Differential locomotor interactions between dopamine D1/D2 receptor agonists and the NMDA antagonist dizocilpine in monoamine-depleted mice

The mGlu7 receptor in schizophrenia - An update and future perspectives

The mGlu₇ receptor belongs to the III group of metabotropic glutamatergic (mGlu) receptors and physiologically serves as an "emergency" receptor that is activated by high, almost pathological, glutamate concentrations. Of all mGlu receptors, this receptor is most highly expressed in the brain. Additionally, relatively intense expression of the receptor was found at the periphery, for example in the bowels or in the reproductive system of male mice, but this review will be focused predominantly on its role in the brain. In the CNS, the receptor is expressed presynaptically, in the center of the synaptic cleft, at the terminals of both excitatory glutamatergic and inhibitory GABAergic neurons. Thus, it may regulate the release of both glutamate and GABA. Schizophrenia is thought to develop as a consequence of a disturbed glutamatergic-GABAergic balance in different parts of the brain. Thus, the mGlu₇ receptor may be involved in the pathophysiology of schizophrenia and consequently constitute the target for antipsychotic drug discovery. In this review, we summarize the available data about mGlu₇ receptor ligands and their activity in animal models of schizophrenia. At present, only a few ligands are available, and negative allosteric modulators (NAMs) appear to exert antipsychotic-like efficacy, indicating that the inhibition of the receptor could constitute a promising target in the search for novel drugs. Additionally, the data concerning the expression of the receptor in the CNS and putative mechanisms by which its inhibition may contribute to the treatment of schizophrenia will be discussed. Finally, the polymorphisms of genes encoding the receptor in schizophrenic patients will also be provided.

MK801-induced locomotor activity in preweanling and adolescent male and female rats: role of the dopamine and serotonin systems

RATIONALE

MK801, like other NMDA receptor open-channel blockers (e.g., ketamine and phencyclidine), increases the locomotor activity of rats and mice. Whether this behavioral effect ultimately relies on monoamine neurotransmission is of dispute.

OBJECTIVE

The purpose of this study was to determine whether these psychopharmacological effects and underlying neural mechanisms vary according to sex and age.

METHODS

Across four experiments, male and female preweanling and adolescent rats were pretreated with vehicle, the monoamine-depleting agent reserpine (1 or 5 mg/kg), the dopamine (DA) synthesis inhibitor ∝-methyl-_DL-p-tyrosine (AMPT), the serotonin (5-HT) synthesis inhibitor 4-chloro-_DL-phenylalanine methyl ester hydrochloride (PCPA), or both AMPT and PCPA. The locomotor activity of preweanling and adolescent rats was then measured after saline or MK801 (0.3 mg/kg) treatment.

RESULTS

As expected, MK801 increased the locomotor activity of all age groups and both sexes, but the stimulatory effects were significantly less pronounced in male adolescent rats. Preweanling rats and adolescent female rats were more sensitive to the effects of DA and 5-HT synthesis inhibitors, as AMPT and PCPA caused only small reductions in the MK801-induced locomotor activity of male adolescent rats. Co-administration of AMPT+PCPA or high-dose reserpine (5 mg/kg) treatment substantially reduced MK801-induced locomotor activity in both age groups and across both sexes.

CONCLUSIONS

These results, when combined with other recent studies, show that NMDA receptor open-channel blockers cause pronounced age-dependent behavioral effects that can vary according to sex. The neural changes underlying these sex and age differences appear to involve monoamine neurotransmission.

Effects of dopamine and serotonin synthesis inhibitors on the ketamine-, d-amphetamine-, and cocaine-induced locomotor activity of preweanling and adolescent rats: sex differences

The pattern of ketamine-induced locomotor activity varies substantially across ontogeny and according to sex. Although ketamine is classified as an NMDA channel blocker, it appears to stimulate the locomotor activity of both male and female rats via a monoaminergic mechanism. To more precisely determine the neural mechanisms underlying ketamine's actions, male and female preweanling and adolescent rats were pretreated with vehicle, the dopamine (DA) synthesis inhibitor ∝-methyl-_DL-p-tyrosine (AMPT), or the serotonin (5-HT) synthesis inhibitor 4-chloro-_DL-phenylalanine methyl ester hydrochloride (PCPA). After completion of the pretreatment regimen, the locomotor activating effects of saline, ketamine, d-amphetamine, and cocaine were assessed during a 2 h test session. In addition, the ability of AMPT and PCPA to reduce dorsal striatal DA and 5-HT content was measured in male and female preweanling, adolescent, and adult rats. Results showed that AMPT and PCPA reduced, but did not fully attenuate, the ketamine-induced locomotor activity of preweanling rats and female adolescent rats. Ketamine (20 and 40 mg/kg) caused a minimal amount of locomotor activity in male adolescent rats, and this effect was not significantly modified by AMPT or PCPA pretreatment. When compared to ketamine, d-amphetamine and cocaine produced different patterns of locomotor activity across ontogeny; moreover, AMPT and PCPA pretreatment affected psychostimulant- and ketamine-induced locomotion differently. When these results are considered together, it appears that both dopaminergic and serotonergic mechanisms mediate the ketamine-induced locomotor activity of preweanling and female adolescent rats. The dichotomous actions of ketamine relative to the psychostimulants in vehicle-, AMPT-, and PCPA-treated rats, suggests that ketamine modulates DA and 5-HT neurotransmission through an indirect mechanism.

Differential effects of 7-OH-DPAT and apomorphine on hyperactivity induced by MK-801 (dizocilpine) in rats

Recent experiments from this laboratory demonstrated synergistic locomotor depressant effects of AMPA/kainate receptor blockade and D(2/3) dopamine (DA) receptor stimulation. This study explored functional interactions between DA and glutamate (Glu) systems using the NMDA receptor antagonist MK-801 and the DA receptor agonists 7-OH-DPAT and apomorphine. Using photocell locomotor activity boxes, systemic effects of MK-801 in combination with 7-OH-DPAT (0.03 mgkg(-1) SC, n=8) or a pre-synaptically effective dose of apomorphine (0.05 mgkg(-1) SC, n=6) were measured in male Sprague-Dawley rats. Effects of bilateral applications of MK-801 and 7-OH-DPAT into the nucleus accumbens (NAS) shell subregion were also investigated (n=7). When given alone, MK-801 (0.13 mgkg(-1) or 0.66 microg intra-NAS shell) increased horizontal locomotor activity, while 7-OH-DPAT (0.03 mgkg(-1)) or apomorphine (0.05 mgkg(-1)) decreased this measure. Co-administration of 7-OH-DPAT (systemically or into the NAS shell) completely blocked MK-801 induced hyperactivity. In contrast, MK-801 and apomorphine demonstrated additive effects. Stimulation of D(3) DA receptors may therefore block the hyperactivity induced by NMDA receptor antagonism, and the NAS shell is an important site for this interaction. The differential effects of the DA agonists on hyperactivity induced by NMDA receptor blockade support the proposal that 7-OH-DPAT may induce hypoactivity by stimulation of postsynaptic D(3) DA receptors.

