Glycine site antagonists abolish dopamine D2 but not D1 receptor mediated catalepsy in rats

Haloperidol-induced catalepsy as an animal model for parkinsonism: A systematic review of experimental studies

Several useful animal models for parkinsonism have been developed so far. Haloperidol-induced catalepsy is often used as a rodent model for the study of motor impairments observed in Parkinson's disease and related disorders and for the screening of potential antiparkinsonian compounds. The objective of this systematic review is to identify publications that used the haloperidol-induced catalepsy model for parkinsonism and to explore the methodological characteristics and the main questions addressed in these studies. A careful systematic search of the literature was carried out by accessing articles in three different databases: Web of Science, PubMed and SCOPUS. The selection and inclusion of studies were performed based on the abstract and, subsequently, on full-text analysis. Data extraction included the objective of the study, study design and outcome of interest. Two hundred and fifty-five articles were included in the review. Publication years ranged from 1981 to 2020. Most studies used the model to explore the effects of potential treatments for parkinsonism. Although the methodological characteristics used are quite varied, most studies used Wistar rats as experimental subjects. The most frequent dose of haloperidol used was 1.0 mg/kg, and the horizontal bar test was the most used to assess catalepsy. The data presented here provide a framework for an evidence-based approach to the design of preclinical research on parkinsonism using the haloperidol-induced catalepsy model. This model has been used routinely and successfully and is likely to continue to play a critical role in the ongoing search for the next generation of therapeutic interventions for parkinsonism.

Glutamate receptors as therapeutic targets for Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by motor symptoms including tremor and bradykinesia. The primary pathophysiology underlying PD is the degeneration of dopaminergic neurons of the substantia nigra pars compacta. Loss of these neurons causes pathological changes in neurotransmission in the basal ganglia motor circuit. The ability of ionotropic and metabotropic glutamate receptors to modulate neurotransmission throughout the basal ganglia suggests that these receptors may be targets for reversing the effects of altered neurotransmission in PD. Studies in animal models suggest that modulating the activity of these receptors may alleviate the primary motor symptoms of PD as well as side effects induced by dopamine replacement therapy. Moreover, glutamate receptor ligands may slow disease progression by delaying progressive dopamine neuron degeneration. Antagonists of NMDA receptors have shown promise in reversing motor symptoms, levodopa-induced dyskinesias, and neurodegeneration in preclinical PD models. The effects of drugs targeting AMPA receptors are more complex; while antagonists of these receptors exhibit utility in the treatment of levodopa-induced dyskinesias, AMPA receptor potentiators show promise for neuroprotection. Pharmacological modulation of metabotropic glutamate receptors (mGluRs) may hold even more promise for PD treatment due to the ability of mGluRs to fine-tune neurotransmission. Antagonists of mGluR5, as well as activators of group II mGluRs and mGluR4, have shown promise in several animal models of PD. These drugs reverse motor deficits in addition to providing protection against neurodegeneration. Glutamate receptors therefore represent exciting targets for the development of novel pharmacological therapies for PD.

Cataleptic activity of the denatured venom of the social wasp Agelaia vicina (Hymenoptera, Vespidae) in Rattus norvegicus (Rodentia, Muridae)

Catalepsy is a state of immobility, commonly experienced by patients with chronic use of many antiparkisonism and neuroleptic drugs. Recently, catatonia has been considered as an evolutionary-based fear response. The cataleptogenic effects of the low molecular weight compounds from the venom of the social wasp A. vicina (AdAv) were reported in rodents. Intracerebroventricular injections of AdAv in highest dose reduced the locomotor activity of Wistar rats in the open field and induced ataxia and catalepsy within 10 min. This effect could be observed up to 30 min after injections. Lower doses of denatured venom injected in brain ventricles also reduced the locomotor activity of the rats but did not induce catalepsy. The cataleptic effects of the intracerebrally administered AdAv were antagonized by the peripheral (intraperitoneal) pretreatment with theophylline and ketamine. Moreover, the central effects of the AdAv were compared to those elicited by the neuroleptic drug haloperidol (intraperitoneally administered), whose cataleptic effects were also antagonized by theophylline and ketamine. However, the association of haloperidol and denatured venom was reverted by theophylline but not by ketamine. These findings suggest that A. vicina venom may affect neural substrates involved with catalepsy in the central nervous system.

