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

Involvement of GABAA receptors in the regulation of the prefrontal cortex on dopamine release in the rat dorsolateral striatum

The characteristics of the prefrontal cortex regulation of dopamine release in the rat dorsolateral striatum were investigated in focusing on the corticostriatal pathway, using dual-probe microdialysis in combination with simple behavioral procedures. Intracortical perfusion of the GABA(A) receptor antagonist bicuculline (0.03 and 0.1 mM) increased the striatal dopamine release, whereas the GABA(A) receptor agonist muscimol (0.1 and 1 mM) reduced the release dose dependently. Co-perfusion of muscimol (0.01 mM), which did not by itself affect the dopamine release, completely prevented the glutamate (1 mM)-stimulated dopamine release and behavioral activities, including locomotion and rearing. Muscimol (0.01 mM) also prevented the N-methyl-D-aspartate (NMDA; 1 mM)-induced increases in glutamate, as well as dopamine levels. These findings suggest that the prefrontal cortex functionally regulates dopamine release in the dorsolateral striatum, which is mediated via GABA(A) receptors. This cortical GABAergic system underlying the prefrontal cortex regulatory mechanism appears to be associated with the corticostriatal pathway.

Disruption of endocannabinoid release and striatal long-term depression by postsynaptic blockade of endocannabinoid membrane transport

Activation of the CB1 cannabinoid receptor inhibits neurotransmission at numerous synapses in the brain. Indeed, CB1 is essential for certain types of both short- and long-term synaptic depression. It was demonstrated recently that CB1 is critical for activity-dependent long-term depression (LTD) at glutamatergic corticostriatal synapses in acute brain slice preparations. Here, we show that CB1 activation is necessary, but not solely sufficient, for induction of LTD and that the requisite signaling by endocannabinoids (eCBs) occurs during a time window limited to the first few minutes after high-frequency stimulation delivery. In addition, we have applied intracellularly anandamide membrane transporter inhibitors to provide novel evidence that postsynaptic transport mechanisms are responsible for the release of eCBs from striatal medium spiny neurons. These findings shed new light on the mechanisms by which transient eCB formation participates in the induction of long-lasting changes in synaptic efficacy that could contribute to brain information storage.

Effects of periadolescent versus adult cocaine exposure on cocaine conditioned place preference and motor sensitization in mice

RATIONALE

Age of initial exposure to addictive substances is inversely proportional to risk of developing drug dependence. There is debate, however, as to whether intake at a young age causes dependency or whether young people who experiment with addictive substances are predisposed to dependency by other factors. OBJECTIVES. We tested the relationship between cocaine exposure at two different ages in mice and the development of subsequent drug-seeking behavior to test for age-specific exposure effects.

METHODS

We performed dose-response analysis of cocaine conditioned place preference (CPP) and locomotor activity in periadolescent and adult C57Bl/6J mice. In addition, we pretreated periadolescent and adult C57Bl/6J mice with cocaine or saline in the home cage or a drug-associated context, and then examined their behavior in a biased CPP procedure in adulthood.

RESULTS

Dose-response relationships were similar between the two age groups. In the pretreatment experiments, we observed locomotor sensitization during the pretreatment in periadolescent but not adult mice. We also observed an enhanced aversion to the non-preferred side of the chamber in periadolescent mice compared to adult mice, which was alleviated by cocaine association with that side. Third, we observed that after further conditioning in adulthood, there were no pretreatment-specific effects.

CONCLUSIONS

Our results are consistent with a "vulnerable brain" hypothesis for responses to cocaine based on our findings that periadolescent mice exhibit greater locomotor sensitization to cocaine, and greater baseline anxiety responses that are alleviated by cocaine exposure compared to adult mice.

Neural mechanisms of reward-related motor learning

The analysis of the neural mechanisms responsible for reward-related learning has benefited from recent studies of the effects of dopamine on synaptic plasticity. Dopamine-dependent synaptic plasticity may lead to strengthening of selected inputs on the basis of an activity-dependent conjunction of sensory afferent activity, motor output activity, and temporally related firing of dopamine cells. Such plasticity may provide a link between the reward-related firing of dopamine cells and the acquisition of changes in striatal cell activity during learning. This learning mechanism may play a special role in the translation of reward signals into context-dependent response probability or directional bias in movement responses.

