Hypertension Induced by Angiotensin II and a High Salt Diet Involves Reduced SK Current and Increased Excitability of RVLM Projecting PVN Neurons

Exercise Science, Health and Physical Education Department, Michigan Technological University, Houghton, Michigan
Journal of Neurophysiology (Impact Factor: 3.04). 11/2010; 104(5):2329-37. DOI: 10.1152/jn.01013.2009
Source: PubMed

ABSTRACT Although evidence indicates that activation of presympathetic paraventricular nucleus (PVN) neurons contributes to the pathogenesis of salt-sensitive hypertension, the underlying cellular mechanisms are not fully understood. Recent evidence indicates that small conductance Ca(2+)-activated K(+) (SK) channels play a significant role in regulating the excitability of a key group of sympathetic regulatory PVN neurons, those with axonal projections to the rostral ventrolateral medulla (RVLM; i.e., PVN-RVLM neurons). In the present study, rats consuming a high salt (2% NaCl) diet were made hypertensive by systemic infusion of angiotensin II (AngII), and whole cell patch-clamp recordings were made in brain slice from retrogradely labeled PVN-RVLM neurons. To determine if the amplitude of SK current was altered in neurons from hypertensive rats, voltage-clamp recordings were performed to isolate SK current. Results indicate that SK current amplitude (P < 0.05) and density (P < 0.01) were significantly smaller in the hypertensive group. To investigate the impact of this on intrinsic excitability, current-clamp recordings were performed in separate groups of PVN-RVLM neurons. Results indicate that the frequency of spikes evoked by current injection was significantly higher in the hypertensive group (P < 0.05-0.01). Whereas bath application of the SK channel blocker apamin significantly increased discharge of neurons from normotensive rats (P < 0.05-0.01), no effect was observed in the hypertensive group. In response to ramp current injections, subthreshold depolarizing input resistance was greater in the hypertensive group compared with the normotensive group (P < 0.05). Blockade of SK channels increased depolarizing input resistance in normotensive controls (P < 0.05) but had no effect in the hypertensive group. On termination of current pulses, a medium afterhyperpolarization potential (mAHP) was observed in most neurons of the normotensive group. In the hypertensive group, the mAHP was either small or absent. In the latter case, an afterdepolarization potential (ADP) was observed that was unaffected by apamin. Apamin treatment in the normotensive group blocked the mAHP and revealed an ADP resembling that seen in the hypertensive group. We conclude that diminished SK current likely underlies the absence of mAHPs in PVN-RVLM neurons from hypertensive rats. Both the ADP and greater depolarizing input resistance likely contribute to increased excitability of PVN-RVLM neurons from rats with AngII-Salt hypertension.

