CB1 receptor-deficient mice as a model for depression

Cannabinoids prevent depressive-like symptoms and alterations in BDNF expression in a rat model of PTSD

Posttraumatic stress disorder (PTSD) is a debilitating condition highly comorbid with depression. The endocannabinoid (eCB) system and brain-derived neurotrophic factor (BDNF) are suggestively involved in both disorders. We examined whether cannabinoids can prevent the long-term depressive-like symptoms induced by exposure to the shock and situational reminders (SRs) model of PTSD. The CB1/2 receptor agonist WIN55,212-2 (0.5 mg/kg; i.p.), the fatty acid hydrolase (FAAH) inhibitor URB597 (0.3 mg/kg, i.p.) or vehicle were administered 2 h after severe shock. Cannabinoids prevented the shock/SRs-induced alterations in social recognition memory, locomotion, passive coping, anxiety-like behavior, anhedonia, fear retrieval, fear extinction and startle response as well as the decrease in BDNF levels in the hippocampus and prefrontal cortex (PFC). Furthermore, significant correlations were found between depressive-like behaviors and BDNF levels in the brain. The findings suggest that cannabinoids may prevent both depressive- and PTSD-like symptoms following exposure to severe stress and that alterations in BDNF levels in the brains' fear circuit are involved in these effects.

Circulating endocannabinoids and affect regulation in human subjects

The endocannabinoid (EC) system influences a wide variety of neurobiological processes including affect and emotionality as well as other neuropsychiatric functions. In this study we examined the relationship of circulating endocannabinoids [anandamide (AEA) and 2-arachidonoylglycerol (2-AG)] with affect and emotionality in 175 individuals with (n = 115) and without (n = 60) mood, anxiety, and/or personality disorders. Circulating AEA levels displayed a modest, though statistically significant, inverse relationship with a composite measure of affect regulation (β = - 0.264, p = 0.009), due to its relationship with affect intensity (β = - 0.225, p = 0.021) across all study participants. Neither AEA nor 2-AG level differed as a function of any syndromal/personality disorder and neither correlated significantly with state depression or state anxiety scores. These data suggest that circulating levels endocannabinoids may play a role in emotionality across individuals regardless of defined psychiatric disorder.

2-Arachidonoylglycerol: A signaling lipid with manifold actions in the brain

2-Arachidonoylglycerol (2-AG) is a signaling lipid in the central nervous system that is a key regulator of neurotransmitter release. 2-AG is an endocannabinoid that activates the cannabinoid CB₁ receptor. It is involved in a wide array of (patho)physiological functions, such as emotion, cognition, energy balance, pain sensation and neuroinflammation. In this review, we describe the biosynthetic and metabolic pathways of 2-AG and how chemical and genetic perturbation of these pathways has led to insight in the biological role of this signaling lipid. Finally, we discuss the potential therapeutic benefits of modulating 2-AG levels in the brain.

CB1 cannabinoid receptor ligands augment the antidepressant-like activity of biometals (magnesium and zinc) in the behavioural tests

OBJECTIVE

During the last few decades, endocannabinoid system has emerged as a novel possible target for antidepressant treatment. Although the medical literature provides information on the mood-changing effects of CB₁ ligands, little is known about the possible interaction between the simultaneous activation or inhibition of the CB₁ receptor and administration of other agents that possess antidepressant potential. The main goal of our study was to evaluate the influence of the CB₁ cannabinoid receptor ligands (oleamide - an endogenous agonist and AM251 - an inverse agonist/antagonist) on the antidepressant-like activity of biometals (i.e. magnesium and zinc).

METHODS

The forced swim test and the tail suspension test in mice were used to determine the antidepressant-like activity.

KEY FINDINGS

Concomitant intraperitoneal administration of per se inactive doses of oleamide (5 mg/kg) or AM251 (0.25 mg/kg) and the tested biometals (i.e. magnesium, 10 mg/kg or zinc, 5 mg/kg) shortened the immobility time of animals in the forced swim test and the tail suspension test. The observed effect was not associated with an increase in spontaneous locomotor activity of mice.

CONCLUSIONS

The simultaneous modulation of the cannabinoid system and supplementation of magnesium or zinc produce at least additive antidepressant-like effect.

CB1 Agonism Alters Addiction-Related Behaviors in Mice Lacking Mu or Delta Opioid Receptors

Opioids are powerful analgesics but the clinical utility of these compounds is reduced by aversive outcomes, including the development of affective and substance use disorders. Opioid systems do not function in isolation so understanding how these interact with other neuropharmacological systems could lead to novel therapeutics that minimize withdrawal, tolerance, and emotional dysregulation. The cannabinoid system is an obvious candidate as anatomical, pharmacological, and behavioral studies point to opioid-cannabinoid interactions in the mediation of these processes. The aim of our study is to uncover the role of specific cannabinoid and opioid receptors in addiction-related behaviors, specifically nociception, withdrawal, anxiety, and depression. To do so, we tested the effects of a selective CB1 agonist, arachidonyl-2-chloroethylamide (ACEA), on mouse behavior in tail immersion, naloxone-precipitated withdrawal, light-dark, and splash tests. We examined cannabinoid-opioid interactions in these tests by comparing responses of wildtype (WT) mice to mutant lines lacking either Mu or Delta opioid receptors. ACEA, both acute or repeated injections, had no effect on nociceptive thresholds in WT or Mu knockout (KO) mice suggesting that analgesic properties of CB1 agonists may be restricted to chronic pain conditions. The opioid antagonist, naloxone, induced similar levels of withdrawal in all three genotypes following ACEA treatment, confirming an opioidergic contribution to cannabinoid withdrawal. Anxiety-like responses in the light-dark test were similar across WT and KO lines; neither acute nor repeated ACEA injections modified this behavior. Similarly, administration of the Delta opioid receptor antagonist, naltrindole, alone or in combination with ACEA, did not alter responses of WT mice in the light-dark test. Thus, there may be a dissociation in the effect of pharmacological blockade vs. genetic deletion of Delta opioid receptors on anxiety-like behavior in mice. Finally, our study revealed a biphasic effect of ACEA on depressive-like behavior in the splash test, with a prodepressive state induced by acute exposure, followed by a shift to an anti-depressive state with repeated injections. The initial pro-depressive effect of ACEA was absent in Mu KO mice. In sum, our findings confirm interactions between opioid and cannabinoid systems in withdrawal and reveal reduced depressive-like symptoms with repeated CB1 receptor activation.

