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

Differential effects of group III metabotropic glutamate receptors on spontaneous inhibitory synaptic currents in spine-innervating double bouquet and parvalbumin-expressing dendrite-targeting GABAergic interneurons in human neocortex

Diverse neocortical GABAergic neurons specialize in synaptic targeting and their effects are modulated by presynaptic metabotropic glutamate receptors (mGluRs) suppressing neurotransmitter release in rodents, but their effects in human neocortex are unknown. We tested whether activation of group III mGluRs by L-AP4 changes GABAA receptor-mediated spontaneous inhibitory postsynaptic currents (sIPSCs) in 2 distinct dendritic spine-innervating GABAergic interneurons recorded in vitro in human neocortex. Calbindin-positive double bouquet cells (DBCs) had columnar "horsetail" axons descending through layers II-V innervating dendritic spines (48%) and shafts, but not somata of pyramidal and nonpyramidal neurons. Parvalbumin-expressing dendrite-targeting cell (PV-DTC) axons extended in all directions innervating dendritic spines (22%), shafts (65%), and somata (13%). As measured, 20% of GABAergic neuropil synapses innervate spines, hence DBCs, but not PV-DTCs, preferentially select spine targets. Group III mGluR activation paradoxically increased the frequency of sIPSCs in DBCs (to median 137% of baseline) but suppressed it in PV-DTCs (median 92%), leaving the amplitude unchanged. The facilitation of sIPSCs in DBCs may result from their unique GABAergic input being disinhibited via network effect. We conclude that dendritic spines receive specialized, diverse GABAergic inputs, and group III mGluRs differentially regulate GABAergic synaptic transmission to distinct GABAergic cell types in human cortex.

Expression of group II and III mGluRs in the carotid body and its role in the carotid chemoreceptor response to acute hypoxia

The carotid body (CB) contributes significantly to oxygen sensing. It is unclear, however, whether glutamatergic signaling is involved in the CB response to hypoxia. Previously, we reported that ionotropic glutamate receptors (iGluRs) and multiple glutamate transporters are present in the rat CB. Except for iGluRs, glutamate receptors also include metabotropic glutamate receptors (mGluRs), which are divided into the following groups: Group I (mGluR1/5); group II (mGluR2/3); group III (mGluR4/6/7/8). We have studied the expression of group I mGluRs in the rat CB and its physiological function response to acute hypoxia. To further elucidate the states of mGluRs in the CB, this study’s aim was to investigate the expression of group II and III mGluRs and the response of rat CB to acute hypoxia. We used reverse transcription-polymerase chain reaction (RT-PCR) to observed mRNA expression of GRM2/3/4/6/7/8 subunits by using immunostaining to show the distribution of mGluR2 and mGluR8. The results revealed that the GRM2/3/4/6/7/8 mRNAs were expressed in both rat and human CB. Immunostaining showed that mGluR2 was localized in the type I cells and mGluR8 was localized in type I and type II cells in the rat CB. Moreover, the response of CB to acute hypoxia in rats was recorded by in vitro carotid sinus nerve (CSN) discharge. Perfusion of group II mGluRs agonist or group III mGluRs agonist (LY379268 or L-SOP) was applied to examine the effect of group II and III mGluRs on rat CB response to acute hypoxia. We found that LY379268 and L-SOP inhibited hypoxia-induced enhancement of CSN activity. Based on the above findings, group II and III mGluRs appear to play an inhibitory role in the carotid chemoreceptor response to acute hypoxia.

mGluR1-Dependent Long Term Depression in Rodent Midbrain Dopamine Neurons Is Regulated by Neuregulin 1/ErbB Signaling

Increasing evidence demonstrates that the neurotrophic factor Neuregulin 1 (NRG1) and its receptors, ErbB tyrosine kinases, modulate midbrain dopamine (DA) transmission. We have previously reported that NRG1/ErbB signaling is essential for proper metabotropic glutamate receptors 1 (mGluR1) functioning in midbrain DA neurons, thus the functional interaction between ErbB receptors and mGluR1 regulates neuronal excitation and in vivo striatal DA release. While it is widely recognized that mGluR1 play a pivotal role in long-term modifications of synaptic transmission in several brain areas, specific mGluR1-dependent forms of synaptic plasticity in substantia nigra pars compacta (SNpc) DA neurons have not been described yet. Here, first we aimed to detect and characterize mGluR1-dependent glutamatergic long-term depression (LTD) in SNpc DA neurons. Second, we tested the hypothesis that endogenous ErbB signaling, by affecting mGluR1, fine-tunes glutamatergic synaptic plasticity in DA cells. We found that either pharmacological or synaptic activation of mGluR1 causes an LTD of AMPAR-mediated transmission in SNpc DA neurons from mice and rat slices, which is reliant on endogenous NRG1/ErbB signaling. Indeed, LTD is counteracted by a broad spectrum ErbB inhibitor. Moreover, the intracellular injection of pan-ErbB- or ErbB2 inhibitors inside DA neurons reduces mGluR1-dependent LTD, suggesting an involvement of ErbB2/ErbB4-containing receptors. Interestingly, exogenous NRG1 fosters LTD expression during minimal mGluRI activation. These results enlarge our cognizance on mGluR1 relevance in the induction of a novel form of long-term synaptic plasticity in SNpc DA neurons and describe a new NRG1/ErbB-dependent mechanism shaping glutamatergic transmission in DA cells. This might have important implications either in DA-dependent behaviors and learning/memory processes or in DA-linked diseases.

The role of dopaminergic midbrain in Alzheimer's disease: Translating basic science into clinical practice

Mammalian brain cortical functions, from executive and motor functioning to memory and emotional regulation, are strictly regulated by subcortical projections. These projections terminate in cortical areas that are continuously influenced by released neurotransmitters and neuromodulators. Among the subcortical structures, the dopaminergic midbrain plays a pivotal role in tuning cortical functions that commonly result altered in many neurological and psychiatric disorders. Incidentally, extensive neuropathological observations support a strong link between structural alterations of the dopaminergic midbrain and significant behavioural symptomatology observed in patients suffering from Alzheimer 's disease(AD). Here, we will review recent progress on the involvement of the dopaminergic system in the pathophysiology of AD as well as the current therapeutic strategies targeting this system.

Mechanisms regulating spill-over of synaptic glutamate to extrasynaptic NMDA receptors in mouse substantia nigra dopaminergic neurons

N‐Methyl‐d‐aspartate glutamate receptors (NMDARs) contribute to neural development, plasticity and survival, but they are also linked with neurodegeneration. NMDARs at synapses are activated by coincident glutamate release and depolarization. NMDARs distal to synapses can sometimes be recruited by ‘spill‐over’ of glutamate during high‐frequency synaptic stimulation or when glutamate uptake is compromised, and this influences the shape of NMDAR‐mediated postsynaptic responses. In substantia nigra dopamine neurons, activation of NMDARs beyond the synapse during different frequencies of presynaptic stimulation has not been explored, even though excitatory afferents from the subthalamic nucleus show a range of firing frequencies, and these frequencies change in human and experimental Parkinson's disease. This study reports that high‐frequency stimulation (80 Hz/200 ms) evoked NMDAR‐excitatory postsynaptic currents (EPSCs) that were larger and longer lasting than those evoked by single stimuli at low frequency (0.1 Hz). MK‐801, which irreversibly blocked NMDAR‐EPSCs activated during 0.1‐Hz stimulation, left a proportion of NMDAR‐EPSCs that could be activated by 80‐Hz stimulation and that may represent activity of NMDARs distal to synapses. TBOA, which blocks glutamate transporters, significantly increased NMDAR‐EPSCs in response to 80‐Hz stimulation, particularly when metabotropic glutamate receptors (mGluRs) were also blocked, indicating that recruitment of NMDARs distal to synapses is regulated by glutamate transporters and mGluRs. These regulatory mechanisms may be essential in the substantia nigra for restricting glutamate diffusion from synaptic sites and keeping NMDAR‐EPSCs in dopamine neurons relatively small and fast. Failure of glutamate transporters may contribute to the declining health of dopamine neurons during pathological conditions.

