Characterizing pinprick evoked brain potentials before and after ...

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Sep 2, 2015 - ERPs and pinprick-evoked pain ratings were recorded in 19 healthy ...... 411 stimulation (512 mN), the P240 is almost absent but the P500 is ...
Articles in PresS. J Neurophysiol (September 2, 2015). doi:10.1152/jn.00444.2015

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Characterizing pinprick evoked brain potentials before and after experimentally-induced secondary hyperalgesia

Emanuel N. van den Broeke 1*, André Mouraux 1, Antonia H. Groneberg 2, Doreen B. Pfau 2, Rolf-Detlef Treede 2 and Thomas Klein 2 Affiliations: 1 2

Institute of Neuroscience, Université catholique de Louvain, B-1200, Brussels, Belgium Division of Neurophysiology, Center for Biomedicine and Medical Technology Mannheim (CBTM), Heidelberg University, 68167, Mannheim, Germany.

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*Corresponding author: Emanuel N van den Broeke Institute of Neuroscience (IoNS) Faculty of Medicine Université catholique de Louvain B-1200, Brussels Belgium E-mail: [email protected]

Keywords: capsaicin, secondary hyperalgesia, central sensitization, pinprick, evoked potentials.

1 Copyright © 2015 by the American Physiological Society.

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ABSTRACT Secondary hyperalgesia is believed to be a key feature of “central sensitization” and is characterized by enhanced pain to mechanical nociceptive stimuli. Using EEG, the aim of the present study was to characterize the effects of pinprick stimulation intensity on the magnitude of pinprick elicited brain potentials (event-related potentials; ERPs), before and after secondary hyperalgesia induced by intradermal capsaicin in humans. Pinprick elicited ERPs and pinprick-evoked pain ratings were recorded in 19 healthy volunteers, using mechanical pinprick stimuli of varying intensities (0.25 mm probe applied with a force extending between 16 and 512 mN). The recordings were performed before (T0) and 30 minutes after (T1) intradermal capsaicin injection. The contralateral non-injected arm served as control. ERPs elicited by stimulation of untreated skin were characterized by (1) an earlylatency negative-positive complex peaking between 120 and 250 ms after stimulus onset (N120-P240) and maximal at the vertex, and (2) a long-lasting positive wave peaking 400-600 ms after stimulus onset and maximal more posterior (P500) which was correlated to perceived pinprick pain. After capsaicin injection, pinprick stimuli were perceived as more intense in the area of secondary hyperalgesia and this effect was stronger for lower compared to higher stimulus intensities. In addition, there was an enhancement of the P500 elicited by stimuli of intermediate intensity, which was significant for 64 mN. The other components of the ERPs were unaffected by capsaicin. Our results suggest that the increase in P500 magnitude after capsaicin is mediated by facilitated mechanical nociceptive pathways.

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INTRODUCTION

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Cutaneous tissue injury is often associated with the development of increased pain

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sensitivity in the area of actual tissue injury (referred to as primary hyperalgesia) and in the

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surrounding uninjured skin (referred to as secondary hyperalgesia).

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A hallmark of secondary hyperalgesia is enhanced pain to mechanical nociceptive stimuli

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(e.g. pinprick stimuli; Raja et al. 1984; Ali et al. 1996; Magerl et al. 1998). It can be induced

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experimentally by activating nociceptors in a sustained and intense fashion, for example,

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using intradermal injection or topical application of capsaicin, a substance that selectively

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activates primary nociceptive afferents via the TRPV1 receptor (LaMotte et al. 1991; Magerl 2

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et al. 1998; Magerl et al. 2001; Ziegler et al. 1999). Some studies suggest that secondary

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pinprick hyperalgesia is primarily mediated by capsaicin-insensitive A-fibers, which include

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high-threshold mechanoreceptors (HTM) and Type I A-fiber mechano-heat nociceptors

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(AMH-I) (Magerl et al. 2001; Ziegler et al. 1999), and results from “central sensitization”

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(Baumann et al. 1991; LaMotte et al. 1991; Latremoliere and Woolf 2009; Simone et al.

