Whole cell patch recordings from forebrain slices demonstrate angiotensin II inhibits potassium currents in subfornical organ neurons

doi:10.1016/0167-0115(96)00049-3

Regulatory Peptides

Volume 66, Issues 1–2, 8 October 1996, Pages 55-58

https://doi.org/10.1016/0167-0115(96)00049-3 Get rights and content

Abstract

Whole cell patch clamp recordings have been obtained from SFO neurons in a forebrain slice preparation. Basic electrophysiological characteristics recorded from these cells in current clamp mode showed a mean resting membrane potential of −57.0 ± 2.5 mV (± SEM, n = 7, mean input resistance of 900 ± 110 MΩ (n = 7), and a mean spike amplitude of 68.6 ± 4.1 mV (n = 7), accompanied by either irregular or no spontaneous activity. All cells also demonstrated a delayed return to baseline membrane potential following large hyperpolarizing current pulses indicative of the presence of a rapidly activated transient potassium current in these neurons. Voltage clamp recordings identified both rapid transient, and a sustained outward currents which demonstrated the characteristics of I_A and I_K respectively. While bath administration of angiotensin II (Ang) (10⁻⁷ M) was without effect on I_K in 4 of 4 neurons tested, I_A was reduced by between 20 and 100% in the same 4 neurons. These data provide the first description of the basic electrophysiological characteristics of SFO neurons recorded in forebrain slice preparations. They also provide the first direct evidence suggesting that Ang may exert its control over the excitability of SFO neurons through modulation of I_A in these cells.

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Cited by (43)

