Absence of antinociceptive effect of alpha-2 agonists microinjected in the periaqueductal gray of the rat

[Systemic clonidine versus opioids in postoperative analgesia-A randomized double-blind study.]

INTRODUCTION

alpha(2)-Adrenozeptoragonisten agonists have shown antinociceptive and analgesic effects, which are not antagonized by naloxone. Therefore, the mechanism of action should be independent of opioid receptors. Most studies on this topic have been performed using clonidine. Experimentally the analgesic effect of clonidine can be suppressed by the inhibition of central adrenergic receptors. Furthermore, clonidine has analgesic effects at the spinal level. During recent years numerous studies have shown the analgesic effect of spinally or epidurally administered clonidine in humans. However, only very few studies have investigated the analgesic effect of parenterally administered clonidine in humans.

METHODS

After the approval of the local ethical committee had been obtained, 60 patients (ASA I-III, age 18-65 years) scheduled for elective orthopaedic procedures were included in this double-blind randomized study. All patients gave their written consent on the day before the operation. Premedication was standardized and involved benzodiazepines. Isoflurane was used as the sole anaesthetic. Postoperatively the pain level of the patients was controlled by a visual analogue scale (VAS 0-10). When the VAS reached at least 5 and the patients requested an analgesic, they were randomly assigned to either the morphine, tramadol or clonidine group. Twenty patients received 5 mg morphine i.v., 20 patients received 50 mg tramadol and 20 patients received 150 clonidine i.v. If the analgesic effect was insufficient, the above-mentioned dosage was repeated after 30 min. The therapy was classified as a failure if no sufficent analgesia could be achieved within 60 min. These patients received 7.5 mg piritramide i.v. VAS and sedation were measured at 10-min intervals during the 1 st h and at 15-min intervals during the following 2h. Heart rate, blood pressure and oxygen saturation were measured at 5-min intervals during the whole study period. Statistical analysis of the data was performed by ANOVA, Wilcoxon test, Student'st-test and chi-square test using a level of significance ofP<0.05.

RESULTS

All groups were comparable as regards their basic clinical parameters. Morphine, tramadol and clonidine significantly reduced the VAS within 20 min. During the whole study period the analgesic effect of clonidine was comparable with that of morphine and tramadol. No significant differences were observed in the number of repetitions after 30 min or in the failure rate. After 2 h sedation was significantly higher in the morphine group. No clinically relevant cardiovascular or respiratory side-effects were observed in any of the patients.

DISCUSSION

In our study the analgesic effect of 150 mug clonidine i.v. was equivalent to that of 5 mg morphine i.v. and 50 mg tramadol. Our results in humans confirm the dosage relationship of 1ratio30 found by Eisenach in sheep. Further studies on the use of parenteral clonidine for postoperative analgesia seem to be warranted.

Noradrenergic pain modulation

Norepinephrine is involved in intrinsic control of pain. Main sources of norepinephrine are sympathetic nerves peripherally and noradrenergic brainstem nuclei A1-A7 centrally. Peripheral norepinephrine has little influence on pain in healthy tissues, whereas in injured tissues it has variable effects, including aggravation of pain. Its peripheral pronociceptive effect has been associated with injury-induced expression of novel noradrenergic receptors, sprouting of sympathetic nerve fibers, and pronociceptive changes in the ionic channel properties of primary afferent nociceptors, while an interaction with the immune system may contribute in part to peripheral antinociception induced by norepinephrine. In the spinal cord, norepinephrine released from descending pathways suppresses pain by inhibitory action on alpha-2A-adrenoceptors on central terminals of primary afferent nociceptors (presynaptic inhibition), by direct alpha-2-adrenergic action on pain-relay neurons (postsynaptic inhibition), and by alpha-1-adrenoceptor-mediated activation of inhibitory interneurons. Additionally, alpha-2C-adrenoceptors on axon terminals of excitatory interneurons of the spinal dorsal horn possibly contribute to spinal control of pain. At supraspinal levels, the pain modulatory effect by norepinephrine and noradrenergic receptors has varied depending on many factors such as the supraspinal site, the type of the adrenoceptor, the duration of the pain and pathophysiological condition. While in baseline conditions the noradrenergic system may have little effect, sustained pain induces noradrenergic feedback inhibition of pain. Noradrenergic systems may also contribute to top-down control of pain, such as induced by a change in the behavioral state. Following injury or inflammation, the central as well as peripheral noradrenergic system is subject to various plastic changes that influence its antinociceptive efficacy.

