Comparison of the antinociceptive effect of morphine, methadone, buprenorphine and codeine in two substrains of Sprague-Dawley rats

Changes in fentanyl demand following naltrexone, morphine, and buprenorphine in male rats

BACKGROUND

Individuals with opioid use disorder (OUD) exhibit high levels of economic demand for opioids, with high levels of consumption and relative insensitivity to changes in price. Because the medications used to treat OUD in medication-assisted therapy (MAT) act as antagonists or agonists at μ opioid receptors, they may alter the relationship between price and opioid intake.

METHODS

This study examined demand for a commonly abused synthetic prescription opioid, fentanyl, in male rats following s.c. pre-treatment with naltrexone (0.1-1.0 mg/kg), morphine (0.3-3.0 mg/kg) or buprenorphine (0.3-3.0 mg/kg). We normalized demand curves to intake at the lowest price and estimated effects on elasticity (sensitivity to changes in price). Rats were first trained to earn fentanyl (5 μg/kg/infusion) on a fixed ratio schedule, then they underwent daily training under a threshold procedure designed to produce within-session demand curve estimates. Rats received 14 threshold sessions before undergoing a series of tests encompassing each drug, at each dose.

RESULTS

Elasticity was increased by pretreatment with naltrexone, morphine or buprenorphine. Morphine also decreased initial intake, when the price for fentanyl was lowest. In contrast, initial intake was increased by naltrexone (according to an inverted-U shaped curve). The effects of naltrexone did not persist after the test session, but morphine and buprenorphine continued affecting demand elasticity 24 h or 48 h after the test, respectively.

CONCLUSIONS

These results indicate that fentanyl demand is sensitive to blockade or activation of opioid receptors by the drug classes used for MAT in humans.

Comparing the Antinociceptive Effects of Methamphetamine, Buprenorphine, or Both After Chronic Treatment and Withdrawal in Male Rats

Introduction:

Methamphetamine (Meth) and Buprenorphine (BUP) modulate pain perception. However, the antinociceptive effects of their interactions, which affect through different systems, are unclear in rats. This study aimed to compare the analgesic effects of Meth, BUP, and their coadministration, as well as the effect of withdrawal from these substances on nociception in male rats.

Methods:

In this experiment, 40 male Wistar rats (weight: 250–300 g) were categorized into four groups: control, Meth, BUP, or BUP+Meth. After seven days of treatments, the antinociceptive effects were assessed using the hot plate and the tail flick tests. The differences among the groups were analyzed with ANOVA and Tukey’s post hoc tests. P values less than 0.05 were considered significant.

Results:

Meth and BUP increased the reaction times during the hot plate and tail flick tests. The combination of Meth and BUP increased reaction time more than Meth or BUP alone.

Conclusion:

The significantly high reaction times in rats treated with Meth and BUP indicate that these substances have antinociceptive effects. In addition, Meth enhanced the antinociceptive effects of BUP. These synergistic effects might occur through the dopaminergic, serotonergic, and or adrenergic systems.

The analgesic efficacy of morphine varies with rat strain and experimental pain model: implications for target validation efforts in pain drug discovery

Background

Translating efficacy of analgesic drugs from animal models to humans remains challenging. Reasons are multifaceted, but lack of sufficiently rigorous preclinical study design criteria and phenotypically relevant models may be partly responsible. To begin to address this fundamental issue, we assessed the analgesic efficacy of morphine in three inbred rat strains (selected based on stress reactivity and affective/pain phenotypes), and outbred Sprague Dawley (SD) rats supplied from two vendors.

Methods

Sensitivity to morphine (0.3–6.0 mg/kg, s.c.) was evaluated in the hot plate test of acute thermal nociception, the Complete Freund's Adjuvant (CFA) model of inflammatory‐induced mechanical hyperalgesia, and in a locomotor motility assay in male rats from the following strains; Lewis (LEW), Fischer (F344), Wistar Kyoto (WKY), and SD's from Envigo and Charles River.

