Mechanisms of neuroprotection by capsaicin, a red pepper extract

Abstract

Background: Capsaicin is a natural compound isolated from red peppers that is currently used in the management of pain due to its ability to desensitise TRPV1 channels to further noxious stimuli following high or repeated doses (De Silva et al. 2011; Derry et al. 2013; Sharma et al. 2013). Recent studies have shown that capsaicin can also upregulate expression of the neuroprotective protein neuroglobin (Ngb), activate cell survival signalling pathways and diminish oxidative stress and inflammation (Kim et al. 2003; Dairam et al. 2008; Guo et al. 2008; Luqman et al. 2011; Lee et al. 2012). Therefore, it was hypothesised that capsaicin pre-treatment could protect neurons in a model of Parkinson’s Disease (PD). Aim: To test whether capsaicin prevents neuronal loss and restores motor function in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of PD. Methods: Male C57BL/6J mice were pre-treated with vehicle or capsaicin (1 mg/kg) prior to saline or MPTP (25 mg/kg/day) treatment over two consecutive days. Gait was assessed on a subset of mice six days after the onset of treatment. Serum and various organs were harvested for biochemical and histological analyses that assessed dopaminergic neuron (DN) counts, apoptosis and necrosis (using caspase-3/7 and LDH assays, respectively), alterations in kinase signalling (PI3K/Akt, ERK, mTOR and p38 MAPK), markers of inflammation (IL-1β, IL-6, TNF-α, IFN-γ and MCP-1), antioxidant activity (SOD, GPx and CAT) and oxidative damage (3-nitrotyrosine (3-NT)). The ability for capsaicin to cross the blood-brain barrier (BBB) and accumulate within the brain was also assessed by matrix assisted laser desorption/ionisation imaging mass spectrometry (MALDI-IMS). Results: The major daughter fragment (m/z 137.03 a.m.u.) of capsaicin appeared abundantly in capsaicin-treated brains and predominantly accumulated in the cerebral cortex. Mice exposed to MPTP experienced a 25% loss in DN viability with a concomitant increase in xi | P a g e caspase-3/7 activity. MPTP treatment also induced phosphorylation of Akt, ERK and p38 while mTOR remained unchanged. MPTP treatment increased SOD and decreased CAT activity and elevated 3-NT expression in the substantia nigra (SN). Cerebral levels of the pro-inflammatory markers TNF-α, IFN-γ, IL-1β and MCP-1 and expression of leukocytes markers, CD45 and Iba-1, were also elevated in MPTP-treated mice. In contrast, capsaicin pre-treatment elevated Akt and ERK phosphorylation and reduced p38 activation. Capsaicin pre-treatment also reversed the MPTP-induced increase in SOD and decrease in CAT activity and concomitantly reduced 3-NT expression. Furthermore, capsaicin pre-treatment reversed the MPTP-induced increase in pro-inflammatory cytokine expression, however no obvious improvement in neuronal viability was observed. Conclusions: Capsaicin was able to cross the BBB and accumulate within the brain. Although capsaicin pre-treatment did not improve neuronal viability in this MPTP model of PD, it did reverse the MPTP-induced increase in pro-inflammatory cytokine expression and decrease in antioxidant activity. Capsaicin-mediated protection may involve both TRPV1-dependent and -independent mechanisms and activation of various kinase signalling pathways, such as PI3K/Akt and p38 MAPK. Further studies are required to elucidate the precise mechanism of capsaicin-mediated protection but should also consider the possibility that capsaicin may generate toxic metabolites under oxidative stress