Activity-dependent activation of presynaptic metabotropic glutamate receptors in locus coeruleus

Astrocytic Endocannabinoids Mediate Hippocampal Transient Heterosynaptic Depression

Astrocytes are highly dynamic cells that modulate synaptic transmission within a temporal domain of seconds to minutes in physiological contexts such as Long-Term Potentiation (LTP) and Heterosynaptic Depression (HSD). Recent studies have revealed that astrocytes also modulate a faster form of synaptic activity (milliseconds to seconds) known as Transient Heterosynaptic Depression (tHSD). However, the mechanism underlying astrocytic modulation of tHSD is not fully understood. Are the traditional gliotransmitters ATP or glutamate released via hemichannels/vesicles or are other, yet, unexplored pathways involved? Using various approaches to manipulate astrocytes, including the Krebs cycle inhibitor fluoroacetate, connexin 43/30 double knockout mice (hemichannels), and inositol triphosphate type-2 receptor knockout mice, we confirmed early reports demonstrating that astrocytes are critical for tHSD. We also confirmed the importance of group II metabotropic glutamate receptors (mGluRs) in astrocytic modulation of tHSD using a group II agonist. Using dominant negative SNARE mice, which have disrupted glial vesicle function, we also found that vesicular release of gliotransmitters and activation of adenosine A1 receptors are not required for tHSD. As astrocytes can release lipids upon receptor stimulation, we asked if astrocyte-derived endocannabinoids are involved in tHSD. Interestingly, a cannabinoid receptor 1 (CB1R) antagonist blocked and an inhibitor of the endogenous endocannabinoid 2-arachidonyl glycerol (2-AG) degradation potentiates tHSD in hippocampal slices. Taken together, this study provides the first evidence for group II mGluR-mediated astrocytic endocannabinoids in transiently suppressing presynaptic neurotransmitter release associated with the phenomenon of tHSD.

Group II/III metabotropic glutamate receptors exert endogenous activity-dependent modulation of TRPV1 receptors on peripheral nociceptors

There is pharmacological evidence that group II and III metabotropic glutamate receptors (mGluRs) function as activity-dependent autoreceptors, inhibiting transmission in supraspinal sites. These receptors are expressed by peripheral nociceptors. We investigated whether mGluRs function as activity-dependent autoreceptors inhibiting pain transmission to the rat CNS, particularly transient receptor potential vanilloid 1 (TRPV1)-induced activity. Blocking peripheral mGluR activity by intraplantar injection of antagonists LY341495 [(2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid] (LY) (20, 100 μm, group II/III), APICA [(RS)-1-amino-5-phosphonoindan-1-carboxylic acid] (100 μm, group II), or UBP1112 (α-methyl-3-methyl-4-phosphonophenylglycine) (30 μm, group III) increased capsaicin (CAP)-induced nociceptive behaviors and nociceptor activity. In contrast, group II agonist APDC [(2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate] (0.1 μm) or group III agonist l-(+)-2-amino-4-phosphonobutyric acid (l-AP-4) (10 μm) blocked the LY-induced increase. Ca(2+) imaging in dorsal root ganglion (DRG) cells confirmed LY enhanced CAP-induced Ca(2+) mobilization, which was blocked by APDC and l-AP-4. We hypothesized that excess glutamate (GLU) released by high intensity and/or prolonged stimulation endogenously activated group II/III, dampening nociceptor activation. In support of this, intraplantar GLU + LY produced heat hyperalgesia, and exogenous GLU + LY applied to nociceptors produced enhanced nociceptor activity and thermal sensitization. Intraplantar Formalin, known to elevate extracellular GLU, enhanced pain behaviors in the presence of LY. LY alone produced no pain behaviors, no change in nociceptor discharge rate or heat-evoked responses, and no change in cytosolic Ca(2+) in DRG cells, demonstrating a lack of tonic inhibitory control. Group II/III mGluRs maintain an activity-dependent autoinhibition, capable of significantly reducing TRPV1-induced activity. They are endogenously activated after high-frequency and/or prolonged nociceptor stimulation, acting as built-in negative modulators of TRPV1 and nociceptor function, reducing pain transmission to the CNS.

Impaired expression and function of group II metabotropic glutamate receptors in pilocarpine-treated chronically epileptic rats

Group II metabotropic (mGlu II) receptor subtypes mGlu2 and mGlu3 are important modulators of synaptic plasticity and glutamate release in the brain. Accordingly, several pharmacological ligands have been designed to target these receptors for the treatment of neurological disorders characterized by anomalous glutamate regulation including epilepsy. In this study, we examine whether the expression level and function of mGlu2 and mGlu3 are altered in experimental epilepsy by using immunohistochemistry, Western blot analysis, RT-PCR and extracellular recordings. A down-regulation of mGlu2/3 protein expression at the mossy fiber pathway was associated with a significant reduction in mGlu2/3 protein expression in the hippocampus and cortex of chronically epileptic rats. Moreover, a reduction in mGlu2 and mGlu3 transcripts levels was noticed as early as 24 h after pilocarpine-induced status epilepticus (SE) and persisted during subsequent "latent" and chronic periods. In addition, a significant impairment of mGlu II-mediated depression of field excitatory postsynaptic potentials at mossy fiber-CA3 synapses was detected in chronically epileptic rats. Application of mGlu II agonists (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) induced a significant reduction of the fEPSP amplitude in control rats, but not in chronic epileptic rats. These data indicate a long-lasting impairment of mGlu2/3 expression that may contribute to abnormal presynaptic plasticity, exaggerate glutamate release and hyperexcitability in temporal lobe epilepsy.

AMN082, an allosteric mGluR7 agonist that inhibits afferent glutamatergic transmission in rat basolateral amygdala

Glutamatergic neurotransmission has been implicated in the pathophysiology of psychiatric disorders, such as anxiety and depression. The possible contribution of group III metabotropic glutamate receptors has been poorly investigated, due to the lack of selective pharmacological tools. However, a selective agonist of mGLUR7, AMN082 has been identified recently, and shown to act through an allosteric mechanism in recombinant cells expressing the receptor. Thus, using AMN082, we examined the role of mGLUR7 in modulating synaptic transmission in the rat basolateral amygdala (BLA), a brain region known to be important for the genesis of anxious states. We found that bath application of AMN082 (1-10microM) produced a concentration-dependent inhibition of synaptic transmission evoked at 2Hz, but had no effect on transmission evoked at 0.05Hz. However, at this lower frequency, AMN082 (10microM) significantly increased the synaptic inhibition produced by a group III mGLUR agonist, L-AP4 (100microM). This effect was blocked by pre-application of CPPG (500microM), a group III mGLUR-preferring antagonist, consistent with the involvement of mGLUR7. Thus, we have shown that AMN082 can modulate high frequency synaptic transmission in the BLA, in vitro, and appears to act on the receptor via an allosteric mechanism. These results suggest that mGLUR7 has a unique role in regulating neuronal activity in the BLA and may be a target for novel drugs for the treatment of anxiety.

