Activation of metabotropic glutamate receptors induces an inward current in rat dopamine mesencephalic neurons

Repeated effects of the neurotensin receptor agonist PD149163 in three animal tests of antipsychotic activity: assessing for tolerance and cross-tolerance to clozapine

Neurotensin is an endogenous neuropeptide closely associated with the mesolimbic dopaminergic system and shown to possess antipsychotic-like effects. In particular, acute neurotensin receptor activation can inhibit conditioned avoidance response (CAR), attenuate phencyclidine (PCP)-induced prepulse inhibition (PPI) disruptions, and reverse PCP-induced hyperlocomotion. However, few studies have examined the long term effects of repeated neurotensin receptor activation and results are inconsistent. Since clinical administration of antipsychotic therapy often requires a prolonged treatment schedule, here we assessed the effects of repeated activation of neurotensin receptors using an NTS1 receptor selective agonist, PD149163, in 3 behavioral tests of antipsychotic activity. We also investigated whether reactivity to the atypical antipsychotic clozapine was altered following prior PD149163 treatment. Using both normal and prenatally immune activated rats generated through maternal immune activation with polyinosinic:polycytidylic acid, we tested PD149163 in CAR, PCP (1.5mg/kg)-induced PPI disruption, and PCP (3.2mg/kg)-induced hyperlocomotion. For each paradigm, rats were first repeatedly tested with vehicle or PD149163 (1.0, 4.0, 8.0mg/kg, sc) along with vehicle or PCP for PPI and hyperlocomotion tests, then challenged with PD149163 after 2 drug-free days. All rats were then challenged with clozapine (5.0mg/kg, sc). During the repeated test period, PD149163 exhibited antipsychotic-like effects in all three models. On the PD149163 challenge day, prior drug treatment only caused a tolerance effect in CAR. This tolerance in CAR was transferrable to clozapine, as it enhanced clozapine tolerance in the same group of animals. Although no tolerance effect was seen in the PD149163 challenge for the PCP-induced hyperlocomotion test, the clozapine challenge showed increased sensitivity in groups previously exposed to repeated PD149163 treatment. Our findings suggest that repeated exposure to NTS1 receptor agonists can induce a dose-dependent tolerance and cross-tolerance to clozapine to some of its behavioral effects but not others.

PINK1 heterozygous mutations induce subtle alterations in dopamine-dependent synaptic plasticity

Homozygous or compound heterozygous mutations in the phosphatase and tensin homolog-induced putative kinase 1 (PINK1) gene are causative of autosomal recessive, early onset Parkinson's disease. Single heterozygous mutations have been detected repeatedly both in a subset of patients and in unaffected individuals, and the significance of these mutations has long been debated. Several neurophysiological studies from non-manifesting PINK1 heterozygotes have demonstrated the existence of neural plasticity abnormalities, indicating the presence of specific endophenotypic traits in the heterozygous state. We performed a functional analysis of corticostriatal synaptic plasticity in heterozygous PINK1 knockout (PINK1(+/-) ) mice using a multidisciplinary approach and observed that, despite normal motor behavior, repetitive activation of cortical inputs to striatal neurons failed to induce long-term potentiation (LTP), whereas long-term depression was normal. Although nigral dopaminergic neurons exhibited normal morphological and electrophysiological properties with normal responses to dopamine receptor activation, a significantly lower dopamine release was measured in the striatum of PINK1(+/-) mice compared with control mice, suggesting that a decrease in stimulus-evoked dopamine overflow acts as a major determinant for the LTP deficit. Accordingly, pharmacological agents capable of increasing the availability of dopamine in the synaptic cleft restored normal LTP in heterozygous mice. Moreover, monoamine oxidase B inhibitors rescued physiological LTP and normal dopamine release. Our results provide novel evidence for striatal plasticity abnormalities, even in the heterozygous disease state. These alterations might be considered an endophenotype to this monogenic form of Parkinson's disease and a valid tool with which to characterize early disease stage and design possible disease-modifying therapies.

Reversal of quinpirole inhibition of ventral tegmental area neurons is linked to the phosphatidylinositol system and is induced by agonists linked to G(q)

Putative dopaminergic (pDAergic) ventral tegmental area neurons play an important role in brain pathways related to addiction. Extended exposure of pDAergic neurons to moderate concentrations of dopamine (DA) results in a time-dependent decrease in sensitivity of pDAergic neurons to DA inhibition, a process called dopamine inhibition reversal (DIR). We have shown that DIR is mediated by phospholipase C and conventional protein kinase C through concurrent stimulation of D2 and D1-like receptors. In the present study, we further characterized this phenomenon by using extracellular recordings in brain slices to examine whether DIR is linked to phosphatidylinositol (PI) or adenylate cyclase (AC) second-messenger pathways. A D1-like dopaminergic agonist associated with PI turnover (SKF83959), but not one linked to AC (SKF83822), promoted reversal of inhibition produced by quinpirole, a dopamine D2-selective agonist. Other neurotransmitter receptors linked to PI turnover include serotonin 5-HT(2), α(1)-adrenergic, neurotensin, and group I metabotropic glutamate (mGlu) receptors. Both serotonin and neurotensin produced significant reversal of quinpirole inhibition, but agonists of α(1)-adrenergic and group I mGlu receptors failed to significantly reverse quinpirole inhibition. These results indicate that some agonists that stimulate PI turnover can facilitate desensitization of D2 receptors but that there may be other factors in addition to PI that control that interaction.

Basal ganglia control of substantia nigra dopaminergic neurons

Although substantia nigra dopaminergic neurons are spontaneously active both in vivo and in vitro, this activity does not depend on afferent input as these neurons express an endogenous calcium-dependent oscillatory mechanism sufficient to drive action potential generation. However, afferents to these neurons, a large proportion of them GABAergic and arising from other nuclei in the basal ganglia, play a crucial role in modulating the activity of dopaminergic neurons. In the absence of afferent activity or when in brain slices, dopaminergic neurons fire in a very regular, pacemaker-like mode. Phasic activity in GABAergic, glutamatergic, and cholinergic inputs modulates the pacemaker activity into two other modes. The most common is a random firing pattern in which interspike intervals assume a Poisson-like distribution, and a less common pattern, often in response to a conditioned stimulus or a reward in which the neurons fire bursts of 2-8 spikes time-locked to the stimulus. Typically in vivo, all three firing patterns are observed, intermixed, in single nigrostriatal neurons varying over time. Although the precise mechanism(s) underlying the burst are currently the focus of intensive study, it is obvious that bursting must be triggered by afferent inputs. Most of the afferents to substantia nigra pars compacta dopaminergic neurons comprise monosynaptic inputs from GABAergic projection neurons in the ipsilateral neostriatum, the globus pallidus, and the substantia nigra pars reticulata. A smaller fraction of the basal ganglia inputs, something less than 30%, are glutamatergic and arise principally from the ipsilateral subthalamic nucleus and pedunculopontine nucleus. The pedunculopontine nucleus also sends a cholinergic input to nigral dopaminergic neurons. The GABAergic pars reticulata projection neurons also receive inputs from all of these sources, in some cases relaying them disynaptically to the dopaminergic neurons, thereby playing a particularly significant role in setting and/or modulating the firing pattern of the nigrostriatal neurons.

Metabotropic glutamate 1 receptor: current concepts and perspectives

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

Functional interactions between somatodendritic dopamine release, glutamate receptors and brain-derived neurotrophic factor expression in mesencephalic structures of the brain

Dopaminergic nigrostriatal neurons may be considered as bipolar functional entities since they are endowed with the ability to synthesize, store and release the transmitter dopamine (DA) at the somatodendritic level in the substantia nigra (SN). Such dendritic DA release seems to be distinct from the transmitter release occurring at the axon terminal and seems to rely preferentially on volume transmission to exert its physiological effects. An increased glutamatergic (Gluergic) transmission into the SN facilitates such dendritic DA release via activation of NMDA-receptors (NMDA-Rs) and to a lesser extent through group II metabotropic glutamate receptors (mGluRs). In addition, nigral mGluRs functionally interact with NMDA-Rs in the SN, further modulating the NMDA-R-mediated increase of DA release from dendrites in the SN. In turn, dendritically released DA may exert, via D1 receptors, a tonic inhibitory control upon nigral glutamate (Glu). Furthermore, released DA, via D2/D3 autoreceptors, produces an autoinhibitory effect upon DA cell firing and its own release process. An increased Gluergic transmission into the SN may also induce, via activation of NMDA-Rs, an augmented expression of different brain-derived neurotrophic factor (BDNF) gene transcripts in this brain area. Pharmacological evidence suggests that non-NMDA-Rs could also participate in the regulation of BDNF gene expression in the SN. Glu-mediated changes of nigral BDNF expression could regulate, in turn, the expression of important transmitter-related proteins in the SN, such as different NMDA-R subunits, mGluRs and DA-D3 receptors. In conclusion, Glu-DA-BDNF interactions in the SN may play an important role in modulating the flow of neuronal information in this brain structure under normal conditions, as well as during adaptive and plastic responses associated with various neurological and psychiatric disorders.

