Localisation of the neuropeptide PACAP and its receptors in the rat parathyroid and thyroid glands

PACAP (pituitary adenylate cyclase activating polypeptide) is widely distributed neuropeptide acting via three subtypes of receptors, PAC1, VPAC1 and VPAC2. Here we examined the localisation and nature of PACAP-immunoreactive nerves in the rat thyroid and parathyroid glands and defined the distribution of PAC1, VPAC1 and VPAC2 receptor mRNA’s. In the parathyroid gland a large number of nerve fibres displaying PACAP-immunoreactivity were distributed beneath the capsule, around blood vessels and close to glandular cells. Most of the PACAP-nerves were sensory, since they co-stored CGRP (calcitonin-gene-related peptide) and were sensitive to capsaicin-treatment. mRNA’s for PAC1 and VPAC2 receptors occurred in the parathyroid gland, mainly located in the glandular cells. In the thyroid gland PACAP-immunoreactive nerve fibres were associated with blood vessels, thyroid follicles and parafollicular C-cells. A high degree of co-existence between PACAP and VIP (vasoactive intestinal polypeptide) was observed in the intrathyroid nerve fibres and cell bodies of the thyroid ganglion indicating a common origin for the two peptides. A minor population of PACAP-immunoreactive nerve fibres with relation to blood vessels co-stored NPY (neuropeptide Y), whereas only a few fibres co-stored CGRP. PAC1 and VPAC1 receptor mRNA’s occurred in follicular cells and blood vessels, whereas the expression of the VPAC2 receptor was low. The findings suggest that PACAP plays a role in the regulation of parathyroid and thyroid blood flow and hormone secretion.

Research highlights
► PACAP-immunoreactive nerve fibres innervate the rat parathyroid and thyroid glands.
► The nerve fibres are associated with blood vessels and endocrine cells.
► PACAP-receptors (PAC1 and VPAC1) are present in the innervated structures.