Diminished Intracellular Calcium in the Hypothalamic Paraventricular Nucleus Augments Neuronal Excitability and Sympathetic Nerve Activity
Authors: Larson Robert A , Chapp Andrew D , Cheng Zixi , Shan Zhiying , Chen Qing‐Hui 
Published: November 2023

We have demonstrated that hypertension induced by high salt diet combined with chronic low‐dose infusion of angiotensin II (AngII‐Salt HTN) involves dysfunction of small conductance calcium‐activated potassium (SK) channels in the hypothalamic paraventricular nucleus (PVN). Diminished current conducted through SK channels augments neuronal excitability, and inhibition of SK channel function in the PVN augments sympathetic outflow and arterial blood pressure (ABP); however, the underlying mechanisms responsible for inhibition of SK channel function among PVN neurons in AngII‐Salt HTN remain unclear. It is well‐known that intracellular calcium is one of the key regulators gating SK channel conductance. Here, we demonstrated the importance of intracellular calcium homeostasis in controlling neuronal excitability in PVN neurons with axon projections to the rostral ventrolateral medulla (PVN‐RVLM) in vitro, and sympathetic nerve activity (SNA) and ABP in vivo. In whole‐cell current‐clamp recordings under brain slice preparation, graded current injections (0, 50, 100, 150, 200, 250 and 300 pA) evoked graded increases in spike frequency in pre‐sympathetic PVN‐RVLM neurons identified by retrograde labelling. Maximum discharge was evoked by +200 pA current injections and averaged 20 ± 2 Hz (n=10) in the presence of a low level of the calcium chelator EGTA (0.1mM). In contrast, neuronal excitability in response to +200pA current injection was significantly greater (P<0.05) in the presence of high EGTA (5.0mM) (31±4 Hz, n=7) or BAPTA (10mM) (30±1 Hz, n=8), a fast calcium chelator, loaded through the intracellular pipette solution. In order to examine the effects of diminished intracellular calcium on sympathetic outflow, in vivo recordings of splanchnic SNA (SSNA), renal SNA (RSNA) and mean arterial pressure (MAP) were assessed via bilateral PVN microinjection of BAPTA‐AM, a cell membrane permeable calcium chelator compared to membrane‐impermeable BAPTA, in anesthetized rats. PVN microinjection of BAPTA‐AM (20nmol/100nl) in the PVN augmented (P<0.05) SSNA (66±11% vs 17±12%, n=3), RSNA (52±13% vs. 11±8%, n=3) and MAP (8±1 vs. 1±2 mmHg, n=3) compared to membrane‐impermeable BAPTA control (20nmol/100nl). Additionally, blockade of calcium channels via PVN microinjection of the non‐selective calcium channel blocker cadmium chloride (CdCl2, 10nmol/100nl) significantly (P<0.05) augmented SSNA (61±16%, n=3), RSNA (80±21%, n=3) and MAP (12±1 mmHg, n=3). Collectively, our data indicate that disruption of cytoplasmic calcium significantly augmented excitability of pre‐sympathetic PVN neurons in vitro, and increased sympathetic outflow and ABP in vivo. Normal intracellular calcium homeostasis among PVN neurons plays an important role in limiting neuronal excitability and regulating sympathetic outflow, likely through activation of calcium activated potassium channels including SK channels.

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11SDG7420029 (ZYS) and R15‐HL122952 (QHC)