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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.

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

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.

International audience

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.

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.

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.

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.

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.

Background Pre-clinical research has shown β2-adrenoceptors to be essential for the antiallodynic action of antidepressant drugs in murine models of neuropathic pain and that sustained treatment with β2-agonists has an antiallodynic action. Here, we clinically investigated whether chronic β2-agonist treatments may influence the incidence of post-thoracotomy chronic pain, defined as pain that recurs or persists along a thoracotomy scar more than 2 months after surgery, either neuropathic or non-neuropathic. Methods We conducted an epidemiological study on patients operated by thoracotomy. Demographic data, medical history and treatments concomitant to the surgery were recorded at a follow-up visit. Information on perioperative treatments was collected from the anaesthesia records and confirmed by the patients. In patients with pain at the surgery level, post-thoracotomy chronic pain was assessed by clinical examination and numeric scale. Physical examination and DN4 questionnaire were used to discriminate neuropathic and non-neuropathic chronic pain at scar level. Results One hundred and eighty-nine patients were included. Eighty-one patients reported persisting thoracic pain, with neuropathic characteristics in 58 of them (30% of the 189 patients). The most common chronic drugs during the perioperative period were inhaled β2-agonists (28.6%). The chronic use of β2-agonists was an independent predictor of thoracic neuropathic pain (but not of non-neuropathic pain) and was associated with a five-fold decrease in the relative incidence of neuropathic pain [OR = 0.19 (0.06–0.45)]. Conclusions These data suggest a possible influence of chronic β2-agonist treatments on neuropathic pain secondary to thoracotomy. This apparent preventive effect of β2-agonist treatments should warrant controlled clinical trials.

The transcription factor cAMP response element (CRE)-binding protein (CREB) has been shown to regulate neural plasticity. Drugs of abuse activate CREB in the nucleus accumbens, an important part of the brain's reward pathways, and local manipulations of CREB activity have been shown to affect cocaine reward, suggesting an active role of CREB in adaptive processes that follow exposure to drugs of abuse. Using CRE-LacZ reporter mice, we show that not only rewarding stimuli such as morphine, but also aversive stimuli such as stress, activate CRE-mediated transcription in the nucleus accumbens shell. Using viral-mediated gene transfer to locally alter the activity of CREB, we show that this manipulation affects morphine reward, as well as the preference for sucrose, a more natural reward. We then show that local changes in CREB activity induce a more general syndrome, by altering reactions to anxiogenic, aversive, and nociceptive stimuli as well. Increased CREB activity in the nucleus accumbens shell decreases an animal's responses to each of these stimuli, whereas decreased CREB activity induces an opposite phenotype. These results show that environmental stimuli regulate CRE-mediated transcription within the nucleus accumbens shell, and that changes in CREB activity within this brain area subsequently alter gating between emotional stimuli and their behavioral responses. This control appears to be independent of the intrinsic appetitive or aversive value of the stimulus. The potential relevance of these data to addiction and mood disorders is discussed.

International audience; Midbrain dopamine neurons are implicated in various psychiatric and neurological disorders. The GABAergic tail of the ventral tegmental area (tVTA), also named the rostromedial tegmental nucleus (RMTg), displays dense projections to the midbrain and exerts electrophysiological control over dopamine cells of the VTA. However, the influence of the tVTA on the nigrostriatal pathway, from the substantia nigra pars compacta (SNc) to the dorsal striatum, and on related functions remains to be addressed. The present study highlights the role played by the tVTA as a GABA brake for the nigrostriatal system, demonstrating a critical influence over motor functions. Using neuroanatomical approaches with tract tracing and electron microscopy, we reveal the presence of a tVTA-SNc-dorsal striatum pathway. Using in vivo electrophysiology, we prove that the tVTA is a major inhibitory control center for SNc dopamine cells. Using behavioral approaches, we demonstrate that the tVTA controls rotation behavior, motor coordination, and motor skill learning. The motor enhancements observed after ablation of the tVTA are in this regard comparable with the performance-enhancing properties of amphetamine, a drug used in doping. These findings demonstrate that the tVTA is a major GABA brake for nigral dopamine systems and nigrostriatal functions, and they raise important questions about how the tVTA is integrated within the basal ganglia circuitry. They also warrant further research on the tVTA's role in motor and dopamine-related pathological contexts such as Parkinson's disease.