Kynurenines, Parkinson's disease and other neurodegenerative disorders: preclinical and clinical studies

The kynurenine pathway is the main pathway of tryptophan metabolism. L-kynurenine is a central compound of this pathway since it can change to the neuroprotective agent kynurenic acid or to the neurotoxic agent quinolinic acid. The break-up of these endogenous compounds' balance can be observable in many disorders. It can be occur in neurodegenerative disorders, such as Parkinson's disease, Huntington's and Alzheimer's disease, in stroke, in epilepsy, in multiple sclerosis, in amyotrophic lateral sclerosis, and in mental failures, such as schizophrenia and depression. The increase of QUIN concentration or decrease of KYNA concentration could enhance the symptoms of several diseases. According to numerous studies, lowered KYNA level was found in patients with Parkinson's disease. It can be also noticeable that KYNA-treatment prevents against the QUIN-induced lesion of rat striatum in animal experiments. Administrating of KYNA can be appear a promising therapeutic approach, but its use is limited because of its poorly transport across the blood-brain barrier. The solution may be the development of KYNA analogues (e.g. glucoseamine-kynurenic acid) which can pass across this barrier and disengaging in the brain, then KYNA can exert its neuroprotective effects binding at the excitatory glutamate receptors, in particular the NMDA receptors. Furthermore, it seems hopeful to use kynurenine derivatives (e.g. 4-chloro-kynurenine) or enzyme inhibitors (e.g. Ro-61-8048) to ensure an increased kynurenic acid concentration in the central nervous system.

Calcium-dependent subthreshold oscillations determine bursting activity induced by N-methyl-D-aspartate in rat subthalamic neurons in vitro

We used whole-cell patch recordings in current clamp to investigate the ionic dependence of burst firing induced by N-methyl-d-aspartate (NMDA) in neurons of the subthalamic nucleus (STN) in slices of rat brain. NMDA (20 microm) converted single-spike firing to burst firing in 87% of STN neurons tested. NMDA-induced bursting was blocked by AP5 (50 microm), and was not mimicked by the non-NMDA receptor agonist AMPA (0.6 microm). Tetrodotoxin (1 microm) converted bursts to oscillations of membrane potential, which were most robust when oscillations ranged between -50 and -70 mV. The NMDA bursts were blocked by an elevated extracellular concentration of Mg(2+), but superfusate containing no added Mg(2+) either reduced or increased burst firing, depending upon the amount of intracellular current injection. Block of K(+) conductances by apamin and tetraethylammonium prolonged burst duration, but iberiotoxin had no effect. NMDA-induced burst firing and membrane oscillations were completely blocked by superfusate containing no added Ca(2+), and they were significantly reduced when patch pipettes contained BAPTA. Selective antagonists for T-type (mibefradil, 10 microm), L-type (nifedipine, 3 microm), and N-type (omega-conotoxin GVIA, 1 micro m) Ca(2+) channels had no effect on NMDA burst firing. Superfusate containing a low concentration of Na(+) (20 mm) completely abolished NMDA-induced burst firing. Flufenamic acid (10 microm), which blocks current mediated by Ca(2+)-activated nonselective cation channels (I(CAN)), reversibly abolished NMDA-depended bursting. These results are consistent with the hypothesis that NMDA-induced burst firing in STN neurons requires activation of either an I(CAN) or a Na(+)-Ca(2+) exchanger.

Antagonists and agonists at the glycine site of the NMDA receptor for therapeutic interventions

For decades neuroreceptor research has focused on the development of NMDA glycine-site antagonists, after Johnson and Ascher found out in 1987 about the co-agonistic character of this achiral amino acid at the NMDA receptor. Contrary to the inhibitory glycine receptor (glycine(A)) the glycine binding site on the NMDA receptor (glycine(B)) is strychnine-insensitive. A great diversity of diseases showing a disturbed glutamate neurotransmission have been linked to the NMDA receptor. Glycine site antagonists have been investigated for acute diseases like stroke and head trauma as well as chronic ones like dementia and chronic pain.