Group III metabotropic glutamate receptors: pharmacology, physiology and therapeutic potential

Glutamate, the primary excitatory neurotransmitter in the central nervous system (CNS), exerts neuromodulatory actions via the activation of metabotropic glutamate (mGlu) receptors. There are eight known mGlu receptor subtypes (mGlu1-8), which are widely expressed throughout the brain, and are divided into three groups (I-III), based on signalling pathways and pharmacological profiles. Group III mGlu receptors (mGlu4/6/7/8) are primarily, although not exclusively, localised on presynaptic terminals, where they act as both auto- and hetero-receptors, inhibiting the release of neurotransmitter. Until recently, our understanding of the role of individual group III mGlu receptor subtypes was hindered by a lack of subtype-selective pharmacological tools. Recent advances in the development of both orthosteric and allosteric group III-targeting compounds, however, have prompted detailed investigations into the possible functional role of these receptors within the CNS, and revealed their involvement in a number of pathological conditions, such as epilepsy, anxiety and Parkinson's disease. The heterogeneous expression of group III mGlu receptor subtypes throughout the brain, as well as their distinct distribution at glutamatergic and GABAergic synapses, makes them ideal targets for therapeutic intervention. This review summarises the advances in subtype-selective pharmacology, and discusses the individual roles of group III mGlu receptors in physiology, and their potential involvement in disease.

Glutamate spillover drives endocannabinoid production and inhibits GABAergic transmission in the Substantia Nigra pars compacta

Endocannabinoids (eCBs) modulate synaptic transmission in the brain, but little is known of their regulatory role in nigral dopaminergic neurons, and whether transmission to these neurons is tonically inhibited by eCBs as seen in some other brain regions. Using whole-cell recording in midbrain slices, we observed potentiation of evoked IPSCs (eIPSCs) in these neurons after blocking CB1 receptors with rimonabant or LY-320,135, indicating the presence of an eCB tone reducing inhibitory synaptic transmission. Increased postsynaptic calcium buffering and block of mGluR1 or postsynaptic G-protein coupled receptors prevented this potentiation. Increasing spillover of endogenous glutamate by inhibiting uptake attenuated eIPSC amplitude, while enhancing the potentiation by rimonabant. Group I mGluR activation transiently inhibited eIPSCs, which could be prevented by GDP-β-S, increased calcium buffering or rimonabant. We explored the possibility that the dopamine-derived eCB N-arachidonoyl dopamine (NADA) is involved. The eCB tone was abolished by preventing dopamine synthesis, and enhanced by l-DOPA. It was not detected in adjacent non-dopaminergic neurons. Preventing 2-AG synthesis did not affect the tone, while inhibition of NADA production abolished it. Quantification of ventral midbrain NADA suggested a basal level that increased following prolonged depolarization or mGluR activation. Since block of the tone was not always accompanied by attenuation of depolarization-induced suppression of inhibition (DSI) and vice versa, our results indicate DSI and the eCB tone are mediated by distinct eCBs. This study provides evidence that dopamine modulates the activity of SNc neurons not only by conventional dopamine receptors, but also by CB1 receptors, potentially via NADA.

Regulation of cocaine-induced reinstatement by group II metabotropic glutamate receptors in the ventral tegmental area

RATIONALE

A high rate of relapse is a daunting challenge facing clinical treatment of cocaine addiction. Recent studies have shown that drugs of abuse enhance glutamate neurotransmission in dopamine neurons in the ventral tegmental area (VTA) and such enhancement may contribute to the risk of relapse.

OBJECTIVES

Given the important role of group II metabotropic glutamate receptors (mGluR2/3s) in regulating glutamate release from the glutamatergic terminals, this study aimed to test whether activation of mGluR2/3s in the VTA can inhibit cocaine-induced reinstatement of cocaine-seeking behavior, a model of relapse to drug-seeking behavior.

METHODS

Rats were trained to self-administer intravenous cocaine (0.25 mg/infusion) under a modified fixed-ratio 5 schedule. After rats reached the training criteria, they went through extinction training to extinguish cocaine-seeking behavior. Then the dose-response effects of a selective mGluR2/3 agonist LY 379268 microinjected into the VTA on cocaine-induced reinstatement of cocaine-seeking behavior were assessed.

RESULTS

LY 379268 (0.032-0.1 μg/side) dose-dependently decreased cocaine-induced reinstatement. The effect could not be fully attributed to diffusion of the drug to the neighboring substantia nigra or to motor impairment. Interestingly, LY 379268 has a less potent effect on cocaine-induced reinstatement than on sucrose-induced reinstatement of sucrose-seeking behavior.

CONCLUSIONS

Our data support the idea that glutamate release in the VTA is critically involved in cocaine-induced reinstatement and indicate that loss of mGluR2/3-mediated regulation of glutamate release in the VTA may critically contribute to the risk of relapse.

Pain-related increase of excitatory transmission and decrease of inhibitory transmission in the central nucleus of the amygdala are mediated by mGluR1

Neuroplasticity in the central nucleus of the amygdala (CeA), particularly its latero-capsular division (CeLC), is an important contributor to the emotional-affective aspects of pain. Previous studies showed synaptic plasticity of excitatory transmission to the CeLC in different pain models, but pain-related changes of inhibitory transmission remain to be determined. The CeLC receives convergent excitatory inputs from the parabrachial nucleus in the brainstem and from the basolateral amygdala (BLA). In addition, feedforward inhibition of CeA neurons is driven by glutamatergic projections from the BLA area to a cluster of GABAergic neurons in the intercalated cell masses (ITC). Using patch-clamp in rat brain slices we measured monosynaptic excitatory postsynaptic currents (EPSCs) and polysynaptic inhibitory currents (IPSCs) that were evoked by electrical stimulation in the BLA. In brain slices from arthritic rats, input-output functions of excitatory synaptic transmission were enhanced whereas inhibitory synaptic transmission was decreased compared to control slices from normal untreated rats. A non-NMDA receptor antagonist (NBQX) blocked the EPSCs and reduced the IPSCs, suggesting that non-NMDA receptors mediate excitatory transmission and also contribute to glutamate-driven feed-forward inhibition of CeLC neurons. IPSCs were blocked by a GABAA receptor antagonist (bicuculline). Bicuculline increased EPSCs under normal conditions but not in slices from arthritic rats, which indicates a loss of GABAergic control of excitatory transmission. A metabotropic glutamate receptor subtype 1 (mGluR1) antagonist (LY367385) reversed both the increase of excitatory transmission and the decrease of inhibitory transmission in the arthritis pain model but had no effect on basal synaptic transmission in control slices from normal rats. The inhibitory effect of LY367385 on excitatory transmission was blocked by bicuculline suggesting the involvement of a GABAergic mechanism. An mGluR5 antagonist (MTEP) inhibited both excitatory and inhibitory transmission in slices from normal and from arthritic rats. The analysis of spontaneous and miniature EPSCs and IPSCs showed that mGluR1 acted presynaptically whereas mGluR5 had postsynaptic effects. In conclusion, mGluR1 rather than mGluR5 can account for the pain-related changes of excitatory and inhibitory synaptic transmission in the CeLC through a mechanism that involves inhibition of inhibitory transmission (disinhibition).

