Development of the fast, transient outward K⁺ current in embryonic sympathetic neurones

Voltage-activated outward potassium (K⁺) currents in developing sympathetic neurones, dissociated from the rat superior cervical ganglion (SCG), were studied using the whole-cell patch clamp recording technique. In voltage-clamped neonatal SCG cells, two voltage-dependent K⁺ currents were measured: the fast, transient K⁺ current, I_A; and, the slower activating, non-inactivating delayed rectifier, I_K. Only I_K, however, appeared to be present in SCG neurones isolated from early embryonic (E14.5-16.5) rat pups: I_A was not observed in these cells. When these embryonic neurones were maintained in cell culture, I_A developed over a time course (≈ 4-6 days) similar to that seen in vivo. I_A, therefore, which appears to facilitate the fast repolarization phase of the action potential in rat SCG neurones, is the last voltage-activated current to develop in these cells.