Dopamine modulation of state-dependent endocannabinoid release and long-term depression in the striatum

Group I metabotropic glutamate receptors generate two types of intrinsic membrane oscillations in hippocampal oriens/alveus interneurons

GABAergic interneurons in the hippocampus are critically involved in almost all hippocampal circuit functions including coordinated network activity. Somatostatin-expressing oriens-lacunosum moleculare (O-LM) interneurons are a major subtype of dendritically projecting interneurons in hippocampal subregions (e.g., CA1), and express group I metabotropic glutamate receptors (mGluRs), specifically mGluR₁ and mGluR₅. Group I mGluRs are thought to regulate hippocampal circuit functions partially through GABAergic interneurons. Previous studies suggest that a group I/II mGluR agonist produces slow supra-threshold membrane oscillations (<0.1 Hz), which are associated with high-frequency action potential (AP) discharges in O-LM interneurons. However, the properties and underlying mechanisms of these slow oscillations remain largely unknown. We performed whole-cell patch-clamp recordings from mouse interneurons in the stratum oriens/alveus (O/A interneurons) including CA1 O-LM interneurons. Our study revealed that the selective mGluR_1/5 agonist (S)-3,5-dihydroxyphenylglycine (DHPG) induced slow membrane oscillations (<0.1 Hz), which were associated with gamma frequency APs followed by AP-free perithreshold gamma oscillations. The selective mGluR₁ antagonist (S)-(+)-α-Amino-4-carboxy-2-methylbenzeneacetic acid (LY367385) reduced the slow oscillations, and the selective mGluR₅ antagonist 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP) partially blocked them. Blockade of nonselective cation-conducting transient receptor potential channels, L-type Ca²⁺ channels, or ryanodine receptors all abolished the slow oscillations, suggesting the involvement of multiple mechanisms. Our findings suggest that group I mGluR activation in O/A interneurons may play an important role in coordinated network activity, and O/A interneuron vulnerability to excitotoxicity, in disease states like seizures, is at least in part due to an excessive rise in intracellular Ca²⁺.

Review of the neurological benefits of phytocannabinoids

Background:

Numerous physical, psychological, and emotional benefits have been attributed to marijuana since its first reported use in 2,600 BC in a Chinese pharmacopoeia. The phytocannabinoids, cannabidiol (CBD), and delta-9-tetrahydrocannabinol (Δ9-THC) are the most studied extracts from cannabis sativa subspecies hemp and marijuana. CBD and Δ9-THC interact uniquely with the endocannabinoid system (ECS). Through direct and indirect actions, intrinsic endocannabinoids and plant-based phytocannabinoids modulate and influence a variety of physiological systems influenced by the ECS.

Methods:

In 1980, Cunha et al. reported anticonvulsant benefits in 7/8 subjects with medically uncontrolled epilepsy using marijuana extracts in a phase I clinical trial. Since then neurological applications have been the major focus of renewed research using medical marijuana and phytocannabinoid extracts.

Results:

Recent neurological uses include adjunctive treatment for malignant brain tumors, Parkinson's disease, Alzheimer's disease, multiple sclerosis, neuropathic pain, and the childhood seizure disorders Lennox-Gastaut and Dravet syndromes. In addition, psychiatric and mood disorders, such as schizophrenia, anxiety, depression, addiction, postconcussion syndrome, and posttraumatic stress disorders are being studied using phytocannabinoids.

Conclusions:

In this review we will provide animal and human research data on the current clinical neurological uses for CBD individually and in combination with Δ9-THC. We will emphasize the neuroprotective, antiinflammatory, and immunomodulatory benefits of phytocannabinoids and their applications in various clinical syndromes.

Oxytocin functions as a spatiotemporal filter for excitatory synaptic inputs to VTA dopamine neurons

The experience of rewarding or aversive stimuli is encoded by distinct afferents to dopamine (DA) neurons of the ventral tegmental area (VTA). Several neuromodulatory systems including oxytocin regulate DA neuron excitability and synaptic transmission that process socially meaningful stimuli. We and others have recently characterized oxytocinergic modulation of activity in mouse VTA DA neurons, but the mechanisms underlying oxytocinergic modulation of synaptic transmission in DA neurons remain poorly understood. Here, we find that oxytocin application or optogenetic release decrease excitatory synaptic transmission, via long lasting, presynaptic, endocannabinoid-dependent mechanisms. Oxytocin modulation of excitatory transmission alters the magnitude of short and long-term depression. We find that only some glutamatergic projections to DA neurons express CB1 receptors. Optogenetic stimulation of three major VTA inputs demonstrates that oxytocin modulation is limited to projections that show evidence of CB1R transcripts. Thus, oxytocin gates information flow into reward circuits in a temporally selective and pathway-specific manner.

The mammalian brain contains millions of nerve cells or neurons that communicate with each other via a process called neurotransmission. To send a message to its neighbor, a neuron releases a chemical called a neurotransmitter into the space between the cells. The neurotransmitter then binds to receiver proteins on the target cell. Another group of chemicals, known as neuromodulators, regulate this process, adjusting the way that neurons respond to neurotransmitters. In doing so, they help regulate many types of behavior in mammals.

The neuromodulator oxytocin, for example, has earned the nickname ‘the love hormone’ because it promotes social behavior and bonding. It does this in part by altering the activity of neurons in a brain region called the ventral tegmental area (VTA). These neurons produce the brain’s main reward signal, dopamine, which is itself a neuromodulator. But exactly how oxytocin affects the activity of dopamine-producing neurons is unclear.

By recording from individual neurons in slices of mouse brain tissue, Xiao et al. show that oxytocin filters inputs to dopamine neurons in the VTA. It does this by making the dopamine neurons release another group of reward signals, known as endocannabinoids. These are the brain’s own version of the chemicals found inside cannabis plants. The endocannabinoids bind to neurons that provide input to the VTA dopamine neurons. Some of these input neurons normally activate the VTA by releasing a neurotransmitter called glutamate. However, the binding of endocannabinoids decreases their ability to do this, and thereby lowers the activation of the VTA dopamine neurons.

But not all glutamate neurons are sensitive to endocannabinoids. Moreover, oxytocin affects glutamate neurons that fire repeatedly less than it affects those that fire only occasionally. Oxytocin thus acts as a filter. It allows certain inputs – those that are repeatedly active and those that are insensitive to endocannabinoids – to continue activating VTA dopamine neurons. At the same time, it weakens the influence of other inputs. Dopamine release in the VTA drives drug abuse and addiction. Understanding how oxytocin affects VTA neurons may thus open up new avenues for the treatment of addiction disorders.

Kainate Receptors Inhibit Glutamate Release Via Mobilization of Endocannabinoids in Striatal Direct Pathway Spiny Projection Neurons

Kainate receptors are members of the glutamate receptor family that function by both generating ionotropic currents through an integral ion channel pore and coupling to downstream metabotropic signaling pathways. They are highly expressed in the striatum, yet their roles in regulating striatal synapses are not known. Using mice of both sexes, we demonstrate that GluK2-containing kainate receptors expressed in direct pathway spiny projection neurons (dSPNs) inhibit glutamate release at corticostriatal synapses in the dorsolateral striatum. This inhibition requires postsynaptic kainate-receptor-mediated mobilization of a retrograde endocannabinoid (eCB) signal and activation of presynaptic CB1 receptors. This pathway can be activated during repetitive 25 Hz trains of synaptic stimulation, causing short-term depression of corticostriatal synapses. This is the first study to demonstrate a role for kainate receptors in regulating eCB-mediated plasticity at the corticostriatal synapse and demonstrates an important role for these receptors in regulating basal ganglia circuits.SIGNIFICANCE STATEMENT The GRIK2 gene, encoding the GluK2 subunit of the kainate receptor, has been linked to several neuropsychiatric and neurodevelopmental disorders including obsessive compulsive disorder (OCD). Perseverative behaviors associated with OCD are known to result from pathophysiological changes in the striatum and kainate receptor knock-out mice have striatal-dependent phenotypes. However, the role of kainate receptors in striatal synapses is not known. We demonstrate that GluK2-containing kainate receptors regulate corticostriatal synapses by mobilizing endocannabinoids from direct pathway spiny projection neurons. Synaptic activation of GluK2 receptors during trains of synaptic input causes short-term synaptic depression, demonstrating a novel role for these receptors in regulating striatal circuits.

