Effects of thyroidectomy on thyrotropin-releasing hormone (TRH) and somatotropin release-inhibiting factor (SRIF) patterns in intrahypophysial microdialysates in rats.

The effects of thyroidectomy on patterns of TRH and somatotropin release-inhibiting factor (SRIF) release from the hypothalamus were investigated by using a microdialysis technique. Thyroidectomized and sham-operated rats underwent placement of a guide cannula into the anterior pituitary gland to obtain dialysates, or implantation of an intravenous cannula into the right atrium for blood sampling. Seven days postoperatively dialysates were collected at a flow rate of 2 microliters/min every 1 h. TRH concentrations in dialysates from thyroidectomized rats (0.43 +/- 0.2 pg/h) were significantly higher than those from control rats (0.17 +/- 0.02 pg/h). In contrast, SRIF concentrations in dialysates from thyroidectomized rats (2.45 +/- 0.05 pg/h) were significantly lower than those from control rats (3.80 +/- 0.22 pg/h). In addition, plasma TSH concentrations in thyroidectomized rats (24.8 +/- 0.5 ng/ml) were increased compared with those in control rats (2.5 +/- 0.1 ng/ml), and plasma GH concentrations were decreased from 68.6 +/- 6.4 ng/ml in control rats to 21.2 +/- 0.6 ng/ml in thyroidectomized rats. These findings indicate that TRH and SRIF releases from the hypothalamus are detectable by microdialysis method, and directly show the increase in TRH secretion and the decrease in SRIF secretion from hypothalamus in the hypothyroid state.