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

The endocannabinoid 2-arachidonoylglycerol mediates D1 and D2 receptor cooperative enhancement of rat nucleus accumbens core neuron firing

Many motivated and addiction-related behaviors are sustained by activity of both dopamine D1- and D2-type receptors (D1Rs and D2Rs) as well as CB1 receptors (CB1Rs) in the nucleus accumbens (NAc). Here, we use in vitro whole-cell patch-clamp electrophysiology to describe an endocannabinoid (eCB)-dopamine receptor interaction in adult rat NAc core neurons. D1R and D2R agonists in combination enhanced firing, with no effect of a D1R or D2R agonist alone. This D1R+D2R-mediated firing increase required CB1Rs, since it was prevented by the CB1R antagonists AM251 and Rimonabant. The D1R+D2R firing increase also required phospholipase C (PLC), the major synthesis pathway for the eCB 2-arachidonoylglycerol (2-AG) and one of several pathways for anandamide. Further, inhibition of 2-AG hydrolysis with the monoglyceride lipase (MGL) inhibitor JZL184 allowed subthreshold levels of D1R+D2R receptor agonists to enhance firing, while inhibition of anandamide hydrolysis with the fatty acid amide hydrolase (FAAH) inhibitors URB597 or AM3506 did not. Filling the postsynaptic neuron with 2-AG enabled subthreshold D1R+D2R agonists to increase firing, and the 2AG+D1R+D2R increase in firing was prevented by a CB1R antagonist. Also, the metabotropic glutamate receptor 5 (mGluR5) blocker MPEP prevented the ability of JZL184 to promote subthreshold D1R+D2R enhancement of firing, while the 2-AG+D1R+D2R increase in firing was not prevented by the mGluR5 blocker, suggesting that mGluR5s acted upstream of 2-AG production. Thus, our results taken together are consistent with the hypothesis that NAc core eCBs mediate dopamine receptor (DAR) enhancement of firing, perhaps providing a cellular mechanism underlying the central role of NAc core D1Rs, D2Rs, CB1Rs, and mGluR5s during many drug-seeking behaviors.

Serotonin induces long-term depression at corticostriatal synapses

The striatum has important roles in motor control and action learning and, like many brain regions, receives multiple monoaminergic inputs. We have examined serotonergic modulation of rat and mouse corticostriatal neurotransmission and find that serotonin (5-HT) activates 5-HT(1b) receptors resulting in a long-term depression (LTD) of glutamate release and striatal output that we have termed 5-HT-LTD. 5-HT-LTD is presynaptically mediated, cAMP pathway dependent, and inducible by endogenous striatal 5-HT, as revealed by application of a selective 5-HT reuptake inhibitor. 5-HT-LTD is mutually occlusive with dopamine/endocannabinoid-dependent LTD, suggesting that these two forms of LTD act on the same corticostriatal terminals. Thus, serotonergic and dopaminergic mechanisms exist that may interact to persistently sculpt corticostriatal circuits, potentially influencing action learning and striatal-based disorders.

Striatal gating through up states and oscillations in the basal ganglia: Implications for Parkinson's disease

Up states are a hallmark of striatal physiology. Spontaneous activity in the thalamo-cortical network drives robust plateau depolarizations in the medium spiny projection neurons of the striatum. Medium spiny neuron firing is only possible during up states and is very tightly regulated by dopamine and NMDA receptors. In a rat model of Parkinson's disease the medium spiny neurons projecting to the globus pallidus (indirect pathway) show more depolarized up states and increased firing. This is translated into abnormal patterns of synchronization between the globus pallidus and frontal cortex, which are believed to underlie the symptoms of Parkinson's disease. Here we review our work in the field and propose a mechanism through which the lack of D2 receptor stimulation in the striatum allows the establishment of fixed routes of information flow in the cortico-striato-pallidal network.

The Basal Ganglia as a Substrate for the Multiple Actions of Amphetamines

Amphetamines are psychostimulant drugs with high abuse potential. Acute and chronic doses of amphetamines affect dopamine (DA) neurotransmission in the basal ganglia. The basal ganglia are a group of subcortical nuclei that are anatomically positioned to integrate cognitive, motor and sensorimotor inputs from the cortex. Amphetamines can differentially alter the functioning of specific BG circuits to produce neurochemical changes that affect cognition, movement, and drug seeking behavior through their effects on DA neurotransmission. This review focuses on how alterations in dopaminergic neurotransmission within distinct basal ganglia pathways can modify their functional output to predict and explain the acute and long term behavioral consequences of amphetamine exposure.

Selective inactivation of adenosine A(2A) receptors in striatal neurons enhances working memory and reversal learning

The adenosine A(2A) receptor (A(2A)R) is highly enriched in the striatum where it is uniquely positioned to integrate dopaminergic, glutamatergic, and other signals to modulate cognition. Although previous studies support the hypothesis that A(2A)R inactivation can be pro-cognitive, analyses of A(2A)R's effects on cognitive functions have been restricted to a small subset of cognitive domains. Furthermore, the relative contribution of A(2A)Rs in distinct brain regions remains largely unknown. Here, we studied the regulation of multiple memory processes by brain region-specific populations of A(2A)Rs. Specifically, we evaluated the cognitive impacts of conditional A(2A)R deletion restricted to either the entire forebrain (i.e., cerebral cortex, hippocampus, and striatum, fb-A(2A)R KO) or to striatum alone (st-A(2A)R KO) in recognition memory, working memory, reference memory, and reversal learning. This comprehensive, comparative analysis showed for the first time that depletion of A(2A)R-dependent signaling in either the entire forebrain or striatum alone is associated with two specific phenotypes indicative of cognitive flexibility-enhanced working memory and enhanced reversal learning. These selective pro-cognitive phenotypes seemed largely attributed to inactivation of striatal A(2A)Rs as they were captured by A(2A)R deletion restricted to striatal neurons. Neither spatial reference memory acquisition nor spatial recognition memory were grossly affected, and no evidence for compensatory changes in striatal or cortical D(1), D(2), or A(1) receptor expression was found. This study provides the first direct demonstration that targeting striatal A(2A)Rs may be an effective, novel strategy to facilitate cognitive flexibility under normal and pathologic conditions.

Endocannabinoid-mediated synaptic plasticity and addiction-related behavior

Endogenous cannabinoids (eCBs) are retrograde messengers that provide feedback inhibition of both excitatory and inhibitory transmission in brain through the activation of presynaptic CB₁ receptors. Substantial evidence indicates that eCBs mediate various forms of short- and long-term plasticity in brain regions involved in the etiology of addiction. The present review provides an overview of the mechanisms through which eCBs mediate various forms of synaptic plasticity and discusses evidence that eCB-mediated plasticity is disrupted following exposure to a variety of abused substances that differ substantially in pharmacodynamic mechanism including alcohol, psychostimulants and cannabinoids. The possible involvement of dysregulated eCB signaling in maladaptive behaviors that evolve over long-term drug exposure is also discussed, with a particular focus on altered behavioral responses to drug exposure, deficient extinction of drug-related memories, increased drug craving and relapse, heightened stress sensitivity and persistent affective disruption (anxiety and depression).

Endocannabinoids and Retrograde Modulation of Synaptic Transmission

Since the first reports of endocannabinoid-mediated retrograde signaling in 2001, great advances have been made toward understanding the molecular basis and functions of the endocannabinoid system. Electrophysiological studies have revealed that the endocannabinoid system is functional at various types of synapses throughout the brain. Basic mechanisms have been clarified as to how endocannabinoids are produced and released from postsynaptic neurons and regulate neurotransmitter release through activating presynaptic cannabinoid CB1 receptors, although there remain unsolved questions and some discrepancies. In addition to this major function, recent studies suggest diverse functions of endocannabinoids, including control of other endocannabinoid-independent forms of synaptic plasticity, regulation of neuronal excitability, stimulation of glia-neuron interaction, and induction of CB1R-independent plasticity. Using recently developed pharmacological and genetic tools, behavioral studies have elucidated the roles of the endocannabinoid system in various aspects of neural functions. In this review, we make a brief overview of molecular mechanisms underlying the endocannabinoid-mediated synaptic modulation and also summarize recent findings, which shed new light on a diversity of functional roles of endocannabinoids.

The distinct role of medium spiny neurons and cholinergic interneurons in the D₂/A₂A receptor interaction in the striatum: implications for Parkinson's disease

A(2A) adenosine receptor antagonists are currently under investigation as potential therapeutic agents for Parkinson's disease (PD). However, the molecular mechanisms underlying this therapeutic effect is still unclear. A functional antagonism exists between A(2A) adenosine and D(2) dopamine (DA) receptors that are coexpressed in striatal medium spiny neurons (MSNs) of the indirect pathway. Since this interaction could also occur in other neuronal subtypes, we have analyzed the pharmacological modulation of this relationship in murine MSNs of the direct and indirect pathways as well in striatal cholinergic interneurons. Under physiological conditions, endogenous cannabinoids (eCBs) play a major role in the inhibitory effect on striatal glutamatergic transmission exerted by the concomitant activation of D(2) DA receptors and blockade of A(2A) receptors in both D(2)- and D(1)-expressing striatal MSNs. In experimental models of PD, the inhibition of striatal glutamatergic activity exerted by D(2) receptor activation did not require the concomitant inhibition of A(2A) receptors, while it was still dependent on the activation of CB(1) receptors in both D(2)- and D(1)-expressing MSNs. Interestingly, the antagonism of M1 muscarinic receptors blocked the effects of D(2)/A(2A) receptor modulation on MSNs. Moreover, in cholinergic interneurons we found coexpression of D(2) and A(2A) receptors and a reduction of the firing frequency exerted by the same pharmacological agents that reduced excitatory transmission in MSNs. This evidence supports the hypothesis that striatal cholinergic interneurons, projecting to virtually all MSN subtypes, are involved in the D(2)/A(2A) and endocannabinoid-mediated effects observed on both subpopulations of MSNs in physiological conditions and in experimental PD.

Cocaine-induced plasticity in the nucleus accumbens is cell specific and develops without prolonged withdrawal

Cocaine induces plasticity at glutamatergic synapses in the nucleus accumbens (NAc). Withdrawal was suggested to play an important role in the development of this plasticity by studies showing that some changes only appear several weeks after the final cocaine exposure. In this study, the requirement for prolonged withdrawal was evaluated by comparing the changes in glutamatergic transmission induced by two different noncontingent cocaine treatments: a short treatment followed by prolonged withdrawal, and a longer treatment without prolonged withdrawal. Recordings were performed from mouse medium spiny neurons (MSNs) in the NAc at the same time after the first cocaine injection under both treatments. A similar increase in the frequency of glutamate-mediated miniature EPSCs was observed in D(1)-expressing MSNs after both cocaine treatments, demonstrating that prolonged withdrawal was not required. Furthermore, larger AMPA receptor-to-NMDA receptor ratios, higher spine density, and enlarged spine heads were observed in the absence of withdrawal after a long cocaine treatment. These synaptic adaptations expressed in D(1)-containing MSNs of the NAc core were not further enhanced by protracted withdrawal. In conclusion, a few repeated cocaine injections are enough to trigger adaptations at glutamatergic synapses in D(1)-expressing MSNs, which, although they take time to develop, do not require prolonged cocaine withdrawal.

Dopaminergic modulation of corticostriatal responses in medium spiny projection neurons from direct and indirect pathways

Suprathreshold corticostriatal responses recorded from medium spiny neurons (MSNs) from the direct and indirect pathways of the basal ganglia are different. Their differences readily distinguish D₁- and D₂-type receptor expressing MSNs in both bacterial artificial chromosome-transgenic mice and their control littermates as well as in rats: indirect pathway neurons are more excitable than direct pathway neurons revealing autoregenerative spikes underlying their spike trains, whereas direct pathway neurons exhibit more prolonged plateau potentials and spike trains. SFK 81297, a selective agonist for D₁-class receptors enhanced corticostriatal responses in direct pathway neurons, while quinelorane, a selective agonist for D₂-class receptors reduced orthodromic and autoregenerative responses in indirect pathway neurons thus making both neuron classes similarly excitable. Because dopaminergic postsynaptic actions target Ca_V1 (L) class voltage-gated calcium channels in MSNs, we hypothesized that these channels are involved and can explain a part of the dopaminergic actions on corticostriatal integration. Both 2.5 μM nicardipine and 400 nM calciseptine, selective Ca_V1 channel blockers, reduced corticostriatal responses in both D₁- and D₂-receptor expressing neurons, respectively. A previous blockade of Ca_V1 channels occluded the actions of dopamine agonists in both neuronal classes. In contrast, a Ca_V1 (L) channel activator, 2.5 μM Bay K 8644, enhanced corticostriatal responses in neurons from both pathways. It is concluded that Ca_V1 intrinsic currents mediate a part of the dopaminergic modulation during orthodromic synaptic integration of cortical inputs in both classes of MSNs.

Involvement of the AATn polymorphism of the CNR1 gene in the efficiency of procedural learning in humans

Procedural learning refers to the acquisition of motor skills and the practice that refines their performance. The striatum participates in this learning through a function regulated by endocannabinoid signaling and other systems. This study relates the efficiency in learning a procedural task with the AATn polymorphism of the CNR1 gene, which encodes for the CB1 receptor. The mirror-drawing star task was solved by 99 healthy young subjects in three trials. The sample was divided into high- and low-performance groups based on performance efficiency. AAT12/14 carriers were more frequent in the former group, while there were more AAT12/13 carriers in the latter, which also made more errors/min. Therefore, we characterized two efficiency phenotypes: high- vs. low-performers associated with the two AATn genotypes, AAT12/14 vs. AAT12/13. The findings suggest that AATn polymorphism modifies CNR1 translation, indicating a different modulation of CB1.

Neuromodulatory control of striatal plasticity and behavior

Excitatory synapses onto projection neurons in the striatum, the input nucleus of the basal ganglia, play a key role in regulating basal ganglia circuit function and are a major site of long-term synaptic plasticity. Here, we review the mechanisms and regulation of both long-term potentiation and long-term depression at these synapses. In particular, we highlight the role that neuromodulators play in determining the strength and direction of plasticity, which ultimately shapes the balance of activity in basal ganglia circuits and regulates motor behavior.

mGluR5 in cortical excitatory neurons exerts both cell-autonomous and -nonautonomous influences on cortical somatosensory circuit formation

Glutamatergic neurotransmission plays important roles in sensory map formation. The absence of the group I metabotropic glutamate receptor 5 (mGluR5) leads to abnormal sensory map formation throughout the mouse somatosensory pathway. To examine the role of cortical mGluR5 expression on barrel map formation, we generated cortex-specific mGluR5 knock-out (KO) mice. Eliminating mGluR5 function solely in cortical excitatory neurons affects, not only the whisker-related organization of cortical neurons (barrels), but also the patterning of their presynaptic partners, the thalamocortical axons (TCAs). In contrast, subcortical whisker maps develop normally in cortical-mGluR5 KO mice. In the S1 cortex of cortical-mGluR5 KO, layer IV neurons are homogenously distributed and have no clear relationship to the location of TCA clusters. The altered dendritic morphology of cortical layer IV spiny stellate neurons in cortical-mGluR5 KO mice argues for a cell-autonomous role of mGluR5 in dendritic patterning. Furthermore, morphometric analysis of single TCAs in both cortical- and global-mGluR5 KO mice demonstrated that in these mice, the complexity of axonal arbors is reduced, while the area covered by TCA arbors is enlarged. Using voltage-clamp whole-cell recordings in acute thalamocortical brain slices, we found that KO of mGluR5 from cortical excitatory neurons reduced inhibitory but not excitatory inputs onto layer IV neurons. This suggests that mGluR5 signaling in cortical excitatory neurons nonautonomously modulates the functional development of GABAergic circuits. Together, our data provide strong evidence that mGluR5 signaling in cortical principal neurons exerts both cell-autonomous and -nonautonomous influences to modulate the formation of cortical sensory circuits.

