Excitatory amino acid receptor antagonists modify regional cerebral metabolic responses to levodopa in 6-hydroxydopamine-lesioned rats

NMDA receptor antagonism potentiates the L-DOPA-induced extracellular dopamine release in the subthalamic nucleus of hemi-parkinson rats

Long term treatment with L-3,4-dihydroxyphenylalanine (L-DOPA) is associated with several motor complications. Clinical improvement of this treatment is therefore needed. Lesions or high frequency stimulation of the hyperactive subthalamic nucleus (STN) in Parkinson's disease (PD), alleviate the motor symptoms and reduce dyskinesia, either directly and/or by allowing the reduction of the L-DOPA dose. N-methyl-D-aspartate (NMDA) receptor antagonists might have similar actions. However it remains elusive how the neurochemistry changes in the STN after a separate or combined administration of L-DOPA and a NMDA receptor antagonist. By means of in vivo microdialysis, the effect of L-DOPA and/or MK 801, on the extracellular dopamine (DA) and glutamate (GLU) levels was investigated for the first time in the STN of sham and 6-hydroxydopamine-lesioned rats. The L-DOPA-induced DA increase in the STN was significantly higher in DA-depleted rats compared to shams. MK 801 did not influence the L-DOPA-induced DA release in shams. However, MK 801 enhanced the L-DOPA-induced DA release in hemi-parkinson rats. Interestingly, the extracellular STN GLU levels remained unchanged after nigral degeneration. Furthermore, administration of MK 801 alone or combined with L-DOPA did not alter the STN GLU levels in both sham and DA-depleted rats. The present study does not support the hypothesis that DA-ergic degeneration influences the STN GLU levels neither that MK 801 alters the GLU levels in lesioned and non-lesioned rats. However, NMDA receptor antagonists could be used as a beneficial adjuvant treatment for PD by enhancing the therapeutic efficacy of l-DOPA at least in part in the STN.

The effects of systemic, intrastriatal, and intrapallidal injections of caffeine and systemic injections of A2A and A1 antagonists on forepaw stepping in the unilateral 6-OHDA-lesioned rat

RATIONALE AND OBJECTIVES

Given that adenosine A2A antagonists appear to be therapeutic in several animal models of Parkinson's disease (PD), we examined the extent to which caffeine and selective A2A and A1 antagonists could enhance contralateral forepaw stepping in the unilateral 6-OHDA-lesioned rat.

MATERIALS AND METHODS

Following unilateral injections of 12 microg 6-OHDA into the medial forebrain bundle (MFB), frequency of stepping with both front paws was counted separately as the paws were dragged anteriorally and laterally by a treadmill.

RESULTS

The MFB lesions decreased contralateral stepping by 74-83%, and 8 mg/kg 3,4-dihydroxy-L-phenylalanine (L-DOPA) increased contralateral stepping by 25-26%. Caffeine given systemically (15 mg/kg) or into the dorsal striatum or external globus pallidus (GPE; 20-40 microg) increased contralateral forepaw stepping by 14%, 27%, and 26%, respectively, and enhanced the effect of 8 mg/kg L-DOPA on stepping. The selective A(2A) antagonist SCH-58261 (2 mg/kg) also increased stepping by 13% and enhanced the therapeutic effect of L-DOPA, whereas the selective A(1) [corrected] antagonist 8-cyclopentyltheophylline (3-7 mg/kg) and A(1) agonist N(6)-cyclopentyladenosine (0.03-0.2 mg/kg) had no effect. None of these drugs appeared to produce dyskinesic effects.

CONCLUSIONS

In this well-validated animal model of the akinesic effects of PD, caffeine and a selective A2A, but not an A1, antagonist were able to provide both monotherapeutic and adjunctive therapeutic effects. These data are consistent with the hypothesis that A2A antagonists may be therapeutic in human PD patients and indicate that the dorsal striatum and GPE are critical sites of therapeutic action.

