The Therapeutic Potential of theEndocannabinoid System for theDevelopment of a Novel Class ofAntidepressantsMatthew N. Hill1,2, Cecilia J. Hillard3, Francis R. Bambico4, Sachin Patel3,5,Boris B. Gorzalka2 and Gabriella Gobbi4

1 Laboratory of Neuroendocrinology, The Rockefeller University, New York, NY, USA2 Department of Psychology, University of British Columbia, Vancouver, B.C. Canada3 Department of Pharmacology and Toxicology, Medical College of Wisconsin, Milwaukee, WI, USA4 Neurobiological Psychiatry Unit, Department of Psychiatry and McGill University Health Center, McGill University,

Montreal, QC, Canada5 Department of Psychiatry, Vanderbilt University, Nashville, TN, USA

Review

The endocannabinoid system is a neuromodulatory sys-tem which is known to regulate emotional, cognitive,neurovegetative and motivational processes. Substan-tial evidence has accumulated implicating a deficit inendocannabinoid in the etiology of depression; accord-ingly, pharmacological augmentation of endocannabi-noid signaling could be a novel target for thepharmacotherapy of depression. Within preclinicalmodels, facilitation of endocannabinoid neurotrans-mission evokes both antidepressant and anxiolyticeffects. Similar to the actions of conventional anti-depressants, enhancement of endocannabinoid sig-naling can enhance serotonergic and noradrenergictransmission; increase cellular plasticity and neurotro-phin expression within the hippocampus; and dampenactivity within the neuroendocrine stress axis. Further-more, limbic endocannabinoid activity is increased byboth pharmacological and somatic treatments fordepression, and, in turn, appears to contribute to someof the neuroadaptive alterations elicited by these treat-ments. These preclinical findings support the rationalefor the clinical development of agents which inhibit thecellular uptake and/or metabolism of endocannabinoidsin the treatment of mood disorders.

The Endocannabinoid SystemThe endocannabinoid system is a modulatory system pre-sent in both the brain and the periphery. At the signalinglevel, two cannabinoid receptors have been characterizedto date [1]. The cannabinoid CB1 receptor is expressed athigh density throughout the brain [2,3] and is also presentat lower density in peripheral tissues, including the liver,adipocytes, the exocrine pancreas, the GI tract, skeletalmuscle and circulating immune cells [4]. The CB2 receptoris located predominately in peripheral immune cells andorgans in physiological conditions [5], but it is also

Corresponding author: Gobbi, G. (gabriella.gobbi@mcgill.ca).

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expressed by microglial cells in injured, infected orinflamed CNS tissue [6]. There is recent evidence thatcannabinoid CB2 receptors exhibit limited neuronalexpression [7,8]. Both CB1 and CB2 are G-protein coupledreceptors. The CB1 receptor couples to both Gi/o proteinswhich function to inhibit adenylyl cyclase activity, activatepotassium channels and inhibit voltage-gated calciumchannels, while the CB2 receptor is only known to coupleto Gi proteins [1]. The CB1 receptor is located predomi-nantly on presynaptic axon terminals, and is capable ofregulating calcium influx, and hence neurotransmitterrelease [9].

The endogenous ligands for cannabinoid receptors arethe arachidonate derived molecules N-arachidonylethano-lamine (anandamide; AEA) and 2-arachidonylglycerol (2-AG) [9–11]. It is believed that both AEA and 2-AG areformed from phospholipid precursors post-synapticallythrough activity-dependent activation of specific phospho-lipase enzymes [10]. Termination of endocannabinoid sig-naling occurs through a re-uptake mechanism andenzymatic hydrolysis; fatty acid amide hydrolase (FAAH)is the primary catabolic enzyme of AEA, and monoacylgly-ceride lipase (MAG lipase) is the primary catabolic enzymefor 2-AG [10].

While it is not known why there are two endogenousligands for one receptor, these molecules do exhibit slightpharmacokinetic differences which could result in differ-ential signaling patterns. Specifically, AEA exhibits a highaffinity for the CB1 receptor (approximately 50-100 nM),but has poor efficacy as an agonist at inducing intracellularsignal transduction [11]. By contrast, 2-AG has a loweraffinity for the CB1 receptor (approximately 1-10 mM), butis very potent, inducing a robust intracellular response asassessed by CB1 receptor mediated guanonucleotideexchange [11]. Thus, it is possible that 2-AG induces arapid and robust CB1 receptor response, while AEA evokesmore of a tonic, but mild, stimulation of the CB1 receptor,as we have previously suggested [12]. This phenomenon

– see front matter � 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tips.2009.06.006

Box 1. Major depression and antidepressants.

The Diagnostic and Statistical Manual of Mental Disorders IV-TR

(2000) [107] defines the symptomatology of major depressive

disorder as involving a complex mix of cognitive, affective, vegeta-

tive, somatic and neuroendocrine manifestations that include

depressed mood, diminished interest or pleasure in nearly all day-

to-day activities, significant weight loss or weight gain, insomnia or

hypersomnia, psychomotor agitation or retardation, fatigue, feelings

of worthlessness or guilt, diminished ability to think and concentrate,

and suicidal ideation. Anxiety is also a common feature of depressive

symptomatology. The prevalence of major depression in population

is estimated about 3.3-6.5 % of the population with the higher

prevalence in women (NIH, Health Canada). Approximately half of

them undergo a second episode, and more episodes increases the

risk of relapse to a high 90%. The condition is extremely disabling and

at its worst, can lead to premature fatality, of which it ranks as a

leading cause just second to cardiovascular disease.

The research on new antidepressant drug targets represents a

major challenge not only for the high prevalence of the disease,

but also because the antidepressants currently available are

effective in a limited percentage of patients following large clinical

trial reports. In the first level of the Sequenced Treatment

Alternatives to Relieve Depression (STAR*D) trial, only about 30%

of patients were in remission following a 12-week therapy with the

selective serotonin receptor inhibitor (SSRI) citalopram [108]. In

addition, 15.8% of patients developed an intolerable adverse event,

38.6% moderate-to-severe impairment due to an adverse event,

8.6% discontinued treatment due to adverse events, and 4%

developed a serious adverse event, findings that underscore

efficacy and tolerability limitations of treatment with a typical

first-line antidepressant agent. The same study showed that

people without a good response to the first antidepressant, that

received the addition of a second antidepressant or cognitive

psychotherapy, still had a low rate of clinical improvement

[109]. These data clearly indicate the importance and emergency

to find novel therapeutic targets for the treatment of major

depression.

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may be particularly relevant for the induction of retrogradeendocannabinoid signaling following depolarization, inwhich the current body of evidence argues that 2-AG,and not AEA, is the primary ligand involved in activity-induced synaptic plasticity. However, the differential pat-tern of signaling remains to be better elucidated.

