Insect Control: Biological and Synthetic Agents - Index of

Figure 13 Diagrammatic representation of the location of the
binding sites associated with the S6 segments of the Na þ channel.
The extracellular mouth of the channel is at the top of the
figure. Two sets of the S4, S5, and S6 transmembrane helices
are shown. Bidirectional arrows show the movements of the S4
and S6 segments associated with activation gating. The thick
solid curves show the outline of the channel protein in the
activated or open conformation, and the thick dotted line
shows the activation gate at the cytoplasmic end of the S6 segments
in the closed position. There is one tetrodotoxin (TTX) and
one Na þ channel blocker insecticide (SCBI) binding site per
channel, but the number of batrachotoxin (BTX) and pyrethroid
sites is not certain, although only one of each is shown. All
three drug sites are in the middle of the S6 segments, and the
pyrethroid site also includes residues in segment 5.
Table 2 Action of DCJW and indoxacarb on the abdominal
stretch receptor organ of Spodoptera frugiperda
Average percent block
(five preparations, in exposure)
Concentration a (M) DCJW Indoxacarb
3e-9 0 0
1e-8 40 0
3e-8 100 25
1e-7 100 60
3e-7 100 100
a In a saline containing, in mM, 28 NaCl, 16 KCl, 9 CaCl2, 1.5
NaH 2PO 4, 1.5 mgCl 2, and 175 sucrose, pH 6.5, the stretch
receptor fired continuously for several hours between 25 and
75 Hz.
The actions of indoxacarb and DCJW were also
compared quantitatively on the abdominal stretch
receptor organs of S. frugiperda (Table 2). In this
preparation, the insecticides in solution are perfused
rapidly over the body wall containing the
stretch receptor, a small, thin organ directly exposed
to the saline. Significant metabolism in this preparation
is unlikely, but cannot be excluded. The organ
fires continually at a rate of between 25 and 75 Hz
for many hours, and is very sensitive to block by
SCBIs. DCJW blocked activity consistently within
20–40 min at 3 10 8 M, but in five tests at 10 8 M
2: Indoxacarb and the Sodium Channel Blocker Insecticides 49
the average percent block within 1 h was 40%.
Indoxacarb was also intrinsically active, but 3- to
10-fold weaker than DCJW, blocking consistently at
3 10 7 M, but only 60% within 1 h at 1 10 7 M
and 25% at 3 10 8 M.
In conclusion, it appears that indoxacarb is weaker
(about three to ten times) than DCJW on Na þ channels,
and that the block is readily reversible. Furthermore,
the much slower effect of indoxacarb on the
compound action potentials in M. sexta ganglia in
comparison with DCJW indicates that indoxacarb
enters the ganglion much more slowly than does
DCJW. Based on the appearance of insect symptoms
concurrent with the appearance of high levels of metabolically
formed DCJW in larval Lepidoptera (Wing
et al., 1998, 2000) and the greater potency and irreversibility
of DCJW Na þ channel block, it appears
that the toxicologically significant compound after
indoxacarb or DPX-JW062 administration is DCMP
or DCJW, respectively.
2.4.9. Effects of SCBIs on Alternative Target Sites
Other possible target sites for SCBIs have been
considered. The voltage-gated Ca 2þ channel a subunits
belong to the same protein superfamily as the
voltage-gated Na þ channels, and have a similar
structure, composed of 24 transmembrane segments
arranged into four domains of six segments each.
Not unexpectedly, Ca 2þ channels were also found
to be blocked by pyrazolines, although at higher
concentrations than Na þ channels (Zhang and
Nicholson, 1993; Zhang et al., 1996). So far, there
is no evidence that Ca 2þ channel effects are significant
in insect poisoning. In the initial work
on the pyrazolines, it was observed in paralyzed
cockroaches and M. sexta larvae that the heart
continued to beat normally, and conduction across
the cholinergic cercal–giant fiber synapses as well
as g-aminobutyric acid (GABA) and glutamate
synapses on muscle, all Ca 2þ channel-dependent processes,
functioned normally (Salgado, 1990). Furthermore,
DCJW had no effect on low-voltage activated
and high-voltage activated calcium currents in isolated
cockroach neurons (Lapied et al., 2001).
A strong hyperpolarization of M. sexta muscle,
on the order of 20 mV, was observed after poisoning
by pyrazolines (Salgado, 1990). The effect was not
reversible with prolonged washing and could not
be reproduced by treatment with pyrazoline in solution,
and therefore appears to be secondary and not
related to the hyperpolarization of DUM neurons
observed by Lapied et al. (2001). Muscles of Periplaneta
or Musca were not hyperpolarized during
pyrazoline poisoning (Salgado, 1990).