Activation of D1, but not D2 receptors potentiates dizocilpine-mediated disruption of prepulse inhibition of the startle

Although substantial evidence has shown interactions between glutamatergic and dopaminergic systems play a cardinal role in the regulation of attentional processes, their involvement in informational filtering has been poorly investigated. Chiefly, little research has focused on functional correlations between the dopaminergic system and the mechanism of action of N-methyl-D-aspartate (NMDA) receptor antagonists on sensorimotor gating. The present study was targeted at evaluating whether the activation of D1 and D2 receptors is able to interact with the disruption of prepulse inhibition (PPI) of startle mediated by dizocilpine, a selective, noncompetitive NMDA receptor antagonist. We tested the effects of SKF 38393 ((+/-)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol) (10 mg/kg, s.c.), a selective D1 agonist, and quinpirole (0.3, 0.6 mg/kg, s.c.), a D2 agonist, in rats, per se and in cotreatment with different doses of dizocilpine, ranging from 0.0015 to 0.15 mg/kg (s.c.). Subsequently, the effect of the D1 antagonist SCH 23390 ((R)-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine) (0.05, 0.1 mg/kg, s.c.) on PPI disruptions mediated by dizocilpine and by combination of dizocilpine and SKF 38393 was tested. Two further experiments were performed to verify whether the synergic effect of the D1 agonist with dizocilpine was counteracted by effective doses of haloperidol (0.1, 0.5 mg/kg, i.p.) and clozapine (5, 10 mg/kg, i.p.). All experiments were carried out using standard procedures for the assessment of PPI of the acoustic startle reflex. SKF 38393, while unable to impair sensorimotor gating alone, induced PPI disruption in cotreatment with 0.05 and 0.15 mg/kg of dizocilpine, both ineffective per se. Furthermore, this effect was reversed by SCH 23390, but not by haloperidol or clozapine. Conversely, no synergistic effect was exhibited between quinpirole and dizocilpine, at any given dose. These findings suggest that D1, but not D2 receptors, enhance the disruptive effect of dizocilpine on PPI.

N-methyl-D-aspartate glutamate receptors and alcoholism: reward, dependence, treatment, and vulnerability

This review takes a translational neuroscience perspective on the role of glutamate systems in human ethanol abuse and dependence. Ethanol is a simple molecule with profound effects on many chemical systems in the brain. Glutamate is the primary excitatory neurotransmitter in the brain. Glutamatergic systems are targets for the actions of ethanol via its antagonism of the N-methyl-D-aspartate (NMDA) subtype of the glutamate receptor and other mechanisms. The modulation of glutamatergic function by ethanol contributes to both euphoric and dysphoric consequences of ethanol intoxication. Adaptations within glutamatergic systems appear to contribute to ethanol tolerance and dependence and to both acute and protracted features of ethanol withdrawal. Perhaps because of the important glutamatergic mediation of the behavioral effects of ethanol, glutamatergic systems appear to contribute to the vulnerability to alcoholism, and novel glutamatergic agents may play a role in the treatment of ethanol abuse and dependence.

Memantine is a clinically well tolerated N-methyl-D-aspartate (NMDA) receptor antagonist--a review of preclinical data

N-methyl-D-aspartate (NMDA) receptor antagonists have therapeutic potential in numerous CNS disorders ranging from acute neurodegeneration (e.g. stroke and trauma), chronic neurodegeneration (e.g. Parkinson's disease, Alzheimer's disease, Huntington's disease, ALS) to symptomatic treatment (e.g. epilepsy, Parkinson's disease, drug dependence, depression, anxiety and chronic pain). However, many NMDA receptor antagonists also produce highly undesirable side effects at doses within their putative therapeutic range. This has unfortunately led to the conclusion that NMDA receptor antagonism is not a valid therapeutic approach. However, memantine is clearly an uncompetitive NMDA receptor antagonist at therapeutic concentrations achieved in the treatment of dementia and is essentially devoid of such side effects at doses within the therapeutic range. This has been attributed to memantine's moderate potency and associated rapid, strongly voltage-dependent blocking kinetics. The aim of this review is to summarise preclinical data on memantine supporting its mechanism of action and promising profile in animal models of chronic neurodegenerative diseases. The ultimate purpose is to provide evidence that it is indeed possible to develop clinically well tolerated NMDA receptor antagonists, a fact reflected in the recent interest of several pharmaceutical companies in developing compounds with similar properties to memantine.

NMDA receptor antagonists inhibit apomorphine-induced climbing behavior not only in intact mice but also in reserpine-treated mice

The present study showed that the noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists, MK-801 {(+)-5-methyl-10,11-dihydroxy-5H-dibenzo-[a,d]-cyclohepten-5,10-im ine hydrogen maleate}, ketamine, dextrorphan and dextromethorphan attenuated apomorphine-induced climbing behavior in reserpine-treated mice. In addition, the competitive NMDA receptor antagonists, D(-)-2-amino-5-phosphonopentanoic acid (AP-5) and D(-)-3-(2-carboxypipera-zine-4-yl)-propyl-1-phosphonic acid (CPP), also inhibited the apomorphine-induced climbing behavior in reserpine-treated mice as well as in intact mice. Previous work in our laboratory had shown that the noncompetitive NMDA receptor antagonists, MK-801, ketamine, dextrorphan and dextromethorphan cause a pronounced inhibition of apomorphine-induced cage climbing behavior in intact mice, suggesting the involvement of NMDA receptors in the glutamatergic modulation of dopaminergic function at the postsynaptic dopamine (DA) receptors. Therefore, the present results strongly support our previous conclusion that the NMDA receptors play important roles in the glutamatergic modulation of dopaminergic function at the postsynaptic DA receptors.

The locomotor effects of MK801 in the nucleus accumbens of developing and adult rats

This developmental study was an investigation of locomotion induced by the NMDA receptor antagonist, (+)MK-801 hydrogen maleate [(5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine], at doses of 0, 3 or 10 microg injected bilaterally into the nucleus accumbens of rats at 11, 21, 31, or 61-66 days of age. During a 2-h test session, only a few 11-day-old pups responded to either dose of MK801; they displayed short bouts of obstinate progression. In contrast, 21- and 31-day-olds were not affected by 3 microg MK801 but exhibited robust activation after 10 microg MK801. The activation was greatest in 21-day-olds and also occurred after mid-striatal injections in 21- but not 31-day-old rats. Adult rats injected with MK801 were not robustly activated, but they maintained their initial level of activity throughout the test session, instead of habituating to the test monitor, as controls did. Ontological changes in MK801-induced activity are likely to reflect maturation of glutamate transmission in the nucleus accumbens.

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.

Dopamine D1- and D2-dependent catalepsy in the rat requires functional NMDA receptors in the corpus striatum, nucleus accumbens and substantia nigra pars reticulata

This study investigated the anticataleptic activity of MK-801 versus the D1 antagonist SCH 23390 and the D2 antagonist raclopride, using the horizontal bar test in the rat. MK-801, 0.2 mg/kg i.p., strongly opposed the cataleptogenic actions of SCH 23390 and raclopride administered systemically (1 and 3 mg/kg i.p., respectively), or directly into the corpus striatum (CS) or nucleus accumbens (NAc; 1 and 10 microg, respectively). Conversely, intraCS and intraNAc pretreatment with MK-801 (10 microg) markedly attenuated the cataleptic response to a systemic injection of SCH 23390 or raclopride. In the latter experiments the anticataleptic effect of MK-801 was pronounced and sustained (> 2 h), except with intraCS MK-801 versus raclopride, where it was initially profound but only short-lived (15 min). Stereotaxic injection of MK-801 (1 microg) into the substantia nigra pars reticulata (SNr) prevented catalepsy developing to either dopamine D1 or D2 receptor antagonism. These results indicate there must be unimpeded glutamate neurotransmission in the CS and NAc before catalepsy can develop fully to D1 and D2 dopamine receptor blockade in these structures. The weaker glutamate-D2 interaction in the CS than in the NAc may be related to differences in the N-methyl-D-aspartate receptor subpopulations in these nuclei. Finally, the ability of intranigral MK-801 to diminish both D1- and D2-dependent catalepsy suggests the SNr acts as a common output pathway for the expression of both forms of catalepsy in the rat.

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.

