Regional and postnatal heterogeneity of activity-dependent long-term changes in synaptic efficacy in the dorsal 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.

Forskolin Enhances Synaptic Transmission in Rat Dorsal Striatum through NMDA Receptors and PKA in Different Phases

The effect of forskolin on corticostriatal synaptic transmission was examined by recording excitatory postsynaptic currents (EPSCs) in rat brain slices using the whole-cell voltage-clamp technique. Forskolin produced a dose-dependent increase of corticostriatal EPSCs (1, 3, 10, and 30 microM) immediately after its treatment, and the increase at 10 and 30 microM was maintained even after its washout. When the brain slices were pre-treated with (DL)-2-amino-5-phosphonovaleric acid (AP-V, 100 microM), an NMDA receptor antagonist, the acute effect of forskolin (10 microM) was blocked. However, after washout of forskolin, an increase of corticostriatal EPSCs was still observed even in the presence of AP-V. When KT 5720 (5 microM), a protein kinase A (PKA) inhibitor, was applied through the patch pipette, forskolin (10 microM) increased corticostriatal EPSCs, but this increase was not maintained. When forskolin was applied together with AP-V and KT 5720, both the increase and maintenance of the corticostriatal EPSCs were blocked. These results suggest that forskolin activates both NMDA receptors and PKA, however, in a different manner.

Extracellular recordings of rodents in vivo: their contribution to integrative neuroscience

The prevalent theory in learning and memory processes is that they are underlain by short and long-term changes in synaptic weight, which continuously modulates neural networks during acquisition and recall. This synaptic plasticity has been revealed by recording extracellular field potentials. The enhancement of synaptic transmission was primarily noted in the hippocampus and was named long-term potentiation (LTP). The opposite mechanism, long-term depression (LTD), a reduction of synaptic transmission, was first discovered in the cerebellum. Since then, the LTP-model has been studied mainly using in vitro and acute anesthetized in vivo preparations. This approach has led to remarkable progress in the comprehension of intracellular molecular processes during LTP and LTD. In this review, we focus mainly on what we can learn about molecular events using extracellular field potential recordings with a more ecological model, i.e., studies using the freely behaving animal, with animals that are genetically modified or not, in several behavioral paradigms aimed at gaining insight into some of the conflicting results obtained with in vitro and in vivo preparations.

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.

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.

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.

Decreased striatal dopamine release underlies increased expression of long-term synaptic potentiation at corticostriatal synapses 24 h after 3-nitropropionic-acid-induced chemical hypoxia

The striatum is particularly sensitive to the irreversible inhibitor of succinate dehydrogenase 3-nitropropionic acid (3-NP). In the present study, we examined early changes in behavior and dopamine and glutamate synaptic physiology created by a single systemic injection of 3-NP in Fischer 344 rats. Hindlimb dystonia was seen 2 h after 3-NP injections, and rats performed poorly on balance beam and rotarod motor tests 24 h later. Systemic 3-NP increased NMDA receptor-dependent long-term potentiation (LTP) at corticostriatal synapses over the same time period. The 3-NP-induced corticostriatal LTP was not attributable to increased NMDA receptor number or function, because 3-NP did not change MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine] binding or NMDA/AMPA receptor current ratios. The LTP seen 24 h after 3-NP was D(1) receptor dependent and reversed by exogenous addition of dopamine or a D(2) receptor agonist to brain slices. HPLC and fast-scan cyclic voltammetry revealed a decrease in dopamine content and release in rats injected 24 h earlier with 3-NP, and much like the enhanced LTP, dopamine changes were reversed by 48 h. Tyrosine hydroxylase expression was not changed, and there was no evidence of striatal cell loss at 24-48 h after 3-NP exposure. Sprague Dawley rats showed similar physiological responses to systemic 3-NP, albeit with reduced sensitivity. Thus, 3-NP causes significant changes in motor behavior marked by parallel changes in striatal dopamine release and corticostriatal synaptic plasticity.

Spontaneous neuronal burst discharges as dependent and independent variables in the maturation of cerebral cortex tissue cultured in vitro: a review of activity-dependent studies in live 'model' systems for the development of intrinsically generated bioelectric slow-wave sleep patterns

A survey is presented of recent experiments which utilize spontaneous neuronal spike trains as dependent and/or independent variables in developing cerebral cortex cultures when synaptic transmission is interfered with for varying periods of time. Special attention is given to current difficulties in selecting suitable preparations for carrying out biologically relevant developmental studies, and in applying spike-train analysis methods with sufficient resolution to detect activity-dependent age and treatment effects. A hierarchy of synchronized nested burst discharges which approximate early slow-wave sleep patterns in the intact organism is established as a stable basis for isolated cortex function. The complexity of reported long- and short-term homeostatic responses to experimental interference with synaptic transmission is reviewed, and the crucial role played by intrinsically generated bioelectric activity in the maturation of cortical networks is emphasized.

Impaired instrumental choice in crib-biting horses (Equus caballus)

Horses displaying an oral stereotypy were tested on an instrumental choice paradigm to examine differences in learning from non-stereotypic counterparts. Stereotypic horses are known to have dysfunction of the dorsomedial striatum, and lesion studies have shown that this region may mediate response-outcome learning. The paradigm was specifically applied in order to examine learning that requires maintenance of response-outcome judgements. The non-stereotypic horses learned, over three sessions, to choose a more immediate reinforcer, whereas the stereotypic horses failed to do so. This suggests an initial behavioural correlate for dorsomedial striatum dysregulation in the stereotypy phenotype.

Dopamine D1 receptors and group I metabotropic glutamate receptors contribute to the induction of long-term potentiation in the nucleus accumbens

Long-term changes in the efficacy of glutamatergic synaptic transmission in the striatal complex are proposed to underlie motor learning and neuroadaptations leading to addiction. Dopamine and glutamate play key roles in the induction of long-term potentiation (LTP) and long-term depression (LTD) in the dorsal striatum, but their contribution to synaptic plasticity in the ventral striatum (nucleus accumbens, NAc) has been less extensively studied. We have examined the role of dopamine, glutamate and GABA in the induction of LTP in mouse brain slices containing the NAc. High-frequency stimulation of glutamatergic inputs elicited LTP of field excitatory postsynaptic potentials/population spikes (fEPSP/PSs) in the core region of the NAc. GABA did not seem to participate in LTP induction because LTP was not altered in the presence of either a GABA(A)- (bicuculline) or a GABA(B)- (CGP 55845) receptor antagonist. However, the dopamine D1 receptor antagonist SCH 23390, but not the dopamine D2 receptor antagonist sulpiride, impaired LTP. The dopamine reuptake blocker nomifensine also inhibited LTP induction. We found that group I metabotropic glutamate receptors (mGluRs) contribute to LTP induction because the mGluR1 antagonist LY 367385, or the mGluR5 antagonist MPEP, blocked LTP induction. Furthermore, the glutamate reuptake blocker DL-TBOA also impaired LTP. The present results demonstrate that dopamine and glutamate play critical roles in the mechanisms of induction of LTP in the NAc through the activation of dopamine D1 receptors and group I mGluRs. However, LTP is negatively regulated when endogenous levels of dopamine or glutamate are elevated.

