Antinociceptive activity of clonidine and its potentiation of morphine analgesia

Interactive Effects of µ-Opioid and Adrenergic-α 2 Receptor Agonists in Rats: Pharmacological Investigation of the Primary Kratom Alkaloid Mitragynine and Its Metabolite 7-Hydroxymitragynine

The primary kratom alkaloid mitragynine is proposed to act through multiple mechanisms, including actions at µ-opioid receptors (MORs) and adrenergic-α 2 receptors (Aα 2Rs), as well as conversion in vivo to a MOR agonist metabolite (i.e., 7-hydroxymitragynine). Aα 2R and MOR agonists can produce antinociceptive synergism. Here, contributions of both receptors to produce mitragynine-related effects were assessed by measuring receptor binding in cell membranes and, in rats, pharmacological behavioral effect antagonism studies. Mitragynine displayed binding affinity at both receptors, whereas 7-hydroxymitragynine only displayed MOR binding affinity. Compounds were tested for their capacity to decrease food-maintained responding and rectal temperature and to produce antinociception in a hotplate test. Prototypical MOR agonists and 7-hydroxymitragynine, but not mitragynine, produced antinociception. MOR agonist and 7-hydroxymitragynine rate-deceasing and antinociceptive effects were antagonized by the opioid antagonist naltrexone but not by the Aα 2R antagonist yohimbine. Hypothermia only resulted from reference Aα 2R agonists. The rate-deceasing and hypothermic effects of reference Aα 2R agonists were antagonized by yohimbine but not naltrexone. Neither naltrexone nor yohimbine antagonized the rate-decreasing effects of mitragynine. Mitragynine and 7-hydroxymitragynine increased the potency of the antinociceptive effects of Aα 2R but not MOR reference agonists. Only mitragynine produced hypothermic effects. Isobolographic analyses for the rate-decreasing effects of the reference Aα 2R and MOR agonists were also conducted. These results suggest mitragynine and 7-hydroxymitragynine may produce antinociceptive synergism with Aα 2R and MOR agonists. When combined with Aα 2R agonists, mitragynine could also produce hypothermic synergism. SIGNIFICANCE STATEMENT: Mitragynine is proposed to target the µ-opioid receptor (MOR) and adrenergic-α2 receptor (Aα2R) and to produce behavioral effects through conversion to its MOR agonist metabolite 7-hydroxymitragynine. Isobolographic analyses indicated supra-additivity in some dose ratio combinations. This study suggests mitragynine and 7-hydroxymitragynine may produce antinociceptive synergism with Aα2R and MOR agonists. When combined with Aα2R agonists, mitragynine could also produce hypothermic synergism.

Novel Approaches, Drug Candidates, and Targets in Pain Drug Discovery

Because of the problems associated with opioids, drug discovery efforts have been employed to develop opioids with reduced side effects using approaches such as biased opioid agonism, multifunctional opioids, and allosteric modulation of opioid receptors. Receptor targets such as adrenergic, cannabinoid, P2X3 and P2X7, NMDA, serotonin, and sigma, as well as ion channels like the voltage-gated sodium channels Nav1.7 and Nav1.8 have been targeted to develop novel analgesics. Several enzymes, such as soluble epoxide hydrolase, sepiapterin reductase, and MAGL/FAAH, have also been targeted to develop novel analgesics. In this review, old and recent targets involved in pain signaling and compounds acting at these targets are summarized. In addition, strategies employed to reduce side effects, increase potency, and efficacy of opioids are also elaborated. This review should aid in propelling drug discovery efforts to discover novel analgesics.

Synergistic interaction between butorphanol and dexmedetomidine in antinociception

Opioid analgesics and the α2-adrenergic receptor (α2AR) agonists are found to produce synergistic antinociception when administered in combination. In this study interactions between butorphanol and dexmedetomidine were investigated in the thermal pain and autonomous locomotor activity. Butorphanol and dexmedetomidine were administered subcutaneously alone and in combination in a fixed-dose ratio (3:1) to assess the antinociceptive and sedative responses. Butorphanol produced antinociception in the hot-plate test via three major opioid receptor subtypes, i.e. MORs, KORs and DORs, while in the tail-immersion test the antinociception was produced by MORs and KORs, whereas dexmedetomidine exhibited antinociception by α2ARs in both tests. They exhibited dose- and time-dependent antinociception and inhibition of locomotor activity when administered alone, while their combination displayed enhanced therapeutic effects. Isobolographic analysis revealed that combined butorphanol and dexmedetomidine produced synergistic interactions in the hot-plate, tail-immersion and locomotor activity tests. Furthermore, the analgesic synergy was also approved to be modulated by MORs, KORs, DORs and α2ARs. Hence we concluded from this study that combined butorphanol and dexmedetomidine produced synergistic antinociception that may be helpful in facilitating clinical management of acute nociceptive pain.

The Effect of Dexmedetomidine on Oral Mucosal Blood Flow and the Absorption of Lidocaine

Dexmedetomidine (DEX) is a sedative and analgesic agent that acts via the alpha-2 adrenoreceptor and is associated with reduced anesthetic requirements, as well as attenuated blood pressure and heart rate in response to stressful events. A previous study reported that cat gingival blood flow was controlled via sympathetic alpha-adrenergic fibers involved in vasoconstriction. In the present study, experiment 1 focused on the relationship between the effects of DEX on alpha adrenoreceptors and vasoconstriction in the tissues of the oral cavity and compared the palatal mucosal blood flow (PMBF) in rabbits between general anesthesia with sevoflurane and sedation with DEX. We found that the PMBF was decreased by DEX presumably because of the vasoconstriction of oral mucosal vessels following alpha-2 adrenoreceptor stimulation by DEX. To assess if this vasoconstriction would allow decreased use of locally administered epinephrine during DEX infusion, experiment 2 in the present study monitored the serum lidocaine concentration in rabbits to compare the absorption of lidocaine without epinephrine during general anesthesia with sevoflurane and sedation with DEX. The depression of PMBF by DEX did not affect the absorption of lidocaine. We hypothesize that this is because lidocaine dilates the blood vessels, counteracting the effects of DEX. In conclusion, despite decreased palatal blood flow with DEX infusion, local anesthetics with vasoconstrictors should be used in implant and oral surgery even with administered DEX.