Effects of acute repetitive transcranial magnetic stimulation on dopamine release in the rat dorsolateral striatum

Animal studies have shown that descending pathways from the frontal cortex modulate dopamine (DA) release in the striatum. This modulation is thought to be relevant to the pathophysiology of Parkinson's disease. In human, repetitive transcranial magnetic stimulation (rTMS) can result in functional changes in the cortex. The present study intended to clarify the effects of acute rTMS treatment using various stimulation intensities on the extracellular DA concentrations in the rat dorsolateral striatum. The frontal brain of each rat received acute rTMS treatment, which consisted of 500 stimuli from 20 trains in a day. Each train was applied at 25 Hz for 1 s with 1-min intervals between trains. The neurochemical effects of acute rTMS treatment were investigated by determining the extracellular concentrations of DA in the rat dorsolateral striatum using in vivo microdialysis. Acute rTMS treatment of the frontal brain using the stimulation intensity of almost 110% motor threshold (MT) markedly and continuously increased the extracellular DA concentrations in the rat dorsolateral striatum. The present study demonstrates that acute rTMS treatment of the frontal brain affects the DAergic neuronal system in the rat dorsolateral striatum, and may have therapeutic implications for Parkinson's disease.

Plastic Control of Striatal Glutamatergic Transmission by Ensemble Actions of Several Neurotransmitters and Targets for Drugs of Abuse

Long-lasting alterations in the efficacy of glutamatergic synapses, such as long-term potentiation (LTP) and long-term depression (LTD), are prominent models for mechanisms of information storage in the brain. It has been suggested that exposure to drugs of abuse produces synaptic plasticity at glutamatergic synapses that shares many features with LTP and LTD, and that these synaptic changes may play roles in addiction. We have examined the involvement of particular neurotransmitters in synaptic plasticity at glutamatergic synapses within the striatum, a brain region with prominent roles in initiation and sequencing of actions, as well as habit formation. Our studies indicate that multiple neurotransmitters interact to produce striatal synaptic plasticity, and that the relative strength and patterning of the afferent inputs that release the various neurotransmitters determines whether LTP or LTD is activated. Drugs of abuse interact with glutamatergic synaptic plasticity in multiple ways, including alterations in dopamine release and more direct effects on glutamate release and glutamate receptors. We hypothesize that these effects contribute to addiction by facilitating the formation of new, drug-centered habits, and by disruption of more adaptive behaviors.

Distinct roles of D1 and D5 dopamine receptors in motor activity and striatal synaptic plasticity

Stimulation of dopamine (DA) receptors in the striatum is essential for voluntary motor activity and for the generation of plasticity at corticostriatal synapses. In the present study, mice lacking DA D1 receptors have been used to investigate the involvement of the D1-like class (D1 and D5) of DA receptors in locomotion and corticostriatal long-term depression (LTD) and long-term potentiation (LTP). Our results suggest that D1 and D5 receptors exert distinct actions on both activity-dependent synaptic plasticity and spontaneous motor activity. Accordingly, the ablation of D1 receptors disrupted corticostriatal LTP, whereas pharmacological blockade of D5 receptors prevented LTD. On the other side, genetic ablation of D1 receptors increased locomotor activity, whereas the D1/D5 receptor antagonist SCH 23390 decreased motor activity in both control mice and mice lacking D1 receptors. Endogenous DA stimulated D1 and D5 receptors in distinct subtypes of striatal neurons to induce, respectively, LTP and LTD. In control mice, in fact, LTP was blocked by inhibiting the D1-protein kinase A pathway in the recorded spiny neuron, whereas the striatal nitric oxide-producing interneuron was presumably the neuronal subtype stimulated by D5 receptors during the induction phase of LTD. Understanding the role of DA receptors in striatal function is essential to gain insights into the neural bases of critical brain functions and of dramatic pathological conditions such as Parkinson's disease, schizophrenia, and drug addiction.

Neocortical LTD via coincident activation of presynaptic NMDA and cannabinoid receptors

There is a consensus that NMDA receptors (NMDARs) detect coincident pre- and postsynaptic activity during induction of long-term potentiation (LTP), but their role in timing-dependent long-term depression (tLTD) is unclear. We examine tLTD in neocortical layer 5 (L5) pyramidal pairs and find that tLTD is expressed presynaptically, implying retrograde signaling. CB1 agonists produce depression that mimics and occludes tLTD. This agonist-induced LTD requires presynaptic activity and NMDAR activation, but not postsynaptic Ca(2+) influx. Further experiments demonstrate the existence of presynaptic NMDARs that underlie the presynaptic activity dependence. Finally, manipulating cannabinoid breakdown alters the temporal window for tLTD. In conclusion, tLTD requires simultaneous activation of presynaptic NMDA and CB1 receptors. This novel form of coincidence detection may explain the temporal window of tLTD and may also impart synapse specificity to cannabinoid retrograde signaling.

