Activation of the hypothalamic paraventricular nucleus by forebrain hypertonicity selectively increases tonic vasomotor sympathetic nerve activity

Paraventricular nucleus projections to the nucleus tractus solitarii are essential for full expression of hypoxia-induced peripheral chemoreflex responses

The hypothalamic paraventricular nucleus (PVN) is essential to peripheral chemoreflex neurocircuitry, but the specific efferent pathways utilized are not well defined. The PVN sends dense projections to the nucleus tractus solitarii (nTS), which exhibits neuronal activation following a hypoxic challenge. We hypothesized that nTS-projecting PVN (PVN-nTS) neurons contribute to hypoxia-induced nTS neuronal activation and cardiorespiratory responses. To selectively target PVN-nTS neurons, rats underwent bilateral nTS nanoinjection of retrogradely transported adeno-associated virus (AAV) driving Cre recombinase expression. We then nanoinjected into PVN AAVs driving Cre-dependent expression of Gq or Gi designer receptors exclusively activated by designer drugs (DREADDs) to test the degree that selective activation or inhibition, respectively, of the PVN-nTS pathway affects the hypoxic ventilatory response (HVR) of conscious rats. We used immunohistochemistry for Fos and extracellular recordings to examine how DREADD activation influences PVN-nTS neuronal activation by hypoxia. Pathway activation enhanced the HVR at moderate hypoxic intensities and increased PVN and nTS Fos immunoreactivity in normoxia and hypoxia. In contrast, PVN-nTS inhibition reduced both the HVR and PVN and nTS neuronal activation following hypoxia. To further confirm selective pathway effects on central cardiorespiratory output, rats underwent hypoxia before and after bilateral nTS nanoinjections of C21 to activate or inhibit PVN-nTS terminals. PVN terminal activation within the nTS enhanced tachycardic, sympathetic and phrenic (PhrNA) nerve activity responses to hypoxia whereas inhibition attenuated hypoxia-induced increases in nTS neuronal action potential discharge and PhrNA. The results demonstrate the PVN-nTS pathway enhances nTS neuronal activation and is necessary for full cardiorespiratory responses to hypoxia. KEY POINTS: The hypothalamic paraventricular nucleus (PVN) contributes to peripheral chemoreflex cardiorespiratory responses, but specific PVN efferent pathways are not known. The nucleus tractus solitarii (nTS) is the first integration site of the peripheral chemoreflex, and the nTS receives dense projections from the PVN. Selective GqDREADD activation of the PVN-nTS pathway was shown to enhance ventilatory responses to hypoxia and activation (Fos immunoreactivity (IR)) of nTS neurons in conscious rats, augmenting the sympathetic and phrenic nerve activity (SSNA and PhrNA) responses to hypoxia in anaesthetized rats. Selective GiDREADD inhibition of PVN-nTS neurons attenuates ventilatory responses, nTS neuronal Fos-IR, action potential discharge and PhrNA responses to hypoxia. These results demonstrate that a projection from the PVN to the nTS is critical for full chemoreflex responses to hypoxia.

The damaging duo: Obesity and excess dietary salt contribute to hypertension and cardiovascular disease

Hypertension is a primary risk factor for cardiovascular disease. Cardiovascular disease is the leading cause of death among adults worldwide. In this review, we focus on two of the most critical public health challenges that contribute to hypertension-obesity and excess dietary sodium from salt (i.e., sodium chloride). While the independent effects of these factors have been studied extensively, the interplay of obesity and excess salt overconsumption is not well understood. Here, we discuss both the independent and combined effects of excess obesity and dietary salt given their contributions to vascular dysfunction, autonomic cardiovascular dysregulation, kidney dysfunction, and insulin resistance. We discuss the role of ultra-processed foods-accounting for nearly 60% of energy intake in America-as a major contributor to both obesity and salt overconsumption. We highlight the influence of obesity on elevated blood pressure in the presence of a high-salt diet (i.e., salt sensitivity). Throughout the review, we highlight critical gaps in knowledge that should be filled to inform us of the prevention, management, treatment, and mitigation strategies for addressing these public health challenges.

Effects of Voluntary Sodium Consumption during the Perinatal Period on Renal Mechanisms, Blood Pressure, and Vasopressin Responses after an Osmotic Challenge in Rats

Cardiovascular control is vulnerable to forced high sodium consumption during the per-inatal period, inducing programming effects, with anatomical and molecular changes at the kidney, brain, and vascular levels that increase basal and induce blood pressure. However, the program- ming effects of the natriophilia proper of the perinatal period on blood pressure control have not yet been elucidated. In order to evaluate this, we studied the effect of a sodium overload challenge (SO) on blood pressure response and kidney and brain gene expression in adult offspring exposed to voluntary hypertonic sodium consumption during the perinatal period (PM-NaCl group). Male PM-NaCl rats showed a more sustained increase in blood pressure after SO than controls (PM-Ctrol). They also presented a reduced number of glomeruli, decreased expression of TRPV1, and increased expression of At1a in the kidney cortex. The relative expression of heteronuclear vaso- pressin (AVP hnRNA) and AVP in the supraoptic nucleus was unchanged after SO in PM-NaCl in contrast to the increase observed in PM-Ctrol. The data indicate that the availability of a rich source of sodium during the perinatal period induces a long-term effect modifying renal, cardiovascular, and neuroendocrine responses implicated in the control of hydroelectrolyte homeostasis.