Singular Location and Signaling Profile of Adenosine A2A-Cannabinoid CB1 Receptor Heteromers in the Dorsal Striatum

The dorsal striatum is a key node for many neurobiological processes such as motor activity, cognitive functions, and affective processes. The proper functioning of striatal neurons relies critically on metabotropic receptors. Specifically, the main adenosine and endocannabinoid receptors present in the striatum, ie, adenosine A_2A receptor (A_2AR) and cannabinoid CB₁ receptor (CB₁R), are of pivotal importance in the control of neuronal excitability. Facilitatory and inhibitory functional interactions between striatal A_2AR and CB₁R have been reported, and evidence supports that this cross-talk may rely, at least in part, on the formation of A_2AR-CB₁R heteromeric complexes. However, the specific location and properties of these heteromers have remained largely unknown. Here, by using techniques that allowed a precise visualization of the heteromers in situ in combination with sophisticated genetically modified animal models, together with biochemical and pharmacological approaches, we provide a high-resolution expression map and a detailed functional characterization of A_2AR-CB₁R heteromers in the dorsal striatum. Specifically, our data unveil that the A_2AR-CB₁R heteromer (i) is essentially absent from corticostriatal projections and striatonigral neurons, and, instead, is largely present in striatopallidal neurons, (ii) displays a striking G protein-coupled signaling profile, where co-stimulation of both receptors leads to strongly reduced downstream signaling, and (iii) undergoes an unprecedented dysfunction in Huntington's disease, an archetypal disease that affects striatal neurons. Altogether, our findings may open a new conceptual framework to understand the role of coordinated adenosine-endocannabinoid signaling in the indirect striatal pathway, which may be relevant in motor function and neurodegenerative diseases.

Endogenous dopamine and endocannabinoid signaling mediate cocaine-induced reversal of AMPAR synaptic potentiation in the nucleus accumbens shell

Repeated exposure to drugs of abuse alters the structure and function of neural circuits mediating reward, generating maladaptive plasticity in circuits critical for motivated behavior. Within meso-corticolimbic dopamine circuitry, repeated exposure to cocaine induces progressive alterations in AMPAR-mediated glutamatergic synaptic transmission. During a 10-14 day period of abstinence from cocaine, AMPAR signaling is potentiated at synapses on nucleus accumbens (NAc) medium spiny neurons (MSNs), promoting a state of heightened synaptic excitability. Re-exposure to cocaine during abstinence, however, rapidly reverses and depotentiates enhanced AMPAR signaling. To understand how re-exposure to cocaine alters AMPAR synaptic transmission, we investigated the roles of dopamine and endocannabinoid (eCB) signaling in modifying synaptic strength in the NAc shell. Using patch-clamp recordings from NAc slices prepared after 10-14 days of abstinence from repeated cocaine, we found that AMPAR-mediated depotentiation is rapidly induced in the NAc shell within 20 min of cocaine re-exposure ex vivo, and persists for up to five days before synapses return to levels of potentiation observed during abstinence. In cocaine-treated animals, global dopamine receptor activation was both necessary and sufficient for the cocaine-evoked depotentiation of AMPAR synaptic function. Additionally, we identified that CB1 receptors are engaged by endogenous endocannabinoids (eCBs) during re-exposure to cocaine ex vivo. Overall, these results indicate the central role that dopamine and eCB signaling mechanisms play in modulating cocaine-induced AMPAR plasticity in the NAc shell.

To Do or Not to Do: Dopamine, Affordability and the Economics of Opportunity

Five years ago, we introduced the thrift hypothesis of dopamine (DA), suggesting that the primary role of DA in adaptive behavior is regulating behavioral energy expenditure to match the prevailing economic conditions of the environment. Here we elaborate that hypothesis with several new ideas. First, we introduce the concept of affordability, suggesting that costs must necessarily be evaluated with respect to the availability of resources to the organism, which computes a value not only for the potential reward opportunity, but also the value of resources expended. Placing both costs and benefits within the context of the larger economy in which the animal is functioning requires consideration of the different timescales against which to compute resource availability, or average reward rate. Appropriate windows of computation for tracking resources requires corresponding neural substrates that operate on these different timescales. In discussing temporal patterns of DA signaling, we focus on a neglected form of DA plasticity and adaptation, changes in the physical substrate of the DA system itself, such as up- and down-regulation of receptors or release probability. We argue that changes in the DA substrate itself fundamentally alter its computational function, which we propose mediates adaptations to longer temporal horizons and economic conditions. In developing our hypothesis, we focus on DA D2 receptors (D2R), arguing that D2R implements a form of “cost control” in response to the environmental economy, serving as the “brain’s comptroller”. We propose that the balance between the direct and indirect pathway, regulated by relative expression of D1 and D2 DA receptors, implements affordability. Finally, as we review data, we discuss limitations in current approaches that impede fully investigating the proposed hypothesis and highlight alternative, more semi-naturalistic strategies more conducive to neuroeconomic investigations on the role of DA in adaptive behavior.

Alterations in mGlu5 receptor expression and function in the striatum in a rat depression model

Major depressive disorder is a common form of mental illness. Many brain regions are implicated in the pathophysiology and symptomatology of depression. Among key brain areas is the striatum that controls reward and mood and is involved in the development of core depression-like behavior in animal models of depression. While molecular mechanisms in this region underlying depression-related behavior are poorly understood, the glutamatergic input to the striatum is believed to play a role. In this study, we investigated changes in metabotropic glutamate (mGlu) receptor expression and signaling in the striatum of adult rats in response to prolonged (10-12 weeks) social isolation, a pre-validated animal paradigm modeling depression in adulthood. We found that mGlu5 receptor protein levels in the striatum were increased in rats that showed typical depression- and anxiety-like behavior after chronic social isolation. This increase in mGlu5 receptor expression was seen in both subdivisions of the striatum, the nucleus accumbens and caudate putamen. At subcellular and subsynaptic levels, mGlu5 receptor expression was elevated in surface membranes at synaptic sites. In striatal neurons, the mGlu5-associated phosphoinositide signaling pathway was augmented in its efficacy after prolonged social isolation. These data indicate that the mGlu5 receptor is a sensitive substrate of depression. Adulthood social isolation leads to the up-regulation of mGlu5 receptor expression and function in striatal neurons.

Dopamine's Effects on Corticostriatal Synapses during Reward-Based Behaviors

Many learned responses depend on the coordinated activation and inhibition of synaptic pathways in the striatum. Local dopamine neurotransmission acts in concert with a variety of neurotransmitters to regulate cortical, thalamic, and limbic excitatory inputs to drive the direct and indirect striatal spiny projection neuron outputs that determine the activity, sequence, and timing of learned behaviors. We review recent advances in the characterization of stereotyped neuronal and operant responses that predict and then obtain rewards. These depend on the local release of dopamine at discrete times during behavioral sequences, which, acting with glutamate, provides a presynaptic filter to select which excitatory synapses are inhibited and which signals pass to indirect pathway circuits. This is followed by dopamine-dependent activation of specific direct pathway circuits to procure a reward. These steps may provide a means by which higher organisms learn behaviors in response to feedback from the environment.

Exercise-Induced Fatigue Impairs Bidirectional Corticostriatal Synaptic Plasticity

Exercise-induced fatigue (EF) is a ubiquitous phenomenon in sports competition and training. It can impair athletes’ motor skill execution and cognition. Corticostriatal synaptic plasticity is considered to be the cellular mechanism of movement control and motor learning. However, the effect of EF on corticostriatal synaptic plasticity remains elusive. In the present study, using field excitatory postsynaptic potential recording, we found that the corticostriatal long-term potentiation (LTP) and long-term depression (LTD) were both impaired in EF mice. To further investigate the cellular mechanisms underlying the impaired synaptic plasticity in corticostriatal pathway, whole-cell patch clamp recordings were carried out on striatal medium spiny neurons (MSNs). MSNs in EF mice exhibited increased spontaneous excitatory postsynaptic current (sEPSC) frequency and decreased paired-pulse ratio (PPR), while with normal basic electrophysiological properties and normal sEPSC amplitude. Furthermore, the N-methyl-D-aspartate (NMDA)/α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) ratio of MSNs was reduced in EF mice. These results suggest that the enhanced presynaptic glutamate (Glu) release and downregulated postsynaptic NMDA receptor function lead to the impaired corticostriatal plasticity in EF mice. Taken together, our findings for the first time show that the bidirectional corticostriatal synaptic plasticity is impaired after EF, and suggest that the aberrant corticostriatal synaptic plasticity may be involved in the production and/or maintenance of EF.

