Fernand Anton

Selective excitation by capsaicin of mechano-heat sensitive nociceptors in rat skin

The effect of close-by arterial injections of capsaicin on single afferent fibers of the saphenous nerve was studied on 82 units from control rats and on 44 units from rats pretreated with capsaicin (total dose 200 mg/kg applied subcutaneously under anesthesia 3 days before the experiment). In control rats low doses of capsaicin selectively excited mechano-heat sensitive cutaneous nociceptors (polymodal C fiber nociceptors and MH-A delta nociceptors). The median threshold dose for polymodal nociceptors was 0.1 micrograms. Repeated injections of capsaicin in near-threshold doses evoked reproducible effects without obvious signs of desensitization. In contrast A delta high-threshold mechanoreceptors, hair follicle receptors, cold receptors and C fiber mechanoreceptors were not excited by capsaicin even at doses of 5 micrograms. These high doses activated, however, some SA-II mechanoreceptors after a time lag, probably due to increased tissue turgor induced by plasma extravasation. Systemic capsaicin pretreatment of adult rats resulted in a selective decrease in the proportion of polymodal nociceptors among the afferent C-units, and an increment in the threshold dose of capsaicin of the responding polymodal nociceptors. It is concluded that in the adult rat capsaicin exerts a selective stimulatory and blocking effect on cutaneous mechano-heat sensitive nociceptors conducting both in the C fiber and A delta fiber range.

SI nociceptive neurons participate in the encoding process by which monkeys perceive the intensity of noxious thermal stimulation

The activity of primary somatosensory (SI) cortical nociceptive neurons was recorded while the monkeys performed a psychophysical task in which they detected small increases in skin temperature superimposed on noxious levels of thermal stimulation. The detection latency to these stimuli, expressed as detection speed, was used as a measure of the perceived intensity of sensation. Two-thirds of the neurons that responded to noxious thermal stimulation increased their discharge in response to graded increases in stimulus intensity. The remaining neurons responded to noxious thermal stimulation, but did not grade their response with the intensity of the stimulus. The response of SI nociceptive neurons that encode the intensity of noxious thermal stimulation was significantly correlated with the monkeys detection speed. We conclude that SI nociceptive neurons are involved in the encoding process by which monkeys perceive the intensity of noxious thermal stimulation.

Spinal lamina I projection neurons in the rat: Collateral innervation of parabrachial area and thalamus

A major ascending nociceptive pathway from spinal lamina I to the mesencephalon has previously been reported in the cat, rat and monkey. In the present paper, we have used single and double retrograde labeling techniques to describe this projection system and its collateralization to the thalamus in the rat. Injections of wheat germ agglutinin-horseradish peroxidase into the pontomesencephalic parabrachial area labeled cell bodies bilaterally in lamina I and deeper laminae of the spinal cord. Bilateral lesions of the dorsolateral funiculi at thoracic levels reduced labeling of lamina I neurons caudal to the lesions. Combined injections of fluorescent retrograde tracers into the lateral thalamus and parabrachial area resulted in double labeling of projection neurons in lamina I, lamina IV VIII and the lateral spinal nucleus of the cervical and lumbar enlargements. Double-labeled neurons were especially abundant in lamina I. Thus, we have demonstrated a major lamina I projection through the dorsolateral funiculi to the parabrachial area with significant collateralization to the thalamus. Moreover, since more than 80% of retrogradely labeled lamina I spinothalamic tract cells had collaterals to the parabrachial area we have indirectly demonstrated the presence of a dorsolateral funicular pathway for lamina I spinothalamic neurons in the rat. More lamina I neurons were retrogradely labeled from midbrain injections as compared to thalamic injections. The significance of these findings rest on previous work in this and other laboratories and concerns the understanding of spinal nociceptive mechanisms. Lamina I projection neurons are primarily nociceptive-specific in their response properties and have been shown to project to both the midbrain and thalamus via the dorsolateral funiculus in a number of species. The role of this projection system in nociceptive transmission may lie in its ability to distribute precise information to multiple brain stem sites which in turn activate autonomic or affective responses or descending pain modulatory mechanisms.