Long-Lasting NMDA Receptor-Mediated EPSCs in Mouse Striatal Medium Spiny Neurons

Excitatory postsynaptic currents (EPSCs) from dorsolateral medium spiny neurons (MSNs) were recorded in cortico-striatal slice preparations from postnatal day 6–8 (P6-8) and >P12 wild-type mice and mice that were lacking either the NR2A or the NR2C subunit of the N-methyl-d-aspartate (NMDA) receptor. EPSCs were elicited by stimulation of the excitatory afferents and the NMDA and non-NMDA receptor-mediated components were pharmacologically isolated. The ratio of these components decreased with development and was significantly reduced only between age-matched +/+ and NR2A −/− neurons. In many MSNs, the NMDA-EPSC decay was characterized by the presence of a slow exponential component with a time constant lasting >1 s regardless of genotype or age. In the NR2A −/−, no developmental increase in the decay time (Tw) of the NMDA-EPSCs was observed although it was almost twofold longer than in +/+ MSNs. NR1/NR2B antagonists were ineffective in reducing the slow NMDA-EPSCs at all ages. Input-output studies revealed differences in stimulation threshold sensitivity of MSNs based on stimulus location. High-threshold responders were preferentially identified with stimulation from intracortical locations that produced considerably faster NMDA-EPSCs, whereas low-threshold responders were mainly elicited with stimulation more proximal to the striatum and exhibited slower NMDA-EPSCs. A low-affinity competitive antagonist of NMDA receptors failed to alter the decay of NMDA-EPSCs elicited from either location, suggesting that glutamate spillover is not responsible for the long-lasting NMDA-EPSCs. Our data are consistent with the expression of a unique NMDA receptor complex in MSNs with very slow deactivation kinetics.

The role of the dorsal striatum in reward and decision-making

Although the involvement in the striatum in the refinement and control of motor movement has long been recognized, recent description of discrete frontal corticobasal ganglia networks in a range of species has focused attention on the role particularly of the dorsal striatum in executive functions. Current evidence suggests that the dorsal striatum contributes directly to decision-making, especially to action selection and initiation, through the integration of sensorimotor, cognitive, and motivational/emotional information within specific corticostriatal circuits involving discrete regions of striatum. We review key evidence from recent studies in rodent, nonhuman primate, and human subjects.

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.

MPTP-induced deficits in striatal synaptic plasticity are prevented by glial cell line-derived neurotrophic factor expressed via an adeno-associated viral vector

This study determined the consequences of dopamine denervation of the striatum on synaptic plasticity and prevention of these changes with gene therapy using an adeno-associated viral vector (AAV) expressing glial cell line-derived neurotrophic factor (GDNF). C57BL6/J mice were injected with the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine(MPTP); long-term depression (LTD) or potentiation (LTP) were measured in vitro. Fast-scan cyclic voltammetry measured electrically released dopamine from a functionally relevant pool in these same striatal slices. After MPTP, dopamine release and uptake were greatly diminished, and LTP and LTD were blocked in the striatal slices. The loss of plasticity resulted directly from the loss of dopamine since its application rescued synaptic plasticity. Striatal GDNF expression via AAV, before MPTP, significantly protected against the loss of dopamine and prevented the blockade of corticostriatal LTP. These data demonstrate that dopamine plays a role in supporting several forms of striatal plasticity and that GDNF expression via AAV prevents the loss of dopamine and striatal plasticity caused by MPTP. We propose that impairment of striatal plasticity after dopamine denervation plays a role in the symptomology of Parkinson's disease and that AAV expression of neurotrophic factors represents a tenable approach to protecting against or slowing these neurobiological deficits.

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.

Spiking neurons, dopamine, and plasticity: timing is everything, but concentration also matters

While both dopamine (DA) fluctuations and spike-timing-dependent plasticity (STDP) are known to influence long-term corticostriatal plasticity, little attention has been devoted to the interaction between these two fundamental mechanisms. Here, a theoretical framework is proposed to account for experimental results specifying the role of presynaptic activation, postsynaptic activation, and concentrations of extracellular DA in synaptic plasticity. Our starting point was an explicitly-implemented multiplicative rule linking STDP to Michaelis-Menton equations that models the dynamics of extracellular DA fluctuations. This rule captures a wide range of results on conditions leading to long-term potentiation and depression in simulations that manipulate the frequency of induced corticostriatal stimulation and DA release. A well-documented biphasic function relating DA concentrations to synaptic plasticity emerges naturally from simulations involving a multiplicative rule linking DA and neural activity. This biphasic function is found consistently across different neural coding schemes employed (voltage-based vs. spike-based models). By comparison, an additive rule fails to capture these results. The proposed framework is the first to generate testable predictions on the dual influence of DA concentrations and STDP on long-term plasticity, suggesting a way in which the biphasic influence of DA concentrations can modulate the direction and magnitude of change induced by STDP, and raising the possibility that DA concentrations may inverse the LTP/LTD components of the STDP rule.

What are the mechanisms underlying the involvement of different subtypes of NMDA receptors in inducing long-term potentiation and depression in the hippocampus?

It has been hypothesized that the sign (direction) of modification of the efficiency of synaptic transmission depends on the subtype of N-methyl-D-aspartate (NMDA)-sensitive receptors involved. Activation of NMDA receptors with 2A subunits facilitates the induction of long-term potentiation, while receptors with 2B subunits facilitate the induction of long-term depression. However, experimental data have been obtained which contradict this hypothesis. We suggest an alternative hypothesis to explain currently available data. According to this hypothesis, activation of NMDA receptors containing different subunits can lead both to long-term potentiation and long-term depression, depending on the post-tetanic increase in the postsynaptic Ca(2+) concentration relative to the increase induced by preceding stimulation. Activation of NMDA receptors containing 2B subunits can lead to long-term depression of the excitatory input to pyramidal neurons because of the presence of these receptors on inhibitory interneurons, with induction of long-term potentiation on the interneuron and potentiation of inhibitory transmission between interneurons and pyramidal cells.

