- Email: [email protected]
Brain TRPV1: a depressing TR(i)P down memory lane?
Opinion
Brain TRPV1: a depressing TR(i)P down memory lane? Vincenzo Di Marzo1, Gabriella Gobbi2,3 and Arpad Szallasi4 1
Endocannabinoid Research Group, Istituto di Chimica Biomolecolare, Consiglio Nazionale delle Ricerche, Via Campi Flegrei 34, Comprensorio Olivetti, 80078, Pozzuoli, NA, Italy 2 Neurobiological Psychiatry Unit, Department of Psychiatry, McGill University, Montre´al, H3A 2T5, Que´bec, Canada 3 Centre de Recherche Fernand Seguin, Department of Psychiatry, Universite´ de Montre´al, Montre´al, H1N 3V2, Que´bec, Canada 4 Department of Pathology, Monmouth Medical Center, Long Branch, NJ 07740, USA
On sensory neurons, the capsaicin receptor TRPV1 (transient receptor potential, vanilloid subfamily, member 1) functions as a molecular integrator of noxious stimuli and represents a novel target for analgesic drugs. The presence of TRPV1 in the brain is now well established but, despite intensive research, its function is only beginning to be understood. New evidence implies an unexpected role for hippocampal TRPV1 in neuropsychiatric disorders. For instance, it was hypothesized that the effects of the cannabinoid-receptor antagonist rimonabant on mood might be due to its capability to antagonize TRPV1 receptors at high doses. Most studies, however, imply a positive (e.g. anxiolytic) outcome for TRPV1 antagonism. With potent small-molecule TRPV1 antagonists undergoing clinical trials, the effect of brain TRPV1 blockade might determine the future of this class of novel analgesic drugs. Clearly, more research is needed to delineate the biological role of brain TRPV1 receptors. Introduction Our ability to detect noxious stimuli is mediated by six temperature-sensitive transient receptor potential (TRP) channels, collectively referred to as ‘thermoTRPs’, expressed by polymodal sensory neurons [1]. Targeting these TRP channels represents a new strategy in pain relief [2]. Unlike traditional analgesic agents that either suppress inflammation (e.g. non-steroidal anti-inflammatory drugs) or inhibit pain transmission (e.g. opioids), TRPchannel inhibitors aim to prevent pain by blocking a receptor where the pain is generated [2]. The archetypal TRP receptor is the capsaicin receptor TRPV1 (transient receptor potential, vanilloid subfamily, member 1) [3]. TRPV1 is now recognized as a molecular integrator of painful stimuli ranging from noxious heat through to pungent plant products (e.g. capsaicin) and animal venoms (e.g. jellyfish and spider) and to endovanilloids in inflammation [4]. TRPV1 is also involved in various reflex responses (such as cough and micturition), neuro-immune regulations (most notably type-1 diabetes) and appetite control [4]. TRPV1 was discovered in 1997 [4] and it took less than a decade from channel cloning through antagonist proof of concept to clinical trials [4]. Potent, small-molecule TRPV1 antagonists are currently being Corresponding author: Szallasi, A. ([email protected]).
594
evaluated as a treatment for chronic pain, migraine, cough, urinary incontinence and inflammatory bowel disease [2,4]. Capsaicin also affects thermoregulation via TRPV1 [3]. Dietary capsaicin induces perspiration, known as ‘gustatory sweating’, in humans (leading to indirect heat loss), whereas systemic capsaicin directly lowers body temperature in experimental animals predominantly via vasodilatation [3]. Not unexpectedly, some TRPV1 antagonists (e.g. AMG517) turned out to induce a febrile reaction, implying an endogenous tone for TRPV1 in body-temperature regulation [5]. This is, however, not an obstacle for drug development: TRPV1 antagonists that do not affect thermoregulation can be synthesized [2]. Brain TRPV1 has a long and convoluted history. Despite claims on the internet that capsaicin is a natural aphrodisiac (medical history has it that in Mexico early missionaries forbade their parishioners to eat chili peppers because Aztecs used it to increase their sexual drive) and is addictive by releasing endorphins in the brain, until recently, there was little experimental evidence to support the existence of TRPV1-expressing neurons in the central nervous system (CNS). The only exception was the area postrema in the hypothalamus, firmly linked to the action of capsaicin on body temperature in the 1970s [6]. Capsaicin was shown to induce ultrastructural changes (e.g. ‘swollen’ mitochondria) in area postrema neurons similar to those seen in the perikarya of dorsal root ganglia (DRG) neurons [7]. Furthermore, capsaicin microinjected into the vicinity of the area postrema mimicked the hypothermic action of systemic capsaicin treatment [8]. In the 1980s, using a silver impregnation method, degenerative changes were described throughout the whole neuroaxis of the rat after neonatal [9], but not adult [10], capsaicin administration. These findings are difficult to interpret. One might argue that brain neurons lose their capsaicin sensitivity during development. A more plausible explanation is, however, an indirect neurotoxic action by capsaicin due to hypoperfusion and resultant hypoxia (capsaicin can cause respiratory arrest via activation of the pulmonary chemoreflex [11]). Indeed, other than the expected effect on thermoregulation, no behavioral changes were noted after capsaicin microinjections (although these studies were not designed to detect such changes).
0165-6147/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.tips.2008.09.004 Available online 22 October 2008
Opinion The quest for capsaicin-sensitive brain structures using capsaicin as a tool was further hindered by the recognition of the widespread action of the compound on membrane proteins other than its receptor, such as, for example, the activation (e.g. adenylate cyclase) or blockade (e.g. NAHD oxidoreductase, voltage-sensitive sodium channels) of various enzymes and receptors and the influence on membrane fluidity (for review, see Ref. [3]). Methodological advances after the cloning of TRPV1 [12] finally resolved the controversy surrounding the existence of brain TRPV1: both TRPV1 mRNA and protein were clearly identified in several brain structures [13]. These developments have sparked immense interest in academia and drug discovery companies alike towards brain TRPV1 as a potential friend (i.e. therapeutic target for neurological and psychiatric diseases) and/or foe (i.e. mediator of unwanted side-effects). TRPV1 in the brain: only ‘hotheaded’ or something worse? The presence of TRPV1 in the brain is now well established [14]. Nuclease protection-solution hybridization studies demonstrated the existence of TRPV1 mRNA in various brain structures (e.g. hypothalamus, basal ganglia and cerebral cortex), albeit at much lower levels than in the DRG [13,14]. In situ hybridization studies revealed the presence of TRPV1 throughout the whole CNS, from the olfactory bulb through to the cortex, the basal ganglia and the limbic system down to the brainstem and cerebellum [13,14]. This was unexpected because these structures have little in common with each other or with DRG neurons [14]. Importantly, TRPV1-like immunoreactivity was demonstrated in the corresponding brain areas of wildtype, but not TRPV1-null, mice [15], and specific resiniferatoxin (RTX) binding implied that the TRPV1 protein corresponds to a functional vanilloid receptor [16]. The presence of TRPV1 was also demonstrated in the human brain [13]. In the basal ganglia, confocal microscopy studies showed the colocalization of TRPV1 immunoreactivity with the enzyme tyrosine hydroxylase, indicating its presence in dopaminergic neurons [13]. Despite intensive research, the function of brain TRPV1 in health and disease is still unclear. In rat brain slices, capsaicin was reported to increase the rate of firing in dopaminergic neurons of the midbrain [17]. Accordingly, microinjection of capsaicin into the midbrain ventral tegmentum transiently increased dopamine release in the nucleus accumbens [17]. At the whole-animal level, systemic capsaicin was observed to suppress spontaneous locomotion [18]. The observation that I-RTX, a TRPV1 antagonist, reduces the frequency of spontaneous excitatory postsynaptic currents in dopaminergic neurons of the substantia nigra pars compacta was interpreted to imply an endogenous TRPV1 tone in the CNS [19], similar to that already suggested in neurons that mediate the effects of TRPV1 agonists on body-temperature regulation [5]. Additionally, TRPV1 activation in mesencephalic neurons by a high concentration of capsaicin results in cell death [20]. Based on these findings, it was speculated that brain TRPV1 might play a part in the pathogenesis of disease states as diverse as Parkinson’s disease, Alzhei-
Trends in Pharmacological Sciences
Vol.29 No.12
mer’s disease and schizophrenia (for review, see Refs [2,21]). These theories were met by skepticism in the neuroscience community mostly because TRPV1-null (‘knockout’) mice have a fairly unremarkable behavioural phenotype (although one might counter-argue that such animals compensate for the absence of TRPV1) (for review, see Ref. [2]). Furthermore, neither capsaicin nor the firstgeneration TRPV1 antagonist capsazepine is selective for TRPV1, casting a cloud over the interpretation of pharmacological studies (for review, see Refs [3,4]). Indeed, capsaicin induces cell death by direct activation of pathways of apoptosis in various cell lines that do not express TRPV1 [22]. Whereas it is easy to visualize why a receptor involved in body-temperature regulation is heat sensitive [5], it is far more complicated to assign a role for a polymodal nociceptor such as TRPV1 in hippocampal or ventral tegmentum neurons. It was postulated that brain TRPV1 is regulated by endogenous ligands, referred to as ‘endovanilloids’ [23]. Of putative endovanilloids, arachidonic acid derivatives have attracted the most attention [24]. The possibility that anandamide, originally isolated as an endogenous cannabinoid CB1-receptor agonist (Box 1), is an important ligand that regulates brain TRPV1 was reviewed elsewhere [14,23]. Subsequently, N-arachidonyol dopamine (NADA) was isolated from brain nuclei where TRPV1 is also present [25]. In vitro, NADA activates TRPV1, although it is unclear whether it can reach high Box 1. A brief overview of the endocannabinoid system The endocannabinoid system comprises three components: (i) Two cloned G-protein-coupled receptors, known as cannabinoid receptor type-1 (CB1) and type-2 (CB2), discovered while looking for a mechanism of action for the cannabis psychotropic principle, D9-tetrahydrocannabinol. (ii) Endogenous agonists of these two receptors, known as endocannabinoids, of which anandamide (N-arachidonoylethanolamine) and 2-arachidonoylglycerol (2-AG) are the two most studied members. (iii) Proteins for the regulation of endocannabinoid levels and actions. Enzymes responsible for endocannabinoid biosynthesis and inactivation have been cloned and include: (i) N-arachidonoylphosphatidylethanolamine-specific phospholipase D (NAPE-PLD), a,b-hydrolase 4 and protein tyrosine phosphatase PTN22, which convert N-arachidonoyl-phosphatidylethanolamine into anandamide via various corresponding routes together with other, as yet uncharacterized, enzymes; (ii) diacylglycerol lipases (DAGL)-a and b, which convert sn-2-arachidonate-containing diacylglycerols into 2-AG; (iii) fatty acid amide hydrolase (FAAH), which hydrolyses anandamide to arachidonic acid and ethanolamine; and (iv) the monoacylglycerol lipase (MAGL), which hydrolyses 2-AG to arachidonic acid and glycerol [64]. Pharmacological tools for the study of the proteins of the endocannabinoid system have been developed and include cannabinoid-receptor-specific agonists and antagonists and DAGL-, MAGL- and FAAH- selective inhibitors [65]. In the CNS, endocannabinoids often inhibit both excitatory and inhibitory neurotransmitter release and mediate neuroplasticity (Box 2) by acting as retrograde signals released from postsynaptic neurons and activating presynaptic CB1 receptors coupled to inhibition of voltage-activated Ca2+ channels or stimulation of inwardly-rectifying K+ channels [66].
595
Opinion enough concentrations in vivo to gate the channel. However, ligand affinity for TRPV1 is regulated by various factors such as the phosphorylation state of the protein (for review, see Refs [2,4]). Consequently, affinities determined in vitro do not necessarily reflect the in vivo situation. Moreover, the binding site on TRPV1 for anandamide (and also probably NADA) is intracellular (for review, see Ref. [4]). Because anandamide is biosynthesized inside the cell, its intracellular concentration might considerably exceed that in the extracellular matrix and be sufficient to activate TRPV1, as shown for DRG neurons [26]. Importantly, enzymes that biosynthesize or degrade endovanilloids are co-localized with TRPV1 in several cerebellar and hippocampal neurons [27]. Brain endovanilloids were suggested to tonically increase TRPV1 activity, thereby regulating, among other things, glutamate release in a large variety of brain nuclei and, consequently, dopaminergic activity in the substantia nigra [19], descending antinociception in the periaqueductal grey [28] and emesis in the nucleus of the solitary tract [29]. Brain TRPV1: a role in affective disorders and in rimonabant central side effects? TRPV1-null mice were shown to exhibit impaired longterm potentiation (LTP; Box 2) in the hippocampus [30]. In keeping with this earlier observation, a novel facilitatory role for TRPV1 in synaptic plasticity has been described, that is, long-term depression (LTD; Box 2) of excitatory inputs from pyramidal neurons onto g-aminobutyric acid (GABA)-ergic interneurons in this brain area [31]. Based on some of this data [31] together with previous findings that the cannabinoid CB1-receptor antagonist rimonabant (sold in the EU under the brand name Acomplia) antagonizes TRPV1 receptors at mM concentrations [32], it was Box 2. Long-term potentiation (LTP); long-term depression (LTD); neuroplasticity and neurogenesis First observed by Bliss and Lomo in 1973 [67], LTP is operationally defined as a long-lasting increase in synaptic efficacy, which follows high-frequency stimulation of afferent fibers. A stimulating electrode was placed at the level of perforant path fibers of the hippocampus and another recording electrode in the postsynaptic neuron of the dentate area. A single electrical pulse produced a response at the postsynaptic level (baseline); later, a brief burst of many rapidly repeated pulses, or ‘potentiating stimuli’, were given, producing enhanced synaptic responses and lasting for many hours. Slow, weak electrical stimulation of CA1 neurons also causes long-term changes in the synapses: in this case, a reduction in their sensitivity. This is called long-term depression (LTD). Both LTP and LTD are presumed to play a part in the establishment of stable memories. Moreover, they are thought to form a part of the physiological mechanisms underlying neuronal plasticity or neuroplasticity; that is, the ability of neurons and brain organization to change after environmental and/or pharmacological stimuli. Neuroplasticity includes different neuronal phenomena, for example, in the young brain developmental plasticity occurs when neurons rapidly sprout branches and form synapses. In the adult brain, neurogenesis (which means the ‘birth of new neurons’) occurs mostly in the dentate gyrus of the hippocampus and in the subventricular zone of the olfactory system. Recently, it has been demonstrated that antidepressant drugs induce neurogenesis in the dentate gyrus and this effect is necessary for the antidepressant response [68,69].
596
Trends in Pharmacological Sciences Vol.29 No.12
speculated that facilitation of depression by rimonabant in some predisposed patients [33] might be due to antagonism of hippocampal TRPV1 receptors and resultant LTD blockade. Rimonabant has been tested in four obesity phase III trials involving >2500 obese patients and, as of mid 2008 [33], is available in 55 countries worldwide for patients who are obese and have associated risk factors for cardiovascular disease such as type-2 diabetes or dyslipidemia. In the United States, however, an advisory panel to the Food and Drug Administration (www.fda.gov) concluded that the safety of rimonabant, particularly with regard to increased occurrence of psychiatric disorders (e.g. anxiety and depression), had not been fully demonstrated. Rimonabant improves the profile of cardiometabolic risk factors in obese subjects largely by reducing the overactivity of peripheral CB1 receptors, whereas its worrisome sideeffects are centrally mediated [34]. But, is it likely that these effects involve central TRPV1 blockade? Because the answer to this question is of great practical importance (also for the future of the whole class of TRPV1 antagonist drugs), one should be cautious and use scrutiny in interpreting the findings. Several lines of evidence argue against a link between suppression of LTD by rimonabant via TRPV1 blockade in vitro and the mood disorders that are worsened by rimonabant in the clinic. (i) The connection between LTD at hippocampal-interneuron synapses in the CA1 region of the hippocampus (the form of LTD studied in Ref. [31]) and the development of mood disorders, particularly depression, is not supported by the available literature. In fact, the involvement of long-term synaptic plasticity in the pathogenesis of major depression and in the mechanism of action of antidepressants is still debated. Exposure to chronic mild stress, a reliable animal model of depression, facilitates LTD but has no effect on LTP in the CA1 region. The antidepressant fluvoxamine increases LTP in stressed and nonstressed animals, thus preventing the stress-induced facilitation of LTD [35]. However, this effect cannot be considered a hallmark of all antidepressant drugs because the tryciclic desipramine and mianserine enhance LTP in the dentate gyrus [36], whereas chronic fluoxetine and electroshock treatments, by contrast, attenuate LTP in the same area [37]. Conversely, electroconvulsive stimulation for one week, and chronic lithium treatment, impairs LTD in the striatum [38] indicating that antidepressant treatment blocks LTD. For all these reasons, the effects of antidepressants on LTP and LTD have been attributed to indirect effects on Ca2+ influx [39] or to increases in brain-derived neurotropic factor and serotonin levels. (ii) Even in the unlikely possibility that inhibition of LTD via TRPV1 leads to clinical depression, it is very unlikely that rimonabant reaches high enough concentrations in vivo to exert this effect. Rimonabant displays a >100-fold selectivity for CB1 over TRPV1 receptors, and its reported IC50 for TRPV1 in vitro is an order of magnitude higher than the Cmax
Opinion (400 nM) estimated in the plasma of patients on the usual dose of rimonabant (20 mg daily) [32]. (iii) There is no evidence that pharmacological blockade or genetic deletion of TRPV1 leads to mood disorders or
Trends in Pharmacological Sciences
Vol.29 No.12
anxiety. TRPV1-null mice are active, have a healthy appetite and reproduce (for review, see Ref. [4]). On the contrary, TRPV1 antagonism or genetic inactivation seems to produce anxiolytic effects in rodents
Figure 1. (a) In vivo effects of rimonabant on 5-HT (5-hydroxytryptamine; serotonin) firing activity after sub-chronic treatment with vehicle (once daily, 4 days) (i) and URB 597 (0.1 mg/kg, once daily) (ii). Note that URB 597 (a selective inhibitor of the enzyme fatty acid amide hydrolase [FAAH] that is involved in the hydrolysis of endogenous cannabinoid CB1-receptor agonists [Box 1]) increases the tonic activation of 5-HT activity and has antidepressant-like activity, whereas rimonabant (0.1 mg/kg, intravenously [i.v.]) dramatically suppresses the 5-HT activity. This decrease is believed to correspond to the worsening of depression in humans [48] (figure adapted, with permission, from Ref. [48]). (b) (i) At low doses, the selective CB1 agonist WIN 55 212 (0.2–0.4 mg/kg, i.v.) increases 5-HT firing activity in vivo, and this effect is blocked by rimonabant (1 mg/kg) but not by the TRPV1 antagonist capzazepine (0.02 mg/kg, i.v.). (ii) However, at higher doses (0.4 mg/kg, i.v) the WIN 55 212 decreases 5-HT firing activity and this effect is blocked by capzazepine but not by rimonabant (figure adapted, with permission, from Ref. [43]). It should be noted that raising anandamide levels by blocking FAAH, especially when using high doses of URB597, will increase the likelihood of activating both CB1 and TRPV1 in various brain regions [63]. However, the antidepressant-like effect of URB597 is reversed by a dose of rimonabant that does not reverse the effect of a high dose of the compound WIN 55 212–2 (a) (ii) and (b) (ii). Altogether, these observations indicate that the antidepressant-like effects of URB597 are due uniquely to indirect activation of cannabinoid CB1 receptors and that the dose of rimonabant used in these experiments is not sufficient to antagonize TRPV1 receptors.
597
Opinion [30,40,41]. By contrast, chronic use of rimonabant in some obese patients is associated not only with increased depression but also with anxiety [33]. (iv) During the past 20 years, a hypoactivity of serotonin function has been firmly linked to dampening of mood and depression [42]. Indeed, drugs that increase serotonin neurotransmission act as antidepressants. Interestingly, it was demonstrated that low doses of the CB1 agonist WIN 55 212 have antidepressant-like properties in the forced swim test and increase in vivo serotonin firing activity; this effect is reversed by rimonabant but not by the TRPV1 antagonist capsazepine [43]. Conversely, higher doses of WIN 55 212 dramatically decrease serotonin firing and this effect was reversed by capsazepine but not rimonabant [43]. Capsazepine also increased serotonin activity over the baseline, thus indicating a facilitatory role of TRPV1 in depression. This hypothesis is supported by the evidence that the TRPV1 agonist olvanil has depressive-like properties in the forced swim test [40]. These data indicate that the antidepressant-like activity of CB1 agonists in rodents, and likewise the prodepressant-like activity of CB1 antagonists, is mediated by CB1 rather than TRPV1 receptors. (It was suggested that WIN 55 212 might also act on a yet to be identified target that is distinct from both CB1 and TRPV1 [44].) In fact, CB1-null-mutant mice exhibit depressive-like and anxiety-like behaviors in several behavioral paradigms [45,46]. (v) Last, taranabant, a CB1 receptor antagonist that is chemically very different from rimonabant, also worsens depression and anxiety [47]. Combined, these observations indicate that mood disorders by rimonabant and taranabant are due to a reduction of CB1-receptor activity in the CNS. However, the effects of CB1 antagonists on depression are complex. Rimonabant at low doses (<2 mg/kg which are ineffective on the basal activity of serotoninergic neurons) is inactive in animal models of depression-like behaviour [43,48], indicating that in normal conditions the endocannabinoid system does not exert a tonic activity over serotoninergic neurons. However, in animals repeatedly injected with the fatty acid amide hydrolase (FAAH) inhibitor URB597, which probably causes an increase in endocannabinoid levels in the brain, an increased serotonin tone was observed, and 1 mg/kg of rimonabant dramatically decreased serotonin neuronal firing activity below control levels (Figure 1). This situation might mimic what could happen in a subgroup of obese patients with increased brain endocannabinoid and serotonin activity, in whom rimonabant could similarly induce an abrupt drop in serotonin electrical activity, thereby enabling the development of depressive symptoms. It is also worth mentioning that a modest increase in serotonin activity in the prefrontal cortex has been observed after high doses of rimonabant [49] and that a decreased immobility in the forced swim test (a sign of antidepressant activity) has been observed after oral injection of high doses of rimonabant [50], although these results are not yet supported by clinical data. 598
Trends in Pharmacological Sciences Vol.29 No.12
Although, in our opinion, the cons with regard to TRPV1 being the target for the psychiatric adverse effects of rimonabant outweigh the pros, this theory cannot be disproved beyond any reasonable doubt. The affinity of TRPV1 for rimonabant might be considerably higher in vivo than in vitro. In fact, some central neuroprotective effects of this compound (e.g. against global cerebral ischemia in gerbils and in favour of adult neurogenesis in mice) have been ascribed to interactions with TRPV1 rather than CB1 receptors [51]. Furthermore, taranabant, to our best knowledge, is yet to be tested on TRPV1. Lastly, at least in principle, TRPV1 antagonists can be anxiolytic in rodents, yet, they might induce other types of mood disorders in humans, as indicated by the finding that TRPV1 in mice facilitates LTP and suppresses another type of hippocampal LTD, thereby protecting hippocampal synaptic plasticity from the influence of acute stress [52]. Until these questions are resolved, patients who participate in clinical trials with TRPV1 antagonists should be carefully watched for signs of mood disorders. As for rimonabant, several authorities agree that, especially if administered to the right patient, the benefits of this drug on cardiometabolic risk factors should, in the long run, outweigh its effects on depression. Brain TRPV1: friend or foe? There is good evidence that activation of TRPV1 is neuroprotective in a model of brain ischemia [53]. This action might be mediated by TRPV1 expressed in endothelial cells [54]. Interestingly, activation of TRPV1 in brain neurons might have the opposite action. In addition to neurons, TRPV1 is expressed in microglia, astrocytes and pericytes and it has been suggested that TRPV1 might play an important part in Alzheimer’s-disease-related neuroinflammatory processes [55]. TRPV1 is also an important player in neuro-immunological regulation and, given the emerging role for TRPV1 in the pathogenesis of autoimmune (type-1) diabetes (for review, see Ref. [56]), the possibility exists that TRPV1 might be involved in other autoimmune diseases including multiple sclerosis. Exposure of mesencephalic dopaminergic or hippocampal neurons to capsaicin triggers cell death or apoptosis, an effect prevented by TRPV1 antagonists, and is accompanied by increasing intracellular Ca2+ levels and/ or mitochondrial damage, mitochondrial cytochrome c release and formation of caspase-3, a mediator of apoptosis [20,57]. These effects, together with a direct action on locomotion, might be relevant to a possible role of TRPV1 in neurodegenerative motor and cognitive disorders. Indeed, agonists of this receptor were found to reduce hyperkinesias in animal models of hyperdopaminergia [58] and Huntington’s chorea [59] and to ameliorate levodopa-induced dyskinesia in a model of Parkinson’s disease [60]. However, opposite effects might exist with regard to levodopa-induced dyskinesia because it was recently suggested that TRPV1 receptors might limit, rather than enhance, the beneficial action of elevated anandamide levels against levodopa-induced dyskinesia [61]. Thus, it is not impossible that, in the brain, both TRPV1 agonists and antagonists exert the same beneficial effect, as shown in a model of excitotoxicity [62] and for some antipsychotics
Opinion Box 3. Outstanding questions What is the role of brain TRPV1 receptors in health and disease? What are the endogenous activators of brain TRPV1 receptors? Considering that anandamide has affinity for both CB1 and TRPV1 receptors, under what neurophysiological circumstances does it activate one or the other? Is pharmacological blockade of TRPV1 by small-molecule antagonists given as analgesic drugs depressive or, by contrast, anxiolytic (or, possibly, neither)? Might TRPV1 ligands (agonists or antagonists) be of clinical value in the treatment of neurological or psychiatric diseases? Do CB1 and TRPV1 receptors have similar, opposite or complementary roles in CNS functions? What is the neurobiological mechanism that renders a subgroup of obese patients more susceptible to the pro-depressant effect of CB1 antagonism? Do they have an elevated endogenous tone of brain endocannabinoids and, consequently, an elevated monoaminergic tone?
(for review, see Ref. [3]). This would not be unprecedented because both TRPV1 desensitization by agonists and blockade by antagonists have a similar outcome (i.e. analgesia) also in nociceptive neurons [3]. Therefore, it is possible that, similar to DRG neurons, also activation of brain TRPV1 is followed by desensitization. Because TRPV1 gating in the brain seems to be mostly coupled to glutamate release, pharmacological TRPV1 overactivation, if followed by desensitization, might result in reduced neuronal excitability, exactly as it would be with TRPV1 antagonism in the presence of an endovanilloid tone. Conclusions Much research is still needed to fully appreciate the role of TRPV1 in the CNS and, hence, the potential central consequences of the pharmacological targeting of this channel with either agonists or antagonists with therapeutic activity (Box 3). Although there is now little doubt about the functional activity of brain TRPV1 receptors, future efforts will have to be aimed at understanding the exact mechanisms leading to their activation under physiological and pathological conditions and their consequences on neurotransmitter release and action. References 1 Bandell, M. et al. (2007) From chills to chilies: mechanisms for thermosensation and chemesthesis via thermoTRPs. Curr. Opin. Neurobiol. 17, 490–497 2 Cortright, D.N. and Szallasi, A. TRP channels and pain. Curr. Pharm. Des. (in press) 3 Szallasi, A. and Blumberg, P.M. (1999) Vanilloid (capsaicin) receptors and mechanisms. Pharmacol. Rev. 51, 159–211 4 Szallasi, A. et al. (2007) The vanilloid receptor TRPV1: 10 years from channel cloning to antagonist proof-of-concept. Nat. Rev. Drug Discov. 6, 357–372 5 Gavva, N. Body-temperature maintenance as the predominant function of the vanilloid receptor TRPV1. Trends Pharmacol. Sci. (in press) 6 Jancso´-Ga´bor, A. et al. (1970) Stimulation and desensitization of the hypothalamic heat-sensitive structure by capsaicin in rats. J. Physiol. 208, 449–459 7 Szolcsa´nyi, J. et al. (1971) Mitochondrial changes in preoptic neurons after capsaicin desensitization of the hypothalamic thermodetectors in rats. Nature 229, 116–117 8 Hori, T. et al. (1988) Responses of anterior hypothalamic-preoptic thermosensitive neurons to locally applied capsaicin. Neuropharmacology 27, 135–142
Trends in Pharmacological Sciences
Vol.29 No.12
9 Ritter, S. and Dinh, T.T. (1990) Capsaicin-induced neuronal degeneration in the brain and retina of preweanling rats. J. Comp. Neurol. 296, 447–461 10 Ritter, S. and Dinh, T.T. (1992) Age-related changes in capsaicininduced neurodegeneration in rat brain. J. Comp. Neurol. 318, 103–116 11 Szolcsa´nyi, J. et al. (1990) Resiniferatoxin: an ultrapotent selective modulator of capsaicin-sensitive primary afferent neurons. J. Pharmacol. Exp. Ther. 255, 923–928 12 Caterina, M.J. et al. (1997) The capsaicin receptor: a heat-activated ion channel in the pain pathway. Nature 389, 816–824 13 Mezey, E. et al. (2000) Distribution of mRNA for vanilloid receptor subtype 1 (VR1), and VR1-like immunoreactivity, in the central nervous system of the rat and human. Proc. Natl. Acad. Sci. U. S. A. 97, 3655–3660 14 Szallasi, A. and Di Marzo, V. (2000) New perspectives on enigmatic vanilloid receptors. Trends Neurosci. 23, 491–497 15 Cristino, L. et al. (2006) Immunohistochemical localization of cannabinoid type 1 and transient receptor potential vanilloid type 1 receptors in the mouse brain. Neuroscience 139, 1405–1416 16 Acs, G. et al. (1996) Specific binding [3H]resiniferatoxin by human and rat preoptic area, locus ceruleus, medial hypothalamus, reticular formation and ventral thalamus membrane preparations. Life Sci. 59, 1899–1908 17 Marinelli, S. et al. (2005) Activation of TRPV1 in the VTA excites dopaminergic neurons and increases chemical and noxious-induced dopamine release in the nucleus accumbens. Neuropsychopharmacology 30, 864–870 18 Di Marzo, V. et al. (2001) Hypolocomotor effects in rats of capsaicin and two long chain capsaicin homolgues. Eur. J. Pharmacol. 