Centrality of striatal cholinergic transmission in Basal Ganglia function

Work over the past two decades revealed a previously unexpected role for striatal cholinergic interneurons in the context of basal ganglia function. The recognition that these interneurons are essential in synaptic plasticity and motor learning represents a significant step ahead in deciphering how the striatum processes cortical inputs, and why pathological circumstances cause motor dysfunction. Loss of the reciprocal modulation between dopaminergic inputs and the intrinsic cholinergic innervation within the striatum appears to be the trigger for pathophysiological changes occurring in basal ganglia disorders. Accordingly, there is now compelling evidence showing profound changes in cholinergic markers in these disorders, in particular Parkinson's disease and dystonia. Based on converging experimental and clinical evidence, we provide an overview of the role of striatal cholinergic transmission in physiological and pathological conditions, in the context of the pathogenesis of movement disorders.

Drug-induced psychosis: how to avoid star gazing in schizophrenia research by looking at more obvious sources of light

The prevalent view today is that schizophrenia is a syndrome rather than a specific disease. Liability to schizophrenia is highly heritable. It appears that multiple genetic and environmental factors operate together to push individuals over a threshold into expressing the characteristic clinical picture. One environmental factor which has been curiously neglected is the evidence that certain drugs can induce schizophrenia-like psychosis. In the last 60 years, improved understanding of the relationship between drug abuse and psychosis has contributed substantially to our modern view of the disorder suggesting that liability to psychosis in general, and to schizophrenia in particular, is distributed trough the general population in a similar continuous way to liability to medical disorders such as hypertension and diabetes. In this review we examine the main hypotheses resulting from the link observed between the most common psychotomimetic drugs (lysergic acid diethylamide, amphetamines, cannabis, phencyclidine) and schizophrenia.

Endocannabinoid signaling in neural plasticity

Plasticity refers to a physiologically measured change that may last for short or long periods of time. Endocannabinoids (ECBs) are prevalent throughout most of the brain, and modulate synaptic transmission in many ways. This chapter will focus on the roles of ECBs in neural plasticity in the mammalian brain. The topics covered can be divided loosely into two themes: how ECBs regulate synaptic plasticity, and how ECBs' actions themselves are regulated by neuronal activity. Because ECBs regulate synaptic plasticity, the modifiability of ECB mobilization constitutes a form of "metaplasticity" (as reported by Abraham and Bear (Trends Neurosci 19:126-130, 1996)), i.e., an upstream process that determines the nature and extent of synaptic plasticity. Many of their basic functions are still being discovered, and while there is consensus on large issues, many points of divergence exist as well. This chapter concentrates on developments in the roles of ECBs in synaptic plasticity that have come to light since the major review by Chevaleyre et al. (Annu Rev Neurosci 29:37-76, 2006).

Endocannabinoids mediate synaptic plasticity at glutamatergic synapses on spiny neurons within a basal ganglia nucleus necessary for song learning

Activation of type 1 cannabinoid receptors (CB(1)R) in many central nervous system structures induces both short- and long-term changes in synaptic transmission. Within mammalian striatum, endocannabinoids (eCB) are one of several mechanisms that induce synaptic plasticity at glutamatergic terminals onto medium spiny neurons. Striatal synaptic plasticity may contribute a critical component of adaptive motor coordination and procedural learning. Songbirds are advantageous for studying the neural mechanisms of motor learning because they possess a neural pathway necessary for song learning and adult song plasticity that includes a striato-pallidal nucleus, area X (homologous to a portion of mammalian basal ganglia). Recent findings suggest that eCBs contribute to vocal development. For example, dense CB(1)R expression in song control nuclei peaks around the closure of the sensori-motor integration phase of song development. Also, systemic administration of a CB(1)R agonist during vocal development impairs song learning. Here we test whether activation of CB(1)R alters excitatory synaptic input on spiny neurons in area X of adult male zebra finches. Application of the CB(1)R agonist WIN55212-2 decreased excitatory postsynaptic current (EPSC) amplitude; that decrease was blocked by the CB(1)R antagonist AM251. Guided by eCB experiments in mammalian striatum, we tested and verified that at least two mechanisms indirectly activate CB(1)Rs through eCBs in area X. First, activation of group I metabotropic glutamate receptors with the agonist 3,5-dihydroxyphenylglycine (DHPG) induced a CB(1)R-mediated reduction in EPSC amplitude. Second, we observed that a 10 s postsynaptic depolarization induced a calcium-mediated, eCB-dependent decrease in synaptic strength that resisted rescue with late CB(1)R blockade. Together, these results show that eCB modulation occurs at inputs to area X spiny neurons and could influence motor learning and production.

Dopamine Release and Uptake Impairments and Behavioral Alterations Observed in Mice that Model Fragile X Mental Retardation Syndrome

In this study we evaluated the relationship between amphetamine-induced behavioral alterations and dopamine release and uptake characteristics in Fmr1 knockout (Fmr1 KO) mice, which model fragile X syndrome. The behavioral analyses, obtained at millisecond temporal resolution and 2 mm spatial resolution using a force-plate actometer, revealed that Fmr1 KO mice express a lower degree of focused stereotypy compared to wild type (WT) control mice after injection with 10 mg/kg (ip) amphetamine. To identify potentially related neurochemical mechanisms underlying this phenomenon, we measured electrically-evoked dopamine release and uptake using fast-scan cyclic voltammetry at carbon-fiber microelectrodes in striatal brain slices. At 10 weeks of age, dopamine release per pulse, which is dopamine release corrected for differences in uptake, was unchanged. However, at 15 (the age of behavioral testing) and 20 weeks of age, dopamine per pulse and the maximum rate of dopamine uptake was diminished in Fmr1 KO mice compared to WT mice. Dopamine uptake measurements, obtained at different amphetamine concentrations, indicated that dopamine transporters in both genotypes have equal affinities for amphetamine. Moreover, dopamine release measurements from slices treated with quinpirole, a D2-family receptor agonist, rule out enhanced D2 autoreceptor sensitivity as a mechanism of release inhibition. However, dopamine release, uncorrected for uptake and normalized against the corresponding pre-drug release peaks, increased in Fmr1 KO mice, but not in WT mice. Collectively, these data are consistent with a scenario in which a decrease in extracellular dopamine levels in the striatum result in diminished expression of focused stereotypy in Fmr1 KO mice.

