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While depression and chronic pain are frequently comorbid, underlying neuronal circuits, and their relevance for the understanding of psychopathology, remain poorly defined. Here we show in mice that hyperactivity of the neuronal pathway linking the basolateral amygdala to the anterior cingulate cortex is essential for chronic pain-induced depression. In naive animals, we demonstrate that activation of this pathway is sufficient to trigger depressive-like behaviors, as well as transcriptomic alterations that recapitulate core molecular features of depression in the human brain. These alterations notably impact gene modules related to myelination and the oligodendrocyte lineage. Among these, we show that Sema4a, a hub gene significantly upregulated in both mice and humans in the context of altered mood, is necessary for the emergence of depressive-like behaviors. Overall, these results place the BLA-ACC pathway at the core of pain and depression comorbidity, and unravel the role of impaired myelination and Sema4a in mood control.

Food intake, which is a highly reinforcing behavior, provides nutrients required for survival in all animals. However, when fat and sugars consumption goes beyond the daily needs, it can favor obesity. The prevalence and severity of this health problem has been increasing with time. Besides covering nutrient and energy needs, food and in particular its highly palatable components, such as fats, also induce feelings of joy and pleasure. Experimental evidence supports a role of the striatal complex and of the mesolimbic dopamine system in both feeding and food-related reward processing, with the nucleus accumbens as a key target for reward or reinforcing-associated signaling during food intake behavior. In this review, we provide insights concerning the impact of feeding, including fat intake, on different types of receptors and neurotransmitters present in the striatal complex. Reciprocally, we also cover the evidence for a modulation of palatable food intake by different neurochemical systems in the striatal complex and in particular the nucleus accumbens, with a focus on dopamine, GABA and the opioid system.

The International Association for the Study of Pain defines neuropathic pain as "pain arising as a direct consequence of a lesion or disease affecting the somatosensory system" (Treede et al., 2008). The associated changes can be observed in the peripheral as well as the central nervous system. The available literature discusses a wide variety of causes as predisposing for the development and amplification of neuropathic pain. Further, key interactions within sensory pathways have been discovered, but no common molecular mechanism leading to neuropathic pain has been identified until now. In the first part of this review, the pain mediating lateral spinothalamic tract is described. Different in vivo models are presented that allow studying trauma-, chemotherapy-, virus-, and diabetes-induced neuropathic pain in rodents. We furthermore discuss approaches to assess neuropathic pain in these models. Second, the current knowledge about cellular and molecular mechanisms suggested to underlie the development of neuropathic pain is presented and discussed. A summary of established therapies that are already applied in the clinic and novel, promising approaches closes the paper. In conclusion, the established animal models are able to emulate the diversity of neuropathic pain observed in the clinics. However, the assessment of neuropathic pain in the presented in vivo models should be improved. The determination of common molecular markers with suitable in vitro models would simplify the assessment of neuropathic pain in vivo. This would furthermore provide insights into common molecular mechanisms of the disease and establish a basis to search for satisfying therapeutic approaches.

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International audience; Mood disorders such as depression and anxiety are frequently observed in patients suffering from chronic pain. Over time, different tests and models have been developed in rodents to study the anxiodepressive‐like consequences of chronic pain. This review describes these preclinical tools (models and tests) used for studying behavioural aspects of the comorbid relationship between chronic pain and anxiety and/or major depressive disorder. Three major types of chronic pain strongly associated with anxiodepressive‐like comorbidity as well as their animal models are presented: neuropathic pain, inflammatory pain and fibromyalgia. After a description of chronic pain animal models and of the tests that allow determining nociceptive responses, this review presents and discusses the various behavioural tests that have been used to assess anxiety and depressive‐like behaviours in these models of chronic pain. Finally, this review highlights the progress that remains to be made to homogenize the results in the field of pain‐induced mood disorders and summarizes the recent advances achieved through these tests and models.

Central dopamine signaling regulates reward-related aspects of feeding behavior, and during diet-induced obesity dopamine receptor signaling is altered. Yet, the influence of dopamine signaling on the consumption of specific dietary components remains to be elucidated. We have previously shown that 6-hydroxydopamine-mediated lesions of dopamine neuron terminals in the lateral shell of the nucleus accumbens promotes fat intake in rats fed a multi-component free-choice high-fat high-sugar (fcHFHS) diet. It is however not yet determined which dopamine receptors are responsible for this shift towards fat preference. In this study, we assess the effects of D1-or D2 receptor acute inhibition in the lateral shell of the nucleus accumbens on fcHFHS diet consumption. We report that infusion of the D1 receptor antagonist SCH2 3390, but not the D2 receptor antagonist raclopride, promotes dietary fat consumption in male Sprague Dawley rats on a fcHFHS diet during 2 h after infusion. Furthermore, anatomical analysis of infusion sites revealed that the rostral region, but not the caudal region, of the lateral shell of the nucleus accumbens is sensitive to the D1 receptor inhibition effects on fat consumption. Our data highlight a role for D1 receptors in the rostral region of the lateral shell of the nucleus accumbens to control dietary fat consumption.

Background and objective Neuropathic pain arises as a direct consequence of a lesion or disease affecting the somatosensory system. A number of preclinical studies have provided evidence for the involvement of cytokines, predominantly secreted by a variety of immune cells and by glial cells from the nervous system, in neuropathic pain conditions. Clinical trials and the use of anti-cytokine drugs in different neuropathic aetiologies support the relevance of cytokines as treatment targets. However, the use of such drugs, in particularly biotherapies, can provoke notable adverse effects. Moreover, it is challenging to select one given cytokine as a target, among the various neuropathic pain conditions. It could thus be of interest to target other proteins, such as growth factors, in order to act more widely on the neuroinflammation network. Thus, platelet-rich plasma (PRP), an autologous blood concentrate, is known to contain a natural concentration of growth factors and immune system messengers and is widely used in the clinical setting for tissue regeneration and repair. Database and data treatment In the present review, we critically assess the current knowledge on cytokines in neuropathic pain by taking into consideration both human studies and animal models. Results This analysis of the literature highlights the pathophysiological importance of cytokines. We particularly highlight the concept of time- and tissue-dependent cytokine activation during neuropathic pain conditions. Results Conclusion: Thus, direct or indirect cytokines modulation with biotherapies or growth factors appears relevant. In addition, we discuss the therapeutic potential of localized injection of PRP as neuropathic pain treatment by pointing out the possible link between cytokines and the action of PRP. Significance Preclinical and clinical studies highlight the idea of a cytokine imbalance in the development and maintenance of neuropathic pain. Clinical trials with anticytokine drugs are encouraging but are limited by a 'cytokine candidate approach' and adverse effect of biotherapies. PRP, containing various growth factors, is a new therapeutic used in regenerative medicine. Growth factors can be also considered as modulators of cytokine balance. Here, we emphasize a potential therapeutic effect of PRP on cytokine imbalance in neuropathic pain. We also underline the clinical interest of the use of PRP, not only for its therapeutic effect but also for its safety of use.

Dopamine influences food intake behavior. Reciprocally, food intake, especially of palatable dietary items, can modulate dopamine-related brain circuitries. Among these reciprocal impacts, it has been observed that an increased intake of dietary fat results in blunted dopamine signaling and, to compensate this lowered dopamine function, caloric intake may subsequently increase. To determine how dopamine regulates food preference we did 6-hydroxydopamine (6-OHDA) lesions, depleting dopamine in specific brain regions in male Sprague Dawley rats. The food preference was assessed by providing the rats with free choice access to control diet, fat, 20% sucrose and tap water. Rats with midbrain lesions targeting the substantia nigra (which is also a model of Parkinson’s disease) consumed fewer calories, as reflected by a decrease in control diet intake, but they surprisingly displayed an increase in fat intake, without change in the sucrose solution intake compared to sham animals. To determine which of the midbrain dopamine projections may contribute to this effect, we next compared the impact of 6-OHDA lesions of terminal fields, targeting the dorsal striatum, the lateral nucleus accumbens and the medial nucleus accumbens. We found that 6-OHDA lesion of the lateral nucleus accumbens, but not of the dorsal striatum or the medial nucleus accumbens, led to increased fat intake. These findings indicate a role for lateral nucleus accumbens dopamine in regulating food preference, in particularly the intake of fat.HIGHLIGHTSDopamine influences fat intakeAnimal model of Parkinson’s disease display lower kcal intake but increased fat choiceDopamine depletion in the lateral nucleus accumbens favors fat intake

The tail of the ventral tegmental area (tVTA) or rostromedial tegmental nucleus (RMTg) receives lateral habenula inputs and projects heavily to midbrain dopamine neurons. Midbrain dopamine and lateral habenula neurons participate in learning processes predicting the outcomes of actions, placing the tVTA in a critical location into prediction error pathways. tVTA GABA neurons show electrophysiological inhibition or activation after reward and aversive stimuli, respectively, and their predictive cues. tVTA molecular recruitment, however, is not elicited by all aversive stimuli. Indeed, precipitated opioid withdrawal, repeated footshocks or food restriction raise tVTA Fos expression, whereas various other unpleasant, stressful or painful stimuli does not elicit that molecular response. However, the basis of that difference remains unknown. In the present study, we tried to disentangle whether the tVTA c-Fos induction observed after food restriction was due to the aversive state of food restriction or to procedure-related reward prediction error. To this end, male Sprague-Dawley rats were food-restricted for 7-8 days. During this period, animals were handled and weighed every day before feeding. On the test day, rats underwent several behavioral procedures to explore the impact of food restriction and food-predictive cue exposure on tVTA c-Fos expression. We showed that food restriction per se was not able to recruit c-Fos in the tVTA. On the contrary, the food-predicting cues induced c-Fos locally in the absence of feeding, whereas the food-predicting cues followed by feeding evoked lower c-Fos expression. Overall, our results support the proposed involvement of the tVTA in reward prediction error.

Supplemental Digital Content is Available in the Text. Mefloquine/amitriptyline THN101 combination reduces cuff-induced neuropathic hypersensitivity by recruiting noradrenergic descending pathways and α2 adrenoceptors and by blocking connexin in mice., Tricyclic antidepressants that inhibit serotonin and noradrenaline reuptake, such as amitriptyline, are among the first-line treatments for neuropathic pain, which is caused by a lesion or disease affecting the somatosensory nervous system. These treatments are, however, partially efficient to alleviate neuropathic pain symptoms, and better treatments are still highly required. Interactions between neurons and glial cells participate in neuropathic pain processes, and importantly, connexins—transmembrane proteins involved in cell–cell communication—contribute to these interactions. In a neuropathic pain model in rats, mefloquine, a connexin inhibitor, has been shown to potentiate the antihyperalgesic effect of amitriptyline, a widely used antidepressant. In this study, we further investigated this improvement of amitriptyline action by mefloquine, using the cuff model of neuropathic pain in mice. We first observed that oral mefloquine co-treatment prolonged the effect of amitriptyline on mechanical hypersensitivity by 12 hours after administration. In addition, we showed that this potentiation was not due to pharmacokinetic interactions between the 2 drugs. Besides, lesional and pharmacological approaches showed that the prolonged effect was induced through noradrenergic descending pathways and the recruitment of α2 adrenoceptors. Another connexin blocker, carbenoxolone, also improved amitriptyline action. Additional in vitro studies suggested that mefloquine may also directly act on serotonin transporters and on adenosine A1 and A2A receptors, but drugs acting on these other targets failed to amplify amitriptyline action. Together, our data indicate that pharmacological blockade of connexins potentiates the therapeutic effect of amitriptyline in neuropathic pain.

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Neuropathic pain arises as a consequence of a lesion or disease affecting the somatosensory nervous system. It is accompanied by neuronal and non-neuronal alterations, including alterations in intracellular second messenger pathways. Cellular levels of 3',5'-cyclic adenosine monophosphate (cAMP) and 3',5'-cyclic guanosine monophosphate (cGMP) are regulated by phosphodiesterase (PDE) enzymes. Here, we studied the impact of PDE inhibitors (PDEi) in a mouse model of peripheral nerve injury induced by placing a cuff around the main branch of the sciatic nerve. Mechanical hypersensitivity, evaluated using von Frey filaments, was relieved by sustained treatment with the non-selective PDEi theophylline and ibudilast (AV-411), with PDE4i rolipram, etazolate and YM-976, and with PDE5i sildenafil, zaprinast and MY-5445, but not by treatments with PDE1i vinpocetine, PDE2i EHNA or PDE3i milrinone. Using pharmacological and knock-out approaches, we show a preferential implication of delta opioid receptors in the action of the PDE4i rolipram and of both mu and delta opioid receptors in the action of the PDE5i sildenafil. Calcium imaging highlighted a preferential action of rolipram on dorsal root ganglia non-neuronal cells, through PDE4B and PDE4D inhibition. Rolipram had anti-neuroimmune action, as shown by its impact on levels of the pro-inflammatory cytokine tumor necrosis factor-α (TNFα) in the dorsal root ganglia of mice with peripheral nerve injury, as well as in human peripheral blood mononuclear cells (PBMCs) stimulated with lipopolysaccharides. This study suggests that PDEs, especially PDE4 and 5, may be targets of interest in the treatment of neuropathic pain.