The neuropharmacology of relapse to food seeking: methodology, main findings, and comparison with relapse to drug seeking

Relapse to old, unhealthy eating habits is a major problem in human dietary treatments. The mechanisms underlying this relapse are unknown. Surprisingly, until recently this clinical problem has not been systematically studied in animal models. Here, we review results from recent studies in which a reinstatement model (commonly used to study relapse to abused drugs) was employed to characterize the effect of pharmacological agents on relapse to food seeking induced by either food priming (non-contingent exposure to small amounts of food), cues previously associated with food, or injections of the pharmacological stressor yohimbine. We also address methodological issues related to the use of the reinstatement model to study relapse to food seeking, similarities and differences in mechanisms underlying reinstatement of food seeking versus drug seeking, and the degree to which the reinstatement procedure provides a suitable model for studying relapse in humans. We conclude by discussing implications for medication development and future research. We offer three tentative conclusions: (1)The neuronal mechanisms of food-priming- and cue-induced reinstatement are likely different from those of reinstatement induced by the pharmacological stressor yohimbine. (2)The neuronal mechanisms of reinstatement of food seeking are possibly different from those of ongoing food-reinforced operant responding. (3)The neuronal mechanisms underlying reinstatement of food seeking overlap to some degree with those of reinstatement of drug seeking.

Endocannabinoid release from midbrain dopamine neurons: a potential substrate for cannabinoid receptor antagonist treatment of addiction

Substantial evidence suggests that all commonly abused drugs act upon the brain reward circuitry to ultimately increase extracellular concentrations of the neurotransmitter dopamine in the nucleus accumbens and other forebrain areas. Many drugs of abuse appear to increase dopamine levels by dramatically increase the firing and bursting rates of dopamine neurons located in the ventral mesencephalon. Recent clinical evidence in humans and behavioral evidence in animals indicate that cannabinoid receptor antagonists such as SR141716A (Rimonabant) can reduce the self-administration of, and craving for, several commonly addictive drugs. However, the mechanism of this potentially beneficial effect has not yet been identified. We propose, on the basis of recent studies in our laboratory and others, that these antagonists may act by blocking the effects of endogenously released cannabinoid molecules (endocannabinoids) that are released in an activity- and calcium-dependent manner from mesencephalic dopamine neurons. It is hypothesized that, through the antagonism of cannabinoid CB1 receptors located on inhibitory and excitatory axon terminals targeting the midbrain dopamine neurons, the effects of the endocannabinoids are occluded. The data from these studies therefore suggest that the endocannabinoid system and the CB1 receptors located in the ventral mesencephalon may play an important role in regulating drug reward processes, and that this substrate is recruited whenever dopamine neuron activity is increased.

Presynaptic metabotropic glutamate receptors regulate glutamatergic input to dopamine neurons in the ventral tegmental area

The ventral tegmental area is part of the midbrain dopamine system and is crucially involved in reward, motivation and drug abuse. The activity of dopamine neurons within this region is controlled by synaptic input. In particular, excitatory glutamatergic inputs are important for the switch from regular firing into burst firing. In the present manuscript we determined the role of presynaptic metabotropic glutamate receptors (mGluRs) in the regulation of spontaneous glutamate release of terminals projecting to dopamine cells in the ventral tegmental area of mice. We show that group III mGluRs regulate spontaneous glutamate release and this effect is most likely mediated by mGluR7. The presynaptic dampening of glutamatergic input might open new perspectives in the treatment of drug addiction.

Brief ischemia causes long-term depression in midbrain dopamine neurons

Degeneration of dopamine neurons in the substantia nigra pars compacta (SNc) plays an important role in the pathophysiology of neurodegenerative diseases like Parkinsonism and vascular dementia. SNc dopamine neurons both in vitro and in vivo show sensitivity to hypoxic/ischemic conditions and undergo degeneration. In acute brain slices, these dopamine neurons undergo hyperpolarization during hypoxia and hypoglycemia, which results in silencing of the neurons. However, the role that SNc excitatory synapses play in this process is poorly understood. Here we examined the effect of oxygen/glucose deprivation (OGD) on glutamatergic synaptic transmission in the SNc in a rat midbrain slice preparation. OGD for 5 min caused pre-synaptic ischemic long-term depression (iLTD) of glutamate transmission, as both alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid- and N-methyl-D-aspartate receptor-mediated synaptic currents in SNc dopamine neurons were depressed to a similar extent. This depression began immediately after exposure to OGD and was not recovered upon washout of OGD. Pharmacological studies revealed that the iLTD was triggered by a rise in post-synaptic intracellular calcium and mediated by activation of pre-synaptic adenosine A(1) receptors, which reduced glutamate-dependent synaptic transmission by activating ATP-dependent potassium channels. Furthermore, we observed that iLTD did not occlude tetanic long-term depression (LTD) at the SNc excitatory synapses, suggesting that these two forms of LTD involve different pathways. Taken together, our results showed that brief exposure to hypoxia and hypoglycemia results in LTD of synaptic activity at glutamatergic synapses onto SNc neurons and this phenomenon could represent a protective mechanism by reducing ischemia-induced excitotoxic injury to dopamine neurons.

The role of dopamine for the pathophysiology of schizophrenia

Since decades, experimental approaches and clinical experience have suggested a dopaminergic system's dysregulation playing an important role within the pathophysiology of schizophrenia. This paper summarizes the actual standard of knowledge of the physiological fundamentals and hypothesized dysbalances of the dopamine (DA) system with respect to schizophrenia including interaction with other neurotransmitter systems (glutamate, GABA). The assumed functional role of DA with respect to physiological and illness-associated cognitive performance, especially working memory, reward, and motivation, as it was assessed by fMRI studies, is presented. A third focus concentrates on giving a short survey of SPECT and PET studies measuring the amount of the striatal and extrastriatal DA, the striatal and extrastriatal dopamine D2 receptor, and the dopamine transporter (DAT) comparing first-episode, drug-naïve, treated, and relapsing schizophrenic patients and healthy control persons.

Modulation of excitatory synaptic transmission by endogenous glutamate acting on presynaptic group II mGluRs in rat substantia nigra compacta

Excitatory synaptic inputs from the subthalamic nucleus (STN) have been proposed to underlie burst firing of substantia nigra pars compacta (SNc) dopamine (DA) neurons in Parkinson's disease. Given the potential importance of the STN-SNc synapse in health and disease, our goal was to study how transmission at this synapse is regulated. We tested the hypothesis that neurotransmission at STN-SNc synapses is tonically inhibited by endogenous glutamate acting on presynaptic group II metabotropic glutamate receptors (mGluRs). By using whole-cell recording techniques in brain slices, we examined the effect of LY341495, a mGluR antagonist that is most potent at group II mGluRs, on excitatory postsynaptic currents (EPSCs) that either were evoked in SNc DA neurons by stimulation of the STN or were spontaneously occurred in the presence of tetrodotoxin (miniature EPSCs; mEPSCs). LY341495 increased the evoked EPSC amplitude and mEPSC frequency without changing mEPSC amplitude. In contrast, the group III mGluR antagonist UBP1112 failed to increase the evoked EPSC amplitude. An elevation of extracellular glutamate concentration by a glutamate transporter inhibitor, TBOA, suppressed the evoked EPSCs. LY341495, but not UBP1112, partially reversed the TBOA action. The modulations of EPSCs by TBOA and LY341495 were associated with changes in paired-pulse facilitation ratio. Furthermore, TBOA decreased mEPSC frequency, which was partially reversed by LY341495, without affecting mEPSC amplitude. The results indicate that presynaptic group II mGluRs at STN-SNc synapses appear to be partially activated by a basal level of extracellular glutamate and able to sense the change in extracellular glutamate concentration, subsequently modulating synaptic glutamate release.

