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Plant-Parasitic Nematode Acetylcholinesterase Inhibition by Carbamate and Organophosphate Nematicides 1

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Plant-Parasitic Nematode Acetylcholinesterase Inhibition by Carbamate and Organophosphate Nematicides 1 Journal of Nematology 22(4):481-488. 1990. © The Society of Nematologists 1990. Plant-parasitic Nematode Acetylcholinesterase Inhibition by Carbamate and Organophosphate Nematicides 1 C. H. OPPERMAN AND S. CHANG':' Abstract: The sensitivity of acetylcholinesterases (ACHE) isolated from the plant-parasitic nema- todes Meloidogyne arenaria, M. incognita, and Heterodera glycines and the free-living nematode Cae- norhabditis elegans to carbamate and organophosphate nematicides was examined. The AChE from plant-parasitic nematode species were more sensitive to carbamate inhibitors than was AChE from C. elegans, but response to the organophosphates was approximately equivalent. The sulfur-con- taining phosphate nematicides were poor inhibitors of nematode acetylcholinesterase, but treatment with an oxidizing agent greatly improved inhibition. Behavioral bioassays with living nematodes revealed a poor relationship between enzyme inhibition and expression of symptoms in live nema- todes. Key words: acetylcholinesterase, Caenorhabditis elegans, carbamate, Heterodera glycines, Meloidogyne arenaria, M. incognita, nematicide, organophosphate. Nematode locomotion depends upon an ilarity in nervous system structure among array of approximately 90 motor neurons diverse groups of nematodes (16), this and interneurons that employ the neuro- scenario probably holds true for plant- transmitters acetylcholine (excitatory) and parasitic nematodes as well. gamma-aminobutyric acid (GABA) (inhib- Carbamate and OP nematicide mode-of- itory) (9,16,17). Acetylcholine causes an in- action is presumed to be inhibition of AChE crease in the rhythmic frequency of muscle (22,26). The presence of AChE in plant- cell depolarizations. The action of acetyl- parasitic nematodes has been demonstrat- choline at the neuromuscular synapse is ed (1,12,20,21,24,27,29,31,32,36), but few terminated by acetylcholinesterase (ACHE, investigations concerning its physiological E.C. 3.1.1.7). The effects of acetylcholine function have been performed. Aphelen- can be potentiated by numerous AChE in- choides ritzemabosi cholinesterase did not be- hibitors, including carbamate and organo- have as a true acetylcholinesterase (33) but phosphate (OP) nematicides (8,16,30). rather had a broader substrate affinity than Genetic mosaic analysis of Caenorhabditis mammalian forms of the enzyme. This elegans has demonstrated that the only vital finding is supported by data generated with function of AChE in nematodes is to hy- C. elegans (15). drolyze acetylcholine at the neuromuscu- There are several conflicting reports lar synapse (19). Mutant strains that lack concerning nematode sensitivity to AChE AChE in neurons behave as the wild type, inhibitors (2,20,24,28). AChE isolated from provided AChE is present in the muscu- Aphetenchus avenae was sensitive to AChE lature. Those nematodes lacking AChE in inhibitors in vitro, but the compounds the muscles are nonviable. Given the sim- tested were ineffective at comparable levels on A. avenae in bioassays (28). However, A. avenae was extremely sensitive to AChE in- Received for Publication 1 August 1989. i Paper No, 12304 of the Journal Series of the North Car- hibitors at low dosages in a different bioas- olina Agricultural Research Service, Raleigh, NC 27695-7643. say (2). Laboratory tests with other nema- Use of trade names in this publication does not imply en- dorsement by the North Carolina Agricultural Research ser- tode species also have shown nematodes to vice of the products named nor criticism of similar ones not be extremely sensitive to AChE inhibition mentioned. Assistant Professor and Research Technician, Depart- (5). ment of Plant Pathology, North Carolina State University, We have studied AChE in Meloidogyne Raleigh, NC 27695-7616. We thank the Caenorhabditis Genetics Center, which is arenaria, M. incognita, and Heterodera glyci- supported by Contract NOI-AG-9-2113 between the NIH nes as part of a project to examine nervous and the Curators of the University of Missouri, for some of the nematode strains. We also thank Carl D. Johnson and system function in plant-parasitic nema- Stephen P. Schmidt for invaluable counsel and assistance. todes. We used C. elegans as a model be- 481 482 Journal of Nematology, Volume 22, No. 4, October 1990 cause its AChE is very well characterized were aliquoted and stored as decribed (7,15,18,19,23). In this paper we report above. the development of methods to analyze Nematode preparation: In initial experi- plant-nematode enzyme activity, compar- ments, fresh suspensions of packed nema- ative effects of carbamate and OP nema- todes were homogenized by