trans

NEUROTRANSMITTERS IN NEMATODES 369
mixture of which levamisole is the laevoisomer).
Tetramisole produced depolarization, an
increase in spike frequency and contraction.
He described the anti-nematodal drug action
and pharmacological properties of pyrantel
and some of its analogs in a variety of vertebrate
and helminth preparations.
Single-channel currents activated by
cholinergic anthelmintics
Figure 15.10 shows a kinetic diagram of the
way in which levamisole-activated channel
currents are thought to behave during the
binding of the drug and opening of the channel.
Also illustrated are some sample openings
of levamisole-activated channels from Ascaris
suum. The ion channels are cation-selective and
have similar kinetic properties to acetylcholineactivated
channels at low levamisole concentrations,
but show evidence of channel block at
higher concentrations. The receptor-operated
ion channels have mean open times in the
range 0.8–2.85 ms with conductances in the
range 20–45 pS.
Likely structure of nematode nicotinic
acetylcholine receptors (nAChR)
The detailed structural studies on vertebrate
nicotinic receptor channels have been carried
out on the nAChR derived from the Torpedo
FIGURE 15.10 Diagram of the operation and kinetics of agonist binding, A, to a ligand-gated ion channel, and the
opening and ion-channel blocking process. Two agonist molecules bind reversibly and in the sequences through
the states, 0, 1 and 2, with the defined rate constants shown. The doubly bound ion channel then may open (3, rate
constant ) briefly before closing (rate constant ), or the channel may become blocked with an agonist molecule
entering the open channel pore to enter the state 4.
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS