Activation of metabotropic glutamate receptors in the hippocampus increases cyclic AMP accumulation

Neuroprotective effect of sigma(1)-receptor ligand 4-phenyl-1-(4-phenylbutyl) piperidine (PPBP) is linked to reduced neuronal nitric oxide production

BACKGROUND AND PURPOSE

The potent final sigma(1)-receptor ligand 4-phenyl-1-(4-phenylbutyl) piperidine (PPBP) provides neuroprotection in experimental stroke. We tested the hypothesis that PPBP attenuates striatal tissue damage after middle cerebral artery occlusion (MCAO) by a mechanism involving reduction of ischemia-evoked nitric oxide (NO) production. Furthermore, we determined whether the agent fails to protect ischemic brain when neuronal nitric oxide synthase (nNOS) is genetically deleted or pharmacologically inhibited (selective nNOS inhibitor, 7-nitroindazole [7-NI]).

METHODS

Halothane-anesthetized adult male Wistar rats were subjected to 2 hours of MCAO by the intraluminal filament occlusion technique. All physiological variables were controlled during the ischemic insult. In vivo striatal NO production was estimated via microdialysis by quantification of local, labeled citrulline recovery after labeled arginine infusion. In a second series of experiments, nNOS null mutants (nNOSKOs) and the genetically matched wild-type (WT) strain were treated with 90 minutes of MCAO. Brains were harvested at 22 hours of reperfusion for measurement of infarction volume by triphenyltetrazolium chloride histology.

RESULTS

PPBP attenuated infarction volume at 22 hours of reperfusion in cerebral cortex and striatum and markedly attenuated NO production in ischemic and nonischemic striatum during occlusion and early reperfusion. Treatment with 7-NI mimicked the effects of PPBP. In WT mice, infarction volume was robustly decreased by both PPBP and 7-NI, but the efficacy of PPBP was not altered by pharmacological nNOS inhibition in combined therapy. In contrast, PPBP did not decrease infarction volume in nNOSKO mice.

CONCLUSIONS

These data suggest that the mechanism of neuroprotection of PPBP in vivo is through attenuation of nNOS activity and ischemia-evoked NO production. Neuroprotective effects of PPBP are lost when nNOS is not present or is inhibited; therefore, PPBP likely acts upstream from NO generation and its subsequent neurotoxicity.

Synaptic control of motoneuronal excitability

Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K(+) current, cationic inward current, hyperpolarization-activated inward current, Ca(2+) channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

Role of metabotropic glutamate receptor subclasses in modulation of adenylyl cyclase activity by a nootropic NS-105

The involvement of metabotropic glutamate (mGlu) receptors in the modulatory actions of a novel cognition enhancer, (+)-5-oxo-D-prolinepiperidinamide monohydrate (NS-105), on adenylyl cyclase activity in rat cerebrocortical membranes and primary neuronal cultures was investigated using selective antagonists and antisense oligodeoxynucleotides for mGlu receptor subclasses. In rat cerebrocortical membranes, the inhibitory action of NS-105 (0.1 microM) on forskolin-stimulated cAMP formation was blocked by a group II mGlu receptor antagonist, (+/-)-alpha-ethylglutamic acid, and by a group III antagonist, (+)-2-amino-2-methyl-4-phosphonobutanoic acid (MAP-4), but not by a group I antagonist, (+/-)-1-aminoindan-1,5-dicarboxylic acid (AIDA), whereas the facilitation of cAMP formation by NS-105 (1 microM) in pertussis toxin-pretreated membranes was abolished by AIDA but not by (+/-)-alpha-ethylglutamic acid or MAP-4. In primary cultured neurons of mouse cerebral cortex, the inhibitory action of NS-105 on adenylyl cyclase activity disappeared after treatment with antisense oligodeoxynucleotides for group II (mGlu(2) and mGlu(3) receptors) and group III (mGlu(4) and mGlu(7) receptors) but not group I (mGlu(5) receptor) mGlu receptor subclasses. These findings suggest that the inhibitory action of NS-105 on adenylyl cyclase activity is mediated through group II and group III mGlu receptor subclasses while the facilitatory action is dependent on the group I mGlu receptor subclass.

Long-lasting increase in cellular excitability associated with the priming of LTP induction in rat hippocampus

The mechanisms underlying the facilitation (priming) of long-term potentiation (LTP) by prior activation of metabotropic glutamate receptors (mGluRs) were investigated in area CA1 of rat hippocampal slices. In particular, we focused on whether a long-lasting increase in postsynaptic excitability could account for the facilitated LTP. Administration of the mGluR agonist 1S, 3R-aminocyclopentanedicarboxylic acid (ACPD) produced rapid decreases in the amplitude of both the slow spike afterhyperpolarization (AHP(slow)) and spike frequency adaptation recorded intracellularly from CA1 pyramidal cells. These changes persisted after drug washout, showing only a slow decay over 20 min. ACPD also caused a leftward shift of the field EPSP-population spike relation and an overall increase in population spike amplitude, but this effect was not as persistent as the intracellularly measured alterations in cell excitability. ACPD-treated cells showed increased spike discharges during LTP-inducing tetanic stimulation, and the amplitude of the AHP(slow) was negatively correlated with the degree of initial LTP induction. The beta-adrenergic agonist isoproterenol also caused excitability changes as recorded intracellularly, whereas in extracellular experiments it weakly primed the induction but not the persistence of LTP. ACPD primed both LTP measures. Isoproterenol administration during the tetanus occluded the priming effect of ACPD on initial LTP induction but not its effect on LTP persistence. We conclude that the persistent excitability changes elicited by ACPD contributes to the priming of LTP induction but that other ACPD-triggered mechanisms must account for the facilitated persistence of LTP in the priming paradigm.

Effects of guanine nucleotides on adenosine and glutamate modulation of cAMP levels in optic tectum slices from chicks

Glutamate and adenosine both modulate adenylyl cyclase activity through interaction of their specific receptors with stimulatory or inhibitory G-proteins. Guanine nucleotides (GN), which modulate G-protein activity intracellularly, are also involved in the inhibition of glutamate responses, acting from the outside of the cells. We had previously reported that glutamate inhibits adenosine-induced cyclic AMP (cAMP) accumulation in slices obtained from the optic tectum of chicks. In the present study we investigated the interaction of GN with these two neurotransmitters and found that GN inhibit the inhibitory effect of glutamate on adenosine-induced cAMP accumulation and potentiate adenosine-induced cAMP accumulation. These effects were observed with 5'-guanylylimidodiphosphate (GppNHp) or GMP, but not with guanosine (the nucleoside). Besides, these interactions of GN occur via a metabotropic glutamate receptor (mGluR) sensitive to (1 S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1 S,3R-ACPD) but not to L-2-amino-4-phosphonobutyrate (L-AP4). These effects were partially modulated by a mGluR antagonist, (RS)-alpha-methyl-4-carboxyphenylglycine ((RS)M-CPG), and by an adenosine receptor antagonist, 8-phenyltheophylline. GN only potentiated the adenosine response when adenosine was acting through its receptor positively linked to adenylyl cyclase. Therefore, the data show that guanine nucleotides not only inhibit glutamate-induced responses, but also stimulate adenosine-induced responses, a fact that may contribute to the understanding of the physiological functions of guanine nucleotides.

Intrathecal administration of the mGluR compound, (S)-4CPG, attenuates hyperalgesia and allodynia associated with sciatic nerve constriction injury in rats

The present study examined the effects of intrathecal (i.t.) treatment (twice-daily injections on post-operative (PO) days 0-8) with the metabotropic glutamate receptor (mGluR) compound, (S)-4-carboxyphenylglycine ((S)-4CPG), or the non-competitive N-methyl-D-aspartate (NMDA) antagonist, dizocilipine maleate (MK-801), on mechanical allodynia and cold hyperalgesia associated with chronic constriction injury (CCI) of the sciatic nerve in rats. Also, the effects of early (twice-daily injections on days 0-3) or late (twice-daily injections on days 8-11) (S)-4CPG treatment on the injury-related mechanical allodynia and cold hyperalgesia were assessed in CCI rats. Results demonstrated that 8-day (S)-4CPG or MK-801 treatment attenuated mechanical allodynia (up to PO days 12 or 16, respectively) and cold hyperalgesia (up to PO days 8 or 16, respectively). Results also demonstrated that early (S)-4CPG treatment significantly attenuated the development of mechanical allodynia (90 and 270 nmol) and cold hyperalgesia (270 nmol). However, late treatment with (S)-4CPG did not reduce the nociceptive behaviours in either behavioural task. These data not only confirm that the NMDA receptor plays a role in chronic nociception, but also suggest that Group I mGluRs are more critically involved in the development, and not the maintenance, of mechanical allodynia and cold hyperalgesia associated with CCI in rats.

