The depressive effect of intrathecal clonidine on the spinal flexor reflex is enhanced after sciatic nerve section in rats

Comparison of the Effects of OPRM1 A118G Polymorphism Using Different Opioids: A Prospective Study

CONTEXT

μ-opioid receptor gene (OPRM1) A118G polymorphism (rs1799971) causes loss of N-glycosylation sites at the extracellular domain of μ-opioid receptors. G-allele carriers show a limited response to morphine; however, studies investigating the impact of A118G polymorphism on the efficacy of opioids other than morphine are limited.

OBJECTIVE

To compare the impact of A118G polymorphism on the efficacy of various opioids.

METHODS

This prospective cohort study enrolled 222 in-patients administered one of the following opioid therapies for cancer pain as part of an opioid introduction or rotation strategy: tapentadol extended-release tablets, methadone tablets, hydromorphone controlled-release tablets, oxycodone controlled-release tablets, or transdermal fentanyl patches. The impact of A118G polymorphism on the difference in the Brief Pain Inventory-Short Form score on days three, seven, and 14 from baseline was compared among the groups.

RESULTS

Overall, 81, 74, and 67 patients had the AA, AG, and GG genotypes, respectively, with an OPRM1 A118G G-allele variant frequency of 0.47. The reduction in the Brief Pain Inventory-Short Form score after opioid therapy initiation did not differ significantly among the patients with the three A118G genotypes treated with tapentadol (p = 0.84) or methadone (p = 0.97), whereas it was significantly smaller in G-allele carriers than that in AA homozygous patients treated with hydromorphone (p < 0.001), oxycodone (p = 0.031), or fentanyl (p < 0.001).

CONCLUSION

Tapentadol and methadone may be more suitable than hydromorphone, oxycodone, and fentanyl for G-allele carriers due to their dual mechanism of action and low susceptibility to OPRM1 A118G polymorphism.

Selective deficiencies in descending inhibitory modulation in neuropathic rats: implications for enhancing noradrenergic tone

Supplemental Digital Content is Available in the Text.

Descending noradrenergic pathways modulate spontaneous but not evoked thalamic neuronal hyperexcitability in neuropathic pain states. Spinal clonidine inhibits evoked and spontaneous firing, whereas reboxetine selectively inhibits evoked firing.

Pontine noradrenergic neurones form part of a descending inhibitory system that influences spinal nociceptive processing. Weak or absent descending inhibition is a common feature of chronic pain patients. We examined the extent to which the descending noradrenergic system is tonically active, how control of spinal neuronal excitability is integrated into thalamic relays within sensory-discriminative projection pathways, and how this inhibitory control is altered after nerve injury. In vivo electrophysiology was performed in anaesthetised spinal nerve–ligated (SNL) and sham-operated rats to record from wide dynamic range neurones in the ventral posterolateral thalamus (VPL). In sham rats, spinal block of α₂-adrenoceptors with atipamezole resulted in enhanced stimulus-evoked and spontaneous firing in the VPL, and produced conditioned place avoidance. However, in SNL rats, these conditioned avoidance behaviours were absent. Furthermore, inhibitory control of evoked neuronal responses was lost, but spinal atipamezole markedly increased spontaneous firing. Augmenting spinal noradrenergic tone in neuropathic rats with reboxetine, a selective noradrenergic reuptake inhibitor, modestly reinstated inhibitory control of evoked responses in the VPL but had no effect on spontaneous firing. By contrast, clonidine, an α₂ agonist, inhibited both evoked and spontaneous firing, and exhibited increased potency in SNL rats compared with sham controls. These data suggest descending noradrenergic inhibitory pathways are tonically active in sham rats. Moreover, in neuropathic states, descending inhibitory control is diminished, but not completely absent, and distinguishes between spontaneous and evoked neuronal activity. These observations may have implications for how analgesics targeting the noradrenergic system provide relief.

Effect of clonidine on the cutaneous silent period during spinal anesthesia

AIM

To investigate the effect of clonidine on the cutaneous silent period (CSP) during spinal anesthesia.

METHODS

A total of 67 adult patients were included in this randomized, prospective, single-center, double-blind trial. They did not have neurological disorders and were scheduled for inguinal hernia repair surgery. This trial was registered on ClinicalTrials.gov (NTC03121261). The patients were randomized into two groups with regards to the intrathecally administered solution: (1) 15 mg of 0.5% levobupivacaine with 50 µg of 0.015% clonidine, or (2) 15 mg of 0.5% levobupivacaine alone. There were 34 patients in the levobupivacaine-clonidine (LC) group and 33 patients in the levobupivacaine (L) group. CSP and its latency were measured four times: prior to the subarachnoid block (SAB), after motor block regression to the 0 level of the Bromage scale, with ongoing sensory blockade, and both 6 and 24 h after SAB.

RESULTS

Only data from 30 patients in each group were analyzed. There were no significant differences between the groups investigated preoperatively and after 24 h. The CSP of the L group at the time point when the Bromage scale was 0 was 44.8 ± 8.1 ms, while in the LC group it measured 40.2 ± 3.8 ms (P = 0.007). The latency in the L group at the time point when the Bromage scale was 0 was 130.3 ± 10.2 ms, and in the LC group it was 144.7 ± 8.3 ms (P < 0.001). The CSP of the L group after 6 h was 59.6 ± 9.8 ms, while in the LC group it was 44.5 ± 5.0 ms (P < 0.001). The latency in the L group after 6 h was 110.4 ± 10.6 ms, while in LC group it was 132.3 ± 9.7 ms (P < 0.001).

