Fentanyl-induced hyperalgesia and analgesic tolerance in male rats: common underlying mechanisms and prevention by a polyamine deficient diet.

Opioids are a mainstay of pain management but can induce unwanted effects, including analgesic tolerance and paradoxical hyperalgesia, either of which leads to increased pain. Clinically, however, the relationship between these two phenomena remains elusive. By evaluating changes in mechanical nociceptive threshold in male rats, we found that in contrast to a purely analgesic control response to a single subcutaneous administration of fentanyl (25 µg/kg), in rats subjected to inflammatory pain 2 weeks previously (Day0), the same test dose (D13) induced a bi-phasic response: initial decreased analgesia (tolerance) followed by hyperalgesia lasting several hours. Both the tolerance and hyperalgesia were further enhanced in rats that had additionally received fentanyl on D0. The dose-response profiles (5 fg to 50 µg/kg) of pain- and opioid-experienced rats were very different from pain/drug-naive rats. At ultra-low fentanyl doses (<5 ng/kg and <500 ng/kg for naïve control and pain/drug-experienced rats, respectively), solely hyperalgesia was observed in all cases. At higher doses, which now produced analgesia alone in naive rats, reduced analgesia (tolerance) coupled with hyperalgesia occurred in pain/fentanyl-experienced rats, with both phases increasing with dose. Transcriptomic and pharmacological data revealed that an overactivation of the spinal N-methyl-D-aspartate receptor-inducible NO synthase cascade plays a critical role in both acute tolerance and hyperalgesia, and together with the finding that the magnitudes of analgesia and associated hyperalgesia are negatively correlated, is indicative of closely related phenomena. Finally, a polyamine deficient diet prevented inducible NO synthase transcript upregulation, restored fentanyl's analgesic efficacy and suppressed the emergence of hyperalgesia.

Aminoguanidine Hydrazone Derivatives as Nonpeptide NPFF1 Receptor Antagonists Reverse Opioid Induced Hyperalgesia.

Neuropeptide FF receptors (NPFF1R and NPFF2R) and their endogenous ligand neuropeptide FF have been shown previously to display antiopioid properties and to play a critical role in the adverse effects associated with chronic administrations of opiates including the development of opioid-induced hyperalgesia and analgesic tolerance. In this work, we sought to identify novel NPFF receptors ligands by focusing our interest in a series of heterocycles as rigidified nonpeptide NPFF receptor ligands, starting from already described aminoguanidine hydrazones (AGHs). Binding experiments and functional assays highlighted AGH 1n and its rigidified analogue 2-amino-dihydropyrimidine 22e for in vivo experiments. As shown earlier with the prototypical dipeptide antagonist RF9, both 1n and 22e reduced significantly the long lasting fentanyl-induced hyperalgesia in rodents. Altogether these data indicate that AGH rigidification maintains nanomolar affinities for both NPFF receptors, while improving antagonist character toward NPFF1R.

RF313, an orally bioavailable neuropeptide FF receptor antagonist, opposes effects of RF-amide-related peptide-3 and opioid-induced hyperalgesia in rodents.

Although opiates represent the most effective analgesics, their use in chronic treatments is associated with numerous side effects including the development of pain hypersensitivity and analgesic tolerance. We recently identified a novel orally active neuropeptide FF (NPFF) receptor antagonist, RF313, which efficiently prevents the development of fentanyl-induced hyperalgesia in rats. In this study, we investigated the properties of this compound into more details. We show that RF313 exhibited a pronounced selectivity for NPFF receptors, antagonist activity at NPFF1 receptor (NPFF1R) subtype both in vitro and in vivo and no major side effects when administered in mice up to 30 mg/kg. When co-administered with opiates in rats and mice, it improved their analgesic efficacy and prevented the development of long lasting opioid-induced hyperalgesia. Moreover, and in marked contrast with the dipeptidic NPFF receptor antagonist RF9, RF313 displayed negligible affinity and no agonist activity (up to 100 µM) toward the kisspeptin receptor. Finally, in male hamster, RF313 had no effect when administered alone but fully blocked the increase in LH induced by RFRP-3, while RF9 per se induced a significant increase in LH levels which is consistent with its ability to activate kisspeptin receptors. Altogether, our data indicate that RF313 represents an interesting compound for the development of therapeutic tools aiming at improving analgesic action of opiates and reducing adverse side effects associated with their chronic administration. Moreover, its lack of agonist activity at the kisspeptin receptor indicates that RF313 might be considered a better pharmacological tool, when compared to RF9, to examine the regulatory roles of RF-amide-related peptides and NPFF1R in reproduction.