Enhanced reduction in hyperalgesia by combined administration of clonidine and TENS

Transcutaneous electrical nerve stimulation (TENS) partially reduces primary hyperalgesia and is frequency dependent such that high frequency TENS produces approximately a 30% reduction in hyperalgesia whereas low frequency TENS has no effect. Both high and low frequency TENS completely reduce secondary hyperalgesia by activation of mu and delta- opioid receptors in the spinal cord and rostral-ventral medulla suggesting an opiate mediated analgesia. Clonidine in combination with opiates produces a synergistic interaction such that there is a potentiated reduction in hyperalgesia. Thus, we tested if combined application of clonidine with TENS would enhance the reduction in primary hyperalgesia. Male Sprague-Dawley rats were inflamed by subcutaneous injection of 3% carrageenan into one hindpaw. Withdrawal latency to radiant heat and withdrawal threshold to mechanical stimuli were assessed before and after inflammation and after administration of clonidine (0.002-2 mg/kg, intraperitoneal (i.p.)) with either low (4 Hz) or high (100 Hz) frequency TENS. Clonidine alone reduced both heat and mechanical hyperalgesia with ED50s of 0.02 and 1.0 mg/kg, respectively. In combination with either low or high frequency TENS, the dose-response curve shifted to the left and was significantly different from clonidine alone. The ED50s for heat and mechanical hyperalgesia following low frequency TENS with clonidine were 0.002 and 0.2 mg/kg, respectively and those following high frequency TENS with clonidine were 0.005 and 0.15 mg/kg, respectively. Thus, combined use of clonidine and TENS enhances the reduction in analgesia produced by TENS and enhances the potency of clonidine. It would thus be expected that one would reduce the side effects of clonidine and enhance analgesic efficacy with combinations of pharmaceutical and non-pharmaceutical treatments.

Pretreatment with dexmedetomidine: altered indices of anesthetic depth for halothane in the neuraxis of cats

The sedative and anesthetic-sparing ability of the alpha2-adrenergic agonist dexmedetomidine is well documented. In this study, we identified the effects of halothane, with and without dexmedetomidine, on hemodynamic and electroencephalographic (EEG) variables and quantified the concentration of halothane resulting in various anesthetic depth indices mediated through the central nervous system (CNS) in chronically instrumented cats. Halothane was given alone or after dexmedetomidine (15 microg/kg p.o.). In both groups, four indices of anesthetic depth--minimum alveolar anesthetic concentration (MAC; no movement to noxious stimuli), MAC(BAR) (no autonomic response to noxious stimuli), MAC(BS) (EEG burst suppression), and MAC(ISOELECTRIC) (EEG isoelectricity)--were determined. Halothane decreased arterial blood pressure, heart rate, and higher frequency components of the EEG before the onset of burst suppression and isoelectricity. Dexmedetomidine pretreatment augmented the actions of halothane on arterial pressure, heart rate, and the EEG. Dexmedetomidine reduced the halothane concentrations resulting in MAC (from 1.22% +/- 0.06% to 0.89% +/- 0.08%) and MAC(BAR) (from 1.81% +/- 0.05% to 1.1% +/- 0.10%), but not those resulting in MAC(BS) (3.01% +/- 0.17% vs 3.14% +/- 0.10%) or MAC(ISOELECTRIC) (4.39% +/- 0.26% vs 4.65% +/- 0.12%). These results suggest that dexmedetomidine does not alter various CNS-mediated indices of anesthetic action to equivalent degrees and that there are dissimilar degrees of an anesthetic-sparing action at different levels of the neuraxis.

IMPLICATIONS

The anesthetic adjuvant dexmedetomidine seems to differentially alter central nervous system-mediated indices of anesthetic action. Lower brainstem or spinal determinants of anesthetic depth (movement and hemodynamic responses) are more attenuated than those of higher brain functions, such as the electroencephalogram.