Endocannabinoids: A Role in Mood Regulation?The idea that the endocannabinoid system may beinvolved in emotional behavior, and in particular depres-sion (see Box 1 for summary of depression and anti-depressants), is rooted in the fact that consumption ofcannabis sativa in humans has profound effects uponmood. This system was first characterized as the targetof the psychoactive constituent of cannabis sativa, D9-tetrahydrocannabinol (THC). For centuries, culturesaround the world have used cannabis sativa for its moodelevating and euphoric effects [13]. A large scale epi-demiological study found that frequent users of cannabisexhibit less depressed mood and more positive affect thannon-consumers of cannabis [14], and case study reportssupport that cannabis use exerts antidepressant effects insome clinically depressed individuals [15]. Given that thepsychoactive effects of cannabis consumption aremediated by activation of the CB1 receptor [16], thesefindings suggest the hypothesis that facilitation of CB1

receptor signaling promotes positive mood and couldthereby exert antidepressant effects.

The cannabinoid receptor and enzymes involved in thesynthesis and degradation of endocannabinoid ligands areprevalent throughout the neuroanatomical structures andcircuits implicated in depression, including the prefrontalcortex, hippocampus, amygdala, hypothalamus and fore-brain monoaminergic circuits [2] (Figure 1). Furthermore,the generation of transgenic mice lacking the CB1 receptorhighlighted the role of this signaling system in mood andemotional regulation, as deletion of this receptor producesa phenotype that is strikingly similar to the symptomaticprofile of severe, typical depression [17]. Specifically, thesetransgenic mice exhibit notable increases in anxiety anddepressive-like behaviors, impairments inmotivated beha-vior and reward salience, altered neurovegetative func-tioning and cognitive deficits in higher order tasks [17].

Furthermore, CB1 receptor-null mice exhibit increasedsusceptibility to the anhedonic effects of chronic stress[18] and an increased neuroendocrine response to stress[19,20]. Collectively, these data indicate that the endocan-nabinoid system is widely distributed throughout the braincircuitry involved in emotional processing, and lead to thehypothesis that deficits in endocannabinoid signaling pro-duce a depressive-like profile.

Endocannabinoids and Depression: Preclinical andClinical StudiesThe endocannabinoid deficiency hypothesis predicts thatimpairments in endocannabinoid signaling will occur indepression; both rodent model and human studies supportthis hypothesis. Chronic stress has been shown to producebehavioral and neurochemical changes in rodents thatmodel those in human depression. Chronic, mild stressresults in widespread reductions in AEA concentrationsthroughout the brain; together with reductions in CB1

receptor binding site density and/or signal transductionin subcortical structures such as the hippocampus, hypo-thalamus and striatum [21–23]. These reductions in endo-cannabinoid signaling, in turn, have been found tocontribute to alterations in reward salience and cognitiveprocessing [21,24,25], indicating that stress-induced down-regulation of limbic endocannabinoid signaling has func-tional manifestations that are relevant to behavioralcharacteristics of human depression.

In humans, there are several lines of evidence thatendocannabinoid signaling is an important regulator ofmood and emotions. First, clinical trials of the CB1 receptorantagonist, rimonabant, for the treatment of obesityrevealed significantly greater numbers of anxiety anddepression adverse events in patients taking rimonabantcompared to those taking placebo [26]. This effect was ofgreat enough concern that it resulted in the suspension ofRimonabant both within North America and Europe [27].This finding also suggests that tonic endocannabinoidsignaling regulate mood and anxiety in a subset of humanpopulation [27]. Second, circulating levels of endocannabi-noid ligands have been found to be significantly reduced intwo independent populations diagnosed with majordepression [28,29]. Third, recent genetic studies have

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Figure 1. CB1 receptors are located in areas involved in the control of mood.

Glutamatergic pyramidal neurons of the prefrontal cortex (PFC), expressing CB1 receptors (CB-1), send their axons to serotonergic (5-HT) neurons and GABAergic

interneurons of the Dorsal Raphe (DR) nucleus (see large window). The activation of both of them increases the 5-HT firing activity and the 5-HT release. In their turn, 5-HT

neurons innervate the amygdala, PFC and hippocampus, which are brain areas strictly involved in depressive symptoms. Hyppocampus sends afferences to the Pituitary

gland regulating the Hypothalamus-Pituitary-adrenal axis (HPA) notably perturbed in depression. CB1 receptors are also expressed in the noradrenergic (NE) neurons of the

Locus Coerules (LC) and in the nucleus prepositus hypoglossi (NPH) whose axons terminate on LC NE neurons (see large window). In its turn, LC innervates DR as well as

Hyppocampus, PFC, Amygdala (afferences not shown). Dopaminergic (DA) neurons of the Ventral tegmental area (VTA) send also their axons to the DR.

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identified that individuals with certain CB1 receptorhaplotypes have increased vulnerability to depression fol-lowing adverse life events [30] and increase the risk ofantidepressant resistance, by modulating subcorticalresponsiveness to social reward stimuli [31]. Collectively,

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these studies support the hypothesis that deficient endo-cannabinoid signaling in humans is sufficient to promotethe development of depressive illness and that impair-ments in this system are present in at least a subset ofdepressed individuals.

Box 2. Monoamines and major depression

Although achieved through different mechanisms, all antidepres-

sant drugs ultimately increase 5-HT neurotransmission. This

suggests that antidepressants ameliorate the underactivity of the

5-HT system associated with depression or augment the function of

intact monoamine systems that convey antidepressant effects.

Similarly after chronic treatment, antidepressants decrease or

increase norepinephrine neurotransmission, which has been hy-

pothesized to be related to the anxioloytic or anti-fatigue effects,

respectively (for more details see 32, 110).

The ability of antidepressant treatments to increase 5-HT neuro-

transmission is achieved through various possible mechanisms. For

example, the Selective Serotonin Reuptake Inhibitors (SSRIs) at first

increase the release of 5-HT in the synaptic cleft, blocking the 5-HT

transporter, decreasing 5-HT firing through the activation of 5-HT1A

autoreceptor (that exert a negative feed-back). After at least two

weeks, the autoreceptors are desensitized and the 5-HT neurotrans-

mission is potentiated. This neurobiological delay in the potentia-

tion of 5-HT neurotransmission has been correlated with the clinical

delay of response to SSRI treatment. Other classes of antidepres-

sants, such as the alpha-2 blocker mirtazapine, increase 5-HT firing

since they block the terminal alpha 2 receptors, located in the

norepinephrine terminal projecting to the dorsal raphe (source of 5-

HT neurons), leading to a stimulation of 5-HT firing through alpha-1

receptors that are stimulatory receptors located on 5-HT neurons.

Concurrently, blockade of the alpha-2 autoreceptor located on the

noradrenergic soma, increases noradrenergic firing activity [32,110].

For all these reasons, it is important to test the effects of

antidepressants in 5-HT and NE neurotransmissions in the assess-

ment of novel antidepressant targets such as endocannabinoids

enhancers.

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Endocannabinoid Neurotransmission and theBiochemical Signatures of AntidepressantsMonoaminergic Neurotransmission

All known agents and regimens that provide effectivetreatment for depression increase central monoaminergic(i.e. norepinephrine (NE) and serotonin (5-HT)) neurotrans-mission (see Box 2 for further description and Table 1).Several classes of antidepressant drugs inhibit the synapticre-uptake or enzymatic metabolism of monoamines, whilesomatic treatments such as electroconvulsive shock orvagus nerve stimulation increase monoaminergic synaptictransmission through an indirect mechanism [32]. Several

Table 1. Effects of different classes of antidepressant treatment onhippocampal 5-HT1A receptors, in comparison with endocannabin

Antidepressant class ACUTE

DR 5-HT

firing

CHRONIC

DR 5-HT

firing

ACUTE

LC NE

firing

MAOI # $ (recovered)* #TCA $# $ (recovered) * #SSRI # $ (recovered) * $NRI $ $ #SNRI # $ (recovered) * #Bupropion #$ #$ #Mirtazapine " " "NK1 antagonist " " "Sigma ligands $ " n.d.

