N-methyl-D-aspartate acutely increases proenkephalin mRNA in the rat striatum

The Modulation of Pain by Metabotropic Glutamate Receptors 7 and 8 in the Dorsal Striatum

The dorsal striatum, apart from controlling voluntary movement, displays a recently demonstrated pain inhibition. It is connected to the descending pain modulatory system and in particular to the rostral ventromedial medulla through the medullary dorsal reticular nucleus. Diseases of the basal ganglia, such as Parkinson's disease, in addition to being characterized by motor disorders, are associated with pain and hyperactivation of the excitatory transmission. A way to counteract glutamatergic hyperactivation is through the activation of group III metabotropic glutamate receptors (mGluRs), which are located on presynaptic terminals inhibiting neurotransmitter release. So far the mGluRs of group III have been the least investigated, owing to a lack of selective tools. More recently, selective ligands for each mGluR of group III, in particular positive and negative allosteric modulators, have been developed and the role of each subtype is starting to emerge. The neuroprotective potential of group III mGluRs in pathological conditions, such as those characterized by elevate glutamate, has been recently shown. In the dorsal striatum, mGluR7 and mGluR8 are located at glutamatergic corticostriatal terminals and their stimulation inhibits pain in pathological conditions such as neuropathic pain. The two receptors in the dorsal striatum have instead a different role in pain control in normal conditions. This review will discuss recent results focusing on the contribution of mGluR7 and mGluR8 in the dorsal striatal control of pain. The role of mGluR4, whose antiparkinsonian activity is widely reported, will also be addressed.

Nucleus accumbens mu opioid receptors mediate immediate postictal decrease in locomotion after an amygdaloid kindled seizure in rats

Postictal movement dysfunction is a common symptom in patients with epilepsy. We investigated the involvement of opioid receptors in the nucleus accumbens (NAC) in amygdaloid kindling-induced postictal decrease in locomotion (PDL) in rats. Seizures were induced by daily electrical stimulation of the basolateral amygdala until four consecutive stage 5 seizures were elicited. Locomotion was quantified before and after infusion of an opioid receptor antagonist or saline into the NAC. Whereas PDL was induced after a stage 5 seizure in saline-infused rats, pre-infusion of the mu opioid receptor antagonist H-D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP, 5 microg/1 microL/side) into the NAC prevented PDL. Pre-infusion of delta (naltrindole, 30 microg/1 microL/side), kappa (nor-binaltorphimine, 1.8 microg/1 microL/side), or nonselective (naloxone, 10 microg/1 microL/side) opioid receptor antagonists did not block PDL, but late postictal hyperactivity was blocked by naltrindole. None of the antagonists affected amygdaloid evoked afterdischarge duration. It is suggested that mu opioid receptors in the NAC participate in amygdaloid seizure-induced PDL without affecting seizure duration.

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.

Effects of 2,3-benzodiazepine AMPA receptor antagonists on dopamine turnover in the striatum of rats with experimental parkinsonism

Although levodopa is the current "gold standard" for treatment of Parkinson's disease, there has been disputation on whether AMPA receptor antagonists can be used as adjuvant therapy to improve the effects of levodopa. Systemic administration of levodopa, the precursor of dopamine, increases brain dopamine turnover rate and this elevated turnover is believed to be essential for successful treatment of Parkinson's disease. However, long-term treatment of patients with levodopa often leads to development of dyskinesia. Therefore, drugs that feature potentiation of dopamine turnover rate and are able to reduce daily levodopa dosages might be used as adjuvant in the treatment of patients suffering from Parkinson's disease. To investigate such combined treatment, we have examined the effects of two non-competitive AMPA receptor antagonists, GYKI-52466 and GYKI-53405, alone or in combination with levodopa on dopamine turnover rate in 6-hydroxydopamine-lesioned striatum of the rat. We found here that repeated administration of levodopa, added with the peripheral DOPA decarboxylase inhibitor carbidopa, increased dopamine turnover rate after lesioning the striatum with 6-hydroxydopamine. Moreover, combination of levodopa with GYKI-52466 or GYKI-53405 further increased dopamine turnover enhanced by levodopa administration while the AMPA receptor antagonists by themselves failed to influence striatal dopamine turnover. We concluded from the present data that potentiation observed between levodopa and AMPA receptor antagonists may reflect levodopa-sparing effects in clinical treatment indicating the therapeutic potential of such combination in the management of Parkinson's disease.

