Interplay between synaptic endocannabinoid signaling and metaplasticity in neuronal circuit function and dysfunction.

European Journal of Neuroscience, Vol. 39, pp. 1189–1201, 2014

doi:10.1111/ejn.12501

Interplay between synaptic endocannabinoid signaling and metaplasticity in neuronal circuit function and dysfunction Miriam Melis,1 Barbara Greco2 and Raffaella Tonini2 1 2

Department of Biomedical Sciences, Division of Neuroscience and Clinical Pharmacology, University of Cagliari, Cagliari, Italy Department of Neuroscience and Brain Technologies, Istituto Italiano di Tecnologia (IIT), via Morego 30, 16163 Genoa, Italy

Keywords: behavioral plasticity, endocannabinoids, experience-dependent plasticity, metaplasticity, synapse modulation

Abstract Synaptic neuromodulation acts across different functional domains to regulate cognitive processing and behavior. Recent challenges are related to elucidating the molecular and cellular mechanisms through which neuromodulatory pathways act on multiple time scales to signal state-dependent contingencies at the synaptic level or to stabilise synaptic connections during behavior. Here, we present a framework with the synaptic neuromodulators endocannabinoids (eCBs) as key players in dynamic synaptic changes. Modulation of various molecular components of the eCB pathway yields interconnected functional activation states of eCB signaling (prior, tonic, and persistent), which may contribute to metaplastic control of synaptic and behavioral functions in health and disease. The emerging picture supports aberrant metaplasticity as a contributor to cognitive dysfunction associated with several pathological states in which eCB signaling, or other neuromodulatory pathways, are deregulated.

Introduction Behavior reflects the current activity states of large-scale neuronal networks. However, major questions relate to the ways in which network dynamics during cognition and behavior integrate animal internal states, such as attention or motivation, and the ways in which this process is influenced by experience or learning. Increasing experimental evidence indicates that synaptic neuromodulation may serve this purpose. At the system level, synaptic neuromodulators such as dopamine, adenosine, acetylcholine, endorphins and endocannabinoids (eCBs) may transduce salient environmental cues and/or integrate internal states into patterns of behavior (Arnsten et al., 2012; Lee & Dan, 2012). For example, dopamine is a key regulator of behaviors directed towards the acquisition and consumption of natural rewards (Schultz, 1998). Patterns of motor behavior may be context-dependent. When an animal is hungry, it actively seeks food, but it may not show this behavior when sated. In the first case, extracellular concentrations of dopamine are expected to be large, leading to increased locomotor behavior to forage for food. The opposite scenario would occur in a sated animal without motivation to ingest more food. Under this condition, dopamine concentrations are expected to be low, resulting in reduced locomotor activity (Szczypka et al., 1999). At the cellular level, neuromodulators typically act through G-protein-coupled mechanisms to modify ion channel activity and affect neuronal excitability in response to synaptic inputs and neurotransmitter receptors and release processes that shape synaptic strength (Scheiderer et al., 2004; Choi et al., 2005; Seol et al., 2007). Because receptors for neuromodulators are functionally coupled to the activation of intracellular signaling cascades, they can influence

Correspondence: Raffaella Tonini, as above. E-mail: [email protected] Received 16 October 2013, revised 23 December 2013, accepted 7 January 2014

nuclear transcription mechanisms. Hence, neuromodulatory pathways have the potential to modify the polarity and gain of synaptic connections (i.e. synaptic efficacy) in a synchronised manner with the dynamics of biochemical cascades to signal state-dependent contingencies at the synaptic level. Not only can synaptic neuromodulators shape the instantaneous synaptic strength during behaviors based on the animal’s current needs, but they can also control long-term forms of synaptic plasticity to trigger behavioral changes. This control may occur in response to changes in the animal’s needs and upon learning or exposure to environmental cues (Hasselmo, 1995; Arnsten et al., 2012; Lee & Dan, 2012; Oleson et al., 2013). In pathological states, altered synaptic neuromodulation may lead to the aberrant assignment of salience to environmental events and internal representations during information processing, which may lead to cognitive and behavioral dysfunction. Neuromodulatory pathways regulate long-term potentiation (LTP) or long-term depression (LTD) by directly contributing to the cellular mechanisms that are necessary for the induction of plasticity. Furthermore, activation of neuromodulatory receptors can modify the threshold for plasticity induction (Scheiderer et al., 2004; Choi et al., 2005), influence the state in which a synapse resides (Montgomery & Madison, 2002), or exert ‘pull–push’ control of bidirectional synaptic plasticity (Montgomery & Madison, 2004; Huang et al., 2012). According to the Bienenstock–Cooper–Munro computational model of synaptic plasticity (Bienenstock et al., 1982), afferent activity will be less effective in generating LTP if time-averaged levels of postsynaptic firing have recently been high, thereby reducing the threshold needed to generate LTD. Neuromodulation may modify plasticity threshold activity by affecting the degree of postsynaptic cell firing during afferent activation. These changes in firing rates can occur over the course of minutes, and last for hours or days.

