| Parkinson's disease (PD) is characterized by extensive degeneration of dopamine (DA) neurons residing in the substantia nigra pars compacta (SNc) and the accompanying neuroinflammatory response. Important cellular mediators of neuroinflammation include activated microglia and reactive astrocytes. Together, these events (i.e. midbrain DA cell loss and neuroinflammation) appear to be progressive, resulting in a self-perpetuating cycle of neurodegeneration. Although current pharmacotherapies provide symptomatic relief, they fail to target the progressive nature of PD. Cannabinoid drugs modulate inflammatory processes and have been pursued as novel therapeutics in a variety of models of neurodegeneration. In this study, we investigated the protective effects of the non-selective cannabinoid receptor agonist WIN 55,212-2 (WIN) in mice treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a toxin-based model of PD that replicates the loss of midbrain DA neurons and glial activation observed in the human disease. WIN protected against loss of tyrosine hydroxylase positive (TH+) neurons from the SNc when administered as either: (1) a single pre-treatment 30 min before MPTP or (2) a short-term post-treatment over a 5 day period, beginning 24 hr after the last injection of MPTP. The protective effect of WIN (1 mg/kg, i.p.) pre-treatment was independent of its ability to inhibit the uptake of the neurotoxic metabolite 1-methyl-4-phenylyridinium (MPP+) via the dopamine transporter (DAT) as this dose of WIN failed to alter striatal DA clearance in vivo. Post-treatment with WIN (4 mg/kg, i.p.), 24 hr after the last injection of MPTP---a time point when MPP+ has been eliminated from the brain---significantly protected against MPTP-induced loss of midbrain DA neurons. Interestingly, genetic ablation of the CB1 receptor also significantly protected against midbrain DA cell loss, suggesting that: (1) WIN may exert its neuroprotective actions via targets other than CB1 receptors and (2) agonism at the CB1 receptor may actually be deleterious to these DA neurons and, thus, the concomitant stimulation of CB1 receptors by WIN may mask its "full" neuroprotective potential. In an effort to identify the molecular mechanism(s) responsible for the beneficial effects of WIN, we found that the CB2 receptor, which is normally not expressed in the ventral midbrain, is up-regulated in this region 3 days post-MPTP treatment and it co-localizes with activated microglia, which stain positive for MAC-1. Agonism of the CB2 receptor with WIN (4 mg/kg, i.p.) or the CB 2 agonist JWH 015 (JWH; 4 mg/kg, i.p.) reduced MPTP-induced elevation of MAC-1 back to control levels in ventral midbrain. This effect was reversed by the CB2 antagonist JTE 907 (JTE; 4 mg/kg, i.p., 20 min before WIN). In addition, WIN administration significantly reduced MPTP-induced up-regulation of the pro-inflammatory enzyme cyclooxygenase-2 (COX-2). Thus, WIN protects against MPTP neurotoxicity via (1) a DAT-dependent mechanism involving inhibition of MPP+ uptake and (2) a DAT-independent mechanism involving inhibition of microglial activation and COX-2 expression. In conclusion, cannabinoid drugs may represent a new therapeutic for slowing the progressive neurodegeneration that is observed in PD. |
