Transient expression of [D-Ala2] deltorphin I-like immunoreactivity in prenatal rat small intestine

We studied the distribution of immunoreactive elements for [D-Ala2] deltorphin I (DADTI), a delta-opioid receptor ligand, in fetal and postnatal rat small intestine. DADTI-like immunoreactive cells were detected transiently on embryonic Days 20 and 21. Electron microscopic examination revealed that positive staining occurred in mucous epithelial cells, either mature goblet cells or undifferentiated cells containing only a few mucous granules. Positive immunoreaction products in mature goblet cells were confined in their apical cytoplasm to the luminal parts of mucous granule aggregates. The result suggests that a DADTI-like molecule(s) is synthesized in rat intestinal goblet cells and is secreted in a diacrine fashion into the intestinal lumen at a late fetal period. The molecule(s) thus secreted may be important for the intestine of rats just before birth, because DADTI-like immunopositive goblet cells are no longer seen at any postnatal period.