Motor behavior induced by bergamot essential oil in experimental tasks is differentially modulated by pretreatment with metabotropic glutamate receptor 2/3 or 5 antagonists.

Bergamot essential oil shows anxiolytic-relaxant effects devoid of sedative action and motor impairment typical of benzodiazepines. Considering the potential for clinical of these effects, it is important to understand the underlying mechanisms of the phytocomplex. Modulation of glutamate group I and II metabotropic receptors is involved in stress and anxiety disorders, in cognition and emotions and increases locomotor activity and wakefulness. Interestingly, early data indicate that bergamot essential oil modulates glutamatergic transmission in specific manifestations of the central nervous system. The aim of this work is to investigate if selective antagonists of metabotropic glutamate 2/3 and 5 receptors affect behavioral parameters modulated by the phytocomplex. Male Wistar rats were used to measure behavioral parameters to correlate anxiety and motor activity using elevated plus maze (EPM), open field (OF), and rotarod tasks. Bergamot essential oil increases in EPM the time spent in open/closed arms and reduces total number of entries. The essential oil also increases immobility in EPM and OF and not affect motor coordination in rotarod. Pretreatment with the metabotropic glutamate antagonists does not affect the time spent in open/close arms, however, differently affects motor behavior measured after administration of phytocomplex. Particularly, glutamate 2/3 antagonist reverts immobility and glutamate 5 antagonist potentiates this parameter induced by the phytocomplex. Our data show that modulation of both metabotropic glutamate receptors is likely involved in some of behavioral effects of bergamot essential oil.

Requirements for Translation in Clinical Trials of Aromatherapy: The Case of the Essential Oil of Bergamot (BEO) for Management of Agitation in Severe Dementia.

Due to the tight link between undertreated pain and agitation in dementia patients, aromatherapy can be a useful approach if an essential oil (EO) with powerful analgesic activity is used. The methodological difficulties of most aromatherapy trials have not allowed any definitive conclusion about the effectiveness of aromatherapy in dementia. The objective of the present perspective is to illustrate the long rigorous process leading from preclinical research to clinical translation of the EO of bergamot (BEO) for the management of agitation in dementia. A nanotechnology-based delivery system consisting of odorless alpha-tocopheryl stearate solid lipid nanoparticles (SLN) loaded with BEO (NanoBEO), has been proven active in acute and neuropathic pain models confirming the strong antinociceptive and anti-allodynic efficacy reported for BEO in preclinical studies. In particular, prolonged physicochemical stability of NanoBEO and titration in its main components are remarkable advantages allowing reproducible antinociceptive and anti-itch responses to be measured. Furthermore, the possibility to perform double-blind clinical trials made impossible so far because of the strong smell of essential oils used in aromatherapy. Demented patients receive limited treatment for chronic pain, particularly neuropathic. The BRAINAID (NCT04321889) trial will assess the effectiveness of NanoBEO on agitation and pain in severely demented patients to offer a safe tool able to provide relief to this fragile population. This double-blind clinical trial will be the first to assess the efficacy and safety of an engineered essential oil and will provide the rationale for the safer treatment of neuropsychiatric symptoms of dementia and pain in clinic.

Preclinical Characterization of Antinociceptive Effect of Bergamot Essential Oil and of Its Fractions for Rational Translation in Complementary Therapy.

Bergamot essential oil (BEO) is endowed with consistent and reproducible antinociceptive and anti-allodynic properties when administered via an inhalation route. However, the effects of its main constituents and of its decolored (DEC) and deterpenated (DET) fractions, which are enriched in limonene or in linalool and linalyl acetate, respectively, on spontaneous motor activity related to anxiety and on formalin-induced licking/biting biphasic behavior have never been investigated before. Therefore, the present research aims to characterize the role of BEO components on an experimental pain model that is relevant to clinical translation. Under our present experimental conditions, a paper filter disc soaked with different volumes of the phytocomplex and of its fractions that was applied at the edge of the observation chamber allowed the effects on the spontaneous motor activity and on the formalin-induced nocifensive response in ddY-strain mice to be studied. The present research demonstrated the effects of the DEC fraction of BEO on motor activity and on formalin-induced licking/biting behavior for the first time, proving that limonene is implicated in reduced motor activity and that it is important for the analgesic effect.