Dopamine-dependent long term potentiation in the dorsal striatum is reduced in the R6/2 mouse model of Huntington's disease

The striatum is critically important in motor, cognitive and emotional functions, as highlighted in neurological disorders such as Huntington's disease (HD) where these functions are compromised. The R6/2 mouse model of HD shows progressive motor and cognitive impairments and alterations in striatal dopamine and glutamate release. To determine whether or not dopamine-dependent neuronal plasticity is also altered in the dorsolateral striatum of R6/2 mice, we compared long term potentiation (LTP) and long term depression (LTD) in striatal slices from R6/2 mice with that seen in slices from wild type (WT) mice. In adult WT mice (aged 8-19 weeks), frequency-dependent bidirectional plasticity was observed. High frequency stimulation (four 0.5 s trains at 100 Hz, inter-train interval 10 s) induced LTP (134+/-5% of baseline), while low frequency stimulation (4 Hz for 15 min) induced LTD (80+/-5% of baseline). LTP and LTD were significantly blocked by the N-methyl-D-aspartic acid (NMDA) receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (D-AP5) (to 93+/-6% and 103+/-8% of baseline respectively), indicating that they are both dependent on NMDA glutamate receptor activation. LTP was significantly blocked by the dopamine D1 receptor antagonist R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH-23390) (98+/-8% of baseline), indicating that LTP is dependent on activation of dopamine D(1)-type receptors, whereas LTD was not significantly different (90+/-7%). In adult R6/2 mice (aged 8-19 weeks), LTP was significantly reduced (to 110+/-4% of baseline), while LTD was not significantly different from that seen in WT mice (85+/-6%). These data show that R6/2 mice have impaired dopamine-dependent neuronal plasticity in the striatum. As dopamine-dependent plasticity is a proposed model of striatum-based motor and cognitive functions, this impairment could contribute to deficits seen in R6/2 mice.

Differential anxiogenic, aversive, and locomotor effects of THC in adolescent and adult rats

RATIONALE

Unpleasant side effects of drugs of abuse often limit their repeated use; however, such effects may be attenuated in adolescents compared to adults.

OBJECTIVES

We investigated whether the anxiogenic, aversive, or locomotor effects of delta-9-tetrahydrocannabinol (THC) differ between adolescent and adult rats.

METHODS

We used the elevated plus maze (EPM) and light-dark tests of anxiety, the conditioned taste aversion and conditioned place aversion (CPA) tests of generalized aversion, and measures of stress hormone levels in serum to examine effects of THC in adolescent and adult rats. Locomotor activity was also recorded in the EPM, light-dark task, and CPA association sessions.

RESULTS

In the EPM and light-dark tasks, THC was anxiogenic in both age groups, but the drug was more anxiogenic in adults than in adolescents. In the place and taste aversion tasks, THC was aversive in both ages, and at 1.25 and 5 mg/kg, was more aversive in adults than in adolescents. The locomotor response to THC, as measured in the anxiety tasks and CPA, affected adults more than adolescents. Multiple measures revealed a locomotor-decreasing effect in adults, whereas some measures suggested a small locomotor-increasing effect in adolescent rats.

CONCLUSIONS

These results suggest that THC can have greater anxiogenic, aversive, and locomotor-reducing effects in adult rats than in adolescent rats. These findings suggest an explanation for reduced marijuana use in adult humans compared to teenagers.

Ethanol induces long-term facilitation of NR2B-NMDA receptor activity in the dorsal striatum: implications for alcohol drinking behavior

Addiction is characterized by compulsive alcohol or drug taking and seeking, and the dorsal striatum has been implicated in such maladaptive persistent habits. The NMDA receptor (NMDAR), which is a major target of alcohol, is implicated in striatal-based habit learning. We found that, in the dorsal striatum, alcohol (ethanol) exposure produced an increase in the phosphorylation of the NR2B subunit of the NMDAR, and a corresponding increase in the activity of Fyn kinase, which phosphorylates NR2B. We further observed an ethanol-mediated long-term facilitation (LTF) of the activity of NR2B-containing NMDARs (NR2B-NMDARs) in the dorsal striatum. This LTF is Fyn kinase dependent, because it was observed in Fyn wild-type but not in Fyn knock-out mice. Importantly, none of these biochemical and physiological changes was observed in the ventral striatum. Finally, dorsal but not ventral striatum infusion of a Fyn or NR2B-NMDAR inhibitor reduced rat operant self-administration of ethanol. Our results suggest that the Fyn-mediated phosphorylation and LTF of NR2B-NMDAR activity in the dorsal striatum after exposure to ethanol may underlie aberrant plasticity that contributes to mechanisms underlying alcohol drinking behavior.

Striatal Contributions to Reward and Decision Making: Making Sense of Regional Variations in a Reiterated Processing Matrix

The striatum is the major input nucleus of the basal ganglia. It is thought to play a key role in learning on the basis of positive reinforcement and in action selection. One view of the striatum conceives it as comprising a reiterated matrix of processing units that perform common operations in different striatal regions, namely synaptic plasticity according to a three-factor rule, and lateral inhibition. These operations are required for reinforcement learning and selection of previously reinforced actions. Analysis of the behavioral effects of circumscribed lesions of the striatum, however, suggests regional specialization of learning and decision-making operations. We consider how a basic processing unit may be modified by regional variations in neurochemical parameters, for example, by the gradient in density of dopamine terminals from dorsal to ventral striatum. These variations suggest subtle differences between dorsolateral and ventromedial striatal regions in the temporal properties of dopamine signaling, which are superimposed on regional differences in connectivity. We propose that these variations make sense in relation to the temporal structure of activity in striatal inputs from different regions, and the requirements of different learning operations. Dorsolateral striatal (DLS) regions may be subject to brief, precisely timed pulses of dopamine, whereas ventromedial striatal regions integrate dopamine signals over a longer time course. These differences may be important for understanding regional variations in the contribution to reinforcement of habits, versus incentive processes that are sensitive to the value of expected rewards.

Plastic Corticostriatal Circuits for Action Learning: What's Dopamine Got to Do with It?