Dopamine/glutamate interaction as studied by combining turning behaviour and c-Fos expression

Dopamine (DA) and glutamate N-methyl-D-aspartate (NMDA) receptors extensively interact in the mediation of motor behaviours originated in basal ganglia. In unilaterally 6-hydroxydopamine (6-OHDA) lesioned rats, this interaction is of a different sign depending on whether D1 or D2 receptors are stimulated. Contralateral turning behaviour induced by D1 agonists is potentiated by the NMDA antagonist MK 801, while turning behaviour induced by D2 agonists is decreased. NMDA receptors not only modulate the acute turning behaviour induced by DA agonists but also the long-term effects induced by stimulation of DA receptors. MK 801, in fact, prevents the sensitization (priming) of D1-mediated turning behaviour induced by a single exposure to a DA agonist. Prevention of priming by MK 801, also appears to be different depending on whether D1 or D1/D2 receptors are stimulated. Studies on c-fos expression induced by DA D1 agonists in the 6-OHDA lesioned striatum show that detection of Fos-like immunoreactivity correlates to the long-term but not the acute effects induced by DA receptor stimulation and NMDA receptor blockade.

Aminoadamantanes as NMDA receptor antagonists and antiparkinsonian agents--preclinical studies

Aminoadamantanes such as 1-aminoadamantane (amantadine) and 1-amino-3,5-dimethyladamantane (memantine) are N-methyl-D-aspartate (NMDA) receptor antagonists which show antiparkinsonian-like activity in animal models and in Parkinson's patients. The issue of whether NMDA antagonism plays a role in the symptomatological antiparkinsonian activity of amantadine and memantine is addressed by comparing: behaviourally effective doses, serum/brain levels, and their potency as NMDA receptor antagonists. In the case of memantine, blockade of NMDA receptors is probably the only mechanism responsible for antiparkinsonian activity, whereas for amantadine the situation is clearly far more complex. There are a number of differences between memantine and amantadine both in vitro and in vivo, and although NMDA receptor antagonism certainly participates in the antiparkinsonian activity of amantadine, other effects, some of which are elusive, also play a role. Moreover, it has been suggested that the pathomechanism of Parkinson's disease involves excitotoxic processes and that treatment with NMDA receptor antagonists might also slow the progression of neurodegeneration. If this claim is true, such an effect could be achieved with amantadine and memantine which show neuroprotective activity in animals at therapeutically relevant doses.

Neurotransmitter aberrations in schizophrenia: new perspectives and therapeutic implications

The dopamine hypothesis has dominated schizophrenia research for decades but is now yielding to a more diversified view, where the interaction of several neurotransmitters in complex circuitries is under scrutiny. Especially, glutamatergic and serotonergic mechanisms are attracting attention. However, the role of dopamine also needs further exploration and may still turn out to have novel therapeutic applications. In the present minireview an attempt is made to integrate preclinical and clinical data on neurotransmitter aberrations in schizophrenia and to discuss their therapeutic implications.

Intrastriatal AP5 differentially affects behaviors induced by local infusions of D1 vs. D2 dopamine agonists

Using bilateral infusions into the rat striatum, the effects of the competitive NMDA receptor antagonist 2-amino-5-phosphopentanoic acid (AP5) on behaviors induced by the dopamine (DA) D1 receptor agonist SKF 82526 (fenoldopam) or the D2 receptor agonist quinpirole were determined. These effects were tested in DA-replete (intact) rats and in rats that were receiving injections of the monoamine-depleting drug reserpine. In both intact and reserpinized rats, fenoldopam induced significant sniffing. This effect was attenuated by simultaneous co-infusion of AP5 in the reserpinized rats. Quinpirole induced locomotion, sniffing, and oral behaviors, all of which were attenuated by AP5 co-infusion in the intact rats. In contrast, AP5 enhanced the quinpirole-induced sniffing of reserpinized rats. These findings suggest that distinct D1/glutamate and D2/glutamate relationships exist in the striatum, and that the nature of the latter is influenced by DA tone.

The competitive NMDA receptor antagonist SDZ 220-581 reverses haloperidol-induced catalepsy in rats

The present studies investigated whether SDZ 220-581 ((S)-alpha-amino 2'chloro-5-(phosphonomethyl)[1,1'-biphenyl]-3-propanoic acid), a potent, competitive antagonist at the NMDA glutamate receptor subtype, reversed haloperidol-induced catalepsy in rats, a widely used model of Parkinson's disease. SDZ 220-581 (0.32-3.2 mg/kg i.p.) dose- and time-dependently reduced the time spent in an abnormal position induced by haloperidol (1.0 mg/kg s.c.). Compared to other NMDA receptor antagonists the rank order of potency was MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine) > SDZ 220-581 > SDZ EAA 494 (D-CPPene: (S)-(E)-4-(3-phosphonoprop-2-enyl)-piperazine-2-carboxylic acid) > SDZ EAB 515 ((S)-alpha-amino-5-(phosphonomethyl)[1,1'-biphenyl]-3-propanoic acid). Since it has been demonstrate that SDZ 220-581 counters the effects of L-dihydroxyphenylalanine (L-DOPA) on the motor disturbances of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-pre-treated primates, the results suggest that the reversal of haloperidol-induced catalepsy by competitive NMDA receptor antagonists may not be predictive of efficacy in other models of Parkinson's disease.

Biphenyl-derivatives of 2-amino-7-phosphono-heptanoic acid, a novel class of potent competitive N-methyl-D-aspartate receptor antagonists--II. Pharmacological characterization in vivo

A selection of biphenyl-analogues of 2-amino-7-phosphonoheptanoic acid (AP7), N-methyl-D-aspartate (NMDA) receptor antagonists with high affinity in vivo efficacy. The lead compound SDZ EAB 515 was found to inhibit L-phenylalanine uptake by the large neutral amino acid carrier in vitro and in vivo; active transport may thus confer a good bioavailability to this class of compounds. CNS effects were demonstrated by significant changes in 2-deoxyglucose-uptake in various brain regions at doses from 1 to 10 mg/kg i.p. With the most active agent, SDZ 220-581, full protection against maximal electroshock seizures (MES) was obtained at oral doses of 10 mg/kg in rats and in mice. The compound had a fast onset (< or = 1 hr) and a long duration (> or = 24 hr) of action. Motor-debilitating effects (impairment of rotarod performance) occurred at doses about 10 times higher than those required for protection against MES. Neuroprotective activity was demonstrated by the ability of the compounds to reduce the extent of quinolinic acid-induced striatal lesions in rats, in the dose range of 3-15 mg/kg (i.p.) or 10-50 mg/kg (p.o.). In the middle cerebral artery occlusion (MCAO) model of focal cerebral ischemia in rats, the test compounds reduced the infarct size by 40-50% when given i.v. before or by 20-30% when given i.v. 1 hr after MCAO. SDZ 220-581 provided 20-30% protection at > or = 2 x 10 mg/kg p.o. This compound also showed analgesic activity at low oral doses in a model of neuropathic pain, although higher doses were required in model of mechanical inflammatory hyperalgesia. Unexpectedly, SDZ 220-581 at low s.c. doses counteracted the antiparkinsonian effects of L-DOPA in MPTP-treated marmosets. (Sub)chronic administration of SDZ 220-581 did not reduce its ability to protect against quinolinic acid neurotoxicity, and no upregulation of NMDA receptors was detected using a [3H]CGP-39653 binding assay. In conclusion, from a series of biphenyl-AP7-derivatives, SDZ 220-581 is clearly the most active compound in vivo. Its pharmacological profile with a good, long-lasting oral activity might open up novel therapeutic applications for competitive NMDA receptor antagonists.