Nitrous oxide persistently alleviates pain hypersensitivity in neuropathic rats: A dose-dependent effect.

BACKGROUND: Despite numerous pharmacological approaches, there are no common analgesic drugs that produce meaningful relief for the majority of patients with neuropathic pain. Although nitrous oxide (N2O) is a weak analgesic that acts via opioid-dependent mechanisms, it is also an antagonist of the N-methyl-D-aspartate receptor (NMDAR). The NMDAR plays a critical role in the development of pain sensitization induced by nerve injury. OBJECTIVE: Using the chronic constriction injury of the sciatic nerve in male rats as a preclinical model of neuropathic pain, the first aim of the present study was to evaluate the lowest N2O concentration and the shortest time of N2O postinjury exposure that would produce persistent relief of neuropathic pain. The second aim was to compare the effects of N2O with gabapentin, a reference drug used in human neuropathic pain relief. METHODS: Changes in the nociceptive threshold were evaluated using the paw pressure vocalization test in rats. RESULTS: Among the various N2O concentrations tested, which ranged from 25% to 50%, only 50% N2O single exposure for 1 h 15 min induced a persistent (minimum of three weeks) and significant (60%) reduction in pain hypersensitivity. A single gabapentin dose (75 mg/kg to 300 mg/kg, intraperitoneally) induced an acute (1 h to 1 h 30 min) dose-dependent effect, but not a persistent effect such as that observed with N2O. CONCLUSIONS: These preclinical results suggest that N2O is advantageous for long-lasting neuropathic pain relief after sciatic nerve injury compared with other drugs used in humans such as gabapentinoids or NMDAR antagonists. The present preclinical study provides a rationale for developing comparative clinical studies.

Development of a peptidomimetic antagonist of neuropeptide FF receptors for the prevention of opioid-induced hyperalgesia.

Through the development of a new class of unnatural ornithine derivatives as bioisosteres of arginine, we have designed an orally active peptidomimetic antagonist of neuropeptide FF receptors (NPFFR). Systemic low-dose administration of this compound to rats blocked opioid-induced hyperalgesia, without any apparent side-effects. Interestingly, we also observed that this compound potentiated opioid-induced analgesia. This unnatural ornithine derivative provides a novel therapeutic approach for both improving analgesia and reducing hyperalgesia induced by opioids in patients being treated for chronic pain.

Exogenous and endogenous opioid-induced pain hypersensitivity in different rat strains.

BACKGROUND: Opioid-induced hyperalgesia (OIH) is a recognized complication of opioid use that may facilitate the development of exaggerated postoperative pain. OBJECTIVE: To examine the role of genetic factors on OIH by comparing four rat strains. Because the authors previously reported that the endogenous opioids released during non-nociceptive environmental stress induce latent pain sensitization, genetic and environmental factor interactions were also evaluated. METHODS: First, the propensity of Sprague Dawley, Wistar, Lewis and Fischer rats to develop OIH following single or repeated fentanyl exposures was compared by measuring the nociceptive threshold using the paw pressure vocalization test. Second, Sprague Dawley and Fischer rats were exposed to a series of three non-nociceptive environmental stress sessions to evaluate the ability of endogenous opioids to enhance hyperalgesia associated with a carrageenan-induced hind-paw inflammation test performed two weeks later. RESULTS: Sprague Dawley, Wistar and Lewis rats exhibited OIH, although differences were observed. OIH was not observed in Fischer rats. Inflammatory hyperalgesia enhancement induced through previous stress in Sprague Dawley rats was not observed in Fischer rats. CONCLUSIONS: The pain level not only reflects nociceptive inputs but also depends on both the history and genetic factors of the individual. Genetic and environmental models may provide new insights into the mechanisms that underlie individual differences observed in postoperative pain.

