Modulation of long-term synaptic plasticity at excitatory striatal synapses

Dopaminergic Modulation of Forced Running Performance in Adolescent Rats: Role of Striatal D1 and Extra-striatal D2 Dopamine Receptors

Improving exercise capacity during adolescence impacts positively on cognitive and motor functions. However, the neural mechanisms contributing to enhance physical performance during this sensitive period remain poorly understood. Such knowledge could help to optimize exercise programs and promote a healthy physical and cognitive development in youth athletes. The central dopamine system is of great interest because of its role in regulating motor behavior through the activation of D1 and D2 receptors. Thus, the aim of the present study is to determine whether D1 or D2 receptor signaling contributes to modulate the exercise capacity during adolescence and if this modulation takes place through the striatum. To test this, we used a rodent model of forced running wheel that we implemented recently to assess the exercise capacity. Briefly, rats were exposed to an 8-day period of habituation in the running wheel before assessing their locomotor performance in response to an incremental exercise test, in which the speed was gradually increased until exhaustion. We found that systemic administration of D1-like (SCH23390) and/or D2-like (raclopride) receptor antagonists prior to the incremental test reduced the duration of forced running in a dose-dependent manner. Similarly, locomotor activity in the open field was decreased by the dopamine antagonists. Interestingly, this was not the case following intrastriatal infusion of an effective dose of SCH23390, which decreased motor performance during the incremental test without disrupting the behavioral response in the open field. Surprisingly, intrastriatal delivery of raclopride failed to impact the duration of forced running. Altogether, these results indicate that the level of locomotor response to incremental loads of forced running in adolescent rats is dopamine dependent and mechanistically linked to the activation of striatal D1 and extra-striatal D2 receptors.

Physiological aging at striatal synapses

Mike Levine's body of work guides thinking on how the basal ganglia process information to create coordinated movements and skill learning throughout the life span and in disease. This special issue is a nod to Mike's career and a well-deserved gesture by the neuroscience community thanking him for the impact he has made on many people's careers and the field of basal ganglia physiology. This paper reviews how aging impacts basal ganglia processing with a focus on single cell and synaptic physiology. This review begins with the work Mike did with his collaborators Nat Buchwald, Chester Hull and Jay Schneider. These early studies paved the way for subsequent studies on changes in synaptic processing that occur with aging in the basal ganglia. The primary focus of this review is aging at corticostriatal synapses. Corticostriatal synapses show reduced expression of both short-term and long-term synaptic potentiation. The roles of age-related changes in calcium homeostasis, vesicle cycling, dopamine modulation, and NMDA receptor function in aging's effect on synaptic plasticity are discussed. The article ends with a review of mitochondrial aging theory as it applies to age-induced changes in corticostriatal synaptic function.

Cyclic AMP and afferent activity govern bidirectional synaptic plasticity in striatopallidal neurons

Recent experimental evidence suggests that the low dopamine conditions in Parkinson's disease (PD) cause motor impairment through aberrant motor learning. Those data, along with computational models, suggest that this aberrant learning results from maladaptive corticostriatal plasticity and learned motor inhibition. Dopaminergic modulation of both corticostriatal long-term depression (LTD) and long-term potentiation (LTP) is proposed to be critical for these processes; however, the regulatory mechanisms underlying bidirectional corticostriatal plasticity are not fully understood. Previously, we demonstrated a key role for cAMP signaling in corticostriatal LTD. In this study, mouse brain slices were used to perform a parametric experiment that tested the impact of varying both intracellular cAMP levels and the strength of excitatory inputs on corticostriatal plasticity. Using slice electrophysiology in the dorsolateral striatum, we demonstrate that both LTP and LTD can be sequentially induced in the same D2-expressing neuron and that LTP was strongest with high intracellular cAMP and LFS, whereas LTD required low intracellular cAMP and high-frequency stimulation. Our results provide a molecular and cellular basis for regulating bidirectional corticostriatal synaptic plasticity and may help to identify novel therapeutic targets for blocking or reversing the aberrant synaptic plasticity that likely contributes to motor deficits in PD.

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.

