Is the Hypotensive Effect of Clonidine and Related Drugs Due to Imidazoline Binding Sites?

Clonidine and related alpha 2-adrenergic receptor (alpha 2AR) agonists lower arterial pressure primarily by an action within the central nervous system. These drugs also have varying degrees of affinity for other cellular components called nonadrenergic imidazoline binding sites (NAIBS). For over 20 years, the alpha 2AR agonist activity of clonidine-like drugs was thought to account for their therapeutic effects (alpha 2 theory). However, several groups have recently proposed a competing "imidazoline theory" according to which the hypotensive effect of clonidine-like drugs would in fact owe more to their affinity for one type of NAIBS, called I1 receptors. The alpha 2-theory is strongly supported by four main types of congruent data. First, the hypotensive effect of systemically administered clonidine is blocked by alpha 2AR antagonists that are without affinity for I1 NAIBs. Second, the hypotensive effect of intravenous clonidine is absent in genetically engineered mice in which a defective alpha 2AAR has been substituted for the normal one. Third, the sympatholytic effect of clonidine is consistent with the presence of conventional inhibitory alpha 2ARs on sympathetic preganglionic neurons and on their main excitatory inputs in the medulla oblongata. Fourth, the first I1 ligand without affinity for alpha 2ARs was found to be biologically inactive. The imidazoline theory is supported by a limited repertoire of whole animal "in vivo" pharmacological experiments that remain open to a wide range of interpretations. In conclusion, the bulk of the evidence strongly supports a largely predominant role of alpha 2AR mechanisms in the action of most clonidine-like agents at therapeutically relevant doses or concentrations. Even the small pharmacological differences between these agents cannot yet be linked with certainty to their relative affinity for I1 NAIBS.