The intermediate conductance calcium‐dependent K ⁺ channel does not contribute to the slow after hyperpolarization in oxytocin and vasopressin hypothalamic magnocellular neurons

Vasopressin (VP) magnocellular neurosecretory neurons of the hypothalamic supraoptic nucleus (SON) are critical regulators of renal water retention and vascular tone. VP neurons undergo detrimental plastic changes in cardiovascular diseases such as heart failure (HF), resulting in hyperexcitability and thus altered fluid/electrolyte balance. A major intrinsic mechanism that regulates the firing activity of VP neurons is the slow afterhyperpolarization (sAHP), a phenomenon underlain by a calcium-dependent K⁺ current (I_sAHP). The sAHP is activated by Ca²⁺ and results in an efflux of K⁺ from the cell, hyperpolarizing it and throttling firing. Importantly, we previously reported that a blunted sAHP contributes to hyperexcitability of VP neurons in heart failure rats. While the features of the sAHP are well characterized, the identity of the channel underlying the I_sAHP remains unknown. Combining patch clamp electrophysiology, pharmacology and immunohistochemistry in Wistar rats, we investigated Intermediate conductance Ca²⁺-dependent K⁺ (IK) channels as a potential candidate responsible for carrying the I_sAHP. We generated and measured the I_sAHP in voltage clamp via 20 Hz trains of 20 square voltage pulses (from -50 to +10) once per minute. After 4 min of baseline recording, we bath applied TRAM-34 (1 μM), a specific IK channel blocker. Blocking IK with TRAM-34 failed to inhibit I_sAHP peak amplitude, amplitude at 1 s after stimulus end, or area. Post hoc immunohistochemistry was performed to identify the phenotype of the recorded cell. We observed no inhibitory effect of TRAM-34 on the I_sAHP in either VP or OT neurons. We also saw no inhibition of I_sAHP (voltage clamp) or sAHP (current clamp) in slices preincubated in TRAM-34 for at least 1 h prior to recording. Conversely, we found that TRAM-34 inhibited isolated whole cell K⁺ currents, supporting the presence of functional, TRAM-34-sensitive IK channels in SON neurons. Taken together, our results indicate that despite the expression of IK in SON neurons and astrocytes, we observed no evidence of a significant contribution to the sAHP in either OT or VP SON neurons. Future studies will be needed to determine other potential K⁺ channel candidates contributing to the sAHP in SON neurons.