ECS $ $ $VNS $ " "CB1 agonists "#** # "FAAH inhibitor URB597 " " "

MAOI: monoamine oxidase inhibitors; TCA: tricyclic antidepressants; SSRI: selective sero

norepinephrine reuptake inhibitors; NK1: Neurokinin 1 receptors; ECS; electroconvulsive

determined.

* after chronic treatment, the initial decrease in 5-HT firing recovers due to a desensitizati

doses (33).

studies demonstrate that activation of cannabinoid recep-tors increasesmonoaminergic neurotransmission (Table 1).Both direct CB1 receptor agonists and inhibitors of AEAhydrolysis increase the firing activity of neurons in thedorsal raphe, the major source of 5-HT neurons [33–35].While CB1 receptors are expressed within the dorsal raphe[36], combined trans-sectional and microinjection studieshave revealed that activation of CB1 receptors within theprefrontal cortex increase neuronal activity of dorsal rapheneurons throughamulti-synaptic circuit linking thesebrainstructures [33]. Support for this notion comes from studiesdemonstrating that antidepressant-like behavioralresponses evoked by activation of CB1 receptors in theprefrontal cortex require intact serotonergic signaling[33]. Furthermore, CB1 receptors are required for seroto-nergic mediated negative feedback and deletion of the CB1

receptor blunts the increase in synaptic serotonin followingadministration of an SSRI [37].

Cannabinoid receptor activation has also been found toincrease firing activity of neurons in the locus coeruleus,the major source of noradrenergic neurons, and con-sequently the NE efflux [34,38,39], in the forebrain(Figure 1). Unlike the effects on dorsal raphe neuronalfiring, this effectmay be due to an ability of cannabinoids todirectly increase the firing activity of locus coeruleusneurons [38].

CB1 receptor agonists also inhibit the reuptake ofmonoamines. In synaptosomal preparations, cannabi-noids have been found to inhibit the reuptake of serotonin,norepinephrine and dopamine [40,41]. A recent reportextends these findings by revealing that cannabinoidscan similarly inhibit the reuptake of serotonin by humanplatelets [42]. This indicates that cannabinoids sharesome integral pharmacological properties of conventionalantidepressants.

Several class of antidepressant drugs desensitize ordown-regulate the serotonin type 2A (5-HT2A) receptors[43] and drugs that block these receptors, such asthe atypical antipsychotics quetiapine or olanzapine, arecurrently used to potentiate the effect of antidepressants.

serotonin (5-HT) and norepinephrine (NE) firing activity, and onoid enhancers. Modified from [32,109].

CHRONIC

LC NE firing

Tonic activation

of hippocampal

5-HT1A heteroreceptors

Hippocampal

Neurogenesis

# " "# " "# " "# " n.d.

# " "" (bursts) " "" " "" (bursts) " "n.d. n.d. n.d.

# " "" n.d. n.d.

" " "" " "

tonin re-uptake inhibitors; NRI: norepinephrine reuptake inhibitors; SNRI: selective

shocks; VNS: vagus nerve stimulation; FAAH: fatty acid amide hydrolase; n.d.: not

on of 5-HT1A autoreceptors (see Box 2); ** increase at low doses, decrease at higher

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Table 2. Antidepressant and Anxiolytic Effects Detected Through the Inhibition of Endocannabinoid Uptake or Metabolism

Tests of Antidepressant Efficacy

Drug Dose Species Test Reference

URB597 0.1 and 0.3 mg/kg Mouse Tail Suspension Test [34]

URB597 0.1 mg/kg Mouse Tail Suspension Test [65]

URB597 0.1 and 0.3 mg/kg Rat Forced Swim Test [34]

URB597 0.1 and 0.3 mg/kg Rat Forced Swim Test [63]

URB597 0.1 and 0.3 mg/kg Rat (Female) Forced Swim Test [64]

URB597 0.3 mg/kg Rat Stress-induced Anhedonia [24]

URB597 0.3 mg/kg Mouse Stress-induced Anhedonia [25]

AM404 5 mg/kg Rat Forced Swim Test [61]

AM404 0.3, 1 and 3 mg/kg Rat Forced Swim Test [63]

Genotype Test Reference

FAAH knockout mice Tail Suspension Test [32]

Tests of Anxiolytic Efficacy

Drug Dose Species Test Reference

URB597 0.1 mg/kg Mouse Elevated Plus Maze [65]

URB597 0.1 and 0.3 mg/kg Mouse Elevated Plus Maze [67]

URB597 1 mg/kg Mouse Elevated Plus Maze [70]

URB597 1 mg/kg Mouse Elevated Plus Maze [72]

URB597 0.1 and 0.3 mg/kg Rat (Female) Elevated Plus Maze [64]

URB597 0.1 mg/kg Rat Elevated Zero Maze [66]

URB597 0.1 and 0.3 mg/kg Hamster Elevated Plus Maze [71]

URB597 0.1 and 0.3 mg/kg Rat Light Dark Test [73]

URB597 0.1 mg/kg Rat (Pup) Isolation-induced USV [66]

AM404 1 and 3 mg/kg Mouse Elevated Plus Maze [67]

AM404 0.75 and 1.25 mg/kg Rat Elevated Plus Maze [69]

AM404 5 mg/kg Rat Elevated Plus Maze [68]

AM404 1 and 2 mg/kg Mouse Elevated Plus Maze [74]

AM404 5 mg/kg Rat Defensive Withdrawal [68]

AM404 1 and 2 mg/kg Rat (Pup) Isolation-induced USV [68]

AA-5HT 0.1, 0.5, 1, 2.5 mg/kg Mouse Elevated Plus Maze [72]

AACOCF3 4 mg/kg Mouse Light Dark Test [75]

Genotype Test Reference

FAAH knockout mice Elevated Plus Maze [70]

FAAH knockout mice Light-Dark Test [70]

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Interestingly, cannabinoids (including AEA), reduce ligandbinding to the5-HT2Areceptor [44]; reduce5-HT2Amediatedsignal transduction [45] and 5-HT2A receptor-mediatedbehavioral responses [46,47]. Moreover, another commonfeature of antidepressant drugs is the enhancement of thetonic activity of hippocampal 5-HT1A receptors, which med-iate hippocampal neurogenesis [32] (Table 2). Thus, there isevidence that agents which enhance endocannabinoid sig-naling could produce all of the desired pharmacologicalproperties of conventional antidepressants onmonoaminer-gic signaling and share the pharmacological attributes ofdrugs used to augment the efficacy and dampen adverseeffects of SSRIs.

