SUMMARY1. The properties of single calcium-activated potassium channels (or C-channels) were studied in cell-attached patches using the patch-clamp technique. Experiments were performed on identified Ca2+-dependent U cells in juvenile specimens (1-2 months old) of Helix aspersa.2. The criteria used to identify C-channels were based on comparison between macroscopic C-currents and currents reconstructed from unitary recordings. Both currents had a slow activation rate at large positive potentials which turned into fast activation after large Ca2+ entries. Both currents were blocked by intracellularly injected EGTA. 4. C-channels in U cells were distributed in clusters of three to ten channels (mean 5-05 channels in seventy-five patches). Calcium channels were present in patches containing clustered C-channels. C-channels within clusters behaved independently. 5. With patch electrode containing 8 mM-calcium, C-channels opened transiently upon patch depolarization. Reopenings in quiescent depolarized patches were induced by whole-cell spikes triggered by current pulses applied to an intracellular electrode. Apparent inactivation of C-channels in depolarized patches was in fact due to a decrease in [Ca2+]i resulting from inactivation of Ca2+ channels.6. Calcium-free saline solutions in the patch electrodes prevented C-channels from opening upon patch depolarization. Entry of calcium through the surrounding membrane induced delayed openings in the patch. Peak opening probability PO occurred 330 + 30 ms after a brief Ca2+ entry with a lag period of 50-80 ms. With patch electrodes filled with Ca2+-containing saline solutions and under conditions which maximized C-channel opening, peak P1 was reached in 20-50 ms. The same value was observed for the whole-cell C-current.7. The peak P. at a given patch potential and in response to a whole-cell spike was not altered by a previous long-lasting patch depolarization, or by producing several * To whom correspondence should be addressed. MS 7550M. GOLA, C. DUCREUX AND H. CHAGNEUX successive Ca2+ entries. Thus, C-channels did not appear to be inactivated by depolarization or increase in [Ca2+]i.8. C-channels were found to be relatively highly voltage dependent, with an e-fold increase in PO per 14-9 mV increase in potential. The location of the PO-V curve on the voltage axis, but not its shape, was variable along a 60 mV span. Openings were never observed at potentials below -30 mV.9. Increasing Ca2+ influxes shifted the PO-V relationship towards negative potentials. Reverse shifts were observed during the PO decay following a brief Ca2+ entry.10. The latency to first opening was of 80-100 ms when patch depolarizations were applied immediately after a brief Ca2+ entry. The latency decreased to 10-20 ms when the depolarization was applied 300-600 ms after the Ca2+ entry. These data would indicate that the C-current activation was rate limited by calcium while the voltage-dependent step had fast kinetics.11. The distributions of open time, closed time and burst duration were con...

A time‐ and voltage‐dependent Na+‐selective current termed INa,M is activated by muscarinic agonists or splanchnic nerve stimulation in sympathetic neurones of rabbit coeliac and superior mesenteric ganglia. The firing patterns induced by INa,M were investigated in patch‐clamped neurones within intact ganglia, and compared with those generated by a neuronal model including INa,M. I Na,M was characterized by voltage‐dependent low‐threshold activation and high‐threshold inactivation functions. The overlapping functions produced a persistent U‐shaped current between −100 and −20 mV, which peaked at the cell resting potential. The activation and inactivation kinetics were fitted to single exponentials with time constants of ≈100 and 400 ms, respectively. Activating INa,M with muscarinic agonists or nerve stimulation depolarized and fired the neurones. The depolarization was paralleled by an apparent increase in input membrane resistance. The model showed that this paradox resulted from the turning off of INa,M during resistance tests, which also accounted for the all‐or‐none slow hyperpolarizing responses to current pulses. I Na,M gave the neurones an N‐shaped I‐V relationship capable of producing complex firing patterns. Under given conditions, carbachol‐treated neurones could either fire regularly or remain silent at ≈‐80 mV, i.e. they displayed bistability. Transitions from one state to the other were triggered with short current pulses. The transitions resulted from the turning on and off of INa,M. Firing reduced INa,M, an effect abolished by blocking Ca2+ channels or adding BAPTA (40 mM) to the pipette. The Ca2+‐related negative regulation of INa,M may have mediated endogenous bursting activity. Burst firing was generated by the model upon introducing Ca2+ regulation of INa,M. The results demonstrate that INa,M gives prevertebral sympathetic neurones a wide repertoire of firing patterns: pacemaker‐like properties, bistability and burst firing capability. They suggest that the INa,M‐related encoding properties may provide sympathetic neurotransmission with new potentialities.

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