Stimulation of phosphoinositide hydrolysis by ibotenic acid in cerebellar neurons via two glutamate receptor subtypes

Antagonist-induced supersensitivity of mGlu1 receptor signalling in cerebellar granule cells

We investigated the effect of 24 h sustained treatment with the mGlu1 receptor antagonist CPCCOEt on mGlu1 receptor signalling in primary cultures of rat cerebellar granule cells. In the absence of ionotropic glutamate (iGlu) blockers, the maximal inositol phosphate (IP) response (E(max)) but not the potency of glutamate was significantly increased when cells were pre-exposed for 24 h with CPCCOEt. When the contribution of iGlu receptors to the glutamate-induced IP response was eliminated with the use of DNQX, the E(max) was again increased but also the concentration eliciting 50% of the maximal glutamate stimulus was significantly decreased. In the absence of iGlu receptor inhibitors, the E(max) of quisqualate, which likely mediates IP accumulation only via the mGlu1 receptor, was significantly increased in CPCCOEt-pretreated cells. Also, less quisqualate was needed to reach the same IP effect. The potency of R193845, a selective mGlu1 receptor antagonist, was significantly decreased in antagonist-pretreated cells. These findings demonstrate that 24 h sustained antagonist treatment can render mGlu1 receptors in neurons supersensitive to agonists, with a concomitant decrease in the effectiveness of antagonists.

The neural mechanisms underlying cholinergic cell death within the basal forebrain

The basal forebrain region includes a large group of cholinergic neurons within the medial septal area and nucleus basalis magnocellularis (NBM) that project to the hippocampus and throughout the neocortex, respectively. This chapter will consider the mechanisms that influence why cholinergic cells within the NBM die and discuss studies that have manipulated the features of these cells that could make them differentially vulnerable to degeneration with aging and Alzheimer's Disease (AD). This chapter will focus upon the NBM cholinergic system because this regions typically demonstrates a greater degree of cell loss with aging and AD.

Prenatal ethanol exposure enhances glutamate release stimulated by quisqualate in rat cerebellar granule cell cultures

Effects of prenatal ethanol exposure on extracellular glutamate accumulation stimulated by glutamate receptor agonists were studied in rat cerebellar granule cell cultures. The prenatal exposure to ethanol was achieved via maternal consumption of a Sustacal liquid diet containing either 5% ethanol or isocaloric sucrose (pair-fed) substituted for ethanol from gestation d 11 until the day of parturition. Neither the basal level of extracellular glutamate nor the increased accumulation of glutamate stimulated by KCl (40 mM) or by ionotropic glutamate receptor agonists, N-methyl-D-aspartate (NMDA) or kainate (KA) (100 microM each), in cells prepared from the ethanol-fed group was significantly different from that in cells prepared from the pair-fed group. Glutamate accumulation stimulated by quisqualate (QA, 100 microM) or by trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (t-ACPD, 250 microM) in the ethanol-fed group was higher than that in the pair-fed group by 116 and 36%, respectively. In the presence of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 100 microM), an ionotropic QA receptor antagonist, the QA-induced accumulation of glutamate in the ethanol-fed group was still higher than that in the pair-fed group. In the presence of MK-801 (5 microM), an antagonist of the NMDA receptor, the enhanced accumulation of glutamate stimulated by either QA or t-ACPD was still observable in the ethanol-fed group as compared to the pair-fed group. Addition of (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG, 500 microM), a selective antagonist of the metabotropic glutamate receptor, abolished the enhanced accumulation of glutamate stimulated by either QA or t-ACPD in the ethanol-fed group. Although immunoblotting of mGluR1 and mGluR2/3 did not show apparent differences between the pair-fed and the ethanol-fed groups, the overall results suggest that the effect of prenatal ethanol exposure was selectively through a pathway mediated by the metabotropic glutamate receptor.