Results

F344 and SD rats were similarly sensitive to morphine in hot plate and CFA‐induced inflammatory hyperalgesia (Minimum Effective Dose (MED) = 3.0 mg/kg). WKY rats developed a less robust mechanical hypersensitivity after CFA injection, and were less sensitive to morphine in both pain tests (MED = 6.0 mg/kg). LEW rats were completely insensitive to morphine in the hot plate test, in contrast to the reversal of CFA‐induced hyperalgesia (MED = 3.0 mg/kg). All strains exhibited a dose‐dependent reduction in locomotor activity at 3.0–6.0 mg/kg.

Conclusion

Sensory phenotyping in response to acute thermal and inflammatory‐induced pain, and sensitivity to morphine in various inbred and outbred rat strains indicates that different pathophysiological mechanisms are engaged after injury. This could have profound implications for translating preclinical drug discovery efforts into pain patients.

Significance

The choice of rat strain used in preclinical pain research can profoundly affect the outcome of experiments in relation to (a) nociceptive threshold responses, and (b) efficacy to analgesic treatment, in assays of acute and tonic inflammatory nociceptive pain.

In vivo profiling of four centrally administered opioids for antinociception, constipation and respiratory depression: Between-colony differences in Sprague Dawley rats

Outbred rodent stocks including Sprague Dawley rats, are known for their genetic diversity and so they are often used to develop animal models of human disease. Although between-colony differences in pharmaco-behavioural studies have been published previously, a direct head-to-head comparison study, whereby all research was performed in the same laboratory by the same experimenter utilising the supraspinal route of drug administration in the same strain of rat, is lacking. Herein, we report our head-to-head comparison study, involving assessment of antinociception, constipation and respiratory depression evoked by single bolus intracerebroventricular (ICV) doses of morphine, buprenorphine, DPDPE and U69,593 using male Sprague Dawley rats sourced from a different breeding colony (BC2) from that (BC1) used by us previously. Our data show that there are marked differences in the potency rank order for morphine and buprenorphine between rats sourced from BC2 and BC1. Although ICV morphine evoked a bell-shaped dose-response curve in the constipation test for rats from both colonies, this occurred at higher doses for rats from BC2. In conclusion, our head-to-head comparison shows considerable between-colony differences for the same rat strain, in the potency rank order of two clinically important strong opioid analgesics given by the ICV route.

Subcutaneous Implants of a Cholesterol-Triglyceride-Buprenorphine Suspension in Rats

A Target Animal Safety protocol was used to examine adverse events in male and female Fischer F344/NTac rats treated with increasing doses of a subcutaneous implant of a lipid suspension of buprenorphine. A single injection of 0.65 mg/kg afforded clinically significant blood levels of drug for 3 days. Chemistry, hematology, coagulation, and urinalysis values with 2- to 10-fold excess doses of the drug-lipid suspension were within normal limits. Histopathology findings were unremarkable. The skin and underlying tissue surrounding the drug injection were unremarkable. Approximately 25% of a cohort of rats given the excess doses of 1.3, 3.9, and 6.5 mg/kg displayed nausea-related behavior consisting of intermittent and limited excess grooming and self-gnawing. These results confirm the safety of cholesterol-triglyceride carrier systems for subcutaneous drug delivery of buprenorphine in laboratory animals and further demonstrate the utility of lipid-based carriers as scaffolds for subcutaneous, long-acting drug therapy.