Dysfunctional play and dopamine physiology in the Fischer 344 rat

Juvenile Fischer 344 rats are known to be less playful than other inbred strains, although the neurobiological substrate(s) responsible for this phenotype is uncertain. In the present study, Fischer 344 rats were compared to the commonly used outbred Sprague-Dawley strain on several behavioral and physiological parameters in order to ascertain whether the lack of play may be related to compromised activity of brain dopamine (DA) systems. As expected, Fischer 344 rats were far less playful than Sprague-Dawley rats, with Fischer 344 rats less likely to initiate playful contacts with a playful partner and less likely to respond playfully to these contacts. We also found that Fischer 344 rats showed less of a startle response and greater pre-pulse inhibition (PPI), especially at higher pre-pulse intensities. The increase in PPI seen in the Fischer 344 rat could be due to reduced DA modulation of sensorimotor gating and neurochemical measures were consistent with Fischer 344 rats releasing less DA than Sprague-Dawley rats. Fast scan cyclic voltammetry (FSCV) revealed Fischer 344 rats had less evoked DA release in dorsal and ventral striatal brain slices and high-performance liquid chromatography revealed Fischer 344 rats to have less DA turnover in the striatum and prefrontal cortex. We also found DA-dependent forms of cortical plasticity were deficient in the striatum and prefrontal cortex of the Fischer 344 rat. Taken together, these data indicate that deficits in play and enhanced PPI of Fischer 344 rats may be due to reduced DA modulation of corticostriatal and mesolimbic/mesocortical circuits critical to the execution of these behaviors.

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

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

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

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

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

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

The corticostriatal pathway in Huntington's disease

The corticostriatal pathway provides most of the excitatory glutamatergic input into the striatum and it plays an important role in the development of the phenotype of Huntington's disease (HD). This review summarizes results obtained from genetic HD mouse models concerning various alterations in this pathway. Evidence indicates that dysfunctions of striatal circuits and cortical neurons that make up the corticostriatal pathway occur during the development of the HD phenotype, well before there is significant neuronal cell loss. Morphological changes in the striatum are probably primed initially by alterations in the intrinsic functional properties of striatal medium-sized spiny neurons. Some of these alterations, including increased sensitivity of N-methyl-D-aspartate receptors in subpopulations of neurons, might be constitutively present but ultimately require abnormalities in the corticostriatal inputs for the phenotype to be expressed. Dysfunctions of the corticostriatal pathway are complex and there are multiple changes as demonstrated by significant age-related transient and more chronic interactions with the disease state. There also is growing evidence for changes in cortical microcircuits that interact to induce dysfunctions of the corticostriatal pathway. The conclusions of this review emphasize, first, the general role of neuronal circuits in the expression of the HD phenotype and, second, that both cortical and striatal circuits must be included in attempts to establish a framework for more rational therapeutic strategies in HD. Finally, as changes in cortical and striatal circuitry are complex and in some cases biphasic, therapeutic interventions should be regionally specific and take into account the temporal progression of the phenotype.

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

Endocannabinoids are important mediators of short- and long-term synaptic plasticity, but the mechanisms of endocannabinoid release have not been studied extensively outside the hippocampus and cerebellum. Here, we examined the mechanisms of endocannabinoid-mediated long-term depression (eCB-LTD) in the dorsal striatum, a brain region critical for motor control and reinforcement learning. Unlike other cell types, strong depolarization of medium spiny neurons was not sufficient to yield detectable endocannabinoid release. However, when paired with postsynaptic depolarization sufficient to activate L-type calcium channels, activation of postsynaptic metabotropic glutamate receptors (mGluRs), either by high-frequency tetanic stimulation or an agonist, induced eCB-LTD. Pairing bursts of afferent stimulation with brief subthreshold membrane depolarizations that mimicked down-state to up-state transitions also induced eCB-LTD, which not only required activation of mGluRs and L-type calcium channels but also was bidirectionally modulated by dopamine D2 receptors. Consistent with network models, these results demonstrate that dopamine regulates the induction of a Hebbian form of long-term synaptic plasticity in the striatum. However, this gating of plasticity by dopamine is accomplished via an unexpected mechanism involving the regulation of mGluR-dependent endocannabinoid release.

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.

