Acetylcholine receptors (AChRs) are pentameric
ligand-gated ion channels involved in the neurotransmission of both vertebrates and invertebrates. A number of
anthelmintic compounds like
levamisole and
pyrantel target the AChRs of nematodes producing
spastic paralysis of the worms. The muscle AChRs of nematode parasites fall into three pharmacological classes that are preferentially activated by the
cholinergic agonists levamisole (L-type),
nicotine (N-type) and bephenium (B-type), respectively. Despite a number of studies of the B-type AChR in parasitic species, this receptor remains to be characterized at the molecular level. Recently, we have reconstituted and functionally characterized two distinct L-AChR subtypes of the gastro-intestinal parasitic nematode Haemonchus contortus in the Xenopus laevis oocyte expression system by providing the cRNAs encoding the receptor subunits and three ancillary
proteins (Boulin et al. in Br J Pharmacol 164(5):1421-1432, 2011). In the present study, the effect of the bephenium
drug on Hco-L-AChR1 and Hco-L-AChR2 subtypes was examined using the two-
microelectrode voltage-clamp technique. We demonstrate that bephenium selectively activates the Hco-L-AChR1 subtype made of Hco-UNC-29.1, Hco-UNC-38, Hco-UNC-63, Hco-ACR-8 subunits that is more sensitive to
levamisole than
acetylcholine. Removing the Hco-ACR-8 subunit produced the Hco-L-AChR2 subtype that is more sensitive to
pyrantel than
acetylcholine and partially activated by
levamisole, but which was bephenium-insensitive indicating that the bephenium-binding site involves Hco-ACR-8. Attempts were made to modify the subunit stoichiometry of the Hco-L-AChR1 subtype by injecting five fold more
cRNA of individual subunits. Increased Hco-unc-29.1
cRNA produced no functional receptor. Increasing Hco-unc-63, Hco-unc-38 or Hco-acr-8 cRNAs did not affect the pharmacological characteristics of Hco-L-AChR1 but reduced the currents elicited by
acetylcholine and the other agonists. Here, we provide the first description of the molecular composition and functional characteristics of any invertebrate bephenium-sensitive receptor.