Stimulation of delta opioid receptors located in substantia nigra reticulata but not globus pallidus or striatum restores motor activity in 6-hydroxydopamine lesioned rats: new insights into the role of delta receptors in parkinsonism

Homology modeling and 3D-QSAR study of benzhydrylpiperazine δ opioid receptor agonists

The binding affinity of a series of benzhydrylpiperazine δ opioid receptor agonists were pooled and evaluated by using 3D-QSAR and homology modeling/molecular docking methods. Ligand-based CoMFA and CoMSIA 3D-QSAR analyses with 46 compounds were performed on benzhydrylpiperazine analogues by taking the most active compound BW373U86 as the template. The models were generated successfully with q² value of 0.508 and r² value of 0.964 for CoMFA, and q² value of 0.530 and r² value of 0.927 for CoMSIA. The predictive capabilities of the two models were validated on the test set with R²_pred value of 0.720 and 0.814, respectively. The CoMSIA model appeared to work better in this case. A homology model of active form of δ opioid receptor was established by Swiss-Model using a reported crystal structure of active μ opioid receptor as a template, and was further optimized using nanosecond scale molecular dynamics simulation. The most active compound BW373U86 was docked to the active site of δ opioid receptor and the lowest energy binding pose was then used to identify binding residues such as s Gln105, Lys108, Leu125, Asp128, Tyr129, Leu200, Met132, Met199, Lys214, Trp274, Ile277, Ile304 and Tyr308. The docking and 3D-QSAR results showed that hydrogen bond and hydrophobic interactions played major roles in ligand-receptor interactions. Our results highlight that an approach combining structure-based homology modeling/molecular docking and ligand-based 3D-QSAR methods could be useful in designing of new opioid receptor agonists.

The delta-opioid receptor and Parkinson's disease

Parkinson's disease (PD) is a common degenerative neurological disease leading to a series of familial, medical, and social problems. Although it is known that the major characteristics of PD pathophysiology are the dysfunction of basal ganglia due to injury/loss of dopaminergic neurons in the substantia nigra pars compacta dopaminergic and exhaustion of corpus striatum dopamine, therapeutic modalities for PD are limited in clinical settings up to date. It is of utmost importance to better understand PD pathophysiology and explore new solutions for this serious neurodegenerative disorder. Our recent work and those of others suggest that the delta-opioid receptor (DOR) is neuroprotective and serves an antiparkinsonism role in the brain. This review summarizes recent progress in this field and explores potential mechanisms for DOR-mediated antiparkinsonism.

Opioidergic Modulation of Striatal Circuits, Implications in Parkinson's Disease and Levodopa Induced Dyskinesia

The functional organization of the dorsal striatum is complex, due to the diversity of neural inputs that converge in this structure and its subdivision into direct and indirect output pathways, striosomes and matrix compartments. Among the neurotransmitters that regulate the activity of striatal projection neurons (SPNs), opioid neuropeptides (enkephalin and dynorphin) play a neuromodulatory role in synaptic transmission and plasticity and affect striatal-based behaviors in both normal brain function and pathological states, including Parkinson's disease (PD). We review recent findings on the cell-type-specific effects of opioidergic neurotransmission in the dorsal striatum, focusing on the maladaptive synaptic neuroadaptations that occur in PD and levodopa-induced dyskinesia. Understanding the plethora of molecular and synaptic mechanisms underpinning the opioid-mediated modulation of striatal circuits is critical for the development of pharmacological treatments that can alleviate motor dysfunctions and hyperkinetic responses to dopaminergic stimulant drugs.

Delta Opioid Pharmacology in Parkinson's Disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder that compromises multiple neurochemical substrates including dopamine, norepinephrine, serotonin, acetylcholine, and glutamate systems. Loss of these transmitter systems initiates a cascade of neurological deficits beginning with motor function and ending with dementia. Current therapies primarily address the motor symptoms of the disease via dopamine replacement therapy. Exogenous dopamine replacement brings about additional challenges since after years of treatment it almost invariably gives rise to dyskinesia as a side effect. Therefore there is a clear unmet clinical need for improved PD therapeutics. Opioid receptors and their respective peptides are expressed throughout the basal ganglia and cortex where monoaminergic denervation strongly contributes to PD pathology. Delta opioid receptors are of particular interest because of their dense localization in basal ganglia and because activating this system is known to enhance locomotor activity under a variety of conditions. This chapter will outline much of the work that has demonstrated the effectiveness of delta opioid receptor activation in models of PD and its neuroprotective properties. It also discusses some of the challenges that must be addressed before moving delta opioid receptor agonists into a clinical setting.

