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

Brief mitochondrial inhibition causes lasting changes in motor behavior and corticostriatal synaptic physiology in the Fischer 344 rat

The striatum is particularly vulnerable to mitochondrial dysfunction and this problem is linked to pathology created by environmental neurotoxins, stimulants like amphetamine, and metabolic disease and ischemia. We studied the course of recovery following a single systemic injection of the mitochondrial complex II inhibitor 3-nitropropionic acid (3-NP) and found 3-NP caused lasting changes in motor behavior that were associated with altered activity-dependent plasticity at corticostriatal synapses in Fischer 344 rats. The changes in synapse behavior varied with the time after exposure to the 3-NP injection. The earliest time point studied, 24h after 3-NP, revealed 3-NP-induced an exaggeration of D1 Dopamine (DA) receptor dependent long-term potentiation (LTP) that reversed to normal by 48 h post-3-NP exposure. Thereafter, the likelihood and degree of inducing D2 DA receptor dependent long-term depression (LTD) gradually increased, relative to saline controls, peaking at 1 month after the 3-NP exposure. NMDA receptor binding did not change over the same post 3-NP time points. These data indicate even brief exposure to 3-NP can have lasting behavioral effects mediated by changes in the way DA and glutamate synapses interact.

RGS4 is required for dopaminergic control of striatal LTD and susceptibility to parkinsonian motor deficits

Plasticity of excitatory synapses onto striatal projection neurons (MSNs) has the potential to regulate motor function by setting the gain on signals driving both direct- and indirect-pathway basal ganglia circuits. Endocannabinoid-dependent long-term depression (eCB-LTD) is the best characterized form of striatal plasticity, but the mechanisms governing its normal regulation and pathological dysregulation are not well understood. We characterized two distinct signaling pathways mediating eCB production in striatal indirect-pathway MSNs and found that both pathways were modulated by dopamine D2 and adenosine A2A receptors, acting through cAMP/PKA. We identified regulator of G protein signaling 4 (RGS4) as a key link between D2/A2A signaling and eCB mobilization pathways. In contrast to wild-type mice, RGS4⁻/⁻ mice exhibited normal eCB-LTD after dopamine depletion and were significantly less impaired in the 6-OHDA model of Parkinson's disease. Taken together, these results suggest that inhibition of RGS4 may be an effective nondopaminergic strategy for treating Parkinson's disease.

Significance of F3/Contactin gene expression in cerebral cortex and nigrostriatal development

F3/Contactin is a neuronal surface glycoprotein, which plays a general role in neural development and, in particular, in neuronal and oligodendrocyte differentiation. In a previous study using the F3/EGFP transgenic mice, which express an EGFP reporter under control of the regulatory region from the mouse F3/Contactin gene, the activation of the F3/Contactin promoter was found to correlate with granule and Purkinje neuron differentiation in developing cerebellar cortex. Here we report that in developing cerebral cortex and basal ganglia the F3/Contactin gene is mostly activated during early commitment of neuronal precursors, thus indicating a region-specific profile of its developmental activation. We also report that, in the same structures of F3/EGFP mice, a downregulation of the endogenous F3/Contactin gene occurs, which correlates with upregulation of the dopaminergic phenotype and with locomotor pattern abnormalities. Therefore, F3/EGFP transgenic mice exhibit morphological and functional phenotypes recapitulating those arising from imbalance of the striatal dopaminergic pathway. As for the underlying mechanisms, we postulate that in F3/EGFP mice F3/Contactin downregulation results from the ability of transgene promoter sequences to interfere with the activation of the endogenous gene, thus realizing an F3/Contactin knockdown model, while dopaminergic upregulation is consistent with a general F3/Contactin inhibitory effect on the neuronal phenotype.

Frequency selectivity and dopamine-dependence of plasticity at glutamatergic synapses in the subthalamic nucleus

In Parkinson's disease, subthalamic nucleus (STN) neurons burst fire with increased periodicity and synchrony. This may entail abnormal release of glutamate, the major source of which in STN is cortical afferents. Indeed, the cortico-subthalamic pathway is implicated in the emergence of excessive oscillations, which are reduced, as are symptoms, by dopamine-replacement therapy or deep brain stimulation (DBS) targeted to STN. Here we hypothesize that glutamatergic synapses in the STN may be differentially modulated by low-frequency stimulation (LFS) and high-frequency stimulation (HFS), the latter mimicking deep brain stimulation. Recordings of evoked and spontaneous excitatory post synaptic currents (EPSCs) were made from STN neurons in brain slices obtained from dopamine-intact and chronically dopamine-depleted adult rats. HFS had no significant effect on evoked (e) EPSC amplitude in dopamine-intact slices (104.4±8.0%) but depressed eEPSCs in dopamine-depleted slices (67.8±6.2%). Conversely, LFS potentiated eEPSCs in dopamine-intact slices (126.4±8.1%) but not in dopamine-depleted slices (106.7±10.0%). Analyses of paired-pulse ratio, coefficient of variation, and spontaneous EPSCs suggest that the depression and potentiation have a presynaptic locus of expression. These results indicate that the synaptic efficacy in dopamine-intact tissue is enhanced by LFS. Furthermore, the synaptic efficacy in dopamine-depleted tissue is depressed by HFS. Therefore the therapeutic effects of DBS in Parkinson's disease appear mediated, in part, by glutamatergic cortico-subthalamic synaptic depression and implicate dopamine-dependent increases in the weight of glutamate synapses, which would facilitate the transfer of pathological oscillations from the cortex.

Drosophila Dopamine2-like receptors function as autoreceptors

Dopaminergic signaling pathways are conserved between mammals and Drosophila and D2 receptors have been identified in Drosophila. However, it has not been demonstrated whether Drosophila D2 receptors function as autoreceptors and regulate the release of dopamine. The goal of this study was to determine if Drosophila D2 receptors act as autoreceptors by probing the extent to which D2 agonists and antagonists affect evoked dopamine release. Fast-scan cyclic voltammetry was used to measure stimulated dopamine release at a carbon-fiber microelectrode implanted in an intact, larval Drosophila nervous system. Dopamine release was evoked using 5-second blue light stimulations that open Channelrhodopsin-2, a blue light activated cation channel that was specifically expressed in dopaminergic neurons. In mammals, administration of a D2 agonist decreases evoked dopamine release by increasing autoreceptor feedback. Similarly, we found that the D2 agonists bromocriptine and quinpirole decreased stimulated dopamine release in Drosophila. D2 antagonists were expected to increase dopamine release and the D2 antagonists flupenthixol, butaclamol, and haloperidol did increase stimulated release. Agonists did not significantly modulate dopamine uptake although the modulatory effects of D2 drugs on release were affected by prior administration of the uptake inhibitor nisoxetine. These results demonstrate that the D2 receptor functions as an autoreceptor in Drosophila. The similarities in dopamine regulation validate Drosophila as a model system for studying the basic neurobiology of dopaminergic signaling.

Amygdala-prefrontal pathways and the dopamine system affect nociceptive responses in the prefrontal cortex

Background

We previously demonstrated nociceptive discharges to be evoked by mechanical noxious stimulation in the prefrontal cortex (PFC). The nociceptive responses recorded in the PFC are conceivably involved in the affective rather than the sensory-discriminative dimension of pain. The PFC receives dense projection from the limbic system. Monosynaptic projections from the basolateral nucleus of the amygdala (BLA) to the PFC are known to produce long-lasting synaptic plasticity. We examined effects of high frequency stimulation (HFS) delivered to the BLA on nociceptive responses in the rat PFC.

Results

HFS induced long lasting suppression (LLS) of the specific high threshold responses of nociceptive neurons in the PFC. Microinjection of N-methyl-D-aspartic acid (NMDA) receptor antagonists (2-amino-5-phosphonovaleric acid (APV), dizocilpine (MK-801)) and also metabotropic glutamate receptor (mGluR) group antagonists (α-methyl-4-carboxyphenylglycine (MCPG), and 2-[(1S,2S)-2-carboxycyclopropyl]-3-(9H-xanthen-9-yl)-D-alanine (LY341495)), prevented the induction of LLS of nociceptive responses. We also examined modulatory effects of dopamine (DA) on the LLS of nociceptive responses. With depletion of DA in response to 6-hydroxydopamine (6-OHDA) injection into the ipsilateral forebrain bundle, LLS of nociceptive responses was decreased, while nociceptive responses were normally evoked. Antagonists of DA receptor subtypes D2 (sulpiride) and D4 (3-{[4-(4-chlorophenyl) piperazin-1-yl] methyl}-1H-pyrrolo [2, 3-b] pyridine (L-745,870)), microinjected into the PFC, inhibited LLS of nociceptive responses.

Conclusions

Our results indicate that BLA-PFC pathways inhibited PFC nociceptive cell activities and that the DA system modifies the BLA-PFC regulatory function.

Genetically engineered mouse models of Parkinson's disease

Parkinson's disease (PD) is the most common neurodegenerative movement disorder, affecting more than 1% of the population over age 60. The most common feature of PD is a resting tremor, though there are many systemic neurological effects, such as incontinence and sleep disorders. PD is histopathologically identified by the presence of Lewy bodies (LB), proteinaceous inclusions constituted primarily by α-synuclein. To date, there is no effective treatment to slow or stop disease progression. To help understand disease pathogenesis and identify potential therapeutic targets, many genetic mouse models have been developed. By far the most common of these models are the wildtype and mutant α-synuclein transgenic mice, because α-synuclein was the first protein shown to have a direct effect on PD pathogenesis and progression. There are many other gene-disrupted or -mutated models currently available, which are based on genetic anomalies identified in the human disease. In addition, there are also models which examine genes that may contribute to disease onset or progression but currently have no identified causative PD mutations. These genes are part of signaling pathways important for maintaining neuronal function in the nigrostriatal pathway. This review will summarize the most commonly used of the genetic mouse models currently available for PD research. We will examine how these models have expanded our understanding of PD pathogenesis and progression, as well as aided in identification of potential therapeutic targets in this disorder.

Dopamine receptors and Parkinson's disease

Parkinson's disease (PD) is a progressive extrapyramidal motor disorder. Pathologically, this disease is characterized by the selective dopaminergic (DAergic) neuronal degeneration in the substantia nigra. Correcting the DA deficiency in PD with levodopa (L-dopa) significantly attenuates the motor symptoms; however, its effectiveness often declines, and L-dopa-related adverse effects emerge after long-term treatment. Nowadays, DA receptor agonists are useful medication even regarded as first choice to delay the starting of L-dopa therapy. In advanced stage of PD, they are also used as adjunct therapy together with L-dopa. DA receptor agonists act by stimulation of presynaptic and postsynaptic DA receptors. Despite the usefulness, they could be causative drugs for valvulopathy and nonmotor complication such as DA dysregulation syndrome (DDS). In this paper, physiological characteristics of DA receptor familyare discussed. We also discuss the validity, benefits, and specific adverse effects of pharmaceutical DA receptor agonist.