Regulation of Striatal Neuron Activity by Cyclic Nucleotide Signaling and Phosphodiesterase Inhibition: Implications for the Treatment of Parkinson's Disease

Cyclic nucleotide phosphodiesterase (PDE) enzymes catalyze the hydrolysis and inactivation of cyclic nucleotides (cAMP/cGMP) in the brain. Several classes of PDE enzymes with distinct tissue distributions, cyclic nucleotide selectivity, and regulatory factors are highly expressed in brain regions subserving cognitive and motor processes known to be disrupted in neurodegenerative diseases such as Parkinson's disease (PD). Furthermore, small-molecule inhibitors of several different PDE family members alter cyclic nucleotide levels and favorably enhance motor performance and cognition in animal disease models. This chapter will explore the roles and therapeutic potential of non-selective and selective PDE inhibitors on neural processing in fronto-striatal circuits in normal animals and models of DOPA-induced dyskinesias (LIDs) associated with PD. The impact of selective PDE inhibitors and augmentation of cAMP and cGMP signaling on the membrane excitability of striatal medium-sized spiny projection neurons (MSNs) will be discussed. The effects of cyclic nucleotide signaling and PDE inhibitors on synaptic plasticity of striatonigral and striatopallidal MSNs will be also be reviewed. New data on the efficacy of PDE10A inhibitors for reversing behavioral and electrophysiological correlates of L-DOPA-induced dyskinesias in a rat model of PD will also be presented. Together, these data will highlight the potential of novel PDE inhibitors for treatment of movement disorders such as PD which are associated with abnormal corticostriatal transmission.

Endocannabinoid Actions on Cortical Terminals Orchestrate Local Modulation of Dopamine Release in the Nucleus Accumbens

Dopamine (DA) transmission mediates numerous aspects of behavior. Although DA release is strongly linked to firing of DA neurons, recent developments indicate the importance of presynaptic modulation at striatal dopaminergic terminals. The endocannabinoid (eCB) system regulates DA release and is a canonical gatekeeper of goal-directed behavior. Here we report that extracellular DA increases induced by selective optogenetic activation of cholinergic neurons in the nucleus accumbens (NAc) are inhibited by CB1 agonists and eCBs. This modulation requires CB1 receptors on cortical glutamatergic afferents. Dopamine increases driven by optogenetic activation of prefrontal cortex (PFC) terminals in the NAc are similarly modulated by activation of these CB1 receptors. We further demonstrate that this same population of CB1 receptors modulates optical self-stimulation sustained by activation of PFC afferents in the NAc. These results establish local eCB actions on PFC terminals within the NAc that inhibit mesolimbic DA release and constrain reward-driven behavior.

Endocannabinoid-Specific Impairment in Synaptic Plasticity in Striatum of Huntington's Disease Mouse Model

Huntington's disease (HD) is an inherited neurodegenerative disease affecting predominantly striatum and cortex that results in motor and cognitive disorders. Before a motor phenotype, animal models of HD show aberrant cortical-striatal glutamate signaling. Here, we tested synaptic plasticity of cortical excitatory synapses onto striatal spiny projection neurons (SPNs) early in the YAC128 mouse model of HD. High-frequency stimulation-induced long-term depression, mediated by the endocannabinoid anandamide and cannabinoid receptor 1 (CB1), was significantly attenuated in male and female YAC128 SPNs. Indirect pathway SPNs, which are more vulnerable in HD, were most affected. Our experiments show metabotropic glutamate receptor and endocannabinoid 2-arachidonoylglycerol-dependent plasticity, as well as direct CB1 activation by agonists, was similar in YAC128 and FVB/N wild-type SPNs suggesting that presynaptic CB1 is functioning normally. These results are consistent with a specific impairment in postsynaptic anandamide synthesis in YAC128 SPN. Strikingly, although suppression of degradation of anandamide was not effective, elevating 2-arachidonoylglycerol levels restored long-term depression in YAC128 striatal neurons. Together, these results have potential implications for neuroprotection and ameliorating early cognitive and motor deficits in HD.SIGNIFICANCE STATEMENT Huntington's disease (HD) is an inherited neurodegenerative disease with no cure. Recent studies find impairment of the endocannabinoid system in animal models but the functional implication for synaptic plasticity in HD remains unclear. Sepers et al. show a selective deficit in synaptic plasticity mediated by the endocannabinoid anandamide, but not 2-arachidonoylglycerol in a mouse model of HD. The deficit is rescued by selectively elevating levels of 2-arachidonoylglycerol produced on-demand. This mechanism could be targeted in the development of future therapeutics for HD.

Accumbens Mechanisms for Cued Sucrose Seeking

Many studies support a perspective that addictive drugs usurp brain circuits used by natural rewards, especially for the dopamine-dependent reinforcing qualities of both drugs and natural rewards. Reinstated drug seeking in animal models of relapse relies on glutamate spillover from cortical terminals synapsing in the nucleus accumbens core (NAcore) to stimulate metabotropic glutamate receptor5 (mGluR5) on neuronal nitric oxide synthase (nNOS) interneurons. Contrasting the release of dopamine that is shared by sucrose and drugs of abuse, reinstated sucrose seeking does not induce glutamate spillover. We hypothesized that pharmacologically promoting glutamate spillover in the NAcore would mimic cocaine-induced adaptations and potentiate cued reinstatement of sucrose seeking. Inducing glutamate spillover by blocking astroglial glutamate transporters (GLT-1) had no effect on reinstated sucrose seeking. However, glutamate release probability is negatively regulated by presynaptic mGluR2/3, and sucrose reinstatement was potentiated following mGluR2/3 blockade. Potentiated sucrose reinstatement by mGluR2/3 blockade was reversed by antagonizing mGluR5, but reinstated sucrose seeking in the absence of mGluR2/3 blockade was not affected by blocking mGluR5. In cocaine-trained rodents mGluR5 stimulation reinstates drug seeking by activating nNOS, but activating mGluR5 did not promote reinstated sucrose seeking, nor was potentiated reinstatement after mGluR2/3 blockade reduced by blocking nNOS. However, chemogenetic activation of nNOS interneurons in the NAcore reinstated sucrose seeking. These data indicate that dysregulated presynaptic mGluR2/3 signaling is a possible site of shared signaling in drug seeking and potentiated reinstated sucrose seeking, but that downregulated glutamate transport and subsequent activation of nNOS by synaptic glutamate spillover is not shared.

The neurobiological bases of autism spectrum disorders: the R451C-neuroligin 3 mutation hampers the expression of long-term synaptic depression in the dorsal striatum

Autism spectrum disorders (ASDs) comprise a heterogeneous group of disorders with a complex genetic etiology. Current theories on the pathogenesis of ASDs suggest that they might arise from an aberrant synaptic transmission affecting specific brain circuits and synapses. The striatum, which is part of the basal ganglia circuit, is one of the brain regions involved in ASDs. Mouse models of ASDs have provided evidence for an imbalance between excitatory and inhibitory neurotransmission. Here, we investigated the expression of long-term synaptic plasticity at corticostriatal glutamatergic synapses in the dorsal striatum of the R451C-NL3 phenotypic mouse model of autism. This mouse model carries the human R451C mutation in the neuroligin 3 (NL3) gene that has been associated with highly penetrant autism in a Swedish family. The R451C-NL3 mouse has been shown to exhibit autistic-like behaviors and alterations of synaptic transmission in different brain areas. However, excitatory glutamatergic transmission and its long-term plasticity have not been investigated in the dorsal striatum so far. Our results indicate that the expression of long-term synaptic depression (LTD) at corticostriatal glutamatergic synapses in the dorsal striatum is impaired by the R451C-NL3 mutation. A partial rescue of LTD was obtained by exogenous activation of cannabinoid CB1 receptors or enhancement of the endocannabinoid tone, suggesting that an altered cannabinoid drive might underlie the deficit of synaptic plasticity in the dorsal striatum of R451C-NL3 mice.

