Activation of AMPA/Kainate Receptors but Not Acetylcholine Receptors Causes Mg(2+) 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 Ca(2+). 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 Mg(2+) concentration ([Mg(2+)](i)). Experiments were designed to investigate whether this kainate-induced [Mg(2+)](i) increase was due to a direct Mg(2+) 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 [Mg(2+)](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 [Mg(2+)](i) increases depended on the extracellular Mg(2+) concentration. (d) A reduction of the extracellular Ca(2+) concentration increased kainate-induced [Mg(2+)](i) increases, excluding possible Ca(2+) interference at the Mg(2+)-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 [Mg(2+)](i) increases. (f) Inhibitors of voltage-dependent Ca(2+) channels did not affect the kainate-induced [Mg(2+)](i) increases. Moreover, previous experiments had already shown that intracellular acidification evoked by the application of 20 mM propionate did not cause changes in [Mg(2+)](i). The results indicate that kainate-induced [Mg(2+)](i) increases in leech Retzius neurones are due to an influx of extracellular Mg(2+) through the AMPA/kainate receptor–coupled cation channel. Mg(2+) may thus act as an intracellular signal to distinguish between glutamatergic and cholinergic activation of leech Retzius neurones.