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High Salt Intake Augments Excitability of PVN Neurons in Rats: Role of the Endoplasmic Reticulum Ca 2+ Store

View Article: PubMed Central - PubMed

ABSTRACT

High salt (HS) intake sensitizes central autonomic circuitry leading to sympathoexcitation. However, its underlying mechanisms are not fully understood. We hypothesized that inhibition of PVN endoplasmic reticulum (ER) Ca2+ store function would augment PVN neuronal excitability and sympathetic nerve activity (SNA). We further hypothesized that a 2% (NaCl) HS diet for 5 weeks would reduce ER Ca2+ store function and increase excitability of PVN neurons with axon projections to the rostral ventrolateral medulla (PVN-RVLM) identified by retrograde label. PVN microinjection of the ER Ca2+ ATPase inhibitor thapsigargin (TG) increased SNA and mean arterial pressure (MAP) in a dose-dependent manner in rats with a normal salt (NS) diet (0.4%NaCl). In contrast, sympathoexcitatory responses to PVN TG were significantly (p < 0.05) blunted in HS treated rats compared to NS treatment. In whole cell current-clamp recordings from PVN-RVLM neurons, graded current injections evoked graded increases in spike frequency. Maximum discharge was significantly augmented (p < 0.05) by HS diet compared to NS group. Bath application of TG (0.5 μM) increased excitability of PVN-RVLM neurons in NS (p < 0.05), yet had no significant effect in HS rats. Our data indicate that HS intake augments excitability of PVN-RVLM neurons. Inhibition of the ER Ca2+-ATPase and depletion of Ca2+ store likely plays a role in increasing PVN neuronal excitability, which may underlie the mechanisms of sympathoexcitation in rats with chronic HS intake.

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Effect of ER Ca2+ uptake inhibition with TG on excitability of PVN-RVLM neurons from NS and HS rats. (A) Voltage traces illustrating neuronal excitability in response to a 200 pA current injection in PVN-RVLM neurons from NS (left) and HS (right) rats in the absence (top, control) and presence (bottom left) of the ER Ca2+ ATPase inhibitor TG. (B) Linear response demonstrating the slope of firing frequency in response to graded current injection (0–200 pA) in PVN-RVLM neurons in the absence and presence of TG in NS (left) and HS (right) neurons. TG increased firing frequency in the NS group, but not HS. Note that firing frequency in response to 200 pA depolarizing current injection was increased in HS (right) compared to NS (left). (C) Summary data showing slope of firing frequency in response to graded current injection before and after bath application of TG in NS and HS rats. Inhibition of the ER Ca2+ store with TG augmented the slope in NS, but not HS neurons. Note that in control conditions, the slope was significantly greater in HS neurons. NS-normal salt; HS-high salt; TG-thapsigargin. †P < 0.05 NS vs. NS –TG; &P < 0.05 NS vs. HS; *P < 0.05 NS vs. NS-TG; #p < 0.05 NS vs. HS (1-way ANOVA Newman-Keuls multiple-comparison test).
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Figure 4: Effect of ER Ca2+ uptake inhibition with TG on excitability of PVN-RVLM neurons from NS and HS rats. (A) Voltage traces illustrating neuronal excitability in response to a 200 pA current injection in PVN-RVLM neurons from NS (left) and HS (right) rats in the absence (top, control) and presence (bottom left) of the ER Ca2+ ATPase inhibitor TG. (B) Linear response demonstrating the slope of firing frequency in response to graded current injection (0–200 pA) in PVN-RVLM neurons in the absence and presence of TG in NS (left) and HS (right) neurons. TG increased firing frequency in the NS group, but not HS. Note that firing frequency in response to 200 pA depolarizing current injection was increased in HS (right) compared to NS (left). (C) Summary data showing slope of firing frequency in response to graded current injection before and after bath application of TG in NS and HS rats. Inhibition of the ER Ca2+ store with TG augmented the slope in NS, but not HS neurons. Note that in control conditions, the slope was significantly greater in HS neurons. NS-normal salt; HS-high salt; TG-thapsigargin. †P < 0.05 NS vs. NS –TG; &P < 0.05 NS vs. HS; *P < 0.05 NS vs. NS-TG; #p < 0.05 NS vs. HS (1-way ANOVA Newman-Keuls multiple-comparison test).