Probenecid, an agonist of transient receptor vanilloid (TRPV) type 2, was used to evaluate the effects of TRPV2 activation on excitatory and inhibitory synaptic transmission in the dorsal horn (DH) of the rat spinal cord and on nociceptive reflexes induced by thermal heat and mechanical stimuli. The effects of probenecid were compared with those of capsaicin, a TRPV1 agonist. Calcium imaging experiments on rat dorsal root ganglion (DRG) and DH cultures indicated that functional TRPV2 and TRPV1 were expressed by essentially non-overlapping subpopulations of DRG neurons, but were absent from DH neurons and DH and DRG glial cells. Pretreatment of DRG cultures with small interfering RNAs against TRPV2 suppressed the responses to probenecid. Patch-clamp recordings from spinal cord slices showed that probenecid and capsaicin increased the frequencies of spontaneous excitatory postsynaptic currents (sEPSCs) and spontaneous inhibitory postsynaptic currents in a subset of laminae III–V neurons. In contrast to capsaicin, probenecid failed to stimulate synaptic transmission in lamina II. Intrathecal or intraplantar injections of probenecid induced mechanical hyperalgesia/allodynia without affecting nociceptive heat responses. Capsaicin induced both mechanical hyperalgesia/allodynia and heat hyperalgesia. Activation of TRPV1 or TRPV2 in distinct sets of primary afferents increased the sEPSC frequencies in a largely common population of DH neurons in laminae III–V, and might underlie the development of mechanical hypersensitivity following probenecid or capsaicin treatment. However, only TRPV1-expressing afferents facilitated excitatory and/or inhibitory transmission in a subpopulation of lamina II neurons, and this phenomenon might be correlated with the induction of thermal heat hyperalgesia.

International audience; The ventral tegmental area (VTA) is a brain region processing salient sensory and emotional information, controlling motivated behaviors, natural or drug-related reward, reward-related learning, mood, and participating in their associated psychopathologies. Mostly studied for its dopamine neurons, the VTA also includes functionally important GABA and glutamate cell populations. Behavioral evidence supports the presence of functional differences between the anterior VTA (aVTA) and the posterior VTA (pVTA), which is the topic of this review. This antero-posterior heterogeneity concerns locomotor activity, conditioned place preference and intracranial self-administration, and can be seen in response to ethanol, acetaldehyde, salsolinol, opioids including morphine, cholinergic agonists including nicotine, cocaine, cannabinoids and after local manipulation of GABA and serotonin receptors. It has also been observed after viral-mediated manipulation of GluR1, phospholipase Cγ (PLCγ) and cAMP response element binding protein (CREB) expression, with impact on reward and aversion-related responses, on anxiety and depression-related behaviors and on pain sensitivity. In this review, the substrates potentially underlying these aVTA/pVTA differences are discussed, including the VTA sub-nuclei and the heterogeneity in connectivity, cell types and molecular characteristics. We also review the role of the tail of the VTA (tVTA), or rostromedial tegmental nucleus (RMTg), which may also participate to the observed antero-posterior heterogeneity of the VTA. This region, partly located within the pVTA, is an inhibitory control center for dopamine activity. It controls VTA and substantia nigra dopamine cells, thus exerting a major influence on basal ganglia functions. This review highlights the need for a more comprehensive analysis of VTA heterogeneity.

Neuropathic pain arises as a consequence of a lesion or a disease affecting the somatosensory system. This syndrome results from maladaptive changes in injured sensory neurons and along the entire nociceptive pathway within the central nervous system. It is usually chronic and challenging to treat. In order to study neuropathic pain and its treatments, different models have been developed in rodents. These models derive from known etiologies, thus reproducing peripheral nerve injuries, central injuries, and metabolic-, infectious- or chemotherapy-related neuropathies. Murine models of peripheral nerve injury often target the sciatic nerve which is easy to access and allows nociceptive tests on the hind paw. These models rely on a compression and/or a section. Here, the detailed surgery procedure for the "cuff model" of neuropathic pain in mice is described. In this model, a cuff of PE-20 polyethylene tubing of standardized length (2 mm) is unilaterally implanted around the main branch of the sciatic nerve. It induces a long-lasting mechanical allodynia, i.e., a nociceptive response to a normally non-nociceptive stimulus that can be evaluated by using von Frey filaments. Besides the detailed surgery and testing procedures, the interest of this model for the study of neuropathic pain mechanism, for the study of neuropathic pain sensory and anxiodepressive aspects, and for the study of neuropathic pain treatments are also discussed.

The amygdala is a structure of the temporal lobe thought to be involved in assigning emotional significance to environmental information and triggering adapted physiological, behavioral and affective responses. A large body of literature in animals and human implicates the amygdala in fear. Pain having a strong affective and emotional dimension, the amygdala, especially its central nucleus (CeA), has also emerged in the last twenty years as key element of the pain matrix. The CeA receives multiple nociceptive information from the brainstem, as well as highly processed polymodal information from the thalamus and the cerebral cortex. It also possesses the connections that allow influencing most of the descending pain control systems as well as higher centers involved in emotional, affective and cognitive functions. Preclinical studies indicate that the integration of nociceptive inputs in the CeA only marginally contributes to sensory-discriminative components of pain, but rather contributes to associated behavior and affective responses. The CeA doesn’t have a major influence on responses to acute nociception in basal condition, but it induces hypoalgesia during aversive situation, such as stress or fear. On the contrary, during persistent pain states (inflammatory, visceral, neuropathic), a long-lasting functional plasticity of CeA activity contributes to an enhancement of the pain experience, including hyperalgesia, aversive behavioral reactions and affective anxiety-like states.