The Hypothalamic-Pituitary-Adrenal (HPA) Axis

A large body of research supports a strong causativerelationship between hyperactivity of the HPA axisand depression in a subset of depressed humans [48].Conventional antidepressants dampen HPA axis acti-vation in response to aversive stimuli, supporting thecausative link [49]. Over the past few years, endocanna-binoid activation of CB1 receptors has been shown tofunction as a negative regulator of the HPA axis(Figure 1). For example, impairments in CB1 receptoractivity increase HPA axis activity and enhancement ofendocannabinoid neurotransmission decreases HPA axis

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responses to stress [19,50,51]. Rodent data indicate thatincreased limbic endocannabinoid signaling is a necessarycomponent of themechanismbywhich tricyclic antidepress-ants attenuate stress-induced activation of the HPA axis[52]. Therefore, there is an overlap in the effects of theendocannabinoids and some antidepressants on HPA axisactivation; and increased endocannabinoid signaling couldcontribute to the mechanism by which antidepressantsmodulate HPA axis function.

Neurotrophins and Neurogenesis in Hippocampus

Volumetric studies have revealed significant atrophy of thehippocampus in depressed patients [53] and it is wellestablished that all classes of antidepressants increasehippocampal neurotrophin expression and neurogenesis[54]. These and other data have lead to the hypothesisthat antidepressants support neurogenesis and cellularresilience in the hippocampus, thus curbing the neurode-generation that can occur in this disease [54].

There is evidence that endocannabinoid signaling in thehippocampus promotes both neurotrophin expression andneurogenesis. CB1 receptor activation with exogenous ago-nists can induce brain-derived neurotrophic factor (BDNF)expression and promote hippocampal neurogenesis[55,56]. Genetic deletion of FAAH, which increasesendogenous AEA signaling [57], increases cell proliferation

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in the hippocampus [58] (Table 2). In particular, it has beenreported that the CB1 agonist HU210 promotes the genesisof new neurons (neural stem/progenitor cell, NS/PCs) andnot gliogenesis [56], but other authors have reported thatWIN 55,212-2 as well as URB 597 and anandamide mayincrease both BrdU+ cells (neurons) as well as nestin+ cells(non-neurons) [58]. This difference could be linked to thedifferent treatment schedule and drugs used.

Conversely, deficiencies in endocannabinoid signalingare associated with impaired cell proliferation, neurogen-esis and reduced levels of BDNF [20,58,59]. Thus, endo-cannabinoid signaling mimics the effects of conventionalantidepressants on neurotrophic signaling and neurogen-sis in the hippocampus. It is not known whether augmen-tation of endocannabinoid signaling is sufficient to reversethe suppression of BDNF or neurogenesis in an animalmodel of depression. However, recent data indicate thatthe mechanism of voluntary exercise to increase cell pro-liferation in the hippocampus (which is associated with theantidepressant effect of this regimen) requires an increasein hippocampal endocannabinoid signaling [60].

Endocannabinoid Signaling and Preclinical Tests ofAntidepressant EfficacyWhereas deficient endocannabinoid signaling has beenfound to produce a behavioral phenotype reminiscent ofdepression in rodents, pharmacological augmentation ofthis system has been shown to evoke antidepressantbehavioral responses in several rodent paradigms. Directpharmacological activation of the cannabinoid CB1 re-ceptor produces antidepressant behavioral responses inthe forced swim test under both acute [33,61–63] andchronic [56] dosing conditions. Similarly, inhibition ofAEA hydrolysis or endocannabinoid uptake have beenfound to produce antidepressant responses in the forcedswim test in both male [34,61,63] and female [64] rodents(see Table 2). Analogous antidepressant-like responseshave also been found following inhibition of AEAhydrolysis and genetic deletion of the FAAH gene (whichresults in tonically elevated AEA levels in the brain [57])in the tail suspension test, a rodent model that examinesbehavioral coping responses to stress [34,65] (Gobbiet al., unpublished findings; see Table 2). These findingshave recently been extended by the documentation thatlong-term inhibition of AEA hydrolysis reverses the de-velopment of anhedonia following chronic stress in amanner comparable to a conventional antidepressant[24,25] (see Table 2).

In addition to antidepressant efficacy in preclinicalmodels, repeated studies have demonstrated that pharma-cological facilitation of endocannabinoid signaling pro-duces anxiolytic effects (see Table 2). Specifically,inhibition of endocannabinoid uptake, inhibition of AEAhydrolysis and genetic deletion of FAAH all produce sig-nificant reductions in rodent indices of anxiety in tests suchas the elevated plus maze and light-dark box tests [64–75].Similarly, the extinction of aversive emotional memorieshas also been linked to an increase in amygdalar endo-cannabinoid signaling [76] and inhibition of endocannabi-noid uptake can aid in the clearance of emotionallyaversive memories [77,78].

The employment of intracerebral microinjection studieshas furthered our understanding of the neural circuitsubserving the ability of cannabinoid receptor stimulationto produce antidepressant and anxiolytic behavioralresponses. Local administration of CB1 receptor ligandsinto the ventromedial prefrontal cortex has been found toproduce both antidepressant and anxiolytic behavioralresponses [33,79]. Similarly, inhibition of AEA hydrolysiswithin the prefrontal cortex exclusively reduces anxiety-like behaviors in rodents [80]. Additionally, facilitation ofcannabinoid receptor signaling in the hippocampusappears to produce both antidepressant and anxiolyticbehavioral responses [79,81].

Paradoxically, a few preclinical studies have alsoreported that CB1 receptor antagonists, alone or in com-bination with conventional antidepressants, [82–84] haveantidepressant-like effects and increase monoaminesrelease. While these data suggest that high doses of CB1

antagonists may have antidepressant properties in someanimal paradigms, they are inconsistent with the results ofhuman clinical studies. As mentioned, administration ofCB1 receptor antagonists for the treatment of obesityresulted in the development of indices of anxiety anddepression in a large proportion of individuals (approxi-mately 1 in 13 individuals on the CB1 antagonist rimona-bant relative to the 1 in 47 individuals on placebo in theSTRADIVARIUS trial [26]). As far, there are no documen-ted reports of antidepressant responses in humans follow-ing administration of CB1 receptor antagonists, whilethere are several observations reporting an elevated moodand reduced depression following short-term cannabisconsumption [14,15], that is blocked by CB1 antagonism[16].

In summary, pharmacological augmentation of endo-cannabinoid neurotransmission produces antidepressantand anxiolytic effects. The current data are limited to theeffects of increased AEA signaling as FAAH inhibition hasbeen the only pharmacological tool available to date toexamine the in vivo hydrolysis inhibition of endocannabi-noids. However, with the recent development of JZL-184, aselective inhibitor of MAG lipase [85], it may soon bepossible to learn if inhibition of 2-AG hydrolysis producescomparable effects on emotional behavior. These effectsappear to be mediated primarily by activation of CB1

receptors within the prefrontal cortex and the hippo-campus; however, further studies are required to deter-mine if other neural circuits are involved in thesebehavioral phenomena. Given the high prevalence ofco-morbidity of depression and anxiety, therapeutic agentswith both antidepressant and anti-anxiety efficacy areparticularly useful. Preclinical studies suggest thatactivators of endocannabinoid signaling could have thesedesired therapeutic characteristics in humans.

