Abstract
The effects of exogenous vasoactive intestinal polypeptide (VIP) involvement in the regulation of thyroid function were detected in the lizard Podarcis sicula. Variations in thyroid gland morphology, thyroid‐stimulating hormone (TSH), 3,5,3'‐triiodo‐L‐thyronine (T3) and L‐thyroxine (T4) plasma levels, were studied in this species after treatment with VIP, during winter and summer. The polypeptide had a stimulatory effect on thyroid hormone secretion throughout the year. Plasma T4 and T3 levels significantly increased; maximal values were reached in specimens exposed longer to treatment. No difference in plasma TSH levels was observed between control and treated specimens, indicating that, in P. sicula, the site of VIP action might be better identified at thyroidal rather than hypophyseal level. Consistently with these results, the morphology of the thyroid gland also appeared stimulated after treatment with VIP, showing an increase in thyroid‐cell height, numerous intracellular colloid droplets PAS‐positive, and enlarged blood vessels around the follicles.
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