Whole-cell, voltage-clamp recordings were obtained from neurones of the supraoptic area of neonatal rats in dissociated cell culture. Recordings were made from neurones having the same morphology as those which were vasopressin or oxytocin immunoreactive. 2. Three types of voltage-activated K+ current were identified on the basis of their kinetics, voltage sensitivities, Ca2+ dependence and pharmacology. The currents corresponded to the delayed rectifier current (IK), the A-current (IA), and the Ca2+-dependent current (IK(Ca] described in other neurones. 3. IK had a threshold of -40 mV, a sigmoidal time course of activation, and was sustained during voltage steps lasting less than 300 ms. The underlying conductance was voltage dependent reaching a maximum at +30 mV (mean maximum conductance 4.09 nS). The activation time constant was also voltage dependent declining exponentially from 4.5 ms at -30 mV to 1.8 ms at +50 mV. 4. IA was transient, and was activated from holding potentials negative to -70 mV; the maximum conductance (mean 5.9 nS) underlying the current was obtained at +10 mV. The activation and inactivation time constants were voltage dependent: the activation time constant declined exponentially between -40 mV (2.2 ms) and +40 mV (0.65 ms). 5. IK and IA were attenuated by the K+ channel blockers tetraethylammonium (TEA) and 4-aminopyridine (4-AP). TEA blocked the conductance underlying IK but appeared to alter the kinetics of IA. In contrast, 4-AP blocked the conductance underlying IA and, to a lesser extent, IK. 6. IK and IA were activated independently of external Ca2+ and the voltage activation of Ca2+ channels since these currents were recorded in the presence of Co2+, a Ca2+ channel blocker. 7. IK(Ca) was recorded only when Ca2+ (2 mM) was present in the external medium. From a holding potential of -30 mV, IK(Ca) had a threshold of -20 mV, was maximal at about +20 mV and declined at more positive potentials. This current was sustained during voltage steps lasting 100 ms and was abolished by addition of Co2+ (2 mM) to the medium. 8. The possible roles of the three K+ currents in regulating the characteristic firing behaviour of supraoptic neurones previously recorded in vivo and in vitro are discussed.