The “motor complication syndrome” in rats with 6-OHDA lesions treated chronically with l-DOPA: Relation to dose and route of administration

L-DOPA-induced motor complications can be modelled in rats with 6-hydroxydopamine (6-OHDA) lesions by chronic injections of L-DOPA. We have compared the sensitisation and duration of rotational responses, and the occurrence of dose-failure episodes and abnormal involuntary movements (AIMs) in 6-OHDA-lesioned rats with regard to the dose and route of administration of L-DOPA. Rats were treated with either low (6mg/kg) or high (25mg/kg) doses of L-DOPA twice daily for 21 days whereas control animals received injections of either saline or bromocriptine (2.5mg/kg). A dose-dependent and gradual development of AIMs and contralateral turning was observed in rats treated chronically with l-DOPA. Rats treated with bromocriptine exhibited rotational sensitisation but no AIMs. A shortening of motor response duration was not seen in any of the drug-treated groups. In contrast, dose-failure episodes occurred frequently in both L-DOPA- and bromocriptine-treated animals. Changing the route of L-DOPA administration from intraperitoneal to subcutaneous completely abolished failures in motor response without affecting the development of dyskinesia. Based on the hypothesis that higher doses of L-DOPA may be toxic to dopaminoceptive structures, we compared the total number of neurons and the levels of activated microglia in the striatum. No signs of neurodegenerative changes could be seen in any of the treatment groups. In conclusion, both body AIMs and rotations were dose-dependently evoked by L-DOPA. Only AIMs, however, provided a specific measure of dyskinesia since rotations also were induced by bromocriptine, a drug with low dyskinesiogenic potential. Dose-failure episodes were not specific to L-DOPA treatment and could be attributed to erratic drug absorption from the peritoneal route.

Evidence of a breakdown of corticostriatal connections in Parkinson's disease

Dendritic spines are important structures which receive synaptic inputs in many regions of the CNS. The goal of this study was to test the hypothesis that numbers of dendritic spines are significantly reduced on spiny neurones in basal ganglia regions in Parkinson's disease as we had shown them to be in a rat model of the disease [Exp Brain Res 93 (1993) 17]. Postmortem tissue from the caudate and putamen of patients suffering from Parkinson's disease was compared with that from people of a similar age who had no neurological damage. The morphology of Golgi-impregnated projection neurones (medium-sized spiny neurones) was examined quantitatively. The numerical density of dendritic spines on dendrites was reduced by about 27% in both nuclei. The size of the dendritic trees of these neurones was also significantly reduced in the caudate nucleus from the brains of PD cases and their complexity was changed in both the caudate nucleus and the putamen. Dendritic spines receive crucial excitatory input from the cerebral cortex. Reduction in both the density of spines and the total length of the remaining dendrites is likely to have a grave impact on the ability of these neurones to function normally and may partly explain the symptoms of the disorder.

Role of NMDA and AMPA glutamate receptors in the induction and the expression of dopamine-mediated sensitization in 6-hydroxydopamine-lesioned rats

Rats with unilateral 6-hydroxydopamine (6-OHDA) lesions exhibit behavioral sensitization following repeated treatment with dopamine agonists, a phenomenon called "priming." Priming has two distinct phases: induction and expression. Priming induction using three injections with D1/D2 agonist apomorphine (0.5 mg/kg) or D1 agonist SKF38393 (10 mg/kg) allows priming expression, robust contralateral rotational behavior and striatal Fos expression, following a challenge with the D2 agonist quinpirole (0.25 mg/kg). We examined the roles of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) glutamate receptors on dopamine agonist priming. Administration of the NMDA antagonist (+)5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine maleate (MK801) (0.5 mg/kg) blocked apomorphine-priming of quinpirole-mediated responses, while MK801 dose-dependently attenuated SKF38393-priming of quinpirole-mediated striatal Fos expression and had no effect on SKF38393-priming of quinpirole-mediated rotational behavior. In contrast, administration of the AMPA antagonist 2,3-dihydroxy-6-nitro-7sulfamoyl-benzo[f]quinoxaline (NBQX) (5 or 10 mg/kg) potentiated apomorphine- and SKF38393-priming of quinpirole-mediated striatal Fos expression, but had no effect on their priming of quinpirole-mediated rotational behavior. In SKF38393-primed 6-OHDA rats, administration of MK801 (0.5 mg/kg) blocked the expression of quinpirole-mediated responses, while administration of NBQX (10 mg/kg) or the noncompetitive AMPA antagonist 4-(8-methyl-9H-1,3-dioxolo[4,5-h][2,3]benzodiazepin-5-yl)-benzenamine dihydrochloride (GYKI52466) (5 or 15 mg/kg) had no effect. These results suggest that NMDA and AMPA glutamate receptors have differing roles in dopamine agonist priming-with NMDA receptors required for D1/D2 priming induction and D2-mediated priming expression, and AMPA receptors inhibiting priming induction of D2-mediated immediate early gene expression in the striatum, but not affecting priming induction of D2-mediated rotational behavior or the expression of D2-mediated responses.