Impact of nutraceuticals on glaucoma: A systematic review.

BACKGROUND: glaucoma is a remarkable social issue being the leading cause of irreversible blindness worldwide. It is a progressive neuropathy characterized by the death of the retinal ganglion cells, of which the most important risk factor is represented by the increase of intraocular pressure (IOP). The role of nutraceutical supplementations with anti-oxidant activity has been extensively tested in preclinical models of glaucoma. The clinical efficacy of nutraceuticals in glaucoma is still controversial. OBJECTIVES: the aim of this systematic review is to assess the efficacy of nutraceuticals with anti-oxidant activity in glaucoma through the PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analyses) rigorous criteria. DATA SOURCES: the literature search has been performed on the electronic databases currently recognized of most relevance for medical scientific literature, i.e. PubMed, MEDLINE, The Cochrane Central Register of Controlled Trials (CENTRAL) with access to EMBASE, ClinicalTrials.gov and Scopus. The date of last search is April 8th, 2020. Study eligibility criteria, participants, and interventions: prospective randomized clinical trials assessing the effects of nutraceuticals and anti-oxidants on IOP and/or visual field in patients with glaucoma. The eligible papers must be published in English and available in full text. STUDY APPRAISAL AND SYNTHESIS METHODS: the evaluation of the eligibility of the studies has been carried out independently by two authors. The selection process has followed the PRISMA flow diagram, assessing the quality of the body of evidence and the risk of bias. RESULTS: the search of literature has retrieved 1615 papers and 2 clinical trials with results, among which only 6 are eligible for inclusion in the present systematic review to address the preset participants, interventions, comparisons, outcomes and study design (PICOS) "are the nutraceuticals effective in glaucoma?". In 5 out of 6 studies the nutraceutical supplementation is effective in providing additional decrease of IOP to current usual therapy, without the occurrence of side effects. However, all the studies present high heterogeneity and some concerns in terms of risk of bias, apart from one trial for which the risk of bias is low. CONCLUSIONS: the evidence of effectiveness of nutraceutical formulations is still uncertain and inconclusive. Therefore, large double-blind randomized clinical trials with adequate design, methodology and statistical power are needed to support the use of nutraceuticals in glaucoma.

Role of 5-HT1A Receptor in the Anxiolytic-Relaxant Effects of Bergamot Essential Oil in Rodent.

The essential oil obtained by the fresh fruit of Citrus bergamia Risso et Poiteau is used worldwide in aromatherapy to reduce pain, facilitate sleep induction, and/or minimize the effects of stress-induced anxiety. Preclinical pharmacological data demonstrate that bergamot essential oil (BEO) modulates specific neurotransmissions and shows an anxiolytic-relaxant effect not superimposable to that of the benzodiazepine diazepam, suggesting that neurotransmissions, other than GABAergic, could be involved. Several studies on essential oils indicate a role for serotonergic (5-HT) neurotransmission in anxiety. Interestingly, among serotonergic receptors, the 5-HT1A subtype seems to play a key role in the control of anxiety. Here, we report that modulation of the 5-HT1A receptor by selective agonist ((±)8-OH-DPAT) or antagonist (WAY-100635) may influence some of the anxiolytic-relaxant effects of BEO in Open Field and Elevated Plus Maze tests.

Anxiolytic-Like Effects of Bergamot Essential Oil Are Insensitive to Flumazenil in Rats.

