LRRK2-G2019S mice display alterations in glutamatergic synaptic transmission in midbrain dopamine neurons.

The progressive degeneration of dopamine (DA) neurons in the substantia nigra compacta (SNc) leads to the emergence of motor symptoms in patients with Parkinson's disease (PD). To propose neuroprotective therapies able to slow or halt the progression of the disease, it is necessary to identify cellular alterations that occur before DA neurons degenerate and before the onset of the motor symptoms that characterize PD. Using electrophysiological, histochemical, and biochemical approaches, we have examined if glutamatergic synaptic transmission in DA neurons in the SNc and in the adjacent ventral tegmental area (VTA) was altered in middle-aged (10-12 months old) mice with the hG2019S point mutation (G2019S) in the leucine-rich repeat kinase 2 (LRRK2) gene. G2019S mice showed increased locomotion and exploratory behavior compared with wildtype (WT) littermates, and intact DA neuron integrity. The intrinsic membrane properties and action potential characteristics of DA neurons recorded in brain slices were similar in WT and G2019S mice. Initial glutamate release probability onto SNc-DA neurons, but not VTA-DA neurons, was reduced in G2019S mice. We also found reduced protein amounts of the presynaptic marker of glutamatergic terminals, VGLUT1, and of the GluA1 and GluN1 subunits of AMPA and NMDA receptors, respectively, in the ventral midbrain of G2019S mice. These results identify alterations in glutamatergic synaptic transmission in DA neurons of the SNc and VTA before the onset of motor impairments in the LRRK2-G2019S mouse model of PD.

The GABAA receptor agonist muscimol induces an age- and region-dependent form of long-term depression in the mouse striatum.

Several forms of long-term depression (LTD) of glutamatergic synaptic transmission have been identified in the dorsal striatum and in the nucleus accumbens (NAc). Such experience-dependent synaptic plasticity might play important roles in reward-related learning. The GABAA receptor agonist muscimol was recently found to trigger a long-lasting depression of glutamatergic synaptic transmission in the NAc of adolescent mice, but the mechanisms that underlie this novel form of LTD were not studied. Here we examined the effect of muscimol applied in the perfusion solution on the amplitude of field excitatory postsynaptic potentials/population spikes (fEPSP/PSs) in mouse brain slices. We found that muscimol depressed the fEPSP/PS in the NAc of adolescent mice but not adult mice, through both postsynaptic and presynaptic mechanisms. Indeed, muscimol altered the fEPSP/PS paired-pulse ratio, depolarized the membrane of projection neurons, and decreased the frequency, but not amplitude, of spontaneous excitatory postsynaptic currents in the NAc of adolescent mice. The LTD induced by muscimol likely involved endocannabinoids, metabotropic glutamate receptors (mGluRs), but not TRPV1 receptors. Muscimol-LTD was occluded by prior induction of LTD through low-frequency stimulation (LFS) of the slice, demonstrating a common pathway in the induction of LFS-LTD and muscimol-LTD. We also found that muscimol induced a form of LTD in the dorsolateral striatum of adult but not adolescent mice. This LTD was mediated by endocannabinoids but did not involve mGluRs or TRPV1 receptors. These results identify a novel form of synaptic plasticity, and its mechanisms of induction, which is age and region dependent. These findings may contribute to a better understanding of the increased susceptibility of the adolescent brain to long-term synaptic changes in regions associated with reward mechanisms.

Induction of cannabinoid- and N-methyl-D-aspartate receptor-mediated long-term depression in the nucleus accumbens and dorsolateral striatum is region and age dependent.

