[D-ALA(2)]DELTORPHIN-I-LIKE IMMUNOREACTIVITY IN THE ADULT-RAT BRAIN -IMMUNOHISTOCHEMICAL LOCALIZATION

Using a specific antiserum recently raised against [D-Ala2]deltorphin I (DADTI: Tyr-D-Ala-Phe-Asp-Val-Val-Gly-NH2), a highly selective ligan d for delta-opioid receptors, we have previously demonstrated the occu rrence of positive immunostaining in several structures of mouse brain . We describe here the neuroanatomical distribution patterns of DADTI- immunoreactive neuronal bodies, axons, and tanycytes in rat brain. Pos itive neuronal somata were localized mainly in the ventral mesencephal on, including the ventral tegmental area and the pars compacta of the substantia nigra. A minor population of positive somata was found in t he pars reticulata and pars lateralis of the substantia nigra, raphe n uclei, supramammillary nucleus, and retrorubral reticular nucleus. All these regions, except for the supramammillary nucleus, contain dopami ne cell bodies. Intensely stained positive nerve fibers could be trace d along the medial forebrain bundle. Dense positive terminals were see n in the neostriatum, nucleus accumbens shell, olfactory tubercle, sep tal areas, cingulate, and medial prefrontal cortex. Double-immunostain ing study revealed that, in the substantia nigra, almost all (97.8%) D ADTI-positive neurons colocalized with tyrosine hydroxylase (TH), and the doubly stained cells occupied about one-third (29.1 %) of the tota l population of TH-positive neurons. Only a few DADTI/TH-positive cell s also stained for 28-kDa calbindin D, although many neurons double-st ained for 28-kDa calbindin D and TH. In contrast, the supramammillary nucleus contained a number of DADTI-positive cells, which nearly alway s stained positively for 28-kDa calbindin D but did not stain for TH. The association of DADTI-like immunoreactivity with certain dopaminerg ic pathways seems of particular interest. A small population of DADTI- immunostained tanycytes was present in the ventral part of the third v entricle wall.