The effect of carrageenan-induced inflammation on the sensitivity of unmyelinated skin nociceptors in the rat

&NA; Carrageenan was subcutaneously injected in the area innervated by the saphenous nerve. Part of the mechano‐heat sensitive C‐fiber receptors (CHM) located inside or at the border of the inflamed area showed an enhanced responsiveness to heat stimulation (sensitization). Those CMH units exhibited spontaneous activity; their mechanical thresholds (von Frey) were higher than those of not spontaneously active fibers. None of the units located outside of the inflamed area displayed sensitization. The data reveal that only part of the CMH units in a uniformly inflamed skin area shows signs of sensitization. Our results are compared to those obtained in other inflammatory processes. The relation to inflammatory pain and to hyperalgesia and the contribution of endogenous substances to sensitization of CMH units are discussed.

Psychoneuroimmunological correlates of persisting sciatic pain in patients who underwent discectomy.

Patients suffering from persisting sciatic pain 8 weeks following discectomy were compared with patients displaying low complaints and healthy, pain-free volunteers regarding their interleukin-6 (IL-6) levels, morning cortisol levels and degree of psychological distress. Whereas serum concentrations of IL-6 were measured by collecting blood samples between 0945 and 2400 h in intervals of 45 min, morning cortisol levels were obtained by sampling saliva on five ensuing measurements, beginning immediately after awakening. In addition, questionnaires aimed at measuring depressive mood, somatic symptoms, coping and chronic stress were filled out by the subjects. The patients with ongoing pain displayed significantly elevated IL-6 levels and an attenuated elevation of cortisol secretion after awakening compared to the two other groups. Patients with persisting pain were also suffering more frequently from depressive mood and ongoing work-related strains. In addition, maladaptive coping strategies were favoured by these patients. The presented data support the hypothesis that the persistence of pain in many of the concerned patients may significantly be related to dysfunctional reciprocal relations between neural, endocrine and immune function.

c-FOS-like immunoreactivity in rat brainstem neurons following noxious chemical stimulation of the nasal mucosa

It has previously been shown that noxious and non-noxious peripheral stimuli induce c-fos expression in spinal dorsal horn neurons. In the present study we have examined the expression of c-fos in brainstem neurons following noxious chemical stimulation of the respiratory region of the nasal mucosa. In urethane-anaesthetized rats we injected mustard oil or applied CO2 pulses to the right nasal cavity. In control animals we applied paraffin oil or a continuous flow of air. A further group of control animals was anaesthetized and not subjected to any experimental treatment. Two hours after the first stimulus the rats were perfused with 4% phosphate-buffered paraformaldehyde. Brainstem sections were incubated with primary antiserum against the FOS protein and processed according to the ABC method. Only the mustard oil-treated rats had obvious signs of rhinitis and displayed FOS-positive cells in laminae I and II of the subnucleus caudalis and in the subnucleus interpolaris of the trigeminal brainstem nuclear complex as well as in the medullary lateral reticular nucleus. These areas are known to be involved in the processing of nociceptive information. Although CO2 pulses applied to the nasal mucosa are known to evoke pain sensations in man we did not observe any FOS-positive neurons in trigeminal and reticular brainstem areas of CO2-treated rats. This lack of c-fos expression probably results from the fact that unlike mustard oil, CO2 did not induce any apparent inflammatory reactions. In all animals c-fos expression was found in the nucleus of the solitary tract and in the area postrema. Staining in these areas might partly result from factors related to anaesthesia, changed respiration parameters and stress. Since the mustard oil-treated rats displayed the highest levels of immunoreactivity in the nucleus of the solitary tract and in the area postrema, additional effects specifically related to nociceptive input are very likely.

Discharge patterns of afferent cutaneous nerve fibers from the rat's tail during prolonged noxious mechanical stimulation

SummaryFeedback controlled constant force stimuli of 4, 6 and 8 N intensities and of 120 s duration were applied to the receptive fields of cutaneous afferent fibers in the rats tail. Two types of nociceptive units showed sustained discharges during these stimuli: “polymodal” unmyelinated C-units (MH-C units, N = 18, c.v. 0.5–0.9 m/s) and high-threshold mechanoreceptive A-delta-units(HTM-units, N=10, c.v. 1.9–11.2 m/s). In addition two classes of sensitive low threshold mechanoreceptors, SA I (N=6) and SA II (N=5) units, responded to the prolonged mechanical stimuli. At the onset of a noxious pressure, 11 of the 18 polymodal nociceptors exhibited dynamic responses (lasting about 10 s) which were followed by slowly adapting tonic discharges that lasted for the duration of the stimuli. The remaining polymodal C-fiber units (8/18) did not show dynamic discharges at 4 and 6 N. Phasic and tonic discharges were positively correlated with stimulus strength. The HTM-units encoded stimulation intensity mainly by their dynamic discharges. The tonic discharges of these units displayed faster adaptation rates with stronger mechanical stimuli, i.e. encoding of stimulation intensity became progressively weaker during the tonic phase. The discharges of sensitive SA I and SA II units with A beta axons were not positively correlated with the strength of noxious pressure stimuli. Tonic discharge rates of SA I units were negatively correlated to stimulus strength, whereas SA II units usually stopped firing in the course of a stimulus and became reversibly irresponsive to mechanical stimulation. Possible afferent mechanisms underlying the induction of pain by sustained noxious mechanical stimulation are discussed.

