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The primary aim of the study was to describe and correlate pain behavior and changes in bone morphology in animal models of arthritis both in rats and guinea pigs. Either complete Freund's adjuvant (CFA) or mono-iodoacetate (MIA) solution was injected into the left knee joint to obtain a model for rheumatoid arthritis and osteoarthritis, respectively. Subsequently, animals were behaviorally tested during a period of 12 days after CFA injection and at least 19 days after MIA injection. During these observation periods increasing pain behavior was observed, characterized by decreased von Frey mechanical thresholds and weight bearing on the affected limb. In Hargreaves' paw flick test slightly increased thermal hypersensitivity was observed in some instances in guinea pigs. In rats there was also decreased limb-use during forced ambulation. To evaluate bone destruction mu-computed tomography scans of the arthritic knee were taken on the last experimental day. Different bone parameters indicative of osteolysis and decreased trabecular connectivity were significantly correlated with the observed pain behavior. Detailed description of morphological changes in arthritic joints better characterizes the different animal models and might add to the knowledge on the working mechanisms of analgesic compounds that have an influence on bone structures in arthritis.

Chronic pain resulting from metastasis into skeleton of certain neoplastic diseases remains poorly understood and relatively resistant to analgesic treatment. Opioids are the principal axis in drug therapy for this type of pain, especially at the end stage of cancer. Our aim was to examine whether, fentanyl as well as morphine, two potent analgesic opioids commonly used to treat cancer pain, would inhibit pain and bone lesion-related responses in a murine model of bone cancer pain. Repeated administration of equianalgesic doses of fentanyl (0.16 mg/kg s.c. once a day) and morphine (20 mg/kg s.c. once a day) initiated at day 1 (prophylactic treatment) or at day 7 (curative treatment) after tumor cell inoculation in the femoral cavity consistently decreased bone pain symptoms and tumor growth-induced bone destruction (micro-CT bone structure parameters). Both fentanyl and morphine treatments resulted in clear antinociceptive properties as well as reductions in cancer cell-induced bone lesions. The present results demonstrate that fentanyl, and to some lesser degree morphine, has potential benefits in the treatment and development of bone cancer pain. As such, chronic administration of high doses of certain opioids like fentanyl may have clinical utility in the management of bone cancer pain.

The intramedulary injection of osteosarcoma cells in the mouse femur has served as a laboratory model to study bone cancer pain. However, the efficacy of different classes of analgesics has not fully been analyzed in this model. Therefore, the acute antinociceptive properties of different classes of drugs were evaluated on post-inoculation day 15 when the degrees of spontaneous pain and mechanical hypersensitivity in the ipsilateral inoculated hind paw reached almost their maximal effects. At high doses, the opioids fentanyl, morphine, and tramadol had full efficacies for all pain parameters tested. Antagonism experiments with naloxone (10 mg/kg s.c.) or its peripheral analogue methylnaltrexone (10 mg/kg s.c.), suggest that the analgesic effects of fentanyl were predominantly mediated by centrally located mu-opiate receptors. Acetaminophen, the non-steroidal anti-inflammatory drug indomethacin, and the COX-2-inhibitor celecoxib did not significantly improve pain behavior. The tricyclic antidepressants amitriptyline and desipramine significantly reduced spontaneous pain behavior but this only at sedative doses; the serotonin reuptake inhibitor fluoxetine had limited efficacy. Also with the anticonvulsants lamotrigine, topiramate, and gabapentin limited or no efficacies were found. In conclusion, the present study provided integrated information about the tumor-induced bone pain in mice, and clarified acute efficacies of different categories of analgesics for the spontaneous lifting, limb-use impairment, and mechanical hypersensitivity. Moreover, the finding that bone cancer-pain behaviors are attenuated by various established compounds further supports the validity of the murine bone cancer model for the study of bone cancer pain and its use for the identification of novel treatments.

