Joel C. Geerling

Summary: Sodium deficiency increases aldosterone, which signals through the mineralocorticoid receptor (MR) to restrict sodium excretion and promote salt appetite. Despite aldosterone’s importance in maintaining hydromineral balance, its role in regulating neuronal function is poorly understood. Here, we define the transcriptional response to aldosterone in neurons and demonstrate that MR signaling in HSD2 neurons is essential for aldosterone-induced sodium appetite. Disruption of MR in HSD2 neurons impaired sodium appetite following deficiency and abolished saline intake in response to aldosterone. Single-nucleus RNA sequencing of the caudal nucleus of the solitary tract (NTS) revealed that aldosterone selectively regulates gene expression in HSD2 neurons, with enrichment for transcripts associated with synaptic function. Consistent with this, aldosterone increased the paired-pulse ratio at HSD2 neuron synapses, indicating reduced synaptic depression during repeated stimulation. Notably, Sv2c , encoding synaptic vesicle glycoprotein 2C, exhibited the greatest increase in expression after aldosterone treatment, and its disruption in HSD2 neurons reduced saline ingestion, identifying SV2C as a mediator of mineralocorticoid-induced sodium appetite. Together, these findings reveal a selective aldosterone-induced transcriptional program associated with enhanced synaptic transmission in HSD2 neurons and identify a molecular mechanism through which aldosterone promotes sodium appetite.

Understanding how the brain processes bodily signals requires mapping the circuits that transform interoceptive information into coordinated responses. Visceral signals converge in the nucleus of the solitary tract (NTS), which coordinates appetite, breathing, cardiovascular reflexes, and digestion. The NTS contains many intermingled subpopulations of neurons, and deciphering their functions requires understanding their connections. Here, we used cell‐type‐specific tracing to test whether molecularly distinct NTS neurons exhibit unique connectivity patterns. First, we found that Lmx1b ‐expressing excitatory neurons provide output to a broad array of NTS target regions in both the brainstem and forebrain, while inhibitory neurons in this region project predominantly within the brainstem. Next, we found that several genetically defined excitatory subpopulations—catecholaminergic ( Th ), neuropeptidergic ( Cck , Npff , or Pdyn ), and aldosterone‐sensitive ( Hsd11b2 )—exhibit unique output patterns across multiple targets. As examples, the ventrolateral medulla receives moderate Th , Cck , and Pdyn , light Npff , and no Hsd11b2 input. The outer rim of the external lateral parabrachial subnucleus receives concentrated Th , Cck , and Npff input, contrasting a more uniform Pdyn input and a lack of Hsd11b2 input. The subcommissural bed nucleus of the stria terminalis receives broad Th , light Cck , sparse Pdyn , and virtually no Npff input, contrasting the focal Hsd11b2 input to its fusiform subnucleus. These divergent patterns demonstrate that molecular identity predicts connectivity and define the organizational logic by which interoceptive signals are transformed into coordinated autonomic and behavioral responses.

Cold-activated neurons in the PB relay thermosensory signals that drive autonomic and behavioral cold-defense responses, but their molecular identity has remained unclear. Here, we show that these neurons are selectively identified by coexpression of Foxp2 and Grp, a combination that distinguishes them from surrounding PB neurons. This intersectional genetic identity provides a practical strategy for selectively targeting this population, enabling future mechanistic studies of thermoregulatory circuits.

Neurons that express Nps send output to brain regions implicated in circadian function and threat responses, but less is known about the afferent control of these neurons. In this study, we used a sensitive retrograde tracer, cholera toxin beta subunit (CTb), to identify afferents to the rostral–lateral parabrachial region that contains the main concentration of Nps ‐expressing neurons. We then used Cre‐dependent rabies retrograde tracing in Nps ‐2A‐Cre mice to identify inputs to Nps ‐expressing neurons within this region. These neurons receive heavy input from auditory brainstem structures, including the inferior colliculus, ventral nucleus of the lateral lemniscus, superior olivary complex, and cochlear nucleus. Due to a discrepancy between sparse rabies and prominent CTb labeling extending from the ipsilateral insular to auditory areas of the cerebral cortex, we also performed anterograde labeling and found many close contacts between cortical boutons and Nps‐ expressing neurons. These findings suggest an unexpected role for auditory information in controlling the activity of Nps‐ expressing neurons and add to existing evidence suggesting that rabies is relatively insensitive to retrograde labeling of cortical afferents.