A polyamine-deficient diet opposes hyperalgesia, tolerance and the increased anxiety-like behaviour associated with heroin withdrawal in rats.

In humans, hyperalgesia, tolerance and anxiety disorders are common symptoms during heroin withdrawal syndrome. Significant evidence supports a role of NMDA receptors in these phenomena. Because polyamines may positively modulate the functioning of NMDA receptors and mainly originate from dietary intake, one hypothesis is that a polyamine deficient diet (PD diet) may reduce withdrawal symptoms. To address this question, we investigated the ability of a PD diet to prevent or to alleviate some symptoms of withdrawal syndrome as hyperalgesia, and increased anxiety-like behaviour in rats receiving 14 once daily subcutaneous heroin injections. Here, we show that a PD diet has both preventive and curative properties for reducing certain signs of withdrawal such as hyperalgesia, tolerance and increased anxiety-like behaviour observed in rats fed with a standard diet. Moreover, in heroin-withdrawn rats which were returned to basal pain sensitivity level, hyperalgesia following acute analgesia induced by a single heroin dose was observed in heroin-treated rats fed with standard diet, not in rats fed with a PD diet. Similarly, a stress-induced hyperalgesia induced by a non-nociceptive environmental stress session was observed in heroin-treated rats fed with standard diet. In contrast, a stress-induced analgesia was observed in heroin-treated rats fed with a PD diet, as it was observed in non heroin-treated rats. Since a PD diet for several weeks did not induce appreciable side-effects in rats, these preclinical results suggest that a PD diet could be an effective strategy for improving the relief of certain negative emotional states of heroin withdrawal syndrome and to allow reducing other medications generally used, such as opioid maintenance drugs.

Development of sub-nanomolar dipeptidic ligands of neuropeptide FF receptors.

Based on our earlier reported neuropeptide FF receptors antagonist (RF9), we carried out an extensive structural exploration of the N-terminus part of the amidated dipeptide Arg-Phe-NH(2) in order to establish a structure-activity relationships (SAR) study towards both NPFF receptor subtypes. This SAR led to the discovery of dipeptides (12, 35) with subnanomolar affinities towards NPFF1 receptor subtype, similar to endogenous ligand NPVF. More particularly, compound 12 exhibited a potent in vivo preventive effect on opioid-induced hyperalgesia at low dose. The significant selectivity of 12 toward NPFF1-R indicates that this receptor subtype may play a critical role in the anti-opioid activity of NPFF-like peptides.

A high-dose of fentanyl induced delayed anxiety-like behavior in rats. Prevention by a NMDA receptor antagonist and nitrous oxide (N(2)O).

Postoperative negative affects such as anxiety need to be better understood and treated to improve patient recovery. The present study evaluates the effect of a single exposure to a high-dose of opioid (4×100 µg/kg fentanyl injections in 15 min intervals resulting in a total dose of 400 µg/kg), as used for surgery in humans, on anxiety-like behavior (ALB) in rats. First, the level of anxiety was evaluated 24-h after a high-dose of fentanyl using an elevated plus-maze apparatus. Second, the preventive effect of BN2572, a N-methyl-D-aspartate receptor (NMDA-R) antagonist, and 50% nitrous oxide (N(2)O), a gas with NMDA-R antagonist properties, was assessed. A significant increase in ALB was observed in fentanyl-treated rats. Interestingly, fentanyl-induced ALB was prevented by BN2572, suggesting a NMDA-dependant pathway. Fentanyl-induced ALB was also prevented by a single 50% N(2)O exposure. The present study provides evidence that deleterious outcomes of opioid use, referred to as "post-opioid syndrome", include not only pain hypersensitivity as previously described, but also negative affects such as anxiety. Since N(2)O prevents elements of this post-opioid syndrome, we speculate that N(2)O could be a good therapeutic agent to facilitate postoperative rehabilitation.

