The research team, led by investigators at Central South University’s Xiangya Hospital, began with the human brain. Using resting-state functional MRI in 43 patients with rotator cuff injury and 30 healthy controls, they measured spontaneous neural activity across the whole brain via the amplitude of low-frequency fluctuations. While classic pain and emotion regions — the dorsal anterior cingulate cortex, the amygdala, and the posterior insula — all showed altered activity in patients, only one region tracked symptom severity at the individual level: the hypothalamus. Activity in both the left and right hypothalamus was significantly suppressed in patients, and the degree of suppression correlated inversely with anxiety scores on the Hospital Anxiety and Depression Scale. Notably, anxiety, not depression, was the dominant psychological phenotype in the patient cohort, with 27.9 percent of patients showing moderate to severe anxiety compared with 10 percent of controls.

To pinpoint which hypothalamic compartments were responsible, the team applied a high-resolution human hypothalamic atlas that divides the structure into seven functional subnuclei. The suppression was not diffuse. It concentrated in the paraventricular nucleus, the medial preoptic nucleus, and the superior compartment of the supraoptic nucleus — with the paraventricular nucleus, or PVN, standing out as the most affected. The PVN is a tiny but strategically vital hub: it contains oxytocin-producing neurons that participate in anxiety regulation and send long-range projections to autonomic centers controlling sympathetic nervous system output throughout the body.

The next step was molecular. By integrating the brain-imaging differences with gene expression data from the Allen Human Brain Atlas, and correcting for spatial autocorrelation using BrainSMASH permutation testing, the researchers performed a genome-wide pathway screen. Among all KEGG pathways tested, the oxytocin signaling pathway emerged as one of the most significantly enriched hits spatially concordant with the hypothalamic suppression. This was not a hypothesis the authors brought to the data — it emerged from an unbiased screen, providing a molecular entry point for the circuit-level investigations that followed.

To establish causality, the team turned to mice. Rotator cuff injury in mice produced a time-dependent anxiety-like phenotype that peaked at four weeks post-injury, visible in both the open field test and the elevated plus maze. Strikingly, this behavioral trajectory closely mirrored the dynamics of norepinephrine — the primary sympathetic neurotransmitter — at the tendon–bone interface, which also peaked at four weeks before declining. Central affective disturbance and peripheral sympathetic hyperactivation appeared to be temporally coupled responses to the same injury, not independent sequelae. Whole-brain activity mapping in TRAP2 mice, combined with tissue clearing and light-sheet microscopy, then confirmed that the PVN showed the most significant reduction in activated neurons among all hypothalamic subnuclei, concentrated precisely in the medial and posterior PVN — the same subregions implicated in the human imaging data.

The question became which neuronal population within the PVN could simultaneously regulate emotion and peripheral autonomic tone. Reanalysis of publicly available single-neuron connectome data revealed that PVN clusters concentrated in the medial and posterior subregions contain both long-range spinal-projecting neurons and local intra-hypothalamic projection neurons. Among the 153 spinal-projecting PVN neurons identified, 46.4 percent expressed oxytocin, making oxytocinergic neurons the predominant genotype capable of engaging spinal autonomic circuitry. The team then injected retrograde pseudorabies virus into the subchondral bone of the supraspinatus enthesis — the critical attachment point where tendon meets bone — and found robust labeling in the medial and posterior PVN, with essentially no labeling in the nearby supraoptic nucleus. Spatial transcriptomics and immunofluorescence confirmed that roughly half of all retrogradely labeled PVN neurons were oxytocin-positive, far exceeding the proportions of vasopressinergic or corticotropin-releasing neurons.

With the anatomical map in hand, the researchers manipulated the circuit directly. Using chemogenetic DREADDs targeted to PVN oxytocinergic neurons with nearly 90 percent specificity, they could silence or activate this population at will. Silencing induced robust anxiety-like behavior in injured mice and devastated tendon–bone healing: biomechanical testing revealed reduced stiffness, failure load, and ultimate tensile strength; histology showed disorganized fibrocartilage and reduced proteoglycan deposition; micro-CT revealed deteriorated bone microarchitecture at the humeral greater tuberosity; and molecular markers of cartilage and bone formation — Aggrecan, Sox9, Ocn, and Runx2 — were all suppressed. Activation produced the opposite effects, preserving or even enhancing healing quality. Critically, these manipulations altered norepinephrine levels locally at the tendon–bone interface without changing circulating serum norepinephrine, indicating site-specific autonomic control rather than a generalized shift in systemic sympathetic tone.

