AMPA receptors with high Ca2+ permeability mediate synaptic transmission in the avian auditory pathway.
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%.
Otis
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Department of Neurophysiology, University of Wisconsin, Madison 53706, USA.
Raman
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Trussell
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D000818:Animals; D001306:Auditory Pathways / Q000737:chemistry / Q000502:physiology* / Q000648:ultrastructure; D002118:Calcium / Q000493:pharmacokinetics*; D015220:Calcium Channels / Q000502:physiology*; D002463:Cell Membrane Permeability / Q000502:physiology; D002642:Chick Embryo; D004594:Electrophysiology; D016057:Evoked Potentials, Auditory, Brain Stem; D018698:Glutamic Acid / Q000494:pharmacology; D007700:Kinetics; D018091:Receptors, AMPA / Q000502:physiology*; D009435:Synaptic Transmission / Q000502:physiology*
D015220:Calcium Channels; D018091:Receptors, AMPA; D018698:Glutamic Acid; D002118:Calcium