Sciatic nerve block fails in preventing the development of late stress-induced hyperalgesia when high-dose fentanyl is administered perioperatively in rats.

UNLABELLED: : Sciatic nerve block fails in preventing the development of late stress-induced hyperalgesia (SIH) when high-dose fentanyl is administered perioperatively in rats. BACKGROUND AND OBJECTIVES: The aim of our study was to evaluate the effect of regional anesthesia (RA) on hyperalgesia and long-term pain vulnerability after surgery in rats exposed or not to high doses of fentanyl intraoperatively. METHODS: Experiment 1 evaluated the effects of D0 RA on hyperalgesia after incision and on the variations of nociceptive threshold (NT) after non-nociceptive environmental stress (NNES) at D10. Four groups were compared: control K1 (saline in sciatic nerve catheter, no plantar surgery), I (incision: saline in sciatic nerve catheter and plantar surgery), ISSR (incision-single-shot ropivacaine: single-shot ropivacaine, plantar surgery), and IMSR (incision-multiple-shot ropivacaine: 1 shot of ropivacaine, plantar surgery, and then 3 more ropivacaine injections every 2 h). Experiment 2 evaluated the effects of D0 RA (4 injections) on NT variations after surgery (D1-D10) and after stress (D10) in rats treated with fentanyl at the time of surgery (FI and FIMSR groups). RESULTS: Postoperative hyperalgesia lasted for 7, 4, and 2 days for groups I, ISSR, and IMSR, respectively. Non-nociceptive environmental stress at D10 showed analgesia during stress in K1 (Dunnett, P < 0.05). Poststress area of hyperalgesia showed that I group developed greater hyperalgesia after NNES than ISSR and IMSR did (Mann-Whitney, P < 0.05). In experiment 2 in the FIMSR group, NT was significantly higher at postoperative D1 and D2 (Dunnett, P < 0.05), but no difference was shown from D3 to D10 (Dunnett, P > 0.05). Hyperalgesic indices calculated for FI and FIMSR groups after NNES at D10 did no show any significant difference (Dunnett, P > 0.05). CONCLUSIONS: Perioperative use of long-lasting RA reduced both acute postoperative hyperalgesia and the development of long-term pain vulnerability. However, high doses of fentanyl for intraoperative analgesia induce central sensitization that cannot be reversed by using long-lasting RA.

Pulsed radiofrequency enhances morphine analgesia in neuropathic rats.

This study examined the effects of pulsed radiofrequency (PRF) on sciatic nerve ligation-induced mechanical pain hypersensitivity in rats. The nociceptive threshold was evaluated using the paw pressure vocalization test. Seven days after nerve ligation, animals receiving a single PRF session (120 s/2 Hz/45 V/42°C) on L4-5-6 dorsal root ganglia ipsilateral to a chronic constriction injury (CCI) showed a reduced sensory hypersensitivity at H4 6 and 1 day after PRF as compared with animals without PRF. One day after PRF, the effect of morphine (2 mg/kg, subcutaneous) increased the nociceptive threshold in the no PRF/CCI group and more extensively in PRF/CCI animals. These results showed that PRF might represent an interesting strategy not only to reduce neuropathic pain but also to enhance the efficacy of morphine in patients with neuropathic pain, well known to be opioid resistant.

Endogenous opioids released during non-nociceptive environmental stress induce latent pain sensitization Via a NMDA-dependent process.