Regulation of Endocannabinoid Signaling byAntidepressant TreatmentsEvidence for a role of endocannabinoid signaling in theregulation of mood begs the question of whether currentlyused antidepressant therapies enhance endocannabinoidsignaling. The currently available data regarding thisquestion indicate that some conventional antidepressant

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treatments do enhance endocannabinoid signaling in somebrain regions; however, increased endocannabinoid sig-naling is not a common feature of all antidepressanttherapies. Chronic treatment of rats with the tricyclicantidepressant desipramine increased CB1 receptor bind-ing in the hippocampus and hypothalamus [52], while anincrease in CB1 receptor binding in the amygdala was seenfollowing treatment with imipramine [22]. Neither of theseantidepressants altered endocannabinoid ligand content[22,52]. The monoamine oxidase inhibitor, tranylcypro-mine, also increased CB1 receptor binding in the hippo-campus and prefrontal cortex, but reduced AEA content inthese same structures [86]. Prolonged treatment of ratswith fluoxetine, a selective serotonin reuptake inhibitor(SSRI), increased CB1 receptor binding and CB1 receptorsignal transduction within the prefrontal cortex [86]; how-ever, fluoxetine and a second SSRI, citalopram, have beenfound to reduce CB1 receptor transcription and signaltransduction, respectively, within the caudate-putamenand hypothalamus [87,88].

The effects of somatic regimens used to treat depressionon endocannabinoid signaling have also been studied inrodents. Electroconvulsive shock treatment, a highly effec-tive treatment for depression, significantly increases CB1

receptor mediated signal transduction in the amygdala[89]. Sleep deprivation, which provides rapid, but transi-ent, alleviation of depressive symptoms increases 2-AGcontent in the hippocampus [90]. Moreover, voluntaryexercise, which has found to be as effective as chemicalantidepressant treatment in clinical trials, dramaticallyup-regulates CB1 receptor binding, G protein signaling andAEA content within the hippocampus [60].

While some inconsistencies do exist in the literature,there is strong evidence from rodent studies that conven-tional treatments for depression enhance endocannabinoidsignaling in the corticolimbic structures that are involvedin mood regulation and depression. Given the ability ofendocannabinoid signaling in these structures to produceantidepressant behavioral responses, it is possible thatincreased endocannabinoid signaling could contribute totheir effects on mood and emotion.

Potential warnings and Side effectsWhile targeting the endocannabinoid system for the de-velopment of a novel class of antidepressant drugs clearlyhas its merits, there are several potential warnings andcaveats. First and foremost, the efficacy, tolerability andsafety of agents which increase endocannabinoid signalingmust be established in human studies. The current datademonstrated that there is a convincing amount of pre-clinical evidence to indicate that the clinical efficacy ofthese compounds should be examined; however, to datethere are not yet clinical data demonstrating that theinhibition of endocannabinoid uptake or hydrolysis is effec-tive in treating mood disorders or is free of adverseresponses following long-term administration.

Specifically, while these preclinical data on direct CB1receptor agonist may suggest the hypothesis that theymight be a useful class of pharmacologic drugs for thetreatment of depression, the adverse effect profile of thesedrugs is likely unacceptable. For example, there is every

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reason to expect that CB1 receptor agonists will mimic thepsychoactive effects and impairments in cognition, bal-ance, and time perception that occur with THC exposure[91]. In addition, it is possible that CB1 receptor agonistswould have also abuse liability [92] and could producepsychotic-like symptoms and increase panic and paranoiain some individuals [91,93]. On the other hand, it is likelythat indirect agonists of the CB1 receptor, such as inhibi-tors of FAAH or MGL inhibitors, will not share the psy-choactive or adverse effects of THC. Indeed, animal studiesof cannabinoid reinforcement suggest that, unlike canna-bis, FAAH inhibitors lack reinforcing properties in non-human primates and rodents [34,92]. Furthermore, FAAHinhibitors do not appear to exert comparable adverseeffects on cognition as THC [94,95]. However, very highdoses of FAAH inhibitors can increase anxiety levels[73,80], an effect which is believed to be through thepromiscuous activation of AEA at TRPV1 (transient re-ceptor potential, vanilloid subfamily, member 1) receptors[80]. Interestingly, it has been recently suggested that thecombined FAAH inhibitors/TRPV1 antagonists, are moreeffective in the reduction of anxiety than FAAH inhibitorsalone [72]. The recent discovery of allosteric modulators ofCB1 receptor signaling [96] could also represent a validalternative, although this pharmacological strategy stillrequires more research to be validated.

Endocannabinoids are also known to be involved innumerous developmental processes, ranging from blasto-cyst implantation [97] to neuronal development and axonalguiding [98,99], suggesting that agents which increaseendocannabinoid signaling should not be employed duringperiods of critical development, such as during pregnancyor prior to the conclusion of adolescence.

An additional potential warning that may arise with theuse of FAAH is the fact that FAAHmediates the hydrolysisof not only AEA, but other fatty acid ethanolamides, suchas palmitoylethanolamide (PEA) and oleoylethanolamide(OEA), thus the inhibition of FAAH results in the elevationof these compounds as well [71,100]. However, the non-specificity of this response is not necessarily negative asPEA and OEA are known to exert potent anti-inflamma-tory properties, reduce appetite and promote energyexpenditure [101–103], all of which could be beneficial inpatients with somatic depressive symptoms such as hyper-phagia or pain and inflammatory diseases.

An analysis of all substrates of both FAAH and MAGlipase is required to understand the putative effects ofthese agents following inhibition of their hydrolysis. Inline with this, it should be noted that a second FAAHenzyme (FAAH-2) has recently been discovered in placen-tal mammals, including humans [104]. These enzymeshave been found to have slightly different substrate selec-tivity, in that FAAH-2 was found to hydrolyze AEA at alower rate than FAAH (or FAAH-1) [104]. Accordingly,these distinct enzymes should be taken into considerationin the development of FAAH inhibitors, such that prefer-ential inhibition of FAAH-1 would theoretically result in agreater induction of AEA than FAAH-2.

However, it is worth noting that conventional anti-depressants exhibit their own plethora of adverse sideeffects, which are not necessarily present in agents which

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increase endocannabinoid signaling. For example, inhi-bition of FAAH does not induce impairments in malesexual activity [105], as reported with many current anti-depressants [106].

Only extensive clinical trials will be able to assess thesecondary as well as adversive effects of endocannabinoidsenhancers in a large population scale.

Concluding RemarksThe endocannabinoid system is widely distributed through-out the corticolimbic circuits in the brain that are implicatedin the etiology and treatment of depressive illness. Impair-ments in endocannabinoid signaling produce behavioraldisturbances reminiscent of the symptom clusters associ-ated with depression and both rodent models of depressionand human subjects diagnosed with major depression exhi-bit hypoactive endocannabinoid signaling. On the otherhand, augmentation of endocannabinoid signaling evokesantidepressant and anxiolytic-like behavioral responses inrodent models. Similarly, several pharmacological andsomatic treatment regimens for depression increase endo-cannabinoid neurotransmission and, in some cases, thisincrease is required for the neurobiological adaptationselicited by these treatments. Preclinical data suggest thataugmentation of endocannabinoid signaling is capable ofproducingall of themajorneurochemical changes elicited byconventional antidepressants such as the increase of ser-otonergic and noradrenergic neurotransmission, enhance-ment of hippocampal 5-HT1A receptor tonic activity, andneurogenesis promotion. Increased endocannabinoid sig-naling can reduce signaling through the 5HT2A receptor,which is a mechanism shared by some adjunctive treat-ments for depression. Collectively, these data argue thatpharmacological agents, which enhance endocannabinoidsignaling, could prove to be a novel class of antidepressantdrug, with equal or greater efficacy than drugs which arecurrently available. Clinical trials seem the next logical stepand will help to establish the clinical validity of these novelagents for the treatment of depressive disorders.