Functional neuroanatomy of the nigrostriatal and striatonigral pathways as studied with dual probe microdialysis in the awake rat--I. Effects of perfusion with tetrodotoxin and low-calcium medium

In the present study we employed the dual probe approach to investigate functional interactions between the nigrostriatal dopaminergic and striatonigral GABAergic pathways in the awake, freely moving rat and their role in motor function. One microdialysis probe of concentric design was implanted in the substantia nigra pars reticulata and another in the ipsilateral dorsolateral striatum. Perfusion with a low-Ca2+ (0.1 mM) medium and with the voltage-dependent Na(+)-channel blocker tetrodotoxin (10 microM) was alternatively performed in both brain regions and the dialysate dopamine, glutamate and GABA levels were simultaneously measured in the dorsolateral striatum, whereas GABA levels alone were monitored in the substantia nigra. Perfusion with a low-Ca2+ medium in the substantia nigra pars reticulata did not affect local GABA levels, but transiently increased striatal dopamine release (+40%) without modifying striatal glutamate and GABA levels. Conversely, intranigral perfusion with tetrodotoxin transiently increased local GABA levels (+40%), while it decreased striatal dopamine (-60%) and increased glutamate (+70%) and GABA (+50%) levels. Perfusion with a low-Ca2+ medium in the dorsolateral striatum reversibly decreased local dopamine (-70%), glutamate (-20%) and GABA (-20%) levels, while local perfusion with tetrodotoxin decreased dopamine (-70%), increased glutamate (+30%) but did not affect dialysate GABA levels in this brain area. Neither of these intrastriatal treatments significantly affected GABA levels in the substantia nigra. Intranigral but not intrastriatal perfusion with tetrodotoxin was also associated with an increase in spontaneous locomotor activity as expressed by contralateral turning. Intranigral and intrastriatal perfusion with low-Ca2+ medium did not influence locomotor activity. On the basis of these neurochemical and behavioural findings, we propose a new dynamic model for the study of motor behaviour as mediated by basal ganglia circuitry.

Functional neuroanatomy of the nigrostriatal and striatonigral pathways as studied with dual probe microdialysis in the awake rat--II. Evidence for striatal N-methyl-D-aspartate receptor regulation of striatonigral GABAergic transmission and motor function

In the present study we used the dual probe approach to investigate striatal N-methyl-D-aspartate receptor regulation of GABA release from the substantia nigra pars reticulata of the awake, freely moving rat. One microdialysis probe of concentric design was implanted in the dorsolateral striatum and another in the ipsilateral substantia nigra pars reticulata. Perfusion with N-methyl-D-aspartate (100 microM) in the dorsolateral striatum decreased local dopamine release (-25%) and increased both glutamate (+40%) and GABA (+35%) release. Moreover, perfusion with N-methyl-D-aspartate (100 microM) in the dorsolateral striatum increased GABA release (+20%) in the substantia nigra pars reticulata. Perfusion with the lower (10 microM) N-methyl-D-aspartate concentration in the dorsolateral striatum did not affect striatal dopamine, glutamate and GABA release or nigral GABA release. Intrastriatal perfusion with the N-methyl-D-aspartate receptor antagonist dizocilpine maleate (10 microM), at a dose which by itself did not affect basal striatal or nigral neurotransmitter levels, prevented the effects of striatal perfusion with N-methyl-D-aspartate on both striatal and nigral neurotransmitter release. Intrastriatal dizocilpine maleate was also perfused concurrently with intranigral tetrodotoxin (10 microM) (see accompanying paper). Intrastriatal perfusion with dizocilpine maleate prevented the tetrodotoxin-induced rise in both striatal and nigral GABA levels and profoundly reduced the tetrodotoxin-induced contralateral turning. In addition, intrastriatal dizocilpine maleate delayed the increase in striatal glutamate release evoked by intranigral tetrodotoxin without affecting the associated decrease in striatal dopamine release. The present study demonstrates that N-methyl-D-aspartate receptors in the dorsolateral striatum regulate GABA release in the substantia nigra pars reticulata of the awake rat and provides evidence that this regulation plays a key role in motor function.

Mechanisms of action of atypical antipsychotic drugs: a critical analysis

Various criteria used to define atypical antipsychotic drugs include: 1) decrease, or absence, of the capacity to cause acute extrapyramidal motor side effects (acute EPSE) and tardive dyskinesia (TD); 2) increased therapeutic efficacy reflected by improvement in positive, negative, or cognitive symptoms; 3) and a decrease, or absence, of the capacity to increase prolactin levels. The pharmacologic basis of atypical antipsychotic drug activity has been the target of intensive study since the significance of clozapine was first appreciated. Three notions have been utilized conceptually to explain the distinction between atypical versus typical antipsychotic drugs: 1) dose-response separation between particular pharmacologic functions; 2) anatomic specificity of particular pharmacologic activities; 3) neurotransmitter receptor interactions and pharmacodynamics. These conceptual bases are not mutually exclusive, and the demonstration of limbic versus extrapyramidal motor functional selectivity is apparent within each arbitrary theoretical base. This review discusses salient distinctions predominantly between prototypic atypical and typical antipsychotic drugs such as clozapine and haloperidol, respectively. In addition, areas of common function between atypical and typical antipsychotic drug action may also be crucial to our identification of pathophysiological foci of the different dimensions of schizophrenia, including positive symptoms, negative symptoms, and neurocognitive deficits.

Neurochemical bases of locomotion and ethanol stimulant effects

The locomotor stimulant effect produced by alcohol (ethanol) is one of a large number of measurable ethanol effects. Ethanol-induced euphoria in humans and locomotor stimulation in rodents, a potential animal model of human euphoria, have long been recognized and the latter has been extensively characterized. Since the euphoria produced by ethanol may influence the development of uncontrolled or excessive alcohol use, a solid understanding of the neurochemical substrates underlying such effects is important. Such an understanding for spontaneous locomotion and for ethanol's stimulant effects is beginning to emerge. Herein we review what is known about three neurochemical substrates of locomotion and of ethanol's locomotor stimulant effects. Several lines of research have implicated dopaminergic, GABAergic, and glutamatergic neurotransmitter systems in determining these behaviors. A large collection of work is cited, which strongly implicates the above-mentioned neurotransmitter substances in the control of spontaneous locomotion. A smaller, but persuasive, body of evidence suggests that central nervous system processes utilizing these transmitters are involved in determining the effects of ethanol on locomotion. Particular emphasis has been placed on the mesolimbic ventral tegmental area to nucleus accumbens dopaminergic pathway, and on the ventral pallidum/substantia innominata, where GABA and glutamate have been found to play a role in altering the activity of this dopaminergic pathway. Research on ethanol and drug locomotor sensitization, increased responsiveness to the substance with repeated administration, is also reviewed as a process that may be important in the development of drug addiction.

The behavioral effects of MK-801 injected into nucleus accumbens and caudate-putamen of rats

In this study, we investigated the behavioral effects of MK-801 (1-20 micrograms) injected into the posterior parts of nucleus accumbens (ACC) and caudate-putamen (CP) in rats. Interactions of diazepam (DZP, 10 micrograms), haloperidol (HPD, 2 micrograms), and scopolamine (SCOP, 10 micrograms) with 20 micrograms of MK-801 were also studied. All injections were done in 2 microliters. In ACC, MK-801 increased locomotion, rearing, and head shakes. The effect of MK-801 especially at 20 micrograms was accompanied by a motor syndrome: head weaves, circling, body rolls, and ataxia. DZP nonsignificantly reduced the locomotion but it significantly (p < 0.05) reduced head shakes, weaves, circling, and body rolls produced by MK-801. HPD reduced grooming and head shakes. SCOP potentiated MK-801 hyperlocomotion, whereas it decreased body rolls, head shakes, and weaves. In CP, MK-801 increased locomotion, but less than in ACC (p < 0.05). The effect of MK-801 was significantly increased by SCOP. MK-801 also increased grooming (reduced by HPD and increased by SCOP) and at 5-20 micrograms induced oral movements that were decreased by HPD. These results indicate that the posterior part of ACC is involved in MK-801 hyperlocomotion and motor syndromes, whereas CP is involved in mediating grooming and oral movements. Blockade of the muscarinic cholinergic receptors seems to facilitate hyperlocomotion and decrease head shakes produced by MK-801. Mechanisms influenced by DZP and HPD appear to be involved in motor syndrome and oral movement, respectively, induced by MK-801, but not in hyperlocomotion.