UNLABELLED: Although stress induces analgesia, there is evidence that stressful events may exacerbate pain syndromes. Here, we studied the effects of 1 to 3 prestressful events (days 0, 2, and 7), such as non-nociceptive environmental stress, on inflammatory hyperalgesia induced by a carrageenan injection (day 14) in 1 rat hind paw. Changes in nociceptive threshold were evaluated by the paw pressure vocalization test. The higher the number of stress sessions presented to the rats, the greater was the inflammatory hyperalgesia. Blockade of opioid receptors by naltrexone before each stress inhibited stress-induced analgesia and suppressed the exaggerated inflammatory hyperalgesia. Stressed versus nonstressed animals could be discriminated by their response to a fentanyl ultra-low dose (fULD), that produced hyperalgesia or analgesia, respectively. This pharmacological test permitted the prediction of the pain vulnerability level of prestressed rats because fULD analgesic or hyperalgesic indices were positively correlated with inflammatory hyperalgesic indices (r(2) = .84). In prestressed rats, fULD-induced hyperalgesia and the exaggerated inflammatory hyperalgesia were prevented NMDA receptor antagonists. This study provides some preclinical evidence that pain intensity is not only the result of nociceptive input level but is also dependent on the individual history, especially prior life stress events associated with endogenous opioid release. PERSPECTIVE: Based on these preclinical data, it would be of clinical interest to evaluate whether prior stressful events may also affect further pain sensation in humans. Moreover, this preclinical model could be a good tool for evaluating new therapeutic strategies for relieving pain hypersensitivity.

Xenon prevents inflammation-induced delayed pain hypersensitivity in rats.

Rats received an intraplantar carrageenan injection for inducing hind paw inflammation. After 1 h 45 min, they were exposed to medical air (air group), xenon 25% (Xe-25 group) or 50% (Xe-50 group) for 1 h 45 min. Mechanical nociceptive threshold was evaluated on experimental day and once daily for 1 week. Beyond the well-known antinociceptive effect of xenon, the delayed hyperalgesia observed for 4 days after carrageenan injection was strongly reduced in Xe-25 group and totally suppressed in Xe-50 group on the inflamed hind paw. Moreover, delayed hyperalgesia on the noninflamed hind paw was totally suppressed for both the xenon concentrations. These results show that xenon, beyond its antinociceptive effects, may be a fruitful therapeutic strategy to limit the development of pain sensitization after tissue injury.

A single nitrous oxide (N2O) exposure leads to persistent alleviation of neuropathic pain in rats.

UNLABELLED: Using the rat chronic constriction injury (CCI) pain model, we evaluated whether nitrous oxide (N2O), a gas shown to have potent anti-hyperalgesic properties, may alleviate neuropathic pain. Mechanical nociceptive threshold was estimated using the paw pressure vocalization test. Thermal allodynia was challenged by measuring the struggle latency by immersion of the hind paw in a 10 degrees C water bath. A single 50% N2O exposure for 1 hour, 15 minutes not only induced potent anti-nociception during N2O exposure but also provoked a delayed and sustained reduction (37% to 46%) of pain hypersensitivity of the injured hind paw and abolished pain hypersensitivity of the contralateral uninjured hind paw for at least 1 month. Thermal allodynia was completely prevented by a single N2O exposure. A preadministration of naltrexone, which markedly reduced acute N2O-induced anti-nociception, did not affect the persistent reduction of hyperalgesia. The administration of naltrexone in N2O-treated rats, 1 week after the gas exposure, did not induce any effect. This suggests that the long-lasting effect of N2O was not due to its prior acute analgesic effect and was independent of endogenous opioid systems. These data suggest that 50% N2O exposure could be an efficient and safe strategy for alleviating neuropathic pain in a persistent manner. PERSPECTIVE: Because a single 50% N2O exposure induced a persistent reduction of hyperalgesia-allodynia in a rat neuropathic pain model, clinical trials must be developed for evaluating the N2O effects in patients with neuropathic pain. The ability of N2O to potentiate analgesic effects of other drugs also must be evaluated.

Long-term pain vulnerability after surgery in rats: prevention by nefopam, an analgesic with antihyperalgesic properties.

