Insect Control: Biological and Synthetic Agents - Index of

known to affect blocker affinity, without alterations
in the voltage dependence or kinetics of gating that
would otherwise account for an increase in the IC50
for block. The S6 segments can be arranged so that
all such residues, shown in red, face into the pore.
A total of 10 residues affecting BTX action have also
been identified scattered among the middle of all
four S6 segments (Wang and Wang, 2003), but it is
not yet clear if these are all associated with the
binding site.
Residues in the S6 segments of domains 1, 2, and 3
that are known to affect pyrethroid sensitivity are
shown in blue in Figures 11 and 12. These are on
the sides thought to face away from the pore. Mutations
conferring pyrethroid resistance have also
been identified in the S5 segments, indicating that
the pyrethroid receptors may be located at the interfaces
between S5 and S6 segments. The number of
pyrethroid receptors per channel is not certain, but
the locations of the resistance mutations in the
S6 segments suggests that there could be as many
as four receptors. Interestingly, quantitative studies
with tetramethrin isomers on squid axon Na þ
channels indicated that there may be at least three
pyrethroid receptors in each channel (Lund and
Narahashi, 1982).
2: Indoxacarb and the Sodium Channel Blocker Insecticides 47
Figure 11 Sequence alignment of the pore-forming S6 transmembrane segments from various arthropod pest species, to show
the location of residues known to be important for binding of local anesthetics (red), batrachotoxin (underscored), and pyrethroids
(blue). Residues conforming with the consensus sequence are shown with a gray background.
The extremely high conservation of the S6 segments
among the arthropods compared in Figure 11
is striking. The residues associated with the binding
sites are all absolutely conserved. However, the
P loops in the extracellular S5–S6 connecting segments
dip deep into the external mouth of the barrel
formed by the S6 segments to form the selectivity
filter and the outer boundary of the inner chamber
containing site 10 (Cronin et al., 2003). The
contributions of the P loops to the binding of blockers
in site 10 have not yet been explored. Figure 13
shows a diagram summarizing current knowledge of
the locations of the S4, S5, and S6 segments and the
three receptors associated with the S6 segments.
TTX is also shown, binding to site 1 at the external
mouth of the channel.
2.4.7. Biochemical Measurements of
the Effects of SCBIs
While electrophysiology and in particular voltageclamp
measurements are essential for studying the
detailed interactions of drugs with ion channels,
certain receptor binding techniques are also useful,
especially when quantitative potency data on many
compounds is needed. Displacement of [ 3 H]BTX-B