Density of transient K+ current influences excitability in acutely isolated vasopressin and oxytocin neurones of rat hypothalamus.

1. The transient outward K+ current (ITO) was studied using whole-cell recording in immunocytochemically identified oxytocin (OT; n = 23) and vasopressin (VP; n = 67) magnocellular neurosecretory cells (MNCs) acutely isolated from the supraoptic nucleus of adult rats. 2. The peak density of ITO during steps to -10 mV was 26 % smaller in OT-MNCs (355 +/- 23 pA pF-1; mean +/- s.e. m.; n = 18) than in VP-MNCs (478 +/- 17 pA pF-1; n = 52). No differences were observed in the voltage dependence of activation or inactivation. 3. Kinetic analysis revealed two components of ITO inactivation in both OT-MNCs (tau1 = 9.2 +/- 0.4 ms and tau2 = 41.2 +/- 1.6 ms; n = 18) and VP-MNCs (tau1 = 12.4 +/- 0.4 ms and tau2 = 37.1 +/- 1.2 ms; n = 52). Although the density of the rapid component (tau1) was not different (275 +/- 13 versus 265 +/- 16 pA pF-1, respectively), the slow component (tau2) was markedly smaller in OT-MNCs (183 +/- 19 versus 331 +/- 16 pA pF-1 in VP-MNCs). 4. In unidentified MNCs, 0.5 mM 4-aminopyridine reduced ITO amplitude by 29% and decreased the latency to spike discharge by about 70% during depolarization from -70 mV. Latency to discharge from potentials less negative than -60 mV, where ITO is inactivated, was unaffected. 5. Comparison of latency to spike discharge in identified cells showed that OT-MNCs achieve spike threshold twice as fast as VP-MNCs when depolarized from -70 mV. The lower density of ITO in OT-MNCs, therefore, accelerates the rate at which excitation can occur in response to depolarizing stimuli and may facilitate the occurrence of higher frequency discharges in OT-MNCs during physiological activation.