Mentions: Similar to previous reports, PVN-RVLM neurons lacked spontaneous discharge at resting Vm (Chen and Toney, 2009; Chen et al., 2010); however, depolarizing current injections consistently evoked repetitive action potential firing. The role of the ER Ca2+ store in regulating excitability of PVN-RVLM neurons was examined by comparing the relationship between graded current injections and the evoked discharge in the absence and presence of TG (0.5 μM). Figure 4A shows representative discharge responses to a 200 pA depolarizing current pulse in the absence (top) and presence of the ER Ca2+ ATPase inhibitor, TG (bottom). Under control conditions, firing frequency in response to a 200 pA current injection in neurons from NS rats (n = 8, 22 ± 2 Hz) was significantly lower than HS (n = 7, 34 ± 5 Hz, P < 0.05 vs. NS control) (Figures 4A,B, left vs. right). Interestingly, inhibition of the ER Ca2+ store via bath application of TG significantly increased firing frequency in the NS group (n = 6, 30 ± 4 Hz, P < 0.05 vs. NS control), but not HS (n = 6, 32 ± 6 Hz, P > 0.05 vs. HS control) (Figures 4A,B). Furthermore, the slope of firing frequency in response to graded current injection was significantly greater in HS neurons (0.16 ± 0.01, P < 0.05 vs. NS control) compared to NS group (0.10 ± 0.01) (Figures 4B,C). Bath application of TG significantly increased the slope of firing frequency in response to graded current injections in NS group (0.14 ± 0.01, P < 0.05 vs. NS control), yet had no significant effects on HS neurons (0.15 ± 0.01, P > 0.05 vs. HS control) (Figures 4B,C).