Drug-Induced Alterations of Endocannabinoid-Mediated Plasticity in Brain Reward Regions

The endocannabinoid (eCB) system has emerged as one of the most important mediators of physiological and pathological reward-related synaptic plasticity. eCBs are retrograde messengers that provide feedback inhibition, resulting in the suppression of neurotransmitter release at both excitatory and inhibitory synapses, and they serve a critical role in the spatiotemporal regulation of both short- and long-term synaptic plasticity that supports adaptive learning of reward-motivated behaviors. However, mechanisms of eCB-mediated synaptic plasticity in reward areas of the brain are impaired following exposure to drugs of abuse. Because of this, it is theorized that maladaptive eCB signaling may contribute to the development and maintenance of addiction-related behavior. Here we review various forms of eCB-mediated synaptic plasticity present in regions of the brain involved in reward and reinforcement and explore the potential physiological relevance of maladaptive eCB signaling to addiction vulnerability.

Combining Dopaminergic Facilitation with Robot-Assisted Upper Limb Therapy in Stroke Survivors: A Focused Review

Despite aggressive conventional therapy, lasting hemiplegia persists in a large percentage of stroke survivors. The aim of this article is to critically review the rationale behind targeting multiple sites along the motor learning network by combining robotic therapy with pharmacotherapy and virtual reality–based reward learning to alleviate upper extremity impairment in stroke survivors. Methods for personalizing pharmacologic facilitation to each individual’s unique biology are also reviewed. At the molecular level, treatment with levodopa was shown to induce long-term potentiation-like and practice-dependent plasticity. Clinically, trials combining conventional therapy with levodopa in stroke survivors yielded statistically significant but clinically unconvincing outcomes because of limited personalization, standardization, and reproducibility. Robotic therapy can induce neuroplasticity by delivering intensive, reproducible, and functionally meaningful interventions that are objective enough for the rigors of research. Robotic therapy also provides an apt platform for virtual reality, which boosts learning by engaging reward circuits. The future of stroke rehabilitation should target distinct molecular, synaptic, and cortical sites through personalized multimodal treatments to maximize motor recovery.

Endocannabinoid modulation of homeostatic and non-homeostatic feeding circuits

The endocannabinoid system has emerged as a key player in the control of eating. Endocannabinoids, including 2-arachidonoylglycerol (2-AG) and anandamide (AEA), modulate neuronal activity via cannabinoid 1 receptors (CB1Rs) in multiple nuclei of the hypothalamus to induce or inhibit food intake depending on nutritional and hormonal status, suggesting that endocannabinoids may act in the hypothalamus to integrate different types of signals informing about the animal's energy needs. In the mesocorticolimbic system, (endo)cannabinoids modulate synaptic transmission to promote dopamine release in response to palatable food. In addition, (endo)cannabinoids act within the nucleus accumbens to increase food's hedonic impact; although this effect depends on activation of CB1Rs at excitatory, but not inhibitory inputs in the nucleus accumbens. While hyperactivation of the endocannabinoid system is typically associated with overeating and obesity, much evidence has emerged in recent years suggesting a more complicated system than first thought - endocannabinoids promote or suppress feeding depending on cell and input type, or modulation by various neuronal or hormonal signals. This review presents our latest knowledge of the endocannabinoid system in non-homeostatic and homeostatic feeding circuits. In particular, we discuss the functional role and cellular mechanism of action by endocannabinoids within the hypothalamus and mesocorticolimbic system, and how these are modulated by neuropeptide signals related to feeding. In light of recent advances and complexity in the field, we review cannabinoid-based therapeutic strategies for the treatment of obesity and how peripheral restriction of CB1R antagonists may provide a different mechanism of weight loss without the central adverse effects. This article is part of the Special Issue entitled "A New Dawn in Cannabinoid Neurobiology".

CB1 receptors down-regulate a cAMP/Epac2/PLC pathway to silence the nerve terminals of cerebellar granule cells

Cannabinoid receptors mediate short-term retrograde inhibition of neurotransmitter release, as well as long-term depression of synaptic transmission at excitatory synapses. The responses of individual nerve terminals in VGLUT1-pHluorin transfected cerebellar granule cells to cannabinoids have shown that prolonged activation of cannabinoid type 1 receptors (CB1Rs) silences a subpopulation of previously active synaptic boutons. Adopting a combined pharmacological and genetic approach to study the molecular mechanisms of CB1R-induced silencing, we found that adenylyl cyclase inhibition decreases cAMP levels while it increases the number of silent synaptic boutons and occludes the induction of further silencing by the cannabinoid agonist HU-210. Guanine nucleotide exchange proteins directly activated by cAMP (Epac proteins) mediate some of the presynaptic effects of cAMP in the potentiation of synaptic transmission. ESI05, a selective Epac2 inhibitor, and U-73122, the specific inhibitor of phospholipase C (PLC), both augment the number of silent synaptic boutons. Moreover, they abolish the capacity of the Epac activator, 8-(4-chlorophenylthio)-2'-O-methyladenosine 3',5'-cyclic monophosphate monosodium hydrate, to prevent HU-210-induced silencing consistent with PLC signaling lying downstream of Epac2 proteins. Furthermore, Rab3-interacting molecule (RIM)1α KO cells have many more basally silent synaptic boutons (12.9 ± 3.5%) than wild-type cells (1.1 ± 0.5%). HU-210 induced further silencing in these mutant cells, although 8-(4-chlorophenylthio)-2'-O-methyladenosine 3',5'-cyclic monophosphate monosodium hydrate only awoke the HU-210-induced silence and not the basally silent synaptic boutons. This behavior can be rescued by expressing RIM1α in RIM1α KO cells, these cells behaving very much like wild-type cells. These findings support the hypothesis that a cAMP/Epac/PLC signaling pathway targeting the release machinery appears to mediate cannabinoid-induced presynaptic silencing.

Striatopallidal dysfunction underlies repetitive behavior in Shank3-deficient model of autism

The postsynaptic scaffolding protein SH3 and multiple ankyrin repeat domains 3 (SHANK3) is critical for the development and function of glutamatergic synapses. Disruption of the SHANK3-encoding gene has been strongly implicated as a monogenic cause of autism, and Shank3 mutant mice show repetitive grooming and social interaction deficits. Although basal ganglia dysfunction has been proposed to underlie repetitive behaviors, few studies have provided direct evidence to support this notion and the exact cellular mechanisms remain largely unknown. Here, we utilized the Shank3B mutant mouse model of autism to investigate how Shank3 mutation may differentially affect striatonigral (direct pathway) and striatopallidal (indirect pathway) medium spiny neurons (MSNs) and its relevance to repetitive grooming behavior in Shank3B mutant mice. We found that Shank3 deletion preferentially affects synapses onto striatopallidal MSNs. Striatopallidal MSNs showed profound defects, including alterations in synaptic transmission, synaptic plasticity, and spine density. Importantly, the repetitive grooming behavior was rescued by selectively enhancing the striatopallidal MSN activity via a Gq-coupled human M3 muscarinic receptor (hM3Dq), a type of designer receptors exclusively activated by designer drugs (DREADD). Our findings directly demonstrate the existence of distinct changes between 2 striatal pathways in a mouse model of autism and indicate that the indirect striatal pathway disruption might play a causative role in repetitive behavior of Shank3B mutant mice.

A Feedforward Inhibitory Circuit Mediated by CB1-Expressing Fast-Spiking Interneurons in the Nucleus Accumbens

The nucleus accumbens (NAc) gates motivated behaviors through the functional output of principle medium spiny neurons (MSNs), whereas dysfunctional output of NAc MSNs contributes to a variety of psychiatric disorders. Fast-spiking interneurons (FSIs) are sparsely distributed throughout the NAc, forming local feedforward inhibitory circuits. It remains elusive how FSI-based feedforward circuits regulate the output of NAc MSNs. Here, we investigated a distinct subpopulation of NAc FSIs that express the cannabinoid receptor type-1 (CB1). Using a combination of paired electrophysiological recordings and pharmacological approaches, we characterized and compared feedforward inhibition of NAc MSNs from CB1⁺ FSIs and lateral inhibition from recurrent MSN collaterals. We observed that CB1⁺ FSIs exerted robust inhibitory control over a large percentage of nearby MSNs in contrast to local MSN collaterals that provided only sparse and weak inhibitory input to their neighboring MSNs. Furthermore, CB1⁺ FSI-mediated feedforward inhibition was preferentially suppressed by endocannabinoid (eCB) signaling, whereas MSN-mediated lateral inhibition was unaffected. Finally, we demonstrated that CB1⁺ FSI synapses onto MSNs are capable of undergoing experience-dependent long-term depression in a voltage- and eCB-dependent manner. These findings demonstrated that CB1⁺ FSIs are a major source of local inhibitory control of MSNs and a critical component of the feedforward inhibitory circuits regulating the output of the NAc.