Neuropathic pain is a challenging condition for which current therapies often remain unsatisfactory. Chronic administration of β2 adrenergic agonists, including formoterol currently used to treat asthma and chronic obstructive pulmonary disease, alleviates mechanical allodynia in the sciatic nerve cuff model of neuropathic pain. The limited clinical data currently available also suggest that formoterol would be a suitable candidate for drug repurposing. The antiallodynic action of β2 adrenergic agonists is known to require activation of the delta-opioid (DOP) receptor but better knowledge of the molecular mechanisms involved is necessary. Using a mouse line in which DOP receptors were selectively ablated in neurons expressing Nav1.8 sodium channels (DOP cKO), we showed that these DOP peripheral receptors were necessary for the antiallodynic action of the β2 adrenergic agonist formoterol in the cuff model. Using a knock-in mouse line expressing a fluorescent version of the DOP receptor fused with the enhanced green fluorescent protein (DOPeGFP), we established in a previous study, that mechanical allodynia is associated with a smaller percentage of DOPeGFP positive small peptidergic sensory neurons in dorsal root ganglia (DRG), with a reduced density of DOPeGFP positive free nerve endings in the skin and with increased DOPeGFP expression at the cell surface. Here, we showed that the density of DOPeGFP positive free nerve endings in the skin is partially restored and no increase in DOPeGFP translocation to the plasma membrane is observed in mice in which mechanical pain is alleviated upon chronic oral administration of formoterol. This study, therefore, extends our previous results by confirming that changes in the mechanical threshold are associated with changes in peripheral DOP profile. It also highlights the common impact on DOP receptors between serotonin noradrenaline reuptake inhibitors such as duloxetine and the β2 mimetic formoterol.

Parkinson's disease is a neurodegenerative disorder partly caused by the loss of the dopamine neurons of the nigrostriatal pathway. It is accompanied by motor as well as non-motor symptoms, including pain and depression. The tail of the ventral tegmental area (tVTA) or rostromedial tegmental nucleus (RMTg) is a GABAergic mesopontine structure that acts as a major inhibitory brake for the substantia nigra pars compacta (SNc) dopamine cells, thus controlling their neuronal activity and related motor functions. The present study tested the influence of suppressing this tVTA brake on motor and non-motor symptoms in a rat model of Parkinson's disease. Using behavioral approaches, we showed that male Sprague-Dawley rats with bilateral and partial 6-hydroxydopamine SNc lesion displayed motor impairments in the rotarod test, impairments that were no more present following a co-lesion of the tVTA. Using a larger set of behavioral tests, we then showed that such SNc lesion also led to non-motor symptoms, including lower body weight, lower mechanical nociceptive thresholds in the forceps test and lower thermal nociceptive thresholds in the incremented hot-plate test, and a decreased sucrose preference in a 2-bottle choice paradigm. The excitotoxic co-lesion of the tVTA led to compensation of body weight, mechanical nociceptive thresholds and anhedonia-like behavior. These findings illustrate the major influence that the tVTA exerts on the dopamine system, modulating the motor and non-motor symptoms related to a partial loss of dopamine cells.

Neuropathic pain arises as a consequence of a lesion or disease affecting the somatosensory system. It is generally chronic and challenging to treat. The recommended pharmacotherapy for neuropathic pain includes the use of some antidepressants, such as tricyclic antidepressants (TCAs) (amitriptyline…) or serotonin and noradrenaline re-uptake inhibitors (duloxetine…), and/or anticonvulsants such as the gabapentinoids gabapentin or pregabalin. Antidepressant drugs are not acute analgesics but require a chronic treatment to relieve neuropathic pain, which suggests the recruitment of secondary downstream mechanisms as well as long-term molecular and neuronal plasticity. Noradrenaline is a major actor for the action of antidepressant drugs in a neuropathic pain context. Mechanistic hypotheses have implied the recruitment of noradrenergic descending pathways as well as the peripheral recruitment of noradrenaline from sympathetic fibers sprouting into dorsal root ganglia; and importance of both α2 and β2 adrenoceptors have been reported. These monoamine re-uptake inhibitors may also indirectly act as anti-proinflammatory cytokine drugs; and their therapeutic action requires the opioid system, particularly the mu (MOP) and/or delta (DOP) opioid receptors. Gabapentinoids, which target the voltage-dependent calcium channels α2δ-1 subunit, inhibit calcium currents, thus decreasing the excitatory transmitter release and spinal sensitization. Gabapentinoids also activate the descending noradrenergic pain inhibitory system coupled to spinal α2 adrenoceptors. Gabapentinoid treatment may also indirectly impact on neuroimmune actors, like proinflammatory cytokines. These drugs are effective against neuropathic pain both with acute administration at high dose and with repeated administration. This review focuses on mechanistic knowledge concerning chronic antidepressant treatment and gabapentinoid treatment in a neuropathic pain context.

