Circadian variation in the distribution of Hymenolepis diminuta (Cestoda) and 5-hydroxytryptamine levels in the gastro-intestinal tract of the laboratory rat.

The circadian migration of Hymenolepis diminuta in the small intestine of the rat may be correlated with a circadian variation in 5-hydroxytryptamine levels present in worm tissue, in the intestinal lumen, in the intestinal mucosa, with the amount of fold present in the small intestine and in arterial blood. 5-HT and food levels in uninfected animals were also determined. The 16.00 h stage in the circadian cycle marks both the commencement of host feeding, followed by rising 5-HT levels in both worm and host tissues, and initiation of an anteriad migration of worm biomass. It was found that 5-HT levels in the intestine of parasitized animals were significantly higher than in the intestine of uninfected controls. This is the first report of circadian variation in mucosal and luminal 5-HT levels. The similarity in the circadian patterns of worm migration and worm luminal, mucosal and blood 5-HT levels were striking. Fasting eliminated the circadian rise in intestinal 5-HT levels and the worms did not migrate. Luminal 5-HT levels were significantly lower in fasted animals than in the comparable rats fed ad libitum. When the intestine was ligatured at the pyloric sphincter, worm anteriad migration still occurred after feeding, indicating that the presence of exogenous food in the intestine is not a factor in the initial migration of the worms.