Probuphine® (buprenorphine implant): a promising candidate in opioid dependence

Opioid dependence leads to physical dependence and addiction which finally results in profound medical, psychological and social dysfunction. One of the useful medications for opioid dependence is buprenorphine, the partial opioid agonist, which is used alone or in combination with naloxone. However, buprenorphine is the victim of its own success due to its illicit use and accidental poisoning in children. Also, buprenorphine typically requires daily self-administration and its effectiveness heavily depends on patient adherence. So, poor treatment adherence results in ineffective treatment manifesting as craving and withdrawal symptoms. Short-term use of buprenorphine in opioid dependence is also often followed by relapse. Buprenorphine when used sublingually often results in inadequate or fluctuating blood concentrations and poorer treatment retention compared with methadone. All of these led to the development of Probuphine®, a polymeric matrix composed of ethylene vinyl acetate and buprenorphine in the form of implants, that are implanted subdermally in office practice and deliver the active drug over 6 months. Buprenorphine release from such implant is fairly consistent, avoiding plasma peaks and troughs, and the implant is also reported to be safe. In this review article, we have highlighted these aspects of treatment of opioid addiction, stressing on the pharmacology of buprenorphine and Probuphine®, and relevant clinical trials addressing the efficacy and safety of Probuphine®. This sustained-release implantable formulation of buprenorphine has the potential to be a suitable alternative to daily or alternate day sublingual buprenorphine which can thereby eliminate the need for daily supervision, minimizing fluctuations in plasma concentrations, and allowing these patients to reduce clinic or pharmacy visits.

Antinociceptive effects of voluntarily ingested buprenorphine in the hot-plate test in laboratory rats

Researchers performing experiments on animals should always strive towards the refinement of experiments, minimization of stress and provision of better animal welfare. An adequate analgesic strategy is important to improve post-operative recovery and welfare in laboratory rats and mice. In addition, it is desirable to provide post-operative analgesia using methods that are minimally invasive and stressful. This study investigated the antinociceptive effects of orally administered buprenorphine ingested in Nutella® in comparison with subcutaneous buprenorphine administration. By exposing the animal to a thermal stimulus using a hot plate, significant antinociceptive effects of voluntarily ingested buprenorphine administered in Nutella® were demonstrated. This was evident at doses of 1.0 mg/kg 60 and 120 min post administration ( P < 0.01), although antinociceptive effects were not as marked as with subcutaneous administration, and had a later onset. It is advised to administer the oral formulation of buprenorphine in Nutella® in a 10-fold higher dose, as well as approximately 60 min earlier, than with the more commonly employed subcutaneous route of administration.

Long-term administration of high doses of transdermal buprenorphine in cancer patients with severe neuropathic pain

Background

Buprenorphine is often administered by the transdermal route (transdermal buprenorphine [TB]) in cancer patients with severe neuropathic pain. However, high doses of TB of 140 µg/h are rarely used.

Patients and methods

Three cancer patients with severe neuropathic Numeric Rating Scale (NRS) pain scores of 8–10 who were successfully treated with high doses of TB up to 140 µg/h along with other opioids and adjuvant analgesics.

Results

TB was administered for a long period of follow-up (9 months to 4 years, including 34–261 days of treatment with the dose of 140 µg/h), which allowed achievement of satisfactory analgesia (NRS 3–5) and good treatment tolerance. In all three patients, TB dose was gradually titrated from 35 to 140 µg/h, and all patients used morphine at least for some time for breakthrough and background pain management along with adjuvant analgesics. Two patients continued the treatment with TB until the end of life, and one patient is still receiving the treatment.

Conclusion

TB at doses of up to 140 µg/h in cancer patients with severe neuropathic pain seems to be effective and safe in combination with other opioids and with adjuvant analgesics, and may significantly improve patients’ quality of life. Clinical studies may explore higher than maximal 140 µg/h TB doses recommended by a manufacturer, and also in combination with other opioids and adjuvant analgesics.

HIV-gp120 and physical dependence to buprenorphine

BACKGROUND

Opioids are among the most effective and commonly used analgesics in clinical practice for severe pain. However, the use of opioid medications is clinically limited by several adverse properties including dependence. While opioid dependence is a complex health condition, the treatment of HIV-infected individuals with opioid dependence presents additional challenges. The goal of this study was to examine the physical dependence to buprenorphine in the context of HIV.

METHODS

Young adult male rats (Sprague-Dawley) were pretreated with HIV-1 envelope glycoprotein 120 (gp120) injected into the periaqueductal gray area (PAG) and we examined the impact on physical dependence to opioid.