Mechanisms of GABA-mediated inhibition of the angiotensin II-induced cytosolic Ca2+ increase in rat subfornical organ neurons
2021, Brain Research
Citation Excerpt :
The inhibitory action of galanin in other areas of the brain, such as the locus coeruleus and supraoptic nucleus neurons, is thought to be due to increased K+ conductance across the cell membrane (Papas and Bourque, 1997; Ren et al., 2001). AII suppresses outward-rectifying K+ current in SFO neurons (Ferguson and Li, 1996); therefore, it is possible that galanin hyperpolarizes the cell membrane and suppresses SFO neuronal activation by opening K+ channels which were closed by AII or other K+ channels on which AII did not act. As we found that the AII-induced persistent [Ca2+]i increase has voltage-dependent component (Izumisawa et al., 2019b); the inhibition by galanin may be explained, at least in part, by the above-mentioned effects on K+ channels.
Neurons in the subfornical organ (SFO) sense both neurotransmitters and circulating humoral factors such as angiotensin II (AII) and atrial natriuretic peptide (ANP), and regulate multiple physiological functions including drinking behavior. We recently reported that AII at nanomolar concentrations induced a persistent [Ca²⁺]_i increase in acutely dissociated SFO neurons and that this effect of AII was reversibly inhibited by GABA. In the present study, we studied the inhibitory mechanism of GABA using Ca²⁺ imaging and patch-clamp electrophysiology. The AII-induced persistent [Ca²⁺]_i increase was inhibited by GABA in more than 90% of AII-responsive neurons and by other two SFO inhibitory ligands, ANP and galanin, in about 60 and 30% of neurons respectively. The inhibition by GABA was mimicked by the GABA_A and GABA_B receptor agonists muscimol and baclofen. The involvement of both GABA receptor subtypes was confirmed by reversal of the GABA-mediated inhibition only when the GABA_A and GABA_B receptors antagonists bicuculline methiodide and CGP55845 were both present. The GABA_B agonist baclofen rapidly and reversibly inhibited voltage-gated Ca²⁺ channel (VGCC) currents recorded in response to depolarizing pulses in voltage-clamp electrophysiology using Ba²⁺ as a charge carrier (I_Ba). Baclofen inhibition of I_Ba was antagonized by CGP55845, confirming GABA_B receptor involvement; was reduced by N-ethylmaleimide, suggesting downstream Gi-mediated actions; and was partially removed by a large prepulse, indicating voltage-dependency. The magnitude of I_Ba inhibition by baclofen was reduced by the application of selective blockers for N-, P/Q-, and L-type VGCCs (ω-conotoxin GVIA, ω-agatoxin IVA, and nifedipine respectively). Overall, our study indicates that GABA inhibition of the AII-induced [Ca²⁺]_i increase is mediated by both GABA_A and GABA_B receptors, and that GABA_B receptors associated with Gi proteins suppress Ca²⁺ entry through VGCCs in SFO neurons.
Persistent cytosolic Ca2+ increase induced by angiotensin II at nanomolar concentrations in acutely dissociated subfornical organ (SFO)neurons of rats
2019, Brain Research
Citation Excerpt :
The former is independent of CaMK, while the latter is dependent on phosphorylation by CaMK. The initial phase thus may be explained by the reported actions of AT1 receptor activation, such as non-selective cation channel activation, K+ channel inhibition and Ca2+ channel potentiation (Ferguson and Li, 1996; Wang et al., 2013; Washburn and Ferguson, 2001). CaMK is a multifunctional protein kinase and highly expressed in central nervous system neuronal cells.
It is known that angiotensin II (AII) is sensed by subfornical organ (SFO) to induce drinking behaviors and autonomic changes. AII at picomolar concentrations have been shown to induce Ca²⁺ oscillations and increase in the amplitude and frequency of spontaneous Ca²⁺ oscillations in SFO neurons. The present study was conducted to examine effects of nanomolar concentrations of AII using the Fura-2 Ca²⁺-imaging technique in acutely dissociated SFO neurons. AII at nanomolar concentrations induced an initial [Ca²⁺]_i peak followed by a persistent [Ca²⁺]_i increase lasting for longer than 1 hour. By contrast, [Ca²⁺]_i responses to 50 mM K⁺, maximally effective concentrations of glutamate, carbachol, and vasopressin, and AII given at picomolar concentrations returned to the basal level within 20 min. The AII-induced [Ca²⁺]_i increase was blocked by the AT1 antagonist losartan. However, losartan had no effect when added during the persistent phase. The persistent phase was suppressed by extracellular Ca2+ removal, significantly inhibited by blockers of L and P/Q type Ca²⁺ channels , but unaffected by inhibition of Ca²⁺ store Ca²⁺ ATPase. The persistent phase was reversibly suppressed by GABA and inhibited by CaMK and PKC inhibitors. These results suggest that the persistent [Ca²⁺]_i increase evoked by nanomolar concentrations of AII is initiated by AT1 receptor activation and maintained by Ca²⁺ entry mechanisms in part through L and P/Q type Ca²⁺ channels, and that CaMK and PKC are involved in this process. The persistent [Ca²⁺]_i increase induced by AII at high pathophysiological levels may have a significant role in altering SFO neuronal functions.
The cytosolic Ca2+ concentration in acutely dissociated subfornical organ (SFO) neurons of rats: Spontaneous Ca2+ oscillations and Ca2+ oscillations induced by picomolar concentrations of angiotensin II
2019, Brain Research
Citation Excerpt :
However, the result that extracellular Ca2+ removal inhibited AII-induced Ca2+ increases clearly indicates that voltage-dependent Ca2+ entry is the major source of the [Ca2+]i increase in response to AII in SFO neurons. AII can activate voltage-dependent Ca2+ entry in two ways: activation of non-selective cation channels (Ono et al., 2001) and inhibition of the outwardly rectifying K+ currents (Ferguson and Li, 1996). The findings that AII-induced Ca2+ oscillations were susceptible to inhibition by Ca2+ removal, Na+ replacement, and TTX, as were spontaneous Ca2+ oscillations, suggest that a common mechanism underlies the two Ca2+ oscillations.