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

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

Increased excitability in cortico-striatal synaptic pathway in a model of paroxysmal dystonia

Dystonias are movement disorders whose pathomechanism is largely unknown. Dystonic dt(sz) hamsters represent a model of primary dystonias, where alterations of striatal interneuron density and sodium channel function in projection neurones were described. Here, using cortico-striatal slices, we explore whether also the communication between neocortex and striatum is altered in dt(sz) hamsters. Field and intracellular recordings were done in dorsomedial striatum. Electrical stimulation was used to mimic neocortical afferents. Neuronal characteristics, synaptic connections, input-output relations and short- and long-term plasticity were analysed. Regarding cellular properties, striatal neurons of affected animals showed no alterations. Concerning network properties, evoked responses at threshold stimulation were mediated by (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptors. In dt(sz) slices, field responses, paired-pulse accentuation and LTP were larger than in control, possibly by an increase in presynaptic release probability at glutamatergic synapses. In summary, the study indicates that a change of cortico-striatal communication is involved in the manifestation of paroxysmal dystonia in the dt(sz) mutant.

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

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

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

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

Metabotropic glutamate receptor 5 (mGluR5)-mediated phosphoinositide hydrolysis and NMDA-potentiating effects are blunted in the striatum of aged rats: a possible additional mechanism in striatal senescence

The aim of the present work was to verify whether an impairment of subtype 5 metabotropic glutamate receptor-mediated neurotransmission did occur in the aged striatum. To this end, the ability of the subtype 5 metabotropic glutamate receptor agonist, RS-2-chloro-5-hydroxyphenylglycine, to stimulate phosphoinositide hydrolysis and to potentiate N-methyl-d-aspartate-induced effects in striatal slices from young (3 months) and aged (24 months) rats was compared. The ability of RS-2-chloro-5-hydroxyphenylglycine to induce maximal phosphoinositide turnover and to potentiate N-methyl-d-aspartate effects was significantly reduced in slices from old vs. young rats. These changes were associated with a significant reduction in the expression of subtype 5 metabotropic glutamate receptor protein (-28.8%) and phospholipase C-beta1 (-55.8%) in old striata, while receptor messenger ribonucleic acid expression was unchanged. These results show that the signalling associated with subtype 5 metabotropic glutamate receptors undergoes significant age-related changes and that a reduced expression of subtype 5 metabotropic glutamate receptors and, more importantly, phospholipase C-beta1 may account for the functional decline of subtype 5 metabotropic glutamate receptors.

Dopamine: a potential substrate for synaptic plasticity and memory mechanisms

It is only recently that a number of studies on synaptic plasticity in the hippocampus and other brain areas have considered that a heterosynaptic modulatory input could be recruited as well as the coincident firing of pre- and post-synaptic neurons. So far, the strongest evidence for such a regulation has been attributed to dopaminergic (DA) systems but other modulatory pathways have also been considered to influence synaptic plasticity. This review will focus on dopamine contribution to synaptic plasticity in different brain areas (hippocampus, striatum and prefrontal cortex) with, for each region, a few lines on the distribution of DA projections and receptors. New insights into the possible mechanisms underlying these plastic changes will be considered. The contribution of various DA systems in certain forms of learning and memory will be reviewed with recent advances supporting the hypothesis of similar cellular mechanisms underlying DA regulation of synaptic plasticity and memory processes in which the cyclic adenosine monophosphate/protein kinase A (cAMP/PKA) pathway has a potential role. To summarize, endogenous DA, which depends on the activity patterns of DA midbrain neurons in freely moving animals, appears as a key regulator in specific synaptic changes observed at certain stages of learning and memory and of synaptic plasticity.

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

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

Cortico-striatal synaptic plasticity in endothelial nitric oxide synthase deficient mice

Nitric oxide (NO) is a retrograde messenger involved in the processes of learning and memory. The role of the endothelial isoform of nitric oxide synthase (eNOS) in striatal synaptic plasticity was investigated in eNOS-deficient (eNOS(-/-)) and wild type (WT) mice. Tetanic stimulation of cortical afferents in WT mice evoked either long-term potentiation (LTP), or long-term depression (LTD) of cortico-striatal transmission. Both these plasticity related phenomena were NMDA-receptor-dependent; LTD was blocked by sulpiride, a dopamine D2-receptor antagonist. LTP occurrence in slices from eNOS(-/-) mice was significantly reduced when compared with WT mice. The NOS inhibitor NL-ARG reduced the occurrence of LTP and increased the occurrence of LTD in WT mice, resembling the balance of LTP/LTD in eNOS(-/-) mice. Impairment of NO-synthesis thus shifts striatal plasticity towards LTD. This indicates a possible involvement of eNOS from endothelia in neuronal modulation.

