The Influence of Intranasal Insulin on Hypothalamic-Pituitary-Thyroid Axis in Normal and Diabetic Rats

Abstract

The functions of hypothalamic-pituitary-thyroid axis are attenuated in type 1 diabetes mellitus due to insulin deficiency. The use of intranasally administered insulin is of considerable interest for treatment of diabetes and cognitive disorders, but its effect on the thyroid system has not been investigated yet. We studied the influence of long-term treatment with intranasal insulin on the hypothalamic-pituitary-thyroid axis of nondiabetic rats and diabetic animals with streptozotocin models of acute and mild type 1 diabetes mellitus. This treatment was carried out for 28 days in acute (daily does of 0.3, 0.6, and 1.5 IU of insulin per rat) and for 135 days in mild diabetes (daily dose of 0.45 IU/rat). Nondiabetic rats were treated in a similar manner. Intranasal insulin in both models of diabetes resulted in the improvement of thyroid status; manifested as increase of thyroid hormones levels and restoration of response to thyroliberin. In acute diabetes, a daily dose of 0.6 IU/rat was the most effective. Twenty eight days treatment of nondiabetic rats with intranasal insulin at a dose of 0.3 IU/rat resulted in a significant increase of free and total thyroxine levels. Longer treatment of rats with mild diabetes and nondiabetic animals significantly increased thyrotropin level. Thus, long-term intranasal insulin treatment restored the hypothalamic-pituitary-thyroid axis function in type 1 diabetes, but led to a significant increase in the thyrotropin level, which must be considered when designing a strategy for the use of intranasal insulin in clinical applications.