The somatodendritic release of dopamine in the ventral tegmental area and its regulation by afferent transmitter systems

The release of dopamine in the ventral tegmental area (VTA) plays an important role in the autoinhibition of the dopamine neurons of the mesocorticolimbic system through the activation of somatodendritic dopamine D2 autoreceptors. Accordingly, the intra-VTA application of dopamine D2 receptor agonists reduces the firing rate and release of dopamine in the VTA, and this control appears to possess a tonic nature because the corresponding antagonists enhance the somatodendritic release of the transmitter. In addition, the release of dopamine in the VTA is increased by potassium or veratridine depolarization and abolished by tetrodotoxin and calcium omission. Overall, it appears that the somatodendritic release of dopamine is consistently lower than that in nerve endings. Apart from intrinsic dopaminergic mechanisms, other transmitter systems such as serotonin, noradrenaline, acetylcholine, GABA and glutamate play a role in the control of the activity of dopaminergic neurons of the VTA, although the final action depends on the particular receptor involved as well as the neuronal type where it is localized. Given the involvement of the mesocorticolimbic dopaminergic systems in the pathogenesis of severe neuropsychiatric disorders such as schizophrenia, the knowledge of the factors that regulate the release of dopamine in the VTA could provide new insight into the ethiogenesis of the disease as well as its implication on the mechanisms of action of therapeutic drugs.

Learning mechanisms in addiction: synaptic plasticity in the ventral tegmental area as a result of exposure to drugs of abuse

One of the central questions in neurobiology is how experience modifies neural function, and how changes in the nervous system permit an animal to adapt its behavior to a changing environment. Learning and adaptation to a host of different environmental stimuli exemplify processes we know must alter the nervous system because the behavioral output changes after experience. Alterations in behavior after exposure to addictive drugs are a striking example of chemical alterations of nervous system function producing long-lasting changes in behavior. The alterations produced in the central nervous system (CNS) by addictive drugs are of interest because of their relationship to human substance abuse but also because these CNS alterations produce dramatic, easily observed alterations in behavior in response to discrete stimuli. Considerable study has been given to behavioral and biochemical correlates of addiction over the past 50 or more years; however, our understanding of the cellular physiological responses of affected CNS neurons is in its infancy. This review focuses on alterations in cellular and synaptic physiology in the ventral tegmental area (VTA) in response to addictive drugs.

Modulation of cellular activity and synaptic transmission in the ventral tegmental area

The mesolimbic dopamine system, of which the cell bodies are located in the ventral tegmental area, has been implicated in the physiology of reward and the related pathophysiology of drug abuse. This area has been a site of significant interest to study the effects of drugs of abuse and neurotransmitter systems implicated in the rewarding effects of these compounds. One important aspect of synaptic transmission is the ability of synapses to strengthen or weaken their connection as a consequence of synaptic activity. Recently, it has become apparent that this phenomenon is also present in the ventral tegmental area and that this may bear important functional consequences for the ways in which drugs of abuse assert their effect. Here, we will review the effects of neurotransmitter systems and drugs of abuse on cellular activity and synaptic transmission in the ventral tegmental area.

Dual modulation of gabaergic transmission by metabotropic glutamate receptors in rat ventral tegmental area

The effects of metabotropic glutamate receptor (mGluR) activation on non-dopamine (putative GABAergic) neurons and inhibitory synaptic transmission in the ventral tegmental area were examined using intracellular recordings from rat midbrain slices. Perfusion of (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD; agonist for group I and II mGluRs), but not L-amino-4-phosphonobutyric acid (L-AP4; agonist for group III mGluRs), produced membrane depolarization (current clamp) and inward current (voltage clamp) in non-dopamine neurons. The t-ACPD-induced depolarization was concentration-dependent (concentration producing 50% maximal depolarization [EC(50)]=6.1+/-2.5 microM), and was blocked by the antagonist (+/-)-alpha-methyl-4-carboxyphenylglycine, but not by tetrodotoxin and ionotropic glutamate-receptor antagonists. The t-ACPD-evoked responses were mimicked comparably by selective group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG). Furthermore, the DHPG-induced depolarization in non-dopamine neurons was greatly reduced by mGluR1-specific antagonist 7(hydroxyimino)cyclopropachromen-1a-carboxylate ethyl ester. When recorded in dopamine neurons, the frequency of spontaneous GABA(A) receptor-mediated inhibitory postsynaptic potentials was increased by t-ACPD but not L-AP4. However, the amplitude of evoked inhibitory postsynaptic currents in dopamine neurons was reduced by all three group mGluR agonists. These results reveal a dual modulation of mGLuR activation on inhibitory transmission in midbrain ventral tegmental area: enhancing putative GABAergic neuronal excitability and thus potentiating tonic inhibitory synaptic transmission while reducing evoked synaptic transmission at inhibitory terminals.

Metabotropic glutamate and muscarinic cholinergic receptor-mediated preferential inhibition of N-methyl-D-aspartate component of transmissions in rat ventral tegmental area

Presynaptic inhibition is one of the major control mechanisms in the CNS. Our laboratory recently reported that presynaptic GABA(B) and adenosine A(1) receptors mediate a preferential inhibition on N-methyl-D-aspartate receptor-mediated excitatory postsynaptic currents recorded in rat midbrain dopamine neurons. Here we extended these findings to metabotropic glutamate and muscarinic cholinergic receptors. Intracellular voltage clamp recordings were made from dopamine neurons in rat ventral tegmental area in slice preparations. (+/-)-1-Aminocyclopentane-trans-1,3-dicarboxylic acid (agonist for groups I and II metabotropic glutamate receptors) and L(+)-2-amino-4-phosphonobutyric acid (L-AP4; agonist for group III metabotropic glutamate receptors) were significantly more potent for inhibiting N-methyl-D-aspartate receptor-mediated excitatory postsynaptic currents, as compared with inhibition of excitatory postsynaptic currents mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors. Such preferential inhibition of the N-methyl-D-aspartate component was also observed for muscarine (agonist for muscarinic cholinergic receptors). Inhibitory effects of (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid, L-AP4, and muscarine were blocked reversibly by their respective antagonists [(RS)-alpha-methyl-4-carboxyphenylglycine, (RS)-alpha-methyl-4-phosphonophenylglycine, and 1,1-dimethyl-4-diphenylacetoxypiperidinium iodide]. In addition, all three agonists increased the ratio of excitatory postsynaptic currents in paired-pulse studies and did not reduce currents induced by exogenous N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid. Interestingly, the glutamate release stimulator 4-aminopyridine (30 microM) and the glutamate uptake inhibitor L-anti-endo-3,4-methanopyrrolidine dicarboxylate (300 microM) preferentially increased the amplitude of N-methyl-D-aspartate excitatory postsynaptic currents.Thus, agonists for metabotropic glutamate and muscarinic cholinergic receptors act presynaptically to cause a preferential reduction in the N-methyl-D-aspartate component of excitatory synaptic transmissions. Together with the evidence for GABA(B) and adenosine A(1) receptor-mediated preferential inhibition of the N-methyl-D-aspartate component, the present results suggest that limiting glutamate spillover onto postsynaptic N-methyl-D-aspartate receptors may be a general rule for presynaptic modulation in midbrain dopamine neurons.

Characterization of pre- and post-synaptic metabotropic glutamate receptor-mediated inhibitory responses in substantia nigra dopamine neurons

Two inhibitory responses mediated by both pre- and post-synaptic metabotropic glutamate receptors (mGluRs) were investigated in dopamine neurons of the substantia nigra using whole-cell patch recordings. (2R,4R)-APDC, a group II mGluR agonist, and L-2-amino-4-phosphonobutyrate (L-AP4), a group III mGluR agonist, reversibly suppressed the amplitude of excitatory postsynaptic currents (EPSCs). However, (S)-3,5-DHPG, a group I mGluR agonist, exhibited less inhibitory action on the EPSCs. LY341495, a highly potent group II mGluR antagonist, antagonized the broad spectrum mGluR agonist, 1S,3R-ACPD-induced suppression of EPSCs. In acutely dissociated dopamine neurons, glutamate (Glu) in the presence of CNQX and AP-5 evoked an outward current accompanied by an increase in K(+) conductance. (S)-3,5-DHPG, but not (2R,4R)-APDC or L-AP4, also induced an outward current. Glu-induced outward current (I(Glu-out)) was partially inhibited by LY367385, a selective mGluR1 antagonist, but not by MPEP, a selective mGluR5 antagonist. Ryanodine and cyclopiazonic acid blocked the I(Glu-out). In the presence of caffeine, Glu failed to induce a current. Charybdotoxin, but not apamin or iberiotoxin, inhibited the I(Glu-out). Taken together, both group II and III mGluRs are mainly involved in the presynaptic inhibition of Glu release to dopamine neurons, while group I mGluRs, including at least mGluR1, participate in the hyperpolarization of dopamine neurons mediated by the opening of charybdotoxin-sensitive Ca(2+)-activated K(+) channels.

