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

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.

Striatal plasticity and basal ganglia circuit function

The dorsal striatum, which consists of the caudate and putamen, is the gateway to the basal ganglia. It receives convergent excitatory afferents from cortex and thalamus and forms the origin of the direct and indirect pathways, which are distinct basal ganglia circuits involved in motor control. It is also a major site of activity-dependent synaptic plasticity. Striatal plasticity alters the transfer of information throughout basal ganglia circuits and may represent a key neural substrate for adaptive motor control and procedural memory. Here, we review current understanding of synaptic plasticity in the striatum and its role in the physiology and pathophysiology of basal ganglia function.

Neurochemical changes in the striatum of dyskinetic rats after administration of the cannabinoid agonist WIN55,212-2

Chronic use of levodopa, the most effective treatment for Parkinson's disease, causes abnormal involuntary movements named dyskinesias, which are linked to maladaptive changes in plasticity and disturbances of dopamine and glutamate neurotransmission in the basal ganglia. Dyskinesias can be modeled in rats with unilateral 6-hydroxydopamine lesions by repeated administration of low doses of levodopa (6 mg/kg, s.c.). Previous studies from our lab showed that sub-chronic treatment with the cannabinoid agonist WIN55,212-2 attenuates levodopa-induced dyskinesias at doses that do not interfere with physiological motor function. To investigate the neurochemical changes underlying WIN55,212-2 anti-dyskinetic effects, we used in vivo microdialysis to monitor extracellular dopamine and glutamate in the dorsal striatum of both the hemispheres of freely moving 6-hydroxydopamine-treated, SHAM-operated and intact rats receiving levodopa acutely or chronically (11 days), and studied how sub-chronic WIN55,212-2 (1 injection x 3 days, 20 min before levodopa) affected these neurochemical outputs. Our data indicate that: (1) the 6-hydroxydopamine lesion decreases dopamine turnover in the denervated striatum; (2) levodopa injection reduces extracellular glutamate in the side ipsilateral to the lesion of dyskinetic rats; (3) sub-chronic WIN55,212-2 prevents levodopa-induced glutamate volume transmission unbalances across the two hemispheres; and (4) levodopa-induced dyskinesias are inversely correlated with glutamate levels in the denervated striatum. These data indicate that the anti-dyskinetic properties of WIN55,212-2 are accompanied by changes of dopamine and glutamate outputs in the two brain hemispheres of 6-hydroxydopamine-treated rats.

High on habits

The neural circuits involved in learning and executing goal-directed actions, which are governed by action-outcome contingencies and sensitive to changes in the expected value of the outcome, have been shown to be different from those mediating habits, which are less dependent on action-outcome relations and changes in outcome value. Extended training, different reinforcement schedules, and substances of abuse have been shown to induce a shift from goal-directed performance to habitual performance. This shift can be beneficial in everyday life, but can also lead to loss of voluntary control and compulsive behavior, namely during drug seeking in addiction. Although the brain circuits underlying habit formation are becoming clearer, the molecular mechanisms underlying habit formation are still not understood. Here, we review a recent study where Hilario et al. (2007) established behavioral procedures to investigate habit formation in mice in order to investigate the molecular mechanisms underlying habit formation. Using those procedures, and a combination of genetic and pharmacological tools, the authors showed that endocannabinoid signaling is critical for habit formation.

Computational model of extracellular glutamate in the nucleus accumbens incorporates neuroadaptations by chronic cocaine

Chronic cocaine administration causes instability in extracellular glutamate in the nucleus accumbens that is thought to contribute to the vulnerability to relapse. A computational framework was developed to model glutamate in the extracellular space, including synaptic and nonsynaptic glutamate release, glutamate elimination by glutamate transporters and diffusion, and negative feedback on synaptic release via metabotropic glutamate receptors (mGluR2/3). This framework was used to optimize the geometry of the glial sheath surrounding excitatory synapses, and by inserting physiological values, accounted for known stable extracellular, extrasynaptic concentrations of glutamate measured by microdialysis and glutamatergic tone on mGluR2/3. By using experimental values for cocaine-induced reductions in cystine-glutamate exchange and mGluR2/3 signaling, and by predicting the down-regulation of glutamate transporters, the computational model successfully represented the experimentally observed increase in glutamate that is seen in rats during cocaine-seeking. This model provides a mathematical framework for describing how pharmacological or pathological conditions influence glutamate transmission measured by microdialysis.

Synaptic plasticity in the basal ganglia

Activity-dependent synaptic plasticity occurs in several parts of the basal ganglia. Increasing evidence supports the hypothesis that activity-dependent plasticity underlies the acquisition, maintenance, and extinction of certain types of learning in the basal ganglia. This review focuses on synaptic plasticity in the corticostriatal pathway. As in other systems, both long-term potentiation and long-term depression have been described, and intracellular calcium signalling plays an important role in the induction of plasticity. However, intracellular calcium levels do not appear to be the dominating control factor. Dopamine, via intracellular signalling cascades, also plays a crucial role in determining the magnitude and direction of plasticity, and in modulating the requirements for induction. Endocannabinoids also play an important role in mediating presynaptic expression of synaptic depression. Recent studies have highlighted spike-timing dependent plasticity phenomena, which also involve dopamine and endocannabinoid signalling. Despite significant progress in recent years, many important questions remain unanswered, especially in relation to long-term potentiation. Of particular interest is the question of how to link the molecular and cellular mechanisms of synaptic plasticity to learning operations at the systems level, which are expressed behaviourally as reinforcement-related learning.

In vivo metabotropic glutamate receptor 5 (mGluR5) antagonism prevents cocaine-induced disruption of postsynaptically maintained mGluR5-dependent long-term depression

Metabotropic glutamate receptor 5 (mGluR5) plays a critical role in psychostimulant-induced behavior, yet it is unclear whether mGluR5 is activated by psychostimulant administration, or whether its role is constitutive. We previously reported that activation of mGluR5 with the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) can induce a long-term depression (DHPG-LTD) of glutamatergic transmission in the bed nucleus of the stria terminalis (BNST), and that ex vivo induction of this LTD is disrupted by repeated in vivo administration of cocaine. Here we demonstrate that DHPG-LTD is not maintained by alterations in glutamate release, and that postsynaptic endocytosis is necessary. Furthermore, we find that a single administration of cocaine produces a transient disruption of DHPG-LTD, and the duration of this disruption was increased by repeated days of cocaine administration. The disruption produced by cocaine was not permanent, because DHPG-LTD could be induced 10 d after cocaine administration. To test the role of mGluR5 in vivo in the cocaine-induced disruption of DHPG-LTD, we injected mice with the mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine before cocaine. mGluR5 antagonism during in vivo cocaine administration rescued subsequent ex vivo induction of DHPG-LTD. The effects of in vivo cocaine could be mimicked by application of cocaine to BNST-containing slices, suggesting that the actions of cocaine are local. Thus, using a novel strategy of in vivo antagonist-induced rescue of ex vivo agonist effects for the same receptor, we provide evidence suggesting that mGluR5 activation is actively recruited by in vivo cocaine.