References1 Howlett, A.C. (2002) The cannabinoid receptors.Prostaglandins Other

Lipid Mediat 68–69, 619–6312 Herkenham, M. et al. (1991) Characterization and localization of

cannabinoid receptors in rat brain: a quantitative in vitroautoradiographic study. J Neurosci 11, 563–583

3 Cristino, L. et al. (2006) Immunohistochemical localization ofcannabinoid type 1 and vanilloid transient receptor potentialvanilloid type 1 receptors in the mouse brain. Neuroscience 139,1405–1415

4 Matias, I. et al. (2006) Endogenous cannabinoids in the brain andperipheral tissues: regulation of their levels and control of food intake.Int J Obes (Lond) 30 (Suppl 1), S7–S12

5 Munro, S. et al. (1993) Molecular characterization of a peripheralreceptor for cannabinoids. Nature 365, 61–65

6 Benito, C. et al. (2008) Cannabinoid CB2 receptors in human braininflammation. Br J Pharmacol 153, 277–285

7 Gong, J.P. et al. (2006) Cannabinoid CB2 receptors:immunohistochemical localization in rat brain.Brain Res 1071, 10–23

8 Van Sickle, M.D. et al. (2005) Identification and functionalcharacterization of brainstem cannabinoid CB2 receptors. Science310, 329–332

9 Freund, T.F. et al. (2003) Role of endogenous cannabinoids in synapticsignaling. Physiol Rev 83, 1017–1066

10 Ahn, K. et al. (2008) Enzymatic pathways that regulateendocannabinoid signaling in the nervous system. Chem Rev 108,1687–1707

11 Hillard, C.J. (2000) Biochemistry and pharmacology of theendocannabinoids arachidonylethanolamide and 2-arachi-donylglycerol. Prostaglandins Other Lipid Mediat 61, 3–18

12 Gorzalka, B.B. et al. (2008) Regulation of endocannabinoid signalingby stress: implications for stress-related affective disorders. NeurosciBiobehav Rev 32, 1152–1160

13 Williamson, E.M. and Evans, F.J. (2000) Cannabinoids in clinicalpractice. Drugs 60, 1303–1314

14 Denson, T.F. and Earleywine, M. (2006) Decreased depression inmarijuana users. Addict Behav 31, 738–742

15 Gruber, A.J. et al. (1996) Do patients use marijuana as anantidepressant? Depression 4, 77–80

16 Huestis, M.A. et al. (2001) Blockade of effects of smoked marijuana bythe CB1-selective cannabinoid receptor antagonist SR141716. ArchGen Psychiatry 58, 322–328

17 Hill, M.N. and Gorzalka, B.B. (2005) Is there a role for theendocannabinoid system in the etiology and treatment ofmelancholic depression? Behav Pharmacol 16, 333–352

18 Martin, M. et al. (2002) Involvement of CB1 cannabinoid receptors inemotional behaviour. Psychopharmacology (Berl) 159, 379–387

19 Steiner, M.A. et al. (2008) Antidepressant-like behavioral effects ofimpaired cannabinoid receptor type 1 signaling coincide withexaggerated corticosterone secretion in mice. Psychoneuroendocrino-logy 33, 54–67

20 Aso, E. et al. (2008) BDNF impairment in the hippocampus is relatedto enhanced despair behavior in CB1 knockout mice. J Neurochem105, 565–572

21 Hill, M.N. et al. (2005) Downregulation of endocannabinoid signalingin the hippocampus following chronic unpredictable stress.Neuropsychopharmacology 30, 508–515

22 Hill, M.N. et al. (2008) Regional alterations in the endocannabinoidsystem in an animal model of depression: effects of concurrentantidepressant treatment. J Neurochem 106, 2322–2336

23 Rossi, S. et al. (2008) Chronic psychoemotional stress impairscannabinoid-receptor-mediated control of GABA transmission inthe striatum. J Neurosci 28, 7284–7292

24 Bortolato, M. et al. (2007) Antidepressant-like activity of the fatty acidamide hydrolase inhibitor URB597 in a rat model of chronic mildstress. Biol Psychiatry 62, 1103–1110

25 Rademacher, D.J. and Hillard, C.J. (2007) Interactions betweenendocannabinoids and stress-induced decreased sensitivity tonatural reward. Prog Neuropsychopharmacol Biol Psychiatry 31,633–641

26 Nissen, S.E. et al. (2008) Effect of rimonabant on progression ofatherosclerosis in patients with abdominal obesity and coronaryartery disease: the STRADIVARIUS randomized controlled trial.JAMA 299, 1547–1560

27 Hill, M.N. and Gorzalka, B.B. (2009) Impairments inendocannabinoid signaling and depressive illness. JAMA 301,1165–1166

28 Hill, M.N. et al. (2008) Serum endocannabinoid content is altered infemales with depressive disorders: a preliminary report.Pharmacopsychiatry 41, 48–53

29 Hill, M.N. et al. (2009) Circulating endocannabinoids and N-acylethanolamines are differentially regulated in major depression andfollowing exposure to social stress. Psychoneuroendocrinology

30 Juhasz, G. et al. (2009) CNR1 Gene is Associated with HighNeuroticism and Low Agreeableness and Interacts with RecentNegative Life Events to Predict Current Depressive Symptoms.Neuropsychopharmacology

31 Domschke, K. et al. (2008) Cannabinoid receptor 1 (CNR1) gene:impact on antidepressant treatment response and emotionprocessing in major depression. Eur Neuropsychopharmacol 18,751–759

32 Bambico, F.R. andGobbi, G. (2008) The cannabinoid CB1 receptor andthe endocannabinoid anandamide: possible antidepressant targets.Expert Opin Ther Targets 12, 1347–1366

33 Bambico, F.R. et al. (2007) Cannabinoids elicit antidepressant-likebehavior and activate serotonergic neurons through the medialprefrontal cortex. J Neurosci 27, 11700–11711

491

Review Trends in Pharmacological Sciences Vol.30 No.9

34 Gobbi, G. et al. (2005) Antidepressant-like activity and modulation ofbrain monoaminergic transmission by blockade of anandamidehydrolysis. Proc Natl Acad Sci U S A 102, 18620–18625

35 Palazzo, E. et al. (2006) Neuropathic pain and the endocannabinoidsystem in the dorsal raphe: pharmacological treatment andinteractions with the serotonergic system. Eur J Neurosci 24,2011–2020

36 Haring, M. et al. (2007) Identification of the cannabinoid receptor type1 in serotonergic cells of raphe nuclei inmice.Neuroscience 146, 1212–

121937 Aso, E. et al. (2009) Lack of CB(1) receptor activity impairs

serotonergic negative feedback. J Neurochem38 Muntoni, A.L. et al. (2006) Cannabinoids modulate spontaneous

neuronal activity and evoked inhibition of locus coeruleusnoradrenergic neurons. Eur J Neurosci 23, 2385–2394

