Evidence for a striatal NMDA receptor modulation of nigral glutamate release. A dual probe microdialysis study in the awake freely moving rat

Dual probe microdialysis was employed to characterize dialysate glutamate levels from the substantia nigra pars reticulata of awake freely moving rats, and to test its sensitivity to alterations in striatal neurotransmission including striatal N-methyl-d -aspartic acid (NMDA) receptor stimulation and blockade. Intranigral perfusion with low (0.1 mm ) Ca²⁺ medium (60 min) did not affect nigral glutamate levels, whereas intranigral perfusion with tetrodotoxin (10 μm , 60 min) increased nigral glutamate levels. Perfusion of KCl (100 mm , 10 min) in the dorsolateral striatum transiently stimulated nigral glutamate levels (maximal increase + 60%), whereas intrastriatal perfusion (60 min) with low Ca²⁺ medium and tetrodotoxin gradually increased nigral glutamate levels. Intrastriatal perfusion with NMDA (0.1–100 μm , 10 min) dose-dependently stimulated glutamate levels in the substantia nigra pars reticulata. The NMDA (1 μm )-induced increase in nigral glutamate release was transient and maximal (+60% within 20 min), whereas that for NMDA (10 μm ) had a slow onset but was long lasting (+35% after 60 min). Lower (0.1 μm ) and higher (100 μm ) NMDA concentrations were ineffective. The effect of intrastriatal NMDA (1 μm ) was prevented by coperfusion with MK-801 (1 μm ). Intrastriatal MK-801 (10 μm ) alone gradually increased glutamate levels up to +50% after 60 min of perfusion. The present results suggest that glutamate levels in the substantia nigra pars reticulata are sensitive to changes in neuronal transmission in the dorsolateral striatum, and that striatal NMDA receptors regulate nigral glutamate release in both a tonic and phasic fashion.     

Relationship between the presence of dopaminergic neurons and GABA receptors in substantia nigra: Effects of lesions

Submaximal destruction of nigrostriatal dopaminergic projections resulted in a significant (25%) decrease in specific GABA binding in substantia nigra; under these conditions, striatal tyrosine hydroxylase activity was 15–44% of control. In rats with lesions which caused maximal destruction of nigrostriatal dopamine neurons (striatal tyrosine hydroxylase was less than 15% of control), specific GABA binding in substantia nigra was apparently not different from that obtained in intact controls. Two distinct processes may be occurring in response to the destruction of dopamine neurons: (1) the loss of GABA binding sites physically associated with nigral dopamine neurons; and (2) an increase in nigral GABA receptors associated with non-dopaminergic neurons. The latter process may result from a decrease in nigral GABA transmission secondary to the complete loss of dopaminergic synaptic activity in striatum.     

Substantia nigra glutamate antagonists produce contralateral turning and basal ganglia Fos expression: interactions with D1 and D2 dopamine receptor agonists

Experiments measuring behavior and immediate-early gene expression in the basal ganglia can reveal interactions between dopamine (DA) and glutamate neurotransmission. Nigrostriatal DA projections influence two striatal efferent pathways that, in turn, directly and indirectly influence the activity of the substantia nigra pars reticulata (SNr). This report tests the interactions between striatal DA receptors and nigral glutamate receptors on basal ganglia function by examining both contralateral turning and Fos immunoreactivity in striatum and pallidum following unilateral intranigral microinfusions of glutamate antagonists given to intact and 6-OHDA-lesioned rats. The NMDA antagonist AP5 (1 microg), or the AMPA/kainate antagonist DNQX (0.015-1.5 microg), injected into the SNr (0.5 microl) elicited contralateral turning as well as both striatal and pallidal Fos expression. Moreover, intranigral DNQX elicited more turning and greater numbers of Fos-positive striatal neurons in 6-OHDA-lesioned animals than in unlesioned controls, suggesting that the 6-OHDA injection induces functional changes in nigral glutamate transmission. In 6-OHDA-lesioned rats, systemic injections of the DA D1 receptor agonist SKF38393 (0.5 mg/kg, i.p.) increased striatal Fos expression due to intranigral DNQX. In contrast, the D2 agonist quinpirole (0.1 mg/kg, i.p.) decreased striatal Fos expression but increased the pallidal Fos arising from intranigral AP5. In additional experiments, both intact and 6-OHDA-lesioned rats were given simultaneous intranigral and intrastriatal infusions and turning and pallidal Fos expression were measured. 6-OHDA-lesioned rats given 5 microg of intrastriatal quinpirole exhibited both turning and pallidal Fos that was significantly increased by intranigral AP5. These results indicate that the opposing influences of D2 agonists and endogenous nigral glutamate transmission are mediated by striatal D2 receptors. Finally, the behavioral effects of intranigral glutamate antagonism can be dissociated from the effects on striatal or pallidal immediate-early gene expression.     