420, 123–131 19 Marinelli, S. et al. (2003) Postsynaptic facilitation of glutaminergic synapses to dopaminergic neurons of the rat substantia nigra by endogenous stimulation of vanilloid receptors. J. Neurosci. 23, 3136– 3144 20 Kim, S.R. et al. (2005) Transient receptor potential vanilloid subtype-1 mediates cell death of mesencephalic dopaminergic neurons in vitro and in vivo. J. Neurosci. 25, 662–671 21 Starowicz, K. et al. (2008) TRPV1 receptors in the central nervous system: potential for previously unforeseen therapeutic applications. Curr. Pharm. Des. 14, 42–54 22 Athanasiou, A. et al. (2007) Vanilloid receptor agonists and antagonists are mitochondrial inhibitors: how vanilloids cause non vanilloid receptor-mediated cell death. Biochem. Biophys. Res. Commun. 354, 50–55 23 Di Marzo, V. et al. (2002) Endovanilloid signaling in pain. Curr. Opin. Neurobiol. 12, 372–379 24 Starowicz, K. et al. (2007) Biochemistry and pharmacology of endovanilloids. Pharmacol. Ther. 114, 13–33 25 Huang, S.M. et al. (2002) An endogenous capsaicin-like substance with high potency at recombinant and native vanilloid VR1 receptors. Proc. Natl. Acad. Sci. U. S. A. 99, 8400–8405 26 Van der Stelt, M. et al. (2005) Anandamide acts as an intracellular messenger amplifying Ca2+ influx via TRPV1 channels. EMBO J. 24, 3026–3037 27 Cristino, L. et al. (2008) Immunohistochemical localization for anabolic and catabolic enzymes for anandamide and other putative endovanilloids in the hippocampus and cerebellar cortex of the mouse brain. Neuroscience 151, 955–968 28 Starowicz, K. et al. (2007) Tonic endovanilloid facilitation of glutamate release and brainstem descending antinociceptive pathways. J. Neurosci. 27, 13739–13749 29 Sharkey, K.A. et al. (2007) Arvanil, anandamide, and N-arachidonoyldopamine (NADA) inhibit emesis through cannabinoid CB1 and vanilloid TRPV1 receptors in the ferret. Eur. J. Neurosci. 25, 2773– 2782 30 Marsch, M. et al. (2007) Reduced anxiety, conditioned fear, and hippocampal long term potentiation in transient receptor potential vanilloid subtype 1 receptor -deficient mice. J. Neurosci. 27, 832– 839 31 Gibson, H.E. et al. (2008) TRPV1 channels mediate long term depression at synapses on hippocampal interneurons. Neuron 57, 746–759 32 De Petrocellis, L. et al. (2001) The activity of anandamide at vanilloid VR1 receptors requires facilitated transport across the cell membrane
599
Opinion
33
34 35 36
37
38
39
40
41
42
43
44
45
46 47
48
49
50
600
and is limited by intracellular metabolism. J. Biol. Chem. 276, 12856– 12863 Van Gaal, L. et al. (2008) Efficacy and safety of rimonabant for improvement of multiple cardiometabolic risk factors in overweight/ obese patients. Pooled 1-year data from the Rimonabant in Obesity (RIO) program. Diabetes Care 31, S229–S240 Di Marzo, V. (2008) Targeting the endocannabinoid system: to enhance or reduce? Nat. Rev. Drug Discov. 7, 438–445 Holderbach, R. et al. (2007) Enhanced long-term synaptic depression in an animal model of depression. Biol. Psychiatry 62, 92–100 Levkovitz, Y. et al. (2001) Transcranial magnetic stimulation and antidepressive drugs share similar cellular effects in rat hippocampus. Neuropsychopharmacology 24, 608–616 Stewart, C.A. and Reid, I.C. (2000) Repeated ECS and fluoxetine administration have equivalent effects on hippocampal synaptic plasticity. Psychopharmacology (Berl.) 148, 217–223 De Murtas, M. et al. (2004) Repeated electroconvulsive stimulation impairs long-term depression in the neostriatum. Biol. Psychiatry 55, 472–476 Dea´k, F. et al. (2000) Inhibition of voltage-gated calcium channels by fluoxetine in rat hippocampal pyramidal cells. Neuropharmacology 39, 1029–1036 Kasckow, J.W. et al. (2004) Effects of the vanilloid agonist olvanil and antagonist capsazepine on rat behaviors. Prog. Neuropsychopharmacol. Biol. Psychiatry 28, 291–295 Micale, V. et al. Anxiolytic effects in mice of a dual blocker of fatty acid amide hydrolase and transient receptor potential vanilloid type-1 channel. Neuropsychopharmacology (in press) Mann, J.J. et al. (2001) The neurobiology and genetics of suicide and attempted suicide: a focus on the serotonergic system. Neuropsychopharmacology 24, 467–477 Bambico, F. et al. (2007) Cannabinoids elicit antidepressant-like behavior and activate serotonergic neurons through the medial prefrontal cortex. J. Neurosci. 27, 11700–11711 Accorsi-Mendonca, D. et al. (2008) Inhibition of spontaneous neurotransmission in the nucleus of the solitary tract of the rat by the cannabinoid agonist WIN 55212-2 is not via CB1 or CB2 receptors. Brain Res. 1200, 1–9 Haller, J. et al. (2004) Context-dependent effects of CB1 cannabinoid gene disruption on anxiety-like and social behaviour in mice. Eur. J. Neurosci. 19, 1906–1912 Martin, M. et al. (2002) Involvement of CB1 cannabinoid receptors in emotional behaviour. Psychopharmacology (Berl.) 159, 379–387 Carr, R.D. et al. (2008) A two-year study to assess the efficacy, safety, and tolerability of taranabant in obese patients. Atherosclerosis Supplements 9, 12 Gobbi, G. et al. (2005) Antidepressant-like activity and modulation of brain monoaminergic transmission by blockade of anandamide hydrolysis. Proc. Natl. Acad. Sci. U. S. A. 102, 18620–18625 Tzavara, E.T. et al. (2001) The cannabinoid CB1 receptor antagonist SR141716A increases norepinephrine outflow in the rat anterior hypothalamus. Eur. J. Pharmacol. 426, R3–R4 Griebel, G. et al. (2005) Effects of the cannabinoid CB1 receptor antagonist rimonabant in models of emotional reactivity in rodents. Biol. Psychiatry 57, 261–267
Trends in Pharmacological Sciences Vol.29 No.12 51 Pegorini, S. et al. (2006) Vanilloid VR1 receptor is involved in rimonabant-induced neuroprotection. Br. J. Pharmacol. 147, 552–559 52 Li, H.B. et al. Antistress effect of TRPV1 channel on synaptic plasticity and spatial memory. Biol. Psychiatry (in press) 53 Pegorini, S. et al. (2005) Capsaicin exhibits neuroprotective effects in a model of transient global cerebral ischemia in Mongolian gerbils. Br. J. Pharmacol. 144, 727–735 54 Golech, S.A. et al. (2004) Human brain endothelium: coexpression and function of vanilloid and endocannabinoid receptors. Brain Res. Mol Brain Res. 132, 87–92 55 Yamamoto, S. et al. (2007) Transient receptor potential channels in Alzheimer’s disease. Biochim. Biophys. Acta 1772, 958–967 56 Suri, A. and Szallasi, A. (2008) The emerging role of TRPV1 in diabetes and obesity. Trends Pharmacol. Sci. 29, 29–36 57 Kim, S.R. et al. (2006) Transient receptor potential vanilloid subtype 1 mediates microglial cell death in vitro and in vivo via Ca2+-mediated mitochondrial damage and cytochrome-c release. J. Immunol. 177, 4322–4329 58 Tzavara, E.T. et al. (2006) Endocannabinoids activate transient receptor potential vanilloid 1 receptors to reduce hyperdopaminergia-related hyperactivity: therapeutic implications. Biol. Psychiatry 59, 508–515 59 de Lago, E. et al. (2005) Arvanil, a hybrid endocannabinoid and vanilloid compound, behaves as an antiohyperkinetic agent in a rat model of Huntington’s disease. Brain Res. 1050, 210–216 60 Lee, J. et al. (2006) A role for vanilloid receptor (TRPV1) and endocannabinoid signaling in the regulation of spontaneous and LDOPA-induced locomotion in normal and reserpine-treated rats. Neuropharmacology 51, 557–565 61 Morgese, M.G. et al. (2007) Anti-dyskinetic effects of cannabinoids in a rat model of Parkinson’s disease: role of CB1 and TRPV1 receptors. Exp. Neurol. 208, 110–119 62 Veldhuis, W.B. et al. (2003) Neuroprotection by the endogenous cannabinoid anandamide and arvanil against in vivo excitotoxicity in the rat: role of vanilloid receptors and lipoxigenases. J. Neurosci. 23, 4127–4133 63 Rubino, T. et al. (2008) Role in anxiety behavior of the endocannabinoid system in the prefrontal cortex. Cereb. Cortex 18, 1292–1301 64 Di Marzo, V. and Petrosino, S. (2007) Endocannabinoids and the regulation of their levels in health and disease. Curr. Opin. Lipidol. 18, 129–140 65 Fowler, C.J. (2008) The tools of the trade’’ – an overview of the pharmacology of the endocannabinoid system. Curr. Pharm. Des. 14, 2254–2265 66 Katona, I. and Freund, T.F. (2008) Endocannabinoid signaling as a synaptic circuit breaker in neurological disease. Nat. Med. 14, 923– 930 67 Bliss, T.V. and Lomo, T. (1973) Long-lasting potentiation of synaptic transmission in the dentate area of the anaesthetized rabbit following stimulation of the perforant path. J. Physiol. 232, 331–356 68 Malberg, J.E. et al. (2000) Chronic antidepressant treatment increases neurogenesis in adult rat hippocampus. J. Neurosci. 20, 9104–9110 69 Santarelli, L. et al. (2003) Requirement of hippocampal neurogenesis for the behavioral effects of antidepressants. Science 301, 805–809
Brain TRPV1: a depressing TR(i)P down memory lane? Vincenzo Di Marzo1, Gabriella Gobbi2,3 and Arpad Szallasi4 1
Endocannabinoid Research Group, Istituto di Chimica Biomolecolare, Consiglio Nazionale delle Ricerche, Via Campi Flegrei 34, Comprensorio Olivetti, 80078, Pozzuoli, NA, Italy 2 Neurobiological Psychiatry Unit, Department of Psychiatry, McGill University, Montre´al, H3A 2T5, Que´bec, Canada 3 Centre de Recherche Fernand Seguin, Department of Psychiatry, Universite´ de Montre´al, Montre´al, H1N 3V2, Que´bec, Canada 4 Department of Pathology, Monmouth Medical Center, Long Branch, NJ 07740, USA
On sensory neurons, the capsaicin receptor TRPV1 (transient receptor potential, vanilloid subfamily, member 1) functions as a molecular integrator of noxious stimuli and represents a novel target for analgesic drugs. The presence of TRPV1 in the brain is now well established but, despite intensive research, its function is only beginning to be understood. New evidence implies an unexpected role for hippocampal TRPV1 in neuropsychiatric disorders. For instance, it was hypothesized that the effects of the cannabinoid-receptor antagonist rimonabant on mood might be due to its capability to antagonize TRPV1 receptors at high doses. Most studies, however, imply a positive (e.g. anxiolytic) outcome for TRPV1 antagonism. With potent small-molecule TRPV1 antagonists undergoing clinical trials, the effect of brain TRPV1 blockade might determine the future of this class of novel analgesic drugs. Clearly, more research is needed to delineate the biological role of brain TRPV1 receptors. Introduction Our ability to detect noxious stimuli is mediated by six temperature-sensitive transient receptor potential (TRP) channels, collectively referred to as ‘thermoTRPs’, expressed by polymodal sensory neurons [1]. Targeting these TRP channels represents a new strategy in pain relief [2]. Unlike traditional analgesic agents that either suppress inflammation (e.g. non-steroidal anti-inflammatory drugs) or inhibit pain transmission (e.g. opioids), TRPchannel inhibitors aim to prevent pain by blocking a receptor where the pain is generated [2]. The archetypal TRP receptor is the capsaicin receptor TRPV1 (transient receptor potential, vanilloid subfamily, member 1) [3]. TRPV1 is now recognized as a molecular integrator of painful stimuli ranging from noxious heat through to pungent plant products (e.g. capsaicin) and animal venoms (e.g. jellyfish and spider) and to endovanilloids in inflammation [4]. TRPV1 is also involved in various reflex responses (such as cough and micturition), neuro-immune regulations (most notably type-1 diabetes) and appetite control [4]. TRPV1 was discovered in 1997 [4] and it took less than a decade from channel cloning through antagonist proof of concept to clinical trials [4]. Potent, small-molecule TRPV1 antagonists are currently being Corresponding author: Szallasi, A. ([email protected]).