Driving cellular plasticity and survival through the signal transduction pathways of metabotropic glutamate receptors

Metabotropic glutamate receptors (mGluRs) share a common molecular morphology with other G protein-linked receptors, but there expression throughout the mammalian nervous system places these receptors as essential mediators not only for the initial development of an organism, but also for the vital determination of a cell's fate during many disorders in the nervous system that include amyotrophic lateral sclerosis, Parkinson's disease, Alzheimer's disease, Huntington's disease, Multiple Sclerosis, epilepsy, trauma, and stroke. Given the ubiquitous distribution of these receptors, the mGluR system impacts upon neuronal, vascular, and glial cell function and is activated by a wide variety of stimuli that includes neurotransmitters, peptides, hormones, growth factors, ions, lipids, and light. Employing signal transduction pathways that can modulate both excitatory and inhibitory responses, the mGluR system drives a spectrum of cellular pathways that involve protein kinases, endonucleases, cellular acidity, energy metabolism, mitochondrial membrane potential, caspases, and specific mitogen-activated protein kinases. Ultimately these pathways can converge to regulate genomic DNA degradation, membrane phosphatidylserine (PS) residue exposure, and inflammatory microglial activation. As we continue to push the envelope for our understanding of this complex and critical family of metabotropic receptors, we should be able to reap enormous benefits for both clinical disease as well as our understanding of basic biology in the nervous system.

Independent presynaptic and postsynaptic mechanisms regulate endocannabinoid signaling at multiple synapses in the ventral tegmental area

Dopamine (DA) neurons in the ventral tegmental area have been implicated in psychiatric disorders and drug abuse. Understanding the mechanisms through which their activity is regulated via the modulation of afferent input is imperative to understanding their roles in these conditions. Here we demonstrate that endocannabinoids liberated from DA neurons activate cannabinoid CB1 receptors located on glutamatergic axons and on GABAergic terminals targeting GABA(B) receptors located on these cells. Endocannabinoid release was initiated by inhibiting either presynaptic type-III metabotropic glutamate receptors or postsynaptic calcium-activated potassium channels, two conditions that also promote enhanced DA neuron excitability and bursting. Thus, activity-dependent release of endocannabinoids may act as a regulatory feedback mechanism to inhibit synaptic inputs in response to DA neuron bursting, thereby regulating firing patterns that may fine-tune DA release from afferent terminals.

Distinct mechanisms of presynaptic inhibition at GABAergic synapses of the rat substantia nigra pars compacta

We investigated the mechanisms of presynaptic inhibition of GABAergic neurotransmission by group III metabotropic glutamate receptors (mGluRs) and GABA(B) receptors, in dopamine (DA) neurons of the substantia nigra pars compacta (SNc). Both the group III mGluRs agonist L-(+)-2-amino-4-phosphonobutyric acid (AP4, 100 microM) and the GABA(B) receptor agonist baclofen (10 microM) reversibly depressed the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) to 48.5 +/- 2.7 and 79.3 +/- 1.6% (means +/- SE) of control, respectively. On the contrary, the frequency of action potential-independent miniature IPSCs (mIPSCs), recorded in tetrodotoxin (TTX, 1 microM) and cadmium (100 microM) were insensitive to AP4 but were reduced by baclofen to 49.7 +/- 8.6% of control. When the contribution of voltage-dependent calcium channels (VDCCs) to synaptic transmission was boosted with external barium (1 mM), AP4 became effective in reducing TTX-resistant mIPSCs to 65.4 +/- 3.9% of control, thus confirming a mechanism of presynaptic inhibition involving modulation of VDCCs. Differently from AP4, baclofen inhibited to 58.5 +/- 6.7% of control the frequency mIPSCs recorded in TTX and the calcium ionophore ionomycin (2 microM), which promotes Ca2+-dependent, but VDCC-independent, transmitter release. Moreover, in the presence of alpha-latrotoxin (0.3 nM), to promote a Ca2+-independent vesicular release of GABA, baclofen reduced mIPSC frequency to 48.1 +/- 3.2% of control, while AP4 was ineffective. These results indicate that group III mGluRs depress GABA release to DA neurons of the SNc through inhibition of presynaptic VDCCs, while presynaptic GABA(B) receptors directly impair transmitter exocytosis.

Glutamate suppresses GABA release via presynaptic metabotropic glutamate receptors at baroreceptor neurones in rats

The nucleus tractus solitarii (NTS) is essential for coordinating arterial baroreflex control of blood pressure. The primary baroreceptor afferent fibres make their first excitatory synaptic contact at second-order NTS neurones with glutamate as the major neurotransmitter. Glutamate regulates its own release by activating presynaptic metabotropic glutamate autoreceptors (mGluRs) on the baroreceptor central terminals to suppress its further release in frequency-dependent manner. Gamma-aminobutyric acid (GABA) interneurones provide the major inhibitory synaptic input. It is the integration of excitatory and inhibitory inputs that shapes the NTS output of baroreceptor signals. We hypothesized that glutamate released from the primary central afferent terminals can spill over to presynaptic mGluRs on GABA interneurones to suppress GABA release at the second-order baroreceptor neurones. We assessed GABA transmission in second-order baroreceptor neurones identified by attached aortic depressor nerve (ADN) boutons. The medial NTS was stimulated to evoke GABA inhibitory postsynaptic currents (eIPSCs). Glutamate spillover, generated by brief 2 s, 25 Hz trains of stimuli applied to the tractus solitarius (TS), induced a small (10%) but significant reduction in the eIPSC amplitudes. The depression was enhanced to a 25% decrease by increasing glutamate in the cleft with a glutamate-uptake inhibitor (M-trans-pyrrolidine-2,4-dicarboxylic acid, 1 mum), blocked by a Group II mGluR antagonist (LY341495, 200 nm) and mimicked by a Group II agonist ((2S,3S,4S)-CCG/(2S,1'S,2'S)-2-carboxycyclopropyl; L-CCG-I). A presynaptic mGluR locus was established by the mGluR agonist-mediated increase in the paired-pulse ratio of two consecutive eIPSCs in conjunction with the decrease in the first eIPSC, and a decrease in the frequency (39-46% reduction at EC(50) concentration), but not amplitude, of spontaneous and miniature GABA IPSCs. The data indicate that endogenous glutamate activation of Group II presynaptic mGluRs can decrease GABA release at the first central synapses, suggesting a heterosynaptic role for the Group II mGluRs in shaping baroreceptor signal transmission.

