Morphological and biochemical changes in the thyroid gland of the lizard Podarcis sicula (Reptilia, Squamata) induced by VIP treatment

The effects of exogenous vasoactive intestinal polypeptide (VIP) involvemen t in the regulation of thyroid function were detected in the lizard Podarci s sicula. Variations in thyroid gland morphology, thyroid-stimulating hormo ne (TSH), 3,5,3'-triiodo-L-thyronine (T3) and L-thyroxine (T4) plasma level s, were studied in this species after treatment with VIP, during winter and summer. The polypeptide had a stimulatory effect on thyroid hormone secret ion throughout the year. Plasma T4 and T3 levels significantly increased; m aximal values were reached in specimens exposed longer to treatment. No dif ference in plasma TSH levels was observed between control and treated speci mens, indicating that, in P. sicula, the site of VIP action might be better identified at thyroidal rather than hypophyseal level. Consistently with t hese results, the morphology of the thyroid gland also appeared stimulated after treatment with VIP, showing an increase in thyroid-cell height, numer ous intracellular colloid droplets PAS-positive, and enlarged blood vessels around the follicles.