Aldosterone-Sensing Neurons in the NTS Exhibit State-Dependent Pacemaker Activity and Drive Sodium Appetite via Synergy with Angiotensin II Signaling

Abstract

Summary Sodium deficiency increases angiotensin II (ATII) and aldosterone, synergistically stimulating its retention and consumption. Recently, ATII-responsive neurons in the subfornical organ (SFO) and aldosterone-sensitive neurons in the nucleus of the solitary tract (NTSHSD2 neurons) were shown to drive sodium appetite. Here we investigate the basis for NTSHSD2 neuron activation, identify the circuit by which NTSHSD2 neurons drive appetite, and uncover an interaction between the NTSHSD2 circuit and ATII signaling. NTSHSD2 neurons respond to sodium deficiency with spontaneous pacemaker-like activity – the consequence of "cardiac" HCN and Nav1.5 channels. Remarkably, NTSHSD2 neurons are necessary for sodium appetite, and with concurrent ATII signaling their activity is sufficient to produce rapid consumption. Importantly, NTSHSD2 neurons stimulate appetite via projections to the vlBNST, which is also the effector site for ATII-responsive SFO neurons. The interaction between angiotensin signaling and NTSHSD2 neurons provides a neuronal context for the long-standing "synergy hypothesis" of sodium appetite regulation.

Aldosterone-sensitive HSD2 neurons in mice

“…Next, to understand how cells inside the brain could detect a blood-borne peptide (angiotensin II) and a steroid that poorly penetrates the blood-brain barrier (aldosterone; Pardridge and Mietus, 1979; Funder and Myles, 1996; Geerling and Loewy, 2009), we find that the HSD2 neuron distribution overlaps an NTS subregion with enhanced blood-brain barrier (BBB) permeability as shown by tissue infiltration of a blood-borne dye and endogenous proteins. We then confirm and expand our understanding of genetic markers identified using single-cell RNA-Seq (Resch et al, 2017) by immunolabeling proteins and using fluorescence in situ hybridization (FISH) to label mRNA transcripts relating HSD2 neurons to or distinguishing them from surrounding NTS neurons. Finally, after two studies showing the major HSD2 axon terminal fields (Jarvie and Palmiter, 2017; Resch et al, 2017), we confirm the basic findings of our conventional tracing experiments in rats (Geerling and Loewy, 2006b) using Cre-conditional genetic tracing in two strains of Hsd11b2 Cre-driver mice to label the full axonal arbors of HSD2 neurons and characterizing each major terminal field in detail.…”

“…We then confirm and expand our understanding of genetic markers identified using single-cell RNA-Seq (Resch et al, 2017) by immunolabeling proteins and using fluorescence in situ hybridization (FISH) to label mRNA transcripts relating HSD2 neurons to or distinguishing them from surrounding NTS neurons. Finally, after two studies showing the major HSD2 axon terminal fields (Jarvie and Palmiter, 2017; Resch et al, 2017), we confirm the basic findings of our conventional tracing experiments in rats (Geerling and Loewy, 2006b) using Cre-conditional genetic tracing in two strains of Hsd11b2 Cre-driver mice to label the full axonal arbors of HSD2 neurons and characterizing each major terminal field in detail. We also use dual retrograde tracing to test whether their axons branch or project separately to their two major target regions.…”

“…These sodium-depletion-activated neurons induce a behavioral state known as sodium appetite. Activating them causes mice to consume salt (Jarvie and Palmiter, 2017; Resch et al, 2017). While their sources of synaptic input are unclear (Geerling and Loewy, 2009; Resch et al, 2017), these neurons appear to sense two hormonal signals of sodium deficiency: aldosterone and angiotensin II (Geerling et al, 2006a; Geerling and Loewy, 2006c; 2009; Resch et al, 2017).…”

“…HSD2, in addition to MR, is therefore a hallmark of all aldosterone-sensitive cells. In adult mice and rats, we refer to the subpopulation of NTS neurons expressing this enzyme as “NTS HSD2 neurons” (Resch et al, 2017) or simply “HSD2 neurons” (Geerling and Loewy, 2006b; Jarvie and Palmiter, 2017). …”

“…A decade after identifying and characterizing HSD2 neurons in rats, we and others switched to using Cre-conditional techniques in mice (Jarvie and Palmiter, 2017; Resch et al, 2017). The opportunity for cell-type-specific experiments outweighed several disadvantages of studying mice, including that sodium appetite in mice may be less mineralocorticoid-dependent than in rats (Rowland and Fregly, 1988).…”