Opioid-Induced Signaling and Antinociception Are Modulated by the Recently Deorphanized Receptor, GPR171

ProSAAS is one of the most widely expressed proteins throughout the brain and was recently found to be upregulated in chronic fibromyalgia patients. BigLEN is a neuropeptide that is derived from ProSAAS and was recently discovered to be the endogenous ligand for the orphan G protein-coupled receptor GPR171. Although BigLEN-GPR171 has been found to play a role in feeding and anxiety behaviors, it has not yet been explored in pain and opioid modulation. The purpose of this study was to evaluate this novel neuropeptide-receptor system in opioid-induced antinociception. We found that GPR171 is expressed in GABAergic neurons within the periaqueductal gray, which is a key brain area involved in pain modulation and opioid functions. We also found that, although the GPR171 agonist and antagonist do not have nociceptive effects on their own, they oppositely regulate morphine-induced antinociception with the agonist enhancing and antagonist reducing antinociception. Lastly, we showed that the GPR171 antagonist or receptor knockdown decreases signaling by the mu-opioid receptor, but not the delta-opioid receptor. Taken together, these results suggest that antagonism of the GPR171 receptor reduces mu opioid receptor signaling and morphine-induced antinociception, whereas the GPR171 agonist enhances morphine antinociception, suggesting that GPR171 may be a novel target toward the development of pain therapeutics. SIGNIFICANCE STATEMENT: GPR171 is a recently deorphanized receptor that is expressed within the periaqueductal gray and can regulate mu opioid receptor signaling and antinociception. This research may contribute to the development of new therapeutics to treat pain.

Pharmacologic Treatment of Opioid Use Disorder: a Review of Pharmacotherapy, Adjuncts, and Toxicity

Opioid use disorder continues to be a significant source of morbidity and mortality in the USA and the world. Pharmacologic treatment with methadone and buprenorphine has been shown to be effective at retaining people in treatment programs, decreasing illicit opioid use, decreasing rates of hepatitis B, and reducing all cause and overdose mortality. Unfortunately, barriers exist in accessing these lifesaving medications: users wishing to start buprenorphine therapy require a waivered provider to prescribe the medication, while some states have no methadone clinics. As such, users looking to wean themselves from opioids or treat their opioid dependence will turn to alternative agents. These agents include using prescription medications, like clonidine or gabapentin, off-label, or over the counter drugs, like loperamide, in supratherapeutic doses. This review provides information on the pharmacology and the toxic effects of pharmacologic agents that are used to treat opioid use disorder. The xenobiotics reviewed in depth include buprenorphine, clonidine, kratom, loperamide, and methadone, with additional information provided on lofexidine, akuamma seeds, kava, and gabapentin.

Using ecological momentary assessment to examine the relationship between craving and affect with opioid use in a clinical trial of clonidine as an adjunct medication to buprenorphine treatment

BACKGROUND

In a recent clinical trial (NCT00295308), we demonstrated that clonidine decreased the association between opioid craving and moderate levels of stress and affect in patients receiving buprenorphine-based opioid agonist therapy.

OBJECTIVES

To examine the relationship between illicit opioid use and craving and affect during the evaluation of clonidine as an adjunct medication in buprenorphine treatment for opioid use disorder. Secondarily, to examine whether those relationships are driven by within- or between-participant factors.

METHODS

This was a secondary data analysis from our original trial. Participants (N = 108, female: n = 23, male n = 85) receiving buprenorphine were randomized to receive adjunct clonidine or placebo. Participants used portable electronic devices to rate stress, mood, and craving via ecological momentary assessment (EMA) four times randomly each day. To associate the EMA data with illicit opioid use, each EMA report was linked to participants' next urine drug screen (thrice weekly). We used generalized linear mixed models to examine the interaction between treatment group and illicit opioid use, as well as to decompose the analysis into within- and between-participant effects.

RESULTS

Craving for opioids and cocaine was increased when participants were using illicit opioids; this effect was greater in the clonidine group. For affect, mood was poorer during periods preceding opioid-positive urines than opioid-negative urines for clonidine-treated participants, whereas there was no difference for placebo participants.

CONCLUSION

This secondary analysis provides evidence that for participants maintained on opioid agonist therapy, clonidine minimized the behavioral impact of moderate levels of negative affect and craving.

Analgesic synergy between opioid and α2 -adrenoceptors

Opioid and α2 -adrenoceptor agonists are potent analgesic drugs and their analgesic effects can synergize when co-administered. These supra-additive interactions are potentially beneficial clinically; by increasing efficacy and/or reducing the total drug required to produce sufficient pain relief, undesired side effects can be minimized. However, combination therapies of opioids and α2 -adrenoceptor agonists remain underutilized clinically, in spite of a large body of preclinical evidence describing their synergistic interaction. One possible obstacle to the translation of preclinical findings to clinical applications is a lack of understanding of the mechanisms underlying the synergistic interactions between these two drug classes. In this review, we provide a detailed overview of the interactions between different opioid and α2 -adrenoceptor agonist combinations in preclinical studies. These studies have identified the spinal cord as an important site of action of synergistic interactions, provided insights into which receptors mediate these interactions and explored downstream signalling events enabling synergy. It is now well documented that the activation of both μ and δ opioid receptors can produce synergy with α2 -adrenoceptor agonists and that α2 -adrenoceptor agonists can mediate synergy through either the α2A or the α2C adrenoceptor subtypes. Current hypotheses surrounding the cellular mechanisms mediating opioid-adrenoceptor synergy, including PKC signalling and receptor oligomerization, and the evidence supporting them are presented. Finally, the implications of these findings for clinical applications and drug discovery are discussed.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

On the g-protein-coupled receptor heteromers and their allosteric receptor-receptor interactions in the central nervous system: focus on their role in pain modulation

The modulatory role of allosteric receptor-receptor interactions in the pain pathways of the Central Nervous System and the peripheral nociceptors has become of increasing interest. As integrators of nociceptive and antinociceptive wiring and volume transmission signals, with a major role for the opioid receptor heteromers, they likely have an important role in the pain circuits and may be involved in acupuncture. The delta opioid receptor (DOR) exerts an antagonistic allosteric influence on the mu opioid receptor (MOR) function in a MOR-DOR heteromer. This heteromer contributes to morphine-induced tolerance and dependence, since it becomes abundant and develops a reduced G-protein-coupling with reduced signaling mainly operating via β-arrestin2 upon chronic morphine treatment. A DOR antagonist causes a return of the Gi/o binding and coupling to the heteromer and the biological actions of morphine. The gender- and ovarian steroid-dependent recruitment of spinal cord MOR/kappa opioid receptor (KOR) heterodimers enhances antinociceptive functions and if impaired could contribute to chronic pain states in women. MOR1D heterodimerizes with gastrin-releasing peptide receptor (GRPR) in the spinal cord, mediating morphine induced itch. Other mechanism for the antinociceptive actions of acupuncture along meridians may be that it enhances the cross-desensitization of the TRPA1 (chemical nociceptor)-TRPV1 (capsaicin receptor) heteromeric channel complexes within the nociceptor terminals located along these meridians. Selective ionotropic cannabinoids may also produce cross-desensitization of the TRPA1-TRPV1 heteromeric nociceptor channels by being negative allosteric modulators of these channels leading to antinociception and antihyperalgesia.

