The ultrastructural localization of serotonin (5-HT)-like immunoreactivity was investigated in human gut endocrine cells by the block-staining method using horseradish peroxidase (HRP)-labeled anti-5-HT rabbit (Fab')2. The reactive products against 5-HT were satisfactorily demonstrated on the membrane-bound and/or non-membrane-bound ribosomal granules as well as on the basally accumulated biconcave granules as seen in typical enterochromaffin (EC) cells.
The cross-reactivity of the anti-5-HT antisera to various tryptophan related compounds and the other monoamines was examined by the 125I labeled protein A (IPA) assay system. The anti-5-HT antisera were completely neutralized by serotonin-bovine serum albumin (BSA) and N-acetylserotonin-BSA conjugates and incompletely by free serotonin creatininesulfate. No inhibition of the antibody binding activity was observed when the antisera were preincubated with tryptophan and its related compounds or other monoamines.
In conclusion, our results confirmed the ultrastructural localization of 5-HT-like immunoreactivity in EC cells and the availability of the blockstaining method using HRP-labeled anti-5-HT (Fab')2 for ultrastructural immunohistochemistry.