594
evaluated as a treatment for chronic pain, migraine, cough, urinary incontinence and inflammatory bowel disease [2,4]. Capsaicin also affects thermoregulation via TRPV1 [3]. Dietary capsaicin induces perspiration, known as ‘gustatory sweating’, in humans (leading to indirect heat loss), whereas systemic capsaicin directly lowers body temperature in experimental animals predominantly via vasodilatation [3]. Not unexpectedly, some TRPV1 antagonists (e.g. AMG517) turned out to induce a febrile reaction, implying an endogenous tone for TRPV1 in body-temperature regulation [5]. This is, however, not an obstacle for drug development: TRPV1 antagonists that do not affect thermoregulation can be synthesized [2]. Brain TRPV1 has a long and convoluted history. Despite claims on the internet that capsaicin is a natural aphrodisiac (medical history has it that in Mexico early missionaries forbade their parishioners to eat chili peppers because Aztecs used it to increase their sexual drive) and is addictive by releasing endorphins in the brain, until recently, there was little experimental evidence to support the existence of TRPV1-expressing neurons in the central nervous system (CNS). The only exception was the area postrema in the hypothalamus, firmly linked to the action of capsaicin on body temperature in the 1970s [6]. Capsaicin was shown to induce ultrastructural changes (e.g. ‘swollen’ mitochondria) in area postrema neurons similar to those seen in the perikarya of dorsal root ganglia (DRG) neurons [7]. Furthermore, capsaicin microinjected into the vicinity of the area postrema mimicked the hypothermic action of systemic capsaicin treatment [8]. In the 1980s, using a silver impregnation method, degenerative changes were described throughout the whole neuroaxis of the rat after neonatal [9], but not adult [10], capsaicin administration. These findings are difficult to interpret. One might argue that brain neurons lose their capsaicin sensitivity during development. A more plausible explanation is, however, an indirect neurotoxic action by capsaicin due to hypoperfusion and resultant hypoxia (capsaicin can cause respiratory arrest via activation of the pulmonary chemoreflex [11]). Indeed, other than the expected effect on thermoregulation, no behavioral changes were noted after capsaicin microinjections (although these studies were not designed to detect such changes).
0165-6147/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.tips.2008.09.004 Available online 22 October 2008
Opinion The quest for capsaicin-sensitive brain structures using capsaicin as a tool was further hindered by the recognition of the widespread action of the compound on membrane proteins other than its receptor, such as, for example, the activation (e.g. adenylate cyclase) or blockade (e.g. NAHD oxidoreductase, voltage-sensitive sodium channels) of various enzymes and receptors and the influence on membrane fluidity (for review, see Ref. [3]). Methodological advances after the cloning of TRPV1 [12] finally resolved the controversy surrounding the existence of brain TRPV1: both TRPV1 mRNA and protein were clearly identified in several brain structures [13]. These developments have sparked immense interest in academia and drug discovery companies alike towards brain TRPV1 as a potential friend (i.e. therapeutic target for neurological and psychiatric diseases) and/or foe (i.e. mediator of unwanted side-effects). TRPV1 in the brain: only ‘hotheaded’ or something worse? The presence of TRPV1 in the brain is now well established [14]. Nuclease protection-solution hybridization studies demonstrated the existence of TRPV1 mRNA in various brain structures (e.g. hypothalamus, basal ganglia and cerebral cortex), albeit at much lower levels than in the DRG [13,14]. In situ hybridization studies revealed the presence of TRPV1 throughout the whole CNS, from the olfactory bulb through to the cortex, the basal ganglia and the limbic system down to the brainstem and cerebellum [13,14]. This was unexpected because these structures have little in common with each other or with DRG neurons [14]. Importantly, TRPV1-like immunoreactivity was demonstrated in the corresponding brain areas of wildtype, but not TRPV1-null, mice [15], and specific resiniferatoxin (RTX) binding implied that the TRPV1 protein corresponds to a functional vanilloid receptor [16]. The presence of TRPV1 was also demonstrated in the human brain [13]. In the basal ganglia, confocal microscopy studies showed the colocalization of TRPV1 immunoreactivity with the enzyme tyrosine hydroxylase, indicating its presence in dopaminergic neurons [13]. Despite intensive research, the function of brain TRPV1 in health and disease is still unclear. In rat brain slices, capsaicin was reported to increase the rate of firing in dopaminergic neurons of the midbrain [17]. Accordingly, microinjection of capsaicin into the midbrain ventral tegmentum transiently increased dopamine release in the nucleus accumbens [17]. At the whole-animal level, systemic capsaicin was observed to suppress spontaneous locomotion [18]. The observation that I-RTX, a TRPV1 antagonist, reduces the frequency of spontaneous excitatory postsynaptic currents in dopaminergic neurons of the substantia nigra pars compacta was interpreted to imply an endogenous TRPV1 tone in the CNS [19], similar to that already suggested in neurons that mediate the effects of TRPV1 agonists on body-temperature regulation [5]. Additionally, TRPV1 activation in mesencephalic neurons by a high concentration of capsaicin results in cell death [20]. Based on these findings, it was speculated that brain TRPV1 might play a part in the pathogenesis of disease states as diverse as Parkinson’s disease, Alzhei-
Trends in Pharmacological Sciences
Vol.29 No.12
mer’s disease and schizophrenia (for review, see Refs [2,21]). These theories were met by skepticism in the neuroscience community mostly because TRPV1-null (‘knockout’) mice have a fairly unremarkable behavioural phenotype (although one might counter-argue that such animals compensate for the absence of TRPV1) (for review, see Ref. [2]). Furthermore, neither capsaicin nor the firstgeneration TRPV1 antagonist capsazepine is selective for TRPV1, casting a cloud over the interpretation of pharmacological studies (for review, see Refs [3,4]). Indeed, capsaicin induces cell death by direct activation of pathways of apoptosis in various cell lines that do not express TRPV1 [22]. Whereas it is easy to visualize why a receptor involved in body-temperature regulation is heat sensitive [5], it is far more complicated to assign a role for a polymodal nociceptor such as TRPV1 in hippocampal or ventral tegmentum neurons. It was postulated that brain TRPV1 is regulated by endogenous ligands, referred to as ‘endovanilloids’ [23]. Of putative endovanilloids, arachidonic acid derivatives have attracted the most attention [24]. The possibility that anandamide, originally isolated as an endogenous cannabinoid CB1-receptor agonist (Box 1), is an important ligand that regulates brain TRPV1 was reviewed elsewhere [14,23]. Subsequently, N-arachidonyol dopamine (NADA) was isolated from brain nuclei where TRPV1 is also present [25]. In vitro, NADA activates TRPV1, although it is unclear whether it can reach high Box 1. A brief overview of the endocannabinoid system The endocannabinoid system comprises three components: (i) Two cloned G-protein-coupled receptors, known as cannabinoid receptor type-1 (CB1) and type-2 (CB2), discovered while looking for a mechanism of action for the cannabis psychotropic principle, D9-tetrahydrocannabinol. (ii) Endogenous agonists of these two receptors, known as endocannabinoids, of which anandamide (N-arachidonoylethanolamine) and 2-arachidonoylglycerol (2-AG) are the two most studied members. (iii) Proteins for the regulation of endocannabinoid levels and actions. Enzymes responsible for endocannabinoid biosynthesis and inactivation have been cloned and include: (i) N-arachidonoylphosphatidylethanolamine-specific phospholipase D (NAPE-PLD), a,b-hydrolase 4 and protein tyrosine phosphatase PTN22, which convert N-arachidonoyl-phosphatidylethanolamine into anandamide via various corresponding routes together with other, as yet uncharacterized, enzymes; (ii) diacylglycerol lipases (DAGL)-a and b, which convert sn-2-arachidonate-containing diacylglycerols into 2-AG; (iii) fatty acid amide hydrolase (FAAH), which hydrolyses anandamide to arachidonic acid and ethanolamine; and (iv) the monoacylglycerol lipase (MAGL), which hydrolyses 2-AG to arachidonic acid and glycerol [64]. Pharmacological tools for the study of the proteins of the endocannabinoid system have been developed and include cannabinoid-receptor-specific agonists and antagonists and DAGL-, MAGL- and FAAH- selective inhibitors [65]. In the CNS, endocannabinoids often inhibit both excitatory and inhibitory neurotransmitter release and mediate neuroplasticity (Box 2) by acting as retrograde signals released from postsynaptic neurons and activating presynaptic CB1 receptors coupled to inhibition of voltage-activated Ca2+ channels or stimulation of inwardly-rectifying K+ channels [66].