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.

Effects of chronic dizocilpine on acute pain and on mRNA expression of neuropeptides and the dopamine and glutamate receptors

The mesocorticolimbic circuitry has been implicated in the pathophysiology of several neuropsychiatric syndromes like chronic pain and addiction. The aim of this study was to evaluate the effects of dizocilpine (MK-801), a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, on sensorimotor behaviors and the consequent changes in the dopamine, glutamate, and opiate systems in rats. Five groups of rats were subjected to acute tests for nociception (hot plate and paw pressure) before and after MK-801 (0.05, 0.1, 0.2 and 0.4 mg/kg, i.p.) or saline. Another two groups received daily i.p. saline or MK-801 (0.4 mg/kg) for 15 days. The nociceptive tests were performed on days 1, 7, and 14. On day 15 the rats received the last injection and were immediately sacrificed. We measured the mRNA expression, by in situ hybridization (ISH), of various dopamine and glutamate receptors, and enkephalin (Enk), dynorphin (Dyn), and substance P (SP) in the striatum, nucleus accumbens (NAC), piriform and cingulate cortex. Acute MK-801, dose-dependently, resulted in hyperalgesia. The chronic effects of 0.4 mg/kg MK-801 showed an extinction of the acute hyperalgesic effects especially with the hot plate test. The ISH studies revealed a decrease in mRNA expression of Enk and SP in the striatum and NAC. Our results indicate that the reversal of acute MK-801-induced hyperalgesia, with repeated exposure to systemic MK-801, is not directly related to changes in dopamine and glutamate receptors and might involve alteration of the striatal neuropeptide system.

Contribution of ionotropic glutamate receptors to acute amphetamine-stimulated preproenkephalin mRNA expression in the rat striatum in vivo

The present study evaluated the possible role of ionotropic glutamate receptors in mediating the inducible preproenkephalin (PPE) mRNA expression in the rat striatum in response to systemic administration of a psychostimulant amphetamine. A single injection of amphetamine (5 mg/kg, i.p.) increased PPE mRNA levels in the dorsal striatum as revealed via quantitative in situ hybridization. Pretreatment with the NMDA receptor antagonist, MK801 (0.5 mg/kg, i.p.) or 3-((R)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (10 mg/kg, i.p.), attenuated the amphetamine-induced PPE mRNA expression. A similar attenuation of the amphetamine-induced PPE expression, although to a lesser extent, was also observed following pretreatment with a kainate/AMPA receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (100-200 mg/kg, i.p.). Three antagonists alone had no significant effect on basal levels of PPE mRNAs in the dorsal striatum. These data indicate that both NMDA and kainate/AMPA receptors are involved in coupling extracellular signals evoked by acute stimulant exposure to PPE mRNA expression in striatal neurons.

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.

Glutamate receptors participate in the nicotine-induced changes of met-enkephalin in striatum

A single dose of nicotine given to mice induces first a rapid decrease (presumed release/enhanced degradation) and then a rise (presumed synthesis/enhanced accumulation) of met-enkephalin (Met-Enk) in dorsal and ventral striatum observed at 30 and 60 min post-treatment, respectively. These studies investigated whether the nicotine effect on Met-Enk was mediated indirectly, in part, via other neurotransmitters known to be released by nicotine. Based on the ability of selective antagonists of dopamine (Sch 23390, D1; Sulpiride, D2), glutamate (CPP, competitive NMDA; dizocilpine, non-competitive NMDA; NBQX, AMPA) and GABA (bicuculline, GABA(A); Sch 50911, GABA(B)) receptors, to inhibit or enhance the response to nicotine, we conclude that nicotine alters striatal Met-Enk, in part, via glutamate NMDA and AMPA receptors. These findings further support the notion that glutamate might play a role in the pharmacology of nicotine.

