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Effect of thyroidectomy on hypothalamic concentration and in vitro release of thyrotropin-releasing hormone in ovariectomized rats.

This investigation was designed to study the effect of thyroidectomy on the release of thyrotropin-releasing hormone (TRH) from the mediobasal hypothalamus (MBH) of ovariectomized (Ovx) rats in vitro. Immediately after ovariectomy, rats were thyroidectomized (Tx) or sham Tx, and decapitated 10 weeks later. The blood samples were collected, mixed with or without 2,3-dimercaptopropanol (BAL). The serum samples with BAL were assayed for immunoreactive (IR) TRH and those without BAL were assayed for thyroid-stimulating hormone (TSH) by radioimmunoassay (RIA). The mediobasal hypothalamus was extracted by 0.1 N HC1, or superfused with Krebs-Ringer phosphate buffer at 37 degrees C in a superfusion chamber, and stimulated by potassium ion (30 mM). The superfusate flowing from the superfusion chamber was collected during successive 10-min intervals at a rate of 0.5 ml/10 min and acidified with 5 N HC1. Concentrations of TRH in medium and tissue samples were determined by RIA. Thyroidectomy increased TSH concentration in the serum, but decreased TRH concentration in the serum and MBH. Both the spontaneous release and the K(+)-stimulated release of TRH from MBHs in vitro were lower in Ovx-Tx than in Ovx rats. These results suggest that the increased secretion of TSH in ovariectomized rats following thyroidectomy can not be attributed to changes of TRH secretion from mediobasal hypothalamus.