On the origin(s) of 5-hydroxytryptamine (5ht) in the tapeworm, Hymenolepis diminuta
It was the purpose of this investigation to determine the origin(s) of 5-hydroxytryptamine (5HT) in the rat tapeworm, Hymenolepis diminuta, where 5HT has been identified as a major neurotransmitter. Three questions were addressed in this study relative to the source(s) of 5HT in these helminths: (1) the ability or inability of these worms to synthesize 5HT via its precursors, i.e., tryptophan; and 5-hydroxytryptophan; (2) to establish whether or not 5HT is acquired directly from the host, i.e., uptake capability; (3) to establish the presence or absence of a 5HT carrier-mediated transport mechanism for 5HT absorption by these worms It was found that, in vitro, H. diminuta were severely limited in their ability to hydroxylate the indole ring of tryptophan. The majority of the exogenous tryptophan absorbed by these worms went directly into polypeptides and/or proteins. However, incubations of these helminths in 5-hydroxytryptophan (50HT) indicated that these worms are capable of decarboxylating 50HT, thereby producing an increase in 5HT These worms in vitro, after 30 minutes incubation, evidence an efficient absorption of 5HT at physiological ambient concentrations that correspond to and in some cases exceed the circadian variation of the amine in vivo. Tissue section autoradiography for ('3)H-5HT absorbed by H. diminuta indicated that there were significant concentrations of 5HT absorbed within the worm internal tissues Five-hydroxytryptamine uptake by these worms, in vitro, was significantly inhibited by its structural analogues, imipramine, tryptophan, 5-hydroxyindole acetic acid and tryptamine. However, 5HT uptake inhibition by its precursor, 5-hydroxytryptophan, was determined not significant. Further, it was found that radiolabeled 5HT uptake is inhibited by chemical 5HT yielding a histogram which suggests a carrier-mediated transport mechanism which seems to operate at the extremely low molar range In summary, it is suggested tht 5HT in H. diminuta is supplied by the host; that the parasite, incapable of manufacturing this compound (5HT) through limitations of its environment (anaerobiasis) and biochemistry, otherwise has developed absorptive mechanisms to acquire this needed compound, synthesized in the intestine by its host