Areas 24a and 24b of the anterior cingulate cortex (ACC) play a major role in cognition, emotion and pain. While their connectivity has been studied in primate and in rat, a complete mapping was still missing in the mouse. Here, we analyzed the afferents to the mouse ACC by injecting retrograde tracers in the ventral and dorsal areas of the ACC (areas 24a/b) and of the midcingulate cortex (MCC; areas 24a'/b'). Our results reveal inputs from five principal groups of structures: (1) cortical areas, mainly the orbital, medial prefrontal, retrosplenial, parietal associative, primary and secondary sensory areas and the hippocampus, (2) basal forebrain, mainly the basolateral amygdaloid nucleus, the claustrum and the horizontal limb of the diagonal band of Broca, (3) the thalamus, mainly the anteromedial, lateral mediodorsal, ventromedial, centrolateral, central medial and reuniens/rhomboid nuclei, (4) the hypothalamus, mainly the lateral and retromammillary areas, and (5) the brainstem, mainly the monoaminergic centers. The neurochemical nature of inputs from the diagonal band of Broca and brainstem centers was also investigated by double-labeling, showing that only a part of these afferents were cholinergic or monoaminergic. Comparisons between the areas indicate that areas 24a and 24b receive qualitatively similar inputs, but with different densities. These differences are more pronounced when comparing the inputs to ACC's areas 24a/24b to the inputs to MCC's areas 24a'/24b'. These results provide a complete analysis of the afferents to the mouse areas 24a/24b and 24a'/24b', which shows important similarity with the connectivity of homologous areas in rats, and brings the anatomical basis necessary to address the roles of cingulate areas in mice.

Since the work of Johnson and North, it is known that opiates increase the activity of dopaminergic neurons by a GABA neuron-mediated desinhibition. This model should however be updated based on recent advances. Thus, the neuroanatomical location of the GABA neurons responsible for this desinhibition has been recently detailed: they belong to a brain structure in continuity with the posterior part of the ventral tegmental area and discovered this past decade. Other data also highlighted the critical role played by glutamatergic transmission in the opioid regulation of dopaminergic neuron activity. During protracted opiate withdrawal, the inhibitory/excitatory balance exerted on dopaminergic neurons is altered. These results are now leading to propose an original hypothesis for explaining the impact of protracted opiate withdrawal on mood.

Resume La douleur chronique a des repercussions sur divers aspects de la qualite de vie et des consequences thymiques. La depression et l’anxiete sont ainsi souvent observees chez les patients douloureux chroniques. Alors que cette comorbidite est cliniquement bien etablie, les mecanismes sous-jacents restent a identifier. Depuis quelques annees, le developpement de modeles precliniques animaux pertinents a permis d’etablir un lien de causalite entre douleur chronique et troubles de l’humeur et de commencer a apporter des informations mecanistiques. Des alterations morphofonctionnelles qui surviennent dans le cortex cingulaire anterieur, le noyau accumbens et l’hippocampe dans les conditions de douleur neuropathique, certains polymorphismes genetiques et facteurs environnementaux dans la fibromyalgie ou le desequilibre des cytokines pro- et anti-inflammatoires et le role de l’amygdale dans la douleur inflammatoire, contribuent ainsi aux symptomes anxiodepressifs qui accompagnent la douleur chronique. Ces travaux suggerent notamment que des mecanismes differents peuvent exister en fonction du type de douleur chronique. Les travaux precliniques completent aussi les donnees cliniques pour mieux comprendre la segregation corticale des divers aspects de la douleur chronique, notamment de ses composantes sensorielles et emotionnelles. Entre clinique et laboratoire, l’effort actuel de recherche translationnelle fait progresser notre comprehension de la comorbidite douleur/depression.

In addition to treating depression, antidepressant drugs are also a first-line treatment for neuropathic pain, which is pain secondary to lesion or pathology of the nervous system. Despite the widespread use of these drugs, the mechanism underlying their therapeutic action in this pain context remains partly elusive. The present study combined data collected in male and female mice from a model of neuropathic pain and data from the clinical setting to understand how antidepressant drugs act. We show two distinct mechanisms by which the selective inhibitor of serotonin and noradrenaline reuptake duloxetine and the tricyclic antidepressant amitriptyline relieve neuropathic allodynia. One of these mechanisms is acute, central, and requires descending noradrenergic inhibitory controls and α2Aadrenoceptors, as well as the mu and delta opioid receptors. The second mechanism is delayed, peripheral, and requires noradrenaline from peripheral sympathetic endings and β2adrenoceptors, as well as the delta opioid receptors. We then conducted a transcriptomic analysis in dorsal root ganglia, which suggested that the peripheral component of duloxetine action involves the inhibition of neuroimmune mechanisms accompanying nerve injury, including the downregulation of the TNF-α–NF-κB signaling pathway. Accordingly, immunotherapies against either TNF-α or Toll-like receptor 2 (TLR2) provided allodynia relief. We also compared duloxetine plasma levels in the animal model and in patients and we observed that patients' drug concentrations were compatible with those measured in animals under chronic treatment involving the peripheral mechanism. Our study highlights a peripheral neuroimmune component of antidepressant drugs that is relevant to their delayed therapeutic action against neuropathic pain.SIGNIFICANCE STATEMENTIn addition to treating depression, antidepressant drugs are also a first-line treatment for neuropathic pain, which is pain secondary to lesion or pathology of the nervous system. However, the mechanism by which antidepressant drugs can relieve neuropathic pain remained in part elusive. Indeed, preclinical studies led to contradictions concerning the anatomical and molecular substrates of this action. In the present work, we overcame these apparent contradictions by highlighting the existence of two independent mechanisms. One is rapid and centrally mediated by descending controls from the brain to the spinal cord and the other is delayed, peripheral, and relies on the anti-neuroimmune action of chronic antidepressant treatment.

International audience; Peripheral delta opioid (DOP) receptors are essential for the antiallodynic effect of the tricyclic antidepressant nortriptyline. However, the population of DOP-expressing cells affected in neuropathic conditions or underlying the antiallodynic activity of antidepressants remains unknown. Using a mouse line in which DOP receptors were selectively ablated in cells expressing Nav1.8 sodium channels (DOP cKO), we established that these DOP peripheral receptors were mandatory for duloxetine to alleviate mechanical allodynia in a neuropathic pain model based on sciatic nerve cuffing. We then examined the impact of nerve cuffing and duloxetine treatment on DOP-positive populations using a knock-in mouse line expressing a fluorescent version of the DOP receptor fused with the enhanced green fluorescent protein (DOPeGFP). Eight weeks postsurgery, we observed a reduced proportion of DOPeGFP-positive small peptidergic sensory neurons (calcitonin gene-related peptide (CGRP) positive) in dorsal root ganglia and a lower density of DOPeGFP-positive free nerve endings in the skin. These changes were not present in nerve-injured mice chronically treated with oral duloxetine. In addition, increased DOPeGFP translocation to the plasma membrane was observed in neuropathic conditions but not in duloxetine-treated neuropathic mice, which may represent an additional level of control of the neuronal activity by DOP receptors. Our results therefore established a parallel between changes in the expression profile of peripheral DOP receptors and mechanical allodynia induced by sciatic nerve cuffing.

