trans

NEUROTRANSMITTERS IN NEMATODES 371
receptors, phosphatases should decrease activation
and depress responses to nicotinic
anthelmintics.
The presence of nAChR subunit genes
in nematodes predict heterogeneity
of receptors
Fleming recognized 11 genes associated with
levamisole resistance in C. elegans. Three of
these genes encode nAChR subunits in the
nematode C. elegans, and are associated with
strong levamisole resistance (Figure 15.11).
These genes are unc-38, unc-29 (both on chromosome
I) and lev-1 (on chromosome IV).
Unc-38 encodes an subunit while unc-29
and lev-1 encode subunits. It is also possible
that there are additional genes (e.g. acr-2 and
acr-3 on the X chromosome) encoding nAChR
subunits associated with weaker levamisole
resistance.
Other genes encoding subunits for the
nAChR in C. elegans have been identified: the
non- subunits, acr-2 and acr-3 (on chromosome
X); and the -subunits deg-3 and Ce21
(on chromosome V). The varying combination
of different subunits may produce significant
heterogeneity of the ion-channel receptors
activated by acetylcholine.
In Oesophagostomum dentatum the muscle
nAChRs have conductances and mean open
times that show variation between patches,
and sometimes more than one subtype of
nAChR is observed in one patch (Figure 15.12).
We interpreted these data as indicating four
different subtypes of levamisole receptor: G25,
G35, G40 and G45 (Figure 15.13). nAChR channel
currents from Ascaris suum with different
conductances ranged between 19 and 50 pS
with distinguishable peaks at 24 pS and 42 pS,
but clearer separation of subtypes was more
difficult, perhaps because of variations in the
source of the Ascaris, which were derived from
field infections. The functions of the different
FIGURE 15.12 (A) Single-channel current recording
from levamisole-activated channels from Oesophagostomum
dentatum. Single-channel currents activated
by 10 M levamisole in a cell-attached patch at 75 mV.
Note the presence of two types of channel: the larger
openings (L) and the smaller openings (S). (B) The
open-channel current histogram showing clear separation
of the two channel openings (L & S).
subtypes of nAChR remain to be evaluated,
but because the pharmacology of each subtype
is predicted to be slightly different we
might predict changes in the proportion of
receptor subtypes associated with the development
of resistance to nicotinic agonists.
Recently we have been able to compare the
levamisole subtypes in levamisole-sensitive and
levamisole-resistant isolates of Oesophagostomum
dentatum. The G35 subtype was lost with
the development of resistance, and resistance
was associated with a reduction in the mean
proportion of time the nAChR channel spent
in the open state. Quantitatively the resistance
is explained by the combination of the
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS