The antinociceptive effect of the mu-opioid fentanyl is reduced in the presence of the alpha(2)-adrenergic antagonist idazoxan in inflammation

The clinical and laboratorial evaluation of transdermal ketamine, fentanyl, clonidine or their combination in chronic low back pain

OBJECTIVES: chronic low back pain may result in central sensitization, with involvement of different receptors. The aim of this study was to evaluate the analgesic action of transdermal (T) ketamine (a NMDA antagonist), clonidine (an α2-agonist), fentanyl (an opioid agonist), or their combination in chronic low back pain. METHODS: after the institutional approval and informed consent signature, 54 patients were prospectively randomized into 6 groups. Each patient had two of the T preparations applied in different arms. The effect of either T ketamine (1 mg/h), T clonidine (25 µg/h) or T fentanyl (25 µg/h), combined with T placebo (CloG, KetG and FenG); or the combination of T ketamine and clonidine (Ket-CloG), T fentanyl and ketamine (Fen-KetG), or T fentanyl and clonidine (Fen-CloG) was searched for pain and adverse effects. Pain was evaluated by: 1) VAS pain scores, and 2) noradrenaline plasma levels at 0-h (just prior to T application), 3- and 6-h after the T application of two medications, by HPLC. RESULTS: clinically, the pain VAS score at 6-h was smaller in comparison to the 0-h in all groups (p<0.02), and lower when compared to the Fen-CloG and Fen-KetG at the 6-h in relation to the administration of each correspondent T drug alone (p<0.05). The laboratorial data revealed that administration of T fentanyl alone (FenG) resulted in plasma noradrenaline decrease at 6-h (p<0.01), while the association of T fentanyl with clonidine resulted in plasma noradrenaline decrease at 3- and 6-h as compared to the others (p<0.01). The combination of both T ketamine and clonidine (Ket-CloG) did not result in a better analgesic profile and resulted in excessive sedation during the evaluation (p<0.02). CONCLUSIONS: all the studied drugs resulted in clinical analgesia (VAS) at 6-h. However, T fentanyl analgesia was corroborated by lower plasma noradrenaline levels at 6-h when applied alone or at 3-h when combined with T clonidine.

The antinociceptive effect of systemic gabapentin is related to the type of sensitization-induced hyperalgesia

Background

Gabapentin is a structural analogue of gamma-aminobutyric acid with strong anticonvulsant and analgesic activities. Important discrepancies are observed on the effectiveness and potency of gabapentin in acute nociception and sensitization due to inflammation and neuropathy. There is also some controversy in the literature on whether gabapentin is only active in central areas of the nervous system or is also effective in the periphery. This is probably due to the use of different experimental models, routes of administration and types of sensitization. The aim of the present study was to investigate the influence of the spinal cord sensitization on the antinociceptive activity of gabapentin in the absence and in the presence of monoarthritis and neuropathy, using the same experimental protocol of stimulation and the same technique of evaluation of antinociception.

Methods

We studied the antinociceptive effects of iv. gabapentin in spinal cord neuronal responses from adult male Wistar rats using the recording of single motor units technique. Gabapentin was studied in the absence and in the presence of sensitization due to arthritis and neuropathy, combining noxious mechanical and repetitive electrical stimulation (wind-up).

Results

The experiments showed that gabapentin was effective in arthritic (max. effect of 41 ± 15% of control and ID50 of 1,145 ± 14 micromol/kg; 200 mg/kg) and neuropathic rats (max. effect of 20 ± 8% of control and ID50 of 414 ± 27 micromol/kg; 73 mg/kg) but not in normal rats. The phenomenon of wind-up was dose-dependently reduced by gabapentin in neuropathy but not in normal and arthritic rats.

Conclusion

We conclude that systemic gabapentin is a potent and effective antinociceptive agent in sensitization caused by arthritis and neuropathy but not in the absence of sensitization. The potency of the antinociception was directly related to the intensity of sensitization in the present experimental conditions. The effect is mainly located in central areas in neuropathy since wind-up was significantly reduced, however, an action on inflammation-induced sensitized nociceptors is also likely.