NMDA receptor antagonists ameliorate the stepping deficits produced by unilateral medial forebrain bundle injections of 6-OHDA in rats

RATIONALE AND OBJECTIVES

To test the hypothesis that excess glutamatergic transmission at NMDA receptors may contribute to the pathogenesis of Parkinson's disease (PD), we examined the effects of various NMDA receptor antagonists on a recently developed rat model of PD.

METHODS

Following unilateral injections of 12 microg 6-OHDA into the medial forebrain bundle of male Long Evans rats, stepping with both front paws was measured separately as the paws were dragged backwards and laterally. The effects of i.p. injections of varying doses of L-dopa, the non-competitive NMDA receptor antagonist dizocilpine [(+)-MK-801], the competitive NMDA receptor antagonist CPP, and combinations of L-dopa and NMDA receptor antagonists were then examined on stepping in three separate groups of rats.

RESULTS

The lesioned rats stepped less often with their contralateral paw than with their ipsilateral paw, and the magnitude of this stepping deficit was positively correlated with the amount of DA depletion in the ipsilateral dorsal striatum. L-dopa (1-25 mg/kg) dose dependently enhanced stepping with the contralateral paw, and 0.15-0.3 mg/kg dizocilpine and 1.5-6.25 mg/kg CPP enhanced stepping with the contralateral paw as much as did 8 mg/kg L-dopa. The combinations of L-dopa and each of the NMDA receptor antagonists did not significantly improve stepping more than either drug alone. Moreover, none of the drugs completely eliminated the stepping deficits, and high doses began to impair stepping with the ipsilateral paw by inducing turning.

CONCLUSIONS

These data indicate that deficits in contralateral stepping are a reliable and sensitive measure of akinesia in unilateral 6-OHDA-lesioned rats, and they support the hypothesis that excess glutamatergic transmission at NMDA receptors may play a role in the expression of PD symptomology.

Rasagiline enhances l-DOPA-induced contralateral turning in the unilateral 6-hydroxydopamine-lesioned guinea-pig

The modification of L-3,4-dihydrooxyphenylalanine- (L-DOPA-) induced turning response by the new selective monoamine oxidase type B (MAO-B) inhibitor rasagiline was studied in guinea-pigs bearing a unilateral 6-hydroxydopamine-induced lesion in the substantia nigra. In an initial experiment, it was established that contralateral turning is induced in lesioned guinea-pigs in response to apomorphine (18 mg/kg i.p.) and L-DOPA/carbidopa (15/3.5 mg/kg i.p.), while ipsilateral turning is induced by S(+)-methamphetamine (7 mg/kg i.p.). The effect of rasagiline was studied in a chronic treatment regimen, in which animals were treated with rasagiline (0.05 mg/kg s.c.) or saline s.c. daily commencing 2 weeks after lesioning, and L-DOPA/carbidopa (4:1 mg/kg) was administered once daily for 21 days. Only guinea-pigs with 95% or more depletion of striatal dopamine were included in this study. Guinea-pigs treated with rasagiline had a significantly increased intensity and duration of turning in response to L-DOPA (P <0.05 by repeated measures ANOVA) over the 21-day period. On day 21, turning averaged 806+/-105 (n=10) vs 442+/-123 (n=11) turns per 180 min for rasagiline and vehicle treated animals, respectively (P <0.05); turning duration half-time averaged 81+/-15.4 (n=10) as opposed to 33+/-7.6 (n=11) min for rasagiline and vehicle treatments (P <0.01). Concentration of dopamine in intact striatum was significantly increased (69.3+/-2.1 and 60.3+/-2.4 pmol/mg tissue for rasagiline and vehicle, P <0.05) and levels of dihydroxyphenylacetic acid and homovanillic acid were decreased by the rasagiline treatment. Activity of brain MAO-B was 8.6+/-2.9% and MAO-A was 71+/-1.5% that of control in rasagiline-treated animals. Chronic, selective inhibition of MAO-B by rasagiline potentiated L-DOPA-induced turning in this rodent model.