4-phenylpyridin-2-yl-guanidine (5b): a new inhibitor of the overproduction of pro-inflammatory cytokines (TNFα and Il1β) was identified from a high-throughput screening of a chemical library on human peripheral blood mononuclear cells (PBMCs) after LPS stimulation. Derivatives, homologues and rigid mimetics of 5b were designed and synthesized, and their cytotoxicity and ability to inhibit TNFα overproduction were evaluated. Among them, compound 5b and its mimetic 12 (2-aminodihydroquinazoline) showed similar inhibitory activities, and were evaluated in vivo in models of lung inflammation and neuropathic pain in mice. In particular, compound 12 proved to be active (5 mg/kg, ip) in both models.

The anterior cingulate cortex (ACC), constituted by areas 25, 32, 24a and 24b in rodents, plays a major role in cognition, emotion and pain. In a previous study, we described the afferents of areas 24a and 24b and those of areas 24a' and 24b' of midcingulate cortex (MCC) in mice and highlighted some density differences among cingulate inputs (Fillinger et al., Brain Struct Funct 222:1509-1532, 2017). To complete this connectome, we analyzed here the efferents of ACC and MCC by injecting anterograde tracers in areas 24a/24b of ACC and 24a'/24b' of MCC. Our results reveal a common projections pattern from both ACC and MCC, targeting the cortical mantle (intracingulate, retrosplenial and parietal associative cortex), the non-cortical basal forebrain, (dorsal striatum, septum, claustrum, basolateral amygdala), the hypothalamus (anterior, lateral, posterior), the thalamus (anterior, laterodorsal, ventral, mediodorsal, midline and intralaminar nuclei), the brainstem (periaqueductal gray, superior colliculus, pontomesencephalic reticular formation, pontine nuclei, tegmental nuclei) and the spinal cord. In addition to an overall denser ACC projection pattern compared to MCC, our analysis revealed clear differences in the density and topography of efferents between ACC and MCC, as well as between dorsal (24b/24b') and ventral (24a/24a') areas, suggesting a common functionality of these two cingulate regions supplemented by specific roles of each area. These results provide a detailed analysis of the efferents of the mouse areas 24a/24b and 24a'/24b' and achieve the description of the cingulate connectome, which bring the anatomical basis necessary to address the roles of ACC and MCC in mice.

Background Besides chronic stress, chronic pain is a prevalent determinant for depression. Changes induced in specific brain regions by sustained pain may alter the processing of affective information, thus resulting in anxiodepressive disorders. Here, we compared the role of the anterior cingulate cortex (ACC) and the posterior insular cortex in the anxiodepressive, sensory, and affective aspects of chronic pain. Methods Neuropathic pain was induced by cuffing the right sciatic nerve of C57BL/6J mice. Lesions were performed by local injection of ibotenic acid and chronic activation of the ACC by optogenetic stimulation. Anxiodepressive-related behaviors were evaluated through the novelty suppressed feeding, marble burying, splash, and forced swimming tests. Mechanical thresholds were determined using von Frey filaments, and the relief of spontaneous pain was determined by using place conditioning. Results The ACC lesion prevented the anxiodepressive consequences of chronic pain without affecting the sensory mechanical allodynia. Conversely, the tonic or spontaneous pain and the anxiodepressive consequences of pain remained present after posterior insular cortex lesion, even though the mechanical allodynia was suppressed. Furthermore, optogenetic stimulation of the ACC was sufficient to induce anxiety and depressive-like behaviors in naive animals. Conclusions Our results show that, at cortical level, the sensory component of chronic pain remains functionally segregated from its affective and anxiodepressive components. Spontaneous tonic pain and evoked allodynia can be experimentally dissociated. Furthermore, the ACC appears as a critical hub for mood disorders, including for the anxiodepressive consequences of chronic pain, and thus constitutes an important target for divulging the underlying mechanism.

Background and Purpose Tricyclic antidepressants are used clinically as first-line treatments for neuropathic pain. Opioid receptors participate in this pain-relieving action, and preclinical studies in receptor-deficient mice have highlighted a critical role for δ-, but not μ-opioid receptors. In this study, we investigated whether κ-opioid (KOP) receptors have a role in the antiallodynic action of tricyclic antidepressants.

Projection systems arising from the ventral tegmental area (VTA) and the substantia nigra (SN) have a critical role in a broad range of functions, as well as in the etiology, symptoms and treatment of neurological and psychiatric diseases. Mostly studied for its dopamine neurons, the ventral tegmentum is in fact heterogeneous at cellular and functional levels. This special issue of Neuroscience gathered some experts in the field to review the connectivity of the ventral mesencephalic dopaminergic complex, its cellular heterogeneity with attention given to glutamate neurons, the D2 autoreceptor and the cholinergic controls of dopamine activity, the influence of neurotrophins, the controls of bursting activity and the heterogeneity of neuronal activity across traits and states, the pedunculopontine tegmental and the sensory controls of dopamine activity, the sex-dependent diversity, the links between circadian and dopamine systems, the functional antero-posterior heterogeneity of the VTA and the role of its GABA tail (tVTA/rostromedial tegmental nucleus (RMTg)), the functional heterogeneity of the VTA outputs, the place of dopamine in cortico-basal ganglia circuitry, the different roles of the D1 and D2 striatal pathways and the role of dopamine in associative learning and memory. Recent progress also highlights the need for molecular markers of functional subpopulations within the ventral tegmentum, for deeper developmental knowledge of this region, and for a single cell level of connectomic. It also raises the question of inter-individual, sex, strain and species heterogeneity, and conversely the question of data generalization in a context of human pathology models, which warrant comparative studies and translational effort.

Mood disorders such as depression and anxiety are frequently observed in patients suffering from chronic pain, including neuropathic pain. While this comorbidity is clinically well established, the underlying mechanism(s) remained unclear. The recent development of animal models now allows addressing the consequences of neuropathic pain. In this review, we report the preclinical evidences from anatomical, neuroimaging, behavioral, pharmacological and biochemical studies that address the anxiodepressive consequences of neuropathic pain. We present an overview of rodent models of these consequences and we discuss the challenges and parameters to consider for generating these models. We then discuss the possible mechanism(s) underlying anxiodepressive consequences by describing morphological and functional changes. Information is provided concerning neuroanatomical changes and plasticity, including LTP and LTD, in the anterior cingulate cortex, the insula, the hippocampus, the amygdala and the mesolimbic system, neuroendocrine parameters concerning the hypothalamo-pituitary-adrenal axis, neuroimmune response including the role of glial cells and cytokines, monoamine systems and changes in locus coeruleus noradrenergic system, and neurotrophic factors such as BDNF.

