Endocannabinoids in striatal plasticity

Sex specific effects of dorsomedial striatal cannabinoid receptor-1 signaling on Pavlovian outcome devaluation

Cannabinoid-1 receptor (CB1R) signaling in the dorsal striatum regulates the shift from flexible to habitual behavior in instrumental outcome devaluation. Based on prior work establishing individual, sex, and experience-dependent differences in Pavlovian behaviors, we predicted a role for dorsomedial striatum CB1R signaling in driving sign-tracking and rigid responding in Pavlovian outcome devaluation. We trained male and female rats in Pavlovian Lever Autoshaping to determine sign-, or goal- or intermediate tracking groups. After extended Pavlovian training, we gave intra-DMS infusions of the CB1R inverse agonist, rimonabant, before satiety-induced outcome devaluation test sessions, in which we sated rats on training pellets (devalued) or home cage chow (valued) and examined responding to cues in brief nonreinforced Pavlovian Lever Autoshaping sessions. Overall, DMS CB1R signaling inhibition blocked Pavlovian outcome devaluation. After extended training, male rats were sensitive to devaluation while female rats were not. Inhibition of DMS CB1R signaling impaired Pavlovian outcome devaluation in male sign-tracking rats making their behavior more rigid but had no effects in female sign-tracking rats. Intra-DMS rimonabant infusions before reinforced sessions had no effects on Pavlovian sign- or goal-tracking in either sex. The sex-specific and CB1R-dependent effects were specific to outcome devaluation and were consistent between sign- and goal-tracking groups. Our results demonstrate that DMS endocannabinoid receptor signaling regulates behavioral flexibility in a sex-specific manner, suggesting differences in CB1R signaling in DMS between male and female rats.

Deficiency in endocannabinoid synthase DAGLB contributes to early onset Parkinsonism and murine nigral dopaminergic neuron dysfunction

Endocannabinoid (eCB), 2-arachidonoyl-glycerol (2-AG), the most abundant eCB in the brain, regulates diverse neural functions. Here we linked multiple homozygous loss-of-function mutations in 2-AG synthase diacylglycerol lipase β (DAGLB) to an early onset autosomal recessive Parkinsonism. DAGLB is the main 2-AG synthase in human and mouse substantia nigra (SN) dopaminergic neurons (DANs). In mice, the SN 2-AG levels were markedly correlated with motor performance during locomotor skill acquisition. Genetic knockdown of Daglb in nigral DANs substantially reduced SN 2-AG levels and impaired locomotor skill learning, particularly the across-session learning. Conversely, pharmacological inhibition of 2-AG degradation increased nigral 2-AG levels, DAN activity and dopamine release and rescued the locomotor skill learning deficits. Together, we demonstrate that DAGLB-deficiency contributes to the pathogenesis of Parkinsonism, reveal the importance of DAGLB-mediated 2-AG biosynthesis in nigral DANs in regulating neuronal activity and dopamine release, and suggest potential benefits of 2-AG augmentation in alleviating Parkinsonism.

Chronic and Acute Manipulation of Cortical Glutamate Transmission Induces Structural and Synaptic Changes in Co-cultured Striatal Neurons

