The striatum as a target for anti-rigor effects of an antagonist of mGluR1, but not an agonist of group II metabotropic glutamate receptors

Effect of the metabotropic glutamate antagonist MPEP on striatal expression of the Homer family proteins in levodopa-treated hemiparkinsonian rats

RATIONALE

Striatal glutamatergic hyperactivity through the metabotropic receptors and their intracellular signaling pathways is considered critical in the development of levodopa-induced dyskinesias in Parkinson's disease and in experimental parkinsonism.

OBJECTIVE

We investigated whether the administration of the metabotropic glutamate antagonist, MPEP, modifies striatal expression of Homer family proteins which are involved in the intracellular mechanisms mediated by these receptors.

MATERIALS AND METHODS

Sprague-Dawley rats were unilaterally lesioned in the nigrostriatal pathway with 6-hydroxydopamine (8 microg) and treated with: levodopa (12 mg/kg, i.p.) plus vehicle (n=10) divided in two daily injections; levodopa plus MPEP (1.5 and 3 mg/kg, i.p.; n=6-13) divided in two daily injections; or saline (n=7) for 10 consecutive days. Axial, limb, and orolingual dyskinesias were evaluated. Striatal expression of tyrosine hydroxylase (TH), Homer 1a, 1b/c, and deltaFosB were measured by Western Blot.

RESULTS

Animals treated with levodopa showed an increase of dyskinesia score (p<0.01) that was attenuated by the administration of MPEP (p<0.01). In the ipsilateral side of the lesion, striatal TH expression was decreased (p<0.01). No significant differences in striatal Homer 1a or b/c expression were observed between the groups of treatment. Striatal deltaFosB expression increased in the animals treated with levodopa (p<0.05) being attenuated after MPEP administration (p<0.05). MPEP effect was not paralleled by any modification of striatal Homer proteins expression.

CONCLUSIONS

These results suggest that Homer protein family is not causally involved in the development of dyskinetic movements induced by levodopa treatment in this animal model of parkinsonism.

Role of the metabotropic glutamate receptor subtype 1 in the harmaline-induced tremor in rats

The present study shows that JNJ 16259685-a selective antagonist of glutamate metabotropic receptor subtype 1 (mGluR1) injected in doses of 0.16 and 0.32 mg/kg sc strongly enhances tremor of forelimbs, head and trunk, hypolocomotion, and ataxia induced by harmaline (7.5 mg/kg ip) in rats. JNJ 16259685 inhibited locomotor and exploratory activity per se. These results may suggest an inhibitory influence of mGluR1 on the harmaline-induced motor disturbances.

Metabotropic glutamate 1 receptor: current concepts and perspectives

Almost 25 years after the first report that glutamate can activate receptors coupled to heterotrimeric G-proteins, tremendous progress has been made in the field of metabotropic glutamate receptors. Now, eight members of this family of glutamate receptors, encoded by eight different genes that share distinctive structural features have been identified. The first cloned receptor, the metabotropic glutamate (mGlu) receptor mGlu1 has probably been the most extensively studied mGlu receptor, and in many respects it represents a prototypical subtype for this family of receptors. Its biochemical, anatomical, physiological, and pharmacological characteristics have been intensely investigated. Together with subtype 5, mGlu1 receptors constitute a subgroup of receptors that couple to phospholipase C and mobilize Ca(2+) from intracellular stores. Several alternatively spliced variants of mGlu1 receptors, which differ primarily in the length of their C-terminal domain and anatomical localization, have been reported. Use of a number of genetic approaches and the recent development of selective antagonists have provided a means for clarifying the role played by this receptor in a number of neuronal systems. In this article we discuss recent advancements in the pharmacology and concepts about the intracellular transduction and pathophysiological role of mGlu1 receptors and review earlier data in view of these novel findings. The impact that this new and better understanding of the specific role of these receptors may have on novel treatment strategies for a variety of neurological and psychiatric disorders is considered.

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.

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.

The striopallidal pathway is involved in antiparkinsonian-like effects of the blockade of group I metabotropic glutamate receptors in rats

The aim of the study was to examine the influence of the blockade of group I metabotropic glutamate receptors (mGluRs) on the haloperidol-induced catalepsy and proenkephalin mRNA expression in the rat striatum. Bilateral, intrastriatal injection of AIDA ((RS)-1-aminoindan-1,5-dicarboxylic acid, 3-15 microg/0.5 microl), a selective antagonist of group I mGluRs, inhibited catalepsy induced by haloperidol (0.5 mg/kg i.p.). Repeated intrastriatal AIDA administrations (3 x 15 microg/0.5 microl, 3 h apart) counteracted the haloperidol-induced (3 x 1.5 mg/kg s.c., 3 h apart) increase in the proenkephalin mRNA expression in that structure. The present study indicates that the blockade of the striatal group I mGluRs may inhibit parkinsonian akinesia by normalizing the function of the striopallidal pathway.

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.