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.

Presynaptic group II metabotropic glutamate receptors reduce stimulated and spontaneous transmitter release in human dentate gyrus

Metabotropic glutamate receptors (mGluRs) control excitatory neurotransmission as inhibitory autoreceptors at many synapses throughout the CNS. Since pharmacological activation of mGluRs potently depresses excitatory transmission, anticonvulsive effects were found in a number of experimental epilepsies. However, although native rodent mGluRs and heterologously expressed human mGluRs have so far been investigated in great detail, our knowledge about native human mGluRs in situ is limited. Here we used acute human hippocampal slices prepared from hippocampi surgically removed for the treatment of temporal lobe epilepsy in order to investigate the modulation of glutamatergic transmission by human mGluRs at the perforant path-granule cell synapse. The broad spectrum mGluR agonist (1S, 3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) profoundly and reversibly reduced field EPSPs (fEPSPs) with an EC(50) of 30+/-7.4 microM. Paired-pulse depression of fEPSPs was converted into strong facilitation. The inhibition of fEPSPs by ACPD was mimicked by the specific group II mGluR agonist (2S, 2'R, 3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV), while the specific group I agonist (S)-3,5-dihydroxyphenylglycine (DHPG) was ineffective. The effect of ACPD was blocked by group II antagonist (2S,3S,4S)-2methyl-2-(carboxycyclopropyl)glycine (MCCG) but was not changed by coapplication of the specific group III antagonist (S)2 amino2methyl4phosphonobutanoic acid (MAP4). ACPD reduced pharmacologically isolated intracellular EPSPs in granule cells to the same extent as fEPSPs, whereas a specific group III agonist had no effect on EPSPs. Whole-cell recordings from morphologically identified granule cells revealed that DCG-IV significantly reduced the frequency of miniature EPSCs (mEPSCs) in granule cells while the mean amplitude of mEPSCs was not affected. We conclude that in human dentate gyrus mGluR2/3 can almost completely depress glutamate release by a presynaptic mechanism which acts downstream of presynaptic voltage gated calcium-entry and most likely involves a direct modulation of the release machinery.

Postsynaptic induction and presynaptic expression of group 1 mGluR-dependent LTD in the hippocampal CA1 region

Activation of metabotropic glutamate receptors (mGluRs) with the group I mGluR selective agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) induces a long-term depression (LTD) of excitatory synaptic transmission in the CA1 region of the hippocampus. Here we investigated the potential roles of pre- and postsynaptic processes in the DHPG-induced LTD at excitatory synapses onto hippocampal pyramidal cells in the mouse hippocampus. Activation of mGluRs with DHPG, but not ACPD, induced LTD at both Schaffer collateral/commissural fiber synapses onto CA1 pyramidal cells and at associational/commissural fiber synapses onto CA3 pyramidal cells. DHPG-induced LTD was blocked when the G-protein inhibitor guanosine-5'-O-(2-thiodiphosphate) was selectively delivered into postsynaptic CA1 pyramidal cells via an intracellular recording electrode, suggesting that DHPG depresses synaptic transmission through a postsynaptic, GTP-dependent signaling pathway. The effects of DHPG were also strongly modulated, however, by experimental manipulations that altered presynaptic calcium influx. In these experiments, we found that elevating extracellular Ca(2+) concentrations ([Ca(2+)](o)) to 6 mM almost completely blocked the effects of DHPG, whereas lowering [Ca(2+)](o) to 1 mM significantly enhanced the ability of DHPG to depress synaptic transmission. Enhancing Ca(2+) influx by prolonging action potential duration with bath applications of the K(+) channel blocker 4-aminopyridine (4-AP) also strongly reduced the effects of DHPG in the presence of normal [Ca(2+)](o) (2 mM). Although these findings indicate that alterations in Ca(2+)-dependent signaling processes strongly regulate the effects of DHPG on synaptic transmission, they do not distinguish between potential pre- versus postsynaptic sites of action. We found, however, that while inhibiting both pre- and postsynaptic K(+) channels with bath-applied 4-AP blocked the effects of DHPG; inhibition of postsynaptic K(+) channels alone with intracellular Cs(+) and TEA had no effect on the ability of DHPG to inhibit synaptic transmission. This suggests that presynaptic changes in transmitter release contribute to the depression of synaptic transmission by DHPG. Consistent with this, DHPG induced a persistent depression of both AMPA and N-methyl-D-aspartate receptor-mediated components of excitatory postsynaptic currents in voltage-clamped pyramidal cells. Together our results suggest that activation of postsynaptic mGluRs suppresses transmission at excitatory synapses onto CA1 pyramidal cells through presynaptic effects on transmitter release.