In contrast to the prenatal topographic development of sensory cortices, striatal circuit organization is slow and requires the functional maturation of cortical and thalamic excitatory inputs throughout the first postnatal month. While mechanisms regulating synapse development and plasticity are quite well described at excitatory synapses of glutamatergic neurons in the neocortex, comparatively little is known of how this translates to glutamate synapses onto GABAergic neurons in the striatum. Here we investigate excitatory striatal synapse plasticity in an in vitro system, where glutamate can be studied in isolation from dopamine and other neuromodulators. We examined pre-and post-synaptic structural and functional plasticity in GABAergic striatal spiny projection neurons (SPNs), co-cultured with glutamatergic cortical neurons. After synapse formation, medium-term (24 h) TTX silencing increased the density of filopodia, and modestly decreased dendritic spine density, when assayed at 21 days in vitro (DIV). Spine reductions appeared to require residual spontaneous activation of ionotropic glutamate receptors. Conversely, chronic (14 days) TTX silencing markedly reduced spine density without any observed increase in filopodia density. Time-dependent, biphasic changes to the presynaptic marker Synapsin-1 were also observed, independent of residual spontaneous activity. Acute silencing (3 h) did not affect presynaptic markers or postsynaptic structures. To induce rapid, activity-dependent plasticity in striatal neurons, a chemical NMDA receptor-dependent “long-term potentiation (LTP)” paradigm was employed. Within 30 min, this increased spine and GluA1 cluster densities, and the percentage of spines containing GluA1 clusters, without altering the presynaptic signal. The results demonstrate that the growth and pruning of dendritic protrusions is an active process, requiring glutamate receptor activity in striatal projection neurons. Furthermore, NMDA receptor activation is sufficient to drive glutamatergic structural plasticity in SPNs, in the absence of dopamine or other neuromodulators.

Discovering the Lost Reward: Critical Locations for Endocannabinoid Modulation of the Cortico-Striatal Loop That Are Implicated in Major Depression

Depression, the most prevalent psychiatric disorder in the Western world, is characterized by increased negative affect (i.e., depressed mood, cost value increase) and reduced positive affect (i.e., anhedonia, reward value decrease), fatigue, loss of appetite, and reduced psychomotor activity except for cases of agitative depression. Some forms, such as post-partum depression, have a high risk for suicidal attempts. Recent studies in humans and in animal models relate major depression occurrence and reoccurrence to alterations in dopaminergic activity, in addition to other neurotransmitter systems. Imaging studies detected decreased activity in the brain reward circuits in major depression. Therefore, the location of dopamine receptors in these circuits is relevant for understanding major depression. Interestingly, in cortico-striatal-dopaminergic pathways within the reward and cost circuits, the expression of dopamine and its contribution to reward are modulated by endocannabinoid receptors. These receptors are enriched in the striosomal compartment of striatum that selectively projects to dopaminergic neurons of substantia nigra compacta and is vulnerable to stress. This review aims to show the crosstalk between endocannabinoid and dopamine receptors and their vulnerability to stress in the reward circuits, especially in corticostriatal regions. The implications for novel treatments of major depression are discussed.

LRRK2 at the pre-synaptic site: A 16-years perspective

Parkinson's disease is a common neurodegenerative disorder and is clinically characterized by bradykinesia, rigidity, and resting tremor. Missense mutations in the leucine-rich repeat protein kinase-2 gene (LRRK2) are a recognized cause of inherited Parkinson's disease. The physiological and pathological impact of LRRK2 is still obscure, but accumulating evidence indicates that LRRK2 orchestrates diverse aspects of membrane trafficking, such as membrane fusion and vesicle formation and transport along actin and tubulin tracks. In the present review, we focus on the special relation between LRRK2 and synaptic vesicles. LRRK2 binds and phosphorylates key actors within the synaptic vesicle cycle. Accordingly, alterations in dopamine and glutamate transmission have been described upon LRRK2 manipulations. However, the different modeling strategies and phenotypes observed require a critical approach to decipher the outcome of LRRK2 at the pre-synaptic site.

Cannabinoids, Endocannabinoids and Sleep

Sleep is a vital function of the nervous system that contributes to brain and bodily homeostasis, energy levels, cognitive ability, and other key functions of a variety of organisms. Dysfunctional sleep induces neural problems and is a key part of almost all human psychiatric disorders including substance abuse disorders. The hypnotic effects of cannabis have long been known and there is increasing use of phytocannabinoids and other formulations as sleep aids. Thus, it is crucial to gain a better understanding of the neurobiological basis of cannabis drug effects on sleep, as well as the role of the endogenous cannabinoid system in sleep physiology. In this review article, we summarize the current state of knowledge concerning sleep-related endogenous cannabinoid function derived from research on humans and rodent models. We also review information on acute and chronic cannabinoid drug effects on sleep in these organisms, and molecular mechanisms that may contribute to these effects. We point out the potential benefits of acute cannabinoids for sleep improvement, but also the potential sleep-disruptive effects of withdrawal following chronic cannabinoid drug use. Prescriptions for future research in this burgeoning field are also provided.