Cannabinoid Modulation of the Stressed Hippocampus

Exposure to stressful situations is one of the risk factors for the precipitation of several psychiatric disorders, including Major Depressive Disorder, Posttraumatic Stress Disorder and Schizophrenia. The hippocampal formation is a forebrain structure highly associated with emotional, learning and memory processes; being particularly vulnerable to stress. Exposure to stressful stimuli leads to neuroplastic changes and imbalance between inhibitory/excitatory networks. These changes have been associated with an impaired hippocampal function. Endocannabinoids (eCB) are one of the main systems controlling both excitatory and inhibitory neurotransmission, as well as neuroplasticity within the hippocampus. Cannabinoids receptors are highly expressed in the hippocampus, and several lines of evidence suggest that facilitation of cannabinoid signaling within this brain region prevents stress-induced behavioral changes. Also, chronic stress modulates hippocampal CB₁ receptors expression and endocannabinoid levels. Moreover, cannabinoids participate in mechanisms related to synaptic plasticity and adult neurogenesis. Here, we discussed the main findings supporting the involvement of hippocampal cannabinoid neurotransmission in stress-induced behavioral and neuroplastic changes.

Low empathy-like behaviour in male mice associates with impaired sociability, emotional memory, physiological stress reactivity and variations in neurobiological regulations

Deficits in empathy have been proposed to constitute a hallmark of several psychiatric disturbances like conduct disorder, antisocial and narcissistic personality disorders. Limited sensitivity to punishment, shallow or deficient affect and reduced physiological reactivity to environmental stressors have been often reported to co-occur with limited empathy and contribute to the onset of antisocial phenotypes. Empathy in its simplest form (i.e. emotional contagion) is addressed in preclinical models through the evaluation of the social transmission of emotional states: mice exposed to a painful stimulus display a higher response if in the presence of a familiar individual experiencing a higher degree of discomfort, than in isolation. In the present study, we investigated whether a reduction of emotional contagion can be considered a predictor of reduced sociality, sensitivity to punishment and physiological stress reactivity. To this aim, we first evaluated emotional contagion in a group of Balb/cJ mice and then discretised their values in four quartiles. The upper (i.e. Emotional Contagion Prone, ECP) and the lower (i.e. Emotional Contagion Resistant, ECR) quartiles constituted the experimental groups. Our results indicate that mice in the lower quartile are characterized by reduced sociability, impaired memory of negative events and dampened hypothalamic-pituitary-adrenocortical reactivity to external stressors. Furthermore, in the absence of changes in oxytocin receptor density, we show that these mice exhibit elevated concentrations of oxytocin and vasopressin and reduced density of BDNF receptors in behaviourally-relevant brain areas. Thus, not only do present results translate to the preclinical investigation of psychiatric disturbances, but also they can contribute to the study of emotional contagion in terms of its adaptive significance.

Is cannabis treatment for anxiety, mood, and related disorders ready for prime time?

Anxiety and related disorders are the most common mental conditions affecting the North American population. Despite their established efficacy, first-line antidepressant treatments are associated with significant side effects, leading many afflicted individuals to seek alternative treatments. Cannabis is commonly viewed as a natural alternative for a variety of medical and mental health conditions. Currently, anxiety ranks among the top five medical symptoms for which North Americans report using medical marijuana. However, upon careful review of the extant treatment literature, the anxiolytic effects of cannabis in clinical populations are surprisingly not well-documented. The effects of cannabis on anxiety and mood symptoms have been examined in healthy populations and in several small studies of synthetic cannabinoid agents but there are currently no studies which have examined the effects of the cannabis plant on anxiety and related disorders. In light of the rapidly shifting landscape regarding the legalization of cannabis for medical and recreational purposes, it is important to highlight the significant disconnect between the scientific literature, public opinion, and related policies. The aim of this article is to provide a comprehensive review of the current cannabis treatment literature, and to identify the potential for cannabis to be used as a therapeutic intervention for anxiety, mood, and related disorders. Searches of five electronic databases were conducted (PubMed, MEDLINE, Web of Science, PsychINFO, and Google Scholar), with the most recent in February 2017. The effects of cannabis on healthy populations and clinical psychiatric samples will be discussed, focusing primarily on anxiety and mood disorders.

Effects of the antipsychotic paliperidone on stress-induced changes in the endocannabinoid system in rat prefrontal cortex

Objectives There is a need to explore novel mechanisms of action of existing/new antipsychotics. One potential candidate is the endocannabinoid system (ECS). The present study tried to elucidate the effects of the antipsychotic paliperidone on stress-induced ECS alterations. Methods Wister rats were submitted to acute/chronic restraint stress. Paliperidone (1 mg/kg) was given prior each stress session. Cannabinoid receptors and endocannabinoids (eCBs) synthesis and degradation enzymes were measured in prefrontal cortex (PFC) samples by RT-PCR and Western Blot. Results In the PFC of rats exposed to acute stress, paliperidone increased CB1 receptor (CB1R) expression. Furthermore, paliperidone increased the expression of the eCB synthesis enzymes N-acylphosphatidylethanolamine- hydrolysing phospholipase D and DAGLα, and blocked the stress-induced increased expression of the degrading enzyme fatty acid amide hydrolase. In chronic conditions, paliperidone prevented the chronic stress-induced down-regulation of CB1R, normalised DAGLα expression and reverted stress-induced down-regulation of the 2-AG degrading enzyme monoacylglycerol lipase. ECS was analysed also in periphery. Acute stress decreased DAGLα expression, an effect prevented by paliperidone. Contrarily, chronic stress increased DAGLα and this effect was potentiated by paliperidone. Conclusions The results obtained described a preventive effect of paliperidone on stress-induced alterations in ECS. Considering the diverse alterations on ECS described in psychotic disease, targeting ECS emerges as a new therapeutic possibility.

Dual Depression: A Sex Perspective

Mood and substance use disorders (SUDs) are mental conditions that are highly prevalent in the general population. Cooccurrence of major depression and SUD, also known as dual depression, is very common in the field of substance addiction. Sex differences are found in both major depression and SUD. This review, after presenting the state of the art of dual depression as regards prevalence, ethiopathologic mechanisms, and clinical aspects, is focused on dual depression in women. An overview of some potential factors associated with the sex gap in dual depression such as injecting, sexual risk behavior, intimate partner violence, and the reproductive cycle is presented.

