Minna M. Hämäläinen

Lidocaine in the rostroventromedial medulla and the periaqueductal gray attenuates allodynia in neuropathic rats

In the present study we attempted to find out if the rostroventromedial medulla (RVM) or the periaqueductal gray (PAG) might contribute to chronic allodynia induced by unilateral ligation of two spinal nerves in the rat. Lidocaine was microinjected in the RVM or PAG and allodynia was quantitatively determined by measuring the hindlimb withdrawal thresholds to mechanical stimulation of the paw. For comparison, lidocaine was also injected systemically (s.c.). Lidocaine in the RVM produced a dose-related (20 and 40 micrograms) antiallodynic effect. Lidocaine (20 micrograms) in the PAG produced identical antiallodynic effect as in the RVM. With systemic administrations of lidocaine, a considerably higher dose (> > 40 micrograms) was needed to produce a significant antiallodynic effect. Naloxone, an opioid-antagonist (1 mg/kg s.c.), did not attenuate the antiallodynic effect of lidocaine in the RVM. An antiallodynic dose of lidocaine (20 micrograms) in the RVM or the PAG did not influence the withdrawal response in the unoperated hindlimb nor the heat-induced tail-flick reflex. The results indicate that the RVM and the PAG have a facilitatory influence on the spinal segmental mechanisms underlying chronic allodynia. The selective attenuation of allodynia induced by lidocaine in the RVM and the PAG is independent of opiate receptors, and it can not be explained by a systemic spread of the drug.

Attenuation of Mechanical Hypersensitivity by an Antagonist of the TRPA1 Ion Channel in Diabetic Animals

Background:The TRPA1 ion channel modulates excitability of nociceptors, and it may be activated by compounds resulting from oxidative insults. Diabetes mellitus produces oxidative stress and sensory neuropathy. The authors tested the hypothesis that diabetes-induced endogenous compounds acting on the TRPA1 ion channel contribute to development and maintenance of mechanical hypersensitivity. Methods:Diabetes mellitus was induced by streptozotocin. Mechanical hypersensitivity was assessed by the monofilament and paw pressure tests. Chembridge-5861528 (CHEM; a TRPA1 channel antagonist, a derivative of HC-030031) or vehicle was administered acutely or twice daily for 10 days in diabetic animals. For comparison, effects of CHEM were assessed in a group of healthy control animals. Results:Acute administration of CHEM attenuated mechanically induced withdrawal responses in diabetic and control groups. The maximal effect (over 50% elevation of the paw pressure threshold) by acute administration of CHEM was obtained in 30 min. The lowest dose producing a significant attenuation was 10 mg/kg in the diabetic group and 30 mg/kg in the healthy controls. Chronic administration of CHEM (30 mg/kg twice daily) for a week in the diabetic group attenuated development of mechanical hypersensitivity. Conclusions:Reduction of pain-related behavior by a lower dose of the TRPA1 channel antagonist in the diabetic than in the control group suggests that endogenous compounds resulting from diabetes mellitus and acting on the TRPA1 channel contribute to diabetic hypersensitivity. Prolonged antihypersensitivity effect after chronic treatment suggests that daily administration of a TRPA1 channel antagonist may prevent development of diabetic hypersensitivity.

Dissociation of the α2-adrenergic antinociception from sedation following microinjection of medetomidine into the locus coeruleus in rats

