Effects of MPEP on avoidance learning in rats

In recent years metabotropic glutamate receptors (mGluRs) have received considerable attention as a potential target for psychotropic drugs, but their influence on learning and memory is still unclear. The aim of the present study was to examine whether intraperitoneal (i.p.) administration of selective mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyrydine (MPEP), injected prior to, immediately after or 30 min after training, affects acquisition and/or retrieval of the inhibitory step-down and active shuttle-box avoidance in rats. Our results indicate that 5 or 10 mg/kg i.p. MPEP in all tested groups impaired memory consolidation of step-down training without affecting acquisition and had no effect on learning and retention in shuttle-box trained rats. The data are in agreement with the statement that mGluR5s may contribute very little and task-dependently to the actual acquisition of new information, but memory formation, appears to require mGluR5s through modulation of consolidation and/or recall.

The influence of group III metabotropic glutamate receptor stimulation by (1S,3R,4S)-1-aminocyclo-pentane-1,3,4-tricarboxylic acid on the parkinsonian-like akinesia and striatal proenkephalin and prodynorphin mRNA expression in rats

Group III metabotropic glutamate receptors (mGluRs) are widely distributed in the basal ganglia, especially on the terminals of pathways which seem to be overactive in Parkinson's disease. The aim of the present study was to determine whether (1S,3R,4S)-1-aminocyclo-pentane-1,3,4-tricarboxylic acid (ACPT-1), an agonist of group III mGluRs, injected bilaterally into the globus pallidus (GP), striatum or substantia nigra pars reticulata (SNr), can attenuate the haloperidol-induced catalepsy in rats, and whether that effect was related to modulation of proenkephalin (PENK) or prodynorphin (PDYN) mRNA expression in the striatum. Administration of ACPT-1 (0.05-1.6 microg/0.5 microl/side) caused a dose-and-structure-dependent decrease in the haloperidol (0.5 mg/kg i.p. or 1.5 mg/kg s.c.)-induced catalepsy whose order was as follows: GP>striatum>SNr. ACPT-1, given alone to any of those structures, induced no catalepsy in rats. Haloperidol (3 x 1.5 mg/kg s.c.) significantly increased PENK mRNA expression in the striatum, while PDYN mRNA levels were not affected by that treatment. ACPT-1 (3 x 1.6 microg/0.5 microl/side) injected into the striatum significantly attenuated the haloperidol-increased PENK mRNA expression, whereas administration of that compound into the GP or SNr did not influence the haloperidol-increased striatal PENK mRNA levels. Our results demonstrate that stimulation of group III mGluRs in the striatum, GP or SNr exerts antiparkinsonian-like effects in rats. The anticataleptic effect of intrastriatally injected ACPT-1 seems to correlate with diminished striatal PENK mRNA expression. However, since the anticataleptic effect produced by intrapallidal and intranigral injection of ACPT-1 is not related to a simultaneous decrease in striatal PENK mRNA levels, it is likely that a decrease in enkephalin biosynthesis is not a necessary condition to obtain an antiparkinsonian effect.

Degeneration of dopaminergic mesocortical neurons and activation of compensatory processes induced by a long-term paraquat administration in rats: implications for Parkinson's disease

A deficiency of the dopaminergic transmission in the mesocortical system has been suggested to contribute to cognitive disturbances in Parkinson's disease. Therefore, the aim of the present study was to examine whether the long-term administration of a commonly used herbicide, paraquat, which has already been found to induce a slowly progressing degeneration of the nigrostriatal neurons, influences mesocortical dopaminergic neurons in rats. Paraquat at a dose of 10 mg/kg i.p. was injected either acutely or once a week for 4, 8, 12 and 24 weeks. Acute treatment with this pesticide increased the level of homovanillic acid (HVA) and HVA/dopamine ratio in the prefrontal cortex. After 8 weeks of administration paraquat increased the number of stereologically counted tyrosine hydroxylase-immunoreactive (TH-ir) neurons and their staining intensity in the ventral tegmental area (VTA), which is a source of the mesocortical dopaminergic projection. At the same time, few TH-ir neurons appeared in different regions of the cerebral cortex: in the frontal, cingulate, retrosplenial and parietal cortices. Chronic paraquat administration did not influence the level of dopamine in the prefrontal cortex but increased the levels of its metabolites: 3,4-dihydroxyphenylacetic acid (after 8-12 weeks), HVA (after 4 and 12 weeks) and HVA/dopamine ratio (4 weeks). After 24 weeks this pesticide reduced the number of TH-ir neurons in the VTA by 42% and of the Nissl-stained neurons by 26%, and induced shrinkage of this structure by ca. 25%. Moreover, TH-ir neurons in the cortex were no more visible after such a long period of administration and levels of dopamine metabolites returned to control values. The present results suggest that the long-term paraquat administration destroys dopaminergic neurons of the VTA. However, compensatory activation of the VTA neurons and cortex overcomes progressing degeneration and maintains cortical dopaminergic transmission.

An influence of ligands of metabotropic glutamate receptor subtypes on parkinsonian-like symptoms and the striatopallidal pathway in rats

Several data indicate that inhibition of glutamatergic transmission may be important to alleviate of parkinsonian symptoms. Therefore, the aim of the present paper is to review recent studies on the search for putative antiparkinsonian-like effects of mGluR ligands and their brain targets. In order to inhibit glutamatergic transmission, the group I mGluRs (mGluR1 and mGluR5) were blocked, and group II (mGluR2/3) or III (mGluR4/7/8) mGluRs were activated. Systemic or intrastriatal administration of group I mGluR antagonists (mGluR5 - MPEP, MTEP; mGluR1 - AIDA) was found to inhibit parkinsonian-like symptoms (catalepsy, muscle rigidity) in rats. MPEP administered systemically and mGluR1 antagonists (AIDA, CPCCOEt, LY367385) injected intrastriatally reversed also the haloperidol-increased proenkephalin (PENK) mRNA expression in the striatopallidal pathway. Similarly, ACPT-1, a group III mGluR agonist, administered into the striatum, globus pallidus or substantia nigra inhibited the catalepsy. Intrastriatal injection of this compound reduced the striatal PENK expression induced by haloperidol. In contrast, a group II mGluR agonist (2R,4R-APDC) administered intrastriatally reduced neither PENK expression nor the above-mentioned parkinsonian-like symptoms. Moreover, a mixed mGluR8 agonist/AMPA antagonist, (R,S)-3,4-DCPG, administered systemically evoked catalepsy and enhanced both the catalepsy and PENK expression induced by haloperidol. The results reviewed in this article seem to indicate that group I mGluR antagonists or some agonists of group III may possess antiparkinsonian properties, and point at the striatopallidal pathway as a potential target of therapeutic intervention.