Cortical adrenoceptor expression, function and adaptation under conditions of cannabinoid receptor deletion

A neurochemical target at which cannabinoids interact to have global effects on behavior is brain noradrenergic circuitry. Acute and repeated administration of a cannabinoid receptor synthetic agonist is capable of increasing multiple indices of noradrenergic activity. This includes cannabinoid-induced 1) increases in norepinephrine (NE) release in the medial prefrontal cortex (mPFC); 2) desensitization of cortical α2-adrenoceptor-mediated effects; 3) activation of c-Fos in brainstem locus coeruleus (LC) noradrenergic neurons; and 4) increases in anxiety-like behaviors. In the present study, we sought to examine adaptations in adrenoceptor expression and function under conditions of cannabinoid receptor type 1 (CB1r) deletion using knockout (KO) mice and compare these to wild type (WT) controls. Electrophysiological analysis of α2-adrenoceptor-mediated responses in mPFC slices in WT mice showed a clonidine-induced α2-adrenoceptor-mediated increase in mPFC cell excitability coupled with an increase in input resistance. In contrast, CB1r KO mice showed an α2-adrenoceptor-mediated decrease in mPFC cell excitability. We then examined protein expression levels of α2- and β1-adrenoceptor subtypes in the mPFC as well as TH expression in the locus coeruleus (LC) of mice deficient in CB1r. Both α2- and β1-adrenoceptors exhibited a significant decrease in expression levels in CB1r KO mice when compared to WT in the mPFC, while a significant increase in TH was observed in the LC. To better define whether the same cortical neurons express α2A-adrenoceptor and CB1r in mPFC, we utilized high-resolution immunoelectron microscopy. We localized α2A-adrenoceptors in a knock-in mouse that expressed a hemoagglutinin (HA) tag downstream of the α2A-adrenoceptor promoter. Although the α2A-adrenoceptor was often identified pre-synaptically, we observed co-localization of CB1r with α2-adrenoceptors post-synaptically in the same mPFC neurons. Finally, using receptor binding, we confirmed prior results showing that α2A-adrenoceptor is unchanged in mPFC following acute or chronic exposure to the synthetic cannabinoid receptor agonist, WIN 55,212-2, but is increased, following chronic treatment followed by a period of abstinence. Taken together, these data provide convergent lines of evidence indicating cannabinoid regulation of the cortical adrenergic system.

Reliability of the chronic mild stress model of depression: A user survey

The chronic mild stress (CMS) model of depression is considered by many to be the animal model of depression that has the greatest validity and translational potential, but it has often been criticized for a perceived lack of reliability. The aims of this study were to establish the extent to which the procedure is reproducible, and to identify experimental variables relevant to its reliability. Because failures to replicate frequently remain unpublished, a survey methodology was used. A questionnaire was circulated to 170 labs identified from a PubMed search as having published a CMS study in the years 2010 or 2015 (with no selection in respect of the results reported). Responses were returned by 71 (42%) of the recipients, followed by further correspondence with some of them. Most of the respondents (n = 53: 75%) reported that the CMS procedure worked reliably in their hands. Of the others, 15 (21%) reported that the procedure was usually reliable, but not always (n = 9: 13%) or not for all measures (n = 6: 8%). Only three respondents (4%) reported being unable to reproduce the characteristic effects, two of whom may be using an insufficient duration of CMS exposure. A series of analyses compared the 75% of 'reliable' labs with the 25% of 'less reliable' labs on a range of experimenter, subject, stress and outcome variables. Few if any significant differences between these two samples were identified, possibly because of the small size and diversity of the 'less reliable' sample. Two other limitations of the study include the (unavoidable) omission of labs that may have worked with the model but not published their data, and the use of ad hoc measures to compare the severity of different stress regimes. The results are discussed in relation to relevant published observations. It is concluded that CMS is in fact a rather robust model, but the factors that result in a less effective implementation in a minority of laboratories remain to be firmly established.

The chronic mild stress (CMS) model of depression: History, evaluation and usage

Now 30 years old, the chronic mild stress (CMS) model of depression has been used in >1300 published studies, with a year-on-year increase rising to >200 papers in 2015. Data from a survey of users show that while a variety of names are in use (chronic mild/unpredictable/varied stress), these describe essentially the same procedure. This paper provides an update on the validity and reliability of the CMS model, and reviews recent data on the neurobiological basis of CMS effects and the mechanisms of antidepressant action: the volume of this research may be unique in providing a comprehensive account of antidepressant action within a single model. Also discussed is the use of CMS in drug discovery, with particular reference to hippocampal and extra-hippocampal targets. The high translational potential of the CMS model means that the neurobiological mechanisms described may be of particular relevance to human depression and mechanisms of clinical antidepressant action.

Cannabinoid signalling inhibits sarcoplasmic Ca2+ release and regulates excitation-contraction coupling in mammalian skeletal muscle

KEY POINTS

Marijuana was found to cause muscle weakness, although the exact regulatory role of its receptors (CB1 cannabinoid receptor; CB1R) in the excitation-contraction coupling (ECC) of mammalian skeletal muscle remains unknown. We found that CB1R activation or its knockout did not affect muscle force directly, whereas its activation decreased the Ca²⁺ -sensitivity of the contractile apparatus and made the muscle fibres more prone to fatigue. We demonstrate that CB1Rs are not connected to the inositol 1,4,5-trisphosphate pathway either in myotubes or in adult muscle fibres. By contrast, CB1Rs constitutively inhibit sarcoplasmic Ca²⁺ release and sarcoplasmic reticulum Ca²⁺ ATPase during ECC in a G_i/o protein-mediated way in adult skeletal muscle fibres but not in myotubes. These results help with our understanding of the physiological effects and pathological consequences of CB1R activation in skeletal muscle and may be useful in the development of new cannabinoid drugs.

ABSTRACT

Marijuana was found to cause muscle weakness, although it is unknown whether it affects the muscles directly or modulates only the motor control of the central nervous system. Although the presence of CB1 cannabinoid receptors (CB1R), which are responsible for the psychoactive effects of the drug in the brain, have recently been demonstrated in skeletal muscle, it is unclear how CB1R-mediated signalling affects the contraction and Ca²⁺ homeostasis of mammalian skeletal muscle. In the present study, we demonstrate that in vitro CB1R activation increased muscle fatigability and decreased the Ca²⁺ -sensitivity of the contractile apparatus, whereas it did not alter the amplitude of single twitch contractions. In myotubes, CB1R agonists neither evoked, nor influenced inositol 1,4,5-trisphosphate (IP₃ )-mediated Ca²⁺ transients, nor did they alter excitation-contraction coupling. By contrast, in isolated muscle fibres of wild-type mice, although CB1R agonists did not evoke IP₃ -mediated Ca²⁺ transients too, they significantly reduced the amplitude of the depolarization-evoked transients in a pertussis-toxin sensitive manner, indicating a G_i/o protein-dependent mechanism. Concurrently, on skeletal muscle fibres isolated from CB1R-knockout animals, depolarization-evoked Ca²⁺ transients, as well qas Ca²⁺ release flux via ryanodine receptors (RyRs), and the total amount of released Ca²⁺ was significantly greater than that from wild-type mice. Our results show that CB1R-mediated signalling exerts both a constitutive and an agonist-mediated inhibition on the Ca²⁺ transients via RyR, regulates the activity of the sarcoplasmic reticulum Ca²⁺ ATPase and enhances muscle fatigability, which might decrease exercise performance, thus playing a role in myopathies, and therefore should be considered during the development of new cannabinoid drugs.

