Biology Reference
In-Depth Information
commissures included a small area of surrounding hypodermis, so
pieces of hypodermis of equal area were assayed as controls, and low
levels of ChAT were found in hypodermis. The presence of an excit-
atory commissure increased the ChAT level dramatically, whereas there
was no increase contributed by an inhibitory commissure. An inter-
esting finding that came out of these experiments was that the con-
centration of ChAT in the hypodermis progressively increased the
further anteriorly it was sampled. Analysis of serial frozen sections
showed that the peak activity was at the tip of the head, anterior to the
nerve ring. Tentatively this was ascribed to the arcade cells of the head.
The role of ChAT in the head has not been further explored, although it
may be responsible for release of ACh into the PCF, and contribute,
along with ACh tonically released from excitatory motorneurons, to
basic muscle tone, possibly by interacting with non-synaptic ACh
receptors on muscle.
58
The issue of whether there are chemical components of the PCF that
affect the properties of neurons and/or muscle cells also arises when
considering the role of neuromodulators in the control of behavior, and
needs to be more fully addressed.
Since del Castillo and colleagues showed that muscle was hyper-
polarized by exogenous GABA,
5
this was believed to be the transmitter in
inhibitory motor neurons of A. suum. A GABA-specific antibody showed
that GABA is concentrated in the commissures, cell bodies, and nerve cord
processes of DI and VI motor neurons, in strong support of del Castillo's
hypothesis.
59
There are other neurons that are GABA-immunoreactive in
A. suum,
60
and comparison with C. elegans GABAergic neurons revealed
a robust difference in cellular expression in the two species. In both species
the dorsal and ventral inhibitory motor neurons (DI and VI in A. suum, and
DD and VD in C. elegans) are GABA-immunoreactive, as are the four RME
neurons of the nerve ring and the DVB neuron in the tail. In each species
there are two GABA-immunoreactive neurons in the ventral ganglion.
However, in C. elegans these neurons are the unpaired RIS and AVL
neurons,
61
whereas in A. suum the GABA-immunoreactive neurons are the
paired AIY or AIM neurons (these neuronal types have not been distin-
guished in A. suum).
There is a further feature of the GABAergic motorneurons that is not
yet understood functionally, namely the presence of branches from the
DI and VI commissures that spread along the sublateral cords as they
cross them.
59
These branches receive synapses from excitatory axons in
the sublateral cords. Remarkably, there are different connections with DI
and VI motor neurons, such that VI neurons receive synapses from dorsal
sublateral axons, and DI neurons from ventral sublaterals. It is not at all
clear how important these connections are in affecting the control of
movement by the DI and VI neurons.
