Delta opioid receptors regulate calcium-dependent, amphetamine-evoked glutamate levels in the rat striatum: an in vivo microdialysis study

The role of enkephalinergic systems in substance use disorders

Enkephalin, an endogenous opioid peptide, is highly expressed in the reward pathway and may modulate neurotransmission to regulate reward-related behaviors, such as drug-taking and drug-seeking behaviors. Drugs of abuse also directly increase enkephalin in this pathway, yet it is unknown whether or not changes in the enkephalinergic system after drug administration mediate any specific behaviors. The use of animal models of substance use disorders (SUDs) concurrently with pharmacological, genetic, and molecular tools has allowed researchers to directly investigate the role of enkephalin in promoting these behaviors. In this review, we explore neurochemical mechanisms by which enkephalin levels and enkephalin-mediated signaling are altered by drug administration and interrogate the contribution of enkephalin systems to SUDs. Studies manipulating the receptors that enkephalin targets (e.g., mu and delta opioid receptors mainly) implicate the endogenous opioid peptide in drug-induced neuroadaptations and reward-related behaviors; however, further studies will need to confirm the role of enkephalin directly. Overall, these findings suggest that the enkephalinergic system is involved in multiple aspects of SUDs, such as the primary reinforcing properties of drugs, conditioned reinforcing effects, and sensitization. The idea of dopaminergic-opioidergic interactions in these behaviors remains relatively novel and warrants further research. Continuing work to elucidate the role of enkephalin in mediating neurotransmission in reward circuitry driving behaviors related to SUDs remains crucial.

Endogenous enkephalin is necessary for cocaine-induced alteration in glutamate transmission within the nucleus accumbens

Altered glutamate transmission within the nucleus accumbens (NAc) has been proposed as a central mechanism underlying behavioural sensitisation associated with repeated cocaine exposure. In addition to glutamate, enkephalin, an endogenous opioid peptide derived from proenkephalin, is necessary for the neuroadaptations associated with chronic cocaine. However, the influence of enkephalin on long-term changes in glutamate transmission within the NAc associated with cocaine-induced sensitisation has not been described. This study used knockout proenkephalin mice (KO) to study the influence of endogenous enkephalin on the adaptations in glutamate neurotransmission associated with repeated cocaine treatment. Wild-type (WT) and KO mice were treated with daily cocaine injections for 9 days to induce sensitisation. On days 15 and 21, the animals received a cocaine challenge and locomotor sensitisation was evaluated, and microdialysis was performed to determine accumbens glutamate content on day 21. No expression of behavioural sensitisation to cocaine was evidenced in the KO mice. Consistently, these showed no changes in glutamate transmission in the NAc associated with repeated cocaine. This study reveals the central role of enkephalin in regulating the glutamate mechanisms associated with cocaine sensitisation.

Loss of Non-Apoptotic Role of Caspase-3 in the PINK1 Mouse Model of Parkinson's Disease

Caspases are a family of conserved cysteine proteases that play key roles in multiple cellular processes, including programmed cell death and inflammation. Recent evidence shows that caspases are also involved in crucial non-apoptotic functions, such as dendrite development, axon pruning, and synaptic plasticity mechanisms underlying learning and memory processes. The activated form of caspase-3, which is known to trigger widespread damage and degeneration, can also modulate synaptic function in the adult brain. Thus, in the present study, we tested the hypothesis that caspase-3 modulates synaptic plasticity at corticostriatal synapses in the phosphatase and tensin homolog (PTEN) induced kinase 1 (PINK1) mouse model of Parkinson’s disease (PD). Loss of PINK1 has been previously associated with an impairment of corticostriatal long-term depression (LTD), rescued by amphetamine-induced dopamine release. Here, we show that caspase-3 activity, measured after LTD induction, is significantly decreased in the PINK1 knockout model compared with wild-type mice. Accordingly, pretreatment of striatal slices with the caspase-3 activator α-(Trichloromethyl)-4-pyridineethanol (PETCM) rescues a physiological LTD in PINK1 knockout mice. Furthermore, the inhibition of caspase-3 prevents the amphetamine-induced rescue of LTD in the same model. Our data support a hormesis-based double role of caspase-3; when massively activated, it induces apoptosis, while at lower level of activation, it modulates physiological phenomena, like the expression of corticostriatal LTD. Exploring the non-apoptotic activation of caspase-3 may contribute to clarify the mechanisms involved in synaptic failure in PD, as well as in view of new potential pharmacological targets.

Targeting opioid dysregulation in depression for the development of novel therapeutics

Since the serendipitous discovery of the first class of modern antidepressants in the 1950's, all pharmacotherapies approved by the Food and Drug Administration for major depressive disorder (MDD) have shared a common mechanism of action, increased monoaminergic neurotransmission. Despite the widespread availability of antidepressants, as many as 50% of depressed patients are resistant to these conventional therapies. The significant length of time required to produce meaningful symptom relief with these medications, 4-6 weeks, indicates that other mechanisms are likely involved in the pathophysiology of depression which may yield more viable targets for drug development. For decades, no viable candidate target with a different mechanism of action to that of conventional therapies proved successful in clinical studies. Now several exciting avenues for drug development are under intense investigation. One of these emerging targets is modulation of endogenous opioid tone. This review will evaluate preclinical and clinical evidence pertaining to opioid dysregulation in depression, focusing on the role of the endogenous ligands endorphin, enkephalin, dynorphin, and nociceptin/orphanin FQ (N/OFQ) and their respective receptors, mu (MOR), delta (DOR), kappa (KOR), and the N/OFQ receptor (NOP) in mediating behaviors relevant to depression and anxiety. Finally, putative opioid based antidepressants that are under investigation in clinical trials, ALKS5461, JNJ-67953964 (formerly LY2456302 and CERC-501) and BTRX-246040 (formerly LY-2940094) will be discussed. This review will illustrate the potential therapeutic value of targeting opioid dysregulation in developing novel therapies for MDD.