CNS Target Identification and Validation: Avoiding the Valley of Death or Naive Optimism?

There are many challenges along the path to the approval of new drugs to treat CNS disorders, one of the greatest areas of unmet medical need with a large societal burden and health-care impact. Unfortunately, over the past two decades, few CNS drug approvals have succeeded, leading many pharmaceutical companies to deprioritize this therapeutic area. The reasons for the failures in CNS drug discovery are likely to be multifactorial. However, selecting the most biologically plausible molecular targets that are relevant to the disorder is a critical first step to improve the probability of success. In this review, we outline previous methods for identifying and validating novel targets for CNS drug discovery, and, cognizant of previous failures, we discuss potential new strategies that may improve the probability of success of developing novel treatments for CNS disorders.

Anxiety, Stress, and Fear Response in Mice With Reduced Endocannabinoid Levels

BACKGROUND

Disruption of the endocannabinoid system through pharmacological or genetic invalidation of cannabinoid CB1 receptors has been linked to depression in humans and depression-like behaviors in mice. The two main endogenous cannabinoids, anandamide and 2-arachidonoyl glycerol (2-AG), are produced on demand from phospholipids. The pathways and enzymes involved in endocannabinoid biosynthesis thus play a major role in regulating the activity of this system. This study investigates the role of the main 2-AG producing enzyme diacylglycerol lipase α (DAGL-α).

METHODS

We generated and used knockout mice lacking DAGL-α (Dagla(-/-)) to assess the behavioral consequences of reduced endocannabinoid levels in the brain. We performed different behavior tests to determine anxiety- and depression-related behavioral changes in Dagla(-/-) mice. We also analyzed expression of genes related to the endocannabinoid system via real-time polymerase chain reaction and used the mitotic marker 5-bromo-2'-deoxyuridine to analyze adult neurogenesis.

RESULTS

Dagla(-/-) animals show an 80% reduction of brain 2-AG levels but also a reduction in cortical and amygdalar anandamide. The behavioral changes induced by Dagla deletion include a reduced exploration of the central area of the open field, a maternal neglect behavior, a fear extinction deficit, increased behavioral despair, increased anxiety-related behaviors in the light/dark box, and reduced hippocampal neurogenesis. Some of these behavioral changes resemble those observed in animals lacking the CB1 receptor.

CONCLUSIONS

Our findings demonstrate that the deletion of Dagla adversely affects the emotional state of animals and results in enhanced anxiety, stress, and fear responses.

Crosstalk between endocannabinoid and immune systems: a potential dysregulation in depression?

BACKGROUND

The endocannabinoid (eCB) system, an endogenous lipid signaling system, appears to be dysregulated in depression. The role of endocannabinoids (eCBs) as potent immunomodulators, together with the accumulating support for a chronic low-grade inflammatory profile in depression, suggests a compelling hypothesis for a fundamental impairment in their intercommunication, in depression.

OBJECTIVE

We aim to review previous literature on individual associations between the immune and eCB systems and depression. It will focus on peripheral and central mechanisms of crosstalk between the eCB and immune systems. A potential dysregulation in this crosstalk will be discussed in the context of depression.

RESULTS

Investigations largely report a hypoactivity of the eCB system and increased inflammatory markers in individuals with depression. Findings depict a multifaceted communication whereby immunocompetent and eCB-related cells can both influence the suppression and enhancement of the other's activity in both the periphery and central nervous system. A dysregulation of the eCB system, as seen in depression, appears to be associated with central and peripheral concentrations of inflammatory agents implicated in the pathophysiology of this illness.

CONCLUSION

The eCB and immune systems have been individually associated with and implicated in pathogenic mechanisms of depression. Both systems tightly regulate the other's activity. As such, a dysregulation in this crosstalk has potential to influence the onset and maintenance of this neuropsychiatric illness. However, few studies have investigated both systems and depression conjointly. This review highlights the demand to consider joint eCB-immune interactions in the pathoetiology of depression.

Keep off the grass? Cannabis, cognition and addiction

In an increasing number of states and countries, cannabis now stands poised to join alcohol and tobacco as a legal drug. Quantifying the relative adverse and beneficial effects of cannabis and its constituent cannabinoids should therefore be prioritized. Whereas newspaper headlines have focused on links between cannabis and psychosis, less attention has been paid to the much more common problem of cannabis addiction. Certain cognitive changes have also been attributed to cannabis use, although their causality and longevity are fiercely debated. Identifying why some individuals are more vulnerable than others to the adverse effects of cannabis is now of paramount importance to public health. Here, we review the current state of knowledge about such vulnerability factors, the variations in types of cannabis, and the relationship between these and cognition and addiction.

Sevoflurane prevents stroke-induced depressive and anxiety behaviors by promoting cannabinoid receptor subtype I-dependent interaction between β-arrestin 2 and extracellular signal-regulated kinases 1/2 in the rat hippocampus

