Immunohistochemical localization of substance P receptors in the midline glia of the developing rat medulla oblongata with special reference to the formation of raphe nuclei

Abstract

Immunohistochemical localization of the substance P receptor (SPR) was examined in the developing rat medulla oblongata, with special reference to the development of substance P (SP)-immunoreactive neurons which form the medullary raphe nuclei. During development, SPR immunoreactivity was detected in cells lying lateral to the medullary midline from embryonic day 13 (E13) to postnatal day 5 (P5). The SPR-positive cell bodies were located close to the fourth ventricle, and bore long processes extending to the ventral pial surface. This SPR immunoreactivity co-localized with staining for monoclonal antibody 1D11, a specific marker of immature astrocytes. Substance P (SP)-immunoreactive neurons were first detected at E14 in the ventrolateral part of the medulla. By E16 their number had increased and they were arrayed in two rows closely parallel to the SPR-immunoreactive processes of non-neuronal cells. By P1, two separate SP-immunoreactive cell clusters could be recognized at the midline, representing dorsally the nascent raphe pallidus and ventrally the raphe obscurus. In addition, many SP-immunoreactive fibers traveled rostrocaudally in the medulla oblongata, juxtaposed to the midline sheets of SPR-immunoreactive long processes. SPR-immunoreactive processes at the midline were also immunoreactive for S-100, a glia-specific calcium-binding protein that is known to promote axonal growth of raphe neurons. These results suggest that SPR-expressing immature glial cells at the medullary midline are involved in the development of SP-immunoreactive raphe neurons, both in the formation of the medullary raphe nuclei and in axon guidance and growth.