The efferent pathway was traced to a classical sympathetic relay: the PVN projects to sympathetic preganglionic neurons in the intermediolateral column of the spinal cord, which in turn drive the superior cervical ganglion. In vivo microelectrode recordings from the superior cervical ganglion showed that silencing PVN oxytocinergic neurons dramatically increased sympathetic firing — elevated spike density, burst rate, and high-frequency local field potential power — while activation suppressed it. When the researchers selectively ablated superior cervical ganglion neurons with diphtheria toxin, the healing deficits caused by PVN silencing were substantially rescued, and ganglion ablation alone improved bone microarchitecture and biomechanical performance in injured mice while lowering local norepinephrine. The peripheral relay was thus established as functionally necessary for translating central oxytocinergic dysregulation into impaired tissue repair.

The model that emerges is a complete brain–body axis: rotator cuff injury suppresses PVN oxytocinergic neuron activity, which couples to anxiety-like affective state and, through disinhibition of the spinal sympathetic relay, drives elevated local norepinephrine at the tendon–bone interface that compromises structural and biomechanical repair. Anxiety, in this framework, is not a secondary psychological burden but a mechanistically coupled component of the healing process itself. The authors caution that the clinical neuroimaging data are cross-sectional and cannot establish whether hypothalamic suppression precedes or follows anxiety onset, and that all mouse experiments were conducted in adult males, leaving generalizability to females an open question. The oxytocin-promoter-driven manipulation also cannot distinguish parvocellular spinal-projecting neurons from magnocellular neuroendocrine cells, and vasopressin and corticotropin-releasing neurons within the PVN may contribute in parallel.

Even with those caveats, the clinical implications are provocative. Interventions that engage hypothalamic oxytocinergic signaling — whether pharmacological, behavioral, or through non-invasive neurostimulation — might concurrently alleviate anxiety and improve healing outcomes after rotator cuff repair. The study suggests a shift away from purely tissue-focused orthopedic treatment toward integrated strategies that address the nervous system as an active participant in musculoskeletal repair. For the millions of patients facing shoulder surgery each year, the mind may matter far more than previously believed — not as a matter of willpower or positive thinking, but as a matter of neuroanatomy, with oxytocinergic neurons in the hypothalamus standing guard over both emotional state and the structural integrity of the healing tendon.

Subject of Research: A hypothalamic oxytocinergic–sympathetic neural axis linking anxiety to impaired tendon–bone healing after rotator cuff injury

Article Title: A Hypothalamic Oxytocinergic–Sympathetic Axis Couples Anxiety Dysregulation With Impaired Tendon–Bone Repair

Article References: Wan, L., Huang, T., Zeng, L., Du, R., Zhang, H., Liu, S., Zhang, T., Hu, J., & Lu, H. (2026). A Hypothalamic Oxytocinergic–Sympathetic Axis Couples Anxiety Dysregulation With Impaired Tendon–Bone Repair. Advanced Science, Article e78190. https://doi.org/10.1002/advs.78190

Image Credits: AI Generated

DOI: 10.1002/advs.78190

Keywords: rotator cuff injury, anxiety, hypothalamus, paraventricular nucleus, oxytocin, sympathetic nervous system, tendon–bone healing, norepinephrine, chemogenetics, superior cervical ganglion, resting-state fMRI, psychological comorbidity

The team deployed an unusually broad technical arsenal to interrogate the question. They combined electrical stimulation of the SCN region in brain slices with visual stimulation of light-exposed animals, perforated multielectrode array recordings that capture the firing of many neurons simultaneously over hours, c-Fos mapping to visualise which cells were activated in the living brain, and detailed neuroanatomical tracing of retinal projections. This multi-pronged strategy matters because no single method could disentangle the two influences at play: the time-of-day signal emanating from the SCN and the moment-to-moment photic signal arriving from the eye.

The first surprise was the sheer diversity of responses. Across the midline hypothalamus—encompassing regions such as the paraventricular nucleus, the dorsomedial and anterior dorsomedial hypothalamus, the anterior hypothalamic area, the anterior ventromedial hypothalamus, the arcuate nucleus, the medial preoptic area, the lateral hypothalamic area and the retrochiasmatic area—the researchers found distinct cell populations that fell into different functional classes. Some received inhibitory input of SCN origin, some excitatory input, and some both. When the SCN region was stimulated electrically in slices, hypothalamic neurons responded with a mixture of fast excitatory postsynaptic currents and inhibitory currents, revealing that the master clock does not simply broadcast a uniform suppressive message to its hypothalamic targets.

