Activation of group I mGluRs is necessary for induction of long-term depression at striatal synapses

Endogenous activation of mGlu5 metabotropic glutamate receptors contributes to the development of nigro-striatal damage induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in mice

We combined the use of knock-out mice and subtype-selective antagonists [2-methyl-6-(phenylethynyl)pyridine (MPEP) and (E)-2-methyl-6-(2-phenylethenyl)-pyridine (SIB1893)] to examine whether endogenous activation of mGlu5 metabotropic glutamate receptors contributes to the pathophysiology of nigro-striatal damage in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of parkinsonism. High doses of MPTP (four injections of 20 mg/kg, i.p., every 2 hr) induced a high mortality rate and a nearly total degeneration of the nigro-striatal pathway in wild-type mice. mGlu5 knock-out mice were less sensitive to MPTP toxicity, as shown by a higher survival and a milder nigro-striatal damage. Protection against MPTP (80 mg/kg) toxicity was also observed after MPEP injections (four injections of 5 mg/kg, i.p., 30 min before each MPTP injection). MPEP treatment did not further increase neuroprotection against 80 mg/kg of MPTP in mGlu5 knock-out mice, indicating that the drug acted by inhibiting mGlu5 receptors. In wild-type mice, MPEP was also neuroprotective when challenged against lower doses of MPTP (either 30 mg/kg, single injection, or four of 10 mg/kg injections). The action of MPEP was mimicked by SIB1893 but not by the mGlu1 receptor antagonist 7-hydroxyiminocyclopropan[b]chromen-1a-carboxylic acid ethyl ester. MPEP did not change the kinetics of 1-methyl-4-phenylpyridinium ion formation in the striatum of mice injected with MPTP. We conclude that mGlu5 receptors act as amplifiers of MPTP toxicity and that mGlu5 receptor antagonists may limit the extent of nigro-striatal damage in experimental models of parkinsonism.

Repeated electroconvulsive stimulation impairs long-term depression in the neostriatum

BACKGROUND

Increasing and consistent findings of structural and functional abnormalities in patients with mood disorders demonstrate a clear involvement of the neostriatum. Therefore, the beneficial effect of electroconvulsive stimulation (ECS) treatment of acute state of mood disorder may relate to changes in striatal synaptic plasticity.

METHODS

We studied the effect of ECS treatment on the reported long-term depression (LTD) of synaptic excitatory afferents to striatal medium spiny neurons. Stimulation of the white matter between the cortex and the striatum elicited excitatory postsynaptic potentials (EPSPs) in medium spiny neurons in rat corticostriatal slices while recording using the whole-cell patch-clamp technique.

RESULTS

The EPSPs evoked in striatal neurons undergo LTD with repeated stimulation trains, and LTD of this pathway is impaired in rats after ECS treatment for 1 week, similar to what is reported in chronic lithium treatment. Electroconvulsive stimulation did not affect intrinsic membrane properties or the occurrence of spontaneous EPSCs. Dose-dependent inhibition of the EPSPs by a nonselective agonist of metabotropic glutamate receptor did not change in rats after ECS treatment.

CONCLUSIONS

Our data suggest that the effectiveness of electroconvulsive therapy in mood disorders may be a consequence of LTD impairment of excitatory cortical afferents to striatal projecting neurons.

Cannabinoids decrease corticostriatal synaptic transmission via an effect on glutamate uptake

Activation of cannabinoid CB1 receptors reduces glutamatergic synaptic transmission in the rodent striatum and is involved in the normal control of motor function by the basal ganglia. Here we investigated CB1 receptor regulation of glutamate release and uptake and synaptic transmission in the rat striatum. We show that CB1 receptor activation reduces both the release and uptake of [3H]glutamate in striatal slices. We also demonstrate that both activation of CB1 receptors and inhibition of glutamate uptake reduce corticostriatal synaptic transmission in a mutually occlusive manner and that both forms of depression are dependent on metabotropic glutamate receptor (mGluR) activation. We propose that CB1 receptor activation in the striatum decreases glutamate transporter activity and that the resulting increase in synaptic cleft glutamate concentration causes the activation of presynaptic mGluRs, which then decrease glutamate release.

