Light microscopic autoradiographic localization of delta opioid receptors in the rat brain using a highly selective bis-penicillamine cyclic enkephalin analog

Light microscopic autoradiography was used to visualize the neuroanatomical distribution of rat brain delta opioid receptors. Slide-mounted sections of rat brain were labeled with [³H]-[2-D-penicillamine, 5-D-penicillamine]enkephalin ([³H]DPDPE), a highly selective delta opioid agonist. Saturation isotherms of [³H]DPDPE binding to thaw-mounted brain slices gave a maximal number of binding sites of 79.9 fmol/mg of protein and an apparent dissociation constant (K(d)) of 6.3 nM. DPDPE and met-enkephalin inhibited [³H]DPDPE binding with high affinity (IC₅₀ values of 6.3 and 13.8 nM, respectively). Putative mu opioid receptor selective ligands such as morphine sulfate, Tyr-D-Ala-Gly-NMePhe-Gyl-ol and [N-MePhe³, D-Pro⁴]morphiceptin (PL017) were less potent inhibitors of [³H]DPDPE binding. The rat brain areas containing the highest densities of receptors were the claustrum, basolateral amygdaloid nucleus, the caudate-putamen and nucleus accumbens, the external plexiform layer of the olfactory bulb and the olfactory tubercle. Moderate receptor density was characteristic of the hippocampal formation in which grains were seen over the molecular layer of the dentate gyrus and stratum oriens (CA1), and of the different layers of cerebral cortex. Generally, low density of binding was found over the thalamus and the septal nuclei. Low specific binding was also seen in the cerebellum, medulla oblongata and in the dorsal horn of the spinal cord. There was little specific [³H]DPDPE binding over the white matter areas.