International audience; A new mesopontine structure exerting a strong influence on dopamine systems has recently been defined: the tail of the ventral tegmental area/rostromedial tegmental nucleus (tVTA/RMTg). This review presents a neuroanatomical, physiological, and behavioral overview of some of the recent and ongoing research on this brain region and its relationship with dopamine systems. The tVTA/RMTg sends dense GABA projections to VTA and substantia nigra neurons. The inhibitory influence of tVTA/RMTg on dopamine neurons is supported by both neuroanatomical and electrophysiology data. The latter studies also reveal the tVTA/RMTg as a substrate for morphine and cannabinoid action on dopamine cells. In primates, the tVTA/RMTg has been implicated in reward prediction error signals, through a basal ganglia-lateral habenula-tVTA/RMTg-dopamine-basal ganglia circuit. In rodents, the tVTA/RMTg has been shown to play a critical role in aversive behaviors, particularly those involving behavioral inhibition, such as freezing and avoidance. These findings highlight the functional importance of the tVTA/RMTg as a major GABA brake for dopamine systems.

International audience; Morphine is a highly potent analgesic with high addictive potential in specific contexts. Although dopamine neurons of the ventral tegmental area (VTA) are widely believed to play an essential role in the development of drug addiction, neuronal circuits underlying morphine action on dopamine neurons have not been fully elucidated. Here we combined in vivo electrophysiology, tract-tracing experiments, and targeted neuronal inactivation to dissect a neural circuit for acute morphine action on dopamine neurons in rats. We found that in vivo, morphine targets the GABAergic tail of the VTA, also called the rostromedial tegmental nucleus, to increase the firing of dopamine neurons through the activation of VTA μ opioid receptors expressed on tail of the VTA/rostromedial tegmental nucleus efferents. Our data also reveal that in the absence of VTA glutamatergic tone, there is no morphine-induced activation of dopamine neurons. These results define the anatomical organization and functional role of a neural circuit for acute morphine action on dopamine neurons.

Background Chronic pain is clinically associated with the development of affective disorders. However, studies in animal models of neuropathic pain are contradictory and the relationship with mood disorders remains unclear. In this study, we aimed to characterize the affective consequences of neuropathic pain over time and to study potential underlying mechanisms. Methods Neuropathic pain was induced by inserting a polyethylene cuff around the main branch of the right sciatic nerve in C57BL/6J mice. Anxiety- and depression-related behaviors were assessed over 2 months, using a battery of tests, such as elevated plus maze, marble burying, novelty suppressed feeding, splash test, and forced swimming test. Plasma corticosterone levels were assessed by radioimmunoassay. We also investigated changes in cyclic adenosine monophosphate response element (CRE) activity using CRE-LacZ transgenic mice. Results Mice developed anxiety-related behavior 4 weeks after induction of the neuropathy, and depression-related behaviors were observed after 6 to 8 weeks. Control and neuropathic mice did not differ for basal or stress-induced levels of corticosterone or for hypothalamic-pituitary-adrenal axis negative feedback. After 8 weeks, the CRE-mediated activity decreased in the outer granule layer of dentate gyrus of neuropathic mice but not in the amygdala or in the anterior cingulate cortex. Conclusions Our results demonstrate that the affective consequences of neuropathic pain evolve over time, independently from the hypothalamic-pituitary-adrenal axis, which remains unaffected. CRE-mediated transcription within a limbic structure was altered at later time points of the neuropathy. These experiments provide a preclinical model to study time-dependent development of mood disorders and the underlying mechanism in a neuropathic pain context.

