In vivo striatal measurement of hydroxytyrosol, and its metabolite (homovanillic alcohol), compared with its derivative nitrohydroxytyrosol

Phenolic compounds were measured by in vivo brain microdialysis in rat striatum. Basal extracellular levels of hydroxytyrosol, homovanillic alcohol and nitro-hydroxytyrosol were not detectable by HPLC with electrochemical detection. However, systemic administration of hydroxytyrosol (20 and 40 mg/kg, i.p.) showed a clear increase in the extracellular level of this compound. This increase was accompanied by an increase in the extracellular level of homovanillic alcohol, a metabolite of hydroxytyrosol formed by catechol-O-methyltransferase activity. Perfusion of hydroxytyrosol (20 μM) through the microdialysis cannula also produced an increase in the extracellular level of homovanillic alcohol. Systemic administration of nitro-hydroxytyrosol (20 and 40 mg/kg, i.p.) produced a small increase in the extracellular level of this compound. Our data show that hydroxytyrosol is a more brain penetrant phenolic compound than nitro-hydroxytyrosol. Accordingly, there is high cerebral metabolism of hydroxytyrosol to produce homovanillic alcohol by catechol-O-methyltransferase activity, that is saturated at the higher administered dose of hydroxytyrosol.