Modulation of GABAergic inhibition in the rat superior colliculus by a presynaptic group II metabotropic glutamate receptor

Previous work has indicated that metabotropic glutamate receptors (mGluRs) modulate visual responses of superior colliculus (SC) neurones in vivo in a variety of ways, in a manner that can be dependent upon visual stimulus properties. How this occurs remains unclear. In this study we aimed to determine how activation of mGluR2 and mGluR3 receptors (Group II) might modulate visual responses, by using field potential and whole-cell patch clamp recording techniques in rat SC slice. Stimulation within the superficial layers of the SC, in the presence of ionotropic glutamate receptor antagonists, evoked IPSCs that were blocked by bicuculline indicating that they are mediated via GABAA receptors. It is likely that these IPSCs were of heterogeneous origin as they showed substantial variation in paired-pulse behaviour. Nevertheless, activation of Group II mGluRs with the group-selective agonist LY354740 (300 nM, bath application) resulted in a reduction of these IPSCs (to 56% of control amplitude), and this was associated with a decrease in paired-pulse depression. At the same concentration, LY354740 did not reduce the EPSC or field-EPSP evoked by stimulation of the retinal input to the SC. The effects of LY354740 on IPSCs were not mimicked by the mGluR3-selective agonist N-acetyl-aspartyl-glutamate (NAAG, 200-500 microM). Stimulation of IPSCs with trains of impulses (10 at 20 Hz) in order to mimic natural activation patterns resulted in sequences of IPSCs that were reduced in amplitude towards the end of the stimulus train. Application of the Group II antagonist LY341495 (100 nM) under these conditions resulted in an increase in later IPSCs in a third of neurones tested. These findings indicate that mGluR2 (but not mGluR3) can selectively modulate GABAergic inhibition in SC, probably via a presynaptic mechanism. Furthermore, these receptors may be activated by synaptically released transmitter during patterns of activation similar to those seen during visual processing. Thus mGluR2 receptors could have a function in activity-dependent modulation of inhibitory processing during visual responses.

Abnormal mGluR2/3 expression in the perforant path termination zones and mossy fibers of chronically epileptic rats

Epilepsy is characterized by hyperexcitability of hippocampal networks, excessive release of glutamate, and progressive neurodegeneration. Presynaptic group II metabotropic receptors (mGluR2 and mGluR3) are among different mechanisms that modulate presynaptic release of glutamate, especially at the mossy fibers in the hippocampus. Here, we explore whether mGluR2/3 expression is affected in a rat model of temporal lobe epilepsy obtained via pilocarpine-induced status epilepticus (SE). Immunohistochemical assays were performed in age-matched controls and two groups of epileptic rats sacrificed at 25-35 days (1 month post-SE) and at 55-65 days (2 months post-SE) following SE onset. A dramatic lessening of mGluR2/3 immunofluorescence was observed at CA1 and CA3 stratum lacunosum/molecular (SLM) declining to 60% and 68% of control values in 1-month and 2-month post-SE, respectively. Additionally, thickness of mGluR2/3-stained SLM layer narrowed up to 70% of controls indicating atrophy at this branch of the perforant path. Epileptic rats exhibited a marked and progressive down-regulation of mGluR2/3 expression in mossy fiber at hilus and CA3 stratum lucidum in contrast with an enhanced expression of vesicular glutamate transporter type 1 (VGluT1) at the mossy fibers. Intense VGluT1 punctated staining was detected at the inner third molecular layer indicating glutamatergic sprouting. In the molecular layer, mGluR2/3 labeling slightly declined in the 1-month post-SE group but then increased in the 2-month post-SE group although it was diffusely distributed. Down-regulation of mGluR2/3 at the mossy fibers and the SLM may render epileptic hippocampal networks hyperexcitable and susceptible to glutamate-mediated excitotoxicity and neurodegeneration.

Role of spinal cord glutamate transporter during normal sensory transmission and pathological pain states

Glutamate is a neurotransmitter critical for spinal excitatory synaptic transmission and for generation and maintenance of spinal states of pain hypersensitivity via activation of glutamate receptors. Understanding the regulation of synaptically and non-synaptically released glutamate associated with pathological pain is important in exploring novel molecular mechanisms and developing therapeutic strategies of pathological pain. The glutamate transporter system is the primary mechanism for the inactivation of synaptically released glutamate and the maintenance of glutamate homeostasis. Recent studies demonstrated that spinal glutamate transporter inhibition relieved pathological pain, suggesting that the spinal glutamate transporter might serve as a therapeutic target for treatment of pathological pain. However, the exact function of glutamate transporter in pathological pain is not completely understood. This report will review the evidence for the role of the spinal glutamate transporter during normal sensory transmission and pathological pain conditions and discuss potential mechanisms by which spinal glutamate transporter is involved in pathological pain.

Brain stem excitatory and inhibitory signaling pathways regulating bronchoconstrictive responses

This review summarizes recent work on two basic processes of central nervous system (CNS) control of cholinergic outflow to the airways: 1) transmission of bronchoconstrictive signals from the airways to the airway-related vagal preganglionic neurons (AVPNs) and 2) regulation of AVPN responses to excitatory inputs by central GABAergic inhibitory pathways. In addition, the autocrine-paracrine modulation of AVPNs is briefly discussed. CNS influences on the tracheobronchopulmonary system are transmitted via AVPNs, whose discharge depends on the balance between excitatory and inhibitory impulses that they receive. Alterations in this equilibrium may lead to dramatic functional changes. Recent findings indicate that excitatory signals arising from bronchopulmonary afferents and/or the peripheral chemosensory system activate second-order neurons within the nucleus of the solitary tract (NTS), via a glutamate-AMPA signaling pathway. These neurons, using the same neurotransmitter-receptor unit, transmit information to the AVPNs, which in turn convey the central command to airway effector organs: smooth muscle, submucosal secretory glands, and the vasculature, through intramural ganglionic neurons. The strength and duration of reflex-induced bronchoconstriction is modulated by GABAergic-inhibitory inputs and autocrine-paracrine controlling mechanisms. Downregulation of GABAergic inhibitory influences may result in a shift from inhibitory to excitatory drive that may lead to increased excitability of AVPNs, heightened airway responsiveness, and sustained narrowing of the airways. Hence a better understanding of these normal and altered central neural circuits and mechanisms could potentially improve the design of therapeutic interventions and the treatment of airway obstructive diseases.