Mortalin inhibition in experimental Parkinson's disease

Among heat shock proteins, mortalin has been linked to the pathogenesis of Parkinson's disease. In the present work a rat model of Parkinson's disease was used to analyze the expression of striatal proteins and, more specifically, mortalin expression. The possible involvement of mortalin in Parkinson's disease pathogenesis was further investigated by utilizing an electrophysiological approach and pharmacological inhibition of mortalin in both the physiological and the parkinsonian states. Proteomic analysis was used to investigate changes in striatal protein expression in the 6-hydroxydopamine rat model of Parkinson's disease. The electrophysiological effects of MKT-077, a rhodamine-123 analogue acting as an inhibitor of mortalin, were measured by field potential recordings from corticostriatal brain slices obtained from control, sham-operated, and 6-hydroxydopamine-denervated animals. Slices in the presence of rotenone, an inhibitor of mitochondrial complex I, were also analyzed. Proteomic analysis revealed downregulation of mortalin in the striata of 6-hydroxydopamine-treated rats in comparison with sham-operated animals. MKT-077 reduced corticostriatal field potential amplitude in physiological conditions, inducing membrane depolarization and inward current in striatal medium spiny neurons. In addition, we observed that concentrations of MKT-077 not inducing any electrophysiological effect in physiological conditions caused significant changes in striatal slices from parkinsonian animals as well as in slices treated with a submaximal concentration of rotenone. These findings suggest a critical link between mortalin function and mitochondrial activity in both physiological and pathological conditions mimicking Parkinson's disease.

The NMDA receptor/nitric oxide pathway: a target for the therapeutic and toxic effects of lithium

Although lithium has largely met its initial promise as the first drug discovered in the modern era of psychopharmacology, to date no definitive mechanism for its effects has been established. It has been proposed that lithium exerts its therapeutic effects by interfering with signal transduction through G-protein-coupled receptor (GPCR) pathways or direct inhibition of specific targets in signaling systems, including inositol monophosphatase and glycogen synthase kinase-3 (GSK-3). Recently, increasing evidence has suggested that N-methyl-D-aspartate receptor (NMDAR)/nitric oxide (NO) signaling could mediate some lithium-induced responses in the brain and peripheral tissues. However, the probable role of the NMDAR/NO system in the action of lithium has not been fully elucidated. In this review, we discuss biochemical, preclinical/behavioral and physiological evidence that implicates NMDAR/NO signaling in the therapeutic effect of lithium. NMDAR/NO signaling could also explain some of side effects of lithium.

Inhibition of phosphodiesterases rescues striatal long-term depression and reduces levodopa-induced dyskinesia

The aim of the present study was to evaluate the role of the nitric oxide/cyclic guanosine monophosphate pathway in corticostriatal long-term depression induction in a model of levodopa-induced dyskinesia in experimental parkinsonism. Moreover, we have also analysed the possibility of targeting striatal phosphodiesterases to reduce levodopa-induced dyskinesia. To study synaptic plasticity in sham-operated rats and in 6-hydroxydopamine lesioned animals chronically treated with therapeutic doses of levodopa, recordings from striatal spiny neurons were taken using either intracellular recordings with sharp electrodes or whole-cell patch clamp techniques. Behavioural analysis of levodopa-induced abnormal involuntary movements was performed before and after the treatment with two different inhibitors of phosphodiesterases, zaprinast and UK-343664. Levodopa-induced dyskinesia was associated with the loss of long-term depression expression at glutamatergic striatal synapses onto spiny neurons. Both zaprinast and UK-343664 were able to rescue the induction of this form of synaptic plasticity via a mechanism requiring the modulation of intracellular cyclic guanosine monophosphate levels. This effect on synaptic plasticity was paralleled by a significant reduction of abnormal movements following intrastriatal injection of phosphodiesterase inhibitors. Our findings suggest that drugs selectively targeting phosphodiesterases can ameliorate levodopa-induced dyskinesia, possibly by restoring physiological synaptic plasticity in the striatum. Future studies exploring the possible therapeutic effects of phosphodiesterase inhibitors in non-human primate models of Parkinson's disease and the involvement of striatal synaptic plasticity in these effects remain necessary to validate this hypothesis.

Aberrant striatal plasticity is specifically associated with dyskinesia following levodopa treatment

Chronic levodopa treatment for Parkinson's disease often results in the development of abnormal involuntary movement, known as L-dopa-induced dyskinesia (LIDs). Studies suggest that LIDs may be associated with aberrant corticostriatal plasticity. Using in vivo extracellular recordings from identified Type I and Type II medium spiny striatal neurons, chronic L-dopa treatment was found to produce abnormal corticostriatal information processing. Specifically, after chronic L-dopa treatment in dopamine-depleted rats, there was a transition from a cortically evoked long-term depression (LTD) to a complementary but opposing form of plasticity, long-term potentiation, in Type II "indirect" pathway neurons. In contrast, LTD could still be induced in Type I neurons. Interestingly, the one parameter that correlated best with dyskinesias was the inability to de-depress established LTD in Type I medium spiny striatal neurons. Taken as a whole, we propose that the induction of LIDs is due, at least in part, to an aberrant induction of plasticity within the Type II indirect pathway neurons combined with an inability to de-depress established plastic responses in Type I neurons. Such information is critical for understanding the cellular mechanisms underlying one of the major caveats to L-dopa therapy.