Bifunctional opioid receptor ligands as novel analgesics

Prolonged treatment of chronic severe pain with opioid analgesics is frought with problematic adverse effects including tolerance, dependence, and life-threatening respiratory depression. Though these effects are mediated predominately through preferential activation of μ opioid peptide (μOP) receptors, there is an emerging appreciation that actions at κOP and δOP receptors contribute to the observed pharmacologic and behavioral profile of μOP receptor agonists and may be targeted simultaneously to afford improved analgesic effects. Recent developments have also identified the related nociceptin opioid peptide (NOP) receptor as a key modulator of the effects of μOP receptor signaling. We review here the available literature describing OP neurotransmitter systems and highlight recent drug and probe design strategies.

Enkephalin as a Pivotal Player in Neuroadaptations Related to Psychostimulant Addiction

Enkephalin expression is high in mesocorticolimbic areas associated with psychostimulant-induced behavioral and neurobiological effects, and may also modulate local neurotransmission in this circuit network. Psychostimulant drugs, like amphetamine and cocaine, significantly increase the content of enkephalin in these brain structures, but we do not yet understand the specific significance of this drug-induced adaptation. In this review, we summarize the neurochemical and molecular mechanism of psychostimulant-induced enkephalin activation in mesocorticolimbic brain areas, and the contribution of this opioid peptide in the pivotal neuroadaptations and long-term behavioral changes underlying psychostimulant addiction. There is evidence suggesting that adaptive changes in enkephalin content in the mesocorticolimbic circuit, induced by acute and chronic psychostimulant administration, may represent a key initial step in the long-term behavioral and neuronal plasticity induced by these drugs.

Enkephalin is essential for the molecular and behavioral expression of cocaine sensitization

Behavioral sensitization to cocaine is associated to neuroadaptations that contribute to addiction. Enkephalin is highly expressed in mesocorticolimbic areas associated with cocaine-induced sensitization; however, their influence on cocaine-dependent behavioral and neuronal plasticity has not been explained. In this study, we employed a knockout (KO) model to investigate the contribution of enkephalin in cocaine-induced behavioral sensitization. Wild-type (WT) and proenkephalin KO mice were treated with cocaine once daily for 9 days to induce sensitization. Additionally, to clarify the observations in KO mice, the same procedure was applied in C57BL/6 mice, except that naloxone was administered before each cocaine injection. All animals received a cocaine challenge on days 15 and 21 of the treatment to evaluate the expression of locomotor sensitization. On day 21, microdialysis measures of accumbal extracellular dopamine, Western blotting for GluR1 AMPA receptor (AMPAR), phosphorylated ERK2 (pERK2), CREB (pCREB), TrKB (pTrkB) were performed in brain areas relevant for sensitization from KO and WT and/or naloxone- and vehicle pre-treated animals. We found that KO mice do not develop sensitization to the stimulating properties of cocaine on locomotor activity and on dopamine release in the nucleus accumbens (NAc). Furthermore, pivotal neuroadaptations such as the increase in pTrkB receptor, pERK/CREB and AMPAR related to sensitized responses were absent in the NAc from KO mice. Consistently, full abrogation of cocaine-induced behavioral and neuronal plasticity after naloxone pre-treatment was observed. We show for first time that the proenkephalin system is essential in regulating long-lasting pivotal neuroadaptations in the NAc underlying behavioral sensitization to cocaine.

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.

Control of striatal signaling by g protein regulators

Signaling via heterotrimeric G proteins plays a crucial role in modulating the responses of striatal neurons that ultimately shape core behaviors mediated by the basal ganglia circuitry, such as reward valuation, habit formation, and movement coordination. Activation of G protein-coupled receptors (GPCRs) by extracellular signals activates heterotrimeric G proteins by promoting the binding of GTP to their α subunits. G proteins exert their effects by influencing the activity of key effector proteins in this region, including ion channels, second messenger enzymes, and protein kinases. Striatal neurons express a staggering number of GPCRs whose activation results in the engagement of downstream signaling pathways and cellular responses with unique profiles but common molecular mechanisms. Studies over the last decade have revealed that the extent and duration of GPCR signaling are controlled by a conserved protein family named regulator of G protein signaling (RGS). RGS proteins accelerate GTP hydrolysis by the α subunits of G proteins, thus promoting deactivation of GPCR signaling. In this review, we discuss the progress made in understanding the roles of RGS proteins in controlling striatal G protein signaling and providing integration and selectivity of signal transmission. We review evidence on the formation of a macromolecular complex between RGS proteins and other components of striatal signaling pathways, their molecular regulatory mechanisms and impacts on GPCR signaling in the striatum obtained from biochemical studies and experiments involving genetic mouse models. Special emphasis is placed on RGS9-2, a member of the RGS family that is highly enriched in the striatum and plays critical roles in drug addiction and motor control.

The Role of BDNF/TrkB Signaling in Acute Amphetamine-Induced Locomotor Activity and Opioid Peptide Gene Expression in the Rat Dorsal Striatum

Exposure to psychostimulants increases brain-derived neurotrophic factor (BDNF) mRNA and protein levels in the cerebral cortex and subcortical structures. Because BDNF is co-localized with dopamine and glutamate in afferents to the striatum of rats, it may be co-released with those neurotransmitters upon stimulation. Further, there may be an interaction between the intracellular signaling cascades activated by dopamine, glutamate, and TrkB receptors in medium spiny striatal neurons. In the present study, the effect of acute amphetamine administration on TrkB phosphorylation, as an indirect indicator of activation, and striatal gene expression, was evaluated. In Experiment 1, 15 min or 2 h after a single saline or amphetamine (2.5 mg/kg, i.p.) injection, the caudate–putamen (CPu), nucleus accumbens (NAc), and dorsomedial prefrontal cortex (dmPFC) were extracted and processed for phospho (p)-TrkB immunoreactivity. Immunoprecipitation analyses indicated that neither the tyrosine phosphorylation (p-Tyr) or autophosphorylation sites of TrkB (706) were changed in NAc, CPu, or dmPFC 15 min after amphetamine administration. In contrast, p-Tyr and the PLCγ phosphorylation site of TrkB (816) were increased in the NAc and CPu 2 h after amphetamine. In Experiment 2, intra-striatal infusion of the tyrosine kinase inhibitor, K252a, increased amphetamine-induced vertical activity but not total distance traveled. In addition, K252a inhibited amphetamine-induced preprodynorphin, but not preproenkephalin, mRNA expression in the striatum. These data indicate that acute amphetamine administration induces p-TrkB activation and signaling in a time- and brain region-dependent manner and that TrkB/BDNF signaling plays an important role in amphetamine-induced behavior and striatal gene expression.