Reentrant corticobasal ganglia circuits are important for voluntary action and for action selection. In vivo and ex vivo studies show that these circuits can exhibit a plethora of short- and long-lasting plastic changes. Convergent evidence at the molecular, cellular, and circuit levels indicates that corticostriatal circuits are involved in the acquisition and automatization of novel actions. There is strong evidence that activity in corticostriatal circuits changes during the learning of novel actions, but the plastic changes observed during the early stages of learning a novel action are different than those observed after extensive training. A variety of studies indicate that the neural mechanisms and the corticostriatal subcircuits involved in the initial acquisition of actions and skills differ from those involved in their automatization or in the formation of habits. Dopamine, a critical modulator of short- and long-term plasticity in corticostriatal circuits, is differentially involved in early and late stages of action learning. Changes in dopaminergic transmission have several concomitant effects in corticostriatal function, which may be important for action selection and action learning. These diverse effects may subserve different roles for dopamine in reinforcement and action learning.

Reliable long-lasting depression interacts with variable short-term facilitation to determine corticostriatal paired-pulse plasticity in young rats

Synaptic plasticity at corticostraital synapses is proposed to fine tune movment and improve motor skills. We found paired-pulse plasticity at corticostriatal synapses reflected variably expressed short-term facilitation blended with a consistent background of longer-lasting depression. Presynaptic modulation via neuotransmitter receptor activation was ruled out as a mechanism for long-lasting paired-pulse depression by examining the effect of selective receptor antagonists. EPSC amplitude and paired-pulse plasticity, however, was influenced by block of D2 dopamine receptors. Block of glutamate transport with l-transdicarboxylic acid (PDC) reduced EPSCs, possibly through a mechanism of AMPA receptor desensitization. Removal of AMPA receptor desensitization with cyclothiazide reduced the paired-pulse depression at long-duration interstimulus intervals (ISIs), indicating that AMPA receptor desensitization participates in corticostriatal paired-pulse plasticity. The low-affinity glutamate receptor antagonist cis-2,3-piperidine dicarboxylic acid (PDA) increased paired-pulse depression, suggesting that a presynaptic component also exists for long-lasting paired-pulse depression. Low Ca(2+)-high Mg(2+) or BAPTA-AM dramatically reduced the amplitude of corticostriatal EPSCs and both manipulations increased the expression of facilitation and, to a lesser extent, they reduced long-lasting paired-pulse depression. EGTA-AM produced a smaller reduction in EPSC amplitude and it did not alter paired-pulse facilitation, but in contrast to low Ca(2+) and BAPTA-AM, EGTA-AM increased long-lasting paired-pulse depression. These experiments suggest that facilitation and depression are sensitive to vesicle depletion, which is dependent upon changes in peak Ca(2+) (i.e. low Ca(2+)-high Mg(2+) or BAPTA-AM). In addition, the action of EGTA-AM suggests that basal Ca(2+) regulates the recovery from long-lasting paired-pulse depression, possibly thourgh a Ca(2+)-sensitive process of vesicle delivery.

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.

The phospholipid mediator platelet-activating factor mediates striatal synaptic facilitation

The phospholipid mediator platelet-activating factor (PAF), an endogenous modulator of glutamatergic neurotransmission, can also be secreted by brain mononuclear phagocytes during HIV-1 infection. Platelet-activating factor can induce neuronal apoptosis by NMDA receptor-dependent and independent mechanisms. We now demonstrate that acute administration of sublethal doses of PAF to striatal slices augments synaptic facilitation in striatal neurons following high-frequency stimulation, which can be blocked by PAF receptor antagonists, suggesting that striatal synaptic facilitation can be augmented by PAF receptor agonism. We also demonstrate that repeated sublethal doses of PAF during tetanic stimulation can greatly increase the magnitude of postsynaptic potentials and action potentials, but a lethal dose of PAF destroys the capacity of corticostriatal synapses to achieve this augmented synaptic facilitation. Thus, the relative concentration and temporal pattern of PAF expression at glutamatergic synapses may govern whether it acts in a physiologic or pathophysiologic manner during striatal neurotransmission.

Methamphetamine-induced structural plasticity in the dorsal striatum

Repeated exposure to psychostimulant drugs produces long-lasting changes in dendritic structure, presumably reflecting a reorganization in patterns of synaptic connectivity, in brain regions that mediate the psychomotor activating and incentive motivational effects of these drugs, including the nucleus accumbens and prefrontal cortex. However, repeated exposure to psychostimulant drugs also facilitates a transition in the control of some behaviors from action-outcome associations to behavior controlled by stimulus-response (S-R) habits. This latter effect is thought to be due to increasing engagement and control over behavior by the dorsolateral (but not dorsomedial) striatum. We hypothesized therefore that repeated exposure to methamphetamine would differentially alter the density of dendritic spines on medium spiny neurons (MSNs) in the dorsolateral vs. dorsomedial striatum. Rats were treated with repeated injections of methamphetamine, and 3 months later dendrites were visualized using Sindbis virus-mediated green fluorescent protein (GFP) expression in vivo. We report that prior exposure to methamphetamine produced a significant increase in mushroom and thin spines on MSNs in the dorsolateral striatum, but a significant decrease in mushroom spines in the dorsomedial striatum. This may be due to changes in the glutamatergic innervation of these two subregions of the dorsal striatum. Thus, we speculate that exposure to psychostimulant drugs may facilitate the development of S-R habits because this reorganizes patterns of synaptic connectivity in the dorsal striatum in a way that increases control over behavior by the dorsolateral striatum.

Facilitation of saccadic eye movements by postsaccadic electrical stimulation in the primate caudate

Sensorimotor experience followed by positive feedback leads to motor learning. Although the striatum, an input channel of the basal ganglia, has been implicated to play a key role in motor learning, little is known about how reward information modulates the neuronal processes in the striatum that causes behavioral changes. Here, we report that direct manipulation of the neuronal signal in the primate caudate yields behavioral changes comparable with those induced by natural reward. Electrical stimulation in the oculomotor region of the caudate immediately after saccades to a fixed direction led to selective facilitation of saccades in that direction. The facilitation remained even after stimulation was stopped, indicating a plastic change. These effects were observed when stimulation was applied after, not before, saccades. We propose that the caudate plays a causal role in behavioral changes by integrating selective sensorimotor and reward information in a temporally specific manner.