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

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

Slow excitatory synaptic potentials evoked by distension in myenteric descending interneurones of guinea-pig ileum

The functional significance of the slow excitatory synaptic potentials (EPSPs) in myenteric neurones is unknown. We investigated this using intracellular recording from myenteric neurones in guinea-pig ileum, in vitro. In all, 121 neurones responded with fast EPSPs to distension of the intestine oral to the recording site. In 28 of these neurones, distension also evoked depolarizations similar to the slow EPSPs evoked by electrical stimulation in the same neurones. Intracellular injection of biocytin and immunohistochemistry revealed that neurones responding to distension with slow EPSPs were descending interneurones, which were immunoreactive for nitric oxide synthase (NOS). Other neurones, including inhibitory motor neurones and interneurones lacking NOS, did not respond to distension with slow EPSPs, but many had slow EPSPs evoked electrically. Slow EPSPs evoked electrically or by distension in NOS-immunoreactive descending interneurones were resistant to blockade of NK(1) or NK(3) tachykinin receptors (SR 140333, 100 nM; SR 142801, 100 nM, respectively) and group I metabotropic glutamate receptors (PHCCC, 10-30 microM), when the antagonists were applied in the recording chamber of a two-chambered organ bath. However, slow EPSPs evoked electrically in inhibitory motor neurones were substantially depressed by SR 140333 (100 nM). Blockade of synaptic transmission in the stimulation chamber of the organ bath abolished slow EPSPs evoked by distension, indicating that they arose from activity in interneurones, and not from anally directed, intrinsic sensory neurones. Thus, distension evokes slow EPSPs in a subset of myenteric neurones, which may be important for intestinal motility.

Chronic ethanol treatment and withdrawal alter ACPD-evoked calcium signals in developing Purkinje neurons

BACKGROUND

Alcohol exposure during human fetal development can result in fetal alcohol syndrome, a condition characterized by central nervous system dysfunction. Detailed studies in animal models of fetal alcohol syndrome show that the cerebellar region is particularly sensitive to alcohol exposure during early development; however, the cellular mechanisms underlying the alcohol sensitivity of the immature cerebellum are poorly understood.

METHODS

Primary neuronal cultures of cerebellar cells were prepared from embryonic day 20 rat pups. Cultures were exposed to ethanol (33 mM; 150 mg/100 ml) during the main period of morphological development of the Purkinje neurons, from 6 to 17 days in vitro. After the ethanol treatment, the response of Purkinje neurons to the selective metabotropic glutamate receptor (mGluR) agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD; 300 microM) was examined in parallel fura-2 Ca2+ imaging and current-clamp experiments. In an additional set of experiments, ethanol-treated cultures were allowed to withdraw from ethanol overnight before recordings were performed.

RESULTS

In Ca2+ imaging studies, the mean peak amplitude of ACPD-evoked Ca2+ signals was depressed in the dendritic region of chronic ethanol-treated Purkinje neurons compared with control neurons (p < 0.05, unpaired t test), whereas there was no apparent difference in the somatic region. In contrast, peak ACPD-evoked Ca2+ signals were enhanced in both the somatic and dendritic regions of withdrawn Purkinje neurons compared with control neurons. Parallel current-clamp studies showed no consistent effect of chronic ethanol treatment or ethanol withdrawal on the membrane response to ACPD.

CONCLUSIONS

These results show that prolonged ethanol exposure and early withdrawal lead to alterations in mGluR-evoked Ca2+ signaling in cerebellar Purkinje neurons. Metabotropic GluRs in the Purkinje neuron play important roles in cerebellar development and function, suggesting that alterations of mGluR signaling pathways by ethanol may play a key role in the actions of ethanol on the developing cerebellum.