Unbalanced Inhibitory/Excitatory Responses in the Substantia Nigra Pars Reticulata Underlie Cannabinoid-Related Slowness of Movements

The substantia nigra pars reticulata (SNr), where the basal ganglia (BG) direct and indirect pathways converge, contains among the highest expression of cannabinoid receptor type 1 (CB1r) in the brain. Hence, SNr is an ideal locus to study pathway interactions and cannabinergic modulations. The objective of this study was to characterize the effects of systemic injections of the CB1r agonist (CP55940) on the balanced activity of the direct/indirect pathways in the SNr and its associated behaviors. To this aim, we recorded somatosensory and pathway-specific representations in the spiking activity of the SNr of male rats under CP55940. CB1r activation mainly decreased the inhibitory, potentially direct pathway component while sparing the excitatory, potentially indirect pathway component of somatosensory responses. As a result, cutaneous stimulation produced unbalanced responses favoring increased SNr firing rates, suggesting a potential locus for cannabinergic motor-related effects. To test this hypothesis, we implemented an ad hoc behavioral protocol for rats in which systemic administration of CP55940 produced kinematic impairments that were completely reverted by nigral injections of the CB1r antagonist (AM251). Our data suggest that cannabinoid-related motor effects are associated with unbalanced direct/indirect pathway activations that may be reverted by CB1r manipulation at the SNr.SIGNIFICANCE STATEMENT The cannabinergic system has been the target of multiple studies to master its potential use as a therapeutic agent. However, significant advances have been precluded by the lack of mechanistic explanations for the variety of its desirable/undesirable effects. Here, we have combined electrophysiological recordings, pharmacological and optogenetic manipulations, and an ad hoc behavioral protocol to understand how basal ganglia (BG) is affected by cannabinoids. We found that cannabinoids principally affect inhibitory inputs, potentially from the direct pathway, resulting in unbalanced responses in the substantia nigra pars reticulata (SNr) and suggesting a mechanism for the cannabinoid-related slowness of movements. This possibility was confirmed by behavioral experiments in which cannabinoid-related slowness of purposeful movements was reverted by cannabinoid receptor type 1 (CB1r) manipulations directly into the SNr.

Dopamine D1 receptor signaling and endocannabinoid cooperate to fuel striatal plasticity: An Editorial Highlight for "Cyclic AMP-dependent protein kinase and D1 dopamine receptors regulate diacylglycerol lipase-α and synaptic 2-arachidonoyl glycerol signaling" on page 334

Endocannabinoids (eCBs) play key roles in short-term and long-term synaptic plasticity in the corticostriatal circuit. By activating cannabinoid receptors expressed in the central nervous system, eCBs regulate several neural functions and behaviors. The major eCB 2-arachidonoyl-glycerol (2-AG) is particularly important for triggering a short-term form of synaptic plasticity (depolarization-induced suppression of excitatory transmission or DSE) on cortical glutamatergic afferents inputting the striatum. The neurotransmitter dopamine, through the action of D1 and D2 receptors, is also critically involved in corticostriatal plasticity. This Editorial highlights the study by Shonesy et al., which presents evidence that activation of dopamine D1 receptor and its classical downstream target cAMP-dependent protein kinase (PKA) are involved in increasing the synthesis of 2-AG in striatal medium spiny neurons (MSN) to drive DSE in the corticostriatal circuit, as schematically outlined in Figure 1. The authors used a set of complementary approaches and identified a putative serine (Ser) residue phosphorylated by PKA in diacylglycerol lipase (DGL) alpha that is required for generating 2-AG, providing a mechanistic clue into how the canonical D1 pathway in MSN might fine-tune short-term plasticity in the corticostriatal circuit. Besides, the work by Shonesy et al. may pave the way for further studies exploring the signaling interplay between canonical dopamine D1 receptor pathway and eCBs to control other forms of synaptic plasticity in different brain circuits with possible pathological relevance.

