N-Methyl-D-aspartate (NMDA) and cannabinoid CB₂ receptors form functional complexes in cells of the central nervous system: insights into the therapeutic potential of neuronal and microglial NMDA receptors

Background: The cannabinoid CB₂ receptor (CB₂R), which is a target to afford neuroprotection, and N-methyl-D-aspartate (NMDA) ionotropic glutamate receptors, which are key in mediating excitatory neurotransmission, are expressed in both neurons and glia. As NMDA receptors are the target of current medication in Alzheimer’s disease patients and with the aim of finding neuromodulators of their actions that could provide benefits in dementia, we hypothesized that cannabinoids could modulate NMDA function. Methods: Immunocytochemistry was used to analyze the colocalization between CB₂ and NMDA receptors; bioluminescence resonance energy transfer was used to detect CB₂-NMDA receptor complexes. Calcium and cAMP determination, mitogen-activated protein kinase (MAPK) pathway activation, and label-free assays were performed to characterize signaling in homologous and heterologous systems. Proximity ligation assays were used to quantify CB₂-NMDA heteromer expression in mouse primary cultures and in the brain of APP_Sw/Ind transgenic mice, an Alzheimer’s disease model expressing the Indiana and Swedish mutated version of the human amyloid precursor protein (APP). Results: In a heterologous system, we identified CB₂-NMDA complexes with a particular heteromer print consisting of impairment by cannabinoids of NMDA receptor function. The print was detected in activated primary microglia treated with lipopolysaccharide and interferon-γ. CB₂R activation blunted NMDA receptor-mediated signaling in primary hippocampal neurons from APP_Sw/Ind mice. Furthermore, imaging studies showed that in brain slices and in primary cells (microglia or neurons) from APP_Sw/Ind mice, there was a marked overexpression of macromolecular CB₂-NMDA receptor complexes thus becoming a tool to modulate excessive glutamate input by cannabinoids. Conclusions: The results indicate a negative cross-talk in CB₂-NMDA complexes signaling. The expression of the CB₂-NMDA receptor heteromers increases in both microglia and neurons from the APP_Sw/Ind transgenic mice, compared with levels in samples from age-matched control mice.