39 Oropeza, V.C. et al. (2005) Systemic administration of WIN 55,212-2increases norepinephrine release in the rat frontal cortex. Brain Res1046, 45–54

40 Steffens, M. and Feuerstein, T.J. (2004) Receptor-independentdepression of DA and 5-HT uptake by cannabinoids in ratneocortex–involvement of Na(+)/K(+)-ATPase. Neurochem Int 44,529–538

41 Banerjee, S.P. et al. (1975) Cannabinoids: influence on neuro-transmitter uptake in rat brain synaptosomes. J Pharmacol ExpTher 194, 74–81

42 Velenovska, M. and Fisar, Z. (2007) Effect of cannabinoids on plateletserotonin uptake. Addict Biol 12, 158–166

43 Yamauchi, M. et al. (2006) Desensitization of 5-HT2A receptorfunction by chronic administration of selective serotonin reuptakeinhibitors. Brain Res 1067, 164–169

44 Kimura, T. et al. (1998) Anandamide, an endogenous cannabinoidreceptor ligand, also interacts with 5-hydroxytryptamine (5-HT)receptor. Biol Pharm Bull 21, 224–226

45 Boger, D.L. et al. (1998) Structural requirements for 5-HT2A and5-HT1A serotonin receptor potentiation by the biologically active lipidoleamide. Proc Natl Acad Sci U S A 95, 4102–4107

46 Gorzalka, B.B. et al. (2005) Functional role of the endocannabinoidsystem and AMPA/kainate receptors in 5-HT2A receptor-mediatedwet dog shakes. Eur J Pharmacol 516, 28–33

47 Egashira, N. et al. (2004) Anandamide inhibits the DOI-induced head-twitch response in mice. Psychopharmacology (Berl) 171, 382–389

48 Parker, K.J. et al. (2003) Neuroendocrine aspects of hypercortisolismin major depression. Horm Behav 43, 60–66

49 Holsboer, F. and Barden, N. (1996) Antidepressants andhypothalamic-pituitary-adrenocortical regulation. Endocr Rev 17,187–205

50 Patel, S. et al. (2004) Endocannabinoid signaling negativelymodulates stress-induced activation of the hypothalamic-pituitary-adrenal axis. Endocrinology 145, 5431–5438

51 Cota,D. et al. (2007)Requirement of cannabinoid receptor type 1 for thebasal modulation of hypothalamic-pituitary-adrenal axis function.Endocrinology 148, 1574–1581

52 Hill, M.N. et al. (2006) Involvement of the endocannabinoid system inthe ability of long-term tricyclic antidepressant treatment to suppressstress-induced activation of the hypothalamic-pituitary-adrenal axis.Neuropsychopharmacology 31, 2591–2599

53 McKinnon, M.C. et al. (2009) A meta-analysis examining clinicalpredictors of hippocampal volume in patients with majordepressive disorder. J Psychiatry Neurosci 34, 41–54

54 Pittenger, C. and Duman, R.S. (2008) Stress, depression, andneuroplasticity: a convergence of mechanisms. Neuropsychophar-macology 33, 88–109

55 Derkinderen, P. et al. (2003) Regulation of extracellular signal-regulated kinase by cannabinoids in hippocampus. J Neurosci 23,2371–2382

56 Jiang, W. et al. (2005) Cannabinoids promote embryonic andadult hippocampus neurogenesis and produce anxiolytic- andantidepressant-like effects. J Clin Invest 115, 3104–3116

57 Cravatt, B.F. et al. (2001) Supersensitivity to anandamide andenhanced endogenous cannabinoid signaling in mice lacking fattyacid amide hydrolase. Proc Natl Acad Sci U S A 98, 9371–9376

58 Aguado, T. et al. (2005) The endocannabinoid system drives neuralprogenitor proliferation. FASEB J 19, 1704–1706

492

59 Jin, K. et al. (2004) Defective adult neurogenesis in CB1 cannabinoidreceptor knockout mice. Mol Pharmacol 66, 204–208

60 Hill, M.N., Titterness, A.K., Morrish, A.C., Carrier, E.J., Lee, T.T.,Gil-Mohapel, J., Gorzalka, B.B., Hillard, C.J. and Christie, B.R. (2009)Endogenous Cannabinoid Signaling Is Required for VoluntaryExercise-induced Enhancement of Progenitor Cell Proliferation inthe Hippocampus. Hippocampus

61 Hill, M.N. andGorzalka, B.B. (2005) Pharmacological enhancement ofcannabinoid CB1 receptor activity elicits an antidepressant-likeresponse in the rat forced swim test. Eur Neuropsychopharmacol15, 593–599

62 Rutkowska, M. and Jachimczuk, O. (2004) Antidepressant–likeproperties of ACEA (arachidonyl-2-chloroethylamide), the selectiveagonist of CB1 receptors. Acta Pol Pharm 61, 165–167

63 Adamczyk, P. et al. (2008) Activation of endocannabinoidtransmission induces antidepressant-like effects in rats. J PhysiolPharmacol 59, 217–228

64 Hill, M.N. et al. (2007) Estrogen recruits the endocannabinoid systemto modulate emotionality. Psychoneuroendocrinology 32, 350–357

65 Naidu, P.S. et al. (2007) Evaluation of fatty acid amide hydrolaseinhibition in murine models of emotionality. Psychopharmacology(Berl) 192, 61–70

66 Kathuria, S. et al. (2003) Modulation of anxiety through blockade ofanandamide hydrolysis. Nat Med 9, 76–81

67 Patel, S. and Hillard, C.J. (2006) Pharmacological evaluation ofcannabinoid receptor ligands in a mouse model of anxiety: furtherevidence for an anxiolytic role for endogenous cannabinoid signaling.J Pharmacol Exp Ther 318, 304–311

68 Bortolato, M. et al. (2006) Anxiolytic-like properties of theanandamide transport inhibitor AM404. Neuropsychopharmacology31, 2652–2659

69 Braida, D. et al. (2007) 5-HT1A receptors are involved in the anxiolyticeffect of Delta9-tetrahydrocannabinol and AM 404, the anandamidetransport inhibitor, in Sprague-Dawley rats. Eur J Pharmacol 555,156–163

70 Moreira, F.A. et al. (2008) Reduced anxiety-like behaviour induced bygenetic and pharmacological inhibition of the endocannabinoid-degrading enzyme fatty acid amide hydrolase (FAAH) is mediatedby CB1 receptors. Neuropharmacology 54, 141–150

71 Moise, A.M. et al. (2008) An endocannabinoid signaling systemmodulates anxiety-like behavior in male Syrian hamsters.Psychopharmacology (Berl) 200, 333–346

72 Micale, V. et al. (2009) Anxiolytic effects in mice of a dual blocker offatty acid amide hydrolase and transient receptor potential vanilloidtype-1 channels. Neuropsychopharmacology 34, 593–606

73 Scherma, M. et al. (2008) The endogenous cannabinoid anandamidehas effects on motivation and anxiety that are revealed by fattyacid amide hydrolase (FAAH) inhibition. Neuropharmacology 54,129–140