&NA; It is well established that &agr;‐adrenoceptor agonists have sedative and antinociceptive properties. In the current behavioral study we tried to find out if the &agr;‐adrenergic sedative and antinociceptive effects can be dissociated. We tested the hypothesis that &agr; adrenergic sedation is mediated by the locus coeruleus (LC) and antinociception by spinal &agr;‐adrenoceptors. Also, we addressed the possibility that intracerebral injection of an &agr;‐agonist might produce its antinociceptive effect by an action directly at the spinal cord. Medetomidine, an &agr;‐adrenergic agonist, or atipamezole, an &agr;‐adrenergic antagonist, were microinjected bilaterally into the LC through chronic cannulae in unanesthetized Han‐Wistar rats. The effect on locomotor activity (/vigilancc). tail‐flick and hot‐plate response, and on formalin‐induced pain behavior was determined. Medetomidine microinjected into the LC (1–10 &mgr;g/cannula) produced dose‐dependently hypolocomotion (/sedation), increase of response latencies in the hot‐plate and the tail‐flick tests, and a decrease in the formalin‐induced pain behavior. Hypolocomotion (/sedation) was obtained at a lower medetomidine dose (1 &mgr;g/cannula) than antinociception (3–10 &mgr;g/cannula). The lowest medetomidine dose used (1 &mgr;g/cannula), which induced significant hypolocomotion (/sedation), produced either no antinociception (hot‐plate and tail‐flick tests) or even a slight hyperalgesia (formalin test). The hypolocomotion (/sedation) but not antinociception (tail‐flick test) induced by systemic administration of medetomidine (100 &mgr;g/kg s.c.) could be reversed by atipamezole (10 &mgr;g/cannula) microinjected into the LC. Only a high systemic dose of atipamezole (1 mg/kg s.c.) reversed the antinociceptive effects of medetomidine. Microinjection of medetomidine into the LC (3 &mgr;g/cannula) produced antinociception (tail‐flick test) also in spinal rats. which effect was completely reversed by atipamezole (l mg/kg s.c.). Following administration of medetomidine at the dose of 1 &mgr;g/cannula into the central or cortical nucleus of the amygdala (control sites) there was no significant effect on locomotor activity, hot‐plate response, or formalin‐induced pain. The results indicate that &agr;‐adrenergic sedative and pain‐modulating effects can be dissociated following microinjection of medetomidine into the LC. The antinociceptive effect of the supraspinally microinjected medetomidine in Han‐Wistar rats could be explained by direct activation of the spinal &agr;‐adrenoceptors, whereas the sedation/hypolocomotion could he explained by an action on the &agr;‐adrenoceptors located in the LC, or its immediate vicinity.

Influence of skin temperature on heat pain threshold in humans

We compared the effect of skin temperature on the critical threshold temperature eliciting heat pain with the effect of skin temperature on the response latency to the first heat pain sensation in healthy human subjects. Also, we determined the effect of the duration of a heat stimulus ramp on pain threshold. Furthermore, we determined the effect of skin temperature on mechanically induced pain. We found that the latency to the first pain sensation induced by a radiant heat stimulus was significantly decreased with an increase in the skin temperature (25–35 °C). However, independent of the rate of the stimulus rise (3–10 °C/s) and independent of the stimulus location (hairy vs glabrous skin), the threshold temperature for eliciting the heat pain sensation, determined with a contact thermostimulator, was not changed by a change in the skin temperature in the same subjects. With a fast rate of stimulus rise, a higher pain threshold was obtained than with a slow rise of stimulus temperature. However, this difference was found only with subject-controlled ascending stimuli (method of limits) but not with experimenter-controlled, predetermined stimulus ramps (method of levels). The heat pain threshold was higher in the glabrous skin of the hand than in the hairy skin of the forearm. With increasing stimulus duration (2.5–10s), the threshold temperature eliciting the heat pain sensation was significantly decreased. The mechanically induced pain threshold was not influenced by the skin temperature. The results indicate that the critical temperature for eliciting heat pain is independent of the skin temperature in humans. However, a change in skin temperature is an important source of an artefactual change in heat pain sensitivity when the radiant heat method (latency or energy) is used as an index of pain sensitivity. With a method dependent on reaction time (the method of limits), the heat pain threshold was artefactually increased, with fast rates of stimulus rise due to the long delay of slowly conducting heat pain signals in reaching the brain. With an increase in the duration of the heat stimulus, the critical temperature for eliciting pain sensation was significantly decreased, which may be explained by central neuronal mechanisms (temporal summation).

Carrageenan-induced changes in spinal nociception and its modulation by the brain stem.

