The effect of foreign cations, pH and pharmacological agents on the ionic permeability of an excitatory glutamate synapse.

Publisher Website
Google Scholar
Abstract
Voltage clamp studies of the post-synaptic membrane of the insect [Schistocerca gregaria] neuromuscular junction showed that normal amplitude glutamate currents could be recorded for a limited time when external Na was completely replaced by Ca, Li, NH4+, methylamine and guanidine. No change in the reversal potential of the glutamate current was observed when Na was replaced by these ions. The glutamate ionic channel has a similar permeability to Na and to these foreign cations, although the foreign ions cause a longer-term block of the permeability increase or receptor function. Procaine, pentobarbitone, 2-4-6-triaminopyrimidine, high and low pH and low temperature reduced the synaptic ionic permeability but did not alter the ratio of the conductance increase of Na to K (.DELTA.gNa/.DELTA.gK). 4-Aminopyridine and TEA [tetraethylammonium] did not reduce the synaptic ionic permeability. The properties of the synaptic ionic channels resemble the tight junction transepithelial ionic channels of the mammalian gall-bladder, but are very different from the Na and K non-synaptic channels of axons. Probably, Na and K normally pass through a single relatively large channel containing strong proton accepting acidic ligands which render the channel cation selective.