Thyrotropin-releasing hormone release in normal and hyperthyroid rats as measured by microdialysis

To measure the thyrotropin-releasing hormone (TRH) release from the hypothalamus in euthyroid and hyperthyroid states, we investigated the changes in TRH levels in the anterior pituitary of conscious male rats using an in vivo microdialysis technique. In the euthyroid rats (n = 18), TRH levels in the extracellular dialysates of the anterior pituitary varied from 1.0 to 101.0 pg/ml in a pulsatile fashion: 15.9 +/- 13.9 (mean +/- SD) pg/ml with 5.8 +/- 1.5 pulses/24 h. In the hyperthyroid rats (n = 5) who received L-thyroxine at 10 micrograms/100 g body weight for 7 days, TRH levels in the dialysates during 6 h was 3.6 +/- 1.7 pg/ml, and significantly lower (P < 0.05) than those of the control rats (15.5 +/- 14.1 pg/ml); the pulse frequency was unchanged. These findings demonstrated for the first time the existence of pulsatile TRH release from the hypothalamus, and showed that thyroid hormone inhibits TRH release by the reduction of pulse amplitude, but not of pulse frequency.