Anxiety disorders are one of the most common mental disorders, and benzodiazepines (BDZs), acting on gamma-aminobutyric acid type A (GABA-A) receptor complex, represent the most common antianxiety medications in the world. However, chronic BDZ use elicits several adverse reactions. Reportedly, aromatherapy is safer for the management of anxiety. Bergamot essential oil (BEO) extracted from Citrus bergamia Risso et Poiteau fruit, like other essential oils, is widely used in aromatherapy to relieve symptoms of stress-induced anxiety. Interestingly, preclinical data indicate that BEO induces anxiolytic-like/relaxant effects in animal behavioural tasks not superimposable to those of benzodiazepine diazepam. To better elucidate the involvement of GABAergic transmission, the present study examines the effects of pretreatment with flumazenil (FLZ), a benzodiazepine site antagonist, on BEO effects using open-field task (OFT) in rats. The data yielded show that FLZ does not significantly affect behavioural effects of the phytocomplex. These results demonstrate the lack of overlapping between BEO and BDZ behavioural effects, contributing to the characterization of the neurobiological profile of the essential oil for its rational use in aromatherapy.

Neuropharmacology of the Neuropsychiatric Symptoms of Dementia and Role of Pain: Essential Oil of Bergamot as a Novel Therapeutic Approach.

Aging of the population makes of dementia a challenge for health systems worldwide. The cognitive disturbance is a serious but not the only issue in dementia; behavioral and psychological syndromes known as neuropsychiatric symptoms of dementia remarkably reduce the quality of life. The cluster of symptoms includes anxiety, depression, wandering, delusions, hallucinations, misidentifications, agitation and aggression. The pathophysiology of these symptoms implicates all the neurotransmitter systems, with a pivotal role for the glutamatergic neurotransmission. Imbalanced glutamatergic and GABAergic neurotransmissions, over-activation of the extrasynaptic N-methyl-D-aspartate (NMDA) receptors and alterations of the latter have been linked to the development of neuropsychiatric symptoms experienced by almost the entire demented population. Drugs with efficacy and safety for prevention or long term treatment of these disorders are not available yet. Aromatherapy provides the best evidence for positive outcomes in the control of agitation, the most resistant symptom. Demented patients often cannot verbalize pain, resulting in unrelieved symptoms and contributing to agitation. Bergamot essential oil provides extensive preclinical evidence of analgesic properties. Incidentally, the essential oil of bergamot induces anxyolitic-like effects devoid of sedation, typical of benzodiazepines, with a noteworthy advantage for demented patients. These data, together with the reported safety profile, form the rational basis for bergamot as a neurotherapeutic to be trialed for the control of behavioral and psychological symptoms of dementia.

Neuropharmacological Properties of the Essential Oil of Bergamot for the Clinical Management of Pain-Related BPSDs.

BACKGROUND: Alzheimer's Disease (AD) accounts for approximately 50% of all cases of dementia and, in spite of the great effort for the development of disease-modifying drugs, a definitive treatment of cognitive impairment is not available yet. A perfect adherence to the current therapy of cognitive decline is needed for a better control of the disease and this is proven to reduce, though not completely abolish, the associated Behavioural and Psychological Symptoms of Dementia (BPSDs) from occurring. This cluster of symptoms, remarkably affecting patients' health-related quality of life (HRQL), is tightly associated with pain states. Antipsychotics are the only treatment for BPSDs. However, these drugs are more effective and safer in the short-term (6-12 weeks), they are able to manage aggression but not agitation and they cannot control pain. Aromatherapy with Melissa officinalis and Lavandula officinalis has been employed to handle BPSDs, but it has not provided strong evidence to offer relief from pain. OBJECTIVE: Bergamot Essential Oil (BEO) exerts antinociceptive activity through several pharmacological mechanisms: in particular, it is able to enhance autophagy, a process undergoing derangement in chronic pain. Thus, the sound pharmacological basis for clinical translation of aromatherapy with BEO in the treatment of BPSDs has been pointed out. CONCLUSION: The antinociceptive effects elicited by BEO in experimental pain models make it a possible candidate for the pharmacological management of pain-related BPSDs.