D1 dopamine receptor-mediated induction of zif268 and c-fos in the dopamine-depleted striatum: Differential regulation and independence from NMDA receptors

Excitatory amino acid afferents from cerebral cortex and dopamine afferents from the substantia nigra synapse on common projection neurons in the striatum. Activation of D1 dopamine receptors increases immediate early gene expression in the striatum and conductance through the N-methyl-d-aspartate (NMDA) receptor. To examine the contribution of NMDA receptor activation to dopamine receptor-mediated responses, we determined the effects of intrastriatal administration of NMDA antagonists on immediate early gene expression in the striatum and rotational behavior induced by stimulation of the D1 receptor in rats with unilateral dopamine depletions. Systemic administration of SKF 38393 increased c-fos and zif268 mRNAs in the striatum and induced contralateral rotation. Intrastriatal infusion of the competitive NMDA receptor antagonist (±)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid caused a dose-dependent attenuation of SKF 38393-induced rotation and partially decreased c-fos mRNA expression. However, D1-mediated increases in zif268 mRNA were not affected, except by the highest concentration of antagonist used (10 mM). Another competitive antagonist, 2-amino-5-phosphonovaleric acid, had similar effects. Like the competitive antagonists, intrastriatal infusion of the non-competitive NMDA antagonist MK-801 partially decreased c-fos, but not zif268, mRNA in the area around the microdialysis probe. However, unlike competitive antagonists, local infusion of 1 mM MK-801 potentiated D1-mediated increases in c-fos and zif268 mRNAs in lateral striatum. These data suggest that 1) some D1 dopamine receptor-mediated effects on striatal function are independent of ongoing NMDA receptor activation, whereas other effects are at least partially mediated by NMDA receptor activity in the striatum, and 2) competitive and non-competitive antagonists of the NMDA receptor differently affect D1-mediated immediate early gene expression in the striatum. © 1996 Wiley-Liss, Inc.     

Intranigral antagonism of neurokinin 1 and 3 receptors reduces intrastriatal dopamine D1 receptor-stimulated locomotion in the rat

Stimulation of striatal dopamine (DA) D1 receptors increases the activity of the direct striatonigral pathway resulting in movement. While GABA has long been considered the primary effector of this pathway, co-released tachykinin peptides and their respective nigral tachykinin receptors are also in position to influence movement. Therefore, the present studies determined to what extent nigral tachykinin receptor subtypes contribute to striatal D1-mediated locomotion. Adult male Sprague-Dawley rats bearing chronic cannulae in the dorsal striatum and/or substantia nigra (SN) were tested for locomotor responses to various drug infusions. Unilateral intranigral infusions of the neurokinin-1 (NK1) antagonist LY306740 (0 and 50 nmol) but not the neurokinin-3 (NK3) antagonist SR142801 (0 and 50 nmol) led to ipsilateral rotations. Bilateral intrastriatal infusions of the full D1 agonist SKF 82958 (0, 1.2 and 12.0 nmol) dose-dependently increased locomotion. Prior bilateral intranigral infusions of LY306740 or SR142801 (0, 5.0 and 50 nmol) dose-dependently attenuated locomotor activity induced by intrastriatal SKF 82958 (12.0 nmol). These findings indicate that NK1, but not NK3, receptors within the SN may be tonically stimulated. However, activation of both nigral NK1 and NK3 receptors appears to be required for increased locomotion in response to striatal D1 receptor stimulation.     

In vivo evidence for a differential contribution of striatal and nigral D1 and D2 receptors to L-DOPA induced dyskinesia and the accompanying surge of nigral amino acid levels

Evidence for an involvement of striatal D1 receptors in levodopa-induced dyskinesia has been presented whereas the contribution of striatal D2 receptors remains controversial. In addition, whether D1 and D2 receptors located in the substantia nigra reticulata shape the response to levodopa remains unknown. We therefore used dual probe microdialysis to unravel the impact of striatal and nigral D1 or D2 receptor blockade on abnormal involuntary movements (AIMs) and striatal output pathways in unilaterally 6-hydroxydopamine lesioned dyskinetic rats. Regional perfusion of D1/D5 (SCH23390) and D2/D3 (raclopride) receptor antagonists was combined with systemic administration of levodopa. Levodopa-induced AIMs coincided with a prolonged surge of GABA and glutamate levels in the substantia nigra reticulata. Intrastriatal SCH23390 attenuated the levodopa-induced AIM scores (~ 50%) and prevented the accompanying neurochemical response whereas raclopride was ineffective. When perfused in the substantia nigra, both antagonists attenuated AIM expression (~ 21-40%). However, only intranigral SCH23390 attenuated levodopa-induced nigral GABA efflux, whereas raclopride reduced basal GABA levels without affecting the response to levodopa. In addition, intranigral raclopride elevated amino acid release in the striatum and revealed a (mild) facilitatory effect of levodopa on striatal glutamate. We conclude that both striatal and nigral D1 receptors play an important role in dyskinesia possibly via modulation of the striato-nigral direct pathway. In addition, the stimulation of nigral D2 receptors contributes to dyskinesia while modulating glutamate and GABA efflux both locally and in the striatum.     

Protection of malonate-induced GABA but not dopamine loss by GABA transporter blockade in rat striatum

Previous work has shown that overstimulation of GABA(A) receptors can potentiate neuronal cell damage during excitotoxic or metabolic stress in vitro and that GABA(A) antagonists or GABA transport blockers are neuroprotective under these situations. Malonate, a reversible succinate dehydrogenase/mitochondrial complex II inhibitor, is frequently used in animals to model cell loss in neurodegenerative diseases such as Parkinson's and Huntington's diseases. To determine if GABA transporter blockade during mitochondrial impairment can protect neurons in vivo as compared with in vitro studies, rats received a stereotaxic infusion of malonate (2 micromol) into the left striatum to induce a metabolic stress. The nonsubstrate GABA transport blocker, NO711 (20 nmol) was infused in some rats 30 min before and 3 h following malonate infusion. After 1 week, dopamine and GABA levels in the striata were measured. Malonate caused a significant loss of striatal dopamine and GABA. Blockade of the GABA transporter significantly attenuated GABA, but not dopamine loss. In contrast with several in vitro reports, GABA(A) receptors were not a downstream mediator of protection by NO711. Intrastriatal infusion of malonate (2 micromol) plus or minus the GABA(A) receptor agonist muscimol (1 micromol), the GABA(A) Cl- binding site antagonist picrotoxin (50 nmol) or the GABA(B) receptor antagonist saclofen (33 nmol) did not modify loss of striatal dopamine or GABA when examined 1 week following infusion. These data show that GABA transporter blockade during mitochondrial impairment in the striatum provides protection to GABAergic neurons. GABA transporter blockade, which is currently a pharmacological strategy for the treatment of epilepsy, may thus also be beneficial in the treatment of acute and chronic conditions involving energy inhibition such as stroke/ischemia or Huntington's disease. These findings also point to fundamental differences between immature and adult neurons in the downstream involvement of GABA receptors during metabolic insult.     

NMDA receptor blockade attenuates locomotion elicited by intrastriatal dopamine D1-receptor stimulation

Previous behavioral studies suggest that the striatum mediates a hyperactive response to systemic NMDA receptor antagonism in combination with systemic D1 receptor stimulation. However, many experiments conducted at the cellular level suggest that inhibition of NMDA receptors should block D1 receptor-mediated locomotor activity. Therefore, we investigated the consequences of NMDA receptor blockade on the ability of striatal D1 receptors to elicit locomotor activity using systemic and intrastriatal injections of the NMDA antagonist MK-801 combined with intrastriatal injections of the D1 full agonist SKF 82958. Following drug treatment locomotor activity was measured via computerized activity monitors designed to quantify multiple parameters of rodent open-field behavior. Both systemic (0.1 mg/kg) and intrastriatal (1.0 microg) MK-801 pretreatments completely blocked locomotor and stereotypic activity elicited by 10 microg of SKF 82958 directly infused into the striatum. Further, increased activity triggered by intrastriatal SKF 82958 was attenuated by a posttreatment with intrastriatal infusion of 1 microg MK-801. These data suggest that D1-stimulated locomotor behaviors controlled by the striatum require functional NMDA channels.     