Glutamate spillover in the striatum depresses dopaminergic transmission by activating group I metabotropic glutamate receptors

Cortical glutamate and substantia nigra dopamine (DA) afferents converge onto the dendritic spines of medium spiny neurons (MSNs) in the striatum where they act to modulate motor and cognitive functions. The released DA spills over from its synapse and is thought to regulate glutamatergic input by acting on distal DA receptors located on corticostriatal axon terminals. By monitoring evoked DA release directly using fast-scan cyclic voltammetry, we report a reciprocal modulation by glutamate spillover on evoked striatal DA release, induced by either glutamate uptake blockade or high-frequency stimulation of corticostriatal tracts. We demonstrate that this modulation is attributable to the activation of group I metabotropic glutamate receptors. Thus, under conditions in which glutamate escapes the confines of its synapse, it can elicit the presynaptic suppression of dopaminergic neurotransmission.

Plastic Control of Striatal Glutamatergic Transmission by Ensemble Actions of Several Neurotransmitters and Targets for Drugs of Abuse

Long-lasting alterations in the efficacy of glutamatergic synapses, such as long-term potentiation (LTP) and long-term depression (LTD), are prominent models for mechanisms of information storage in the brain. It has been suggested that exposure to drugs of abuse produces synaptic plasticity at glutamatergic synapses that shares many features with LTP and LTD, and that these synaptic changes may play roles in addiction. We have examined the involvement of particular neurotransmitters in synaptic plasticity at glutamatergic synapses within the striatum, a brain region with prominent roles in initiation and sequencing of actions, as well as habit formation. Our studies indicate that multiple neurotransmitters interact to produce striatal synaptic plasticity, and that the relative strength and patterning of the afferent inputs that release the various neurotransmitters determines whether LTP or LTD is activated. Drugs of abuse interact with glutamatergic synaptic plasticity in multiple ways, including alterations in dopamine release and more direct effects on glutamate release and glutamate receptors. We hypothesize that these effects contribute to addiction by facilitating the formation of new, drug-centered habits, and by disruption of more adaptive behaviors.

Group I metabotropic glutamate receptors in the monkey striatum: subsynaptic association with glutamatergic and dopaminergic afferents

Group I metabotropic glutamate receptors (mGluRs) are involved in long-term synaptic plasticity and neuroprotection in the striatum, but the specific role(s) of mGluR1 and mGluR5 remain poorly understood. In this study, we used electron-microscopic immunocytochemistry to compare the pattern of subsynaptic and subcellular distribution of mGluR1a and mGluR5 in relation to putative glutamatergic and dopaminergic inputs to the monkey striatum. At the light-microscopic level, both group I mGluRs are expressed in the striatal neuropil. In addition, numerous perikarya of striatal output neurons are immunostained for mGluR5, but much less frequently for mGluR1a. At the electron-microscopic level, immunoreactivity for both receptor subtypes is primarily expressed postsynaptically in dendrites and spines, although presynaptic mGluR1a labeling of glutamatergic thalamostriatal boutons and, less frequently, dopaminergic and corticostriatal terminals is also seen. In contrast to mGluR1a, mGluR5 immunoreactivity is rarely encountered presynaptically. In postsynaptic elements, 40-70% of immunoreactivity for both receptor subtypes is expressed intracellularly, whereas 30-60% is apposed to the plasma membrane. More than 80% of the labeling apposed to the plasma membrane is extrasynaptic. The remaining 20% is located at the edges of putative glutamatergic synapses or in the active zone of symmetric synapses. In mGluR5-, but not mGluR1a-immunostained sections, approximately 70% of dopaminergic symmetric synapses are labeled perisynaptically. These data emphasize the differential pattern of subsynaptic localization of the two group I mGluRs and provide various presynaptic and postsynaptic sites whereby mGluR1 and mGluR5 could mediate different, but complementary, effects on glutamatergic and dopaminergic transmission in the primate striatum.

Molecular mechanisms and therapeutical implications of intramembrane receptor/receptor interactions among heptahelical receptors with examples from the striatopallidal GABA neurons

The molecular basis for the known intramembrane receptor/receptor interactions among G protein-coupled receptors was postulated to be heteromerization based on receptor subtype-specific interactions between different types of receptor homomers. The discovery of GABAB heterodimers started this field rapidly followed by the discovery of heteromerization among isoreceptors of several G protein-coupled receptors such as delta/kappa opioid receptors. Heteromerization was also discovered among distinct types of G protein-coupled receptors with the initial demonstration of somatostatin SSTR5/dopamine D2 and adenosine A1/dopamine D1 heteromeric receptor complexes. The functional meaning of these heteromeric complexes is to achieve direct or indirect (via adapter proteins) intramembrane receptor/receptor interactions in the complex. G protein-coupled receptors also form heteromeric complexes involving direct interactions with ion channel receptors, the best example being the GABAA/dopamine D5 receptor heteromerization, as well as with receptor tyrosine kinases and with receptor activity modulating proteins. As an example, adenosine, dopamine, and glutamate metabotropic receptor/receptor interactions in the striatopallidal GABA neurons are discussed as well as their relevance for Parkinson's disease, schizophrenia, and drug dependence. The heterodimer is only one type of heteromeric complex, and the evidence is equally compatible with the existence of higher order heteromeric complexes, where also adapter proteins such as homer proteins and scaffolding proteins can exist. These complexes may assist in the process of linking G protein-coupled receptors and ion channel receptors together in a receptor mosaic that may have special integrative value and may constitute the molecular basis for some forms of learning and memory.