The selective 5-HT2A receptor antagonist MDL 100,907 counteracts the psychomotor stimulation ensuing manipulations with monoaminergic, glutamatergic or muscarinic neurotransmission in the mouse--implications for psychosis

The present study has shown that a subthreshold dose of the uncompetitive N-methyl-D-aspartate (NMDA) antagonist MK-801, combined with a subthreshold dose of LSD, produces marked locomotor stimulation in monoamine-depleted mice. Likewise, MK-801, as well as the muscarine receptor antagonist atropine and the alpha-adrenoceptor agonist clonidine, were found to interact synergistically with the putative 5-HT2 receptor agonist UH-232 to produce locomotor activation in monoamine-depleted mice. All these responses were effectively blocked by the highly selective 5-HT2A receptor antagonist MDL 100,907. On the other hand, MDL 100,907 did not antagonize the hyperactivity response produced by clonidine given in combination with MK-801 or atropine in monoamine-depleted mice, nor the response produced by the mixed DA receptor agonist apomorphine, underlining the selectivity in the antagonistic action of MDL 100,907. Furthermore, MDL 100,907 attenuated the hyperactivity produced in intact mice by such disparate agents as MK-801, atropine or the DA uptake inhibitor GBR 12,909. A putative "permissive" role of the 5-HT2 receptor in the context of psychomotor activation is discussed, as well as its possible importance as target for antipsychotic therapy.

Modulation of dopamine D1-mediated turning behavior and striatal c-fos expression by the substantia nigra

In order to study the possible contribution of the substantia nigra (SN) in the positive interaction between dopamine D1 receptor agonists and glutamate antagonists in unilaterally 6-hydroxydopamine (6-OHDA) lesioned rats, the effect of the D1 agonist, SKF 38393, was studied in combination with intranigral infusions of glutamate antagonists of the NMDA (MK 801, CPP) or AMPA (NBQX) type of receptor. Local infusion into the SN of the 6-OHDA lesioned side of MK 801, CPP or NBQX at doses inducing no or minimal behavioral effects significantly increased the turning behavior and the expression of c-fos induced, in the lesioned caudate-putamen (CPu), by a parenteral administration of SKF 38393. The same result was obtained after intra-SN infusion of the GABA agonist, muscimol. High doses of MK 801, CPP or muscimol infused into the SN produced intense contralateral turning per se and induced a sparse c-fos expression in the lesioned CPu which was antagonized by parenteral administration of MK 801. The results indicate that a depression of SN pars reticulata efferent neurons potentiates D1-mediated responses and suggest that this area may play a role in the positive interaction between glutamate antagonists and D1 receptor agonists.

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.

Locomotor effects of amantadine in the mouse are not those of a typical glutamate antagonist

Amantadine has been shown to displace [3H]MK 801 from its binding site on the NMDA receptor. We have therefore studied the motor effects of amantadine in normal and 24 h reserpine-treated mice to determine whether the behavioural profile of this drug resembles that of other NMDA receptor antagonists (e.g. MK 801). In common with the latter, amantadine (5-40 mg/kg IP) produced a modest dose-dependent sedation in dopamine-intact mice, with a reduction in locomotion and other species-typical behaviours (e.g. rearing and grooming), but with no signs of the hyperactivity, stereotypy, ataxia or loss of muscle tone commonly seen with MK 801. Amantadine (5-80 mg/kg IP) effected a small increase in motility in akinetic reserpine-treated mice by itself, but this response was highly variable and not statistically significant. As with MK 801, amantadine significantly inhibited the locomotion induced by the selective D2 agonist RU 24213 (5 mg/kg SC) and the mixed D1/D2 agonist apomorphine (0.5 mg/kg SC) in monomine-depleted mice, without altering the animals' responsiveness to threshold doses of these drugs. However, amantadine did not facilitate the locomotion induced by threshold (3 mg/kg IP) or fully active doses (30 mg/kg IP) of the selective D1 agonist SKF 38393, which distinguishes amantadine from other NMDA receptor blockers. Since the potentiation of dopamine D1-dependent locomotion may be a major factor in the antiparkinson activity of MK 801 and other glutamate receptor antagonists, the inability of amantadine to potentiate SKF 38393 in this study suggests the mechanism of its anti-akinetic activity differs from that of conventional glutamate blocking drugs.

The muscarine antagonist methscopolamine and the NMDA antagonist AP-5 injected unilaterally into the nucleus accumbens cause mice to rotate in opposite directions

Previously, we have reported that the NMDA antagonist AP-5, injected unilaterally into the nucleus accumbens of mice, induces ipsilateral rotation in monoaminergically intact mice, but contralateral rotation in monoamine-depleted animals. In this paper we report that the muscarine antagonist methscopolamine, injected unilaterally into the nucleus accumbens, induced predominantly contralateral rotation in monoaminergically intact mice. In monoamine-depleted animals intra-accumbens methscopolamine induced only a weak stimulation of rotational behaviour (not significant), but the direction of the rotation was exclusively contralateral, and the animals showed contralateral body deviation. Moreover, when these animals received additional treatment with the alpha-adrenergic agonist clonidine, which potentiates the motor effects of cholinergic antagonists in monoamine-depleted mice (Carlsson et al., 1991), a clear-cut contralateral rotation was induced. The observed behavioural effects are discussed in relation to the positive and negative feedback circuits, which link the nucleus accumbens with the cerebral cortex and thalamus. In previous papers, we have suggested that the ipsilateral rotation induced by AP-5 is mediated primarily by the positive feedback circuit, whereas the AP-5-induced contralateral rotation is due to interference with primarily the negative feedback circuit. Applying this reasoning to the rotational effects of methscopolamine, it seems that methscopolamine interferes primarily with the negative feedback circuit.

Effect of coadministration of glutamate receptor antagonists and dopaminergic agonists on locomotion in monoamine-depleted rats

Combinations of dopaminergic agonists with glutamate receptor antagonists have been suggested to be a possible alternative treatment of Parkinson's disease. To gain further insights into this possibility, the antagonist of the competitive AMPA-type glutamate receptor NBQX and the ion-channel blocker of the NMDA glutamate receptor (+)-MK-801 in combination with the dopamine D1 receptor agonists: SKF 38393, SKF 82958 and dihydrexidine; the dopamine D2 receptor agonist bromocriptine and the dopamine-precursor L-DOPA were tested in rats pretreated with reserpine and alpha-methyl-p-tyrosine. MK-801 on its own induced locomotor behaviour and potentiated the antiakinetic effects of dihydrexidine and L-DOPA but not of the other dopamine agonists tested. NBQX neither on its own nor coadministered with the dopamine agonists tested had an antiakinetic effect. These results indicate that agents, blocking the ion-channel of the NMDA receptor, might be useful adjuvants to some but not all dopaminomimetics in therapy of Parkinson's disease. The same does not seem to be true for the AMPA-antagonist NBQX.

