Brain Clock Neurons Tied to Ghrelin Control When and How Much You Eat
SCN neurons expressing ghrelin's receptor regulate food intake and body weight only during the mid-rest phase, revealing a circadian-metabolic circuit.
Summary
Researchers at UT Southwestern discovered that neurons in the suprachiasmatic nucleus (SCN) — the brain's master circadian clock — that express the ghrelin receptor (GHSR) govern food intake and body weight in a strictly time-of-day-dependent manner. Chemogenetically activating these neurons at mid-rest (ZT4, roughly 4 hours into daylight for mice) increased food intake by 144%, while the same stimulation at other times of day had no effect. Conversely, repeatedly inhibiting these neurons each day at ZT4 for 15 days reduced food intake by ~35% during that window, cut cumulative body weight by ~6.8% relative to controls, and lowered feed efficiency — suggesting increased energy expenditure. Equivalent inhibition during the active phase produced no metabolic changes. These findings establish GHSR-expressing SCN neurons as a time-gated circuit linking circadian biology, hunger hormone signaling, and energy balance.
Detailed Summary
Why does eating at the wrong time of day promote weight gain even when total calories are matched? This study from UT Southwestern Medical Center's Center for Hypothalamic Research offers a compelling neural answer: a discrete population of suprachiasmatic nucleus (SCN) neurons that express GHSR — the receptor for the hunger hormone ghrelin — appears to act as a gatekeeper that amplifies or suppresses metabolic consequences of eating specifically during the mid-rest phase. This has direct relevance to the rising fields of time-restricted eating and chrono-nutrition, areas with growing clinical trial evidence in humans.
The investigators used Ghsr-IRES-Cre transgenic mice and stereotaxic delivery of Cre-dependent DREADDs (designer receptors exclusively activated by designer drugs) bilaterally into the SCN. Stimulatory hM3Dq and inhibitory hM4Di constructs were used in separate cohorts. Cases were rigorously classified post-hoc by histology into 'hits' (confirmed SCN/SPZ targeting), 'misses/ARC' (arcuate nucleus only), and 'misses/other' (neither region), ensuring anatomical specificity. Clozapine-N-oxide (CNO, 1 mg/kg i.p.) or saline was administered in a crossover design across multiple zeitgeber times spanning the full 24-hour cycle.
Chemogenetic activation of GHSR-expressing SCN neurons at ZT4 (mid-rest phase) increased 4-hour food intake by 144% versus saline controls. The same manipulation at ZT0, ZT12, or ZT16 produced no significant change in food intake in SCN-targeted mice. Strikingly, GHSR-expressing ARC neurons (misses/ARC group) increased food intake at all four time points tested — including a 530% increase at ZT4 — confirming that the time-specificity is a property of the SCN circuit specifically, not of ghrelin signaling in general. C-fos immunoreactivity confirmed that 59.5% of hM3Dq-expressing SCN neurons were activated by CNO at ZT4, validating the chemogenetic approach.
Repeated daily inhibition of GHSR-expressing SCN neurons at ZT4 for 15 consecutive days reduced food intake during the ZT4–8 window by approximately 35% each day, while food intake during the remaining 20 hours was unaffected — indicating a highly localized temporal effect rather than general anorexia. Cumulatively, 15-day total food intake was lower in hits versus misses. Body weight fell by 4.3% in hits while misses gained ~2.5%, producing a net between-group difference of ~6.8%. Feed efficiency (body weight gained per calorie consumed) was also significantly reduced in hits, suggesting that the SCN circuit influences not just intake but energy expenditure or metabolic rate. None of these effects were observed when inhibition was applied during ZT16 (active phase), confirming temporal specificity of the circuit.
Transcriptomic profiling revealed that GHSR-expressing SCN neurons represent subpopulations within six distinct SCN neuronal clusters. These neurons are predominantly GABAergic, exhibit light-sensitive gene expression, and show time-of-day-dependent transcriptomic profiles — consistent with their role as bona fide circadian clock neurons. Approximately 27.5% co-express arginine vasopressin (AVP), a canonical SCN neuropeptide. Projection mapping showed dense innervation of the paraventricular hypothalamus (PVH), dorsomedial hypothalamus (DMH), and ventromedial hypothalamus (VMH) — regions centrally involved in energy homeostasis. These anatomical findings provide a plausible downstream circuit through which SCN ghrelin signaling could regulate feeding behavior and metabolic rate in a time-gated fashion.
For human health, this work provides a neural mechanistic basis for why late eating — analogous to eating during the rest phase — promotes weight gain independent of caloric quantity, a finding previously documented in clinical trials. It raises the possibility that the SCN-ghrelin axis could be targeted pharmacologically or through timed feeding strategies to exploit this time-of-day metabolic sensitivity. A key caveat is that all experiments were conducted in mice, and translating the specific zeitgeber time windows to human chronobiology requires further work. The small sample sizes (4–6 hits per experiment) and reliance on chemogenetics in a non-disease model also limit immediate clinical inference.
Key Findings
- Chemogenetic activation of GHSR-expressing SCN neurons at ZT4 (mid-rest phase) increased 4-hour food intake by 144% versus saline; no effect was seen at ZT0, ZT12, or ZT16
- GHSR-expressing ARC neurons increased food intake at all four time points tested, including a 530% increase at ZT4, confirming that time-specificity is unique to the SCN circuit
- 59.5% of DREADD-expressing SCN neurons showed c-fos immunoreactivity after CNO at ZT4, confirming successful neuronal activation
- Repeated daily inhibition of GHSR-expressing SCN neurons at ZT4 over 15 days reduced food intake during the ZT4–8 window by ~35% per day without affecting intake during the remaining 20 hours
- 15-day repeated inhibition at ZT4 caused a net ~6.8% body weight difference: hits lost 4.3% while misses gained ~2.5%
- Cumulative feed efficiency was significantly reduced in SCN-targeted inhibition mice, suggesting increased energy expenditure beyond food intake reduction alone
- GHSR-expressing SCN neurons map to six distinct transcriptomic clusters, are predominantly GABAergic, and project densely to PVH, DMH, and VMH; ~27.5% co-express arginine vasopressin (AVP)
Methodology
The study used Ghsr-IRES-Cre transgenic mice with bilateral stereotaxic delivery of Cre-dependent stimulatory (hM3Dq) or inhibitory (hM4Di) DREADDs to the SCN. CNO (1 mg/kg i.p.) or saline was administered in a crossover design across zeitgeber times spanning the full 24-hour cycle, with n = 4–6 confirmed SCN hits per experiment verified post-hoc by histological mCherry mapping. Long-term metabolic effects were assessed over 15-day repeated daily inhibition periods separated by a 7-day recovery. Transcriptomic characterization of GHSR-expressing SCN neuron clusters used single-cell RNA sequencing analysis.
Study Limitations
All experiments were performed in mice, and translating specific zeitgeber time windows (e.g., ZT4 mid-rest) to equivalent human chronobiological phases requires further validation in human studies. Sample sizes were small (4–6 confirmed SCN hits per experiment), limiting statistical power and generalizability. The authors acknowledge that the chemogenetic approach is non-physiological and cannot fully replicate the nuanced temporal dynamics of endogenous ghrelin-SCN signaling. No conflicts of interest were declared by the authors.
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