Enhanced activity of angiotensin II-sensitive neurons in the anterior hypothalamic area of spontaneously hypertensive rats

Cardiovascular responses elicited by a new endogenous angiotensin in the nucleus tractus solitarius of the rat

Cardiovascular effects of angiotensin-(1-12) [ANG-(1-12)] were studied in the medial nucleus of the tractus solitarius (mNTS) in anesthetized, artificially ventilated, adult male Wistar rats. Microinjections (100 nl) of ANG-(1-12) (0.06 mM) into the mNTS elicited maximum decreases in mean arterial pressure (MAP; 34 ± 5.8 mmHg) and heart rate (HR; 39 ± 3.7 beats/min). Bilateral vagotomy abolished ANG-(1-12)-induced bradycardia. Efferent greater splanchnic nerve activity was decreased by microinjections of ANG-(1-12) into the mNTS. Blockade of ANG type 1 receptors (AT(1)Rs; using ZD-7155 or L-158,809), but not ANG type 2 receptors (AT(2)Rs; using PD-123319), significantly attenuated ANG-(1-12)-induced cardiovascular responses. Simultaneous inhibition of both angiotensin-converting enzyme (ACE; using captopril) and chymase (using chymostatin) completely blocked the effects of ANG-(1-12). Microinjections of A-779 [ANG-(1-7) antagonist] did not attenuate ANG-(1-12)-induced responses. Pressure ejection of ANG-(1-12) (0.06 mM, 2 nl) caused excitation of barosensitive mNTS neurons, which was blocked by prior application of the AT(1)R antagonist. ANG-(1-12)-induced excitation of mNTS neurons was also blocked by prior sequential applications of captopril and chymostatin. These results indicate that 1) microinjections of ANG-(1-12) into the mNTS elicited depressor and bradycardic responses by exciting barosensitive mNTS neurons; 2) the decreases in MAP and HR were mediated via sympathetic and vagus nerves, respectively; 3) AT(1)Rs, but not AT(2)Rs, mediated these actions of ANG-(1-12); 4) the responses were mediated via the conversion of ANG-(1-12) to ANG II and both ACE and chymase were involved in this conversion; and 5) ANG-(1-7) was not one of the metabolites of ANG-(1-12) in the mNTS.

Involvement of angiotensin-(1-7) in the hypothalamic hypotensive effect of captopril in sinoaortic denervated rats

The role of anterior hypothalamic angiotensin-(1-7) (Ang-(1-7)) on blood pressure regulation was studied in sinoaortic denervated (SAD) rats. Since angiotensin-converting enzyme inhibitors increase endogenous levels of Ang-(1-7), we addressed the involvement of Ang-(1-7) in the hypotensive effect induced by captopril in SAD rats. Wistar rats 7 days after SAD or sham operation (SO) were anaesthetized and the carotid artery was cannulated for monitoring mean arterial pressure (MAP). A needle was inserted into the anterior hypothalamus for drug administration. Intrahypothalamic administration of Ang-(1-7) (5 pmol) was without effect in SO rats but reduced MAP in SAD rats by 15.5+/-3.2 mm Hg and this effect was blocked by 250 pmol [D-Ala(7)]-Ang-(1-7), a Mas receptor antagonist. Angiotensin II (Ang II) induced an increase in MAP in both groups being the effect greater in SAD rats (DeltaMAP=15.8+/-1.4 mm Hg) than in SO rats (DeltaMAP=9.6+/-1.0 mm Hg). Ang-(1-7) partially abolished the pressor response caused by Ang II in SAD rats. Whilst the captopril intrahypothalamic injection did not affect MAP in SO animals, it significantly reduced MAP in SAD rats (DeltaMAP=-13.3+/-1.9 mm Hg). Either [D-Ala(7)]-Ang-(1-7) or an anti-Ang-(1-7) polyclonal antibody partially blocked the MAP reduction caused by captopril. In conclusion, whilst Ang-(1-7) does not contribute to hypothalamic blood pressure regulation in SO normotensive animals, in SAD rats the heptapeptide induces a reduction of blood pressure mediated by Mas receptor activation. Although Ang-(1-7) is not formed in enough amount in the AHA of SAD animals to exert cardiovascular effects in normal conditions, our results suggest that enhancement of hypothalamic Ang-(1-7) levels by administration of captopril is partially involved in the hypotensive effect of the ACE inhibitor.