2,9-Dimethyl-β-carbolinium, a neurotoxin occurring in human brain, is a potent inducer of apoptosis as 1-methyl-4-phenylpyridinium

The causes of neurodegeneration are not well understood. However, the role of environmental and endogenous toxins is receiving much attention. In this study, we compared the synthetic neurotoxin 1-methyl-4-phenyl-pyridinium with beta-carbolines occurring in human brain. Methylation of both nitrogens is necessary to convert a beta-carboline into a potent inhibitor of mitochondrial complex I. The respective beta-carboline, 2,9-dimethyl-beta-carbolinium ion is neurotoxic in rats. To investigate the underlying mechanisms, we incubated mouse neuroblastoma 2A cells with 2,9-dimethyl-beta-carbolinium ion, and compared the findings with effects of norharman, the precursor beta-carboline of methylated derivatives, and with 1-methyl-4-phenyl-pyridinium. 2,9-Dimethyl-beta-carbolinium ion caused a significant increase of reactive oxygen species (higher efficiency than 1-methyl-4-phenyl-pyridinium) and of mitochondrial membrane potential within the first minutes. After 60 min, the membrane potential dissipated. Concomitantly, the levels of glutathione increased in 2,9-dimethyl-beta-carbolinium ion but not in 1-methyl-4-phenyl-pyridinium treated cells. After 24 h effector caspases 3 and 7 were activated and the number of apoptotic cells increased as revealed by fluorescence-activated cell sorting cytometry. When incubated longer (48 h), cells underwent late apoptosis/secondary necrosis as shown by fluorescence-activated cell sorting analysis and confirmed qualitatively by an electron microscopy study. The effects of 2,9-dimethyl-beta-carbolinium ion on apoptotic changes were similar to those induced by 1-methyl-4-phenyl-pyridinium(,) while norharman showed only a weak potency at the very high doses. To investigate whether 2,9-dimethyl-beta-carbolinium ion is neurotoxic under in vivo conditions and whether only dopaminergic neurones are affected we conducted a dose-response study. Three weeks after injection of 2,9-dimethyl-beta-carbolinium ion in the substantia nigra we found a dose-dependent decrease of dopamine and its metabolites in the striatum of rats. The levels of 5-hydroxytryptamine were diminished although the decrease was less. The levels of noradrenaline increased after some doses. The findings strongly suggest an important role of endogenous beta-carbolines in neurodegeneration with apoptosis as the predominant mechanism.

A slowly developing dysfunction of dopaminergic nigrostriatal neurons induced by long-term paraquat administration in rats: an animal model of preclinical stages of Parkinson's disease?

The aim of the present study was to examine the influence of the long-term paraquat administration on the dopaminergic nigrostriatal system in rats. Paraquat was injected at a dose of 10 mg/kg i.p. for 4-24 weeks. We found that this pesticide reduced the number of tyrosine hydroxylase-immunoreactive neurons of the substantia nigra; after the 4-week treatment the reduction (17%, nonsignificant) was confined to the rostrocentral region of this structure but, after 24 weeks, had spread along its whole length and was approximately 37%. Moreover, it induced a biphasic effect on dopaminergic transmission. First, levels of dopamine, its metabolites and turnover were elevated (4-8 weeks) in the caudate-putamen, then all these parameters returned to control values (12 weeks) and dropped by 25-30% after 24 weeks. The binding of [3H]GBR 12,935 to dopamine transporter in the caudate-putamen was decreased after 4-8 weeks, then returned to control values after 12 weeks but was again decreased after 24 weeks. Twenty-four-week paraquat administration also decreased the level of tyrosine hydroxylase (Western blot) in the caudate-putamen. In addition, paraquat activated serotonin and noradrenaline transmission during the first 12 weeks of treatment but no decreases in levels of these neurotransmitters were observed after 24 weeks. The above results seem to suggest that long-term paraquat administration produces a slowly progressing degeneration of nigrostriatal neurons, leading to delayed deficits in dopaminergic transmission, which may resemble early, presymptomatic, stages of Parkinson's disease.

MTEP, a new selective antagonist of the metabotropic glutamate receptor subtype 5 (mGluR5), produces antiparkinsonian-like effects in rats

The aim of the present study was to examine a potential antiparkinsonian-like action of 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP), a new non-competitive antagonist of mGluR5, in the rat models. This compound has affinity for mGluR5 in a nanomolar concentration range and seems to be superior to the earlier known antagonists in terms of its specificity and bioavailability. Catalepsy and muscle rigidity induced by haloperidol administered at doses of 0.5 and 1 mg/kg were regarded as models of parkinsonian akinesia and muscle rigidity, respectively. MTEP at doses between 0.5 and 3 mg/kg i.p. decreased the haloperidol-induced muscle rigidity measured as an increased muscle resistance of the rat's hind leg in response to passive extension and flexion at the ankle joint. The strongest and the longest effect was observed after the dose of 1 mg/kg. MTEP (0.5-3 mg/kg i.p.) also reduced the haloperidol-induced increase in electromyographic (EMG) activity recorded in the gastrocnemius and tibialis anterior muscles. MTEP (3 and 5 mg/kg i.p.) inhibited the catalepsy induced by haloperidol. The present study confirms earlier suggestions that the antagonists of mGluR5 may possess antiparkinsonian properties. However, selective mGluR5 antagonists may be more effective in inhibiting parkinsonian muscle rigidity than parkinsonian akinesia.

Opposite influence of MPEP, an mGluR5 antagonist, on the locomotor hyperactivity induced by PCP and amphetamine

Potential antipsychotic effects of a selective non-competitive antagonist of metabotropic glutamate receptor 5 (mGluR5), 2-methyl-6-phenylethynylpyridine (MPEP), was examined in two commonly used screening tests: (1) the hyperactivity induced by an NMDA receptor antagonist phencyclidine (PCP), and (2) the hyperactivity induced by an indirect dopamine agonist, D-amphetamine. PCP was administered at a dose of 2.5 mg/kg s.c. and D-amphetamine was given at a dose of 1 mg/kg s.c. MPEP (5 mg/kg i.p.) significantly enhanced the locomotor activity increased by PCP, but inhibited amphetamine-induced hyperactivity. The opposite effect of MPEP in the two above-mentioned models questions significance of the blockade of mGluR5 receptors to antipsychotic effects.

The role of striatal metabotropic glutamate receptors in degeneration of dopamine neurons: review article

Degeneration of dopaminergic nigrostriatal neurons is a primary cause of Parkinson's disease. Oxidative stress, excitotoxicity and mitochondrial failure are thought to be key mechanisms responsible for degeneration of dopaminergic cells. We found that the selective antagonist of the mGluR5 subtype MPEP in a dose of 5 mg/kg diminished basal and veratridine (100 microM)-stimulated dopamine release in rat striatum in an in vivo model of microdialysis. In contrast, MPEP given intrastriatally in a high concentration (500 microM) enhanced the striatal extracellular concentration of dopamine. DCG-IV (100 microM), a non-selective agonist of group II mGluRs, inhibited the veratridine-stimulated striatal dopamine release. In an animal model of neuroxicity in vivo, methamphetamine (5 x 10 mg/kg, injected at 2 h intervals) produced deficits in the striatal content of dopamine and its metabolites DOPAC and HVA 72 h after the treatment. MPEP (5 x 5 mg/kg) given before each methamphetamine injection reversed the decrease in the striatal content of dopamine and diminished the methamphetamine-induced dopamine outflow from nigrostriatal terminals. It is concluded that the MPEP-produced blockade of mGluR5 situated on dopaminergic cells, or the suppression of glutamate release in the subthalamic nucleus or substantia nigra pars reticulata may directly and indirectly cause a decrease in striatal dopamine release. However, inhibitory effect of DCG-IV on dopamine release can be induced by attenuation of excitatory input from corticostriatal terminals by activation of mGluR2/3. Regulation of dopamine carriers by MPEP, an antagonist of group I mGluRs may be responsible for the reversal of toxicity induced by methamphetamine.