595
Opinion enough concentrations in vivo to gate the channel. However, ligand affinity for TRPV1 is regulated by various factors such as the phosphorylation state of the protein (for review, see Refs [2,4]). Consequently, affinities determined in vitro do not necessarily reflect the in vivo situation. Moreover, the binding site on TRPV1 for anandamide (and also probably NADA) is intracellular (for review, see Ref. [4]). Because anandamide is biosynthesized inside the cell, its intracellular concentration might considerably exceed that in the extracellular matrix and be sufficient to activate TRPV1, as shown for DRG neurons [26]. Importantly, enzymes that biosynthesize or degrade endovanilloids are co-localized with TRPV1 in several cerebellar and hippocampal neurons [27]. Brain endovanilloids were suggested to tonically increase TRPV1 activity, thereby regulating, among other things, glutamate release in a large variety of brain nuclei and, consequently, dopaminergic activity in the substantia nigra [19], descending antinociception in the periaqueductal grey [28] and emesis in the nucleus of the solitary tract [29]. Brain TRPV1: a role in affective disorders and in rimonabant central side effects? TRPV1-null mice were shown to exhibit impaired longterm potentiation (LTP; Box 2) in the hippocampus [30]. In keeping with this earlier observation, a novel facilitatory role for TRPV1 in synaptic plasticity has been described, that is, long-term depression (LTD; Box 2) of excitatory inputs from pyramidal neurons onto g-aminobutyric acid (GABA)-ergic interneurons in this brain area [31]. Based on some of this data [31] together with previous findings that the cannabinoid CB1-receptor antagonist rimonabant (sold in the EU under the brand name Acomplia) antagonizes TRPV1 receptors at mM concentrations [32], it was Box 2. Long-term potentiation (LTP); long-term depression (LTD); neuroplasticity and neurogenesis First observed by Bliss and Lomo in 1973 [67], LTP is operationally defined as a long-lasting increase in synaptic efficacy, which follows high-frequency stimulation of afferent fibers. A stimulating electrode was placed at the level of perforant path fibers of the hippocampus and another recording electrode in the postsynaptic neuron of the dentate area. A single electrical pulse produced a response at the postsynaptic level (baseline); later, a brief burst of many rapidly repeated pulses, or ‘potentiating stimuli’, were given, producing enhanced synaptic responses and lasting for many hours. Slow, weak electrical stimulation of CA1 neurons also causes long-term changes in the synapses: in this case, a reduction in their sensitivity. This is called long-term depression (LTD). Both LTP and LTD are presumed to play a part in the establishment of stable memories. Moreover, they are thought to form a part of the physiological mechanisms underlying neuronal plasticity or neuroplasticity; that is, the ability of neurons and brain organization to change after environmental and/or pharmacological stimuli. Neuroplasticity includes different neuronal phenomena, for example, in the young brain developmental plasticity occurs when neurons rapidly sprout branches and form synapses. In the adult brain, neurogenesis (which means the ‘birth of new neurons’) occurs mostly in the dentate gyrus of the hippocampus and in the subventricular zone of the olfactory system. Recently, it has been demonstrated that antidepressant drugs induce neurogenesis in the dentate gyrus and this effect is necessary for the antidepressant response [68,69].
596
Trends in Pharmacological Sciences Vol.29 No.12
speculated that facilitation of depression by rimonabant in some predisposed patients [33] might be due to antagonism of hippocampal TRPV1 receptors and resultant LTD blockade. Rimonabant has been tested in four obesity phase III trials involving >2500 obese patients and, as of mid 2008 [33], is available in 55 countries worldwide for patients who are obese and have associated risk factors for cardiovascular disease such as type-2 diabetes or dyslipidemia. In the United States, however, an advisory panel to the Food and Drug Administration (www.fda.gov) concluded that the safety of rimonabant, particularly with regard to increased occurrence of psychiatric disorders (e.g. anxiety and depression), had not been fully demonstrated. Rimonabant improves the profile of cardiometabolic risk factors in obese subjects largely by reducing the overactivity of peripheral CB1 receptors, whereas its worrisome sideeffects are centrally mediated [34]. But, is it likely that these effects involve central TRPV1 blockade? Because the answer to this question is of great practical importance (also for the future of the whole class of TRPV1 antagonist drugs), one should be cautious and use scrutiny in interpreting the findings. Several lines of evidence argue against a link between suppression of LTD by rimonabant via TRPV1 blockade in vitro and the mood disorders that are worsened by rimonabant in the clinic. (i) The connection between LTD at hippocampal-interneuron synapses in the CA1 region of the hippocampus (the form of LTD studied in Ref. [31]) and the development of mood disorders, particularly depression, is not supported by the available literature. In fact, the involvement of long-term synaptic plasticity in the pathogenesis of major depression and in the mechanism of action of antidepressants is still debated. Exposure to chronic mild stress, a reliable animal model of depression, facilitates LTD but has no effect on LTP in the CA1 region. The antidepressant fluvoxamine increases LTP in stressed and nonstressed animals, thus preventing the stress-induced facilitation of LTD [35]. However, this effect cannot be considered a hallmark of all antidepressant drugs because the tryciclic desipramine and mianserine enhance LTP in the dentate gyrus [36], whereas chronic fluoxetine and electroshock treatments, by contrast, attenuate LTP in the same area [37]. Conversely, electroconvulsive stimulation for one week, and chronic lithium treatment, impairs LTD in the striatum [38] indicating that antidepressant treatment blocks LTD. For all these reasons, the effects of antidepressants on LTP and LTD have been attributed to indirect effects on Ca2+ influx [39] or to increases in brain-derived neurotropic factor and serotonin levels. (ii) Even in the unlikely possibility that inhibition of LTD via TRPV1 leads to clinical depression, it is very unlikely that rimonabant reaches high enough concentrations in vivo to exert this effect. Rimonabant displays a >100-fold selectivity for CB1 over TRPV1 receptors, and its reported IC50 for TRPV1 in vitro is an order of magnitude higher than the Cmax
Opinion (400 nM) estimated in the plasma of patients on the usual dose of rimonabant (20 mg daily) [32]. (iii) There is no evidence that pharmacological blockade or genetic deletion of TRPV1 leads to mood disorders or
Trends in Pharmacological Sciences
Vol.29 No.12
anxiety. TRPV1-null mice are active, have a healthy appetite and reproduce (for review, see Ref. [4]). On the contrary, TRPV1 antagonism or genetic inactivation seems to produce anxiolytic effects in rodents
Figure 1. (a) In vivo effects of rimonabant on 5-HT (5-hydroxytryptamine; serotonin) firing activity after sub-chronic treatment with vehicle (once daily, 4 days) (i) and URB 597 (0.1 mg/kg, once daily) (ii). Note that URB 597 (a selective inhibitor of the enzyme fatty acid amide hydrolase [FAAH] that is involved in the hydrolysis of endogenous cannabinoid CB1-receptor agonists [Box 1]) increases the tonic activation of 5-HT activity and has antidepressant-like activity, whereas rimonabant (0.1 mg/kg, intravenously [i.v.]) dramatically suppresses the 5-HT activity. This decrease is believed to correspond to the worsening of depression in humans [48] (figure adapted, with permission, from Ref. [48]). (b) (i) At low doses, the selective CB1 agonist WIN 55 212 (0.2–0.4 mg/kg, i.v.) increases 5-HT firing activity in vivo, and this effect is blocked by rimonabant (1 mg/kg) but not by the TRPV1 antagonist capzazepine (0.02 mg/kg, i.v.). (ii) However, at higher doses (0.4 mg/kg, i.v) the WIN 55 212 decreases 5-HT firing activity and this effect is blocked by capzazepine but not by rimonabant (figure adapted, with permission, from Ref. [43]). It should be noted that raising anandamide levels by blocking FAAH, especially when using high doses of URB597, will increase the likelihood of activating both CB1 and TRPV1 in various brain regions [63]. However, the antidepressant-like effect of URB597 is reversed by a dose of rimonabant that does not reverse the effect of a high dose of the compound WIN 55 212–2 (a) (ii) and (b) (ii). Altogether, these observations indicate that the antidepressant-like effects of URB597 are due uniquely to indirect activation of cannabinoid CB1 receptors and that the dose of rimonabant used in these experiments is not sufficient to antagonize TRPV1 receptors.