Blockade of NMDA receptors by MK-801 reverses the changes in striatal glutamate immunolabeling in 6-OHDA-lesioned rats

A lesion of the dopamine (DA)-containing nigrostriatal pathway with 6-hydroxydopamine (6-OHDA) results in an increase in the density of nerve terminal glutamate immunolabeling and in the mean percentage of asymmetrical synapses containing a discontinuous postsynaptic density [Meshul et al. (1999) Neuroscience 88:1-16]. Similar alterations in striatal glutamate synapses have been reported following blockade of striatal DA D-2 receptors with subchronic haloperidol treatment [Meshul et al. (1994) Brain Res 648:181-195]. The haloperidol-induced change in glutamate synapses was blocked by coadministration of the N-methyl-D-aspartate (NMDA) noncompetitive receptor antagonist MK-801. In order to determine if blockade of NMDA receptors could alter the density of nerve terminal glutamate immunolabeling following a 6-OHDA lesion of the nigrostriatal pathway, MK-801 was administered to lesioned animals for 14 days. In addition, the number of apomorphine-induced contralateral rotations was determined prior to and following the administration of MK-801. MK-801 administration reversed the increase in the density of nerve terminal glutamate immunolabeling due to a 6-OHDA lesion. There was a small but significant decrease in the number of apomorphine-induced contralateral rotations following administration of MK-801 compared to the number of rotations prior to treatment with the NMDA antagonist. These results demonstrate that blockade of postsynaptic NMDA receptors affects the density of presynaptic glutamate immunolabeling and that this change in nerve terminal glutamate density is associated with a decreased behavioral response to direct DA receptor stimulation. Whether the effect of MK-801 is directly on the striatum or acts through other excitatory pathways of the basal ganglia remains unclear.

Effects of dopaminergic agents and of an NMDA receptor antagonist on motor coordination in Lurcher mutant mice

Lurcher mutant mice, characterized by an ataxic gait and olivocerebellar degeneration, were evaluated for motor coordination in the coat-hanger test after peripheral injections of two doses of dextromethorphan, a noncompetitive N-methyl-D-aspartate receptor antagonist, L-dopa/carbidopa, and SKF 77434, a dopamine D1 receptor agonist. There was an improvement in the distance traveled on the suspended horizontal string after 25 and 50 mg/kg of dextromethorphan and 37.5 mg/kg of L-dopa/carbidopa, but not after SKF 77434. None of the drugs reduced movement times or increased latencies before falling. These results indicate that NMDA receptor antagonism or stimulation of some dopaminergic mechanisms partially improve genetically determined cerebellar ataxia in mice.

Enhanced tyrosine phosphorylation of striatal NMDA receptor subunits: effect of dopaminergic denervation and L-DOPA administration

