Insect Control: Biological and Synthetic Agents - Index of

activated (Table 6). This represents the sensitivity of
the most sensitive cells. This value appears relatively
large, suggesting that cells have homeostatic mechanisms
to compensate for a relatively large amount of
inward current. In fact, DUM neurons first altered
their firing pattern when the spinosyn A concentration
reached 200 nM (Figure 9), at which 80% of
nAChN receptors are activated. This may be related
to the fact that DUM neurons fire spontaneously
and have a relatively low input resistance.
6.4.3.3. Correlation of spinosyn biological activity
with potency on nAChN receptors It has already
been shown (Table 6) that symptoms appear when
the internal aqueous concentration of spinosyn A
reaches the range of 20 nM, which is also near the
EC50 for activation of nAChN receptors, and is
also in the range where significant nervous system
stimulation is measurable. These results indicate
that activation of nAChN receptors can account
for the insecticidal activity of spinosyn A.
Potency in activating nAChN receptors was determined
for 15 spinosyns and spinosoids, and the
data are shown in Table 7 and plotted in Figure 13.
There is a clear relation between activity on cockroach
nicotinic receptors and insecticidal activity
against first instar H. virescens larvae. These results
provide further support for the importance of
nAChR activation in spinosyn poisoning.
Table 7 Spinosyns and spinosoids tested for nicotinic
activity. Each cell was calibrated with 100 nM spinosyn A and
the potency of the test compound was determined by increasing
the concentration gradually to bracket the magnitude of the
response to 100 nM spinosyn A. The potency of the test
compound relative to spinosyn A was determined by dividing
100 nM by the interpolated equi-effective concentration of the
test compound. At least two determinations were made for each
compound. The data are plotted in Figure 13
Number Spinosyn or spinosoid
Relative
nicotinic
potency
4 00 -N-Me Quat 0.071 0.039
13,14-Epoxide 0.112 0.18
74 5,6-b-Epoxide 0.225 0.32
8 Spinosyn H 0.26 0.088
73 5,6-a-Epoxide 0.45 0.089
72 5,6-H2 0.79 0.50
1 Spinosyn A 1.0 1.0
5,6-H2, 4 00 -N-deMe 1.49 0.18
4 Spinosyn D 1.73 0.33
2 Spinosyn B 2.0 0.91
23 Spinosyn K 2.3 0.22
3 0 -O-Et, 4 00 -N-deMe 3.3 0.83
77 5,6-H2, 3 0 -O-ethyl 5.9 3.8
62 2 0 ,3 0 ,4 0 -tri-O-ethyl A 8.2 12.5
46 3 0 -O-ethyl A 15 8.3
6: The Spinosyns: Chemistry, Biochemistry, Mode of Action, and Resistance 227
Relative Heliothis
activity
Figure 13 Relationship between relative insecticidal activity
against first instar Heliothis virescens larvae in a drench assay
and relative potency against nAChN receptors, for 15 spinosyns
and spinosoids that were tested and had measurable activity in
both assays. The regression line had a slope of 0.91, and
r 2 ¼ 0.72. The data and structural information for the spinosyns
included in this figure are shown in Table 7. (Adapted from
Salgado, V.L., Watson, G.B., Sheets, S.S., 1997. Studies on the
mode of action of spinosad, the active ingredient in Tracer Õ
insect control. Proc. 1996 Beltwide Cotton Production Conf.,
pp. 1082–1086.)
6.4.4. Effects of Spinosyns on GABA
Receptors in Small Diameter Neurons
of Cockroach
Watson (2001), using the whole-cell patch clamp
method, found that in cockroach neurons with a
diameter less than 20 mm, the GABA-activated chloride
current was completely and irreversibly antagonized
by as little as 5 nM spinosyn A (Figure 14),
whereas GABA-activated chloride currents in cells
larger than 25 mm were not affected. Over the same
concentration range, a picrotoxinin-sensitive chloride
current was activated by spinosyn A. The
chloride current activated by spinosyn A is small in
relation to the GABA response, an average of
124 pA in cells with GABA-activated chloride currents
generally exceeding 2 nA, but because of the
occurrence of GABA receptor antagonism and chloride
current activation in the same concentration
range, both of these effects might be due to a low
efficacy partial agonist action of spinosyn A on
a subtype of GABA receptor that appears to be
found specifically in small neurons. This would presumably
be an allosteric effect, due to binding of
spinosyns at a modulatory site analogous to that
for spinosyns on nicotinic receptors or avermectins