NMDA and AMPA/kainate receptors are involved in the anticonvulsant activity of riluzole in DBA/2 mice

The anticonvulsant activity of riluzole against sound-induced seizures was studied in the DBA/2 mouse model. Riluzole (0.1-4 mg kg(-1), intraperitoneal (i.p.)) produced dose-dependent effects with ED(50) values for the suppression of tonic, clonic and wild running phases of 0.72, 1.38 and 2.71 mg kg(-1), respectively. Riluzole also protected DBA/2 mice from seizures induced by an intracerebroventricular (i.c.v.) injection of N-methyl-D-aspartate (NMDA) with ED(50) values of 3.03 and 5.0 mg kg(-1) for tonus and clonus, respectively. Pretreatment with glycine, an agonist to the glycine/NMDA receptors, shifted the dose-response effect of riluzole to the right (ED(50)=6.53 against tonus and 9.34 mg kg(-1) vs. clonus). Similarly, D-serine, an agonist at the glycine site, shifted the ED(50) of riluzole against the tonic component of audiogenic seizures from 0.72 to 1.97, and that against clonus from 1.38 to 2.77 mg kg(-1). Riluzole was also potent to prevent seizures induced by administration of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), an AMPA/kainate receptor agonist (ED(50)=1.80 and 3.35 mg kg(-1), against tonus and clonus, respectively). Pretreatment with aniracetam, a positive allosteric modulator of AMPA/kainate receptors, shifted the dose-response curve of riluzole to the right (ED(50)=1.78 against tonus and 2.58 mg kg(-1) vs. clonus). The data indicate that riluzole is an effective anticonvulsant drug in the genetic model of seizure-prone DBA/2 mice. Our findings suggest that the anticonvulsant properties of riluzole depend upon its interaction with neurotransmission mediated by both the glycine/NMDA and the AMPA/kainate receptor complex.

The neuroprotectant properties of glutamate antagonists and antiglutamatergic drugs

In the slowly progressive neurodegenerative disorders like Parkinson's disease and Alzheimer's disease very different neuronal populations undergo degenerative processes, although the cascades of cellular events leading to death are supposed to be similar. We suggest that the complex pattern of degeneration in Parkinson's disease depends on two processes, a 'primary neurodegeneration' that takes place in the striato-nigral dopamine neurons and a 'secondary degeneration', occurring in distant structures of the basal ganglia network. For the purpose of explaining the regionally different expression of 'primary neurodegeneration' in different diseases, we postulate that the origin of neurodegeneration is associated with the local release of a neurotransmitter. For Parkinson's disease this would mean that the metabolism of dopamine in the striatum, nucleus accumbens and presumably the pedunculopontine tegmental nucleus, together with one or more pathological factors contribute to the initial neurodegeneration. There are recent studies indicating that a transneuronal retrograde degeneration of the substantia nigra pars compacta neurons might be induced by a loss of function of dopaminergic synapses in the striatum. We have recently established an animal model of retrograde striato-nigral degeneration, where the assessment of markers for cellular stress is possible. In Parkinson's disease, several structures distal from the substantia nigra pars compacta undergo neuropathological changes, characterizing the 'secondary neurodegeneration. Our recent studies provide experimental evidence for a chronic cellular stress in these structures because of a relative or absolute glutamatergic overactivity due to the initial loss of dopaminergic innervation. Thus, a loss of dopamine transforms the basal ganglia to a 'destructive network'. Both processes, the 'primary' and 'secondary neurodegeneration', affecting each other, characterize the progress of chronic neurodegeneration. From this point of view, we would further like to develop strategies for symptomatic amendment. Excitatory amino acids seem to be involved not only in the secondary processes of neurodegeneration, but also in initiation of the 'primary degeneration' of the substantia nigra pars compacta. Therefore, a reduction of glutamatergic overactivity constitutes a promising neuroprotective strategy. Especially the new antagonists of the NMDA-receptors with high affinity to the NR2B subunit of the receptor are in focus of our interest, since they reveal a favourable profile of side effects, therefore providing a promising tool for neuroprotection.

