Activation of AMPA/kainate receptors but not acetylcholine receptors causes Mg2+ influx into Retzius neurones of the leech Hirudo medicinalis.
In Retzius neurones of the medicinal leech, Hirudo medicinalis, kainate activates ionotropic glutamate receptors classified as AMPA/kainate receptors. Activation of the AMPA/kainate receptor-coupled cation channels evokes a marked depolarization, intracellular acidification, and increases in the intracellular concentrations of Na+ ([Na+]i) and Ca2+. Qualitatively similar changes are observed upon the application of carbachol, an activator of acetylcholine receptor-coupled cation channels. Using multibarrelled ion-selective microelectrodes it was demonstrated that kainate, but not carbachol, caused additional increases in the intracellular free Mg2+ concentration ([Mg2+]i). Experiments were designed to investigate whether this kainate-induced [Mg2+]i increase was due to a direct Mg2+ influx through the AMPA/kainate receptor-coupled cation channels or a secondary effect due to the depolarization or the ionic changes. It was found that: (a) Similar [Mg2+]i increases were evoked by the application of glutamate or aspartate. (b) All kainate-induced effects were inhibited by the glutamatergic antagonist DNQX. (c) The magnitude of the [Mg2+]i increases depended on the extracellular Mg2+ concentration. (d) A reduction of the extracellular Ca2+ concentration increased kainate-induced [Mg2+]i increases, excluding possible Ca2+ interference at the Mg2+-selective microelectrode or at intracellular buffer sites. (e) Neither depolarizations evoked by the application of 30 mM K+, nor [Na+]i increases induced by the inhibition of the Na+/K+ ATPase caused comparable [Mg2+]i increases. (f) Inhibitors of voltage-dependent Ca2+ channels did not affect the kainate-induced [Mg2+]i increases. Moreover, previous experiments had already shown that intracellular acidification evoked by the application of 20 mM propionate did not cause changes in [Mg2+]i. The results indicate that kainate-induced [Mg2+]i increases in leech Retzius neurones are due to an influx of extracellular Mg2+ through the AMPA/kainate receptor-coupled cation channel. Mg2+ may thus act as an intracellular signal to distinguish between glutamatergic and cholinergic activation of leech Retzius neurones.
Muller
Anja
A
Institut für Neurobiologie, Heinrich-Heine-Universität Düsseldorf, Germany.
Gunzel
Dorothee
D
Schlue
Wolf-Rudiger
WR
D000818:Animals; D002217:Carbachol / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D007608:Kainic Acid / Q000494:pharmacology; D007865:Leeches / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D008274:Magnesium / Q000378:metabolism*; D009474:Neurons / Q000187:drug effects / Q000378:metabolism; D018091:Receptors, AMPA / Q000819:agonists / Q000378:metabolism*; D011950:Receptors, Cholinergic / Q000378:metabolism*; D018092:Kainic Acid Receptors / Q000819:agonists / Q000378:metabolism*
D002217:Carbachol; D007608:Kainic Acid; D008274:Magnesium; D018091:Receptors, AMPA; D011950:Receptors, Cholinergic; D018092:Kainic Acid Receptors