Although cholinergic drugs are known to modulate nociception, the role of endogenous acetylcholine in nociceptive processing remains unclear. In the current study, we evaluated the role of cholinergic transmission through spinal β(2)-subunit-containing nicotinic acetylcholine receptors in the control of nociceptive thresholds. We show that mechanical and thermal nociceptive thresholds are significantly lowered in β(2)(∗)-knockout (KO) mice. Using nicotinic antagonists in these mice, we demonstrate that β(2)(∗)-nAChRs are responsible for tonic inhibitory control of mechanical thresholds at the spinal level. We further hypothesized that tonic β(2)(∗)-nAChR control of mechanical nociceptive thresholds might implicate GABAergic transmission since spinal nAChR stimulation can enhance inhibitory transmission. Indeed, the GABA(A) receptor antagonist bicuculline decreased the mechanical threshold in wild-type but not β(2)(∗)-KO mice, and the agonist muscimol restored basal mechanical threshold in β(2)(∗)-KO mice. Thus, β(2)(∗)-nAChRs appeared to be necessary for GABAergic control of nociceptive information. As a consequence of this defective inhibitory control, β(2)(∗)-KO mice were also hyperresponsive to capsaicin-induced C-fiber stimulation. Our results indicate that β(2)(∗)-nAChRs are implicated in the recruitment of inhibitory control of nociception, as shown by delayed recovery from capsaicin-induced allodynia, potentiated nociceptive response to inflammation and neuropathy, and by the loss of high-frequency transcutaneous electrical nerve stimulation (TENS)-induced analgesia in β(2)(∗)-KO mice. As high-frequency TENS induces analgesia through Aβ-fiber recruitment, these data suggest that β(2)(∗)-nAChRs may be critical for the gate control of nociceptive information by non-nociceptive sensory inputs. In conclusion, acetylcholine signaling through β(2)(∗)-nAChRs seems to be essential for setting nociceptive thresholds by controlling GABAergic inhibition in the spinal cord.

Here, we characterized behavioral abnormalities induced by prolonged social isolation in adult rodents. Social isolation induced both anxiety- and anhedonia-like symptoms and decreased cAMP response element–binding protein (CREB) activity in the nucleus accumbens shell (NAcSh). All of these abnormalities were reversed by chronic, but not acute, antidepressant treatment. However, although the anxiety phenotype and its reversal by antidepressant treatment were CREB-dependent, the anhedonia-like symptoms were not mediated by CREB in NAcSh. We found that decreased CREB activity in NAcSh correlated with increased expression of certain K 1 channels and reduced electrical excitability of NAcSh neurons, which was sufficient to induce anxietylike behaviors and was reversed by chronic antidepressant treatment. Together, our results describe a model that distinguishes anxiety- and depression-like behavioral phenotypes, establish a selective role of decreased CREB activity in NAcSh in anxiety-like behavior, and provide a mechanism by which antidepressant treatment alleviates anxiety symptoms after social isolation. Depression and anxiety are common forms of mental illness in the general population. Although they are classified as distinct syndromes by the Diagnostic and Statistical Manual (American Psychiatric Association), symptoms of depression and anxiety often occur together and to widely varying extents in different subtypes of the illnesses. Despite the importance of these clinical phenomena, very little is known about the distinctions between depression- and anxiety-like symptoms in animal models 1 . Models of ‘active’ stress, such as foot shock, restraint stress, social defeat and learned helplessness, produce depressionand anxiety-like phenotypes; the molecular mechanisms of these models have been extensively studied, but specific molecular mediators of depression versus anxiety symptoms have not yet been described 2‐4 . Even less well studied, however, is a ‘passive’ model of stress and social isolation in adulthood, which, as with active stress, mimics aspects of human depression and anxiety 5,6 . This lack of attention is unfortunate, as social isolation would appear to be particularly relevant to certain subtypes of human depression and anxiety disorders 7,8 . Although social isolation has been studied, most models to date have focused on adulthood behaviors after isolation rearing early in life, either as pups or adolescents, which is a very different model than adulthood social isolation 5 . Reports on adulthood isolation provide

In animal studies, thermal sensitivity is mostly evaluated on the basis of nociceptive reaction latencies in response to a given thermal aversive stimulus. However, these techniques may be inappropriate to differentiate allodynia from hyperalgesia or to provide information differentiating the activation of nociceptor subtypes. The recent development of dynamic hot and cold plates, allowing computer-controlled ramps of temperature, may be useful for such measures. In this study, we characterized their interest for studying thermal nociception in freely moving mice and rats. We showed that escape behavior (jumps) was the most appropriate parameter in C57Bl/6J mice, whereas nociceptive response was estimated by using the sum of paw lickings and withdrawals in Sprague-Dawley rats. We then demonstrated that this procedure allows the detection of both thermal allodynia and hyperalgesia after peripheral pain sensitization with capsaicin in mice and in rats. In a condition of carrageenan-induced paw inflammation, we observed the previously described thermal hyperalgesia, but we also revealed that rats exhibit a clear thermal allodynia to a cold or a hot stimulus. These results demonstrate the interest of the dynamic hot and cold plate to study thermal nociception, and more particularly to study both thermal allodynia and hyperalgesia within a single paradigm in awake and freely moving rodents.Despite its clinical relevance, thermal allodynia is rarely studied by researchers working on animal models. As shown after stimulation of capsaicin-sensitive fibers or during inflammatory pain, the dynamic hot and cold plate validated in the present study provides a useful tool to distinguish between thermal allodynia and thermal hyperalgesia in rodents.