CONCLUSION

Intrathecal addition of clonidine to levobupivacaine for SAB in comparison with levobupivacaine alone results in a diminished inhibitory tonus and shortened CSP.

The effects of dexmedetomidine pretreatment on the pro- and anti-inflammation systems after spinal cord injury in rats

Excessive inflammatory responses play important roles in the aggravation of secondary damage to an injured spinal cord. Dexmedetomidine (DEX), a selective α2-adrenoceptor agonist, has recently been implied to be neuroprotective in clinical anesthesia, but the underlying mechanism is elusive. As signaling through Toll-like receptor 4 (TLR4) and nicotinic receptors (nAChRs, notably α7nAChR) play important roles in the pro- and anti-inflammation systems in the central nervous system, respectively, this study investigated whether and how they were modulated by DEX pretreatment in a rat model of spinal cord compression. The model was used to mimic perioperative compressive spinal cord injury (SCI) during spinal correction. DEX preconditioning improved locomotor scores after SCI, which was accompanied by increased α7nAChR and acetylcholine (Ach, an endogenous ligand of α7nAChR) expression as well as PI3K/Akt activation. However, there was a decrease in Ly6h (a negative regulator for α7nAChR trafficking), TLR4, PU.1 (a critical transcriptional regulator of TLR4), HMGB1 (an endogenous ligand of TLR4), and caspase 3-positive cells, which was prevented by intrathecal preconditioning with antagonists of either α2R, α7nAChR or PI3K/Akt. In addition, application of an α7nAChR agonist produced effects similar to those of DEX after SCI, while application of an α7nAChR antagonist reversed these effects. Furthermore, both α7nAChR and TLR4 were mainly co-expressed in NeuN-positive cells of the spinal ventral horn, but not in microglia or astrocytes after SCI. These findings imply that the α2R/PI3K/Akt/Ly6h and α7nAChR/PI3K/Akt/PU.1 cascades are required for upregulated α7nAChR and downregulated TLR4 expression by DEX pretreatment, respectively, which provided a unique insight into understanding DEX-mediated neuroprotection.

Preclinical assessment of pain: improving models in discovery research

To date, animal models have not sufficiently "filtered" targets for new analgesics, increasing the failure rate and cost of drug development. Preclinical assessment of "pain" has historically relied on measures of evoked behavioral responses to sensory stimuli in animals. Such measures can often be observed in decerebrated animals and therefore may not sufficiently capture affective and motivational aspects of pain, potentially diminishing translation from preclinical studies to the clinical setting. Further, evidence indicates that there are important mechanistic differences between evoked behavioral responses of hypersensitivity and ongoing pain, limiting evaluation of mechanisms that could mediate aspects of clinically relevant pain. The mechanisms underlying ongoing pain in preclinical models are currently being explored and may serve to inform decisions towards the transition from drug discovery to drug development for a given target.

Opioid and noradrenergic contributions of tapentadol in experimental neuropathic pain

Tapentadol is a dual action molecule with mu opioid agonist and norepinephrine (NE) reuptake blocking activity that has recently been introduced for the treatment of moderate to severe pain. The effects of intraperitoneal (i.p.) morphine (10mg/kg), tapentadol (10 or 30 mg/kg) or duloxetine (30 mg/kg), a norepinephrine/serotonin (NE/5HT) reuptake inhibitor, were evaluated in male, Sprague-Dawley rats with spinal nerve ligation (SNL) or sham surgery. Additionally, the effects of these drugs on spinal cerebrospinal fluid (CSF) NE levels were quantified. Response thresholds to von Frey filament stimulation decreased significantly from baseline in SNL, but not sham, operated rats. Duloxetine, tapentadol and morphine produced significant and time-related reversal of tactile hypersensitivity. Duloxetine significantly increased spinal CSF NE levels in both sham and SNL rats and no significant differences were observed in these groups. Tapentadol (10 mg/kg) produced a significant increase in spinal NE levels in SNL, but not in sham, rats. At the higher dose (30 mg/kg), tapentadol produced a significant increase in spinal CSF NE levels in both SNL and sham groups; however, spinal NE levels were elevated for an extended period in the SNL rats. This could be detected 30 min following tapentadol (30 mg/kg) in both sham and SNL groups. Surprisingly, while the dose of morphine studied reversed tactile hypersensitivity in nerve-injured rats, CSF NE levels were significantly reduced in both sham- and SNL rats. The data suggest that tapentadol elicits enhanced elevation in spinal NE levels in a model of experimental neuropathic pain offering a mechanistic correlate to observed clinical efficacy in this pain state.

Mast cell stabilization promotes antinociceptive effects in a mouse model of postoperative pain

BACKGROUND

Nerve injury and consequent inflammatory responses produced by surgical incision result in a complicated pain status which still affects half of all surgical patients. Therefore, it is essential for anesthesiologists to identify the mechanisms of postoperative pain. Mast cells are resident cells of connective tissue and the mucosa that participate in the immune response. Degranulation of mast cells is involved in the development of postoperative pain and can be induced by surgical incision. The aim of this study was to investigate whether stabilization of mast cells causes an antinociceptive effect in a mouse model of postoperative pain.