Hyperactive mice show elevated D2(High) receptors, a model for schizophrenia: Calcium/calmodulin-dependent kinase II alpha knockouts

The cerebral frontal cortex of patients who had schizophrenia shows elevated levels of RNA for calcium/calmodulin-dependent protein kinase II beta (CaMKIIbeta). In addition, recent research shows that animal models for schizophrenia, such as amphetamine-sensitized rats, consistently show elevated levels of D2 receptors in their high-affinity state (D2(High)), the major target for antipsychotic medication. The present study was done, therefore, to examine whether an alteration in the levels of CaMKIIbeta could lead to altered levels of D2(High) receptors. We found that the CaMKII inhibitor, KN-93, markedly reduced D2(High) states in rat striatum. In addition, we studied heterozygous CaMKIIalpha knock-out mice that show features analogous to schizophrenia. The striata of these mice revealed a 2.8-fold increase in D2(High) receptors. In frontal cortex of the heterozygous CaMKIIalpha knock-out mice, CaMKIIalpha mRNA levels were reduced by 50%, while CaMKIIbeta mRNA levels were unaltered. In striatum, CaMKIIbeta mRNA levels were increased by 29%, suggesting the presence of a new CaMKIIbeta regulatory pathway not previously described. The elevated levels of CaMKIIbeta mRNA in the striatum suggest that this enzyme may increase D2(High) in animals and possibly in schizophrenia itself.

Sensitization to amphetamine occurs simultaneously at immune level and in met-enkephalin of the nucleus accumbens and spleen: an involved NMDA glutamatergic mechanism

Administration of psychostimulants can elicit a sensitized response to the stimulating and reinforcing properties of the drugs, although there is scarce information regarding their effects at immune level. We previously demonstrated that an acute exposure to amphetamine (5 mg/kg, i.p.) induced an inhibitory effect on the splenic T-cell proliferative response, along with an increase in met-enkephalin at limbic and immune levels, 4 days following drug administration. In this study, we evaluated the amphetamine-induced effects at weeks one and three after the same single dose treatment (5 mg/kg, i.p.) on the lymphoproliferative response and on the met-enkephalin in the nucleus accumbens (NAc), prefrontal cortex (PfC), spleen and thymus. It was demonstrated that these effects disappeared completely after three weeks, although re-exposure to an amphetamine challenge induced the expression of sensitization to the effects of amphetamine on the lymphoproliferative response and on the met-enkephalin from NAc, spleen and thymus, but not in the PfC. Pre-treatment with MK-801 (0.1 mg/kg, i.p.), an N-methyl-d-aspartate (NMDA) glutamatergic receptor antagonist, blocked the effects of a single amphetamine exposure on the lymphoproliferative response and on met-enkephalin in the NAc and spleen. Furthermore, the NMDA receptor antagonist administered prior to amphetamine challenge also blocked the expression of sensitization in both parameters evaluated. These findings show a long-lasting amphetamine-induced sensitization phenomenon at the immune level in a parallel way to that occurring in the limbic and immune enkephalineric system. A glutamate mechanism is implied in the long-term amphetamine-induced effects at immune level and in the met-enkephalin from NAc and spleen.

Changes in Proenkephalin mRNA expression in forebrain areas after amphetamine-induced behavioural sensitization

Acute and repeated psychostimulant administration induces a long-lasting enhanced behavioural response to a subsequent drug challenge, known as behavioural sensitization. This phenomenon involves persistent neurophysiological adaptations, which may lead to drug addiction. Brain dopaminergic pathways have been implicated as the main neurobiological substrates of behavioural sensitization, although other neurotransmitters and neuromodulators may also participate. In order to investigate a possible involvement of opioid systems in amphetamine (AMPH) behavioural sensitization, we studied the AMPH-induced changes in Proenkephalin (Pro-Enk) mRNA expression in forebrain areas in both drug-naïve and AMPH-sensitized rats. Male Sprague-Dawley rats were sensitized to AMPH by means of a single AMPH (1 mg/kg s.c.) injection and the same dose was injected 7 days later to assess the expression of sensitization. Pro-Enk mRNA levels were evaluated by in situ hybridization in coronal brain sections. AMPH injection induced an increase in Pro-Enk mRNA expression in the nucleus accumbens and the medial-posterior caudate-putamen in drug-naïve rats. Challenge with AMPH to rats injected 1 week earlier with AMPH induced motor sensitization and increased and decreased Pro-Enk mRNA expression in the prefrontal cortex and the anterior medial caudate-putamen, respectively. Our results suggest that alterations in cortical and striatal enkephalinergic systems could contribute to the expression of AMPH behavioural sensitization.

Nalfurafine, the kappa opioid agonist, inhibits icilin-induced wet-dog shakes in rats and antagonizes glutamate release in the dorsal striatum

Icilin, a cooling compound, produces vigorous wet-dog shakes in rats. We have reported previously that icilin-induced wet-dog shakes are blocked by the kappa opioid receptor agonists, nalfurafine and U50,488H, and that icilin evokes a dose- and time-dependent increase in glutamate within the dorsal striatum. Since activation of kappa opioid receptors inhibits glutamate release intrastriatally, we targeted glutamate release within the dorsal striatum using nalfurafine and examined the role of the dorsal striatum in icilin-induced wet-dog shakes, more specifically, the effect that icilin-evoked intrastriatal glutamate release has on the overt stimulant behavior. We report that nalfurafine (0.04mg/kg) inhibits icilin (0.50mg/kg)-induced wet-dog shakes and that this inhibition is reversed by intrastriatal perfusion of the kappa opioid receptor antagonist, norbinaltorphimine (100nM). Furthermore,we antagonized icilin-evoked glutamate release with nalfurafine (0.04mg/kg), and reversed inhibition of glutamate release with intrastriatal norbinaltorphimine (100nM). These findings support a central component in the behavioral response to icilin and suggest that activation of kappa opioid receptors antagonizes icilin-induced wet-dog shakes in rats by inhibiting glutamate release within the dorsal striatum.