mGluR-evoked augmentation of receptor-mediated cyclic AMP formation in neonatal and adult rat striatum

1. The effects of selective agonists at group I, II and III metabotropic glutamate receptors (mGluRs) on adenosine A2 receptor-mediated cyclic AMP formation were compared in cross-chopped slices of adult and neonatal (8 days old) rat striatum, in the presence of 1 u ml(-1) adenosine deaminase. 2. The group II selective agonist, (2S,1R,2R,3R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV), elicited a potentiation of 5'-N-ethylcarboxamidoadenosine (NECA)-stimulated cyclic AMP production with similar potencies in adult (EC50 value 122 +/- 35 nM) and neonatal (EC50 value 285 +/-6 nM) brain. In contrast, the group I selective agonist (S)-dihydroxyphenylglycine ((S)-DHPG) augmented the NECA cyclic AMP response in neonatal striatum (EC50 value 9 +/- 1 microM), but at a concentration of 100 microM, (S)-DHPG failed to affect the NECA response in adult striatal slices. 3. The potentiation evoked by (S)-DHPG was specific for group I mGluRs as (2S,3S,4S,)-2-methyl-2-(carboxycyclopropyl)glycine (MCCG), a group II antagonist, was ineffective on the (S)-DHPG (100 microM) response at a concentration (500 microM) which reversed a similar augmentation elicited by DCG-IV (300 nM). Furthermore, a protein kinase C inhibitor (Ro 31-8220, 10 microM) markedly reversed the effect of (S)-DHPG without affecting the response to DCG-IV. 4. The mGluR agonist (2S,3S,4S,)-alpha-(carboxycyclopropyl)glycine (L-CCG-I), elicited a greater potentiation of NECA-stimulated cyclic AMP production in neonatal striatum in comparison with that observed in adult rat brain. Moreover, EC50 values obtained from adult and neonatal striatum were 2 +/-1 microM and 9 +/-1 microM, respectively. These differences in potency might reflect co-activation of both group I and group II mGluRs by L-CCG-I in neonatal striatum. 5. Distinct patterns of mGluR expression in various brain areas might account for previous conflicting data on the nature of the mGluR able to evoke such potentiated responses.

Metaplasticity: a new vista across the field of synaptic plasticity

Over the past 20 years there has been an increasing understanding of the properties and mechanisms underlying long-term potentiation (LTP) and long-term depression (LTD) of synaptic efficacy, putative learning and memory mechanisms in the mammalian brain. More recently, however, it has become apparent that synaptic activity can also elicit persistent neuronal responses not manifest as changes in synaptic strength. Some of these changes may nonetheless modify the ability of synapses to undergo strength changes in response to subsequent episodes of synaptic activity. This kind of activity-dependent modulatory plasticity we have termed "metaplasticity". Metaplasticity has been observed physiologically as an inhibition of LTP and concomitant facilitation of LTD by prior N-methyl-D-aspartate receptor activation or, conversely, a facilitation of LTP induction by prior metabotropic glutamate receptor activation. The examples of metaplasticity described to date are input specific, and last as long as several hours. The mechanisms underlying such phenomena remain to be fully characterized, although some likely possibilities are an altered N-methyl-D-aspartate receptor function, altered calcium buffering, altered states of kinases or phosphatases, and a priming of protein synthesis machinery. While some details vary, experimentally observed metaplasticity bears some similarity to the "sliding threshold" feature of the Bienenstock, Cooper and Munro model of experience-dependent synaptic plasticity. Metaplasticity may serve several functions including (1) providing a way for synapses to integrate a response across temporally spaced episodes of synaptic activity and (2) keeping synapses within a dynamic functional range, and thus preventing them from entering states of saturated LTP or LTD.

The pharmacology of amino-acid responses in septal neurons

Septal cholinergic neurons are known to play an important role in cognitive processes including learning and memory through afferent innervation of the hippocampal formation and cerebral cortex. The septum contains not only cholinergic neurons but also various types of neurons including GABA (gamma-aminobutyric acid)-ergic neurons. Although synaptic transmission in the septum is mediated primarily by the activation of excitatory and inhibitory amino-acid receptors, it is possible that a distinct phenotype of neuron is endowed with a different type for each of the amino-acid receptors and thus they play different roles from each other, since it has been demonstrated within the septum that there is a regional distribution of various types of amino-acid receptor subunits, their expression as different combinations within a specific cell may produce receptor channels with disparate functional properties. As a first step towards knowing the various functions of septal cholinergic neurons, we characterized the functional properties of glutamate, GABA (type A; GABAA) and glycine receptor channels on cultured rat septal neurons which were histologically identified to be cholinergic. These were similar to those of receptor channels on other types of neurons, except for the actions of some neuromodulators. The septal N-methyl-D-aspartate receptor channel was distinct in being less sensitive to Mg2+ and in a voltage-dependent action of Zn2+. The septal GABAA receptor channel exhibited a lanthanide site whose activation resulted in a positive allosteric interaction with a binding site of pentobarbital. The septal glycine receptor channel was only positively modulated by Zn2+; this action of Zn2+ was not accompanied by an inhibitory effect. Our data suggest that the amino-acid receptors on septal cholinergic neurons may play a distinct role compared to other types of neurons; this difference depends on the actions of neuromodulators and metal cations. It would be interesting to compare these effects recorded in tissue culture to those observed with septal cholinergic neurons in slice preparations.

Characterization of metabotropic glutamate receptors in rat C6 glioma cells

Metabotropic glutamate receptors in rat C6 glioma cells have been characterized by pharmacological and kinetic binding experiments, using both L-[3H]glutamate and [3H(+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid ([3H](+/-)-trans-ACPD) radioligands. Saturation experiments revealed a single binding site with a Kd = 1250 +/- 101 nM and Bmax = 12.1 +/- 1.8 pmol/mg protein when the assays were performed with L-[3H]glutamate as radioligand in the presence of AMPA, kainate, NMDA and DL-threo-beta-hydroxyaspartic acid. When [3H](+/-)-trans-ACPD was used as radioligand, the kinetic parameters obtained were Kd = 2605 +/- 1042 nM and Bmax = 13.66 +/- 5.01 pmol/mg protein. Pharmacological characterization indicated that specific binding of L-[3H]glutamate was sensitive to different agonists of mGlu receptors, showing a rank order of affinity L-glutamate > L-quisqualic acid > (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD) > ibotenic acid >>> (2S, 'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I). Specific binding of L-[3H]glutamate to mGlu receptors is regulated by guanine nucleotides. Guanylyl imidodiphosphate (Gpp(NH)p) causes an affinity shift on the L-glutamate dose-response curve, increasing the IC50 value. These results support the evidence that metabotropic glutamate receptors are present in rat C6 glioma cells and they are coupled to a G-protein.

Microinfusion of the metabotropic glutamate receptor agonist 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid into the nucleus accumbens induces dopamine-dependent locomotor activation in the rat

Although the striatum has one of the highest densities of metabotropic glutamate receptor (mGluR) binding sites in the brain, little is known about their physiological role. In this study we characterized the contribution of mGluRs located in the ventral part of the striatum (the nucleus accumbens) to the control of extrapyramidal motor function. Activation of mGluRs by local infusion of the selective agonist 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD; 25, 50 and 100 nmol/0.5 microl) into the nucleus accumbens induced a dose-dependent increase in locomotor activity in rats. Intra-accumbens infusion of a selective antagonist of mGluRs, alpha-methyl-4-carboxyphenylglycine (MCPG) did not modify spontaneous locomotion but decreased the locomotor response to 1S,3R-ACPD. This effect appeared to be mediated by dopamine, since blockade of dopamine receptors with haloperidol (0.05 and 0.1 mg/kg i.p.) dose-dependently reduced 1S,3R-ACPD-induced locomotor activation. Furthermore, D-amphetamine (0.5 mg/kg, i.p.) combined with intra-accumbens infusion of 1S,3R-ACPD (100 nmol) potentiated the locomotor hyperactivity response to a higher level than that seen with a single treatment with either drug. In contrast, D-amphetamine-induced hypermotility was abolished by infusion of MCPG (100 nmol) into the nucleus accumbens. These results demonstrate that glutamate may control extrapyramidal motor function through metabotropic receptors. Furthermore, activation of metabotropic glutamate receptors appears to act in synergy with the dopamine system at the level of the nucleus accumbens to produce a motor stimulant response.

