Characterization of a new mouse line triggering transient oligodendrocyte progenitor depletion.

Oligodendrocyte progenitor cells (OPC) are the main proliferative cells in the healthy adult brain. They produce new myelinating oligodendrocytes to ensure physiological myelin remodeling and regeneration after various pathological insults. Growing evidence suggests that OPC have other functions. Here, we aimed to develop an experimental model that allows the specific ablation of OPC at the adult stage to unravel possible new functions. We generated a transgenic mouse expressing a floxed human diphtheria toxin receptor under the control of the PDGFRa promoter, crossed with an Olig2Cre mouse to limit the recombination to the oligodendrocyte lineage in the central nervous system. We determined a diphtheria toxin dose to substantially decrease OPC density in the cortex and the corpus callosum without triggering side toxicity after a few daily injections. OPC density was normalized 7 days post-treatment, showing high repopulation capacity from few surviving OPC. We took advantage of this strong but transient depletion to show that OPC loss was associated with behavioral impairment, which was restored by OPC recovery, as well as disruption of the excitation/inhibition balance in the sensorimotor cortex, reinforcing the hypothesis of a neuromodulatory role of OPC in the adult brain.

Animal models of Parkinson's disease: An updated overview.

Parkinson's disease (PD) is a progressive neurodegenerative disorder whose etiology, besides a minority of genetic cases, is still largely unknown. Animal models have contributed to elucidate PD etiology and pathogenesis, as well as its cellular and molecular mechanisms, leading to the general hypothesis that this neurological disorder is due to complex interactions between environmental and genetic factors. However, the full understanding of PD is still very far from being achieved, and new potential treatments need to be tested to further improve patients' quality of life and, possibly, slow down the neurodegenerative process. In this context, animal models of PD are required to address all these issues. "Classic" models are based on neurotoxins that selectively target catecholaminergic neurons (such as 6-hydroxydopamine, 1-methyl-1,2,3,6-tetrahydropiridine, agricultural pesticides, etc.), while more recent models employ genetic manipulations that either introduce mutations similar to those find in familial cases of PD (α-synuclein, DJ-1, PINK1, Parkin, etc.) or selectively disrupt nigrostriatal neurons (MitoPark, Pitx3, Nurr1, etc.). Each one of these models has its own advantages and limitations, thus some are better suited for studying PD pathogenesis, while others are more pertinent to test therapeutic treatments. Here, we provide a critical and updated review of the most used PD models.

Role of glutamate transporters in corticostriatal synaptic transmission.

High-affinity glutamate transporters (GTs) play a major role in controlling the extracellular level of this excitatory neurotransmitter in the CNS. Here we have characterized, by means of in vitro patch-clamp recordings from medium spiny neurons (MSNs), the role of GTs in regulating corticostriatal glutamatergic synaptic transmission in the adult rat. Charge transfer and decay-time, but not amplitude, of excitatory postsynaptic currents (EPSCs) were enhanced by dl-threo-beta-benzyloxyaspartate (TBOA), a broad inhibitor of GTs. Moreover, TBOA also potentiated currents induced by high-frequency stimulation (HFS) protocols. Interestingly, the effect of TBOA on EPSCs was lost when MSNs were clamped at +40 mV, a condition in which neuronal GTs, that are voltage-dependent, are blocked. However, in this condition TBOA was still able to enhance HFS-induced currents, suggesting that glial GT's role is to regulate synaptic transmission when glutamate release is massive. These data suggest that neuronal GTs, rather than glial, shape EPSCs' kinetics and modulate glutamate transmission at corticostriatal synapse. Moreover, the control of glutamate concentration in the synaptic cleft by GTs may play a role in a number of degenerative disorders characterized by the hyperactivity of corticostriatal pathway, as well as in synaptic plasticity.

Deficits of glutamate transmission in the striatum of experimental hemiballism.