Changes in the Brain Endocannabinoid System in Rat Models of Depression

A growing body of evidence implicates the endocannabinoid (eCB) system in the pathophysiology of depression. The aim of this study was to investigate the influence of changes in the eCB system, such as levels of neuromodulators, eCB synthesizing and degrading enzymes, and cannabinoid (CB) receptors, in different brain structures in animal models of depression using behavioral and biochemical analyses. Both models used, i.e., bulbectomized (OBX) and Wistar Kyoto (WKY) rats, were characterized at the behavioral level by increased immobility time. In the OBX rats, anandamide (AEA) levels were decreased in the prefrontal cortex, hippocampus, and striatum and increased in the nucleus accumbens, while 2-arachidonoylglycerol (2-AG) levels were increased in the prefrontal cortex and decreased in the nucleus accumbens with parallel changes in the expression of eCB metabolizing enzymes in several structures. It was also observed that CB1 receptor expression decreased in the hippocampus, dorsal striatum, and nucleus accumbens, and CB2 receptor expression decreased in the prefrontal cortex and hippocampus. In WKY rats, the levels of eCBs were reduced in the prefrontal cortex (2-AG) and dorsal striatum (AEA) and increased in the prefrontal cortex (AEA) with different changes in the expression of eCB metabolizing enzymes, while the CB1 receptor density was increased in several brain regions. These findings suggest that dysregulation in the eCB system is implicated in the pathogenesis of depression, although neurochemical changes were linked to the particular brain structure and the factor inducing depression (surgical removal of the olfactory bulbs vs. genetic modulation).

Metabotropic glutamate receptor 2 inhibits thalamically-driven glutamate and dopamine release in the dorsal striatum

The striatum plays critical roles in action control and cognition, and activity of striatal neurons is driven by glutamatergic input. Inhibition of glutamatergic inputs to projection neurons and interneurons of the striatum by presynaptic G protein-coupled receptors (GPCRs) stands to modulate striatal output and striatum-dependent behaviors. Despite knowledge that a substantial number of glutamatergic inputs to striatal neurons originate in the thalamus, most electrophysiological studies assessing GPCR modulation do not differentiate between effects on corticostriatal and thalamostriatal transmission, and synaptic inhibition is frequently assumed to be mediated by activation of GPCRs on corticostriatal terminals. We used optogenetic techniques and recently-discovered pharmacological tools to dissect the effects of a prominent presynaptic GPCR, metabotropic glutamate receptor 2 (mGlu₂), on corticostriatal vs. thalamostriatal transmission. We found that an agonist of mGlu₂ and mGlu₃ induces long-term depression (LTD) at synapses onto MSNs from both the cortex and the thalamus. Thalamostriatal LTD is selectively blocked by an mGlu₂-selective negative allosteric modulator and reversed by application of an antagonist following LTD induction. Activation of mGlu_2/3 also induces LTD of thalamostriatal transmission in striatal cholinergic interneurons (CINs), and pharmacological activation of mGlu_2/3 or selective activation of mGlu₂ inhibits CIN-mediated dopamine release evoked by selective stimulation of thalamostriatal inputs. Thus, mGlu₂ activation exerts effects on striatal physiology that extend beyond modulation of corticostriatal synapses, and has the potential to influence cognition and striatum-related disorders via inhibition of thalamus-derived glutamate and dopamine release.

URB597 and the Cannabinoid WIN55,212-2 Reduce Behavioral and Neurochemical Deficits Induced by MPTP in Mice: Possible Role of Redox Modulation and NMDA Receptors

Several physiological events in the brain are regulated by the endocannabinoid system (ECS). While synthetic cannabinoid receptor (CBr) agonists such as WIN55,212-2 act directly on CBr, agents like URB597, a fatty acid amide hydrolase (FAAH) inhibitor, induce a more "physiological" activation of CBr by increasing the endogenous levels of the endocannabinoid anandamide (AEA). Herein, we compared the pre- and post-treatment efficacy of URB597 and WIN55,212-2 on different endpoints evaluated in the toxic model produced by the mitochondrial toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in mice. MPTP (40 mg/kg, s.c., single injection) decreased locomotor activity, depleted the striatal and nigral levels of dopamine (DA), augmented the levels of lipid peroxidation and protein carbonylation in both regions, decreased the striatal protein levels of tyrosine hydroxylase, and increased the striatal protein content of the subunit 1 (NR1) of the N-methyl-D-aspartate receptor (NMDAr). Both URB597 (0.3 mg/kg, i.p., once a day) and WIN55,212-2 (10 μg/kg, i.p., twice a day), administered for five consecutive days, either before or after the MPTP injection, prevented the alterations elicited by MPTP and downregulated NMDAr. Our results support a modulatory role of the ECS on the toxic profile exerted by MPTP in mice via the stimulation of antioxidant activity and the induction of NMDAr downregulation and hypofunction, and favor the stimulation of CBr as an effective experimental therapeutic strategy.

The GABAA receptor agonist muscimol induces an age- and region-dependent form of long-term depression in the mouse striatum

Several forms of long-term depression (LTD) of glutamatergic synaptic transmission have been identified in the dorsal striatum and in the nucleus accumbens (NAc). Such experience-dependent synaptic plasticity might play important roles in reward-related learning. The GABAA receptor agonist muscimol was recently found to trigger a long-lasting depression of glutamatergic synaptic transmission in the NAc of adolescent mice, but the mechanisms that underlie this novel form of LTD were not studied. Here we examined the effect of muscimol applied in the perfusion solution on the amplitude of field excitatory postsynaptic potentials/population spikes (fEPSP/PSs) in mouse brain slices. We found that muscimol depressed the fEPSP/PS in the NAc of adolescent mice but not adult mice, through both postsynaptic and presynaptic mechanisms. Indeed, muscimol altered the fEPSP/PS paired-pulse ratio, depolarized the membrane of projection neurons, and decreased the frequency, but not amplitude, of spontaneous excitatory postsynaptic currents in the NAc of adolescent mice. The LTD induced by muscimol likely involved endocannabinoids, metabotropic glutamate receptors (mGluRs), but not TRPV1 receptors. Muscimol-LTD was occluded by prior induction of LTD through low-frequency stimulation (LFS) of the slice, demonstrating a common pathway in the induction of LFS-LTD and muscimol-LTD. We also found that muscimol induced a form of LTD in the dorsolateral striatum of adult but not adolescent mice. This LTD was mediated by endocannabinoids but did not involve mGluRs or TRPV1 receptors. These results identify a novel form of synaptic plasticity, and its mechanisms of induction, which is age and region dependent. These findings may contribute to a better understanding of the increased susceptibility of the adolescent brain to long-term synaptic changes in regions associated with reward mechanisms.

Endocannabinoids and Synaptic Function in the CNS

Marijuana affects neural functions through the binding of its active component (Δ9-THC) to cannabinoid receptors in the CNS. Recent studies have elucidated that endogenous ligands for cannabinoid receptors, endocannabinoids, serve as retrograde messengers at central synapses. Endocannabinoids are produced on demand in activity-dependent manners and released from postsynaptic neurons. The released endocannabinoids travel backward across the synapse, activate presynaptic CB1 cannabinoid receptors, and modulate presynaptic functions. Retrograde endocannabinoid signaling is crucial for certain forms of short-term and long-term synaptic plasticity at excitatory or inhibitory synapses in many brain regions, and thereby contributes to various aspects of brain function including learning and memory. Molecular identities of the CB1 receptor and enzymes involved in production and degradation of endocannabinoids have been elucidated. Anatomical studies have demonstrated unique distributions of these molecules around synapses, which provide morphological bases for the roles of endocannabinoids as retrograde messengers. CB1-knockout mice exhibit various behavioral abnormalities and multiple defects in synaptic plasticity, supporting the notion that endocannabinoid signaling is involved in various aspects of neural function. In this review article, the authors describe molecular mechanisms of the endocannabinoid-mediated synaptic modulation and its possible physiological significance. NEUROSCIENTIST 13(2):127—137, 2007.

Calcium dynamics predict direction of synaptic plasticity in striatal spiny projection neurons

The striatum is a major site of learning and memory formation for sensorimotor and cognitive association. One of the mechanisms used by the brain for memory storage is synaptic plasticity - the long-lasting, activity-dependent change in synaptic strength. All forms of synaptic plasticity require an elevation in intracellular calcium, and a common hypothesis is that the amplitude and duration of calcium transients can determine the direction of synaptic plasticity. The utility of this hypothesis in the striatum is unclear in part because dopamine is required for striatal plasticity and in part because of the diversity in stimulation protocols. To test whether calcium can predict plasticity direction, we developed a calcium-based plasticity rule using a spiny projection neuron model with sophisticated calcium dynamics including calcium diffusion, buffering and pump extrusion. We utilized three spike timing-dependent plasticity (STDP) induction protocols, in which postsynaptic potentials are paired with precisely timed action potentials and the timing of such pairing determines whether potentiation or depression will occur. Results show that despite the variation in calcium dynamics, a single, calcium-based plasticity rule, which explicitly considers duration of calcium elevations, can explain the direction of synaptic weight change for all three STDP protocols. Additional simulations show that the plasticity rule correctly predicts the NMDA receptor dependence of long-term potentiation and the L-type channel dependence of long-term depression. By utilizing realistic calcium dynamics, the model reveals mechanisms controlling synaptic plasticity direction, and shows that the dynamics of calcium, not just calcium amplitude, are crucial for synaptic plasticity.

Electrophysiological Measurement of Cannabinoid-Mediated Synaptic Modulation in Acute Mouse Brain Slices

Endocannabinoids (eCBs) are a class of bioactive lipids that mediate retrograde synaptic modulation at central and peripheral synapses. The highly lipophilic nature of eCBs and the pharmacological tools available to interrogate this system require unique methodological consideration, especially when applied to ex vivo systems such as electrophysiological analysis in acute brain slices. This unit provides protocols for measuring cannabinoid and eCB-mediated synaptic signaling in mouse brain slices, including analysis of short-term, long-term, and tonic eCB signaling modes, and the unique considerations for working with eCBs and TRPV1/cannabinoid ligands in acute brain slices.

Modulation by Trace Amine-Associated Receptor 1 of Experimental Parkinsonism, L-DOPA Responsivity, and Glutamatergic Neurotransmission

Parkinson's disease (PD) is a movement disorder characterized by a progressive loss of nigrostriatal dopaminergic neurons. Restoration of dopamine transmission by l-DOPA relieves symptoms of PD but causes dyskinesia. Trace Amine-Associated Receptor 1 (TAAR1) modulates dopaminergic transmission, but its role in experimental Parkinsonism and l-DOPA responses has been neglected. Here, we report that TAAR1 knock-out (KO) mice show a reduced loss of dopaminergic markers in response to intrastriatal 6-OHDA administration compared with wild-type (WT) littermates. In contrast, the TAAR1 agonist RO5166017 aggravated degeneration induced by intrastriatal 6-OHDA in WT mice. Subchronic l-DOPA treatment of TAAR1 KO mice unilaterally lesioned with 6-OHDA in the medial forebrain bundle resulted in more pronounced rotational behavior and dyskinesia than in their WT counterparts. The enhanced behavioral sensitization to l-DOPA in TAAR1 KO mice was paralleled by increased phosphorylation of striatal GluA1 subunits of AMPA receptors. Conversely, RO5166017 counteracted both l-DOPA-induced rotation and dyskinesia as well as AMPA receptor phosphorylation. Underpinning a role for TAAR1 receptors in modulating glutamate neurotransmission, intrastriatal application of RO5166017 prevented the increase of evoked corticostriatal glutamate release provoked by dopamine deficiency after 6-OHDA-lesions or conditional KO of Nurr1. Finally, inhibition of corticostriatal glutamate release by TAAR1 showed mechanistic similarities to that effected by activation of dopamine D2 receptors. These data unveil a role for TAAR1 in modulating the degeneration of dopaminergic neurons, the behavioral response to l-DOPA, and presynaptic and postsynaptic glutamate neurotransmission in the striatum, supporting their relevance to the pathophysiology and, potentially, management of PD.

SIGNIFICANCE STATEMENT

Parkinson's disease (PD) is characterized by a progressive loss of nigrostriatal dopaminergic neurons. Restoration of dopamine transmission by l-DOPA relieves symptoms of PD but causes severe side effects. Trace Amine-Associated Receptor 1 (TAAR1) modulates dopaminergic transmission, but its role in PD and l-DOPA responses has been neglected. Here, we report that TAAR1 potentiates the degeneration of dopaminergic neurons and attenuates the behavioral response to l-DOPA and presynaptic and postsynaptic glutamate neurotransmission in the striatum, supporting the relevance of TAAR1 to the pathophysiology and, potentially, management of PD.

Roles for Regulator of G Protein Signaling Proteins in Synaptic Signaling and Plasticity

The regulator of G protein signaling (RGS) family of proteins serves critical roles in G protein-coupled receptor (GPCR) and heterotrimeric G protein signal transduction. RGS proteins are best understood as negative regulators of GPCR/G protein signaling. They achieve this by acting as GTPase activating proteins (GAPs) for Gα subunits and accelerating the turnoff of G protein signaling. Many RGS proteins also bind additional signaling partners that either regulate their functions or enable them to regulate other important signaling events. At neuronal synapses, GPCRs, G proteins, and RGS proteins work in coordination to regulate key aspects of neurotransmitter release, synaptic transmission, and synaptic plasticity, which are necessary for central nervous system physiology and behavior. Accumulating evidence has revealed key roles for specific RGS proteins in multiple signaling pathways at neuronal synapses, regulating both pre- and postsynaptic signaling events and synaptic plasticity. Here, we review and highlight the current knowledge of specific RGS proteins (RGS2, RGS4, RGS7, RGS9-2, and RGS14) that have been clearly demonstrated to serve critical roles in modulating synaptic signaling and plasticity throughout the brain, and we consider their potential as future therapeutic targets.

Interneuronal Nitric Oxide Signaling Mediates Post-synaptic Long-Term Depression of Striatal Glutamatergic Synapses

Experience-driven plasticity of glutamatergic synapses on striatal spiny projection neurons (SPNs) is thought to be essential to goal-directed behavior and habit formation. One major form of striatal plasticity, long-term depression (LTD), has long appeared to be expressed only pre-synaptically. Contrary to this view, nitric oxide (NO) generated by striatal interneurons was found to induce a post-synaptically expressed form of LTD at SPN glutamatergic synapses. This form of LTD was dependent on signaling through guanylyl cyclase and protein kinase G, both of which are abundantly expressed by SPNs. NO-LTD was unaffected by local synaptic activity or antagonism of endocannabinoid (eCb) and dopamine receptors, all of which modulate canonical, pre-synaptic LTD. Moreover, NO signaling disrupted induction of this canonical LTD by inhibiting dendritic Ca(2+) channels regulating eCb synthesis. These results establish an interneuron-dependent, heterosynaptic form of post-synaptic LTD that could act to promote stability of the striatal network during learning.

Promising cannabinoid-based therapies for Parkinson's disease: motor symptoms to neuroprotection

Parkinson’s disease (PD) is a slow insidious neurological disorder characterized by a loss of dopaminergic neurons in the midbrain. Although several recent preclinical advances have proposed to treat PD, there is hardly any clinically proved new therapeutic for its cure. Increasing evidence suggests a prominent modulatory function of the cannabinoid signaling system in the basal ganglia. Hence, use of cannabinoids as a new therapeutic target has been recommended as a promising therapy for PD. The elements of the endocannabinoid system are highly expressed in the neural circuit of basal ganglia wherein they bidirectionally interact with dopaminergic, glutamatergic, and GABAergic signaling systems. As the cannabinoid signaling system undergoes a biphasic pattern of change during progression of PD, it explains the motor inhibition typically observed in patients with PD. Cannabinoid agonists such as WIN-55,212-2 have been demonstrated experimentally as neuroprotective agents in PD, with respect to their ability to suppress excitotoxicity, glial activation, and oxidative injury that causes degeneration of dopaminergic neurons. Additional benefits provided by cannabinoid related compounds including CE-178253, oleoylethanolamide, nabilone and HU-210 have been reported to possess efficacy against bradykinesia and levodopa-induced dyskinesia in PD. Despite promising preclinical studies for PD, use of cannabinoids has not been studied extensively at the clinical level. In this review, we reassess the existing evidence suggesting involvement of the endocannabinoid system in the cause, symptomatology, and treatment of PD. We will try to identify future threads of research that will help in the understanding of the potential therapeutic benefits of the cannabinoid system for treating PD.