Investigating the molecular and functional identity of specific neuronal populations is a critical component of neuroscience research. Specific brain regions can be targeted with stereotaxic injections of viral vectors that deliver recombinase‐dependent genetic tools, but this approach cannot target widely distributed neuronal populations. Peripheral injection is a potential strategy to access widely distributed populations, and unlike most viral vectors, PHP.eB penetrates the blood–brain barrier. However, many neurotropic viruses have tropism for specific neuronal subtypes, and it is unclear if PHP.eB transduces neurons equally across brain regions. To evaluate tropism and transduction efficiency, we examined the brain after retro‐orbital injections of a PHP.eB vector that delivers a Cre‐dependent reporter in a variety of Cre mice. We observed widely variable transduction efficiency across brain regions and cell types. Additionally, we found scattered neurons and astrocytes throughout the brain in all Cre‐driver strains, as well as Cre‐negative control mice, indicating that a low rate of spontaneous recombination had occurred. These results confirm that peripheral delivery of PHP.eB can transduce widely distributed neuronal populations. However, variations in tropism, sensitivity, and specificity merit careful consideration for each population of interest.

Neurons that produce NPS send output to brain regions implicated in circadian function and threat responses, but less is known about the afferent control of NPS neurons. In this study, we used a conventional retrograde tracer, cholera toxin beta subunit (CTb), to identify afferents to the rostral-lateral parabrachial region that contains the main concentration of NPS neurons. We then used Cre-dependent rabies retrograde tracing in Nps -2A-Cre mice to identify inputs specifically to NPS neurons. Nps -expressing neurons receive heavy input from auditory brainstem structures, including the inferior colliculus, nucleus of the lateral lemniscus, superior olivary complex, and cochlear nucleus. These findings suggest an unexpected role for auditory information in controlling the activity of NPS neurons.

This study demonstrates a fundamental role for serotonergic neurons in limiting sodium intake. Mice with genetic deletion of serotonin-producing neurons consume more salt, indicating that serotonergic neurons act like a brake to restrain sodium appetite. These findings advance our understanding of how the brain controls salt-seeking behavior.

Identifying lesion sites associated with central poststroke pain (CPSP) may facilitate targeted screening for early symptoms, possibly even paving the way for preventive measures and earlier treatment initiation. Here, we test the hypothesis that damage to a nociceptive pathway extending from the brainstem to the cerebral cortex, and including white matter tracts, is associated with CPSP. We investigated the lesion locations of 72 patients with CPSP relative to poststroke comparison subjects without pain (n = 123), divided into a discovery and independent validation data set. The study included three main analyses: (1) we compared lesion intersection with our a priori region of interest (ROI) between groups with and without CPSP, (2) we performed lesion-symptom mapping to evaluate whether lesions associated with CPSP localize to the a priori ROI, and (3) we used lesion network mapping to infer the broader structural and functional connectivity patterns associated with CPSP lesions. CPSP lesions overlapped the nociceptive pathway ROI to a greater extent than comparison lesions. Lesion-symptom mapping identified a CPSP-associated region overlapping with the ventrocaudal thalamus and adjacent white matter, which was located mostly within the a priori ROI. Lesion network mapping demonstrated that lesions associated with CPSP disrupt nodes and tracts of the nociceptive pathway ROI. Interestingly, the CPSP lesion network results demonstrated connectivity to intereffector nodes of the primary motor cortex, providing a novel link between CPSP and the somato–cognitive action network. Together, these findings indicate that CPSP can be conceptualized as a lesion-associated network disruption of the nociceptive pathway and somato–cognitive action network.