Hypothalamo-pituitary regulation of thyrotrophin secretion in chronically catheterized Brattleboro rats

Abstract

ABSTRACT Plasma TSH rhythms were measured in Brattleboro (DI) and control Long–Evans (LE) rats with an intracardiac catheter allowing repeated sampling in conscious unstressed animals. The TSH response to thyrotrophin-releasing hormone (TRH; 500 ng/100 g body weight) was also determined. Finally, hypothalamic and pancreatic TRH concentrations and TRH-degrading activity (TRH-DA) were measured by specific radioimmunoassay. Long–Evans rats had a 24-h rhythm with a major modulatory 8-h component. In DI rats, only the 24-h rhythm was detected. The mean 24-h rhythm-adjusted mean TSH level was higher in DI than in LE rats (1·38±0·05 and 1·14 ± 0·06 μg/l respectively, P<0·01). The peak TSH response to TRH was significantly increased in DI rats while the pituitary concentration of TSH was also higher (0·93 ± 0·09 vs 0·39± 0·06 μg/mg wet weight in LE, P<0·001). Hypothalamic TRH and TRH-DA were similar in both strains. The response to propyl-thiouracil-induced hypothyroidism was identical in both strains. We conclude that DI rats have a normal pituitary sensitivity to tri-iodothyronine but a central dysfunction in the pituitary environment leading to some alterations of TSH secretion. J. Endocr. (1985) 105, 277–283