Modulation of striatal single units by expected reward: a spiny neuron model displaying dopamine-induced bistability

Single-unit activity in the neostriatum of awake monkeys shows a marked dependence on expected reward. Responses to visual cues differ when animals expect primary reinforcements, such as juice rewards, in comparison to secondary reinforcements, such as tones. The mechanism of this reward-dependent modulation has not been established experimentally. To assess the hypothesis that direct neuromodulatory effects of dopamine on spiny neurons can account for this modulation, we develop a computational model based on simplified representations of key ionic currents and their modulation by D1 dopamine receptor activation. This minimal model can be analyzed in detail. We find that D1-mediated increases of inward rectifying potassium and L-type calcium currents cause a bifurcation: the native up/down state behavior of the spiny neuron model becomes truly bistable, which modulates the peak firing rate and the duration of the up state and introduces a dependence of the response on the past state history. These generic consequences of dopamine neuromodulation through bistability can account for both reward-dependent enhancement and suppression of spiny neuron single-unit responses to visual cues. We validate the model by simulating responses to visual targets in a memory-guided saccade task; our results are in close agreement with the main features of the experimental data. Our model provides a conceptual framework for understanding the functional significance of the short-term neuromodulatory actions of dopamine on signal processing in the striatum.

Altered long-term corticostriatal synaptic plasticity in transgenic mice overexpressing human CU/ZN superoxide dismutase (GLY(93)-->ALA) mutation

Apart from the extensive loss of motor neurons, degeneration of midbrain dopaminergic cells has been described in both familial and sporadic forms of amyotrophic lateral sclerosis (ALS). Mice overexpressing the mutant human Cu/Zn superoxide dismutase (SOD1) show an ALS-like phenotype in that they show a progressive death of motor neurons accompanied by degeneration of dopaminergic cells. To describe the functional alterations specifically associated with this dopaminergic dysfunction, we have investigated the corticostriatal synaptic plasticity in mice overexpressing the human SOD1 (SOD1+) and the mutated (Gly(93)-->Ala) form (G93A+) of the same enzyme. We show that repetitive stimulation of the corticostriatal pathway generates long-term depression (LTD) in SOD1+ mice and in control (G93A-/SOD1-) animals, whereas in G93A+ mice the same stimulation generates an N-methyl-D-aspartic acid receptor-dependent long-term potentiation. No significant alterations were found in the intrinsic membrane properties of striatal medium spiny neurons and basal corticostriatal synaptic transmission of G93A+ mice. Bath perfusion of dopamine or the D(2) dopamine receptor agonist quinpirole restored LTD in G93A+ mice. Consistent with these in vitro results, habituation of locomotor activity and striatal-dependent active avoidance learning were impaired in G93A+ mice. Thus, degeneration of dopaminergic neurons in the substantia nigra of G93A+ mice causes substantial modifications in striatal synaptic plasticity and related behaviors, and may be a cellular substrate of the extrapyramidal motor and cognitive disorders observed in familial and sporadic ALS.

Phorbol ester uncouples adenosine inhibition of presynaptic Ca2+ transients at hippocampal synapses

Synaptic transmission involves Ca2+ influx at presynaptic terminals. Adenosine receptors inhibit transmission, and this effect can be abolished by activation of PKC with phorbol esters. Whether protein kinase C (PKC) acts via alterations in Ca2+ entry at the presynaptic terminal is unknown. In the present study, we recorded the presynaptic Ca2+ transients (preCa(delta)) in hippocampal stratum radiatum, using fluorescence photometry. The calcium dye Fura-2 AM was used to load the Schaffer collateral/commissural tract and its terminals. Tetrodotoxin (TTX)-sensitive Na+ channels and Cd2+-sensitive, high-voltage activated Ca2+ channels (HVACCs) were required to elicit the preCa(delta). Application of the phorbol ester phorbol-12,13-dibutyrate (PDBu) abolished the adenosine inhibition of both preCa(delta) and the field excitatory postsynaptic potentials (fEPSPs). PDBu consistently potentiated fEPSPs, and also increased preCa(delta) in a large majority of the slices examined. Regardless of whether potentiation was observed, PDBu always prevented adenosine inhibition of preCa(delta). In contrast, the inactive phorbol ester, 4alpha-phorbol, did not alter adenosine inhibition of preCa(delta), indicating that PKC activation is necessary for the occurrence of the observed effects. Our findings suggest that PKC activation abolishes adenosine's inhibitory effect on synaptic activity involving presynaptic Ca2+ entry.

