M. Dusch

Predominant CB2 receptor expression in endothelial cells of glioblastoma in humans.

BACKGROUND AND OBJECTIVES The most abundant malignant brain tumor in human is glioblastoma and patients with this type of tumor have a poor prognosis with high mortality. Glioblastoma are characterized particularly by fast growth and a dependence on blood vessel formation for survival. Cannabinoids (CBs) inhibit tumor growth by inducing apoptosis of tumor cells and impairing tumor angiogenesis. The distribution of CB1 and CB2 receptors in glioblastoma and associated endothelial vessels is still unknown. METHODS Tissue samples were collected consecutively after neurosurgery of 19 patients suspected glioblastoma and examined immunohistochemically for CB1 and CB2 receptor expression. Vessel endothelial cells of the sections were immunocytochemically identified by using a primary antibody against PECAM-1. Double labelling was performed for CB receptors and endothelial cells of the vessels by DAPI staining. RESULTS In endothelia of control tissue, about 24% and 45% of the cells were positive for CB1 and CB2 receptors. In glioblastoma endothelial cells, CB1 and CB2 receptors were present in about 38% and 54% of the cells respectively. In comparison to CB1, an elevated CB2 receptor expression was identified in glioblastoma. CONCLUSIONS The abundant expression and distribution of CB2 receptors in glioblastoma and particularly endothelial cells of glioblastoma indicate that impaired tumor growth in presence of CB may be associated with CB2 activation. Selective CB2 agonists might become important targets attenuating vascular endothelial growth factor (VEGF) signalling and thereby diminishing neoangiogenesis and glioblastoma growth.

Nociceptor sensitization to mechanical and thermal stimuli in pig skin in vivo

Primary hyperalgesia to mechanical and thermal stimuli are major clinical symptoms of inflammatory pain and can be induced experimentally by ultraviolet‐B (UV‐B) irradiation in humans. We set‐up a pig model in order to have more options for pharmacological intervention on primary hyperalgesia.

Rapid flare development evoked by current frequency-dependent stimulation analyzed by full-field laser perfusion imaging.

We analyzed, with a new imaging technique, the rapid axon reflex flare responses in human skin upon transcutaneous delivery of electrical stimuli at 1, 5, 10 and 50 Hz in single bursts of five pulses each. Two-dimensional perfusion images covering an area of 8×8 cm2 were captured at 25 Hz and their averages saved at 0.5 Hz. The stimulation caused an axon reflex flare (maximum 3 cm2, 20 s after stimulation) that gradually resolved within 2 min. Maximum flare responses developed at 5 Hz, whereas pain ratings increased with stimulation frequency. The highest neuropeptide release at 5 Hz correlates to the discharge characteristics of mechanoinsensitive C-fibers, whereas the maximum pain intensity at 50 Hz may be attributed to the activation of A-delta fibers.

Skin innervation at different depths correlates with small fibre function but not with pain in neuropathic pain patients

Neuropathy can lead not only to impaired function but also to sensory sensitization. We aimed to link reduced skin nerve fibre density in different levels to layer‐specific functional impairment in neuropathic pain patients and tried to identify pain‐specific functional and structural markers.

Nerve growth factor‐evoked nociceptor sensitization in pig skin in vivo

Peripheral sensitization of skin nociceptors by nerve growth factor (NGF) was explored in pig skin in vivo. As an objective output measure, the area of axon‐reflex‐mediated erythema was assessed upon mechanical, thermal, chemical, and electrical stimuli delivered at 1, 3, and 7 days after i.d. injection of 1 μg NGF into the pigs back skin (n = 8). Pretreatment with NGF provoked a sensitization to mechanical (600 mN), thermal (10 sec 49°C) and chemical (15 μl, pH 3) stimuli that lasted for 7 days. No sensitization, however, was found in response to weak mechanical (100 mN), weak thermal (10 sec 45°C), or electrical stimuli. Irrespective of the skin pretreatment (NGF or PBS vehicle control), the area of electrically induced erythema decreased upon repetition (days 1–7) by 70% (P < 0.05). Sensitization of sensory endings by NGF upon mechanical, heat, and chemical stimuli suggests recruitment of sensory transducer molecules [e.g., TRPV1, acid‐sensing ion channels (ASICs)]. In contrast, the gradual decrease in electrically induced erythema over 7 days might be attributable to axonal desensitization and possibly activity‐dependent down‐regulation of sodium channels. Thus, long‐lasting sensitization processes of nociceptor endings or axonal sodium channel desensitization mechanisms can be explored in the pig as a translational experimental animal model.

The Effect of Intravenous Infusion of Adenosine on Electrically Evoked Hyperalgesia in a Healthy Volunteer Model of Central Sensitization

Human pain models invoking central sensitization, one of the key mechanisms of chronic pain, may be useful for characterizing new analgesics. A new model of electrical hyperalgesia can detect the efficacy of several analgesic mechanisms. Because IV adenosine can alleviate neuropathic pain, we investigated its effect on experimental sensitization. This was a double-blinded, randomized, two-period crossover study in 20 healthy volunteers. Current pulses (0.5 ms; 1 Hz) were applied intracutaneously to achieve pain rating of ~5 on a 0–10 numeric rating scale. Pain, areas of pinprick hyperalgesia, and tactile allodynia were assessed during the 2.5-h stimulation period. Adenosine (50 μg · kg−1 · min−1) and placebo were infused IV over 60 min. Additional testing was performed 24 h after each treatment. Adenosine reduced the area of pinprick hyperalgesia during the infusion compared with placebo; there was no significant effect on tactile allodynia or pain rating. The effect on hyperalgesia developed over 15 min and was significant (P ≤ 0.05) for the rest of the infusion period. There was no difference between treatments at 24 h. Thus, in accordance with reports on neuropathic pain, adenosine reduced central sensitization in the human model of electrical hyperalgesia. However, adenosine did not have the long-term effects seen in patients. The model can investigate mechanisms of drugs for the treatment of chronic pain.