BACKGROUND: The adolescent brain is sensitive to experience-dependent plasticity and might be more vulnerable than the adult brain to the effects of some drugs of abuse. The factors that contribute to these differences are not fully identified. We have examined the ability of cannabinoids to induce a form of synaptic plasticity, long-term depression, in the nucleus accumbens and dorsolateral striatum of adolescent and adult mice. METHODS: We measured field excitatory postsynaptic potentials/population spikes in brain slices. RESULTS: We found that the cannabinoid receptor agonist WIN 55,212-2 (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate) induced long-term depression in the nucleus accumbens of adolescent but not adult mice and failed to induce long-term depression in the dorsolateral striatum of adolescent or adult mice. Similar results were obtained with the group I metabotropic glutamate receptor agonist (S)-3,5- dihydroxyphenylglycine, which has previously been shown to promote the release of endocannabinoids. These age-related differences were associated with reduced protein levels of the cannabinoid type 1 receptor and metabotropic glutamate receptor 1 in adult nucleus accumbens and dorsolateral striatum and with an increased tone of endocannabinoids in the dorsolateral striatum of adult mice. We also found that N-methyl-D-aspartate receptor-dependent long-term depression, which was induced in the nucleus accumbens of adolescent mice, was blunted in adult mice, possibly because of decreased levels of GluN1, the obligatory subunit of N-methyl-D-aspartate receptors. CONCLUSIONS: This study identifies region- and age-specific differences in the ability of endogenous and exogenous cannabinoids, and of N-methyl-D-aspartate receptors, to induce long-term depression in the striatal complex. These observations might contribute to a better understanding of the increased sensitivity of the adolescent brain to drug induced-plasticity.

GluN2D-containing NMDA receptors inhibit neurotransmission in the mouse striatum through a cholinergic mechanism: implication for Parkinson's disease.

The GluN2 subunits that compose NMDA receptors (NMDARs) determine functional and pharmacological properties of the receptor. In the striatum, functions and potential dysfunctions of NMDARs attributed to specific GluN2 subunits have not been clearly elucidated, although NMDARs play critical roles in the interactions between glutamate and dopamine. Through the use of amperometry and field potential recordings in mouse brain slices, we found that NMDARs that contain the GluN2D subunit contribute to NMDA-induced inhibition of evoked dopamine release and of glutamatergic neurotransmission in the striatum of control mice. Inhibition is likely mediated through increased firing in cholinergic interneurons, which were shown to express GluN2D. Indeed, NMDA-induced inhibition of both dopamine release and glutamatergic neurotransmission is reduced in the presence of muscarinic receptor antagonists and is mimicked by a muscarinic receptor agonist. We have also examined whether this function of GluN2D-containing NMDARs is altered in a mouse model of Parkinson's disease. We found that the inhibitory role of GluN2D-containing NMDARs on glutamatergic neurotransmission is impaired in the 6-hydroxydopamine lesioned striatum. These results identify a role for GluN2D-containing NMDARs and adaptive changes in experimental Parkinsonism. GluN2D might constitute an attractive target for the development of novel pharmacological tools for therapeutic intervention in Parkinson's disease.

Claustrum and dorsal endopiriform cortex complex cell-identity is determined by Nurr1 and regulates hallucinogenic-like states in mice.

The Claustrum/dorsal endopiriform cortex complex (CLA) is an enigmatic brain region with extensive glutamatergic projections to multiple cortical areas. The transcription factor Nurr1 is highly expressed in the CLA, but its role in this region is not understood. By using conditional gene-targeted mice, we show that Nurr1 is a crucial regulator of CLA neuron identity. Although CLA neurons remain intact in the absence of Nurr1, the distinctive gene expression pattern in the CLA is abolished. CLA has been hypothesized to control hallucinations, but little is known of how the CLA responds to hallucinogens. After the deletion of Nurr1 in the CLA, both hallucinogen receptor expression and signaling are lost. Furthermore, functional ultrasound and Neuropixel electrophysiological recordings revealed that the hallucinogenic-receptor agonists' effects on functional connectivity between prefrontal and sensorimotor cortices are altered in Nurr1-ablated mice. Our findings suggest that Nurr1-targeted strategies provide additional avenues for functional studies of the CLA.

Ethanol inhibits excitatory neurotransmission in the nucleus accumbens of adolescent mice through GABAA and GABAB receptors.

Age-related differences in various acute physiological and behavioral effects of alcohol have been demonstrated in humans and in other species. Adolescents are more sensitive to positive reinforcing properties of alcohol than adults, but the cellular mechanisms that underlie such a difference are not clearly established. We, therefore, assessed age differences in the ability of ethanol to modulate glutamatergic synaptic transmission in the mouse nucleus accumbens (NAc), a brain region importantly involved in reward mechanisms. We measured field excitatory postsynaptic potentials/population spikes (fEPSP/PS) in NAc slices from adolescent (22-30 days old) and adult (5-8 months old) male mice. We found that 50mM ethanol applied in the perfusion solution inhibits glutamatergic neurotransmission in the NAc of adolescent, but not adult, mice. This effect is blocked by the gamma-aminobutyric acid (GABA)A receptor antagonist bicuculline and by the GABAB receptor antagonist CGP 55845. Furthermore, bicuculline applied alone produces a stronger increase in the fEPSP/PS amplitude in adult mice than in adolescent mice. Activation of GABAA receptors with muscimol produces a stronger and longer lasting depression of neurotransmission in adolescent mice as compared with adult mice. Activation of GABAB receptors with SKF 97541 also depresses neurotransmission more strongly in adolescent than in adult mice. These results demonstrate that an increased GABA receptor function associated with a reduced inhibitory tone underlies the depressant action of ethanol on glutamatergic neurotransmission in the NAc of adolescent mice.

Aberrant somatic calcium channel function in cNurr1 and LRRK2-G2019S mice.

In Parkinson's disease (PD), axons of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNc) degenerate before their cell bodies. Calcium influx during pacemaker firing might contribute to neuronal loss, but it is not known if dysfunctions of voltage-gated calcium channels (VGCCs) occur in DA neurons somata and axon terminals. We investigated T-type and L-type VGCCs in SNc-DA neurons of two mouse models of PD: mice with a deletion of the Nurr1 gene in DA neurons from an adult age (cNurr1 mice), and mice bearing the G2019S mutation in the gene coding for LRRK2 (G2019S mice). Adult cNurr1 mice displayed motor and DA deficits, while middle-aged G2019S mice did not. The number and morphology of SNc-DA neurons, most of their intrinsic membrane properties and pacemaker firing were unaltered in cNurr1 and G2019S mice compared to their control and wild-type littermates. L-type VGCCs contributed to the pacemaker firing of SNc-DA neurons in G2019S mice, but not in control, wild-type, and cNurr1 mice. In cNurr1 mice, but not G2019S mice, the contribution of T-type VGCCs to the pacemaker firing of SNc-DA neurons was reduced, and somatic dopamine-D2 autoreceptors desensitized more. Altered contribution of L-type and T-type VGCCs to the pacemaker firing was not observed in the presence of a LRRK2 kinase inhibitor in G2019S mice, and in the presence of a flavonoid with antioxidant activity in G2019S and cNurr1 mice. The role of L-type and T-type VGCCs in controlling dopamine release from axon terminals in the striatum was unaltered in cNurr1 and G2019S mice. Our findings uncover opposite changes, linked to oxidative stress, in the function of two VGCCs in DA neurons somata, but not axon terminals, in two different experimental PD models.

Prosaposin maintains lipid homeostasis in dopamine neurons and counteracts experimental parkinsonism in rodents.

Prosaposin (PSAP) modulates glycosphingolipid metabolism and variants have been linked to Parkinson's disease (PD). Here, we find altered PSAP levels in the plasma, CSF and post-mortem brain of PD patients. Altered plasma and CSF PSAP levels correlate with PD-related motor impairments. Dopaminergic PSAP-deficient (cPSAPDAT) mice display hypolocomotion and depression/anxiety-like symptoms with mildly impaired dopaminergic neurotransmission, while serotonergic PSAP-deficient (cPSAPSERT) mice behave normally. Spatial lipidomics revealed an accumulation of highly unsaturated and shortened lipids and reduction of sphingolipids throughout the brains of cPSAPDAT mice. The overexpression of α-synuclein via AAV lead to more severe dopaminergic degeneration and higher p-Ser129 α-synuclein levels in cPSAPDAT mice compared to WT mice. Overexpression of PSAP via AAV and encapsulated cell biodelivery protected against 6-OHDA and α-synuclein toxicity in wild-type rodents. Thus, these findings suggest PSAP may maintain dopaminergic lipid homeostasis, which is dysregulated in PD, and counteract experimental parkinsonism.