NMDA-dependent facilitation of corticostriatal plasticity by the amygdala

Emotions generally improve memory, and the basolateral amygdala (BLA) is believed to mediate this effect. After emotional arousal, BLA neurons increase their firing rate, facilitating memory consolidation in BLA targets. The enhancing effects of BLA activity extend to various types of memories, including motor learning, which is thought to involve activity-dependent plasticity at corticostriatal synapses. However, the underlying mechanisms are unknown. Here we show that the NMDA-to-AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) ratio is nearly twice as high at BLA as compared with cortical synapses onto principal striatal neurons and that activation of BLA inputs greatly facilitates long-term potentiation induction at corticostriatal synapses. This facilitation was NMDA-dependent, but it occurred even when BLA and cortical stimuli were 0.5 s apart during long-term potentiation induction. Overall, these results suggest that BLA activity opens long time windows during which the induction of corticostriatal plasticity is facilitated.

Alterations of rat corticostriatal synaptic plasticity after chronic ethanol exposure and withdrawal

BACKGROUND

The purpose of this study was to investigate the effects of chronic ethanol exposure (CEE) and withdrawal on corticostriatal plasticity in rats.

METHODS

We established an animal model of alcoholism using the method of Turchan et al. (1999). A synaptic model of long-term memory (long-term depression, LTD) was used as an index and the striatum, which is related to habit learning, was selected as a target region in the present study. The effects of CEE and withdrawal on the LTD were studied in striatal slices of ethanol-dependent rats using the extracellular recording method.

RESULTS

A stable LTD can be induced after high-frequency stimulation (HFS) in the slices of control rats. Chronic ethanol exposure and withdrawal suppressed the induction of corticostriatal LTD to different extents, with the strongest suppressive effects on LTD occurring in the slices of rats exposed to ethanol for 10 days and in those withdrawn from ethanol for 1 day. Notably, 3 days of withdrawal resulted in the shift of corticostriatal synaptic plasticity from LTD to long-term potentiation, and the peak latencies of the population spikes were obviously shortened compared with those of control rats. After 7 days of withdrawal, ethanol's effects tended to disappear.

CONCLUSIONS

These results suggest that the alterations of corticostriatal synaptic plasticity produced by CEE and withdrawal may play a prominent role in alcohol abuse and alcoholism.

Disrupted motor learning and long-term synaptic plasticity in mice lacking NMDAR1 in the striatum

Much research has implicated the striatum in motor learning, but the underlying mechanisms have not been identified. Although NMDA receptor (NMDAR)-dependent long-term potentiation has been observed in the striatum, its involvement in motor learning remains unclear. To examine the role of striatal NMDAR in motor learning, we created striatum-specific NMDAR1 subunit knockout mice, analyzed the striatal anatomy and neuronal morphology of these mice, evaluated their performance on well established motor tasks, and performed electrophysiological recordings to assay striatal NMDAR function and long-term synaptic plasticity. Our results show that deleting the NMDAR1 subunit of the NMDAR specifically in the striatum, which virtually abolished NMDAR-mediated currents, resulted in only small changes in striatal neuronal morphology but severely impaired motor learning and disrupted dorsal striatal long-term potentiation and ventral striatal long-term depression.

Chronic cocaine prevents depotentiation at corticostriatal synapses

BACKGROUND

The advanced stages of addiction are characterized by compulsive drug-seeking and drug-taking behaviors despite the loss of the hedonic effect of drug consumption. A pathology of habit forming systems might underlie these features of addiction.

METHODS

We have compared use-dependent plasticity of corticostriatal synapses in saline- and cocaine-treated rats by means of single neuron electrophysiological recordings.

RESULTS

High-frequency stimulation of cortical afferents induced long-term potentiation (LTP) of corticostriatal synapses in treated and untreated animals. Saline- and acute-cocaine-treated rats, however, showed synaptic depotentiation in response to subsequent low-frequency stimulation of the same pathway, whereas chronic cocaine-treated animals were refractory to this process. Depotentiation was also absent in control slices bathed with cocaine, dopamine, or with the D1 receptor agonist SKF38393. The effect of cocaine on depotentiation was prevented by D1 but not D2 dopamine receptor antagonists and was mimicked by pharmacological inhibition of cyclin-dependent kinase 5, to enhance D1-receptor-associated intracellular signaling.

CONCLUSIONS

These results provide the first evidence that cocaine blocks the reversal of LTP in brain circuits. This alteration might be important for the persistence of addictive behavior despite efforts to abstain.

Post- versus presynaptic plasticity in L-DOPA-induced dyskinesia

L-3,4-dihydroxyphenylalanine (L-DOPA) remains the most efficacious drug for the treatment of Parkinson's disease (PD), but causes adverse effects that limit its utility. L-DOPA-induced dyskinesia (abnormal involuntary movements) is a significant clinical problem that attracts growing scientific interest. Current notions attribute the development of dyskinesia to two main factors, viz. the loss of nigrostriatal dopamine (DA) projections and the maladaptive changes produced by L-DOPA at sites postsynaptic to the nigrostriatal neuron. Basic research in the past 15 years has placed a lot of emphasis on the postsynaptic plasticity associated with dyskinesia, but recent experimental work shows that also some presynaptic factors, involving the regulation of L-DOPA/DA release and metabolism in the brain, may show plasticity during treatment. This review summarizes significant studies of L-DOPA-induced dyskinesia in patients and animal models, and outlines directions for future experiments addressing mechanisms of presynaptic plasticity. These investigations may uncover clues to the varying susceptibility to L-DOPA-induced dyskinesia among PD patients, paving the way for tailor-made treatments.

The role of the basal ganglia in habit formation

Many organisms, especially humans, are characterized by their capacity for intentional, goal-directed actions. However, similar behaviours often proceed automatically, as habitual responses to antecedent stimuli. How are goal-directed actions transformed into habitual responses? Recent work combining modern behavioural assays and neurobiological analysis of the basal ganglia has begun to yield insights into the neural basis of habit formation.