Δ-Tetrahydrocannabinol Increases Dopamine D1-D2 Receptor Heteromer and Elicits Phenotypic Reprogramming in Adult Primate Striatal Neurons

Long-term cannabis users manifest deficits in dopaminergic functions, reflecting Δ9-tetrahydrocannabinol (THC)-induced neuroadaptive dysfunctional dopamine signaling, similar to those observed upon dopamine D1-D2 heteromer activation. The molecular mechanisms remain largely unknown. We show evolutionary and regional differences in D1-D2 heteromer abundance in mammalian striatum. Importantly, chronic THC increased the number of D1-D2 heteromer-expressing neurons, and the number of heteromers within individual neurons in adult monkey striatum. The majority of these neurons displayed a phenotype co-expressing the characteristic markers of both striatonigral and striatopallidal neurons. Furthermore, THC increased D1-D2-linked calcium signaling markers (pCaMKIIα, pThr75-DARPP-32, BDNF/pTrkB) and inhibited cyclic AMP signaling (pThr34-DARPP-32, pERK1/2, pS845-GluA1, pGSK3). Cannabidiol attenuated most but not all of these THC-induced neuroadaptations. Targeted pathway analyses linked these changes to neurological and psychological disorders. These data underline the importance of the D1-D2 receptor heteromer in cannabis use-related disorders, with THC-induced changes likely responsible for the reported adverse effects observed in heavy long-term users.

Endocannabinoid Signaling from 2-Arachidonoylglycerol to CB1 Cannabinoid Receptor Facilitates Reward-based Learning of Motor Sequence

The endocannabinoid system modulates synaptic transmission, controls neuronal excitability, and is involved in various brain functions including learning and memory. 2-arachidonoylglycerol, a major endocannabinoid produced by diacylglycerol lipase-α (DGLα), is released from postsynaptic neurons, retrogradely activates presynaptic CB₁ cannabinoid receptors, and induces short-term or long-term synaptic plasticity. To examine whether and how the endocannabinoid system contributes to reward-based learning of a motor sequence, we subjected male CB₁-knockout (KO) and DGLα-KO mice to three types of operant lever-press tasks. First, we trained mice to press one of three levers labeled A, B, and C for a food reward (one-lever task). Second, we trained mice to press the three levers in the order of A, B, and C (three-lever task). Third, the order of the levers was reversed to C, B, and A (reverse three-lever task). We found that CB₁-KO mice and DGLα-KO mice exhibited essentially the same deficits in the operant lever-press tasks. In the one-lever task, both strains of knockout mice showed a slower rate of learning to press a lever for food. In the three-lever task, both strains of knockout mice showed a slower rate of learning of the motor sequence. In the reverse three-lever task, both strains of knockout mice needed more lever presses for reversal learning. These results suggest that the endocannabinoid system facilitates reward-based learning of a motor sequence by conferring the flexibility with which animals can switch between strategies.

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.

CB1-Dependent Long-Term Depression in Ventral Tegmental Area GABA Neurons: A Novel Target for Marijuana

The VTA is necessary for reward behavior with dopamine cells critically involved in reward signaling. Dopamine cells in turn are innervated and regulated by neighboring inhibitory GABA cells. Using whole-cell electrophysiology in juvenile-adolescent GAD67-GFP male mice, we examined excitatory plasticity in fluorescent VTA GABA cells. A novel CB1-dependent LTD was induced in GABA cells that was dependent on metabotropic glutamate receptor 5, and cannabinoid receptor 1 (CB1). LTD was absent in CB1 knock-out mice but preserved in heterozygous littermates. Bath applied Δ⁹-tetrahydrocannabinol depressed GABA cell activity, therefore downstream dopamine cells will be disinhibited; and thus, this could potentially result in increased reward. Chronic injections of Δ⁹-tetrahydrocannabinol occluded LTD compared with vehicle injections; however, a single exposure was insufficient to do so. As synaptic modifications by drugs of abuse are often tied to addiction, these data suggest a possible mechanism for the addictive effects of Δ⁹-tetrahydrocannabinol in juvenile-adolescents, by potentially altering reward behavioral outcomes.SIGNIFICANCE STATEMENT The present study identifies a novel form of glutamatergic synaptic plasticity in VTA GABA neurons, a currently understudied cell type that is critical for the brain's reward circuit, and how Δ⁹-tetrahydrocannabinol occludes this plasticity. This study specifically addresses a potential unifying mechanism whereby marijuana could exert rewarding and addictive/withdrawal effects. Marijuana use and legalization are a pressing issue for many states in the United States. Although marijuana is the most commonly abused illicit drug, the implications of legalized, widespread, or continued usage are speculative. This study in juvenile-adolescent aged mice identifies a novel form of synaptic plasticity in VTA GABA cells, and the synaptic remodeling that can occur after Δ⁹-tetrahydrocannabinol use.

