A randomized phase I trial evaluating the effects of inhaled 50-50% N2 O-O2 on remifentanil-induced hyperalgesia and allodynia in human volunteers.

BACKGROUND: Opioids are known to relieve pain, and also aggravate pre-existing hyperalgesia. In animal studies, the N-methyl-d-aspartate-receptor antagonist nitrous oxide (N2 O) was able to prevent hyperalgesia. The present study evaluated the effect of N2 O on hyperalgesia after remifentanil infusion in healthy volunteers. METHODS: Twenty-one healthy volunteers were enrolled in this placebo-controlled cross-over study. Transcutaneous electrical stimulation at high current densities induced spontaneous acute pain and stable areas of hyperalgesia. Each volunteer underwent the following four sessions: (1) 50-50% N2 -O2 and i.v. saline; (2) 50-50% N2 -O2 and i.v. remifentanil 0.1 µg/kg/min; (3) 50-50% N2 O-O2 and i.v. saline; (4) 50-50% N2 O-O2 and i.v. remifentanil 0.1 µg/kg/min. Inhaled gas mixtures lasted for 60 min, i.v. drug administration for 30 min. Visual analogue scale pain intensity, areas of pinprick hyperalgesia and touch-evoked allodynia were assessed repeatedly for 160 min. RESULTS: Data of 19 volunteers were analysed. There were significant time and treatment effects regarding areas of hyperalgesia and allodynia (p < 0.02). The area of hyperalgesia was significantly reduced in the N2 O + remifentanil session compared to the remifentanil session (35.88 ± 22.37 vs. 43.55 ± 18.48 cm(2) , p = 0.004). The area of allodynia was significantly reduced in the N2 O + remifentanil session compared to the remifentanil session (29.95 ± 16.15 vs. 34.80 ± 15.35 cm(2) , p = 0.008). The pain intensity was significantly reduced in the N2 O + remifentanil session compared to the remifentanil session (37.96 ± 12.78 vs. 42.15 ± 13.34 mm, p < 0.0001). CONCLUSIONS: Nitrous oxide significantly reduced hyperalgesia, allodynia and pain intensity aggravated after remifentanil administration in a human volunteer model. WHAT DOES THIS STUDY ADD?: This study brings the evidence that N2 O reduces the remifentanil aggravated secondary hyperalgesia in human volunteers exposed to a well-known model of electrical pain. N2 O was able to oppose the hyperalgesia, the allodynia and the pain intensity consecutive to remifentanil use in this specific pain model.

[Low doses ketamine: antihyperalgesic drug, non-analgesic]. / Kétamine à faibles doses: antihyperalgésique, non analgésique.

Recent data in animal experiments as in clinical trials have clearly reported that pain modulation is related to an equilibrium between antinociceptive and pronociceptive systems. Therefore, the apparent pain level could not only be a consequence of a nociceptive input increase but could also result from a pain sensitization process. Glutamate, via NMDA receptors, plays a major role in the development of such a neuronal plasticity in the central nervous system, leading to a pain hypersensitivity that could facilitate chronic pain development. By an action on NMDA receptors opioids also induce, in a dose dependent manner, an enhancement of this postoperative hypersensitivity. "Antihyperalgesic" doses of ketamine, an NMDA receptor antagonist, were able to decrease this central sensitization not only in painful animal but also in human volunteers exposed to different pain models, or in the postoperative period. Many studies have reported that ketamine effects are elicited when this drug is administered the following manner: peroperative bolus (0.1 to 0.5 mg/kg), followed by a constant infusion rate (1 to 2 microg/kg per min) during the peroperative period and for 48 to 72 hours after anaesthesia. Those ketamine doses improved postoperative pain management by reducing hyperalgesia due to both surgical trauma and high peroperative opioid doses. This antihyperalgesic action of ketamine also limited the postoperative morphine tolerance leading to a decrease in analgesic consumption and an increase in the analgesia quality.