Background Depression is frequently associated with chronic pain or chronic stress. Among cortical areas, the anterior cingulate cortex (ACC, areas 24a and 24b) appears to be important for mood disorders and constitutes a neuroanatomical substrate for investigating the underlying molecular mechanisms. The current work aimed at identifying ACC molecular factors subserving depression. Methods Anxiodepressive-like behaviors in C57BL/6J male mice were induced by neuropathic pain, unpredictable chronic mild stress, and optogenetic ACC stimulation and were evaluated using novelty suppressed feeding, splash, and forced swim tests. ACC molecular changes in chronic pain–induced depression were uncovered through whole-genome expression analysis. Further mechanistic insights were provided by chromatin immunoprecipitation, Western blot, and immunostaining. The causal link between molecular changes and depression was studied using knockout, pharmacological antagonism, and local viral-mediated gene knockdown. Results Under chronic pain–induced depression, gene expression changes in the ACC highlighted the overexpression of a regulator of the mitogen-activated protein kinase pathway, mitogen-activated protein kinase phosphatase-1 (MKP-1). This upregulation is associated with the presence of transcriptionally active chromatin marks (acetylation) at its proximal promoter region as well as increased cyclic adenosine monophosphate response element–mediated transcriptional activity and phosphorylation of cyclic adenosine monophosphate response element binding protein and activating transcription factor. MKP-1 overexpression is also observed with unpredictable chronic mild stress and repeated ACC optogenetic stimulation and is reversed by fluoxetine. A knockout, an antagonist, or a local silencing of MKP-1 attenuates depressive-like behaviors, pointing to an important role of this phosphatase in depression. Conclusions These data point to ACC MKP-1 as a key factor in the pathophysiology of depression and a potential target for treatment development.

Sciatica with or without low back pain is not easily managed,1 and few treatments have established efficacy for these conditions.2,3 Although the antiepileptic drug pregabalin has been recommended for the treatment of neuropathic pain (i.e., the broad category of pain associated with a lesion or disease of the somatosensory system, such as postherpetic neuralgia, polyneuropathy, or nerve trauma), a randomized, placebo-controlled trial of pregabalin involving patients with chronic lumbosacral painful radiculopathy did not show a benefit with pregabalin.4 However, the trial did not allow definitive conclusions because of methodologic limitations. In this issue of the Journal, Mathieson and colleagues . . .

International audience; The GABAergic tail of the ventral tegmental area (tVTA), also named rostromedial tegmental nucleus (RMTg), exerts an inhibitory control on dopamine neurons of the VTA and substantia nigra. The tVTA has been implicated in avoidance behaviors, response to drugs of abuse, reward prediction error, and motor functions. Stimulation of the lateral habenula (LHb) inputs to the tVTA, or of the tVTA itself, induces avoidance behaviors, which suggests a role of the tVTA in processing aversive information. Our aim was to test the impact of aversive stimuli on the molecular recruitment of the tVTA, and the behavioral consequences of tVTA lesions. In rats, we assessed Fos response to lithium chloride (LiCl), β-carboline, naloxone, lipopolysaccharide (LPS), inflammatory pain, neuropathic pain, foot-shock, restraint stress, forced swimming, predator odor, and opiate withdrawal. We also determined the effect of tVTA bilateral ablation on physical signs of opiate withdrawal, and on LPS- and LiCl-induced conditioned taste aversion (CTA). Naloxone-precipitated opiate withdrawal induced Fos in μ-opioid receptor-positive (15%) and -negative (85%) tVTA cells, suggesting the presence of both direct and indirect mechanisms in tVTA recruitment during withdrawal. However, tVTA lesion did not impact physical signs of opiate withdrawal. Fos induction was also present with repeated, but not single, foot-shock delivery. However, such induction was mostly absent with other aversive stimuli. Moreover, tVTA ablation had no impact on CTA. Although stimulation of the tVTA favors avoidance behaviors, present findings suggest that this structure may be important to the response to some, but not all, aversive stimuli.

Neuropathic pain is pain arising as a direct consequence of a lesion or disease affecting the somatosensory system. It is usually chronic and challenging to treat. Some antidepressants are first-line pharmacological treatments for neuropathic pain. The noradrenaline that is recruited by the action of the antidepressants on reuptake transporters has been proposed to act through β2-adrenoceptors (β2-ARs) to lead to the observed therapeutic effect. However, the complex downstream mechanism mediating this action remained to be identified. In this study, we demonstrate in a mouse model of neuropathic pain that an antidepressant's effect on neuropathic allodynia involves the peripheral nervous system and the inhibition of cytokine tumor necrosis factor α (TNFα) production. The antiallodynic action of nortriptyline is indeed lost after peripheral sympathectomy, but not after lesion of central descending noradrenergic pathways. More particularly, we report that antidepressant-recruited noradrenaline acts, within dorsal root ganglia, on β2-ARs expressed by non-neuronal satellite cells. This stimulation of β2-ARs decreases the neuropathy-induced production of membrane-bound TNFα, resulting in relief of neuropathic allodynia. This indirect anti-TNFα action was observed with the tricyclic antidepressant nortriptyline, the selective serotonin and noradrenaline reuptake inhibitor venlafaxine and the β2-AR agonist terbutaline. Our data revealed an original therapeutic mechanism that may open novel research avenues for the management of painful peripheral neuropathies.

Climbing fibers, the projections from the inferior olive to the cerebellar cortex, carry sensorimotor error and clock signals that trigger motor learning by controlling cerebellar Purkinje cell synaptic plasticity and discharge. Purkinje cells target the deep cerebellar nuclei, which are the output of the cerebellum and include an inhibitory GABAergic projection to the inferior olive. This pathway identifies a potential closed loop in the olivo-cortico-nuclear network. Therefore, sets of Purkinje cells may phasically control their own climbing fiber afferents. Here, using in vitro and in vivo recordings, we describe a genetically modified mouse model that allows the specific optogenetic control of Purkinje cell discharge. Tetrode recordings in the cerebellar nuclei demonstrate that focal stimulations of Purkinje cells strongly inhibit spatially restricted sets of cerebellar nuclear neurons. Strikingly, such stimulations trigger delayed climbing-fiber input signals in the stimulated Purkinje cells. Therefore, our results demonstrate that Purkinje cells phasically control the discharge of their own olivary afferents and thus might participate in the regulation of cerebellar motor learning.