Group I metabotropic glutamate receptor-mediated enhancement of dopamine cell burst firing in rat ventral tegmental area in vitro

Metabotropic glutamate receptor (mGluR) agonist t-ACPD produced concentration-dependent enhancement of NMDA/apamin-induced burst firing and membrane oscillations in dopamine cells, recorded intracellularly from ventral tegmental area in the rat midbrain slice. Such effects were blocked reversibly by mGluR antagonist MCPG, and mimicked by the selective agonist for group I (but not group II and III) mGluRs. Our results point out a selective involvement of group I mGluRs in facilitating burst firing of midbrain dopamine cells.

Haloperidol-induced alteration in the physiological actions of group I mGlus in the subthalamic nucleus and the substantia nigra pars reticulata

Excitatory glutamatergic inputs to the subthalamic nucleus (STN), and subthalamic afferents to the substantia nigra pars reticulata (SNr) are believed to play a key role in the pathophysiology of Parkinson's disease (PD). Previously, we have shown that activation of the group I mGlus in the STN and SNr induces a direct depolarization of the neurons in these nuclei. Surprisingly, although both group I mGlus were present in the STN and SNr, mGlu5 alone mediated the DHPG-induced depolarization of the STN, and mGlu1 alone mediated the DHPG-induced depolarization of the SNr. We now report that both mGlu1 and mGlu5 are coexpressed in the same cells in both of these brain regions, and that both receptors play a role in mediating the DHPG-induced increase in intracellular calcium. Furthermore, we demonstrate that the induction of an acute PD-like state using a 16 h haloperidol treatment produces an alteration in the coupling of the group I receptors, such that post-haloperidol, DHPG-induced depolarizations are mediated by both mGlu1 and mGlu5 in the STN and SNr. Therefore, the pharmacology of the group I mGlu-mediated depolarization depends on the state of the system, and alterations in receptor coupling may be evident in pathological states such as PD.

Distinct physiological roles of the Gq-coupled metabotropic glutamate receptors Co-expressed in the same neuronal populations

The group I metabotropic glutamate receptors, mGluR1 and mGluR5, exhibit a high degree of sequence homology, and are often found co-expressed in the same neuronal populations. These receptors couple to a broad array of effector systems, and are implicated in diverse physiological and pathophysiological functions. Due to the high degree of sequence homology, and the findings that these receptors couple identically in recombinant systems, it has been generally assumed that these two group I mGluR subtypes would exhibit redundant function when coexpressed in the same neurons. With the advent of subtype-selective pharmacological tools, it has become possible to tease apart the functions of mGluR1 and mGluR5 in the same neuron. The emerging picture is one of diverse function, which implies differential regulation. Interestingly, the group I mGluRs are modulated by a rich variety of regulatory systems, which may explain how these receptors can mediate divergent actions when present in the same cell.

Modulation of dendritic release of dopamine by metabotropic glutamate receptors in rat substantia nigra

A superfusion system was used to study the effects of metabotropic glutamate receptor (mGluR) ligands upon the release of [(3)H]dopamine ([(3)H]DA) previously taken up by rat substantia nigra (SN) slices. trans-(+/-)-1-Amino-(1S,3R)-cyclopentane dicarboxylic acid (trans-ACPD; 100 and 600 microM), a group I and II mGluR agonist, evoked the release of [(3)H]DA from nigral slices. This last effect was reduced significantly by (2S,3S,4S)-2-methyl-2-(carboxycyclopropyl)-glycine (MCCG; 300 microM), an antagonist of group II mGluR, or by the addition of tetrodotoxin (D-APV; 1 microM) to the superfusion medium. D-(-)-2-Amino-5-phosphono-valeric acid (100 microM), an N-methyl-D-aspartate receptor antagonist, or the presence of Mg(2+) (1.2mM) in the superfusion medium did not modify trans-ACPD-induced [(3)H]DA release. In addition, a group II mGluR agonist such as (2S,1'R,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)-glycine (DCG-IV; 100 microM) significantly induced the release of [(3)H]DA from nigral slices, whereas a group I mGluR agonist such as (RS)-3,5-dihydroxyphenylglycine (DHPG; 50 and 100 microM) did not modify the release of the [(3)H]-amine. Further experiments showed that the NMDA (100 microM)-evoked release of [(3)H]DA was decreased significantly by prior exposure of SN slices to trans-ACPD. Finally, partial denervation of the DA nigro-striatal pathway with 6-hydroxydopamine (6-OH-DA) increased trans-ACPD-induced release of [(3)H]DA, whereas it decreased trans-ACPD inhibitory effects on NMDA-evoked release of [(3)H]DA from nigral slices. The present results suggest that the dendritic release of DA in the SN is regulated by mGluR activation. Such nigral mGluR activation may produce opposite effects upon basal and NMDA-evoked release of DA in the SN. In addition, such mGluR-induced effects in the SN are modified in response to partial denervation of the DA nigro-striatal pathway.

Intrinsic membrane properties and synaptic inputs regulating the firing activity of the dopamine neurons

Dopamine (DA) neurones of the ventral mesencephalon are involved in the control of reward related behaviour, cognitive functions and motor performances, and provide a critical site of action for major categories of neuropsychiatric drugs, such as antipsychotic agents, dependence producing drugs and anti-Parkinson medication. The midbrain DA neurones are mainly located in the substantia nigra pars compacta (SNPC) and the ventral tegmental area (VTA). Intrinsic membrane properties regulate the activity of these neurones. In fact, they possess several conductances that allow them to fire in a slow pacemaker-like mode. The internal set of membrane currents interact with afferent synaptic inputs which, especially in in vivo conditions, contribute to accelerate or decelerate the firing activity of the cells in accordance with the necessity to optimise the release of dopamine in the terminal fields. In particular, discrete excitatory and inhibitory inputs transform the firing from a low regular into a bursting pattern. The bursting activity promotes dopamine release being very important in cognition and motor performances. In the present paper we review electrophysiological data regarding the role of glutamatergic and cholinergic and GABAergic afferent inputs in regulating the midbrain DAergic neuronal activity.

Group I metabotropic glutamate receptors activate burst firing in rat midbrain dopaminergic neurons

We have investigated the changes in the spontaneous firing pattern induced by DHPG ((S)-3,5-dihydroxyphenylglycine) and NMDA (N-methyl-d-aspartic acid) on rat dopaminergic neurons in substantia nigra pars compacta (SNc) using sharp microelectrode recordings in in vitro conditions. Twenty-five out of 33 cells modified the regular single-pacemaker activity in burst firing when exposed to the Group I metabotropic glutamate receptor (mGluR) agonist DHPG (30 microM) and d-tubocurarine (500 microM) (d-TC), whereas they all fired in bursts during NMDA (20 microM) plus d-TC application. The blockade of SK-channels by d-TC and apamin was essential for the production of both types of bursts. Although the two drugs induced a similar number of action potentials per burst, the DHPG-induced bursts had a lower frequency, a longer duration and a longer plateau period without spikes. In addition, the DHPG-induced bursting had a longer wash-out, could be reduced or blocked by the mGluR 1 selective, non-competitive antagonist CPCCOEt (7-cyclopropan[b]chromen-1a-carboxylic acid ethyl ester) (100 microM) while it was not affected by the mGluR 5 selective antagonist MPEP (2-methyl-6-(phenylethynyl)-pyridine (10 microM). These results suggest that both the activation of glutamate metabotropic type 1 and NMDA ionotropic receptors induce burst firing in the dopaminergic cells of the ventral midbrain when the activity of the SK-channels is reduced.

Group I mGluRs coupled to G proteins are regulated by tyrosine kinase in dopamine neurons of the rat midbrain

Metabotropic glutamate receptors (mGluRs) modulate neuronal function via different transduction mechanisms that are either dependent or independent on G-protein function. Here we investigated, using whole cell patch-clamp recordings in combination with fluorimetric measurements of intracellular calcium concentration ([Ca(2+)](i)), the metabolic pathways involved in the responses induced by group I mGluRs in dopamine neurons of the rat midbrain. The inward current and the [Ca(2+)](i) increase caused by the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG, 100 microM) were permanently activated and subsequently abolished in cells loaded with the nonhydrolizable GTP-analogue GTP-gamma-S (600 microM). In addition, when GDP-beta-S (600 microM) was dialyzed into the cells to produce the blockade of the G proteins, the DHPG-dependent responses were reduced. When the tissue was bathed with the phospholipase C inhibitor 1-[6[[(17 beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]exyl]-1H-pyrrole-2,5-dione (10 microM), the DHPG-induced calcium transients slightly diminished but the associated inward currents were not affected. Interestingly, a substantial depression of the DHPG-induced inward current and transient increase of [Ca(2+)](i) was caused by the protein tyrosine kinase inhibitors tyrphostin B52 (40 microM) and 4',5,7-trihydroxyisoflavone (genistein; 40 microM), whereas genistein's inactive analogue 4',5,7-trihydroxyisoflavone-7-glucoside (40 microM) was ineffective. The blockade of the Src family of tyrosine kinase by 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (20 microM), mitogen-activated protein kinase by 2'-amino-3' methoxyflavone (50 microM), and protein kinase C by staurosporine (1 microM) had no effect on the cellular responses caused by DHPG. The mGluR5-selective antagonist 2-methyl-6-(phenylethynyl)-pyridine (10--100 microM) did not affect the actions of DHPG. Thus our results indicate that the responses, mainly mediated by mGluRs1 in dopamine neurons, are activated by intracellular mechanisms coupled to G proteins and regulated by tyrosine kinases.