Research on mechanisms of drug action, and preclinical screening of molecules with a potential activity on neuropathic pain requires extensive animal work. The chronic constriction injury model is one of the best-characterized models of neuropathic pain behavior in rats, but requires extensive time consuming operations and animal handling. The formalin test is easier to perform, and a well validated model. The latter may serve as an effective prescreening test of molecules and may facilitate drug targeting. In the present study the activity of different pharmacological reference compounds was tested in rats and gerbils on the cold plate for animals that had undergone chronic constriction injury and in the second phase of the formalin test. In rats, a comparable outcome in both test conditions was observed for morphine, fentanyl, MK-801 and flunarizine. Clonidine had more activity in the second phase of the formalin test, whereas baclofen, tramadol, amitryptiline, ketamine and topiramate showed more activity in the cold plate. In gerbils, both test conditions yielded comparable results for fentanyl and ketoprofen. Tramadol and CP-96345 tended to have more activity in the second phase of the formalin test, whereas morphine, SR-48968, SR-142801 and R116301 demonstrated more activity in the cold plate test. This study demonstrates a good correlation between the second phase of the formalin test and the cold allodynia in the CCI model for, both for rats and gerbils. Drugs with a proven activity in humans, used as reference compounds, also showed good pharmacological activity in this animal study.

The present study was conducted to evaluate the pain development and bone destruction during bone cancer growth in a murine model of bone cancer pain and to evaluate the analgesic efficacy of fentanyl, sufentanil, and morphine in this model. C3H/HeNCrl mice were inoculated into the intramedullary space of the femur with osteolytic NCTC 2472 fibrosarcoma cells, and followed during a 3-week period to assess pain behaviors (spontaneous lifting and limb-use during forced ambulation on rotarod) and bone destruction (parameters indicative of bone lesions determined by microCT-scans of the tumor-bearing bones) during bone cancer growth. The results showed that in this murine model of cancer-induced bone pain, behavioural manifestations of pain emerge in parallel with the progression of bone destruction. The subcutaneous administration of fentanyl (0.025-0.64 mg/kg), sufentanil (0.005-0.04 mg/kg), and morphine (2.5-40 mg/kg) on the test days 15 and 22 post-inoculation reduced pain-related behaviors in a dose dependent manner. A complete relief from pain-related behaviors was achieved with the following doses: > or =0.16 mg/kg fentanyl, 0.02 mg/kg sufentanil, and 20 mg/kg morphine. In conclusion, the results showed a clear link between tumor growth-induced bone destruction and behavioral pain manifestations, the latter was effectively controlled by the opioids fentanyl, sufentanil, and morphine.

Reduced side-effect liability of opioids may enhance the patient's quality of life and decrease the incidence of opioid-insensitive pain. Literature offers few comparative data between different opioids at equianalgesic doses. Therefore morphine, fentanyl, buprenorphine, codeine, hydrocodone and oxycodone were compared for analgesic properties and side-effect profiles in rats. Analgesic efficacy was analysed using a tail withdrawal test for acute thermal nociception, a formalin test for chemically induced inflammatory pain and a von Frey test for mechanical hypersensitivity. For side-effect profiling inhibition of gastrointestinal activity was evaluated in a charcoal and ricinus oil test, arterial PCO(2) was determined for measuring respiratory depression, the discriminative stimulus properties linked to the narcotic cue were assessed using a drug discrimination learning test, and motor coordination was tested through rotarod performance. ED(50)'s for the occurrence of side-effects were compared to ED(50)'s in behavioural pain tests. Fentanyl had a strong analgesic potency and, compared to other opioids, an acceptable side-effect profiling at analgesic ED(50)'s. Also consistent was the ceiling effect of buprenorphine implying an increased safety margin for side-effects, but a decreased analgesic efficacy. Differences between opioids as observed in this study can have important indications for their use in acute as well as in the onset of chronic treatments.

Described here is a comparison of five peripheral sciatic nerve injury models in rats which all result in various degrees of neuropathic pain symptoms. They are the chronic constriction injury (CCI), the spinal nerve ligation (SNL), the partial sciatic ligation (PSL), the tibial and sural transection (TST), and the complete sciatic transection (CST) model. Behavioral testing was performed on these models over a 56-day period under strict experimental conditions to minimise variability between the surgical models and to allow for an accurate evaluation of the sensory deficits produced by each model. Overall, all five models of neuropathic pain produced signs of allodynia and hyperalgesia to various stimuli. However, the duration and magnitude of the evoked responses varied considerably between the different models.

The primary aim of the study was to correlate pain development during bone cancer growth with objectively obtained tumor-induced changes in bone morphology. Additionally morphine sensitivity of this bone pain was evaluated. Mice were injected into the femur with osteolytic NCTC2472 cells, and behaviorally followed during a 3-week period. During the observation period increasing pain behavior was observed in tumor-bearing animals. Tumor mice exhibited spontaneous and movement-evoked lifting, the latter evoked through non-noxious palpation of the tumor. Limb use during forced ambulation on a rotarod decreased to substantial non-use of the affected limb by day 23. On day 23, micro-computer tomography scans of the tumor-bearing bones were evaluated for bone destruction. Different bone parameters indicative of osteolysis or fragmentation were significantly correlated with pain behavior. In a separate group of mice the effects of different morphine doses on pain behavior were evaluated on days 17 and 21 of tumor growth. Spontaneous lifting and movement-evoked lifting were sensitive to morphine treatment, although stress-induced analgesia due to repeated restraint might minimize movement-evoked lifting in mice. Limb use during forced ambulation was only slightly ameliorated by high morphine doses.