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1991), which is defined by the International Association for the Study of Pain (IASP) as

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“increased responsiveness of nociceptive neurons in the central nervous system to their

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normal or subthreshold afferent input” (Loeser and Treede 2008).

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Experimentally induced secondary pinprick hyperalgesia seems very similar to the observed

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hyperalgesia in patients with pain following lesions of the peripheral and central nervous

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system (Jensen and Finnerup 2014). Until now the measurement of pinprick hyperalgesia

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relies predominantly on the subjective report of pain intensity. It would thus be of great

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value to have a non-invasive clinical and research laboratory tool that is able to reliably

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assess sensitization of mechanical nociceptive pathways. One of these tools could be the

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recording of pinprick event-related brain potentials (ERPs).

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Recently, Iannetti et al. (2013) recorded for the first time ERPs elicited by mechanical

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pinprick stimulation before and after intradermal injection of capsaicin in the area of

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secondary hyperalgesia, in healthy volunteers. The mechanical pinprick stimulation was

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performed by quickly applying and removing a sharp-tipped probe (0.25 mm diameter). The

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probe could slide freely inside a tube, handheld by the experimenter. A calibrated weight on

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top of the load was used such that the normal force was 128 mN when the needle is

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maintained against the skin. The stimulus elicited a biphasic ERP waveform (N120 and P240

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waves) with latencies compatible with the conduction of myelinated Aβ or Aδ fiber 3

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afferents. After capsaicin injection, they observed an enhancement of both the pain ratings

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and the magnitude of the pinprick-evoked N120 wave.

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However, only one force was used in this seminal study, leaving unexplored the possibility

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that pinprick evoked potentials are modulated differently depending on the applied force,

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both in untreated skin and in the area of secondary hyperalgesia. Therefore, the aim of the

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present study was to characterize the ERPs elicited by different intensities of pinprick

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stimulation, before and after inducing secondary hyperalgesia. Different weights were used

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to deliver a range of stimulation intensities, extending between 16 and 512 mN. Secondary

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hyperalgesia was induced by intradermal injection of capsaicin.

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2. METHODS

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2.1 Participants

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Nineteen healthy volunteers took part in the experiment (9 men and 10 women; aged 19 –

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29 years; mean ± SD age: 23.6 ± 2.3 years, 18 right handed, 1 left handed). Approval for the

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experiment was obtained from the local Ethical Committee. All participants signed an

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informed consent form.

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2.2 Experimental design

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Participants were comfortably seated in a reclining chair with their arms resting on armrests.

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Two concentric circles were drawn on the left and right ventral forearm, with a diameter of

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1.5 cm and 2.5 cm, respectively.

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2.3 Intradermal injection of capsaicin

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A 10 mM solution of capsaicin (40 µg capsaicin dissolved in 12.5 µl of 16% Tween80 in

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normal saline; Magerl et al. 2001) was injected in the dermis, at the center of the circles

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drawn on the ventral forearm of the non-dominant arm (Fig. 1). Immediately after the

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injection, participants were asked to rate the magnitude of the capsaicin-induced pain every

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10 sec. for the first minute and then every 30 sec. until 15 minutes post injection, using a

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numeric rating scale (NRS) ranging from 0 (no pain) to 100 (most intense pain imaginable). A

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placebo saline injection was not included in this study, as the aim was not to ascertain

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whether capsaicin induces hyperalgesia, but rather to examine the relationship between this

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well-known phenomenon and pinprick ERPs.

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2.4 Pinprick stimulation

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Pinprick elicited ERPs were recorded before (T0) and 30 minutes after (T1) capsaicin

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injection, using six different intensities of mechanical pinprick stimulation: 16, 32, 64, 128,

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256 and 512 mN. The pinprick stimuli were delivered to the capsaicin-treated and control

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arm, within the 1-cm wide area defined by the two circles drawn on the forearms at a 5

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distance ranging between 1.5 and 2.5 cm from the site of capsaicin injection on the treated

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arm (Fig. 1).