Ethanol disrupts the mechanisms of induction of long-term potentiation in the mouse nucleus accumbens.

BACKGROUND: Long-term changes in the efficacy of glutamatergic synaptic transmission in reward-related brain regions such as the nucleus accumbens (NAc) are proposed to contribute to neuroadaptations that lead to drug addiction. Although alcohol is a widely used addictive substance, the cellular mechanisms by which it influences synaptic plasticity in the NAc are not elucidated. We therefore examined whether acute ethanol (EtOH) alters long-term potentiation (LTP) in the core region of the NAc and investigated the possible underlying mechanisms. METHODS: We measured field excitatory postsynaptic potential/population spike (fEPSP/PS) amplitude in mouse brain slices containing the NAc. We also used amperometry to detect, with carbon fiber electrode, evoked dopamine release in brain slices. RESULTS: In control slices, high-frequency stimulation (HFS) induced a stable LTP. LTP was reduced in slices perfused with EtOH (50 mM). Given that induction of LTP is dependent on glutamate acting on N-methyl-d-aspartate (NMDA) receptors and group I metabotropic glutamate receptors (mGluRs), we studied the ability of EtOH to modulate these 2 classes of receptors. NMDA (20 µM) depressed the amplitude of the fEPSP/PS, but this effect was not altered by EtOH in our experimental conditions. However, EtOH reversed the ability of the group I mGluR agonist (S)-3,5-Dihydroxyphenylglycine (DHPG) (50 µM) to potentiate the depressant action of NMDA on the fEPSP/PS. We also examined whether EtOH could modulate dopamine release given that dopamine plays important roles in mediating the reinforcing actions of abused drugs and in the induction of LTP in the NAc. We found that EtOH reversibly decreased action potential-dependent dopamine release evoked by single stimulation pulses and by HFS trains in NAc slices. CONCLUSIONS: These results show that EtOH impairs the induction of LTP possibly through several mechanisms that include inhibition of group I mGluR-mediated potentiation of NMDA receptor function and of evoked dopamine release. This study provides additional support for a key role of glutamatergic and dopaminergic neurotransmission in the NAc in mediating the reinforcing effects of acute alcohol.

Neuronal Firing and Glutamatergic Synapses in the Substantia Nigra Pars Reticulata of LRRK2-G2019S Mice.

Pathogenic mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are frequent causes of familial Parkinson's Disease (PD), an increasingly prevalent neurodegenerative disease that affects basal ganglia circuitry. The cellular effects of the G2019S mutation in the LRRK2 gene, the most common pathological mutation, have not been thoroughly investigated. In this study we used middle-aged mice carrying the LRRK2-G2019S mutation (G2019S mice) to identify potential alterations in the neurophysiological properties and characteristics of glutamatergic synaptic transmission in basal ganglia output neurons, i.e., substantia nigra pars reticulata (SNr) GABAergic neurons. We found that the intrinsic membrane properties and action potential properties were unaltered in G2019S mice compared to wild-type (WT) mice. The spontaneous firing frequency was similar, but we observed an increased regularity in the firing of SNr neurons recorded from G2019S mice. We examined the short-term plasticity of glutamatergic synaptic transmission, and we found an increased paired-pulse depression in G2019S mice compared to WT mice, indicating an increased probability of glutamate release in SNr neurons from G2019S mice. We measured synaptic transmission mediated by NMDA receptors and we found that the kinetics of synaptic responses mediated by these receptors were unaltered, as well as the contribution of the GluN2B subunit to these responses, in SNr neurons of G2019S mice compared to WT mice. These results demonstrate an overall maintenance of basic neurophysiological and synaptic characteristics, and subtle changes in the firing pattern and in glutamatergic synaptic transmission in basal ganglia output neurons that precede neurodegeneration of dopaminergic neurons in the LRRK2-G2019S mouse model of late-onset PD.

Age- and Sex-Dependent Behavioral and Neurochemical Alterations in hLRRK2-G2019S BAC Mice.

