Heterogeneity of endogenous opiates: H-endorphin is not correlated with enkephalin or with β-endorphin

H-endorphin, an endogenous opiate present in brain, blood and cerebrospinal fluid of rats and humans is distinguishable from both enkephalin and β-endorphin. Its distribution within the rats' brain resembles, but does not overlap that of enkephalin. Its stability and presence in blood is similar to that of β-endorphin; however, its blood level is not altered under extreme conditions (stress, hypophysectomy) which change dramatically the level of β-endorphin. H-endorphin is almost equally potent in three bio-assay systems (guinea pig ileum, mouse vas deferans and rat vas deferens) thus dissociable from both typical μ (morphine-like) and δ (enkephalin-like) ligands. The distinct characteristics of H-endorphin, enkephalin and β-endorphin, differing in their anatomical distribution, physiological response and pharmacological effect, indicate the heterogeneity of the internal opioid system which consists of at least three different mediators.