The Interplay Between CB₂ and NMDA Receptors in Parkinson's Disease

Parkinson's disease (PD) is a progressive neurological disorder that affects movement, causing symptoms such as tremors, stiffness, slowness, and balance problems due to the degeneration of dopamine-producing neurons in the brain. Nowadays there is no cure for PD. Alpha synuclein (α-syn) aggregates, which are a hallmark of PD, are known to induce microglial activation, specifically the detrimental M1 microglial phenotype, which contributes to neuroinflammation and disease progression. Cannabinoid receptor 2 (CB₂R) activation has been shown to counteract neuroinflammation. CB₂R is able to interact with N-methyl-D-aspartate (NMDA) receptors (NMDAR), which has also attracted attention in PD research due to its role in excitotoxicity. Here we aimed to study the interaction between CB₂R and NMDAR in a PD context in rat tissue. We observed that α-syn fibrils alter CB₂R activation and CB₂R-NMDAR heteromerization in a heterologous expression system. Furthermore, activation of CB₂R counteracted NMDAR signaling. In microglia, α-syn fibrils decreased CB₂R-NMDAR heteromer expression while increasing CB₂R signaling. Importantly, CB₂R activation counteracted the α-syn fibrils-induced increase in M1-activated microglia, while it favored the polarization of microglia to the beneficial M2 phenotype. These results reinforce the idea of using cannabinoids for treating PD, as they provide not only the anti-inflammatory effects of cannabinoids but also counteract the detrimental increase in NMDAR signaling present in this disease.