Receptor-mediated activation of protein kinase C in hippocampal long-term potentiation: facts, problems and implications

During the last decade hippocampal long-term potentiation has become one of the most frequently used models to study cellular mechanisms of learning and memory. Receptor-mediated activation of protein kinase C is thought to be involved in LTP stabilisation. In the present review, 1. the molecular structure and activation mechanisms of PKC isoenzymes, 2. the biochemical evidences for PKC activation after induction of LTP using different stimulation paradigms as well as 3. the involvement of metabotropic glutamate receptors in PKC activation after induction of LTP are critically discussed.

Effect of guanine nucleotides on [3H]glutamate binding and on adenylate cyclase activity in rat brain membranes

GMP-PNP, a non-hydrolyzable analog of GTP binds tightly to G-protein in the presence of Mg2+, so that the binding is stable even after exhaustive washings. This property was exploited to prepare membrane samples of rat brain where G-protein GTP-binding sites were saturated with GMP-PNP. Experiments carried out with these membranes showed that GTP, GMP-PNP, GDP-S and GMP (1 mM) inhibit the sodium-independent [3H]glutamate binding by 30-40% [F(4,40) = 5.9; p < .001], whereas only GMP-PNP activates adenylate cyclase activity [F(6,42) = 3.56; p < .01]. The inhibition of sodium-independent [3H]glutamate binding occurred in the absence of Mg2+. These findings suggest that guanine nucleotides may inhibit glutamate binding and activate adenylate cyclase through distinct mechanisms by acting on different sites.

Characterization of metabotropic glutamate receptor-mediated nitric oxide production in vivo

We tested the hypothesis that stimulation of metabotropic glutamate receptors (mGluRs) increases nitric oxide (NO) production in the hippocampus in vivo. Microdialysis probes were placed bilaterally into the CA3 region of the hippocampus of adult Sprague-Dawley rats under pentobarbital anesthesia. Probes were perfused for 5 h with artificial cerebrospinal fluid (CSF) containing 3 microM [14C]-L-arginine. Recovery of [14C]-L-citrulline in the effluent was used as a marker of NO production. In nine groups of rats, increases in [14C]-L-citrulline recovery were compared between right- and left-sided probes perfused with various combinations of the selective mGluR agonist, trans-(1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid (ACPD); the mGluR antagonist, (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG); the NO synthase inhibitor, N-nitro-L-arginine (LNNA); the ryanodine sensitive calcium-release channel inhibitor dantrolene, the non-N-methyl-D-aspartate (NMDA); receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX); the NMDA receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d] cyclohepten-5,10-imine (MK-801); and the Na+ channel blocker, tetrodotoxin. Recovery of [14C]-L-citrulline during perfusion with artificial CSF progressively increased to 90 +/- 21 fmol/min (+/-SD) over 5 h. Perfusion in the contralateral hippocampus with 1 mM ACPD augmented [14C]-L-citrulline recovery to 250 +/- 81 fmol/min. Perfusion of 1 mM nitroarginine + ACPD inhibited [14C]-L-citrulline recovery compared to that with ACPD alone. Perfusion with 1 mM MCPG + ACPD attenuated ACPD enhanced [14C]-L-citrulline recovery. Perfusion of 1 mM dantrolene + ACPD inhibited the ACPD-evoked increase in [14C]-L-citrulline recovery. Perfusion of 1 mM MCPG or dantrolene without ACPD did not decrease [14C]-L-citrulline recovery as compared to CSF alone. ACPD-enhanced [14C]-L-citrulline recovery was not attenuated by CNQX, MK-801, or tetrodotoxin (TTX). Using an indirect method of assessing NO production in vivo, these data demonstrate that mGluR stimulation enhances NO production in rat hippocampus. Inhibition with dantrolene suggests that calcium-induced calcium release amplifies the inositol triphosphate-mediated calcium signal associated with mGluR stimulation, thereby resulting in augmented calcium-dependent NO production.

Multiple actions of 1S,3R-ACPD in modulating endogenous synaptic transmission to spinal respiratory motoneurons

To determine physiological roles of metabotropic glutamate receptors (mGluRs) affecting breathing, we examined the effects of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) on synaptic transmission and excitability of phrenic motoneurons (PMNs) in an in vitro neonatal rat brainstem/spinal cord preparation. The effects of 1S,3R-ACPD were multiple, including reduction of inspiratory-modulated synaptic currents and increase of neuronal excitability via an inward current (Iacpd) associated with a decrease of membrane conductance. The mechanism underlying synaptic depression was examined. We found that 1S,3R-ACPD reduced the frequency but not the amplitude of miniature excitatory postsynaptic currents. The current induced by exogenous AMPA was not significantly affected by 1S,3R-ACPD. These results suggest that 1S,3R-ACPD-induced reduction of inspiratory synaptic currents is mediated by presynaptic mGluRs. We also examined the ionic basis for Iacpd. We found that Iacpd had a reversal potential of approximately -100 mV, close to the estimated, EK+ (-95 mV). Elevating extracellular [K+] to 9 mM reduced the Iacpd reversal potential to -75 mV. The K+ channel blocker Ba2+ induced an inward current with a reversal potential at -93 mV associated with a decrease of membrane conductance, closely resembling the effect of 1S,3R-ACPD. Moreover, Ba2+, occluded 1S,3R-ACPD effects. In the presence of Ba2+, Iacpd and the 1S,3R-ACPD-induced decrease of membrane conductance were diminished. Our data indicate that the dominant component of Iacpd results from the blockade of a Ba(2+)-sensitive resting K+ conductance. We conclude that the activation of mGluRs affects the inspiratory-modulated activity of PMNs via distinct mechanisms at pre- and postsynaptic sites.

Potentiation of spontaneous acetylcholine release from motor nerve terminals by glutamate in Xenopus tadpoles

Extracellular application of glutamate (100 microM) increased the spontaneous secretion of acetylcholine, as well as the amplitude and decay time of miniature endplate potentials at developing neuromuscular synapses in Xenopus tadpoles. Kainate, quisqualate and N-methyl-D-aspartate (100 microM each) increased miniature endplate potential frequency by 26-, 13- and four-fold, respectively. The rank order of efficacy at 100 microM was kainate > quisqualate > N-methyl-D-aspartate > glutamate. The effect of kainate on miniature endplate potential frequency was inhibited by 6-cyano-2,3-dihydroxy-7-nitroquinoxaline (20 microM), but not by (+/-)-2-amino-5-phosphonovalerate (20 microM). Treatment with the voltage-dependent Ca2+ channel blockers verapamil (10 microM), Cd2+ (100 microM) or omega-conotoxin (1 microM) inhibited the potentiating action of kainate on miniature endplate potential frequency. On the other hand, 1S,3R-1-aminocyclopentane-1,3-dicarboxylate (300 microM), a glutamate metabotropic receptor agonist, inhibited the spontaneous acetylcholine release, which was antagonized by the application of 2-amino-3-phosphonopropionate (500 microM). The potentiating effect of glutamate receptor agonists on the miniature endplate potential frequency declined or disappeared in older Xenopus tadpoles. Quisqualate (100 microM) and N-methyl-D-aspartate (100 microM) but not kainate (30 microM) increased the amplitude and decay time of miniature endplate potential, whereas 1S, 3R-1-aminocyclopentane-1, 3-dicarboxylate (300 microM) only increased the decay time of miniature endplate potentials. These results suggest that there are kainate/quisqualate and N-methyl-D-aspartate receptors existing in the motor nerve terminals of younger Xenopus tadpoles and the activation of these receptors potentiates spontaneous acetylcholine release through increasing Ca2+ influx. Our data suggest that the presynaptic glutamate receptors on cholinergic terminals may be involved in feedback regulation of acetylcholine secretion at earlier embryonic stages.