METHODS

Postoperative pain was induced by making an incision in the hind paw of BALB/c mice. The mast cell membrane stabilizer cromoglycate (200 μg/20 μL) was injected before incision of the paw, and postoperative pain responses were measured by assessing guarding behavior, withdrawal threshold to mechanical stimuli, and latency of heat pain behavior 1, 2, and 7 days after the incision.

RESULTS

The incision produced guarding pain, mechanical allodynia, and heat hypersensitivity. Cromoglycate decreased the guarding pain score (day 1) and the withdrawal threshold to mechanical stimuli (days 1, 2, and 7). However, the withdrawal latency to heat was not affected by cromoglycate treatment.

CONCLUSION

Cromoglycate significantly attenuated the pain response expressed as guarding pain and mechanical allodynia in a mouse model of postoperative pain. Thus, mast cell activation is likely a mechanism of postoperative pain and is an interesting target for the development of new therapies.

Antinociceptive effects of neurotropin in a rat model of central neuropathic pain: DSP-4 induced noradrenergic lesion

Neurotropin is a nonprotein extract isolated from inflamed skin of rabbits inoculated with vaccinia virus, and used for treatment of neuropathic pain. In the present study, we have determined whether neurotropin could exert antinociceptive action using the central neuropathic pain model that we recently established. Rats were randomly allocated to 3 groups: Sham group (n=20), DSP-4 [N-(-2-chloroethyl)-N-ethyl-2-bromobenzylamine] group (50mg/kg ip, n=18), and DSP-4+5,7-DHT [5,7-dihydroxytryptamine] group (ip DSP-4 50mg/kg+icv 5,7-DHT 200μg, n=18). In Sham, DSP-4 and DSP-4+5,7-DHT groups, the effects of ip neurotropin (100NU/Kg) on hot-plate latency in rats with no lesion, noradrenergic neuron depletion and both noradrenergic and serotonergic neuronal depletion were studied, respectively. Rats in each group were subdivided equally to 2 subgroups: saline and neurotropin. After completion of the hot-plate tests, each rat was decapitated, the cerebral cortex was dissected from its internal structure for measurement of norepinephrine contents. Hot-plate latency significantly decreased by ∼40% 10 days after ip DSP-4 or after ip DSP-4 and 5,7-DHT. Norepinephrine contents in DSP-4 treated rats (55.6±6.3ng/ng tissue) and DSP-4+5,7-DHT treated rats (35.3±6.3ng/ng tissue) were significantly lower than those in intact rats (131.6±5.7ng/ng tissue, p<0.01). Neurotropin significantly increased the area under the curve (AUC) of the hot-plate latency in the DSP-4 and DSP-4+5,7-DHT groups but not in the Sham group. There was a significant correlation between AUC and norepinephrine contents in saline subgroup (p<0.01, r=0.597) but not in neurotropin subgroup in DSP-4 group. Neurotropin exerted an antinociceptive effect in DSP-4 induced central neuropathic pain. The present data suggest neuronal pathways other than descending inhibitory noradrenergic and serotonergic systems may be involved in neurotropin mediated antinociception.

The antinociceptive and antihyperalgesic effect of tapentadol is partially retained in OPRM1 (μ-opioid receptor) knockout mice

Activation of the μ-opioid receptor (MOR) and noradrenaline reuptake inhibition (NRI) are well recognized as analgesic principles in acute and chronic pain indications. The novel analgesic tapentadol combines MOR agonism and NRI in a single molecule. The present study used OPRM1 (MOR) knockout (KO) mice to determine the relative contribution of MOR activation to tapentadol-induced analgesia in models of acute (nociceptive) and chronic (neuropathic) pain. Antinociceptive efficacy was inferred from paw withdrawal latencies on a 48 °C hot plate in naive animals. Antihyperalgesic efficacy was inferred from the number of nocifensive reactions in diabetic animals (streptozotocin-induced) and non-diabetic controls on a 50 °C hot plate. The effect of tapentadol (0.316-31.6 mg/kg IP) and the MOR agonist morphine (3-10 mg/kg IP) was determined in OPRM1 KO- and congenic wildtype mice. At baseline, diabetic OPRM1 KO mice showed reduced nocifensive reactions as compared to diabetic wildtype mice. In both pain models, morphine and tapentadol were effective in wildtype mice. In the KO mice, however, morphine failed to produce analgesia in either model. On the other hand, tapentadol still had clear effects, and when tested at a dose that was fully efficacious in wildtype mice, showed reduced but still significant antinociceptive efficacy in non-diabetic, and antihyperalgesic efficacy in diabetic OPRM1 KO mice. The remaining antinociceptive activity of tapentadol in OPRM1 KO mice was abolished by the α₂-adrenoceptor antagonist yohimbine. In OPRM1 wildtype mice, the antihyperalgesic effect of tapentadol was 10 times more potent in diabetic animals (ED₅₀=1.10 mg/kg) than its antinociceptive effect in naïve animals (ED₅₀=10.8 mg/kg). This study supports the conclusion that the analgesic effect of tapentadol is only partly due to the activation of MOR, both under acute and chronic pain conditions, and that the efficacy of tapentadol against acute and chronic pain is based on its combined mechanism of action.