Combination of oral clonidine and intravenous low-dose ketamine reduces the consumption of postoperative patient-controlled analgesia morphine after spine surgery

OBJECTIVE

Because ketamine, clonidine, and morphine modulate nociceptive pain, coadministration of these drugs would augment the activity of postoperative analgesic drugs. The purpose of this study was to evaluate the effects of coadministration of ketamine and clonidine on postoperative morphine consumption in patients after spine surgery.

METHODS

The patients undergoing spine surgery were allocated randomly to one of the four study groups, which are as follows: group M (n = 12), intravenously (IV) administered patient-controlled analgesia (PCA) morphine alone; group MK (n = 12), IV-PCA morphine plus intra- and postoperative ketamine; group MC (n = 13), IV-PCA morphine plus oral clonidine premedication; group MCK (n = 12), IV-PCA morphine plus intra- and postoperative ketamine and clonidine premedication. The patients in the MC and MCK groups received 4 μg/kg clonidine orally, whereas those in the MK and MCK groups received IV bolus of ketamine (10 mg) at a rate of 2 mg/kg/hour during anesthesia. Patients were arranged to use IV-PCA mode for administration of drugs, which was programmed to deliver a bolus dose of 2-mg morphine (groups M and MC), or boluses of 2-mg morphine and 2-mg ketamine (groups MK and MCK). Scores of visual analog scale (VAS) for pain, morphine requirement, vital signs, nausea, sedation, and other side effects were followed up to 60 hours after surgery.

RESULTS

Although there were significant differences in VAS pain scores at rest 24-48 hours after the surgery, the VAS pain score at movement was similar among the groups. The number of PCA request and cumulative morphine requirement were significantly lower in the MCK group than in the M group.

CONCLUSION

This study results show that the administration of perioperative low-dose ketamine combined with clonidine premedication could reduce the consumption of postoperative PCA morphine following spine surgery.

Receptor and channel heteromers as pain targets

Recent discoveries indicate that many G-protein coupled receptors (GPCRs) and channels involved in pain modulation are able to form receptor heteromers. Receptor and channel heteromers often display distinct signaling characteristics, pharmacological properties and physiological function in comparison to monomer/homomer receptor or ion channel counterparts. It may be possible to capitalize on such unique properties to augment therapeutic efficacy while minimizing side effects. For example, drugs specifically targeting heteromers may have greater tissue specificity and analgesic efficacy. This review will focus on current progress in our understanding of roles of heteromeric GPCRs and channels in pain pathways as well as strategies for controlling pain pathways via targeting heteromeric receptors and channels. This approach may be instrumental in the discovery of novel classes of drugs and expand our repertoire of targets for pain pharmacotherapy.

Further studies on the anti-nociceptive effect of vasopressin

The possible interactions of pathways which mediate anti-nociception when stimulated by (α2-adrenoceptor agonists and arginine vasopressin (AVP) were investigated. Yohimbine, an α2-antagonist, failed to modify the anti-nociceptive response of AVP. However, clonidine pretreatment, in sub-effective and effective doses, potentiated the anti-nociceptive response of a sub-effective dose of AVP. This potentiation was attenuated by yohimbine and completely anatagonized by naloxone. These studies suggest that pathways related to the opioidergic system and those stimulated by α2-agonists may be utilized by AVP in eliciting the anti-nociceptive response.

Study of adrenergic, imidazoline, and endothelin receptors in clonidine-, morphine-, and oxycodone-induced changes in rat body temperature

OBJECTIVES

The potentiation of morphine or oxycodone analgesia by endothelin-A (ET(A)) receptor antagonists and imidazoline/α(2)-adrenergic agonists is well documented. However, the effect of morphine or oxycodone in combination with an ET(A) receptor antagonist or an imidazoline/α(2) adrenergic agonist on body temperature is not known. The present study was carried out to study the role of ET(A) and imidazoline/α(2) adrenergic receptors in body temperature effects of morphine, oxycodone, and clonidine in rats.

METHODS

Body temperature was determined in male Sprague-Dawley rats treated with morphine, oxycodone, or clonidine. Yohimbine, idazoxan, and BMS182874 were used to determine the involvement of α(2)-adrenergic, imidazoline, and ET(A) receptors, respectively.

KEY FINDINGS

Morphine and oxycodone produced hyperthermia which was not affected by α(2)-adrenergic antagonist yohimbine, imidazoline/α(2)-adrenergic antagonist idazoxan, or ET(A) receptor antagonist BMS182874. Clonidine alone produced hypothermia that was comparable to the hypothermia observed with clonidine plus morphine or oxycodone. The hypothermic effect of clonidine was blocked by idazoxan and yohimbine. The blockade by idazoxan was more pronounced compared to yohimbine. Clonidine hypothermia was not affected by BMS182874.

CONCLUSIONS

This is the first report demonstrating that ET(A) receptors do not influence morphine- and oxycodone- induced hyperthermia or clonidine-induced hypothermia. Imidazoline receptors and α(2)-adrenergic receptors are involved in clonidine-induced hypothermia, but not in morphine- and oxycodone-induced hyperthermia.

Heteromerization of G protein-coupled receptors: relevance to neurological disorders and neurotherapeutics

Because G protein-coupled receptors (GPCRs) are numerous, widely expressed and involved in major physiological responses, they represent a relevant therapeutic target for drug discovery, particularly regarding pharmacological treatments of neurological disorders. Among the biological phenomena regulating receptor function, GPCR heteromerization is an important emerging area of interest and investigation. There is increasing evidence showing that heteromerization contributes to the pharmacological heterogeneity of GPCRs by modulating receptor ontogeny, activation and recycling. Although in many cases the physiological relevance of receptor heteromerization has not been fully established, the unique pharmacological and functional properties of heteromers are likely to lead to new strategies in clinical medicine. This review describes the main GPCR heteromers and their implications for major neurological disorders such as Parkinson's disease, schizophrenia and addiction. A better understanding of molecular mechanisms underlying drug interactions related to the targeting of receptor heteromers could provide more specific and efficient therapeutic agents for the treatment of brain diseases.

