The Neurobiology of Ascaris and Other Parasitic Nematodes - Ascaris: The Neglected Parasite - page 128

oscillations ( Figure 6.2 A). Supporting this hypothesis, pairwise recordings
from DI and VI, and of DI and DE neurons, showed that they oscillated in
antiphase, as predicted by the hypothesis. These recordings were made in
pieces of worm 3

5 cm long, continuously perfused with Ascaris artificial
physiological saline solution at 37 C( Figure 6.2 C).
A more rigorous test was aimed at correlating the oscillatory potentials
with the propagating behavioral waveform. A semi-intact preparation
of a whole worm was made. 30 A small slit in the body wall allowed
microelectrode penetration of chosen motor neurons, while the rest of the
worm was free to move within a narrow trough ( Figure 6.2 D). The results
were compelling in that the individual oscillations in the motor neurons
do not correspond with separate body bends

e

the oscillations occur at
about 10 times the frequency of the propagating waveform, and it is bouts
of oscillations rather than individual oscillations that correlate with
behavior. 30 The conclusion is that the description of the properties of the
neurons and their synapses that were incorporated into the hypothesis is
incomplete. The same neurons in the fully dissected and perfused prep-
aration, and in the semi-intact preparation, have different activities. One
obvious source of these differences is the presence and activity of neu-
romodulators, which would most likely be washed away in the fully
dissected and perfused preparation. Many neuromodulators have potent
activity on Ascaris motorneurons, 39 so it is clear that their effects must be
incorporated into the next hypothetical model to be tested. Developing
models that accurately encompass the diverse effects of a changeable mix
of neuromodulators remains a major challenge. For example, it is already
known that single neurons can contain multiple peptides or combinations
of peptides with other modulators, and these are likely to be co-released
to act on distinct receptors on the target cell(s). Little is known about the
physiological consequences of such a mix of neuromodulators acting on
individual nerve cells. Furthermore, the release of modulators into the
circulating pseudocoelomic fluid (PCF), where they would act as neuro-
hormones, is strongly supported by the morphology of certain neurons.
Clearly, understanding which neuromodulators and/or neurohormones
are actually released onto the target neurons and establishing the impact
of these physiological interactions needs to be investigated in the future.

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NEU ROMODULATORS AND SIGNAL ING

Classical Transmitters and their Receptors
While much data on classical neurotransmitter occurrence in nematode
parasites are available, it has been reviewed previously and will not
be repeated here. 40 e 42 The largest and most diverse group of classical