γ-Aminobutyric acid in the lateral septal area is involved in mediation of the inhibition of hypothalamic angiotensin II-sensitive neurons induced by blood pressure increases in rats

Previously, we have demonstrated that intravenous phenylephrine-induced increases in blood pressure inhibit angiotensin II-sensitive neurons via gamma-aminobutyric acid (GABA) inputs in the anterior hypothalamic area (AHA). The lateral septal area (LSV) is also demonstrated to be involved in mediation of the baroreceptor reflex. To investigate central mechanisms involved in mediating the baroreceptor reflex, we examined whether GABA in the LSV is involved in mediation of the phenylephrine-induced inhibition of AHA angiotensin II-sensitive neurons. Microinjection of GABA into the LSV inhibited angiotensin II-sensitive neurons in the AHA of rats. The LSV GABA-induced inhibition of AHA neurons was abolished by pressure application of bicuculline onto the same AHA neurons. Intravenous injection of phenylephrine also inhibited AHA angiotensin II-sensitive neurons and the phenylephrine-induced inhibition of AHA neurons was abolished by microinjection of the GABAA receptor antagonist bicuculline into the LSV. In contrast, the LSV microinjection of bicuculline did not affect the inhibition of firing of AHA neurons induced by GABA pressure-applied in the AHA. These findings suggest that intravenous phenylephrine inhibits AHA angiotensin II-sensitive neurons via release of GABA in the LSV.

[Mechanisms of hypertension in the central nervous system]

This article reviews studies by the author on central mechanisms of hypertension. Spontaneously hypertensive rats (SHR) have been developed as a rat model of genetic hypertension, and central acetylcholine has been implicated in hypertension in SHR. The rostral ventrolateral medulla (RVL), a major source of efferent sympathetic activity, has cholinergic pressor systems. The release of acetylcholine is enhanced in the RVL of SHR, leading to hypertension. The alteration of the RVL cholinergic system in SHR results from enhanced angiotensin systems in the anterior hypothalamic area (AHA). Angiotensin II-sensitive neurons are present in the AHA and they are tonically activated by endogenous angiotensins. The basal activity of AHA angiotensin II-sensitive neurons is enhanced in SHR, mainly due to enhanced sensitivity of AHA neurons to angiotensin II. The AHA angiotensin system is also responsible for hypertension induced by emotional stress and central Na(+) increases. These findings suggest that the AHA angiotensin system may play a critical role in the development of hypertension.

Angiotensin II sensitivity of anterior hypothalamic area neurons is enhanced in both spontaneously hypertensive rats and Dahl salt-sensitive rats

We have previously demonstrated that some neurons in the anterior hypothalamic area (AHA) are tonically activated by endogenous angiotensins. Furthermore, we have demonstrated that intracerebroventricular injection of hypertonic saline increases the firing rate of AHA angiotensin II-sensitive neurons via angiotensins and that the central sodium-induced activation of AHA neurons is enhanced in spontaneously hypertensive rats (SHR) and Dahl salt-sensitive (Dahl S) rats. In this study, we examined whether sensitivities of AHA angiotensin II-sensitive neurons to angiotensin II are enhanced in SHR and Dahl S rats as compared with their respective controls. Male 15- to 16-week-old SHR and age-matched Wistar Kyoto rats (WKY), and male 15- to 16-week-old Dahl S rats and Dahl R rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. In SHR, pressure application of angiotensin II (3 x 10(-9) to 3 x 10(-8) M) onto AHA angiotensin II-sensitive neurons increased their firing rate in a concentration-dependent manner. In WKY, only the highest concentration of angiotensin II increased the firing rate, while the lower concentrations of angiotensin II did not affect it. In Dahl S rats, pressure application of angiotensin II (10(-8) and 3 x 10(-8) M) onto AHA neurons increased their firing rate, while angiotensin II (3 x 10(-9) M) did not affect it. In Dahl R rats, the highest concentration of angiotensin II increased the firing rate, while the lower concentrations of angiotensin II did not affect it. These findings indicate that the sensitivity of AHA neurons to angiotenisn II is enhanced in SHR and Dahl S rats as compared with their respective controls.

Enhanced central hypertonic saline-induced activation of angiotensin II-sensitive neurons in the anterior hypothalamic area of spontaneously hypertensive and Dahl S rats