Synaptic functions of endocannabinoid signaling in health and disease

Endocannabinoids (eCBs) are a family of lipid molecules that act as key regulators of synaptic transmission and plasticity. They are synthetized "on demand" following physiological and/or pathological stimuli. Once released from postsynaptic neurons, eCBs typically act as retrograde messengers to activate presynaptic type 1 cannabinoid receptors (CB₁) and induce short- or long-term depression of neurotransmitter release. Besides this canonical mechanism of action, recent findings have revealed a number of less conventional mechanisms by which eCBs regulate neural activity and synaptic function, suggesting that eCB-mediated plasticity is mechanistically more diverse than anticipated. These mechanisms include non-retrograde signaling, signaling via astrocytes, participation in long-term potentiation, and the involvement of mitochondrial CB₁. Focusing on paradigmatic brain areas, such as hippocampus, striatum, and neocortex, we review typical and novel signaling mechanisms, and discuss the functional implications in normal brain function and brain diseases. In summary, eCB signaling may lead to different forms of synaptic plasticity through activation of a plethora of mechanisms, which provide further complexity to the functional consequences of eCB signaling. This article is part of the Special Issue entitled "A New Dawn in Cannabinoid Neurobiology".

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.

Using In Vitro Electrophysiology to Screen Medications: Accumbal Plasticity as an Engram of Alcohol Dependence

The nucleus accumbens (NAc) is a central component of the mesocorticolimbic reward system. Increasing evidence strongly implicates long-term synaptic neuroadaptations in glutamatergic excitatory activity of the NAc shell and/or core medium spiny neurons in response to chronic drug and alcohol exposure. Such neuroadaptations likely play a critical role in the development and expression of drug-seeking behaviors. We have observed unique cell-type-specific bidirectional changes in NAc synaptic plasticity (metaplasticity) following acute and chronic intermittent ethanol exposure. Other investigators have also previously observed similar metaplasticity in the NAc following exposure to psychostimulants, opiates, and amazingly, even following an anhedonia-inducing experience. Considering that the proteome of the postsynaptic density likely contains hundreds of biochemicals, proteins and other components and regulators, we believe that there is a large number of potential molecular sites through which accumbal metaplasticity may be involved in chronic alcohol abuse. Many of our companion laboratories are now engaged in identifying and screening medications targeting candidate genes and its products previously linked to maladaptive alcohol phenotypes. We hypothesize that if manipulation of such target genes and their products change NAc plasticity, then that observation constitutes an important validation step for the development of novel therapeutics to treat alcohol dependence.

Cannabinoid-dopamine interactions in the physiology and physiopathology of the basal ganglia