Modulatory effects mediated by metabotropic glutamate receptor 5 on lateral geniculate nucleus relay cells

Glutamate is thought to be the excitatory neurotransmitter in the lateral geniculate nucleus (LGN) of the cat, mediating visual transmission from the retina via ionotropic receptors of both D,L-alpha-amino-3-hydroxy-5-alpha-methyl-4-isoxazolepropionate and N-methyl-D-aspartate subtypes. Moreover, glutamate also exerts an important modulatory influence on LGN cells, where metabotropic glutamate receptors (mGluRs) seem to play a crucial role. Here we show in anesthetized adult cats that iontophoretic application of the specific mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) produced two, distinctly different, effects on LGN neurons. Visual responses to flashing spots and drifting gratings were attenuated (decreased by an average of 59%) in 13 of 23 of the cells but augmented (increased by an average of 60%) in 10 of 23 of the cells. Further, in each case when the specific mGluR5 agonist (R,S)-2-chloro-5-hydroxyphenylglycine was applied, the effects obtained were the opposite to those of MPEP. Data obtained in a second group of experiments to determine a possible interaction between mGluR5 blockade by MPEP and glutamate ionotropic receptors show that, in the majority of neurons (11 of 15, 73%), the MPEP-mediated effects seem to be independent of N-methyl-D-aspartate and D,L-alpha-amino-3-hydroxy-5-alpha-methyl-4-isoxazolepropionate receptor activity. Our results demonstrate a physiological role for mGluR5 in controlling retinal input and show, in vivo, a more intricate scenario than previously suggested, highlighting the complexity of metabotropic receptor interactions with excitatory and inhibitory elements in the thalamus.

Identification and functional roles of metabotropic glutamate receptor-interacting proteins

In the mammalian brain, a majority of excitatory synapses use glutamate as a neurotransmitter. Glutamate activates ligand-gated channels (ionotropic receptors) and G protein-coupled (metabotropic) receptors. During the past decade, a number of intracellular proteins have been described to interact with these receptors. These proteins not only scaffold the glutamate receptors at the pre- and post-synaptic membranes, but also regulate their subcellular targeting and intracellular signaling. Thus, identification of these proteins has been essential for further understanding the functions of glutamate receptors. Here we will focus on those proteins that interact with the subgroup of metabotropic glutamate (mGlu) receptors, and review the methods used for their identification, as well as their functional roles in neurons.

Activation of Group II and Group III metabotropic glutamate receptors by endogenous ligand(s) and the modulation of synaptic transmission in the superficial superior colliculus

Previous work from this laboratory indicates that Group II/III metabotropic glutamate (mGlu) receptors modulate responses of SC neurones to visual stimuli in vivo. It is thought that tonic levels of glutamate may be sufficient to activate some mGlu receptors. We wished to investigate if these receptors are activated under ambient conditions in SC. Field excitatory postsynaptic potentials (fEPSPs) evoked by optic tract stimulation were recorded from 300 microm slices of the adult pigmented rat superior colliculus at 34 degrees C. The Group II receptor selective agonist LY354740 (100-300 nM) had no significant effect on the peak amplitude of the fEPSP, although it did enhance the late phase of the fEPSP. In order to test for activation of Group II receptors by endogenous ligand, the selective antagonists LY341495 (50 nM) or EGLU (200 microM) were applied: these either enhanced or reduced the fEPSP amplitude. In similar experiments carried out at 22 degrees C, no effect was seen. The fEPSP enhancements, but not the fEPSP reductions, could be occluded by GABA antagonists. Application of higher concentrations of LY341495 (300, 600 nM-known to also affect Group III receptors, particularly mGlu8), or co-application of 50 nM LY341495 and the Group III-selective antagonist CPPG (100 microM) produced enhancements of responses, or counteracted response reductions over those seen with 50 nM LY341495 alone. The predominant Group II receptor in SC is mGlu3. It is known that this can be located presynaptically on GABAergic and glutamatergic terminals, postsynaptically, and on glia. Our results indicate that such receptors are tonically activated by endogenous transmitter, have distinct effects, and influence retino-collicular transmission. Furthermore, there is a segregation of effects where receptors exert some of their effects via modulation of GABAergic circuitry.

Group III metabotropic glutamate receptors maintain tonic inhibition of excitatory synaptic input to hypocretin/orexin neurons

Hypocretin/orexin neurons play an important role in hypothalamic arousal. Synaptic glutamate input to hypocretin neurons regulates cell firing. We studied the actions of group III metabotropic glutamate receptors (mGluRs) in modulating the activity of hypocretin neurons using whole-cell voltage- and current-clamp recording in mouse whole hypothalamic slices or minislices consisting only of the lateral hypothalamus. Selective green fluorescent protein expression was used to detect live hypocretin neurons. The mGluR agonist l-(+)-2-amino-4-phosphonobutyric acid (l-AP-4) inhibited synaptic input to hypocretin neurons in a dose-dependent manner; both spontaneous glutamate and GABA-mediated synaptic currents were reduced in frequency. l-AP-4 also reduced the amplitude of postsynaptic potentials evoked by a stimulating electrode placed medial or lateral to the recorded cell. No postsynaptic effect of l-AP-4 was found relative to membrane potential, input resistance, or AMPA-evoked currents. l-AP-4 appeared to act by a presynaptic mechanism and reduced the frequency of both glutamate- and GABA-mediated miniature events recorded in the presence of tetrodotoxin, with no change in amplitude. (RS)-phosphonopentanoic acid (CPPG), a group III mGluR antagonist, suppressed the actions of l-AP-4. Of substantial interest, CPPG by itself increased synaptic activity recorded in hypocretin neurons, suggesting an ongoing inhibitory tone attributable to activation of group III mGluRs. Glutamatergic interneurons have been suggested to play a role in a positive feedback recruitment of hypocretin on hypocretin neurons. l-AP-4 blocked hypocretin-mediated increases in EPSCs and attenuated the hypocretin-mediated increase in spike frequency. Together, these data suggest that tonically active inhibitory mGluRs are expressed on local hypocretin-sensitive glutamate neurons within the lateral hypothalamus that modulate the output of the hypocretin arousal system.

