Activation of a Large-conductance Ca2+-Dependent K+ Channel by Stimulation of Glutamate Phosphoinositide-coupled Receptors in Cultured Cerebellar Granule Cells

Properties of GABA and glutamate responses in identified glial cells of the mouse hippocampal slice

In this study, the patch-clamp technique was applied to brain slices to test for the presence of GABAA and glutamate receptors in glial cells of an intact tissue preparation, the hippocampus from 9-12 day old mice. Two types of glial cells were studied in the CA1 stratum pyramidale, termed passive and complex cells, which were distinct by their characteristic pattern of voltage-dependent currents. Both cell types were previously identified as glial by combining electrophysiology with ultrastructural inspection (Steinhüser et al., 1992, Eur J Neurosci 4:472-484). A subpopulation of passive cells was positive, all complex cells were negative for immunocytochemical staining against glial fibrillary acidic protein, a marker of mature astrocytes. In both cell types, GABA activated currents compatible with GABAA-receptor mediated responses. The glutamate response in complex and in most of the passive cells was mediated by a ligand-gated ion channel and closely matched the pharmacology of the kainate receptor. Activation of glutamate receptors led to a transient decrease of the resting K+ conductance in complex cells and to an irreversible decrease in the passive cells. In three passive cells, glutamate-activated currents were most likely dominated by an electrogenic uptake. In a small group of passive cells NMDA-activated currents were observed. This study provides evidence that glial cells from an intact tissue express receptors for the most abundant transmitters in the central nervous system, glutamate, and GABA.

Metabotropic glutamate receptors: an original family of G protein-coupled receptors

In 1985, we discovered a new glutamate receptor which was coupled to phospholipase C via a G protein and which was later termed metabotropic glutamate receptor (mGluR). In this review, both the diversity of mGluRs and the cellular events they control are discussed, as well as their roles in physiological regulation and brain function.

Activation of metabotropic glutamate receptors induces an inward current in rat dopamine mesencephalic neurons

To investigate the electrophysiological effects of the stimulation of the metabotropic excitatory amino acid receptors, we applied trans-1-amino-cyclopentane-1,3-dicarboxylate, an agonist of this type of receptors, on presumed rat dopamine cells intracellularly recorded in vitro. Trans-1-amino-cyclopentane-1,3-dicarboxylate (3-30 microM, t-ACPD) caused a sustained increase of the spontaneous firing rate and a depolarization. When the membrane potential was held at about the resting level (-50, -60 mV), by the single-electrode voltage-clamp technique, t-ACPD induced an inward current. In 57% of the tested cells the inward current was associated with a decrease of the apparent input conductance. In the remaining cells no obvious changes in membrane conductance were observed. The active form of t-ACPD, (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylate [3-50 microM, (1S,3R)-ACPD] also produced a reversible inward current on the dopaminergic cells and this was antagonized by (S)-4-carboxy-3-hydroxyphenylglycine (300 microM), a selective antagonist of the (1S,3R)-ACPD-induced depolarization on central neurons. The (1S,3R)-ACPD-induced inward current was not antagonized by L-2-amino-3-phosphonopropionic acid (100 microM), an antagonist of the t-ACPD-induced activation of inositide synthesis. 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM), an alfa-amino-3-hydroxy-5- methyl-isoxazole propionic acid/kainate antagonist, DL-amino-5-phosphonopentanoic acid (30 microM), an N-methyl-D-aspartate antagonist, and scopolamine (10 microM), a muscarinic antagonist, did not significantly affect the actions of t-ACPD. A block of synaptic transmission obtained by applying tetrodotoxin failed to prevent the action of t-ACPD.(ABSTRACT TRUNCATED AT 250 WORDS)

Single-channel activity in cultured cortical neurons of the rat in the presence of a toxic dose of glutamate