Crucial role of the accumbens nucleus in the neurotransmitter interactions regulating motor control in mice

Previous work, based on systemic drug administration, has shown that neurotransmitter interactions between dopaminergic, adrenergic, glutamatergic and cholinergic systems are involved in locomotor control in mice. In an attempt to identify the target sites in the brain of these interactions, we have started a series of experiments, where the drugs are administered intracerebrally in mice. The locomotor threshold doses of the competitive NMDA antagonist AP-5 and the noncompetitive NMDA antagonist MK-801 were investigated by means of local application in the accumbens nucleus of monoamine-depleted and monoaminergically intact mice, respectively. The threshold dose of AP-5 was lower in depleted than in intact animals, whereas the threshold dose of MK-801 was lower in monoaminergically intact than monoamine-depleted mice. The locomotor effects of AP-5 and the AMPA-kainate receptor antagonist CNQX were registered in monamine-depleted mice after local application in the accumbens or entopeduncular nucleus (= medial pallidum). Both AP-5 and CNQX stimulated locomotor activity in the accumbens, but had no effects in the entopeduncular nucleus. We have previously shown synergistic interactions with regard to locomotor stimulation in monoamine-depleted mice, between an NMDA antagonist and an alpha 2-adrenoceptor agonist or a dopamine D1 agonist (all drugs given systemically). In the present study the alpha 2-adrenoceptor agonist alpha-methylnoradrenaline was applied intracerebrally in combination with a subthreshold dose of MK-801 given intraperitoneally: Locomotor stimulation was produced after alpha-methyl-noradrenaline injection into the accumbens nucleus, but not after injection into the dorsal striatum, prefrontal cortex or thalamus. Likewise, local application of the D1 agonist SKF 38393, in combination with a subthreshold dose of MK-801 given intraperitoneally, point to an important role of the accumbens nucleus in motor control. Previous experiments based on systemic drug administration have also shown a synergistic interaction between a muscarine antagonist and an alpha 2-adrenoceptor agonist in monoamine-depleted mice. Local application of the muscarine antagonist methscopolamine, in combination with the alpha 2-adrenoceptor agonist clonidine given intraperitoneally, showed that the striatum, in this case both the ventral and dorsal parts of the striatum, is an important target for the muscarine antagonist. Unilateral injection of AP-5 into the accumbens nucleus of mice induces rotational behaviour: Previous findings have shown that the rotation is ipsilateral in monoaminergically intact animals, whereas monoamine-depleted animals rotate contralaterally. In addition, these findings have shown that dopamine D2 receptor stimulation seems to determine whether AP-5 will induce ipsilateral or contralateral rotation. In the present study we report further evidence for a crucial role of the D2 receptor in this respect. Finally, the rotational effects of AP-5 injected into the dorsal striatum or hippocampus were investigated: As after AP-5 application into the accumbens nucleus, monoaminergically intact mice rotated ipsilaterally, whereas monoamine-depleted animals rotated contralaterally, following AP-5 application in the dorsal striatum or the hippocampus. The present data show that the accumbens nucleus has an important role in motor control. Both glutamatergic, muscarine cholinergic, dopaminergic and alpha-adrenergic systems are involved in the control of motor functions in the accumbens nucleus.

Do NMDA receptor-mediated changes in motor behaviour involve nitric oxide?

Nitric oxide (NO) synthase inhibitors were investigated for their effects on motor behaviour. In normal mice, NG-nitro-L-arginine (5-125 mg/kg i.p.) and 7-nitroindazole (10-50 mg/kg i.p.), but not aminoguanidine (60-150 mg/kg i.p.) suppressed species-typical behaviours. In 24 h reserpine-treated mice, akinesia was reversed with the dopamine D1 receptor agonist 2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine hydrochloride (SKF 38393, 3-30 mg/kg i.p.) and by the dopamine D2 receptor agonist N-n-propyl-N-phenylethyl-p-(3-hydroxyethyl) ethylamine hydrochloride (RU 24213, 0.5-5 mg/kg s.c.), but not by any of the NO synthase inhibitors. NG-Nitro-L-arginine and 7-nitroindazole (not aminoguanidine) suppressed D1 and D2 receptor agonist-induced locomotion, but L-arginine (500 mg/kg i.p.) was not always able to prevent this effect. These results suggest that continued activity of constitutive NO synthase is necessary for normal body movements to occur. The difference in the interaction profiles of constitutive NO synthase inhibitors and NMDA antagonists with dopaminergic drugs, indicates that inhibition of NO generation is not a factor in the well-known D1-facilitatory effect of glutamate receptor blockade.

Differential effect of MK 801 and scopolamine on c-fos expression induced by L-dopa in the striatum of 6-hydroxydopamine lesioned rats

In rats with a unilateral 6-hydroxydopamine lesion of the dopaminergic nigro-striatal pathway, striatal D1-receptor-stimulated c-fos expression and turning behavior are positively modulated by D2 receptor stimulation and by blockade of N-methyl-D-aspartate (NMDA) or muscarinic receptors. Combined D1/D2 receptor stimulation by L-dopa activates c-fos in a manner not additive with muscarinic receptor blockade by scopolamine. On the other hand, blockade of NMDA receptors by MK 801 reduced c-fos expression induced by L-dopa while, depending on the dose of L-dopa, differentially affecting contralateral turning behavior. The results are interpreted to suggest that D2 receptor stimulation amplifies D1-receptor-mediated c-fos expression by two mechanisms differentially related to muscarinic and NMDA receptors.

Glutamatergic neurons projecting to the nucleus accumbens can affect motor functions in opposite directions depending on the dopaminergic tone

1. Intracerebral cannulas were implanted stereotactically into the nucleus accumbens, dorsal striatum or nucleus entopeduncularis of male NMRI mice. 2. Monoamine-depleted mice were injected intracerebrally with the competitive NMDA receptor antagonist AP-5, the non-competitive NMDA antagonist MK-801 or the AMPA-kainate receptor antagonist CNQX. A marked locomotor stimulation was produced when AP-5 was injected into the nucleus accumbens, but not when injected into the dorsal striatum. Likewise, CNQX stimulated locomotor activity when injected into the nucleus accumbens. Neither AP-5 nor CNQX produced behavioral stimulation following injection into the nucleus entopeduncularis. 3. The tone in the monoaminergic systems influences the potency of competitive and non-competitive NMDA antagonists differently with regard to stimulation of locomotor activity. In the case of the competitive NMDA antagonist AP-5 the potency was higher in monoamine-depleted than in monoaminergically intact mice. In contrast, the potency of the non-competitive NMDA antagonist MK-801 was higher in monoaminergically intact than in monoamine-depleted animals. 4. A unilateral injection of AP-5 into the nucleus accumbens caused the animals to rotate: The rotation was predominantly ipsilateral in monoaminergically intact animals, whereas monoamine-depleted mice rotated exclusively contralaterally. When AP-5 was given to monoamine-depleted mice treated with the D-2 agonist quinpirole the animals rotated ipsilaterally, whereas monoamine-depleted mice treated with the D-1 agonist SKF 38393 still rotated contralaterally after AP-5 treatment. These data show that glutamatergic neurons projecting to the nucleus accumbens can affect behavior in different directions depending on the degree of dopamine D-2 receptor stimulation.