FGF acts as a co-transmitter through adenosine A(2A) receptor to regulate synaptic plasticity

Abnormalities of striatal function have been implicated in several major neurological and psychiatric disorders, including Parkinson's disease, schizophrenia and depression. Adenosine, via activation of A(2A) receptors, antagonizes dopamine signaling at D2 receptors and A(2A) receptor antagonists have been tested as therapeutic agents for Parkinson's disease. We found a direct physical interaction between the G protein-coupled A(2A) receptor (A(2A)R) and the receptor tyrosine kinase fibroblast growth factor receptor (FGFR). Concomitant activation of these two classes of receptors, but not individual activation of either one alone, caused a robust activation of the MAPK/ERK pathway, differentiation and neurite extension of PC12 cells, spine morphogenesis in primary neuronal cultures, and cortico-striatal plasticity that was induced by a previously unknown A(2A)R/FGFR-dependent mechanism. The discovery of a direct physical interaction between the A(2A) and FGF receptors and the robust physiological consequences of this association shed light on the mechanism underlying FGF functions as a co-transmitter and open new avenues for therapeutic interventions.

Short-term and long-term plasticity at corticostriatal synapses: implications for learning and memory

The striatum is the major division of the basal ganglia, representing the input station of the circuit and arguably the principal site within the basal ganglia where information processing occurs. Striatal activity is critically involved in motor control and learning. Many parts of the striatum are involved in reward processing and in various forms of learning and memory, such as reward-association learning. Moreover, the striatum appears to be a brain center for habit formation and is likely to be involved in advanced stages of addiction. The critical role played by the striatum in learning and cognitive processes is thought to be based on changes in neuronal activity when specific behavioral tasks are being learned. Accordingly, excitatory corticostriatal synapses onto both striatal projecting spiny neurons and interneurons are able to undergo the main forms of synaptic plasticity, including long-term potentiation, long-term depression, short-term forms of intrinsic plasticity and spike timing-dependent plasticity. These specific forms of neuroplasticity allow the short-term and long-term selection and differential amplification of cortical neural signals modulating the processes of motor and behavioral selection within the basal ganglia neural circuit.

Context-specific reversal of cocaine sensitization by the CB1 cannabinoid receptor antagonist rimonabant

The CB(1) cannabinoid receptor is implicated in the rewarding properties of many drugs of abuse, including cocaine. While CB(1) receptor involvement in the acute rewarding properties of cocaine is controversial, CB(1) antagonists such as SR141716 (rimonabant) have clearly been found to prevent cue- and cocaine-elicited reinstatement of cocaine self-administration in rodents. Here we demonstrate the novel involvement of CB(1) receptors in the maintenance of behavioral sensitization to cocaine in C57BL/6 mice. Consistent with previous reports, the induction of locomotor sensitization following repeated daily cocaine was not prevented by systemic pretreatment of either rimonabant, Delta(9)-tetrahydrocannabinol (THC), or a 1:1 mixture of THC and cannabidiol (CBD). In contrast, established cocaine sensitization was markedly disrupted following subchronic treatment with rimonabant alone. This effect was notably context-dependent, in that rimonabant did not diminish established cocaine sensitization if delivered in the home cage, but only if the rimonabant-injected mice were exposed to activity chambers previously paired with cocaine. These findings are consistent with CB(1) receptor involvement in conditioned cocaine-seeking behaviors, and further suggest that endocannabinoid (eCB)-mediated synaptic plasticity may act specifically within drug-paired environments to maintain cocaine-directed behavioral responses.

Looking for the role of cannabinoid receptor heteromers in striatal function

The introduction of two concepts, "local module" and "receptor heteromer", facilitates the understanding of the role of interactions between different neurotransmitters in the brain. In artificial cell systems, cannabinoid CB(1) receptors form receptor heteromers with dopamine D2, adenosine A2A and mu opioid receptors. There is indirect but compelling evidence for the existence of the same CB1 receptor heteromers in striatal local modules centered in the dendritic spines of striatal GABAergic efferent neurons, particularly at a postsynaptic location. Their analysis provides new clues for the role of endocannabinoids in striatal function, which cannot only be considered as retrograde signals that inhibit neurotransmitter release. Recent studies using a new method to detect heteromerization of more than two proteins, which consists of sequential BRET-FRET (SRET) analysis, has demonstrated that CB1, D2 and A2A receptors can form heterotrimers in transfected cells. It is likely that functional CB1-A2A-D2 receptor heteromers can be found where they are highly co-expressed, in the dendritic spines of GABAergic enkephalinergic neurons. The functional properties of these multiple receptor heteromers and their role in striatal function need to be determined.

Interneuron activity controls endocannabinoid-mediated presynaptic plasticity through calcineurin

Retrograde signaling by endocannabinoids (eCBs) mediates a widely expressed form of long-term depression at excitatory and inhibitory synapses (eCB-LTD), involving a reduction in neurotransmitter release. In the hippocampus, eCB-LTD occurs at interneuron (IN)-pyramidal cell (PC) synapses (I-LTD), and its induction requires a presynaptic reduction of cAMP/PKA signaling resulting from minutes of type 1 cannabinoid receptor (CB1R) activation. Although repetitive activity of glutamatergic synapses initiates the eCB mobilization required for I-LTD, it is unclear whether CB1R-containing GABAergic terminals are passive targets of eCBs or whether they actively contribute to induction. Here, we show that the minutes-long induction period for I-LTD may serve as a window to integrate associated spontaneous activity in the same IN receiving the retrograde eCB signal. Indeed, reducing spontaneous IN firing blocked I-LTD, which could be rescued with extra stimulation of inhibitory afferents. Moreover, cell pair recordings showed that a single IN expressed LTD onto a PC only if it was active during eCB signaling. Several methods of disrupting presynaptic Ca(2+) dynamics all blocked I-LTD, strongly suggesting that IN spikes regulate I-LTD by raising Ca(2+) at the nerve terminal. Finally, inhibiting the Ca(2+)-activated phosphatase, calcineurin, fully blocked I-LTD, but blocking another phosphatase did not. Our findings support a model where both CB1R signaling and IN activity shift the balance of kinase and phosphatase activity in the presynaptic terminal to induce I-LTD.

AMPA receptor-dependent H2O2 generation in striatal medium spiny neurons but not dopamine axons: one source of a retrograde signal that can inhibit dopamine release

Dopamine-glutamate interactions in the striatum are critical for normal basal ganglia-mediated control of movement. Although regulation of glutamatergic transmission by dopamine is increasingly well understood, regulation of dopaminergic transmission by glutamate remains uncertain given the apparent absence of ionotropic glutamate receptors on dopaminergic axons in dorsal striatum. Indirect evidence suggests glutamatergic regulation of striatal dopamine release is mediated by a diffusible messenger, hydrogen peroxide (H2O2), generated downstream from glutamatergic AMPA receptors (AMPARs). The mechanism of H2O2-dependent inhibition of dopamine release involves activation of ATP-sensitive K+ (KATP) channels. However, the source of modulatory H2O2 is unknown. Here, we used whole cell recording, fluorescence imaging of H2O2, and voltammetric detection of evoked dopamine release in guinea pig striatal slices to examine contributions from medium spiny neurons (MSNs), the principal neurons of striatum, and dopamine axons to AMPAR-dependent H2O2 generation. Imaging studies of H2O2 generation in MSNs provide the first demonstration of AMPAR-dependent H2O2 generation in neurons in the complex brain-cell microenvironment of brain slices. Stimulation-induced increases in H2O2 in MSNs were prevented by GYKI-52466, an AMPAR antagonist, or catalase, an H2O2 metabolizing enzyme, but amplified by mercaptosuccinate (MCS), a glutathione peroxidase inhibitor. By contrast, dopamine release evoked by selective stimulation of dopamine axons was unaffected by GYKI-52466 or MCS, arguing against dopamine axons as a significant source of modulatory H2O2. Together, these findings suggest that glutamatergic regulation of dopamine release via AMPARs is mediated through retrograde signaling by diffusible H2O2 generated in striatal cells, including medium spiny neurons, rather than in dopamine axons.