[Complexity and physiology of the analgesic effects of opioids]. / Comptexité et logique physiologiques du mode d'action des substances opioïdes.

Nowadays, it is considered that clinical pain is not only a reflexion of the nociceptive stimulus intensity, but is to a large extent the expression of neural plasticity including both peripheral and central sensitization. N-Methyl-D-aspartate receptors (NMDA-Rs) play a critical role in the central pain sensitization process leading to exaggerated pain and chronic pain. Although opioids are unsurpassed analgesics, we observed that opioids induced delayed and sustained NMDA-related hyperalgesia after an initial analgesia suggesting a pain sensibilisation process. It is shown that NMDA receptor antagonists and specific diets able to negatively modulate NR2B subunit containing NMDA receptors prevented abnormal pain hypersensitivity, partially reversed chronic pain and restored the opioid effectiveness on opioid-resistant pain models.

Interaction between neuropeptide FF and opioids in the ventral tegmental area in the behavioral response to novelty.

Considerable evidence has focused on the interaction between endogenous opioid peptides and the dopaminergic mesocorticolimbic system in behavioral responses to stress. Recently, it has been proposed that the CNS synthesizes and secretes neuropeptides that act as part of a homeostatic system to attenuate the effects of morphine or endogenous opioid peptides. Among these antiopioids, neuropeptide FF (NPFF) is particularly interesting since both NPFF immunoreactive-like terminals and NPFF binding sites are located in the vicinity of the dopaminergic cell bodies within the ventral tegmental area (VTA) suggesting an interaction at this level. The purpose of the present study was to evaluate the respective implication of opioid and antiopioid peptides at the level of the VTA in the locomotor response to novelty in rats. The results indicate that s.c. naloxone pretreatment, an opiate receptor antagonist, reduced locomotor activity in rats placed in a novel environment without having any effect in a familiar environment. This effect takes place in the VTA since intra-VTA administration of naloxone methobromide diminished similarly and dose-dependently the motor response to novelty. This effect is mainly dependent on opioid peptides released at VTA level since local injections of thiorphan, an inhibitor of enkephalin degradation, strongly increased locomotor response to novelty and this effect is completely prevented by the co-administration of naloxone methobromide. When injected in the VTA, NPFF is acting as an antiopioid compound, i.e. it reduces the locomotor activity triggered by exposure to novelty to the level recorded in a familiar environment. Moreover, NPFF decreased dose-dependently the potentiation of novelty-induced locomotor response produced by VTA injection of thiorphan. Taken together, these results suggest that NPFF neurons may participate at the level of the VTA to a homeostatic regulating process counteracting opioid effects induced by a mild stress such as novelty.

The neuropeptide FF analogue, 1DMe, acts as a functional opioid autoreceptor antagonist in the rat spinal cord.

We assessed the possible influence of a neuropeptide FF analogue, 1DMe ([D-Tyr(1),(NMe)Phe(3)]neuropeptide FF), on the inhibitory action of endogenous and exogenous partial differential-opioid receptor agonists on K(+)-evoked [Met(5)]-enkephalin release from superfused rat spinal cord slices. 1DMe (0.1-10 microM) dose-dependently enhanced the increase in superfusate [Met(5)]-enkephalin content due to the peptidase inhibitors thiorphan (1 microM) and bestatin (20 microM), and prevented the reduction in [Met(5)]-enkephalin release due to stimulation of partial differential receptors by 1 microM deltorphin I. Because it had the same effects as partial differential-opioid receptor antagonists, 1DMe might act through the functional blockade of presynaptically located partial differential-opioid autoreceptors.

Lessons from an unpleasant surprise: a biochemical strategy for the diagnosis of pheochromocytoma.