Zinc transport from the striatum and substantia nigra

⁶⁵Zn was unilaterally injected into the striatum or substantia nigra of rats to see the transport of intracerebral zinc (Zn). In the case of intrastriatal injection, ⁶⁵Zn was densely distributed in the ipsilateral medial forebrain bundle (MFB) and the substantia nigra. On unilateral colchicine injection into the MFB, ⁶⁵Zn distribution in the ipsilateral substantia nigra decreased significantly compared to that of the contralateral one after ⁶⁵Zn injection into the bilateral striata. These results suggest the presence of axonal transport of ⁶⁵Zn taken up by striatonigral γ-aminobutyric acid (GABA)-ergic and/or nigro-striatal dopaminergic neurons. On the other hand, in the case of intranigral injection, ⁶⁵Zn was distributed in the ipsilateral MFB, striatum, globus pallidus, pontine reticular nuclei, and pontine nuclei. ⁶⁵Zn distribution in the pons 1 day after intranigral injection was very similar to that 6 days after intrastriatal injection, suggesting that, in the case of intrastriatal injection of ⁶⁵Zn, nigral ⁶⁵Zn, which was transported anterogradely and/or retrogradely from the striatum, was transported to the postsynaptic neurons through the synapse and then transported to the pons.     

Effects of KCl-induced depolarization on the GABA concentration in the corpus striatum and in the substantia nigra

Summary The effects of potassium ion depolarization on the concentration, the synthesis and the utilization of GABA as well as the effects on the concentrations of glutamine, dopamine (DA) and DOPAC in the substantia nigra and in the corpus striatum of the rat were investigated.An intranigral KCl injection did not influence the GABA concentration or the synthesis and utilization of GABA in the substantia nigra. Following an intrastriatal KCl injection the GABA concentration increased and the glutamine concentration decreased in the corpus striatum. A marked increase in the DA concentration in the corpus striatum was seen following an intranigral KCl injection, whereas the DA concentration decreased and the DOPAC concentration increased following an intrastriatal KCl injection. It is concluded that the GABA in the substantia nigra and in the corpus striatum is affected differently by potassium ions, perhaps due to the different cellular localization of GABA in the two structures. The increase of GABA in the corpus striatum was accompanied by a decreased glutamine concentration indicating that glutamine might be a precursor of GABA.     

Preservation of a functional nigrostriatal dopamine pathway by GDNF in the intrastriatal 6-OHDA lesion model depends on the site of administration of the trophic factor

Here we studied whether glial cell line-derived neurotrophic factor (GDNF), given as a single bolus injection before an intrastriatal 6-hydroxydopamine (6-OHDA) lesion, can protect the nigrostriatal dopamine neurons against the toxin-induced damage and preserve normal motor functions in the lesioned animals. GDNF or vehicle was injected in the striatum (25 microg), substantia nigra (25 microg) or lateral ventricle (50 microg) 6 h before the 6-OHDA lesion (20 microg/3 microL). Motor function was evaluated by the stepping and drug-induced motor asymmetry tests. Lesioned animals given vehicle alone showed a clear ipsilateral-side bias in response to amphetamine (13 turns/min), a moderate contralateral-side bias to apomorphine (4.5 turns/min) and a moderate to severe stepping deficit on the contralateral forepaw (three to four steps, as compared with 11-13 steps on the unimpaired side). Injection of GDNF into the striatum had a significant protective effect both on nigrostriatal function (1-2 turns/min in the rotation tests and seven to eight steps in the stepping test), and the integrity of the nigrostriatal pathway, seen as a protection of both the cell bodies in the substantia nigra and the dopamine innervation in the striatum. Injection of GDNF in the nigra had a protective effect on the nigral cell bodies, but not the striatal innervation, and failed to provide any functional benefit. In contrast, intranigral GDNF had deleterious effects on both the striatal TH-positive fibre density and on drug-induced rotation tests. Intraventricular injection had no effect. We conclude that preservation of normal motor functions in the intrastriatal 6-OHDA lesion model requires protection of striatal terminal innervation, and that this can be achieved by intrastriatal, but not nigral or intraventricular, administration of GDNF.     

