Authentication of the cholinergic system in the cattle tick, Boophilus microplus

Abstract

Acid-aqueous and organic-solvent extracts of organophosphate-susceptible larval cattle ticks yielded a biologically active substance which co-chromatographed with [ 14C]acetylcholine (ACh) on a molecular-sieve gel, and migrated identically with [ 14C]ACh during paper electrophoresis. The response of the rat fundus and the frog rectus abdominis muscle to these extracts was abolished after they were incubated with the specific ACh-hydrolysing enzyme, acetylcholinesterase (acetylcholine hydrolase, E.C. 3.1.1.7), or subjected to alkaline hydrolysis. Extracts of the isolated brains of adult ticks also showed biological activity which co-electrophoresed identically with [ 14C]ACh. No comparable amounts of an active substance, other than that which moved coincident with [ 14C]ACh, was found in the extracts. In terms of ACh chloride equivalents the ACh content of larvae was estimated by bioassay as 12.6 μg. per g. and that of adult brains as 47 μg. per g. Consistent with the model for the operation of the cholinergic system, and with the hypothesis for the lethal action of organophosphates, the ACh content of larvae treated with a lethal dose of Coumaphos increased significantly.