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

The primary cause of Parkinson's disease is a loss of dopamine in the corpus striatum. It has been postulated that this effect leads to disinhibition of the striopallidal pathway and secondarily, to a functional shift towards glutamatergic stimulation. The aim of the present study was to find out whether inhibition of glutamatergic transmission at a level of metabotropic glutamate receptors (mGluRs) in the striatum may alleviate parkinsonian-like symptoms in rats. The non-competitive antagonist of receptor subtype 5 (mGluR5), MPEP (1.0-10 mg/kg ip), or the agonist of group II mGluRs, LY354,740 (5-10 mg/kg ip), reduced haloperidol-induced muscle rigidity and catalepsy. Intrastriatal injections of the mGluR1 antagonist, (RS) AIDA (7.5-15 microg/0.5 microl), but not of the agonist of group II mGluRs, 2R,4R-APDC (7.5-15 microg/0.5 microl), inhibited the muscle rigidity induced by haloperidol. In order to search for an influence of mGluRs on the striopallidal pathway, the effect of MPEP or of the agonist of group II mGluRs, DCG-IV, on the proenkephalin (PENK) mRNA expression in the dorso-lateral striatum was examined by an in situ hybridization. Repeated MPEP (6 x 10 mg/kg ip) administration did not influence PENK expression in naïve rats, but diminished that increased by haloperidol. In contrast, repeated DCG-IV (3 x 1 nmol/4 microl icv) injections enhanced both the control and the haloperidol-increased levels of PENK expression. The obtained results suggest that blockade of group I mGluRs, or stimulation of group II mGluRs may be important to ameliorate parkinsonian symptoms. Striatal mGluRs may contribute to at least some of these effects.

Neuroprotective action of MPEP, a selective mGluR5 antagonist, in methamphetamine-induced dopaminergic neurotoxicity is associated with a decrease in dopamine outflow and inhibition of hyperthermia in rats

The aim of this study was to examine the role of metabotropic glutamate receptor 5 (mGluR5) in the toxic action of methamphetamine on dopaminergic neurones in rats. Methamphetamine (10 mg/kg sc), administered five times, reduced the levels of dopamine and its metabolites in striatal tissue when measured 72 h after the last injection. A selective antagonist of mGluR5, 2-methyl-6-(phenylethynyl)pyridine (MPEP; 5 mg/kg ip), when administered five times immediately before each methamphetamine injection reversed the above-mentioned methamphetamine effects. A single MPEP (5 mg/kg ip) injection reduced the basal extracellular dopamine level in the striatum, as well as dopamine release stimulated either by methamphetamine (10 mg/kg sc) or by intrastriatally administered veratridine (100 microM). Moreover, it transiently diminished the methamphetamine (10 mg/kg sc)-induced hyperthermia and reduced basal body temperature. MPEP administered into the striatum at high concentrations (500 microM) increased extracellular dopamine levels, while lower concentrations (50-100 microM) were devoid of any effect. The results of this study suggest that the blockade of mGluR5 by MPEP may protect dopaminergic neurones against methamphetamine-induced toxicity. Neuroprotection rendered by MPEP may be associated with the reduction of the methamphetamine-induced dopamine efflux in the striatum due to the blockade of extrastriatal mGluR5, and with a decrease in hyperthermia.

The role of metabotropic glutamate receptors in regulation of striatal proenkephalin expression: implications for the therapy of Parkinson′s disease

Overactivity of the striatopallidal pathway, associated with an enhancement of enkephalin expression, has been suggested to contribute to the development of parkinsonian symptoms. The aim of the present study was to examine whether the blockade of group I metabotropic glutamate receptors: subtypes 1 and 5 (mGluR1/5), or stimulation of group II: subtypes 2 and 3 (mGluR2/3) may normalize enkephalin expression in the striatopallidal pathway in an animal model of parkinsonism. The proenkephalin mRNA level measured by in situ hybridization in the striatum was increased by pretreatments with haloperidol (1.5 mg/kg s.c., three times, 3 h apart). Triple (3 h apart), bilateral, intrastriatal administration of selective antagonists of mGluR1: (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid (3 x 5 microg/0.5 microl) or 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate (3 x 2.5 microg/0.5 microl), reversed the haloperidol-induced increases in proenkephalin mRNA levels in the rostral and central regions of the striatum. Similarly, repeated (6 times, 1.5 h apart), systemic injections of an antagonist of mGluR5, 2-methyl-6-(phenylethynyl)pyridine (6 x 10 mg/kg i.p.) counteracted an increase in the striatal proenkephalin mRNA expression elicited by haloperidol. None of the abovementioned antagonists of mGluR1 and mGluR5 per se influenced the proenkephalin expression. Differential effects were induced by agonists of the group II mGluRs, viz. (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine administered intraventricularly (3 times at 0.1-0.2 microg/4 microl, 3 h apart) increased both the normal and haloperidol-increased proenkephalin mRNA level, whereas (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate injected intrastriatally (3 times at 15 microg/0.5 microl, 3 h apart) was ineffective. The present study indicates that the blockade of striatal glutamate receptors belonging to the group I (mGluR1 and mGluR5) but not stimulation of the group II mGluRs may normalize the function of the striatopallidal pathway in an animal model of parkinsonism, which may be important for future antiparkinsonian therapy in humans.

Blockade of the metabotropic glutamate receptor subtype 5 (mGluR5) produces antiparkinsonian-like effects in rats

The aim of the present study was to examine a potential beneficial effect of the blockade of metabotropic glutamate receptor subtype 5 (mGluR5) by the selective non-competitive antagonist, 2-methyl-6-(phenylethynyl)pyridine (MPEP), in models of parkinsonian symptoms in rats. Haloperidol, 0.25, 0.5 and 1mg/kg ip, was used to induce hypolocomotion, catalepsy and muscle rigidity, respectively. The locomotor activity was estimated by an open-field test, the catalepsy -- by a 9-cm cork test. The muscle rigidity was measured as an increased resistance of a hind leg to passive extension and flexion at the ankle joint. Additionally, increases in the electromyographic activity were recorded in the gastrocnemius and tibialis anterior muscles. MPEP (1.0-10mg/kg ip) inhibited the muscle rigidity, electromyographic activity, hypolocomotion and catalepsy induced by haloperidol. MPEP administered alone (5mg/kg ip) did not induce catalepsy, nor did it influence the muscle tone or locomotor activity in rats. The present results suggest that blockade of mGluR5 receptors may be important to amelioration of both parkinsonian akinesia and muscle rigidity.