  • [Show abstract] [Hide abstract]
    ABSTRACT: Small conductance calcium-activated K(+) (SK) channels regulate neuronal excitability. However, little is known about changes in SK channel activity of presympathetic neurons in the hypothalamic paraventricular nucleus (PVN) in essential hypertension. SK channels, calmodulin, and casein kinase II (CK2) form a molecular complex. Because CK2 is upregulated in the PVN in spontaneously hypertensive rats (SHRs), we hypothesized that CK2 increases calmodulin phosphorylation and contributes to diminished SK channel activity in PVN presympathetic neurons in SHRs. Perforated whole-cell recordings were performed on retrogradely labeled spinally projecting PVN neurons in Wistar-Kyoto (WKY) rats and SHRs. Blocking SK channels with apamin significantly increased the firing rate of PVN neurons in WKY rats but not in SHRs. CK2 inhibition restored the stimulatory effect of apamin on the firing activity of PVN neurons in SHRs. Furthermore, apamin-sensitive SK currents and depolarization-induced medium after-hyperpolarization potentials of PVN neurons were significantly larger in WKY rats than in SHRs. CK2 inhibition significantly increased the SK channel current and medium after-depolarization potential of PVN neurons in SHRs. In addition, CK2-mediated calmodulin phosphorylation level in the PVN was significantly higher in SHRs than in WKY rats. Although SK3 was detected in the PVN, its expression level did not differ significantly between SHRs and WKY rats. Our findings suggest that CK2-mediated calmodulin phosphorylation is increased and contributes to diminished SK channel function of PVN presympathetic neurons in SHRs. This information advances our understanding of the mechanisms underlying hyperactivity of PVN presympathetic neurons and increased sympathetic vasomotor tone in hypertension. This article is protected by copyright. All rights reserved.
    Journal of Neurochemistry 05/2014; 130(5). DOI:10.1111/jnc.12758 · 4.24 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Despite the availability of several antihypertensive medications, the morbidity and mortality caused by hypertension is on the rise, suggesting the need for investigation of novel signaling pathways involved in its pathogenesis. Recent evidence suggests the role of toll-like receptor (TLR) 4 in various inflammatory diseases, including hypertension. The role of the brain in the initiation and progression of all forms of hypertension is well established, but the role of brain TLR4 in progression of hypertension has never been explored. Therefore, we investigated the role of TLR4 within the paraventricular nucleus (PVN; an important cardioregulatory center in the brain) in an animal model of human essential hypertension. We hypothesized that a TLR4 blockade within the PVN causes a reduction in mean arterial blood pressure (MAP), inflammatory cytokines and sympathetic drive in hypertensive animals. Spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto (WKY) rats were administered either a specific TLR4 blocker, viral inhibitory peptide (VIPER), or control peptide in their PVN for 14 days. MAP was recorded continuously by radiotelemetry. PVN and blood were collected for the measurement of pro-inflammatory cytokines (Tumor Necrosis Factor (TNF)-α, interleukin (IL)-1β), anti-inflammatory cytokine IL-10, inducible nitric oxide synthase (iNOS), TLR4, nuclear factor (NF) κB activity and plasma norepinephrine (NE) and high mobility group box (HMGB)1 expression, respectively. Hypertensive rats exhibited significantly higher levels of TLR4 in the PVN. TLR4 inhibition within the PVN attenuated MAP, improved cardiac hypertrophy, reduced TNF-α, IL-1β, iNOS levels, and NFκB activity in SHR but not in WKY rats. These results were associated with a reduction in plasma NE and HMGB1 levels and an increase in IL-10 levels in SHR. This study demonstrates that TLR4 upregulation in PVN plays an important role in hypertensive response. Our results provide mechanistic evidence that hypertensive response in SHR are mediated, at least in part, by TLR4 in the PVN and that inhibition of TLR4 within the PVN attenuates blood pressure and improves inflammation, possibly via reduction in sympathetic activity.
    Journal of Neuroinflammation 12/2015; 12(1). DOI:10.1186/s12974-015-0242-7 · 4.90 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Development of angiotensin II (Ang II)-dependent hypertension involves microglial activation and proinflammatory cytokine actions in the hypothalamic paraventricular nucleus (PVN). Cytokines activate receptor signaling pathways that can both acutely grade neuronal discharge and trigger long-term adaptive changes that modulate neuronal excitability through gene transcription. Here, we investigated contributions of PVN cytokines to maintenance of hypertension induced by subcutaneous infusion of Ang II (150 ng/kg per min) for 14 days in rats consuming a 2% NaCl diet. Results indicate that bilateral PVN inhibition with the GABA-A receptor agonist muscimol (100 pmol/50 nL) caused significantly greater reductions of renal and splanchnic sympathetic nerve activity (SNA) and mean arterial pressure in hypertensive than in normotensive rats (P<0.01). Thus, ongoing PVN neuronal activity seems required for support of hypertension. Next, the role of the prototypical cytokine tumor necrosis factor-α was investigated. Whereas PVN injection of tumor necrosis factor-α (0.3 pmol/50 nL) acutely increased lumbar and splanchnic SNA and mean arterial pressure, interfering with endogenous tumor necrosis factor-α by injection of etanercept (10 μg/50 nL) was without effect in hypertensive and normotensive rats. Next, we determined that although microglial activation in PVN was increased in hypertensive rats, bilateral injections of minocycline (0.5 μg/50 nL), an inhibitor of microglial activation, failed to reduce lumbar or splanchnic SNA or mean arterial pressure in hypertensive or in normotensive rats. Collectively, these findings indicate that established Ang II-salt hypertension is supported by PVN neuronal activity, but short term maintenance of SNA and arterial blood pressure does not depend on ongoing local actions of tumor necrosis factor-α.
    Hypertension 12/2013; 63(3). DOI:10.1161/HYPERTENSIONAHA.113.02429 · 7.63 Impact Factor