© 2014 Federation of European Neuroscience Societies and John Wiley & Sons Ltd

1190 M. Melis et al. In the ‘state model’, synapses may exist in various discrete states that depend on synaptic activation history (Montgomery & Madison, 2002). Receptors for synaptic neuromodulators, and their downstream intracellular targets, may influence the properties of the state in which the synapses reside, or the states recently visited by synapses. This may determine the ability of a synapse to undergo future synaptic plasticity and the mechanisms by which this process would occur. In the pull–push regulation of LTP and LTD, the polarity (i.e. facilitation or suppression) of the neuromodulatory effect depends on the signaling cascade downstream from G-protein activation (Huang et al., 2012). Receptors that are coupled to Gs-proteins can simultaneously promote LTP and suppress LTD, whereas receptors coupled to Gq11 can promote LTD and suppress LTP. This mechanism, which operates on a time scale of minutes, may be particularly suitable for signaling the behavioral states of an animal at the synaptic level. Regulation of plasticity thresholds, state-dependent plasticity and pull–push control of LTP/LTD have been collectively defined as metaplasticity mechanisms (Abraham & Bear, 1996; Abraham, 2008). Metaplasticity has been conceptualised as an activity-dependent change in physiological and biochemical neuronal states, synapses and networks that alters their ability to generate synaptic plasticity (Abraham & Bear, 1996). An essential property of metaplasticity is that there must be a change in neuronal function upon priming stimulation (presynaptic or postsynaptic activity). This effect must persist after the removal of the priming stimulus, and influence the ability of a subsequent stimulus to induce synaptic plasticity. Emerging experimental evidence suggests that metaplasticity may act across different functional domains to control cognitive processing, under both physiological and pathological conditions. In processes triggered by repeated training trials, learning-induced metaplastic mechanisms may induce synaptic states in which subthreshold stimuli can trigger synaptic changes, thus facilitating subsequent learning (Zelcer et al., 2006). Metaplasticity may also be a means of signaling within the network which synapses are conveying salient information and which synapses are not, depending on the level of activation in repetitive stimulation. For instance, a weak priming bout of activity that results in the metaplastic inhibition of LTP upon a subsequent stimulation may indicate that a specific synapse is encoding non-behavioral relevant information; therefore, its potentiation during repetitive training must be prevented. In this scenario, aberrant metaplasticity may contribute to the cognitive dysfunction associated with several pathological states in which neuromodulatory pathways are deregulated, such as in drug or alcohol addiction, compulsive behaviors, and autism. In this review, we summarise evidence suggesting that not only does synaptic neuromodulation represent a mechanism for triggering metaplasticity, but that it may be dynamically regulated through metaplastic phenomena. This may provide an additional level of complexity to the metaplastic control of neuronal circuit function and behavioral outputs in health and disease. Neuromodulators do not typically segregate functionally; rather, they often interact to regulate synaptic plasticity, as supported by anatomical evidence for neuromodulatory pathways converging on the same brain region [for example, see Bear & Singer (1986) and Zhou et al. (2001)]. Here, we focus on the lipid molecules eCBs, which are key neuromodulators of plasticity in long-range connected neuronal circuits, including the corticomesolimbic and corticostriatal pathways implicated in the control of motivated and motor behaviors.

eCBs are synthesised in the postsynaptic neurons, and mostly exert retrograde actions on presynaptic cannabinoid (CB) receptors to inhibit neurotransmitter release. As compared with other neuromodulatory pathways, eCB signaling shows two unique features: (i) it integrates chemical signals generated by different neurotransmitters and synaptic neuromodulators (e.g. glutamate, dopamine, and acetylcholine) with changes in neuronal cell excitability; and (ii) by acting mainly as a retrograde messenger, eCB signaling represents a fundamental means by which postsynaptic neuronal activity finetunes the synaptic gain at neuronal afferents, and can influence the transmission of information relevant to behavior.

eCB signaling eCBs constitute a family of lipid molecules that belong to an unconventional neurotransmitter system comprising synthesising and inactivating enzymes, a transport protein, and the CB receptors (Jonsson et al., 2006; Marsicano & Lutz, 2006; Katona & Freund, 2012). The most well-described eCBs are anandamide (AEA) (Devane et al., 1992) and 2-arachidonoyl glycerol (2-AG) (Mechoulam et al., 1995; Sugiura et al., 1995). Among the other eCBs, 2-arachidonylglyceryl ether (nolandin), O-arachidonoyl-ethanolamine (virodhamine) and N-arachidonoyl-dopamine are the most studied; however, their physiological role remains elusive. Increased levels of eCBs usually form part of an on demand response, which occurs upon physiological and/or pathological stimulation in several brain regions (Di Marzo et al., 1994; Cadas et al., 1996). Once released, eCBs activate mainly type 1 CB (CB1) receptors, which primarily inhibit neurotransmitter release and are located in the presynaptic compartment. By selectively reducing synaptic inputs onto the releasing neuron(s), eCBs influence diverse forms of synaptic plasticity. eCBs are rapidly cleared away from their extracellular targets by a specific uptake system (Beltramo et al., 1997; Hillard & Jarrahian, 2000), named the AEA membrane transporter, which is widely distributed throughout the brain (Giuffrida et al., 2001). AEA and 2-AG are subsequently and very efficiently degraded mainly by two enzymes: fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL), respectively (Cravatt et al., 1996; Sugiura & Waku, 2000; Ueda et al., 2000; Dinh et al., 2002). Although MAGL is regarded as the main degrading enzyme of 2-AG, the activity of a/b-hydrolase domain 6 and a/b-hydrolase domain 12 in the brain have been shown to account for a proportion of 2-AG hydrolysis (Blankman et al., 2007; Marrs et al., 2010; Savinainen et al., 2012). Along with AEA, other N-acylethanolamines (NAEs), such as oleoylethanolamide and palmitoylethanolamide, are degradation targets of FAAH (Ueda et al., 2000). NAEs act as ligands for a subtype of a-peroxisome proliferator-activated receptor, and are devoid of CB-like activity (O’Sullivan, 2007). Thus, both pharmacological blockade of FAAH (e.g. with the compound URB597) and genetic deletion of FAAH not only enhance NAE levels, but also amplify eCB effects (Cravatt et al., 2001; Kathuria et al., 2003; Fegley et al., 2005) via activation of a-peroxisome proliferator-activated receptor (Jhaveri et al., 2008; Melis et al., 2008; Sagar et al., 2008; Mazzola et al., 2009). This raises the possibility that many eCB and CB effects in the CNS may be attributable to varying levels of these diverse receptor-mediated pathways, which result in/from an interplay between these parallel endogenous systems. A great deal of research has unraveled the different biological mechanisms involved in the AEA and 2-AG metabolic pathways (Paradisi et al., 2006; Di Marzo, 2009; Petrosino & Di Marzo, 2010; Karl et al., 2012; Marco & Laviola, 2012; Pertwee, 2012).

© 2014 Federation of European Neuroscience Societies and John Wiley & Sons Ltd European Journal of Neuroscience, 39, 1189–1201

Endocannabinoid signaling in metaplasticity 1191 AEA is derived from the cleavage of N-arachidonoylphosphatidylethanolamine (NAPE), a precursor synthesised by the enzyme Nacyltransferase, which requires the presence of Ca2+ and is regulated by cAMP (Cadas et al., 1996; Piomelli, 2003). Its release is catalyzed by a specific phospholipase D (Hansen et al., 2000; Okamoto et al., 2004) downstream from the depolarisation and/or activation of ionotropic [e.g. N-methyl-D-aspartate (NMDA) and acetylcholine nicotinic a7-neuronal receptors] or metabotropic receptors (Giuffrida et al., 1999; Stella & Piomelli, 2001; Kim et al., 2002; Varma et al., 2002; Piomelli, 2003). 2-AG originates from the metabolism of triacylglycerols, via receptor-dependent activation of phosphatidylinositol-specific phospholipase A1 and/or phospholipase C (PLC) (Sugiura & Waku, 2000). The canonical model posits that activation of metabotropic receptors [e.g. group I metabotropic glutamate receptor (mGluR), dopamine D2 receptor, and muscarinic acetylcholine receptor (mAChR) types M1 and M3] coupled to the PLC and diacylglycerol (DAG) lipase pathways yields 2-AG (Stella et al., 1997; Piomelli, 2003) (Fig. 1). Irrespective of their different metabolic pathways, binding properties, and intrinsic activity at CB receptors, both AEA and 2-AG activate CB receptors (Stella et al., 1997; Hillard, 2000; Howlett, 2002). The CB receptors form part of the superfamily of G-protein-