Decline in motor functions in aging is related to the loss of NMDA receptors

The aim of the study was to assess the contribution of central dopaminergic and glutamatergic systems to the age-dependent loss of motor functions in rats. Rats of three age groups were compared: young (3-5-month-old), middle-aged (20-21-month-old) and old (29-31-month-old). The obtained results showed an age-dependent decline in the electromyographic (EMG) resting and reflex activities in the gastrocnemius and tibialis anterior muscles, as well as in the T-maze performance. Although these disturbances were accompanied with significant age-dependent decreases in the binding to NMDA, AMPA and dopamine D2 receptors, and a decline in the number of nigral dopamine neurons, they were significantly correlated with the loss of the binding to NMDA receptors only. The reduction in T-maze performance with aging was additionally correlated with a decrease in motor functions (EMG activity). The study suggests a crucial role of the loss of NMDA receptors in age-dependent motor disabilities, as well as in disturbances measured in the T-maze.

The role of metabotropic glutamate receptor (mGluR) ligands in parkinsonian muscle rigidity

It has been shown that the primary striatal dopaminergic hypofunction which is at the origin of Parkinson's disease, results in a secondary hyperactivity of glutamatergic neurotransmission. In the search for a therapy of Parkinson's disease, ionotropic, mainly NMDA, receptor antagonists were found to have moderately beneficial, yet also some undesirable side-effects. Therefore the present study was aimed at determining whether some metabotropic glutamate receptor (mGluR) ligands may have antiparkinsonian effects in the haloperidol-induced muscle rigidity. To this end three mGluR ligands were used: the potent and selective mGluR I antagonist (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA), the mixed group II agonist/ group I antagonist (S)-4-carboxy-3-hydroxyphenyl-glycine ((S)-4-C3HPG), and the potent group II agonist (+)-2-aminobicyclo[3.1.0.]hexane-2,6,-dicarboxylic acid (LY354740). Only LY354740 penetrated the brain from the periphery; for this reason other drugs were injected bilaterally into the rostral striatum or nucleus accumbens. The muscle tone was recorded by a mechanomyographic/electromyographic (MMG/EMG) method which measured the resistance of a rat's hind foot and the EMG reflex response of its muscles to passive movements. (S)-4C3HPG (5 and 15 microg/0.5 microl) and LY354740 (5 and 10mg/kg i.p.) diminished the muscle rigidity induced by haloperidol (1 mg/kg i.p.). AIDA (0.5-15 microg/0.5 microl) injected into the striatum was only slightly effective in the highest dose used. However, when injected into the nucleus accumbens AIDA (15microg/0.5microl) significantly and strongly counteracted the haloperidol-induced muscle rigidity. Our results suggest that stimulation of group II striatal mGluRs seems to play a major role in diminution of parkinsonian-like muscle rigidity. However, it seems that the antagonism of group I mGluRs located in the nucleus accumbens may also be of importance to the antiparkinsonian effect.

The role of glutamate receptors in antipsychotic drug action

It has recently been postulated that disturbances in glutamatergic neurotransmission may contribute to the pathophysiology of schizophrenia. Therefore the aim of the present study was to evaluate the role of glutamate NMDA and group II metabotropic receptors in the antipsychotic drug action. To this aim the influence of some well-known neuroleptics on cortical NMDA receptors was examined. Furthermore, their behavioral effects were compared with those of the novel agonist of group II glutamate metabotropic receptors, LY 354740, in some animal models of schizophrenic deficits. We found that long-term administration of the typical neuroleptic haloperidol and the atypical one clozapine increased the number of NMDA receptors labelled with [3H]CGP 39653 in different cortical areas. Long-, but not short-term, treatment with haloperidol and raclopride diminished the deficit of prepulse inhibition produced by phencyclidine, which is a model of sensorimotor gating deficit in schizophrenia. In contrast, neither short- nor long-term treatment with clozapine influenced the phencyclidine effect in that model. Acute treatment with LY 354740 reversed neither (1) the deficit of prepulse inhibition produced by phencyclidine or apomorphine, nor (2) the impairment in a delayed alternation task induced by MK-801, which is commonly used to model the frontal lobe deficits associated with schizophrenia. The present study suggests that an increase in the density of cortical NMDA receptors may be important to a longterm neuroleptic therapy. Conversely, the results do not support the role of group II metabotropic glutamate receptors in the antipsychotic drug action.

L-701,324, a selective antagonist at the glycine site of the NMDA receptor, counteracts haloperidol-induced muscle rigidity in rats

RATIONALE

It has recently been suggested that the overactivity of glutamatergic neurotransmission may contribute to the pathophysiology of Parkinson's disease. Therefore, a search for new compounds which block glutamatergic receptors and show antiparkinsonian properties in animal models of this disease seems to be justified.

OBJECTIVE

The aim of this study was to determine whether L-701,324 [7-chloro-4-hydroxy-3(3-phenoxy) phenylquinoline-2-(H)-one], a selective and full antagonist at the glycine site of the NMDA receptor, counteracts parkinsonian-like muscle rigidity and catalepsy induced by haloperidol in rats.

METHODS

The muscle tone was measured as the resistance developed to passive flexion and extension of the hind limb. Electromyographic (EMG) activity was additionally recorded in the gastrocnemius and tibialis anterior muscles.

RESULTS

L-701,324 (2.5-40 mg/kg IP) dose-dependently decreased the muscle tone enhanced by haloperidol (1-5 mg/kg IP). Likewise, the haloperidol-enhanced resting EMG activity and the EMG reflex response to passive movements were diminished by lower and almost abolished by higher doses of L-701,324. However, up to a dose of 20 mg/kg IP, L-701,324 did not influence haloperidol (0.5 mg/kg IP)-induced catalepsy. Moreover, L-701,324 (1.25-5 mg/kg IP) given alone or together with haloperidol (0.5-1 mg/kg IP) disturbed rotarod performance. Gross observation of behaviour indicated that rats injected with L-701,324 in doses equal to or higher than 5 mg/kg, alone or in combination with haloperidol, were markedly ataxic, i.e. rats showed signs of disturbed balance and loss of control over their hind limbs.

CONCLUSIONS

The present study suggests that L-701,324 exhibits a beneficial action in the animal model of parkinsonian rigidity, but not that of parkinsonian akinesia. Nonetheless, this compound is not devoid of motor side-effects.

Chronic haloperidol and clozapine administration increases the number of cortical NMDA receptors in rats

The aim of the present study was to examine the influence of 3-month administration of haloperidol (1 mg/kg per day) and clozapine (30 mg/kg per day) in drinking water on cortical NMDA (N-methyl-D-aspartate) receptors in rats. On day 5 of withdrawal, the animals were killed and their brains were removed. The binding of [3H]MK-801 ([3H](5R, 10S)-(+)-5-methyl-10,1 11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine) and [3H]CGP 39653([3H]D,L-(E)-2-amino-4-propyl-5-phosphono-3-pentenoic acid) to NMDA receptors in different cortical areas, as well as the binding of [3H]spiperone to dopamine D2 receptors in the striatum, were analysed by quantitative autoradiography. Haloperidol increased the binding of [3H]CGP 39653 in frontal, insular and parietal cortices. Clozapine increased the binding of [3H]CGP 39653 in insular and parietal cortices. Haloperidol, but not clozapine, increased the binding of [3H]spiperone in the striatum. None of the neuroleptics influenced the binding of [3H]MK-801 to cortical NMDA receptors. An additional assay revealed an increase in the Bmax value, with no significant changes in the K(D) of [3H]CGP 39653 binding in parieto-insular cortical homo-genates as a result of haloperidol and clozapine administration. The present results suggest that long-term treatments with haloperidol and clozapine increase the number of NMDA receptors in different cortical regions.