Chronic cocaine administration reduces striatal dopamine terminal density and striatal dopamine release which leads to drug-seeking behaviour

Drug addiction is associated with altered dopamine (DA) neurotransmission in the basal ganglia. We have previously shown that chronic stimulation of the dopamine D2 receptor (D(2)R) with cocaine results in reduced striatal DA terminal density. The aims of this study were to establish whether this reduction in DA terminal density results in reduced striatal DA release and increased cocaine-seeking behaviour and whether D(2)R antagonism can restore the cocaine-induced alterations in DA neurotransmission and drug-seeking behaviour. Rats were housed individually and either control, cocaine, haloperidol (D(2)R antagonist), or cocaine and haloperidol was administered in the drinking water for 16 weeks. Chronic cocaine treatment, which reduced striatal DA terminal density by 20%, resulted in a reduction in basal (-34%) and cocaine-evoked (-33%) striatal DA release and increased cocaine-seeking behaviour. These cocaine-mediated effects on striatal DA terminal density, DA release and drug-seeking could be prevented by co-administration with haloperidol. Basal and cocaine-evoked DA release in the striatum directly correlated with DA terminal density and with preference for cocaine. We conclude that striatal DA terminal density and DA release is an important factor in maintaining drug preference and should be considered as a factor in drug-seeking behaviour and relapse.

Behavioral, neurochemical, and electrophysiological changes in an early spontaneous mouse model of nigrostriatal degeneration

In idiopathic Parkinson's disease, clinical symptoms do not emerge until consistent neurodegeneration has occurred. The late appearance of symptoms implies the existence of a relatively long preclinical period during which several disease-induced neurochemical changes take place to mask the existence of the disease and delay its clinical manifestations. The aim of this study was to examine the neurochemical, neurophysiological, and behavioral changes induced by the loss of nigrostriatal innervation in the En1+/-;En2-/- mouse, in the 10 months following degeneration, compared to En2 null mutant mice. Behavioral analysis (Pole-test, Beam-walking test, and Inverted grid test) and field potential recordings in the striatum indicated that loss of ~70% of nigrostriatal neurons produced no significant functional effects until 8 months of age, when En1+/-;En2-/- animals started to show frank motor deficits and electrophysiological alterations in corticostriatal plasticity. Similarly, alterations in dopamine homeostasis, dopamine turnover, and dopamine innervation were observed in aged animals compared to young En1+/-;En2-/- mice. These data suggests that in En1+/-;En2-/- mice nigrostriatal degeneration in the substantia nigra is functionally compensated.

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

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

GPI-1046 increases presenilin-1 expression and restores NMDA channel activity

BACKGROUND

Previously we showed that 6-hydroxydopamine lesions of the substantia nigra eliminate corticostriatal LTP and that the neuroimmunolophilin ligand (NIL), GPI-1046, restores LTP.

METHODS

We used cDNA microarrays to determine what mRNAs may be over- or under-expressed in response to lesioning and/or GPI-1046 treatment. Patch clamp recordings were performed to investigate changes in NMDA channel function before and after treatments.

RESULTS

We found that 51 gene products were differentially expressed. Among these we found that GPI-1046 treatment up-regulated presenilin-1 (PS-1) mRNA abundance. This finding was confirmed using QPCR. PS-1 protein was also shown to be over-expressed in the striatum of lesioned/GPI-1046-treated rats. As PS-1 has been implicated in controlling NMDA-receptor function and LTP is reduced by lesioning we assayed NMDA mediated synaptic activity in striatal brain slices. The lesion-induced reduction of dopaminergic innervation was accompanied by the near complete loss of NDMA receptor-mediated synaptic transmission between the cortex and striatum. GPI-1046 treatment of the lesioned rats restored NMDA-mediated synaptic transmission but not the dopaminergic innervation. Restoration of NDMA channel function was apparently specific as the sodium channel current density was also reduced due to lesioning but GPI-1046 did not reverse this effect. We also found that restoration of NMDA receptor function was also not associated with either an increase in NMDA receptor mRNA or protein expression.

CONCLUSION

As it has been previously shown that PS-1 is critical for normal NMDA receptor function, our data suggest that the improvement of excitatory neurotransmission occurs through the GPI-1046-induced up-regulation of PS-1.

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

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

Synaptic dysfunction in Parkinson's disease

In neuronal circuits, memory storage depends on activity-dependent modifications in synaptic efficacy, such as LTD (long-term depression) and LTP (long-term potentiation), the two main forms of synaptic plasticity in the brain. In the nucleus striatum, LTD and LTP represent key cellular substrates for adaptive motor control and procedural memory. It has been suggested that their impairment could account for the onset and progression of motor symptoms of PD (Parkinson's disease), a neurodegenerative disorder characterized by the massive degeneration of dopaminergic neurons projecting to the striatum. In fact, a peculiar aspect of striatal plasticity is the modulation exerted by DA (dopamine) on LTP and LTD. Our understanding of these maladaptive forms of plasticity has mostly come from the electrophysiological, molecular and behavioural analyses of experimental animal models of PD. In PD, a host of cellular and synaptic changes occur in the striatum in response to the massive loss of DA innervation. Chronic L-dopa therapy restores physiological synaptic plasticity and behaviour in treated PD animals, but most of them, similarly to patients, exhibit a reduction in the efficacy of the drug and disabling AIMs (abnormal involuntary movements) defined, as a whole, as L-dopa-induced dyskinesia. In those animals experiencing AIMs, synaptic plasticity is altered and is paralleled by modifications in the postsynaptic compartment. In particular, dysfunctions in trafficking and subunit composition of NMDARs [NMDA (N-methyl-D-aspartate) receptors] on striatal efferent neurons result from chronic non-physiological dopaminergic stimulation and contribute to the pathogenesis of dyskinesias. According to these pathophysiological concepts, therapeutic strategies targeting signalling proteins coupled to NMDARs within striatal spiny neurons could represent new pharmaceutical interventions for PD and L-dopa-induced dyskinesia.