Hemiballism (HB) is a quite rare disorder, generally secondary to stroke, neoplasms or demyelinating plaques, classically considered as almost pathognomonic of a lesion in the subthalamic nucleus (STN). This alteration causes involuntary movements in the chorea-ballism spectrum. One theory is that the output nuclei of the basal ganglia are overinhibited in HB, while little is known about the physiological state of the striatum, the major input structure of the basal ganglia. In the present study, we recorded spontaneous and miniature excitatory and inhibitory postsynaptic currents (sEPSCs, mEPSCs, sIPSCs, mIPSCs) from projection neurons of the striatum of experimental HB. We found a selective reduction of striatal sEPSC and mEPSC frequency following chemical lesion of the STN of the rat, suggesting that reduced synaptic excitation of the input structure of the basal ganglia represents a physiological correlate of HB.

Subthalamic nucleus lesion reverses motor abnormalities and striatal glutamatergic overactivity in experimental parkinsonism.

Subthalamic nucleus (STN) is a target of choice for the neurosurgical treatment of Parkinson's disease (PD). The therapeutic effect of STN lesion in PD is classically ascribed to the rescue of physiological activity in the output structures of the basal ganglia, and little is known about the possible involvement of the striatum. In the present study, therefore, we electrophysiologically recorded in vitro single striatal neurons of DA-depleted rats unilaterally lesioned by 6-hydroxydopamine, treated or not with therapeutic doses of levodopa (l-DOPA), or with a consecutive ipsilateral STN lesion. We show that the beneficial motor effects produced in parkinsonian rats by STN lesion or l-DOPA therapy were paralleled by the normalization of overactive frequency and amplitude of striatal glutamate-mediated spontaneous excitatory postsynaptic currents (sEPSCs). Since neither l-DOPA treatment nor STN lesion affected sEPSCs kinetic properties, the reversal of these abnormalities in striatal excitatory synaptic transmission can be attributable to the normalization of glutamate release.

Differential contribution of dopamine D2S and D2L receptors in the modulation of glutamate and GABA transmission in the striatum.

Compelling evidence indicates that the long (D2L) and the short (D2S) isoform of dopamine (DA) D2 receptors serve distinct physiological functions in vivo. To address the involvement of these isoforms in the control of synaptic transmission in the striatum, we measured the sensitivity to D2 receptor stimulation of glutamate- and GABA-mediated currents recorded from striatal neurons of three mutant mice, in which the expression of D2L and D2S receptors was either ablated or variably altered. Our data indicate that both isoforms participate in the presynaptic inhibition of GABA transmission in the striatum, while the D2-receptor-dependent modulation of glutamate release preferentially involves the D2S receptor. Accordingly, the inhibitory effects of the DA D2 receptor agonist quinpirole (10 microM) on GABA(A)-mediated spontaneous inhibitory postsynaptic currents (IPSCs)correlate with the total number of D2 receptor sites in the striatum, irrespective of the specific receptor isoform expressed. In contrast, glutamate-mediated spontaneous excitatory postsynaptic currents (EPSCs) were significantly inhibited by quinpirole only when the total number of D2 receptor sites, normally composed by both D2L and D2S receptors in a ratio favoring the D2L isoform, was modified to express only the D2S isoform at higher than normal levels. Understanding the physiological roles of DA D2 receptors in the striatum is essential for the treatment of several neuropsychiatric conditions, such as Parkinson's disease, Tourette's syndrome, schizophrenia, and drug addiction.

Targeting striatal cholinergic interneurons in Parkinson's disease: focus on metabotropic glutamate receptors.

