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366 NEUROTRANSMITTERS
the surrounding medium. A tenfold increase
in the extracellular potassium concentration
causes a 1.5 mV decrease in the Ascaris membrane
potential. This small effect is in marked
contrast to the equivalent situation in vertebrate
muscle, where the membrane potential
is dependent on potassium ions. Del Castillo
and his co-workers considered that organic
anions contributed to the membrane potential
in Ascaris muscle, but ascribed the maintenance
of the membrane potential to the intracellular
chloride battery. Brading and Caldwell
hypothesized that an electrogenic pump, operating
across the muscle cell membrane, was
responsible for maintaining the membrane
potential. Our studies have led us to propose
that there is a proton pump that allows the
movement of carboxylic acids across the Ascaris
muscle membrane and cuticle, in a manner
closely linked to the membrane potential.
Neurotransmitters
Despite the topographical simplicity of the
nematode nervous system, a large number of
A. suum neurotransmitters have been identified.
These include: acetylcholine; -aminobutyric
acid (GABA); glutamate; serotonin (5HT); and
FMRFamide-related peptides (FaRPs).
Acetylcholine
Initially, experiments with bath-applied acetylcholine
showed that acetylcholine produces
contraction of muscle strips of Ascaris suum,
demonstrating the presence of acetylcholine
receptors on muscle. Electrophysiological
observations followed with bath-applied acetylcholine
that was shown to produce depolarizations
and changes in spike frequency and
amplitude in muscle. The receptors responsible
for these effects are located synaptically at
the syncytial region, and extrasynaptically on
the bag region of the muscle. The ionic basis of
electrical responses to acetylcholine is due to
the ability of acetylcholine to increase the nonselective
cation conductance of the membrane:
that is, acetylcholine opens ion channels permeable
to both Na and K .
Single-channel currents activated by
acetylcholine
We have recorded single-channel currents
activated by acetylcholine from cell-attached
and isolated inside-out patches. The channels
activated by acetylcholine have at least two
conductances: the larger was 40–50 pS and the
smaller 25–35 pS. The average open time was
similar to that of the nicotinic channel at the
frog neuromuscular junction and had a mean
value of 1.3 ms. High concentrations of acetylcholine
(25–100 m) produced a reduction in
open probability and caused single-channel
currents to occur in clusters; this behavior has
also been reported for the frog neuromuscular
junction and described as desensitization.
Biochemistry of acetylcholine
Bueding was the first to demonstrate that
cholinesterase, the enzyme inactivating acetylcholine,
was present in A. suum. In the head
region, activity is associated with the contractile
region of the muscle in the extracellular
matrix; enzyme activity was also observed in
muscle arms near their endings on the nerve
cords but not on the bag region or the nervous
tissue. Johnson and Stretton showed that there
are two types of cholinesterase, a 13 S and a 5 S
form, which have different distributions in
the body of Ascaris. These two forms of acetylcholinesterase
are also found in C. elegans,
where they are products of separate genes.
Although cholinesterase is responsible for the
breakdown of acetylcholine released from
excitatory motoneurons, it is also secreted
into the external environment by A. suum and
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS