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The physiology and pharmacology of neuromuscular transmission in the nematode parasite, Ascaris suum

Published online by Cambridge University Press:  06 April 2009

R. J. Martin ,
A. J. Pennington ,
A. H. Duittoz ,
S. Robertson  and
J. R. Kusel
R. J. Martin
Affiliation:
Department of Pre-clinical Veterinary Sciences, R.(D).S.V.S. University of Edinburgh, Edinburgh EH9 1QH
A. J. Pennington
Affiliation:
Department of Pre-clinical Veterinary Sciences, R.(D).S.V.S. University of Edinburgh, Edinburgh EH9 1QH
A. H. Duittoz
Affiliation:
Department of Pre-clinical Veterinary Sciences, R.(D).S.V.S. University of Edinburgh, Edinburgh EH9 1QH
S. Robertson
Affiliation:
Department of Pre-clinical Veterinary Sciences, R.(D).S.V.S. University of Edinburgh, Edinburgh EH9 1QH
J. R. Kusel
Affiliation:
Department of Biochemistry, University of Glasgow, Glasgow G12 8QQ
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Extract

The organization of Ascaris motoneurones and nervous system is summarized. There is an anterior nerve ring and associated ganglia, main dorsal and ventral nerve cords which run longitudinally, and a small set of posterior ganglia. Cell bodies of motoneurones are found in the ventral nerve cord and occur in 5 repeating ‘segments’; each contains 11 motoneurones. Seven morphological types of excitatory or inhibitory motoneurone are recognized.

Each Ascaris somatic muscle cell is composed of the contractile spindle; the bag region, containing the nucleus; the arm; and the syncytial region, the location of neuromuscular junctions. The resting membrane potential of muscle is approximately — 30 mV and shows regular depolarizing, Ca-dependent ‘spike potentials’ superimposed on smaller Na+- and Ca2+-dependent ‘slow waves’ and even slower ‘modulation waves’. The membrane shows high Cl- permeability. Adjacent cells are electrically coupled so that electrical activity in the cells is synchronized. Acetylcholine (ACh) and γ-aminobutyric acid (GABA) affect the electrical activity. Bath-applied ACh increases membrane cation conductance, depolarizes the cells, alters the frequency and amplitude of spike potentials and produces contraction. Bath-applied GABA increases Cl- conductance, decreases spike activity and causes hyperpolarization and muscle relaxation.

The extra-synaptic ACh receptors on the bag region of Ascaris muscle can be regarded as a separate subtype of nicotinic receptor. ACh and anthelmintic agonists (pyrantel, morantel, levamisole) produce a dose-dependent increase in cation conductance and membrane depolarization which is blocked by tubocurarine, mecamylamine but not by hexamethonium. The potency, of GABA agonists, with the exception of sulphonic acid derivatives, correlates with the vertebrate GABAa receptor. The potency of antagonists does not. Thus, bicuculline, securinine, pitrazepine, SR95531 and RU5135 are potent vertebrate GABAa antagonists but have little effect on GABA receptors. The potency order of the arylaminopyridazine GABA antagonists: SR95103, SR95132, SR42666, SR95133, SR95531, SR42627 and SR42640 at the Ascaris GABA receptors contrasts with that at vertebrate GABAa receptors. It has been suggested that the receptor is referred to as a GABAn receptor.

Patch-clamp studies show that ACh activates a non-selective cation channel which has a main conductance of 40–50pS and apparent mean open time of 1·3 ms; a smaller channel of 20–30 pS with a similar open-time is also activated. Pyrantel and levamisole also produce openings with similar conductances and open-times. GABA activates a Cl- channel with a main state conductance of 22 pS and an apparent mean open duration of 32 ms; conductance states of 10 and 15 pS are also seen. Piperazine similarly activates this channel but the mean open-time is shorter (14 ms). Ivermectin in high doses, is an antagonist which reduces the GABA channel conductance and Popen; it does, however, open ‘small’ Cl- channels when applied to the outside surface of membrane. These channels have a conductance of 9–15 pS and very long open times (> 100 mS). 5-HT does not have a direct effect on membrane potential or conductance but acts on cAMP levels and glycogen metabolism. Dopamine, octopamine and AF1 may act as neurotransmitters or neuromodulators.

Keywords

Ascaris suumneuromuscular transmissionelectrophysiologypharmacologyanthelmintics.
Type
Research Article
Information
Parasitology , Volume 102 , supplement S1: Symposia of the British Society for Parasitology Volume 28:Parasite neurobiology , January 1991 , pp. S41 - S58
Copyright
Copyright © Cambridge University Press 1991

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