Sensitization of striatal N-methyl-D-aspartate receptors (NMDAR) has been linked to events leading to the motor response changes associated with the administration of dopaminomimetics to parkinsonian animals and patients. To determine whether tyrosine phosphorylation of NMDAR subunits contributes to the apparent long-term enhancement in synaptic efficacy of these receptors, we examined the effect of unilateral nigrostriatal dopamine system ablation with 6-hydroxydopamine followed by twice-daily treatment with l-DOPA on the phosphorylation state of rat striatal NR2A and NR2B subunits. Three weeks of intermittent l-DOPA administration produced a shortening in the duration of the rotational response to dopaminergic challenge and other changes mimicking those occurring in patients with Parkinson's disease. Concurrently, tyrosine phosphorylation of NR2A and especially of NR2B subunits increased ipsilateral to the lesion (20+/-5% and 46+/-7% of intact striatum, respectively; p<0.01) without attendant changes in subunit protein levels. Selective blockade of NR2B subunits with ACEA 10-1244, but not of NR2A subunits with MDL 100,453, reversed the l-DOPA-induced response alterations. The intrastriatal injection of a tyrosine kinase inhibitor, genistein, at a dose (2.0 microg) that normalized the response shortening, attenuated the NR2A and NR2B phosphorylation increase by about 12% and 24%, respectively (p<0.01). Taken together, these results suggest that augmented tyrosine phosphorylation of NR2B subunits, alone or in combination with the smaller rise in NR2A subunit phosphorylation, contributes to the apparent enhancement in striatal NMDAR sensitivity and thus to the plastic alterations in dopaminergic responses in l-DOPA-treated parkinsonian rats.

Antiparkinsonian actions of glutamate antagonists--alone and with L-DOPA: a review of evidence and suggestions for possible mechanisms

There has been much speculation of late as to whether antagonists of glutamate receptors can be used to combat the motor difficulties of Parkinson's disease, either as monotherapy, or as polytherapy to boost the effects of conventional L-DOPA treatment. The latter seems to be the more practical approach and the therapeutic implications of such treatment have been discussed in some detail. However, the mechanisms by which glutamate antagonists potentiate the antiparkinsonian actions of L-DOPA, remain cryptic. In this review we have explored the evidence and considered the practicality of using NMDA and non-NMDA receptor blockers to treat parkinsonism, as well as focusing on the ways in which the behavioural synergy between dopamine and glutamate systems could conceivably arise at the cellular level. Particular attention has been paid to the differential interaction between glutamate antagonists and postsynaptic dopamine D1 and D2 receptory mechanisms, since these are currently believed to reflect the activity of the two major basal ganglia output circuits: the so-called direct pathway to the substantia nigra and the indirect pathway to the globus pallidus. Finally, we have considered the new proposal, that inhibiting glutamate transmission in the basal ganglia accelerates the enzymic conversion of L-DOPA to dopamine at presynaptic sites.

Differential interaction of competitive NMDA and AMPA antagonists with selective dopamine D-1 and D-2 agonists in a rat model of Parkinson's disease

Stimulation of the dopamine (DA) D-2 and D-1 receptors results in behavioural activation (i.e., induction of contralateral rotations) in 6-hydroxydopamine (6-OHDA) substantia nigra lesioned rats. Competitive N-methyl-D-aspartate (NMDA) antagonists as well as alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) antagonists potentiate the stimulatory responses to threshold doses of L-DOPA or the mixed dopamine D-1/D-2 agonist apomorphine in this model, indicating the potential of such combinations for the management of Parkinson's disease. Neuroanatomic and electrophysiologic data indicate a differential distribution of DA D-1 and DA D-2 receptors within motor loops of the basal ganglia. DA D-1 receptors are preferentially located on GABAergic neurones projecting to the substantia nigra compacta (SNc) and to the substantia nigra reticulata (SNr), whereas DA D-2 receptors are preferentially located on neurones that innervate the external pallidum. NMDA receptors are present in high densities within the striatum, whereas AMPA receptors are enriched in the entopeduncular nucleus/internal pallidum and the SNr. To further characterise the functional interaction between DA and glutamate receptors, we tested the competitive NMDA antagonist 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) and the AMPA antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f] quinoxaline (NBQX) following systemic administration in combination with the DA D-2 selective agonist quinpirole or the DAD-1 selective agonist A 68 930 (1R,3S)-1-aminomethyl-5,6-dihydroxy-3-phenylisochroman) in rats with chronic 6-OHDA lesions of the SNc. CPP potentiated quinpirole-induced rotations and did not affect those induced by the D-1 agonist A 68930. By contrast, NBQX had no effect on quinpirole-induced rotations, whereas synergism was seen with A 68930. These results suggest that rotations induced by combined treatment with glutamate antagonists and DA agonists are mediated by different pathways within the basal ganglia, depending on which subtype of receptor is involved. AMPA antagonists could act preferentially by activating the direct motor pathway, whereas NMDA antagonists could modulate the indirect loop.

Stimulation of basal and L-DOPA-induced motor activity by glutamate antagonists in animal models of Parkinson's disease

In parkinsonism, glutamate pathways within the basal ganglia become overactive, leading to the suggestion that glutamate antagonists might possess antiparkinsonian qualities. This report examines the motor properties of antagonists of NMDA and AMPA-type glutamate receptors, as well as some inhibitors of glutamate release, in animal models of idiopathic Parkinson's disease. High affinity NMDA open-channel blockers (e.g. MK 801, phencyclidine), are highly potent antagonists with inconsistent antiakinetic and strong myorelaxant activity. Other compounds are better tolerated and are capable of relieving immobility and muscular rigidity by themselves (e.g. 1-aminoadamantanes, polyamine site antagonists, kappa agonists, riluzole). Yet others do not restore movements alone (e.g. dextromethorphan, ketamine), but may interact with and strengthen the antiparkinsonian action of L-DOPA (e.g. competitive NMDA and AMPA antagonists, lamotrigine). They may do this by potentiating dopaminergic behaviours mediated by D1 or D2 receptors, or by some other mechanism.

Modulation of levodopa-induced motor response complications by NMDA antagonists in Parkinson's disease

The complex dopamine-glutamate interactions within the basal ganglia are disrupted by chronic nigrostriatal denervation and standard replacement therapy with levodopa. Acute N-methyl-D-aspartate (NMDA) receptor blockade is able to overcome the changes in dopamine D1- and D2-dependent responses and the progressive shortening in the duration of response induced by long-term exposure to levodopa in 6-hydroxydopamine-lesioned rats. Preliminary results further suggest that NMDA receptor blockade can counteract levodopa-induced dyskinesias in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned non-human primates and parkinsonian patients without substantially altering the motor benefit derived from levodopa. These results appear to be in accordance with our 2-deoxyglucose studies in 6-hydroxydopamine-lesioned rats showing that NMDA receptor blockade can attenuate many of the changes in synaptic activity induced by levodopa, particularly in the striatopallidal complex. Taken together, our observations suggest that abnormal glutamate transmission or dysregulation of NMDA receptor-mediated mechanisms contribute to levodopa-induced motor response complications. Additional preclinical and clinical experiments need to be completed with well tolerated glutamate antagonists to determine the full potential of glutamate receptor blockade as a long-term strategy against levodopa-related motor response complications in Parkinson's disease.

Unilateral 6-hydroxydopamine lesions of meso-striatal dopamine neurons and their physiological sequelae

One of the primary approaches in experimental brain research is to investigate the effects of specific destruction of its parts. Here, several neurotoxins are available which can be used to eliminate neurons of a certain neurochemical type or family. With respect to the study of dopamine neurons in the brain, especially within the basal ganglia, the neurotoxin 6-hydroxydopamine (6-OHDA) provides an important tool. The most common version of lesion induced with this toxin is the unilateral lesion placed in the area of mesencephalic dopamine somata or their ascending fibers, which leads to a lateralized loss of striatal dopamine. This approach has contributed to neuroscientific knowledge at the basic and clinical levels, since it has been used to clarify the neuroanatomy, neurochemistry, and electrophysiology of mesencephalic dopamine neurons and their relationships with the basal ganglia. Furthermore, unilateral 6-OHDA lesions have been used to investigate the role of these dopamine neurons with respect to behavior, and to examine the brain's capacity to recover from or compensate for specific neurochemical depletions. Finally, in clinically-oriented research, the lesion has been used to model aspects of Parkinson's disease, a human neurodegenerative disease which is neuronally characterized by a severe loss of the meso-striatal dopamine neurons. In the present review, which is the first of two, the lesion's effects on physiological parameters are being dealt with, including histological manifestations, effects on dopaminergic measures, other neurotransmitters (e.g. GABA, acetylcholine, glutamate), neuromodulators (e.g. neuropeptides, neurotrophins), electrophysiological activity, and measures of energy consumption. The findings are being discussed especially in relation to time after lesion and in relation to lesion severeness, that is, the differential role of total versus partial depletions of dopamine and the possible mechanisms of compensation. Finally, the advantages and possible drawbacks of such a lateralized lesion model are discussed.