Pharmacological profiles of alpha 2 adrenergic receptor agonists identified using genetically altered mice and isobolographic analysis

Endogenous, descending noradrenergic fibers impose analgesic control over spinal afferent circuitry mediating the rostrad transmission of pain signals. These fibers target alpha 2 adrenergic receptors (alpha(2)ARs) on both primary afferent terminals and secondary neurons, and their activation mediates substantial inhibitory control over this transmission, rivaling that of opioid receptors which share a similar pattern of distribution. The terminals of primary afferent nociceptive neurons and secondary spinal dorsal horn neurons express alpha(2A)AR and alpha(2C)AR subtypes, respectively. Spinal delivery of these agents serves to reduce their side effects, which are mediated largely at supraspinal sites, by concentrating the drugs at the spinal level. Targeting these spinal alpha(2)ARs with one of five selective therapeutic agonists, clonidine, dexmedetomidine, brimonidine, ST91 and moxonidine, produces significant antinociception that can work in concert with opioid agonists to yield synergistic antinociception. Application of several genetically altered mouse lines had facilitated identification of the primary receptor subtypes that likely mediate the antinociceptive effects of these agents. This review provides first an anatomical description of the localization of the three subtypes in the central nervous system, second a detailed account of the pharmacological history of each of the six primary agonists, and finally a comprehensive report of the specific interactions of other GPCR agonists with each of the six principal alpha(2)AR agonists featured.

Analgesia induced by perineural clonidine is enhanced in persistent neuritis

Alpha 2-adrenoceptors are concentrated near sites of peripheral nerve injury or inflammation, primarily on immune cells, and their activation reduces inflammation and hypersensitivity to tactile stimuli. These results were obtained during acute inflammation, but the efficacy of alpha2-adrenoceptor stimulation in persistent inflammation has not been tested. Here, we show that perineural injection of the alpha2-adrenoceptor agonist, clonidine, reduces hypersensitivity in persistent sciatic neuritis with an onset more rapid than acute neuritis. Perineural clonidine reduces microglial activation in the spinal cord in persistent, but not acute neuritis, and does not change the number of spinal neurons with phosphorylated transcription factor, cyclic adenosine monophosphate response element binding protein. These data support treatment strategies with alpha2-adrenoceptor agonists in persistent neuritis.

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.

Clonidine maintains intrathecal self-administration in rats following spinal nerve ligation

Clonidine is approved for spinal administration against neuropathic pain, and reverses both spontaneous and elicited pain in humans following spinal administration. Rodent studies that seek to model pharmacology in pain states have historically relied on reflexive withdrawal from noxious stimuli as the primary endpoint. Drug self-administration studies have face validity in the drug abuse field for modeling drug abuse in humans, however, this methodology has not been applied to address issues related to drug seeking behaviors that may be relevant for other human populations, such as patients with neuropathic pain. Rats without spinal nerve ligation (SNL) failed to acquire intrathecal clonidine self-administration over 10 days of access. Rats were found to self-administer intrathecal infusions of clonidine following SNL in a stable and dose-responsive manner, however, and clonidine was self-administered throughout the day with 66% of total drug intake occurring during the dark cycle. Substitution of clonidine with saline or with clonidine and the alpha2-adrenoceptor antagonist idazoxan resulted in extinction of responding in SNL animals. Food reinforcement was initially decreased in SNL rats self-administering clonidine compared to normal animals, however, tolerance developed to this effect of clonidine in SNL rats after 5 days. These data demonstrate that drug self-administration can be applied to questions other than drug abuse, and provides an additional measure for development of novel therapeutic strategies for chronic pain treatment.

Spinal nerve ligation increases alpha2-adrenergic receptor G-protein coupling in the spinal cord

Intrathecal and epidural administration of the alpha2-adrenergic receptor agonist clonidine in humans results in analgesia to both acute nociceptive and chronic neuropathic pain. The potency of clonidine increases with hypersensitivity to mechanical stimuli after nerve injury, although the reasons for this change are unknown. In the present study, we tested the hypothesis that peripheral nerve injury alters either spinal alpha2-adrenergic receptor-mediated G-protein activity or alpha2-adrenergic receptor number. Rats were randomized to left spinal nerve ligation (SNL) or sham surgery. Tactile hypersensitivity in the hindpaw was confirmed and lumbar spinal cords were removed for binding assays. To examine agonist-induced G-protein coupling, [35S]GTP gamma S binding experiments were performed in spinal cord membranes and sections using norepinephrine as an alpha2-adrenergic agonist. SNL was associated with an increase in maximal efficacy, but not potency, of norepinephrine-stimulated [35S]GTP gamma S binding in dorsal horn. SNL had no effect on basal [35S]GTP gamma S binding or on muscarinic cholinergic-stimulated [35S]GTP gamma S binding. [35S]GTP gamma S autoradiography showed that this increase in alpha2-adrenergic-activated G-proteins occurred both ipsilateral and contralateral to SNL surgery. SNL did not alter total alpha2-adrenergic receptor number or affinity to [3H]-rauwolscine binding, and displacement studies with the alpha2A-adrenergic antagonist BRL44408 revealed that most of the binding was associated with the alpha2A-adrenergic subtype. These data suggest that the increased potency of clonidine in neuropathic pain could reflect increased efficiency of G-protein coupling from spinal alpha2-adrenergic receptors.

Opioid effectiveness and side effects in chronic pain

Opioids can provide effective analgesia by way of different routes of administration without limiting side effects for most patients suffering from chronic pain when clinicians properly manage the pertinent patient-, pain-, and drug-centered characteristics. Randomized, placebo-controlled, prospective studies are needed to establish a causal relationship between opioids and hypogonadism. Many of the current studies are retrospective, which only lead to suggestive associations between opioids and hypogonadism and incorporate bias. Clinicians may incorporate available tools, including urine toxicology tests, to assess any aberrant behavior on the part of patients using opioids and to maximize compliance with an opioid regimen.