Corticostriatal LTP requires combined mGluR1 and mGluR5 activation

Metabotropic glutamate receptors (mGluRs) have been demonstrated to play a role in synaptic plasticity. It has been recently shown that mGluR1 is involved in corticostriatal long-term depression, by means of pharmacological approach and by using mGluR1-knockout mice. Here, we report that both mGluR1 and mGluR5 are involved in corticostriatal long-term potentiation (LTP). In particular, the mGluR1 antagonist LY 367385, as well as the mGluR5 antagonist MPEP, reduce LTP amplitude. Moreover, blockade of both mGluR1 and mGluR5 by LY 367385 and MPEP co-administration fully suppresses LTP. Accordingly, group II and group III mGluRs antagonists fail to affect LTP induction. Interestingly, LTP amplitude is also significantly reduced in both mGluR1- and mGluR5-knockout mice. The differential function of mGluR1 and mGluR5 in corticostriatal synaptic plasticity may play a role in the modulation of the motor activity mediated by the basal ganglia, thus providing a substrate for the pharmacological treatment of motor disorders involving the striatum.

Dopamine-dependent plasticity of corticostriatal synapses

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

A possible mechanism for the dopamine-evoked synergistic disinhibition of thalamic neurons via the "direct" and "indirect" pathways in the basal ganglia

The mechanism of synaptic plasticity which we have previously proposed for striatal spiny neurons, along with published data on the predominance of dopamine-sensitive D1/D2 receptors on strionigral/striopallidal neurons, was used as the basis to propose the hypothesis that the induction of long-term potentiation/depression of the efficiency of the cortical inputs to these cells may result from the excitatory/inhibitory actions of dopamine on the activity of the neurons originating the "direct" and "indirect" pathways through the basal ganglia. Thus, the action of dopamine increases disinhibition of thalamic neurons via the "direct" pathway and decreases their inhibition via the "indirect" pathway. Both effects lead to increases in the activity of thalamic cells and in the activity of the efferent neocortical neurons which they excite. The actions of dopamine on striosomal neurons, which mainly have D1 receptors, may also be to induce long-term potentiation of cortical inputs. This effect should lead to increased inhibition of dopaminergic cells and decreases in their dopamine release, which may promote the maintenance of a stable dopamine concentration in the cortex-basal ganglia-thalamus-cortex neural network.

Corticostriatal combinatorics: the implications of corticostriatal axonal arborizations

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

Regional differences in the expression of corticostriatal synaptic plasticity

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

Selective involvement of mGlu1 receptors in corticostriatal LTD

Although metabotropic glutamate receptors (mGluRs) have been proposed to play a role in corticostriatal long-term depression (LTD), the specific receptor subtype required for this form of synaptic plasticity has not been characterized yet. Thus, we utilized a corticostriatal brain slice preparation and intracellular recordings from striatal spiny neurons to address this issue. We observed that both AIDA (100 microM) and LY 367385 (30 microM), two blockers of mGluR1s, were able to fully prevent the induction of this form of synaptic plasticity, whereas MPEP (30 microM), a selective antagonist of the mGluR5 subtype, did not significantly affect the amplitude and time-course of corticostriatal LTD. Both AIDA and LY 367385 were ineffective on LTD when applied after its induction. The critical role of mGluR1s in the formation of corticostriatal LTD was confirmed in experiments performed on mice lacking mGluR1s. In these mice, in fact, a significant reduction of the LTD amplitude was observed in comparison to the normal LTD measured in their wild-type counterparts. We found that neither acute pharmacological blockade of mGluR1s nor the genetic disruption of these receptors affected the presynaptic modulation of corticostriatal excitatory postsynapic potentials (EPSPs) exerted by DCG-IV and L-SOP, selective agonists of group II and III mGluRs, respectively. Our data show that the induction of corticostriatal LTD requires the activation of mGluR1 but not mGluR5. mGluR1-mediated control of this form of synaptic plasticity may play a role in the modulatory effect exerted by mGluRs in the basal ganglia-related motor activity.

