Evaluating the antidepressant-like properties of Sceletium tortuosum, alone and as adjunctive treatment

Abstract

BACKGROUND: Millions of people worldwide suffer from major depressive disorder (MDD) which can cause physical and emotional suffering for patients and their loved-ones. Unfortunately current treatment fails in approximately a third of MDD patients. This may be attributed to numerous side effects, delayed onset of action, and ineffective targeting of the many different, interrelated biological systems. This includes structural and functional brain alterations driven especially by decreased brain-derived neurotrophic factor (BDNF), and neurotransmitter abnormalities such as reduced monoamines (serotonin (5-HT), dopamine (DA) and norepinephrine (NE)). Evidently, new and more effective treatments are urgently needed. Fortunately, complementary medicines like plants and herbs with antidepressant effects offer untapped potential. This study set out to evaluate the antidepressant potential of the South African plant, Sceletium tortuosum (L.) N.E.Br. (Zembrin®) (ST), which displays diverse pharmacological attributes that offer potential antidepressant activity, e.g. 5-HT transporter (SERT) inhibition, upregulation of vesicular monoamine transporter 2 (VMAT-2), mild MAO-A inhibition, and inhibition of phosphodiesterase 4 (PDE4). These mechanisms have almost all been studied in vitro, leaving an urgent need for in vivo studies. Furthermore, ST has not been tested in combination with known antidepressant compounds to evaluate its potential as a possible augmentative therapy for MDD. AIM: To use acute and sub-chronic treatment paradigms with biobehavioural parameters to evaluate the antidepressant-like properties of ST, alone and in combination with the selective 5-HT reuptake inhibitor (SSRI), escitalopram (ESC), in the FSL rat. METHODOLOGY: 1) A fingerprint analysis of the alkaloid profile of ST was done using an ultra-performance liquid chromatography - tandem mass spectrometer (UPLC-MS). 2) Behavioural confirmation of the FSL model of MDD. 12 saline-treated FSL and 6 Flinders Resistant Line (FRL) rats (reference control) were used to confirm the depressive phenotype of the FSL rat, using the open field test (OFT) and forced swim test (FST). 3) Acute dose response studies in FSLs, using a 3-tier dose response with escitalopram oxalate (ESC) (3 groups (n = 10); 5, 10 or 20 mg/kg), and a 5-tier dose response with ST (5 groups (n = 10); 5, 10, 25, 50 or 100 mg/kg). Treatment spanned over 24 hours, followed by the OFT and FST. The results were used to establish a low-dose of ESC, and a therapeutic dose of ST. 4) A sub-chronic treatment response study wherein four groups of FSL rats (n = 12) received a) saline, b) low dose of ESC, c) a therapeutic dose of ST or d) combination therapy of ESC + ST, over 15 days. After the OFT and FST on days 13 and 15, animals were decapitated, with hippocampus and frontal cortex samples harvested for analysis of monoamines and BDNF. RESULTS: Four main alkaloids were identified and quantified in an UPLSC-MS chromatogram. FSL rats presented with significantly decreased swimming and struggling, and increased immobility versus FRL controls, thus reaffirming its face validity as a rodent model of MDD. ST and ESC showed dose-dependent antidepressant responses following acute treatment. ESC 5 mg/kg (ESC 5) was chosen as a low dose of ESC and ST 50 mg/kg (ST 50) was selected as the therapeutic dose of ST. Sub-chronically, ESC 5 and ST 50 alone displayed similar neurochemical changes with no significant antidepressant-like effects. ESC+ST significantly increased hippocampal 5-HT and NE, and locomotor activity, which is normally deemed positive in MDD treatment. However, increased immobility and decreased struggling are indicative of depressogenic effects. Furthermore, hippocampal 5-HT was significantly increased, possibly due to synergistic serotonergic effects of ST 50 and ESC 5. A hypersensitive inhibitory 5-HT1A response, characteristic of FSL rats, could have prompted 5-HT1A receptor-mediated “5-HT behavioural syndrome” presenting as passive coping in the FST and increased locomotion in the OFT. The apparent lack of antidepressant effects in the alone treatment groups may be due to delayed response attributed to delayed desensitization of inhibitory 5-HT1A receptors, typical of SSRIs. ST 50 alone increased hippocampal BDNF indicating potential of ST in the treatment of impaired cognition, ESC + ST reduced frontal cortical BDNF levels. CONCLUSION: ST and ESC induced dose-dependent antidepressant-like effects after acute treatment, however not after sub-chronic treatment. Combined sub-chronic treatment with ESC + ST appears to be depressogenic, displaying significant serotonergic activity at behavioural and neurochemical levels. ST 50 increasing hippocampal BDNF levels suggests promise in the treatment of mood and cognitive disorders.