Effect of antisense knock-down of alpha(2a)- and alpha(2c)-adrenoceptors on the antinociceptive action of clonidine on trigeminal nociception in the rat

Although activation of alpha(2)-adrenoceptors is known to play an important role in mediating antinociception, the contribution of various alpha(2)-adrenoceptor subtypes in modulating trigeminal nociception remains unknown since subtype specific agonists and antagonists are not available. The present study investigated the functional role of alpha(2)-adrenoceptor subtypes in modulating the N-methyl-D-aspartate-induced nociceptive behavior in the medullary dorsal horn by using antisense oligodeoxynucleotides to selectively knock-down the receptor subtypes. Microinjection of N-methyl-D-aspartate (2 nmol in 10 microl) through a cannula implanted dorsal to the medullary dorsal horn produced a total of 164.9+/-8.8 scratches in the facial region (n=14), and the scratching behavior lasted for 77.8+/-5.2s (n=14). Microinjection of clonidine, an alpha(2)-agonist (7 microg in 5 microl), 15 min prior to administration of N-methyl-D-aspartate, produced a reduction of 71.6% (n=12) in the number of scratches and a reduction of 57.5% (n=12) in the duration. The inhibitory effect of clonidine was blocked by idazoxan (n=4) and yohimbine (n=4), alpha(2) antagonists. In rats pretreated with the antisense probe to the alpha(2A) adrenoceptor, clonidine only produced a reduction of 7.3% in the number of scratches (n=12) and a reduction of 9% in the duration (n=12). The antinociceptive effect of clonidine recovered completely 4 days after termination of the alpha(2A) antisense oligodeoxynucleotide treatment. In contrast to the alpha(2A) antisense-treated animals, clonidine reduced the number of scratches and the duration by 85.5% (n=9) and 82.1% (n=9), respectively, in rats pretreated with the sense probe to the alpha(2A) adrenoceptor. The effect of clonidine was not altered in rats pretreated with the antisense or the sense probes to the alpha(2C) adrenoceptor. In the alpha(2C) antisense pretreated rats, clonidine reduced the number of scratches and the duration by 60.8% (n=11) and 44.5 % (n=11), respectively. In the sense-pretreated rats, clonidine produced a reduction of 69.1% in the number of scratches (n=9) and a reduction of 55.1% in the duration (n=9). In order to assess the effectiveness of the antisense treatment, the receptor expression was examined by immunohistochemistry. Antisense treatment reduced alpha(2A) and alpha(2C) receptor immunoreactivity in the medullary dorsal horn compared to the sense and the vehicle-treated animals. Quantitative image analysis revealed a significant decrease in pixel intensity following the antisense treatment. These results indicate that activation of alpha(2A) adrenoceptor plays an important role in mediating the antinociceptive effect of clonidine in the medullary dorsal horn in the rat.

Opioid poorly-responsive cancer pain. Part 3. Clinical strategies to improve opioid responsiveness

Some pain syndromes may be difficult to treat due to a poor response to opioids. This situation demands a range of alternative measures, including the use of adjuvant drugs with independent effects, such as antidepressants, sodium channel-blocking agents, steroids and anti-inflammatory drugs (NSAIDs); drugs that reduce opioid side effects; and drugs that enhance analgesia produced by opioids, such as N-methyl-D-aspartate (NMDA) antagonists, calcium channel antagonists, and clonidine. Other approaches, including opioid trials, neural blockade when necessary, and psychological interventions, also may be useful.

The alpha(2A) adrenoceptor and the sympathetic postganglionic neuron contribute to the development of neuropathic heat hyperalgesia in mice

We have addressed the role of the sympathetic nervous system in the development and maintenance of neuropathic pain. Using a new neuropathic mouse model, we examined the development of hyperalgesia in transgenic mice lacking functional alpha(2A) adrenoceptors and in sympathectomized wild-type mice, to determine if sympathetic-sensory coupling generates hyperalgesia. The development of neuropathic heat hyperalgesia required the presence of both the alpha(2A) adrenoceptor and the sympathetic postganglionic neuron (SPGN), but the development of mechanical hyperalgesia did not require either the alpha(2A) adrenoceptor or the SPGN, indicating different mechanisms of sensitization. These results suggest that the development of neuropathic heat hyperalgesia, but not mechanical hyperalgesia, requires sympathetic-sensory coupling in the peripheral nervous system. Nerve injury enhanced the analgesic efficacy of the alpha(2) adrenoceptor agonist dexmedetomidine, and paradoxically also induced an analgesic response to alpha(2) adrenoceptor antagonists. The alpha(2) agonist-evoked analgesia to mechanical stimuli was mediated by activating central alpha(2A) adrenoceptors, possibly at the spinal level. The peripherally restricted alpha(2) antagonist L659,066 evoked analgesia for heat, but not for mechanical stimuli, findings which support the hypothesis that the peripheral alpha(2) adrenoceptor plays a role in both the development and the maintenance of neuropathic heat hyperalgesia. The alpha(2) antagonist-evoked analgesia for heat stimuli was mediated by blocking peripheral and probably central alpha(2) adrenoceptors, while the analgesia for mechanical stimuli was mediated by blocking central alpha(2A) adrenoceptors. Intradermal injections with an alpha(2) agonist or antagonist had no effect on nociceptive thresholds, indicating that sympathetic-sensory coupling at the level of the cutaneous nociceptor did not contribute to the maintenance of neuropathic hyperalgesia.