High dietary salt intake activates the brain renin-angiotensin system in spontaneously hypertensive rats (SHR) and Dahl S rats, resulting in sympathetic hyperactivity and hypertension. Increases of sodium concentration in cerebrospinal fluid (CSF) and/or enhanced responses to CSF sodium are considered to be involved in the high dietary salt-induced activation of central nervous system pathways in those rats. Previously we have demonstrated that intracerebroventricular injection of hypertonic saline increases the neural activity of angiotensin II-sensitive neurons trans-synaptically via endogenous angiotensins in the anterior hypothalamic area (AHA) of rats. In the present study, we examined whether the AHA angiotensin II-sensitive neuron response to hypertonic saline would differ in SHR and Dahl S rats from those of their controls. Male 15- to 16-week-old SHR and age-matched Wistar Kyoto rats (WKY), Dahl S rats and Dahl R rats and Wistar rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Intracerebroventricular injection of hypertonic saline increased the firing rate of AHA angiotensin II-sensitive neurons. The threshold sodium concentration for the central sodium-induced increase of neural firing was lower in SHR than those of WKY, Dahl S rats, Dahl R rats and Wistar rats. The increase in neural firing induced by hypertonic saline (250 mM) was greater in SHR than those of other four kinds of rats. Similarly, the threshold sodium concentration was lower in Dahl S rats than those of WKY, Dahl R rats and Wistar rats and the increase in neural firing induced by hypertonic saline (250 mM) was greater in Dahl S rats than those of WKY, Dahl R rats and Wistar rats. In SHR, intracerebroventricular injection of the amiloride-sensitive sodium channel blocker benzamil abolished the hypertonic saline (250 mM)-induced increase in neural firing, but the sodium channel blocker itself did not affect the basal firing of these neurons. These findings indicate that central sodium-induced activation of AHA angiotensin II-sensitive neurons is enhanced in SHR and Dahl S rats.

Activities of hypothalamic angiotensin II-sensitive neurons are greately enhanced even in prehypertensive spontaneously hypertensive rats

We have previously demonstrated that some neurons in the anterior hypothalamic area (AHA) of rats are tonically activated by endogenous angiotensins and that reactivities of these neurons to angiotensin II are enhanced in 15- to 16-week-old spontaneously hypertensive rats (SHR). To investigate whether the enhanced reactivity of SHR AHA neurons to angiotensin II is secondary to raised blood pressure, we examined whether the enhanced reactivity to angiotensin II also occurs in prehypertensive SHR. We also examined whether reactivities of AHA angiotensin II-sensitive neurons to intracerebroventricular hypertonic saline are enhanced in prehypertensive SHR, since intracerebroventricular injection of hypertonic saline increases the firing rate of AHA neurons via release of angiotensins at AHA neuron levels. Male 4-week-old SHR and age-matched Wistar Kyoto rats (WKY) were used in this study. There was no difference in systolic blood pressure between both rats. They were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Pressure application of angiotensin II onto some AHA neurons increased their firing rate. The basal firing rate of angiotensin II-sensitive neurons was increased in SHR as compared with WKY. The increase of unit firing by angiotenisn II was enhanced in SHR as compared with WKY. Intracerebroventricular injection of hypertonic saline increased the firing rate of AHA angiotensin II-sensitive neurons. The average threshold sodium concentration for the saline-induced increase of neural firing was lower in SHR than in WKY. These findings demonstrate that basal activities and responsiveness to angiotensin II in AHA angiotensin II-sensitive neurons are enhanced in prehypertensive SHR as compared with age-matched WKY. In addition, these findings indicate that central saline-induced activation of AHA angiotensin II-sensitive neurons is also enhanced in SHR. It appears that the enhanced reactivity of SHR AHA neurons to angiotensin II occurs primarily in nature but not secondarily to raised blood pressure in SHR.

Posterior hypothalamus cholinergic stimulation-induced activation of anterior hypothalamic area neurons is enhanced in spontaneously hypertensive rats

We have previously demonstrated that some neurons in the anterior hypothalamic area (AHA) are tonically activated by endogenous angiotensins in rats and that activities of these AHA neurons are enhanced in spontaneously hypertensive rats (SHR). In addition, we have demonstrated that cholinergic mechanisms in the posterior hypothalamic nucleus (PHN) are involved in the activation of AHA angiotensin-II-sensitive neurons. It has been suggested that cholinergic function in the posterior hypothalamus is enhanced in SHR and that this hyperactivity plays a role in hypertension in SHR. In the present study, we examined whether the PHN cholinergic stimulation-induced activation of AHA angiotensin-II-sensitive neurons is altered in SHR. Male 15- to 16-week-old SHR and age-matched Wistar Kyoto rats (WKY) were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Microinjection of the cholinoceptor agonist carbachol, the cholinesterase inhibitor physostigmine and the excitatory amino acid glutamate into the PHN caused increases in firing rate of AHA angiotensin-II-sensitive neurons in anesthetized WKY and SHR. The increase in firing rate of AHA neurons induced by these drugs was enhanced in SHR as compared to WKY. The enhancement of the physostigmine-induced activation of AHA neurons in SHR was similar to that of the carbachol-induced activation of AHA neurons in SHR. The enhancement of the glutamate-induced activation of AHA neurons in SHR was similar to that of the carbachol-induced activation of AHA neurons in SHR. Microinjection of scopolamine, a cholinoceptor antagonist, into the PHN caused a small but significant decrease of firing rate of AHA angiotensin-II-sensitive neurons in SHR but not in WKY. These findings indicate that the PHN cholinergic stimulation-induced activation of AHA angiotensin-II-sensitive neurons is enhanced in SHR and that PHN cholinergic mechanisms are involved in tonic activation of angiotensin-II-sensitive neurons in the AHA of SHR. It appears that the enhancement of the PHN cholinergic stimulation-induced activation of AHA neurons in SHR results mainly from the enhanced neural reactivity to angiotensins in AHA neurons of SHR.

