Evaluation of GluR2 subunit involvement in AMPA receptor function of neonatal rat hypoglossal motoneurons

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Abstract
AMPA receptors (AMPAr) mediate fast synaptic responses to glutamate and, when they lack the GluR2 subunit, are strongly Ca2+ permeable and may increase intracellular Ca2+ levels. Because hypoglossal motoneurons possess restricted ability to buffer internal Ca2+ and are vulnerable to Ca2+ excitotoxicity, we wondered if, in these cells, any significant Ca2+ influx could be generated via AMPAr activity. Using whole cell patch‐clamp recording from neonatal rat hypoglossal motoneurons, we tested the AMPAr properties conferred by GluR2 subunits, namely Ca2+ permeability, current rectification and sensitivity to pentobarbital or to the subunit‐specific channel blockers, IEM‐1460 and IEM‐1925. We recorded membrane currents generated by the agonist, kainate, and compared them with those obtained from hippocampal pyramidal neurons (expressing GluR2‐containing AMPAr) and from striatal giant aspiny or hippocampal interneurons (with GluR2‐lacking AMPAr). Ca2+ vs. Na+ permeability of motoneuron AMPAr was relatively low (0.25 ± 0.05), although higher than that of pyramidal neurons. With intracellularly applied spermine, significant inward rectification was absent from motoneurons. These data indicated the prevalence of functional GluR2 subunits. However, the sensitivity of motoneuron AMPAr to pentobarbital did not differ from that of GluR2‐lacking AMPAr on interneurons. Motoneurons possessed sensitivity to IEM‐1460 (IC50 = 90 ± 10 µm) approximately 10‐fold lower than striatal interneurons, although 10‐fold higher than hippocampal pyramidal cells. IEM‐1925 also reduced the amplitude of excitatory synaptic currents in brainstem slice motoneurons. We hypothesize that hypoglossal motoneuron AMPAr (moderately Ca2+ permeable because they contain few GluR2 subunits) may contribute to intracellular Ca2+ rises especially if persistent AMPAr activation (or the pathological GluR2 down‐regulation) occurs.

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