Rotifer neuropharmacology--I. Cholinergic drug effects on oviposition of Philodina acuticornis (Rotifera, Aschelminthes).

The presence of acetylcholine in the culture medium was found to result in egg retention in the bdelloid rotifer Philodina acuticornis in a dose-dependent fashion, with no other discernible physiological effect. When six to eight eggs accumulate, the animals distend, burst and die. No other cholinergic agonist has been shown to produce a similar effect. The antagonistic effect of six anticholinergic drugs and five acetylcholinesterase-inhibitor insecticides was investigated on egg retention. All compounds were found to inhibit this phenomenon to varying degrees, but on the basis of their EC50 neuromuscular blockers appear to be most active in inhibiting egg retention. We suggest, therefore, that egg retention is caused by a spasm or increased tone of the cloacal sphincter rather than a paralysis of muscles needed in the peristaltic expulsion of the egg. Five acetylcholine-esterase inhibitory insecticides also inhibit egg retention.