Background: The active ingredients of essential plant oils appear as potentially effective antinematodal drugs or substances that can potentiate the action of already-existing anthelmintics. So far, we have verified that, aside from the direct effect on the neuromuscular system of nematodes, some of them can potentiate the effects of drugs that are agonists or antagonists of nematode cholinergic receptors. Methods: In this study, the antinematodal effects of geraniol and carvacrol were compared, as well as their interaction in the experimental model Caenorhabditis elegans, on the contractile properties of Ascaris suum neuromuscular preparations and on the ACR-16 nicotinic acetylcholine receptor (nAChR) of A. suum expressed in Xenopus leavis oocytes. Results: The combination of geraniol and carvacrol showed a synergistic nematocidal effect in the tests on C. elegans, reducing the value of individual LC₅₀ by almost 10-times. This combination also exerted a synergistic inhibitory effect on the contractions of A. suum, significantly increased the EC₅₀ of ACh and reduced the maximal contractile effect. The synergistic interaction of these two monoterpenes on Asu-ACR-16 nAChR expressed in Xenopus oocytes resulted in a significant decrease in the maximum current, while the ACh EC₅₀ value remained unchanged. Conclusions: Our findings provide a better understanding of the mode of action of monoterpene plant compounds. The possible antiparasitic application of active ingredients of essential plant oils that exhibit a synergistic anthelmintic effect represents an important basis for the development of new drugs and new therapeutic procedures.

Background: The active ingredients of essential plant oils appear as potentially effective antinemtodal drugs or substances that can potentiate the action of already existing anthelmintics. So far, we have verified that, aside from the direct effect on the neuromuscular system of nematodes (by inhibiting or potentiating contractility), some of them can potentiate the effects of drugs that are agonists or antagonists of nematode cholinergic receptors. Methods: In this study the antinematodal effects of geraniol and carvacrol was compared, as well as their interaction in the experimental model Caenorhabditis elegans, on the contractile properties of Ascaris suum neuromuscular preparations and on the ACR-16 nicotinic acetylcholine receptor (nAChR) of A. suum expressed in Xenopus leavis oocytes. Results: Combination of geraniol and carvacrol, showed a synergistic nematocidal effect in the tests on C. elegans, reducing the value of individual LC50 for almost 10 times. This combination also exerted a synergistic inhibitory effect on the contractions of A. suum, and significantly increased the EC50 of ACh and reduced the maximal contractile effect. The synergistic interaction of these two monoterpenes on Asu-ACR-16 nAChR expressed in Xenopus oocytes resulted in a significant decrease of the maximum current while the ACh EC50 value remained unchanged. Conclusions: Our findings bring a better understanding on the mode of action of monoterpene plant compounds. The possible application of active ingredients of essential plant oils that exhibit a synergistic anthelmintic effect represents an important basis for the development of new drugs and new therapeutic procedures.

Based on the previously known therapeutic properties of the active principles from essential plant oils, we investigated the antinociceptive effects of p-cymene (PC) and cinnamaldehyde (CNA) on carrageenan (CG)-induced inflammatory hyperalgesia in female Wistar rats, as well as their relationship with the L-arginine-nitric oxide pathway. Hyperalgesia was induced by intraplantar administration of CG (500 µg) into the rat hind paw, lasting 6 hours. The electronic von Frey apparatus measured the paw withdrawal threshold induced by pressure. Motor coordination in PC-treated animals was assessed using the Rota-rod test. PC and CNA (5, 25, 50 mg/kg BW), administered orally 50 minutes before CG, reduced CG-induced hyperalgesia, with PC showing a significantly dose-dependent (p<0.05, p<0.01, p<0.001) and stronger antinociceptive effect (p<0.001) than CNA. Compared to diclofenac (10 mg/kg), PC (50 mg/kg) demonstrated superior antinociceptive activity (p<0.001), while CNA (50 mg/kg) had a lower effect (p<0.05, p<0.01, p<0.001). Co-administration of PC (5 mg/kg) or CNA (5 mg/kg) with NOS inhibitors L-NAME (5 mg/kg) or AG (0.3 mg/kg) significantly enhanced the antinociceptive effect (p<0.01, p<0.001), with PC+L-NAME showing greater potentiation (p<0.001) than PC+AG. L-ARG (10 mg/kg), an NO donor, significantly reduced or reversed the antinociceptive effect of PC/CNA+NOS inhibitors (p<0.05, p<0.01, p<0.001). In the Rota-rod test, PC (100 mg/kg BW) did not impair motor coordination or cause CNS depression in female rats. Given their significant antinociceptive effects on CG-induced hyperalgesia, the close relationship between the L-arginine-NO system and their mechanism of action, and the fact that PC does not adversely affect the CNS, both PC and CNA are promising candidates for the development of new analgesic drugs in veterinary practice

The parasitic nematode Ascaris suum is an economically significant worm that infects pigs and causes health issues and economic losses in pig farming. The intestine of A. suum is a long, straight tube that runs from the mouth to the anus and can be colonized by bacteria. Several bacteria have been studied for their potential antinematocidal effects. Bacillus thuringiensis (Bt), a soil bacterium known to produce many toxic crystal proteins (Cry), is emerging as a promising candidate for nematode control. Treatment of A. suum with all tested Bt strains (SS_26.2, SS_29.2, SS_35.1, SS_37.7) resulted in histopathological changes in the parasite intestine. Our research highlights the anthelmintic effect of Cry proteins and emphasizes the potential of Bt as an alternative tool for controlling parasitic nematodes of domestic animals. The studied strains are promising for the eradication of A. suum as a novel, environmentally friendly and cost-effective One Health approach, but further testing in pigs is needed to confirm these findings.

The active constituents of essential plant oils (EOAIs), monoterpenoid carvacrol and monoterpene p-cymene, are widely distributed in many aromatic plants and their products. They differ in that carvacrol has a phenolic functional group. The numerous pharmacological effects of these two EOAIs are well known. In different doses/concentrations, they exhibit analgesic, neuroprotective, vasorelaxant, anti-inflammatory, antiviral, antibacterial and antiparasitic effects. The acute toxicity of carvacrol and p-cymene in rats and the free-living nematode Caenorhabditis elegans was investigated. Furthermore, the impact of subacute administration of these two terpenes on general health, CNS integration, i.e., motor coordination and balance of rats, as well as their effects on the movement of adult C. elegans, was also examined. The aim was to compare the effects and describe in more detail the selective toxicity of carvacrol and p-cymene. The calculated LD₅₀ value of carvacrol was 790.15 ± 1.15 mg/kg, while the LD₅₀ value of p-cymene is above 3000 mg/kg. Tested doses of carvacrol and p-cymene administered for 28 days (50, 100, and 200 mg/kg) did not exert any effect on the CNS of rats or cause any clinical disorders. LC₅₀ value of carvacrol for adult C. elegans was 184.13 ± 1.51 μM and for p-cymene 1268 ± 1.65 μM. In subacute testing, carvacrol showed negative effects on C. elegans reproduction, distance traveled, movement speed and rotational index at lower concentrations than p-cymene, indicating higher toxicity, which may be due to its phenolic structure. On the other hand, although less toxic to C. elegans, p-cymene exhibited a specific effect on worm motility, with more rolling which should be further investigated, and can be a consequence of cuticle damage or loss of orientation.