Possible involvement of the peripheral Mu-opioid system in antinociception induced by bergamot essential oil to allodynia after peripheral nerve injury.

The essential oil of bergamot (BEO) is one of the most common essential oils and is most familiar to the general public. The aims of this study were to investigate the effect of intraplantar (i.pl.) BEO on neuropathic allodynia induced by partial sciatic nerve ligation (PSNL) in mice and the opioid receptor subtypes involved in the antiallodynic effects of BEO. Our findings showed that a single dose of i.pl. administration of BEO significantly inhibited the PSNL-induced neuropathic pain using the von Frey test. The i.pl pretreatment with naloxone methiodide, a peripherally acting µ-opioid receptor preferring antagonist, ß-funaltrexamine hydrochloride (ß-FNA), a selective µ-opioid receptor antagonist, and ß-endorphin antiserum significantly reversed the antiallodynic effect of BEO in the von Frey test, but not by naltrindole, the nonselective δ-opioid receptor antagonist and nor-binaltorphimine, the selective κ-opioid receptor antagonist. Furthermore, in the western blotting analysis, i.pl. administration of BEO resulted in a significant blockage of spinal extracellular signal-regulated protein kinase (ERK) activation induced by PSNL. Naloxone methiodide and ß-FNA significantly reversed the blockage of spinal ERK activation induced by BEO. These results suggest that i.pl. injection of BEO-induced antiallodynic effect and blockage of spinal ERK activation may be triggered by activation of peripheral µ-opioid receptors.

Antinociceptive effect of inhalation of the essential oil of bergamot in mice.

Bergamot essential oil (BEO) has proven wide evidence of pharmacological antinociceptive effectiveness both in nociceptive and in neuropathic pain models. The antinociceptive properties of BEO for inhalation have not been investigated. The purpose of this study is to evaluate the effects of the inhalation of BEO on formalin-induced nociceptive response in mice. Male ddY-strain mice (Japan SLC, Hamamatsu, Japan) of 23-25 g of weight at the time the experiments underwent the formalin test. Twenty µl of formalin (2% in saline) were administered into the plantar surface of the mice hindpaw and the time of licking/biting was observed and recorded at intervals of 5 min. The device for BEO inhalatory delivery consisted in a filter paper disc soaked with known volume of BEO placed on the edge of the cage. Inhalation of BEO exerted antinociceptive activity. In particular, it reduced the formalin-induced licking/biting behaviour in a manner that was dependent on the volume of BEO used in the device for its release and on the time of exposure to the phytocomplex. The results support the use of BEO in aromatherapy for complementary management of chronic pain relief in a stepwise therapeutic programme.

Aromatherapy and Aromatic Plants for the Treatment of Behavioural and Psychological Symptoms of Dementia in Patients with Alzheimer's Disease: Clinical Evidence and Possible Mechanisms.

The treatment of agitation and aggression, typical Behavioural and Psychological Symptoms of Dementia (BPSDs) of Alzheimer's Disease (AD), is one of the most complicated aspects of handling patients suffering from dementia. Currently, the management of these symptoms often associated with an increased pain perception, which notably reduces the patients' quality of life (QoL), relies on the employment of antipsychotic drugs. Unfortunately, the use of these pharmacological agents has some limits: in the long term, they do not result in being equally effective as in the first weeks of treatment and they present important side effects. Therefore, there is growing interest, supported by clinical evidence, in aromatherapy for the control of agitation, aggression, and psychotic symptoms. Some molecular mechanisms have been proposed to explain the behavioural effects of essential oils, as the whole phytocomplex or the single components, but important basic research effort is still needed. For this reason, rigorous preclinical studies are necessary in order to understand the pharmacological basis of aromatherapy in the treatment of BPSDs and to widen the cluster of effective essential oils in pharmacotherapeutic practice.

Bergamot Essential Oil Attenuates Anxiety-Like Behaviour in Rats.