Cholinergic stimulation in the posterior hypothalamic nucleus activates angiotensin II-sensitive neurons in the anterior hypothalamic area of rats

We have previously reported that some neurons in the anterior hypothalamic area (AHA) are tonically activated by endogenous angiotensins in rats and that activities of these AHA angiotensin II-sensitive neurons are enhanced in spontaneously hypertensive rats (SHR). Acetylcholine in the posterior hypothalamic nucleus (PHN) has been implicated in hypertension in SHR. It is suggested that there exist neuronal projections from the PHN to the AHA in rats. In this study, we examined whether cholinergic stimulation in the PHN activates AHA angiotensin II-sensitive neurons. Male Wistar rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Microinjection of carbachol, physostigmine and glutamate into the PHN caused an increase in firing rate of AHA angiotensin II-sensitive neurons in anesthetized rats. The carbachol-induced increase of firing rate was inhibited by pressure application of the AT1 receptor antagonist losartan onto AHA angiotensin II-sensitive neurons. The glutamate-induced increase of firing rate was also inhibited by the pressure application of losartan. PHN microinjections of carbachol and glutamate did not affect blood pressure in these anesthetized rats. In conscious rats, PHN microinjection of carbachol produced an increase of blood pressure and the carbachol-induced pressor response was inhibited by bilateral microinjections of losartan into the AHA. These findings indicate that cholinergic stimulation in the PHN activates AHA angiotensin II-sensitive neurons. It seems likely that the activation of AHA angiotensin II-sensitive neurons induced by PHN cholinergic stimulation is partly mediated via release of angiotensins at AHA angiotensin II-sensitive neuron levels.

Anterior hypothalamic neurons respond to blood pressure changes via γ-aminobutyric acid and angiotensins in rats

It has been suggested that neurons in the hypothalamus respond to baroreflex activation and deactivation. In this study, we examined whether angiotensin II-sensitive neurons in the anterior hypothalamic area (AHA) respond to baroreflex activation and deactivation, and which neurotransmitters are involved in mediating the baroreflex responses. Male Wistar rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Increases in blood pressure induced by intravenous phenylephrine completely inhibited the firing of AHA angiotensin II-sensitive neurons. The phenylephrine-induced inhibition of neuronal firing was blocked and enhanced by the pressure application of bicuculline and nipecotic acid, respectively, onto the same neurons. In contrast, decreases in blood pressure induced by intravenous nitroprusside increased the firing of angiotensin II-sensitive neurons. The nitroprusside-induced increase of neuronal firing was blocked by the pressure application of losartan onto the same neurons. These findings suggest that angiotensin II-sensitive neurons in the AHA respond to blood pressure changes via gamma-aminobutyric acid and angiotensins in rats.

Role of protein kinase Cβ in phorbol ester-induced c-fos gene expression in neurons of normotensive and spontaneously hypertensive rat brains

We have previously demonstrated that pressure application of the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) onto some neurons in the anterior hypothalamic area of rats increases neural activity in vivo and that this PKC activation-induced increase of neural activity is enhanced in spontaneously hypertensive rats (SHR), an animal model for genetic hypertension. Activation of PKC increases expression of the c-fos gene, an important transcription factor and proto-oncogene thought to be a marker of neural activity. To evaluate PKC isoforms responsible for neural activation, we examined which isoforms of PKC are involved in the PKC activation-induced c-fos gene expression in neuronal cultures of Wistar rat and spontaneously hypertensive rat (SHR) brains. PMA increased c-fos gene expression in neuronal cultures of Wistar rat brain and the PMA-induced c-fos gene expression was inhibited by the PKC inhibitor 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7). The PKCalpha,beta,gamma activator thymeleatoxin also increased c-fos gene expression, while the PKCdelta,epsilon activator ingenol did not affect it. In addition, the PMA-induced c-fos gene expression was inhibited by PKCbetaantisense oligonucleotides (AON) but not by PKCalpha and PKCgammaAONs. In SHR brain neuronal cultures, the PMA-induced c-fos gene expression was enhanced as compared with that of Wistar Kyoto rats (WKY), while basal c-fos gene expression was almost the same in both neuronal cultures. The enhancement of PMA-induced c-fos gene expression in SHR brain cultures was abolished by PKCbetaAON. These findings suggest that in rat brain neuronal cultures, PMA increases c-fos gene expression via activation of PKC and that PKCbetaisoforms are partly involved in the PMA-induced c-fos gene expression. In neuronal cultures of SHR brain, it appears that the PMA-induced c-fos gene expression is also enhanced via PKCbeta.

