Conserved photoperiodic communication between the pars tuberalis and mediobasal hypothalamus across mammals and birds: 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 showed that the only conserved photoperiodic signalling pathway from the PT to the MBH was TSH signalling. In addition our data suggest that, in both species, MBH expression of the somatostatin encoding gene, Sst , may signal to the PT via SSTR1 / GPR107 receptors. Together, these findings deepen our understanding of the molecular dialogue between the PT and MBH in the seasonal regulation of physiology and offer new insights into the extent to which photoperiodic signalling mechanisms are conserved across mammals and birds.