Novel glial-neuronal signalling by coactivation of metabotropic glutamate and beta-adrenergic receptors in rat hippocampus

1. We have previously reported that activation of group II-like metabotropic glutamate receptors (mGluRs) in rat hippocampus results in a potentiation of the accumulation of cAMP elicited by activation of G-protein Gs-coupled receptors. This large increase in cAMP levels results in release of cAMP or a cAMP metabolite and depression of synaptic transmission at the Schaffer collateral-CA1 pyramidal cell synapse through activation of A1 adenosine receptors. 2. Consistent with these studies, we report that antagonists of group II mGluRs block both the potentiation of cAMP accumulation elicited by activation of mGluRs and the depression of synaptic transmission induced by coactivation of mGluRs and beta-adrenergic receptors. 3. In situ hybridization studies suggest that of the cloned group II mGluRs only mGluR-3 mRNA is present in area CA1. Interestingly, mGluR-3 appears to be present predominantly in glia in this region. Thus, we tested the hypothesis that mGluRs coupled to potentiation of cAMP accumulation were present on glia rather than neurons in area CA1. 4. The selective group II mGluR agonist 2S,1'R,2'R,3'R-2(2,3-dicarboxycyclo-propyl)glycine (DCG-IV) failed to enhance cAMP-mediated electrophysiological responses to the beta-adrenergic receptor agonist isoprenaline (Iso) in CA1 pyramidal cells, suggesting that mGluRs coupled to potentiation of cAMP accumulation may not be present in these cells. 5. Pre-incubation of hippocampal slices with either of the selective glial toxins L-alpha-aminoadipic acid (L-AA) or fluorocitrate (FC) blocked mGluR-mediated potentiation of cAMP accumulation. However, L-AA and FC had no discernible effects on viability of CA1 pyramidal cells, or cAMP-mediated electrophysiological effects in these neurons. 6. Pre-incubation of hippocampal slices with the neurotoxin kainate resulted in disruption of neuronal transmission and degeneration of neurons in area CA1, but had no effect on mGluR-mediated potentiation of cAMP accumulation. 7. Pre-incubation of hippocampal slices with the cAMP/cAMP metabolite transport blocker probenicid blocked the depression of synaptic transmission elicited by coapplication of Iso and DCG-IV, while having no significant effect on cAMP accumulation elicited by these agonists. 8. Taken together, these data suggest that mGluRs coupled to potentiation of cAMP accumulation are present on glia rather than neurons in area CA1 of hippocampus. This suggests that a novel form of glial-neuronal communication may exist, since activation of these mGluRs in concert with beta-adrenergic receptors results in depression of synaptic transmission.

Effects of excitatory amino acids on phosphoinositide metabolism in frog retina

The effects of excitatory amino acid receptor agonists on the hydrolysis of phosphoinositides were examined using frog retinal membranes prelabeled in vitro with either 32PO4 or [3H]inositol. Glutamate stimulated release of [3H]inositol phosphates (IPs) from the retinas and altered the 32P-labeling pattern of phosphatidylinositol (PI) cycle intermediates. This indicates that glutamate affects not only the hydrolysis of phosphoinositides but possibly other steps involved in the PI cycle. Among glutamate analogs, kainate (KA), quisqualate (QA), and, to a lesser extent, N-methyl-D-aspartate (NMDA) mimicked the glutamate effect, whereas L-2-amino-4-phosphonobutyrate (L-AP4) was not effective in causing either the accumulation of [3H]IPs or the alteration of the 32P-labeling pattern of PI cycle intermediates. Among QA specific agonists, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), but not ibotenate (IBO) or trans-1-aminocyclopentane-1,3-dicarboxylate (ACPD) was active in stimulating IPs formation. KA effect on IPs formation may be due to indirect (polysynaptic) activation of receptor(s) other than L-AP4, IBO, or ACPD specific QA receptors. To avoid activating polysynaptic pathways, retinal synaptoneurosomes prelabeled with [3H]inositol were used to examine the hydrolysis of phosphoinositides. As in whole retinas, KA, carbachol (CARB), and NMDA stimulated the release of IPs while L-AP4 had minimal effect. Glycine (GLY) had no effect. Our results show CARB and KA to be the most effective in stimulating the production of IPs. Their effects were exerted directly through separate receptors and not through polysynaptic pathways. ACPD and IBO were the least effective in eliciting the release of IPs. Our studies provide evidence that ionotropic and not metabotropic glutamate receptors are involved in PI metabolism in the retina.

Pharmacological characterization of metabotropic glutamate receptors coupled to phospholipase D in the rat hippocampus

1. Phospholipase D (PLD) is the key enzyme in a signal transduction pathway leading to the formation of the second messengers phosphatidic acid and diacylglycerol. In order to define the pharmacological profile of PLD-coupled metabotropic glutamate receptors (mGluRs), PLD activity was measured in slices of adult rat brain in the presence of mGluR agonists or antagonists. Activation of the phospholipase C (PLC) pathway by the same agents was also examined. 2. The mGluR-selective agonist (1S,3R)-l-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD] induced a concentration-dependent (10-300 microM) activation of PLD in the hippocampus, neocortex, and striatum, but not in the cerebellum. The effect was particularly evident in hippocampal slices, which were thus used for all subsequent experiments. 3. The rank order of potencies for agonists stimulating the PLD response was: quisqualate > ibotenate > (2S,3S,4S)-alpha-(carboxycyclopropyl)-glycine > (1S,3R)-ACPD > L-cysteine sulphinic acid > L-aspartate > L-glutamate. L-(+)-2-Amino-4-phosphonobutyric acid and the ionotropic glutamate receptor agonists N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, and kainate failed to activate PLD. (RS)-3,5-dihydroxyphenylglycine (100300 microM), an agonist of mGluRs of the first group, stimulated PLC but inhibited the PLD response elicited by 100 microM (1S,3R)-ACPD. 4. (+)-alpha-Methyl-4-carboxyphenylglycine (0.1-1 mM), a competitive antagonist of mGluRs of the first and second group, elicited a significant PLD response. L-(+)-2-Amino-3-phosphonopropionic acid (1 mM), an antagonist of mGluRs of the first group, inhibited the 100 microM (1S,3R)-ACPD-induced PLC response but produced a robust stimulation of PLD. 5. 12-O-Tetradecanoylphorbol 13-acetic acid and phorbol 12,13-dibutyrate (PDBu), activators of protein kinase C, at 1 microM had a stimulatory effect on mGluRs linked to PLD but depressed (1S,3R)-ACPD-induced phosphoinositide hydrolysis. The protein kinase C inhibitor, staurosporine (1 and 10 microM) reduced PLD activation induced by 1 microM PDBu but not by 100 microM (1S,3R)-ACPD. 6. Our results suggest that PLD-linked mGluRs in rat hippocampus may be distinct from any known mGluR subtype coupled to PLC or adenylyl cyclase. Moreover, they indicate that independent mGluRs coupled to the PLC and PLD pathways exist and that mGluR agonists can stimulate PLD through a PKC-independent mechanism.

The enhancement and the inhibition of noradrenaline-induced cyclic AMP accumulation in rat brain by stimulation of metabotropic glutamate receptors

1. The actions of several metabotropic glutamate receptor and antagonists on noradrenaline (NA)-stimulated [3H]-cyclic AMP accumulation were investigated in rat cerebral cortical slices. 2. Quisqualate (QUIS), L-2-amino-3-phosphonopropionic acid (L-AP3) and glutamate (GLU) elicited concentration-dependent inhibition of (NA)-stimulated [3H]-cyclic AMP accumulation, with IC50 values of 105 +/- 29, 275 +/- 36 and 944 +/- 150 microM respectively. In contrast (Rs)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) (0.5 mM) and N-methyl-D-aspartic acid (NMDA) (0.5 mM) had no effect. 3. (2S,3S,4S)-alpha-(Carboxycyclopropyl)glycine (L-CCGI), 1-Aminocyclo-pentane-1S,3R-dicarbo-xylate (1S,3R-ACPD), ibotenate (IBO) and (RS)-4-carboxy-3-hydroxy-phenylglycine (CHPG)elicited a concentration-dependent enhancement of NA-stimulated [3H]-cyclic AMP accumulation, with EC50 values of 2.5 +/- 0.11, 42 +/- 1.3, 97.8 +/- 2.1 and 157 +/- 13.4 microM, respectively. 4. (S)-3-carboxy-4-hydroxyphenylglycine (3C4HPG) and (S)-4-carboxy-3-hydroxyphenyl-glycine (4C3HPG) produced a biphasic effect, at concentrations up to 100 and 500 microM, respectively, they significantly enhanced the action of NA (100 microM), at 1mM concentration both compounds as well as alpha-methyl-4-carboxyphenylglycine (MCPG) produced a significant inhibition of NA-stimulated cyclic AMP accumulation. 5. A putative mGluR antagonist-L-AP3, inhibited the 1S,3R-ACPD (100 microM) induced enhancement of the action of NA (100 microM) on [3H]-cyclic AMP accumulation in a biphasic manner with an IC50 of 4.5 microM for the high affinity site, which represented 65% of the total and an IC50 of 283 microM for the low affinity site. 6. beta-adrenoceptor antagonist propranolol inhibited the interaction between 1S,3R-ACPD (100 microM) and NA (100 microM) on [3H]-cyclic AMP accumulation by about 80%, with an IC50 of 0.52 +/- 0.011 microM, to the level observed after 1S,3R-ACPD alone. Prazosin, an alpha 1-adrenoceptor antagonist was more potent (IC50 of 0.091 +/- 0.012 microM) but less efficacious (60% inhibition) as an inhibitor of the interaction either between NA and 1S,3R-ACPD while yohimbine, na alpha 2-adrenoceptor antagonist (up to 1 microM) had no effect. 7. Neither the protein kinase C inhibitor - staurosporine (10 microM) nor thapsigargin (1 microM), which depletes IP3 sensitive calcium stores, inhibited significantly the 1S,3R-ACPD (100 microM)-induced enhancement of the action of NA (100 microM) on [3H]-cyclic AMP accumulation. 8. Adenosine deaminase (0.5 U/ml) abolished both the 1S,3R-ACPD (100 microM)-induced [3H]-cyclic AMP accumulation and the synergistic interaction of this compound with NA (100 microM). 9. These results indicate the existence of different subtypes of metabotropic glutamate receptors in rat brain which either inhibit or enhance the NA-stimulated [3H]-cyclic AMP accumulation. The enhancement in cerebral cortical slices is mediated via receptors which are blocked with high affinity by L-AP3 and occurs via interactions with endogenous adenosine; the inhibition is mediated by receptors sensitive to quisqualate, L-AP3 and glutamate and may represent a predominant interaction between NA and excitatory amino acids (EAA), which in cerebral cortical slices is masked by excitatory effects.

