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+.