Activity modes and modulation of the peptide-gated Na+ channel of Helix neurones

Unitary currents activated by steady-state applications of FMRFamide analogues have been recorded in outside-out patches from the C2 and F2 neurones of Helix aspersa. The bimodal conductance distribution ranged from 4 pS to 6 pS in patches in each neurone. Application of up to 10 muM cytochalasin B or D to 12 of the 41 inside-out patches tested stimulated activity of the larger conductance events and/or increased the amplitude of smaller conductance events, indicating a cytoskeletal influence on the channels. FMRFamide-gated currents were blocked by external Ca2+ and Mg2+, both with an IC50 of about 1 mM. During continuous application of FMRFamide, clustering of events and/or the appearance of a mode of activity with a lower probability of being open (P-open) suggested two different types of desensitization. More rarely, exceptionally high P-open activity with low FMRFamide concentrations was seen. In some cases, there were long periods of high activity during which the channels gated normally with regular short sojourns in the fully closed state. In others, isolated long openings were seen which interrupted the normal gating pattern of the channels. Amiloride and its analogue 5-(N-ethyl-N-isopropyl) amiloride (EIPA) consistently caused channel block, but in some patches with a low background activity an increased P-open was also observed in the presence of each drug.