Activation of group I metabotropic glutamate receptors enhances efficacy of glutamatergic inputs to neonatal rat hypoglossal motoneurons in vitro

Group I metabotropic glutamate receptors (mGluRs) are the main class of metabotropic receptors expressed in the hypoglossus nucleus. Their role in glutamatergic transmission was investigated using patch-clamp recording from motoneurons in a neonatal rat brainstem slice preparation. After pharmacological block of gamma-aminobutyric acid and glycine-mediated inhibition, under voltage-clamp, the selective group I agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) induced a motoneuron inward current by depressing a leak conductance, and strongly facilitated spontaneous glutamatergic synaptic currents. This effect was blocked by 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) and unaffected by 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP), indicating a role for subtype 1 mGluRs. The frequency but not the amplitude of miniature glutamatergic currents was also enhanced by DHPG. Currents elicited by puffer application of (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) in the presence of tetrodotoxin were also unchanged, suggesting that DHPG facilitated release of glutamate. Glutamatergic currents evoked by electrically stimulating the dorsomedullary reticular column premotoneurons were, however, depressed by DHPG in a CPCCOEt-sensitive fashion. Neither CPCCOEt nor MPEP per se changed glutamatergic transmission. Under current-clamp, even if DHPG depressed excitatory postsynaptic potentials, motoneuron spike threshold and time to peak were reduced so that facilitation of synaptic potential/spike coupling became apparent. We propose a wiring diagram to account for the differential action by DHPG on spontaneous and evoked transmission, based on the discrete distribution of subtype 1 mGluRs on glutamatergic afferents. Although under standard recording conditions there was insufficient ambient glutamate to activate mGluRs, such receptors were a powerful target to upregulate excitatory synaptic transmission and enhance signalling by hypoglossal motoneurons to tongue muscles.

Extracellular taurine in the substantia nigra: Taurine-glutamate interaction

Taurine has been proposed as an inhibitory transmitter in the substantia nigra (SN), but the mechanisms involved in its release and uptake remain practically unexplored. We studied the extracellular pool of taurine in the rat's SN by using microdialysis methods, paying particular attention to the taurine-glutamate (GLU) interaction. Extracellular taurine increased after cell depolarization with high-K(+) in a Ca(2+)-dependent manner, being modified by the local perfusion of GLU, GLU receptor agonists, and zinc. Nigral administration of taurine increased the extracellular concentration of gamma-aminobutyric acid (GABA) and GLU, the transmitters of the two main inputs of the SN. The modification of the glial metabolism with fluocitrate and L-methionine sulfoximine also changed the extracellular concentration of taurine. The complex regulation of the extracellular pool of taurine, its interaction with GABA and GLU, and the involvement of glial cells in its regulation suggest a volume transmission role for taurine in the SN.

Group II metabotropic glutamate receptor modulation of excitatory transmission in rat subthalamic nucleus

Patch pipettes were used to record currents in whole-cell configuration to study the effects of group II metabotropic glutamate receptor (mGluR) stimulation on synaptic transmission in slices of rat subthalamic nucleus. Evoked glutamatergic excitatory postsynaptic currents (EPSCs) were reversibly reduced by the selective group II mGluR agonist (2'S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) in a concentration-dependent manner, with an IC50 of 0.19 +/- 0.05 microM. DCG IV (1 microM) had no effect on inhibitory postsynaptic currents mediated by GABA. DCG IV-induced inhibition of EPSCs was reversed by the selective group II mGluR antagonist LY 341495 (100 nM) and mimicked by another selective group II agonist (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I). Inhibition of EPSC amplitude by DCG IV and L-CCG-I was associated with an increase in the paired-pulse ratio of EPSCs. The protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (2 microM) reduced the inhibitory effect of DCG IV on EPSCs. However, the response to DCG IV was not affected by the protein kinase A (PKA) activator forskolin (20 microM), by the adenylyl cyclase inhibitor MDL 12230A (20 microM), or by the phosphodiesterase inhibitor Ro 20-1724 (50 microM). DCG IV-induced inhibition of EPSCs was reduced by the non-selective protein kinase inhibitors H-7 (100 microM), H-8 (50 microM) and HA-1004 (100 microM). These results suggest that group II mGluR stimulation acts presynaptically to inhibit glutamate release by a PKC-dependent mechanism in the subthalamic nucleus.

Group II metabotropic and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainate glutamate receptors regulate the deficit in brain reward function associated with nicotine withdrawal in rats

This study investigated the role of ionotropic and metabotropic glutamate receptors in the deficits in brain reward function, as measured by elevations in intracranial self-stimulation (ICSS) reward thresholds, associated with nicotine withdrawal. The group II metabotropic glutamate (mGluII) receptor agonist LY314582 [a racemic mixture of LY354740 ([+]-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid])] (2.5-7.5 mg/kg) precipitated withdrawal-like elevations in ICSS thresholds, a sensitive measure of reward function, in nicotine-dependent but not control rats. LY314582 did not affect response latencies, a measure of performance in the ICSS paradigm. Bilateral microinfusion of LY314582 (10-100 ng/side) into the ventral tegmental area likewise precipitated dose-dependent threshold elevations in nicotine-dependent rats. Furthermore, a single injection of the mGluII receptor antagonist LY341495 (2S-2-amino-2-[1S,2S-2-carboxycyclopropan-1-yl]-3-[xanth-9-yl]propionic acid) (1 mg/kg) attenuated the threshold elevations observed in rats undergoing spontaneous nicotine withdrawal. mGluII receptors are primarily located on glutamatergic terminals throughout the mesocorticolimbic system, where they act as inhibitory autoreceptors. To investigate whether mGluII receptors contributed to nicotine withdrawal by decreasing glutamatergic transmission, we next examined whether direct blockade of postsynaptic glutamate receptors precipitated withdrawal-like reward deficits in nicotine-dependent rats. The alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainate receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f)quinoxaline (NBQX; 0.01-1 mg/kg) precipitated withdrawal-like threshold elevations in nicotine-dependent but not control rats, whereas 6-methyl-2-[phenylethynyl]-pyridine (MPEP; 0.01-3 mg/kg) and dizocilpine (MK-801; 0.01-0.2 mg/kg), antagonists at metabotropic glutamate 5 and N-methyl-d-aspartate receptors, respectively, did not. Overall, these data demonstrate that mGluII receptors play an important role in the reward deficits associated with nicotine withdrawal. Furthermore, it is likely that mGluII receptors generate this reward deficit, at least in part, by decreasing glutamate transmission at AMPA/kainate receptors.

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.

Group I metabotropic glutamate receptors modulate glutamate and gamma-aminobutyric acid release in the periaqueductal grey of rats

In this study, we investigated the effects of group I metabotropic glutamate (mglu) receptor ligands on glutamate and gamma-aminobutyric acid (GABA) extracellular concentrations at the periaqueductal grey level by using in vivo microdialysis. An agonist of group I mglu receptors, (S)-3,5-dihydroxyphenylglycine [(S)-3,5-DHPG, 1 and 2 mM], as well as a selective agonist of mglu(5) receptors, (RS)-2-chloro-5-hydroxyphenylglycine (CHPG, 2 and 4 mM), both increased dialysate glutamate and GABA concentrations. 7-(Hydroxyimino)cyclopropa-[b]-chromen-1alpha-carboxylate ethyl ester (CPCCOEt, 1 mM), a selective mglu(1) receptor antagonist, and 2-methyl-6-(phenylethynyl)pyridine (MPEP, 0.5 mM), a selective mglu(5) receptor antagonist, perfused in combination with DHPG, antagonized the effect induced by DHPG on the extracellular glutamate and GABA concentrations. MPEP (0.5 mM), perfused in combination with CHPG, antagonized the increased glutamate and GABA extracellular levels induced by CHPG. MPEP (1 mM) decreased the extracellular concentrations of glutamate but did not modify the dialysate GABA concentrations. Moreover, as the intra-periaqueductal grey perfusion of (RS)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid [(RS)-CPP, 100 microM], a selective N-methyl-D-aspartate (NMDA) glutamate receptor antagonist, did not change the extracellular concentrations of glutamate, this suggests that the MPEP-induced decrease in glutamate is not a consequence of NMDA receptor blockade. These data show that group I mglu receptors in the periaqueductal grey may modulate the release of glutamate and GABA in awake, freely moving rats. In particular, mglu(5), but not mglu(1), receptors seem to be functionally active on glutamate terminals.