Determination of α(2)-adrenoceptor and imidazoline receptor involvement in augmentation of morphine and oxycodone analgesia by agmatine and BMS182874

Studies have demonstrated that clonidine (α(2)-adrenoceptor and imidazoline receptor agonist) and BMS182874 (endothelin ET(A) receptor antagonist) potentiate morphine and oxycodone analgesia. Agmatine, an endogenous clonidine-like substance, enhances morphine analgesia. However, its effect on oxycodone analgesia and its interaction with endothelin ET(A) receptor antagonists are not known. The present study was performed to determine the effect of agmatine on morphine and oxycodone analgesia and the involvement of α(2)-adrenoceptors, imidazoline receptors, opioid receptors, and endothelin receptors. Antinociception at various time intervals was determined by the tail-flick latency method in mice. Agmatine produced dose-dependent increase in tail-flick latency, while BMS182874 did not produce any change over the 360-min observation period. Agmatine significantly potentiated morphine as well as oxycodone analgesia which was not altered by BMS182874. BMS182874 pretreatment did not increase the analgesic effect produced by agmatine alone. Agmatine-induced potentiation of morphine and oxycodone analgesia was blocked by idazoxan (imidazoline receptor/α(2)-adrenoceptor antagonist) and yohimbine (α(2)-adrenoceptor antagonist). BMS182874-induced potentiation of morphine or oxycodone analgesia was not affected by yohimbine. However, idazoxan blocked BMS182874-induced potentiation of oxycodone but not morphine analgesia. This is the first report demonstrating that agmatine potentiates not only morphine but also oxycodone analgesia in mice. Potentiation of morphine and oxycodone analgesia by agmatine appears to involve α(2)-adrenoceptors, imidazoline receptors, and opioid receptors. In addition, imidazoline receptors may be involved in BMS182874-induced potentiation of oxycodone but not morphine analgesia. It is concluded that agmatine may be used as an adjuvant in opiate analgesia.

A comparison of intravenous sedation with two doses of dexmedetomidine: 0.2 µg/kg/hr Versus 0.4 µg/kg/hr

The present study investigated the physiologic and sedative effects between two different continuous infusion doses of dexmedetomidine (DEX). Thirteen subjects were separately sedated with DEX at a continuous infusion dose of 0.2 µg/kg/hr for 25 minutes after a loading dose of 6 µg/kg/hr for 5 minutes (0.2 group) and a continuous infusion dose of 0.4 µg/kg/hr for 25 minutes after a loading dose of 6 µg/kg/hr for 5 minutes (0.4 group). The recovery process was then observed for 60 minutes post infusion. The tidal volume, mean arterial pressure, and heart rate in both groups decreased significantly during infusion, but they were within a clinically acceptable level. A Trieger dot test plot error ratio in the 0.4 group was significantly higher than that in the 0.2 group until 15 minutes post infusion. Sedation appears to be safe at the infusion doses of DEX studied. However, increasing maintenance infusion doses of DEX from 0.2 µg/kg/hr to 0.4 µg/kg/hr delays some recovery parameters.

Involvement of imidazoline and opioid receptors in the enhancement of clonidine-induced analgesia by sulfisoxazole

Clonidine, an α2-adrenergic agonist, has been demonstrated to produce significant analgesia and potentiate morphine analgesia. Endothelin (ETA) receptor antagonists have also been found to potentiate the antinociceptive response to morphine. Clonidine and ET have been reported to have cardiovascular interactions involving the sympathetic nervous system, but it is not known whether ETA receptor antagonist affects clonidine analgesia. This study examined the influence of sulfisoxazole (ETA receptor antagonist) on clonidine analgesia. Male Swiss Webster mice were used to determine antinociceptive response of drugs by measuring tail-flick latency. The effect of clonidine (0.3, 1.0, and 3.0 mg/kg, i.p.) alone or in combination with sulfisoxazole (25, 75, and 225 mg/kg, p.o.) on analgesia and body temperature was determined. Clonidine produced a dose-dependent analgesia and hypothermia. Sulfisoxazole (25, 75, and 225 mg/kg), when administered with clonidine (0.3 mg/kg), significantly potentiated (31% increase in area under the curve (AUC)) the analgesic effect of clonidine. Yohimbine (α2-adrenergic receptor antagonist) did not affect analgesic effect of clonidine plus sulfisoxazole. Idazoxan (I1-imidazoline and α2-adrenergic receptor antagonist) reduced (47% decrease in AUC) the analgesic effect of clonidine plus sulfisoxazole. Treatment with naloxone reduced (46% decrease in AUC) the analgesic effect of clonidine plus sulfisoxazole. The effect of another ETA receptor antagonist, BMS-182874 (2, 10, and 50 µg, i.c.v.) was studied, and it was found that the dose of 10 µg significantly potentiated (26% increase in AUC) the analgesic effect of clonidine. These results indicate that sulfisoxazole, an ETA receptor antagonist, potentiates the analgesic effect of clonidine, which could be mediated through I1-imidazoline receptors and opioid receptors.

Interaction of morphine with a new alpha2-adrenoceptor agonist in mice

Finding new chemicals or adjuvants with analgesic effects in the central nervous system is clinically relevant due to the limited number of drugs with these properties. Here, we present PT-31, which is chemically related to 3-benzyl-imidazolidine, with an analgesic profile that results from alpha(2)-adrenoceptor activation. Intraperitoneal administration of PT-31 dose-dependently produced antinociception in the hot plate test, and interacted synergistically with morphine. This effect was completely reversed by yohimbine, a non-selective antagonist of alpha(2)-adrenoceptors, and by BRL 44408, a selective alpha(2A)-adrenoceptor antagonist. The combination of morphine and PT-31 produced greater antinociceptive activity than either alone, and isobolographic analysis revealed a synergistic interaction between these compounds. Docking results confirm the high affinity of the PT-31 ligand at the alpha(2A)-adrenoceptor.

PERSPECTIVE

This study introduces a new analgesic compound (PT-31) that acts via alpha(2A)-adrenoceptor activation. A significant increase in analgesia was observed when co-administered with morphine. PT-31 is an interesting new substance for pain therapy.

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.

