1. The permeability of AMPA (alpha‐amino‐3‐hydroxy‐5‐methyl‐4‐ isoxazolepropionate) receptors in the chick cochlear nucleus, the nucleus magnocellularis (nMAG), was examined by measuring the shift in reversal potential (Erev) of current through glutamate or neurotransmitter‐gated channels in solutions of different ionic composition. 2. Outwardly rectifying glutamate‐activated currents in outside‐out membrane patches showed rapid activation and desensitization. The Erev of glutamate‐evoked current in zero sodium solutions was dependent on the extracellular Ca2+ concentration. The relation between Erev and Ca2+ ionic activities could be described by the Goldman‐Hodgkin‐Katz equation with a permeability ratio, PCa/PCs, of 3.3. The PNa/PCs was estimated as 0.66, indicating a PCa/PNa of 5. 3. Evoked excitatory postsynaptic currents (EPSCs) could be recorded during local perfusion of the auditory nerve‐nMAG synapse with isotonic Ca2+. The Erev of the EPSC shifted in the positive direction in high‐Ca2+ solution as predicted from the preceding analysis. The fraction of current carried by Ca2+ during the AMPA receptor EPSC was estimated as 18%.
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