Endoplasmic Reticulum and Mitochondrial Calcium Handling Dynamically Shape Slow Afterhyperpolarizations in...

Many neurons including vasopressin (VP) magnocellular neurosecretory cells (MNCs) of the hypothalamic supraoptic nucleus (SON) generate afterhyperpolarizations (AHPs) during spiking to slow firing, a phenomenon known as spike frequency adaptation. The AHP is underlain by Ca²⁺-activated K⁺ currents, and while slow component (sAHP) features are well described, its mechanism remains poorly understood. Previous work demonstrated that Ca²⁺ influx through N-type Ca²⁺ channels is a primary source of sAHP activation in SON oxytocin neurons, but no obvious channel coupling was described for VP neurons. Given this, we tested the possibility of an intracellular source of sAHP activation, namely, the Ca²⁺-handling organelles endoplasmic reticulum (ER) and mitochondria in male and female Wistar rats. We demonstrate that ER Ca²⁺ depletion greatly inhibits sAHPs without a corresponding decrease in Ca²⁺ signal. Caffeine sensitized AHP activation by Ca²⁺ In contrast to ER, disabling mitochondria with CCCP or blocking mitochondria Ca²⁺ uniporters (MCUs) enhanced sAHP amplitude and duration, implicating mitochondria as a vital buffer for sAHP-activating Ca²⁺ Block of mitochondria Na⁺-dependent Ca²⁺ release via triphenylphosphonium (TPP⁺) failed to affect sAHPs, indicating that mitochondria Ca²⁺ does not contribute to sAHP activation. Together, our results suggests that ER Ca²⁺-induced Ca²⁺ release activates sAHPs and mitochondria shape the spatiotemporal trajectory of the sAHP via Ca²⁺ buffering in VP neurons. Overall, this implicates organelle Ca²⁺, and specifically ER-mitochondria-associated membrane contacts, as an important site of Ca²⁺ microdomain activity that regulates sAHP signaling pathways. Thus, this site plays a major role in influencing VP firing activity and systemic hormonal release.