Abnormal synaptic plasticity in the striatum of mice lacking dopamine D2 receptors

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

High-frequency activation of excitatory striatal synapses produces lasting changes in synaptic efficacy that may contribute to motor and cognitive functions. While some of the mechanisms responsible for the induction of long-term potentiation (LTP) and long-term depression (LTD) of excitatory synaptic responses at striatal synapses have been characterized, much less is known about the factors that govern the direction of synaptic plasticity in this brain region. Here we report heterogeneous activity-dependent changes in the direction of synaptic strength in subregions of the developing rat striatum. Neurons in the dorsolateral region of the anterior striatum tended to express LTD after high-frequency afferent stimulation (HFS) in slices from animals aged P15-P34. However, HFS in dorsolateral striatum from P12-P14 elicited an N-methyl-D-aspartate (NMDA) receptor-dependent form of LTP. Synapses in the dorsomedial anterior striatum exhibited a propensity to express an NMDA-receptor dependent form of LTP across the entire developmental time period examined. The NMDA receptor antagonist (+/-)-2-amino-5-phosphopentanoic acid (APV) inhibited evoked excitatory postsynaptic potentials recorded in striatum obtained from P12-P15 rats but had little effect in striatum from older animals. The expression of multiple forms of synaptic plasticity in the striatum suggests mechanisms by which this brain region plays pivotal roles in the acquisition or encoding of some forms of motor sequencing and stereotypical behaviors.

The cortico-basal ganglia-thalamocortical circuit with synaptic plasticity. I. Modification rules for excitatory and inhibitory synapses in the striatum

It is pointed out that Ca(2+)-dependent modification rules for NMDA-dependent (NMDA-independent) synaptic plasticity in the striatum are similar to those in the neocortex and hippocampus (cerebellum). A unitary postsynaptic mechanism of synaptic modification is proposed. It is based on the assumption that, in diverse central nervous system structures, long-term potentiation/depression (LTP/LTD) of excitatory transmission (depression/potentiation of inhibitory transmission, LTDi/LTPi) is the result of an increasing/decreasing the number of phosphorylated AMPA and NMDA (GABA(A)) receptors. According to the suggested mechanism, Ca(2+)/calmodulin-dependent protein kinase II and protein kinase C, whose activity is positively correlated with Ca(2+) enlargement, together with cAMP-dependent protein kinase A (cGMP-dependent protein kinase G, whose activity is negatively correlated with Ca(2+) rise) mainly phosphorylate ionotropic striatal receptors, if NMDA channels are opened (closed). Therefore, the positive/negative post-tetanic Ca(2+) shift in relation to a previous Ca(2+) rise must cause NMDA-dependent LTP+LTDi/LTD+LTPi or NMDA-independent LTD+LTPi/LTP+LTDi. Dopamine D(1)/D(2) or adenosine A(2A)/A(1) receptor activation must facilitate LTP+LTDi/LTD+LTPi due to an augmenting/lowering PKA activity. Activation of muscarinic M(1)/M(4) receptors must enhance LTP+LTDi/LTD+LTPi as a consequence of an increase/decrease in the activity of protein kinase C/A. The proposed mechanism is in agreement with known experimental data.

Linking Hebb's coincidence-detection to memory formation

The theoretical foundations of learning and memory were laid by Donald Hebb 50 years ago. Recent genetic experiments that enhanced coincidence-detection of the NMDA receptor (a molecular master-switch in implementing Hebb's rule) and that led to better learning and memory in adult animals have substantially validated Hebb's rule in memory formation in the brain.

Haloperidol-induced catalepsy is absent in dopamine D(2), but maintained in dopamine D(3) receptor knock-out mice