Cross-over evaluation of electrically induced pain and hyperalgesia

Abstract Background Anewexperimental protocol of electrically induced pain and hyperalgesia was established to examine orally administered drugs. In a randomized, double-blind, placebo-controlled cross-over study this experimental protocol was used to assess the effects of paracetamol. Methods Twenty-four subjects were enrolled in this study. The magnitude of pain, axon reflex flare, and areas of pin-prick hyperalgesia and touch-evoked allodynia were assessed in two consecutive sessions; prior to, and 2 h after drug administration. This protocol was repeated after 1 week. Subjects were randomized to receive either paracetamol (2 g) or a placebo. Results In comparison to the placebo arm there were no significant effects of paracetamol on pain, hyperalgesia, allodynia, or axon reflex flare. Pain and flare responses were highly reproducible on the same day (r = 0.77 and r = 0.79, respectively), and after 1 week (r = 0.6 and r = 0.71, respectively). The correlation between areas of hyperalgesia and allodynia was, however, significantly improved when the protocol was repeated on the same day (r = 0.8 and r = 0.75), as opposed to after a week (r = 0.54 and r = 0.53). Discussion The electrical pain model is a well established method for the assessment of intravenously applied analgesics. In order to assess effects of orally applied drugs the model had to be modified: for the assessment of hyperalgesia and allodynia a protocol repeating the model within 1 day proved to have advantages over repetition after 1 week.

Mechano-sensitive nociceptors are required to detect heat pain thresholds and cowhage itch in human skin

Mechano‐sensitive and mechano‐insensitive C‐nociceptors in human skin differ in receptive field sizes and electrical excitation thresholds, but their distinct functional roles are yet unclear.

Discriminative sensory characteristics of the lateral femoral cutaneous nerve after mepivacaine-induced block

Abstract Background and objectives Unmyelinated C-fibres comprise the largest group of somatic afferents and have demonstrated a crucial role not only in the perception of high-threshold mechanically, thermally or chemically induced pain, but also in non-harmful low-threshold mechanical stimuli [1,2]. The objective of our study was to characterize differential sensitivity changes of C-fibre related subclasses of high-threshold and low-threshold polymodal nociceptors and low-threshold mechanoreceptors to the local anaesthetic (LA) mepivacaine during nerve block of the purely sensory lateral femoral cutaneous nerve (LFCN) in human. We assumed a diverse response of different classes of afferents to the two different concentrations of the LA mepivacaine (Scandicaine). Methods In a double-blind randomized experimental setting, an ultrasound-guided nerve block of the LFCN was performed in 10 healthy male subjects, each with two different concentrations of mepivacaine (0.5 and 1%). Responsiveness of afferent nerve fibres to different noxious and non-noxious stimuli was tested by Quantitative Sensory Testing (QST) 30, 180, and 300 min after nerve block. Both LA concentrations of mepivacaine were compared for time course of the areas of anaesthesia for the tested sensory modalities. Results Initial extension of anaesthetic areas at 30 min did not differ between both LA concentrations. At 180 min only the anaesthetic areas to nociceptive stimuli were reduced at the site of lower mepivacaine injection (260mN: 204mm2 (18; 244; median difference and 95% confidence interval; p < 0.05), heat: 276mm2 (3; 305)). In contrast, no significant differences were found between the two concentration when non-nociceptive stimuli were used (100mN: 187mm2 (4; 240), p >0.05, brush: 159mm2 (–59; 202)). Conclusion Equal initial sizes of anaesthesia areas for all sensory modalities can be explained by supramaximal perineural LA molecule concentration in both administered mepivacaine dosages. Upon washout of the LA nociceptive function is restored faster as compared to non-nociceptive sensation and higher concentration of the LA are required to maintain the analgesia. Quantitative sensory testing is able to detect different susceptibility of low threshold mechanosensors and subtypes of nociceptive C-fibres to mepivacaine. Using painful mechanical, heat and electrical stimulation different classes of nociceptors will be activated. The analgesic areas to electrical stimulation were particularly small; one might therefore hypothesize that the proposed protocol allows to also differentiate mechano-insensitive (“silent”) and mechanosensitive (“polymodal”) nociceptors. Implications QST is a non-invasive method to functionally examine sensory modalities and their pharmacological modulation in humans. The method is sufficiently sensitive to differentiate the analgesic properties of mepivacaine at 0.5 and 1% and might also be adequate to different classes of nociceptors. Further development of nociceptive stimuli including supra-threshold encoding characteristics will enable to investigate peripheral analgesic effects more specifically and thus might help to design new analgesics with preferential effect on high frequency discharge of nociceptors.

Laser‐evoked potentials mediated by mechano‐insensitive nociceptors in human skin

Laser‐evoked potentials (LEP) were assessed after peripheral nerve block of the lateral femoral cutaneous nerve (LFCN) in healthy volunteers from partially anesthetized skin areas to differentially stimulate mechano‐insensitive nociceptors.