Intranigral transplants of GABA-rich striatal tissue induce behavioral recovery in the rat Parkinson model and promote the effects obtained by intrastriatal dopaminergic transplants

Intrastriatal transplantation of fetal ventral mesencephalon (VM) is currently explored as a potential clinical therapy in Parkinson's disease (PD). Although providing substantial benefit for the patient, behavioral recovery so far obtained with intrastriatal VM grafts is not complete. Using the 6-hydroxydopamine lesion model of PD, we show here that near-complete restoration of the striatal dopamine (DA) innervation can be achieved by multiple intrastriatal microtransplants of fetal DA cells; nevertheless, complete recovery in complex sensorimotor behaviors was not obtained in these animals. In line with the current model of basal ganglia function, this suggests that the lesion-induced overactivity of the basal ganglia output structures, i.e., the substantia nigra (SN) and the entopeduncular nucleus, may not be completely reversed by intrastriatal VM grafts. In the present study, we have transplanted fetal VM tissue or fetal striatal tissue, as a source of DA and GABA neurons, respectively, into the SN of DA-depleted rats. Intranigral VM grafts induced behavioral recovery in some sensorimotor behaviors (forelimb akinesia and balance tests), but the effect did not exceed the recovery observed after intrastriatal VM grafts. Intranigral grafts of striatal tissue induced a pattern of functional recovery which was distinctly different from that observed after intranigral VM grafts, and recovery in coordinated forelimb use in the paw-reaching test was even more pronounced than after intrastriatal transplantation of VM cells. Combined transplantation of DA neurons into the striatum and GABA-rich striatal neurons into the SN induced additive effects of behavioral recovery observed in the forelimb akinesia test. We propose that intranigral striatal transplants, by a GABA-mediated inhibitory action, can reduce the overactivity of the host SN projection neurons and can induce significant recovery in complex motor behavior in the rat PD model and that such grafts may be used to increase the overall functional efficacy of intrastriatal VM grafts.     

(+)MK-801 prevents the DDC-induced enhancement of MPTP toxicity in mice

In order to reach deeper insight into the mechanism of diethyldithiocarbamate (DDC)-induced enhancement of MPTP toxicity in mice, MK-801, a non-competitive antagonist of NMDA receptors, has been used as a tool to study the role of excitatory amino acids. In agreement with previous reports, (+)MK-801 did not significantly affect either striatal dopamine (DA) or tyrosine-hydroxylase (TH) activity in MPTP-treated animals. On the contrary (+)MK-801, but not (−)MK-801 significantly reduced the DDC + MPTP-induced fall in striatal DA and TH activity. A similar preventing effect on DA metabolites (DOPAC and HVA) and HVA/DA ratio was observed. The number of TH⁺Mneurons in the substantia nigra (SN) of (+)MK-801-pretreated mice was not significantly different from that of control animals, indicating that this treatment specifically antagonized the extensive DDC-induced lesion of dopaminergic cell bodies in this brain area. (+)MK-801 treatment did not affect the DDC-induced changes of striatal MPP⁺ levels, suggesting that the observed antagonism of MK-801 against DDC is not due to MPP⁺ kinetic modifications. Pretreatment with the MAO-B inhibitor,l-deprenyl, or with the DA uptake blocker, GBR 12909, completely prevented the marked DA depletion elicited by DDC + MPTP within the striatum. Both treatments also protected from the fall in DA metabolites and TH activity as well. This indicates that DDC-induced potentiation is dependent upon MPP⁺ production and its uptake by the dopaminergic nerve terminals. All these findings suggest that NMDA receptors play a crucial role in the DDC-induced enhancement of MPTP toxicity.     

