NA+-ACTIVATED K+ CHANNELS AND VOLTAGE-EVOKED IONIC CURRENTS IN BRAIN-STEM AND PARASYMPATHETIC NEURONS OF THE CHICK

1. Patch-clamp and computer-modelling techniques were used to study the activation of Na+ -activated K+ channels (I(K)(Na)) in dissociated neurones from the embryonic chick ciliary ganglion and the embryonic chick brain stem. 2. Numerical solutions of diffusion equations suggested that Na+ accumulation as a result of Na+ influx through voltage-sensitive Na+ channels (I(Na)) is insufficient to allow for alteration in the gating of I(K)(Na) channels. 3. Whole-cell recordings using two independent micropipettes were made from chick ciliary-ganglion neurones. These showed that transient outward currents were present only when there were clear indications of incomplete voltage clamp. 4. Single-electrode whole-cell recordings from ciliary-ganglion neurones showed that transient tetrodotoxin (TTX)-sensitive outward currents were present, but only when partial TTX blockade produced significant alterations in the kinetics of I(Na). In cells that were properly voltage clamped, there was no effect of TTX on the kinetics of I(Na) or on voltage-evoked outward currents. 5. Examination of the relationship between peak I(Na) and the command potential showed that transient outward currents were only present in neurones that showed sharp deviations from the behaviour expected of a cell that is adequately voltage clamped. Transient outward currents were not present in cells that were adequately voltage clamped. 6. Application of TTX to isolated outside-out patches obtained from ciliary ganglion neurones eliminated voltage-evoked inward currents but had no effect on outward currents. 7. Isolated inside-out patches obtained from ciliary-ganglion neurones did not contain I(K)(Na) channels. These patches usually contained Ca2+ -activated K+ channels (I(K)(Ca) with a unitary conductance of around 200 pS when [K+]o = 150 mM and [K+]i = 75 mM. 8. Two-electrode whole-cell recordings from cultured brain stem neurones showed that transient outward currents were present only when there were clear indications of incomplete voltage clamp. 9. Application of TTX caused blockade of inward but not outward currents in brain stem neurones voltage clamped with a single whole-cell pipette. TTX had no effect on the kinetics of I(Na). Application of TTX to outside-out patches isolated from the same cells blocked only the inward currents. 10. Isolated inside-out patches obtained from brain stem neurones contained I(K)(Na) channels that could be activated by exposure of the cytoplasmic face of the patch membrane to 75 mM-Na+. These channels had a predominant conductance state of around 100 pS when [K+]o = 150 mM and [K+]i = 75 mM. The I(K)(Na)) channels were not activated by 1 mM-Ca2+. 11. I(K)(Na)) channels cannot be activated by Na+ influx through voltage-sensitive Na+ channels in cultured chick brain stem neurones and cannot be detected in isolated ciliary-ganglion neurones. Therefore, they are unlikely to alter their gating significantly during the course of a single action potential.