Properties of the transient K+ current in acutely isolated supraoptic neurons from adult rat

The transient outward current (I-TO) in magnocellular neurosecretory cells was studied using whole cell patch clamp recordings made from supraoptic neurons acutely isolated from the adult rat. In the presence of tetrodotoxin, depolarizing steps applied from a negative holding potential evoked a rapidly activating and inactivating outward current followed by a slowly activating and sustained outward current. The I-TO was unaffected by tetraethylammonium (TEA), but was blocked by 4-aminopyridine. The I-TO evoked by depolarization from negative potentials in the presence of 40 mM TEA was not different from that revealed by digital subtraction of current traces recorded with and without a negative conditioning prepulse in control solutions. Tail current analysis during perfusion of media containing different external [K+] indicated that I-TO is selective for K+ ions. Exposure to Ca2+-free solutions containing divalent inorganic cations such as Cd2+ and Ni2+ could cause shifts in voltage-dependency and, thereby, reduce the amplitude of I-TO recorded at fixed submaximal potentials, The maximal I-TO achievable under these conditions, however, was reduced compared to control. Moreover, the I-TO was also reduced by application of organic Ca2+ channel antagonists such as nifedipine and omega-conotoxin GVIA, indicating that a component of the I-TO is somehow dependent on Ca2+ influx.