Mode of action of praziquantel on Schistosoma japonicum

The stimulatory properties of praziquantel on Schistosoma japonicum were demonstrated: 1) the stimulating effect of 5-hydroxytryptamine on schistosomes was increased by low concentrations of praziquantel; 2)stimulation of the motor activity of the worms by praziquantel could be blocked by a 5-HT blocking agent, cinanserin; 3) praziquantel could stimulate the motor activity of the parasites which had been exposed previously to a 5-HTdepleting agent, imipramine-HCl; and 4) the paralyzing effect of arecoline and metrifonate on schistosomes was antagonized by low concentrations of praziquantel. It is suggested that praziquantel may display a 5-HT like action or may be a 5-HT receptor agonist. Despite a lengthening effect on schistosomes, catecholamines did not block the praziquantel-induced spasmodic contraction of the worms. When the worms were incubated in a medium containing a high concentration of Ca²⁺ , strong spasmodic contraction of the worms caused by praziquantel was seen. Pre-incubation of the worms in a Ca²⁺-free medium resulted in a blockage of the action of praziquantel on schistosome musculature. On the contrary, in a Mg²⁺-free medium the praziquantel-induced spasmodic contraction was augmented. But K⁺ exerted negligible effects on the action of praziquantel. These results indicate that Ca²⁺ may play an essential role in the contractile activity of the musculature caused by praziquantel, which is controlled to a certain degree by Mg²⁺.