Altered extracellular signal-regulated kinase signal transduction by the muscarinic acetylcholine and metabotropic glutamate receptors after cerebral ischemia

To determine whether muscarinic acetylcholine receptors (mAChR) in the post-ischemic hippocampus may be involved in altered extracellular signal-regulated kinases (ERK) signal transduction, we have investigated changes in the activity of ERK1/2 induced by a muscarinic agonist, carbachol. Cerebral ischemia was produced in the rat by injecting 900 microspheres (48 microm in diameter) into the right internal carotid artery. Applying carbachol to the contralateral hippocampal slices from ischemic rats increased the phosphorylation of ERK1/2 but did not increase phosphorylation in the ipsilateral hippocampus. Analysis of M(1) mAChR binding showed that there was no significant difference in the number and K(d) values between the hippocampi from naïve and ischemic rats. Immunoblotting analysis showed no significant difference in the amount of M(1) mAChR in both hemispheres. In contrast to carbachol stimulation, the protein kinase C activator induced an activation of ERK1/2 in the ipsilateral hippocampus. This increase was shown to occur in neurons by immunofluorescence colocalization study. Carbachol-stimulated tyrosine phosphorylation of the G alpha(q/11), inositol 1,4,5-trisphosphate formation, and association of G alpha(q/11) with phospholipase C beta 1 were attenuated in the ipsilateral hippocampus. We also found that stimulation of group I metabotropic glutamate receptors, which are linked to G alpha(q/11), failed to increase in phosphorylation of ERK1/2 in the ipsilateral hippocampus. These results suggest that failure in receptor-mediated tyrosine phosphorylation of the G alpha(q/11) subunit and a defect in receptor-G alpha(q/11-)effector coupling in the ischemic hippocampus may be involved in alterations of ERK signal transduction.

Group II metabotropic glutamate receptor activation is required for normal hippocampal mossy fibre development in the rat

Glutamate is the main neurotransmitter at hippocampal mossy fibre (MF) terminals. Because neurotransmitters have been proposed as regulating factors of neural network formation and neurite morphogenesis in the developing CNS, we examined the possible contribution of glutamate to MF pathfinding. Entorhino-hippocampal slices prepared from early postnatal rats were cultivated in the presence of glutamate receptor antagonists. Timm histochemical staining revealed that pharmacological blockade of metabotropic glutamate receptors (mGluR), but not of ionotropic glutamate receptors, induced abnormal outgrowth of the MFs. When slices were cultured in the presence of mGluR antagonists, DiI-labelled MF axons displayed a great degree of defasciculation, and MF-mediated EPSPs in the CA3 pyramidal cells were altered. Similar results were obtained for a selective antagonist of group II mGluR, but not of group I or III mGluR. Glutamate is, therefore, likely to regulate MF outgrowth via activation of group II mGluR. The present study may provide a novel role of glutamate in hippocampal development.

Dendritic spines elongate after stimulation of group 1 metabotropic glutamate receptors in cultured hippocampal neurons

Changes in the morphology of dendritic spines are correlated with synaptic plasticity and may relate mechanistically to its expression and stabilization. Recent work has shown that spine length can be altered by manipulations that affect intracellular calcium, and spine length is abnormal in genetic conditions affecting protein synthesis in neurons. We have investigated how ligands of group 1 metabotropic glutamate receptors (mGluRs) affect spine shape; stimulation of these receptors leads both to calcium release from intracellular stores and to dendritic protein synthesis. Thirty-minute incubation of cultured hippocampal slices and dissociated neurons with the selective group 1 mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) induced a significant increase in the average length of dendritic spines. This elongation resulted mainly from the growth of existing spines and was also seen even in the presence of antagonists of ionotropic receptors, indicating that activation of these receptors by mGluR-induced glutamate release was not required. Prolonged antagonism of group 1 mGluRs with (S)-alpha-methyl-4-carboxyphenylglycine (MCPG) did not result in shorter average spine length. Elongation of dendritic spines induced by DHPG was blocked by calcium chelation and by preincubation with the protein synthesis inhibitor puromycin. The results suggest that in vivo activation of group 1 mGluRs by synaptically released glutamate affects spine shape in a protein synthesis-dependent manner.

Metabotropic glutamate and GABA(B) receptors contribute to the modulation of glucose-stimulated insulin secretion in pancreatic beta cells

AIMS/HYPOTHESIS

The neurotransmitters glutamate and gamma-aminobutyric acid (GABA) could participate in the regulation of the endocrine functions of islets of Langerhans. We investigated the role of the metabotropic glutamate (mGluRs) and GABA(B) (GABA(B)Rs) receptors in this process.

