Transient calcium-dependent potassium current in magnocellular neurosecretory cells of the rat supraoptic nucleus.

1. Magnocellular neurosecretory neurones were impaled in the supraoptic nucleus of perfused explants of rat hypothalamus. Membrane currents were studied at 35 degrees C using the single-microelectrode voltage-clamp technique. 2. Depolarizing voltage steps applied from -100 mV evoked a transient outward current (TOC) from a threshold of -75 mV. From this potential, the amplitude of the current increased non-linearly with voltage. 3. Following its activation TOC reached a peak within 7 ms and subsequently decayed monotonically with a time constant of 30 ms. This time constant did not vary significantly with voltage between -75 and -55 mV. 4. The TOC showed complete steady-state inactivation at potentials positive to -55 mV. Inactivation was removed by hyperpolarization, with a mid-point near -80 mV. The removal of inactivation followed a complex time course with distinct fast (tens of milliseconds) and slow (hundreds of milliseconds) components. 5. Tail current measurements revealed that the TOC equilibrium potential (ETOC) lies near -97 mV in the presence of 3 mM [K+]o. Increasing [K+]o caused a decrease of TOC amplitude and a shift in ETOC of 57 mV/log [K+]o. The TOC is therefore predominantly a K+ current. 6. The TOC was unaffected by tetraethylammonium (up to 12 mM) but was reversibly reduced by 4-aminopyridine (ca. 50% block at 1.0 mM) and dendrotoxin (ca. 50% block at 4 nM). 7. The TOC was strongly inhibited (greater than 70%) by adding Co2+ or Mn2+ (1-3 mM) or Cd2+ (50-400 microM) to Ca-containing solutions, or by removal of Ca2+ from the perfusate. These effects were not accompanied by detectable changes in threshold voltage. The amplitude of TOC was also depressed by the organic Ca2+ channel blocker methoxyverapamil (D600). Finally replacement of Ca2+ by Ba2+ in the perfusate completely and reversibly abolished the TOC. 8. These findings suggest that neurosecretory neurones of the rat supraoptic nucleus display a transient K+ current which is strongly dependent on the presence of external Ca2+. The possible role of this current is briefly discussed.