We have previously found that mice homozygous for the deletion of the dopamine D(2) receptor gene (D(2)(-/-) mice) do not present spontaneous catalepsy when tested in a "bar test". In the present study, we sought to analyse the reactivity of D(2) receptor mutant mice to the cataleptogenic effects of dopamine D(2)-like or D(1)-like receptor antagonists. In parallel, we assessed the cataleptogenic effects of these antagonists in dopamine D(3) receptor mutant mice. D(2)(-/-) mice were totally unresponsive to the cataleptogenic effects of the dopamine D(2)-like receptor antagonist haloperidol (0.125-2 mg/kg i.p.), while D(2)(+/-) mice, at the highest haloperidol doses tested, showed a level of catalepsy about half that of wild-type controls. The degree of haloperidol-induced catalepsy was thus proportional to the level of striatal dopamine D(2) receptor expression (0.50, 0.30 and 0.08 pmol/mg protein as measured at 0.25 nM [3H]spiperone for D(2)(+/+), D(2)(+/-) and D(2)(-/-) mice, respectively). However, D(2)(-/-) and D(2)(+/-) mice were as sensitive as their wild-type counterparts to the cataleptogenic effects of the dopamine D(1)-like receptor antagonist R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4, 5-tetrahydro-1H-3-benzazepine hydrochloride (SCH 23390: 0.03-0.6 mg/kg s.c.). Striatal dopamine D(1) receptor expression (as measured using [3H]SCH 23390 binding) was not significantly affected by the genotype. The ability of SCH 23390 to induce catalepsy in D(2)(-/-) mice suggests that their resistance to haloperidol-induced catalepsy is due to the absence of dopamine D(2) receptors, and not to the abnormal striatal synaptic plasticity that has been shown by others to occur in these mice. In agreement with the observation that dopamine D(2) and dopamine D(1) receptor expression was essentially identical in D(3)(+/+), D(3)(+/-) and D(3)(-/-) mice, dopamine D(3) receptor homozygous and heterozygous mutant mice, on the whole, did not differ from their controls in the time spent in a cataleptic position following administration of either haloperidol (0.5-2 mg/kg i.p.) or SCH 23390 (0.03-0.6 mg/kg s.c.). Also, dopamine D(3) receptor mutant mice were no more responsive than wild-type controls when co-administered subthreshold doses of haloperidol (0.125 mg/kg) and SCH 23390 (0.03 mg/kg), suggesting that dopamine D(3) receptor knock-out mice are not more sensitive than wild-types to the synergistic effects of concurrent blockade of dopamine D(2) and dopamine D(1) receptors in this model. These results suggest that the dopamine D(2) receptor subtype is necessary for haloperidol to produce catalepsy, and that the dopamine D(3) receptor subtype appears to exert no observable control over the catalepsy produced by dopamine D(2)-like, D(1)-like and the combination of D(1)-like and D(2)-like receptor antagonists.

Tissue plasminogen activator controls multiple forms of synaptic plasticity and memory

Induction of long-term depression (LTD) in rat striatal slices revealed that this form of synaptic plasticity is coupled to an increased expression of tissue-plasminogen activator (t-PA) mRNA, as detected by the mRNA differential display technique. To further investigate the involvement of this gene in synaptic remodelling following striatal LTD, we recorded electrical activity from mice lacking the gene encoding t-PA (t-PA-KO) and from wild-type (WT) mice. Tetanic stimulation induced LTD in the large majority of striatal neurons recorded from WT mice. Conversely, LTD was absent in a significant proportion of striatal neurons obtained from mice lacking t-PA. Electrophysiological recordings obtained from hippocampal slices in the CA1 area showed that mainly the late phase of long-term potentiation (LTP) was reduced in t-PA-KO mice. Learning and memory-related behavioural abnormalities were also found in these transgenic mice. Disruption of the t-PA gene, in fact, altered both the context conditioning test, a hippocampus-related behavioural task, and the two-way active avoidance, a striatum-dependent task. In an open field object exploration task, t-PA-KO mice expressed deficits in habituation and reactivity to spatial change that are consistent with an altered hippocampal function. Nevertheless, decreased rearing and poor initial object exploration were also observed, further suggesting an altered striatal function. These data indicate that t-PA plays a critical role in the formation of various forms of synaptic plasticity and memory.

In vivo NMDA/dopamine interaction resulting in Fos production in the limbic system and basal ganglia of the mouse brain

Glutamatergic and dopaminergic effects on molecular processes have been extensively investigated in the basal ganglia. It has been demonstrated that NMDA and dopamine D(1) and D(2) receptors interact in the regulation of signal transduction and induction of transcription factors. In the present experiments, NMDA/dopamine interactions were investigated in the normosensitive caudate nucleus, hippocampus and amygdala by monitoring Fos production. We demonstrated that NMDA and the D(1) receptor agonist SKF 38393 triggered Fos levels in a distinct, non-overlapping and region-specific pattern. NMDA injected intraperitoneally (i.p.) elevated Fos levels in all hippocampal subfields and the central amygdala, whereas SKF 38393 triggered Fos production in basomedial, cortical, medial amygdala and caudate nucleus. The NMDA receptor antagonist CGS 19755 prevented NMDA- and SKF 38393-triggered Fos production in all investigated brain areas. Similarly, the D(1) receptor antagonist SCH 23390 inhibited the effects produced by SKF 38393 or NMDA. The D(2) receptor antagonist sulpiride exerted synergistic and antagonistic effects on NMDA- and SKF 38393-triggered Fos production, in a region specific manner. These data suggest that NMDA and dopamine receptors regulate Fos production within the limbic system and basal ganglia through regionally differentiated but interdependent actions.