OBJECTIVE: To audit the performances of the analytes used in the diagnosis of pheochromocytoma and to present a graphical guideline to help the diagnosis. DESIGN: A 5 year retrospective study. SETTINGS: Laboratory and departments of a university hospital. PARTICIPANTS: In-patients, suspected of bearing a pheochromocytoma, were investigated for urinary metanephrines and catecholamines (photometric method) and vanillylmandelic acid, fractionated catecholamines and metanephrines [high pressure liquid chromatography (HPLC) coupled to electrochemical detection (ED)] urinary excretion. MAIN OUTCOME: Patients with a pheochromocytoma (24 out of 2003 patients) were diagnosed by the combination of normetanephrine and metanephrine determination. RESULTS: All analytes but dopamine were significantly elevated in patients with a pheochromocytoma. The area under the receiver operating characteristics (ROC) curves were the highest for total metanephrines, normetanephrine and metanephrine determinations. Because of analytical interferences in the metanephrines determination, the normetanephrine and metanephrine performed better. It is noteworthy that all pheochromocytomas had either normetanephrine or metanephrine levels above their respective optimal threshold (sensitivity 100%). The best optimal threshold performance was reached by the mean of three daily samples. Total or fractionated catecholamines or vanillylmandelic acid were less accurate tools. CONCLUSION: Amongst urinary tests, the combined use of HPLC/ED determination of normetanephrine and metanephrine seems the most effective screening strategy for the diagnosis of pheochromocytoma. The older total metanephrine photometric assay is grieved by analytical interferences.

Progressive enhancement of delayed hyperalgesia induced by repeated heroin administration: a sensitization process.

It is difficult to conceive that tolerance and sensitization processes, two apparently opposite phenomena, can concomitantly modify one given biological process, i.e., the processing of pain. We have shown recently that opiates produce not only analgesia but also long-lasting hyperalgesia in rats. This suggests that tolerance to the analgesic effect of an opiate, especially heroin, could be in part the result of an actual sensitization of pronociceptive systems. Here, we show that both magnitude and duration of heroin-induced delayed hyperalgesia increase with intermittent heroin administrations, leading to an apparent decrease in the analgesic effectiveness of a given heroin dose. Our observation that a small dose of heroin which is ineffective for triggering a delayed hyperalgesia in non-heroin-treated rats induced an enhancement in pain sensitivity for several days after a series of heroin administrations is in agreement with the sensitization hypothesis. The effectiveness of the opioid receptor antagonist naloxone to precipitate hyperalgesia in rats that had recovered their pre-drug nociceptive value after single or repeated heroin administrations indicates that heroin-deprived rats were in a new biological state associated with a high level balance between opioid-dependent analgesic systems and pronociceptive systems. Because the NMDA receptor antagonist dizocilpine maleate (MK-801) prevented both heroin-induced long-lasting enhancement in pain sensitivity and naloxone-precipitated hyperalgesia, these findings further suggest that tolerance, sensitization, and one withdrawal symptom, hyperalgesia, are issued from a neuroadaptive process in which NMDA systems play a critical role.

Long-lasting hyperalgesia induced by fentanyl in rats: preventive effect of ketamine.

BACKGROUND: It has been reported that mu-opioid receptor activation leads to a sustained increase in glutamate synaptic effectiveness at the N-methyl-D-aspartate (NMDA) receptor level, a system associated with central hypersensitivity to pain. One hypothesis is that postoperative pain may result partly from the activation of NMDA pain facilitatory processes induced by opiate treatment per se. The authors tested here the effectiveness of the opiate analgesic fentanyl for eliciting a delayed enhancement in pain sensitivity. METHODS: The consequences of four bolus injections (every 15 min) of fentanyl (20-100 microg/kg per injection, subcutaneously) on immediate (for several hours) and long-term (for several days) sensitivity to nociceptive stimuli in the rat (paw-pressure vocalization test) were evaluated. The effects of the combination of the NMDA-receptor antagonist ketamine (10 mg/kg, subcutaneously) with fentanyl also were assessed. RESULTS: Fentanyl administration exhibited a biphasic time-dependent effect: first, an early response (for 2-5 h) associated with a marked increase in nociceptive threshold (analgesia), and second, a later response associated with sustained lowering of the nociceptive threshold (5 days for the longest effect) below the basal value (30% of decrease for the maximal effect) indicative of hyperalgesia. The higher the fentanyl dose used, the more pronounced was the fentanyl-induced hyperalgesia. Ketamine pretreatment, which had no analgesic effect on its own, enhanced the earlier response (analgesia) and prevented the development of long-lasting hyperalgesia. CONCLUSIONS: Fentanyl activates NMDA pain facilitatory processes, which oppose analgesia and lead to long-lasting enhancement in pain sensitivity.