Intravenous Sedation with Low-Dose Dexmedetomidine: Its Potential for Use in Dentistry

This study investigated the physiologic and sedative parameters associated with a low-dose infusion of dexmedetomidine (Dex). Thirteen healthy volunteers were sedated with Dex at a loading dose of 6 mcg/kg/h for 5 minutes and a continuous infusion dose of 0.2 mcg/kg/h for 25 minutes. The recovery process was observed for 60 minutes post infusion. The tidal volume decreased significantly despite nonsignificant changes in respiratory rate, minute ventilation, oxygen saturation, and end-tidal carbon dioxide. The mean arterial pressure and heart rate also decreased significantly but within clinically acceptable levels. Amnesia to pin prick was present in 69% of subjects. A Trieger dot test plot error ratio did not show a significant change at 30 minutes post infusion despite a continued significant decrease in bispectral index. We conclude that sedation with a low dose of Dex appears to be safe and potentially efficacious for young healthy patients undergoing dental procedures.

Conformational cross-talk between alpha2A-adrenergic and mu-opioid receptors controls cell signaling

Morphine, a powerful analgesic, and norepinephrine, the principal neurotransmitter of sympathetic nerves, exert major inhibitory effects on both peripheral and brain neurons by activating distinct cell-surface G protein-coupled receptors-the mu-opioid receptor (MOR) and alpha2A-adrenergic receptor (alpha2A-AR), respectively. These receptors, either singly or as a heterodimer, activate common signal transduction pathways mediated through the inhibitory G proteins (G(i) and G(o)). Using fluorescence resonance energy transfer microscopy, we show that in the heterodimer, the MOR and alpha2A-AR communicate with each other through a cross-conformational switch that permits direct inhibition of one receptor by the other with subsecond kinetics. We discovered that morphine binding to the MOR triggers a conformational change in the norepinephrine-occupied alpha2A-AR that inhibits its signaling to G(i) and the downstream MAP kinase cascade. These data highlight a new mechanism in signal transduction whereby a G protein-coupled receptor heterodimer mediates conformational changes that propagate from one receptor to the other and cause the second receptor's rapid inactivation.

Deep sedation with dexmedetomidine in a porcine model does not compromise the viability of free microvascular flap as depicted by microdialysis and tissue oxygen tension

BACKGROUND

Deep sedation is often necessary after major reconstructive plastic surgery in the face and neck regions to prevent sudden spontaneous movements capable of inflicting mechanical injury to the transplanted musculocutaneous flap(s). An adequate positioning may help to optimize oxygenation and perfusion of the transplanted tissues. We hypothesized that dexmedetomidine, a central alpha2-agonist and otherwise potentially ideal postoperative sedative drug, may induce vasoconstriction in denervated flaps, and thus increase the risk of tissue deterioration.

METHODS

Two symmetrical myocutaneous flaps were raised on each side of the upper abdomen in 12 anesthetized pigs. The sympathetic nerve fibers were stripped from the arteries in one of the flaps (denervated flap), while nerve fibers were kept untouched in the other (innervated flap). After simulation of ischemia and reperfusion periods, the animals were randomized to deep postoperative sedation with either propofol (n = 6) or dexmedetomidine (n = 6). Flap tissue metabolism was monitored by microdialysis and tissue-oxygen partial pressure. Glucose, lactate, and pyruvate concentrations were analyzed from the dialysate every 30 min for 4 h.

RESULTS

Mean arterial blood pressure was higher in the dexmedetomidine group (P = 0.036). Flap tissue metabolism remained stable throughout the experiment as measured by lactate-pyruvate and lactate-glucose ratios (median ranges 14.3-24.5 for lactate-pyruvate and 0.3-0.6 for lactate-glucose) and by tissue-oxygen partial pressure, and no differences were found between groups.

CONCLUSIONS

Our data suggest that dexmedetomidine, even if used for deep sedation, does not have deleterious effects on local perfusion or tissue metabolism in denervated musculocutaneous flaps.

Perioperative pain management

The under-treatment of postoperative pain has been recognised to delay patient recovery and discharge from hospital. Despite recognition of the importance of effective pain control, up to 70% of patients still complain of moderate to severe pain postoperatively. The mechanistic approach to pain management, based on current understanding of the peripheral and central mechanisms involved in nociceptive transmission, provides newer options for clinicians to manage pain effectively. In this article we review the rationale for a multimodal approach with combinations of analgesics from different classes and different sites of analgesic administration. The pharmacological options of commonly used analgesics, such as opioids, NSAIDs, paracetamol, tramadol and other non-opioid analgesics, and their combinations is discussed. These analgesics have been shown to provide effective pain relief and their combinations demonstrate a reduction in opioid consumption. The basis for using non-opioid analgesic adjuvants is to reduce opioid consumption and consequently alleviate opioid-related adverse effects. We review the evidence on the opioid-sparing effect of ketamine, clonidine, gabapentin and other novel analgesics in perioperative pain management. Most available data support the addition of these adjuvants to routine analgesic techniques to reduce the need for opioids and improve quality of analgesia by their synergistic effect. Local anaesthetic infiltration, epidural and other regional techniques are also used successfully to enhance perioperative analgesia after a variety of surgical procedures. The use of continuous perineural techniques that offer prolonged analgesia with local anaesthetic infusion has been extended to the care of patients beyond hospital discharge. The use of nonpharmacological options such as acupuncture, relaxation, music therapy, hypnosis and transcutaneous nerve stimulation as adjuvants to conventional analgesia should be considered and incorporated to achieve an effective and successful perioperative pain management regimen.

The impact of opioid-induced hyperalgesia for postoperative pain

Clinical evidence suggests that--besides their well known analgesic activity - opioids can increase rather than decrease sensitivity to noxious stimuli. Based on the observation that opioids can activate pain inhibitory and pain facilitatory systems, this pain hypersensitivity has been attributed to a relative predominance of pronociceptive mechanisms. Acute receptor desensitization via uncoupling of the receptor from G-proteins, upregulation of the cAMP pathway, activation of the N-methyl-D-aspartate (NMDA)-receptor system, as well as descending facilitation, have been proposed as potential mechanisms underlying opioid-induced hyperalgesia. Numerous reports exist demonstrating that opioid-induced hyperalgesia is observed both in animal and human experimental models. Brief exposures to micro-receptor agonists induce long-lasting hyperalgesic effects for days in rodents, and also in humans large-doses of intraoperative micro-receptor agonists were found to increase postoperative pain and morphine consumption. Furthermore, the prolonged use of opioids in patients is often associated with a requirement for increasing doses and the development of abnormal pain. Successful strategies that may decrease or prevent opioid-induced hyperalgesia include the concomitant administration of drugs like NMDA-antagonists, alpha2-agonists, or non-steroidal anti-inflammatory drugs (NSAIDs), opioid rotation or combinations of opioids with different receptor/selectivity.

