Phox2b expression in the aldosterone-sensitive HSD2 neurons of the NTS

Control of sodium appetite by hindbrain aldosterone-sensitive neurons

Mineralocorticoids play a key role in hydromineral balance by regulating sodium retention and potassium wasting. Through favoring sodium, mineralocorticoids can cause hypertension from fluid overload under conditions of hyperaldosteronism, such as aldosterone-secreting tumors. An often-overlooked mechanism by which aldosterone functions to increase sodium is through stimulation of salt appetite. To drive sodium intake, aldosterone targets neurons in the hindbrain which uniquely express 11β-hydroxysteroid dehydrogenase type 2 (HSD2). This enzyme is a necessary precondition for aldosterone-sensing cells as it metabolizes glucocorticoids - preventing their activation of the mineralocorticoid receptor. In this review, we will consider the role of hindbrain HSD2 neurons in regulating sodium appetite by discussing HSD2 expression in the brain, regulation of hindbrain HSD2 neuron activity, and the circuitry mediating the effects of these aldosterone-sensitive neurons. Reducing the activity of hindbrain HSD2 neurons may be a viable strategy to reduce sodium intake and cardiovascular risk, particularly for conditions of hyperaldosteronism.

Sex-Specific Effects of Stress on Respiratory Control: Plasticity, Adaptation, and Dysfunction

As our understanding of respiratory control evolves, we appreciate how the basic neurobiological principles of plasticity discovered in other systems shape the development and function of the respiratory control system. While breathing is a robust homeostatic function, there is growing evidence that stress disrupts respiratory control in ways that predispose to disease. Neonatal stress (in the form of maternal separation) affects "classical" respiratory control structures such as the peripheral O₂ sensors (carotid bodies) and the medulla (e.g., nucleus of the solitary tract). Furthermore, early life stress disrupts the paraventricular nucleus of the hypothalamus (PVH), a structure that has emerged as a primary determinant of the intensity of the ventilatory response to hypoxia. Although underestimated, the PVH's influence on respiratory function is a logical extension of the hypothalamic control of metabolic demand and supply. In this article, we review the functional and anatomical links between the stress neuroendocrine axis and the medullary network regulating breathing. We then present the persistent and sex-specific effects of neonatal stress on respiratory control in adult rats. The similarities between the respiratory phenotype of stressed rats and clinical manifestations of respiratory control disorders such as sleep-disordered breathing and panic attacks are remarkable. These observations are in line with the scientific consensus that the origins of adult disease are often found among developmental and biological disruptions occurring during early life. These observations bring a different perspective on the structural hierarchy of respiratory homeostasis and point to new directions in our understanding of the etiology of respiratory control disorders. © 2021 American Physiological Society. Compr Physiol 11:1-38, 2021.

A neural basis for tonic suppression of sodium appetite

Sodium appetite is a powerful form of motivation that can drive ingestion of high, yet aversive concentrations of sodium in animals that are depleted of sodium. However, in normal conditions, sodium appetite is suppressed to prevent homeostatic deviations. Although molecular and neural mechanisms underlying the stimulation of sodium appetite have received much attention recently, mechanisms that inhibit sodium appetite remain largely obscure. Here we report that serotonin 2c receptor (Htr2c)-expressing neurons in the lateral parabrachial nucleus (LPBNHtr2c neurons) inhibit sodium appetite. Activity of these neurons is regulated by bodily sodium content, and their activation can rapidly suppress sodium intake. Conversely, inhibition of these neurons specifically drives sodium appetite, even during euvolemic conditions. Notably, the physiological role of Htr2c expressed by LPBN neurons is to disinhibit sodium appetite. Our results suggest that LPBNHtr2c neurons act as a brake against sodium appetite and that their alleviation is required for the full manifestation of sodium appetite.

