Holger Holthusen

Nitric oxide evokes pain in humans on intracutaneous injection

To test the hypothesis that nitric oxide (NO) acts algetically in humans, we determined pain intensity/dose relations for intracutaneously applied NO solutions. NO, dissolved in isoosmolar phosphate buffer, was injected in the forearm of six volunteers and the subjects rated NO-evoked pain continuously with the help of an electronically controlled visual analogue scale. Pain always occurred at a NO dose of 12 nmol, increased with dose and reached the tolerance maximum at 50 nmol. This shows for the first time the genuine pain evoking properties of NO.

Subjective Ratings of Pain Correlate with Subcortical-Limbic Blood Flow: An fMRI Study

Studies investigating the cerebral representations of pain using functional imaging techniques failed to elucidate the affective aspects of pain. This investigation used functional magnetic resonance imaging to measure pain-related changes in cerebral activity during painful stimulation with a strong affective component. Vascular pain was induced via balloon dilatation of a dorsal foot vein of healthy volunteers. The subjects rated their perceived pain uninterruptedly during imaging, allowing cerebral activity to be correlated with both stimulus function (boxcar) and, more importantly, subjective ratings reflecting individual pain experience. The findings indicated signal increases in subcortical-limbic regions, particularly in the amygdala. This region is suggested to be involved in the affective dimension of pain.

Involvement of the NO/cyclic GMP pathway in bradykinin-evoked pain from veins in humans

Abstract Cyclic GMP is probably a second messenger in vascular nociceptors that are excited by nitric oxide (NO), because NO is known to activate the guanylate cyclase. If so, inhibition of this enzyme should render nociceptors insensitive to algesics that act via NO. To test this hypothesis, the effect of the specific guanylate cyclase inhibitor methylene blue was studied on bradykinin‐evoked, i.e. NO‐mediated pain and, for control, on mechanically‐evoked pain, which is probably not mediated by NO. In eight subjects, pain was evoked from isolated hand vein segments by either injection of bradykinin (1×10−6 M) or noxious balloon distention. Pretreatment of the vein segments with methylene blue inhibited bradykinin‐evoked pain in a concentration‐dependent manner and abolished pain at a concentration of 1×10−3 M. Methylene blue had no effect on mechanically evoked pain. Tachyphylaxis to intravenously applied bradykinin was not observed. These results are consistent with the hypothesis that cyclic GMP plays a role in the transduction of NO‐mediated noxious stimuli in vascular nociceptors in humans.

Preemptive analgesia: No relevant advantage of preoperative compared with postoperative intravenous administration of morphine, ketamine, and clonidine in patients undergoing transperitoneal tumor nephrectomy☆

Background and Objectives Preemptive analgesia often failed in the clinical arena because application of a single intravenously applied drug may not prevent nociceptive input and spinal pain processing sufficiently. We therefore used an intravenous (IV), multireceptor approach and tested the preemptive analgesic effect of the antinociceptive drugs morphine, ketamine, and clonidine given before or immediately after surgery. Methods A double-blind, randomized, prospective study was performed in 30 patients undergoing transperitoneal tumor nephrectomy (via median laparotomy). Standard general anesthesia procedure without opioids was used. After induction, patients were randomly allocated to receive 150 μg · kg−1 of morphine, 150 μg · kg−1 of ketamine, and 5 μg · kg−1 of clonidine intravenously via a motor-driven pump within 15 minutes, either before or immediately after surgery. Patient-controlled analgesia (PCA) with the opioid piritramide (IV) was used for postoperative analgesia. Postoperative pain at rest and during induced cough was quantified by analgesic requirement and pain scores (visual analog scale [VAS]) within 48 hours. Results There was no significant difference in analgesic requirement of piritramide and pain scores at rest or during induced cough. Conclusions In contrast to encouraging observations on the combination of antinociceptive drugs, the multireceptor approach tested here failed to exert a clinically relevant effect.

