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

Abstract

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.

Hypothyroidism Reduces Content and Increases in vitro Release of Pro-Thyrotropin-Releasing Hormone Peptides from the Median Eminence

“…A vari ety of experimental approaches ranging from passive immuni zation against TRH [6,19] ablation of the PVN [20], hypophy sial portal blood collection for the direct measurement of TRH [8,9], determination of TRH in ME tissue [7] or characteriza tion of anterior pituitary TRH binding sites [21 ] have resulted in opposing views as to whether hypothalamic TRH release is regulated by thyroid hormone. Since direct measurement of TRH in vivo is complicated by the dilTicult technical nature of the approach (either hypophysial portal blood collection or push-pull perfusion experiments) and by the extremely short half-life time of TRH [22,23], we tested the effect of altered thyroid status on the in vitro release o f TRH and another proTRH derived peptide which is not recognized in the TRH RIA.…”

“…Szabo et al [6] concluded from their in vivo studies that the hypersecretion of TSH following thyroidectomy is largely independent of TRH control, while Mori and Yamada [7], on the other hand, reported changes in the median eminence (ME) con tent of TRH following thyroidectomy consistent with in creased TRH secretion. Further, studies involving the direct Received: April 2, 1990 Accepted after revision: October 24, 1990 measurement of TRH in hypophysial portal plasma have not clarified whether hypothalamic TRH secretion is regulated by thyroid hormone [8,9].Since it was possible that changes in thyroid status might affect the posttranslational processing of proTRH and thereby result in an altered pattern of synthesis and release of TRH vis-a-vis other proTRH-derived peptides, we studied the effects of hypothyroidism and hyperthyroidism on the hypothalamic content and release of preproTRH2s-so, the N-terminal se quence of proTRH, as well as TRH itself. …”

Effect of Cold Exposure on the Hypothalamic Release of Thyrotropin-Releasing Hormone and Catecholamines

“…Rats with signs of infection, continued weight loss or incorrect placement of the cannula [22] were excluded from the experiment (n = 21). Three weeks later, the mediobasal hypo thalamus was perfused as described before [22][23][24], Artificial cerebro spinal fluid was delivered at a rate of 20 ¡.tl/min and perfusate was col lected continuously for 120 min into tubes kept on melting ice. Transit time from the tip of the cannula to the sample tube was 5-7 min, and every 15 min a new sample was begun.…”

“…In the present study, push-pull per fusion of the mediobasal hypothalamus was used to determine the in vivo release of TRH and catecholamines in male rats ex posed to cold. Previous evidence has suggested that push-pull perfusion of the mediobasal hypothalamus is a valid method for the estimation of in vivo release of TRH, dopamine and adrena line [22][23][24]. …”

Collision of Basic and Applied Approaches to Risk Assessment of Thyroid Toxicants

“…This may be a mechanism by which T 4 can be adequately maintained at a developmental time when it is uniformly important. THs (T 4 and T 3 ) exert a negative feedback effect on the release of pituitary TSH and on the activity of hypothalamic TRH neurons 18, 19, 124 . Although it is clear that TH regulates the expression of TSH 153–155 and TRH 18, 123, 124, 156 in a negative feedback manner, it is also clear that the functional characteristics of negative feedback must include more than simply the regulation of the gene encoding the secreted protein/peptide.…”