N-methyl-D-aspartate receptor blockade impairs behavioural performance of rats in a reaction time task: new evidence for glutamatergic-dopaminergic interactions in the striatum

The effects of blocking glutamate transmission at the N-methyl-D-aspartate receptor subtype were studied in rats performing a conditioned reaction time motor task. Rats were trained to release a lever after the onset of a visual stimulus within a time limit to obtain food reward. The results showed that the performances of the groups receiving the N-methyl-D-aspartate receptor antagonists dizocilpine maleate (0.1 mg/kg) injected systemically or DL-2-amino-5-phosphonovaleric acid at the highest dose tested (5.0 micrograms/microliter/side) injected locally into the striatum changed significantly as compared to controls. The effects of these antagonists, consisting of an increase in the number of lever releases occurring before the visual stimulus onset ("anticipated responses"), were similar to those induced by injecting dopamine into the same striatal location. Both dizocilpine maleate and DL-2-amino-5-phosphonovaleric acid (5.0 micrograms/microliter) reversed the motor deficits, resulting in an increase in the number of lever releases after the time limit ("delayed responses") that were induced by the D2 dopamine receptor antagonist raclopride. Although these results partly confirm the existence of a functional antagonism between the glutamatergic and the dopaminergic systems in the striatum, opposite findings were obtained with the group that received intrastriatal DL-2-amino-5-phosphonovaleric acid at the lowest dose (0.5 micrograms/microliter/side). When given alone, 0.5 micrograms/microliter DL-2-amino-5-phosphonovaleric acid had no behavioural effects, but when jointly administered with dopamine or raclopride, it was found to reverse the effects of dopamine and to potentiate the motor deficits induced by raclopride. These opposite effects on the reaction time task observed after the intrastriatal injection of DL-2-amino-5-phosphonovaleric acid, depending on the dose tested, occurred only after a combined treatment with a dopaminergic agonist or antagonist and suggest that the level of the striatal dopaminergic activity may play a critical role in regulating the glutamate transmission via the N-methyl-D-aspartate receptors during the performance of complex sensorimotor tasks of this kind.

Reversal of the adaptive response of neuropeptide Y neurons in the rat striatum to nigrostriatal dopamine deafferentation by the N-methyl-D-aspartate antagonist dizocilpine maleate

This study examined the effects of systemic treatments with dizocilpine maleate alone or in combination with unilateral 6-hydroxydopamine-induced lesion of the nigrostriatal dopaminergic neurons on the number and staining intensity of neuropeptide Y-immunoreactive neurons in the rat striatum. In the combined condition, short-term and long-term treatments with dizocilpine maleate were started 19 days and 12 days after the lesion of the nigrostriatal dopaminergic pathway, respectively. As reported previously, the unilateral dopaminergic lesion elicited an increase in both the number and staining intensity of neuropeptide Y-immunoreactive neurons in the ipsilateral striatum. Short-term treatment with dizocilpine maleate at the dose of 0.2 mg/kg (four injections, 6 h apart, sacrifice 2 h after the final dose), which by itself did not modify neuropeptide Y immunostaining, totally suppressed the effect of the dopaminergic deafferentation on the number of neuropeptide Y-positive neurons but not that on the intraneuronal amount of labelling. When administered twice a day for eight days at the same dose of 0.2 mg/kg, dizocilpine maleate by itself elicited an increase in the number of neuropeptide Y-immunodetectable cells, paradoxically concomitant with a decrease in the levels of intraneuronal labelling. After combination of this treatment with unilateral lesion of the nigrostriatal dopaminergic pathway, the changes related to either the dizocilpine maleate treatment or the 6-hydroxydopamine-induced lesion totally disappeared, so that the number and staining intensity of neuropeptide Y-immunoreactive neurons in that condition did not differ from control values.(ABSTRACT TRUNCATED AT 250 WORDS)

On the roles of dopamine D-1 vs. D-2 receptors for the hyperactivity response elicited by MK-801

The present study was aimed at clarifying to what extent the hypermotility induced by the uncompetitive N-methyl-D-aspartate (NMDA) antagonist MK-801 depends on dopamine (DA) D-1 compared to D-2 receptor tone. The D-1 receptor antagonist SCH 23390 was found to reduce locomotion to a greater extent in MK-801-treated than in vehicle-treated mice, whereas the reverse appeared to be the case for the DA D-2 receptor antagonist raclopride. In other words, MK-801-induced hyperactivity was more readily antagonized by SCH 23390 than by raclopride and, thus, DA D-1 receptors seem to be more important than D-2 receptors for MK-801-induced hyperactivity. These results are in line with our previous observation that MK-801 generally interacts synergistically with a DA D-1 but not with a D-2 receptor agonist in monoamine-depleted mice. In view of the possible role of deficient glutamatergic neurotransmission in schizophrenia, our findings underline the importance of investigating the efficacy of selective DA D-1 antagonists in this disorder.

Modulatory functions of neurotransmitters in the striatum: ACh/dopamine/NMDA interactions

The striatum is viewed as a structure performing fast neurotransmitter-mediated operations through somatotopically organized projections to medium-size spiny neurons. This view is contrasted with another view that depicts the striatum as a site of diffuse modulatory influences mediated by cholinergic interneurons and by dopamine and N-methyl-D-aspartate receptors. These two operational and organizational modes both contribute, through their mutual interaction, to the function of basal ganglia. Detailed knowledge of the neural mechanisms by which such interactions take place and are expressed into behaviour, can provide new insight into the physiopathology and new clues for therapy of disorders of basal ganglia.

Potentiation of dopamine-dependent locomotion by clonidine in reserpine-treated mice is restricted to D2 agonists

Mice treated with reserpine (5 mg/kg IP), 24 h beforehand, were completely akinetic. Fluent locomotion was reinstated with the D1-selective agonist SKF 38393 (3-30 mg/kg IP), the D2-selective agonist RU 24213 (0.5-5 mg/kg SC) and the mixed D1/D2 agonist apomorphine (0.025-0.5 mg/kg SC). Clonidine (0.03125-1 mg/kg IP) caused a dose-dependent sedation in dopamine-intact mice, but had no effect by itself on the locomotor activity of monoamine-depleted mice. In drug interaction experiments, clonidine did not modify the motor stimulant action of SKF 38393, but greatly enhanced the motor responses to RU 24213 and apomorphine. These results support the hypothesis that alpha-adrenoceptor agonists facilitate dopamine D2 but not dopamine D1 motor responding in the reserpine-treated mouse model of Parkinson's disease.

Motor activation in short- and long-term reserpinized mice: role of N-methyl-D-aspartate, dopamine D1 and dopamine D2 receptors

The effects of dopamine D1 and dopamine D2 receptor agonists and of subconvulsant doses of N-methyl-D-aspartate (NMDA) and the non-competitive NMDA receptor antagonist, dizocilpine (MK-801), alone and in combination, on the motor activity of short- and long-term reserpinized mice (mice pretreated with 5 mg/kg reserpine 4 h or 20 h before, respectively) were analyzed. With short-term reserpinization, the dopamine D2 receptor agonist, quinpirole (1.5 mg/kg), but not the dopamine D1 receptor agonist, SKF-38393 (15 mg/kg), increased motor activity. The effect of quinpirole in short-term reserpinized mice was potentiated by the simultaneous administration of SKF-38393 (15 mg/kg) and was counteracted by the previous administration of the dopamine D2 receptor antagonist, raclopride (1 mg/kg), or by the simultaneous administration of NMDA (25 mg/kg) or MK-801 (0.5 mg/kg). Neither NMDA (25-100 mg/kg) nor MK-801 (0.5-3 mg/kg) induced motor activation in short-term reserpinized mice. With long-term reserpinization, either quinpirole (1.5 mg/kg) or SKF-38393 (15 mg/kg) increased motor activity. The effect of quinpirole in long-term reserpinized mice was not potentiated by the concurrent administration of SKF-38393 (15 mg/kg), was inhibited by the simultaneous administration of MK-801 (0.5 mg/kg) and was not modified by NMDA (25 mg/kg). The effect of SKF-38393 (15 mg/kg) in long-term reserpinized mice was inhibited by the concomitant administration of MK-801 (0.5 mg/kg) and was slightly antagonized by NMDA (25 mg/kg). NMDA induced motor activation in long-term reserpinized mice at doses which were similar to those causing motor activation in non-reserpinized mice (75 and 100 mg/kg), while MK-801 induced motor activation at a dose which was associated with motor depression in non-reserpinized mice (2 mg/kg). The NMDA-induced motor activation in long-term reserpinized mice was counteracted by the previous administration of a low dose of MK-801 (0.5 mg/kg) and was still present when a stronger dopamine-depleting pretreatment was used. These results are interpreted on the basis of changes in sensitivity of the direct striatal efferent pathway after long-term reserpinization.