Metaplastic control of the endocannabinoid system at inhibitory synapses in hippocampus

The modifiability of neuronal response plasticity is called "metaplasticity." In suppressing synaptic inhibition and facilitating induction of long-term excitatory synaptic plasticity, endocannabinoids (eCBs) act as agents of metaplasticity. We now report the discovery of a calcium-dependent mechanism that regulates eCB mobilization by metabotropic glutamate receptor (mGluR) activation. The switch-like mechanism primes cells to release eCBs and requires a transient rise in intracellular Ca2+ concentration ([Ca2+]i) but not concurrent activation of mGluRs. Conversely, short-term, [Ca2+]i-dependent eCB release can be persistently enhanced by mGluR activation. Hence, eCBs are also objects of metaplasticity, subject to higher levels of physiological control.

TRPV1 channels mediate long-term depression at synapses on hippocampal interneurons

TRPV1 receptors have classically been defined as heat-sensitive, ligand-gated, nonselective cation channels that integrate nociceptive stimuli in sensory neurons. TRPV1 receptors have also been identified in the brain, but their physiological role is poorly understood. Here we report that TRPV1 channel activation is necessary and sufficient to trigger long-term synaptic depression (LTD). Excitatory synapses onto hippocampal interneurons were depressed by either capsaicin, a potent TRPV1 channel activator, or the endogenously released eicosanoid, 12-(S)-HPETE, whereas neighboring excitatory synapses onto CA1 pyramidal cells were unaffected. TRPV1 receptor antagonists also prevented interneuron LTD. In brain slices from TRPV1-/- mice, LTD was absent, and neither capsaicin nor 12-(S)-HPETE elicited synaptic depression. Our results suggest that, in the hippocampus, TRPV1 receptor activation selectively modifies synapses onto interneurons. Like other forms of hippocampal synaptic plasticity, TRPV1-mediated LTD may have a role in long-term changes in physiological and pathological circuit behavior during learning and epileptic activity.

Endocannabinoid signaling mediates cocaine-induced inhibitory synaptic plasticity in midbrain dopamine neurons

Drugs that increase GABA levels in the brain reduce cocaine seeking in rodents and humans, suggesting that GABAergic inhibition regulates cocaine-seeking behavior. We previously reported that repeated cocaine exposure in vivo facilitates long-term potentiation by reducing the strength of GABAergic inhibition in dopamine neurons of the ventral tegmental area (VTA). Selective blockade of cocaine-induced reduction of GABAergic inhibition in the VTA might diminish cocaine-induced aberrant synaptic plasticity and addictive behavior. Here, we investigated the mechanism for cocaine-induced reduction of GABAergic inhibition. We show that a pathophysiologically relevant concentration of cocaine enables a normally ineffective stimulus to induce long-term depression (LTD) of IPSCs (I-LTD) in VTA dopamine neurons of midbrain slices. Activation of D2 dopamine receptors and group I metabotropic glutamate receptors and subsequent recruitment of endocannabinoid signaling are required for I-LTD induction. We further demonstrate that in vivo pretreatment with antagonists to these receptors blocks cocaine-induced reduction of GABAergic inhibition and that repeated cocaine exposure in vivo occludes the subsequent induction of I-LTD ex vivo. Together, these results suggest that repeated cocaine exposure reduces the strength of GABAergic inhibition in dopamine neurons by inducing I-LTD-like modification in vivo.

Regulation of excitability and plasticity by endocannabinoids and PKA in developing hippocampus

The activity-dependent strengthening and weakening of synaptic transmission are hypothesized to be the basis of not only memory and learning but also the refinement of neural circuits during development. Here we report that, in the developing CA1 area of the hippocampus, endocannabinoid (eCB)-mediated heterosynaptic long-term depression (LTD) of glutamatergic excitatory synaptic transmission is associated with PKA-mediated homosynaptic long-term potentiation (LTP). This form of LTD was dominant at postnatal days 2-10 (P2-P10), attenuated during development, and finally disappeared in the mature hippocampus. Heterosynaptic LTD of excitatory postsynaptic currents in the developing hippocampus was expressed presynaptically, spread to neighboring neurons, and was mediated by eCBs. Heterosynaptic LTD of field excitatory postsynaptic potentials was associated with a decrease in fiber volley amplitude with a similar time course. Depression of fiber volleys was blocked by K(+) channel blockers, suggesting the involvement of the decrease in presynaptic excitability in heterosynaptic LTD. In the P2-P5 hippocampus, eCBs also attenuate LTP and fiber volleys in homosynaptic pathways and help to prevent too much excitability in the neonatal hippocampus where the GABAergic system is poorly developed and even excitatory. In the hippocampus older than P6 (P > 6), however, LTP is protected from eCB-mediated depression by PKA activated at presynaptic sites by high-frequency stimulation, serving to highlight PKA-mediated LTP by weakening inactive synapses even in adjacent cells. Thus, eCBs and PKA make synapses plastic without changing excitability homeostasis in the developing hippocampus.

Synaptic plasticity: multiple forms, functions, and mechanisms

Experiences, whether they be learning in a classroom, a stressful event, or ingestion of a psychoactive substance, impact the brain by modifying the activity and organization of specific neural circuitry. A major mechanism by which the neural activity generated by an experience modifies brain function is via modifications of synaptic transmission; that is, synaptic plasticity. Here, we review current understanding of the mechanisms of the major forms of synaptic plasticity at excitatory synapses in the mammalian brain. We also provide examples of the possible developmental and behavioral functions of synaptic plasticity and how maladaptive synaptic plasticity may contribute to neuropsychiatric disorders.

Presynaptic modulation by endocannabinoids

Modulation of neurotransmitter release by G-protein-coupled receptors (GPCRs) is a prominent presynaptic mechanism for regulation of synaptic transmission. Activation of GPCRs located at the presynaptic terminal can decrease the probability of neurotransmitter release. This presynaptic depression involves activation of Gi/o-type G-proteins that mediate different inhibitory mechanisms, including inhibition of voltage-gated calcium channels, activation of potassium channels, and direct inhibition of the vesicle fusion process. A variety of neurotransmitters and modulatory agents can activate GPCRs that produce presynaptic depression. Among these are lipid metabolites that serve as agonists for GPCRs. The discovery of endocannabinoids and their cognate receptors, including the CB1 receptor, has stimulated intense investigation into the neurophysiological roles of these lipid metabolites. It is now clear that presynaptic depression is the major physiological role for the CB1 receptor. Endocannabinoids activate this receptor mainly via a retrograde signaling process in which these compounds are synthesized in and released from postsynaptic neuronal elements, and travel back to the presynaptic terminal to act on the CB1 receptor. This retrograde endocannabinoid modulation has been implicated in short-term synaptic depression, including suppression of excitatory or inhibitory transmission induced by postsynaptic depolarization and transient synaptic depression induced by activation of postsynaptic GPCRs during agonist treatment or synaptic activation. Endocannabinoids and the CB1 receptor also play a key role in one form of long-term synaptic depression (LTD) that involves a longlasting decrease in neurotransmitter release.