Oxytocin Receptor Activation Rescues Opioid-Induced Respiratory Depression by Systemic Fentanyl in the Rat

Opioid overdose intervention by naloxone, a high affinity receptor antagonist, reverses opioid-induced respiratory depression (OIRD) and analgesia by displacing opioids. Systemic naloxone stimulates release of the hypothalamic neuropeptide oxytocin, which has analgesic properties and participates in cardiorespiratory homeostasis. To test the hypothesis that oxytocin can reverse OIRD, we assessed the rescue potential of graded doses (0, 0.1, 2, 5, 10, 50 nmol/kg, i.v.) of oxytocin to counter fentanyl (60 nmol/kg, i.v.)-induced depression of neural inspiration indexed by recording phrenic nerve activity (PNA) in anesthetized (urethane/α-chloralose), vagotomized, and artificially ventilated rats. Oxytocin dose-dependently rescued fentanyl OIRD by almost immediately reversing PNA burst arrest (P = 0.0057) and restoring baseline burst frequency (P = 0.0016) and amplitude (P = 0.0025) at low but not high doses, resulting in inverted bell-shaped dose-response curves. Oxytocin receptor antagonism (40 nmol/kg, i.v.) prevented oxytocin reversal of OIRD (arrest: P = 0.0066, frequency: P = 0.0207, amplitude: P = 0.0022). Vasopressin 1A receptor (V1aR) antagonism restored high-dose oxytocin efficacy to rescue OIRD (P = 0.0170 to P < 0.0001), resulting in classic sigmoidal dose-response curves, and prevented (P = 0.0135) transient hypertension from V1aR cross-activation (P = 0.0275). Alone, vasopressin (5 nmol/kg, i.v.) failed to reverse fentanyl respiratory arrest (P = 0.6184). The nonpeptide oxytocin receptor agonist WAY-267464 (75 nmol/kg, i.v.), which has V1aR antagonist properties, quickly reversed fentanyl OIRD (P < 0.0001), with rapid recovery of PNA frequency (P = 0.0011) and amplitude (P = 0.0044) without adverse hemodynamic consequences (P = 0.9991). Findings indicate that peptide and nonpeptide agonist activation of oxytocin receptors without V1aR cross-activation rescues fentanyl OIRD. Oxytocin receptor agonists could be lifesaving resuscitation agents that enhance rather than interrupt opioid analgesia. SIGNIFICANCE STATEMENT: Oxytocin receptor activation produces analgesia. Here, we demonstrate that activation by the US Food and Drug Administration-approved agonist oxytocin and the nonpeptide partial agonist WAY-267464 can each reverse fentanyl cardiorespiratory depression. Selective targeting of oxytocin receptors for resuscitation from opioid overdose, alone or in combination with an opioid antagonist, could eliminate or attenuate negative side effects associated with traditional opioid receptor antagonism.

The subfornical organ drives hypertension in polycystic kidney disease via the hypothalamic paraventricular nucleus

AIMS

Hypertension is a prevalent yet poorly understood feature of polycystic kidney disease. Previously we demonstrated that increased glutamatergic neurotransmission within the hypothalamic paraventricular nucleus produces hypertension in the Lewis Polycystic Kidney rat model of polycystic kidney disease. Here we tested the hypothesis that augmented glutamatergic drive to the paraventricular nucleus in Lewis Polycystic Kidney rats originates from the forebrain lamina terminalis, a sensory structure that relays blood-borne information throughout the brain.

METHODS AND RESULTS

Anatomical experiments revealed that 38% of paraventricular nucleus-projecting neurons in the subfornical organ of the lamina terminalis expressed Fos/Fra, an activation marker, in Lewis Polycystic Kidney rats while <1% of neurons were Fos/Fra+ in Lewis control rats (P = 0.01, n = 8). In anaesthetised rats, subfornical organ neuronal inhibition using isoguvacine produced a greater reduction in systolic blood pressure in the Lewis Polycystic Kidney versus Lewis rats (-21 ± 4 vs. -7 ± 2 mmHg, P < 0.01; n = 10), which could be prevented by prior blockade of paraventricular nucleus ionotropic glutamate receptors using kynurenic acid. Blockade of ionotropic glutamate receptors in the paraventricular nucleus produced an exaggerated depressor response in Lewis Polycystic Kidney relative to Lewis rats (-23 ± 4 vs. -2 ± 3 mmHg, P < 0.001; n = 13), which was corrected by prior inhibition of the subfornical organ with muscimol but unaffected by chronic systemic angiotensin II type I receptor antagonism or lowering of plasma hyperosmolality through high-water intake (P > 0.05); treatments that both nevertheless lowered blood pressure in Lewis Polycystic Kidney rats (P < 0.0001).

CONCLUSION

Our data reveal multiple independent mechanisms contribute to hypertension in polycystic kidney disease, and identify high plasma osmolality, angiotensin II type I receptor activation and, importantly, a hyperactive subfornical organ to paraventricular nucleus glutamatergic pathway as potential therapeutic targets.

