Wolfgang Greffrath

Peripheral and central components of habituation of heat pain perception and evoked potentials in humans

Abstract For the neurophysiological examination of nociceptive pathways, contact‐heat evoked potentials (contact‐heat EPs) are elicited by repetitive brief noxious heat stimuli. Suppression of heat responses in primary nociceptive neurons during repetitive stimulation has been shown in animal models in vivo and in vitro. We now investigated whether heat pain and contact‐heat EPs in humans display equivalent signs of habituation. Heat pain and EPs were elicited in 16 volunteers with a contact thermode (30 °C s−1). Heat pulses at three intensities (pain threshold, moderate noxious and maximum available) were applied to the right forearm either by moving the thermode after each pulse to variable locations or when fixed to one location (inter‐stimulus intervals 8–10 s). Contact‐heat EPs consisted of an early negativity in temporal leads (N1), followed by a biphasic response at the vertex (N2‐P2). Pain ratings and contact‐heat EPs (N1 and N2‐P2 components) displayed significant temperature dependence. N2‐P2 correlated positively with ratings. With stimulation at variable locations, both measures slowly decreased with time constants τ of 2 min (ratings) and 12 min (EPs). With stimulation at a fixed location, habituation was much faster for both, ratings (τ = 10 s) and EPs (τ = 33 s). As a consequence, both measures were significantly reduced (p < 0.005) leading to a rightward shift of the stimulus–response function by 5 °C. In conclusion, human heat pain perception and contact‐heat EPs display signs of rapid habituation when stimulation is restricted to a fixed location and thus, reflect fatigue of peripheral nociceptive neurons. Habituation within the central nervous system is slower and less pronounced.

Differential nociceptive deficits in patients with borderline personality disorder and self-injurious behavior: laser-evoked potentials, spatial discrimination of noxious stimuli, and pain ratings

&NA; Approximately 70–80% of women meeting criteria for borderline personality disorder (BPD) report attenuated pain perception or analgesia during non‐suicidal, intentional self‐mutilation. The aim of this study was to use laser‐evoked potentials (LEPs) and psychophysical methods to differentiate the factors that may underlie this analgesic state. Ten unmedicated female patients with BPD (according to DSM‐IV) and 14 healthy female control subjects were investigated using brief radiant heat pulses generated by a thulium laser and five‐channel LEP recording. Heat pulses were applied as part of a spatial discrimination task (two levels of difficulty) and during a mental arithmetic task. BPD patients had significantly higher heat pain thresholds (23%) and lower pain ratings (67%) than control subjects. Nevertheless, LEP amplitudes were either normal (N1, P2, P3) or moderately enhanced in BPD patients (N2). LEP latencies and task performance did not differ between patients and control subjects. The P3 amplitudes, the vertex potential (N2–P2), and the N1, which is generated near the secondary somatosensory cortex, were significantly reduced during distraction by mental arithmetic in both groups. In addition, P3 amplitudes reflected task difficulty. This study confirms previous findings of attenuated pain perception in BPD. Normal nociceptive discrimination task performance, normal LEPs, and normal P3 potentials indicate that this attenuation is neither related to a general impairment of the sensory‐discriminative component of pain, nor to hyperactive descending inhibition, nor to attention deficits. These findings suggest that hypoalgesia in BPD may primarily be due to altered intracortical processing similar to certain meditative states.

Components of after-hyperpolarization in magnocellular neurones of the rat supraoptic nucleus in vitro