Evidence for adenosine- and serotonin-mediated antihyperalgesic effects of cizolirtine in rats suffering from diabetic neuropathy

Cizolirtine is a novel non-opioid drug which demonstrated antinociceptive activity in numerous pain models in rodents. Yet, its mechanism of action remains unknown. Several lines of evidence support the idea that adenosine (ADO) and serotonin (5-HT) modulate nociceptive signaling. Our study aimed at investigating whether these neuroactive molecules could be implicated in the mechanism of action of cizolirtine. Cizolirtine-induced antihyperalgesia was compared before and after pretreatment with ADO A(1)-A(2A) and 5-HT(1B/1D) receptor ligands in rats rendered diabetic by streptozotocin pretreatment and suffering from neuropathic pain. Cizolirtine alone (30-80 mg/kg, i.p.) significantly increased mechanical nociceptive thresholds. Acute pretreatment with the A(1)-A(2A) receptor antagonist caffeine (5 mg/kg, i.p.) or the 5-HT(1B/1D) receptor antagonist GR-127,935 (3 mg/kg, i.p.) significantly reduced the antihyperalgesic effects of cizolirtine. Conversely, cizolirtine-induced antihyperalgesia was promoted by pretreatment with either the selective A(1) receptor agonist CPA (0.3 mg/kg, i.p.) or the selective 5-HT(1B) receptor agonist CP-94,253 (3mg/kg, i.p.), and this potentiation was totally prevented by acute pretreatment with respective antagonists. Interestingly, A(1) receptor blockade by DPCPX inhibited the promoting effect of CP-94,253 on cizolirtine-induced antihyperalgesia, suggesting that the adenosine A(1)-mediated step takes place downstream the serotonin 5-HT(1B)-mediated step in the neurobiological mechanisms underlying cizolirtine action.

The analgesic efficacy of partial opioid agonists is increased in mice with targeted inactivation of the alpha2A-adrenoceptor gene

Alpha(2A)-Adrenoceptors mediate the antinociceptive effects of alpha(2)-adrenoceptor agonists in mice, and analgesic synergism between noradrenergic and opioidergic mechanisms has been reported to be lacking in mice devoid of functional alpha(2A)-adrenoceptors. We investigated whether the antinociceptive actions of opioid agonists with different efficacy would be altered in mice with targeted inactivation of the alpha(2A)-adrenoceptor gene (alpha(2A)-KO mice). The antinociceptive effects of fentanyl, morphine, buprenorphine and tramadol were assessed using conventional tail-flick and hot-plate assays. Antinociceptive responses to fentanyl were unaltered in the alpha(2A)-KO animals. Morphine analgesia was slightly accentuated in the tail-flick test. The naloxone-sensitive antinociceptive responses to both tested weak partial agonists were very markedly accentuated in both tests. For example, after 40 mg/kg tramadol administration, the tramadol-induced prolongation of tail-flick latency was 86+/-6% of the maximal possible effect (MPE) in alpha(2A)-KO and 22+/-2% of MPE in control mice; prolongation of hot-plate latency was 93+/-5% of MPE in alpha(2A)-KO mice and 8+/-2% of MPE in the controls (P<0.001 for both). The effects of alpha(2A)-KO were mimicked by pretreatment of wild-type control mice with the alpha(2)-adrenoceptor antagonists, atipamezole and yohimbine; the apparent efficacy of tramadol and buprenorphine now approached that of morphine and fentanyl. Other behavioural effects of the tested opioid agonists were not similarly influenced by alpha(2A)-KO. Antagonists of alpha(2A)-adrenoceptors may offer a novel mechanism to augment the antinociceptive actions of partial opioid agonists.

The antinociceptive effects of the systemic adenosine A1 receptor agonist CPA in the absence and in the presence of spinal cord sensitization

Adenosine A1 receptor agonists are effective antinociceptive agents in neuropathic and inflammatory pain, though they appear to be weak analgesics in acute nociception. Important discrepancies are observed on the effectiveness and potency of adenosine analogues when comparing different studies, probably due to the use of different ligands, models of antinociception, routes of administration and types of sensitization. We studied the systemic antinociceptive effects of the adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA) in spinal cord neuronal responses from adult male rats in acute nociception and in sensitization due to arthritis and neuropathy. The experiments showed that CPA was effective in the three experimental conditions, with a similar potency in reducing responses to noxious mechanical stimulation (ID50s: 20 +/- 1.2 microg/kg in acute nociception, 18 +/- 1.1 microg/kg in arthritis, 17.4 +/- 2 microg/kg in neuropathy). The phenomenon of wind-up was also dose-dependently reduced by CPA in the three experimental situations although the main action was seen in arthritis. Depression of blood pressure by CPA was not dose-dependent. We conclude that systemic CPA is a potent and effective analgesic in sensitization due to arthritis and neuropathy but also in acute nociception. The effect is independent of the cardiovascular activity and is centrally mediated since wind-up was inhibited.

