WIDESPREAD DISTRIBUTION OF HISTAMINE IN THE NERVOUS-SYSTEM OF A TREMATODE FLATWORM

In general, most flatworms contain very little histamine (HA) and thei r nervous systems often lack, or contain very few, histaminergic eleme nts. However, preliminary studies in our laboratory have revealed that the frog lung parasite, Haplometra cylindracea (Trematoda: Digenea), contains HA in a very high concentration. For this reason, the present study was undertaken to study the localization and synthesis of HA in this worm by using immunocytochemistry and high-pressure liquid chrom atography (HPLC). Essentially all parts of the nervous system of H. cy lindracea showed HA-like immunoreactivity. The paired cerebral ganglia and nerves emanating from these, including the longitudinal nerve cor ds, were intensely immunoreactive. The musculature of the pharynx, ora l and ventral suckers, and those of the reproductive organs were all i nnervated by HA-immunoreactive fibers. Fiber plexuses beneath the tegu ment and throughout the parenchyma also showed HA-like immunoreactivit y. HPLC studies revealed one of the highest HA concentrations in the a nimal kingdom, 6.49 +/- 1.36 nmole/mg protein, in the worm. The frog l ung and blood contained very low concentrations of HA and could be exc luded as sources for HA, while an enzyme assay revealed that the worm produces HA by decarboxylation of histidine. Thus, it is likely that H . cylindracea uses HA as a neurotransmitter or modulator. (C) 1996 Wil ey-Liss, Inc.