The roles of dopamine oxidative stress and dopamine receptor signaling in aging and age-related neurodegeneration

Aging is accompanied by a decline of functions controlled by the central dopaminergic system, such as reduced locomotor activity, motivation, impairment of memory formation, and learning deficits. The molecular mechanisms underlying age-related impairment of dopaminergic functions are unknown. Current literature and our own recent work, which are reviewed and summarized in the present paper, suggest that dopamine oxidative stress and its subsequent signaling may contribute to the aging of dopaminergic system.

Involvement of ryanodine receptor type 3 in dopamine release from the striatum: evidence from mutant mice lacking this receptor

Although it is known that ryanodine receptor type 3 is expressed in the striatum, the function of this receptor has not been elucidated. Therefore, we examined whether caffeine- and ryanodine-induced dopamine release in striatal slices is affected in mice lacking ryanodine receptor type 3. Pretreatment with thapsigargin, an inhibitor of the Ca(2+) ATPase pump of the endoplasmic reticulum, abolished caffeine- or ryanodine-induced dopamine release in slices from normal mice. Dopamine concentration in the striatum and KCl-induced dopamine release were unaffected by a ryanodine receptor type 3 deficiency. Ryanodine-induced dopamine release was significantly attenuated in mice lacking ryanodine receptor type 3, whereas caffeine-induced dopamine release was partially attenuated. Caffeine produced a similar hyper-motor activity in both wild and homozygous mice. The present results suggest the involvement of ryanodine receptor type 3 in dopamine release from the striatum.

Unilateral dopamine denervation blocks corticostriatal LTP

The nigrostriatal dopaminergic projection is crucial for the striatal processing of motor information received from the cortex. Lesion of this pathway in rats causes locomotor alterations that resemble some of the symptoms of Parkinson's disease and significantly alters the excitatory transmission in the striatum. We performed in vitro electrophysiological recordings to study the effects of unilateral striatal dopamine (DA) denervation obtained by omolateral nigral injection of 6-hydroxydopamine (6-OHDA) in the formation of corticostriatal long-term potentiation (LTP). Unilateral nigral lesion did not affect the intrinsic membrane properties of striatal spiny neurons. In fact, these cells showed similar pattern of firing discharge and current-voltage relationship in denervated striata and in naive controlateral striata. Moreover, excitatory postsynaptic potentials (EPSPs) evoked by stimulating corticostriatal fibers and recorded from DA-denervated slices showed a pharmacology similar to that observed in slices obtained from controlateral intact striata. Conversely, in magnesium-free medium, high-frequency stimulation (HFS) of corticostriatal fibers produced LTP in slices from nondenervated striata but not in slices from 6-OHDA-denervated rats. After denervation, in fact, no significant changes in the amplitude of extra- and intracellular synaptic potentials were recorded after the conditioning HFS. The absence of corticostriatal LTP in DA-denervated striata might represent the cellular substrate for some of the movement disorders observed in Parkinson's disease.

Cortically driven Fos induction in the striatum is amplified by local dopamine D2-class receptor blockade

Dopamine D2-class receptors have been shown to control the excitability of striatal neurons in response to cortical activation. It has been unclear, however, whether such receptors could regulate the number of striatal neurons activated by cortical stimulation, and thus affect the population response of the striatum to its cortical inputs. We used Fos induction as a readout to measure the ensemble response of striatal neurons to localized stimulation of the frontal cortex and tested for the effects of D2-class dopamine receptor blockade on this response. In freely moving rats, we stimulated the frontal cortex by local epidural application of a dose of a GABAA receptor antagonist (picrotoxin) just threshold for inducing Fos in the striatum. We combined this treatment with D2-class dopamine receptor antagonist treatments at dose levels also just threshold for inducing Fos, using either (i) systemic haloperidol or (ii) intrastriatal (-)sulpiride. Both systemic and intrastriatal blockade of D2-class receptors sharply increased the numbers of striatal neurons exhibiting cortically evoked Fos induction. These findings suggest that local activation of intrastriatal D2-class dopamine receptors can regulate the number of striatal neurons responsive to cortical inputs, thus dynamically shaping the flow of information through the striatum.

