Biology Reference
In-Depth Information
Other Transmitters in Ascaris
In female A. suum, serotonin (5-HT) is present in one pair of neurons
(NSM neurons) in the pharynx. 62,63 These neurons are very atypical of
nematode neurons in that they are highly branched. They extend multiple
varicose endings to the outside surface of the pharynx; by electron
microscopy these varicosities are packed with dense-core vesicles, and
have no postsynaptic partners. Morphologically they seem to be neuro-
secretory neurons, releasing serotonin into the PCF. They have an espe-
cially high density of endings immediately under the nerve ring. 63 When
5-HT is injected into intact worms, it produces a rapid paralysis. An
unresolved question is whether there is tonic release of 5-HT, and there-
fore whether in the circulating PCF there is a basal level of 5-HT that
contributes to muscle tone, or other aspects of behavior.
Glutamate is another small molecular mass transmitter that is impor-
tant in A. suum; 64,65 DE2 motor neurons receive glutamatergic excitatory
input from small interneurons that conduct calcium-mediated action
potentials in the VC. 66 The pharmacology of this system explains the
potency of kainic acid, domoic acid, and quisqualate as anthelmintics that
have long been used in Asia. 67 These compounds are also well-known
tools in the study of glutamatergic mechanisms in vertebrates.
Neuropeptides and their Receptors
Nematode neuropeptides have been divided into three groupings, two
of which form distinct peptide families (the FMRFamide-like peptides or
FLPs and the insulin-like peptides or INSs) and one which includes at
least 11 distinct families (the neuropeptide-like proteins or NLPs). The
theme of structural diversity that pervades the components of nematode
classical neurotransmitter signaling is mirrored within nematode neuro-
peptide signaling systems with each family encompassing large numbers
of distinct peptides (~70 FLPs, ~40 INSs, and ~70 NLPs). Assuming that
all of these ligands are involved in intercellular communication from
neurons, there is much scope for a complex signaling milieu. In contrast
to the ligand diversity inherent to neuropeptide signaling in nematodes,
the numbers of known neuropeptide receptors are more limited with
initial trawls recovering 54 candidates from the C. elegans genome, 68
although the impact of splice variation (whereby a single gene encodes
multiple proteins through alternative splicing) could provide for addi-
tional complexity. 69
Ascaris Neuropeptides
Discovery and Cellular Localization
The variety of neuropeptides in A. suum is impressive. The first
nematode peptides to be characterized were the AF peptides (Ascaris
FMRFamide-like peptides or FLPs),
e
isolated chemically from crude
Search WWH ::




Custom Search