Electrophysiological Properties of Identified Oxytocin and Vasopressin Neurons

Abstract

To understand the contribution of intrinsic membrane properties to the different firing patterns of oxytocin (OT) and vasopressin (VP) neurons in vivo, in vitro studies are needed, where stable intracellular recordings can be made. Combining immunochemistry for OT and VP and intracellular dye injections allows characterization of identified OT and VP neurons, and several differences between the two cell types have emerged. These include a greater transient K⁺ current that delays spiking to stimulus onset, and a higher Na⁺ current density leading to greater spike amplitude and a more stable spike threshold, in VP neurons. VP neurons also show a greater incidence of both fast and slow Ca²⁺‐dependent depolarizing afterpotentials, the latter of which summate to plateau potentials and contribute to phasic bursting. In contrast, OT neurons exhibit a sustained outwardly rectifying potential (SOR), and a consequent depolarizing rebound potential, not found in VP neurons. The SOR makes OT neurons more susceptible to spontaneous inhibitory synaptic inputs and correlates with a longer period of spike frequency adaptation in these neurons. While both types exhibit prominent Ca²⁺‐dependent afterhyperpolarizing potentials (AHPs) that limit firing rate and contribute to bursting patterns, Ca²⁺‐dependent AHPs in OT neurons selectively show significant increases during pregnancy and lactation. In OT, but not VP neurons, AHPs are highly dependent on the constitutive presence of the second messenger, phosphatidylinositol 4,5‐bisphosphate, which permissively gates N‐type channels that contribute the Ca²⁺ during spike trains that activates the AHP. In contrast to the intrinsic properties supporting phasic bursting in VP neurons, the synchronous bursting of OT neurons has only been demonstrated in vitro in cultured hypothalamic explants and is completely dependent on synaptic transmission. Additional differences in Ca²⁺ channel expression between the two neurosecretory terminal types suggests these channels are also critical players in the differential release of OT and VP during repetitive spiking, in addition to their importance to the potentials controlling firing patterns.

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