597
Opinion [30,40,41]. By contrast, chronic use of rimonabant in some obese patients is associated not only with increased depression but also with anxiety [33]. (iv) During the past 20 years, a hypoactivity of serotonin function has been firmly linked to dampening of mood and depression [42]. Indeed, drugs that increase serotonin neurotransmission act as antidepressants. Interestingly, it was demonstrated that low doses of the CB1 agonist WIN 55 212 have antidepressant-like properties in the forced swim test and increase in vivo serotonin firing activity; this effect is reversed by rimonabant but not by the TRPV1 antagonist capsazepine [43]. Conversely, higher doses of WIN 55 212 dramatically decrease serotonin firing and this effect was reversed by capsazepine but not rimonabant [43]. Capsazepine also increased serotonin activity over the baseline, thus indicating a facilitatory role of TRPV1 in depression. This hypothesis is supported by the evidence that the TRPV1 agonist olvanil has depressive-like properties in the forced swim test [40]. These data indicate that the antidepressant-like activity of CB1 agonists in rodents, and likewise the prodepressant-like activity of CB1 antagonists, is mediated by CB1 rather than TRPV1 receptors. (It was suggested that WIN 55 212 might also act on a yet to be identified target that is distinct from both CB1 and TRPV1 [44].) In fact, CB1-null-mutant mice exhibit depressive-like and anxiety-like behaviors in several behavioral paradigms [45,46]. (v) Last, taranabant, a CB1 receptor antagonist that is chemically very different from rimonabant, also worsens depression and anxiety [47]. Combined, these observations indicate that mood disorders by rimonabant and taranabant are due to a reduction of CB1-receptor activity in the CNS. However, the effects of CB1 antagonists on depression are complex. Rimonabant at low doses (<2 mg/kg which are ineffective on the basal activity of serotoninergic neurons) is inactive in animal models of depression-like behaviour [43,48], indicating that in normal conditions the endocannabinoid system does not exert a tonic activity over serotoninergic neurons. However, in animals repeatedly injected with the fatty acid amide hydrolase (FAAH) inhibitor URB597, which probably causes an increase in endocannabinoid levels in the brain, an increased serotonin tone was observed, and 1 mg/kg of rimonabant dramatically decreased serotonin neuronal firing activity below control levels (Figure 1). This situation might mimic what could happen in a subgroup of obese patients with increased brain endocannabinoid and serotonin activity, in whom rimonabant could similarly induce an abrupt drop in serotonin electrical activity, thereby enabling the development of depressive symptoms. It is also worth mentioning that a modest increase in serotonin activity in the prefrontal cortex has been observed after high doses of rimonabant [49] and that a decreased immobility in the forced swim test (a sign of antidepressant activity) has been observed after oral injection of high doses of rimonabant [50], although these results are not yet supported by clinical data. 598
Trends in Pharmacological Sciences Vol.29 No.12
Although, in our opinion, the cons with regard to TRPV1 being the target for the psychiatric adverse effects of rimonabant outweigh the pros, this theory cannot be disproved beyond any reasonable doubt. The affinity of TRPV1 for rimonabant might be considerably higher in vivo than in vitro. In fact, some central neuroprotective effects of this compound (e.g. against global cerebral ischemia in gerbils and in favour of adult neurogenesis in mice) have been ascribed to interactions with TRPV1 rather than CB1 receptors [51]. Furthermore, taranabant, to our best knowledge, is yet to be tested on TRPV1. Lastly, at least in principle, TRPV1 antagonists can be anxiolytic in rodents, yet, they might induce other types of mood disorders in humans, as indicated by the finding that TRPV1 in mice facilitates LTP and suppresses another type of hippocampal LTD, thereby protecting hippocampal synaptic plasticity from the influence of acute stress [52]. Until these questions are resolved, patients who participate in clinical trials with TRPV1 antagonists should be carefully watched for signs of mood disorders. As for rimonabant, several authorities agree that, especially if administered to the right patient, the benefits of this drug on cardiometabolic risk factors should, in the long run, outweigh its effects on depression. Brain TRPV1: friend or foe? There is good evidence that activation of TRPV1 is neuroprotective in a model of brain ischemia [53]. This action might be mediated by TRPV1 expressed in endothelial cells [54]. Interestingly, activation of TRPV1 in brain neurons might have the opposite action. In addition to neurons, TRPV1 is expressed in microglia, astrocytes and pericytes and it has been suggested that TRPV1 might play an important part in Alzheimer’s-disease-related neuroinflammatory processes [55]. TRPV1 is also an important player in neuro-immunological regulation and, given the emerging role for TRPV1 in the pathogenesis of autoimmune (type-1) diabetes (for review, see Ref. [56]), the possibility exists that TRPV1 might be involved in other autoimmune diseases including multiple sclerosis. Exposure of mesencephalic dopaminergic or hippocampal neurons to capsaicin triggers cell death or apoptosis, an effect prevented by TRPV1 antagonists, and is accompanied by increasing intracellular Ca2+ levels and/ or mitochondrial damage, mitochondrial cytochrome c release and formation of caspase-3, a mediator of apoptosis [20,57]. These effects, together with a direct action on locomotion, might be relevant to a possible role of TRPV1 in neurodegenerative motor and cognitive disorders. Indeed, agonists of this receptor were found to reduce hyperkinesias in animal models of hyperdopaminergia [58] and Huntington’s chorea [59] and to ameliorate levodopa-induced dyskinesia in a model of Parkinson’s disease [60]. However, opposite effects might exist with regard to levodopa-induced dyskinesia because it was recently suggested that TRPV1 receptors might limit, rather than enhance, the beneficial action of elevated anandamide levels against levodopa-induced dyskinesia [61]. Thus, it is not impossible that, in the brain, both TRPV1 agonists and antagonists exert the same beneficial effect, as shown in a model of excitotoxicity [62] and for some antipsychotics
Opinion Box 3. Outstanding questions What is the role of brain TRPV1 receptors in health and disease? What are the endogenous activators of brain TRPV1 receptors? Considering that anandamide has affinity for both CB1 and TRPV1 receptors, under what neurophysiological circumstances does it activate one or the other? Is pharmacological blockade of TRPV1 by small-molecule antagonists given as analgesic drugs depressive or, by contrast, anxiolytic (or, possibly, neither)? Might TRPV1 ligands (agonists or antagonists) be of clinical value in the treatment of neurological or psychiatric diseases? Do CB1 and TRPV1 receptors have similar, opposite or complementary roles in CNS functions? What is the neurobiological mechanism that renders a subgroup of obese patients more susceptible to the pro-depressant effect of CB1 antagonism? Do they have an elevated endogenous tone of brain endocannabinoids and, consequently, an elevated monoaminergic tone?
(for review, see Ref. [3]). This would not be unprecedented because both TRPV1 desensitization by agonists and blockade by antagonists have a similar outcome (i.e. analgesia) also in nociceptive neurons [3]. Therefore, it is possible that, similar to DRG neurons, also activation of brain TRPV1 is followed by desensitization. Because TRPV1 gating in the brain seems to be mostly coupled to glutamate release, pharmacological TRPV1 overactivation, if followed by desensitization, might result in reduced neuronal excitability, exactly as it would be with TRPV1 antagonism in the presence of an endovanilloid tone. Conclusions Much research is still needed to fully appreciate the role of TRPV1 in the CNS and, hence, the potential central consequences of the pharmacological targeting of this channel with either agonists or antagonists with therapeutic activity (Box 3). Although there is now little doubt about the functional activity of brain TRPV1 receptors, future efforts will have to be aimed at understanding the exact mechanisms leading to their activation under physiological and pathological conditions and their consequences on neurotransmitter release and action. References 1 Bandell, M. et al. (2007) From chills to chilies: mechanisms for thermosensation and chemesthesis via thermoTRPs. Curr. Opin. Neurobiol. 17, 490–497 2 Cortright, D.N. and Szallasi, A. TRP channels and pain. Curr. Pharm. Des. (in press) 3 Szallasi, A. and Blumberg, P.M. (1999) Vanilloid (capsaicin) receptors and mechanisms. Pharmacol. Rev. 51, 159–211 4 Szallasi, A. et al. (2007) The vanilloid receptor TRPV1: 10 years from channel cloning to antagonist proof-of-concept. Nat. Rev. Drug Discov. 6, 357–372 5 Gavva, N. Body-temperature maintenance as the predominant function of the vanilloid receptor TRPV1. Trends Pharmacol. Sci. (in press) 6 Jancso´-Ga´bor, A. et al. (1970) Stimulation and desensitization of the hypothalamic heat-sensitive structure by capsaicin in rats. J. Physiol. 208, 449–459 7 Szolcsa´nyi, J. et al. (1971) Mitochondrial changes in preoptic neurons after capsaicin desensitization of the hypothalamic thermodetectors in rats. Nature 229, 116–117 8 Hori, T. et al. (1988) Responses of anterior hypothalamic-preoptic thermosensitive neurons to locally applied capsaicin. Neuropharmacology 27, 135–142
Trends in Pharmacological Sciences
Vol.29 No.12
