Vesicular glutamate transporter 3 (VGLUT3) identifies spatially segregated excitatory terminals in the rat substantia nigra

The excitability of dopaminergic (DA) neurons in the substantia nigra is controlled by the convergent activity of multiple glutamatergic afferents. Here, we show that vesicular glutamate transporter 3 (VGLUT3)-immunoreactive (ir) terminals segregate to the perisomatic region of DA neurons in the substantia nigra pars compacta, and VGLUT3 decorates a synapse population distinct from those marked by vesicular glutamate transporters 1 and 2. VGLUT3-ir nerve endings form asymmetric terminals on DA neurons. Retrograde tracing suggests the superior colliculus as an origin of excitatory VGLUT3-ir afferents. Collectively, our data indicate that VGLUT3 identifies a novel excitatory terminal subset that contributes to the tuning of DA cell excitability in the substantia nigra.     

Modulation of GABAergic transmission by endogenous glutamate in the rat supraoptic nucleus

The presence of group III metabotropic glutamate receptors on GABAergic terminals in the supraoptic nucleus suggests that the level of glutamate in the extracellular space may regulate synaptic strength at inhibitory synapses. To test this hypothesis we examined the consequences of increasing ambient glutamate on GABA-mediated synaptic activity in supraoptic neurons. The concentration of the excitatory amino acid in the extracellular space was increased pharmacologically by blocking glutamate transporters. Inhibition of the astrocyte-specific GLT-1 glutamate transporter led to a reversible decrease in evoked inhibitory postsynaptic current amplitude. This modulation had a presynaptic origin as revealed by analysis of paired-pulse ratio and miniature inhibitory currents. Furthermore, blocking group III metabotropic glutamate receptors with the specific antagonist MAP4 prevented the depression of GABAergic transmission induced by glutamate transporter blockade. Thus, presynaptic metabotropic glutamate receptors located on inhibitory terminals in the supraoptic nucleus appear to sense changes in ambient glutamate and modify GABA release accordingly. However, it seems that such changes need to reach a certain magnitude because the discrete deficit in glutamate clearance which occurs in the supraoptic nucleus of lactating rats is not sufficient to modulate GABA-mediated transmission. These results suggest that ambient glutamate contributes to the modulation of synaptic efficacy not only at glutamatergic synapses but also at inhibitory GABAergic synapses.     

Depression of mGluR-mediated IPSCs by 5-HT in dopamine neurons of the rat substantia nigra pars compacta

Dopamine neurons of the substantia nigra pars compacta receive a prominent serotonin (5-HT) projection from the dorsal raphe nucleus and important functional interactions between the serotonergic and the dopaminergic system have been postulated. In the present report we examined the role of 5-HT in the modulation of the metabotropic glutamate receptor-mediated inhibitory postsynaptic current (mGluR-IPSC) in midbrain dopamine neurons, and we found a reversible depression of this synaptic response at concentrations of 5-HT ranging from 100 nm to 30 microm (EC50 1.06 microm). This resulted in a shift towards excitation of the overall dopamine neuron response to glutamatergic synaptic input. This effect was not because of a direct modulation of the Ca2+-sensitive K+ conductances underlying the mGluR-IPSC, but was associated with a decrease in the intracellular calcium signal triggered by mGluR stimulation. Similar results were obtained with alpha-methyl-5-hydroxytryptamine and 5-methoxytryptamine, but not with 5-carboxamidotryptamine or 1-(3-chlorophenyl) piperazine. No significant depression of the mGluR-IPSC by 5-HT was observed in the presence of the 5-HT2 antagonist cinanserin or the 5-HT4 receptor antagonist RS 23597-190, whereas the 5-HT2C antagonist RS 102221 was ineffective. Our results demonstrate a powerful inhibition of the mGluR-IPSC by 5-HT in midbrain dopamine neurons, most probably through stimulation of 5-HT2A and 5-HT4 receptors.     