Intrauterine hypoxia-ischemia reduces phosphoinositide hydrolysis stimulated by metabotropic glutamate receptor agonists in cultured rat cerebellar granule cells

Effects of intrauterine hypoxia-ischemia (HI) on receptor-stimulated phosphoinositide (PPI) hydrolysis were studied in rat cerebellar granule cell cultures prepared from an in utero HI model. On gestation day 17, HI conditions were achieved by complete clamping of the uterine vasculature for 30 min followed by removal of the clamps to permit reperfusion. Sham operation (SH, surgery without vasculature ligation) was performed as the control. Intrauterine HI did not affect the basal level of PPI hydrolysis (in the absence of stimulants) in cells prepared from either the SH or the HI group. PPI hydrolysis stimulated by quisqualate (QA) or trans-(1S,3R)-1-amino-1,3-cyclo-pentanedicarboxylic acid (trans-ACPD) was significantly reduced in cells prepared from the HI group, whereas intrauterine HI did not affect the PPI hydrolysis induced by ionotropic glutamate receptor agonists or by norepinephrine or serotonin. At a dose range of 100-300 microM, QA-stimulated PPI hydrolysis in cells prepared from the SH group increased by 3-to 4.5-fold, while this increase was only 2- to 2.5-fold in cells prepared from the HI group. Presence of L-NG-monomethyl-arginine (L-NMMA), a nitric oxide (NO) synthase inhibitor, did not increase QA-stimulated PPI hydrolysis in cells prepared from either the SH or the HI group, indicating that stimulation of NO formation is unlikely involved in the suppressive effects of intrauterine HI on QA-induced PPI hydrolysis. The QA-stimulated PPI hydrolysis in cells prepared from the HI group, but not from the SH group, was further inhibited by L-(+)-2-amino-3-phosphono-propionic acid (L-AP3). The overall results suggest that intrauterine HI has long-lasting suppressive effects on metabotropic glutamate receptor agonist-stimulated PPI hydrolysis and these effects might be associated with alterations in expression of metabotropic glutamate receptor subtypes.

Neuroprotection and selective vulnerability of neurons within the nucleus basalis magnocellularis

Neurons within the nucleus basalis may die due to their selective vulnerability to endogenous excitatory amino acid neurotransmitters, nitric oxide and free radicals. The factors influencing the selective vulnerability of neurons within the nucleus basalis depend upon many different factors related to the presence of these agents and the neuron's ability to defend itself against the consequences of exposure. Many different mechanisms have been investigated to provide neuroprotection for neurons within the nucleus basalis and throughout the central nervous system. This review summarizes the results of studies that have investigated our current capability to either attenuate the neurotoxicity of endogenous excitatory amino acids, such as glutamate, or to provide effective neuroprotection during circumstances of neurotoxin exposure.

Prenatal ethanol exposure reduces phosphoinositide hydrolysis stimulated by quisqualate in rat cerebellar granule cell cultures

Prenatal ethanol exposure-induced alteration in poly-phosphoinositide (PPI) hydrolysis stimulated by excitatory amino acids (EAA) was studied in rat cerebellar granule cells previously labeled with [3H]myoinositol. The prenatal exposure to ethanol was achieved via maternal consumption of a Sustacal (chocolate flavored) liquid diet containing either 5% ethanol (w/v, 35% of calories) or isocaloric sucrose (pair-fed) substituted for ethanol from gestation d 11 until the day of parturition. The ionotropic glutamate receptor agonists, N-methyl-D-aspartate, kainate or (+/-)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) (100 microM each) induced a two- to four-fold increase in PPI hydrolysis over the basal level, regardless of the liquid dietary treatment. Stimulation with quisqualate (QA), an agonist activating both metabotropic and ionotropic glutamate receptors, resulted in a much stronger and dose-dependent response in PPI hydrolysis and exposure in utero to ethanol significantly reduced this response. Tetrodotoxin, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), or (+/-)-3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid (CPP) had no effect on QA-stimulated PPI hydrolysis nor on the suppression of this hydrolysis by ethanol. Exposure in utero to ethanol did not affect PPI hydrolysis stimulated by a selective metabotropic glutamate receptor agonist, trans-(+/-)-l-amino-1,3-cyclopentanedicarboxylic acid (t-ACPD). Although the PPI hydrolysis stimulated by t-ACPD could be blocked by (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG), an antagonist of the metabotropic glutamate receptor, MCPG was incapable of affecting QA-induced PPI hydrolysis and the suppressive effects of prenatal ethanol exposure on this hydrolysis. Taken together, the data suggest that the long-lasting suppressive effects of prenatal ethanol exposure on QA-stimulated PPI hydrolysis in cerebellar granule cell cultures is through a metabotropic QA receptor pathway that may be different from the one activated by t-ACPD.