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.

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.

GABA-mediated synchronization in the human neocortex: elevations in extracellular potassium and presynaptic mechanisms

Field potential and extracellular [K(+)] ([K(+)](o)) recordings were made in the human neocortex in an in vitro slice preparation to study the synchronous activity that occurs in the presence of 4-aminopyridine (50 microM) and ionotropic excitatory amino acid receptor antagonists. Under these experimental conditions, negative or negative-positive field potentials accompanied by rises in [K(+)](o) (up to 4.1 mM from a baseline of 3.25 mM) occurred spontaneously at intervals of 3-27 s. Both field potentials and [K(+)](o) elevations were largest at approximately 1000 microm from the pia. Similar events were induced by neocortical electrical stimuli. Application of medium containing low [Ca(2+)]/high [Mg(2+)] (n=3 slices), antagonism of the GABA(A) receptor (n=7) or mu-opioid receptor activation (n=4) abolished these events. Hence, they represented network, GABA-mediated potentials mainly reflecting the activation of type A receptors following GABA release from interneurons. The GABA(B) receptor agonist baclofen (10-100 microM, n=11) reduced and abolished the GABA-mediated potentials (ID(50)=18 microM). Baclofen effects were antagonized by the GABA(B) receptor antagonist CGP 35348 (0.1-1 mM, n=6; ID(50)=0.19 mM). CGP 38345 application to control medium increased the amplitude of the GABA-mediated potentials and the concomitant [K(+)](o) rises without modifying their rate of occurrence. The GABA-mediated potentials were not influenced by the broad-spectrum metabotropic glutamate agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (100 microM, n=10), but decreased in rate with the group I receptor agonist (S)-3,5-dihydroxyphenylglycine (10-100 microM, n=9). Our data indicate that human neocortical networks challenged with 4-aminopyridine generate glutamatergic-independent, GABA-mediated potentials that are modulated by mu-opioid and GABA(B) receptors presumably located on interneuron terminals. These events are associated with [K(+)](o) elevations that may contribute to interneuron synchronization in the absence of ionotropic excitatory synaptic transmission.

Glutamate receptor stimulation induces a persistent rhythmicity of the GABAergic inputs to rat midbrain dopaminergic neurons

The substantia nigra pars compacta and the ventral tegmental area are part of a complex network in the basal ganglia involved in behaviours as diverse as motor planning, generation of pleasure and drug addiction. Here we report that in the dopaminergic neurons of the rat ventral midbrain a brief coactivation of group I metabotropic and NMDA glutamate receptors may transform a temporally dispersed synaptic GABAergic input into a rhythmic pattern (range 4.5-22.5 Hz), probably through a mechanism involving electrotonic couplings. The plastic and long-lasting modification in the temporal code of the inhibitory synaptic activity induced by glutamate may be a key element in determining the function of midbrain dopaminergic neurons in both normal and pathological behaviour.

Activation of groups I or III metabotropic glutamate receptors inhibits excitatory transmission in the rat subthalamic nucleus

The subthalamic nucleus (STN) is a key nucleus in the basal ganglia motor circuit that provides the major glutamatergic excitatory input to the basal ganglia output nuclei. The STN plays an important role in the normal motor function, as well as in pathological conditions such as Parkinson's disease. Development of a complete understanding of the role of the STN in motor control will require a detailed understanding of the mechanisms involved in the regulation of excitatory and inhibitory synaptic transmission in this nucleus. Here, we report that activation of groups I or III metabotropic glutamate (mGlu) receptors, but not group II, causes a depression of excitatory transmission in the STN. In contrast, mGlu receptor activation has no effect on the inhibitory transmission in this nucleus. Further characterization of the group I mGlu receptor-induced effect on EPSCs suggests that this response is mediated by mGlu1 and not mGlu5. Further, paired pulse studies suggest that both the mGlu1 receptor and the group III mGlu receptor-mediated effects are due to a presynaptic mechanism. If these receptors are involved in endogenous synaptic transmission in the STN, these results raise the exciting possibility that selective agents targeting mGlu receptors may provide a novel approach for the treatment of motor disorders involving the STN.

Ionotropic and metabotropic GABA and glutamate receptors in primate basal ganglia

The functions of glutamate and GABA in the CNS are mediated by ionotropic and metabotropic, G protein-coupled, receptors. Both receptor families are widely expressed in basal ganglia structures in primates and nonprimates. The recent development of highly specific antibodies and/or cDNA probes allowed the better characterization of the cellular localization of various GABA and glutamate receptor subtypes in the primate basal ganglia. Furthermore, the use of high resolution immunogold techniques at the electron microscopic level led to major breakthroughs in our understanding of the subsynaptic and subcellular localization of these receptors in primates. In this review, we will provide a detailed account of the current knowledge of the localization of these receptors in the basal ganglia of humans and monkeys.

Differential subcellular localization of mGluR1a and mGluR5 in the rat and monkey Substantia nigra

Neurons in the rat substantia nigra (SN) are enriched in group I metabotropic glutamate receptor (mGluR) subtypes and respond to group I mGluR activation. To better understand the mechanisms by which mGluR1 and mGluR5 mediate these effects, the goal of this study was to elucidate the subsynaptic localization of these two receptor subtypes in the rat and monkey substantia nigra. At the light microscope level, neurons of the SN pars reticulata (SNr) displayed moderate to strong immunoreactivity for both mGluR1a and mGluR5 in rats and monkeys. However, mGluR1a labeling was much stronger in monkey than in rat SN pars compacta (SNc) neurons, whereas a moderate level of mGluR5 immunoreactivity was found in both species. At the electron microscope level, the immunoreactivity for both group I mGluR subtypes was primarily expressed postsynaptically, although light mGluR1a labeling was occasionally seen in axon terminals in the rat SNr. Immunogold studies revealed a striking difference in the subcellular distribution of mGluR1a and mGluR5 immunoreactivity in SNr and SNc neurons. Although the bulk of mGluR1a was attached to the plasma membrane, >80% of mGluR5 immunoreactivity was intracellular. Plasma membrane-bound immunoreactivity for group I mGluRs in both SNc and SNr neurons was mostly extrasynaptic or in the main body of symmetric, putative GABAergic synapses. On the other hand, asymmetric synapses either were nonimmunoreactive or displayed perisynaptic labeling. These data raise important questions about the trafficking, internalization, and potential functions of group I mGluRs at extrasynaptic sites or symmetric synapses in the substantia nigra.

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.