Molecular and behavioral effects mediated by Gs-coupled adenosine A2a, but not serotonin 5-Ht4 or 5-Ht6 receptors following antipsychotic administration

Typical antipsychotic agents are potent antagonists of Gi-coupled dopamine D2 receptors, but their mechanisms of action following this initial blockade remain poorly understood. We hypothesized that in striatal neurons, interruption of this inhibitory dopamine D2 input would unmask endogenous striatal Gs-coupled receptors. An increase in cAMP levels generated by these unopposed receptors would then lead to the well-described behavioral and molecular effects of antipsychotic administration such as catalepsy and striatal c-fos and neurotensin gene transcription. We examined three striatal Gs-coupled receptor systems (serotonin 5-HT4, serotonin 5-HT6 and adenosine A2a) to assess their potential involvement in the mechanism of action of the typical antipsychotic haloperidol. Antagonists of each of these three receptor systems together with a 1 mg/kg dose of haloperidol were co-administered to Sprague-Dawley rats, and both the degree of catalepsy produced in the animals and the induction of striatal c-fos and neurotensin messenger RNAs were measured. Both the specific adenosine A2a antagonist 8-(3-chlorostyryl)-caffeine and the general adenosine antagonist theophylline reduced haloperidol-dependent induction of striatal neurotensin and c-fos messenger RNA. Administration of these agents also greatly reduced the degree of catalepsy induced by haloperidol. Antagonists of the 5-HT6 receptor failed to block the induction of striatal messenger RNAs, but the 5-HT6 antagonist clozapine (an important atypical antipsychotic agent in its own right) was a potent inhibitor of catalepsy. 5-HT4 agents were unable to alter haloperidol's effects on striatal messenger RNA levels or catalepsy. We conclude that the striatal Gs-coupled adenosine A2a receptor is an important mediator of the molecular and behavioral sequelae following haloperidol administration.

GlycineB antagonists and partial agonists in rodent models of Parkinson's disease--comparison with uncompetitive N-methyl-D-aspartate receptor antagonist

Antiparkinsonian-like activity of glutamate receptor antagonists (mostly of N-methyl-D-aspartate (NMDA) receptors) has been demonstrated in animals and for uncompetitive agents, also in humans. In the present study we investigated the potential antiparkinsonian-like activity of compounds acting at the glycine site of the NMDA receptor complex in three animal models of Parkinson's disease and compared them with the new uncompetitive NMDA receptor antagonist MRZ 2/579. Haloperidol-induced catalepsy was inhibited by the Merz glycine site antagonists MRZ 2/570, MRZ 2/571 and MRZ 2/576 but not by another antagonist L-701,324 or the glycine site partial agonists ACPC and D-CS. None of the tested glycine site antagonists or partial agonists increased locomotor activity or potentiated L-DOPA responses in reserpine and alpha-MT treated rats. In rats with a unilateral 6-OHDA medial forebrain bundle lesion neither glycine site antagonists nor partial agonists affected rotations on their own or enhanced the contralateral rotations induced by L-DOPA. In contrast, the uncompetitive NMDA receptor antagonist MRZ 2/579 was active in all antiparkinsonian tests used in this study. Based on the present data the therapeutic potential of the glycine site antagonists and partial agonists tested for the treatment of Parkinson's disease is rather doubtful. Uncompetitive NMDA receptor antagonists seem to possess a better profile as antiparkinsonian agents.

Ligands of the NMDA receptor-associated glycine recognition site and motor behavior

Motor behavior critically depends on glutamatergic functions in the basal ganglia (BG). The dorsal and ventral striatum--the main input structures of the BG--are involved in modulation of stereotyped sniffing behavior, locomotion, catalepsy and prepulse inhibition. The effects of the NMDA receptor have been well characterized in respect to motor behavior in the past. The function of the allosteric glycine site was however disregarded until now, because brain penetrating ligands were missing. The present study summarized the motor behavioral profile of several glycine site ligands (7-chlorokynurenate, ACEA 1021, MRZ-2/576, (+) HA-966, D-cycloserine and felbamate). It is shown that through blockade of the glycine site of the NMDA receptor a distinct behavioral profile can be obtained.

