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

1 The transient outward K + current ( I TO ) 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 I TO 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 I TO inactivation in both OT‐MNCs (τ 1 = 9.2 ± 0.4 ms and τ 2 = 41.2 ± 1.6 ms; n = 18) and VP‐MNCs (τ 1 = 12.4 ± 0.4 ms and τ 2 = 37.1 ± 1.2 ms; n = 52). Although the density of the rapid component (τ 1 ) was not different (275 ± 13 versus 265 ± 16 pA pF −1 , respectively), the slow component (τ 2 ) 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 I TO 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 I TO 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 I TO 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.