coupled receptors. The CB1 receptor is the most abundant G-protein-coupled receptor in the brain (Howlett et al., 1990; Herkenham et al., 1991). Additionally, increasing pharmacological evidence suggests that type 2 CB (CB2) receptors (Van Sickle et al., 2005; Ashton et al., 2006; Onaivi, 2006; Xi et al., 2011), transient receptor potential vanilloid-1 (TRPV1) receptor (Szallasi et al., 1995; Zygmunt et al., 1999; Szabo et al., 2002; Toth et al., 2005; Cristino et al., 2006; Marinelli et al., 2006) and at least two non-CB1, nonCB2 receptors (Hajos et al., 2001; Howlett et al., 2002; Kunos et al., 2002) are present in the brain. CB1 and CB2 receptors are coupled to similar transduction systems. CB receptor activation was initially reported to inhibit cAMP formation via coupling to Gi-proteins (Devane et al., 1988; Howlett et al., 1990), and thus result in reduced protein kinase A (PKA)dependent phosphorylation processes. CB receptors are also coupled to ion channels through Golf-protein, which ultimately leads to the inhibition of Ca2+ influx through N-type (Mackie & Hille, 1992), P/ Q-type (Twitchell et al., 1997) and L-type (Gebremedhin et al., 1999) Ca2+ channels, and to the activation of inwardly rectifying potassium conductance and A currents (Mackie et al., 1995; Childers & Deadwyler, 1996). CB1 and CB2 receptors have also been shown to couple to other intracellular cascades, including the mitogen-activated protein kinase cascade, phosphatidylinositol 3-kinase, Arachidonic acid + Glycerol O OH OH

O

cAMP ATP

2-AG AC

Ca2+ CB1 receptor

2-AG GIRK

AEA

K+

A AMP or pt e c e r

a2+ L-Type C e chann l

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mGluR1/5 mACh M1/3

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Gq

Src

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A2A recep tor

Ca2+

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Gs Gi

RGS4 cAMP

PKA O O

CaN/Dynamin

NAPE-PLD

OH OH

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AEA O

N H

OH

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Arachidonic acid + Ethanolamide

Fig. 1. Schematic summary of eCB signaling at the synapse. AC, adenilyl cyclase; MAGL, monoacylglycerol lipase; PLD, phospholipase D. © 2014 Federation of European Neuroscience Societies and John Wiley & Sons Ltd European Journal of Neuroscience, 39, 1189–1201

1192 M. Melis et al. focal adhesion kinase, ceramide signaling, and nitric oxide production (Derkinderen et al., 1996; Bouaboula et al., 1997; MolinaHolgado et al., 1997; Galve-Roperh et al., 2000; Howlett, 2002; Rueda et al., 2002).

eCB signaling in synaptic plasticity eCBs mainly serve as retrograde messengers, and are key modulators of synaptic function in various brain regions of the CNS. Recent evidence indicates that eCB signaling may also act in a nonretrograde manner by activating postsynaptic TRPV1 receptors (Chavez et al., 2010; Grueter et al., 2010; Puente et al., 2011) or by interacting with postsynaptic CB1 receptors to provide an autocrine signal (Bacci et al., 2004; Marinelli et al., 2008, 2009). Through these discrete mechanisms, eCB signaling contributes to different forms of short-term and long-term synaptic plasticity at both excitatory and inhibitory synapses [for detailed reviews, see Heifets & Castillo (2009) and Castillo et al. (2012)]. LTP and LTD are widely considered to be indicative of long-lasting adaptations of individual synapses, circuits or neural networks underlying behavioral changes. Nonetheless, short-term forms of synaptic plasticity, which are rapid means for the bidirectional and reversible modulation of synaptic strength, serve as important mechanisms to modify synaptic and circuit function during computation (Abbott & Regehr, 2004). eCB-mediated short-term synaptic plasticity Often viewed as a dynamic filter to transmit specific input frequencies or patterns, short-term plasticity is a key control mechanism for the proper scaling of synaptic inputs (Abbott et al., 1997; Dittman et al., 2000; Fortune & Rose, 2001; Lewis & Maler, 2002; Abbott & Regehr, 2004; Klyachko & Stevens, 2006). The most well-studied and characterised form of eCB-mediated short-term plasticity is depolarisation-induced suppression of inhibition (DSI)/depolarisation-induced suppression of excitation (DSE), depending on whether the input that is transiently silenced is GABAergic or glutamatergic (Kreitzer & Regehr, 2001; Ohno-Shosaku et al., 2001; Wilson & Nicoll, 2001). DSI/DSE persists for tens of seconds, and requires specific stimulation patterns to depolarise postsynaptic cells to activate voltage-gated Ca2+ channels (VGCCs) and enhance intracellular Ca2+ levels. A separate form of eCB-mediated short-term plasticity is driven by the activation of postsynaptic metabotropic receptors coupled to Gq-like G-proteins (e.g. group I mGluRs, M1/M3 mAChRs, and dopamine D2 receptors) (Varma et al., 2001; Kim et al., 2002; Ohno-Shosaku et al., 2002, 2003; Melis et al., 2004a,b; Edwards et al., 2006; Uchigashima et al., 2007). eCB mobilisation downstream from Gq-coupled receptor activation depends on PLC and DAGL activity in many brain regions, including the hippocampus, cortex, amygdala, and ventral tegmental area (VTA) (Melis et al., 2004a; Hashimotodani et al., 2005; Isokawa & Alger, 2005; Jung et al., 2005; Zhu & Lovinger, 2005; Edwards et al., 2006; Uchigashima et al., 2007; De-May & Ali, 2013). Thus, eCB-mediated DSI/DSE is able to influence the diverse neurotransmitter systems that are involved in information processing (e.g. reward-related stimuli). eCB-mediated long-term synaptic plasticity eCBs induce persistent changes in synaptic connection strength, which depends on both the pattern of afferent stimulation [Hebbian high-frequency stimulation (HFS), moderate-frequency stimulation,