Effects of opioid receptor and alpha2-adrenoceptor agonists on slow ventral root potentials and on capsaicin and formalin tests in neonatal rats

The inhibitory effects of morphine and alpha2-adrenoceptor agonists on slow ventral root potentials (slow VRP) following ipsilateral dorsal root stimulation in neonatal rat spinal cord were compared with the analgesic effects of these drugs on formalin and capsaicin tests in neonatal rats. Morphine, (D-Phe2, D-Pen5)-enkephalin (DPDPE), dexmedetomidine, clonidine and xylazine showed concentration-related inhibition of slow VRP. The order of potency was dexmedetomidine>morphine=DPDPE>clonidine>xylazine. The inhibitory effects of opioid agonists and alpha2-adrenoceptor agonists were abolished by naloxone, an opioid antagonist, and atipamezole, an alpha2-adrenoceptor antagonist, respectively. There was no cross antagonism. Morphine, dexmedetomidine and xylazine dose-dependently inhibited body movement induced by formalin or capsaicin. The order of potency was dexmedetomidine>morphine>xylazine. Although morphine and dexmedetomidine inhibited formalin- and capsaicin-induced body movement in the same dose range, xylazine inhibited formalin-induced body movement at lower concentrations than capsaicin-induced one. The inhibitory potency for slow VRP by these drugs seems to be correlated with that for capsaicin-induced body movement but not that for formalin-induced one. Dexmedetomidine and morphine in combination inhibited slow VRP and body movement induced by capsaicin in an additive manner. It is suggested that the antinociceptive effects of dexmedetomidine and morphine but not xylazine on the capsaicin test are mainly due to spinal effects and that there is no synergistic interaction between dexmedetomidine and morphine in the neonatal rat.

Enhancement of fentanyl antinociception by subeffective doses of nitroparacetamol (NCX-701) in acute nociception and in carrageenan-induced monoarthritis

We have reported that subanalgesic doses of new generation non-steroidal anti-inflammatory drugs (NSAIDs) enhance the antinociceptive activity of the mu-opiate fentanyl, and the duration of its effect, in acute nociception. Since this therapy is intended for situations of hyperalgesia, we have compared the antinociceptive activity of fentanyl in the absence and in the presence of subeffective doses of NCX-701 (nitroparacetamol) in normal animals and in animals with carrageenan-induced monoarthritis. Subanalgesic dose of NCX-701 did not modify any of the nociceptive responses on its own but reduced the ID50 of fentanyl more than two-fold in both the normal and sensitized states. When administered alone, full recovery from fentanyl was always observed within 15 to 20 minutes, however, full recovery was not observed in the presence of NCX-701. Naloxone was unable to reverse the effect, suggesting a possible reduction of other opiate-mediated secondary effects. We therefore studied the possibility that combining administration of fentanyl and nitroparacetamol (NCX-701) would reduce the development of acute tolerance to fentanyl in behavioral experiments. Acute tolerance to fentanyl in behavioral nociceptive reflexes was developed within 72 h after the constant infusion of the drug, whereas in animals treated with small doses of NCX-701 tolerance was not observed. In summary, our results, both in normal animals and in animals with hyperalgesia, show that fentanyl antinociception can be strongly potentiated with subanalgesic doses of the NSAID NCX-701 and that the development of acute tolerance to fentanyl in normal animals is prevented by this combination of drugs.

Low doses of nitroparacetamol or dexketoprofen trometamol enhance fentanyl antinociceptive activity

We have reported that subeffective doses of the nonsteroidal anti-inflammatory drug (NSAID) dexketoprofen trometamol enhances micro-opioid receptor agonist fentanyl antinociception. The aim of this study was to assess if this effect can also be observed with other new cyclooxygenase-inhibitors such as nitroparacetamol, and in responses to high intensity electrical stimulation (wind-up). Single motor units were recorded in male Wistar rats under alpha-chloralose anaesthesia. The antinociceptive effect of fentanyl was studied alone and in the presence of subeffective doses of dexketoprofen trometamol or nitroparacetamol. In responses to noxious mechanical stimulation, the potency of fentanyl was enhanced by more than threefold in the presence of the NSAIDs and no significant recovery was observed after 45 min. The opioid antagonist naloxone and the alpha(2)-adrenoceptor antagonist atipamezol did not reverse the effect. The enhancement of the effect of fentanyl in wind-up was lower though significant. We conclude that the co-administration of subeffective doses of new cyclooxygenase-inhibitors and the micro-opioid receptor agonist fentanyl should be considered as a potential pain therapy.