BACKGROUND: Tissue damage associated with surgery often produces peripheral and central sensitization that may outlast the stimuli, leading to exaggerated postoperative pain. Paradoxically, the use of opioid analgesia, which is essential for surgical pain management may induce pain sensitization leading to enhanced postoperative pain and an increased risk of developing chronic pain. We studied whether a surgical incision in the rat hindpaw may favor the development of long-term pain vulnerability by estimating hyperalgesia induced by an inflammatory stimulation of the unlesioned contralateral hindpaw 3 wk later. We also evaluated the ability of nefopam, an analgesic drug commonly used in postoperative pain management, to prevent not only exaggerated postoperative pain but also long-term pain vulnerability. The efficacy of morphine was assessed 1 day after surgical incision. METHODS: On Day 0, a surgical plantar incision was performed in one hindpaw of rats treated or untreated with fentanyl (4 x 100 microg/kg, one injection every 15 min). Nefopam (10 mg/kg) or saline was subcutaneously injected 30 min before injury. Three weeks later, once pain measures had returned to basal values, a subsequent nociceptive stimulus, specifically intraplantar carrageenan injection, was performed to evaluate pain sensitivity in incision- and fentanyl-experienced rats. Pain was measured by the paw-pressure vocalization test and the weight bearing test. RESULTS: Surgical incision in rats induced latent and long-term pain hypersensitivity, which was manifested by exaggerated hyperalgesia on carrageenan injection. Administering fentanyl in association with the surgical incision induced exaggerated postoperative pain. When injected before incision, nefopam reduced the exaggerated postoperative pain induced by perioperative fentanyl treatment and prevented the development of long-term pain hypersensitivity. Preoperative nefopam administration also improved morphine analgesic efficacy in the context of fentanyl-induced postoperative hyperalgesia. CONCLUSIONS: Given preemptively, nefopam may be effective at improving postoperative pain management and at reducing the risk of developing postoperative chronic pain, because the drug has both analgesic and antihyperalgesic properties.

Polyamine deficient diet to relieve pain hypersensitivity.

There is a compelling body of evidence that N-methyl-d-aspartate receptors (NMDA-R) play a critical role in the development and maintenance of pain hypersensitivity. However, long-term treatments with NMDA-R antagonists are limited by unacceptable side effects. Since polyamines modulate the functioning of NMDA-R and mainly originate from normal dietary intake and bacterial metabolism in the gut, we developed a nutritional therapy based on dietary polyamine deficiency. Here, we reported that a polyamine deficient diet (PD diet) for 7 days prevented the enhancement of tyrosine phosphorylation of the spinal NR2B subunit-containing NMDA-R associated with inflammation in rats. Based on these data, we studied the ability of PD diet to prevent long-lasting pain hypersensitivity associated with tissue injury on one hind paw by evaluating long-lasting changes in both mechanical nociceptive threshold and weight bearing. A PD diet strongly reduced long-lasting hyperalgesia induced by inflammation or incision, especially in fentanyl-treated rats. Moreover a PD diet also prevented the exaggerated hyperalgesia induced by a second inflammation performed 7 days after the first one. A PD diet also opposed paradoxical hyperalgesia induced by non-nociceptive environmental stress in rats with pain and opioid experiences. A PD diet reversed pain hypersensitivity associated with monoarthritis or neuropathy and restored the analgesic effect of morphine. Since PD diet was devoid of any noticeable side effects, this nutritional therapy could be part of an effective and safe strategy for pre-emptive analgesia and for reducing the transition from acute to chronic pain and its outcomes in various pain syndromes.

Nitrous oxide (N2O) prevents latent pain sensitization and long-term anxiety-like behavior in pain and opioid-experienced rats.