CARRAGEENAN was used to study inflammation-induced changes in spinal nociception and its brain stem modulation in the pentobarbitone-anesthetized rat. Carrageenan was administered intraplantarly into one hindpaw 2 h before the start of electrophysiological single unit recordings of wide-dynamic range (WDR) neurons of the spinal dorsal horn. Carrageenan produced a significant leftward shift in the stimulus–response function for mechanical stimuli, whereas that for noxious heat stimuli was short of statistical significance. Conditioning electrical stimulation in the rostroventromedial medulla (RVM) significantly attenuated noxious heat-evoked, but not mechanically evoked, responses to spinal dorsal horn WDR neurons in the control (contralateral) side. However, in the carrageenan-treated side RVM stimulation had no significant effect on mechanically or noxious heat-evoked responses. Following direct spinal administration of neuropeptide FF (NPFF), noxious heat-evoked responses, but not mechanically evoked responses, were attenuated by RVM-stimulation also in the carrageenan-treated side. This selective NPFF-induced enhancement of brain stem–spinal inhibition was not reversed by naloxone. The results indicate that carrageenan-induced inflammation significantly changes the response properties of spinal nociceptive neurons and their brain stem-spinal modulation. During inflammation, NPFF in the spinal cord produces a submodality-selective potentiation of the antinociceptive effect induced by brain stem–spinal pathways, independent of naloxone-sensitive opioid receptors.

Spinal potentiation and supraspinal additivity in the antinociceptive interaction between systemically administered α2-adrenoceptor agonist and cocaine in the rat

We studied the antinociceptive interaction of systemically administered medetomidine, a selective alpha 2-adrenoceptor agonist, with cocaine in the tailflick and hotplate tests in rats. In intact rats, both medetomidine alone (25-300 micrograms/kg subcutaneously [SC]) and cocaine alone (10-30 mg/kg intraperitoneally [IP]) produced a dose-dependent antinociception in the supraspinally mediated hotplate test, and their combination (medetomidine 25 micrograms/kg+cocaine 10-20 mg/kg) produced an additive antinociceptive effect. In the spinally mediated tailflick test, only medetomidine alone produced a significant antinociceptive effect both in intact and spinal rats, whereas cocaine alone produced a marginally significant antinociceptive effect in the tailflick test only at the highest dose used (30 mg/kg) and only in intact rats. The combination of medetomidine (25 micrograms/kg SC) with cocaine (10-20 mg/kg IP) produced potentiation of antinociception in the tailflick test both in intact and spinal rats. The results indicate that at the supraspinal level there is an additive antinociceptive interaction between cocaine and medetomidine, whereas at the spinal level cocaine potentiates the medetomidine-induced antinociception independent of supraspinal mechanisms. The results further support previous evidence indicating that antinociception produced by a systemically administered alpha 2-adrenergic drug alone is mainly due to spinal mechanisms, whereas that by systemically administered cocaine alone is mainly due to supraspinal mechanisms.

The rostroventromedial medulla is not involved in α2-adrenoceptor-mediated antinociception in the rat

The aim of the current study was to investigate the role of the rostroventromedial medulla (RVM) in alpha 2-adrenoceptor-mediated antinociception. Medetomidine or clonidine, selective alpha 2-adrenoceptor agonists were microinjected into the RVM in unanesthetized rats with a chronic guide cannula. The antinociceptive effects were evaluated using the tail-flick and hot-plate tests. For comparison, medetomidine was microinjected into the cerebellum or the periaqueductal gray (PAG). To study the role of medullospinal pathways, the tail-flick latencies were also measured in spinalized rats. The reversal of the antinociception induced by intracerebral microinjections of medetomidine was attempted by s.c. atipamezole, a selective alpha 2-adrenoceptor antagonist. The reversal of the antinociception induced by systemic administration of medetomidine was attempted by microinjections of 5% lidocaine or atipamezole into the RVM. When administered into the RVM, medetomidine produced a dose-dependent (1-30 micrograms) antinociception in the tail-flick and hot-plate tests, which antinociceptive effect was completely reversed by atipamezole (1 mg/kg, s.c.). Also clonidine produced a dose-dependent (3-30 micrograms) antinociception following microinjection into the RVM. Microinjections of medetomidine into the cerebellum or the PAG produced an identical dose-response curve in the tail-flick test as that obtained following microinjection into the RVM. In spinalized rats the antinociceptive effect (tail-flick test) induced by medetomidine microinjected into the RVM was not less effective than in intact rats. Lidocaine (5%) or atipamezole (5 micrograms) microinjected into the RVM did not attenuate the antinociception induced by systemically administered medetomidine (100 micrograms/kg, s.c.).(ABSTRACT TRUNCATED AT 250 WORDS)