Dopamine receptor supersensitivity in rat subthalamus after 6-hydroxydopamine lesions

The subthalamic nucleus (STN) receives direct dopaminergic innervation from the substantia nigra pars compacta, but the importance of this input in the pathophysiology of parkinsonism remains to be determined. We used whole-cell patch-clamp recordings in brain slices to study presynaptic dopaminergic modulation of synaptic inputs to the STN in unilateral 6-hydroxydopamine (6-OHDA)-lesioned rats. Here, we report that dopamine was more potent for inhibiting GABA IPSCs and glutamate EPSCs in the STN ipsilateral to the lesion, and was less potent for suppressing IPSCs and EPSCs in the STN contralateral to the lesion, compared with the effects of dopamine in control STN. Dopamine reduced IPSCs with an IC50 value of 20.9 +/- 3.6 microM in control STN, whereas IC50 values were 0.83 +/- 0.15 and 55.1 +/- 11.1 microM in STN ipsilateral and contralateral to 6-OHDA lesions, respectively. Dopamine also inhibited EPSCs with an IC50 value of 12.8 +/- 2.8 microM in control STN, whereas IC50 values were 4.5 +/- 0.9 and 41.6 +/- 9.8 microM in STN ipsilateral and contralateral to 6-OHDA lesions, respectively. Results with paired stimuli to evoke EPSCs and IPSCs suggest that endogenous dopamine acts presynaptically to inhibit transmitter release in the STN. These results show that chronic dopamine denervation significantly alters the regulation of synaptic input to the STN. Our results also suggest that the STN may be an important target for levodopa therapy in Parkinson's disease.     

Mechanisms regulating spill‐over of synaptic glutamate to extrasynaptic NMDA receptors in mouse substantia nigra dopaminergic neurons

N‐Methyl‐d ‐aspartate glutamate receptors (NMDARs) contribute to neural development, plasticity and survival, but they are also linked with neurodegeneration. NMDARs at synapses are activated by coincident glutamate release and depolarization. NMDARs distal to synapses can sometimes be recruited by ‘spill‐over’ of glutamate during high‐frequency synaptic stimulation or when glutamate uptake is compromised, and this influences the shape of NMDAR‐mediated postsynaptic responses. In substantia nigra dopamine neurons, activation of NMDARs beyond the synapse during different frequencies of presynaptic stimulation has not been explored, even though excitatory afferents from the subthalamic nucleus show a range of firing frequencies, and these frequencies change in human and experimental Parkinson's disease. This study reports that high‐frequency stimulation (80 Hz/200 ms) evoked NMDAR‐excitatory postsynaptic currents (EPSCs) that were larger and longer lasting than those evoked by single stimuli at low frequency (0.1 Hz). MK‐801, which irreversibly blocked NMDAR‐EPSCs activated during 0.1‐Hz stimulation, left a proportion of NMDAR‐EPSCs that could be activated by 80‐Hz stimulation and that may represent activity of NMDARs distal to synapses. TBOA, which blocks glutamate transporters, significantly increased NMDAR‐EPSCs in response to 80‐Hz stimulation, particularly when metabotropic glutamate receptors (mGluRs) were also blocked, indicating that recruitment of NMDARs distal to synapses is regulated by glutamate transporters and mGluRs. These regulatory mechanisms may be essential in the substantia nigra for restricting glutamate diffusion from synaptic sites and keeping NMDAR‐EPSCs in dopamine neurons relatively small and fast. Failure of glutamate transporters may contribute to the declining health of dopamine neurons during pathological conditions.     

Functional heterogeneity of NMDA receptors in rat substantia nigra pars compacta and reticulata neurones

The nigra substantia nigra pars compacta (SNc) and substantia pars reticulata (SNr) form two major basal ganglia components with different functional roles. SNc dopaminergic (DA) neurones are vulnerable to cell death in Parkinson’s disease, and NMDA receptor activation is a potential contributing mechanism. We have investigated the sensitivity of whole‐cell and synaptic NMDA responses to intracellular ATP and GTP application in the SNc and SNr from rats on postnatal day (P) 7 and P28. Both NMDA current density (pA/pF) and desensitization to prolonged or repeated NMDA application were greater in the SNr than in the SNc. When ATP levels were not supplemented, responses to prolonged NMDA administration desensitized in P7 SNc DA neurones but not at P28. At P28, SNr neurones desensitized more than SNc neurones, with or without added ATP. Responses to brief NMDA applications and synaptic NMDA currents were not sensitive to inclusion of ATP in the pipette solution. To investigate these differences between the SNc and SNr, NR2 subunit‐selective antagonists were tested. NMDA currents were inhibited by ifenprodil (10 μm ) and UBP141 (4 μm ), but not by Zn²⁺ (100 nm ), in both the SNr and SNc, suggesting that SNc and SNr neurones express similar receptor subunits; NR2B and NR2D, but not NR2A. The different NMDA response properties in the SNc and SNr may be caused by differences in receptor modulation and/or trafficking. The vulnerability of SNc DA neurones to cell death is not correlated with NMDA current density or receptor subtypes, but could in part be related to inadequate NMDA receptor desensitization.     

