*2.1.1 Compounds acting via glutamate-gated chloride channels*

• Avermectins and milbemycins:

Avermectins and milbemycins (**Figure 4**) are two families of hemisynthetic macrolides that have been widely used as pesticides in agriculture. The hemisynthesis of these molecules produces an increase of the chemical diversity starting from the natural ones produces by bacteria from the order Actinomycetes (as for example *Streptomyces avermitilis*). As an example of these molecules, we can mention aglycone milbemycin (**Figure 4**), abamectin, emamectin, doramectin,

#### **Figure 4.** *Milbemycin aglycone synthesis [32].*

milbemycin oxime, latidectin etc. These molecules attack the nervous system of insects [33]. This macrocyclic lactones exert their parasiticide (anthelmintic) and insecticidal effects mainly by potentiating the agonistic action of glutamate on GluCls or by directly activating GluCls, as in the case of *Drosophila melanogaster* [31], [34]. They act by linking to glutamate-dependent chloride channels common to invertebrate nerve and muscle cells. This binding causes the opening of the channels, increasing the flow of chloride ions and hyperpolarizing the cell membranes, paralyzing and killing the invertebrate. **Figure 4** shows the hemisynthesis of aglycone milbemycin by solubilizing ivermectin in a concentrated sulfuric acid solution [32]. However, the milbemycins have characteristics that are harmless to the environment.

#### **2.2 Voltage-gated sodium channels (VGSC)**

The voltage-gated sodium channel (VGSC) mediates the increase in sodium conductance during the rapid depolarization phase of the membrane action potential (high concentration of sodium ions (Na<sup>+</sup> ) and a low concentration of potassium ions (K<sup>+</sup> )). Therefore, this channel represents a key structural element that controls cellular excitability in biological systems [35]. Mammalian sodium channels are composed of a pore-forming α-subunit and one or more β-subunits. Sodium channel α-subunits have four homologous repeat domains (I–IV), each possessing six α-helical transmembrane segments (**Figure 5**). There are no orthologs of mammalian β-subunits in insects. Instead, the non-orthologous proteins TipE and three to four TipE-homologs (TEH1–4) seem to serve as auxiliary subunits of sodium channels *in vivo*. Structurally, both TipE and TEH1 have intracellular N- and C-termini and two transmembrane segments connected by a large extracellular loop [37]. In physiological function, the flow of sodium ions into and out of the insect synapse occurs through the sodium channel present on the cell membrane of the neuron. This flow is controlled by the normal movements of the insect's muscles. When the sodium channel is open, the muscle is activated, when it is closed, the muscle can relax.

#### **Figure 5.**

*The α subunit of the voltage-gated sodium channel. A. Structure of the subunit (four domains (I-IV) and their six transmembrane segments (1–6)). B. Four states of the VGSC (channel closed: at resting membrane potentials; activated or opened channel: During the rising phase of an action potential; inactivated channel: Falling phase; deactivated channel: During the undershoot phase prior to returning to the closed phase) [36].*

## *2.2.1 Compounds acting via Voltage-Gated Sodium Channels*

• Pyrethroids

Pyrethroids are biomimetic molecules adapted from natural pyrethrins isolated from the flowers of *Chrysanthemum cinerariifolium* [38]. Cypermethrin and deltamethrin are two examples of pyrethroid compounds. Their synthetical pathway consists of a cyclopropanation reaction of an α,β-unsaturated ester derived from D-glyceraldehyde, giving a hemicaronaldehyde, which subsequently leads to deltamethrin (**Figure 6**) [39].

Pyrethroids have been used in pest control as the main insecticides. The mode of action of pyrethroids consists in the binding and modulation of the activity of the VGSC, leading to a prolonged opening of sodium channels, and a continuous firing of action potential [40]. This neurotoxic action produces in the insect hyperactivity and convulsions, followed by lethargy, paralysis and death [41, 42].

#### **2.3 Transient receptor potential vanilloid (TRPV) channels**

The transient receptor potential vanilloid (TPRV) channel is a subfamily of 6 cationic channels. They are tetrameric and each subunit is composed of 6 transmembrane domains with 3 to 5 N-terminal ankyrin repeats and a TRP box in

**Figure 6.** *Deltametrin synthesis.* their C-terminal (**Figure 7**). The activation of TRPV1 primarily permits an influx of extracellular Ca2+, which is involved in a number of essential physiological functions, such as neurotransmitter release, membrane excitability, and muscle cell contraction [44].