Endocannabinoids and their receptors play a modulatory role in the control of dopamine transmission in the basal ganglia. However, this influence is generally indirect and exerted through the modulation of GABA and glutamate inputs received by nigrostriatal dopaminergic neurons, which lack cannabinoid CB1 receptors although they may produce endocannabinoids. Additional evidence suggests that CB2 receptors may be located in nigrostriatal dopaminergic neurons, and that certain eicosanoid-related cannabinoids may directly activate TRPV1 receptors, which have been found in nigrostriatal dopaminergic neurons, thus allowing in both cases a direct regulation of dopamine transmission by specific cannabinoids. In addition, CB1 receptors form heteromers with dopaminergic receptors which provide another pathway to direct interactions between both systems, in this case at the postsynaptic level. Through these direct mechanisms or through indirect mechanisms involving GABA or glutamate neurons, cannabinoids may interact with dopaminergic transmission in the basal ganglia and this is likely to have important effects on dopamine-related functions in these structures (i.e. control of movement) and, particularly, on different pathologies affecting these processes, in particular, Parkinson's disease, but also dyskinesia, dystonia and other pathological conditions. The present review will address the current literature supporting these cannabinoid-dopamine interactions at the basal ganglia, with emphasis on aspects dealing with the physiopathological consequences of these interactions.

LINKED ARTICLES

This article is part of a themed section on Updating Neuropathology and Neuropharmacology of Monoaminergic Systems. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.13/issuetoc.

Deep brain stimulation of the subthalamic nucleus preferentially alters the translational profile of striatopallidal neurons in an animal model of Parkinson's disease

Deep brain stimulation targeting the subthalamic nucleus (STN-DBS) is an effective surgical treatment for the motor symptoms of Parkinson's disease (PD), the precise neuronal mechanisms of which both at molecular and network levels remain a topic of debate. Here we employ two transgenic mouse lines, combining translating ribosomal affinity purification (TRAP) with bacterial artificial chromosome expression (Bac), to selectively identify changes in translational gene expression in either Drd1a-expressing striatonigral or Drd2-expressing striatopallidal medium spiny neurons (MSNs) of the striatum following STN-DBS. 6-hydroxydopamine lesioned mice received either 5 days stimulation via a DBS electrode implanted in the ipsilateral STN or 5 days sham treatment (no stimulation). Striatal polyribosomal RNA was selectively purified from either Drd2 or Drd1a MSNs using the TRAP method and gene expression profiling performed. We identified eight significantly altered genes in Drd2 MSNs (Vps33b, Ppp1r3c, Mapk4, Sorcs2, Neto1, Abca1, Penk1, and Gapdh) and two overlapping genes in Drd1a MSNs (Penk1 and Ppp1r3c) implicated in the molecular mechanisms of STN-DBS. A detailed functional analysis, using a further 728 probes implicated in STN-DBS, suggested an increased ability to receive excitation (mediated by increased dendritic spines, increased calcium influx and enhanced excitatory post synaptic potentials) accompanied by processes that would hamper the initiation of action potentials, transport of neurotransmitters from soma to axon terminals and vesicular release in Drd2-expressing MSNs. Finally, changes in expression of several genes involved in apoptosis as well as cholesterol and fatty acid metabolism were also identified. This increased understanding of the molecular mechanisms induced by STN-DBS may reveal novel targets for future non-surgical therapies for PD.

Weeding out bad waves: towards selective cannabinoid circuit control in epilepsy

Endocannabinoids are lipid-derived messengers, and both their synthesis and breakdown are under tight spatiotemporal regulation. As retrograde signalling molecules, endocannabinoids are synthesized postsynaptically but activate presynaptic cannabinoid receptor 1 (CB1) receptors to inhibit neurotransmitter release. In turn, CB1-expressing inhibitory and excitatory synapses act as strategically placed control points for activity-dependent regulation of dynamically changing normal and pathological oscillatory network activity. Here, we highlight emerging principles of cannabinoid circuit control and plasticity, and discuss their relevance for epilepsy and related comorbidities. New insights into cannabinoid signalling may facilitate the translation of the recent interest in cannabis-related substances as antiseizure medications to evidence-based treatment strategies.