Retrograde endocannabinoid signaling at striatal synapses requires a regulated postsynaptic release step

Endocannabinoids (eCBs) mediate short- and long-term depression of synaptic strength by retrograde transsynaptic signaling. Previous studies have suggested that an eCB mobilization or release step in the postsynaptic neuron is involved in this retrograde signaling. However, it is not known whether this release process occurs automatically upon eCB synthesis or whether it is regulated by other synaptic factors. To address this issue, we loaded postsynaptic striatal medium spiny neurons (MSNs) with the eCBs anandamide (AEA) or 2-arachidonoylglycerol and determined the conditions necessary for presynaptic inhibition. We found that presynaptic depression of glutamatergic excitatory postsynaptic currents (EPSCs) and GABAergic inhibitory postsynaptic currents (IPSCs) induced by postsynaptic eCB loading required a certain level of afferent activation that varied between the different synaptic types. Synaptic depression at excitatory synapses was temperature-dependent and blocked by the eCB membrane transport blockers, VDM11 and UCM707, but did not require activation of metabotropic glutamate receptors, l-calcium channels, nitric oxide, voltage-activated Na(+) channels, or intracellular calcium. Application of the CB(1)R antagonist, AM251, after depression was established, reversed the decrease in EPSC, but not in IPSC, amplitude. Direct activation of the CB(1) receptor by WIN 55,212-2 initiated synaptic depression that was independent of afferent stimulation. These findings indicate that retrograde eCB signaling requires a postsynaptic release step involving a transporter or carrier that is activated by afferent stimulation/synaptic activation.

Endocannabinoid signaling is critical for habit formation

Extended training can induce a shift in behavioral control from goal-directed actions, which are governed by action-outcome contingencies and sensitive to changes in the expected value of the outcome, to habits which are less dependent on action-outcome relations and insensitive to changes in outcome value. Previous studies in rats have shown that interval schedules of reinforcement favor habit formation while ratio schedules favor goal-directed behavior. However, the molecular mechanisms underlying habit formation are not well understood. Endocannabinoids, which can function as retrograde messengers acting through presynaptic CB1 receptors, are highly expressed in the dorsolateral striatum, a key region involved in habit formation. Using a reversible devaluation paradigm, we confirmed that in mice random interval schedules also favor habit formation compared with random ratio schedules. We also found that training with interval schedules resulted in a preference for exploration of a novel lever, whereas training with ratio schedules resulted in less generalization and more exploitation of the reinforced lever. Furthermore, mice carrying either a heterozygous or a homozygous null mutation of the cannabinoid receptor type I (CB1) showed reduced habit formation and enhanced exploitation. The impaired habit formation in CB1 mutant mice cannot be attributed to chronic developmental or behavioral abnormalities because pharmacological blockade of CB1 receptors specifically during training also impairs habit formation. Taken together our data suggest that endocannabinoid signaling is critical for habit formation.

Corticostriatal synaptic function in mouse models of Huntington's disease: early effects of huntingtin repeat length and protein load

Huntington's disease (HD) is an autosomal dominant, late onset, neurodegenerative disease characterized by motor deficits and dementia that is caused by expansion of a CAG repeat in the HD gene. Clinical manifestations result from selective neuronal degeneration of predominantly GABAergic striatal medium-sized spiny neurons (MSNs). A growing number of studies demonstrate that personality, mood and cognitive disturbances are some of the earliest signs of HD and may reflect synaptic dysfunction prior to neuronal loss. Previous studies in striatal MSNs demonstrated early alterations in NMDA-type glutamate receptor currents in several HD mouse models, as well as evidence for presynaptic dysfunction prior to disease manifestations in the R6/2 HD fragment mouse model. We have compared corticostriatal synaptic function in full-length, human HD gene-carrying YAC transgenic mice expressing a non-pathogenic CAG repeat (YAC18; control) with three increasingly severe variants of pathogenic HD gene-expressing mice (YAC72 and two different lines of YAC128), at ages that precede any detectable disease phenotype. We report presynaptic dysfunction and a propensity towards synaptic depression in YAC72 and YAC128 compared to YAC18 mice, and, in the most severe model, we also observed altered AMPA receptor function. When normalized to evoked AMPAR currents, postsynaptic NMDAR currents are augmented in all three pathogenic HD YAC variants. These findings demonstrate multiple perturbations to corticostriatal synaptic function in HD mice, furthering our understanding of the early effects of the HD mutation that may contribute to cognitive dysfunction, mood disorders and later development of more serious dysfunction. Furthermore, this study provides a set of neurophysiological sequelae against which to test and compare other mouse models and potential therapies in HD.

Relating mode of action to clinical practice: dopaminergic agents in Parkinson's disease

Most treatment advances in PD have been based on restoring dopaminergic input. The development of levodopa was the first breakthrough and, since then, other compounds have been developed. Each antiparkinsonian medication has its own profile of efficacy and adverse effects, and these can largely be explained by their modes of action. As patients receive a number of different compounds, physicians should be aware of the differences of agents and understand how these differences may relate to clinical practice. This article reviews the three main classes of dopaminergic PD therapy (levodopa, monoamine oxidase inhibitors and dopamine agonists).

Shift in induction mechanisms underlies an age-dependent increase in DHPG-induced synaptic depression at CA3 CA1 synapses

Several forms of log-term synaptic plasticity have been identified and the mechanisms for induction and expression of synaptic modifications change over development and maturation. The present study examines age-related changes in the induction of group I metabotropic receptor selective agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) induced long-term synaptic depression (DHPG-LTD) at CA3-CA1 synapses. The results demonstrate that the magnitude of DHPG-LTD is enhanced in male aged Fischer 344 rats compared with young adults. The role of mGluR1 in the induction of DHPG-LTD was increased with advanced age and, in contrast to young adults, induction involved a significant contribution of NMDA receptors and L-type Ca(2+) channels. Moreover, the protein tyrosine phosphatase inhibitor sodium orthovanadate significantly attenuated DHPG-LTD only in young adults. The expression of DHPG-LTD in aged animals was dependent on protein synthesis and the enhanced expression was associated with an increase in paired-pulse facilitation. The results provide evidence that DHPG-LTD is one of the few forms of synaptic plasticity that increases with advanced age and suggest that DHPG-LTD may contribute to age-related changes in hippocampal function.

Long-term depression of cortico-striatal synaptic transmission by DHPG depends on endocannabinoid release and nitric oxide synthesis

In models of early stage Parkinson's disease (PD), motor deficits are accompanied by excessive activation of striatal glutamate receptors. Metabotropic glutamate group I receptors (mGluR I) play an important but not well-understood role in PD progression. In mouse brain slices, bath application of the mGluR I agonist (RS)-DHPG (3,5-dihydroxyphenylglycine, 100 microm for 20 min) caused a long-term depression of corticostriatal transmission (LTD(DHPG)), which was reversed by three mGluR I antagonists: LY 367385, CPCCOEt and MPEP. LTD(DHPG) required nitric oxide (NO) synthesis as it was blocked by the broad-spectrum NO synthase (NOS) inhibitor Nomega-nitro-l-arginine (NL-Arg) and impaired under blockade of neuronal NOS and in endothelial NOS-deficient mice. Release of endocannabinoids (eCB) was critically involved in this form of striatal plasticity givem that the CB1 receptor antagonist AM251 prevented LTD(DHPG), while the CB1 agonist ACEA elicited LTD. The NO synthesis necessary for LTD(DHPG) induction occurred downstream of CB1 activation as ACEA-evoked LTD was also abolished by NL-Arg. These findings are relevant for the pathophysiology of PD, as they link the overactivation of group I mGluRs and striatal NO production.