It could be habit forming: drugs of abuse and striatal synaptic plasticity

Drug addiction can take control of the brain and behavior, activating behavioral patterns that are directed excessively and compulsively toward drug usage. Such patterns often involve the development of repetitive and nearly automatic behaviors that we call habits. The striatum, a subcortical brain region important for proper motor function as well as for the formation of behavioral habits, is a major target for drugs of abuse. Here, we review recent studies of long-term synaptic plasticity in the striatum, emphasizing that drugs of abuse can exert pronounced influences on these processes, both in the striatum and in the dopaminergic midbrain. Synaptic plasticity in the ventral striatum appears to play a prominent role in early stages of drug use, whereas dopamine- and endocannabinoid-dependent synaptic plasticity in the dorsal striatum could contribute to the formation of persistent drug-related habits when casual drug use progresses towards compulsive drug use and addiction.

Evidence for functionally distinct synaptic NMDA receptors in ventromedial versus dorsolateral striatum

N-methyl-D-aspartate receptors (NMDARs) are comprised of different subunits. NR2 subunits confer different pharmacological and biophysical properties to NMDARs. Although NR2B subunit expression is uniform throughout striatum, NR2A subunit expression is greater laterally. Pharmacologically isolated NMDAR-mediated excitatory postsynaptic currents (NMDAR-EPSCs) were elicited using minimal local stimulation and recorded in the whole cell configuration to test the hypothesis that biophysical and pharmacological properties of NMDAR-EPSCs of striatal neurons would vary as a function of their location in adult rat striatum. We observed that the decay-time kinetics of NMDAR-EPSCs are significantly slower in neurons of ventromedial versus dorsolateral striatum. Whereas ifenprodil did not differentially affect NMDAR-EPSCs in these regions, application of either glycine or D-serine increased the peak current of NMDAR-EPSCs selectively in dorsolateral striatum. These data provide evidence for functionally distinct NMDARs in the same neuron type in the same brain region of the adult rodent brain. These data thus suggest that the nature of synaptic processing of excitatory input is different in the ventromedial and dorsolateral striatum of the adult rodent brain, regions differentially involved in limbic versus sensorimotor processes, respectively.

LTP in the mouse nucleus accumbens is developmentally regulated

Glutamatergic transmission in the nucleus accumbens (NAc) has been shown to be important for behavioral adaptations in response to drugs of abuse. NMDA-receptor dependent long-term potentiation (LTP) of glutamatergic synaptic transmission has been hypothesized to underlie many lasting alterations in behavior. Thus, we examined LTP in NAc core and find that it is developmentally regulated. Specifically, tetanus-evoked, NMDA receptor-dependent LTP is observed in the NAc of "adolescent" (3-week-old) mice more frequently than in adult (6-20-week-old) mice. In contrast, cAMP-dependent enhancement of transmission is not developmentally regulated. Removal of extracellular Mg(2+) restores LTP in adult NAc core, suggesting developmental regulation of induction processes rather than maintenance mechanisms. These findings are discussed in the context of behavioral changes elicited in response to drugs of abuse, which differ in adolescent vs. adult rodents and humans.

Activation of cyclic AMP-dependent protein kinase is required for long-term enhancement at corticostriatal synapses in rats

The induction of long-term potentiation (LTP) at corticostriatal synapses is dependent on the activation of postsynaptic NMDA receptors, but the mechanisms involved in the maintenance of LTP are not known. We report here that forskolin, an activator of adenylyl cyclase, induces a lasting enhancement of the corticostriatal synaptic response. This enhancement is associated with a lasting decrease in paired-pulse ratio, and is blocked by inhibitors of adenylyl cyclase and cyclic AMP-dependent protein kinase (PKA), but not by a PKA inhibitor injected into the postsynaptic cell. Tetanically-induced LTP is also associated with a decrease in paired-pulse ratio and partially occludes the subsequent action of forskolin. Our results suggest that activation of presynaptic PKA can enhance neurotransmission at corticostriatal synapses; a mechanism required for the expression of LTP at these synapses.