Deficiency in endocannabinoid signaling in the nucleus accumbens induced by chronic unpredictable stress

The nucleus accumbens (NAc) is a critical component of the reward circuitry, and dysfunction of the NAc may account for anhedonia and other symptoms of depression. Here, we investigated whether alterations in endocannabinoid (eCB) signaling in the NAc contribute to depression-like behaviors induced by chronic unpredictable stress (CUS) in mice. We compared three types of eCB/CB1 receptor-mediated synaptic plasticity in slices prepared from the NAc core of control and stress-exposed mice: depolarization-induced suppression of excitation, long-term depression, and the depression of field excitatory postsynaptic potentials (fEPSPs) induced by group I metabotropic glutamate receptor agonist DHPG. CUS (5-6-week exposure to stressors), but not sub-CUS (1 week exposure to stressors), induces depression-like behaviors and impairs these forms of eCB/CB1 receptor-mediated plasticity examined in the NAc core. Neither sub-CUS nor CUS altered the tissue contents of the eCBs, anandamide and 2-arachidonoylglycerol in the striatum. However, exposure to CUS, but not to sub-CUS, attenuated the depression of fEPSPs induced by the CB1 receptor agonist WIN 55 212-2. CUS exposure reduced the maximal effect without affecting the EC(50) of WIN 55 212-2 to induce fEPSP depression. Thus, impaired CB1 receptor function could account for CUS-induced deficiency in eCB signaling in the NAc. Both CUS-induced deficiency in eCB signaling and depression-like behaviors were reversed by in vivo administration of antidepressant fluoxetine. These results suggest that downregulation of eCB signaling in the NAc occurs after CUS and contributes to the pathophysiology of depression.

Does dopamine mediate the psychosis-inducing effects of cannabis? A review and integration of findings across disciplines

General population epidemiological studies have consistently found that cannabis use increases the risk of developing psychotic disorders in a dose-dependent manner. While the epidemiological signal between cannabis and psychosis has gained considerable attention, the biological mechanism whereby cannabis increases risk for psychosis remains poorly understood. Animal research suggests that delta-9-tetrahydrocannabinol (THC, the main psychoactive component of cannabis) increases dopamine levels in several regions of the brain, including striatal and prefrontal areas. Since dopamine is hypothesized to represent a crucial common final pathway between brain biology and actual experience of psychosis, a focus on dopamine may initially be productive in the examination of the psychotomimetic effects of cannabis. Therefore, this review examines the evidence concerning the interactions between THC, endocannabinoids and dopamine in the cortical as well as subcortical regions implicated in psychosis, and considers possible mechanisms whereby cannabis-induced dopamine dysregulation may give rise to delusions and hallucinations. It is concluded that further study of the mechanisms underlying the link between cannabis and psychosis may be conducted productively from the perspective of progressive developmental sensitization, resulting from gene-environment interactions.

Competitive regulation of synaptic Ca2+ influx by D2 dopamine and A2A adenosine receptors

Striatal D2-type dopamine receptors (D2Rs) have been implicated in the pathophysiology of neuropsychiatric disorders, including Parkinson's disease and schizophrenia. Although these receptors regulate striatal synaptic plasticity, the mechanisms underlying dopaminergic modulation of glutamatergic synapses are unclear. We combined optogenetics, two-photon microscopy and glutamate uncaging to examine D2R-dependent modulation of glutamatergic synaptic transmission in mouse striatopallidal neurons. We found that D2R activation reduces corticostriatal glutamate release and attenuates both synaptic- and action potential-evoked Ca2+ influx into dendritic spines by approximately 50%. Modulation of Ca2+ signaling was mediated by a protein kinase A (PKA)-dependent regulation of Ca2+ entry through NMDA-type glutamate receptors that was inhibited by D2Rs and enhanced by activation of 2A-type adenosine receptors (A2ARs). D2Rs also produced a PKA- and A2AR-independent reduction in Ca2+ influx through R-type voltage-gated Ca2+ channels. These findings reveal that dopamine regulates spine Ca2+ by multiple pathways and that competitive modulation of PKA controls NMDAR-mediated Ca2+ signaling in the striatum.

Distinct coincidence detectors govern the corticostriatal spike timing-dependent plasticity

Corticostriatal projections constitute the main input to the basal ganglia, an ensemble of interconnected subcortical nuclei involved in procedural learning. Thus, long-term plasticity at corticostriatal synapses would provide a basic mechanism for the function of basal ganglia in learning and memory. We had previously reported the existence of a corticostriatal anti-Hebbian spike timing-dependent plasticity (STDP) at synapses onto striatal output neurons, the medium-sized spiny neurons. Here, we show that the blockade of GABAergic transmission reversed the time dependence of corticostriatal STDP. We explored the receptors and signalling mechanisms involved in the corticostriatal STDP. Although classical models for STDP propose NMDA receptors as the unique coincidence detector, the involvement of multiple coincidence detectors has also been demonstrated. Here, we show that corticostriatal STDP depends on distinct coincidence detectors. Specifically, long-term potentiation is dependent on NMDA receptor activation, while long-term depression requires distinct coincidence detectors: the phospholipase Cbeta (PLCbeta) and the inositol-trisphosphate receptor (IP3R)-gated calcium stores. Furthermore, we found that PLCbeta activation is controlled by group-I metabotropic glutamate receptors, type-1 muscarinic receptors and voltage-sensitive calcium channel activities. Activation of PLCbeta and IP3Rs leads to robust retrograde endocannabinoid signalling mediated by 2-arachidonoyl-glycerol and cannabinoid CB1 receptors. Interestingly, the same coincidence detectors govern the corticostriatal anti-Hebbian STDP and the Hebbian STDP reported at cortical synapses. Therefore, LTP and LTD induced by STDP at corticostriatal synapses are mediated by independent signalling mechanisms, each one being controlled by distinct coincidence detectors.

Glutamatergic synaptic transmission in neuroendocrine cells: Basic principles and mechanisms of plasticity

Glutamate synapses drive the output of neuroendocrine cells in the hypothalamus, but until recently, relatively little was known about the fundamental properties of transmission at these synapses. Here we review recent advances in the understanding of glutamate signals in magnocellular neurosecretory cells (MNCs) in the paraventricular (PVN) and supraoptic nuclei (SON) of the hypothalamus that serve as the last step in synaptic integration before neurohormone release. While these synapses exhibit many similarities with other glutamate synapses described throughout the brain, they also exhibit a number of unique properties that are particularly well suited to the physiology of this system and will be discussed here. In addition, a number of recent studies begin to provide insights into new forms of synaptic plasticity that may be common in other brain regions, but in these cells, may serve important adaptive roles.

Dopaminergic modulation of endocannabinoid-mediated plasticity at GABAergic synapses in the prefrontal cortex

Similar to dopamine (DA), cannabinoids strongly influence prefrontal cortical functions, such as working memory, emotional learning, and sensory perception. Although endogenous cannabinoid receptors (CB(1)Rs) are abundantly expressed in the prefrontal cortex (PFC), very little is known about endocannabinoid (eCB) signaling in this brain region. Recent behavioral and electrophysiological evidence has suggested a functional interplay between the dopamine and cannabinoid receptor systems, although the cellular mechanisms underlying this interaction remain to be elucidated. We examined this issue by combining neuroanatomical and electrophysiological techniques in PFC of rats and mice (both genders). Using immunoelectron microscopy, we show that CB(1)Rs and dopamine type 2 receptors (D(2)Rs) colocalize at terminals of symmetrical, presumably GABAergic, synapses in the PFC. Indeed, activation of either receptor can suppress GABA release onto layer 5 pyramidal cells. Furthermore, coactivation of both receptors via repetitive afferent stimulation triggers eCB-mediated long-term depression of inhibitory transmission (I-LTD). This I-LTD is heterosynaptic in nature, requiring glutamate release to activate group I metabotropic glutamate receptors. D(2)Rs most likely facilitate eCB signaling at the presynaptic site as disrupting postsynaptic D(2)R signaling does not diminish I-LTD. Facilitation of eCB-LTD may be one mechanism by which DA modulates neuronal activity in the PFC and regulates PFC-mediated behavior in vivo.