Muscarinic agonist carbachol depresses excitatory synaptic transmission in the rat basolateral amygdala in vitro

Intracellular recordings in slice preparations of the basolateral amygdala were used to test which excitatory amino acid receptors mediate the excitatory postsynaptic potentials due to stimulation of the external capsule. These recordings were also used to examine the action of muscarinic agonists on the evoked excitatory potentials. Intracellular recordings from amygdaloid pyramidal neurons revealed that carbachol (2-20 microM) suppressed, in a dose-dependent manner, excitatory postsynaptic responses evoked by stimulation of the external capsule (EC). This effect was blocked by atropine. The estimated effective concentration to produce half-maximal response (EC(50)) was 6.2 microM. Synaptic suppression was observed with no changes in the input resistance of the recorded cells, suggesting a presynaptic mechanism. In addition, the results obtained using the paired-pulse protocol provided additional support for a presynaptic action of carbachol. To identify which subtype of cholinergic receptors were involved in the suppression of the EPSP, four partially selective muscarinic receptor antagonists were used at different concentrations: pirenzepine, a compound with a similar high affinity for muscarinic M1 and M4 receptors; gallamine, a noncompetitive antagonist for M2; methoctramine, an antagonist for M2 and M4; and 4-diphenylacetoxy-N-methylpiperidine, a compound with similar high affinity for muscarinic receptors M1 and M3. None of them independently antagonized the suppressive effect of carbachol on the evoked EPSP completely, suggesting that more than one muscarinic receptor subtype is involved in the effect. These experiments provide evidence that in the amygdala muscarinic agonists block the excitatory synaptic response, mediated by glutamic acid, by acting on several types of presynaptic receptors.

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.

2R,4R-4-Aminopyrrolidine-2,4-dicarboxylate (APDC) attenuates cortical EPSPs

We studied the effect of the Type II metabotropic glutamate receptor (mGluR 2,3) agonist APDC on the response of neurons in slices of rat visual cortex. In all cortical layers, APDC attenuated the EPSP produced by stimulation of the predominant excitatory input. This EPSP attenuation was seen in both younger and older rat slices and was present with G-protein blockade in the cell recorded, demonstrating that it was a presynaptic effect. Further, this EPSP attenuation was blocked by the mGluR 2,3 antagonist EGLU. A postsynaptic depressive effect of APDC on the NMDA response was seen in layers 2 and 3, but not in layers 5 and 6. Thus, the predominant action of Type II mGluRs in the visual cortex is a presynaptic reduction of glutamate release which persists through development. This regulation may be important in the setting of excitatory tone in visual cortex and in the extraction/processing of visual information.

The cortico-basal ganglia-thalamocortical circuit with synaptic plasticity. I. Modification rules for excitatory and inhibitory synapses in the striatum

It is pointed out that Ca(2+)-dependent modification rules for NMDA-dependent (NMDA-independent) synaptic plasticity in the striatum are similar to those in the neocortex and hippocampus (cerebellum). A unitary postsynaptic mechanism of synaptic modification is proposed. It is based on the assumption that, in diverse central nervous system structures, long-term potentiation/depression (LTP/LTD) of excitatory transmission (depression/potentiation of inhibitory transmission, LTDi/LTPi) is the result of an increasing/decreasing the number of phosphorylated AMPA and NMDA (GABA(A)) receptors. According to the suggested mechanism, Ca(2+)/calmodulin-dependent protein kinase II and protein kinase C, whose activity is positively correlated with Ca(2+) enlargement, together with cAMP-dependent protein kinase A (cGMP-dependent protein kinase G, whose activity is negatively correlated with Ca(2+) rise) mainly phosphorylate ionotropic striatal receptors, if NMDA channels are opened (closed). Therefore, the positive/negative post-tetanic Ca(2+) shift in relation to a previous Ca(2+) rise must cause NMDA-dependent LTP+LTDi/LTD+LTPi or NMDA-independent LTD+LTPi/LTP+LTDi. Dopamine D(1)/D(2) or adenosine A(2A)/A(1) receptor activation must facilitate LTP+LTDi/LTD+LTPi due to an augmenting/lowering PKA activity. Activation of muscarinic M(1)/M(4) receptors must enhance LTP+LTDi/LTD+LTPi as a consequence of an increase/decrease in the activity of protein kinase C/A. The proposed mechanism is in agreement with known experimental data.

GABA(B) and group I metabotropic glutamate receptors in the striatopallidal complex in primates

Glutamate and GABA neurotransmission is mediated through various types of ionotropic and metabotropic receptors. In this review, we summarise some of our recent findings on the subcellular and subsynaptic localisation of GABA(B) and group I metabotropic glutamate receptors in the striatopallidal complex of monkeys. Polyclonal antibodies that specifically recognise GABA(B)R1, mGluR1a and mGluR5 receptor subtypes were used for immunoperoxidase and pre-embedding immunogold techniques at the light and electron microscope levels. Both subtypes of group I mGluRs were expressed postsynaptically in striatal projection neurons and interneurons where they aggregate perisynaptically at asymmetric glutamatergic synapses and symmetric dopaminergic synaptic junctions. Moreover, they are also strongly expressed in the main body of symmetric synapses established by putative intrastriatal GABAergic terminals. In the globus pallidus, both receptor subtypes are found postsynaptically in the core of striatopallidal GABAergic synapses and perisynaptically at subthalamopallidal glutamatergic synapses. Finally, extrasynaptic labelling was commonly seen in the globus pallidus and the striatum. Moderate to intense GABA(B)R1 immunoreactivity was observed in the striatopallidal complex. At the electron microscope level, GABA(B)R1 immunostaining was commonly found in neuronal cell bodies and dendrites. Many striatal dendritic spines also displayed GABA(B)R1 immunoreactivity. Moreover, GABA(B)R1-immunoreactive axons and axon terminals were frequently encountered. In the striatum, GABA(B)R1-immunoreactive boutons resembled terminals of cortical origin, while in the globus pallidus, subthalamic-like terminals were labelled. Pre-embedding immunogold data showed that postsynaptic GABA(B)R1 receptors are concentrated at extrasynaptic sites on dendrites, spines and somata in the striatopallidal complex, perisynaptically at asymmetric synapses and in the main body of symmetric striatopallidal synapses in the GPe and GPi. Consistent with the immunoperoxidase data, immunoparticles were found in the presynaptic grid of asymmetric synapses established by cortical- and subthalamic-like glutamatergic terminals. These findings indicate that both GABA and glutamate metabotropic receptors are located to subserve various modulatory functions of the synaptic transmission in the primate striatopallidal complex. Furthermore, their pattern of localisation raises issues about their roles and mechanisms of activation in normal and pathological conditions. Because of their 'modulatory' functions, these receptors are ideal targets for chronic drug therapies in neurodegenerative diseases such as Parkinson's disease.

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.