The alpha2A-adrenoceptor subtype is not involved in inflammatory hyperalgesia or morphine-induced antinociception

The purpose of the present study was to investigate the role of the alpha(2A)-adrenoceptor subtype in inflammatory hyperalgesia, and in adrenergic-mu-opioid interactions in acute pain and inflammatory hyperalgesia. Behavioral responses to mechanical and thermal stimuli were studied in alpha(2A)-adrenoceptor knockout mice and their wild-type controls. Thermal nociception was evaluated as paw withdrawal latencies to radiant heat applied to the hindpaws. Mechanical nociception was measured using von Frey monofilament applications to the hindpaws. Mechanical and thermal hyperalgesia, induced with intraplantar carrageenan (1 mg/40 microl) were compared in alpha(2A)-adrenoceptor knockout and wild-type mice. The effects of the systemically administered mu-opioid receptor agonist morphine (1-10 mg/kg) were evaluated on mechanical withdrawal responses under normal and inflammatory conditions in knockout and wild-type mice. Withdrawal responses to radiant heat and von Frey monofilaments were similar in alpha(2A)-adrenoceptor knockout and wild-type mice before and after the carrageenan-induced hindpaw inflammation. Also, the antinociceptive effects of morphine in mechanical nociceptive tests were similar before and after carrageenan-induced hindpaw inflammation. Our observations indicate that alpha(2A)-adrenoceptors are not tonically involved in the modulation of inflammation-induced mechanical and thermal hyperalgesia. In addition, alpha(2A)-adrenoceptors do not appear to play an important role in mu-opioid receptor-mediated antinociception or antihyperalgesia.

RX 821002 as a tool for physiological investigation of alpha(2)-adrenoceptors

RX 821002 is the 2-methoxy congener of idazoxan. In binding and tissue studies it behaves as a selective antagonist of alpha(2)-adrenoceptors, with at least 5 times greater affinity for these receptors than any other binding site. It does not select between the different types of alpha(2)-receptor. Although this drug probably has no future as a therapeutic agent, it remains a good probe for physiological activity at alpha(2)-adrenoceptors in animal experiments. A particularly useful feature of this compound is its lack of binding at I(1) and I(2) imidazoline receptors. However, it has relatively high affinity for 5-HT(1A) receptors (at which it acts as an antagonist) and a tendency to behave as an inverse agonist at alpha(2A)-adrenoceptors in some cell culture systems. These potential drawbacks may be overcome by careful design of experiments, and the greater selectivity of RX 821002 renders it much superior to yohimbine or idazoxan as a tool for probing physiological actions at alpha(2)-receptors. It can be compared favorably with other selective antagonists such as atipamezole. In physiological studies, RX 821002 augments norepinephrine release in the frontal cortex and increases drinking behavior in rat. In rabbit, intrathecal administration of this drug enhances somatic and autonomic motor outflows, showing that tonic adrenergic descending inhibition of withdrawal reflexes and sympathetic pre-ganglionic neurons is strong in this species. The potentiation of reflexes may be considered a pro-nociceptive action. In the same model, RX 821002 antagonizes the inhibitory effects of the mu opioid fentanyl, indicating that exogenous opioids synergize with endogenously released norepinephrine in the spinal cord. Thus, the careful use of RX 821002 has revealed several aspects of the physiological activity of alpha(2)-adrenoceptors in rabbit spinal cord and rat brain. We recommend that RX 821002 and/or compounds with similar selectivity for alpha(2)-adrenoceptors (atipamezole, MK-912, RS-79948) should be used in preference to yohimbine or idazoxan in all future studies of this type.

Subeffective doses of dexketoprofen trometamol enhance the potency and duration of fentanyl antinociception

The combination of classic non-steroidal antiinflammatory drugs (NSAIDs) with opiates induces more analgesia than the summed effect of each drug given separately. No studies have been performed using new generation NSAIDs and fentanyl nor on the duration of this effect. We have studied the analgesic effect of fentanyl alone and after the administration of subeffective doses of dexketoprofen trometamol in rat nociceptive responses. The responses were evoked by noxious mechanical stimulation and were recorded as single motor units in male Wistar rats anaesthetized with alpha-chloralose. The effective dose 50 (ED(50)) observed with fentanyl was 22.4 +/- 1.5 microg kg(-1) and full recovery was apparent 20 min later. The administration of a total dose of 40 microg kg(-1) of dexketoprofen trometamol did not induce any significant effect on the nociceptive responses. In the presence of dexketoprofen trometamol, the ED(50) for fentanyl was 5 fold lower than before: 3.8 +/- 1.1 microg kg(-1) and no significant recovery was observed 45 min later. The opioid antagonist naloxone (200 microg kg(-1)) did not reverse the effect, although in control experiments the same dose was able to prevent any action of fentanyl given alone. We conclude that the combination of fentanyl and subeffective doses of dexketoprofen trometamol induces a more potent and longer lasting analgesic effect than that observed with fentanyl alone, and that this is not an opioid mediated action.