Dopamine-dependent plasticity of corticostriatal synapses

Knowledge of the effect of dopamine on corticostriatal synaptic plasticity has advanced rapidly over the last 5 years. We consider this new knowledge in relation to three factors proposed earlier to describe the rules for synaptic plasticity in the corticostriatal pathway. These factors are a phasic increase in dopamine release, presynaptic activity and postsynaptic depolarisation. A function is proposed which relates the amount of dopamine release in the striatum to the modulation of corticostriatal synaptic efficacy. It is argued that this function, and the experimental data from which it arises, are compatible with existing models which associate the reward-related firing of dopamine neurons with changes in corticostriatal synaptic efficacy.

Postsynaptic endocannabinoid release is critical to long-term depression in the striatum

The striatum functions critically in movement control and habit formation. The development and function of cortical input to the striatum are thought to be regulated by activity-dependent plasticity of corticostriatal glutamatergic synapses. Here we show that the induction of a form of striatal synaptic plasticity, long-term depression (LTD), is dependent on activation of the CB1 cannabinoid receptor. LTD was facilitated by blocking cellular endocannabinoid uptake, and postsynaptic loading of anandamide (AEA) produced presynaptic depression. The endocannabinoid necessary for striatal LTD is thus likely to be released postsynaptically as a retrograde messenger. These findings demonstrate a new role for endocannabinoids in the induction of long-term synaptic plasticity in a circuit necessary for habit formation and motor control.

Nicotinic acetylcholine receptors interact with dopamine in induction of striatal long-term depression

The dorsal striatum participates in motor function and stimulus-response or "habit" learning. Acetylcholine (ACh) is a prominent neurotransmitter in the striatum and exerts part of its actions through nicotinic cholinergic receptors. Activation of these receptors has been associated with the enhancement of learning and certainly is instrumental in habitual use of nicotine. Nicotinic receptors have also been suggested to be a possible therapeutic target for disorders of the basal ganglia. In this report we show that the activation of nicotinic acetylcholine receptors in the dorsal striatum contributes to dopamine (DA)- and activity-dependent changes in synaptic efficacy. High-frequency activation of glutamatergic synapses onto striatal neurons results in a long-term depression (LTD) of synaptic efficacy that is dependent on the activation of dopamine receptors. This stimulation also produces robust increases in extracellular dopamine concentration as well as strong activation of cholinergic striatal interneurons. Antagonists of nicotinic acetylcholine receptors inhibit striatal LTD. However, on coapplication of dopamine reuptake inhibitors with nicotinic receptor antagonists, activity-induced striatal LTD is restored. Dopamine release is modulated by activation of nicotinic receptors in the dorsal striatum, and activation of nicotinic receptors during high-frequency synaptic activation appears to be capable of interacting with dopaminergic actions that lead to striatal LTD. Our results suggest that stimulation of mechanisms involved in striatal synaptic plasticity is an important role for striatal nicotinic acetylcholine receptors and that these mechanisms may contribute to the enhancement of learning and habit formation produced by nicotine intake.

Synaptic regulation of somatodendritic dopamine release by glutamate and GABA differs between substantia nigra and ventral tegmental area

Midbrain dopamine (DA) cells of the substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA) exhibit somatodendritic release of DA. To address how somatodendritic release is regulated by synaptic glutamatergic and GABAergic input, we examined the effect of ionotropic-receptor antagonists on locally evoked extracellular DA concentration ([DA]o) in guinea pig midbrain slices. Evoked [DA]o was monitored with carbon-fiber microelectrodes and fast-scan cyclic voltammetry. In SNc, evoked [DA]o was 160% of control in the presence of the AMPA-receptor antagonist, GYKI-52466, or the NMDA-receptor antagonist, AP5. Similar increases were seen with the GABAA-receptor antagonist, picrotoxin, or the GABA(B)-receptor antagonist, saclofen. The increase seen with GYKI-52466 was prevented when both picrotoxin and saclofen were present, consistent with normal, AMPA-receptor mediated activation of GABAergic inhibition. The increase with AP5 persisted, however, implicating NMDA-receptor mediated activation of another inhibitory circuit in SNc. In the VTA, by contrast, evoked [DA]o was unaffected by GYKI-52466 and fell slightly with AP5. Neither picrotoxin nor saclofen alone or in combination had a significant effect on evoked [DA]o. When GABA receptors were blocked in the VTA, evoked [DA]o was decreased by 20% with either GYKI-52466 or AP5. These data suggest that in SNc, glutamatergic input acts predominantly on GABAergic or other inhibitory circuits to inhibit somatodendritic DA release, whereas in VTA, the timing or strength of synaptic input will govern whether the net effect on DA release is excitatory or inhibitory.

