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Svalbard Ptarmigan and Golden Hamsters Shared a TSH Pathway for Seasonal Timing

Male Svalbard rock ptarmigan standing on tundra vegetation
Research · Molecular biology & biochemistry

Svalbard ptarmigan (Lagopus muta hyperborea) and golden hamsters (Mesocricetus auratus) showed the same conserved core TSH signalling mechanism for seasonal timing, despite being a bird and a mammal with different ways of sensing day length. The study compared gene expression in two connected neuroendocrine regions in experimentally produced summer- and winter-like states. RNA sequencing measured which genes were active in the pars tuberalis and tanycyte-enriched mediobasal hypothalamus, and matching ligand–receptor expression was used to identify possible communication pathways between them.

A conserved TSH pathway

The pars tuberalis, a small part of the pituitary gland, acts as a seasonal relay. It releases thyroid-stimulating hormone (TSH) toward tanycytes — specialised glial cells lining the third ventricle in the mediobasal hypothalamus. TSH signalling in these cells changes local thyroid-hormone metabolism, which in turn helps regulate seasonal changes in neural circuits controlling energy metabolism and reproduction.

This pathway stood out strongly in both species. Among the peptide-ligand genes shared between the two animals, Tshb, which encodes the beta subunit of TSH, was the only one whose expression changed by about 100-fold between seasonal states in both. The result reinforces the TSH–TSHR pathway as an evolutionarily conserved mechanism linking photoperiod to seasonal physiology in birds and mammals.

Different routes into the same seasonal system

The upstream signal differed, however. The melatonin receptor gene Mtnr1a was expressed in the hamster pars tuberalis but fell below detection in the Svalbard ptarmigan. This fits the broader model in which mammals relay day-length information through melatonin, whereas birds can use deep-brain photoreception before converging on the same downstream TSH system.

Candidate feedback signals from the hypothalamus

The comparison also identified possible signals in the opposite direction, from the hypothalamus back toward the pars tuberalis. Somatostatin, chemerin and neuropeptide Y were among the shared candidates. Somatostatin regulates pituitary hormone secretion, while chemerin and neuropeptide Y are linked to energy balance and feeding. That makes them plausible routes by which metabolic state could feed back into the seasonal timing system. The study identified matching ligand–receptor expression, however, not the functional signalling itself.

The two species came from separate experiments with different sexes and light protocols, so the authors caution against treating differences in expression magnitude as direct species contrasts. Their main conclusion is instead that the core TSH pathway is conserved and that several additional feedback routes now have clear candidates for functional testing.

Sources and image credit

  1. Hofinger, A., West, A., Appenroth, D., Cázarez-Márquez, F., Melum, V., Wood, S., & Hazlerigg, D. (2026). Conserved communication between the pars tuberalis and mediobasal hypothalamus in the Golden hamster and Svalbard ptarmigan: a genome enabled ligand-receptor pairing analysis.Primary open-access research article published 2 September 2026. Comparative transcriptomics of pars tuberalis and tanycyte-enriched mediobasal hypothalamus samples from summer- and winter-state Svalbard ptarmigan and golden hamsters reaffirmed conserved TSH–TSHR signalling. Tshb was the only shared peptide-ligand gene with an approximately 100-fold seasonal shift in both species; somatostatin, chemerin and NPY emerged as candidate reciprocal signals from the hypothalamus toward the pars tuberalis.

Image: Wild male Svalbard rock ptarmigan. Photo: Per Harald Olsen (Perhols) / Wikimedia Commons. CC BY-SA 3.0. Representative species image; not a bird from the laboratory study. · CC BY-SA 3.0 · Source

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Molecular biology & biochemistry

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