THE NEUROPEPTIDES, VIP AND NPY, THAT ARE PRESENT IN THE THYROID NERVES ARE NOT RELEASED INTO THE THYROID VEIN

In the present study, we tested the hypothesis that the neuropeptides, vasoactive intestinal peptide (VIP) and neuropeptide Y (NPY), which a re present in the thyroid nerves, act as physiological neurotransmitte rs involved in the regulation of thyroid hormone secretion and thyroid blood flow. Specifically, we examined whether these neuropeptides can be released into thyroid blood vessels by electrical stimulation of t he major thyroid nerves or whether their expression is altered by chan ges in iodine intake. Sprague-Dawley rats were used in this study, The cervical sympathetic trunk or the superior laryngeal nerve was stimul ated by bipolar electrodes in anesthetized rats. During nerve stimulat ion, blood samples were withdrawn from the thyroid vein. Thyroid blood now was monitored by laser Doppler blood flowmetry. Sympathetic stimu lation caused a marked decrease in thyroid blood flow, which was assoc iated with a significant increase in release of norepinephrine. Howeve r, these effects were not accompanied by any change in NPY release int o the thyroid vein. Stimulation of the superior laryngeal nerve was no t associated with changes in thyroid blood flow or VIP release into th e thyroid vein. In a separate experiment, rats were fed a diet contain ing low-, high-, or normal iodine concentrations. Triiodothyronine (T- 3) and thyroxine (T-4) levels in thyroid venous plasma were significan tly reduced in rats fed a low-iodine diet but not in a separate group of rats fed a high iodine diet. However, these treatments had no effec t on VIP or NPY concentrations in thyroid venous plasma or in thyroid ganglia. Thus, our results indicate that VIP and NPY, which are presen t in the thyroid nerves, may not be directly involved in the regulatio n of thyroid function.