Cocaine increases endoplasmic reticulum stress protein expression in striatal neurons

Cocaine administration upregulates the levels of extracellular glutamate and dopamine in the striatum. Activation of the receptors alters calcium homeostasis in striatal neurons leading to the expression of the endoplasmic reticulum (ER) stress proteins. It was therefore hypothesized that cocaine upregulates the expression of the ER stress proteins, immunoglobulin heavy chain binding protein (BiP), Ire1alpha and perk via glutamate and dopamine receptor activation. A novel glutamate microbiosensor and Western immunoblot analyses were mainly performed to test the hypothesis in the rat dorsal striatum. The results showed that i.p. injection of repeated cocaine (20 mg/kg) for nine consecutive days significantly increased extracellular glutamate levels while acute cocaine injection did not. However, the immunoreactivities (IR) of the ER stress proteins in the dorsal striatum were significantly increased by either acute or repeated cocaine injections as compared with saline controls. Intrastriatal injection (i.s.) of the selective group I metabotropic glutamate receptor (mGluR) antagonist N-phenyl-7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxamide (PHCCC; 25 nmol) or the mGluR5 subtype antagonist 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP; 2 and 25 nmol) significantly decreased repeated cocaine-induced increases in the IR of the ER stress proteins in the injected dorsal striatum. Similarly, the selective D1 antagonist (R)-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH23390; 0.1 mg/kg, i.p.) or the N-methyl-d-aspartate antagonist dizocilpine/(5S,10R)-(+)-5-methyl-10,11-dihydro-5H-ibenzo[a,d]cyclohepten-5,10-imine maleate (MK801; 2 nmol, i.s.) decreased acute or repeated cocaine-induced the IR of the ER stress proteins in the dorsal striatum. These data suggest that cocaine upregulates expression of the ER stress proteins in striatal neurons via a mechanism involving activation of glutamate and dopamine receptors.

Co-localization of GABA with other neuroactive substances in the basal ganglia

The dorsal striatum (caudate putamen) contains two types of GABAergic medium spiny neurons (MSNs) that are distinguished by the expression of either the opioid peptide, enkephalin, or the opioid peptide, dynorphin, as well as the tachykinin substance P. Pharmacological studies suggest that these peptides modulate local neurotransmission in the striatum in response to direct and indirect dopamine agonists. In contrast, GABA appears to have minimal impact within the striatum under these conditions. The actions of the peptide cocktail are dependent on the cellular distribution of their receptors in the striatal network. The net result of their actions is a homeostatic response that regulates striatal output and balances dopamine and glutamate receptor stimulation.

Extracellular signal-regulated mitogen-activated protein kinase inhibitors decrease amphetamine-induced behavior and neuropeptide gene expression in the striatum

The aim of this study was to determine whether inhibition of the extracellular-regulated kinase signaling pathway decreases acute amphetamine-induced behavioral activity and neuropeptide gene expression in the rat striatum. Western blotting revealed that extracellular-regulated kinase1/2 phosphorylation was highly induced in the rat striatum 15 min after an acute amphetamine (2.5 mg/kg, i.p.) injection without altering the total amount of extracellular-regulated kinase protein. In a separate experiment, the systemic injection of SL327, a selective inhibitor of extracellular regulated kinase kinase that crosses the blood-brain barrier, 1 h prior to amphetamine administration decreased amphetamine-induced vertical and horizontal activity. Quantitative in situ hybridization histochemistry showed that SL327 abolished the high levels of preproenkephalin and preprodynorphin mRNA induced by amphetamine in the striatum with no alteration of their basal levels. In another set of experiments, the hyperlocomotor activity induced by amphetamine was reduced by pretreatment with intra-striatal infusion of U0126. U0126 also blocked the amphetamine-induced increases in phospho-extracellular-regulated kinase and preproenkephalin and preprodynorphin gene expression in the striatum. These data indicate that activation of the extracellular-regulated kinase cascade contributes to the behavioral effects and changes in striatal neuropeptide gene expression induced by acute amphetamine.

Intracerebral baclofen administration decreases amphetamine-induced behavior and neuropeptide gene expression in the striatum

In a previous study, systemic administration of the GABA(B) receptor agonist, R-(+)-baclofen (2.5 mg/kg, i.p.) blocked acute amphetamine (2.5 mg/kg, i.p.)-induced rearing and neuropeptide (preprodynorphin (PPD), preprotachykinin (PPT), preproenkephalin (PPE), and secretogranin II (SGII)) mRNA expression in the striatum (Zhou et al, 2004). The purpose of the present study was to investigate the site(s) of action of these baclofen effects in the dorsal and ventral striatal circuitries. Infusion of baclofen (75 ng/side) into the ventral tegmental area (VTA), substantia nigra (SN), nucleus accumbens (NA), caudate-putamen (Cpu), or medial prefrontal cortex (mPFC) had no effect on behavioral activity in saline-treated rats habituated to a photocell apparatus. However, intra-VTA infusion of baclofen (75 ng/side) completely blocked, whereas intra-NA and intra-SN infusion of baclofen attenuated, amphetamine-induced vertical activity without affecting amphetamine-induced total distance traveled. In contrast, intramedial PFC and intra-CPu infusion of baclofen had no effect on behavioral activity in amphetamine-treated rats. Infusion of baclofen into the VTA, NA, or SN decreased amphetamine-induced neuropeptide gene expression in the striatum. These results indicate that GABA(B) receptor stimulation within the ventral striatal circuitry is involved in mediating acute amphetamine-induced behaviors and neuropeptide gene expression in the dorsal and ventral striatum. The present study provides information on the potential targets in the brain for baclofen in the initial behavioral and genomic response to amphetamine.