In the early sixties, anticholinergic drugs were introduced in the pharmacological treatment of Parkinson's disease (PD). The rationale behind their utilisation in the treatment of the disease was based on the evidence of an imbalance between the dopaminergic inputs and the intrinsic cholinergic innervation within the striatum. Metabotropic glutamate (mGlu) receptors have been shown to play a key role in striatal function both in physiological conditions and in experimental models of diseases affecting this brain area. Indeed, compelling electrophysiological and morphological evidence shows that mGlu receptors are highly expressed at cellular level and exert a profound modulatory role on cholinergic interneurons excitability. This review will provide a brief survey of studies on the localization and function of mGlu receptors in cholinergic interneurons. The potential relevance of these findings in the control of motor function and in the treatment of PD will be discussed.

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.

Impaired excitatory transmission in the striatum of rats chronically intoxicated with manganese.

Chronic exposure to manganese (Mn) is known to produce a parkinsonian or dystonic state in humans caused by a rather selective involvement of the basal ganglia. Experimental observations suggest that secondary excitotoxic mechanisms play a crucial role in the development of Mn-induced neurodegeneration in the striatum, although the site of interference of Mn with glutamatergic transmission in this brain area is still unknown. To answer this question, in the present in vitro study, we investigated the physiological characteristics of striatal excitatory synaptic transmission in a rat model of Mn intoxication. We found that chronic Mn greatly increased both frequency and amplitude of spontaneous excitatory postsynaptic potentials, in the absence of appreciable changes of intrinsic membrane properties of striatal cells. The sensitivity of striatal neurons to glutamate AMPA and NMDA receptor stimulation was unaffected by Mn poisoning, as demonstrated by comparing the membrane responses produced in control and treated rats to the application of selective agonists of these receptors and to the direct activation of corticostriatal glutamatergic fibers. In addition, also paired-pulse facilitation was unaltered by Mn treatment, indicating that this toxin does not affect the pre- and postsynaptic mechanisms responsible for the appearance of this short-term form of synaptic plasticity at corticostriatal synapses. It is concluded, therefore, that hyperactivity of corticostriatal neurons, rather than increased postsynaptic sensitivity to glutamate, accounts for the abnormal excitation of striatal neurons in the course of Mn intoxication.

Metabotropic glutamate receptor 5 mediates the potentiation of N-methyl-D-aspartate responses in medium spiny striatal neurons.

Medium spiny neurons were recorded from striatal slices obtained from mice lacking the group I metabotropic glutamate receptor (mGluR) subtype 1 or subtype 5. In wild-type animals, N-methyl-D-aspartate (NMDA)-induced membrane depolarization/inward currents were potentiated in the presence of both the group I mGluR agonist 3,5-dihydroxyphenylglycine (3,5-DHPG) and the mGluR5 selective agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG). Likewise, in mGluR1 knockout mice, both 3,5-DHPG and CHPG were able to potentiate NMDA responses. Conversely, in neurons recorded from mGluR5-deficient mice, the enhancement of NMDA responses by both 3,5-DHPG and CHPG was absent. Pharmacological analysis performed from rat slices confirmed the data obtained with mice. In the presence of the competitive mGluR1 antagonist LY367385, the NMDA responses were potentiated in the presence of CHPG, whereas the CHPG-induced enhancement was not observed in slices treated with the non-competitive mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine. As in wild-type mice, in neither of the mGluR1- and mGluR5-deficient mice did (2S,1'R,2'R,3'R)-2-(2,3-dicarboxylcyclopropyl)-glycine (1 microM), nor L-serine-O-phosphate (30 microM) (agonists for group II and III mGluRs, respectively) affect the NMDA-evoked responses. In striatal medium spiny neurons, NMDA responses are potentiated by endogenous acetylcholine via M1-like muscarinic receptors. Since the enhancement of NMDA responses by 3,5-DHPG and by M1-like muscarinic agonists was shown to share common post-receptor mechanisms, we verified whether the muscarinic potentiation of NMDA responses was affected in these group I mGluR-deficient mice. Both in mGluR1 and mGluR5 knockout animals, in the presence of either muscarine or the M1-like muscarinic receptor agonist McN-A-343, the positive modulation of the NMDA-induced membrane depolarization persisted.These results confirm the permissive role of group I mGluRs on NMDA responses in striatal neurons and reveal that this functional interplay occurs exclusively through the mGluR5 subtype. The NMDA-mGluR5 interaction might play an important modulatory role in the final excitatory drive from corticostriatal afferents and suggests that drugs acting at mGluR5 might prove useful for the treatment of movement disorders involving the striatum.

