Effects of cholinergic drugs on longitudinal contraction in levamisole-susceptible and -resistant Haemonchus contortus.

A novel force transducer was used to measure the effects of cholinergic agonists on longitudinal contraction in Haemonchus contortus. Drugs were applied to whole worms or injected via a cannula in the pseudocoelomic cavity. A number of agonists, including nicotine and the anthelmintics m-aminolevamisole, levamisole and morantel, caused contractions in whole worms. Four- to 25-fold increases in concentration of the active compounds were required to cause contractions in each of two levamisole-resistant strains of H. contortus. Of the other compounds tested, bephenium had equivalent activity against susceptible and resistant strains. Anticholinesterase compounds caused contractions after a slight delay in susceptible, but not resistant worms. Numerous cholinergic agonists and other compounds did not cause contraction when applied to whole worms. One of these, acetylcholine, caused contractions in cannulated worms. Compared with the susceptible strain, five- to six-fold higher concentrations of acetylcholine were required to cause equivalent contractions in the resistant strains. Levamisole resistance in adult H. contortus is likely to be due to a change in the characteristics of the cholinergic receptor(s).