Aldosterone-sensitive HSD2 neurons in mice

Sodium deficiency elevates aldosterone, which in addition to epithelial tissues acts on the brain to promote dysphoric symptoms and salt intake. Aldosterone boosts the activity of neurons that express 11-beta-hydroxysteroid dehydrogenase type 2 (HSD2), a hallmark of aldosterone-sensitive cells. To better characterize these neurons, we combine immunolabeling and in situ hybridization with fate mapping and Cre-conditional axon tracing in mice. Many cells throughout the brain have a developmental history of Hsd11b2 expression, but in the adult brain one small brainstem region with a leaky blood-brain barrier contains HSD2 neurons. These neurons express Hsd11b2, Nr3c2 (mineralocorticoid receptor), Agtr1a (angiotensin receptor), Slc17a6 (vesicular glutamate transporter 2), Phox2b, and Nxph4; many also express Cartpt or Lmx1b. No HSD2 neurons express cholinergic, monoaminergic, or several other neuropeptidergic markers. Their axons project to the parabrachial complex (PB), where they intermingle with AgRP-immunoreactive axons to form dense terminal fields overlapping FoxP2 neurons in the central lateral subnucleus (PBcL) and pre-locus coeruleus (pLC). Their axons also extend to the forebrain, intermingling with AgRP- and CGRP-immunoreactive axons to form dense terminals surrounding GABAergic neurons in the ventrolateral bed nucleus of the stria terminalis (BSTvL). Sparse axons target the periaqueductal gray, ventral tegmental area, lateral hypothalamic area, paraventricular hypothalamic nucleus, and central nucleus of the amygdala. Dual retrograde tracing revealed that largely separate HSD2 neurons project to pLC/PB or BSTvL. This projection pattern raises the possibility that a subset of HSD2 neurons promotes the dysphoric, anorexic, and anhedonic symptoms of hyperaldosteronism via AgRP-inhibited relay neurons in PB.

Functional and neurochemical characterization of angiotensin type 1A receptor-expressing neurons in the nucleus of the solitary tract of the mouse

Angiotensin II acts via two main receptors within the central nervous system, with the type 1A receptor (AT_1AR) most widely expressed in adult neurons. Activation of the AT₁R in the nucleus of the solitary tract (NTS), the principal nucleus receiving central synapses of viscerosensory afferents, modulates cardiovascular reflexes. Expression of the AT₁R occurs in high density within the NTS of most mammals, including humans, but the fundamental electrophysiological and neurochemical characteristics of the AT_1AR-expressing NTS neurons are not known. To address this, we have used a transgenic mouse, in which the AT_1AR promoter drives expression of green fluorescent protein (GFP). Approximately one-third of AT_1AR-expressing neurons express the catecholamine-synthetic enzyme tyrosine hydroxylase (TH), and a subpopulation of these stained for the transcription factor paired-like homeobox 2b (Phox2b). A third group, comprising approximately two-thirds of the AT_1AR-expressing NTS neurons, showed Phox2b immunoreactivity alone. A fourth group in the ventral subnucleus expressed neither TH nor Phox2b. In whole cell recordings from slices in vitro, AT_1AR-GFP neurons exhibited voltage-activated potassium currents, including the transient outward current and the M-type potassium current. In two different mouse strains, both AT_1AR-GFP neurons and TH-GFP neurons showed similar AT_1AR-mediated depolarizing responses to superfusion with angiotensin II. These data provide a comprehensive description of AT_1AR-expressing neurons in the NTS and increase our understanding of the complex actions of this neuropeptide in the modulation of viscerosensory processing.

Neural circuits underlying thirst and fluid homeostasis

Thirst motivates animals to find and consume water. More than 40 years ago, a set of interconnected brain structures known as the lamina terminalis was shown to govern thirst. However, owing to the anatomical complexity of these brain regions, the structure and dynamics of their underlying neural circuitry have remained obscure. Recently, the emergence of new tools for neural recording and manipulation has reinvigorated the study of this circuit and prompted re-examination of longstanding questions about the neural origins of thirst. Here, we review these advances, discuss what they teach us about the control of drinking behaviour and outline the key questions that remain unanswered.

