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

AV-101, a Pro-Drug Antagonist at the NMDA Receptor Glycine Site, Reduces L-Dopa Induced Dyskinesias in MPTP Monkeys

N-methyl-D-aspartate (NMDA) receptors have been implicated in L-Dopa-induced dyskinesias (LID) in Parkinson's disease patients, but the use of antagonists that directly inhibit this receptor is associated with severe side effects. L-4-chlorokynurenine (4-Cl-KYN or AV-101) is a pro-drug of 7-chlorokynurenic acid (7-Cl-KYNA), a potent and specific antagonist of the glycine (GlyB) co-agonist site of NMDA receptors. The 7-Cl-KYNA has limited ability to cross the blood-brain barrier, whereas AV-101 readily accesses the brain. We investigated if AV-101 reduces LID in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys while maintaining the antiparkinsonian activity of L-Dopa. A first pilot study using three dyskinetic MPTP monkeys showed that acute AV-101 treatment (250 and 450 mg/kg) reduced LID and maintained the antiparkinsonian activity of L-Dopa. The main study using six additional dyskinetic MPTP monkeys showed that repeated AV-101 treatment (250 mg/kg, b.i.d. for 4 consecutive days) maintained their L-Dopa antiparkinsonian response. We measured significantly less LID when AV-101 was combined with L-Dopa treatment. AV-101 alone or with L-Dopa had no non-motor adverse effects in MPTP monkeys. Our study showed antidyskinetic activity of AV-101 in MPTP monkeys was comparable to amantadine tested previously in our laboratory in this model. We observed no adverse effects with AV-101, which is an improvement over amantadine, with its known side effects.

Ameliorative effects of histamine on 7-chlorokynurenic acid-induced spatial memory deficits in rats

RATIONALE

Histamine plays an important role in modulating acquisition and retention in learning and memory process in experimental animals.

OBJECTIVES

We examined the effects of polyamine and histamine on the N-methyl- d-aspartate (NMDA) receptor glycine site antagonist 7-chlorokynurenic acid-induced spatial memory deficits in radial maze performance in rats.

METHOD

Effects of histamine (0.5 or 1 nmol/site intracerebroventricularly), spermidine (1 nmol/site, intracerebroventricularly) and spermine (1 nmol/site, intracerebroventricularly) on spatial memory deficit in 9-week-old-male Wistar rats were observed. Both reference and working memory errors occurred in radial maze performance in rats, following intracerebroventricular injection of 7-chlorokynurenic acid (10 nmol/site).

RESULTS

Spermidine (1 nmol/site, intracerebroventricularly) or spermine (1 nmol/site, intracerebroventricularly) antagonized 7-chlorokynurenic acid-induced deficits on working memory but not on reference memory errors. Intracerebroventricular histamine (0.5 or 1 nmol/site) or thioperamide (100 nmol/site) also ameliorated 7-chlorokynurenic acid-induced working memory deficits. To determine whether the effects of histamine involve histamine receptors, the effects of some methylhistamines were examined. The effects of R-alpha-methylhistamine on radial maze performance were mimicked by histamine. N(alpha)-methylhistamine had no effect on 7-chlorokynurenic acid-induced memory deficits, whereas 1-methylhistamine, but not 3-methylhistamine reversed 7-chlorokynurenic acid-induced working memory deficits.

CONCLUSION

These results suggest that the amelioration of 7-chlorokynurenic acid-induced working memory deficits by histamine may involve a direct action of histamine at the polyamine sites on NMDA receptors.

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.

The role of striatal glutamate receptors in models of Parkinson's disease

The aim of the study was to examine the effect of antagonists of the NMDA receptor on the parkinsonian-like muscle rigidity in rats. Reserpine and haloperidol increased the muscle resistance of the hind foot to passive movements, as well as the reflex electromyographic (EMG) activity in the gastroenemius and tibialis anterior muscles. MK-801 (0.32-1.28 mg/kg s.c.), an uncompetitive antagonist of the NMDA receptor, and L-701,324 (5-40 mg/kg i.p.), an antagonist of the glycine site, reduced the muscle tone and the reflex EMG activity enhanced by reserpine or haloperidol. AP-5 (2 and 5 micrograms/0.5 microliter), a competitive antagonist of the NMDA receptor, and 5,7-dichlorokynurenic acid (1.0-4.5 micrograms/0.5 microliter), the glycine site antagonist injected bilaterally into the rostral striatum, inhibited the muscle rigidity induced by haloperidol. In contrast, AP-5, injected alone bilaterally into the intermediate-caudal striatum induced muscle rigidity. The present results suggest that: (1) the inhibitory effect of the NMDA receptor antagonists on the parkinsonian-like muscle rigidity depends, at least partly, on their action on the rostral striatum; (2) the blockade of NMDA receptors in the intermediate-caudal striatum may reduce the beneficial impact of these compounds.

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.

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.

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.

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.