The opioid receptor triple agonist DPI-125 produces analgesia with less respiratory depression and reduced abuse liability

Opioid analgesics remain the first choice for the treatment of moderate to severe pain, but they are also notorious for their respiratory depression and addictive effects. This study focused on the pharmacology of a novel opioid receptor mixed agonist DPI-125 and attempted to elucidate the relationship between the δ-, μ- and κ-receptor potency ratio and respiratory depression and abuse liability. Five diarylmethylpiperazine compounds (DPI-125, DPI-3290, DPI-130, KUST202 and KUST13T02) were selected for this study. PKA fluorescence redistribution assays in CHO cells individually expressing δ-, μ- or κ-receptors were used to measure the agonist potency. The respiratory safety profiles were estimated in rats by the ratio of ED₅₀ (pCO₂ increase)/ED₅₀ (antinociception). The abuse liability of DPI-125 was evaluated with a self-administration model in rhesus monkeys. The observed agonist potencies of DPI-125 for δ-, μ- and κ-opioid receptors were 4.29±0.36, 11.10±3.04, and 16.57±4.14 nmol/L, respectively. The other four compounds were also mixed agonists with varying potencies. DPI-125 exhibited a high respiratory safety profile, clearly related to its high δ-receptor potency. The ratio of the EC₅₀ potencies for the μ- and δ-receptors was found to be positively correlated with the respiratory safety ratio. DPI-125 has similar potencies for μ- and κ-receptors, which is likely the reason for its reduced abuse potential. Our results demonstrate that the opioid receptor mixed agonist DPI-125 is safer and less addictive than traditional μ-agonist analgesics. These findings suggest that the development of δ>μ∼κ opioid receptor mixed agonists is feasible, and such compounds could represent a promising class of potent analgesics with wider therapeutic windows.

G Protein-coupled Receptor Biased Agonism

G protein-coupled receptors are the largest family of targets for current therapeutics. The classic model of their activation was binary, where agonist binding induced an active conformation and subsequent downstream signaling. Subsequently, the revised concept of biased agonism emerged, where different ligands at the same G protein-coupled receptor selectively activate one downstream pathway versus another. Advances in understanding the mechanism of biased agonism have led to the development of novel ligands, which have the potential for improved therapeutic and safety profiles. In this review, we summarize the theory and most recent breakthroughs in understanding biased signaling, examine recent laboratory investigations concerning biased ligands across different organ systems, and discuss the promising clinical applications of biased agonism.

Using iPSC-derived human DA neurons from opioid-dependent subjects to study dopamine dynamics

INTRODUCTION

The dopaminergic (DA) system plays important roles in addiction. However, human DA neurons from drug-dependent subjects were not available for study until recent development in inducible pluripotent stem cells (iPSCs) technology.

METHODS

In this study, we produced DA neurons differentiated using iPSCs derived from opioid-dependent and control subjects carrying different 3' VNTR (variable number tandem repeat) polymorphism in the human dopamine transporter (DAT or SLC6A3). In addition, the effects of valproic acid (VPA) exposures on iPSC-derived human DA neurons are also examined.

RESULTS

We present the first evidence suggesting that the 3' VNTR polymorphism in the hDAT gene affects DAT expression level in iPSC-derived human DA neurons. In human DA neurons, which provide an appropriate cellular milieu, VPA treatment alters the expression of several genes important for dopaminergic neuron function including DAT, Nurr1, and TH; this might partly explain its action in regulating addictive behaviors. VPA treatment also significantly increased DA D2 receptor (Drd2) expression, especially in the opioid-dependent iPSC cell lines.