Simultaneous intrastriatal and intranigral grafting (double grafts) in the rat model of Parkinson's disease

Experimental and clinical studies of neural transplantation in Parkinson's disease have focused on the placement of fetal dopaminergic grafts not in their ontogenic site (substantia nigra) but in the main nigral target area (striatum). The reason for this is the apparent inability of intranigral nigral grafts to extend axons for long distances reinnervating the ipsilateral striatum. This review presents previous work by our laboratory [I. Mendez, M. Hong, Reconstruction of the striato-nigro-striatal circuitry by simultaneous double dopaminergic grafts: a tracer study using fluorogold and horseradish peroxidase, Brain Res. 778 (1997) 194–205; I. Mendez, D. Sadi, M. Hong., Reconstruction of the nigrostriatal pathway by simultaneous intrastriatal and intranigral dopaminergic transplants, J. Neurosci. 16 (1996) 7216–7227] using a new transplantation strategy aimed at restoring dopaminergic innervation of the nigra and striatum by simultaneous dopaminergic transplants placed in the substantia nigra and ipsilateral striatum (double grafts) in the 6-hydroxydopamine lesioned adult rat brain. These double grafts achieve not only greater striatal reinnervation than the standard intrastriatal grafts but also produce a faster and more complete behavioural recovery six weeks after transplantation. Injection of the retrograde tracer fluorogold into the striatum and nigra resulted in fluorescent labeled cells within the intranigral graft and the intrastriatal graft and surrounding striatum, respectively suggesting that these double grafts promote at least partial reconstruction of the nigrostriatal dopaminergic pathway. This double graft strategy may have potential implications in clinical neural transplantation for Parkinson's disease.     

Disinhibition as a basic process in the expression of striatal functions. I. The striato-nigral influence on tecto-spinal/tecto-diencephalic neurons

The striato-nigro-collicular pathway is one of the neuronal circuits through which basal ganglia can influence ocular and cephalic motricity. Through this pathway striatal signals are conveyed to a major collicular efferent system i.e. the tecto-spinal/tecto-diencephalic neurons (TSD). A striking particularity of the striato-nigro-collicular connection is to present an assemblage of two successive GABAergic inhibitory links. To analyze how striatal information is expressed through this double inhibitory system we made use of concomitant unitary recordings of substantia nigra pars reticulata (SNR) and TSD activity while nigral and/or striatal activity was changed by local applications of pharmacological agents. We observed (1) that an intranigral application of GABA which transiently abolishes the tonic discharge of SNR cells, causes the TSD cells to discharge vigorously. During this period of increased excitability the responsiveness of TSD cells to somatosensory input is significantly enhanced; (2) that an intrastriatal injection of glutamate which silences the nigro-collicular cells, causes the TSD cells to discharge. This striatally induced tectal firing being sensitive to intranigral application of a GABA antagonist (bicuculline), led us to the conclusion that striatum exerts a facilitatory influence on TSD cells by releasing these neurons from the tonic inhibitory nigral influence. The functional implication of such a mechanism is discussed in the light of the current knowledge on the involvement of basal ganglia in eye/head orienting movements.     

Regulation of neurotensin-containing neurons in the rat striatum and substantia nigra. Effects of unilateral nigral lesion with 6-hydroxydopamine on neurotensin content and its binding site density

The effect of unilateral lesion of the rat substantia nigra with 6-hydroxydopamine (6-OHDA) was investigated on the endogenous contents of neurotensin (NT) and its binding site densities in the striatum and substantia nigra. Tyrosine hydroxylase (T-OH) activity, γ-aminobutyric acid (GABA) content, binding site densities of dihydrotetrabenazine (TBZOH), a marker of dopaminergic synaptic vesicles, and of iodosulpride, a ligand for dopamine D₂ receptors, were also determined. Fourteen days following nigral lesions, these markers were analyzed by means of radioimmunoassay for NT levels, fluoremetric method for GABA content, radiochemical method for T-OH activity and quantitative autoradiography for NT, TBZOH and iodosulpiride binding site densities. Unilateral nigral lesion with 6-OHDA provoked only ipsilateral modifications in dopamine markers. T-OH activity and TBZOH binding site densities significantly decreased in both the ipsilateral striatum and substantia nigra. Iodosulpiride binding sites decreased in the substantia nigra and increased in the striatum on the ipsilateral side. In contrast to these unilateral changes observed for dopamine markers, dramatic increases in NT contents were found in both the ipsi- and contralateral striata. No change was found in nigral NT levels on either side. On the other hand, NT binding sites decreased in the ipsilateral striatum and substantia nigra, which reflected the destruction of dopaminergic elements in these regions. The present results strongly suggest a dopaminergic control of striatal NT systems and demonstrate that a unilateral loss of this control may lead to strong bilateral alterations in NT levels.     