Mu-, kappa- and delta-opioid receptor agonists are reported to attenuate the acetic-acid-induced abdominal constriction response in mice. NK-1, -2 and -3 receptor antagonists also display activity in several visceral pain models. As the gerbil NK-1 receptor is comparable to the human receptor, we evaluated the efficacy of NK-1, -2 and -3 receptor antagonists and opioids (both alone and in combination) in the writhing test in this species. The effects of a selective L-type calcium (Ca2+) channel antagonist on the writhing response were also assessed to determine the contribution of Ca2+ channel antagonism to the antinociceptive effects of the NK-1 antagonists. Gerbils received subcutaneous injections of either the mu-opioids morphine or fentanyl, the kappa-opioid U50,488-H, the delta-opioid SNC80, NK-1 antagonists R116301, CP-96,345 or GR203040, the NK-2 antagonist SR-48968, the NK-3 antagonist SR-142801 or the Ca2+ channel antagonist nimodipine. Writhing was evoked 1 h after treatment by intraperitoneal injection of 0.2 ml 1% acetic acid solution and the frequency was recorded. Morphine, fentanyl and U50,488-H attenuated the writhing response dose dependently with complete inhibition occurring at the highest doses. SNC80 did not significantly attenuate the writhing response even at a dose of 40 mg/kg. The tachykinin NK-1 antagonists CP-96,345 and GR203040, the NK-2 antagonist SR-48968 and the NK-3 antagonist SR-142801 reduced the writhing frequency although without complete inhibition. The NK-1 antagonist R116301 displayed limited activity at doses up to 40 mg/kg. Nimodipine did not exhibit any antinociceptive efficacy in this assay. Adding the NK-1, -2 or -3 antagonists to the opioids did not improve the efficacy of the opioids. Selective NK antagonists may therefore be effective in a visceral nociception assay in gerbils but do not modulate opioid action.

Substance P (SP) acts as a transmitter of nociception in both the peripheral and the central nervous system. Because the NK-1 receptors in gerbils are comparable to those in humans, gerbil models could be used to study the role of SP in neuropathic pain. A modification of the rat chronic constriction injury (CCI) model of neuropathic pain was produced in male gerbils by placing four loose chromic catgut ligatures around the sciatic nerve. This procedure clearly resulted in mechanical hypersensitivity. Intraplantar injections of SP and the selective NK-1 receptor agonist, [Sar(9)-Met(O(2))(11)]-substance P (Sar-SP), to the paw ipsilateral to the nerve injury and intrathecal administration of these peptides produced paw-lifting behavior in the CCI gerbils in thermoneutral conditions. In sham-operated and nonoperated controls, no such effects were observed. Systemic administration of the NK-1 antagonist R116301 attenuated the SP and the Sar-SP-induced paw-lifting behavior in the CCI gerbils indicating the role of NK-1 receptors in these effects. Intraplantar injection of the highest dose of SP (200 ng) to the paw contralateral to the CCI produced lifting of the paw ipsilateral to the injury, indicative for spinal mechanisms especially since administration of SP to the ipsilateral front paw or even intracardially did not have any effect at all. The SP-induced responses were not antagonized by the NMDA antagonist MK801. These results indicate that the peripheral and spinal SP reveal an increased reactivity in a neuropathic pain model. This increased pain sensitivity seems to involve spinal NK-1 mechanisms.

