(+)-Borneol Protects Dopaminergic Neuronal Loss in Methyl-4-phenyl-1,2,3,6-tetrahydropyridine-Induced Parkinson’s Disease Mice: A Study of Dopamine Level using In Vivo Brain Microdialysis

Considerable research efforts have been directed toward the symptom relief of Parkinson's disease (PD) by attenuating dopamine (DA) depletion. One common feature of these existing therapies is their unavailability of preventing the neurodegenerative process of dopaminergic neurons. (+)-Borneol, a natural highly lipid-soluble bicyclic monoterpene, has been reported to regulate the levels of monoamine neurotransmitters in the central nervous system and exhibit neuroprotective effects. However, the effect of (+)-borneol on the dopaminergic neuronal loss of methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD mice is not defined. Herein, we first report that 30 mg/kg (+)-borneol significantly attenuated the motor deficits of PD mice, which benefits from markedly increasing the level of DA and decreasing the metabolic rate of DA in the striatum of conscious and freely moving mouse detected by ultraperformance liquid chromatography tandem mass spectrometry online combined with in vivo brain microdialysis sampling. It is worth noting that the enhanced level of DA by (+)-borneol was enabled by the reduction in loss of tyrosine hydroxylase-immunoreactive dopaminergic neurons in the substantia nigra and striatum and promotion of reserpine- or nomifensine-induced DA release in PD mice. Interestingly, (+)-borneol evidently inhibited the decreased expression levels of DA transporter (DAT) and vesicular monoamine transporter 2 (VMAT2) on the MPTP mouse model of PD. Moreover, (+)-borneol suppressed the neuroinflammation by inhibiting the production of IL-1β, IL-6, and TNF-α and attenuated oxidative stress by decreasing the level of MDA and increasing the activities of SOD and GSH-px in PD mice. These findings demonstrate that (+)-borneol protects DA neurons by inhibiting neuroinflammation and oxidative stress. Further research work for the neuroprotection mechanism of (+)-borneol will focus on reactive oxygen species-mediated apoptosis. Therefore, (+)-borneol is a potential therapeutic candidate for retarding the neurodegenerative process of PD.