The alpha-1 (a₁) adrenergic receptor is a G protein-coupled receptor (GPCR) associated with the G_q heterotrimeric G-protein. It consists of three highly homologous subtypes, including a_1A-, a_1B-, and a_1D-adrenergic. Catecholamines like norepinephrine (noradrenaline) and epinephrine (adrenaline) signal through the a₁-adrenergic receptor in the central and peripheral nervous systems.