74 Naderi, N. et al. (2008) Interaction between cannabinoid compoundsand diazepam on anxiety-like behaviour of mice. Pharmacol BiochemBehav 89, 64–75

75 Rutkowska,M. et al. (2006) Effects of cannabinoids on the anxiety-likeresponse in mice. Pharmacol Rep 58, 200–206

76 Marsicano, G. et al. (2002) The endogenous cannabinoid systemcontrols extinction of aversive memories. Nature 418, 530–534

77 Chhatwal, J.P. et al. (2005) Enhancing cannabinoidneurotransmission augments the extinction of conditioned fear.Neuropsychopharmacology 30, 516–524

78 Bitencourt, R.M. et al. (2008) Facilitation of contextual fear memoryextinction and anti-anxiogenic effects of AM404 and cannabidiol inconditioned rats. Eur Neuropsychopharmacol 18, 849–859

79 Rubino, T. et al. (2008) CB1 receptor stimulation in specific brain areasdifferently modulate anxiety-related behaviour. Neuropharmacology54, 151–160

80 Rubino, T. et al. (2008) Role in anxiety behavior of theendocannabinoid system in the prefrontal cortex. Cereb Cortex 18,1292–1301

81 McLaughlin, R.J. et al. (2007) Local enhancement of cannabinoidCB1 receptor signalling in the dorsal hippocampus elicits anantidepressant-like effect. Behav Pharmacol 18, 431–438

82 Tzavara, E.T. et al. (2003) The CB1 receptor antagonist SR141716Aselectively increasesmonoaminergic neurotransmission in themedial

Review Trends in Pharmacological Sciences Vol.30 No.9

prefrontal cortex: implications for therapeutic actions. Br JPharmacol 138, 544–553

83 Griebel, G. et al. (2005) Effects of the cannabinoid CB1 receptorantagonist rimonabant in models of emotional reactivity inrodents. Biol Psychiatry 57, 261–267

84 Takahashi, E. et al. (2008) Additive subthreshold dose effects ofcannabinoid CB(1) receptor antagonist and selective serotoninreuptake inhibitor in antidepressant behavioral tests. Eur JPharmacol 589, 149–156

85 Long, J.Z. et al. (2009) Selective blockade of 2-arachidonoylglycerolhydrolysis produces cannabinoid behavioral effects. Nat Chem Biol 5,37–44

86 Hill, M.N. et al. (2008) Differential effects of the antidepressantstranylcypromine and fluoxetine on limbic cannabinoid receptorbinding and endocannabinoid contents. J Neural Transm 115,1673–1679

87 Oliva, J.M. et al. (2005) Time course of opioid and cannabinoid genetranscription alterations induced by repeated administration withfluoxetine in the rat brain. Neuropharmacology 49, 618–626

88 Hesketh, S.A. et al. (2008) Effects of chronic treatment withcitalopram on cannabinoid and opioid receptor-mediated G-proteincoupling in discrete rat brain regions.Psychopharmacology (Berl) 198,29–36

89 Hill, M.N. et al. (2007) Electroconvulsive shock treatmentdifferentially modulates cortical and subcortical endocannabinoidactivity. J Neurochem 103, 47–56

90 Chen, C. and Bazan, N.G. (2005) Lipid signaling: sleep, synapticplasticity, and neuroprotection. Prostaglandins Other Lipid Mediat77, 65–76

91 D’Souza, D.C. et al. (2008) Effects of haloperidol on the behavioral,subjective, cognitive, motor, and neuroendocrine effects of Delta-9-tetrahydrocannabinol in humans. Psychopharmacology (Berl) 198,587–603

92 Justinova, Z. et al. (2008) Fatty acid amide hydrolase inhibitionheightens anandamide signaling without producing reinforcingeffects in primates. Biol Psychiatry 64, 930–937

93 D’Souza, D.C. et al. (2004) The psychotomimetic effects of intravenousdelta-9-tetrahydrocannabinol in healthy individuals: implications forpsychosis. Neuropsychopharmacology 29, 1558–1572

94 Mazzola, C. et al. (2009) Fatty acid amide hydrolase (FAAH)inhibition enhances memory acquisition through activation ofPPAR-alpha nuclear receptors. Learn Mem 16, 332–337

95 Varvel, S.A. et al. (2007) Inhibition of fatty-acid amide hydrolaseaccelerates acquisition and extinction rates in a spatial memorytask. Neuropsychopharmacology 32, 1032–1041

96 Ross, R.A. (2007) Allosterism and cannabinoid CB(1) receptors: theshape of things to come. Trends Pharmacol Sci 28, 567–572

97 Battista, N. et al. (2007) Regulation of female fertility by theendocannabinoid system. Hum Fertil (Camb) 10, 207–216

98 Harkany, T. et al. (2007) The emerging functions of endocannabinoidsignaling during CNS development. Trends Pharmacol Sci 28,83–92

99 Berghuis, P. et al. (2007) Hardwiring the brain: endocannabinoidsshape neuronal connectivity. Science 316, 1212–1216

100 Fegley, D. et al. (2005) Characterization of the fatty acid amidehydrolase inhibitor cyclohexyl carbamic acid 30-carbamoyl-biphenyl-3-yl ester (URB597): effects on anandamide andoleoylethanolamide deactivation. J Pharmacol Exp Ther 313, 352–

358101 Lo Verme, J. et al. (2005) The nuclear receptor peroxisome

proliferator-activated receptor-alpha mediates the anti-inflammatory actions of palmitoylethanolamide. Mol Pharmacol 67,15–19

102 Rodriguez de Fonseca, F. et al. (2001) An anorexic lipid mediatorregulated by feeding. Nature 414, 209–212

103 Guzman, M. et al. (2004) Oleoylethanolamide stimulates lipolysisby activating the nuclear receptor peroxisome proliferator-activated receptor alpha (PPAR-alpha). J Biol Chem 279, 27849–

27854104 Wei, B.Q. et al. (2006) A second fatty acid amide hydrolase with

variable distribution among placental mammals. J Biol Chem 281,36569–36578

105 Gorzalka, B.B. et al. (2008) Endocannabinoid modulation of male ratsexual behavior. Psychopharmacology (Berl) 198, 479–486

106 Haberfellner, E.M. (2007) A review of the assessment ofantidepressant-induced sexual dysfunction used in randomized,controlled clinical trials. Pharmacopsychiatry 40, 173–182

107 American Psychiatric Association. Diagnostic and Statistical MAnualof mental disorders (DSM-IV-TR). 4th edition. Washington, DC:American Psychiatric Association; 2000

108 Trivedi, M.H. et al. (2006) Evaluation of outcomes with citalopram fordepression using measurement-based care in STAR*D: implicationsfor clinical practice. Am J Psychiatry 163, 28–40

109 Thase, M.E. et al. (2007) Cognitive therapy versus medication inaugmentation and switch strategies as second-step treatments: aSTAR*D report. Am J Psychiatry 164, 739–752

110 Bambico, R.F., Duranti, A., Tontini, A., Tarzia, G. and Gobbi, G.(2009) Endocannabinoids for the Treatment of Mood Disorders:Evidence from Animal Models. Current Pharmaceutical Design 15-14, 1623–1646

493