Rat cortical neurons grown in cell culture were exposed to 500 microM glutamate for 5 min during continuous current recording from cell-attached patches. The Ca(2+-dependence and ion selectivity of the membrane channels activated during and after glutamate application were studied in inside-out patches. Glutamate blocked spontaneous action potential firing. In 77% of the experiments glutamate activated several types of ion channels indirectly, i.e. via a change of cytoplasmic factors. Channel activity did not disappear after removing glutamate from the bath. A K+ channel requiring intracellular calcium ([Ca2+]i) was activated in 44% of the experiments (conductance for inward currents in cell-attached patches 118 +/- 6 pS; 'BK channel'). Another Ca(2+)-dependent channel permeable for Cl- (conductance for outward currents in cell-attached patches 72 +/- 17 pS), acetate and methanesulphonate appeared in 26% of the patches. Other K+ channels of smaller conductance were infrequently observed. During and after glutamate application the activity of the BK channel showed an initial increase followed by a transient decay and a second rise to a plateau, probably reflecting a similar time course of changes in [Ca2+]i. Both phases of increasing channel activity required the presence of extracellular Ca2+ suggesting that [Ca2+]i was mainly increased by Ca2+ influx. The N-methyl-D-aspartate (NMDA) antagonists dizocilpine (MK-801, 10 microM) and DL-2-amino-5-phosphonovaleric acid (AP5; 100 microM), added within 5 min after glutamate application, stopped BK channel activity and restored the spontaneous action potential firing. We conclude that the influx of Ca2+ through NMDA receptor channels causes a strong activation of Ca(2+)-dependent K+ channels, which is likely to result in pronounced loss of intracellular K+. NMDA receptor channels seem to remain active for a long time (> 10 min) after the end of glutamate application.

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.

Alternative splicing generates metabotropic glutamate receptors inducing different patterns of calcium release in Xenopus oocytes

A splice variant of the metabotropic glutamate receptor (mGluR) 1a, named mGluR1c, was isolated. Compared to mGluR1a, the predicted mGluR1c protein is 302 amino acids shorter at its C-terminal end. Despite this difference, mGluR1c activates phospholipase C in Xenopus oocytes with a pharmacological profile identical to that of mGluR1a. However, in contrast to the large fast transient responses induced by mGluR1a, mGluR1c receptors elicit a small more slowly generated long-lasting oscillatory current, suggesting that these two receptors do not generate the same pattern of Ca2+ release in Xenopus oocytes. In situ hybridization data show that mGluR1c mRNA is expressed at a lower level than the other splice variants of mGluR1. Some differences in the regional distribution of these transcripts were observed in the cerebellum, the olfactory bulb, and the striatum.

Characterization of a metabotropic glutamate receptor: direct negative coupling to adenylyl cyclase and involvement of a pertussis toxin-sensitive G protein

We have characterized a G-protein-coupled glutamate receptor in primary cultures of striatal neurons. Glutamate, quisqualate, or trans-1-aminocyclopentane-1,3-dicarboxylate inhibited by 30-40% either forskolin-stimulated cAMP production in intact cells or forskolin plus vasoactive intestinal peptide-activated adenylyl cyclase assayed in neuronal membrane preparations. These inhibitory effects were suppressed after treatment of striatal neurons with Bordetella pertussis toxin, suggesting the involvement of a heterotrimeric guanine nucleotide-binding protein (G protein) of the G(i)/G(o) subtype. The pharmacological profile of this glutamate receptor negatively coupled to adenylyl cyclase was different from that of the metabotropic Qp glutamate receptor coupled to phospholipase C in striatal neurons and from that of the recently cloned "mGluR2" glutamate receptor, which is negatively coupled to adenylyl cyclase when expressed in non-neuronal cells.

Actions of a metabotropic glutamate receptor agonist in immature and adult rat cerebellum

The electrophysiological actions of the metabotropic glutamate receptor agonist 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) on Purkinje and granule cells were studied in immature and adult cerebellar slices. ACPD elicited a depolarising response when applied to Purkinje cells (EC50 approximately 20 microM). Granule cells hyperpolarised when exposed to low (3-10 microM) concentrations of ACPD; higher concentrations produced a depolarisation (EC50 approximately 40 microM) that was rapidly curtailed by a hyperpolarisation. The hyperpolarisation was abolished when Ca2+ was removed. In Purkinje cells, the amplitude of the depolarisation was greater in adult slices compared to those in immature slices. The responses were not blocked by ionotropic glutamate receptor antagonists or (L)-2-amino-3-phosphonopropionate (AP3).