TRANSLATIONAL PERSPECTIVE

Hypertension is a significant comorbidity for all forms of chronic kidney disease and for individuals with polycystic kidney disease, often an early presenting feature. Nevertheless, the cause(s) of hypertension in polycystic kidney disease are poorly defined. Here we define the contribution of a neural pathway that contributes to hypertension in polycystic kidney disease. Critically, targeting this pathway may provide an additional antihypertensive effect beyond that achieved with current conventional antihypertensive therapies. Future work identifying the drivers of this neural pathway will aid in the development of newer generation antihypertensive medication.

Intermittent hypercapnic hypoxia induces respiratory hypersensitivity to fentanyl accompanied by tonic respiratory depression by endogenous opioids

KEY POINTS

Sleep apnoea increases susceptibility to opioid-induced respiratory depression (OIRD). Endogenous opioids are implicated as a contributing factor in sleep apnoea. Rats exposed to sleep-phase chronic intermittent hypercapnic hypoxia (CIHH) for 7 days exhibited exaggerated OIRD to systemic fentanyl both while anaesthetized and artificially ventilated and while conscious and breathing spontaneously, implicating heightened CNS inhibitory efficacy of fentanyl. CIHH also induced tonic endogenous opioid suppression of neural inspiration. Sleep-related episodes of hypercapnic hypoxia, as in sleep apnoea, promote hypersensitivity to OIRD, with tonic respiratory depression by endogenous opioids implicated as a potential underlying cause.

ABSTRACT

Sleep apnoea (SA) increases opioid-induced respiratory depression (OIRD) and lethality. To test the hypothesis that this results from chronic intermittent bouts of hypercapnic hypoxia (CIHH) accompanying SA, we compared OIRD across continuously normoxic control rats and rats exposed to sleep-phase (8 h/day) CIHH for 1 week. OIRD sensitivity was first assessed in anaesthetized (urethane/α-chloralose), vagotomized and artificially ventilated rats by recording phrenic nerve activity (PNA) to index neural inspiration and quantify PNA burst inhibition to graded doses (0, 2, 20, 50 μg kg-1 , i.v.) of the synthetic opioid fentanyl. Fentanyl dose-dependently reduced PNA burst frequency (P = 0.0098-0.0001), while increasing the duration of burst quiescence at 50 μg kg-1 (P < 0.0001, n = 5-6/group/dose). CIHH shifted the fentanyl dose-phrenic burst frequency response curve to the left (P = 0.0163) and increased the duration of burst quiescence (P < 0.0001). During fentanyl recovery, PNA burst width was increased relative to baseline in normoxic and CIHH rats. Systemic naloxone (1 mg kg-1 , i.v.) reversed fentanyl-induced PNA arrest in both groups (P = 0.0002), and increased phrenic burst amplitude above baseline (P = 0.0113) in CIHH rats only. Differential sensitivity to anaesthesia as a cause of CIHH-related OIRD hypersensitivity was excluded by observing in conscious spontaneously breathing rats that fentanyl at 20 μg kg-1 (i.v.), which silenced PNA in anaesthetized rats, differentially increased breathing variability in normoxic versus CIHH rats (P = 0.0427), while significantly reducing breathing frequency (P < 0.0001) and periodicity (P = 0.0003) in CIHH rats only. Findings indicate that CIHH increased OIRD sensitivity, with tonic inspiratory depression by endogenous opioids as a likely contributing cause.

Central AT1 receptor signaling by circulating angiotensin II is permissive to acute intermittent hypoxia-induced sympathetic neuroplasticity

Acute intermittent hypoxia (AIH) triggers sympathetic long-term facilitation (sLTF), a progressive increase in sympathetic nerve activity (SNA) linked to central AT1 receptor (AT1R) activation by circulating angiotensin II (ANG II). Here, we investigated AIH activation of the peripheral renin-angiotensin system (RAS) and the extent to which the magnitude of RAS activation predicts the magnitude of AIH-induced sLTF. In anesthetized male Sprague-Dawley rats, plasma renin activity (PRA) increased in a linear fashion in response to 5 (P = 0.0342) and 10 (P < 0.0001) cycles of AIH, with PRA remaining at the 10th cycle level 1 h later, a period over which SNA progressively increased. On average, SNA ramping began at the AIH cycle 4.6 ± 0.9 (n = 12) and was similar in magnitude 1 h later whether AIH consisted of 5 or 10 cycles (n = 6/group). Necessity of central AT1R in post-AIH sLTF was affirmed by intracerebroventricular (icv) losartan (40 nmol, 2 µL; n = 5), which strongly attenuated both splanchnic (P = 0.0469) and renal (P = 0.0018) sLTF compared with vehicle [artificial cerebrospinal fluid (aCSF), 2 µL; n = 5]. Bilateral nephrectomy largely prevented sLTF, affirming the necessity of peripheral RAS activation. Sufficiency of central ANG II signaling was assessed in nephrectomized rats. Whereas ICV ANG II (0.5 ng/0.5 µL, 30 min) in nephrectomized rats exposed to sham AIH (n = 4) failed to cause SNA ramping, it rescued sLTF in nephrectomized rats exposed to five cycles of AIH [splanchnic SNA (SSNA), P = 0.0227; renal SNA (RSNA), P = 0.0390; n = 5]. Findings indicate that AIH causes progressive peripheral RAS activation, which stimulates an apparent threshold level of central AT1R signaling that plays a permissive role in triggering sLTF.NEW & NOTEWORTHY Acute intermittent hypoxia (AIH) triggers sympathetic long-term facilitation (sLTF) that relies on peripheral renin-angiotensin system (RAS) activation. Here, increasing AIH cycles from 5 to 10 proportionally increased RAS activity, but not the magnitude of post-AIH sLTF. Brain angiotensin II (ANG II) receptor blockade and nephrectomy each largely prevented sLTF, whereas central ANG II rescued it following nephrectomy. Peripheral RAS activation by AIH induces time-dependent neuroplasticity at an apparent central ANG II signaling threshold, triggering a stereotyped sLTF response.

