Conserved communication between the pars tuberalis and mediobasal hypothalamus in the Golden hamster and Svalbard ptarmigan: a genome enabled ligand-receptor pairing analysis.

Many seasonal vertebrates rely on photoperiod to synchronise their seasonal physiology and behaviour with annual changes in the environment. In both mammals and birds, the pars tuberalis (PT) of the pituitary gland decodes photoperiodic information and regulates seasonal timing. It achieves this through thyroid-stimulating hormone (TSH) signalling to neighbouring hypothalamic glial cells known as tanycytes, which line the ventricular walls of the mediobasal hypothalamus (MBH) and express TSH receptors (TSHR). In turn, tanycytes govern the seasonal state of neural circuits controlling energy metabolism and reproduction. PT - MBH signalling interactions in addition to the TSH / TSHR axis have been suggested but conservation between mammals and birds has not been demonstrated. Here, we performed laser capture microdissection and transcriptomic analysis of PT and tanycyte-enriched MBH samples taken from one highly seasonal mammal, the Golden hamster, and from one highly seasonal bird, the Svalbard ptarmigan. Comparison of bidirectional G-protein coupled receptor (GPCR) - peptidergic ligand expression levels across summer and winter states reconfirmed the central role of TSH signalling as a conserved mechanism within the seasonal neuroendocrine pathway. In both species, ligand-receptor pairing analysis provides evidence for MBH signalling to the PT through somatostatin (Sst), chemerin (Rarres2) and neuropeptide Y (Npy) signalling pathways. Further analysis of cellular expression patterns and signalling function is warranted to determine whether these pathways play conserved roles in the expression of seasonal changes in metabolic or reproductive physiology.