Dopamine D1 activation potentiates striatal NMDA receptors by tyrosine phosphorylation-dependent subunit trafficking

Interactions between dopaminergic and glutamatergic afferents in the striatum are essential for motor learning and the regulation of movement. An important mechanism for these interactions is the ability of dopamine, through D1 receptors, to potentiate NMDA glutamate receptor function. Here we show that, in striatal neurons, D1 receptor activation leads to rapid trafficking of NMDA receptor subunits, with increased NR1 and NR2B subunits in dendrites, enhanced coclustering of these subunits with the postsynaptic density scaffolding molecule postsynaptic density-95, and increased surface expression. The dopamine D1 receptor-mediated NMDA receptor trafficking is blocked by an inhibitor of tyrosine kinases. Blockers of tyrosine phosphatases also induce NMDA subunit trafficking, but this effect is nonselective and alters both NR2A- and NR2B-containing receptors. Furthermore, tyrosine phosphatase inhibition leads to the clustering of tyrosine-phosphorylated NR2B subunit along dendritic shafts. Our findings reveal that D1 receptor activation can potentiate striatal NMDA subunit function by directly promoting the surface insertion of the receptor complexes. This effect is regulated by the reciprocal actions of protein tyrosine phosphatases and tyrosine kinases. Modification of these pathways may be a useful therapeutic target for Parkinson's disease and other basal ganglia disorders in which abnormal function of striatal NMDA receptors contributes to the symptoms of the diseases.

Coincident signaling in mesolimbic structures underlying alcohol reinforcement

The medium spiny neurons (MSNs) of the nucleus accumbens function in a critical regard to examine and integrate information in the processing of rewarding behaviors. These neurons are aberrantly affected by drugs of abuse, including alcohol. However, ethanol is unlike any other common drug of abuse, due to its pleiotropic actions on intracellular and intercellular signaling processes. Intracellular biochemical pathways appear to critically contribute to long-term changes in the level of synaptic activation of these neurons, which have been implicated in ethanol dependence. Additionally, these neurons also display a fascinating pattern of up/down activity, which appears to be, at least in part, regulated by convergent activation of dopaminergic and glutamatergic (NMDA) inputs. Thus, dopaminergic and NMDA receptor-mediated synaptic transmission onto these neurons may constitute a critical site of ethanol action in mesolimbic structures. For instance, dopaminergic inputs alter the ability of ethanol to regulate NMDA receptor-mediated synaptic transmission onto accumbal MSNs. Prior activation of D1-signaling cascade through the cAMP-regulated phosphoprotein-32kD (DARPP-32) and protein phosphatase-1 (PP-1) pathway significantly attenuates ethanol inhibition of NMDA receptor function. Therefore, the interaction of D1-signaling and NMDA receptor signaling may alter NMDA receptor-dependent long-term synaptic plasticity, contributing to the development of ethanol-induced neuroadaptation of the reward pathway.

Pre- and postsynaptic contributions to age-related alterations in corticostriatal synaptic plasticity

Aging creates deficits in motor performance related to changes in striatal processing of cortical information. This study describes age-related changes in corticostriatal snaptic plasticity and associated mechanisms, which may contribute to declines in motor behavior. Intracellular recordings revealed an age-related decrease in the expression of paired-pulse, posttetanic, and long-term potentiation (LTP). The age-related difference in LTP was associated with reduced sensitivity to block of N-methyl-D-aspartate (NMDA) receptors in the aged population. These age-related changes could not be explained by increased L-type Ca(2+)channel activity, since block of L-type Ca(2+) channels with nifedipine increased rather than decreased the age-related difference in long-term plasticity. Age-related increases in reactive oxygen species (ROS) modulation were also ruled out, since application of H(2)O(2) produced changes in synaptic function that were opposite to trends seen in aging, and addition of the antioxidant Trolox-C had a larger effect on long-term plasticity in young rats than in older rats. A robust age-related difference in long-term synaptic plasticity was found by studying synaptic plasticity following the blocking of D2 receptors with l-sulpiride, which may involve age-difference in NMDA receptor function. l-sulpiride consistently enabled a slow development of LTP at young (but not aged) corticostriatal synapses. However, No age differences were found in the sensitivity to the addition of the D2 receptor agonist quinpirole. These findings provide evidence for age-induced changes in the release properties of cortical terminals and in the functioning of postsynaptic striatal NMDA receptors, which may contribute to age-related deficits in striatum control of movement.

Bidirectional activity-dependent plasticity at corticostriatal synapses

Corticostriatal projections originate from the entire cerebral cortex and provide the major source of glutamatergic inputs to the basal ganglia. Despite the importance of corticostriatal connections in sensorimotor learning and cognitive functions, plasticity forms at these synapses remain strongly debated. Using a corticostriatal slice preserving the connections between the somatosensory cortex and the target striatal cells, we report the induction of both non-Hebbian and Hebbian forms of long-term potentiation (LTP) and long-term depression (LTD) on striatal output neurons (SONs). LTP and LTD can be induced selectively by different stimulation patterns (high-frequency trains vs low-frequency pulses) and were evoked with similar efficiency in non-Hebbian and Hebbian modes. Combination of LTP-LTD and LTD-LTP sequences revealed that bidirectional plasticity occurs at the same SONs and provides efficient homeostatic mechanisms leading to a resetting of corticostriatal synapses avoiding synaptic saturation. The effect of temporal relationship between cortical stimulation and SON activity was assessed using spike-timing-dependent plasticity (STDP) protocols. An LTP was observed when an action potential was triggered in the striatal neuron before the cortical stimulus, and, conversely, an LTD was induced when the striatal neuron discharge was triggered after the cortical stimulation. Such STDP was reversed when compared with those described so far in other mammalian brain structures. This mechanism may be essential for the role of the striatum in learning of motor sequences in which sensory and motor events are associated in a precise time sequence.

Blockade of NMDA receptors in the dorsomedial striatum prevents action-outcome learning in instrumental conditioning

Although there is consensus that instrumental conditioning depends on the encoding of action-outcome associations, it is not known where this learning process is localized in the brain. Recent research suggests that the posterior dorsomedial striatum (pDMS) may be the critical locus of these associations. We tested this hypothesis by examining the contribution of N-methyl-D-aspartate receptors (NMDARs) in the pDMS to action-outcome learning. Rats with bilateral cannulae in the pDMS were first trained to perform two actions (left and right lever presses), for sucrose solution. After the pre-training phase, they were given an infusion of the NMDA antagonist 2-amino-5-phosphonopentanoic acid (APV, 1 mg/mL) or artificial cerebral spinal fluid (ACSF) before a 30-min session in which pressing one lever delivered food pellets and pressing the other delivered fruit punch. Learning during this session was tested the next day by sating the animals on either the pellets or fruit punch before assessing their performance on the two levers in extinction. The ACSF group selectively reduced responding on the lever that, in training, had earned the now devalued outcome, whereas the APV group did not. Experiment 2 replicated the effect of APV during the critical training session but found no effect of APV given after acquisition and before test. Furthermore, Experiment 3 showed that the effect of APV on instrumental learning was restricted to the pDMS; infusion into the dorsolateral striatum did not prevent learning. These experiments provide the first direct evidence that, in instrumental conditioning, NMDARs in the dorsomedial striatum are involved in encoding action-outcome associations.