Timing and location of synaptic inputs determine modes of subthreshold integration in striatal medium spiny neurons

Medium spiny neurons (MSNs) are the principal cells of the striatum and perform a central role in sensorimotor processing. MSNs must integrate many excitatory inputs located across their dendrites to fire action potentials and enable striatal function. However, the dependence of synaptic responses on the temporal and spatial distribution of these inputs remains unknown. Here, we use whole-cell recordings, two-photon microscopy, and two-photon glutamate uncaging to examine subthreshold synaptic integration in MSNs from acute rat brain slices. We find that synaptic responses can summate sublinearly, linearly, or supralinearly depending on the spatiotemporal pattern of activity. Repetitive activity at single inputs leads to sublinear summation, reflecting long-lived AMPA receptor desensitization. In contrast, asynchronous activity at multiple inputs generates linear summation, with synapses on neighboring spines functioning independently. Finally, synchronous activity at multiple inputs triggers supralinear summation at depolarized potentials, reflecting activation of NMDA receptors and L-type calcium channels. Thus, the properties of subthreshold integration in MSNs are determined by the distribution of synaptic inputs and the differential activation of multiple postsynaptic conductances.

Ethanol reverses the direction of long-term synaptic plasticity in the dorsomedial striatum

The striatum is a critical structure for the control of voluntary behaviour, and striatal synaptic plasticity has been implicated in instrumental learning. As ethanol consumption can cause impairments in cognition, learning, and action selection, it is important to understand the effects of this drug on striatal function. In this study we examined the effects of ethanol on long-term synaptic plasticity in the dorsomedial striatum (DMS), a striatal subregion that plays a central role in the acquisition and selection of goal-directed actions. Ethanol was found to impair N-methyl-d-aspartic acid receptor (NMDAR)-dependent long-term potentiation (LTP) dose-dependently in the DMS, and to promote long-term depression (LTD) at the highest concentration (50 mm) used. These results suggest that ethanol, at a concentration usually associated with mild intoxication, could significantly change experience-dependent modification of corticostriatal circuits underlying the learning of goal-directed instrumental actions.

Molecular components and functions of the endocannabinoid system in mouse prefrontal cortex

Background

Cannabinoids have deleterious effects on prefrontal cortex (PFC)-mediated functions and multiple evidences link the endogenous cannabinoid (endocannabinoid) system, cannabis use and schizophrenia, a disease in which PFC functions are altered. Nonetheless, the molecular composition and the physiological functions of the endocannabinoid system in the PFC are unknown.

Methodology/Principal Findings

Here, using electron microscopy we found that key proteins involved in endocannabinoid signaling are expressed in layers V/VI of the mouse prelimbic area of the PFC: presynaptic cannabinoid CB1 receptors (CB1R) faced postsynaptic mGluR5 while diacylglycerol lipase α (DGL-α), the enzyme generating the endocannabinoid 2-arachidonoyl-glycerol (2-AG) was expressed in the same dendritic processes as mGluR5. Activation of presynaptic CB1R strongly inhibited evoked excitatory post-synaptic currents. Prolonged synaptic stimulation at 10Hz induced a profound long-term depression (LTD) of layers V/VI excitatory inputs. The endocannabinoid -LTD was presynaptically expressed and depended on the activation of postsynaptic mGluR5, phospholipase C and a rise in postsynaptic Ca²⁺ as predicted from the localization of the different components of the endocannabinoid system. Blocking the degradation of 2-AG (with URB 602) but not of anandamide (with URB 597) converted subthreshold tetanus to LTD-inducing ones. Moreover, inhibiting the synthesis of 2-AG with Tetrahydrolipstatin, blocked endocannabinoid-mediated LTD. All together, our data show that 2-AG mediates LTD at these synapses.

Conclusions/Significance

Our data show that the endocannabinoid -retrograde signaling plays a prominent role in long-term synaptic plasticity at the excitatory synapses of the PFC. Alterations of endocannabinoid -mediated synaptic plasticity may participate to the etiology of PFC-related pathologies.

Endocannabinoid-mediated long-term plasticity requires cAMP/PKA signaling and RIM1alpha

Endocannabinoids (eCBs) have emerged as key activity-dependent signals that, by activating presynaptic cannabinoid receptors (i.e., CB1) coupled to G(i/o) protein, can mediate short-term and long-term synaptic depression (LTD). While the presynaptic mechanisms underlying eCB-dependent short-term depression have been identified, the molecular events linking CB1 receptors to LTD are unknown. Here we show in the hippocampus that long-term, but not short-term, eCB-dependent depression of inhibitory transmission requires presynaptic cAMP/PKA signaling. We further identify the active zone protein RIM1alpha as a key mediator of both CB1 receptor effects on the release machinery and eCB-dependent LTD in the hippocampus. Moreover, we show that eCB-dependent LTD in the amygdala and hippocampus shares major mechanistic features. These findings reveal the signaling pathway by which CB1 receptors mediate long-term effects of eCBs in two crucial brain structures. Furthermore, our results highlight a conserved mechanism of presynaptic plasticity in the brain.

Group I mGluRs and long-term depression: potential roles in addiction?

Addiction is an enormous societal problem. A number of recent studies have focused on adaptations at glutamatergic synapses that may play a role in the behavioral responses to drugs of abuse. These studies have largely focused on NMDA receptor-dependent forms of synaptic plasticity such as NMDA receptor-dependent long-term potentiation (LTP) and long-term depression (LTD). A growing body of evidence, however, suggests that metabotropic glutamate receptors (mGluRs) also play important roles in the behavioral responses to drugs of abuse and participate in producing synaptic plasticity at glutamate synapses. In this review, we focus first on the evidence supporting a role for mGluRs in addiction and then on the properties of mGluR-dependent forms of synaptic plasticity, focusing in particular on Gq-linked receptor-induced LTD.

Inhibition of striatal dopamine release by CB1 receptor activation requires nonsynaptic communication involving GABA, H2O2, and KATP channels

The main psychoactive component of marijuana, Delta9-tetrahydrocannabinol (THC), acts in the CNS via type 1 cannabinoid receptors (CB1Rs). The behavioral consequences of THC or synthetic CB1R agonists include suppression of motor activity. One explanation for movement suppression might be inhibition of striatal dopamine (DA) release by CB1Rs, which are densely localized in motor striatum; however, data from previous studies are inconclusive. Here we examined the effect of CB1R activation on locally evoked DA release monitored with carbon-fiber microelectrodes and fast-scan cyclic voltammetry in striatal slices. Consistent with previous reports, DA release evoked by a single stimulus pulse was unaffected by WIN55,212-2, a cannabinoid receptor agonist. However, when DA release was evoked by a train of stimuli, WIN55,212-2 caused a significant decrease in evoked extracellular DA concentration ([DA]o), implicating the involvement of local striatal circuitry, with similar suppression seen in guinea pig, rat, and mouse striatum. Pulse-train evoked [DA]o was not altered by either AM251, an inverse CB1R agonist, or VCHSR1, a neutral antagonist, indicating the absence of DA release regulation by endogenous cannabinoids with the stimulation protocol used. However, both CB1R antagonists prevented and reversed suppression of evoked [DA]o by WIN55,212-2. The effect of WIN55,212-2 was also prevented by picrotoxin, a GABAA receptor antagonist, and by catalase, a metabolizing enzyme for hydrogen peroxide (H2O2). Furthermore, blockade of ATP-sensitive K+ (KATP) channels by tolbutamide or glybenclamide prevented the effect of WIN55,212-2 on DA release. Together, these data indicate that suppression of DA release by CB1R activation within striatum occurs via a novel nonsynaptic mechanism that involves GABA release inhibition, increased generation of the diffusible messenger H2O2, and activation of KATP channels to inhibit DA release. In addition, the findings suggest a possible physiological substrate for the motor effects of cannabinoid agonist administration.

Combined activation of L-type Ca2+ channels and synaptic transmission is sufficient to induce striatal long-term depression

Changes in synaptic strength at striatal synapses, such as long-term depression (LTD), may be involved in striatal-based learning and memory. Several molecular mechanisms have been implicated in striatal LTD, but it is not clear which mechanisms are crucial for LTD induction. We found that the activation of L-type calcium channels by 2,5-dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylic acid methylester (FPL64176), combined with modest postsynaptic depolarization and synaptic activation, is sufficient to induce robust LTD (FPL-LTD). The L-channel activator 1,4-dihydro-2,6-dimethyl-5-nitro-4-[2(trifluoromethyl)phenyl]pyridine-3-carboxylic acid methyl ester (Bay K 8644) has a similar action. FPL-LTD occludes LTD induced by high-frequency stimulation (HFS-LTD) and requires elevated postsynaptic calcium and retrograde endocannabinoid signaling, properties similar to those of HFS-LTD. In contrast, FPL-LTD does not require the activation of metabotropic glutamate receptors (mGluRs), phospholipase C, or dopamine D2 receptors. FPL-LTD induction also requires afferent stimulation. These findings suggest a scenario in which L-type calcium channel activation is a crucial switch for LTD induction, and mGluRs and D2 receptors can be bypassed if this channel is activated.

Input-specific plasticity at excitatory synapses mediated by endocannabinoids in the dentate gyrus

Endocannabinoids (eCBs) mediate transient and long-lasting synaptic plasticity in several brain structures. In the dentate gyrus, activation of the type 1 cannabinoid receptor (CB1R) by exogenous ligands reportedly depresses excitatory synaptic transmission. However, direct evidence of eCB signaling at excitatory synapses in this region has been lacking. Here, we demonstrate that eCB release can be induced by a brief postsynaptic depolarization of dentate granule cells (DGCs), which potently and transiently suppresses glutamatergic inputs from mossy cell interneurons (MCs) but not from entorhinal cortex via the lateral and medial perforant paths. This input-specific depolarization-induced suppression of excitation (DSE) is calcium-dependent and can be modulated by agonists of cholinergic and group I metabotropic glutamate receptors. Inhibiting the synthesis of 2-arachidonoyl glycerol (2-AG), one of the most abundant eCBs in the brain, by diacyglycerol lipase (DGL) does not abolish DSE. Moreover, preventing the breakdown of anandamide, the other main eCB, does not potentiate DSE. Thus, eCB signaling underlying DSE in the dentate does not require DGL activity and is unlikely to be mediated by anandamide. Finally, we find that manipulations known to induce eCB-LTD at other central synapses do not trigger LTD at MCF-DGC synapses.

Corticolimbic expression of TRPC4 and TRPC5 channels in the rodent brain

The canonical transient receptor potential (TRPC) channels are a family of non-selective cation channels that are activated by increases in intracellular Ca(2+) and G(q)/phospholipase C-coupled receptors. We used quantitative real-time PCR, in situ hybridization, immunoblots and patch-clamp recording from several brain regions to examine the expression of the predominant TRPC channels in the rodent brain. Quantitative real-time PCR of the seven TRPC channels in the rodent brain revealed that TRPC4 and TRPC5 channels were the predominant TRPC subtypes in the adult rat brain. In situ hybridization histochemistry and immunoblotting further resolved a dense corticolimbic expression of the TRPC4 and TRPC5 channels. Total protein expression of HIP TRPC4 and 5 proteins increased throughout development and peaked late in adulthood (6-9 weeks). In adults, TRPC4 expression was high throughout the frontal cortex, lateral septum (LS), pyramidal cell layer of the hippocampus (HIP), dentate gyrus (DG), and ventral subiculum (vSUB). TRPC5 was highly expressed in the frontal cortex, pyramidal cell layer of the HIP, DG, and hypothalamus. Detailed examination of frontal cortical layer mRNA expression indicated TRPC4 mRNA is distributed throughout layers 2-6 of the prefrontal cortex (PFC), motor cortex (MCx), and somatosensory cortex (SCx). TRPC5 mRNA expression was concentrated specifically in the deep layers 5/6 and superficial layers 2/3 of the PFC and anterior cingulate. Patch-clamp recording indicated a strong metabotropic glutamate-activated cation current-mediated depolarization that was dependent on intracellular Ca(2+)and inhibited by protein kinase C in brain regions associated with dense TRPC4 or 5 expression and absent in regions lacking TRPC4 and 5 expression. Overall, the dense corticolimbic expression pattern suggests that these Gq/PLC coupled nonselective cation channels may be involved in learning, memory, and goal-directed behaviors.

Mechanisms for synapse specificity during striatal long-term depression

Endocannabinoid (eCB)-mediated forms of long-term synaptic plasticity occur in several brain regions, but much remains unknown about their basic properties and underlying mechanisms. Here, we present evidence that eCB-mediated long-term depression (eCB-LTD) at excitatory synapses on medium spiny neurons in the striatum requires presynaptic activity coincident with CB1 receptor activation. This dual requirement for CB1 activation and presynaptic activity is a mechanism by which eCB-LTD may be made synapse specific.

Neurotensin reduces glutamatergic transmission in the dorsolateral striatum via retrograde endocannabinoid signaling

Neurotensin is a peptide that has been suggested to mimic the actions of antipsychotics, but little is known about how it affects synaptic transmission in the striatum, the major input nucleus of the basal ganglia. In this study we measured the effects of neurotensin on EPSCs from medium spiny projection neurons in the sensorimotor striatum, a region implicated in habit formation and control of motor sequences. We found that bath-applied neurotensin reduced glutamate release from presynaptic terminals, and that this effect required retrograde endocannabinoid signaling, as it was prevented by the CB1 cannabinoid receptor antagonist AM251. Neurotensin-mediated inhibition of striatal EPSCs was also blocked by antagonists of D2-like dopamine receptors and group I metabotropic glutamate receptors, as well as by intracellular calcium chelation and phospholipase C inhibition. These results suggest that neurotensin can indirectly engage an endocannabinoid-mediated negative feedback signal to control glutamatergic input to the basal ganglia.

Endocannabinoid Liberation from Neurons in Transsynaptic Signaling

Endocannabinoids are fatty acid derivatives that have a variety of biological actions, most notably via activation of the cannabinoid receptors. These receptors are also targets for drugs derived from Cannabis sativa. In the nervous system, endocannabinoids act as neuromodulators that depress neurotransmitter release at the presynaptic terminal. In most instances of neural endocannabinoid signaling, the compounds appear to be released from the postsynaptic neuron to act on the presynaptic terminal in a "retrograde" manner. Several common mechanisms involved in postsynaptic endocannabinoid production and presynaptic depression produced via activation of the CB1 cannabinoid receptor have been identified. However, significant problems remain in defining the mechanisms underlying endocannabinoid production, release, and movement across the membrane. These issues are discussed in the present review.