Expression of mineralocorticoid and glucocorticoid receptors in preautonomic neurons of the rat paraventricular nucleus

Activation of mineralocorticoid receptors (MR) of the hypothalamic paraventricular nucleus (PVN) increases sympathetic excitation. To determine whether MR and glucocorticoid receptors (GR) are expressed in preautonomic neurons of the PVN and how they relate to endogenous aldosterone levels in healthy rats, retrograde tracer was injected into the intermediolateral cell column at T4 to identify preautonomic neurons in the PVN. Expression of MR, GR, 11-β hydroxysteroid dehydrogenase1 and 2 (11β-HSD1, 2), and hexose-6-phosphate dehydrogenase (H6PD) required for 11β-HSD1 reductase activity was assessed by immunohistochemistry. RT-PCR and Western blot analysis were used to determine MR gene and protein expression. Most preautonomic neurons were in the caudal mediocellular region of PVN, and most expressed MR; none expressed GR. 11β-HSD1, but not 11β-HSD2 nor H6PD immunoreactivity, was detected in the PVN. In rats with chronic low or high sodium intakes, the low-sodium diet was associated with significantly higher plasma aldosterone, MR mRNA and protein expression, and c-Fos immunoreactivity within labeled preautonomic neurons. Plasma corticosterone and sodium and expression of tonicity-responsive enhancer binding protein in the PVN did not differ between groups, suggesting osmotic adaptation to the altered sodium intake. These results suggest that MR within preautonomic neurons in the PVN directly participate in the regulation of sympathetic nervous system drive, and aldosterone may be a relevant ligand for MR in preautonomic neurons of the PVN under physiological conditions. Dehydrogenase activity of 11β-HSD1 occurs in the absence of H6PD, which regenerates NADP(+) from NADPH and may increase MR gene expression under physiological conditions.

Neuronal control of breathing: sex and stress hormones

There is a growing public awareness that hormones can have a significant impact on most biological systems, including the control of breathing. This review will focus on the actions of two broad classes of hormones on the neuronal control of breathing: sex hormones and stress hormones. The majority of these hormones are steroids; a striking feature is that both groups are derived from cholesterol. Stress hormones also include many peptides which are produced primarily within the paraventricular nucleus of the hypothalamus (PVN) and secreted into the brain or into the circulatory system. In this article we will first review and discuss the role of sex hormones in respiratory control throughout life, emphasizing how natural fluctuations in hormones are reflected in ventilatory metrics and how disruption of their endogenous cycle can predispose to respiratory disease. These effects may be mediated directly by sex hormone receptors or indirectly by neurotransmitter systems. Next, we will discuss the origins of hypothalamic stress hormones and their relationship with the respiratory control system. This relationship is 2-fold: (i) via direct anatomical connections to brainstem respiratory control centers, and (ii) via steroid hormones released from the adrenal gland in response to signals from the pituitary gland. Finally, the impact of stress on the development of neural circuits involved in breathing is evaluated in animal models, and the consequences of early stress on respiratory health and disease is discussed.

Sex, hormones, and stress: how they impact development and function of the carotid bodies and related reflexes

Progesterone and corticosterone are key modulators of the respiratory control system. While progesterone is widely recognized as an important respiratory stimulant in adult and newborn animals, much remains to be described regarding the underlying mechanisms. We review the potential implication of nuclear and membrane progesterone receptors in adults and in newborns. This raises intriguing questions regarding the contribution of progesterone as a protective factor against some respiratory control disorders during early life. We then discuss our current understanding of the central integration of stressful stimuli and the responses they elicit. The fact that this system interacts with the respiratory control system, either because both share some common neural pathways in the brainstem and hypothalamus, or because corticosterone directly modulates the function of the respiratory control network, is a fascinating field of research that has emerged over the past few years. Finally, we review the short- and long-term consequences of disruption of stress circuitry during postnatal development on these systems.

