Changes in the Electrical Properties of Supraoptic Nucleus Oxytocin and Vasopressin Neurons during Lactation

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Abstract
Magnocellular oxytocin (OT) and vasopressin (VP) neurons adopt different firing patterns in response to relevant physiological stimuli. OT neurons selectively display short (2–4 sec), high-frequency bursts of action potentials that are highly synchronized and correlated with OT release during lactation. The present experiments were done to determine whether the electrophysiological properties of OT neurons differ from those of VP neurons, and whether these properties are modulated during lactation to support short bursting activity. Intracellular recordingsin vitrowere obtained from immunochemically identified supraoptic neurons of diestrous or lactating female rats. Resting membrane potential, input resistance, membrane time constant, and the depolarizing afterpotential did not differ among groups. However, near spike threshold, OT, but not VP, neurons expressed a sustained outward rectification that was removed by small hyperpolarizing pulses and a rebound depolarization that occurred at the offset of these hyperpolarizing pulses. The rebound depolarization was short (2+-dependent afterhyperpolarization, and the degree of spike broadening of OT, but not VP, neurons were also larger during lactation, suggesting an increase in Ca2+influx per spike. The results indicate that OT neurons possess properties favoring the expression of short spike trains, and that some of these properties are enhanced during lactation. In addition, spikes in OT neurons may promote more Ca2+influx in this state.

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