The G2019S mutation in the leucine-rich repeat kinase 2 (LRRK2) gene is associated with late-onset Parkinson's disease (PD). Although PD affects men and women differently, longitudinal studies examining sex- and age-dependent alterations in mice carrying the G2019S mutation are limited. We examined if behavioral and neurochemical dysfunctions, as well as neurodegeneration, occur in male and female BAC LRRK2-hG2019S (G2019S) mice, compared to their age-matched wild type littermates, at four age ranges. In the open field test, hyperlocomotion was observed in 10-12 month old male and 2-4.5 months old female G2019S mice. In the pole test, motor coordination was impaired in male G2019S mice from 15 months of age and in 20-21 months old female G2019S mice. In the striatum of G2019S male and female mice, the amounts of tyrosine hydroxylase (TH), measured with Western blotting, were unaltered. However, we found a decreased expression of the dopamine transporter in 20-21 month old male G2019S mice. The number of TH-positive neurons in the substantia nigra compacta was unaltered in 20-21 month old male and female G2019S mice. These results identify sex- and age-dependent differences in the occurrence of motor and neurochemical deficits in BAC LRRK2-hG2019S mice, and no degeneration of DA neurons.

PRC2-mediated repression is essential to maintain identity and function of differentiated dopaminergic and serotonergic neurons.

How neurons can maintain cellular identity over an entire life span remains largely unknown. Here, we show that maintenance of identity in differentiated dopaminergic and serotonergic neurons is critically reliant on the Polycomb repressive complex 2 (PRC2). Deletion of the obligate PRC2 component, Eed, in these neurons resulted in global loss of H3K27me3, followed by a gradual activation of genes harboring both H3K27me3 and H3K9me3 modifications. Notably, H3K9me3 was lost at these PRC2 targets before gene activation. Neuronal survival was not compromised; instead, there was a reduction in subtype-specific gene expression and a progressive impairment of dopaminergic and serotonergic neuronal function, leading to behavioral deficits characteristic of Parkinson's disease and anxiety. Single-cell analysis revealed subtype-specific vulnerability to loss of PRC2 repression in dopamine neurons of the substantia nigra. Our study reveals that a PRC2-dependent nonpermissive chromatin state is essential to maintain the subtype identity and function of dopaminergic and serotonergic neurons.

Ketamine induces opposite changes in AMPA receptor calcium permeability in the ventral tegmental area and nucleus accumbens.

Ketamine elicits rapid and durable antidepressant actions in treatment-resistant patients with mood disorders such as major depressive disorder and bipolar depression. The mechanisms might involve the induction of metaplasticity in brain regions associated with reward-related behaviors, mood, and hedonic drive, particularly the ventral tegmental area (VTA) and the nucleus accumbens (NAc). We have examined if ketamine alters the insertion of the GluA2 subunit of AMPA receptors (AMPAR), which determines calcium permeability of the channel, at glutamatergic synapses onto dopamine (DA) neurons in the VTA and spiny projection neurons (SPNs) in the Core region of the NAc. Mice received one injection of either saline or a low dose of ketamine 24 h before electrophysiological recordings were performed. We found that GluA2-lacking calcium-permeable (CP) AMPARs were present in DA neurons in the VTA of mice treated with saline, and that ketamine-induced the removal of a fraction of these receptors. In NAc SPNs, ketamine induced the opposite change, i.e., GluA2-lacking CP-AMPARs were inserted at glutamatergic synapses. Ketamine-induced metaplasticity was independent of group I metabotropic glutamate receptors (mGluRs) because an agonist of these receptors had similar effects on glutamatergic transmission in mice treated with saline and in mice treated with ketamine in both VTA DA neurons and in the NAc. Thus, ketamine reduces the insertion of CP-AMPARs in VTA DA neurons and induces their insertion in the NAc. The mechanism by which ketamine elicits antidepressant actions might thus involve an alteration in the contribution of GluA2 to AMPARs thereby modulating synaptic plasticity in the mesolimbic circuit.

Restricted cortical and amygdaloid removal of vesicular glutamate transporter 2 in preadolescent mice impacts dopaminergic activity and neuronal circuitry of higher brain function.

A major challenge in neuroscience is to resolve the connection between gene functionality, neuronal circuits, and behavior. Most, if not all, neuronal circuits of the adult brain contain a glutamatergic component, the nature of which has been difficult to assess because of the vast cellular abundance of glutamate. In this study, we wanted to determine the role of a restricted subpopulation of glutamatergic neurons within the forebrain, the Vglut2-expressing neurons, in neuronal circuitry of higher brain function. Vglut2 expression was selectively deleted in the cortex, hippocampus, and amygdala of preadolescent mice, which resulted in increased locomotor activity, altered social dominance and risk assessment, decreased sensorimotor gating, and impaired long-term spatial memory. Presynaptic VGLUT2-positive terminals were lost in the cortex, striatum, nucleus accumbens, and hippocampus, and a downstream effect on dopamine binding site availability in the striatum was evident. A connection between the induced late-onset, chronic reduction of glutamatergic neurotransmission and dopamine signaling within the circuitry was further substantiated by a partial attenuation of the deficits in sensorimotor gating by the dopamine-stabilizing antipsychotic drug aripiprazole and an increased sensitivity to amphetamine. Somewhat surprisingly, given the restricted expression of Vglut2 in regions responsible for higher brain function, our analyses show that VGLUT2-mediated neurotransmission is required for certain aspects of cognitive, emotional, and social behavior. The present study provides support for the existence of a neurocircuitry that connects changes in VGLUT2-mediated neurotransmission to alterations in the dopaminergic system with schizophrenia-like behavioral deficits as a major outcome.

NMDA receptors are altered in the substantia nigra pars reticulata and their blockade ameliorates motor deficits in experimental parkinsonism.

In Parkinson's disease (PD) reduced levels of dopamine (DA) in the striatum lead to an abnormal circuit activity of the basal ganglia and an increased output through the substantia nigra pars reticulata (SNr) and the globus pallidus internal part. Synaptic inputs to the SNr shape its activity, however, the properties of glutamatergic synaptic transmission in this output nucleus of the basal ganglia in control and DA-depleted conditions are not fully elucidated. Using whole-cell patch-clamp recordings and pharmacological tools, we examined alterations in glutamatergic synaptic transmission in the SNr of a mouse model of PD, i.e. mice with unilateral 6-OHDA lesion of DA neurons in the substantia nigra pars compacta, as compared to control mice. We found that AMPA receptor (AMPAR)-mediated spontaneous and evoked excitatory postsynaptic currents (sEPSCs and eEPSCs) were not altered. The AMPA/NMDA ratio was significantly decreased in 6-OHDA-lesioned mice, suggesting an increased synaptic function of NMDA receptors (NMDARs) in DA-depleted mice. The decay kinetics of NMDAR-eEPSCs were faster in 6-OHDA-lesioned mice, indicating a possible change in the subunit composition of synaptic NMDARs. In control mice NMDAR-eEPSCs were mediated by diheteromeric NMDARs made of GluN2A, GluN2B and GluN2D. In 6-OHDA-lesioned mice the function of diheteromeric NMDARs containing either GluN2B or GluN2D was dramatically decreased, whereas the function of diheteromeric NMDARs made of GluN2A was preserved. Microinjections of an NMDAR antagonist into the SNr of 6-OHDA-lesioned mice resulted in significant improvements in spontaneous locomotion. This study identifies novel alterations occurring at excitatory synapses in the basal ganglia output nucleus following DA depletion. An increased synaptic NMDAR function, due to an altered subunit composition, might contribute to hyperactivation of SNr neurons in the DA depleted state and to motor impairments in PD.

Deficits in Motor Performance, Neurotransmitters and Synaptic Plasticity in Elderly and Experimental Parkinsonian Mice Lacking GPR37.

