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

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Abstract
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. 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. Kinetic analysis revealed two components of ITO inactivation in both OT‐MNCs (τ1= 9.2 ± 0.4 ms and τ2= 41.2 ± 1.6 ms; n= 18) and VP‐MNCs (τ1= 12.4 ± 0.4 ms and τ2= 37.1 ± 1.2 ms; n= 52). Although the density of the rapid component (τ1) was not different (275 ± 13 versus 265 ± 16 pA pF−1, respectively), the slow component (τ2) was markedly smaller in OT‐MNCs (183 ± 19 versus 331 ± 16 pA pF−1 in VP‐MNCs). 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. 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.

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