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      Anesthetic Agents of Plant Origin: A Review of Phytochemicals with Anesthetic Activity

      Molecules
      MDPI

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          Thymol, a constituent of thyme essential oil, is a positive allosteric modulator of human GABA(A) receptors and a homo-oligomeric GABA receptor from Drosophila melanogaster.

          The GABA-modulating and GABA-mimetic activities of the monoterpenoid thymol were explored on human GABAA and Drosophila melanogaster homomeric RDLac GABA receptors expressed in Xenopus laevis oocytes, voltage-clamped at -60 mV. The site of action of thymol was also investigated. Thymol, 1-100 microm, resulted in a dose-dependent potentiation of the EC20 GABA response in oocytes injected with either alpha1beta3gamma2s GABAA subunit cDNAs or the RDLac subunit RNA. At 100 microm thymol, current amplitudes in response to GABA were 416+/-72 and 715+/-85% of controls, respectively. On both receptors, thymol, 100 microm, elicited small currents in the absence of GABA. The EC50 for GABA at alpha1beta3gamma2s GABAA receptors was reduced by 50 microm thymol from 15+/-3 to 4+/-1 microm, and the Hill slope changed from 1.35+/-0.14 to 1.04+/-0.16; there was little effect on the maximum GABA response. Thymol (1-100 microm) potentiation of responses to EC20 GABA for alpha1beta1gamma2s, alpha6beta3gamma2s and alpha1beta3gamma2s human GABAA receptors was almost identical, arguing against actions at benzodiazepine or loreclezole sites. Neither flumazenil, 3-hydroxymethyl-beta-carboline (3-HMC), nor 5alpha-pregnane-3alpha, 20alpha-diol (5alpha-pregnanediol) affected thymol potentiation of the GABA response at alpha1beta3gamma2s receptors, providing evidence against actions at the benzodiazepine/beta-carboline or steroid sites. Thymol stimulated the agonist actions of pentobarbital and propofol on alpha1beta3gamma2s receptors, consistent with a mode of action distinct from that of either compound. These data suggest that thymol potentiates GABAA receptors through a previously unidentified binding site.
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            Anti-inflammatory activity of linalool and linalyl acetate constituents of essential oils.

            Linalool and linalyl acetate are the principal components of many essential oils known to possess several biological activities, attributable to these monoterpene compounds. In this work, we evaluated individually the anti-inflammatory properties of (-) linalool, that is, the natural occurring enantiomer, and its racemate form, present in various amounts in distilled or extracted essential oils. Because in the linalool-containing essential oils, linalyl acetate, is frequently present, we also examined the anti-inflammatory action of this monoterpene ester. Carrageenin-induced edema in rats was used as a model of inflammation. The experimental data indicate that both the pure enantiomer and its racemate induced, after systemic administration, a reduction of edema. Moreover, the pure enantiomer, at a dose of 25 mg/kg, elicited a delayed and more prolonged effect, while the racemate form induced a significant reduction of the edema only one hour after carrageenin administration. At higher doses, no differences were observed between the (-) enantiomer and the racemate; a further increase in the dose of both forms did not result in an increased effect at any time of observation. The effects of equi-molar doses of linalyl acetate on local edema were less relevant and more delayed than that of the corresponding alcohol. These finding suggest a typical pro-drug behavior of linalyl acetate. The results obtained indicate that linalool and the corresponding acetate play a major role in the anti-inflammatory activity displayed by the essential oils containing them, and provide further evidence suggesting that linalool and linalyl acetate-producing species are potentially anti-inflammatory agents.
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              Flavonoids and the CNS

              Flavonoids are present in almost all terrestrial plants, where they provide UV-protection and colour. Flavonoids have a fused ring system consisting of an aromatic ring and a benzopyran ring with a phenyl substituent. The flavonoids can be divided into several classes depending on their structure. Flavonoids are present in food and medicinal plants and are thus consumed by humans. They are found in plants as glycosides. Before oral absorption, flavonoids undergo deglycosylation either by lactase phloridzin hydrolase or cytosolic β-glucocidase. The absorbed aglycone is then conjugated by methylation, sulphatation or glucuronidation. Both the aglycones and the conjugates can pass the blood-brain barrier. In the CNS several flavones bind to the benzodiazepine site on the GABAA-receptor resulting in sedation, anxiolytic or anti-convulsive effects. Flavonoids of several classes are inhibitors of monoamine oxidase A or B, thereby working as anti-depressants or to improve the conditions of Parkinson’s patients. Flavanols, flavanones and anthocyanidins have protective effects preventing inflammatory processes leading to nerve injury. Flavonoids seem capable of influencing health and mood.
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                Journal
                10.3390/molecules22081369
                https://creativecommons.org/licenses/by/4.0/

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