Evidence that the metabotropic glutamate receptor 5 antagonist MPEP may act as an inhibitor of the norepinephrine transporter in vitro and in vivo

The mechanisms through which blockade of metabotropic glutamate receptors 5 (mGluR5) results in anxiolytic and antidepressant effects are currently unknown. In the present study, we therefore hypothesized that the anxiolytic- and antidepressant-like profile of the noncompetitive mGluR5 receptor antagonist 2-ethyl-6-(phenylethynyl)-pyridine (MPEP) may be mediated by inhibition of the norepinephrine transporter (NET). Accordingly, we first examined the potency of MPEP to bind to or inhibit uptake at the NET as well as the dopamine and serotonin transporters (DAT and SERT, respectively). We also examined the simultaneous in vivo effects of MPEP and desipramine (DMI) on both NE-like oxidation current in the amygdala (AMY) and cell firing in the locus coeruleus (LC) by means of differential pulse voltammetry (DPV) coupled with electrophysiology. MPEP completely displaced the binding of [3H]-nisoxetine on human NET with a pKi of 6.63 +/- 0.02. In addition, MPEP was able to inhibit [3H]-NE uptake in LLCPK cells expressing human NET, with a pIC50 of 5.55 +/- 0.09. In vivo DPV data revealed that both MPEP (30 mg/kg i.p.) and DMI (10 mg/kg i.p.) significantly increased NE-like voltammetric responses levels in the AMY, whereas both compounds also significantly decreased cell firing monitored concomitantly from the second microelectrode in the LC. Collectively, the results of the present study provide potential new mechanisms through which MPEP exerts its anxiolytic and antidepressant effects.

Increased tonic activation of presynaptic metabotropic glutamate receptors in the rat supraoptic nucleus following chronic dehydration

Chronic dehydration induces structural changes in the hypothalamic supraoptic nucleus (SON), including increased glutamate synapses and retraction of astroglial processes. We performed whole-cell recordings in acute hypothalamic slices to determine whether these changes increase tonic activation of presynaptic metabotropic glutamate receptors (mGluRs) by increasing ambient glutamate in the SON. Activation of presynaptic group III mGluRs caused a decrease in the frequency of miniature excitatory postsynaptic currents (mEPSCs) in SON neurones that was significantly attenuated in slices from dehydrated rats (-27.8 %) compared with untreated rats (-41.7 %), suggesting a higher basal occupancy of mGluRs by ambient glutamate during dehydration. Blocking group III mGluRs caused an increase in the frequency of mEPSCs that was significantly higher in slices from dehydrated rats (+42.8 %) than untreated rats (+31.4 %), suggesting greater tonic activation of presynaptic mGluRs by ambient glutamate during dehydration. Increasing ambient glutamate levels by inhibiting astrocyte glutamate uptake resulted in a decrease in mEPSC frequency due to increased activation of presynaptic mGluRs. This was attenuated in slices from dehydrated rats (-35.4 %) compared with slices from untreated rats (-48.8 %), suggesting diminished astrocytic glutamate uptake during dehydration. Immunochemical analyses revealed a robust expression of the GLT-1 transporter protein in the SON, which was diminished in SON punches from dehydrated rats compared with untreated controls. Thus, dehydration leads to increased tonic activation of presynaptic mGluRs on glutamate terminals, consistent with a decrease in glutamate buffering capacity. The resulting reduction in glutamate release probability may compensate for the increase in glutamate release sites that occurs during dehydration.

Modulation of spinal nociceptive processing through the glutamate transporter GLT-1

GLT-1 is the predominant glutamate transporter in most brain regions and therefore plays a major role in terminating synaptic transmission and protecting neurons from glutamate neurotoxicity. In the present study we assessed (i) the regulation of GLT-1 expression in the spinal cord after peripheral nociceptive stimulation and (ii) the nociceptive behavior of rats following inhibition or transient knockdown of spinal GLT-1. Formalin injection into one hindpaw caused a rapid transient upregulation of GLT-1 protein expression in the spinal cord which did not occur when rats were pretreated with morphine (10 mg/kg, i.p.) suggesting that the nociceptive input specifically caused the increase of GLT-1 transcription. Inhibition of GLT-1 by the transportable inhibitor trans-pyrrolidine-2,4-dicarboxylic acid resulted in a significant reduction of nociceptive behavior in the rat formalin assay. Similar results were obtained with a transient reduction of GLT-1 protein expression by antisense oligonucleotides. These data suggest that inhibition of GLT-1 activity or expression reduces excitatory synaptic efficacy and thereby nociception. Mechanisms that might explain this phenomenon may include activation of inhibitory metabotropic glutamate receptors, postsynaptic desensitization or disturbance of glutamate recycling.

Antidepressant and anxiolytic-like effects in mice lacking the group III metabotropic glutamate receptor mGluR7

Glutamatergic neurotransmission has been strongly implicated in the pathophysiology of affective disorders, such as major depression and anxiety. Of all glutamate receptors, the role of group III metabotropic glutamate receptors (mGluR4, mGluR6, mGluR7, mGluR8) in such disorders is the least investigated because of the lack of specific pharmacological tools. To this end, we examined the behavioural profiles of mice with a targeted deletion of the gene for mGluR7 (mGluR7-/-) in animal models of depression and anxiety. mGluR7-/- mice were compared with wild-type (mGluR7+/+) littermates and showed substantially less behavioural immobility in both the forced swim test and the tail suspension test. Both behavioural paradigms are widely used to predict antidepressant-like activity. Further, mGluR7-/- mice displayed anxiolytic activity in four different behavioural tests, i.e. the light-dark box, the elevated plus maze, the staircase test, and the stress-induced hyperthermia test, while their cognitive performance was normal in the passive avoidance paradigm. Analysis of locomotor activity in a novel environment demonstrated that mGluR7-/- mice were slightly more active in the initial minutes following placement in the chamber only. Together, these data suggest that mGluR7 may play a pivotal role in mechanisms that regulate behavioural responses to aversive states. Therefore, drugs acting at mGluR7 may provide novel treatments for psychiatric disorders such as depression and anxiety.

Reduction of excitatory postsynaptic responses by persistently active metabotropic glutamate receptors in the hippocampus

The release of glutamate from axon terminals is under the control of a variety of presynaptic receptors, including several metabotropic glutamate receptors (mGluRs). Synaptically released glutamate can activate mGluRs within the same synapse where it was released and also at a distance following its diffusion from the synaptic cleft. It is unknown, however, whether the release of glutamate is under the control of persistently active mGluRs. We tested the contribution of mGluR activation to the excitatory postsynaptic responses recorded from several types of GABAergic interneuron in strata oriens/alveus of the mouse hippocampus. The application of 1 microM (alphaS)-alpha-amino-alpha-[(1S,2S)-2-carboxycyclopropyl]xanthine-9-propanoic acid (LY341495), a broad-spectrum mGluR (subtypes 2/3/7/8) antagonist at this concentration, increased evoked-excitatory postsynaptic current (eEPSC) amplitudes by 60% (n = 33). On identified cell types, LY341495 had either no effect (7 of 14 basket and 7 of 13 oriens-lacunosum moleculare, O-LM cells) or resulted in a 32 +/- 30% (mean +/- SD) increase in EPSC amplitudes recorded from basket cells and a seven-times greater (216 +/- 102%) enhancement of EPSCs in O-LM cells. The enhancement of the first EPSC of a high-frequency train indicates persistent mGluR activation. During antagonist application, the relative increase in EPSC amplitude evoked by the second and subsequent pulses in the train was not larger than that of the first EPSC, showing no further receptor activation by the released transmitter. The effect of mGluR subtype selective agonists [3 microM L(+)-2-amino-4-phosphonobutyric acid (L-AP4): mGluR4/8; 600 microM L-AP4: mGluR4/7/8; 1 microM (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IU): mGluR2/3] and an antagonist (0.2 microM LY341495: mGluR2/3/8) suggests that persistently active mGluR2/3/8 control the excitability of hippocampal network.

Presynaptic effects of group III metabotropic glutamate receptors on excitatory synaptic transmission in the retina

Metabotropic glutamate receptors (mGluRs) are located in both plexiform layers in the retina and may modulate transmission between photoreceptors and bipolar cells and between bipolar and ganglion cells. We investigated whether mGluR activation modulates excitatory synaptic input to bipolar cells and ganglion cells in the salamander retinal slice preparation. The group III mGluR agonist L-2-amino-4-phosphonobutyric acid (AP4) inhibited monosynaptic excitatory postsynaptic currents (EPSCs) in ganglion cells evoked by electrical stimuli, whereas group I and group II agonists had no significant effect. AP4 reduced the frequency but not the amplitude of ganglion cell miniature EPSCs, suggesting a presynaptic action at bipolar cell terminals. AP4 also reduced ganglion cell EPSCs evoked by the offset of a light stimulus, suggesting that group III mGluRs modulate release from OFF bipolar cells. Comparison of light-evoked EPSCs in OFF bipolar cells and ganglion cells indicated that AP4 reduced ganglion cell EPSCs by acting primarily at bipolar cell terminals, and to a lesser extent at photoreceptor terminals. The group II/III mGluR antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG) blocked the effect of AP4 at bipolar cell terminals, consistent with localization of group III mGluRs at these sites. However, CPPG did not increase EPSCs at light offset, indicating that activation of group III mGluRs by synaptic glutamate does not play a large role in modulating transmission from bipolar cells to ganglion cells.

Group III metabotropic glutamate receptors as autoreceptors in the cerebellar cortex

1. Group III metabotropic glutamate receptors (mGluRs) of the subtype 4a are localized within presynaptic active zones of cerebellar parallel fibre (PF)-Purkinje cell (PC) synapses. In order to investigate the conditions necessary for group III mGluR autoreceptor-activation by synaptically released glutamate, we characterized the effects of selective agonists and antagonists on excitatory postsynaptic currents (EPSCs) evoked by several distinct PF stimulation patterns. 2. The group III mGluR-selective agonist L-AP4 depressed evoked EPSCs at PF-PC synapses in rat brain slices with an EC(50) of 2.4 microM and maximum inhibition of 80%. This L-AP4-induced depression was antagonized by the group III mGluR-selective antagonist MSOP with an estimated equilibrium dissaciation constant of 12.5 microM. 3. Paired-pulse or four-pulse PF stimulations did not activate presynaptic group III mGluRs as revealed by the lack of effect of 1 mM MSOP on relative test EPSC amplitudes with latencies of 250-500 ms. The potentiation of a test EPSC evoked 200-500 ms after a short tetanic burst (100 Hz for 60 ms), was also unchanged in the presence of MSOP. 4. Endogenous autoreceptor-activation was revealed only during prolonged stimulation trains (10 Hz for 4.4 s), where, in the presence of 1 mM MSOP, the EPSC amplitudes were enhanced by 15%. 5. These observations support an autoreceptor function of group III mGluRs and a role in short-term synaptic plasticity at PF synapses. However, the low to moderate activation levels observed, despite the close spatial relation with glutamate release sites, suggests that additional mechanisms regulate receptor activation.

Different metabotropic glutamate receptors play opposite roles in synaptic plasticity of the rat medial vestibular nuclei