Acute immediate-early gene response to 6-hydroxydopamine infusions into the medial forebrain bundle

Although the long-term neurobiological and behavioral effects of nigrostriatal lesions are well characterized, the events occurring soon after injury are not. These acute events can provide insight into the mechanisms underlying long-term adaptations to nigrostriatal lesions. The present experiments examined the basal ganglia immediate-early gene response to infusions of the catecholamine neurotoxin 6-hydroxydopamine into the nigrostriatal pathway in rats. Following 6-hydroxydopamine infusions into the medial forebrain bundle in awake, behaving rats, there was a rapid and transient induction of striatal c-fos and zif/268 messenger RNAs. Both immediate-early genes were maximally induced by 45min post-infusion, and returned to control levels by 1.5h (c-fos) or 3h (zif/268) post-infusion. Double-labeling experiments revealed that striatal c-fos expression occurred preferentially in preproenkephalin-expressing neurons. 6-Hydroxydopamine-induced c-fos messenger RNA was also observed in the substantia nigra pars reticulata and entopeduncular nucleus, but not the globus pallidus, 45 min after medial forebrain bundle 6-hydroxydopamine infusions. Finally, the role of ionotropic striatal glutamate receptors in nigrostriatal injury-induced striatal c-fos was examined by combining medial forebrain bundle 6-hydroxydopamine infusions with intrastriatal glutamate antagonist infusions. Both the N-methyl-D-aspartate antagonist, (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid, and the non-N-methyl-D-aspartate antagonist, 6,7-dinitroquinoxaline-2, 3-dione, blocked striatal induction of c-fos messenger RNA following 6-hydroxydopamine infusions into the medial forebrain bundle. These results provide evidence of rapidly developing, glutamate-dependent molecular responses in the basal ganglia which may contribute to some of the well-described long-term adaptations of this system to nigrostriatal injury.

Distribution of glutamate and preproenkephalin messenger RNAs following transient focal cerebral ischemia

Middle cerebral artery occlusion may result in increased activation of N-methyl-D-aspartate- or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-type receptors by glutamate and lead to neuronal cell death. To characterize molecular events that precede cell death following transient focal ischemia, in situ hybridization histochemistry was used to measure levels of glutamate receptor subunit 1 (GluR1), GluR2, GluR3, N-methyl-D-aspartate receptor subunit 1 (NR1) and preproenkephalin messenger RNAs in adult rats at various recirculation times (1.5, 3 and 24 h) following a 90-min period of middle cerebral artery occlusion. At 1.5 and 3 h recirculation, autoradiography showed pronounced but differential decreases in AMPA, NR1 and preproenkephalin messenger RNA expression throughout the infarcted ipsilateral striatum. Non-uniform patterns of in situ hybridization grains emerged such that many striatal neurons were depleted of AMPA and preproenkephalin messenger RNAs, while others retained control levels. In cortical regions destined to undergo infarction, GluR2 and NR1 messenger RNAs were preferentially reduced relative to the contralateral side (to 75+/-8.5% and 66+/-4.5%, respectively); GluR1, GluR3 and preproenkephalin messenger RNAs were unaltered. At 24 h recirculation, depletion of striatal and cortical messenger RNAs became less selective. GluR3 and preproenkephalin messenger RNAs were up-regulated in ipsilateral spared regions of the striatum, and GluR1 and GluR2 messenger RNAs increased bilaterally in the cingulate cortex and in selective nuclei of the amygdala. Histological cell death or neurodegeneration was not detected in areas of reduced glutamate and preproenkephalin messenger RNA expression in either the ipsilateral striatum or cortex before 24 h. These findings suggest that complex and long-lasting decreases in messenger RNA expression occur prior to significant cell loss in regions destined to undergo infarction. Increased formation of Ca2+-permeable AMPA receptor assemblies may occur in "unspared" and "spared" regions via different mechanisms and contribute to alterations in post-ischemic synaptic activity. The possibility arises that there may be altered relationships between glutamatergic and enkephalin synapses, since the dorsolateral striatum, where preproenkephalin messenger RNA expression is acutely reduced, receives innervation by the affected ipsilateral cortical region.