Characteristics of subfornical organ (SFO) neurons were examined by measuring the cytosolic Ca²⁺ concentration ([Ca²⁺]_i) in acutely dissociated neurons of the rat. SFO neurons, defined by the responsiveness to 50 mM K⁺ (n = 67) responded to glutamate (86%), angiotensin II (AII) (50%), arginine vasopressin (AVP) (66%) and/or carbachol (CCh) (61%), at their maximal concentrations, with marked increases in [Ca²⁺]_i. More than a half (174/307) of SFO neurons examined exhibited spontaneous Ca²⁺ oscillations, while the remainder showed a relatively stable baseline under unstimulated conditions. Spontaneous Ca²⁺ oscillations were suppressed when extracellular Ca²⁺ was removed and were inhibited when extracellular Na⁺ was replaced with equimolar N-methyl-D-glucamine. Ca²⁺ oscillations were unaffected by the inhibitor of Ca²⁺-dependent ATPases cyclopiazonic acid, the N-type Ca²⁺ channel blocker ω-conotoxin GVIA and the P/Q-type Ca²⁺ channel blocker ω-agatoxin IVA, but significantly inhibited by the high-voltage-activated Ca²⁺ channel blocker Cd²⁺ and the L-type Ca²⁺ channel blocker nicardipine. Ca²⁺ oscillations were also completely arrested by the voltage-gated Na⁺ channel blocker tetrodotoxin in 50% of SFO neurons but only partially in the remaining neurons. These results suggest that SFO neurons exhibit spontaneous membrane Ca²⁺ oscillations that are dependent in part on Ca²⁺ entry through L-type Ca²⁺ channels, whose activation may result from burst firing. Moreover, AII at picomolar concentrations induced Ca²⁺ oscillations in neurons showing no spontaneous Ca²⁺ oscillations, while spontaneous Ca²⁺ oscillations were arrested by gamma-aminobutyric acid (10 μM), suggesting that rises in [Ca²⁺]_i during Ca²⁺ oscillations may play an important role in the modulation of SFO neuron function.
Circumventricular organs: Targets for integration of circulating fluid and energy balance signals?
2013, Physiology and Behavior
Citation Excerpt :
It is also interesting to note the expression levels of AT1 receptors in the SFO have been reported to be decreased in hypertensive states [46], thus highlighting the dynamic nature of angiotensin II effects on SFO neurons in accordance with the cardiovascular status of the organism. Finally, patch-clamp recordings have revealed that angiotensin II modulates numerous conductances to exert its central effects, namely by inhibiting the transient potassium conductance IA [47], and potentiating both a non-selective cation conductance [48] and voltage-gated calcium channels [49]. Since these studies demonstrating SFO neurons to be critical sensors of circulating angiotensin II, considerable additional evidence has highlighted roles for this CVO in responding to many other circulating signals of importance to cardiovascular regulation and fluid balance.
The subfornical organ (SFO), as one of the sensory circumventricular organs (CVOs), is among the only central nervous system structures which interfaces directly with circulating substances that do not cross the blood brain barrier. Here we describe a growing literature showing that circulating indicators of cardiovascular (angiotensin II, osmolarity, calcium, sodium) and metabolic (adiponectin, amylin, glucose, ghrelin, leptin) statuses influence the excitability of single SFO neurons. Single cell electrophysiological studies from our laboratory have demonstrated excitatory effects of angiotensin II on individual SFO neurons, and changes in angiotensin II receptor expression in this CVO in hypertensive states emphasize the dynamic contribution of SFO neurons to the regulation of fluid balance. Furthermore, we have shown both depolarizing and hyperpolarizing effects of the adipokines adiponectin and leptin in SFO cells, and highlight that conditions of fasting in the case of adiponectin, and obesity in the case of leptin, alter the sensitivity of SFO neurons to these circulating factors. The results examined in this review provide evidence for a role of the SFO as a mediator and integrative structure in the maintenance of cardiovascular and metabolic functions.
AT2 receptor signaling and sympathetic regulation
2011, Current Opinion in Pharmacology
There is a growing consensus that the balance between Angiotensin Type 1 (AT1R) and Angiotensin Type 2 (AT2R) signaling in many tissues may determine the magnitude and, in some cases the direction, of the biological response. Sympatho-excitation in cardiovascular diseases is mediated by a variety of factors and is, in part, dependent on Angiotensin II signaling in the central nervous system. Recent data have provided evidence that the AT2R can modulate sympatho-excitation in animals with hypertension and heart failure. The evidence for this concept is reviewed and a model is put forward to support the rationale that therapeutic targeting of the central AT2R may be beneficial in the setting of chronic heart failure.
The attenuation of central angiotensin II-dependent pressor response and intra-neuronal signaling by intracarotid injection of nanoformulated copper/zinc superoxide dismutase
2010, Biomaterials
Adenoviral-mediated overexpression of the intracellular superoxide (O₂^·−) scavenging enzyme copper/zinc superoxide dismutase (CuZnSOD) in the brain attenuates central angiotensin II (AngII)-induced cardiovascular responses. However, the therapeutic potential for adenoviral vectors is weakened by toxicity and the inability of adenoviral vectors to target the brain following peripheral administration. Therefore, we developed a non-viral delivery system in which CuZnSOD protein is electrostatically bound to a synthetic poly(ethyleneimine)–poly(ethyleneglycol) (PEI–PEG) polymer to form a polyion complex (CuZnSOD nanozyme). We hypothesized that PEI–PEG polymer increases transport of functional CuZnSOD to neurons, which inhibits AngII intra-neuronal signaling. The AngII-induced increase in O₂^·−, as measured by dihydroethidium fluorescence and electron paramagnetic resonance spectroscopy, was significantly inhibited in CuZnSOD nanozyme-treated neurons compared to free CuZnSOD- and non-treated neurons. CuZnSOD nanozyme also attenuated the AngII-induced inhibition of K⁺ current in neurons. Intracarotid injection of CuZnSOD nanozyme into rabbits significantly inhibited the pressor response of intracerebroventricular-delivered AngII; however, intracarotid injection of free CuZnSOD or PEI–PEG polymer alone failed to inhibit this response. Importantly, neither the PEI–PEG polymer alone nor the CuZnSOD nanozyme induced neuronal toxicity. These findings indicate that CuZnSOD nanozyme inhibits AngII intra-neuronal signaling in vitro and in vivo.