International audience; We previously showed that chronic psychostimulant exposure induces the transcription factor DeltaFosB in gamma-aminobutyric acid (GABA)ergic neurons of the caudal tier of the ventral tegmental area (VTA). This subregion was defined as the tail of the VTA (tVTA). In the present study, we showed that tVTA can also be visualized by analyzing FosB/DeltaFosB response following acute cocaine injection. This induction occurs in GABAergic neurons, as identified by glutamic acid decarboxylase (GAD) expression. To characterize tVTA further, we mapped its inputs by using the retrograde tracers Fluoro-Gold or cholera toxin B subunit. Retrogradely labeled neurons were observed in the medial prefrontal cortex, the lateral septum, the ventral pallidum, the bed nucleus of the stria terminalis, the substantia innominata, the medial and lateral preoptic areas, the lateral and dorsal hypothalamic areas, the lateral habenula, the intermediate layers of the superior colliculus, the dorsal raphe, the periaqueductal gray, and the mesencephalic and pontine reticular formation. Projections from the prefrontal cortex, the hypothalamus, and the lateral habenula to the tVTA were also shown by using the anterograde tracer biotinylated dextran amine (BDA). We showed that the central nucleus of the amygdala innervates the anterior extent of the VTA but not the tVTA. Moreover, the tVTA mainly receives non-aminergic inputs from the dorsal raphe and the locus coeruleus. Although the tVTA has a low density of dopaminergic neurons, its afferents are mostly similar to those targeting the rest of the VTA. This suggests that the tVTA can be considered as a VTA subregion despite its caudal location.

International audience; Neuropathic pain is a disease caused by a lesion or dysfunction of the nervous system. Antidepressants or anticonvulsants are presently the best available treatments. The mechanism by which antidepressants relieve neuropathic pain remains poorly understood. Using pharmacological and transgenic approaches in mice, we evaluated adrenergic receptor (AR) implication in the action of the tricyclic antidepressant desipramine, the noradrenaline and serotonin reuptake inhibitor venlafaxine, and the noradrenaline reuptake inhibitor reboxetine. Neuropathy was induced by cuff insertion around the sciatic nerve. We showed that chronic antidepressant treatment suppressed cuff-induced allodynia in wild-type mice but not in beta(2)-AR deficient mice, and/or that this antiallodynic action was blocked by intraperitoneal or intrathecal injection of the beta(2)-AR antagonist ICI 118,551 but not by the alpha(2)-AR antagonist yohimbine. We also showed that the anticonvulsant gabapentin was still effective in beta(2)-AR deficient mice. Our results demonstrate that beta(2)-ARs are essential for the antiallodynic action of antidepressant drugs.

International audience; BACKGROUND: The therapeutic effect of antidepressant drugs against depression usually necessitates a chronic treatment. A large body of clinical evidence indicates that antidepressant drugs can also be highly effective against chronic neuropathic pain. However, the mechanism by which these drugs alleviate pain is still unclear. METHODS: We used a murine model of neuropathic pain induced by sciatic nerve constriction to study the antiallodynic properties of a chronic treatment with the tricyclic antidepressants nortriptyline and amitriptyline. Using knockout and pharmacological approaches in mice, we determined the influence of delta-opioid receptors in the therapeutic action of chronic antidepressant treatment. RESULTS: In our model, a chronic treatment by tricyclic antidepressant drugs totally suppresses the mechanical allodynia in neuropathic C57Bl/6J mice. This therapeutic effect can be acutely reversed by an injection of the delta-opioid receptor antagonist naltrindole. Moreover, the antiallodynic property of antidepressant treatment is absent in mice deficient for the delta-opioid receptor gene. CONCLUSIONS: The antiallodynic effect of chronic antidepressant treatment is mediated by a recruitment of the endogenous opioid system acting through delta-opioid receptors.

International audience; Antidepressant drugs act mainly by blocking the noradrenaline and/or serotonin uptake sites and require a chronic treatment. Tricyclic antidepressants are among the first line treatments clinically recommended against neuropathic pain. As observed against depression, a chronic treatment is required for a therapeutic effect. However, both in depression-related and pain-related research in rodents, it is difficult to design models that reproduce the clinical conditions and are sensitive to chronic but not to acute treatment by antidepressant drugs. In this study, we used a murine neuropathic pain model induced by the unilateral insertion of a polyethylene cuff around the main branch of the sciatic nerve. This model induced a long-lasting ipsilateral mechanical allodynia. We evidenced that chronic, but not acute, treatment with the tricyclic antidepressants nortriptyline or amitriptyline suppressed the cuff-induced mechanical allodynia. On the contrary, fluoxetine, a selective serotonin reuptake inhibitor, remained ineffective. To understand which mechanism is recruited downstream in order to alleviate the allodynia, we tested the opioid receptor antagonist naloxone, the delta-opioid receptor antagonist naltrindole and the kappa-opioid receptor antagonist nor-BNI. We show that the therapeutic effect of notriptyline implicates the endogenous opioid system, in particular the delta- and the kappa-opioid receptors. For comparison, we tested the anticonvulsant gabapentin and showed that it alleviates neuropathic allodynia after 3 days of treatment. Naloxone had no effect on gabapentin therapeutic benefit, showing that antidepressants and anticonvulsants alleviate neuropathic allodynia through independent mechanisms. Our work provides a clinically relevant model to understand the mechanism by which chronic antidepressant treatment can alleviate neuropathic pain.