A preliminary study of the effect of a high-salt diet on transcriptome dynamics in rat hypothalamic forebrain and brainstem cardiovascular control centers

BACKGROUND

High dietary salt intake is strongly correlated with cardiovascular (CV) diseases and it is regarded as a major risk factor associated with the pathogenesis of hypertension. The CV control centres in the brainstem (the nucleus tractus solitarii (NTS) and the rostral ventrolateral medulla (RVLM)) and hypothalamic forebrain (the subfornical organ, SFO; the supraoptic nucleus, SON and the paraventricular nucleus, PVN) have critical roles in regulating CV autonomic motor outflows, and thus maintaining blood pressure (BP). Growing evidence has implicated autonomic regulatory networks in salt-sensitive HPN (SSH), but the genetic basis remains to be delineated. We hypothesized that the development and/ or maintenance of SSH is reliant on the change in the expression of genes in brain regions controlling the CV system.

METHODOLOGY

We used RNA-Sequencing (RNA-Seq) to describe the differential expression of genes in SFO, SON, PVN, NTS and RVLM of rats being chronically fed with high-salt (HS) diet. Subsequently, a selection of putatively regulated genes was validated with quantitative reverse transcription polymerase chain reaction (qRT-PCR) in both Spontaneously Hypertensive rats (SHRs) and Wistar Kyoto (WKY) rats.

RESULTS

The findings enabled us to identify number of differentially expressed genes in SFO, SON, PVN, NTS and RVLM; that are either up-regulated in both strains of rats (SON- Caprin2, Sctr), down-regulated in both strains of rats (PVN- Orc, Gkap1), up-regulated only in SHRs (SFO- Apopt1, Lin52, AVP, OXT; SON- AVP, OXT; PVN- Caprin2, Sclt; RVLM- A4galt, Slc29a4, Cmc1) or down-regulated only in SHRs (SON- Ndufaf2, Kcnv1; PVN- Pi4k2a; NTS- Snrpd2l, Ankrd29, St6galnac6, Rnf157, Iglon5, Csrnp3, Rprd1a; RVLM- Ttr, Faim).

CONCLUSIONS

These findings demonstrated the adverse effects of HS diet on BP, which may be mediated via modulating the signaling systems in CV centers in the hypothalamic forebrain and brainstem.

Glutamate and GABA neurotransmission are increased in paraventricular nucleus of hypothalamus in rats induced to 6-OHDA parkinsonism: Involvement of nNOS

AIM

Parkinson's disease (PD) is a progressive neurodegenerative disease that manifests itself clinically after reaching an advanced pathological stage. Besides motor signals, PD patients present cardiovascular and autonomic alterations. Recent data showed that rats induced to Parkinsonism by 6-hydroxydopamine (6-OHDA) administration in the substantia nigra pars compacta (SNpc) showed lower mean arterial pressure (MAP) and heart rate (HR), as reduction in sympathetic modulation. The paraventricular nucleus of the hypothalamus (PVN) is an important site for autonomic and cardiovascular control, and amino acid neurotransmission has a central role. We evaluate PVN amino acid neurotransmission in cardiovascular and autonomic effects of 6-OHDA Parkinsonism.

METHODS

Male Wistar rats were submitted to guide cannulas implantation into the PVN. 6-OHDA or sterile saline (sham) was administered bilaterally in the SNpc. After 7 days, cardiovascular recordings in conscious state was performed.

RESULTS

Bicuculline promoted an increase in MAP and HR in sham group and exacerbated those effects in 6-OHDA group. NBQX (non-NMDA inhibitor) did not promote changes in sham as in 6-OHDA group. On the other hand, PVN microinjection of LY235959 (NMDA inhibitor) in sham group did not induced cardiovascular alterations, but decreased MAP and HR in 6-OHDA group. Compared to Sham group, 6-OHDA lesion increased the number of neuronal nitric oxide synthase (nNOS)-immunoreactive neurons in the PVN and, nNOS inhibition promoted higher increases in MAP and HR.

CONCLUSION

Our data suggest that the decreased baseline blood pressure and heart rate in animals with Parkinsonism may be due to an increased GABAergic tone via nNOS in the PVN.

