Innervation of the pineal gland in the Arctic fox (Vulpes lagopus) by nerve fibers immunoreactive to substance P and calcitonin gene-related peptide.

Abstract

The study demonstrates, for the first time, the presence of substance P (SP) and calcitonin gene-related peptide (CGRP) in the nerve fibers supplying the pineal gland in the Arctic fox. The expression and distribution pattern of the studied substances were examined by double-labeling immunofluorescence technique. The SP-positive fibers enter into the pineal gland through the capsule as the nervi conari. The fibers formed thick bundles in the capsule and connective tissue septa, from where they penetrated into the pineal parenchyma. Inside the parenchyma, the nerve fibers created basket-like structures surrounding clusters of pinealocytes. The density of intrapineal SP positive fibers was slightly higher in the distal and middle parts of the gland than in the proximal one. Double immunostaining with antibodies against SP and CGRP revealed that the vast majority of SP positive fibers was also CGRP positive. The fibers showing a positive reaction to SP and negative to CGRP were scattered within the whole gland. The fibers immunopositive to CGRP and immunonegative to SP were not observed. In the habenular and posterior commissural areas adjoining to the pineal gland the immunoreactive nerve fibers were not found. Moreover, no immunopositive cell bodies were observed in both the pineal gland and the commissural areas. These results reveal that SP and CGRP are involved in the innervation of pineal gland in carnivores. In turn we suggest that these peptides can regulate/modulate melatonin secretion.