Immunization against vasoactive intestinal peptide does not affect thyroid hormone secretion or thyroid blood flow

Vasoactive intestinal peptide (VIP) is present in thyroid parasympathetic nerves. To assess the involvement of endogenous VIP in the regulation of thyroid function, blood levels of thyroid hormones and thyroid blood flows (TBF) were measured after systemic immunization against VIP or after transection of the superior laryngeal nerves in male rats, which reduced the thyroid content of VIP but did not affect blood levels of thyroid hormones or TBF. Anti-VIP monoclonal antibody or anti-VIP serum was used for immunization against VIP in normal rats. In addition, VIP antibody was given to rats fed an iodine-deficient diet for 5 days to examine the involvement of this peptide in iodine deficiency-induced increases in TBF. Effects were measured at different times (90 s, 30 min, 1 h, and 5 days) after immunoneutralization, but none of these treatments changed blood levels of thyroid hormones or TBF in normal or iodine-deficient rats. However, passive immunization against VIP was associated with a high binding capacity of rat plasma to VIP, and this treatment reduced blood levels of prolactin as well as blood flows to the duodenum, stomach, and lung. These findings suggest that the VIP present in thyroid nerves is not involved in maintaining basal thyroid hormone secretion or TBF and that this neuropeptide does not mediate thyroid vascular adjustments to dietary iodine deficiency.