Pituitary receptors during development in the rat. I. TRH binding capacity

In order to study the role of thyroid releasing hormone (TRH) in the control of thyroid stimulating hormone (TSH) secretion during the neonatal period, we measured the binding of [3H]-TRH to pituitary homogenate of rats at various stages of development. In 2-day-old animals, the number of [3H]-TRH binding sites were similar to that of adult male animals (23 +/- 8 fmole/mg protein versus 21 +/- 7 fmole/mg protein, respectively). Between 5-21 days of age, the number of [3H]-TRH binding sites was significantly higher than that of adult animals (P less than 0.01). Hypothyroidism induced a 2-fold increase in the number of [3H]-TRH binding sites in all the age groups studied. This effect was reversed by administration of L-T4 (0.4 microgram/g body weight for 3 days). These studies thus indicate that TRH binding sites are present during the neonatal period in the rat and suggest that TRH may be an important modulator of TSH secretion during this period in the rat and that these effects are mediated by postreceptor mechanisms.