The medial amygdaloid area is involved in activation of angiotensin II-sensitive neurons in the anterior hypothalamic area

We have previously reported that some neurons in the anterior hypothalamic area (AHA) are tonically activated by endogenous angiotensins in rats and that the activities of these AHA angiotensin II-sensitive neurons are enhanced in spontaneously hypertensive rats. It is suggested that there exist neural projections from the medial amygdala to the AHA in rats. In this study, we examined whether neurons in the medial amygdaloid area (MeA) are involved in the activation of AHA angiotensin II-sensitive neurons. Male Wistar rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Microinjection of glutamate into the MeA caused an increase in the firing rate of AHA angiotensin II-sensitive neurons. The glutamate-induced increase of firing rate was inhibited by pressure application of the AT1 receptor antagonist losartan onto AHA angiotensin II-sensitive neurons. The microinjection of glutamate into the central amygdaloid area also increased the firing rate of AHA angiotensin II-sensitive neurons, but the glutamate-induced increase of firing rate was not affected by pressure application of losartan onto AHA angiotensin II-sensitive neurons. The microinjection of corticotropin-releasing factor (CRF) into the MeA also increased the firing rate of AHA angiotensin II-sensitive neurons, but the CRF-induced increase of firing rate was not inhibited by pressure application of losartan onto AHA angiotensin II-sensitive neurons. Repeated microinjection of glutamate into the MeA caused an increase in the release of angiotensins in the AHA. These findings indicate that neurons in the MeA are involved in the activation of AHA angiotensin II-sensitive neurons. It seems likely that the activation of AHA angiotensin II-sensitive neurons induced by glutamate but not CRF is partly mediated via the release of angiotensins at AHA angiotensin II-sensitive neuron levels.

Protein kinase C activation-induced increases of neural activity are enhanced in the hypothalamus of spontaneously hypertensive rats

We have previously reported that some neurons in the anterior hypothalamic area (AHA) are tonically activated by endogenous angiotensins in rats and that activities of these angiotensin II-sensitive neurons in the AHA are enhanced in spontaneously hypertensive rats (SHR). In addition, neural activations induced by both angiotensin II and glutamate were enhanced in the AHA of SHR. In this study, we examined whether intracellular neural activation mechanisms via protein kinase C (PKC) and a potassium channel are altered in angiotensin II-sensitive neurons in the AHA of SHR. Male 15- to 16-week-old SHR and age-matched Wistar-Kyoto rats (WKY) and Wistar rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Pressure application of the PKC activator phorbol 12-myristate 13-acetate (PMA) onto angiotensin II-sensitive neurons in the AHA of Wistar rats increased their firing rate. The increase of unit activity by PMA was inhibited by the potent inhibitor of PKC, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7), but not by the weak PKC inhibitor, N-(2-guanidinoethyl)-5-isoquinolinesulfonamide hydrochloride (HA1004). The increase of unit firing by PMA was enhanced in SHR as compared with WKY. Pressure application of H-7 alone decreased the basal firing activity of angiotensin II-sensitive neurons in SHR but not in WKY. HA1004 did not affect the basal firing activity of angiotensin II-sensitive neurons in SHR. Angiotensin II-induced increases of firing rate in AHA neurons were inhibited by H-7 and the inhibition by H-7 was enhanced in SHR as compared with WKY. Pressure application of 4-aminopyridine, a blocker of the transient potassium current, onto angiotensin II-sensitive neurons increased their firing rate and the increase of unit firing rate was almost the same in WKY and SHR. These findings indicate that activation of PKC increases neural activity in angiotensin II-sensitive neurons in the AHA and that this PKC activation-induced increase of neural activity is enhanced in the AHA of SHR. It seems likely that the enhanced PKC activation effect is responsible for the enhanced basal neural activity seen in the AHA of SHR.