Contribution of the glycine site of NMDA receptors in rostral and intermediate-caudal parts of the striatum to the regulation of muscle tone in rats

The aim of the present study was to assess the contribution of the glycine site of NMDA receptors in the striatum to the regulation of muscle tone. Muscle tone was examined using a combined mechanoand electromyographic method, which measured simultaneously the muscle resistance (MMG) of the rat's hind foot to passive extension and flexion in the ankle joint and the electromyographic activity (EMG) of the antagonistic muscles of that joint: gastrocnemius and tibialis anterior. Muscle rigidity was induced by haloperidol (2.5 mg/kg i.p.). 5,7-dichlorokynurenic acid (5,7-DCKA), a selective glycine site antagonist, injected in doses of 2.5 and 4.5 microg/0.5 microl bilaterally, into the rostral region of the striatum, decreased both the haloperidol-induced muscle rigidity (MMG) and the enhanced electromyographic activity (EMG). 5,7-DCKA injected bilaterally in a dose of 4.5 microg/0.5 microl into the intermediate-caudal region of the striatum of rats not pretreated with haloperidol had no effect on the muscle tone. The present results suggest that blockade of the glycine site of NMDA receptors in the rostral part of the striatum may be mainly responsible for the antiparkinsonian action of this drug.

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.

ACEA 1021, a glycine site antagonist with minor psychotomimetic and amnestic effects in rats

Antagonists of the allosteric glycine site of the NMDA receptor complex have been suggested to be beneficial in the treatment of neurodegenerative disorders. However, unwanted side effects like psychomotor stimulation and amnesia must be expected. ACEA 1021 (5-nitro-6,7-dichloro-1,4-dihydroquioxaline-2,3dione) is one of the first high-selective glycine site antagonists which passes the blood-brain barrier and which has promising anticonvulsive and neuroprotective properties. In the present study the effects of ACEA 1021 (5, 7.5, 8, 10, 15 and 20 mg/kg i.p.) on sniffing stereotypy, locomotor activity, prepulse inhibition of the acoustic startle response, the anti-cataleptic properties and spatial learning were tested. Only 7.5 mg/kg ACEA 1021 induced a sniffing stereotypy which was antagonized by the partial glycine site agonist D-cycloserine (D-4-amino-3-isoxazolidinone). ACEA 1021 had neither an effect on motor behavior measured in the open field nor on the acoustic startle response in the prepulse inhibition paradigm nor on the acquisition of spatial learning in the 8-arm-radial maze. Anti-cataleptic properties of ACEA 1021 in dopamine D2 (haloperidol (4'fluoro-4-(1-(4-hydroxy-4-p-chlorophenyl-piperidino)-butyrophe non)) or D1 (SCH 23390 (7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzaze pin e hydrochloride)) receptor antagonist-pretreated rats were only minor. Thus, ACEA 1021 is a glycine site antagonist with minimal psychotomimetic side effects and with no amnesia properties. However, it has only minor anti-parkinsonian effects.

Reversal of parkinsonian symptoms in primates by antagonism of excitatory amino acid transmission: potential mechanisms of action

Parkinsonism is characterised by overactive glutamatergic transmission in the cortico-striatal and subthalamo-medial pallidal pathways. Local blockade of glutamatergic transmission in these pathways can alleviate parkinsonian symptoms. The effectiveness of the treatment, however, is often limited by the simultaneous appearance of unwanted side-effects. These side-effects, including ataxia and dissociative anaesthesia, are particularly problematic when N-methyl-D-aspartate (NMDA) antagonists are used. In an attempt to overcome these problems we have attempted to manipulate excitatory amino acid (EAA)-mediated neurotransmission indirectly by targeting the NMDA receptor associated modulatory sites. We review evidence which demonstrates that antagonists for both the NMDA associated glycine and polyamine sites can reverse parkinsonian symptoms when injected intra-cerebrally in both MPTP-treated and bilateral 6-OHDA lesioned marmosets without eliciting unwanted side-effects. We further review preliminary data which suggest that ifenprodil, a polyamine site antagonist, has striking anti-parkinsonian actions in the marmoset. Potential mechanisms of action underlying these effects are discussed in terms of NMDA receptor subtypes and the neuroanatomical locus of action. The anti-parkinsonian efficacy of intra-striatally administered EAA antagonists leads us to question the view of dopamine acting in the striatum as a simple neuromodulator.