Involvement of corticostriatal glutamatergic terminals in striatal dopamine release elicited by stimulation of delta-opioid receptors

We have previously shown that striatal dopamine release induced locally by a delta-opioid receptor agonist was totally inhibited by a glutamate N-methyl-D-aspartate receptor antagonist, indicating the involvement of glutamatergic receptors in this effect. The aim of the present study was to specify this mechanism. Firstly, we investigated the effect of [D-Pen2,D-Pen5]-enkephalin (DPDPE) on glutamate release in rats by intrastriatal microdialysis. The infusion of DPDPE (10 microm) enhanced the glutamate content in dialysate by approximately 34%, an effect which did not appear to result from inhibition of glutamate uptake. We then considered the consequences of a unilateral thermocoagulation of the frontal cortex on either glutamate or dopamine release induced by stimulation of delta-opioid receptors 2 days later. This lesion, which decreased the glutamate content in ipsilateral striatum by approximately 30%, totally prevented the increase in dialysate levels of glutamate induced by DPDPE. Moreover, whereas DPDPE (10 microm) was found to increase the striatal dopamine release in intact animals by approximately 59%, this effect was also completely suppressed by the cortical lesion. Finally, we studied the effect of the lesion on the [3H]-DPDPE binding to striatal membranes prepared from the whole striatum. In the ipsilateral striatum a significant decrease in this [3H]-DPDPE binding (by approximately 18%) was found 2 days after the lesion. Our results indicate that the increase in striatal dopamine release induced by DPDPE probably depends on glutamate release from corticostriatal glutamatergic afferents in response to the stimulation of delta-opioid receptors located on terminals of these neurons.

mGluR5-dependent increases in immediate early gene expression in the rat striatum following acute administration of amphetamine

Metabotropic glutamate receptor 5 (mGluR5) is densely expressed in medium-sized spiny projection neurons of the rat striatum. Activation of mGluR5 increases intracellular Ca2+, resulting in Ca(2+)-dependent cellular responses. Acute administration of the psychostimulant amphetamine (AMPH) induces immediate early gene (IEG) expression in the striatum, which is considered an important molecular event for the development of striatal neuroplasticity related to the addictive properties of drugs of abuse. This study investigated the role of mGluR5 in the mediation of IEG expression in the rat striatum induced by a single dose of AMPH (4 mg/kg, i.p.) in vivo. We found that systemic administration of the mGluR5-selective antagonist 2-methyl-6-(phenylethynyl) pyridine hydrochloride (MPEP) at a dose of 10 mg/kg, i.p. reduced AMPH-stimulated c-fos mRNA levels in the dorsal (caudoputamen) and ventral (nucleus accumbens) striatum as revealed by quantitative in situ hybridization. Similar results were observed in the three areas of cerebral cortex (cingulate, sensory, and piriform cortex). In contrast to c-fos mRNAs, AMPH-stimulated mRNA expression of another IEG, zif/268, was not significantly altered by the blockade of mGluR5 with MPEP in the entire striatum and the three areas of cortex. Treatment with MPEP alone had no effect on basal levels of c-fos and zif/268 mRNAs in the striatal and cortical areas. These results indicate that an mGluR5-dependent mechanism selectively contributes to c-fos expression in the striatum and cortex in response to acute exposure to AMPH.

GABAB receptor stimulation decreases amphetamine-induced behavior and neuropeptide gene expression in the striatum

The purpose of this study was to investigate whether GABA(B) receptor activation blocks acute amphetamine-induced behavioral activity, dopamine release, and neuropeptide mRNA expression in the striatum. Systemic administration of R-(+)-baclofen (1.25 mg/kg, i.p.) did not alter total distance traveled or vertical rearing induced by amphetamine (2.5 mg/kg, i.p.). At 2.5 mg/kg, baclofen did not alter spontaneous motor activity or total distance traveled, but completely blocked vertical rearing induced by amphetamine. At 5.0 mg/kg, baclofen completely blocked both total distance traveled and vertical rearing induced by amphetamine. Quantitative in situ hybridization histochemistry revealed that baclofen (2.5 mg/kg, i.p.) decreased the ability of amphetamine to increase preprodynorphin (PPD), preprotachykinin (PPT), preproenkephalin (PPE), and secretogranin II (SGII) mRNA levels in the striatum without altering the basal levels of these signals. Baclofen also blocked the amphetamine-induced rise in SGII mRNA in the core and shell of the nucleus accumbens and cingulate cortex. In a separate experiment, systemic baclofen (2.5 mg/kg) decreased the amphetamine-induced increase in dialysate dopamine levels in the striatum. These results suggest that reduced striatal dopamine release contributes to the ability of GABA(B) receptor activation to decrease acute amphetamine-induced behavioral activity and striatal neuropeptide gene expression.

Activation of metabotropic glutamate receptor 5 is associated with effect of amphetamine on brain neurons

The role of metabotropic glutamate receptor 5 (mGluR5) was explored in mechanisms underlying the action of amphetamine (AMPH). The activity of mGluR5 was monitored by measuring the level of [3H]inositol monophosphates in brain neurons, in response to stimulation of 2-choloro-5-hydroxyphenylglycine (CHPG), a selective agonist of mGluR5. Treatment with 1 microM of AMPH for 1 h or 7 days increased the CHPG (1 mM, 30 min)-evoked phosphoinositide turnover by 46% or 92% and 26% or 84% in cultured cortical and hippocampal neurons, respectively, from that of CHPG-only treated cells. When AMPH was present during CHPG application post-1 h or 7 day AMPH incubation, the rate of phosphoinositide hydrolysis in cortical neurons became 121% or 142% higher than that treated with CHPG only. The postnatal day (P) 21 (juvenile) and P60 (adult) rats received three intraperitoneal injections of 5 mg/kg of AMPH or saline daily for 6 days. They were challenged on the eighth day with one dosage and sacrificed 3 h later. Reversible 3H-glutamate binding detected increases of 22-89% in the binding levels of cortex and hippocampus of both ages following the AMPH injections. Increases of 13-18% in the levels of mGluR5 mRNA were seen in the juvenile pyramidal neurons of hippocampal CA1-4, granular cells of dentate gyrus, and ventral thalamic nuclei, as shown by in situ hybridization. The AMPH-induced altered activity of mGluR5 is probably associated with changes in the expression of the glutamate receptors, including mGluR5. AMPH may modify the sensitivity of mGluR5 or interact with the receptor itself.