Endocannabinoids in synaptic plasticity and neuroprotection

Endocannabinoids (eCBs) are endogenous lipid mediators involved in a variety of physiological, pharmacological, and pathological processes. While activation of the eCB system primarily induces inhibitory effects on both GABAergic and glutamatergic synaptic transmission and plasticity through acting on presynaptically expressed CB1 receptors in the brain, accumulated information suggests that eCB signaling is also capable of facilitating or potentiating excitatory synaptic transmission in the hippocampus. Recent studies show that a long-lasting potentiation of excitatory synaptic transmission at Schaffer collateral (SC)-CA1 synapses is induced by spatiotemporally primed inputs, accompanying with a long-term depression of inhibitory synaptic transmission (I-LTD) in hippocampal CA1 pyramidal neurons. This input timing-dependent long-lasting synaptic potentiation at SC-CA1 synapses is mediated by 2-arachidonoylglycerol (2-AG) signaling triggered by activation of postsynaptic N-methyl-D-aspartate receptors, group I metabotropic glutamate receptors (mGluRs), and a concurrent rise in intracellular Ca(2+). Emerging evidence now also indicates that 2-AG is an important signaling mediator keeping brain homeostasis by exerting its anti-inflammatory and neuroprotective effects in response to harmful insults through CB1/2 receptor-dependent and/or -independent mechanisms. Activation of the nuclear receptor protein peroxisome proliferator-activated receptor-γ apparently is one of the important mechanisms in resolving neuroinflammation and protecting neurons produced by 2-AG signaling. Thus, the information summarized in this review suggests that the role of eCB signaling in maintaining integrity of brain function is greater than what we thought previously.

Long-term depression at distinct glutamatergic synapses in the basal ganglia

Long-term adaptations of synaptic transmission are believed to be the cellular basis of information storage in the brain. In particular, long-term depression of excitatory neurotransmission has been under intense investigation since convergent lines of evidence support a crucial role for this process in learning and memory. Within the basal ganglia, a network of subcortical nuclei forming a key part of the extrapyramidal motor system, plasticity at excitatory synapses is essential to the regulation of motor, cognitive, and reward functions. The striatum, the main gateway of the basal ganglia, receives convergent excitatory inputs from cortical areas and transmits information to the network output structures and is a major site of activity-dependent plasticity. Indeed, long-term depression at cortico-striatal synapses modulates the transfer of information to basal ganglia output structures and affects voluntary movement execution. Cortico-striatal plasticity is thus considered as a cellular substrate for adaptive motor control. Downstream in this network, the subthalamic nucleus and substantia nigra nuclei also receive glutamatergic innervation from the cortex and the subthalamic nucleus, respectively. Although these connections have been less investigated, recent studies have started to unravel the molecular mechanisms that contribute to adjustments in the strength of cortico-subthalamic and subthalamo-nigral transmissions, revealing that adaptations at these synapses governing the output of the network could also contribute to motor planning and execution. Here, we review our current understanding of long-term depression mechanisms at basal ganglia glutamatergic synapses and emphasize the common and unique plastic features observed at successive levels of the network in healthy and pathological conditions.

Modulation of the glutamatergic transmission by Dopamine: a focus on Parkinson, Huntington and Addiction diseases

Dopamine (DA) plays a major role in motor and cognitive functions as well as in reward processing by regulating glutamatergic inputs. In particular in the striatum the release of DA rapidly influences synaptic transmission modulating both AMPA and NMDA receptors. Several neurodegenerative and neuropsychiatric disorders, including Parkinson, Huntington and addiction-related diseases, manifest a dysregulation of glutamate and DA signaling. Here, we will focus our attention on the mechanisms underlying the modulation of the glutamatergic transmission by DA in striatal circuits.

Induction of cannabinoid- and N-methyl-D-aspartate receptor-mediated long-term depression in the nucleus accumbens and dorsolateral striatum is region and age dependent

BACKGROUND

The adolescent brain is sensitive to experience-dependent plasticity and might be more vulnerable than the adult brain to the effects of some drugs of abuse. The factors that contribute to these differences are not fully identified. We have examined the ability of cannabinoids to induce a form of synaptic plasticity, long-term depression, in the nucleus accumbens and dorsolateral striatum of adolescent and adult mice.

METHODS

We measured field excitatory postsynaptic potentials/population spikes in brain slices.

RESULTS

We found that the cannabinoid receptor agonist WIN 55,212-2 (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate) induced long-term depression in the nucleus accumbens of adolescent but not adult mice and failed to induce long-term depression in the dorsolateral striatum of adolescent or adult mice. Similar results were obtained with the group I metabotropic glutamate receptor agonist (S)-3,5- dihydroxyphenylglycine, which has previously been shown to promote the release of endocannabinoids. These age-related differences were associated with reduced protein levels of the cannabinoid type 1 receptor and metabotropic glutamate receptor 1 in adult nucleus accumbens and dorsolateral striatum and with an increased tone of endocannabinoids in the dorsolateral striatum of adult mice. We also found that N-methyl-D-aspartate receptor-dependent long-term depression, which was induced in the nucleus accumbens of adolescent mice, was blunted in adult mice, possibly because of decreased levels of GluN1, the obligatory subunit of N-methyl-D-aspartate receptors.

CONCLUSIONS

This study identifies region- and age-specific differences in the ability of endogenous and exogenous cannabinoids, and of N-methyl-D-aspartate receptors, to induce long-term depression in the striatal complex. These observations might contribute to a better understanding of the increased sensitivity of the adolescent brain to drug induced-plasticity.

Input- and cell-type-specific endocannabinoid-dependent LTD in the striatum

Changes in basal ganglia plasticity at the corticostriatal and thalamostriatal levels are required for motor learning. Endocannabinoid-dependent long-term depression (eCB-LTD) is known to be a dominant form of synaptic plasticity expressed at these glutamatergic inputs; however, whether eCB-LTD can be induced at all inputs on all striatal neurons is still debatable. Using region-specific Cre mouse lines combined with optogenetic techniques, we directly investigated and distinguished between corticostriatal and thalamostriatal projections. We found that eCB-LTD was successfully induced at corticostriatal synapses, independent of postsynaptic striatal spiny projection neuron (SPN) subtype. Conversely, eCB-LTD was only nominally present at thalamostriatal synapses. This dichotomy was attributable to the minimal expression of cannabinoid type 1 (CB1) receptors on thalamostriatal terminals. Furthermore, coactivation of dopamine receptors on SPNs during LTD induction re-established SPN-subtype-dependent eCB-LTD. Altogether, our findings lay the groundwork for understanding corticostriatal and thalamostriatal synaptic plasticity and for striatal eCB-LTD in motor learning.

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

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.

The initiation of synaptic 2-AG mobilization requires both an increased supply of diacylglycerol precursor and increased postsynaptic calcium

On-demand postsynaptic synthesis and release of endocannabinoid lipids and subsequent binding to presynaptic CB1 receptors (CB1Rs) mediates short and long-term depression (LTD) of excitatory transmission in many brain regions. However, mechanisms involved in the synthesis of the endocannabinoid 2-arachidonoylglycerol (2-AG) by diacylglycerol lipase α (DGLα) are poorly understood. Since Gq-coupled receptor activation can stimulate production of a major DGL substrate 1-stearoyl-2-arachidonoyl-sn-glycerol (SAG) by PLCβ, we sought to determine if 2-AG biosynthesis was limited only by a lack of substrate availability, or if other pathways, such as Ca(2+) signaling, also need to be simultaneously engaged. To address this question, we loaded medium spiny neurons of the dorsolateral striatum with SAG while monitoring excitatory synaptic inputs. SAG-loading had no significant effect on evoked excitatory synaptic currents when cells were voltage-clamped at -80 mV. However, depolarization of MSNs to -50 mV revealed a SAG-loading dependent decrease in the amplitude of excitatory currents that was accompanied by an increase in paired pulse ratio, consistent with decreased glutamate release. Both effects of loading SAG at -50 mV were blocked by chelation of postsynaptic Ca(2+) using BAPTA or by bath application of tetrahydrolipstatin (THL), a DGL inhibitor. Loading of SAG into glutamatergic pyramidal neurons of the amygdala similarly inhibited excitatory synaptic inputs and increased the PPR. SAG-induced depression was absent in both regions from mice lacking CB1Rs. These data show that increasing substrate availability alone is insufficient to drive 2-AG mobilization and that DGL-dependent synaptic depression via CB1R activation requires postsynaptic Ca(2+) signals.

