Abstract

1. Spinal cord and hippocampal neurones in cell culture were voltage clamped using the tight‐seal, whole‐cell recording technique. The concentration of sodium and a series of divalent cations in the extracellular media was varied to study permeation through excitatory amino acid receptor channels activated by the selective agonists N‐methyl‐D‐aspartic acid (NMDA), kainic acid and quisqualic acid. 2. On raising the extracellular calcium concentration, with [Na+]o held constant at 105 mM, the reversal potential of responses to NMDA shifted in the depolarizing direction. This shift was adequately described by the extended constant‐field equation over the range 0.3‐50 mM‐calcium. Using ionic activity coefficients we calculate a value of PCa/PNa = 10.6. Under the same experimental conditions the reversal potential of responses to kainic and quisqualic acids was much less affected by raising the calcium concentration, such that PCa/PNa = 0.15. A depolarizing shift of the NMDA reversal potential was also recorded during application of 20 mM‐barium, strontium or manganese, suggesting permeation of these ions. The permeability sequence was Ca2+ greater than Ba2+ greater than Sr2+ much greater than Mn2+. No depolarizing shift of the NMDA reversal potential occurred during application of 20 mM‐cobalt, magnesium or nickel. 3. In experiments in which the extracellular Na+ concentration was varied the extended constant‐field equation was adequate in predicting shifts of the NMDA reversal potential recorded on varying [Na+]o over the range 50‐150 mM, but failed to accurately predict the reversal potential of responses to NMDA with 10 mM‐[Ca2+]o and only 10 or 20 mM‐[Na+]o. These results imply an apparent increase in PCa/PNa on lowering [Na+]o and may result from interaction of permeant ions within the channel. 4. Barium and to a lesser extent calcium, but not strontium (all 20 mM), reduced the slope conductance of responses to NMDA recorded within +/‐ 15 mV of the reversal potential; over this limited range of membrane potential the current‐voltage relationship remained linear in the presence of each of these ions. In contrast manganese produced a strong, voltage‐dependent block of responses to NMDA, similar to that produced by magnesium, such that even close to the reversal potential the NMDA current‐voltage relationship was highly non‐linear. Thus manganese both permeates and blocks the NMDA receptor channel. 5. Raising the extracellular calcium concentration, from 0.1 to 5 mM, had two effects on the conductance mechanism activated by NMDA.(ABSTRACT TRUNCATED AT 400 WORDS)