METHODS

We studied the expression of mGluRs and GABA(B)Rs in rat and human islets of Langerhans and in pancreatic alpha-cell and beta-cell lines using RT-PCR and immunoblot analysis. Effects of mGluR and GABA(B) R agonists on insulin secretion were determined by radioimmunoassays and enzyme-linked immunoadsorbent assays (ELISAs).

RESULTS

We detected mGluR3 and mGluR5 (but not mGluR1, 6 and 7) mRNAs in all of the samples examined. Trace amount of mGluR2 was found in MIN6 beta cells; mGluR4 was identified in rat islets; and mGluR8 expression was detected in rat islets, RINm5F and MIN6 cells. GABA(B)R1 a/b and 2 mRNAs were identified in islets of Langerhans and MIN6 cells. The expression of mGluR3, mGluR5, GABA(B)R1 a/b and GABA(B)R2 proteins was confirmed using specific antibodies. Group I (mGluR1/5) and group II (mGluR2/3) specific mGluR agonists increased the release of insulin in the presence of 3 to 10 mmol/l or 3 to 25 mmol/l glucose, respectively, whereas a group III (mGluR4/6-8) specific agonist inhibited insulin release at high (10-25 mmol/l) glucose concentrations. Baclofen, a GABA(B)R agonist, also inhibited the release of insulin but only in the presence of 25 mmol/l glucose.

CONCLUSION/INTERPRETATION

These data suggest that mGluRs and GABA(B)Rs play a role in the regulation of the endocrine pancreas with mechanisms probably involving direct activation or inhibition of voltage dependent Ca(2+)-channels, cAMP generation and G-protein-mediated modulation of K(ATP) channels.

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.

Allosteric modulators of group I metabotropic glutamate receptors: novel subtype-selective ligands and therapeutic perspectives

Recent drug discovery programs aimed at identifying selective metabotropic mGlu receptor ligands by high-throughput functional screening efforts have revealed subtype-selective allosteric modulators of mGlu1 and mGlu5 receptors that are structurally unrelated to glutamate. In contrast to competitive ligands, which bind to the glutamate binding site located in the large N-terminal extracellular domain, these modulators act as non-competitive antagonists, inverse agonists or positive modulators by binding to specific residues in the seven-transmembrane domain. More recent studies to assess the potential of these compounds in in vivo models of nervous system disorders have implicated the mGlu5 receptor subtype as a potentially important therapeutic target for inflammatory pain, anxiety, Parkinson's disease and drug abuse, and mGlu1 and mGlu5 receptors as potential targets for anticonvulsant and neuroprotective therapies. Very recent findings indicate an important regulatory role for intracellular proteins interacting with metabotropic glutamate receptors, which might constitute novel drug targets for modulating metabotropic glutamate receptor activity.

Acute administration of antipsychotics modulates Homer striatal gene expression differentially

Typical and atypical antipsychotics, the mainstay of schizophrenia pharmacotherapy, have been demonstrated to affect differently neuronal gene expression in several preclinical paradigms. Here we report the differential gene expression of the glutamatergic post-synaptic density proteins Homer and PSD-95 in rat forebrain following acute haloperidol or olanzapine treatment. Moreover, considering the extensive interactions between dopaminergic and opioidergic systems we also measured striatal preproenkephalin mRNA. Male Sprague-Dawley rats were treated with haloperidol 1 mg/kg or olanzapine 0.5 mg/kg or vehicle, i.p. and sacrificed 3 h after the injection. Homer gene expression was significantly increased in caudate putamen and nucleus accumbens of rats treated with haloperidol and in the core of accumbens of rats treated with olanzapine. No changes were detected for Homer in prefrontal and parietal cortex in any of the experimental groups. PSD-95 gene expression was not modulated in our paradigm by administration of either typical or atypical antipsychotics. These results (1) suggest a differential modulation of Homer by typical and atypical antipsychotics; (2) confirm that Homer can be induced as an early gene with putative direct effect on neuronal plasticity and (3) demonstrate different response to antipsychotics by different classes of postsynaptic density proteins at glutamatergic synapses.

