Medicinal Lamiaceae

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There is an increasing body of evidence to suggest that flavonoids and diterpenes of Lamiaceae might be of value as source of original antagonists of the GABAergic neurotransmission. Flavonoids in particular possess partial allosteric modulatory action at the GABAa receptor complex and, therefore, constitute a possibly promising group of naturally occurring agents for the treatment of anxiety disorders. Huen et al. made the interesting discovery that 2'-hydroxyl substitution is a critical moiety on flavonoids with regard to benzodiazepine receptor affinities. Benzodiazepine receptor binding assay-guided fractionation of the methanol extract from sage leaves (Salvia officinalis L.), for instance, revealed flavones and abietane diterpenes functioning as benzodia-zepine receptor-active components. Apigenin, hispidulin, and cirsimaritin competitively inhibit the binding of 3H-flumazenil to the benzodiazepine receptor with respective IC50 values of 30, 1.3, and 350 (iM (46,47).

The medicinal Lamiaceae, with about 250 species, represents a vast source of material to explore when looking for anxiolytic agents of clinical value. Among these medicinal species are Scutellaria baicalensis Georgi and Leonotis nepetifolia, the properties of which are described here.

Scutellaria baicalensis Georgi (Scutellaria micrantha Fisch, Scutellaria lanceolaria Miq.), or Baical scullcap, Chinese scullap, or huang ch'in (Chinese), is a perennial herb that grows from a rhizome to a height of 1.20 m. It is common on sunny, grassy slopes, waste and cultivated areas, from 100 to 2000 m in China, Japan, Korea, Mongolia, and the steppes of Siberia. The stems are erect, much branched, quadrangular, and puberulent. The leaves are simple, decussate, and linear-lanceolate. The influorescences are terminal racemes of up to 15 cm long. The flowers are tubular, labiate, dark blue, purple-red to blue, and up to 3 cm long (Fig. 50).

The drug consists of the roots, usually cut into slices is used to promote urination, to quiet pregnant uterus, stimulate respiratory organs, reduce fever, treat jaundice, diarrhea, cancer of the breast, and heal boils. In Korea, the plant is used to treat bacterial infection of respiratory and gastrointestinal tracts and fever. The plant has attracted a great deal of interest as it elaborates a series of flavones: baicalein, wooing, and oroxy-lina, which bind antagonistly to the benzodiazepine binding site GABAA receptor (48-50).

5,7-Dihydroxy-6-methoxyflavone (oroxylin A) inhibits the binding of [3H]fluni-trazepam to rat cerebral cortical membrane with a IC50 value of 1.09 (iM. oral administration of oroxylin A (3.75-60 mg/kg) attenuated the anxiolytic, myorelaxant, and motor incoordination elicited by diazepam. Oroxylin A or wogonin given orally to animals (7.5-30 mg/kg) is anxiolytic similarly to diazepam (Valium®) through positive allosteric modulation of the GABAA receptor complex via interaction at the benzodi-azepine site (Fig. 51).

Fig. 50. Scutellaria baicalensis Georgi.

It is interesting to note that GABA and glutamine have been characterized from extracts of Scutellaria lateriflora L. (American skullcap) at 1.6 and 31 mg/g, respectively (51). Leonotis nepetifolia R.Br, or annual lion's ear, is an annual herb that grows to a height of 2 m. The plant is native to Africa and is commonly grown as an ornamental. The stems are quadrangular. The leaves simple, decussate, and up to 12 cm long. The internodes are 20 cm long. The influorescences are axillary, globular, and spiny (Fig. 52). The flowers are tubular, red, and about 2.5 cm long. In Malaysia, a paste of leaves is applied to wounds. The neuropharmacological potential of this plant is unexplored. Note that an aqueous extract of Leonotis leonurus in the doses of 200 and 400 mg/kg, respectively, abbrogated the onset of tonic seizures produced by picrotoxin, hence some GABAergic activities (52). Aqueous extracts of the plant showed some levels of activity on guinea pig smooth muscle and rat cardiac muscle (53). Is 4,6,7-trimethoxy-5-methylchromen-2-one involved here (54)? An interesting development from these observations is the fact that flavonoids bind to GABAA receptors.

Diazepam (Valiumfi)

Oroxylin A

Oroxylin A

Fig. 51. Valium® and GABAergic flavonoids of Scutellaria baicalensis Georgi.

Baicalein Wogonin

Fig. 51. Valium® and GABAergic flavonoids of Scutellaria baicalensis Georgi.

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