One of the most frequent psychological consequences of stroke is depression. Previous animal studies have demonstrated that post-conditioning with sevoflurane protects against focal cerebral ischemia and reperfusion injury. Thus, we hypothesized that repeated exposure to sevoflurane after transient ischemia can prevent the development of depressive-like behavior. To test this hypothesis, we induced transient cerebral ischemia via transient occlusion of bilateral common carotid arteries and examined the effects of subsequent repeated exposure to sevoflurane on sucrose preference, locomotor activity, and rearing activity in rats. To explore the putative neurobiological mechanisms, we further investigated the roles of hippocampal CB1 receptor in the behavioral effects of sevoflurane. We found that repeated sevoflurane exposures reversed ischemia-induced depressive-like behaviors. Furthermore, CB1 receptor inhibition in the dorsal hippocampus (DH) abolished the effects of sevoflurane exposures on ischemia-induced depressive-like behaviors. In addition, repeated sevoflurane exposures increased CB1 receptor expression and endocannabinoids levels in the DH of ischemic rats. Moreover, repeated sevoflurane exposures enhanced the expression of β-arrestin 2, increased the activation of extracellular signal-regulated kinases (ERK)1/2, and promoted the interaction of β-arrestin 2 and ERK1/2 in the DH, and such effects were reversed by CB1 receptor antagonism in the DH. Finally, β-arrestin 2 expression and ERK1/2 activation in the DH were critical for the preventative effects of sevoflurane exposures on ischemia-induced depressive-like behaviors. Taken together, our results suggested that sevoflurane exposure after brain ischemia may prevent the development of depression, and such preventative effects of sevoflurane are likely ascribed to the activation of CB1 receptor-mediated β-arrestin 2-ERK1/2 signaling pathways. We propose that the following mechanisms are critical for the preventative effects of sevoflurane against post-stroke depressive and anxiety behaviors: repeated sevoflurane exposure after transient brain ischemia enhances N-arachidonoylethanolamine (AEA) and 2-Arachidonoylglycerol (2-AG) levels and normalize cannabinoid receptor type 1 (CB1) receptor expression in the dorsal hippocampus, which results in enhanced interaction of β-arrestin 2 and extracellular signal-regulated kinases (ERK1/2) and increased ERK1/2 activation, leading to decreased depressive and anxiety behaviors. We think these findings should provide a new strategy for treatment of post-stroke depression.

Treatment-resistant depression: are animal models of depression fit for purpose?

BACKGROUND

Resistance to antidepressant drug treatment remains a major health problem. Animal models of depression are efficient in detecting effective treatments but have done little to increase the reach of antidepressant drugs. This may be because most animal models of depression target the reversal of stress-induced behavioural change, whereas treatment-resistant depression is typically associated with risk factors that predispose to the precipitation of depressive episodes by relatively low levels of stress. Therefore, the search for treatments for resistant depression may require models that incorporate predisposing factors leading to heightened stress responsiveness.

METHOD

Using a diathesis-stress framework, we review developmental, genetic and genomic models against four criteria: (i) increased sensitivity to stress precipitation of a depressive behavioural phenotype, (ii) resistance to chronic treatment with conventional antidepressants, (iii) a good response to novel modes of antidepressant treatment (e.g. ketamine; deep brain stimulation) that are reported to be effective in treatment-resistant depression and (iv) a parallel to a known clinical risk factor.

RESULTS

We identify 18 models that may have some potential. All require further validation. Currently, the most promising are the Wistar-Kyoto (WKY) and congenital learned helplessness (cLH) rat strains, the high anxiety behaviour (HAB) mouse strain and the CB1 receptor knockout and OCT2 null mutant mouse strains.

CONCLUSION

Further development is needed to validate models of antidepressant resistance that are fit for purpose. The criteria used in this review may provide a helpful framework to guide research in this area.

Role of CB2 receptors in social and aggressive behavior in male mice

RATIONALE

Male CB1KO mice exhibit stronger aggressive responses than wild-type mice.

OBJECTIVE

This study was designed to examine the role of cannabinoid CB2r in social and aggressive behavior.

METHODS

The social interaction test and resident-intruder paradigm were performed in mice lacking CB2r (CB2KO) and in wild-type (WT) littermates. The effects of the CB2r selective agonist JWH133 (1 and 2 mg/kg) on aggression were also evaluated in Oncins France 1 (OF1) mice. Gene expression analyses of monoamine oxidase-A (MAO-A), catechol-o-methyltransferase (COMT), 5-hydroxytryptamine transporter (5-HTT), and 5-HT1B receptor (5HT1Br) in the dorsal raphe nuclei (DR) and the amygdala (AMY) were carried out using real-time PCR.

RESULTS

Group-housed CB2KO mice exhibited higher levels of aggression in the social interaction test and displayed more aggression than resident WT mice. Isolation increased aggressive behavior in WT mice but did not affect CB2KO animals; however, the latter mice exhibited higher levels of social interaction with their WT counterparts. MAO-A and 5-HTT gene expression was significantly higher in grouped CB2KO mice. The expression of 5HT1Br, COMT, and MAO-A in the AMY was more pronounced in CB2KO mice than in WT counterparts. Acute administration of the CB2 agonist JWH133 significantly reduced the level of aggression in aggressive isolated OF1 mice, an effect that decreased after pretreatment with the CB2 receptor antagonist AM630.

CONCLUSION

Our results suggest that CB2r is implicated in social interaction and aggressive behavior and deserves further consideration as a potential new target for the management of aggression.

Endocannabinoid system activation may be associated with insulin resistance in women with polycystic ovary syndrome

OBJECTIVE

To assess the levels of endocannabinoids and cannabinoid receptors (CB) 1 and 2 in women with polycystic ovary syndrome (PCOS).

DESIGN

Case-control study.

SETTING

University teaching hospital.

PATIENT(S)

In total, 20 women with PCOS and 20 healthy women in a control group, who were matched for body mass index and age, were enrolled in this study.

INTERVENTION(S)

The homeostasis model index was used to assess insulin resistance.

MAIN OUTCOME MEASURE(S)

Omental adipose tissue and human peripheral blood mononuclear cells (PBMCs) from PCOS and the controls were analyzed using real-time polymerase chain reactions for the expressions of CB1 and CB2. The levels of endocannabinoids were analyzed using high-performance liquid chromatography.

RESULT(S)

The levels of anandamide and 2-arachidonoylglycerol, and the expression of CB1 and CB2 mRNA (messenger ribonucleic acid) in the PBMCs were significantly higher in the women with PCOS than in the women serving as controls. We found that expression of CB1, but not CB2, in adipose tissue was significantly higher in the women with, vs. without, PCOS. The expressions of CB1 mRNA and endocannabinoids showed a significant positive correlation with 2-hour glucose and insulin levels 2 hours after glucose loading in the PBMCs and adipose tissue.

CONCLUSION(S)

Activation of endocannabinoids and overexpression of cannabinoid receptors, especially CB1, may be associated with insulin resistance in women with PCOS.

Reinforcement learning in depression: A review of computational research

Despite being considered primarily a mood disorder, major depressive disorder (MDD) is characterized by cognitive and decision making deficits. Recent research has employed computational models of reinforcement learning (RL) to address these deficits. The computational approach has the advantage in making explicit predictions about learning and behavior, specifying the process parameters of RL, differentiating between model-free and model-based RL, and the computational model-based functional magnetic resonance imaging and electroencephalography. With these merits there has been an emerging field of computational psychiatry and here we review specific studies that focused on MDD. Considerable evidence suggests that MDD is associated with impaired brain signals of reward prediction error and expected value ('wanting'), decreased reward sensitivity ('liking') and/or learning (be it model-free or model-based), etc., although the causality remains unclear. These parameters may serve as valuable intermediate phenotypes of MDD, linking general clinical symptoms to underlying molecular dysfunctions. We believe future computational research at clinical, systems, and cellular/molecular/genetic levels will propel us toward a better understanding of the disease.