An abnormal striatal synaptic plasticity may account for the selective neuronal vulnerability in Huntington's disease.

A marked decrease in the activity of mitochondrial complex II (succinate dehydrogenase, SD) has been found in the brains of Huntington's disease (HD) patients. Here we have examined the possibility that SD inhibitors might produce their toxic action by increasing corticostriatal glutamatergic transmission. We report that SD inhibitors produce a durable augmentation of NMDA-mediated corticostriatal excitation (DANCE) in striatal spiny neurons, but not in striatal cholinergic interneurons. DANCE involves increased intracellular calcium, activation of MAP kinase ERK and is critically dependent upon endogenous dopamine (DA) acting via D2-like receptors. This pathological form of corticostriatal synaptic plasticity might play a key role in the regional and cell-type specific neuronal death observed in HD.

Inhibition of mitochondrial complex II induces a long-term potentiation of NMDA-mediated synaptic excitation in the striatum requiring endogenous dopamine.

Abnormal involuntary movements and cognitive impairment represent the classical clinical symptoms of Huntington's disease (HD). This genetic disorder involves degeneration of striatal spiny neurons, but not striatal large cholinergic interneurons, and corresponds to a marked decrease in the activity of mitochondrial complex II [succinate dehydrogenase (SD)] in the brains of HD patients. Here we have examined the possibility that SD inhibitors exert their toxic action by increasing glutamatergic transmission. We report that SD inhibitors such as 3-nitroproprionic acid (3-NP), but not an inhibitor of mitochondrial complex I, produce a long-term potentiation of the NMDA-mediated synaptic excitation (3-NP-LTP) in striatal spiny neurons. In contrast, these inhibitors had no effect on excitatory synaptic transmission in striatal cholinergic interneurons and pyramidal cortical neurons. 3-NP-LTP involves increased intracellular calcium and activation of the mitogen-activated protein kinase extracellular signal-regulated kinase and is critically dependent on endogenous dopamine acting via D2 receptors, whereas it is negatively regulated by D1 receptors. Thus 3-NP-LTP might play a key role in the regional and cell type-specific neuronal death observed in HD.

A synaptic mechanism underlying the behavioral abnormalities induced by manganese intoxication.

In the present study we have characterized a rat model of manganese (Mn) intoxication leading to behavioral disinhibition in the absence of major motor alterations. These behavioral changes were associated with significantly increased brain Mn levels but were uncoupled to anatomical lesions of the striatum or to morphological and cytochemical changes of the nigrostriatal dopaminergic pathway. The analysis of this model at cellular level showed an enhanced dopaminergic inhibitory control of the corticostriatal excitatory transmission via presynaptic D2-like dopamine (DA) receptors in slices obtained from Mn-treated rats. Conversely, the use of agonists acting on presynaptic purinergic, muscarinic, and glutamatergic metabotropic receptors revealed a normal sensitivity. Moreover, membrane responses recorded from single dopaminergic neurons following activation of D2 DA autoreceptors were also unchanged following Mn intoxication. Thus, our findings indicate a selective involvement of the D2-like DA receptors located on glutamatergic corticostriatal terminals in this pathological condition and suggest that the behavioral symptoms described in the "early" clinical phase of manganism may be caused by an abnormal dopaminergic inhibitory control on corticostriatal inputs. The identification of the synaptic mechanism underlying the "early" phase of Mn intoxication might have a critical importance to understand the causes of the progression of this pathological condition towards an "established" phase characterized by motor abnormalities and anatomical lesions of the basal ganglia.