Inhibitory avoidance memory deficit induced by scopolamine: Interaction of cholinergic and glutamatergic systems in the ventral tegmental area

Interaction of cholinergic and glutamatergic inputs in the ventral tegmental area (VTA) influencing a learned behavior is a topic of great interest. In the present study the effect of intra-VTA administration of a nonselective muscarinic acetylcholine antagonist, scopolamine, and N-methyl-d-aspartate (NMDA) receptor agents by themselves as well as their interactions on consolidation and retrieval of inhibitory avoidance (IA) memory have been investigated. A step-through inhibitory avoidance task was used for memory assessment in male Wistar rats. The results showed that intra-VTA administration of scopolamine (1 and 2microg/rat) and NMDA receptor antagonist, MK-801 (0.75 and 1microg/rat) immediately after training, impaired consolidation of IA memory. Interestingly, co-administration of an ineffective dose of MK-801 (0.5microg/rat) with ineffective doses of scopolamine (0.25 and 0.5microg/rat) significantly decreased the consolidation process. Post-training intra-VTA injections of NMDA (0.001 and 0.01microg/rat) had no effects by itself, whereas its co-administration with scopolamine (2microg/rat) prevented the effect of the later drug. The results also showed that pre-test intra-VTA administration of scopolamine (3 and 4microg/rat) and MK-801 (1 and 2microg/rat) impaired retrieval of the IA memory. Moreover, co-administration of an ineffective dose of MK-801 (0.5microg/rat) with ineffective doses of scopolamine (1 and 2microg/rat) increasingly reduced the retrieval of the IA memory. On the contrary to its post-training treatment, pre-test administration of NMDA either alone or in combination with scopolamine caused no significant effect on retrieval of IA memory. It can be concluded that muscarinic acetylcholine and NMDA glutamate receptors in the VTA are involved in the mechanism(s) underlying consolidation and retrieval of the IA memory.

The endocannabinoid system and nondrug rewarding behaviours

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

Dopamine D2 receptor dysfunction is rescued by adenosine A2A receptor antagonism in a model of DYT1 dystonia

DYT1 dystonia is an inherited disease linked to mutation in the TOR1A gene encoding for the protein torsinA. Although the mechanism by which this genetic alteration leads to dystonia is unclear, multiple lines of clinical evidence suggest a link between dystonia and a reduced dopamine D2 receptor (D2R) availability. Based on this evidence, herein we carried out a comprehensive analysis of electrophysiological, behavioral and signaling correlates of D2R transmission in transgenic mice with the DYT1 dystonia mutation. Electrophysiological recordings from nigral dopaminergic neurons showed a normal responsiveness to D2-autoreceptor function. Conversely, postsynaptic D2R function in hMT mice was impaired, as suggested by the inability of a D2R agonist to re-establish normal corticostriatal synaptic plasticity and supported by the reduced sensitivity to haloperidol-induced catalepsy. Although an in situ hybridization analysis showed normal D1R and D2R mRNA expression levels in the striata of hMT mice, we found a significant decrease of D2R protein, coupled to a reduced ability of D2Rs to activate their cognate Go/i proteins. Of relevance, we found that pharmacological blockade of adenosine A2A receptors (A2ARs) fully restored the impairment of synaptic plasticity observed in hMT mice. Together, our findings demonstrate an important link between torsinA mutation and D2R dysfunction and suggest that A2AR antagonism is able to counteract the deficit in D2R-mediated transmission observed in mutant mice, opening new perspectives for the treatment of this movement disorder.

Neural plasticity and treatment across the lifespan for motor deficits in cerebral palsy

The past decade of research in neuroscience and stroke rehabilitation has demonstrated that the adult brain is capable of recovery through physiological processes (often called 'plasticity'). Some of the recovery is spontaneous and some is a result of experience, including interventions such as physical therapy, which probably enhance or activate changes in brain structure and function. There is virtually no literature on physiological changes in the brains of children or adults with cerebral palsy (CP) after an intervention. It is unclear whether the principles of plasticity that have been deduced from animal models of stroke might also apply to children and adults with CP. But children with CP should have the potential to respond to experience in a similar way to adults, with the additional potential of regulation of neuronal development in response to injury. This article describes mechanisms of plasticity and a rehabilitation strategy to preserve the substrates for motor control in CP and then to apply later therapies for more refinement of motor control.

Endocannabinoid signaling and long-term synaptic plasticity

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

Roles for nigrostriatal--not just mesocorticolimbic--dopamine in reward and addiction

Forebrain dopamine circuitry has traditionally been studied by two largely independent specialist groups: students of Parkinson's disease who study the nigrostriatal dopamine system that originates in the substantia nigra (SN), and students of motivation and addiction who study the role of the mesolimbic and mesocortical dopamine systems that originate in the ventral tegmental area (VTA). The anatomical evidence for independent nigrostriatal and mesolimbic dopamine systems has, however, long been obsolete. There is now compelling evidence that both nominal "systems" participate in reward function and addiction. Electrical stimulation of both SN and VTA is rewarding, blockade of glutamatergic or cholinergic input to either SN or VTA attenuates the habit-forming effects of intravenous cocaine, and dopamine in both nigrostriatal and mesocorticolimbic terminal fields participates in the defining property of rewarding events: the reinforcement of memory consolidation. Thus, the similarities between nigrostriatal and mesolimbic dopamine systems can be as important as their differences.

