Pituitary-Thyroid Function during Acute Immobilization Stress in Rats

Summary

In nine experiments a total of 394 male Wistar rats weighing about 300 g was used. In each experiment, one intact control group was used and several other groups were subjected to immobilization (IMO) for 2 to 240 min in a prone position by inserting their heads through steel wire loops fixed on a board and by fasting their limbs to four metal strips by adhesive tape. All animals were killed by decapitation and the levels of thyrotropin (TSH), thyroxine (T₄), 3,5,3′-triiodothyronine (T₃) and 3,3′, 5′-triiodothyronine (rT₃) in plasma were estimated by radioimmunoassay. In one experiment, significant changes in the level of all hormones measured were found within 2 to 15 min of IMO, e.g. an increase of TSH, T₄ and rT₃ and a decrease of T₃. Later (i.e. between 30 and 240 min) the level of TSH, T₄ and T₃ was repeatedly found to be significantly decreased almost in all experiments as compared to controls. In contrast, a significant increase of rT₃ was found only in one out of 7 experiments, the values in other ones being unchanged. Corresponding changes of hormone levels were observed when IMO for 150 min was repeated daily for 7 or 40 days and the animals were sacrificed immediately after the last stress, while one day of rest after such procedures as well as after a single IMO for 150 min appeared to be sufficient for the levels to return to control values.