RESULTS

It was found that the physical dependence to methadone occurred earlier than that to buprenorphine, and that gp120 did not enhance or precipitate the buprenorphine withdrawal.

CONCLUSION

The results suggest that buprenorphine could be the better therapeutic option to manage opioid dependence in HIV.

Differences in basal and ethanol-induced levels of opioid peptides in Wistar rats from five different suppliers

One major cause for discrepancies in results from animal experimental studies is the use of different animal strains and suppliers. We have previously reported that Wistar rats from five different suppliers display profound differences in ethanol intake and behavior. One of the neurobiological processes that could be underlying these differences is the endogenous opioid system, which has been implicated in the rewarding and reinforcing effects of alcohol. We therefore hypothesized that the differences between the supplier groups would also be evident in the endogenous opioid system. Radioimmunoassay was used to determine the levels of the opioid peptides Met-enkephalin-Arg(6)Phe(7) and dynorphin B in several brain areas of ethanol-drinking and ethanol naïve Wistar rats from five different suppliers. In the ethanol naïve animals, differences between the supplier groups were found in the pituitary gland, hypothalamus, frontal cortex, dorsal striatum and hippocampus. In the ethanol-drinking rats, differences were found in the same structures, with the addition of medial prefrontal cortex and substantia nigra. Correlations between ethanol intake and peptide levels were also found in several of the areas examined. The structures in which differences were found have all been implicated in the transition from drug use to addiction and these differences may lead to different propensities and vulnerability to this transition. Because the endogenous opioids have been suggested to be involved in a number of neurobiological disorders the results do not only have implications for research on alcohol or drug addiction, but many other fields as well.

Transfer of neuroplasticity from nucleus accumbens core to shell is required for cocaine reward

It is well established that cocaine induces an increase of dendritic spines density in some brain regions. However, few studies have addressed the role of this neuroplastic changes in cocaine rewarding effects and have often led to contradictory results. So, we hypothesized that using a rigorous time- and subject-matched protocol would demonstrate the role of this spine increase in cocaine reward. We designed our experiments such as the same animals (rats) were used for spine analysis and behavioral studies. Cocaine rewarding effects were assessed with the conditioned place preference paradigm. Spines densities were measured in the two subdivisions of the nucleus accumbens (NAcc), core and shell. We showed a correlation between the increase of spine density in NAcc core and shell and cocaine rewarding effects. Interestingly, when cocaine was administered in home cages, spine density was increase in NAcc core only. With anisomycin, a protein synthesis inhibitor, injected in the core we blocked spine increase in core and shell and also cocaine rewarding effects. Strikingly, whereas injection of this inhibitor in the shell immediately after conditioning had no effect on neuroplasticity or behavior, its injection 4 hours after conditioning was able to block neuroplasticity in shell only and cocaine-induced place preference. Thus, it clearly appears that the neuronal plasticity in the NAcc core is essential to induce plasticity in the shell, necessary for cocaine reward. Altogether, our data revealed a new mechanism in the NAcc functioning where a neuroplasticity transfer occurred from core to shell.

Analgesic efficacy of buprenorphine in the presence of high levels of SDF-1α/CXCL12 in the brain

Although morphine is often the best option for treating acute and chronic severe pain, its analgesic activity can be blocked in situations in which there are elevated levels of chemokines. Indeed, recently we have shown that elevated brain levels of the chemokine stromal cell-derived growth factor-1alpha (SDF-1α/CXCL12, the ligand of the HIV co-receptor CXCR4) diminish the antinociceptive effect of morphine. The purpose of the present study was to investigate whether such an effect is restricted to morphine or extends to other opioid medications such as buprenorphine. A sterilized stainless-steel C313G guide cannula was implanted into the periaqueductal grey (PAG), a brain region critical to the processing of pain signals, and a primary site of action of many analgesic compounds. The cold-water (-3°C) tail-flick test (CWT) was used to measure antinociception. Rats were pretreated with SDF-1α/CXCL12 administered into the PAG, and the antinociceptive actions of buprenorphine were measured. Direct infusion of SDF-1α/CXCL12 into the PAG failed to alter the antinociceptive action of buprenorphine. The presence of SDF-1α/CXCL12 in the PAG differentially alters the antinociceptive function of opioid medications. While it was able to diminish the antinociception induced by morphine (Adler et al., 2006), SDF-1α/CXCL12 did not affect the buprenorphine-induced antinociception. Buprenorphine appears to be more effective in the presence of high levels of SDF-1α/CXCL12 in the brain (which frequently occurs during neuroinflammatory conditions).