Presynaptic regulation of glutamate release in the ventral tegmental area during morphine withdrawal

The regulation of glutamate (Glu) release from the excitatory input to dopamine cells in the ventral tegmental area (VTA) during acute withdrawal from morphine was studied in slices from animals treated for 6-7 d with morphine. EPSCs were inhibited by opioid agonists acting at micro-subtype receptors but not by selective delta- or kappa-subtype agonists. The opioid inhibition was reduced by 65% with the potassium channel blocker 4-aminopyridine (4-AP; 100 microM) and a 12-lipoxygenase inhibitor, baicalein (5 microM), suggesting that opioids acted via a transduction pathway involving activation of a voltage-dependent potassium conductance by lipoxygenase metabolites as has been shown in the periaqueductal gray (). During withdrawal, neither the potency nor the efficacy of D-Ala-Met-enkephalin-Gly-ol (DAMGO) were changed; however, the blockade of micro-opioid inhibition by both 4-AP and baicalein was reduced. In addition, the potency of baclofen to depress EPSCs by GABA-B receptors and the effects of the GABA-uptake inhibitor NO-711 (10 microM) were increased in withdrawn rats. Finally, group 2 (but not group 4 or 1) metabotropic glutamate receptor-mediated presynaptic inhibition was also enhanced in morphine-withdrawn rats. These results suggest that one of the consequences of withdrawal from chronic morphine is an enhanced presynaptic inhibition of the excitatory inputs to the dopamine cells of the VTA. Inhibition of glutamate release during acute withdrawal would add to the inhibition of dopamine cells that is mediated by an augmented release of GABA ().

Group I metabotropic glutamate receptors at GABAergic synapses in monkeys

Recent data showed that group I metabotropic glutamate receptors (mGluRs) are located perisynaptic to the postsynaptic specializations of asymmetric glutamatergic synapses in the cerebellum and hippocampus in rats. In the present study, we used immunogold labeling to elucidate the subsynaptic localization of group I mGluRs (mGluR1a and mGluR5) in the internal and external segments of the globus pallidus in monkeys. In contrast to hippocampal and cerebellar neurons, which receive massive glutamatergic inputs, dendrites of pallidal neurons are covered with GABAergic boutons from the striatum intermingled with a small proportion of glutamatergic terminals arising largely from the subthalamic nucleus. In line with previous data, mGluR1a and mGluR5 immunoreactivity was found at the edge of the postsynaptic specializations of asymmetric synapses established by subthalamic-like boutons in the monkey pallidum. However, a large proportion of gold particles were also seen in the main body of the postsynaptic specializations of symmetric synapses formed by striatal GABAergic terminals. These data raise questions about the possible sources of activation of these receptors and the potential roles of group I mGluRs in modulating GABAergic neurotransmission at striatopallidal synapses.

Glutamate inhibits GABA excitatory activity in developing neurons

In contrast to the mature brain, in which GABA is the major inhibitory neurotransmitter, in the developing brain GABA can be excitatory, leading to depolarization, increased cytoplasmic calcium, and action potentials. We find in developing hypothalamic neurons that glutamate can inhibit the excitatory actions of GABA, as revealed with fura-2 digital imaging and whole-cell recording in cultures and brain slices. Several mechanisms for the inhibitory role of glutamate were identified. Glutamate reduced the amplitude of the cytoplasmic calcium rise evoked by GABA, in part by activation of group II metabotropic glutamate receptors (mGluRs). Presynaptically, activation of the group III mGluRs caused a striking inhibition of GABA release in early stages of synapse formation. Similar inhibitory actions of the group III mGluR agonist L-AP4 on depolarizing GABA activity were found in developing hypothalamic, cortical, and spinal cord neurons in vitro, suggesting this may be a widespread mechanism of inhibition in neurons throughout the developing brain. Antagonists of group III mGluRs increased GABA activity, suggesting an ongoing spontaneous glutamate-mediated inhibition of excitatory GABA actions in developing neurons. Northern blots revealed that many mGluRs were expressed early in brain development, including times of synaptogenesis. Together these data suggest that in developing neurons glutamate can inhibit the excitatory actions of GABA at both presynaptic and postsynaptic sites, and this may be one set of mechanisms whereby the actions of two excitatory transmitters, GABA and glutamate, do not lead to runaway excitation in the developing brain. In addition to its independent excitatory role that has been the subject of much attention, our data suggest that glutamate may also play an inhibitory role in modulating the calcium-elevating actions of GABA that may affect neuronal migration, synapse formation, neurite outgrowth, and growth cone guidance during early brain development.

Metabotropic glutamate receptors prevent programmed cell death through the modulation of neuronal endonuclease activity and intracellular pH

Metabotropic glutamate receptor (mGluR) activation prevents neurodegeneration against nitric oxide (NO)-induced programmed cell death (PCD). We therefore investigated whether specific neuronal endogenous deoxyribonucleases, enzymes recently identified to be responsible for the maintenance of DNA integrity, mediated mGluR protection against NO. In rat primary hippocampal neurons, injury was assessed by using a 0.4% trypan blue dye exclusion method and TUNEL assay 24 h following treatment with the NO generators sodium nitroprusside (300 microM) or SIN-1 (300 microM). DNA digestion studies using neuronal cell extracts were employed to assess specific endonuclease activity. Individual application of aurintricarboxylic acid (ATA) (10 microM), an endonuclease inhibitor, or the mGluR agonists 1S,3R-ACPD (750 microM), DHPG (750 microM), L-CCG-I (750 microM), or L-AP4 (750 microM) prior to NO exposure significantly increased neuronal survival. Yet, combination therapy with ATA (10 microM) and the mGluR agonists did not synergistically improve neuronal survival, suggesting a common pathway of protection for ATA and the mGluRs that is dependent upon the modulation of neuronal endonuclease activity. In further support of this premise, protection by the mGluR agonists 1S,3R-ACPD, DHPG, L-CCG-I, and L-AP4 was significantly decreased during enhancement of endonuclease activity with the zinc chelator, N,N,N',N',-tetrakis (2-pyridylmethyl) ethylenediamine. Antagonism of the mGluR system was ineffective against endonuclease induced DNA destruction. Further assessment with DNA digestion assays identified two distinct mechanisms to maintain DNA integrity, a Ca2+/Mg2+-dependent endonuclease inhibited by L-AP4 and a magnesium dependent endonuclease inhibited by 1S,3R-ACPD. These neuroprotective mechanisms during activation of the mGluR system were also intricately linked to the active reversal of the biphasic intracellular pH changes induced by NO. Further investigation into the molecular pathways modulated by mGluRs may identify specific mechanisms that can maintain DNA integrity during adverse cellular environments.

Metabotropic glutamate receptors: electrophysiological properties and role in plasticity

Electrophysiological research on mGluRs is now very extensive, and it is clear that activation of mGluRs results in a large number of diverse cellular actions. Studies of mGluRs and on ionic channels has clearly demonstrated that mGluR activation has a widespread and potent inhibitory action on both voltage-gated Ca2+ channels and K+ channels. Inhibition of N-type Ca2+ channels, and inhibition of Ca(++)-dependent K+ current, IAHP, and IM being particularly prominent. Potentiation of activation of both Ca2+ and K+ channels has also been observed, although less prominently than inhibition, but mGluR-mediated activation of non-selective cationic channels is widespread. In a small number of studies, generation of an mGluR-mediated slow excitatory postsynaptic potential has been demonstrated as a consequence of the effect of mGluR activation on ion channels, such as activation of a non-selective cationic channels. Although certain mGluR-modulation of channels is a consequence of direct G-protein-linked action, for example, inhibition of Ca2+ channels, many other effects occur as a result of activation of intracellular messenger pathways, but at present, little progress has been made on the identification of the messengers. The field of study of the involvement of mGluRs in synaptic plasticity is very large. Evidence for the involvement of mGluRs in one form of LTD induction in the cerebellum and hippocampus is now particularly impressive. However, the role of mGluRs in LTP induction continues to be a source of dispute, and resolution of the question of the exact involvement of mGluRs in the induction of LTP will have to await the production of more selective ligands and of selective gene knockouts.