The transcriptome of the ovine choroid plexus is regulated by thyroid hormone but not by photoperiod.

The photoperiodic control of seasonal functions requires the action of melatonin at the pars tuberalis of the pituitary and subsequent control of local thyroid hormone (TH) signaling by tanycytes lining the basolateral part of the third ventricle. Therefore, TH supply of tanycytes through the cerebrospinal fluid (CSF) produced by the choroid plexuses (CP) is central to photoperiodism. Here, the transcriptome of the CP of the lateral ventricles was established by RNAseq in ewes maintained under three experimental conditions: ewes exposed to a short photoperiod (SP; 8.5 h of light), intact ewes submitted to an acute 3-week exposure to a long photoperiod (LP-Sham; 15.5 h of light) and ewes thyroidectomized prior to the LP exposure (LP-THX). Photoperiod impacted the expression of 1169 genes (SP vs LP-Sham) while 575 genes were sensitive to TH (LP vs LP-THX). Compared to TH-responsive genes, photoperiod-responsive genes displayed rather weak transcriptional changes. In line with this, RT-qPCR for select candidate genes validated the impact of TH, but not that of photoperiod. We demonstrate weak expression of the melatonin MT1 receptor in the CP, which provides a functional rationale for this. In conclusion, the CP appears as a permissive tissue to the expression of TH-dependent seasonality governed by tanycytes rather than being an integral component of the melatonin-dependent photoperiodic response.