Clinical interpretation of pharmacokinetic and pharmacodynamic data in zoologic companion animal species

The treatment and prevention of pain in zoologic companion animals is difficult because of the lack of data available on the safety and efficacy of analgesics. Pharmacokinetic (PK)-pharmacodynamic (PD) studies integrate changes in drug concentrations and changes in the drug's effect. All experimental studies assessing the PDs of analgesics have limitations in animals, but the data provided by experimental studies are valuable in designing dosages. Placebo-controlled, randomized, and blinded clinical trials provide the best PK and PD data, but are rarely performed in major veterinary species because of the number of animals required for the study, lack of preliminary PK and PD data in a given species, species-specific differences in PK and PD, and ethical and toxicologic concerns. The usefulness and limitations as well as considerations for interpreting PK, PD, and controlled clinical studies are discussed. An example of allometric analysis of buprenorphine in mammals is also included.

Buprenorphine for Neuropathic Pain—Targeting Hyperalgesia

Opioids are well known to relieve severe, acute, and chronic nociceptive pain, but neuropathic pain shows a relatively poor response to opioids. Buprenorphine, a partial mu and ORL-1-receptor agonist, kappa-delta receptor antagonist, interacts with different G proteins than potent mu agonists and hence is not cross-tolerant to standard opioids. Buprenorphine blocks central sensitization (hyperalgesia) that is commonly found with neuropathic pain. We present a patient with neuropathic pain and tactile allodynia in which buprenorphine alleviated the hyperalgesia to a greater extent than pain severity. We found buprenorphine to be effective in reducing hypersensitivity in neuropathic pain when pure mu agonists fail to produce a response or in individuals who are intolerant to pure mu agonists.

The importance of genetic background on pain behaviours and pharmacological sensitivity in the rat spared serve injury model of peripheral neuropathic pain

Neuropathic pain conditions can encompass a diverse constellation of signs and symptoms consisting of sensory deficits, allodynia and hyperalgesia. Animal models of neuropathic pain have enabled the identification of key pathophysiological changes occurring within nociceptive pathways as a result of injury, and serve an invaluable role for preclinical screening of novel analgesic candidates. We have produced the first systematic description of the development and maintenance, and the pharmacological sensitivity of nociceptive behaviours in four rat strains with different genetic background (outbred Sprague-Dawley and inbred Brown Norway, Lewis and Fischer 344 rats), using the spared nerve injury model of peripheral neuropathic pain. Hindpaw mechanical hypersensitivity was evident from 7 to 30 days post-injury in all four strains, developing most quickly and severely in Fischer 344 rats; Lewis rats were least affected. Morphine (6 but not 3 mg/kg, s.c.) and gabapentin (100 but not 50 mg/kg, s.c.) had significant antiallodynic and antihyperalgesic actions in all four strains after spared nerve injury, although marked differences in potency across strains were observed. Two strains (Fischer 344 rats and Lewis) were insensitive to the antihyperalgesic properties of gaboxadol (15 mg/kg) whereas gaboxadol (6 mg/kg) was equipotent to morphine (6 mg/kg) in two other strains (Sprague-Dawley and Brown Norway). The observed pharmacogenetic variations have important implications for the preclinical testing of drugs, and provide a basis for development of pharmacogenomics in neuropathic pain.