Non-calyceal excitatory inputs mediate low fidelity synaptic transmission in rat auditory brainstem slices

Principal neurons of the medial nucleus of the trapezoid body (MNTB) receive a synaptic input from a single giant calyx terminal that generates a fast-rising, large excitatory postsynaptic current (EPSC), each of which are supra-threshold for postsynaptic action potential generation. Here, we present evidence that MNTB principal neurons receive multiple excitatory synaptic inputs generating slow-rising, small EPSCs that are also capable of triggering postsynaptic action potentials but are of non-calyceal origin. Both calyceal and non-calyceal EPSCs are mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and N-methyl-d-aspartate (NMDA) receptor activation; however, the NMDA receptor-mediated response is proportionally larger at the non-calyceal synapses. Non-calyceal synapses generate action potentials in MNTB principal neurons with a longer latency and a lower reliability than the large calyceal input. They constitute an alternative low fidelity synaptic input to the fast and secure relay transmission via the calyx of Held synapse.     

Kainate receptor-mediated presynaptic inhibition converges with presynaptic inhibition mediated by Group II mGluRs and long-term depression at the hippocampal mossy fiber-CA3 synapse

Summary Kainate receptors (KARs) effect depression of glutamate release at hippocampal mossy fiber-CA3 (MF-CA3) synapses by a metabotropic action involving adenylyl cyclase (AC) inhibition, cAMP reduction, and diminished protein kinase A (PKA) activation. Using hippocampal slices, we show here that KAR activation interferes with the depression of glutamate release produced by Group II metabotropic glutamate receptor stimulation and low frequency stimulation (LFS)-induced long-term depression (LTD), also expressed through presynaptic AC/cAMP/PKA at MF-CA3 synapses. The mutual occlusion of depression mediated by presynaptic KARs, Group II mGluR and LFS-induced LTD suggests their mechanistic convergence at the MF-CA3 synapse and thus invokes KARs in synaptic plasticity manifest in LTD.     

Turning behavior induced by injections of glutamate receptor antagonists into the substantia nigra of the rat

We have found recently that muscimol microinjections into the subthalamic nucleus produce contralateral turning activity [Murer and Pazo (1993) NeuroReport, 4:1219–1222]. To test the hypothesis that a reduced glutamate action on substantia nigra pars reticulata neurons mediates this turning response, we examined the effect of unilateral intranigral microinjections of the AMPA/kainate receptor antagonist 6,7-dinitro-quinoxaline-2,3-dione (DNQX) and the competitive N-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-5-phosphonovaleric acid (AP-5). DNQX and AP-5 both produced a dose-dependent contralateral turning response, while vehicle administration did not induce turning activity. Application of glutamate receptor antagonist at adjacent regions of the mesencephalic tegmentum were also ineffective. Coadministration of NMDA or AMPA significantly reduced the turning response induced by AP-5 or DNQX, respectively. Lesions of the nigrostriatal pathway by 6-hydroxydopamine did not modify the response to DNQX or AP-5 administration into the nigra. However, their behavioral effects were significantly reduced by a lesion of the ipsilateral subthalamic nucleus. Our results show that the blockade of a tonic input acting on AMPA/kainate and NMDA receptors located at the substantia nigra produces contralateral turning behavior. The effect seems to involve pars reticulata cells since this area remains unchanged after destruction of dopaminergic neurons. The subthalamic nucleus seems to be the endogenous source of the agonist acting on the nigral glutamate receptors related to turning behavior. © 1996 Wiley-Liss, Inc.     

Dual modulation of excitatory synaptic transmission by agonists at group I metabotropic glutamate receptors in the rat spinal dorsal horn

The effects of group I metabotropic glutamate (mGlu) receptors on excitatory transmission in the rat dorsal horn, but mostly substantia gelatinosa, neurons were investigated using conventional intracellular recording in slices. The broad spectrum mGlu receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S, 3R-ACPD), the group I mGlu receptor selective agonist (S)-3, 5-dihydroxyphenylglycine (DHPG), and the selective mGlu subtype 5 agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), all induce long-lasting depression of A primary afferent fibers-mediated monosynaptic excitatory postsynaptic potential (EPSP), and long-lasting potentiation of polysynaptic EPSP, and EPSP in cells receiving C-afferent fiber input. The DHPG potentiation of polysynaptic EPSP was partially or fully reversed by (S)-4-carboxyphenylglycine (S-4CPG), the mGlu subtype 1 preferring antagonist. 2-Methyl-6-(phenylethynyl)-pyridine, the potent and selective mGlu subtype 5 antagonist, partially reversed the CHPG potentiation of polysynaptic EPSP. The effects of DHPG on monosynaptic and polysynaptic EPSPs were reduced, or abolished, by the N-methyl-D-aspartate (NMDA) receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (AP5). A clear and pronounced facilitation of the expression of DHPG- and CHPG-induced enhancement of polysynaptic EPSP, and EPSP evoked at C-fiber strength, was seen in the absence of gamma-aminobutyric acid subtype A receptor- and glycine-mediated synaptic inhibition. Besides dual modulation of excitatory synaptic transmission, DHPG induces depression of inhibitory postsynaptic potentials evoked by primary afferent stimulation in dorsal horn neurons. In addition, group I mGlu receptor agonists produced a direct persistent excitatory postsynaptic effect consisting of a slow membrane depolarization, an increase in input resistance, and an intense neuronal discharge. Cyclothiazide and (S)-4-CPG, the mGlu receptor subtype 1 preferring antagonists, significantly attenuated the DHPG-induced depolarization. These results demonstrate that the pharmacological activation of group I metabotropic glutamate receptors induces long-term depression (LTD) and long-term potentiation (LTP) of synaptic transmission in the spinal dorsal horn. These types of long-term synaptic plasticity may play a functional role in the generation of post-injury hypersensitivity (LTP) or antinociception (LTD).     

Down-regulation of metabotropic glutamate receptor 1alpha in globus pallidus and substantia nigra of parkinsonian monkeys

Enhanced glutamatergic neurotransmission via the subthalamopallidal or subthalamonigral projection seems crucial for developing parkinsonian motor signs. In the present study, the possible changes in the expression of metabotropic glutamate receptors (mGluRs) were examined in the basal ganglia of a primate model for Parkinson's disease. When the patterns of immunohistochemical localization of mGluRs in monkeys administered systemically with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) were analysed in comparison with normal controls, we found that expression of mGluR1alpha, but not of other subtypes, was significantly reduced in the internal and external segments of the globus pallidus and the substantia nigra pars reticulata. To elucidate the functional role of mGluR1 in the control of pallidal neuron activity, extracellular unit recordings combined with intrapallidal microinjections of mGluR1-related agents were then performed in normal and parkinsonian monkeys. In normal awake conditions, the spontaneous firing rates of neurons in the pallidal complex were increased by DHPG, a selective agonist of group I mGluRs, whereas they were decreased by AIDA, a selective antagonist of group I mGluRs, or LY367385, a selective antagonist of mGluR1. These electrophysiological data strongly indicate that the excitatory mechanism of pallidal neurons by glutamate is mediated at least partly through mGluR1. The effects of the mGluR1-related agents on neuronal firing in the internal pallidal segment became rather obscure after MPTP treatment. Our results suggest that the specific down-regulation of pallidal and nigral mGluR1alpha in the parkinsonian state may exert a compensatory action to reverse the overactivity of the subthalamic nucleus-derived glutamatergic input that is generated in the disease.     

Depression of retinogeniculate synaptic transmission by presynaptic D(2)-like dopamine receptors in rat lateral geniculate nucleus

Extraretinal projections onto neurons in the dorsal lateral geniculate nucleus (dLGN) play an important role in modifying sensory information as it is relayed from the visual thalamus to neocortex. The dLGN receives dopaminergic innervation from the ventral tegmental area; however, the role of dopamine in synaptic transmission in dLGN has not been explored. In the present study, whole cell recordings were obtained to examine the actions of dopamine on glutamatergic synaptic transmission. Dopamine (2-100 microm) strongly suppressed excitatory synaptic transmission in dLGN relay neurons that was evoked by optic tract stimulation and mediated by both N-methyl-d-aspartate and non-N-methyl-d-aspartate glutamate receptors. In contrast, dopamine did not alter inhibitory synaptic transmission arising from either dLGN interneurons or thalamic reticular nucleus neurons. The suppressive action of dopamine on excitatory synaptic transmission was mimicked by the D(2)-like dopamine receptor agonist bromocriptine (2-25 microm) but not by the D(1)-like receptor agonist SKF38393 (10-25 microm). In addition, the dopamine-mediated suppression was antagonized by the D(2)-like receptor antagonist sulpiride (10-20 microm) but not by the D(1)-like receptor antagonist SCH23390 (5-25 microm). The dopamine-mediated decrease in evoked excitatory postsynaptic current amplitude was accompanied by an increase in the magnitude of paired-pulse depression. Furthermore, dopamine also reduced the frequency but not the amplitude of miniature excitatory postsynaptic currents. Taken together, these data suggest that dopamine may act presynaptically to regulate the release of glutamate at the retinogeniculate synapse and modify transmission of visual information in the dLGN.     

Lesion of the substantia nigra pars compacta downregulates striatal glutamate receptor subunit mRNA expression

This is a study of the effect of the unilateral administration of dopamine (DA) in the pars compacta of the substantia nigra (SN) of the rat on striatal glutamate receptor subunit (GluR1, GluR2 and NMDAR1) gene expression determined by in situ hybridization. The location of the nigral lesion was determined by tyrosine hydroxylase (TH) immunohistochemistry and its extent by the striatal DA and 3,4-dihydroxyphenylacetic acid (DOPAC) concentrations. The DA-induced lesions produce significant bilateral reductions in the expression of GluR1 and NMDAR1 subunit mRNA in the medio-lateral striatum, whereas the expression of striatal GluR2 receptors was not changed. The reduction in GluR1 and NMDAR1 subunit mRNA may be the consequence of glutamatergic hyperactivity developed in the presence of a damaged nigro-striatal system and these may be associated with the genesis of some neurodegenerative diseases.     

Suppression of presynaptic calcium influx by metabotropic glutamate receptor agonists in neonatal rat hippocampus

Mechanisms of presynaptic inhibition by metabotropic glutamate receptor (mGluR) agonists were investigated in neonatal rat hippocampal CA1 region using the optical recording technique recently developed. Following selective loading of presynaptic terminals with a fluorescent Ca²⁺ indicator dye rhod-2 AM, changes in Ca²⁺ signals and the corresponding field excitatory postsynaptic potentials (EPSPs) induced by single electrical stimuli to the Schaffer collateral-commissural (SCC) pathway were recorded simultaneously. Application of a mGluR agonist, 1 S,3 R-1-aminocyclopentane-1,3-dicar?ylic acid (1 S,3 R-ACPD; 100 μM) or (±)-1-aminocyclopentane-trans-1,3-dicar?ylic acid (trans-ACPD; 100 μM), reversibly reduced both the field EPSP and the presynaptic Ca²⁺ transient, and the quantitative relationship between them was quite similar to that observed during application of Cd²⁺, a non-selective Ca²⁺ channel blocker, or in a Ca²⁺-free solution. Application of 4-aminopyridine (4-AP; 1 mM), a blocker of certain subtypes of voltage-dependent K⁺ channels, significantly inhibited the 1 S,3 R-ACPD effect. Application of DCG-IV, a novel mGluR2/mGluR3-selective agonist, suppressed field EPSPs only slightly even at a high dose (3 μM). These results suggest that activation of presynaptic mGluR different from mGluR2/mGluR3 suppresses the action potential-triggered Ca²⁺ influx, probably via 4-AP-sensitive mechanisms, and thereby reduces glutamate release in neonatal rat hippocampal CA1 region.     

