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Magnetic resonance imaging is a very powerful ap-proach to perform anatomical and functional imaging explorations. Large animal models are perfectly suited for MRI studies using clinical devices and these tools are cur-rently being developed and improved. Here we highlight an example of MRI to demonstrate the efficacy of gene therapy in sheep models of Batten disease, a pediatric neurodegenerative disease.

Human and animal imaging studies demonstrated that chronic pain profoundly alters the structure and the functionality of several brain regions. In this article, we conducted a longitudinal and multimodal study to assess how chronic pain affects the brain. Using the spared nerve injury model which promotes both long-lasting mechanical and thermal allodynia/hyperalgesia but also pain-associated comorbidities, we showed that neuropathic pain deeply modified the intrinsic organization of the brain functional network 1 and 2 months after injury. We found that both functional metrics and connectivity of the part A of the retrosplenial granular cortex (RSgA) were significantly correlated with the development of neuropathic pain behaviours. In addition, we found that the functional RSgA connectivity to the subiculum and the prelimbic system are significantly increased in spared nerve injury animals and correlated with peripheral pain thresholds. These brain regions were previously linked to the development of comorbidities associated with neuropathic pain. Using a voxel-based morphometry approach, we showed that neuropathic pain induced a significant increase of the gray matter concentration within the RSgA, associated with a significant activation of both astrocytes and microglial cells. Together, functional and morphological imaging metrics of the RSgA could be used as a predictive biomarker of neuropathic pain., This work was supported by Inserm, Clermont Auvergne University, grants from Agence Nationale de la Recherche (ANR13-BSV1-0006-03), the Société Française d’Étude et de Traitement de la Douleur.

Rheumatic diseases are often associated to debilitating chronic joint pain, which remains difficult to treat and requires new therapeutic strategies. Here, we describe increased content of lysophosphatidyl-choline (LPC) 16:0 in the knee synovial fluids of two independent cohorts of patients with painful joint diseases. If LPC16:0 levels correlated with pain in patients with osteoarthritis (OA), they do not appear to be the hallmark of a particular joint disease. We found that intra-articular injections of LPC16:0 in mouse produce chronic pain and anxiety-like behaviors in both males and females with no apparent inflammation, peripheral nerve sprouting and damage, nor bone alterations. LPC16:0-induced persistent pain state is dependent on peripheral Acid-Sensing Ion Channel 3 (ASIC3), ultimately leading to central sensitization. LPC16:0 and ASIC3 thus appear as key players of chronic joint pain with potential implications in OA and possibly across others rheumatic diseases.

International audience; Background and Purpose We previously demonstrated that paracetamol has to be metabolised in the brain by fatty acid amide hydrolase enzyme into AM404 (N-(4-hydroxyphenyl)-5Z,8Z,11Z,14Z-eicosatetraenamide) to activate CB1 receptors and TRPV1 channels, which mediate its analgesic effect. However, the brain mechanisms supporting paracetamol-induced analgesia remain unknown. Experimental Approach The effects of paracetamol on brain function in Sprague-Dawley rats were determined by functional MRI. Levels of neurotransmitters in the periaqueductal grey (PAG) were measured using in vivo H-1-NMR and microdialysis. Analgesic effects of paracetamol were assessed by behavioural tests and challenged with different inhibitors, administered systemically or microinjected in the PAG. Key Results Paracetamol decreased the connectivity of major brain structures involved in pain processing (insula, somatosensory cortex, amygdala, hypothalamus, and the PAG). This effect was particularly prominent in the PAG, where paracetamol, after conversion to AM404, (a) modulated neuronal activity and functional connectivity, (b) promoted GABA and glutamate release, and (c) activated a TRPV1 channel-mGlu(5) receptor-PLC-DAGL-CB1 receptor signalling cascade to exert its analgesic effects. Conclusions and Implications The elucidation of the mechanism of action of paracetamol as an analgesic paves the way for pharmacological innovations to improve the pharmacopoeia of analgesic agents.