Opiate receptors in the periaqueductal gray mediate analgesic effect of nitrous oxide in rats

The site of action and the pathways which are activated by nitrous oxide (N2O) to produce an analgesic effect are not well defined. Experiments were designed to determine whether N2O produces analgesia by activating opiate receptors or alpha2-adrenoceptors in periaqueductal gray. The analgesic effect of N2O was determined using the tail flick response to noxious radiant heat in lightly anesthetized rats. Different antagonists were bilaterally microinjected into ventrolateral periaqueductal gray to determine whether the analgesic effect produced by N2O was reversed. The increase in the tail flick latencies produced by N2O was reversed by bilateral microinjection into the ventrolateral part of periaqueductal gray with the opiate receptor antagonist naloxone 2.5 microg/0.5 microl, but not with the alpha2-adrenoceptors antagonist yohimbine 1.5 microg/0.5 microl. These results indicate that the N2O analgesic effect is mediated by activation of opiate receptors, but not alpha2-adrenoceptors, in the periaqueductal gray. Combined with the previous experiments that the N2O analgesic effect is reversed by intrathecal injection of an alpha2-adrenoceptor antagonist but not by an opiate receptor antagonist, it seems likely that N2O causes activation of the opiate receptors in the periaqueductal gray, which in turn activate the noradrenergic descending pathways to the spinal cord to produce the analgesic effect.

Effects of clonidine on a C-fibre reflex in the rat

A C-fibre reflex elicited by electrical stimulation within the territory of the sural nerve, was recorded from the ipsilateral biceps femoris muscle in anaesthetized rats. The temporal evolution of the response was studied using a constant stimulus intensity (3 x threshold) and recruitment curves were built by varying stimulus intensity from 0 to 7 x threshold. The intravenous administration of 0.02-0.2 mg/kg clonidine resulted in a dose-dependent depression of the C-fibre reflex. The alpha 2-adrenoceptor antagonist idazoxan completely prevented this depressive effect of clonidine. The effects of clonidine on the C-fibre reflex elicited by a wide range of stimulus intensities were investigated using recruitment curves: following 0.16 mg/kg clonidine, a dramatic shift of the recruitment curve to the right was seen with both an increase in the threshold and a decrease in the slope. Clonidine also produced a dose-dependent increase in blood pressure, but this was not correlated with the depression of the nociceptive reflex.

The antinociceptive action of an alpha 2-adrenoceptor agonist in the spinal dorsal horn is due to a direct spinal action and not to activation of descending inhibition

In this electrophysiological study we tried to find out whether the spinal antinociceptive effect of a supraspinally administered alpha 2-adrenoceptor agonist is due to a direct spinal effect or to activation of descending inhibition. The responses to wide-dynamic range (WDR) neurons of the spinal dorsal horn were studied following application of medetomidine, a selective alpha 2-adrenergic agonist, into the rostroventromedial medulla (RVM) or directly onto the spinal cord of the intact and in spinal rats. The noxious electrical stimuli were applied to the ipsilateral receptive field in the plantar region of the hind paw, and responses mediated by A- and C-fibers to WDR neurons were separately evaluated. The reversal of medetomidine-induced effects was attempted by a systemic administration of atipamezole, a selective alpha 2-adrenoceptor antagonist. Medetomidine injection into the RVM produced a dose-dependent, atipamezole-reversible attenuation of the C-fiber-mediated responses to WDR neurons of the spinal dorsal horn in both intact and spinal rats. Paradoxically, the spinal antinociceptive effect of supraspinally administered medetomidine was stronger in spinal rats. The A-fiber-mediated responses were significantly less attenuated by medetomidine than the C-fiber-mediated responses to the WDR neurons. Also a direct application of medetomidine onto the spinal cord produced a dose-dependent, atipamezole-reversible attenuation of the C-fiber-mediated responses, and this effect was identical in intact and in spinal rats. The medetomidine doses producing spinal antinociception were considerably lower with a direct spinal application than with a supraspinal application.(ABSTRACT TRUNCATED AT 250 WORDS)

The rostroventromedial medulla is not involved in alpha 2-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)

Antinociceptive effects of intrathecally administered F8Famide and FMRFamide in the rat

The effects of intrathecal injections of F8Famide (Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2, 0.05-17.5 nmol) and FMRF-amide (Phe-Met-Arg-Phe-NH2, 0.002-25 nmol), known as anti-opioid agents, were investigated by using noxious thermal (tail flick) and mechanical (paw pressure) tests in the rat. Both peptides produced significant long-lasting (24-48 h) analgesia in both tests without causing detectable motor dysfunction. Pretreatment with systemic naloxone (5.5 mumol/kg i.p.) attenuated the initial antinociceptive effects (first hour) induced by both peptides (8.8 nmol) in the tail flick test and only by FMRFamide in the paw pressure test. A subeffective dose of F8Famide (0.05 nmol) enhanced both the intensity and the duration of spinal morphine (6.6 nmol) analgesia in both tests. In contrast, a subanalgesic dose of FMRFamide (0.002 nmol) decreased the intensity and enhanced the duration of the effect of morphine. These results show that, besides acting as antinociceptive agents in the spinal cord, F8Famide and FMRFamide could differentially modulate spinal opioid functions.

