Nervous systems of helminths are highly peptidergic. Species
in the phylum Nematoda (roundworms) possess at least 50 FMRFamide-related
peptides (FaRPs), with more yet to be identified. To date, few
non-FaRP neuropeptides have been identified in these organisms,
though evidence suggests that other families are present. FaRPergic
systems have important functions in nematode neuromuscular control.
In contrast, species in the phylum Platyhelminthes (flatworms)
apparently utilize fewer FaRPs than do nematodes; those species
examined possess one or two FaRPs. Other neuropeptides, such
as neuropeptide F (NPF), play key roles in flatworm physiology.
Although progress has been made in the characterization of FaRP
pharmacology in helminths, much remains to be learned. Most
studies on nematodes have been done with
Ascaris suum because
of its large size. However, thanks to the
Caenorhabditis elegans genome project, we know most about the FaRP complement of this
free-living animal. That essentially all
C. elegans FaRPs are
active on at least one
A. suum neuromuscular system argues for
conservation of ligand-receptor recognition features among the
Nematoda. Structure-activity studies on nematode FaRPs have
revealed that structure-activity relationship (SAR) "rules"
differ considerably among the FaRPs. Second messenger studies,
along with experiments on ionic dependence and anatomical requirements
for activity, reveal that FaRPs act through many different mechanisms.
Platyhelminth FaRPs are myoexcitatory, and no evidence exists
of multiple FaRP receptors in flatworms. Interestingly, there
are examples of cross-phylum activity, with some nematode FaRPs
being active on flatworm muscle. The extent to which other invertebrate
FaRPs show cross-phylum activity remains to be determined. How
FaRPergic nerves contribute to the control of behavior in helminths,
and are integrated with non-neuropeptidergic systems, also remains
to be elucidated.