Altered corticostriatal neurotransmission and modulation in dopamine transporter knock-down mice

Dopamine (DA) modulates glutamate neurotransmission in the striatum. Abnormal DA modulation has been implicated in neurological and psychiatric disorders. The development of DA transporter knock-down (DAT-KD) mice has permitted modeling of these disorders and has shed new light on DA modulation. DAT-KD mice exhibit increased extracellular DA, hyperactivity, and alterations in habituation. We used whole cell patch-clamp recordings from visually identified striatal neurons in slices to examine the effects of DAT-KD on corticostriatal transmission. Electrophysiological recordings from medium-sized spiny neurons in the dorsal striatum revealed alterations in both amplitude and frequency, of spontaneous glutamate receptor-mediated synaptic currents in cells from DAT-KD mice. Furthermore, kinetic analyses revealed that these currents had shorter half-amplitude durations and faster decay times. In contrast, GABA-receptor-mediated synaptic currents were not altered. Striatal neurons from DAT-KD mice also responded differently to amphetamine, cocaine, and DA D2-receptor agonists or antagonists compared with wildtype (WT) littermate controls. In WTs amphetamine and cocaine reduced the frequency of spontaneous glutamate currents and these effects appeared to be mediated by activation of D2 receptors. In contrast, in DAT-KD mice either no changes or only small increases in frequency occurred. D2-receptor agonists or antagonists also had opposing effects in WT and DAT-KD mice. Together, these results indicate that chronically increased extracellular DA produces long-lasting changes in corticostriatal communication that may be mediated by changes in D2-receptor function. These findings have implications for understanding mechanisms underlying attention deficit hyperactivity disorder and Tourette's syndrome and may provide insights into novel therapeutic approaches.

Role of the cyclic-AMP/PKA cascade and of P/Q-type Ca++ channels in endocannabinoid-mediated long-term depression in the nucleus accumbens

Glutamate transmission between prefrontal cortex (PFC) and accumbens (NAc) plays a crucial role in the establishment and expression of addictive behaviors. At these synapses exogenous cannabinoid receptor 1 (CB1R) agonists reversibly inhibit excitatory transmission, and the sustained release of endogenous cannabinoids (eCB) following prolonged cortical stimulation leads to long-term depression (LTD). Activation of presynaptic K(+) channels mediates the effects of exocannabinoids, but the transduction pathway underlying the protracted phase of eCB-LTD is unknown. Here we report that the maintenance of eCB-LTD does not involve presynaptic K(+) channels: eCB-LTD was not affected by blockade of K(+) channels with 4-AP (100 microM) and BaCl(2) (300 microM) (fEPSP=78.9+/-5.4% of baseline 58-60 min after tetanus, compared to 78.9+/-5.9% in control slices). In contrast, eCB-LTD was blocked by treatment of the slices with the adenylyl cyclase (AC) activator forskolin (10 microM), and with the protein kinase A (PKA) inhibitor KT5720 (1 microM) (fEPSP=108.9+/-5.7% in forskolin and 110.5+/-7.7% in KT5720, compared to 80.6+/-3.9% in control conditions). Additionally, selective blockade of P/Q-type Ca(2+) channels with omega-agatoxin-IVA (200 nM) occluded the expression of eCB-LTD (fEPSP=113.4+/-15.9% compared to 78.6+/-4.4% in control slices), while blockade of N- with omega-conotoxin-GVIA (1 microM) or L-type Ca(2+) channels with nimodipine (1 microM), was without effect (fEPSP was 83.7+/-5.3% and 87+/-8.9% respectively). These data show that protracted inhibition of AC/PKA activity and P/Q-type Ca(2+) channels are necessary for expression of eCB-LTD at NAc synapses.

Subcellular arrangement of molecules for 2-arachidonoyl-glycerol-mediated retrograde signaling and its physiological contribution to synaptic modulation in the striatum

Endogenous cannabinoids (endocannabinoids) mediate retrograde signals for short- and long-term suppression of transmitter release at synapses of striatal medium spiny (MS) neurons. An endocannabinoid, 2-arachidonoyl-glycerol (2-AG), is synthesized from diacylglycerol (DAG) after membrane depolarization and Gq-coupled receptor activation. To understand 2-AG-mediated retrograde signaling in the striatum, we determined precise subcellular distributions of the synthetic enzyme of 2-AG, DAG lipase-alpha (DAGLalpha), and its upstream metabotropic glutamate receptor 5 (mGluR5) and muscarinic acetylcholine receptor 1 (M1). DAGLalpha, mGluR5, and M1 were all richly distributed on the somatodendritic surface of MS neurons, but their subcellular distributions were different. Although mGluR5 and DAGLalpha levels were highest in spines and accumulated in the perisynaptic region, M1 level was lowest in spines and was rather excluded from the mGluR5-rich perisynaptic region. These subcellular arrangements suggest that mGluR5 and M1 might differentially affect endocannabinoid-mediated, depolarization-induced suppression of inhibition (DSI) and depolarization-induced suppression of excitation (DSE) in MS neurons. Indeed, mGluR5 activation enhanced both DSI and DSE, whereas M1 activation enhanced DSI only. Importantly, DSI, DSE, and receptor-driven endocannabinoid-mediated suppression were all abolished by the DAG lipase inhibitor tetrahydrolipstatin, indicating 2-AG as the major endocannabinoid mediating retrograde suppression at excitatory and inhibitory synapses of MS neurons. Accordingly, CB1 cannabinoid receptor, the main target of 2-AG, was present at high levels on GABAergic axon terminals of MS neurons and parvalbumin-positive interneurons and at low levels on excitatory corticostriatal afferents. Thus, endocannabinoid signaling molecules are arranged to modulate the excitability of the MS neuron effectively depending on cortical activity and cholinergic tone as measured by mGluR5 and M1 receptors, respectively.

D1 and D2 dopamine-receptor modulation of striatal glutamatergic signaling in striatal medium spiny neurons

Dopamine shapes a wide variety of psychomotor functions. This is mainly accomplished by modulating cortical and thalamic glutamatergic signals impinging upon principal medium spiny neurons (MSNs) of the striatum. Several lines of evidence suggest that dopamine D1 receptor signaling enhances dendritic excitability and glutamatergic signaling in striatonigral MSNs, whereas D2 receptor signaling exerts the opposite effect in striatopallidal MSNs. The functional antagonism between these two major striatal dopamine receptors extends to the regulation of synaptic plasticity. Recent studies, using transgenic mice in which cells express D1 and D2 receptors, have uncovered unappreciated differences between MSNs that shape glutamatergic signaling and the influence of DA on synaptic plasticity. These studies have also shown that long-term alterations in dopamine signaling produce profound and cell-type-specific reshaping of corticostriatal connectivity and function.

