Repeated local administration of noradrenaline or saline inhibits thermal hyperalgesia in pain-sensitized human skin

The role of the endogenous neurotransmitters associated with neuropathic pain and in the opioid crisis: The innate pain-relieving system

Neuropathic pain is a chronic pain caused by central and peripheral nerve injury, long-term diabetes or treatment with chemotherapy drugs, and it is dissimilar to other chronic pain conditions. Chronic pain usually seriously affects the quality of life, and its drug treatment may result in increased costs of social and medical care. As in the USA and Canada, in Europe, the demand for pain-relieving medicines used in chronic pain has also significantly increased, but most European countries are not experiencing an opioid crisis. In this review, the role of various endogenous transmitters (noradrenaline, dopamine, serotonin, met- and leu-enkephalins, β-endorphin, dynorphins, cannabinoids, ATP) and various receptors (α₂, μ, etc.) in the innate pain-relieving system will be discussed. Furthermore, the modulation of pain processing pathways by transmitters, focusing on neuropathic pain and the role of the sympathetic nervous system in the side effects of excessive opioid treatment, will be explained.

Repeated cycles of electrical stimulation decrease vasoconstriction and axon-reflex vasodilation to noradrenaline in the human forearm

AIM

To investigate whether desensitization to the vasomotor effects of noradrenaline is a specific effect of electrical stimulation.

METHODS

Three sites on the forearm of 10 healthy volunteers were stimulated with 0.2 mA direct current for 2 min twice daily for 10 days. Noradrenaline and histamine were then displaced from ring-shaped iontophoresis chambers into two of the pretreated sites and two untreated sites on the contralateral forearm. Axon-reflex vasodilation was measured from the centre of the ring described by the iontophoresis chamber with a laser Doppler flowmeter. One or two days later, noradrenaline and vasopressin were introduced into pretreated and untreated sites by iontophoresis, and vasoconstriction at sites of administration was measured in the heated forearm.

RESULTS

The pretreatment blocked vasoconstriction to noradrenaline [median increase in flow 1%, interquartile range (IR) -41 to 52%; median decrease at the untreated site 53%, IR. -70 to -10%; P < 0.05], but did not block vasoconstriction to vasopressin (median decrease 42% at the untreated site and 45% at the pretreated site). Axon-reflex vasodilation to noradrenaline was diminished at the pretreated site (median increase in flow 33%, IR 2-321%; untreated site 247%, IR 31-1087%; P < 0.05). However, axon-reflex vasodilation to histamine did not differ significantly between the pretreated site (median increase 1085%) and the untreated site (median increase 1345%).

CONCLUSIONS

The conditioning pretreatment appears to evoke a specific decrease in responsiveness to noradrenaline. Repeated cycles of electrical stimulation may downregulate neural and vascular responses to noradrenaline by repetitively activating cutaneous sympathetic nerve fibres.

Cell therapy of pain: Characterization of human fetal chromaffin cells at early adrenal medulla development

Adult adrenal chromaffin cells are being utilized for therapeutic transplantation. With the prospect of using fetal chromaffin cells in pain therapy, we studied their phenotype, proliferative power, function, and growth in vitro and in situ in order to determine the optimal time for implantation. Between 7 and 10 gestational weeks (GW), we isolated, in vitro, two types of chromaffin cells with a noradrenergic phenotype akin to that observed, in situ. Among the adherent chromaffin cells first observed in vitro, only a few samples expressed met-enkephalin, whereas almost all the neurosphere-like colonies, which appeared later, expressed it. However, neither of the two types of populations expressed an adrenergic phenotype in line with that observed in situ. At the upper limits of the voluntary abortion period authorized in France, this phenotype (12 GW) and met-enkephalin expression (13 GW) were evidenced in situ. For the first time in man, we demonstrate the secretion of noradrenaline in vitro by the two populations of cells. Consistent with this result, we also noted dopamine beta hydroxylase (DbetaH) mRNA expression in vitro and in situ within this period. These observations on the expression of these biological factors indicate that 9-10 GW would be the best stage for sampling these cells for preclinical transplantation experiments.

Personality traits associated with perception of noxious stimuli in women with vulvar vestibulitis syndrome

Vulvar vestibulitis syndrome (VVS) is associated with enhanced pain sensitivity. The present study explores the role of personality on the perception of noxious stimuli among women with VVS. More specifically, the aim of the study was to explore whether the personality traits assessed by Cloninger's Tridimensional Personality Questionnaire (TPQ) (harm avoidance [HA], novelty seeking [NS], and reward dependence [RD]) are associated with the augmented pain perception in women with VVS. Quantitative sensory tests were applied to the forearm of 98 women with VVS and 135 control subjects, all of whom completed the TPQ. The women with VVS scored higher than the control subjects on HA and RD with no significant differences in NS. Linear regression analyses revealed that in the VVS group, lower pain thresholds and higher magnitude estimations of suprathreshold pain stimuli were associated with higher HA and RD scores. The enhanced pain perception among women with VVS might reflect their tendency to respond intensely to signals of reward and to elevate the perceived risk. This might lead them to avoid hazards by overestimating the level of potential harm, as represented by greater pain sensitivity. The association between personality traits assessed by Cloninger's Tridimensional Personality Questionnaire, ie, harm avoidance, novelty seeking, and reward dependence, and the enhanced perception of noxious stimuli in vulvar vestibulitis syndrome might suggest neurochemical mechanisms of pain experience affected by personality, with possible application for future treatment approaches toward pain disorders.