CERVOXY COLL; International audience; In utero exposure to ionizing radiation can lead to cerebral alterations during adulthood. Using anatomical magnetic resonance imaging (MRI), it is possible to assess radiation-induced structural brain damage noninvasively. However, little is currently known about microstructure alterations in brain tissue. Therefore, the goal of this study was to establish, based on an original and robust pipeline of MRI image analysis, whether the long-term effects of in utero radiation exposure on brain tissue microstructure could be detected noninvasively. Pregnant C57BL/6N mice received a single dose of 1 Gy on gestation day 14.5, which led to behavioral impairments in adults. At 3 months old, in vivo MRI data were acquired from in utero irradiated and nonirradiated male mice. An MRI protocol was designed to assess the effects of radiation on the parameters of brain volume, non-Gaussian diffusion (ADC0, kurtosis and signature index) and anisotropic diffusion (fractional anisotropy and mean, axial, radial diffusivities and anisotropic signature index) in 10 key cerebral structures defined using an in-house atlas of the mouse brain. Based on the relative amplitude of these anatomical and microstructural changes, maps of the radiosensitivity of the brain to in utero irradiation were created. We observed microcephaly in irradiated mice with noticeably larger volume changes in the cortex and the corpus callosum. We also observed significantly lower ADC0, anisotropy fraction (sFA), radial diffusivity (sRD), as well as signature index (S-index and SI3) values, which are original markers sensitive to tissue microstructure alterations. All these changes together are in favor of a decreased cellular “imprint” and in some regions a reduced density in myelinated axons. A reduction in the number and complexity of myelinated axons was further revealed by myelin basic protein immunostaining. Combining anatomical and diffusion MRI is a promising approach to noninvasively investigate the radiosensitivity of local brain areas in adult mice after in utero irradiation in terms of microstructure.

Reproduction induces changes within the brain to prepare for gestation and motherhood. However, the dynamic of these central changes and their relationships with the development of maternal behavior remain poorly understood. Here, we describe a longitudinal morphometric neuroimaging study in female mice between pre-gestation and weaning, using new magnetic resonance imaging (MRI) resources comprising a high-resolution brain template, its associated tissue priors (60-μm isotropic resolution) and a corresponding mouse brain atlas (1320 regions of interest). Using these tools, we observed transient hypertrophies not only within key regions controlling gestation and maternal behavior (medial preoptic area, bed nucleus of the stria terminalis), but also in the amygdala, caudate nucleus and hippocampus. Additionally, unlike females exhibiting lower levels of maternal care, highly maternal females developed transient hypertrophies in somatosensory, entorhinal and retrosplenial cortices among other regions. Therefore, coordinated and transient brain modifications associated with maternal performance occurred during gestation and lactation.

A major current trend in stress research is to consider both the spectrum of responses by different individuals to the same stressful stimulus or environment and the dynamics of the stress response within the individual. The motivation for such an approach is the desire for, and need of, developing methods, which are able to identify individuals susceptible/vulnerable to overall stress or to specific stressors and, in that way, to identify those at risk of developing stress-related disorders. Herein, we briefly review and discuss a number of approaches used in this field of research and describe how they led to our current state of knowledge. We also provide a perspective for the future development of the field to produce relevant findings, while simultaneously generating sensitive and specific predictive biomarkers of stress resilience.

Improvements in the survival of breast cancer patients have led to the emergence of bone health and pain management as key aspects of patient’s quality of life. Here, we used a female rat MRMT-1 model of breast cancer-induced bone pain to compare the effects of three drugs used clinically morphine, nabilone and zoledronate on tumor progression, bone remodeling and pain relief. We found that chronic morphine reduced the mechanical hypersensitivity induced by the proliferation of the luminal B aggressive breast cancer cells in the tumor-bearing femur and prevented spinal neuronal and astrocyte activation. Using MTT cell viability assay and MRI coupled to 18FDG PET imaging followed by ex vivo 3D µCT, we further demonstrated that morphine did not directly exert tumor growth promoting or inhibiting effects on MRMT-1 cancer cells but induced detrimental effects on bone healing by disturbing the balance between bone formation and breakdown. In sharp contrast, both the FDA-approved bisphosphonate zoledronate and the synthetic cannabinoid nabilone prescribed as antiemetics to patients receiving chemotherapy were effective in limiting the osteolytic bone destruction, thus preserving the bone architecture. The protective effect of nabilone on bone metabolism was further accompanied by a direct inhibition of tumor growth. As opposed to zoledronate, nabilone was however not able to manage bone tumor-induced pain and reactive gliosis. Altogether, our results revealed that morphine, nabilone and zoledronate exert disparate effects on tumor growth, bone metabolism and pain control. These findings also support the use of nabilone as an adjuvant therapy for bone metastases.

