Peptide histidine isoleucine amide stimulates thyroid hormone secretion and coexists with vasoactive intestinal polypeptide in intrathyroid nerve fibers from laryngeal ganglia.

Peptide histidine isoleucine amide (PHI) and vasoactive intestinal polypeptide (VIP) are fragments of the same precursor molecule, prepro-VIP, and coexistence of the two peptides is, therefore, to be expected. Nerve fibers displaying PHI and VIP immunoreactivity occurred around blood vessels and follicles in the thyroid gland of several species. Sequential staining with antibodies against PHI and VIP revealed coexistence of the two peptides in the same population of nerve cell bodies in ganglia situated along the laryngeal nerves and in intrathyroid nerve fibers. Chemical sympathectomy (6-hydroxydopamine treatment), surgical sympathectomy (removal of the superior cervical ganglia), and unilateral cervical vagotomy (removal of the nodose ganglion) failed to affect the number and distribution of PHI/VIP fibers in the thyroid gland. Taken together, the findings suggest that both the perivascular and interfollicular PHI/VIP fibers originate in laryngeal ganglia. PHI weakly stimulated basal thyroid hormone secretion in mice in vivo, but did not influence the response to TSH or VIP. PHI had no effect on calcitonin secretion in rats. Like VIP, PHI may play a physiological role in the regulation of thyroid hormone secretion.