GABAB receptors in the hypothalamic paraventricular nucleus mediate β-adrenoceptor-induced elevations of plasma noradrenaline in rats

In the brain, various neurotransmitters such as noradrenaline and GABA regulate peripheral sympathetic functions. Previously, it has been reported that both β-adrenoceptor activation and GABA_B receptor activation in the brain are involved in the elevation of plasma noradrenaline levels. However, it is unknown whether these pathways interact with each other. In the present study, we examined the relationship between the central actions of β-adrenoceptor activation and GABA_B receptor activation with regard to plasma noradrenaline responses using urethane-anesthetized rats. Intracerebroventricular pretreatment with the GABA_A receptor antagonist bicuculline did not affect the β-adrenoceptor agonist isoproterenol-induced elevation of plasma noradrenaline levels. In contrast, pretreatment with the GABA_B receptor antagonist CGP 35348 suppressed the isoproterenol-induced elevation of noradrenaline levels. Intracerebroventricular pretreatment with the β-adrenoceptor antagonist propranolol did not alter the GABA_B receptor agonist baclofen-induced elevation of plasma noradrenaline levels. We next examined the central effects of β-adrenoceptor activation on GABA release in the paraventricular hypothalamic nucleus (PVN), the major integrative center for sympathetic regulation in the brain. Intracerebroventricular administration of isoproterenol increased GABA content in PVN dialysates. In addition, baclofen microinjected unilaterally into the PVN resulted in elevated plasma levels of noradrenaline, but not adrenaline. Finally, unilateral blockade of GABA_B receptors in the PVN suppressed the isoproterenol-induced elevation of plasma noradrenaline level. Our results suggest that activation of β-adrenoceptors in the brain, likely in the PVN, induces GABA release in the PVN, which in turn activates GABA_B receptors in the PVN, leading to elevated plasma noradrenaline.

Glutamatergic neurons of the paraventricular nucleus are critical contributors to the development of neurogenic hypertension

KEY POINTS

Recurrent periods of over-excitation in the paraventricular nucleus (PVN) of the hypothalamus could contribute to chronic over-activation of this nucleus and thus enhanced sympathetic drive. Stimulation of the PVN glutamatergic population utilizing channelrhodopsin-2 leads to an immediate frequency-dependent increase in baseline blood pressure. Partial lesions of glutamatergic neurons of the PVN (39.3%) result in an attenuated rise in blood pressure following Deoxycorticosterone acetate (DOCA)-salt treatment and reduced index of sympathetic activity. These data suggest that stimulation of PVN glutamatergic neurons is sufficient to cause autonomic dysfunction and drive the increase in blood pressure during hypertension.

ABSTRACT

Neuro-cardiovascular dysregulation leads to increased sympathetic activity and neurogenic hypertension. The paraventricular nucleus (PVN) of the hypothalamus is a key hub for blood pressure (BP) control, producing or relaying the increased sympathetic tone in hypertension. We hypothesize that increased central sympathetic drive is caused by chronic over-excitation of glutamatergic PVN neurons. We tested how stimulation or lesioning of excitatory PVN neurons in conscious mice affects BP, baroreflex and sympathetic activity. Glutamatergic PVN neurons were unilaterally transduced with channelrhodopsin-2 using an adeno-associated virus (CamKII-ChR2-eYFP-AAV2) in wildtype mice (n = 7) to assess the impact of acute stimulation of excitatory PVN neurons selectively on resting BP in conscious mice. Stimulation of the PVN glutamatergic population resulted in an immediate frequency-dependent (2, 10 and 20 Hz) increase in BP from baseline by ∼9 mmHg at 20 Hz stimulation (P < 0.001). Additionally, in vGlut2-cre mice glutamatergic neurons of the PVN were bilaterally lesioned utilizing a cre-dependent caspase (AAV2-flex-taCASP3-TEVp). Resting BP and urinary noradrenaline (norepinephrine) levels were then recorded in conscious mice before and after DOCA-salt hypertension. Partial lesions of glutamatergic neurons of the PVN (39.3%, P < 0.05) resulted in an attenuated rise in BP following DOCA-salt treatment (P < 0.05 at 7 day time point, n = 8). Noradrenaline levels as an index of sympathetic activity between the lesion and wildtype groups showed a significant reduction after DOCA-salt treatment in the lesioned animals (P < 0.05). These experiments suggest that stimulation of PVN glutamatergic neurons is sufficient to cause autonomic dysfunction and drive the increase in BP.

Carotid bodies contribute to sympathoexcitation induced by acute salt overload

NEW FINDINGS

What is the central question of this study? Does carotid body input contribute to the hyperosmotic responses? What is the main finding and its importance? The response to NaCl overload is sympathorespiratory excitation. Eliminating the carotid body input reduced sympathoexcitation but did not affect the increase in phrenic burst frequency, whereas eliminating the hypothalamus prevented the tachypnoea and sympathoexcitation. We conclude that the carotid body inputs are essential for the full expression of the sympathetic activity during acute NaCl overload, whereas the tachypnoea depends on hypothalamic mechanisms.