Evidence for regulation of vasopressin gene transcription by the NMDA receptor

We investigated the effects of dizocilpine maleate (MK-801), an NMDA receptor antagonist, on arginine vasopressin heterogeneous nuclear RNA expression in the supraoptic nucleus in the rat hypothalamus. MK-801 treatment completely blocked the osmotic stimulus-induced increase in AVP hnRNA expression, but had no effect on basal AVP hnRNA expression in the SON. These observations indicate that the NMDA receptor is essential for regulation of AVP gene transcription in response to osmotic stimulation, but has no effect on steady-state AVP gene transcription.

Subcellular and subsynaptic distribution of the NR1 subunit of the NMDA receptor in the neostriatum and globus pallidus of the rat: co-localization at synapses with the GluR2/3 subunit of the AMPA receptor

Glutamatergic neurotransmission in the neostriatum and the globus pallidus is mediated through NMDA-type as well as other glutamate receptors and is critical in the expression of basal ganglia function. In order to characterize the cellular, subcellular and subsynaptic localization of NMDA receptors in the neostriatum and globus pallidus, multiple immunocytochemical techniques were applied using antibodies that recognize the NR1 subunit of the NMDA receptor. In order to determine the spatial relationship between NMDA receptors and AMPA receptors, double labelling was performed with the NR1 antibodies and an antibody that recognizes the GluR2 and 3 subunits of the AMPA receptor. In the neostriatum all neurons with characteristics of spiny projection neurons, some interneurons and many dendrites and spines were immunoreactive for NR1. In the globus pallidus most perikarya and many dendritic processes were immunopositive. Immunogold methods revealed that most NR1 labelling is associated with asymmetrical synapses and, like the labelling for GluR2/3, is evenly spread across the synapse. Double immunolabelling revealed that in neostriatum, over 80% of NR1-positive axospinous synapses are also positive for GluR2/3. In the globus pallidus most NR1-positive synapses are positive for GluR2/3. In both regions many synapses labelled only for GluR2/3 were also detected. These results, together with previous data, suggest that NMDA and AMPA receptor subunits are expressed by the same neurons in the neostriatum and globus pallidus and that NMDA and AMPA receptors are, at least in part, colocalized at individual asymmetrical synapses. The synaptic responses to glutamate in these regions are thus likely be mediated by both AMPA and NMDA receptors at the level of individual synapses.

Increased striatal proenkephalin mRNA subsequent to production of spreading depression in rat cerebral cortex: activation of corticostriatal pathways?

Cortical Spreading Depression (CSD) is a slowly propagating wave of depolarization and negative interstitial DC potential, that when induced in the rat brain extends across the entire homolateral hemisphere. Despite evidence that CSD does not penetrate into subcortical regions, neurochemical changes in areas anatomically connected to cortex have been reported. In this study in situ hybridization histochemistry was used to examine the levels of cholecystokinin (CCK), proenkephalin (ENK) and prodynorphin (DYN) mRNA in cortex and forebrain basal ganglia following KCl-induced CSD. Unilateral CSD was induced by topical application of 3 M KCl ( approximately 10 microliter) onto the right parietal cortex for 10 min and rats were then killed 1-6 h and 1-28 days later. CCK mRNA levels were increased (P<0.01) in the ipsilateral neocortex 3 h after CSD (13% above levels in contralateral side), reached a peak at 2 days ( approximately 70%) and were still elevated at 7 (30%) but not, 14 or 28 days later. Unilateral CSD also produced a rapid and sustained increase (P<0.05) in ENK mRNA in ipsilateral piriform cortex (from 3 h to 2 days; 70-250% above contralateral), and a delayed increase in caudate putamen and olfactory tubercle at 1 and 2 days ( approximately 25% in both regions), but levels were again equivalent to control at 7 days and beyond. In contrast, no marked changes in neocortical ENK mRNA, or DYN mRNA in both cortex and basal ganglia, were observed under these conditions. These findings demonstrate that CSD has specific, rapid and long-lasting effects on neuropeptide expression in neocortex and subcortical areas. CSD-induced changes in mesostriatal ENK mRNA are proposed to reflect synaptic activation of local neurons via cortical afferent projections.

Quantitative comparison of pretreatment regimens used to sensitize in situ hybridization using oligonucleotide probes on paraffin-embedded brain tissue

Paraffin embedding of tissue is generally perceived to dramatically reduce RNA detectability. As a consequence, in situ hybridization on paraffin-embedded tissue is largely confined to detection of high-copy RNA species (e.g., viral RNA) and/or to detection using typically more sensitive cDNA probes or riboprobes. In this study, several procedures for in situ hybridization on paraffin-embedded rat tissue using oligonucleotide probes complementary to cellular transcripts were developed and quantitatively compared. Certain pretreatments showed marked increases in sensitivity compared to untreated sections. Furthermore, through quantitative assessment using image analysis, sensitivity of optimal pretreatments was equal to that of routinely used fresh-frozen, postfixed tissue sections. The development of such techniques permitting in situ hybridization to be carried out on paraffin-embedded tissue allows a comparison of protein and mRNA distribution to be made in adjacent sections and provides the potential for double labeling by in situ hybridization and immunohistochemistry which may not be possible on post-fixed frozen sections.

Preproenkephalin mRNA expression in the caudate-putamen of MPTP monkeys after chronic treatment with the D2 agonist U91356A in continuous or intermittent mode of administration: comparison with L-DOPA therapy

The effect of chronic treatment with the D2 dopamine agonist U91356A or L-DOPA therapy on the regulation of preproenkephalin (PPE) mRNA was investigated in the caudate-putamen of previously drug-naive cynomolgus monkeys Macaca fascicularis rendered parkinsonian by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). In MPTP monkeys, pulsatile treatment with either L-DOPA or U91356A relieved parkinsonian symptoms but caused progressive sensitization to treatment and, as expected, induced choreic dyskinesias. In contrast, U91356A given in a continuous mode led to partial behavioral tolerance without appearance of dyskinesias. Using in situ hybridization histochemistry, lesioning was shown to produce elevation of PPE mRNA levels in the lateral and medial parts of the putamen and in the lateral part of the caudate nucleus compared to control animals at the three rostrocaudal regions analyzed. In general, no change of PPE mRNA levels were observed in the medial caudate after MPTP lesioning with or without L-DOPA or U91356A treatments in the three rostrocaudal regions measured except for an increase in the caudal part of L-DOPA-treated MPTP monkeys. In the putamen and lateral caudate nucleus, elevated PPE mRNA expression by MPTP generally was not corrected (or only partially corrected) by chronic L-DOPA treatment except for the rostral medial putamen where correction to control values was observed. In general, pulsatile administration of U91356A partially corrected the lesion-induced elevation of PPE mRNA levels in the putamen and lateral caudate nucleus whereas the correction was more pronounced and widespread when MPTP monkeys received the continuous administration of this drug. These results indicate that the mode of administration of a D2 dopamine receptor agonist, such as U91356A, although at a roughly equivalent dosage influences the extent of inhibition of the expression of PPE in the denervated striatum of monkeys. In addition, the general lack of correction of the MPTP-induced increase of PPE mRNA in the striatum of L-DOPA-treated monkeys compared to the decreases observed with the D2 agonist treatments suggest that the D1 agonist component of L-DOPA therapy opposes the D2 agonist activity. Hence, D1 receptor agonist activity would stimulate PPE mRNA expression whereas D2 receptor agonists inhibit the expression of this peptide. Increases in PPE expression in the striatum may be implicated in the induction of dyskinesias since both groups of treated MPTP monkeys displaying dyskinesias had elevated striatal PPE mRNA levels whereas the MPTP monkeys with the lowest striatal PPE mRNA levels developed tolerance without dyskinesias.