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.

Activity- and age-dependent GABAergic synaptic plasticity in the developing rat hippocampus.

Activity-dependent plasticity of GABAergic synaptic transmission was investigated in rat hippocampal slices obtained between postnatal day (P) 0-15 using the whole-cell patch-clamp recording technique. Spontaneous GABA(A) receptor-mediated postsynaptic currents (sGABA(A)-PSCs) were isolated in the presence of ionotropic glutamate receptor antagonists. A conditioning protocol relevant to the physiological condition, consisting of repetitive depolarizing pulses (DPs) at 0.1 Hz, was able to induce long-lasting changes in both frequency and amplitude of sGABA(A)-PSCs between P0 and P8. Starting from P12, DPs were unable to induce any form of synaptic plasticity. The effects of DPs were tightly keyed to the frequency at which they were delivered. When delivered at a lower (0.05 Hz) or higher (1 Hz) frequency, DPs failed to induce any long-lasting change in the frequency or amplitude of sGABA(A)-PSCs. In two cases, DPs were able to activate sGABA(A)-PSCs in previously synaptically silent cells at P0-1. These results show that long-term changes in GABAergic synaptic activity can be induced during a restricted period of development by a conditioning protocol relevant to the physiological condition. It is suggested that such activity-induced modifications may represent a physiological mechanism for the functional maturation of GABAergic synaptic transmission.

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.

Tissue plasminogen activator controls multiple forms of synaptic plasticity and memory.

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

Synaptic transmission in the striatum: from plasticity to neurodegeneration.

Striatal neurones receive myriad of synaptic inputs originating from different sources. Massive afferents from all areas of the cortex and the thalamus represent the most important source of excitatory amino acids, whereas the nigrostriatal pathway and intrinsic circuits provide the striatum with dopamine, acetylcholine, GABA, nitric oxide and adenosine. All these neurotransmitter systems interact each other and with voltage-dependent conductances to regulate the efficacy of the synaptic transmission within this nucleus. The integrative action exerted by striatal projection neurones on this converging information dictates the final output of the striatum to the other basal ganglia structures. Recent morphological, immunohistochemical and electrophysiological findings demonstrated that the striatum also contains different interneurones, whose role in physiological and pathological conditions represents an intriguing challenge in these years. The use of the in vitro brain slice preparation has allowed not only the detailed investigation of the direct pre- and postsynaptic electrophysiological actions of several neurotransmitters in striatal neurones, but also the understanding of their role in two different forms of corticostriatal synaptic plasticity, long-term depression and long-term potentiation. These long-lasting changes in the efficacy of excitatory transmission have been proposed to represent the cellular basis of some forms of motor learning and are altered in animal models of human basal ganglia disorders, such as Parkinson's disease. The striatum also expresses high sensitivity to hypoxic-aglycemic insults. During these pathological conditions, striatal synaptic transmission is altered depending on presynaptic inhibition of transmitter release and opposite membrane potential changes occur in projection neurones and in cholinergic interneurones. These ionic mechanisms might partially explain the selective neuronal vulnerability observed in the striatum during global ischemia and Huntington's disease.

Acetylcholine-mediated modulation of striatal function.

Striatal spiny neurones serve as a major anatomical locus for the relay of cortical information flow through the basal ganglia. these projection neurones also represent the main synaptic target of cholinergic interneurones, whose physiological role in striatal activity still remains largely enigmatic. The striatal cholinergic system has been implicated in the pathophysiology of movement disorders such as Parkinson's disease, but the cellular mechanisms underlying cholinergic-neurone function are still unknown. On the basis of in vitro electrophysiological evidence, obtained from a rat corticostriatal-slice preparation, we propose that endogenous ACh exerts a complex modulation of striatal synaptic transmission, which produces both short-term and long-term effects. ACh-mediated mechanisms might be of crucial importance in processing the cortical inputs to the striatum.