1 The pharmacological sensitivity of hyperpolarizing components of spike train after‐potentials was examined in sixty‐one magnocellular neurones of the rat supraoptic nucleus using intracellular recording techniques in a brain slice preparation. 2 In 26 % of all neurones a slow after‐hyperpolarization (AHP) was observed in addition to a fast AHP. In 31 % of all neurones a depolarizing after‐potential (DAP) was observed. 3 The fast AHP was blocked by apamin whereas the slow AHP was blocked by charybdotoxin (ChTX). The DAP was enhanced by ChTX or a DAP was unmasked if not present during the control period. 4 Low concentrations of TEA (0.15–1.5 mm) induced effects on the slow AHP and the DAP essentially resembling those of ChTX. The same was true for the effects of CoCl2 (1 mm). 5 Spike train after‐potentials were not affected by either iberiotoxin (IbTX), a selective high‐conductance potassium (BK) channel antagonist, or margatoxin (MgTX), a Kv1.3 α‐subunit antagonist. 6 Kv1.3 α‐subunit immunohistochemistry revealed that these units are not expressed in the somato‐dendritic region of supraoptic neurones. 7 The effects of ChTX, IbTX, MgTX, TEA, CoCl2 and CdCl2 on spike train after‐potentials are interpreted in terms of an induction of the slow AHP by the activation of calcium‐dependent potassium channels of intermediate single channel conductance (IK channels). 8 The results suggest that at least the majority of supraoptic magnocellular neurones share the capability of generating both a slow AHP and a DAP. The slow AHP may act to control the expression of the DAP, thus regulating the excitability of magnocellular neurones. The interaction of the slow AHP and the DAP may be important for the control of phasic discharge.

A cross-sectional investigation of discontinuation of self-injury and normalizing pain perception in patients with borderline personality disorder

Objective:  Several studies have shown reduced pain perception in patients with borderline personality disorder (BPD) and current self‐injurious behavior (SIB). The aim of the present study was to test whether pain perception in patients with current SIB is different from that of patients who had stopped SIB, and whether pain perception of the latter group differs from healthy controls (HC).

Co-expression of the voltage-gated potassium channel Kv1.4 with transient receptor potential channels (TRPV1 and TRPV2) and the cannabinoid receptor CB1 in rat dorsal root ganglion neurons.

Potassium channels contribute to basic neuronal excitability and modulation. Here, we examined expression patterns of the voltage-gated potassium channel Kv1.4, the nociceptive transduction channels TRPV1 and TRPV2 as well as the putative anti-nociceptive cannabinoid receptor CB1 by immunofluorescence double-labelings in sections of rat dorsal root ganglia (DRGs). Kv1.4, TRPV1 and CB1 were each detected in about one third of neurons (35.7+/-0.5%, 29.4+/-1.1% and 36.4+/-0.5%, respectively, mean diameter 19.1+/-0.3 microm). TRPV2 was present in 4.4+/-0.4% of all neurons that were significantly larger in diameter (27.4+/-0.7 microm; P < 0.001). Antibody double-labeling revealed that the majority of Kv1.4-positive neurons co-expressed TRPV1 (73.9+/-1.5%) whereas none expressed TRPV2. The largest overlap was found with CB1 (93.1+/-0.1%). CB1 expression resembled that seen for Kv1.4 since the majority of neurons expressing CB1-protein also expressed TRPV1 (69.4+/-6.5%) but not TRPV2 (0.6+/-0.3%). When CB1-mRNA was detected using in situ hybridizations an additional subset of larger neurons was labeled including 82.4+/-17.7% of the TRPV2 expressing neurons. However, co-localization of Kv1.4 with CB1-mRNA (92%, mean diameter: 18.5 microm) was essentially the same as with CB1-protein. The almost complete overlap of CB1 and Kv1.4 in nociceptive DRG neurons suggests a functional synergistic action between Kv1.4 and CB1. The potassium channel may have two important roles in nociception. As the molecular basis of A-type current it could be involved in the control of repetitive discharges at peripheral terminals and as a downstream signal transduction site of CB1 in the control of presynaptic transmitter release at central terminals.

Inward currents in primary nociceptive neurons of the rat and pain sensations in humans elicited by infrared diode laser pulses.