Difference in time course of modulation of synaptic transmission by group II versus group III metabotropic glutamate receptors in region CA1 of the hippocampus

We investigated the time course of modulation of synaptic transmission by group II and group III metabotropic glutamate receptors in region CA1 of the hippocampus. In the presence of 50 microM picrotoxin, pressure pulse application of 1 mM glutamate resulted in a fast onset of suppression of synaptic transmission in stratum lacunosum moleculare and a slower onset of suppression in stratum radiatum, with both effects returning to baseline over the course of several minutes. Application of 50 microM of the group II agonist (2R,4R)-APDC in stratum lacunosum moleculare resulted in the same fast onset of suppression while having no effect in stratum radiatum. Pressure pulse application of 100 microM DL-AP4 in stratum lacunosum moleculare and stratum radiatum resulted in a much slower onset of suppression of synaptic transmission than (2R,4R)-APDC. Suppression by (2R,4R)-APDC was accompanied by a rapid enhancement of paired pulse facilitation, indicative of a presynaptic mechanism. This demonstrates that activation of group II mGluRs in the hippocampus causes a fast onset of suppression in stratum lacunosum moleculare, while activation of group III mGluRs causes a slower onset of suppression. The difference in time course for group II vs. group III mGluRs suggests a different functional role, with group II playing a potential role in making synapses act as low pass filters.     

Muscarinic receptors on dopamine terminals in the cat caudate nucleus: Neuromodulation of [H]dopamine release in vitro by endogenous acetylcholine

The directly acting muscarinic receptor agonist oxotremorine (1.8–10 μM) produced an increase in electrically evoked [³H]dopamine release from slices of the cat caudate. The maximal increase caused by oxotremorine was 40%, and was antagonized by the muscarinic receptor blocking agent atropine (0.1 μM). Exposure to the acetylcholinesterase (AChE) inhibitor physostigmine (1 μM) resulted in a 50% increase in electrically evoked [³H]dopamine release. The increase caused by physostigmine was also antagonized by atropine (0.1 μM).Atropine did not, however, alter the modulations in [³H]dopamine release mediated by the dopamine autoreceptor: the increase in electrically evoked [³H]dopamine release caused by the dopamine receptor antagonist S-sulpiride (0.1 μM) and the decrease caused by the dopamine receptor agonist pergolide (30 nM) were unaffected by atropine (0.1 μM). These results indicate that the muscarinic receptor-mediated and dopamine autoreceptor-mediated presynaptic effects on [³H]dopamine release are independent.The present results suggest that in the electrically depolarized caudate slice in vitro, released endogenous acetylcholine may interact with muscarinic receptors faciliting depolarization-evoked [³H]dopamine release,ifAChE is inhibited. These muscarinic receptors may be located on dopamine nerve terminals. In the context of present neuroanatomical knowledge, the action of released endogenous acetylcholine on dopamine terminals may be a non-synaptic neuromodulation.     

Age-related changes in the expression of axonal and glial group I metabotropic glutamate receptor in the rat substantia nigra pars reticulata

Neuronal systems undergo many significant changes during the course of brain development. To characterize the developmental changes in the substantia nigra pars reticulata (SNr) associated with the expression of group I metabotropic glutamate receptors (mGluRs), we used the immunoperoxidase and immunogold methods at the electron microscope level to determine whether the subcellular and subsynaptic patterns of distribution of mGluR1a and mGluR5 differ between young (P14-P18) and adult (>2 months) rats. The SNr of young rats contained a significantly higher density of labeled unmyelinated axons for both receptor subtypes. In addition, mGluR5-immunoreactive glial processes were very abundant in young rats but absent in the adults. On the other hand, the relative proportion of immunoreactive dendrites was the same for both age groups. Analysis of immunogold-labeled rat SNr revealed similar proportions of plasma membrane-bound mGluR1a and mGluR5 in adult (59.8 and 19.4%, respectively) and young (60.6 and 18.4%, respectively) rats. The pattern of subsynaptic localization of mGluR1a also remained the same between young and adults. However, the proportion of extrasynaptic mGluR5 decreased, whereas proportions of gold particles associated with symmetric synapses increased in adults. The results of this study demonstrate significant differences in the expression of group I mGluRs in the SNr of young and adult rats. These findings support a role for group I mGluRs during development and emphasize the importance of using brain tissue from age-matched subjects when attempting to correlate functional data from young rat brain slices with immunocytochemical localization of group I mGluRs.     

