Boyle, J. P., Zaide, J. V., and Yoshino T. P. 2000. Schistosoma mansoni: Effects of serotonin and serotonin receptor antagonists on motility and length of primary sporocysts in vitro. Experimental Parasitology94, 217–226. The effects of serotonin (5-hydroxytryptamine; 5-HT) on in vitro transformed primary sporocysts of Schistosoma mansoni were investigated. Serotonin treatment significantly increased parasite motility (percentage of motile sporocysts) and length at concentrations as low as 1 μM. These effects were mimicked by the 5-HT agonist tryptamine, albeit with 10- to 100-fold less potency. The effects of 10 μM 5-HT on sporocyst motility were observed within 15 min posttreatment and on parasite length by 6 h posttreatment, and both effects were stable for up to 48 h. Receptor antagonists with varying affinities for defined vertebrate neurotransmitter receptor subtypes were examined for their effects on parasite behavior in the absence and presence of 10 μM 5-HT. In the absence of 5-HT, only methiothepin significantly inhibited normal parasite growth after 48 h of incubation. In the presence of 10 μM 5-HT, the serotonin receptor antagonists mianserin, ketanserin (both at 100 μM), and methiothepin (at 10 μM) significantly inhibited 5-HT-induced lengthening of primary sporocysts, while 3-tropanyl-indole-3-carboxylate and chlorpromazine had no significant effect. The effects of these same drugs on parasite motility were also examined. In the absence of 5-HT, 10 μM chlorpromazine increased parasite motility, while the other antagonists had no effect. When sporocysts were treated with 10 μM 5-HT for 2 h in the continued presence of antagonist, 100 μM mianserin, ketanserin, 3-tropanyl-indole-3-carboxylate, and 10 μM methiothepin inhibited 5-HT induced increases in parasite motility, while 10 μM chlorpromazine had no effect. These results show that primary sporocysts of S. mansoni exhibit behavioral responses to serotonin much like adult stages of this parasite. Furthermore, these responses appear to be mediated via receptors with pharmacological similarities to those previously described in adult worms.