1. Intracellular recordings in vitro from immunochemically identified oxytocin (OT) and vasopressin (VP) neurones in the supraoptic nucleus (SON) of virgin or lactating female rats revealed no differences between neurone types in membrane potential (Vm), input resistance and current‐voltage relationships (I‐V), when taken at resting membrane potentials. 2. When OT (94%), but not VP, neurones (93%) were current clamped at depolarized voltages (above ‐50 mV), small hyperpolarizing pulses revealed a time‐ and voltage‐dependent outward rectification that was present above ‐75 mV and that decreased in amplitude as Vm approached the equilibrium potential for potassium (EK). The rectification was more pronounced when the neurones were held at a more depolarized membrane potential, and was larger the longer the neurone was held depolarized, reaching a maximum at 0.6‐0.9 s. 3. A rebound depolarization followed the offset of hyperpolarizing pulses that were associated with the rectification. The peak amplitude of the rebound showed a time and a voltage dependence. It followed a bell‐shaped curve as the hyperpolarizing commands were made larger, attaining a peak at ‐65 +/‐ 1.5 mV. The rebound amplitude increased with pulse duration, achieving a half‐maximal amplitude at 0.5 +/‐ 0.1 s. 4. The expression of the sustained outward rectification and the rebound in OT neurones was similar in virgin and lactating female rats. 5. These results indicate the presence of significant differences in the intrinsic membrane properties, probably K+ currents, between OT and VP neurones in both lactating and virgin female rats.
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