Exposure to chronic stress leads to an array of anatomical, functional, and metabolic changes in the brain that play a key role in triggering psychiatric disorders such as depression. The hippocampus is particularly well known as a target of maladaptive responses to stress. To capture stress-induced changes in metabolic and functional connectivity in the hippocampus, stress-resistant (low-responders) or -susceptible (high-responders) rats exposed to a chronic unpredictable stress paradigm (categorized according to their hormonal and behavioral responses) were assessed by multimodal neuroimaging; the latter was achieved by using localized 1H MR spectroscopy and resting-state functional MRI (fMRI) at 11,7T data from stressed (n = 25) but also control (n = 15) male Wistar rats.Susceptible animals displayed increased GABA-glutamine (+19%) and glutamate-glutamine (+17%) ratios and decreased levels of macromolecules (-11%); these changes were positively correlated with plasma corticosterone levels. In addition, the neurotransmitter levels showed differential associations with functional connectivity between the hippocampus and the amygdala, the piriform cortex and thalamus between stress-resistant and -susceptible animals. Our observations are consistent with previously reported stress-induced metabolomic changes that suggest overall neurotransmitter dysfunction in the hippocampus. Their association with the fMRI data in this study reveals how local adjustments in neurochemistry relate to changes in the neurocircuitry of the hippocampus, with implications for its stress-associated dysfunctions.SIGNIFICANCE STATEMENT Chronic stress disrupts brain homeostasis, which may increase the vulnerability of susceptible individuals to neuropsychiatric disorders such as depression. Characterization of the differences between stress-resistant and -susceptible individuals on the basis of noninvasive imaging tools, such as magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI), contributes to improved understanding of the mechanisms underpinning individual differences in vulnerability and can facilitate the design of new diagnostic and intervention strategies. Using a combined functional MRI/MRS approach, our results demonstrate that susceptible- and non-susceptible subjects show differential alterations in hippocampal GABA and glutamate metabolism that, in turn, associate with changes in functional connectivity., This work is part of the Sigma project (FCT-ANR/NEU-OSD/0258/2012) cofinanced by the French ANR (Agence National pour la Recherche, APP Blanc International II 2012), the Portuguese FCT (Fundacao para a Ciencia e Tecnologia) and by the project NORTE-01-0124-FEDER-000021, supported by the Portuguese North Regional Operational Program (ON.2-O Novo Norte), under the National Strategic Reference Framework (QREN) through the European Regional Development Fund (FEDER), by the project NORTE-01-0145-FEDER-000013, supported by the Northern Portugal Regional Operational Programme (NORTE 2020), under the Portugal 2020 Partnership Agreement through the FEDER as well as the Competitiveness Factors Operational Programme (COMPETE), and the Foundation for Science and Technology (FCT) under the scope of the project POCI-01-0145-FEDER-007038, and performed on a platform of the France Life Imaging network partly funded by Grant ANR-11-INBS-0006; R.M. was supported by Grant PDE/BDE/113604/2015 from the PhD-iHES program of the School of Medicine of Universidade do Minho. We thank Douglas L. Rothman, Grame F. Mason, and Osborne Almeida for their feedback.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

International audience; Neurotoxicity remains the most common adverse effect of oxaliplatin, limiting its clinical use. In the present study, we developed a mouse model of chronic oxaliplatin-induced neuropathy, which mimics both sensory and motor deficits observed in patients, in a clinically relevant time course. Repeated oxaliplatin administration in mice induced both cephalic and extracephalic long lasting mechanical and cold hypersensitivity after the first injection as well as delayed sensorimotor deficits and a depression-like phenotype. Using this model, we report that riluzole prevents both sensory and motor deficits induced by oxaliplatin as well as the depression-like phenotype induced by cumulative chemotherapeutic drug doses. All the beneficial effects are due to riluzole action on the TREK-1 potassium channel, which plays a central role in its therapeutic action. Riluzole has no negative effect on oxaliplatin antiproliferative capacity in human colorectal cancer cells and on its anticancer effect in a mouse model of colorectal cancer. Moreover, riluzole decreases human colorectal cancer cell line viability in vitro and inhibits polyp development in vivo. The present data in mice may support the need to clinically test riluzole in oxaliplatin-treated cancer patients and state for the important role of the TREK-1 channel in pain perception.