and low-frequency stimulation (LFS)] and the relative timing of neuronal output and input spikes [i.e. spike-timing-dependent plasticity (STDP)]. The expression and maintenance of eCB-mediated LTD may require coincident presynaptic mechanisms (Yin et al., 2006), including changes in the release machinery via the cAMP/PKA/type a Rab3 interacting molecule 1 signaling cascade (Chevaleyre et al., 2007). The most well-characterised form of long-term synaptic plasticity mediated by eCB is LTD triggered by patterns (in the range of 10– 100 Hz) of afferent stimulation [eCB-mediated long-term depression (eCB-LTD)]. eCB-LTD is typically induced in response to a transient increase in the activity of glutamatergic inputs, which results in the activation of postsynaptic group I mGluRs and/or increased intracellular Ca2+ concentrations. This type of synaptic plasticity is expressed in many brain regions, including the dorsal striatum, nucleus accumbens (NAc), amygdala, hippocampus, prefrontal cortex (PFC), and VTA. To produce an enduring reduction in glutamate release, eCBs can activate either presynaptic CB1 receptors on the same afferents (i.e. homosynaptic eCB-LTD) or afferents in close proximity (i.e. heterosynaptic eCB-LTD) (Heifets & Castillo, 2009). Dorsal striatum In the dorsal striatum, eCB-LTD is mediated by the synergistic activation of group I mGluRs and D2 receptors (receptor-driven eCB release) and by Ca2+ entry via VGCCs and NMDA receptors (Ca2+dependent eCB release) (Uchigashima et al., 2007). At excitatory cortical afferents to striatal medium spiny neurons (MSNs), 2-AG and AEA may be produced and released from defined cellular compartments in response to precisely timed and localised stimuli (Puente et al., 2011; Nazzaro et al., 2012). Depending on the frequency of afferent stimulation, eCB-LTD may engage different biosynthetic pathways and lead to the production of 2-AG or AEA. The prolonged moderate-frequency stimulation (10–20 Hz, 10 min) of excitatory afferents paired with postsynaptic depolarisation triggers the production and release of 2-AG from the precursor DAG via activation of Gq-coupled mGluR1/5 and stimulation of PLCb (Ronesi & Lovinger, 2005; Lerner et al., 2010; Lerner & Kreitzer, 2012) This process is Ca2+-independent. Conversely, LTD induced by HFS (100 Hz) paired with postsynaptic depolarisation requiring increased intracellular Ca2+ is mediated by L-type VGCCs and Ca2+-induced Ca2+ release from intracellular stores. During HFS, the activation of group I mGluRs together with enhanced intracellular Ca2+ stimulates the enzyme phospholipase D, which then leads to the production and release (i.e. mobilisation) of eCBs (Ade & Lovinger, 2007; Lerner & Kreitzer, 2012). eCB-LTD also occurs at GABAergic synapses impinging on MSNs [inhibitory LTD (iLTD)] (Adermark et al., 2009). GABAergic afferents appear to be more sensitive to eCBs released from MSNs after L-type VGCC activation and in the absence of afferent stimulation. This observation may suggest different threshold levels of neuronal activity for induction of iLTD and LTD (Adermark et al., 2009). Additionally, eCB signaling contributes to synaptic depotentiation at excitatory afferents to the lateral part of the dorsal striatum (Nazzaro et al., 2012). Synaptic depotentiation is a form of synaptic plasticity occurring selectively at previously potentiated synapses following LFS (2 Hz) of excitatory afferents (Picconi et al., 2003). In contrast to LTD, eCB modulation of synaptic depotentiation does not involve VGCCs; rather, it requires the activation of mGluR5 (Nazzaro et al., 2012).


Endocannabinoid signaling in metaplasticity 1193 Nucleus accumbens eCB-LTD also occurs at PFC?NAc synapses (Robbe et al., 2001). In contrast to eCB-LTD in the dorsal striatum, eCB-LTD in the NAc is induced by activation of mGluR5 and increases in postsynaptic Ca2+ released from ryanodine-sensitive Ca2+ stores. However, it does not involve dopamine D2 receptor activation (Robbe et al., 2002, 2003). Additionally, strong postsynaptic mGluR5 activation in dopamine D2 receptor-positive MSNs of the NAc mobilises eCBs that simultaneously activate presynaptic CB1 and postsynaptic TRPV1 receptors to induce LTD (Grueter et al., 2010). As in the dorsolateral striatum (DLS), AEA appears to mediate this form of LTD, considering that the FAAH inhibitor URB597 enhances both CB1 receptor-dependent and TRPV1 receptor-dependent forms of plasticity (Grueter et al., 2010). Another form of synaptic plasticity, i.e. heterosynaptic iLTD, is expressed in the NAc at VTA?NAc synapses (Ishikawa et al., 2013). This form of iLTD is triggered by dopaminergic activity at presynaptic D1 receptors, revealing a novel biological function of dopamine (Ishikawa et al., 2013), which has long been considered to not directly mediate long-term forms of synaptic plasticity in this brain region (Calabresi et al., 2007). Whether eCBs play a role in this type of LTD has not yet been established. Investigating this issue would be valuable, given that the disruption of iLTD may potentially lead to compromised ability of the NAc to process incentive stimuli. Basolateral amygdala (BLA) and hippocampus In the BLA, the stimulation of afferent neurons induces eCB-LTD of GABAergic (Marsicano et al., 2002) and glutamatergic (Huang et al., 2003; Chevaleyre et al., 2007; Shin et al., 2010) afferents. eCB release appears to be Ca2+-independent, and occurs downstream from mGluR1 activation, suggesting a role for AEA in this phenomenon (Azad et al., 2004). Similarly, repetitive stimulation of hippocampal cornu ammonis 1 (CA1) pyramidal cells induces heterosynaptic depression at neighboring GABAergic synapses, which requires the activation of group I mGluRs. However, this mechanism does not require increased intracellular Ca2+ levels (Chevaleyre & Castillo, 2003). Unlike LTD in the amygdala, hippocampal LTD is mediated by 2-AG (Chevaleyre & Castillo, 2003; Edwards et al., 2006). Prefrontal cortex In the PFC, repetitive stimulation of cortical layer 5 pyramidal cells can elicit eCB-LTD (Lafourcade et al., 2007). This form of eCBLTD appears to be mediated mainly by 2-AG, as suggested by different observations: first, eCB-LTD depends on the activation of postsynaptic mGluR5 and PLC, with subsequently increased intracellular Ca2+ concentrations; second, eCB-LTD is reduced following DAG lipase (DAGL) inhibition and enhanced following MAGL inhibition (Lafourcade et al., 2007). In addition, a specific form of heterosynaptic iLTD is expressed in the PFC and controlled by eCBs (Chiu et al., 2010). This type of iLTD requires glutamate release and group I mGluR activation, and is facilitated by dopamine D2 receptors. Notably, CB1 and dopamine D2 receptors co-localise at GABAergic terminals in the PFC; activation of either receptor can suppress GABA release onto layer 5 pyramidal cells (Chiu et al., 2010). Thus, the facilitation of eCB-LTD via dopamine may be one mechanism by which dopamine modulates neuronal activity within the PFC, and its subsequent behavior. At PFC?

