Prenatal thyroxine treatment disparately affects peripheral and amygdala thyroid hormone levels

Abstract

A prenatal hypothyroid state is associated with behavioral abnormalities in adulthood. Wistar Kyoto (WKY) rats exhibit hypothyroidism and increased depressive and anxiety-like behaviors. Thus, the WKY could illuminate the mechanisms by which the reversal of developmental hypothyroidism in humans and animals results in adult behavioral improvement. We examined the outcome of maternal thyroxine (T4) treatment on thyroid hormone-regulated functions and adult behavior of the WKY offspring. Pregnant WKY dams completed gestation with and without T4 administration and their adult male offspring were tested. Measures included depressive and anxiety-like behaviors, and thyroid hormone (TH) concentrations in both plasma and specific brain regions. In addition, the expression of two proteins affecting thyroid hormone trafficking and metabolism, monocarboxylate transporter 8 (MCT-8) and iodothyronine deiodinase type III (Dio3), and of several behavior-altering molecules, glucocorticoid receptor (GR), prepro-thyrotropin releasing hormone (prepro-TRH) and corticotrophin releasing hormone (CRH), were determined in the hippocampus and amygdala of the offspring. Prenatal T4 treatment of WKYs did not affect adult depressive behavior but increased anxiety-like behavior and decreased plasma levels of THs. In the hippocampus of males treated with T4 in utero, Dio3 and MCT-8 protein levels were increased, while in the amygdala, there were increases of free T4, MCT-8, GR, prepro-TRH protein and CRH mRNA levels. These results show that T4 administration in utero programs adult peripheral and amygdalar thyroid hormone levels divergently, and that the resulting upregulation of anxiety-related genes in the amygdala could be responsible for the exacerbated anxiety-like behavior seen in WKYs after prenatal T4 treatment.

Figure 1. Increased anxiety after prenatal T4 treatment

T4-treated males exhibited (A) decreased latency to bury the prod and (B) increased grooming in the DB test compared to controls (n=6-11 animals/group). No significant effect of prenatal T4 was observed in other DB measures: approaches, rears, or time spent burying the prod. One-way ANOVA, *p<0.05, **p<0.01.

Figure 2. Brain region-specific effects of prenatal T4 on thyroid hormone levels in adult offspring

Free T4 and T3 concentrations were both non-significantly decreased in the hippocampus (A), but in the amygdala, T4 was significantly increased and T3 showed a similar trend (B). Student's t-test, n=6/group, **p<0.01.

Figure 3. Prenatal T4 treatment alters protein levels in the hippocampus and amygdala

(A) Hippocampus: Levels of the TH transporter monocarboxylate transporter 8 (MCT-8) were increased and the TH-inactivating enzyme deiodinase-III (Dio3) was also elevated. (B) Amygdala: MCT-8 levels were increased along with glucocorticoid receptor (GR) and prepro-thyrotropin-releasing hormone (prepro-TRH) levels. Student's t-test, n=4/group, *p<0.05. Inserts are representative immunoblots.