Actions of epinephrine on neurons in the rat midbrain periaqueductal gray maintained in vitro

The effect of epinephrine (EPI) on the activity of 150 periaqueductal gray (PAG) neurons was examined using extracellular recordings in an in vitro slice preparation. Drop application of EPI inhibited 45%, excited 35%, and had no effect on 20% of PAG neurons. Both the excitatory and inhibitory effects of EPI were of long duration; excitatory responses averaged 17 min and inhibitory responses averaged 11 min in duration. EPI responses could be blocked by specific alpha-1 and alpha-2 receptor antagonists. In 35% of the neurons tested, blockade of synaptic transmission by perfusion with low calcium-high magnesium physiological saline blocked responses to EPI. The effects of EPI were site specific: 77% of the cells in the caudal ventrolateral region of the PAG were inhibited by EPI; in all other regions of PAG equal numbers of cells were excited and inhibited by EPI. It is concluded that: (a) EPI has potent effects on a majority (80%) of PAG neurons; (b) EPI responses are mediated by presynaptic as well as postsynaptic mechanisms; (c) EPI preferentially inhibits neurons in the ventrolateral subdivision of caudal PAG. As this part of PAG contains many neurons that project to the ventral medulla, it is possible that EPI modulates the PAG-medullary functions such as analgesia, autonomic regulation, defense reactions, and sexual behaviors.

Reversal of mu-opioid-mediated respiratory depression by alpha 2-adrenoceptor antagonism

The present study was performed in order to evaluate the effects of the selective alpha 2-adrenoceptor antagonist 6-chloro-2,3,4,5-tetrahydro-3-methyl-1H-3-benzazepine (SK&F 86466) on dermorphin-induced analgesia, respiratory depression and inhibition of locomotor activity in the conscious rat. Intracerebroventricular (icv) administration of dermorphin (3 nmol/rat) decreased respiration rate and relative ventilatory minute volume maximally by 38% and 50% of baseline respectively. SK&F 86466 dose-dependently reversed the dermorphin-induced depression of ventilatory parameters, while SK&F 86466 exerted no effect on dermorphin-induced analgesia or depression of locomotor activity due to catalepsia. It appears, therefore, that alpha 2-adrenoceptors selectively interact with mu 2-opioid-receptor mediated effects, such as respiratory depression, but are not involved in the modulation of mu 1-opioid-related effects, such as supraspinal analgesia and depression of locomotor activity.

Antagonism of stimulation-produced antinociception from ventrolateral pontine sites by intrathecal administration of alpha-adrenergic antagonists and naloxone

Focal electrical stimulation of the ventrolateral pontine tegmentum in conscious rats induced antinociception in approximately one-half of the animals screened, as indicated by a marked suppression of the thermally evoked tail-flick flexion reflex. The effectiveness of ventrolateral pontine stimulation in elevating tail-flick latency was significantly reduced by intrathecal microinjection of 30 micrograms of the non-selective alpha-adrenergic antagonist phentolamine, and was largely abolished by a 60-micrograms dose of this drug. The blockade of ventrolateral pontine stimulation-produced antinociception by phentolamine was maximal by 15 min postinjection, and was still evident 60 min after drug microinjection. Ventrolateral pontine stimulation-produced antinociception was also attenuated by intrathecal administration of the alpha 2-selective antagonist yohimbine (37 micrograms) and the opioid antagonist naloxone (30 micrograms), but not the alpha 1 antagonist WB-4101 (37 micrograms), the beta-adrenergic antagonist propranolol (111.6 micrograms) nor the serotonergic antagonist methysergide (30 micrograms). However, the antagonism of pontine stimulation-produced antinociception by naloxone was unlike that of phentolamine and yohimbine, in that it developed slowly and was only evident at 60 min postinjection. Hence naloxone's site of action may be distant from the injection site. These data indicate that the thermal antinociception produced by stimulation of the ventrolateral pons is mediated through spinal alpha 2-receptors and opioid receptors of uncertain location. The close proximity of many of the effective electrode placements to the rostral A5 and ventral subcoerulear A7 noradrenergic cell groups suggests that noradrenergic spinopetal projections arising from these groups are involved in mediating the antinociception induced by stimulating these sites.