Corticostriatal combinatorics: the implications of corticostriatal axonal arborizations

The complete striatal axonal arborizations of 16 juxtacellularly stained cortical pyramidal cells were analyzed. Corticostriatal neurons were located in the medial agranular or anterior cingulate cortex of rats. All axons were of the extended type and formed synaptic contacts in both the striosomal and matrix compartments as determined by counterstaining for the mu-opiate receptor. Six axonal arborizations were from collaterals of brain stem-projecting cells and the other 10 from bilaterally projecting cells with no brain stem projections. The distribution of synaptic boutons along the axons were convolved with the average dendritic tree volume of spiny projection neurons to obtain an axonal innervation volume and innervation density map for each axon. Innervation volumes varied widely, with single axons occupying between 0.4 and 14.2% of the striatum (average = 4%). The total number of boutons formed by individual axons ranged from 25 to 2,900 (average = 879). Within the innervation volume, the density of innervation was extremely sparse but inhomogeneous. The pattern of innervation resembled matrisomes, as defined by bulk labeling and functional mapping experiments, superimposed on a low background innervation. Using this sample as representative of all corticostriatal axons, the total number of corticostriatal neurons was estimated to be 17 million, about 10 times the number of striatal projection neurons.

Regional differences in the expression of corticostriatal synaptic plasticity

Field recordings of responses to activation of corticostriatal afferents were made in coronally sectioned rat brain slices. Each recording site was categorized according to its medial to lateral and rostral to caudal position to investigate anatomical differences in synaptic plasticity. Individual responses were highly variable exhibiting extremes of tetanus induced depression and potentiation. Consequently, averaging masked the capacity of these synapses to express long-term forms of plasticity. Block of GABA(A) inhibition and elimination of dopaminergic input with 6-hydroxydopamine lesions both acted to increase the expression of potentiation, but again considerable variability was observed. Separation of recordings into medial and lateral groups revealed clear anatomical trends which contributed to the variability observed in the total sample. Paired-pulse, post-tetanic and long-term potentiation was greater in medial than in lateral groups in normal artificial cerebral spinal fluid. Similar tendencies were seen after block of GABA(A) receptors with bicuculline. 6-Hydroxydopamine lesions in combination with bicuculline treatment reduced medial to lateral differences. Factoring in medial to lateral trends revealed block of GABA(A) receptor mediated inhibition had its greatest effect on medial corticostriatal responses and 6-hydroxydopamine lesions had their greatest effect on lateral responses. From these data we suggest anatomical variation in striatal circuitry may underlie regional differences in synaptic plasticity evoked by corticostriatal activation.

Molecular mechanisms of memory acquisition, consolidation and retrieval

Memory is often considered to be a process that has several stages, including acquisition, consolidation and retrieval. Memory can be modified further through reconsolidation and performance can change during extinction trials while the original memory remains intact. Recent studies of the molecular basis of these processes have found that many signaling molecules are involved in several stages of memory but, in some cases, molecular pathways may be selectively recruited only during certain stages of memory.

Dopaminergic control of synaptic plasticity in the dorsal striatum

Cortical glutamatergic and nigral dopaminergic afferents impinge on projection spiny neurons of the striatum, providing the most significant inputs to this structure. Isolated activation of glutamate or dopamine (DA) receptors produces short-term effects on striatal neurons, whereas the combined stimulation of both glutamate and DA receptors is able to induce long-lasting modifications of synaptic excitability. Repetitive stimulation of corticostriatal fibres causes a massive release of both glutamate and DA in the striatum and, depending on the glutamate receptor subtype preferentially activated, produces either long-term depression (LTD) or long-term potentiation (LTP) of excitatory synaptic transmission. D1-like and D2-like DA receptors interact synergistically to allow LTD formation, while they operate in opposition during the induction phase of LTP. Corticostriatal synaptic plasticity is severely impaired after chronic DA denervation and requires the stimulation of DARPP-32, a small protein expressed in dopaminoceptive spiny neurons which acts as a potent inhibitor of protein phosphatase-1. In addition, the formation of LTD and LTP requires the activation of PKG and PKA, respectively, in striatal projection neurons. These kinases appear to be stimulated by the activation of D1-like receptors in distinct neuronal populations.