International audience; BACKGROUND: Brain-derived neurotrophic factor (BDNF) plays an important role in neural plasticity in the adult nervous system and has been suggested as a target gene for antidepressant treatment. The neurotrophic hypothesis of depression suggests that loss of BDNF from the hippocampus contributes to an increased vulnerability for depression, whereas upregulation of BDNF in the hippocampus is suggested to mediate antidepressant efficacy. METHODS: We have used a viral-mediated gene transfer approach to assess the role of BDNF in subregions of the hippocampus in a broad array of behavioral paradigms, including depression-like behavior and antidepressant responses. We have combined the adeno-associated virus (AAV) with the Cre/loxP site-specific recombination system to induce the knockout of BDNF selectively in either the CA1 or dentate gyrus (DG) subregions of the hippocampus. RESULTS: We show that the loss of BDNF in either the CA1 or the DG of the hippocampus does not alter locomotor activity, anxiety-like behavior, fear conditioning, or depression-related behaviors. However, the selective loss of BDNF in the DG but not the CA1 region attenuates the actions of desipramine and citalopram in the forced swim test. CONCLUSIONS: These data suggest that the loss of hippocampal BDNF per se is not sufficient to mediate depression-like behavior. However, these results support the view that BDNF in the DG might be essential in mediating the therapeutic effect of antidepressants.

Background Indirect evidence suggests that loss of brain-derived neurotrophic factor (BDNF) from forebrain regions contributes to an individual's vulnerability for depression, whereas upregulation of BDNF in these regions is suggested to mediate the therapeutic effect of antidepressants. Methods We have tested this hypothesis by generating two independent lines of conditional BDNF knockout mice in which the BDNF gene is deleted selectively in forebrain. Results We show that male conditional knockouts exhibit hyperactivity but normal depression-related behaviors. In contrast, female conditional knockouts display normal locomotor activity but a striking increase in depression-like behavior. We also demonstrate that loss of BDNF in both male and female mice attenuates the actions of the antidepressant desipramine in the forced swim test. Conclusions These gender differences in depression-related behaviors in BDNF conditional knockout mice provide direct evidence for a role of BDNF in depression. The results also support the view that forebrain BDNF may be essential in mediating antidepressant efficacy.

Devices designed for mechanical pain threshold studies are often difficult to implement. The purpose of this study was to investigate a simple tool based on calibrated forceps to induce quantifiable mechanical stimulation in the rat on a linear scale. The most suitable protocol was tested by determining the effects of 3 repetitive measurements on both hind paws, respectively, during long-term (9 days), mid-term (1 day), and short-term (2 hours). Only threshold increase related to weight gain over long-term was observed, suggesting that moderate rat training can be used. The capacity of the device to reveal hyperalgesia was tested in a model of carrageenan-induced inflammation in the hind paw. The hyperalgesia was maximal 6 hours after carrageenan injection and progressively decreased. Similar, although more variable, responses were observed with von Frey filaments. Morphine-induced analgesia resulted in a dose-dependent increase of paw threshold. Tolerance to morphine administrated on a once daily schedule (10 mg/kg) during 5 days was revealed by a significant decrease in analgesia by day 3. Taken together, these results demonstrated accuracy of this device for easy, fast, and reproducible measure of mechanical pain threshold on rat limbs. Moreover, it allows the performance of rat testing with minimal constraint, which reduces data variability. Perspective The calibrated forceps is an easy to use device well-suited to rapidly test mechanical pain threshold with accuracy. It is well-designed for preclinical behavioral screening of noxious or analgesic properties of molecules.

