Adrenergic receptors (also called adrenoceptors) are the receptors for the catecholamines adrenaline and noradrenaline (called epinephrine and norepinephrine in the United States). Adrenaline and noradrenaline play important roles in the control of blood pressure, myocardial contractile rate and force, airway reactivity, and a variety of metabolic and central nervous system functions.
 
Adrenergic receptors are members of the G-protein coupled receptor (GPCR) superfamily of membrane proteins. They share a common structure of seven putative transmembrane domains, an extracellular N-terminus, and a cytoplasmic C- terminus.

Adrenoceptors are divided into three types: α₁, α₂ and β-adrenoceptors. Each type is further divided into at least three subtypes: α_1A, α_1B, α_1D, α_2A, α_2B, α_2C, β₁, β₂, β₃^1,2. Adrenoceptors are expressed in nearly all peripheral tissues and in the central nervous system^1,2.
 
In general, the pharmacological properties of each subtype are quite homogenous across different species. However, this is not the case for the α_2A subtype which was first isolated from human platelets³. This subtype shows different pharmacological properties from that of mouse and rat. For this reason, until molecular techniques significantly advanced, it was believed that human α_2A and that of mouse and rat (termed α_2D) were two different subtypes. Today, it is accepted that these two subtypes are in fact one gene product and is generally termed α_2A. One of the main differences between this subtype from the different organisms is its affinity for yohimbine and rauwolscine; rat and mouse α_2A displays less affinity compared to human^3,4.