Evidence for opiate-activated NMDA processes masking opiate analgesia in rats.

The acute interaction between opioid receptors and N-methyl-D-aspartate (NMDA) receptors on nociception was examined in rats using tail-flick and paw-pressure vocalisation tests. When injected at various times (1 to 6 h) after morphine (5 to 20 mg/kg, i.v.) or fentanyl (4x40 microgram/kg, i.v.), the opioid receptor antagonist naloxone (1 mg/kg, s.c.) not only abolished the opiate-induced increase in nociceptive threshold, but also reduced it below the basal value (hyperalgesia). The noncompetitive NMDA receptor antagonist MK-801 (0.15 or 0.30 mg/kg, s.c.) prevented the naloxone-precipitated hyperalgesia and enhanced the antinociceptive effects of morphine (7.5 mg/kg, i.v.) and fentanyl (4x40 microgram/kg, i.v.). These results indicate that the antinociceptive effects of morphine and fentanyl, two opiate analgesics widely used in humans in the management of pain, are blunted by concomitant NMDA-dependent opposing effects which are only revealed when the predominant antinociceptive effect is sharply blocked by naloxone. This study provides new rationale for beneficial adjunction of NMDA receptor antagonists with opiates for relieving pain by preventing pain facilitatory processes triggered by opiate treatment per se.

Opiate tolerance to daily heroin administration: an apparent phenomenon associated with enhanced pain sensitivity.

From a classical viewpoint, tolerance to analgesic effects of opiates refers to the decreased effectiveness of a given opiate following its repeated use. Despite much research, it has not been conclusively demonstrated in vivo that functional changes observed at the opioid receptor level in the responsiveness to opiates account for development of tolerance. An alternative hypothesis is that opioid receptors remain operative following repeated opiate administration but that opioid receptor activation rapidly induces a prolonged increase in pain sensitivity which opposes the predominant opiate analgesic effect following repeated opiate administration. We recently showed that a single heroin administration induces an enhanced pain sensitivity for several days, a phenomenon which is prevented by the non-competitive N-methyl-D aspartate receptor antagonist MK-801. Herein we report that repeated once-daily heroin injections induced a gradual lowering of the nociceptive threshold which progressively masked a sustained heroin analgesic functional effect. MK-801 prevented such opiate-induced allodynia and thereby prevented development of an apparent decrease in the effectiveness of heroin. These results indicate that intermittent heroin administration induced a persistent increase in the basal pain sensitivity which, if not taken into account gives the impression of less analgesia, i.e. apparent tolerance.

Long-lasting increased pain sensitivity in rat following exposure to heroin for the first time.

Acute dependence, defined as a precipitation of somatic signs by an antagonist, may occur after a single administration of an opiate drug. Because hyperalgesia is a consistent sign of the withdrawal syndrome, we tested the effectiveness of heroin, an opiate used by addicts, to induce pain facilitation even after a first exposure to the drug. In opiate-naive rats, subcutaneous injection of heroin induced analgesia followed by allodynia, a decrease in pain threshold. This latter phenomenon was observed in the absence of noxious stimuli and lasted several days. An N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801 prevented such long-lasting allodynia. These results suggest that allodynia is an early sign reflecting neural plasticity associated with the development of dependence.

