An electron microscopical study of calcitonin gene-related peptide-like immunoreactive innervation of the anterior pituitary in the dog.

Our previous studies have demonstrated the presence of a considerable number of substance P-, calcitonin gene-related peptide (CGRP)-, and galanin-like immunoreactive (LI) nerve fibers in the anterior pituitary in several mammalian species. The present study investigated the ultrastructure of the CGRP-LI innervation of this gland in the dog. The CGRP-LI nerve fibers were unmyelinated, with a wealth of varicosities containing both small clear synaptic vesicles and large dense-cored vesicles. They were found to be in direct contact with every cell type of the anterior pituitary. However, only on corticotropes and somatotropes were CGRP-LI synaptic contacts identified. Most of them were asymmetrical in type. Occasional symmetrical synaptic contacts were also found. It is considered likely that direct neural factors may play a role in the regulation of the anterior pituitary.