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For each intensity of pinprick stimulation and for each arm, a total of twenty stimuli were

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administered in separate blocks. Stimulation of the control and capsaicin-treated arm was

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performed in an alternating fashion (block-by-block design), but the capsaicin-treated arm

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was always stimulated first. The order of the different stimulation intensities was balanced

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across participants. The stimuli were delivered using a random inter-stimulus interval

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ranging from 7 to 10 sec. To avoid sensitization of the stimulated skin, the target of the

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pinprick stimulus was displaced by the experimenter after each stimulus.

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The pinprick stimuli were generated by a set of six probes consisting of a cylindrical stainless-

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steel 0.25 mm diameter flat-tip (uniform tip geometry) mounted on a plastic rod (MRC

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Systems GmbH, Heidelberg, Germany). The plastic rod moved freely inside a polished hand-

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held stainless steel tube. Each pinprick stimulus was delivered by applying the needle in

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vertical direction onto the skin and moving the tube downwards and upwards with a total

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duration of approximately one second. The normal force applied by the needle against the

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skin (16, 32, 64, 128, 256 and 512 mN) was adjusted by placing inside the tube and on top of

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the plastic rod, one of a set of six calibrated cylindrical weights.

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Instead of applying the pinprick stimulus as fast as possible such as in the study of Iannetti et

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al. (2013), the pinprick stimulus was applied and removed slowly, and maintained against

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the skin for at least one second. The reason for doing so is that the actual force that is

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applied onto the skin is dependent on the velocity of which the stimulus is delivered. To be

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sure that the actual delivered force is the same as the planned one, the pinprick stimulus 6

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should

be

applied

onto

the

skin

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systems.de/englisch/products/pinprick.html).

in

a

slowly

fashion

(www.mrc-

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2.5 Intensity of perception

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The effect of capsaicin on the intensity of perception elicited by pinprick stimulation was

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assessed by asking participants to rate the intensity of “pinprick pain” on a NRS ranging from

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0 (no pain) to 100 (most intense pain imaginable). Participants were instructed to distinguish

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the perception of pain (i.e. sharp or slightly pricking or burning sensation) from touch or

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pressure by rating the stimulus > 0 (Rolke et al. 2006). Subjects were free to use integers as

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well as fractions. Ratings of the intensity of perception were obtained 3 s after each

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individual pinprick stimulus.

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2.6 Electro-encephalography (EEG)

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The EEG was recorded using five Ag-AgCl electrodes (Easycap, Brain Products GmbH,

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Germany) mounted in an elastic electrode-cap and located at Fz, Cz, Pz, T3 and T4 according

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to the International 10/10 system. Linked earlobes (A1A2) served as reference. Eye blinks

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were recorded using a pair of surface electrodes placed at the upper and lower sides of the

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right eye. Impedance was kept below 5kΩ for all leads. The signals were amplified and

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digitized at a 200 Hz sampling rate using a BrainAmp system (Brain Products GmbH,

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Germany). During the recording, participants were instructed to keep their gaze fixed on a

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reference point at a distance of approximately 1 m at eye level, and to sit as still as possible

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without making any movements.

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The experiment was carried out with a modified set of pinprick probes (MRC Systems GmbH,

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Heidelberg, Germany). The stimulators were equipped with an electrically conductive

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connection from the needle tips via the internal sliding body to an external cable. As soon as

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the probe came in contact with the skin, the electrical circuit detected the change in circuit

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impedance and generated a trigger signal.

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2.7 Data analysis

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2.7.1 Intensity of perception

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Intensity ratings of capsaicin- and pinprick-evoked pain were transformed into decadic

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logarithmic values to obtain a secondary normal distribution. A small constant of 0.1 was

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added to pain ratings to avoid a loss of zero values (Magerl et al. 1998).

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To characterize the effect of capsaicin on the intensity of the percept elicited by pinprick

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stimulation, a General Linear Model (GLM) repeated measures ANOVA analysis (Statistica

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4.5) was performed on log-transformed pain ratings using three within-subject factors: time

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(two levels: T0, T1), treatment (two levels: control arm, capsaicin arm) and stimulation

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intensity (six levels: 16, 32, 64, 128, 256, 512 mN). The level of significance was set at p