Selective blocking action of tetraethylammonium ion on three types of acetylcholine receptors in Aplysia ganglion cells.

The ganglion cells of Aplysia have three types of acetylcholine (ACh)-receptors in their membranes. The activation of one type produces Na+-dependent depolarization (DNa-type). The other receptors are HK- and HC1-types, the activation of which produces either K+- and Cl- dependent hyperpolarization, respectively. ACh-induced membrane current (IACh) was measured under the voltage clamp at different holding potential levels (V) of the membrane. The effects of tetraethylammonium (TEA) were analyzed quantitatively on each type of receptor activity by observing the changes in dose-inhibition curves and IACh-V relations obtained under voltage clamp. TEA exhibited a dose dependent block on both DNa- and HK-type of receptor activities but no effect on HCl-type. The mode of blocking action was found to be competitive in HK-type but noncompetitive in DNa-type. In the DNa-type receptor, 3 mM TEA shifted the reversal potential of ACh-induced current response from +5 to +40 mV, and caused a decrease in the slope of the IACh-V curve. In HK-type, TEA simply decreased the slope of IACh-V curve without changing the reversal potential of the ACh-induced response. It was concluded that TEA competed that with ACh for common binding sites at the receptor of the HK-type, but not of the DNa-type, and that in DNa-type, TEA made a binding with an allosteric site of the ACh-receptor and depressed the increase in membrane permeability toward Na+ as well as K+.