Experience-dependent plasticity without long-term depression by type 2 metabotropic glutamate receptors in developing visual cortex

Synaptic depression is thought to underlie the loss of cortical responsiveness to an eye deprived of vision. Here, we establish a fundamental role for type 2 metabotropic glutamate receptors (mGluR2) in long-term depression (LTD) of synaptic transmission within primary visual cortex. Direct mGluR2 activation by (2S,2'R,3'R-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) persistently depressed layer 2/3 field potentials in slices of mouse binocular zone when stimulated concomitantly. Chemical LTD was independent of N-methyl-d-aspartate (NMDA) receptors but occluded conventional LTD by low-frequency stimulation, indicating shared downstream events. Antagonists or targeted disruption of mGluR2 conversely prevented LTD induction by electrical low-frequency stimulation to layer 4. In contrast, Schaeffer collateral synapses did not exhibit chemical LTD, revealing hippocampal area CA1, naturally devoid of mGluR2, to be an inappropriate model for neocortical plasticity. Moreover, monocular deprivation remained effective in mice lacking mGluR2, and receptor expression levels were unchanged during the critical period in wild-type mice, indicating that experience-dependent plasticity is independent of LTD induction in visual cortex. Short-term depression that was unaffected by mGluR2 deletion may better reflect circuit refinement in vivo.

Activation of metabotropic glutamate receptor 1 inhibits glutamatergic transmission in the substantia nigra pars reticulata

The substantia nigra pars reticulata is a primary output nucleus of the basal ganglia motor circuit and is controlled by a fine balance between excitatory and inhibitory inputs. The major excitatory input to GABAergic neurons in the substantia nigra arises from glutamatergic neurons in the subthalamic nucleus, whereas inhibitory inputs arise mainly from the striatum and the globus pallidus. Anatomical studies revealed that metabotropic glutamate receptors (mGluRs) are highly expressed throughout the basal ganglia. Interestingly, mRNA for group I mGluRs are abundant in neurons of the subthalamic nucleus and the substantia nigra pars reticulata. Thus, it is possible that group I mGluRs play a role in the modulation of glutamatergic synaptic transmission at excitatory subthalamonigral synapses. To test this hypothesis, we investigated the effects of group I mGluR activation on excitatory synaptic transmission in putative GABAergic neurons in the substantia nigra pars reticulata using the whole cell patch clamp recording approach in slices of rat midbrain. We report that activation of group I mGluRs by the selective agonist (R,S)-3,5-dihydroxyphenylglycine (100 microM) decreases synaptic transmission at excitatory synapses in the substantia nigra pars reticulata. This effect is selectively mediated by presynaptic activation of the group I mGluR subtype, mGluR1. Consistent with these data, electron microscopic immunocytochemical studies demonstrate the localization of mGluR1a at presynaptic sites in the rat substantia nigra pars reticulata. From this finding that group I mGluRs modulate the major excitatory inputs to GABAergic neurons in the substantia nigra pars reticulata we suggest that these receptors may play an important role in basal ganglia functions. Studying this effect, therefore, provides new insights into the modulatory role of glutamate in basal ganglia output nuclei in physiological and pathophysiological conditions.

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.

Group I metabotropic glutamate receptors in spinal cord injury: roles in neuroprotection and the development of chronic central pain

Spinal cord injury (SCI) initiates a cascade of biochemical events that leads to an increase in extracellular excitatory amino acid (EAA) concentrations, which results in glutamate receptor-mediated excitotoxic events. An important division of these glutamate receptors is the metabotropic glutamate receptor (mGluR) class, which is divided into three groups. Of these three groups, group I (mGluR1 and mGluR5) activation can initiate a number of intracellular pathways that lead to increased extracellular EAA concentrations. To evaluate subtypes of group I mGluRs in SCI, we administered AIDA (group I antagonist), LY 367385 (mGluR1 specific antagonist), or MPEP (mGluR5 specific antagonist) by interspinal injection to adult male Sprague-Dawley rats (175-200 g) immediately following injury at T10 with an NYU impactor (12.5-mm drop, 10-g rod, 2 mm in diameter). AIDA- and LY 367385-treated subjects had improved locomotor scores and demonstrated an attenuation in the development of mechanical allodynia as measured by von Frey stimulation of the forelimbs; however, LY 367385 potentiated the development of thermal hyperalgesia. MPEP had no effect on locomotor recovery or mechanical allodynia, but attenuated the development of thermal hyperalgesia. AIDA and LY 367385 treatment resulted in a significant increase in tissue sparing compared to the vehicle-treated group at 4 weeks following SCI. These results suggest that mGluRs play an important role in EAA toxicity and have different acute pathophysiological roles following spinal cord injury.

