Chemical profiling, anticonvulsant and anxiolytic effects of the smoke constituents isolated from Leonotis leonurus (L.) R.Br.

Ethnopharmacological Relevance: The use of medicinal plants for central nervous system (CNS)-related ailments, such as epilepsy and anxiety, is prevalent in South Africa. Plants from the Lamiaceae family are commonly used for their therapeutic benefits. Leonotis leonurus (L.) R.Br. has been reported in ethnobotanical literature to have anticonvulsant and anxiolytic effects through the inhalation of pyrolysis products obtained by combustion of the aerial parts.

Aim And Objectives: To explore the chemical profiles and CNS activity of the smoke extract and isolated constituents of L. leonurus in zebrafish larvae, through anticonvulsive and anxiolytic activity assays.

Materials And Methods: The smoke extract of L. leonurus was obtained through the combustion of the aerial parts of the plant using a custom-built smoke recovery apparatus. The chemical profile of the smoke constituents was determined using Ultra-Performance Liquid Chromatography coupled with Mass Spectrometry (UPLC-MS). Targeted compounds were subjected to preparative High-Performance Liquid Chromatography for separation before structure elucidation using Nuclear Magnetic Resonance (NMR). The maximum tolerated concentrations, as well as the anxiolytic activity of the smoke extract were determined in five days post fertilisation zebrafish larvae. Reverse-thigmotaxis and locomotor activity of larvae in the light/dark transition assay were used to determine anxiolytic activity. Zebrafish larvae at six days post fertilisation (dpf) were subjected to several concentrations of the smoke constituents of L. leonurus. The baseline locomotor activity of the larvae was tracked for 30 min, prior to addition of pentylenetetrazole (PTZ) to induce seizure-like behaviour in the larvae, after which the locomotor activity of the larvae was once again tracked for an additional 30 min.

Results: The UPLC-MS profiles of the smoke extract revealed the presence of two main compounds, leoleorin A and leoleorin B, which were targeted and isolated. Upon subjection to NMR spectroscopy for structure elucidation, the compounds were confirmed to be labdane diterpenoids. Both leoleorin A and leoleorin B, and the smoke extract displayed suppression of the PTZ induced seizure-like behaviour in zebrafish larvae. Under light and dark conditions, the smoke extract and compounds displayed potential anxiolytic activity at different concentrations.

Conclusion: Our results suggest that the smoke constituents of L. leonurus may exert anticonvulsant and anxiolytic effects which align with the traditional indications and the mode of administration.