Hypothyroidism reduces ObRb–STAT3 leptin signalling in the hypothalamus and pituitary of rats associated with resistance to leptin acute anorectic action

Aug 10, 2012The Journal of endocrinology

Hypothyroidism lowers leptin signaling in brain hunger and hormone control areas linked to resistance to leptin’s appetite-suppressing effects in rats

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Abstract

Hypothyroid rats showed decreased ObRb and phosphorylated STAT3 levels in both the hypothalamus and pituitary gland.

Hypothyroid rats had lower levels of ObRb and phosphorylated STAT3 in the hypothalamus compared to hyperthyroid rats.
In the pituitary gland of hypothyroid rats, there were reductions in ObRb, total STAT3, phosphorylated STAT3, and SOCS3.
Hyperthyroid rats showed minimal changes in STAT3 signaling compared to hypothyroid counterparts.
Hypothyroid rats did not reduce food intake after leptin injection, unlike euthyroid rats.
Decreased ObRb-STAT3 signaling in hypothyroid rats may contribute to diminished leptin effects on food intake and TSH secretion.

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Leptin has been shown to regulate the hypothalamus-pituitary-thyroid axis, acting primarily through the STAT3 pathway triggered through the binding of leptin to the long-chain isoform of the leptin receptor, ObRb. We previously demonstrated that although hyperthyroid rats presented leptin effects on TSH secretion, those effects were abolished in hypothyroid rats. We addressed the hypothesis that changes in the STAT3 pathway might explain the lack of TSH response to leptin in hypothyroidism by evaluating the protein content of components of leptin signalling via the STAT3 pathway in the hypothalamus and pituitary of hypothyroid (0·03% methimazole in the drinking water/21 days) and hyperthyroid (thyroxine 5 μg/100 g body weight /5 days) rats. Hypothyroid rats exhibited decreased ObRb and phosphorylated STAT3 (pSTAT3) protein in the hypothalamus, and in the pituitary gland they exhibited decreased ObRb, total STAT3, pSTAT3 and SOCS3 (P<0·05). Except for a modest decrease in pituitary STAT3, no other alterations were observed in hyperthyroid rats. Moreover, unlike euthyroid rats, the hypothyroid rats did not exhibit a reduction in food ingestion after a single injection of leptin (0·5 mg/kg body weight). Therefore, hypothyroidism decreased ObRb-STAT3 signalling in the hypothalamus and pituitary gland, which likely contributes to the loss of leptin action on food intake and TSH secretion, as previously observed in hypothyroid rats.

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Hypothyroidism reduces ObRb–STAT3 leptin signalling in the hypothalamus and pituitary of rats associated with resistance to leptin acute anorectic action

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