Metabotropic glutamate receptor subtype 1A activates adenylate cyclase when expressed in baby hamster kidney cells

1. The coupling of subtype mGluR1a of the metabotropic glutamate receptor family to phosphoinositide hydrolysis, cAMP-formation and arachidonic acid release was characterized when the receptor was expressed in baby hamster kidney (BHK) cells. 2. When measuring cAMP-formation in BHK cells expressing mGluR1a, the rank order of agonist potency as well as the efficacy of mGluR1a antagonists was comparable to what has been observed when measuring mGluR1a-mediated phosphoinositide (PI)-hydrolysis. 3. However, while the presence of extracellular calcium increased the efficacy of quisqualate to stimulate phosphoinositide hydrolysis no significant effects were observed when measuring quisqualate-induced cAMP-formation. 4. Pretreatment of mGluR1a-expressing cells with pertussis toxin increased quisqualate-induced cAMP-formation in contrast to the observed partial inhibition of PI-hydrolysis by pertussis toxin. Cholera toxin increased cAMP-formation in BHK cells but showed no effects on PI-hydrolysis. 5. While quisqualate also stimulated [3H]-arachidonic acid release from BHK cells expressing mGluR1a this effect may be secondary to activation of phospholipase C. 6. These data further suggest that mGluR1a is coupled to PI-hydrolysis as well as cAMP-formation via different G-proteins which can be discriminated by their sensitivity to pertussis toxin.

Differentiation of group 2 and group 3 metabotropic glutamate receptor cAMP responses in the rat hippocampus

The effects of group 2- versus group 3-selective metabotropic glutamate (mGlu) receptor agonists were examined against forskolin (10 microM)-, vasoactive intestinal peptide (VIP; 1 microM)- and 5'-N-ethylcarboxamidoadenosine (NECA; 10 microM)-stimulated cAMP accumulations in adult rat hippocampal slices (in the presence of adenosine deaminase). Group 2 mGlu receptor-selective ((1S,3R)-1-aminocyclopentane-1, 3-dicarboxylic acid (1S,3R-ACPD) and (2S,3S,4S)-alpha-(carboxycyclopropyl)-glycine (L-CCG I)) and group 3 mGlu receptor-selective (L-2-amino-4-phosphonobutyric acid (L-AP4) and L-serine-O-phosphate) agonists greatly inhibited forskolin-stimulated cAMP formation ( > 80% at maximally effective concentrations). In contrast, stimulation of cAMP by VIP or NECA was inhibited by group 3, but not by group 2, mGlu receptor agonists. In fact, group 2 mGlu receptor agonists greatly potentiated cAMP accumulation evoked by NECA. Both the inhibitory effects of 1S,3R-ACPD on forskolin-stimulated cAMP and the potentiating effects on NECA-stimulated cAMP accumulation were reversed by the competitive group 1/2 mGlu receptor antagonist (+)-alpha-methyl-4-carboxyphenylglycine ((+)-MCPG). However, (+)-MCPG had no effects on L-AP4 inhibition of cAMP. Thus, the effects of group 2 versus group 3 mGlu receptor agonists on cAMP coupling can be pharmacologically as well as functionally differentiated in the rat hippocampus.

Glutamate receptors and development of the visual cortex: effect of metabotropic agonists on cAMP

Glutamate receptors are more active in several respects in young animals than in adults. Here we examine the effect of metabotropic glutamate agonists on rat cortical cAMP during and after the critical period for visual cortex plasticity. Quisqualate produced a substantial increase in cAMP, which was larger during the critical period than in the adult. The increase was not affected by CNQX or APV, showing that it was not due to the action of quisqualate on ionotropic glutamate receptors. Both Type I mGluRs (mGluRs 1 and/or 5) and Type II mGluRs (mGluRs 2 and/or 3) probably contributed to the cAMP increase because (i) ACPD and L-CCG-I, which are more active on Type II mGluRs, were more effective than DHPG, which is more active on Type I mGluRs; and (ii) there was a significant difference in the effect of ACPD on the increase in cAMP, comparing mGluR1 knockout mice with control mice. Agonists which produce large stimulation of cAMP production (ACPD, L-CCG-I), as well as L-AP4, also produced small attenuations of forskolin-stimulated cAMP, but only at high concentrations. Thus, we conclude that it is the stimulation and/or potentiation of cAMP production that is significant, rather than the attenuation of forskolin-stimulated cAMP. Since this stimulation and/or potentiation is higher during the critical period than in the adult, and the cAMP second messenger system has been implicated in hippocampal plasticity, it may also play a role in visual cortex plasticity.

Adenosine A2 receptor-mediated modulation of contralateral rotation induced by metabotropic glutamate receptor activation

Systemic pretreatment with the adenosine receptor antagonist theophylline significantly decreases contralateral rotation induced by unilateral intrastriatal 1-aminocyclopentane-1S,3R-dicarboxylic acid (1S,3R-ACPD). Intrastriatal or intrasubthalamic nucleus coadministration of theophylline and 1S,3R-ACPD significantly decreases contralateral rotation suggesting that metabotropic glutamate (mGlu) receptors and adenosine receptors interact locally. These appear to be adenosine A2 receptor effects as the adenosine A2 receptor antagonist 8-(3-chlorostyryl)caffeine (CSC) also decreases contralateral rotation induced by unilateral intrastriatal and intrasubthalamic nucleus administration of 1S,3R-ACPD, while the adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) has no effect. Pretreatment with the adenosine A2 receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamido adenosine hydrochloride (CGS 21680) potentiates contralateral rotation induced by unilateral striatal 1S,3R-ACPD, whereas pretreatment with the adenosine A1 receptor agonist N6-cyclopentyl-adenosine (CPA) has no effect. These results suggest that mGlu receptor effects may be due, in part, to modulation of adenosine action.

Modulation of adenosine-induced cAMP accumulation via metabotropic glutamate receptors in chick optic tectum

Changes on cyclic adenosine monophosphate (cAMP) levels in response to adenosine and glutamate and the subtype of glutamate receptors involved in this interaction were studied in slices of optic tectum from 3-day-old chicks. cAMP accumulation mediated by adenosine (100 microM) was abolished by 8-phenyltheophylline (15 microM). Glutamate and the glutamatergic agonists kainate or trans-D, L-1-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD) did not evoke cAMP accumulation. Glutamate blocked the adenosine response in a dose-dependent manner. At 100 microM, glutamate did not inhibit the effect of adenosine. The 1 mM and 10 mM doses of glutamate inhibited adenosine-induced cAMP accumulation by 55% and 100%, respectively. When glutamatergic antagonists were used, this inhibitory effect was not affected by 200 microM 6,7-dihydroxy-2,3,dinitroquinoxaline (DNQX), an ionotropic antagonist, and was partially antagonized by 1 mM (RS)-alpha-methyl-4-carboxyphenylglycine [(RS)M-CPG], a metabotropic antagonist, while 1 mM L-2-amino-3-phosphonopropionate (L-AP3) alone, another metabotropic antagonist, presented the same inhibitory effect of glutamate. Kainate (10 mM) and trans-ACPD (100 microM and 1 mM) partially blocked the adenosine response. This study indicates the involvement of metabotropic glutamate receptors in adenylate cyclase inhibition induced by glutamate and its agonists trans-ACPD and kainate.