Regulation of plasticity of glutamate synapses by endocannabinoids and the cyclic-AMP/protein kinase A pathway in midbrain dopamine neurons

Endocannabinoids (eCBs) are lipid signalling molecules which play a key role in the regulation of synaptic transmission and plasticity in the central nervous system. Previous studies have reported that eCBs are released 'on demand' in the ventral tegmental area (VTA), a brain region critical for reward learning. However, their role in modulating the long-term plasticity of glutamate synapses of VTA dopamine (DA) neurons remains unknown. In the present study, we showed that low frequency afferent stimulation paired with moderate postsynaptic depolarization elicited an N-methyl-d-aspartate (NMDA) receptor-independent long-term depression (LTD) at glutamate synapses of VTA DA neurons. This form of LTD was caused by a decrease in the probability of glutamate release. Examination of the mechanisms underlying this form of LTD revealed that it was mediated by retrograde eCB signalling. In addition, we found that inhibition of 2-arachidonoyl glycerol biosynthesis blocked LTD induction, suggesting that 2-arachidonoyl glycerol is the most likely retrograde eCB messenger mediating LTD. The eCB-LTD induced at glutamate synapses of VTA DA neurons also required the inhibition of the presynaptic cAMP/PKA pathway. Taken together, these results reveal a critical role of eCBs in controlling the long-term plasticity of glutamate synapses in VTA DA neurons.

The small-conductance calcium-activated potassium channel is a key modulator of firing and long-term depression in the dorsal striatum

The striatum is considered to be critical for the control of goal-directed action, with the lateral dorsal striatum (latDS) being implicated in modulation of habits and the nucleus accumbens thought to represent a limbic-motor interface. Although medium spiny neurons from different striatal subregions exhibit many similar properties, differential firing and synaptic plasticity could contribute to the varied behavioral roles across subregions. Here, we examined the contribution of small-conductance calcium-activated potassium channels (SKs) to action potential generation and synaptic plasticity in adult rat latDS and nucleus accumbens shell (NAS) projection neurons in vitro. The SK-selective antagonist apamin exerted a prominent effect on latDS firing, significantly decreasing the interspike interval. Furthermore, prolonged latDS depolarization increased the interspike interval and reduced firing, and this enhancement was reversed by apamin. In contrast, NAS neurons exhibited greater basal firing rates and less regulation of firing by SK inhibition and prolonged depolarization. LatDS neurons also had greater SK currents than NAS neurons under voltage-clamp. Importantly, SK inhibition with apamin facilitated long-term depression (LTD) induction in the latDS but not the NAS, without alterations in glutamate release. In addition, SK activation in the latDS prevented LTD induction. Greater SK function in the latDS than in the NAS was not secondary to differences in sodium or inwardly rectifying potassium channel function, and apamin enhancement of firing did not reflect indirect action through cholinergic interneurons. Thus, these data demonstrate that SKs are potent modulators of action potential generation and LTD in the dorsal striatum, and could represent a fundamental cellular mechanism through which habits are regulated.

Juxtacellular labeling of tonically active neurons and phasically active neurons in the rat striatum

Tonically active neurons (TANs) and phasically active neurons (PANs) are widely believed to be the cholinergic interneurons and GABAergic projection neurons, respectively, in the striatum based on in vivo intracellular recordings coupled with morphological examinations of anesthetized rats, and on histochemical, electrophysiological, and labeling studies of in vitro slice preparations. TANs of alert behaving animals exhibit prolonged pause responses to behaviorally significant events. PANs, on the other hand, are mostly inactive when subjects are quiet and not performing any actions, but exhibit burst discharges in response to external stimuli and/or voluntary actions. Several other types of interneurons have also been identified in the striatum, such as parvalbumin-containing GABAergic interneurons (fast-spiking cells), somatostatin-containing interneurons, and calretinin-containing interneurons. To identify the neurochemical and morphological characteristics of TANs and PANs in a more direct manner, we conducted juxtacellular labeling, combining electrophysiology with immunohistochemistry and morphology in anesthetized rats. All of the juxtacellularly labeled TANs (n=3) among those recorded (n=10) were ChAT-positive and had large cell somata with aspiny dendrites. Thus, although our observations are based on a limited number of neurons, our findings provide the most convincing evidence to date that TANs in the striatum are cholinergic neurons. We also found that the majority of PANs are GABA-immunoreactive (46 of 48 tested) and approximately two-thirds had spiny dendrites (30 of 48 tested), indicating that the majority are medium-sized, spiny, GABAergic projection neurons, consistent with general beliefs. Conversely, the remaining one-third of PANs had aspiny dendrites (n=18), indicating that they were interneurons. Therefore, the present study reveals that TANs are cholinergic neurons and that the majority of PANs are medium-sized, spiny, GABAergic projection neurons, while a smaller number are GABAergic interneurons.

Synaptic plasticity in the mesolimbic system: therapeutic implications for substance abuse

In an ever-changing environment, animals must learn new behavioral strategies for the successful procurement of food, sex, and other needs. Synaptic plasticity within the mesolimbic system, a key reward circuit, affords an animal the ability to adapt and perform essential goal-directed behaviors. Ironically, drugs of abuse can also induce synaptic changes within the mesolimbic system, and such changes are hypothesized to promote deleterious drug-seeking behaviors in lieu of healthy, adaptive behaviors. In this review, we will discuss drug-induced neuroadaptations in excitatory transmission in the ventral tegmental area and the nucleus accumbens, two critical regions of the mesolimbic system, and the possible role of dopamine receptors in the development of these neuroadaptations. In particular, we will focus our discussion on recent studies showing changes in AMPA receptor function as a common molecular target of addictive drugs, and the possible behavioral consequences of such neuroadaptations.

The endocannabinoid system and nondrug rewarding behaviours

Rewarding behaviours such as sexual activity, eating, nursing, parenting, social interactions, and play activity are conserved strongly in evolution, and they are essential for development and survival. All of these behaviours are enjoyable and represent pleasant experiences with a high reward value. Remarkably, rewarding behaviours activate the same brain circuits that mediate the positive reinforcing effects of drugs of abuse and of other forms of addiction, such as gambling and food addiction. Given the involvement of the endocannabinoid system in a variety of physiological functions of the nervous system, it is not surprising that it takes part in the complex machinery that regulates gratification and perception of pleasure. In this review, we focus first on the role of the endocannabinoid system in the modulation of neural activity and synaptic functions in brain regions that are involved in natural and nonnatural rewards (namely, the ventral tegmental area, striatum, amygdala, and prefrontal cortex). Then, we examine the role of the endocannabinoid system in modulating behaviours that directly or indirectly activate these brain reward pathways. More specifically, current knowledge of the effects of the pharmacological manipulation of the endocannabinoid system on natural (eating, sexual behaviour, parenting, and social play) and pathological (gambling) rewarding behaviours is summarised and discussed.

Transient receptor potential vanilloid 1 channels control acetylcholine/2-arachidonoylglicerol coupling in the striatum

The neurotransmitter acetylcholine (Ach) controls both excitatory and inhibitory synaptic transmission in the striatum. Here, we investigated the involvement of the endocannabinoid system in Ach-mediated inhibition of striatal GABA transmission, and the potential role of transient receptor potential vanilloid 1 (TRPV1) channels in the control of Ach-endocannabinoid coupling. We found that inhibition of Ach degradation and direct pharmacological stimulation of muscarinic M1 receptors reduced striatal inhibitory postsynaptic currents (IPSCs) through the stimulation of 2-arachidonoylglicerol (2AG) synthesis and the activation of cannabinoid CB1 receptors. The effects of M1 receptor activation on IPSCs were occlusive with those of metabotropic glutamate receptor 5 stimulation, and were prevented in the presence of capsaicin, agonist of TRPV1 channels. Elevation of anandamide (AEA) tone with URB597, a blocker of fatty acid amide hydrolase, mimicked the effects of capsaicin, indicating that endogenous AEA acts as an endovanilloid substance in the control of M1-dependent 2AG-mediated synaptic effects in the striatum. Accordingly, both capsaicin and URB597 effects were absent in mice lacking TRPV1 channels. Pharmacological interventions targeting AEA metabolism and TRPV1 channels might be considered alternative therapeutic routes in disorders of striatal cholinergic or endocannabinoid neurotransmission.

Neurotransmitter roles in synaptic modulation, plasticity and learning in the dorsal striatum

The dorsal striatum is a large forebrain region involved in action initiation, timing, control, learning and memory. Learning and remembering skilled movement sequences requires the dorsal striatum, and striatal subregions participate in both goal-directed (action-outcome) and habitual (stimulus-response) learning. Modulation of synaptic transmission plays a large part in controlling input to as well as the output from striatal medium spiny projection neurons (MSNs). Synapses in this brain region are subject to short-term modulation, including allosteric alterations in ion channel function and prominent presynaptic inhibition. Two forms of long-term synaptic plasticity have also been observed in striatum, long-term potentiation (LTP) and long-term depression (LTD). LTP at glutamatergic synapses onto MSNs involves activation of NMDA-type glutamate receptors and D1 dopamine or A2A adenosine receptors. Expression of LTP appears to involve postsynaptic mechanisms. LTD at glutamatergic synapses involves retrograde endocannabinoid signaling stimulated by activation of metabotropic glutamate receptors (mGluRs) and D2 dopamine receptors. While postsynaptic mechanisms participate in LTD induction, maintained expression involves presynaptic mechanisms. A similar form of LTD has also been observed at GABAergic synapses onto MSNs. Studies have just begun to examine the roles of synaptic plasticity in striatal-based learning. Findings to date indicate that molecules implicated in induction of plasticity participate in these forms of learning. Neurotransmitter receptors involved in LTP induction are necessary for proper skill and goal-directed instrumental learning. Interestingly, receptors involved in LTP and LTD at glutamatergic synapses onto MSNs of the "indirect pathway" appear to have important roles in habit learning. More work is needed to reveal if and when synaptic plasticity occurs during learning and if so what molecules and cellular processes, both short- and long-term, contribute to this plasticity.

A role for 2-arachidonoylglycerol and endocannabinoid signaling in the locomotor response to novelty induced by olfactory bulbectomy

Bilateral olfactory bulbectomy (OBX) in rodents produces behavioral and neurochemical changes associated clinically with depression and schizophrenia. Most notably, OBX induces hyperlocomotion in response to the stress of exposure to a novel environment. We examined the role of the endocannabinoid system in regulating this locomotor response in OBX and sham-operated rats. In our study, OBX-induced hyperactivity was restricted to the first 3 min of the open field test, demonstrating the presence of novelty (0-3 min) and habituation (3-30 min) phases of the open field locomotor response. Levels of the endocannabinoids 2-arachidonoylglycerol (2-AG) and anandamide were decreased in the ventral striatum, a brain region deafferented by OBX, whereas cannabinoid receptor densities were unaltered. In sham-operated rats, 2-AG levels in the ventral striatum were negatively correlated with distance traveled during the novelty phase. Thus, low levels of 2-AG are reflected in a hyperactive open field response. This correlation was not observed in OBX rats. Conversely, 2-AG levels in endocannabinoid-compromised OBX rats correlated with distance traveled during the habituation phase. In OBX rats, pharmacological blockade of cannabinoid CB(1) receptors with either AM251 (1 mg kg(-1) i.p.) or rimonabant (1 mg kg(-1) i.p.) increased distance traveled during the habituation phase. Thus, blockade of endocannabinoid signaling impairs habituation of the hyperlocomotor response in OBX, but not sham-operated, rats. By contrast, in sham-operated rats, effects of CB(1) antagonism were restricted to the novelty phase. These findings suggest that dysregulation in the endocannabinoid system, and 2-AG in particular, is implicated in the hyperactive locomotor response induced by OBX. Our studies suggest that drugs that enhance 2-AG signaling, such as 2-AG degradation inhibitors, might be useful in human brain disorders modeled by OBX.

mGluR-mediated and endocannabinoid-dependent long-term depression in the hilar region of the rat dentate gyrus

We report that bath application of the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) causes acute inhibition of evoked IPSCs recorded from hilar mossy cells, and that significant long-term depression (LTD) of synaptic transmission remains following washout of DHPG. Subsequent experiments using minimal stimulation techniques revealed that expression of both acute and long-term effects of DHPG are restricted to a subset of GABAergic afferents that are also sensitive to depolarization-induced suppression of inhibition (DSI). Experiments with a selective CB1 antagonist and with transgenic animals lacking CB1 receptors indicate that all effects of DHPG, like DSI, depend on activation of CB1 receptors. Further work with selective mGluR antagonists suggests a direct involvement of mGluR1 receptors. Interestingly, we also report that induction of LTD under our experimental conditions does not require prior direct somatic depolarization via the patch pipette and does not appear to depend critically on the level of activity in incoming GABAergic afferents. Collectively, these results represent the first characterization of mGluR-mediated and endocannabinoid-dependent LTD in the hilar region of the dentate gyrus. The dentate gyrus is thus one of relatively few areas where this mechanism has clearly been demonstrated to induce long-term modulation of inhibitory synaptic transmission.

The role of dopamine in modulating the structure and function of striatal circuits

Dopamine (DA) is a key regulator of action selection and associative learning. The striatum has long been thought to be a major locus of DA action in this process. Although all striatal cell types express G protein-coupled receptors for DA, the effects of DA on principal medium spiny neurons (MSNs) understandably have received the most attention. In the two principal classes of MSN, DA receptor expression diverges, with striatonigral MSNs robustly expressing D(1) receptors and striatopallidal MSNs expressing D(2) receptors. In the last couple of years, our understanding of how these receptors and the intracellular signalling cascades that they couple to modulate dendritic physiology and synaptic plasticity has rapidly expanded, fuelled in large measure by the development of new optical and genetic tools. These tools also have enabled a rapid expansion of our understanding of the striatal adaptations in models of Parkinson's disease. This chapter highlights some of the major advances in these areas.

Dopamine modulation of excitatory currents in the striatum is dictated by the expression of D1 or D2 receptors and modified by endocannabinoids

Striatal medium-sized spiny neurons (MSSNs) receive glutamatergic inputs modulated presynaptically and postsynaptically by dopamine. Mice expressing the gene for enhanced green fluorescent protein as a reporter gene to identify MSSNs containing D1 or D2 receptor subtypes were used to examine dopamine modulation of spontaneous excitatory postsynaptic currents (sEPSCs) in slices and postsynaptic N-methyl-d-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) currents in acutely isolated cells. The results demonstrated dopamine receptor-specific modulation of sEPSCs. Dopamine and D1 agonists increased sEPSC frequency in D1 receptor-expressing MSSNs (D1 cells), whereas dopamine and D2 agonists decreased sEPSC frequency in D2 receptor-expressing MSSNs (D2 cells). These effects were fully (D1 cells) or partially (D2 cells) mediated through retrograde signaling via endocannabinoids. A cannabinoid 1 receptor (CB1R) agonist and a blocker of anandamide transporter prevented the D1 receptor-mediated increase in sEPSC frequency in D1 cells, whereas a CB1R antagonist partially blocked the decrease in sEPSC frequency in D2 cells. At the postsynaptic level, low concentrations of a D1 receptor agonist consistently increased NMDA and AMPA currents in acutely isolated D1 cells, whereas a D2 receptor agonist decreased these currents in acutely isolated D2 cells. These results show that both glutamate release and postsynaptic excitatory currents are regulated in opposite directions by activation of D1 or D2 receptors. The direction of this regulation is also specific to D1 and D2 cells. We suggest that activation of postsynaptic dopamine receptors controls endocannabinoid mobilization, acting on presynaptic CB1Rs, thus modulating glutamate release differently in glutamate terminals projecting to D1 and D2 cells.

Striatal action-learning based on dopamine concentration

The reinforcement learning hypothesis of dopamine function predicts that dopamine acts as a teaching signal by governing synaptic plasticity in the striatum. Induced changes in synaptic strength enable the cortico-striatal network to learn a mapping between situations and actions that lead to a reward. A review of the relevant neurophysiology of dopamine function in the cortico-striatal network and the machine reinforcement learning hypothesis reveals an apparent mismatch with recent electrophysiological studies. It was found that in addition to the well-described reward-related responses, a subpopulation of dopamine neurons also exhibits phasic responses to aversive stimuli or to cues predicting aversive stimuli. Obviously, actions that lead to aversive events should not be reinforced. However, published data suggest that the phasic responses of dopamine neurons to reward-related stimuli have a higher firing rate and have a longer duration than phasic responses of dopamine neurons to aversion-related stimuli. We propose that based on different dopamine concentrations, the target structures are able to decode reward-related dopamine from aversion-related dopamine responses. Thereby, the learning of actions in the basal-ganglia network integrates information about both costs and benefits. This hypothesis predicts that dopamine concentration should be a crucial parameter for plasticity rules at cortico-striatal synapses. Recent in vitro studies on cortico-striatal synaptic plasticity rules support a striatal action-learning scheme where during reward-related dopamine release dopamine-dependent forms of synaptic plasticity occur, while during aversion-related dopamine release the dopamine concentration only allows dopamine-independent forms of synaptic plasticity to occur.

Dopamine and synaptic plasticity in dorsal striatal circuits controlling action selection

The striatum is thought to play a central role in learning how to choose acts that lead to reward and avoid punishment. Dopamine-dependent modification of striatal synapses in the action selection circuitry has long been thought to be a key step toward this type of learning. The development of new genetic and optical tools has pushed this field forward in the last couple of years, demanding a re-evaluation of models of how experience controls dopamine-dependent synaptic plasticity and how disease states like Parkinson's disease affect the striatal circuitry.

Adenylyl cyclase type 5 contributes to corticostriatal plasticity and striatum-dependent learning

Dopamine (DA)-dependent corticostriatal plasticity is thought to underlie incremental procedural learning. A primary effector of striatal DA signaling is cAMP, yet its role in corticostriatal plasticity and striatum-dependent learning remains unclear. Here, we show that genetic deletion of a striatum-enriched isoform of adenylyl cyclase, AC5 knock-out (AC5KO), impairs two forms of striatum-dependent learning and corticostriatal synaptic plasticity. AC5KO mice were severely impaired in acquisition of a response strategy in the cross maze, a striatum-dependent task requiring a correct body turn to find a goal arm. In addition, AC5KO mice were impaired in acquisition of a motor skill, as assessed by the accelerated rotarod. Slice electrophysiology revealed a deficit in corticostriatal long-term depression (LTD) after high-frequency stimulation of tissue from AC5KO mice. LTD was rescued by activation of either presynaptic cannabinoid type 1 (CB(1)) receptors or postsynaptic metabotropic glutamate receptors (mGluRs), suggesting a postsynaptic role of AC5-cAMP, upstream of endocannabinoid release. In striatopallidal-projecting medium spiny neurons, DA D(2) receptors are negatively coupled to cAMP production, and activation of these receptors is required for endocannabinoid release and corticostriatal LTD. Recordings from striatopallidal neurons indicated that this is mediated by AC5, because coactivation of D(2) and mGluRs could induce LTD in wild-type but not in AC5KO neurons. To further examine the role of cAMP in corticostriatal plasticity, we elevated cAMP in striatal neurons of wild-type mice via the recording electrode. Under these conditions, corticostriatal LTD was eliminated. Together, these data suggest an AC5-cAMP-endocannabinoid-CB(1) signaling pathway in corticostriatal plasticity and striatum-dependent learning.

Endocannabinoid signaling and long-term synaptic plasticity

Endocannabinoids (eCBs) are key activity-dependent signals regulating synaptic transmission throughout the central nervous system. Accordingly, eCBs are involved in neural functions ranging from feeding homeostasis to cognition. There is great interest in understanding how exogenous (e.g., cannabis) and endogenous cannabinoids affect behavior. Because behavioral adaptations are widely considered to rely on changes in synaptic strength, the prevalence of eCB-mediated long-term depression (eCB-LTD) at synapses throughout the brain merits close attention. The induction and expression of eCB-LTD, although remarkably similar at various synapses, are controlled by an array of regulatory influences that we are just beginning to uncover. This complexity endows eCB-LTD with important computational properties, such as coincidence detection and input specificity, critical for higher CNS functions like learning and memory. In this article, we review the major molecular and cellular mechanisms underlying eCB-LTD, as well as the potential physiological relevance of this widespread form of synaptic plasticity.

Developmental alterations of DHPG-induced long-term depression of corticostriatal synaptic transmission: switch from NMDA receptor-dependent towards CB1 receptor-dependent plasticity

In animal models of early Parkinson's disease (PD), motor deficits are accompanied by excessive striatal glutamate release. Blockade of group I metabotropic glutamate receptors (mGluRs), endocannabinoid degradation and nitric oxide (NO) synthesis combats PD symptoms. Activation of group I mGluRs with the specific agonist 3,5-dihydroxyphenylglycine (DHPG) induces long-term depression of corticostriatal transmission (LTD(DHPG)) in the adult mouse striatum requiring NO synthesis downstream to cannabinoid CB1 receptor (CB1R) activation suggesting a dual role for LTD(DHPG): neuroprotective by down-regulation of glutamatergic transmission and, under certain circumstances, neurotoxic by release of NO. We report now that LTD(DHPG) undergoes a developmental switch from N-methyl-D-aspartate (NMDA)-receptor-dependent/CB1R-independent to NMDA receptor-independent/CB1R-dependent plasticity with NO playing an essential role for LTD(DHPG) at all developmental stages. The gain in function of CB1R is explained by their developmental up-regulation evaluated with real-time reverse transcription-polymerase chain reaction. These findings are relevant for the pathophysiology and therapy of PD as they link the activation of group I mGluRs, endocannabinoid release, and striatal NO production.

