Endocrine Abstracts

Fasting produces a rapid reduction of the activity of the central arm of the hypothalamic–pituitary–thyroid (HPT) axis, with a drop in biosynthesis of thyrotropin releasing hormone (TRH) in periventricular and medial parvocellular neurons of the paraventricular nucleus (PVN), and decreased TRH release into the pituitary portal capillaries. These events contribute to fasting-induced suppression of serum thyroid hormone levels, an adaptive mechanism to conserve energy. Pyroglutamyl peptidase II (PPII), the TRH degrading ectoenzyme, is expressed by the β2 tanycytes, in close apposition with TRH nerve endings in the external layer of the median eminence, and controls TRH levels in the extracellular space and serum TSH levels. To test the hypothesis that tanycyte PPII activity is regulated during fasting, we measured PPII expression and activity in the median eminence of 250 g BW male Wistar rats. Rats were transferred into individual cages and submitted to fasting for 24–72 h, with water ad libitum. Control animals were submitted to isolation for the same time than food-deprived animals. Animals were sacrificed at various time points after fasting initiation, but always 3–5 h after lights were on. Compared to isolation, fasting reduced animal weight, PVN TRH mRNA levels, and concentration of total T₄ (but not T₃) in serum; a delayed drop in serum TSH concentration was observed between 48–60 h. Semi-quantitative in situ hybridization indicated that PPII mRNA levels increased in β2 tanycytes 48 h after fasting initiation. This increase was transitory (not detected at 72 h), and followed by an increase of PPII activity in the median eminence, with a peak response between 60 and 72 h. We conclude that during fasting a delayed increase in median eminence PPII activity may facilitate the maintenance of the profound reduction of HPT axis activity.

Declaration of interest: The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.

Funding: This work was supported, however funding details unavailable.