Enhancement of N-methyl-D-aspartate (NMDA) immunoreactivity in residual dendritic spines in the caudate-putamen nucleus after chronic haloperidol administration

Glutamate receptors of the N-methyl-D-aspartate (NMDA) subtype in the caudate-putamen nucleus (CPN) have been implicated in the adverse motor effects produced by chronic administration of the typical antipsychotic drug haloperidol. To determine the functionally relevant sites, we examined the electron microscopic immunocytochemical localization of the R1 receptor subunit (NMDAR1) in the dorsolateral CPN of rats receiving 4 months of biweekly depot intramuscular injections of either haloperidol or vehicle. In all animals, NMDAR1 immunoreactivity was seen mainly in dendritic spines, but was also present in a few somata and dendrites of spiny neurons, axon terminals, and glia. In comparison with controls, the dissector stereological analysis showed a significant reduction in the numerical density of total NMDAR1-labeled and unlabeled dendritic spines in the dorsolateral CPN after haloperidol administration. When labeled spines were identified separately based exclusively on the presence of immunoreactivity within a single plane of section, there was, however, a significant increase in the numerical density of NMDAR1-containing spines in haloperidol vs. control animals. This increase was not seen using a classic dissector, suggesting that the enhancement was mainly attributed to more frequent detection of spines having higher levels of NMDA immunoreactivity. Our results are the first to identify dendritic spines in the dorsolateral CPN as preferential sites for the regulated expression of NMDA receptors following chronic administration of haloperidol.

Modulation of gene expression following long-term synaptic depression in the striatum

A number of behavioural and cellular studies have suggested that activity-dependent synaptic plasticity associated with learning and memory may lead to the expression of various genes whose protein products can play a critical role in memory acquisition and consolidation. Long-term potentiation (LTP) and long-term depression (LTD) represent two forms of synaptic plasticity which have been widely studied by electrophysiological techniques. However, the molecular mechanisms at target gene involved in the generation of long term depression remain to be determined. To elucidate the molecular mechanism underlying activity dependent synaptic remodeling in striatal long term depression, we used the mRNA differential display technology to isolate genes that are induced or modulated by high frequency stimulation of the corticostriatal pathway in a rat brain slice preparation. We have differentially displayed, by means of reverse transcriptase-polymerase chain reaction, mRNA species isolated from striatal slices in which long term depression was induced by tetanic stimuli as well as from slices stimulated at low frequency. We then compared radio-labeled RT-PCR banding patterns to isolate cDNAs that are differentially expressed. Three independent cDNAs were isolated and identified whose mRNA level were enhanced by tetanic stimulation inducing long term depression. We provide evidence that two of these genes encode proteins involved in synaptic vesicle trafficking (dynamin I and amphiphysin II). Moreover, expression of tissue plasminogen activator (t-PA) gene was also increased following striatal long term depression. Our data suggest that a complex pattern of genes acting at presynaptic level and extracellularly may be involved in LTD-associated synaptic remodeling.

Glutamate-triggered events inducing corticostriatal long-term depression

Repetitive activation of corticostriatal fibers produces long-term depression (LTD) of excitatory synaptic potentials recorded from striatal spiny neurons. This form of synaptic plasticity might be considered the possible neural basis of some forms of motor learning and memory. In the present study, intracellular recordings were performed from rat corticostriatal slice preparations to study the role of glutamate and other critical factors underlying striatal LTD. In current-clamp, but not in voltage-clamp experiments, brief focal applications of glutamate, as well as high-frequency stimulation (HFS) of corticostriatal fibers, induced LTD. This pharmacological LTD and the HFS-induced LTD were mutually occlusive, suggesting that both forms of synaptic plasticity share common induction mechanisms. Isolated activation of either non-NMDA-ionotropic glutamate receptors (iGluRs) or metabotropic glutamate receptors (mGluRs), respectively by AMPA and t-ACPD failed to produce significant long-term changes of corticostriatal synaptic transmission. Conversely, LTD was obtained after the simultaneous application of AMPA plus t-ACPD. Moreover, also quisqualate, a compound that activates both iGluRs and group I mGluRs, was able to induce this form of pharmacological LTD. Electrical depolarization of the recorded neurons either alone or in the presence of t-ACPD and dopamine (DA) failed to mimic the effects of the activation of glutamate receptors in inducing LTD. However, electrical depolarization was able to induce LTD when preceded by coadministration of t-ACPD, DA, and a low dose of hydroxylamine, a compound generating nitric oxide (NO) in the tissue. None of these compounds alone produced LTD. Glutamate-induced LTD, as well as the HFS-induced LTD, was blocked by L-sulpiride, a D2 DA receptor antagonist, and by 7-nitroindazole monosodium salt, a NO synthase inhibitor. The present study indicates that four main factors are required to induce corticostriatal LTD: (1) membrane depolarization of the postsynaptic neuron; (2) activation of mGluRs; (3) activation of DA receptors; and (4) release of NO from striatal interneurons.