The epidemic of type 2 diabetes mellitus (T2DM) is fueled by added fructose consumption. Here, we thus combined high-fat/high-fructose diet, with multiple low-dose injections of streptozotocin (HF/HF/Stz) to emulate the long-term complications of T2DM. HF/HF/Stz rats, monitored over 56 weeks, exhibited metabolic dysfunctions associated with the different stages of the T2DM disease progression in humans: an early prediabetic phase characterized by an hyperinsulinemic period with modest dysglycemia, followed by a late stage of T2DM with frank hyperglycemia, normalization of insulinemia, marked dyslipidemia, hepatic fibrosis and pancreatic β-cell failure. Histopathological analyses combined to [18F]-FDG PET imaging further demonstrated the presence of several end-organ long-term complications, including reduction in myocardial glucose utilization, renal dysfunction as well as microvascular neuropathy and retinopathy. We also provide for the first time a comprehensive µ-PET whole brain imaging of the changes in glucose metabolic activity within discrete cerebral regions in HF/HF/Stz diabetic rats. Altogether, we developed and characterized a unique non-genetic preclinical model of T2DM adapted to the current diet and lifestyle that recapitulates the major metabolic features of the disease progression, from insulin resistance to pancreatic β-cell dysfunction, and closely mimicking the target-organ damage occurring in type 2 diabetic patients at advanced stages.

Stress is a well-established trigger for a number of neuropsychiatric disorders, as it alters both structure and function of several brain regions and its networks. Herein, we conduct a longitudinal neuroimaging study to assess how a chronic unpredictable stress protocol impacts the structure of the rat brain and its functional connectome in both high and low responders to stress. Our results reveal the changes that stress triggers in the brain, with structural atrophy affecting key regions such as the prelimbic, cingulate, insular and retrosplenial, somatosensory, motor, auditory and perirhinal/entorhinal cortices, the hippocampus, the dorsomedial striatum, nucleus accumbens, the septum, the bed nucleus of the stria terminalis, the thalamus and several brain stem nuclei. These structural changes are associated with increasing functional connectivity within a network composed by these regions. Moreover, using a clustering based on endocrine and behavioural outcomes, animals were classified as high and low responders to stress. We reveal that susceptible animals (high responders) develop local atrophy of the ventral tegmental area and an increase in functional connectivity between this area and the thalamus, further spreading to other areas that link the cognitive system with the fight-or-flight system. Through a longitudinal approach we were able to establish two distinct patterns, with functional changes occurring during the exposure to stress, but with an inflection point after the first week of stress when more prominent changes were seen. Finally, our study revealed differences in functional connectivity in a brainstem-limbic network that distinguishes resistant and susceptible responders before any exposure to stress, providing the first potential imaging-based predictive biomarkers of an individual's resilience/vulnerability to stressful conditions., This work is part of the Sigma project with the reference FCT-ANR/NEU-OSD/ 0258/2012 co-financed by the French public funding agency ANR (Agence National pour la Recherche, APP Blanc International II 2012), the Portuguese FCT (Fundação para a Ciência e Tecnologia) and by the Portuguese North Regional Operational Program (ON.2 – O Novo Norte) under the National Strategic Reference Framework (QREN), through the European Regional Development Fund (FEDER) as well as the Projecto Estratégico co-funded by FCT (PEst-C/SAU/LA0026-/2013) and the European Regional Development Fund COMPETE (FCOMP-01-0124-FEDER-037298). DAB and AN were funded by grants from FCT-ANR/NEU-OSD/0258/2012. RM is supported by the FCT fellowship grant with the reference PDE/BDE/113604/2015 from the PhDiHES program; AC was supported by a grant from the foundation NRJ. PM was funded by Fundação Calouste Gulbenkian (Portugal; ‘Better mental health during ageing based on temporal prediction of individual brain ageing trajectories (TEMPO)’), Grant Number P-139977. We thank Drs Patrício Costa and Pedro Moreira for support on the various statistical analyses., info:eu-repo/semantics/publishedVersion