Parkinson's disease (PD) etiology is attributed to aging and the progressive neurodegeneration of dopamine (DA) neurons of substantia nigra pars compacta (SNc). GPR37 is an orphan G-protein Coupled Receptor (GPCR) that is linked to the juvenile form of PD. In addition, misfolded GPR37 has been found in Lewy bodies. However, properly folded GPR37 found at the cell membrane appears to exert neuroprotection. In the present study we investigated the role of GPR37 in motor deficits due to aging or toxin-induced experimental parkinsonism. Elderly GPR37 knock out (KO) mice displayed hypolocomotion and worse fine movement performance compared to their WT counterparts. Striatal slice electrophysiology reveiled that GPR37 KO mice show profound decrease in long term potentiation (LTP) formation which is accompanied by an alteration in glutamate receptor subunit content. GPR37 KO animals exposed to intrastriatal 6-hydroxydopamine (6-OHDA) show poorer score in the behavioral cylinder test and more loss of the DA transporter (DAT) in striatum. The GPR37 KO striata exhibit a significant increase in GABA which is aggravated after DA depletion. Our data indicate that GPR37 KO mice have DA neuron deficit, enhanced striatal GABA levels and deficient corticostriatal LTP. They also respond stronger to 6-OHDA-induced neurotoxicity. Taken together, the data indicate that properly functional GPR37 may counteract aging processes and parkinsonism.

NR2A-containing NMDA receptors depress glutamatergic synaptic transmission and evoked-dopamine release in the mouse striatum.

NMDA receptors play essential roles in the physiology and pathophysiology of the striatum, a brain nucleus involved in motor control and reward-motivated behaviors. NMDA receptors are composed of NR1 and NR2A-D subunits. Functional properties of NMDA receptors are determined by the type of NR2 subunit they contain. In this study, we have examined the involvement of NR2B and NR2A in the modulatory effect of NMDA on glutamatergic and dopaminergic synaptic transmission in the striatum. We found that bath application of NMDA decreased the amplitude of the field excitatory post-synaptic potential/population spike (fEPSP/PS) measured in corticostriatal mouse brain slices. This depression was not affected by the NR2B-selective antagonists Ifenprodil and Ro 25-6981, but was abolished by the NR2A antagonist NVP-AAM077. Activation of corticostriatal neurons by NMDA did not contribute to synaptic depression because similar results were obtained in decorticated striatal slices. Synaptic depression was not dependent on GABA release because the GABA(A) receptor antagonist bicuculline did not affect NMDA-induced decrease of the fEPSP/PS. NMDA also depressed evoked-dopamine release through NR2A- but not NR2B-containing NMDA receptors. Our results identify an important role for NR2A-containing NMDA receptors intrinsic to the striatum in regulating glutamatergic synaptic transmission and evoked-dopamine release.

Long-term potentiation in the nucleus accumbens requires both NR2A- and NR2B-containing N-methyl-D-aspartate receptors.

N-methyl-d-aspartate (NMDA) receptors play crucial roles in several forms of long-term changes in the efficacy of glutamatergic synaptic transmission. The suggestion that the NR2A subunit of the NMDA receptor may be selectively involved in the induction of long-term potentiation (LTP) in the hippocampus and cortex has been challenged. However, the contribution of NR2B in the induction of LTP is not always clearly established. The present study investigates the role of NR2A and NR2B in the induction of LTP in the nucleus accumbens (NAc), a brain region that expresses high levels of NR2B and an NMDA-dependent form of LTP. We recorded extracellular field excitatory postsynaptic potentials/population spikes in slices of mouse NAc. High-frequency stimulation of glutamatergic fibers consistently induced LTP of the field excitatory postsynaptic potential/population spike in the NAc. LTP was abolished in the presence of selective antagonists of either NR2B [R-(R*,S*)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenyl-methyl)-1-piperidine propanol and Ifenprodil] or NR2A ([(R)-[(S)-1-(4-bromo-phenyl)-ethylamino]-(2,3-dioxo-1,2,3,4-tetrahydroquinoxalin-5-yl)-methyl]-phosphonic acid) subunits. Recordings performed in a low concentration of Mg(2+) ions in the perfusion solution did not reveal a selective involvement of a particular NMDA receptor subunit because either NR2A or NR2B antagonists were able to block LTP. LTP was also abolished in the presence of a low concentration of the non-subunit-selective NMDA receptor antagonist dl-2-amino-5-phosphonopentanoic acid in normal Mg(2+) and low Mg(2+) in the perfusion solution. These results show that the degree of NMDA receptor activation, and not their subunit composition, determines whether LTP is induced in the NAc.