Antinociception: Mechanistic studies on the action of MD-354 and clonidine. Part 1. The 5-HT3 component

MD-354 (m-chlorophenylguanidine) is a 5-HT3/alpha2B-adrenoceptor ligand. Both receptors play a role in antinociception. In the mouse tail-flick assay, subcutaneously administered MD-354 was inactive as an analgesic. However, a combination of an inactive dose of clonidine (0.25 mg/kg) with an inactive dose of MD-354 (6 mg/kg) produced a substantial antinociceptive effect (maximal possible effect=66%). Considering the 5-HT3 receptor partial agonist properties of MD-354, the analgesia enhancing effect of MD-354 on clonidine might be associated, at least in part, with its 5-HT3 receptor agonist or antagonist activity. Combinations of an inactive dose of clonidine (0.25 mg/kg) with 5-HT3 receptor antagonists (tropisetron, zacopride and ondansetron) were examined. Saline-like doses of tropisetron, zacopride and ondansetron significantly enhanced the antinociceptive effect of clonidine (combinations: maximal possible effect=86%, 82% and 79% respectively), suggesting that MD-354 may enhance the analgesic actions of clonidine, at least in part, through a 5-HT3 receptor antagonist mechanism.

The effect of pre-anaesthetic administration of intravenous dexmedetomidine on postoperative pain in patients receiving patient-controlled morphine

BACKGROUND AND OBJECTIVE

This prospective, randomized, double-blind, controlled study was designed to test the effect of pre-anaesthetic administration of dexmedetomidine, given as a single intravenous (i.v.) dose, on postoperative pain scores and morphine consumption in patients receiving patient-controlled morphine after abdominal surgery.

METHODS

Sixty patients were randomly allocated to receive dexmedetomidine (1 microg kg(-1)) or saline 10 min before induction of anaesthesia. Twenty minutes before the end of surgery, all patients received a standardized (0.1 mg kg(-1)) loading dose of morphine. They were then allowed to use a patient-controlled analgesia (PCA) device giving bolus doses of morphine (0.02 mg kg(-1)). Pain, discomfort and sedation scores; cumulative morphine consumption; time to extubation; time to recovery; and any side-effects were recorded after recovery and at 1, 2, 6, 12 and 24 h after the start of PCA.

RESULTS

The mean time to extubation at the end of anaesthesia and recovery time were similar in both groups. There were no significant differences between groups with regard to mean pain, discomfort, sedation and nausea scores. Cumulative morphine consumption was significantly lower in the dexmedetomidine group at 6, 12 and 24 h (P < 0.05). The incidence of side-effects did not differ between the groups.

CONCLUSIONS

A single i.v. dose of dexmedetomidine (1 microg kg(-1)) given 10 min before induction of anaesthesia significantly reduced postoperative morphine consumption at identical pain scores compared to control, but had no effect on postoperative recovery time.

MD-354 potentiates the antinociceptive effect of clonidine in the mouse tail-flick but not hot-plate assay

Albeit conflicting, evidence suggests that 5-HT3 receptor partial agonists as well as alpha2NON-A-adrenoceptor agonists might be involved in antinociception. MD-354 (m-chlorophenylguanidine) can be viewed as the first example of a rather selective 5-HT3/alpha2B-adrenergic ligand. In a tail-flick test in mice, subcutaneous administration of MD-354 doses up to 30 mg/kg did not produce antinociception and failed to antagonize the effect of clonidine (ED50=0.5 mg/kg), but a combination of an inactive de of clonidine (0.25 mg/kg) that produced only 13% maximal possible effect (MPE) with an inactive dose of MD-354 (10 mg/kg, MPE=8%) produced an antinociceptive effect (MPE=83%). In the hot-plate assay, neither subcutaneous administration of MD-354 (3 to 30 mg/kg) alone nor in combination with clonidine (ED50=0.8 mg/kg) produced an antinociceptive effect. MD-354 was demonstrated to potentiate the antinociceptive effect of clonidine in the tail-flick assay, but its underlying mechanism remains to be determined.

Topical clonidine antinociception

Clonidine, an alpha-2 adrenergic agonist, is an extremely potent antinociceptive agent. However, the therapeutic utility of systemic clonidine for the treatment of pain is limited by centrally mediated side effects including sedation, hypotension and rebound hypertension. Given that alpha-2 adrenoceptors are expressed on the peripheral and central terminals of nociceptive fibers, we administered clonidine topically in order to avoid central effects. Here, we demonstrate that topical administration of clonidine to mice (via tail immersion) elicited antinociception in the radiant heat tail-flick test. The magnitude of antinociception was dependent upon the duration of exposure to the clonidine solution. Further, the antinociceptive activity of clonidine was limited to the portion of the tail exposed to drug solution suggesting that the actions of clonidine were locally mediated. Systemic pretreatment with the alpha-2 receptor antagonist, yohimbine, blocked the antinociceptive activity of topical clonidine. Concentrations of clonidine administered locally that were antinociceptive did not impair motor coordination as measured by the rota-rod test. However, doses of clonidine administered systemically that produced antinociception significantly impaired motor coordination. Repeated daily topical administration of clonidine resulted in antinociceptive tolerance. Tolerance to the antinociceptive actions of clonidine was not blocked by topical administration of the NMDA antagonist, ketamine. In conclusion, topical administration of clonidine elicits antinociception by blocking the emerging pain signals at peripheral terminals via alpha-2 adrenoceptors without producing the undesirable central side effects observed following the systemic administration. The ineffectiveness of topical ketamine to block topical clonidine antinociceptive tolerance suggests that peripheral NMDA receptors do not mediate local clonidine antinociceptive tolerance.

Use of oral clonidine for sedation in ventilated paediatric intensive care patients

OBJECTIVES

We aimed to document our experience with oral clonidine when used as a sedative in combination with intravenous morphine and lorazepam in a group of mechanically ventilated children with single-organ, respiratory failure. In particular, our objectives were to establish the relationship between oral dose, plasma concentration, and sedative effect, and second, to document the side-effect profile.

DESIGN

Prospective, cohort study over a 72-h period.

SETTING

Regional paediatric intensive care unit.