Behavioural pharmacology of glutamate receptors in the basal ganglia

Glutamate receptors play a major role in the transmitter balance within the basal ganglia (BG). N-methyl-D-aspartate (NMDA) receptor stimulation within the striatum acts behaviourally depressant while intrastriatal as well as systemic administration of NMDA receptor-antagonists have rather stimulatory effects despite the different profiles of non-competitive-, competitive NMDA receptor- and glycine site-antagonists. In animal models of Parkinson's disease all these NMDA receptor antagonists counteract parkinsonian symptoms or act synergistically with L-3,4-dihydroxyphenylalanine (L-DOPA). The strong locomotion-inducing effect of the non-competitive NMDA receptor antagonists is partly, but not fully, mediated by a dopamine (DA) release in the nucleus accumbens. Manipulations at alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors produce poor behavioural effects. These, however, are different or even opposed to NMDA receptor mediated effects. Local infusions of AMPA receptor-antagonists into the BG output nuclei have an anti-parkinsonian effect but systemic injections are ineffective. These drugs even counteract the anti-parkinsonian effect of DA agonists and of non-competitive NMDA receptor antagonists as well as the DA releasing effects of the latter drugs. Only few data on the role of metabotropic receptors exist but the different receptor subtypes with different regional distribution represent a promising target for pharmacological interventions.

Neuroprotective effects of the strychnine-insensitive glycine site NMDA antagonist (R)-HA-966 in an experimental model of Parkinson's disease

The neuroprotective effects of (R)-HA-966 and (S)-HA-966 (3-amino-1-hydroxy-2-pyrrolidinone) were examined in an MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine)-induced animal model of Parkinson's disease. Systemic pretreatment of C57 black mice with the strychnine-insensitive glycine site antagonist, (R)-HA-966 (3-30 mg/kg, i.p.), dose-dependently attenuated MPTP-induced depletion of striatal dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC). Pretreatment with (R)-HA-966 also significantly protected the degeneration of tyrosine hydroxylase-positive neurons in the substantia nigra of mice treated with MPTP and alleviated the acute behavioral changes caused by the neurotoxin. In contrast, the other racemic form, (S)-HA-966, neither prevented the neurochemical depletions nor the neuronal injury caused by MPTP. These results indicate that excitatory mechanisms of neurodegeneration are involved in the pathophysiology of Parkinson's disease, and that strychnine-insensitive glycine site NMDA antagonists may serve as dopaminoprotective agents which intervene in the progressive neurodegeneration in Parkinson's disease.

Antiparkinsonian and other motor effects of flupirtine alone and in combination with dopaminergic drugs

In this study we attempted to specify the behavioural profile of the analgesic flupirtine (1, 10 and 20 mg/kg p.o.) in the rat with respect to (i) its antiparkinsonian potential alone and as an adjunct to L-dihydroxyphenylalanine (L-DOPA) in the haloperidol-induced catalepsy (0.5 mg/kg haloperidol i.p.), (ii) locomotion and exploratory behaviour in the open field with holeboard, and (iii) possible psychomotor stimulating effects in the experimental chamber. In the two latter tests, behaviour was additionally challenged by D-amphetamine (2 mg/kg i.p.). In the catalepsy tests (horizontal bar, podium, grid) flupirtine alone was anticataleptic at doses of 10 and 20 mg/kg p.o., and the antiparkinsonian potential of a subthreshold dose of L-DOPA (50 mg/kg p.o.) was potentiated by 1 and 10 mg/kg p.o. flupirtine. On spontaneous forward locomotion in the open field with holeboard, flupirtine (1 and 10 mg/kg p.o.) had no marked effect but increased the frequency and duration of head dips, indicative for augmenting exploratory behaviour. Spontaneous rearing was reduced and D-amphetamine-induced rearing was enhanced by 1 mg/kg p.o. flupirtine. Grooming was reduced by 1 and 10 mg/kg p.o. flupirtine. In contrast, turning and grooming behaviour (spontaneous as well as D-amphetamine-induced) was not markedly influenced by flupirtine in the experimental chamber. Sniffing was increased in this test by 1 mg/kg p.o. flupirtine but not by the higher dose. Flupirtine is highly effective in antagonising neuroleptic-induced catalepsy as well as in potentiating L-DOPA treatment in the rat, suggesting it is a prospective new candidate for the therapy of Parkinson's disease.