Dopaminergic control of synaptic plasticity in the dorsal striatum

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

Dopamine and cAMP-regulated phosphoprotein 32 kDa controls both striatal long-term depression and long-term potentiation, opposing forms of synaptic plasticity

A complex chain of intracellular signaling events, critically important in motor control, is activated by the stimulation of D1-like dopamine (DA) receptors in striatal neurons. At corticostriatal synapses on medium spiny neurons, we provide evidence that the D1-like receptor-dependent activation of DA and cyclic adenosine 3',5' monophosphate-regulated phosphoprotein 32 kDa is a crucial step for the induction of both long-term depression (LTD) and long-term potentiation (LTP), two opposing forms of synaptic plasticity. In addition, formation of LTD and LTP requires the activation of protein kinase G and protein kinase A, respectively, in striatal projection neurons. These kinases appear to be stimulated by the activation of D1-like receptors in distinct neuronal populations.

Functional state of corticostriatal synapses determines their expression of short- and long-term plasticity

Relationships between presynaptic function and short- and long-term plasticity were investigated at adult corticostriatal synapses. Wide variability was observed in the expression of short- and long-term synaptic plasticity. Intracellular records from 47 cells produced 17 examples of LTD (<90% of control), 10 examples of no long-term change (between 90-110% of control), and 20 examples of LTP (>110% of control). Similar variation existed in paired-pulse and posttetanic plasticities. The variability expressed in all three forms of plasticity appears to be related, based on correlations found between the paired-pulse ratio (PPR) and tetanus-induced short- (3 min posttetanus) and long-term plasticities (16-20 min posttetanus). These data suggest that tetanus-induced changes in synaptic strength are related to the intrinsic, preconditioned behavior of synapses. Every cell showing paired-pulse depression also expressed LTD in response to high-frequency activation of its afferents. Those synapses showing paired-pulse potentiation tended to express LTP, although exceptions did exist. Similar relationships were found in a parallel analysis of population spikes. PPR also changed in association with the expression of posttetanic and long-term depression. Greater paired-pulse potentiation was observed in medial intracellular recordings, but no medial to lateral differences were seen in posttetanic plasticities. Field recordings also showed a medial bias toward paired-pulse and posttetanic potentiation, but not in long-term plasticity. Block of postsynaptic L-type Ca(2+) channels with nifedipine eliminated LTD expression, but overall no differences were found between nifedipine and control cells.

Age-related decline in the functional response of striatal group I mGlu receptors

In order to verify whether striatal group I metabotropic glutamate (mGlu) receptors undergo functional alteration in ageing, the effects induced by the selective agonist 3,5-dihydroxyphenylglycine (DHPG) in the striatum of young (3 months) and aged (24-25 months old) rats were compared. The ability of DHPG to stimulate phosphoinositide (PI) hydrolysis (striatal slices), to influence striatal dopamine release (in vivo microdialysis) and to potentiate the effects of NMDA on extracellular field potential amplitude (extracellular recordings on striatal slices) was reduced in the striatum of old vs young rats. These results show an age-dependent reduction in the functional response of striatal group I mGlu receptors, which may be one of the factors underlying the reduced ability aged striatum to integrate information.

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.

Role of pertussis toxin-sensitive G-proteins in synaptic transmission and plasticity at corticostriatal synapses

The role of pertussis toxin (PTX)-sensitive G-proteins in corticostriatal synaptic transmission and long-term synaptic depression (LTD) was examined using extracellular field potential and whole cell voltage-clamp recordings in striatal slices. High-frequency stimulation (HFS) produced LTD, defined as long-lasting decreases both in synaptically driven population spikes (PSs) measured with field potential recording and in excitatory postsynaptic currents (EPSCs) measured with whole cell recording. Striatal LTD could not be induced in slices obtained from rats that had received a unilateral intrastriatal injection of PTX. However, LTD could be induced in slices obtained from paired control slices. Furthermore, striatal LTD was prevented by pretreatment with N-ethylmaleimide (NEM), another compound that disrupts the function of PTX-sensitive G-proteins. NEM, itself, also potentiated PS and EPSC amplitudes. In addition, NEM increased the frequency and amplitude of both spontaneous and miniature EPSCs and decreased the paired-pulse facilitation ratio, suggesting that it may act on both pre- and postsynaptic sites. The findings suggest that PTX-sensitive G-proteins have multiple roles at corticostriatal synapses, including regulation of synaptic transmission at both pre- and postsynaptic sites, and a key role in striatal LTD.