Protective role of group-II metabotropic glutamate receptors against nigro-striatal degeneration induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in mice

To examine how mGlu2/3 metabotropic glutamate receptors affect nigro-striatal degeneration, we used the agonist, LY379268, and the antagonist, LY341495, in mice challenged with the nigro-striatal toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). In control mice, high doses of MPTP (20 mg/kg, i.p., injected four times with 2 h of interval) induced a nearly total degeneration of the nigro-striatal pathway, as shown by measurements of striatal dopamine (DA) levels and by immunohistochemical analysis of tyrosine hydroxylase, high affinity dopamine transporter, and glial fibrillary acidic protein in the corpus striatum and substantia nigra. Lower cumulative doses of MPTP (30 mg/kg, i.p., injected only once) produced a partial lesion of the nigro-striatal pathway (about 50% reduction of striatal DA content). Systemic injection of LY379268 (1 mg/kg, i.p., 30 min prior to each injection of MPTP) partially reduced the extent of nigro-striatal degeneration induced by high doses of MPTP. Similar results were obtained by continuously delivering LY379268 (1 mg/kg/d for 7 d) by means of a subcutaneous osmotic minipump. The protective effect of LY379268 was antagonized by LY341495 (also delivered by the osmotic minipump). In mice challenged with the lower cumulative dose of MPTP, injection of LY379268 did not produce a significant neuroprotective effect. In contrast, the lesion was amplified by the antagonist, LY341495. Neither LY379268 nor LY341495 influenced the central bioavailability and the local half-life of MPTP, as shown by measurements of the toxin and its active metabolite, MPP(+), in the striatum. We conclude that mGlu2/3 receptors play a protective role against MPTP toxicity, and that the efficacy of the agonist, LY379268, critically depends on the extent of the nigro-striatal lesion.

Metabotropic glutamate receptor 5-regulated Elk-1 phosphorylation and immediate early gene expression in striatal neurons

The Galphaq protein-coupled metabotropic glutamate receptor subtype-5 (mGluR5) is densely expressed in medium spiny projection neurons of striatum. Emerging evidence suggests a significant role of mGluR5 in the addictive plasticity of striatal neurons that is likely derived from inducible cellular gene expression related to stimulation of mGluR5 and associative signaling proteins. In this study, we found that activation of mGluR5 with a selective agonist (RS)-2-chloro-5-hydroxy-phenylglycine (CHPG) induced a rapid and transient phosphorylation of a transcription regulator Elk-1 in cultured striatal neurons from rat E19 embryos or neonatal day-1 pups. The Elk-1 phosphorylation was dose-dependent and occurred in neurochemically identified GABAergic neurons, but not glia. A series of experiments further demonstrated that the CHPG-stimulated Elk-1 phosphorylation was mediated through selective activation of mGluR5-regulated phospholipase C and associative second messenger system, i.e. 1,4,5,-triphosphate-sensitive Ca2+ release. Moreover, the Elk-1 phosphorylation was partially dependent on mGluR5-mediated co-activation of NMDA, but not kainate/AMPA receptors and L-type voltage-operated Ca2+ channels. Using an immediate early gene c-fos as a report of inducible gene expression, we found that CHPG induced marked c-fos mRNA expression. The c-fos induction kinetically corresponded to the Elk-1 phosphorylation and was attenuated by antisense oligonucleotides that selectively knocked down Elk-1 proteins. These results indicate that glutamatergic tone on mGluR5 is positively coupled to Elk-1 phosphorylation in striatal neurons via multiple signaling mechanisms involving Ca2+ release and NMDA activation, and the mGluR5-mediated Elk-1 phosphorylation facilitates gene transcription.

