Developmental iodine deficiency resulting in hypothyroidism reduces hippocampal ERK1/2 and CREB in lactational and adolescent rats

Abstract

Background: Developmental iodine deficiency (ID) leads to inadequate thyroid hormone that impairs learning and memory with an unclear mechanism. Here, we show that hippocampal extracellular signal-regulated kinase (ERK1/2) and cAMP response element-binding protein (CREB) are implicated in the impaired learning and memory in lactational and adolescent rat hippocampus following developmental ID and hypothyroidism.

Methods: Three developmental rat models were created by administrating dam rats with either iodine-deficient diet or propylthiouracil (PTU, 5 ppm or 15 ppm)-added drinking water from gestational day (GD) 6 till postnatal day (PN) 28. Then, the total and phorsporylated ERK1/2 and total and phorsporylated CREB in the hippocampus were detected with western blot on PN14, PN21, PN28 and PN42.

Results: The iodine-deficient and hypothyroid pups showed lower serum FT3 and FT4 levels, smaller body size, and delayed eyes opening. The mean number of surviving cells in the hippocampus of the iodine-deficient and 15 ppm PTU-treated rats was significantly reduced compared to controls (P < 0.05). Iodine-deficient and 15 ppm PTU-treatment groups demonstrated significantly lower level of total and phosphorylated ERK1/2 and CREB than the controls on PN14, PN21 and PN28 (P < 0.05, respectively). The reduction of ERK1/2 and CREB was not reversible with the restoration of serum thyroid hormone concentrations on PN42.

Conclusions: Developmental ID and hypothyroidism down-regulate hippocampal ERK1/2 and CREB in lactational and adolescent rats.

Figure 1

ID and PTU-induced hypothyroidism delayed the eye opening in offspring. The ratio of iodine-deficient and 15 ppm PTU-treatment pups within a litter with both eyes open was significantly lower than controls on PN17 and PN18. Each bar represents the mean ± SEM (control group: n = 6; 5 ppm treatment group: n = 6; 15 ppm treatment group: n = 6; iodine-deficient group: n = 7). *P < 0.05, as compared to control. ^#P < 0.05, as compared to 5 ppm PTU-treatment group. ^ΔP < 0.05, as compared to 15 ppm PTU-treatment group.

Figure 2

Western blot analysis shows the decreased t-ERK1/2 following ID and hypothyroidism (n = 3 for each group). The levels of t-ERK1/2 in ID and hypothyroidism groups were significantly lower than controls in CA1 and CA3 regions on PN14 (A), PN21 (B), PN28 (C) and PN42 (D). (a) Representative blots of t-ERK1/2 in hipocampal CA1, CA3 and DG regions of control, 5 ppm PTU-treatment, 15 ppm PTU-treatment and iodine-deficient groups. (b and c) t-ERK1 and t-ERK2 levels. Each bar represents the mean ± SEM. *P < 0.05, as compared to control. ^#P < 0.05, as compared to 5 ppm PTU-treatment group. ^ΔP < 0.05, as compared to 15 ppm PTU-treatment group.

Figure 3

Western blot analysis shows the decreased p-ERK1/2 following ID and hypothyroidism (n = 3 for each group). The levels of p-ERK1/2 in ID and hypothyroidism groups were significantly lower than controls in CA1 and CA3 regions on PN21 (A), PN28 (B) and PN42 (C). (a) Representative blots of p-ERK1/2 in hipocampal CA1, CA3 and DG regions of control, 5 ppm PTU-treatment, 15 ppm PTU-treatment and iodine-deficient groups. (b and c) p-ERK1 and p-ERK2 levels. Each bar represents the mean ± SEM. *P < 0.05, as compared to control. ^#P < 0.05, as compared to 5 ppm PTU-treatment group. ^ΔP < 0.05, as compared to 15 ppm PTU-treatment group.

Figure 4

Western blot analysis shows the decrease levels of t-CREB following ID and Hypothyroidism (n = 3 for each group). The levels of t-CREB in ID and hypothyroidism groups were significantly lower than controls on PN14 (A), PN21 (B), PN28 (C) and PN42 (D). (a) Representative blots of t- CREB in hipocampal CA1, CA3 and DG regions of control, 5 ppm PTU-treatment, 15 ppm PTU-treatment and iodine-deficient groups. (b) t- CREB levels. Each bar represents the mean ± SEM. *P < 0.05, as compared to control. ^#P < 0.05, as compared to 5 ppm PTU-treatment group. ^ΔP < 0.05, as compared to 15 ppm PTU-treatment group.

Figure 5

Western blot analysis shows the decrease levels of p-CREB following ID and Hypothyroidism (n = 3 for each group). The levels of p-CREB in ID and hypothyroidism groups were significantly lower than controls on PN21 (A), PN28 (B) and PN42 (C). (a) Representative blots of p-CREB in hipocampal CA1, CA3 and DG regions of control, 5 ppm PTU-treatment, 15 ppm PTU-treatment and iodine-deficient groups. (b) p-CREB levels. Each bar represents the mean ± SEM. *P < 0.05, as compared to control. ^#P < 0.05, as compared to 5 ppm PTU-treatment group. ^ΔP < 0.05, as compared to 15 ppm PTU-treatment group.