Preclinical studies have recently highlighted that bergamot essential oil (BEO) is endowed with remarkable neurobiolological effects. BEO can affect synaptic transmission, modulate electroencephalographic activity and it showed neuroprotective and analgesic properties. The phytocomplex, along with other essential oils, is also widely used in aromatherapy to minimize symptoms of stress-induced anxiety and mild mood disorders. However, only limited preclinical evidences are actually available. This study examined the anxiolytic/sedative-like effects of BEO using an open field task (OFT), an elevated plus-maze task (EPM), and a forced swimming task (FST) in rats. This study further compared behavioural effects of BEO to those of the benzodiazepine diazepam. Analysis of data suggests that BEO induces anxiolytic-like/relaxant effects in animal behavioural tasks not superimposable to those of the DZP. The present observations provide further insight to the pharmacological profile of BEO and support its rational use in aromatherapy.

Involvement of spinal glutamate in nociceptive behavior induced by intrathecal administration of hemokinin-1 in mice.

The most recently identified tachykinin, hemokinin-1, was cloned from mouse bone marrow. While several studies indicated that hemokinin-1 is involved in pain and inflammation, the physiological functions of hemokinin-1 are not fully understood. Our previous research demonstrated that the intrathecal (i.t.) administration of hemokinin-1 (0.00625-1.6 nmol) dose-dependently induced nociceptive behaviors, consisting of scratching, biting and licking in mice, which are very similar with the nociceptive behaviors induced by the i.t. administration of substance P. Low-dose (0.0125 nmol) hemokinin-1-induced nociceptive behavior was inhibited by a specific NK1 receptor antagonist; however, high-dose (0.1 nmol) hemokinin-1-induced nociceptive behavior was not affected. In the present study, we found that the nociceptive behaviors induced by hemokinin-1 (0.1 nmol) were inhibited by the i.t. co-administration of MK-801 or D-APV, which are NMDA receptor antagonists. Moreover, we measured glutamate in the extracellular fluid of the mouse spinal cord using microdialysis. The i.t. administration of hemokinin-1 produced a significant increase in glutamate in the spinal cord, which was significantly reduced by co-administration with NMDA receptor antagonists. These results suggest that hemokinin-1-induced nociceptive behaviors may be mediated by the NMDA receptor in the spinal cord.

Peripherally injected linalool and bergamot essential oil attenuate mechanical allodynia via inhibiting spinal ERK phosphorylation.

Bergamot essential oil (BEO) is one of the most common essential oil containing linalool and linalyl acetate as major volatile components. This study investigated the effect of intraplantar (i.pl.) bergamot essential oil (BEO) or linalool on neuropathic hypersensitivity induced by partial sciatic nerve ligation (PSNL) in mice. The i.pl. injection of BEO or linalool into the ipsilateral hindpaw to PSNL reduced PSNL-induced mechanical allodynia in a dose-dependent manner. Peripheral (i.pl.) injection of BEO or linalool into the contralateral hindpaw did not yield anti-allodynic effects, suggesting a local anti-mechanical allodynic effect of BEO or linalool in PSNL mice. Anti-mechanical hypersensitivity of morphine was enhanced by the combined injection of BEO or linalool at an ineffective dose when injected alone. We also examined the possible involvement of spinal extracellular signal-regulated protein kinase (ERK) in BEO or linalool-induced anti-mechanical allodynia. In western blotting analysis, i.pl. injection of BEO or linalool resulted in a significant blockade of spinal ERK activation induced by PSNL. These results suggest that i.pl. injection of BEO or linalool may reduce PSNL-induced mechanical allodynia followed by decreasing spinal ERK activation.

Intraplantar injection of bergamot essential oil induces peripheral antinociception mediated by opioid mechanism.