Crucially, the responses were not confined to SCN-derived signals. The recordings revealed excitatory and inhibitory responses of retinal origin in hypothalamic cells, indicating that photic information arrives through routes that bypass, or at least run in parallel to, the classical SCN relay. The pharmacology supported this: blocking ionotropic glutamate receptors abolished the fast excitatory components, while the GABA receptor antagonist bicuculline unmasked inhibitory drive, consistent with glutamate and GABA acting as the principal transmitters carrying both circadian and retinal messages into these downstream nuclei.

Ex vivo recordings of hypothalamic slices maintained in artificial cerebrospinal fluid added a temporal dimension. Even when isolated from the body, different hypothalamic cell populations displayed divergent circadian activity patterns—some firing vigorously at what would be daytime in the donor animal, others peaking at night, and others showing phase relationships that shifted depending on where in the hypothalamus the cells resided. This differential phase resetting means the hypothalamus is not a passive uniform follower of the SCN clock; it hosts multiple local oscillators with distinct timing signatures, each potentially tuned to different physiological outputs such as hormone secretion, thermoregulation, feeding or sleep-wake control.

Back in the living animal, the picture sharpened further. Light-evoked activity in the hypothalamus in vivo was dominated by melanopsin—the photopigment found in intrinsically photosensitive retinal ganglion cells, the specialised neurons that also drive circadian photoentrainment and the pupil reflex. The melanopsin-dominated character of the hypothalamic visual responses aligned neatly with the distribution of direct retinal projections traced by the anatomical experiments: the regions that received the densest retinal innervation were the ones showing the strongest light-driven activation. Neutral density filters that attenuated light intensity, and comparisons across Zeitgeber times, allowed the team to separate the photic component from the circadian component of hypothalamic firing.

The c-Fos mapping experiments then delivered the study’s most visually compelling result. When the researchers quantified c-Fos expression—the classic molecular fingerprint of recent neuronal activation—across hypothalamic subregions, they found a striking mosaic. Some discrete subregions were dominated by circadian signals, lighting up according to time of day regardless of illumination. Others were dominated by light, responding to illumination irrespective of clock phase. And a third set responded to both, integrating the internal clock with the external light environment. This anatomical segregation of circadian, photic and combined influences means the hypothalamus is organised into parallel channels rather than a single SCN-gated pipeline.

The implications ripple outward well beyond rodent neuroanatomy. The hypothalamic nuclei implicated in this study are major effector sites for physiology: the paraventricular nucleus orchestrates the hormonal stress axis and autonomic output, the dorsomedial hypothalamus helps coordinate sleep-wake and feeding rhythms, the arcuate nucleus governs appetite and energy balance, and the ventromedial hypothalamus is central to defensive behaviour and metabolic regulation. If light can modulate these circuits directly, through melanopsin-driven retinal input, then environmental illumination has a far more immediate and granular influence on physiology than the SCN-suppression model allowed. It also offers new substrates for understanding how light can be adaptive—sharpening daily rhythms—and how it can be disruptive, as in the metabolic and mood disturbances associated with irregular light exposure, shift work and excessive evening screen use.

The study also reframes how we think about the SCN itself. Rather than a despotic conductor whose output wholly determines hypothalamic state, the master clock emerges as one player in a distributed network, contributing inhibitory and excitatory signals that interact with independent retinal channels. The finding that SCN-derived responses include both inhibitory and excitatory components challenges the textbook framing of daytime SCN output as uniformly suppressive on downstream hypothalamic effectors. Instead, the SCN appears to shape hypothalamic activity with the same kind of push-pull synaptic logic found elsewhere in the brain, while the retina delivers a second, parallel stream of information that can act on the same nuclei on its own timescale.

Methodologically, the work sets a benchmark for how to disentangle circadian and photic influences on neural activity. By pairing long-duration ex vivo multielectrode recordings—capable of tracking slice activity across the circadian cycle—with in vivo visual stimulation, pharmacological dissection of glutamatergic and GABAergic transmission, and quantitative c-Fos mapping validated against anatomical templates, the Manchester team built a converging chain of evidence in which each line of attack compensates for the limitations of the others. The result is a coherent map of where, when and how light and time-of-day signals converge in the hypothalamus. What remains to be explored is how these parallel channels influence specific physiological outputs—whether direct retinal drive to the paraventricular nucleus, for instance, acutely modulates stress hormones, or whether melanopsin input to feeding circuits contributes to the appetite-suppressing effects of bright light. The authors’ data establish that light-dependent activity across the anterior midline hypothalamus is surprisingly extensive, and in doing so they open a rich new seam for research into how the light environment shapes, and sometimes scrambles, the daily biology of mammals—including, very likely, ourselves.