Dopamine D1 receptors and group I metabotropic glutamate receptors contribute to the induction of long-term potentiation in the nucleus accumbens.

Long-term changes in the efficacy of glutamatergic synaptic transmission in the striatal complex are proposed to underlie motor learning and neuroadaptations leading to addiction. Dopamine and glutamate play key roles in the induction of long-term potentiation (LTP) and long-term depression (LTD) in the dorsal striatum, but their contribution to synaptic plasticity in the ventral striatum (nucleus accumbens, NAc) has been less extensively studied. We have examined the role of dopamine, glutamate and GABA in the induction of LTP in mouse brain slices containing the NAc. High-frequency stimulation of glutamatergic inputs elicited LTP of field excitatory postsynaptic potentials/population spikes (fEPSP/PSs) in the core region of the NAc. GABA did not seem to participate in LTP induction because LTP was not altered in the presence of either a GABA(A)- (bicuculline) or a GABA(B)- (CGP 55845) receptor antagonist. However, the dopamine D1 receptor antagonist SCH 23390, but not the dopamine D2 receptor antagonist sulpiride, impaired LTP. The dopamine reuptake blocker nomifensine also inhibited LTP induction. We found that group I metabotropic glutamate receptors (mGluRs) contribute to LTP induction because the mGluR1 antagonist LY 367385, or the mGluR5 antagonist MPEP, blocked LTP induction. Furthermore, the glutamate reuptake blocker DL-TBOA also impaired LTP. The present results demonstrate that dopamine and glutamate play critical roles in the mechanisms of induction of LTP in the NAc through the activation of dopamine D1 receptors and group I mGluRs. However, LTP is negatively regulated when endogenous levels of dopamine or glutamate are elevated.

Neurokinin B/NK3 receptors exert feedback inhibition on L-DOPA actions in the 6-OHDA lesion rat model of Parkinson's disease.

Neurokinin B (NKB) and substance P (SP) act via NK(3) and NK(1) receptors. Using the unilateral 6-hydroxydopamine (6-OHDA) lesion rat model of Parkinson's disease (PD), it was found that chronic, but not acute, administration of L-DOPA increases striatal NKB expression in the dopamine-depleted hemisphere. In contrast, both acute and chronic administrations of L-DOPA restore reduced levels of SP mRNA. Co-treatment with the NK(3) receptor antagonist, SB222200, and L-DOPA increased contralateral rotations compared to L-DOPA alone in L-DOPA primed rats. The NK(3)R agonist, senktide, increased the phosphorylation of tyrosine hydroxylase (TH) at Ser(19)-TH, a CaMKII site, and of Thr(286)-CaMKII in striatal slices. Senktide had no effect on P-Ser(31)-TH, a MAPK site, but reduced P-Ser(217/221)-MEK. Amperometry demonstrated that senktide increased evoked dopamine release. SB222200 blocked the effects of senktide. In striatal slices prepared from 6-OHDA-lesioned rats repeatedly treated with L-DOPA, senktide no longer increased P-Thr(286)-CaMKII, suggesting a role of NK(3)R on dopamine terminals under normal conditions. SB222200 increased P-Ser(217/221)-MEK only in dopamine-depleted slices, indicating an increased NK(3)R tone under Parkinsonism conditions. Altogether, these data demonstrate a differential regulation of NKB and SP by L-DOPA in an animal model of PD and indicate a unique role of NKB in long-term effects of L-DOPA. Behavioural, biochemical and amperometric data indicate that NKB/NK(3)R signalling stimulates dopamine transmission at the presynaptic site, but inhibits it at the postsynaptic site. The inhibitory influence of NKB/NK(3)R on dopamine transmission dominates in an animal model of PD and provides a feedback inhibition on actions mediated via L-DOPA.