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Regulatory Peptides

Abstract

References (19)

Angiotensin acts at the subfornical organ to increase plasma oxytocin concentrations in the rat

Regul. Pept.

Effect of transection of subfornical organ efferent projections on vasopressin release induced by angiotensin or isoprenaline in the rat

Brain Res.

The efferent projections of the subfornical organ of the rat: A circumventricular organ within a neural network subserving water balance

Brain Res.

Actions of endothelin at the subfornical organ

Brain Res.

Angiotensin II responsiveness of rat paraventricular and subfornical organ neurons in vitro

Neuroscience

Angiotensin II sensitive neurons in the supraoptic nucleus, subfornical organ and anteroventral third ventricle of rats in vitro

Brain Res.

Autoradiographic localization of [¹²⁵I]-endothelin-1 binding sites in rat brain

Neurosci. Res.

Subfornical organ lesions reduce intravenous angiotensin-induced drinking

Brain Res.

Effect of nonpeptide angiotensin receptor antagonists on water intake and salt appetite in rats

Brain Res. Bull.

Cited by (43)

Mechanisms of GABA-mediated inhibition of the angiotensin II-induced cytosolic Ca<sup>2+</sup> increase in rat subfornical organ neurons

Persistent cytosolic Ca<sup>2+</sup> increase induced by angiotensin II at nanomolar concentrations in acutely dissociated subfornical organ (SFO)neurons of rats

The cytosolic Ca<sup>2+</sup> concentration in acutely dissociated subfornical organ (SFO) neurons of rats: Spontaneous Ca<sup>2+</sup> oscillations and Ca<sup>2+</sup> oscillations induced by picomolar concentrations of angiotensin II

Circumventricular organs: Targets for integration of circulating fluid and energy balance signals?

AT2 receptor signaling and sympathetic regulation

The attenuation of central angiotensin II-dependent pressor response and intra-neuronal signaling by intracarotid injection of nanoformulated copper/zinc superoxide dismutase

Regular paperWhole cell patch recordings from forebrain slices demonstrate angiotensin II inhibits potassium currents in subfornical organ neurons

Abstract

Regul. Pept.

Brain Res.

Brain Res.

Brain Res.

Neuroscience

Brain Res.

Neurosci. Res.

Brain Res.

Brain Res. Bull.

Regular paper
Whole cell patch recordings from forebrain slices demonstrate angiotensin II inhibits potassium currents in subfornical organ neurons