Anxiety-Reducing Tropical Plants: Phytochemical and Pharmacological Characterization of Souroubea sympetala and Piper amalago

Abstract

This thesis investigates the phytochemistry and pharmacology of two neotropical plants used traditionally to treat anxiety and stress, Souroubea sympetala (Marcgraviaceae) and Piper amalago (Piperaceae). A method of phytochemical analysis was developed to characterize S. sympetala extracts, identifying and quantifying four triterpenes, which were present in higher amounts in bark as compared to leaf. Subsequently, a standardized supercritical CO2 extraction procedure for S. sympetala was developed and compared favourably with conventional extraction methods in terms of its anxiety-reducing effects in a behavioural assay of anxiety and content of the active principle, betulinic acid (BA). All of these materials demonstrated anxiolytic properties. The pharmacological mode of action of S. sympetala raw plant, extracts and isolated active principle were examined in rodent behavioural models of anxiety. The extracts were shown to have affinity for the γ-amino butyric acid (GABA)a benzodiazepine (GABAa- BZD) receptor of the central nervous system in vitro, in a competitive binding assay. Pre-treatment of animals with the GABAa-BZD antagonist flumazenil, followed by plant extract and pure compound extinguished the anxiety-reducing effect, demonstrating that S. sympetala and BA act at the GABAa- BZD receptor in vivo. The effect of S. sympetala in stressed animals, specifically its cortisol-lowering ability was investigated in vitro and in vivo in rainbow trout. Both leaf extract and BA significantly lowered cortisol in response to an adrenocorticotropic hormone (ACTH) challenge in vitro and a standardized net restraint assay in vivo. The anxiety-reducing effect of P. amalago was examined and the bioactive principle identified by bioassay-guided fractionation. P. amalago extract significantly reduced anxiety-like behaviour in rats and demonstrated affinity for the GABAa-BZD receptor in vitro. The bioactive molecule was determined to be a furofuran lignan. Together these results provide a pharmacological basis for the traditional use of S. sympetala and P. amalago to treat anxiety and elucidate their mode of action and active principles. S. sympetala is now thoroughly characterized and represents an excellent candidate plant for development as a natural health product.