Influence of long-lasting administration of neuroleptics on cortical NMDA receptors and phencyclidine-induced deficit in the sensorimotor gating in rats

The aim of this study was to examine the role of cortical NMDA receptors in the antipsychotic action of neuroleptics. Haloperidol (1 mg/kg/day) and clozapine (30 mg/kg/day) were administered to rats in drinking water. Autoradiographic and saturation binding analyses showed that a 3-month treatment with both haloperidol and clozapine increased the density of NMDA receptors labelled with [3H]CGP 39653 (a competitive antagonist) in the parietal and insular cortices. Haloperidol additionally increased the binding of that ligand in the frontal cortex. None of those neuroleptics influenced the binding of [3H]MK-801, an uncompetitive antagonist of NMDA receptors, in the frontal, parietal or insular cortices. A 6-week and a 3-month treatment with haloperidol antagonized the deficit of prepulse inhibition induced by phencyclidine (5 mg/kg s.c.). In contrast, short-term (4-day) administration of that neuroleptic was ineffective. The present study suggests that the increased density of cortical NMDA receptors, induced by long-term neuroleptic administration, may overcome the deficit of sensorimotor gating induced by phencyclidine. However, contribution of such an effect to the antipsychotic activity needs to be established.

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.

LY354740, a group II metabotropic glutamate receptor agonist with potential antiparkinsonian properties in rats

The aim of this study was to examine whether (+)-2-aminobicyclo[3.1.0]-hexane-2,6-dicarboxylate monohydrate (LY354740), a selective agonist of group II metabotropic glutamate receptors, possesses antiparkinsonian properties. Parkinsonian-like muscle rigidity was induced by pretreatment with haloperidol (1 mg/kg i.p.). It was measured as increased resistance developed by the rat's hind leg to passive extension and flexion. LY354740 (5 and 10 mg/kg i.p.) dose-dependently diminished the haloperidol-induced muscle rigidity. The present results suggest that LY354740 counteracts the muscle rigidity in an animal model of parkinsonism.

Muscle rigidity induced by fluphenazine in rats is antagonized by L-DOPA, an antiparkinsonian drug

The aim of the present study was to find out whether the classic neuroleptic fluphenazine is a good model compound for inducing parkinsonian-like muscle rigidity in rats. The muscle tone was measured as resistance developed by the rat's hind foot to passive flexion and extension. Fluphenazine in doses of 0.4-3.0 mg/kg i.p. induced a dose-dependent increase in the hind foot resistance to passive movements. The muscle rigidity induced by fluphenazine 1.5 mg/kg i.p.) was counteracted in a dose-dependent manner by the main antiparkinsonian drug L-DOPA (25-75 mg/kg i.p.). The present results suggest that the fluphenazine-induced muscle rigidity may be a useful model of parkinsonian rigidity.

Chronic treatment with haloperidol diminishes the phencyclidine-induced sensorimotor gating deficit in rats

Prepulse inhibition is a model in which a weak subthreshold stimulus (prepulse), presented to an individual before a strong stimulus (pulse), inhibits a startle response to the latter. A deficit of prepulse inhibition induced by dopaminomimetics and antagonists of NMDA receptors has been suggested as an animal model of the sensorimotor deficit in schizophrenia. The aim of the present study was to examine the effect of chronic treatment with the classic neuroleptic haloperidol on the disruption of prepulse inhibition induced by the uncompetitive antagonist of NMDA receptors phencyclidine (PCP, 5 mg/kg sc). Haloperidol in a dose of 1 mg/kg/day was given to rats in drinking water for 3 months. The PCP-induced reduction in prepulse inhibition was not reversed by short-term (4-day) haloperidol administration. In contrast, long-term treatment with haloperidol (6 weeks or 3 months) diminished the PCP-induced effect. The present study suggests that the improvement in sensorimotor gating in the PCP model in rats by prolonged treatment with haloperidol may reflect its antipsychotic action.

Age-related changes in glutamate receptors: an autoradiographic analysis

The aim of the present study was to evaluate age-related changes in NMDA and AMPA receptors in old female rats. To this end a quantitative autoradiography of [3H]-MK-801 and [3H]-AMPA binding was performed in the brain of young (3-month-old), middle-aged (12-month-old) and old (36-month-old) rats. Moreover, the binding of [3H]-spiperone to D2 dopamine receptors was also examined. No changes were observed in the binding of [3H]-MK-801 or [3H]-AMPA in middle-aged rats compared to young ones. In the caudate-putamen and shell and core of the nucleus accumbens septi of old rats, a pronounced decrease in the [3H]-MK-801 binding and a decreasing tendency in the [3H]-AMPA binding were observed. Furthermore, the binding of [3H]-MK-801 and [3H]-AMPA was reduced in the hippocampal formation and, additionally, a marked decline in the [3H]-MK-801 binding in different parts of the cerebral cortex including the frontal, parietal, cingulate, pyriform and insular cortices was found. The [3H]-spiperone binding progressively decreased with age in the dorsolateral, ventrolateral and medial caudate-putamen. The present results show that aging processes lead to changes in the binding of both [3H]-MK-801 to NMDA and [3H]-AMPA to AMPA receptors in a number of structures, a phenomenon which may reflect motor and memory disturbances found in old rats and elderly humans.

Age-related muscle stiffness: predominance of non-reflex factors

This study was aimed at assessing the contribution of reflex and non-reflex factors to the muscle tone of old female Wistar rats. The hind foot of a rat was flexed or extended at the ankle joint by 25 degrees over 250 ms. The resistance of the foot to passive movements (torque, mechanomyogram), as well as the reflex electromyographic activity in the gastrocnemius and tibialis anterior muscles, were recorded simultaneously. Moreover, the impact of the blockade of the reflex activity caused by the local anesthetic lignocaine (1-2 ml of a 2% solution, injected in the vicinity of the sciatic nerve) on the muscle tone was investigated. Additionally, old rats' hind leg muscle samples were analysed using fluorescent microscopy for the expression of fibronectin, which is an early marker of connective tissue formation. It has been shown that old rats are characterized by (i) a substantially increased resistance of flexor muscle stiffness (measured during extension) and unchanged resistance of extensors (measured during flexion), (ii) the loss of a major part of the reflex electromyographic activity and (iii) the increased content of fibronectin in muscles. Moreover, it has been shown that lignocaine, which completely blocked the electromyographic reflex activity in the gastrocnemius and tibialis anterior muscles in young animals, was unable to counteract the resistance of these muscles to passive movements in old rats. The present results suggest that the muscle stiffness seen in old rats is not due to a reflex response, but depends mainly on non-reflex factors--chiefly on a large overgrowth of non-elastic connective tissue replacing degenerated active muscle fibers.