Glutamate/dopamine D1/D2 balance in the basal ganglia and its relevance to Parkinson's disease

The recent availability of selective ligands for NMDA and AMPA receptors has enabled neuroscientists to test the hypothesis that Parkinson's disease is a glutamate hyperactivity disorder and hence treatable with glutamate antagonists. This review takes a critical look at the motor characteristics of this new class of drugs in rodent and primate models of parkinsonism and assesses the clinical potential and pitfalls of this radical new approach. Monotherapy of Parkinson's disease with glutamate antagonists appears impractical at the present time, due to their low efficacy and unacceptable side effects, but polypharmacy with L-DOPA and a glutamate antagonist as adjuvant is a more realistic prospect. This review will focus on the ways in which glutamate receptor blockade facilitates motor recovery with L-DOPA and will examine whether the basis for this beneficial effect can be traced to a specific interaction with dopamine at D1 or D2 receptors, and therefore to discrete motor pathways within the basal ganglia.

A role for non-NMDA excitatory amino acid receptors in regulating the basal activity of rat globus pallidus neurons and their activation by the subthalamic nucleus

We have investigated the hypothesis that excitatory amino acid (EAA) receptors in the globus pallidus (GP) play a significant role in maintaining the firing rates of GP neurons under basal conditions and following activation of the subthalamic nucleus (STN). Drugs were infused directly into the GP and/or STN while the extracellular single unit activity of Type II GP neurons was recorded in ketamine-anesthetized rats. Local infusions of the EAA agonists NMDA (30-300 pmol/200 nl) or AMPA (0.1-1 pmol/200 nl) elicited increases in the firing rate of GP neurons in a dose-dependent fashion. Infusion of the GABAA receptor antagonist bicuculline methiodide (1-10 pmol/100 nl) into the STN also elicited dose-related increases in the firing rate of GP neurons. Intrapallidal infusion of the non-NMDA (AMPA/kainate) receptor antagonist NBQX (0.1-1.0 nmol) reduced the basal firing rate of GP neurons by 40%. In contrast, the NMDA antagonist MK-801 (0.01-0.1 nmol) produced no significant effect on basal firing rate. Intrapallidal infusion of the non-selective EAA receptor antagonist kynurenic acid or NBQX reversed or blocked the increase in firing rate of GP neurons following bicuculline-induced activation of the STN. Similar treatment with MK-801, however, had no significant effect on this response. These results indicate that tonic stimulation of non-NMDA receptors plays an important role in maintaining the basal activity of GP neurons and in mediating the effects of increased excitatory input from subthalamic afferent neurons.

Role of D1 receptor mechanisms in the potentiation of motor responses to L-dopa and apomorphine by MK 801 in the reserpine-treated mouse

In 24 h reserpine-treated akinetic mice, locomotion was induced by the D1-selective agonist SKF 38393 (30 mg/kg IP), or by the mixed D1/D2 agonists L-dopa (150 mg/kg IP, plus benserazide 100 mg/kg IP) and apomorphine (0.5 mg/kg SC). The non-competitive NMDA receptor antagonist MK 801 (0.01-1.6 mg/kg IP) did not induce motor activity by itself, but potentiated the motor responses to L-dopa and apomorphine at roughly 10-fold lower doses than those which facilitated D1 responding. These data cast doubt on the notion that glutamate antagonists enhance the antiparkinsonian efficacy of mixed D1/D2 agonists solely through a D1 receptor mechanism.