Metabotropic glutamate receptor 5 (mGluR5)-mediated phosphoinositide hydrolysis and NMDA-potentiating effects are blunted in the striatum of aged rats: a possible additional mechanism in striatal senescence

The aim of the present work was to verify whether an impairment of subtype 5 metabotropic glutamate receptor-mediated neurotransmission did occur in the aged striatum. To this end, the ability of the subtype 5 metabotropic glutamate receptor agonist, RS-2-chloro-5-hydroxyphenylglycine, to stimulate phosphoinositide hydrolysis and to potentiate N-methyl-d-aspartate-induced effects in striatal slices from young (3 months) and aged (24 months) rats was compared. The ability of RS-2-chloro-5-hydroxyphenylglycine to induce maximal phosphoinositide turnover and to potentiate N-methyl-d-aspartate effects was significantly reduced in slices from old vs. young rats. These changes were associated with a significant reduction in the expression of subtype 5 metabotropic glutamate receptor protein (-28.8%) and phospholipase C-beta1 (-55.8%) in old striata, while receptor messenger ribonucleic acid expression was unchanged. These results show that the signalling associated with subtype 5 metabotropic glutamate receptors undergoes significant age-related changes and that a reduced expression of subtype 5 metabotropic glutamate receptors and, more importantly, phospholipase C-beta1 may account for the functional decline of subtype 5 metabotropic glutamate receptors.

It could be habit forming: drugs of abuse and striatal synaptic plasticity

Drug addiction can take control of the brain and behavior, activating behavioral patterns that are directed excessively and compulsively toward drug usage. Such patterns often involve the development of repetitive and nearly automatic behaviors that we call habits. The striatum, a subcortical brain region important for proper motor function as well as for the formation of behavioral habits, is a major target for drugs of abuse. Here, we review recent studies of long-term synaptic plasticity in the striatum, emphasizing that drugs of abuse can exert pronounced influences on these processes, both in the striatum and in the dopaminergic midbrain. Synaptic plasticity in the ventral striatum appears to play a prominent role in early stages of drug use, whereas dopamine- and endocannabinoid-dependent synaptic plasticity in the dorsal striatum could contribute to the formation of persistent drug-related habits when casual drug use progresses towards compulsive drug use and addiction.

Corticostriatal LTP requires combined mGluR1 and mGluR5 activation

Metabotropic glutamate receptors (mGluRs) have been demonstrated to play a role in synaptic plasticity. It has been recently shown that mGluR1 is involved in corticostriatal long-term depression, by means of pharmacological approach and by using mGluR1-knockout mice. Here, we report that both mGluR1 and mGluR5 are involved in corticostriatal long-term potentiation (LTP). In particular, the mGluR1 antagonist LY 367385, as well as the mGluR5 antagonist MPEP, reduce LTP amplitude. Moreover, blockade of both mGluR1 and mGluR5 by LY 367385 and MPEP co-administration fully suppresses LTP. Accordingly, group II and group III mGluRs antagonists fail to affect LTP induction. Interestingly, LTP amplitude is also significantly reduced in both mGluR1- and mGluR5-knockout mice. The differential function of mGluR1 and mGluR5 in corticostriatal synaptic plasticity may play a role in the modulation of the motor activity mediated by the basal ganglia, thus providing a substrate for the pharmacological treatment of motor disorders involving the striatum.

Retrograde signaling in the regulation of synaptic transmission: focus on endocannabinoids

This review covers recent developments in the cellular neurophysiology of retrograde signaling in the mammalian central nervous system. Normally at a chemical synapse a neurotransmitter is released from the presynaptic element and diffuses to the postsynaptic element, where it binds to and activates receptors. In retrograde signaling a diffusible messenger is liberated from the postsynaptic element, and travels "backwards" across the synaptic cleft, where it activates receptors on the presynaptic cell. Receptors for retrograde messengers are usually located on or near the presynaptic nerve terminals, and their activation causes an alteration in synaptic transmitter release. Although often considered in the context of long-term synaptic plasticity, retrograde messengers have numerous roles on the short-term regulation of synaptic transmission. The focus of this review will be on a group of molecules from different chemical classes that appear to act as retrograde messengers. The evidence supporting their candidacy as retrograde messengers is considered and evaluated. Endocannabinoids have recently emerged as one of the most thoroughly investigated, and widely accepted, classes of retrograde messenger in the brain. The study of the endocannabinoids can therefore serve as a model for the investigation of other putative messengers, and most attention is devoted to a discussion of systems that use these new messenger molecules.