Endocannabinoids and striatal function: implications for addiction-related behaviours

Since the identification and cloning of the major cannabinoid receptor expressed in the brain almost 25 years ago research has highlighted the potential of drugs that target the endocannabinoid system for treating addiction. The endocannabinoids, anandamide and 2-arachidonoyl glycerol, are lipid-derived metabolites found in abundance in the basal ganglia and other brain areas innervated by the mesocorticolimbic dopamine systems. Cannabinoid CB1 receptor antagonists/inverse agonists reduce reinstatement of responding for cocaine, alcohol and opiates in rodents. However, compounds acting on the endocannabinoid system may have broader application in treating drug addiction by ameliorating associated traits and symptoms such as impulsivity and anxiety that perpetuate drug use and interfere with rehabilitation. As a trait, impulsivity is known to predispose to addiction and facilitate the emergence of addiction to stimulant drugs. In contrast, anxiety and elevated stress responses accompany extended drug use and may underlie the persistence of drug intake in dependent individuals. In this article we integrate and discuss recent findings in rodents showing selective pharmacological modulation of impulsivity and anxiety by cannabinoid agents. We highlight the potential of selective inhibitors of endocannabinoid metabolism, directed at fatty acid amide hydrolase and monoacylglycerol lipase, to reduce anxiety and stress responses, and discuss novel mechanisms underlying the modulation of the endocannabinoid system, including the attenuation of impulsivity, anxiety, and drug reward by selective CB2 receptor agonists.

Presynaptic long-term depression mediated by Gi/o-coupled receptors

Long-term depression (LTD) of the efficacy of synaptic transmission is now recognized as an important mechanism for the regulation of information storage and the control of actions, as well as for synapse, neuron, and circuit development. Studies of LTD mechanisms have focused mainly on postsynaptic AMPA-type glutamate receptor trafficking. However, the focus has now expanded to include presynaptically expressed plasticity, the predominant form being initiated by presynaptically expressed Gi/o-coupled metabotropic receptor (Gi/o-GPCR) activation. Several forms of LTD involving activation of different presynaptic Gi/o-GPCRs as a 'common pathway' are described. We review here the literature on presynaptic Gi/o-GPCR-mediated LTD, discuss known mechanisms, gaps in our knowledge, and evaluate whether all Gi/o-GPCRs are capable of inducing presynaptic LTD.

Modeling intracellular signaling underlying striatal function in health and disease

Striatum, which is the input nucleus of the basal ganglia, integrates cortical and thalamic glutamatergic inputs with dopaminergic afferents from the substantia nigra pars compacta. The combination of dopamine and glutamate strongly modulates molecular and cellular properties of striatal neurons and the strength of corticostriatal synapses. These actions are performed via intracellular signaling networks, containing several intertwined feedback loops. Understanding the role of dopamine and other neuromodulators requires the development of quantitative dynamical models for describing the intracellular signaling, in order to provide precise unambiguous descriptions and quantitative predictions. Building such models requires integration of data from multiple data sources containing information regarding the molecular interactions, the strength of these interactions, and the subcellular localization of the molecules. Due to the uncertainty, variability, and sparseness of these data, parameter estimation techniques are critical for inferring or constraining the unknown parameters, and sensitivity analysis evaluates which parameters are most critical for a given observed macroscopic behavior. Here, we briefly review the modeling approaches and tools that have been used to investigate biochemical signaling in the striatum, along with some of the models built around striatum. We also suggest a future direction for the development of such models from the, now becoming abundant, high-throughput data.

Closing the gate in the limbic striatum: prefrontal suppression of hippocampal and thalamic inputs

Many brain circuits control behavior by integrating information arising from separate inputs onto a common target neuron. Neurons in the ventral striatum (VS) receive converging excitatory afferents from the prefrontal cortex (PFC), hippocampus (HP), and thalamus, among other structures, and the integration of these inputs is critical for goal-directed behaviors. Although HP inputs have been described as gating PFC throughput in the VS, recent data reveal that the VS desynchronizes from the HP during epochs of burst-like PFC activity related to decision making. It is therefore possible that PFC inputs locally attenuate responses to other glutamatergic inputs to the VS. Here, we found that delivering trains of stimuli to the PFC suppresses HP- and thalamus-evoked synaptic responses in the VS, in part through activation of inhibitory processes. This interaction may enable the PFC to exert influence on basal ganglia loops during decision-making instances with minimal disturbance from ongoing contextual inputs.