Influence of 6-hydroxydopamine lesion of the dopaminergic nigrostriatal pathway on the muscle tone and electromyographic activity measured during passive movements

The aim of the present study was to find out whether a 6-hydroxydopamine-induced lesion of the substantia nigra in rats would evoke muscular rigidity of the parkinsonian type. Simultaneous measurements of muscle resistance (mechanomyogram) of the hind foot to passive flexion and extension at the ankle joint, as well as of the electromyographic activity of the antagonistic muscles of the ankle joint--the gastrocnemius and tibialis anterior--in rats were carried out one, two and four weeks after bilateral injections of 6-hydroxydopamine (6.5 micrograms/microliter) into the substantia nigra. After immunohistochemical staining of brain sections for tyrosine hydroxylase, the rats were divided into two groups in which, on average, either 70% (63-80%) or 89% (81-96%) of nigral cells degenerated. Larger lesions increased the resistance (mechanomyogram) of the rat's hind leg to passive movements two weeks after 6-hydroxydopamine injection, whereas smaller lesions did not. Muscle rigidity was accompanied by an increase in the movement-induced reflex electromyographic activity in both muscles, mainly in long-latency components which are most probably influenced by supraspinal mechanisms. However, in spite of relatively large lesions of nigral dopamine cells, already four weeks after the lesion, muscle rigidity and the respective electromyographic activity diminished dramatically, which seems to result from very effective compensatory mechanisms operating in young lesioned rats. The results suggest that the muscle rigidity induced by the 6-hydroxydopamine nigral lesion seems to be a good model of parkinsonian rigidity.

Further evidence for the subsensitivity of striatal AMPA receptors, induced by chronic haloperidol administration: an autoradiographic study

The aim of the present study was to investigate the influence of chronic treatment with haloperidol on the striatal N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxasole-propionic acid (AMPA) and dopamine D2 receptors using a quantitative autoradiography in rats. Haloperidol was given to animals in a dose of ca. 1 mg/kg/day in drinking water for 6 weeks or 3 months and was afterwards withdrawn for 5 days. Haloperidol increased by 20-50% the binding of [3H]spiperone in different regions of the caudate-putamen. Haloperidol decreased by ca. 30% the binding of [3H]AMPA in the ventrolateral region of intermediate part of the caudate-putamen, but did not influence the binding of [3H]MK-801. The present results suggest that, apart from supersensitivity to dopamine, chronic treatment with haloperidol also induces subsensitivity of striatal AMPA receptors.

Haloperidol-increased muscle tone in rats as a model of parkinsonian rigidity

The aim of the present study was to find out whether haloperidol-induced rigidity was similar to that seen in parkinsonism. Simultaneous measurements of the muscle resistance (mechanomyogram, MMG) of the hind foot to passive flexion and extension in the ankle joint, as well as determination of the electromyographic (EMG) activity of the gastrocnemius and tibialis anterior muscles of rats were carried out. Haloperidol was injected in doses of 0.5-10 mg/kg 1 h before the start of measurements. Haloperidol increased, in a dose-dependent manner, the muscle resistance of the rat's hind leg to passive movements. Muscle rigidity was accompanied with an increase resting, as well as in the stretch-induced long-latency EMG activity (in which supraspinal reflexes are most probably involved) in both those muscles, whereas the short-latency EMG activity (first large bursts of EMG activity, beginning ca. 9 ms after the start of a movement, probably of a spinal origin) was significantly decreased. The obtained results suggest that the haloperidol-increased MMG/EMG activity might be a good model of parkinsonian rigidity.

NMDA receptors in the rostral and intermediate-caudal striatum play an opposite role in regulation of the muscle tone in rats

The aim of the study was to assess the contribution of the NMDA receptors in the caudate-putamen to the regulation of the muscle tone. The experiment was carried out on male Wistar rats. The hind foot of a rat was flexed or extended at the ankle joint by 25 degrees and the resistance of the foot to passive movements was measured. Haloperidol (1 mg/kg ip) induced the muscle rigidity. The competitive antagonist of NMDA receptors, (+/-)-2-amino-5-phosphonopentanoic acid (AP-5), injected in doses of 2 and 5 micrograms/0.5 microliter bilaterally into rostral regions of the caudate-putamen, inhibited the muscle rigidity induced by haloperidol. In contrast, AP-5 injected bilaterally in the same doses into the intermediate-caudal region of the caudate-putamen in rats not pretreated with haloperidol, induced muscle rigidity. The present results seem to suggest that NMDA receptors localized in the rostral and intermediate-caudal regions of the caudate-putamen play an opposite role in regulation of the muscle tone in rats.

The role of reflex activity in the regulation of muscle tone in rats

The aim of the study was to assess the contribution of reflex activity to the regulation of muscle tone in rats. The experiment was carried out on young Wistar male and female rats. The hindfoot of a rat was flexed or extended at the ankle joint by 25 deg over 250 ms. The resistance of the foot to passive movements, as well as the electromyographic (EMG) activity in the gastrocnemius and the tibialis anterior muscles, were recorded simultaneously. During passive movements, reflex EMG activity developed simultaneously in both antagonistic muscles of the foot. Three components were distinguished: a short-latency EMG-A (within the first 0-20 ms of a movement), long-latency EMG-B (within 60-160 ms), and EMG-C (within 220-340 ms). When the amplitudes of EMG-B and EMG-C components of the gastrocnemius muscle reflex response were greater than 50 microV, a significant correlation was found between them and the maximum resistance of the hindfoot (MMGmax) during flexion, whereas no such correlation was observed for the tibialis anterior muscle. No correlation was found when the amplitudes of the log-latency components of the gastrocnemius muscle were less than 50 microV. Moreover, no correlation was observed between the EMG-A and the MMG(max). The above results suggest that: (1) the muscle tone of the gastrocnemius muscle in rats seems to be regulated by long-latency (supraspinal) reflexes only when the level of EMG activity exceeds a critical threshold of ca 50 microV; (2) when the level of EMG activity is lower, a major role in the resistance of hindlimb muscles is played by some non-neuronal factors; and (3) the proposed animal model emphasizes new aspects of the reflex which may be useful in a search for basic mechanisms underlying changes in the muscle tone.

The influence of dizocilpine (MK-801) on the reserpine-enhanced electromyographic stretch reflex in rats

The electromyographic (EMG) stretch reflex in the gastrocnemius and tibialis anterior muscles was elicited by passive bending and stretching of the rat's hind foot in the ankle joint. The EMG stretch reflex was increased by pretreatment with reserpine (10 mg/kg ip) which is a model compound commonly used to induce parkinsonian rigidity in laboratory animals. Dizocilpine (MK-801) (0.32, 0.64 and 1.28 mg/kg sc) inhibited long-latency supraspinal components of the reserpine-increased EMG stretch reflex, whereas a short-latency spinal component was not diminished. The present results suggest that MK-801 exhibits an antiparkinsonian action against reserpine-induced rigidity.