High Salt Intake Augments Excitability of PVN Neurons in Rats: Role of the Endoplasmic Reticulum Ca 2+ Store
Effect of ER Ca2+ uptake inhibition with TG on excitability of PVN-RVLM neurons from NS and HS rats. (A) Voltage traces illustrating neuronal excitability in response to a 200 pA current injection in PVN-RVLM neurons from NS (left) and HS (right) rats in the absence (top, control) and presence (bottom left) of the ER Ca2+ ATPase inhibitor TG. (B) Linear response demonstrating the slope of firing frequency in response to graded current injection (0–200 pA) in PVN-RVLM neurons in the absence and presence of TG in NS (left) and HS (right) neurons. TG increased firing frequency in the NS group, but not HS. Note that firing frequency in response to 200 pA depolarizing current injection was increased in HS (right) compared to NS (left). (C) Summary data showing slope of firing frequency in response to graded current injection before and after bath application of TG in NS and HS rats. Inhibition of the ER Ca2+ store with TG augmented the slope in NS, but not HS neurons. Note that in control conditions, the slope was significantly greater in HS neurons. NS-normal salt; HS-high salt; TG-thapsigargin. †P < 0.05 NS vs. NS –TG; &P < 0.05 NS vs. HS; *P < 0.05 NS vs. NS-TG; #p < 0.05 NS vs. HS (1-way ANOVA Newman-Keuls multiple-comparison test).
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Figure 4: Effect of ER Ca2+ uptake inhibition with TG on excitability of PVN-RVLM neurons from NS and HS rats. (A) Voltage traces illustrating neuronal excitability in response to a 200 pA current injection in PVN-RVLM neurons from NS (left) and HS (right) rats in the absence (top, control) and presence (bottom left) of the ER Ca2+ ATPase inhibitor TG. (B) Linear response demonstrating the slope of firing frequency in response to graded current injection (0–200 pA) in PVN-RVLM neurons in the absence and presence of TG in NS (left) and HS (right) neurons. TG increased firing frequency in the NS group, but not HS. Note that firing frequency in response to 200 pA depolarizing current injection was increased in HS (right) compared to NS (left). (C) Summary data showing slope of firing frequency in response to graded current injection before and after bath application of TG in NS and HS rats. Inhibition of the ER Ca2+ store with TG augmented the slope in NS, but not HS neurons. Note that in control conditions, the slope was significantly greater in HS neurons. NS-normal salt; HS-high salt; TG-thapsigargin. †P < 0.05 NS vs. NS –TG; &P < 0.05 NS vs. HS; *P < 0.05 NS vs. NS-TG; #p < 0.05 NS vs. HS (1-way ANOVA Newman-Keuls multiple-comparison test).
Mentions: Similar to previous reports, PVN-RVLM neurons lacked spontaneous discharge at resting Vm (Chen and Toney, 2009; Chen et al., 2010); however, depolarizing current injections consistently evoked repetitive action potential firing. The role of the ER Ca2+ store in regulating excitability of PVN-RVLM neurons was examined by comparing the relationship between graded current injections and the evoked discharge in the absence and presence of TG (0.5 μM). Figure 4A shows representative discharge responses to a 200 pA depolarizing current pulse in the absence (top) and presence of the ER Ca2+ ATPase inhibitor, TG (bottom). Under control conditions, firing frequency in response to a 200 pA current injection in neurons from NS rats (n = 8, 22 ± 2 Hz) was significantly lower than HS (n = 7, 34 ± 5 Hz, P < 0.05 vs. NS control) (Figures 4A,B, left vs. right). Interestingly, inhibition of the ER Ca2+ store via bath application of TG significantly increased firing frequency in the NS group (n = 6, 30 ± 4 Hz, P < 0.05 vs. NS control), but not HS (n = 6, 32 ± 6 Hz, P > 0.05 vs. HS control) (Figures 4A,B). Furthermore, the slope of firing frequency in response to graded current injection was significantly greater in HS neurons (0.16 ± 0.01, P < 0.05 vs. NS control) compared to NS group (0.10 ± 0.01) (Figures 4B,C). Bath application of TG significantly increased the slope of firing frequency in response to graded current injections in NS group (0.14 ± 0.01, P < 0.05 vs. NS control), yet had no significant effects on HS neurons (0.15 ± 0.01, P > 0.05 vs. HS control) (Figures 4B,C).

View Article: PubMed Central - PubMed

ABSTRACT

High salt (HS) intake sensitizes central autonomic circuitry leading to sympathoexcitation. However, its underlying mechanisms are not fully understood. We hypothesized that inhibition of PVN endoplasmic reticulum (ER) Ca2+ store function would augment PVN neuronal excitability and sympathetic nerve activity (SNA). We further hypothesized that a 2% (NaCl) HS diet for 5 weeks would reduce ER Ca2+ store function and increase excitability of PVN neurons with axon projections to the rostral ventrolateral medulla (PVN-RVLM) identified by retrograde label. PVN microinjection of the ER Ca2+ ATPase inhibitor thapsigargin (TG) increased SNA and mean arterial pressure (MAP) in a dose-dependent manner in rats with a normal salt (NS) diet (0.4%NaCl). In contrast, sympathoexcitatory responses to PVN TG were significantly (p &lt; 0.05) blunted in HS treated rats compared to NS treatment. In whole cell current-clamp recordings from PVN-RVLM neurons, graded current injections evoked graded increases in spike frequency. Maximum discharge was significantly augmented (p &lt; 0.05) by HS diet compared to NS group. Bath application of TG (0.5 &mu;M) increased excitability of PVN-RVLM neurons in NS (p &lt; 0.05), yet had no significant effect in HS rats. Our data indicate that HS intake augments excitability of PVN-RVLM neurons. Inhibition of the ER Ca2+-ATPase and depletion of Ca2+ store likely plays a role in increasing PVN neuronal excitability, which may underlie the mechanisms of sympathoexcitation in rats with chronic HS intake.

No MeSH data available.


Related in: MedlinePlus