CONCLUSIONS

Our data suggest that human iPSC-derived DA neurons may be useful in in vitro experimental model to examine the effects of genetic variation in gene regulation, to examine the underlying mechanisms in neurological disorders including drug addiction, and to serve as a platform for therapeutic development.

Loss of the preferential control over the striato-nigral direct pathway by striatal NMDA receptors in a rat model of Parkinson's disease

By using multi-probe microdialysis we previously demonstrated that endogenous glutamate differentially regulates the activity of the striatal output pathways in vivo, through N-methyl-d-aspartate (NMDA) receptors containing the GluN2A or GluN2B subunits. Using the same approach, we presently investigate whether reverse dialysis of NMDA in the striatum differentially affects GABA release in the striatum and in striatal target areas, i.e. globus pallidus (GP) and substantia nigra reticulata (SNr). Moreover, we ask whether this control is altered under parkinsonian conditions. Intrastriatal NMDA perfusion (10 min) evoked GABA release more potently in SNr (1-100 μM) than in other regions (10-100 μM), suggesting preferential control over striato-nigral projection neurons. Intrastriatal NMDA more potently stimulated glutamate levels in the striatum (1-100 μM) and SNr (1-10 μM) than in GP (10 μM). Striatal dopamine denervation with 6-hydroxydopamine caused a leftward shift in the NMDA concentration-response curve. Intrastriatal NMDA elevated GABA levels at 0.1 μM (all regions) and 1 μM (striatum and GP only), but not at higher concentrations, indicating that, compared to naïve animals, the GABA response in SNr was attenuated. Attenuation of the glutamate response was also observed in SNr (NMDA effective only at 0.1 μM). Conversely, the glutamate response in GP was widened (NMDA effective in the 0.1-1 μM range). We conclude that NMDA preferentially stimulates the activity of the striato-nigral direct pathway under physiological conditions. In Parkinson's disease, dopamine loss compromises the NMDA ability to stimulate striato-nigral neurons, thus shifting the NMDA control towards the striato-pallidal ones.

Acupuncture improves locomotor function by enhancing GABA receptor expression in transient focal cerebral ischemia rats

Stroke is the major cause of long-term disability among adults. Recent studies have found that GABAergic inhibitory neurotransmission plays a vital role in ameliorate locomotor damage after ischemic injury. Acupuncture has been widely used to improve locomotor function. However, the underlying mechanisms remain unclear. The present study is designed to investigate whether GABA and GABA receptors are involved in the mechanism underlying acupuncture treatment in rats with middle cerebral artery occlusion (MCAO). One week after acupuncture at JiaJi acupoint, the locomotor function and infarct volumes were tested. Then level of GABA and the expressions of GABAAγ2 and GABABR2 were assessed by high-performance liquid chromatography, immunofluorescence and immunohistochemistry, respectively. Compared with normal group, GABAAγ2 and GABABR2 expressions were decreased in striatum and spinal cord of the MCAO group. After acupuncture, the expressions of the two receptors were increased, but levels of GABA and trafficking protein, kinesin binding 1 (TRAK1), which plays a role in the intracellular transport of GABA receptors, were unchanged. The present study suggests that acupuncture could reverse locomotor function by modulating the expressions of GABA receptors in MCAO rats.

Stimulation of δ opioid receptor and blockade of nociceptin/orphanin FQ receptor synergistically attenuate parkinsonism