LRRK2 overexpression alters glutamatergic presynaptic plasticity, striatal dopamine tone, postsynaptic signal transduction, motor activity and memory

Mutations in leucine-rich repeat kinase 2 (Lrrk2) are the most common genetic cause of Parkinson's disease (PD), a neurodegenerative disorder affecting 1-2% of those >65 years old. The neurophysiology of LRRK2 remains largely elusive, although protein loss suggests a role in glutamatergic synapse transmission and overexpression studies show altered dopamine release in aged mice. We show that glutamate transmission is unaltered onto striatal projection neurons (SPNs) of adult LRRK2 knockout mice and that adult animals exhibit no detectable cognitive or motor deficits. Basal synaptic transmission is also unaltered in SPNs of LRRK2 overexpressing mice, but they do exhibit clear alterations to D2-receptor-mediated short-term synaptic plasticity, behavioral hypoactivity and impaired recognition memory. These phenomena are associated with decreased striatal dopamine tone and abnormal dopamine- and cAMP-regulated phosphoprotein 32 kDa signal integration. The data suggest that LRRK2 acts at the nexus of dopamine and glutamate signaling in the adult striatum, where it regulates dopamine levels, presynaptic glutamate release via D2-dependent synaptic plasticity and dopamine-receptor signal transduction.

Humanized Foxp2 accelerates learning by enhancing transitions from declarative to procedural performance

The acquisition of language and speech is uniquely human, but how genetic changes might have adapted the nervous system to this capacity is not well understood. Two human-specific amino acid substitutions in the transcription factor forkhead box P2 (FOXP2) are outstanding mechanistic candidates, as they could have been positively selected during human evolution and as FOXP2 is the sole gene to date firmly linked to speech and language development. When these two substitutions are introduced into the endogenous Foxp2 gene of mice (Foxp2(hum)), cortico-basal ganglia circuits are specifically affected. Here we demonstrate marked effects of this humanization of Foxp2 on learning and striatal neuroplasticity. Foxp2(hum/hum) mice learn stimulus-response associations faster than their WT littermates in situations in which declarative (i.e., place-based) and procedural (i.e., response-based) forms of learning could compete during transitions toward proceduralization of action sequences. Striatal districts known to be differently related to these two modes of learning are affected differently in the Foxp2(hum/hum) mice, as judged by measures of dopamine levels, gene expression patterns, and synaptic plasticity, including an NMDA receptor-dependent form of long-term depression. These findings raise the possibility that the humanized Foxp2 phenotype reflects a different tuning of corticostriatal systems involved in declarative and procedural learning, a capacity potentially contributing to adapting the human brain for speech and language acquisition.

Selective loss of bi-directional synaptic plasticity in the direct and indirect striatal output pathways accompanies generation of parkinsonism and l-DOPA induced dyskinesia in mouse models

Parkinsonian symptoms arise due to over-activity of the indirect striatal output pathway, and under-activity of the direct striatal output pathway. l-DOPA-induced dyskinesia (LID) is caused when the opposite circuitry problems are established, with the indirect pathway becoming underactive, and the direct pathway becoming over-active. Here, we define synaptic plasticity abnormalities in these pathways associated with parkinsonism, symptomatic benefits of l-DOPA, and LID. We applied spike-timing dependent plasticity protocols to cortico-striatal synapses in slices from 6-OHDA-lesioned mouse models of parkinsonism and LID, generated in BAC transgenic mice with eGFP targeting the direct or indirect output pathways, with and without l-DOPA present. In naïve mice, bidirectional synaptic plasticity, i.e. LTP and LTD, was induced, resulting in an EPSP amplitude change of approximately 50% in each direction in both striatal output pathways, as shown previously. In parkinsonism and dyskinesia, both pathways exhibited unidirectional plasticity, irrespective of stimulation paradigm. In parkinsonian animals, the indirect pathway only exhibited LTP (LTP protocol: 143.5±14.6%; LTD protocol 177.7±22.3% of baseline), whereas the direct pathway only showed LTD (LTP protocol: 74.3±4.0% and LTD protocol: 63.3±8.7%). A symptomatic dose of l-DOPA restored bidirectional plasticity on both pathways to levels comparable to naïve animals (Indirect pathway: LTP protocol: 124.4±22.0% and LTD protocol: 52.1±18.5% of baseline. Direct pathway: LTP protocol: 140.7±7.3% and LTD protocol: 58.4±6.0% of baseline). In dyskinesia, in the presence of l-DOPA, the indirect pathway exhibited only LTD (LTP protocol: 68.9±21.3% and LTD protocol 52.0±14.2% of baseline), whereas in the direct pathway, only LTP could be induced (LTP protocol: 156.6±13.2% and LTD protocol 166.7±15.8% of baseline). We conclude that normal motor control requires bidirectional plasticity of both striatal outputs, which underlies the symptomatic benefits of l-DOPA. Switching from bidirectional to unidirectional plasticity drives global changes in striatal pathway excitability, and underpins parkinsonism and dyskinesia.

Cyclic AMP and afferent activity govern bidirectional synaptic plasticity in striatopallidal neurons

Recent experimental evidence suggests that the low dopamine conditions in Parkinson's disease (PD) cause motor impairment through aberrant motor learning. Those data, along with computational models, suggest that this aberrant learning results from maladaptive corticostriatal plasticity and learned motor inhibition. Dopaminergic modulation of both corticostriatal long-term depression (LTD) and long-term potentiation (LTP) is proposed to be critical for these processes; however, the regulatory mechanisms underlying bidirectional corticostriatal plasticity are not fully understood. Previously, we demonstrated a key role for cAMP signaling in corticostriatal LTD. In this study, mouse brain slices were used to perform a parametric experiment that tested the impact of varying both intracellular cAMP levels and the strength of excitatory inputs on corticostriatal plasticity. Using slice electrophysiology in the dorsolateral striatum, we demonstrate that both LTP and LTD can be sequentially induced in the same D2-expressing neuron and that LTP was strongest with high intracellular cAMP and LFS, whereas LTD required low intracellular cAMP and high-frequency stimulation. Our results provide a molecular and cellular basis for regulating bidirectional corticostriatal synaptic plasticity and may help to identify novel therapeutic targets for blocking or reversing the aberrant synaptic plasticity that likely contributes to motor deficits in PD.

Adaptive gene regulation in the Striatum of RGS9-deficient mice

BACKGROUND

RGS9-deficient mice show drug-induced dyskinesia but normal locomotor activity under unchallenged conditions.

RESULTS

Genes related to Ca2+ signaling and their functions were regulated in RGS9-deficient mice.

CONCLUSION

Changes in Ca2+ signaling that compensate for RGS9 loss-of-function can explain the normal locomotor activity in RGS9-deficient mice under unchallenged conditions.

SIGNIFICANCE

Identified signaling components may represent novel targets in antidyskinetic therapy. The long splice variant of the regulator of G-protein signaling 9 (RGS9-2) is enriched in striatal medium spiny neurons and dampens dopamine D2 receptor signaling. Lack of RGS9-2 can promote while its overexpression prevents drug-induced dyskinesia. Other animal models of drug-induced dyskinesia rather pointed towards overactivity of dopamine receptor-mediated signaling. To evaluate changes in signaling pathways mRNA expression levels were determined and compared in wild-type and RGS9-deficient mice. Unexpectedly, expression levels of dopamine receptors were unchanged in RGS9-deficient mice, while several genes related to Ca2+ signaling and long-term depression were differentially expressed when compared to wild type animals. Detailed investigations at the protein level revealed hyperphosphorylation of DARPP32 at Thr34 and of ERK1/2 in striata of RGS9-deficient mice. Whole cell patch clamp recordings showed that spontaneous synaptic events are increased (frequency and size) in RGS9-deficient mice while long-term depression is reduced in acute brain slices. These changes are compatible with a Ca2+-induced potentiation of dopamine receptor signaling which may contribute to the drug-induced dyskinesia in RGS9-deficient mice.