Metaplasticity of hypothalamic synapses following in vivo challenge

Neural networks that regulate an organism's internal environment must sense perturbations, respond appropriately, and then reset. These adaptations should be reflected as changes in the efficacy of the synapses that drive the final output of these homeostatic networks. Here we show that hemorrhage, an in vivo challenge to fluid homeostasis, induces LTD at glutamate synapses onto hypothalamic magnocellular neurosecretory cells (MNCs). LTD requires the activation of postsynaptic alpha2-adrenoceptors and the production of endocannabinoids that act in a retrograde fashion to inhibit glutamate release. In addition, both hemorrhage and noradrenaline downregulate presynaptic group III mGluRs. This loss of mGluR function allows high-frequency activity to potentiate these synapses from their depressed state. These findings demonstrate that noradrenaline controls a form of metaplasticity that may underlie the resetting of homeostatic networks following a successful response to an acute physiological challenge.

From real-world events to psychosis: the emerging neuropharmacology of delusions

The earliest stages of delusion are characterized by an overabundance of meaningful coincidences impinging on the sufferer's existing worldview. Successive events are seen by him as pointing to, and then confirming, a fundamentally new reality that takes him over and engulfs his everyday life. Research over the last 4 decades has revealed the importance of dopamine (DA), D2 receptors, and the basal ganglia in psychotic thinking. Recent work has implicated the aberrant reward learning initiated by the excess release of striatal DA in the attribution of excessive importance or "salience" to insignificant stimuli and events. But our knowledge of what is happening beyond D2 receptors has remained scant. The gap is especially apparent at the cellular and microcircuit levels, encompassing the plastic changes, which are believed to be essential for new learning, and whose processes may go awry in major mental illness. Now new pharmacological findings are advancing our understanding of information processing and learning within the striatum. DA has an important role in setting the strength of individual striatal connections, but it does not act in isolation. Two other modulator systems are critical, the endocannabinoids and adenosine. Thus, at medium spiny neurons belonging to the indirect pathway, D2 stimulation evokes endocannabinoid-mediated depression of cortical inputs. Adenosine acting at A2A receptors elicits the opposite effect. Remarkably, drugs that target the endocannabinoid and purinergic systems also have pro- or antipsychotic properties. Here, we discuss how the 3 modulators regulate learning within the striatum and how their dysfunction may lead to delusional thinking.

Physiology and pharmacology of striatal neurons

The basal ganglia occupy the core of the forebrain and consist of evolutionarily conserved motor nuclei that form recurrent circuits critical for motivation and motor planning. The striatum is the main input nucleus of the basal ganglia and a key neural substrate for procedural learning and memory. The vast majority of striatal neurons are spiny GABAergic projection neurons, which exhibit slow but temporally precise spiking in vivo. Contributing to this precision are several different types of interneurons that constitute only a small fraction of total neuron number but play a critical role in regulating striatal output. This review examines the cellular physiology and modulation of striatal neurons that give rise to their unique properties and function.

D2 dopamine receptor activation facilitates endocannabinoid-mediated long-term synaptic depression of GABAergic synaptic transmission in midbrain dopamine neurons via cAMP-protein kinase A signaling

Endocannabinoid (eCB) signaling mediates short-term and long-term synaptic depression (LTD) in many brain areas. In the ventral tegmental area (VTA) and striatum, D(2) dopamine receptors cooperate with group I metabotropic glutamate receptors (mGluRs) to induce eCB-mediated LTD of glutamatergic excitatory and GABAergic inhibitory (I-LTD) synaptic transmission. Because D(2) receptors and group I mGluR agonists are capable of inducing the release of eCBs, the predominant hypothesis is that the cooperation between these receptors to induce eCB-mediated synaptic depression results from the combined activation of type I cannabinoid (CB(1)) receptors by the eCBs. By determining the downstream effectors for D(2) receptor and group I mGluR activation in VTA dopamine neurons, we show that group I mGluR activation contributes to I-LTD induction by enhancing eCB release and CB(1) receptor activation. However, D(2) receptor activation does not enhance CB(1) receptor activation, but facilitates I-LTD induction via direct inhibition of cAMP-dependent protein kinase A (PKA) signaling. We further demonstrate that cAMP/PKA signaling pathway is the downstream effector for CB(1) receptors and is required for eCB-mediated I-LTD induction. Our results suggest that D(2) receptors and CB(1) receptors target the same downstream effector cAMP/PKA signaling pathway to induce I-LTD and D(2) receptor activation facilitates eCB-mediated I-LTD in dopamine neurons not by enhancing CB(1) receptor activation, but by enhancing its downstream effects.

Endocannabinoid-mediated control of synaptic transmission

The discovery of cannabinoid receptors and subsequent identification of their endogenous ligands (endocannabinoids) in early 1990s have greatly accelerated research on cannabinoid actions in the brain. Then, the discovery in 2001 that endocannabinoids mediate retrograde synaptic signaling has opened up a new era for cannabinoid research and also established a new concept how diffusible messengers modulate synaptic efficacy and neural activity. The last 7 years have witnessed remarkable advances in our understanding of the endocannabinoid system. It is now well accepted that endocannabinoids are released from postsynaptic neurons, activate presynaptic cannabinoid CB(1) receptors, and cause transient and long-lasting reduction of neurotransmitter release. In this review, we aim to integrate our current understanding of functions of the endocannabinoid system, especially focusing on the control of synaptic transmission in the brain. We summarize recent electrophysiological studies carried out on synapses of various brain regions and discuss how synaptic transmission is regulated by endocannabinoid signaling. Then we refer to recent anatomical studies on subcellular distribution of the molecules involved in endocannabinoid signaling and discuss how these signaling molecules are arranged around synapses. In addition, we make a brief overview of studies on cannabinoid receptors and their intracellular signaling, biochemical studies on endocannabinoid metabolism, and behavioral studies on the roles of the endocannabinoid system in various aspects of neural functions.

Endocannabinoid-dependent plasticity at GABAergic and glutamatergic synapses in the striatum is regulated by synaptic activity

Long-term depression (LTD) at striatal synapses is mediated by postsynaptic endocannabinoid (eCB) release and presynaptic cannabinoid 1 receptor (CB(1)R) activation. Previous studies have indicated that eCB mobilization at excitatory synapses might be regulated by afferent activation. To further address the role of neuronal activity in synaptic plasticity we examined changes in synaptic strength induced by the L-type calcium channel activator 2,5-dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylic acid methyl ester (FPL 64176, FPL) at glutamatergic and gamma-aminobutyric acid (GABA)ergic synapses in the striatum. We found that the basic mechanisms for FPL-mediated eCB signaling are the same at glutamatergic and GABAergic synapses. FPL-induced LTD (FPL-LTD) was blocked in slices treated with the CB(1)R antagonist AM251 (2 microm), but established depression was not reversed by AM251. FPL-LTD was temperature dependent, blocked by protein translation inhibitors and prevented by intracellular loading of the anandamide transporter inhibitor VDM11 (10 microm) at both glutamatergic and GABAergic synapses. FPL-LTD at glutamatergic synapses required paired-pulse afferent stimulation, while FPL-LTD at GABAergic synapses could be induced even in the absence of explicit afferent activation. By evaluating tetrodotoxin-insensitive spontaneous inhibitory postsynaptic currents we found that neuronal firing is vital for eCB release and LTD induction at GABAergic synapses, but not for short-term depression induced by CB(1)R agonist. The data presented here suggest that the level of neuronal firing regulates eCB signaling by modulating release from the postsynaptic cell, as well as interacting with presynaptic mechanisms to induce LTD at both glutamatergic and GABAergic synapses in the striatum.