Interactions of the opioid and cannabinoid systems in reward: Insights from knockout studies

The opioid system consists of three receptors, mu, delta, and kappa, which are activated by endogenous opioid peptides (enkephalins, endorphins, and dynorphins). The endogenous cannabinoid system comprises lipid neuromodulators (endocannabinoids), enzymes for their synthesis and their degradation and two well-characterized receptors, cannabinoid receptors CB1 and CB2. These systems play a major role in the control of pain as well as in mood regulation, reward processing and the development of addiction. Both opioid and cannabinoid receptors are coupled to G proteins and are expressed throughout the brain reinforcement circuitry. Extending classical pharmacology, research using genetically modified mice has provided important progress in the identification of the specific contribution of each component of these endogenous systems in vivo on reward process. This review will summarize available genetic tools and our present knowledge on the consequences of gene knockout on reinforced behaviors in both systems, with a focus on their potential interactions. A better understanding of opioid-cannabinoid interactions may provide novel strategies for therapies in addicted individuals.

Diacylglycerol Lipase α Knockout Mice Demonstrate Metabolic and Behavioral Phenotypes Similar to Those of Cannabinoid Receptor 1 Knockout Mice

After creating >4,650 knockouts (KOs) of independent mouse genes, we screened them by high-throughput phenotyping and found that cannabinoid receptor 1 (Cnr1) KO mice had the same lean phenotype published by others. We asked if our KOs of DAG lipase α or β (Dagla or Daglb), which catalyze biosynthesis of the endocannabinoid (EC) 2-arachidonoylglycerol (2-AG), or Napepld, which catalyzes biosynthesis of the EC anandamide, shared the lean phenotype of Cnr1 KO mice. We found that Dagla KO mice, but not Daglb or Napepld KO mice, were among the leanest of 3651 chow-fed KO lines screened. In confirmatory studies, chow- or high fat diet-fed Dagla and Cnr1 KO mice were leaner than wild-type (WT) littermates; when data from multiple cohorts of adult mice were combined, body fat was 47 and 45% lower in Dagla and Cnr1 KO mice, respectively, relative to WT values. By contrast, neither Daglb nor Napepld KO mice were lean. Weanling Dagla KO mice ate less than WT mice and had body weight (BW) similar to pair-fed WT mice, and adult Dagla KO mice had normal activity and VO2 levels, similar to Cnr1 KO mice. Our Dagla and Cnr1 KO mice also had low fasting insulin, triglyceride, and total cholesterol levels, and after glucose challenge had normal glucose but very low insulin levels. Dagla and Cnr1 KO mice also showed similar responses to a battery of behavioral tests. These data suggest: (1) the lean phenotype of young Dagla and Cnr1 KO mice is mainly due to hypophagia; (2) in pathways where ECs signal through Cnr1 to regulate food intake and other metabolic and behavioral phenotypes observed in Cnr1 KO mice, Dagla alone provides the 2-AG that serves as the EC signal; and (3) small molecule Dagla inhibitors with a pharmacokinetic profile similar to that of Cnr1 inverse agonists are likely to mirror the ability of these Cnr1 inverse agonists to lower BW and improve glycemic control in obese patients with type 2 diabetes, but may also induce undesirable neuropsychiatric side-effects.

Towards a personalized treatment in depression: endocannabinoids, inflammation and stress response

The complex nature of depression is mirrored by difficulties in tailoring its treatment. Key underlying mechanisms of this mental disorder include elevated inflammation and a dysregulated hypothalamic-pituitary-adrenal (HPA) axis. More recently, the endocannabinoid system has been proposed as another important component in the pathogenesis of depression, and strong evidence suggests that all three systems communicate with each other. A growing number of genetic studies have investigated polymorphisms in depression in each of these systems separately. However, no study to date has looked at these genes in conjunction. In this article we will review the crosstalk between the endocannabinoid system, immune system and HPA axis; and discuss the evidence of gene polymorphisms and their relation to the risk of depression and its treatment. We propose future directions where genes of these three systems are considered from a joint perspective to improve prediction of treatment response, taking into account potentially overlooked genetic variations.

The endocannabinoid/endovanilloid system and depression

Depression is one of the most frequent causes of disability in the 21st century. Despite the many preclinical and clinical studies that have addressed this brain disorder, the pathophysiology of depression is not well understood and the available antidepressant drugs are therapeutically inadequate in many patients. In recent years, the potential role of lipid-derived molecules, particularly endocannabinoids (eCBs) and endovanilloids, has been highlighted in the pathogenesis of depression and in the action of antidepressants. There are many indications that the eCB/endovanilloid system is involved in the pathogenesis of depression, including the localization of receptors, modulation of monoaminergic transmission, inhibition of the stress axis and promotion of neuroplasticity in the brain. Preclinical pharmacological and genetic studies of eCBs in depression also suggest that facilitating the eCB system exerts antidepressant-like behavioral responses in rodents. In this article, we review the current knowledge of the role of the eCB/endovanilloid system in depression, as well as the effects of its ligands, models of depression and antidepressant drugs in preclinical and clinical settings.

Estrous cycle and sex affect cocaine-induced behavioural changes in CD1 mice

RATIONALE

Several findings on sex differences in cocaine response suggest a role for hormonal milieu in modulating the subjective effects of cocaine. Nitric oxide (NO) has been involved in the neurochemical, hormonal, and behavioral changes related to stress and anxiety. Within the brain, the anteroventral subdivision of the medial amygdala (MeAV) is an important area involved in processing emotional responses such as anxiety and a high density of NO-producing neurons is observed in this area.

OBJECTIVES

In this study, we hypothesize the possibility of sex/hormonal differences in response to cocaine and that these differences may reflect a change in the MeAV nitrergic system. We have examined cocaine's acute effects on nicotinamide adenine dinucleotide phosphate diaphorase (nadph-d) expression, as well as its effect on motor activity and anxiety in male and estrus and diestrus females.

RESULTS

Our results show that acute cocaine administration produces an increase in both anxiety behaviors and nadph-d expression in the MeAV. Male and diestrus female mice were more susceptible to these effects of cocaine than estrus female mice in which no differences were detected. In addition, we examined individual differences in male and female mice responding to intravenous cocaine reinforcement in a self-administration paradigm. Female mice acquired cocaine self-administration at a faster rate than males and showed a higher motivation to self-administer cocaine under a progressive ratio schedule of reinforcement.