dropwise ad- ticides on isolated nematode AChE com- dition to liquid nitrogen in a chilled mortar pared with living nematodes, and and the liquid nitrogen was allowed to boil experiments addressing bioactivation of away. The remaining frozen beads were parent nematicides to their toxic metabo- crushed to powder with a chilled pestle. lites. The preparation was thawed and then son- icated with a Virtis 50 ultrasonic cell dis- rupter (Gardner, NY) at 50% power for MATERIALS AND METHODS three 5-second bursts. The resulting ho- Nematode culture: Meloido~,ne arenaria mogenate was used immediately for en- and M. incognita were maintained on roots zyme assays and protein determinations (3). of tomato (Lycopersicon esculentum cv. Rut- This method proved to be an inefficient gers) in the greenhouse. Egg masses were use of tissue; therefore, most experiments dissolved with 0.5% sodium hypochlorite utilized a microassay. (13) and the eggs were collected for hatch- Samples (ca. 100 nematodes) were pre- ing. Heterodera glycines was cultured on soy- pared for AChE microassay by subjecting bean (Glycine max cv. Ransom) in the green- them to six freeze-thaw cycles. After the house. Mature cysts were collected by final cycle, samples were incubated at room sieving and gently cracked in a glass ho- temperature overnight. This treatment ef- mogenizer to release the eggs. The eggs fectively disrupts nematode cuticle per- were placed on 25-/zm-pore screens in meability without grossly disrupting the water, and hatched second-stage juveniles morphology and makes the AChE within were collected at 24-hour intervals. The the nematode accessible to substrate. collected nematodes were separated from AChE activity assay: AChE assays were debris by flotation on 20-40% sucrose and carried out using a modification of the sin- rinsed to remove residual sucrose. The gle vial radiometric method (14). Test nematodes were pelleted by centrifugation compounds were solubilized in acetone (718 g, 5 minutes) and then resuspended such that the solvent concentration in the in 1-2 volumes of 100 mM Na borate, 1 reaction mixture did not exceed 2.5%. The mM NaN3 (as a preservative), 1 mg/ml BSA AChE inhibitors tested included the nat- (to prevent denaturative losses), pH 8.0. ural carbamates eserine and neostigmine Packed nematodes were bulk homoge- (Sigma Chemicals, St. Louis, MO); the syn- nized in 1-ml fractions and used immedi- thetic carbamates aldicarb, carbofuran, and ately, or aliquots of this suspension con- oxamyl; the thiophosphoryl phosphates taining approximately 100 nematodes were parathion, phorate, and terbufos; the stored in 1.5-ml tubes at -80 C for mi- phosphorothioate ethoprop; the phos- croassay. phoramide fenamiphos; and the phos- Caenorhabditis elegans was cultured on phoryl phosphate paraoxon (Chem Ser- Escherichia coli 'OP50', on nematode growth vice, West Chester, PA). Nematode medium (NGM) (4). Nematode egg sus- homogenates (40 tzl) (ca. 1.5-2.0 mg/ml pensions prepared by a hypochlorite meth- protein for C. elegans; ca. 0.5-1.0 mg/ml od (11) were added to the plate and incu- for all plant-parasitic nematode species) or bated at 20 C for 4-5 days. Nematodes freeze-thawed nematodes (ca. 100 nema- were harvested by rinsing the plates with todes per tube) were preincubated for 10 borate buffer and were separated from de- minutes with 2 td inhibitor before addition bris and bacteria by flotation on 20-40% of substrate. Final inhibitor concentrations sucrose. Caenorhabditis elegans suspensions in the reaction mixture ranged from 10 -~ Nematode Acetylcholinesterases: Opperman, Chang 483 M to 10 -8 M. Bioactivation of OP inhibi- havior was observed with a 40 x dissecting tors was performed with meta-chlorper- microscope daily for 4-7 days. Controls benzoic acid (mCPBA) (10,34). Inhibitor both with and without solvent were in- and mCPBA were preincubated with the cluded in each assay to determine baseline nematode preparation for 60 minutes in 1 responses and to insure lack of solvent ef- x 10 -4 M concentrations before addition fects on nematode behavior. Results are a of the substrate, 22.9 ~tM [SH]acetylcholine summation of observations from two to ten (TRA-244, Amersham, Arlington Heights, separate repetitions of the assays, with six IL). After a 20-60-minute incubation pe- replications per treatment for C. elegans riod for C. elegans, or 60-360 minutes for and four replications per treatment with the plant-parasitic nematode species, the M. incognita. reaction was terminated by addition of 20 ~tl stop buffer (1 M trichloroacetic acid, 0.5 RESULTS M NaOH, 2 M NaC1) and then 1.3 ml of Incubation of nematode homogenates a toluene-n-butanol scintillation cocktail with [3H]acetylcholine showed a linear in- (0.5% PPO, 0.03% POPOP in toluene). crease in substrate hydrolysis between 10 Substitution of n-butanol for isoamyl al- and 60 minutes. Fractional conversion rates
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