Neuropeptide FF in the rat adrenal gland: presence, distribution and pharmacological effects.

Neuropeptide FF (NPFF, FLFQPQRFamide) is an FMRFamide-like octapeptide exhibiting antiopiate activity. The presence of both NPFF-immunoreactivity (NPFF-IR) and NPFF-specific receptors has been described in the mammalian central nervous system (CNS). The peripheral effects of NPFF indicate that NPFF-IR material is present outside the CNS. Biochemical and immunohistochemical methods enabled us to determine the presence and distribution of NPFF-IR in the rat adrenal gland. The amount of NPFF-IR material in whole gland was estimated by radioimmunoassay to be 19.00 +/- 4.00 fmol/gland. High performance liquid chromatography analysis of adrenal extracts revealed a single molecular form which coeluted with authentic NPFF. Demedullation decreased adrenal NPFF-IR content, indicating that NPFF-IR was present in both cortex and medulla. Light microscopy revealed NPFF-IR in beaded fibers confined in the outer part of the cortex and in medullary cells. Double-labeling with antityrosine-hydroxylase and anti-NPFF antibodies showed NPFF-IR in cortical catecholaminergic postganglionic fibers restricted to the subcapsular and glomerulosa zonae. NPFF-IR was also located in medullary chromaffin cells and in rays and islets of chromaffin cells dispersed throughout the cortex. Insulin-induced hypoglycemia did not alter NPFF-IR content. Denervation lowered adrenal NPFF-IR content. These data indicate that this peptide is present in nerve fibers of extrinsic origin. In vitro approaches using adrenal slices have shown that NPFF inhibited aldosterone release in a dose-dependent manner. Taken together, these data suggest that NPFF may participate in the control of aldosterone production and adrenal blood supply.

Acute tolerance associated with a single opiate administration: involvement of N-methyl-D-aspartate-dependent pain facilitatory systems.

Mechanisms underlying the development of acute tolerance to the analgesic effect of opiates were investigated. In the rat tail-flick test, administration of naloxone (1 mg/kg, s.c.) 40 min after heroin (1 mg/kg, s.c.) was shown to induce hyperalgesia, indicative of a short-onset, opiate-activated pain facilitatory systems masking the opiate analgesia. Pretreatment with the N-methyl-D-aspartate receptor antagonist dizocilpine maleate blocked, in a dose-dependent manner, the naloxone-induced hyperalgesia and potentiated the heroin-induced analgesia. Using a schedule of two successive injections of 1 mg/kg heroin, acute tolerance was indicated by a marked reduction (-52%) in analgesia induced by the second dose. After pretreatment with dizocilpine maleate, the acute tolerance was abolished and the analgesic effects of both injections of heroin were strongly potentiated. These observations indicate that acute tolerance appears after the first exposure to opiates and stems from opiate activation of N-methyl-D-aspartate-dependent pain facilitatory systems.

Evidence for multiple opioid receptors in the human posterior pituitary.

The distribution and quantification of opioid receptor types in post-mortem human pituitary cryostat sections was determined by quantitative in vitro receptor autoradiography. Highly specific radioligands were used for each opioid receptor type i.e. [125l]-FK-33-824 for mu-opioid sites, [125l][D.Ala2]-Deltorphin-l for delta-opioid sites and 3H-U69,593 for kappa-opioid sites. None of the five specimens tested exhibited any labeling in the anterior lobe of the pituitary for the three radioligands. As for the posterior pituitary, all 5 specimens contained both mu and kappa-opioid binding sites whereas none of them showed delta-binding sites. The presence of both mu- and kappa-opioid binding sites in the human posterior pituitary contrasts with previous findings in the rat where only kappa-sites have been found. The present study could contribute to understanding of the functional action of opioids in the human pituitary.

Simultaneous activation of spinal antiopioid system (neuropeptide FF) and pain facilitatory circuitry by stimulation of opioid receptors in rats.