In the medial vestibular nuclei (MVN) of rat brainstem slices, the role of group II and III metabotropic glutamate receptors (mGluRs) and of the subtypes of group I mGluRs: mGluR1, mGluR5, was investigated in basal synaptic transmission and in the induction and maintenance of long-term potentiation (LTP). We used selective antagonists and agonists for mGluRs and we analysed the field potentials evoked by vestibular afferent stimulation before and after high-frequency stimulation (HFS) to induce LTP. The group II and III mGluR antagonist, (R,S)-alpha-2-methyl-4sulphonophenylglycine (MSPG), induced LTP per se and caused a reduction of the paired-pulse facilitation (PPF) ratio indicating an enhancement of glutamate release. This suggests that group II and III mGluRs are activated under basal conditions to limit glutamate release. Both the group II and III mGluR selective antagonists, 2S-2-amino-2-(1S,2S-2-carboxycycloprop-1-yl)-3-(xanth-9-yl)propanoate (LY341495) and (R,S)-alpha-methylserine-O-phosphate (MSOP), induced LTP, and the selective agonists, (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (APDC) and L(+)-2-amino-4-phosphonobutyric acid (L-AP4) depressed the field potentials and prevented HFS-LTP, with a prevailing contribution of group II mGluRs over that of group III mGluRs. The mGluR1 antagonist, 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) prevented the full development and maintenance of HFS-LTP. By contrast, the mGluR5 antagonist, 2-methyl-6-phenylethynylpyridine (MPEP) induced LTP per se, which was impeded by CPCCOEt, and it had no effect on LTP once induced by HFS. The PPF analysis showed an enhancement of glutamate release during MPEP potentiation. The group I mGluR agonist, (R,S)-3,5-dihydroxyphenylglycine (DHPG) induced LTP per se, which was blocked by CPCCOEt. By contrast the mGluR5 agonist, (R,S)-2-chloro-5-hydroxypheylglycine (CHPG) prevented LTP elicited by HFS and DHPG as well. In conclusion vestibular LTP is inhibited by group II and III mGluRs during the early induction phase while it is facilitated by mGluR1 for achieving its full expression and consolidation. An additional inhibitory control is exerted by mGluR5 at the level of this facilitatory phase.

PICK1 is required for the control of synaptic transmission by the metabotropic glutamate receptor 7

Both postsynaptic density and presynaptic active zone are structural matrix containing scaffolding proteins that are involved in the organization of the synapse. Little is known about the functional role of these proteins in the signaling of presynaptic receptors. Here we show that the interaction of the presynaptic metabotropic glutamate (mGlu) receptor subtype, mGlu7a, with the postsynaptic density-95 disc-large zona occludens 1 (PDZ) domain-containing protein, PICK1, is required for specific inhibition of P/Q-type Ca(2+) channels, in cultured cerebellar granule neurons. Furthermore, we show that activation of the presynaptic mGlu7a receptor inhibits synaptic transmission and this effect also requires the presence of PICK1. These results indicate that the scaffolding protein, PICK1, plays an essential role in the control of synaptic transmission by the mGlu7a receptor complex.

Vesicle pool partitioning influences presynaptic diversity and weighting in rat hippocampal synapses

Hippocampal synapses display a range of release probabilities. This is partially the result of scaling of release probability with the total number of releasable vesicles at each synapse. We have compared synaptic release and vesicle pool sizes across a large number of hippocampal synapses using FM 1-43 and confocal fluorescence microscopy. We found that the relationship between the number of recycling vesicles at a synapse and its release probability is dependent on firing frequency. During firing at 10 Hz, the release probability of each synapse is closely related to the number of recycling vesicles that it contains. In contrast, during firing at 1 Hz, different synapses turn over their recycling vesicle pools at different rates leading to an indirect relationship between recycling vesicle pool size and release probability. Hence two synapses may release vesicles at markedly different rates during low frequency firing, even if they contain similar numbers of vesicles. Both further kinetic analyses and manipulation of the number of vesicles in the readily releasable pool using phorbol ester treatment suggested that this imprecise scaling observed during firing at 1 Hz resulted from synapse-to-synapse differences in the proportion of recycling vesicles partitioned into the readily releasable pool. Hence differential partitioning between vesicle pools affects presynaptic weighting in a frequency-dependent manner. Since hippocampal single unit firing rates shift between 1 Hz and 10 Hz regimes with behavioural state, differential partitioning may be a mechanism for encoding information in hippocampal circuits.

Long-term potentiation and contextual fear conditioning increase neuronal glutamate uptake

Induction and expression of long-term potentiation (LTP) in area CA1 of the hippocampus require the coordinated regulation of several cellular processes. We found that LTP in area CA1 was associated with an N-methyl-D-aspartate (NMDA) receptor-dependent increase in glutamate uptake. The increase in glutamate uptake was inhibited by either removal of Na+ or addition of D,L-threo-beta-hydroxyaspartate. Dihydrokainate (DHK), a specific inhibitor of the glial glutamate transporter GLT-1, did not block the increase in glutamate uptake. LTP was also associated with a translocation of the EAAC1 glutamate transporter from the cytosol to the plasma membrane. Contextual fear conditioning increased the maximum rate (Vmax) of glutamate uptake and membrane expression of EAAC1 in area CA1. These results indicate that regulation of glutamate uptake may be important for maintaining the level of synaptic strength during long-term changes in synaptic efficacy.

Synaptic transmission in nucleus tractus solitarius is depressed by Group II and III but not Group I presynaptic metabotropic glutamate receptors in rats

Presynaptic metabotropic glutamate receptors (mGluRs) serve as autoreceptors throughout the CNS to inhibit glutamate release and depress glutamatergic transmission. Both presynaptic and postsynaptic mGluRs have been implicated in shaping autonomic signal transmission in the nucleus tractus solitarius (NTS). We sought to test the hypothesis that activation of presynaptic mGluRs depresses neurotransmission between primary autonomic afferent fibres and second-order NTS neurones. In second-order NTS neurones, excitatory postsynaptic currents (EPSCs) synaptically evoked by stimulation of primary sensory afferent fibres in the tractus solitarius (ts) and currents postsynaptically evoked by alpha-amino-3-hydroxy-4-isoxazoleproprionic acid (AMPA) were studied in the presence and absence of mGluR agonists and antagonists. Real-time quantitative RT-PCR (reverse transcription-polymerase chain reaction) was used to determine whether the genes for the mGluR subtypes were expressed in the cell bodies of the primary autonomic afferent fibres. Agonist activation of Group II and III but not Group I mGluRs reduced the peak amplitude of synaptically (ts) evoked EPSCs in a concentration-dependent manner while having no effect on postsynaptically (AMPA) evoked currents recorded in the same neurones. At the highest concentrations, the Group II agonist, (2S,3S,4S)-CCG/(2S,1'S,2'S)-2-carboxycyclopropyl (L-CCG-I), decreased the amplitude of the ts-evoked EPSCs by 39 % with an EC50 of 21 microM, and the Group III agonist, L(+)-2-amino-4-phosphonobutyric acid (L-AP4), decreased the evoked EPSCs by 71 % with an EC50 of 1 mM. mRNA for all eight mGluR subtypes was detected in the autonomic afferent fibre cell bodies in the nodose and jugular ganglia. Group II and III antagonists ((2S,3S,4S)-2-methyl-2-(carboxycyclopropyl)glycine (MCCG) and (RS)-alpha-methylserine-O-phosphate (MSOP)), at concentrations that blocked the respective agonist-induced synaptic depression, attenuated the frequency-dependent synaptic depression associated with increasing frequencies of ts stimulation by 13-34 % and 13-19 %, respectively (P < 0.05, for each). We conclude that Group II and III mGluRs (synthesized in the cell bodies of the primary autonomic afferent fibres and transported to the central terminals in the NTS) contribute to the depression of autonomic signal transmission by attenuating presynaptic release of glutamate.