PATIENTS AND PARTICIPANTS

Twenty-four children were enrolled (median age 3 months) of whom ten were excluded (six due to extubation before 72 h, three sedation failures, one protocol violation).

MEASUREMENTS AND RESULTS

Plasma clonidine was measured using gas chromatography mass spectrometry, and sedation assessed using the COMFORT score. Using a dose of 3-5 microg/kg every 8 h, plasma concentrations appeared to plateau at approximately 41 h giving a mean value of 1.38 ng/ml (95% confidence interval 1.0-1.8). Adequate sedation was achieved during 82% (837/1022 h) of the study period; however, this decreased to 70.3% when analysed on an intention-to-treat basis. There was a concomitant overall decrease in the average hourly requirements for both morphine ( P = 0.02) and lorazepam ( P = 0.003). There were no documented episodes of bradycardia, hypotension or hyperglycaemia.

CONCLUSIONS

Oral clonidine may be a safe and effective sedative in combination with morphine and lorazepam for young children with single-organ, respiratory failure. This agent may also exhibit opioid and benzodiazepine sparing effects in this patient group. A full pharmacokinetic study is warranted.

The heritability of antinociception: common pharmacogenetic mediation of five neurochemically distinct analgesics

The heritability of nociception and antinociception has been well established in the mouse. The pharmacogenetics of morphine analgesia are fairly well characterized, but far less is known about other analgesics. The purpose of this work was to begin the systematic genetic study of non-mu-opioid analgesics. We tested mice of 12 inbred mouse strains for baseline nociceptive sensitivity (49 degrees C tail-withdrawal assay) and subsequent antinociceptive sensitivity to systemic administration of (trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide methane-sulfonate hydrate (U50,488; 10-150 mg/kg), a kappa-opioid receptor agonist; (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone (WIN55,212-2; 0.5-480 mg/kg), a synthetic cannabinoid receptor agonist; epibatidine (7.5-150 microg/kg), a nicotinic receptor agonist; clonidine (0.1-5 mg/kg), an alpha(2)-adrenergic receptor agonist; and, for purposes of comparison, the prototypic mu-opioid receptor agonist, morphine (5-200 mg/kg). Robust interstrain variability was observed in nociceptive sensitivity and in the antinociceptive effects of each of the drugs, with extreme-responding strains exhibiting antinociceptive potencies differing up to 37-fold. Unexpectedly, we observed moderate-to-high genetic correlations of strain sensitivities to the five drugs (r = 0.39-0.77). We also found moderate-to-high correlations between baseline nociceptive sensitivity and subsequent analgesic response to each drug (r = 0.33-0.68). The generalizability of these findings was established in follow-up experiments investigating morphine and clonidine inhibition of formalin test nociception. Despite the fact that each drug activates a unique receptor, our results suggest that the potency of each drug is affected by a common set of genes. However, the genes in question may affect antinociception indirectly, via a primary action on baseline nociceptive sensitivity.

Implantable devices for pain control: spinal cord stimulation and intrathecal therapies

Untreated chronic pain is costly to society and to the individual suffering from it. The treatment of chronic pain, a multidimensional disease, should rely on the expertise of varying health care providers and should focus not only on the neurobiological mechanisms of the process but also on the psychosocial aspects of the disease. Implantable devices are costly and invasive, and such efficacious therapies should be used only when more conservative and less costly therapies have failed to provide relief of pain and suffering. Spinal cord stimulation provides neuromodulation of neuropathic, but not nociceptive, pain signals and when used for appropriate indications in the right individuals provides approximately 60-80% long-term pain relief in 60-80% of patients trialled for efficacy. Intrathecal therapies with opioids such as morphine, fentanyl, sufentanil or meperidine--or non-opioids such as clonidine or bupivacaine--provide analgesia in patients with nociceptive or neuropathic pain syndromes. Baclofen, intrathecally, provides profound relief of muscle spasticity due to multiple sclerosis, spinal cord injuries, brain injuries or cerebral palsy.

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.

Long-term intrathecal infusion of drug combinations for chronic back and leg pain

Continuous intrathecal infusion of analgesic drugs by implantable pumps is recognized as an established treatment option for patients with chronic pain resistant to oral or parenteral medication. Polyanalgesia, the simultaneous use of more than one intrathecal analgesic drug, is practiced relatively often, but there are only a few published clinical studies on intrathecal polyanalgesia for chronic nonmalignant pain. This pilot study represents a long-term evaluation of a treatment regimen consisting of intrathecal morphine admixed with bupivacaine, clonidine, or midazolam in patients with chronic nonmalignant back and leg pain due to degenerative lumbar spinal disease. Twenty-six adult patients have been treated by intrathecal programmable pump-controlled infusion of analgesic drugs and followed for up to 3.5 years (27 +/- 11 months). Combination of morphine with a second drug was used in 10 cases, morphine with 2 additional drugs in 12 cases, and morphine with 3 additional drugs in 4 cases. Mean daily doses at 24 months after pump implantation were 6.2 +/- 2.8 mg for morphine, 2.5 +/- 1.5 mg for bupivacaine, 0.06 +/- 0.03 mg for clonidine, and 0.8 +/- 0.4 mg for midazolam. Nineteen patients reported excellent or good long-term treatment results, 6 patients had sufficient results, and only 1 patient complained of poor therapeutic efficacy. No long-term clinical side effects of intrathecal polyanalgesia were noted. Mean morphine dose had to be increased from 1.2 mg at baseline to 5.1 mg at 24 months due to tolerance development and disease progression. This experience suggests that intrathecal polyanalgesia employing morphine combined with additional nonopioid drugs can have a favorable analgesic efficacy in patients with complex chronic pain of spinal origin, and lacks major drug-related complications.

Postoperative analgesia for outpatient arthroscopic knee surgery with intraarticular clonidine and/or morphine

Both clonidine, an alpha(2) agonist, and morphine, an opioid agonist, provide enhanced patient analgesia after arthroscopic knee surgery when administered via the intraarticular (IA) route. Clonidine potentiates morphine analgesia in the animal model. We designed this study to determine whether clonidine or morphine results in better analgesia and whether their combination would provide superior analgesia to either drug alone. We evaluated 60 patients undergoing arthroscopic knee meniscus repair under local anesthesia with sedation. After surgery, patients were randomized into four IA groups: Group B received 30 mL 0.25% bupivacaine; Group BC received 30 mL 0.25% bupivacaine and clonidine 1 microg/kg; Group BM received 30 mL 0.25% bupivacaine and morphine 3 mg; and Group BCM received 30 mL 0.25% bupivacaine, clonidine 1 microg/kg, and morphine 3 mg. This study revealed a significant benefit from the individual IA administration of both clonidine and morphine. The combination of these drugs resulted in decreased postoperative pain and analgesic use, as well as an increased analgesic duration compared with either drug alone. We conclude that IA clonidine and morphine improved comfort compared with either drug alone in patients undergoing knee arthroscopy.