This study investigated the effect of bergamot essential oil (BEO) containing linalool and linalyl acetate as major volatile components in the capsaicin test. The intraplantar injection of capsaicin (1.6 µg) produced a short-lived licking/biting response toward the injected paw. The nociceptive behavioral response evoked by capsaicin was inhibited dose-dependently by intraplantar injection of BEO. Both linalool and linalyl acetate, injected into the hindpaw, showed a significant reduction of nociceptive response, which was much more potent than BEO. Intraperitoneal (i.p.) and intraplantar pretreatment with naloxone hydrochloride, an opioid receptor antagonist, significantly reversed BEO- and linalool-induced antinociception. Pretreatment with naloxone methiodide, a peripherally acting µ-opioid receptor preferring antagonist, resulted in a significant antagonizing effect on antinociception induced by BEO and linalool. Antinociception induced by i.p. or intrathecal morphine was enhanced by the combined injection of BEO or linalool. The enhanced effect of combination of BEO or linalool with morphine was antagonized by pretreatment with naloxone hydrochloride. Our results provide evidence for the involvement of peripheral opioids, in the antinociception induced by BEO and linalool. Combined administration of BEO or linalool acting at the peripheral site, and morphine may be a promising approach in the treatment of clinical pain.

The cannabinoid 1 receptor antagonist AM251 produces nocifensive behavior via activation of ERK signaling pathway.

Intrathecal (i.t.) injection of AM251, a cannabinoid 1 (CB(1)) receptor antagonist, into the spinal lumbar space of mice elicited a behavioral response consisting of biting and licking with a few scratchings. In this study, we investigated to determine whether i.t. AM251 could influence the activity of extracellular signal-regulated kinase-1 and -2 (ERK1/2), a mitogen-activated protein kinase (MAPK) in neuronal nitric oxide synthase (nNOS) and inducible NOS (iNOS) activation. The CB(1) receptor agonist ACEA, neurokinin 1 (NK(1)) receptor antagonists and NMDA receptor antagonists, inhibited i.t. AM251-induced behavioral response in a dose-dependent manner. The CB(2) receptor agonist, JWH-133 gave no effect on response elicited by i.t. AM251. Both non-selective NOS inhibitors, L-NAME and 7-NI, and N(ω)-propyl-L-arginine, a selective inhibitor of nNOS resulted in a dose-dependent inhibition of i.t. AM251-induced behavioral response. The selective iNOS inhibitor, 1400W, in relatively large doses, inhibited in a non dose-dependent manner. The i.t. injection of AM251 produced a definite activation of ERK1/2 in the lumbar dorsal spinal cord. Behavioral experiments showed that U0126, a MAPK/ERK kinase (MEK) inhibitor, dose-dependently attenuated the behavioral response to i.t. AM251. Spinal activation of ERK1/2 following i.t. AM251 was reduced clearly by N(ω)-propyl-L-arginine and U0126, while 1400W gave a significant effect on only ERK1 activation. These findings suggest that the nNOS-ERK pathway in spinal cord neurones plays an important role in AM251-induced nocifensive behavior and its inhibition may provide significant anti-nociception.

Characterization of intrathecally administered hemokinin-1-induced nociceptive behaviors in mice.

Hemokinin-1 is a novel mammalian tachykinin cloned from mouse bone marrow. At present, pharmacological profile and physiological role of hemokinin-1 are still unclear. In the present study, we found that intrathecal (i.t.) administration of hemokinin-1 (0.00625-1.6 nmol) induced nociceptive responses consisting of scratching, biting and licking, which resemble substance P-induced behavioral responses in mice. The behaviors evoked by low-dose of hemokinin-1 (0.0125 nmol) were dose-dependently inhibited by i.t. co-administration of CP-99,994, a non-peptidic tachykinin NK(1) receptor antagonist, whereas high-dose of hemokinin-1 (0.1 nmol)-induced behaviors were not affected. Moreover, sendide, a peptidic tachykinin NK(1) receptor antagonist, failed to reduce the behavioral responses of both low- and high-dose of hemokinin-1. In contrast, substance P-induced behaviors were completely suppressed by both CP-99,994 and sendide. These results suggest that hemokinin-1 plays an important role in pain transmission at spinal cord. Moreover, the mechanism of hemokinin-1-induced nociceptive behaviors may be dose-dependent, and distinct from substance P-induced nociceptive behaviors.