A neural network model with dopamine-like reinforcement signal that learns a spatial delayed response task

This study investigated how the simulated response of dopamine neurons to reward-related stimuli could be used as reinforcement signal for learning a spatial delayed response task. Spatial delayed response tasks assess the functions of frontal cortex and basal ganglia in short-term memory, movement preparation and expectation of environmental events. In these tasks, a stimulus appears for a short period at a particular location, and after a delay the subject moves to the location indicated. Dopamine neurons are activated by unpredicted rewards and reward-predicting stimuli, are not influenced by fully predicted rewards, and are depressed by omitted rewards. Thus, they appear to report an error in the prediction of reward, which is the crucial reinforcement term in formal learning theories. Theoretical studies on reinforcement learning have shown that signals similar to dopamine responses can be used as effective teaching signals for learning. A neural network model implementing the temporal difference algorithm was trained to perform a simulated spatial delayed response task. The reinforcement signal was modeled according to the basic characteristics of dopamine responses to novel stimuli, primary rewards and reward-predicting stimuli. A Critic component analogous to dopamine neurons computed a temporal error in the prediction of reinforcement and emitted this signal to an Actor component which mediated the behavioral output. The spatial delayed response task was learned via two subtasks introducing spatial choices and temporal delays, in the same manner as monkeys in the laboratory. In all three tasks, the reinforcement signal of the Critic developed in a similar manner to the responses of natural dopamine neurons in comparable learning situations, and the learning curves of the Actor replicated the progress of learning observed in the animals. Several manipulations demonstrated further the efficacy of the particular characteristics of the dopamine-like reinforcement signal. Omission of reward induced a phasic reduction of the reinforcement signal at the time of the reward and led to extinction of learned actions. A reinforcement signal without prediction error resulted in impaired learning because of perseverative errors. Loss of learned behavior was seen with sustained reductions of the reinforcement signal, a situation in general comparable to the loss of dopamine innervation in Parkinsonian patients and experimentally lesioned animals. The striking similarities in teaching signals and learning behavior between the computational and biological results suggest that dopamine-like reward responses may serve as effective teaching signals for learning behavioral tasks that are typical for primate cognitive behavior, such as spatial delayed responding.

Expression of N-methyl-D-aspartate receptor-dependent long-term potentiation in the neostriatal neurons in an in vitro slice after ethanol withdrawal of the rat

To examine changes in corticostriatal synaptic transmission in rats with ethanol withdrawal syndrome, intracellular and extracellular responses to subcortical white matter stimulation were recorded in neostriatal slice preparations. The resting membrane potential, input resistance and depolarizing postsynaptic potentials to single cortical white matter stimulation were similar in the neostriatum of naive and ethanol withdrawal rats. Repetitive stimulation of the white matter induced more pronounced N-methyl-D-aspartate receptor-mediated postsynaptic potentials in ethanol withdrawal than naive rat neostriatum. In intracellular recording, tetanic stimulation (50 Hz, 20 s) induced more pronounced post-tetanic potentiation of depolarizing postsynaptic potentials in the neostriatum of ethanol withdrawal than naive rats. However, in extracellular recording, tetanic stimulation induced smaller post-tetanic depression of population spikes in the neostriatum of ethanol withdrawal than naive rats. Tetanic stimulation of the subcortical white matter induced long-term potentiation of postsynaptic potentials and population spikes in the ethanol withdrawal rat neostriatum, while long-term depression was evoked in the naive rat neostriatum. The induction of long-term potentiation was blocked by D-2-amino-5-phosphonovaleric acid or 7-chlorokynurenic acid, N-methyl-D-aspartate receptor antagonists, but not by (RS)-methyl-4-carboxyphenyl-glycine, a metabotropic glutamate receptor antagonist. Dopamine also significantly depressed the induction of long-term potentiation in ethanol withdrawal rat neostriatum and this depressant effect was antagonized by the D2 antagonist L-sulpiride but not by the D1 antagonist SCH23390. These results indicate that the N-methyl-D-aspartate component of the corticostriatal glutamatergic responses, which might be necessary for induction of long-term potentiation, was enhanced in ethanol withdrawal rats. The depression of long-term potentiation induction by activation of D2 receptor suggests that corticostriatal N-methyl-D-aspartate response or intracellular mechanisms involving in the induction of the long-term potentiation can be suppressed by D2 activation and that the D2 effects are inhibited in the neostriatum of ethanol withdrawal rats.

New insights into dopaminergic receptor function using antisense and genetically altered animals

Dopaminergic receptors are widespread throughout the central and peripheral nervous systems, where they regulate a variety of physiological, behavioral, and endocrine functions. These receptors are also clinically important drug targets for the treatment of a number of disorders, such as Parkinson's disease, schizophrenia, and hyperprolactinemia. To date, five different dopamine receptor subtypes have been cloned and characterized. Many of these subtypes are pharmacologically similar, making it difficult to selectively stimulate or block a specific receptor subtype in vivo. Thus, the assignment of various physiological or behavioral functions to specific dopamine receptor subtypes using pharmacological tools is difficult. In view of this, a number of investigators have--in order to elucidate functional roles--begun to use highly selective genetic approaches to alter the expression of individual dopamine receptor subtypes in vivo. This review discusses recent studies involving the use of genetic approaches for the study of dopaminergic receptor function.

