Antihistamines as neuroprotective agents / Hester Elizabeth Badenhorst

Abstract

Biomolecules are continuously exposed to potentially harmful oxidative stress as a consequence of free radical formation. Increased free radical generation has been associated with aging as well as neurodegenerative disorders. Antioxidants affect processes associated with oxidative stress by quenching free radicals and acting as oxygen scavengers. Parkinson's disease results from a deficiency in the neurotransmitter, dopamine and it is also evident that damage caused by free radicals play an important role in the progression of this neurodegenerative disorder. The histamine HI antagonist, diphenhydramine is used to treat mild cases of Parkinson's disease and cimetidine can scavenge hydroxyl radicals. The histamine H3 antagonists are known to promote the release of dopamine. Together with a free radical scavenging activity, these compounds might have a dual therapeutic effect in the reduction of the progression of Parkinson's disease. The aim of this study was thus to determine whether histamine antagonists could act as free radical scavengers and to compare the results with aspirin, a known free radical scavenger. Ibuprofen was included to compare the free radical scavenging activities of aspirin to another non-steroidal anti-inflammatory drug. The free radical scavenging activities of the following compounds were determined and compared: diphenhydramine; cimetidine; roxatidine; clobenpropit; impentarnine; thioperamide; aspirin and ibuprofen. The ORAC assay was used to determine whether the test compounds were able to scavenge peroxyl radicals and the FRAP assay was used to determine the reducing abilities of the compounds. No meaningll results were obtained from these two assays, suggesting that the compounds were not able to act as direct antioxidants or as peroxyl radical scavengers. The comet assay was used to determine whether the compounds were able to reduce oxidative damage after MPTP administration. No damage was however obtained after a single dose of MPTP and it is suggested that one year old mice and chronic rather than acute treatment is used. Using a cyanide model to induce neurotoxic effects in rat brain homogenates, the neuroprotective properties of the histamine antagonists were examined and compared to aspirin. Superoxide anion levels and malondialdehyde concentrations were assessed using the nitrobluetetrazolium and lipid peroxidation assays. Clobenpropit and thioperamide significantly reduced superoxide anion generation and lipid peroxidation. At a concentration of lmM, these two histamine Hj antagonists reduced lipid peroxidation to values lower than that of the control. Impentamine reduced lipid peroxidation at all concentrations used and superoxide anion generation at a concentration of 1mM. Diphenhydramine (0.25 and 0.5mM) significantly reduced both variables at lower concentrations. Cimetidine (1mM) was able to reduce superoxide anion generation and significantly reduced lipid peroxidation at all concentrations Abstract used. Roxatidine (0.5mM and 1mM) significantly reduced the rise in superoxide anion generation and significantly reduced malondialdehyde concentration in a dose dependent manner. Ibuprofen significantly decreased superoxide anions in a dose dependant manner and lipid peroxidation at a concentration of 1mM. In the lipid peroxidation assay, all the drugs compared favourably to aspirin. The in vivo free radical scavenging effects of the selected compounds were also examined with the nitroblue tetrazoliurn and lipid peroxidation assays. MPP' was used to induce a Parkinsonian like condition. Diphenhydramine, ibuprofen, thioperamide and clobenpropit significantly reduced free radical generation in both assays. Thioperamide and clobenpropit were able to reduce lipid peroxidation and superoxide anion generation to values lower than that of the control, suggesting that these two compounds could be able to attenuate normal free radical processes in the brain. Cimetidine did not have the expected in vivo scavenging effects and it is suggested that blood-brain barrier permeability might play a role. All the compounds, except diphenhydramine had significantly lower in vivo values than aspirin. The superoxide anion plays an important role in the formation of further free radicals. It leads to the formation of peroxyl radicals during the inititation step of lipid peroxidation and also leads to the generation of hydroxyl radicals, where transition metals like iron, is a key factor. Diphenhydramine at lower concentrations, and the newly discovered histamine Hj antagonists, clobenpropit and thioperamide significantly reduced superoxide anion generation and lipid peroxidation. Although the compounds did not have meaningful effects in the FRAY and ORAC assay, their significant ability to reduce lipid peroxidation and superoxide levels make them promising tools to attenuate oxidative damage. This study demonstrates the potential of these agents to be neuroprotective with a dual therpeutic effect by exerting a scavenging effect on superoxide anions and increasing dopamine levels.