G protein-coupled receptors (GPRs) are a protein family of transmembrane receptors that transmit an extracellular signal (ligand binding) into an intracellular signal (G protein activation). GPR signaling is an evolutionarily ancient mechanism used by all eukaryotes to sense environmental stimuli and mediate cell-cell communication. All of the receptors have seven membrane-spanning domains and the extracellular parts of the receptor can be glycosylated. These extracellular loops also contain two highly conserved cysteine residues which create disulfide bonds to stabilize the receptor structure. GPR45, also known as PSP24-1 or PSP24-α, is a 372 amino acid orphan receptor that is suspected to play a role in brain function. Expressed in brain, GPR45 has been detected in the basal forebrain, frontal cortex and caudate, but not in thalamus, hippocampus or putamen.