Correlation between autotomy-behavior and current theories of neuropathic pain

The past 10 years have brought several new experimental models with which to study chronic neuropathic pain in animals. Consequently, our knowledge about the mechanisms subserving neuropathic pain in humans has improved. However, the first animal model that was used for studying this type of chronic pain was the autotomy-model which can still be considered as a useful tool for pain studies. The present review assesses some of the similarities and differences between autotomy-model and more recent models of experimental traumatic mononeuropathy. In addition, it considers some of the similarities between the results obtained in clinical studies and in autotomy studies.

alpha2-adrenoceptors modulate NMDA-evoked responses of neurons in superficial and deeper dorsal horn of the medulla

Extracellular single unit recordings were made from neurons in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis) in 21 male rats anesthetized with urethan. NMDA produced an antagonist-reversible excitation of 46 nociceptive as well as nonnociceptive neurons. Microiontophoretic application of a preferential alpha2-adrenoceptor (alpha2AR) agonist, (2-[2, 6-dichloroaniline]-2-imidazoline) hydrochloride (clonidine), reduced the NMDA-evoked responses of 86% (6/7) of nociceptive-specific (NS) neurons, 82% (9/11) of wide dynamic range (WDR) neurons, and 67% (4/6) of low-threshold (LT) neurons in the superficial dorsal horn. In the deeper dorsal horn, clonidine inhibited the NMDA-evoked responses of 94% (16/17) of NS and WDR neurons and 60% (3/5) of LT neurons. Clonidine facilitated the NMDA-evoked responses in 14% (1/17) of NS, 9% (1/11) of WDR, and 33% (2/6) of LT neurons in the superficial dorsal horn. Idazoxan, an alpha2AR antagonist, reversed the inhibitory effect of clonidine in 90% (9/10) of neurons, whereas prazosin, an alpha1-adrenoceptor antagonist with affinity for alpha2BAR, and alpha2CAR, were ineffective. We suggest that activation of alpha2ARs produces a predominantly inhibitory modulation of the NMDA-evoked responses of nociceptive neurons in the medullary dorsal horn.

Peripherally administered alpha2-adrenoceptor agonist in the modulation of chronic allodynia induced by spinal nerve ligation in the rat

We studied whether a peripherally administered alpha2-adrenoceptor agonist modulates mechanical allodynia caused by unilateral ligation of two spinal nerves in the rat. Medetomidine, an alpha2-adrenoceptor agonist, atipamezole, an alpha2-adrenoceptor antagonist, or saline (control) was administered into the footpad of either the allodynic or the contralateral hindpaw. Medetomidine (1-10 microg/kg in 50 microL) reversed the unilateral allodynia in a dose-dependent fashion independent of the site of administration. At this dose range, medetomidine did not influence the heat-induced tail-flick response. The antiallodynic effect of medetomidine was completely reversed by a dose of atipamezole that alone was ineffective (30 microg/kg). At the largest dose used (100 microg/kg in 50 microL), atipamezole decreased the latency of the heat-induced tail flick and had an ipsilateral allodynic effect in the injected paw when administered into the control side. The atipamezole-induced mechanical allodynia was not attenuated by medetomidine. The results indicate that an alpha2-adrenoceptor agonist at a subantinociceptive dose may significantly attenuate allodynia produced by spinal nerve ligation. This antiallodynic effect is not due to a peripheral action, but rather to action on central (presumably spinal) alpha2-adrenoceptors. The allodynic effect of a high concentration of atipamezole (100 microg/kg in 50 microL) in the injected paw can be explained by peripheral nonadrenergic mechanisms (e.g., local irritation).

IMPLICATIONS

The present behavioral results indicate that a selective alpha2-adrenoceptor agonist at a subantinociceptive dose effectively attenuates mechanical allodynia induced by an experimental model of chronic neuropathy in the rat. This antiallodynic action can be explained by central, rather than peripheral, alpha2-adrenergic mechanisms.

Sympatholysis after neuron-specific, N-type, voltage-sensitive calcium channel blockade: first demonstration of N-channel function in humans

SNX-111 is the first neuronal N-type, voltage-sensitive calcium channel (VSCC) blocker to enter clinical drug development. Areas of potential therapeutic utility include treatment of nociceptive and neuropathic pain and neuroprotection after ischemic brain injury. The data presented demonstrate that SNX-111 is biologically active in humans and indicate for the first time a neurophysiologic function of N-type VSCCs in humans.

Effects of intrathecal vs. systemic clonidine in treating chronic allodynia-like response in spinally injured rats

A chronic pain-like response to innocuous mechanical stimuli (allodynia) was observed in rats after severe spinal cord ischemia, which resembled some painful conditions observed in spinally injured patients. The present studies examined the effects of clonidine, an alpha 2-adrenoceptor agonist, on this allodynia-like response. Intrathecal (i.t.) clonidine dose-dependently relieved allodynia and doses up to 10 micrograms did not induce motor deficits or sedation, but slightly increased systemic blood pressure. The anti-allodynic effect of i.t. clonidine was reversed by the selective alpha 2-adrenoceptor antagonist atipamezole. In contrast, 50 and 100 micrograms/kg intraperitoneal (i.p.) clonidine did not relieve the chronic allodynia, although the higher dose induced some motor deficits and sedation. Allodynic behavior was abolished after 200 micrograms/kg, i.p. clonidine, which, however, caused strong sedative and motor impairment. The present data suggested that spinal, but not systemic, alpha 2-adrenoceptor agonists may have therapeutic value in treating mechanical allodynia in patients with neuropathic pain of spinal origin.