Neuropharmacology of the essential oil of bergamot.

Bergamot (Citrus bergamia, Risso) is a fruit most knowledgeable for its essential oil (BEO) used in aromatherapy to minimize symptoms of stress-induced anxiety and mild mood disorders and cancer pain though the rational basis for such applications awaits to be discovered. The behavioural and EEG spectrum power effects of BEO correlate well with its exocytotic and carrier-mediated release of discrete amino acids endowed with neurotransmitter function in the mammalian hippocampus supporting the deduction that BEO is able to interfere with normal and pathological synaptic plasticity. The observed neuroprotection in the course of experimental brain ischemia and pain does support this view. In conclusion, the data yielded so far contribute to our understanding of the mode of action of this phytocomplex on nerve tissue under normal and pathological experimental conditions and provide a rational basis for the practical use of BEO in complementary medicine. The opening of a wide venue for future research and translation into clinical settings is also envisaged.

Effects of systemic administration of the essential oil of bergamot (BEO) on gross behaviour and EEG power spectra recorded from the rat hippocampus and cerebral cortex.

Bergamot (Citrus bergamia Risso et Poiteau) is a citrus fruit growing almost exclusively in the South of Italy. Its essential oil is obtained by cold pressing of the epicarp and, partly, of the mesocarp of the fresh fruit. Although this phytocomplex has been used for centuries, reputedly effectively, as a traditional medicine, there is very little verified scientific evidence to support this use. This paper reports original data on the systemic effects of the essential oil of bergamot (BEO) on gross behaviour and EEG activity recorded from the hippocampus and cerebral cortex of the rat. The Fast Fourier Transformation (FFT) was used to analyse and quantify the energy in single frequency bands of the EEG spectrum. The results obtained indicate that systemic administration of increasing volumes of BEO produces dose-dependent increases in locomotor and exploratory activity that correlate with a predominant increase in the energy in the faster frequency bands of the EEG spectrum. These data contribute to our understanding of the neurobiological profile of BEO.

Mechanism of allodynia evoked by intrathecal morphine-3-glucuronide in mice.

Morphine-3-glucuronide (M3G), a main metabolite of morphine, has been proposed as a responsible factor when patients present with the neuroexcitatory side effects (allodynia, hyperalgesia, and myoclonus) observed following systemic administration of large doses of morphine. Indeed, both high-dose morphine (60 nmol/5 microl) and M3G (3 nmol/5 microl) elicit allodynia when administered intrathecally (i.t.) into mice. The allodynic behaviors are not opioid receptor mediated. This chapter reviews the potential mechanism of spinally mediated allodynia evoked by i.t. injection of M3G in mice. We discuss a possible presynaptic release of nociceptive neurotransmitters/neuromodulators such as substance P, glutamate, and dynorphin in the primary afferent fibers following i.t. M3G. It is possible to speculate that i.t. M3G injection could activate indirectly both NK(1) receptor and glutamate receptors that lead to the release of nitric oxide (NO) in the dorsal spinal cord. The NO plays an important role in M3G-induced allodynia. The phosphorylation of extracellular signal-regulated protein kinase (ERK) in the dorsal spinal cord evoked via NO/cGMP/PKG pathway contributes to i.t. M3G-induced allodynia. Furthermore, the increased release of NO observed after i.t. injection of M3G activates astrocytes and induces the release of the proinflammatory cytokine, interleukin-1beta. Taken together, these findings suggest that M3G may induce allodynia via activation of NO-ERK pathway, while maintenance of the allodynic response may be triggered by NO-activated astrocytes in the dorsal spinal cord. The demonstration of the cellular mechanisms of neuronal-glial interaction underlying M3G-induced allodynia provides a fruitful strategy for improved pain management with high doses of morphine.