1S,3R-ACPD has cataleptogenic effects and reverses MK-801-, and less pronounced, D,L-amphetamine-induced locomotion

The purpose of this study was to examine the motor effects of (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), an agonist at metabotropic glutamate receptors, its interaction with dizocilpine (MK-801), a NMDA receptor antagonist, and with D,L-amphetamine, an indirect dopamine receptor agonist. 1S,3R-ACPD (20, 30, 40, 80 micrograms) evoked prominent locomotor and exploratory deficits in an open-field hole-board test and a moderate akinesia and rigidity in a catalepsy test (30, 40, 80 micrograms). MK-801 (0.08, 0.16, 0.32 mg/kg i.p.) as well as D,L-amphetamine (1.0, 2.0, 3.0 mg/kg i.p.) potently reversed 1S,3R-ACPD-induced (80 micrograms) catalepsy. MK-801 and D,L-amphetamine, administered alone, induced motor stimulation. 1S,3R-ACPD (80 micrograms) reversed the effects of the two lower doses of MK-801. 1S,3R-ACPD reversed D,L-amphetamine-induced motor stimulation to a minor extent than that of MK-801. Thus motor deficits induced by 1S,3R-ACPD were reversed by both, NMDA receptor blockade and dopamine receptor activation. 1S,3R-ACPD reversed motor stimulation, induced by NMDA receptor blockade and, however less pronounced, that by dopamine receptor activation.

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.

Behavioral and neurochemical actions of the strychnine-insensitive glycine receptor antagonist, 7-chlorokynurenate, in rats

The present study investigated if blockade of the modulatory glycine receptor of the NMDA receptor complex influences the expression of behavior (sniffing stereotypy and locomotion) and dopamine metabolism in rats as it has been shown for NMDA receptor antagonists. The glycine receptor antagonist, 7-chlorokynurenate (7-chloro-4-hydroxyquinoline-2-carboxylic acid), induced a dose-dependent sniffing stereotypy but had no effect on locomotion when it was given i.c.v. The glycine receptor agonist, D-cycloserine (D-4-amino-3-isoxazolidinone), antagonized the sniffing stereotypy. 7-Chlorokynurenate had no influence on dopamine metabolism in the striatum and the nucleus accumbens, but moderately decreased the metabolism in the prefrontal cortex. Comparison of behavioral and neurochemical outcomes suggests that the failure to induce locomotion correlates with the unchanged dopamine metabolism in the basal ganglia, while sniffing stereotypy does not. These results show that blockade of the glycine receptor of the NMDA receptor complex induces a behavioral and neurochemical profile similar to that of competitive NMDA receptor antagonists.

Felbamate, an anti-convulsive drug, has anti-parkinsonian potential in rats

The potency of felbamate, an anti-convulsive drug, to influence dopamine D1 (SCH 23390) and D2 (haloperidol) receptor-mediated catalepsy (akinesia and bradykinesia) was studied in rats. In the catalepsy test, felbamate antagonized dopamine D2 receptor- but not D1 receptor-induced akinesia. Bradykinesia in the open field was never influenced. The results demonstrate that felbamate has similar anti-parkinsonian potential as glycine site antagonists blocking the N-methyl-D-aspartate (NMDA) receptor complex.