Putative Mode of Action of the Monoterpenoids Linalool, Methyl Eugenol, Estragole, and Citronellal on Ligand-Gated Ion Channels

Essential oil has been used as sedatives,anticonvulsants,and local anesthetics in traditional medical remedies;as preservatives for food,fruit,vegetable,and grain storage;and as bio-pesticides for food production.Linalool(LL),along with a few other major components such as methyl eugenol(ME),estragole(EG),and citronellal,are the active chemicals in many essential oils such as basil oil.Basil oil and the aforementioned monoterpenoids are potent against insect pests.However,the molecular mechanism of action of these chemical constituents is not well understood.It is well-known that the c-aminobutyric acid type A receptors(GABAARs)and nicotinic acetylcholine receptor(nAChR)are primary molecular targets of the synthetic insecticides used in the market today.Furthermore,the GABAAR-targeted therapeutics have been used in clinics for many decades,including barbiturates and benzodiazepines,to name just a few.In this research,we studied the electrophysiological effects of LL,ME,EG,and citronellal on GABAAR and nAChR to further understand their versatility as therapeutic agents in traditional remedies and as insecticides.Our results revealed that LL inhibits both GABAAR and nAChR,which may explain its insecticidal activity.LL is a concentration-dependent,noncompetitive inhibitor on GABAAR,as the half-maximal effective concentration(EC50)values of c-aminobutyric acid(GABA)for the rat a1b3c2L GABAAR were not affected by LL:(36.2±7.9)lmol-1 and(36.1±23.8)μmol·L-1 in the absence and presence of 5 mmol·L-1 LL,respectively.The half-maximal inhibitory concentration(IC50)of LL on GABAAR was approximately 3.2 mmol·L-1.Considering that multiple monoterpenoids are found within the same essential oil,it is likely that LL has a synergistic effect with ME,which has been previously characterized as both a GABAAR agonist and a positive allosteric modulator,and with other monoterpenoids,which offers a possible explanation for the sedative and anticonvulsant effects and the insecticidal activities of LL.

The language of GABA in pollen tube growth and guidance

The achievement of double fertilization in higher plants requires the successful transport of sperm cells to the female gametes,the ovules.Pollen tubes,the tubular structure protruding from pollens,carrying the sperms play an important role in this process.How a pollen tube precisely guides its direction to gain its goal is of mystery.Previous investigation indicated that multiple signal clues from the pistils function as the route signs to regulate the pathway of pollen tube growth.Among the signal clues,c-aminobutyric acid(GABA)seems to be universal.Its gradient concentration has been found both in tobacco and Arabidopsis pollen tube guidance.In the communication of pollens and pistils,what on earth GABA tells pollen tubes is of great interest.The GABA receptors on the pollen membrane are thought to be the hinge in the language conversation.In this review,the mechanism of GABA gradient formation is investigated.The possible GABA receptor on the pollen membrane is examined and its function is discussed.To decipher the possible language of GABA in pollen tube growth and guidance,multiple methods are needed.The combination of transcriptome and proteomics assay is expected to unveil the secret.