Neuropeptide Y (NPY) is one of the most abundant and ubiquitously expressed neuropeptides in both the central and peripheral nervous systems, and its regulatory effects on feed intake and appetite have been extensively studied in a wide variety of animals, including mammalian and non-mammalian species. Recent studies have shown that this neuropeptide and its receptors are expressed in various peripheral tissues; however, research investigating the distribution and function of peripherally expressed NPY in avian (non-mammalian vertebrates) species are limited.
1. Structure of Neuropeptide Y
The Neuropeptide Y (NPY) system is an ancient signaling pathway, as it is found in both vertebrates and invertebrates, highlighting a potential evolutionarily conserved function 
. Structurally similar to PYY and PP, the amino acid sequence of NPY is one of the most highly conserved neuropeptides. As shown in Table 1
, there is over 90% identity in the amino acid sequence among mammalian species, and greater than 80% identity between chicken and other species (Figure 1
. Additionally, phylogenic analysis indicates that the non-mammalian species share a common ancestor that diverged from mammals in their NPY sequence 
B). The molecular structure contains numerous hydrophobic interactions, as well as and N-terminal polyproline-II-like helix and a C-terminal α-helix 
. The N-terminal portion is responsible for interactions with various receptors, as studies have shown this segment interacts with Y1 but not Y2 
. Additionally, NPY contains two translation initiation sequences, allowing for the production of both full-length and a truncated NPY, containing only peptides 17–36 
, which can further differentially bind to receptors.
NPY amino acid sequence alignments (A
) and phylogeny (B
). Amino acid sequences were aligned using Clustal Omega 1.2.4 
. * positions with a single, fully conserved residue. “:” (colon) conservation between groups of strongly similar properties. “.” (period) conservation between groups of weakly similar properties. Phylogenetic tree generated with MEGA7: Molecular Evolutionary Genetics Analysis version 7.0 for bigger datasets