trans

362 NEUROTRANSMITTERS
TABLE 15.1
Anti-nematodal drugs and their mode of action
Chemical Class Mode of action
group
1 BZ benzimidazoles, Bind to the nematode structural protein,
probenzimidazoles -tubulin, adversely affecting nematode feeding
and movement
2 LM levamisole, morantel, Nicotinic receptor agonists, which activate
pyrantel
nematode acetylcholine receptors to open sodium
channels, causing muscular paralysis
3 AV avermectins, Agonists at the glutamate-gated chloride
milbemycin
channel in nematode nerve and muscle, causing
inhibition
organophosphates Inhibit acetylcholinesterase, leading to increased
stimulation of acetylcholine receptors and muscular
paralysis
piperazine
Agonist at GABA-gated chloride channels, resulting
in muscular paralysis
HO
HO
N
H
CH 2 CH 2 NH 2
Serotonin
Dopamine
CH 2 CH 2 NH 2
Phenylalanine-X-Arginine-Phenylalanine-NH 2
Original FMRFamide, FaRP
Now: Any Peptide with RFamide modif at the C terminal
including FLFQPQRFamide
FIGURE 15.4 The structure of the neurotransmitters
serotonin and dopamine, and general amino acid
sequence of the FaRP peptides. X is any amino acid.
Ascaris and a diagram of an individual muscle
cell are shown in Figure 15.6.
The muscle cell
Ascaris somatic muscle cells differ in structure
from their vertebrate counterparts. As can be
seen in Figure 15.6B, the muscle cells are composed
of three distinct regions: the spindle
or fiber; the muscle bag or belly; and the arm.
The structure of the muscle cells is dictated
by function, and was investigated initially by
Rosenbluth in 1965.
Stretton described the innervation of the
somatic muscle cells. They are arranged longitudinally
and can be divided into two groups.
The dorsal mass of somatic muscle is innervated
by the motor axons in the dorsal nerve
cord; the ventral mass receives innervation
from the motor component of the ventral nerve
cord. Due to this arrangement, waves of contraction
are propagated along the nematode’s
body in the dorsoventral plane. By this means,
the worm can move forwards or backwards.
The spindle
The spindle contains actin and myosin and
is the contractile part of the cell. Actin and
myosin are arranged in overlapping rows that
produce striations, as in vertebrate skeletal
muscle. However, in nematode body wall
muscle, the striations are not aligned perpendicularly
to the long axis of the muscle.
Instead, the muscle is obliquely striated,
which confers extensibility with high-speed
contraction.
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS