Neuropeptide Y (NPY) is a 36 amino acid peptide neurotransmitter present in the central and peripheral nervous systems. NPY is involved in the regulation of a broad range of functions such as eating behavior, hypertension, and reproduction. NPY exerts its effects via five receptor subtypes known as the neuropeptide Y receptors (NPYR): Y₁ (NPY₁R), Y₂ (NPY₂R), Y₄ (NPY₄R), Y₅ (NPY₅R), and Y₆ (NPY₆R). Four of these (Y₁, Y₂, Y₄, and Y₅) are expressed in humans. The NPY receptors belong to the G protein-coupled receptor superfamily. Activation of these receptors by NPY affects [Ca²⁺]_i, adenylate cyclase signalling, and a number of ion channel functions.