Calcium-dependent fast depolarizing afterpotentials in vasopressin neurons in the rat supraoptic nucleus

Oxytocin (OT) and vasopressin (VP) synthesizing magnocellular cells (MNCs) in the supraoptic nucleus (SON) display distinct firing patterns during the physiological demands for these hormones. Depolarizing afterpotentials (DAPs) in these neurons are involved in controlling phasic bursting in VP neurons. Our whole cell recordings demonstrated a Cs+-resistant fast DAP (fDAP; decay tau = similar to 200 ms), which has not been previously reported, in addition to the well-known Cs+-sensitive slower DAP (sDAP; decay tau = similar to 2 s). Immunoidentification of recorded neurons revealed that all VP neurons, but only 20% of OT neurons, expressed the fDAP. The activation of the fDAP required influx of Ca2(+) through voltage-gated Ca2+ channels as it was strongly suppressed in Ca2+ -free extracellular solution or by bath application of Cd2+. Additionally, the current underlying the fDAP (I-fDAP) is a Ca2+-activated current rather than a Ca2+ current per se as it was abolished by strongly buffering intracellular Ca2+ with BAPTA. The I-V relationship of the I fDAP was linear at potentials less than -60 mV but showed pronounced outward rectification near -50 mV. I-fDAP is sensitive to changes in extracellular Na+ and K+ but not Cl-. A blocker of Ca2+-activated nonselective cation (CAN) currents, flufenamic acid, blocked the fDAP, suggesting the involvement of a CAN current in the generation of fDAP in VP neurons. We speculate that the two DAPs have different roles in generating after burst discharges and could play important roles in determining the distinct firing properties of VP neurons in the SON neurons.