[Intrathecal and epidural administration of alpha adrenergic receptor agonists]

Alpha-agonists are frequently added to local anaesthetic agents to prolong the duration of spinal or extradural anaesthesia. Adrenaline and phenylephrine have been employed most commonly for this purpose. Recent controlled studies indicated that the alpha-adrenoceptor agonist clonidine, when administered spinally, has a dose-dependent antinociceptive effect. Clonidine seems to be as effective as adrenaline to prolong the duration of local anaesthetic blocks and is useful to decrease the incidence of tourniquet pain under spinal anaesthesia. As they improve the intensity and duration of opioid analgesia, intraspinal alpha-agonists have also a synergic analgesic effect with spinal opioids. Alpha-agonist effects are due: 1) to an activation of the post and/or presynaptic alpha 2-adrenoceptors in the substantia gelatinosa of the spinal cord, 2) to a local vasoconstriction by stimulating vascular smooth muscle alpha-receptors which decrease the rate of absorption of local anaesthetics from the subarachnoid or extradural space, 3) to a co-activation of the spinal opioid and alpha-adrenergic receptors at the spinal cord level. However, spinally administered alpha-agonists have side effects, which include vasoconstriction in the spinal cord, hypotension, bradycardia or tachycardia, somnolence and respiratory depression. To minimize such complications, great care may be needed, which is described in this review, assessing the minimal required amount of alpha-agonists and effective clinical monitoring. The development of this technique in the management of subarachnoid and extradural anaesthesia and of chronic pain is discussed.

Enhancement of clonidine-induced analgesia by lesions induced with spinal and intracerebroventricular administration of 5,7-dihydroxytryptamine

The role of serotonin (5-HT) in analgesia induced by clonidine was examined by determining the effect of intraspinal (i.s.) and intracerebroventricular (i.c.v.) injections of 5,7-dihydroxytryptamine (5,7-DHT) on analgesia produced by clonidine in the tail-flick and hot plate tests. Depletion of amines was verified by high performance liquid chromatography analysis. Intraspinal injections of 5,7-DHT potentiated the action of clonidine in both tests for analgesia and caused depletion of 5-HT in the spinal cord. Intracerebroventricularly injected 5,7-DHT also increased the action of clonidine and depleted 5-HT in brain as well as in the spinal cord. In the groups given intracerebroventricular injections, there appeared to be a biphasic increase in the action of the clonidine. Significant hyperalgesia from pretreatment with neurotoxin was observed only on a limited number of occasions. The present results indicate that 5-HT mechanisms in the CNS are important mediators of the analgesic action of clonidine. Interactions between clonidine and 5-HT systems at both spinal and supraspinal sites are considered.

Locus coeruleus lesions in the rat enhance the antinociceptive potency of centrally administered clonidine but not morphine

The nucleus locus coeruleus (LC) has been implicated in the descending inhibition of spinal nociceptive dorsal horn neurons, spinal nociceptive reflexes and in the antinociception produced by morphine. To further explore the involvement of the LC in antinociception, bilateral electrolytic lesions in the LC were made in adult male Sprague-Dawley rats. Lesions in the LC did not alter the antinociception produced by morphine (2.5 and 5 micrograms) administered in the periaqueductal gray in either the tail-flick (TF) or hot-plate (HP) tests when tested 7 and 14 days after the lesions. Baseline nociceptive thresholds in the TF and HP tests likewise were not affected at 7 or 14 days post-lesion. In contrast, the antinociceptive potency of clonidine administered intrathecally on day 13 post-lesion was enhanced significantly in the TF test; the antinociceptive ED50 of the LC lesion group was 0.52 micrograms whereas that of the sham lesion group was 2.29 micrograms. The antinociceptive potency of clonidine administered systemically (750 and 500 micrograms/kg, s.c.) was also enhanced in the LC lesion group in the TF but not the HP test. Norepinephrine (NE) in the lumbar spinal cord was correlated negatively and significantly with the extent of destruction of the LC. The lumbar spinal content of NE was reduced maximally at 12 days post-lesion (to 56% of control). The binding of [3H]clonidine in the lumbar spinal cord was slightly greater in the LC lesion than sham lesion group; the Bmax values were 42.4 fmol/mg protein and 35.5 fmol/mg protein for the LC lesion and sham lesion groups, respectively. It is suggested that the LC participates in the descending inhibition of spinal nociceptive transmission and that this inhibition may be mediated in the spinal cord by alpha-2 adrenoceptors located postsynaptically with respect to the NE terminals of the spinopetal LC efferents.