Permissive effect of voltage on mGlu 7 receptor subtype signaling in neurons

G protein-coupled receptors mobilize neuronal signaling cascades which until now have not been shown to depend on the state of membrane depolarization. Thus we have previously shown that the metabotropic glutamate receptor type 7 (mGlu7 receptor) blocks P/Q-type Ca(2+) channels via activation of a G(o) protein and PKC, in cerebellar granule cells. We show here that the transient depolarizations used to evoke the studied Ca(2+) current were indeed permissive to activate this pathway by a mGlu7 receptor agonist. Indeed, sustained depolarization to 0 mV was sufficient to inhibit P/Q-type Ca(2+) channels. This effect involved a conformational change in voltage-gated sodium channel independently of Na(+) flux, activation of a pertussis toxin-sensitive G-protein, inositol trisphosphate formation, intracellular Ca(2+) release, and PKC activity. Subliminal sustained membrane depolarization became efficient in inducing inositol trisphosphate formation, release of intracellular Ca(2+) and in blocking Ca(2+) channels, when applied concomitantly with the mGlu7a receptor agonist, d,l-aminophosphonobutyrate. This synergistic effect of membrane depolarization and mGlu7 receptor activation provides a mechanism by which neuronal excitation could control action of the mGlu7 receptor in neurons.

mGlu1 receptors mediate a post-tetanic depression at parallel fibre-Purkinje cell synapses in rat cerebellum

Metabotropic glutamate (mGlu) receptors are located pre- and postsynaptically at central synapses. Activation of the receptors by exogenous agonists usually results in a reversible depression of fast glutamatergic neurotransmission. Evidence that synaptically released glutamate has such an action, however, is scarce. Sharp microelectrode recordings were used to investigate the modulatory role of mGlu receptors at a well-studied glutamatergic synapse, the one between parallel fibres and Purkinje cells in rat cerebellar slices. Brief, tetanic stimulation of the parallel fibres caused a depression of subsequent fast EPSPs. This post-tetanic depression (PTD) reached its maximum 4.5 s after the tetanus. Measured at this point, PTD was frequency-dependent; 10 stimuli at 20 Hz produced no significant depression, whereas, at 100 Hz the same number of stimuli was maximally effective (approximately 50% depression). The nonselective mGlu antagonist, (S)-alpha-methyl-4-carboxyphenylglycine 1 mm or the GABAB antagonist, CGP35348 (1 mm), both decreased the magnitude of the PTD. In the presence of CGP35348 the mGlu1 antagonist, 7-hydroxyiminocyclopropan[b]chromen-1a-carboxylic acid ethyl ester (300 microm), inhibited PTD further. A group II/III mGlu antagonist had no effect. These observations indicate that synaptically activated mGlu1 receptors not only generate a slow EPSP and induce Ca2+ mobilization in Purkinje cells, as reported previously, but also produce a transient depression of fast synaptic transmission. This short-term plasticity may be important for shaping the output of cerebellar circuits and/or it could provide a substrate for long-term depression when additional mechanisms are superimposed.

Frequency dependence of endocytosis in aortic baroreceptor neurons and role of group III mGluRs

Synaptic transmission between baroreceptor afferents and the nucleus tractus solitarius (NTS) is known to exhibit frequency-dependent depression. Reductions in neurotransmitter release and alterations in mechanisms regulating synaptic transmission are hypothesized to be involved in the activity-dependent depression observed in baroreceptor afferent neurons. The present study utilized cultured aortic baroreceptor neurons and the fluorescent dyes FM1-43 and FM2-10 to characterize the process of endocytosis or vesicle retrieval and its dependence on 1) frequency of neuronal activation, 2) metabotropic glutamate receptor (mGluR) activation, and 3) calcium concentrations inside and outside the cell. Endocytosis per spike, measured in fluorescence units after a 10-s stimulus applied at frequencies of 0.5 (53 +/- 4), 1.0 (23 +/- 1), and 10.0 Hz (2.7 +/- 0.2), was significantly depressed at higher frequencies. Blockade of group III mGluRs with (RS)-cyclopropyl-4-phosphonophenylglycine (CPPG) facilitated endocytosis at all frequencies, suggesting that this receptor subtype may be involved in the inhibition of endocytosis. Manipulating the extracellular and intracellular calcium concentrations subsequent to exocytosis had no effect on endocytosis. These results suggest that frequency-dependent depression of endocytosis observed in vitro could contribute to the frequency-dependent depression of baroreceptor afferent neurotransmission and that group III mGluRs inhibit endocytosis.

Group II and III metabotropic glutamate receptors contribute to different aspects of visual response processing in the rat superior colliculus

1. Neurones in the superior colliculus (SC) respond to novel sensory stimuli and response habituation is a key feature of this. It is known that both ionotropic and metabotropic glutamate (mGlu) receptors participate in visual responses of superficial SC neurones. A feature of Group II and Group III mGlu receptors is that they may modulate specific neural pathways, possibly via presynaptic mechanisms. However, less is known about how this may relate to functions of systems in whole animals. We have therefore investigated whether these receptors affect specific attributes of visual responses in the superficial SC. 2. Recordings were made from visually responsive neurones in anaesthetised rats, and agonists and antagonists of Group II and III mGlu receptors were applied iontophoretically at the recording site. 3. We found that application of the Group III metabotropic glutamate receptor agonist L-2-amino-4-phosphonobutyric acid (L-AP4) produced an increase in visual response habituation, whilst Group III antagonists decreased habituation. These effects were independent of the response habituation mediated via GABA(B) receptors. In contrast, modulation of Group II mGlu receptors with the specific agonist LY354740 or the antagonist LY341495 did not affect response habituation, although these compounds did modulate visual responses. This suggests a specific role for Group III mGlu receptors in visual response habituation. 4. The magnitude of Group II effects was smaller during presentation of low contrast stimuli compared with high contrast stimuli. This suggests that activation of Group II receptors may be activity dependent and that these receptors can translate this into a functional effect in adapting to high contrast stimuli.