Subject of Research: Parallel retinal and circadian signalling pathways regulating light-dependent activity in the mouse hypothalamus

Article Title: Parallel retinal and circadian signalling channels organise mouse hypothalamic activity

Article References: Gurung, B., Harding, C., Mouland, J. W., Sahid, N., Ray, H., Bechtold, D. A., & Brown, T. M. (2026). Parallel retinal and circadian signalling channels organise mouse hypothalamic activity. BMC Biology. https://doi.org/10.1186/s12915-026-02746-w

Image Credits: AI Generated

DOI: 10.1186/s12915-026-02746-w

Keywords: suprachiasmatic nucleus, hypothalamus, circadian rhythms, melanopsin, intrinsically photosensitive retinal ganglion cells, retinohypothalamic tract, c-Fos mapping, multielectrode recordings, paraventricular nucleus, dorsomedial hypothalamus, light exposure, electrophysiology

The research, published in Physiological Reports by a team led by Takahiro Masuda and Masahide Yoshida of Jichi Medical University together with colleagues in Japan and the United States, examined whether the gene encoding SGLT2, known formally as Slc5a2, is switched on in the neurons that govern thirst and vasopressin synthesis. For years, physiologists have observed that SGLT2 inhibitors stimulate water intake and vasopressin secretion even when blood volume and plasma osmolality remain normal, which suggests that something beyond simple dehydration is driving these responses. Because the drugs can cross the blood-brain barrier and SGLT2 immunoreactivity had previously been reported in brain tissue, the team suspected that the transporter might be present in the very neurons that control drinking behavior and hormone release.

To find out, the researchers studied male Sprague-Dawley rats and combined two complementary techniques: droplet digital PCR, an ultra-sensitive method for quantifying gene transcripts, and in situ hybridization, which reveals exactly which cells express a given gene. Using droplet digital PCR on micro-punched tissue samples, they detected Sglt2 messenger RNA in four key regions: the organum vasculosum of the lamina terminalis (OVLT) and the subfornical organ (SFO), two sensory circumventricular organs that form the brain’s thirst center, and the supraoptic nucleus (SON) and paraventricular nucleus (PVN) of the hypothalamus, where vasopressin is synthesized. The relative abundance of Sglt2 transcripts in these regions, normalized to the housekeeping gene Polr2a, fell in the range of 0.02 to 0.04, a level comparable to that of Agtr1a, the angiotensin II receptor type 1a gene that is well established as a central regulator of drinking and vasopressin release.

Brain expression was, unsurprisingly, far lower than in the kidney, the body’s dominant site of SGLT2 expression. Depending on which of three internal control genes was used for normalization, Sglt2 expression in the paraventricular nucleus, the region with the strongest signal, was roughly 76 to 205 times lower than in kidney tissue. Yet the authors argue that low abundance does not mean functional irrelevance. The fact that Sglt2 transcripts appear at levels similar to those of Agtr1a, a gene whose central role in thirst regulation is beyond dispute, suggests that even modest neuronal expression could be physiologically meaningful, particularly if the neurons involved are the direct targets of circulating drug.

The anatomical story became far more compelling when the team turned to double fluorescent in situ hybridization, a technique that can visualize two genes simultaneously within single cells. In the OVLT and the SFO, Sglt2 messenger RNA was clearly present in neurons that also express Nos1, the gene encoding neuronal nitric oxide synthase. These NOS1-expressing neurons are known from landmark studies to be the excitatory, thirst-promoting cells of the circumventricular organs; when activated, they drive drinking behavior, and they send direct projections to the vasopressin-producing nuclei of the hypothalamus. Quantification showed that approximately 48.1 percent of NOS1-positive neurons in the OVLT and 60.3 percent in the SFO expressed Sglt2. In other words, the molecular target of SGLT2 inhibitors sits within roughly half of the very neurons that command an animal to drink.

The co-localization in the hypothalamus was even more striking. In the supraoptic nucleus, 80.8 percent of vasopressin-expressing neurons carried Sglt2 transcripts, and in the paraventricular nucleus the figure reached 84.0 percent. The researchers also examined oxytocin neurons, the other major neuroendocrine population in these nuclei, and found Sglt2 expression in 58.9 percent of oxytocin neurons in the SON and 65.7 percent in the PVN. Chromogenic in situ hybridization confirmed the pattern at the tissue level, with Sglt2 signals appearing not only in the four fluid-regulating regions but also in the choroid plexus, consistent with earlier reports in human and mouse brains that identified the transporter in choroid plexus epithelial cells and ependymal cells. Prior work had also located SGLT2 in brain pericytes and in neurons of the rostral ventrolateral medulla, a master controller of sympathetic nerve activity, but this study is the first to pin the transporter down at single-cell resolution within the thirst and vasopressin circuits themselves.