Effect of pre- or post-traumatically applied i.v. lidocaine on primary and secondary hyperalgesia after experimental heat trauma in humans

&NA; Hyperalgesia on intradermal capsaicin application can be attenuated by systemic application of local anesthetics. We tested whether low doses of local anesthetics applied pre‐ or post‐traumatically can reduce heat trauma‐induced primary and secondary hyperalgesia in humans. Six healthy volunteers consented to the randomized, double‐blind, and cross‐over designed study. In each subject, a first‐degree burn injury was induced three times (corresponding to a pre‐traumatic, post‐traumatic and control group) at an interval of at least 3 weeks. Heat was applied by a computer‐controlled Peltier thermode (47°C, 5 min). In the pre‐traumatic group, lidocaine infusion was commenced 30 min prior to heat trauma, and in the post‐traumatic group immediately after heat trauma for a total infusion time of 60 min each. Volunteers rated pain on a visual analogue scale (VAS) between threshold and tolerance maximum (0–100% VAS). Primary hyperalgesia was quantified by determining mechanical (von Frey hairs) and thermal (Peltier thermode) pain thresholds. Secondary hyperalgesia was quantified by determining the area in which normally unpleasant von Frey hairs evoked pain or tenderness. Baseline thermal and mechanical pain thresholds did not differ between groups. Heat application always resulted in a first‐degree burn injury including both primary and secondary hyperalgesia. The former remained by and large stable for about 4 h whereas the latter continuously increased within the first 2 h. Lidocaine did not affect primary hyperalgesia, irrespective of pre‐ or post‐traumatic application, but substantially reduced the development of secondary hyperalgesia on pre‐traumatic, and for tendency on post‐traumatic infusion (treatment groups did not differ significantly). Burn injury‐induced erythema was smallest in the pre‐traumatic group and largest in the control group; however, the level of significance was not reached. Plasma concentrations of lidocaine were always higher than 1.5 &mgr;g/ml 30 min after bolus application of lidocaine and reached a peak of 2–3 &mgr;g/ml after about 1 h. Thus, local anesthetics at concentrations that do not block nerve conduction substantially affect ongoing central changes in pain processing that are induced by a real tissue trauma. A significant preemptive effect could not be demonstrated. The anti‐hyperalgesic effect of lidocaine is likely based on action of central (spinal) sites, but peripheral sites may also be addressed.

Tachyphylaxis to Local Anesthetics Does Not Result from Reduced Drug Effectiveness at the Nerve Itself

Possible development of tachyphylaxis to local anesthetics in the nerve itself (time-dependent change in axonal conduction properties) was studied in the aortic nerve of eight rabbits anesthetized with urethane. The nerve was immersed in Tyrode solution with or without bupivacaine at pH 7.4 and 38 degrees C in a trough molded from the surrounding tissues. After control measurements the nerve was exposed to increasing bupivacaine concentrations until complete nerve block at minimal blocking concentration. Subsequently, bupivacaine concentrations were reduced and kept constant for 4 h (partial block). Finally, intact nerve function was confirmed after bupivacaine washout with Tyrode solution. For quantification total nerve activity was recorded continuously and related to drug concentrations. Two findings argue against the occurrence of tachyphylaxis at the nerve itself: 1) nerve activity decreased rather than increased over time in the presence of constant bupivacaine concentrations during partial block; and 2) for the same bupivacaine concentration, nerve activity during partial block was always lower than during the initial blocking experiments. Thus, drug effectiveness increased rather than decreased over time, which cannot be reconciled with the theory that tachyphylaxis might be mediated by changes in axonal conduction properties.