Dopamine-mediated regulation of corticostriatal synaptic plasticity

The striatum represents the main input into the basal ganglia. Neurons projecting from the striatum receive a large convergence of afferents from all areas of the cortex and transmit neural information to the basal ganglia output structures. Corticostriatal transmission is essential in the regulation of voluntary movement, in addition to behavioural control, cognitive function and reward mechanisms. Long-term potentiation (LTP) and long-term depression (LTD), the two main forms of synaptic plasticity, are both represented at corticostriatal synapses and strongly depend on the activation of dopamine receptors. Here, we discuss possible feedforward and feedback mechanisms by which striatal interneurons, in association with striatal spiny neurons and endogenous dopamine, influence the formation and maintenance of both LTP and LTD. We also propose a model in which the spontaneous membrane oscillations of neurons projecting from the striatum (named 'up' and 'down' states), in addition to the pattern of release of endogenous dopamine, bias the synapse towards preferential induction of LTP or LTD. Finally, we discuss how endogenous dopamine crucially influences changes in synaptic plasticity induced by pathological stimuli, such as energy deprivation.

Anandamide regulates postnatal development of long-term synaptic plasticity in the rat dorsolateral striatum

Long-term changes in synaptic efficacy produced by high-frequency stimulation (HFS) of glutamatergic afferents to the rat dorsolateral striatum exhibit heterogeneity during early stages of postnatal development. Whereas HFS most often induces striatal long-term potentiation (LTP) in rats postnatal day 12 (P12)-P14, the same stimulation tends to induce long-term depression (LTD) at ages P16-P34. Previous studies have shown that striatal LTD induction depends on retrograde endocannabinoid signaling and activation of the CB1 cannabinoid receptor. It is also known that levels of one of the primary endogenous CB1 receptor agonists, anandamide (AEA), increases during development in whole-brain samples. In the present study, we sought to determine whether this developmental increase in AEA also takes place in striatal tissue and whether increased AEA levels contribute to the postnatal switch in the response to HFS. We observed a pronounced increase in striatal levels of AEA, but not the other major endogenous cannabinoid 2-arachidonoylglycerol (2-AG), during the postnatal period characterized by the switch from LTP to LTD. Furthermore, application of synthetic AEA during HFS in field recordings of slices from P12-P14 rats allowed for induction of LTD whereas blocking the CB1 receptor during HFS in animals P16-P34 resulted in expression of LTP. However, blocking 2-AG synthesis with the DAG-lipase inhibitor tetrahydrolipstatin did not alter HFS-induced striatal LTD. In addition, synaptic depression produced by a synthetic CB1 agonist was similar across development. Together, these findings suggest that the robust developmental increase in striatal AEA may be the key factor in the emergence of HFS-induced striatal LTD.

Endocannabinoid-mediated rescue of striatal LTD and motor deficits in Parkinson's disease models

The striatum is a major forebrain nucleus that integrates cortical and thalamic afferents and forms the input nucleus of the basal ganglia. Striatal projection neurons target the substantia nigra pars reticulata (direct pathway) or the lateral globus pallidus (indirect pathway). Imbalances between neural activity in these two pathways have been proposed to underlie the profound motor deficits observed in Parkinson's disease and Huntington's disease. However, little is known about differences in cellular and synaptic properties in these circuits. Indeed, current hypotheses suggest that these cells express similar forms of synaptic plasticity. Here we show that excitatory synapses onto indirect-pathway medium spiny neurons (MSNs) exhibit higher release probability and larger N-methyl-d-aspartate receptor currents than direct-pathway synapses. Moreover, indirect-pathway MSNs selectively express endocannabinoid-mediated long-term depression (eCB-LTD), which requires dopamine D2 receptor activation. In models of Parkinson's disease, indirect-pathway eCB-LTD is absent but is rescued by a D2 receptor agonist or inhibitors of endocannabinoid degradation. Administration of these drugs together in vivo reduces parkinsonian motor deficits, suggesting that endocannabinoid-mediated depression of indirect-pathway synapses has a critical role in the control of movement. These findings have implications for understanding the normal functions of the basal ganglia, and also suggest approaches for the development of therapeutic drugs for the treatment of striatal-based brain disorders.

Motor Deficits in Homozygous and Heterozygous P/Q-Type Calcium Channel Mutants

P/Q-type voltage-dependent Ca2+channels (VDCCs) are highly expressed in the cerebellum, and mutations of these channels are associated with disrupted motor function. Several allelic variants of the α1A pore-forming subunit of P/Q-type VDCCs have been described, and mice homozygous for these mutations exhibit gait ataxia, as do α1A knockout mice. Here we report that heterozygous α1A mutants also have a motor phenotype. Mice heterozygous for the leaner and α1A knockout mutations exhibit impaired motor learning in the vestibulo-ocular reflex (VOR), suggesting that subtle disruption of P/Q Ca2+currents is sufficient to disrupt motor function. Basal VOR and optokinetic reflex performance were normal in the heterozygotes but severely impaired in the leaner and α1A knockout homozygotes.

Synaptic activation of dendritic AMPA and NMDA receptors generates transient high-frequency firing in substantia nigra dopamine neurons in vitro

Transient high-frequency activity of substantia nigra dopamine neurons is critical for striatal synaptic plasticity and associative learning. However, the mechanisms underlying this mode of activity are poorly understood because, in contrast to other rapidly firing neurons, high-frequency activity is not evoked by somatic current injection. Previous studies have suggested that activation of dendritic N-methyl-d-aspartate (NMDA) receptors and/or G-protein-coupled receptor (GPCR)-mediated reduction of action potential afterhyperpolarization and/or activation of cation channels underlie high-frequency activity. To address their relative contribution, transient high-frequency activity was evoked using local electrical stimulation (1 s, 10-100 Hz) in brain slices prepared from p15-p25 rats in the presence of GABA and D2 dopamine receptor antagonists. The frequency, pattern, and morphology of action potentials evoked under these conditions were similar to those observed in vivo. Evoked activity and reductions in action potential afterhyperpolarization were diminished greatly by application of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or NMDA receptor selective antagonists and abolished completely by co-application of AMPA and NMDA antagonists. In contrast, application of glutamatergic and cholinergic GPCR antagonists moderately enhanced evoked activity. Dendritic pressure-pulse application of glutamate evoked high-frequency activity that was similarly sensitive to antagonism of AMPA or NMDA receptors. Taken together, these data suggest that dendritic AMPA and NMDA receptor-mediated synaptic conductances are sufficient to generate transient high-frequency activity in substantia nigra dopamine neurons by rapidly but transiently overwhelming the conductances underlying action potential afterhyperpolarization and/or engaging postsynaptic voltage-dependent ion channels in a manner that overcomes the limiting effects of afterhyperpolarization.

The role of protein synthesis in striatal long-term depression

Long-term depression (LTD) at the corticostriatal synapse is postsynaptically induced but presynaptically expressed, the depression being a result of retrograde endocannabinoid signaling that activates presynaptic cannabinoid CB1 receptors and reduces the probability of glutamate release. To study the role of protein synthesis in striatal LTD, we used a striatum-only preparation in which the presynaptic cell body is cut off, leaving intact only its axons, whose terminals synapse on medium spiny neurons. LTD (duration >150 min) was induced in this preparation, thus providing evidence that transcription in the presynaptic cell nucleus is not necessary for this form of plasticity. The maintenance of striatal LTD, however, was blocked by bath application of protein translation inhibitors but not by the same inhibitors loaded into the postsynaptic cell. These results suggest that local translation is critical for the expression of striatal LTD, distinguishing this form of mammalian synaptic plasticity from other forms that require postsynaptic protein synthesis. Possible roles of axonal or glial translation in striatal LTD are considered.