Modulation of the neuronal dopamine transporter activity by the metabotropic glutamate receptor mGluR5 in rat striatal synaptosomes through phosphorylation mediated processes

There is considerable evidence that the activity of the neuronal dopamine transporter (DAT) is dynamically regulated and a putative implication of its phosphorylation in this process has been proposed. However, there is little information available regarding the nature of physiological stimuli that contribute to the endogenous control of the DAT function. Based on the close relationship between glutamatergic and dopaminergic systems in the striatum, we investigated the modulation of the DAT activity by metabotropic glutamate receptors (mGluRs). Short-term incubations of rat striatal synaptosomes with micromolar concentrations of the group I mGluR selective agonist (S)-3,5-dihydroxyphenylglycine were found to significantly decrease the DAT capacity and efficiency. This alteration was completely prevented by a highly selective mGluR5 antagonist, 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP). The effect of (S)-3,5-dihydroxyphenylglycine was also inhibited by staurosporine and by selective inhibitors of protein kinase C and calcium calmodulin-dependent protein kinase II. Co-application of okadaic acid prolonged the transient effect of the agonist, supporting a critical role for phosphorylation in the modulation of the DAT activity by mGluRs. In conclusion, we propose that striatal mGluR5 contribute to the control of the DAT activity through concomitant activation of both protein kinase C and calcium calmodulin-dependent protein kinase II.

Hypoglycemia enhances ionotropic but reduces metabotropic glutamate responses in substantia nigra dopaminergic neurons

It is widely accepted that energy deprivation causes a neuronal death that is mainly determined by an increase in the extracellular level of glutamate. Consequently an excessive membrane depolarization and a rise in the intracellular concentration of sodium and calcium are produced. In spite of this scenario, the function of excitatory and inhibitory amino acids during an episode of energy failure has not been studied yet at a cellular level. In a model of cerebral hypoglycemia in the rat substantia nigra pars compacta, we measured neuronal responses to excitatory amino acid agonists. Under single-electrode voltage-clamp mode at -60 mV, the application of the ionotropic glutamate receptor agonists N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, kainate, and the metabotropic group I agonist (S)-3,5-dihydroxyphenilglycine (DHPG) produced reversible inward currents in the dopaminergic cells. In addition, an outward current was caused by the superfusion of the metabotropic GABA(B) agonist baclofen. Glucose deprivation enhanced the inward responses caused by each ionotropic glutamate agonist. In contrast, hypoglycemia depressed the DHPG-induced inward current and the baclofen-induced outward current. These effects of hypoglycemia were reversible. To test whether a failure of the Na(+)/K(+) ATPase pump could account for the modification of the agonist-induced currents during hypoglycemia, we treated the midbrain slices with strophanthidin (1-3 microM). Strophanthidin enhanced the inward currents caused by glutamate agonists. However, it did not modify the GABA(B)-induced outward current. Our data suggest that glucose deprivation enhances the inward current caused by the stimulation of ionotropic glutamate receptors while it dampens the responses caused by the activation of metabotropic receptors. Thus a substantial component of the augmented neuronal response to glutamate, during energy deprivation, is very likely due to the failure of Na(+) and Ca(2+) extrusion and might ultimately favor excitotoxic processes in the dopaminergic cells.

Distribution and roles of metabotropic glutamate receptors in the basal ganglia motor circuit: implications for treatment of Parkinson's disease and related disorders

The basal ganglia (BG) are a set of interconnected subcortical structures that play a critical role in motor control. The BG are thought to control movements by a delicate balance of transmission through two BG circuits that connect the input and output nuclei: the direct and the indirect pathways. The BG are also involved in a number of movement disorders. Most notably, the primary pathophysiological change that gives rise to the motor symptoms of Parkinson's Disease (PD) is the loss of dopaminergic neurons of the substantia nigra pars compacta (SNc) that are involved in modulating function of the striatum and other BG structures. This ultimately results in an increase in activity of the indirect pathway relative to the direct pathway and the hallmark PD symptoms of rigidity, bradykinesia, and akinesia. A great deal of effort has been dedicated to finding treatments for this disease. The current pharmacotherapies are aimed at replacing the missing dopamine, while the current surgical treatments are aimed at reducing transmission through the indirect pathway. Dopamine replacement therapy has proven to be helpful, but is associated with severe side effects that limit treatment and a loss of efficacy with progression of the disease. Recently developed surgical therapies have been highly effective, but are highly invasive, expensive, and assessable to a small minority of patients. For these reasons, new effort has been dedicated to finding pharmacological treatment options that will be effective in reducing transmission through the indirect pathway. Members of the metabotropic glutamate receptor (mGluR) family have emerged as interesting and promising targets for such a treatment. This review will explore the most recent advances in the understanding of mGluR localization and function in the BG motor circuit and the implications of those findings for the potential therapeutic role of mGluR-targeted compounds for PD.

Behavioural activity of (S)-3,5-DHPG, a selective agonist of group I metabotropic glutamate receptors

The influence of intracerebroventricular (i.c.v.) injections of (S)-3,5-dihydroxyphenyl-glycine (S)-3,5-DHPG, a selective agonist of group I metabotropic glutamate receptors (mGluRs), on the activity of the central nervous system was examined in male rats. (S)-3,5-DHPG at doses of 25, 50 and 100 nmol significantly attenuated crossings of squares and rearings, but not bar approaches, in an 'open field' test and failed to change apomorphine-induced stereotypy. (S)-3,5-DHPG at the above doses, given immediately after the learning trial, significantly facilitated the consolidation process in a passive avoidance situation, but given before the learning trial and before the retention testing did not have any influence on acquisition and retrieval processes, respectively. Moreover, (S)-3,5-DHPG did not influence recognition memory evaluated in an object recognition test. These results may suggest that activation of group I mGluRs takes part in the consolidation process in affectively-motivated memory, but is probably not necessary for processing of recognition memory, and that (S)-3,5-DHPG memory facilitation seems to be independent of glutamatergic and dopaminergic interaction.

Regulation of neurotransmitter release by metabotropic glutamate receptors

The G protein-coupled metabotropic glutamate (mGlu) receptors are differentially localized at various synapses throughout the brain. Depending on the receptor subtype, they appear to be localized at presynaptic and/or postsynaptic sites, including glial as well as neuronal elements. The heterogeneous distribution of these receptors on glutamate and nonglutamate neurons/cells thus allows modulation of synaptic transmission by a number of different mechanisms. Electrophysiological studies have demonstrated that the activation of mGlu receptors can modulate the activity of Ca(2+) or K(+) channels, or interfere with release processes downstream of Ca(2+) entry, and consequently regulate neuronal synaptic activity. Such changes evoked by mGlu receptors can ultimately regulate transmitter release at both glutamatergic and nonglutamatergic synapses. Increasing neurochemical evidence has emerged, obtained from in vitro and in vivo studies, showing modulation of the release of a variety of transmitters by mGlu receptors. This review addresses the neurochemical evidence for mGlu receptor-mediated regulation of neurotransmitters, such as excitatory and inhibitory amino acids, monoamines, and neuropeptides.

Group I metabotropic glutamate receptors mediate an inward current in rat substantia nigra dopamine neurons that is independent from calcium mobilization

Metabotropic glutamate receptors modulate neuronal excitability via a multitude of mechanisms, and they have been implicated in the pathogenesis of neurodegenerative processes. Here we investigated the responses mediated by group I metabotropic glutamate receptors (mGluRs) in dopamine neurons of the rat substantia nigra pars compacta, using whole cell patch-clamp recordings in combination with microfluorometric measurements of [Ca(2+)](i) and [Na(+)](i). The selective group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (3,5-DHPG) was bath-applied (20 microM, 30 s to 2 min) or applied locally by means of short-lasting (2-4 s) pressure pulses, delivered through an agonist-containing pipette positioned close to the cell body of the neuron. 3,5-DHPG evoked an inward current characterized by a transient and a sustained component, the latter of which was uncovered only with long-lasting agonist applications. The fast component coincided with a transient elevation of [Ca(2+)](i), whereas the total current was associated with a rise in [Na(+)](i). These responses were not affected either by the superfusion of ionotropic excitatory amino acid antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D-2-amino-5-phosphono-pentanoic acid (D-APV), nor by the sodium channel blocker tetrodotoxin (TTX). (S)-alpha-methyl-4-carboxyphenylglycine (S-MCPG) and the more selective mGluR1 antagonist 7(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate (CPCCOEt) depressed both 3,5-DHPG-induced inward current components and, although less effectively, the associated [Ca(2+)](i) elevations. On repeated agonist applications the inward current and the calcium transients both desensitized. The time constant of recovery from desensitization differed significantly between these two responses, being 67.4+/-4.4 s for the inward current and 28.6+/-2.7 s for the calcium response. Bathing the tissue in a calcium-free/EGTA medium or adding thapsigargin (1 microM) to the extracellular medium prevented the generation of the [Ca(2+)](i) transient, but did not prevent the activation of the inward current. These electrophysiological and fluorometric results show that the 3, 5-DHPG-induced inward current and the [Ca(2+)](i) elevations are mediated by independent pathways downstream the activation of mGluR1.

