Axonal localization of an excitatory post-synaptic potential in a molluscan neurone.

The excitatory post-synaptic potentials (EPSPs) of molluscan neurones presumably result, as in other cells (Fatt & Katz, 1951; Eccles, 1964), from an increase in membrane conductance to one or more ions which tends to drive the membrane toward a reversal potential near zero. Consequently, membrane hyperpolarization should increase the amplitude of an EPSP, whereas depolarization should have the opposite effect. In molluscan giant neurones, however, two factors may complicate the analysis of the conductance changes associated with the EPSP. First, the membrane conductance may be markedly voltage-dependent (anomalous rectification); consequently, hyperpolarization of the membrane may decrease the amplitude of EPSPs (Kandel & Tauc, 1966). Secondly, most molluscan synapses appear to be located on the axon and axonal branches (Gerschenfeld, 1963; Tauc, 1966), at some distance from the soma where intracellular micro-electrodes are usually inserted; thus, the analysis of EPSPs is complicated by the electrotonic spread of potential from distant regions to the soma and vice versa (Rail, 1967; Rail et al. 1967; Kehoe & Ascher, 1970). In a preceding study of the synaptic organization of neurones in the marine mollusc, Anisodoris nobilis (Gorman, & Mirolli, 1969), we showed that EPSPs were decreased or not changed by moderate hyperpolarization and suggested that this paradoxical behaviour might be due to the anomalous conductance of the cell membrane. In this paper we show that the EPSP elicited in the gastro-oesophageal giant cell (G cell) by stimulation of the lateral nerve occurs on the cell axon at a site distant from the soma. Thus, the apparent change in conductance during the EPSP is small by comparison with changes in the cell input conductance produced by applied currents. It is the relative magnitude of the changes of membrane conductance produced by the synaptic event and by the applied currents that determine the amplitude of the EPSP.