Although exogenous opioids alter the responses of animals to tissue-damaging stimuli and therefore are the cornerstone in the treatment of acute antinociception, they have profound side effects on ventilation. To diminish ventilatory effects, combination therapies have been advocated. Recent studies reported the effectiveness of the addition of N-methyl-D-aspartate (NMDA) receptor antagonists such as ketamine to morphine in the treatment of acute pain. However, NMDA receptors, together with non-NMDA receptors are known to be involved in the neurotransmission of inspiratory drive to phrenic motoneurons. Co-administration of NMDA and non-NMDA receptor antagonists has been shown to be deleterious to respiratory function. The present study investigated the hypothesis that the association of opioids and NMDA receptor antagonists may add to the impairment of respiratory parameters. In male Wistar rats, combinations of opioids (fentanyl or morphine) at antinociceptive doses and NMDA receptor antagonists (ketamine, 40 mg/kg, or dextromethorphan, 10 mg/kg) at subanesthetic doses were administered intraperitoneally. Antinociception was tested with the tail-withdrawal reaction (TWR) test, while the effect on respiratory parameters was investigated with blood-gas analysis. We found that, in rats, co-administration of NMDA receptor antagonists and opioids may result in an increased respiratory depression as compared to the opioids alone. The effect of the NMDA receptor antagonists on opioid-induced antinociception was limited.

Animal and clinical studies have reported potentiation of opioid antinociception by NMDA receptor antagonists such as ketamine and dextromethorphan. The aim of this study was to compare these clinically available NMDA antagonists in combination with classical morphine, mu-selective fentanyl-like opioids, the delta-opioid agonist SNC80 and the kappa-opioid agonist U50,488H. Using a mouse hot-plate test, dose-response relationships were first determined for all compounds individually and then for opioids co-administered with fixed doses of ketamine or dextromethorphan. All compounds were administered intraperitoneally ED(50) values were calculated from the proportion of animals failing to exhibit any response within a fixed cut-off criterion of 30 s. To varying degrees, all compounds produced increases in response latencies over time. Dextromethorphan produced lower ED(50) values for morphine, fentanyl and sufentanil but exerted no effect on the potency of SNC80 or U50,488H. Similarly, ketamine potentiated the antinociceptive potency of morphine, fentanyl and sufentanil but not SNC80 or U50,488H. In summary, these results support the use of mu-opioid agonists in combination with NMDA antagonists, but suggest that there may be no advantage in combining dextromethorphan or ketamine with delta- or kappa-opioids in the management of acute pain.

In this study the possible antagonistic effects of five different antipsychotics on the discriminative stimulus properties of 10 mg/kg cocaine were evaluated by use of a two-lever food-reinforced drug discrimination procedure in rats. To do so, rats were treated with several doses of haloperidol, risperidone, seroquel, sertindole, and olanzapine, either at 60 or 120 min prior to testing. With all compounds tested, no substantial antagonism of the cocaine cue was observed. Only with haloperidol (maximum 60%), risperidone (maximal 20%), and olanzapine (maximal 20%) a partial antagonism without clearcut dose-response was observed. Clozapine, seroquel, and sertindole did not influence the discriminative stimulus properties of cocaine. These results indicate that antipsychotics with different pharmacological profiles are unable to antagonize more than partially the cueing properties of 10 mg/kg cocaine in rats, pointing to the unique underlying stimulus properties of this stimulant.

Although the interactions between the mu- and the delta-opiate receptor subtypes are well documented with regard to supraspinal analgesia, less is known about the mutual interactions on respiratory depression. To clarify the functional interactions between both opiate receptor subtypes with regard to antinociception and respiratory depression, male Wistar rats were intravenously injected with 2.5 microg/kg of the mu-opiate agonist sufentanil and subsequently intravenously challenged with the delta antagonist naltrindole (NTI) or naltrindole 5'-isothiocyanate (5'-NTII), a delta-2 antagonist. Antinociception was measured by means of the tail-flick latency, and respiratory depression was evaluated by means of analysis of PaCO2, PaO2, and oxygen saturation. To quantify the antagonistic properties of NTI and 5'-NTII, mean areas under the curve were calculated for groups treated with sufentanil, control vehicle, and sufentanil plus a dose of the antagonists. NTI, but not 5'-NTII, antagonized the sufentanil-induced antinociception at 10 mg/kg NTI. Below this dose the effects were inconsistent. The sufentanil-induced hypercapnia and hypoxia were diminished with 10 mg/kg NTI or 5'-NTII. These data indicate that NTI antagonizes the sufentanil-induced antinociception and respiratory depression in rats. A dissociation between the antinociception and respiratory depression following intravenous sufentanil could be obtained with 10 mg/kg 5'-NTII pointing to different regulatory effects of opiate delta receptor subtypes on mu-opiate agonist-induced behavioral effects.