In addition to its neurotrophic role, brain-derived neurotrophic factor (BDNF) is involved in a wide array of functions, including anxiety and pain. The central amygdaloid nucleus (CeA) contains a high concentration of BDNF in terminals, originating from the pontine parabrachial nucleus. Since the spino-parabrachio-amygdaloid neural pathway is known to convey nociceptive information, we hypothesized a possible involvement of BDNF in supraspinal pain-related processes. To test this hypothesis, we generated localized deletion of BDNF in the parabrachial nucleus using local bilateral injections of adeno-associated viruses in adult floxed-BDNF mice. Basal thresholds of thermal and mechanical nociceptive responses were not altered by BDNF loss and no behavioural deficit was noticed in anxiety and motor tests. However, BDNF-deleted animals displayed a major decrease in the analgesic effect of morphine. In addition, intra-CeA injections of the BDNF scavenger TrkB-Fc in control mice also decreased morphine-induced analgesia. Finally, the number of c-Fos immunoreactive nuclei after acute morphine injection was decreased by 45% in the extended amygdala of BDNF-deleted animals. The absence of BDNF in the parabrachial nucleus thus altered the parabrachio-amygdaloid pathway. Overall, our study provides evidence that BDNF produced in the parabrachial nucleus modulates the functions of the parabrachio-amygdaloid pathway in opiate analgesia.

Circadian rhythms are repeating patterns of physiological and other parameters that recur with periods of approximately 24 h, and are generated by an endogenous circadian timekeeping mechanism. Such circadian rhythms, and their underlying molecular mechanisms, are known to be altered by a number of central nervous system acting pharmacological compounds, as well as becoming perturbed in a number of common psychiatric and neurological conditions. The psychostimulant methylphenidate and the non-stimulant atomoxetine are used in the pharmacotherapy of attention deficit hyperactivity disorder, a common condition in which circadian rhythms have been reported to be altered. In the present study we have examined the effects of daily methylphenidate or atomoxetine treatment across 7 days on circadian clock gene product expression across numerous brain regions in the male mouse to test the potential impact of such compounds on circadian timing. We report drug, brain region and molecular specific effects of such treatments, including alterations in expression profiles in the suprachiasmatic nucleus, the master circadian pacemaker. These results indicate that drugs used in the clinical management of attention deficit hyperactivity disorder can alter molecular factors that are believed to underpin circadian timekeeping, and such effects may be of importance in both the therapeutic and side effect profiles of such drugs.

The tail of the ventral tegmental area (tVTA), also named the rostromedial tegmental nucleus (RMTg), is a recently defined midbrain structure considered to exert a major inhibitory drive on dopamine systems. In view of its connectivity, tVTA is well placed to convey salient positive and negative signals to dopamine cells and participate in adaptative behavioral responses. This structure could act as a hub converging and integrating widespread multimodal signals toward dopamine systems. The tVTA participates in prediction error, motor control, and responses to aversive stimuli and drugs of abuse. In light of the crucial role of the tVTA in the opiate control of dopamine activity, a neuroanatomical update of the disinhibition model of morphine action is proposed.

Since the work of Johnson and North, it is known that opiates increase the activity of dopaminergic neurons by a GABA neuron-mediated desinhibition. This model should however be updated based on recent advances. Thus, the neuroanatomical location of the GABA neurons responsible for this desinhibition has been recently detailed: they belong to a brain structure in continuity with the posterior part of the ventral tegmental area and discovered this past decade. Other data also highlighted the critical role played by glutamatergic transmission in the opioid regulation of dopaminergic neuron activity. During protracted opiate withdrawal, the inhibitory/excitatory balance exerted on dopaminergic neurons is altered. These results are now leading to propose an original hypothesis for explaining the impact of protracted opiate withdrawal on mood.

BACKGROUND: Clinical management of neuropathic pain, which is pain arising as a consequence of a lesion or a disease affecting the somatosensory system, partly relies on the use of anticonvulsant drugs such as gabapentinoids. Therapeutic action of gabapentinoids such as gabapentin and pregabalin, which act by the inhibition of calcium currents through interaction with the α2δ-1 subunit of voltage-dependent calcium channels, is well documented. However, some aspects of the downstream mechanisms are still to be uncovered. Using behavioral, genetic, and pharmacological approaches, we tested whether opioid receptors are necessary for the antiallodynic action of acute and/or long-term pregabalin treatment in the specific context of neuropathic pain. RESULTS: Using the cuff model of neuropathic pain in mice, we show that acute pregabalin administration at high dose has a transitory antiallodynic action, while prolonged oral pregabalin treatment leads to sustained antiallodynic action, consistent with clinical observations. We show that pregabalin remains fully effective in μ-opioid receptor, in δ-opioid receptor and in κ-opioid receptor deficient mice, either female or male, and its antiallodynic action is not affected by acute naloxone. Our work also shows that long-term pregabalin treatment suppresses tumor necrosis factor-α overproduction induced by sciatic nerve constriction in the lumbar dorsal root ganglia. CONCLUSIONS: We demonstrate that neither acute nor long-term antiallodynic effect of pregabalin in a context of neuropathic pain is mediated by the endogenous opioid system, which differs from opioid treatment of pain and antidepressant treatment of neuropathic pain. Our data are also supportive of an impact of gabapentinoid treatment on the neuroimmune aspect of neuropathic pain.

Nociceptive reflexes are predominant in traditional “pain” tests using rodents. However, pain is not a nociceptive reflex, but an unpleasant personal experience associated with a complex sensory, emotional and cognitive phenomenology. Modern research methods in behavioral neuroscience have now enabled preclinical pain researchers to develop procedures which differentially combine reflex and non-reflex based measures, thus making various aspects of pain experience experimentally accessible in rodents. These aspects include the aversive component, anxiodepressive and cognitive consequences of pain as well as pain empathy. Rodents are also highly sensitive to their environmental context, and can accordingly attribute different contextual meaning to different experimental situations. This chapter summarizes classically used nociceptive tests, but mainly focuses on the recent developments highlighting the possibility to address the complex experience of pain in rodents.

