Light microscopic autoradiographic localization of μ and δ opioid binding sites in the mouse central nervous system

Abstract

Much work has been done on opioid systems in the rat CNS. Although the mouse is widely used in pharmacological studies of opioid action, little has been done to characterize opioid systems in this species. In the present study the distribution of μ and δ opioid binding sites in the mouse CNS was examined using a quantitative in vitro autoradiography procedure. Tritiated dihydromorphine was used to visualize μ sites and [3H-d-Ala2-d-Leu5]enkephalin with a low concentration of morphine was used to visualize δ sites. μ and δ site localizations in the mouse are very similar to those previously described in the rat (Goodman, R. R., S. H. Snyder, M. J. Kuhar, and W.S. Young III (1980) Proc. Natl. Acad. Sci. U.S.A. 77:6239-6243), with certain exceptions and additions. μ and δ sites were observed in sensory processing areas, limbic system, extrapyramidal motor system, and cranial parasympathetic system. Differential distributions of μ and δ sites were noted in many areas. μ sites were prominent in laminae I, IV, and VI of the neocortex, in patches in the striatum, and in the ventral pallidum, nucleus accumbens, medial and midline thalamic nuclei, medial habenular nucleus, interpeduncular nucleus, and laminae I and II of the spinal cord. In contrast, δ sites were prominent in all laminae of the neocortex, olfactory tubercle, diffusely throughout the striatum, and in the basal, lateral, and cortical nuclei of the amygdala. The determination of the differential distributions of opioid binding sites should prove useful in suggesting anatomical substrates for the actions of opiates and opioids.