Despite tremendous progress in the management of breast cancer, the survival rate of this disease is still correlated with the development of metastases-most notably, those of the bone. Diagnosis of bone metastasis requires a combination of multiple imaging modalities. MR imaging remains the best modality for soft-tissue visualization, allowing for the distinction between benign and malignant lesions in many cases. On the other hand, PET imaging is frequently more specific at detecting bone metastasis by measuring the accumulation of radiotracers, such as (18)F-sodium fluoride ((18)F-NaF) and (18)F-FDG. Thus, the main purpose of this study was to longitudinally monitor bone tumor progression using PET/MR image coregistration to improve noninvasive imaging-assisted diagnoses.After surgical implantation of mammary MRMT-1 cells in a rat femur, we performed minimally invasive imaging procedures at different time points throughout tumor development. The procedure consisted of sequential coregistered MR and PET image acquisition, using gadolinium-diethylenetriaminepentaacetic acid (DTPA) as a contrast agent for MR imaging and (18)F-FDG, (11)C-methionine, and (18)F-NaF as molecular tracers for PET imaging. The animals were then euthanized, and complementary radiologic (micro-CT scans) and histologic analyses were performed.In this preclinical study, we demonstrated that coregistered MR and PET images provide helpful information in a rat mammary-derived bone cancer model. First, MR imaging provided a high-definition anatomic resolution that made the localization of bone resorption and tumor extension detectable between days 9 and 18 after the injection of cancer cells in the medullary channel of the femur. Indeed, the calculation of mean standardized uptake value (SUVmean) and maximal SUV (SUVmax) in bone and soft-tissue regions, as defined from the gadolinium-DTPA contrast-enhanced MR images, showed (18)F-NaF uptake modifications and increased (18)F-FDG or (11)C-methionine uptake in the bone and surrounding soft tissues. (18)F-FDG and (11)C-methionine were compared in terms of the magnitude of change in their uptake and variability. We observed that (11)C-methionine SUVmean variations in the tumor were more important than those of (18)F-FDG. We also found fewer interindividual variations using SUVmean as a quantitative parameter than SUVmax.This preclinical evaluation demonstrated that a PET/MR image coregistration protocol provided a powerful tool to evaluate bone tumor progression in a rat model of bone metastasis and that this protocol could be translated to improve the clinical outcome for metastatic breast cancer management.

Resume Plus d’un siecle apres sa synthese, le paracetamol demeure l’antalgique le plus utilise dans le monde. Malgre son grand âge, ses effets benefiques (antalgique, antipyretique) autant qu’indesirables ainsi que sa toxicite sont une source intarissable de publications. En effet, les mecanismes sous-tendant ces differentes actions sont loin d’etre elucides. Nous pensons qu’une meilleure comprehension de son mecanisme d’action antalgique permettra d’identifier la ou les cible(s) moleculaire(s) a l’origine de son effet et ainsi de concevoir de nouveaux antalgiques plus efficaces presentant une tolerance au moins comparable a celle du paracetamol. Plusieurs systemes semblent etre impliques dont le systeme serotoninergique et celui des prostaglandines, auxquels s’ajoute desormais la participation des systemes endogenes cannabinoidergique et vanilloidergique. Ainsi, de recentes etudes devoilent une nouvelle vision du paracetamol : celle d’un pro-medicament qui necessite d’etre biotransforme pour exercer son effet antalgique. Ces dernieres donnees laissent presager que le paracetamol n’a pas fini de nous etonner.

International audience; Lithium is the first-line mood stabilizer for the treatment of patients with bipolar disorder. However, its mechanisms of action and transport across the blood–brain barrier remain poorly understood. The contribution of lithium-7 magnetic resonance imaging ($^7$Li MRI) to investigate brain lithium distribution remains limited because of the modest sensitivity of the lithium nucleus and the expected low brain concentrations in humans and animal models. Therefore, we decided to image lithium distribution in the rat brain ex vivo using a turbo-spin-echo imaging sequence at 17.2 T. The estimation of lithium concentrations was performed using a phantom replacement approach accounting for $B_1$ inhomogeneities and differential $T_1$ and $T_2$ weighting. Our MRI-derived lithium concentrations were validated by comparison with inductively coupled plasma-mass spectrometry (ICP-MS) measurements ([Li]$_{MRI}$ = 1.18[Li]$_{MS}$, $R$ = 0.95). Overall, a sensitivity of 0.03 mmol/L was achieved for a spatial resolution of 16 μL. Lithium distribution was uneven throughout the brain (normalized lithium content ranged from 0.4 to 1.4) and was mostly symmetrical, with consistently lower concentrations in the metencephalon (cerebellum and brainstem) and higher concentrations in the cortex. Interestingly, low lithium concentrations were also observed close to the lateral ventricles. The average brain-to-plasma lithium ratio was 0.34 ± 0.04, ranging from 0.29 to 0.39. Brain lithium concentrations were reasonably correlated with plasma lithium concentrations, with Pearson correlation factors ranging from 0.63 to 0.90.