Endocannabinoid-mediated control of synaptic transmission

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

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.

Learning processing in the basal ganglia: a mosaic of broken mirrors

In the present review we propose a model to explain the role of the basal ganglia in sensorimotor and cognitive functions based on a growing body of behavioural, anatomical, physiological, and neurochemical evidence accumulated over the last decades. This model proposes that the body and its surrounding environment are represented in the striatum in a fragmented and repeated way, like a mosaic consisting of the fragmented images of broken mirrors. Each fragment forms a functional unit representing articulated parts of the body with motion properties, objects of the environment which the subject can approach or manipulate, and locations the subject can move to. These units integrate the sensory properties and movements related to them. The repeated and widespread distribution of such units amplifies the combinatorial power of the associations among them. These associations depend on the phasic release of dopamine in the striatum triggered by the saliency of stimuli and will be reinforced by the rewarding consequences of the actions related to them. Dopamine permits synaptic plasticity in the corticostriatal synapses. The striatal units encoding the same stimulus/action send convergent projections to the internal segment of the globus pallidus (GPi) and to the substantia nigra pars reticulata (SNr) that stimulate or hold the action through a thalamus-frontal cortex pathway. According to this model, this is how the basal ganglia select actions based on environmental stimuli and store adaptive associations as nondeclarative memories such as motor skills, habits, and memories formed by Pavlovian and instrumental conditioning.

Long-term administration of cocaine or serotonin reuptake inhibitors results in anatomical and neurochemical changes in noradrenergic, dopaminergic, and serotonin pathways

The catechol and indole pathways are important components underlying plasticity in the frontal cortex and basal ganglia. This study demonstrates that administering rats either cocaine or a selective serotonin (or 5-hydroxytryptamine; 5-HT) reuptake inhibitor (SSRI) for 16 weeks results in reduced density of dopaminergic and noradrenergic terminals in the striatum and olfactory bulb, respectively, reflecting pruning of the terminal arbor of ventral midbrain dopaminergic and locus coeruleus noradrenergic neurones. In the striatum of cocaine-treated animals, basal dopamine levels, as well as cocaine-induced dopamine release, is diminished compared with controls. In contrast, serotonergic fibers, projecting from the raphe, sprout and have increased terminal density in the lateral septal nucleus and frontal cortex, following long-term cocaine or SSRI treatment. This is associated with elevated basal 5-HT and enhanced cocaine-induced 5-HT release in the frontal cortex. The anatomical and neurochemical changes in serotonergic fibers following cocaine or SSRI treatment may be explained by attenuated 5-HT(1A) autoreceptor function in the raphe. This study demonstrates extensive plasticity in the morphology and neurochemistry of the catechol and indole pathways that contribute to drug-induced plasticity of the corticostriatal (and other) projections. Moreover, our data suggest that drug-induced plastic adaptation is anatomically widespread and consequently, likely to have multiple and complex consequences.

The effects of positive versus negative feedback on information-integration category learning

A number of studies have shown that in category learning, providing feedback about errors allows faster learning than providing feedback about correct responses. However, these previous studies used explicit, rule-based tasks in which the category structures could be separated by a simple rule that was easily verbalized. Here, the results of the first experiment known to compare the efficacy of positive versus negative feedback during information-integration category learning are reported. Information-integration tasks require participants to integrate perceptual information from incommensurable dimensions, and evidence suggests that optimal responding recruits procedural learning. The results show that although nearly all of the full-feedback control participants demonstrated information-integration learning, participants receiving either positive-only or negative-only feedback generally used explicit, rule-based strategies. It thus appears that, unlike rule-based learning, consistent information-integration learning requires full feedback. The theoretical implications of these findings for current models of information-integration learning are discussed.

Paradoxical modulation of short-term facilitation of dopamine release by dopamine autoreceptors

Electrophysiological studies have demonstrated that dopaminergic neurons burst fire during certain aspects of reward-related behavior; however, the correlation between dopamine release and cell firing is unclear. When complex stimulation patterns that mimic intracranial self-stimulation were employed, dopamine release was shown to exhibit facilitated as well as depressive components (Montague et al. 2004). Understanding the biological mechanisms underlying these variations in dopamine release is necessary to unravel the correlation between unit activity and neurotransmitter release. The dopamine autoreceptor provides negative feedback to dopamine release, inhibiting release on the time scale of a few seconds. Therefore, we investigated this D(2) receptor to see whether it is one of the biological mechanisms responsible for the history-dependent modulation of dopamine release. Striatal dopamine release in anesthetized rats was evoked with stimulus trains that were designed to promote the variability of dopamine release. Consistent with the well established D(2)-mediated autoinhibition, the short-term depressive component of dopamine release was blocked by raclopride, a D(2) antagonist, and enhanced by quinpirole, a D(2)-receptor agonist. Surprisingly, these same drugs exerted a similar effect on the short-term facilitated component: a decrease with raclopride and an increase with quinpirole. These data demonstrate that the commanding control exerted by dopamine autoreceptors over short-term neuroadaptation of dopamine release involves both inhibitory and paradoxically, facilitatory components.