Central projections of trigeminal primary afferents innervating the nasal mucosa: a horseradish peroxidase study in the rat.

The respiratory region of the nasal mucosa is innervated by the ethmoidal nerve. Chemical nociceptive stimulation of this area leads to upper airway reflexes that prevent access of noxious substances to the respiratory tract and the lungs. In the present study we examined the localization of the cell bodies of the respective primary afferent fibres within the trigeminal ganglion, as well as their central projections. In 25 rats a horseradish peroxidase-wheat germ agglutinin gel was applied to the right nasal cavity. The animals were killed after 48-72 h. For visualization of the tracer the tissue was processed according to the tetramethylbenzidine method. In the trigeminal ganglion almost all labelled cell bodies were localized in a medial band immediately caudal to the entrance of the ophthalmomaxillary branch. Transganglionic projections to the trigeminal brainstem nuclear complex were only localized in the superficial laminae of the subnucleus caudalis and in the subnucleus interpolaris, areas known to be involved in processing of nociceptive information. An additional labelled terminal field was observed in the interstitial subnucleus of the nucleus tractus solitarius, which is involved in respiratory control. These results are in favour of the hypothesis that the ethmoidal nerve in rat constitutes the afferent limb of protective upper airway reflexes since it transmits mainly nociceptive information.

Morphine, 5-HT2 and 5-HT3 receptor antagonists reduce c-fos expression in the trigeminal nuclear complex following noxious chemical stimulation of the rat nasal mucosa.

Noxious chemical stimulation of the rat nasal mucosa induces the expression of the immediate early gene c-fos in trigeminal brainstem neurons. In the present study, we applied the irritant mustard oil (1%) into the left nostril of urethane anesthetized rats. Immunohistochemical methods were used to evaluate the expression of Fos protein in the trigeminal subnuclei interpolaris and caudalis and to test the effects of putative analgesics that might depress synaptic transmission in neurons related to nociception. For this purpose, morphine (3 mg/kg and 10 mg/kg), the 5-HT2 antagonist ketanserin (0.5 mg/kg and 5 mg/kg) and the 5-HT3 antagonist ICS 205-930 (0.1 mg/kg and 1 mg/kg) were administered intravenously prior to noxious stimulation. Pretreatment with any of the three compounds reduced Fos-like immunoreactivity. The effect of morphine was reversible with naloxone. The reduction of the expression of Fos-like immunoreactivity by exogenous morphine speaks in favour of an opioidergic link in the modulation of orofacial pain in the trigeminal nuclei. The effects of the 5-HT receptor antagonists are most likely mediated via 5-HT2 and 5-HT3 receptors located on primary afferent fibres.

Differential physiological effects during tonic painful hand immersion tests using hot and ice water

The cold pressor test (CPT) is an empirically validated test commonly used in research on stress, pain and cardiovascular reactivity. Surprisingly, the equivalent test with water heated to noxious temperatures (hot water immersion test, HIT) has not been thoroughly investigated. The aim of the present study was to characterize the physiological effects and psychophysics of both tests and to analyze whether the autonomic responses are mainly induced by baroreflexes or a consequence of the pain experience itself. The study consisted of a single session including one CPT (4 ± 0.2 °C) and one HIT (47 ± 0.5 °C; cut‐off point 5 min) trial performed on 30 healthy drug free volunteers aged 19–57 (median 24) yrs. The sequence of both trials was alternated and participants were randomly assigned to sequence order and parallelized with respect to gender. Physiological parameters (cardiovascular, respiratory and electrodermal activity) and subjective pain intensity were continuously monitored. In addition, pain detection and tolerance thresholds as well as pain unpleasantness were assessed. Both tests were comparable with regard to the time course and intensity of subjective pain. However, a significantly higher increase of blood pressure could be observed during the CPT when compared to the HIT. The HIT appears less confounded with thermoregulatory baroreflex activity and therefore seems to be a more appropriate model for tonic pain.