Multiple opiate receptors and their functional significance.

There is ample evidence from studies of peripheral isolated organs, binding of opiate receptor ligands to brain membranes and behavioural experiments that opiate receptors are not homogeneous but consist of different types such as the mu- (= morphine), delta- (= enkephalin), kappa- (= ketocyclazocine) and the epsilon- (= beta-endorphin) receptors. In addition, there are indications for subtypes within the particular classes of receptors, at least in the case of mu- and kappa-receptors. In one organ or particular brain structure, several types of receptors can be present-though big differences in the relative concentrations between various brain areas exist. The precise nature of the relationship between the multiplicity of opiate receptors and the multiplicity of endogenous opioids is not clear. The enkephalins and dynorphin appear to be, however, related to, respectively, delta- and kappa-receptors. According to our present knowledge, there is no strict relationship between certain physiological functions (or pharmacological actions) and the activation of a particular receptor type; for example, mu-, delta- and kappa-receptors are involved in pain modulation.