Novel depots of buprenorphine have a long-acting effect for the management of physical dependence to morphine

Buprenorphine is a promising new pharmacotherapy for the management of physical dependence to opioids. The aim of the study was to evaluate the duration of action of several novel depots of buprenorphine in the treatment of physical dependence to morphine in mice. Following intramuscular injection, the duration of action of several novel oil-based depots of buprenorphine base in morphine-dependent mice were evaluated. The traditional dosage form of buprenorphine hydrochloride in saline was used as control. We found that the depot of buprenorphine base in sesame oil produced a dose-related long-lasting effect. On an equimolar basis of 6 micromol kg(-1), its effect was 5.7-fold longer than that of buprenorphine hydrochloride in saline. When prepared in several other oleaginous vehicles (castor oil, cottonseed oil, peanut oil and soybean oil), buprenorphine base also produced a long-lasting effect, which was similar to buprenorphine base in sesame oil. In conclusion, buprenorphine base, when prepared in oleaginous vehicles and injected intramuscularly in mice, produced a long-lasting effect on physical dependence to morphine.

Influence of low doses of naltrexone on morphine antinociception and morphine tolerance in male and female rats of four strains

In a recently proposed bimodal opioid receptor model, the inhibitory actions of opioids on action potential duration in dorsal root ganglion neurons have been proposed to produce antinociception, and the excitatory actions of hyperalgesia. Recent studies indicate that selectively blocking these excitatory actions with low doses of opioid antagonists enhances opioid antinociception and attenuates the development of opioid tolerance. To determine if the excitatory actions of opioids contribute to sex as well as strain differences in opioid sensitivity, the effects of morphine alone and in combination with low doses of naltrexone were examined in male and female rats of four strains. The strains examined differed in their sensitivity to opioid antinociception and magnitude of sex differences in opioid sensitivity. All testing was conducted using a thermal tail-flick procedure with the nociceptive stimulus intensity adjusted so that baseline latencies were comparable across strains/sexes. In chronic studies, the morphine dosing regimen was adjusted in each strain/sex to produce comparable levels of tolerance. In each of the strains tested, morphine produced dose-dependent increases in antinociception, with differences in morphine potency observed across strains and sexes. In male and female Sprague-Dawley and Long-Evans rats, naltrexone enhanced morphine antinociception and attenuated the development of morphine tolerance. These effects were not observed in F344 and Lewis rats, even when tests were conducted across a range of morphine and naltrexone doses. These results suggest that the ability of low doses of naltrexone to enhance opioid antinociception does not contribute to sex or rat strain differences in opioid sensitivity.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Eye-wiping test: a sensitive animal model for acute trigeminal pain studies

The possibility of introducing eye-wiping test as a model of acute pain was examined in rat, and it was compared with the well-known hot plate test. One drop of NaCl 5 M was placed into the animal eye, and the number of eye wipes with the ipsilateral forelimb was counted during 30 s. The withdrawal latency in hot plate test was also examined. Afterward, animals were treated with morphine (1, 2, 4, 6, 8 or 10 mg/kg), imipramine (25 mg/kg), sodium salicylate (250 mg/kg) or saline (i.p). After 30 min, the animals were tested again with eye-wiping and hot plate tests. Our results showed that morphine injection dose dependently decreased the number of eye wipes and increased the response latency to hot plate tests. There was a good correlation between the analgesic effects of morphine on responses to both tests, however, morphine produced more pain relief in eye-wiping test. Imipramine significantly decreased the number of eye wipes and increased the response latency to hot plate test, while sodium salicylate and saline injection did not. It may be concluded that the eye-wiping test can be used as a reliable method in trigeminal pain studies, which is sensitive to opioid and tricyclic antidepressant in rat.