Group I metabotropic glutamate receptors: implications for brain diseases

Glutamate is the major excitatory neurotransmitter in the brain and plays a unique role in a variety of central nervous system (CNS) functions. The discovery of the metabotropic receptors (mGluRs), a family of G-protein coupled receptors than can be activated by glutamate, has led to an impressive number of studies in recent years aimed at understanding their biochemical, physiological and pharmacological characteristics. The eight mGluRs now known are divided into three groups according to their sequence homology, signal transduction mechanisms, and agonist selectivity. Group I mGluRs include mGluR1 and mGluR5, which are linked to the activation of phospholipase C; Groups II and III include all others and are negatively coupled to adenylyl cyclases. The availability in recent years of agents selective for Group I mGluRs has made possible the study of the physiological roles of these receptors in the CNS. In addition to mediating glutamatergic neurotransmission, Group I mGluRs can modulate other neurotransmitter receptors, including GABA and the ionotropic glutamate receptors. Group I mGluRs are involved in many CNS functions and may participate in a variety of disorders such as pain, epilepsy, ischemia, and chronic neurodegenerative diseases. This class of receptor may provide important pharmacological therapeutic targets and elucidating its functions will be relevant to develop new treatments for neurological and psychiatric disorders in which glutamatergic neurotransmission is abnormally regulated. In this review anatomical, physiological and pharmacological results are presented with a special emphasis on the role of Group I mGluRs in functional and pathological processes.

Glutamate inhibits thalamic reticular neurons

Activation of metabotropic glutamate receptors (mGluRs) can result in long-lasting modulation of neuronal excitability. Multiple mGluR subtypes are localized within the rat thalamic reticular nucleus (TRN), and we have examined the effects of activating these different receptor subtypes on the excitability of these neurons using an in vitro slice preparation. Typical of most mGluR-sensitive preparations, the general mGluR agonist, (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD) produced a robust, long-lasting excitatory response. Surprisingly, ACPD produced a membrane hyperpolarization in some neurons. Using selective mGluR agonists, we found that activation of group II mGluRs produces the hyperpolarization, whereas the depolarization is mediated by group I mGluRs. While the polarity of the postsynaptic response (hyperpolarization vs depolarization) was dependent on the mGluR subtype activated, both actions appear to result from modification of a linear K(+) conductance. The inhibitory action of Glutamate, via group II mGluRs, provides an avenue for a disinhibitory effect that could have interesting consequences upon a well-investigated, model neuronal circuit, turning its assumed functional role upside down.

Effects of metabotropic glutamate receptor activation in auditory thalamus

Metabotropic glutamate receptors (mGluRs) are expressed predominantly in dendritic regions of neurons of auditory thalamus. We studied the effects of mGluR activation in neurons of the ventral partition of medial geniculate body (MGBv) using whole cell current- and voltage-clamp recordings in brain slices. Bath application of the mGluR-agonist, 1S,3R-1-aminocyclopentan-1,3-dicarboxylic acid or 1S,3R-ACPD (5-100 microM), depolarized MGBv neurons (n = 67), changing evoked response patterns from bursts to tonic firing as well as frequency responses from resonance ( approximately 1 Hz) to low-pass filter characteristics. The depolarization was resistant to Na(+)-channel blockade with tetrodotoxin (TTX; 300 nM) and Ca(2+)-channel blockade with Cd(2+) (0.1 mM). The application of 1S, 3R-ACPD did not change input conductance and produced an inward current (I(ACPD)) with an average amplitude of 84.2 +/- 5.3 pA (at -70 mV, n = 22). The application of the mGluR antagonist, (RS)-alpha-methyl-4-carboxyphenylglycine (0.5 mM), reversibly blocked the depolarization or I(ACPD). During intracellular application of guanosine 5'-O-(3-thiotriphosphate) from the recording electrode, bath application of 1S,3R-ACPD irreversibly activated a large amplitude I(ACPD). During intracellular application of guanosine 5'-O-(2-thiodiphosphate), application of 1S, 3R-ACPD evoked only a small I(ACPD). These results implicate G proteins in mediation of the 1S,3R-ACPD response. A reduction of external [Na(+)] from 150 to 26 mM decreased I(ACPD) to 32.8 +/- 10. 3% of control. Internal applications of a Ca(2+) chelator, 1, 2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA; 10 mM), suppressed I(ACPD), implying a contribution of a Ca(2+) signal or Na(+)/Ca(2+) exchange. However, partial replacement of Na(+) with Li(+) (50 mM) did not significantly change I(ACPD). Therefore it seemed less likely that a Na(+)/Ca(2+) exchange current was a major participant in the response. A reduction of extracellular [K(+)] from 5.25 to 2.5 mM or external Ba(2+) (0.5 mM) or Cs(+) (2 mM) did not significantly change I(ACPD) between -40 and -85 mV. Below -85 mV, 1S,3R-ACPD application reversibly attenuated an inward rectification, displayed by 11 of 20 neurons. Blockade of an inwardly rectifying K(+) current with Ba(2+) (1 mM) or Cs(+) (2-3 mM) occluded the attenuation. In the range positive to -40 mV, 1S, 3R-ACPD application activated an outward current which Cs(+) blocked; this unmasked a voltage dependence of the inward I(ACPD) with a maximum amplitude at approximately -30 mV. The I(ACPD) properties are consistent with mGluR expression as a TTX-resistant, persistent Na(+) current in the dendritic periphery. We suggest that mGluR activation changes the behavior of MGBv neurons by three mechanisms: activation of a Na(+)-dependent inward current; activation of an outward current in a depolarized range; and inhibition of the inward rectifier, I(KIR). These mechanisms differ from previously reported mGluR effects in the thalamus.

Group I mGlu receptor modulation of dopamine release in the rat striatum in vivo

The present study investigated the ability of mGlu (metabotropic glutamate) receptor to modulate dopamine release in the striatum of freely moving rats assessed using the microdialysis technique. The group I and II mGlu receptor agonist (1S,3R)-ACPD (1-amino-cyclopentane-1,3-dicarboxylate; 1-3 mM) increased dopamine release (367% of basal levels) which was prevented by the non-selective mGlu receptor antagonist, (+)-MCPG (alpha-methyl-4-carboxyphenylglycine; 10 mM). The group I mGlu receptor agonist, DHPG (3,5-dihydroxyphenylglycine; 0.3-1 mM), also increased dopamine release (maximum increase 229%) which was also antagonised by (+)-MCPG (10 mM). In contrast, the group II mGlu receptor agonist, DCG-IV (2-(2,3-dicarboxycyclopropyl)glycine; 3-50 microM), induced a more modest increase in dopamine release (156% of basal levels). Combined administration of DHPG (1 mM) and DCG-IV (50 microM) maximally increased dopamine release by 252% of basal levels which was antagonised completely by (+)-MCPG (10 mM). Such findings indicate that group I (and possibly group II) mGlu receptors facilitate rat striatal dopamine release in vivo.

Metabotropic glutamate receptors and the generation of locomotor activity: interactions with midbrain dopamine

Interactions between excitatory amino acid (EAA) and dopamine (DA) pathways in the basal ganglia have been known for some time to contribute importantly to the generation of motor behaviors. In particular, the role played by ionotropic glutamate receptors (iGluRs) in such interactions and in the production of locomotion has received considerable attention, particularly in brain areas such as the ventral tegmental area (VTA) where EAA afferants are known to modulate the activity of DA neurons and the nucleus accumbens (NAcc) where descending EAA projections and ascending DA mesencephalic projections come in close apposition to each other and co-innervate intrinsic neurons projecting to motor output regions. Recently, the growing importance of the metabotropic glutamate receptor (mGluR) in the generation of motor behaviors and various forms of plasticity has begun to emerge. The known coupling of the mGluR to second messenger systems and its demonstrated role in the long-term modulation of synaptic transmission make it a logical candidate not only for the generation of locomotion involving EAA-DA interactions, but also for the induction and expression of locomotor plasticity involving these neurotransmitters. In this review, we examine the evidence supporting a role for mGluRs in the generation of DA-dependent locomotion as well as in one form of locomotor plasticity: the sensitization of locomotor activity by psychomotor stimulant drugs.

