Avermectin and avermectin derivatives are antagonists at the 4-aminobutyric acid (GABA) receptor on the somatic muscle cells of Ascaris; is this the site of anthelmintic action?

Molecular aspects of drug resistance in parasitic helminths

Parasitic helminths (worms) cause serious infectious diseases in humans and animals. As control of these infections relies mostly on chemotherapeutics, the anthelmintics, resistance has developed against most of these drugs in several parasite species. These resistant parasites are being used to elucidate the molecular mechanisms of drug action.

Physiological and pharmacological studies on annelid and nematode body wall muscle

1. This review covers the pharmacology and physiology of the body wall muscle systems of nematodes and annelids. 2. Both acetylcholine and gamma-aminobutyric acid (GABA) play important roles in the control of body wall muscle in both phyla. In annelids and nematodes, acetylcholine is the excitatory neuromuscular transmitter while GABA is the inhibitory neuromuscular transmitter. In addition, 5-hydroxytryptamine (5-HT) has a modulatory role at annelid body wall muscle but little if any effect on nematode body wall muscle. 3. The acetylcholine receptor of the body wall muscle can be classified as nicotinic-like in both phyla though the annelid receptor has not been analysed in detail. In nematodes, vertebrate ganglionic nicotinic agonists were the most effective of those so far examined while mecamylamine and benzoquinonium were the most effective antagonists. Both neuronal bungarotoxin and neosurugatoxin were potent antagonists of acetylcholine excitation at the nematode receptor. 4. The GABA receptor of the body wall muscle exhibits similarities with the vertebrate GABA-A receptor in both phyla. Picrotoxin is a very weak or inactive antagonist at leech and nematode GABA receptors, while bicuculline methiodide blocks leech GABA receptors but is inactive on nematode GABA receptors. Picrotoxin does block GABA responses of earthworm body wall muscle. All these GABA responses are chloride mediated. 5. Neuroactive peptides of the RFamide family occur in both phyla and FMRFamide has been identified in leeches. RFamides probably have an important role in heart regulation in leeches and in modulation of their body wall muscles. RFamides also modulate nematode body wall muscle activity with KNEFIRFamide raising muscle tone while SDPNFLRFamide relaxes the muscle. It is likely that this family and other neuroactive peptides play an important role in the physiology of body wall muscle throughout both phyla.

An ion-sensitive microelectrode study on the effect of a high concentration of ivermectin on chloride balance in the somatic muscle bag cells of Ascaris suum

Ivermectin has been shown to increase chloride conductances of invertebrate cells. On the muscle cells of the parasitic nematode Ascaris, ivermectin acts as both a GABA receptor antagonist and a chloride channel opener. In this study, ion-sensitive microelectrodes were used to investigate the effect of ivermectin on intracellular Cl- concentration of the somatic muscle bag cells of Ascaris suum. Incubation of muscle cells with ivermectin (10 microM in 1% dimethyl sulphoxide vehicle for 60 min) increased intracellular Cl- by 2.9 mM or 15% compared to controls (P < 0.01, n = 6).

Efficacy of ivermectin in the treatment of concomitant Mansonella perstans infections in onchocerciasis patients

As part of an ivermectin dose-ranging study of onchocerciasis patients in Togo, 55 onchocerciasis patients with concomitant mansonelliasis received single oral doses either of ivermectin (100 to 200 micrograms/kg body weight) or placebo. As expected, Onchocerca volvulus microfilariae in the skin were greatly reduced in number soon after drug treatment, but microfilariae of Mansonella perstans reacted differently. Microfilarial densities of M. perstans were assessed with a filtration technique both before, and 4 times after, treatment. In untreated patients microfilarial densities were stable until the end of the study at 6 months. In patients receiving ivermectin, microfilarial densities dropped on average to less than 60% of the pre-treatment level and remained there until the final post-treatment examination. This partial reduction was probably not caused by a microfilaricidal effect of ivermectin, but rather by an altered distribution of microfilariae in the peripheral blood and in a suspected microfilarial reservoir.

Experimental chemotherapy of filariasis: comparative evaluation of the efficacy of filaricidal compounds in Mastomys coucha infected with Litomosoides carinii, Acanthocheilonema viteae, Brugia malayi and B. pahangi

Eleven types/classes of compound with antifilarial activity were comparatively evaluated in Mastomys coucha infected with Litomosoides carinii, Acanthocheilonema viteae, Brugia malayi or B. pahangi. The paper deals with the efficacy of (i) predominantly microfilaricidal compounds [diethylcarbamazine, levamisole, avermectins (ivermectin, milbemycin), nitrofurans (nitrofurantoin, hydroxymethylnitrofurantoin, nifurtimox, furazolidone, furapyrimidone), organophosphorals (metrifonate, haloxon), and aminophenyl-amidines], (ii) predominantly macrofilaricidal compounds [suramin, benzimidazoles (flubendazole, mebendazole, oxfendazole, ciclobendazole, albendazole, cambendazole, fenbendazole), and arsenicals (thiacetarsamide, Mel PH, R7/45)], and (iii) micro- and macrofilaricidal compounds [benzazole derivatives (CGP 20376 and other benzothiazoles) and nitrophenylamines (amoscanate, CGP 6140)]. Minimum effective doses against microfilariae and minimum curative doses against adult filariae as well as detailed data on dose-efficacy relationships are reported for the various drugs. The results obtained in M. coucha are compared with those published for other experimental in vivo filarial systems, thus attempting to describe a general status of in vivo antifilarial activity of the compounds.

Neuromuscular transmission in nematode parasites and antinematodal drug action

Some anthelmintic drugs interfere selectively with nematode neuromuscular transmission. These drugs include: the nicotinic agonists, e.g. levamisole, the gamma-amino butyric acid agonist piperazine, and the avermectins which open Cl- channels. The physiology and pharmacology of neuromuscular transmission in nematodes is reviewed and the actions of antinematodal drugs which interfere with the transmission described. The results of experiments on the large porcine-intestinal nematode parasite, Ascaris suum, form the basis of the account presented but experiments on other nematodes suggest that these observations may be generalized. Results of some experiments on the small free living nematode Caenorhabditis elegans are also included.