VTA synapses, dopamine D2 receptors cooperate with group I mGluRs to induce eCB-LTD of inhibitory inputs and facilitate LTP (Pan et al., 2008a,b). The mediator of these phenomena is most likely 2-AG, given that this eCB has been shown to act as a negative regulator of spike-timing-dependent LTP induction within the VTA, as opposed to inducing spike-timing-dependent LTP in this brain area (Kortleven et al., 2011). eCBs also regulate spike-timing-dependent LTD (tLTD), in which postsynaptic back-propagating action potentials precede cortical stimulation within a critical time window. tLTD has been mainly described at corticocortical and corticostriatal synapses. In the visual and sensory cortices, tLTD requires CB1 and presynaptic NMDA receptor activation (Sjostrom et al., 2003; Nevian & Sakmann, 2006; Rodriguez-Moreno & Paulsen, 2008; Bender et al., 2006). Additionally, CB1 receptor expressed on cortical astrocytes has been identified as a major player in tLTD occurring at layer 4 to layer 2/3 synapses onto pyramidal neurons of the developing barrel cortex (Min & Nevian, 2012). At excitatory synapses within the dorsal striatum, the eCB signaling that is required for the induction of tLTD involves PLCb and inositol trisphosphate receptor as coincident detectors (Fino et al., 2010), coupled with mGluR5 and VGCC activation. Striatal dopamine exerts dichotomous control of tLTD at striatopallidal and striatonigral MSNs. In identified striatopallidal MSNs, dopamine D2 receptors gate tLTD when cortical and thalamic afferents are activated by intrastriatal stimulation. Conversely, dopamine D1 receptor activation prevents tLTD at excitatory synapses to striatonigral MSNs (Shen et al., 2008).

Functional regulation of eCB signaling: metaplasticity and metaplastic control eCB release and the ensuing eCB-mediated plasticity are highly dependent on the relative levels of the neuromodulators present in defined circuits at any given time. This finding suggests that the plasticity of eCB signaling provides an additional dimension to the neuromodulatory mechanisms underlying the processing of behavioral states at the synaptic level. The neurotransmitters/synaptic neuromodulators acetylcholine and dopamine can directly influence eCB release. The mechanisms underlying this type of modulation differ according to the brain region, and can be cell-type specific. In the hippocampus, Ca2+-dependent release of eCB is enhanced by the activation of M1/M3 mAChRs (Hashimotodani et al., 2005; Ohno-Shosaku et al., 2005). Conversely, acetylcholine released from tonically active cholinergic interneurons hinders eCB biosynthesis in MSNs of the dorsal striatum. This mechanism involves the M1 receptor-mediated inhibition of 1.3 Ca2+ channel, voltage-dependent (Cav 1.3 channel) activity, which is necessary for eCB production (Wang et al., 2006). In MSNs of the striatopallidal pathway, the activation of postsynaptic Gi/o-coupled dopamine D2 receptors gates eCB biosynthesis by inhibiting the PKA-downstream protein regulator of G-protein signaling 4 (RGS4) that normally exerts an inhibitory effect on Gq-dependent eCB production (Lerner & Kreitzer, 2012). In the same cell type, the activation of Gs-coupled type 2 adenosine receptors plays an opposite role by enhancing RGS4 activity, thus inhibiting eCB signaling (Fig. 1). The regulation of synaptic eCB signaling may also vary according to the ongoing and recent levels of neuronal activity, and it may occur on multiple time scales. On a short time scale, voltage oscillations can gate circuit-specific eCB release. In the dorsal striatum, mGluR-mediated eCB release is enhanced by subthreshold depolar-


1194 M. Melis et al. isation (Kreitzer & Malenka, 2005). Furthermore, striatal up-states and down-states of activity may differentially trigger the release of 2-AG and AEA from MSNs (Mathur et al., 2013). On a long time scale, persistent changes in network activity induced by in vivo experience can lead to homeostatic adaptations of eCB signaling, which affect either eCB production/degradation or CB1 expression/ function. For example, glucocorticoid elevations upon repeated stress enhance eCB breakdown at inhibitory synapses of the BLA. Similarly, prolonged alterations in energy balance, which occur in animal models of obesity, trigger upregulation of hippocampal DAGL and 2-AG levels (Sumislawski et al., 2011). Conversely, food deprivation (Crosby et al., 2011) and nutritional omega-3 deficiencies (Lafourcade et al., 2011) abolish eCB signaling in feeding circuits by triggering CB1 downregulation and desensitisation. The persistent activation of eCB signaling upon chronic exposure to D9-tetrahydrocannabinol (D9-THC), the main psychoactive component of marijuana, results in homeostatic adaptations at presynaptic CB1 receptors and deficits in eCB-mediated synaptic plasticity in several brain regions (Hoffman et al., 2003; Mato et al., 2005; Tonini et al., 2006). To elucidate the cellular processes underlying homeostatic modulation of eCB signaling, we must also consider that adaptive changes in CB1 receptor function may be secondary to alterations in the endogenous tone of eCBs. Indeed, prolonged tonic eCB signaling can lead to CB1 receptor downregulation (Chanda et al., 2010) and deficits in eCB-mediated plasticity. The mechanisms outlined above suggest that modulation of eCB pathway activity gives rise to at least three interconnected functional states by which the ensuing changes in eCB-mediated synaptic plasticity can dynamically affect neuronal circuits and behavior: prior, tonic and persistent engagement of eCB signaling (summarised in Table 1). These activation states involve different molecular components of eCB signaling, and can either trigger metaplasticity or represent a means for metaplastic control of the eCB system.