A metabotropic glutamate receptor regulates transmitter release from cone presynaptic terminals in carp retinal slices

The role of group III metabotropic glutamate receptors (mGluRs) in photoreceptor-H1 horizontal cell (HC) synaptic transmission was investigated by analyzing the rate of occurrence and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) in H1 HCs uncoupled by dopamine in carp retinal slices. Red light steps or the application of 100 microM cobalt reduced the sEPSC rate without affecting their peak amplitude, which is consistent with hyperpolarization or the suppression of Ca(2+) entry into cone synaptic terminals reducing vesicular transmitter release. Conversely, postsynaptic blockade of H1 HC AMPA receptors by 500 nM CNQX reduced the amplitude of sEPSCs without affecting their rate. This analysis of sEPSCs represents a novel methodology for distinguishing between presynaptic and postsynaptic sites of action. The selective agonist for group III mGluRs, l-2-amino-4-phosphonobutyrate (L-APB or L-AP4; 20 microM), reduced the sEPSC rate with a slight reduction in amplitude, which is consistent with a presynaptic action on cone synaptic terminals to reduce transmitter release. During L-APB application, recovery of sEPSC rate occurred with 500 microM (s)-2-methyl-2-amino-4-phosphonobutyrate (MAP4), a selective antagonist of group III mGluR, and with 200 microM 4-aminopyridine (4-AP), a blocker of voltage-dependent potassium channels. Whole-cell recordings from cones in the retinal slice showed no effect of L-APB on voltage-activated Ca(2+) conductance. These results suggest that the activation of group III mGluRs suppresses transmitter release from cone presynaptic terminals via a 4-AP-sensitive pathway. Negative feedback, operating via mGluR autoreceptors, may limit excessive glutamate release from cone synaptic terminals.

Control of Ca(2+) influx by cannabinoid and metabotropic glutamate receptors in rat cerebellar cortex requires K(+) channels

1. In the rodent cerebellum, both presynaptic CB1 cannabinoid receptors and presynaptic mGluR4 metabotropic glutamate receptors acutely depress excitatory synaptic transmission at parallel fibre-Purkinje cell synapses. Using rat cerebellar slices, we have analysed the effects of selective CB1 and mGluR4 agonists on the presynaptic Ca(2+) influx which controls glutamate release at this synapse. 2. Changes in presynaptic Ca(2+) influx were determined with the Ca(2+)-sensitive dyes fluo-4FF AM or fluo-3 AM. Five stimulations delivered at 100 Hz or single stimulations of parallel fibres evoked rapid and reproducible transient increases in presynaptic fluo-4FF or fluo-3 fluorescence, respectively, which decayed to prestimulus levels within a few hundred milliseconds. Bath application of the selective CB1 agonist WIN55,212-2 (1 microM) markedly reduced the peak amplitude of these fluorescence transients. This effect was fully reversed by the selective CB1 antagonist SR141716-A (1 microM). 3. Bath application of the selective mGluR4 agonist L-AP4 (100 microM) also caused a transient decrease in the peak amplitude of the fluorescence transients evoked by parallel fibre stimulation. 4. Bath application of the potassium channel blocker 4-AP (1 mM) totally prevented both the WIN55,212-2- and the L-AP4-induced inhibition of peak fluorescence transients evoked by parallel fibre stimulation. 5. The present study demonstrates that activation of CB1 and mGluR4 receptors inhibits presynaptic Ca(2+) influx evoked by parallel fibre stimulation via the activation of presynaptic K(+) channels, suggesting that the molecular mechanisms underlying this inhibition involve an indirect inhibition of presynaptic voltage-gated Ca(2+) channels rather than their direct inhibition.