A critical role of the nitric oxide/cGMP pathway in corticostriatal long-term depression

High-frequency stimulation (HFS) of corticostriatal glutamatergic fibers induces long-term depression (LTD) of excitatory synaptic potentials recorded from striatal spiny neurons. This form of LTD can be mimicked by zaprinast, a selective inhibitor of cGMP phosphodiesterases (PDEs). Biochemical analysis shows that most of the striatal cGMP PDE activity is calmodulin-dependent and inhibited by zaprinast. The zaprinast-induced LTD occludes further depression by tetanic stimulation and vice versa. Both forms of synaptic plasticity are blocked by intracellular 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxalin-1-one (ODQ), a selective inhibitor of soluble guanylyl cyclase, indicating that an increased cGMP production in the spiny neuron is a key step. Accordingly, intracellular cGMP, activating protein kinase G (PKG), also induces LTD. Nitric oxide synthase (NOS) inhibitors N(G)-nitro-L-arginine methyl ester hydrochloride (L-NAME) and 7-nitroindazole monosodium salt (7-NINA) block LTD induced by either HFS or zaprinast, but not that induced by cGMP. LTD is also induced by the NO donors S-nitroso-N-acetylpenicillamine (SNAP) and hydroxylamine. SNAP-induced LTD occludes further depression by HFS or zaprinast, and it is blocked by intracellular ODQ but not by L-NAME. Intracellular application of PKG inhibitors blocks LTD induced by HFS, zaprinast, and SNAP. Electron microscopy immunocytochemistry shows the presence of NOS-positive terminals of striatal interneurons forming synaptic contacts with dendrites of spiny neurons. These findings represent the first demonstration that the NO/cGMP pathway exerts a feed-forward control on the corticostriatal synaptic plasticity.

Modulation of long-term synaptic plasticity at excitatory striatal synapses

Modulation of long-term plasticity by both the intrinsic activation of metabotropic glutamate receptors and dopamine released from the nigrostriatal pathway was investigated at excitatory striatal synapses. Intracellular recordings demonstrated that tetanic stimulation at an intensity equal to that used for synaptic sampling produced, on average, a slight long-term depression of excitatory postsynaptic potentials. The long-term response pattern was variable, however, with some cells showing potentiation and others no plasticity. Block of metabotropic glutamate receptors with 3-aminophosphonovaleric acid changed the pattern of responses, increasing the percentage of cells showing long-term potentiation. Similarly, 6-hydroxydopamine lesions to the substantia nigra changed the pattern of response to tetanic stimulation, increasing the expression of long-term potentiation. These data indicate that metabotropic glutamate receptor and dopamine receptor activation may function to regulate the expression of activity-dependent plasticity at corticostriatial synapses. Paired-pulse stimulation revealed that post-tetanic plasticity was negatively correlated with changes in paired-pulse plasticity in the control and 6-hydroxydopamine-lesioned groups, suggesting that the expression of long-term plasticity has a presynaptic component at corticostriatal synapses.

Endogenous ACh enhances striatal NMDA-responses via M1-like muscarinic receptors and PKC activation

Cortical glutamatergic fibres and cholinergic inputs arising from large aspiny interneurons converge on striatal spiny neurons and play a major role in the control of motor activity. We have investigated the interaction between excitatory amino acids and acetylcholine (ACh) on striatal spiny neurons by utilizing intracellular recordings, both in current- and in voltage-clamp mode in rat brain slices. Muscarine (0.3-10 microM) produced a reversible and dose-dependent increase in the membrane depolarizations/inward currents induced by brief applications of N-methyl-D-aspartate (NMDA), while it did not affect the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-induced responses. These concentrations of muscarine did not alter the membrane potential and the current-voltage relationship of the recorded cells. Neostigmine (0.3-10 microM), an ACh-esterase inhibitor, mimicked this facilitatory effect. The facilitatory effects of muscarine and neostigmine were antagonized either by scopolamine (3 microM) or by pirenzepine (10-100 nM), an antagonist of M1-like muscarinic receptors, but not by methoctramine (300 nM), an antagonist of M2-like muscarinic receptor. Accordingly, these facilitatory effects were mimicked by McN-A-343 (1-10 microM), an agonist of M1-like muscarinic receptors, but not by oxotremorine (300 nM), an agonist of M2-like receptors. Tetrodotoxin (TTX) did not block the facilitatory effect produced by the activation of muscarinic receptors suggesting that this effect is postsynaptically mediated. The action of neostigmine was prevented either by the intracellular calcium (Ca2+) chelator BAPTA (200 mM) or by preincubating the slices with inhibitors of protein kinase C (PKC) (staurosporine 100 nM or calphostin C 1 microM). McN-A-343 did not alter the excitatory post synaptic potentials (EPSPs) evoked by corticostriatal stimulation in the presence of physiological concentration of magnesium (Mg2+ 1.2 mM), while it enhanced the duration of these EPSPs recorded in the absence of external magnesium. Our data show that endogenous striatal ACh exerts a positive modulatory action on NMDA responses via M1-like muscarinic receptors and PKC activation.