Dopamine depletion alters phosphorylation of striatal proteins in a model of Parkinsonism

Nigrostriatal dopamine depletion disrupts striatal medium spiny neuron morphology in Parkinson's disease and modulates striatal synaptic plasticity in animal models of parkinsonism. We demonstrate that long-term nigrostriatal dopamine depletion in the rat induces evolving changes in the phosphorylation of striatal proteins critical for synaptic plasticity. Dopamine depletion increased the phosphorylation of the alpha isoform of calcium-calmodulin-dependent protein kinase II (CaMKIIalpha) at Thr286, a site associated with enhanced autonomous kinase activity, but did not alter total levels of CaMKIIalpha or other synaptic proteins. Dopamine depletion decreased CaMKIIalpha levels in postsynaptic density-enriched fractions without significant changes in other proteins. The activity of protein phosphatase 1 (PP1), a postsynaptic phosphatase that dephosphorylates CaMKII, is regulated by DARPP-32 (dopamine- and cAMP-regulated phosphoprotein of 32 kDa). Dopamine depletion had no effect on DARPP-32 phosphorylation at Thr34, but increased DARPP-32 phosphorylation at Thr75. Levodopa administration reversed the increased phosphorylation of both CaMKIIalpha and DARPP-32. Normal ageing increased the levels of PP1(gamma1 isoform) but decreased levels of the PP1gamma1-targeting proteins spinophilin and neurabin. Elevated phosphorylations of CaMKIIalpha and DARPP-32 were maintained for up to 20 months after dopamine depletion. However, phosphorylation of the CaMKII-PP1 substrate, Ser831 in the glutamate receptor GluR1 subunit, was increased only after sustained (9-20 months) dopamine depletion. Interaction of ageing-related changes in PP1 with the dopamine depletion-induced changes in CaMKIIalpha may account for enhanced GluR1 phosphorylation only after long-term dopamine depletion. These evolving changes may impact striatal synaptic plasticity, Parkinson's disease progression and the changing efficacy and side-effects associated with dopamine replacement therapy.

Chronic haloperidol promotes corticostriatal long-term potentiation by targeting dopamine D2L receptors

Reduced glutamate-mediated synaptic transmission has been implicated in the pathophysiology of schizophrenia. Because antipsychotic agents might exert their beneficial effects against schizophrenic symptoms by strengthening excitatory transmission in critical dopaminoceptive brain areas, in the present study we have studied the effects of acute and chronic haloperidol treatment on striatal synaptic plasticity. Repetitive stimulation of corticostriatal terminals in slices induced either long-term depression or long-term potentiation (LTP) of excitatory transmission in control rats, whereas it invariably induced NMDA receptor-dependent LTP in animals treated chronically with haloperidol. Haloperidol effects were mimicked and occluded in mice lacking both D2L and D2S isoforms of dopamine D2 receptors (D2R-/-), in mice lacking D2L receptors and expressing normal levels of D2S receptors (D2R-/-;D2L-/-), and in mice lacking D2L receptors and overexpressing D2S receptors (D2L-/-). These data indicate that the blockade of D2L receptors was responsible for the LTP-favoring action of haloperidol in the striatum. In contrast, overexpression of D2S receptors uncovered a facilitatory role of this receptor isoform in LTP formation because LTP recorded from D2L-/- mice, but not those recorded from wild-type, D2R-/-, and D2R-/-;D2L-/- mice, was insensitive to the pharmacological blockade of D1 receptors. The identification of the cellular, molecular, and receptor mechanisms involved in the action of haloperidol in the brain is essential to understand how antipsychotic agents exert their beneficial and side effects.

Altered distribution of striatal activity-dependent synaptic plasticity in the 3-nitropropionic acid model of Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder characterized by involuntary choreiform movements, neuropsychiatric disturbances and cognitive decline. The hyperkinetic phenomenology has commonly been attributed to a disturbance of the basal ganglia function, mainly the neostriatum, but its pathophysiology mechanisms remain unclear. Activity-dependent long-term changes in synaptic efficacy, such as long-term potentiation (LTP) and long-term depression (LTD), are widely considered to be the cellular models for acquisition and storage of information in neuronal networks. Both LTP and LTD have been described at the corticostriatal pathway and they might be probably involved not only in physiological motor behavior processing but also in disease states affecting that pathway. Systemic injection of 3-nitropropionic acid (3-NP) induces excitotoxic striatal lesions and abnormal movements in rodents, resembling those seen in HD. We examined synaptic plasticity in dorsolateral striatum slices prepared from both control and 3-NP-treated rats by recording extracellular field potentials. Our results reinforce the idea that both forms of activity-dependent synaptic plasticity can be recorded at the dorsolateral region of striatum by the same stimulating protocol in control rats and suggest that 3-NP-induced striatal lesions may be associated with suppression of LTD expression in the sensorimotor striatum.

Temporal sequence learning, prediction, and control: a review of different models and their relation to biological mechanisms

In this review, we compare methods for temporal sequence learning (TSL) across the disciplines machine-control, classical conditioning, neuronal models for TSL as well as spike-timing-dependent plasticity (STDP). This review introduces the most influential models and focuses on two questions: To what degree are reward-based (e.g., TD learning) and correlation-based (Hebbian) learning related? and How do the different models correspond to possibly underlying biological mechanisms of synaptic plasticity? We first compare the different models in an open-loop condition, where behavioral feedback does not alter the learning. Here we observe that reward-based and correlation-based learning are indeed very similar. Machine control is then used to introduce the problem of closed-loop control (e.g., actor-critic architectures). Here the problem of evaluative (rewards) versus nonevaluative (correlations) feedback from the environment will be discussed, showing that both learning approaches are fundamentally different in the closed-loop condition. In trying to answer the second question, we compare neuronal versions of the different learning architectures to the anatomy of the involved brain structures (basal-ganglia, thalamus, and cortex) and the molecular biophysics of glutamatergic and dopaminergic synapses. Finally, we discuss the different algorithms used to model STDP and compare them to reward-based learning rules. Certain similarities are found in spite of the strongly different timescales. Here we focus on the biophysics of the different calcium-release mechanisms known to be involved in STDP.