ABSTRACT

Acute salt excess activates central osmoreceptors, which trigger an increase in sympathetic and respiratory activity. The carotid bodies also respond to hyperosmolality of the extracellular compartment, but their contribution to the sympathoexcitatory and ventilatory responses to NaCl overload remains unknown. To evaluate their contribution to acute NaCl overload, we recorded thoracic sympathetic (tSNA), phrenic (PNA) and carotid sinus nerve activities in decorticate in situ preparations of male Holtzman rats (60-100 g) while delivering intra-arterial infusions of hyperosmotic NaCl (0.17, 0.3, 0.7, 1.5 and 2.0 mol l^-1 ; 200 μl infusion over 25-30 s, with a 10 min time interval between solutions) or mannitol (0.3, 0.5, 1.0, 2.7 and 3.8 mol l^-1 ) progressively. The cumulative infusions of hyperosmotic NaCl increased the perfusate osmolality to 341 ± 5 mosmol (kg water)^-1 and elicited an immediate increase in PNA and tSNA (n = 6, P < 0.05) in sham-denervated rats. Carotid body removal attenuated sympathoexcitation (n = 5, P < 0.05) but did not affect the tachypnoeic response. A precollicular transection disconnecting the hypothalamus abolished the sympathoexcitatory and tachypnoeic responses to NaCl overload (n = 6, P < 0.05). Equi-osmolar infusions of mannitol did not alter the PNA and tSNA in sham-denervated rats (n = 5). Sodium chloride infusions increased carotid sinus nerve activity (n = 10, P < 0.05), whereas mannitol produced negligible changes (n = 5). The results indicate that carotid bodies are activated by acute NaCl overload, but not by mannitol. We conclude that the carotid bodies contribute to the increased sympathetic activity during acute NaCl overload, whereas the ventilatory response is mainly mediated by hypothalamic mechanisms.

Sympathoexcitation following intermittent hypoxia in rat is mediated by circulating angiotensin II acting at the carotid body and subfornical organ

KEY POINTS

In anaesthetized rats, acute intermittent hypoxia increases sympathetic nerve activity, sympathetic peripheral chemoreflex sensitivity and central sympathetic-respiratory coupling. Renin-angiotensin system inhibition prevents the sympathetic effects of intermittent hypoxia, with intermittent injections of angiotensin II into the systemic circulation replicating these effects. Bilateral carotid body denervation reduces the sympathetic effects of acute intermittent hypoxia and eliminates the increases in chemoreflex sensitivity and sympathetic-respiratory coupling. Pharmacological inhibition of the subfornical organ also reduces the sympathetic effects of acute intermittent hypoxia, although it has no effect on the increases in chemoreflex sensitivity and central sympathetic-respiratory coupling. Combining both interventions eliminates the sympathetic effects of both intermittent hypoxia and angiotensin II.

ABSTRACT

Circulating angiotensin II (Ang II) is vital for arterial pressure elevation following intermittent hypoxia in rats, although its importance in the induction of sympathetic changes is unclear. We tested the contribution of the renin-angiotensin system to the effects of acute intermittent hypoxia (AIH) in anaesthetized and ventilated rats. There was a 33.7 ± 2.9% increase in sympathetic nerve activity (SNA), while sympathetic chemoreflex sensitivity and central sympathetic-respiratory coupling increased by one-fold following AIH. The sympathetic effects of AIH were prevented by blocking angiotensin type 1 receptors with systemic losartan. Intermittent systemic injections of Ang II (Int.Ang II) elicited similar sympathetic responses to AIH. To identify the neural pathways responsible for the effects of AIH and Int.Ang II, we performed bilateral carotid body denervation, which reduced the increase in SNA by 56% and 45%, respectively. Conversely, pharmacological inhibition of the subfornical organ (SFO), an established target of circulating Ang II, reduced the increase in SNA following AIH and Int.Ang II by 65% and 59%, respectively, although it did not prevent the sensitization of the sympathetic peripheral chemoreflex, nor the increase in central sympathetic-respiratory coupling. Combined carotid body denervation and inhibition of the SFO eliminated the enhancement of SNA following AIH and Int.Ang II. Repeated systemic injections of phenylephrine caused an elevation in SNA similar to AIH, and this effect was prevented by a renin inhibitor, aliskiren. Our findings show that the sympathetic effects of AIH are the result of RAS-mediated activations of the carotid bodies and the SFO.