This study investigates the influence of possible stress due to housing in Bolman cages on antinociception and on respiratory depression following opioid administration. To evaluate the functional role of this stressor and to modulate it, rats were subcutaneously pretreated with the anxiolytic chlordiazepoxide (CDP; 10 mg/kg) or saline (SAL) before the immobilization in the Bolman cages and before the intravenous administration of small doses of morphine (MOR), sufentanil (SUF), or vehicle (VEH). Antinociception, respiratory impairment and stress were evaluated by means of the tail-flick latency, blood gas analysis, and serum corticosterone (CS), adrenocorticotropic hormone (ACTH), and prolactin (PRL) determinations. The results demonstrated that 10 mg/kg CDP did not alter the antinociceptive effects of low doses of morphine and sufentanil. CDP pretreatment differentially affected the various blood gas parameters. Compared to vehicle pretreatment, there was a larger decrease in PaO2 following MOR and SUF in the CDP-pretreated rats. The effects were most pronounced at the lowest doses of both opioids. A CDP potentiation was also observed for the short-lasting raises in PaCO2 with the lowest concentrations of the opioids. At higher concentrations of the opioids, CDP was without any effect. With regard to the stress hormones, immobilization and an intravenous injection resulted in increases in CS and PRL in both CDP- and VEH-pretreated rats. ACTH did not change in these controls. SUF prevented the CS raises independent of a CDP pretreatment, while ACTH only increased in the SUF plus CDP groups, pointing to a stress-reducing effect of SUF. Also, MOR without CDP prevented the increases in CS, but the opioid intrinsically increased ACTH. These results indicate that restraint in Bolman cages in the present setup, with animals recovering for several hours in these cages after being equipped with an arterial catheter, is stressful but without any significant effect on the opioid-induced antinociception. Pretreatment with an anxiolytic benzodiazepine only minimally affected the outcome of the opioids on respiratory depression and pointed to a stress-reducing effect of low doses of the opioids, especially sufentanil.

The present study was performed to evaluate the nature of the interaction between epidurally administered sufentanil and bupivacaine in producing antinociception in rats. Rats in which epidural catheters had been inserted received epidural injections with bupivacaine and sufentanil. Nociception was tested by use of the tail-withdrawal reaction (TWR) test and the hot-plate test. Isobolographic analyses were performed with fixed and variable dose ratio treatment schedules based on the ED50s and the highest inactive concentrations of the compounds in both tests. In the TWR test, a synergistic interaction was obtained between the two compounds independent of whether a variable dose ratio regimen (with either 0.08 microgram/rat sufentanil or 80 micrograms/rat bupivacaine as the preset component) or a fixed dose ratio of 1/1,000 sufentanil/bupivacaine (based on the individual ED50s) was used. In the hot-plate test, a synergistic interaction was observed only in the variable dose ratio regimen with 0.08 microgram/rat sufentanil as the preset component and in the fixed dose ratio regimen of 1/1,000 sufentanil/bupivacaine (a ratio based on the ED50 values of the TWR test) but not with a ratio of 1/200, as demonstrated by the ED50s of both drugs in the hot-plate test. The interaction between epidurally administered bupivacaine and sufentanil seems to be synergistic for both tests when variable and fixed dose ratios are used. The synergism could be more easily demonstrated in the TWR test. For drugs with a segmental action, the hot-plate test seems to be less optimal. The necessity of a minimal critical amount of bupivacaine to obtain synergism may have clinical implications.

The present experiments were designed to investigate the role of housing and handling conditions during testing, as well as data analysis, on the outcome of antinociceptive testing of alpha 2-adrenoceptor agonists, fentanyl, and a high dose of chlordiazepoxide in the tail withdrawal reaction test (TWR test) in rats. Dose-response curve data were obtained with fentanyl, clonidine, xylazine, dexmedetomidine, and 40.00 mg/kg chlordiazepoxide and were compared under normal TWR test conditions and during immobilization or immobilization with continuous painful stimulation. Data were analyzed in terms of all-or-none criteria as well as percentage maximum possible effect (%MPE) analysis over the total measurement period or at any specific time point during testing. The results indicate that stress, induced by immobilization and immobilization with long-term-applied paw pressure, unmasked possible antinociceptive properties of the various alpha 2-adrenoceptor agonists and potentiated the effects of fentanyl. Stress also unmasked the positive effects of benzodiazepines. The manner of data analysis was shown to significantly affect the outcome measured in stress and nonstress conditions. The MPE analysis, particularly at one time point, appeared much more sensitive than the all-or-none criteria. The data indicate that the housing and handling conditions of animals during testing, together with data analysis, may affect the outcome of different classes of compounds in the TWR test, and this knowledge may help control for false positive results.