Area postrema projects to FoxP2 neurons of the pre-locus coeruleus and parabrachial nuclei: brainstem sites implicated in sodium appetite regulation

The area postrema (AP) is a circumventricular organ located in the dorsal midline of the medulla. It functions as a chemosensor for blood-borne peptides and solutes, and converts this information into neural signals that are transmitted to the nucleus tractus solitarius (NTS) and parabrachial nucleus (PB). One of its NTS targets in the rat is the aldosterone-sensitive neurons which contain the enzyme 11 β-hydroxysteroid dehydrogenase type 2 (HSD2). The HSD2 neurons are part of a central network involved in sodium appetite regulation, and they innervate numerous brain sites including the pre-locus coeruleus (pre-LC) and PB external lateral-inner (PBel-inner) cell groups of the dorsolateral pons. Both pontine cell groups express the transcription factor FoxP2 and become c-Fos activated following sodium depletion. Because the AP is a component in this network, we wanted to determine whether it also projects to the same sites as the HSD2 neurons. By using a combination of anterograde axonal and retrograde cell body tract-tracing techniques in individual rats, we show that the AP projects to FoxP2 immunoreactive neurons in the pre-LC and PBel-inner. Thus, the AP sends a direct projection to both the first-order medullary (HSD2 neurons of the NTS) and the second-order dorsolateral pontine neurons (pre-LC and PB-el inner neurons). All three sites transmit information related to systemic sodium depletion to forebrain sites and are part of the central neural circuitry that regulates the complex behavior of sodium appetite.

Regional differences in estradiol effects on numbers of HSD2-containing neurons in the nucleus of the solitary tract of rats

Estrogens affect body fluid balance, including sodium ingestion. Recent findings of a population of neurons in the hindbrain nucleus of the solitary tract (NTS) of rats that are activated during sodium need suggest a possible central substrate for this effect of estrogens. We used immunohistochemistry to label neurons in the NTS that express 11-β-hydroxysteroid dehydrogenase type 2 (HSD2), an enzyme that promotes aldosterone binding, in male rats, and in ovariectomized (OVX) rats given estradiol benzoate (EB) or oil vehicle (OIL). During baseline conditions, the number of HSD2 immunoreactive neurons in the NTS immediately rostral to the area postrema was greater in EB-treated OVX rats compared to those in OIL-treated OVX and male rats. A small number of HSD2 immunoreactive neurons was also labeled for dopamine-β-hydroxylase (DBH), an enzyme involved in norepinephrine biosynthesis. Double-labeled neurons in the NTS were located primarily in the more lateral portion of the HSD2 population, at the level of the area postrema in all three groups, with no sex or estrogen-mediated differences in the number of double-labeled neurons. These results suggest that two subpopulations of HSD2 neurons are present in the NTS. One subpopulation, which does not colocalize with DBH and is increased during conditions of elevated estradiol, may contribute to the effects of estrogens on sodium ingestion. The role of the other, smaller subpopulation, which colocalizes with DBH and is not affected by estradiol, remains to be determined, but one possibility is that these latter neurons are part of a larger network of catecholaminergic input to neuroendocrine neurons in the hypothalamus.

Aldosterone in the brain

Pharmacological and physiological phenomena suggest that cells somewhere inside the central nervous system are responsive to aldosterone. Here, we present the fundamental physiological limitations for aldosterone action in the brain, including its limited blood-brain barrier penetration and its substantial competition from glucocorticoids. Recently, a small group of neurons with unusual sensitivity to circulating aldosterone were identified in the nucleus of the solitary tract. We review the discovery and characterization of these neurons, which express the enzyme 11beta-hydroxysteroid dehydrogenase type 2, and consider alternative proposals regarding sites and mechanisms for mineralocorticoid action within the brain.