These findings provide an anatomical basis for a hypothesis that has been gathering force in the literature: that SGLT2 inhibitors act directly on the brain, not merely through the kidney. Two features of the circumventricular organs make this plausible. First, the OVLT and SFO sit outside a functional blood-brain barrier, meaning drugs circulating in the bloodstream can reach them directly. Second, SGLT2 inhibitors as a class have been shown to penetrate the blood-brain barrier, and central administration of the drug tofogliflozin has already been demonstrated to stimulate water intake in non-diabetic rats, just as peripheral administration of ipragliflozin does in both diabetic and non-diabetic animals. If SGLT2 inhibitors enhance the excitability of NOS1 thirst neurons in the OVLT and SFO, that could trigger drinking independently of any actual fluid deficit, which would explain the paradoxical observation that these drugs increase vasopressin secretion and copeptin levels, a stable clinical surrogate for vasopressin, even under normal osmotic conditions.

The authors go further, weaving in evidence from glucose-sensing physiology. In the SFO, a subset of so-called glucose-inhibited neurons, cells that become excited when intracellular glucose falls, shares properties with NOS1 neurons, and many glucose-inhibited neurons also respond to angiotensin II, a potent dipsogenic hormone. Roughly 27 percent of SFO neurons show this glucose-inhibited character, and the majority of them respond to angiotensin II. The team speculates that by reducing intracellular glucose uptake through SGLT2, the inhibitors might directly stimulate a NOS1-positive, glucose-sensing population, initiating drinking before osmotic diuresis has had time to deplete body fluids. Intriguingly, a parallel logic operates in the kidney: in macula densa cells, which behave in many ways like neurons, SGLT1-mediated glucose uptake supports NOS1 activity that modulates tubuloglomerular feedback. The authors suggest that SGLT-dependent glucose sensing linked to nitric oxide synthase may thus push systemic fluid balance in a negative direction in both organs, a conceptual echo of coupled hunger-and-thirst sensing described in the fly brain.

Important caveats remain, and the authors are careful about them. This study mapped gene expression; it did not directly test function. Whether blocking SGLT2 in these neurons actually changes their electrical activity, drinking behavior, or vasopressin release has not yet been demonstrated, and global SGLT2 knockout mice, which drink more and secrete more vasopressin than wild-type animals, cannot disentangle central from renal mechanisms because fluid balance disturbances confound the interpretation. Brain-region-specific knockout models and intracerebroventricular drug administration will be needed to establish causality, along with electrophysiological recordings and activity markers such as Fos in SGLT2-expressing neurons. It is also uncertain how much of the vasopressin response during SGLT2 inhibition reflects direct central action versus classical osmotic pathways, since clinical studies show copeptin remains strongly tied to plasma osmolality during treatment. Still, the finding that Sglt2 is expressed in the majority of vasopressin neurons and roughly half of thirst-promoting NOS1 neurons reframes how scientists think about these drugs. Beyond fluid balance, vasopressin and oxytocin neurons influence social behavior, stress responses, and learning and memory, raising the speculative but testable possibility that SGLT2 inhibitors touch neuroendocrine circuits far beyond hydration. If the central mechanisms are confirmed, they could deepen understanding of the brain-kidney axis and help explain why a drug class designed to squeeze glucose out of urine has turned out to be one of the most versatile cardiorenal therapies ever developed.

Subject of Research: SGLT2 gene expression in thirst-promoting NOS1 neurons and vasopressin-synthesizing neurons in the rat brain

Article Title: SGLT2 gene expression in thirst‐promoting NOS1 neurons and vasopressin‐synthesizing neurons in male rats

Article References: Masuda, T., Yoshida, M., Kim, Y. C., Vallon, V., Morishita, Y., Nagata, D., & Onaka, T. (2026). SGLT2 gene expression in thirst‐promoting NOS1 neurons and vasopressin‐synthesizing neurons in male rats. Physiological Reports, 14(17), Article e71100. https://doi.org/10.14814/phy2.71100

Image Credits: AI Generated

DOI: 10.14814/phy2.71100

Keywords: SGLT2, Slc5a2, thirst, vasopressin, NOS1 neurons, subfornical organ, organum vasculosum laminae terminalis, supraoptic nucleus, paraventricular nucleus, fluid homeostasis, in situ hybridization, cardiorenal