Multiple Dopamine Functions at Different Time Courses

Many lesion studies report an amazing variety of deficits in behavioral functions that cannot possibly be encoded in great detail by the relatively small number of midbrain dopamine neurons. Although hoping to unravel a single dopamine function underlying these phenomena, electrophysiological and neurochemical studies still give a confusing, mutually exclusive, and partly contradictory account of dopamine's role in behavior. However, the speed of observed phasic dopamine changes varies several thousand fold, which offers a means to differentiate the behavioral relationships according to their time courses. Thus dopamine is involved in mediating the reactivity of the organism to the environment at different time scales, from fast impulse responses related to reward via slower changes with uncertainty, punishment, and possibly movement to the tonic enabling of postsynaptic motor, cognitive, and motivational systems deficient in Parkinson's disease.

Dopamine-mediated regulation of corticostriatal synaptic plasticity

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

In vivo presynaptic and postsynaptic striatal dopamine functions in idiopathic normal pressure hydrocephalus

Differentiation of impaired gait seen in idiopathic normal pressure hydrocephalus (iNPH) from parkinsonian gait is sometimes a great challenge and important for future medication in the clinical setting. To investigate dopaminergic contribution to its pathophysiology, two aspects of the trans-synaptic dopamine functions in the striatal region in eight iNPH patients naïve to dopaminergic drugs were examined using positron emission tomography with a presynaptic marker [11C]CFT ([11C]2-beta-carbomethoxy-3beta-(4-fluorophenyl) tropane) that binds to dopamine transporter and a postsynaptic marker [11C]raclopride that binds to D2 receptor. Quantitative values of binding potentials (BPs) for [11C]CFT and [11C]raclopride were compared between patients and eight age-matched healthy subjects. The BPs and magnetic resonance imaging-based morphometric measures in iNPH were used for correlation analyses between the magnitude of binding of these in vivo markers and clinical severity of the patients. Analysis of variance showed significant reduction in [11C]raclopride binding in the putamen and nucleus accumbens (P<0.05, corrected for multiple comparison) and unchanged striatal [11C]CFT binding in iNPH. The dorsal putamen [11C]raclopride binding correlated negatively with gait severity (r=0.720, P<0.05), and the nucleus accumbens [11C]raclopride binding correlated positively with emotional recognition score (r=0.727, P<0.05) in the disease group. No significant relationship was observed between BPs and morphometric measures. The current result of the postsynaptic D2 receptor reduction along with preserved presynaptic activity in the nigrostriatal dopaminergic system reflects a pathophysiology of iNPH. Postsynaptic D2 receptor hypoactivity in the dorsal putamen may predict the severity of gait impairment in iNPH.

A "good parent" function of dopamine: transient modulation of learning and performance during early stages of training

While extracellular dopamine (DA) concentrations are increased by a wide category of salient stimuli, there is evidence to suggest that DA responses to primary and conditioned rewards may be distinct from those elicited by other types of salient events. A reward-specific mode of neuronal responding would be necessary if DA acts to strengthen behavioral response tendencies under particular environmental conditions or to set current environmental inputs as goals that direct approach responses. As described in this review, DA critically mediates both the acquisition and expression of learned behaviors during early stages of training, however, during later stages, at least some forms of learned behavior become independent of (or less dependent upon) DA transmission for their expression.

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

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

Insights into the Role of Dopamine Receptor Systems in Learning and Memory

It is well established that learning and memory are complex processes involving and recruiting different brain modulatory neurotransmitter systems. Considerable evidence points to the involvement of dopamine in various aspects of cognition, and interest has been focused on investigating the clinical relevance of dopamine systems to age-related cognitive decline and manifestations of cognitive impairment in schizophrenia, Alzheimer's disease, Parkinson's disease and other neurodegenerative diseases. In the past decade or so, in spite of the molecular cloning of the five dopamine receptor subtypes, their specific roles in brain function remained inconclusive due to the lack of completely selective ligands that could distinguish between the members of the D1-like and D2-like dopamine receptor families. One of the most important advances in the field of dopamine research has been the generation of mutant mouse models permitting evaluation of the dopaminergic system using gene targeting technologies. These mouse models represent an important approach to explore the functional roles of closely related receptor subtypes. In this review, we present and discuss evidence on the role of dopamine receptors in different aspects of learning and memory at the cellular, molecular and behavioral levels. We compare evidence using conventional pharmacological, lesion or electrophysiological studies with results from mice with targeted deletions of different subtypes of dopamine receptor genes. We particularly focus on dopamine D1 and D2 receptors in an effort to delineate their specific roles in various aspects of cognitive function. We provide strong evidence, from our own recent work as well as others, that dopamine is part of the network that plays a very important role in cognitive function, and that although multiple dopamine receptor subtypes contribute to different aspects of learning and memory, the D1 receptor seems to play a more prominent role in mediating plasticity and specific aspects of cognitive function, including spatial learning and memory processes, reversal learning, extinction learning, and incentive learning.

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.

Altered cortico-striatal synaptic plasticity and related behavioural impairments in reeler mice

Reelin-deficient mice have been used to investigate the role of this extracellular protein in cortico-striatal plasticity and striatum-related behaviours. Here we show that a repetitive electrical stimulation of the cortico-striatal pathway elicited long-term potentiation (LTP) in homozygous reeler (rl/rl) mice, while causing long-term depression in their wild-type (+/+) littermates. The N-methyl-D-aspartic acid (NMDA) receptor antagonist D-(-)-2 amino-5-phosphonopentanoic acid prevented the induction of LTP in (rl/rl) mice, thus confirming that this form of synaptic plasticity was NMDA receptor-dependent. Interestingly, in the presence of tiagabine, a blocker of gamma-aminobutyric acid (GABA) re-uptake system, the probability that (rl/rl) mice showed LTP decreased significantly, thus suggesting an impaired GABAergic transmission in reeler mutants. Consistent with this view, a decreased density of parvalbumin-positive GABAergic striatal interneurons was found in (rl/rl) mice in comparison to (+/+) mice. Finally, compatible with their abnormal striatal function (rl/rl) mice exhibited procedural learning deficits. Our data, showing alterations in cortico-striatal plasticity largely depending on a depressed GABAergic tone, delineate a mechanism whereby the lack of reelin may affect cognitive functions.