State-dependent calcium signaling in dendritic spines of striatal medium spiny neurons

Striatal medium spiny neurons (MSNs) in vivo undergo large membrane depolarizations known as state transitions. Calcium (Ca) entry into MSNs triggers diverse downstream cellular processes. However, little is known about Ca signals in MSN dendrites and spines and how state transitions influence these signals. Here, we develop a novel approach, combining 2-photon Ca imaging and 2-photon glutamate uncaging, to examine how voltage-sensitive Ca channels (VSCCs) and ionotropic glutamate receptors contribute to Ca signals in MSNs. We find that upstate transitions switch the VSCCs available in dendrites and spines, decreasing T-type while enhancing L-type channels. Moreover, these transitions change the dominant synaptic Ca source from Ca-permeable AMPA receptors to NMDA receptors. Finally, pairing bAPs with synaptic inputs generates additional synaptic Ca signals due to enhanced Ca influx through NMDA receptors. By altering the sources, amplitude, and kinetics of spine Ca signals, state transitions may gate synaptic plasticity and gene expression in MSNs.

GABAergic microcircuits in the neostriatum

The vast majority of neostriatal neurons and intrinsic intrastriatal synapses are GABAergic, the latter arising from axon collaterals of spiny projection neurons and from GABAergic interneurons. An important feature of the functional organization of the neostriatum has long been assumed to be the existence of a widespread lateral inhibitory network mediated by the axon collaterals of spiny projection neurons. However, these collateral connections have recently been demonstrated electrophysiologically to be relatively weak--in contrast to feedforward interneuronal inhibition, which exerts strong effects on spike timing in spiny neurons. These new data are incompatible with current "winner-take-all" models of lateral inhibitory function in the neostriatum, and they force a modification of established concepts of the functional roles of feedback inhibition in this nucleus.

Dual synaptic sites of D1-dopaminergic regulation of ethanol sensitivity of NMDA receptors in nucleus accumbens

Regulation of NMDAreceptor-mediated synaptic transmission onto accumbal medium spiny neurons (MSN) may constitute an important site in drug reward and reinforcement in mesolimbic structures. Previously, we reported that D(1)-like dopamine receptors activate a postsynaptic cAMP/PKA/DARPP-32 signaling cascade culminating in phosphorylation of SER897-NR1 subunits and a reduction in the sensitivity to ethanol of NMDA receptor-mediated synaptic transmission. Here, we use a detailed electrophysiological analysis of D(1)-like receptor regulation of the ethanol sensitivity of accumbal NMDA receptors (NMDARs) through recordings of quantal Sr(2+)-supported NMDA miniature synaptic currents (mEPSCs) in reduced Mg(2+) (0.6 mM) and report dual presynaptic and postsynaptic components of D(1)-like regulation of ethanol sensitivity of NMDARs. Ethanol inhibited NMDA mEPSC amplitude and frequency in a dose-dependent manner (25-75 mM), indicating inhibitory effects on presynaptic and postsynaptic components NMDA receptor-mediated synaptic transmission. The presynaptic inhibitory effect was corroborated by analysing the ratio of paired-pulse facilitation (PPF) of Ca(2+)-supported NMDA EPSCs. Activation of D(1) receptors with the agonist, SKF 38393 (25 microM), reversed ethanol suppression of NMDA mEPSC frequency and amplitude. Furthermore, the Mg(2+)-dependent decay off-rate of NMDA mEPSCs was substantially reduced by ethanol in a manner strongly reversed by the D(1) agonist. D(1) receptor-mediated attenuation of both the presynaptic and postsynaptic actions of ethanol was completely blocked by a D(1) selective antagonist (SCH 23390). These data suggest that D(1)-like receptors modulate both the presynaptic and postsynaptic effects of ethanol on NMDA receptor-mediated synaptic transmission in nucleus accumbens (NAc) and that these interactions may contribute to ethanol-induced neuroadaptation of the reward pathway.

Corticostriatal plasticity: life after the depression

Experience-dependent changes in corticostriatal transmission efficacy are likely to support the role of the striatum in reinforcement-based motor learning. Whereas long-term depression at glutamatergic corticostriatal synapses has long been regarded as the normal form of striatal plasticity, recent work provides evidence that use-dependent potentiation can naturally occur at these connections through an increase in both synaptic efficacy and postsynaptic intrinsic excitability. By decreasing the weight of cortical inputs required to fire striatal output neurons, short-term and long-term potentiation at corticostriatal connections can jointly participate in the formation of sensorimotor links by which specific context-dependent patterns of cortical activity can engage selected motor programs.

Induction of striatal long-term synaptic depression by moderate frequency activation of cortical afferents in rat

The striatum regulates motor output, and it is thought that changes in the synaptic efficacy of inputs to the striatum contribute to motor learning and habit formation. Previously, several laboratories have observed that brief high frequency stimulation (HFS) of cortical afferents innervating the dorsolateral striatum induces a long-term decrease in synaptic efficacy called long-term depression (LTD). We recently showed that HFS-induced striatal LTD requires retrograde signalling involving postsynaptic release of endocannabinoids and activation of presynaptic CB1 cannabinoid receptors. In the present study we have employed whole-cell recording in brain slices to examine a new form of LTD at corticostriatal synapses that can be induced by a 10 Hz, 5 min train. The decrease in synaptic efficacy is associated with a decrease in presynaptic release probability, as demonstrated by a decrease in frequency but not amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) and an increase in the paired pulse ratio (PPR). This form of LTD is blocked by antagonists for CB1 and D2 dopamine receptors and impaired by blockers of L-type calcium channels. However, 10 Hz-induced LTD does not depend on postsynaptic depolarization, unlike HFS-induced LTD. Furthermore, this new form of LTD is not prevented by treatments known to block HFS-induced LTD, including antagonism of metabotropic glutamate receptors (mGluRs), chelation of postsynaptic calcium, or intracellular application of an anandamide membrane transport inhibitor (VDM11). From these findings it is not clear that the endocannabinoid responsible for this form of LTD acts in a retrograde fashion, and the cellular source of endocannabinoid necessary for 10 Hz-induced LTD is as yet unknown. Our results demonstrate that a prolonged moderate frequency train induces cannabinoid-dependent LTD, further supporting the idea that endocannabinoids play a prominent role in the regulation of long-lasting changes in striatal output.