Effect of TRH on the secretion of prolactin in ewes at various stages of pregnancy and in non-pregnant ewes during the breeding season and seasonal anoestrus

The influence of TRH and TSH injections on plasma concentrations of tri-iodothyronine (T3) and thyroxine (T4) was investigated in neonatal (injection within 0.5 h after delivery) and growing lambs and in normal, pregnant and lactating adult ewes (all 2 years old and originating from Suffolk, Milksheep and Texal crossbreeds). Neonatal lambs had higher levels of T3, T4 and GH compared with all other groups, whereas prolactin and TSH were higher in lactating ewes. In all animals, injections of TRH increased plasma concentrations of prolactin and TSH after 15 min but not of GH at any time. Small increases in T3 and T4 were observed in neonatal lambs, without any effect on the T3 and T4 ratio, after prolactin administration, whereas prolactin did not influence plasma concentrations of T3 or T4 in all other experimental groups. Similar results for thyroid hormones were obtained after TRH or TSH injections. It was therefore concluded that the effects observed after TRH challenge were mediated by the release of TSH. With the possible exception of neonatal lambs, plasma concentrations of T3 after administration of TRH or TSH were always increased before those of T4; the increase in T3 occurred within 0.5-1 h compared with 2-4 h for T4 in all experimental groups. This resulted in an increased ratio of plasma T3 to T4 up to 4 h after injection. It is concluded that, in sheep, TRH and TSH preferentially release T3 from the thyroid gland probably by a stimulatory effect of TSH on the intrathyroidal conversion of T3 to T4.

Section: Basal Levels Of Hormonessupporting

confidence: 92%

Preferential release of tri-iodothyronine following stimulation by thyrotrophin or thyrotrophin-releasing hormone in sheep of different ages

Peeters

Buys²,

Vanmontfort³

et al. 1992

“…In rats, pro¬ lactin secretion is associated with increased secretion of TRH into portal blood during suckling (de Greef & Visser, 1981) and oestrogen stimulation (de Greef, Klootwijk, Karels & Visser, 1985). In sheep, TRH is known to release pituitary prolactin both in vitro (Ray & Wallis, 1984) and in vivo (Fell, Findlay, Cumming & Goding, 1973;Lamming, Moseley & McNeilly, 1974;Fitzgerald, Evins & Cunningham, 1981). Whether this stimu¬ latory effect of TRH is of physiological significance remains to be determined.…”

Section: Introductionmentioning

confidence: 99%

Release of prolactin is independent of the secretion of thyrotrophin-releasing hormone into hypophysial portal blood of sheep

Thomas

Cummins²,

Yao³

et al. 1988

In order to determine whether pituitary prolactin release was directly related to the secretion of TRH into hypophysial portal blood, serial portal and jugular venous blood samples were collected from seven lactating and three non-lactating ewes. In another experiment, samples were collected from five ovariectomized ewes while being exposed to an audio-visual stress and then later administered with chlorpromazine. Secretion of TRH was pulsatile in all ewes and independent of prolactin secretion; TRH pulses coincided with significant increases in prolactin secretion in only 15% of cases and only 29% of prolactin pulses were associated with TRH pulses. Sixty-seven per cent of suckling bouts were associated with increases in prolactin secretion, but only 22% of these were associated with significant increases in TRH secretion. Chlorpromazine increased prolactin levels fourfold but did not affect portal concentrations of TRH. Audio-visual stress was not a reliable method of causing prolactin release in this model. Mean portal concentrations of TRH and jugular concentrations of prolactin were not significantly correlated. These results show that hypothalamic TRH and pituitary prolactin are secreted independently in the sheep, implying that increases in prolactin release caused by suckling or chlorpromazine are not the direct result of increased TRH secretion.

“…TRH also causes the release of prolactin in the ewe (Lamming et al 1974). In pregnant and non-pregnant ewes, basal prolactin secre¬ tion and the prolactin response to TRH administration change with season (Fitzgerald et al 1981). Basal prolactin secretion and the response to TRH (Table 2) were unchanged over the period of experiment 2, so seasonal variation over the period of the experiment (approximately 90 days) is unlikely to be a factor in the recovery of GH responsiveness to GRF.…”

Section: Discussionmentioning

confidence: 99%

Effects of hypothyroidism on the growth hormone axis in sheep

Fletcher¹,

Clarke

1994

This study examined the effect of thyroidectomy (TX) on the GH axis in sheep. The secretion of GH was monitored 10 and 77 days after TX or sham-TX when the effects on plasma GH and prolactin levels of the injection of 0.5 micrograms GH-releasing factor (GRF)/kg and 1 microgram thyrotrophin-releasing hormone (TRH)/kg were also assessed. There were no significant differences in GH pulse amplitude, pulse frequency, inter-pulse interval and GH secreted/h between sham-TX and TX animals at 10 or 77 days after TX. There was no difference in the GH response to GRF injection in sham-TX sheep at any time but in TX sheep the GH response was significantly (P < 0.05) attenuated 10 days after TX. After 77 days the GH response was similar to the response before TX. There was no measurable GH response to injection of TRH in sham-operated or TX sheep at any time. The prolactin response to TRH was not affected by TX or sham-TX. These results suggest that TX in sheep does not affect GH secretion but paradoxically the response to GRF is attenuated in hypothyroid sheep in the short term. TRH causes release of prolactin but not GH in sheep.