The discovery that paracetamol is metabolized to the potent TRPV1 activator N-(4-hydroxyphenyl)-5Z, 8Z, 11Z, 14Z-eicosatetraenamide (AM404) and that this metabolite contributes to paracetamol's antinociceptive effect in rodents via activation of TRPV1 in the central nervous system (CNS) has provided a potential strategy for developing novel analgesics. Here we validated this strategy by examining the metabolism and antinociceptive activity of the de-acetylated paracetamol metabolite 4-aminophenol and 4-hydroxy-3-methoxybenzylamine (HMBA), both of which may undergo a fatty acid amide hydrolase (FAAH)-dependent biotransformation to potent TRPV1 activators in the brain. Systemic administration of 4-aminophenol and HMBA led to a dose-dependent formation of AM404 plus N-(4-hydroxyphenyl)-9Z-octadecenamide (HPODA) and arvanil plus olvanil in the mouse brain, respectively. The order of potency of these lipid metabolites as TRPV1 activators was arvanil = olvanil >> AM404. HPODA. Both 4-aminophenol and HMBA displayed antinociceptive activity in various rodent pain tests. The formation of AM404, arvanil and olvanil, but not HPODA, and the antinociceptive effects of 4-aminophenol and HMBA were substantially reduced or disappeared in FAAH null mice. The activity of 4-aminophenol in the mouse formalin, von Frey and tail immersion tests was also lost in TRPV1 null mice. Intracerebroventricular injection of the TRPV1 blocker capsazepine eliminated the antinociceptive effects of 4-aminophenol and HMBA in the mouse formalin test. In the rat, pharmacological inhibition of FAAH, TRPV1, cannabinoid CB1 receptors and spinal 5-HT3 or 5-HT1A receptors, and chemical deletion of bulbospinal serotonergic pathways prevented the antinociceptive action of 4-aminophenol. Thus, the pharmacological profile of 4-aminophenol was identical to that previously reported for paracetamol, supporting our suggestion that this drug metabolite contributes to paracetamol's analgesic activity via activation of bulbospinal pathways. Our findings demonstrate that it is possible to construct novel antinociceptive drugs based on fatty acid conjugation as a metabolic pathway for the generation of TRPV1 modulators in the CNS.

Diabetic peripheral neuropathy (DPN) is the most common clinical complication of diabetes mellitus, and can be related to type 1 as well as type 2. To date, this highly invalidating neurological impairment is insufficiently known, understood and the treatments proposed by physicians are still empirical and poorly efficient. Animal rodent modeling of clinical DPN offers a powerful tool in order to understand diabetes-mediated peripheral nerve injury. The majority of studies which have investigated DPN in rodent used the streptozotocin-induced rat model which reproduces metabolic lesional mechanisms of Type 1 Diabetes Mellitus (T1DM) and usual symptoms of evoked pain. Although the clinical relevance of this model is challenged due to 1) a high prevalence of type 2compared to type 1-diabetes in the adult population, 2) the important alteration of the general clinical state of the animals and 3) the lack of morphological changes in peripheral nerves, many studies have contributed to a better pathophysiological and pharmacological understanding of the DPN. In this review we investigated rodent models of T1DM and T2DM, their contributions for a better understanding of DPN, molecular targets and pharmacological strategies, which could be used for the enhancement of clinical care. Finally, we proposed possible ways to improve animal modeling.

BackgroundAcetaminophen, the major active metabolite of acetanilide in man, has become one of the most popular over-the-counter analgesic and antipyretic agents, consumed by millions of people daily. However, its mechanism of action is still a matter of debate. We have previously shown that acetaminophen is further metabolized to N-(4-hydroxyphenyl)-5Z,8Z,11Z,14Z -eicosatetraenamide (AM404) by fatty acid amide hydrolase (FAAH) in the rat and mouse brain and that this metabolite is a potent activator of transient receptor potential vanilloid 1 (TRPV(1)) in vitro. Pharmacological activation of TRPV(1) in the midbrain periaqueductal gray elicits antinociception in rats. It is therefore possible that activation of TRPV(1) in the brain contributes to the analgesic effect of acetaminophen.Methodology/principal findingsHere we show that the antinociceptive effect of acetaminophen at an oral dose lacking hypolocomotor activity is absent in FAAH and TRPV(1) knockout mice in the formalin, tail immersion and von Frey tests. This dose of acetaminophen did not affect the global brain contents of prostaglandin E(2) (PGE(2)) and endocannabinoids. Intracerebroventricular injection of AM404 produced a TRPV(1)-mediated antinociceptive effect in the mouse formalin test. Pharmacological inhibition of TRPV(1) in the brain by intracerebroventricular capsazepine injection abolished the antinociceptive effect of oral acetaminophen in the same test.ConclusionsThis study shows that TRPV(1) in brain is involved in the antinociceptive action of acetaminophen and provides a strategy for developing central nervous system active oral analgesics based on the coexpression of FAAH and TRPV(1) in the brain.