δ opioid peptide (DOP) receptors are considered a therapeutic target in Parkinson's disease, although the use of DOP agonists may be limited by side effects, including convulsions. To circumvent this issue, we evaluated whether blockade of nociceptin/orphanin FQ (N/OFQ) tone potentiated the antiparkinsonian effects of DOP agonists, thus allowing for reduction of their dosage. Systemic administration of the N/OFQ receptor (NOP) antagonist J-113397 [(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H benzimidazol-2-one] and the DOP receptor agonist SNC-80 [(+)-4-[(αR)-α-(2S,5R)-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxy-benzyl]-N-N-diethylbenzamide] revealed synergistic attenuation of motor deficits in 6-hydroxydopamine hemilesioned rats and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated mice. In this model, repeated administration of the combination produced reproducible antiparkinsonian effects and was not associated with rescued striatal dopamine terminals. Microdialysis studies revealed that either systemic administration or local intranigral perfusion of J-113397 and SNC-80 led to the enhancement of nigral GABA, reduction of nigral Glu, and reduction of thalamic GABA levels, consistent with the view that NOP receptor blockade and DOP receptor stimulation caused synergistic overinhibition of nigro-thalamic GABA neurons. Whole-cell recording of GABA neurons in nigral slices confirmed that NOP receptor blockade enhanced the DOP receptor-induced effect on IPSCs via presynaptic mechanisms. Finally, SNC-80 more potently stimulated stepping activity in mice lacking the NOP receptor than wild-type controls, confirming the in vivo occurrence of an NOP-DOP receptor interaction. We conclude that endogenous N/OFQ functionally opposes DOP transmission in substantia nigra reticulata and that NOP receptor antagonists might be used in combination with DOP receptor agonists to reduce their dosage while maintaining their full therapeutic efficacy.

Automated touch screen device for recording complex rodent behaviors

BACKGROUND

Monitoring mouse behavior is a critical step in the development of modern pharmacotherapies.

NEW METHOD

Here we describe the application of a novel method that utilizes a touch display computer (tablet) and software to detect, record, and report fine motor behaviors. A consumer-grade tablet device is placed in the bottom of a specially made acrylic cage allowing the animal to walk on the device (MouseTrapp). We describe its application in open field (for general locomotor studies) which measures step lengths and velocity. The device can perform light-dark (anxiety) tests by illuminating half of the screen and keeping the other half darkened. A divider is built into the lid of the device allowing the animal free access to either side.

RESULTS

Treating mice with amphetamine and the delta opioid peptide receptor agonist SNC80 stimulated locomotor activity on the device. Amphetamine increased step velocity but not step length during its peak effect (40-70min after treatment), thus indicating detection of subtle amphetamine-induced effects. Animals showed a preference (74% of time spent) for the darkened half compared to the illuminated side.

COMPARISON WITH EXISTING METHOD

Animals were videotaped within the chamber to compare quadrant crosses to detect motion on the device. The slope, duration and magnitude of quadrant crosses tightly correlated with overall locomotor activity as detected by MouseTrapp.

CONCLUSIONS

We suggest that modern touch display devices such as MouseTrapp will be an important step toward automation of behavioral analyses for characterizing phenotypes and drug effects.

Synergistic activity between the delta-opioid agonist SNC80 and amphetamine occurs via a glutamatergic NMDA-receptor dependent mechanism

Glutamate is known to cause the release of dopamine through a Ca(2+)-sensitive mechanism that involves activation of NMDA ionotropic glutamate receptors. In the current study, we tested the hypothesis that the delta opioid agonist SNC80 acts indirectly, via the glutamatergic system, to enhance both amphetamine-stimulated dopamine efflux from striatal preparations and amphetamine-stimulated locomotor activity. SNC80 increased extracellular glutamate content, which was accompanied by a concurrent decrease in GABA levels. Inhibition of NMDA signaling with the selective antagonist MK801 blocked the enhancement of both amphetamine-induced dopamine efflux and hyperlocomotion observed with SNC80 pretreatment. Addition of exogenous glutamate also potentiated amphetamine-stimulated dopamine efflux in a Mg(2+)- and MK801-sensitive manner. After removal of Mg(2+) to relieve the ion conductance inhibition of NMDA receptors, SNC80 both elicited dopamine release alone and produced a greater enhancement of amphetamine-evoked dopamine efflux. The action of SNC80 to enhance amphetamine-evoked dopamine efflux was mimicked by the GABA(B) antagonist 2-hydroxysaclofen. These cumulative findings suggest SNC80 modulates amphetamine-stimulated dopamine efflux through an intra-striatal mechanism involving inhibition of GABA transmission leading to the local release of glutamate followed by subsequent activation of NMDA receptors.