The Spinal Antinociceptive Effect of Nocistatin in Neuropathic Rats Is Blocked by D-Serine

Background: The neuropeptide nocistatin (NST) has been implicated in the modulation of nociceptive responses in the spinal cord. Depending on the dose, both pronociceptive and antinociceptive effects have repeatedly been reported. The pronociceptive effect is most likely attributable to inhibition of synaptic glycine and &ggr;-aminobutyric acid release and a subsequent reduction in the activation of inhibitory glycine and &ggr;-aminobutyric acid receptors, but the mechanisms of its antinociceptive action have hitherto remained elusive. It has recently been demonstrated that synaptically released glycine contributes to N-methyl-d-aspartate receptor activation. The authors therefore investigated whether a reduction in glycine release might also account for the antinociceptive action of NST in neuropathic rats. Methods: The authors analyzed the effects of spinally applied NST in the chronic constriction injury model of neuropathic pain. NST was injected intrathecally from nanomolar to picomolar doses and its effects on thermal paw withdrawal latencies were monitored. Furthermore, we tested whether D-serine (100 &mgr;g per rat), a full agonist at the glycine binding site of the N-methyl-d-aspartate receptor, would interfere with the effects of NST. Results: At high doses (10 nmol/rat), intrathecally injected NST was pronociceptive, whereas lower doses (1 pmol/rat) elicited antinociception. The antinociceptive, but not the pronociceptive, action was occluded by intrathecal pretreatment with D-serine. L-serine, which does not bind to N-methyl-d-aspartate receptors, affected neither the pronociceptive nor the antinociceptive effect. Conclusions: These results demonstrate that NST produces a biphasic dose-dependent effect on neuropathic pain. The spinal antinociception by NST is most likely attributable to inhibition of glycine-dependent N-methyl-d-aspartate receptor activation.

Substance P is not involved in vascular nociception in humans

Blood vessels are similar to skin invested with substance P-containing nerves and the binding sites for substance P, which has been shown to be involved in nociception from skin. No attempts have been made so far to see whether substance P is also involved in vascular nociception. We therefore tested substance P for its algesic properties in human veins because their innervation exclusively subserves nociception. Substance P never evoked pain from veins at concentrations of 10(-8)-10(-4) M but did so in a concentration-related fashion from skin. Therefore, substance P is not involved in the generation of acute vascular pain.

Nitric oxide is not involved in vascular nociception of noxious physical stimuli in humans

We tested the hypothesis that nitric oxide (NO) is involved in vascular nociception of physical stimuli in humans. Vascularly isolated hand vein segments of six healthy volunteers were pretreated with the NO synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME; 10(-7)-10(-4) M) and repeatedly subjected to noxious thermal (2 degrees C, 52 degrees C) or mechanical stimuli (balloon distention) and, for control, to the endogenous algetic bradykinin (10(-6) M). L-NAME prevented in a concentration-related manner the algesic action of bradykinin, but had no effect on pain evoked by heat, cold, or stretch. NO is therefore not a general chemical link in nociception.

The role of pain from veins for the formation of perivenous edema in humans

&NA; To test the hypothesis that nociception from veins plays a role in the formation of perivenous edema, we looked at edema along hand veins of humans during painful noxious stimulation in the presence and absence of nerve conduction block. Pain from vascularly isolated hand vein segments was evoked by perfusion with hyperosmolar saline and rated with the help of an electronically controlled visual analogue scale. Perivenous edema measured as changes in skin altitude was continuously recorded by means of infrared reflection. To alternately block the innervation of skin and vein segment we used a perivenous block (vein but not skin numbed), a distal ulnar nerve block (skin but not vein numbed), and a proximal ulnar nerve block (both vein and skin numbed). Without nerve block, hyperosmolar saline always evoked both pain and a continuous increase in perivenous edema to a maximum of 2.0–3.2 mm after 30 min. On painless control perfusions with isoosmolar saline, edema increased slightly (0.2–0.8 mm) to a plateau which was maintained until the end of perfusion. When the vein was denervated by perivenous or proximal ulnar nerve block, hyperosmolar saline evoked a slight increase in edema which resembled that of control perfusions in both extent and time course. On distal ulnar nerve block, which numbed the skin but not the vein, both pain and substantial edema were evoked. These observations show that nociception from veins is a prerequisite for perivenous edema to occur.