We have investigated the effects of the prolonged administration (2 or 8 days) of arginine-vasopressin (AVP), alone or with antagonists of its V1 and V2 receptors (V1-Ra and V2-Ra), on the rat pituitary-thyroid axis. In the 8-day, but not 2-day experiments, AVP raised thyroid weight, and the effect was prevented by V1-Ra. Morphometry showed that the AVP-induced increase in thyroid weight was mainly due to a rise in the stroma volume. In the 2-day, but not 8-day experiments, AVP lowered TSH plasma concentration, and the effect was annulled by V1-Ra. V1-Ra was ineffective per se, while V2-Ra lowered TSH blood level. AVP administration increased the level of circulating thyroid hormones, especially thyroxine (free and total T4). In the 2-day, but not 8-day experiments, this effect of AVP was blocked by both V1-Ra and V2-Ra. When administered alone V1-Ra and V2-Ra induced a small, but significant rise in T4 plasma concentration in both the 2-day and 8-day experiments. Collectively, these findings indicate that: i) AVP administration exerts a transient inhibitory effect on TSH secretion, and a more prolonged stimulatory action on thyroid secretion and growth, both of which are mainly mediated by the V1-R subtype; and ii) endogenous AVP exerts a clearcut tonic inhibitory effect on pituitary TSH release and doubtful regulatory actions on the thyroid gland.

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