Selective stimulation of kainate but not quisqualate or NMDA receptors in substantia nigra evokes limbic motor seizures

Bilateral microinjection of kainic acid (30-117 pmol) into the substantia nigra induced convulsive seizures resembling those elicited from limbic system structures. The convulsive seizures, which consisted of facial and forelimb clonus with rearing and falling, developed after a latency of more than 30 min and were preceded by wet dog shakes and non-convulsive seizure activity registered electroencephalographically. The convulsant effect of intranigral kainic acid was strictly dose-dependent (ED50 = 60 pmol) and anatomically site-specific. Stimulation of nigral neurons by focal application of agonists for NMDA or quisqualate receptors, or by focal application of the GABA antagonist, bicuculline, was without convulsant effects. The convulsant action of intranigral kainic acid was prevented by the focal application of kynurenic acid (100 nmol) but not by 2-amino-7-phosphonoheptanoic acid (AP-7) (25 nmol) or 7-chlorokynurenic acid (20 nmol), suggesting that the convulsant effect of kainic acid in the substantia nigra does not depend upon activation of NMDA receptors in this region.     

Striatal NTS1 , dopamine D2 and NMDA receptor regulation of pallidal GABA and glutamate release--a dual-probe microdialysis study in the intranigral 6-hydroxydopamine unilaterally lesioned rat

The current microdialysis study elucidates a functional interaction between the striatal neurotensin NTS(1) receptor and the striatal dopamine D(2) and N-methyl-d-aspartic acid (NMDA) receptors in the regulation of striatopallidal gamma-aminobutyric acid (GABA) and glutamate levels after an ipsilateral intranigral 6-hydroxydopamine-induced lesion of the ascending dopamine pathways to the striatum. Lateral globus pallidus GABA levels were higher in the lesioned group while no change was observed in striatal GABA and glutamate levels. The 6-hydroxydopamine-induced lesion did not alter the ability of intrastriatal NT (10 nm) to counteract the decrease in pallidal GABA and glutamate levels induced by the dopamine D(2) -like receptor agonist quinpirole (10 μm). A more pronounced increase in the intrastriatal NMDA- (10 μm) induced increase in pallidal GABA levels was observed in the lesioned group while it attenuated the increase in striatal glutamate levels and amplified the increase in pallidal glutamate levels compared with that observed in the controls. NT enhanced the NMDA-induced increase in pallidal GABA and glutamate and striatal glutamate levels; these effects were counteracted by the NTS(1) antagonist SR48692 (100 nm) in both groups. These findings demonstrate an inhibitory striatal dopamine D(2) and an excitatory striatal NMDA receptor regulation of striatopallidal GABA transmission in both groups. These actions are modulated by NT via antagonistic NTS(1) /D(2) and facilitatory NTS(1) /NMDA receptor-receptor interactions, leading to enhanced glutamate drive of the striatopallidal GABA neurons associated with motor inhibition, effects which all are counteracted by SR48692. Thus, NTS(1) antagonists in combination with conventional treatments may provide a novel therapeutic strategy in Parkinson's disease.     

Do NMDA receptor antagonists protect against MPTP-toxicity? Biochemical and immunocytochemical analyses in black mice.

We investigated whether excitatory amino acids acting at the N-methyl-D-aspartate (NMDA) subtype of the L-glutamate receptor contribute to the dopaminergic neurotoxicity induced by systemic administration of the Parkinson's syndrome-inducing toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in C57Bl/6 mice. The MPTP-regimen chosen (30-40 mg/kg body weight subcutaneously) resulted a 60-70% depletion of striatal dopamine (DA) content and a 20% reduction of tyrosine hydroxylase immunoreactive (TH-IR) cells in the substantia nigra pars compacta 20 days after administration. Repeated systemic coadministration of the non-competitive NMDA receptor antagonist MK-801 or of the novel competitive NMDA receptor antagonist CGP 40116 did not protect against MPTP-induced striatal DA depletion 20 days after toxin administration. Additionally, no short-term protective effects of MK-801 on striatal DA content were observed 24, 48, and 96 h, respectively, after exposure to MPTP. A slight and non-significant attenuation (approximately 10%) of the MPTP-induced decrease in the number of nigral TH-IR cells was observed after MK-801- and CGP 40116-treatment. We conclude that neurotoxicity of systemically administered MPTP is not substantially antagonized by NMDA receptor antagonists in mice.