Evolutionary conservation of neuropeptide expression in the thymus of different species.

Evidence suggests that the immune and neuroendocrine systems cross talk by sharing ligands and receptors. Hormones and neuropeptides produced by the neuroendocrine system often modulate the function of lymphoid organs and immune cells. We have previously reported the intrathymic expression of somatostatin (SOM) in the mouse and that several neuropeptides, most notably calcitonin-gene-related peptide (CGRP), neuropeptide Y (NPY), SOM and substance P (SP), can modulate thymocyte development. However, little is known about the intrathymic expression of these neuropeptides either in the mouse or in other species. Moreover, a comparative analysis of the expression of these molecules would highlight the evolutionary importance of intrathymic neuroendocrine interactions in T-cell development. We have studied the expression of different neuropeptides in the thymus of zebrafish, Xenopus, avians, rodent, porcine, equine and human by immunohistochemistry and reverse transcription-polymerase chain reaction. We found that CGRP, NPY, SOM, SP and vasointestinal polypeptide (VIP) are expressed in the thymus of all species investigated. The thymic location of many of these neuropeptides was conserved and appears to be within the stromal compartments. Interestingly, in the avian thymus the expression of CGRP, SOM and SP appears to change depending on the age of the tissue. These findings suggest that neuropeptides may play an important role in T-cell development and provide further evidence of cross talk between the immune and neuroendocrine systems.