9 Ritter, S. and Dinh, T.T. (1990) Capsaicin-induced neuronal degeneration in the brain and retina of preweanling rats. J. Comp. Neurol. 296, 447–461 10 Ritter, S. and Dinh, T.T. (1992) Age-related changes in capsaicininduced neurodegeneration in rat brain. J. Comp. Neurol. 318, 103–116 11 Szolcsa´nyi, J. et al. (1990) Resiniferatoxin: an ultrapotent selective modulator of capsaicin-sensitive primary afferent neurons. J. Pharmacol. Exp. Ther. 255, 923–928 12 Caterina, M.J. et al. (1997) The capsaicin receptor: a heat-activated ion channel in the pain pathway. Nature 389, 816–824 13 Mezey, E. et al. (2000) Distribution of mRNA for vanilloid receptor subtype 1 (VR1), and VR1-like immunoreactivity, in the central nervous system of the rat and human. Proc. Natl. Acad. Sci. U. S. A. 97, 3655–3660 14 Szallasi, A. and Di Marzo, V. (2000) New perspectives on enigmatic vanilloid receptors. Trends Neurosci. 23, 491–497 15 Cristino, L. et al. (2006) Immunohistochemical localization of cannabinoid type 1 and transient receptor potential vanilloid type 1 receptors in the mouse brain. Neuroscience 139, 1405–1416 16 Acs, G. et al. (1996) Specific binding [3H]resiniferatoxin by human and rat preoptic area, locus ceruleus, medial hypothalamus, reticular formation and ventral thalamus membrane preparations. Life Sci. 59, 1899–1908 17 Marinelli, S. et al. (2005) Activation of TRPV1 in the VTA excites dopaminergic neurons and increases chemical and noxious-induced dopamine release in the nucleus accumbens. Neuropsychopharmacology 30, 864–870 18 Di Marzo, V. et al. (2001) Hypolocomotor effects in rats of capsaicin and two long chain capsaicin homolgues. Eur. J. Pharmacol. 420, 123–131 19 Marinelli, S. et al. (2003) Postsynaptic facilitation of glutaminergic synapses to dopaminergic neurons of the rat substantia nigra by endogenous stimulation of vanilloid receptors. J. Neurosci. 23, 3136– 3144 20 Kim, S.R. et al. (2005) Transient receptor potential vanilloid subtype-1 mediates cell death of mesencephalic dopaminergic neurons in vitro and in vivo. J. Neurosci. 25, 662–671 21 Starowicz, K. et al. (2008) TRPV1 receptors in the central nervous system: potential for previously unforeseen therapeutic applications. Curr. Pharm. Des. 14, 42–54 22 Athanasiou, A. et al. (2007) Vanilloid receptor agonists and antagonists are mitochondrial inhibitors: how vanilloids cause non vanilloid receptor-mediated cell death. Biochem. Biophys. Res. Commun. 354, 50–55 23 Di Marzo, V. et al. (2002) Endovanilloid signaling in pain. Curr. Opin. Neurobiol. 12, 372–379 24 Starowicz, K. et al. (2007) Biochemistry and pharmacology of endovanilloids. Pharmacol. Ther. 114, 13–33 25 Huang, S.M. et al. (2002) An endogenous capsaicin-like substance with high potency at recombinant and native vanilloid VR1 receptors. Proc. Natl. Acad. Sci. U. S. A. 99, 8400–8405 26 Van der Stelt, M. et al. (2005) Anandamide acts as an intracellular messenger amplifying Ca2+ influx via TRPV1 channels. EMBO J. 24, 3026–3037 27 Cristino, L. et al. (2008) Immunohistochemical localization for anabolic and catabolic enzymes for anandamide and other putative endovanilloids in the hippocampus and cerebellar cortex of the mouse brain. Neuroscience 151, 955–968 28 Starowicz, K. et al. (2007) Tonic endovanilloid facilitation of glutamate release and brainstem descending antinociceptive pathways. J. Neurosci. 27, 13739–13749 29 Sharkey, K.A. et al. (2007) Arvanil, anandamide, and N-arachidonoyldopamine (NADA) inhibit emesis through cannabinoid CB1 and vanilloid TRPV1 receptors in the ferret. Eur. J. Neurosci. 25, 2773– 2782 30 Marsch, M. et al. (2007) Reduced anxiety, conditioned fear, and hippocampal long term potentiation in transient receptor potential vanilloid subtype 1 receptor -deficient mice. J. Neurosci. 27, 832– 839 31 Gibson, H.E. et al. (2008) TRPV1 channels mediate long term depression at synapses on hippocampal interneurons. Neuron 57, 746–759 32 De Petrocellis, L. et al. (2001) The activity of anandamide at vanilloid VR1 receptors requires facilitated transport across the cell membrane
599
Opinion
33
34 35 36
37
38
39
40
41
42
43
44
45
46 47
48
49
50
600
and is limited by intracellular metabolism. J. Biol. Chem. 276, 12856– 12863 Van Gaal, L. et al. (2008) Efficacy and safety of rimonabant for improvement of multiple cardiometabolic risk factors in overweight/ obese patients. Pooled 1-year data from the Rimonabant in Obesity (RIO) program. Diabetes Care 31, S229–S240 Di Marzo, V. (2008) Targeting the endocannabinoid system: to enhance or reduce? Nat. Rev. Drug Discov. 7, 438–445 Holderbach, R. et al. (2007) Enhanced long-term synaptic depression in an animal model of depression. Biol. Psychiatry 62, 92–100 Levkovitz, Y. et al. (2001) Transcranial magnetic stimulation and antidepressive drugs share similar cellular effects in rat hippocampus. Neuropsychopharmacology 24, 608–616 Stewart, C.A. and Reid, I.C. (2000) Repeated ECS and fluoxetine administration have equivalent effects on hippocampal synaptic plasticity. Psychopharmacology (Berl.) 148, 217–223 De Murtas, M. et al. (2004) Repeated electroconvulsive stimulation impairs long-term depression in the neostriatum. Biol. Psychiatry 55, 472–476 Dea´k, F. et al. (2000) Inhibition of voltage-gated calcium channels by fluoxetine in rat hippocampal pyramidal cells. Neuropharmacology 39, 1029–1036 Kasckow, J.W. et al. (2004) Effects of the vanilloid agonist olvanil and antagonist capsazepine on rat behaviors. Prog. Neuropsychopharmacol. Biol. Psychiatry 28, 291–295 Micale, V. et al. Anxiolytic effects in mice of a dual blocker of fatty acid amide hydrolase and transient receptor potential vanilloid type-1 channel. Neuropsychopharmacology (in press) Mann, J.J. et al. (2001) The neurobiology and genetics of suicide and attempted suicide: a focus on the serotonergic system. Neuropsychopharmacology 24, 467–477 Bambico, F. et al. (2007) Cannabinoids elicit antidepressant-like behavior and activate serotonergic neurons through the medial prefrontal cortex. J. Neurosci. 27, 11700–11711 Accorsi-Mendonca, D. et al. (2008) Inhibition of spontaneous neurotransmission in the nucleus of the solitary tract of the rat by the cannabinoid agonist WIN 55212-2 is not via CB1 or CB2 receptors. Brain Res. 1200, 1–9 Haller, J. et al. (2004) Context-dependent effects of CB1 cannabinoid gene disruption on anxiety-like and social behaviour in mice. Eur. J. Neurosci. 19, 1906–1912 Martin, M. et al. (2002) Involvement of CB1 cannabinoid receptors in emotional behaviour. Psychopharmacology (Berl.) 159, 379–387 Carr, R.D. et al. (2008) A two-year study to assess the efficacy, safety, and tolerability of taranabant in obese patients. Atherosclerosis Supplements 9, 12 Gobbi, G. et al. (2005) Antidepressant-like activity and modulation of brain monoaminergic transmission by blockade of anandamide hydrolysis. Proc. Natl. Acad. Sci. U. S. A. 102, 18620–18625 Tzavara, E.T. et al. (2001) The cannabinoid CB1 receptor antagonist SR141716A increases norepinephrine outflow in the rat anterior hypothalamus. Eur. J. Pharmacol. 426, R3–R4 Griebel, G. et al. (2005) Effects of the cannabinoid CB1 receptor antagonist rimonabant in models of emotional reactivity in rodents. Biol. Psychiatry 57, 261–267
Trends in Pharmacological Sciences Vol.29 No.12 51 Pegorini, S. et al. (2006) Vanilloid VR1 receptor is involved in rimonabant-induced neuroprotection. Br. J. Pharmacol. 147, 552–559 52 Li, H.B. et al. Antistress effect of TRPV1 channel on synaptic plasticity and spatial memory. Biol. Psychiatry (in press) 53 Pegorini, S. et al. (2005) Capsaicin exhibits neuroprotective effects in a model of transient global cerebral ischemia in Mongolian gerbils. Br. J. Pharmacol. 144, 727–735 54 Golech, S.A. et al. (2004) Human brain endothelium: coexpression and function of vanilloid and endocannabinoid receptors. Brain Res. Mol Brain Res. 132, 87–92 55 Yamamoto, S. et al. (2007) Transient receptor potential channels in Alzheimer’s disease. Biochim. Biophys. Acta 1772, 958–967 56 Suri, A. and Szallasi, A. (2008) The emerging role of TRPV1 in diabetes and obesity. Trends Pharmacol. Sci. 29, 29–36 57 Kim, S.R. et al. (2006) Transient receptor potential vanilloid subtype 1 mediates microglial cell death in vitro and in vivo via Ca2+-mediated mitochondrial damage and cytochrome-c release. J. Immunol. 177, 4322–4329 58 Tzavara, E.T. et al. (2006) Endocannabinoids activate transient receptor potential vanilloid 1 receptors to reduce hyperdopaminergia-related hyperactivity: therapeutic implications. Biol. Psychiatry 59, 508–515 59 de Lago, E. et al. (2005) Arvanil, a hybrid endocannabinoid and vanilloid compound, behaves as an antiohyperkinetic agent in a rat model of Huntington’s disease. Brain Res. 1050, 210–216 60 Lee, J. et al. (2006) A role for vanilloid receptor (TRPV1) and endocannabinoid signaling in the regulation of spontaneous and LDOPA-induced locomotion in normal and reserpine-treated rats. Neuropharmacology 51, 557–565 61 Morgese, M.G. et al. (2007) Anti-dyskinetic effects of cannabinoids in a rat model of Parkinson’s disease: role of CB1 and TRPV1 receptors. Exp. Neurol. 208, 110–119 62 Veldhuis, W.B. et al. (2003) Neuroprotection by the endogenous cannabinoid anandamide and arvanil against in vivo excitotoxicity in the rat: role of vanilloid receptors and lipoxigenases. J. Neurosci. 23, 4127–4133 63 Rubino, T. et al. (2008) Role in anxiety behavior of the endocannabinoid system in the prefrontal cortex. Cereb. Cortex 18, 1292–1301 64 Di Marzo, V. and Petrosino, S. (2007) Endocannabinoids and the regulation of their levels in health and disease. Curr. Opin. Lipidol. 18, 129–140 65 Fowler, C.J. (2008) The tools of the trade’’ – an overview of the pharmacology of the endocannabinoid system. Curr. Pharm. Des. 14, 2254–2265 66 Katona, I. and Freund, T.F. (2008) Endocannabinoid signaling as a synaptic circuit breaker in neurological disease. Nat. Med. 14, 923– 930 67 Bliss, T.V. and Lomo, T. (1973) Long-lasting potentiation of synaptic transmission in the dentate area of the anaesthetized rabbit following stimulation of the perforant path. J. Physiol. 232, 331–356 68 Malberg, J.E. et al. (2000) Chronic antidepressant treatment increases neurogenesis in adult rat hippocampus. J. Neurosci. 20, 9104–9110 69 Santarelli, L. et al. (2003) Requirement of hippocampal neurogenesis for the behavioral effects of antidepressants. Science 301, 805–809