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. 2021 Jul 5;11:373. doi: 10.1038/s41398-021-01494-5

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© The Author(s) 2021, corrected publication 2021

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 2 — A A mid-sagittal section of the human brain, with an expanded diagram of the basal portion of the hypothalamus and third ventricle (V3) and the infundibular stalk that connects the pituitary gland to the hypothalamus. B Circulating levels of melatonin at night are reduced during the long days (LD) of summer and increase with the short days (SD) of winter. This diagram depicts a coronal section through the V3, PT, and the anterior pituitary. The PT wraps around the infundibular stalk and contains a high density of melatonin (MT1) receptors on PT-specific thyrotrophs. A retrograde pathway from the PT involves the release of thyroid stimulating hormone (TSHβ) that stimulates receptors on β2 tanycytes that line the base of V3. Under long-day conditions of summer, type 2 deiodinase is upregulated to facilitate the conversion of the prohormone thyroxine (T4) to the bioactive triiodothyronine (T3). T3 is taken up into brain areas to increase reproductive functions, energy levels and mood. PT-specific thyrotrophs also communicate through an anterograde pathway to the pars distalis (anterior pituitary) by releasing the endocannabinoids, N-arachidonoylethanolamine (anandamide, AEA) and 2-arachidonoylglycerol (2-AG), which stimulate folliculo-stellate (FS) cells in the anterior pituitary, leading to the release of ACTH from corticotrophs and prolactin from lactrotrophs [70–74].