Interaction between striatal excitatory amino acid and gamma-aminobutyric acid (GABA) receptors in the turning behaviour of rats

N-Methyl-D-aspartate (NMDA, 500 ng/0.5 microliters), alpha-amino-3-hydroxy-5-methyl-4-isoxasole-propionic acid (AMPA, 1000 ng/0.5 microliters), or kainic acid (50 ng/0.5 microliters) injected into intermediate and caudal parts of the caudate-putamen induced contralateral head turns and rotations. Picrotoxin (250 ng/0.5 microliters) injected into the same striatal region 30 min before NMDA, AMPA, or kainic acid strongly increased the contralateral turning induced by each of those compounds. The present results suggest that blockade of gamma-aminobutyric acid (GABA)A receptor complex by picrotoxin rendered striatal neurons more sensitive to the action of glutamate on NMDA and non-NMDA receptors.

The subsensitivity of striatal glutamate receptors induced by chronic haloperidol in rats

The aim of the present study was to investigate the influence of chronic treatment with haloperidol on the contralateral head turns and rotations induced by intrastriatal agonists of NMDA and non-NMDA receptors in rats. N-Methyl-D-aspartate (NMDA, 500 ng/0.5 mu l), alpha-amino-3-hydroxy-5-methyl-4-isoxasole-propionic acid (AMPA, 1000 ng/0.5 mu l) or kainic acid (50 ng/0.5 mu l), injected into the intermediate and caudal parts of the caudate-putamen, induced contralateral head turns and rotations. Haloperidol was given to animals in a dose of ca. 1 mg/kg per day in drinking water for 6 weeks. On day 5 of withdrawal, haloperidol decreased the number of contralateral head turns, but did not significantly influence the contralateral rotations induced by NMDA, AMPA and kainic acid. At the same time, haloperidol enhanced the stereotypy induced by apomorphine (0.25 mg/kg s.c.). The present results seem to suggest that, apart from supersensitivity to dopamine, chronic treatment with haloperidol also induces subsensitivity of striatal NMDA and non-NMDA receptors.

Specific involvement of striatal D1 and D2 dopamine receptors in the neuroleptic catalepsy in rats

Since highly specific antagonists of D1 (SCH 39166) and D2 (raclopride) dopamine receptors have recently become available, we decided to investigate the role of striatal populations of these receptors in catalepsy - an animal model of neuroleptic-induced parkinsonism in humans. Injections of raclopride (2.5, 5 and 10 micrograms/0.5 microliters) into the ventro-rostral part of the striatum induced a strong, dose-dependent and long-lasting catalepsy. Intrastriatal injections of SCH 39166 (1.5 and 3.6 micrograms/ 0.5 microliters) also evoked a dose-dependent, but short-lasting catalepsy. The present results suggest, that neuroleptic side-effects are specifically dependent on the blockade of D2 and D1 dopamine receptors in the striatum.

Does reserpine induce parkinsonian rigidity?

The aim of the study was to find out whether the reserpine-induced rigidity is similar to that seen in parkinsonism. Simultaneous measurements of the muscle resistance of the hind foot to passive bending and stretching in the ankle joint, as well as of the electromyographic (EMG) activity of the gastrocnemius and tibialis anterior muscles of rats were carried out. Reserpine was injected in a dose of 10 mg/kg alone or with alpha-methyl-p-tyrosine (250 mg/kg) 1, 4 and 27.5 h before the measurements. Reserpine increased the muscle resistance of the rat's hind leg to passive movements. That effect was the strongest at 1-2 h after the injections, and diminished markedly afterwards. The rigidity was accompanied with an increase in the resting, as well as in the stretch-induced short- and long-latency EMG activity in the gastrocnemius muscle. However, the intensity of the latter symptom did not change for a long period of time, which seems to correlate with the striatal dopamine depletion. The results suggest that the reserpine-increased EMG activity is a good model of parkinsonian rigidity.

Dopamine receptors--the present state of research and perspectives

The aim of the review is a brief description of the present status of research and perspectives in the field of multiple dopamine receptors, as well as of the importance of new discoveries for the treatment of schizophrenic psychoses and Parkinson's disease. Since the discovery of D1 and D2 dopamine receptors in the seventhies, evidence has accumulated for the existence of some new dopamine receptors. Among them the D3, D4 and D5 receptors, as well as their isoforms have been cloned. Molecular biology and pharmacology of these receptors have been exhaustively studied in recent years. These studies have prompted a number of hypotheses which open new perspectives for investigation into pathophysiology and therapy of schizophrenia and Parkinson's disease.

Lack of a strong influence of neuroleptic decanoates on dopaminergic and GABAergic functions

Data concerning the incidence of extrapyramidal symptoms and the development of the supersensitivity to dopamine after administration of depot neuroleptics are controversial. The aim of the study was to examine the influence of depot neuroleptics on the sensitivity of dopamine receptors and GABA nigral receptors. Haloperidol decanoate (30 or 60 mg/kg im) and fluphenazine decanoate (12.5 or 25 mg/kg im) were injected twice at a 15 day interval. These treatments induced weak but very long-lasting catalepsy (60-105 days depending on the neuroleptic and its dose). The only significant enhancement of the apomorphine (0.25 mg/kg sc) stereotypy was observed 135 days after the lower dose of haloperidol and 230 days after the lower dose of fluphenazine. Haloperidol decanoate (30 mg/kg) did not influence the number of contralateral rotations induced by muscimol (10 or 25 ng/0.5 microliter) injected into the substantia nigra pars reticulata 35, 55 and 135 days after the first injection. Present results indicate that the dopaminergic supersensitivity after administration of depot neuroleptics is weak and appears very late, and that haloperidol decanoate does not induce nigral supersensitivity to GABA. It is suggested that the depot neuroleptics might induce less extrapyramidal symptoms in the clinic than the daily neuroleptic treatment.

Stimulation of glutamate receptors in the intermediate/caudal striatum induces contralateral turning

The aim of the present study was to investigate the role of striatal NMDA, kainate and AMPA receptors in the turning behaviour of rats. N-methyl-D-aspartate (NMDA, 500 ng/0.5 microliters), kainic acid (50 ng/0.5 microliters) or alpha-amino-3-hydroxy-5-methyl-4-isoxasole- propionic acid (AMPA, 1000 ng/0.5 microliters), injected into the intermediate and caudal parts of the caudate-putamen, induced contralateral head turns and rotations. This effect was delayed or was not observed after administration of the compounds into the globus pallidus. The antagonist of non-NMDA receptors, 6,7-dinitroquinoxaline-2,3-dione (DNQX, 1000 ng/0.5 microliter), antagonized the contralateral head turns and rotations induced by AMPA (1000 ng/0.5 microliter) or kainic acid (50 ng/0.5 microliter), and evoked per se (2000 ng/0.5 microliter) the ipsilateral head turns and rotations. The NMDA receptor antagonist, (+/-)-2-amino-5-phosphonopentanoic acid (AP5, 1000 ng/0.5 microliter), induced mainly ipsilateral head turns and rotations; when injected in a dose of 500 ng/0.5 microliters, it inhibited the contralateral head turns and rotations after NMDA. The results seem to suggest that the contralateral head turns and rotations induced by stimulation of NMDA, AMPA and kainate receptors in the intermediate and caudal parts of the caudate-putamen may result from activation of the gamma-aminobutyrate (GABA)-ergic strionigral pathway.