Dopamine neurons report an error in the temporal prediction of reward during learning

Many behaviors are affected by rewards, undergoing long-term changes when rewards are different than predicted but remaining unchanged when rewards occur exactly as predicted. The discrepancy between reward occurrence and reward prediction is termed an 'error in reward prediction'. Dopamine neurons in the substantia nigra and the ventral tegmental area are believed to be involved in reward-dependent behaviors. Consistent with this role, they are activated by rewards, and because they are activated more strongly by unpredicted than by predicted rewards they may play a role in learning. The present study investigated whether monkey dopamine neurons code an error in reward prediction during the course of learning. Dopamine neuron responses reflected the changes in reward prediction during individual learning episodes; dopamine neurons were activated by rewards during early trials, when errors were frequent and rewards unpredictable, but activation was progressively reduced as performance was consolidated and rewards became more predictable. These neurons were also activated when rewards occurred at unpredicted times and were depressed when rewards were omitted at the predicted times. Thus, dopamine neurons code errors in the prediction of both the occurrence and the time of rewards. In this respect, their responses resemble the teaching signals that have been employed in particularly efficient computational learning models.

Predictive reward signal of dopamine neurons

The effects of lesions, receptor blocking, electrical self-stimulation, and drugs of abuse suggest that midbrain dopamine systems are involved in processing reward information and learning approach behavior. Most dopamine neurons show phasic activations after primary liquid and food rewards and conditioned, reward-predicting visual and auditory stimuli. They show biphasic, activation-depression responses after stimuli that resemble reward-predicting stimuli or are novel or particularly salient. However, only few phasic activations follow aversive stimuli. Thus dopamine neurons label environmental stimuli with appetitive value, predict and detect rewards and signal alerting and motivating events. By failing to discriminate between different rewards, dopamine neurons appear to emit an alerting message about the surprising presence or absence of rewards. All responses to rewards and reward-predicting stimuli depend on event predictability. Dopamine neurons are activated by rewarding events that are better than predicted, remain uninfluenced by events that are as good as predicted, and are depressed by events that are worse than predicted. By signaling rewards according to a prediction error, dopamine responses have the formal characteristics of a teaching signal postulated by reinforcement learning theories. Dopamine responses transfer during learning from primary rewards to reward-predicting stimuli. This may contribute to neuronal mechanisms underlying the retrograde action of rewards, one of the main puzzles in reinforcement learning. The impulse response releases a short pulse of dopamine onto many dendrites, thus broadcasting a rather global reinforcement signal to postsynaptic neurons. This signal may improve approach behavior by providing advance reward information before the behavior occurs, and may contribute to learning by modifying synaptic transmission. The dopamine reward signal is supplemented by activity in neurons in striatum, frontal cortex, and amygdala, which process specific reward information but do not emit a global reward prediction error signal. A cooperation between the different reward signals may assure the use of specific rewards for selectively reinforcing behaviors. Among the other projection systems, noradrenaline neurons predominantly serve attentional mechanisms and nucleus basalis neurons code rewards heterogeneously. Cerebellar climbing fibers signal errors in motor performance or errors in the prediction of aversive events to cerebellar Purkinje cells. Most deficits following dopamine-depleting lesions are not easily explained by a defective reward signal but may reflect the absence of a general enabling function of tonic levels of extracellular dopamine. Thus dopamine systems may have two functions, the phasic transmission of reward information and the tonic enabling of postsynaptic neurons.

The basal ganglia and chunking of action repertoires

The basal ganglia have been shown to contribute to habit and stimulus-response (S-R) learning. These forms of learning have the property of slow acquisition and, in humans, can occur without conscious awareness. This paper proposes that one aspect of basal ganglia-based learning is the recoding of cortically derived information within the striatum. Modular corticostriatal projection patterns, demonstrated experimentally, are viewed as producing recoded templates suitable for the gradual selection of new input-output relations in cortico-basal ganglia loops. Recordings from striatal projection neurons and interneurons show that activity patterns in the striatum are modified gradually during the course of S-R learning. It is proposed that this recoding within the striatum can chunk the representations of motor and cognitive action sequences so that they can be implemented as performance units. This scheme generalizes Miller's notion of information chunking to action control. The formation and the efficient implementation of action chunks are viewed as being based on predictive signals. It is suggested that information chunking provides a mechanism for the acquisition and the expression of action repertoires that, without such information compression would be biologically unwieldy or difficult to implement. The learning and memory functions of the basal ganglia are thus seen as core features of the basal ganglia's influence on motor and cognitive pattern generators.

