separate groups of animals were then treated with an intraplantar (i.pl.) injection of vehicle or the cannabinoid receptor agonist WIN 55,212‐2 at doses of 1, 10, or 30 &mgr;g in 100 &mgr;l. WIN 55,212‐2 attenuated both heat and mechanical hyperalgesia dose‐dependently. The inactive enantiomer WIN 55,212‐3 did not alter mechanical or heat hyperalgesia, suggesting the effects of WIN 55,212‐2 were receptor‐mediated. The CB1 receptor antagonist AM 251 (30 &mgr;g) co‐injected with WIN 55,212‐2 (30 &mgr;g) attenuated the antihyperalgesic effects of WIN 55,212‐2. The CB2 receptor antagonist AM 630 (30 &mgr;g) co‐injected with WIN 55,212‐2 attenuated only the early antihyperalgesic effects of WIN 55,212‐2. I.pl. injection of WIN 55,212‐2 into the contralateral paw did not alter the heat‐injury induced hyperalgesia, suggesting that the antihyperalgesia occurred through a peripheral mechanism. These data demonstrate that cannabinoids primarily activate peripheral CB1 receptors to attenuate hyperalgesia. Activation of this receptor in the periphery may attenuate pain without causing unwanted side effects mediated by central CB1 receptors.

Hypothesis for novel classes of chemoreceptors mediating chemogenic pain and itch

cf, Mon 1:30 N.C. Brecha*, P.W. Mantyh*, and C. Sternini*, Departments of Room 6 Anatomy, Anesthesiology, and Medicine and Brain Research Ins i u e, UCLA Center for Health Sciences, Los Angeles, CA 90024, USA Aim of Investigation: The presence of CGRP in small and medium diameter sensory ganglion cells is restricted to a portion of neurons emitting thin axons subserving nociception, distinct from a similar size population displaying fluoride resistant acid phosphatase (FRAP). The peripheral terminals related to CGRP, was examined in a variety of tissues known to exhibit specialization of nociceptive innervation. Methods: Immunohistochemistry was carried out in a variety of rat tissues including skin, teeth, joint capsules, periosteum, tympanic membrane, testis, cornea, mesentery, etc. FRAP was studied in rat but also demonstrated in sensory ganglia and dorsal horn of several mammals including human. Specific high-affinity peptide receptor binding sites were studied autoradiographically in a variety of peripheral tissues known to receive sensory peptidergic innervation. Results: CGRP immunoreactive axons terminate in a variety of specialized patterns in tissues known to possess a segregated nociceptor innervation, e.g. cornea, dentinal tubules, tympanic membrane, cornea, periosteum, but also to structures presumably unrelated to afferent function e.g. lymph nodes, blood vessels, mast cells, etc. The distribution of peptide receptor binding sites does not routinely match that of immunoreactive axons. Conclusion: A variety of biochemical labels may provide the basis for a new taxonomy of sensory C fibers when correlated with morphological criteria. Nociceptive axons can be categorized into distinct populations using enzyme, peptide and receptor binding site markers which may be unrelated to their afferent role. (Supported by NIH grant NS-5685.)

Influence of thermode size for detecting heat pain dysfunction in a capsaicin model of epidermal nerve fiber loss.

&NA; Quantitative sensory testing of heat pain sensation has become an important tool to evaluate small caliber afferent nerve function in peripheral neuropathy. In earlier studies, we found that topical application of capsaicin in humans results in the loss of epidermal nerve fibers (ENFs) with a corresponding decrease in detection of heat pain sensation. Capsaicin may therefore be a useful model for developing optimal psychophysical testing procedures for detection of neuropathy in its early stages. Here we determined the influence of thermal probe (thermode) size in detecting the diminished heat pain sensation following capsaicin application. Twelve healthy volunteers applied 0.075% capsaicin topically to the volar forearm four times daily for 7 days. Psychophysical measures of heat pain, mechanical (sharp) pain, and tactile threshold were obtained daily from untreated control skin and from capsaicin‐treated skin during capsaicin application, and once weekly for 5 weeks following discontinuation of capsaicin. Heat pain sensation was assessed using a large (30×30 mm) and small (3×3 mm) thermode and different algorithms to assess pain threshold and suprathreshold heat pain. Skin biopsies were obtained and were processed for immunohistochemical localization of (ENFs) using the pan neuronal marker protein gene product 9.5. Capsaicin produced a rapid decrease in the number of ENFs, with nearly complete disappearance after 3 days of treatment. Heat pain evoked by the small, but not the large, thermode decreased dramatically after capsaicin treatment. The sensation of heat pain returned toward normal after 2–3 weeks following discontinuation of capsaicin treatment concordant with gradual reinnervation of the epidermis. Regression analysis indicated that the sensation of heat pain evoked by the small thermode correlated much better with the number of ENFs than heat pain evoked by the large thermode. The detection of sharp pain decreased moderately after capsaicin treatment. Assessment of heat pain sensation using small thermodes has potential for detecting sensory deficits in early stages of small fiber neuropathy.