Local μ and δ opioid receptors regulate amphetamine-induced behavior and neuropeptide mRNA in the striatum

The purpose of this study was to investigate the role that mu and delta opioid receptor blockade has upon stimulant-induced behavior and neuropeptide gene expression in the striatum. Acute administration of amphetamine (2.5 mg/kg i.p.) caused an increase in behavioral activity and preprodynorphin, substance P, and preproenkephalin mRNA expression. Intrastriatal infusion of the mu opioid antagonist, H-D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP), or the delta opioid antagonist, H-Tyr-Tic[CH(2)NH]-Phe-Phe-OH (TIPPpsi), significantly decreased amphetamine-induced vertical activity. However, only CTAP reduced amphetamine-induced distance traveled. Quantitative in situ hybridization histochemistry revealed that CTAP blocked amphetamine-induced preprodynorphin and substance P mRNA. However, preproenkephalin mRNA levels in the dorsal striatum were increased to the same extent by CTAP, amphetamine, or a combination of the two drugs. In contrast, TIPPpsi significantly decreased amphetamine-induced mRNA expression of all three neuropeptides. These data indicate that both mu and delta receptor subtypes differentially regulate amphetamine-induced behavior and neuropeptide gene expression in the rat striatum.

Neurotensin: dual roles in psychostimulant and antipsychotic drug responses

Central administration of neurotensin (NT) results in a variety of neurobehavioral effects which, depending upon the administration site, resemble the effects of antipsychotic drugs (APDs) and psychostimulants. All clinically effective APDs exhibit significant affinities for dopamine D(2) receptors, supporting the hypothesis that an increase in dopaminergic tone contributes to schizophrenic symptoms. Psychostimulants increase extracellular dopamine (DA) levels and chronics administration can produce psychotic symptoms over time. APDs and psychostimulants induce Fos and NT expression in distinct striatal subregions, suggesting that changes in gene expression underlie some of their effects. To gain insight into the functions of NT, we analyzed APD and psychostimulant induction of Fos in NT knockout mice and rats pretreated with the NT antagonist SR 48692. In both NT knockout mice and rats pretreated with SR 48692, haloperidol-induced Fos expression was markedly attenuated in the dorsolateral striatum; amphetamine-induced Fos expression was reduced in the medial striatum. These results indicate that NT is required for the activation of specific subpopulations of striatal neurons in distinct striatal subregions in response to both APDs and psychostimulants. This review integrates these new findings with previous evidence implicating NT in both APD and psychostimulant responses.

Brain neurotransmitter turnover rates during rat intravenous cocaine self-administration

The turnover rates of dopamine, norepinephrine, serotonin, aspartate, glutamate and GABA were measured in 27 brain regions of rats self-administering cocaine and in yoked cocaine- and yoked vehicle-infused controls using radioactive pulse-labeling procedures to identify brain neuronal systems underlying self-administration. Changes in the activity of heretofore unrecognized dopamine, norepinephrine, serotonin, glutamate and GABA innervations of the forebrain specific to cocaine self-administration were found. This included innervations of the nucleus accumbens, ventral pallidum, lateral hypothalamus and the anterior and posterior cingulate, entorhinal-subicular and visual cortices. Turnover rates also were calculated using metabolite/neurotransmitter ratios which were inconsistent with the pulse-label technologies indicating that ratio procedures are not accurate measures of neurotransmitter utilization. Results with the pulse-label technique provide evidence of the involvement of neuronal systems in cocaine self-administration not previously known, some of which may have a broader role in brain reinforcement processes for natural reinforcers (i.e. food, water, etc.) since drugs of abuse are thought to produce reinforcing effects by modulating activity in these endogenous systems.

Expression of amphetamine-induced behavioral sensitization after short- and long-term withdrawal periods: participation of mu- and delta-opioid receptors

Repeated amphetamine administration results in behavioral sensitization, an enduring behavioral transformation expressed after short and long periods of withdrawal. To investigate the participation of the opioid system in amphetamine-induced behavioral sensitization, we studied the effect of naloxone, an opioid receptor antagonist, on the expression of behavioral sensitization tested after short- (2 days) and long-term (14 days) withdrawal periods. In addition, using quantitative competitive RT-PCR, we examined the levels of mu-opioid receptor (MOR) and delta-opioid receptor (DOR) mRNA in the nucleus accumbens shell (NAcSh) and ventral tegmental area (VTA) of behaviorally sensitized rats, at these two withdrawal times. This study showed that whereas naloxone did not modify the expression of behavioral sensitization tested after 2 days of withdrawal, it completely blocked the expression when tested after 14 days of withdrawal. DOR and MOR mRNA levels were not modified in the NAcSh of rats expressing behavioral sensitization after 2 or 14 days of withdrawal. Conversely, DOR and MOR mRNA levels were elevated in the VTA of animals expressing behavioral sensitization after 2 days of withdrawal. However, whereas DOR mRNA returned to control levels, MOR mRNA levels remained elevated in animals expressing behavioral sensitization after 14 days of withdrawal. These results indicate a striking difference between the role played by opioid receptors in the expression of amphetamine-induced behavioral sensitization, when tested after short- or long-term withdrawal periods. In addition, our results support the notion that repeated amphetamine-induced changes in opioid receptor expression may contribute to the perpetuation of psychostimulant abuse and/or relapse.

CaMKII regulates amphetamine-induced ERK1/2 phosphorylation in striatal neurons

Amphetamine activates extracellular signal-regulated kinase 1 and 2 (ERK1/2) resulting in cAMP response element-binding protein (CREB) and Elk-1 phosphorylation in striatal neurons. In the present study we investigated whether calcium and calmodulin-dependent protein kinase II (CaMKII) regulates amphetamine-induced ERK1/2 pathways in striatal neurons using Western blot and immunohistochemical analysis. Acute administration of amphetamine (5 mg/kg, i.p.) increased phosphorylated (p)CaMKII immunoreactivity. Inhibition of CaMKII by intrastriatal infusion of KN62 (2, 10, or 25 nmol) attenuated amphetamine-induced increases in pERK1/2, pCREB, and pElk-1 immunoreactivity in the ipsilateral dorsal striatum in a dose-dependent manner. These data suggest that CaMKII controls amphetamine-activated ERK1/2 pathways in striatal neurons in vivo.

