Effects of vasoactive intestinal polypeptide on hypothalamic-pituitary-thyroid axis in rats

The effects of vasoactive intestinal polypeptide (VIP) on the hypothalamic-pituitary-thyroid axis in rats were studied. VIP (250 micrograms kg-1) was injected i.v. and the rats were serially decapitated. Thyrotropin-releasing hormone (TRH), thyrotropin (TSH) and thyroid hormone were measured by radioimmunoassay. The hypothalamic immunoreactive TRH (ir-TRH) content significantly decreased after VIP injection, whereas its plasma concentration significantly increased. The plasma TSH level increased significantly in a dose-related manner with a zenith at 20 min after the injection. The plasma thyroid hormone level also increased significantly. The plasma ir-TRH and TSH responses to cold as well as the plasma TSH response to TRH were significantly enhanced by VIP. Naloxone partially blocked VIP induced TSH release. In 5-hydroxytryptophan pretreated group the stimulatory effect of VIP on TSH release was prevented, but not in that pretreated with para-chlorophenylalanine, L-DOPA or haloperidol. These drugs alone did not affect plasma TSH levels in terms of the dose used. The inactivation of TRH immunoreactivity by plasma or hypothalamus did not differ from that of the control. These findings suggest that VIP acts on the hypothalamus, pituitary and thyroid gland to stimulate TRH, TSH and thyroid hormone release, respectively, and that its effect may be at least partially modified by the serotonergic system and opioid peptides.