Synaptic and behavioral implications of prior, tonic and persistent eCB signaling

lular mechanisms, these observations not only further support the role of prior eCB signaling activation in triggering metaplasticity, but also suggest that the eCB system itself undergoes plastic changes to mediate metaplasticity. The degree of complexity in the dynamic regulation of synaptic eCB signaling is further increased by the fact that eCBs can also be metaplastic targets. Indeed, mGluR-mediated release of eCBs can be primed by a Ca2+-dependent process at hippocampal CA1 synapses. Reciprocally, prior activation of mGluRs enhances Ca2+-dependent release of eCBs (Edwards et al., 2008). These observations suggest metaplastic control of eCB signaling. As proposed by Edwards et al. (2008), prior eCB signaling in the hippocampus may significantly contribute to subcortical attentional function, thereby gating subsequent eCB-assisted associational learning processes (Edwards et al., 2008). In support of behavioral implications of prior eCB signaling activation, Depoy et al. (2013) recently reported significantly higher levels of 2-AG, but not AEA, in the DLS of animals exposed to chronic intermittent alcohol treatment. In this animal model, changes in 2-AG levels were associated with downregulation of CB1 signaling, followed by deficits in eCB-LTD at corticostriatal synapses of the DLS. Animals exposed to chronic intermittent alcohol showed marked facilitation in behavioral tasks specifically encoded by the DLS. These animals also showed task-related increases in the ensemble neuronal activity in the DLS. This phenomenon did not mirror hyperexcitability of DLS neurons. Thus, the authors proposed a model of dysfunctional DLS circuit activity, in which alcoholinduced increases in 2-AG levels lead to compensatory downregulation of CB1 signaling and plasticity. This may unmask a form of NMDA receptor-mediated LTP. Altogether, these adaptations would contribute to the enhanced neuronal encoding of DLS-dependent behavior by reshaping synaptic plasticity to drive proper learning and regulate rewarding behaviors. Therefore, the canonical interpretation of on-demand mobilisation of eCBs appears to need revision, at least with regard to brain areas such as the striatum and hippocampus (Depoy et al., 2013; Edwards et al., 2008). Tonic eCB signaling activation

Prior eCB signaling activation The first evidence suggesting that eCB signaling could be involved in metaplasticity processes was provided by Carlson et al. (2002). In particular, eCB-mediated DSI gated homosynaptic LTP in CA1 pyramidal neurons. This increased gain of excitatory signaling, induced by transient suppression of local inhibitory tone, might underlie behavioral learning. Indeed, the establishment of place fields during maze learning has been shown to be dependent on temporal asymmetry and associative properties of NMDA-dependent LTP (Ekstrom et al., 2001). Accordingly, in the hippocampus, synaptically driven release of eCBs depresses inhibitory inputs in a restricted area of the dendritic tree of CA1 neurons (Chevaleyre & Castillo, 2004). By removing synaptic inhibition in this compartmentalised region of the neuron, eCBs prime nearby excitatory synapses, thereby facilitating subsequent LTP induction. Similarly, in the same neuronal cell population, repetitive LFS produces the enduring potentiation of eCB-mediated DSI at inhibitory inputs, thus facilitating LTP (Zhu & Lovinger, 2007). This effect is mediated by mGluR5 activation during LFS. The authors of this work did not address whether this form of activity-dependent modulation of eCB signaling was caused by enhanced mGluR5-mediated eCB release, increased CB1 receptor expression, or increased signal transduction downstream of the CB1 receptor. Irrespective of the underlying cel-

In addition to canonical activity-dependent eCB mobilisation, under some experimental conditions tonic eCB signaling can regulate basal neurotransmission (Hentges et al., 2005; Neu et al., 2007; Oleson et al., 2012a,b; Oliet et al., 2007). Growing evidence suggests that tonic eCB signaling also plays a prominent role in the metaplastic control of neuronal circuits. A suitable example is the role played by the fragile X mental retardation protein (FMRP) in coupling DAGLa and mGluR5 into a functional signalosome (Jung et al., 2012), which is essential for retrograde 2-AG transmission (Busquets-Garcia et al., 2013; Jung et al., 2012; Maccarrone et al., 2010; Zhang & Alger, 2010). In mice lacking FMRP [Fmr1 knockout (KO)], the mGluR-dependent mobilisation of eCBs is enhanced, suggesting that their constitutive regulation of diverse forms of synaptic plasticity, and ultimately metaplasticity, may underlie the cognitive dysfunction associated with fragile X syndrome (Zhang & Alger, 2010). Specifically, increased tonic eCB release from CA1 hippocampal neurons led to greatly increased mGluR-induced eCB-dependent inhibitory short-term depression, unaffected eCB-mediated DSI, and greater eCB-mediated iLTD. This resulted in long-lasting increased neuronal excitability in Fmr1 KO mice (i.e. excitatory postsynaptic potentialspike coupling potentiation) (Zhang & Alger, 2010). The observation that eCB-mediated DSI was not modified in Fmr1 KO mice



Persistent

Tonic

Prior

Functional activation states of eCB signaling

eCB-mediated DSI gates homosynaptic LTP in CA1 neurons (Carlson et al., 2002) Synaptically driven release of eCB depresses inhibitory inputs in compartmentalised dendritic regions of CA1 neurons, facilitating LTP induction at nearby excitatory synapses (Chevaleyre & Castillo, 2004) Repetitive LFS triggers enduring potentiation of eCB-mediated DSI at inhibitory inputs, thus facilitating LTP (Zhu & Lovinger, 2007). mGluR-mediated release of eCBs can be primed by Ca2+-dependent processeses at CA1 synapses (Edwards et al., 2008) In CA1 neurons of mice lacking FMRP, mGluR-dependent mobilisation of eCBs is tonically enhanced, leading to greater eCB-mediated iLTD (Zhang & Alger, 2010) Chronic hippocampal infusion of the CB1 receptor antagonist rimonabant normalises cognitive deficits shown by mice lacking FMRP (Busquets-Garcia et al., 2013) Tonic eCB signaling at inhibitory synapses between cholecystokinin-positive basket cells and CA1 pyramidal neurons is impaired in autism associated with neuroligin-3 mutations (Foldy et al., 2013)

Hippocampus

Persistent activation of eCB signaling induced by chronic D9-THC administration leads to deficits in eCB-mediated plasticity (LTD and synaptic depotentiation) and a bias towards habitual behavior (Nazzaro et al., 2012)

FMRP deficiency is linked to deregulation of the coupling between mGluRs and eCB mobilisation, which is secondary to constitutively enhanced DAGL activity (Maccarrone et al., 2010)

In animals exposed to chronic alcohol treatment, changes in 2-AG levels correlate with deficits in eCB-LTD (unmasking NMDA receptor-mediated LTP) and with facilitation of visual discrimination and reversal learning (Depoy et al., 2013)

DLS

Persistent eCB signaling, resulting from a high-fat diet, triggers CB1 receptor desensitisation and loss of eCB-LTD at excitatory synapses (Lafourcade et al., 2011) In cocaine self-administering rats, persistent loss of eCB-LTD is specifically associated with the drug abuse condition but not with the addictive behavioral state (Kasanetz et al., 2012)