Nuclear transcription factors in the hippocampus

In the mammalian hippocampus, there is a trisynaptic loop that has been often referred to in studies on learning and memory mechanisms and their physiological correlate, the long-term potentiation (LTP). The three sets of synapses are formed by the fibers of perforant pathway terminating on granule cells and by the mossy fibers and Schaeffer collaterals making connections with the pyramidal cells. Each of the three types of synapses can develop LTP. LTP is accompanied by changes in gene expression and it is the nuclear transcription, involving specific transcription factors, that is the starting point for the series of biological amplifications and consolidations both necessary for such sustained changes. The transcription factors are proteins that control gene expression, development and functional formation in every eukaryotic cell. Two categories of transcription factors have been defined to date: general factors that comprise at least 20 proteins to form multiple preinitiation complex at the TATA box (TATA rich sequence) or regulatory factors that bind to promoter or enhancer regions at specific sites on the DNA close to, or distant from, the TATA box. Transcription factors have been divided into five different major classes according to unique protein motifs. These include basic domain, zinc-finger, helix-turn-helix, beta-Scaffold factors with minor groove contacts and other transcription factors not specifically classified. Much evidence has been accumulating in favor of the participation of several transcription factors in the consolidation of memory in the mammalian hippocampus following a spatial memory task. It is, therefore, of great importance that the involvement of transcription factors in de novo protein synthesis relevant to the synaptic mechanisms that mediate the formation of long-term memory should be summarized and discussed. No specific correlation between transduction of extracellular signals and expression of nuclear transcription factors, however, has been demonstrated to date.

Synergistic interaction between adenosine A2A and glutamate mGlu5 receptors: implications for striatal neuronal function

The physiological meaning of the coexpression of adenosine A2A receptors and group I metabotropic glutamate receptors in gamma- aminobutyric acid (GABA)ergic striatal neurons is intriguing. Here we provide in vitro and in vivo evidence for a synergism between adenosine and glutamate based on subtype 5 metabotropic glutamate (mGluR5) and adenosine A2A (A2AR) receptor/receptor interactions. Colocalization of A2AR and mGluR5 at the membrane level was demonstrated in nonpermeabilized human embryonic kidney (HEK)-293 cells transiently cotransfected with both receptors by confocal laser microscopy. Complexes containing A2AR and mGluR5 were demonstrated by Western blotting of immunoprecipitates of either Flag-A2AR or hemagglutinin-mGluR5 in membrane preparations from cotransfected HEK-293 cells and of native A2AR and mGluR5 in rat striatal membrane preparations. In cotransfected HEK-293 cells a synergistic effect on extracellular signal-regulated kinase 1/2 phosphorylation and c-fos expression was demonstrated upon A2AR/mGluR5 costimulation. No synergistic effect was observed at the second messenger level (cAMP accumulation and intracellular calcium mobilization). Accordingly, a synergistic effect on c-fos expression in striatal sections and on counteracting phencyclidine-induced motor activation was also demonstrated after the central coadministration of A2AR and mGluR5 agonists to rats with intact dopaminergic innervation. The results suggest that a functional mGluR5/A2AR interaction is required to overcome the well-known strong tonic inhibitory effect of dopamine on striatal adenosine A2AR function.

Postsynaptic endocannabinoid release is critical to long-term depression in the striatum

The striatum functions critically in movement control and habit formation. The development and function of cortical input to the striatum are thought to be regulated by activity-dependent plasticity of corticostriatal glutamatergic synapses. Here we show that the induction of a form of striatal synaptic plasticity, long-term depression (LTD), is dependent on activation of the CB1 cannabinoid receptor. LTD was facilitated by blocking cellular endocannabinoid uptake, and postsynaptic loading of anandamide (AEA) produced presynaptic depression. The endocannabinoid necessary for striatal LTD is thus likely to be released postsynaptically as a retrograde messenger. These findings demonstrate a new role for endocannabinoids in the induction of long-term synaptic plasticity in a circuit necessary for habit formation and motor control.

Induction mechanisms for L-LTP at thalamic input synapses to the lateral amygdala: requirement of mGluR5 activation

L-LTP (late-phase long-term potentiation) at thalamo-amygdala synapses is thought to be critical for auditory fear conditioning, but it has not been clear what kinds of surface receptors and channels are involved in the induction phase of the L-LTP. Here we report that the NMDA receptor antagonist D-AP5 (50 microM), the L-type calcium channel antagonist nifedipine (30 microM) and the metabotropic glutamate receptor 5 antagonist MPEP (10 microM) prevented L-LTP induction when each antagonist was separately applied at saturating concentrations before and during repeated tetanus. By contrast, the mGluR1 antagonist CPCCOEt (80 microM) failed to show any effects on L-LTP induction. Neither D-AP5 nor MPEP produced any significant effects on potentiated synaptic responses when applied after L-LTP had been established. Thus, our data suggest that NMDA receptors, L-type calcium channels and mGluR5 are involved in L-LTP induction in the thalamo-amygdala pathway.