Moxonidine, a selective imidazoline/alpha(2) adrenergic receptor agonist, synergizes with morphine and deltorphin II to inhibit substance P-induced behavior in mice

The alpha(2) adrenergic receptor (AR) class of catecholamine/imidazoline (I) agonists, such as norepinephrine and clonidine, produce spinal antinociceptive synergy when co-administered with opioids. We have observed that intrathecally administered moxonidine, a selective I(1)/alpha(2) (AR) agonist, produces antinociception. The present experiments tested moxonidine for ability to synergize with morphine, deltorphin II, and DAMGO (Tyr-D-Ala-NMe-Phe-Gly(ol)) to inhibit substance P-elicited nociceptive behavior in Institute of Cancer Research mice. Moxonidine, morphine, deltorphin II, and DAMGO inhibited substance P-elicited nociceptive behavior with full efficacy. Effective dose 50% (ED(50)) values were calculated and equi-effective dose ratios of the combinations moxonidine-morphine, moxonidine-deltorphin II, and moxonidine-DAMGO were determined. The interactions were tested by isobolographic analysis. The observed ED(50) values of the combinations were statistically compared against their respective calculated theoretical additive ED(50) values. The combinations of moxonidine-morphine and moxonidine-deltorphin II resulted in significant leftward shifts in the dose-response curves compared to those of each agonist administered separately. The ED(50) values of the dose-response curves of these combinations were significantly less than the corresponding calculated theoretical additive ED(50) values; these results indicated that moxonidine synergizes with both morphine and deltorphin II. In contrast, combining moxonidine with DAMGO did not increase the potencies of the agonists (in combination) when compared to the potencies of each agonist administered separately. These results indicated that the moxonidine-DAMGO interaction is subadditive. Collectively, these data demonstrate that moxonidine combined with some opioid agonists produces spinal antinociceptive synergy. Spinally administered moxonidine-opioid combinations may prove an effective therapeutic strategy to manage pain.

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

Interactions between alpha(2)-adrenergic and mu-opioid systems play an important role in the modulation of hyperalgesic states. The antinociceptive effects of alpha(2)-adrenergic agonists and mu-opioids are potentiated when co-administered; however, attempts to induce cross reversal of the antinociceptive effects of alpha(2)-adrenergic and mu-opioid systems have produced contradictory results. We have studied the possible endogenous tonic control of the alpha(2)-adrenergic systems in the modulation of pain in inflammation, and the interactions between the two antinociceptive systems in rat spinal cord nociceptive reflexes activated by both natural and electrical stimulation. The facilitatory actions of the alpha(2)-adrenergic antagonist idazoxan were compared in control rats and in animals with carrageenan-induced paw inflammation. The antinociceptive effect of the mu-opioid fentanyl was tested alone and in the presence of idazoxan. In agreement with some previous observations, idazoxan i.v. produced no change in responses to natural and electrical stimulation in normal animals. In animals with inflammation, idazoxan only induced facilitation of responses evoked by noxious thermal stimulation but not by mechanical or electrical stimulation. Fentanyl reduced the responses to either stimuli with lower potency in the presence of idazoxan, but only in animals with inflammation. Its dose-response curve was shifted to the right between 1.8- and 3. 5-fold depending on the stimulus used. It is concluded that the increase of thermal responses by idazoxan in animals with inflammation is probably due to changes in the peripheral blood flow. Nevertheless, since an interaction with mu-opioids is clear in inflammation, endogenous alpha(2)-adrenergic systems play an important role in the modulation of the effectiveness of opioids during inflammation.

Alteration of spinal protein kinase C expression and kinetics in morphine, but not clonidine, tolerance

Antinociceptive synergism between spinally administered morphine and clonidine decreases to an additive interaction in morphine- and clonidine-tolerant mice. Spinally administered protein kinase C (PKC) inhibitors also decrease the synergism to addition. To determine whether chronic morphine or clonidine treatment alters spinal PKC activity, the present studies measured PKC activity and expression of PKC isoform proteins in spinal cord cytosol and membrane fractions. Mice were treated for 4 days with either placebo pellets, morphine pellets, s.c. saline, or s.c. clonidine. Morphine pellet-implanted mice were tolerant to morphine-induced tail flick antinociception, but not cross-tolerant to clonidine. Clonidine-pretreated mice were tolerant to clonidine, but not cross-tolerant to morphine. Induction of morphine tolerance produced a 2-fold lower Km value for PKC (8.24 +/- 1.67 microM in placebo pellet vs 4.43 +/- 1.24 microM in morphine pellet) in cytosol, but not membrane fractions from spinal cord. Vmax values were not different. No difference in Km or Vmax values was found between proteins from saline- and clonidine-pretreated animals. Immunoreactive cPKCalpha, betaI, and gamma isoforms decreased 14, 26, and 17%, respectively, in cytosol from morphine-tolerant animals. No difference in PKC isoforms was found in the membranes or in fractions from clonidine-tolerant mice. Morphine tolerance, but not clonidine tolerance, enhanced PKC activity while decreasing protein expression.

Epidural and spinal agents for postoperative analgesia

The discovery of opioid receptors and the subsequent development of the technique of epidural and intrathecal opioid administration are undoubtedly two of the most significant advances in pain management in recent decades. The use of spinal opioids is widespread and increasing. The technique is used widely to treat intraoperative, postoperative, traumatic, obstetric, chronic, and cancer pain. Newer developments include the increasing use of combined local anesthetics and opioids or nonopioids and also PCEA, particularly in the obstetric population. Meta-analysis of controlled trials has demonstrated improved pulmonary outcome in patients receiving epidural postoperative analgesia. Although rare, respiratory depression continues to be a major problem of the technique. None of the currently available opioids is completely safe; however, extensive international experience has shown that patients receiving spinal opioids for postoperative analgesia can be safely nursed on regular wards, provided that trained personnel and appropriate guidelines are available. The importance of a good acute pain service to provide the safe and effective use of spinal opioids cannot be overemphasized.