Immediate increase of thyroid hormone release during acute stress in rats: effect of biogenic amines rather than that of TSH?

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Abstract
An increase of thyroxine (T4; 36% in average) and 3,5,3''-triiodothyronine (T3; 9% in average) levels in plasma was found in rats after 2 min of immobilization stress (IMO), while later (from 5 to 240 min of IMO) the level of both hormones decreased sharply. This increase was prevented by i.p. injection of phenoxybenzamine (4 or 16 mg kg-1), prazosin (2.5 mg kg-1), yohimbine (4 or 16 mg kg-1) and fluoxetine (10 mg kg-1) at 60 min before IMO, but similar injections of phentolamine (4 or 16 mg kg-1), tolazoline (4 or 16 mg kg-1), methysergide (10 mg kg-1), cyproheptadine (10 mg kg-1) as well as of lower doses of prazosin (0.6 or 1.2 mg kg-1) did not show such an effect. The level of TSH was increased after 2 min IMO too; however, this was considered to be a separate phenomenon from the increase of T4 and T3, since in several groups injected with some of the above drugs the level of TSH was decreased together with increased levels of T4 and T3. Apparently the acute release of thyroid hormone might be due to the effect of biogenic amines which may be blocked by some .alpha.-adrenergic blocking agents. Since neither adrenalectomy nor adrenal medullectomy prevented such a release, it may be concluded that circulating catecholamines of adrenal origin did not play any role in this process. The possibility of participation of intrathyroidal sympathetic nerve endings was also repeatedly tested after pharmacological sympathectomy achieved by a long-term administration of guanethidine, but data obtained to date have been inconsistent.