Antiparkinsonian action of MK-801 on the reserpine-induced rigidity: a mechanomyographic analysis

MK-801, a non-competitive antagonist of NMDA receptors, is known to exhibit a beneficial action in many animal models of Parkinson's disease. The aim of this study was to examine the influence of MK-801 on the reserpine-induced muscle rigidity. The rigidity was estimated by a direct mechanomyographic method. This method consists in successive bending and straightening of a rat's hind foot in the ankle joint and measuring the resistance of the foot to passive movements. Reserpine in doses of 5-10 mg/kg ip, given alone or in combination with alpha-methyl-p-tyrosine (alpha MT, 250 mg/kg ip), induced rigidity. The strongest muscle rigidity was induced by 10 mg/kg of reserpine 1 hour after administration. MK-801 (0.32-1.28 mg/kg sc) injected 70 min after reserpine (10 mg/kg ip) decreased the rigidity induced by the latter compound. Similarly, MK-801 (1.28 mg/kg sc), administered 27 h 40' after joint treatment with reserpine (10 mg/kg ip) and alpha MT (250 mg/kg ip), strongly inhibited the reserpine-induced muscle rigidity. The obtained results show that the glutamatergic hyperactivity plays a significant role in the reserpine-induced rigidity. As the reserpine-induced motor disturbances are commonly accepted to be an animal model of parkinsonian symptoms, it may be assumed that the NMDA receptor blocking component may contribute substantially to the therapeutic action of antiparkinsonian drugs.

Involvement of the nucleus accumbens in the myorelaxant effect of baclofen in rats

The study was designed to search for brain structures responsible for the myorelaxant action of baclofen. Rats were chronically implanted with cannulae in the medial nucleus accumbens (NAS). The muscle tone was increased by reserpine (10 mg/kg i.p.) and measured as a resistance of the hind foot, developed in response to successive passive bendings and straightenings in the ankle joint. (+/-)Baclofen (1 or 2.5 mg/0.5 microliter), injected bilaterally into the NAS 1.5 h after pretreatment with reserpine, decreased the muscle tone of flexors and extensors enhanced by that compound. It is suggested that GABAB receptors of the NAS are involved in the myorelaxant action of baclofen.

Genotypic differences in locomotor stimulation and dopaminergic activity following acute ethanol administration

The effects of increasing doses of ethanol on locomotor activity and on the metabolism of dopamine (DA) in the limbic forebrain and in the striatum of CBA, C57, and NMRI mice were studied. In NMRI mice, low doses of ethanol produced locomotor stimulation which was followed by inhibition of locomotor activity at higher doses. In C57 and CBA animals, ethanol caused only reduction of locomotor activity. A low dose of ethanol (2.25 g/kg, i.p.) produced a significant enhancement of the release of dopamine (measured as the ratio DOPAC/DA) in limbic brain structures of all animals with no corresponding effect in the striatum. A high dose of ethanol (4.5 g/kg, i.p.) significantly increased the DOPAC/DA ratio both in the limbic forebrain and in the striatum of all animals. The significance of these behavioral and biochemical observations with regard to genetic aspects of the role of DA in the stimulatory/reinforcing properties of ethanol and to previously observed genetic differences in other neurotransmitter systems, especially GABA and glutamate, is discussed.

The role of excitatory amino acids in experimental models of Parkinson's disease

The aim of this article was to review the recent literature on the role of excitatory amino acids in Parkinson's disease and in animal equivalents of parkinsonian symptoms. Effects of NMDA and AMPA antagonists on the reserpine-induced akinesia, catalepsy and rigidity, on the neuroleptic-induced catalepsy, on the turning behaviour of 6-OHDA-lesioned rats, as well as on the parkinsonian symptoms evoked by MPTP in monkeys were analysed. Moreover, the role of NMDA antagonists in Parkinson's disease was discussed. Data concerning the protective influence of these drugs on degenerative properties of methamphetamine, MPTP and 6-OHDOPA were also presented. On the basis of the above findings, the following conclusions may be drawn: (1) disturbances in the glutamatergic transmission in various brain structures seem to play a significant role in the development of symptoms of Parkinson's disease; (2) the NMDA-receptor blocking component may make a substantial contribution to the therapeutic effect of antiparkinsonian drugs; a similar contribution of AMPA-receptor blocking component has not been sufficiently documented, so far; (3) compounds blocking NMDA receptors may possibly prevent the development of Parkinson's disease; this presumption needs, however further studies; (4) side effects of NMDA receptor antagonists may be a limiting factor in the use of these compounds in humans.

Muscle rigidity induced by the opioid analgesic tramadol, but not by the non-opioid flupirtine

The influence of high doses of two analgesic drugs, tramadol and flupirtine on the electromyographic activity in the gastrocnemius soleus muscles was examined. Tramadol (100-200 mg/kg po) dose-dependently induced a tonic electromyographic activity, which is generally accepted as a model of the opiate-induced muscle rigidity. That effect was antagonized by intraperitoneal injection of naloxone (0.8 mg/kg ip). On the other hand, flupirtine even in the high doses (100-200 mg/kg po) did not induce any tonic electromyographic activity. The obtained results confirm an opiate-like action of tramadol, but not that of flupirtine, on the muscle tension.

Contralateral rotations induced by intrastriatal injections of agonists of excitatory amino acid receptors

NMDA (100, 250 and 500 ng/0.5 microliters), kainic acid (50 and 100 ng/0.5 microliters) and AMPA (500 and 1000 ng/0.5 microliters), injected unilaterally into the ventrolateral part of the intermediate region of the caudate-putamen of rats, induced contralateral rotations. It is proposed that stimulation of NMDA, kainate and AMPA receptors present on strionigral neurons evokes an "antiparkinsonian effect".

Zona incerta-lateral hypothalamus as an output structure for impulses involved in neuroleptic drug-induced catalepsy

Our previous studies showed that the neuronal impulses connected with catalepsy, which have their origin at dopamine D2 receptors in the ventro-rostral part of the nucleus caudatus-putamen in rats, are conveyed to the zona incerta-lateral hypothalamic region. The aim of the present study was to investigate the route of the neuronal impulses between these structures. The experiments were carried out on rats with cannulae chronically implanted in the brain structures. We showed that (1) bilateral injection of bicuculline methiodide (5-50 ng) into the ventro-medial part of the globus pallidus (GPv) and (2) bilateral injection of muscimol (2.5-25 ng) into the substantia nigra pars reticulata (SNR) inhibit, in a dose dependent manner, the catalepsy induced by sulpiride (1 microgram) administered bilaterally into the ventro-rostral part of the nucleus caudatus-putamen. It was also demonstrated that muscimol (25 ng), injected bilaterally into the ventro-medial part of the globus pallidus, induces catalepsy which, in turn, is dose-dependently inhibited by either (1) muscimol (5-25 ng) injected into the substantia nigra pars reticulata, or (2) bicuculline (1.0-2.5 ng) injected into the zona incerta-lateral hypothalamus (ZI-LH). Moreover, even a dose as high as 50 ng of bicuculline, injected into the ventro-medial part of the globus pallidus, had no significant effect on the locomotor activity of rats.(ABSTRACT TRUNCATED AT 250 WORDS)