Comparative actions of the opioid analgesics morphine, methadone and codeine in rat models of peripheral and central neuropathic pain

Controversy persists in relation to the analgesic efficacy of opioids in neuropathic pain. In the present study the effects of acute, subcutaneous administration of the mu-opioid receptor agonists morphine, methadone and codeine were examined in rat models of peripheral and central neuropathic pain. In the spared nerve injury (SNI) and chronic constriction injury (CCI) models of peripheral neuropathic pain, both morphine (6mg/kg) and methadone (3mg/kg) attenuated mechanical allodynia, mechanical hyperalgesia and cold allodynia for up to 1.5h post-injection (P<0.05); codeine (30mg/kg) minimally alleviated mechanical hypersensitivity in SNI, but not CCI rats. When administered to rats with photochemically-induced spinal cord injury (SCI), morphine (2 and 6mg/kg) and methadone (0.5-3mg/kg) robustly attenuated mechanical and cold allodynia for at least 2h post-injection (P<0.05). Codeine (10 and 30mg/kg) also attenuated mechanical and cold allodynia in this model for at least 3h after injection. The magnitude of opioid-mediated antinociception was similar between SNI, SCI and non-injured rats as measured in the tail flick test. At antinociceptive doses, no motor impairment as determined by the rotarod test was observed. The therapeutic window (based on antiallodynia versus ataxia) obtained for codeine, was vastly superior to that obtained with morphine or methadone in SNI and SCI rats. Furthermore, the therapeutic window for codeine in SCI rats was 4-fold greater than in SNI rats. Our results further support the efficacy of mu-opioid receptor agonists in alleviating signs of neuropathic pain in animal models of peripheral and especially central nerve injury.

Buprenorphine in cancer pain

Buprenorphine is a broad spectrum, highly lipophilic, and long-acting partial mu opioid receptor agonist that is noncross tolerant to other opioids. Buprenorphine can be given by several routes. Metabolism is through CYP3A4 and CYP2C8 and by conjugases. Constipation and sexual dysfunction appear to be less with buprenorphine than with other opioids. The recent development of a polymer matrix patch delivery system for buprenorphine prevents "dose dumping" and facilitates pain management in those unable to take oral analgesics. Sublingual buprenorphine has been combined with naloxone to prevent illicit conversion to parenteral administration. Buprenorphine has been used extensively to control cancer pain. In certain clinical situations, buprenorphine may have particular advantages over other opioids.

Centrally-mediated antinociceptive actions of GABAA receptor agonists in the rat spared nerve injury model of neuropathic pain

Gamma aminobutyric acid (GABA) plays a major role in the central hyperexcitabilty associated with nerve damage. The precise antinociceptive actions mediated by GABA(A) receptor agonists remain unclear as previous studies have shown mixed results in neuropathic pain models. Thus, various drugs which modulate GABA(A) receptor function were tested in the rat spared nerve injury (SNI) model of neuropathic pain. The selective GABA(A) receptor agonist gaboxadol dose-dependently (6 and 15 mg/kg, s.c.) reversed hindpaw mechanical allodynia and hyperalgesia for at least 150 min after administration. The GABA(A) receptor agonist muscimol (0.02-2 mg/kg, s.c.) also dose-dependently reversed mechanical allodynia, although the maximal effect achieved was less than that observed for gaboxadol. Mechanical hyperalgesia was attenuated only by the highest dose of muscimol. In contrast, the selective GABA(A) receptor agonist isoguvacine (20 mg/kg, s.c.) which has poor central nervous system penetration, and the benzodiazepine-site ligand zolpidem (20 mg/kg, s.c.) were ineffective against either nociceptive behaviour. In the rotarod test, both gaboxadol (15 mg/kg) and zolpidem impaired motor function for at least 60 min after injection; muscimol (2 mg/kg) and gaboxadol (6 mg/kg) were ineffective. Importantly, the ataxic effects induced by gaboxadol resolved 1-2 h after administration, a time point where clear antiallodynic and antihyperalgesic actions still occurred. Thus, systemic administration of blood-brain penetratable selective GABA(A) receptor agonists attenuate nociceptive behaviours in the SNI rat model of neuropathic pain that can be considered to occur independently of other effects on motor function.