Different roles of alpha 2-adrenoceptors of the medulla versus the spinal cord in modulation of mustard oil-induced central hyperalgesia in rats

We attempted to determine the roles of spinal versus medullary alpha 2-adrenoceptors in modulation of central hyperalgesia in rats. Central hyperalgesia was produced by applying mustard oil (50%) to the skin of the ankle of one hindpaw. The threshold for eliciting a hindlimb flexion reflex was determined by applying a series of calibrated monofilaments to the glabrous skin of the hindpaw contralaterally (= control) or ipsilaterally to the mustard oil-treated ankle (= outside the area of primary hyperalgesia). Medetomidine (an alpha 2-adrenoceptor agonist; 1 micrograms), atipamezole (an alpha 2-adrenoceptor antagonist; 2.5 micrograms) or saline was microinjected into the lateral reticular nucleus of the medulla, the nucleus raphe magnus, or intrathecally to the lumbar spinal cord 12 min before the mustard oil treatment. Following saline injections, mustard oil produced a significant decrease of the hindlimb withdrawal threshold in the mustard oil-treated limb but not in the contralateral limb. Atipamezole in the lateral reticular nucleus produced a complete reversal of the hyperalgesia but no effect on the threshold of the intact limb. However, atipamezole in the raphe magnus nucleus or in the lumbar spinal cord did not produce a significant attenuation of the hyperalgesia. Medetomidine in the spinal cord, but not in the lateral reticular nucleus, reversed the hyperalgesia. At this dose range (up to 3 micrograms), medetomidine in the spinal cord of nonhyperalgesic control rats did not produce any significant change in the withdrawal response of hindlimbs or in the tail-flick latency. The results indicate that neurogenic inflammation induces significant plastic changes in the function of alpha 2-adrenergic pain regulatory mechanisms. In rats with mustard oil-induced central hyperalgesia, an alpha 2-adrenoceptor antagonist produces an antihyperalgesic effect due to an action on the caudal ventrolateral medulla, whereas an alpha 2-adrenoceptor agonist produces an enhanced antinociceptive effect due to a direct action on the spinal cord.

[Antinociceptive effects of alpha(2)-adrenoceptor agonists ("analgesic" actions in animal experiments)agonists ("analgesic" actions in animal experiments).]

alpha(2)-Adrenoceptor agonists like clonidine, dexmedetomidine, and ST-91, inhibit nociceptive reflex activity predominantly by a spinal mode of action. They mimic the action of the inhibitory transmitter noradrenaline, which is released from the terminals of bulbospinal monoaminergic pathways. The inhibition by noradrenaline is due partly to hyperpolarization of the postsynaptic neuronal membrane; however, the selective antinociceptive effect of the alpha(2)-adrenoceptor agonists results from reduction of the release of the excitatory transmitters such as glutamate and substance P, blockade of the binding of substance P to spinal neurones, and enhancement of the action of the inhibitory transmitter, 5-hydroxytryptamine. Clonidine and dexmedetomidine stimulate adrenoceptors of the alpha(2A) subtype, while ST-91 stimulates alpha(2B) adrenoceptors. Antinociception is manifested not only by depression of nociceptive reflexes and behaviour, but also by inhibition of the expression of immediate early genes in dorsal horn neurones following noxious stimulation. The inhibitory control from the brain stem of spinal nociceptive activity can be triggered by alpha(2)-adrenoceptor agonists. Moreover, impulse conduction in C fibres of peripheral nerves is far more reduced by these compounds than that in A fibres. Antinociceptive effects are reported to occur in various models of clinical pain, e.g. the formalin test, adjuvans-induced arthritis, autotomy following deafferentation, and "hyperalgesia" after nerve ligation. Therefore, the mechanisms involved in antinociception may also be responsible for the analgesia produced by alpha(2)-adrenoceptor agonists.

Systemic clonidine differentially modulates the abnormal reactions to mechanical and thermal stimuli in rats with peripheral mononeuropathy

The antinociceptive action of the systemically administered alpha 2-adrenoceptor agonist clonidine was evaluated in a rat model of peripheral unilateral mononeuropathy produced by loose ligatures around the common sciatic nerve, using nociceptive tests based on mechanical (vocalization threshold to paw pressure) or thermal (struggle latency to paw immersion in a cold (10 degrees C) or hot (44 degrees C) water bath) stimuli. Experiments were performed 2-3 weeks after surgery when pain-related behavior was fully developed. We demonstrated a dissociative action depending on the test used: clonidine (30-100 micrograms/kg i.v.) had a moderate effect on the abnormal reactions to the mechanical stimulus. By contrast it dramatically increased the struggle latency to hot or cold stimuli. These latter effects were completely prevented by prior administration of the alpha 2-adrenoceptor antagonist idazoxan (0.5 mg/kg i.v.).

