Differences in magnesium and calcium effects on N-methyl-D-aspartate- and quisqualate-induced decreases in extracellular sodium concentration in rat hippocampal slices.
Decreases in extracellular sodium concentration [( Na+]o) and associated slow negative field potentials (fp's) were monitored with double barreled sodium sensitive/reference microelectrodes in area CA1 of rat hippocampal slices during iontophoretic application of the glutamate receptor agonists N-methyl-D-aspartate (NMDA) and quisqualate (quis). The effects of lowering [Ca2+]o on these signals were compared to those of lowering [Mg2+]o. Both NMDA- and quis-induced decreases in [Na+]o of up to 60 mM and in the fp's of up to 8 mV. Decreasing [Mg2+]o enhanced NMDA-induced signals, whereas quis-induced signals were unaffected. Lowering [Ca2+]o also enhanced NMDA signals, although somewhat less than lowering [Mg2+]o. This effect was still present, even when voltage dependent Na+ currents were blocked by 10(-7) tetrodotoxin. Interestingly, quis-induced signals could be enhanced in a low Ca2+ medium as well, but only when high quis concentrations were used. The results suggest that, during the sorts of large decreases of [Ca2+]o observed during seizure activity, activation of NMDA receptors is facilitated.
Köhr
G
G
Institut für normale und pathologische Physiologie, Universität Köln, Federal Republic of Germany.
Heinemann
U
U
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D001224:Aspartic Acid / Q000031:analogs & derivatives* / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology* / Q000502:physiology; D004558:Electric Stimulation; D006624:Hippocampus / Q000187:drug effects / Q000502:physiology*; D066298:In Vitro Techniques; D008274:Magnesium / Q000494:pharmacology* / Q000502:physiology; D016202:N-Methylaspartate; D010069:Oxadiazoles / Q000494:pharmacology*; D016318:Quisqualic Acid; D051381:Rats; D011919:Rats, Inbred Strains; D017470:Receptors, Glutamate; D017981:Receptors, Neurotransmitter / Q000187:drug effects / Q000502:physiology*; D012964:Sodium / Q000494:pharmacology* / Q000502:physiology
D010069:Oxadiazoles; D017470:Receptors, Glutamate; D017981:Receptors, Neurotransmitter; D001224:Aspartic Acid; D016202:N-Methylaspartate; D016318:Quisqualic Acid; D012964:Sodium; D008274:Magnesium; D002118:Calcium