Hypothalamic PVN contributes to acute intermittent hypoxia-induced sympathetic but not phrenic long-term facilitation

Blackburn MB, Andrade MA, Toney GM. Hypothalamic PVN contributes to acute intermittent hypoxia-induced sympathetic but not phrenic long-term facilitation. J Appl Physiol 124: 1233-1243, 2018. First published December 19, 2017; doi: 10.1152/japplphysiol.00743.2017 .- Acute intermittent hypoxia (AIH) repetitively activates the arterial chemoreflex and triggers a progressive increase of sympathetic nerve activity (SNA) and phrenic nerve activity (PNA) referred to as sympathetic and phrenic long-term facilitation (S-LTF and P-LTF), respectively. Neurons of the hypothalamic paraventricular nucleus (PVN) participate in the arterial chemoreflex, but their contribution to AIH-induced LTF is unknown. To determine this, anesthetized rats were vagotomized and exposed to 10 cycles of AIH, each consisting of ventilation for 3 min with 100% O₂ followed by 3 min with 15% O₂. Before AIH, rats received bilateral PVN injections of artificial cerebrospinal fluid (aCSF; vehicle) or the GABA-A receptor agonist muscimol (100 pmol in 50 nl) to inhibit neuronal activity. Thirty minutes after completing the AIH protocol, during which rats were continuously ventilated with 100% O₂, S-LTF and P-LTF were quantified from recordings of integrated splanchnic SNA and PNA, respectively. PVN muscimol attenuated increases of SNA during hypoxic episodes occurring in later cycles (6-10) of AIH ( P < 0.03) and attenuated post-AIH S-LTF ( P < 0.001). Muscimol, however, did not consistently affect peak PNA responses during hypoxic episodes and did not alter AIH-induced P-LTF. These findings indicate that PVN neuronal activity contributes to sympathetic responses during AIH and to subsequent generation of S-LTF. NEW & NOTEWORTHY Neural circuits mediating acute intermittent hypoxia (AIH)-induced sympathetic and phrenic long-term facilitation (LTF) have not been fully elucidated. We found that paraventricular nucleus (PVN) inhibition attenuated sympathetic activation during episodes of AIH and reduced post-AIH sympathetic LTF. Neither phrenic burst patterning nor the magnitude of AIH-induced phrenic LTF was affected. Findings indicate that PVN neurons contribute to AIH-induced sympathetic LTF. Defining mechanisms of sympathetic LTF could improve strategies to reduce sympathetic activity in cardiovascular and metabolic diseases.

Burst patterning of hypothalamic paraventricular nucleus-driven sympathetic nerve activity in ANG II-salt hypertension

ANG II-salt hypertension selectively increases splanchnic sympathetic nerve activity (sSNA), but the extent to which this reflects increased respiratory versus cardiac rhythmic bursting is unknown. Here, integrated sSNA was elevated in ANG II-infused rats fed a high-salt (2% NaCl) diet (ANG II-HSD) compared with vehicle-infused rats fed a normal-salt (0.4% NaCl) diet (Veh-NSD; P < 0.01). Increased sSNA was not accompanied by increased inspiratory or expiratory bursting, consistent with no group difference in central inspiratory drive. Consistent with preserved inhibitory baroreflex entrainment of elevated sSNA in ANG II-HSD rats, the time integral ( P < 0.05) and amplitude ( P < 0.01) of cardiac rhythmic sSNA were increased. Consistent with activity of hypothalamic paraventricular nucleus (PVN) neurons supporting basal SNA in ANG II-salt hypertension, inhibition of PVN with the GABA-A receptor agonist muscimol reduced mean arterial pressure (MAP) and integrated sSNA only in the ANG II-HSD group ( P < 0.001). PVN inhibition had no effect on respiratory rhythmic sSNA bursting in either group but reduced cardiac rhythmic sSNA in ANG II-HSD rats only ( P < 0.01). The latter likely reflected reduced inhibitory baroreflex entrainment subsequent to the fall of MAP. Of note is that MAP as well as integrated and rhythmic burst patterns of sSNA were similar in vehicle-infused rats whether they were fed a normal or high-salt diet. Findings indicate that PVN neurons support elevated sSNA in ANG II-HSD rats by driving a tonic component of activity without altering respiratory or cardiac rhythmic bursting. Because sSNA was unchanged in Veh-HSD rats, activation of PVN-driven tonic sSNA appears to require central actions of ANG II. NEW & NOTEWORTHY ANG II-salt hypertension is strongly neurogenic and depends on hypothalamic paraventricular nucleus (PVN)-driven splanchnic sympathetic nerve activity (sSNA). Here, respiratory and cardiac bursts of sSNA were preserved in ANG II-salt rats and unaltered by PVN inhibition, suggesting that PVN neurons drive a tonic component of sSNA rather than modulating dominant patterns of burst discharge.

Sympathetic neural and hemodynamic responses to head-up tilt during isoosmotic and hyperosmotic hypovolemia