Reciprocal dopamine-glutamate modulation of release in the basal ganglia

Dopaminergic and glutamatergic transmissions have long been known to interact at multiple levels in the basal ganglia to modulate motor and cognitive functions. One important aspect of their interactions is represented by the reciprocal modulation of release. This topic has been the object of interest since the late 70's, particularly in the striatum and in midbrain dopaminergic areas (substantia nigra and ventral tegmental area). Analysis of glutamate-dopamine interactions in the control of each other's release is complicated by the fact that both glutamate and dopamine act on multiple receptor subtypes which can exert different effects. Therefore, glutamatergic modulation of dopamine release has been reviewed by analyzing the effects of glutamatergic selective receptor agonists and antagonists in the striatum (both motor and limbic portions) and in midbrain dopaminergic areas, as revealed by in vitro (slices, cell cultures, synaptosomes) and in vivo (push-pull, microdialysis and voltammetry techniques) experimental approaches. The same approach has been followed for dopaminergic modulation of glutamate release. The facilitatory nature of glutamate modulating both presynaptic and dendritic dopamine release has clearly emerged from in vitro studies. However, evidence is presented that, at least in the striatum and in the nucleus accumbens of awake rats, glutamate-mediated inhibitory effects may also occur. In vitro and in vivo experiments in the striatum and midbrain dopaminergic areas mainly depict dopamine as an inhibitory modulator of glutamate release. However, in vivo studies reporting dopamine D1 receptor mediated facilitatory effects are also considered. Therefore, the general notion that glutamate and dopamine act oppositely to regulate each other's release, is only partly supported by the available data. Conversely, the nature of the interaction between the two neurotransmitters seems to vary depending on the experimental approach, the brain area considered and the subtype of receptor involved.

Intracellular sodium and calcium homeostasis during hypoxia in dopamine neurons of rat substantia nigra pars compacta

We investigated the hypoxia-induced disturbance of cytosolic sodium concentration ([Na+]i) and of cytosolic calcium concentration ([Ca2+]i) in dopamine neurons of the substantia nigra pars compacta in rat midbrain slices, by combining whole cell patch-clamp recordings and microfluorometry. Transient hypoxia (3-5 min) induced an outward current (118.7 +/- 15.1 pA, mean +/- SE; VH = -60 mV). The development of this outward current was associated with an elevation in [Na+]i and in [Ca2+]i. The hypoxia-induced outward current as well as the elevations in [Na+]i and [Ca2+]i were not affected by the ionotropic and metabotropic glutamate receptor antagonists -amino-phosphonovalerate (50 microM), 6nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione (10 microM) and S-(alpha)-methyl-4-carboxyphenylglycine (500 microM). Tolbutamide, a blocker of ATP-dependent K+ channels, depressed the hypoxia-induced outward current but did not affect the increases in [Na+]i or [Ca2+]i. Increasing the concentration of ATP in the internal solution from 2 to 10 mM strongly reduced the hypoxia-induced outward current but did not reduce the rise in [Na+]i. Decreasing the concentration of extracellular Na+ to 19.2 mM depressed the hypoxia-induced outward current and resulted in a decrease in resting [Na+]i. Under this condition hypoxia still increased [Na+]i, albeit to levels not exceeding those of resting [Na+]i observed under control conditions. We conclude that 1) a major component of the hypoxia-induced outward current of these cells is caused by a depletion of intracellular ATP in combination with an increase in [Na+]i, 2) that the [Na+]i and [Ca2+]i responses are not mediated by glutamate receptors, 3) that the [Na+]i and [Ca2+]i responses are not depressed by activation of sulfonylurea receptors, and 4) that the rise in [Na+]i induced by short-lasting hypoxia is not due to a ATP depletion-induced failure of Na+ extrusion.

Expression of metabotropic glutamate receptor 1 isoforms in the substantia nigra pars compacta of the rat

Metabotropic glutamate receptors, which are linked via G-proteins to second messenger systems, have been implicated in the physiological regulation of dopaminergic neurons of the substantia nigra pars compacta as well as in neurodegeneration. Of the eight known metabotropic glutamate receptors, metabotropic glutamate receptor 1 is the most abundant subtype in the substantia nigra pars compacta. Metabotropic glutamate receptor 1 is alternatively spliced at the carboxy terminal region to yield five variants: 1a, 1b, 1c, 1d and a form recently identified in human brain, 1g. We used an antibody recognizing metabotropic glutamate receptor 1, and another recognizing the splice form la only, to study the localization of these receptors in dopaminergic neurons identified by the presence of tyrosine hydroxylase. Metabotropic glutamate receptor immunoreactivity was present within the somata, axons, and dendrites of substantia nigra pars compacta neurons. The 1a splice form specific antibody, however, did not label these cells, suggesting that they express a metabotropic glutamate receptor 1 splice form different from 1a. In situ hybridization with splice form-specific oligonucleotide probes was used to determine which of the other known metabotropic glutamate receptor 1 splice forms might be present in the substantia nigra pars compacta. Each probe produced a very distinct labelling pattern in the rat brain with the exception of the 1g specific probe which produced only background signal. Substantia nigra pars compacta neurons were most intensely labelled by the metabotropic glutamate receptor 1d splice form specific probe. Metabotropic glutamate receptor 1a was expressed weakly whereas there was no detectable 1b, c, or g signal in the substantia nigra pars compacta. These data demonstrate that metabotropic glutamate receptor 1 protein is present within the perikarya and processes of dopaminergic neurons in the substantia nigra pars compacta. The majority of this protein is not the 1a splice form, which is abundant in other brain regions, and may be the 1d isoform. Since splicing alters the carboxy terminus of the receptor, it is likely to affect the interaction of the receptor with intracellular signalling systems.

Glutamate mediates an inhibitory postsynaptic potential in dopamine neurons

Rapid information transfer within the brain depends on chemical signalling between neurons that is mediated primarily by glutamate and GABA (gamma-aminobutyric acid), acting at ionotropic receptors to cause excitatory or inhibitory postsynaptic potentials (EPSPs or IPSPs), respectively. In addition, synaptically released glutamate acts on metabotropic receptors to excite neurons on a slower timescale through second-messenger cascades, including phosphoinositide hydrolysisl. We now report a unique IPSP mediated by the activation of metabotropic glutamate receptors. In ventral midbrain dopamine neurons, activation of metabotropic glutamate receptors (mGluR1) mobilized calcium from caffeine/ryanodine-sensitive stores and increased an apamin-sensitive potassium conductance. The underlying potassium conductance and dependence on calcium stores set this IPSP apart from the slow IPSPs described so far. The mGluR-induced hyperpolarization was dependent on brief exposure to agonist, because prolonged application of exogenous agonist desensitized the hyperpolarization and caused the more commonly reported depolarization. The rapid rise and brief duration of synaptically released glutamate in the extracellular space can therefore mediate a rapid excitation through activation of ionotropic receptors, followed by inhibition through the mGluR1 receptor. Thus the idea that glutamate is solely an excitatory neurotransmitter must be replaced with a more complex view of its dual function in synaptic transmission.

Regulation of striatal dopamine release by metabotropic glutamate receptors

In vivo microdialysis in conscious rats was used to assess the effect of metabotropic glutamate receptor stimulation on striatal dopamine release. Local application of the metabotropic glutamate agonist (+/-)-trans-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), via a microdialysis probe, produced a concentration-dependent response: infusion of 50 microM ACPD did not produce a significant effect on extracellular dopamine levels, while application of 100 microM or 500 microM ACPD increased dopamine release by approximately 50% or 100%, respectively. To examine the contribution of impulse flow and multisynaptic mechanisms to the ACPD-induced increase in dopamine release, 500 microM ACPD were coapplied with 2 microM tetrodotoxin (TTX). An increase in extracellular dopamine levels was observed after the application of 500 microM ACPD, despite the presence of TTX. To further study the actions of metabotropic glutamate receptor-stimulation on terminal release characteristics of dopamine, the effect of ACPD on 40 mM K+-stimulated dopamine release was investigated. It was found that application ofACPD reduces dopamine release in response to K+ stimulation. These data suggest that during basal conditions, metabotropic glutamate receptor activation facilitates striatal dopamine release, possibly through presynaptic, impulse-independent mechanisms. However, during conditions of hyperstimulation, activation of metabotropic receptors, in contrast to ionotropic receptors, reduces excess dopamine release.

Metabotropic glutamate receptors depress glutamate-mediated synaptic input to rat midbrain dopamine neurones in vitro

1. Glutamate (AMPA receptor-mediated) excitatory postsynaptic potentials (e.p.s.ps.), evoked by electrical stimulation rostral to the recording site, were examined by intracellular microelectrode recording from dopamine neurones in parasagittal slices of rat ventral midbrain. 2. The e.p.s.p. was depressed by the group III metabotropic glutamate (mGlu) receptor agonist L-2-amino-4-phosphonobutyric acid (L-AP4; 0.01-30 microM) by up to 60% with an EC50 of 0.82 microM. The depression induced by L-AP4 (3 microM) was reversed by the group III preferring mGlu receptor antagonist, alpha-methyl-4-phosphonophenylglycine (MPPG; 250 microM). 3. The group I and II mGlu agonist, 1S,3R-aminocyclopentanedicarboxylic acid (ACPD; 3-30 microM) also depressed the e.p.s.p. in a concentration-dependent manner. The effect of ACPD (10 microM) was reversed by (+)-alpha-methyl-4-carboxyphenylglycine (MCPG; 1 mM; 4 cells). This effect of ACPD was also partially antagonized (by 50.3+/-15.7%, 4 cells) by MPPG (250 microM). 4. The selective agonist at group I mGlu receptors, dihydroxyphenylglycine (DHPG; 100 microM), decreased e.p.s.p. amplitude by 27.1+/-8.2% (7 cells), as did the group II mGlu receptor-selective agonist (1S,1R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV; 1 microM) by 26.7+/-4.3% (5 cells). 5. DHPG (10-100 microM) caused a depolarization of the recorded cell, as did ACPD (3-30 microM), whereas no such postsynaptic effect of either L-AP4 or DCG-IV was observed. 6. These results provide evidence for the presence of presynaptic inhibitory metabotropic glutamate autoreceptors from the mGlu receptor groups II and III on descending glutamatergic inputs to midbrain dopamine neurones. Group I mGlu receptors mediate a postsynaptic depolarization, and can also depress glutamatergic transmission, but may not necessarily be localized presynaptically. These sites represent novel drug targets for treatment of schizophrenia and movement disorders of basal ganglia origin.