Prominence of the dopamine D2 short isoform in dopaminergic pathways

As a result of alternative splicing, the D2 gene of the dopamine receptor family exists in two isoforms. The D2 long is characterized by the insertion of 29 amino acids in the third cytoplasmic loop, which is absent in the short isoform. We have produced subtype-specific antibodies against both the D2 short and D2 long isoforms and found a unique compartmentalization between these two isoforms in the primate brain. The D2 short predominates in the cell bodies and projection axons of the dopaminergic cell groups of the mesencephalon and hypothalamus, whereas the D2 long is more strongly expressed by neurons in the striatum and nucleus accumbens, structures targeted by dopaminergic fibers. These results show that the splice variants of the dopamine D2 receptor are differentially distributed and possess distinct functions. The strategic localization of the D2 short isoform in dopaminergic cell bodies and axons strongly suggests that this isoform is the likely dopamine autoreceptor, whereas the D2 long isoform is primarily a postsynaptic receptor.

Lack of autoreceptor-mediated inhibitory control of dopamine release in striatal synaptosomes of D2 receptor-deficient mice

Mouse purified striatal synaptosomes were used to study the release of newly synthesised [3H]-dopamine ([3H]-DA) or of previously taken up [3H]-DA. Quinpirole (QP, 10 microM), a D2/D3 dopaminergic agonist, was found to reduce the release of newly synthesised [3H]-DA with a larger amplitude when 4-aminopyridine (100 microM) instead than veratridine (1 microM) or potassium (25 mM) was used to evoke DA release. Among the different D2/D3 dopaminergic agonists tested R(-)-propylnorapomorphine (NPA) and quinpirole were the most potent. These compounds reduced, in a concentration-dependent manner, the 4-aminopyridine-evoked release of [3H]-DA previously taken up by synaptosomes (50% maximal inhibition). In contrast, the D3 agonist PD-128,907 had little effect even when used at 100 nM. The QP (100 nM)-induced response was completely antagonised by sulpiride (1 microM). Strikingly, the NPA (100 nM) and PD-128,907 (100 nM)-evoked responses were completely suppressed in D2 receptor-deficient mice. This data strongly suggest that only D2 but not D3 receptors are involved in the autoreceptor-mediated inhibition of the evoked release of [3H]-DA. Interestingly, while amphetamine-induced release of [3H]-DA was not modified, a slight reduction of [3H]-DA efflux induced by the dopamine (DA) uptake inhibitor cocaine was observed in D2 receptor-deficient mice.

D1/D5 dopamine receptors inhibit depotentiation at CA1 synapses via cAMP-dependent mechanism

Recent work has shown that D1/D5 dopamine receptors can enhance long-term potentiation (LTP). We investigated whether D1/D5 receptors also affect depotentiation, the reversal of LTP by low-frequency stimulation. D1/D5 agonists greatly reduced depotentiation, an effect that was inhibited by a D1/D5 antagonist. The D1/D5 effect appears to be mediated by adenylyl cyclase (AC) and cAMP-dependent protein kinase (PKA), because it was mimicked by the AC activator forskolin and was inhibited by the AC and PKA inhibitors. In vivo studies show that dopamine is released when a reward occurs. Our results raise the possibility that the memory of events before reward might be retained selectively, because dopamine blocks their erasure.

Dopamine D3 receptor mutant mice exhibit increased behavioral sensitivity to concurrent stimulation of D1 and D2 receptors

The dopamine D3 receptor is expressed primarily in regions of the brain that are thought to influence motivation and motor functions. To specify in vivo D3 receptor function, we generated mutant mice lacking this receptor. Our analysis indicates that in a novel environment, D3 mutant mice are transiently more active than wild-type mice, an effect not associated with anxiety state. Moreover, D3 mutant mice exhibit enhanced behavioral sensitivity to combined injections of D1 and D2 class receptor agonists, cocaine and amphetamine. However, the combined electrophysiological effects of the same D1 and D2 agonists on single neurons within the nucleus accumbens were not altered by the D3 receptor mutation. We conclude that one function of the D3 receptor is to modulate behaviors by inhibiting the cooperative effects of postsynaptic D1 and other D2 class receptors at systems level.