1S,3R-ACPD-preferring inward current in rat dorsolateral septal neurons is mediated by a novel excitatory amino acid receptor

Metabotropic glutamate receptors (mGluRs) form a receptor family that consists of diverse receptor subtypes; now, numbering 8--exclusive of splice variants. (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) has been suggested to be a selective agonist for the mGluRs. We have recently reported that, in rat dorsolateral septal nucleus (DLSN) neurones, a 1S,3R-ACPD-preferring inward current (ACPDi) persists in pertussis toxin-treated rats. We now report that this ACPDi-current: (1) persists in DLSN neurones dialyzed with a stable analog of GTP, namely, GTP gamma S; (2) exhibits a negative slope region with inward rectification in its I-V relationship; (3) persists in neurones superfused with tetrodotoxin or low calcium solutions; (4) is dependent upon both sodium and calcium ions; and (5) is independent of a reduction in temperature. Furthermore, pharmacological data suggest that this current may be activated by a unique type of excitatory amino acid (EAA) receptor, i.e. a receptor which prefers "metabotropic" EAA agonists and is insensitive to AP5 or CNQX. Activation by ACPD of inward currents associated with a conductance increase have also been reported at cultured mouse cerebellar Purkinje neurones; in slices of rat hippocampal CA1 neurones and slice cultures of hippocampal CA3 neurones. We suggest that this ACPDi current may play an important role within the CNS in the induction of long-term potentiation and other neurological processes; processes attributed previously to currents associated with NMDA receptor activation.

Intrastriatal and intrasubthalamic stimulation of metabotropic glutamate receptors: a behavioral and Fos immunohistochemical study

Prior work has shown that intrastriatal injection of the metabotropic glutamate receptor agonist 1S,3R-ACPD results in pronounced contralateral rotation, and the basis for this effect is thought to be increased activity of dopaminergic nigrostriatal neurons. We tested this hypothesis by determining the expression of Fos-like immunoreactivity after intrastriatal injection of 1S,3R-ACPD. Intense Fos-like immunoreactivity was noted in the globus pallidus, entopeduncular nucleus, subthalamic nucleus and substantia nigra pars reticulata. Ablation of the subthalamic nucleus 10 days prior to intrastriatal injection of 1S,3R-ACPD abolished rotational behaviour but not Fos-like immunoreactivity in the globus pallidus, entopeduncular nucleus and substantia nigra. Intrasubthalamic injection of 1S,3R-ACPD produced marked contralateral rotation and a pattern of Fos-like immunoreactivity similar to that seen after intrastriatal 1S,3R-ACPD injection. These results suggest that stimulation of striatal metabotropic glutamate receptors inhibits striatal projection neuron activity, while stimulation of subthalamic metabotropic glutamate receptors increases subthalamic nucleus activity. Increased subthalamic nucleus activity is necessary and sufficient for the expression of rotational behavior. These results also suggest that metabotropic glutamate receptor antagonists may be useful in the treatment of Parkinson's disease.

Pharmacological differentiation of the effects of co-activation of beta-adrenergic and metabotropic glutamate receptors in rat hippocampus

Activation of metabotropic glutamate receptors (mGluRs) can potentiate the cAMP response elicited by activation of beta-adrenergic receptors (beta ARs) in the hippocampus. We have shown that co-activation of mGluRs and beta ARs induces both an acute depression of excitatory synaptic transmission and a long-lasting excitation of CA1 pyramidal cells. However, these studies were performed using a non-selective mGluR agonist. We have now used subtype selective mGluR agonists, and report that while the acute depression of transmission exhibits a pharmacology consistent with mediation by this mGluR subtype, the lasting excitation of CA1 pyramidal cells may be mediated by an interaction between beta ARs and mGluRs that are coupled to phosphoinositide hydrolysis.

The metabotropic glutamate receptors: structure and functions

Glutamate is the main excitatory neurotransmitter in the brain. For many years it has been considered to act only on ligand-gated receptor channels--termed NMDA, AMPA and kainate receptors--involved in the fast excitatory synaptic transmission. Recently, glutamate has been shown to regulate ion channels and enzymes producing second messengers via specific receptors coupled to G-proteins. The existence of these receptors, called metabotropic glutamate receptors, is changing our views on the functioning of fast excitatory synapses.

Stimulation of postsynaptic and inhibition of presynaptic adenylyl cyclase activity by metabotropic glutamate receptor activation

To determine the subcellular distribution of cyclic AMP-coupled metabotropic glutamate receptors (mGluRs), the effects of glutamate agonists on adenylyl cyclase activity were examined using two hippocampal membrane preparations. These were synaptosomes (SY), which are composed of presynaptic terminals, and synaptoneurosomes (SN), which are composed of both pre- and postsynaptic elements. In SY, a water-soluble analogue of forskolin (7 beta-forskolin) increased enzyme activity approximately 10-fold at the highest concentration tested. The selective metabotropic receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) inhibited enzyme activity as did glutamate and quisqualate. L-Amino-4-phosphobutanoate (L-AP4) had no effect on enzyme activity at any concentration tested. The metabotropic receptor antagonist L-2-amino-3-phosphopropionic acid (L-AP3) was not effective in the SY in antagonizing the agonist-induced decreases in adenylyl cyclase activity by glutamate or 1S,3R-ACPD. It was, however, effective at antagonizing quisqualate-induced decreases in enzyme activity. In SN, at the highest concentration tested, 7 beta-forskolin produced a 60-fold increase in adenylyl cyclase activity. As was observed in SY, glutamate decreased adenylyl cyclase activity in SN. In contrast, 1S,3R-ACPD, quisqualate, and L-AP4 increased adenylyl cyclase activity. In the SN, L-AP3 was ineffective in antagonizing any agonist-induced increases (1S,3R-ACPD, L-AP4, and quisqualate) or decreases (glutamate) in adenylyl cyclase activity. The data suggest that postsynaptic metabotropic glutamate receptor activation results in stimulation of adenylyl cyclase activity, whereas inhibition of this enzyme appears to be mediated at least partly through presynaptic mechanisms.

Metabotropic glutamate receptor agonist ACPD inhibits some, but not all, muscarinic-sensitive K+ conductances in basolateral amygdaloid neurons

Muscarinic agonists produce membrane depolarization and losses of spike frequency accommodation and the slow afterhyperpolarization (AHP) when applied to neurons of the basolateral amygdala (BLA). Underlying these changes are the muscarinic-induced inhibitions of several K+ conductances, including the voltage-activated M-current (IM), a slowly decaying Ca(2+)-activated current (IAHP), a voltage-insensitive leak current (ILeak), and the hyperpolarization-activated inward rectifier current (IIR). Similar depolarizations and losses of the slow AHP have been observed in other neuronal cell types following stimulation of metabotropic glutamate receptors. Therefore, we tested the effects of the metabotropic glutamate receptor agonist, 1-aminocyclopentane-1s,3R-dicarboxylic acid (ACPD), on pyramidal neurons impaled with a single microelectrode for current- and voltage-clamp recordings in a brain slice preparation of the rat BLA. Application of ACPD (20 or 100 microM) to BLA neurons inhibited IM and IAHP, resulting in membrane depolarization and reductions in the amplitude and duration of the slow AHP. However, ACPD did not inhibit the muscarinic-sensitive current IIR, nor was ILeak blocked in the majority of neurons examined. These findings suggest the possibility that muscarinic cholinergic and metabotropic glutamatergic receptor agonists may activate separate intracellular transduction pathways which have convergent inhibitory effects onto IM and IAHP in BLA pyramidal neurons.

The cholinergic inhibition of afterhyperpolarization in rat hippocampus is independent of cAMP-dependent protein kinase

The possible involvement of protein kinase A (PKA) in the muscarinic inhibition of the slow afterhyperpolarizing current (IAHP) was investigated in rat hippocampal pyramidal cells. IAHP was recorded using the whole cell method in hippocampal slices, and Rp-cAMPS, a PKA antagonist, was applied intracellularly. The inhibition of IAHP by carbachol was not affected by Rp-cAMPS. In contrast, Rp-cAMPS reduced the cAMP-dependent inhibition of IAHP by norepinephrine. The results show that phosphorylation by PKA does not contribute to the muscarinic effect on IAHP.