Burst firing in midbrain dopaminergic neurons

Midbrain dopaminergic (DA) neurons fire bursts of activity in response to sensory stimuli, including those associated with primary reward. They are therefore conditional bursters - the bursts conveying, amongst other things, motivationally relevant information to the forebrain. In the forebrain, bursts give rise to a supra-additive release of dopamine, and possibly favour the release of co-localised neuropeptides. Evidence is presented that in rat DA neurons, bursts are engendered by the activity of cortically-regulated afferents. Certain factors are identified which, in combination, lead to burst production: (1) A burst of activity in EAAergic afferents to DA neurons arising from non-cortical sources, but controlled by the medial prefrontal cortex; (2) N-methyl-D-aspartate receptor activation, producing a slow depolarising wave in the recipient neuron; (3) activation of a high threshold, dendritically located calcium conductance which produces a 'plateau potential'; (4) activation of a calcium-activated potassium conductance, which terminates the burst. These factors are argued to operate in the context of an 'optimal' level of intracellular calcium buffering for bursting. Other factors which appear to be involved in bursting in other systems, in particular a low threshold calcium conductance, are rejected as being necessary for bursting in DA neurons. The factors which do play a crucial role in burst production in DA neurons are integrated into a theory from which arises a series of hypotheses amenable to empirical investigation. Additional factors are discussed which may modulate bursting. These may either act indirectly through changes in membrane potential (or intracellular calcium concentration), or they may act directly through an interaction with certain conductances, which appear to promote or inhibit burst firing in DA neurons.

Glutamate metabotropic receptor agonists depress excitatory and inhibitory transmission on rat mesencephalic principal neurons

Intracellular and whole-cell patch-clamp recordings were used to evaluate the actions of different metabotropic glutamate receptor (mGluR) agonists on the synaptic inputs evoked on principal cells of the rat mesencephalon. Bath application of the group III mGluR agonists L-2-amino-4-phosphonobutyric acid (L-AP4) and L-serine-O-phosphonobutanoate (L-SOP) did not change the holding current of the cells held at resting potential (-60 mV) but produced a dose-dependent inhibition of the amplitude of the excitatory and inhibitory events. L-AP4 and L-SOP were more effective at inhibiting the excitatory postsynaptic currents (EPSCs) than the GABA(A) and GABA(B) inhibitory postsynaptic currents (IPSCs). The suppressing effects of L-AP4 and L-SOP were antagonized by (S)-2-amino-2-methyl-4-phosphonobutanoic acid (MAP-4) but not by +/- -alpha-methyl-4-carboxyphenylglycine (MCPG). Moreover, the group II agonist (2S,1'S,2'S)-(carboxycyclopropyl)glycine (L-CCG1) and the group I agonist (RS)-3,5-dihydrophenylglycine (3,5-DHPG) depressed in a dose-related manner the EPSC, the GABA(A) IPSC and the GABA(B) IPSC. The suppressing effect of the two mGluRs agonists was partially antagonized by MCPG but not by MAP-4. In addition, both L-CCG1 and 3,5-DHPG caused an inward shift of the holding current. To characterize the site of action of the metabotropic receptor agonists, experiments were performed to examine the amplitude and ratio of EPSC and GABA(A) IPSC pairs. The increase of the s2/s1 ratio caused by the agonists suggests that the location of the inhibitory mGluRs was presynaptic. These results indicate that the activation of presynaptic mGluRs controls the release of excitatory and inhibitory transmitters on presumed dopaminergic cells within the ventral mesencephalon.

Excitation of rat subthalamic nucleus neurones in vitro by activation of a group I metabotropic glutamate receptor

The subthalamic nucleus (SThN) provides a glutamate mediated excitatory drive to several other component nuclei of the basal ganglia, thereby significantly influencing locomotion and control of voluntary movement. We have characterised functionally the metabotropic glutamate (mGlu) receptors in the SThN using extracellular single unit recording from rat midbrain slices. SThN neurones fired action potentials spontaneously at a rate of 10 Hz which was increased by the group I/II mGlu receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (1S,3 R-ACPD; 1-30 microM) and the group I selective agonist (S, R)-dihydroxyphenylglycine (DHPG; 1-30 microM). However, both the group II selective agonist (1S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV; 1 microM) and the group III selective agonist (S)-2-amino-4-phosphonobutanoic acid (L-AP4; 10 microM) were without effect, indicating that the excitation was mediated by a group I mGlu receptor. The excitation caused by DHPG (3 microM) was reversed by co-application of the mGlu receptor antagonist (+)-alpha-methyl-4-carboxyphenylglycine (MCPG; 500 microM). Thus a group I mGlu receptor mediates excitation of SThN neurones, and suggests a use for group I mGlu receptor ligands for treatment of both hypo- and hyperkinetic disorders of basal ganglia origin, such as Parkinson's disease and Huntington's disease.

A long-lasting calcium-activated nonselective cationic current is generated by synaptic stimulation or exogenous activation of group I metabotropic glutamate receptors in CA1 pyramidal neurons

We have shown previously that a selective metabotropic glutamate receptor (mGluR) agonist, 1S,3R-1-aminocyclo-pentane-1, 3-dicarboxylate (1S,3R-ACPD), evokes an inward current in CA1 pyramidal neurons of rat hippocampal slices in the presence of K+ channel blockers (). This current has been characterized as a Ca2+-activated nonselective cationic (CAN) current. Using whole-cell patch-clamp recordings and intracellular dialysis, we now have identified the mGluR subtype and the mechanisms underlying the CAN current (ICAN) and report for the first time the presence of a synaptic ICAN in the mammalian CNS. First, we have shown pharmacologically that activation of ICAN by 1S,3R-ACPD involves the group I mGluRs (and not the groups II and III) and a G-protein-dependent process. We also report that ICAN is modulated by the divalent cations (Mg2+, Cd2+, and Zn2+). Second, we have isolated a slow synaptic inward current evoked by a high-frequency stimulation in the presence of K+ channel blockers, ionotropic glutamate, and GABAA receptor antagonists. This current shows similar properties to the exogenously evoked ICAN: its reversal potential is close to the reversal potential of the 1S, 3R-ACPD-evoked ICAN, and it is G-protein- and Ca2+-dependent. Because the amplitude and duration of ICAN increased in the presence of a glutamate uptake blocker, we suggest that this synaptic current is generated via the activation of mGluRs. We propose that the synaptic ICAN, activated by a brief tetanic stimulation and leading to a long-lasting inward current, may be involved in neuronal plasticity and synchronized network-driven oscillations.

Modulation of dopamine neuronal activity by glutamate receptor subtypes

In vitro and in vivo electrophysiological studies have been used to assess the effects of glutamate, as well as specific agonists and antagonists for ionotropic, N-methyl-D-aspartate (NMDA), (R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and kainate, and metabotropic subtypes of the glutamate receptor, on the neuronal firing activity of midbrain, substantia nigra zona compacta (A9) and ventral tegmental area (A10), dopamine neurons. In in vitro experiments, agonists for all glutamate receptor subtypes depolarize the membrane and increase firing rate. In in vivo experiments, iontophoretic application of these agonists increases the firing rate and induces burst-firing. Studies with subtype selective antagonists suggest that a tonic glutamate tone, acting via NMDA receptors, may modulate the firing activity of some dopamine neurons. Glutamatergic afferents from the subthalamus, pedunculopontine nucleus and frontal cortex can modulate the firing activity of dopamine neurons. The role(s) of the different glutamate receptor subtypes and pathways in mediating the physiological and pathological effects on dopamine systems is an area for further investigation.