Endocannabinoid-mediated synaptic plasticity in the CNS

Changes in synaptic efficacy are thought to be crucial to experience-dependent modifications of neural function. The diversity of mechanisms underlying these changes is far greater than previously expected. In the last five years, a new class of use-dependent synaptic plasticity that requires retrograde signaling by endocannabinoids (eCB) and presynaptic CB1 receptor activation has been identified in several brain structures. eCB-mediated plasticity encompasses many forms of transient and long-lasting synaptic depression and is found at both excitatory and inhibitory synapses. In addition, eCBs can modify the inducibility of non-eCB-mediated forms of plasticity. Thus, the eCB system is emerging as a major player in synaptic plasticity. Given the wide distribution of CB1 receptors in the CNS, the list of brain structures and synapses expressing eCB-mediated plasticity is likely to expand.

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.

Anatomy of a decision: striato-orbitofrontal interactions in reinforcement learning, decision making, and reversal

The authors explore the division of labor between the basal ganglia-dopamine (BG-DA) system and the orbitofrontal cortex (OFC) in decision making. They show that a primitive neural network model of the BG-DA system slowly learns to make decisions on the basis of the relative probability of rewards but is not as sensitive to (a) recency or (b) the value of specific rewards. An augmented model that explores BG-OFC interactions is more successful at estimating the true expected value of decisions and is faster at switching behavior when reinforcement contingencies change. In the augmented model, OFC areas exert top-down control on the BG and premotor areas by representing reinforcement magnitudes in working memory. The model successfully captures patterns of behavior resulting from OFC damage in decision making, reversal learning, and devaluation paradigms and makes additional predictions for the underlying source of these deficits.

Dopaminergic control of corticostriatal long-term synaptic depression in medium spiny neurons is mediated by cholinergic interneurons

Long-term depression (LTD) of the synapse formed between cortical pyramidal neurons and striatal medium spiny neurons is central to many theories of motor plasticity and associative learning. The induction of LTD at this synapse is thought to depend upon D(2) dopamine receptors localized in the postsynaptic membrane. If this were true, LTD should be inducible in neurons from only one of the two projection systems of the striatum. Using transgenic mice in which neurons that contribute to these two systems are labeled, we show that this is not the case. Rather, in both cell types, the D(2) receptor dependence of LTD induction reflects the need to lower M(1) muscarinic receptor activity-a goal accomplished by D(2) receptors on cholinergic interneurons. In addition to reconciling discordant tracts of the striatal literature, these findings point to cholinergic interneurons as key mediators of dopamine-dependent striatal plasticity and learning.

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.

Dopaminergic modulation of long-lasting direct current-induced cortical excitability changes in the human motor cortex

Dopaminergic mechanisms participate in N-methyl-D-aspartate (NMDA) receptor-dependent neuroplasticity, as animal experiments have shown. This may be similar in humans, where dopamine influences learning and memory. We tested the role of dopamine in human cortical neuroplasticity. Changes of excitability were induced by transcranial direct current stimulation (tDCS). D2 receptor blocking by sulpiride abolished the induction of after-effects nearly completely. D1 activation alone in the presence of D2 receptor blocking induced by co-administration of sulpiride and pergolide did not re-establish the excitability changes induced by tDCS. This suggests that D2 receptors play a major supporting role in inducing neuroplasticity in the human motor cortex. Enhancement of D2 and, to a lesser degree, D1 receptors by pergolide consolidated tDCS-generated excitability diminution until the morning after stimulation. The readiest explanation for this pattern of results is that D2 receptor activation has a consolidation-enhancing effect on tDCS-induced changes of excitability in the human cortex. The results of this study underscore the importance of the dopaminergic system for human neuroplasticity, suggest a first pharmacological add-on mechanism to prolong the excitability-diminishing effects of cathodal tDCS for up to 24 h after stimulation, and thus render the application of tDCS practicable in diseases displaying enhanced cortical excitability, e.g. migraine and epilepsy.

Induction and expression mechanisms of postsynaptic NMDA receptor-independent homosynaptic long-term depression

The induction of long-term depression (LTD) can be divided into two main forms, one dependent upon activation of postsynaptic NMDAR, and another independent of postsynaptic NMDAR. Non-postsynaptic NMDAR-LTD (non-NMDAR-LTD) occurs in many regions of the brain, and encompasses a wide variety of induction and expression mechanisms. In this article, the induction and expression mechanisms of such LTD in over 10 brain regions are described, with a number of common mechanisms compared across a large range of types of LTD. The article describes the involvement of different presynaptic or postsynaptic receptors in the induction of non-NMDAR-LTD, especially metabotropic glutamate receptors, cannabinoid receptors and dopamine receptors. An increase in presynaptic or postsynaptic intracellular Ca concentration is a key event in induction, commonly followed by activation of certain kinases, especially PKC, p38 MAPK and ERK. Expression mechanisms are either presynaptic via a reduction in release probability, or postsynaptic involving a decrease in AMPAR via phosphorylation of a glutamate receptor subunit, especially GluR2, followed by clathrin-mediated endocytosis. Retrograde signalling from postsynaptic to presynaptic occurs when induction is postsynaptic and expression is presynaptic.

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.