Interaction between metabotropic receptors and purinergic transmission in rat hippocampal slices

Inhibition of forskolin-stimulated cAMP formation by (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) in rat hippocampal slices was partially obliterated by the adenosine-depleting enzyme, adenosine deaminase, or by the adenosine receptor agonist, 5'-(N-ethylcarboxamido)-adenosine, suggesting that activation of metabotropic glutamate receptors (mGluRs) modulates the release of endogenous adenosine. Consistent with this hypothesis, forskolin stimulated the release of purines from rat hippocampal slices, and this effect was reduced by 1S,3R-ACPD. To establish which transduction pathway is involved in the modulation of forskolin-stimulated purine release, we have tested the novel mGluR2 agonist, (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV), which reduced forskolin-stimulated cAMP formation but, as opposed to 1S,3R-ACPD, did not stimulate polyphosphoinositide hydrolysis. DCG-IV was highly potent and more efficacious than 1S,3R-ACPD in inhibiting forskolin-stimulated purine release. Neither DCG-IV nor 1S,3R-ACPD reduced the release of purines stimulated by depolarizing concentrations of K+, suggesting that their effect was stimulus-specific. These results indicate that, in rat hippocampal slices, activation of mGluR2 receptors attenuates the release of purines induced by forskolin, a process that amplifies the final effect of forskolin on cAMP formation as a result of A2 purinergic receptor activation. Thus, the final effect of mGluR agonists on forskolin-stimulated cAMP formation in hippocampal slices depends on both a direct inhibition of adenylyl cyclase and the inhibition of adenosine release.

Potentiation of cAMP responses by metabotropic glutamate receptors depresses excitatory synaptic transmission by a kinase-independent mechanism

Coactivation of metabotropic glutamate receptors (mGluRs) and beta-adrenergic receptors causes a synergistic increase in cAMP formation in the rat hippocampus. Increases in cAMP are known to have many actions in the hippocampus via activation of cAMP-dependent protein kinase. We now report that coactivation of mGluRs and beta-adrenergic receptors induces an acute depression of EPSCs at the Schaffer collateral-CA1 synapse. Interestingly, this depression of EPSCs is dependent upon increases in cAMP levels but independent of protein kinase activity. A series of studies suggests that cAMP-mediated depression of EPSCs is dependent on metabolism of cAMP and release of adenosine or 5'-AMP into the extracellular space with resultant activation of presynaptic adenosine receptors. These studies suggest that cAMP can have local hormone-like effects in the hippocampal formation which are independent of cAMP-dependent protein kinase.

Trans-(+-)-1-amino-1,3-cyclopentanedicarboxylate (trans-ACPD) stimulates cAMP accumulation in rat cerebral cortical slices but not in glial or neuronal cultures

Recent cloning experiments indicate that multiple metabotropic receptors for excitatory amino acids (EAAs) exist, which are coupled to adenylate cyclase. Trans-(+-)-1-amino-1,3-cyclopentanedicarboxylate (trans-ACPD) is a selective agonist of metabotropic receptors for EAAs. One of the effects of trans-ACPD is stimulation of cAMP accumulation. In the present experiments, cAMP accumulation was measured using a [3H]adenine-prelabelling technique. It has been found that trans-AC-PD was able to induce significant stimulation of cAMP accumulation in rat cerebral cortical slices, with ED50 of 47.8 microM, which value is similar to that described earlier for hippocampal slices. However, trans-ACPD had no effect on cAMP accumulation either in primary neuronal or glial cell cultures. The reason for the lack of effects of trans-ACPD on cAMP accumulation in primary cultures from glial cells and neurons is discussed.

Metabotropic glutamate receptor modulation of cAMP accumulation in the neonatal rat hippocampus

The pharmacology and cellular mechanism by which metabotropic glutamate receptor (mGluR) activation modulates cAMP formation was studied in cross-chopped hippocampal slices from neonatal (7 day old) rats. The selective mGluR agonist 1S,3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), and other non-selective mGluR agonists produced concentration-related stimulation of basal cAMP formation in this tissue. The relative agonist potency order was 1S,3R-ACPD = quisqualate > ibotenate >> 1R,3S-ACPD. 1S,3R-ACPD stimulated cAMP accumulation was antagonized in a stereoselective manner by L-2-amino-3-phosphonopropionate (L-AP3), but not by higher chain homologues such as L-2-amino-4-phosphonobutyrate (L-AP4) and 2-amino-5-phosphonopentanoate (AP5). 1S,3R-ACPD-enhanced cAMP formation was greatly inhibited by incubation with adenosine deaminase. In the adult rat hippocampus, 1S,3R-ACPD did not appreciably increase basal cAMP, but inhibited forskolin-stimulated cAMP formation, and this effect was observed with or without adenosine deaminase. In the presence of the adenosine receptor antagonist and cAMP phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (IBMX), 1S,3R-ACPD did not enhance cAMP formation in the neonatal hippocampus, but inhibited forskolin-stimulated cAMP (like in the adult tissue). These results demonstrate that mGluRs that increase cAMP in the neonatal hippocampus have a unique pharmacology when compared to mGluRs that decrease cAMP accumulation and increase phosphoinositide hydrolysis. 1S,3R-ACPD stimulation of cAMP in the neonatal rat hippocampal slice involves potentiation of responses to endogenous adenosine. Negatively coupled cAMP linked mGluRs are also present in the neonatal tissue, but are masked by the predominance of the positively coupled mGluR cAMP response.

(1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid attenuates N-methyl-D-aspartate-induced neuronal cell death in cortical cultures via a reduction in delayed Ca2+ accumulation

The effects of (1S,3R)-ACPD, a selective metabotropic glutamate receptor agonist, on NMDA-induced 45Ca2+ accumulation and delayed neuronal cell death were determined using primary cerebrocortical cultures. Exposure to (1S,3R)-ACPD alone, although causing small increases in 45Ca2+ accumulation, was not neurotoxic. The presence of (1S,3R)-ACPD during exposure to NMDA attenuated the resulting sustained accumulation of 45Ca2+ and delayed neuronal cell death. Reductions in sustained Ca2+ accumulation were associated both with Ca2+ efflux, in the absence of cell death, and inhibition of delayed intracellular Ca2+ accumulation. The protective effects of (1S,3R)-ACPD on NMDA-induced cell death were inhibited by pretreatment of cultures with pertussis toxin. These results suggest that activation of metabotropic glutamate receptors may stimulate intracellular processes capable of limiting sustained elevations in intracellular calcium and the resulting excitotoxic neuronal damage.

Intracerebral 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) produces limbic seizures that are not blocked by ionotropic glutamate receptor antagonists

The functional role of metabotropic glutamate receptor (mGluR) activation was investigated following intracerebral administration of 1S,3R-ACPD in mice. Injections of 1S,3R-ACPD (50-800 nmol in 5 microliters) into the thalamus produced a dose-dependent increase in limbic seizures. These effects were stereoselective since 1R,3S-ACPD, did not elicit seizure activity. Pharmacologically, limbic seizures were attenuated by the mGluR partial agonist/antagonist L-2-amino-3-phosphonopropionate (L-AP3) and dantrolene, an inhibitor of intracellular calcium mobilization, but not by D-AP3 or ionotropic glutamate receptor antagonists (MK-801 or GYKI-52466). Thus, activation of mGluRs by 1S,3R-ACPD in mice, induces limbic seizures that may involve the mobilization of intracellular calcium stores.

Daily variation in active glycogen phosphorylase patches in the molecular layer of rat dentate gyrus

A larger number of discrete patches of active glycogen phosphorylase (alpha GP) were found in the molecular layer of the hippocampal dentate gyrus in rats sacrificed during the daily dark phase (mean = 28.7/section) than during the light phase (mean = 7.8/section). Light-dark differences in the patterns of alpha GP may reflect circadian differences in metabolic demand in the hippocampus. Patch sizes were consistent with increased activation of single astrocytes or perisynaptic astrocyte clusters by focal input at night.