Uncoupling FMRP from DAGL and mGluR5 results in reduced 2-AG levels and loss of eCB-LTD at excitatory synapses (Jung et al., 2012)

PFC

Table 1. Summary of functional activation states of eCB signaling (prior, tonic and persistent) and their impact on synaptic and behavioral plasticity in distinct brain regions

Chronic CB1 receptor activation following exposure to D9-THC impairs eCB-LTD (Hoffman et al., 2003) and triggers homeostatic changes in eCB signaling at excitatory synapses (Mato et al., 2005)

NAc

Endocannabinoid signaling in metaplasticity 1195

1196 M. Melis et al. suggested that Ca2+-dependent mobilisation of eCBs and signaling downstream from CB1 receptor activation at inhibitory synapses remained functional (Zhang & Alger, 2010). This finding further supports the role of FMRP in modulating the coupling between mGluR5 and eCB mobilisation. The importance of eCB tone at hippocampal inhibitory synapses has been associated with some of the cognitive dysfunctions related to fragile X syndrome. Specifically, chronic hippocampal infusion of the CB1 receptor antagonist rimonabant normalised the cognitive deficits (e.g. impaired object recognition) shown by mice lacking FMRP (Busquets-Garcia et al., 2013). In the dorsal striatum, FMRP deficiency has been directly linked to deregulation of the coupling between mGluRs and eCB mobilisation, which is secondary to enhanced DAGL activity (Maccarrone et al., 2010). The resulting increased tonic release of 2-AG at corticostriatal synapses promoted iLTD and affected activity-dependent MSN excitability. These cellular mechanisms may provide a plausible explanation for the hyperactivity and perseverative motor behavior that are frequently observed in patients affected by this disease (Freund & Reiss, 1991). Conversely, uncoupling FMRP from DAGL and mGluR5 results in reduced 2-AG levels and a loss of eCB-LTD at excitatory synapses of the ventral striatum and PFC (Jung et al., 2012). Altered tonic eCB signaling has also been recently implicated in the pathophysiology of autism. Foldy et al. (2013) investigated the interaction between the postsynaptic cell-adhesion molecule neuroligin (isoform 3) and tonic eCB signaling. Neuroligin is expressed at both inhibitory and excitatory synapses, and mutations of neuroligin have been associated with autism (Sanders et al., 2011). These authors demonstrated that neuroligin is required for tonic, but not phasic, eCB signaling specifically at inhibitory synapses formed by cholecystokinin-positive basket cells and CA1 pyramidal neurons (Foldy et al., 2013). Increased eCB tone can occur after changes in key molecular players of eCB signaling. For instance, diminished activity of degradative eCB enzymes or changes in CB1 receptor expression and/or function have been linked to reduced anxiety-like behaviors, enhanced serotoninergic tone in the PFC (Cassano et al., 2011) and enhanced LTP in the BLA in CB1 KO mice (Marsicano et al., 2002). However, to date, a plausible explanation other than ‘between-systems compensatory adaptations to life-long absence of CB1 receptors’ (Marsicano et al., 2002) has not been found. It is tempting to speculate that tonic eCB modulation of metaplastic mechanisms may account for these behavioral and synaptic effects. Additionally, deficient clearance of eCBs may lead to input-specific regulation of a given synapse, setting different thresholds for gating subsequent synaptic plasticity, and ultimately altering postsynaptic cell excitability. Persistent eCB signaling activation Persistent eCB signaling such as that resulting from an unbalanced diet (i.e. low levels of essential omega-3 n-3 polyunsaturated fatty acids) results in the loss of eCB-LTD at excitatory synapses of the PFC without homeostatic modulation of other forms of LTD (Lafourcade et al., 2011); see also Schlosburg et al. (2010). These functional changes depend on CB1 receptor desensitisation but not on tonically enhanced eCB levels (Lafourcade et al., 2011). Additionally, persistent loss of eCB-LTD associated with a lack of group II mGluR-dependent LTD in the PFC (Kasanetz et al., 2012) may prime synapses for LTP-like phenomena under conditions of high neuronal activity, such as those observed following repeated

cocaine exposure (Huang et al., 2007). Thus, altered eCB signaling during cocaine self-administration (Kasanetz et al., 2012) may contribute to metaplastic mechanisms leading to enhanced neuronal activity within the PFC (Sun & Rebec, 2006) that can be ascribed to impaired negative regulation of glutamate release. Drug abuse and addiction are terms that are often used interchangeably, thus adding confusion regarding the diagnostic implications and neurobiological underpinnings of these conditions. It is therefore compelling to analyse – and, for us, to underscore – the synaptic modifications associated with these discrete behaviors. Kasanetz et al. (2012) elegantly dissected the synaptic impairments associated with these two behavioral phenomena in a preclinical model of addiction. These authors demonstrated that group II mGluR-dependent LTD was selectively abolished at excitatory synapses of the prelimbic PFC of addicted rats. Conversely, eCB-LTD was impaired in both addicted and non-addicted animals, irrespective of the amount of drug to which the animals were exposed. Hence, persistent activation of eCB signaling pathways cannot account for the anaplastic mechanisms aimed at counteracting the initial detrimental drug-induced effects. Additionally, this activation may trigger the loss of executive control over drug intake. Thus, it is tempting to speculate that persistently unbalanced eCB signaling, by modulating metaplasticity mechanisms at these synapses, may allow for behavioral flexibility that would prevent loss of control over drug seeking and taking, and therefore transition to addiction. Similar persistent changes in eCB signaling have been reported in the NAc after chronic CB1 receptor activation (Hoffman et al., 2003). Notably, these changes also affected endogenous opioidergic signaling. Moreover, the persistent impairment of eCB signaling, which has been ascribed to decreased sensitivity to CB1 receptor activation of both excitatory and inhibitory inputs, prevented eCBLTD in the NAc. Whether these changes reflected the uncoupling of CB1 receptors from their effectors (e.g. VGCCs) or CB1 receptor downregulation at presynaptic terminals has not yet been elucidated. Nonetheless, the authors could rule out the involvement of tonically enhanced eCB levels (Hoffman et al., 2003). While a single in vivo exposure to D9-THC abolished eCB-LTD and eCB-mediated iLTD in the NAc and hippocampus, respectively, owing to the functional tolerance of CB1 receptors (Mato et al., 2004), repeated exposure to D9-THC produced homeostatic changes in eCB signaling in the NAc (Mato et al., 2005). Hence, persistent eCB signaling triggers both CB1 receptor internalisation and desensitisation, results in uncoupling from downstream effectors, and enables homeostatic modifications, ultimately leading to the recovery of activity-dependent LTD (Mato et al., 2005). Because Mato et al. (2005) demonstrated that CB1 receptors and group II mGluRs are co-localised at the same axon terminals in the NAc, and that the two forms of LTD are mutually occluded, it is plausible that these receptors may share the same G-protein pool. If so, persistent eCB signaling might enhance the number of G-proteins available for group II mGluRs, and therefore rescue mGluR-dependent LTD. This tempting hypothesis has not been tested to date, although it could explain additional forms of adaptation (Kasanetz et al., 2012) involving other neurotransmitter systems (Hoffman et al., 2003, 2010). Along these lines, the persistent engagement of eCB signaling via the triggering of homeostatic neuroadaptations may enable metaplastic mechanisms. In support of this hypothesis, we recently demonstrated that persistent activation of the eCB pathway triggered downregulation of CB1 receptors and desensitisation of CB1 receptor-activated G-proteins in the DLS of mice that were chronically exposed to D9-THC (Nazzaro et al., 2012). This was accompanied by the loss of eCB-LTD at corticostriatopallidal synapses and synap-