Metabotropic glutamate receptor subtypes modulating neurotransmission at parallel fibre-Purkinje cell synapses in rat cerebellum

The actions of reportedly group-selective metabotropic glutamate (mGlu) receptor agonists and antagonists on neurotransmission at parallel fibre-Purkinje cell synapses in the rat cerebellum have been characterised using sharp microelectrode recording and an in vitro slice preparation. Application of the group I agonist (S)-3,5-dihydroxyphenylglycine (DHPG) or the group III selective agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4) depressed synaptic transmission in a reversible and concentration-dependent manner (EC(50)=18 and 5 microM, respectively). The depression produced by DHPG was unrelated to the depolarisation observed in some Purkinje cells. The group II agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV, 1 microM) had no effect. The effects of DHPG were inhibited by the group I-selective antagonist 7-hydroxyiminocyclopropan[b]chromen-1a-carboxylic acid ethyl ester (CPCCOEt), but not by the group II/III antagonist alpha-methyl-4-phosphonophenylglycine (MPPG). The effect of L-AP4 was inhibited by MPPG, but not by the group I/II antagonist (S)-alpha-methyl-4-carboxyphenylglycine (MCPG). By themselves, the antagonists did not affect the EPSPs, suggesting that neither receptor is activated during low frequency neurotransmission. It is concluded that, in addition to the excitatory role for group I receptors described previously, both group I and III (but not group II) mGlu receptors operate at this synapse to inhibit synaptic transmission. The specific receptor subtypes involved are likely to be mGlu1 and mGlu4.

Intensity-dependent, rapid activation of presynaptic metabotropic glutamate receptors at a central synapse

Synaptic signals from retinal bipolar cells were monitored by measuring EPSCs in ganglion cells voltage-clamped at -70 mV. Spontaneous EPSCs were strongly suppressed by l-2-amino-4-phosphonobutyrate (AP-4), an agonist at group III metabotropic glutamate receptors (mGluRs). Agonists of group I or II mGluRs were ineffective. AP-4 also suppressed ganglion cell EPSCs evoked by bipolar cell stimulation using potassium puffs, sucrose puffs, or zaps of current (0.5-1 microA). In addition, AP-4 suppressed Off EPSCs evoked by dim-light stimuli. This indicates that group III mGluRs mediate a direct suppression of bipolar cell transmitter release. An mGluR antagonist, (RS)-alpha-cyclopropyl-4-phosphonophenylyglycine (CPPG), blocked the action of AP-4. When bipolar cells were weakly stimulated, AP-4 produced a large suppression of the EPSC, but CPPG alone had little effect. Conversely, when bipolar cells were strongly stimulated, CPPG produced an enhancement of the EPSC, but AP-4 alone had little effect. This indicates that endogenous feedback regulates bipolar cell transmitter release and that the dynamic range of the presynaptic metabotropic autoreceptor is similar to that of the postsynaptic ionotropic receptor. Furthermore, the feedback is rapid and intensity-dependent. Hence, concomitant activation of presynaptic and postsynaptic glutamate receptors shapes the responses of ganglion cells.

Selective blockade of P/Q-type calcium channels by the metabotropic glutamate receptor type 7 involves a phospholipase C pathway in neurons

Although presynaptic localization of mGluR7 is well established, the mechanism by which the receptor may control Ca(2+) channels in neurons is still unknown. We show here that cultured cerebellar granule cells express native metabotropic glutamate receptor type 7 (mGluR7) in neuritic processes, whereas transfected mGluR7 was also expressed in cell bodies. This allowed us to study the effect of the transfected receptor on somatic Ca(2+) channels. In transfected neurons, mGuR7 selectively inhibited P/Q-type Ca(2+) channels. The effect was mimicked by GTPgammaS and blocked by pertussis toxin (PTX) or a selective antibody raised against the G-protein alphao subunit, indicating the involvement of a G(o)-like protein. The mGuR7 effect did not display the characteristics of a direct interaction between G-protein betagamma subunits and the alpha1A Ca(2+) channel subunit, but was abolished by quenching betagamma subunits with specific intracellular peptides. Intracellular dialysis of G-protein betagamma subunits did not mimic the action of mGluR7, suggesting that both G-protein betagamma and alphao subunits were required to mediate the effect. Inhibition of phospholipase C (PLC) blocked the inhibitory action of mGluR7, suggesting that a coincident activation of PLC by the G-protein betagamma with alphao subunits was required. The Ca(2+) chelator BAPTA, as well as inhibition of either the inositol trisphosphate (IP(3)) receptor or protein kinase C (PKC) abolished the mGluR7 effect. Moreover, activation of native mGluR7 induced a PTX-dependent IP(3) formation. These results indicated that IP(3)-mediated intracellular Ca(2+) release was required for PKC-dependent inhibition of the Ca(2+) channels. Possible control of synaptic transmission by the present mechanisms is discussed.

Long-term regulation of neuronal high-affinity glutamate and glutamine uptake in Aplysia

An increase in transmitter release accompanying long-term sensitization and facilitation occurs at the glutamatergic sensorimotor synapse of Aplysia. We report that a long-term increase in neuronal Glu uptake also accompanies long-term sensitization. Synaptosomes from pleural-pedal ganglia exhibited sodium-dependent, high-affinity Glu transport. Different treatments that induce long-term enhancement of the siphon-withdrawal reflex, or long-term synaptic facilitation increased Glu uptake. Moreover, 5-hydroxytryptamine, a treatment that induces long-term facilitation, also produced a long-term increase in Glu uptake in cultures of sensory neurons. The mechanism for the increase in uptake is an increase in the V(max) of transport. The long-term increase in Glu uptake appeared to be dependent on mRNA and protein synthesis, and transport through the Golgi, because 5,6-dichlorobenzimidazole riboside, emetine, and brefeldin A inhibited the increase in Glu uptake. Also, injection of emetine and 5,6-dichlorobenzimidazole into Aplysia prevented long-term sensitization. Synthesis of Glu itself may be regulated during long-term sensitization because the same treatments that produced an increase in Glu uptake also produced a parallel increase in Gln uptake. These results suggest that coordinated regulation of a number of different processes may be required to establish or maintain long-term synaptic facilitation.