A reappraisal of the relative sensitivity of nematode pharyngeal and somatic musculature to macrocyclic lactone drugs

Abstract

Macrocyclic lactone (ML) drugs inhibit pharyngeal pumping, motility and egg laying in parasitic nematodes. Previous work has indicated that in vitro effects on worm feeding occurred at lower ivermectin concentrations than effects on worm motility, suggesting that the pharynx musculature was a more important target site for the ML drugs than somatic musculature. We have reassessed this issue of relative sensitivity by examining the response of drug-susceptible and -resistant adult Haemonchus contortus worms to abamectin in vitro using both feeding and motility assays. The motility assay involved observation of changes in the form and degree of movement of individual worms in response to the drug. A comparison of the data from the two assays indicated that worm motility was affected at drug concentrations below those required to inhibit feeding. Analysis of the motility data using different levels of sensitivity (varying in the degree to which they accounted for subtle vs. more profound changes in worm motility) provided an explanation as to why earlier reports had observed feeding to be the more sensitive target. Motility IC50 values shifted from being less than feeding IC50s to being greater than the feeding IC50s as the motility assay analysis method became less sensitive. The present study indicates that when sensitive worm motility assessment methods are utilised, worm motility is affected at lower abamectin concentrations than worm feeding, and hence highlights somatic musculature as a more important target site for this ML drug, and most likely for ML drugs in general.

Thymus vulgaris L. essential oil and its main component thymol: Anthelmintic effects against Haemonchus contortus from sheep

“…Ivermectin acts primarily at invertebrate glutamate receptors associated with chloride (GluCl), enhancing chloride ion influx, which results in cell hyperpolarization. This action causes the inhibition of parasite development and of larval and adult motility, in addition to paralysis of the pharyngeal muscles, which affects parasite feeding (Forrester et al, 2003;Wolstenholme, 2011;Kotze et al, 2012b;Wolstenholme, 2012). Hibbs and Gouaux (2011) proposed a model for ivermectin binding at the GLC-1 subunit of the GluCl receptor using the freeliving nematode Caenorhabditis elegans.…”

Utilization of computer processed high definition video imaging for measuring motility of microscopic nematode stages on a quantitative scale: “The Worminator”

“…To our knowledge there are no similar published data to compare these results to, however, these data are consistent with electrophysiological responses of muscle from B. malayi adults, which shows that paralysis is the primary mode of action of nicotinic agonist anthelmintics ( Robertson et al, 2013 ). It is also widely accepted that a primary mechanism of action for the macrocyclic lactone anthelmintics is paralysis of somatic musculature, due to irreversible binding at glutamate gated chloride channels that produces a long-lasting hyperpolarization of the neuron or muscle cell, therefore blocking further function ( Wolstenholme and Rogers, 2005; Kotze et al, 2012 ). Consistent with this belief, in vitro measurements of muscle contraction and motility-based assays such as the LMIA can be used to detect resistance to the macrocyclic lactones in strongylid species ( Demeler et al, 2010a,b, 2014 ).…”

How do the macrocyclic lactones kill filarial nematode larvae?

“…At those higher concentrations other nematode ligand-gated chloride channels are also affected by the drugs, including those gated by GABA and dopamine (Holden-Dye and Walker, 1990; Accardi et al 2012; Beech et al 2010a; Rao et al 2009), and it is possible that these channels affect performance in the migration assay in addition to, or instead of, the GluCls. It is worth emphasizing that for gastro-intestinal nematodes it is the adult worms rather than the larvae that are targeted by the drug and there is considerable evidence that inhibition of feeding, via paralysis of the pharyngeal muscle, may be the most important mechanism of action at this stage (Geary et al 1993; Sherriff et al 2005; Kotze et al 2012). …”