We hypothesized that muscle sympathetic nerve activity (MSNA) during head-up tilt (HUT) would be augmented during exercise-induced (hyperosmotic) dehydration but not isoosmotic dehydration via an oral diuretic. We studied 26 young healthy subjects (7 female, 19 male) divided into three groups: euhydrated (EUH, n = 7), previously exercised in 40°C while maintaining hydration; dehydrated (DEH, n = 10), previously exercised in 40°C during which ~3% of body weight was lost via sweat loss; and diuretic (DIUR, n = 9), a group that did not exercise but lost ~3% of body weight via diuresis (furosemide, 80 mg by mouth). We measured MSNA, heart rate (HR), and blood pressure (BP) during supine rest and 30° and 45° HUT. Plasma volume (PV) decreased similarly in DEH (-8.5 ± 3.3%) and DIUR (-11.4 ± 5.7%) (P > 0.05). Plasma osmolality was similar between DIUR and EUH (288 ± 4 vs. 284 ± 5 mmol/kg, respectively) but was significantly higher in DEH (299 ± 5 mmol/kg) (P < 0.05). Mixed-model ANOVA was used with repeated measures on position (HUT) and between-group analysis on condition. HR and MSNA increased in all subjects during HUT (main effect of position; P < 0.05). There was also a significant main effect of group, such that MSNA and HR were higher in DEH compared with DIUR (P < 0.05). Changes in HR with HUT were larger in both hypovolemic groups compared with EUH (P < 0.05). The differential HUT response "strategies" in each group suggest a greater role for hypovolemia per se in controlling HR responses during dehydration, and a stronger role for osmolality in control of SNA.NEW & NOTEWORTHY Interactions of volume regulation with control of vascular sympathetic nerve activity (SNA) have important implications for blood pressure regulation. Here, we demonstrate that SNA and heart rate (HR) during hyperosmotic hypovolemia (exercise-induced) were augmented during supine and tilt compared with isoosmotic hypovolemia (diuretic), which primarily augmented the HR response. Our data suggest that hypovolemia per se had a larger role in controlling HR responses, whereas osmolality had a stronger role in control of SNA.

Neuronal circuits involved in osmotic challenges

The maintenance of plasma sodium concentration within a narrow limit is crucial to life. When it differs from normal physiological patterns, several mechanisms are activated in order to restore body fluid homeostasis. Such mechanisms may be vegetative and/or behavioral, and several regions of the central nervous system (CNS) are involved in their triggering. Some of these are responsible for sensory pathways that perceive a disturbance of the body fluid homeostasis and transmit information to other regions. These regions, in turn, initiate adequate adjustments in order to restore homeostasis. The main cardiovascular and autonomic responses to a change in plasma sodium concentration are: i) changes in arterial blood pressure and heart rate; ii) changes in sympathetic activity to the renal system in order to ensure adequate renal sodium excretion/absorption, and iii) the secretion of compounds involved in sodium ion homeostasis (ANP, Ang-II, and ADH, for example). Due to their cardiovascular effects, hypertonic saline solutions have been used to promote resuscitation in hemorrhagic patients, thereby increasing survival rates following trauma. In the present review, we expose and discuss the role of several CNS regions involved in body fluid homeostasis and the effects of acute and chronic hyperosmotic challenges.

Hypothalamic signaling mechanisms in hypertension

The etiology of hypertension, a critical public health issue affecting one in three US adults, involves the integration of the actions of multiple organ systems, including the central nervous system. Increased activation of the central nervous system, driving enhanced sympathetic outflow and increased blood pressure, has emerged as a major contributor to the pathogenesis of hypertension. The hypothalamus is a key brain site acting to integrate central and peripheral inputs to ultimately impact blood pressure in multiple disease states that evoke hypertension. This review highlights recent advances that have identified novel signal transduction mechanisms within multiple hypothalamic nuclei (e.g., paraventricular nucleus, arcuate nucleus) acting to drive the pathophysiology of hypertension in neurogenic hypertension, angiotensin II hypertension, salt-sensitive hypertension, chronic intermittent hypoxia, and obesity-induced hypertension. Increased understanding of hypothalamic activity in hypertension has the potential to identify novel targets for future therapeutic interventions designed to treat hypertension.

ATP stimulates rat hypothalamic sympathetic neurons by enhancing AMPA receptor-mediated currents

We have previously shown that ATP within the paraventricular nucleus (PVN) induces an increase in sympathetic activity, an effect attenuated by the antagonism of P2 and/or glutamatergic receptors. Here, we evaluated precise cellular mechanisms underlying the ATP-glutamate interaction in the PVN and assessed whether this receptor coupling contributed to osmotically driven sympathetic PVN neuronal activity. Whole-cell patch-clamp recordings obtained from PVN-rostral ventrolateral medulla neurons showed that ATP (100 μM, 1 min, bath applied) induced an increase in firing rate (89%), an effect blocked by kynurenic acid (1 mM) or 4-[[4-Formyl-5-hydroxy-6-methyl-3-[(phosphonooxy)methyl]-2-pyridinyl]azo]-1,3-benzenedisulfonic acid tetrasodium salt (PPADS) (10 μM). Whereas ATP did not affect glutamate synaptic function, α-amino-3-hydroxy-5-methylisoxazole propionic acid (AMPA) receptor-mediated currents evoked by focal application of AMPA (50 μM, n = 13) were increased in magnitude by ATP (AMPA amplitude: 33%, AMPA area: 52%). ATP potentiation of AMPA currents was blocked by PPADS (n = 12) and by chelation of intracellular Ca(2+) (BAPTA, n = 10). Finally, a hyperosmotic stimulus (mannitol 1%, +55 mosM, n = 8) potentiated evoked AMPA currents (53%), an effect blocked by PPADS (n = 6). Taken together, our data support a functional stimulatory coupling between P2 and AMPA receptors (likely of extrasynaptic location) in PVN sympathetic neurons, which is engaged in response to an acute hyperosmotic stimulus, which might contribute in turn to osmotically driven sympathoexcitatory responses by the PVN.