&NA; Radiant heat is often used to study nociception in vivo. We now used infrared radiation generated by a diode laser stimulator (wavelength 980 nm) to investigate transduction mechanisms for noxious heat stimuli in acutely dissociated dorsal root ganglion (DRG) neurons of rats in vitro. The laser stimulator offered the unique opportunity to test whether the same stimuli also elicit pain sensations in humans. A specific heat‐induced current (Iheat) was elicited in six of 13 small DRG neurons (diameter ≤30 &mgr;m) tested in the whole‐cell configuration of the patch–clamp mode. Current responses in the seven heat‐insensitive neurons were within the range explainable by the temperature dependence of the recording setup. Iheat was characterized by: (1) non‐linearity of its amplitude during a suprathreshold heat ramp as well as with stimuli of increasing intensity with an estimated threshold of 42±1°C; (2) fast rise time and even faster decay time (t1/2=96.5±5.9 and 27.7±1.5 ms, respectively); and (3) rate dependence of its induction. All three heat‐sensitive neurons tested were also sensitive to capsaicin. The mean threshold for the induction of Iheat was 2.8±0.3 J mm−2. The threshold for the induction of action potentials by depolarizing current pulses was significantly reduced after laser stimulation, suggesting a sensitization at the transformation stage. No such change was seen in heat‐insensitive neurons that underwent the same heat stimuli. The same diode laser elicited pain sensations and laser‐evoked potentials in human subjects. No significant differences were seen between the pain thresholds in hairy and in glabrous skin, probably due to the deep penetration of this laser radiation. The mean pain threshold for stimuli ≥200 ms in humans was 2.5±0.2 J mm−2 (n=11), and did not differ from the thresholds for the induction of Iheat in vitro. Our results indicate that Iheat in primary sensory neurons can be activated by infrared laser pulses that are painful in humans.

Differentiation of pain ratings in combat-related posttraumatic stress disorder.

ABSTRACT Although posttraumatic stress disorder (PTSD) is associated with chronic pain, preliminary evidence suggests reduced experimental pain sensitivity in this disorder. The questions addressed in the present study were whether pain perception would also be reduced in PTSD patients who are not suffering from chronic pain symptoms, and whether a reduction in pain sensitivity would also be present in combat veterans who did not develop PTSD. For this, we determined thermal detection and pain thresholds in 10 male combat‐related PTSD patients, 10 combat control subjects (no PTSD) and 10 healthy controls without combat experience. All subjects were pain free. First, we measured thermal sensory thresholds with ramped heat and cold stimuli using the method of limits. Ramped thermal sensory stimulation revealed no deficits for the detection of (non‐noxious) f2.1thermal stimuli between groups. In contrast, heat and cold pain thresholds in both combat groups (PTSD and combat controls) were significantly increased compared to healthy controls. However, these stimuli could not distinguish between the two groups due to ceiling effects. When using longer‐lasting heat stimulation at different temperatures (30 s duration; method of fixed stimuli), we found significantly lower frequency of pain reports in PTSD patients compared with both combat and healthy controls, as well as significantly lower pain ratings. Our results suggest an association of PTSD with reduced pain sensitivity, which could be related to PTSD‐related (neuro‐)psychological alterations or to a pre‐existing risk factor for the disorder.

Contact heat and cold, mechanical, electrical and chemical stimuli to elicit small fiber-evoked potentials: merits and limitations for basic science and clinical use.

Laser-evoked potentials are the most extensively validated method to objectively assess nociceptive pathway function in humans. Here, we review merits and shortcomings of alternative techniques using different principles of stimulus generation to stimulate Aδ- or C-fibers. Fast ramp contact heat stimuli yield reproducible responses; however, stimulus location needs to be changed to reduce peripheral habituation, and the limited steepness of temperature ramps may result in response jitter and absence of averaged responses even in some healthy subjects. Inverse temperature ramps can serve to evoke cool evoked potentials to specifically test the cold pathway; the clinical impact of such findings is promising but uncertain to date, and availability of devices optimized for this purpose is currently limited. Mechanical stimuli excite low- or high-threshold mechanoreceptors depending on both the probe surface and the applied force. Electrical stimuli can be used to excite nerve fibers directly in the epidermis, the mucosa of the gut, or the tooth pulp. Principle limitation of the applicability of mechanical and electrical stimuli is the inevitable co-excitation of tactile (Aβ-) fibers. The nasal mucosa can be stimulated using pulsed-CO(2) air streams, which excite chemo-nociceptors; although these stimuli are specific to excite thin trigeminal afferents, their use is limited as it is restricted to a relatively small region. Current data do not allow a comparative analysis on their respective diagnostic values. Quantification of analgesic efficacy in healthy subjects has been established and may be useful in phase I and IIa clinical trials.