The Meissner corpuscle revised: a multiafferented mechanoreceptor with nociceptor immunochemical properties

Meissner corpuscles (MCs) in the glabrous skin of monkey digits have at least three types of innervation as revealed by immunofluorescence. The previously well known Aalphabeta-fiber terminals are closely intertwined with endings from peptidergic C-fibers. These intertwined endings are segregated into zones that alternate with zones containing a third type of ending supplied by nonpeptidergic C-fibers. Although MCs are widely regarded as low-threshold mechanoreceptors, all three types of innervation express immunochemical properties associated with nociception. The peptidergic C-fiber endings have readily detectable levels of immunoreactivity (IR) for calcitonin gene-related peptide (CGRP) and substance P (SP). The Aalphabeta endings have relatively lower levels of IR for CGRP and SP as well as the SP neurokinin 1 receptor and vanilloid-like receptor 1. Both the Aalphabeta and peptidergic C-fiber endings were also labeled with antibodies for different combinations of adrenergic, opioid, and purinergic receptors. The nonpeptidergic C-fiber endings express IR for vanilloid receptor 1, which has also been implicated in nociception. Thus, MCs are multiafferented receptor organs that may have nociceptive capabilities in addition to being low-threshold mechanoreceptors.

Endogenous adrenergic control of reflexes evoked by mechanical stimulation of the heel in the decerebrated rabbit

In decerebrated rabbits, reflexes were evoked in medial gastrocnemius motoneurones by mechanical stimulation of the heel, using four pinch strengths from 183 (innocuous) to 4577 (noxious) mN. The alpha(2)-adrenoceptor antagonist idazoxan (1-156 microg intrathecal (i.th.) significantly increased responses to pinch strengths of 607 mN and above. Subsequent administration of the alpha(1) adrenoceptor selective antagonist prazosin (200 microg i.th.) decreased reflexes to 4577 mN pinches but had no other significant effects. The opioid antagonist (-)-quadazocine (25 microg i.th.) caused no further changes in reflexes. Spinal section at L1 in the presence of this drug combination enhanced gastrocnemius responses to 183 and 607 mN stimuli, had no effect on reflexes to 1866 mN and significantly decreased responses to 4577 mN pinches. These data confirm that reflexes evoked by 'natural' stimulation of heel mechanoreceptors are subject to powerful tonic descending inhibition mediated by alpha(2)-adrenoceptors. For the highest strength stimulus, the results of alpha(2) blockade involved enabling of descending facilitation as well as reduction of descending inhibition.

Effects of yohimbine on the antinociceptive and place conditioning effects of opioid agonists in rodents

1. The pharmacological modulation of opioid actions by drugs acting on heterologous mechanisms could be useful to overcome some of the main problems associated with the use of opiate agonists. Based on previous findings on the interactions between yohimbine and opioid drugs, we have further studied the effects of yohimbine on the antinociceptive and positive-negative reinforcing effects of morphine (mu opioid receptor-preferring agonist), U-50,488 (kappa agonist) and SNC80 (delta agonist). 2. Pretreatment with yohimbine completely blocked the antinociception provided by the three opioid agonists in the mouse tail-immersion test. 3. A similar blockade of SNC80 and U-50,488-induced antinociception was observed with yohimbine in the mouse hot plate test at the same doses. In this paradigm, the effect of the kappa agonist was very slight and the actions of yohimbine rather variable. 4. In place conditioning experiments with SD (Sprague -- Dawley) male rats, yohimbine alone was inactive but it limited the preference induced by morphine and SNC80 and the aversive effect of U-50,488. Impaired novelty preference was also observed with the combination of yohimbine and U-50,488. 5. It is concluded that yohimbine tends to limit opioid antinociception and the addictive potential of mu and delta opioid agonists. More selective drugs could help to understand the mechanisms involved in these actions.