The spinal analgesic role of alpha 2-adrenoceptor subtypes in rats after peripheral nerve section

Two putative agonists of subtypes of alpha 2-adrenoceptors, guanfacine (alpha 2A) and ST-91 ([2,6-dicthylphenylamino]-2-imidazoline, alpha 2C), were applied intrathecally and their effects on autotomy behaviour and on the flexor reflex before and after sciatic nerve section were examined. Neither drug influenced autotomy during a 17-day observation period. Both drugs dose dependently depressed the flexor reflex in rats with intact sciatic nerves. After axotomy, the sensitivity of the flexor reflex to guanfacine and ST-91 was moderately increased compared to normals. ST-91 i.t. at high doses evoked motor discharges, an effect which was reversed by the alpha 1-adrenoceptor antagonist, WB4101 (2-[2,6-dimethyoxyphenoxyethyl]-aminomethyl-1,4-benzodioxane). Thus, the effect of i.t. clonidine on the flexor reflex and autotomy behaviour observed previously may not involve its action on alpha 2A- and alpha 2C-adrenoceptors. Furthermore, due to its motor effect which may involve activation of alpha 1-adrenoceptors, ST-91 may not be a suitable tool to study the physiological function of spinal alpha 2C-adrenoceptors.

The effects of intrathecal morphine and clonidine on the prevention and reversal of spinal cord hyperexcitability following sciatic nerve section in the rat

We have previously shown that intrathecal (i.t.) morphine, but not the alpha 2-adrenoreceptor agonist clonidine, administered prior to sciatic nerve section, reduced the level of autotomy in rats, which is a behavioural model of neuropathic pain. Neither drug was effective when administered 15 min after nerve section. We now examined the effects of i.t. morphine and clonidine on the development of flexor reflex hyperexcitability following sciatic nerve section in acute physiological experiments. The flexor reflex was recorded from the hamstring muscles in decerebrate, spinalized, unanesthetized rats. The effect of sciatic nerve section on the flexor reflex without drugs was compared with axotomy performed 60 min after i.t. injection of 3 micrograms or 30 micrograms morphine, as well as 50 micrograms clonidine. The effect of these drugs on reversing reflex hyperexcitability was also examined. Both doses of morphine administered prior to sciatic nerve section profoundly depressed the baseline reflex and the higher dose almost completely abolished reflex hyperexcitability following nerve section. In contrast, clonidine pre-administration was less effective in depressing the baseline reflex and blocked reflex hyperexcitability less than morphine. Both morphine and clonidine administered 15 min after nerve section reversed spinal hyperexcitability. Thus, the ability of morphine to prevent the occurrence of autotomy may be related to its effectiveness in blocking axotomy-induced hyperexcitability. These physiological data suggest that even a short period of spinal cord hyperexcitability following nerve injury may lead to the development of neuropathic pain.

Neither cholecystokinin nor galanin modulate intrathecal clonidine-induced depression of the nociceptive flexor reflex in the rat

Previous studies have established that the antinociceptive effect of morphine was subjected to peptidergic modulation at spinal level. Intrathecal (i.t.) galanin (GAL) potentiated morphine-induced analgesia, whereas i.t. cholecystokinin (CCK) antagonized morphine's hypoalgesic effect. In the present study, we examined the possible interaction between GAL, CCK and clonidine, an alpha 2 adrenoceptor agonist and potent spinal antinociceptive agent, in the spinal nociceptive flexor reflex. I.t. clonidine dose-dependently depressed the flexor reflex similarly to i.t. morphine. However, unlike morphine, the reflex depressive effect of i.t. clonidine was neither potentiated by i.t. GAL nor blocked by i.t. CCK. The present results suggested that the analgesia elicited by activation of spinal alpha 2 adrenoceptors is not subjected to the modulatory effect of CCK and GAL and therefore may be mediated through different mechanisms than opioids.

The effect of intrathecal guanfacine and clonidine on the flexor reflex in rats with intact and sectioned sciatic nerves

We examined the effect of intrathecal (i.t.) administration of the non-selective alpha 2-adrenoceptor agonist clonidine and the alpha 2A-adrenoceptor selective agonist guanfacine on flexor reflex excitability in decerebrate, spinalized, unanesthetized rats before and after unilateral section of the sciatic nerve. Both guanfacine and clonidine dose dependently depressed the flexor reflex in rats with intact nerves. There was a dramatic increase in the sensitivity of the flexor reflex to the depressive effect of i.t. clonidine 4 to 18 days after sciatic nerve section. In contrast, the sensitivity to i.t. guanfacine increased only slightly. The present results suggest that the markedly increased sensitivity to the analgesic effect of the non-selective alpha 2-adrenoceptor agonist does not involve up-regulation of alpha 2A-adrenoceptors. Since radioligand binding data suggested that about 4% of spinal cord alpha 2-adrenoceptors are of the alpha 2C type, whereas alpha 2B-adrenoceptors are not detectable, alpha 2C-adrenoceptors seem to be a good candidate to mediate the up-regulated response to clonidine in axotomized rats. Thus, alpha 2C-adrenoceptor agonists may be useful in the treatment of neuropathic pain.

Low-dose intrathecal clonidine releases tachykinins in rat spinal cord

In decerebrate, spinalized, unanesthetized rats, 37 pmol (10 ng) intrathecally injected clonidine facilitated the flexor reflex. This effect was blocked by the specific tachykinin antagonists CP-96,345, which acts at the NK1 receptor, and Men 10207, which acts at the NK2 receptor. Thus, low-dose intrathecal clonidine releases the tachykinins substance P and neurokinin A in the spinal cord.