Investigations of neurotoxicity and neuroprotection within the nucleus basalis of the rat

The present study investigated the specific ways by which cytotoxicity due to glutamate receptor stimulation could be attenuated by the administration of agonists and antagonists of the ionotropic and metabotropic glutamate receptors within the nucleus basalis magnocellularis (NBM) of rats as measured by cortical choline acetyltransferase activity. The results of these studies suggest that (1) the cytotoxicity of ibotenate to NBM cholinergic cells is not dependent upon stimulation of metabotropic glutamate receptors, but results from activation of N-methyl-D-aspartate (NMDA) receptors, (2) the cytotoxicity of quisqualate to cholinergic cells within the NBM is not dependent upon stimulation of NMDA or metabotropic receptors, and (3) the cytotoxicity of NMDA was prevented by administration (i.p.) of the un-competitive NMDA antagonist memantine (30 mg/kg), resulting in plasma levels of 2.5 micrograms/ml, a concentration known to block efficiently NMDA receptors in vitro. Finally, performance of a food-motivated, delayed-alternation task on a T-maze was impaired by injections of NMDA into the NBM, but was prevented by co-administration of NMDA with memantine.

Acute and long-term inhibition of agonist-stimulated phosphoinositide hydrolysis by pulse treatment of cerebellar granule cells with TPA

Acute pretreatment (30 min) of primary cultures of cerebellar granule cells with TPA (10 nM) resulted in a decrease in carbachol-and glutamate-stimulated phosphoinositide hydrolysis, but not in basal levels of PI hydrolysis. To investigate the mechanism of TPA action, phospholipase C was assayed in membranes prepared from cerebellar granule cells acutely treated with TPA. TPA had no effect on basal, GTP gamma S-, NaF-, and calcium-stimulated phospholipase C when compared with membranes prepared from vehicle-treated cells. The effects of pulsing with TPA (30-min pulse, 10 nM) on agonist-stimulated PI hydrolysis were studied 1, 3, and 5 or 6 d after TPA treatment. TPA treatment results in a statistically significant decrease in glutamate-stimulated PI hydrolysis, and a slight reduction of carbachol-stimulated PI hydrolysis when compared to temporally matched controls. Measurements in membranes prepared from TPA-treated vs control cells 1, 3, and 5 d after treatment showed that calcium- and NaF-stimulated phospholipase C activity was significantly decreased at all days tested, whereas GTP gamma S-stimulated phospholipase C activity was significantly decreased only at d 3. These data demonstrate differences in the acute vs long-term effects of TPA treatment on agonist-stimulated PH hydrolysis, and suggest that the acute effects may be mediated at the level of the receptor, whereas long-term effects of TPA on PI hydrolysis may be mediated by deficits in effector function.

Sulphur-containing excitatory amino acid-stimulated inositol phosphate formation in primary cultures of cerebellar granule cells is mediated predominantly by N-methyl-D-aspartate receptors