CNS penetration of the opioid glycopeptide MMP-2200: a microdialysis study

Endogenous opioid peptides enkephalin and dynorphin are major co-transmitters of striatofugal pathways of the basal ganglia. They are involved in the genesis of levodopa-induced dyskinesia and in the modulation of direct and indirect striatal output pathways that are disrupted in Parkinson's disease. One pharmacologic approach is to develop synthetic glycopeptides closely resembling endogenous peptides to restore their normal functions. Glycosylation promotes penetration of the blood-brain barrier. We investigated CNS penetration of the opioid glycopeptide MMP-2200, a mixed δ/μ-agonist based on leu-enkephalin, as measured by in vivo microdialysis and subsequent mass spectrometric analysis in awake, freely moving rats. The glycopeptide (10 mg/kg) reaches the dorsolateral striatum (DLS) rapidly after systemic (i.p.) administration and is stably detectable for the duration of the experiment (80 min). The detected level at the end of the experiment (around 250 pM) is about 10-fold higher than the level of the endogenous leu-enkephalin, measured simultaneously. This is one of the first studies to directly prove that glycosylation of an endogenous opioid peptide leads to excellent blood-brain barrier penetration after systemic injection, and explains robust behavioral effects seen in previous studies by measuring how much glycopeptide reaches the target structure, in this case the DLS.

Current research on opioid receptor function

The use of opioid analgesics has a long history in clinical settings, although the comprehensive action of opioid receptors is still less understood. Nonetheless, recent studies have generated fresh insights into opioid receptor-mediated functions and their underlying mechanisms. Three major opioid receptors (μ-opioid receptor, MOR; δ-opioid receptor, DOR; and κ-opioid receptor, KOR) have been cloned in many species. Each opioid receptor is functionally sub-classified into several pharmacological subtypes, although, specific gene corresponding each of these receptor subtypes is still unidentified as only a single gene has been isolated for each opioid receptor. In addition to pain modulation and addiction, opioid receptors are widely involved in various physiological and pathophysiological activities, including the regulation of membrane ionic homeostasis, cell proliferation, emotional response, epileptic seizures, immune function, feeding, obesity, respiratory and cardiovascular control as well as some neurodegenerative disorders. In some species, they play an essential role in hibernation. One of the most exciting findings of the past decade is the opioid-receptor, especially DOR, mediated neuroprotection and cardioprotection. The upregulation of DOR expression and DOR activation increase the neuronal tolerance to hypoxic/ischemic stress. The DOR signal triggers (depending on stress duration and severity) different mechanisms at multiple levels to preserve neuronal survival, including the stabilization of homeostasis and increased pro-survival signaling (e.g., PKC-ERK-Bcl 2) and antioxidative capacity. In the heart, PKC and KATP channels are involved in the opioid receptor-mediated cardioprotection. The DOR-mediated neuroprotection and cardioprotection have the potential to significantly alter the clinical pharmacology in terms of prevention and treatment of life-threatening conditions like stroke and myocardial infarction. The main purpose of this article is to review the recent work done on opioids and their receptor functions. It shall provide an informative reference for better understanding the opioid system and further elucidation of the opioid receptor function from a physiological and pharmacological point of view.

Microdialysis and mass spectrometric monitoring of dopamine and enkephalins in the globus pallidus reveal reciprocal interactions that regulate movement

Pallidal dopamine, GABA and the endogenous opioid peptides enkephalins have independently been shown to be important controllers of sensorimotor processes. Using in vivo microdialysis coupled to liquid chromatography-mass spectrometry and a behavioral assay, we explored the interaction between these three neurotransmitters in the rat globus pallidus. Amphetamine (3 mg/kg i.p.) evoked an increase in dopamine, GABA and methionine/leucine enkephalin. Local perfusion of the dopamine D(1) receptor antagonist SCH 23390 (100 μM) fully prevented amphetamine stimulated enkephalin and GABA release in the globus pallidus and greatly suppressed hyperlocomotion. In contrast, the dopamine D(2) receptor antagonist raclopride (100 μM) had only minimal effects suggesting a greater role for pallidal D(1) over D(2) receptors in the regulation of movement. Under basal conditions, opioid receptor blockade by naloxone perfusion (10 μM) in the globus pallidus stimulated GABA and inhibited dopamine release. Amphetamine-stimulated dopamine release and locomotor activation were attenuated by naloxone perfusion with no effect on GABA. These findings demonstrate a functional relationship between pallidal dopamine, GABA and enkephalin systems in the control of locomotor behavior under basal and stimulated conditions. Moreover, these findings demonstrate the usefulness of liquid chromatography-mass spectrometry as an analytical tool when coupled to in vivo microdialysis.

Endogenous nociceptin/orphanin FQ (N/OFQ) contributes to haloperidol-induced changes of nigral amino acid transmission and parkinsonism: a combined microdialysis and behavioral study in naïve and nociceptin/orphanin FQ receptor knockout mice

The contribution of endogenous nociceptin/orphanin FQ (N/OFQ) to neuroleptic-induced parkinsonism has been evaluated in haloperidol-treated mice. Pharmacological blockade of N/OFQ receptors (NOP) via systemic administration of 1-[(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H benzimidazol-2-one (J-113397, 0.01-10 mg/kg i.p.) or central injection of [Nphe(1),Arg(14),Lys(15)]N/OFQ-NH(2) (UFP-101, 10 nmol i.c.v.) attenuated (0.8 mg/kg) haloperidol-induced motor deficits as evaluated by a battery of behavioral tests providing complementary information on motor parameters: the bar, drag and rotarod tests. A combined neurochemical and behavioral approach was then used to investigate whether the substantia nigra reticulata could be involved in antiakinetic actions of J-113397. Microdialysis combined to the bar test revealed that haloperidol (0.3 and 0.8 mg/kg i.p.) caused a dose-dependent and prolonged elevation of immobility time (i.e. akinesia) which was associated with an increase in nigral glutamate and a reduction in GABA release. Conversely, J-113397 (1 mg/kg) alone reduced glutamate and elevated nigral GABA release, and when challenged against haloperidol, counteracted its behavioral and neurochemical effects. Microdialysis coupled to behavioral testing also demonstrated that NOP receptor knockout mice were resistant to haloperidol (0.3 mg/kg) compared to wild-type mice, lack of response being associated with a reversal of glutamate release facilitation into inhibition and no change in nigral GABA release. This study provides pharmacological and genetic evidence that endogenous N/OFQ contributes to haloperidol-induced akinesia and changes of amino acid transmission in mice. Moreover, it confirms the view that NOP receptor antagonists are capable of reversing akinesia across species and genotypes and may prove effective in relieving neuroleptic-induced parkinsonism.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Zonisamide enhances delta receptor-associated neurotransmitter release in striato-pallidal pathway

A recent randomized control study demonstrated that zonisamide (ZNS), an antiepileptic drug, is effective in Parkinson's disease at the lower than the therapeutic doses against epilepsy (25-50 mg/day); however, the detailed mechanism of antiparkinsonian effects of ZNS remains to be clarified. To determine the mechanism of antiparkinsonian effect of ZNS, we investigated the effects of ZNS on extracellular levels of dopamine in the striatum (STR), glutamate in substantia nigra pars reticulata (SNr), GABA in globus pallidus (GP), subthalamic nucleus (STN) and SNr, using multiple microdialysis probes. Striatal perfusion of 1000 microM ZNS (within therapeutic-relevant concentration against epilepsy) increased extracellular levels of dopamine in STR, whereas 100 microM ZNS (lower than the therapeutic-relevant concentration against epilepsy but within the therapeutic rage against Parkinson's disease) did not affect it. Striatal perfusion of ZNS (100 and 1000 microM) decreased the extracellular levels of GABA in STN and glutamate in SNr, but decreased extracellular GABA level in GP without affecting GABA level in SNr. These concentration-dependent effects of ZNS on extracellular neurotransmitter levels were independent of dopamine and delta(2) receptors; however, blockade of delta(1) receptor inhibited the effects of ZNS. Furthermore, activation of delta(1) receptor enhanced the effects of ZNS on neurotransmitter level. These results suggest that ZNS does not affect the direct pathway but inhibits the indirect pathway, which is mediated by delta(1) receptor. Therefore, the antiparkinsonian effects of ZNS seem to be mediated through the interaction between lower than therapeutically-relevant concentration against epilepsy of ZNS (100 microM) and delta(1) receptor.