The tail of the ventral tegmental area (tVTA) or rostromedial tegmental nucleus (RMTg) is a recently described brainstem group of gamma-amino butyric acid cells that densely innervate the VTA and the substantia nigra pars compacta, exerting tonic and phasic inhibitory control over the activity of dopamine cells. The molecular impact of psychostimulants on the tVTA/RMTg, the expression of µ-opioid receptors in these neurons, and the dense input from the lateral habenula, together with the brake activity exerted on dopamine systems, are fostering research on tVTA/RMTg functions. This nucleus has already been shown to encode prediction error, control freezing behavior and locomotor activity, influence motor coordination performance and motor skill learning, promote avoidance behaviors, participate in responses to aversive stimuli and drugs of abuse, and to be a key structure for opioid-induced disinhibition of dopamine neurons. This chapter details the current physiological and behavioral knowledge on the tVTA/RMTg.

Protracted opiate withdrawal is accompanied by altered responsiveness of midbrain dopaminergic (DA) neurons, including a loss of DA cell response to morphine, and by behavioral alterations, including affective disorders. GABAergic neurons in the tail of the ventral tegmental area (tVTA), also called the rostromedial tegmental nucleus, are important for behavioral responses to opiates. We investigated the tVTA–VTA circuit in rats after chronic morphine exposure to determine whether tVTA neurons participate in the loss of opiate-induced disinhibition of VTA DA neurons observed during protracted withdrawal. In vivo recording revealed that VTA DA neurons, but not tVTA GABAergic neurons, are tolerant to morphine after 2 weeks of withdrawal. Optogenetic stimulation of tVTA neurons inhibited VTA DA neurons similarly in opiate-naive and long-term withdrawn rats. However, tVTA inactivation increased VTA DA activity in opiate-naive rats, but not in withdrawn rats, resembling the opiate tolerance effect in DA cells. Thus, although inhibitory control of DA neurons by tVTA is maintained during protracted withdrawal, the capacity for disinhibitory control is impaired. In addition, morphine withdrawal reduced both tVTA neural activity and tonic glutamatergic input to VTA DA neurons. We propose that these changes in glutamate and GABA inputs underlie the apparent tolerance of VTA DA neurons to opiates after chronic exposure. These alterations in the tVTA–VTA DA circuit could be an important factor in opiate tolerance and addiction. Moreover, the capacity of the tVTA to inhibit, but not disinhibit, DA cells after chronic opiate exposure may contribute to long-term negative affective states during withdrawal.

Neuropathic pain is often a chronic condition, disabling and difficult to treat. Using a murine model of neuropathic pain induced by placing a polyethylene cuff around the main branch of the sciatic nerve, we have shown that chronic treatment with β-AR agonists is effective against neuropathic allodynia. β-mimetics are widely used against asthma and chronic obstructive pulmonary disease and may offer an interesting option for neuropathic pain management. The most prominent adverse effects of chronic treatment with β-mimetics are cardiovascular. In this study, we compared the action of low doses of the selective β 2 -AR agonist terbutaline and of a high dose of the mixed β 1 /β 2 -AR agonist isoproterenol on cardiovascular parameters in a neuropathic pain context. Isoproterenol was used as a positive control for some heart-related changes. Cardiac functions were studied by echocardiography, hemodynamic measurements, histological analysis of fibrosis and cardiac hypertrophy, and by quantitative real time PCR analysis of atrial natriuretic peptide ( Nppa ), periostin ( Postn ), connective tissue growth factor ( Ctgf ) and β-myosin heavy chain ( Myh7 ). Our data show that a chronic treatment with the β 2 -AR agonist terbutaline at low antiallodynic dose does not affect cardiovascular parameters, whereas the mixed β 1 /β 2 -AR agonist isoproterenol induces cardiac hypertrophy. These data suggest that low doses of β 2 -AR agonists may provide a suitable treatment with rare side effects in neuropathic pain management. This study conducted in an animal model requires clinical confirmation in humans.

Tricyclic antidepressants (TCAs) are among the first line treatments clinically recommended against neuropathic pain. However, the mechanism by which they alleviate pain is still unclear. Pharmacological and genetic approaches evidenced a critical role of delta-opioid receptors (DORs) in the therapeutic action of chronic TCA treatment. It is however unclear whether mu-opioid receptors (MORs) are also necessary to the pain-relieving action of TCAs. The lack of highly selective MOR antagonists makes difficult to conclude based on pharmacological studies. In the present work, we thus used a genetic approach and compared mutant mice lacking MORs and their wild-type littermates. The neuropathy was induced by unilateral sciatic nerve cuffing. The threshold for mechanical response was evaluated using von Frey filaments. MOR-deficient mice displayed the same baseline for mechanical sensitivity as their wild-type littermates. After sciatic nerve cuffing, both wild-type and MOR-deficient mice displayed an ipsilateral mechanical allodynia. After about 10 days of treatment, nortriptyline suppressed this allodynia in both wild-type and MOR-deficient mice. MORs are thus not critical for nortriptyline action against neuropathic pain. An acute injection of the DOR antagonist naltrindole induced a relapse of neuropathic allodynia in both wild-type and MOR-deficient mice, thus confirming the critical role of DORs in nortriptyline action. Moreover, morphine induced an acute analgesia in control and in neuropathic wild-type mice, but was without effect in MOR-deficient mice. While MORs are crucial for morphine action, they are not critical for nortriptyline action. Our results highlight the functional difference between DORs and MORs in mechanisms of pain relief.

Neuropathic pain arises as a direct consequence of a lesion or disease affecting the somatosensory system. Among the recommended first-line treatments are antidepressant drugs – that is, molecules that were initially developed to treat other disorders of the nervous system. While their clinical efficacy against neuropathic pain was established more than 30 years ago, there is little information on the mechanism underlying their antidepressant action. However, understanding the therapeutic mechanism of these treatments could help to improve them, or even lead to new therapeutic approaches. In this article, we discuss the difficulties in conducting relevant preclinical research on neuropathic pain treatment with antidepressant drugs and we present the most recent findings on the putative mechanism, which highlight the role of β2-adrenoceptors and δ-opioid receptors.