In VivoHypothalamic Release of Thyrotropin-Releasing Hormone after Electrical Stimulation of the Paraventricular Area: Comparison between Push-Pull Perfusion Technique and Collection of Hypophysial Portal Blood

Unilateral electrical stimulation for 15 min of the paraventricular area of anesthetized rats induced a 2- to 3- fold increase in plasma TSH levels and caused an increased release of TRH into hypophysial stalk blood from 217 +/- 25 to 530 +/- 90 pg/15 min (n = 6). This experimental model was then used to determine the in vivo hypothalamic release of TRH by push-pull perfusion of either the mediobasal hypothalamus (MBH) or anterior pituitary (AP). Before stimulation, TRH release per 15 min was 4.2 +/- 0.7 pg from the MBH (n = 18) and 3.5 +/- 0.3 pg from the AP (n = 13). Unilateral electrical stimulation of the paraventricular area led to higher plasma TSH levels in 27 of 31 rats, and levels during stimulation increased from 0.89 +/- 0.04 to 1.86 +/- 0.10 ng/ml (n = 31). No significant increase in TRH in the perfusates was observed when push-pull perfusion was done in the MBH contralateral to the site of stimulation (n = 6). However, TRH release increased 2- to 3-fold during the perfusion of the MBH ipsilateral to the site of stimulation (15.4 +/- 4.3 pg/15 min; n = 13). In conclusion, push-pull perfusion of the MBH or AP can be used to estimate hypothalamic TRH release. However, the output of TRH by push-pull perfusion is low and varies considerably between individual rats. Thus, the practical value of push-pull perfusion for measurement of in vivo TRH release seems limited.