CONCLUSIONS

Our data suggest a complex interaction between hormonal milieu and the behavioral and reinforcing effects of cocaine.

Prenatal poly I:C age-dependently alters cannabinoid type 1 receptors in offspring: a longitudinal small animal PET study using [(18)F]MK-9470

Evidence suggests that there is a link between the endocannabinoid system (ECS) and neuropsychiatric illnesses, including schizophrenia. Whilst the ECS has been shown to be involved in immune system regulation in various ways, it is known that infections during pregnancy can modulate the immune system of the mother and increase the risk for schizophrenia in offspring. In animal studies, maternal immune activation following administration of viral or bacterial mimics has been shown to reproduce many key structural, behavioural, and pharmacological abnormalities in offspring that resemble schizophrenia. In the present study, we used Positron Emission Tomography (PET) and [(18)F]MK-9470, a selective high-affinity inverse agonist radioligand for cannabinoid type 1 receptors (CB1R), to longitudinally assess CB1R expression in the progeny of female rats exposed to the viral mimic polyriboinosinic-polyribocytidilic acid (poly I:C) (4mg/kg i.v.) or vehicle at gestational day 15 (GD 15). PET scans were performed in offspring at postnatal days (PND) 32-42 (adolescence) and in the same animals again at PNDs 75-79 (adulthood). Sixteen regions of interest were assessed, encompassing the whole rat brain. At adolescence, offspring exposed prenatally to poly I:C had significantly lower CB1R relative Standard Uptake Values (rSUV) compared to controls in the globus pallidus (p=0.046). In adulthood, however, poly I:C exposed offspring had higher levels of CB1R rSUV in sensory cortex (p=0.034) and hypothalamus (p=0.032) compared to controls. Our results suggest that prenatal poly I:C leads to long term alterations in the integrity of the ECS that are age and region-specific. The increased CB1R expression in adulthood following poly I:C mirrors the increased CB1R observed in patients with schizophrenia in post-mortem and in vivo PET studies.

Disease-specific heteromerization of G-protein-coupled receptors that target drugs of abuse

Drugs of abuse such as morphine or marijuana exert their effects through the activation of G-protein-coupled receptors (GPCRs), the opioid and cannabinoid receptors, respectively. Moreover, interactions between either of these receptors have been shown to be involved in the rewarding effects of drugs of abuse. Recent advances in the field, using a variety of approaches, have demonstrated that many GPCRs, including opioid, cannabinoid, and dopamine receptors, can form associations between different receptor subtypes or with other GPCRs to form heteromeric complexes. The formation of these complexes, in turn, leads to the modulation of the properties of individual protomers. The development of tools that can selectively disrupt GPCR heteromers as well as monoclonal antibodies that can selectively block signaling by specific heteromer pairs has indicated that heteromers involving opioid, cannabinoid, or dopamine receptors may play a role in various disease states. In this review, we describe evidence for opioid, cannabinoid, and dopamine receptor heteromerization and the potential role of GPCR heteromers in pathophysiological conditions.

Stress, serotonin, and hippocampal neurogenesis in relation to depression and antidepressant effects

Chronic stressful life events are risk factors for developing major depression, the pathophysiology of which is strongly linked to impairments in serotonin (5-HT) neurotransmission. Exposure to chronic unpredictable stress (CUS) has been found to induce depressive-like behaviours, including passive behavioural coping and anhedonia in animal models, along with many other affective, cognitive, and behavioural symptoms. The heterogeneity of these symptoms represents the plurality of corticolimbic structures involved in mood regulation that are adversely affected in the disorder. Chronic stress has also been shown to negatively regulate adult hippocampal neurogenesis, a phenomenon that is involved in antidepressant effects and regulates subsequent stress responses. Although there exists an enormous body of data on stress-induced alterations of 5-HT activity, there has not been extensive exploration of 5-HT adaptations occurring presynaptically or at the level of the raphe nuclei after exposure to CUS. Similarly, although hippocampal neurogenesis is known to be negatively regulated by stress and positively regulated by antidepressant treatment, the role of neurogenesis in mediating affective behaviour in the context of stress remains an active area of investigation. The goal of this review is to link the serotonergic and neurogenic hypotheses of depression and antidepressant effects in the context of stress. Specifically, chronic stress significantly attenuates 5-HT neurotransmission and 5-HT1A autoreceptor sensitivity, and this effect could represent an endophenotypic hallmark for mood disorders. In addition, by decreasing neurogenesis, CUS decreases hippocampal inhibition of the hypothalamic-pituitary-adrenal (HPA) axis, exacerbating stress axis overactivity. Similarly, we discuss the possibility that adult hippocampal neurogenesis mediates antidepressant effects via the ventral (in rodents; anterior in humans) hippocampus' influence on the HPA axis, and mechanisms by which antidepressants may reverse chronic stress-induced 5-HT and neurogenic changes. Although data are as yet equivocal, antidepressant modulation of 5-HT neurotransmission may well serve as one of the factors that could drive neurogenesis-dependent antidepressant effects through these stress regulation-related mechanisms.

Translational evidence for the involvement of the endocannabinoid system in stress-related psychiatric illnesses

Accumulating evidence over the past decade has highlighted an important role of the endocannabinoid (eCB) system in the regulation of stress and emotional behavior across divergent species, from rodents to humans. The general findings from this work indicate that the eCB system plays an important role in gating and buffering the stress response, dampening anxiety and regulating mood. Work in rodents has allowed researchers to determine the neural mechanisms mediating this relationship while work in human populations has demonstrated the possible importance of this system in stress-related psychiatric diseases, such as post-traumatic stress disorder, generalized anxiety and major depression. These stress-protective effects of eCB signaling appear to be primarily mediated by their actions within corticolimbic structures, particularly the amygdala and the prefrontal cortex. The aim of this review is to provide an up-to-date discussion of the current level of knowledge in this field, as well as address the current gaps in knowledge and specific areas of research that require attention.