The Time Course of Photoadaptation and Pigmentation Studied Using a Novel Method to Distinguish Pigmentation from Erythema

The dynamics of human pigmentation in response to ultraviolet radiation (UVR) remain poorly characterized. In part, this is attributable to methodological issues relating to the overlap in spectra of hemoglobin and melanin. We describe a new method, based on the recording of reflectance properties following iontophoresis of a potent vasoconstrictor, noradrenaline. This removes the influence of blood, allowing measurement of pigmentation, represented as L* on the L*a*b* scale. Blood flow was separately assessed using laser Doppler flowmetry. We show that there is a clear dose response with the dose of UVR administered, that pigmentation peaks at 1 wk and declines over the following 10 wk, but does not return to baseline within this period. We show clear differences in the degree, but not the temporal pattern of pigmentation between different pigmentary groups. We also report that the relation between facultative pigment and constitutive pigment is incomplete, with a wide scatter of responses for the development of pigmentation irrespective of constitutive levels. For comparison we also document overall photoadaptation and relate changes in pigmentation to the overall changes in photoadaptation.

No alpha-adrenoreceptor-induced C-fiber activation in healthy human skin

In healthy volunteers, flare responses induced by norepinephrine (NE) iontophoresis have been observed. However, as NE iontophoresis is a combined electrical and chemical stimulus axon, reflexes cannot be directly linked to pharmocological activity of NE. Different concentrations of NE, clonidine (CL), and phenylephrine (PE) (NE: 10(-10)-10(-3) M; CL and PE: 10(-8)-10(-3) M) were applied via intradermal microdialysis fibers into the skin of healthy volunteers. Simultaneously, skin blood flow was visualized by laser-Doppler imaging scans and quantified in a vasoconstriction skin area directly above the membranes to control drug effects and in expected axon reflex vasodilation areas that were 0.75 cm apart. NE, PE, and CL caused dose-dependent vasoconstriction. However, neither in the presumed axon reflex areas (quantitative analysis) nor on laser-Doppler imaging pictures (qualitative analysis) were any vasodilation observed. Even at concentrations causing maximum vasoconstriction (10(-3) M for any drug), no vasodilation was induced. Our results indicate that, in healthy human skin, exogenously supplied alpha-adrenoreceptor agonists alone do not activate nociceptors sufficiently to induce axon reflex flare.

Break excitation alone does not explain the delay and amplitude of anodal current-induced vasodilatation in human skin

In iontophoresis experiments, a 'non-specific' current-induced vasodilatation interferes with the effects of the diffused drugs. This current-induced vasodilatation is assumed to rely on an axon reflex due to excitation of cutaneous nociceptors and is weaker and delayed at the anode as compared to the cathode. We analysed whether these anodal specificities could result from a break excitation of nociceptors. Break excitation is the generation of action potentials at the end of a square anodal DC current application, which are generally weaker than those observed at the onset of a same application at the cathode. In eight healthy volunteers, we studied forearm cutaneous laser Doppler flow (LDF) responses to: (1) anodal and cathodal 100 microA current applications of 1, 2, 3, 4 or 5 min; (2) 100 microA anodal applications of 3 min with a progressive ending over 100 s (total charge 23 mC); these were compared to square-ended 100 microA anodal applications of the same total charge (23 mC) or duration (3 min); (3) a 4 min 100 microA anodal application with a 333 msec break at half time. Results (mean +/- S.D.) are expressed as percentage of heat-induced maximal vasodilatation (%MVD). Onset (T(vd)) and amplitude (LDF(peak)) of vasodilatation were determined. We observed that: T(vd) was linearly related to the duration of current application at the anode (slope = 1.01, r(2) = 0.99, P < 0.0001) but not at the cathode (slope = 0.03, r(2) = 0.02, n.s.). Progressive ending of anodal current did not decrease LDF(peak) (63.3 +/- 24.6 %MVD) as compared to square-ending of current application of the same duration (36.9 +/- 22.2 %MVD) or the same total charge (57.1 +/- 23.5 %MVD). A transient break of anodal current did not allow for the vasodilatation to develop until current was permanently stopped. We conclude that, during iontophoresis, anodal break excitation alone cannot account for the delay and amplitude of the vascular response.