DeltaFosB, a stable splice variant of FosB, has been proposed to mediate persistent brain adaptation in response to several chronic perturbations, but it has not yet been considered in the context of sustained pain. Inflammatory pain induces neuronal plasticity that can result in persistent alteration of nociceptive pathways. This neuronal plasticity can partly result from changes in gene expression controlled by transcription factors. In the present study, we analyse the capacity of carrageenan-mediated inflammation to induce DeltaFosB in the spinal cord. We found that hind-paw inflammation increases FosB-like immunoreactivity in the superficial layers of rat lumbar spinal cord for at least 7 days. This induction parallels mechanical hyperalgesia and is maximal in the dorsal horn of segment L4 of the spinal cord which corresponds to the primary nociceptive afferent regions from the hind paw. We identified this FosB-like signal as DeltaFosB by comparing data obtained with antibodies raised against either an epitope present in both FosB and DeltaFosB, or the FosB C-terminal region that is deleted in DeltaFosB. The week-lasting changes in DeltaFosB highlight the interest in this protein as a molecular marker of sustained pain, and suggest a role of this transcription factor in pain-related plasticity within the spinal cord.

Sexual deficits and other behavioral disturbances such as anxiety-like behaviors can be observed in animals that have undergone social isolation, especially in species having important social interactions. Using a model of protracted social isolation in adult rats, we observed increased anxiety-like behavior and deficits in both the latency to initiate sexual behavior and the latency to ejaculate. We show, using transgenic cAMP response element (CRE)-LacZ reporter mice, that protracted social isolation also reduces CRE-dependent transcription within the nucleus accumbens. This decrease in CRE-dependent transcription can be mimicked in nonisolated animals by local viral gene transfer of a dominant negative mutant of CRE-binding protein (CREB). We previously showed that this manipulation increases anxiety-like behavior. We show here that it also impairs initiation of sexual behavior in nonisolated animals, a deficit that can be corrected by anxiolytic drug treatment. This local reduction in CREB activity, however, has no influence on ejaculation parameters. Reciprocally, we used the viral transgenic approach to overexpress CREB in the nucleus accumbens of isolated animals. We show that this local increase in CREB activity completely rescued the anxiety phenotype of the isolated animals, as well as their deficit in initiating sexual behavior, but failed to rescue the deficit in ejaculation. Our data suggest a role for the nucleus accumbens in anxiety responses and in specific aspects of sexual behavior. The results also provide insight into the molecular mechanisms by which social interactions affect brain plasticity and behavior.

International audience; BACKGROUND: We recently identified a facilitory control exerted by serotonin4 (5-HT4) receptors on the in vivo firing activity of dorsal raphe nucleus (DRN) serotonergic (5-HT) neurons. However, these findings were based on acute administrations of 5-HT4 receptor agonists and antagonists, which were active only in a subpopulation of 5-HT neurons. We had no evidence that this influence was significant when considering the entire DRN, nor if it was persistent after chronic treatments. In addition, the poor distribution of 5-HT4 receptors within the DRN raised the question of the neuroanatomical bases underlying this control. METHODS AND RESULTS: Here we show that the subacute intraperitoneal (IP) injection of the 5-HT4 receptor agonists prucalopride (2.5 mg/kg) and RS 67333 (1.5 mg/kg) 30 minutes before the beginning of recordings augment the mean firing rate of DRN neurons by 40% and 66%, respectively. These increases remain stable when the compounds are administered continuously during 3 and 21 days; the effects of the 3-day treatment are blocked by the 5-HT4 receptor antagonist GR 125487 (1000 microg/kg, intravenous [i.v.]). In addition, stereotaxic microinjections of herpes simplex viruses, transformed to overexpress 5-HT4 receptors, increase DRN 5-HT neuronal mean activity when performed in the medial prefrontal cortex (mPFC) but not in the striatum or in the hippocampus. CONCLUSIONS: This finding suggests the existence of a 5-HT(4)-dependent activation of DRN that may involve the mPFC, unveiling the 5-HT4 receptor as a putative player in the physiopathology of several disorders related to central 5-HT dysfunction.

Inhibitory synaptic transmission in the dorsal horn (DH) of the spinal cord plays an important role in the modulation of nociceptive messages because pharmacological blockade of spinal GABAAreceptors leads to thermal and mechanical pain symptoms. Here, we show that during the development of thermal hyperalgesia and mechanical allodynia associated with inflammatory pain, synaptic inhibition mediated by GABAAreceptors in lamina II of the DH was in fact markedly increased. This phenomenon was accompanied by an upregulation of the endogenous production of 5α-reduced neurosteroids, which, at the spinal level, led to a prolongation of GABAAreceptor-mediated synaptic currents and to the appearance of a mixed GABA/glycine cotransmission. This increased inhibition was correlated with a selective limitation of the inflammation-induced thermal hyperalgesia, whereas mechanical allodynia remained unaffected. Our results show that peripheral inflammation activates an endogenous neurosteroid-based antinociceptive control, which discriminates between thermal and mechanical hyperalgesia.