Improving rehabilitation after a severe tissue injury does not only require a reduction in pain, but also requires alleviation of negative affects, particularly anxiety. Although opioids remain unsurpassed analgesics to relieve moderate to severe pain, it has been shown that they also induce latent pain sensitization leading to long-lasting hyperalgesia via N-methyl-D-aspartate-(NMDA)-dependent pronociceptive systems. The present study evaluated the ability of nitrous oxide (N2O), a gas with NMDA antagonist properties, to prevent latent pain sensitization and long-term anxiety-like behavior (ALB) in rats with pain and opioid experiences. On D0, the pro-inflammatory drug carrageenan was injected in one hind paw of rats treated with fentanyl (4x100 microg/kg subcutaneously). Nociceptive threshold was evaluated with the paw pressure vocalization test. Rats were re-exposed to carrageenan or exposed to repeated non-nociceptive environmental stress (NNES) 2-3 weeks later. Rats were also challenged in the elevated plus-maze 2 weeks after fentanyl administration for evaluating ALB. The preventive effects of a single 4 h 50/50% N2O-O2 exposure performed on D0 was evaluated. Fifty percent N2O strongly reduced hyperalgesia induced by a first inflammation and its enhancement by fentanyl, and prevented exaggerated hyperalgesia induced by second inflammatory pain or NNES. Moreover, we provide first evidence that a high fentanyl dose induces long-term ALB 2 weeks after its administration. When associated with fentanyl, 50% N2O prevented such long-term ALB. These results suggest that a single exposure to N2O could improve post-injury pain management and facilitate rehabilitation especially when potent analgesics as opioids have to be used.

Non-nociceptive environmental stress induces hyperalgesia, not analgesia, in pain and opioid-experienced rats.

It is well admitted that stress induces analgesia (SIA) via endogenous opioid release. However, there is evidence that stressful events play a role in the pathogenesis of pain, but little is known about mechanisms underlying such pain vulnerability. Previous studies reported that a single opioid exposure activates NMDA-dependent pronociceptive systems leading to long-term pain vulnerability after analgesia. Here, we studied whether prior inflammatory pain or/and opioid experiences may favour the development of pain vulnerability after non-nociceptive environmental stress (NNES). Nociceptive threshold (NT) changes were evaluated by paw pressure vocalization test. By contrast to discrete SIA observed in naive rats, 1 h stress induced hyperalgesia (SIH) for several hours (15-65% NT decrease) in pain and opioid experienced rats. Repetition of NNES induced an 18- to 22-fold SIH enhancement (3-4 days), whereas SIA decreased. SIH was still observed 4 months after pain and opioid experiences. This phenomenon is referred to as latent pain sensitization. Furthermore, a fentanyl ultra-low dose (ULD, 50 ng/kg) administration, mimicking SIA in naive rats, induced hyperalgesia (65% NT decrease, 4 h), not analgesia, in pain and opioid-experienced rats. This indicates that low levels of opioids induce opposite effects, that is analgesia vs hyperalgesia dependent on prior life events. In pain and opioid-experienced rats, NMDA receptor antagonists, ketamine or BN2572, completely prevented hyperalgesia when injected just before NNES or fentanyl ULD. This latent pain sensitization model may be important for studying the transition from acute to chronic pain and individual differences in pain vulnerability associated with prior life events.

RF9, a potent and selective neuropeptide FF receptor antagonist, prevents opioid-induced tolerance associated with hyperalgesia.

Neuropeptide FF (NPFF) has been proposed to play a role in pain modulation, opioid tolerance, and several other physiological processes. However, pharmacological agents that would help define physiological roles for this peptide are still missing. Here we report the discovery of a potent and selective NPFF receptor antagonist, RF9, that can be administered systemically. This compound does not show any effects by itself but can block efficiently the increase in blood pressure and heart rate evoked by NPFF. When chronically coinjected with heroin, RF9 completely blocks the delayed and long-lasting paradoxical opioid-induced hyperalgesia and prevents the development of associated tolerance. Our data indicate that NPFF receptors are part of a bona fide antiopioid system and that selective antagonists of these receptors could represent useful therapeutic agents for improving the efficacy of opioids in chronic pain treatment.