Neuropeptide FF (NPFF) is a mammalian FMRFamide-like octapeptide with antiopioid properties that inhibits morphine-induced analgesia but also produces hyperalgesia. In the present study, a series of three experiments was carried out to investigate the interactions between opioid receptor stimulation and antiopioid systems. First, by using in vitro superfusion system with rat spinal cord slices, we showed that morphine stimulated NPFF release in a dose-dependent manner. The stimulating effect which was observed with morphine concentrations as low as 100 fM reached a maximum at 0.1 nM, then decreased and was ineffective at 10 microM. The morphine-induced release of NPFF was abolished by naloxone (1 microM) but unaltered by tetrodotoxin. Second, by an in vivo approach, we showed that a single heroin administration (2.5 mg/kg, s.c.) elicited in 30 min a drastic drop (38%) in spinal NPFF content. In a third experiment, we evaluated the capacity of naloxone in revealing an antiopioid component associated with opioid receptor stimulation. The administration of naloxone (1 mg/kg, s.c..) 25 min following that of heroin (2.5 mg/kg, s.c.) not only abolished the heroin-induced increase of tail-flick latency, but also lowered it under the basal value by 30%. These results indicate that opioid receptor stimulation activates both pain inhibitory and pain facilitatory systems in which NPFF may play a significant role and that opiate-induced analgesia is always partly masked.

Distribution of neuropeptide FF (FLFQPQRFamide) receptors in the adult rat spinal cord: effects of dorsal rhizotomy and neonatal capsaicin.

By using quantitative autoradiography and highly selective iodinated ligands, we quantified modifications in neuropeptide FF binding sites in the superficial layers (laminae I and II) of the cervical (C6-C8 segments) and lumbar (L3-L5 segments) enlargements in two models: (i) rats neonatally treated with capsaicin; (ii) rat submitted 15 days before to unilateral dorsal rhizotomies. We comparatively analysed the distribution of mu-opioid binding sites in the same animals. We have shown that the [125I]YLFQPQRFamide (neuropeptide FF sites) labelling is not significantly modified following selective damage of fine afferent fibres by neonatal capsaicin treatment. In the cervical and lumbar enlargements, capsaicin-treated/control binding ratios for [125I]YLFQPQRFamide were 0.90 and 0.86, respectively. While unilateral dorsal rhizotomy induced a drastic decrease in [125I]FK-33-824 labelling in the side ipsilateral to the lesion as compared to the intact side of (yielding ratios of 0.29 and 0.31 for cervical and lumbar levels, respectively), [125I]YLFQPQRFamide labelling was not significantly modified, yielding ratios of 0.98 and 0.91 for cervical and lumbar levels, respectively. These data suggest that, in contrast with a majority of mu-opioid receptors, neuropeptide FF receptors are not located on fine primary afferent fibers carrying nociceptive information from the fore- or hindlimb in the rat. This preferential postsynaptic localization, together with the reported "morphine modulating" action of this peptide, support the proposal of a role for neuropeptide FF in intraspinal modulation of nociceptive input.

[Neurobiology of dependence]. / Neurobiologie de la dépendance.

Investigations concerning dependence to drug of abuse are of pluridisciplinary nature. It is clear that to understand this complex phenomenon will not be the privilege of one particular level of psychobiological organizations. Whatever the theories, it must be stated that correlational relations exist between behavior, mental states, and brain-neuronal states whatever the transduction principles in cause. An important neglected problem to be investigated concerns the vulnerability presented by some organisms and not by other, either for the search of a drug-stimulus or to repeat the first encounter with the pharmacological agent, potentially a drug of abuse. Such vulnerability corresponds to a neurobiological state considered far from the equilibrium, i.e. homeostasis. Whether the drug, as many other agents, environmental, nutritional, sexual, tends to reduce the imbalance within the neuronal systems is a matter of debate.