Metabotropic glutamate receptor-mediated inhibition and excitation of substantia nigra dopamine neurons

Microiontophoretic drug application and extracellular recording techniques were used to evaluate the effects of the selective metabotropic glutamate receptor (mGluR) agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate(1S,3R-ACPD) on dopamine (DA) neurons in the substantia nigra zona compacta (SNZC) of chloral hydrate-anesthetized rats. 1S,3R-ACPD had a biphasic effect on the firing rate of DA cells, initially decreasing, then increasing the firing rate. 1S,3R-ACPD also increased the burst-firing activity of DA neurons. Application of the ionotropic receptor (iGluR) agonists (R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) or N-methyl-D-aspartate (NMDA) increased the firing rates of neurons which had responded to 1S,3R-ACPD, indicating that mGluRs and iGluRs reside on the same neurons. The initial inhibitory period was not antagonized by systemic haloperidol or iontophoretic bicuculline, indicating a lack of DA or gamma-amino-n-butyric acid (GABA) involvement in this effect. Combined application of the AMPA antagonist, 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f)quinoxaline (NBQX), and the NMDA antagonist, (I)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphoric acid (CPP), at currents which antagonized AMPA and NMDA, did not antagonize either the inhibitory or excitatory effects of 1S,3R-ACPD. Application of the metabotropic antagonist (S)-4-carboxy-phenylglycine antagonized both the inhibitory and excitatory effects of 1S,3R-ACPD. These results indicate that mGluRs may play a role in the modulation of dopaminergic activity in the SNZC.

Neuromedin C decreases potassium conductance and increases a non-specific conductance in rat suprachiasmatic neurones in brain slices in vitro

Whole-cell recordings were made, in both current and voltage clamp, from suprachiasmatic neurones maintained in coronal rat brain slices. In current clamp doses of 10 and 100 nM neuromedin C (NMC) were shown to increase basal firing rate in 9 out of 32 neurones. The excitatory responses to 100 nM NMC were accompanied by small increases in neuronal input resistance (25.0 +/- 9.9% in 4 out of 7 neurones tested) and depolarisations of membrane potential (9.8 +/- 3.4 mV in 4 out of 7 neurones tested). However, 10 nM NMC caused no changes in either neuronal input resistance or membrane potential despite the clear increases in neuronal firing rate. When voltage-clamped at -60 mV, 100 nM NMC induced an inward current of 14.8 +/- 1.2 pA in 46 of 210 neurones. The NMC-induced inward current was shown to be unaffected by perfusion with 1 microM tetrodotoxin (TTX). The inward current recorded at -60 mV was typically associated with a decrease in membrane conductance. Construction of current-voltage relationships in the absence and presence of 100 nM NMC showed that with the majority of the NMC-sensitive neurones the inward current either reversed polarity close to the potassium reversal potential or decreased at hyperpolarised potentials. This reversal potential was shifted to more depolarised potentials when the extracellular concentration of potassium was increased. The NMC-induced inward current was unaffected by reduction of the extracellular concentration of sodium or by addition of 0.2 mM cadmium. In potassium-free conditions, in both the dialysing pipette solution and perfusing saline, NMC was still able to induce an inward current. The additional reduction of the extracellular concentration of sodium, whilst recording in potassium-free conditions, was also unable to abolish the inward current. Recordings made with an electrode containing the non-hydrolysable guanosine triphosphate analogue, guanosine 5'-thio-triphosphate, resulted in NMC-induced inward currents which failed to recover to baseline. It is concluded that NMC excites a subpopulation of suprachiasmatic neurones by decreasing a resting potassium conductance and increasing a non-specific conductance, via a G-protein link.

Quisqualate-preferring metabotropic glutamate receptor activates Na(+)-Ca2+ exchange in rat basolateral amygdala neurones

1. Inward currents evoked by metabotropic glutamate receptor (mGlu) agonists quisqualate and 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) were characterized in the basolateral nucleus of the amygdala. Currents were recorded with whole-cell patch electrodes in the presence of D-2-amino-5-phosphonovaleric acid (D-APV, 50 microM), 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX, 30 microM) and tetrodotoxin (TTX, 1 microM). 2. When recording with K+ electrodes, quisqualate (10-50 microM) produced an inward current which was not associated with a significant change in membrane slope conductance (Gm) and was insensitive to Ba2+ (0.2 mM) and Cs+ (2 mM). The 1S,3R-ACPD (50-200 microM)-induced inward current was associated with a decreased Gm and reversed polarity around -95 mV. However, in Ba2+ and Cs+, the 1S,3R-ACPD inward current amplitude was enhanced and was not accompanied by a change in Gm, a response similar to that evoked by quisqualate. 3. Glutamate (1 mM) and the group I mGlu specific agonist (S)-3,5-dihydroxyphenylglycine (DHPG, 100 microM) also evoked currents not associated with a change in Gm. 4. When recorded with Cs+ electrodes in external Ba2+ and Cs+ solution, quisqualate activated an inward current more potently than 1S,3R-ACPD, suggesting that this current is preferentially activated by quisqualate. The mGlu agonist-induced inward current was not accompanied by a Gm change under these conditions. 5. Substitution of extracellular Na+ with Li+ (117 or 50 mM) or with 100 mM choline reduced the quisqualate- and 1S,3R-ACPD-induced inward currents, results consistent with mediation by Na(+)-Ca2+ exchange. 6. The quisqualate- and 1S,3R-ACPD-induced inward currents were reduced in Ca(2+)-free EGTA (1 mM) solution and prevented by including the Ca2+ chelating agent BAPTA (10 mM) in the recording electrode. In low-Ca2+ (100 microM)- and Cd2+ (200 microM)-containing solution to block voltage-gated Ca2+ currents, the quisqualate-induced current was not altered, but the 1S,3R-ACPD inward current was blocked. These data suggest that the quisqualate- and 1S,3R-ACPD-induced currents are mediated through a rise in intracellular Ca2+ and require extracellular Ca2+, but that the 1S,3R-ACPD current may depend on Ca2+ influx via voltage-gated Ca2+ channels. 7. The quisqualate current with no Gm change was inhibited by including the Na(+)-Ca2+ exchange inhibitory peptide (XIP; 10 microM) in the K+ recording electrode. XIP did not prevent the outward current evoked by baclofen (10 microM) or the 1S,3R-ACPD-induced inward current associated with decreased conductance. 8. These data are consistent with the hypothesis that quisqualate and 1S,3R-ACPD in Ba2+ and Cs+ solution activate a Na(+)-Ca2+ exchange current not associated with a conductance change. The quisqualate exchange current mediated through a group I mGlu may result from mobilization of Ca2+ from intracellular stores. The 1S,3R-ACPD exchange current requires extracellular Ca2+ passing through voltage-gated Ca2+ channels and may be mediated through a different receptor.

Pharmacology and functions of metabotropic glutamate receptors

In the mid to late 1980s, studies were published that provided the first evidence for the existence of glutamate receptors that are not ligand-gated cation channels but are coupled to effector systems through GTP-binding proteins. Since those initial reports, tremendous progress has been made in characterizing these metabotropic glutamate receptors (mGluRs), including cloning and characterization of cDNA that encodes a family of eight mGluR subtypes, several of which have multiple splice variants. Also, tremendous progress has been made in developing new highly selective mGluR agonists and antagonists and toward determining the physiologic roles of the mGluRs in mammalian brain. These findings have exciting implications for drug development and suggest that the mGluRs provide a novel target for development of therepeutic agents that could have a significant impact on neuropharmacology.

A slow excitatory postsynaptic current mediated by G-protein-coupled metabotropic glutamate receptors in rat ventral tegmental dopamine neurons

Dopamine neurons in the substantia nigra and ventral tegmental area express metabotropic glutamate receptors, but activation of these receptors by synaptic release of neurotransmitter has not been demonstrated thus far. Patch pipettes were used to record membrane currents under voltage clamp from presumed dopamine-containing neurons in the whole-cell configuration in the rat brain slice. A short train of electrical stimuli delivered to bipolar electrodes placed in the slice evoked a slow excitatory postsynaptic current (EPSC; 50-300 pA at -70 mV) which peaked 560 ms after onset and lasted several seconds, with a decay time-constant of 630 ms. This slow EPSC was voltage-dependent, and was abolished by tetrodotoxin (0.5 microM) or by perfusate containing low calcium (0.5 mM) and high magnesium (10 mM). The metabotropic glutamate receptor antagonist (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG; 300 microM) blocked the slow EPSC, but L(+)-2-amino-3-phosphonopropionic acid (AP3; 300 microM) had no effect. The slow EPSC was largely occluded by inward current produced by the metabotropic receptor agonist trans-(+/-)-1-amino-1, 3-cyclopentanedicarboxylic acid (t-ACPD; 300 microM), and the EPSC was reduced > 90% during acute desensitization produced by prolonged perfusion with t-ACPD. (+/-)-2-Amino-4-phosphonobutyric acid (AP4; 300 microM), another metabotropic receptor agonist, reduced the slow EPSC but had no effect on currents evoked by t-ACPD applied by pressure-ejection from micropipettes. The slow EPSC was progressively reduced in amplitude when pipettes contained the G-protein inhibitor GDP-beta-S (0.5 mM). When pipettes contained GTP-gamma-S (0.5 mM), a non-hydrolysable analogue of GTP, onset of the slow EPSC was more rapid and its decay was significantly prolonged. These results demonstrate that a slow EPSC mediated by G-protein-coupled metabotropic glutamate receptors can be evoked in dopamine neurons.