Alpha 1-adrenergic modulation of metabotropic glutamate receptor-induced calcium oscillations and glutamate release in astrocytes

Astrocytic responses to activation of metabotropic glutamate receptors group I (mGluRs I) and alpha(1)-adrenoreceptors in cultured cells have been assessed using spectral analyzes and calcium imaging. Concentration-dependent changes were observed after stimulation with the mGluR I agonist (S)-3,5-dihydroxyphenylglycine (DHPG). These responses changed from a regular low frequency signal with sharp peaks at 1 microm to a pronounced stage of irregularity at 10 microm. After stimulation with 100 microm the signal was again homogenous in shape and regularity but occurred at a higher frequency. In contrast, the spectral properties after stimulation with the alpha(1)-adrenoreceptor agonist phenylephrine, exhibited considerable variation for all investigated concentrations. DHPG-induced increases in [Ca(2+)](i) were also associated with astroglial glutamate release, whereas no release was observed after noradrenergic stimulation. Both DHPG-mediated calcium signaling and glutamate release were inhibited by preincubation with 10 or 100 microm phenylephrine. Collectively, the present investigation provides new information about the spatial-temporal characteristics of astroglial intracellular calcium responses and demonstrates distinct differences between noradrenergic and glutamatergic receptors regarding intracellular calcium signaling and coupling to glutamate release. The noradrenergic modulation of DHPG-induced responses indicates that intracellular astroglial processes can be regulated in a bi-directional feedback loop between closely connected astrocytes and neurons in the central nervous system.

Magnesium decreases arterial pressure and inhibits cardiovascular responses induced by N-methyl-D-aspartate and metabotropic glutamate receptors stimulation in rostral ventrolateral medulla

OBJECTIVE

Magnesium sulfate (MgSO4) is widely used for the treatment of eclampsia. However, effects of Mg2+ in central cardiovascular regulation remain unclear. In the present study, the role of Mg2+ on cardiovascular regulation in the rostral ventrolateral medulla (RVLM) of rats was examined.

METHODS

Adult male Wistar rats were anesthetized with urethane, and artificially ventilated. The ventral surface of the medulla was exposed, and the RVLM was identified by microinjection (50 nl) of l-glutamate (l-Glu; 2 nmol). Then, MgSO4 (1, 3, 10 nmol, n = 7 for each dose) and magnesium chloride (MgCl2; 10 nmol, n = 7) were microinjected into the RVLM. l-Glu (2 nmol), N-methyl-D-aspartate (NMDA; 20 pmol), alpha-amino-3-hydroxy-5-methyl isoxazole-4-propionic acid (AMPA; 5 pmol) and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD, metabotropic glutamate receptor agonist; 1 nmol] were also microinjected with or without pretreatment of MgSO4 (10 nmol; n = 7 for each drug).

RESULTS

MgSO4 dose-dependently decreased mean arterial pressure (MAP) and heart rate (HR). The high dose of MgSO4 (10 nmol) significantly decreased MAP and HR (-25 +/- 4 mmHg and -43 +/- 6 bpm). Similarly, MgCl2 decreased MAP and HR (-27 +/- 4 mmHg and -30 +/- 6 bpm). The pressor response evoked by NMDA or (1S,3R)-ACPD was significantly attenuated by the pretreatment with MgSO4. In contrast, pressor response caused by l-Glu or AMPA was not affected by pretreatment with MgSO4.

CONCLUSIONS

These results suggest that Mg2+ has an inhibitory role on the RVLM neurons, and inhibits cardiovascular responses induced by NMDA and metabotropic glutamate receptor agonists.

A characterisation of long-term depression induced by metabotropic glutamate receptor activation in the rat hippocampus in vitro

1. In the CA1 region of hippocampal slices prepared from juvenile (12- to 18-day-old) rats, activation of group I metabotropic L-glutamate (mGlu) receptors by the specific agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) induces a form of long-term depression (LTD) of excitatory synaptic transmission. 2. We have used a variety of electrophysiological techniques applied to CA1 neurones in hippocampal slices and from pyramidal cells in dissociated hippocampal cultures to investigate the Ca2+ dependence and locus of expression of DHPG-induced LTD. 3. In patch-clamp experiments from hippocampal slices, bath application of DHPG induced a depression of synaptically evoked responses that persisted for the duration of the recording (up to 2 h after commencing washout of DHPG) in 27 of 29 neurones investigated. 4. DHPG-induced LTD was associated with an increase in both the paired-pulse facilitation ratio and the coefficient of variation of EPSCs. 5. Using dendritic recording, there was a decrease in EPSC success rate (number of trials that elicited a detectable response) but no change in potency (mean EPSC amplitude excluding failures) associated with DHPG-induced LTD. 6. In experiments using dissociated hippocampal cultures, application of DHPG elicited a persistent decrease in the frequency of tetrodotoxin-resistant miniature EPSCs but no change in the amplitude of such events. 7. DHPG-induced LTD was not blocked by intracellular application of the calcium chelator BAPTA. It was also unaffected when intracellular calcium stores were depleted by perfusion with thapsigargin. Furthermore, when synaptic transmission was blocked by perfusing with Ca2+-free medium, DHPG application reliably induced LTD. 8. These data suggest that DHPG-induced LTD is Ca2+ independent and is expressed presynaptically.

