Insect Control: Biological and Synthetic Agents - Index of

deflections resulting from stresses due to the body’s
own movements or to external stimuli, and internally
in the form of chordotonal organs or muscle
stretch receptor organs to report on joint angles.
Mechanoreceptors in general have both phasically
and tonically responding sensory cells or units. The
phasic units respond with a burst of activity at the
start or end of a stimulus, but rapidly adapt in
the presence of constant stimuli, because their function
is to sensitively detect changes in stimulus level.
The tonic units maintain a steady firing rate during
stimulus presentation and give rise to slowly adapting
sensations. In insect muscle stretch receptor organs,
both phasic and tonic functions are subserved by a
single neuron. A sudden elongation of this receptor
induces firing initially at a high rate, which then
declines to a constant rate that is proportional to
static elongation, so that both velocity and position
are encoded (Finlayson and Lowenstein, 1958).
The complete absence of neural activity in poisoned
insects indicates that SCBIs block not only
tonic sensory activity, but also pacemaker activity
in the CNS. Both of these effects involve action
potential generation in regions of neurons that are
able to generate action potentials repetitively in
response to constant stimuli. The ability of phasic
receptors to respond long after paralysis at a high
dose (Figure 6), suggests that phasic receptors are not
as sensitive as the tonic ones, although they may also
be affected at higher doses in Lepidoptera, when
insects are completely paralyzed, as mentioned earlier.
At this point in the electrophysiological analysis,
the pseudoparalysis resulting from SCBI poisoning is
apparently due to inhibition of spontaneous activity
2: Indoxacarb and the Sodium Channel Blocker Insecticides 41
Figure 6 Sensory and motor nerve activity could still be elicited long after the onset of paralysis. These recordings, from nerve 5
(crural nerve) of a cockroach injected 24 h earlier with 5 mgg 1 RH-3421, show that although there was no background activity
(trace a), tactile stimulation of the ipsilateral trochanter elicited activity in several axons (trace b). Furthermore, cutting nerve 5
proximal to the recording site abolished the larger spikes (trace c), showing that the remaining smaller spikes were in primary
sensory neurons while the larger ones in trace (b) were reflexly evoked motor spikes. (Reproduced with permission from Salgado,
V.L., 1990. Mode of action of insecticidal dihydropyrazoles: selective block of impulse generation in sensory nerves. Pestic. Sci. 28,
389–411; ß Society of Chemical Industry, permission is granted by John Wiley & Sons Ltd. on behalf of the SCI.)
in tonic sensory receptors and pacemaker neurons.
However, excitatory symptoms are also seen during
the course of poisoning; a plausible hypothesis is that
early in poisoning, before the compound reaches the
CNS, sensory receptors in the periphery become
blocked. In the absence of adequate sensory feedback
to the CNS in response to attempted movements,
there would be a tendency to overaccentuate
these movements, resulting in the altered posture
and gait that is observed. The quiet periods that
predominate later in pseudoparalysis may be due to
block of pacemaker activity in the CNS. An explanation
of the tremors will be proposed below, after
the cellular effects of the compounds that cause the
observed block are considered. What is noteworthy,
however, about the excitatory symptoms arising
from a blocking action is the long time period over
which apparent excitatory effects can be seen. Compounds
with primary excitatory effects on neurons
lead to continuous excitation, and within a few
hours to complete paralysis due to, among other
things, neuromuscular block, as seen for example
with pyrethroids (Schouest et al., 1986) or spinosad
(Salgado, 1998). Insects poisoned with SCBIs,
alternatively, do not suffer this pysiological exhaustion,
and can therefore show strong tremors
intermittently for several days after poisoning.
2.4.3. Block of Na + Channels in Sensory Neurons
A deeper understanding of the mode of action of
the SCBIs required the investigation of their cellular
effects on sensory neurons. Abdominal stretch
receptors from cockroach adults and M. sexta larvae
were shown to be blocked by various pyrazolines