Characterization of metabotropic glutamate receptors negatively linked to adenylyl cyclase in brain slices

We have characterized the pharmacological profile of activation of metabotropic glutamate receptors negatively linked to adenylyl cyclase (mGluR decreases cAMP) in brain slices. Among the putative mGluR agonists, (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), were the most potent inhibitors of forskolin-stimulated cAMP formation in hippocampal slices, followed by ibotenate, L-2-amino-3-phosphonopropionate (AP3), quisqualate, L-glutamate and beta-N-methylamino-L-alanine (BMAA). Inhibition of forskolin-stimulated cAMP formation by DL-2-amino-4-phosphonobutanoate (AP4) was biphasic, suggesting that the drug interacts with more than one mGluR decreases cAMP subtype. Both L-AP4 and L-serine-O-phosphate (a restricted analogue of AP4) were much more effective in inhibiting forskolin-stimulated cAMP formation than their D-isomers, indicating that interaction of these drugs with the mGluR decreases cAMP is stereoselective. Despite the fact that DCG-IV and ibotenate behave as NMDA receptor agonists, their effect was insensitive to MK-801. The regional pattern of expression of mGluR decreases cAMPS, as estimated by using 1S,3R-ACPD as an agonist, did not correlate with the steady-state levels of mGluR2 mRNA. Thus, 1S,3R-ACPD inhibited forskolin-stimulated cAMP in slices from hippocampus, cerebral cortex, corpus striatum, olfactory tubercle or hypothalamus, but not in slices from olfactory bulb or cerebellum; in contrast, mGluR2 mRNA levels were high in the olfactory bulb and very low in the corpus striatum. 1S,3R-ACPD also inhibited forskolin-stimulated cAMP formation in cortical membranes, excluding the involvement of trans-synaptic mechanisms in the activity of mGluR decreases cAMPS.(ABSTRACT TRUNCATED AT 250 WORDS)

Excitotoxic neuronal damage and neuropsychiatric disorders

Excitatory amino acids (EAA) serve important physiological functions in the vertebrate CNS, including participation in fast excitatory synaptic transmission, modulation of synaptic plasticity and regulation of neuronal morphology during development. However, paradoxically they also harbor neurotoxic (excitotoxic) potential, which, if unleashed, can cause widespread degeneration of CNS neurons. Accumulating evidence suggests a role for excitotoxins in a variety of human neuropsychiatric disorders. This paper reviews the classes of EAA receptors in the CNS, the mechanisms underlying EAA-mediated neuronal damage and the role of EAA in specific human disorders.

Functional evidence for a L-AP3-sensitive metabotropic receptor different from glutamate metabotropic receptor mGluR1

The efficacy of mGluR agonists quisqualate and 1S,3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) in stimulating the inositol phosphate (IP) formation in primary cultures of cerebellar granule neurons correlated with mGluR1 mRNA expression and was affected by the medium KCl content. L-2-Amino-3-phosphonopropionic acid (L-AP3) mimicked the stimulatory action of mGluR agonists. Maximal stimulatory doses of mGluR agonist 1S,3R-ACPD and L-AP3 were additive, suggesting the action of L-AP3 on a receptor different from mGluR1. Indeed, in embryonic kidney 293 cells transfected with mGluR1 cDNA quisqualate and 1S,3R-ACPD but not L-AP3 stimulated the IP formation.

A postsynaptic G-protein in hippocampal long-term potentiation

The effects of guanosine triphosphate (GTP)-binding protein (G-protein) blockade on hippocampal LTP at stratum radiatum-CA1 synapses was studied. Bath application of 20 mM lithium chloride (LiCl) inhibited long-term potentiation (LTP) of extracellularly-recorded excitatory postsynaptic potentials (EPSPs). Inclusion of 100 mM LiCl in intracellular recording electrodes was shown to block postsynaptic G-proteins by bath-application of baclofen, an agonist at the G-protein linked gamma-aminobutyric acid (GABAB) receptor. Under normal conditions, GABAB receptor activation causes a hyperpolarization postsynaptically, and a decrease in neurotransmitter release presynaptically. With LiCl in the recording electrodes, the postsynaptically-mediated hyperpolarization was blocked, while the presynaptically-mediated depression of EPSPs was unaffected. With postsynaptic G-proteins blocked in this manner, LTP at these synapses was inhibited. These studies provide evidence for the involvement of a postsynaptic G-protein in LTP of stratum radiatum-CA1 synapses.

Endogenous adenosine regulates the apparent efficacy of 1-aminocyclopentyl-1S,3R-dicarboxylate inhibition of forskolin-stimulated cyclic AMP accumulation in rat cerebral cortical slices

In rat cerebral cortical slices, the 1-aminocyclopentyl-1S,3R-dicarboxylate (1S,3R-ACPD) isomer of the selective metabotropic excitatory amino acid agonist ACPD inhibited forskolin-stimulated cyclic AMP (cAMP) accumulation in a concentration-dependent manner with a maximal inhibition of 51 +/- 3% and a half-maximally effective concentration of 8.8 +/- 3.4 microM. Similarly, 1R,3S-ACPD inhibited the forskolin response in a concentration-dependent manner, but with an inhibition of 80 +/- 5% at 3 mM. In addition to inhibiting forskolin-stimulated cAMP levels, 1S,3R-ACPD, but not 1R,3S-ACPD, enhanced the cAMP response to A2b adenosine receptor activation. In the presence of 1.2 U/ml of adenosine deaminase (included to reduce the contribution of endogenous adenosine), the efficacy of 1S,3R-ACPD was increased (88 +/- 3% inhibition), but the potency was unchanged. The adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine also increased the inhibitory effect of 100 microM 1S,3R-ACPD, from 57 +/- 1 to 78 +/- 5%. These results indicate that endogenous adenosine plays an important role in regulating the apparent efficacy of 1S,3R-ACPD inhibition of forskolin-stimulated cAMP accumulation in rat cerebral cortical slices and that previous studies in rat hippocampus and hypothalamus in the absence of added adenosine deaminase may have underestimated the efficacy of this compound.

In rats, the metabotropic glutamate receptor-triggered hippocampal neuronal damage is strain-dependent

The effect of intrahippocampal (i.h.) and intraocular (i.o.) administration of the selective metabotropic glutamate receptor (mGluR) agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) was studied in different rat strains. A massive hippocampal damage was observed in CD/SD and Fischer 344 but not in SD/Rij and Brown Norway rats 7 days following the i.h. injection of 1S,3R-ACPD, while no retinal damage was observed following its i.o. administration. Moreover, 1S,3R-ACPD reduced the N-methyl-D-aspartate (NMDA) toxicity in the retina of both CD/SD and SD/Rij rats. Regardless of its toxic action on hippocampal neurons the i.h. injection of 1S,3R-ACPD caused an acute stimulation of motor activity in both CD/SD and SD/Rij rats. This effect was blocked by the intracerebroventricular (i.c.v.) administration of the putative mGluR antagonist L-2-amino-3-phosphono-propionic acid (L-AP3). It is suggested that the differential expression of mGluR subtypes might determine their role in brain pathology.

Metabotropic glutamate receptors in brain function and pathology

Metabotropic glutamate receptors (mGluRs) are a novel family of recently cloned G protein-coupled receptors. These receptors are heterogeneous and coupled to multiple second messenger systems that include increases in phosphoinositide hydrolysis, activation of phospholipase D, decreases in cAMP formation, increases in cAMP formation, and changes in ion channel function. Using the selective mGluR agonist 1-aminocyclopentane-1,3-dicarboxylic acid (1s,3R-ACPD), considerable progress has been made towards understanding the role of this glutamate receptor class in the central nervous system. This article reviews the molecular aspects and pharmacology of mGluRs, and recent studies elucidating their role in brain function and pathology.

Metabotropic excitatory amino acid receptor activation stimulates phospholipase D in hippocampal slices

Metabotropic excitatory amino acid (EAA) receptors are coupled to effector systems through G proteins. Because various G protein-coupled receptors stimulate the hydrolysis of phosphatidylcholine by phospholipase D (PLD), we examined the possibility that metabotropic EAA receptors exist that are coupled to the activation of PLD. We found that the selective metabotropic glutamate receptor (mGluR) agonists 1S,3R-amino-1,3-cyclopentanedicarboxylic acid (ACPD) and 1S,3S-ACPD, but not the inactive isomer, 1R,3S-ACPD, induce a concentration-dependent increase in PLD activity in hippocampal slices. Selective ionotropic glutamate receptor (iGluR) antagonists did not block 1S,3R-ACPD-induced PLD stimulation. Furthermore, although selective iGluR agonists did not activate this response, the nonselective mGluR-iGluR agonists, ibotenate and quisqualate, caused significant increases in PLD activity (all in the presence of iGluR antagonists). L-2-Amino-3-phosphonopropionic acid, which blocks the mGluR that is coupled to phosphoinositide hydrolysis in various brain regions, activates PLD to the same extent as the active isomers of ACPD. These data suggest that metabotropic EAA receptors exist in hippocampus that are coupled to PLD activation and are pharmacologically distinct from phosphoinositide hydrolysis-coupled mGluRs.