Amphetamine-induced Fos expression is evident in gamma-aminobutyric acid neurons in the globus pallidus and entopeduncular nucleus in rats treated with intrastriatal c-fos antisense oligodeoxynucleotides

Double immunostaining for Fos and gamma-aminobutyric acid (GABA) was used in a previously established animal model of striatal dysfunction to examine whether GABA-immunoreactive neurons in the globus pallidus (GP) and entopeduncular nucleus (EP) are activated to express Fos immunoreactivity by intraperitoneal injection of amphetamine. Striatal efferent activity was suppressed by intrastriatal infusions of antisense oligodeoxynucleotide targeted to the messenger RNA of the immediate early gene, c-fos. This suppression produced robust rotational behavior and expression of Fos in the ipsilateral GP and EP following amphetamine challenge. The expression of Fos in the ipsilateral GP and EP following amphetamine challenge is not observed in naïve or control antisense-treated animals. Quantitative analysis revealed that a majority of the amphetamine-activated (Fos-immunoreactive) neurons in the GP and EP express GABA. The present results suggest that inhibitory GABAergic projection neurons within these two nuclei are regulated by inhibitory striatal output and suggests that decreased inhibitory striatal output may contribute to the motor dysfunction observed in patients with Huntington's disease.

NK-1 receptor blockade decreases amphetamine-induced behavior and neuropeptide mRNA expression in the striatum

The effect of intrastriatal administration of LY306740, a specific NK-1 receptor antagonist, on the behavior and changes in gene expression elicited by the psychomotor stimulant, amphetamine, was studied. Acute administration of amphetamine (2.5 mg/kg, i.p.) caused an increase in behavioral activity and preproenkephalin, preprodynorphin and substance P mRNA expression in the striatum. When amphetamine-treated rats were pretreated with LY306740 (35 and 20 nmoles per side, intrastriatally), there was a significant decrease in amphetamine-induced behavioral activity. Quantitative in situ hybridization histochemistry revealed that both concentrations of LY306740 significantly decreased amphetamine-induced mRNA expression of all three neuropeptides. These data indicate that striatal NK-1 receptors modulate amphetamine-induced behavior and mRNA expression of neuropeptides in the rat striatum.

Augmented motor activity and reduced striatal preprodynorphin mRNA induction in response to acute amphetamine administration in metabotropic glutamate receptor 1 knockout mice

Metabotropic glutamate receptor 1 (mGluR1) is a G-protein-coupled receptor and is expressed in the medium spiny projection neurons of mouse striatum. To define the role of mGluR1 in actions of psychostimulant, we compared both motor behavior and striatal neuropeptide mRNA expression between mGluR1 mutant and wild-type control mice after a single injection of amphetamine. We found that acute amphetamine injection increased motor activity in both mutant and control mice in a dose-dependent manner (1, 4, and 12 mg/kg, i.p.). However, the overall motor responses of mGluR1 -/- mice to all three doses of amphetamine were significantly greater than those of wild-type +/+ mice. Amphetamine also induced a dose-dependent elevation of preprodynorphin mRNA in the dorsal and ventral striatum of mutant and wild-type mice as revealed by quantitative in situ hybridization. In contrast to behavioral responses, the induction of dynorphin mRNA in both the dorsal and ventral striatum of mutant mice was significantly less than that of wild-type mice in response to the two higher doses of amphetamine. In addition, amphetamine elevated basal levels of substance P mRNA in the dorsal and ventral striatum of mGluR1 mutant mice to a similar level as that of wild-type mice. There were no differences in basal levels and distribution patterns of the two mRNAs between the two genotypes of mice treated with saline. These results demonstrate a clear augmented behavioral response of mGluR1 knockout mice to acute amphetamine exposure that is closely correlated with reduced dynorphin mRNA induction in the same mice. It appears that an intact mGluR1 is specifically critical for full dynorphin induction, and impaired mobilization of inhibitory dynorphin system as a result of lacking mGluR1 may contribute to an augmentation of motor stimulation in response to acute administration of psychostimulant.

Endogenous opiates: 2000

This paper is the twenty-third installment of the annual review of research concerning the opiate system. It summarizes papers published during 2000 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Effects of sydnocarb and D-amphetamine on the extracellular levels of amino acids in the rat caudate-putamen

The neurotoxic effects of psychostimulants at high dosages limit their clinical applicability but the mechanism of neurotoxicity is still unsettled. We now studied by microdialysis how acute and subchronic (four times at 2-h intervals) administrations of D-amphetamine and sydnocarb [3-(beta-phenylisopropyl)-N-phenylcarbamoylsydnonimine], an original novel Russian psychostimulant, affected the extracellular levels of amino acids in the caudate-putamen of halothane-anesthetized male Sprague-Dawley rats. Acute D-amphetamine administration (5.0 mg/kg, i.p.) produced a moderate accumulation of extracellular glutamate and aspartate. Sydnocarb (23.8 mg/kg, i.p., a dose equimolar to 5.0 mg/kg D-amphetamine) also increased extracellular glutamate and alanine. Subchronic D-amphetamine administration (5.0 mg/kg x 4, i.p.) caused gradual fivefold increases in the glutamate and taurine levels and moderate increases in the aspartate and alanine levels. Subchronic sydnocarb administration (23.8 mg/kg x 4, i.p.) elicited a marked increase in the aspartate level and a small increase in the level of glutamate. The alanine level increased temporarily after each administration of sydnocarb, while the taurine level increased only after the last injection. We conclude that the mode of action of sydnocarb differs from that of D-amphetamine. Sydnocarb also seems to be less neurotoxic than D-amphetamine, since it elicits lesser changes in the extracellular level of glutamate.

Gliogenesis in the striatum of the adult rat: alteration in neural progenitor population after psychostimulant exposure

Cytogenesis from proliferating progenitor cells is present in the rat brain throughout adulthood, and is regulated by a variety of environmental stimuli. To determine whether adult cytogenesis occurs in the intact rat striatum and to explore the possible regulatory role of psychostimulant exposure on striatal cytogenesis, immunohistochemistry with the thymidine analog bromodeoxyuridine (BrdU), a marker of DNA synthesis that labels dividing cells and their terminal progeny, was performed on the brain sections of normal adult rats and rats treated with a psychostimulant, amphetamine (AMPH). Scattered cells that incorporated BrdU were consistently seen throughout the dorsal (caudate putamen) and ventral (nucleus accumbens) striatum 24 h after BrdU injection. Three to four weeks after BrdU injection, approximately 10-20% of surviving newborn cells differentiated into astroglia according to their radial morphology of glia and co-expression of an astroglial marker, S100beta. However, none of BrdU-positive cells were found to co-localize with a neuronal marker, neuronal nuclear antigen (NeuN). Acute injection of AMPH at a behaviorally active dose (10 mg/kg, i.p.) produced a rapid and transient decrease in the number of BrdU-labeled cells in both the dorsal (70.6% of control) and ventral (66.7% of control) striatum, but not in the subventricular zone and the hippocampal dentate gyrus. However, the fraction of differentiated astrocytes was not altered 3-4 weeks after AMPH treatment. These results indicate an existence of active gliogenesis (both proliferation and differentiation) in the adult rat striatum. Vulnerability of striatal cytogenesis to psychostimulant exposure indicates a new approach to elucidate brain mechanisms responsible for addictive properties of drugs of abuse.

Differentially altered mGluR1 and mGluR5 mRNA expression in rat caudate nucleus and nucleus accumbens in the development and expression of behavioral sensitization to repeated amphetamine administration

Altered glutamatergic transmission in the striatum may be implicated in behavioral sensitization to repeated amphetamine (AMPH) administration. Quantitative in situ hybridization histochemistry was performed to define the effects of acute and chronic AMPH exposures on mRNA expression of Group I metabotropic glutamate receptors (mGluRs) in the striatum. Behavioral ratings indicated that the motor activity of rats was significantly higher after the final of five daily AMPH injections (4 mg/kg, i.p.) than that after the first of five daily AMPH, indicative of the development of behavioral sensitization. Moreover, the motor activity of rats treated with five daily AMPH was significantly greater than that of rats treated with five daily saline in response to a 2 mg/kg challenge dose of AMPH 7, 14, 28, and 60 days after the discontinuation of drug treatments, indicative of the persistent expression of behavioral sensitization. Three hours after acute administration of AMPH to naive rats, mGluR1 and mGluR5 mRNA expression in the dorsal (caudatoputamen) and ventral (nucleus accumbens) striatum showed no change as compared to acute saline injection. In rats that developed behavioral sensitization to repeated AMPH, mGluR1 levels in the dorsal and ventral striatum were increased by 53% and 43%, respectively, 3 h after the final AMPH treatment. However, this change did not persist during withdrawal since it was not observed 7, 14, and 28 days after the discontinuation of AMPH treatment. Conversely, mGluR5 levels were markedly reduced 3 h after the final of five daily AMPH treatments in the entire striatum of sensitized rats (34% and 77% of controls in the dorsal and ventral striatum, respectively). The reduction persisted at 7, 14, and 28 days of withdrawal. These results reveal a close linkage between striatal Group I mGluR gene expression and behavioral sensitization to AMPH. This may indicate functional implications of the two subtypes of Group I mGluRs in the regulation of behavioral sensitization to the dopamine stimulant.

Contrasting responses by basal ganglia met-enkephalin systems to low and high doses of methamphetamine in a rat model

The influence of methamphetamine (METH) on basal ganglia met-enkephalin (Menk) was studied by determining levels of this peptide in striatal, pallidal and nigral regions after administering a single low (0.5 mg/kg) or high (10 mg/kg) dose of this stimulant. The Menk levels in the striatal and pallidal areas were reduced and increased after the low- and high-dose METH treatments, respectively, 12 h after drug administration in all striatal and pallidal regions examined. The low-dose effect appeared to be principally influenced by increased activation of the dopamine D2-like receptor, while the high-dose effect seemed to result from dominance of D1-like receptor activation. However, both effects required coactivation of D1- and D2-like receptors. For the most part, both low- and high-dose METH-induced changes in Menk tissue content were fully recovered by 24 h. The Menk levels were not significantly altered in the substantia nigra 3-24 h after either METH treatment. Results reported herein indicated that striatal and pallidal Menk pathways respond differently after acute treatment with low or high doses of METH.

Activation of group III metabotropic glutamate receptors inhibits basal and amphetamine-stimulated dopamine release in rat dorsal striatum: an in vivo microdialysis study

Group III metabotropic glutamate (mGlu) receptors are negatively coupled to adenylate cyclase and are distributed pre-synaptically in the striatum. A behavioral study previously conducted in this laboratory shows that activation of this group of mGlu receptors attenuates acute amphetamine-stimulated motor activity. By administering a group III selective agonist or antagonist via the dialysis probe, the present study employed in vivo microdialysis to evaluate the capacity of the group III selective agents to alter extracellular levels of dopamine in the dorsal striatum of normal and amphetamine-treated rats. It was found that the group III agonist L-2-amino-4-phosphonobutyrate (L-AP4) dose-dependently (1, 10 and 100 microM) reduced basal levels of extracellular dopamine. In contrast, the group III antagonist alpha-methyl-4-phosphonophenylglycine (MPPG) dose-dependently (10, 50 and 250 microM) elevated the basal release of extracellular dopamine. This elevation was antagonized by co-perfusion of L-AP4. Perfusion of 5-microM amphetamine through the dialysis probe increased extracellular dopamine in the dorsal striatum. Co-perfusion of L-AP4 (100 microM) significantly reduced amphetamine-stimulated dopamine levels, whereas co-perfusion of L-AP4 (100 microM) and MPPG (100 microM) did not alter the capacity of amphetamine to elicit dopamine release. The data obtained from this study demonstrate the presence of a tonically active glutamatergic tone on group III mGlu receptors in the dorsal striatum to pre-synaptically regulate basal dopamine release in an inhibitory fashion. Moreover, activation of L-AP4-sensitive group III mGlu receptors can suppress the phasic release of dopamine induced by a dopamine stimulant amphetamine.