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.

Memantine, amantadine, and L-deprenyl potentiate the action of L-dopa in monoamine-depleted rats

Some treatments used for Parkinson's disease attenuate locomotor depression in rats treated with reserpine and alpha-methyl-p-tyrosine. In the present study memantine (2.5, 5.0 mg/kg), amantadine (10, 20 mg/kg) (both uncompetitive NMDA antagonists), and L-deprenyl (1.0, 5.0 mg/kg; MAO-B inhibitor) were tested for possible synergistic interactions with the dopamine agonists: bromocriptine (2.5, 5.0 mg/kg) and L-dopa (50, 100 mg/kg, +benserazide, 100 mg/kg). At higher doses, memantine (10 mg/kg), amantadine (40 mg/kg), bromocriptine (5 and 10 mg/kg) and L-dopa (100, 200 mg/kg) but not L-deprenyl (up to 10 mg/kg) produced a pronounced increase in locomotor activity when given alone. The combination of memantine, amantadine and L-deprenyl with bromocriptine did not result in synergism of action and, at best, an additive effect was seen. On the other hand the combination of these agents with L-dopa produced a pronounced synergistic effect. Hence, the clinical observation that coadministration of L-dopa with either memantine or amantadine results in enhancement of their action is also reflected in an animal model of Parkinson's disease. Such a combination therapy should allow the use of lower doses of both drugs which may reduce the occurrence of side effects and may also be predicted to have additional benefits related to the neuroprotective properties of memantine, amantadine, and L-deprenyl.

Locomotor activation induced by MK-801 in the rat: postsynaptic interactions with dopamine receptors in the ventral striatum

The effects of bilateral 6-hydroxydopamine-induced destruction of the dopamine nerve terminals in the ventral striatum (nucleus accumbens) or pharmacological blockade of dopamine receptors with haloperidol injected locally into this area were examined on the locomotor hyperactivity induced by systemic administration of the non-competitive NMDA receptor antagonist, MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d) cyclohepten-5,10-imine hydrogen maleate salt). The locomotor stimulation induced by two doses of MK-801 (0.15 and 0.3 mg/kg, i.p.) was not attenuated by 6-hydroxydopamine bilateral lesions to the ventral striatum, either 7 or 14 days after the operation. The same lesion however reduced the locomotor activation induced by 0.5 mg/kg d-amphetamine 14 days after surgery. Bilateral intra-accumbens injection of haloperidol at a dose (2.5 micrograms/side) that blocked d-amphetamine-induced hypermotility did not reduce the locomotor response to 0.3 mg/kg MK-801, while 5 micrograms/side haloperidol decreased the MK-801-induced locomotor stimulation. These results suggest that the locomotor response to MK-801 is dependent on an interaction between dopaminergic and excitatory amino acid transmission occurring postsynaptically rather than presynaptically in the ventral striatum.

The non-NMDA glutamate receptor antagonist GYKI 52466 counteracts locomotor stimulation and anticataleptic activity induced by the NMDA antagonist dizocilpine

The effects of the non-NMDA glutamate receptor antagonist GYKI 52466 (2.4 and 4.8 mg/kg, i.p.) on spontaneous locomotor activity and haloperidol-induced catalepsy (0.5 mg/kg, i.p.) were assessed in naive rats and in rats pretreated with the NMDA antagonist dizocilpine (0.08 mg/kg, i.p.). GYKI 52466 given alone did not alter locomotor activity and haloperidol-induced catalepsy, but significantly antagonized the dizocilpine-induced locomotor stimulation and counteracted the anti-cataleptic effects of dizocilpine on haloperidol-induced catalepsy. Thus blockade of non-NMDA glutamate receptors antagonized the behavioural stimulant effects of a NMDA receptor blockade.

Glutamate antagonists modify the motor stimulant actions of D1 and D2 agonists in reserpine-treated mice in complex ways that are not predictive of their interactions with the mixed D1/D2 agonist apomorphine

In 24h reserpine-treated mice, the locomotion induced by the D1 dopamine agonist SKF 38393 (30 mg/kg IP) was facilitated by the NMDA antagonists MK 801 (0.4 mg/kg IP), CPP (1 mg/kg IP), CGP 40116 (1 mg/kg IP) and HA 966 (2 mg/kg IP), and by the AMPA antagonist NBQX (0.2 mg/kg IP). By contrast, CPP, CGP 40116 and NBQX had no effect on, while MK 801 and HA 966 suppressed, the locomotion elicited by the selective D2 agonist RU 24213 (5 mg/kg SC). When these same doses of glutamate antagonists were tested against the locomotion induced by a threshold (0.025 mg/kg SC), intermediate (0.1 mg/kg SC) or large dose (0.5 mg/kg SC) of the mixed D1/D2 agonist apomorphine, CPP, CGP 40116 and HA 966 were found to have no significant effect, whilst MK 801 was strongly inhibitory and NBQX potentiated the response to 0.1 mg/kg apomorphine only. It is evident from these data that the behavioural interaction profiles between glutamate antagonists and dopamine agonists are complex and depend on the receptor selectivities of the drugs concerned. The manner of the interaction between these glutamate antagonists and selective D1 or D2 agonists, is not predictive of the way that blockade of glutamate transmission interferes with the actions of drugs which have combined D1 and D2 motor stimulant properties.

Are the disparate pharmacological profiles of competitive and un-competitive NMDA antagonists due to different baseline activities of distinct glutamatergic pathways? (Hypothesis)

Corticostriatal glutamatergic neurons impinging on the so-called "direct" striato-thalamic pathways appear to act as a driving force with respect to psychomotor functions, whereas corticostriatal glutamatergic neurons projecting to the "indirect" striato-thalamic route appear to mediate inhibition of the thalamus and thus act as a "brake" with respect to psychomotor functions. The GABAergic striatal projection neurons pertaining to the "direct" pathway mediating behavioural stimulation appear to be phasically activated, whereas GABAergic striatal projection neurons pertaining to the "indirect" pathway mediating suppression of behaviour must be assumed to display a high tonic activity. Such an organization could explain some of the behavioural differences between competitive and un-competitive NMDA antagonists, since the binding of competitive NMDA antagonists is inhibited by glutamate, whereas the binding of un-competitive NMDA antagonists is enhanced by the presence of NMDA receptor agonists, a phenomenon called use/agonist dependence.

Interaction between glutamatergic and dopaminergic tone in the nucleus accumbens of mice: evidence for a dual glutamatergic function with respect to psychomotor control

The rotation induced by a unilateral injection of the competitive NMDA receptor antagonist AP-5 was studied in mice with different tone in the central dopaminergic systems. AP-5 induced contralateral rotation in monoamine-depleted mice and in monoamine-depleted mice treated with a dopamine D-1 receptor agonist. In contrast, AP-5 induced predominantly ipsilateral rotation in monoamine-depleted mice treated with a mixed D-1/D-2 or a D-2 selective dopamine agonist and in mice with intact monoaminergic systems.