Capsaicin differentially modulates voltage-activated calcium channel currents in dorsal root ganglion neurones of rats

It is discussed whether capsaicin, an agonist of the pain mediating TRPV1 receptor, decreases or increases voltage-activated calcium channel (VACC) currents (I(Ca(V))). I(Ca(V)) were isolated in cultured dorsal root ganglion (DRG) neurones of rats using the whole cell patch clamp method and Ba2+ as charge carrier. In large diameter neurones (>35 micorm), a concentration of 50 microM was needed to reduce I(Ca(V)) (activated by depolarizations to 0 mV) by 80%, while in small diameter neurones (< or =30 microm), the IC50 was 0.36 microM. This effect was concentration dependent with a threshold below 0.025 microM and maximal blockade (>80%) at 5 microM. The current-voltage relation was shifted to the hyperpolarized direction with an increase of the current between -40 and -10 mV and a decrease between 0 and +50 mV. Isolation of L-, N-, and T-type calcium channels resulted in differential effects when 0.1 microM capsaicin was applied. While T-type channel currents were equally reduced over the voltage range, L-type channel currents were additionally shifted to the hyperpolarized direction by 10 to 20 mV. N-type channel currents expressed either a shift (3 cells) or a reduction of the current (4 cells) or both (3 cells). Thus, capsaicin increases I(Ca(V)) at negative and decreases I(Ca(V)) at positive voltages by differentially affecting L-, N-, and T-type calcium channels. These effects of capsaicin on different VACCs in small DRG neurones, which most likely express the TRPV1 receptor, may represent another mechanism of action of the pungent substance capsaicin in addition to opening of TRPV1.

Inactivation and tachyphylaxis of heat-evoked inward currents in nociceptive primary sensory neurones of rats

1 Membrane currents evoked by repeated noxious heat stimuli (43–47 °C) of 3 s duration were investigated in acutely dissociated dorsal root ganglion (DRG) neurones of adult rats. The heat stimuli generated by a fast solution exchanger had a rise time of 114 ± 6 ms and a fall time of 146 ± 13 ms. 2 When heat stimuli were applied to heat‐sensitive small (≤ 32·5 μm) DRG neurones, an inward membrane current (Iheat) with a mean peak of 2430 ± 550 pA was observed (n= 19). This current started to activate and deactivate with no significant latency with respect to the heat stimulus. The peak of Iheat was reached with a rise time of 625 ± 115 ms. When the heat stimulus was switched off Iheat deactivated with a fall time of 263 ± 17 ms. 3 During constant heat stimulation Iheat decreased with time constants of 4–5 s (inactivation). At the end of a 3 s heat stimulus the peak current was reduced by 44 ± 5 % (n= 19). 4 Current‐voltage curves revealed outward rectifying properties of Iheat and a reversal potential of −6·3 ± 2·2 mV (n= 6). Inactivation was observed at all membrane potentials investigated (−80 to 60 mV); however, inactivation was more pronounced for inward currents (37 ± 5 %) than for outward currents (23 ± 6 %, P < 0·05). 5 When neurones were investigated with repeated heat stimuli (3 to 5 times) of the same temperature, the peak current relative to the first Iheat declined by 48 ± 6 % at the 3rd stimulus (n= 19) and by 54 ± 18 % at the 5th stimulus (n= 4; tachyphylaxis). 6 In the absence of extracellular Ca2+ (buffered with 10 mm EGTA) inactivation (by 53 ± 6 %) and tachyphylaxis (by 42 ± 7 % across three stimuli) were still observed (n= 8). The same was true when intracellular Ca2+ was buffered by 10 mm BAPTA (inactivation by 49 ± 4 %, tachyphylaxis by 52 ± 7 % across three stimuli; n= 13). Thus, inactivation and tachyphylaxis were mainly independent of intra‐ and extracellular Ca2+. 7 These results indicate that inactivation and tachyphylaxis of heat‐evoked inward currents can be observed in vitro, similar to adaptation and suppression of action potential discharges elicited by comparably fast heat stimuli in vivo. Whereas the voltage dependence of Iheat resembles that of capsaicin‐induced membrane currents (ICaps), the independence of inactivation and tachyphylaxis of Iheat from calcium is in clear contrast to ICaps. A similar difference in calcium dependence of inactivation has been reported between heat‐evoked and capsaicin‐induced currents through the cloned capsaicin receptor channel VR1. Thus, the properties of Iheat and of VR1 largely account for the adaptation and suppression of heat‐evoked nociceptor discharges.