Endocannabinoid signaling in metaplasticity 1197 tic depotentiation. Given that activation of CB1 receptors contributes to the induction of synaptic depotentiation (Nazzaro et al., 2012), deficits in this form of plasticity after repeated D9-THC administration may result from the functional tolerance of presynaptic CB1 receptors. The concomitant loss of eCB-LTD and synaptic depotentiation compromised the dynamic range of synaptic activity in the DLS. This was associated with a switch from goal-directed actions to inflexible habitual strategies. Deficits in eCB-LTD and behavioral alterations could be rescued by in vivo pharmacological inhibition of small-conductance Ca2+-activated potassium channels of the DLS, thus relating the loss of DLS eCB-LTD to a bias toward habitual behavior. We can speculate that persistent activation of eCB signaling may induce forms of homeostatic metaplasticity that can influence the control of goal-directed behavior, at least after chronic exposure to D9-THC. However, whether and how synaptic eCB signaling and eCB-mediated metaplasticity are engaged during the physiological switch from goal-directed to habitual behaviors remain unclear.

Concluding remarks Growing evidence suggests that eCB signaling participates in the adaptive responses of many central synapses in the brain via mechanisms involving diverse signaling cascades, in both presynaptic and postsynaptic compartments. The emerging picture is that eCBs act not only as retrograde intercellular messengers mediating diverse forms of synaptic plasticity, but also as important modulators of homo-synaptic and hetero-synaptic metaplasticity. Indeed, by regulating the initial probability of neurotransmitter release, eCBs guarantee the dynamic accuracy of input information and modify the sensitivity of the cell to subtle changes in firing patterns of its inputs. This may ultimately change circuit functions during computation. As a result, the plethora of eCB action spans from restricted synapse-specific control to wide (i.e. heterosynaptic) regulation of long-lasting changes in the neural circuits or networks used in information processing. Thus, any homeostatic change in the eCB system could possibly influence any subsequent plasticity induction. To date, important questions remain unaddressed. Are AEA or 2-AG similarly involved in metaplasticity? Is there any cell type or regional specificity of their contributions to metaplastic phenomena? Equally important, is there a bias for one of the two eCBs to be mobilised upon prior neuronal activity, and does this depend on synaptic states? Are these unique features of eCB signaling in metaplasticity limited to brain regions that are cytoarchitectonically related, or is this a common mechanism of their action beyond that in conventional forms of plasticity? Because the CB1 receptor is one of the most widely expressed G-protein-coupled receptors in the brain, it is tempting to speculate that eCB neuromodulatory functions may be instrumental in maintaining an apt and timely synaptic dynamic range to support learning processes. Synaptic dynamic ranges can vary across different neuronal populations and/or brain regions, and they can be modified by the balance between interoceptive stimuli and sensory information from the environment. Thus, the eCB system appears to be in a unique position to contribute to and be modulated by the activitydependent plasticity of synaptic plasticity and cognitive processing. In pathological settings, understanding whether different states of engagement of eCB signaling (i.e. prior, tonic, and persistent) represent adaptive or maladaptive changes is imperative, both to gain knowledge about pathophysiological mechanisms and to identify and exploit new therapeutic targets.

Competing financial interests The authors declare no competing financial interests.

Acknowledgements We wish to thank all members of the Tonini laboratory and all of the scientists whose data were reviewed in this article. We are grateful to Riccardo Brambilla for helpful comments and suggestions regarding this manuscript. This research was supported by the Fondazione Istituto Italiano di Tecnologia, and by the Compagnia di San Paolo, with the Grant ‘Progetti di Ricerca Scientifica e Iniziative in Campo Sanitario’ (to R. Tonini) and with the Grant ‘Progetto Giovani Ricercatori Ministero della Salute’ (to R. Tonini).

Abbreviations AEA, anandamide; 2-AG, 2-arachidonoyl glycerol; BLA, basolateral amygdala; BMC, Bienenstock–Cooper–Munro; CA1, cornu ammonis 1; CB, cannabinoid; CB1, type 1 cannabinoid; CB2, type 2 cannabinoid; DAG, diacylglycerol; DAGL, diacylglycerol lipase; DLS, dorsolateral striatum; DSE, depolarisation-induced suppression of excitation; DSI, depolarisationinduced suppression of inhibition; eCB, endocannabinoid; eCB-LTD, endocannabinoid-mediated long-term depression; FAAH, fatty acid amide hydrolase; FMRP, fragile X mental retardation protein; HFS, high-frequency stimulation; iLTD, inhibitory long-term depression; KO, knockout; LFS, lowfrequency stimulation; LTD, long-term depression; LTP, long-term potentiation; mAChR, muscarinic acetylcholine receptor; MAGL, monoacylglycerol lipase; mGluR, metabotropic glutamate receptor; MSN, medium spiny neuron; NAc, nucleus accumbens; NAE, N-acylethanolamine; NAPE N-arachidonoylphosphatidylethanolamine; PLD, phospholipase D; NMDA, N-methyl-D-aspartate; PFC, prefrontal cortex; PLC, phospholipase C; PKA, protein kinase A; RGS4, regulator of G-protein signaling 4; tLTD, spike-time dependent long-term depression; TRPV1, transient receptor potential vanilloid-1; VGCC, voltage-gated Ca2+ channel; VTA, ventral tegmental area; D9-THC, D-9-tetrahydrocannabinol.

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