The Effect of a Selective α2-Adrenoceptor Antagonist on Pain Behavior of the Rat Varies, Depending on Experimental Parameters

Effects of atipamezole, an alpha2-adrenoceptor antagonist, in various acute pain tests were studied in the rat. Atipamezole (at doses > or = 0.1 mg/kg I.P.) and idazoxan, another alpha2-adrenoceptor antagonist (2.5 mg/kg, I.P.), increased licking latency in the hot-plate test. Bilateral administration of atipamezole (10 microg) into the locus coeruleus did not increase licking latency in the hot-plate test. Medetomidine (an alpha2-adrenoceptor agonist; 1-3 mg/kg) or repeated pre-exposures to the testing apparatus reversed the effect of atipamezole (1.5 mg/kg) in the hot-plate test. Atipamezole also increased the latency to mechanically induced licking/biting response at a dose of 1.5 mg/kg, but not at lower doses. In the heat-induced tail-flick test, in contrast, atipamezole at doses of 0.1 and 1.5 mg/kg produced a medetomidine-reversible decrease of response latencies. This facilitation of the tail-flick response disappeared if the intensity of the heat stimulus was high. At a dose range from 0.03 to 1.5 mg/kg atipamezole did not significantly alter the paw withdrawal latency to noxious mechanical stimulation, nor pain behavior in the formalin test. Responses to nociceptive spinal dorsal horn neurons were not modulated by atipamezole (1 mg/kg) in anesthetized spinalized rats. The results indicate that an alpha2-adrenoceptor antagonist may have variable effects in behavioral pain tests, depending on habituation of the experimental animals to the testing conditions, the dose of the drug, the type of behavioral response and the submodality or the intensity of the noxious test stimulus. The atipamezole-induced changes in pain behavior observed in this study may rather be explained due to action on motor expression of pain than due to modulation of nociception.

A noninvasive method for studying quantitatively heat-evoked nocifensive hindlimb withdrawal reflexes in lightly anesthetized rats.

We have developed a noninvasive method for studying quantitatively the magnitude of hindlimb withdrawal reflexes induced by noxious heat in lightly anesthetized rats. The amplitude, latency, and duration of the hindlimb withdrawal was determined by a very small piezoceramic device placed on the hamstring muscle while the glabrous skin of the hindpaw was stimulated using a feedback-controlled contact thermostimulator. An increase in the amplitude and duration of the withdrawal response, concomitant with a decrease in the response latency, was found with increasing stimulus temperature. The sensitivity of the method was verified using morphine, which produced a dose-related (3.5-7.0 mg/kg) attenuation of all these response components. The use of a piezoceramic device for measuring the withdrawal response provides a quantitative, noninvasive method for evaluating the magnitude of various components of the nocifensive withdrawal reflexes induced by noxious stimuli in lightly anesthetized rats.

Can the α2-adrenoceptor agonist-mediated suppression of nocifensive reflex responses be due to action on motoneurons or peripheral nociceptors? ☆

To exclude the possibility that the suppression of nocifensive reflex responses induced by alpha 2-adrenergic agents is due to action on alpha-motoneurons or peripheral nociceptors, we studied the effect of medetomidine, an alpha 2-adrenoceptor agonist, on the monosynaptic reflex and on the primary afferent nociceptor-mediated antidromic vasodilator response in rats. Additionally, the effect on the dorsal root potential, an index of a transient excitability change in the central terminals of primary afferent fibers, was determined. Medetomidine was applied systemically at doses (100 and 300 micrograms/kg) which have proven strongly antinociceptive in previous studies. The amplitudes of a submaximal monosynaptic reflex volley or a submaximal dorsal root potential were not changed by medetomidine. Medetomidine induced a decrease of cutaneous blood flow but did not abolish the vasodilatatory response to antidromic stimulation of the sciatic nerve at C-fiber intensity as determined by the laser Doppler flow method. The results indicate that the alpha 2-adrenoceptor-mediated suppression of nocifensive reflex responses is not caused by a decreased excitability of motoneurons or peripheral nociceptors. An alpha 2-adrenoceptor agonist does not modulate the transient stimulus-evoked change in the excitability of central terminals of primary afferent fibers.