International audience; Brain-derived neurotrophic factor (BDNF) regulates neuronal development and function. However, it has been difficult to discern its role in the adult brain in influencing complex behavior. Here, we use a recently developed inducible knockout system to show that deleting BDNF in broad forebrain regions of adult mice impairs hippocampal-dependent learning and long-term potentiation. We use the inducible nature of this system to show that the loss of BDNF during earlier stages of development causes hyperactivity and more pronounced hippocampal-dependent learning deficits. We also demonstrate that the loss of forebrain BDNF attenuates the actions of desipramine, an antidepressant, in the forced swim test, suggesting the involvement of BDNF in antidepressant efficacy. These results establish roles for BDNF in the adult, and demonstrate the strength of this inducible knockout system in studying gene function in the adult brain.

The lateral hypothalamus (LH) is implicated in the behavioral actions of drugs of abuse, but the cellular and molecular basis of this role is unclear. Recent identification of neuropeptides localized in LH neurons has allowed for more specific studies of LH function. The LH-specific peptide orexin (hypocretin) has been shown to be important in arousal and sleep regulation. However, orexin cells of the LH project broadly throughout the brain such that orexin may influence other behaviors as well. In this study, we show that orexin neurons, and not nearby LH neurons expressing melanin-concentrating hormone (MCH), have mu-opioid receptors and respond to chronic morphine administration and opiate antagonist-precipitated morphine withdrawal. cAMP response element-mediated transcription is induced in a subset of orexin cells, but not MCH cells, after exposure to chronic morphine or induction of withdrawal. Additionally, c-Fos and the orexin gene itself are induced in orexin cells in the LH during morphine withdrawal. Finally, we show that orexin knock-out mice develop attenuated morphine dependence, as indicated by a less severe antagonist-precipitated withdrawal syndrome. Together, these studies support a role for the orexin system in molecular adaptations to morphine, and demonstrate dramatic differences in molecular responses among different populations of LH neurons.

Chronic opiate exposure is associated with upregulation of the cAMP signaling pathway and the transcription factor cAMP response element-binding protein in the locus ceruleus (LC) and certain other brain areas. To determine whether these adaptations ultimately affect transcription mediated by the cAMP response element (CRE), we induced morphine dependence in CRE-LacZ transgenic mice and performed a regional and cellular mapping of beta-galactosidase (beta-gal) expression during naltrexone-precipitated withdrawal. Consistent with our model of opiate dependence, beta-gal expression increased in the LC, but decreased in the lateral ventral tegmental area (VTA) and dorsal raphe nucleus (DRN). In addition, withdrawal increased beta-gal expression in the continuum of the extended amygdala and nucleus accumbens, macrostructures associated with the coupling of emotional stimuli to motor and autonomic responses. At the cellular level, in the central nucleus of the amygdala, beta-gal was found in cells both with and without mu opioid receptors as well as in corticotropin-releasing factor-expressing cells. In nucleus accumbens, beta-gal was expressed in several major subpopulations of neurons. In LC, beta-gal expression was induced predominantly in tyrosine hydroxylase-expressing cells, whereas in the VTA and DRN the majority of cells expressing beta-gal were nonmonoaminergic. These results show that molecular adaptations to chronic morphine alter CRE-mediated transcription during opiate withdrawal in physiologically salient regions involved in arousal, reward, mood, and affective responses. We propose that CRE-mediated transcription serves as a functional marker for neuronal plasticity during withdrawal. CRE-mediated transcription may itself contribute to re-establishing homeostasis in the organism through target gene regulation in these regions.

Recent work implicates regulation of neurogenesis as a form of plasticity in the adult rat hippocampus. Given the known effects of opiates such as morphine and heroin on hippocampal function, we examined opiate regulation of neurogenesis in this brain region. Chronic administration of morphine decreased neurogenesis by 42% in the adult rat hippocampal granule cell layer. A similar effect was seen in rats after chronic self-administration of heroin. Opiate regulation of neurogenesis was not mediated by changes in circulating levels of glucocorticoids, because similar effects were seen in rats that received adrenalectomy and corticosterone replacement. These findings suggest that opiate regulation of neurogenesis in the adult rat hippocampus may be one mechanism by which drug exposure influences hippocampal function.

The active zone protein RIM1alpha is required both for maintaining normal probability of neurotransmitter release and for long-term presynaptic potentiation at brain synapses. We now demonstrate that RIM1alpha(-/-) mice exhibit normal coordination and anxiety-related behaviors but display severely impaired learning and memory. Mice with a synaptotagmin 1 mutation, which selectively lowers release probability, and mice with Rab3A deletion, which selectively abolishes presynaptic long-term potentiation, do not exhibit this abnormality. Our data suggest that a decrease in release probability or a loss of presynaptic LTP alone is not sufficient to cause major behavioral alterations, but the combination of presynaptic abnormalities in RIM1alpha(-/-) mice severely alters learning and memory.