NMDA-type glutamatergic modulation in dopaminergic activation measured by apomorphine-induced cage climbing behaviors

The present study examined the hypothesis that NMDA, AMPA/Kainate, and metabotropic (mGlu) glutamate receptors contribute to a behavioral stimulation induced by activation of dopamine receptors by comparing responses in apomorphine-induced cage climbing behaviors in mice. MK-801, CNQX, and MCPG were served as the NMDA receptor, AMPA/Kainate receptor, and mGlu receptor antagonist, respectively, to elucidate the glutamatergic modulation in apomorphine-induced dopaminergic activation in mice. Drugs were administered intracerebroventricularly (i.c.v.) into the mouse brain 15 min before the apomorphine treatment (2 mg/kg, s.c.). I.c.v. injection of MK-801 inhibited the apomorphine-induced cage climbing behavior dose-dependently. However, treatments with CNQX and MCPG did not any significant change in apomorphine-induced cage climbing behavior in mice. These results suggest that stimulation of NMDA type of glutamate receptors could contribute to the dopaminergic stimulation, but not AMPA/Kainate and mGlu type glutamate receptors.

Modulation of metabotropic glutamate receptors fails to prevent the loss of adult rat retinal ganglion cells following axotomy or N-methyl-D-aspartate lesion in vivo

Both optic nerve (ON) transection and intraocular injection of N-methyl-D-aspartate (NMDA) are established lesion models to cause death of retinal ganglion cells (RGCs) in the adult rat. Excitotoxic effects via glutamate receptors resulting in secondary neuronal death are discussed as possible initiators in both types of RGC damage. We examined whether modulating glutamatergic transmission through metabotropic glutamate receptors rescues RGCs from lesion-induced degeneration in vivo. Unexpectedly, repeated intraocular injection of four different agonists/antagonists on the various subtypes of mGluRs did not decrease retinal damage in both lesion paradigms as revealed by measurement of visual performance and RGC survival. We conclude that activation/inactivation of retinal mGluRs does not play an important role for the initiation and execution of secondary RGC loss after ON transection and NMDA lesion in the adult rat.

Retrograde modulation of transmitter release by postsynaptic subtype 1 metabotropic glutamate receptors in the rat cerebellum

1. The aim of the study was to elucidate the mechanisms underlying the depressant effect of the group I/II metabotropic glutamate receptor (mGluR) agonist 1S,3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) on parallel fibre (PF) to Purkinje cell (PC) synaptic transmission. Experiments were performed in rat cerebellar slices using the whole-cell patch-clamp technique and fluorometric measurements of presynaptic calcium variation 2. Analysis of short-term plasticity, fluctuation of EPSC amplitude and responses of PCs to exogenous glutamate showed that depression caused by 1S,3R-ACPD is presynaptic. 3. The effects of 1S,3R-ACPD were blocked and reproduced by group I mGluR antagonists and agonists, respectively. 4. These effects remained unchanged in mGluR5 knock-out mice and disappeared in mGluR1 knock-out mice. 5. 1S,3R-ACPD increased calcium concentration in PFs. This effect was abolished by AMPA/kainate (but not NMDA) receptor antagonists and mimicked by focally applied agonists of these receptors. Thus, it is not directly due to mGluRs but to presynaptic AMPA/kainate receptors indirectly activated by 1S,3R-ACPD. 6. Frequencies of spontaneous and evoked unitary EPSCs recorded in PCs were respectively increased and decreased by mGluR1 agonists. Similar results were obtained when mGluR1s were activated by tetanic stimulation of PFs. 7. Injecting 30 mM BAPTA into PCs blocked the effects of 1S,3R-ACPD on unitary EPSCs. 8. In conclusion, 1S,3R-ACPD reduces evoked release of glutamate from PFs. This effect is triggered by postsynaptic mGluR1s and thus implies that a retrograde messenger, probably glutamate, opens presynaptic AMPA/kainate receptors and consequently increases spontaneous release of glutamate from PF terminals and decreases evoked synaptic transmission.