Biology Reference
In-Depth Information
further posteriorly than in a control worm.
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When extended to the effects
of peptides on the electrophysiology of motor neurons, the number of
categories of different effects is raised to about seven.
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However, taken
together, the total number of categories of different biological effects
observed is considerably less than the number of peptides, suggesting
either that there may be functional redundancy, or that the number of
biological tests is still too small. There are many potential targets that have
not yet been tested electrophysiologically, and to test all neurons with all
known peptides is scarcely feasible. It is also certain that the concentration
of the ligand and the affinity of receptor(s) are important functionally.
Among the many aspects of neuropeptide function that are currently
unknown are the time course of the release of peptides from the cells that
synthesize them. In some cases, e.g. the effects of AF1 (KNEFIRFamide)
on inhibitory motor neurons, there is close contact (synapse-like) between
the interneurons in the DC and VC that contain AF1 and the inhibitory
motor neurons that are short-circuited by the peptide.
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In other cases
there is apparently secretion into the PCF, as mentioned above for the
serotonin-containing neurons of the pharynx. It would be interesting to
assay the PCF for its levels of neuromodulators, including peptides and
biogenic amines, and to determine whether they change with behavioral
state, or developmental state.
The locomotory behavior of the L3 larva and the adult are very
different, with the L3 larva behaving like C. elegans: posteriorly propa-
gating waves drive the animal forwards, and anteriorly propagating
waves drive it backwards. This is the opposite in adult A. suum; in a tube
of dimensions like that of a pig small intestine, it is anteriorly propagat-
ing waves that lead to anterior locomotion. In addition, the locomotory
waveform of adult A. suum contains three waves, whereas that of
C. elegans has a fraction more than a single wave. The A. suum L3 has only
a single wave, although the number of neurons is similar to that of the
adult. One possibility is that there are changes in modulator levels that
alter the physiological properties of parts of the motor nervous system or
of muscle. Another enigma is that the anteriorly propagating waves in
adult A. suum are initiated near the vulva; the mechanism is unclear.
Neuropeptides in Other Parasites
The peptide extraction and biochemical characterization approach that
worked so well in A. suum was much less evident in other parasites, with
discovery confined to a total of three FLPs (KHEYLRFamide, KSAYMR-
Famide, and LQPNFLRFamide) that were structurally characterized from
the third larval stage of H. contortus with KHEYLRFamide being the most
abundant peptide detected.
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e
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A second method of parasite neuropeptide discovery relied on the
identification of predicted peptides from cloned and sequenced gene
