Differentiation of delta and mu opiate receptor localizations by light microscopic autoradiography

We have observed two discrete populations of opiate receptors that are differently localized in rat brain. Morphine-like (mu) receptors, labeled by ¹²⁵I-labeled [D-Ala-²MePhe⁴Met(O)⁵-ol]enkephalin, are concentrated selectively in lamina IV of the cerebral cortex, certain thalamic nuclei, and the periaqueductal grey, while delta receptors, labeled by ¹²⁵I-labeled [D-Ala²D-Leu⁵]enkephalin, are more diffused, having high densities in cerebral cortex, corpus striatum, amygdala, and olfactory tubercle. Because of similarities in their localizations, we propose that mu and delta receptors are respectively the physiologic receptors for [Met]- and [Leu]enkephalin neurons. These distributions reflect the different physiological functions attributed to mu and delta receptors and thus represent discrete functions of [Met]- and [Leu]enkephalin neurons.