The stimulatory effect of excitatory sulphur-containing amino acids on inositol phosphate formation was investigated in primary cultures of cerebellar granule cells. L-Cysteine sulphinate (CSA), L-cysteate (CA), L-homocysteine sulphinate (HCSA), L-homocysteate (HCA) and S-sulpho-L-cysteine (SSC) dose-dependently stimulated the formation of [3H]inositol phosphates exhibiting EC50 values in the range 60-200 microM and maximal effects of six- to 17-fold that of basal [3H]inositol phosphate levels. Endogenous L-glutamate spontaneously released into the extracellular medium or following exposure of cells to HCSA, HCA or SSC did not contribute significantly to formation of [3H]inositol phosphates, whereas 10% of the total [3H]inositol phosphates accumulated following exposure to CSA and CA was due to released L-glutamate. The selective N-methyl-D-aspartate receptor antagonist, D,L-2-amino-5-phosphonopentanoic acid (APV, 500 microM) attenuated by 20% (HCSA) to between 80 and 100% (CSA, CA, SSC, HCA) the formation of [3H]inositol phosphates induced by 1 mM sulphur-containing amino acids. When, however, HCSA was used at 100 microM (a concentration near to its EC50 for phosphoinositide hydrolysis), APV inhibited induced responses by 70%. Sulphur-containing amino acid-stimulated [3H]inositol phosphate formation was unaffected by the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM). Inhibition of sulphur-containing amino acid-stimulated [3H]inositol phosphate formation by co-administration of APV and CNQX was similar to that obtained in the presence of APV alone. CSA-, CA-, SSC- and HCA-stimulated [3H]inositol phosphate formation was markedly reduced by removal of Ca2+ from the extracellular medium whereas that stimulated by HCSA was less affected. A similar inhibitory profile was observed when the levels of sulphur-containing amino acid-induced increases in intracellular free calcium ([Ca2+]i) were measured in the presence of 500 microM APV; 1 mM HCSA-induced responses being inhibited by only 30% whereas responses to the remaining sulphur-containing amino acid (also at 1 mM) were inhibited by > 45%. When the sulphur-containing amino acids were used at concentrations approximating their EC50 values for phosphoinositide hydrolysis, APV inhibited the induced increases in [Ca2+]i by 70-100%. HCA and SSC co-administered with the less efficacious but selective metabotropic glutamate receptor agonist, (+-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD) at maximally effective concentrations (1 mM) of each agonist stimulated [3H]inositol phosphate formation in an additive manner.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacological characterization of metabotropic glutamate receptors in cultured cerebellar granule cells

A detailed pharmacological characterization of metabotropic glutamate receptors (mGluR) was performed in primary cultures of cerebellar granule cells at 6 days in vitro (DIV). The rank order of agonists induced polyphosphoinositide (PPI) hydrolysis (after correcting for the ionotropic component in the response) was as follows: in terms of efficiency, Glu > quisqualate (quis) = ibotenate (ibo) > (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylic acid (ACPD) > beta-methyl-amino-L-alanine (BMAA) and in terms of potency, quis > ACPD > Glu > ibo = BMAA. Ionotropic excitatory amino acid (EAA) receptor agonists, such as alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) were relatively inactive (in the presence of Mg2+). Quis and ACPD-induced PPI hydrolysis was unaffected by ionotropic Glu receptor antagonists, but was inhibited, in part by L-2-amino-3-phosphonopropionate (AP3). In contrast, Glu-or ibo- induced PPI hydrolysis was reduced, in part, by both AP3 and NMDA receptor antagonists. Characteristic interactions involving different transmitter receptors were noted. PPI hydrolysis evoked by quis and 1S,3R-ACPD was not additive. In contrast, PPI hydrolysis stimulated by quis/ACPD and carbamylcholine was additive (indicating different receptors/transduction pathways). In the presence of Mg2+, the metabotropic response to quis/AMPA and NMDA was synergistic (this being consistent with AMPA receptor-induced depolarization activating NMDA receptor). On the other hand, in Mg(2+)-free buffer the effects of quis and NMDA, at concentrations causing maximal PPI hydrolysis, were additive (indicating that PPI hydrolysis was effected by two different mechanisms).(ABSTRACT TRUNCATED AT 250 WORDS)

(S)-homoquisqualate: a potent agonist at the glutamate metabotropic receptor

The synthetic quisqualate analogue, (S)-homoquisqualate was examined for activity at the glutamate metabotropic receptor, in relation to its ability to stimulate phosphoinositide hydrolysis in rat pup cerebro-cortical slices. The compound produced a robust increase in hydrolysis (EC50 = 50.2 +/- 1.6 microM), which, in common with responses to quisqualate and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate ((1S,3R)-ACPD), was antagonized uncompetitively by L-2-amino-3-phosphonopropionate (L-AP3). In contrast to quisqualate which exhibits low efficacy, (S)-homoquisqualate behaves as a full agonist at the metabotropic receptor.