NMDA receptors modulate dopamine loss due to energy impairment in the substantia nigra but not striatum

Defects in energy metabolism have been detected in patients with Parkinson's disease and have been proposed as a contributing factor in the disease. Previous in vitro studies showed that NMDA receptors contribute to the loss of dopamine neurons caused by the metabolic inhibitor malonate. In vivo, it is not known whether this interaction occurs through a postsynaptic action on the cell body in the substantia nigra or through a presynaptic action at the dopamine terminal in the striatum. So we could discern the anatomical level of NMDA receptor involvement, rats were infused with malonate, either into the left striatum or into the left substantia nigra. NMDA receptors were locally blocked by an intranigral or intrastriatal coinfusion of malonate plus MK-801 followed by a second infusion of MK-801 3 h later. Animals were examined at 1 week for striatal and nigral dopamine and GABA levels. Intranigral infusion of malonate (0.5 micromol) produced an approximate 50% loss of both nigral dopamine and GABA. MK-801 (0.1 micromol) provided significant protection against both nigral dopamine and GABA loss and against anterograde damage to dopamine terminals in the striatum. Intrastriatal administration of malonate (2 micromol) produced a 68 and 35% loss of striatal dopamine and GABA, respectively. In contrast to intranigral administration, intrastriatal blockade of NMDA receptors did not protect against striatal dopamine loss, although GABA loss was significantly attenuated. Core body temperature monitored several hours throughout the experiment was unchanged. Consistent with a lack of effect of NMDA antagonists on malonate-induced toxicity to dopamine neurons in striatum, intrastriatal infusion of NMDA had a pronounced effect on long-term GABA toxicity with little effect of dopamine loss. These findings are consistent with a postsynaptic action of NMDA receptors on mediating toxicity to dopamine neurons during impaired energy metabolism.     

Effects of high frequency stimulation of subthalamic nucleus on extracellular glutamate and GABA in substantia nigra and globus pallidus in the normal rat

High frequency stimulation (130 Hz) of the subthalamic nucleus has dramatic beneficial motor effects in severe parkinsonian patients. However, the mechanisms underlying these clinical results remain obscure. The objective of the present work was to study the neurochemical changes induced in rats by high frequency stimulation of the subthalamic nucleus by using intracerebral microdialysis within its target structures. Our results show that high frequency stimulation of the subthalamic nucleus induces a significant increase of extracellular glutamate levels in the ipsilateral globus pallidus and substantia nigra while GABA was augmented only in the substantia nigra. These data suggest that functional effects induced by high frequency stimulation of the subthalamic nucleus might imply distal mechanisms involving the synaptic relationships with the subthalamic efferences. They question the current view that the direct inhibition of the subthalamic neurons is induced by high frequency stimulation.     

Glycine receptors in the striatum, globus pallidus, and substantia nigra of the human brain: an immunohistochemical study

Glycine receptors (GlyRs) are heteropentameric chloride ion channels that facilitate fast-response, inhibitory neurotransmission in the mammalian spinal cord and brain. GlyRs have four functional subunits, alpha1-3 and beta, which likely exist in heteromeric alphabeta combinations. Mutations in GlyR alpha1 and beta subunits are well known for their involvement in hyperekplexia, a paroxysmal motor disorder. In this study we present the first detailed immunohistochemical investigation at the regional, cellular, and subcellular levels of GlyRs in the human basal ganglia. The results show that GlyRs are present at the regional level in low concentrations in the striatum and globus pallidus and are present in the highest concentrations in the substantia nigra. At the cellular level, GlyRs are present only in discrete populations of neurons immunoreactive for choline acetyltransferase (ChAT), parvalbumin, and calretinin in the human striatum, on a subpopulation of parvalbumin- and calretinin-positive neurons in the globus pallidus, and in the substantia nigra GlyRs are present on approximately three-fourths of all pars compacta and one-third of all pars reticulata neurons. They also form a distinct band of immunoreactive neurons in the intermedullary layers of the globus pallidus. At the subcellular level in the substantia nigra pars reticulata (SNr), GlyRs show a localized distribution on the soma and dendrites that partially complements but does not overlap with the distribution of gamma-aminobutyric acid (GABA)A receptors. Our results demonstrate the precise cellular and subcellular localization of GlyRs in the human basal ganglia and suggest that glycinergic receptors may play an important complementary role to other inhibitory receptors in modulating cholinergic, dopaminergic, and GABAergic neuronal pathways in the basal ganglia.