Endocannabinoid receptor deficiency affects maternal care and alters the dam's hippocampal oxytocin receptor and brain-derived neurotrophic factor expression

Maternal care is the newborn's first experience of social interaction, and this influences infant survival, development and social competences throughout life. We recently found that postpartum blocking of the endocannabinoid receptor-1 (CB1R) altered maternal behaviour. In the present study, maternal care was assessed by the time taken to retrieve pups, pups' ultrasonic vocalisations (USVs) and pup body weight, comparing CB1R deleted (CB1R KO) versus wild-type (WT) mice. After culling on postpartum day 8, hippocampal expression of oxytocin receptor (OXTR), brain-derived neurotrophic factor (BDNF) and stress-mediating factors were evaluated in CB1R KO and WT dams. Comparisons were also performed with nulliparous (NP) CB1R KO and WT mice. Compared to WT, CB1R KO dams were slower to retrieve their pups. Although the body weight of the KO pups did not differ from the weight of WT pups, they emitted fewer USVs. This impairment of the dam-pup relationship correlated with a significant reduction of OXTR mRNA and protein levels among CB1R KO dams compared to WT dams. Furthermore, WT dams exhibited elevated OXTR mRNA expression, as well as increased levels of mineralocorticoid and glucocorticoid receptors, compared to WT NP mice. By contrast, CB1R KO dams showed no such elevation of OXTR expression, alongside lower BDNF and mineralocorticoid receptors, as well as elevated corticotrophin-releasing hormone mRNA levels, when compared to CB1R KO NP. Thus, it appears that the disruption of endocannabinoid signalling by CB1R deletion alters expression of the OXTR, apparently leading to deleterious effects upon maternal behaviour.

Opposing local effects of endocannabinoids on the activity of noradrenergic neurons and release of noradrenaline: relevance for their role in depression and in the actions of CB(1) receptor antagonists

There is strong evidence that endocannabinoids modulate signaling of serotonin and noradrenaline, which play key roles in the pathophysiology and treatment of anxiety and depression. Most pharmacological and genetic, human and rodent studies suggest that the presence of under-functioning endocannabinoid type-1 (CB(1)) receptors is associated with increased anxiety and elevated extracellular serotonin concentration. In contrast, noradrenaline is presumably implicated in the mediation of depression-type symptoms of CB(1) receptor antagonists. Evidence shows that most CB(1) receptors located on axons and terminals of GABA-ergic, serotonergic or glutamatergic neurons stimulate the activity of noradrenergic neurons. In contrast, those located on noradrenergic axons and terminals inhibit noradrenaline release efficiently. In this latter process, excitatory ionotropic or G protein-coupled receptors, such as the NMDA, alpha1 and beta1 adrenergic receptors, activate local endocannabinoid synthesis at postsynaptic sites and stimulate retrograde endocannabinoid neurotransmission acting on CB(1) receptors of noradrenergic terminals. The underlying mechanisms include calcium signal generation, which activates enzymes that increase the synthesis of both anandamide and 2-arachidonoylglycerol, while G(q/11) protein activation also increases the formation of 2-arachidonoylglycerol from diacylglycerol during the signaling process. In addition, other non-CB(1) receptor endocannabinoid targets such as CB(2), transient receptor potential vanilloid subtype, peroxisome proliferator-activated receptor-alpha and possibly GPR55 can also mediate some of the endocannabinoid effects. In conclusion, both neuronal activation and neurotransmitter release depend on the in situ synthesized endocannabinoids and thus, local endocannabinoid concentrations in different brain areas may be crucial in the net effect, namely in the regulation of neurons located postsynaptically to the noradrenergic synapse.

Associations of depressive symptoms with serum proportions of palmitic and arachidonic acids, and α-tocopherol effects among male population--a preliminary study

BACKGROUND & AIMS

Recent years, inflammation and oxidative stress have been addressed in relation to interactions between fatty acid (FA) and depression. To study the associations between FAs and depressive symptoms in men, serum FA proportion was compared with perceived depression. We also measured α-tocopherol (a-Toc) levels to investigate the associations with FA functions.

METHODS

A cross-sectional study was performed on 113 male workers recruited from a software development company in Japan. Depressive symptoms were assessed according to the 20-item Center for Epidemiologic Studies Depression (CES-D) scale. Twenty-four FAs in the serum from the peripheral blood were examined.

RESULTS

CES-D scores were significantly positively correlated with the serum percentage of palmitic acid (PA), while they were negatively correlated with arachidonic acid (AA). The CES-D scores were not correlated with the serum ratio of docosahexaenoic acid or eicosapentaenoic acid (n-3 poly-unsaturated FAs (PUFAs)). CES-D scores were significantly negatively correlated with a-Toc/PA ratio (correlation: adjusting for possible confounders).

CONCLUSIONS

Although no associations were found between depressive symptoms and n-3 PUFA proportions in male population, depressive symptoms were positively correlated with PA percentages and negatively correlated with AA percentages. PA may increase neural vulnerability and AA may decrease the severity of depression. Moreover, a-Toc may have protective effects against depressive symptoms.

Introduction to the special issue on stress, emotional behavior, and the endocannabinoid system: a decade of research

This article is part of a Special Issue entitled:Stress, Emotional Behavior, and the Endocannabinoid System.

Effect of delta-9-tetrahydrocannabinol on behavioral despair and on pre- and postsynaptic serotonergic transmission

Preclinical and clinical studies suggest that direct and indirect cannabinoid agonists, including enhancers of endocannabinoids, engender stress-relieving, anxiolytic and antidepressant effects, mediated by central CB(1) receptors (CB(1)Rs). The effect of the main pharmacologically active principle in cannabis, (-)-trans-Δ(9)-tetrahydrocannabinol (delta-9-THC), on depressive behavior and on the serotonin (5-HT) system, which is implicated in the mechanism of action of antidepressants, has not been extensively clarified. Here, we showed that repeated (5 days), but not single (acute) intraperitoneal (ip) treatment with delta-9-THC (1mg/kg) exerts antidepressant-like properties in the rat forced swim test (FST). This effect was CB(1)R-dependent because it was blocked by the CB(1)R antagonist rimonabant (1mg/kg, ip). Using in vivo electrophysiology, we demonstrated that delta-9-THC modulated dorsal raphe (DR) 5-HT neuronal activity through a CB(1)R-dependent mechanism. Acute intravenous delta-9-THC administration (0.1-1.5mg/kg) elicited a complex response profile, producing excitatory, inhibitory and inert responses of 5-HT neurons. Only excitatory responses were blocked by rimonabant. Finally, repeated but not single delta-9-THC administration (1mg/kg, ip) enhanced tonic 5-HT(1A) receptor activity in the hippocampus, a postsynaptic event commonly elicited by standard antidepressants. These results suggest that delta-9-THC, like other CB(1)R agonists and endocannabinoid enhancers, may possess antidepressant properties at low doses, and could modulate 5-HT transmission in the DR and hippocampus as standard antidepressants such as selective serotonin reuptake inhibitors.